Silicon Oxycarbide Glasses

Journal of Sol-Gel Science and Technology 14, 7–25 (1999)c© 1999 Kluwer Academic Publishers. Manufactured in The Netherlands.

Silicon Oxycarbide Glasses

CARLO G. PANTANO, ANANT K. SINGH∗ AND HANXI ZHANG ∗∗

Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA

Received August 4, 1998; Accepted October 22, 1998

Abstract. The first attempts to introduce carbon into glass date back to 1951. But up until recently, the use of carbonor carbide raw materials, and the oxidation, volatilization and decomposition that accompany high temperaturemelting, have limited the synthesis of truesilicon oxycarbideglasses. Here, the termsilicon-oxycarbiderefersspecifically to a carbon-containing silicate glass wherein oxygen and carbon atoms share bonds with silicon inthe amorphous, network structure. Thus, there is a distinction betweenblack glass, which contains only a second-phase dispersion ofelemental carbon, and oxycarbide glasses which usually contain bothnetwork carbonandelemental carbon. In addition to exploring the unique properties and applications of these glasses, per se, they arealso of interest for developing models of the residual amorphous phases in polymer-derived silicon-carbide andsilicon-nitride ceramics.

The application of sol/gel techniques to glass synthesis has significantly advanced the development and charac-terization of silicon oxycarbide glasses. In this approach, alkyl-substituted silicon alkoxides, which are molecularprecursors containing oxygen and carbon functionalities on the silicon, can be hydrolyzed and condensed withoutdecomposition or loss of the carbon functional group. A low-temperature(<1000◦C) heat-treatment of the gel cre-ates a glassy silicate material whose molecular structure consists of an oxygen/carbon anionic network. In addition,there is always a blackening of the material due to elemental carbon, which forms during pyrolysis and densificationof the gel. The nature of thenetwork carbon, and especially the distribution and form of theelemental carbon, arefundamental to the structure and properties of these novel materials. Their chemical and physical characteristics asrevealed by NMR, Raman and TEM are discussed in the overview. In addition, the high temperature stability ofthese glasses (up to 1750◦C), and the effect of hot-pressing, are described.

It will be shown that the silicon oxycarbide network is stable up to 1000–1200◦C. The network carbon isterminated with hydrogen (i.e., CH,CH2 and CH3), and with polyaromatic carbon (i.e.,nC6Hx) wherein most ofthe elemental carbon resides. These glasses can be described as molecular composites of polyaromatic graphene-rings dispersed in a silicon oxycarbide network. After heating to temperatures in excess of 1000–1200◦C, theoxycarbide network decomposes through the loss of hydrogen, and a two- or three-phase glass-ceramic consistingof nanocrystalline graphite, silicon carbide, and amorphous silica or cristobalite, is created. Some of the propertiesand applications of these glasses/glass-ceramics for coatings, composites and porous solids are summarized.

Keywords: black glass, silicon oxycarbide, Nicalon, NMR, Raman, TEM, high temperature stability, surfacechemistry, network carbon, elemental carbon, structure, free carbon, FTIR, nanocomposite, silicon carbide

∗Now with Triton Systems, Inc.∗∗Now with Sun Microsystems, Inc.

8 Pantano, Singh and Zhang

Introduction

Silicon oxycarbide is a term used to denote the chemi-cal structure in which silicon is simultaneously bondedwith carbon and oxygen. These tetrahedral networkspecies can be generally described as [CxSiO4−x]wherex= 1, 2, or 3. The incorporation of carbon insilicate glasses presents the possibility of replacingsome oxygen, which is only two-coordinated, withcarbon which can be four-coordinated. This increa-sed bonding per anion is expected to strengthen themolecular structure of the glass network, and thereby,to improve the thermal and mechanical properties.

The melt synthesis of glasses with carbon substitu-tions is difficult due to oxidation. Thus, it should notbe surprising that carbon incorporation in glasses hasbeen most successful by the sol/gel process. The useof alkyl-substituted silicon-alkoxide precursors (e.g.,RSi(OR′)3 where R is CH3, C2H5, C3H7 or C6H6 andR′ is usually CH3 or C2H5) is a common feature of themethod. The SiC bond in the precursor is preservedduring hydrolysis, condensation, and drying. The re-sulting gels contain Si atoms bonded simultaneouslyto carbon and oxygen atoms, thus creating asiliconoxycarbidestructure. These oxycarbide gels have ap-plications themselves, or they can be further processedto obtain dense glasses, porous glasses, powders, glass-ceramics or composites.

The relationship between ‘silicon oxycarbides’ and‘organically modified silicates’ deserves comment.Fundamentally, they are uniquely related to one an-other through the local bonding environment of Si.An almost infinite number of materials with differentproperties and applications can be obtained by varying(i) the degree and nature of the organic functionalitieson the Si, and (ii) the temperature and atmosphere ofheat treatment. In this sense, the term ‘silicon oxycar-bide’ can be considered to encompass a wider classof materials. For most applications, though, the or-ganically modified silicates are not pyrolyzed abovethe decomposition temperature of the organic groups.Hence, they can be considered to be silicon oxycar-bide gels or silicon oxycarbide polymers. To synthe-size silicon oxycarbide glasses, however, pyrolysis athigher temperatures is required in order to dehydro-genate and cross-link the terminal organic groups, andto achieve the original goal of coordinating the carbonatoms to two, three or four Si atoms. The resultingsilicon oxycarbide glassesare shiny, black, X-rayamorphous materials. The black color is most certainlydue to the presence of an elemental form of carbon, butthe unique characteristic of these silicon oxycarbide

glasses is due to the presence of [CSiO3], [C2SiO2],[C3SiO] and [C4Si] molecular species in the glass net-work structure.

In general, the chemical characteristics of theseglasses change continuously with the temperature usedto process the precursor silicon oxycarbide gel. Thesechanges are associated with the thermal instability ofthe organic component and its chemical interactionwith the inorganic component of the material and theprocessing atmosphere. Herein lies the difficulty indefining these materials rigorously as ‘glasses’; i.e.,as noncrystalline solids displaying a glass transition.Indeed, these sol-gel derived silicon oxycarbides havebeen found to be amorphous (by X-ray and electron-diffraction patterns), at least up to pyrolysis temper-atures of 1200◦C. Yet, any attempt to measure theglass transition temperature (Tg) of the material willinevitably change the material itself. For example, agel pyrolyzed at 600◦C has to be heated to a tempe-rature higher than 600◦C in order to measure itsTg;this causes irreversible chemical changes in the mate-rial. These include redistribution and decompositionreactions as well as crystallization. Also, there is nodistinct pyrolysis temperature that separates gels fromglasses. It is noteworthy here that Brinker and Scherer[1] regarded the solid-phase of a non-crystalline gelas a glass, and they used the terms glassy, amorphousand noncrystalline interchangeably; Schmidt [2] con-cluded that organically-modified silicates are more likeinorganic glasses than organic polymers. It is in thiscontext that we refer to these materials assilicon oxy-carbide glasses.

