Curable organopolysiloxanes filled with silicon carbide powders and highly densified sintered bodies therefrom

Abstract: The preparation of highly densified ceramic bodies by sintering certain curable organopolysiloxanes filled with silicon carbide powders, metal-containing sintering aids and organopolysiloxane curing agents is described. Such highly densified ceramic bodies can be prepared by either a pressureless or a hot press sintering process. The compositions of this invention can be formed into desired shapes and then sintered to form ceramic, shaped bodies with high densities. One advantage of the present invention is that the green bodies have relative high strengths and thus can be easily handled and, if desired, machined before sintering. The curable organopolysiloxanes useful in this invention must yield a ceramic char which contains free or excess carbon in addition to carbon in the form of silicon carbide upon pyrolysis to elevated temperatures.

Description: This invention relates to the preparation of highly densified ceramic bodies by the pyrolysis of certain curable organopolysiloxanes filled with silicon carbide powders, organopolysiloxane curing agents and certain metal-containing sintering aids. Such highly densified ceramic bodies can be prepared by either a pressureless sintering process or a hot press sintering process. This invention also relates to novel compositions of matter consisting essentially of silicon carbide powder, a metal-containing sintering aid, an organopolysiloxane curing agent and a preceramic organopolysiloxane. These compositions of matter can be formed into desired shapes and then sintered to form ceramic, shaped bodies with high densities. One advantage of the present invention is that the green bodies have relatively high strengths and thus can be easily handled and, if desired, machined before sintering. The green strength of the shaped bodies can be further increased by curing the formed body prior to sintering. The present invention provides high density ceramic products from a pressureless sintering process using curable organopolysiloxanes as binders. Generally, high density ceramic materials will possess high strengths. EP-A-0 148 277 describes a process for producing a sintered silicon carbide which comprises allowing an organosilicon polymer to be present on the surface of starting silicon carbide particles of 1 micrometer or 5 less in size so as thereby to precipitate superfine silicon carbide particles produced from the organo-silicon polymer onto the starting silicon carbide particles in a heating step prior to a sintering step. The organic silicon polymer is defined as an high molecular organic silicon compound whose skeleton is mainly composed of silicon and carbon, side-chains being decomposed almost entirely at a temperature of 800°C to form a nearly amorphous continuous structure mainly composed of Si-C bonds and which deposit extremely fine silicon carbide of less than 1,000 Angstroms in size at a temperature above 1,000°C., the amount of the organic silicon polymer added being from 1.5 to 30 parts by weight based on 100 parts by weight of the powdery silicon carbide. Compounds containing boron, aluminum and beryllium as well as carbonaceous materials are recommended as sintering promoting agents, in particular aluminium and aluminium compounds such as aluminium nitride, the amount of an aluminium compound to be added being preferably from 0.5 to 5 parts by weight calculated as aluminium based on 100 parts by weight of the total silicon carbide content in the resulting silicon carbide sintered product. FR-A-2 317 039 describes shaped molded silicon carbide articles and a method for making them in which a mixture of silicon carbide powder with an organosilicon polymer used as a binder is molded together and thereafter heated. A variety of organosilicon polymers are described. The addition of oxides of aluminium, boron and silicon is indicated to be disadvantageous. Furthermore, although there is a teaching of preliminary condensation of the organic silicon polymer in the presence of a polymerization catalyst, for instance, the French Patent Publication does not teach the curing of a handleable green body formed by molding but on the contrary recommends the cured organosilicon polymers in admixture with uncured other organic silicon polymer after powdering of the cured product to prepare the molded product which is to be sintered. Furthermore, the emphasis in the case of curing is placed on compounds having Si-C, Si-Hal and Si-Si linkages as opposed to organopolysiloxanes. GB-A-2 002 738 discloses the addition of siloxane polymers amongst other organic silicon polymers to preceramic molding mixtures, including silicon carbide-containing mixtures before sintering to improve the quality of the sintered product. Cross-linking of the polymers by means of catalysts, including organic peroxide catalysts and also by means of irradiation, such as with gamma rays is also disclosed. The organosilicon polymer is recommended as an additive to improve sintering in substitution for the additives previously used to this effect, for example, the boron, silicon and carbon additives employed in the case of silicon carbide sintered products. This is to avoid the existence of different phases occurring in the production of the high density fired body as a result of solid phase reactions between such previously proposed additives and the ceramic substrates, which are stated to be apt to cause plastic deformation in the grain boundaries at elevated temperatures, whereby the production of ceramic fired bodies having an improved strength at high temperature is made difficult. DE-A-3 007 384 also discloses the combination of an organosilicon polymer with silicon carbide preceramic mixtures amongst others, and exemplifies amongst other organic silicon polymers, polyborosiloxanes, such as polyborodiphenylsiloxane, polyborodimethylsiloxane and polyboromethylphenyl siloxane. Sintering aids such as magnesium and beryllium oxides are mentioned by way of example. This invention relates to a composition of matter for producing a highly densified ceramic body, said composition matter comprising, in intimate admixture, an organic silicon polymer, silicon carbide powder and a metal-containing sintering aid, characterised in that: (1) said organic silicon polymer is a curable organopolysiloxane: (i) containing units of general structure [R 3 SiO 0.5 ], [R 2 SiO], [RSiO 1.5 ], [SiO 2 ], where each R is independently selected from the group consisting of hydrogen, alkyl radicals containing 1 to 20 carbon atoms, phenyl radicals and vinyl radicals. (ii) capable of yielding by pyrolysis in an inert atmosphere a stable ceramic char, which does not significantly decrease in weight upon further exposure to said pyrolysis, in an amount greater than 20 weight percent; (iii) containing in the ceramic char an amount of free or excess carbon, determined by pyrolysis of the organopolysiloxane, in the absence of any silicon carbide powder or sintering aid, to an elevated temperature, under an inert atmosphere until a stable ceramic char is obtained of at least 10 weight percent; and (iv) present at such a level that the free carbon value of the mixture is greater than 0.2 weight percent, and not greater than 3 weight percent based on the total weight of the silicon carbide powder and the char derived from the preceramic organopolysiloxane, (2) said metal-containing sintering aid is selected from iron, Fe 3 C, boron, boron-containing compounds aluminum, aluminum-containing compounds and metal oxides selected from thorium oxide, yttrium oxide, lanthanum oxide and cerium oxide, and is present in the mixture in an amount of from 0.1 to 3.0 weight percent of the metal based on the weight of silicon carbide powder; and (3) an organopolysiloxane curing agent is additionally present in an effective amount sufficient to induce crosslinking in the organopolysiloxane. This invention also relates to a method of forming a handleable green body, which method comprises: (a) preparing a composition of matter as defined just above; and (b) forming said composition of matter into the desired shape under pressure at a temperature below 500°C. This invention further relates to a method of preparing a sintered body of silicon carbide, which method comprises (a) forming a handleable green body by the method defined just above, and (b) thereafter sintering the handleable green body in an inert atmosphere at a temperature above 1900°C to obtain a sintered body of silicon carbide with a density above 2.4 g/cm 3 . The sintered bodies produced from the practice of this invention have densities greater than 75% of theoretical (i.e., densities greater than 2.4 g/cm 3 ). Such highly densified bodies are useful as light weight refractory ceramics. The theoretical density of silicon carbide is 3.21 g/cm 3 . The