Carbothermal Reaction of Silica–Phenol Resin Hybrid Gels to ProduceSilicon Nitride/Silicon Carbide Nanocomposite Powders

Jinwang Li*,w,z and Ralf Riedel**

Fachgebiet Disperse Feststoffe, Fachbereich Material und Geowissenschaften, Technische Universitat Darmstadt,Petersenstrasse 23, 64287 Darmstadt, Germany

A carbothermal reaction of silica–phenol resin hybrid gels pre-pared from a two-step sol–gel process was conducted in atmo-spheric nitrogen. The gels were first pyrolyzed into homogeneoussilica–carbon mixtures during heating and subsequently under-went a carbothermal reaction at higher temperatures. Using agel-derived precursor with a C/SiO2 molar ratio higher than 3.0,Si3N4/SiC nanocomposite powders were produced at 15001–15501C, above the Si3N4–SiC boundary temperature. The pre-dominant phase was Si3N4 at 15001C, and SiC at 15501C. TheSi3N4 and SiC phase contents were adjustable by varying thetemperature in this narrow range. The phase contents could alsobe adjusted by changing the starting carbon contents, or by itscombination with varying reaction temperature. A two-stageprocess, i.e., a reaction first at 15501C and then at 15001C,offered another means of simple and effective control of thephase composition: the Si3N4 and SiC contents varied almostlinearly with the variation of the holding time at 15501C. TheSiC was nanosized (B13 nm, Scherrer method) formed via asolid–gas reaction, while the Si3N4 has two morphologies: elon-gated microsized crystals and nanosized crystallites, with theformer crystallized via a gaseous reaction, and the latter formedvia a solid–gas reaction. The addition of a Si3N4 powder as aseed to the starting gel effectively reduced the size of the Si3N4

produced.

I. Introduction

IT has been reported that micro/nanograined Si3N4/SiC com-posites have significantly higher mechanical properties and

thermal oxidation resistance over the monolithic Si3N4 ceram-ics.1–4 Therefore, this type of materials, generally known asSi3N4/SiC nanocomposites, has attracted considerable scientificinterest. The fabrication routes for these composites can be di-vided into two classes: the conventional powder-sintering routesand the recently developed powderless routes. The powderlessroutes produce composite ceramics typically via direct thermoly-sis of polymeric organosilicon precursors without the use of asintering additive.1,4,5 The resultant composites can be highlyoxidation- and creep-resistant at high temperatures.1,4 In con-trast, the powder-sintering routes can produce dense and thushigh-strength composites more easily because of the use of sinte-ring additives.

In the present study, we focus on the preparation of Si3N4/SiC nanocomposite powders used in the powder-sinteringroutes. To obtain high-quality final products, the SiC nanopar-

ticles must be uniformly dispersed in the composite powder. Themethods reported for the fabrication of the nanocompositepowders have included chemical vapor deposition6 from orpyrolysis4,7 of organic precursors (to make amorphous Si–N–C powders, which crystallize into Si3N4/SiC nanocompositesduring sintering), mechanical mixing of a Si3N4 micro/nano-powder with a SiC nanopowder8 or an Si–N–C amorphouspowder,9 adding carbon to a Si3N4 powder (SiC nanoparticlesare produced in situ during sintering through the reaction be-tween the carbon and the silica located on the surface of theSi3N4 particles),10,11 partial reaction of a Si3N4 powder withpyrolyzed carbon,12–14 nitridation of SiC,15 and carbothermalreaction of a mixture of silica and carbon powders in a nitrogenatmosphere.16,17 Among these approaches, the use of amor-phous Si–N–C powders has achieved the most uniform distri-bution of SiC, and consequently excellent mechanicalproperties. Unfortunately, the production of high-quality Si–N–C powders entails high costs. The method of mechanicalmixing is simple and cost effective, but does not result in a uni-form distribution of SiC.

