CA1250130A - Aluminum oxide powders and products - Google Patents
Aluminum oxide powders and productsInfo
- Publication number
- CA1250130A CA1250130A CA000520385A CA520385A CA1250130A CA 1250130 A CA1250130 A CA 1250130A CA 000520385 A CA000520385 A CA 000520385A CA 520385 A CA520385 A CA 520385A CA 1250130 A CA1250130 A CA 1250130A
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- CA
- Canada
- Prior art keywords
- powder
- alpha alumina
- micron
- alumina
- alpha
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
- C04B35/10—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on aluminium oxide
- C04B35/111—Fine ceramics
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F7/00—Compounds of aluminium
- C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
- C01F7/44—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water
- C01F7/441—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by calcination
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/60—Particles characterised by their size
- C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/60—Particles characterised by their size
- C01P2004/62—Submicrometer sized, i.e. from 0.1-1 micrometer
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/10—Solid density
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/14—Pore volume
Abstract
ABSTRACT
Fine alumina powders for making alumina bodies are produced by seeding of a non-alpha alumina with submicron alpha alumina particles, firing, and crushing and milling the fired product to a fine powder. The powder so produced may be formed by conventional techniques such as pressing, granulating, slip casting, tape casting, and extrusion, and sintered to produce the desired fired product
Fine alumina powders for making alumina bodies are produced by seeding of a non-alpha alumina with submicron alpha alumina particles, firing, and crushing and milling the fired product to a fine powder. The powder so produced may be formed by conventional techniques such as pressing, granulating, slip casting, tape casting, and extrusion, and sintered to produce the desired fired product
Description
~Z5~)~3(~
BACKGRO~ND OF THE INVENTION
Most current commercial methods for producing anhydrous alumina powders involve comminution steps whereby the initial powder particles or agglomerates are broken down to a desired size range.
The most commonly desired form of alumina for ceramic article fabrication is a very fine powder. The crystals o alpha alumina are extremely hard and durable and hence quite difficult to mill to fine sizes; the process requires long milling times and is energy intensive. It is possible to mill hydrated aluminas (such as are produced in the Bayer process) more easily than the alpha form, however, sodium contamination is generally a problem and agglomeration arises when the hydrate is calcined to alpha and further milling is required. A major reason that this agglomeration occurs is that ~1100-C is required to convert the hydrated form to alpha; at this temperature partial sintering and grain growth occur.
A key feature in the millability of alpha aluminas is the size of the ultimate crystals since sintered bonds are weaker than atomic crystalline bonds. It follows then that in milling alpha powders, it is desirable to comminute the agglomerated systems rather than ultimate crystals. Clearly then, the ultimate crystal size should be equal to or less than the desired milled particle size to minimize time and energy requirements for the size reduction process.
One of the major problems with current comminution technology of alpha aluminas is this ultimate crystal size factor. Bayer process derived alpha alumina i8 produced from the hydrated form and is therefore necessarily calcined at 1100-C or more. It is difficult to obtain submicron ultimate crystals in this process, particularly if sodium removal at high temperatures is r 1~`
3(~
~ 3 --also carried out. In order to manufacture a submicron alpha powder, the ultimate crystals themselves must be ground to a finer size, a difficult task.
According to the present invention we have developed techniques for producing extremely fine (<.5 micron) ultimate crystals of alpha alumina; such aluminas have the desirous properties of being relatively easy to reduce in size to their ultimate crystal size and, being highly reactive, sintèr at lower than conventional temperatures by virtue of their particle small size (high surface energy).
It is, accordingly, an object of the present invention to produce alumina (preferably the alpha form) with ultimate crystal size less than one micron (prsferably less than 0.5 micron) which can be deagglomerated, as by milling, at much less than conventional milling times and energies and will be much more thermally reaction (i.e. easily sinterable) than conventional powders, and to produce fired monolithic bodies therefrom.
DESCRI PTI ON OF THE I NVENTI ON
In published E.P.O. application No. 0152768 (August 28, 1985) has been described the seeding of an alumina gel with very fine t<.5 micron) alpha alumina to produce a fine grained (<1 micron) ceramic which can be sintered to >95% density at <1400-C in 5 minutes.
Differential thermal analysis data has shown that the seeding lowers the temperature of transition to alpha alumina by about 100-C. This fact, coupled with the further grain size reduction encouraged by seeds themselves, provides a means to produce fine alpha at lower than conventional temperatures.