novel composition of the present invention consists essentially of an organopolysiloxane filled with silicon carbide powder, a metal-containing sintering aid and an organopolysiloxane curing agent. These novel compositions can be used to prepare handleable green bodies. By "handleable green bodies", we mean green bodies which have sufficient green strength to be handled or machined prior to sintering. Additionally, the compositions may be cured prior to sintering to obtain even stronger green bodies. High green strength is one significant advantage of the present invention; the high green strength allows the green bodies to be further processed and shaped prior to the final sintering or pyrolysis step. Generally, green strengths of 3,448 kPa (500 psi) or more may be obtained in the practice of this invention. The green bodies may be formed by conventional techniques known in the art. Such methods include pressure molding, uniaxial pressing, isopressing, extrusion, transfer molding, injection molding and the like. The formed green bodies may be further shaped by machining if desired. Once shaped, the green bodies are fired to an elevated temperature under an inert atmosphere to convert the green bodies into ceramic articles having densities greater than 75% of theoretical. It is preferred that the density of the formed ceramic article be greater than 85% of theoretical (about 2.7 g/cm 3 ). It is more preferred that the density of the produced ceramic article be greater than 2.9 g/cm 3 (90% of theoretical); it is most preferred that the density be greater than 3.05 g/cm 3 (95% of theoretical). The sintering may be carried out using either a pressureless sintering process or a hot press sintering process. Either process will produce a highly densified ceramic article using the compositions of this invention. The hot press sintering process will generally produce higher density ceramic articles. Therefore, if the maximum density is desired, a hot press sintering process is preferred. Generally, however, the pressureless sintering process is preferred because of the simplified operations involved. The sintering step is carried out under an inert atmosphere such as nitrogen or argon. The growth of alpha-SiC grains can be reduced by sintering under a nitrogen-containing atmosphere. The sintering to the final ceramic product is generally carried out at a temperature of 1900°C. or higher. Lower temperatures can be used but the ceramic product may not possess the desired density. The preferred sintering temperature is from 2000 to 2200°C. with 2150°C. being the most preferred. The organopolysiloxanes useful in this invention are generally well known in the art. The organopolysiloxanes must be capable of being converted to ceramic materials with a significant ceramic char yield. The ceramic char yield must be greater than 20 weight percent. Naturally, there will be less shrinkage if the ceramic char yield is higher. Therefore, it is preferred that organopolysiloxanes with ceramic char yields greater than 40 weight percent be employed in the practice of this invention. The organopolysiloxane must also yield a ceramic char containing free carbon. Using a rule of mixtures, the ceramic char must contain more than 30 weight percent total carbon in order for free carbon to be present. Organopolysiloxanes which yield ceramic chars with more than 40 weight percent total carbon are preferred. A 40 weight percent carbon-containing ceramic material contains, based on a rule of mixtures, 86 weight percent SiC and 14 weight percent free carbon. Most preferred are organopolysiloxanes which yield ceramic chars with more than 50 weight percent total carbon; a 50 weight percent carbon-containing ceramic material contains, based on a rule of mixtures, 72 weight percent SiC and 28 weight percent free carbon. The ceramic char should contain at least 10 weight percent free carbon. It is more preferred that the ceramic char should contain at least 25 weight percent free carbon. So long as the organopolysiloxane can be converted to a ceramic char with sufficient char yield and the derived ceramic char contains sufficient free carbon, the structure of the organopolysiloxane is not critical. The organopolysiloxane contain units of general structure [R 3 SiO 0.5 ], [R 2 SiO], [RSiO 1.5 ] and [SiO 2 ] where each R is independently selected from the group consisting of hydrogen, alkyl radicals containing 1 to 20 carbon atoms, phenyl radicals and vinyl radicals. Generally, organopolysiloxanes which contain phenyl groups are preferred as such groups easily allow for the formation of ceramic chars with sufficient free carbon. Organopolysiloxanes which contain vinyl groups are also preferred since vinyl groups attached to silicon provide a mechanism whereby the organopolysiloxane can be cured prior to sintering. Preferred organopolysiloxanes contain varying amounts of [PhSiO 1.5 ], [MeSiO 1.5 ] and [PhViSiO] units. Especially preferred organopolysiloxanes are described by the unit formula [PhSiO 1.5 ][MeSiO 1.5 ][PhViSiO] where there are 10 to 90 mole percent [PhSiO 1.5 ] units, 0 to 50 mole percent [MeSiO 1.5 ] units and 10 to 90 mole percent [PhViSiO] units; more preferably, there are 20 to 30 mole percent [PhSiO 1.5 ] units, 30 to 50 mole percent [MeSiO 1.5 ] units and 30 to 50 mole percent [PhViSiO] units. Organopolysiloxanes useful in this invention may contain other siloxane units in addition to, or in place of, the siloxane units just mentioned. Examples of such siloxane units include [ViSiO 1.5 ], [PhMeSiO], [MeHSiO], [MeViSiO], [Ph 2 SiO], [Me 2 SiO], [Me 3 SiO 0.5 ] and the like. Mixtures of organopolysiloxanes may also be employed. Generally, organopolysiloxanes where R is almost exclusively methyl are not suitable for use in this invention as there is insufficient free carbon in the resulting ceramic char. The organopolysiloxanes of this invention can be prepared by techniques well known in the art. The actual method used to prepare the organopolysiloxanes is not critical. Most commonly, the organopolysiloxanes are prepared by the hydrolysis of organochlorosilanes. Such methods, as well as others, are described in Noll, Chemistry and Technology of Silicones , chapter 5 (translated 2d Ger. Ed., Academic Press, 1968). Specific methods for preparation of suitable organopolysiloxanes are illustrated in the examples included in the present specification. In addition to the organopolysiloxane, other components in the compositions of this invention include silicon carbide powder, a metal-containing sintering aid and an organopolysiloxane curing agent. The silicon carbide powders useful in this invention are commercially available. Both alpha-SiC and beta-SiC powders, as well as mixtures, can be used. Generally, SiC powders with an average particle size below five micrometres (microns) are preferred; powders with an average particle size below one micrometre (micron) are more preferred. Suitable metal-containing sintering aids include iron, Fe 3 C, magnesium, MgC 3 , lithium Li 2 C 2 , beryllium, Be 2 C, boron, boron-containing compounds, aluminum, aluminum-containing compounds and metal oxides such as thorium oxide, yttrium oxide, lanthanum oxide and cerium oxide. Many of these metal-containing sintering aids are described in Negita, "Effective Sintering Aids for Silicon Carbide Ceramics: Reactivities of Silicon Carbide with Various Additives," 69 J. Am. Ceram. Soc. C-308 (1986). Other metal-containing sintering aids suggested by Negita might also be effective in the practice of this invention. Generally, the sintering aid should be present in an amount equivalent to from 0.1 to 3.0 weight percent of the metal based on the weight of the silicon carbide powder. Preferred sintering aids are selected from the group consisting of boron, boron-containing compounds, aluminum and aluminum-containing compounds. Examples of boron-containing sintering aids include, for example, boron carbide, lithium borohydride, trivinylboron, triphenylboron, silicon hexaboride, H 3 BO 3 and B 2 O 3 . Examples of aluminum-containing sintering aids include, for example, aluminum oxide, aluminum nitride and aluminum diboride. The most preferred sintering aids are boron and boron carbide. Mixtures of sintering aids may also be used. The compositions of this invention also contain organopolysiloxane curing agents. Such curing agents can be used to cure (via crosslinking the organopolysiloxanes) the shaped articles prior to sintering. Such cured articles generally have higher strengths than the noncured articles and, thus, can better withstand any handling or machining processes prior to sintering. Conventional organopolysiloxane curing agents which are useful in the present invention are well known in the art. Examples include organic peroxides such as dibenzoyl peroxide, bis- p -chlorobenzoyl peroxide, bis-2,4-dichlorobenzoyl peroxide, di- t -butyl peroxide, dicumyl