The synthesis of Si3N4/SiC nanocomposite powders via thecarbothermal reaction of silica is attractive because it can easilybe integrated into the industrial process of Si3N4 productionfrom silica. The carbothermal reaction of silica is a major indus-trial route for the production of both Si3N4 and SiC powders.18,19

The SiC nanoparticles can be in situ produced in the Si3N4 matrixin the carbothermal reaction, and can thus be inherently welldistributed. The Si3N4/SiC nanocomposite ceramics sinteredfrom the carbothermally prepared Si3N4/SiC nanocompositepowders have shown a dramatic improvement in the high-tem-perature strength and creep resistance over the components pre-pared from mechanically mixed Si3N4/SiC powders.16

The kinetics of the carbothermal reaction of silica are slow. Ahomogeneous mixing state of the two solid reactants, i.e., silicaand carbon, is highly preferred. The sol–gel process is capable ofmixing reactants on the molecular scale. This process has beenused to obtain inorganic–organic hybrid gels, in which the or-ganic compound acts as a carbon source. Carbothermal reac-tions of the hybrid gels containing silica have led to SiCpowders,20–29 Si3N4 powders,30 SiC and Si3N4 fibers,31–33 andSi–Ti–C and Si–Zr–C fibers.34–36

We have developed a two-step sol–gel process for the prep-aration of silica–phenol resin hybrid gels.29,37 In contrast toother reported processes, in our preparation, organic catalysts,namely, oxalic acid and hexamethylenetetramine (HMTA), areused instead of inorganic basic and acidic catalysts that maycontain elements such as sulfur and chlorine, which are detri-mental to the mechanical properties of the final products. Theresulting hybrid gels have been converted to SiC powders invacuum.29 The use of excess carbon, which is required in theconventional mechanically mixed reactant mixtures to improvethe conversion of silica, is not necessary because of the enhancedcontact of carbon and silica in the present gels. The reaction at16501C for 30 min resulted in the formation of a SiC powderthat was almost free of unreacted silica and residual carbon(oxygen 0.43 wt%, residual carbon 0.50 wt%).29

G. Soraru—contributing editor

*Member, American Ceramic Society.**Fellow, American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: li.j.aa@m.titech.ac.jpzPresent address: Materials and Structures Laboratory, Tokyo Institute of Technology,

4259-R3-20Nagatsuta,Midori-ku, Yokohama 226-8503, Japan. e-mail: li.j.aa@m.titech.ac.jpor jinwangli@yahoo.com.

Manuscript No. 23264. Received May 25, 2007; approved July 31, 2007.

Journal

J. Am. Ceram. Soc., 90 [12] 3786–3792 (2007)

DOI: 10.1111/j.1551-2916.2007.02046.x

r 2007 The American Ceramic Society

3786

In this paper, we report on the carbothermal reaction of ourhybrid gels in a nitrogen atmosphere to produce Si3N4/SiCnanocomposite powders. We demonstrate, for the first time,controlling of the SiC content in the composite simply by ad-justing the reaction temperature and/or the starting carbon con-tent. Moreover, a two-stage process is also introduced here toproduce Si3N4/SiC powders with a controlled amount of SiC.

II. Experimental Procedure

The phenol resin was of novolac type and provided by BakeliteAG, Iserlohn, Germany (product No. Bakelite Harz 0790 K03).Other chemicals were of analytical grade. The silica–phenol res-in hybrid gels were prepared using a two-step sol–gel processdescribed elsewhere.29,37 In brief, in a solution of phenol resin,deioned water, and ethanol, TEOS was first hydrolyzed (prehy-drolysis) under the catalysis of oxalic acid, and subsequentlygelated in the presence of HMTA. The molar ratio of oxalic acidto TEOS was 0.005, and the weight ratio of HMTA to phenolresin was 0.09. The preparation was conducted at room tem-perature with a prehydrolysis time of 20 h. A transparent gel wasobtained after gelation. The gel was broken into pieces less than5 mm in size and aged at room temperature in air for at least 2days, during which the volatile components evaporated gradu-ally. The volatile components were further removed at 901C for24 h in air. The gel was then milled and sieved through a 63-mmmesh screen. The milled gel was treated at 1501C in air for 2 h tocure the resin. Two gels, denoted as C2 and C3, were preparedusing 48.0 and 72.0 g resin per mole of TEOS, respectively. The-rmogravimetry (TG) analyses (Fig. 1) indicated that the decom-position of the resin into carbon was completed below 8001C.The carbon/SiO2 molar ratios after pyrolysis at 10401C in argonwere 2.51 and 3.69 for C2 and C3, respectively. The composi-tions of the gels used in this study are summarized in Table I.

