FR2654718A1 - COMPOSITION OF SILICON-OXY-CARBIDE GLASS PREPARED FROM COLLOUIDAL SILICA TREATED WITH SILOXANOL AND PROCESS FOR ITS PREPARATION. - Google Patents

Abstract

The invention relates to glass compositions, consisting of silicon, oxygen and carbon, in which the silicon atoms are chemically bonded to carbon and oxygen atoms in substantially the absence of a chemical bond between the carbon atoms. carbon and oxygen, which is prepared by pyrolysis of colloidal silica, treated with a siloxanol, in a non-oxidizing atmosphere; the silicon-oxy-carbide glasses of the invention are resistant to devitrification and decomposition in oxidizing or reducing atmospheres at temperatures up to about 1600 degrees C.

Description

The present invention relates to a silicon-oxy- glass composition.

  carbide prepared from colloidal silica

  treated with a siloxanol, and a process for its preparation.

  The vitreous silica is a refractory glass, however it devitrifies at approximately 1 1000 C The devitrification consists in the transformation of the random structures of which the glasses are made into a crystallized structure The crystallization considerably reduces one of the main characteristics of vitreous silicas, it is i.e. their low thermal expansion as well as many other desirable properties. Consequently, significant research has aimed at increasing the resistance to

  devitrification of silica glass compositions.

  The reactions between silicon, carbon and oxygen have been much studied Some of the known reactions, in a silicon, carbon and oxygen system, include the combination of oxygen with silicon to form silica, mainly in the form of silicon dioxide At temperatures above 1,100 IC, silica begins to crystallize to form cristobalite, one of the common mineral forms of silica Carbon can react with silicon to form crystalline silicon carbide or can react-with oxygen to form

carbon monoxide.

  The thermodynamics of the reactions of silicon, carbon and oxygen is exposed in "The High-Temperature Oxidation, Reduction, and Volatilization Reactions of Silicon and Silicon Carbide", Gulbransen EA and Jansson SA Oxidation of Metals, Volume 4, number 3 , 1972 The thermodynamic analysis of Gulbransen et al shows that at 1200 C silica and carbon form silicon monoxide and gaseous carbon monoxide or solid silicon carbide, Si C However, formation is not expected

  of a material containing the three elements Gulbransen et al.

  conclude that it is not recommended to use silica in reducing atmospheres above 1,125 C, as a result of the formation of gaseous silicon monoxide Also, it is not recommended to use carbide silicon in environments containing oxygen as a result of the oxidation of silicon carbide. There is known a material described as vitreous silica modified by carbon, which is called here "black glass", in which 1 to 3% of carbon have been added & silica Le

  process for preparing black glass is discussed by Smith et al.

  in US Patent 3,378,431 Carbonaceous organic materials, such as a Carbowax, are added to silica and the mixture is hot pressed at about 1,200 C to form black glass Smith CF Jr characterized black glass by infrared spectroscopy in "The Vibrational Spectra of High Purity and Chemically Substituted Vitreous Silicas", Thesis of Doctor of Philosophy, Alfred University, Alfred, NY, May 1973 Smith indicates that, in addition to the elementary carbon dispersed in black glass, carbon is associated with oxygen in carbonato groups The term carbonato designates a radical having a carbon atom linked to three oxygen atoms and having the structure

\ C = O

  0 / The mechanical resistance of black glass is similar to the resistance of conventional carbon-free silica glass, however black glass has an increased resistance to devitrification compared to conventional silica glass which begins to devitrify at around 1100 C, while black glass begins to devitrify at around 12501 C The increased thermal stability of black glass allows it to be used at higher temperatures

  which conventional silica devitrifies.

  In the continuous silicon carbide ceramic fiber marketed under the trademark "Nicalon", approximately 10% of oxygen is introduced into the fiber to crosslink it After crosslinking, the fibers are pyrolyzed and it seems that oxygen is incorporated to the fiber in the form of an amorphous contaminant, probably in the form of silica The degrading behavior of such fibers, after heat treatment in various environments, is presented in the article "Thermal Stability of Si C Fibers (Nicalon @)", Mah T et al, Journal of Material Science, Vol 19, pp 1191-1201 (1984) Mah et al have observed that, whatever the environmental conditions during the heat treatment, the resistance of "Nicalon" fibers degrades

  when subjected to temperatures above 1,200 WC.