The preferred method for production is to prepare a sol or gel of pseudoboehmite (microcrystalline boehmite) and to mix in the seeding agent. In ~eneral, the finer 1~
~25~30 the seeds are, the finer the ultimate crystals will be and hence the quality of seeds used is critical in determining the microstructure of the product alumina.
One method of producing seeds is to simply allow alpha alumina grinding media to wear against themselves in a vibratory mill and hence produce fines (seeds). We have found such seeds extremely useful when added in about 1% by weight (weight of seeds in finished product alumina). The relationship of grain size to seed size has been derived approximately as 3.6 times the seed size (when a spherical seed and a cubic grain shape are assumed) for a 1% addition of seed.
It is obvious from the above relation that to obtain submicron grain sizes, seeds less than 0.3 micron in size are required. In a distribution of seeds, the finest fraction does the major portion of promoting nucleation to the alpha phase.
Once the seeded gel has been prepared it is simply dried and then may be calcined to alpha. After conversion, the alpha is milled to a desired size range.
~ he preferred method for conversion to alpha is to use the lowest temperature which is convenient to obtain the desired ratio of alpha/gamma (or other intermediate anhydrous form). Generally 95% or greater alpha is desired but in some applications the gamma or other anhydrous f G rm is useful. By utilizing the lowest temperatures, the degree of sintering and densification is minimized and comminution is made much easier. For example, if the dried gel is calcined at 1350-C 90%
theoretical density is approached and millin~ becomes difficult even though ultimate crystal size is fine, whereas at ~1250'C (depending on seed qualities) milling is relatively easy.
The milled product may be utilized as is in the size range produced or further classified to narrow the size distribution. When milling is carried out to reduce ~SV~3~
essentially all agglomerates, an essentially monosized powder is produced. The milling may be carried out dry or wet. If desired, various organic solvents may be used instead of water. Bimodal powders may be produced by mixing a coarser monosized powder of say 0.6 microns with a finer powder of say 0.07 microns in suitable proportions. Monosized powders are useful for making dense smooth surfaces, for example, for electronic substrates and for high green density and for very fine controlled pore size membranes or tubes for microfiltration.
The slurries after milling/classification may be used directly for tape casting applications, for slip casting, or may be dried and processed by dry powders as in isostatic pressing, hot pressing, hot isostatic pressing, and the like.
EXAMPLE I
This illustrates the conversion of the seeded gel to alpha at low temperatures.
An alumina gel was produced by stirring 200 grams of Condea PURAL SB alumina monohydrate into 1700 ml of distilled water to which 100 ml of 14 wt% nitric acid had been added. The sample was split in half and to one half was added "milled water'~ so that the alpha alumina content of the seeds relative to the alpha alumina content of the gel was 1% by weight.
The samples were placed in a drying oven at 90 C for 4 days and then crushed with a rolling pin into powder of -54 mesh. Samples were then placed into a preheated muffle furnace at 900 C plus or minus 10 (static air) and aliquots removed periodically and examined for alpha alumina content by x-ray diffraction. The following results were obtained:
Heating Time % of alpha A1203 (~3%) thrs. @ 900 C) Seeded Unseedad 3~
0. 67 4 ND
- Not Determined Clearly the act of seeding allows formation of alpha to proceed smoothly at 900 C whereas the unseeded gel converts only extremely slowly.
The X-ray diffraction patterns also exhibited line broadening in the alpha cases, which calculated to ultimate crystal size of less than 0.1 micron.
Studies at other temperatures and DTA investigations led to an empirical formula for the rate of conversion of the 1% seeded gel, using seed made by vibratory milling, in water, of alumina media:
Ln (gamma) = -6. 67t X 1012 x e exp [42~275 where Ln i~; the natural logarithm (gamma) is the fraction of "gamma" remaining e is the base of natural logarithms T is the absolute temperature (-K) t is the time in minutes Thus to produce 95% alpha at 1000'C requires 2 hours and 1100'C requires 11 minutes, for 1% of seed having an estimated si3e of about 0.02 microns. For minimum size of ultimate crystals and for maximum ease in milling minimum sintering to get the desired results should be used.
EXAMPLE II
This example illustrates ease o-E milling of the seeded calcined gel.