A gel (denoted as C3S) containing a Si3N4 seed was preparedwith the same resin to TEOS ratio of C3. The Si3N4 powder(H.C. Starck, Goslar, Germany; Grade M 11. 93.5% a-Si3N4,particle size distribution: 90% 1.26 mm, 50% 0.59 mm, 10%0.27 mm (data as provided by the producer)) used as the seedwas added at the beginning of the sol–gel process, and the mix-ture was mechanically milled until the completion of gelationto prevent sedimentation of the seed. Based on the Si3N4 pow-der and the TEOS used, the calculated weight ratio of seed toSiO2 was 0.3.

The carbothermal reaction was performed in a horizontaltube furnace (LORA model, HTM Reetz GmbH, Berlin, Ger-many) equipped with an alumina tube of 32-mm inner diameter.The silica–resin hybrid gel in an alumina boat was loaded intothe tube, with a sample thickness of B5 mm. The tube was first

evacuated and then purged with nitrogen gas. This evacuation–purging cycle was performed three times before heating. Thecarbothermal reaction was conducted at 14001–15501C in a ni-trogen flow (10 L/h if not otherwise specified). The heating andcooling rates were 101 and 201C/min, respectively.

The TG analyses for the gels C2 and C3 were conducted on aNetzsch STA 429 analyzer (Selb, Germany) in an argon flow(0.1 MPa) at a heating rate of 51C/min up to 10401C. To esti-mate the carbon and silica contents after pyrolysis, the sampleswere later heated at 6001C in an argon flow to remove the ad-sorbed moisture, followed by firing up to 10001C in an oxygenflow to burn out the carbon. The mass change during the burnout of carbon and the final remaining mass were assigned tocarbon and silica, respectively. All nitrogen, oxygen, and carbonin the samples after the high-temperature reaction were deter-mined using the hot gas extraction method (Leco C-analyzer C-200 with the accuracy of 0.5% and Leco N/O-analyzer TC-432with accuracies of 5 ppm for nitrogen and 1% of the measuredamount for oxygen; Leco, St. Joseph, MI). The phase compo-sitions were estimated using the measured nitrogen, oxygen, andcarbon contents, assuming that only Si3N4, SiC, SiO2, and re-sidual carbon were present in the products. The phases wereidentified using X-ray powder diffraction (XRD) on a STOEdiffractometer (STOE & Cie GmbH, Darmstadt, Germany)with a CuKa1 radiation (l5 1.5406 A). The average crystallitesize t of SiC was estimated from the width B of the XRD lineusing the Scherrer equation t5 0.9l/(B cos y), where the XRDlines were approximated by the Lorentzian shape and the mi-crometer-sized Si3N4 sample was used to calibrate the instru-mental broadening. The microstructure was observed usinghigh-resolution scanning electron microscopy (SEM) coupledwith energy-dispersive X-ray spectroscopy (EDS) on a PhilipsXL30 FEG microscope (Eindhoven, the Netherlands). TheSEM observation for the C3 gel after the TG analyses wasalso performed on an SEM S-4500 microscope (Hitachi, Tokyo,Japan). For some SEM analyses, the residual carbon after car-bothermal reaction was burned out at 5501C for 3–6 h in air.Burning in air is the typical method to remove the residual car-bon after a carbothermal reaction.16,18,38

III. Results and Discussion

(1) Microstructure of the Gel-Derived Silica–CarbonMixture

The microstructure of the silica–carbon mixture derived fromthe hybrid gel at 10401C in argon was analyzed using XRD andSEM coupled with EDS. The XRD pattern showed that themixtures were X-ray amorphous, and the SEM studies revealedthat the mixtures had a glass-like fracture surface (Fig. 2(a)). Nodiscernable particles could be found by high-resolution SEManalysis (Fig. 2(b)). The element mapping analysis by EDS in-dicated that the entire surface was composed of uniformly dis-tributed silicon, oxygen, and carbon. However, an elementdistribution analysis with high space resolution was impossiblebecause the diameter of the electron beam was B1 mm. Accord-ing to the morphology of the microstructure, the carbon andsilica should have distributed homogeneously within each other.