This overview describes the sol-gel processing ofsilicon oxycarbides, and the evolution and characteri-zation of their chemical structure from the gel state upto 1750◦C. The dependence of the chemical structureand composition on precursor chemistry is presented,and models of the final (micro)structure are proposed.The fundamental aspects of carbon substitution in thesilicate network, and its thermal stability, are empha-sized throughout. Finally, the properties and potentialapplications of sol-gel derived silicon oxycarbides aresummarized (and some are described with more detailin other chapters of this special volume).

Background

Ellis was one of the first to attempt the incorporationof carbon in glass [3]. He added aqueous solutions ofcarbohydrates to porous Vycor (a high silica glass).Heating in inert atmospheres to 1200◦C decomposedthe carbohydrate to carbon, whose presence in the

Silicon Oxycarbide Glasses 9

interconnected porosity of the glass made it electri-cally conductive. The low-temperature resistivity ofthe glass was found to depend on the amount, sizeand shape of the carbon particles. Smith and Crandall[4], hot-pressed a mixture of fine colloidal silica andcarbowax at 1000–1150◦C and 2000 psi to obtain adense mass of glass containing 1.2 wt% carbon. Thisglass, wherein the carbon was “physically insepara-ble and microscopically indistinguishable from silica”,was found to exhibit a greater resistance to devitrifica-tion and a higher viscosity than pure vitreous silica.This result was significant because the usefulness offused silica is limited at temperatures over 1000◦C dueto the formation of cristobalite. Elmer and Meissner [5]incorporated carbon into porous (high-silica) glassesby impregnation with furfuryl alcohol and heat treat-ment in nitrogen at 1250◦C. A marked increase in theannealing point of the glasses was observed as a re-sult of the carbon incorporation. This increase wasattributed to the removal of hydroxyl groups from theporous glass by carbon.

Homeny et al. [6] used SiC as the carbon source andsynthesized glasses containing up to 2.5% carbon inthe Mg-Al-Si-O-C system by melting at 1750–1800◦Cunder nitrogen. Due to the high temperature meltingand consequent loss of CO, CO2 and SiO gases, thefinal compositions were difficult to control. The synthe-sized glasses were shown to be homogeneous (exceptfor a few metallic inclusions) and free of crystallitesby X-ray diffraction and electron microscopy exami-nation. The Mg-Al-Si-O-C glasses showed an increasein density, Young’s modulus, Shear modulus, Vicker’shardness and fracture toughness with increasing carboncontent. Coon [7] studied the effect of silicon carbideadditions on the crystallization behavior of a Mg-Li-Al-Si-O glass. SiC was mixed into a Mg-Li-Al-Si-Oglass melt and then the black glass was heat treated tostudy its crystallization behavior. The carbon contain-ing glass was found to be much more refractory thanthe parent glass.

In all of these studies of carbon-containing glasses,separate sources for silica and carbon were mixed andpyrolyzed at high temperature to allow them to reactto yield intimate mixtures of the two phases. Therewas no verification of carbon substitution in the glassnetwork in any of these studies. In some cases, theproperty changes supported the idea of a silicon oxy-carbide structure, but it is also possible that reduction ordehydroxylation of the glass (by the carbon additions)caused the property variations.

It is also important to point out that there are noknown thermodynamically stable phases of silicon

oxycarbide. This is in contrast to the situation withsilicon oxynitride glasses where there exists theequilibrium crystalline phase Si2N2O. This probablyaccounts for the greater ease of melt synthesis of siliconoxynitride glasses. Nevertheless, the existence ofamorphous,metastablesilicon oxycarbide phases hasbeen confirmed in a number of studies [8–20].

In a study of the oxidation products of SiC, Pampuchet al. [8, 9] obtained evidence for the existence of aternary Si-O-C phase situated between the silica layerand the SiC. The SiC powder was cleaned with HF so-lution, and was then subjected to varying degrees of ox-idation in the temperature range of 675 to 1775 K. Thesolid oxidation products were analyzed using SEM,X-ray photoelectron spectroscopy (XPS), infraredspectroscopy, X-ray microanalysis and petrographicanalysis. The presence of a considerable amount ofSi-O-C phase was indicated by all the analytical meth-ods. An extra Si2p peak, with binding energy higherthan that of the Si2p peak in SiC but lower than thatof the Si2p peak in SiO2, was found in samples heattreated below 1575 K. The additional peak was claimedto be due to the formation of Si-O-C phase. The infraredspectra showed some unique peaks, and suggested thatthis ternary phase was present at intermediate stagesof oxidation of SiC, and was not stable above oxida-tion temperatures of 1400–1500 K. Petrographic ana-lysis revealed that the refractive index of the Si-O-Cinterphase was between 2.58 and 2.63. More recently,Yurkov and Polyak [10] re-examined the oxide/silicon-carbide interface and confirmed the formation of siliconoxycarbide phase.

Lipowitz et al. [11, 12] studied the synthesis, com-position, structure and properties of polymer-derivedceramic fibers (NiCALON). They performed extensivestructural characterization by X-ray diffraction,29Siand 13C NMR, IR, Raman, XPS and Auger depth-profile analysis. They found that the microstructurecontained a continuous, amorphous Si-O-C phase withnanocrystallineβ-SiC. TEM micrographs revealed thattheβ-SiC crystals, in the size range of 4.0 to 2.6 nm,were embedded in this amorphous matrix.29Si NMRproved to be the most useful method for characterizingthe oxycarbide phase. It showed a broad peak coveringthe range of SiC (−15 to−25 ppm), CSiO3 (−70 ppm),C2Si2O2 (−30 ppm) and C3SiO (10 ppm).13C NMRof the fibers showed that free carbon was also presentin the fibers. The Raman spectra showed two broadbands at 1575–1610 cm−1 and 1350−1; these verifiedthe presence of free carbon. XPS analysis showed aSi2p peak which did not correspond to SiO2 or SiC. Thebinding energy value was between that of SiO2 and SiC


and was considered to be the binding energy of Si2pphotoelectrons in the oxycarbide amorphous phase.

