CA2021614A1

CA2021614A1 - Preparation of silicon-oxy-carbide glasses from siloxanol treated colloidal silica

Info

Publication number: CA2021614A1
Application number: CA002021614A
Authority: CA
Inventors: Judith Stein; Larry N.. Lewis; Gary M. Renlund
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1989-11-20
Filing date: 1990-07-19
Publication date: 1991-05-21
Also published as: IT1243923B; FR2654718A1; GB2238307A; JPH03197334A; IT9022117A1; IT9022117A0; DE4034996A1; GB9023473D0

Abstract

PREPARATION OF SILICON-OXY-CARBIDE
GLASSES FROM SILOXANOL TREATED
COLLOIDAL SILICA
Abstract Siloxanol treated colloidal silicas are pyrolized in a non-oxidizing atmosphere to form a glass comprised of silicon, oxygen, and carbon where silicon atoms are chemically bonded to carbon and oxygen atoms, but there are essentially no chemical bonds between carbon and oxygen atoms. The silicon-oxy-carbide glasses of this invention resist devitrification and decomposition in oxidizing or reducing atmospheres at temperatures up to about 1600°C.

Description

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Cross Reference to Related Applications: The subject application relates ~o copending applications for RD-17,455, Serial No. 359,619; RD-19,110, Ser:ial No. 386,327;
and RD-19,549, Serial NoO 428~711.

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The present invention relates to glass compositions and in particular to glass compositions comprising silicon, oxygen, and carbon made from a siloxanol treated colloidal silica.
Vitreous silica is a refractory glass, however, it devitrifies at about 1100 C. Devitrification refers to the transition from the random structures that glasses are made of to a crystallized structure. Crystallization drastically reduces one of the predominant attributes of vitreous silicat i.e., its low thermal expansion, as well as many of its other desirable properties. As a result, much research has been directed towards increasing the resistance to devitrification in silica glass compositions.
Reactions between silicon, carbon, and oxygen have been studied extensively. Known reactions in a silicon, carbon, oxygen system include oxygen combining with silicon to form silica, predominantly as silicon dioxide. At temperatures in excess of 1100 C silica begins to crystalliæe to form cristobalite, one of the common mineral forms of silica. Carbon can react with silicon to form crystalline silicon carbide or it can react wi~h oxygen to form carbon monoxide.

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RD-19,818 The thermodynamics of silicon, carbon, and oxygen reactions is discussed in l'The High-Temperature Oxidation, Reduction, and Volatilization Reactions of Silicon and Sili-con Carbide", Gulbransen, E.A., and Jansson, S.A. Oxidation 5 of Metals, Volume 4, Number 3, 1972. The thermodynamic analysis of Gulbransen et al. shows that at 1200 C silica and carbon should form gaseous silicon ~onoxide and carbon monoxide or sol~d silicon ¢arbide, SiC. Howevex, no single material containing all three elements would be expected to form. Gulbransen et al. conclude that silica was not recommended for use in reducing atmospheres above 1125 C due to the formation of silicon monoxide gas. Also silicon carbide was not recommended for use in oxygen containing environments due to oxidation of the silicon carbide.
lS There is a material described as carbon modified ~itreous silica and herein referred to as "black glass" where 1-3 percent carbon has been added to silica.
The method for making black glass is disclosed by Smith et al. in U.S patent 3,378,431. Carbonaceous organics such as carbowax are added to silica and the mixture is hot pressed at about 1200 C to form black glass. Smith, C.F., Jr. has characterized black glass by infrared spectroscopy in "The Vibrational Spectra of High Purity and Chemically Substituted Vitreous Silicas", PhD Thesis, Alfred University, Alfred, N.Y., May 1973. Smith discloses that in addition to elemental carbon dispersed in the black glass, some carbon is associated with oxygen in carbonato type groups. The term carbonato describes a radical having a carbon atom bonded to three oxygen atoms and having the structure, o /~=o :