Fig. 1. Thermogravimetric analyses for gels C2 and C3 in an argonflow.

Table I. Compositions of the Starting Silica-Phenol ResinHybrid Gels and Their Derived Silica–Carbon Mixtures after

Pyrolysis in Argon at 10401C

Gel No.

Starting gels:

After pyrolysis

Phenol resin (g)

per mole of SiO2

C

(wt%)

SiO2

(wt%)

C/SiO2 molar

ratio

C2 48.0 33.3 66.7 2.51C3 72.0 42.4 57.6 3.69C3S 72.0 n.d. n.d. n.d.

n.d.: not detected.

December 2007 Si3N4/SiC Synthesis from Silica–Phenol Resin Hybrid Gels 3787

(2) Effect of Temperature on the Product’s PhaseComposition

The XRD patterns for the products after a carbothermal reac-tion of the C3 gel at different temperatures are shown in Fig. 3.At 14001C, no crystalline phases were produced. Increasing thetemperature to 14501C led to the formation of Si3N4. At 15001C,Si3N4 was the dominant phase, and a very small amount of SiCbegan to appear. With a further increase in the temperature, theamount of SiC increased while Si3N4 decreased. At 15501C, SiCwas the major crystalline phase accompanied by only a verysmall amount of Si3N4. Thus, at 15001–15501C, Si3N4/SiC com-posite powders were produced, with the SiC content determinedby the temperature. An elemental analysis performed for theproduct obtained at 15251C resulted in 53.3 wt% (11.7 mol%)

Si3N4, 16.9 wt% (13.0 mol%) SiC, 0.58 wt% (0.30 mol%)SiO2 (0.31 wt% oxygen), and 29.2 wt% (74.9 mol%) residualcarbon (Table II, sample C3-1525). Note that in ourproducts, the Si3N4 polymorph was mostly the a phase, whileSiC mainly appeared in the b phase (cubic) accompaniedby some a phase (possibly 2H or 6H polytype). It should benoted that the line at 2y5 33.91 could also have arisen fromstacking faults in b-SiC.39 The broad line at 2y5 151–301 wasattributed to the amorphous carbon and silica that remainedunreacted.

In the presence of sufficient carbon (C/Si molar ratio43) andunder atmospheric nitrogen pressure, the thermodynamicSi3N4–SiC boundary temperature (reaction (1)) was estimatedto be B14001–14351C,40,41 depending on the thermodynamicdata used.

Si3N4ðsÞ þ 3CðsÞ23SiCðsÞ þ 2N2ðgÞ (1)

The equilibrium phases below and above the boundary tem-perature are Si3N4 and SiC, respectively. However, due to thekinetic factors, the boundary temperature observed varied from14001C to above 15001C.16,30,42 Also due to the kinetic factors,Si3N4/SiC composite powders could be produced slightly abovethe boundary temperature in our study. This can be explained asfollows: in the carbothermal synthesis, gaseous SiO was firstformed via the following reactions38,43:

SiO2ðsÞ þ CðsÞ ¼ SiOðgÞ þ COðgÞ (2)

SiO2ðsÞ þ COðgÞ ¼ SiOðgÞ þ CO2ðgÞ (3)

CO2ðgÞ þ CðsÞ ¼ 2COðgÞ (4)

SiC might nucleate and grow via a reaction between solid car-bon and gaseous SiO

2CðsÞ þ SiOðgÞ ¼ SiCðsÞ þ COðgÞ (5)

Fig. 2. Scanning electron microscopy images of the gel C3 after pyrolysis at 10401C in argon, at a low (a) and a high (b) magnification, respectively.

Fig. 3. X-ray diffraction patterns for the products obtained after thereaction of the C3 gel at 14001, 14501, 15001, 15251, and 15501C, re-spectively, for 5 h.

