Method of production of a cast part made out of a composite material, as a cast part consisting out of a ceramic or glass composite

0001The invention relates to a method for producing a casting from a Composite by providing a Kompositschlickers, the material particles in the size range below 10 microns and material grain in includes size range above 10 microns, homogenizing the Kompositschlikkers, Casting the Kompositschlickers in a mold to form a Blue body, drying the blue body to form a porous green body, and thermal bonding of the green body.

0002Furthermore, the invention relates to a cast part made of a ceramic or glassy Composite material comprising a homogeneous matrix of the material, in embedded which grains of the material with a particle size of greater than 10 microns are, and having a wall, inside of a casting axis extends which runs during casting in the vertical.

0003The so-called slip casting is in the ceramic process engineering for producing ceramic or glassy components in use, in particular also for the production of quartz glass components. is DE 101 14 484 A1 Slip casting for producing a composite material with a high SiO₂-Salary known. Here, a silica grain which serves as a filler in a SiO₂embedded -containing matrix entirely from synthetic produced SiO₂ will be produced. The matrix has a SiO₂Content of at least 99 wt .-% and consists of at least two particle fractions, each as granules of nanoscale, amorphous, synthetically produced SiO₂primary particles present with an average primary particle size of less than 100 nm, formed will.

0004The composite material obtained by the known method has only closed pores, has no crystalline fractions, and its density is 2.1 g / cm³, It is characterized by high thermal shock resistance and chemical resistance. The composite is therefore for use suitable as a mold for melting solar silicon. The synthetic raw materials However, for the production of composite material are expensive.

0005Another method for producing a quartz glass crucible by means Schlikkergießverfahren is described in DE 199 43 103 A1. Here is a Suspension of high-purity, amorphous SiO₂assumed particles whose degree of filling at least 83,96Gew .-%. The high SiO₂-Füllgrad Is used of SiO₂Particles with bimodal particle size distribution obtained, with about 95% of SiO used₂ as produced in the oxyhydrogen flame SiO₂powder with present an average particle size of 30 microns.

0006A process for the production of a refractory sintered casting of glassy Material and a casting according to the above-mentioned type of DE 693 06 169 T2. In the fused silica glass article described therein form two fractions of fine-grained SiO₂Powder, a binder phase for a more SiO₂-containing component in the form of coarse SiO₂Grains having a Particle size between 40 microns and 1000 microns. One of the more fine-grained SiO₂-Powder is present as fumed silica of substantially spherical particles is formed with particle sizes in the range between 0.2 microns and 0.6 microns. Of the Fumed silica preferably used comes from the melting process and the Reduction of zirconia. The quartz glass starting components are in a dry-premixed and then is it under Addition of a stabilizer, a slurry produced. The weight of the individual Components be 54% (gross SiO₂Grains), 33% (fine-grained SiO₂particles) and 13% (fumed silica). The slurry is degassed in vacuum and in a plaster mold cast. The green body thus produced, we dried and in an oven sintered at 1050 ° C to a composite component. For production reasons , the cast body on a casting axis, which during casting in the vertical and in the growth direction of the wall of the forming Blue body runs.

0007For the micro-structure of the component are coarse quartz grains in a relatively continuous matrix of finer particles and of spherical particles Fumed embedded characteristic. The component has an open porosity from 13% to, and its density is bei1,91 g / cm³, The crystallographic Analysis gives a cristobalite content of less than 2%.

0008Due to its open - that is through - porosity is known Composite material for components, which require for leaks or high purity, not universally applicable. Metallic melts can penetrate the pores in the component wall and lead to leaks.

0009In one process variant, according to DE 693 06 169 T2, instead of the Melting process and the reduction of zirconium dioxide resulting fumed silica milled or ground quartz micro used. The thus obtained However, fused silica glass article showed a low density and a very low Flexural strength.

0010Problems arise in particular through the slush Shrinkage of the green body during drying and during sintering. It can Shrinkage cracks occur and the dimensional stability of the components is often low.

0011The drying shrinkage also complicates the manufacture of components by so-called Core casting, wherein the slip is cast around a core, the is removed after drying. By shrinkage on drying shrinking the green body to the core and thereby entraining, or the core may not be questioned without damaging the green body of this.

