CZ169394A3 - Silicon granulate for silicification of building elements made of carbon or mixtures of silicon carbide and carbon, process for producing such granulate and process for producing silicon-infiltrated building elements of silicon carbide - Google Patents

Abstract

The proposed process for producing structural element from silicon infiltrated silicon carbide is characterized in that shaped bodies of carbon or a mixture of silicon carbide and carbon are put onto a burning furnace baking plate a by baking they are infiltrated with silicon, wherein silicon granulate is used as a silicon source. Said silicon granulate particles are provided with a coating that contains carbon or a carbon-releasing compound, a binding agent and boron nitride.

Description

The invention relates to a silicon granulate for silicon molding of carbon or silicon carbide moldings, a process for producing a coated silicon granulate and a process for producing silicon-infiltrated silicon carbide components using a coated silicon granulate.

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BACKGROUND OF THE INVENTION

Among non-oxidic high-quality ceramic materials, silicon carbide materials represent a particularly important and promising group of materials. To combine their excellent fabric properties such as hardness, abrasion resistance, high temperature stability, resistance to most acids and alkalis and good sliding properties, silicon carbide materials are used as a material for sliding rings, bearings, sliding rings, burners, thermal heat exchangers and other purposes.

In the silicon process, the carbon molding may react wholly or partially by infiltration, often referred to as reaction sintering, at high temperatures in the presence of silicon to silicon carbide, and by further infiltration of silicon the pores of the molding may be closed. Furthermore, the

-2-silicon process molded from a mixture of silicon carbide and carbon to infiltrate in the presence of silicon.

Liquid silicon penetrates into the shaped body and reacts with carbon to form a new silicon carbide. The infiltration may be carried out under vacuum at a temperature of from about 1400 ° C, preferably at a temperature of 1600 ° C to 1800 ° C, or using a shielding gas, for example argon, at temperatures up to 2200 ° C. The infiltration process has the advantage that there is no or almost no shrinkage.

In a frequently used silicon process, usually the silicon shaped bodies are deposited on a firing aid, for example, discs, plates or prisms of the same or similar already siliconized material. The firing aid is located on the firing plate of the firing furnace; silicon granulate is distributed between the firing aids. Silicon melts at temperatures above about 1400 ° C. The liquid silicon enters the pores of the auxiliary firing agent into the interior of the molding so that the moldings placed on the firing auxiliary are infiltrated with liquid silicon.

This procedure is subject to significant disadvantages. Despite the use of the firing aid, direct contact of the liquid silicon and the silicon molded body is not always excluded. This can lead to cracks in the shaped body. In addition, direct contact of the liquid silicon and the moldings can lead to large-area adhesion of the silicon to the moldings or the bonding of the moldings, firing aids and firing plates, which are removed only with a great waste of time and increased rejects.

Furthermore, liquid silicon leaking from the firing plate can damage the firing furnace, particularly its insulation. In addition, the effluent silicon is no longer available for molding the silicon body, so that the components are incomplete siliconized. By accumulating liquid silicon on the firing plate, it is heavily stressed and easily breaks. The same applies if the firing aid is baked onto the firing plate.

After completion of the silicon process, the components may be damaged or completely destroyed by breaking off, especially at the edges, when they are separated from the firing aids. Furthermore, this separation is very laborious. The necessity of using auxiliary firing agents brings further disadvantages. Firstly, the production of firing aids is necessary, which is relatively expensive. Secondly, the firing auxiliaries are space-intensive in the firing furnace, so that optimum utilization of the furnace capacity is impossible. Third, it is costly to place the moldings on the baking plates.

SUMMARY OF THE INVENTION

SUMMARY OF THE INVENTION It is an object of the present invention to provide for use a silicon granulate which allows for higher yields of perfect siliconized structural elements and which protects the baking furnace from damage by liquid silicon. It is further intended to allow a siliconing process in which the use of a firing aid is not necessary.

