DE1139427B - Process for siliconizing refractory bodies - Google Patents

Description

Method of siliconizing refractory bodies The invention relates to on a new process for impregnating moldings with metallic silicon as well as processes for the production of bodies, which silicon in metallic and contained in chemically bound form.

A known method of impregnating a body with silicon consists in covering the body with fine particles of metallic silicon relatively low temperature surrounds, then the body and the silicon like that highly heated so that the molten metal flows quickly through the pores of the body. This process is used, for example, for the manufacture of siliconized silicon carbide objects used.

This siliconization process was unsatisfactory for certain purposes, but a process which was useful for all purposes was not known. For example, large preforms have been immersed in molten silicon, but then uneven heat distribution often occurs in the preform, so that the body sometimes warps. In addition, erosion occurs due to the flow of the molten silicon, and therefore the method of siliconizing bodies with precise dimensions is not satisfactory. In addition, this method has very little control over the manufacture of bodies with precise amounts of free silicon. If fine metallic silicon is used to siliconize an object, an indefinite amount of silicon is always lost because the silicon evaporates below the siliconization temperature. Some silicon always evaporates and so is lost before siliconization occurs. For example, when siliconizing a preformed body made of silicon carbide with metallic silicon, the temperature must be at least 1600.degree . No siliconization occurs below this temperature. The metallic silicon melts at around 1420'C, i.e. earlier than the body reaches the siliconization temperature, so that some silicon evaporates and is lost.

The purpose of the invention is to avoid warping of the preformed body during siliconization. Furthermore, according to the invention, a method is to be developed in which the molten silicon reacts immediately in the desired manner at a sufficiently high temperature. The method should also be avoided during siliconization of bodies having precise dimensions erosion and generally allow the preparation of Siliziumkarbidkörpern with very exact dimensions. Furthermore, according to the method of the invention, the silicon content of the siliconized body should be precisely controlled and the silicon content required for siliconization should be precisely determined in advance. The method should also be cheap and in particular allow mass production.

The method according to the invention allows the production of dense carbide bodies, which contain silicon carbide and optionally free silicon and which with regard to the degree of their porosity are to be controlled.

According to the invention, silicon nitride is used as the source of silicon during siliconization. Silicon nitride is a solid material that decomposes at temperatures above about 1900'C and releases molten silicon and gaseous nitrogen. Silicon nitride is particularly suitable for siliconizing, since the silicon is released at a temperature where it wets the body, so that in this way siliconizing takes place immediately. In practice there is little or no loss.

When siliconized with silicon nitride, the silicon nitride does not change until a temperature of around 1900 ° C is reached. The silicon nitride then decomposes into silicon and nitrogen, and the molten silicon is supplied to the body, which is at such a high temperature that it reacts instantly with the molten silicon. In the presence of carbon, silicon nitride decomposes at temperatures above 1600 ° C, especially if a temperature of 1600 ° C or higher is maintained for a long time, and forms silicon carbide. If the temperature rises quickly to 1900 ° C. or higher, this has no effect on the silicon nitride, even in the presence of carbon, until the decomposition temperature is reached and only then is free silicon released.

These properties make silicon nitride a source of silicon recovery particularly suitable for many purposes. In the examples below are several types of application and the drawings show this schematically.

Fig. 1 shows schematically a section through the interior of a high-frequency detector in which a rocket nozzle is located. There is granular silicon nitride inside the furnace. The rocket nozzle is silicized by burning; 2 shows schematically the process stages of the siliconizing process; Fig. 3 shows another sequence of steps shows schematically the method according to the granular silicon carbide and granular silicon are fired and siliconized Sfliziumkarbidkörper with controllable density forms), 0 Fig. 4 shows a further embodiment in which a mixture of granular silicon carbide, silicon and carbon, hot pressed and results in a dense silicon carbide body; 5 schematically shows a further exemplary embodiment of the invention, according to which a specially produced mixture of carbon and silicon nitride is formed, from which silicon carbide bodies with complex dimensions are formed.

