Thermoelectric Properties Optimization Of Anisotropic Layered Bismuth Telluride And Selenide Tin Based Materials

Thermoelectric material is a kind of functional materials that can generate electric power from waste heat.Improving the comprehensive properties of thermoelectric materials is of great significance for the utilization of energy.In this thesis,the performance optimization of layered-like structure thermoelectric materials is studied.The details are as following:1.Bismuth telluride based thermoelectric materials are widely used in commercial applications.However,the largest figure of merit?ZT?is still near 1.It is necessary to further improve the thermoelectric performance.In this thesis,p type Bi_0.48Sb_1.52Te₃/PbTe composites are prepared by zone melting.The preparation process is simple and the experimental period is short.And the sample can be produced in a large scale.The conductivity and power factors of Bi_0.48Sb_1.52Te₃composites are greatly enhanced by incorporating PbTe.The power factor of Bi_0.48Sb_1.52Te₃/PbTe composites reaches 50?Wcm^–1K^–22 at 300 K.When the amount of PbTe is 0.05 wt%,the power factor reaches a peak value of 55?Wcm^–1K^–2 at 300 K.The reason for the increase of electrical conductivity is that PbTe enters the lattice of the Bi_0.48Sb_1.52Te₃ matrix and introduces more carriers.Due to the additional phonon scattering by PbTe,the lattice thermal conductivity of Bi_0.48Sb_1.52Te₃/PbTe composites is decreased.However,due to the increase of carrier,the electron thermal conductivity is increased,and the total thermal conductivity is increased,too,which makes the enhancement of the figure of merit not so obvious.When the amount of PbTe is 0.05wt%,the maximum ZT value is about 1 at 350 K.The average ZT value of Bi_0.48Sb_1.52Te₃ is about 0.7,which is consistent with previous literatures.After compositing PbTe,the average ZT value is effectively improved.When the amount of PbTe is 0.05 wt%,the average ZT value is about 0.8,which is increased by about14%.2.The tin selenide based thermoelectric materials have attracted wide attention because of their ultra-low thermal conductivity(0.23 Wm^–1K^–1 at 923 K),and the extraordinary high ZT value of 2.6 in 923 K in the single crystal.But the mechanical properties for single crystal limited its practical application.In this thesis,textured oriented polycrystalline Na_0.03Sn_0.97Se is prepared by hot deformation,where Na doping is to increase the concentration of carrier.The Na_0.03Sn_0.97Se oriented polycrystalline prepared by hot deformation are preferable along the crystal surfaces of?210?and?400?.The orientation of the sample with two times of hot deformation?HD2?is better than that of one time of hot deformation.And the degree of texturing for hot deformation samples is better than that of hot pressing.The increase of texture degree improves carrier mobility and then increases the conductivity substantially while the Seebeck coefficients remain.And the power factor is doubled.The peak value of the power factor of the HD2 samples is over 10?Wcm^–1K^–2 at about 825 K.Due to the grain growth,the lattice thermal conductivity increases.And the total thermal conductivity also increases.But with the increase of temperature,the lattice thermal conductivity and total thermal conductivity are increasing smaller,making the ZT value promotes substantially.The ZT value of hot pressed sample is about 0.76 near 800 K,which is consistent with literature.Compared with hot pressing,the peak of ZT values of hot deformation samples are increased by about 70%.At 800 K,the peak of the ZT value is obtained for hot deformation samples,about 1.3.It is higher than those prepared by hot pressing,spark plasma sintering,zone melting and zone melting combined with spark plasma sintering in literature.