DE4418945A1 - Silicon granules used in mfr. of silicon carbide components - Google Patents

Abstract

Si granules contain particles with a coating contg. C or C cpd. and binder. Prodn. of the granules is also claimed. Pref., the coated Si granules are produced by mixing the granules with soln. of organic binder or liq. binder, with C, and opt. with B-nitride, and drying. The granules are pref. mixed with an aq. binder soln. or liq. binder, before adding the C and opt. B-nitride.

Description

The invention relates to silicon granules for siliconizing Shaped bodies made of carbon or silicon carbide / carbon Mixtures, a process for the production of coated Silicon granules and a process for the production of silicon-infiltrated silicon carbide components with the be layered silicon granules.

Place among the non-oxide high-performance ceramics Silicon carbide materials are particularly important and made visible group of materials. Because of the combination their outstanding material properties, such as hardness, ver wear resistance, high temperature resistance, durability against most acids and bases as well as good glide properties, are silicon carbide materials for sliding rings, bearings, mechanical seals, burners, heat exchangers and other uses.

In the siliconization process, a molded article made of carbon by infiltration, often also as reaction sintering referred to, at high temperatures in the presence of silicon fully or partially react to silicon carbide, and by Further infiltration of silicon can damage the pores Shaped body are closed. Furthermore, in the silication process a molded body from a mixture of silicon carbide and carbon by infiltration in the presence of Silicon are burned. Liquid silicon penetrates into the Molded body and reacts with the carbon below Formation of new silicon carbide. The infiltration can under vacuum from about 1400 ° C, advantageously at 1600 ° C to 1800 ° C or under protective gas, e.g. B. argon, at temperatures up to 2200 ° C. Infiltration has that Advantage that there is no or almost no shrinkage.

In a commonly used siliconization process usually the moldings to be siliconized on firing  tools, e.g. B. rings, plates or prisms, from the same or a similar already siliconized material. The kiln furniture is on a firing plate of the kiln arranged; between the kiln furniture is the silicon distributed in granules. At temperatures above about 1400 ° C melts the silicon. The liquid silicon gets through the pores of the kiln furniture into the moldings, so that the moldings placed on the kiln furniture with liquid silicon can be infiltrated.

This process has some serious disadvantages is liable. This is not the case despite the use of kiln furniture always a direct contact of liquid silicon and the to siliconized moldings excluded. This can crack formations in the molded body. The direct contact of Liquid silicon and moldings can also be too large flat silicon build-up on the molded body or gluing of moldings, firing aids and firing plates, which only with greater expenditure of time and increased out fall rate are to be removed.

Furthermore, liquid flowing down from the fuel plate Silicon the furnace, especially its insulation, be damage. The run down silicon is also not more available for siliconizing the moldings, so that components that are not completely siliconized can be obtained. Due to larger accumulations of liquid silicon on the Burning plates are subject to heavy wear and break light. The same applies if the kiln furniture on the Bake the firing plate.

After the siliconization process, when separating the burrow parts of the kiln furniture the components by flaking especially damaged at the edges or completely destroyed. Furthermore, this separation is labor intensive. The absolutely he required use of kiln furniture brings more  Disadvantages with it. First is the manufacture of distillery tools necessary, which is relatively expensive. Secondly the kiln furniture take up space in the kiln, so that optimal utilization of the furnace capacity is not possible is. Thirdly, the process of setting the shaped bodies onto the Firing plates more complex.

The object of the present invention is a silicon to provide granules with which a higher Aus loot on perfectly siliconized components can and that the kiln from damage by liquid Silicon protects. It is also said to be a siliconization ver enable driving, in which kiln furniture is not required are.

This task is solved by a silicon granulate, the Particles are provided with a coating, the coals fabric or a carbon-providing compound and a Contains binders. Preferred embodiments are Claims 2 to 7.

In the process for producing the sili according to the invention cium granules, the silicon granules with a solution of organic binder or a liquid binder, mixed with carbon and optionally with boron nitride and then dried. Preferred embodiments the method can be found in claims 9 to 13.

The coated silicon granules according to the invention are used in Siliconization process for the production of silicon infiltrators th silicon carbide components are used, in which the too sili Forming body made of carbon or silicon carbide / carbon Mixtures of substances on the burning plates of a kiln arranged and infiltrated by firing with silicon.  

The silicon granulate to be coated can vary depending on the purity degree of pure silicon, a mixture of Silicon and silicides or a mixture of silicon and Contain metals for silicide formation. In the case of mixtures The silicon granules can contain up to contain about 50% by weight of silicides. With mixtures of silicon and metals, the maximum content of metals is clearly below 50% by weight. Preferred metals for silicide formation and Chromium, tungsten, hafnium, Zirconium, vanadium, boron, manganese, niobium, tantalum, molybdenum and Iron. The silicide-containing silicon granulate can be made from a Mixing of solid silicon and silicide or by melting of silicon and silicide and subsequent breaking and Grinding of the melting body can be produced. The metal Containing silicon granules is made from a mixture of sili cium and metal made.