  Fiber degradation is associated with loss of carbon monoxide by the fibers and growth of 63-carbide grain.

silicon in the fibers.

  Ceramic materials generally exhibit a brittle behavior characterized by their high mechanical strength and their weak brittle breaking strength The brittle breaking strength is the resistance to the propagation of cracks in materials. Composite ceramics have been developed to reduce the fragile behavior of ceramics The "Nicalon" ceramic fiber is excellent, but it degrades at temperatures above 1 2001 C The integration of "Nicalon" fibers into a protective ceramic matrix, having desirable mechanical properties and capable of withstanding temperatures appreciably higher than 1 2001 C, would make it possible to form an improved ceramic composite However it emerges from the account above that the properties of known ceramics or glasses, and particularly those containing silicon, oxygen and carbon, are degraded for example by decomposition n silicon carbide or devitrification of conventional glass. An object of the invention is therefore to form a glassy glass comprising silicon, oxygen and carbon, in which a substantial proportion of the carbon atoms are chemically bonded to silicon atoms, and the remaining carbon is carbon. elementary dispersed in the glass matrix These glass compositions resist decomposition in oxidizing or reducing atmospheres and devitrification at temperatures reaching

approximately 1600 WC.

  Another object of the invention is to provide a method for forming a glass, consisting of silicon, oxygen and carbon, by

  pyrolysis of colloidal silicas treated with a siloxanol.

  Another object of the invention is to form articles of

  glass from a colloidal silica treated with a siloxanol.

  The Applicant has discovered that colloidal silicas treated with a siloxanol can be pyrolyzed in a non-oxidizing atmosphere to form particular glass compositions Surprisingly, the Applicant has discovered that colloidal silicas treated with a siloxanol pyrolysed in a non-oxidizing atmosphere - below approximately 1 6000 C do not form silica, cristobalite, silicon carbide, monoxide

  carbon or mixtures of silica and elemental carbon.

  The non-crystalline glasses with a stable structure of the invention are prepared by pyrolysis of a colloidal silica treated with a siloxanol to form a glass composition comprising silicon, oxygen and carbon, in which a major proportion of the atoms of carbon are chemically bonded to silicon atoms These glasses resist crystallization and decomposition in oxidizing or reducing atmospheres at temperatures reaching about 1 6000 C In addition, a major proportion of the carbon present in the glasses of the invention is bound to silicon, the rest being present in the form of elementary carbon dispersed in the glass matrix, so that there are no groups

detectable carbonato.

  The carbon-silicon bonds, discovered in the glasses of the invention, were not previously known in silica glasses In silica glasses, and in particular in black glass, carbon is only present in the form of an element not bound in the silica matrix or in carbonato groups o the carbon is bound to oxygen The glasses of the invention, characterized by the bond

  chemical described above, here are called silicon glass-oxy-

  carbide. The glasses of the invention are produced from a colloidal silica treated with a siloxanol, heated in a non-oxidizing atmosphere, to pyrolyze the treated silica As used herein the term "non-oxidizing atmosphere" designates an atmosphere substantially free of oxygen which removes pyrolysis by-products from the resin during pyrolysis, without affecting the reactions which occur during pyrolysis Examples of non-oxidizing atmospheres include a vacuum of less than about 4 bar, inert atmospheres, such as helium, argon or

  nitrogen, and reducing atmospheres such as hydrogen.

  In the present description, a colloidal silica treated

  with a siloxanol comprises a dispersion of colloidal silica in a partial condensate of a silanol of formula R Si (OH) 3 The partial condensate is formed by hydrolysis of an organotrialkoxysilane in which R is a monovalent hydrocarbon radical containing approximately 1 to 12 atoms carbon, such as a halohydrocarbon radical, C 1-12 alkyl, C 6-12 aryl, or an ester, such as methacrylate or acrylate; at least 70% by weight of the silanol consisting of methyltrihydroxysilane The partial condensate of the silanol is siloxanol The colloidal silica treated with the siloxanol is

  sometimes called here treated silica or treated colloidal silica.

