DE10344189A1 - Manufacture of opaque quartz glass composite material, used as starting material of permanent shaping-die manufacture of solar silicon melting, involves forming composite slip by mixing quartz glass granules and homogenous base slip - Google Patents

Abstract

The silica particles having specific particle size are obtained by grain refining non-crystal silica granules in a liquid. The composite slip having density of at least 1.85 g/cm 3>is obtained by mixing quartz glass granules with the prepared homogenous base slip. The composite slip is introduced into a shaping die and formation of porous substrate is performed, to obtain an opaque quartz glass composite material. The silica particles having particle size (D 50) of 15 Microm or less and D 90of 50 Microm or less are obtained by performing grain refining of non-crystal silica granules in a liquid. The homogenous base slip having solid content of at least 75% is manufactured by homogenizing the slip by pH reduction. A composite slip having density of at least 1.85 g/cm 3>is obtained by mixing the quartz glass granules with the homogenous base slip. The composite slip is introduced into a shaping die, then a porous substrate is formed by drying and sintering the substrate, to obtain an opaque quartz glass composite material. The slip contains silica particles of size less than 100 Microm and quartz glass granules of size greater than 100 Microm. Independent claims are included for; (1) A composite material; and (2) Use of the above composite material as starting material of permanent shaping-die manufacture of solar silicon melting.

Description

The The invention relates to a method for the production of a casting from a granular material by providing a composite slurry, the material particles in the size range below 10 μm as well as material grain in the size range above 10 μm contains Homogenize the composite slip, pour the composite slip into a mold to form a blue body, drying the blue body below Formation of a porous Green body, and thermal solidification of the green body.

Farther The invention relates to a casting of a ceramic or glassy Composite material that has a homogeneous matrix of the material, in which grains of the material with a grain size of more than 10 μm embedded, and having a wall, within the one casting axis runs, the while of the casting runs in the vertical.

The so-called Schlickergießverfahren is in ceramic processing technology for the production of ceramic or glassy components, in particular for the production of quartz glass components, common. From the DE 101 14 484 A1 is a Schlickergießverfahren for producing a composite material with high SiO ₂ content known. Here, a quartz glass grain, which serves as a filler, embedded in a SiO ₂ -containing matrix, which is completely made of synthetically produced SiO ₂ . The matrix has an SiO ₂ content of at least 99% by weight and consists of at least two particle fractions, each of which is formed as granules of nanoscale, amorphous, synthetically produced SiO ₂ primary particles having an average primary particle size of less than 100 nm ,

The composite material obtained by the known method has only closed pores, has no crystalline components, and its density is 2.1 g / cm ³ . It is characterized by high thermal shock resistance and chemical resistance. The composite material is therefore suitable for use as a mold for the melting of solar grade silicon. However, the synthetic starting materials for making the composite are expensive.

Another method for the production of a quartz glass crucible by slip casting is in the DE 199 43 103 A1 described. This is based on a suspension of high-purity, amorphous SiO ₂ particles, the degree of filling is at least 83.96 wt .-%. The high SiO ₂ filling level is achieved by using SiO ₂ particles with a bimodal particle size distribution, wherein about 95% of the SiO _{2 used is present} as SiO ₂ powder produced in the oxyhydrogen flame with an average particle size of 30 μm.

A method for producing a refractory, sintered casting of glassy material and a casting according to the aforementioned type are known from DE 693 06 169 T2 known. In the silica glass article described therein, two fractions of fine-grained SiO ₂ powder form a binder phase for a further SiO ₂ -containing component in the form of coarse SiO ₂ grains having a particle size between 40 μm and 1000 μm. One of the two finer-grained SiO ₂ powders is present as fumed silica, which is formed from substantially spherical particles with particle sizes in the range between 0.2 μm and 0.6 μm. The preferably used quartz dust originates from the melting process and the reduction of zirconium dioxide. The quartz glass starting components are premixed in a dry grinding process and then a slip is produced therefrom with the addition of a stabilizer. The weight proportions of the individual components are 54% (coarse SiO ₂ grains), 33% (fine-grained SiO ₂ particles) and 13% (fumed silica). The slurry is degassed in vacuum and poured into a plaster mold. The green body thus produced is dried and sintered in a furnace at 1050 ° C to form a composite component. Due to the manufacturing process, the cast body has a casting axis that scales during casting in the vertical and that runs in the direction of growth of the wall of the forming blue body.

For the microstructure of the component, coarse quartz glass grains embedded in a relatively continuous matrix of finer particles and spherical particles of fumed silica are characteristic. The component has an open porosity of 13% and its density is 1.91 g / cm ³ . The crystallographic analysis shows a cristobalite content of less than 2%.

by virtue of his open - that is called continuous - porosity is the known grain material for Components that require tightness or high purity, not unrestricted used. Metallic melts can pass through the pores in the Part wall penetrate and lead to leaks.

In a variant of the method according to DE 693 06 169 T2 For example, fused or micromilled fused silica is used instead of the fumed silica resulting from the melting process and the reduction of zirconium dioxide. However, the silica glass article thus obtained showed a low density and a very low flexural strength.

Problems with the Schlickergießverfahren arise in particular by the shrinkage of the Green body during drying and sintering. Shrinkage cracks can occur and the dimensional accuracy of the components is often low.

The Dry shrinkage also complicates the manufacture of components so-called core casting, in which the slip is poured around a core, which is removed after drying. By shrinkage during drying the green body shrinks the core up and tearing doing so, or the core can not without damaging the green body of this to be pulled.

