Synthesis And Annealing Of Large Single Diamonds Using Hydrazine Organics As Additive Under High Pressure And High Temperature

As we all know,natural diamond contains a lot of impurity elements,one of the most important impurity elements such as nitrogen,hydrogen,oxygen,boron,what role do these existence of impurity elements in the process of diamond nucleation and growth and which elements are the essential components in the process of diamond nucleation and growth,these has been the hot topics in the study of domestic and foreign scientific research workers.Nitrogen is the major impurity in natural and synthetic diamonds.The nitrogen concentration in natural diamond is generally above2000 ppm,which exists in the aggregation form of A-center or B-center in diamond lattice.Since,the synthesis of diamond with high content nitrogen is always been the important research direction.In the laboratory,high nitrogen concentration diamond crystals with a maximum nitrogen concentration of 2100 ppm have been synthesized.High nitrogen concentration of type-Ib diamond can be transformed into type-IaA diamond monocrystal under high pressure and high temperature annealing.Hydrogen,after nitrogen,is another important impurity element which are existence into natural and synthetic diamond lattice.Due to the limitation of synthetic technology,the research on hydrogen doped diamond started late.Until recent years,hydrogen doped diamond has attracted extensive attention in the mechanism of diamond formation,soon hydrogen,nitrogen-hydrogen and hydrogen-oxygen doped diamond have been synthesized under high pressure and high temperature,and the hydrogen doped high nitrogen concentration diamond was convert to the type-IaA diamond via annealing under high pressure and high temperature.Last,oxygen has long been found in natural diamonds,and in many inclusions of natural diamonds contained oxygen,for example H₂O,solid CO₂ and carbonate,silicate and oxide.This indicated that oxygen play an important role in natural diamond nucleation and growth process.Based on the study of natural diamond inclusions and geology,a widely accepted C-H-O fluids model for diamond formation is proposed,that is,most of the carbon bearing geological fluids in the Earth's crust and mantle can be classified as C-H-O fluids,including H₂O,CO₂,CH₄ and their mixtures.Study of diamond synthesis in C-H-O fluids under high pressure and high temperature started 1990s,for example?COOH?₂)₂·H₂O,H₂O?C₆H₈O.However,it is imperfect that the C-H-O fluids of the formation environment of natural diamond,because the C-H-O fluids is reckon without the role of nitrogen.Therefore,the formation environment of natural diamond can be changed into C-N-H-O fluid by adding nitrogen element on the basis of C-H-O fluid.Up to now,in the research of synthetic diamond in the laboratory,there has not been any report about nitrogen,hydrogen and oxygen co-doping diamond.In summary,it is meaningful that synthesis diamond in C-N-H-O fluid environment to study the formation mechanism of diamond and material circulation in the mantle environment by synthesizing diamond crystals under conditions suitable for diamond nucleation and growth.In this work,via the temperature gradient growth method,we study the synthesis nitrogen,hydrogen and oxygen co-doping diamond crystals in the NiMnCo-C system by adding C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O as additives by using China type large volume cubic high pressure apparatus?CHPA??SPD-6×1200?.The advantage of selecting C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O as additives is that only carbon,nitrogen,hydrogen and oxygen atoms are released and diffusion into NiMnCo catalytic melt to form C-N-H-O fluid melt under high temperature and high pressure,and no other impurity elements was released that affect the growth of diamond.We carried out the high pressure and high temperature annealing study for the N-H-O co-doped diamonds which synthesized in NiMnCo-C system with C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O as additives and successfully obtained the type IaA diamond under 3.5 GPa.To study the crystal ine quality,the concentration and distribution and existence of nitrogen,hydrogen and oxygen impurities in diamond crystals,we carried by optical microscope,Raman spectra,Fourier transform infrared?FTIR?and XPS.The main content and achievement are as follow:1.Diamond crystals were successfully synthesized in a NiMnCo-C system by adding C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O as organic additives,investigate the different atomic ratio of nitrogen,hydrogen and oxygen in the environment of synthetic diamond for the influence of diamond growth habit.We found that the diamond crystals of nitrogen,hydrogen and oxygen co-doping are mainly composited of the{111}faces.In addition,the color of diamonds was gradually changed from yelow to green with increasing concentrations of the C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O additives.Furthermore,when the C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O was added in the NiMnCo-C system,the inside of synthetic diamonds was prone to“burrs”,and with the content of C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O increasing,the phenomenon of"burr"is serious.But increasing the synthetic pressure and temperature can effectively reduce the"burr"of the crystals.From the infrared spectrum,as the content of C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O increases,the nitrogen concentration of synthesis diamonds also gradually increases and mainly in the form of C-center,but the absorption intensity of characteristic peaks of hydrogen(2850 cm^-1 and 2920 cm^-1)is weakened.The higher the additive content is,the weaker the absorption intensity of hydrogen characteristic peaks 2850 cm^-1 and 2920 cm^-1 is.Raman spectra show that nitrogen,hydrogen and oxygen co-doped diamond crystals with have high quality sp³ hybrid structure.The synthesis of diamond with C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O as additives can help the synthesis of higher quality diamonds with more suitable compounds of nitrogen,hydrogen and oxygen atom ratio,which is beneficial to reveal the atom ratio of nitrogen,hydrogen and oxygen in the formation environment of natural diamond,further conducive to understanding the formation mechanism of natural diamond.2.The diamond crystals synthesized by using C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O as additives was annealing experiment under high pressure and high temperature,investigate the synthesis diamonds after annealing under high pressure and high temperature.Under lower pressure of 3.5 GPa,the color of the N-H-O co-doped diamond was changed from green to yellow when the annealing temperature up to 1800oC for 2 h,and the crystals was shown uniform color distribution after annealing.And the nitrogen aggregate state was changed from C-center to A-center,the synthesis diamond became type-IaA diamond.When the annealing temperature was reduced to1600oC for 4 h,the color of the diamond crystal turned light yelow,and feeble graphitization occurred at the"burr"region of the diamonds.Part of C-center was converted to A-center,the degree of nitrogen aggregation was different due to the C₃H₅N₃O,C₃H₈N₄O₂ and CH₆N₄O.