  Pyrolysis of the treated silica forms a glass of silicon-

  oxy-carbide which is characterized by continuous pooling

  of electrons between the silicon, oxygen and carbon atoms.

  In a silicon-oxy-carbide glass, the silicon atoms are present up to four polyatomic units In a unit, called here tetraoxysilicon, one atom of silicon is linked to four oxygen atoms In a second unit, called here, monocarbosiloxane, a silicon atom is linked to three oxygen atoms and a carbon atom In a third motif, called here dicarbosiloxane, a silicon atom is linked to two oxygen atoms and two carbon atoms In a fourth motif, here called tetracarbosilicon, a silicon atom is linked to four carbon atoms. When a treated silica consisting of the partial condensate of methyltrimethoxysiloxane and colloidal silica is pyrolysed in a ratio of approximately 1.8 / 1, there is formed a silicon-oxy-carbide glass having a distribution of polyatomic units consisting of approximately 76 to 86% by weight of tetraoxysilicon, approximately 11 to 21% by weight of monocarbosiloxane, up to approximately 8% by weight of dicarbosiloxane with at least approximately 3% by weight of elementary carbon dispersed in the glass matrix The polyatomic units are mainly linked by silicon-oxygen bonds with a small and negligible number of bonds between carbon and oxygen atoms.

  The following description of the invention will be better understood at

  the examination of the figures briefly described below.

  Figure 1 is a graph showing weight loss during

  pyrolysis of a colloidal silica treated with a siloxanol.

  Figure 2 is a graphical representation of the spectrum of

  29 nuclear magnetic resonance of a silicon glass-

  oxy-carbide formed by pyrolysis of a colloidal silica treated with

a siloxanol.

  FIG. 3 is a graphic representation of the nuclear magnetic resonance spectrum of silicon 29 of silicon carbide "Nicalon '.

  The invention will now be described in detail.

  The glasses are generally formed from an extremely viscous supercooling liquid and have a crosslinked structure polymerized with a short range order. The glasses of the invention are not made from supercooling liquids, but have a crosslinked structure at order at short distance Instead of the supercooling of a liquid, the glasses of the invention are formed by pyrolysis of a colloidal silica treated with a siloxanol in a non-oxidizing atmosphere However, the glasses of the invention have

  the short distance order characteristic of classic glasses.

  The colloidal silica treated with a siloxanol, which can be used in the practice of the invention, comprises a dispersion of colloidal silica in the partial condensate of a silanol of formula R Si (OH) 3, in which R is a a monovalent hydrocarbon radical of about 1 to about 12 carbon atoms, such as a halohydrocarbon radical, C 1 12 alkyl, C 6 12 aryl, or an ester, such as a methacrylate or acrylate; at least 70% by weight of the silanol consisting of methyltrihydroxysilane The colloidal silica treated with the partial condensate can be dried to form a powder or the treated silica can be diluted in a solution of water and an aliphatic alcohol containing approximately 10 to 50% by weight of dry matter, the dry matter consisting essentially of 10 to 70% by weight of colloidal silica and 30 to% by weight of the partial condensate, the composition having a p H of 3.5

at 8.0.

  The treated colloidal silica can be prepared by hydrolysis of a trialcoxysilane or of a mixture of trialcoxysilanes of formula R Si (OR ') 3, in which R is as defined above and R' represents C 1-8 alkyl radicals, in the presence of a dispersion

aqueous colloidal silica.

  Suitable colloidal aqueous silica dispersions generally have particles with a diameter of 5 to 150 nm. These silica dispersions are well known in the art and those sold under the trade names Ludox can for example be obtained commercially ( Du Pont) and Nalcoag (NALCO Chemical Co) These colloidal silicas can be available in the form of acidic and basic hydrosols. Colloidal silicas are preferred, the mean particle size of which is 5 to 25 nm.

  Ludox AS-40 sold by Du Pont Company.

