DE102013206993B4 - Process for coating shaped bodies of fused silica - Google Patents

Abstract

Method for applying a protective layer of silicon nitride at least in regions to molded articles made of quartz material, in which an aqueous suspension is applied to the surface of the molded article at least once at least in regions and the molded article is then thermally treated at least once, the aqueous suspension a) silicon nitride particles, b) at least an organic cationic or pseudocationic dispersing / liquefying agent, and / or at least one anionic basic dispersing / liquefying agent, c) at least one organic binder, d) at least one plasticizer, e) ad 100% by weight water, and the pH Value of the aqueous suspension is between 7.5 and 11.

Description

A method for coating fused silica moldings is proposed, in which a suspension containing silicon nitride particles, dispersants / plasticizers, binders, plasticizers and water and having a pH in the range of 7.5 to 11 is used. In this method for coating shaped bodies of fused silica, the suspension is applied to the surface of a shaped body at least once and at least in some areas, and then the molding is thermally treated at least once. In addition, moldings of fused silica are described, which have a silicon nitride protective layer with a layer thickness of at least 700 microns and can be produced by the novel process.

In the photovoltaic industry and in its development laboratories as well as in the laboratories of the crystal growing institutes, quartz vitreous crucibles of various dimensions are used for the growth of Si crystals according to the method of "directional solidification". These crucibles must be provided with a separating layer before use. This separating layer prevents the reaction of the silicon melt with the quartz material and also the penetration of the low-viscosity melt into the pores of the quartz material.

The separation layer must not be wettable by the silicon melt and must allow, due to the anomaly of the silicon, volume growth of the crystal during crystallization. Thus, the possible formation of material stresses within the grown ingot is reduced and prevents cracking.

Furthermore, the separation layer must have a high degree of purity, since it is in direct contact with the Si melt or with the Si crystal during the entire crystallization process.

Another special problem of the release layer or coating is the diffusion permeability to impurities, which originate mainly from the crucible. During the crystal growth, oxygen, transition metals and other impurities diffuse from the inside of the crucible into the already existing solid phase of the Si crystal, which significantly affects the quality of the Si crystal in the edge regions.

Some crystal growers in the photovoltaic industry, but especially some of their development laboratories, as well as crystal growing institutes, use simple suspensions to coat the silica gobs, which usually consist of an aqueous suspension of high purity Si ₃ N ₄ powder and additives for better processing. These suspensions are usually applied by spraying on the inside of the crucible and then fired at different temperatures. The resulting Si ₃ N ₄ coating is suitable for immediate use on site, ie the coating is carried out by the user before the crucible for crystallization.

A disadvantage of this type of coating is, first, that the coating is not transportable, since it tends due to the relatively lower adhesion to slight shocks to flake off the crucible wall and second, that it has a low cohesion and surface hardness and when loading the crucible by touching With the Si pieces flake off parts of the coating and later get into the Si melt.

The ready-to-use crucible already provided by the crucible manufacturers generally has a good, transportable surface adhesion of the coating. However, the coating itself is often deficient. It has a rough, slightly crumbling surface or a weak consistency and firmness, which, as already mentioned, often leads to the popping of the coating in the Tiegelbestockung.

Certain properties are also achieved by the addition of additives or by the use of methods that leave impurities in the coating. Some of the methods used or proposed are technically very complicated and expensive.

Further solutions are in the WO 2004/053207 A1 . EP 0 963 464 A1 . DE 10 2007 053 284 A1 . WO 2007/039310 A1 and DE 10 2005 050 593 A1 disclosed, but have so far found little application in photovoltaic.

The US 6,165,425 A describes crucibles equipped with a protective layer of silicon and a method for applying the silicon protective layer and uses thereof.

From the US 2013/0026469 A1 For example, a silicon nitride coated crucible for molten semiconductor material and its use in the production of multicrystalline silicon is known.