This object is achieved by a silicon granulate whose particles are provided with a coating comprising a carbon or carbon-releasing compound and a binder. A preferred embodiment is

The silicon granulate, the coating of which contains carbon in the form of carbon black, graphite or other types of carbon, further comprises an organic binder or boron nitride, and the coating has a thickness of from 1 µm to 0.5 mm.

The silicon granulate preferably consists of silicon, a mixture of silicon and a silicide or a mixture of silicon and a silicide-forming metal, and its coating comprises a water-soluble cellulose derivative as a binder.

In the process for producing the silicon granulate according to the invention, the silicon granulate is mixed with a solution of an organic binder or a liquid binder, with carbon and optionally boron nitride and subsequently dried. In a preferred embodiment, the silicon granulate is mixed with an aqueous solution of an organic binder or a liquid binder before carbon and optionally boron nitride are added, the binder solution containing 3 to 60% by weight binder and 0.1 to 10% by weight binder. %, preferably 0.5 to 5% by weight, based on the silicon granulate and the coating composition.

The amount of carbon is 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the silicon granulate and the coating composition.

The amount of boron nitride is 0 to 5% by weight, preferably 0 to 1% by weight, based on the silicon granulate and the coating composition.

The coated silicon granulate of the invention is used in a silicon process to produce silicon-infiltrated silicon carbide components in which the moldings are carbon or silicon carbide mixtures

The carbon to be siliconized is placed on the firing plates of the kiln and is fired by silicon. The silicon granulate to be coated may contain, depending on the degree of purity, more or less pure silicon, a mixture of silicon and silicides or a mixture of silicon and metals to form silicides.

In the case of mixtures of silicon and silicides, the silicon granulate may contain up to 50% by weight of silicides. For mixtures of silicon and metals, the maximum metal content is well below 50% by weight. Preferred metals for forming silicides and as a component of silicides are chromium, tungsten, hafnium, zirconium, vanadium, boron, manganese, niobium, tantalum, molybdenum and iron. Silicide containing silicon granules can be made from a mixture of solid silicon and silicides, followed by crushing and grinding the bar. The metal-containing silicon granulate is made from a mixture of silicon and metal.

During the first firing stage during siliconization at temperatures up to about 400 ° C, the organic binder of the coated silicon granulate according to the invention is decomposed. Carbon and boron nitride remain as the coating on the particles when boron nitride is added to the coating, as envisaged in a preferred embodiment. When the silicon begins to melt, the carbon to be coated with the silicon from the interior of the particles reacts to the silicon carbide, which, optionally together with boron nitride, forms a shell to trap the molten silicon. The molten silicon permeates a heat-resistant but permeable coating and can penetrate either directly or through the porous firing aid into the shaped body. At the end of the silicon process, coatings remain as empty packages, hereinafter referred to as slag. If the silicon granulate contains not only silicon, but also silicides or metals that react with silicon to form silicides during silicon firing, the silicides are found in molten silicon firing

-6or solid form. The molten silicide penetrates the molten silicon through the particle coating. The solid silicide is carried by the molten silicon through the coating of the particles when the particle size is sufficiently small.

The method of producing the coated silicon granulate of the invention avoids direct contact of the molten silicon and the silicon moldings during firing. The molten silicon penetrates the coating slowly into the shaped body and reacts belatedly. Both results in the formation of cracks in the silicon-based structural elements. An even silicon uptake in the shaped bodies is achieved. Furthermore, large-area adhesion of silicon, which is very time-consuming to remove in the subsequent cleaning process, is avoided. A further advantage of using the granulate according to the invention is that no silicon can escape from the baking plate. This prevents damage to the baking furnace and also does not produce any imperfectly siliconized components which would have to be subjected to repeated firing. It is further advantageous that no firing auxiliaries are required in the process for producing the silicon granulate according to the invention. This simplifies the batch process, the capacity of the kiln can be better utilized; there is no need to produce a firing aid and to separate the firing aid from the silicon-based structural elements, which can lead to damage or even destruction of the individual structural elements. The empty slags formed as slag in the production of the silicon granulate according to the invention are easily separated from the components.