  To prepare the treated colloidal silica composition,

  adds the aqueous dispersion of colloidal silica to an alkyl tri

  alkoxysilane or an aryltrialcoxysilane which may contain a buffering agent such as acetic acid, otherwise an alkyltriacetoxysilane can be used instead of alkoxysilanes and acid If desired, small amounts of a dialkyldialcoxysilane can also be used reaction mixture The temperature of the reaction mixture is maintained between about 201 C and about 400 C, preferably below 250 C It has been found that in about 6 to 8 hours, a sufficient amount of trialkoxysilane has reacted to reduce the liquid mixture initial two phase into a liquid phase

  unique in which the silica is dispersed.

  Generally, the hydrolysis reaction is allowed to continue for a total of about 12 hours to 48 hours depending on the desired viscosity of the final product. The more the hydrolysis reaction is allowed to continue, the higher the viscosity of the colloidal silica dispersion.

is high.

  After completion of the hydrolysis, the dry matter content is adjusted by the addition of an alcohol, preferably isopropanol, to the dispersion of colloidal silica. Other alcohols suitable for this purpose include lower aliphatic alcohols, such as methanol, ethanol, isobutanol, isopropanol, n-butyl alcohol and tert-butyl alcohol or their mixtures When it is desirable to use a treated colloidal silica, in which the partial condensate is in solution, the solvent system can contain from about 20 to 75% in

alcohol weight.

  The p H of the reacted composition obtained is in the range of about 3.5 to 8.0, preferably from about 6.6 to about 7.8 or from 3.8 to 5.7 If necessary, can add a dilute base, such as ammonium hydroxide, or a weak acid, such as acetic acid,

  to the composition to adjust the p H in the desired range.

  The acid is used to buffer the alkalinity of the initial two-phase liquid reaction mixture and thus moderate the rate of hydrolysis. Glacial acetic acid can be used, as well as other acids such as organic acids, such as propionic, butyric, citric, benzoic, formic, oxalic acids and the like. Alkyltriacetoxysilanes can be used in which the alkyl group contains from 1 to 6 carbon atoms, the alkyl groups having 1 to 3 carbon atoms being preferred.

  particularly methyltriacetoxysilane.

  The aforementioned silanetriols R Si (OH) 3 are formed by hydrolysis of the corresponding trialcoxysilanes with the aqueous medium, that is to say the aqueous dispersion of colloidal silica. Examples of the trialcoxysilanes are those containing methoxy, ethoxy, isopropoxy and nbutoxy substituents. which, by hydrolysis, form the silanetriol and the corresponding alcohol When a mixture of trialkoxysilanes is used, a mixture of different silanetriols and different alcohols is obtained During the production of silanetriol or mixtures of silanetriols in the basic aqueous medium , hydroxyl substituents condense to form bonds

-Si-O-Si-

I I

  This condensation evolves over time and is not a complete condensation, the siloxane retaining an appreciable quantity of hydroxyl groups linked to silicon which make the polymer soluble in the alcohol-water co-solvent. It seems that this partial soluble condensate can be characterized as a siloxanol polymer having at least one hydroxyl group bonded to silicon for three units

-If O-

  I The non-volatile solids of the colloidal silica treated here consist of a mixture of colloidal silicas and the partial condensate or siloxanol of a silanol Most or all of the partial condensate or siloxanol results from the condensation of methyltrihydroxysilane and, depending on the ingredients introduced into the hydrolysis reaction, minor proportions of the partial condensate can come, for example, from the condensation of methyltrihydroxysilane with ethyltrihydroxysilane, or propyltrihydroxysilane or methyltrihydroxysilane with C 6 H 5 Si (OH) 3, or their mixtures On prefers to use only methyltrimethoxysilane and thus produce only methylsilanetriol in the preparation of treated colloidal silica compositions which can be dried to form a powder or dissolved in a solvent so that the partial condensate is present in an amount of about 55 to 75% of total dry matter in a cosolvent f has alcohol and water, the alcohol constituting

  about 50% to 95% of the weight of the co-solvent.