Five 80 lb. lots of dried gel were crushed to finer than 132 microns. The gel was prepared from Condea PUR~L
~2S~3~
NG with water and nitric acid similar to example I. The seeding level in this case was 0.6% wt%. Each 80 pound lot was processed through an electrically heated rotary kiln at various temperatures for about 5 minutes. The 5 results are tabulated as follows:
Approximate % alpha alumina Firing Tem~erature by X-ra~ deffraction The two samples which contained 100% alpha were characterized further:
Firing Temp. ('C) Ultimate Crystal Size Chemistry 1200 .07 microns Na20 .01 1250 0.1 microns MgO .02 Sio2 .06 CaO .02 TiO2 .23 Fe203 .04 - by SEM
The impurities, except for most of the titania, were from the seed material produced from commercial grinding media. Higher purity seed can be achieved by employing ve.y high purity grinding media, such as high purity fused alumina, or dense sintered bodies of the present invention. These powders were wet vibratory milled and the empirical observation was that they were much more friable and took only a fraction of the time to mill than conventional alpha powders took.
The process described herein produces an alumina powder which is more easily mi~lable than conventional powders and which has an ultimate alpha crystal size of less than 1 micron. This fine ultimate crystal size makes the powder more reactive (i.e. easily sinterable) ~;~S~D~3V
and is an important factor in providing smoother surface finishes in ceramic articles/substrates. The product is thus of superior quality and is made more economically than current commercially practised ceramic articles/substrates. The product is thus of superior quality and is made more economically than current commercially practised.
Since the seeding process is dependent on the actual number of seeding sites, the amount of seed added should ideally be specified in terms of numbers rather than weight %. Such numbers may be calculated if the particle si2e distribution is accurately known. It can bs calculated that for submicron product microstructure, seeding numbers on the order of greater than 1012 per cubic centimeter are required. However, often it i5 impractical to obtain an accurate particle size distribution of the seeds and the empirical approach of seeding by weight % addition of a specific batch of seed material to obtain the desired submicron microstructure ~0 may be employed. For example, a specific lot of seed material generated by attrition of grinding media, observed approximately as having most particles less than 0.05 microns, is effective in producing submicron grain when added as 0.1 weight % or more.
~5 If the seed size is diminished, lesser weights can be used. The optimum effectiveness of seeds would be observed when the seeds are at their smallest possible physical size, i.e. at the critical nucleation size.
This size is currently unknown but is certainly less than 0.02 microns.
It is recognized that the product microstructure may be controlled as described above and approximate values may be obtained by using the following approximate equations.
N = 6 w G/~d3[(100-w)S ~ wG]
Grain diameter = 1/cube root of N
~5~30 Where: N = no. of seeds per cuhic centimeter w = weight % of seeds G = grain density, grams/cc S = seed density, grams/cc and d = diameter of seeds in centimeters For all practical purposes G and S are equal to 4.0, the theoretical density of alpha alumina.
Many assumptions are used in the derivation of the above equation and, as such, they are merely meant to provide general guide li~es and recourse to emperical tests much generally be carried out to verify structure.
It is to be noted also that the grain size equations given represent minimum grain sizes, i.e. assume no growth after 100% conversion to alpha alumina. If it is p.referred to have larger grains, a combination of the seed size, seed amount, time, and temperature control can be varied to generate desired structures.
Polycrystalline granular or pelletized material may be made from the powders of this invention, in addition to shaped articles made by tape casting, molding, pressing, extruding, and slip casting. Granular products have utility as abrasives, proppants, milling media, and refractories, in addition to use as fillers in fine ceramics.
~he alumina of the present invention may be used as an ingredient in mixes containing other ceramic materials such as zirconia or stabilized zirconia, Si3N4, AlN, etc.
EXAMPLE III
A smooth alpha alumina electronic substrate with very fine microstructure was produced by the following procedure. A seeded dried gel crushed to pass through a sieve with 132 micron sized openings was fired at 1200-C
for five minutes, containing about 1% of seed produced by vibratory milling of a mass of ~ x ~ inch cylindrical alpha alumina grinding media in water for several hours.
, ,.
