CA2010675A1

CA2010675A1 - Silicon-oxy-carbide glass and articles

Info

Publication number: CA2010675A1
Application number: CA002010675A
Authority: CA
Inventors: Gary M. Renlund
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1989-06-01
Filing date: 1990-02-22
Publication date: 1990-12-01
Also published as: IT9020456A0; DE4016569A1; GB2232979A; JPH0365527A; FR2647777A1; IT1248657B; GB9009108D0; IT9020456A1

Abstract

RD-17,455 SILICON-OXY-CARBIDE GLASS
AND ARTICLES
Abstract Methyl-silicone resins are pyrolized in a non-oxi-dizing atmosphere to form a glass comprised of silicon, oxy-gen and carbon in a mass of silicon-oxy-carbide glass, wherein about 52 to 66 percent of the silicon atoms are bonded to at least one carbon atom, and about 3 to 9 weight percent carbon is dispersed atomically or in small clusters within the glass matrix. The silicon-oxy-carbide glasses of this invention resist devitrification and decomposition in oxidizing or reducing atmospheres at temperatures up to at least 1650°C. Methods for forming silicon-oxy-carbide glass articles are disclosed, along with specific methods for form-ing silicon-oxy-carbide glass fibres and composites.

Description

2~)1067~
RD-1~,455 SILICON-OXY-CARBIDE GLASS
AND AR~IC~ES

B~sk9~=L~s- of t~ Inv~ntio~
The present invention relates to glass compositions and in particular to glass compositions comprising silicon, oxygen, and carbon.
Amorphous silica is a refractory glass, however, it devitrifies readily at temperatures greater than 1100 C. De-vitrification refers to the ordering or crystallization of the random structures that glasses are made of. Crystal-lization drastically reduces one of vitreous silicas predom-inant attributes, i.e., its low thermal expansion, as well as many of its other desirable properties. As a result, much research has been directed to seeking ways to increase the resistance to devitrification in silica glass compositions.
Reactions between silicon, carbon, and oxygen have been studied extensively. Some of the known reactions in a silicon, carbon, and oxygen system include oxygen combining with silicon to form silica, SiO2. Then at temperatures in excess of 1100 C silica begins to crystallize to form cristo-balite. Cristobalite is one of the common mineral forms of silica. Carbon can react with available silica to form crys-talline silicon carbide or escape as carbon monoxide gas.Any carbon remaining as elemental carbon readily oxidizes above 600 C when exposed to air.
The thermodynamics of silicon, carbon 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 RD-17,455 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 monoxide and carbon mono-xide or solid silicon carbide, SiC. ~owever, no material containing silicon, oxygen and carbon would be expected to form Gulbransen et al. conclude that silica was not recom-mended for use in reducing atmospheres above 1125-C due to the formation of volatile silicon monoxide gas. Also silicon carbide was not recommended for use in oxygen containing en-vironments where active oxidation may occur due to oxidationof the silicon carbide.
There is a material functionally described as car-bon 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. ~n U.S patent 3,378,431. Carbonaceous organics such as carbowax are added to sil~ca and the mixture is hot pressed at about 1200-C to form black glass. Smith, C.F., Jr. has further characterized black qlass by infrared spectroscopy in "The Vibrational Spectra of High Purity a~d Chemically Sub-stituted Vitreous Silicas", PhD Thesis, Alfred University, Alfred, N.Y., May 1973. Smith discloses that in addition to elemental carbon dispersed in the glass, carbon in black glass is associated with oxygen in carbonato type groups. A
carbonato group is the description of a particular way that a carbon atom bonds with three oxygen atoms and has the struc-ture, C=0 The mechanical strength of black glass is similar to the strength of carbon free silica glass, however black ~3 ~ 201~675 RD-17,455 glass has an increased resistance 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 temperatures higher than vitreous silica can withstand.
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., 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 1200-C. 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 be-havior as characterized by their high strength and low frac-ture toughness. Fracture toughness is the resistance tocrack 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 1200-C.
Integrating "Nicalon" fibres in a protective ceramic matrix having desirable mechanical properties and capable of with-standing temperatures substantially higher than 1200-C, would 20~06~5 RD-17,455 be one way of forming an improved ceramic composite.
However, from the discussion above, it is apparent that the properties of known ceramic or glass compositions, and specifically those containing silicon, oxygen and carbon, are degraded by decomposition or devitrification of the glass or ceramic at temperatures above 1100 to 1250-C.
Therefore, it is an object of this invention to form a glass, comprising silicon, oxygen and carbon wherein a substantial portion of the carbon atoms are bonded to silicon atoms and the remaining carbon is elemental carbon dispersed in the glass matrix. Such glass compositions remain struc-turally stable and do not decompose in oxidizing or reducing atmospheres at temperatures up to at least 1650-C.
Another object of this invention is a process for formlng such a glass comprised of silicon, oxygen and carbon by pyrolizing methyl silicone resins.
Still another object of this invention is processes for forming such a glass comprised of silicon, oxygen and carbon into articles.
Brief Descri~tion of th~ Invention I have found that some silicone resins can be py-rolized in a non-oxidizing atmosphere to form unique glass compositions. Surprisingly, I have found that these silicone resins when pyrolized in a non-oxidizing atmosphere do not form silica, cristobalite, silicon carbide, carbon monoxide or mixtures of silica and carbon.
Glasses of this invention are made by pyrolizing a methyl silicone resin to form a glass composition, comprising silicon, oxygen, and carbon wherein a significant portion of ~5 ~ 20~0675 RD-17,455 the carbon atoms are chemically bonded to silicon atoms.
According to one method of this invention a methyl silicone resin is heated in a non-oxidizing atmosphere to pyrolize the resin. As used herein, a non-oxidizing atmosphere is an atmosphere that will remove reaction products from the pyrolizing resin without influencing the reactions occurring during pyrolysis. Examples of such non-oxidizing atmospheres are inert atmospheres like helium, argon, or nitrogen, and reducing atmospheres such as hydrogen. ~yrolysis can also occur in a vacuum having a pressure below about 10-4 atmospheres if the resin is crosslinked prior to pyrolysis.
Methyl silicone resins suitable for use in the method of this invention can be prepared by the method described in U.S. Patent 4,026,868 which is incorporated by reference herein.
Methyl silicone~ are made up of siloxane chains with methyl groups attached to the silicon atoms.
Siloxane chains contain an alternating linkage of silicon and oxygen atoms to form the structure;

