silicon glass

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Silicon oxycarbide glasses: Part I. Preparation and chemistryGary M. Renlund and Svante ProchazkaGeneral Electric Corporate Research, Schenectady, New York 12301

Robert H. DoremusMaterials Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180-3590

(Received 10 December 1990; accepted 3 June 1991)

Silicone polymers were pyrolyzed to form silicon oxycarbides that contained onlysilicon, oxygen, and carbon. The starting polymers were mainly methyl trichlorosilanewith a small amount of dimethyl dichlorosilane. NMR showed that the polymers had asilicon-oxygen backbone with branching and ring units. When the polymer was heatedin hydrogen, toluene and isopropyl alcohol, used in production of the polymer, weregiven off in the temperature range 150 °C to 500 °C. Substantial decomposition of thepolymer itself began only above about 700° by evolution of methane. The network ofsilicon-oxygen bonds and silicon-carbon bonds did not react and was preserved; thesilicon-carbon bonds were linked into the silicon-oxygen network. The silicon oxycarbidewas stable above 1000 °C, showing no dimensional changes above this temperature. Theinterior of the silicon oxycarbide was at very low effective oxygen pressure becauseoxygen diffused slowly in it. There was also a protective layer of silicon dioxide on thesurface of the silicon oxycarbide.

I. INTRODUCTION

Metal organic polymers are excellent starting ma-terials for making ceramic powders and bulk materials,both crystalline and amorphous.1"5 For example, siliconepolymers with a silicon-oxygen backbone react withammonia to form silicon nitride. In the present work wefound that certain silicone resins pyrolyze in hydrogenor helium to a black, hard, amorphous solid containingonly silicon, oxygen, and carbon. This silicon oxycarbideglass has exceptional high temperature strength andchemical stability compared even to vitreous silica. Itis resistant to crystallization and oxidation at tempera-tures above 1000 °C. From these properties and detailedstructural studies we conclude that this glass is not justa mixture of silica and silicon carbide or carbon, butone of a series of homogeneous, probably metastable,amorphous solids in the silicon-oxygen-carbon system.

Carbon has been added to silica by others in avariety of ways. Organic compounds or carbon wereincorporated into porous silica that was a precursor toVycor glass.4""6

Fibers made from polycarbosilanes7 under the tradename Nicalon are mainly composed of silicon, oxygen,and carbon in the ratio close to 3 :1 :4 . 8 Their propertiesdegrade above 1000 °C.9

A glass was prepared by pyrolysis of dimethyl di-ethoxysilane/etraethoxysilane copolymers that containedsilicon, oxygen, and carbon.10 At temperatures above1300 °C this glass reacted to form crystalline siliconcarbide.

In this paper the preparation of a stable siliconoxycarbide glass is described, as well as information

about the pyrolytic reactions during its formation and itshigh temperature stability. In a subsequent publicationthe molecular structure and some physical properties ofthe glass will be discussed. More details are in Ref. 11.

II. EXPERIMENTAL METHODS

A. Precursor resins

Two different silicone resins were used to make thesilicon oxycarbide glass. The first was a methyl-siloxanemanufactured by a hydrolysis-condensation polymeriza-tion reaction with trade name SR 350 (General ElectricSilicon Products Div., Waterford, NY). This resin is solidat 20 °C, softens at about 30 °C, and becomes fluid at90 °C. At 20 °C it is clear, hard, and brittle, and hasa density of 1.08 g/cc. This resin is soluble in manypolar and nonpolar liquids such as toluene, xylenes,ketones, alcohols, and di-ethyl ether. Structurally it isa partially branched silicon-oxygen chain with methyland hydroxyl substituents and terminated by methylgroups. The molecular weight is variable and increaseson storage by continuing condensation reactions.