0012As a result of the use of forms, which absorb the liquid of the suspension, often forms a pronounced Gießseele in the form of aligned Pores that lead to a weakening of the structure. Large-sized components can be produced only limited and simple design, often for Facilitating the removal of the blue conical body from the mold walls must be produced.

0013Since the slip casting and for a cost-effective production of components - even with complex geometry - enables it would be highly desirable, to avoid the said disadvantages in the production of quartz glass.

0014The invention is therefore based on the object a procedure for preparing a casting of a composite material by means of slip casting specify with which the problems associated with the drying shrinkage disadvantages reduce or eliminate blank, and with the production of castings cylindrical walls facilitate and especially for the production large-sized castings suitable.

0015Furthermore, the invention addresses the problem of providing a casting of a ceramic provide or glassy composite which by itself high strength distinguished.

0016With regard to the method, this object, starting from the beginning said method according to the invention is achieved in that the Kompositschlikker poured with a solids content of at least 80 wt .-% in a form is having an impermeable mold wall for the liquid, wherein the Kompositschlicker after casting to form a frozen blue body to a cooling temperature below the freezing point of the liquid is cooled, and wherein the drying comprises heating the frozen body Blue to a heating temperature below the boiling point of the liquid includes.

0017The produced composite material consists of a homogeneous matrix of the material, embedded in which coarser grains of the material. at the material is quartz glass, silicon nitride, silicon carbide, zirconium silicate, Silicon, or a mixture of these substances.

0018The inventive method allows crack-free castings from the Composite by slush - by core cast - produce. This requires a high strength and a very low Drying shrinkage of the green body. This requirement is the following Process features achieved: <sl><li>1. The Kompositschlicker contains fine particles of the material and coarser Grain size of the material with an average particle size above 10 microns. It is characterized by a high solids content of at least 80 Wt .-%, preferably at least 90 wt .-% of. The high solids content in Connection with the addition of coarse grain material affects the shrinkage the green body counter and improves its dimensional stability, which a favorable effect on the dimensional accuracy of components of the composite material effect.</li><li>2. The Kompositschlicker is poured into a mold containing at least one for the liquid impervious mold wall. The liquid for the impermeable mold wall prevents loss of fluid from the Kompositschlicker during a phase in which the suspended therein Material particles still move freely. This mobility of the would otherwise material particles on a mass transport towards the absorbent mold wall and thus to the formation of a Gießseele or other material inhomogeneities lead. It has been shown that this soul free blue body with a homogeneous and receiving solid distribution with high strength. This advantageous effect is - albeit to a lesser extent - even a, if the shape other than the at least one impermeable mold wall also comprises wall regions, the entrapped therein for the liquid and for Bubbles are permeable. When completely impermeable mold walls no "sagging" occurs of Kompositschlickers, so that a casting smooth surface is obtained, which no post-processing needed. The mold may be other than the at least one impermeable mold wall and a porous mold wall for gases or have for gases porous Formwandungsbereiche which to vent serve and prevent the emergence of surface bubbles. As Material for forming the mold have Teflon-coated sheet (Such as aluminum or steel) proven. Another preferred form of material is silicone in its use particularly impurity- (Bubble-free) surfaces are obtained when heated before use becomes.</li><li>3. The Kompositschlicker is in the mold to form a frozen blue body to a cooling temperature below the freezing point of the liquid cooled. The mold is either before, during or after the casting to a Temperature below the freezing point of the liquid of the suspension cooled. This leads to a formation of crystals in the liquid, in the case water to ice formation, the coagulation of particles of material especially causes submicron and segregation or sedimentation counteracts the still viscous blue body. To a needle formation largely to prevent, carried out the freezing takes place as soon as possible.</li><li>4. For drying and solidifying, the solidified blue body frozen State at a heating temperature below the boiling point of the liquid heated. The comparatively gentle heating is a achieved slower evaporation of the liquid, so that a strong gas formation as a result of boiling within the frozen blue body liquid avoided and concomitant damage to the body Blue will. At this surprisingly favorable effect of drying the green body by shock freezing and then gently heating frozen carries the low water content of the Kompositschlickers and blue body significantly. The Blue body in a frozen state removed from the mold and then dried. The drying of the blue body can also take place in the mold when This evaporation of the liquid little hindered.</li></sl>

0019The subsequent thermal bonding of the green body (sintering at a temperature below the melting point of the material) is a mechanically get stable casting that a homogeneous pore and Density distribution and a low open porosity, and which is characterized by a low overall shrinkage (drying and sintering shrinkage) distinguishes, less than 4%, and even less than 2.5%, may be.

