CA1235147A - High thermal conductivity ceramic body - Google Patents

Abstract

HIGH THERMAL CONDUCTIVITY CERAMIC BODY

ABSTRACT OF THE DISCLOSURE

A process for producing an aluminum nitride ceramic body having a composition defined and encompassed by polygon PlJFA4 but not including lines JF and A4F of Figure 4, a porosity of less than about 10% by volume, and a thermal conductivity greater than 1.00 W/cm.K at 25°C which comprises forming a mixture comprised of aluminum nitride powder containing oxygen, yttrium oxide, and free carbon, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges between points J and A4 of Figure 4, said compact having an equivalent % composition of Y, A1, O and N outside the composition defined and encompassed by polygon PlJFA4 of Figure 4, heating said compact to a temperature at which its pores remain open reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, O and N is defined and encompassed by polygon PlJFA4 but not including lines JF and A4E of Figure 4, and sintering said deoxidized compact at a temperature of at least about 1850°C producing said ceramic body.

Description

so ROD 16,446 HIGH THRILL CONDUCTIVITY CERAMIC BODY
The present invention relates to the production of a liquid phase sistered polycrystalline aluminum nitride body having a thermal conductivity greater than 1.00 W/cm-K at 25C and preferably at least 1.42 W/cm-K at 25C. In one aspect of the present process, aluminum nitride is deoxidized by carbon to a certain extent, and then it is further deoxidized and/or sistered by utilizing yttrium oxide to produce the present ceramic.
A suitably pure aluminum nitride single crystal, containing 300 ppm dissolved oxygen, has been measured to have a room temperature thermal conductivity of 2.8 W/cm.K, which is almost as high as that of Boo single crystal, which is 3.7 W/cm-K, and much higher than that of d -AYE single crystal, which is 0.44 W/cm.K. The thermal conductivity of an aluminum nitride single crystal is a strong function of dissolved oxygen and decreases with an increase in dissolved oxygen content. For example, the thermal conductivity of aluminum nitride single crystal having 0.8 wit% dissolved oxygen, is about 0.8 W/cm-K.
Aluminum nitride powder has an affinity for oxygen, especially when its surface is not covered by an oxide. The introduction of oxygen into the aluminum nitride lattice in aluminum nitride powder results in the formation of Al vacancies via the equation:

RD-16,~46 1 ~5~7 - `

3N-3 30 2 + V (1) (N-3) No (Al Thus; the insertion of 3 oxygen atoms on 3 nitrogen sites will for one vacancy on an aluminum site. The presence of oxygen atoms on nitrogen sites will probably have a neglig-isle influence on the thermal conductivity of Awn. however, due to the large difference in mass between an aluminum atom and a vacancy, the presence of vacancies on aluminum sites has a strong influence on the thermal conductivity of Awn and, for all practical purposes, is probably responsible for all of the decrease in the thermal conductivity of Awn.
There are usually three different sources of oxygen n nominally pure Awn powder. Source #l is discrete particles of Aye. Source #2 is an oxide coating, perhaps as Allah, coating the Awn powder particles. Source #3 is oxygen in solution in the Awn lattice. The amount of oxygen present in the Awn lattice in Awn powder will depend on the method of preparing the Awn powder. Additional oxygen can be introduced into the Awn lattice by heating the Awn powder at elevated temperatures. Measurements indicate that at ~1900C the Awn lattice can dissolve ~1.2 wit% oxygen. In the present invention, by oxygen content of Awn powder, it is meant to include oxygen present as sources #1, #2 and #3.
Also, in the present invention, the oxygen present with Awn powder as sources #1, #2 and #3 can be removed by utilizing free carbon, and the extent of the removal of oxygen by carbon depends largely on the composition desired in the resulting sistered body.
According to the present invention, aluminum ruptured powder can be processed in air and still produce a

-2-123~ 7 ROD 16,446 ceramic body having a thermal conductivity greater than 1.00 W/cm at 25~C, and preferably at least 1.42 W/cm~K at 25C.
In one embodiment of the present invention, the aluminum nitride in a compact comprised of particulate aluminum nitride of known oxygen content, free carbon and yttrium oxide, is deoxidized by carbon to produce a desired equivalent composition of Al, N, Y and O, and the deoxidized compact is sistered by means of a liquid phase containing mostly Y and O and a smaller amount of Al and N.
Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjunction with the figures accompanying and forming a part of the specification in which:
FIGURE 1 is a composition diagram (also shown as Figure 1 in U.S. Patent No. 4,547,471 issued October 15, 1985 and assigned to the assignee herein showing the subsolidus phase equilibria in the reciprocal ternary system comprises of Awn, YIN, YO-YO and AYE. Figure 1 is plotted in equivalent and along each axis of ordinates the equivalent % of oxygen is shown (the equivalent % of nitrogen is 100~ minus the equivalent of oxygen).
Along the axis of abscissas, the equivalent % of yttrium is shown (the equivalent of aluminum is 100%
minus the equivalent % of yttrium). In Figure 1, line ABCDEF but not lines CUD and EN encompasses and defines the composition of the sistered body of U.S. Patent No. 4,547,471. Figure 1 also shows an example of an ordinates-joining straight line ZZ' joining the oxygen contents of an Yule additive and an aluminum nitride powder. From the given equivalent % of yttrium and Al at any point on an ordinates-joining line passing through the polygon ABCDEF, the required amounts of yttrium additive and Awn for producing the 12~4~
ROD 16,446 composition of that point on the ordinates-joining line can be calculated;
FIGURE 2 is an enlarged view of the section of Figure 1 showing the composition of the polycrystalline body of U.S. Patent No. 4,547,471;
FIGURE 3 is a composition diagram showing the subsolidus phase equilibria in the reciprocal ternary system comprised of Awn, YIN, Yo-yo and Aye. Figure 3 is plotted in equivalent % and along each axis of ordinates the equivalent of oxygen is shown (the equivalent of nitrogen is 100%
minus the equivalent % of oxygen). Along the axis of abscissas, the equivalent % of yttrium is shown (the equivalent % of aluminum is 100~ minus the equivalent % of yttrium). In Figure 3, line, i.e. polygon, PlJFA4 but not including lines JO and A4F encompasses and defines thy composition of the sistered body produced by the present process; and FIGURE 4 is an enlarged view of the section of Figure 3 showing polygon PlJFA4.
Figures 1 and 3 show the same composition diagram showing the subsolidus phase equilibria in the reciprocal ternary system comprised of Awn, YIN, Yo-yo and Aye and differ only in that Figure 1 shows the polygon ABCDEF of U.S. Patent No. 4,547,471 and the line ZZ', whereas Figure 3 shows the polygon PlJFA4. The composition defined and encompassed by the polygon ABCDEF does not include the composition of the present invention.
Figures 1 and 2 were developed algebraically on the basis of data produced by forming a particulate mixture of YIN of predetermined oxygen content and Awn powder of predetermined oxygen content, and in a few instances a mixture of Awn, YIN and Yo-yo powders, under nitrogen gas, shaping the mixture into a compact under nitrogen gas and sistering the compact for time ~2~47 ROD 16,446 periods ranging from 1 to 1.5 hours at sistering temperatures ranging from about 1860C to about 2050C
in nitrogen gas at ambient pressure. More specifically, the entire procedure ranging from mixing of the powders to sistering the compact formed therefrom was carried out in non oxidizing atmosphere of nitrogen.
Polygon PlJFA4 of Figures 3 and 4 also was developed algebraically on the basis of data produced by the examples set forth herein as well as other experiments which included runs carried out in a manner similar to that of the present examples.
The best method to plot phase equilibria that involve oxynitrides and two different metal atoms, where the metal atoms do not change valence, is to plot the compositions as a reciprocal ternary system as is done in Figures 1 and 3. In the particular system of Figures l and 3 there are two types of non-metal atoms (oxygen and nitrogen) and two types of metal atoms (yttrium and aluminum). The Al, Y, oxygen and nitrogen are assumed to have a valence of +3, +3, -2, and -3, respectively. All of the Al, Y, oxygen and nitrogen are assumed to be present as oxides, nitrides or oxynitrides, and to act as if they have the aforementioned valences.
The phase diagrams of Figures 1 to 4 are plotted in equivalent percent. The number of equivalents of each of these elements is equal to the number of moles of the particular element multiplied by its valence. Along the ordinate is plotted the number of oxygen equivalents multiplied by 100% and divided by the sum of the oxygen equivalents and the nitrogen equivalents. Along the abscissa is plotted the number of yttrium equivalents multiplied by 100%
and divided by the sum of the yttrium equivalents and aluminum equivalents. All compositions of Figures 1 to 4 are plotted in this manner.

ROD 16,446 Compositions on the phase diagrams of Figures 1 to 4 can also be used to determine the weight percent and the volume percent of the various phases. For example, a particular point on the polygon PlJFA4 in Figure 3 or 4 can be used to determine the phase composition of the polycrystalline body at that point.
Figures 1 to 4 show the composition and the phase equilibria of the polycrystalline body in the solid state.
In U.S. Patent Jo. 4,547,471 entitled "High Thermal Conductivity Aluminum Nitride Ceramic Body, issued October 15, 1985, in the names of I. C. Huseby and C. F. Bobik and assigned to the assignee hereof there is disclosed the process for producing a polycrysalline aluminum nitride ceramic body having a composition defined and encompassed by line ABCDEF but not including lines CUD and EN of Figure 1 therein (also shown as prior art Figure 1 herein), a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.0 W/cm K at 22~C
which comprises forming a mixture comprised of aluminum nitride powder and an yttrium additive selected from the group consisting of yttrium, yttrium hydrides yttrium nitride and mixtures thereof, said aluminum nitride and yttrium additive having a predetermined oxygen content, said mixture having a composition wherein the equivalent of yttrium, aluminum, nitrogen and oxygen is defined and encompassed by line ABCDEF but not including lines CUD
and EN in Figure 1, shaping said mixture into a compact, and sistering said compact at a temperature ranging from about 1850C to about 2170C in an atmosphere selected from the group consisting of nitrogen, argon, hydrogen and mixtures thereof to produce said polycrystalline body.
U.S. Patent No. 4,547,471 also discloses a ~;~35147 ROD 16,446 polycrystalline body having a composition comprised of from greater than about 1.6 equivalent % yttrium to about 19.75 equivalent yttrium, from about 80.25 equivalent aluminum up to about 98.4 equivalent %
aluminum, from greater than about 4.0 equivalent oxygen to about 15.25 equivalent % oxygen and from about 84.75 equivalent % nitrogen up to about 96 equivalent % nitrogen.
U.S. Patent No. 4,547,471 also discloses a polycrystalline body having a phase composition comprised of Alp and a second phase containing Y and 0 wherein the total amount of said second phase ranges from greater than about 4.2% by volume to about 27.3%
by volume of the total volume of said body, said body having a porosity of less than about 10~ by volume of said body and a thermal conductivity greater than 1.0 W/cm K at 22C.
Briefly stated, the present process for producing the present sistered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by line, i.e. polygon, PlJFA4 but not including lines JO and A4F of Figures 3 or 4, a porosity of less than about 10~ by volume, and preferably less than about I by volume, of said body and a thermal conductivity greater than 1.00 W/cm K
at 25C, and preferably at least 1.4 W/cm K at 25C
comprises the steps.
(a) forming a mixture comprised of aluminum nitride powder containing oxygen, yttrium oxide or precursor therefore and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50C
to about 1000C to free carbon and gaseous product of decomposition which vaporizes away, shaping said , RD-16,446 :lZ3~;14~

mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges between points J and A of Figures 3 or 4, which is from greater than about 0.3 equivalent % to less than about 2.5 equivalent % yttrium and from greater than about 97.5 equivalent % to less than about 99.7 equivalent %
aluminum, said compact having an equivalent % composition of Y, Al, 0 and N outside the composition defined and encompassed by polygon PlJFA4 of Figures 3 or 4, lo (b) heating said compact in a non oxidizing atmosphere at a temperature up to about 1200C thereby providing yttrium oxide and free carbon, (c) heating said compact in a nitrogen-containing non oxidizing atmosphere at a temperature ranging from about 1350C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon PlJFA4 but not including lines JO and A4F of Figure 3 or 4, said free carbon being in an amount which produces said deoxidized compact, and (d) sistering said deoxidized compact in a nitrogen-containing non oxidizing atmosphere at a temperature of at least about 1850C producing said polycrystalline body.
In the present process, the composition of the deoxidized compact in equivalent % is the same as or does not differ significantly from that of the resulting sistered body in equivalent %.
In the present invention, oxygen content can be determined by neutron activation analysis.

