| Diamond as a multifunctional material has been used in various fields due to its exceptional physical and chemical properties.In general,with the increase of diamond crystal size,the application of the diamond has been greatly expanded in various fields.The diamond size less than 1mm are usually used in diamond machining for abrasive and abrasive tools;diamond single crystal in the size of about 1-3mm was used for single crystal processing,wire drawing dies,diamond in the size of about 4-5mm was used for jewelry processing,size of 6-10 mm is used for the production of high-grade knife and diamond anvil.However,natural diamond production is limited,especially high quality natural diamond,which limits the application of diamond in various areas.The success of artificial diamond,not only broke the constraints of natural diamond shortage production,and widens the scope of applications of diamond in various fields.There are mainly two kinds of synthetic methods: one is the crystal vapor deposition(CVD)method,another is the temperature gradient method(TGM).Although the CVD method in the past ten years has been made considerable progress,but the temperature gradient method(TGM)under high temperature and high pressure(HPHT)is still the main method to manufacture synthetic diamond.The temperature gradient method is: graphite converts to diamond and dissolved in catalytic solvent under the high temperature and high pressure conditions,the diamond driven from high temperature region to low temperature region under certain temperature gradient and make the diamond seed grow.Because of the existence of temperature difference in catalyst,lead to the difference catalyst density which produce natural convection in the catalyst under gravity.The transportation of dissolved carbon in catalyst is mainly relies on natural convection caused by catalyst flow.For large single crystal diamonds growth under high temperature and high pressure,the temperature field distribution in synthesis cavity and the convection flow in the catalyst are important for high quality large size diamond crystals growth.Study on cavity temperature distribution and convection flow situation of the catalyst is helpful for our better understanding of diamond growth process in metal catalysts,which has important significance to guide and optimize the synthesis of diamond single crystal.But,the diamond synthetic experiments is established by using a china-type anvil high-pressure apparatus under HPHT conditions,it is very difficult to characterize the temperature field and natural convection in the growth cell under HPHT conditions.It is difficult to observe the impact of physical field's change on the diamond crystal growth.Using FEM method to simulate the physical field in the cavity is the only feasible method to solve this problem.The distributions of temperature field of the synthesis chamber and the catalyst convection flow situation was simulated and analyzed by FEM.The results provide a theoretical reference for further guidance and optimization of the synthesis of diamond single crystal.Research contents and innovative achievements are as follows:1.We have simulated the cavity temperature and convection fields and growth process of the diamond crystal by the application of finite element method(FEM).The traditional method of changing the crystal morphology is mainly according to the corresponding relation between the crystal type and the synthesis temperature.For example,low temperature synthesis of {100} tabular shaped crystals,high temperature synthesis of {111} tower shaped crystals,and so on.The simulation results successfully explained the influence of the carbon convection field on diamond morphology in diamond synthesis process.The simulation results indicated that the morphology of diamond crystals could be adjusted by changing the carbon convection field,different morphology crystals for growth in different convection field.At the same time,the experimental data obtained from the corresponding synthesis experiments were consistent with the simulation results.Based on the simulation and experimental results,We summarize the characteristics of the convection fields which are suitable for the growth of plate and tower shape diamond crystals.At this point,we get a new method to control the crystal morphology.That is,design different carbon convection fields to synthesis different morphology crystals.2.We found that a growth defect with bowl shaped pits always occur on top surface of the tower-shaped diamond during the process of diamond growth in long time synthesis by TGM method under HPHT.In order to explain the formation of the defect,the temperature and convection fields in initial and later growth states of the catalyst have been analyzed by the finite element method(FEM).The mechanism of growth defect on diamond crystals has been explained accurately by the simulated results and a good agreement has been obtained between the calculated results and the observed experimental data.We give a theoretical explanation of the growth defect on tower-shaped diamonds by using the finite element method and provide a theoretical reference and basis for further solve the growth defect problems.3.We proposed a simply and efficient method to eliminate the growth defect by adjusting the catalyst thickness.The transport distance between the carbon source and diamond crystal become longer by increasing the catalyst thickness,and the carbon solubility,concentration and the convection intensity become stronger on top surface of the diamond.Thus,the carbon will be able to transport to the growth surface effectively which can eliminates the defect.The experiment data show that the simulation results are correct.This convenient and efficient method not only provides a method to improve the quality of diamond crystals,but also is helpful to increase the utilization rate in commercial market.4.We developed a new assembly to solve the difficulty.As expected,the convection simulation results show that the new design assembly has a strong convection intensity on the top {100} crystal face of the growth diamond,which not only can effectively eliminate the defect but is also more suitable for tower-shaped diamond growth.Long-time synthetic experimental results confirm the effectiveness of the new design.This design is more suitable for large,gem-quality,tower-shaped diamond growth than the traditional assembly.Man-made diamonds have a broad prospect of application in high-pressure studies,commercial jewellery production,and other fields.We have successfully designed a new type of assembly to solve the problem of surface defect in the synthesis of tower-shape diamond.At the same time,we have obtained a more suitable assembly for the growth of tower-shape large diamond crystals. |
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