Study On Thermoelectric Properties Of Two Dimensional Layered Cr₂Ge₂Te₆-based Materials

Energy crisis and environmental pollution have urged researchers to develop new and clean energy sources.Compared with the existing energy sources,the thermoelectric conversion technology has some unique advantages as following:all solid state process,no mechanical transmission parts,small volume,high reliability,and pollution-free,no noise,no emissions,etc.In addition,as long as there is a temperature gradient applied on thermoelectric materials,the reversible conversion of heat and electricity can be realized,which is not limited by the space.Therefore,thermoelectric technology provides a unique green energy solution for supplementing to the existing energy technology.Thermoelectric energy conversion technology can directly convert waste heat and solar energy into electric energy,and can also create local cooling for thermal management of advanced electronic equipment or human comfort.This technology has been widely used in aerospace,micro electronic devices and medical devices.In addition,it has a broad application prospect in solar heat,industrial waste heat,automobile exhaust waste heat and other recycling power generation.Therefore,thermoelectric material,as an environmental friendly and reliable candidate for solid-state energy application,has attracted global attention.Last decade has witnessed the rapid progress in thermoelectric materials research.The new concept based on“high mobility-low lattice thermal conductivity”in the crystal structures with mixed chemical bonding has recently been purposed,which offers a new perspective for discovering novel thermoelectric materials.Furthermore,to achieve a high thermoelectric performance,the compounds with layered structures associated with the hexagonal,trigonal,or tetragonal symmetry should be the promising candidates due to the potential high degeneracy near band edges and the resultant favorable electrical transport properties.One such case is Cr₂Ge₂Te₆,which is a narrow band gap semiconductor with hexagonal high symmetry structure and mixed chemical bonds.In this paper,the thermoelectric properteis of layered Cr₂Ge₂Te₆ material with intrinsically low lattice thermal conductivity was systematcilly studied.Theoretical calculations of energy band and density of states revealed the internal mechanism of doping effect on electrical properties of Cr₂Ge₂Te₆ based compounds.Experimentally,the thermoelectric performance of these compounds were optimized by selecting the appropriate doping or solid solution elements,and the relationship between which and thermoelectric parameters was studied.The morphology and chemical composition of Cr₂Ge₂Te₆ based compounds were characterized by SEM and TEM.In addition,the preparation of Cr₂Ge₂Te₆ single crystal and the effective doping of Fe and Mn were explored.The influence of the magnetic dopant on the magnetic transition temperature was also investigated.As a supplement,the low temperature electrical properties of Cr₂Ge₂Te₆ single crystal were studied,which laid a foundation for the further research on the thermoelectric transport properteis of the single crystalline Cr₂Ge₂Te₆.The main contents of this thesis are as follows:(1)Cr₂Ge₂Te₆ based polycrystalline materials with high density were successfully prepared by solid state reaction coupled with spark plasma sintering.Based on the fact that the intrinsically low carrier concentration of Cr₂Ge₂Te₆ limited its thermoelectric performance,the enlargement of hole concentration by elemental doping was attempted.By comparing the electrical properties of Cr₂Ge₂Te₆ based compounds with a variety of doping elements,Fe was identified as one of most effective doping element and its doping content was optimized.The influence of Fe doping on the thermoelectric properties of Cr₂Ge₂Te₆ was studied.The relationship between the morphology of the products and the electrical and thermal transport properties was also explored.The band structures of pristine Cr₂Ge₂Te₆ and Fe doped one were given by the first principle calculations,and the change of band structure due to Fe doping was analyzed to reveal the internal physical mechanism of the improvement in its electronical conductivity upon Fe doping.Experimental data showed that the maximum power factor was