Silicon-nitride-containing separating layer having high hardness

Abstract

The invention relates to a shaped body comprising a substrate with a firmly adhering separating layer, wherein the separating layer comprises 92-98 wt. % silicon nitride (Si.sub.3N.sub.4) and 2-8 wt. % silicon dioxide (SiO.sub.2) and wherein the separating layer has a total oxygen content of 8 wt. % and a hardness of at least 10 HB 2.5/3 according to DIN EN ISO 6506-1.

Claims

1. Shaped body comprising a substrate with a firmly adhering separating layer, wherein the separating layer comprises 92-98 wt. % silicon nitride (Si.sub.3N.sub.4) and 2-8 wt. % silicon dioxide (SiO.sub.2) and wherein the separating layer has a total oxygen content of 8 wt. % and a hardness of at least 10 HB 2.5/3 according to DIN EN ISO 6506-1.

2. The shaped body according to claim 1, wherein the separating layer furthermore contains a residual content of a dopant in the form of a flux.

3. The shaped body according to claim 2, wherein the dopant comprises an alkali metal compound.

4. The shaped body according to claim 3, wherein the fraction of the dopant expressed as the alkali metal content of the separating layer is up to 150 ppm.

5. The shaped body according to claim 1, wherein the total oxygen content of the separating layer is <5 wt. %.

6. The shaped body according to claim 1, wherein the separating layer contains 94-98 wt. % Si.sub.3N.sub.4 and 2-6 wt. % SiO.sub.2.

7. The shaped body according to claim 1, wherein the hardness of the separating layer is at least 15 HBW 2.5/3 according to DIN EN ISO 6506-1.

8. The shaped body according to claim 1, wherein the substrate consists of fused silica.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The separating layer according to the invention contains 92-98 wt. % silicon nitride (Si.sub.3N.sub.4) and 2-8 wt. % silicon dioxide (SiO.sub.2), preferably 94-98 wt. % Si.sub.3N.sub.4 and 2-6 wt. % SiO.sub.2 and particularly preferably >95-97 wt. % Si.sub.3N.sub.4 and 3-<5 wt. % (SiO.sub.2).

(2) In the separating layer according to the invention, the SiO.sub.2 is a binder for the silicon nitride. The SiO.sub.2 content of the separating layer can be determined by analysis of the total oxygen content of the coating and converting to SiO.sub.2.

(3) The total oxygen content of the separating layer is 8 wt. %, preferably <5 wt. %. The total oxygen content is composed of the oxygen of the SiO.sub.2 binder, the oxygen unavoidably contained in the silicon nitride powder, and the oxygen content produced during baking of the coating by oxidation of the silicon nitride.

(4) If the dopant contained in the coating suspension to produce the separating layers according to the invention does not vaporise completely free from residue during hardening of the coating suspension by baking, the separating layer can also contain a residual content of a dopant in the form of a flux.

(5) The dopant in the form of a flux is preferably an alkali metal compound, more preferably a sodium compound. The fraction of the dopant, expressed as alkali metal content of the separating layer, is preferably up to 150 ppm, further preferably up to 50 ppm.

(6) The hardness of the separating layer is at least 10 HB 2.5/3 according to DIN EN ISO 6506-1, preferably at least 15 HBW 2.5/3 and particularly preferably at least 20 HBW 2.5/3.

(7) It is also possible to produce layers having a gradient in the oxygen content where the layer in contact with the semiconductor material such as for example solar silicon contains at most 8 wt. % of oxygen and preferably less than 5 wt. % of oxygen and at the same time has a hardness HBW 2.5/3 of at least 10.

(8) The shaped body according to the invention can be produced by a method comprising the steps: a) preparing a coating suspension comprising Si.sub.3N.sub.4 and an SiO.sub.2-based binder as well as a dopant in the form of a flux for producing a firmly adhering separating layer b) providing a substrate c) applying the coating suspension to the substrate and d) hardening the applied coating suspension by baking at an elevated temperature to form a firmly adhering separating layer.

(9) The coating suspension in step a) preferably comprises a suspension of solid particles and the dopant, wherein the solid particles comprise 88-98 wt. % of silicon nitride and 2-12 wt. % of a SiO.sub.2-based binder.