0020To be particularly favorable, a cooling temperature has been found that between minus 20 ° C and minus 80 ° C, preferably between minus 30 ° C and minus 60 ° C, lies.

0021The cooling temperature depends essentially on the freezing temperature of the liquid from. These limits apply to liquids with a Freezing temperature to 0 ° C, such as water or homogeneous mixtures of organic Compounds with water. The more the cooling temperature below the Freezing temperature of the liquid is, the more rapidly solidifies the Kompositschlikker and the higher the strength of the frozen blue body. In the case of Casting made of quartz glass (material) is preferably made of a liquid, the tetraethyl orthosilicate (TEOS) contains, as the proportion of the liquid TEOS to an increased proportion of SiO₂ in Kompositschlicker and a higher strength leads and also contributes to a reduction of frozen structures.

0022Furthermore, it has proven advantageous if the heating temperature in the range between 40 ° C and 90 ° C, preferably in the range between 50 ° C and 80 ° C.

0023It has been shown that the removal of the liquid by heating the shock frozen Blue body at a temperature in the range between 40 ° C and contributes 90 ° C to a high strength of the resulting green body.

0024Preferably the frozen blue body is in a drying chamber, from the humidity is withdrawn, dried.

0025In the drying chamber is, for example, a circulating air drying cabinet or a microwave drying oven. By removing the Moisture from the drying chamber is a condensation of water vapor and a superficial refreezing that disturb the surface structure and can reduce the green strength, is prevented. Particularly, in the microwave drying it must be ensured that the temperature does not exceed 90 ° C.

0026In a first preferred variant of the method is advantageously Kompositschlicker poured into a mold to a temperature below the Freezing point of the liquid is cooled.

0027The mold has been in casting the Kompositschlickers a temperature below the freezing point of the liquid. Thereby, a particularly rapid reached cooling the Kompositschlickers and sedimentation prevented.

0028In an alternative and equally preferred variant of the method of Kompositschlicker poured into a mold to a temperature above the Ambient temperature is heated.

0029This is before pouring the Kompositschlickers condensation of Water from the ambient air to the mold walls avoided. Such Condensations may result in blue body surface damage. These therefore procedure is mainly used for high demands on surface quality and at high humidity are used.

0030The strength of the green body may be formed by growth of very large crystals (Ice crystals) are impaired. In the case of quartz glass (material), although acts dissolved in the suspension SiO₂ already as Eiskristallwachstumsinhibitor. It has proved in any case to be advantageous, the addition Kompositschlicker accompanied by a eiskristallwachstumsinhibierende substance.

0031The addition of eiskristallwachstumsinhibierenden substance causes as many and for the smallest possible crystals are formed. Effective Substances of this type are alcohols such as isopropanol, as well as glycerol, polyethylene glycol, Polyacrylates and organosilicon compounds such as TEOS, wherein also an addition of nanoscale particles serves the same purpose (the latter, however, most 5 wt .-%, preferably at most 1 wt .-%).

0032For producing a cast part made of quartz glass containing Kompositschlicker amorphous SiO₂Particles in the size range below 10 microns and silica grain in the size range above 10 .mu.m, preferably above 100 microns, and it is having a density of at least 1.75 g / cm³ poured into the mold.

0033The Kompositschlicker contains in the case fine-grained SiO₂Particles with particle sizes &lt;10 microns and coarser silica grain with an average particle size above 10 microns, preferably above 100 microns. He stands out a high density of at least 1.75 g / cm³from which high by a Solids content is achieved. This density corresponds approximately to a solids content of at least 80 wt .-% SiO₂, The thermal bonding of the green body takes place in the case preferably in the temperature range between 800 ° C and 1440 ° C. Sintering is a mechanically stable casting of opaque quartz glass obtained that a homogeneous pore and density distribution and a low having open porosity, and by a low overall shrinkage (Dry and sintering shrinkage), which can be less than 2%, and by a high density of at least 1.80 g / cm³ distinguished.

0034The following comments relate to the production of a Kompositschlickers to the case of the material of quartz glass. For other materials, the for quartz glass characteristic parameters based on a few trials and expert be adapted knowledge.