~2~ 3~
The tired powder was then milled in isopropyl alcohol (50 to 60% solids) with glycerol trioleate (1%) to break the agglomerates. The powder was then dried and mixed with a carrier, defloculant, and binder:
5powder 50%
xylene 21.82%
isopropyl alcohol 14.41%
defloculent 0.22% (menhaden ~ish oil) polyethylene glycol 4.73%) lOpolyvinyl butvar 2.97%) plasticizer/binder butyl benzyl phthalate 5.83%) The mix was then homogenized in a Sweco vibratory mill for 20 to 24 hours. The resultant slurry was then de-aired for one hour and cast into a green sheet using a doctor blade setting between 0.035 to 0.050 inches. The green tape was dried at about 70-F plus or minus 4 C in a relative humidity held at 45 to 55%. Squares, 2 inches by 2 inches, were punched out using a steel die. The squares were heated at 600 C to drive off the volatiles, and then sintered at 1370 to 1450-C. The resulting substrate squares were 1-5/8 x 1-5/8 inches in size, had a density of at least 3.65 g/cc and were water impervious. They had a surface finish of 2 to 4 microinches. Scanning electron microscope examination revealed a very uniform microstructure at 20000 x magnification. The particles were uniformly sized at about 0.2 to 0.3 microns, were equiaxed, and at that magnification, generally spherical and softly rounded in shape when observed on a broken surface.
BACKGRO~ND OF THE INVENTION
Most current commercial methods for producing anhydrous alumina powders involve comminution steps whereby the initial powder particles or agglomerates are broken down to a desired size range.
The most commonly desired form of alumina for ceramic article fabrication is a very fine powder. The crystals o alpha alumina are extremely hard and durable and hence quite difficult to mill to fine sizes; the process requires long milling times and is energy intensive. It is possible to mill hydrated aluminas (such as are produced in the Bayer process) more easily than the alpha form, however, sodium contamination is generally a problem and agglomeration arises when the hydrate is calcined to alpha and further milling is required. A major reason that this agglomeration occurs is that ~1100-C is required to convert the hydrated form to alpha; at this temperature partial sintering and grain growth occur.
A key feature in the millability of alpha aluminas is the size of the ultimate crystals since sintered bonds are weaker than atomic crystalline bonds. It follows then that in milling alpha powders, it is desirable to comminute the agglomerated systems rather than ultimate crystals. Clearly then, the ultimate crystal size should be equal to or less than the desired milled particle size to minimize time and energy requirements for the size reduction process.
One of the major problems with current comminution technology of alpha aluminas is this ultimate crystal size factor. Bayer process derived alpha alumina i8 produced from the hydrated form and is therefore necessarily calcined at 1100-C or more. It is difficult to obtain submicron ultimate crystals in this process, particularly if sodium removal at high temperatures is r 1~`
3(~
~ 3 --also carried out. In order to manufacture a submicron alpha powder, the ultimate crystals themselves must be ground to a finer size, a difficult task.
According to the present invention we have developed techniques for producing extremely fine (<.5 micron) ultimate crystals of alpha alumina; such aluminas have the desirous properties of being relatively easy to reduce in size to their ultimate crystal size and, being highly reactive, sintèr at lower than conventional temperatures by virtue of their particle small size (high surface energy).
It is, accordingly, an object of the present invention to produce alumina (preferably the alpha form) with ultimate crystal size less than one micron (prsferably less than 0.5 micron) which can be deagglomerated, as by milling, at much less than conventional milling times and energies and will be much more thermally reaction (i.e. easily sinterable) than conventional powders, and to produce fired monolithic bodies therefrom.
DESCRI PTI ON OF THE I NVENTI ON
In published E.P.O. application No. 0152768 (August 28, 1985) has been described the seeding of an alumina gel with very fine t<.5 micron) alpha alumina to produce a fine grained (<1 micron) ceramic which can be sintered to >95% density at <1400-C in 5 minutes.
Differential thermal analysis data has shown that the seeding lowers the temperature of transition to alpha alumina by about 100-C. This fact, coupled with the further grain size reduction encouraged by seeds themselves, provides a means to produce fine alpha at lower than conventional temperatures.
The preferred method for production is to prepare a sol or gel of pseudoboehmite (microcrystalline boehmite) and to mix in the seeding agent. In ~eneral, the finer 1~
~25~30 the seeds are, the finer the ultimate crystals will be and hence the quality of seeds used is critical in determining the microstructure of the product alumina.
One method of producing seeds is to simply allow alpha alumina grinding media to wear against themselves in a vibratory mill and hence produce fines (seeds). We have found such seeds extremely useful when added in about 1% by weight (weight of seeds in finished product alumina). The relationship of grain size to seed size has been derived approximately as 3.6 times the seed size (when a spherical seed and a cubic grain shape are assumed) for a 1% addition of seed.