(10) The silicon-nitride-containing coating suspension based on water or an organic dispersing medium contains preferably highly pure silicon nitride having a mean particle or agglomerate size (d.sub.50) in the range of 0.5-20 m, preferably of 1-5 m and particularly preferably of 1.5-3 m.

(11) The SiO.sub.2-based binder in the coating suspension is, for example, derived from substances or substance mixtures which contain or form silicon dioxide precursors or is already present as silicon dioxide particles or as a mixture of precursors of silicon dioxide particles and silicon dioxide particles and during pyrolysis at 300 C. form SiO.sub.2 having a purity of >99.95%, preferably >99.99%. In the baked, ready-to-use coating there is thus a high-purity SiO.sub.2 binder. In the coating suspension the SiO.sub.2-based binder according to the invention is preferably present as a nanodisperse phase or as sol or as ceramic precursors or as mixtures thereof (precursor, monomer, fractal accumulations of monomers or polycondensate).

(12) The coating suspension or the SiO.sub.2-based binder in the coating suspension contains a dopant. The dopant is a flux for SiO.sub.2. Fluxes generally reduce the melting point or the softening point of SiO.sub.2 and reduce the glass transition temperature. Preferred are fluxes which particularly substantially reduce the melting point or the softening point of the SiO.sub.2 and can achieve this with only low contents. Substances or substance mixtures which contain alkali metal compounds are preferably used as dopant, for example, alkali carbonates such as potassium or sodium carbonate or also sodium silicate or potassium silicate as well as combinations of such substances. Particularly preferably sodium compounds are used as dopant.

(13) The content of the active component of the dopant such as the alkali metal content in alkali metal compounds, for example, of potassium in potassium carbonate or sodium in sodium silicate in the coating suspension ready for use is preferably between 30 and 500 ppm, further preferably between 50 and 400 ppm and particularly preferably between 80 and 300 ppm, where the quantitative details of the dopant are relative to the total solid content of the coating suspension after pyrolysis of the binder, i.e. the binder was taken into account when determining the total solid content as SiO.sub.2.

(14) It is preferred that the dopant is at least partially transient in a thermal treatment of the coating so that after baking of the coating or during use before the silicon begins to melt, said dopant is only partially present in the coating or has almost or completely disappeared. This prevents the dopant from being transported in undesired quantities as an impurity into the silicon ingot.

(15) The dopant is added to the dispersing medium of the coating suspension and is preferably present there as an insoluble or barely soluble compound.

(16) In the case of porous solar crucibles made of fused silica, a part of the binder is drawn into the crucible wall due to capillary forces and is thus no longer part of the coating. Likewise, when using SiO.sub.2 precursors such as metallorganic compounds (such as, for example, tetraethyl orthosilicate and sol gel systems produced therefrom) low-molecular components can vaporise during drying of the layer so that the effective binder content in the coating is significantly lower than the binder content which is set in the formulation of the coating suspension.

(17) The binding mechanism of the binder modified by adding the dopant differs from coatings in which a low-temperature binder known from the prior art is used. Proof of this is that neither the dopant alone nor the binder alone, together with silicon nitride powder, are capable of producing the functional silicon nitride layers according to the invention having low oxygen content and high hardness and adhesive strength (see Reference examples).

(18) The binder modified by adding dopant differs significantly in its properties from the original undoped binder. By using the dopant with low binder contents at the same time, few stresses are formed in the separating layer with the result that significantly fewer defects occur in the substrate surface or in the separating layer both after the coating or after the baking and also during the process such as possibly so-called chipping (flaky splitting of the coating in the entire depth including possibly crucible material) or cracks and microcracks or the formation of entire crack networks. This is not achieved as in DE 10 2007 053 284 A1 by reducing the sintering activity, but by using the dopant which reduces the glass transition temperature. This already brings about at <400 C. a softening of dopant-rich regions in the coating and therefore a relaxation of stresses which can produce defects in the separating layer.

(19) The coating suspension to produce the shaped body according to the invention with the silicon-nitride-containing separating layers can be produced by means of a method comprising the following process steps: 1) producing a pre-product A by mixing SiO.sub.2-based binder raw materials, dispersing medium and the dopant, 2) producing the coating suspension by dispersion of silicon nitride powder and optional adjuvants in pre-product A.