  The silicon-oxy-carbide glass is formed by pyrolysis of the colloidal silica treated with a siloxanol in a non-oxidizing atmosphere at temperatures between about 9000 ° C. and 1600 ° C. Preferably the heating rate, during pyrolysis, is limited to allow pyrolysis by-products to escape without

  leave voids or bubbles in the silicon-oxy-carbide glass.

  Preferably heating rates below 1 C per minute are used. During pyrolysis, by-products are released and cause weight loss, while the treated silica densifies. Although the treated silica undergoes during pyrolysis a weight loss, the density of the treated silica increases during the pyrolysis due to the decrease in the volume of the treated silica The pyrolysis reactions are essentially completed when a substantially constant weight of the treated silica is obtained course of pyrolysis A substantially constant weight is generally obtained between approximately 9000 C and 1 2500 C An additional densification of the pyrolyzed treated silica may occur after the weight loss has stopped, if the heating is continued It may therefore be desirable to stop the heating and pyrolysis of the treated silica after it is completely densified or, in other words, to stop the reduction in theft ume It has been determined that the weight loss during pyrolysis of a treated silica is

about 14%.

  The glasses of the invention resist devitrification and retain their structural stability at temperatures reaching at least 1600 WC. It is understood by this that all of the material retains essentially the same microstructure from ordinary temperature to approximately 1600 C. La formation of small crystallized zones of at most about 100 angstr 8 ms in an otherwise amorphous matrix, has no undesirable or detrimental effect on the properties of the whole of the material So glasses with a stable structure of the present invention are essentially amorphous, but may contain small crystallized regions, for example graphite, cristobalite or silicon carbide, in the glass or present small amounts of cristobalite on the surface of the glass. Silicon-oxy-carbide glass articles can be formed by powdering the pyrolyzed treated silica using mills well known in the art.

  silicon-oxy-carbide by hot compression to form an article.

  A hot compression process consists in applying a uniaxial pressure of at least about 3.4 da N / mm 2 between about 15501 C and 16001 C to the powder. These pressures and these temperatures are sufficient to

form a densified article.

  Shaped articles can also be formed directly from the treated silica. The treated silica is first dissolved in a solvent such as isopropanol, then poured into the desired shape. Examples of the solvents which have been found suitable when the treated silica is dissolved, are lower aliphatic alcohols, such as methanol, ethanol, isobutanol, n-butyl alcohol and tert-butyl alcohol or their mixtures and preferably isopropanol We can dissolve

  silicas treated in about 20 to 75% by weight of solvent.

  The cast treated silica is dried at room temperature and slowly pyrolysed in a non-oxidizing atmosphere, as described herein. Pyrolysis is carried out by slow heating to avoid the formation of voids and bubbles when the gases are released and cause loss of weight of treated silica When the weight

  of the pyrolyzed treated silica stabilizes, the pyrolysis is completed.

  Otherwise, the treated silica can be shaped by hot compression.

  is normally in the form of a powder.

  The treated silica in solution in isopropanol can also be drawn into fibers. The treated silica solution is treated with a base, such as ammonium hydroxide, to increase the viscosity until a solid object can be immersed in the solution and removed from it by lifting a thread of treated silica from the solution Fibers can be formed by drawing or drawing from the solution of silica treated according to such immersion processes Otherwise, the treated silica can be aspirated in a Teflon tube with a slight vacuum When the isopropanol evaporates and the viscosity of the solution of the treated silica increases, the fiber shrinks and can be removed from the tube The fibers are reinforced to facilitate their handling by heating to about 500 C The fibers are then pyrolyzed in a non-atmospheric

  oxidizing or vacuum, as described above.

  Ceramic composites can be formed having ceramic fibers in a matrix made of silicon-oxy-carbide glass and a ceramic filler The treated silica is dissolved in a solvent and ceramic particles are dispersed in the solution to form a infiltration suspension The load of ceramic particles limits the removal of the matrix from the composite during pyrolysis and can be chosen so that the matrix is compatible with the reinforcing fibers to be used. Some examples of the ceramic fillers are silicon carbide powder, diatomaceous earth and the aluminosilicate of formula 2 Si O 2 3 A 1203 called

mullite.

