High Pressure Synthesis And Thermoelectric Properties Of Caged Ba₈Ga₁₄Ge₃₂, CoSb₃ And Layered Bicuseo Compounds

Thermoelectric materials,which can directly convert thermal energy into electrical energy and vice verse,have appealing prospect in power generation and thermoelectric cooling applications.The cage-structured compounds such as Ba₈Ga₁₄Ge₃₂ and CoSb₃,possessing the feature of phonon-glass electron-crystal?PGEC?,are an important class of thermoelectric materials.High pressure synthesis?HPS?has been proven to be an effective way to prepare these cage-structured compounds with enhanced thermoelectric performance.The layer structured BiCuSeO oxyselenides have also been considered as promising thermoelectric materials due to their low intrinsic thermal conductivity.However,high pressure synthesis of BiCuSeO oxyselenides has not been well studied up to now.In this dissertation,we focus on high pressure syntheses and thermoelectric properties of both cage-structured and BiCuSeO thermoelectric materials.The synthesis procedure of Ba₈Ni_0.1Zn_0.54Ga_13.8Ge_31.56 clathrate was explored under high pressure.Yb-doped Ba_8-xYb_xNi_0.1Zn_0.54Ga_13.8Ge_31.56 samples were prepared with HPS followed by spark plasma sintering?SPS?.Structure and thermoelectric properties were investigated.The electrical resistivity and lattice thermal conductivity reduce with increasing Yb doping level,while the Seebeck coefficient keeps almost constant.For the highest doped?x = 0.3?sample,trace secondary phase Yb₂O₃ coexsits in the sample,which suppress the thermal conductivity further.As a result,the maximum ZT value of 0.91 is achieved in this sample at 900 K.Lanthanum filled CoSb₃ skutterudite was successfully synthesized with high pressure synthesis.The maximal filling fraction of La in CoSb₃ crystal voids is 0.29 in our HPS samples,which is ca.30% higher than that of the ambient-pressure synthesized samples.Consequently,our samples show significantly enhanced thermoelectric properties.The power factor as high as 4630 ?Wm-1K-2 is reached for the optimal La_0.29Co₄Sb₁₂ sample.Combined with the greatly suppressed thermal conductivity,the highest ZT of 1.06 is achieved at 863 K for La_0.29Co₄Sb₁₂.The synthesis procedure of layer-structured Bi CuSeO compounds was explored under high pressure.Lead doped Bi_1-xPb_xCuSeO samples were prepared with HPS followed by SPS.Structure and thermoelectric properties were investigated.The electrical transport properties are clearly improved after Pb doping.The power factor of Bi_0.95Pb_0.05 CuSeO maintains a relatively high value in the whole measurement temperature range,and a ZT value of 0.86 is achieved at 823 K.HPS thus provides us a fast and effective pathway to fabricate BiCuSeO thermoelectric materials.We also investigated the possible texture in the SPS Bi_0.95Pb_0.05 CuSeO bulk,and revealed a preferential orientation of the ab planes perpendicular to the compressing direction of SPS.Correspondingly,higher power factor and higher thermal conductivity is achieved in the direction perpendicular to the SPS compressing,leading to more or less the same ZT as that along the SPS compressing.Lithium doped Bi_1-xLi_xCuSeO samples were prepared with HPS followed by SPS.Structure and thermoelectric properties were investigated.Lithium doping improves the electrical transport properties and significantly enhances the power factor.Combined with the low thermal conductivity,a maximum ZT value of 0.75 is achieved at 873 K for the Bi_0.925Li_0.075CuSeO sample.