peroxide, t -butyl perbenzoate and t-butyl peracetate; and platinum-containing curing agents such as platinum, H 2 PtCl 6 and ((C 4 H 9 ) 3 P) 2 PtCl 2 . Preferred organopolysiloxane curing agents include dicumyl peroxide and t-butyl perbenzoate. Other conventional organopolysiloxane curing agents known in the art may also be used. The organopolysiloxane curing agent is present in an effective amount, i.e. an amount sufficient to induce crosslinking in the organopolysiloxane. Therefore, the actual amount of the curing agent will depend on the activity of the actual agent used. Normally, however, the non-platinum curing agent will be present in an amount of from 0.1 to 5.0 weight percent based on the weight of the organopolysiloxane with the preferred level being about 2.0 weight percent. When the organopolysiloxane contains both vinyl groups and hydrogen atoms attached to silicon, platinum-containing curing agents can be used. For such platinum-containing curing agents, the level of curing agents will normally be such that platinum is present in an amount of from 1 to 1000 ppm based on the weight of the organopolysiloxane with the preferred level at from 50 to 150 ppm platinum. The preceramic organopolysiloxane is present in the compositions of the present invention at such a level that the free carbon value of the composition is greater than 0.2 weight percent based on the total weight of the silicon carbide powder and the char derived from the preceramic organopolysiloxane. What is meant by "free carbon value of the mixture" in this invention is the amount of free or excess carbon derived from the organopolysiloxane during pyrolysis as expressed by a weight percentage based on the total weight of the silicon carbide powder and the char derived from the organopolysiloxane. The total amount of carbon in the ceramic char equals the amount of free or excess carbon plus the amount of carbon in the form of silicon carbide. The amount of free carbon derived from the organopolysiloxane is determined by pyrolysis of the organopolysiloxane, in the absence of any silicon carbide powder or sintering aid, to an elevated temperature under an inert atmosphere until a stable ceramic char is obtained. For purposes of this invention, a "stable ceramic char" is defined as the ceramic char produced at an elevated temperature which will not significantly decrease in weight upon further exposure to heating at the elevated temperature. Normally, a stable ceramic har is produced upon pyrolysis at 1800°C. for 30 minutes. Other elevated temperatures can be used to form the stable ceramic char but the length of exposure to the elevated temperature will need to be increased for temperatures below 1800°C. Both the ceramic yield and the carbon content of the stable ceramic char are then determined. Using a rule of mixtures, the amount of SiC and free carbon of the stable ceramic char can be calculated. The amount of free carbon normally is expressed as the weight of free carbon produced per gram of preceramic organopolysiloxane. Knowing the amount of free carbon produced by pyrolysis of the organopolysiloxane, one can determine how much organopolysiloxane is required to obtain an organopolysiloxane/silicon carbide mixture with the desired free carbon value. Naturally, if one is using the same or a very similar organopolysiloxane to prepare a sintered body, it is not required that the amount of free carbon produced per gram of organopolysiloxane be determined every time. This procedure can perhaps be best illustrated by an example. Assume an organopolysiloxane (100 g) which gives, upon pyrolysis to 1800°C., a char yield of 50 weight percent which contains 40 weight percent carbon and 60 weight percent silicon. Such a char contains 30 g (1.07 moles) of silicon. Using a rule of mixtures, the char also contains 1.07 moles (12.8 g) of carbon in the form of SiC. Since the char contains 20 g carbon, the amount of free carbon in the char is 7.2 g (20 g minus 12.8 g). Thus, each gram of the preceramic organopolysiloxane yields 0.072 g free carbon. If a free carbon value for the mixture of 1.5 weight percent is desired, the following calculations can be performed. Let X equal the amount of organopolysiloxane required. The amount of char derived from the organopolysiloxane is 0.5 X (in grams, based on a 50% char yield); the amount of free carbon formed during the pyrolysis is 0.072 X (in grams). For a mixture containing 100 g of SiC powder, the equation 0.015 = (0.072 X )/(100 + 0.5 X ) is obtained where 0.072 X is the amount of free carbon derived from the organopolysiloxane and (100 + 0.5 X ) is the total weight of SiC powder and char derived from the organopolysiloxane. Solving the above equation for X , it is found that 23.3 g of the organopolysiloxane will give the desired 1.5% free carbon value of the mixture. Using this procedure, the amount of organopolysiloxane required to prepare the compositions of this invention can be determined. This procedure avoids the costly and time-consuming trial and error method which might otherwise be required. The free carbon value of the mixture must be greater than 0.2 weight percent based on the total weight of the silicon carbide powder and the char derived from the organopolysiloxane. For free carbon values less than below about 0.2 weight percent, the density of the sintered body will generally fall below 2.4 g/cm 3 (75% of theoretical). It is generally preferred that the free carbon value of the mixture be greater than 0.5% and that the density of the resulting sintered body be greater than 85% of theoretical. It is more preferred that the free carbon value of the mixture be between 0.5 and 3.0 weight percent with a range of 1.0 to 2.0 weight percent being even more preferred. The optimum density is generally obtained when the free carbon value of the mixture is 1.5 weight percent. Once the amount of organopolysiloxane required in the mixture has been determined, the various components are combined in a manner which ensures an uniform and intimate mixture. Such a mixture is required to avoid areas of varying density throughout the sintered product. Uniform and intimate mixtures can be prepared by using conventional blending techniques. Examples of such techniques include grinding the various powders in either the dry or the wet state. Generally preferred is wet grinding where the various powders are mixed and ground with organic solvents, the solvent is removed and then the resulting mixture is further ground. Other mixing and grinding methods will be apparent to those skilled in the art. The uniform and intimate mixture may then be formed into the desired shape. Preferably, the desired shape is formed under pressure using, for example such methods as injection molding, uniaxial pressing, isopressing, extrusion and transfer molding. Once formed the article may be further shaped by machining. The composition is preferably cured prior to its final shaping. Curing procedures are well known in the art. Generally, such curing can be carried out by heating the article to 50 to 300°C. Once the final shape has been obtained, the article is sintered in an inert atmosphere to a temperature of 1900°C. or more. The preferred sintering temperature is 2000 to 2200°C. with 2150°C. being most preferred. Although not wishing to be limited by theory, it is thought that the free carbon derived from the preceramic organopolysiloxane plays two different roles in the formation of highly densified sintered bodies. First, it helps to remove oxygen present in the silicon carbide powder; and, secondly, it apparently acts as an additional sintering aid. Silicon carbide powders often contain so-called "free carbon." However, the "free carbon" present in the silicon carbide powder does not appear to be as active or effective as free carbon generated in situ from the preceramic organopolysiloxane. It is not clear whether the free carbon produced in situ is more active chemically or whether it is simply more evenly dispersed. In any event, when the free carbon value of the mixture (as defined above) is 1.5 weight percent, sintered bodies with optimum densities are obtained. So that those skilled in the art can better appreciate and understand the invention, the following examples are given (Example 4 being a comparative example). Unless otherwise indicated, all percentages are by weight. Throughout this specification, "Me" represents a methyl group, "Ph" represents a phenyl group and "Vi" represents a vinyl group. In the following examples, the analytical methods used were as follows: Proton NMR spectra were recorded on either a Varian® EM360 or EM390 spectrometer; fourier transform IR spectra were recorded on a Nicolet 5 DX spectrometer. Gel permeation chromatography (GPC) data were obtained on a Waters GPC equipped with a model 600E systems controller, a model 490 UV and model 410 Differential Defractometer