GB2238307A

GB2238307A - Preparation of silicon-oxy carbide glasses from siloxanol treated colloidal silica

Info

Publication number: GB2238307A
Application number: GB9023473A
Authority: GB
Inventors: Judith Stein; Larry Neil Lewis; Gary Mats Renlund
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1989-11-20
Filing date: 1990-10-29
Publication date: 1991-05-29
Also published as: IT9022117A0; JPH03197334A; FR2654718A1; DE4034996A1; CA2021614A1; IT9022117A1; GB9023473D0; IT1243923B

Abstract

Siloxanol treated colloidal silicas are pyrolized in a nonoxidizing 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-oxycarbide glasses of this invention resist devitrification and decomposition in oxidizing or reducing atmospheres at temperatures up to about 1600 DEG C. The glasses contain at least 3% elemental carbon.

Description

1 1 i -1- 3 G1 PRZPARATION or sizicoN-oxy-c.A"iDz GLAsszs rRom SILOXAMOL

TREATED COLLOIDAL SILICA 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.

is Vitreous silica is a refractory glass, however, it devitrifies at about 1100C. 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 silica, 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 1100C 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 it can react with oxygen to form carbon monoxide.

11/15/89 RD-19,818 The thermodynamics of silicon-. and oxygen reactions is discussed in "The High-Temperature Oxidation, Reduction, and Volatilization Reactions of Silicon and Sili con Carbide", Gulbransen, E.A., and Jansson, S.A. oxidation of Metals, Volume 4, Number 3, 1972. The thermodynamic analysis of Gulbransen et al. shows that at 1200C silica and carbon should form gaseous silicon monoxide and carbon monoxide or solid silicon carbide, SiC. However, 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 1125C 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.

There is a material described as carbon modified vitreous 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 1200C 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 structurer 0 0 C=0 11/15/89 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 resistance to devitrification over conventional silica glass which begins 5 to devitrify at about 1100C while black glass begins to devitrify at about 1250C. The increased thermal stability of black glass allows it tobe 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@) 11, Mah, T., et al., Journal of Material Science, Vol. 19, pp. 1191-1201 (1984). Mah et al. found that regardless of the environmental conditions during heat treatment, the "Nicalon" fibre strength degraded when the fibres were subjected to temperatures greater than 1200C. The fibre degradation was associated with loss of carbon monoxide from the fibres and beta-silicon carbide grain growth in the fibres.

Ceramic materials generally exhibit brittle behavior as characterized by their high strength and low fracture toughness. Fracture 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. "Nicalon" is an excellent ceramic fibre but it degrades at temperatures above 1200C. Integrating "Nicalon" fibres in a protective ceramic matrix 11/15/89 RD-191 818 1 having desirable mechanical properties and capable of withstanding temperatures substantially higher than 1200C, 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 tontaining silicon, oxygen, and carbon, are degraded for example by decomposition of silicon carbide or devitrification of conventional glas.

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 1600C.

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.

It has been found that siloxanol treated colloidal silicas can be pyrolized in a Aon-oxidizing atmosphere to form unique glass compositions. Surprisingly,it has been found that siloxanol treated colloidal silicas pyrolized in a non- oxidizing atmosphere below about 1600C do not form silica, cristobalite, silicon carbide, carbon monoxide, or mixtures of silica and elemental carbon.

1 J 11/15/89 RD-1 9, 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 atoms are chemically bonded to silicon atoms. These glasses resist crystallization, and decomposition in oxidizing or reducing atmospheres at temperatures up to about 160CC. 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 detectable 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 treated 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 atmosphere that removes pyrolysis by-products from the pyrolizing resin without influencing the reactions occurring during pyrolysis. Examples of non-oxidizing atmospheres include a vacuum of less than about 10-4 atmospheres, inert atmospheres like helium, argon, or nitrogen, and reducing atmospheres, such as hydrogen.

As referred to herein, a siloxanol treated colloidal silica comprises a dispersion of colloidal silica 11/15/89 RD-19, 818 in a partial condensate of a silanol of the formula Mi(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 weight 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 silica.

Pyrolysis of the treated silica forms a siliconoxy-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 unit, 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 two oxygen atoms and two carbon atoms. In a fourth unit, herein referred to as tetracarbosilicon, a silicon atom is bonded to four carbon atoms.

When a treated silica comprised of the partial condensate of methyltrimethoxysilane and colloidal silica in a ratio of about 1.8:1 is pyrolized, a silicon-oxy-carbide glass is formed having a distribution 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 1 1 1 1 11/15/89 RD-19, 818 silicon-oxygen bonds with a small and insignificant number of bonds between carbon and oxygen atoms.

