Synthesis of beta-SiC powder from alkoxide precursor and study of its sintering behaviour

Abstract

In this study, β-SiC powder was prepared using a pyrolysed spherical precursor derived from the hydrolysis mixture of phenyltrimethoxysilane and tetraethyl orthosilicate. Before the pyrolysed experiment, an alkoxide precursor was characterised using ²⁹Si solid nuclear magnetic resonance, Fourier transform infrared spectroscopy and thermogravimetric analysis. The alkoxide precursor was heated at 1800°C for 4 h under an Ar atmosphere. To examine the pyrolysed residue after heat treatment, the sample was collected and analysed with X-ray diffraction. The X-ray diffraction results for the sample show diffraction peaks at ∼35, 60 and 73°, which correspond to the β-SiC phase. According to the results of chemical analysis, the SiC content of the powder that was prepared at 1800°C was determined to be 99·4%. The sintering behaviour of the prepared β-SiC powder was examined using B₄C and C as sintering additives in the temperature range of 1900–2200°C.

Introduction

Silicon carbide is an excellent material for high temperature structural applications because of its covalent bonding characteristics.1^–5 SiC powder can generally be produced through the Acheson process, and it requires a long time for the carbothermic reaction of SiO₂ with carbon powder at ∼2200–2400°C.6 Because of the high reaction temperature and long reaction time of the process, the powder produced has a large particle size and primarily consists of α-phase SiC. Recently, a large BET surface area is required with reducing particle size for applications of the SiC powder, such as catalyst supporter at high temperature or diesel particulate filter.

The SiC powder is generally produced in two types of crystalline structures, such as hexagonal and cubic. The processing temperature at which the β-phase SiC powder is produced is known to be 1600–1800°C, and it is lower than the temperature at which the α-phase SiC powder is produced.6^–9

Yamauchi et al. reported that a high density and high strength β-type SiC sintered body could be obtained using β-phase SiC powder as starting material and B₄C and C as sintering additives. By adjusting the sintering step and temperature, its crystal grains in the body are made both uniform and fine.10 The density of the β-type SiC sintered body was 3·15 g cm⁻³, and its flexural strength was ∼78 kgf mm⁻².

Hatakeyama and Kanzaki11 and Seog and Kim12 investigated monodispersed spherical β-SiC powder by heating spherical gel powder, which was derived from the hydrolysis of a mixture of phenyltrimethoxysilane (PTMS) and tetraethyl orthosilicate (TEOS). Yoshioka et al.13 and Li et al.14 developed a sol–gel process to mix the reactants in the liquid phase that use ethylsilicate liquid and liquid phenolic resin as sources of SiO₂ and carbon respectively. Yoshioka et al.13 used toluenesulphonic acid, and Li et al.14 used oxalic acid as a catalyst to cure the above liquid mixtures.

Previous studies have investigated methods for the pyrolysis of sol–gel precursors to obtain β-SiC powder, including different heating temperatures and atmospheres. However, the fine dispersion of Si and C in the precursor structure is a very important factor to reduce the reaction temperature and the loss of SiO (gas) during synthesis.

In this study, we perform structural investigation of an alkoxide precursor derived from the hydrolysis of PTMS and TEOS using ²⁹Si solid nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). To optimise the condition for obtaining stoichiometric β-SiC powder, the C/Si mole ratio in the alkoxide precursor was controlled. The alkoxide precursors were heated at 1800°C for 4 h under an Ar atmosphere to convert into β-SiC powder. In addition, the sintering behaviour of the prepared β-SiC powder was examined using B₄C and C as sintering additives in the temperature range of 1900–2200°C.

Experimental

Alkoxide precursors of β-SiC powder were prepared via the hydrolysis of PTMS and TEOS mixtures in ethanol. The PTMS [C₆H₅Si(OCH₃)₃], TEOS [Si(OC₂H₅)₄] and NH₄OH used in this study were all reagent grade. The C/Si ratios in the alkoxide precursors were controlled in the range of 1·88–2·77 by fixing the mole ratios of TEOS and PTMS at 4∶6, 5∶5 and 6∶4. The PTMS and TEOS mixture solution (500 mL) was diluted in 1800 mL ethanol, and 25% NH₄OH solution was added dropwise until the pH reached 9. The solution was then aged for 24 h at room temperature with stirring to make spherical gel particles ripen in the solution. The alkoxide precursors were obtained by centrifugal sedimentation and dried at 60°C in the oven.

To perform structural investigation of the alkoxide precursor, gel powders derived from the hydrolysis mixture of PTMS and TEOS were analysed using ²⁹Si solid NMR, FTIR and TGA. The ²⁹Si solid NMR spectra were obtained using a Varian Unity INOVA600 at 600 MHz by cross-polarisation magic angle spinning (MAS) technique to obtain better signal/noise ratios. The FTIR spectra were obtained using a Jasco FTIR 4100 (Jasco Inc.) in the range of 400–4000 cm⁻¹. The FTIR spectra were obtained using pellets made from the mixture of the solid sample and dried KBr. Thermal decomposition of the alkoxide precursor at temperatures up to 1000°C was measured using a thermal gravimetric analysis instrument (TGA 4000, PerkinElmer) under nitrogen atmosphere.