The DE 10 2008 031 766 A1 describes a method for producing a coated crucible from a crucible green body or an intercalated crucible body and the use of the crucible.

Based on this prior art, it was the object of the present invention to provide an improved method for coating molded articles of fused silica.

The object is achieved by the method for coating shaped bodies made of fused silica according to claim 1. In the dependent claims preferred embodiments are shown.

• a) Siliciumnitrid-Partikel,
• b) mindestens ein organisches (pseudokationisches oder kationisches) Dispergier-/Verflüssigungsmittel, und/oder mindestens ein basisches anionisches Dispergier-/Verflüssigungsmittel
• c) mindestens ein organisches Bindermittel
• d) mindestens ein Plastifizierungsmittel
• e) ad 100 Gew.-% Wasser,

die dadurch gekennzeichnet ist, dass der pH-Wert der Suspension zwischen 7,5 und 11 liegt.The aqueous suspension used in the process according to the invention for coating shaped articles of fused silica comprises

• a) silicon nitride particles,
• b) at least one organic (pseudo cationic or cationic) dispersing / liquefying agent, and / or at least one basic anionic dispersing / liquefying agent
• c) at least one organic binder
• d) at least one plasticizer
• e) ad 100% by weight of water,

characterized in that the pH of the suspension is between 7.5 and 11.

The pH of the suspension in the range of pH 7.5-11 is responsible for the fact that when applying the suspension to the surface of a shaped body of fused silica to a very good binding of the molecules of the organic cationic or pseudokationischen and / or anionic basic Dispersing / liquefying agent on the "acidic" surface of the quartz material as well as on the surface of the Si ₃ N ₄ particles comes. Consequently, good wetting of the fused silica surface by the Si ₃ N ₄ suspension is achieved.

Preferably, the suspension has a pH between 8 and 11, since in this area the binding effect d. H. the adhesion of the suspension components to the surface of the molded article occurs more intensively.

The suspension used according to the invention also has the advantage that a deep infiltration of the suspension into the porous fused silica (> 700 microns infiltration depth) and thus a large-scale chemical / physical bonding of the components of the suspension is made possible with the fused silica. This results in a high adhesion of the constituents to the surface of the quartz material, which leads to a high adhesion of the resulting protective layer after thermal treatment of the quartz. Furthermore, the pores of the quartz material are sealed with Si ₃ N ₄ particles, whereby a smooth, non-powdery surface or protective layer is formed.

The suspension used according to the invention can penetrate into the molding body so strongly that the resulting overall layer in the quartz material can have a depth of at least 700 μm. Due to the enormous depth and a concentration shift in this area, the diffusion of particles of the quartz material through the layer is inhibited. If the coated molded body of fused quartz is used as a crucible for receiving liquid or crystalline silicon, a contamination of the silicon with impurities of the quartz material is thus counteracted.

The silicon nitride particles may be contained at 10 to 40 wt .-%, preferably at 15.0 to 35.0 wt .-%, based on the total suspension.

• a) eine mittlere Teilchengrößenverteilung d₁₀ von 0,20 bis 0,40 μm, d₅₀ von 0,50 bis 1,49 μm, bevorzugt von 0,50 bis 0,90 μm und/oder d₉₀ von 1,10 bis 3,90 μm, bevorzugt von 1,50 bis 2,50 μm auf, bestimmt nach dem Laserbeugungsverfahren gemäß ASTM B 822, und/oder
• b) eine spezifische Oberfläche von 4,67 bis 15,00 m²/g, bevorzugt von 8 bis 13 m²/g auf, bestimmt nach dem Gasadsorptionsverfahren gemäß ASTM D 3663.

