Thermoelectric Efficiency And Superconductivity In Chalcogenide Under Pressure

In recent years,with the rapid development of science and technology,the human demand for new renewable energy is increasing with the more and more serious environmental pollution.Thermoelectric materials,as an energy material,have shown great potential in realizing the conversion between thermal and electrical energy,especially in the current time faced with energy shortage.However,the low conversion efficiency is the critical factor limiting the application of thermoelectric materials.The commonly used methods for improving the conversion efficiency of thermoelectric materials,such as doping,alloying,band engineering,and nanostructures,have shown excellent performance.However,these traditional methods seem to reach a new bottleneck(ZT>3),recently.Thus,it is urgent to find a new way to further improve the thermoelectric conversion efficiency.Pressure is a fundamental thermodynamic variable that can dramatically drive the modifications of the physical and chemical properties.With recent developments in techniques,all TE properties(?,S,?)can now be measured at high pressures.At present,there are an endless variety of thermoelectric materials,among which the chalcogenide compounds are a kind of thermoelectric materials with stable thermoelectric performance and relatively low cost.The thermoelectric performance of this system has been greatly improved,and there are many examples with the ZT value exceeding 2 in the optimal operating temperature range.However,in order to achieve large-scale applications,the thermoelectric conversion efficiency of this system still needs to be further improved,especially for the performance near room temperature.Here,we select the chalcogenide compounds(PdS,PbSe)as the research object,to study the evolution of thermoelectric properties at room temperature under pressure by using the diamond anvil cell.In addition,we also explore the superconductivity of these thermoelectric materials at higher pressures.The main contents of our work are presented as below:(1)For the PdS system.Firstly,we measured the thermoelectric properties of PdS at ambient pressure.The obtained power factor of 27 ?Wcm-1 K-2 at 800 K is the largest value obtained for the transition metal sulfides studied so far.However,the high thermal conductivity limits its thermoelectric performance and technological applications(800 K,ZT=0.33).In order to understand the high thermal conductivity,Raman scattering spectroscopy is used to investigate the thermal transport properties,and find that the three-phonon scattering processes account for the thermal transport in the temperature range of 10-620 K.Then,the phonon dispersion and phonon density of state in PdS are presented by using the first-principles theory.The longitudinal acoustic(LA)branch and the low-frequency optical modes have more contributions to the lattice thermal conductivity.By combining Raman scattering and X-ray diffraction measurements,we obtain the mode-Gruneisen parameters for the detected phonon modes.The small mode-Gruneisen parameters indicate a weak anharmonicity in this material.This offers an explanation for its high thermal conductivity.With recent developments in techniques,we measured each thermoelectric parameters(?,S,?)under pressure up to 10 GPa.a times enhancement of the ZT value has been obtained around room temperature.The largest value of ZT at high pressures near 10 GPa is comparable to the value at ambient pressure near 800 K.The results indicate that pressure as an irreplaceable thermodynamic variable has positively regulated the thermoelectric performance around room temperature.An extended study is carried out at high pressures up to 42.3 GPa for exploring the superconductivity of PdS.With increasing pressure,superconductivity is observed accompanying with a structural phase transition at around 19.5 GPa.The similar evolution between the superconducting transition temperature and carrier concentration with pressure supports the phonon-mediated superconductivity in this material.These results highlight the important role of pressure played in inducing superconductivity from these narrow band-gap semiconductors.(2)For the PbSe system.We begin by measuring the electrical and thermal transport properties of Pb0.99Cr0.01e from 2 to 300 K at ambient pressure by using the PPMS system,and found that the low-and high-temperature regimes show good agreement.Then,all the thermoelectric properties are measured at high pressures up to near 6 GPa.With increasing pressure,ZT first monotonically increases until 2 GPa and then it jumps up and reaches a maximum value of?1.7 at about 2.8 GPa.It then continuously decreases until 4.5 GPa before finally maintaining a nearly constant value until near 6 GPa.The largely increased ZT value is induced by the topological phase transition,which is supported by the sharply increased electrical conductivity,the asymmetrical form of the Seebeck coefficient,the peak of mobility,the maxima in the linewidths of two fundmental Raman-active phonon modes,and the minima of the difference in their frequencies.After that,the material was found to enter a topological crystalline insulator state as a result of band inversion,supported by the linear magnetoresistivity in experiments and the observed surface states by band structure calculations.Then,we study the physical properties of PbO.99Cr0.01Se at higher pressures.Two phase transitions(B1-Pnma-B2)in this sample have been clearly verified by these systematic measurements.The formation of the intermediate phase is suggested to be mediated by the Peierls distortion.Superconductivity has been observed in the B2 phase,accompanying by prodigiously suppressing the broad hump of resistivity at intermediate temperatures.The close correlation between the structural evolution and the associated physical properties has been firmly established for this compound.The present findings can also be expanded to other similar ?-? semiconductors.