(20) It is possible to combine steps 1) and 2) and disperse the silicon nitride power jointly with binder raw materials, dopant and optionally adjuvants in a dispersing medium.

(21) Another possible method of manufacture for the coating suspension which can be used, for example, when using a liquid dopant, comprises the following process steps: 1) producing a pre-product B by precipitating the dopant in the dispersing medium and dispersing silicon nitride powder in the doped dispersing medium by joint grinding, 2) producing the coating suspension by homogenisation of the pre-product B with the SiO.sub.2-based binder raw materials and optionally adjuvants by joint grinding.

(22) The binder raw materials in 1) or 2) of the aforesaid process variants are preferably precursors of the silicon dioxide or silicon nitride such as, for example, silicon organic compounds and hydrolysis products therefrom as well as condensation products thereof as well as mixtures of silicon organic compounds, hydrolysis and condensation products thereof which are produced by means of a sol-gel process and/or salts of silicon such as silicon tetrachloride, optionally with added silicon dioxide nanoparticles or silicon dioxide particles in the submicron range.

(23) Examples for suitable silicon organic compounds are tetraethyl orthosilicate (TEOS) and methyltriethoxy silane (MTEOS). It is also possible to use only SiO.sub.2 nanoparticles as binder raw materials without added silicon organic compounds. The mean particle size of the SiO.sub.2 nanoparticles is preferably 100 nm and less, further preferably 50 nm and less.

(24) The dopant in 1) preferably comprises an alkali metal compound, particularly preferably a sodium compound. For example, an alkali carbonate such as potassium or sodium carbonate can be used as dopant or also potassium or sodium silicate.

(25) The dopant is added to the dispersing medium of the coating suspension and is preferably present there as an insoluble or barely soluble compound.

(26) Preferably between 30 and 500 ppm, further preferably between 50 and 400 ppm and particularly preferably between 80 and 300 ppm of dopant is added, where the quantitative details of the dopant is relative to the total solid content of the coating suspension after pyrolysis of the binder, i.e. the binder has been taken into account when determining the total solid content as SiO.sub.2.

(27) Water and/or an organic solvent, for example, an alcohol such as ethanol can be used as the dispersing medium.

(28) The mixing and the dispersing in step 1) or 2) can be carried out in a wet grinding or other mixing units.

(29) Preferably a high-purity powder is used as silicon nitride powder in 2) or 1). The total content of metal impurities of the silicon nitride powder is preferably less than 100 ppm, particularly preferably less than 50 ppm. The oxygen content of the silicon nitride powder is preferably less than 2 wt. % and the total carbon content is preferably less than 0.35 wt. %. The mean particle or agglomerate size (d.sub.50) of the silicon nitride powder preferably lies in the range of 0.5-20 m, further preferably 1-5 m and particularly preferably 1.5-3 m.

(30) The adjuvants in 2) can be organic compounds such as, for example, polyvinyl butyral (PVB), polyvinyl alcohol (PVA), polyethylene glycol (PEG), wax or ethanol-soluble polymers.

(31) The solid content in the coating suspension is suitably 40 to 65 wt. % for application by flooding, 35 to 55 wt. % for application by wet-on-wet spraying.

(32) The application of the suspension produced in step c) of the method for producing the shaped body according to the invention is accomplished by commonly used coating methods such as spraying (preferably wet on wet) or flooding on an inorganic substrate (such as, for example, fused silica).

(33) The formation of the separating layer of the shaped body according to the invention in step d) suitably takes place by baking the coating at 300-1300 C., preferably at 900 C.-1200 C., further preferably at 1000 C.-1100 C. in air or at low oxygen partial pressure, or in a reducing or inert atmosphere at 800-1750 C., preferably at 1000 C.-1725 C., and further preferably at 1100 C.-1700 C. or at gas pressure (e.g. nitrogen, argon) at 1000-2000 C., preferably at 1500 C.-1900 C., and further preferably at 1600 C.-1800 C. The heating and cooling time is, for example, 8 hours in each case, the holding time at maximum temperature is preferably about one hour. Baking of the coating in a gas-fired furnace at low oxygen partial pressure is preferred since the silicon nitride in the coating is then less strongly oxidized.

(34) The use of highly pure initial chemicals (silicon nitride powder, silicon organic compounds, alcohols etc.) is preferred since extremely pure layers can be obtained in this case which in particular meet the requirements of the solar industry.