In a preferred embodiment, the silicon nitride particles

• a) an average particle size distribution d ₁₀ of 0.20 to 0.40 μm, d ₅₀ of 0.50 to 1.49 μm, preferably of 0.50 to 0.90 μm and / or d ₉₀ of 1.10 to 3 , 90 microns, preferably from 1.50 to 2.50 microns, determined by the laser diffraction method according to ASTM B 822, and / or
• b) a specific surface area of from 4.67 to 15.00 m ² / g, preferably from 8 to 13 m ² / g, determined by the gas adsorption process according to ASTM D 3663.

The liquefying agent may be selected from the group consisting of basic anionic dispersants and non-foaming, alkali-free pseudokationischen / cationic dispersing / liquefying agents in particular be selected from the group consisting of amino alcohols. A particularly preferred liquefying agent is 2-amino-2-methylpropanol.

In a further preferred embodiment, the at least one organic (cationic or pseudo-cationic) dispersing / liquefying agent and / or the at least one anionic basic dispersing / liquefying agent is from 0.01 to 10% by weight, preferably from 0.1 to 2% by weight .-%, more preferably at 0.16 to 0.3 wt .-%, based on the total suspension, in the suspension.

The at least one in the suspension according to the invention contained at least organic binder preferably selected from the group consisting of water-soluble or water-dispersible hydrophilic polymers, in particular polyethylene glycols, polypropylene glycols, polybutylene glycols, polyvinyl alcohols, polyvinyl butyrals, polyvinyl pyrrolidones, polyvinyl acetates and partially or fully saponified polyvinyl acetates, in particular Fully hydrolyzed PVA M145000.

The at least one organic binder may contain from 0.01 to 20% by weight, preferably from 0.1 to 5% by weight, more preferably from 1.0 to 3.4% by weight, based on the total suspension, be included.

The plasticizer is in particular a polyhydric alcohol, in particular propane triol. The plasticizer is preferably present in an amount of from 0.1 to 6.0% by weight, more preferably from 0.5 to 3.0% by weight, based on the total suspension.

Furthermore, it is preferred that the suspension has a dynamic viscosity of 20 to 50 mPa · s, determined by means of a rotational viscometer with an immersion measuring device according to DIN 53019. A viscosity in this range has a particularly advantageous effect on the penetration depth of the suspension according to the invention into a fused silica.

According to the invention, a method is provided for at least partially applying a silicon nitride protective layer on molded articles made of fused quartz, in which the suspension described above is applied at least once at least partially to the surface of a shaped body and the shaped body is subsequently thermally treated at least once.

During the application of the suspension to the shaped body of fused silica, which is preferably in the green state, it is preferable to neutralize the silanol groups of Quarzgutoberfläche by molecules of the basic dispersing / liquefying agent, resulting in the formation of a monomolecular layer of organic molecules of the basic dispersant on the fused silica surface, which may also be modified. As a result of formation of numerous hydrogen bonds and higher affinity between already modified by basic dispersing / liquefying agent Si ₃ N ₄ particles and optionally modified Quarzgutoberfläche then good adhesion of the coating produced to the Quarzgutoberfläche or in the pores instead.

In the thermal treatment, preferably a firing process under oxidizing conditions, a permanent chemical adhesion between the fused silica surface and the coating is produced. During this step Si-O-Si bonds are formed as a result of a chemical polycondensation reaction between reactive silanol groups of the quartz material and the Si ₃ N ₄ powder with elimination of water. This causes a covalent chemical bond of the Si ₃ N ₄ coating with the fused silica surface, resulting in a good abrasion resistance of the coating.

The surface of the coating produced by the method according to the invention is smooth and has a low wettability to Si melt. This is particularly advantageous in contacting the crucible surface with liquid or crystalline silicon. In addition, the method has the advantage that it is easy and inexpensive to carry out.

The fused silica of the molding used in the process may be porous and preferably have a porosity of at least 8%, preferably 8 to 25%, particularly preferably from 10 to 15%. It is further preferred that the fused silica after the thermal treatment has an average pore diameter of 0.05 to 0.8 .mu.m, preferably 0.1 to 0.4 .mu.m, in particular 0.15 to 0.25 microns.