During the first phase of the silicon firing at temperatures up to about 400 ° C the organic binder of the coated silicon granules according to the invention. It stays Carbon and if boron nitride is added to the coating as provided in a preferred embodiment is boron nitride as a coating on the particles. If As the silicon begins to melt, the carbon reacts the coating with the silicon from inside the part Chen to silicon carbide, which as a shell, if necessary together with boron nitride to get the molten silicon remains. The molten silicon penetrates through the high temperature resistant but permeable coating and can ent either directly or via porous kiln furniture into the mold body-length. At the end of the siliconization process Coatings as empty casings, in the following as slag designated. If the silicon granules are not just silicon, but also contains silicides or metals with the Sili cium react in silicon firing to form silicides,  are the silicides melted or in the silicon firing solid form. The molten silicide penetrates with the ge melted silicon by coating the particles. Solid silicide is provided the particle size is sufficient is small, from the molten silicon through the coating tion of the particles carried.

With the coated silicon granules according to the invention becomes a direct contact of the molten silicon and the ver to be siliconized moldings during the Silicierbrands avoided. The molten silicon penetrates through the coating tion slower into the moldings and reacts with a delay. Both lead to the avoidance of rejects due to tears formation in the siliconized components. It will be the same moderate silicon absorption achieved in the moldings. Further Large-scale silicon build-up is prevented, the Removal especially during the subsequent cleaning process is time consuming. Another advantage of using the Silicon granules according to the invention consist in that none Silicon can drain from the fuel plate. So be Damage to the kiln prevented and none incomplete constantly siliconized components that produce a re catch fire would have to be subjected. Fer is advantageous ner that no burning in the silicon granules according to the invention tools must be used. This sets the law process easier, the kiln capacity can be better be used; the production of kiln furniture and the separation of kiln furniture and siliconized components, which damage or even destroy individual construction parts can be omitted. The slag in the invention leave empty casings acc. to silicon granulate move away from the components easily.

With the silicon granulate according to the invention, all porous molded body made of carbon or mixtures of sili  Siliconize silicon carbide and carbon. The moldings can also made of fiber-reinforced carbon, e.g. B. carbon fiber reinforced carbon (CFC). The ones to be siliconized Shaped bodies can by conventional molding processes, such as e.g. B. slip casting or pressing. she can also be made of silicon carbide with the addition of a car chargeable binder and carbon compacted and then be coked. By turning, drilling, etc. Geometry of the molded body produced.

The material of the silicon-infiltrated construction according to the invention parts include RBSiC, SiSiC, with higher proportions of large Carbon inclusions CSiSiC or mixtures of silicon, Silicon carbide and silicides (Siliced-SiSiC), the Materials can be fiber reinforced. Some usage possibilities of components made from these materials already listed as an example at the beginning.

In the method according to the invention for the production of sili cium-infiltrated silicon carbide components are available different possibilities for the arrangement of the moldings and the coated silicon granules according to the invention in the kiln. With a particularly simple arrangement on a possibly coated with boron nitride plate the coated silicon granules poured, and directly on the molded body to be siliconized puts. The moldings can also be in the bed of be layered silicon granules. At tube The shaped silicon can be shaped granules in the moldings arranged on the firing plate to fill. It is also possible to stack the moldings one above the other to arrange, each by layers of the Invention according to silicon granules are separated from each other. The Silication process according to the invention is particularly advantageous adheres to large moldings and large amounts of silicon on a burning plate.  

If you don't see any of the finished siliconized components Want to have slag, different arrangements are possible in which the moldings to be siliconized by porous Material that liquid silicon can penetrate from the coated silicon granules are separated. For example can be silicon granulate coated on the fuel plate be poured, a porous silicon carbide plate placed and the moldings are placed on this plate. Furthermore, the coated silicon granules can be porous Crucibles made of silicon carbide can be contained between the stand to be siliconized moldings on the fuel plate. Finally, it is also possible, as was previously the case with kiln furniture in combination with the coated silicon granulate use on which the moldings lie. In this case will not be the ones related to the through implementation of the process without firing aids mentioned parts reached. All other advantages mentioned above, such as improved yield of perfectly siliconized components and avoid damage to the kiln but also achieved in this embodiment.

The size of the particles of silicon granule to be coated lats is at least 1 mm, preferably 3 to 15 mm and particularly preferably in the range of 7 to 12 mm.