Porte and Sartre [13] also provided evidence forthe presence of silicon oxycarbide phase in polymer-derived SiC (NiCALON) fibers (in addition to SiC,silica and graphitic carbon). XPS yielded a quanti-tative compositional analysis as well as the chemicalstructure of Si in the fibers. The binding energies of theC1s and Si2p photoelectrons unambiguously showedthe presence of SiO2 and SiC in the sample. Deconvo-lution of the Si2p peaks showed a species whose bind-ing energy was between that of Si2p peaks for SiO2

and SiC. This intermediate phase was concluded to besilicon oxycarbide. The molar ratios of the chemicalentities constituting the fibers were calculated to beSiC : SiOxCy : C : SiO2= 1 : 0.5 : 0.75± 0.25 : 0.08.

The formation of a Si-O-C intermediate phase hasalso been found by White et al. [14, 15] in their syn-thesis of high surface area SiC powders. Gels withthe general formula{RSiO1.5} were synthesized fromorganosilicon precursors having various organic groupsusing sol-gel processing. A black “glassy, carbon con-taining silica phase” was observed upon pyrolysis ofthe gels at temperatures close to 1000◦C under inert at-mospheres. Pyrolysis at 1500◦C typically yielded large,shiny black particles containing a partially crystallineand partially amorphous mixture of SiC, carbon andsilica. These particles were glassy, and difficult to oxi-dize or to dissolve with HF. Since the focus of theirwork was to synthesize SiC, the nature of the blackglassy phase was not studied in any detail.

It is now widely recognized that the chemical/polymer-processing of silicon carbide and nitride pow-ders, ceramics, and especially fibers such as Nicalon,can create a silicon oxycarbide phase. The thermalstability of this phase is fundamental to the hightemperature behavior of these important engineeringmaterials. Thus the widespread interest in chemical/polymer-processing of silicon carbide ceramics hasprovided an additional driving force for synthesis andstudy of phase-pure silicon oxycarbide [16–20].

Sol/Gel Processing of Silicon Oxycarbide Glasses

The sol-gel process has enabled the low-temperaturesynthesis of silicon oxycarbide glasses without theproblems of decomposition and oxidation during melt-ing. This is achieved through the use of polymericprecursors containing SiC bonds; viz., organically-modified alkoxysilanes of the general formula—

{RxSi(OR′)4−x} [21–53]. These precursors provide adirect Si C bond in the starting solution which ispreserved in the gel and glass structures; this is in con-trast to the alkoxysilanes{Si(OR)4} which are com-monly used for sol/gel synthesis of silica glass. In thecase of these modified-alkoxysilanes, one or more ofthe alkoxy groups are replaced by saturated (e.g., CH3,C2H5, C3H7) or unsaturated (e.g., C2H3, C6H5) ‘R’group(s). The carbon chain-length, and the number andnature of the ‘R’ group modifications, allow a controlof the amount of carbon introduced. An additional de-gree of compositional control is afforded by mixing theorganically-modified alkoxysilanes, in desired molarratios, with other alkoxysilanes. The gel, obtained afterhydrolysis and condensation of the precursors, containsSi atoms bonded simultaneously to oxygen and carbonatoms. The silicon oxycarbide gels are commonly heattreated at intermediate temperatures (800–1000◦C) toobtain silicon oxycarbide glasses.

Chi [21] reported on the sol/gel processing ofmonolithic glasses from mixtures of tetraethylortho-silicate {Si(OC2H5)4} and methyltrimethoxysilane{CH3Si(OCH3)3}, cohydrolyzed and mixed with col-loidal alumina monohydrate and colloidal silica fillers.Upon pyrolysis in argon at 1200◦C, the gels turnedblack. The maximum carbon content was 12.5%. Onlylimited structural characterization was reported onthese gels and glasses, but the black glass showed veryhigh thermal stability and resistance to crystallization.After 6 hours in argon at 1250◦C, no crystallization wasobserved, while the same heat treatment completely de-vitrified pure silica gels or glasses. Crystallization didnot take place until the carbonized-glass was held at1450◦C for 5 hours in argon. The crystalline phaseswereβ-SiC and cristobalite.

A systematic series of silane precursors were usedto prepare oxycarbide glasses by Zhang and Pantano[23]. Methyl-, ethyl-, propyl- and phenyl-trimetho-xysilanes with chemical formulae CH3Si(OCH3)3,C2H5Si(OCH3)3, C3H7Si(OCH3)3, and C6H5Si(OCH3)3, respectively, were hydrolyzed in H2O (1 : 6molar) and ethanol (1 : 1 volume) using HCl as the cat-alyst. 1H NMR indicated that the majority of SiORgroups had been hydrolyzed to SiOH, while themethyl, ethyl, propyl and phenyl groups bonded di-rectly to Si, were retained. Ammoniated solutionswere added to initiate the gelation which occurredwithin a few days to several weeks.13C and29Si MASNMR of these gels showed that the alkyl groups werestill present in the gels, while the SiOH terminal


bonds had been largely replaced by SiO Si throughcondensation. The dried gels were translucent, butturned black upon heat treatment in argon at 800–1000◦C. X-ray and electron diffraction confirmed theamorphous state of these materials.

