IT9021930A1 - TRANSLUCENT SILICON OXYCARBON GLASS AND OBJECTS MADE WITH THE SAME - Google Patents

Description

DESCRIPTION of the industrial invention

The present invention relates to glass compositions and in particular translucent glass compositions comprising silicon, oxygen and carbon. Amorphous silica is a refractory glass, however, it devitrifies easily at temperatures above 1100 ° C. Devitrification consists in the ordering or crystallization of the random structures of which the glasses are made. Crystallization drastically reduces one of the predominant qualities of vitreous silica, namely its low thermal expansion, as well as several of its other desirable properties. As a result, much research has been engaged in looking for ways to increase devitrification resistance in silica glass compositions.

The reactions between silicon, carbon and oxygen have been studied extensively. Some of the known reactions in a silicon, carbon and oxygen system contain oxygen combined with silicon to form silica, SiO2. At temperatures above 1100 ° C the silica begins to crystallize forming crystobalite, one of the common mineral forms of silica. Carbon can react with available silica to form crystalline silicon carbide or escape as gaseous carbon monoxide. Any remaining carbon as elemental carbon readily oxidizes above 600 ° C when exposed to air.

The thermodynamics of silicon, carbon and oxygen reactions is discussed in "The hign temperature oxidation, reduction, and volatilization reactions of Silicon and Silicon Carbide", Gulbransen, E.A. and Jansson, S.A. Oxidation of Metals, Volume 4, Number 3, 1972. The thermodynamic analysis by Gulbransen and others shows that at 1200 ° C silica and carbon would form gaseous silicon monoxide and carbon monoxide or solid silicon carbide, SiC. However, it would not be expected to form a material containing silicon, oxygen and carbon. Gulbranssen and others conclude that silica was not recommended for use in reducing atmospheres above 1123 ° C due to the formation of gaseous volatile silicon monoxide. Furthermore, silicon carbide was not recommended for use in oxygen-containing environments where active oxidation can occur due to the oxidation of silicon carbide.

There is an opaque black glass, which is described functionally and as carbon-modified vitreous silica and herein called "black glass", where 1 to 3% carbon has been added to the silica. The method of making black glass is described by Smith et al in U.S. Patent No. 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. also characterized black glass for infrared spectroscopy in The vibration spectra of high purity and chemically substituted vitreous silicas ", Alfred University graduate thesis, Alfred, N.Y., May 1973. Smith reveals that in addition to carbon elementary dispersed in glass, carbon in black glass is associated with oxygen in carbonate-like groups. A carbonate group is the description of a particular way in which a carbon atom binds with three oxygen atoms and has the structure,

The mechanical strength of black glass is similar to the strength of carbon-free silica glass, however, black glass has a higher resistance to devitrification than conventional silica glass which begins to devitrify at around 1100 ° C, while black glass begins to devitrify at about 1250 ° C. The greater thermal stability of black glass allows it to be used at temperatures higher than those which vitro silica can resist.

In a commercially produced continuous silicon carbide ceramic fiber sold under the "Nicalon" brand, approximately 10% oxygen is introduced into the fiber to crosslink it. After crosslinking, the fibers are pyrolyzed and oxygen is believed to become part of the fiber as an amorphous contaminant, possibly in the form of silica. The degradation behavior of these fibers after heat treatment in various environments was reported in the article "Thermal Stability of SiC fibers (Nicalon)", Mah, T., et al., Journal of Material Science, Vol. 19, from page 1191 to 1201 (1984). ' Mah et al found that regardless of the environmental conditions during heat treatment, the strength of the "Nicalon" fiber degraded when the fibers were subjected to temperatures greater than 1000 ° C. Fiber degradation was associated with carbon monoxide losses from the fibers and growth of beta silicon carbide grains within the fibers.

In the Italian patent application No. 20456 A / 90 filed on May 29, 1990, a glass composition is described in which the silicon atoms are bonded to oxygen and carbon atoms to form a glass that resists devitrification and decomposition at temperatures up to at least 1650 ° C. The glass in the Italian application No. 20456 A / 90 filed on May 29, 1990 contains in addition from 3 to 9% by weight of elemental carbon dispersed atomically or in small clusters in the glass matrix, and this free carbon makes the glass opaque and black in appearance.

Ceramic materials generally exhibit a brittle behavior characterized by their high strength and low fracture toughness. Fracture toughness is the resistance to the propagation of cracks in materials. The development of ceramic compounds has been investigated as a way to alleviate the brittle behavior of ceramics. "Nicalon" is an excellent ceramic fiber but degrades at temperatures above 1200 ° C. Integrating "Nicalon" fibers into a protective ceramic matrix having desirable mechanical properties and capable of withstanding temperatures substantially greater than 1200 ° C would be one way to form an improved ceramic compound.