0035A particularly high solids content of the green body is achieved when the Kompositschlicker is generated by SiO₂Particles whose particle sizes and Particle size distribution by a D₅₀Value of not more than 15 microns and a D₉₀marked value of at most 50 microns, mixed with liquid , whereby a homogeneous base slip with a solids content of at least 75 is wt .-%, and wherein the homogeneous base slip the silica grain is mixed to form the Kompositschlickers.

0036Preferably, the mixing takes place of the SiO₂Particles and the liquid by by amorphous SiO₂Granules in the liquid to the SiO₂particles is milled.

0037The high solids content of Grundschlickers is firstly by a special Quality of the solid contained in the slip condition, namely finely divided, amorphous SiO₂Particles that are characterized by particle sizes and a particle size distribution distinguished, distinguished by a D₅₀Value of not more than 15 microns and by D₉₀are characterized by a maximum value of 50 .mu.m.

0038This special quality of the solid is preferably prepared by grinding SiO₂Granules generated in the liquid, again a prerequisite for effective grinding a high initial solids content of Grundschlickers is. The milling of SiO₂Granulate contributes to homogenize the Grundschlickers at, and it comes during grinding moreover by gradually Dissolving SiO₂ up to the solubility limit in the liquid to be a lowering of the pH, which in turn further solubility of SiO₂favored.

0039The base slurry thus obtained is sticky and has no sedimentation on. Thus a strong cohesion inclination is connected, the high to a Green density and good homogeneity and fine pores of Blaukörper- and Green body matrix results.

0040The solids content is further increased by the homogeneous base slip Another solid is added in the form of silica grain. Enclosing the Silica grain may not happen until after the mixing and homogenization the Grundschlickers, since otherwise come to a clumping can, and the pourability of the slurry is degraded. The addition of Silica grain is such that a Kompositschlicker with a Density of at least 1.85 g / cm³ results. The addition of the silica grain is not only the solids content of the slurry increases, but it is also improves the dimensional stability of the green body and the shrinkage during Drying and reduced during sintering. is therefore by the addition of such particles the dimensional accuracy of castings from the composite improved.

0041Particle size and particle size distribution of the amorphous SiO₂particles are based on the so-called D₅₀-value And the D₅₀-value Of a particle size distribution curve (Cumulative volume of SiO₂Particles depending characterized by the particle size). The D₉₀Value indicates a Particle size of 90% of the cumulative volume of the SiO₂not particles is achieved, and the D₅₀Value represents a corresponding average particle size. The particle size distribution is scattered light and determined laser diffraction spectroscopy ISO 13320th

0042The amorphous SiO₂Particles of Grundschlickers have a BET specific surface area of more than 1 m²/ G, whereas the BET surface of the silica grain at less than 1 m²/ G. The SiO₂Granules and the silica grain originate from the same raw material or from different materials. Of the Raw materials or raw materials are amorphous and synthetic or natural origin.

0043It has proven desirable if SiO by the grinding₂Particles are generated, their particle sizes and particle size distribution by a D₅₀-Value of maximum of 9 microns and a D₉₀in value of 40 microns are.

0044By this measure, a high green density and a good homogeneity and fine pores of the green body matrix achieved in combination added to the Silica grain to poor drying and sintering shrinkage of Green body contributes totaling less than 4% and even less than 2.5%.

0045Homogenization of Grundschlickers requires some processing time. Be suitable has a least 12 hours lasting homogenization proved the Grundschlickers, preferably take this step at least 24 hours a day. In the case of simultaneous homogenization and Milling of SiO₂Granules are by use of optimized grinding media, for example, from Al₂O₃, Shorter process times possible. The thus generated Base slip is indeed directly usable for the production of castings, its solids content but preferably by the addition of silica grain before elevated.

0046To a small drying and sintering shrinkage also contributes when maximum 10 wt .-% of SiO₂Particles in the base slip a particle size less have than 1 .mu.m.

0047When a small amount (up to a proportion of approximately 10 wt .-%, based on the total mass of SiO₂Particles) is such a finely divided SiO₂particles a binder-like effect in the green body to. They then contribute to increasing of density and mechanical strength of the green body - and thus the produced therefrom composite material - at, by the sintering process, the so-called promote necking and thus act as a sintering aid and the reduce sintering temperature. When the amount is more than 10 wt .-% occurs due to an increased fluid requirement to greater shrinkage. contents of said finely divided SiO₂Particles of more than 10 wt .-% in the base slip should therefore be avoided as a rule.

