Growth And Properties Of Bismuth Telluride Nanocomposites

Thermoelectric materials are a kind of important energy-conversion materials that can realize the direct conversion between thermal and electrical energy.Thermoelectric devices have high stability and long life due to no moving parts involved,which make them have special advantages in a variety of applications such as aerospace and defense.The performance of thermoelectric materials depends on thermal conductivity,electrical conductivity and Seebeck coefficient,which are corelated by carrier transport.It is difficult to improve the overall performance of a material by the single modulation of one parameter.To date,nanostructuring has been widely used as an effective route to increase ZT values of thermoelectric materials through reducing thermal conductivity without detriment to electric transport.As a traditional thermoelectric material,bismuth telluride exhibits excellent thermoelectric performance near room temperature and has received extensive attention.In this thesis,Bi₂Te₃ materials were nanostructured,doped by Se and composited with magnetic nanoparticles,and the microstructures and thermoelectric properties of the prepared materials were characterized.The main results are shown as follows:?1?The low-dimensional Bi₂Te₃ material was grown by a solvothermal method,and N-type ternary alloy Bi₂Te_2.7Se_0.3?BTS?nanomaterial was prepared by Se doping.The phase,morphology and microstructure of the samples were characterized.The results show that the Bi₂Te₃ hexagonal nanopaltes with well facted shape were grown,and Se was successfully doped into the Bi₂Te₃ lattice and remains the hexagonal shape with good crystalinity.The examinations on thermoelectric properties showed that Bi₂Te_2.7Se_0.3 nanomaterial obtained a maximum ZT value of 0.46 along the in-plane direction at 440 K.?2?BTS/xNi nanocomposites with four molar ratios?x=0.0%,0.2%,0.4%and 0.6%?were prepared through the ultrasonic mixing of the Ni nanoparticles and BTS nanoplates.Characterizations on the morphology,composition and microstructure showed that the Ni nanopaticales were successfully incorporated into the BTS matrix.The examination on the thermoelectric performance showed that Bi₂Te_2.7Se_0.3/0.4%Ni nanocomposite exhibited the best performance with the maximum conductivity of 3.0?10⁴Sm^-1 at 295 K,the maximum Seebeck coefficient of-165?VK^-1 at 425 K and the maximum ZT of 0.66 at 420 K along the in-plane direction.Such ZT value is 43%higher than that of the BTS matrix.The enhancement in ZT was mainly ascribed to the increase in the power factor that is caused by the enlarged electrical conductivity and enhanced Seebeck coefficient.The present results demonstrated that incorporation of magnetic nanoparticles could be an effective approach to promote the thermoelectric performance of conventional semiconductors.In summary,Se-doped Bi₂Te₃ nanomaterials were prepared and the thermoelectric performance of Bi₂Te_2.7Se_0.3 was further improved by the incorporation of magnetic Ni nanoparticles to optimize the electrical transport properties.Meanwhile,this study indicates the feasibility to promote the performance of conventional thermoelectric materials by embedding magnetic nanoparticles,which provides a new reference for improving the performance of traditional material systems.