All compounds can be used as binders, which has a good adhesion of the coating to the silicon gray result in nulate. Inorganic binders, e.g. B. Polysilanes, or organic binders can be used. Organic, water-soluble binders are preferred applies because these are applied from aqueous solutions can, which does not cause environmental problems as with or ganic solvents. Are the binders in liquid Form, such as B. some synthetic resins, so no loosening needs agent to be added. The silicon granules can in  this case directly with the one in liquid form Binder are mixed, and the remaining ingredients the coating can then be added.

Water-soluble binders are preferably obtained from the Group of cellulose derivatives, such as. B. sodium carboxymethyl cellulose used. They are particularly suitable chain cellulose derivatives because they dissolve faster. They also act as a condenser, which is a finer one Distribution of carbon in the coating mixture surrender. Are other binders used that are not have this additional effect, so more water can ver be used or a condenser added to the covers the surface charges of the carbon particles.

Depending on the chain length of the cellulose derivatives, the con concentration of the aqueous binder solution used for the production the coating mixture used is in the range of 1 to 60 wt .-%, preferably 3 to 30 wt .-%, the higher Concentrations are used the lower the chain length is. Depending on the concentration of the binder in the aq residual binder solution, the amount of binder solution chosen that ver for the preparation of the coating mixture is applied. Highly concentrated binder solutions are preferred because of its adhesive effect. With a 20% bandage medium solution, for example, in amounts of about 0.5 to 15 wt .-%, preferably 3 to 7 wt .-%, based on the Coating mixture and the silicon granules used become. In this case, about 0.1 to 3% by weight, or 0.6 to 1.4% by weight of binder, based on the coating mixture and the granules. It however, larger amounts of binder may also be preferred up to about 10% by weight. Under coating Mixing is all components with the exception of silicon to understand granules, which in the coating step are essential.  

Carbon black, carbon black, Graphite and / or other types of carbon, such as. B. coke powder can be used. Furthermore, a carbon supplying compound used instead of carbon become. Carbon comes as carbon-providing compounds rich compounds, such as. B. phenolic resins or starch, in question. Since fine graphite powder mostly becomes firmer be stratifications, soot is also preferably used Graphite too. The weight ratio of carbon black to graphite is preferably in the range from 8: 1 to 1: 2, especially in the Range from 3: 1 to 2: 1.

The carbon becomes the coating mixture in an amount added enough to coat the silicon to obtain granules, which in melting the silicon in Inside the coated granulate particles an intact shell forms. The carbon content is about 0.1 to 10% by weight, preferably 0.5 to 5 wt .-%, based on the coating mixture and the silicon granulate. When using coal compounds that supply substances are correspondingly higher Amounts of coating mix added.

Preferably, the coating mixture is boron nitride High temperature release agent added. The boron nitride content is in the range from 0 to 5% by weight, preferably up to 1% by weight based on the coating mixture and the sili cium granules. When melting the silicon inside the be layered particles then a shell made of silicon carbide and boron nitride.

In a preferred embodiment of the invention Process for the preparation of the coated silicon granules The silicon granulate is lats with the aqueous binder solution in a mixer, e.g. B. a concrete mixer, well ver mixes. Then the carbon is added and  further mixed. If boron nitride is also added, this then added and everything together ge mixes. Then the coated silicon granules at room temperature or higher temperatures up to about 100 ° C for example, dried in a convection oven. The The flowability of the silicon granules changes due to the Coating not.

The following examples illustrate the invention explained.

example 1

Silicon granules with a particle size of 7 to 10 mm were used. A 20% aqueous solution of sodium carboxymethyl cellulose with a small chain length was used as the binder solution. The following quantities were used:
4.5% by weight of the 20% binder solution
1.5% by weight of carbon black
0.5% by weight graphite
1.0 wt% boron nitride
Rest of silicon granulate.

The silicon granules were added with the binder solution mixed for 15 minutes in a concrete mixer. Then was added the carbon black and graphite and mixed for another 15 minutes. The boron nitride was then added and another 15 Minutes mixed. The coated silicon granulate was in dried in a forced air drying cabinet at 40 ° C for 24 h.

In a kiln with one coated with boron nitride Burning plates were sliding rings made of 85% by weight silicon carbide and 15% by weight of carbon are placed on the fuel plate and the coated silicon between and in the slide rings  poured granules. The slip rings were under vacuum Silicated at 1800 ° C. The slag made of silicon carbide and boron nitride was easy to separate from the SiSiC sliding rings. A committee through cracks did not appear.

Example 2

The procedure was as in Example 1, but with the following changes. The following quantities were used for the coating.
4.5% by weight of the 20% binder solution
1.5% by weight of carbon black
0.5% by weight graphite
Rest of silicon granulate.

There was no addition of boron nitride in this example. The Coated silicon granules were dried in a forced air cabinet dried at 100 ° C for 10 h.