--si -- o -- si--Several combinations of methyl groups can be present on the siloxane chains to form polymethylpolysiloxanes.
The basic unit structures in polymethylpolysilox-anes are trimethylsiloxy, dimethylsiloxy, and monomethyl siloxane. The trimethylsiloxy monofuctional unit at the end of a siloxone chain has the structure;

-6 - 20~06~
RD-17,455 I

CH3 - 5i - O -I

Dimethylsiloxy is a difunctional unit that builds chains or rings and has the structure;

I
- O - Si - O -I

Monomethyl siloxane is a trifunctional unit and not only extends siloxane chains but also crosslinks between chains and has the structure;

I

-- O -- si -- o --o I

Methyl silicone resins may also contain unsub-stituted tetrafunctional units having the structure;

RD-17,455 -- O -- si -- o --o Polymeric structures can be built from these unit structures to form polymethylpolysiloxanes having a desired number of methyl groups per silicon atom. By varying the ratio of methyl groups to silicon atoms different methyl silicone resins are formed having more or less organic sub-stituent, the organic substituent being the methyl groups.
Methyl silicone resins generally contain a ratio of methyl groups to silicon atoms of about 2:1 or less. The methyl silicone resin used in this invention consists of, by weight percent, about 5 percent dimethylsiloxy and about 95 percent monomethylsiloxy, and is hereafter referred to and claimed as a methyl silicone precursor resin or sometimes as the pre-cursor resin or resin.

During pyrolysis, the resin densifies as gases areevolved causing a weight loss from the resin. The pyrolysis reactions are completed when a substantially constant weight was achieved in the pyrolizing resin. Weight loss during pyrolysis was determined to be from about 11 to 35 percent.
It was found that the methyl silicone precursor resins could be pyrolized at temperatures ranging from about 900 to 1600-C.