The starting reagents for making the SR 350 resinare 2 to 8% dimethyl dichlorosilane and 92 to 98%methyl trichlorosilane.12 In a three component solvent ofwater, toluene, and isopropyl alcohol the chlorine atomsare stripped by hydrolysis to give an unstable mixture ofdi-ols, tri-ols, and HC1 gas, for example by the reaction:

Si(CH3)Cl3 + 3H2O = Si(CH3)(OH)3 + 3HC1

(1)

2716 J. Mater. Res., Vol. 6, No. 12, Dec 1991 1991 Materials Research Society

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G.M. Renlund, S. Prochazka, and R. H. Doremus: Silicon oxycarbide glasses: Part I. Preparation and chemistry

These unstable silanol intermediates react by condensa-tion of —OH groups by eliminating water to form asilicon-oxygen chain polymer with some siloxane rings.

A simple polymer segment contains one methyl andone OH group attached to each silicon atom along thebackbone chain:

CH3 OH

—O—Si —O— Si—O—

OH CH,

The actual polymer is more complicated, withbranching and rings. The resin is not entirely stableat room temperature, and the condensation reactioncontinues, giving variable ratio of OH to CH3

substituents, usually substantially less than one. Aftersynthesis the resin retains several percent solvents suchas toluene and isopropyl alcohol.

The SR 350 resin crosslinks when it is heated up to90 °C; water, toluene, and isopropyl alcohol evolve. Theuncrosslinked resin is soluble in toluene; as it crosslinks,it becomes less soluble in toluene, and the degree ofcrosslinking can be assessed from the toluene solubility.SR 350 is also crosslinked at room temperature byaddition of strong bases such as amines or ammonia tosolutions of the resins. The manufacturer recommendsfor crosslinking 4% of p-amino triethoxysilane.

A second resin, General Electric SR 545, also py-rolyzed to a silicon oxycarbide glass. This resin hasmore oxygen in its structure, and is made from trimethylchlorosilane and polysilicic acid. It is a copolymer oftrimethylsiloxane and silica.

B. Glass formation

Pyrolysis was carried out in molybdenum cruciblesin hydrogen or helium in a molybdenum disilicide fur-nace. Above 90 °C the uncrosslinked SR 350 melts andpyrolytic reactions begin by evolution of gases. Thedensity increases and the color changes from clear toyellow between 200 and 300 °C, from yellow to brownbetween 600 and 750 °C, and to black above 850 °C.At about 1100 °C the resin is completely converted toglass, with minimal weight loss at higher temperatures.

Dense specimens of the silicon oxycarbide glasswere formed by three different methods: hot pressing andhot isostatic pressing of pyrolyzed powder, and directpyrolyzation of thin layers.

Powder was prepared from pyrolyzed pieces bymilling with silicon carbide in isopropyl alcohol. Itssurface area was about 3 m2/gm. The powder was hot-pressed in a Centorr graphite element furnace withgraphite dies lined with Grafoil. About 120 gms ofpowder were heated to 1300 °C without pressure, andat 1400 °C a pressure of 41 MPa was applied; then

the sample was heated to 1650 °C and held there for30 min. The pressure was held on cooling to 1400 °C,after which the pressure was released and the samplecooled to room temperature.

To avoid high processing temperature and conse-quent slight crystallization of silicon carbide, sampleswere isostatically pressed in a Conaway Mini Hipper.Six grams of powder were sealed at 10^ Pa in a vitreoussilica ampoule 20 mm in diameter by 50 mm long andwere heated at 10° per minute to 1300 °C at 170 KPa;then the pressure was increased to 180 MPa. One samplewas heated to 1400 °C and a second to 1550 °C.

In direct consolidation the gel was not made intopowder, and the sample thickness was limited to about1 mm because of cracking in thicker samples. If theinitial resin is crosslinked, its shape is maintainedthroughout pyrolysis. Glass sheets were made frompyrolysis of 50% SR 350 resin dissolved in a mixtureof 90% toluene-10% isopropyl alcohol containing 4%7-aminopropyl triethoxysilane as crosslinking agent. Thesolution was cast into sheets 2 - 3 mm thick, dried, andcured at 50 °C for 4 h. This sheet was heated at 0.2 °Cper h to form 1 mm thick glass.

