Research On The Directionally Solidified Microstructures And Thermoelectric Properties Of Bite Basedalloys

Thermoelectric material (TM) provides a useful solid-state conversiontechnology for refrigeration and power generation. As the best candidatethermoelectric material, bismuth chalcogenide alloys, possessing a unique topologicallayered structure, exhibits lower thermal conductivity but higher electricalconductivity at room temperature, promoting it as a promising thermoelectric materialin a sustainable energy solution.The thermoelectric transport properties of BiTe based alloys depend on theformed microstructures, elements distributions, chemical composition and dopinglevels during the TMs preparation. Understanding the mechanism of the interactionsbetween these factors and the thermoelectric properties is essential to improve theperformance of TMs. Meanwhile, probing the relationship between the carriertransport properties and the nature of the electrical and thermal conduction canprovide a theoretical basis to make a reasonable prediction of the potentialthermoelectric properties. In view of this, we have used high temperature-gradientdirectional growth method to grow a series of single and polycrystalline Bi2Te3andBiSb3Te6crystals at the rate of1-1000μm/s. The effects of grown crystalsmicrostructures, elements distributions, and chemical composition on thethermoelectric transport properties under different growth rates have beensystematically investigated. We have also given theoretical calculations to study theelectrical structures, thermal dynamic properties of Bi2Te3under different temperatureand pressure. Simultaneously, the thermoelectric transport properties of Bi2Te3andBiSb3Te6crystals under different temperature and doping levels have been calculatedand analyzed. We obtain the following results:High quality single crystals of BiTe based alloy can be prepared under hightemperature-gradient directional growth method; the chemical composition of Bi2Te3single crystal can be accurately controlled by tuning the growth rates. With increasingthe growth rate of Bi2Te3single crystal from1μm/s to5μm/s, the thermoelectrictransport parameters of electrical conductivity, carrier concentration, thermalconductivity, power factor (PF) and ZT are improved, but the Seebeck coefficient,Hall coefficient and carrier mobility are reduced.The directionally solidified Bi2Te3polycrystals show that the microstructures of Bi2Te3evolve from planar to cellular and to lamellar and needle-like morphology with increasing growth rate; the grown Bi2Te3crystals perform preferred orientations along (015),(1010) and (110) faces. For the grown Bi2Te3poly crystals, the optimal values of the root-temperature Seebeck coefficient and PF reach-253μV/K and3.83×10-3W/(m-K2), respectively. The lattice thermal conductivity of Bi2Te3crystal reaches0.79W/(m-K) that accounts for70%of the entire room-temperature thermal conductivity, but at500K the electronic thermal conductivity becomes the main contributions to thermal conductivity.Calculated atomic bonds of Bi2Te3present that Bi-Te1is more ionic but Bi-Te2is more covalent; these special atomic bonds predicate the chemical balance bonding of Bi2Te3to be Bi-0.15Tc1-0.14Tc2-0.08. And the calculated lattice dynamic properties of Bi2Te3crystal show that there is a strong anisotropy of Born effective charges and dielectric constants, which lead a greatest LO-TO splitting in the infrared phonon modes. The phonon dispersions are divided into two fields by a phonon gap. In the lower field, atomic vibrations of both Bi and Te contribute the density of phonon states. In the higher field, most contributions come from Te atoms. There are four Raman peaks E1A11g, E2g and A21g; the A11g and A21g atomic modes vibrate along the c axis, but E1g and E2g modes vibrate along a axis.Through theoretical analysis and experimental studies, Bi2Te3crystals present anisotropic transport properties at different doping levels. For p-type doping Bi2Te3crystals, the Seebeck coefficient along c axis is larger than that along a axis as the doping level is lower5.5×1018/cm3; the electrical conductivities along a axis are higher than that along c axis and the maximum σα/σc reaches2.7. For n-type doping Bi2Te3crystals, the Seebeck coefficient along a axis is larger than that along c axis, and the same for electrical conductivity and the maximum σα/σc reaches4.5. Therefore, to obtain the ZT over unity, for the p-type and n-type doping Bi2Te3crystals, the doping levels should be in the ranges of5.39×1018-3.26×1019/cm3and7.71×1018-2.36×1019/cm3, respectively.BiSb3Te6is a narrow gap semiconductor with an indirect band gap of0.113eV and the band structures of the material present multi-valley characteristics. Calculated thermoelectric coefficients of the p-type doping BiSb3Te6are anisotropic and mainly dependent on the doping concentrations. The electrical conductivity along a axis is larger than that along c axis, and the maximum σα/σc reaches1.6. The Seebeck coefficient along a axis is larger than that along c axis as the doping level is lower 1×1019/cm3, but the Seebeck coefficient is isotropy as the doping level is over1×1019/cm3. Therefore, to acquire a high performance BiSb3Te6material, i.e., ZT≥1.5,the best carrier concentration should be in the range of2.32×1019-4.55×1019/cm3.