Example 1 Siliconization of a Shaped Body A rocket nozzle 10 (FIG. 1) was manufactured in such a way that granular silicon carbide was pressed with a small amount of an intermediate binder in a steel mold and dried at about 120.degree. The body 10 was then placed on a base 12 made of silicon carbide, the latter being located in a graphite container 11 . The graphite container 11 was so large that it fit in the chamber of a high-temperature furnace. The container 11 had a gas inlet 13 and a sight glass at the bottom and a gas outlet 14 at the top. The cavity of the body 10 was filled with granular silicon nitride 15. The siliconizing then took place with the process steps shown in FIG. In the container 11 , a stream of helium gas was continuously admitted through the inlet 13. The contents of the chamber were then brought to a maximum temperature of approximately 1950 ° C. gradually over a period of 21/2 hours.

The container and its contents were held at a temperature of 1900 to 1950 ° C for approximately 30 minutes and then allowed to cool.

The rocket nozzle was then taken out and along with it cut up with a diamond disc. The inside of the nozzle was very dense and fine-grained Structure. Physical and chemical tests showed that the impregnation was very good had taken place with silicon.

The exact process of this impregnation cannot be fully explained, but it can be assumed that the process is similar to that when siliconizing silicon carbide bodies. It is known that silicon metal wets the silicon carbide at sufficiently high temperatures. Silicon nitride remains essentially unchanged when heated rapidly in helium or another neutral atmosphere to a decomposition temperature of approximately 1900 ° C. At this temperature silicon nitride decomposes almost suddenly into silicon and nitrogen. The released silicon then wets the silicon carbide object and penetrates the pores with capillary action. This is a phenomenon that is not entirely understandable, but is widely used in siliconizing. This siliconization process can be used to impregnate porous refractory bodies with silicon, provided that the body can be wetted with molten silicon at over 1900 ° C , up to the temperature at which the refractory body decomposes. The siliconization is preferably carried out by quickly heating the body to around 1900 to 2000.degree . A neutral atmosphere must be maintained during firing. This atmosphere can be helium, argon or the like, whereby the appropriate safety measures must be taken. Hydrogen does not affect the siliconization either, nor does nitrogen. As a precautionary measure, after siliconizing the atmosphere should be maintained up to a temperature of around 800 ° C.

In certain cases it may be desirable to use the silicon nitride to hold in a certain shaped body until it is decomposed. In such cases An intermediate binder made of sodium silicate or the like can be used. For the good Siliconization does not matter whether the silicon nitride is granular or solid. In both cases, the process steps according to FIG. 2 are used.

Example 2 Siliconization of a Form Body Made of Silicon Nitride-Bonded Silicon Carbide A rocket nozzle made of silicon nitride-bonded silicon carbide was fired in the presence of silicon nitride. As described above, the rocket nozzle and silicon nitride were held at a temperature between 1900 and 1950 ° C for approximately 30 minutes. The original weight of the rocket nozzle was 874 g and the weight after firing was 863 g. The weight loss is due to the fact that the silicon nitride originally present in the rocket nozzle was decomposed during the firing. The rocket nozzle was impregnated by the silicon nitride originally present and by the granular silicon nitride present within the cavity of the rocket nozzle. A small residue of the silicon nitride remained in the container after firing.

After siliconization, the rocket nozzle had turned light gray in color or blue changed to strong dark gray, almost black. The remainder of the Silicon nitride was a black ash that contained a few metallic granules contained.

Example 3 A process for producing silicided by heat pressing This example corresponds to the method illustrated schematically in Fig. 3. Silicon carbide of selected particle size is mixed with ordinary granular silicon nitride, and the mixture is placed in a graphite mold in a hot press furnace. The pressing takes place in a helium atmosphere at 1900 to 1950'C and at a pressure of about 140 kg / cm2 for 30 minutes. The body is then allowed to cool to about 800 ° C. in the helium atmosphere.

In this way it becomes a dense siliconized silicon carbide body obtain. The elementary element released by the decomposition of silicon nitride Silicon flows into and around the silicon carbide particles and fills so the pores in between.