Foam glass was made from a foam of the SR 350resin dissolved in toluene containing a urethane foam-ing agent, Hypol (W. R. Grace Co.). On pyrolysis theurethane decomposes completely, and a foam with thin-walled cells about 1 mm in diameter is formed.

C. Analytical procedures

Weight loss of the resin during pyrolysis was fol-lowed by thermogravimetric analysis (TGA) in a Netzschthermobalance with 20 /xg resolution on 1 gm sample.The sample was held in an alumina cup in vacuum,inert, oxidizing, or reducing atmosphere; the gas flowrate was 75 to 100 cc per min. The gases evolved duringheating were analyzed with gas chromatography andmass spectroscopy. A carbowax 20M capillary column17 m long with Model 5790A Chromatograph and MassSpectrometer Model 5970A, all Hewlett Packard, wereused. More details are in Ref. 11.

Dimensional changes during pyrolysis were mea-sured on foamed resins in flowing hydrogen at a heatingrate of 0.25 °C/min with a Theta Industries Dilatometerwith sapphire push rods.

Silicon was determined gravimetrically on samplesfused in sodium carbonate. The detailed procedure isin Ref. 11. Carbon was analyzed as CO2 with a Lecocarbon analyzer. Oxygen was determined by neutronactivation by JRT Corp., San Diego, CA.

The local environment of silicon atoms was fol-lowed with 29Si solid state nuclear magnetic resonance(NMR)13 with a General Electric GN-300 spectrometerand a 59.6 MHz Chemagnetics probe. Magic angle spin-

J. Mater. Res., Vol. 6, No. 12, Dec 1991 2717




ning (MAS) narrowed spectral lines, and spin rates of3.2 kHz were typical. Most spectra were obtained withthe Hahn spin echo pulse sequence: 2000 data points,40 kHz spectral width, and 5 min delays between pulses.Line broadening of 50-100 Hz was applied to the 100to 500 scans to obtain adequate signal-to-noise ratios.

III. EXPERIMENTAL RESULTS

A. NMR of resins

The 29Si NMR of the as-received SR 350 resin isshown in Fig. 1. The peak at —68.0 ppm is attributed toa (SiO3)SiCH3 group, and the shoulder at —59.9 ppm toa (SiO)2SiCH3OH group.13 The integrated areas underthese peaks should be proportional to their relativeamounts, showing that there is about three times asmuch of the first group as of the second. These are theonly local environments of silicon revealed in the NMRspectrum; the small peaks at —20 and —120 ppm arespinning side bands not related to the resin structure.The NMR spectrum of SR 350 resin crosslinked byreaction in ammonium hydroxide is shown in Fig. 2.The -70.1 ppm peak is presumably the same as the—68.0 ppm peak in Fig. 1, shifted because the shoulderpeak at —59.9 is reduced in intensity, probably becauseOH has been removed by crosslinking.

B. Weight changes and outgassingduring pyrolysis

The change of weight of the resin during heating isshown in Fig. 3. There are two regions of substantialweight loss, from about 150 °C to 250 °C and from700 °C to 850 °C. The total weight loss of about 25% is

100 0 -100 -200 -300 PPM

FIG. 2. 29Si NMR spectrum of crosslinked SR 350 resin.

complete by about 1000 °C; there is no further weightloss up to 1500 °C.

The total of all ion counts in the mass spectrometeras a function of temperature showed major outgassing at250° to 500 °C and 700 °C to 900 °C. Analysis of themass numbers evolving in the lower temperature regionshowed fragments from toluene at the lowest tempera-tures and then isopropyl alcohol at higher temperatures;these two solvents account for most of the mass loss upto 600 °C. These solvents are used in the making of theSR 350 resin. The gas evolving from 700 °C to 900 °Cis mainly methane, as shown by the predominance ofion masses 14, 15, and 16, as expected for fragmenta-tion of methane. More details of these gas analyses arein Ref. 11.