0048In this context it should be noted that particularly finely divided SiO₂particles having particle sizes in the nanometer range, such as those in colloidal SiO₂Suspensions present, although to a higher basic strength of the green body, but also results in a greater shrinkage. Therefore, advantageously, most 5 wt .-% of SiO₂Particles in the base slurry, preferably a maximum of 1 wt .-% of SiO₂Particles (based on the total mass of SiO₂particles) in the base slurry, to a particle size of less than 100 nm.

0049The addition of such finely divided SiO₂Particles according to the invention to a maximum of 5 wt .-% (based on the mass of the Grundschlickers), more preferably at a maximum of 1 wt .-% limited. For the benefit of a lower total shrinkage is accepted a lower green body strength. is therefore an above Illustrated drying process at comparatively low Drying temperatures required to produce a crack-free drying of the blue body to reach.

0050Preferably, a base slurry is having a solids content between 75 wt .-% and 82 wt .-% produced.

0051At a solids content of less than 75 wt .-% of the base slip tends to sedimentation and it is too small a green body density and uniformity reached. The total shrinkage (drying and sintering shrinkage) of it Composite material produced is then too high to crack-free silica glass components be able to produce. At a solids content above the upper limit mentioned there is a risk that the slip by clumping of the SiO₂-Dimensions loses its pourability.

0052In a first preferred variant of the method the base slip is coarse-grained attached silica grain, wherein the quartz glass granulation has an average Grain size (D₅₀Value) in the range between 200 .mu.m and 1000 .mu.m, preferably in the Range between 50 microns and 500 microns, and more preferably in the range between having 90 .mu.m and 200 .mu.m.

0053In a second equally preferred process variant, the base slip accompanied by fine grained silica grain, in which the silica grain an average grain size (D₅₀Value) preferably in the range between 10 microns and 90 microns, in the range between 15 microns and 60 microns.

0054In each of these preferred process variants causing the silica grain - as already explained - a lower shrinkage on drying and sintering of the Green body, so that the addition of these particles, the dimensional stability and Dimensional stability of a silica glass member made of the composite material is improved. A total shrinkage of less than 4%, even less than 2.5% is achievable. The silica grain is essentially used as a filler, however, the setting of physical or chemical properties of Composite material are specifically selected. With decreasing size of Silica grain decrease the effects described, whereas in the Use coarse silica grain with grain sizes above 1000 microns a Composite with inhomogeneous structure and relatively low strength is obtained.

0055Furthermore, the addition of silica grain to a base slip bimodal distribution of SiO₂achieved particles in the green body, the higher to a Density contributes to give to this in terms of both a coarse silica grain, whose average grain size (D₅₀Value) particularly preferably is between 90 .mu.m and 200 .mu.m, and a finer silica grain have proven particularly useful. The finer silica grain is preferably in the form a spherical grain used.

0056At a high solids content of Kompositschlickers also carries a lowering the pH at the Grundschlickers. Already the grinding of SiO₂granulate results due to the specific surface area enlarging milling of SiO₂Granules to a higher solubility of SiO₂ in the liquid and thus a lowering of the pH of the Grundschlickers.

0057In addition, the pH value, by adding acidifying components, such as by addition of nanoscale silica particles or these Sols are contained is lowered to the solubility of SiO₂ to increase. at a preferred method is by the pH value of the Grundschlickers Addition of an acid set.

0058With regard to the lowest possible total shrinkage of the composite material with high density and strength, it has proven to be favorable, if the proportion of the solid from the base slip of the total mass the solid in Kompositschlicker between 30 wt .-% and 60 wt .-%.

0059In a mass fraction of Grundschlickers of less than 30 wt .-% is a reduced cross-linking and a low basic strength of the green body achieved, while as in a mass fraction of Grundschlickers more 60 wt .-%, a composite material with increased shrinkage and reduced dimensional accuracy results.

0060In general, the density of the composite material is the greater, and the total shrinkage the smaller, the higher the density of the Kompositschlickers is set. Preferably, the Kompositschlicker has a density of at least 1.95 g / cm³ on.