detectors; all values are relative to polystyrene. Carbon analysis was done on a Control Equipment Corporation 240-XA Elemental Analyzer. Oxygen analysis was done on a Leco Oxygen Analyzer equipped with an Oxygen Determinator 316 (Model 783700) and an Electrode Furnace EF100. Silicon was determined by a fusion technique which consisted in converting the silicon material to soluble forms of silicon and analyzing the solute for total silicon by atomic absorption spectrometry. Compounding was done on a Brabender Plasticorder (Model PL-V151) equipped with roller blades. A 10.9 Mg (12 ton) Hull console molding machine (model 359E) was used for transfer molding. Test bars were formed on a Carver laboratory press (Fred S. Carver Inc., Summit, N.J.) using a tungsten carbide lined die. Pyrolysis was carried out in an Astro graphite element tube furnace Model 1000-3060-FP12 equipped with an Eurotherm Controller/Programmer Model 822. Flex strengths (using the four-point bend technique) were determined on either a Model TTC or Model 8562 Instron instrument. Four different SiC powders were employed: Ibiden UF SiC ("Ibiden"), which contains a mixture of 5 to 8% alpha-SiC and 92 to 95% beta-SiC; Superior Graphite (HSC 059) beta-SiC ("Superior Graphite"); Herman C. Starck Co. A10 alpha-SiC ("Starck A10"); and Herman C. Starck Co. B10 SiC ("Starck B10"), which contains a mixture of 15% alpha-SiC and 85% beta-SiC. The boron used was amorphous boron powder from Cerac Inc. The boron carbide (Tetrabor) was from Elektroschmelzwerk Kempten GmbH., Munich, West Germany. The aluminum oxide (Baikalox CR 125) was obtained from Baikowski International Corp., Charlotte, North Carolina. The aluminum nitride (Grade A) was from Herman C. Starck Co. Example 1 - Preparation of Sintered Bodies with (PhMeSiO) 0.72 (MeViSiO) 0.14 (MeHSiO) 0.14 . A -- Polymer Synthesis. A solution of 47.78 g (0.25 moles) PhMeSiCl 2 , 7.06 g (0.05 moles) MeViSiCl 2 and 5.75 g (0.05 moles) MeHSiCl 2 in 120 mL toluene was added dropwise to a stirred mixture of 74.2 g Na 2 CO 3 in 300 mL water. After stirring for about one hour, the organic layer was collected, dried over MgSO 4 and filtered. Concentration of the filtrate gave 31.9 g (77.2% yield) of a cloudy, viscous oil. IR (KBr, thin film): 3073 (w), 3052 (w), 2959 (w), 2903 (w), 2164 (w), 1595 (w), 1427 (w), 1406 (w), 1265 (s), 1124 (s), 1082 (s), 1026 (s), 907 (m), 872 (w), 973 (s), 731 (m) and 694 cm -1 (m). B -- Char Composition Calculations. A sample of the above polymer under argon was heated to 1800°C. at 10°C./min and held at 1800°C. for two hours before cooling to room temperature. The sample had a mass retention of 39.6% and contained 49.0% carbon and 46.7% silicon. The following calculations were made: 100 g of cured polymer gives 39.6 g of a ceramic char consisting of 49.0% carbon and 51.0% silicon (by difference). (In order to simplify the calculations, the amount of oxygen has been ignored.) The char consists of 28.9 g SiC (73%) and 10.7 g carbon (27%). Therefore, each gram of polymer, after pyrolysis, gives 0.289 g SiC and 0.107 g free carbon. C -- Test Bar Fabrication. A series of five siloxane/SiC powder blends were prepared using the following procedure: A weighed sample of the siloxane prepared above and two weight percent benzoyl peroxide (based on weight of the siloxane) were dissolved in 10 to 20 mL toluene. This solution was mixed with Ibiden SiC powder and boron in an hardened alumina mortar and pestle until a thick paste was formed. The residual solvent was removed under vacuum. The dried powder was reground in a mortar and pestle and then passed through a 90 µm mesh sieve. The sieved powder was dry pressed into 35x8x2 mm test bars at approximately 64.8 MPa (9.4 ksi). Each test bar was prepared with 20.000 g of SiC and 0.080 g boron and varying amounts of organopolysiloxane and benzoyl peroxide. The test bars were fired to 2150°C. in an Astro tube furnace (argon atmosphere) using the following temperature program: room temperature to 300°C. at 26.7°C./min, 300 to 700°C. at 25°C./min, 700 to 1350°C. at 20°C./min, 1350°C. for 30 minutes, 1350°C. to 2150°C. at 20°C./min and 2150°C. for 30 minutes. The density of the fired test bars was measured. The following results were obtained: Test Bar Wt. (g) Polymer Wt. (g) Bz202 Green Density [g/cm 3 ] Fired Density [g/cm 3 ] Free Carbon Value (%) Flex Strength [MPa (ksi)] 1 1.000 0.021 1.97 2.54 0.5 133.1 (19.3) 2 2.000 0.040 2.06 2.84 1.0 218.6 (31.7) 3 3.000 0.060 2.16 3.10 1.5 353 (51.2) 4 4.000 0.080 2.23 3.11 2.0 -- 5 6.000 0.124 -- 3.08 2.9 -- Example 2 - Preparation of Sintered Bodies With (PhSiO 1.5 ) 0.30 (PhViSiO) 0.70 . A -- Polymer Synthesis. A solution of 71.8 g (0.35 moles) PhViSiC12 and 31.7 g (0.15 moles) PhSiCl3 in 70 mL toluene was added dropwise to a stirred mixture of 70 g of Na 2 C0 3 in 450 mL water and 200 mL toluene. After the addition was complete, the reaction mixture was stirred at room temperature for 30 minutes. The two phases were separated, and the organic layer collected and then concentrated under vacuum. The residue (71 g) was dissolved in 400 mL toluene and the solution refluxed over 0.5 g KOH. The water was collected in a Dean-Stark trap. The toluene solution was further dried over MgS0 4 and filtered through a sintered glass frit. Concentration of the filtrate under vacuum gave 59.6 g (83.8% yield) of a viscous oil. GPC molecular weight (THF solvent): M n =901, M w =3711. Proton NMR (CDCl3, delta-values): 5.33 to 6.17 (SiVi, broad multiplet) 6.67 to 7.70 (SiPh, broad multiplet); the SiPh/SiVi molar ratio was 1.5/1.0. B -- Calculations. A mixture of 1.672 g of the above polymer and 0.056 g benzoyl peroxide was dissolved in about 5 mL toluene, the toluene removed under vacuum and the residue heated to more than 150°C. for 10 minutes in an argon atmosphere. The residue was weighed into a graphite crucible and transferred into an Astro tube furnace. The furnace was twice evacuated to less than 2.66 kPa (20 torr) and then backfilled with argon. Under an argon purge, the sample was heated to 1800°C. at 10°C./min and held at 1800°C. for two hours before cooling to room temperature. The sample had a mass retention of 42.8% and contained 62.2% carbon, 36.8% silicon and 0.2% oxygen. The following calculations were made: 100 g of cured polymer gives 42.8 g of a ceramic char consisting of 62.2% carbon and 37.8% silicon (by difference). The char consists of 23.1 g SiC (54%) and 19.7 g carbon (46%). Therefore, each gram of polymer, after pyrolysis, gives 0.231 g SiC and 0.197-g free carbon. C -- Test Bar Fabrication. A series of seven siloxane/SiC blends were prepared using the same procedure as in Example 1. Each test bar was prepared with 25.00 g of Ibiden SiC, 0.10 g boron and varying amounts of organopolysiloxane and benzoyl peroxide. The test bars were fired to 2150°C. under an argon atmosphere using the same temperature profile as in Example 1. The following results were obtained: Test Bar Wt. (g) Polymer Wt. (g) Bz202 Green Density [g/cm 3 ] Fired Density [g/cm 3 ] Free Carbon Value (%) Flex Strength [MPa (ksl)] 1 0.250 0.006 1.87 2.41 0.2 115.8 (16.3) 2 0.624 0.014 1.89 2.65 0.5 139.3 (20.2) 3 1.000 0.024 1.95 2.97 0.8 308.2 (44.7) 4 1.500 0.030 1.97 3.11 1.2 370.3 (53.7) 5 2.000 0.042 1.99 3.16 1.5 390.9 (56.7) 6 2.500 0.050 2.03 3.09 1.9 425.4 (61.7) 7 5.000 0.104 2.25 3.09 3.6 -- Example 3 - Effect of Firing Temperature on Density A siloxane/SiC blend with a free carbon value of 1.5% was prepared using the following procedure: 12.00 g of siloxane polymer of Example 2 and 0.252 g benzoyl peroxide (about 2 weight percent based upon the weight of siloxane polymer), dissolved in 150 mL toluene, was mixed with 150 g Ibiden SiC powder and 0.60 g boron. The free carbon value of the blend was 1.5%. The blend was attritor milled for 30 minutes. The resulting slurry was transferred to a resin kettle and the solvent removed under vacuum. The dried powder was ground in a mortar and pestle and then passed through a 90 µm mesh sieve. The sieved powder was dry pressed into test bars (35x8x2 mm) at 324.1 MPa (47 ksi). The test bars were fired to 1950, 2000, 2050 and 2100°C. under an argon atmosphere using the same temperature program as in Example 1 except for the difference in the final temperature. The following results were obtained. Final Temperature (°C.) Density (g/cm 3 ) Percent of Percent of Theoretical 1950 3.05 95.0 2000 3.08 96.0 2050 3.14 97.8 2100 3.15 98.1 Example 4 - Preparation of Sintered Bodies with (PhSiO 1.5 ) 0.27 (MeSiO 1.5 ) 0.09 (PhViSiO) 0.64 . A -- Polymer Synthesis. A solution of 71.76 g (0.35 moles) PhViSiCl 2 , 31.7 g (0.15 moles) PhSiCl 3 and 7.45 g (0.05 moles) MeSiCl 3 in 50 mL toluene was added dropwise to a stirred solution of 85 g Na 2 C0 3 in 400 mL water and 200 mL toluene. After the addition was complete, the reaction mixture was stirred at room temperature for 30 minutes. The two phases