(35) In the shaped bodies according to the invention, the substrate suitably consists of ceramic, including silicon nitride ceramic or SiO.sub.2 (fused silica) or also fibre mats or fabric. In a preferred embodiment the shaped body comprises a crucible having a substrate of SiO.sub.2 (fused silica) which is suitable for the processing of corrosive nonferrous metal metals, in particular silicon melts.

EXAMPLES AND COMPARATIVE EXAMPLES

(36) Brinell Hardness Measurement:

(37) The hardness of the coatings is determined herein as Brinell hardness according to DIN EN ISO 6506-1, using a ball diameter of 2.5 mm and a load of 3 kg.

(38) Peeling Test (Adhesive Strength):

(39) The adhesive strength was determined by measuring the force required to peel off a plate glued to the baked coating of a coated sample (adhesive area 500 mm.sup.2). The plate was glued using a two-component epoxy resin adhesive which as a result of its high viscosity penetrates a maximum of 50-80 m into the coatings. The adhesive strength was determined perpendicular to the layer surface in N/mm.sup.2. The layer thickness of the tested coated samples was between 150 and 250 m.

(40) Determination of Total Oxygen Content and Sodium Content of the Coating:

(41) The coatings were removed using a silicon nitride scraper and the powder thus obtained was used for the analysis without drying, The total oxygen content was determined by means of carrier gas hot extraction. The sodium content was determined by means of optical emission spectrometry with inductively coupled plasma and electrothermal vaporization (ETV-ICP OES).

Comparative Example 1

Standard Suspension

(42) 50 wt. % of silicon nitride powder (UBE E10) is homogenized in ethanol. The suspension is applied to the cleaned, dust-free dry fused silica crucible. Coating of the crucible by means of flooding is not possible since cracks are already formed in the coating from a layer thickness of 150-200 m during drying which results in a flat detachment of the coating before the baking of the coating. Wetting of the crucible with ethanol before the application could not prevent this effect. Also when 2% PVA Celvol E 04/88 (Celanese Emulsions GmbH) was added to the ethanol-silicon nitride suspension, no crack-free layer thicknesses of >250 m could be achieved during an application by flooding. The oxygen content of the coating is found in Table 1, it was not possible to determine the hardness.

Comparative Example 2

Standard Suspension

(43) 50 wt. % of silicon nitride powder (UBE E10) is homogenized in ethanol. The suspension is applied to the cleaned, dust-free dry fused silica crucible. The coating is applied by spraying. It was found that a wet-on-wet spraying of the suspension is not possible since cracks are formed in the coating from a layer thickness of about 200 m. The coatings are sprayed in powder form.

(44) After drying the coatings are baked at about 1000 C. before being used as crucibles. The silicon nitride layer thus produced is only touch-proof to a certain degree and should be handled with appropriate care.

(45) The adhesive strength of the coating is 0.18 N/mm.sup.2, the hardness is 2 HBW 2.5/3 (see Table 1).

Comparative Example 3

(46) A coating suspension was produced according to WO 2007/039310 A1, Example 1b. The coating was applied by powder spraying or flooding and baked at 500 C. (3a) and 750 C. (3b).

(47) The coating is powdery after baking and not touch-proof. For the powder-sprayed coatings the oxygen content, the hardness and the adhesive strength of the coating are given in Table 1.

(48) When the coating is applied by flooding, the coating already cracks during drying or baking at a layer thickness of >80-150 m and becomes detached so that no adhesive strength could be measured here and it was also not possible to measure the hardness.

Comparative Example 4

(49) A coating was prepared in accordance with DE 10 2005 050 593 A1. To this end 600 g of silicon nitride powder (H.C. Starck, M11 h.p., mean particle size d.sub.50<1 m measured using a Mastersizer 2000, Malvern) is dispersed in a mixture of 900 g of ethanol and 95 g of the binder Inosil S38, Inomat GmbH, Germany (having an SiO.sub.2 solid content in the annealing residue of 33 wt. %) on a roller block in a PE vessel containing silicon nitride grinding balls for five hours. The suspension contains 38 wt. % of silicon nitride. The suspension is applied by flooding on a fused silica solar crucible with a layer thickness of 100 m. After drying the layer is baked at 500 C. for 30 minutes. Table 1 gives the oxygen content, the hardness and the adhesive strength of the coating.