Glasses formed by the method of this invention possess unique properties and characteristics. These glasses both resist crystallization and do not decompose in oxidizing RD-17,455 or reducing atmospheres at temperatures up to at least 1650 C. In addition, a significant 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 car-bonato 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 un-bonded element in the silica matrix or in carbonato groupswhere carbon is bonded with oxygen. Glasses formed by the method of this invention and characterized by such unique properties are herein referred to and claimed as silicon-oxy-carbide glass.
Pyrolysis of the methyl silicone precursor resin forms a silicon-oxy-carbide glas~ that is characterlzed by a continued sharing of electrons between atoms of silicon, oxy-gen and carbon. In silicon-oxy-carbide glass silicon atoms are present in four polyatomic units. In one unit herein re-ferred 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 re-ferred to as tetracarbosilicon, a silicon atom is bonded to four carbon atoms. The pyrolized precursor resin forms a glass having a distribution of these polyatomic units in a matrix, comprising in weight percent about 34 to 44 percent tetraoxysilicon, about 19 to 29 percent monocarbosiloxane, about 17 to 27 percent dicarbosiloxane, up to about 6 percent tetracarbosilicon, and about 3 to 9 percent elemental carbon dispersed atomically or in small clusters within the glass - 9 201067~
RD-17,455 matrix. These units are linked primarly by silicon-oxygen bonds with a small and insignificant number of bonds between carbon and oxygen atoms.
This glass can alternatively be described as a com-position of silicon, oxygen, and carbon in a mass of silicon-oxy-carbide glass wherein about 56 to 66 percent of the sill-con atoms are bonded to at least one carbon atom, and about 3 to 9 weight percent carbon is present as elemental carbon dispersed atomically or in small clusters within the glass matrix.
Articles of silicon-oxy-carbide glass can be formed by pulverizing the pyrolized resin into a pGwder. The sili-con-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 about 5 ksi at about 1550- to 1650'C to the powder. Such pressures and temperatures are sufficlent to form a densified article.
Shaped articles can also be formed directly from the methyl silicone precursor resin. First the resin was crosslinked by dissolving the resin in a solvent such as toluene and then adding a curing agent such as gamma amino propyl triethoxy silane. The solution was cast into a de-sired shape and dried and cured at room temperature. The crosslinked resin was slowly pyrolized in a non-oxidizing at-mosphere as described herein. Pyrolysis is performed at alow rate of heating that avoids formation of voids and bubbles as gases evolve and cause a weight loss in the resin.
When the weight of the pyrolizing resin stabilizes, pyrolysis is complete. The crosslinked resin densifies to form a sili-con-oxy-carbide glass having a distribution of polyatomic units, comprising in weight percent about 38 to 48 percent tetraoxysilicon, about 11 to 21 percent monocarbosiloxane, - 1 o - 20~0675 RD-17,455 about 11 to 21 percent dicarbosiloxane, about 12 to 22 per-cent tetracarbosilicon, and about 3 to 9 percent elemental carbon dispersed atomically or in small clusters within the glass matrix. The silicon-oxy-carbide glass formed from a crosslinked precursor resin is herein referred ~o as crosslinked resin silicon-oxy-carbide glass.
Crosslinked resin silicon-oxy-carbide glass can alternatively be described as a composition of silicon, oxygen, and carbon in a mass of silicon-oxy-carbide glass wherein about 52 to 62 percent of the silicon atoms are bonded to at least one carbon atom, and about 3 to 9 weight percent carbon is present as elemental carbon dispersed atomically or in small clusters within the glass matrix.
The methyl silicone precursor resin can be crosslinked to any partial degree of the fully crosslinked state. Such partially crosslinked resins can be pyrolized according to the method of this invention to form silicon-oxy-carbide glass compositions intermediate to the composi-tions described above. Therefore, silicon-oxy-carbide glasses can be formed having a distribution of polyatomic units, comprising in weight percent about 34 to 48 percent tetraoxysilicon, about 11 to 29 percent monocarbosiloxane, about 11 to 27 percent dicarbosiloxane, up to about 22 per-cent tetracarbosilicon, and about 3 to 9 percent elemental carbon dispersed atomically or in small clusters within the glass matrix.
Alternatively, such silicon-oxy-carbide glasses can be described as a composition of silicon, oxygen, and carbon in a mass of silicon-oxy-carbide glass wherein about 52 to 66 percent of the silicon atoms are bonded to at least one car-bon atom, and about 3 to 9 weight percent carbon is dispersed atomically or in small clusters within the glass matrix.

20~0675 ~D-17,455 The crosslinking precursor resin solution can also be drawn into fibres. The precursor resin solution is allowed to crosslink until the viscosity increases to a point where a solid object can be dipped into the solution and S withdrawn pulling a strand of the resin from the solution.
Fibres can then be drawn or pulled from the resin solution by such dipping processes. Alternatively, the resin solution can be drawn into a teflon tube with a slight vacuum. As the resin cures and toluene evaporates the fibre shrinks and can be pushed out of the tube. Fibres can be fully crosslinked for easier handling by heating them to about 50 C. The fibres are then pyrolized 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-carbide glass and ceramic filler. The precursor resin is dissolved in a solvent and ceramic particles are dispersed ln 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 2Si02-3A1203 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 carbi.de fibre and alumino-boro-silicate fibres. The im-pregnated 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 resin matrix RD-17,455 surrounding the ceramic fibres. The composite is then pyrolized in a non-oxidizing atmosphere or a vacuum as described above. The resin 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-filtrated with a solution of precursor resin dissolved in a solvent to reduce porosity in the composite. The composite is placed in the re-infiltrant solution while in a vacuum.
Pressure is applied to the solution to force the solution into the pores of the composite. After re-infiltrating, toluene is allowed to evaporate and the re-infiltrated com-posite is pyrolized in a non-oxidizing atmosphere or vacuum aq described above. Re-infiltration and pyrolysis can be re-peated 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 atmo-spheres at temperatures up to at least 1650 C. 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 any known ceramic fibre.

RD-17,455 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 of weight loss data measured during pyrolysis of methyl silicone precursor resins.
Figure 2 is a graphical presentation of the 29Silicon nuclear magnetic resonance spectrum of silicon-oxy-carbide glass.
Figure 3 is a graphical presentation of the 29Silicon nuclear magnetic resonance spectrum of "Nicalon"
silicon carbide.
Figure 4 is a graphical presentation of the 29Sili-cone nuclear magnetic resonance spectrum of resin cured silicon-oxy-carbide glass.

Detailed DeJcri~tion of the Invention Glasses can be defined by two of their basic fea-tures. One feature being that glasses are formed from an ex-tremely viscous supercooled liquid, and a second feature is that the liquids which form glasses 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 glasses of this inven-tion are formed by pyrolizing a methyl silicone precursor resin in a non-oxidizing atmosphere. However, the glasses of this invention have the short-range ordering characteristic of conventional glasses.