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-ll/L~L~2 RD-19,818 The mechanical strength of black glass is similar to the strength of conventional carbon-free silica glass however black glass has an increased reslstance to devitrification over conventional silica glass which begins to devitrify at about llOO C while black glass begins to devitrify at about 1250 C. The increased thermal stability of black glass allows it to be used at the higher temperatures where conventional silica would devitrify.
In a commercially produced continuous silicon car-bide ceramic fibre sold under the trademark "Nicalon", about 10 percent oxygen is introduced into the fibre to crosslink it. After crosslinking, the fibres are pyrolized and it is believed that the oxygen becomes part of the fibre as an amorphous contaminant, probably in the form of silica. The degradation behavior of such fibres after heat treatment in various environments was reported in the article "Thermal Stability of SiC Fibres (Nicalon~)", Mah, T., et al. r ~ournal of Material Science, Vol. 19, pp. 1191-1201 ~1984). Mah et al. found that regardless of the environmental conditions during heat treatment, the "Nicalon77 fibre strength degraded when the fibres were subjected to temperatures greater than 1200-C. The fibre degradation was associated with loss of carbon monoxide from ~he fibres and beta-silicon carbide grain growth ln the fibr~s.
Ceramic materials generally exhiblt brittle be-havior as characterized by their high strength and low frac-ture toughness. Fra~ture toughness is the resistance to crack propagation in materials. The development of ceramic composites has been investigated as a way to alleviate the brittle behavior of ceramics. 7'Nicalon" is an excellent ceramic fibre but it degrades at temperatures above 1200-C.
Integrating 'INicalon" fibres in a protective ceramic matrix .
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RD-19,818 having desirable mechanical properties and capable of with-standing temperatures substantially higher than 1200-C, would be one way of forming an improved ceramic composite.
However, from the discussion above, it is apparent that the properties of known ceramics or glasses, and specifically those containing silicon, oxygen, and carbon, are degraded for example by decomposition of silicon carbide or devitrification of conventional glass.
Therefore, it is an object of this invention to form a vitreous glass, comprising silicon, oxygen, and carbon in which a substantial portion of the carbon atoms are chemically bonded to silicon atoms and the remaining carbon is elemental carbon dispersed in the glass matrix. Such glass compositions resist decomposition in oxidizing or reducing atmospheres and devitrification at temperatures up to about 1600-C.
Another object of this invention is to provide a process for forming a glass comprised of silicon~ oxygen, and carbon by pyrolizing siloxanol treated colloidal silicas.
Still another object of this invention is forming glass articles from a siloxanol treated colloidal silica.

We have found that siloxanol treated colloidal silicas can be pyrolized in a non-oxidizing atmosphere to form unique glass compositionsO Surprisingly, we have found that iloxanol treated colloidal silicas pyrolized in a non-oxidizing atmosphere below about 1600 C do not form silica,cristobalite, silicon carbide, carbon monoxide, or mixtures of silica and elemental carbon.

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l~/1$~89 RD-19,818 The structurally stable non-crystalline glasses of this invention are made by pyrolizing a siloxanol treated colloidal silica to form a glass composition, comprising silicon, oxygen, and carbon wherein a major portion of the carbon a~oms are chemlcally bonded to silicon atoms. These glasses resist crystallization, and decomposition in oxidizing or reducing atmospheres at temperatures up to about 1600-C. In addition, a major portion of the carbon present in the glasses of this invention is bonded to silicon with the remainder present as elemental carbon dispersed within the glass matrix so that there are no detec~able carbonato groups.
The carbon-silicon bonds discovered in the glasses of this invention have heretofore been unknown in silica glasses. In silica glasses, and specifically in black glass, carbon has only been known to be present as an unbonded element in the silica matrix or in carbonato groups where carbon is bonded with oxygen. The glasses of this invention;
characterized by the chemical bonding described above are herein referred to as silicon-oxy-carbide glass.
Glasses of this invention are produced from a siloxanol trea~ed colloidal silica heated in a non-oxidizing atmosphere to pyrolize the treated silica. As used herein, the term "non-oxidizing atmosphere" means a substantially oxygen-free a~mosphere that removes pyrolysis by-products from the pyrolizing resin without influencing the reactions occurring during pyrolysis. Examples of non-oxidizing atmosphsres include a vacuum of less than about 10-4 atmospheres, inert atmospheres like helium, argon, or nitrogen, and reducing atmospheres, such as hydrogen.
A~ referred to herein, a siloxanol treated colloidal silica comprises a dispersion of colloidal silica :