I

RD-16,~46 1~3S147 By weight % or % by weight of a component herein, it is meant that the total weight % of all the components is 1 00% .
By ambient pressure herein, it is meant atmosphere to or about atmospheric pressure.
By specific surface area or surface area of a powder herein, it is meant the specific surface area act cording to BET surface area measurement.
Briefly stated, in one embodiment, the present process for producing a sistered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by line, i.e. polygon, A3JFA2 but not including lines A3J, JO and A2F of Figures 3 or 4, a porosity of less than about 10% by volume, and preferably less than about 2%
by volume, of said body and a thermal conductivity greater than 1.00 W/cm-K at 25C, and preferably greater than 1.42 W/cm-K at 25C comprises the steps:
(a) forming a mixture comprised of aluminum nitride powder containing oxygen, yttrium oxide or precursor therefore and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50C to about 1000C to free carbon and gaseous product of decomposition which vaporizes away, said free carbon having a specific surface area greater than about 100 mug the aluminum nitride powder in said mixture having a specific surface area ranging from about 3.4 mug to about 6 m go shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent %
of yttrium and aluminum ranges between points J and A of Figures 3 or 4, which is from greater than about 0.65 equivalent % to less than about 2.5 equivalent % yttrium and RD-16,446 12~5~;47 from greater than about 97.5 equivalent % to less than about 99.35 equivalent % aluminum, said compact having an equivalent % composition of Y, Al, 0 and N outside the composition defined and encompassed by polygon PlJFA4 of Figures 3 or 4, the aluminum nitride in said compact con-twining oxygen in an amount ranging from greater than about 1.42% by weight to less than about 4.70% by weight of said aluminum nitride, (b) heating said compact in a non oxidizing atmosphere at a temperature up to about 1200C thereby providing yttrium oxide and free carbon, (c) heating said compact at ambient pressure in a nitrogen-containing non oxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1350C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon A3JFA2 but not including lines A3J, JO and A2F of Figure 3 or 4, the aluminum nitride in said compact before said deoxidation by said carbon having an oxygen content ranging from greater than about 1.42% by weight to less than about 4.70% by weight of said aluminum nitride, said free carbon being in an amount which produces said deoxidized compact, and (d) sistering said deoxidized compact at ambient pressure in a nitrogen-containing non oxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1885C to about 1970C, in one embodiment from about 18~5C to about 1950C, in another embodiment from about 1890C to about 1950C, in another embodiment from about 1895C to about 1950C, and yet in RD-16,446 I

another embodiment from about 1940C to about 1970C, producing said polycrystalline body.
Briefly stated, in another embodiment, the present process for producing the present sistered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by line, i.e. polygon, ~lJFA4 but not including lines JO and A4F of Figures 3 or 4, a porosity of less than about 10% by volume, and preferably less than about 4% by volume, of said body and a thermal conductivity greater than 1.00 W/cm-K at 25C and preferably greater than 1.42 W/cm-K at 25 C comprises the steps:
a) processing an aluminum nitride powder into a compact for deoxidation by free carbon by providing an aluminum nitride powder having an oxygen content ranging up to about 4.4% by weight of said aluminum nitride powder, forming a mixture comprised of said aluminum nitride powder, yttrium oxide or precursor therefore and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50C to about 1000C to free carbon and gaseous product of decomposition which vaporizes away, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent %
of yttrium and aluminum ranges between points J and A of Figures 3 or 4, which is from greater than about 0.3 equiva-lent % to less than about 2.5 equivalent % yttrium and from greater than about 97.5 equivalent % to less than about 99.7 equivalent % aluminum, said compact having an equivalent %
composition of Y, Al, 0 and N outside the composition defined and encompassed by polygon PlJFA4 of Figures 3 or 4, during said processing said aluminum nitride picking up oxygen, the oxygen content of said aluminum nitride in said . .

ROD 16,446 123~47 compact before said deoxidation by carbon ranging from greater than about 1.0% by weight and usually greater than about 1.42% by weight, up to about 4.70% by weight of said aluminum nitride, (b) heating said compact in a non oxidizing atmosphere at a temperature up to about 1200C thereby providing yttrium oxide and free carbon, I heating said compact in a nitrogen-containing non oxidizing atmosphere at a temperature ranging from about 1350C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon PlJFA4 but not including lines JO and A4F of Figure 3 or 4, said free carbon being in an amount which produces said deoxidized compact, and (d) sistering said deoxidized compact in a nitrogen-containing non oxidizing atmosphere at a temperature of at least about 1850C producing said polycrystalline body Briefly stated, in another embodiment, the present process for producing a sistered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon A3JFA2 but not including lines A3J, JO and A2F of Figures 3 or 4, a porosity of less than about 10% by volume, and preferably less than about 2% by volume of said body and a thermal conductivity greater than 1.00 W/cm-K at 25C and preferably greater than 1.42 W/cm-K at 25C comprises the steps:
(a) processing an aluminum nitride powder into a compact for deoxidation by free carbon by providing an aluminum nitride powder having an oxygen content ranging .

.:1 ` ' RD-16,446 ~5147 from greater than about 1.00% by weight to less than about 4.4% by weight of said aluminum nitride powder, forming a mixture comprised of said aluminum nitride powder, yttrium oxide or precursor therefore and a carbonaceous additive selected from the group consisting of free carbon, a carbon-assess organic material and mixtures thereof, said carbon-Swiss organic material thermally decomposing at a tempera-lure ranging from about 50C to about 1000C to free carbon and gaseous product of decomposition which vaporizes away, said free carbon having a specific surface area greater than about 100 mug the aluminum nitride powder in said mixture having a specific surface area ranging from about 3.4 mug to about 6 mug shaping said mixture into a compact, said mixture and said compact having a composition wherein the lo equivalent % of yttrium and aluminum ranges between points J
and A of Figures 3 or 4, which is from greater than about 0.65 equivalent % to less than about 2.5 equivalent %
yttrium and from greater than about 97.5 equivalent % to less than about 99.35 equivalent % aluminum, said compact having an equivalent % composition of Y, Al, 0 and N outside the composition defined and encompassed by polygon PlJFA4 of Figures 3 or 4, during said processing said aluminum nitride picking up oxygen, the oxygen content of said aluminum nitride in said compact before said deoxidation by carbon ranging from greater than about 1.42% by weight up to about 4.70% by weight of said aluminum nitride and being greater than said oxygen content of said starting aluminum nitride powder by an amount ranging from greater than about 0.03% by weight up to about 3.00% by weight of said aluminum nitride, (b) heating said compact in a non oxidizing atmosphere at a temperature up to about 1200C thereby providing yttrium oxide and free carbon, RD-l6,446 4~7 (c) heating said compact at ambient pressure in a nitrogen-containing non oxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1350C to a temperature sufficient to deoxidize the compact but below its pore closing temperature thereby reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon A3JFA2 but not including lines A3J, JO and A2F of Figure 3 or 4, the aluminum nitride in said compact before said deoxidation by said carbon having an oxygen content ranging from greater than about 1.42% by weight to less than about 4.70% by weight of said aluminum nitride, said free carbon being in an amount which produces said deoxidized compact, and (d) sistering said deoxidized compact at ambient pressure in a nitrogen-containing non oxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1885C to about 1970C, in one embodiment from about 1885C to about 1950C, in another embodiment from about 1890C to about 1950C, in another embodiment from about 1895C to about 1950C, and yet in another embodiment from about 1940C to about 1970C, producing said polycrystalline body.
In one embodiment of the present process to produce a sistered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon Ply but excluding lines Play, AYE and AYE of Figure 4, the mixture and compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point A up to point A of Figure 4, i.e. the yttrium ranges from about 0.3 equivalent % to about 0.85 equivalent % and the aluminum ....

RD-16,446 ranges from about 99.15 equivalent % to about 99.7 equivalent %.
In another embodiment of the present process to produce a sistered polycrystalline aluminum nitride ceramic body having a composition defined by line Play of Figure 4, the mixture and compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point Pi to point A, i.e. the yttrium ranges from about 0.35 equivalent % to about 0.85 equivalent % and the aluminum ranges from about 99.15 equivalent % to about 99.65 equivalent %.
In another embodiment of the present process to produce a sistered polycrystalline aluminum nitride ceramic body having a composition defined by line A3J but excluding point J of Figure 4, the mixture and compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point A up to point J, i.e. the yttrium ranges from about 0.85 equivalent % to less than about 2.5 equivalent % and the aluminum ranges from greater than about 97.5 equivalent % to about 99.15 equivalent %.
More specifically, in one embodiment of the present process to produce a sistered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon A3JFA2 but not including lines A3J, JO and A2F of Figure 4 and a porosity of less than 1%
by volume of the body, the free carbon has a specific surface area greater than about 100 mug the aluminum nitride in said mixture has a specific surface area ranging from about 3.5 mug to about 6.0 mug all firing of the compact is carried out in nitrogen, and at a sistering temperature ranging from about 1890C to about 1950C, the resulting sistered body has a thermal conductivity greater than 1.42 W/cm-K at 25~C, and at a sistering temperature ranging from about 1895C to about 1950C, the resulting RD-16,446 14~

sistered body which contains carbon in an amount of less than about .04% by weight of the sistered body has a thermal conductivity greater than about 1.53 W/cm~K at 25DC.
In another embodiment of the present process, to produce a sistered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon A3JFA2 but excluding lines JO and A2F of Figure 4 which contains carbon in an amount of less than about .04% by weight of the sistered body and has a thermal conductivity greater than 1.57 W/cm-K at 25DC and a porosity of less than 1% by volume of the body, the aluminum nitride in said mixture has a specific surface area ranging from about 3.4 mug to about 6.0 mug the free carbon has a specific surface area greater than 100 mug all firing of the compact is carried out in nitrogen and the sistering temperature ranges from about 1940DC to about 1970DC.
In yet another embodiment of the present process, said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point A up to point J of Figure 4, said yttrium in said compact ranges from about 0.85 equivalent % to less than about 2.5 equivalent %, said aluminum in said compact ranges from greater than about 97.5 equivalent % to about 99.15 equivalent %, and said sistered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, 0 and N is defined by line A3J
but excluding point J of Figure 4, said free carbon has a specific surface area greater than about 100 mug said aluminum nitride powder in said mixture has a specific surface area ranging from about 3.5 mug to about 6.0 mug said firing atmosphere is nitrogen, said sistering temperature ranges from about 1890DC to about 1950~C to produce a sistered body having a porosity of less than 2% by RD-16,446 3514~

volume of the sistered body, or said sistering temperature ranges from about 1895C to about 1950C to produce a sistered body having a porosity of less than 1% by volume of the sistered body, and said sistered body has a thermal conductivity greater than 1.43 W/cm-K at 25C.
The calculated compositions of particular points in Figures 3 or 4 in the polygon PlJFA4 are shown in Table I
as follows:
TABLE I
LO Composition (Equivalent I Vow I and (Wit %) of Phases*
Point Y Oxygen Along YO-YO

