silica hydrogels

Carbon Vol. 33, No. 12, pp. 1739-1746, 1995 Copyright 0 1995 Elsevier Science Ltd

Pergamon

Printed in Great Britam. All rights reserved 0008-6223/95 $9.50 + 0.00

0008-6223(95)00146-8

FORMATION OF BLACK GLASSES AND SILICON CARBIDE FROM BINARY CARBONACEOUS/SILICA HYDROGELS HENRY PREISS,~,* LUTZ-MICHAEL BERGER~ and MICHAEL BRAUN~ “TU Bergakademie Freiberg, Institut fiir Energieverfahrenstechnik, 09596 Freiberg, Germany bFraunhofer lnstitut ftir Keramische Technologien und Sinterwerkstoffe, 01277 Dresden, Germany “Technische UniversitLt Berlin, Institut ftir Nichtmetallische Werkstoffe, 12489 Berlin, Germany (Received

14 December 1994; accepted in revised form 11 August 1995)

Abstract-Binary hybrid gels consisting of a carbonaceous hydrogel and a silica gel were prepared as preceramic materials. The pyrolytic conversion into carbon-containing glasses under inert conditions were followed by TG analysis, FTIR spectroscopy and nitrogen adsorption. On pyrolysis up to 7Oo”C, the functional groups of the carbonaceous gel constituent were destroyed and the porosity of the binary gels is nearly completely reduced. Black glasses consisting of amorphous silica and carbon are formed between 700 and 1400°C. TG analysis showed that the dense glasses with a carbon content < 10 wt.% are stable up to 1000°C in air. Heat treatment at 1300-1400°C renders the glasses porous and instable against oxidation by air. On heating above 14OO”C, the SiO, crystallizes and converts into Sic by a carbothermal reduction. At 16OO”C, the glass with a molar ratio C/Si=3 has transformed into Sic crystallites with grain sizes of 1 pm and less. Key Words-Carbonaceous

gel, sol-gel,

black glass, silicon carbide,

1. INTRODUCTION The sol-gel process as a route to glasses and ceramics has been investigated for more than four decades. Starting from molecular precursors, a macromolecular oxide network is obtained through hydroxylationcondensation reactions. The chief advantages of sol-gel routes using alkoxid precursors are very high purity and homogeneity on a molecular or colloidal scale. Metal oxide gel chemistry, however, is also applicable to non-oxide ceramic systems, e.g. for the synthesis of carbides, nitrides and oxycarbides via incorporation of carbon or nitrogen. Many workers have shown that it is possible to synthesize SIC powders from gel-derived precursors as well as from polymer precursors [ l-33. Mostly the polymer routes proceed through a gel stage, thus the two approaches are often grouped together. Synthesis of sinterable SIC powders and incorporation of carbon into a glass network are the most widely used sol&gel techniques in the studies on Si-C-0 systems. Carbon-containing glasses (black glasses) approachable by incorporation of carbon into a glass network are a new class of materials with improved mechanical, electrical and thermal properties. Inhibition of crystallization is considered as a source of the enhancement of mechanical properties. In most cases these glasses were prepared by mixing glassforming oxides and finely dispersed carbon blacks [ 41 or SiC[S], or by impregnating a porous glass with an organic polymer (e.g. furfuryl alkohol or an aqueous solution of sucrose) and subsequent pyrolysis[6,7]. However, decomposition reactions which

*Present Germany. CAR 33-12-o

address:

Kiillnische

StraDe 22a, 12439 Berlin,

porosity.

