Experimental Research On Heat Conduction Of Semiconductor Materials Under Ultrahigh Pressure

Ultrahigh pressure conditions generally exist in the fields of earth sciences,material synthesis and high-temperature superconductivity.Ultrahigh pressure induces the changes of substances in lattice structure,elect:ronic structure and interactions among energy-carrying particles,thus making substances present special and excellent physical and chemical properties such as heat,electricity and magnetism.The thermal transport mechanism under ultrahigh pressure and the pressure dependence of their thermal conductivity are of great significance to the thermal evolution of the Earth's temperature field and the formation of magnetic field,the synthesis of high-performance semiconductor functional materials and the revealing of the mechanism of high-temperature superconductivity.However,the thermal transport mechanism of materials under ultrahigh pressure and the influence of pressure on the thermal conductivity are not clear,the existing researches lacked systematicness,and the theoretical researches were lack of experimental verification.Due to the restriction of the ultrahigh pressure experiment technology,there were a few experimental studies on the thermal conductivity of materials over 10 GPa.The existing researches mostly adopt the contact thermal conductivity measurement methods,which have the disadvantages such as low pressure condition,time-consuming and large experiment error.In this paper,the thermal transport mechanism and thermal conductivity pressure dependence of semiconductor materials under 20 GPa pressure were studied through the combination of experiment and theoretical researches.In this paper,an ultrahigh pressure thermal transport experimental system was set up,which was composed by time-domain thermoreflectance(TDTR)combined with diamond anvil cell(DAC)technology.The preparation of ultrahigh pressure samples and the pressure measurement of samples were introduced in detail.Through the optimization design of modulation module and frequency multiplier module,the interference of the pump pulse to the effective signal in TDTR system was reduced,and the signal-to-noise ratio of the system was greatly improved.Due to the small size(?100?m)and the complex structure of DAC sample cavity,the traditional TDTR experiment system is difficult to accurately locate the sample of this scale.Therefore,through the modification of the high-pressure sample platform and the design of the in-situ imaging system,we have achieved the rapid localization of the sample under the ultrahigh pressure condition,the characterization of the surface morphology of the sensing layer and the selection of test points.These technologies provide technical guarantee for accurately obtaining the real thermal signal of the ultrahigh pressure sample.In this paper,a bidirectional heat transfer model was established for high pressure TDTR experimental data processing,and verified by measuring the thermal conductivity of liquid medium.The TDTR method obtains the unknown measurement parameters by fitting the experimental measurement data with the theoretical heat transfer model,while the accurate values of the free parameters in the heat transfer model under the corresponding pressure are required.However,the free parameters thermophysical data under high pressure are scarce,such as the pressure transmitting medium,sensing layer and interfacial thermal conductivity.Therefore,based on the sensitivity analysis of each parameter to the TDTR signal,the influence weights of each parameter was identified and then the corresponding processing method was selected in this paper,which reduced the difficulty of the TDTR experimental data processing under ultrahigh pressure.Based on the different sensitivity of the thermal conductivity and volume specific heat capacity of the base to TDTR amplitude and phase signals,a two-step fitting method was proposed to obtain these two parameters simultaneously.Based on the problem that there are many unknown free parameters in the heat transfer model of multilayer structure and the TDTR experimental data can not be processed,two simplified heat transfer models were built up and their accuracy and applicability were discussed.These improvements and data processing methods provide technical support for accurate measurement of the thermal conductivity of ultrahigh pressure samples.The TDTR+DAC experimental system was used to study the thermal conductivity of single crystal silicon and gallium arsenide as well as the pressure dependence of Al/Si and Au/GaAs interfacial thermal conductance,respectively.The experimental results showed that in the rang of 20 GPa the thermal conductivity of single crystal silicon and gallium arsenide increases with the pressure increase and the Al/Si and Au/GaAs interfacial thermal conductance increases first and then tends to saturation with the pressure increase.Based on the sensitivity of free parameters to the TDTR signal,the error transfer formula for the TDTR+DAC system measurement was established,and the experimental measurement error was analyzed by estimating the inaccuracy of free parameter values.Finally,the pressure dependence of the lattice thermal conductivity,specific heat capacity,group velocity,relaxation time and phonon density of states of single crystal silicon and gallium arsenide was calculated through the the first principle combined with Boltzmann equation,and the influence mechanism of ultrahigh pressure on the lattice thermal conductivity of single crystal silicon and gallium arsenide was explained.Through the comparison and analysis of theoretical calculation results and experimental test results,it was verified that the TDTR+DAC experimental system built in this paper can measure the thermal conductivity of materials under ultrahigh pressure.