p 0.55 1.159~.7(98.2) 1.3(1.8) A 0.85 1.6 97.9(97.2) 2.1(2.8) J 2.5 4.1 94.0(91.9) 6.0(8.1) F 1.6 4.0 95.8(93.8) 4.2(6.2) A 0.65 2.1 98.3(97.4) 1.7(2.6) Al 0.4 1.6 98.9(98.4) - 1.1(1.6) A 0.3 1.4 99.2(98.8) 0.8(1.2) Pi 0.35 0.8599.2(98.8) 0.8(1.2) * - Wit % is given in parentheses, Vow % is given without parentheses The polycrystalline aluminum nitride body produced by the present process has a composition defined and encom-passed by polygon, i.e. line, Pledge but not including lines JO and A4F of Figures 3 or 4. The sistered polycrystalline body of polygon PlJFA4 but not including lines JO and A4F of Figures 3 or 4 produced by the present process has a RD-16,446 1~i35147 composition comprised of from greater than about 0.3 equivalent % yttrium to less than about 2.5 equivalent %
yttrium, from greater than about 97.5 equivalent % aluminum to less than about 99.7 equivalent % aluminum, from about 0.85 equivalent % oxygen to less than about 4.1 equivalent %
oxygen and from greater than about 95.9 equivalent %
nitrogen to about 99.15 equivalent % nitrogen.
Also, the polycrystalline body having a composition defined and encompassed by polygon PlJFA4 but lo not including lines JO and A4F of Figure 3 or 4 is comprised of an Awn phase and a second phase which ranges in amount from greater than about 0.8% by volume for a composition adjacent, nearest or next to point A, to less than about 6.0% by volume for a composition adjacent, nearest or next to point J of the total volume of the sistered body, and such second phase can be comprised of YO-YO or a mixture of YO-YO and YO-YO. When the second phase is comprised of YO-YO, i.e. at line Ply, it ranges in amount from about 0.85% by volume to less than about 6.0% by volume of the sistered body. However, when the second phase is a mixture of second phases comprised of Yule and Yule, i.e. when the polycrystalline body has a composition defined and encompassed by polygon PlJFA4 excluding lines Ply, JO and A4F, both of these second phases are always present in at least a trace amount, i.e. at least an amount detectable by X-ray diffraction analysis, and in such mixture, the Yule phase can range from a trace amount to less than about 4.2%
by volume of the sistered body, and the YO-YO phase can range from a trace amount to less than about 6.0% by volume of the total volume of the sistered body. More specifically, when a mixture of Yule and YO-YO phases is present, the amount of YO-YO phase decreases and the amount of YO-YO phase increases as the composition moves away R~-16,4~
~3~1~7 from line A4F toward line Ply in Figure 4. Line Ply in Figure 4 is comprised of Awn phase and a second phase comprised of YO-YO.
As can be seen from Table I, the polycrystalline body at point J composition would have the largest amount of second phase present which at point J would be YO-YO.
In another embodiment, the polycrystalline alum-nut nitride body produced by the present process has a composition defined and encompassed by polygon, i.e. line, A3JFA2 but not including lines A3J, JO and A2F of Figures 3 or 4. The sistered polycrystalline body of polygon A3JFA2 but not including lines A3J, JO and A2F of Figures 3 or 4 produced by the present process has a composition comprised of from greater than about 0.65 equivalent % yttrium to less than about 2.5 equivalent % yttrium, from greater than about 97.5 equivalent % aluminum up to about 99.35 equivalent %
aluminum, from about 1.6 equivalent % oxygen to less than about 4.1 equivalent % oxygen and from greater than about 95.9 equivalent % nitrogen to about 9~3.4 e~ivalent %
nitrogen.
Also, the polycrystalline body defined and encom-passed by polygon A3JFA2 but not including lines A3J, JO and A2F of Figure 3 or 4 is comprised of an Awn phase and a second phase which ranges in amount from greater than about 1.7% by volume to less than about 6.0% by volume of the total volume of the sistered body, and such second phase is comprised of a mixture of YO-YO and YO-YO and both of these second phases are always present in at least a trace amount, i.e. at least an amount detectable by X-ray diffraction analysis. Specifically, the YO-YO phase can range from a trace amount to less than about 4.2% by volume of the sistered body, and the YO-YO phase can range from a RD-16,446 1;235147 trace amount to less than about 6.0% by volume of the total volume of the sistered body.
In another embodiment, the polycrystalline aluminum nitride body produced by the present process has a composition defined and encompassed by polygon, i.e. line, Ply but not including lines Play, AYE and AYE of Figure 4 comprised of prom greater than about 0.3 equivalent % to abut 0.85 equivalent % yttrium, from about 99.15 equivalent % to about 99.7 equivalent % aluminum, from greater than about 0.85 equivalent % to less than about 2.1 equivalent % oxygen and from greater than about 97.9 equivalent % nitrogen to less than about 99.15 equivalent %
nitrogen.
Also, the polycrystalline body defined and encompassed by polygon Ply but not including lines Play, AYE and AYE of figure 4 is comprised of an Awn phase and a second phase which ranges in amount from greater than about 0.8% by volume to less than about 2.1% by volume of the total volume of the sistered body, and such second phase is comprised of a mixture of YO-YO and YO-YO and both of these second phases are always present in at least a trade amount, i.e. at least an amount detectable by X-ray diffraction analysis. Specifically, the YO-YO phase can range from a trace amount to less than about 1.7% by volume of the sistered body, and the YO-YO phase can range from a trace amount to less than about 2.1% by volume of the total volume of the sistered body.
In another embodiment, the present process pro-dupes a sistered body defined by line Play of Figure 4 which has a phase composition comprised of Awn and YO-YO wherein the YO-YO phase ranges from about 0.8% by volume to less than about 2.1% by volume of the body. Line Play of Figure 4 has a composition comprised of from about 0.35 equivalent RD-16,446 lZ35~4~

% to about 0.85 equivalent % yttrium, from about 99.15 equivalent % to about 99.65 equivalent % aluminum, from about 0.85 equivalent % to about 1.6 equivalent % oxygen and from about 98.4 equivalent % to about 99.15 equivalent %
nitrogen.
In another embodiment, the present process produces a sistered body defined by line A3J but not including point J of Figure 4 which has a phase composition comprised of Awn and YO-YO wherein the YO-YO phase ranges from about 2.1% by volume to less than about 6.0% by volume of the body. Line A3J but not including point J of Figure 4 has a composition comprised of from about 0.85 equivalent % to less than about 2.5 equivalent % yttrium, from greater than about 97.5 equivalent % to about 99.15 equivalent % aluminum, from about 1.6 equivalent % to less than about 4.1 equivalent % oxygen and from greater than about 95.9 equivalent % to about 98.4 equivalent % nitrogen.
In the present process, the aluminum nitride powder can be of commercial or technical grade. Specifically-lye it should not contain any impurities which would have significantly deleterious effect on the desired properties of the resulting sistered product. The starting aluminum nitride powder used in the present process contains oxygen generally ranging in amount up to about 4.4% by weight and usually ranging from greater than about 1.0% by weight to less than about 4.4% weight, i.e. up to about 4.4% by weight. Typically, commercially available aluminum nitride powder contains from about 1.5 weight % (2.6 equivalent %) to about 3 weight % (5.2 equivalent %) of oxygen and such powders are most preferred on the basis of their substantially lower cost.
The oxygen content of aluminum nitride is deter-mixable by neutron activation analysis.

RD-16,446 ~Z;3S14~

Generally, the present starting aluminum nitride powder has a specific surface area which can range widely, and generally it ranges up to about 10 mug Frequently, it has a specific surface area greater than about 1.0 mug and more frequently of at least about 3.0 mug usually greater than about 3.2 mug and preferably at least about 3.4 mug Generally, the present aluminum nitride powder in the present mixture, i.e. after the components have been mixed, usually by milling, has a specific surface area which can range widely, and generally it ranges to about 10 mug Frequently, it ranges from greater than about 1.0 mug to about 10 mug and more frequently from about 3.2 mug to about lo mug and preferably it ranges from about 1.5 mug to about 5 mug and in one embodiment it ranges from about

3.4 mug to about 5 mug according to BET surface area measurement. Specifically, the minimum sistering temperature of a given composition of the present invention increases with increasing particle size of the aluminum nitride.
Generally, the yttrium oxide (Yo-yo) additive in the present mixture has a specific surface area which can range widely. Generally, it is greater than about 0.4 mug and generally it ranges from greater than about 0.4 mug to about 6.0 mug usually from about 0.6 mug to about 5.0 mug more usually from about 1.0 mug to about 5.0 mug and in one embodiment it is greater than 2.0 mug In the practice of this invention, carbon for deoxidation of aluminum nitride powder is provided in the form of free carbon which can be added to the mixture as elemental carbon, or in the form of a carbonaceous additive, for example, an organic compound which can thermally decompose to provide free carbon.

.
" ` .

RD-16,446 lZ3S147 The present carbonaceous additive is selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof. The carbonaceous organic material pyrolyzes, i.e. thermally decomposes, completely at a temperature ranging from about ;0C to about 1000C to free carbon and gaseous product of decomposition which vaporizes away. In a preferred embodiment, the carbonaceous additive is free carbon, and preferably, it is graphite.
High molecular weight aromatic compounds or materials are the preferred carbonaceous organic materials for making the present free carbon addition since they ordinarily give on pyrolyzes the required yield of portico-late free carbon of sub micron size. Examples of such aromatic materials are a phenol formaldehyde condensate resin known as Novolak which is soluble in acetone or higher alcohols, such as bottle alcohol, as well as many of the related condensation polymers or resins such as those of resorcinol-formaldehyde, aniline-formaldehyde, and crossly-formaldehyde. Another satisfactory group of materials are derivatives of polynuclear aromatic hydrocarbons contained in coal tar, such as dibenzanthracene and chrysene. A
preferred group are polymers of aromatic hydrocarbons such as polyphenylene or polymethylphenylene which are soluble in aromatic hydrocarbons.
I The present free carbon has a specific surface area which can range widely and need only be at least sufficient to carry out the present deoxidation. Generally, it has a specific surface area greater than about lo mug preferably greater than 20 mug more preferably greater than about lo mug and still more preferably greater than 150 mug according to BET surface area measurement to insure intimate contact with the Awn powder for carrying out its deoxidation. Most preferably, the present free carbon RD-16,44~

lZ3514~

has as high a surface area as possible. Also, the finer the particle size of the free carbon, i.e. the higher its surface area, the smaller are the holes or pores it leaves behind in the deoxidized compact. Generally, the smaller the pores of a given deoxidized compact, the lower is the amount of liquid phase which need be generated at sistering temperature to produce a sistered body having a porosity of less than about 1% by volume of the body.
By processing of the aluminum nitride powder into a compact for deoxidation by free carbon, it is meant herein to include all mixing of the aluminum nitride powder to produce the present mixture, all shaping of the resulting mixture to product the compact, as well as handling and storing of the compact before it is deoxidized by carbon.
In the present process, processing of the aluminum nitride powder into a compact for deoxidation by free carbon is at least partly carried out in air, and during such processing of the aluminum nitride powder, it picks up oxygen from air usually in an amount greater than about 0.03% by weight of the aluminum nitride, and any such pick up of oxygen is controllable and reproducible or does not differ signify-gently if carried out under the same conditions. If desire Ed the processing of the aluminum nitride powder into a compact for deoxidation by free carbon can be carried out in air.
In the present processing of aluminum nitride, the oxygen it picks up can be in any form, i.e. it initially may be oxygen, or initially it may be in some other form, such as, for example, water. The total amount of oxygen picked up by aluminum nitride from air-or other media generally is less than about 3.00% by weight, and generally ranges from greater than about 0.03% by weight to less than about 3.00%
by weight, and usually it ranges from about 0.10% by weight , ` RD-16,44~

to about 1.00% by weight, and preferably it ranges from about 0.15% by weight to about 0.70% by weight, of the total weight of the aluminum nitride. Generally, the aluminum nitride in the present mixture and compact prior to deoxidation of the compact have an oxygen content of less than about 4.70% by weight, and generally ranges from greater than about 1.00% by weight, and usually greater than about 1.42% by weight to less than about 4.70% by weight, and more usually it ranges from about 2.00% by weight to about 4.00% by weight, and frequently it ranges from about 2.20% by weight to about 3.50% by weight, of the total weight of aluminum nitride.
The oxygen content of the starting aluminum nitride powder and that of the aluminum nitride in the compact prior to deoxidation can be determined by neutron activation analysis.
In a compact, an aluminum nitride containing oxygen in an amount of about 4.7% by weight or more is not desirable.
In carrying out the present process, a uniform or at least a significantly uniform mixture or dispersion of the aluminum nitride powder, yttrium oxide powder and carbonaceous additive, generally in the form of free carbon powder, is formed and such mixture can be formed by a number of techniques. Preferably, the powders are ball milled preferably in a liquid medium at ambient pressure and temperature to produce a uniform or significantly uniform dispersion. The milling media, which usually are in the form of cylinders or balls, should have no significant deleterious effect on the powders, and preferably, they are comprised of steel or polycrystalline aluminum nitride, preferably made by sistering a compact of milling media size of Awn powder and Yo-yo sistering additive. Generally, the R~-16,~46 ~:~3~4~