Comparative Example 5

(50) A coating was prepared in accordance with DE 10 2005 050 593 A1. To this end 600 g of silicon nitride powder (H.C. Starck, M11 h.p., d.sub.50<1 m) is dispersed in a mixture of 800 g of ethanol and 190 g of the binder Inosil S38, Inomat GmbH, Germany (having an SiO.sub.2 solid content in the annealing residue of 33 wt. %) on a roller block in a PE vessel containing silicon nitride grinding balls for five hours. The suspension contains 38 wt. % of silicon nitride. The suspension is applied by flooding on a fused silica solar crucible with a layer thickness of 100 m. After drying the layer is baked at 500 C. for 30 minutes. Table 1 gives the oxygen content, the hardness and the adhesive strength of the coating.

Comparative Example 6

(51) A coating was prepared in accordance with DE 10 2007 053 284 A1.

(52) 2450 g of Inosil S-B binder (Inomat GmbH having an SiO.sub.2 solid content in the annealing residue of 33 wt. %), 1200 g of ethanol, 4580 g of silicon nitride UBE E10 and 200 g of PVB are homogenized with silicon nitride grinding balls in a PE container in the form of a suspension. The suspension is converted into granules by spray granulation. After annealing the granules in air at 450 C., annealing is carried out for one hour in the closed crucible at 900 C. 750 g of the annealed granules are homogenized in 500 g of ethanol with grinding balls and ground in the form of a suspension to an agglomerate size of 4 m. The suspension is applied by flooding to a fused silica crucible having a layer thickness of 350 m. The layer is dried in air and baked for one hour at 1125 C. in a gas-fired furnace with a heating and cooling time of 8 hours each. Table 1 gives the oxygen content, the hardness and the adhesive strength of the coating.

Reference Example 1

Binder-Free Coating

(53) 12 g of sodium silicate (8% solid, ultrapure) is precipitated by dropping whilst stirring vigorously in 462 g of ethanol. In the dispersion 1150 g of silicon nitride powder (HC Starck M11 h.p. coarse, d.sub.50=1.9 m, measured using a Mastersizer 2000, Malvern) is homogenized with silicon nitride grinding balls in a PE container on a roller block for 6 hours to produce the coating suspension. The doping with sodium is 200 ppm relative to the total oxygen content of the coating suspension. The total oxygen content of the suspension is 66 wt. %. The coating suspension is applied by one-off flooding to a fused silica crucible with a layer thickness of about 200 m. The layer is dried for 24 hours in air and then baked for one hour at 1100 C. with a heating and cooling rate of 2.3 C./min. The coating is not touch-proof.

(54) The total oxygen content of the coating is given in Table 1. It was not possible to measure the hardness of the coating since the baked coating had already detached from the substrate during the measurement. The adhesive strength could not be determined for the same reason.

Reference Example 2

Coating with Binder without Doping

(55) 1090 g of silicon nitride powder (HC Starck M11 h.p. coarse, d.sub.50=1.9 m, measured using a Mastersizer 2000, Malvern) is homogenized with silicon nitride grinding balls in 462 g of ethanol in a PE container on a roller block for 6 hours to produce the coating suspension. The coating suspension is formed by adding 174 g of Inosil S-P (Type Inosil S-P 38, Inomat, Germany) with 34.5 wt. % of resulting SiO.sub.2 solid after drying and pyrolysis (annealing residue) and homogenization for a further 4 hours. The resulting SiO.sub.2 solid content from the binder in the coating suspension is 5 wt. %. The total solid content of the suspension is 66 wt. % relative to the annealing residue of the binder (after drying and pyrolysis). The coating suspension is applied by one-off flooding to a fused silica crucible with a layer thickness of about 180 m. The layer is dried for 24 hours in air and then baked for one hour at 1100 C. with a heating and cooling rate of 2.3 C./min. The coating is not touch-proof.

(56) The total oxygen content of the coating is given in Table 1.

(57) It was not possible to measure the hardness of the coating since the baked coating had already detached from the substrate during the measurement. The adhesive strength could not be determined for the same reason.

Reference Example 3

(58) Comparative example 3 was repeated but the binder content was reduced to 4 wt. %. The coating was applied by powder spraying and baked at 500 C.