The 29Si MAS-NMR spectra of silicon oxycarbideglasses indicate that a distribution of distinct siliconspecies appear in the structure. Moreover, these spec-tra show the creation of new Si-species during the finalheat-treatment. Figure 1 compares the29Si NMR spec-tra for the methyltrimethoxysilane (MTMS) precursor,

Figure 1. 29Si MAS-NMR spectra of the methyl-trimethoxysilane(MTMS) precursor, the dry gel, and the 800◦C oxycarbide glass.

the MTMS gel, and the final black glass. The liquidprecursor exhibits one type of Si-species. The line issharp due to the rotational degrees of freedom providedin the liquid state. The amorphous, solid-state of the geland glass causes the observed broadening of the lines.The dried gel shows two types of Si-species. One is a Sitetrahedra with two non-bridging sites (SiCH3 andSi OH) and two bridging oxygen sites. The other is aSi tetrahedra with one non-bridging site (SiCH3) andthree bridging oxygen sites. Obviously, these speciesconform to the chemical structure that defines asiliconoxycarbide. The final black glass has a wider distribu-tion of Si-species, and clearly, two of these have beencreated during the pyrolysis/densification step. One ofthe new lines is due to a silicon-oxygen tetrahedra-[SiO4]. This is the usual building block of silicateglasses, and its presence here indicates that some de-composition of the oxycarbide species occurred in thegel during pyrolysis. The other new species that formsduring the pyrolysis is a Si tetrahedra with two bridgingoxygens and two carbon bonds, i.e., [C2SiO2]. Clearly,these species were created through condensation and/orexchange reaction during pyrolysis of the gel. It hasto be emphasized that these29Si NMR spectra can-not characterize the functionality of the carbon atomsbonded to the silicon [52]; i.e., they can be non-bridgingmethyl groups ( Si CH3), or bridging groups suchas Si CH2 Si or even [CSi4]. (This issue is dis-cussed further in the next section).

A number of studies have focused on the effects ofprecursor chemistry on the carbon-content of the gelsand glasses. Zhang and Pantano also studied the effectof increasing carbon chain-length of the alkyl-modifiedalkoxysilane precursor, i.e.,{(CxHy)Si(OR)3} on theamount of carbon retained in the gels and glasses [23].Precursors havingsaturatedhydrocarbon group modi-fications, e.g., methyl, ethyl, propyl groups attacheddirectly to the Si atom were subjected to hydrolysisand condensation. The carbon content of thegelswasfound to increase proportionately with increasing car-bon chain length of the alkyl group. Yet, the carbon con-tent of the oxycarbideglassesobtained after pyrolysisat 900◦C did not show a proportional increase with car-bon content of the precursor/gel. Oxycarbide glassesobtained from methyltrimethoxysilane (MTMS) andpropyltrimethoxysilane (PTMS) precursors displayednearly equal carbon contents. And, the29Si MAS NMRspectra showed proportionately, higher amounts of car-bon bonded to the Si atoms in MTMS-glasses. Figure 2compares the29Si NMR spectra of the oxycarbide


Figure 2. 29Si MAS-NMR spectra of oxycarbide glasses synthe-sized with methyl-, ethyl-, propyl-, and phenyl-trimethoxysilanes.

glasses prepared with the various alkyl-substitutedprecursors. There is a clear trend towards increasingoxycarbide fraction with a decreasing content of car-bon in the alkyl group. But in general, about 45% ofthe silicon is bonded with one carbon and 3 oxygens([CSiO3]), about 45% of the silicon is bonded with4 oxygens ([SiO4]), and about 10% of the silicon atoms

are bonded with 2 carbons and 2 oxygens ([C2SiO2]).Thus, it is proposed that only the carbon atom bondeddirectly to the Si atom can be retained in the glass struc-ture. The cleavage of the SiC bonds in the PTMS gelsbefore the high temperature thermal decompositionis attributed to the kinetics ofβ-elimination reactions,which are known to occur at temperatures<450◦C insilicon compounds [55]. Similar results were obtainedby White et al. in their study of oxycarbide gels synthe-sized with a series ofsaturatedhydrocarbon modifyinggroup [14, 15].

Gels and glasses synthesized from precursors hav-ingunsaturatedhydrocarbon modifications (e.g., vinyl,allyl and phenyl groups) behave differently. Compa-risons were made between the carbon contents ofglasses obtained at 900◦C from gels modified by sat-urated and unsaturated organic groups containing thesame number of carbon atoms. Comparison of the re-sults of the ethyl (C2H5) gels vs. vinyl (H2C CH) gels,and propyl (C3H7) gels vs. allyl (H2C CHCH2) gelsshowed much higher carbon-contents in the glassesmade from gels containing unsaturated organic groups[14, 15, 27].

One of the key issues in synthesis is the ability tomaximize retention of the SiC bonds (and thereby tominimize formation of free carbon). This problem hasbeen successfully addressed through the introductionof reactive Si H groups in the precursors; i.e., oneor more of the Si O bonds of the precursor are re-placed by Si H bonds. The SiH bonds initiate earlycross-linking of the network through reactions with theSi CxHy bonds, and therefore polymerize the networkthrough Si C Si bridges. This is represented by thefollowing reaction:

H Si CH3+ H Si CH3

→ H Si CH2 Si CH3+ H2

Singh and Pantano [28] compared oxycarbidegels and glasses made from methyltrimethoxysilane{MTMS—CH3Si(OCH3)3} with those made frommethyldimethoxysilane{MDMS—CH3HSi(OCH3)2}.The MDMS precursor possesses a SiH functionalityin addition to the Si CH3 ligand. MDMS and MTMSwere mixed in various ratios with TEOS in order to varythe carbon content of the gels. Thus, the structures ofMDMS-derived glasses were compared with those ofthe MTMS-derived glasses over a wide range of carbon


concentrations. The effects of the SiH functionalityupon the condensation of the oxycarbide network wereinvestigated using FTIR and NMR and it was foundthat the Si H bonds survived the acidic hydrolysis andremained intact in the gel structures. The FTIR spec-tra in Fig. 3 clearly show the formation of SiCH2 Silinkages through a reaction of the SiH bonds with themethyl groups at temperatures below 500◦C. The NMRspectra in Fig. 4 verify a significantly higher oxycarbidefraction in the MDMS glasses obtained after pyrolysisat 900◦C (compare lower two spectra).

Figure 3. FTIR spectra of glasses synthesized with MDMS; in glasses made with MTMS, SiCH2 Si does not form below 800◦C.

Further enhancements in network crosslinking, andhence enhancements in oxycarbide contents, wereachieved by using SiH bonds in both precursors;i.e., by mixing MDMS with triethoxysilane (TES-HSi(OCH3)3). The uppermost spectra in Fig. 4 veri-fies an even higher oxycarbide fraction in these glasses.Chemical analysis confirmed that thetotal carbon con-tent in each of the glasses (in Fig. 4) were identi-cal. Thus, it could be concluded that SiH function-alities enhance oxycarbide formation and inhibit freecarbon. This is attributed to the fact that redistribution


Figure 4. 29Si MAS NMR spectra showing the effect of SiH bonds in the precursor on the oxycarbide glass structure obtained at 900◦C;the total carbon concentrations (by chemical analysis) are equal in all three; MTMS/TEOS (lower), MDMS/TEOS (middle) and MDMS/TES(upper).

and cross-linking reaction occur at lower temperaturein the gels containing one or more SiH functiona-lities.

