Preparation And Properties Of Multicomponent Thermoelectric Tellurides

Thermoelectric (TE) materials are a kind of semiconductor functional materials, which can be used to convert heat energy directly into electricity or reversely. They are of interest for applications in TE cooling devices and power generators. Bi₂T₃-based and GeTe-based compounds have high dimensionless figure of merit. They are one of the commonly used thermoelectric materials. The preparation of multicomponent alloy can optimize material compositions and improve thermoelectric properties.In this work, Bi₂T₃-based and GeTe-based multicomponent alloys are prepared by vacuum melting from pure Bi, Te, Ge and other alloying or doping elements, and also by vacuum hot pressing from the alloy powders. The structure and morphologies of the compounds were investigated by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The thermoelectric properties of samples have been measured. The main results of the present work are listed as follows.1. Starting from Ag, Bi, Sb, Te elements, quaternary alloys Ag_xBi_0.5Sb_1.5-xTe₃ have been synthesized. Quaternary alloys containing Ag have higher electrical conductivity and lower Seebeck coefficient than Bio.5Sb1.5Te3 ternary alloy. The power factor of quaternary alloy Ag_0.2Bi_01.5Sb_1.3Te₃ reaches 1.7×10^-3 W·m^-1·K^-2 at 550 K.2. The electrical conductivities and Seebeck coefficients of quaternary alloys Zn_xBi_0.5Sb_1.5-xTe₃ have a closer relationship with the content of Zn. When x is 0.2 the sample has best electrical conductivity and Seebeck coefficient among the series. The difference of electrical properties among three alloys is large at room temperature and diminishes when temperature increases. The best power factor of alloy samples is 1.83×l0^-3 W·m^-1·K^-2. The electrical conductivities of Sn_x3i_0.5Sb_1.5-xTe₃ alloys drop with increasing x and the Seebeck coefficient of Sno.5Bio.5Sb1.oTe3 sample is best of all. The power factors of three alloy samples rise while temperature increases.3. Dopant SbI₃ could change the conductive type of Bi₂Te_2.85Se_0.15 sample from p type to n type. The fracture morphologies of the samples by hot-press by ball-milled powders and rapidily-solidified powders are different. The former has big crystal grains and there exists crack between layers. The latter has small crystal grain, just several microns, and compact structure. The thermal conductivities of the samples from ball-milled powder are low and increase with temperature rising. The highest figure of merit is 1.42×10^-3 K^-1 .4. Ge-Sb-Te alloys have p type conductive mechanism. These alloys own high electrical conductivity which arrives at 4.5×10⁵ S·m^-1. When Sb content in alloys increases the electrical conductivity decreases. The power factor of Ge₄₅Sb₅Te₅₀ alloy is 2.49×10^-3 W·m^-1·K^-2 at 640 K. Due to existence of the second phase Ge-Bi-Te alloys have the lower electrical conductivity and the better Seebeck coefficient than Ge-Sb-Te alloys. Quaternary alloys Ge-Sb-Te-Se also have good electrical conductivity. The samples with more Sb exibit better electrical property.5. Bi₂Te₃ compound doped with a little PbTe wouldn't alter the structure of crystal. After comparing the Seebeck coefficients of samples we get the fact that low Seebeck coefficient is better for the samples doped high concentration PbTe. The electrical property at room temperature is better.