Thermoelectric Properties And Microstructure Of PbSe Based Thermoelectric Materials Fabricated With High Pressure Sintering

Thermoelectric (TE) materials are new energy materials which can convert thermal energy into electricity directly. Power generation and refrigeration which made by TE materials have many excellences than conventional machine, for example, no moving parts, no greenhouse gases, no refrigerant or no noise. Nowadays, however, the increasingly exasperating of the environment, for more efficient energy management has focused attention on the potential thermoelectric applications of these materials. PbTe has been found to have higher thermoelectric figure-of-merit ZT at intermediate Temperature, which might find application in the recovery of waste heat from motor vehicles and other sources. However, Te is not only rare in the Earth's crust but also increasingly used in a number of other applications, such as steel metallurgy, solar cells, phase change materials for digital recording, and thermoelectric cooling devices based on Bi2Te3, so looking for Te alternative elements. Selenium is50times more abundant than tellurium, Furthermore, PbSe and PbTe, which importantly have remarkably similar electronic and crystal structural. Melting point of PbSe (1340K) is higher than PbTe (1190K). The higher decomposition temperature makes PbSe more suitable for more of the bad environment application. Lately, however, theoretical and experimental reports have pointed out the appealing characteristics of PbSe for higher temperature thermoelectric applications. Lately, PbSe based TE materials paid more and more attention. The preparation method of bulk TE materials has zone melting method, melting method and powder metallurgy method, however, the preparation method are in ordinary pressure or the low pressure (MPa). However in higher pressure (GPa), the crystal structure and electronic structure of material can change, which affects the electrical and thermal transport properties of material. In our research, utilizing the effect of high pressure sintering (HPS) on phase structural, microstructure and electronic structure, to control electrical and thermal transport properties, the performances of TE materials are expected to be improved.In this research, we focus on p-type PbSe compound. With almost the same power factor, carriers concentration are controlled through the preparation technology and doping. The thermal conductivity of the samples is reduced, especially the lattice conductivity under high temperature by lower the crystallinity and fine grains scale. The high TE performance of nanocomposite TE materials can be obtained. The results show that HPS is a way to improve effectively the performance of TE.Taking into account HPS Pbo.55Teo.45, the influence of pressure on the microstructure and TE performance of Pbo.55Teo.45samples was investigated. The results show that the pressure affects vacancy concentration, band width and grain size, and the performance of Pbo.55Teo.45was improved. The carriers concentration of HPS samples is affected by two factors, one is that pressure reduces vacancy concentration, which thins out the carriers concentration, the other, pressure decreases the forbidden band width of the sample, the carriers concentration increases with pressure rise; the bigger pressure is, the more obvious the effect, especially, the sample which is prepared by6GPa, the carriers concentration increased significantly. Compared with BS (Before Sintering) samples, reduction in vacancy concentration will dominate, reduction in forbidden band width will dominate in all HPS samples. HRTEM were used to analyze the average dimension of the interplanar spacing along to (220) of BS samples and HPS samples. The result shows that the interplanar spacing along to (220) of the HPS samples is longer0.0098nm than the BS samples, indicating Pb atoms can be packed into PbTe lattice internal under certain high pressure, become interstitial atom. The nanocomposite bulk materials were fabricated by HPS, those nanocrystallines significantly enhance boundary phonon scattering, resulting in notably reduction in thermal conductivity, being50%lower than BS samples. The samples prepared by6GPa shows a ZTmax=0.59, at700K, being150%higher than BS samples.PbSe compound was prepared by HPS, and the effects of pressure on PbSe were studied. The results show that the crystallinity and order degree of atom are reduced, resulting in notably reduction in thermal conductivity. The forbidden band width of PbSe is affected by pressure, which first decreases and then increases when the samples are prepared under the2GPa,4GPa and6GPa respectively. The pressure further decreases the carriers concentration of samples whose carriers concentration are low originally, and the samples show intrinsic excitation, type of electric conduction from p to n, and bipolar thermal conductivity at cooler temperatures. So HPS does not improve the performance of Stoichiometric PbSe compound.We developed HPS fabrication technology for p-type Pb0.99Ag0.01Se1.01compound. The effects of pressure on the phase structure and microstructure were investigated. The results show that the crystallinity decreases with increasing pressure, and the lattice constants increase with increasing pressure, and the influences of pressure on phase composition are is stabile. There are randomly distributed layered structures in the HPS samples, and nanocrystalline (20nm) dispersed on the boundary, which has great effect on the depression of the thermal conductivity. The thermal conductivity of HPS samples prepared by6GPa is39.7%and20%lower than SPS samples at room temperature and at870K respectively. For HPS Pbo.99Ag0.01Se1.01specimen, ZT increase with increasing pressure, a maximum ZT=1.16is obtained at723K, which is21%higher than BS specimen. The effects of temperature on the phase structure and microstructure were investigated. The results show that the crystallinity increases with increasing temperature, and the grain size increases with increasing temperature. For HPS Pb0.99Ag0.01Se1.01specimen, Seebeck coefficient decreases with increasing temperature, electric conductivity and thermal conductivity increase with increasing temperature, and ZT>1at776～873K was obtained. The results show that the optimum technological to prepare p-type PbSe based TE materials is6GPa—750℃。The microstructure and phase composition of Pb(1-x) AgxSei.01(x=0.0025～0.02) compound were studied, when the HPS fabricating technology is6GPa—750℃. The results show that the pressure increases the solution limit of Ag in Pb sites. The carrier concentration of HPS samples is lower than SPS samples under the same doping conditions, for example, the carrier concentration of HPS sample is2.8×1019cm-3, SPS sample is4.7×1019cm-3, when x=0.01. HPS samples show the lower carrier concentration and larger effective mass of holes, due to the pressure affects the vacancy concentration and DOS near the Fermi level, so the Seebeck coefficient is higher. In comparison with SPS samples, the power factor of HPS samples is higher obviously, but the thermal conductivity is opposite, resulting in the higher ZT value of HPS samples at the low temperature (300～500K). The power factor of HPS samples is approximately equivalent to the SPS samples at the high temperature (800～900K), but the thermal conductivity is opposite, resulting in the higher ZT value of HPS samples, at the high temperature (800-900K), resulting in the highest ZT=1.2is obtained at780K, which is22%higher than that of SPS ones, and the ZT of HPS samples is higher than that of SPS samples in a wide temperature range680～900K. Both carrier concentration and electrical conductivity increases with Ag increasing within solution limit, which are opposite when above solution limit, the maximum of carrier concentration and electrical conductivity are4.13×1019cm-3and6.5×104S.m-1respectively when the content of Ag is1.5%. Lattice conductivity decreases gradually with increasing Ag content within solution limit, the minimum of lattice conductivity is0.34W.m-1.K-1at734K when the content of Ag is1.5%. The ZT value of HPS samples is basically unchanged with Ag content increasing at temperature300～700K, but increase and then decrease at temperature700～900K, resulting in the highest ZT=1.2is obtained at780K when Ag content is1.5%.