However, from the previous discussion, it is evident that the properties of known ceramic or glass compositions and in particular those containing silicon, oxygen and carbon are degraded by decomposition or devitrification of the glass or ceramic at temperatures between more than 1100 ° C and 1250 °. ° C.

Therefore, it is an object of this invention to form compositions of glass, consisting of chemically bonded silicon, oxygen and carbon in which a substantial portion of the carbon atoms are bonded to oxygen atoms and the glass is formed of selected methyl silicone resins.

Another object of this invention is to form a translucent glass, comprising chemically bonded silicon, oxygen and carbon, in which a substantial portion of the carbon atoms are bonded to silicon atoms with up to trace amounts of elemental carbon dispersed in the carbon matrix. glass. Such glass compositions remain structurally stable and do not decompose in oxidizing or reducing atmospheres at temperatures up to at least 1600 ° C.

Another object of this invention is a process for forming such glasses comprising silicon, oxygen and carbon by pyrolyzing selected methyl silicone resins.

Still another object of this invention is the formation of such a glass, comprising silicon, oxygen and carbon, in articles.

It has been found that some silicone resins can be pyrolyzed in a non-oxidizing atmosphere to form unique glass compositions. Surprisingly, it has been found that these silicone resins when pyrolyzed in a non-oxidizing atmosphere do not form silica, cristobalite, silicon carbide, carbon monoxide, or mixtures of silica or carbon. Furthermore, preselected silicone resins have been found which pyrolyze to form translucent glass compositions containing up to trace amounts of free carbon which allow at least partial transmission of light through the glass so that the glass is not opaque or black in appearance. Glasses of this invention are made by pyrolyzing a methyl silicone resin to form a glassy composition, comprising silicon, oxygen and carbon, in which a significant portion of the carbon atoms are chemically bonded to silicon atoms. According to a method of this invention, a methyl silicone resin is heated in a non-oxidizing atmosphere to pyrolyze the resin. As used herein, a non-oxidizing atmosphere is an atmosphere that removes reaction products from the pyrolyzing resin without affecting the reactions that occur during pyrolysis. Examples of non-oxidizing atmospheres are inert atmospheres such as helium, argon or nitrogen and reducing atmospheres, such as hydrogen. Pyrolysis can also occur in a vacuum having a pressure lower than about 0.101 mbar (10 <-4> atmospheres).

Methyl silicone resins suitable for use in the method of this invention can be prepared by the method described in U.S. Patent No.

2,676,182, which is incorporated herein by reference. In particular, examples 2 and 4 of patent No. 2,676,182 modified by replacing ethanol with alcohols and by replacing dimethylphenylchlorosilane and trimethylethoxysilane with trimethylchlorosilane and the use of toluene to aid in the separation of hydrocarbons is particularly relevant to prepare the methylsilicone resins used in the method of this invention.

Methylsilicones are formed from siloxane chains with methyl groups attached to silicon atoms. Siloxane chains contain an alternating bond of silicon and oxygen atoms. Several combinations of methyl groups can be present in the siloxane chains to form polymethylpolysiloxanes.

The unitary and fundamental structures in polymethylpolysiloxanes are trimethylsiloxy, dimethylsiloxy and monomethylsiloxane. The trimethylsiloxy monofunctional unit at the end of a siloxane chain has the structure;

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

Monomethylsiloxane is a trifunctional unit and not only prolongs siloxane chains, but also crosslinks between chains and has the structure;

The methyl silicone resins can also contain unsubstituted tetrafunctional units, here called Q units and having the structure;

Polymer structures can be constructed from these unitary 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 substituent, the organic substitutions being the methyl groups.

Methyl silicone resins contain a ratio of methyl groups to silicon atoms of about 2: 1 or less. The methylsilicone resins used in this invention consist of unsubstituted trimethylsiloxy units and tetrafunctional Q units in ratios up to the maximum amount of trimethylsiloxy that can be cured with Q units, or up to about 3: 1, preferably in the ratio of about 0.7 : 1 to about 3: 1 and most preferably in the ratio of about 1: 1 to about 3: 1. Such methyl silicone resins are hereinafter referred to and claimed as a methyl silicone precursor resin or sometimes as a precursor resin or resin. It should be understood that the ratio of trimethylsiloxy units to Q units in the precursor resins is specified according to the initial stoichiometry of the resin prepared by the processes described above, however, the ratio in the cured resin between trimethylsiloxy units and Q units may be smaller in the resin.