It is obvious from the above relation that to obtain submicron grain sizes, seeds less than 0.3 micron in size are required. In a distribution of seeds, the finest fraction does the major portion of promoting nucleation to the alpha phase.
Once the seeded gel has been prepared it is simply dried and then may be calcined to alpha. After conversion, the alpha is milled to a desired size range.
~ he preferred method for conversion to alpha is to use the lowest temperature which is convenient to obtain the desired ratio of alpha/gamma (or other intermediate anhydrous form). Generally 95% or greater alpha is desired but in some applications the gamma or other anhydrous f G rm is useful. By utilizing the lowest temperatures, the degree of sintering and densification is minimized and comminution is made much easier. For example, if the dried gel is calcined at 1350-C 90%
theoretical density is approached and millin~ becomes difficult even though ultimate crystal size is fine, whereas at ~1250'C (depending on seed qualities) milling is relatively easy.
The milled product may be utilized as is in the size range produced or further classified to narrow the size distribution. When milling is carried out to reduce ~SV~3~
essentially all agglomerates, an essentially monosized powder is produced. The milling may be carried out dry or wet. If desired, various organic solvents may be used instead of water. Bimodal powders may be produced by mixing a coarser monosized powder of say 0.6 microns with a finer powder of say 0.07 microns in suitable proportions. Monosized powders are useful for making dense smooth surfaces, for example, for electronic substrates and for high green density and for very fine controlled pore size membranes or tubes for microfiltration.
The slurries after milling/classification may be used directly for tape casting applications, for slip casting, or may be dried and processed by dry powders as in isostatic pressing, hot pressing, hot isostatic pressing, and the like.
EXAMPLE I
This illustrates the conversion of the seeded gel to alpha at low temperatures.
An alumina gel was produced by stirring 200 grams of Condea PURAL SB alumina monohydrate into 1700 ml of distilled water to which 100 ml of 14 wt% nitric acid had been added. The sample was split in half and to one half was added "milled water'~ so that the alpha alumina content of the seeds relative to the alpha alumina content of the gel was 1% by weight.
The samples were placed in a drying oven at 90 C for 4 days and then crushed with a rolling pin into powder of -54 mesh. Samples were then placed into a preheated muffle furnace at 900 C plus or minus 10 (static air) and aliquots removed periodically and examined for alpha alumina content by x-ray diffraction. The following results were obtained:
Heating Time % of alpha A1203 (~3%) thrs. @ 900 C) Seeded Unseedad 3~
0. 67 4 ND
- Not Determined Clearly the act of seeding allows formation of alpha to proceed smoothly at 900 C whereas the unseeded gel converts only extremely slowly.
The X-ray diffraction patterns also exhibited line broadening in the alpha cases, which calculated to ultimate crystal size of less than 0.1 micron.
Studies at other temperatures and DTA investigations led to an empirical formula for the rate of conversion of the 1% seeded gel, using seed made by vibratory milling, in water, of alumina media:
Ln (gamma) = -6. 67t X 1012 x e exp [42~275 where Ln i~; the natural logarithm (gamma) is the fraction of "gamma" remaining e is the base of natural logarithms T is the absolute temperature (-K) t is the time in minutes Thus to produce 95% alpha at 1000'C requires 2 hours and 1100'C requires 11 minutes, for 1% of seed having an estimated si3e of about 0.02 microns. For minimum size of ultimate crystals and for maximum ease in milling minimum sintering to get the desired results should be used.
EXAMPLE II
This example illustrates ease o-E milling of the seeded calcined gel.
Five 80 lb. lots of dried gel were crushed to finer than 132 microns. The gel was prepared from Condea PUR~L
~2S~3~
NG with water and nitric acid similar to example I. The seeding level in this case was 0.6% wt%. Each 80 pound lot was processed through an electrically heated rotary kiln at various temperatures for about 5 minutes. The 5 results are tabulated as follows:
Approximate % alpha alumina Firing Tem~erature by X-ra~ deffraction The two samples which contained 100% alpha were characterized further:
Firing Temp. ('C) Ultimate Crystal Size Chemistry 1200 .07 microns Na20 .01 1250 0.1 microns MgO .02 Sio2 .06 CaO .02 TiO2 .23 Fe203 .04 - by SEM
The impurities, except for most of the titania, were from the seed material produced from commercial grinding media. Higher purity seed can be achieved by employing ve.y high purity grinding media, such as high purity fused alumina, or dense sintered bodies of the present invention. These powders were wet vibratory milled and the empirical observation was that they were much more friable and took only a fraction of the time to mill than conventional alpha powders took.