Similar results on the effects of SiH bonds havebeen obtained by Babonneau et al. [29, 34].29Si NMRspectra of the gels were obtained to characterize thepresence of SiH in the gels, and also to measure therelative intensity of the oxycarbide phases in the vari-ous systems. The system with the greatest amount ofSi H in the precursor gel exhibited the highest concen-tration of Si C bonds, and at 1000◦C, [SiC4] specieshad already formed. A rough estimate of the free car-bon content showed that there was a very small amountof free carbon in this system at 1000◦C. Their estimatewas based on the assumption that all bonded carbonis four fold, as in Cx/4SiO(4−x)/2. Thus, it seems cer-tain that the use of reactive groups such as SiH canenhance the formation of stable oxycarbide species,and thereby, limit the creation of free carbon species.

The use of organosilicon precursors containing reac-tive Si Si bonds have likewise been shown to enhanceoxycarbide formation [32, 33, 39, 43].

Multicomponent oxycarbide glasses have also beensynthesized using the sol/gel process [31, 44, 47]. Inone study [31], the composition of the glass was cho-sen to approximate that of Pyrex glass except that theSiO2 was replaced by SiOxC4−x. 29Si NMR showedthat the Si C bonds remained in the gel and glass struc-tures. The Pyrex/oxycarbide glass showed much bettercrystallization resistance and thermal stability at hightemperatures than Pyrex glass. The Pyrex/oxycarbidegel was also used to fabricate carbon-fiber reinforcedglass matrix composites. The mechanical properties ofthese composites were superior to pure Pyrex glassmatrix composites at elevated temperature. This wasattributed to the more refractory nature of the oxycar-bide matrix and its resistance to crystallization. Onestudy has reported the effect of boron in increasing the


overall ceramic yield in a silicon oxycarbide systemmade from precursors containing SiH and Si CH3

bonds [44]. Although the chemical evolution of thesystem is not well understood at this time, the pre-sence of boron appears to have a catalytic effect onthe transformation of SiH bonds to Si O Si bonds,thus forming a more interconnected gel network, andgiving a higher ceramic yield. Multicomponent siliconoxycarbide glasses based on Al-Si-O-C and B-Si-O-Csystems have also been reported [47].

Finally, it should be noted that a variety of polymeric-precursors have been explored for the synthesis ofsilicon-oxycarbide glasses [26, 27, 36]. Renlund et al.[26, 27] synthesized oxycarbide glasses using a com-mercial silicone resin. The resin is a mixture of linearand cyclic siloxanes which are terminated with methyl( Si CH3) and hydroxyl ( Si OH) groups. The resincan be cross-linked with heat, or it can be dissolved inmany polar and nonpolar liquids (such as toluene, xy-lene, ketone, alcohols and di-ethyl ether) to permit so-lution processing. Due to the higher molecular weightof these precursors (relative to the more conventionalmonomeric sol/gel precursors), one can expect lessweight loss and shrinkage, and thereby, the possiblefabrication of monoliths or fibers. In their work,Renlund et al. prepared thin disks (1 mm thick), thinfoils, and fibers (∼1 mm in diameter). These were pro-cessed by spreading the resin directly on a substrateor by casting desired shapes. They were pyrolyzedat 1000–1250◦C in hydrogen or helium, and repor-tedly, they retained their shape without cracks. Fordensification of larger pieces, uniaxial and isostatic hotpressing techniques at 1400–1650◦C were used. Theyperformed a comprehensive study of the pyrolysis pro-cess and the microstructures.

TGA of the silicone resin in H2 showed two weightloss stages during pyrolysis. The first weight loss ofless than 10% occurred around 150◦C, and the majorweight loss (15%) at 750◦C. The measured compo-sition of the pyrolysis product was Si1.7O2.6C. Theirstructural characterization showed no obvious crys-talline phases in the materials pyrolyzed at 1200 and1450◦C while those hot-pressed at 1650◦C showedthe presence of crystalline SiO2 and SiC. TEM mi-crographs of the glasses heat treated at 1400◦C alsoshowed a featureless structure. In the samples hotpressed at 1650◦C, SiC crystals of 4 to 5 nm were foundby TEM. XPS data were reported for the samples py-rolyzed at 1200◦C. The C1s spectra revealed no peaksfor C O and C O. The Si2p lines were intermediate

in binding energy between SiO2 and SiC.29Si NMRspectra of the glass pyrolyzed at 1200◦C showed thepresence of [SiO4], [CSiO3], [C2SiO2] and [SiC4]species. Their Raman spectra were attributed to thepresence of graphite. These investigators also reportedmany properties for their materials.

Altogether, it seems clear that many different precur-sors can be used for sol/gel and/or polymer synthesisof silicon oxycarbide glasses. In the next section, itwill be shown that the pyrolysis of these various pre-cursors yields materials consisting of an oxycarbideamorphous phase, elemental carbon species, and in thecase of high temperature pyrolysis or heat-treatment,crystalline phases. Reactive SiH bonds can be usedto enhance network crosslinking and hence the reten-tion of the Si C bonds. The fraction of carbon in theamorphous oxycarbide phase is thus enhanced in com-parison to the elemental free carbon species.

Structure of Silicon Oxycarbide Glasses

The 29Si NMR analyses of oxycarbide glasses pre-pared by pyrolysis of gels in the range 800–1000◦Cconsist primarily of [SiO4], [CSiO3], [C2SiO2] and[CSiO2(OH)] species; in some cases, [C3SiO] and[SiC4] may also be present. These species undoubtedlyconstitute the basic structural units of the oxycarbidenetwork. There is a significant quantity of hydrogen inthese glasses which is primarily associated with the car-bon atoms in this oxycarbide network. This is verifiedin the13C NMR spectra shown in Fig. 5. The13C NMRspectra shows the presence of [SiCH3] species whichprobably represent terminations in the network whichare analogous to non-bridging oxygens; the breadth ofthis peak suggests that there may be higher order hy-drocarbon terminations (C2H5, C3H7) as well. The13CNMR spectra also shows the presence of free carbonin the form of polyaromatics [nC6Hx].