-14 - 20~0675 RD-17,455 The preferred methyl silicone resin used in this invention is predominantly comprised of monomethylsiloxane units many of which units contain a hydrogen atom on one oxygen atom, i.e., a hydroxyl group. Crosslinking occurs in the resin when hydroxyl units combine to form a bond between silicon and oxygen and generate water. It has been found that other silicone resins made according to the method in the '868 patent can also be pyrolized to form unique carbonato-free glasses comprising silicon, oxygen and carbon wherein carbon is bonded to silicon and some elemental carbon may be present in the glass matrix.
Silicone resins have a three dimensional structure with short-range order and silicone resins can be described in terms of their stoichiometric compositions. The stoichiometric units in silicone resins contain a silicon atom bonded to oxygen atoms and radical groups. The radical group~ are formed from the monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals such as;
alkyl radicals having from 1 to 8 carbon stoms, cycloalkyl radicals of 5 to 10 carbon atoms, alkenyl radicals such as vinyl and alkyl, fluorinated substituted hydrocarbon radicals and halogenerated monovalent hydrogen radicals such as tri-fluoropropyl, and phenyl radicals.
The four basic units in silicone resins are herein referred to as M groups in which a silicon atom is bonded to one oxygen atom and three organic radicals, D groups in which a silicon atom is bonded to two oxygen atoms and two organic radicals, T groups in which a silicon atom is bonded to three oxygen atoms and one organic radical, and Q groups in which the silicon atom i5 bonded to four oxygen atoms. Silicone resins that may be pyrolized to form glasses contain a combination of M, T, D, and Q groups so that the ratio of -l5 - 201067S
RD-17,455 organic radicals to silicon atoms ls between about 0.5:1 and about 1.7:l.
The glasses of this invention resist devitri-fication, and remain structurally stable at temperatures up to at least 1650 C. The term "structurally stable" refers to a bulk material that essentially retains the same micro-structure from room temperature up to the elevated tem-peratures indicated. This means that minor changes may occur in the microstructure. Minor changes, such as the formation of small crystallized areas up to about 100 angstroms in an otherwise amorphous matrix have no adverse or deletereous af-fect 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.
Silicon-oxy-carbide glass articles, can be made ac-cording to several methods in this invention. In one method, the pyrolized resin is pulverized into a powder having a par-ticle size ranging from O.l up to 2 microns. Grinding mills, such as an attritor or planetary mill, have been used to pro-duce si icon-oxy-carbide particle sizes of 0.1 to 2 microns.
Attritor milling is performed by impellor stirring of a solu-tion comprised of about 52 percent liquid, such as water,about 35 percent milling media, such as 1.2 mm diameter balls that are harder than the material to be ground, and the re-mainder is crushed particles of silicon-oxy-carbide glass.
Impellor stirring of the solution at 1000 rpm pulverizes the glass particles into a powder. Planetary milling is per-formed with a similar solution except the milling media is 5 to 8 mm diameter balls and the solution is agitated by ro-RD-17,4~5 tating the milling vessel in a planetaxy fashion at slower speeds.
The milled powder is then dried and consolidated by application of heat and pressure to form a shaped article.
Consolidation can be achieved through application of a uni-axial pressure of at least about 5 ksi at about 1550--1650-C., or application of isostatic pressure of at least about 8 ksi at about 1200--1650-C. Heat and pressure are ap-plied until the article has been densified the desired amount or until fully densified.
In another method for forming silicon-oxy-carbide glass articles from cast or shaped precursor resins, the methyl silicone precursor resin can be dissolved in a solvent and crosslinked with a curing agent. Illustrative of the solvents that have been found suitable for dissolving the precursor resin are toluene and mixtures of toluene with iso-propyl alcohol. The resin can be dissolved in the solvent at ratios up to about eight parts resin to five parts solvent.
Illustrative of the curing agents found suitable for crosslinking the precursor resin are bases such as ammonium hydroxide, commercial silicon containing amines such as gamma amino propyl triethoxy silane and acids such as hydrochloric acid. The curing agent is added in an amount of about 0.1 to 4 percent of the resin. The crosslinked precursor resin is dried and cured at room temperature. Preferably, the crosslinked precursor resin is dried at a rate that allows solvent to evaporate from the resin without forming voids in the resin. The precursor resin is shaped or cast into the desired form either before or during crosslinking.
The cured precursor resin was then pyrolized in a non-oxidizing atmosphere as described herein. Since the pre-cursor resin is crosslinked in this embodiment of the inven-RD-17,455 tion, pyrolysis can also be performed in a vacuum. The heat-ing rate during pyrolysis must be controlled to allow evolu-tion of gases without forming voids or bubbles in the resin.
Preferably heating rates of less than l.O C per minute are used to allow sufficient gas evolution without forming bub-bles, voids or defects in the glass. Pyrolysis was complete when weight loss from the evolution of water, methyl groups and other decomposition products from the precursor resin substantially ended. The precursor resin densifies during pyrolysis and forms the crosslinked resin silicon-oxy-carbide glass.