milling media has a diameter of at least about 1/4 inch and usually ranges from about 1/4 inch to about 1/2 inch in diameter. The liquid medium should have no significantly deleterious effect on the powders and preferably it is non-aqueous. Preferably, the liquid mixing or milling medium can be evaporated away completely at a temperature ranging from above room or ambient temperature to below 300C leaving the present mixture. Preferably, the liquid mixing medium is an organic liquid such as Hutton or hexane. Also, preferably, the liquid milling medium contains a dispersant for the aluminum nitride powder thereby producing a uniform or significantly uniform mixture in a significantly-shorter period of milling time. Such dispersant should be used in a dispersing amount and it should evaporate or decompose and evaporate away completely or leave no significant residue, i.e. no residue which has a significant effect in the present process, at an elevated temperature below lOOODC. Generally, the amount of such dispersant ranges from about 0.1% by weight to less than about 3% by weight of the aluminum nitride powder, and generally it is an organic liquid, preferably oleic acid.
In using steel milling media, a residue of steel or iron is left in the dried dispersion or mixture which can range from a detectable amount up to about 3.0% by weight of the mixture. This residue of steel or iron in the mixture has no significant effect in the present process or on the thermal conductivity of the resulting sistered body.
The liquid dispersion can be dried by a number of conventional techniques to remove or evaporate away the liquid and produce the present particulate mixture. If desired, drying can be carried out in air. Drying of a milled liquid dispersion in air causes the aluminum nitride to pick up oxygen and, when carried out under the same RD-16,446 l~S~4~

conditions, such oxygen pick up is reproducible or does not differ significantly. Also, if desired, the dispersion can be spray dried.
A solid carbonaceous organic material is prefer-by admixed in the form of a solution to coat the aluminum nitride particles. The solvent preferably is non-agueous.
The wet mixture can then be treated to remove the solvent producing the present mixture. The solvent can be removed by a number of techniques such as by evaporation or by freeze drying, i.e. subliming off the solvent in vacuum from the frozen dispersion. In this way, a substantially uniform coating of the organic material on the aluminum nitride powder is obtained which on pyrolyzes produces a sub Stan-tidally uniform distribution of free carbon.
The present mixture is shaped into a compact in air, or includes exposing the aluminum nitride in the mixture to air. Shaping of the present mixture into a compact can be carried out by a number of techniques such as extrusion, injection molding, die pressing, isostatic pressing, slip casting, roll compaction or forming or tape casting to produce the compact of desired shape. Any lubricants, binders or similar shaping aid materials used to aid shaping of the mixture should have no significant deteriorating effect on the compact or the present resulting sistered body. Such shaping-aid materials are preferably of the type which evaporate away on heating at relatively low temperatures, preferably below 400C, leaving no significant residue. Preferably, after removal of the shaping aid materials, the compact has a porosity of less than 60% and more preferably less than 50% to promote densification during sistering.
If the compact contains carbonaceous organic material as a source of free carbon, it is heated at a RD-16,446 _ læ~sl47 temperature ranging from about 50C to about lOOODC to pyrolyze, i.e. thermally decompose, the organic material completely producing the present free carbon and gaseous product of decomposition which vaporizes away. Thermal decomposition of the carbonaceous organic material is carried out, preferably in a vacuum or at ambient pressure, in a non oxidizing atmosphere. Preferably, the non oxidizing atmosphere in which thermal decomposition is carried out is selected from the group consisting of nitrogen, hydrogen, a noble gas such as argon and mixtures thereof, and more preferably it is nitrogen, or a mixture of at least about 25% by volume nitrogen and a gas selected from the group consisting of hydrogen, a noble gas such as argon and mixtures thereof. In one embodiment, it is a mixture of nitrogen and from about 1% by volume to about 5% by volume hydrogen.
The actual amount of free carbon introduced by pyrolyzes of the carbonaceous organic material can be determined by pyrolyzing the organic material alone and determining weight loss. preferably, thermal decomposition of the organic material in the present compact is done in the sistering furnace as the temperature is being raised to deoxidizing temperature, i.e. the temperature at which the resulting free carbon reacts with the oxygen content of the Awn.
Alternately, in the present process, yttrium oxide can be provided by means of an yttrium oxide precursor. The term yttrium oxide precursor means any organic or inorganic compound which decomposes completely at a temperature below about 1200C to form yttrium oxide and by-product gas which vaporizes away leaving no contaminants in the sistered body which would be detrimental to the thermal conductivity.
Representative of the precursors of yttrium oxide useful in I

RD-16,446 I I -the present process is yttrium acetate, yttrium carbonate, yttrium oxalate, yttrium nitrate, yttrium sulfate and yttrium hydroxide.
If the compact contains a precursor for yttrium oxide, it is heated to a temperature up to about 1200C to thermally decompose the precursor thereby providing yttrium oxide. Such thermal decomposition is carried out in a non-oxidizing atmosphere, preferably in a vacuum or at ambient pressure, and preferably the atmosphere is selected from the group consisting of nitrogen, hydrogen, a noble gas such as argon and mixtures thereof. Preferably, it is nitrogen, or a mixture of at least about 25% by volume nitrogen and a gas selected from the group consisting of hydrogen, a noble gas such as argon and mixtures thereof.
In one embodiment, it is a mixture of nitrogen and from about 1% by volume to about 5% by volume hydrogen.
The present deoxidation of aluminum nitride with carbon, i.e. carbon-deoxidation, comprises heating the compact comprised of aluminum nitride, free carbon and yttrium oxide at deoxidation temperature to react the free carbon with at least a sufficient amount of the oxygen contained in the aluminum nitride to produce a deoxidized compact having a composition defined and encompassed by polygon PlJFA4 but not including lines JO and A4F of Figures 3 or 4. This deoxidation with carbon is carried out at a temperature ranging from about 1350C to a temperature at which the pores of the compact remain open, i.e. a temperature which is sufficient to deoxidize the compact but below its pore closing temperature, generally up to about 1800C, and preferably, it is carried out at from about 1600C to 1650C.
The carbon-deoxidation is carried out, preferably at ambient pressure, in a gaseous nitrogen-containing RD-16,446 123514~7 non oxidizing atmosphere which contains sufficient nitrogen to facilitate the deoxidation of the aluminum nitride. In accordance with the present invention, nitrogen is a required component for carrying out the deoxidation of the compact. Preferably, the nitrogen-containing atmosphere is nitrogen, or it is a mixture of at least about 25% by volume of nitrogen and a gas selected from the group consisting of hydrogen, a noble gas such as argon, and mixtures thereof.
Also, preferably, the nitrogen-containing atmosphere is comprised of a mixture of nitrogen and hydrogen, especially a mixture containing up to about 5% by volume hydrogen.
The time required to carry out the present carbon-deoxidation of the compact is determinable empirically and depends largely on the thickness of the compact as well as the amount of free carbon it contains, i.e. the carbon-deoxidation time increases with increasing thickness of the compact and with increasing amounts of free carbon contained in the compact. Carbon-deoxidation can be carried out as the compact is being heated to sistering temperature provide Ed that the heating rate allows the deoxidation to recompleted while the pores of the compact are open and such heating rate is determinable empirically. Also, to some extent, carbon deoxidation time depends on deoxidation temperature, particle size and uniformity of the particulate mixture of the compact i.e. the higher the deoxidation temperature, the smaller the particle size and the more uniform the mixture, the shorter is deoxidation time.
Typically, the carbon-deoxidation time ranges from about hour to about 1.5 hours.
Preferably, the compact is deoxidized in the sistering furnace by holding the compact at deoxidation temperature for the required time and then raising the temperature to sistering temperature. The deoxidation of RD-16,446 lZ3~4'7.
the compact must be completed before sistering closes off pores in the compact preventing gaseous product from vapor-icing away and thereby preventing production of the present sistered body.
In the present deoxidation with carbon, the free carbon reacts with the oxygen of the aluminum nitride producing carbon monoxide gas which vaporizes away. It is believed that the following deoxidation reaction occurs wherein the oxygen content of the aluminum nitride is given lo as Allah:

Allah + 3C + No 3C0(g) + Allen (2) In the oxidation effected by carbon, gaseous carbon-containing product is produced which vaporizes away thereby removing free carbon.
If the compact before deoxidation is heated at too fast a rate through the carbon-deoxidation temperature to sistering temperature, and such too fast rate would depend largely on the composition of the compact and the amount of carbon it contains, the present~carbon-deoxidation does not occur, i.e. an insufficient amount of deoxidation occurs, and a significant amount of carbon is lost by reactions (3) and/or (PA).

C + Awn Alan (3) C + 1/2 No ' ON (AYE

The specific amount of free carbon required to produce the present deoxidized compact can be determined by a number of techniques. It can be determined empirically.

-~31-RD-16,446 :~23S147 Preferably, an initial approximate amount of carbon is calculated from Equation (2), that is the stoichiometric amount for carbon set forth in Equation I and using such approximate amount, the amount of carbon required in the present process to produce the present sistered body would require one or a few runs to determine if too much or too little carbon had been added. Specifically, this can be done by determining the porosity of the sistered body and by analyzing it for carbon and by X-ray diffraction analysis.
If the compact contains too much carbon, the resulting deoxidized compact will be more difficult to stinter and will not produce the present sistered body. If the compact contains too little carbon, X-ray diffraction analysis of the resulting sistered body will not show any YO-YO phase and will show that its composition is not defined or encompassed by the polygon PlJFA4 not including lines JO and A4F of Figure 4.
The amount of free carbon used to carry out the present deoxidation should produce the present deoxidized compact leaving no significant amount of carbon in any form, i.e. no amount of carbon in any form which would have a significantly deleterious effect on the sistered body. More specifically, no amount of carbon in any form should be left in the deoxidized compact which would prevent production of the present sistered body, i.e. any carbon content in the sistered body should be low enough so that the sistered body has a thermal conductivity greater than 1.00 WACO at 25C.
Generally, the present sistered body may contain carbon in some form in a trace amount, i.e. generally less than about .08% by weight, preferably in an amount of less than about .065% by weight, more preferably less than about .04% by weight, and most preferably less than .03% by weight of the total weight of the sistered body.