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RD-19,818 in a partlal condensate of a silanol of the formula RSi(OH)3.
The partial condensate is formed by hydrolysis of an organotrialkoxysilane in which R is a monovalent hydrocarbon radical containing from about 1 to 12 carbon atoms such as a halohydrocarbon/ C(1_12~ alkyl~ C~6_12) aryl, or an ester such as methacrylate or acrylate; at least 70 wleight percent of the silanol being methyltrihydroxysilane~ The partial condensate of the silanol is the siloxanol. The siloxanol treated colloidal silica is sometimes herein referred to as treated silica or treated colloidal si}ica.
Pyrolysis of the treated silica forms a silicon-oxy-carbide glass that is characterized by a continued sharing of electrons between atoms of silicon, oxygen and carbon. In silicon-oxy-carbide glass, silicon atoms are present in up to four polyatomic units. In one unit, herein referred to as tetraoxysilicon, a silicon atom is bonded to four oxygen atoms. In a second unitr herein referred to as monocarbosiloxane, a silicon atom is bonded to three oxygen atoms and one carbon atom. In a third unit, herein referred to as dicarbosiloxane~ a silicon atom is bonded to twa oxygen atoms and two carbon atoms. In a fourth unit, herein re-ferred to as tetracarbosilicon, a silicon atom is bonded to four carbon atoms.
~hen a treated silica comprised of the partial condensate of methyltrimethoxysilane and colloidal silica in a ratio of about 1.8:1 is pyrolized, a sllicon-oxy-carbide ylass is formed having a distri~ution of polyatomic units comprising, about 76 to 86 weight percent tetraoxysilicon, about 11 to 21 weight percent monocarbosiloxane, and up to about 8 weight percent dicarbosiloxane, with at least about 3 weight percent of elemental carbon dispersed within the glass matrix. The polyatomic units are linked primarily by ;

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RD-19,818 silicon-oxygen bonds with a small and insignificant number of bonds betwecn carbon and oxygen atoms.
The following description of the invention will be more easily understood by making reference to the figures briefly described below.

Figure 1 is a graph showiny the weight lost cluring pyrolysis of a siloxanol treated colloidal silica.
Figure 2 is a graphical presentation of the ~9Silicon nuclear magnetic resonance spectrum of the silicon-oxy-carbide glass formed by pyrolizing a siloxanol treated colloidal silica.
Figure 3 is a graphical presentation of the 29Silicon nuclear magnetic resonance spectrum of "Nicalon"
silicon carbide.

Glasses are generally formed from an extremely viscous supercooled liquid and possess a polymerized network stxucture with short-ranse order. The glasses o~ this invention are not made from supercooled liquids, but they do possess a network structure with short-range order. Instead of supercooling a liquid, the glasses of this invention are formed by pyrolizing a siloxanol treated colloidal silica in a non-oxidizing atmosphere. Rowever, the glasses of this invention have the short-range ordering characteristic of conventional glasses.
The siloxanol treated colloidal silica which can be used in the practice of the present invention comprises a . . . , ,.:

:. :: . : . , ~ , RD-19,818 dispersion of colloidal silica in the partlal condensate of a silanol of the formula RSi(O~)3,where R is a monovalent hydrocarbon radical from about 1 to 12 carbon atoms such as a halohydrocarboll, Ctl-l2) alkyl, C~s_12) aryl, or an ester such as methacrylate or acrylate; at least 70 weight percent o~
the silanol being methyltrihydroxysilane. Colloidal silica treated with the partial condensate can be dried to form a powder, or the treated silica can be diluted in an aliphatic alcohol-water solution containing about 10 to 50 weight percent solids, the solids consisting essentially of 10 to 70 weight percent colloidal silica and 30 to 90 weight percent of the partial condensate, the composition having a pH of from 3.5 to 8Ø
The treated colloidal silica can be prepared by hy-drolyzing a trialkoxysilane or a mixture of trialkoxysilanes of the formula RSi~OR')3, wherein R is as previously defined, and R' is C~1_g~ alkyl radicals, in the presence of an aqueous dispersion of colloidal silica.-Suitable aqueous colloidal silica dispersions generally have a particular size of from 5 to 150millimicrons in diameter. These silica dispersions are well known in ~he art and commercially available ones include, for example, those sold under the trademark of Ludox (DuPont) and Nalcoag (NALCO Chemical CoO). Such colloidal silicas are available as both acidic and basic hydrosols. Colloidal silicas having an average partlcle size of from 5 to 25 millimicrons are preferred. A particularly preferred one for the purposes herein is known as Ludox AS-40, sold by the DuPont Company.
In preparing the treated ~olloidal silica composition, the aqueou colloidal silica dispersion is added to an alkyltrialkoxysilanP or aryltrialkoxysilane which may .: ' ,,:
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RD-19,818 contain a buffexing agent such as acetic acid, alternatively alkyltriacetoxysilane may be used in place of the alkoxysilanes and acid. If desired, small amounts of dialkyl dialkoxysilane also can be utilized in the reaction mixture.
The temperature of the reaction mixture is maintained at about 20 C. to about 40 C. and pre~erably below 25 C. It has been found that in about six to eight hours sufficient trialkoxysilane has reacted to redu~e the initial two-phase liquid mixture to a single liquid phase in which the silica is dispersed.
In general~ the hydrolysis reaction is allo~ed to continue for a total of about 12 hours to 48 hours, depending upon the desired viscosity of the final product. The more time the hydrolysis reaction is permitted to continue, the higher will be the viscosity of the colloidal silica dispersion.
After hydrolysis has been completed, the solids content is adjusted by the addition of alcohol, preferably isopropanol, to the colloidal silica dispersion. Other suitablP alcohols for this purpose include lower aliphatic alcohols such as methanol, ethanol, isobutanol, isopropanol, n-butyl alcohol and t-butyl alcohol or mixtures thereof.
When it is desirable to use a treated colloidal silica in which the partial ~ondensate is in solutionl the solvent system can contain from about 20 to 75 weight percent alcohol.
The pH of the resultant reacted composition is in the range of from about 3.5 to 8.0j preferably from about 6.6 to about 7.3 or from 3.8 ~Q 5.7. If necessary, a dilute base, such as ammonium hydroxide, or weak acid, such as acetic acid, may be added to the composition to adjust the pH
to the desired range.

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RD-19,818 The acid is used to buf~er the basicity of the initial two liquid phase reaction mixture and thereby also ~emper the hydrolysis rate. ~lacial acetic acid as well as other acids such as organic acids like propionic, butyr~c, citric, ben20ic, formic, oxalic and the like may be used.
Alkyltriacetoxy silanes wherein the alkyl group contains from 1-6 carbon atoms can be used, alkyl gxoups having from 1 to 3 carbon atoms being preferred. ~ethyltxiacetoxysilane is most pre~erred.
The silanetriols, RSi(OH)3, hereinbefore mentioned, are formed as a result of the hydrolysis of the corresponding trialkoxysilanes with the aqueous medium, i.e., the aqueous dispersion of colloidal silica. Exemplary trialkoxysilanes ar~ those containing methoxy, ethoxy, isopropoxy and n-butoxy substituents which, upon hydrolysis form the silanetriol and the corresponding alcohol. If a mixture of trialkoxysilanes is employed, a mixture of different silanetriols, as well as different alcohols, is produced. Upon the production of the silanetriol or mixtures of silanetriols in the basic aqueous medium, condensation of the hydroxyl suhstituents to form --S i--O--S i--bonding occurs. This condensation takes place over a period of time and is not an exhausting condensation but rather the siloxane retains an appreclable quantity o~ silicon-bonded hydroxyl groups which render the polymer soluble in the alcohol-water cosolvent. It is believed that this soluble partial condensate can be characterized aq a siloxanol polymer having at least one silicon~bonded hydroxyl group per e~ery three --ll--RD-19,81R