The fact that silicon-oxycarbide glasses are preparedwith precursors wherein all the Si atoms have at leastone Si C bond, and yet there are [SiO4] species af-ter pyrolysis at 800–1000◦C, reveals some dissociationof Si C bonds and formation of free carbon duringpyrolysis. The black color of virtually all silicon oxy-carbide glasses prepared atT > 800◦C is, perhaps, themost obvious evidence for free (elemental or polyaro-matic) carbon in the glass. The issue that remains tobe resolved is the distribution of this free carbon andits structural association with the silicon-oxycarbidenetwork.


Figure 5. 13C CP-NMR spectra of oxycarbide glasses synthesizedwith MTMS (upper) and PTMS (lower); these glasses were stabilizedat 800◦C for 10 hours.

Many attempts have been made to characterize thefree carbon phase in these glasses using high-reso-lution transmission electron microscopy (HRTEM).Although glasses heat-treated to temperatures in ex-cess of 1400◦C reveal the presence of amorphousand graphitic carbon phases using HRTEM (see nextsection), the lower temperature, X-ray amorphousglasses prepared at 800–1000◦C show no observablemicrostructure. Figure 6 is a typical bright-field imageof an 800–1000◦C glass wherein only a single-phaseamorphous structure exists.

Raman spectroscopy is very sensitive to the pre-sence and form of disordered elemental carbon. Butwhereas the Raman spectra of the higher temperature(>1400◦C) glasses also show amorphous and graphiticcarbon species in the glasses, the Raman spectra of800–1000◦C glasses are featureless. Since the visualappearance of the glasses, and the13C NMR spectra,verify the presence of elemental carbon in the glasses,the absence of a Raman spectra reveals that the size of

the elemental carbon species is below a critical size.According to both Lespade et al. [57] and Knight andWhite [58], there exists a coherence length (La) thatmust be exceeded to create a Raman signal. In thecase of amorphous and crystalline carbons, this co-herence length is estimated to be∼2.5 nm. Thus, onecan conclude that the free carbon species in the 800–1000◦C glasses are dispersed in the glass at a scale<2.5 nm. To date, there is no direct evidence con-cerning the bonding or other structural association ofthese free carbon species with the silicon-oxycarbidenetwork. It seems likely, though, that some fractionof the Si C bonds detected in the29Si NMR are as-sociated with carbon atoms in polyaromatic (free) car-bon species. Thus, we propose the schematic structureshown in Fig. 7. Most of the carbon in the oxycar-bide network is in the form of Si CH3 non-bridgingsites. These species are clearly evident in the29Si and13C NMR spectra. It is possible that bridging networkcarbon species (Si CH2 Si ) are also present. Thepolyaromatic carbon species are shown bonded to theoxycarbide network, but an equally valid interpreta-tion of the available data would show them as distinctpolyaromatic carbon species (nC6Hx) embedded in theglass structure. It is likely that both situations existin the glasses. These polyaromatic carbon species-bonded to the network or simply embedded-couldrange in size from single carbon rings to polyaromaticswith dimensions up to 2.5 nm.

Thermal Stability of Silicon Oxycarbide Glasses

The oxycarbide glasses synthesized from gels py-rolyzed at 1000 to 1400◦C have several distinct fea-tures compared to the lower temperature (800–1000◦C)glasses. Table 1 presents a typical set of compositionaldata for MTMS-derived glasses after heat-treatmentin this higher temperature range. It shows that from1000 to 1400◦C, the carbon, oxygen and silicon con-centrations are reasonably constant. But, there is asignificant loss of hydrogen. Simultaneously, the dis-tribution of oxycarbide species—observed in the29SiNMR analyses—change dramatically. Figure 8 showsthat the concentration of [CSiO3] decreases relativeto [SiO4], accompanied by an increase in higher or-der oxycarbide and carbide species [CxSiO4−x] withx= 2 and 4. The position of the [CSiO3] peak is alsoshifted up-field by a few ppm, relative to the samepeak at 800◦C, during this decomposition. These small


Figure 6. Bright-field TEM micrograph of a 900◦C oxycarbide glass.

Figure 7. Schematic representation of an amorphous 800–1000◦C silicon oxycarbide glass structure.


Table 1. Composition of the MTMS glasses (by chemicalanalysis) in atomic percent.

PyrolysisT Carbon Silicon Hydrogen Oxygen(◦C) 1 h/2 h 1 h/2 h 1 h/2 h 1 h/2 h

800 16/16 23/23 19/20 43/41

1000 22/21 30/29 6/2 46/44

1200 22/22 30/31 2/1 46/46

1400 22/22 30/31 2/0.3 47/47

Figure 8. 29Si MAS-NMR spectra of MTMS oxycarbide glassesstabilized for 10 hours at various temperatures.

peak shifts can be attributed to a change of the sec-ond nearest neighbor environment [54]. The study ofsilicate minerals with different structural units by29SiNMR has shown that polymerization of silicates intothree-dimensional structures continuously shifts the

Figure 9. The time-dependent decomposition of [CSiO3] oxycar-bide species at various temperatures for MTMS-derived glasses.

corresponding peaks to a higher field. Here, shifts in the29Si NMR spectra of the oxycarbide species indicate theremoval of OH and H, the formation of SiC or Si Obonds, and consequently, further polymerization of theglass network structure.

Figure 9 shows the time and temperature dependenceof the oxycarbide ([CSiO3]) decomposition reaction.There appears to be anequilibriumdistribution of thevarious oxycarbide species at each temperature. Sincethis decomposition occurs without a change in the C, Siand O concentrations (only hydrogen is evolved), it islikely that exchange reactions are playing an importantrole in redistribution of the oxycarbide species (CSiO3].Exchange reactions, or redistribution reactions, havebeen found to be prevalent in liquid state chemical re-actions since the mobility of atoms are higher [55], andhave been verified in polysiloxanes by Belot et al. [56].They are essentially chemical reactions in which two ormore atomic or molecular species exchange sites witheach other, on one or more polyfunctional central atomsor moieties, with the exchange eventually reaching anequilibrium state. They can be acid or base catalyzed,or thermally activated.