During pyrolysis, the precursor resin thickens as gas unfolds causing weight loss from the resin. Although the pyrolyzing resin undergoes a weight loss, the density of the pyrolyzing resin is increased due to the volume reduction of the pyrolyzing resin. The pyrolysis reactions are essentially completed when a substantially constant weight in the pyrolyzing resin has been obtained. Further thickening of the pyrolyzing resin can occur after the weight loss is over, if the heating is continued. Therefore, it may sometimes be desirable to stop the heating and pyrolysis in the resin after it has fully thickened, or in other words, to stop the volume reduction. Weight loss during pyrolysis was determined to be between about 17 and 54%. Precursor methyl silicone resins can be pyrolyzed at temperatures ranging from about 900 ° C to 1600 ° C.

Glasses formed by the method of this invention possess unique properties and characteristics. These glasses resist crystallization and do not decompose in oxidizing and reducing atmospheres at temperatures up to at least 1600 ° C. In addition, a significant portion of the carbon present in the glasses of this invention is bonded to the silicon with the remainder present as elemental carbon dispersed within the glass matrix so that there are no detectable carbonate groups. The bonds between carbon and silicon discovered in the glasses of this invention were previously unknown in silica glasses. In silica glasses, and particularly black glass, carbon has only been known to be present as an unbound element in the silica matrix and in carbonate groups, where carbon is bound with oxygen. Glasses formed by the method of this invention and characterized by such unique properties are herein called silicon oxycarbon glasses.

The pyrolysis of the precursor methyl silicone resin forms a silicon oxycarbon glass which is characterized by a continuous division of electrons between silicon, oxygen and carbon atoms. In silicon oxycarbon glass, silicon atoms are present in four polyatomic units. In a unit, here called tetraoxysilicon, a silicon atom is bonded to four oxygen atoms. In a second unit, here called monocarboosiloxane, a silicon atom is bonded to three oxygen atoms and one carbon. In a third unit, here called dicarboosiloxane, a silicon atom is bonded to two oxygen atoms and two carbon atoms. In a fourth unit, tetracarbosilicon, one silicon atom is bonded to four carbon atoms. A silicon oxycarbon glass is formed by pyrolysis of precursor resins containing trimethylsiloxy units and polymerized Q units in any ratio but, surprisingly, the ratio of polymerized trimethylsiloxy units to Q units in the precursor resin has been found to have an effect on the composition and on the properties of the silicon oxycarbon glass being formed. When precursor resins containing trimethylsiloxy and Q units are used in a ratio below the preferred precursor resins described above or, in other words, precursor resins having a ratio of M to Q of less than about 0.7: 1, a opaque silicon oxycarbon glass which is black in appearance.

When the preferred precursor resins containing trimethylsiloxy and Q units in the ratio of 0.7: 1 or greater are pyrolyzed, a translucent glass of silicon oxycarbon is formed having at least a polyatomic unit distribution comprising in percentages by weight from about 18 to 28 % tetraoxysilicon, about 21 to 31% monocarboosiloxane, about 12 to 22% dicarbosiloxane, about 28 to 38% tetracarbosilicon, with up to trace amounts of elemental carbon dispersed atomically or in small clusters within the glass matrix. A trace amount of elemental carbon is an amount that is insufficient to make the glass opaque or, in other words, allows at least partial transmission of light through the glass. In general, a trace amount of elemental carbon is less than about 0.1% by weight. Polyatomic units are mainly connected by bonds between silicon and oxygen, with a small and insignificant number of bonds between carbon atoms and oxygen atoms.

Translucent glass may alternatively be described as a composition of silicon, oxygen and carbon in a mass of translucent silicon oxycarbon glass in which about 73 to 83 percent silicon atoms are each bonded to at least one single carbon atom, with up to trace amounts of elemental carbon dispersed atomically or in small masses within the glass matrix.

Silicon oxycarbon glass objects can be formed by reducing the pyrolyzed resin to a powder. The silicon oxycarbon powder is then consolidated by hot pressing to form an object. One hot pressing method is to apply an axial pressure of at least about 34.45 MPa (5 ksi) between about ISSO and 1600 ° C to the powder. The ksi unit is equivalent to 1000 pounds per square inch. Such pressures and temperatures are sufficient to form a thickened object. Shaped objects can also be formed directly from the precursor methylsiiicone resin. First, the resin is dissolved in a solvent such as toluene and then cast into a desired shape. The cast resin is dried at room temperature and slowly pyrolyzed in a non-oxidizing atmosphere, as described herein. The pyrolysis is performed at a low heating rate which avoids the formation of voids and bubbles, since gases take place and cause a weight loss in the resin. When the weight loss of the pyrolyzing resin stabilizes, the pyrolysis is complete. When the preferred precursor resins described above are pyrolyzed, the cast resin thickens to form a translucent glass of silicon oxycarbon having at least one distribution of polyatomic units as described above, however when pyrolyzing the precursor resins having a ratio of trimethylsiloxy units to Q unit less than 0.7: 1, the cast resin thickens to form an opaque glass of silicon oxycarbon which is black in appearance.