0061The following statements relate to all of the above, appropriate Materials.

0062It has proven to be particularly favorable, the green body or the casting containing a heat treatment in a nitrogen or a nitrogen compound undergo atmosphere.

0063Characterized a nitrogen enriched surface layer is produced, which a diffusion barrier for the diffusion of gases, in particular oxygen forms. When using the casting into contact with a melt, such as Silicon, the diffusion barrier layer reduces the transport of oxygen from the casting into the melt. To prepare the diffusion barrier layer is containing either the green body in a nitrogen or a nitrogen compound sintered atmosphere, or the already thermally pre-compacted Casting is heated in such an atmosphere. In the case of quartz glass (material) the preferred treatment temperature in the range between 1100 ° C and 1450 ° C. In the case of a green body of metallic silicon this is due to the heat treatment in a nitrogen or a nitrogen compound Atmosphere containing completely or at least for the most part in silicon transferred. Because silicon nitride is not wetted by the silicon melt, in this way a multi-purpose "reusable crucible" for melting generated by solar silicon blocks.

0064It has proven to be advantageous when the heat treatment is a treatment in comprises a nitrogen-containing plasma.

0065By plasma-assisted nitriding is a rapid nutrient nitrogen also reaches deeper layers.

0066Preferably, a mold is used in which a cavity of a cylindrical Inner wall is limited.

0067In the conventional plaster mold technology, the forms have to facilitate removal the blue body on often slightly tapered inner walls. These Measure is not necessary in the present process, so that the Producing exactly cylindrical castings is facilitated. This is especially advantageous when the casting mold itself as (crucible) for the preparation of cylindrical cast body is used as in the case of a post the cast body walls to produce the cylinder geometry not necessary is. In particular, the inventive method that simplifies Producing rectangular castings by a mold is used, the inner wall a rectangular cavity surrounds.

0068In particular for the production of crucible-cast parts it has proven to be proved particularly favorable to use a casting core in the mold cavity, is completely doused by the Kompositschlicker.

0069When completely impermeable mold wall no "sagging" occurs the Kompositschlickers, so that a casting smooth surface is obtained. This smooth Casting surface, which requires no post-processing, forms the bottom (outside) the crucible-cast part.

0070With regard to the casting of a ceramic or a glassy composite is the above mentioned object, starting from the beginning said material according to the invention is achieved in that the wall of a homogeneous pore and density distribution having such a way that the local density , Viewed over the entire wall in the direction perpendicular to the casting axis, deviates by a maximum of 5% from the average density of the composite material.

0071The casting of the invention is characterized by a homogeneous pore and Density distribution. It therefore has no Gießseele as example formed in molds with absorbent walls during casting of slip can. The casting of the present invention therefore shows a high mechanical strength. It can be produced by the inventive process. By rapid freezing of a Kompositschlickers with a high solids content in a Shape with a non-permeable for the liquid of the Kompositschlickers Wall is the formation of density gradients perpendicular to the casting axis largely avoided.

0072The casting invention consists of a composite material, which a homogeneous matrix of the material comprises, in the coarser grains of Material are embedded. When the material is quartz glass, Silicon nitride, silicon carbide, zirconium silicate, silicon, or a mixture of these Substances.

0073To measure the density of cubic samples having an edge length of taken 1 cm from the wall of the casting and by X-ray method measured. The casting axis indicates the orientation of the casting at its Manufacturing features and

0074In the case of quartz glass, the composite material is characterized by a density of at least 1.80 g / cm³ out.

0075Preferably casting according to the invention is as core casting rectangular Cross-section and a cylindrical inner wall formed.

0076The cylindrical inner wall is for use of the casting as a mold (Mold for melting solar silicon) for manufacturing cylindrical Casting particularly suitable because a reworking of the casting-walls is not needed to produce the cylinder geometry.

0077It is an embodiment of the casting according to the invention preferred that the having a nitrogen doped surface layer.

0078Mold for melting solar silicon are usually with a S₁₃N₄separating layer provided. By contrast, the casting is distinguished according to the Invention 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, in particular oxygen. The nitrogen content of the surface layer is located for this purpose at least 1 wt .-%, preferably at least 0.5 wt .-%.

0079The cast part according to the invention is preferably according to the invention receive method. In that regard, reference is made to the above explanations.