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units.
The non-volatile solids portion of the treated colloidal silica herein is a mixture of co]Lloidal silica and the partial condensate or siloxanol, of a silanol. The major portion or all of the par~ial condensate or s~loxanol is ob-tained from the condensation o~ me~hyltrihydroxysilane and, depending upon the input of ingredients ~o the hydrolysis reaction, minor portions of partial condensate can be obtained, for example, from the condensation of methyltrihydroxysilane with e~hyltrihydroxysilane, or propyltrihydroxysilane; methyltrihydroxysîlane wi~h C6HsSi(OH)3, or mixtures o~ the foregoing. It is preferred to use all methyltrimethoxysilane, thus producing all methylsi-lanetriol, in preparing the treated colloidal silica compo-sitions which can be dried to form a powder or dissolved in a solvent so the partial condensate is present in an amount of from about 55 to 75 weight percent of the total solids in a cosolvent of alcohol and water~ the alcohol comprising from about 50~ to 95% by weight of the cosolvent.
Silicon-oxy-carbide glass is formed by pyrolysis of siloxanol treated colloidal silica in a non-oxidizing atmosphere at tempera~ures between about 900 C and 1600 C.
Preferably the heating rate during pyrolysis is controlled ~o allow the pyrolysis by-products to escape without leaving voids or bubbles in the silicon-oxy-carbide glass.
Preferably heating rates of less than l C per minute are used. Durlng pyrolysi~, by products evolve and cause a weight loss as the treated iliea densifies. Although the pyrolizing treated silica experiences a weight loss, the density of the pyrolizing treated silica is increasing due to ..;.

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RD-19,818 a reduction in volume of the pyrolizing treated silica. The pyrolysis reactions are essentially completed when a substantially constant weight was achievecl in the pyroli7ing treated silica. A substantially constant weight is generally achieved at about 900 C to 1250 C. Further densification of the pyrolizing treated silica may occur after weight loss has ended~ if heating is continued. Therefore, i.t may be desirable to stop heating and pyrolizing of the treated silica after it has completely densified, or in other words, stops redu~ing in volume. Weight loss during pyrolysis of one treated silica was determined to be about 14 percent.
The glasses of this invention resist devitri-fication, and remain structurally stable at temperatures up to at least 1600-C. The term "structurally stable" refers to a bulk material that retains essentially the same micro-structure from room temperature up to about 1600 C. The formation of small crystallized areas up to about 100 angstroms in an otherwise amorphous matrix have substantially no adverse or deleterious effect on the properties of the bulk material. Therefore, structurally stable glasses of the present invention are essentially amorphous but may contain small crystallized areas of, for example, graphite, cristo-balite, or silicon carbide witAin the glass, or display minor amounts of cristobalite on the surfaces of the glass.
Articles of silicon-oxy-carbide glass can be formed by pulveri~ing the pyrolized treated silica into a powder using grinding mills well known in the art. The silicon-oxy-carbide powder is then consolidated by hot pressing to form an article. One method for hot pressing is to apply a 30 uniaxial pressure of at least about 5 ksi at about 1550C to 1600 C to the powder. The unit ksi is kips per square inch;
the equivalent of 1000 pounds per square inch. Such RD-19,818 pressures and temperatures are sufficient to ~orm a densified article.
Shaped articles can also be formed directly from the treated silic~. First, the treated si.lica is put in solution in a solvent such as isopropanol and then cast into a desired shape. Illustrative of the solv~nts that have been found suitable for placing the treated sil:ica in solution are lower aliphatic alcohols such as methanol, ethanol, isobutanol, n-butyl alcohol and t-butyl alcohol or mixt~res thereof, or preferably isopropanol. Treated silicas can be dissolved in about 20 to 75 weight percent soIvent.
The cast treated silica is dried at room temperature and slowly pyrolized in a non-oxidizing at-mosphere as described herein. Pyrolysis is performed at a low rate of heating that avoids formation of voids and bubbles as gases evolve and cause a weight loss in the treated silica. When the weight of the pyrolizin~ treated silica stabilizes, pyrolysis is complete. Alternatively, the treated silica, which is normally in the form of a powder, can be shaped by hot pressing.
The treated silica in isopropanol solution can also be drawn into ribres. The treated silica solution is treated with a base such as ammonium hydroxide to increase the viscosity to a point where a solid object can be dipped into the solution and withdrawn, pulling a strand of the treated silica from the solution. Fibres can then be drawn or pulled from the treated silica solution by such dipping processes.
Alternatlvely, the treated silica solution can be drawn into a teflon tube with a slight vacuum. As the isopropanol evaporates and the ~reated silica solution increases in viscosity, khe fibre shrinks and is pushed out of the tube.
Fibres are strengthened for easier handling by heating them , .~ ' , . ' .