enhanced from 0.23 m W/m K² for the pristine sample to 0.37 m W/m K² for the best Fe doped one along the pressure direction at the same temperature.Combined with the intrinsically low thermal conductivity of the Cr₂Ge₂Te₆ material,the final peak z T value was increased from 0.3 to 0.4.In addition,Cr₂Ge₂Te₆ showed obvious anisotropic thermoelectric properties due to its intrinsically two-dimensional layered structure.By comparing the thermoelectric parameters of Cr₂Ge₂Te₆ based compounds along the pressure direction and perpendicular to the pressure direction,the preferred direction for thermoelectric performance is obtained,that is,along the pressure direction.(2)Inspired the effectiveness of Fe doping on improving the carrier concentration of Cr₂Ge₂Te₆,the thermoelectric performance of Mn-doped Cr₂Ge₂Te₆ compounds was investigated.The comparison between these two doping elements showed that Mn is a better doping element.The physical mechanism of the effect of Mn doping on electrical transport was explained by the change in energy band and density of state due to Mn doping.Also,the alteration in the projected density of state(PDOS)of pristine compound with respect to Fe and Mn doping,the influence of doping elements on the total density of state near Fermi level and the origin of impurity level were explored.The essential reason for the higher efficiency of Mn doping in increasing carrier concentration was revealed.Theoretical calculation of energy band combined with single band parabolic model was used to find out the reason for the increase in the carrier mobility upon Mn doping.The microstructure and composition of Cr_1.9Mn_0.1Ge₂Te₆ and the effectiveness of Mn doping were analyzed and verified by SEM and TEM.Also,the relationship between morphology and thermoelectric parameters was explored.The Mn doping content was optimizd to attain higher thermoelectric performance.The peak value of power factor for Cr_1.9Mn_0.1Ge₂Te₆reached 0.57 m W/m K²,which was 2.5 times of that for pristine Cr₂Ge₂Te₆.Furthermore,the peak z T value of Cr_1.9Mn_0.1Ge₂Te₆ was 0.63,which was twice of that of the undoped one.Again,the conclusion about the preferred direction for thermoelectric performance of Cr₂Ge₂Te₆ is still valid for Mn doped samples.(3)The fluctuations of mass field and stress field were introduced to intensify the scattering for a broad spectrum of phonons and reduce the lattice thermal conductivity by the making solid solutions with Pb and Se at Ge and Te sites,respectively.The variation in thermoelectric parameters due to solid solution was studied,and the optimum substitution contents of Pb and Se were explored.The lowest thermal conductivity in the series can reach about 0.5 W/m K.Meanwhile,the electrical performance was not affected and the maximum PF can still be maintained about 0.23m W/m K².Finally,the z T peak was raised from 0.3 to about 0.38.Afterwards,on the basis of the best solid-solutioned Cr₂Ge₂Te₆ compounds with Pb and Se,Mn was also added as doping element to explore the effect of dual element doping on thermoelectric properties.The electrical and thermal properties were optimized and improved independently.Finally,the maximum z T values of Cr_1.9Mn_0.1Ge_1.9Pb_0.1Te₆ and Cr_1.9Mn_0.1Ge₂Te_5.88Se_0.12 can both reach about 0.8 at 830 K,which was a considerable improvement compared with the highest z T value?0.63 in the previous study.As a result,thermoelectric properties of Cr₂Ge₂Te₆ were further optimized.These results further promoted the application of Cr₂Ge₂Te₆ with intrinsically low thermal conductivity.(4)The preparation method of Cr₂Ge₂Te₆ single crystal was explored and improved.The single crystalline Cr₂Ge₂Te₆ samples with high quality were synthesized and effectively doped to further understand this compound by measuring the intrinsic physical transport properties.The effect of Fe and Mn doping on the magnetic transition temperature of Cr₂Ge₂Te₆ was studied to resolve the question whether the doping of magnetic elements would affect the measurement of Hall carrier concentration.In addition,the electrical properties of Cr₂Ge₂Te₆ single crystal at low temperature were investigated,which laied the foundation for the continueous research and application of Cr₂Ge₂Te₆ single crystal,and provides guidance for further thermoelectric performance optimization.