The suspension can be infiltrated into the molding in the process according to the invention at least 500 .mu.m, preferably 700 to 1000 .mu.m.

Particularly preferably, the molded body made of fused silica is a green body. As a green body is understood according to the invention an unfired blank. In this case, it is firstly advantageous that higher infiltration depths of the suspension according to the invention into the shaped body become possible by using a green body. Second, the coating of a green body instead of a fired molded body has the advantage that in only one step green body and coating can be fired (economic advantage). Thirdly, the common firing process of green body and coating better closes the pores of the green body in the area of the coating and thereby firmly anchors the Si ₃ N ₄ particles within the quartz material, ie below the surface of the quartz material.

The thermal treatment can be carried out at temperatures from 1000 to 1300 ° C., preferably from 1100 to 1200 ° C. and / or over a period of from 2 to 5 h, preferably from 3 to 4 h.

The thermal treatment can also be carried out under oxidative conditions, which causes oxidation of the top Si ₃ N ₄ layer of the coating on the molding. This oxidized layer in turn protects the underlying layers from further oxidation so that the full separation function of the coating is maintained. This can ensure that a pure Si ₃ N ₄ layer remains on the inner surface of the molding.

In summary, the silicon nitride particles of the suspension according to the invention become solid and chemically covalent in the fused silica surface anchored. As a result, a well-adherent, non-powdery and touch-resistant layer on the quartz body is formed from the suspension according to the invention applied to the quartz body after the thermal treatment.

The order of the suspension on the molding can be done by brushing, spraying, spraying, dipping the molding in the suspension and / or by electrostatic application. The suspension used according to the invention can also be applied with a specially equipped spraying device with which contamination of the suspension with metals is prevented. The penetration of the coating into the pores can be assisted by a spray-fogging process in wet / wet technique, whereby the depth of infiltration of the suspension according to the invention used in the molding can be further increased.

After application of the suspension and before the thermal treatment is preferably carried out a drying of the shaped body, preferably at temperatures of 20 ° C to 150 ° C and in particular at a humidity of 40 to 60%.

Furthermore, it is preferred that a closed silicon nitride protective layer is produced on the substrate surface of at least 50 μm layer thickness by the method according to the invention.

The fused silica may be selected from the group consisting of amorphous SiO ₂ , preferably melt-cast SiO ₂ having a crystalline phase content of <0.5%.

In a preferred embodiment, the molded article is selected from the group consisting of crucibles, conveyor rollers for treatment furnaces, furnace lining and charging station.

The method according to the invention thus makes it possible to provide shaped bodies of sintered fused silica, in particular a crucible or a roll, comprising a silicon nitride protective layer formed at least in regions on the surface of the molded body with a layer thickness of at least 50 μm, which can be produced by the method according to the invention.

The suspension used according to the invention or the method according to the invention can in principle be used for coating all porous ceramic materials having a porosity in the range of 8-25%, depending on the pore distribution, eg. As well as rolls of fused quartz, aluminosilicates, SiC materials and others on the assumption that silanol groups are located on the component surface or can be artificially produced.

Reference to the following figure and examples, the subject invention is to be explained in more detail, without wishing to limit this to the specific embodiments shown here.

example 1

1. Production of a shaped body

On a non-sintered (green) raw quartz crucible (green body), a specially formulated Si ₃ N ₄ suspension was applied to the inside of the porous raw quartz glass crucible, which infiltrates the porous quartz crucible at least at a certain layer depth and there "chemically "Adsorbed. Subsequently, the produced coating was cured by uniform mono-sintering.

The suspension used for this consisted of powdered silicon nitride, which was dispersed in an aqueous solution, the suspension containing an organic, amino group-containing liquefaction agent, namely 2-amino-2-methylpropanol with a weight fraction of 0.20%. Additional components of the suspension were an organic temporary binder (fully hydrolyzed polyvinyl acetates) with a mass fraction of 3%, and 0.5% plasticizer (propanetriol). The mass fraction of Si ₃ N ₄ powder was 30.0%. The pH of the Si ₃ N ₄ suspension was in the basic range (pH = 9).