Table II. Elemental and Phase Compositions (wt%) of Some Products after Carbothermal Reaction

Sample No. Starting gel and reaction conditions C N O Si3N4 SiC SiO2 Residual carbon

C2-1550 C2, 15501C 5 h 20.77 17.72 2.35 44.3 (14.9) 43.6 (51.4) 4.41 (3.46) 7.69 (30.2)C3-1525 C3, 15251C 5 h 34.28 21.31 0.31 53.3 (11.7) 16.9 (13.0) 0.58 (0.30) 29.2 (74.9)C3-TS0.25 C3, 15501C 0.25 h, then 15001C 4.75 h 37.33 22.07 0.53 55.2 (11.2) 9.28 (6.59) 0.99 (0.47) 34.5 (81.7)C3-TS0.5 C3, 15501C 0.5 h, then 15001C 5 h 37.55 18.60 0.60 46.5 (9.54) 21.2 (15.2) 1.12 (0.54) 31.2 (74.7)C3-TS1.0 C3, 15501C 1 h, then 15001C 4 h 43.88 8.95 1.90 22.4 (4.12) 43.1 (27.8) 3.56 (1.53) 30.9 (66.5)C3S-TS0.5 C3S, 15501C 0.5 h, then 15001C 5 h 26.24 23.96 0.58 59.9 (16.3) 18.3 (17.3) 1.08 (0.68) 20.8 (65.7)

The data in the parentheses present the phase compositions in mol%.

3788 Journal of the American Ceramic Society—Li and Riedel Vol. 90, No. 12

or between two gases

SiOðgÞ þ 3COðgÞ ¼ SiCðsÞ þ 2CO2ðgÞ (6)

On the other hand, Si3N4 might also form via a solid–gas re-action

3SiOðgÞ þ 3CðsÞ þ 2N2ðgÞ ¼ Si3N4ðsÞ þ 3COðgÞ (7)

or via a gaseous reaction

3SiOðgÞ þ 3COðgÞ þ 2N2ðgÞ ¼ Si3N4ðsÞ þ 3CO2ðgÞ (8)

Research showed that reaction (6) was not thermodynamicallyfavored in the carbothermal process.43 The particle shape andsize of the SiC powder produced through carbothermal reduc-tion of silica are determined by the starting carbon particles,which supports the fact that SiC is formed via reaction (5).43 Inthe case of Si3N4, both reactions (7) and (8) can be effective.38

Si3N4 may nucleate through reaction (7), but grows mainly viareaction (8) because the solid carbon is not available everywhere.Therefore, above the boundary temperatures, e.g., at 15251C,where SiC was the equilibrium product and reaction (7) was thusthermodynamically unfavorable compared with reaction (5),Si3N4 was also produced via reaction (8) because of the defi-ciency of carbon for reaction (5) and the sufficiency of CO forreaction (8). This was also evidenced by our observation that,even at 15501C, Si3N4 whiskers were produced above the samplesurface and on the upper part of the wall of the sample boat.

(3) Effect of Starting Carbon Content on the Product’s PhaseComposition

From the reaction mechanism discussed above, it is clear thatreactions (5) and (8) competed with each other above the Si3N4–SiC boundary temperature. Increasing the temperature favoredreaction (8) less, and thus the powder obtained at 15501C fromthe C3 gel contained only a very small amount of Si3N4 (Fig. 3).Less carbon in the starting material can favor reaction (5) lessand thus more SiO is consumed via reaction (8). In addition, athermodynamic calculation without considering the kinetic fac-tors has indicated that, with an increase in the C/SiO2 molarratio from two to three, the SiC content in the Si3N4/SiC com-posite powder varies linearly from 0 to 1 above the Si3N4–SiCboundary temperature.40 These mechanistic and thermodynam-ic considerations were evidenced as indicated in Fig. 4. Theuse of C2 gel led to a much higher content of Si3N4 in the pow-der obtained even at 15501C. Elemental analysis gave 44.3 wt%(14.9 mol%) Si3N4, 43.6 wt% (51.4 mol%) SiC, 4.41 wt% (3.46mol%) SiO2 (2.35 wt% oxygen), and 7.7 wt% (30.2 mol%)

residual carbon for this C2 gel-derived powder (Table II, sampleC2-1550). Therefore, Si3N4/SiC composite powders can be pro-duced above the Si3N4–SiC boundary temperature, with thephase contents controllable simply by changing the carbon con-tent in the starting material.