evolve CO, and CO occur in the melt during the glass-making procedures. Polymer-derived precursors obtainable through hydrolysis of substituted silanes or siloxanes offer the possibility of introducing carbon at a low temperature. Organosilicon gels as precursors of black glasses have been prepared through hydrolysis of alkyl and aryl substituted alkoxy- or chloro-silanes by a number of researchers [ 8- 111. The non-hydrolyzable carbon groups in the substituted polymers give rise to the presence of carbon in the glass as oxycarbide as well as a separate carbon phase. Although no equilibrium solid phases between Sic and SiOZ are known, evidence for the existence of a metastable silicon oxycarbide in black glasses at low temperature and low partial pressure of oxygen has been presented by numerous researchers using i.r. and NMR techniques[ g-131. According to these spectroscopic investigations, the silicon oxycarbide has an amorphous covalent structure described as a combination of Si-C and Si-0 bonds approaching a random distribution. From a survey of the literature, it appears that the upper temperature limit for the existence of oxycarbide glasses is not yet clear. Stability in air up to 1000°C has been reported[%lO]. Between 1200 and 1400°C they begin to crystallize, combined with a small loss of CO and SiO[ 10,13,14]. The beginning of the carbothermal reduction causing SIC formation is reported by numerous authors at different temperatures between 1400 and 1600°C[1,3,10,11,15]. It is to be assumed that thermal stability as well as the mechanical properties of these glasses depends on the Si/O/C ratio. Especially the carbon content is expected to influence these properties. The Si/C ratio, however, is generally fixed if silicon organic polymers are used as precursors. In order to overcome this

1739

1740

H. PREISSet trl.

limitation, polymer mixtures or copolymers were used as precursors [ lo]. The present work focuses on the variation of the carbon content in Sip0-C systems by using a carbonaceous sol as the carbon source. In studies on the functionalization of pitches and mesophase pitches, a sulfonized and oxidized material was developed which is soluble in distilled water forming a hydrosol[ 16,171. The aqueous sol becomes viscous on cooling and concentrating, and it may be turned into a carbonaceous gel. The advantage of using the carbonaceous gel instead of a divided solid carbon or SIC as a glass precursor is the intimate mixing of the reactants on a molecular or colloidal scale. The purpose of this study is: (1) to demonstrate the possibility of producing binary gels consisting of a carbonaceous and a silica-based gel, and (2) to study the evolution of this binary gel with temperature. The binary gel is produced from hydrolyzed tetraethoxysilane and a carbonaceous hydrosol. We thoroughly mix the two preexisting sols at low viscosities and assume “perfect mixing”. A gel is then formed from the mixed sol, dried, and partly heated to an anhydrous (xerogel) condition, and used for further thermal studies. The thermal process was investigated by thermogravimetric analysis; the solid products at different temperatures were characterized by i.r. spectroscopy, X-ray diffraction, nitrogen adsorption, electron microscopy and elemental analysis.

2. EXPERIMENTAL 2.1 Preparation qf samples The tetraethoxysilane (TEOS) was obtained from Fluka Co. and used as received. Transformation into a silica-based hydrosol was carried out by a full hydrolysis of the ethoxide groups with an excess of water and HNO, as a catalyst. 4.16 g TEOS (0.02 mol) were mixed with 4 ml water in a beaker and adjusted to pH 3 using 1 M HNO,. The mixture was stirred at room temperature with a magnetic stirrer for 1 hour; thereafter the solution appeared to be a homogeneous sol. The carbonaceous hydrogel used for this study was prepared by sulfonation and subsequent H,O, oxidation of a coal tar pitch with a KS softening point of 118°C according to Ref. [ 161. The dried gel had an elemental composition of 62.0 wt.% C; 2.8 wt.% H: 0.7 wt.% N, 6.2 wt.% S, and 28.3 wt.% 0. The coke residue after pyrolysis up to 1OOO’C under inert conditions was 55.5 wt.%. In order to prepare the carbonaceous hydrosols, different quantities (A, 0.19 g; B, 0.37 g; C, 0.54 g; and D, 1.4 g) of the dried carbonaceous gel were dissolved in a mixture of 19 ml distilled water and 1 ml 30% H20, under heating. The carbonaceous hydrosols were added to the silica-based sol and stirred in a beaker for 8 minutes. The final hybrid sols were dark brown homogeneous solutions. When the beakers were covered with alumi-