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RD-19,818 to about 50 C. The fibres are then pyroliz.ed in a non oxidizing atmosphere or a vacuum as described above.
Ceramic composites can be formed having ceramic fibres in a matrix of silicon-oxy-carhide ~lass and ceramic filler. Trea~ed silica is put in solution in a solvent, and ceramic particle~ are dispersed in the solution to form an infiltrant slurry. The particulate ceramic filler con~rols shrinkage of the composite ma~rix during pyrolysis and can be chosen so the matrix is compatible with the fibre reinforcement to be used. Some examples of c~ramic fillers are powdered silicon carbide, diatomaceous earth and the 2Si02-3A1203 aluminosilica~e referred to as mullite.
A ceramic fibre or fibres, or a cloth of the fibres is drawn through an agitated bath of the infiltrant slurry.
Some examples of ceramic fibres are carbon fibre, silicon carbide fibre and alumino-boro-silicate fibres. The im-pregnated fihre is then shaped and dried to allow evaporation of the solvent. One shaping method includes winding an impregnated fibre spirally on a drum to form a panel. Layers of the f.i~re can be consolidated ~hrough the application of heat and pressure to form a continuous treated silica matrix surrounding the ceramic fibres. The composite is then pyrolized in a non~oxidizing atmosphere or a vacuum as described above. The treated silica densifies into a substantially amorphous silicon-oxy-carbide glass that binds the ceramic filler, thus forming a continuous matrix around the fibres. Depending on the pyrolysis temperature used, the ceramic filler may be dispersed, partially sintered or fully sintered within the glass.
Optionally, the ceramic composite can be re-in-fil~rated with the infiltrant slurry to reduce porosity in the composite. The composite is placed in the re-infiltrant :, .

RD-19,818 solution while in a vacuum. Pressure is applied to the solution to force the solu~ion into the pores of the composite. After re-infiltrating, the sol~ent is allowed to evaporate and the re-infiltrated composite is pyrolized in a non-oxidizing atmosphere or vacuum as described above. Re-infiltration and pyrolysis can be repeated as often as needed to achieve the desired degree of density in ~he matri~.
The matrix of amorphous silicon-oxy-carbide glass binding a ceramic filler surrounds and protects the ceramic fibres from decomposition in oxidizing and reducing atmo-spheres at temperatures up to about 160Q-C. It was found that the inert natuxe of silicon-oxy-carbide glass readily accepts ceramic fibres without reacting with them and degrading their properties. As a result, silicon-oxy-carbide glass containing appropriate ceramic fillers can be used as a matrix material for many known ceramic fibres.
The following examples are offered to further il-lustrate the silicon-oxy-carbide glass of this invention and methods for producing the glass and glass articles.

The siloxanol treated colloidal silica composition comprising the partial condensate derived from methyltrimethoxysilane, and colloidal silica in a weight ratio of about 1.8:1 was pyrolized while weight loss from the treated silica was measured by thermal gravimetric analysis.
Thermal grav metric analysis is a method for measuring weight loss from a sample while it is being heated. The material was pyrolized in a hydrogen atmosphere by heating at a rate of lO C/minute to a temperature of 1250-C. The measured weight loss for the silicon-oxy-carbide glass formed after pyrolysis was about 14 percent.
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RD-19,818 The weight loss data from pyrolysis of the treated silica, i5 presented in ~.he graph of Figure 1. In the graph of Figure 1, the percent weight loss in the sample is plotted on the ordinate while the increase in heat:lng temperature is plotted on the abscissa. The graph of Figure 1 shows weight loss is essentially complete in the sample at about 900 C.
Substantially no evidence of crystalliza~ion was found by x-ray diffraction of the pyrolizPd material.
The composition of different glasses can be broadly defined by referring to the amount of each element in the glass. However, it is the shor~-range ordering in glasses that give them their differen~ properties. Therefore, b.y characterizing the short-range ordering in glasses different glass compositions can be defined with respect to properties.
The short range ordering of the silicon-oxy-carbide glass prepared in Example 1 is determined by defining the percentage of each of the polyatomic units;
monocarbosiloxane, dicarbosiloxane, and tetraoxysilicon that are present in the glass.
The 29Silicon solid state nuclear magnetic resonance spectrum of the silicon-oxy-carbide glass prepared above was recorded and is presented in Figure 2. Figure 3 is the 29Silicon nuclear magnetic resonance spectrum from a sample of "Nicalon" silicon carbide fibre. On the ordinate of Figures 2 and 3 is plott~d the intensity of radiation measured from the excited sample, and on the abscissa is plotted the parts per million (ppm~ in chemical shift from a tetramethyl silicon standard that fixes the zero point on the abscissa. The chemical shift in ppm are known for many polyatomic units, for example ~etraoxysilicon, dicarbosiloxane and monocarbosiloxane are shown in; "NMR
Basic Principles and Pro~ress 29Si-NMR Spetroscopic Results", ,.~ .

