Tham khảo tài liệu 'silicon carbide materials processing and applications in electronic devices part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 60 Silicon Carbide - Materials Processing and Applications in Electronic Devices Fig. 7. Schematic drawings of a free energy vs. concentration diagram and bconcentration profile of the Ostwald ripening. at the stable eutectic temperature of 1426K. On the other hand the specimens of white cast iron shows the double peaks of endothermic reactions at the slow heating rates. Okada et al. 1981 found that at the first peak the metastable Fe3C melts but soon graphite solidifies and then remelt at the second peak. At the faster heating rates only the melting of the metastable Fe3C phase occurs. For the specimens of mixture cast iron the reactions are complicated but the melting and solidifying occur simultaneously. These experimental results indicate that the metastable phase is so stable that can melt directly. a b Fig. 8. a DTA curves of cast irons Okada et al. 1981 and b the double phase diagram of equiblibrium Fe-Graphite and metastable Fe-Fe3C systems. Speculated mechanism From the experimental result shown in Fig. 4 4H-SiC is expected to be more stable than 3C-SiC. The Si-C system should show a double phase diagram as schematically shown in Fig. 9a. The corresponding free-energy vs. concentration diagram is also illustrated in Fig. 9b. Metastable Solvent Epitaxy of SiC the Other Diamond Synthetics 61 The solubility limit of each phase is determined by the tangent common to the free-energy curves of the coexisting phases. At the temperature indicated by the dotted line the solubility limit of metastable 3C-SiC is 3C which corresponds to the dashed line of the liquidus in Fig. 9a. The solubility limit of stable 4H-SiC is 4H which corresponds to the solid line of the liquidus. The concentration gradient in the layers consisting of 3C-SiC and 4H-SiC and a very thin layer of liquid Si in between is obtained. The schematic carbon concentration profile in the liquid Si layer is shown in Fig. 9c. The concentration gradient at the metastable solubility limit of 3C-SiC .