were separated and the organic layer collected and concentrated under vacuum. The residue (64.9 g) was dissolved in 65 g toluene and the solution refluxed over 0.10 g KOH. The water was collected in a Dean-Stark trap. After all of the water was removed, the toluene solution was cooled to room temperature and 0.3 mL Me 3 SiCl was added. After about one hour, the solution was filtered through a medium glass frit. Concentration of the filtrate under vacuum gave 60.7 g (81.5% yield) of a viscous oil. Proton NMR (CDCl 3 , delta-values): -0.3 to 0.4 (SiMe, broad hump), 5.5 to 6.2 (SiVi, broad multiplet) and 6.8 to 7.8 (SiPh, broad multiplet); the SiMe/SiVi/SiPh molar ratio was 1.0/7.1/10.8. B -- Char Composition Calculation. A sample of the above polymer was heated to 1800°C. using the same procedure as in Example 1, Part B. The sample had a mass retention of 48.1% and contained 62.2% carbon. The following calculations were carried out: 100 g of cured polymers gives 48.1 g of ceramic char consisting of 37.8% (by difference) silicon and 62.2% carbon. The char therefore consists of 26.0 g SiC (54.1%) and 22.1 g carbon (45.9%). Thus, each gram of polymer, after pyrolysis, gives 0.260 g SiC and 0.221 g free carbon. C -- Test Bar Fabrication. Five test bars were prepared from a blend containing 2.001 g of the siloxane polymer, 0.041 g benzoyl peroxide and 25.0 g Ibiden SiC powder using the same procedures as in Example 1. The free carbon value of the blend was 1.7%. The average flex strength of the green test bars was 1627 kPa (236 psi). The test bars were then fired to 2098°C. under argon using the same firing procedure as in Example 1. The average density of the fired test bars was 3.16 g/cm 3 (98.5% of theoretical); the average flex strength was 260.6 ± 104.8 MPa (37.8 ± 15.2 ksi). Example 5 - Preparation of Sintered Bodies with (PhSiO1.5) 0.21 (MeSiO 1.5 ) 0.29 (PhViSiO) 0.50 . A -- Polymer Synthesis. A solution of 71.76 g (0.35 moles) PhViSiCl 2 , 31.7 g (0.15 moles) PhSiCl 3 and 29.8 g (0.20 moles) MeSiCl 3 in 50 mL toluene was added dropwise to a stirred mixture of 180 g Na 2 CO 3 in 400 mL water and 250 mL toluene. After the addition was complete, the reaction mixture was stirred at room temperature for 45 minutes. The two phases were separated and the organic layer collected and concentrated under vacuum. The residue (79.0 g) was dissolved in 88.7 g toluene and refluxed over 0.10 g KOH. After all of the water was removed using a Dean-Stark trap, the toluene solution was cooled to room temperature and 1.0 mL Me 3 SiCl was added. After about one hour, the solution was filtered through a medium glass frit. Concentration of the filtrate under vacuum gave 70.9 g (83.8% yield) of a tacky gum. The polymer was dissolved in 150 mL toluene and poured into 1.5 liters isopropanol. The precipitated polymer was collected and dried at 200°C. and 133 Pa (1 torr). Proton NMR (CDCl 3 , delta-values): -0.36 to 0.40 (SiMe, broad singlet), 5.5 to 6.3 (SiVi, broad singlet) and 6.8 to 7.8 (SiPh, broad singlet); the SiMe/SiVi/SiPh molar ratio was 1.0/1.86/2.78. B -- Calculations. A sample of above pol mer (2.643 g) was mixed with 0.268 g of a (MeHSiO) 0.97 (Me 3 SiO 0.5 ) 0.03 fluid (20-40 x 10 -6 m 2 /s (20-40 centistokes) viscosity) and 2 drops of 0.1 M H 2 PtCl 6 in isopropanol. The mixture was heated at 135°C. for approximately two hours before cooling to room temperature in order to crosslink the polymer. The residue was heated to 1800°C. at 10°/min and held at 1800°C. for two hours under an argon atmosphere before cooling to room temperature. The sample had a mass retention of 56.7% and contained 50.3% carbon. The following calculations were made: 100 g of cured polymer gives 56.7 g of a ceramic char consisting of 49.7% (by difference) silicon and 50.3% carbon. The char consists of 40.25 g SiC (71.0%) and 16.44 g carbon (29.0%). Therefore, each gram of polymer, after pyrolysis, gives 0.403 g SiC and 0.164 g free carbon. C -- Test Bar Fabrication. Using the above data, the following mixture was prepared with an 1.4% free carbon value and 0.3% boron: 2.082 g of the above siloxane polymer dissolved in 10-15 mL toluene were mixed with 23.65 g Ibiden SiC powder, 0.073 g boron, 0.217 g of a (MeHSiO) 0.97 (Me 3 SiO 0.5 ) 0.03 fluid [20-40x10 - 6 m 2 /s (20-40 centistokes) viscosity] and 2-3 drops 0.1 M H 2 PtCl 6 in isopropanol, and ground in an hardened alumina mortar and pestle until a thick paste was obtained. The paste was dried under vacuum to remove residual solvent. The dried powder was ground in a mortar and pestle and passed through a 90 µm mesh sieve. The sieved powder was dry pressed into test bars (35x8x2 mm) at 317.2 MPa (46 ksi). The test bars were heated at 130°C. for one hour to crosslink the polymer. After curing, the test bars had an average flex strength of 7585 ± 517 kPa (1100 ± 75 psi). The cured test bars were fired to 2100°C. at a rate of 10°C./min and held at 2100°C. for 30 minutes under an argon atmosphere. The average density of the six fired test bars was 3.14 ± 0.01 g/cm 3 (97.8% of theoretical). Example 6 - Hot Pressing of Siloxane/SiC Mixtures. A sieved siloxane/SiC blend with a free carbon value of 1.5% was prepared by attritor milling a mixture of 150 g Ibiden SiC powder, 0.6 g boron, 12.0 g of a siloxane of the formula (PhSiO 1.5 ) 0.30 (PhViSiO) 0.70 prepared by the same procedure as in Example 2 and 0.48 g benzoyl peroxide together in toluene. The siloxane, upon pyrolysis, contained 0.197 g free carbon per gram of siloxane. The solvent was removed under vacuum at below 50°C. The dried mixture was ground and collected through a 90 µm sieve. A sample of this mixture was fired to 2100°C. in argon at 20°C./min and held at 2100°C. for one hour. Throughout the firing cycle, the sample was kept under pressure [27.6 MPa (4 ksi)]. The density of the resulting hot pressed body was 3.17 g/cm 3 (98.8% of theoretical). The hot pressed sample was sectioned and machined into test bars according to MIL-STD-1942 (MR) (November 21, 1983). The average flex strength of the machined test bars was 620.6 MPa (90 ksi). Example 7 - Preparation of Sintered Bodies with (PhSiO 1.5 ) 0.21 (MeSiO 1.5 ) 0.43 (PhViSiO) 0.36 . A -- Polymer Synthesis. A solution of 50.8 g (0.25 moles) PhViSiCl 2 , 31.7 g (0.15 moles) PhSiCl 3 and 44.8 g (0.30 moles) MeSiCl 3 in 50 mL toluene was added dropwise to a stirred mixture of 180 g Na 2 C0 3 in 400 mL water and 250 mL toluene. After the addition was complete, the reaction mixture was stirred at room temperature for 45 minutes. The two phases were separated and the organic layer collected and concentrated under vacuum. The residue (75.2 g) was dissolved in 150 g toluene and the solution refluxed over 0.103 g KOH by collecting the water using a Dean-Stark trap. After the water removal was complete, the toluene solution was cooled to room temperature and 1.0 mL Me 3 SiCl was added. After about one hour, the solution was filtered through an 0.2 µm membrane filter. The filtrate was concentrated under vacuum and the residue dissolved in 150 mL toluene. The toluene solution was poured into 1.5 liters isopropanol. The precipitated polymer was collected and dried at 200°C. and 133 Pa (1 torr) . The yield was 41.9 g (54.8%). Proton NMR (CDCl 3 , delta-values): -0.23 to 0.60 (SiMe, broad singlet), 5.6 to 6.4 (SiVi, broad singlet) and 6.9 to 8.0 (SiPh, broad singlet); the SiMe/SiVi/SiPh molar ratio was 2.7/1.0/1.1. GPC molecular weight (THF solvent) : M w =4993 and M n =1215. B -- Calculations. A blend of 1.841 g of the above siloxane polymer and 0.035 g benzoyl peroxide was prepared in a mortar and pestle. The blend was heated to 135°C. for 45 minutes to crosslink the siloxane polymer. The cured siloxane was then fired to 1800°C. (room temperature to 1200°C. at 13°C./min, 1200 to 1800°C. at 5°C./min and 30 minutes at 1800°C.) under argon. The sample had a mass retention of 50.0% and contained 49.0% carbon. The following calculations were made: 100 g of cured polymer gives 50.0 g of a ceramic char consisting of 51.0% (by difference) silicon and 49.0% carbon. The char consists of 36.4 g SiC (72.8%) and 13.6 g carbon (27.2%). Therefore, each gram of polymer, after pyrolysis, gives 0.364 g SiC and 0.136 g free carbon. C -- Test Bar Fabrication. A siloxane/SiC mixture with a free carbon value of 1.8% was prepared using the calculations of Part B above and the following procedure: 3.391 g of above siloxane polymer and 0.068 g benzoyl peroxide were mixed with 23.333 g Ibiden SiC powder and 0.075 g boron in an hardened alumina mortar and pestle until a thick paste was obtained. The paste was dried under vacuum to remove residual solvent. The dried powder was ground in a mortar and pestle and then passed through a 90 µm mesh sieve. The sieved powder was dry pressed into test bars (35x8x2 mm) at 317.2 MPa (46 ksi). The green test bars had an average flex strength of 7240 ± 1145 kPa (1050 ± 166 