Fig. 1. Schematic diagram of thermal decomposition of SiC powders in a high purity graphite crucible. The process takes place in a medium frequency induction furnace under ambient pressure at temperatures starting from 2500°C, typically not exceeding 2800°C. The shape of the CDC layer (a loose layer) was holding fragile stability. The cooled SiC layer (a dense layer) maintains good strength.

Fig. 2. Optical images of the products due to decomposition of α-SiC powders with the same weight after thermal treatment at different temperatures in graphite crucible. (a) All have become loose CDC after 8 minutes of 2800°C treatment. The samples in the graphs (b), (c), (d) were subjected to a high-temperature reaction for 1 h at 2700, 2600 and 2500°C, respectively. The measured thicknesses are 1.3 mm, 3.8 mm, 4.9 mm, respectively, all with a diameter of 34 mm. The effective thickness of the samples, i.e., 5.0 mm was determined by averaging thicknesses of the residuals at 2400-2500°C,

Fig. 3. XPS patterns of CDC26-28 samples synthesized and their sp3/sp2 carbon ratio at different temperatures. (a) sp3C/sp2C ratio of CDC25-28 after reaction for 1h. (b) XPS spectra of CDC-26, its C 1s peaks were fitted into C-Si, sp2 C, sp3 C and C-O sub peaks. CDC-27 (c) and CDC-28 (d) were obtained by pyrolysis at 2700°C and 2800°C, respectively, while their C 1s peaks were fitted into sp2 C, sp3 C and C-O sub peaks.

Fig. 4. (a) Raman microscopy of CDC-28 where the numbers of 1, 2, 3, 4 represents the smooth side, bevel, fault and rugged surface of the sample, respectively. (b) Raman spectra of area 1-4 of which the Micron Diamond powder with a particle size range of 1-2 μm shows a characteristic peak of diamond.

Fig. 5. TEM observations of nanoparticles formed by decomposition of silicon carbide at 2800 °C under ambient pressure. (a) Bright field image showing good dispersibility and integrity. (b) A histogram of the diameter data of 165 nanoparticles analyzed by HRTEM, showing that they are almost all distributed in the range of 2-5 nm. (c) and (e) The d-spacing values of the particles in the two HRTEM images correspond to cubic diamond (111). (d) and (f) The d-spacing values of the particles in the two HRTEM images correspond to n-diamond (200).

Fig. 6. HRTEM analysis of a diamond nanoparticle. (a) HRTEM image of a nanodiamond particle. (b) The nanoparticle in the white dotted line is converted into an FFT diagram and imaged along the <01> zone axis. (c) The inverse FFT image and lattice spacing values confirm that particle is a n-diamond.

Fig. 7. HRTEM analysis of the interface between SiC and carbon layers. (a) SiC powder undecomposed at 2600 °C with its TEM sample prepared by using FIB. (b) The area highlighted by the blue line is the TEM image showing the thermal decomposition of the edge of the SiC substrate, which is part of the bluely circled area of the FIB image, the black spots are the NDs in situ growth layer, light grey areas are SiC layers. (c) A representative HRTEM image of nanoparticles is obtained from the purple circled area in (b), and these nanoparticles emerges with a tendency to deviate from the edge of the substrate. The nanoparticle in the red squared is identified as a n-diamond one with a zone axe of [01] from the FFT image (inset). (d) Determination of the SiC (200) and (111) by measuring the lattice spacing by converting to FFT image. The particles marked by the white circles are nanodiamonds, and they are surrounded by amorphous carbon and SiC, the black dotted area is the amorphous carbon, and the orange dotted area is the C/SiC interface. (e) HRTEM images of C/SiC interfaces showing loss of Si from SiC lattice. Formation of nucleation of diamond at the SiC surface.

Fig. 8. Nucleation process of diamond in SiC (3C-SiC as an example). (a) 3C-SiC is in the thermal field to cause thermal vibration of Si-C. (b) The surface of 3C-SiC thermally separates gaseous small molecules such as Si, SiC2, Si2C, SiC, etc, with only the SiC lattice left. (c) When the temperature rises, more thermally decomposed small molecules break away from the SiC matrix, the lattice cavity, and swing carbon chain were involved in the formation of the sp3 carbon structure. (d) The newly formed carbon skeleton (The diamond core part is sp3 carbon) and continuously collapsing SiC lattice form the C/SiC interface. (e) The sp3 carbon stucture transformed into diamond nucleation and separated from SiC/C interface. (f) The nucleation grows into nanodiamond.