Oxidation of SiC powders for the preparation of SiC/mullite/alumina nanocomposites

Abstract

The oxidation behaviour of two types of SiC powder of differing particle size and morphology distribution has been studied in the present work; one submicron-sized and the other micron-sized. It has been observed that the onset-temperature for significant oxidation of the SiC powder of smaller particle size is much lower than that for the SiC powder of larger particle size; namely, about 760 °C as compared with about 950 °C. Furthermore, the rate and extent of oxidation of the former SiC powder is much higher than that of the latter SiC powder. Interestingly, however, the SiC powder of smaller particle size exhibits more controllable oxidation behaviour in the context of the preparation of SiC/mullite/alumina nanocomposites, i.e., in terms of the extent of oxidation, and hence the amount of silica formed as an encapsulating outer layer and the resulting core SiC particle size, than the SiC powder of larger particle size. The SiO₂ layer formed was amorphous when the SiC powders were oxidized below 1,200 °C, but crystalline in the form of cristobalite when they were oxidized above 1,200 °C. Since the presence of amorphous silica can accelerate the sintering of the nanocomposite, oxidation of the chosen SiC powder should thus take place below 1,200 °C.

Appendices

Appendix A: calculation of the relative weight gain and oxidation mass fraction of SiC particles in SiC powders

If the oxidation behaviour of SiC is in accordance with the following oxidation reaction [2]:

$$\begin{array}{ll}\hbox{SiC}+2\hbox{O}_2\rightarrow & \hbox{SiO}_2+\hbox{CO}_2\\40.1& 60.09\\ X&Y\\\end{array}$$

$$ Y=\frac{M_{{\rm SiO}_2}}{M_{\rm SiC}} X=\frac{60.09X}{40.1}=1.50X$$

(A1)

where X is the mass of SiC which has been oxidized, Y is the corresponding mass of SiO₂ formed;$M_{{\rm SiO}{_2}}$ and M_SiC are the molecular masses of SiO₂ and SiC, respectively.

Assuming the mass of the sample before and after oxidation is M_S and $M_{\rm S}^{\prime}$, respectively, thus,

$$ \frac{Y-X}{M_{\rm S}}=\frac{M_{\rm S}^{\prime}-M_{\rm S}}{M_{\rm S}}=C_{\rm M}$$

(A2)

where C_M is the relative mass change after oxidation. Substituting Eqs. A1 into A2 gives:

$$C_{\rm M}=\frac{1.50X-X}{M_{\rm S}}\therefore\frac{X}{M_{\rm S}}=\frac{C_{\rm M}}{0.5}, $$

(A3)

In fact X/M_S is the oxidation fraction of SiC. If we use f to express it, thus

$$ f=\frac{C_{\rm M}}{0.5}=2C_{\rm M}$$

(A4)

Appendix B: theoretical derivation of the size change of a SiC particle after oxidation

Since the oxidation of SiC is a mass-gain process and the density of amorphous $\hbox{SiO}_{2}$ ($\rho_{{\rm SiO}_{2}}=2.2\,\hbox{g}\,\hbox{cm}^{-3}$) is less than that of SiC $(\rho_{\rm SiC}=3.21\,\hbox{g}\,\hbox{cm}^{-3})$, the particle size will be increased after oxidation. Supposing that all of the SiC particles have the same size and all of them are spherical, this process can be shown in Fig. B1.

Fig. B1

The schematic diagram of the change of SiC particle after oxidation

Here r₁ is the radius of the initial SiC particle, r₂ is the radius of the remaining SiC particle, r₃ is the radius of the particle after oxidation and $r_{3}-r_{2}=\Updelta r$ is the thickness of the SiO₂ formed.

Thus, the initial mass of a SiC particle, M, is $\frac{4}{3}\pi r_1^3 \rho_{\rm SiC}$, and after oxidation, its mass is $\frac{4}{3}\pi r_2^3 \rho_{\rm SiC}$. Thus, the mass of oxidized SiC, X, is:

$$ \frac{4}{3}\pi r_1^3 \rho_{\rm SiC}-\frac{4}{3}\pi r_2^3 \rho_{\rm SiC} $$

(B1)

So the oxidation mass fraction is:

$$ f=\frac{\frac{4}{3}\pi r_1^3 \rho_{\rm SiC}-\frac{4}{3}\pi r_2^3 \rho_{\rm SiC}}{\frac{4}{3}\pi r_1^3 \rho_{\rm SiC}} $$

(B2)

$$ \therefore r_2=r_1 \root{3}\of{1-f} $$

(B3)

From Eq. B1, the mass of SiO₂ formed, Y, is:

$$ Y=\left({\frac{4}{3}\pi r_3^3 -\frac{4}{3}\pi r_2^3} \right)\rho _{{\rm SiO}_2} $$

(B4)

According to Eq. A1 in Appendix A

$$ Y=\frac{M_{{\rm SiO}_2}}{M_{\rm SiC}}X=\frac{60.09X}{40.1}=1.50X $$

Substituting Eqs. B1 and B4 into Eq. A1 above gives:

$$ r_3^3=r_2^3+\left({r_1^3-r_2^3}\right)z $$

(B5)

where $z=\left(\frac{M_{{\rm SiO}_2}}{\rho_{{\rm SiO}_2}}\right)/\left(\frac{M_{\rm SiC}}{\rho_{\rm SiC}}\right) = \left(\frac{60.09}{2.2}\right)/\left(\frac{40.1}{3.21}\right)=2.186$ is the ratio of the molar volume of SiO₂ to that of SiC.

Substituting equation B3 into B5 gives:

$$ r_3=r_1 \root{3}\of{1+f(z-1)}=r_1 \root{3}\of{1+1.186f} $$

(B6)

Thus, the thickness of the SiO₂ formed, $\Updelta r = r_{3} -r_{2}$, is:

$$ \Updelta r=r_3 -r_2 =r_1 \left[{\root{3}\of{1+f(z-1)}} \right.-\left. {\root{3}\of{1-f}} \right]=r_1 \left[ {\root{3}\of{1+1.186f}} \right.-\left. {\root{3}\of{1-f}} \right] $$

(B7)