As described above, the product’s phase composition can alsobe controlled by varying the reaction temperature. However, thephase composition is very sensitive to the variation of the tem-perature: a change in the temperature only by 501C (from 15001to 15501C) brought about a drastic change in the major phasefrom Si3N4 to SiC, as described in the above section. In contrast,controlling the product’s phase composition by adjusting thestarting carbon content can be more effective. The use of bothparameters, temperature and starting carbon content, can beeven more practical, in particular with respect to scaling up theprocess for industrial applications.

(4) Controlling the Product’s Phase Composition via a Two-Stage Process

An alternative method was demonstrated to control the phasecomposition of the Si3N4/SiC composite powders. The temper-ature was first held at 15501C for various periods for SiC for-mation, and then decreased at 101C/min to 15001C and heldisothermally to convert the remaining SiO2 into Si3N4. Here, at15001C, an SiC-Si3N4 conversion (reaction (1)) was not ex-pected because the temperature was above the thermodynamicSi3N4–SiC boundary temperature, as described above. The C3gel was used for these experiments, and the total reaction time atthe two temperatures was about 5 h. The products’ elementalcomposition, phase composition, and allocation of silicon inSi3N4, SiC, and SiO2 are shown in Table II and Fig. 5. Thecontent of SiC was 9.28 wt% (6.59 mol%) for a holding time of0.25 h at 15501C (sample C3-TS0.25), and increased almost lin-early to 43.1 wt% (27.8 mol%) with an increase of the holdingtime at 15501C to 1.0 h (sample C3-TS1.0). Such a simple rela-tionship of the phase contents with the holding time makesphase composition control rather simple and robust.

Because SiC was formed via reaction (5), an increase in theSiC content was accompanied by a decrease in the residual car-bon content, as shown in Fig. 5. Theoretical residual carboncontents were calculated from the data of silicon allocation inSi3N4, SiC, and SiO2 in the products (Fig. 5), the starting C/SiO2

molar ratio (3.69, Table I), and the overall reactions (9) and (10)for the formation of Si3N4 and SiC derived from the reactionmechanisms discussed above:

3SiO2ðsÞ þ 6CðsÞ þ 2N2ðgÞ ! Si3N4ðsÞ þ 6COðgÞ (9)

Fig. 4. Comparison of X-ray diffraction patterns for the productsafter a carbothermal reaction of C2 and C3 gels, respectively, at15501C for 5 h.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10

10

20

30

40

50

60

70

80

90

Allo

catio

n of

sili

con

/%

Dwelling time at 1550°C /h

Si in Si N

Si in SiC Si in SiO

0

10

20

30

40

50

60

70

80 P

hase

com

posi

tion

/mol

%

Si N

SiC SiO

Free C Theor. free C

Fig. 5. Allocation of silicon (solid line) and phase composition (dashedline) for the powders obtained from the reaction of the C3 gel first an-nealed at 15501C and then at 15001C. The theoretical residual carboncontents are also presented for comparison (see the text for details).

December 2007 Si3N4/SiC Synthesis from Silica–Phenol Resin Hybrid Gels 3789

SiO2ðsÞ þ 3CðsÞ ! SiCðsÞ þ 2COðgÞ (10)

The calculated results are also presented in Fig. 5 for com-parison. The carbon contents measured are B4–11 mol% high-er than the theoretical values. This is because some siliconescaped from the samples in the form of gaseous SiO before itcould react to form Si3N4 and SiC, as evidenced by the obser-vation of the formation of some Si3N4 whiskers above the sam-ples and on the wall of the alumina boat. The escape of CO2

(reaction (3)) from the samples before it could react with carbon(reaction (4)) can also lead to an increase in the final residualcarbon contents. By assuming that no CO2 escaped from thesamples, the calculated loss of silicon in the form of gaseous SiOwas B10%–15% in these samples. The use of a more appro-priate reactor, e.g., a fluidized bed reactor, may improve theconversion efficiency of the reactants.