The weight changes on pyrolysis were also mea-sured for samples crosslinked by an organic amine.

250 500 1000 1250TEMPERATURE (°C)

100 0 -100 -200 -300 PPM

FIG. 1. 29Si NMR spectrum of as-received SR 350 resin.

FIG. 3. Weight change and derivation of weight change as a functionof temperature (TGA) for as-received SR 350 resin. Constant heatingrate of 10 °C/min in flowing hydrogen.

2718 J. Mater. Res., Vol. 6, No. 12, Dec 1991



G. M. Renlund, S. Prochazka, and R. H. Doremus: Silicon oxycarbide glasses: Part I. Preparation and chemistry

Closely similar results were found in hydrogen andhelium atmospheres. These crosslinked resins showeda broader low-temperature region of weight loss fromabout 150 °C to 500 °C and a similar high temperatureloss from about 700 °C to 800 °C. The total weight losswas substantially less at about 17% in hydrogen and15% in helium—again, toluene began to evolve above200 °C and isopropyl alcohol at about 400 °C, and onlymethane evolved from 650 °C to 900 °C. Only smallamounts of water were identified by mass spectrometryup to 400 °C.

C. Volume changes during pyrolysis

The dimensional change that occurred during thepyrolysis of a foamed SR 350 resin was measured bydilatometry, as shown in Fig. 4. The sample was foamedwith 4% of the crosslinking agent and cured at 190 °Cfor 16 h, and then cut to a bar 0.25 cm square and1.901 cm long, with ends ground flat and parallel. Thesample expands about 4% up to 300 °C, and begins astrong contraction above about 600 °C to about 1200 °C.Above 1250 °C the sample expands at a rate consistentwith thermal expansion of the stable glass.

The density of SR 350 at room temperature heatedat different times and temperatures is shown in Table I.The density of the as-received resin of 1.08 gm/cm3

increases to about 2.34 gm/cm3 after pyrolysis.

D. Chemical analysis

The analysis of the glass for silicon and carbon isgiven in Table II. The glass from crosslinked resin hadmore silicon and less oxygen than the glass obtained bypyrolysis of thermally set resin. Analysis for impuritiesin the milled powder is given in Table III; most of theseprobably came from the milling process.

TABLE I. Density of silicon-oxy-carbide pyrolyzed at various tem-peratures.

400 600

Temperature ( X )

Temperature( ° Q

As-received220780950

110012251650

Time at temperature(hours)

166610.50.5

Density(grams/cm3)

1.081.231.692.022.352.332.33

E. High temperature oxidation

Two samples of hot-pressed silicon oxycarbidewere examined for oxidation resistance by heating240 h in air at 1420 °C and 1520°. The samples were8.5 x 5.8 x 5.6 mm3. The weight change after thesetreatments was no more than a few milligrams, or lessthan 0.5%.

Layers of oxide about 6 //m thick at 1520 °C and2 /j,m thick at 1420 °C formed at the surface afterthese treatments, as shown in Fig. 5. X-ray diffractionspectra showed that the oxide layer was crystallinea-cristobalite. At the oxidation temperature /3-cristobaliteformed and transformed to a below the a-to-/3 trans-formation temperature of 270 °C. There are cracks inthe oxide layer and also in the silicon oxycarbide atthe interface, probably resulting from the large volumechange in the /3-to-a transformation and also the differ-ence in thermal contraction between the cristobalite andthe oxycarbide.

IV. DISCUSSION

The 29Si NMR results on the SR 350 resin show thatit is composed of about 75% of (SiO^Si CH3 groupsgiving a structural unit bonded with others:

O

_O—Si—O—

CH3

(a)

TABLE II. Elemental analysis of silicon oxycarbide glasses made byheating SR 350 resin at 1300 °C for 1 h in hydrogen.