EXA~PLES

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 silicone resin formed by the method in the '868 patent and having methyl radical groups, and consisting of about 5 weight percent D groups and 95 weisht percent T groups was used in the following examples.
Methyl silicone precursor resins were pyrolized by heating them to temperatures ranging from 900 to 1600 C. in a non-oxidizing atmosphere. During pyrolysis, the precursor resins experienced weight loss as water, methyl groups, and other decomposition products evolved. When the weight of the pyrolizing resin stabilizes pyrolysis is sùbstantially com-plete. Measured weight loss during pyrolysis varied from about 11 to 35 percent. Part of the weight loss can be at-tributed to variations in the amount of retained solvents andthe amount of crosslinking that has occurred prior to the be-ginning of pyrolysis. As explained previously these pre-RD-17,455 cursor resins evolve water as they crosslink. The reslns will crosslink at room temperature or when curing aids are added to increase crosslinking. Therefore, the amount of water evolved from the resin before the beginning of pyroly-sis may vary depending upon the amount of crosslinking thathas occurred prior to pyrolysis. As more crosslin~ing oc-curs more water is lost before pyrolysis and there will be lesser amounts of weight loss from the resin during py-rolysis.
Exa~ples 1-3 Three pyrolysis examples were conducted according to the method of this invention. One uncured precursor resin lS and two cured or crosslinked precursor resins were pyrolized while weight loss from the resins was measured by thermal gravimetric analysis. Thermal gravimetric analysis is a method for measuring weight loss from a sample while it is being heated. Two examples were heated in a hydrogen atmosphere and one example in a helium atmosphere at a rate of l0 C/minute until weight loss ended. The measured weight loss and final composition of the silicon-oxy-carbide glass formed after pyrolysis are shown in Table I.

Table I - Therma1 Gravimetric ~nalysis of Pyrolized Resins ExampleSample Atmoa- Weight Compoaition No.Precuraor phere Loaa ~ Weight ~
Reain Si O C

1 uncured H2 25 q7 41 12 2 cured H2 17.5 51 32 11.22 3 cured He 15.5 51 32 11.22 -19 - 20~0675 RD-17,455 Conventional carbon and silicon values were measured by conventional wet chemistry techniques for dissolved carbon and silicon. Oxygen content was measured by neutron activation.
The weight loss data from Examples 1-3, as deter-mined by thermal gravimetric analysis, is presented in the graph of Figure 1. In the graph of Figure 1, the percent weight loss in each sample is plotted on the ordinate while the increase in heating temperature is plotted on the ab-scissa. The graph of Figure 1 shows that a significant por-tion of the weight loss in each sample has occurred at tem-peratures as low as 900 C while weight loss was essentially completed at 1200-C.

~g~Dl L_~

A sample of consolidated silicon-oxy-carbide glass was produced by pyrolizing a precursor resin in flowing hy-drogen at 1400-C. The precursor resin was placed in a molybdenum boat and pyrolized as described herein. The py-rolized precursor resin waq pulverized in six 25 gram batches in a planetary mill using an agate mortar and an agate media of l/4 inch diameter. This produced 150 grams of silicon-oxy-carbide powder having a surface area of 2.2 m2/gram which is an equivalent spherical diameter of about 1.16 micro-meters.
About 120 grams of the silicon-oxy-carbide powder were hot pressed in a two-inch diameter die that was faced with a graphite sheet separating agent. The graphite sheet prevents the powder from sintering to the die while it is being hot pressed. The sample was heated at a rate of 10 C/minute up to 1650-C and held for 30 minutes at 1650-C

RD-17,455 while a uniaxial pressure of 6 ksi was applied. An essen-tially fully dense sample was produced having the properties shown below in Table II.

S ~h~L
ProR~i~æ,sf_5snsolidat5~L5~ L~
Oxy-Carbi~ Glass Vi~cosity Ela~tic Thermal~racture Den~1ty at 1565-C Modulu~ ExpanqionStrength (gram/CC) ~pol~e~ (p~i) (1/ C) (K3i) 2.35 2xlol2 14.2x106 3.1x106 24.7 High resolution transmission electron microscopy of the hot pressed materlal showed 20 to lO0 angstrom sized par-tlcles of beta silicon carbide in an otherwise amorphousmatrix. Substantially no evidence of crystallization was found by x-ray diffraction of the hot pressed material.

I~L~21~L 5 A sample of crosslinked resin silicon-oxy-carbide glass was produced by slowly pyrolizing a crosslinked precursor resin. Equal portions of toluene and precursor resin were mixed with a crosslinking agent in the amount of about 4 weight percent of the precursor resin. This mixture was poured into a glass dish and the toluene was allowed to slowly evaporate by holding at room temperature for a period of 24 hours. As toluene evaporated, the precursor resin was crosslinking. The crosslinked sample was heated from room temperature to 500 C in lO hours, from 500 C to 800 C in 16 hours, from 800 C to llOO C in 4 hours and held at llOO C for RD-17,455 1 hour. This produced an overall heating rate of about 0.6 C/minute. The sample was then furnace cooled. A fully dense sheet of crosslinked resin silicon-oxy-carbide glass was produced having a thickness of about 2 mm.
s ~3;UL~_ 6 The oxidation resistance and structural stability or resistance to devitrification of silicon-oxy-carbide glass was analyzed by heating hot pressed specimens of the glass for 240 hours at 1420-C and 1520-C in air. No weight loss from decomposition of silicon or carbon in the glass was measured. X-ray diffraction of a sectioned surface revealed no evidence of crystailization in the bulk material of either specimen. X-ray diffraction of exposed surfaces showed evldence of surface crystallizatlon to cristobalite in both specimens in about 0.002 inch of the surface.