The precursor resin in a toluene solution can also be pulled into fibers. The precursor resin solution is treated with a base or acid to increase viscosity to a point where a solid object can be immersed in the solution and withdrawn, pulling a strand of resin from the solution. The fibers can then be sucked 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 increases in viscosity and the toluene evaporates, the fibers shrink and can be pulled out of the tube. The fibers can be reinforced for easier handling by heating them to approximately 50 ° C. The fibers are then pyrolyzed in a non-oxidizing atmosphere or in a vacuum, as described above.

Ceramic composites can be formed having ceramic fibers in a silicon oxycarbon glass matrix and a ceramic filler. The precursor resin is dissolved in a solvent and the ceramic particles are dispersed in the solution to form an infiltrating slurry. The particle ceramic filler controls the shrinkage of the compound matrix during pyrolysis and can be chosen so that the matrix is compatible with the fiber reinforcement to be used. Some examples of ceramic fillers are pulverized silicon carbide, diatomaceous earth and 2SiO2 aluminosilicate.

called mullite.

A ceramic fiber or fibers, or fabric of the fibers is pulled through a stirred bath of the infiltrating slurry. Some examples of ceramic fibers are carbon fibers, silicon carbide fibers, and aluminoborosilicate fibers. The impregnated fiber is then shaped and dried to allow evaporation of the solvent. One shaping method comprises wrapping a spiral impregnated fiber onto a drum to form a panel. Fiber layers can be consolidated through the application of heat and pressure to form a continuous resin matrix surrounding the ceramic fibers. The compound is then pyrolyzed in a non-oxidizing atmosphere or in a vacuum, as described above. The resin thickens into a substantially amorphous silicon oxycarbon glass which binds the ceramic filler, thereby forming a continuous matrix around the fibers. Depending on the pyrolysis temperature used, the ceramic filler can be dispersed, partially sintered or completely sintered within the glass. Optionally, the ceramic compound can be reinfiltered with a precursor resin solution dissolved in a solvent to reduce the porosity of the compound. The compound is placed in the re-infiltrating solution under vacuum. Pressure is applied to the solution to push the solution into the pores of the compound. After re-infiltration, the solvent is allowed to evaporate and the re-infiltrated compound is pyrolyzed in a non-oxidizing atmosphere or under vacuum, as described above. Reinfiltration and pyrolysis can be repeated as many times as necessary to achieve the desired degree of matrix density.

The amorphous silicon oxycarbon glass matrix binding a ceramic charge surrounds and protects the ceramic fibers from decomposition in oxidizing and reducing atmospheres at temperatures up to at least 1600 ° C. It was found that the inert nature of silicon oxycarbon glass readily accepts ceramic fibers without reacting with them and degrading their properties. As a result, a silicon oxycarbon glass containing suitable ceramic fillers can be used as the matrix material for several known ceramic fibers.

The following description of the invention will be more easily understood with reference to the figures briefly described below.

Figure 1 is a graph showing the weight loss during pyrolysis of precursor methyl silicone resins.

Figure 2 is a graphical representation of the nuclear magnetic resonance spectrum of silicon 29 translucent glass of silicon oxycarbide. Figure 3 is a graphical representation of the nuclear magnetic resonance spectrum of silicon 29 silicon carbide "Nicalon".

Glasses can be defined by two of their fundamental characteristics; a feature being that the glasses are formed from an extremely viscous supercooled liquid and a second feature being that the liquids which form the glasses possess a polymerized network structure with short distance order. The glasses of this invention are not made from supercooled liquids, but actually possess a short distance order network structure. Instead of supercooling a liquid, the glasses of this invention are formed by pyrolyzing a precursor methyl silicone resin in a non-oxidizing atmosphere. However, the glasses of this invention have the short range order characteristics of conventional glasses. Silicone resins have a three-dimensional structure with short distance order and silicone resins can be described in terms of their stoichiometric compositions. Stoichiometric units of silicone resins contain a silicon atom bonded to oxygen atoms and radical groups. The radical groups in silicone resins that can be pyrolyzed to form glasses are formed by the monovalent hydrocarbon radicals and by halogenated monovalent hydrocarbon radicals containing alkyls, such as methyl, ethyl, propyl, isopropyl, butyl, octyl, dodecyl and the like; cycloalkyls, such as cyclopentyl, cyclohexyl, cycloeptyl and the like; aryls such as phenyl, naphthyl, tolyl, xylyl and the like; aralkyls such as benzyl, phenylethyl, phenylpropyl and the like; halogenated derivatives of the aforesaid radicals comprising chloromethyl, trifluoromethyl, chloropropyl, chlorophenyl, dibromophenyl, tetrachlorophenyl, difluorophenyl and the like and alkenyls such as vinyl, aythyl, metallyl, butenyl, pentite and the like.