num foil, the sols gelled under ambient conditions for approx. 3 days. In order to accelerate the sol-gel process, the sols were stirred on a hot plate at 60 C in open air. After gelling, the samples were first dried at room temperature and after that at 150-C to a constant weight. This procedure yielded large fragments of shiny black xerogels A, B, C and D of different carbonaceous gel/silica gel ratios. Samples of the xerogels were heat-treated in an alumina tube furnace (Carbolite Furnaces Limited) in flowing argon up to temperatures between 300 and 1600°C with 100°C intervals. The pyrolysis was carried out at a heating rate of IO”C/min up to 1000 ‘C and S”C/min from lOOO-1600°C. The time of annealing at the maximum temperature was 1 hour. The samples were allowed to cool in flowing argon. 2.2

TGIDTA experiments and chemical analysis Prior to chemical analysis and to subsequent investigations, the xerogels as well as their pyrolysis products were pulverized and screened to lOOG600 pm. Thermogravimetric (TG) and differential thermal analysis (DTA) of the xerogels were performed using a Netzsch STA 409 Thermoanalyser. About 20 mg of the samples were heated from room temperature to 1550°C in flowing argon at a rate of lO”C/min. Thermal stability of the black glass and Sic synthesized at 1000, 1400 and 1600’C. respectively, were investigated by using a Derivatograph C (MOM Budapest) in an air flow of IO I/h with a heating rate of lO’C/min from room temperature to 1000’C. The black glasses were analyzed for carbon and nitrogen by combustion using standard organic chemical analysis technique. The content of silica was estimated by extracting the glass with hydrofluoric acid (40 wt.%). The product of the carbothermal reduction obtained at 1600°C was purified prior to the Sic analysis. In order to remove excess carbon, this product was oxidized at 57O’C for 5 hours in air after silica had been removed by boiling with HF solution. Since high-surface area Sic is susceptible to oxidation. the HF treatment was repeated after the carbon burn-off. SIC powders received this way were analyzed for heteroelements: total carbon content with LECO CS-444, and oxygen and nitrogen with LECO TC-436. 2.3 Iqfrured spectroscopy und X-ray d#“ractometrJ The FTIR spectra of the xerogel D and its pyrolysis products were obtained using the KBr wafer technique. 1.5 or 0.5 mg of a sample were ground with 400 mg KBr and compacted at 400 MPa. The spectra were recorded with a BIORAD FTS 60A spectrometer at a resolution of 2 cm-i collecting 256 scans. Standard X-ray diffraction techniques with nickelfiltered Cu K, radiation were used to determine the crystalline phases present in the heat-treated samples.

Formation of black glasses and silicon carbide

2.4 Surface area and porosity BET surface areas were determined by nitrogen adsorption at 77 K according to the method of Haul and Diimbgen[ 181 (simplified single point procedure). Nitrogen isotherms were additionally measured for the products heat-treated at 1300-1600°C using an ASAP 2000 instrument (Micromeritics, U.S.A.). From these isotherms, specific surface areas were calculated using the BET and the t-plot (Harkins and Jura) method, and mesopore size distributions were calculated according to Barret, Joyner and Halenda with the software supplied with the equipment.

3. RESULTS

3.1 Heat treatment of the xerogels TG analyses of two xerogels with different ratios of carbonaceous gel/silica gel are displayed up to 1550°C in Fig. 1. The TG traces show that weight loss takes place mainly in three steps. The first step, which begins almost immediately on heating, is assigned to the loss of water, with the maximum rate occurring at 100-l 10°C. This water derives partially from endothermic condensation of Si-OH groups and partially from evolution of free water which is adsorbed due to swelling of the carbonaceous gel constituent. The second step of weight loss centered at 250-300°C is attributed to the pyrolysis of the carbonaceous gel constituent. In a previous paper [ 171, it has been shown that this weight loss is associated with the destruction of the acidic functional groups releasing CO,, SO, and H,O. At about 7OO”C, the pyrolysis of the carbonaceous constituent is nearly completed. The binary xerogel has been decomposed to an intimate mixture of carbon and amorphous silica. The temperature interval between 800 and 1300°C is a region of almost constant weight for all of the gels. Larger samples of xerogel D which were heated in the tube furnace up to 800°C under