~LLLLQ 7~9 The composition of two different glasses is not al-ways adequately defined by just referring to the amount of each element in the glass. Rather, it is the short-range ordering in glasses that give them their different prop-erties. Therefore, by characterizing the short-range order-ing in glasses different glass compositions can be defined, and the glasses of this invention are defined by their short-range ordering.
A sample of silicon-oxy-carbide glass was prepared by pyrolizing a sample of precursor resin at llOO C in flow-ing hydrogen. A sample of resin cured silicon-oxy-carbide glass was prepared by pyrolizing a sample of crosslinked pre--22 - 2010~75 RD-17,455 cursor resin at 1100-C in flowing hydrogen. 29Silicon solid state nuclear magnetic resonance spectra recorded from these samples are presented in Figures 2 and 4. FIG. 3 is the 29Silicon nuclear magnetic resonance spectrum from a sample of "Nicalon" silicon carbide fibre. On the ordinate is plotted the intensity of radiation measured from the excited sample, and on the absicssa is plotted the parts per million (ppm) in chemical shift from a tetramethyl silicon standard that fixes the zero point on the abscissa. Characteristic ppm in chemical shift are known for many polyatomic units, for example tetraoxysilicon, dicarbosiloxane and mono-carbosiloxane are shown in; "NMR Basic Principles and Progress 29Si-NMR Spetroscopic Results", Editors P. Diehl, R.
Kosfeld, Springer Verlag Berlin Heidelberg 1981 at pp. 186, 184 and 178. Therefore, each peak in Figures 1, 2 and 3 defines the short-range ordering of specific silicon poly-atomic units.
In FIG. 2, the spectra of silicon-oxy-carbide glass containing peaks labeled 1 through 3 is shown. Peak 1, the broadest peak, represents a small amount of tetra-carbosiloxane and a large amount of dicarbosiloxane, peak 2 defines monocarbosiloxane, and peak 3 defines tetraoxy-silicon. By integrating the area under each peak, the frac-tion of each of these polyatomic units can be determined.
The integrated area under each peak in FIG. 2 reveals a composition for silicon-oxy-carbide glass, comprising in weight percent up to about 6 percent tetracarbosilicon and about + 5 percent of the following, about 22 percent dicarbosiloxane, 24 percent mono-carbosiloxane, and 39 percent tetraoxysilicon.
The spectra in Figure 2 can be compared to the silicon carbide spectra in FIG. 3 measured from a "Nicalon"

RD-l7,455 silicon carbide fibre sample. The composition for "Nicalon"
in FIG. 3 is about 75 percent silicon carbide, about 7 percent dicarbosiloxane, about 13 percent monocarbosiloxane, and about 5 percent tetraoxysilicon. From the spectra in FIG. 3, it can be seen that "Nicalon" fibres are comprised principally of silicon carbide with trace amounts of di-carbosiloxane, monocarbosiloxane, and tetraoxysilicon. In contrast, the spectra of FIG. 2 shows that silicon-oxy-carbide is comprised principally of dicarbosiloxane, monocar-bosiloxane, and tetraoxysilicon. It is this latter combina-tion of polyatomic units that bonds carbon to silicon in a heretofore unknown manner in glasses, providing increased devitrification and decomposition resistance and char-acterizes the glasses of this invention.
The spectra of crosslinked resin silicon-oxy-carbide glass shown in FIG. 4 lndicates a composition comprising, in weight percent, about + 5 percent of the following; about 17 percent tetracarbosilicon, about 16 per-cent dicarbosiloxane, about 16 percent monocarbosiloxane, and about 43 percent tetraoxysilicon. Peak l is tetracarbosilicon, peak 2 is dicarbosiloxane, peak 3 is monocarbosiloxane, and peak 4 is tetraoxysilicon. From a comparison of Figures 2, 3 and 4, it can be seen that the resin cured silicon-oxy-carbide glass differs in composition from "Nicalon" fibres, and both resin cured silicon-oxy-carbide glass and "Nicalon" differ in composition from silicon-oxy-carbide glass.
~-~Q
Silicon-oxy-carbide glass fibres were made by the following process. A solution of precursor resin and toluene was mixed in a 1:1 ratio, and gamma amino propyl triethoxy silane was added as a curing agent in the amount of 2 weight -24 - 20~6~5 RD-17,455 percent of the resin. The solution was allowed to crosslink until a strand could be pulled from the solution. The end of Zl fibre blank, 0.5 mm in diameter, was dipped into the resin solution and withdrawn, thereby pulling a fibre of precursor resin from the solution. This procedure was repeated several times and the fibres were heated at 50 C to dry and fully crosslink them for easy handling. The fibres were then pyrolized according to the method described herein, forming silicon-oxy-carbide glass fibres of about 0.3 mm in diameter.
~ le 11 Ceramic composites having an amorphous silicon-oxy-carbide ceramic matrix were made by preparing an infiltrant slurry consisting of by weight; 3 parts precursor resin, 3 parts 0.2 micron silicon carbide powder, and 4 parts toluene.
This slurry was infiltrated into a continuous carbon fibre tow by pulling the tow through an agitated bath of the slurry. A tow is a strand made by weaving individual fibres together. The infiltrated tow was wound on a hexagonal drum to form unidirectional resin impregnated panels. After the toluene evaporated the dried panels were removed from the drum. The panels were cut into tapes and several tapes were stacked in a rectangular die maintaining the unidirectional alignment of the fibres. The layered tapes were pressed a~
300 MPa while the die was slowly heated up to 200 C and held for 15 minutes. The resin flowed to fill gaps between the fibre tows and layers of tape to form a bar having a contin-uous matrix of crosslinked resin and silicon carbide powder surrounding the fibre tows. The bar was removed from the die and pyrolized in an argon atmosphere by heating at 2 C per minute to 1200 C and holding at 1200-C for 30 minutes. A
ceramic composite having a matrix of amorphous silicon-oxy-carbide glass binding a ceramic filler and reinforced by RD-17,455 carbon fibres was formed. The ceramic composite had a den-sity of 1.73 g/cc and contained 19 volume percent open poro-slty. Small bars were machined from the composite panel and mechanical properties were measured using 3-point bend testing. Ultlmate bend strength was 200 MPa and the fracture energy was greater than 2.3 Kj/m2. At fracture the composite showed non-brittle behavior characterized by fibre debonding and pullout.
~ uaL~ 12 A second ceramic composite was formed using the procedure described in Example 11, however, the infiltrant slurry consisted of, by weight, 2 parts precursor resin, 3 parts of 3.5 micron silicon carbide powder, and 5 parts toluene. The ceramic fibre was a boron-nitride coated "Nicalon" silicon carbide fibre. The impregnated and consolidated fibre panels were pyrolized to form a ceramic composite having a density of 2.08 g/cc, 18'~ open porosity, ultimate bend strength of 312 Mpa, and fracture energy of 2.4 Kj/m ~alLULL~__L~