The four basic units of silicone resins are referred to here as M groups in which one silicon atom is bonded to one oxygen atom and three organic radicals, D groups in which one silicon atom is bonded to two silicon atoms and two organic radicals , T groups in which a silicon atom is bonded to three oxygen atoms and an organic radical and Q groups in which the silicon atom is bonded to four silicon atoms. Silicone resins that can be pyrolyzed to form glasses contain a combination of M, T, D and Q groups so that the ratio of organic radicals to silicon atoms is between about 0.6: 1 and less than about 3: 1 .

The glasses of this invention resist devitrification and remain structurally stable at temperatures up to at least 1600 ° C. The term "structurally stable" indicates a bulk material which essentially maintains the same microstructure from room temperature up to the high temperatures indicated. This means that minor variations can occur in the microstructure. Variations, such as the formation of small crystallized zones up to about 100 angstroms in an otherwise amorphous matrix, have substantially no adverse or detrimental effects on the properties of the bulk material. Therefore, structurally stable glasses of the present invention are essentially amorphous but may contain small crystallized zones of, for example, graphite, cristobalite or silicon carbide within the glass, or show minor amounts of cristobalite on the glass surfaces.

Silicon oxycarbon glass objects can be made according to several methods of this invention. In one method, the pyrolyzed resin is reduced to a powder having particle sizes ranging from 0.1 to two microns. Grinding mills, such as a friction mill or planetary mill, have been used to produce silicon oxycarbon particle sizes of 0.1 to 2 microns. Friction grinding is performed by agitating with a propeller a solution made up of about 52% of a liquid, such as water, about 35% of the grinding medium, as spheres with a diameter of 1.2 mm which are harder than the material to be grind and the rest consisting of crumbled particles of silicon oxycarbon glass. The agitation of the solution by helix at 1000 rpm reduces the glass particles into powder. Planetary grinding is performed with a similar solution except that the grinding media are spheres with a diameter of 5 to 8 mm and the solution is agitated by rotating the grinding container planetary at slower speeds.

The ground powder is then dried and consolidated by application of heat and pressure to form a shaped design. Consolidation can be achieved by applying a uniaxial pressure of at least about 34.45 MPa (5 ksi) at about 1500 ° C-1600 ° C or applying isostatic pressure of at least about 55.12 MPa. (8 ksi) at about between 1200 ° C and 1600 ° C. Heat and pressure are applied until the object has been thickened to the desired degree or until completely thickened.

In another method of forming silicon oxycarbon glass objects from cast or shaped precursor resins, the precursor methyl silicone resin is dissolved in a solvent and cast into the desired shape. Illustrative of the solvents that have been found suitable for dissolving the precursor resin are toluene and blends of toluene with isopropyl alcohol. Precursor resins can be dissolved in the solvent in ratios of up to about 8 parts of resin to 5 parts of solvent. The cast precursor resin is dried at room temperature. Preferably, the cast precursor resin is dried at a rate which allows the solvent to evaporate from the resin without forming voids in the resin itself. For example, an evaporation rate that prevents the formation of voids in drying the resin was established by placing the resin solution in a cylindrical plate open at one end and placing a piece of paper over the open end. Alternatively, the precursor resin, which is normally in the form of a powder, can be shaped by hot pressing.

The cast precursor resin is then pyrolyzed in an oxidizing atmosphere, as described herein. The rate of heating during pyrolysis must be controlled to allow gas to unfold without forming voids or bubbles in the resin. Preferably, heating rates of less than 1.0 ° C per minute are used to allow sufficient gas unwinding without forming bubbles, voids or defects in the glass. Pyrolysis is essentially complete when the unwinding weight loss of water, methyl groups and other decomposition products from the precursor resin substantially ends. Pyrolysis can be continued until the glass has completely thickened or stops shrinking in volume. The precursor resin identifies during pyrolysis and forms silicon oxycarbon glass.

EXAMPLES

The following examples are offered to further illustrate the silicon oxycarbon glass of this invention and the methods of making the glass and glassware. In the following examples the precursor resins formed by the method described above in U.S. Patent No. 2,676,182 and having methyl radical groups were used, where a first resin consists of M and Q units in a ratio of about 0.5: 1, a second MQ resin had a 1: 1 ratio, a third MQ resin had a 2: 1 ratio, and a fourth MQ resin had a 3: 1 ratio.