The alkoxide precursors were placed in a BN crucible and heated to 1800°C for 4 h at a heating rate of 20°C min⁻¹ under an Ar atmosphere. X-ray powder diffraction analysis was carried out using an X-ray diffractometer (P/MAX 2200V/PC, Rigaku Corp.) with a Cu target (K_α = 1·54 Å) to identify the crystalline phase of the powder. The shapes and sizes of the powders were observed by scanning electron microscopy (JSM-6700F, JEOL).

The C/Si ratio of the pyrolysed residue after preheat treatment at 1000°C was determined using inductively coupled plasma (Optima 5300 DV, PerkinElmer) and C/S determinator (CS244, LECC). Chemical analysis of the synthesised β-SiC powder was carried out following the JIS R6124 standard.

The sintering behaviour of the prepared β-SiC powder was examined. The β-SiC powder (9·65 g) was fully wet mixed with 0·29 g of carbon black and 0·06 g of B₄C in a mortar using ethanol. The mixed powders were pressed at 98 MPa to produce green bodies in a 16 mm diameter mould. The samples were sintered under vacuum at 1900–2200°C for 60 min. The microstructure of the sintered body was observed by scanning electron microscopy after polishing and chemical etching of the surface using Murakami's solution. The density of the sintered samples was determined using a water immersion method (based on Archimedes’ principle).

Results and discussion

The formation of alkoxide precursors from PTMS and TEOS is confirmed by the FTIR spectra of the samples. As shown in Fig. 1, the FTIR spectra of the synthesised alkoxide precursors exhibit a significant vibration band at 1130 cm⁻¹. This band is assigned to the Si phenyl stretching vibration in the precursor. Additionally, C–C stretching in the phenyl at 1620 cm⁻¹ indicates the formation of a phenylated alkoxide precursor. Other than these peaks, the C–H stretching peak in phenyl appeared at 3071 cm⁻¹ and the C–H bending peak in phenyl appeared at 695 cm⁻¹. The peaks near 1075 and 3416 cm⁻¹ are assigned to Si–O–Si and Si–OH respectively.15

Figure 1

Fourier transform infrared spectroscopy data of alkoxide precursors by changing mole ratios of TEOS and PTMS were a 4∶6, b 5∶5 and c 6∶4

Figure 2 shows the ²⁹Si MAS NMR spectrum of the as synthesised alkoxide precursor. The ²⁹Si MAS NMR spectrum of the synthesised sample shows three significant split peaks at −78, −101·5 and −10·4 ppm. Generally, the peak at −78 ppm is assigned to Ph–Si(O–Si)₃. The peaks at −101·5 and −110·4 ppm are attributed to HOSi(O–Si)₃ and Si(O–Si)₄ respectively.16 Figure 2 also shows small peaks at about −90 ppm, corresponding to residual MeO–Si(O–Si)₃ species.

Figure 2

²⁹Si MAS NMR spectrum of alkoxide precursor (TEOS/PTMS = 4∶6)

Figure 3 shows the proposed structure of the alkoxide precursors, derived from the FTIR and ²⁹Si MAS NMR spectrum data.

Figure 3

Proposed structure of alkoxide precursor

The TGA curve of the as synthesised alkoxide precursor reveals that significant weight loss begins at 500°C. This implies that the phenyl groups in the alkoxide precursor begin to undergo decomposition near this temperature (Fig. 4). Pyrolysis of the alkoxide precursor gives Si, O and C based ceramics a 79% yield after the release of CH₄, CO₂ and H₂ gases.

Figure 4

Thermogravimetric DTA data of alkoxide precursor (TEOS:PTMS = 4∶6)

The theoretical mole ratio of C/Si is 3 in carbothermic reaction, as derived from the following equation However, the loss of SiO(g) cannot be avoided during synthesis. Therefore, high purity β-SiC powder can be obtained in the range of 2·4<C/Si<2·8 with a reduced residual carbon.17

Table 1 lists the C/Si mole ratio of the pyrolysis residue after preburning at 1000°C as which determined using an inductively coupled plasma and C/S determinator. The C/Si ratios of the alkoxide precursors were controlled in the range of 1·88–2·77 by changing the mole ratios of PTMS and TEOS as 4∶6, 5∶5 and 6∶4.

Table 1

C/Si ratios of alkoxide precursors

Source (molar ratio)		Chemical analysis data/wt-%		C/Si (molar ratio)
TEOS	PTMS	SiO2	C
6	4	72·3	27·0	1·88
5	5	69·4	30·2	2·17
4	6	64·3	35·5	2·77

The alkoxide precursors were heated at 1800°C for 4 h in an Ar atmosphere to convert them into β-SiC powder. The X-ray diffraction results for the synthesised β-SiC powder show mainly diffraction peaks near 35, 60 and 73°, which correspond to the β-SiC phase in the Joint Committee on Powder Diffraction Standards card (Fig. 5).