RD-16,446 A significant amount of carbon in any form remain-in in the sistered body significantly reduces its thermal conductivity. An amount of carbon in any form greater than about 0.065% by weight of the sistered body is likely to significantly decrease its thermal conductivity.
The present deoxidized compact is densified, i.e.
liquid-phase sistered, at a temperature which is a sistering temperature for the composition of the deoxidized compact to produce the present polycrystalline body having a porosity of less than about 10% by volume, and preferably less than about 4% by volume, of the sistered body. For the present composition defined and encompassed by polygon PlJFA4 but excluding lines JO and A4F, this sistering temperature generally is at least about 1850C and generally ranges from about 1850C to about 2050C with the minimum sistering temperature increasing generally from about 1850C for a composition represented by a point next to point F to greater than about 1920C but less than about 1990C for the composition at point Pi. Minimum sistering temperature is dependent most strongly on composition and less strongly on particle size.
More specifically, in the present invention, for the present deoxidized compact having a constant particle size, the minimum sistering temperature occurs at a combo-session represented by a point next to point F within the polygon PlJFA4 and such temperature increases as the combo-session moves away from point F toward point Pi.
Specifically, for such a deoxidized compact having a combo-session defined and encompassed by polygon A3JFA2 of Figure 4 excluding lines A3J, JO and A2F, the minimum sistering temperature is generally about 1850C. For a deoxidized compact having a composition defined and encompassed by polygon Ply excluding lines Play, AYE and AYE, the RD-16,446 J.235147 minimum sistering temperature increases generally from about 1850DC at a point adjacent, next or nearest to point A to generally about 1890C at a point adjacent, next or nearest to point P to less than about 1990C at point Pi. The minimum sistering temperature for a composition on line A3J
of Figure 4 generally is about 1860C. The minimum sistering temperature for a composition on line Apple generally ranges from about 1860DC at point A to generally about 1900C at point P to less than about 1990C at point Pi.
More specifically, the minimum sistering tempera-lure is dependent largely on the composition (i.e., position in the Figure 4 phase diagram), the green density of the compact, i.e. the porosity of the compact after removal of shaping aid materials but before deoxidation, the particle size of aluminum nitride, and to a much lesser extent the particle size of yttrium oxide and carbon. The minimum sistering temperature increases as the composition moves from next or nearest to point F to point Pi, and as the green density of the compact decreases, and as the particle size of aluminum nitride and to a much lesser extent, yttrium oxide and carbon increases. For example, for a composition represented by a point within polygon PlJFA4 of Figure 4 and nearest to point F, the minimum sistering temperature is about 1850C for the particle size combine-lion of aluminum nitride, yttrium oxide, and carbon of about 5.0 mug 2.8 mug and 200 mug respectively.
To carry out the present liquid phase sistering, the present deoxidized compact contains sufficient equiva-lent percent of Y and 0 to form a sufficient amount of liquid phase at sistering temperature to density the carbon-deoxidized compact to produce the present sistered body.
The present minimum densification, i.e. sistering, RD-16,446 123S~47 temperature depends on the composition of the deoxidized compact, i.e. the amount of liquid phase it generates.
Specifically, for a sistering temperature to be operable in the present invention, it must generate at least sufficient liquid phase in the particular composition of the deoxidized compact to carry out the present liquid phase sistering to produce the present product. For a given composition, the lower the sistering temperature, the smaller is the amount of liquid phase generated, i.e. densification becomes more difficult with decreasing sistering temperature. However, a sistering temperature higher than about 2050C provides no significant advantage.
In one embodiment of the present invention, the sistering temperature ranges from about 1890C to about 2050C, and in another embodiment from about 1880C to about 1950C, and in another embodiment from about 1890C to about 1950C, and yet in another embodiment from about 1885C to about 1950C, and still in anther embodiment from about 1895C to about 1950C, and still in another embodiment from about 1940C to about 1970C, to produce the present polyp crystalline body.
The deoxidized compact is sistered, preferably at ambient pressure, in a gaseous nitrogen-containing nonoxi-dozing atmosphere which contains at least sufficient vitro-gun to prevent significant weight loss of aluminum nitride.
In accordance with the present invention, nitrogen is a necessary component of the sistering atmosphere to prevent any significant weight loss of Awn during sistering, and also to optimize the deoxidation treatment and to remove carbon. Significant weight loss of the aluminum nitride can vary depending on its surface area to volume ratio, i.e.
depending on the form of the body, for example, whether it is in the form of a thin or thick tape. As a result, RD-16,446 ~2351~7 generally, significant weight loss of aluminum nitride ranges from in excess of about 5% by weight to in excess of about 10% by weight of the aluminum nitride. Preferably the nitrogen-containing atmosphere is nitrogen, or it is a mixture at least about 25% by volume nitrogen and a gas selected from the group consisting of hydrogen, a noble gas such as argon and mixtures thereof. Also, preferably, the nitrogen-containing atmosphere is comprised of a mixture of nitrogen and hydrogen, especially a mixture containing from about 1% by volume to about 5% by volume hydrogen.
Sistering time is determinable empirically.
Typically, sistering time ranges from about 40 minutes to about 90 minutes.
In one embodiment, i.e. the composition defined by lo polygon PlJFA4 but not including lines Ply, JO and A4F of Figure 4, where the aluminum nitride in the carbon-deoxidiz-Ed compact contains oxygen, the yttrium oxide further deoxidizes the aluminum nitride by reacting with the oxygen to form YO-YO and YO-YO, thus decreasing the amount of oxygen in the Awn lattice to produce the present sistered body having a phase composition comprised of Awn and a second phase mixture comprised of YO-YO and YO-YO.
In another embodiment, i.e. line Ply but excluding point J of Figure worry the aluminum nitride in the carbon-deoxidized compact contains oxygen in an amount significantly smaller than that of polygon PlJFA4 but not including lines Ply, JO and A4F of Figure 4, the resulting sistered body has a phase composition comprised of Awn and YO-YO ' The present sistered polycrystalline body is a pressure less sistered ceramic body. By pressure less stinter-in herein it is meant the densification or consolidation of the deoxidized compact without the application of mechanical RD-16,446 1235~47 pressure into a ceramic body having a porosity of less than about 10% by volume, and preferably less than about 4% by volume.
The polycrystalline body of the present invention is liquid-phase sistered. I.e., it stinters due to the presence of a liquid phase, that is liquid at the sistering temperature and is rich in yttrium and oxygen and contains some aluminum and nitrogen. In the present polycrystalline body, the Awn grains have about the same dimensions in all directions, and are not elongated or disk shaped. General-lye the Awn in the present polycrystalline body has an average grain size ranging from about 1 micron to about 20 microns. An inter granular second phase of YO-YO or a mixture of Yule and YO-YO is present along some of the Awn grain boundaries. The morphology of the micro structure of the present sistered body indicates that this inter granular second phase was a liquid at the sistering temperature. As the composition approaches line JO in Figure 4, the amount of liquid phase increases and the Awn grains in the present sistered body become more rounded and have a smoother surface. As the composition moves away from line JO in Figure 4 and approaches line Play, the amount of liquid phase decreases and the Awn grains in the present sistered body become less rounded and the corners of the grains become sharper.
The present sistered body has a porosity of less than about 10% by volume, and generally less than about 4%
by volume of the sistered body. Preferably, the present sistered body has a porosity of less than about 2% and most preferably less than about 1% by volume of the sistered body. Any pores in the sistered body are fine sized, and generally they are less than about 1 micron in diameter.

.

RD-16,44~
lZ35147 Porosity can be determined by standard metallographic procedures and by standard density measurements.
The present process is a control process for producing a sistered body of aluminum nitride having a thermal conductivity greater than 1.00 W/cm-K at 25~C, and preferably at least or greater than 1.42 W/cm-K at 25~C.
Generally, the thermal conductivity of the present polycrystalline body is less than that of a high purity single crystal of aluminum nitride which is about 2.8 W/cm-K
at 25C. If the same procedure and conditions are used throughout the present process, the resulting sistered body has a thermal conductivity and composition which is reproducible or does not differ significantly. Generally, thermal conductivity increases with a decrease in volume %
of second phase, a decrease in porosity and for a given composition with increase in sistering temperature.
In the present process, aluminum nitride picks up oxygen in a controllable or substantially controllable manner. Specifically, if the same procedure and conditions are used in the present process, the amount of oxygen picked up by aluminum nitride is reproducible or does not differ significantly. Also, in contrast to yttrium, yttrium nitride and yttrium hydrides yttrium oxide or the present precursor does not pick up oxygen, or does not pick up any significant amount of oxygen, from air or other media in the present process. More specifically, in the present process, yttrium oxide does not pick up any amount of oxygen in any form from the air or other media which would have any significant effect on the controllability or reproducibility of the present process. Any oxygen which yttrium oxide might pick up in the present process is so small as to have no effect or no significant effect on the thermal conductivity or composition of the resulting sistered body.

D

~235147 Examples of calculations for equivalent % are as follows:
For a starting Awn powder weighing 89.0 grams measured as having 2.3 weight % oxygen, it is assumed that all of the oxygen is bound to Awn as Aye, and that the measured 2.3 weight % of oxygen is present as 4.89 weight %
Aye so that the Awn powder is assumed to be comprised of 84.65 grams Awn and 4.35 grams Aye.
A mixture is formed comprised of 89.0 grams of the 10 starting Awn powder, 3.2 grams of Yo-yo and 1.15 grams free carbon.
During processing, this Awn powder picks up additional oxygen by reactions similar to (4) and now contains 2.6 weight % oxygen.

2 Awn + 3H20 ' Aye + 2NH3 (4) The resulting compact now is comprised of the following composition:

89.11 grams Awn powder containing 2.6 weight % oxygen, (84.19g Awn + 4.92g Aye), 3-2 grams Yo-yo and 1.15 20 grams carbon.

During oxidation of the compact, all the carbon is assumed to react with Aye via reaction (5) Aye + 3C + No ' Allen + KIWI

In the present invention, the carbon will not 25 reduce Yo-yo, but instead, reduces Aye.

.... .

RD-16,4~6 1'~3S~7 After reaction (5) has gone to completion, the deoxidized compact now is comprised of the following combo-session which was calculated on the basis of Reaction (5):

B8.47 grams Awn powder containing 0.89 weight % oxygen (B6.Bl grams Awn 1.67 grams Allah) and 3.2 grams Yo-yo From this weight composition, the composition in equivalent % can be calculated as follows:

Wit (~) Moles Equivalents Awn B6.B1 2.118 6.354 lo Allah 1.67 1.636 x lo 2 0.098 ~23 3.20 1.~17 x 10-2 0.085 TOTAL EQUIVALENTS = 6.537 V = Valence M = Moles = Lowe MY = molecular weight En = Equivalents En = M X V
Valences: Al + 3 y + 3 En % Y in deoxidized compact =
_ nosy equivalents x 100% (6) nosy equivalents + nodal equivalents 0 085 x 100% = 1.30%
6.537 En % O in deoxidized compact =
~40-RD-16, 446 ~Z35147 no. O equivalents x 100% to) no. O equivalents + noun equivalents = 0.098 0-0~5 x 100% = 2.80% (8) 6.537 This deoxidized compact as well as the sistered body contains about 1.30 equivalent % Y and about 2.80 equivalent % Oxygen.
To produce the present sistered body containing 1.5 equivalent % Y and 3.0 equivalent % O, i.e. comprised of 10 1.5 equivalent % Y, 98.5 equivalent % Al, 3.0 equivalent % O
and 97.0 equivalent % N, using an Awn powder measured as having 2.3 weight % Oxygen (4.89 weight % Allah), the following calculations for weight % from equivalent % can be made:

100 grams = weight of Awn powder x grams = weight of Yo-yo powder z grams = weight of Carbon powder Assume that during processing, the Awn powder picks up additional oxygen by reaction similar to (9) and in the compact before deoxidation now contains 2.6 weight %
oxygen (5.52 weight % Allah) and weighs 100.12 grams Allen + 3H20 Allah l 2NH3 (9) After processing, the compact can be considered as having the following composition:

RD-16,446 i'~35~7 Weight Motes _ Equivalents Allen 2.308 6.923 Allah . 0.0542 0.325 Yo-yo x4.429 x 10 3x 0.02657x C z .0833z During deoxidation, 3 moles of carbon reduce 1 mole of Allah and in the presence of No form 2 moles of Awn by the reaction:

~1203 + 3C + No ' Allen + KIWI (10) loafer deoxidation, all the carbon will have reacted and the compact can be considered as having the following composition:

Weight (g) Moles Equivalents Awn 94.59 + 2.275z 2.308 + 0.05551z 6.923 + 0.1665z Allah 5.53 - 2.830z 0.0542 - 0.02775z 0.325 - 0.1665z Yo-yo x 4.429 x 10 3 x 0.02657 x T = Total Equivalents = 7.248 + 0.02657 x Equivalent Fraction of Y = 0.015 = 0.02657 x (11) Equivalent Fraction of 0=0.030=0.325-0.1665z + 0.02657x (12) T

olving Equations (11) and (12) for x and z:
x = 4.15 grams of Yo-yo powder z = 1.29 grams of free carbon RD-16,446 ~;~3514~

A body in a form or shape useful as a substrate, i.e. in the form of a flat thin piece of uniform thickness, or having no significant difference in its thickness, usually referred to as a substrate or tape, may become S non-flat, for example, warp, during sistering and the resulting sistered body may require a heat treatment after sistering to flatten it out and make it useful as a sub-striate. This non-flatness or warping is likely to occur in the sistering of a body in the form of a substrate or tape having a thickness of less than about .070 inch and can be eliminated by a flattening treatment, i.e. by heating the sistered body, i.e. substrate or tape, under a sufficient applied pressure at a temperature in the present sistering temperature range of from about 1850C to about 2050C for a period of time determinable empirically, and allowing the sandwiched body to cool to below its sistering temperature, preferably to ambient or room temperature before recovering the resulting flat substrate or tape.
Specifically, in one embodiment of this flattening process, the non-flat substrate or tape is sandwiched between two plates and is separated from such plates by a thin layer of Awn powder, the sandwiched body is heated to its sistering temperature, i.e. a temperature which is a sistering temperature for the sandwiched sistered body, preferably in the same atmosphere used for sistering, under an applied pressure at least sufficient to flatten the body, generally at least about .03 psi, for a time period sufficient to flatten the sandwiched body, and then the sandwiched body is allowed to cool to below its sistering temperature before it is recovered.
One embodiment for carrying out this flattening treatment of a sistered thin body or substrate tape , . .

.