It is possible to siliconize all porous molded bodies of carbon or a mixture of silicon carbide and carbon with the silicon granulate according to the invention. The shaped bodies may be

Also fiber-reinforced carbon, for example carbon-fiber reinforced carbon (CFC). Molded bodies for silicon can be produced by conventional molding processes, such as casting or pressing. It is also possible to prepare them by concentrating the silicon carbide by admixing the carbonizable binder and carbon and then coking. The geometric shape of the bodies is adjusted by turning, drilling and the like.

The silicon-infiltrated structural material of the present invention comprises RBSiC, SiSiC, with a higher proportion of large CSiSiC carbon inclusions, or a mixture of silicon, silicon carbide and silicides (silicide - SiSiC), whereby the material can be fiber reinforced. Some possibilities of using structural elements made of these materials have already been mentioned in the examples.

In the process for the production of silicon-infiltrated silicon carbide components according to the invention, various possibilities are offered for arranging the moldings and the coated silicon granulate in the kiln. In a particularly simple embodiment, for example, a coated silicon granulate is poured onto a baking plate on which boron nitride is superimposed and the moldings to be silicon are placed directly on it. The shaped bodies can also be inserted into a layer of coated silicon granulate. In the case of tubular shaped bodies, it is possible to fill the coated silicon granulate into a shaped body located on the firing plate. Furthermore, it is possible to arrange the shaped bodies one above the other, each being separated by a layer of the silicon granulate according to the invention.

The silicon process according to the invention is particularly advantageous for large moldings and large amounts of silicon on the firing plate.

If the presence of slag is unacceptable on the finished silicon components, various arrangements are possible in which the moldings to be silicon are separated from the coated silicon granulate by a porous liquid-permeable material. For example, a coated silicon granulate can be poured onto the baking plate, a porous silicon carbide plate can be placed thereon and the shaped bodies deposited thereon. Further, the coated silicon granulate may be embedded in porous silicon carbide crucibles which are placed on the firing plate between the moldings to be siliconized. Finally, it is also possible to use, as hitherto, auxiliary firing agents in combination with the coated silicon granulate on which the shaped bodies lie. In this case, however, the advantages mentioned above with regard to the embodiment without firing aids do not apply. However, all the other advantages mentioned above, such as the improved yield of perfect silicon-based components and the prevention of deterioration of the baking furnace, remain in this embodiment.

The particle size of the silicon granulate to be coated is at least 1 mm, preferably 3 to 15 mm, particularly preferably 7 to 12 mm.

All the compounds which have good adhesion of their thin layer to the silicon granulate can be used as a binder. Inorganic binders, for example polysilanes, or organic binders may be used. Preferably, water-soluble organic binders are used, since they can be applied from aqueous solutions so that no environmental problems such as

In organic solvents. When the binders are present in liquid form, such as some synthetic resins, no solvent is required. In this case, the silicon granulate can be mixed directly with the present binder in liquid form and the other components of the coating can be added subsequently.

Preferably, water-soluble binders of the cellulose derivative group are used, such as sodium carboxymethylcellulose. Particularly suitable are short chain cellulose derivatives as these dissolve more rapidly. At the same time, it also acts as a flow-enhancing means, thereby resulting in a finer carbon distribution in the coating composition. If other binders that do not have this additional effect are used, a greater amount of water or a flow aid may be used to release the surface tension in the carbon particles.