1741

argon showed a C/SiO, ratio of 3.03 which is close to the ratio required for the carbothermal reduction reaction SiO,+3C-+SiC+2CO

(1)

At about 14OO”C, the onset of the carbothermal reduction gives rise to a third step in the TGA trace. The weight loss of the xerogel, however, is higher than expected from eqn (1) (this discrepancy is discussed later). Samples of the xerogels annealed in the tube furnace at different temperatures were subjected to chemical analyses and spectroscopic investigations. Figure 2 shows FTIR spectra of xerogel D and its annealing products at 800, 1400, 1500 and 1600°C. The spectrum of the xerogel shows the characteristic peaks that had been previously found for the carbonaceous gel[ 161, and, in addition, the bands of silica. The peaks at 460,796 and 1093 cm- ’ are characteristic for Si-0 bonds. The peak at 1730 cm-’ and the shoulder at about 1162 cm-l are attributed to carboxy1 groups of the carbonaceous gel constituent, and the peaks at 585 and 957 cm-’ to its sulfonic groups. The band at about 1600 cm-’ may be assigned to aromatic C=C vibrations activated by neighboring oxygen groups. The observed bands of the binary xerogel show that there is a shift in the i.r. absorbance when compared to the values reported for the pure carbonaceous gel [ 161. After pyrolysis at 8OO”C, the i.r. spectrum confirms the destruction of the functional groups; the characteristic peaks of these groups are absent. Between 800 and 1400°C the

“T

10 -

$T

20

: s E ol ‘6 5

30-

-

(a)

(b) 40. 50 60

t 200

600

Temperature

1000

1400

I

t 4000

I

I

3000

I

I

2000

I

I

I

1000

Wavenumber

(“C)

Fig. 1. TGA traces up to 1550°C (under argon) of the carbonaceous/silica gels B (a) and D (b).

Fig. 2. Infrared spectra of the carbonaceous/silica xerogel D (a) and its pyrolysis products at 800°C (b), 1400°C (c), 1500°C (d), and 1600°C (e).

1142

H. PREISSet al.

spectra are nearly identical apart from a low shifting of the peak positions. At 15OO”C, however, a spectral evidence for the carbothermal reduction of silica is obvious. A broad band at about 825-898 cm ’ characteristic for Si-C bonds appears beside the Si-0 vibrations. In addition, the intensity of the aromatic C=C vibration has decreased. In the 1600°C spectrum, only the Si-C vibration centred at about 850 cm-’ is observed, showing that the carbothermal reduction is almost finished. Measurement of surface areas has also been used to follow the pyrolysis process. The BET surface areas of the xerogel D pyrolyzed to final temperatures of 300&16OO”C with 100°C intervals are listed in Table 1 and show that the gels calcined between 300 and 700°C are porous having BET surface areas between 120 and 250 m2/g. Presumably, the observed porosity up to a calcination temperature of about 700°C is a characteristic feature of the silica gel constituent because carbonaceous gels do not produce any measurable surface area in this temperature range[ 161. Transformation into a dense glass reduces the surface area nearly completely. The surface area increases again above 1300°C and reaches a maximum value of 95 m’/g at 1500°C. This increase corresponds to the third weight loss step in the TG trace and reflects the onset of the carbothermal reduction evolving mainly CO. The nitrogen isotherms for 1300&1400 C. the range in which the glass becomes porous, show a broad hysteresis loop (Fig. 3) which refers to the presence of mesoporosity. Since the hysteresis loops do not close at low relative pressures, it is suggested that microporosity is additionally present at these temperatures. The formally calculated BET surface areas and the micropore areas obtained from the tplots are listed in Table 1. The t-plots demonstrate that only a small portion of these surface areas is associated with micropores. The mesopore distribution curves determined from the adsorption branches reveal maxima at a pore diameter of 6 nm ( 13OO’C) Table 1. BET and micropore surface area. and pore diameter maximum (d,), of gel D pyrolyzed at different temperatures (T)