psi). The test bars were then cured at 75°C. for 1.5 hours; the average flex strength increased to 11,860 ± 738 kPa (1720 ± 107 psi). The test bars were then fired to 2050°C. under argon using the following temperature program: room temperature to 300°C. at 11.2°C./min, 300 to 550°C. at 10°C./min, 550 to 700°C. at 6°C./min, 700 to 1350°C. at 32.4°C./min, 1350 to 2050°C./min and 2050°C. for 30 minutes. The average density of the fired test bars was 2.99 g/cm 3 (93.1% of theoretical). The fired test bars had an average flex strength of 237.2 ± 28.96 MPa (34.4 ± 4.2 ksi). Example 8 - Preparation of Sintered Bodies with (PhSiO 1.5 ) 0.31 (MeSiO 1.5 ) 0.46 (PhViSiO) 0.23 . A -- Polymer Synthesis. A solution of 30.5 g (0.15 moles) PhViSiCl 2 , 42.3 g (0.20 moles) PhSiCl 3 and 44.8 g (0.30 moles) MeSiCl 3 in 50 mL toluene was added dropwise to a stirred mixture of 180 g Na 2 CO 3 in 400 mL water and 250 mL toluene. After the addition was complete, the reaction mixture was stirred at room temperature for 45 minutes. The two phases were separated and the organic layer was collected and concentrated under vacuum. The residue (71.3 g) was dissolved in 189.5 g toluene and the solution refluxed over 0.129 g KOH. The water was collected in a Dean-Stark trap. After all of the water had been removed, the toluene solution was cooled to room temperature and 1.0 mL Me 3 SiCl was added. After about one hour, the solution was filtered through an 0.2 µm membrane filter. The filtrate was concentrated under vacuum and the residue dried at 200°C. and 133 Pa (1 torr). The yield was 40.0 g (58.7%). GPC molecular weight (THF solvent): M w =9011, M n =1540. B -- Calculations. A blend of 1.740 g of the above siloxane polymer and 0.068 g benzoyl peroxide was prepared in a mortar and pestle. The blend was heated to 170°C. for 15 minutes to crosslink the polymer. The crosslinked polymer was fired to 1900°C. under an argon purge (room temperature to 1200°C. at 13°C./min, 1200 to 1900°C. at 5°C./min and two hours at 1900°C.). The sample had a mass retention of 44.4% and contained 43.8% carbon. The following calculations were made: 100 g of cured polymer gives 44.4 g of a ceramic char consisting of 56.2% (by difference) silicon and 43.8% carbon. The char consists of 35.6 g SiC (80.3%) and 8.8 g carbon (19.7%). Therefore, each gram of polymer, after pyrolysis, gives 0.356 g SiC and 0.088 g free carbon. C -- Test Bar Fabrication. A siloxane/SiC blend with a free carbon value of 1.5% was prepared using the following procedure: 4.409 g of the above siloxane polymer and 0.176 g benzoyl peroxide dissolved in 10-15 mL toluene were mixed with 23.042 g Ibiden SiC powder and 0.100 g boron in an hardened alumina mortar and pestle until a thick paste was obtained. The paste was dried under vacuum to remove residual solvent. The dried powder was ground in a mortar and pestle and then passed through a 90 µm mesh sieve. The sieved powder was dry pressed into test bars (35x8x2 mm) at 317.2 MPa (46 ksi). The green test bars had an average density of 2.21 g/cm 3 and a flex strength of 9.49 ± 1.54 MPa (1376 ± 224 ksi). The test bars were fired to 2050°C. in an argon atmosphere using the same temperature program as in Example 7 (Part C). The average density of the fired test bars was 3.11 g/cm 3 (96.9% of theoretical). The fired test bars had an average flex strength of 288.2 ± 51 MPa (41.8 ± 7.4 ksi). Example 9 - Preparation of Sintered Bodies with (PhSiO 1.5 ) 0.75 (Me 2 ViSi0 0.5 ) 0.25 and Ph 2 Si(OSiMe 2 H) 2 . A -- Polymer Synthesis. A mixture of 3960 g PhSi(OMe) 3 and 620 g (ViMe 2 Si) 2 O was added to a solution of 3 g trifluoromethane sulfonic acid in 800 g water. The reaction mixture was kept at room temperature for 20 minutes and then refluxed for five hours. The reaction mixture was cooled to room temperature and then neutralized with 2.73 g potassium carbonate. Volatiles were removed by distillation until the temperature of the reaction mixture reached 120°C. The reaction mixture was cooled to room temperature at which time 1500 g toluene and 125.7 g of a 3% aqueous KOH solution were added. Water was then removed using a Dean-Stark trap. After water removal was complete, the reaction mixture was cooled to room temperature at which time 20 mL Me 2 ViSiCl were added. After stirring at room temperature for two hours, the reaction mixture was filtered through an 0.2 µm membrane filter and the filtrate concentrated using a rotary evaporator. The residue was dried for 1-2 hours at 100°C. and 133 Pa (1 torr). The-yield of the resin of unit formula (PhSiO 1.5 ) 0.75 (Me 2 ViSi 0.5 ) 0.25 was 3053.3 g (95.4% yield). GPC molecular weight (THF solvent): M w =1803, M n =806. B -- Calculations. (1). A blend of 14.85 g of the above resin, 5.16 g Ph 2 Si(OSiMe 2 H) 2 and 0.01 g catalyst A was prepared. Catalyst A consisted of 0.09 g octadecyldimethylamine, 0.351 g ((C 4 H 9 ) 3 P) 2 PtCl 2 and 32.7 g toluene. A sample of this blend was crosslinked at 120°C. for one hour. This sample was fired to 1900°C. under argon using the same procedure as in Example 8. The sample had a mass retention of 44.9% and contained 53.4% carbon. The following calculations were made: 100 g of the cured polymer give 44.9 g of a ceramic char consisting of 46.6% (by difference) silicon and 53.4% carbon. The char consists of 29.9 g SiC (66.6%) and 15.0 g carbon (33.4%). Therefore, each gram of polymer, after pyrolysis, gives 0.299 g SiC and 0.150 g free carbon. (2). A blend of 16.21 g of the above resin, 3.80 g Ph 2 Si(OSiMe 2 H) 2 and 0.594 g catalyst A was prepared. A sample was crosslinked at 120°C. for one hour and then fired to 1900°C. under argon using the same procedure as in Example 8. The sample had a mass retention of 47.4% and contained 48.8% carbon. The following calculations were made: 100 g of the cured polymer give 47.4 g of a ceramic char consisting of 48.8% (by difference) silicon and 51.2% carbon. The char consists of 33.0 g SiC (69.7%) and 14.4 carbon (30.3%). Therefore, each gram of polymer, after pyrolysis, gives 0.330 g SiC and 0.144 g free carbon. C -- Test Bar Fabrication. (1). A blend of 29.7 g of the above resin, 10.32 g Ph 2 Si(OSiMe 2 H) 2 and 0.594 g catalyst A was prepared. The blend had a free carbon value of 1.5%. A sample of this blend (5.000 g) was dissolved in 10-15 mL toluene and mixed with 47.6 g Ibiden SiC powder and 0.15 g boron in an hardened alumina mortar and pestle until a thick paste was obtained. The paste was dried under vacuum to remove residual solvent. The dried powder was ground in a mortar and pestle and then passed through a 90 Am mesh sieve. The sieved powder was dry pressed into test bars (35x8x2 mm) at 241.3 MPa (35 ksi). The green test bars had an average density of 2.04 g/cm 3 and an average flex strength of 4.3 ± 1 MPa (622 ± 145 psi). The test bars were heated to 135°C. for 15 hours to crosslink the polymer. The cured test bars had an average density of 2.04 g/cm 3 and an average flex strength of 13.72 ± 2.98 MPa (1990 ± 433 psi). The cured test bars were then fired to 2075°C. under argon at 15°C./min and then held at 2075°C. for 30 minutes. The fired test bars had an average density of 3.15 g/cm 3 (98.1% of theoretical) and an average flex strength of 377.2 ± 59.3 MPa (54.7 ± 8.6 ksi). (2). A sample (3.215 g) of the uncrosslinked blend prepared in Part C(1) above was mixed with 23.4 g Ibiden SiC powder and 0.106 g boron in toluene. The blend had a free carbon value of 1.9%. Test bars were prepared using the same procedure as in Part C(1) above and had a density of 2.15 g/cm 3 in the green state. The green test bars were heated to 135°C. for 3 days to crosslink the polymer. Afterwards, the test bars had an average flex strength of 21.13 ± 2.3 (3065 ± 339 psi). The cured test bars were fired to 2075°C. using the following temperature program: room temperature to 520°C. at 17.3°C./min, 520 to 800°C. at 9.3°C./min, 800 to 1100°C. at 11.1°C./min, 1100 to 1300°C. at 1.7°C./min, 1300 to 2075°C. at 25.8°C./min and 2075°C. for 30 minutes. The average density of the fired test bars was 3.15 g/cm 3 (98.1% of theoretical). The fired test bars had an average flex strength of 335.8 ± 42.75 MPa (48.7 ± 6.2 ksi). (3). A blend of 29.7 g of the above (PhSiO 1.5 ) 0.75 (Me 2 ViSiO 0.5 ) 0.25 resin, 10.32 g Ph 2 Si(OSiMe 2 H) 2 and 0.594 g catalyst B was prepared. Catalyst B consisted of 0.182 g HOCH 2 CH 2 NH 2 , 1.00 g ((C 4 H 9 ) 3 P) 2 PtCl 2 and 100 g toluene. A solution of this uncrosslinked blend (5.25 g) and 2.0 g of a dispersing agent (OLOA 1200 from Chevron Chemical) in hexane was mixed with 50 g Superior Graphite SiC powder and 0.5 g boron in an hardened alumina mortar and pestle until a thick paste was obtained. The blend had a free carbon value of 1.4%. The residual solvent was removed under vacuum. The dried powder was ground and passed through a 90 µm screen. The sieved powder was uniaxially pressed at 344.75 MPa (50 ksi) into