A third ceramic composite was formed using the pro-cedure of Example 11, however, the infiltrant slurry con-sisted of, by weight, 2 parts precursor resin, 3 parts of 2 micron mullite powder, and 5 parts toluene. The ceramic fibre was an alumino-boro-silicate fibre. Mullite is a re-fractory ceramic of aluminosilicate having the chemical for-mula 2SiO2-3Al203. The impregnated and consolidated fibre panels were pyrolized to form a composite ceramic with a den-sity of 2.39 g/cc, 13.5~ open porosity, and ultimate bend strength of 200 MPa.

Claims

1. A glass composition that remains structurally stable at temperatures up to at least 1650°C, comprising silicon, oxygen and carbon in a distribution of polyatomic units comprising in weight percent about 34 to 48 percent tetraoxysilicon, about 11 to 29 percent monocarbosiloxane, about 11 to 27 percent dicarbosiloxane, up to about 22 percent tetracarbosilicon, and about 3 to 9 percent elemental carbon dispersed in the glass matrix.

2. The glass of claim 1 wherein silicon, oxygen and carbon are distributed in polyatomic units, comprising in weight percent about 38 to 48 percent tetraoxysilicon, about 11 to 21 percent monocarbosiloxane, about 11 to 21 percent dicarbosiloxane, about 12 to 22 percent tetracarbosilicon, and about 3 to 9 percent elemental carbon dispersed in the glass matrix.

3. The glass of claim 1 wherein silicon, oxygen and carbon are distributed in polyatomic units comprising in weight percent about 34 to 44 percent tetraoxysilicon, about 19 to 29 percent monocarbosiloxane, about 17 to 27 percent dicarbosiloxane, up to about 6 percent tetracarbosilicon, and about 3 to 9 percent elemental carbon dispersed in the glass matrix.

4. A glass composition that remains structurally stable at temperatures up to at least 1650°C, comprising silicon, oxygen and carbon in a mass of silicon-oxy-carbide glass wherein about 52 to 66 percent of the silicon atoms are bonded to at least one carbon atom, and about 3 to 9 weight percent carbon is present as elemental carbon dispersed within the glass matrix.

RD-17,455

5. A glass composition that remains structurally stable at temperatures up to at least 1650°C, comprising silicon, oxygen and carbon in a mass of resin cured silicon oxy-carbide glass wherein about 56 to 66 percent of the silicon atoms are bonded to at least one carbon atom, and about 3 to 9 weight percent carbon is present as elemental carbon dispersed within the glass matrix.

6. A glass composition that remains structurally stable at temperatures up to at least 1650°C, comprising silicon, oxygen and carbon in a mass of resin cured silicon oxy-carbide glass wherein about 52 to 62 percent of the silicon atoms are bonded to at least one carbon atom, and about 3 to 9 weight percent carbon is present as elemental carbon dispersed within the glass matrix.

7. A process for forming a glass, comprising heating a methyl silicone precursor resin in a non-oxidizing atmosphere at a temperature that will pyrolize the resin, said heating being performed for a period of time ending when weight loss from the pyrolizing resin substantially ends;
said pyrolized resin forming a silicon-oxy-carbide glass that remains structurally stable at temperatures up to at least 1650°C.