Depending on the chain length of the cellulose derivative, the concentration of the aqueous binder solutions used to produce the coating composition is in the range of 1 to 60% by weight, preferably 3 to 30% by weight, the higher the concentration used, the shorter the chain length. Depending on the concentration of binder in the aqueous binder solution, the amount of binder solution to be used to produce the coating composition is selected. Higher concentration binder solutions are preferable for their tack effect. In the case of a 20% binder solution, this solution may, for example, be used in amounts of from about 0.5 to 15% by weight, preferably 3 to 7% by weight, based on the coating composition and the silicon granulate. Thus, about 0.1 to 3% by weight, preferably 0.6 to 1.4% by weight, based on the coating composition and the granulate are used. However, larger amounts may also be used

Binders, preferably up to about 10% by weight. The term coating composition is to be understood as meaning all components, with the exception of the silicon granulate, which are present in the coating process.

Carbon, graphite and / or other types of carbon, such as coke dust, may be used in the coating compositions. Further, a carbon-releasing compound may be used instead of carbon. Carbon-rich compounds such as phenolic resins or starch can be used as carbon-releasing compounds. Since fine graphite powder usually results in stronger coatings, graphite is also preferably added to the carbon black. The weight ratio of carbon black to graphite is preferably in the range of from 8: 1 to 1: 2, especially in the range of from 3 to 2 liters.

The carbon is added to the coating composition in an amount sufficient to form a silicon granulate coating which forms an intact envelope when the silicon melts inside the coated granulate particles. The carbon content is about 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the coating composition and the silicon granulate. When using carbon-releasing compounds, correspondingly higher amounts of coating composition are added.

Preferably, boron nitride is added to the coating composition as a high temperature release agent. The boron nitride content ranges from 0 to 5% by weight, preferably up to 1% by weight, based on the coating composition and the silicon granulate. The melting of the silicon inside the coated particles then results in an envelope of silicon carbide and boron nitride.

In a preferred embodiment of the process for producing the coated silicon granulate according to the invention, the silicon granulate is well mixed with the aqueous binder solution in a mixing device, for example a concrete mixer. Subsequently, carbon is added and mixed further. If boron nitride is used, it is finally added and mixed further. Finally, the coated silicon granulate is dried at room temperature or at higher temperatures up to about 100 ° C, for example in an air circulation oven. The flowability of the silicon granulate does not change by coating.

The invention will be explained in more detail by the following examples.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

A silicon granulate having a particle size of 7 to 10 mm is used. A 20% aqueous solution of sodium carboxymethylcellulose with a low chain length is used as the binder solution. The quantities used are as follows:

4.5% by weight of a 20% binder solution

1.5% by weight of carbon black

0.5% by weight of graphite

1.0% by weight of boron nitride residue silicon granulate

The silicon granulate is added with the addition of a binder solution

The mixture was stirred for 15 minutes in a concrete mixer. The carbon black and graphite were then added and mixed for a further 15 minutes. Finally, boron nitride is added and stirred for a further 15 minutes. The coated silicon granulate is dried in an air circulating oven for 24 hours at 40 ° C.

Sliding rings consisting of 85% by weight of silicon carbide and 15% by weight of carbon are placed in the baking furnace with a baking plate layered with boron nitride layer and the coated silicon granulate is poured therebetween into the sliding rings. The slide rings are then siliconized under vacuum at 1800 ° C. Silicon carbide and boron nitride slag can be easily separated from the SiSiC sliding rings. There was no scrap as a result of the formation of Irhlin.

Example 2

The procedure is as in Example 1, but with the following changes. The following amounts are used in the coating:

4.5% by weight of a 20% binder solution

1.5% by weight of carbon black

0.5% by weight graphite residue silicon granulate

In this example, the addition of boron nitride is omitted. The coated silicon granulate is dried in an air circulating oven for 10 hours at 100 ° C.

This silicon granulate is poured onto the baking plate

-13baking furnaces. A porous silicon carbide plate is placed on top of which silicon carbide and carbon nozzles are placed. In silicon firing, perfect SiSiC silicon nozzles are obtained.

Example 3

The procedure is as described in Example 1 using the following amounts in the coating:

5.5% by weight 20% binder solution% by weight carbon black% by weight graphite

0.5% by weight of boron nitride residue silicon granulate

The technological coating and drying step is carried out as in Example 1.

