The Synthesis Of High Quality Diamond With Super-fine Grain Size

Since diamond was first synthesized by a General Electric research group from graphite and metal catalyst at static high pressures and high temperatures (HPHT) in 1955, diamond synthesis has attracted much attention worldwide because of their potential economical competitiveness for many applications. The subsequent rapid development of superhard materials syntheses to the current level is mainly contributed by the continual improvement and perfection of static high pressure apparatus and the finding of a series of transition metals and alloys. The superhard materials have many applications which mainly rely on their unusual physical characteristics. Among these superhard materials, diamond is of the greatest interest. Diamond is a functional material such as its extreme rigidity, favorable physical and chemical properties. It has high electron vacant-site migration rate, large band gap, high breakdown voltage and large thermal conductivity. Diamond excels all other materials in many aspects. Now, it has being applied in many fields such as industry, scientific research, national defence, medical treatment etc. As a new force, industrial diamonds boost the development of industry society.The technique for the synthesis of diamond has been greatly improved due to its extensive application. In the late 1980s, the technique for the high-strength coarse diamond crystal synthesized by powdered catalyst in the belt apparatus was successfully exploited by Winter Company, Germany, which made a great breakthrough in synthesis of high grade diamond using powdered materials. This synthesizing method had many merits such as increasing the output, decreasing the consumption of raw materials and improving the crystal quality, therefore, the famous diamond companies of the world almost employed this method to synthesize high-strength coarse diamond and cornered chronically the high grade diamond market since the end of 90's. Although China is a large country of the industrial diamond, most of her products belong to the mean grade in the international diamond market. The reason for that lies in the obsolete assembly cell and dated technics. Therefore, the study of the technics for the diamond synthesized by powdered materials independently is a crux to increase competitive ability of our country in international diamond market.The synthesis of coarse grain diamond by powder catalyst had made preliminary progress; the field of fine grain diamond synthesis is still blank. In this paper I researched the synthesie of fine grain diamond by powder catalyst under the guidance of solvent theory that diamond grows under the high temperature high pressure. I adopted powder catalyst and graphite as raw materials, and performed experiments by the cubic apparatus.Factors influencing grain size of artificial diamond are various; carbon source, catalyst and impurity element will play a decisive role to change the nature to diamond. Systematic research of diamond growth is not enough in our contry at present. The understanding about catalyst is not overall. The grain size of synthesized diamond differs significantly, and crystal's quality is low. In order to perfect the theoretical system of the fine grain diamond synthesis and promote the development of industry of diamond in our country, I have carried on research in several following respects in this paper.In this paper we studied and obtained the effective method to synthesize high-quality diamonds with super fine grain size. The separation technology diamonds with fine grain size is different from diamonds with coarse granularity. Because fine grain's specific surface area is large, the surface tension is strong and it is difficult for them to deposit in the liquid, the separation of diamonds with fine grain size needs special method. we find that the hot mixture of H₂SO₄ and HNO₃(1:3) is best for the acid treatment of the synthesis bar. The following step is to separate to the mixture of the graphite and diamond. We have adopted diamond-graphite separator designed ourself, and the result is very good. We also eliminate the graphite effectively by the method of heating in air. At last we simply introduced the machine for selecting and so on and summarized a whole set of method to obtain highgrade diamond with super fine grain size.In this paper we investigated the influence of catalyst with dirrerent to the grain size of synthesized diamond. We got seven kind of Fe₈₀Ni₂₀ powder catalyst by test sieve, and observed the diamonds synthesized by catalyst of dirrerent grain size. I found finally that was coarsest. Granularity's peak value of diamonds that grow in 8 minute was 45-50 mesh, and the coarsest diamond was up to 35-40 mesh. Granularity's peak value of diamond synthesized by the catalyst with grain size of 140-170 mesh was 60-70 mesh, and the fineest diamond was 80-100 mesh. The grain size of diamonds synthesized by the other catalyst was 50-60 mesh.Through the sieving of the catalyst, I have also investigated question of diamond granularity's homogeneity. The result indicated that there are corresponding relations between and homogeneity of catalyst's grain size. Namely the diamonds synthesized by sieved catalyst with even granularity had better granularity's homogeneity. And diamond granularity's homogeneity was better when the catalyst's grain size was fineer. As to this characteristic, the grain size of mostly (62.03%) diamonds synthesized by the catalyst of 400 mesh was 60-70 mesh and the ratio of diamond's maximal grain size and the minimal grain size is minimum, although the catalyst granularity's homogeneity was worst. We conclude that this phenomenon is associated with the pooling of catalyst. That is said that the difference among each point diamond growing is smaller caused by fine grain size catalyst than that caused by big grain size catalystUsing specific craft (increasing pressure in two steps and decreasing pressure in one step), high-quality 1300 mesh diamonds with perfect morphology and smooth surface were synthesized. The result indicated that high quality diamonds with regular shape can be synthesized from regular irregular shape. Batch of 400 mesh diamond were carried out by specific craft application.We studied the impacts of additive Zn on the diamonds synthesized. The result showed that the colour of synthetic diamonds with Zn added are buff or orange; the (111) plane upgrowth and the grain size get smaller when the content of Zn increased; the appearance of Fe₃C is restrained effectively in diamonds with Zn added in. Tokens were carried out with 400-mesh and 100-mesh synthetic diamonds and 400-mesh milled diamond. The SEM results show that diamond with size 3μm have no relugarly shape and relugarly shape appeared when diamond's size reached 10μm; The 400-mesh and 100-mesh synthetic diamonds both have well hex-octahedron shape; however, the 400-mech milled diamond has irregularity crystal shape and accidented crystal surface. The TG-DTA results show that the 400-mesh and 100-mesh synthetic diamonds both have higher thermal stability than that of the 400-mesh milled diamond. The results of magnetization strength show that the 400-mesh milled diamond has much weaker magnetization strength than that of the 400-mena and 100-mesh synthetic diamonds, which indicates that the milled diamond contains few metal inclusions. The irregularity crystal shape, accidented crystal surface and macroscopical disfigurement caused in the milling process may induce its lower thermal stability.