As a result the use of molds which absorb the liquid of the suspension, forms frequently a pronounced Gießseele in the form of juxtaposed pores leading to a weakening of the structure to lead. Large format Components can only conditionally and with a simple design, often to the relief the removal of the blue body must be generated from the form conical walls.

There the slip casting process in and of itself a cost-effective Production of components - too with complex geometry - allows it would be extremely desirable to avoid the mentioned disadvantages in the production of quartz glass.

Of the Invention is therefore the object of a procedure for producing a casting from a composite material by means of slip which is associated with the drying shrinkage Reduce or eliminate disadvantages, and that the production of Castings with cylindrical Walls facilitated and in particular for the production of large format Castings is suitable.

Farther the invention has the object, a casting of a ceramic or glassy composite material to provide characterized by a high strength.

Regarding of the method, this object is based on the above-mentioned Process according to the invention thereby solved, that the composite slip with a solids content of at least 80 wt .-% is poured into a mold, which is one for the liquid impermeable Form wall, wherein the Kompositschlicker after casting under Formation of a frozen blue body to a cooling temperature is cooled below the freezing point of the liquid, and wherein the drying is heating of the frozen blue body to a heating temperature below the boiling point of the liquid includes.

Of the The composite material to be produced consists of a homogeneous matrix from the material into which coarser grains embedded in the material. The material is quartz glass, silicon nitride, silicon carbide, zirconium silicate, silicon, or a mixture of these substances.

• 1. Der Kompositschlicker enthält feinkörnige Teilchen des Werkstoffes und gröbere Körnung des Werkstoffes mit einer mittleren Teilchengröße oberhalb von 10 μm. Er zeichnet sich durch einen hohen Feststoffgehalt von mindestens 80 Gew.-%, bevorzugt mindestens 90 Gew.-% aus. Der hohe Feststoffgehalt in Verbindung mit dem Zusatz der gröberen Werkstoffkörnung wirkt der Schwindung des Grünkörpers entgegen und verbessert dessen Formstabilität, was sich auf die Maßhaltigkeit von Bauteilen aus dem Kompositwerkstoff günstig auswirkt.
• 2. Der Kompositschlicker wird in eine Form gegossen, welche mindestens eine für die Flüssigkeit undurchlässige Formwandung aufweist. Die für die Flüssigkeit undurchlässige Formwandung verhindert einen Verlust von Flüssigkeit aus dem Kompositschlicker während einer Phase, in der die darin suspendierten Werkstoffteilchen noch frei beweglich sind. Diese Beweglichkeit der Werkstoffteilchen würde ansonsten zu einem Materietransport in Richtung auf die saugende Formwandung und damit zur der Ausbildung einer Gießseele oder anderer Material-Inhomogenitäten führen. Es hat sich gezeigt, dass dadurch seelenfreie Blaukörper mit einer homogenen Feststoffverteilung und mit hoher Festigkeit erhalten werden. Dieser vorteilhafte Effekt tritt – wenn auch in vermindertem Umfang – auch dann ein, wenn die Form außer der mindestens einen undurchlässigen Formwandung auch Wandungsbereiche aufweist, die für die Flüssigkeit und für darin eingeschlossene Luftblasen durchlässig sind. Bei vollständig undurchlässigen Formwandungen erfolgt kein „Nachsacken" des Kompositschlickers, so dass eine glatte Gießoberfläche erhalten wird, die keiner Nachbearbeitung bedarf. Die Form kann außer der mindestens einen undurchlässigen Formwandung auch eine für Gase poröse Formwandung oder für Gase poröse Formwandungsbereiche aufweisen, welche zum Entlüften dienen und die Entstehung Oberflächenblasen verhindern. Als Werkstoff für die Ausbildung der Form haben sich teflon-beschichtetes Blech (wie Aluminium- oder Stahlblech) bewährt. Ein weiterer bevorzugter Formwerkstoff ist Silikon, bei dessen Einsatz besonders störstellenfreie (blasenfreie) Oberflächen erhalten werden, wenn er vor dem Einsatz ausgeheizt wird.
• 3. Der Kompositschlicker wird in der Form unter Bildung eines gefrorenen Blaukörpers auf eine Kühltemperatur unterhalb des Gefrierpunktes der Flüssigkeit gekühlt. Die Form wird entweder vor, während oder nach dem Gießen auf eine Temperatur unterhalb des Gefrierpunktes der Flüssigkeit der Suspension gekühlt. Dadurch kommt es zu einer Kristallbildung in der Flüssigkeit, im Fall von Wasser zu einer Eisbildung, die eine Koagulation von Werkstoff-Partikeln vor allem im Submikronbereich bewirkt und einer Entmischung oder Sedimentation im noch viskosen Blaukörper entgegenwirkt. Um eine Nadelbildung weitestgehend zu unterbinden, erfolgt das Gefrieren erfolgt so rasch wie möglich.
• 4. Zum Trocknen und Verfestigen wird der erstarrte Blaukörper im gefrorenen Zustand auf eine Heiztemperatur unterhalb der Siedetemperatur der Flüssigkeit erwärmt. Durch die vergleichsweise schonende Erwärmung wird ein langsameres Abdampfen der Flüssigkeit erreicht, so dass eine starke Gasbildung infolge einer innerhalb des gefrorenen Blaukörpers siedenden Flüssigkeit und damit einhergehende Beschädigungen des Blaukörpers vermieden werden. Zu dieser überraschend günstigen Wirkung des Trocknens des Grünkörpers durch Schockgefrieren und anschließendes schonendes Erwärmen in gefrorenem Zustand trägt der geringe Wassergehalt des Kompositschlickers und des Blaukörpers wesentlich bei.

