Thermoelectric Property Optimization Of Zone-Melted Bi₂Te₃-Based Alloys For Mid-Temperature Applications

Bismuth-telluride（Bi₂Te₃）-based alloys have the best thermoelectric（TE）performance near room temperature and are the only widely used commercial TE materials so far.Both n-type and p-type Bi₂Te₃-based alloys have zT values of～1 near room temperature,which are mainly applied for the solid-state refrigeration.It is of significant value to expand the working temperature of Bi₂Te₃-based alloys,making them more suitable for mid-temperature power generation.The components of the bismuth telluride suitable for power generation are different from those applied for refrigeration and need to be further optimized.Although some Bi₂Te₃-based alloys in the mid-temperature range have been reported,most of them require complex powder metallurgy process,which cannot be applied in large-scale commercial production.Moreover,At present,the commercial zone melting method has become very mature,so it is of great commercial value to prepare Bi₂Te₃-based alloys with excellent TE performance in the mid-temperature range by this simple production process.We prepared Te-free Bi_2-xSb_xSe₃ alloys by zone melting and spark plasma sintering（SPS）,and studied the anisotropy of TE properties of SPS samples.By doping the n-type traditional Bi₂Te₃-based components,the carrier concentration was improved to optimize the TE performance in the mid-temperature region.Meanwhile,Ag and In were used to modify p-type Bi₂Te₃-based alloys.The main results are listed as blew:1、The crystal structure of Bi_2-xSb_xSe₃ alloy is transformed from a rhombohedral phase to an orthorhombic phase due to Sb alloying,and the electronic energy band structure is changed accordingly.The convergence of heavy bands realized in the orthorhombic phase largely increases the effective mass of the density of states,thus enhancing power factors.In addition,phonon softening and substantial lattice anharmonicity caused by structural transition greatly reduce the lattice thermal conductivity.The single-phase BiSbSe₃ and two-phase Bi_1.2Sb_0.8Se₃ alloys prepared by SPS method were doped with SbI3,and their maximum in-plane zT value were both 0.7 at 800 K.Moreover,the conductivity ratio and thermal conductivity ratio of the in-plane and the out-of-plane direction are basically the same,and the Seebeck coefficient shows no anisotropy.As a result,the SPS-Bi_2-xSb_xSe₃ alloys have almost the same zT values in the in-plane and out-of-plane direction.The single-phase BiSbSe₃ and two-phase Bi_1.2Sb_0.8Se₃ alloys were prepared by zone melting and the maximum zT of BiSbSe₃ and Bi_1.2Sb_o.8Se₃ is 0.6 and 0.4 respectively at 800 K via SbI₃ doping.In ZM-BiSbSe₃ samples,the carrier concentration can be further increased by extra In doping,but their zT values cannot be effectively improved due to the high lattice thermal conductivity.2.In n-type ZM-Bi₂Te_2.7Se_0.3 samples,an appropriate amount of Cu was added to reduce the lattice thermal conductivity.The bipolar conduction was inhibited by the increased carrier concentration via Zn doping.As a result,ZM-Zn_0.02Cu_0.015Bi₂Te_2.7Se_0.3 sample shows a zT value of～0.9 at 500 K.In addition,Bi_i.8Sb_0.2Te_2.85Se_0.15 with relatively low lattice thermal conductivity was selected as the matrix composition for further SbI₃ doping and we obtained a maximum zT value of～0.7 at 500 K.Besides,the zT value of the commercial n-type ZM-Bi₂Te₃ alloy is only 0.5 at 500 K.The TE performance of n-type ZM-Bi₂Te₃ alloys in the mid-temperature has been greatly improved after optimization.3、As for p-type ZM-Bi₂Te₃-based alloys,Bi_0.3Sb_1.7Te_2.85Se_0.15 with low lattice thermal conductivity was doped by In and Ag to improve TE performance in the mid-temperature range.In doping can increase the carrier concentration of ZM-Bi_o.3Sb_1.7Te_2.85Se_0.15 samples,but the carrier mobilities drop sharply,resulting in the decreased electrical conductivity.Finally,the obtained maximum zT value is 0.55 at 650 K.The carrier concentrations were also significantly increased by Ag doping in p-type Bi_o.3Sb_1.7Te_2.85Se_0.15,and the carrier mobilities were almost unaffected.The intrinsic conduction was inhibited by the increased carrier concentrations,so the PF remained a high value at 630 K.Besides,the addition of an appropriate amount of Ag can significantly reduce the lattice thermal conductivity of the ZM samples.As a consequence,we obtained a maximum zT value of～0.64 at 630 K.Compared to a zT value of～0.45 at 475 K obtained in p-type ZM-Bi₂Te₃-based alloys,our ZM samples show great TE performance above 500 K.