8. The process of claim 7 wherein said heating is performed between 900°C to 1650°C.

9. The process of claim 7 before the step of heating further comprising the step of crosslinking the methyl silicone precursor resin by dissolving the resin in a solvent and adding a curing agent, whereby a crosslinked resin silicon-oxy-carbide glass is formed after pyrolysis.

RD-17,455

10. The process of claim 7 wherein said heating is performed for a period of time that allows a weight loss from the resin of approximately 11 to 35 percent.

11. The process of claim 7 wherein said heating is performed in a hydrogen gas atmosphere.

12. The process of claim 7 wherein said heating is performed in a helium gas atmosphere.

13. A process for forming a silicon-oxy carbide glass article, comprising:
heating a methyl silicone precursor resin in a non-oxidizing atmosphere at a temperature that will pyrolize the resin, said heating being performed for a period of time ending when weight loss from the pyrolizing resin substantially ends;
pulverizing said deposit into a powder of about 0.1 to 2 microns in size; and consolidating said particles through the application of heat and pressure that will densify the powder into the article.

14. The process of claim 13 wherein said step of heating is performed between 900°C and 1650°C.

15. The process of claim 13 wherein said step of heating is performed in a hydrogen gas atmosphere.

16. The process of claim 13 wherein said step of heating is performed for a period of time that allows a weight loss from the resin of approximately 11 to 35 percent.

RD-17,455

17. The process of claim 13 wherein said step of consolidating comprises applying a uniaxial pressure to the powder of at least about 5 ksi and heating the powder to about 1550°C to 1650°C.

18. The process of claim 13 wherein said step of consolidating comprises applying an isostatic pressure to the powder of at least about 8 ksi and heating the powder to about 1200°C to 1600°C.

19. A process for forming a crosslinked resin silicon-oxy-carbide glass article, comprising:
dissolving a methyl silicone precursor resin in a solvent;
adding a curing agent to crosslink the resin;
shaping the resin to form the article;
evaporating the solvent from the crosslinking resin; and heating the resin in a non-oxidizing atmo-sphere at a temperature that will pyrolize the resin, said heating being performed for a period of time ending when weight loss from the pyrolizing resin substantially ends.

20. The process of claim 19 wherein said curing agent is ammonium hydroxide.

21. The process of claim 19 wherein said curing agent is a silicon containing amine.

22. The process of claim 19 wherein said step of heating is performed between 900°C and 1650°C.

RD-17,455

23. The process of claim 19 wherein said step of heating is performed in a hydrogen gas atmosphere.

24. The process of claim 19 wherein said step of heating is performed in a vaccuum.

25. The process of claim 19 wherein said step of heating is performed at a rate of heating that minimizes the formation of voids in said glass.

26. The process of claim 19 wherein said step of heating is performed for a period of time that allows a weight loss from the resin of approximately 11 to 35 percent.

27. The process of claim 19 wherein said step of heating is performed at a rate of heating less than about 1°C
per minute.

28. The process of claim 19 wherein said step of evaporating is performed at a rate of evaporation that pre-vents the formation of voids in the resin.

29. A glass fibre, comprising silicon, oxygen and carbon in a distribution of polyatomic units comprising in weight percent about 38 to 48 percent tetraoxysilicon, about 11 to 21 percent monocarbosiloxane, about 11 to 21 percent dicarbosiloxane, about 12 to 22 percent tetracarbosilicon, and about 3 to 9 percent elemental carbon dispersed in the glass matrix.

30. A process for forming silicon-oxy-carbide glass fibres, comprising:
dissolving a methyl silicone precursor resin in a solvent;

RD-17,455 adding a curing agent to the dissolved resin and allowing the resin to crosslink to a viscosity where the resin can be formed into a fibre;
drawing fibres from the resin;
evaporating the solvent from the resin; and heating the resin in a non-oxidizing atmo-sphere at a temperature that will pyrolize the resin, said heating being performed for a period of time ending when weight loss from the pyrolizing resin substantially ends.

31. A composite ceramic, comprising at least one ceramic fibre within a matrix of silicon-oxy-carbide glass binding a ceramic filler, the glass comprising silicon, oxygen and carbon in a distribution of polyatomic units comprising in weight percent about 34 to 48 percent tetraoxy-silicon, about 11 to 29 percent monocarbosiloxane, about 11 to 27 percent dicarbosiloxane, and about 3 to 9 percent elemental carbon dispersed in the glass matrix.

32. A process for making a ceramic composite com-prising:
dissolving a precursor resin in a solvent;
adding a particulated ceramic filler to the resin to form a composite resin;
impregnating at least one ceramic fibre with the composite resin;
shaping the impregnated fibre into the com-posite;
evaporating the solvent from the impregnated fibre; and heating the shaped fibre in a non-oxidizing atmosphere at a temperature that will pyrolize the resin, said heating performed for a period of time RD-17,455 ending when weight loss from the pyrolizing resin substantially ends, thereby forming a matrix of silicon-oxy-carbide glass and ceramic filler surrounding the ceramic fibre.

33. The process of claim 32 further comprising be-fore the step of heating, consolidating layers of impregnated fibre through the application of heat and pressure to form a continuous composite resin matrix around the fibre.