0080The invention is based on embodiments and a Drawing explained. <dl tsize="7" compact="compact"><dt><b>figure 1</b></dt><dd>a flow chart for explaining a procedure for preparing the composite material based on the invention Process.</dd></dl>

example 1

0081It is a homogeneous base slip 14 is first prepared. For an approach 10 kg base slip (SiO₂-Water Suspension) be in a by Al₂O₃lined drum mill with about 20 liters capacity, 8.2 kg of an amorphous Silica grain 11 from natural raw materials with particle sizes in the range between 250 microns and 650 microns with 1.8 kg of deionized water with a conductivity 12 blended of less than 3 microseconds.

0082This mixture 13 is by means of grinding balls of Al₂O₃ (Weight: 2.3 kg and 4.5 kg) on a roll at 23 U / min for a period of 3 days until ground that a homogeneous base slip 14 with a solids content forms of 82% and a viscosity of 140 mPas. During the milling it comes as a result of the dissolving SiO₂ to a lowering of the pH to about 5th

0083The SiO obtained after milling the silica grain 11₂particles in Base slip 14 show a particle size distribution with a D₅₀-Value of about 5 microns and a D₉₀in value of about 23 .mu.m is.

0084is from the resulting homogeneous base slip 14 a Kompositschlicker 19 generated by an amorphous SiO₂Granules with a grain size between 63 microns and 180 microns is mixed until a solids content of 90 wt .-% achieved is. To this mixture is also 16 wt .-% glycerol (based on the remaining liquid phase), which acts as a growth inhibitor needle 15 which and the mixture is homogenized for 12 hours. homogenization takes place in the drum mill at a speed of 25 rev / min, but only 1/10 the grinding balls are used, as they are used during grinding.

0085The Kompositschlicker thus generated 19 has a solids content of 90%, a Viscosity of 1531 mPas and a density of 2.0 g / cm³,

0086Then, from the homogeneous Kompositschlicker 19, a green body 20 generated. To this end, the Kompositschlicker 19 in a membrane form of vacuum-formed molded silicone that embedded in carbon dioxide snow (dry ice) is. This rapid freezing of Kompositschlickers a blue body 22 causes.

0087The addition of glycerol to the Kompositschlicker 19 contributes to a homogeneous Structure in which is free of Eisnadelstrukturen. Since the walls of the membrane form for water impermeable, is the formation of a Gießseele avoided and there is no "sagging" of the Kompositschlickers 19 so that a blue body 22 is obtained with a smooth casting surface.

0088The shock-frozen blue body 22 is removed from the membrane and directly form - Frozen - in a pre-heated to 80 ° C Circulation cabinet and introduced in several hours at this Temperature dried. Because of the steady evaporation and removal of moisture from the surface is recondensation of moisture and a again to superficial freezing, the needle with crystal formation and a failure the green body structure would involve, prevented.

0089It is particularly important that when drying to a hardening of the blue body 22 comes to a self-supporting green body 20th The appearance of a liquid phase is avoided by the ice sublimes and just thawed Moisture is instantly vaporized. In addition to the low drying temperature carries the low moisture content of the blue body 22 to the success. Simultaneously with the evaporation of water and the largest part of the added Glycerol converted into the gas phase. Lack Extension nanoscale SiO₂Particles to the Kompositschlicker 19, shows the green body 20 a relatively if it is not, dried as described above low strength, becomes.

0090The dried green bodies 20 is for 2 hours at a temperature of 1250 ° C sintered to form a casting 21 of opaque quartz glass.

0091The composite material of the casting 21 has a density of 1.95 g / cm³ on. He consists of a matrix with a homogeneous microstructure with uniform fine pores and a homogeneous density distribution in which the silica grain is embedded. He is characterized by high thermal shock resistance as well as excellent chemical resistance, especially against a Silicon melt from. His total shrinkage (drying and sintering shrinkage) compared to the dimensions of the mold is 1.5%.

example 2

0092Hereinafter, a further embodiment of the method according to the invention for the manufacture of a mold of opaque quartz glass for receiving a melt of solar silicon by means of a core casting process described in more detail.

0093There the Kompositschlicker prepared according to Example 1, wherein 50% of the there added glycerol are replaced by TEOS. TEOS causes besides Grain refinement of ice to increase the solids content by means of sol-gel reaction and an increase in the green and sintered strength of the green body.