T

( C) 300 400 500 600 700 800 9OOC1200 1300 1400 1500 1600h

BET

Micropore surface area

Al

(m’/g)

(m’,ig)

(nm)

173a 239” 254’ 201” 120a 5a
1.6 5.3 8.7 9.2

-6 9.5 2 100 IO-1ooc

“From single point procedure. bWithout purification. ‘Broad maximum.

and 9.5 nm (1400°C); the samples remain amorphous in this pyrolysis range. Further increase of the pyrolysis temperature changes the shape of the isotherms; those at 1500 and 1600°C are similar and may be indicative of an agglomerate of individual particles. The pore size distribution curves show broad maxima between IO and 100 nm for 1500 and 1600°C. X-ray diffraction demonstrates the presence of crystalline SiO, and SIC at 15OO”C, but only SIC at 1600°C.

3.2 Characterization

of the black glasses

In this paper, the transformation products of xerogels which are shiny black, hard, dense, X-ray amorphous. and composed of silica and pure carbon only (with the exception of traces of hydrogen) are termed as black glasses. Thus, the transformation products in the temperature range 800-1400°C may be considered as black glasses. Above 1400°C the carbothermal reduction destroys the glasses. The gels with a C/SiO, weight ratio >O.l remain amorphous (to X-rays) up to 1400°C. Devitrification, however, is observed for the gels A and B at this temperature since the diffraction patterns show week lines of xc-SiO,. As was shown by nitrogen adsorption, the glasses are non-porous for the pyrolysis range 800-13OO”C, and porous at a pyrolysis temperature of 1400°C. Simultaneous TG/DTA analyses in flowing air were conducted to investigate the stability against oxidation. Figure 4 presents the thermoanalytical characteristics measured for the xerogels A, B, C and D after their pyrolysis at 1OOO’C under argon. No significant weight loss is observed for xerogel A at temperatures up to 1OOO’C over a time period up to 60 minutes, showing its resistance against oxidation. Xerogel B shows a negligible small weight loss (of about 0.7%). and some white silica is observed to form on its surface. Carbon oxidation is more pronounced for the samples C and D. The weight loss begins at about 550°C (the temperature at which free carbon usually begins to oxidize in open air). The weight loss of both xerogel C and D is finished at 1000°C and corresponds to the values of the carbon content (Table 2). The corresponding DTA peaks reveal maxima at about 620 C. Another oxidation behaviour is observed when preexisting pores are present in the glass, i.e. after heat treatment at 14OO’C. Figure4 shows also the thermoanalytical oxidation data for the xerogels A-D annealed at 14OO’C under argon. In comparison to the samples pyrolyzed at IOOO’C, some differences may be observed. The first major observation is that neither xerogel A nor xerogel B is resistant against air oxidation. A second observation from the TG curves is that all the 1400°C xerogels show a higher weight loss rate. Finally. the DTA maxima are shifted to about 650°C. 3.3 Characterization qfSic Lumps of a gray color containing and unreacted SiOZ were obtained

residual carbon after the heat

Formation of black glasses and silicon carbide

3

300.

S b ’

1743

,$? sooE

1300°C

1400°C >$(tO

200.

100.

lOO-

o+ 0

0.6

OT 0

1

P/PO

0.6

@PO

’

P/PI)

’

100

0 0.6

Fig. 3. Nitrogen adsorption

@PO

0.6

’

isotherms of the carbonaceous/silica 1500, and 1600°C.