  One or more ceramic fibers or a tissue of fibers are passed through a stirred bath of the infiltration suspension. Some examples of ceramic fibers are carbon fibers, silicon carbide fibers and aluminum fibers. borosilicate The impregnated fiber is then shaped and dried to allow the solvent to evaporate. A shaping process consists of winding an impregnated fiber in a spiral on a drum to form a panel. The layers of the fibers can be fused by application of heat and pressure to form a continuous matrix of treated silica surrounding the ceramic fibers The composite is then pyrolyzed in a non-oxidizing atmosphere or under vacuum, as described above The resin densifies into a substantially amorphous silicon-oxy-carbide glass which binds the ceramic filler to form a continuous matrix around the fibers Depending on the pyrolysis temperature used, the filler ceramic can be dispersed, partially sintered or completely

sintered in the glass.

  Optionally, the ceramic composite can be reinfiltrated with the infiltration suspension to reduce the porosity of the composite. The composite is placed under vacuum in the reinfiltration solution. Pressure is applied to the solution to force the solution into the pores of the composite. After reinfiltration , the toluene is allowed to evaporate and the reinfiltrated composite is pyrolyzed in a non-oxidizing atmosphere or under vacuum, as described above. The reinfiltration and pyrolysis can be repeated as often as necessary to obtain the desired degree of density. in

the matrix.

  The amorphous silicon-oxy-carbide glass matrix bonding a ceramic filler surrounds the ceramic fibers and protects them from decomposition in oxidizing and reducing atmospheres up to temperatures of about 1600 WC It has been established that nature inert silicon-oxy-carbide glass allows it to tolerate ceramic fibers, without reacting with them or degrading their properties Therefore, a silicon-oxy-carbide glass, containing appropriate ceramic fillers, can be used

  as a matrix of any known ceramic fibers.

  The following examples illustrate in more detail the silicon-oxy-carbide glass of the invention and the methods for

  produce glass and glassware.

  EXAMPLE 1 A colloidal silica composition with a siloxanol comprising the partial condensate derived from methyltrimethoxysilane and colloidal silica in a weight ratio of approximately 1.8 is pyrolysed, by measuring the weight loss of the silica treated by thermogravimetric analysis. 1 Thermogravimetric analysis is a process for measuring the weight loss of a sample during heating. The material is pyrolyzed in a hydrogen atmosphere by heating at the speed of 10 WC / min at a temperature of 1250 WC. The measured weight loss of the silicon-oxy-carbide glass formed after

the pyrolysis is around 14%.

  The weight loss, during the pyrolysis of the treated silica, is represented graphically by FIG. 1 On the graph of FIG. 1, the percentage of weight loss of the sample is represented on the ordinates, while the increase in the heating temperature is shown on the abscissa. The graph in FIG. 1 shows that the weight loss of the sample essentially stops at around 9000 C. The X-ray diffraction of the pyrolyzed material

  practically shows the absence of crystallization.

  The composition of different glasses can be determined, from

  generally, by the quantity of each element in the glass.

  However, it is the order at short distance in the glasses which gives them different properties. Therefore, by characterization of the order at short distance in the glasses, one can determine the compositions corresponding to defined properties. The order at 1 S short distance of the silicon-oxy-carbide glass prepared in Example 1 is determined by the percentage of each of the polyatomic units of monocarbosiloxane, dicarbosiloxane and

  tetraoxysilicon present in the glass.

  The nuclear magnetic resonance spectrum of 29 Si in the solid state of a sample of silicon-oxy-carbide glass prepared above has been recorded and is illustrated in FIG. 2 FIG. 3 is the nuclear magnetic resonance spectrum of 295 i of a sample of silicon carbide fiber "Nicalon" In FIGS. 2 and 3, the intensity of the radiation, measured from the excited sample, is represented on the ordinate and the parts per million (ppm ) of the chemical displacement, relative to a tetramethylsilicon standard fixing the zero point on the abscissa axis, are represented on the abscissa We know the chemical displacement expressed in ppm of many polyatomic units, for example that of tetraoxysilicon, dicarbosiloxane and monocarbosiloxane appear in NMR Basic Principles and Progress 29 SiNMR Spectroscopic Results ", editors P Diehl, R Kosfeld, Springer Verlag Berlin Heidelberg 1981, pages 186, 184 and 178 So each peak in Figures 2 and 3 defines the order at a short distance of the patterns

  specific silicon polyatomics.