test bars (7.7x35.9x15 mm). The density of the green test bars was 2.27 ± 0.02 g/cm 3 . The test bars were fired to 2075°C. under argon using the following temperature profile: room temperature to 1400°C. at 25°C./min, 1400°C. for 30 minutes, 1400 to 2075°C. at 15°C./min and 2075°C. for 30 minutes. The density of the fired samples averaged 3.14 ± 0.02 g/cm 3 (97.8% of theoretical). After machining (according to MIL-STD-1942 (MR) (November 21, 1983)), the flex strength averaged 333.0 ± 51.71 MPa (48.3 ± 7.5 ksi). (4). A solution of 4.14 g of the above resin of unit formula (PhSiO 1.5 ) 0.75 (Me 2 ViSi 0.5 ) 0.25 , 1.11 g Ph 2 Si(OSiMe 2 H) 2 and 5 drops of catalyst C in 150 mL hexane was ultrasonically mixed with 50 g Starck A10 SiC powder and 0.25 g boron for 10 minutes to give a 1.5% free carbon value blend. Catalyst C consisted of 0.11 g HOCH 2 CH 2 NH 2 , 0.10 g ((C 4 H 9 ) 3 P) 2 PtCl 2 and 10 g toluene. The solvent was removed under vacuum and the residue passed through a 60 mesh screen. The sieved powder was uniaxially pressed into test bars at various pressures. Test bars were fired to 2020°C. and 2075°C. using the same temperature program as described in Part C(3) above. The following results were obtained: pressure [MPa (ksi)] green density (g/cm 3 ) 2020°C. fired density (g/cm 3 ) 2075°C. fired density (g/cm 3 ) 68.9 (10) 1.92 - 2.99 137.9 (20) 1.93 2.97 3.02 206.9 (30) 1.96 2.97 3.02 344.75 (50) 1.99 2.99 3.05 413.7 (60) 2.05 3.00 3.07 (5). A solution of 4.14 g of the above resin of unit formula (PhSiO 1.5 ) 0.75 (Me 2 ViSi 0.5 ) 0.25 , 1.11 g Ph 2 Si(OSiMe 2 H) 2 and 5 drops of catalyst C in 150 mL hexane was ultrasonically mixed with 50 g Starck B10 SiC powder and 0.25 g boron for 10 minutes to give a 1.5% free carbon value blend. Catalyst C is described in Part C(4) above. The solvent was removed under vacuum and the residue passed through a 60 mesh screen. The sieved powder was uniaxially pressed into test bars at various pressures. Test bars were fired to 2020°C. and 2075°C. using the same temperature program as used in Part C(4) above. The following results were obtained: pressure [MPa (ksi)] green density (g/cm 3 ) 2020°C. fired density (g/cm 3 ) 2075°C. fired density (g/cm 3 ) 68.95 (10) 2.06 - 3.11 137.9 (20) 2.08 3.04 3.11 206.9 (30) 2.13 3.06 3.11 344.75 (50) 2.15 3.03 3.11 413.7 (60) 2.18 3.08 3.10 (6). Test bars were prepared using the same blend and procedures as in C(5) above except that Ibiden SiC powder was used; the pressure for preparing the test bars was varied. The test bars were cured in an argon atmosphere at 150°C. for 17 hours. The following results were obtained: pressure [MPa (ksi)] green strength [KPa (psi)] cured strength [KPa (psi)] 68.95 (10) 966 (140) 7707 (1117) 137.9 (20) 1794 (260) 12924 (1873) 241.3 (35) 1725 (250) 12820 (1858) 344.75 (50) 2229 (323) 16181 (2345) (7). A solution of 4.57 g of the above resin of unit formula (PhSiO 1.5 ) 0.75 (Me 2 ViSiO 0.5 ) 0.25 1.59 g Ph 2 Si(OSiMe 2 H) 2 and 0.111 g catalyst B in 450 mL hexane was ultrasonically mixed with 44.75 g Ibiden SiC powder and 0.503 g aluminum oxide for 10 minutes. The resulting blend had a free carbon value of 1.9%. Catalyst B is described in Part C(3) above. The solvent was removed under vacuum and the residue passed through a 250 µm screen. The sieved powder was uniaxially pressed into test bars at 324 MPa (47 ksi). The test bars were cured in air at 200°C. for 18 hours. The cured test bars had an average density of 2.19 ± 0.01 g/cm 3 and a flex strength of 17.95 ± 1.55 MPa (2601 ± 224 psi) . The test bars were then fired to 2075°C. at 15°C./min (with 30 minutes at 1400°C. and 2075°C.) under an argon atmosphere. The average density of the fired test bars was 2.76 g/cm 3 (86% theoretical). (8). Test bars were prepared from 3.53 g of the above resin, 1.22 g Ph 2 Si(OSiMe 2 H) 2 , 0.105 g catalyst B, 44.75 g Ibiden SiC powder and 0.50 g boron carbide powder using the same procedure as in C(7) above; the test bars were cured and fired to 2075°C. as in C(7) above. The cured (unfired) test bars had an average density of 2.13 ± 0.01 g/cm 3 and a flex strength of 16.68 ± 1.77 MPa (2417 ± 256 psi). The average density of the fired test bars was 3.14 g/cm 3 (98% theoretical). (9). Test bars were prepared from 3.53 g of the above resin, 1.22 g Ph 2 Si(OSiMe 2 H) 2 , 0.105 g catalyst B, 44.75 g Ibiden SiC powder and 0.503 g aluminum nitride using the same procedure as in C(7) above; the test bars were cured and fired to 2075°C. as in C(7) above. The cured (unfired) test bars had an average density of 2.23 ± 0.02 g/cm 3 and a flex strength of 29.15 ± 0.76 MPa (4224 ± 110 psi). The average density of the fired test bars was 2.70 g/cm 3 (84% theoretical). (10). Test bars were prepared from 3.53 g of the above resin, 1.22 g Ph 2 Si(OSiMe 2 H) 2 , 0.105 g catalyst B, 44.75 g Ibiden SiC powder, 0.255 g aluminum oxide and 0.256 g boron using the same procedure as in C(7) above; the test bars were cured and fired to 2075°C. as in C(7) above. The cured (unfired) test bars had an average density of 2.11 ± 0.02 g/cm 3 and a flex strength of 15.9 ± 1.16 MPa (2300 ± 168 psi). The average density of the fired test bars was 2.76 g/cm 3 (86% theoretical). Example 10 - Preparation of Sintered Bodies with (PhSiO 1.5 ) 0.50 (C 3 H 7 SiO 1.5 ) 0.15 (Me 2 ViSiO) 0.35 and PhSi(OSiMe 2 H) 3 . A -- Polymer Synthesis. A mixture of 152.5 g PhSi(OMe) 3 , 37.9 g C3H7Si(OMe)3 and 50.1 g (ViMe 2 Si) 2 O was added to a solution of 0.16 g trifluoromethane sulfonic acid in 54 g water. The reaction mixture was kept at room temperature for approximately 20 minutes and then refluxed for four hours. The reaction mixture was cooled and then neutralized with 0.18 g potassium carbonate. The volatiles were removed by distillation until the reaction mixture reached 120°C. The reaction mixture was cooled and 70 g toluene and 6.2 g aqueous 3% KOH were added. The water was removed using a Dean-Stark trap. After the water removal was complete, the reaction mixture was cooled and 1 mL Me 2 ViSiCl was added. After stirring at room temperature for about two hours, the reaction mixture was filtered through a sintered glass filter and the filtrate concentrated using rotary evaporation. The residue was dried at 100°C. and 133 Pa (1 torr). The yield was 155.2 g. Proton NMR (d8-toluene, delta-values): 0.04 to 0.6 (SiMe, broad singlet), 0.7 to 2.1 (SiC 3 H7, broad hump), 5.7 to 6.3 (SiVi, broad multiplet) and 6.8 to 8.4 (SiPh, broad multiplet); the molar ratio of SiMe/SiC 3 H 7 /SiVi/SiPh was 2.1/1.4/1.0/3.3. GPC molecular weight (THF solvent): M w =1248, M n =867. B -- Calculations. A blend of 5.00 g of the above resin, 1.73 g PhSi(OSiMe 2 H) 3 and 0.069 g catalyst B was prepared. Catalyst B consisted of 0.182 g HOCH 2 CH 2 NH 2 , 1.00 g ((C 4 H 9 ) 3 P) 2 PtCl 2 and 100 g toluene. A sample of this blend was crosslinked at 200°C. for two hours. The crosslinked polymer was fired to 1900°C. under argon at a rate of about 15°C./min and held at 1900°C. for two hours. The sample had a mass retention of 27.6% and contained 40.8% carbon. The following calculations were made: 100 g of cured polymer gives 27.6 g of a ceramic char consisting of 59.2% (by difference) silicon and 40.8% carbon. the char consists of 23.3 g SiC (84.6%) and 4.3 g carbon (15.4%). Therefore, each gram of polymer, after pyrolysis, gives 0.233 g SiC and 0.043 g free carbon. C -- Test Bar Fabrication. A blend containing 25.00 g of the above resin, 8.65 g PhSi(OSiMe 2 H) 3 and 0.345 g catalyst B was prepared. This blend (25.02 g) was mixed with 74.7 g Ibiden SiC powder and 0.3 g amorphous boron in a Brabender mixing chamber until a thick paste was obtained. The blend had a free carbon value of 1.3%. Several 10 g samples were transfer molded at l10°C. into a spiral flow mold using a 6.9 MPa (1000 psi) ram pressure, a 12.42 MPa (1800 psi) clamping pressure and a five minute transfer cycle. A spiral flow of 216 mm (8.5 inches) was obtained. Two 36 g samples were transfer molded into a 12-cavity test bar mold (each cavity was 6.2x37.8x2.5 mm) using the same molding conditions. The molded test bars were cured at 205°C. for two hours. The cured test bars had an average flex strength of 35.79 ± 5.2 MPa (5187 ± 758 psi). The test bars were fired to 2075°C. at 1 to 2°C./min with 30 minutes at 2075°C. The average density of the fired test bars was 3.09 g/cm 3 (96.3% of theoretical). The fired test bars had an average flex strength of 178.6 ± 115.8 MPa (25.9 ± 16.8 ksi). Example 11 - Preparation of Sintered Bodies with (PhSiO 1.5 ) 0.27 (MeSiO 1.5 ) 0.18 (Me 2 SiO) 0.18 (MeViSiO) 0.25 (Ph 2 SiO) 0.12 A -- Polymer Synthesis. To a three-necked, five liter flask equipped with a drainage stopcock, thermometer, condenser, overhead mechanical stirrer and addition funnel were added 1790 g water and 324 g isopropyl alcohol. A mixture of 134.6 g (0.90 moles) MeSiCl 3 , 116.1 g (0.90 moles) Me 2 SiCl 2 , 