The method according to the invention makes it possible to produce crack-free cast parts from the grain material by means of slip casting - also by core casting. Prerequisite for this is a high strength and a very low drying shrinkage of the green body. This requirement is achieved by the following process characteristics:

• 1. The Kompositschlicker contains fine-grained particles of the material and coarser grain size of the material with a mean particle size above 10 microns. It is characterized by a high solids content of at least 80 wt .-%, preferably at least 90 wt .-%. The high solids content in conjunction with the addition of the coarser material grain counteracts the shrinkage of the green body and improves its dimensional stability, which has a favorable effect on the dimensional stability of components made of the composite material.
• 2. The Kompositschlicker is poured into a mold which has at least one liquid-impermeable mold wall. The liquid-impermeable mold wall prevents loss of liquid from the composite slip during a phase in which the material particles suspended therein are still freely movable. This mobility of the material particles would otherwise lead to a transport of material in the direction of the absorbent mold wall and thus to the formation of a casting core or other material inhomogeneities. It has been found that thereby soul-free blue body are obtained with a homogeneous distribution of solids and high strength. This advantageous effect occurs - albeit to a lesser extent - even if the mold, in addition to the at least one impermeable mold wall, also has wall regions which are permeable to the liquid and to air bubbles trapped therein. In the case of completely impermeable mold walls, there is no "sagging" of the composite slip, so that a smooth casting surface is obtained which requires no further processing Teflon-coated sheet metal (such as aluminum or steel sheet) has proven to be a suitable material for forming the mold Free) surfaces are obtained when it is baked before use.
• 3. The composite slurry is cooled in the mold to a frozen blue body to a cooling temperature below the freezing point of the liquid. The mold is cooled either before, during or after casting to a temperature below the freezing point of the liquid of the suspension. This leads to crystal formation in the liquid, in the case of water to ice formation, which causes coagulation of material particles, especially in the submicron range and counteracts segregation or sedimentation in the still viscous blue body. In order to prevent needle formation as far as possible, freezing takes place as quickly as possible.
• 4. To dry and solidify the solidified blue body is heated in the frozen state to a heating temperature below the boiling temperature of the liquid. By comparatively gentle heating a slower evaporation of the liquid is achieved, so that a strong gas formation due to a boiling within the frozen blue body fluid and associated damage to the blue body can be avoided. The low water content of the composite slip and of the blue body contributes substantially to this surprisingly favorable effect of drying the green body by means of shock freezing and subsequent gentle heating in the frozen state.

Of the blue body demolded in the frozen state and then dried. The drying of the blue body But can also be done in the form, if this evaporation of the liquid little handicapped.

By the subsequent one thermal solidification of the green body (sintering at a temperature below the melting point of the material) obtain a mechanically stable casting which has a homogeneous pore and having a density distribution and a low open porosity, and that by a small total shrinkage (dry and sintering shrinkage) less than 4% and even less than 2,5% can.

When very cheap has a cooling temperature proved to be between minus 20 ° C and minus 80 ° C, preferably between minus 30 ° C and minus 60 ° C, lies.

The cooling temperature depends essentially on the freezing temperature of the liquid. These limits apply to liquids with a freezing temperature around 0 ° C, such as water or homogeneous mixtures of organic compounds with water. The further the cooling temperature is below the freezing temperature of the liquid, the faster the composite slip solidifies and the higher the strength of the frozen blue body. In the case of a casting made of quartz glass (material), a liquid containing tetraethylorthosilicate (TEOS) is preferably used, since the TEOS fraction of the liquid leads to an increased proportion of SiO ₂ in the composite slip and a higher strength and also to a reduction of freeze structures contributes.

Farther has it proven when the heating temperature is in the range between 40 ° C and 90 ° C, preferably in the range between 50 ° C and 80 ° C is.

It It has been shown that the removal of liquid by heating the shock-frozen blue body at a temperature in the range between 40 ° C and 90 ° C to a high strength contributes the resulting green body.

Prefers becomes the frozen blue body in a drying chamber, is removed from the moisture, dried.

at The drying chamber is, for example, a circulating air drying cabinet or a microwave drying oven. By removing the Moisture from the drying chamber will cause condensation of water vapor and moisture a superficial one Refreeze the surface texture to disturb and the green strength can reduce. Especially in microwave drying make sure that the temperature does not exceed 90 ° C.

In A first preferred variant of the method is the composite slip advantageously poured into a mold at a temperature is cooled below the freezing point of the liquid.

The Shape already shows when casting the composite slip a temperature below freezing the liquid on. This results in a particularly rapid cooling of the composite slip achieved and prevents sedimentation.

In an alternative and equally preferred method variant The composite slip is poured into a mold that is at a temperature is heated above the ambient temperature.

As a result, condensation of water from the ambient air on the mold walls is avoided before casting off the Kompositschlickers. Such condensations can lead to surface damage in the blue body. This procedure is therefore particularly important for high demands the surface quality and at high humidity for use.

The strength of the green body can be affected by the growth of very large crystals (ice crystals). In the case of quartz glass (material), although the SiO ₂ dissolved in the suspension already acts as an ice crystal growth inhibitor. However, it has proven to be advantageous in any case to additionally add an ice crystal growth-inhibiting substance to the composite slip.

The Addition of the ice crystal growth-inhibiting substance causes that possible many and for that preferably small crystals are formed. Effective substances of this kind are alcohols, such as isopropanol, as well as glycerol, polyethylene glycol, Polyacrylates and organosilicon compounds, such as TEOS, where An addition of nanoscale particles fulfills the same purpose (the latter however, at most 5% by weight, preferably at most 1% by weight).