The suspension was applied by spraying on the inner surface of a non-sintered fused silica crucible (green body). After coating, the crucible was dried and then sintered under an oxidative atmosphere at a temperature of 1150 ° C. for 4 hours. After the mono-sintering of the coated Si ₃ N ₄ suspension with the substrate, a well-adhering, non-powdery and contact-resistant and mechanically strong layer finally formed.

2. Use of the molding according to the invention

The crucible was filled with silicon and a multicrystalline silicon ingot was grown according to the directional solidification method used in the PV industry. After the procedure, which lasted about 48 hours and reached a temperature of 1480 ° C, the ingot could be removed from the crucible without any problems and without any adhesion.

Example 2

1. Production of a shaped body

A round sintered fused silica blank is machined by turning or by turning and grinding. The result was a Quarzgutrolle with a diameter of 50 mm and a length of 2430 mm, which was suitable for a treatment furnace.

After covering both ends (about 2 × 80 mm), to which later the metallic roller caps are attached, the surface of the quartz glass roll was coated with a Si ₃ N ₄ suspension. The suspension used for this consisted of powdered silicon nitride, which was dispersed in an aqueous solution containing an organic, amino group-containing liquefaction agent, namely 2-amino-2-methylpropanol with a weight fraction of 0.20%. Additional components of the solution were an organic temporary binder (fully hydrolyzed polyvinyl acetates) of 1.0% by mass, and 1.0% propanetriol (plasticizer). The mass fraction of the Si ₃ N ₄ powder was 20.0%. The pH of the Si ₃ N ₄ suspension was in the basic range (pH = 10).

The suspension was applied to the surface of the quartz glass roll by spraying. After the coating, the roll was dried and then fired under an oxidative atmosphere at a temperature of 1100 ° C for 15 minutes. After firing the quartz glass roll with the Si ₃ N ₄ coating, a well-adhering, non-powdery, contact-resistant and mechanically strong layer finally formed.

2. Use of the molding

The coated roll was used in a heat treatment furnace in which Al-Si coated sheets were treated prior to press-hardening at temperatures above 800 ° C.

It was observed that the Si ₃ N ₄ coating prevented a chemical reaction between the liquid Al-Si coating and the rolls. Usually, metal / metal oxide mixtures are formed here, which build up on the quartz glass roll, infiltrate the fused silica and later, due to premature failure of the rolls, lead to transport problems. In addition, occasionally the quality of the sheet coating is affected by contact with the infiltrated quartz glass roll.

Claims (19)