  The spectrum of the silicon-oxy-carbide glass of FIG. 2 contains peaks numbered 1 to 3 Peak 1 is that of dicarbosiloxane, peak 2 that of monocarbosiloxane and peak 3 that of tetraoxysilicon By integrating the area under each peak, we can determine the fraction of each of these polyatomic patterns present in the glass A correction of the background interference before the determination of the area integrated under each peak has been

  performed for the spectra of Figures 2 and 3.

  The area integrated under each peak in FIG. 2 reveals that the composition of the silicon-oxy-carbide glass prepared as described above comprises up to approximately 8% by weight of dicarbosiloxane, approximately 11 to 21% by weight of monocarbosiloxane and about 76 to 86% by weight of tetraoxysilicon Analysis of the nuclear magnetic resonance spectrum and the black appearance of the glass indicate that at least about 3% by weight of elementary carbon are dispersed in the

  glass, either in an atomic state, or in small clusters.

  Treated colloidal silicas having various ratios of siloxanol to colloidal silica can be pyrolyzed to form silicon-oxy-carbide glasses It is expected that treated colloidal silicas having a ratio of siloxanol to colloidal silica greater than 1.8 / 1 have a reduced tetraoxysilicon content lying in the lower part of the 76 to 86% range mentioned above or below Conversely, it is expected that treated colloidal silicas having a siloxanol to colloidal silica ratio of less than 1.8 / 1 have an increased tetraoxysilicon content in the upper part of the range

76 to 86% or above.

  The spectrum of FIG. 2 can be compared with the spectrum of the silicon carbide of FIG. 3 obtained with a sample of the silicon carbide fiber "Nicalon". The composition in weight percentages of the "Nicalon" of FIG. 3 is approximately 68% of silicon carbide, approximately 8% of dicarbosiloxane, approximately 17% of monocarbosiloxane and approximately 7% of tetraoxysilicon The spectrum of figure 3 shows that the "Nicalon" fibers consist mainly of silicon carbide with small quantities of dicarbosiloxane , monocarbosiloxane and tetraoxysilicon On the other hand, the spectrum of figure 2 shows that the silicon-oxy-carbide glass is made up of notable quantities of dicarbosiloxane, monocarbosiloxane and tetraoxysilicon This particular short distance order of the glass of silicon-oxy -carbide, which unites carbide with silicon in a previously unknown way in glasses, ensures the increase in resistance to devitrification and l has decomposition and characterizes the glasses of the invention.

  The compositions of the silicon-oxy- glass sample

  carbide and "Nicalon" sample can also be described

  relative to the molar percentages of each polyatomic unit.

  The following table indicates the conversion between the molar percentages and the weight percentages of each of these.

compositions.

  Glass of silicon c Nicalon "oxy-carbide Si C% wt% mole% wt% mole Tetraoxysilicon 76-86 74-84 7 5 Monocarbosiloxane 11-21 13-23 17 13 Dicarbosiloxane up to 8 to 8 8 7 Tetracarbosilicon 68 75 The molar percentage expresses the percentage of each polyatomic unit in the samples on a molar basis The percentage of silicon atoms in the samples which are bound to oxygen or carbon can be determined from the molar percentage The glass sample of silicon-oxy-carbide of example 1 contains approximately 16 to 26% of the silicon atoms of the glass bonded to at least one individual carbon atom The sample of silicon carbide "Nicalon" contains approximately 90 to 100% of the atoms of silicon carbide silicon sample carbide.

