Thermoelectric Properties And Nanostructures Of Te Based Materials

As one of the options tor the energy conversion between thermal and electrical energy,thermoelectric(TE) materials are widely used as clean,reliable,and sustainable energy sources.Among all the tellurium based TE materials,Bi₂Te₃-based alloys are the state-of-art room temperature TE materials.After decades of research,the dimensionless figure of merit ZT of bulk Bi₂Te₃-based alloys is kept at about 1.On the other hand,for medium temperature range application,PbTe-based alloys are considered to be the most convenient choice,and the best ZT value reported is 2.2.With the development of nano-technique,more and more low-dimensional TE materials are studied and great improvements are made.With the crystal size decreasing from micron-to nano-meter,the carrier and phonon scattering increased,which lead to the increasing of Seebeck coefficient and decreasing of thermal conductivity,and resultantly the improvement of the thermoelectric properties.In the present work,Bi₂Te₃-and PbTe-based TE materials are systemically studied. These telluride-based materials are prepared using solvo-/hydro-thermal method, low-temperature aqueous chemical route and reflux method.The structures and morphologies of these nanopowders were investigated by XRD,SEM,TEM,and HRTEM, and the reaction mechanism were also discussed in detail here.The electrical transport properties can be tuned due to the nano effect in these bulk samples,and the ZT values were improved a lot.Some important results of the present work are listed as follows:During the low-temperature aqueous chemical route of Bi₂Te₃ process,too much complexing agent is not good for the synthesis.The reductant NaBH₄ should be added by several steps,which is good for the synthesis of purer Bi₂Te₃.The lower the concentration of reactions is,the lower the nucleation ratio of Bi₂Te₃ will be,and longer time will be needed.With different Te source,Te powders and Na₂TeO₃ namely,different morphologies of Bi₂Te₃ powders are obtained.The samples prepared with Te powders are mainly about 100nm diameters,0.5～1μm long nanotubes,with the walls about 30nm thick.The powders obtained from the later one are megranate like structures,which were formed by 30nm sized powders gathered together.The Bi₂Te₃ nanopowders synthesized by low-temperature aqueous chemical route have smaller sized than those by reflux method.This can be explained by the different reaction temperature.By ex-situ investigation and analyzing,the mechanism of chemical synthesis of Bi₂Te₃ was assumed to be a mixture of two mechanisms,namely,atom reaction and ion reaction.Ternary alloys have strong scattering effect to short-wave phonon,which can decrease the thermal conductivity and optimize the electron band structure,and lead to a higher thermoelectric properties.By hydrothermal method at 180℃,high purity ternary (Bi,Sb)₂Te₃ nanopowders were prepared successfully.The morphology of these powders was hollow,curve sheets.The size of these sheets was about 100-150nm with a thickness of 20nm.The crystal size in the bulk samples is about 10s to 100s nanometers.The powder sizes had no obvious change before and after sintering,which was maybe due to the hollow,curve morphology.Because of the boundary scattering effect,the thermal conductivity dropped to as low as 0.9Wm^-1K^-1,and the highest ZT reached 1.28 at room temperature.The nanopowders Bi₂Te₃ and Sb₂Te₃ were prepared by low-temperature chemical route at 70℃and solvothermal at 180℃,respectively.With different Bi₂Te₃ to Sb₂Te₃ ratio,these nanopowders are consolidated by Spark Plasma Sintering Method(SPS).At 500K,the thermal conductivity of the sample with a Bi₂Te₃/Sb₂Te₃=1:1 is 0.7Wm^-1K^-1. on the other hand,these powders are also consolidated by hot pressing in vacuum.The sample with Bi₂Te₃/Sb₂Te₃=1:1 performed higher Seebeck coefficient and electrical conductivity,the figure of merit ZT value reached 1.47 at 450K.The electrical conductivity can be effectively improved by Sn doping.The value 1.4 of ZT at 500K was obtained for the sample of 2%doping.The thermal conductivity at room temperature of micron-sized Sb₂Te₃ bulk sample was about 2.0W/(m-K).While the thermal conductivity of the nano-micro composite with a nano/micro=4:6 dropped to 1.1W/(m-K). Considering all the nano effect to Seebeck coefficient,electrical conductivity and thermal conductivity,the optimized ZT of 0.9 was obtained at 450K for the sample with nano/micro=3:7.Using Pb(CH₃COO)₂ and Te powders as precursor,the PbTe powders with 100nm size were prepared by reflux method at 100℃for 48h.The morphology of PbTe powders sampling at 24h is 50-75nm sized sphere.This could be the medium step during the nuclear and growth process.Here,the chemical synthesis of PbTe nanopowders was assumed to be a mixture of two mechanisms,namely atom reaction and ion reaction.Four PbTe bulk samples were consolidated by hot pressing with different parameters.Due to the boundary effect,even after Porosity modification,these samples still showed very low thermal conductivities.And even with the porosity and boundary double effect,the electrical conductivities remained at very high level in virtue of the high carrier concentration.The ZT value of 0.8 was obtained at 580K for the sample hot pressed at 400℃,which is comparable with the optimized binary PbTe alloy in previous reports.