To produce a cast part made of quartz glass, the composite slip contains amorphous SiO ₂ particles in the size range below 10 μm and quartz glass grains in the size range above 10 μm, preferably above 100 μm, and it has a density of at least 1.75 g / cm ³ poured into the mold.

The composite slip contains in this case fine-grained SiO ₂ particles with particle sizes <10 μm and coarser quartz glass grains having a mean particle size above 10 μm, preferably above 100 μm. It is characterized by a high density of at least 1.75 g / cm ³ , which is achieved by a high solids content. This density corresponds approximately to a solids content of at least 80 wt .-% SiO ₂ . The thermal solidification of the green body takes place in the case preferably in the temperature range between 800 ° C and 1440 ° C. By sintering a mechanically stable casting of opaque quartz glass is obtained, which has a homogeneous pore and density distribution and a low open porosity, and by a small Gesamtschwindung (dry and sintering shrinkage), which may be less than 2%, and by a high density of at least 1.80 g / cm ³ .

The following versions for the preparation of a composite slip refer to the case of Material of quartz glass. For other materials can the for Quartz glass characteristic parameters based on fewer trials and expert Knowledge to be customized.

A particularly high solids content of the green body is achieved when the composite slip is produced by SiO ₂ particles whose particle sizes and particle size distribution are characterized by a D ₅₀ value of not more than 15 μm and by a D ₉₀ value of not more than 50 μm Liquid are mixed, whereby a homogeneous base slip having a solids content of at least 75 wt .-% is produced, and wherein in the homogeneous base slip the quartz glass grain is mixed to form the Kompositschlickers.

Preferably, the mixing of the SiO ₂ particles and the liquid takes place thereby by grinding amorphous SiO ₂ grain in the liquid to form the SiO ₂ particles.

The high solids content of the base slip is obtained on the one hand by a special quality of the solid contained in the slip, namely by finely divided, amorphous SiO ₂ particles, which are characterized by particle sizes and a particle size distribution, by a D ₅₀ value of 15 microns and max are characterized by a D ₉₀ value of a maximum of 50 microns.

This particular quality of the solid is preferably produced by grinding SiO ₂ granules in the liquid, again a prerequisite for an effective grinding process being a high initial solids content of the base silt. The grinding of the SiO ₂ grain contributes to the homogenization of the base slip and, in addition, by gradual dissolution of SiO ₂ up to the solubility limit in the liquid, the pH is lowered, which in turn results in a further solubility of SiO ₂ favored.

Of the Base slip so obtained is tacky and has no sedimentation tendency on. This is a strong cohesion tendency connected to a high green density and a good homogeneity and Fine poredness of the blue body and green body matrix leads.

The solids content is additionally increased by adding further solid in the form of quartz glass grain to the homogeneous base slip. However, the addition of quartz glass granules takes place only after mixing and homogenizing the base slip, since otherwise it can lead to lumping, and the pourability of the slip is impaired. The addition of quartz glass grains is such that a composite slip having a density of at least 1.85 g / cm ³ is obtained. The addition of the quartz glass grain not only increases the solids content of the slurry, but also improves the dimensional stability of the green body and reduces shrinkage during drying and sintering. By adding such particles, therefore, the dimensional stability of Castings from the composite material improved.

Particle size and the particle size distribution of the amorphous SiO ₂ particles are characterized on the basis of the so-called D ₅₀ value and the D ₉₀ value of a particle size distribution curve (cumulative volume of the SiO ₂ particles as a function of the particle size). The D ₉₀ value indicates a particle size that is not reached by 90% of the cumulative volume of the SiO ₂ particles, and the D ₅₀ value represents a corresponding average particle size. The particle size distribution is determined by scattered light and laser diffraction spectroscopy according to ISO 13320.

The amorphous SiO ₂ particles of the base slip have a BET specific surface area of more than 1 m ² / g, whereas the BET surface area of the silica grain is less than 1 m ² / g. The SiO ₂ grain and the quartz glass grains come from the same raw material or from different raw materials. The raw material or raw materials are amorphous and of synthetic or natural origin.

It has proven to be useful if SiO ₂ particles are produced by grinding, whose particle sizes and particle size distribution are characterized by a D ₅₀ value of not more than 9 μm and by a D ₉₀ value of not more than 40 μm.

By This measure becomes a high green density and a good homogeneity and fine poredness of the green body matrix achieved, in combination with the added Quarzglerkörnung too a slight drying and sintering shrinkage of the green body of contributes less than 4% and even less than 2.5%.

The homogenization of the base slip requires a certain process time. It has proved suitable to homogenize the base slip for at least 12 hours, and this step preferably lasts for at least 24 hours. In the case of simultaneous homogenization and grinding of SiO ₂ grain, shorter process times are possible by using optimized grinding media, for example Al ₂ O ₃ . Although the basic slip produced in this way can be used directly to produce castings, its solids content is preferably increased beforehand by adding quartz glass grain.

Low dry and sintering shrinkage also contributes when a maximum of 10% by weight of the SiO ₂ particles in the base slip has a particle size of less than 1 μm.

With a small amount (up to a proportion of about 10% by weight, based on the total mass of the SiO ₂ particles), such finely divided SiO ₂ particles have a binder-like effect in the green body. They then contribute to increasing the density and mechanical strength of the green body - and thus of the composite material produced therefrom - by promoting so-called necking during sintering and thus acting as a sintering aid and reducing the sintering temperature. With amounts of more than 10% by weight, greater shrinkage occurs due to an increased liquid requirement. Contents of said finely divided SiO ₂ particles of more than 10 wt .-% in the base slip are therefore to be avoided as a rule.

