Synthesis,Film Preparation And Physical Properties Of Bismuth Selenide(BiSe) Compounds

As one of the ?-? main group compounds,bismuth selenide(BiSe)has low intrinsic thermal conductivity,showing better thermoelectric properties than traditional Bi₂Se₃.Now the mainstream commercial thermoelectric materials are some compounds containing Te such as Bi₂Te₃-based and PbTe-based thermoelectric materials that are difficult to be applied on a large scale due to the low content of Te in the earth and high toxicity.BiSe-based compounds can avoid such shortcomings,and are expected to become a new generation of near room temperature thermoelectric materials.Morever,a variety of layered structures of Bi-Se series compounds will benefit to improve the thermoelectric properties of materials.In this thesis,the synthesis,doping,nano-modification,thin film preparation and doping of BiSe compounds were studied.The physical and chemical properties of the obtained materials were characterized.The main results are as follows.(1)The preparation and physical properties of Sn doped and nanosized BiSe.Bi_1-xSn_xSe compounds(x=0,0.1,0.2)were synthesized by high temperature solid state method using Bi,Se and Sn as raw materials.Some products were treated by high energy ball milling for nanocrystallization.The grain size of the powder before and after ball milling was calculated showing that the grain size of the powder after ball milling decreased to be about 1/7 of the original grain size.The samples were pressed into bulk by spark plasma sintering(SPS),and the thermoelectric properties were investigated.The results show that both Sn doping and nanocrystallization contribute to improving the thermoelectric properties of BiSe.The electrical transport properties indicated that Sn doping reduces the carrier concentration.Nanocrystallization not only effectively improves the Seebeck coefficient of the sample by11.2%,but also affects the anisotropy of the pressed block,which improves the conductivity in the parallel SPS direction and increases the power factor slightly.The thermal transport analyses show that,the phonon scattering was enhanced due to Sn doping,and nanocrystallization leading to the decrease of the thermal conductivity of the sample.The thermal conductivity of the optimal sample is only 0.43 W·m^-1·K^-1 at 673 K,which is 46.2%lower than that of the undoped samples without ball milling treatment.With the optimization of electrical and thermal transport properties,the maximum ZT value of 0.32 at 473K was obtained in BM-Bi_0.9Sn_0.1Se sample,which was 31.6%higher than that in BiSe sample.(2)Preparation and characterization of Sn-doped BiSe films.Bi_1-xSn_xSe thin films(x=0.06,0.11,0.18)were prepared by vacuum thermal evaporation using Bi,Sn and Bi₂Se₃ as raw materials.The formation law of the phase-pure BiSe film was found.The BiSe film was formed when the substrate temperature is above 470?.The BiSe thin films doped with Sn show pure phase,and the degree of prefer orientation decreases with the increase of Sn doping concentration.In addition,Sn doping makes the film rough.The electrical transport properties of BiSe powder have the same trend as that of BiSe powder.The carrier concentration decreases,the conductivity decreases,the mobility increases,and the power factor increases slightly.The above results show that the thermoelectric properties of BiSe can be optimized within a certain range by doping Sn into the BiSe system,and ball milling treatment,filming and other means to optimize the thermoelectric properties of BiSe.Sn doping improves the thermoelectric properties of BiSe materials with different morphologies(powder or film).