Study And Design Of New Catalytic Materials For Diamond Single Crystal Synthesis Under High Pressure And High Temperature

The research on catalytic materials for diamond synthesis has been widely performed by many scientists in the world, and notable achievements have been reported during this research process. Recently, foreign researchers found that some non-transition metal materials and compounds show active catalytic effects under much higher pressure and temperature (P-T) conditions, usually higher than 6.0 GPa. The development of these new catalysts is greatly helpful for the synthesis of special diamonds with potential applications in optics and electrics. In some developed countries, the diamond synthesis using the new catalysts has already been well developed. But the synthesis technologies are not open because of the huge commercial value of diamond industry and the difficulties in improving performance of diamond. Thus, the research on the catalytic materials in our country is very necessary and of significance in diamond industry.Since the limited P-T conditions in the china-type large volume Cubic High-Pressure Apparatus (CHPA) (the maximum pressure ~ 6.0 GPa and temperature ~ 1600℃) prevented further study, we focused on the designing and developing the high-pressure apparatuses and cells for various high pressure and high temperature (HPHT) experimental applications. As well known the maximum pressure of the large volume piston CHPA is mainly determined by the shape of the tungsten carbide anvils, the area ratio between the high-pressure anvils and piston cylinder, and the design of the high-pressure cell. Thus, we have designed several new type of multiple high-pressure anvils and developed the high-pressure cell to obtain much higher P-T conditions ( ~ 7.5 GPa, 2000℃). In this work, we carried out the extensive studies on the new catalysts for the diamond synthesis under the pressure conditions of higher than 7.0 GPa. Most of our studies are as follows:Firstly, we have designed the stable assembling for diamond synthesis by optimizing the pressure transferring and insulation media, and improved the synthetic technology for the high-quality diamond crystallization.It is reported that many non-transition metal materials and compounds show active catalytic effects under much higher P-T conditions (larger than 1600℃). So it is necessary to improve the synthetic assembling to be used in the higerer P-T conditions. Thus, we have designed and optimized the shape of multiple high-pressure cells in a XKY—6×12MN CHPA to obtain much higher pressures (larger than 7.5 GPa). Under these conditions, we can perform the studies on new catalytic materials. Besides, we successfully designed a reasonable assembling to synthesize diamond under such HPHT conditions.Secondly, we examined the properties of low-melting metal catalysts and their catalytic effects on the characteristics and performance of the synthetic diamond.In the diamond industry, how to lower the synthetic temperature of diamond is still topic interests. Some low-melting metals (Zn, Cu, Mg, Al, et al.) and their alloys have attracted much attention. Usually, such metals are stable in the ambient conditions, but active at HPHT. In this work, we have synthesized the diamond crystals in the Fe-Ni-C and Fe-C systems with some low-melting metals additive (ranging from 1 wt.% to 100 wt.%) and their catalytic capability for converting graphite to diamond were also investigated in detail. Based on the analysis of the experimental results, we found that although much higher temperatures are required for these low-melting materials to be used as catalyst, the reaction temperatures can be reduced significantly (~100-150℃) with an appropriate addition of low-melting materials in conventional catalysts. The diamond nucleation and growth rate is also accelerated, which is very favorable for the diamond industry. Besides, we found that with some low-melting metals additive, the nitrogen concentration in the synthetic diamond is very low, thus such catalysts are very suitable for the type-IIb diamond synthesis.Thirdly, we examined the effects of minor elements in catalysts on the catalytic activities and properties of synthetic diamonds.In this paper, we studied the growth characteristics of diamond using the carbonyl iron catalyst under HPHT. We successfully synthesized the diamond single crystals with high nitrogen concentration. Our results show that the diamond morphology is not only determined by P-T conditions, but also significantly influenced by the composition of crystallization medium. The stable growth forms are strip and lamellar shapes at relatively low temperatures. On the other hand, the most important achievement is the successful synthesis of Ia-type diamonds for the first time using a new carbonyl nickel catalyst at temperatures of 1500-1800℃and pressures of 6.2-7.0 GPa. We know that most natural diamonds are dominantly Ia-type, containing aggregated nitrogen. Thus, the man made"natural diamond"may be realized employed a certain nitrogen compounds in the carbonyl nickel catalyst in the further work.Fourthly, we successfully synthesized covalently bonded"BCN"diamond by subjecting graphitic mixtures of C and BN to HPHT conditions.The X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared (FTIR) spectrometer were used to confirm the chemical composition of"BCN"diamond and atomic-level hybrid qualities. Based on the results, we found two possible reaction routes during diamond crystallization as described below: Firstly, in the system of C0.98(BN)0.02 and C0.9(BN)0.1, the h-BN powders are partially decomposed into B and N atoms. Then, certain C-C pairs are replaced by B-N pairs into the crystal structure during diamond crystallization. However, it is difficult to control the incorporation of B and N atoms for the high growth rate, which leads to the phase separation for the different B/N ratio in the diamond crystal structures. The other route for the diamond crystallization: the mixtures of h-BN and graphite are firstly transformed to some graphitic B-C-N compounds under HPHT treatment, and then the diamond crystallize directly from graphitic B-C-N compounds in C0.5(BN)0.5 system. Thus, both the B-N pairs and the N-C sp3 bondings are also found in the crystal structure. No phase separation in diamond crystals is found in this system and these"BCN"diamond crystals with well morphology are nearly transparent because of the uniform distribution of B and N atoms in the crystal structure.At last, we explored the growth mechanism of diamond under HPHT conditions, and presented a reasonable nucleation and growth models.In 1955, Bundy et al. firstly successfully synthesized diamond using metal catalysts and graphite under HPHT. Since then, the research on diamond synthesis and growth mechanism has been widely performed. But during the diamond synthesis process, the formation of diamond shape has not been examined in detail. In this work, we performed extensive studies on the shape-controlled synthesis. To obviously distinguish the seeds from new grown diamonds, we chose boron-doped diamonds, black in color, as seed crystals. We also established the growth model for the diamond grown on several seeds and proposed the possible growth processes by tracking the particular shapes of seeds before and after treated under HPHT conditions. We found the crystal direction and original shape of seed play important roles in the formation of diamond morphology in the early growth stage and the synthetic temperature will further affect the crystal shape in the following growth process. Our current research proposed a new effective way to further establish the diamond growth mechanism under HPHT conditions.