The formation of [SiC4] species is significant. In thecase of oxycarbide glasses made with MTMS only, the[SiC4] does not appear until∼1200◦C (e.g., see Fig. 8),whereas the glasses synthesized with MDMS or TESprecursors containing SiH show [SiC4] formation atlower temperatures. The formation of SiC through car-bothermal reduction of SiO2 (by C) occurs only at much


higher temperature (≥1500◦C). Thus, it is likely thatthese [SiC4] species are created through exchange reac-tions and decomposition of the [SiOxCy] species. Thisis analogous to the formation of SiC through pyrolysisof polycarbosilane which starts at 1000◦C. Since all ofthese glasses are X-ray amorphous, the [SiC4] speciesmust be quite dispersed. Nevertheless, they can be con-sidered nuclei for subsequent crystallization at highertemperatures. The observed decrease in relative con-centration of [SiCO3] and [SiC2O2] species with in-creasing temperature is consistent with this model. At1400◦C, the oxycarbide structure has completely de-composed into an amorphous mixture of [SiO4] and[SiC4] species. A complete microstructural character-ization of theseglass-ceramicsis reported elsewhere[38].

Some selected studies have been performed toexamine the chemical evolution of these glasses at tem-peratures in excess of 1400◦C. In this case, PTMS oxy-carbide glasses were synthesized (via pyrolysis of thegels in Ar) at 1000◦C, and then they were subjectedto further heat-treatment in a graphite furnace (also inargon) at 1600, 1700 and 1750◦C. At 1600 and 1700◦C,little weight loss was observed, while after heat treat-ment at 1750◦C, the glasses exhibited a large weightloss and significant compositional changes. The29SiNMR spectra in Fig. 10 show that in the glasses heattreated at 1600 and 1700◦C, SiC was the major phase,although there was still some amorphous SiO2 present.The narrower [SiC4] peak width indicates that the crys-tallinity of SiC was greatly increased. The [SiC4] peakevolves to that ofα-SiC indicating its coexistencewith β-SiC in the samples. Upon heat treatment at1750◦C, the amorphous SiO2 disappeared, leaving onlya (broad) SiC peak in the spectrum. The broad peak isdue to a mixture ofα-, β- and other SiC polymorphs.

X-ray diffraction patterns confirmed these results byshowing an amorphous hump for SiO2 and the peaks forSiC at 1600 and 1700◦C, and sharpα andβ-SiC peaksat 1750◦C. Although the29Si NMR spectra suggestedthat the crystallinity of SiC had greatly increased at1600 and 1700◦C, compared to the lower temperatureglasses, the X-ray diffraction patterns showed that thesize of the SiC crystals was still very small in the 1600and 1700◦C heat treated glasses. The Raman spectrain Fig. 11 reveal the presence of disordered carbon at1600 and 1700◦C, and SiC at 1750◦C. The absence ofSiC peaks in the Raman spectra at 1600 and 1700◦Cfurther confirms the molecular dimensions of the [SiC4]species detected in the29Si NMR analyses.

Figure 10. 29Si MAS-NMR spectra of PTMS glasses heated to hightemperatures under flowing argon.

The crystallinity of the glass, and the relative com-position of the material, changed abruptly at 1750◦C.There were large crystals of SiC, and no silica or car-bon present in the 1750◦C material. TEM micrographsof the 1750◦C material confirmed that large crystalsof SiC existed at this temperature with little amor-phous phase left. Due to the SiO2 and C in the 1600and 1700◦C materials, and the possible evolution ofgaseous reaction products in this heat-treatment, thecarbothermal reduction of SiO2 by C is expected [59].This reaction proceeds as:

SiO2(s) + C(s) = SiO(g) + CO(g)

SiO(g) + 2C(s) = SiC(s) + CO(g)

Thermodynamic calculations show that at 1600, 1700and 1750◦C, the equilibrium pressure of CO is 2.5, 6.4


Figure 11. Raman spectra of the PTMS glasses heat-treated underflowing argon.

and 9.3 atm, respectively. These pressures represent astrong driving force for the reaction. In the absenceof an applied pressure, and especially under a flowingstream of Ar, the reaction will be displaced towards theformation of SiC and CO. The formation of SiC at theexpense of C and SiO2 is consistent with the chemi-cal and structural analyses of the heat treated glasses.It was further observed that the surface of the glassesheat treated at 1600 and 1700◦C changed to a dull color,while the core of the pieces remained black and unrea-cted. This, too, is consistent with the above mechanismwhich depends on the loss of CO through the surface.The higher the temperature, the higher the equilibriumCO pressure and the faster the reaction proceeds. At1750◦C, the reactions were completed in 10 min, leav-ing only SiC.

Altogether, these studies show that oxycarbideglasses can be hosts for the crystallization of fine-grainsilicon-carbide; similar findings were also reported byWootton et al. [47]. The data suggests that the presence

of amorphous (free) carbon plays an important role insuppressing SiC grain growth. At temperatures greaterthan 1700◦C, however, carbothermal reduction leads tocomplete decomposition of the glass-ceramic into SiCpowder. In an application of these concepts, silicon-oxycarbide xerogels and aerogels were used to synthe-size high surface area SiC [46].

Hot-Pressed Silicon Oxycarbide Glasses

The PTMS oxycarbide glasses have also been hot-pressed to examine evolution of the structure in theabsence of the gaseous decomposition reactions. Inthis case, glasses were prepared at 1200◦C (to permithydrogen evolution), ground to a fine-powder, and thenhot-pressed in the range 1600–1750◦C. The chemicalanalyses of the hot-pressed glasses verified the absenceof composition changes except for some further loss ofhydrogen.

The 29Si NMR spectra of the hot-pressed oxycar-bide glasses are presented in Fig. 12. There is a large[SiC4] peak in all the spectra as well as the [SiO4]peak. This [SiC4] peak is higher in intensity and nar-rower in band width than the [SiC4] peak in the 1200◦Cpyrolyzed glasses. The increased intensity of [SiC4]peak shows that the amount of [SiC4] has increasedduring hot-pressing. The decreased [SiC4] peak widthobserved in these hot pressed materials indicate an in-creased crystallinity of the SiC. The intensity and bandwidth of the [SiO4] peak remains almost the same asthat of as-pyrolyzed glassed indicating that the SiO2

phase is still amorphous. The intensity of the SiC peakis higher than that of SiO2 indicating the formation of alarge amount of SiC during heat treatment under pres-sure. The small [CSiO3] at−70 ppm peak shows thatthe oxycarbide species have decomposed even in theabsence of gas evolution.