The invention will now be described, by way of exwple only, with reference to the accenpanying drawings in which:- Figure 1 is a graph showing the weight lost during pyrolysis of a siloxanol treated colloidal silica.

Figure 2 is a graphical presentation of the 29Silicon nuclear magnetic resonance spectrum of the siliconoxy-carbide glass formed by pyrolizing a siloxanol treated colloidal silica.

Figure 3 is a graphical presentation of the 29Silicon nuclear magnetic resonance spectrum of 11Nicalonll silicon carbide.

Glasses are generally formed from an extremely viscous supercooled liquid and possess a polymerized network structure with short-range order. The glasses of 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 glas ' ses of this invention are formed by pyrolizing a siloxanol treated colloidal silica in a non-oxidizing atmosphere. However, 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 -B- 11/15/89 RD-19, 818 dispersion of colloidal silica in the partial condensate of a silanol of the formula RSI (OH) 3, where R is a monovalent hydrocarbon radical from 1 to 12 carbon atoms such as a halohydro carbon r C(1-12) alkyl, C(6-12) aryl, or an ester such as methacrylate or acrylate; at least 70 weight percent of 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 8.0.

The treated colloidal silica can be prepared by hy- drolyzing a trialkoxysilane or a mixture of trialkoxysilanes of the formula RSi(OR1)3, wherein R is as previously defined, and R' is C(1-8) 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 150 millimicrons in diameter. These silica dispersions are well known in the art and commercially available ones include, for example, those sold under the trademark of Ludox (DuPont) and Nalcoag (NALCO Chemical Co.). Such colloidal silicas are available as both acidic and basic hydrosols. Colloidal silicas having an average particle 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 colloidal silica composition, the aqueous colloidal silica dispersion is added to an alkyltrialkoxysilane or aryltrialkoxysilane which may -g- t 11115/89 RD-19, 818 contain a buffering 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 20C. to about 40C. and preferably below 25C. It has been found that in about six to eight hours sufficient trialkoxysilane has reacted to reduce the initial two-phase liquid mixture to a single liquid phase in which the silica is dispersed.

In general, t:he hydrolysis reaction is allowed 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 suitable alcohols for this purpose include lower aliphatic alcohols such as methanol, ethanolf 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 condensate is in solution, 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 8.0. preferably from about 6.6 to about 7. 8 or from 3.8 to 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.

11/15/89 RD-19P 818 The acid is used.to buffer the basicity of the initial two liquid phase reaction mixture and thereby also temper the hydrolysis rate. Glacial acetic acid as well as other acids such as organic acids like propionic, butyric, 5 citric, benzoic, formic, oxalic-and the like may be used. Alkyltriacetoxy silanes wherein the alkyl group contains from 1-6 carbon atoms can be used, alkyl groups having from 1 to 3 carbon atoms being preferred. Methyltriacetoxysilane is most preferred.

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 are 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 substituents to form 1 1 -si-o-si- 1 1 bonding occurs. This condensation takes place over a period of time and is not an exhausting condensation but rather the siloxane retains an appreciable quantity of 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 as a siloxanol polymer having at least one silicon-bonded hydroxyl group per every three t 1 1 f 1 -sio- 1 11/15/89 RD-19,818 units.

The non-volatile solids portion of the treated colloidal silica herein Is a mixture of colloidal silica and the partial condensate or siloxanol, of a silanol. The major portion or all of the partial condensate or siloxanol is ob tained from the condensation of methyltrihydroxysilane and, depending upon the input of ingredients to the hydrolysis reaction, minor portions of partial condensate can be obtained, for example, from the condensation of methyltrihydroxysilane with ethyltrihydroxysilane, or propyltrihydroxysilane; methyltrihydroxysilane with COSSi(OH)3, or mixtures of the foregoing. It is preferred to use all methyltrimethoxysilaner thus producing all methylsilanetriol, in preparing the treated colloidal silica compositions 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 temperatures between about 900C and 1600C.

Preferably the heating rate during pyrolysis is controlled to allow the pyrolysis by-products to escape without leaving voids or bubbles in the silicon-oxy-carbide glass.

Preferably heating rates of less than VC per minute are used. During pyrolysis, by-products evolve and cause a weight loss as the treated silica densifies. Although the pyrolizing treated silica experiences a weight loss, the density of the pyrolizing treated silica is increasing due to 1 11/15/89 RD-19, 818 a reduction in volume of the pyrolizing treated silica. The pyrolysis reactions are essentially completed when a substantially constant weight was achieved in the pyrolizing treated silica. A substantially constant weight is generally achieved at about 900'C to 1250C. Further densification of the pyrolizing treated silica may occur after weight loss has ended, if heating is continued. Therefore, it may be desirable to stop heating and pyrolizing of the treated silica after it has completely densified, or in other words, stops reducing in volume. Weight loss during pyrolysis of one treated silica was determined to be about 14 percent.

