Design, Preparation And Performances Of Graded Thermoelectric Materials With Large Temperature Span

Thermoelectric materials are functional materials which can be used to convert thermal energy into electrical energy or vice versa directly. The properties of thermoelectric materials are closely related to temperature. The best performance of a given thermoelectric material can be obtained only in a very small temperature region. Considering the fact that thermoelectric materials will work in a relative large temperature span when they are used in a thermoelectric generator, only a small section of the thermoelectric materials can work at the corresponding most favorable temperature. However, the performance of thermoelectric materials can be optimized if various materials with different properties are arranged along the temperature axis to form a graded or laminated thermoelectric material.In the present work some homogeneous materials such as Bi2Te3- and FeSi2-based thermoelectric materials as well as pseudo-binary alloys (PbTe)1-x(SnTe)x (0 x 1) have been prepared, their thermoelectric properties have been measured, and the possibilities for constitution of laminated structures have been discussed. The processes for preparation of laminated thermoelectric materials and the thermal stress buffer layers sandwiched between thermoelectric material segments have been studied. The microstructures, diffusion and chemical reactions near the interfaces have been analyzed using SEM, ED AX, EMPA and XPS. The thermoelectric power outputs of laminated structures with various thermoelectric materials have been mathematically optimized and experimentally measured.Some conclusions have been drawn in the present work as follows:1. The performance, microstructures and the failure mechanism of laminated Bi2Te3/FeSi2 prepared by dip coating using tin-based alloys as bridge materials have been investigated. The maximal power outputs of 37.0 mW/cm2 and 30.0 mW/cm2 for the p- and n-type laminated materials respectively at the temperature difference 490 have been experimentally obtained, which are about 2.5 and 3.0 times those of -FeSi2. Chemical analyses show that the interface failure between the bridge alloy and the semiconductor Bi2Te3 results mainly from the eutectic mixtures with low melting point and brittle compounds formed during welding and long time annealing at 190 . It is found that the electrical properties of a laminated structure are mainly controlled by the wettability of the bridge alloy on the semiconductor surface.2. Powder metallurgy was firstly used to prepare the nickel buffer layer sandwiched between Bi2Te3 and FeSi2 semiconductors. It was found that the buffer layers can effectively hinder the diffusion of elements across the interface and release the thermal stress caused by thermal coefficients mismatch. The thermal stability of graded materials, therefore, has been significantly improved. In the meantime, experimentsshowed that the diffusion of nickel into two matrixes does not worsen the performance of the laminated materials.3. Pseudo-binary alloys (PbTe)1-x(SnTe)x (x=0-1) were firstly prepared by pressureless sintering and the thermoelectric properties were experimentally investigated. Very low thermal conductivities of the alloys were observed, which are about l/4~1/3 at the ambient temperature and 1/5 at 450 those of single crystals. The best figure of merit Zmax equal to 0.4 10-3/K has been measured at 450 for (PbTe)0.6(SnTe)0.4, which is close to that of the single crystal.4. The effect of Ag doping on the properties of pseudo-binary alloys (PbTe)1-x(SnTe)x has been investigated. It was found that 200 mol ppm Ag doping in the PbTe alloy can greatly improve the electrical conductivity at the whole temperature range, especially near room temperature. This effect was gradually decreased with the increase of SnTe content in the pseudo-binary alloys as well as the increase of temperature, which is considered to be caused by the change of scattering mechanism, the saturation of carrier concentration as well as the Ag atoms occ...