Research And Analysis Of Synthesis Process Of Diamond Using A Novel Iron-nickel Catalyst Under HPHT

As a substitute for natural diamond,man-made diamond is a super-hard material with various applications.Generally,the industrial diamond is synthesized from high purity graphite under high-pressure and high-temperature(HPHT)condition,in which the iron-nickel alloy powder is employed as a catalyst.Therefore,it is of great significance to improve the quality of the synthetic diamond by optimizing the composition of the iron-nickel catalyst and developing new catalysts.The metal such as iron or nickel can react with carbon monoxide under certain temperature and pressure to produce to carbonyl compound.And then the ensued decomposition of the carbonyl compound under higher temperature will results in the iron or nickel powder with high purity,fine particle and high activity.The powder generated by this method is called carbonyl powder.Under the guidance of the solvent theory of diamond synthesis,diamond with high quality was manufactured under HPHT condition by a cubic press using the new carbonyl iron and carbonyl nickel powder catalyst as an alternative to the traditional iron-nickel powder prepared by water atomization.The main research and conclusions are summarized as follows:The comparison experiments were conducted on the blending process of carbonyl iron powder and carbonyl nickel powder.The result showed that it needed 6 hours to mix the two kinds of powder uniformly.The mixing process of the iron nickel-catalyst with the graphite was also studied.It turns out that the mixture reaches a homogeneous state after blending them for 90 minutes.The effect of the ratio of carbonyl iron powder to the carbonyl nickel powder on the synthesis of diamond was investigated by three groups of experiment.It was shown the diamond yield of a synthesis cake was almost equal and the peak granularity occurred between 40/45 mesh in all the three groups.The static pressure strength and impact toughness of the diamond increased obviously with the increase of the carbonyl nickel content in the catalyst.In consideration of the cost,the optimal ratio of carbonyl iron powder to the carbonyl nickel powder was set as 7:3.Another three groups of experiment were conducted to examine the effect of the ratio of catalyst to graphite.It was found that the 40/45 mesh was the peak granularity and the diamond grew stably in these three groups.There was a decrease tendency in the 35/40 mesh granularity with the rise of the catalyst content in the synthesis cake.When the ratio of carbonyl catalyst to graphite was chosen to be 3:7,the yield,static pressure and impact toughness of synthetic diamond had the highest value,indicating that 3:7 was the optimal ratio of catalyst to graphite.On the basis of the aforementioned analysis,high quality diamond single crystal particles were produced from the synthetic cakes in which the ratios of carbonyl iron powder to carbonyl nickel powder in the catalyst and of catalyst to graphite were 7:3 and 3:7,respectively.The diamond revealed excellence in color,transparency,regular shape and had few surface flaws and little internal impurity.The yield of a synthetic cake,hydrostatic strength,TI and TTI were as high as 223 ct,434.14 N,83.3%and 81.8%,respectively.And the difference between TI and TTI was only 1.5%,suggesting that the diamond had very good thermal stability.In the case of using the same ratio of catalyst to graphite and processing condition,the diamond synthesis experiments were carried out with carbonyl iron nickel powder catalyst and water atomized iron nickel powder catalyst,respectively.The results showed that the diamond grew more stably during the synthetic stage,the yield of a synthetic cake was higher,the average diamond crystal size was larger,the values of the static pressure strength and impact toughness were larger when the former catalyst was employed.Theses indicate that the carbonyl catalyst superior to the water atomized one due to its finer particle size,higher purity and higher activity.