RD-16,446 ~Z35147 - --comprises sandwiching the sistered non-flat substrate or tape between two plates of a material which has no significant deleterious effect thereon such as molybdenum or tungsten, or an alloy containing at least about 80% by weight of tungsten or molybdenum. The sandwiched substrate or tape is separated from the plates by a thin layer, preferably a discontinuous coating, preferably a discontinuous monolayer, of aluminum nitride powder preferably just sufficient to prevent the body from sticking lo to the surfaces of the plates during the flattening heat treatment. The flattening pressure is determinable empirically and depends largely on the particular sistered body, the particular flattening temperature and flattening time period. The flattening treatment should have no significant deleterious effect on the sistered body. A
decrease in flattening temperature requires an increase in flattening pressure or flattening time. Generally, at a temperature ranging from about 1850C or about 1890C to about 2050C, the applied flattening pressure ranges from about .03 psi to about lo psi, preferably from about .06 psi to about .50 psi, and more preferably from about lo psi to about .30 psi. Typically, for example, heating the sandwiched sistered body at the sistering temperature under a pressure of from about .03 psi to about .5 psi for l hour in nitrogen produces a flat body useful as a substrate, especially as a supporting substrate for a semiconductor such as a silicon chip.
The present invention makes it possible to fabric gate simple, complex and/or hollow shaped polycrystalline aluminum nitride ceramic articles directly. Specifically, the present sistered body can be produced in the form of a useful shaped article without machining or without any significant machining, such as a hollow shaped article for RD-16,446 1235~7 use as a container, a crucible, a thin walled tube, a long rod, a spherical body, a tape, substrate or carrier. It is useful as a sheath for temperature sensors. It is especial-lye useful as a substrate for a semiconductor such as a silicon chip. The dimensions of the present sistered body differ from those of the unsintered body, by the extent of shrinkage, i.e. densification,-which occurs during stinter-in.
The present ceramic body has a number of uses. In the form of a thin flat piece of uniform thickness, or having no significant difference in its thickness, i.e. in the form of a substrate or tape, it is especially useful as packaging for integrated circuits and as a substrate for an integrated circuit, particularly as a substrate for a semi conducting So chip for use in computers.
The invention is further illustrated by the following examples wherein the procedure was as follows, unless otherwise stated:
The starting aluminum nitride powder contained oxygen in an amount of less than 4% by weight.
The starting aluminum nitride powder was greater than 99% pure Awn exclusive of oxygen.
In Examples lo 11, 13, and 15 of Table III, the starting Awn powder had a surface area of 0.5 mug In Example 14 of Table III the Awn powder had a surface area of 1 6 mug In Examples 5 and 6 of Table II, and AYE and 12B
of Table III, the starting aluminum nitride powder had a surface area of 3.84 mug (0.479 micron) and contained 2.10 wit% oxygen as determined by neutron activation analysis.
In the remaining examples of Tables II and III, the starting aluminum nitride powder had a surface area of RD-16,446 1235~47

4.96 mug (0.371 micron) and contained 2.25 wit% oxygen as determined by neutron activation analysis.
In all of the examples of Table II and Examples PA, 9B, AYE and 12B of Table III, the Yo-yo powder, before any mixing, i.e. as received, had a surface area of about 2.75 mug In Examples 10, 11 and 15 of Table III, the Yo-yo powder, before mixing, had a surface area of 0.6 mug In Examples 13 and 14 of Table III, YOKE was added as a precursor for Yo-yo.
The carbon used in all of the examples of Table II
was graphite and in Examples 10, 11 and 15 of Table III, it had a specific surface area of 25 mug as listed by the vender, and in the remaining examples of Tables II and III, it had a specific surface area of 200 mug (0.017 micron) as listed by the vendor.
Non-aqueous Hutton was used to carry out the mixing, i.e. milling, of the powders in all of the examples of Tables II and III.
In all of the examples of Tables II and III, the milling media was hot pressed aluminum nitride in the approximate form of cubes or rectangles having a density of about 100%.
In Examples lay lo, 2, 5 and 6 of Table II and 10, 11, AYE, 12B, 13, 14 and 15 of Table III, the Awn, Yo-yo and carbon powders were immersed in non-agueous Hutton containing oleic acid in an amount of about 0.7% by weight of the aluminum nitride powder in a plastic jar and Libra-tory milled in the closed jar at room temperature for about lo hours in Examples lay lo and 2, and for about 16 hours in Examples 5, 6, 10, 11, AYE, 12B, 13, 14 and 15, producing the given powder mixture. In the remaining examples of Tables II and III, no oleic acid was used, and the Awn, Yo-yo and carbon powders were immersed in non-aqueous Hutton in a . .
. .

RD-1~,446 ~;~35~47 plastic jar and vibratory milled in the closed jar at room temperature for a period of time which for Example 3 was about 91 hours, for Examples PA and 4B, it was about 20 hours, for Examples PA, 7B and 8, it was about 68 hours, and for Examples PA and 9B it was about 46 hours.
In all of the Examples of Tables II and III, the milled liquid dispersion of the given powder mixture was dried in air at ambient pressure under a heat lamp for about 20 minutes and during such drying, the mixture picked up oxygen from the air.
In all of the Examples of Tables II and III, the dried milled powder mixture was die pressed at 5 Kpsi in air at room temperature to produce a compact having a density of roughly 55% of its theoretical density.
In those examples of Tables II and III, wherein the sistered body is given as being of A size or of B size, the compacts were in the form of a disk, in those examples wherein the sistered body is given as being of C size, the compacts were in the form of a bar, and in those examples wherein the sistered body is given as being of D size, the compacts were in the form of a substrate which was a thin flat piece, like a tape, of uniform thickness, or of a thickness which did not differ significantly.
In Table II the composition of the mixture of powders is shown as Powder Mixture whereas in Table III it is shown as Powders Added.
In all of the examples of Table II and Table III
except Examples PA, 4B, 5, 6, PA, 7B, I AYE, 12B, 13, 14 and 15, the given powder mixture as well as the compact formed therefrom had a composition wherein the equivalent %
of yttrium and aluminum ranged between points J and A of Figure 4.

RD-16,446 1~3S147 In Examples PA, 4B, 5, 6, PA, 7B and 3 of Table II, and AYE, 12B, 13, 14 and 15 of Table III, the given powder mixture as well as the compact formed therefrom had a composition wherein the equivalent % of yttrium and aluminum were outside the range of from point J to point A of Figure 4.
The equivalent % composition of Y, Al, 0 and N of the compacts of all of the Examples of Tables II and III, i.e. before deoxidation, was outside the composition defined lo and encompassed by polygon PlJFA4 of Figure 4.
In all of the examples of Tables II and III, the aluminum nitride in the compact before deoxidation contained oxygen in an amount ranging from greater than about 1.42% by weight to less than about 4.70% by weight of the aluminum nitride.
The composition of the deoxidized compacts of all of the Examples of Tables II and III, except Examples PA, 4B, 5, 6, PA, 7B, 8, AYE, 12B, 13, 14 and 15, were defined and encompassed by polygon PlJFA4 of Figure 4 but did not include lines JO and A4F.
In each of the examples of Tables II and III, one compact was formed from the given powder mixture and was given the heat treatment shown in Tables II and III. Also, the examples in Tables II and III having the same number but including the letters A or B indicate that they were carried out in an identical manner, i . e. the powder mixtures were prepared and formed into two compacts in the same manner and the two compacts were heat treated under identical conditions, i.e. the two compacts were placed side by side in the furnace and given the same heat treatment simultaneously, and these examples numbered with an A or B
may be referred to herein by their number only.

RD-16,446 :123514~ -In all of the examples of Tables II and III, the same atmosphere was used to carry out the deoxidation of the compacts as was used to carry out the sistering of the deoxidized compact except that the atmosphere to carry out the deoxidization was fed into the furnace at a rate of 1 SKIFF to promote removal of the gases produced by dockside-lion, and the flow rate during sistering was less than about . 1 SUE.
The atmosphere during all of the heat treatment in all of the examples in Tables II and III was at ambient pressure which was atmospheric or about atmospheric pressure.
The furnace was a molybdenum heat element furnace.
The compacts were heated in the furnace to the given deoxidation temperature at the rate of about 100C per minute and then to the given sistering temperature at the rate of about 50C per minute.
The sistering atmosphere was at ambient pressure, i.e. atmospheric or about atmospheric pressure.
At the completion of heat treatment, the samples were furnace-cooled to about room temperature.
All of the examples of Tables II and III were carried out in substantially the same manner except as indicated in Tables II and III, and except as indicated herein.
Carbon content of the sistered body was determined by a standard chemical analysis technique.
Based on the predetermined oxygen content of the starting Awn powders and the measured compositions of the resulting sistered bodies, as well as other experiments, it was calculated or estimated that in every example in Tables II and III, the aluminum nitride in the compact before RD-16,446 :lZ35~47 deoxidation had an oxygen content of about 0.3% by weight higher than that of the starting aluminum nitride powder.
Measured oxygen content was determined by neutron activation analysis and is given in wit%, which is % by weight of the sistered body.
In Tables II and III, in those examples where the oxygen content of the sistered body was measured, the equivalent % composition of the sistered body was calculated from the starting powder composition and from the given measured oxygen content of the sistered body. The Y, Al, N
and oxygen are assumed to have their conventional valences of: +3, +3, -3, -2, respectively. In the sistered bodies, the equivalent percent amount of Y and Al was assumed to be the same as that in the starting powder. During processing, the amount of oxygen gain and nitrogen loss was assumed to have occurred by the overall reaction:

2 Awn 3/202 Aye No (13) During deoxidation, the amount of oxygen loss and nitrogen gain was assumed to have occurred by the overall reaction:

Aye 3C No (14) The nitrogen content of the sistered body was determined by knowing the initial oxygen content of the starting aluminum nitride powder and measuring the oxygen content of the sistered body and assuming that reactions 13 and 14 have occurred.
In Tables II and III, an approximation sign is used in front of the equivalent percent oxygen for sistered bodies whose oxygen content was not measured Since RD-16,446 ~35~47 examples having the same number but including the letter A
or B were carried out under the same conditions to produce the given pair of sistered bodies simultaneously, this pair of sistered bodies will have the same oxygen content, and therefore, the oxygen content of one such sistered body is assumed to be the same US the measured oxygen content of the other such sistered body. Also, in Tables II and III, the equivalent % oxygen content of the sistered body of Example 2 (Sample 108D) is assumed not to differ significantly from the equivalent % oxygen content of the sistered body of Example lo (Sample AYE) and the equivalent % oxygen content of the sistered body of Example 11 (Sample 175B) is assumed not to differ significantly from the equivalent %
oxygen content of the sistered body of Example 10 (AYE).
The equivalent % oxygen content of the sistered bodies of Example 3 (Sample 94C), Example 5 (Sample 150B) and Example 10 (Sample AYE) was calculated from the X-ray diffraction analysis data.
4 The equivalent % oxygen of Examples 12B (13lD1), AYE), AYE) and AYE) were calculated from the following equation:

O = (2.91 R + 3.82) Y
3.86 where O = equivalent % oxygen Y = equivalent % yttrium R = v/o YO-YO
v/o YO-YO + v/o Yo-yo The equivalent % oxygen in Example 8 (WOK) is assumed to be the same as the equivalent percent oxygen as in another experiment where the powder mixture had the same composition, which was carried out in argon, and where the . .