The silicon carbide and the silicon nitride. can also be in powder form. When the powder mixture is hot-pressed, a dense body of siliconized silicon carbide is obtained. For three such mixtures, the three ingredients were mixed, then the liquid ingredients were added and mixing continued. The three mixtures had the following components: Mixture 1 1 Mixture 21 Mixture 3 Weight in grams Silicon carbide, Grain 1000 ...... 54 54 54 Silicon nitride ...... 17 30 42 Liquid phenolic formaldehyde resin 1.5 1.58 1.92 Pine oil ......... 6 cm3 7 cm3 8 cm.3 The mixtures were then pressed into shaped bars at a pressure of 210 kg / cm 2, which were then dried at 200 ° C. for 2 hours. Then the bars were gradually heated to 1000'C, after which the temperature was suddenly increased to 1900'C. This temperature was then held for 5 minutes to decompose the silicon nitride. The bars then had the following properties: Density in grams / " free per cubic centimeter of silicon. Bar 1 ............ 1.49 5.64 Bar 2 ............ 1.39 14.00 Bar 3 ............ 1.38 17.60 An X-ray examination of the bars showed that they did not contain any detectable silicon nitride. This method therefore makes it possible to manufacture silicon carbide bodies with a verifiable proportion of free silicon.

A mixture of silicon carbide and silicon nitride, which had previously been sintered to form a specific shaped body, was also produced. The body was then placed in a correspondingly shaped graphite mold and pressed in a hot press furnace as described above. The molded body had dimensions after hot pressing his original intentions.

In molding such siliconized silicon carbide bodies, the same neutral atmosphere and pressing temperatures as described above were used. Depending on the desired density, the pressure can be 70 kg / cm2 or even higher, depending on the strength of the graphite form. Pressures between 140 and 700 kg / CM2 are possible. The density or porosity essentially depends on the level of the proportions of silicon carbide or silicon nitride. Since gaseous nitrogen escapes during the decomposition of the silicon nitride, the result is a somewhat porous mass. During hot pressing, the high pressures solidify the mass to a certain extent and thus fill the empty spaces left by the escaping nitrogen gas.

When hot pressing, the application of a neutral atmosphere is not always necessary, as there is little contact with the atmosphere and there the escaping Nitrogen largely protects the silicon.

Example 4 Method of Making Silicon Carbide Bodies This Example illustrates the production of silicon carbide bodies according to the schematic The siliconizing process shown in FIG. 4.

Finely divided silicon carbide, silicon nitride and carbon were mixed together, and the mixture was then poured into a mold, quickly heated and hot-pressed in a neutral atmosphere as described in the preceding Example. When reached temperatures above the decomposition temperature of the silicon nitride metallic silicon, which reacts with the carbon, and silicon carbide were released forms. Then, under the densifying effect of the hot-pressing furnace, the newly formed Silicon combined with the silicon carbide in the raw mix.

The best results are obtained when the silicon carbide is a mixture sorted out particle sizes, then there is the greatest density of the body.

Instead of silicon carbide, it is also possible to use other refractory carbides which do not decompose at around 1900 ° C. , for example boron carbide. This example shows a simple way of producing bodies from any carbide mixture, for example a body with 850 /, silicon carbide and 15 0 /, boron carbide. In the production of such a body, the required proportion of boron carbide replaces the finely divided silicon carbide in the first stage of the process. The following process steps are then the same as described above. Refractory carbides should be understood to mean those carbides which do not decompose at the decomposition temperature of the silicon nitride. Other refractory carbides that can be siliconized by the decomposition of silicon nitride, alone or in a mixture with silicon carbide, are, for example, titanium carbide, zirconium carbide, niobium carbide and tantalum carbide.

According to the method described above, pre-sintered shaped bodies made of silicon nitride-bonded silicon carbide were also used as the starting material. These shaped bodies were produced from a mixture of sorted silicon carbide particles with fine metallic silicon. When these dry materials were mixed, a resinous intermediate binder and a small amount of pine oil were added. Several moldings were produced from this raw mixture by molding under 420 kg / cm2 pressure. The moldings were dried in an oven between 125 and 150 ° C. for one day. The dried moldings were then nitrided for 6 hours at 1400 ° C. in a flowing nitrogen atmosphere. In this way, silicon nitride-bonded silicon carbide bodies were obtained. These bodies were porous.

