| As a kind of extreme functional material with a wide range of applications,diamond is widely used in national defense,science and technology,industry,biology and medical fields.Its excellent performance is manifested in the aspects of high hardness,high thermal conductivity,low compressibility,light transmission bandwidth,corrosion resistance,and radiation resistance.As the size of diamond increases,the application field becomes wider and wider.However,due to the lack of natural diamond resources,its application in the above-mentioned fields is limited.If synthetic large-size high-quality diamond single crystals can replace natural diamonds,it will greatly expand the application fields of diamonds.The artificial synthesis of large-size and high-quality diamond single crystals is one of the key topics in current diamond research.The current theoretical basis of synthetic diamond is based on the temperature gradient method established by G.E.in 1970.Through certain high pressure and high temperature conditions,graphite can be converted into diamond and dissolved in the catalyst catalyst.Driven by the temperature gradient,diamond flows from a high concentration area at a high temperature to a low concentration area at a low temperature,and precipitates near the seed crystal to continuously grow the diamond.For the temperature gradient method to grow gem-grade diamond,the distribution of the pressure field,temperature field,convection field and other physical fields in the high-temperature and high-pressure synthesis chamber has important guiding significance for crystal growth.But,the diamond synthetic experiments is established by using a china-type anvil high-pressure apparatus under HPHT conditions,and conventional test methods cannot measure its in-situ performance.It is impossible to accurately measure the pressure field,temperature field,distribution and strength of the flow field in the high-temperature and high-pressure synthesis cavity,it is difficult to directly observe the physical field changes during the synthesis process and its influence on crystal growth.Many phenomena and results can often only be analyzed based on the long-term accumulated experience of experimenters.Due to the lack of a correct understanding of the distribution and laws of the physical field in the process of crystal growth,there is a lack of clear understanding of the formation mechanism of crystal impurities,crystal defects,crystal cracks and other phenomena,and it is difficult to effectively control its growth.Therefore,an analytical method is urgently needed to solve many problems faced in the process of crystal growth.In this paper,the finite element method and multi-physics coupling technology are used to numerically simulate and analyze the pressure field,temperature field and the distribution of the catalyst convection field in the cavity of the diamond synthesis assembly.The diamond synthesis assembly was optimized and designed to solve the problems of insufficient oil pressure in the cavity,asymmetric physical field distribution in the cavity,selection of catalyst solvent viscosity and convection control,and the influence of the radial temperature of the cavity on crystal growth.It provides an important theoretical basis for improving the synthesis quality of large-size diamond single crystals.The research content and innovation results of this paper are as follows:1.Numerical simulation of the pressure field distribution in the synthesis cavity of different pressurized blocks is carried out.By adjusting the height,diameter and shape of the pressurizing block,the FEM method is used to simulate and simulate,and the change of the pressure field in the cavity is studied.The simulation results show that the center pressure of the cavity mainly depends on the proportion of the material with high compressive strength in the cavity.The center pressure of the synthetic cavity increases with the increase of the volume of the pressurized block.By adjusting the height of the pressurized block,Can effectively reduce the oil pressure required for synthesis.According to the simulation results,a new type of pressurized cavity assembly was designed,and the synthesis experiment proved the accuracy of the simulation results.The experimental results show that every time the height of the pressurized block increases by 1 mm,the oil pressure drops by about 2.25 MPa.So far,this article has obtained a new method of controlling cavity pressure and synthetic oil pressure.The research results can be used to optimize the design of the cavity,improve the safety of high-pressure experiments,reduce the cost of high-temperature and high-pressure synthetic diamond,extend the service life of the WC top hammer,and reduce energy consumption.2.In a large number of diamond synthesis experiments,it is found that long-term growth of large-size diamond crystals is prone to stepped defects on the surface,resulting in a decline in crystal quality and utilization.In order.to study the formation mechanism of this growth defect,this experiment carried out a numerical simulation of the temperature field and the flow field of the catalyst in the synthesis cavity by the finite element method.The simulation results show that the temperature field and convection field of the catalyst in the diamond synthesis cavity are distributed asymmetrically,and the temperature along the sealing edge is higher than the temperature in the direction perpendicular to the hammer surface.The simulation results successfully revealed the formation mechanism of stepped defects on the crystal surface,and the calculated results are consistent with the experimental data.Two new assembly processes that can effectively eliminate diamond growth defects are designed.This work not only improves the quality of large single crystal diamonds,but also helps reduce the cutting cost of commercial diamonds,and provides an important reference for improving the synthesis of high-quality gem-grade diamonds.3.Through the simulation and analysis of the flow field of the catalyst in the diamond synthesis cavity under different viscosity catalyst systems,the influence of the catalyst viscosity on the growth of diamond is studied.The results show that the low-viscosity catalyst has a higher convection speed,a fast carbon flow,and a larger crystal growth volume,but long-term growth is prone to defects;The convection speed in the high-viscosity catalyst is slow,the precipitation speed of carbon is moderate,and the crystal quality is good.It is suitable for long-term growth of high-quality large-size diamond single crystals.The result of the synthesis experiment is consistent with the simulation analysis.This experiment puts forward a new concept of viscosity to control the quality of diamond crystals,and control the crystal morphology by adjusting the convection field changes in the catalyst.It is not just limited to changing the size of the temperature gradient in the diamond synthesis chamber to synthesize crystals with different appearances.Therefore,this paper establishes the characteristics of the convection field suitable for the viscous growth of diamond crystals with different catalysts,which provides an important theoretical reference for better cavity optimization and the design of the flow field with new catalysts.4.The finite element method is used to simulate the temperature field and convection field of crystal growth under symmetric and asymmetric temperature gradient environments,and the influence of the change of the radial temperature gradient of the catalyst on the crystal morphology is studied.The simulation results show that the crystal is in a symmetrical temperature gradient environment,and the physical field is symmetrically distributed.The crystal is in an asymmetric temperature gradient environment,and the temperature and convection of each growth surface of the crystal are different.For this reason,this paper designs a new type of double-layer composite structure crystal bed that can effectively reduce the radial temperature gradient in the cavity,which provides a stable growth environment for the synthesis of high-quality diamond crystals by the poly-seed method.At the same time,it provides an effective solution for realizing the adjustment of the radial temperature of the cavity. |
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