Methylsilicone precursor resins were pyrolyzed by heating them to temperatures ranging from 1100 ° C to 1250 ° C in a non-oxidizing atmosphere. During pyrolysis, the precursor resins underwent weight loss as water, methyl groups and other decomposition products developed. When the weight of the pyrolytic robbery stabilizes, the pyrolysis is substantially complete. However, after the weight loss is over, some thickening of the silicon oxycarbon glass may occur; therefore, heating and pyrolysis can continue until the silicon oxycarbon glass is completely thickened.

Weight losses measured during pyrolysis ranged from approximately 17 to 54%. Part of the weight loss can be attributed to variations in the amount of solvents left over from resin production.

Examples 1-4

The first, second, third and fourth MQ resin described above were pyrolyzed according to the method of this invention while the weight loss of the resins was measured by thermogravimetric analysis. Thermogravimetric analysis is a method for measuring weight loss from a sample while it is heated. The samples were heated in a hydrogen atmosphere at a rate of 10 ° C per minute to a temperature of J250 ° C. The measured weight loss for each silicon oxycarbon glass formed after pyrolysis is shown in Table 1. Unexpectedly, the second, third and fourth precursor resin consisted of M and Q units in a ratio of 1: 1. , 2: 1 and 3: 1 formed a translucent glass after pyrolysis. The first resin having an M to Q ratio of about 0.5: 1 was opaque and black in appearance after pyrolysis. Therefore, precursor resins having an M to Q ratio of about 0.7: 1 or more are believed to form translucent glasses of silicon oxycarbon, while precursor resins having an M to Q ratio of less than about 0.7: 1 form opaque glasses of silicon oxycarbon.

The weight loss data of Example 1, 2 and 3, determined by thermogravimetric analysis, are presented in the graph of Figure 1. In the graph of Figure 1, the percentage weight loss of each sample is plotted on the ordinate, while the The heating temperature rise is plotted on the abscissa. A graph of Figure 1 shows that a significant portion of the weight loss in each sample occurred at temperatures as low as 900 ° C, while the weight loss is essentially complete at 1200 ° C. Substantially no evidence of crystallization by X-ray diffraction of the pyrolyzed material was found and substantially no bond between carbon atoms and oxygen was found by infrared spectroscopy of the same. The weight loss of example 4 followed the same temperature model of examples 1, 2 and 3.

The diffraction index was measured on the silicon oxycarbon glass sample of Example 2 as 1.58, using a sodium light frequency of 5,893 angstroms. Glasses having a diffraction index between about 1.5 and 1.9 at the sodium frequency of 5,893 angstroms are generally known. The diffraction index is the phase velocity of the radiation in free space divided by the phase velocity of the same radiation in a particular medium.

Examples 5-6

The composition of different glasses can be largely defined by considering the amount of each element in the glass. However, it is the short-distance ordering of the glasses that gives them their different properties. Therefore, by characterizing the short-distance ordering of the glasses, different compositions of glasses can be defined with respect to properties. In example 5, the short-distance sorting of the silicon oxycarbon glass of this invention is determined by defining the percentage of each of the polyatomic units; tetracarbosilicon, monocarboosiloxane, dicarboosiloxane and tetraoxysilicon which are present in the glass.

The nuclear magnetic resonance spectrum of silicon 29 in the solid state of a translucent glass sample of silicon oxycarbon of Example 2 was recorded and is presented in Figure 2. Figure 3 is the nuclear magnetic resonance spectrum of silicon 29 from a sample of silicon carbide fiber "Nicalon". On the ordinate of Figures 1 and 3 the intensity of radiation measured by the excited sample is plotted and the parts per million (ppm) of chemical displacement are plotted on the abscissa by a tetramethylsilicon standard that fixes the zero point on the abscissa. Chemical displacements in ppm are known for several polyatomic units, for example, tetraoxysilicon, dicarboosiloxane and monocarbosiloxane are shown in "NMR Basic Principles and Progress Si-NMR Spectroscopic Results", Editori P. Diehl, R. Kosfeld, Springer Yerlag Berlin Heidelberg 1981 on pages 186, 184 and 178. Thus, each peak of Figures 2 and 3 defines the short-range ordering of particular polyatomic units of silicon.

Figure 2 shows the spectrum of the silicon oxycarbon glass prepared in example 2 and containing the peaks indicated with the numbers from 1 to 4. Peak 1 represents tetracarbosilicon, peak 2 represents dicarboosiloxane, peak 3 represents monocarboosiloxane and peak 4 represents tetraoxysilicon. By integrating the area under each peak, the fraction of each of said polyatomic units that is present in the glass can be determined. A correction for the background interference was made for the spectra of figures 2 and 3 to determine the integrated area under each peak.