As a further process step, the pores were covered with carbon impregnated. Different types of impregnation with carbon are known, Preferably, however, the bodies were immersed in a resinous phenol-formaldehyde solution immersed and soaked. Then the bodies were burned to carbonize the resin and to deposit carbon in the pores of the body. Then these were made with carbon impregnated silicon nitride bonded silicon carbide body in a neutral Atmosphere, as described above, hot-pressed.

Contain silicon carbide bodies produced according to this example usually little excess silicon, provided that the carbon components correspond to the silicon nitride components are correct. This method has the advantage that the hot-pressed in a mold Bodies do not warp or warp. The bodies have a smooth surface and can be manufactured in practically any dimension that can be achieved by molding, whereby narrow tolerances can be maintained. Such bodies are particularly suitable as a material for fuel elements of nuclear reactors.

Similarly, almost any refractory body can be networked can be made less porous or denser by pores, either by siliconization by heating in the presence of silicon nitride in a neutral atmosphere the decomposition temperature of the silicon nitride addition, or by filling the Pores with carbon and subsequent reaction of the free silicon with the carbon. Regarding the known method for the deposition of carbon in the pores is to mention the above-mentioned method of impregnation with resin and that Carbonization, continued combustion of methane or other hydrocarbons as well Impregnation with furfural or the like and subsequent carbonization by acidification.

Known proposed methods can also be used to produce dense silicon carbide bodies in such a way that the silicon is allowed to react with the carbon-containing silicon carbide bodies. According to a proposal that has not yet been published, the silicon is allowed to react with the free carbon of a shaped body which has a network of pores and consists mainly of silicon carbide with carbon deposited in pores, the carbon content being about 85 to 950 /, of the stoichiometric amount is required to react with silicon in order to form so much silicon carbide that a completely solid body of silicon carbide is formed. Silicon nitride can also be used as the source of silicon in these proposals.

Example 5 Hot pressed Siliziumkarbidkörper from mixtures of carbon and silicon nitride this example, the method shown schematically in Fig. 5 explained. Finely divided silicon nitride was stirred into a solution of furfural and furfural alcohol until an essentially homogeneous dispersion resulted. A mineral acid, such as hydrochloric acid, was then added to the mixture and this acidified dispersion was poured into a mold. After several hours, the reaction between the solution and the hydrochloric acid had ended. Then the wet carbon body was removed from the mold and calcined in a neutral atmosphere at about 650 ° C. This temperature was maintained until all moisture and volatiles were driven off.

The molded body thus obtained consisted of carbon with silicon nitride uniformly distributed thereon. Essentially the stoichiometric amount of silicon nitride was used as is necessary for the complete siliconization of the carbon to silicon carbide. The body was then placed in a graphite mold and quickly heated to over 1900 ° C to decompose the silicon nitride to release silicon and nitrogen gas. The silicon then reacted with the carbon to form a silicon carbide body.

This process is particularly suitable for the production of bodies with precise dimensions, and the bodies can also have complex shapes, because the shape prevents warping.

Other useful carbonizable mixtures of this type are described in U.S. Patent 2,431,326, particularly Examples 7, 8 and 9 . Glycerine, for example, can also be used as a carbonizable material in the furfural solution in furfural alcohol. Small amounts of graphite can also be added to the mixture, or furfural or furfural alcohol can also be used alone. Other mineral acids, for example sulfuric acid, can also be used.

Similarly, mixtures of graphite or finely divided carbon, synthetic resin, and silicon nitride can be used, especially for bodies of complex dimensions and tight tolerances. Synthetic resins which can be rapidly carbonated are preferred, such as, for example, condensation products of formaldehyde with a phenol, such as phenol or resorcinol oil. Example 6 Mass Production To illustrate the suitability of the methods for mass production, several stones made of silicon carbide and a small amount of finely divided carbon were placed in a specialty silicon carbide furnace, and several other stones made of silicon nitride were placed over the first-mentioned stones. The stones were covered with an electrically conductive compound, and the temperature of the stones was allowed to rise to over 1900 ° C. These process steps correspond to FIG. 2. This resulted in an effective siliconization process for silicon carbide bricks in which essentially all of the carbon in the bricks was converted into silicon carbide, so that the bricks were dense and refractory.