Raman spectra of the hot pressed glasses show thepersistence of disordered carbon. The spectra in Fig. 13show that this carbon phase grows in size over the tem-perature range 1600 to 1750◦C. The intensity ratios ofthe D and G bands suggest an average size of∼5.0 nmfor the disordered carbon phase at 1750◦C. The pres-ence of carbon and silica in these hot-pressed materialsverifies the role of the gaseous decomposition reactionsin the composition changes that occur in the absence ofpressure, and the role of carbon in limiting SiC graingrowth.

The presence and morphology of the SiC, SiO2 andC have been directly confirmed by TEM micrographs.


Figure 12. 29Si MAS-NMR spectra of hot-pressed PTMS glasses.

Some high resolution TEM micrographs of the hotpressed glasses are presented in Fig. 14. These datashow turbostratic (disordered) carbon with a size rang-ing from 1 to 10 nm uniformly distributed in the glassstructure. The lattice images of SiC crystals, with asize of 5 to 10 nm, are also observed. Altogether,the TEM, Raman and diffraction data characterize thestructure of these hot pressed glasses as microcrys-tals of silicon-carbide and turbostratic carbon domains,embedded in an amorphous silica matrix. The uni-form distribution of SiC microcrystals suggest that thesilicon-oxycarbide species (in the glass precursor) maybe the operative nucleating agents for the crystalliza-tion of SiC. Some additional studies of the synthesisand properties of these hot-pressed silicon-oxycarbideglass-ceramics were reported [35, 38].

Figure 13. Raman spectra of the hot-pressed PTMS glasses.

Properties and Applications

There is still only limited data available concerningthe physical properties of silicon oxycarbide glasses.There are probably two reasons for this. The sol/gelprocess does not always yield samples whose size andintegrity are appropriate for property measurements. Inaddition, the details of the synthesis and thermal pro-cessing influence the final structure and composition.Thus, it is difficult at this time to provide a genericset of property data. Nevertheless, it is instructive toexamine the trends.

Renlund et al. [14] have reported on the properties ofoxycarbide glasses synthesized with the silicone-resinprecursor. Their data has been reproduced in Table 2.Although most of the properties are comparable to sil-ica, the differences that do exist are consistent with amore refractory, and a more compact, network structuredue to the carbon substitutions. Unfortunately, the ef-fects of theelemental carbonspecies are superimposed


Figure 14. High-resolution TEM micrograph showing lattice images of disordered-carbon and silicon-carbide dispersed in the amorphousmatrix of a hot-pressed PTMS glass (5 nm).

upon the effects of the oxycarbide network. More re-cently, others have reported on the viscosity [35] andmechanical properties [50, 53] of sol/gel processedsilicon oxycarbide glasses.

Although the intrinsic properties of these materialsare not yet established, practical applications are, nev-ertheless, emerging [60–72]. The stability of the porestructure in silicon oxycarbide glasses [45] has raisedinterest in their evaluation for lightweight structuralapplications [72], (porous) coatings and catalyst sup-ports. The resistance of the oxycarbide to sintering is

an obvious advantage for catalyst supports. The lim-ited data available suggests, further, that the surfacechemistry of porous oxycarbides [52] enhance the dis-persion of metallic catalysts and the rejuvenation ofcatalyst activity through hydrogen treatments. The hy-drophobic characteristic of porous silicon-oxycarbideglasses (processed and used atT ≤ 700◦C) is also anadvantageous property.

There has been a much greater activity in the applica-tion of oxycarbide glasses for composites. In principle,they are ideally suited for fiber-coatings and interphases


Table 2. Properties of silicone-resin derived oxycarbide glasses (from [26]).

Values forProperty Value Comments vitreous silica

Density 2.35 gm/cm3 2.20

Coefficient of the thermal 3.14× 10−6/K Average of many samples on cooling 0.5expansion between 1000 and 100◦C; hot-pressed

Vickers hardness 855 kg/mm2 200 gm load 600 to 700704 kg/mm2 1000 gm load

Critical stress intensity 1.8 MPa m1/2 1000 gm load 1factor

Fracture strength 153 MPa± 20 MPa SD 3-pt. bending of 0.74 mmdiameter fibers

385 MPa± 227 MPa SD 3-pt. bending of bars

Young’s elastic modulus 97.9 GPa 70

Index of refraction 1.58 At 0.5893µm 1.46

Glass transition 1350◦C Viscosity of 1013 P 1190◦

Dielectric constant 4.4 25◦C, 10 to 107 Hz pyrolyzed 4to 1100◦C

Dielectric loss tangent 0.1 25◦C, 10 to 107 Hz pyrolyzedto 1100◦C 10−4

Electrical conductivity 4× 10−13/Ä · cm 25◦C, pyrolyzed to 1100◦C ∼10−22

in carbon and silicon-carbide fiber reinforced glass orceramic matrix composites [60–65]. Moreso, they maybe useful matrix phases in composites because theycan enhance composite processing via solution infil-tration or resin transfer molding. In this regard, thedevelopment of a commercial silicon-oxycarbide pre-cursor (Blackglass by Allied-Signal) for pre-preggingfiber tows and fabric should be noted. But clearly,further development of temperature stable oxycarbideglass-ceramic compositions must emerge before hightemperature applications can be considered. Most re-cently, the use of irradiation to facilitate process-ing [66–69], and the application of silicon-oxycarbideglass for joining SiC ceramics, was also reported[70, 71].

Summary

It is quite clear that through the use of sol/gelor polymer processing methods, metastable silicon-oxycarbide glasses can be synthesized. There are stillfundamental issues that need to be resolved con-cerning the chemical structures and distribution ofthe network and elemental carbon species. Qualita-tively, though, they can be described as molecularcomposites of polyaromatic graphene-rings dispersed

in a silicon-oxycarbide network structure. The high-temperature stability (>1200◦C) of these glasses is lim-ited, but it is quite likely that with further developmentof composition and processing, their controlled heat-treatment could yield useful glass-ceramics containingsilicon-carbide and graphite. The current level of ac-tivity and interest in these materials suggest that theseadvancements can be expected. Now, the primary chal-lenge is to establish properties for these new and uniqueglasses, so that their practical applications can proceed.

Acknowledgments

The authors gratefully acknowledge the National Sci-ence Foundation (DMR-9118797) for their financialsupport. Thanks are also extended to Alan Benesi, ElseBreval, James Hamilton, Paolo Colombo and MikeHammond.

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