# Fabrication of Silicon Carbide from Recycled Silicon Wafer Cutting Sludge and Its Purification
## 3 Results and Discussion
### Purification of SiC  
To remove excess C from the obtained SiC, the product was heat-treated in air at different temperatures. We assumed that C could be removed as CO or CO\({}_{2}\) when it was heated in the presence of air. SiC powder synthesized at 1550 \({}^{\circ}\)C from a Si sludge-to-C molar ratio of 1:1.4 was used in the carbon removal experiments. Figure 8 shows the XRD patterns of SiC powder after carbon removal treatment for 1h at different temperatures. It can be seen in Figure 8 that, at 750 \({}^{\circ}\)C, the significant peak for C (20 = 26.4\({}^{\circ}\)) is small, compared to those obtained for SiC treatments at 550 \({}^{\circ}\)C and 650 \({}^{\circ}\)C. In contrast, there is no significant change in SiC. To confirm the effect of the treatment time on carbon removal from SiC products, heat treatment at 750 \({}^{\circ}\)C was performed at different times. A decrease in weight of the synthesized SiC powder was recognized after the carbon removal treatment. Figure 9 shows the XRD patterns for SiC powder at different carbon removal treatment times. It was found that the carbon peak (20 = 26.4\({}^{\circ}\)) decreased  
Figure 8: XRD patterns of the SiC products after heat treatment for 1 h at different temperatures. Figure 7: XRD patterns of the obtained SiC with different Si sludge-to-C molar ratio at 1550 \({}^{\circ}\)C.  
with the increase of the treatment time. For the complete removal of carbon, SiC was heated for 300 min at 750 \({}^{\circ}\)C, as shown in Figure 9. The trend of C reduction is shown in Figure 10, which represents the change of intensity for the main peak of carbon with the increase of carbon removal treatment time. Apparently, the intensity of carbon decreased with the increase of the treatment time. According to the graph, carbon was removed rapidly up to 180 min but, after that, the process became slow. This may be attributed to the decrease of C.  
A confirmation of the removal of C could be obtained from the ATR spectra that also helped to get more information about the morphology of the samples. Figure 11a shows the comparison of SiC before and after the removal of C, along with the Si sludge. The Si sludge shows peaks at 618 cm\({}^{-1}\),1065 cm\({}^{-1}\), 800 cm\({}^{-1}\), 1134 cm\({}^{-1}\), 1225 cm\({}^{-1}\), and 2124 cm\({}^{-1}\), which belong to Si-O-Si, Si-O, Si-O-C, Si-Si, and C\(\equiv\)C bonds, respectively. This analysis showed similarities with other analyses indicating that the Si sludge contains a little amount of C and O. SiC synthesized at the optimum  
Figure 10: Variation of intensity of the 26.4 degree peak in the XRD patterns for SiC powder products with different carbon removal treatment times. Figure 9: XRD patterns of the SiC powder product at different carbon removal treatment times (T = 750 \({}^{\circ}\)C). condition showed peaks at 760 cm\({}^{-1}\), 1047 cm\({}^{-1}\), and 2110 cm\({}^{-1}\), corresponding to Si-O, Si-O-C, and C\(\equiv\)C bonds [29; 30; 31; 32; 33]. Both Si sludge and synthesized SiC contain carbon in the samples with trace amounts of oxide. However, after 300 min of heat treatment at 750 \({}^{\circ}\)C, the peak for C\(\equiv\)C disappeared, as seen in the enlarged spectra in Figure 11b. Both the XRD and the ATR analyses confirmed the formation of pure SiC. Therefore, the removal of extra carbon in SiC products can be verified through ATR (or FT-IR) analysis. The morphology of the synthesized SiC, before and after C removal, is shown in Figure 12a,b respectively. Both SEM images show a similar surface morphology with faceted features of few microns in size. However, in Figure 12a, there are some large black particles which are estimated to be covered with C particles. From the EDS pattern (inset of Figure 12a), it can be confirmed that carbon-rich parts are present.