In this context, it should be noted that particularly finely divided SiO ₂ particles with particle sizes in the nanometer range, as present for example in colloidal SiO ₂ suspensions, although leading to a higher basic strength of the green body, but also to a greater shrinkage. Therefore, advantageously at most 5% by weight of the SiO ₂ particles in the base slip, preferably at most 1% by weight of the SiO ₂ particles (based on the total mass of the SiO ₂ particles) in the base slip, have a particle size of less than 100 nm on.

According to the invention, the addition of such finely divided SiO ₂ particles is limited to a maximum of 5% by weight (based on the mass of the base slip), more preferably to a maximum of 1% by weight. In favor of a lower overall shrinkage, a lower green body strength is accepted. As a result, a drying process as discussed above is required at relatively low drying temperatures to achieve crack-free drying of the blue body.

Preferably is a base slip with a solids content between 75 wt .-% and 82 wt .-% produced.

At a solids content of less than 75 wt .-%, the base sludge tends to sedimentation and it is achieved too low green compact density and homogeneity. The total shrinkage (dry and sinter shrinkage) of the composite material produced therefrom is then too high to be able to produce crack-free quartz glass components. With a solids content above the stated upper limit there is a risk that the slip loses its pourability due to lumping together of the SiO ₂ mass.

In a first preferred variant of the method, coarse-grained quartz glass grains are added to the base slurry, in which the quartz glass grains have a mean particle size (D ₅₀ value) in the range between 200 μm and 1000 μm, preferably in the range between 50 μm and 500 μm, and particularly preferably in the range between 90 μm and 200 μm has.

In a second equally preferred variant of the method, the basic slip is accompanied by fine-grained quartz glass grains, in which the quartz glass grains have a mean grain size (D ₅₀ value) in the range between 10 μm and 90 μm, preferably in the range between 15 μm and 60 μm.

In Each of these preferred process variants causes the quartz glass grains - as already explained - a lesser Shrinkage during drying and sintering of the green body, so by the addition of these particles the dimensional stability and dimensional accuracy a quartz glass component produced from the composite material is improved. A total shrinkage of less than 4%, even less than 2.5% is achievable. The quartz glass grain essentially serves as a filler, However, it can be used to adjust the physical or chemical properties of the Composite material can be selected specifically. With decreasing Take size of quartz glass grain the effects described, whereas when using coarse Quarzglaskörnung with Grain sizes above of 1000 μm a composite material with inhomogeneous structure and comparatively low strength is obtained.

In addition, a bimodal distribution of the SiO ₂ particles in the green body is achieved by the addition of the quartz glass grain to Grundschlicker, which contributes to a higher density, with regard to both a coarser quartz glass grains whose average grain size (D ₅₀ value) particularly preferred in Range between 90 microns and 200 microns, as well as a finer quartz glass grains have proven particularly useful. The finer quartz glass grain is preferably used in the form of a spherical grain.

Lowering the pH of the base slip also contributes to a high solids content of the composite slip. Already the grinding of the SiO ₂ grain leads to a higher solubility of SiO ₂ in the liquid and thus to a lowering of the pH of the base slip due to the increasing surface area of the SiO ₂ grain during grinding.

In addition, the pH can be lowered by adding acidifying components, such as by adding nanoscale silica particles or a sol containing them, to increase the solubility of SiO ₂ . In a preferred procedure, the pH of the base slip is adjusted by adding an acid.

in the Regard to one as possible low total shrinkage of the composite material at the same time high density and strength, it has proved favorable when the proportion of the solid from the base slip to the total mass of the solid in Kompositschlicker between 30 wt .-% and 60 wt .-% is.

at a mass fraction of the base slip of less than 30% by weight is a reduced cross-linking and a low basic strength of the green body scored, while at a mass fraction of the base slip of more than 60% by weight a composite material with increased Shrinkage and reduced dimensional stability results.

As a rule, the density of the composite material is greater and the total shrinkage is lower, the higher the density of the composite slip is set. Preferably, the composite slurry has a density of at least 1.95 g / cm ³ .

The following versions refer to all the above suitable materials.

It has proved to be particularly favorable proved the green body or the casting of a hot treatment in a nitrogen or a nitrogen compound containing the atmosphere to undergo.

Thereby a nitrogen-enriched surface layer is generated, which is a Diffusion barrier for forms the diffusion of gases, in particular of oxygen. At the Use of the casting in contact with a melt, such as Silicon, the diffusion barrier layer reduces the transport of Oxygen from the casting into the melt. For the production of Diffusion barrier layer is either the green body in a nitrogen or sintered a nitrogen compound containing atmosphere, or already thermally precompressed casting is heated in such an atmosphere. in the Case of quartz glass (material) is the preferred treatment temperature in the range between 1100 ° C and 1450 ° C. In the case of a green body out Metallic silicon this is due to the hot treatment in a nitrogen or a nitrogen compound containing the atmosphere Completely or at least for the most part converted into silicon nitride. There Silicon nitride is not wetted by silicon melts can this way a multipurpose "Mehrwegtiegel" for melting of Solar silicon ingots be generated.

It has been considered favorable proved when the heat treatment a treatment in a nitrogen-containing plasma.

By the plasma assisted Nitridation becomes a rapid nitrogen enrichment of even deeper layers reached.