The integrated area under each peak of Figure 2 reveals a composition for the silicon oxycarbon glass of Example 2 comprising a percentage by weight plus or minus 5% of the following, about 33% of tetracarbosilicon, about 17% of dicarbosiloxane, about 26% of monocarboosiloxane and about 23% of tetraoxysilicon. The analysis of nuclear magnetic resonance spectra and the translucent appearance of the glass indicate that up to traces of elemental carbon of about 0.1% by weight are dispersed in the glass.

The spectrum of figure 2 can be compared with the silicon carbide spectrum of figure 3 measured from a sample of "Nicalon" silicon carbide fiber. The composition for the "Nicalon" of figure 3 in percentage by weight is about 68% of silicon carbide, about 8% of dicarboosiloxane, about 17% of monocarboosiloxane and about 7% of tetraoxysilicon. From the spectrum of figure 3 it can be seen that the "Nicalon" fibers are mainly formed from silicon carbide with minor amounts of dicarboosiloxane, monocarboxy siloxane and tetraoxysilicon. On the contrary, the spectrum of figure 2 shows that a silicon oxycarbon glass is formed from tetracarbosilicon with substantial amounts of dicarboosiloxane, monocarboxyloxane and tetraoxysilicon. This unique short-range sort of silicon oxycarbon glass that binds carbon to silicon in a way previously unknown in glasses, provides the greatest resistance to devitrification and decomposition and characterizes the glasses of this invention.

The composition of the translucent glass sample of silicon oxycarbon of example 2 and of the Nicalon sample can also be described by considering the percentage in moles of each polyatomic unit. Table II below provides the conversion between mol percent and weight percent for each of these compositions. The compositions described in Table II are considered to be within plus or minus 5% by weight or plus or minus 5% by moles for each polyatomic unit.

Since the unit in moles is a molecular weight, the percentage in moles gives the percentage of each polyatomic unit of the samples on a molecular basis. The percentage of silicon atoms in the samples that are bonded to oxygen or carbon can then be determined using the mol percent. The silicon oxycarbon glass sample of example S, originally prepared in example 2, has from about 73 to 83% of the silicon atoms in the glass bonded to at least one single carbon atom. The "Nicalon" silicon carbide sample had about 90 to 100% silicon atoms in the silicon carbide sample bonded to carbon atoms.

Example 7

A silicon oxycarbon glass sample was prepared by pyrolyzing, in accordance with the method of this invention, a methyl silicone resin consisting of 5% D groups and 95% T groups. The resin thickened into a silicon oxycarbon glass comprising, in percentage by weight, about 39% of tetraoxysilicio, about 24% of monocarboosiloxane, about 22% of dicarboosiloxane, about 6% of tetracarbosilicon, with about 3 to 9% of elemental carbon dispersed in the glass. The silicon oxycarbon glass which is made from a DT-type methyl silicone resin is the subject of Italian patent application No. 20456 A / 90 filed on May 29, 1990. Resistance to oxidation and structural stability or resistance to devitrification Silicon oxycarbon glass made from the DT-type methyl silicone resin identified above was analyzed by heating hot pressed samples of glass for 240 hours at 1400 ° 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 crystallization in the bulk material of either sample. X-ray diffraction of exposed surfaces showed evidence of cristobalite surface crystallization in both samples within the depth of about 0.05 mm (0.002 inch) from the surface. Although the silicon oxycarbon glass sample of Example 7 is black in appearance and has a different composition from the translucent silicon oxycarbon glass sample, it contains the chemical bond between silicon and carbon atoms with an absence of chemical bond between carbon and oxygen atoms which characterizes the glasses of this invention. Therefore, the silicon oxycarbon glass of Example 1 and the translucent silicon oxycarbon glasses of Examples 2, 3 and 4 are expected to have substantially the same resistance to devitrification and decomposition as the silicon oxycarbon glass prepared for Example 7.

Claims (22)