(59) The total oxygen content of the baked coating is given in Table 1. The coating is not touch-proof. The hardness and adhesion of the coating could not be measured since the layer flaked during sample preparation.

Example 1

(60) 12 g of sodium silicate (8% solid, ultrapure) is precipitated by dropping whilst stirring vigorously in 462 g of ethanol. In the dispersion 1090 g of silicon nitride powder (HC Starck M11 h.p. coarse, d.sub.50=1.9 m, measured using a Mastersizer 2000, Malvern) is homogenized with silicon nitride grinding balls in a PE container on a roller block for 2 hours. The coating suspension is formed by adding 174 g of Inosil S-P (Type Inosil S-P 38, Inomat GmbH, 34.5 wt. % of resulting SiO.sub.2 solid after drying and pyrolysis) and homogenizing for a further 4 hours. The resulting SiO.sub.2 solid content from the binder in the coating suspension is 5 wt. %. The doping with sodium is 200 ppm relative to the total oxygen content of the coating suspension after pyrolysis of the binder. The total oxygen content of the suspension is 66 wt. % relative to the annealing state of the binder (after drying and pyrolysis).

(61) The coating suspension is applied by one-off flooding to a fused silica crucible with a layer thickness of about 150 m. The layer is dried for 24 hours in air and then baked for one hour at 1100 C. with a heating and cooling rate of 2.3 C./min. The coating is defect-free and not powdery.

Examples 2-23

(62) Further examples according to the invention are prepared similarly to Example 1 with different fractions of binder and dopant and baked at different temperatures. The experimental results are shown in Table 1.

(63) In Table 1 Examples Nos. 1 to 23 are according to the invention, Examples V1 to V6 are comparative examples and Examples R1 to R3 are reference examples.

(64) The values given in Table 1 for binder content (content of SiO.sub.2-based binder) and Na doping each relate to the coating suspension. In addition, Table 1 gives the respective baking temperature of the coating and the values measured on the baked coating for oxygen and sodium content as well as hardness and adhesive strength.

(65) TABLE-US-00001 TABLE 1 Binder Na Baking Na Total oxygen Brinell Adhesive Example content doping temperature content content hardness strength No. [wt. %] [ppm] [ C.] [ppm] [wt. %] [HBW 2.5/3] [N/mm.sup.2] 1 5 200 1100 6 4.9 29 0.92 2 2 150 1100 6 4.8 15 0.30 3 4 150 1100 4 4.6 20 0.47 4 5 50 300 30 1.7 12 0.39 5 5 50 500 26 1.9 14 0.22 6 5 50 750 22 2.1 15 0.38 7 5 50 1000 3 3.1 17 0.26 8 5 50 1100 2 4.5 26 0.16 9 5 120 1100 4 4.5 28 0.74 10 5 150 300 111 1.8 13 0.57 11 5 150 500 94 1.8 16 0.78 12 5 150 750 79 2.3 18 1.02 13 5 150 1000 6 3.6 22 0.71 14 5 150 1100 5 4.9 25 1.29 15 5 200 300 119 1.7 16 0.33 16 5 200 500 108 1.9 18 0.69 17 5 200 750 98 2.1 19 0.95 18 5 200 1000 9 3.9 28 0.71 19 5 350 1100 14 7.0 29 0.21 20 6 150 1100 4 4.7 28 0.71 21 3 150 1100 7 3.7 16 0.23 22 8 150 500 70 4.0 22 1.03 23 8 150 1100 7 4.2 30 1.69 V1 0 0 1000 n.a.*) 2.8 n.m.**) n.m.**) V2 0 0 1000 n.a.*) 2.8 2.0 0.18 V3a 15 0 500 n.a.*) 8.6 2.7 0.23 V3b 15 0 500 n.a.*) 8.9 3.0 0.27 V4 5 0 500 n.a.*) 1.9 6.4 1.77 V5 10 0 500 n.a.*) 2.8 8.0 0.64 V6 15 0 1100 n.a.*) 9.2 6.7 0.56 R1 0 150 1100 22 8.2 n.m.**) n.m.**) R2 5 0 1100 <1 4.0 n.m.**) n.m.**) R3 4 0 500 n.a.*) n.m.**) n.m.**) *)Not analysed **)Not measurable since layer was already detached before or during the measurement