llL15 RD-19,818 Editors P. Diehl, R. Kosfeld, Springer Verlag Berlin Heidelberg 1981 at pp. 186, 184 and 178. Therefore, each peak in Figures 2 and 3 defines the short-range ordering of specific silicon polyatomic units.
The spectra of the silicon-oxy-carbide glass in Figure 2 contains peaks labeled 1 through 3. Peak 1 is dicarbosiloxane, peak 2 is ~onocarbosiloxane, and peak 3 is tetraoxysilicon. By integrating the area under each peak, the fraction of each of these polyatomic units that is present in the glass can be determined. A correction for background interference was made to the spectra in Figures 2 and 3 before determining the integrated area under each peak.
The integrated area under each peak in Figure 2 reveals a composition for the silicon-oxy-carbide glass prepared as described above comprising, up to about 8 weight percent dicarbosiloxane, about 11 to 21 weight percent mono-carbosiloxane, and about 76 to 86 wei~ht percent tetraoxysilicon. ~nalysis of the nuclear magnetic resonance spectra and the black appearance of the glass indicates that at least about 3 weight percent of elemental carbon is dispersed in ~he glass either atomically or in small clusters.
Treated colloidal silicas having various ratios of siloxanol to colloidal silica can be pyroliz-ed to form silicon-oxy-carbide glasses. It is expected that treated colloidal silicas having a ratio of siloxanol to colloidal silica that is greater than 1.8:1 will have a decreased tetraoxysilicon content that is in the lower part of the 76 to 86 percent range described above, or lower. Conversely treated colloidal silicas having a ratio of siloxanol to colloidal silica that is less than 1.8:1 are expected to have ~ '; . '' -:::

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RD-19,818 an increased tetrao~ysilicon content that is in the upper part of the 76 to 86 percent range or higher.
The spectra in Figure 2 can be compared to the silicon carbide spectra in FIG. 3 measured from a 'INicalon"
silicon carbide fibre sample. The composi~ion for "Nicalon"
in FIG. 3, in weight percent, is about 68 percent silicon carbide/ about 8 percent dicarbosiloxane~ about 17 percent monocarbosiloxane, and about 7 percènt tetraoxysilicon. From the spectra in FIG. 3, it can be seen that "Nicalon" fibres are comprised principally of silicon carbide with minor amounts of dicarbosiloxane, monocarbosiloxane, and tetraoxysilicon. In contrast, the spectra of FIG. 2 shows that silicon-oxy-carbid~ glass is comprised of substantial amounts of dicarbosiloxane, monocarbosiloxane, and tetraoxysilicon. This unique short range ordering of silicon-oxy-carbide glass that bonds carbon to silicon in a heretofore unknown manner in glasses, provides the increased devitrification and decomposition resistance and characterizes the glasses of this invention.
The composition of the silicon-oxy-carbide glass sample and Nicalon sample can also be described by referring to the mole percent of each polyatomic unit. Table I helow provides the conversion be~ween mole percent and weight percent for each of these compositions.

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~etraoxy3ilicon 76-86 74 84 7 5 Monocarbo lloxane 11-21 13-23 17 13 Dicarb~iloxaneup to 8 up to 8 ~ 7 Tetracarbo~ilicon - ~68 75 The mole percent gives the percentage of each polyatomic unit in the samples on a molecular basis. The percentage of the silicon atoms in the samples that is bonded to oxygen or carbon can then be determined using the mole percent. The silicon-oxy-carbide glass sample from Example 1, has about 16 to 26 percent of the silicon atoms in the glass bonded to at least an individual carbon atom. The "Nicalon" silicon carbide sample had about 90 to 100 percent of the silicon atoms in the silicon carbide sample bonded to carbon.