Prefers is used a mold in which a cavity of a cylindrical inner wall is limited.

at the usual Gipsformtechnik have the forms to facilitate removal of the blue body often slightly conical inner walls on. This measure is in the method according to the invention not required, allowing the production of exactly cylindrical castings is relieved. This is especially advantageous if the casting itself as a mold (Crucible) used for the production of cylindrical castings is, as in the case, a post-processing of the cast-body walls to Generation of the cylinder geometry is not required. Especially simplifies the process according to the invention the production of rectangular castings by inserting a mold is, whose inner wall encloses a rectangular cavity.

Especially for the Production of crucible-shaped Castings, it has proved to be particularly favorable in the cavity to use a casting core in the mold which is completely doused by the Kompositschlicker.

at Completely impermeable Mold wall is no "sagging" of Kompositschlickers, so that a smooth casting surface is obtained. This smooth casting surface, the requires no post-processing, forms the bottom (outside) of the crucibletype Casting.

Regarding the casting of a ceramic or a glassy composite material the above object is based on the above-mentioned Material according to the invention thereby solved, that the wall has a homogeneous pore and density distribution, such that the local density over the entire wall, viewed in the direction perpendicular to the casting axis, deviates by a maximum of 5% from the average density of the composite material.

The Casting according to the invention is characterized by a homogeneous pore and density distribution. It therefore has no casting soul on, as for example when pouring slip into molds can arise with absorbent walls. The casting according to the invention therefore shows a high mechanical strength. It is according to the method of the invention produced. By quickly freezing a Kompositschlickers with high solids content in a mold with one for the fluid of the composite slip not permeable Wall is the formation of density gradients perpendicular to the casting axis largely avoided.

The Casting according to the invention consists of a composite material, which is a homogeneous matrix from the material, embedded in the coarser grains of the material are. The material is quartz glass, silicon nitride, Silicon carbide, zirconium silicate, silicon, or a mixture of these Substances.

To measure the density, cube-shaped samples with an edge length of 1 cm are removed from the wall of the casting and measured by X-ray techniques. The casting axis identifies the orientation of the casting during its manufacture and identifies
In the case of quartz glass, the composite material is characterized by a density of at least 1.80 g / cm ³ .

Preferably is the casting according to the invention as a core casting with a rectangular cross section and a cylindrical inner wall educated.

The cylindrical Inner wall is for an insert of the casting as a casting mold (mold for melting of solar grade silicon) for producing cylindrical castings particularly suitable because a post-processing of the cast-body walls to produce the cylinder geometry is not required.

It becomes an embodiment the casting according to the invention preferably, that is a nitrogen-doped surface layer having.

Molds for melting solar silicon are usually provided with a Si ₃ N ₄ separating layer. In contrast, the casting according to the invention is characterized by an integral surface layer in which the material is doped with nitrogen. The nitrogen enriched surface layer acts as a diffusion barrier for the diffusion of gases, especially oxygen. The nitrogen content of the surface layer is for this purpose at least 1 wt .-%, preferably at least 0.5 wt .-%.

The Casting according to the invention is preferably obtained by the method according to the invention. in this respect is referred to the above explanations taken.

following the invention is based on embodiments and a Drawing explained in more detail.

1 a flow diagram for explaining a procedure for the preparation of the composite material by the method according to the invention.

example 1

It is first a homogeneous base slip 14 produced. For a batch of 10 kg base slip (SiO ₂ water suspension) are in a lined with Al ₂ O ₃ drum mill with about 20 liters volume content, 8.2 kg of amorphous quartz glass grains 11 from natural raw material with grain sizes in the range between 250 μm and 650 μm with 1.8 kg deionized water 12 mixed with a conductivity of less than 3 μS.

This mixture 13 is ground by means of grinding balls of Al ₂ O ₃ (weight: 2.3 kg and 4.5 kg) on a roller block at 23 U / min for a period of 3 days so far that a homogeneous base slip 14 with a solids content of 82% and a viscosity of 140 mPas forms. During the course of grinding, the pH drops to about 5 as a result of the SiO ₂ going into solution.

The after milling the quartz glass grains 11 obtained SiO ₂ particles in the base slip 14 show a particle size distribution characterized by a D ₅₀ value of about 5 μm and a D ₉₀ value of about 23 μm.

From the thus obtained homogeneous base slip 14 becomes a composite slip 19 produced by mixing an amorphous SiO ₂ grain having a particle size between 63 microns and 180 microns until a solids content of 90 wt .-% is reached. This mixture is also 16 wt .-% glycerol (based on the remaining liquid phase), as a needle growth inhibitor 15 acts, fed and the mixture 12 Homogenized for hours. The homogenization is carried out in the drum mill at a speed of 25 U / min, but only 1/10 of the grinding balls are used, as used in grinding. The composite slip produced in this way 19 has a solids content of 90%, a viscosity of 1531 mPas and a density of 2.0 g / cm ³ .

Subsequently, the homogeneous Kompositschlicker 19 a green body 20 generated. This is the Kompositschlicker 19 poured into a membrane mold made of vacuum-formed silicone, which is embedded in carbon dioxide snow (dry ice). This will cause a rapid freezing of the composite slip to a blue body 22 causes.

The addition of glycerol to the composite slip 19 contributes to a homogeneous structure that is free of ice-needle structures. Since the walls of the membrane mold are impermeable to water, the formation of a casting core is avoided and there is no "sagging" of the Kompositschlickers 19 , leaving a blue body 22 obtained with a smooth casting surface.

The shock frozen blue body 22 is taken from the membrane mold and immediately - when frozen - placed in a pre-heated to 80 ° C convection oven and dried for several hours at this temperature. Continuous evaporation and removal of moisture from the surface will prevent recondensation of moisture and re-surface freezing, which would be associated with needle crystal formation and disruption of the green body structure.

