Different effect of NiMnCo or FeNiCo on the growth of type-IIa large diamonds with Ti/Cu as nitrogen getter

In order to synthesize high-quality type-IIa large diamond, the selection of catalyst is very important, in addition to the nitrogen getter. In this paper, type-IIa large diamonds are grown under high pressure and high temperature (HPHT) by using the temperature gradient method (TGM), with adopting Ti/Cu as the nitrogen getter in Ni₇₀Mn₂₅Co₅ (abbreviated as NiMnCo) or Fe₅₅Ni₂₉Co₁₆ (abbreviated FeNiCo) catalyst. The values of nitrogen concentration (N_c) in both synthesized high-quality diamonds are less than 1 ppm, when Ti/Cu (1.6 wt%) is added in the FeNiCo or Ti/Cu (1.8 wt%) is added in the NiMnCo. The difference in solubility of nitrogen between both catalysts at HPHT is the basic reason for the different effect of Ti/Cu on eliminating nitrogen. The nitrogen-removal efficiency of Ti/Cu in the NiMnCo catalyst is less than in the FeNiCo catalyst. Additionally, a high-quality type-IIa large diamond size of 5.0 mm is obtained by reducing the growth rate and keeping the nitrogen concentration of the diamond to be less than 1 ppm, when Ti/Cu (1.6 wt%) is added in the FeNiCo catalyst.