The stones made of silicon carbide and graphite were made from a mixture with the following components: Silicon carbide, 1.5mm mesh size and finer ............................ 86.0 "/, Fine carbon (15 X silk screen) ... 5,001, Liquid phenolic resin ................. 9.00 /, The discarded silicon carbide was mixed for 30 minutes to achieve uniformity, then the graphite was added and the dry ingredients were then mixed again for 30 minutes. The mixture was then wetted with the liquid phenolic resin and passed through a coarse sieve to break up lumps. The wet mixture was then added to blocks 62 - 224 mm is formed with a wet density of 2.7 g / cms - 110th These stones were pressed at 420 kg / cm2 and dried for one day at 77 ° C and another day at 127 ° C. The average density of the dry stones was 2.64 g / cm3. Silicon nitride bricks were made from a mixture of 100 parts by weight of fine silicon metal having a particle size of 75 microns and smaller and 2 parts by weight of a phenolic intermediate binder. So much pine oil was added that the whole mass was moistened. After achieving a homogeneous mixture kg / cm2 stones were prepared with the same size of upper and 'lower surface as the above-mentioned silicon carbide-carbon bricks, but of reduced thickness of the mass under a pressure of 210th The silicon nitride units were less thick because only the stoichiometric proportion of silicon nitride was used to obtain the silicon required to impregnate the stones. The silicon stones were then dried in an oven at 66 ° C for several hours and then again for 16 hours at 71 ° C. Then the stones were nitrided by heating in a nitrogen atmosphere for 5 hours at 1350 ° C. and a further 5 hours at 14000 ° C.

The use of silicon nitride as a source for siliconizing The silicon used has the particular advantage that in mass production the silicon can be brought to any desired location and in any amount. In addition, as shown in the examples, bodies with complex and precise dimensions can be manufactured with tight tolerances. Use is still economical electric ovens particularly useful for mass production.

The invention is not intended to be limited to the examples described. Instead of the preferred temperatures of 1900 to 2000.degree. C. for siliconizing with silicon nitride, higher temperatures can also be used, for example if the silicon carbide in the siliconized body is to be converted from its cubic crystal form to hexagonal form after siliconizing.

It is also not absolutely necessary that a there is direct contact between the silicon nitride and the refractory body. A porous band can also be inserted between the two in such a way that the silicon nitride and the refractory body are separated from each other so that the siliconizing is complete takes place by silicon vapors. Other modification options result from the Knowledge of professionals. The inventive method can be used to produce known types of refractory bodies can be used, but also for new types refractory bodies. The invention is particularly advantageous for mass production of heating elements made of silicon carbide and those made of siliconized silicon carbide, also for thin tubes made of silicon carbide, cutting tools, high-temperature linings and for nozzles for jet and rocket motors, for crucibles, rods, muffles, Plates, discs, for refractory furnace equipment, etc.

Claims (2)

PATENT CLAIMS: 1. A method for siliconizing a refractory body, characterized in that a refractory body containing silicon nitride or in contact with it is heated in a neutral atmosphere to at least the decomposition temperature of the silicon nitride. 2. The method according to claim 1, characterized in that a mixture of refractory material with silicon nitride is introduced into a mold and hot-pressed at the decomposition temperature of the silicon nitride or a higher temperature. 3. The method according to claims 1 and 2, characterized in that silicon carbide and silicon nitride of different grain sizes are used for the production of bodies. 4. The method according to claims 1 to 3, characterized in that the pre-burned bodies produced from the mixture are impregnated with organic compounds and heated (carbonized) to decompose the organic compounds. 5. The method according to any one of claims 1 to 3, characterized in that a body made of carbon and silicon nitride distributed therein is produced and heated in a neutral atmosphere until the silicon nitride decomposes. 6. The method according to any one of claims 1 to 3, characterized in that a body made of a homogeneous mixture of a refractory carbide with silicon nitride and carbon is heated in the proportions theoretically required for the formation of silicon carbide in a neutral atmosphere. 7. The method according to claim 5, characterized in that in a refractory body with a network of pores carbon is deposited in the pores, and the body is then heated in a neutral atmosphere in the presence of silicon nitride. Documents considered: German Patent No. 173 066.