CLAIMS 1. Clear glass which remains structurally stable at temperatures of about 1250 ° C or more, comprising silicon, oxygen and carbon in a distribution of polyatomic units comprising, by weight percent, from about 12 to 31% monocarboosiloxane and dicarboosiloxane, to be about 28 to 38% tetracarbosilicon, about 18 to 28% tetraoxysilicon, up to a trace amount of elemental carbon dispersed in the glass which allows at least partial light transmission through the glass. 2. The translucent glass of claim 1 comprising, in percentage by weight, from about 18 to 28% of tetraoxysilicon, from about 21 to 31% of monocarboosiloxane, from about 12 to 22% of dicarboxiloxane and from about 28 to 38% tetracarbosilicon. 3. Translucent glass that remains structurally stable at temperatures of about 1250 ° C or more, comprising silicon, oxygen and carbon in a mass of silicon oxycarbon glass in which about 73 to 83% silicon atoms are each bonded to at least a single carbon atom. 4. A process for forming a translucent glass comprising heating a methyl precursor methyl silicone resin formed from M and Q units in a ratio of about 0.7: 1 or more, in a non-oxidizing atmosphere to a temperature that pyrolyzes the resin, said heating being performed to a time period ending when the weight loss from the pyrolyzing resin substantially stabilizes; said pyrolyzed resin forming a translucent glass of silicon oxycarbon which remains structurally stable at temperatures of about 1250 ° C or more. 5. Process of claim 4 wherein said heating is performed between 900 ° C and 1600 ° C. 6. The process of claim 4, wherein said heating is performed for a period of time which allows a weight loss from the resin of about 17 to 54%. 7. The process of claim 4 wherein said heating is performed for a period of time which allows the resin to fully thicken. 8. The process of claim 4, wherein said heating is performed in a gaseous hydrogen atmosphere. 9. The process of claim 4 wherein said precursor resin is formed of M and Q units in a ratio of about 0.7: 1 to about 3: 1. 10. A process for forming a translucent silicon oxycarbon glass object, comprising: dissolving a precursor methyl silicone resin in a solvent, the precursor resin being formed of M and Q units in a ratio of about 0.7: 1 or more; shape the resin to form the object; evaporate the solvent from the molded resin; heating the resin in a non-oxidizing atmosphere to a temperature which pyrolyzes the resin, said heating being carried out for a period of time ending when the weight loss from the pyrolyzing resin substantially ends. 11. The process of claim 10, wherein said heating step is performed between 900 ° C and 1600 ° C. 12. The process of claim 10, in which said heating step is carried out in a gaseous hydrogen atmosphere. 13. The process of claim 10 wherein said heating step is performed at a heating rate which minimizes the formation of voids in the glass. 14. The process of claim 10, wherein said heating step is performed for a period of time which allows a weight loss from the resin between about 17 and 54%. 15. The process of claim 10 wherein said heating is performed for a period of time which allows the pyrolyzing resin to form a glass which is fully thickened. 16. The process of claim 10 wherein said heating step is performed at a heating rate of less than about 1 ° C per minute. 17. The process of claim 10 wherein said precursor resin is formed of M and Q units in a ratio of about 0.7: 1 to about 3: 1. 18. Glass fiber comprising silicon, oxygen and carbon in a distribution of polyatomic units comprising, in percentages by weight, from about 18 to 28% of tetraoxysilicon, from about 21 to 31% of monocarboosiloxane, from about 12 to 22 % dicarbosiloxane, about 28 to 38% tetracarbosilicon. 19. Compound ceramic, comprising at least one ceramic fiber within a silicon oxycarbon glass matrix bonding a ceramic filler, the glass comprising silicon, oxygen and carbon in a distribution of polyatomic units comprising, in percent by weight, from about 18 to 28% tetraoxysilicon, about 21 to 31% monocarboosiloxane, about 12 to 22% dicarbosiloxane and about 28 to 38% tetracarbosilicon. 20. A composition of translucent glass which remains structurally stable at temperatures of about 1250 ° C or more comprising silicon, oxygen and carbon chemically bonded with the glass substantially free of chemical bonds between oxygen and carbon atoms; the glass being produced by a process comprising: heating a precursor methyl silicone resin formed by groups M and Q in a ratio of about 0.7: 1 or more, in a non-oxidizing atmosphere to a temperature that pyrolyzes the resin, said heating being carried out for a period of time ending when the weight loss from the pyrolyzing resin substantially stabilizes. 21. A glass composition which remains structurally stable at temperatures of about 1250 ° C or more comprising silicon, oxygen and carbon chemically bonded with the glass substantially free of chemical bonds between oxygen and carbon atoms; the glass being produced by a process comprising: heating a precursor methyl silicone resin formed by groups M and Q in a ratio of about 0.7: 1 in a non-oxidizing atmosphere to a temperature that pyrolyzes the resin, said heating being performed for a period of time ending when the weight loss from the resin pyrolyzer substantially stabilizes. 22. A process for forming a glass comprising heating a precursor raethyl silicone resin formed from units M and Q in a ratio of up to about 0.7: 1, in a non-oxidizing atmosphere to a temperature that pyrolyzes the resin, said heating being performed for a period of time ending when the weight loss from the pyrolyzing resin substantially stabilizes; said pyrolyzed resin forming a silicon oxycarbon glass which remains substantially stable at temperatures of about 1250 ° C or more.