RD-16,446 lZ35~4~

oxygen content of the sistered body was measured. The equivalent % oxygen in Example 6 (Sample 150C) is assumed to be the same as in another experiment where the powder mixture had the same composition and where the equivalent %
oxygen was calculated from the X-ray diffraction analysis data.
Weight loss in Tables II and III is the difference between the weight of the compact after die pressing and the resulting sistered body.
Density of the sistered body was determined by the Archimedes method.
Porosity in % by volume of the sistered body was determined by knowing the theoretical density of the stinter-Ed body on the basis of its composition and comparing that to the density measured using the following equation:

porosity = (1 - measured density ) 100% (15) theoretical density Phase composition of the sistered body was deter-mined by optical microscopy and X-ray diffraction analysis, and each sistered body was comprised in % by volume of the sistered body of aluminum nitride phase and the given volume % of the given second phases. The X-ray diffraction anal-skis for volume % of each second phase is accurate to about + 20% of the given value.
The thermal conductivity of the sistered body of Example 8 (WOK) was measured by laser flash at about 25~C.
The thermal conductivity of the sistered body of all of the remaining examples was measured at 25~C by a steady state heat-flow method using a rod shaped sample ~0.4 cm x 0.4 cm x 2.2 cm sectioned from the sistered body. This method was originally devised by A. Beret in 1888 and is RD-16,446 -` 1235i4~

described in an article by G. A. Slack in the "Encyclopedic Dictionary of Physics", Ed. by J. Thewlis, Pergamon, Oxford, 1961. In this technique the sample is placed inside a high-vacuum chamber, heat is supplied at one end by an electrical heater, and the temperatures are measured with fine-wire thermocouples. The sample is surrounded by a guard cylinder. The absolute accuracy is about + 3% and the repeatability is about + 1%. As a comparison, the thermal conductivity of an Aye single crystal was measured with a similar apparatus to be 0.44 W/cm~K at about 22C.
In Tables II and III, the size of the resulting sistered body is given as A, B, C or D. The body of A size was in the form of a disk about .17 inch in thickness and about .32 inch in diameter. The body of B size was also in the form of a disk with a thickness of about 0.27 inch and a diameter of about 0.50 inch. The body of C size was in the shape of a bar measuring about 0.16 inch x 0.16 inch x 1.7 inches. The body of D size was in the form of a substrate, i.e. a thin piece of uniform thickness, or of no significant difference in thickness, having a diameter of about 1.5 inch and a thickness of .042 inch.
In all of the examples of Tables II and III, the compacts were placed on a molybdenum plate and then given the heat treatment shown in Tables II and III.
In all of the Examples of Tables II and III
wherein the sistered body was of C size or of D size, the starting compact was separated from the molybdenum plate by a thin discontinuous layer of Awn powder.
The sistered body of Example 2 exhibited some non-flatness, i.e. exhibited some warping, and was subjected to a flattening treatment. Specifically, the sistered body produced in Example 2 was sandwiched between a pair of molybdenum plates. The sandwiched sistered body was RD-16,446 lZ3514~

separated from the molybdenum plates by a thin discontinuous coating or monolayer of aluminum nitride powder which was just sufficient to prevent sticking of the sistered body to the plates during the flattening treatment period. The top molybdenum plate exerted a pressure of about 0.11 psi on the sistered body. The sandwiched sistered body was heated in nitrogen, i.e. the same atmosphere used to stinter it, to about 1900C where it was held for about l hour and then furnace cooled to about room temperature. The resulting lo sistered body was flat and was of uniform thickness, i.e.
its thickness did not differ significantly. This flat sistered body would be useful as a supporting substrate for a semiconductor such as a silicon chip.

Example 1 lo 0.932 grams of Yo-yo powder and 0.237 grams of graphite powder were added to 17.01 grams of aluminum nitride powder and the mixture, along with aluminum nitride milling media, was immersed in non-aqueous Hutton containing oleic acid in an amount of about 0.7% by weight of the aluminum nitride in a plastic jar and vibratory milled in the closed jar at room temperature for about 18 hours. The resulting dispersion was dried in air under a heat lamp for about 20 minutes and during such drying, the aluminum nitride picked up oxygen from the air. During milling, the mixture picked up 0.772 gram Awn due to wear of the Awn milling media.
Equivalent portions of the resulting dried mixture were die pressed producing compacts.
Two of the compacts were placed side by side on a molybdenum plate.

RD-16,446 lZ35147 The compacts were heated in nitrogen to 1500 where they were held for 1/2 hour, then the temperature was raised to 1600C where it was held for 1/2 hour, and then the temperature was raised to 1870DC where it was held for 1 hour.
This example is shown as Examples lo and lo in Table II. Specifically, one of the sistered bodies, Example lo, had a measured oxygen content of 1.75% by weight of the body of the sistered body. Also, it had a phase composition comprised of Awn and 4.6% by volume of the body of YO-YO.
Also, it had an equivalent % composition comprised of 3.10%
0, (100%-3.10%) or 96.90% N, l.B8% Y and (100%-1.8B%) or 98.12% Al.
The compact used in Example 2 was produced in Example 1. Specifically, in Example 2, one compact was heated to 1600C where it was held for 1 hour and then the temperature was raised to l900DC where it was held for 1 hour.
In Example 3, one compact was heated to 1500C
where it was held for 1/2 hour, then the temperature was raised to 1600C where it was held for 1 hour and then it was raised to 1950~C where it was held for 1 hour.
Examples PA, 4B, 5, 6, PA, 7B, PA, 9B, 10, 11, 13, 14 and 15 were carried out in the same manner as Example 2 except as indicated herein and except as shown in Tables II
and III. Also, Examples 8, AYE and 12B were carried out in the same manner as Example 3 except as indicated herein and except as shown in Tables II and III.

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RD-16,445 12~5147 Examples lay lo, 2, 3, PA and 9B illustrate the present invention. The sistered body produced in Examples lay lo, 2, 3, PA and 9B is useful for packaging of integrated circuits as well as for use as a substrate or carrier for a semiconductor such as a silicon chip.
Examples lo and lo illustrate the present invention, and have a composition which is on line Ply of Figure 4. Knowing that the thermal conductivity of the Awn sistered body decreases with increasing content of second phase, and based on other experiments and a comparison of Examples lo and lo with Example 5 where the sistered body contained significantly more second phase, it is known that the sistered body produced in Examples lo and lo had a thermal conductivity greater than 1.42 W/cm-X at 25C.
Example 2 illustrates the present invention.
Based on a comparison of Example 2 with Examples lo and lo which have the same powder mixture, and based on other experiments, it is known that the sistered body of Example 2 had a composition which was the same or which did not differ significantly from that of the sistered bodies of Examples lo and lo, Specifically, the sistered body produced in Example 2 was comprised of Awn phase and about 4.6% by volume of the sistered body of Yule phase, and has a composition which is on about line Ply of Figure 4. Also, knowing that the thermal conductivity of the Awn sistered body decreases with increasing content of second phase, and based on other experiments and a comparison of Example 2 with Example 5 where the sistered body contained significantly more second phase, it is known that the sistered body produced in Example 2 had a thermal conductivity greater than 1.42 W/cm-K at 25~C.
The sistered body produced in Example 3, which illustrates the present invention, has a composition defined RD-16,446 i2~35~47 and encompassed by polygon PlJFA4 excluding lines JO and A4F
of Figure 4.
Examples PA and 9B illustrate the present invention, and have a composition defined and encompassed by polygon PlJFA4 excluding lines JO and A4F of Figure 4.
Knowing that the thermal conductivity of the Awn sistered body decreases with increasing content of second phase, and based on other experiments and a comparison of Example PA
and with Example S where the sistered body contained significantly more second phase, it is known that the sistered body produced in Examples PA and 9B had a thermal conductivity greater than 1.42 W/cm-K at 25C.
The powder mixtures of Examples PA and 4B
contained less than 0.3 equivalent % yttrium. The equiva-lent % composition of the sistered bodies of Examples PA and4B fell outside polygon PlJFA4 of Figure 4 and specifically they fell below point Pi of Figure 4. In Examples PA and 4B, the sistered bodies had a porosity higher than 10% by volume of the body which illustrates the difficulty of sistering in this composition area below point Pi of Figure 4.
In Examples 5 and 6, the sistered bodies had a composition outside polygon Pledge of Figure 4, and more specifically, above line JO.
Examples PA and 7B illustrate that even though there was a deoxidation of the compact, the use of the argon atmosphere resulted in a large amount of carbon being left in the sistered body.
Example 8 illustrates that the use of an argon atmosphere results in a sistered body having a low thermal conductivity.
Examples 10 and 11 illustrate that the minimum sistering temperature increases with an increase in particle ,"

~235147 ROD 16,446 size of Awn. Specifically, at the composition -1.3 equivalent % oxygen and 0.64 equivalent %
yttrium, it is difficult to stinter a compact, even at 2000C, prepared from the particle size combination of Awn, YO-YO and carbon of about 0.5 m go 0.6 m go and 25 m go respectively.
Reference is made to U.S. Patent 4,478,785 entitled HIGH THRILL CONDUCTIVITY ALUMINUM NITRIDE
CERAMIC BODY issued October 23, 1984 in the names of I. C. Huseby and C. F. Bobik and assigned to the assignee hereof, there is disclosed the process comprising forming a mixture comprised of aluminum nitride powder and free carbon wherein the aluminum nitride has a predetermined oxygen content higher than about 0.8% by weight and wherein the amount of free carbon reacts with such oxygen content to produce a deoxidized powder or compact having an oxygen content ranging from greater than about 0.35% by weight to about 1.1% by weight and which is at least 20% by weightAlower than the predetermined oxygen content, heating the mixture of a compact thereof to react the carbon and oxygen producing the deoxidized aluminum nitride, and sistering a compact of the deoxidized aluminum nitride producing a ceramic body having a density greater than 85~ of theoretical and a thermal conductivity greater than 0.5 W/cm K at 22C.

-

Claims (37)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a sintered polycrys-talline aluminum nitride ceramic body having a composition defined and encompassed by polygon P1JFA4 but not including lines JF and A4F of Figure 4, a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C which comprises the steps:
a) forming a mixture comprised of oxygen-containing aluminum nitride powder, yttrium oxide, and free carbon, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges between points J and A4 of Figure 4, said yttrium ranging from greater than about 0.3 equivalent % to less than about 2.5 equivalent %, said aluminum ranging from greater than about 97.5 equivalent %
to less than about 99.7 equivalent %, said compact having an equivalent % composition of Y, Al, O and N outside the composition defined and encompassed by polygon P1JFA4 of Figure 4, (b) heating said compact in a nitrogen-containing nonoxidizing atmosphere at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, O
and N is defined and encompassed by polygon PlJFA4 but not including lines JF and A4F of Figure 4, said free carbon being in an amount which produces said deoxidized compact, and (c) sintering said deoxidized compact in a nitrogen-containing nonoxidizing atmosphere at a temperature of at least about 1850°C producing said polycrystalline body.

2. The process according to claim 1 wherein said nitrogen-containing atmosphere in step (b) contains sufficient nitrogen to facilitate deoxidation of the aluminum nitride to produce said sintered body.

3. The process according to claim 1 wherein said nitrogen-containing atmosphere in step (c) contains sufficient nitrogen to prevent significant weight loss of said aluminum nitride.

4. The process according to claim 1 wherein said process is carried out at ambient pressure.

5. The process according to claim 1 wherein the aluminum nitride in said compact in step (a) before said deoxidation of step (b) contains oxygen in an amount ranging from greater than about 1.0% by weight to less than about 4.7% by weight of said aluminum nitride.

6. The process according to claim 1 wherein said aluminum nitride in step (a) has a specific surface area ranging up to about 10 m2/g and said free carbon has a specific surface area greater than about 10 m2/g.

7. The process according to claim 1 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges between points J
and A2 of Figure 4, said yttrium ranging from greater than about 0.65 equivalent % to less than about 2.5 equivalent %, said aluminum ranging from greater than about 97.5 equivalent % to less than about 99.35 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined and encompassed by polygon A3JFA2 but does not include lines A3J, JF and A2F of Figure 4.

8. The process according to claim 1 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point A3 up to point A4 of Figure 4, said yttrium ranging from about 0.3 equivalent % to about 0.85 equivalent %, said aluminum ranging from about 99.15 equivalent % to about 99.7 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of A1, Y, O and N is defined and encompassed by polygon P1A3A2A4 but does not include lines P1A3, A3A2 and A2A4 of Figure 4.

9. The process according to claim 1 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point P1 to point A3 of Figure 4, said yttrium ranging from about 0.35 equivalent % to about 0.85 equivalent %, said aluminum ranging from about 99.15 equivalent % to about 99.65 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined by line P1A3 of Figure 4, and said sintering temperature is at least about 1860°C.

10. The process according to claim 1 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point A3 up to point J of Figure 4, said yttrium ranging from about 0.85 equivalent % to less than about 2.5 equivalent %, said aluminum ranging from greater than about 97.5 equivalent %
to about 99.15 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined by line A3J but not including point J of Figure 4, and said sintering temperature is at least about 1860°C.