The glasses of this invention resist devitrification, and remain structurally stable at temperatures up to at least 160CC. The term "structurally stable" refers to a bulk material that retains essentially the same microstructure from room temperature up to about 1600C. 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, cristobalite, or silicon carbide within the glass, or display minor amounts of cristobalite on the surfaces of the glass.

Articles of silicon-oxy-carbide glass can be formed by pulverizing 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 uniaxial pressure of at least about5ksi (34.Snpa) at about 1550 to 1600C to the powder. The unit ksi is kips per square inch; the equivalent of 1000 pounds per square inch (or 6.9mpa). Such 11/15/89 RD-19, 818 pressures and temperatures are sufficient to form a densified article.

Shaped articles can also be formed directly from the treated silica. First, the treated silica is put in solution in a solvent such as isopropanol and then cast into a desired shape. Illustrative of the solvents that have been found suitable for placing the treated silica in solution are lower aliphatic alcohols such as methanol, ethanol, isobutanol, n-butyl alcohol and t-butyl alcohol or mixtures thereof, or preferably isopropanol. Treated silicas can be dissolved in about 20 to 75 weight percent solvent.

1 The cast treated silica is dried at room temperature and slowly pyrolized in a non-oxidizing atmosphere 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 pyrolizing treated silica stabilizes, pyrolysis is complete. Alternatively, the treated silicar 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 fibres. 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.

Alternatively, the treated silica solution can be drawn into - (I-J- -709) a 7eflonLube with a slight vacuum. As the Isopropanol evaporates and the treated silica solution increases in viscosity. the fibre shrinks and is pushed out of the tube. Fibres are strengthened for easier handling by heating them 1 11/15/89 RD-19, 818 to about-50'C. The fibres are then pyrolized in a nonoxidizing atmosphere or a vacuum as described above.

Ceramic composites can be formed having ceramic fibres in a matrix of silicon-oxy-carbide glass and ceramic filler. Treated silica is put in solution in a solvent, and ceramic particles are dispersed in the solution to form an infiltrant slurry. The particulate ceramic filler controls shrinkage of the composite matrix during pyrolysis and can be chosen so the matrix is compatible with the fibre reinforcement to be used. Some examples of ceramic fillers are powdered silicon carbide, diatomaceous earth and the 2SiO2-3Al2O3 aluminosilicate 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 impregnated fibre 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 fibre can be consolidated through the application of heat and pressure to form a continuous treated silica matrix surrounding the cqramic fibres. The composite is then pyrolized in a non-oxidizing atmosphere or a vacuum as described above. The treated silica densifies into a substaniially 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 filtrated with the infiltrant slurry to reduce porosity in the composite. The composite is placed in the re-infiltrant 4 J,J./ 1 s 18 9 RD19, 818 solution while in a vacuum. Pressure is applied to the solution to force the solution into the pores of the composite. After re-infiltrating, the solvent is allowed to evaporate and the re-infiltrated composite is pyrolized in a non-oxidizing atmosphere or vacuum as described above. Reinfiltration and pyrolysis can be repeated as often as needed to achieve the desired degree of density in the matrix.

The matrix of amorphous silicon-oxy-carbide glass binding a ceramic filler surrounds and protects the ceramic fibres from decomposition in oxidizing and reducing atmospheres at temperatures up to about 1600C. It was found that the inert nature 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 illustrate the silicon-oxycarbide glass of this invention and methods for producing the glass and glass articles.

ZXA= 10 1 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 gravimetric 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 ICC/minute to a temperature of 1250C. The measured weight loss for the silicon-oxy-carbide glass formed after pyrolysis was about 14 percent.

i 11 115/89 RD-19, 818 The weight loss data from pyrolysis of the treated silica, is presented in the 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 heating temperature is plotted on the abscissa. The graph of Figure I shows weight loss is essentially complete in the sample at about 900C. Substantially no evidence of crystallization was found by xray diffraction of the pyrolized 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 short-range ordering in glasses that give them their different properties. Therefore, by 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 plotted 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 30 polyatomic units, for example tetraoxysilicon, dicarbosiloxane and monocarbosiloxane are shown in; uNMR Basic Principles and Progress 29Si-NMR Spetroscopic Results", R_ 11/15189 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 monocarbosiloxane, 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 monocarbosiloxane, and about 76 to 86 weight percent tetraoxysilicon. Analysis 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 the glass either atomically or in small clusters.