Process for the at least partial application of a silicon nitride protective layer on shaped articles made of fused silica, in which an aqueous suspension is applied at least once at least in some areas to the surface of the molded article and the molded article is subsequently thermally treated at least once, the aqueous suspension a) silicon nitride particles, b) at least one organic cationic or pseudo-cationic dispersing / liquefying agent, and / or at least one anionic basic dispersing / liquefying agent, c) at least one organic binder, d) at least one plasticizer, e) ad 100% by weight of water, and the pH of the aqueous suspension is between 7.5 and 11. Method according to the preceding claim, characterized in that the fused silica of the shaped body is porous and preferably has a porosity of at least 8%, preferably 8 to 25%, particularly preferably from 10 to 15% and / or after the thermal treatment has an average pore diameter of 0 0.05 to 0.8 μm, preferably 0.1 to 0.4 μm, in particular 0.15 to 0.25 μm. Method according to the preceding claim, characterized in that the suspension is infiltrated into the shaped body at least 500 μm, preferably 700 to 1000 μm. Method according to one of the preceding claims, characterized in that the shaped body of fused silica is a green body. Method according to one of the preceding claims, characterized in that the thermal treatment at temperatures of 1000 to 1300 ° C, preferably from 1100 to 1200 ° C and / or over a period of 2 to 5 hours, preferably from 3 to 4 hours performed becomes. Method according to one of the preceding claims, characterized in that the application of the suspension by means of brushing, spraying, spraying, dipping the shaped body in the suspension and / or by electrostatic application. Method according to one of the preceding claims, characterized in that after application of the suspension and before the thermal treatment, a drying of the shaped body, preferably at temperatures of 20 ° C to 150 ° C and in particular at a humidity of 40 to 60%. Method according to one of the preceding claims, characterized in that a closed silicon nitride protective layer on the Substrate surface of at least 50 microns layer thickness is generated. Method according to one of the preceding claims, characterized in that the fused silica is selected from the group consisting of amorphous SiO ₂ , preferably melt-cast SiO ₂ having a crystalline phase content of <0.5%. Method according to one of the preceding claims, characterized in that the shaped body is selected from the group consisting of crucibles, conveyor rollers for treatment ovens, furnace lining and charging point. Method according to one of the preceding claims, characterized in that the pH of the aqueous suspension is between 7.5 and 11, preferably between 8 and 11. Method according to one of the preceding claims, characterized in that the silicon nitride particles to 10 to 40 wt .-%, preferably to 15.0 to 35.0 wt .-%, based on the total suspension are included. Method according to one of the preceding claims, characterized in that the silicon nitride particles a) has an average particle size distribution d ₁₀ of 0.20 to 0.40 microns, d ₅₀ from 0.50 to 1.49 microns, preferably from 0.50 to 0.90 μm and / or d ₉₀ from 1.10 to 3.90 μm, preferably from 1.50 to 2.50 μm, determined by the laser diffraction method according to ASTM B 822, and / or b) has a specific surface area of 4 , 67 to 15.00 m ² / g, preferably from 8 to 13 m ² / g, determined by the Gasadsorptionsverfahren according to ASTM D 3663 have. Method according to one of the preceding claims, characterized in that either at least one organic (cationic or pseudo cationic) dispersing (or liquefaction) agent consisting of amino alcohols (preferably 2 amino-2-methylpropanol) and / or at least one basic anionic dispersant (or liquefaction) agent is selected. Method according to one of the preceding claims, characterized in that in the aqueous suspension at least one organic (cationic or pseudokationisches) dispersing / liquefying agent and / or at least one anionic basic dispersing / liquefying agent to 0.01 to 10 wt .-%, preferably to 0.1 to 2 wt .-%, particularly preferably 0.16 to 0.3 wt .-%, based on the total suspension is included. Method according to one of the preceding claims, characterized in that the at least one organic binder is selected from the group consisting of water-soluble or water-dispersible hydrophilic polymers, in particular polyethylene glycols, polypropylene glycols, polybutylene glycols, polyvinyl alcohols, polyvinyl butyrals, polyvinyl pyrrolidones, polyvinyl acetates and partially or vollverseiften Fully hydrolyzed polyvinyl acetates, in particular PVA M145000. Method according to one of the preceding claims, characterized in that in the aqueous suspension at least one organic binder to 0.01 to 20 wt .-%, preferably from 0.1 to 5 wt .-%, particularly preferably from 1.0 to 3 , 4 wt .-%, based on the total suspension is included. Method according to one of the preceding claims, characterized in that the at least one plasticizer is selected from the group consisting of polyhydric alcohols, in particular propane triol and / or in the aqueous suspension preferably in an amount of 0.1 to 6.0 wt .-% , more preferably from 0.5 to 3.0 wt .-%, based on the total suspension is included. Method according to one of the preceding claims, characterized in that the aqueous suspension has a dynamic viscosity of 20 to 50 mPa · s, determined by means of a rotational viscometer with an immersion measuring device according to DIN 53019.