Claims

1. A process for forming a glass composition, comprising:
pyrolizing a siloxanol treated colloidal silica in a non-oxidizing atmosphere by heating for a period of time sufficient to form a silicon-oxy-carbide glass that remains structurally stable at temperatures up to 1600°C.

2. A process for forming a glass composition, comprising:
pyrolizing a siloxanol treated colloidal silica in a non-oxidizing atmosphere by heating for a period of time sufficient to form a silicon-oxy-carbide glass that remains structurally stable at temperatures of 1250°C or greater.

3. The process of claim 2 wherein pyrolysis is achieved by heating from about 900°C to 1600°C.

4. The process of claim 3 wherein heating is performed for a period of time ending when weight loss from the pyrolizing treated colloidal silica substantially stabilizes.

5. The process of claim 3 wherein heating is performed for a period of time that allows the treated silica to fully densify.

6. The process of claim 2 wherein pyrolysis is performed in a hydrogen gas atmosphere.

7. The process of claim 2 wherein said siloxanol treated colloidal silica is a dispersion of colloidal silica, having a particle size between about 5 to 150 millimicrons, in a partial condensate of a silanol of the formula RSi(OH)3, where R is a monovalent hydrocarbon radical from about 1 to 12 carbon atoms.

8. The process of claim 2 wherein said siloxanol treated colloidal silica is a dispersion of colloidal silica, having a particle size between about 5 to 150 millimicrons, in a partial condensate of a silanol of the formula RSi(OH)3, where R is a methyl group.

9. The process of claim 7 wherein said partial condensate is formed by hydrolysis of methyltrimethoxysilane and the ratio of partial condensate to colloidal silica is about 1.8:1.

10. A glass composition comprising chemically bonded silicon, oxygen, and carbon with the glass substantially free of chemical bonding between oxygen and carbon atoms; the glass being produced by a process comprising, pyrolizing a siloxanol treated colloidal silica in a non-oxidizing atmosphere by heating for a period of time sufficient to form a silicon-oxy-carbide glass that remains structurally stable at temperatures up to 1600°C.

11. A glass composition comprising chemically bonded silicon, oxygen, and carbon with the glass substantially free of chemical bonding between oxygen and carbon atoms; the glass being produced by a process comprising, pyrolizing a siloxanol treated colloidal silica in a non-oxidizing atmosphere by heating for a period of time sufficient to form a silicon-oxy-carbide glass that remains structurally stable at temperatures of about 1250°C or greater.

12. The glass of claim 11 wherein said siloxanol treated colloidal silica is a dispersion of colloidal silica, having a particle size between about 5 to 150 millimicrons, in a partial condensate of a silanol of the formula RSi(OH)3, where R is a monovalent hydrocarbon radical from about 1 to 12 carbon atoms.

13. The process of claim 11 wherein said siloxanol treated colloidal silica is a dispersion of colloidal silica, having a particle size between about 5 to 150 millimicrons, in a partial condensate of a silanol of the formula RSi(OH)3, where R is a methyl group.

14. The glass of claim 11 wherein said partial condensate is formed by hydrolysis of methyltrimethoxysilane and the ratio of partial condensate to colloidal silica is about 1.8:1.

15. A glass that remains structurally stable at temperatures of about 1250°C or greater, comprising polyatomic units of silicon, oxygen, and carbon, in weight percent, of about 11 to 21 percent monocarbosiloxane, up to about 8 percent dicarbosiloxane, about 76 to 86 percent tetraoxysilicon, with at least about 3 percent elemental carbon dispersed in the glass.

16. A glass that remains structurally stable at temperatures up to about 1600°C, comprising polyatomic units of silicon, oxygen, and carbon, in weight percent, of about 11 to 21 percent monocarbosiloxane, up to about 8 percent dicarbosiloxane, about 76 to 86 percent tetraoxysilicon, with at least about 3 percent elemental carbon dispersed in the glass.

17. A glass that remains structurally stable at temperatures of about 1250°C or greater, comprising silicon, oxygen and carbon in a mass of silicon-oxy-carbide glass wherein about 16 to 26 percent of the silicon atoms are each bonded to at least an individual carbon atom.

18. The invention as defined in any of the preceding claims including any further features of novelty dislcosed.