Synthesis Of High Nitrogen Concentration Industrial Diamond By High Temperature High Pressure

It is well known that diamond has many outstanding properties, such as extreme hardness, optical transmission over a wide range, high thermal conductivity and good insulator. There has been a long history for people to make use of natural diamond, but it is still not very clear about the genesis of natural diamond. People have done much to investigate the forming process of natural diamond, which mainly includes the following two research routes.First is to simulate the forming environments of natural diamond. It is usually believed that natural diamond was formed in the magma of upper or lower mantle, where the conditions are both high pressure and high temperature (HPHT). Since diamond was successfully synthesized using transition metals as solvent-catalyst in 1955, it had provided a possibility to simulate the HPHT environments of natural diamond in laboratory. As we know, natural diamond is usually found in kimberlite, which contains many inorganic mineral, such as olivine, picrite and phosphorite. So it is believed that diamond, just as natural diamond, can be synthesized using inorganic mineral catalyst, and provides valuable information about diamond nucleation and crystallization that bring us closer to the understanding of natural diamond formation process. So far many inorganic minerals have been chosen as catalysts to synthesize diamond, including carbonates, oxides, hydroxides, chlorides, phosphorus and numbers of other compounds. But the highest nitrogen concentration of diamond synthesized using inorganic mineral catalyst is only 700 ppm, which is much lower than normal nitrogen concentration of natural diamond (2000 ppm).The other route is to synthesize diamond with the same high concentration nitrogen as that of natural diamond. Diamond can be classified into types of Ia, Ib,Ⅱa andⅡb respectively, based on the concentration and forms of nitrogen and boron. Nitrogen in types Ia and Ib diamond crystals are in the forms of aggregate (A center) and single substitution (C center), respectively. Most of natural diamond is type Ia, containing more than 1000 ppm nitrogen; while the synthetic diamond, is mainly type Ib, whose nitrogen concentration is typically 200-300 ppm. Recent report indicated that the nitrogen in the type Ib diamond can be transformed into the forms of type Ia under high pressure and high temperature conditions. So it provided a means for fabricating diamond with the same nitrogen concentration and form as natural diamond. H. Kanda et al. have successfully synthesized diamond containing 1200-1900ppm nitrogen using Na2SO4 as catalyst and hexagonal boron nitride as container at 7.7GPa. But the synthetic diamond exhibited aggregate form and irregular fracture surfaces. Y. Borzdvo et al. also synthesized high nitrogen diamond in the system of Fe3N-C at the conditions of 7GPa and 1550-1850℃. It is found that the twinning crystal is usually prominent. High nitrogen diamond crystals with cub-octahedral shape or octahedral shape have been synthesized by Liang of our group, in the system of Fe90Ni10-C with 0.7 wt % additive NaN3, and the nitrogen concentration is 2000 ppm.In this work, Ba(N3)2 as a newly discovered catalyst-additive is disclosed for getting the highest level of nitrogen doping (2550ppm). The minimum pressure and temperature for diamond synthesis increase with an increase of the Ba(N3)2 additive, meanwhile the crystal colour changed from yellow to green ,and finally to deep green. Besides normal cub-octahedral shape, the crystals exhibit a special strip shape which may be formed form the stretched cub-octahedron having elongated (100) and (111) crystal faces.The quantity of stripe crystal increased at first and finally decreased. In order to analyze the surface characters of the diamond synthesized with additive Ba(N3)2 more carefully, we took some SEM photographs of synthetic crystals, which are shown in Fig. 5. Many strip shape crystals exhibit twinning structure, most of which looks like stretched cub-octahedron having elongated (100) and (111) crystal faces. Some accidented growth lines take orderly on the surfaces of the diamond crystals synthesized with additive Ba(N3)2,. The width of each line was about 0.3-1μm, the depth is 0.1μm. However, no accidented line exists on the crystal surface synthesized without additive Ba(N3)2.It is also found there ramiform veins can be seen on the triangle cusp of strip crystals, and this form changed to faultage form with an increase of the Ba(N3)2 additive.Diamond crystals have been detected by FTIR spectra, and some new features have been found. The result indicated that the nitrogen concentration increases with the increase of Ba(N3)2 additive. The highest nitrogen concentration is 2550 ppm, which is the highest nitrogen concentration reported so far. There are both types of Ia and Ib appeared when Ba(N3)2 additive is 0.7% in parts of crystals.Photographs of the diamond crystals showed that some of line and point inclusions can be seen in the crystals synthesized without additive, while some foggy inclusions can be seen in the diamond synthesized with additive. In order to analyze the kinds of inclusion in diamond, local states of iron in the inclusions of crystal were investigated on Mo¨ssbauer spectroscopy at room temperature. The result indicated that component of inclusion of crystal synthesized without additive is Fe-Ni alloy, while component of inclusion of crystal synthesized with additive is Fe-Ni alloy and iron nitride, that's to say the addition of additive has changed the component of inclusion in diamond.