It is particularly important that when drying to solidify the blue body 22 to a self-supporting green body 20 comes. The occurrence of a liquid phase is avoided by sublimating the ice and immediately evaporating just thawed moisture. In addition to the low drying temperature, the low moisture content of the blue body 22 contributes to success. Simultaneously with the evaporation of water, most of the added glycerol is transferred into the gas phase. In the absence of the addition of nanoscale SiO ₂ particles to the composite slip 19 , shows the green body 20 a relatively low strength, if it is not dried as described above.

The dried green body 20 becomes a casting for 2 hours at a temperature of 1250 ° C 21 sintered from opaque quartz glass.

The composite material of the casting 21 has a density of 1.95 g / cm ³ . It consists of a matrix with a homogeneous microstructure with uniform fine poredness and homogeneous density distribution, in which the quartz glass grains are embedded. It is characterized by high thermal shock resistance as well as by excellent chemical resistance, especially against a silicon melt. Its total shrinkage (dry and sintering shrinkage) compared to the dimensions of the mold is 1.5%.

Example 2

following becomes another embodiment the method according to the invention for the production a mold made of opaque quartz glass for receiving a melt Solarsiliziums described by means of a core casting process.

The composite slip according to Example 1 is produced, with 50% of the glycerol added thereto being replaced by TEOS. In addition to the grain refining of the ice, TEOS causes an increase in the solids content by means of a sol-gel reaction and an increase in the green and sintering resistance of the green body.

In a square, multi-part aluminum mold with Teflon-coated inner walls with the internal dimensions of 850 mm × 850 mm, a square core with the lateral dimensions 830 mm × 830 mm and a height of 450 mm is used in the middle. The aluminum mold is cooled to a temperature of minus 20 ° C and with the Kompositschlicker 19 filled up so high that it covers the casting core.

Shock freezing prevents sedimentation and segregation of the composite slip 19 and causes rapid coagulation. Since the walls of the metal mold are impermeable to water, there is no formation of a casting soul. There is also no "settling" of the composite slip 19 So that a smooth casting surface is obtained, which requires no post-processing after drying and solidification and the finished mold 21 forms the bottom (outer wall).

The shock frozen blue body 22 is taken from the metal mold and immediately - when frozen - placed in a microwave oven with circulating air and dried therein at a temperature of about 80 ° C for several hours in pulsed operation. The thawing liquid evaporates continuously from the surface until the water and most of the glycerol are evaporated. In addition, a recondensation of moisture and re-superficial freezing, which would be associated with needle crystal formation and a disturbance of the green body structure prevented.

After drying, the green body thus obtained 20 placed in a sintering furnace and treated at a temperature of about 1200 ° C for 3 hours in an atmosphere of a mixture of ammonia and nitrogen, thereby forming a mold 21 sintered from nitrogen-containing, opaque quartz glass according to the invention.

The mold 21 has a rectangular cross section with dimensions of 850 mm × 850 mm × 500 mm and vertical walls. The nitrogen content in the area of the surface is about 2 wt .-% When using the mold 21 For melting silicon, the nitrogen-containing surface layer counteracts the diffusion of oxygen into the melt, so that a silicon block with a low oxygen content is obtained, which has a favorable effect on the efficiency of the solar cells produced therefrom. In comparison to the usual molds with conical walls, reduce the vertical walls of the mold according to the invention 21 Cuttings at the molten silicon block.

Example 3

A "micro-composite slip" is prepared by adding 7900 g of high purity synthetic amorphous silica glass containing 2100 g of deionized water in a polyurethene-lined ball mill with Al ₂ O ₃ milling balls to a finely divided base slip having a D ₅₀ value of 5 , 5 microns and a D .mu.m ₉₀ value of 24 and milled to a solids content of 79 wt .-%. 2000 g of this Grundschlickers are with 1250 g of a substantially spherical amorphous silica grain having a D ₅₀ value of 16 microns homogenized. If the slip density 1.92 g / cm ³ .

14% by weight of glycerol are added to the microcomposite slip. After a homogenization time of 12 hours, the microcomposite slurry is poured into a silicone membrane mold with a vertically oriented tube longitudinal axis, formed as a tube and heated in advance at a temperature of 90 ° C., and frozen at -50 ° C. in the mold. To 3 Hours the frozen blue body is removed from the freezer and removed the support form and the silicone membrane.

The blue body is transported directly to a pre-heated to 80 ° C convection oven and dried with continuous evaporation and removal of moisture from the surface. Finally, the dry green body is sintered at a temperature of 1440 ° C for 2 hours to a quartz glass tube having a density of 2.15 g / cm ³ , which is opaque, and characterized by a uniform homogeneous structure with homogeneous density and pore distribution distinguished. A casting soul is not recognizable. Cube-shaped 1 cm ³ cube samples taken inside, outside and in the middle of the pipe wall (in the area of half the pipe length) show no differences in density.

Example 4

It is a Grundschlicker prepared according to Example 3, and this 2000 g of amorphous synthetic, flame-glazed SiO ₂ grains are added a particle size of 60 microns -125 microns with a D ₅₀ value of 80 microns to a solids content of 90.4 wt. % is reached. After addition of 15% by weight of TEOS (based on the weight of the liquid), the slip is readily pourable. It is poured into a flanged form of Teflon-coated sheet, frozen therein and then sintered as described in Example 1.