5.0 4

0

3

;1 A

1

z

gel D after pyrolysis at 1300, 1400,

,

-

0.0

D

0 2 2

-5.0

$

& -10.0 -1.5

1 400

I 500

600

700

800

900

-15.0

1000

f 400

500

Temperature [“C]

600

700

Temperature 5.0

z

800

900

1000

900

1000

[“Cl

I-

-5.0

D 0 E -15.0 .zE, ii (IJ -25.0 c D

-,400

-35,o 500

600

700

Temperature

800

900

1000

f%]

400

500

600

700

Temperature

800 [“Cl

Fig. 4. Weight loss (1) and DTA (3) obtained in air of the gels A, B, C and D pyrolyzed under argon up to lOOO”C,and weight loss (2) and DTA (4) in air of these gels pyrolyzed under argon up to 1400°C.

treatment of xerogel D at 1600°C. These lumps could be readily crushed, and X-ray as well as chemical analyses were conducted using this as-received powder. X-ray diffraction showed that the powder consisted mainly of /I-Sic with a minor amount of a-Sic (Fig. 5). Purification by HF treatment. subsequent carbon oxidation, and repeated HF treatment

did not change the X-ray pattern. The puri~~ation procedure resulted in an amount of 10 wt.% residual carbon, 2 wt.% SiOz, and an SIC yield of about 10% which varied for different pyrolysis experiments. The yield is defined as the ratio of the actual weight of SE to the theoretical weight of Sic predicted for complete reaction according eqn (1). Chemical analy-

1744

H.

PREISS

Table 2. Chemical composition of the gels obtained at 1400°C and of the SIC obtained at 16OO’C, and TG weight loss in air up to 1000°C of gels pyrolyzed at 1000°C (TG 1000) and 1400’C (TG 1400) respectively Gel

Carbon

Nitrogen

Oxygen

type

wt.%

wt.%

wt.%

A B C D Sic”

5.2 10.0 14.6 31.5 30.6

0.05 0.05 0.1 0.1 0.21

TG 1000 wt.%

0.8 14.9 37.0

TG 1400 wt.%

1.2 6.6 13.1 37.0

0.62

“After purification

28 (degrees)

Fig. 5. XRD pattern of the carbonaceous/silica gel D after heat-treatment at 16OO’C and purification by oxidation and HF treatment.

sis of the purified Sic gave the values listed in Table 2. TG analysis shows stability in air up to IOOO’C. above this temperature the SIC violently oxidizes. Examination by scanning electron microscopy and X-ray diffraction showed that the purified powder consists of submicron Sic particles. An average crystallite size of D = 0.03 /Lrn is obtained from X-ray linebroadening analysis using the Scherrer equation. Typical particles of the powders can be seen from the electron micrograph in Fig. 6. The fine primary particles of 0.10-0.30 ;Lrn diameter are locked together to form agglomerates of different forms and sizes. The surface area was determined as II,= 15.0 rn’!g by nitrogen adsorption. Using this value, an average particle size d of 0.125 itrn was calculated according to d= 6/qu,, where q is the density of SIC. The higher particle sizes in comparison to the average crystallite size indicate that the particles are polycrystalline. Particle size measurement was conducted using the Laser Partikel Sizer “analysette” 22 (Fritsch GmbH). The particle size distribution is depicted in Fig. 7 and shows a maximum at about 0.800 ,nm. These particle sizes include the agglomerates. 4. DISCUSSION

Carbonaceous hydrogels recently described by a number of authors have been hitherto used as novel materials for the manufacture of artificial carbon products[ 17,19,20]. In addition, their hydrophyhc properties offer the possibility for the preparation of hybrid organic-inorganic networks in an aqueous medium. Especially sol-gel derived hybrid materials

