| As is well known, diamond is one kind of multifunctional material with a lot of excellent performance. It is widely applied in many domains such as industry, science, national defense and medical treatment. The ideal pure polycrystalline diamond not only shows the excellent physical properties of single crystal diamond equally, but also exhibits the advantage of isotropy over single crystal diamond. Recently, many researchers have reported that under extreme high pressure and high temperature (pressure above 13 GPa and temperature above 2000℃) by adopting secondary compression of high temperature and high pressure (HPHT) equipment, a variety of carbon materials can be turned into pure polycrystalline diamond directly without any additives. Pure polycrystalline diamond demonstrates a broader application prospect than single crystal diamond, because it shows more superior performances in hardness, elasticity modulus, thermal stability and impact toughness. Although it has a wide range of application demands, the synthetic technique is too rigorous (extreme high pressure and high temperature) to implement large scale production. Therefore, the technology of pure polycrystalline diamond synthesis under the relatively lower pressure and temperature conditions has become a research focus, which is attached great importance by scientists and engineers around the world.In order to lower the synthetic conditions, we design a new assembly on base of the solvent theory of single crystal diamond. Therefore, the pure growth-type polycrystalline diamond is successfully synthesized by melt infiltration method. The detailed process is:1) carbon materials precipitate and grow from the solution in form of diamond by the infiltration of metal, where the low-metal-content polycrystalline diamond can be obtained; 2) the metal in the diamond is removed through special purification process by the micro-channels of infiltration, and then the pure growth-type polycrystalline diamond is achieved eventually.Firstly, we intensively investigate the current available assembly of growth-type polycrystalline diamond synthesis by melt infiltration method on China-type large volume cubic high-pressure apparatus (CHPA). Many theoretical calculations and practice productions clearly testify that the indirect heating technique plays a decisive role in stably controlling the chamber temperature in high pressure synthesis. With the purpose of synthesizing high quality growth-type polycrystalline diamond, we completely design a new elevated assembly, which is suitable for melt infiltration method. This important preliminary work paves the way for the synthesis of growth-type polycrystalline diamond by melt infiltration method. Some related research results have already been authorized invention patents.Further, we also intensively research the synthesis of growth-type polycrystalline diamond by melt infiltration method. Currently, although scholars have done some studies on growth-type polycrystalline diamond, the solvent theory of single crystal diamond has not been applied to guide the synthesis of growth-type polycrystalline diamond. From this point, after the in-depth study of growth-type polycrystalline diamond by melt infiltration method, the solvent theory is firstly introduced to the synthesis process of polycrystalline diamond in this paper, so as to explain the phenomenon in the polycrystalline diamond growth procedure, which plays an important guiding role in the polycrystalline diamond synthesis.The main research contents and innovation results are listed as follows:1. We successfully design a new type of indirectly heated assembly, which is suitable for the synthesis of growth-type polycrystalline diamond by melt infiltration method on China-type large volume cubic high-pressure apparatus (CHPA). Besides, by the special purification process, we successfully synthesized pure growth-type polycrystalline diamond with D-D direct bonding.Almost all the traditional assemblies of polycrystalline diamond synthesis are direct heating type at present in domestic. However, the internal metal will react with graphite heating tube, which leads the unstable temperature in chamber. In order to obtain a stable synthetic environment, after considering the characteristics of melt infiltration synthesis, we design the new type indirect-heating-based assembly, synthesis technology, and purification technology, which can satisfy the demands of high quality pure growth-type polycrystalline diamond synthesis. The detailed technologies include:1) a certain thickness of metal layer is placed at one end of the diamond particles layer in indirect heated chamber, which is beneficial to the infiltration of metal through diamond particles layer and the formation of micro-channels; 2) according to the solvent theory, one-time pressurization and rapid heating technology is adopted to restrain the excessive graphitization; 3) the special purification process is implemented by the micro-channels from metal infiltration to achieve pure growth-type polycrystalline diamond.2. In the pressure range of 6GPa and temperature range of 1500℃, using nickel based metal (Ni7oMn25Co5) as infiltration agent, we systematically study the effects of technological parameters on growth-type polycrystalline diamond, such as pressure, temperature, synthetic time, particle size of diamond powder. In all, various kinds of high quality growth-type polycrystalline diamonds are successfully synthesized, including Φ 6 mmx4mm, Φ10 mmx4.5mm, and Φ15mm×4mm. Through the characterizations and evaluations, plentiful net shaped structures are formed by numerous boundaries of diamond micro crystals connected by with D-D direct bonding in the interior of the obtained growth-type polycrystalline diamond. In addition, the growth-type polycrystalline diamond contains low content of homogeneously distributed metal. After the purification process, the density of the synthesized polycrystalline diamond is 3.5346 g/cm3, which is comparable to that of single crystal diamond (3.5362 g/cm3). By comparing test, the abrasion resistance of synthesized polycrystalline diamond which is isotropy, is higher than that of commercial polycrystalline diamond for 33%. The Vickers hardness measurement at the applied load of 9.8 N reveals that the sample has higher hardness (106 GPa) than that of traditional growth-type polycrystalline diamond(70GP in journal) for 51%.3. Boron-doped polycrystalline diamond is also successfully prepared by employing the same above mentioned technology of growth-type polycrystalline diamond. After the purification process and Hall effect electronic measurement, the obtained Boron-doped polycrystalline diamond shows behavior of P type condition semiconductor with a bulk electrical resistivity range of 1 to 10 Ω·cm. The factors that may affect the electrical properties of Boron-doped polycrystalline diamond include the Boron content in the raw materials of diamond powders and the bonding between the particles.4. It is the first time that the solvent theory is introduced to the synthesis of growth-type polycrystalline diamond, which reasonably explains phenomena and mechanisms in synthesis process, especially the infiltration and growth procedures. Some important mechanisms and models are established:the infiltration force and infiltration mechanism, the graphitization force and graphitization mechanism, the growth force and growth mechanism of growth-type polycrystalline diamond.In conclusion, high quality growth-type polycrystalline diamond and Boron-doped polycrystalline diamond are successfully synthesized by melt infiltration method, based on solvent theory via a new designed indirect heated chamber and advanced high pressure and high temperature synthetic technology. After special purification process, high quality pure growth-type polycrystalline diamond is obtained. The establishment of growth mechanism of growth-type polycrystalline diamond will provide important technical support for the large-scale production of pure growth-type polycrystalline diamond. |
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