0094In a square, multi-part aluminum mold with Teflon-coated inner walls inner size 850 mm x 850 mm is centered a square Casting core with lateral dimensions 830 mm x 830 mm and a height of 450 mm used. The aluminum mold is heated to a temperature of minus 20 ° C cooled and made up so high with the Kompositschlicker 19 that this the casting core covered.

0095The Rapid freezing prevents sedimentation and segregation of Kompositschlickers 19 and causes the rapid coagulation. Since the walls the metal mold for water impermeable, does not form a Gießseele instead of. There is also no "Set" of the Kompositschlickers 19 so that a casting smooth surface is obtained, which after drying and solidification no post-processing needs and in the finished mold 21 the ground (outer wall) forms.

0096The shock-frozen Blue body 22 is taken out of the metal mold and directly - Frozen - in a microwave oven with convection drying and introduced therein at a temperature of about 80 ° C for several hours dried in pulse mode. The thawing liquid evaporated continuously of the ab surface, until the water and de major part of the glycerol is evaporated are. In addition, a re-condensation of moisture and a again to superficial freezing, the needle with crystal formation and a failure the green body structure would involve, prevented.

0097After drying, the thus obtained green body 20 is introduced into a sintering oven and at a temperature of about 1200 ° C for 3 hours in a treated atmosphere from a mixture of ammonia and nitrogen and thereby to a mold 21 of nitrogen containing, opaque quartz glass according to the Invention sintered.

0098The mold 21 has a rectangular cross section with dimensions of 850 mm x 850 mm x 500 mm and vertical walls. The nitrogen content in the range of Surface is about 2 wt .-% when using the mold 21 for melting silicon affects the nitrogen-containing surface layer of the diffusion of oxygen in the contrary to melt so that a silicon block with a low Oxygen content is obtained, which affects the efficiency of the fabricated therefrom Solar cells has a favorable effect. Compared to the conventional molds with conical walls, which reduce the vertical walls of the invention Mold 21 blend in which it molten silicon block.

example 3

0099There is a "micro-Kompositschlicker" prepared. For this, 7900 g of a synthetic amorphous silica glass of high purity with 2,100 g of deionized Water in a lined Polyurtethan ball mill with Al₂O₃grinding balls a finely divided base slip with a D₅₀Value of 5.5 microns and a D₉₀Value of 24 microns and a solids content of 79 wt .-% milled. 2000 g of this Grundschlickers be with 1250 g of a largely spherical amorphous silica grain with a D₅₀Value homogenized by 16 microns. The slip density is 1.92 g / cm³,

0100The micro-Kompositschlicker be 14 wt .-% glycerin added. After 12 hours Homogenization is the micro-Kompositschlicker in an as Pipe-trained and thoroughly heated in advance at a temperature of 90 ° C silicone membrane form poured with a vertically oriented tube longitudinal axis and at -50 ° C frozen in the mold. After 3 hours the frosted Blue body is the freezer removed and the support mold and the silicone membrane removed.

0101The blue body is transported directly into a preheated to 80 ° C convection oven and under continuous evaporation and removal of moisture dried from the surface. Finally the dry green body is at a Temperature of 1440 ° C during 2h sintered into a quartz glass tube having a Density of 2.15 g / cm³ which is opaque, and be consistent with a homogeneous microstructure with homogeneous density and pore distribution is characterized .. A Gießseele is not recognizable. Inside, outside and in the middle of the tube wall (In the region of half the tube length) taken cubic 1cm³-Messproben No measurable differences in density.

example 4

0102There is a base slurry prepared according to Example 3, and this will be 2000 g amorphous synthetic, flame-glazed SiO₂Granules of a particle size of 60 microns -125 microns with a D₅₀Value of 80 microns admixed until a solids content of is 90.4 wt .-% achieved. After addition of 15 wt .-% TEOS (based on the Weight of the fluid) is the slip well pourable. He is in a flange as shaped mold cast from Teflon-coated sheet metal, frozen therein and then sintered, as described with reference to Example. 1

0103The finished flange is made of opaque quartz glass and has a homogeneous Pore ​​and density distribution. The total shrinkage is approximately 2.5%. It is free of calcium contamination as in the conventional technology they plaster mold occur.