et al

may be of interest. This paper reports on examples in Sip0-C systems. As our results show, it is possible to mix a carbonaceous hydrosol with a sol prepared by hydrolysis of an alkoxy silane and to transform the mixed sols in binary hydrogels and xerogels which are distinguished by the intimate mixing of two reactants. This mixing is based on the interpenetration of the two networks and not on the formation of SiK bonds. High-temperature treatment of these binary gels is a novel route for attaining carbon-containing glasses and SIC powders. During the thermal treatment, first the functional groups of the carbonaceous constituent are destroyed and the silica gel constituent calcines. The porous structure of the binary xerogel, which may be mainly associated with the silica constituent, is preserved during calcination up to about 700°C. Above 7OO’C, however, the porosity and surface area are considerably reduced. It is thought that recombination reactions during pyrolysis and calcination processes make the binary network shrink, and the pores collapse. This way a black and non-porous glass is formed which does not reveal any remarkable weight loss up to 1300°C. Some properties of this glass are different from those of both a separate carbon network and a separate SiOZ network. Although no Si-C bonds had been formed (the FTIR spectra did not reveal Si--C vibrations up to 14OOC) the observed crystallization and oxidation behavior are similar to that of silicon oxycarbide glasses[9,10,12]. The xerogel glass was X-ray-amorphous and did not show any diffraction lines up to 1400’C with the exception of the xerogels with a C/Si02 ratio 0.1, however, carbon is oxidized in air, resulting in a DTA peak centered at about 620 ‘C. Porous materials are produced by a high-temperature treatment of the glass in argon at 14OOC. These porous glasses reveal an oxidation reaction with a higher weight loss rate. Moreover, the corresponding DTA peak maximum is shifted by about 30’C to a higher temperature. It is to be assumed that the more graphite-like structure of the 1400°C-carbon as compared to the 1000 C-material is the cause for this shift. On heating from 1200 to 14OO’C, the glass loses about 2% of weight, but it remains black, hard and X-ray amorphous. This small weight loss, which is connected with a generation of pores, may be due to

Formation of black glasses and silicon carbide

Fig. 6. Scanning electron micrograph of the purified Sic powder. 20

10

$$ P

20

0

0 0

0.5

1

10

5

50

100

500

1000

d@m)

Fig. 7. distribution

function (Q) and density of distribtltion (q) of Sic particle sizes.

the onset of the reduction reaction releasing small amounts of CO and SiO. The material is comprised primarily of carbon and amorphous silica. Si-C vibrations have not been found by FTIR spectroscopy. Concerning the absence of Sic at 14Oo”C, our findings are in correspondence with results on silicon oxycarbide pyrolysis[2,11,12]. Only Hurwitzf lo]

reported on the formation of some small amount of SIC at 1400°C. The high temperature range from 1400 to 1600°C is that of the significant carbnthermal reduction with a weight loss of about 56%. The glass transforms into lumps of a gray color which consist mainly of P-Sic with minor parts of a-Sic. FTIR spectroscopy confirms this conversion by the disap-

H. PREISSet al.

1146

pearance of the Si-0 vibration at 1100 cm ’ and the appearance of the SiLC band at 850 cm-‘. The shape of the isotherms changes and approaches the type which is characteristic for agglomerates. The BET surface area passes through a maximum at 15OO”C, and the pore size distribution curve shows a broad pore diameter maximum for the final product. The change in the adsorption behavior demonstrates the breakdown of the hybrid network and the formation of SIC agglomerates with carbon and SiO, impurities. After purification by oxidation in air and HF treatment, Sic results as a finely divided powder with a yield of about 70% of the theoretical yield and a surface area of about 15 m*/g. This dramatic effect of purification suggests that the high surface area in the as-received product is mainly associated with the residual carbon. The SIC yield in our experiments is unexpectedly small and suggests loss of silicon species not predicted by the overall carbothermal reduction according to reaction (1). It should be borne in mind that the carbothermal reduction of silica involves the formation of the volatile suboxide SiO[e.g. 21,221. The overall reaction is a complex and multistep reaction in which the reactions SiO,+C+SiO+CO

(2)

SiO+2C+SiC+CO

(3)

are the most essential steps. Under the soILgel conditions, a very intimate mixing of the reactants is achieved, and due to the short diffusion distances reaction (2) should be very rapid suddenly producing a high SiO pressure. In a flowing gas stream SiO can be transported to cooler parts of the tube, and may there disproportionate to Si and SiO, or form Sic with CO. On the other hand, the sudden increase of SiO pressure may cause a high rate of SIC nucleation homogeneously all over the matrix according to reaction (3). If the rates of temperature increase and

heat transfer are high, a Sic powder with small crystals results. The sinterability of the SIC powder will be studied in further experiments.

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