The process described herein produces an alumina powder which is more easily mi~lable than conventional powders and which has an ultimate alpha crystal size of less than 1 micron. This fine ultimate crystal size makes the powder more reactive (i.e. easily sinterable) ~;~S~D~3V
and is an important factor in providing smoother surface finishes in ceramic articles/substrates. The product is thus of superior quality and is made more economically than current commercially practised ceramic articles/substrates. The product is thus of superior quality and is made more economically than current commercially practised.
Since the seeding process is dependent on the actual number of seeding sites, the amount of seed added should ideally be specified in terms of numbers rather than weight %. Such numbers may be calculated if the particle si2e distribution is accurately known. It can bs calculated that for submicron product microstructure, seeding numbers on the order of greater than 1012 per cubic centimeter are required. However, often it i5 impractical to obtain an accurate particle size distribution of the seeds and the empirical approach of seeding by weight % addition of a specific batch of seed material to obtain the desired submicron microstructure ~0 may be employed. For example, a specific lot of seed material generated by attrition of grinding media, observed approximately as having most particles less than 0.05 microns, is effective in producing submicron grain when added as 0.1 weight % or more.
~5 If the seed size is diminished, lesser weights can be used. The optimum effectiveness of seeds would be observed when the seeds are at their smallest possible physical size, i.e. at the critical nucleation size.
This size is currently unknown but is certainly less than 0.02 microns.
It is recognized that the product microstructure may be controlled as described above and approximate values may be obtained by using the following approximate equations.
N = 6 w G/~d3[(100-w)S ~ wG]
Grain diameter = 1/cube root of N
~5~30 Where: N = no. of seeds per cuhic centimeter w = weight % of seeds G = grain density, grams/cc S = seed density, grams/cc and d = diameter of seeds in centimeters For all practical purposes G and S are equal to 4.0, the theoretical density of alpha alumina.
Many assumptions are used in the derivation of the above equation and, as such, they are merely meant to provide general guide li~es and recourse to emperical tests much generally be carried out to verify structure.
It is to be noted also that the grain size equations given represent minimum grain sizes, i.e. assume no growth after 100% conversion to alpha alumina. If it is p.referred to have larger grains, a combination of the seed size, seed amount, time, and temperature control can be varied to generate desired structures.
Polycrystalline granular or pelletized material may be made from the powders of this invention, in addition to shaped articles made by tape casting, molding, pressing, extruding, and slip casting. Granular products have utility as abrasives, proppants, milling media, and refractories, in addition to use as fillers in fine ceramics.
~he alumina of the present invention may be used as an ingredient in mixes containing other ceramic materials such as zirconia or stabilized zirconia, Si3N4, AlN, etc.
EXAMPLE III
A smooth alpha alumina electronic substrate with very fine microstructure was produced by the following procedure. A seeded dried gel crushed to pass through a sieve with 132 micron sized openings was fired at 1200-C
for five minutes, containing about 1% of seed produced by vibratory milling of a mass of ~ x ~ inch cylindrical alpha alumina grinding media in water for several hours.
, ,.
~2~ 3~
The tired powder was then milled in isopropyl alcohol (50 to 60% solids) with glycerol trioleate (1%) to break the agglomerates. The powder was then dried and mixed with a carrier, defloculant, and binder:
5powder 50%
xylene 21.82%
isopropyl alcohol 14.41%
defloculent 0.22% (menhaden ~ish oil) polyethylene glycol 4.73%) lOpolyvinyl butvar 2.97%) plasticizer/binder butyl benzyl phthalate 5.83%) The mix was then homogenized in a Sweco vibratory mill for 20 to 24 hours. The resultant slurry was then de-aired for one hour and cast into a green sheet using a doctor blade setting between 0.035 to 0.050 inches. The green tape was dried at about 70-F plus or minus 4 C in a relative humidity held at 45 to 55%. Squares, 2 inches by 2 inches, were punched out using a steel die. The squares were heated at 600 C to drive off the volatiles, and then sintered at 1370 to 1450-C. The resulting substrate squares were 1-5/8 x 1-5/8 inches in size, had a density of at least 3.65 g/cc and were water impervious. They had a surface finish of 2 to 4 microinches. Scanning electron microscope examination revealed a very uniform microstructure at 20000 x magnification. The particles were uniformly sized at about 0.2 to 0.3 microns, were equiaxed, and at that magnification, generally spherical and softly rounded in shape when observed on a broken surface.