Treated colloidal silicas having various ratios of siloxanol to colloidal silica can be pyrolized to form silicon-oxy-carbide glasses. it isexpected 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 f 11/15/89 RD-19, 818 an increased tetraoxysilicon 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 11Nicalonll silicon carbide fibre sample. The composition 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 percent tetraoxysilicon. From the spectra in FIG. 3, it can be seen that 11Nicalonll 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-carbide glass is comprised of substantial amounts of dicarbosiloxane, monocarbosiloxane, and is 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 below provides the conversion between mole percent and weight percent for each of these compositions.

j -t X 11 Table TT

11/15/89 RD-19, 818 Silicon-Oxy-Carbide Glass "Nicalon" sir xt.,1 M01P% xt,l Mole% Tetraoxyallicon 76-86 74-84 7 5 Monocarboziloxane 11-21 13-23 17 13 Dicarbosiloxane up to a up to a 8 7 Tetracarbosilicon - 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 20 glass bonded to at leas t an individual carbon atom. The 11Nicalonll silicon carbide sample had about 90 to 100 percent of the silicon atoms in the silicon carbide sample bonded to carbon.

i -

Claims

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 siliconoxy-carbide glass that remains structurally stable at temperatures up to 16000C.
2. A process as claimed in claim 1 wherein the silicon-oxy-carbide glass remains structurally stable at temperatures up to 12500C.
3. A process as claimed in claim 1 or claim 2 wherein pyrolysis is achieved by heating from 9000C to 16000C.
4. A process as claimed in any of the preceding claims wherein heating is performed for a period of time ending when weight loss from the pyrolizing treated colloidal silica substantially stablizes.
5. A process as calimed in any of the preceding claims wherein heating is performed for a period of time that allows the treated silica to fully densify.
6. A process as claimed in any of the preceding claims wherein pyrolysis is performed in a hydrogen gas atmosphere.
7. A process as claimed in any of the preceding claims wherein said siloxanol treated colloidal silica is a dispersion of colloidal silica, in a partial condenstate of a silanol of the formula RSi(OH)3, where R is a monovalent hydrocarbon radical from 1 to 12 carbon P69622.DOC 23-Oct-90 t 1 A atoms.
8. A process as claimed in any of the preceding claims wherein said siloxanol treated colloidal silica is a dispersion of colloidal silica, having a particle size between 5 to 150 millimicrons, in a partial condensate of a silanol of the formula RSi(OH)3, where R is a methyl group.
9. A process as claimed in any of the preceding claims 1 to 7 wherein said partial condensate is formed by hydrolysis of methyltrimethoxysilane and the ratio of partial condensate to colloiddal silica is 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 siliconoxy-carbide glass that remains structurally stable at temperatures up to 16000C.
11. A glass composition as claimed in claim 10 wherein the silicon-oxycarbide glass remains structurally stable at temperatures of 12500C or greater.
12. A glass as claimed in claims 10 or 11 wherein said siloxanol treated colloidal silica is a dispersion of colloidal silica, 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. A process as claimed in claim 10 or 11 wherein said siloxanol treated colloidal silica is a dispersion of colloidal silica, having a particle size between 5 to 150 millimicrons, in a partial condensate of a P69622.DOC 23-Oct-90 silanol of the formula RS'(OH)3, where R is a methyl group.
14. A glass as claimed in any of the preceding claims 10 to 13 wherein said partial condensate is formed by hydrolysis of methyltrinethoxysilane and the ratio of partial condensate to colloidal silica is 1.8:1.
15. A glass that remains structurally stable at temperatures up to 16000C, comprising polyatomic units of silicon, oxygen, and carbon, in weight percent, of 11 to 21 percent monocarbosiloxane, up to 8 percent dicarbosiloxane, 76 to 86 percent tetraoxysilicon, with at least 3 percent elemental carbon dispersed in the glass.
16. A glass as claimed in claim 15 that remains structurally stable at temperatures of 12500C or greater.
17. A glass as claimed in claim 16 that remains structurally stable at temperatures of 12500C or greater, comprising silicon, oxygen and carbon in a mass of silicon-oxy-carbide glass wherein 16 to 26 percent of the silicon atoms are each bonded to at least an individual carbon atom.
18. A process for forming a glass composition as claimed in claim 1 substantially as hereinbefore described in the Example.
19. A glass composition when produced by a process as claimed in any one of claims 1 to 9 and 18.
20. A glass composition as claimed in claim 10 substantially as hereinbefore described in the Example.

P69622.DOC 23-Oct-90 published 1991 atIbe Patent Office. State House. 66/71 High Holborn. London WCIR47P. Further copies may be obtained from Sales Branch. Unit 155. Nine Mile Point Cwmfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques ltd. St Mary Cray. Kent.

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