FIG. 4. Linear dimensional change of foamed SR 350 resin heated0.25°/min in hydrogen.

at

Element

SiliconOxygenCarbon

As-received resin

Wt. % Atomic resin

47 1.041 1.5312 0.60

Wt.

513611

Crosslinked resin

% Atomic resin

1.01.240.50





TABLE III. Chemical analysis of impurities in milled silicon oxy-carbide glass.

Element

AluminumCalciumCopperIronGermaniumPotassiumLithiumMagnesiumSodiumPhosphorusTitaniumNitrogen

Amount, ppm

300021

245

7262

11375218450

Elements not detected were barium, bismuth, boron, cadmium,chromium, cobalt, nickel, strontium, tungsten, vanadium, zinc, andzirconium.

2 0 Mm

(a)

The resin has a silicon-oxygen backbone with branchingand ring units. About 25% of the groups are (SiO)2SiCH3OH units:

OH

—O—Si—0—

CH,

(b)

When this polymer is heated at 10 °C/min in hy-drogen, toluene and then isopropyl alcohol are givenoff at from 150 °C to 500°. These solvents are usedin the production of the polymer and are retained inthe as-received resin. A small amount of crosslinking bycondensation of OH groups also takes place.

The polymer is remarkably stable to decompositionby pyrolysis. During heating at 10 °C/min in hydrogenor helium, substantial decomposition begins only atabout 700 °C by evolution of methane. Carbon dioxide,carbon monoxide, longer chain organics, and siliconcompounds are not evolved in appreciable amounts dur-ing this process. Some methyl side groups are breakingoff the chain by reaction with ambient hydrogen or chainhydrogen on OH groups. The silicon-oxygen bonds donot react in this process; a network of silicon-oxygenand silicon-carbon bonds is preserved. The retention ofa considerable amount of carbon in the final productmeans that many methyl groups give off hydrogen, andthe resulting Si—C bond is linked into the silicon-oxygennetwork. Structural studies to be published showed thatthere were no carbon-oxygen bonds in the final product,suggesting that silicon-carbon bonds are formed by thecarbon atoms from the methyl groups.

The silicon oxycarbide is stable above 1000 °Cand shows no further dimensional changes above thistemperature.

20 um

(b)FIG. 5. Optical photomicrographs of a cross section of the sur-face of silicon oxycarbide treated 240 h in air at (a) 1420 °C and(b) 1520 °C. Lower grey region is silicon oxycarbide; upper, lighterregion is plotting compound.

If the starting resin is crosslinked by a catalystsuch as ammonia below 100 °C, the resulting siliconoxycarbide after pyrolysis contains less oxygen per sili-con atom (Table II). In the oxycarbide derived from the"uncrosslinked resin", each silicon is surrounded by threeoxygens on the average. However, this is not the actualstructure of the final product.

If it is assumed that the uncrosslinked resin is cross-linked during heating so that at 600 °C it is entirely com-posed of groups (a) above, then the atomic compositionof this material has the formula SiOi.5CH3. For it todecompose to the atomic ratio of Table II requires a lossof 0.4 carbons per silicon, or 10.9 wt. % of the resinat 600 °C, if it is assumed that no silicon or oxygenare lost and all the hydrogen is evolved. This amountcompares well with the experimental value of 12.4%from Fig. 3, showing that the assumption of no loss





of silicon or oxygen is consistent with the weight loss.For the crosslinked resin the experimental weight lossis considerably smaller than expected for the formulaSiO15CH3 at 600 °C. Thus this formula probably doesnot represent the crosslinked polymer at 600 °C.