(5) Microstructure of Powders without Using Si3N4-Seeds

SEM images of the C3 gel-derived powders are shown in Fig. 6.The powders obtained at 15001C (Fig. 6(a)) and 15501C(Fig. 6(c)), with residual carbon fired out for each, were main-ly composed of Si3N4 and SiC, respectively. There were twomorphologies for Si3N4 as shown in Fig. 6(a). One of them waselongated crystals with a size of several micrometers, and theother was nanometer sized. In contrast, in the SiC powder, theprimary particles were all o100 nm and could not be unambig-uously identified via SEM; only agglomerates were clearly vis-ible. Estimation from the XRD spectrum using the Scherrerequation resulted in an average grain size of B13 nm for theSiC. The powder produced at 14501C contained, apart fromSi3N4, unreacted silica appearing as large irregular particles sim-ilar to the starting gel in shape as shown in Fig. 2(a). The prod-uct at 15251C (Fig. 6(b)) was a Si3N4/SiC nanocompositepowder. The size of the SiC in this sample was not estimatedfrom the XRD lines because of significant overlapping with themuch stronger Si3N4 lines. Nevertheless, this size must be similarto that of the SiC produced at 15501C (B13 nm) because of the

formation mechanism described above. The micrometer-sizedcrystals were Si3N4, surrounded by nanosized SiC. Fig. 6(d)shows the as-synthesized powder obtained at 15001C before theresidual carbon was removed. The carbon after the carbother-mal reaction retained the appearance of the starting hybrid gel(large irregular particles).

The elongated, micrometer-sized Si3N4 crystals were locatedoutside the large particles of the solid reactants (Fig. 6(d)),which was proof that this type of Si3N4 grew via the vapor phasedeposition according to reaction (8) instead of reaction (7). Thenanometer-sized Si3N4 particles were most likely formed via re-action (7), analogous to SiC, which was formed via a solid–gasreaction (reaction (5)) and thus where size was determined bythe solid reactant (carbon). It was reported that, in the forma-tion of SiC from silica–phenol resin hybrid gels, the generationof SiO was faster than it was consumed and, thus, theoverall process was controlled by the diffusion of SiO and/orcarbon for reaction (5).44 In our study, SiO was able to diffuseto the outside of the reactant particles and was involved in re-action (8). Accordingly, the production of SiO must be fasterthan its consumption by reaction (7), and the diffusion ofSiO could not be the rate-controlling factor. Because silica andcarbon were homogeneously mixed, reaction (7) was mostlikely rate controlled by the nitrogen diffusion. It can be envis-aged that nitrogen was difficult to reach into the reactantparticles because these particles did not have a porous structure(Fig. 2).

Micrometer-sized Si3N4 crystals were not observed by Choiet al.30 in their products from monodisperse spherical organo-silica powders. This was due to the fact that their organo-slicapowders consisted of porous small (0.91 mm) spheres that al-lowed high accessibility of nitrogen within. Therefore, theirSi3N4 could form exclusively via reaction (7). When mixturesof silica and carbon powders were used, as reported by Weimeret al.,38 micrometer-sized Si3N4 powders were produced eventhough nitrogen was passed through the reactants. Nitrogenshould be able to access carbon easily in this case, but the start-ing carbon particles might not be fine enough, resulting in long

Fig. 6. Scanning electron microscopy images of the C3 gel-derived powders after (a–c) and before (d) the residual carbon was burned out in air. Thecarbothermal reaction was conducted for 5 h at: (a and d) 15001C; and (b) 15251C; (c) 15501C.

3790 Journal of the American Ceramic Society—Li and Riedel Vol. 90, No. 12

diffusion paths of carbon, and consequently suppression of re-action (7).

(6) Reducing Si3N4 Particle Size Using Si3N4 Seeds

Efforts were made to reduce the size of the micrometer-sizedSi3N4 in our powders. We observed that, even after short reac-tion periods where the conversion of SiO2 was low, Si3N4 crys-tals of several micrometers were produced. It seemed that thenucleation rate of Si3N4 was too low compared with its growthrate. An increase of nitrogen flow from 10 to 50 L/h, with theexpectation of facilitating the nucleation by achieving a higherconcentration of nitrogen and faster sweeping away of CO, didnot evidently reduce the Si3N4 size.

