Research On Thermoelectric Materials And Devices For Waste Heat Utilization

Thermoelectric material is a kind of function material which can directly achieve theconversion between electricity and heat based on the Seebeck effect and Peltier effect.Thermoelectric material has become the critical materials of thermoelectric cooler and powergeneration application, which are forward-looking and strategic new energy technologies. Asthe most excellent thermoelectric materials near room temperature, bismuth telluride-basedalloys has been widely used in aerospace, microelectronics, special power and other fields. Atthe low-temperature200~300℃, because of poor performance of bismuth telluride-basedthermoelectric material limit its widespread application, thus the research of optimizing lowtemperature thermoelectric properties of P-type bismuth telluride-based thermoelectricmaterial has important significance in practical applications.Firstly, this paper explores the improvement of thermoelectric performance of the matrixcomposite material which consist of P-type bismuth telluride and multi-valence element.Bi0.5Sb1.5Te3/In2Te5composite thermoelectric material is prepared by combination of zonemelting and melt method. With rational design of experiments, the introduction ofmulti-valence indium elements achieve the coordinately regulation of electrical and thermaltransport properties. Secondly, to further optimize the electrical properties of the material, weprepared Bi0.5Sb1.5Te3/SnTe solid solution alloy thermoelectric materials by zone meltingmethod. Meanwhile, the paper studied the influence of preparation technique which combinedzone melting and spark plasma sintering on the performance of thermoelectric materials.Finally, we selected the optimal performance of Bi0.5Sb1.5Te3/In2Te5complex withcommercial N-type bismuth telluride based thermoelectric material to assemble thermo-electric devices for testing performance. The main findings are as follows:Bi0.5Sb1.5Te3/In2Te5composite thermoelectric materials were prepared by Bi0.5Sb1.5Te3composite matrix material with InSb addition. Because of multivalent characteristics of InSb,InSb displace with matrix materials with the precipitation of a variety of sizes of the secondphase In2Te5. The increase of grain boundary and varisized second phase scatter different freepath phonons, effectively reducing the lattice thermal conductivity. With the regulation of theelectron band structure achieving degenerate band, prompting the intrinsic excitation temperature shifts to the high temperature zone.Because of band convergence, Seebeck coefficient increase with increasing temperatureat300-550K. The increase of carrier concentration is in favor of optimizing the electricalproperties. With the coordinated regulation of the electric and thermal transport properties,it greatly improved the composite ZT values at low temperature. At550K, the peak ZTvalue of Bi0.5Sb1.5Te3/In2Te5composite thermoelectric material was increased to1.14,which is three times compared to the matrix material (ZT=0.38).Bi0.5Sb1.5Te3/SnTe solid solution alloy thermoelectric materials were prepared by zonemelting method. The dopant SnTe forms a solid solution alloy with the matrix material,which brings in a large number of structural defects to adjust the carrier concentration. Atthe same time, it is beneficial to enhance the phonon scattering to reduce the latticethermal conductivity. The formation of band degeneracy and solid solution alloys madethe Seebeck coefficient and electrical conductivity have increased greatly. As a result, theZT value of Bi0.5Sb1.5Te3/SnTe solid solution alloy is double compared to the basematerial.We choose the best performance samples to grind and sift for spark plasma sintering(SPS). Due to the introduction of a large number of ground interface by grinding, itenhanced carrier and phonon scattering to reduce electrical and thermal conductivitysignificantly. To some extent, the Seebeck coefficient was increased. As a result the peakZT is almost unchanged. But at300-550K, the average ZT value of Bi0.5Sb1.5Te3/SnTesolid solution alloy is about0.56throughout the test temperature range.On the basis of above experiments, The P-type Bi0.5Sb1.5Te3/In2Te5combined withN-type commercial Bi2Te3thermoelectric materials was assembled into thermoelectricdevices and measured its performance. As experiments shows: the size of single-stagethermoelectric devices is40*40mm. It consists of127pairs of P-N junction. When thedevice was put at temperature difference of270℃（Hot side temperature300℃, cold side30℃）, the no-load power of device is about18.9W, the conversion efficiency is higherthan12%.