Claims (12)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of preparing an alumina powder containing alpha alumina crystals having a size below 1 micron comprising seeding a precursor of alpha alumina with submicron alpha alumina particles, sintering the seeded precursor at 900 to 1350 C for a time sufficient to convert at least a portion of the non-alpha alumina to alpha alumina, and crushing the sintered product to a powder having a particle size of less than 1 micron.
2. A method as in claim 1 in which the seed particles have a size of 0.1 micron or less.
3. A method as in claim 1 in which the seed particles are finer than 0.1 micron and the sintering is stopped when the growing alpha alumina crystals have reached a size of no greater than 0.1 microns.
4. An alpha alumina powder consisting of a mixture of equiaxed single crystals of seeded alpha alumina having a size below 1 micron and sintered porous agglomerates of said single crystals.
5. A material as in claim 4 in which the agglomerates are all finer than 1 micron.
6. A milled powder consisting of seeded alpha alumina particles finer than 1 micron.
7. A milled powder made according to claim 1.
8. A method of making a monolithic polycrystalline body of alpha alumina comprising forming the powder of claim 6 into a body and sintering it at a temperature above 1100°C for a time sufficient to bond the crystals.
9. A method of making a monolithic polycrystalline body comprising forming the powder of claim 1 into a body and sintering to achieve bonding.
10. A ceramic body made according to the method of claim 9.
11. A ceramic body made with the powder of claim 5.
12. An alumina substrate made from the powder of claim 1 having a surface roughness as fired of 4 microinches or finer, a density of at least 3.65, and being made by tape casting a plasticized mix of the seeded powder; drying and firing the casting at 1450°C or less.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US800,494 | 1985-11-21 | ||
| US06/800,494 US4657754A (en) | 1985-11-21 | 1985-11-21 | Aluminum oxide powders and process |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1250130A true CA1250130A (en) | 1989-02-21 |
Family
ID=25178539
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000520385A Expired CA1250130A (en) | 1985-11-21 | 1986-10-14 | Aluminum oxide powders and products |
Country Status (9)
| Country | Link |
|---|---|
| US (1) | US4657754A (en) |
| EP (1) | EP0224118B1 (en) |
| JP (1) | JPH07100608B2 (en) |
| AU (1) | AU591944B2 (en) |
| BR (1) | BR8605726A (en) |
| CA (1) | CA1250130A (en) |
| DE (1) | DE3688775T2 (en) |
| ES (1) | ES2043597T3 (en) |
| ZA (1) | ZA868550B (en) |
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| ATE26819T1 (en) * | 1982-08-24 | 1987-05-15 | Alusuisse | PROCESS FOR THE PRODUCTION OF CRYSTALLINE ALUMINUM. |
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-
1985
- 1985-11-21 US US06/800,494 patent/US4657754A/en not_active Expired - Lifetime
-
1986
- 1986-10-14 CA CA000520385A patent/CA1250130A/en not_active Expired
- 1986-11-11 ES ES86115634T patent/ES2043597T3/en not_active Expired - Lifetime
- 1986-11-11 DE DE86115634T patent/DE3688775T2/en not_active Expired - Lifetime
- 1986-11-11 ZA ZA868550A patent/ZA868550B/en unknown
- 1986-11-11 EP EP86115634A patent/EP0224118B1/en not_active Expired - Lifetime
- 1986-11-12 JP JP61267858A patent/JPH07100608B2/en not_active Expired - Fee Related
- 1986-11-18 AU AU65357/86A patent/AU591944B2/en not_active Expired
- 1986-11-20 BR BR8605726A patent/BR8605726A/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| ES2043597T3 (en) | 1994-01-01 |
| AU591944B2 (en) | 1989-12-21 |
| JPH07100608B2 (en) | 1995-11-01 |
| JPS62128918A (en) | 1987-06-11 |
| ZA868550B (en) | 1987-07-29 |
| EP0224118B1 (en) | 1993-07-28 |
| DE3688775D1 (en) | 1993-09-02 |
| EP0224118A1 (en) | 1987-06-03 |
| BR8605726A (en) | 1987-08-18 |
| US4657754A (en) | 1987-04-14 |
| DE3688775T2 (en) | 1993-12-23 |
| AU6535786A (en) | 1987-05-28 |
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