Claims (13)

RESELL ICATIONS   1 Method for forming a glass composition, characterized in that it comprises the pyrolysis of a colloidal silica, treated with a siloxanol, in a non-oxidizing atmosphere by heating for a period sufficient to form a silicon-oxy-carbide glass which retains its structural stability at temperatures reaching 1 6000 C. 2 Process for forming a glass composition, characterized in that it comprises the pyrolysis of a colloidal silica, treated with a siloxanol, in a non-oxidizing atmosphere by heating for a sufficient period to form a silicon-oxy-carbide glass which retains its structural stability at temperatures of 1,2501 C or more.   3 The method of claim 2, wherein the pyrolysis   is carried out by heating from approximately 9000 C to 1600 WC.   4 The method of claim 3, wherein the heating is carried out for a period which ends when the loss of   weight of the pyrolysed treated colloidal silica practically ceases.   The method of claim 3, wherein the heating is carried out for a period which allows complete densification treated silica.   6 The method of claim 2, wherein the pyrolysis   is carried out in a hydrogen atmosphere.   7 Process according to claim 2, in which the colloidal silica treated with a siloxanol is a dispersion of colloidal silica, having a particle size between approximately 5 and 150 nm, in a partial condensate of a silanol of formula R Si (OH) 3, wherein R is a monovalent hydrocarbon radical of about 1 to 12 carbon atoms. 8 Process according to claim 2, in which the colloidal silica treated with a siloxanol is a dispersion of colloidal silica, having a particle size between approximately 5 to 150 nm, in a partial condensate of a silanol of formula R Si (OH) 3, in which R is a methyl group.   9 The method of claim 7, wherein said partial condensate is formed by hydrolysis of methyltrimethoxysilane, and the ratio of the partial condensate to colloidal silica is about 1.8 / 1.   Glass composition, characterized in that it comprises chemically bonded silicon, oxygen and carbon, the glass being substantially devoid of chemical bond between the oxygen and carbon atoms, the glass being produced according to a process comprising pyrolysis of a colloidal silica, treated with a siloxanol, in a non-oxidizing atmosphere by heating for a period sufficient to form a silicon-oxy-carbide glass which retains its structural stability at temperatures reaching 1600 C.   11 Glass composition, characterized in that it comprises chemically bonded silicon, oxygen and carbon, the glass being substantially devoid of chemical bond between the oxygen and carbon atoms, the glass being produced according to a process comprising pyrolysis of a colloidal silica, treated with a siloxanol, in a non-oxidizing atmosphere by heating for a period sufficient to form a silicon-oxy-carbide glass which retains its structural stability at temperatures of about 1250 C or more.   12 The glass composition according to claim 11, wherein said colloidal silica treated with a siloxanol is a dispersion of colloidal silica, having a particle size between approximately 5 and 150 nm, in a partial condensate of a silanol of formula R Si (OH) 3, in which R is a hydrocarbon radical   monovalent of about 1 to 12 carbon atoms.   13 The glass composition according to claim 11, wherein said colloidal silica treated with a siloxanol is a dispersion of colloidal silica, having a particle size between about 5 and 150 nm, in a partial condensate of a silanol of   formula R Si (OH) 3, in which R is a methyl group.   14 The glass composition according to claim 11, wherein said partial condensate is formed by hydrolysis of methyltrimethoxysilane, and the ratio of the partial condensate to the   colloidal silica is approximately 1.8 / 1.   Glass retaining its structural stability at temperatures of approximately 1250 C or more, characterized in that it comprises polyatomic units of silicon, oxygen and carbon, in weight percentages of approximately 11 to 21% of monocarbosiloxane, at most about 8% dicarbosiloxane, about 76 to 86% tetraoxysilicon, with at least about 3% elemental carbon scattered in the glass.   16 Glass retaining its structural stability up to temperatures of approximately 1600 C, characterized in that it comprises polyatomic units of silicon, oxygen and carbon, in weight percentages of approximately 11 to 21% of monocarbosiloxane , at most about 8% of dicarbosiloxane, about 76 to 86% of tetraoxysilicon, with at least about 3% of elementary carbon scattered in the glass.   17 Glass retaining its structural stability at temperatures of approximately 1250 C or more, characterized in that it comprises silicon, oxygen and carbon in a mass of silicon-oxy-carbide glass, o approximately 16 to 26% of silicon atoms   are each linked to at least one individual carbon atom.