11. A process for producing a sintered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon A3JFA2 but not including lines A3J, JF and A2F of Figure 4, a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C which comprises the steps:
(a) forming a mixture comprised of an oxygen-containing aluminum nitride powder, yttrium oxide, and free carbon, said free carbon having a specific surface area greater than about 100 m2/g, the aluminum nitride powder in said mixture having a specific surface area ranging from about 3.4 m2/g to about 6.0 m2/g, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges between points J
and A2 of Figure 4, said yttrium ranging from greater than about 0.65 equivalent % to less than about 2.5 equivalent %, said aluminum ranging from greater than about 97.5 equivalent % to less than about 99.35 equivalent %, said compact having an equivalent % composition of Y, Al, O and N
outside the composition defined and encompassed by polygon P1JFA4 of Figure 4, the aluminum nitride in said compact containing oxygen in an amount ranging from greater than about 1.42% by weight to less than about 4.70% by weight of said aluminum nitride, (b) heating said compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of A1, Y, O and N is defined and encompassed by polygon A3JFA2 but not including lines A3J, JF and A2F of Figure 4, the aluminum nitride in said compact before said deoxidation by said carbon having an oxygen content ranging from greater than about 1.42% by weight to less than about 4.70% by weight of said aluminum nitride, said free carbon being in an amount which produces said deoxidized compact, and (c) sintering said deoxidized compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1885°C to about 1970°C
producing said polycrystalline body.

12. The process according to claim 11 wherein the sintering temperature ranges from about 1890°C to about 1950°C, said aluminum nitride powder in said mixture has a specific surface area ranging from about 3.5 m2/g to about 6.0 m2/g, and said sintered body has a porosity of less than about 1% by volume of said body.

13. The process according to claim 11 wherein the sintering temperature ranges from about 1940°C to about 1970°C, and said sintered body contains carbon in an amount of less than about .04% by weight of said body and has a porosity of less than about 1% by volume of said body and a thermal conductivity greater than about 1.57 W/cm?K at 25°C.

14. A process for producing a sintered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon P1JFA4 but not including lines JF and A4F of Figure 4, a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C which comprises the steps:
(a) forming a mixture comprised of an oxygen-containing aluminum nitride powder, yttrium oxide or precursor therefor, and a carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50°C to about 1000°C to free carbon and gaseous product of decomposition which vaporizes away, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges between points J and A4 of Figure 4, said yttrium ranging from greater than about 0.3 equivalent % to less than about 2.5 equivalent %, said aluminum ranging from greater than about 97.5 equivalent % to less than about 99.7 equivalent % aluminum, said compact having an equivalent %
composition of Y, Al, O and N outside the composition defined and encompassed by polygon P1JFA4 of Figure 4, (b) heating said compact in a nonoxidizing atmosphere at a temperature up to about 1200°C thereby providing yttrium oxide and free carbon, (c) heating said compact in a nitrogen-containing nonoxidizing atmosphere at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon PlJFA4 but not including lines JF and A4F of Figure 4, said free carbon being in an amount which produces said deoxidized compact, and (d) sintering said deoxidized compact in a nitrogen-containing nonoxidizing atmosphere at a temperature of at least about 1850°C producing said polycrystalline body.

15. The process according to claim 14 wherein said nitrogen-containing atmosphere in step (c) contains sufficient nitrogen to facilitate deoxidation of the aluminum nitride to produce said sintered body.

16. The process according to claim 14 wherein said nitrogen-containing atmosphere in step (d) contains sufficient nitrogen to prevent significant weight loss of said aluminum nitride.

17. The process according to claim 14 wherein said process is carried out at ambient pressure.

18. The process according to claim 14 wherein the aluminum nitride in said compact in step (a) before said deoxidation of step (c) contains oxygen in an amount ranging from greater than about 1.0% by weight to less than about 4.7% by weight of said aluminum nitride.

19. The process according to claim 14 wherein said aluminum nitride in step (a) has a specific surface area ranging up to about 10 m2/g and said free carbon has a specific surface area greater than about 10 m2/g.

20. The process according to claim 14 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges between points J
and A2 of Figure 4, said yttrium ranging from greater than about 0.65 equivalent % to less than about 2.5 equivalent %, said aluminum ranging from greater than about 97.5 equivalent % to less than about 99.35 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, 0 and N is defined and encompassed by polygon A3JFA2 but does not include lines A3J, JF and A2F of Figure 4.

21. The process according to claim 14 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point A3 up to point A4 of Figure 4, said yttrium ranging from about 0.3 equivalent % to about 0.85 equivalent %, said aluminum ranging from about 99.15 equivalent % to about 99.7 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of A1, Y, 0 and N is defined and encompassed by polygon P1A3A2A4 but does not include lines P1A3, A3A2 and A2A4 of Figure 4.

22. The process according to claim 14 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point P1 to point A3 of Figure 4, said yttrium ranging from about 0.35 equivalent % to about 0.85 equivalent %, said aluminum ranging from about 99.15 equivalent % to about 99.65 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined by line P1A3 of Figure 4, and said sintering temperature is at least about 1860°C.

23. The process according to claim 14 wherein said mixture and said compact have a composition wherein the equivalent % of yttrium and aluminum ranges from point A3 up to point J of Figure 4, said yttrium ranging from about 0.85 equivalent % to less than about 2.5 equivalent %, said aluminum ranging from greater than about 97.5 equivalent %
to about 99.15 equivalent %, and wherein said sintered body and said deoxidized compact are comprised of a composition wherein the equivalent percent of Al, Y, O and N is defined by line A3J but not including point J of Figure 4, and said sintering temperature is at least about 1860°C.

24. A process for producing a sintered polycrystalline aluminum nitride ceramic body having a composition defined and encompassed by polygon A3JFA2 but not including lines A3J, JF and A2F of Figure 4, a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C which comprises the steps:
(a) forming a mixture comprised of aluminum nitride powder, yttrium oxide or precursor therefor, and a Claim 24 - Continued carbonaceous additive selected from the group consisting of free carbon, a carbonaceous organic material and mixtures thereof, said carbonaceous organic material thermally decomposing at a temperature ranging from about 50°C to about 1000°C to free carbon and gaseous product of decomposition which vaporizes away, said free carbon having a specific surface area greater than about 100 m2/g, the aluminum nitride powder in said mixture having a specific surface area ranging from about 3.4 m2/g to about 6.0 m2/g, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges between points J and A2 of Figure 4, said yttrium ranging from greater than about 0.65 equivalent % to less than about 2.5 equivalent %, said aluminum ranging from greater than about 97.5% equivalent %
to less than about 99.35 equivalent %, said compact having an equivalent % composition of Y, Al, 0 and N outside the composition defined and encompassed by polygon PlJFA4 of Figure 4, the aluminum nitride in said compact containing oxygen in an amount ranging from greater than about 1.42% by weight to less than about 4.70% by weight of said aluminum nitride, (b) heating said compact in a nonoxidizing atmosphere at a temperature up to about 1200°C thereby providing yttrium oxide and free carbon, (c) heating said compact at ambient pressure in a nitrogen-containing atmosphere containing at least about 25%
by volume nitrogen at a temperature ranging from about 1350°C to a temperature sufficient to deoxidize the compact but below its pore closing temperature reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, 0 and N is defined and encompassed by polygon A3JFA2 but not including lines A3J, JF and A2F of Figure 4, the aluminum nitride in said compact before said deoxidation by said carbon having an oxygen content ranging from greater than about 1.42% by weight to less than about 4.70% by weight of said aluminum nitride, said free carbon being in an amount which produces said deoxidized compact, and (d) sintering said deoxidized compact at ambient pressure in a nitrogen-containing nonoxidizing atmosphere containing at least about 25% by volume nitrogen at a temperature ranging from about 1885°C to about 1970°C
producing said polycrystalline body.

25. The process according to claim 24 wherein said sintering temperature ranges from about 1890°C to about 1950°C, said aluminum nitride powder in said mixture has a specific surface area ranging from about 3.5 m2/g to about 6.0 m2/g, and said sintered body has a porosity of less than about 1% by volume of said body.

26. The process according to claim 24 wherein said sintering temperature ranges from about 1940°C to about 1970°C, and said sintered body contains carbon in an amount of less than about .04% by weight of said body and has a porosity of less than about 1% by volume of said body and a thermal conductivity greater than about 1.57 W/cm.K at 25°C.

27. A polycrystalline body having a composition defined and encompassed by polygon PlJFA4 of Figure 4 but excluding lines JF and A4F which is comprised of from greater than about 0.3 equivalent % yttrium to less than about 2.5 equivalent % yttrium, from greater than about 97.5 equivalent % aluminum to less than about 99.7 equivalent %
aluminum, from about 0.85 equivalent % oxygen to less than about 4.1 equivalent % oxygen and from greater than about 95.9 equivalent % nitrogen to about 99.15 equivalent %
nitrogen, said polycrystalline body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.

28. A polycrystalline body having a composition defined and encompassed by polygon A3JFA2 of Figure 4 but excluding lines A3J, JF and A2F which is comprised of from greater than about 0.65 equivalent % yttrium to less than about 2.5 equivalent % yttrium, from greater than about 97.5 equivalent % aluminum up to about 99.35 equivalent %
aluminum, from about 1.6 equivalent % oxygen to less than about 4.1 equivalent % oxygen and from greater than about 95.9 equivalent % nitrogen to about 98.4 equivalent %
nitrogen, said polycrystalline body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.

29. A polycrystalline body having a composition defined and encompassed by polygon P1A3A2A4 of Figure 4 but excluding lines P1A3, A3A2 and A2A4 which is comprised of from greater than about 0.3 equivalent % yttrium to about 0.85 equivalent % yttrium, from about 99.15 equivalent %
aluminum to about 99.7 equivalent % aluminum, from greater than about 0.85 equivalent % oxygen to less than about 2.1 equivalent % oxygen and from greater than about 97.9 equivalent % nitrogen to less than about 99.15 equivalent %
nitrogen, said polycrystalline body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.

30. A polycrystalline body having a composition defined by line P1A3 of Figure 4 which is comprised of from about 0.35 equivalent % yttrium to about 0.85 equivalent %
yttrium, from about 99.15 equivalent % aluminum to about 99.65 equivalent % aluminum, from about 0.85 equivalent %
oxygen to about 1.6 equivalent % oxygen and from about 98.4 equivalent % nitrogen to about 99.15 equivalent % nitrogen, said polycrystalline body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C.

31. A polycrystalline body having a composition defined by line A3J of Figure 4 but excluding point J which is comprised of from about 0.85 equivalent % yttrium to less than about 2.5 equivalent % yttrium, from greater than about 97.5 equivalent % aluminum to about 99.15 equivalent %
aluminum, from about 1.6 equivalent % oxygen to less than about 4.1 equivalent % oxygen and from greater than about 95.9 equivalent % nitrogen to about 98.4 equivalent %
nitrogen, said polycrystalline body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm?K at 25°C.

32. A polycrystalline body having a phase composition comprised of AlN, YAlO3 and Y4Al2O9 wherein the total amount of said YAlO3 and Y4Al2O9 phases ranges from greater than about 0.8% by volume to less than about 6.0% by volume of the total volume of said body, said YAlO3 phase ranging from a trace amount to less than about 4.2% by volume of said sintered body, said Y4Al2O9 phase ranging from a trace amount to less than about 6.0% by volume of said sintered body, said body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.

33. A polycrystalline body having a phase composition comprised of A1N and Y4A12o9 wherein the amount of said Y4A1209 phase ranges from about 0.8% by volume to less than about 2.1% by volume of the total volume of said body, said body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.

34. A polycrystalline body having a phase composition comprised of A1N and Y4A1209 wherein the amount of said Y4A12O9 phase ranges from about 2.1% by volume to less than about 6.0% by volume of the total volume of said body, said body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.

35. A polycrystalline body having a phase composition comprised of A1N, YA103 and Y4A1209 wherein the total amount of YA103 and Y4A1209 phases ranges from greater than about 0.8% by volume to less than about 2.1% by volume of the total volume of said body, said YA103 phase ranging from a trace amount to less than about 1.7% by volume of the sintered body, said Y4A1209 phase ranging from a trace amount to less than about 2.1% by volume of the sintered body, said body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.

36. A polycrystalline body having a phase composition comprised of A1N, YA103 and Y4A1209 wherein the total amount of YA103 and Y4A1209 phase ranges from greater than about 1.7% by volume to less than about 6.0% by volume of the total volume of said body, said YA103 phase ranging from a trace amount to less than about 4.2% by volume of the sintered body, said Y4A1209 phase ranging from a trace amount to less than about 6.0% by volume of the sintered body, said body having a porosity of less than about 10% by volume of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.

37. The polycrystalline body according to claim 36 wherein said body contains carbon in an amount of less than .04% by weight of said body and has a porosity of less than about 1% by volume of said body and a thermal conductivity greater than 1.57 W/cm.K at 25°C.