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In Situ Thermal Conductivity Measurement And Method In Diamond Anvil Cells Under Stable Thermal Condition

Posted on:2022-05-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:C H JiaFull Text:PDF
GTID:1480306728981889Subject:Condensed matter physics
Abstract/Summary:PDF Full Text Request
Diamond anvil cell(DAC)is the only static high-pressure device that can generate an environment above one million atmospheres.Combined with electrical,optical,magnetic,mechanical and other physical property measurement methods,DAC can realize the characterization of material structure and various physical properties under high pressure.At present,it has been widely used in the research fields of materials science,physics,chemistry and so on.Especially when DAC is combined with high temperature,it can also simulate the temperature and pressure conditions in the earth's interior in the laboratory,which is an important platform for studying the thermal properties of typical minerals and rocks.However,it is extremely difficult to characterize thermophysical properties under high temperature and high pressure(HT-HP).The main performance is that thermal radiation and the deformation of diamond anvil and sample under HT-HP have great influence on the temperature field distribution,which leads to inaccurate measurement of thermal conductivity.Therefore,this paper studies the error sources,error correction methods,and calibration of temperature gradient distribution in DAC,so as to realize in-situ accurate measurement of thermal conductivity of materials under HT-HP conditions.The results are as follows:1.Thermodynamic parameters of main components of DAC device have great influence on the measurement accuracy of thermal conductivity under high temperature and pressure,so it is very important to study the thermodynamic properties of diamond anvil under high temperature and pressure.The thermodynamic properties of diamond up to 100 GPa and 2000 K are studied by first-principles calculation method.Our results are in good agreement with the previous experimental data at normal pressure,and phonon dispersion and most thermodynamic quantities are well reproduced,the rationality of the method is proved.The results show that the thermal expansion coefficient and heat capacity can be divided into three temperature regions in the temperature range from 0 to2000 K,which can be described by Debye model,combination of Debye and Einstein models,and Einstein model respectively.Furthermore,these models can well explain the variation law of Grüneisen coefficient under HT-HP.The effect of pressure increases the upper temperature limit applicable to the Einstein model.In the temperature range where the two models work together,the effect of pressure would increase the contribution of Debye model to the thermal expansion coefficient and heat capacity of diamond,and Grüneisen coefficient has a larger peak.In the meanwhile the position of which shifts to higher temperature with the pressure elevation.Under fixed pressure,the bulk modulus of diamond decreases gradually as the temperature increase.At a fixed temperature,the bulk modulus of diamond increases as the pressure increase.This is because the bond strength between atoms strengthens,so the bulk modulus shows an increasing trend with the increase of pressure.2.The thermal radiation properties of type Ia diamond were characterized by spectrophotometer.The emissivity ?,absorption coefficient ??and complex refractive index n(?)of Ia type diamond in the range of 200 nm to 50000 nm were obtained.Based on the study of thermodynamic properties and thermal radiation characteristics of diamond anvil,a numerical simulation method for analyzing the thermal transfer process in externally heated DAC based on finite volume method(FVM)is proposed by nonlinear three-dimensional(3D)radiation-thermal conduction coupling thermal transfer model.This method considers the radiation thermal transfer in the DAC anvil and the radiation energy consumption on the outer wall.The simulation results indicate that the radiation effect significantly enhances with the increase of anvil base temperature.Combined with the temperature measurements via thermocouples,the rationality of this method is verified by studying the thermal conductivity of Ia type diamond.Compared with the previous heat conduction model,this model combined with multi-thermocouple local temperature measurement can achieve better in-situ thermal conductivity measurements accuracy.3.Using the nonlinear 3D radiation and thermal conduction coupling transfer model,it is found that the radiation effect is the main reason for the obvious error in measuring the local temperature of the anvil by thermocouples through quantitative analysis of the influence of radiation thermal transfer on the temperature field.Therefore,we propose a temperature correction method with high universality,which can be widely used in high temperature experiments with thermocouples as temperature calibration.4.Anvil and sample size are important parameters in numerical analysis of temperature distribution.The deformation of anvil and sample has great affect on the accurate measurement of thermal conductivity at HT-HP.We use finite element method(FEM)to study the influence of anvil and sample deformation on in-situ thermal conductivity measurement in high temperature and pressure environment in DAC.The numerical simulation results show that the thermal load caused by the uneven anvil temperature aggravates the cupping deformation of the DAC,which directly leads to the uneven pressure on the upper and lower surfaces of the sample,and the radial gradient distribution of the sample thickness is more obvious.This deformation will lead to obvious errors in traditional sample thickness measurement in DAC.At the same time,according to Fourier law,the error of sample thickness can lead to the corresponding error of sample thermal conductivity measurement.Therefore,we propose a new method to study the thermal transport properties of samples in DAC with the help of thermal-solid coupling.This method only needs to measure the initial sample thickness in DAC,and effectively improves the thermal measurement error caused by the anvil and the sample deformation under HT-HP.5.The temperature effects of polarons in monolayer graphene(MG)and MG on polar substrates under uniform magnetic field and Coulomb impurity are studied by LLP transformation method,linear combination operator and quantum statistical theory.The excited state energy of MG under magnetic field and Coulomb impurity is studied.The results show that the first excited state energy increases with the increase of temperature under uniform magnetic field.This relationship is mainly due to the increase of temperature,MG would excite more phonons,which makes the number of polarons increase with the increase of temperature.In the temperature range of 500 K,the existence of Coulomb impurity increases the excited state energy.Our study on the ground state properties in MG placed on the substrate under the action of magnetic field shows that the zero Landau energy level splitting phenomenon of graphene on the substrate is mainly caused by the interaction between electrons/holes on the graphene and surface optical phonons on the substrate.What's more,temperature also has a certain regulation effect on the energy gap of MG on the substrate.The existence of Coulomb impurity enhances the temperature effect of energy gap.The research results provide a new idea for regulating the energy band structure of graphene on the basis of retaining the excellent electrical properties of graphene.
Keywords/Search Tags:high pressure, high temperature, DAC, thermal conductivity, mathematical analysis
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