285.5 g (1.35 moles) PhSiCl 3 , 151.8 g (0.60 moles) Ph 2 SiCl 2 and 176 g (1.25 moles) MeViSiCl 2 dissolved in 792 g toluene was added below the water surface over a fifteen minute period. During the addition, the reaction temperature increased to 45°C. The reaction mixture was stirred for five minutes. The collected organic layer was washed twice with warm tap water and then azeotropically dried and bodied with zinc octoate (0.1% based on the total weight of the polymer). The solvent was removed and the resin dried at 150°C. and 2.66 kPa (20 torr). The product yield was 535 g (91.2%). B -- Calculations. A sample of the above polymer mixed with 1% dicumyl peroxide was cured at 200°C. for one hour. The cured polymer was fired to 1900°C. under argon at a rate of 15°C./min and held at 1900°C. for two hours. The sample had a mass retention of 43.7% and contained 40.6% carbon. The following calculations were made: 100 g of cured polymer gives 43.7 g of a ceramic char consisting of 59.4% (by difference) silicon and 40.6% carbon. The char consists of 37.1 g SiC (84.9%) and 6.6 g carbon (15.1%). Therefore, each gram of polymer, after pyrolysis, gives 0.371 g SiC and 0.066 g free carbon. C -- Test Bar Fabrication. Ibiden SiC (76.1 g), 0.34 g boron, 0.51 g zinc stearate and 27.02 g of the above resin were mixed at 110°C. in a Brabender mixing chamber until a homogeneous mix was obtained. The mixture had a 2.0% free carbon value. The mixture was transfer molded into a 12-cavity test bar mold (each cavity was 6.2x37.8x2.5 mm) at 177°C. at a ram pressure of 6.9 MPa ± 4.45 (1000 psi) and a clamping pressure of 12.77 MPa (1850 psi). The test bars were removed from the mold and further cured at 205°C. for 1.5 hours. The cured test bars had an average flex strength of 14.11 ± 4.45 MPa (2045 ± 645 psi). The test bars were fired to 2075°C. using a temperature program of room temperature to 180°C. at 3°C./min, 180 to 700°C. at 0.5°C./min, 700 to 1400°C. at 1.5°C./min, 1400°C. for 30 minutes, 1400 to 2075°C. at 20°C./min and 2075°C. for 30 minutes. The furnace was cooled from 2075°C. to 600°C. at 10°C./min. The fired test bars had an average flex strength of288.9 ± 39.99 MPa (41.9 ± 5.8 ksi) and an average density of 3.07 g/cm 3 (95.6% of theoretical). Example 12 - Preparation of Sintered Bodies with (PhMeSiO) 0.33 (MeHSiO) 0.61 (Me 3 SiO 0.5 ) 0.05 and (PhSiO 1.5 ) 0.34 (ViSiO 1.5 ) 0.33 (Me 2 ViSiO 0.5 ) 0.33 . A -- Polymer Synthesis. Polymer 1: (PhMeSiO) 0.33 (MeHSiO) 0.61 (Me 3 SiO 0.5 ) 0.06 . A mixture of 77.0 g PhSi(OMe) 3 , 63.4 g of a siloxane fluid obtained from the hydrolysis of MeHSiCl 2 , 8.02 g hexamethyldisiloxane and 0.2 g trifluoromethane sulfonic acid was dissolved in 170 g toluene. The reaction mixture was refluxed four hours while the byproduct water was removed by azeotropic distillation. After cooling to room temperature, the reaction mixture was neutralized with K 2 CO 3 and the solution filtered through a sintered glass frit. Removal of the solvent gave 129.8 g (87.0% yield) of a low viscosity fluid. Polymer 2: (PhSiO 1.5 ) 0.33 (ViSiO 1.5 ) 0.33 (Me 2 ViSi 0.5 ) 0.33 . A mixture of 495.1 g (2.5 moles) PhSi(OMe) 3 , 370 g (0.25 moles) ViSi(OMe) 3 , 232.5 g (1.25 moles) (Me 2 ViSi) 2 O, 270 g distilled water and 0.7 g trifluoromethane sulfonic acid was refluxed for four hours. The reaction mixture was cooled to room temperature and neutralized with 0.72 g K 2 CO 3 . The volatiles were removed by distillation until the temperature of the reaction mixture reached 120°C. After cooling to room temperature, 34.0 g of 3% aqueous KOH and 400 g toluene was added. The water was then removed by azeotropic distillation. The reaction was then quenched with 10 mL Me 2 ViSiCl. After filtration and removal of the solvent, 650 g (86.4% yield) of a clear residue was obtained. Proton NMR (CDCl 3 , delta-values): -0.2 to 0.3 (SiMe, overlapping singlets), 5.4 to 6.1 (SiVi, broad multiplet) and 6.8 to 7.9 (SiPh, broad multiplet); SiMe/SiVi/SiPh molar ratio was 1.0/2.1/1.2. GPC molecular weight (THF solvent): M w =863, M n =1641. B -- Calculations. A blend of 4.06 g of Polymer 1, 5.00 g of Polymer 2 and 0.13 g catalyst B (as described in Example 10) was prepared. The polymer mixture was crosslinked at 200°C. for 4 to 5 hours and then fired to 1900°C. in argon using a temperature program of room temperature to 1000°C. at 11°C./min, 1000 to 1900°C., at4.4°C./min and two hours at 1900°C. The sample had a mass retention of 50.8% and contained 38.8% carbon. The following calculations were made: 100 g of cured polymer gives 50.8 g of a ceramic char consisting of 61.2% (by difference) silicon and 38.8% carbon. The char consists of 44.4 g SiC (87.4%) and 6.4 g carbon (12.6%). Therefore, each gram of polymer, after pyrolysis, gives 0.444 g SiC and 0.064 g free carbon. C -- Test Bar Fabrication. A blend of 11.96 g of Polymer 1, 14.73 g of Polymer 2, 0.51 g catalyst B, 79.8 g Ibiden SiC and 0.32 g boron was compounded in a Brabender mixing chamber until a homogeneous paste was obtained. The blend had a free carbon value of 1.8%. A 10 g sample of this blend was transfer molded in a spiral flow mold at 150°C.; at a ram pressure of 6.9 MPa (1000 psi) and a clamping pressure of 28.12 MPa (1850 psi), 343 mm (13.5 inches) of flow were obtained. Test bars (6.2x37.8x6.2 mm) were prepared using the same molding conditions and a 12-cavity test bar mold. The test bars were further cured at 200°C. for 16 hours The cured test bars had an average flex strenght of 26.91 ± 7.94 MPa (3900 ± 1150 psi). The test bars were fired to 2049°C. in argon using the following temperature profile: room temperature to 490°C. at 4.9°C./min, 490 to 1050°C. at 4.7°C./min, 1050 to 2049°C. at 4.3°C./min and 30 minutes at 2049°C. The average density of the fourteen fired test bars was 3.10 g/cm 3 (96.6% of theoretical).

Claims: A composition of matter for producing a highly densified ceramic body, said composition matter comprising, in intimate admixture, an organic silicon polymer, silicon carbide powder and a metal-containing sintering aid, characterised in that: (1) said organic silicon polymer is a curable organopolysiloxane: (i) containing units of general structure [R 3 SiO 0.5 ], [R 2 SiO], [RSiO 1.5 ] , [SiO 2 ], where each R is independently selected from the group consisting of hydrogen, alkyl radicals containing 1 to 20 carbon atoms, phenyl radicals and vinyl radicals. (ii) capable of yielding by pyrolysis in an inert atmosphere a stable ceramic char, which does not significantly decrease in weight upon further exposure to said pyrolysis, in an amount greater than 20 weight percent; (iii) containing in the ceramic char an amount of free or excess carbon, determined by pyrolysis of the organopolysiloxane, in the absence of any silicon carbide powder or sintering aid, to an elevated temperature under an inert atmosphere until a stable ceramic char is obtained, of at least 10 weight percent; and (iv) present at such a level that the free carbon value of the mixture is greater than 0.2 weight percent, and not greater than 3 weight percent based on the total weight of the silicon carbide powder and the char derived from the preceramic organopolysiloxane, (2) said metal-containing sintering aid is selected from iron, Fe 3 C, boron, boron-containing compounds aluminum, aluminum-containing compounds and metal oxides selected from thorium oxide, yttrium oxide, lanthanum oxide and cerium oxide, and is present in the mixture in an amount of from 0.1 to 3.0 weight percent of the metal based on the weight of silicon carbide powder; and (3) an organopolysiloxane curing agent is additionally present in an effective amount sufficient to induce crosslinking in the organopolysiloxane. A composition of matter according to Claim 1, wherein the organopolysiloxane is represented by the unit formula: [PhSiO 1.5 ] [MeSiO 1.5 ] [PhViSiO] having 10 to 90 mole percent [PhSiO 1.5 ] unita, 0 to 50 mole percent [MeSiO 1.5 ] units and 10 to 90 mole percent [PhViSiO] units. A method of forming a handleable green body, which method comprises: (a) preparing a composite of matter in accordance with Claim 1; and (b) forming said composition of matter into the desired shape under pressure at a temperature below 500°C. A method according to Claim 3, wherein said handleable green body is subjected to curing. A method of preparing a sintered body of silicon carbide which method comprises: (a) forming a handleable green body according to Claim 3 or Claim 4, and (b) thereafter sintering the handleable green body in an inert atmosphere at a temperature above 1900°C to obtain a sintered body of silicon carbide with a density above 2.4 g/cm 3 .

Inventors: ATWELL WILLIAM HENRY, BURNS GARY THOMAS, SAHA CHANDAN KUMAR, ATWELL, WILLIAM HENRY, BURNS, GARY THOMAS, SAHA, CHANDAN KUMAR
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1989
Main classification: C04B35
IPC codes: C04B35, C08L83, C08K3