The remarkable chemical stability of the silicon oxy-carbide glass can be explored with the thermodynamicsof the applicable chemical reactions. Three solid phases,amorphous silica, carbon, and silicon carbide, are foundin these glasses. Thus one reaction to consider is:

SiO2 + C = SiC + 0 2 (2)

The standard free energy change for this reaction at1500 °C is about +534 kJ/mole, so that the equilibriumconstant for the reaction at this temperature is about3(10)~16. If the thermodynamic activities of the threesolid phases are assumed to be unity, the equilibriumoxygen pressure in atmospheres is equal to the equilib-rium constant. Therefore if the effective oxygen pressureis less than 3(10)~16 atm in the solid oxycarbide, solidcarbon should coexist with silica. At higher pressuresthe silica-carbon mixtures should be converted to siliconcarbide until all excess carbon is used up. If only silicon-oxygen, silicon-carbon, and carbon-carbon bonds arepossible, and each oxygen is bonded to two silicons,there must be some "free" carbon or carbon-carbonbonds with the compositions of Table II. For the oxy-carbide from as-received resin there are 1.53 oxygensper silica, leaving 0.94 bonds on the average siliconatom to bond to carbon. This leaves more than half thecarbon bonded to carbon. Similarly for the oxycarbidefrom crosslinked resin there are 1.52 bonds per siliconnot bonded to oxygen; if these become Si -C bonds, thereare still about one-quarter of the carbon bonds that mustbe bonded to other carbon atoms.

Another reaction to consider is the formation ofsilicon monoxide, for example by

SiO2 + C = SiO(g) + CO(g). (3)

The standard free energy of this reaction at 1572 °C isabout +35 kJ/mole.14 Schick14 has shown that the ef-fective pressure of SiO for these conditions is controlledby the reaction

SiO = Si + SiO2 (4)

From Eq. (4) the vapor pressure of SiO at 1500 °Cis about 0.05 atm and at 1600 °C is about 0.15 atm.14

However, there is no evidence for solid silicon in theoxycarbide glass, suggesting that the actual SiO pressureis less than these values. Furthermore, structural studiesto be reported in a subsequent publication show that thesilicon oxycarbide contains no detectable carbon-oxygenbonds. Neither carbon monoxide nor carbon dioxide wasdetected during pyrolysis. Thus below about 1600 °C it

appears that reaction (3) does not influence the stabilityof the oxycarbide.

The above considerations show that the interior ofthe silicon oxycarbide remains at a very low effectiveoxygen pressure. We suggest two reasons for this re-sult. One is that oxygen permeates much more slowlythrough the oxycarbide than it does through vitreoussilica, because the oxycarbide is considerably more dense(density of 2.35 gm/cm3) than vitreous silica (density of2.2 gm/cm3). A second reason is that the oxycarbideforms a protective layer of silica on its surface byreaction with oxygen, as shown in Fig. 4. This layer ispartially or entirely crystalline /3-cristobalite, as shownby x-ray diffraction.

If the oxidation is controlled by the diffusion ofmolecular oxygen through the cristobalite, as it is foroxidation of silicon at these temperatures,15 the depen-dence of layer thickness L on time t should follow aparabolic relation:

L2 = Bt (5)

in which B is a parabolic rate constant. From the dataof Fig. 4 of growth of a 6 /im layer after 240 h in airat 1520 °C, B = 4.2(10)-13 cm2/s or 0.15 /xm2/h. Theextrapolated value for the oxidation of silicon in 1 atmof oxygen is about 0.17 ^m3/h. These two values areclosely similar. The diffusion and solubility of oxygen in/3-cristobalite should be similar to that in vitreous silica,since the two have the same density of 2.2 gm/cm3; thepermeation rate of gases in crystalline and amorphoussilicas is closely related to their densities.16 The conclu-sion is that a silica layer on the surface of the siliconoxycarbide helps protect it from oxidation.

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11. G.M. Renlund, "Silicon Oxycarbide Glasses", Ph.D. Thesis, 14. H.L. Schick, Chem. Rev. 60, 331-362 (1960).Rensselaer Polytechnic Institute, Troy, NY, 1989. 15. R.H. Doremus, in The Physics and Chemistry of SiO2 Interface,

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