This silicon granulate was placed on the burning plate of the kiln furnace poured. A porous silicon carbide was Plate placed on the nozzles made of silicon carbide and carbon were laid. The silicon firing was flawless Get siliconized SiSiC nozzles.

Example 3

The procedure was as in Example 1, the following amounts being used in the coating.
5.5% by weight of the 20% binder solution
2% by weight of carbon black
1 wt% graphite
0.5% by weight boron nitride
Rest of silicon granulate.

Coating step and drying were as in Example 1 carried out.

Burner tubes were placed on the burning plate, into which the coated silicon granules was filled. After the sili cierbrand let the slag easily from the SiSiC burner separate pipes.

Example 4

The procedure was as in Example 1, but with the following changes. A 10% aqueous solution of sodium carboxymethyl cellulose with a small chain length was used as the binder solution. The following amounts were used for the coating.
7.5% by weight of the 10% binder solution
1% by weight of carbon black
1 wt% graphite
Rest of silicon granulate from 64% by weight
Silicon and 36% by weight molybdenum.

There was no addition of boron nitride in this example. The Coated silicon granules were dried in a forced air cabinet dried at 100 ° C for 12 h.

The silicon granules were placed on the firing plate of the kiln poured. Cylinders were removed from the leveled bed a silicon carbide / carbon mixture. infiltration and reaction took place at temperatures around 2000 ° C.

Example 5

The procedure was as in Example 1, but with the following changes. A 14% aqueous solution of sodium carboxymethyl cellulose with a small chain length was used as the binder solution. The following amounts were used for the coating.
6.5% by weight of the 14% binder solution
1% by weight of carbon black
1 wt% graphite
1.5% by weight boron nitride
Rest of silicon granulate.

The coated silicon granules were in a circulating air drying cabinet dried at 100 ° C for 12 h.

That was on the fuel plate coated with boron nitride coated silicon granules poured and evenly distributed. The CFC plates were placed on top and siliconized.

Claims (17)

1. Silicon granules for siliconizing moldings made of carbon or silicon carbide / carbon mixtures, characterized in that the particles of the silicon granules are provided with a coating which contains carbon or a carbon- providing compound and a binder. 2. silicon granules according to claim 1, characterized in that the coating as carbon black, graphite or contains other types of carbon. 3. silicon granules according to claim 1 or 2, characterized records that the coating is an organic binder contains. 4. Silicon granules according to one of claims 1 to 3, characterized characterized in that the coating contains boron nitride. 5. Silicon granules according to one of claims 1 to 4, characterized characterized in that the coating has a thickness of 1 µm up to 0.5 mm. 6. Silicon granules according to one of claims 1 to 5, characterized characterized in that the coating is a water-soluble Contains cellulose derivative as a binder. 7. silicon granules according to one of claims 1 to 6, characterized characterized in that the granules of silicon, a Ge mix of silicon and a silicide or a mixture consists of silicon and a silicide-forming metal. 8. Process for the preparation of the coated silicon granules lats according to one of claims 1 to 7, characterized in  net that the silicon granules with the solution of an organ or a liquid binder, with Carbon and optionally mixed with boron nitride and is then dried. 9. The method according to claim 8, characterized in that the silicon granules with an aqueous binder solution or mixed with the liquid binder before the Carbon and optionally the boron nitride added become. 10. The method according to claim 8 or 9, characterized in that the binder solution ent 3 to 60 wt .-% binder ent holds. 11. The method according to any one of claims 8 to 10, characterized ge indicates that the amount of binder 0.1 to 10 wt .-%, preferably 0.5 to 3 wt .-%, based on silicon granules lat and coating mixture. 12. The method according to any one of claims 8 to 11, characterized ge indicates that the amount of carbon 0.1 to 10 wt .-%, preferably 0.5 to 5 wt .-%, based on silicon granules lat and coating mixture. 13. The method according to any one of claims 8 to 12, characterized ge indicates that the amount of boron nitride is 0 to 5% by weight, preferably 0 to 1 wt .-%, based on silicon granules lat and coating mixture. 14. Process for the production of silicon infiltrated Silicon carbide components, in which the molded body is made of Carbon or silicon carbide / carbon mixtures arranged on burning plates of a kiln and by Burn infiltrated with silicon, thereby known  records that a silicon granulate after silicon one of claims 1 to 7 is used. 15. The method according to claim 14, characterized in that the moldings to be siliconized with before infiltration Silicon are fired to reduce. 16. The method according to claim 14 or 15, characterized in that that the moldings to be siliconized on or in a Bulk made of silicon granules that are placed on the fuel plate is applied. 17. The method according to any one of claims 14 or 15, characterized characterized in that the moldings to be siliconized through porous material, through the liquid silicon can penetrate, are separated from the silicon granules.