The addition of a Si3N4 powder as a seed to the starting geleffectively decreased the size of Si3N4 particles. The microstruc-tures of the Si3N4 used as a seed and of the products from theseed-containing gel (C3S gel) are shown in Fig. 7. The Si3N4

powder (Fig. 7(b)) was obtained via a reaction at 15001C for 5 h,and the Si3N4/SiC powder (Fig. 7(c)) was obtained via reactions

first at 15501C for 0.5 h and subsequently at 15001C for 5 h.Particles of the seed had sharp surface edges (Fig. 7(a)), and theC3S gel-derived Si3N4 had a much more round surface, with thesize slightly larger than the Si3N4 seeds. Therefore, it is evidentthat Si3N4 had grown on the surface of the seed particles. Be-cause of this, the size distribution of the final Si3N4 was deter-mined by that of the Si3N4 seed, as can be seen by comparingFigs. 7 (a) and (b). The addition of the seed increased substan-tially the number of grains onto which Si3N4 grew via reaction(8) and consequently reduced the particle size of the Si3N4 ob-tained. The Si3N4/SiC powder (Fig. 7(c)) contained 59.9 wt%Si3N4 and 18.3 wt% SiC before removal of the residual carbon(Table II, sample C3S-TS0.5). The size of the Si3N4 in the Si3N4/SiC powder is similar to that of the S3N4 product. The size of theSiC nanoparticles is expected to be the same as that of the prod-uct without using seed particles.

The seed-containing gel (C3S gel) was found to be fragile andeasily crushed, while the gels without seeds did not show thisbehavior. On comparing their SEM images (Fig. 8(a) andFig. 2), it was found that the C3S gel was less densely struc-

Fig. 7. Scanning electron microscopy images of the Si3N4 powder used as seed (a), the seed containing gel-derived Si3N4 (b), and Si3N4/SiC composite(c) powders after removal of the free carbon.

Fig. 8. Scanning electron microscopy images of the C3S gel (a) and its derived Si3N4 powder obtained at 15001C for 5h before the residual carbon wasremoved (b).

December 2007 Si3N4/SiC Synthesis from Silica–Phenol Resin Hybrid Gels 3791

tured than the C2 or C3 gels. The products derived from the C3Sgel, for example, did not retain the shape of the starting particleseven before removal of the residual carbon; see Fig. 8(b). Theparticles were broken during the reaction process in the furnace,which is clearly different from the behavior of the C3 gel(Fig. 6(d)). The particle disintegration improved the accessibil-ity of nitrogen to the reactants and the Si3N4 seed, resulting in amore favored formation of Si3N4 via reaction (7) and an in-crease in the number of seed particles onto which Si3N4 grew viareaction (8), both contributing to the reduction of the Si3N4

particle size.

IV. Conclusions and Outlook

A carbothermal reaction of the silica–phenol resin hybrid gels ina nitrogen atmosphere can produce Si3N4/SiC nanocompositepowders. The Si3N4 and SiC contents can be easily controlled byadjusting the reaction temperature and/or the starting carboncontent, or using a two-stage process. The size of the obtainedSi3N4 particles can be effectively controlled by adding Si3N4

powder as seed particles to the silica/phenol resin gel. Nitrogendiffusion toward the inside of the reactant particles, SiO2 and C,can be further improved by (i) reducing the starting particle sizeof the dried hybrid gels, (ii) changing the hybrid gels to a porousstructure, or by (iii) applying a fluidized bed reactor. Thesemethods can facilitate reaction (7) and suppress reaction (8),thus serving as alternative methods to reduce the Si3N4 particlesize in the Si3N4/SiC composite powders.

Acknowledgments

The authors thank Bakelite AG, Germany, for providing the phenol resin,Claudia Fasel and Ildiko Balog for TG and elemental analyses, and Jean-Christ-ophe Jaud and Rahul Harshe for assistance with the XRD measurements. J. L.thanks the Alexander von Humboldt Foundation, Germany, for granting a re-search fellowship. R. R. thanks the Deutsche Forschungsgemeinschaft (DFG),Bonn, Germany, and the Fonds der Chemischen Industrie, Frankfurt, Germany,for financial support.

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