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This paper reported the synthesis of high quality sheet cubic diamond crystals by temperature gradient method under high pressure and high temperature conditions with FeNi alloy as solvent catalyst. The growth mechanisms were discussed by finite element simulation. The synthetic sheet cubic diamond crystals had a higher growth rate in radial direction than that in axial direction. Raman spectrum and Fourier transform infrared spectrum indicated that synthesized sheet cubic diamonds had fewer crystal lattice distortions and nitrogen impurities. Finite element simulations indicated that the solvent metal convection field in the radial direction was stronger than that in the axial direction. A mechanism of sheet cubic diamond crystal growth was proposed and discussed.

The shape controlled growth of diamond is beneficial to its subsequent processing. The shape controlled growth for abrasive grade diamond, whose particle size is less than 1 mm, has been studied extensively, while the shape controlled growth of large diamond crystals, which have important commercial and scientific applications, has not been investigated in detail. Therefore, it is necessary to do further researches. In this study, we synthesize large type Ib diamond crystals and investigate their growth shapes at pressures of 5.3-5.9 GPa and temperatures of 1200-1370℃, by using Fe64Ni36 alloy as the catalyst and (100) or (111) face of seed as growth face. Experimental results show that for the diamond crystals grown along the (100) face, the crystal shapes presents plate shape at 1206-1215℃, tower shape at 1216-1260℃, and tower steeple shape at 1261-1360℃; in sequence while for those grown along the (111) face, the crystal shape is of tower at 1233-1238℃ and becomes plate at 1239-1364℃. The ratio of height to diameter, which can provide a standard to quantify the shape of a diamond, is used to describe the crystal shape in detail. For large diamond crystals growing along the (100) face, under a high pressure of 5.6 GPa, the ratio of height to diameter increases with temperature increasing but the ratio of height to diameter, when growing along the (111) face, decreases. The shape distributions of large diamond crystals in the V-shaped region can be determined in the experiments of large diamond crystal synthesis at different temperatures (1200-1370℃) and pressures (5.3 GPa, 5.6 GPa, 5.9 GPa). The lower limit temperature of large diamond crystal growing along the (111) face in the V-shape region is obviously higher than that growing along the (100) face, but the difference between the higher limit temperatures for growing along these two faces is not obvious. The difference between the lower temperature limits of large diamond crystals growing along the (100) and (111) face can be explained by the different energies of the crystal surface and diamond/graphite equilibrium line in the phase diagram of carbon/alloy. Therefore, it has been realized that the shapes for type Ib large diamond crystals are controlled.

Recently, a variety of carbon materials can be turned into pure polycrystalline diamond directly without any additives under extreme high pressures and high temperatures (pressure above 13 GPa and temperature above 2000 ℃). Polycrystalline diamond shows a broad application prospect because of its superior performance. However, it is difficult to realize the industrialization of pure polycrystalline diamond on current high pressure equipment due to the high synthetic conditions. The focus of our work is that the synthesis of pure polycrystalline diamond can be realized in the same synthesis range of single diamond produced from the solvent metal (pressure below 6 GPa and temperature below 1500 ℃). The carbon materials can precipitate from the solution in a form of diamond, and fill into the gaps between the diamond particles. According to some domestic scholars' researches on polycrystalline diamond, the solvent method can reduce the high temperature and high pressure conditions on which carbon may transform into diamond directly, and precipitate from the solution in the form of diamond into the gaps between diamond particles. Through a deep study of the approach, the low addition content, even pure polycrystalline diamond without gaps can be prepared. In this paper we have prepared pure polycrystalline diamonds under relatively lower conditions (the pressure being below 6 GPa and the temperature below 1500 ℃) by the method that the metal solution layer infiltrates into the gaps between the pure diamond particles and then the diamond particles will grow up. We also carry out a research on the mechanism of carbon transformation in the preparation of polycrystalline diamond. Compared with the traditional method of powder mixing technology, the melt infiltration and growth method is more advantageous to prepare high abrasive resistance and high density pure polycrystalline diamond.In order to prepare pure flawless polycrystalline diamonds without additives by China-type large volume cubic high-pressure apparatus (CHPA) (SPD-61200), we study thoroughly on the melt infiltration and growth method under high pressures; and this provides a theoretical guidance for pure polycrystalline diamond synthesis. In this paper, polycrystalline diamond is prepared by melt infiltration and growth method at pressures below 6 GPa and temperatures below 1500 ℃. Mechanism research of carbon transformation is made under high pressure and high temperature (HPHT). Through the analyses of optical microscope, X-ray diffraction, and field emission scanning electron microscope measurements, graphitization occurs on the surface of diamond in the procedure of metal solution infiltrating, and then the generated graphite quickly change into diamond-like carbon under HPHT. Meanwhile, the morphology of diamond particles changes distinctly in the syntheses process. From the analysis of experimental phenomena, carbon may undergo three transformations in the preparation: 1) graphite is generated due to the graphitization on the surface of diamond particles, which is caused by the metal solution infiltrating; 2) the generated graphite quickly fills into the gap with the form of diamond-like carbon during the sintering stage; 3) the diamond-like carbon is dissolved in a metal solution, and then precipitates between particles in the form of diamond. The mechanism research on carbon source transformation plays an important guiding role in the industrialization of no-additive, no-gap pure polycrystalline diamond preparation.

In this paper, gem diamond synthesis is systematically studied using the multiseed method in China-type cubic high-pressure apparatus. High-quality Ib diamond crystals are synthesized in a growth cell with 3-5 diamond seeds, by adjusting the growth cell assembly and investigating the pressure and temperature regions of diamond synthesis. Because of several diamond seeds embedded in a growth cell, the synthesized diamond crystals possess the same morphology and quality. At the same time, the whole growth rate increases apparently. Using the multiseed method of diamond synthesis the growth cell volume can be effectively utilized, the efficiency of diamond synthesis can be enhanced, and the problem of low utilization rate is solved. Meanwhile, those also provide an effective support for the gem diamond industrialization synthesis.