Figure 5

Fourier transform infrared spectroscopy data of β-SiC powders synthesised at 1800°C

When the C/Si mole ratio of the alkoxide precursor is 1·88, the X-ray diffraction result after heat treatment reveals small peaks corresponding to residual Si species. However, at C/Si mole ratios of the alkoxide precursor of 2·17 and 2·77, the X-ray diffraction results primarily corresponded to the β-SiC phase and a small peak at 2θ = 33·6 attributed to the α-SiC phase.

As shown in Fig. 6, the FTIR spectrum of the β-SiC powders synthesised from C/Si with mole ratio >2·17 shows peaks at 890 cm⁻¹, which can be assigned to the Si–C stretching vibration. In the case C/Si mole ratio of the alkoxide precursor is 1·88, Si–C stretching vibration does not appear in the same region.

Figure 6

X-ray diffraction data of β-SiC powders synthesised at 1800°C

Scanning electron microscopy image show the shape and size of the β-SiC powders produced from different alkoxide precursors that were heated at 1800°C for 4 h under an Ar atmosphere (Fig. 7). The particle size of the β-SiC powder is reduced with an increase in the C/Si mole ratio of the alkoxide precursor. When the C/Si ratio of the alkoxide precursor is 2·77, the particle size of the β-SiC powder is <1 μm. However, when the C/Si ratio of the alkoxide precursor is <2·17, the particle size of the produced β-SiC powder is ∼10 μm and exhibits good crystallinity. In the case C/Si ratio of the alkoxide precursor is <1·88, the SEM image indicate that the sample is a mixture of β-SiC and residual Si.

Figure 7

(SEM) images of β-SiC powders synthesised at 1800°C: ((a)(b) TEOS : PTMS = 6:4, (c)(d) TEOS : PTMS  = 5:5, (e)(f) TEOS : PTMS  = 4:6))

The results of the chemical analysis, regarding the SiC content of the powder heated at 1800°C, are listed in Table 2. When the C/Si ratio of the alkoxide precursor is >2·17, the purity of the β-SiC powder is >99·4%. In the case C/Si ratio of the alkoxide precursor is <1·88, the purity of the β-SiC powder is ∼96·7%, and it includes 2·4% residual Si.

Table 2

Chemical analysis data of β-SiC powder synthesised at 1800°C under Ar atmosphere

	Chemical analysis data/wt-%
Source (molar ratio)				Silicon
TEOS	PTMS	SiC	Free C	SiO2	Free Si
6	4	96·7	0·31	0·55	2·44
5	5	99·8	0·12	0·05	0·04
4	6	99·4	0·30	0·16	0·11

The sintering behaviour of the prepared β-SiC powder derived from the alkoxide precursor with a C/Si ratio of 2·77 was examined using B₄C and C as sintering additives in the temperature range of 1900–2200°C. Figure 8 shows that the microstructure of the sintered body after polishing and chemical etching of the surface using Murakami's solution. Between 1900 and 2000°C, porous microstructures were observed with 60% relative density while the β‐SiC phase was maintained. The relative density rapidly increased to 90% at 2100°C. Additionally, X-ray diffraction data show that the phase transition from β- to α‐phase starts to occur at 2100°C (Fig. 9). It is known that the transformation of β-SiC into α‐phase seems to generate many defects, such as stacking faults, which cause the SiC grain to elongate.18 In our experimental data, we observed the elongation of the SiC grain while phase transition occurred from β‐ to α-phase at temperatures above 2100°C.

Figure 8

SEM images of microstructure of β-SiC sintered bodies at a 1900°C, b 2000uC, c 2100°C and d 2200°C

Figure 9

X-ray diffraction data of sintered β-SiC bodies sintered at 1900, 2000, 2100 and 2200°C

Figure 10 shows relative density of SiC sintered bodies using β-SiC powder derived from alkoxide precursor. Up to 2000°C, porous sintered body was obtained with 60% relative density. However, the relative density rapidly increased to 90% at 2100°C.

Figure 10

Relative density of SiC sintered bodies using β-SiC powder derived from alkoxide precursor (PTMS/TEOS = 6∶4)

Conclusion

In this study, β-SiC powder was prepared by pyrolysis of a precursor derived from the hydrolysis mixture of PTMS and TEOS. The alkoxide precursor was heated at 1800°C for 4 h under an Ar atmosphere. According to the results of chemical analysis, when the C/Si mole ratio of the alkoxide precursor was >2·17, the purity of the β-SiC powder was >99·4%. In the case of that the C/Si ratio of the alkoxide precursor was <1·88, the purity of the β-SiC powder was ∼96·7%, and it also included 2·44% residual Si.

The sintering behaviour of the prepared β-SiC powder was examined using B₄C and C as sintering additives in the temperature range of 1900–2200°C. Between 1900 and 2000°C, porous microstructures were observed with 60% relative density while the β-SiC phase was maintained. The relative density rapidly increased to 90% at 2100°C. Elongation of the SiC grain was observed while the transition from β- to α-phase occurred at temperatures above 2100°C.