The burner bodies are placed on the baking plate and filled with coated silicon granulate. After the silicon firing, the slag can be easily separated from the SiSiC burner bodies.

Example 4

The procedure is as in Example 1, but with the following changes. A 10% aqueous solution of sodium carboxymethylcellulose with a low chain length is used as the binder solution. The following amounts are used in the coating:

-147.5% by weight 10% binder solution% by weight carbon black% by weight graphite residue silicon granulate consisting of 64% silicon by weight and 36% molybdenum by weight

In this example, the addition of boron nitride is omitted. The coated silicon granulate is dried in an air circulating oven for 12 hours at 100 ° C.

The silicon granulate is poured onto the baking furnace firing plate. Rollers of silicon carbide / carbon mixture are placed on a leveled embankment. The infiltration and reaction is carried out at a temperature of about 2000 ° C.

Example 5

The procedure is as in Example 1, but with the following changes. A 14% aqueous solution of sodium carboxymethylcellulose with a low chain length is used as the binder solution. The following amounts are used in the coating:

6.5% by weight of a 14% binder solution% by weight carbon black% by weight graphite

1.5% by weight of boron nitride residue silicon granulate

The coated silicon granulate is dried in an oven

-15s with air circulation for 12 hours at 100 ° C.

The coated silicon granulate is spread over the baking plate with the layered boron nitride and evenly distributed. CFC plates are placed on it and siliconized.

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Claims (16)

Silicon granulate for silicon molding of carbon bodies or mixtures of silicon carbide and carbon, characterized in that the silicon granulate particles are coated with a carbon or carbon-releasing compound and a binder. Silicon granulate according to claim 1, characterized in that the coating comprises carbon in the form of carbon black, graphite or other types of carbon. Silicon granulate according to claim 1 or 2, characterized in that the coating comprises an organic binder. Silica granulate according to any one of claims 1 to 3, characterized in that the coating comprises boron nitride. 5. v y od The silicon granulate according to any one of claims 1 to 4, characterized in that the coating has a thickness of 1 μιη to 0.5 mm. -176. Silicon granulate according to any one of claims 1 to 5, characterized in that the coating comprises a water-soluble cellulose derivative as a binder. Silicon granulate according to any one of claims 1 to 6, characterized in that the granulate consists of silicon, a mixture of silicon and a silicide or a mixture of silicon and a silicide-forming metal. A process for producing a coated silicon granulate according to any one of claims 1 to 7, characterized in that the silicon granulate is mixed with an organic binder solution, carbon and optionally boron nitride and subsequently dried. The process according to claim 8, characterized in that the silicon granulate is mixed with an aqueous solution of an organic binder or with a liquid binder before carbon and optionally boron nitride are added. Method according to claim 8 or 9, characterized in that the binder solution contains 3 to 60% by weight of binder. Method according to one of claims 8 to 10, characterized in that the amount of binder is 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the silicon granulate and the coating -18mix. Method according to one of claims 8 to 11, characterized in that the amount of carbon is 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the silicon granulate and the coating mixture. Method according to one of claims 8 to 12, characterized in that the amount of boron nitride is 0 to 5% by weight, preferably 0 to 1% by weight, based on the silicon granulate and the coating composition. 14. A method of making silicon-infiltrated silicon carbide components by placing carbon or silicon carbide moldings on a baking furnace firing plate and infiltrating with silicon by firing, characterized in that silicon granules according to the invention are used as silicon Any of claims 1 to 7. Method according to claim 14, characterized in that the moldings to be siliconized are burned off by reduction before silicon infiltration. A method according to claim 14 or 15, Characterized in that the moldings to be siliconized are laid on a layer or in a layer of bulk silicon granulate which is spread on the baking plate. Method according to either of claims 14 or 15, characterized in that the moldings to be silicon are separated from the silicon granulate by a porous material through which liquid silicon can pass.