The finished flange part consists of opaque quartz glass and it has a homogeneous pore and density distribution. The total shrinkage is at about 2.5%. It is free of calcium contamination, as they occur in the conventional plaster mold technique.

Claims (28)

A method of making a casting from a composite material by providing a composite slurry containing material particles in the size range below 10 μm and material grain size range greater than 10 μm, homogenizing the composite slurry, casting the composite slurry into a mold to form a blue body, drying the blue body forming a porous green body, and thermal solidification of the green body, characterized in that the Kompositschlicker having a solids content of at least 80 wt .-% is poured into a mold having a liquid impermeable mold wall, the Kompositschlicker after casting under Cooling a frozen blue body to a cooling temperature below the freezing point of the liquid, and wherein the drying comprises heating the frozen blue body to a heating temperature below the boiling temperature of the liquid includes. Method according to claim 1, characterized in that that the cooling temperature between minus 20 ° C and minus 80 ° C, preferably between minus 30 ° C and minus 60 ° C, lies. Method according to claim 1, characterized in that that the heating temperature in the range between 40 ° C and 90 ° C, preferably in the range between 50 ° C and 80 ° C is. Method according to one of the preceding claims, characterized characterized in that the frozen blue body in a drying chamber, is removed from the moisture, is dried. Method according to one of the preceding claims, characterized characterized in that the Kompositschlicker poured into a mold which is at a temperature below the freezing point of the liquid chilled is. Method according to one of claims 1 to 4, characterized characterized in that the composite slurry is poured into a mold, which is heated to a temperature above ambient temperature. Method according to one of the preceding claims, characterized characterized in that the Kompositschlicker a the formation of Ice crystal inhibiting substance is added. Method according to one of the preceding claims, characterized in that for producing a casting of quartz glass, the composite slurry contains amorphous SiO ₂ particles in the size range below 10 microns and quartz glass grains in the size range above 10 microns, preferably above 100 microns, and with a Density of at least 1.75 g / cm ^{3 is} poured into the mold. A method according to claim 8, characterized in that the composite slip is obtained by SiO ₂ particles whose particle sizes and particle size distribution by a D ₅₀ value of at most 15 microns and a maximum D ₉₀ value of 50 microns, with liquid are mixed, whereby a homogeneous base slip having a solids content of at least 75 wt .-% is produced, and wherein in the homogeneous base slip the quartz glass grain is mixed to form the Kompositschlickers. A method according to claim 9, characterized in that the SiO ₂ particles are produced by grinding amorphous SiO ₂ grain in the liquid to the SiO ₂ particles. A method according to claim 10, characterized in that SiO ₂ particles are produced by grinding, whose particle sizes and particle size distribution by a D ₅₀ value of not more than 9 microns and by a D ₉₀ value of 40 microns maximum are characterized. Method according to claim 10 or 11, characterized that the grinding and homogenization of the base slip at least 12 hours, preferably at least 24 hours. Method according to one of the preceding claims 9 to 12, characterized in that a maximum of 10 wt .-% of the SiO ₂ particles in the base slurry have a particle size of less than 1 micron Method according to one of claims 9 to 13, characterized in that a maximum of 5 wt .-% of the SiO ₂ particles in the base slurry, preferably at most 1 wt .-% of SiO ₂ particles in the base slurry, have a particle size of less than 100 nm , Method according to one of the preceding claims 9 to 14, characterized in that a base slip having a solids content between 75 wt .-% and 82 wt .-% is generated. Method according to one of the preceding claims 9 to 15, characterized in that the Grundschlicker a coarse-grained Quarzglaskör An average grain size (D ₅₀ value) in the range between 200 .mu.m and 1000 .mu.m, preferably in the range between 50 .mu.m and 500 .mu.m, and particularly preferably in the range between 90 .mu.m and 200 .mu.m attached. Method according to one of the preceding claims 9 to 15, characterized in that the Grundschlicker a fine-grained quartz glass grain is added, which has a mean grain size (D ₅₀ value) in the range between 10 .mu.m and 90 .mu.m, preferably in the range between 15 .mu.m and 60 .mu.m having. Method according to one of the preceding claims 9 to 17, characterized in that the Grundschlicker an acid for Lowering the pH is added. Method according to one of the preceding claims 9 to 18, characterized in that the proportion of the solid the base slip on the total mass of the solid in Kompositschlicker between 30% by weight and 60% by weight Method according to one of the preceding claims, characterized in that the composite slip has a density of at least 1.95 g / cm ³ . Method according to one of the preceding claims, characterized characterized in that the green body or the casting of a hot treatment in a nitrogen or a nitrogen compound containing the atmosphere is subjected. Method according to claim 21, characterized that the heat treatment a treatment in a nitrogen-containing plasma. Method according to one of the preceding claims, characterized characterized in that a mold is used in which a cavity from a cylindrical Inner wall is limited. Method according to claim 23, characterized that the inner wall defines a rectangular cavity. Method according to claim 23 or 24, characterized that in the cavity a casting core is used, the completely doused from the Kompositschlicker becomes. Casting of a ceramic or vitreous composite material, which has a homogeneous matrix of the material in which grains of the material with a grain size of more embedded as 10 μm are, and having a wall within which a casting axis passes, the during the casting runs in the vertical, characterized in that the wall is a homogeneous pore and Has density distribution, such that the local density over the entire wall, viewed in the direction perpendicular to the casting axis, deviates by a maximum of 5% from the average density of the composite material. Casting part according to claim 26, characterized that it as a core casting with rectangular cross-section and cylindrical inner wall is trained. Casting part according to claim 26 or 27, characterized it has a nitrogen-doped surface layer.