| Thermoelectric materials are semiconducting functional materials,which can convert heat energy directly into electricity or reversely.It is of great significance to solve the current problems on environmental pollution and energy crisis.Sn(Te,Se)-based compounds are a class of Ⅳ-Ⅵ lead-free semiconductors which are considered to be one of environmentally friendly thermoelectric materials with a wide range of application prospects.In this thesis,SnTe,SnSe and Sn1-x BixSe samples have been prepared by chemical synthesis,reduction annealing and spark plasma sintering(SPS)processes.The phase composition,micro morphology,basic physical properties and thermoelectric properties were studied.The main results are as follows:Different chemical synthesis methods have been designed to prepare Sn Te materials.The wet chemical method using Sn Cl2 and Te powder as raw materials,NaBH4 as reducing agent,Na OH as p H regulator,and ethylenediamine as stabilizer was selected.The obtained Sn Te powder has pure phase,but aggregating seriously.The thermoelectric properties of the bulk samples can be effectively improved through annealing and SPS.The maximum room temperature electrical conductivity is ~6.2′105 S m-1,and the maximum power factor is ~1.6 μW cm-1 K-2 at 873 K.The minimum thermal conductivity is ~1.8 W m-1 K-1 at 873 K.The maximum ZT value reaches ~0.75 at 873 K,which is nearly twice of that prepared by the traditional melting method.The influence of different stabilizers on the synthesis of SnSe was explored.Single-phase SnSe nanosheets can be obtained by utilizing ethylenediamine or citric acid as stabilizers.Polycrystalline SnSe bulks with obvious anisotropy have been prepared.Based on this,the reaction time was adjusted for the ethylenediamine-stabilized solution,and 2 hours was determined as the best reaction time.Then the thermoelectric performance of the bulk samples was tested in different directions.The maximum power factor in the direction perpendicular to the sintering-pressing direction is ~4.8 μW cm-1 K-2 at 475 K,and the minimum thermal conductivity in the direction parallel to the sintering-pressing direction is ~0.2 W m-1 K-1 at 773 K.The maximum ZT values in the direction perpendicular and parallel to the sinteringpressing direction are 0.6 and 0.73(773 K),respectively.The average ZT values are 0.26 and 0.3 with the cold side temperature 300 K and hot side temperature 773 K,respectively.Bi-doped Sn1-x BixSe(x = 0,0.01,0.02,0.03,0.04,0.05,0.10)was prepared by different methods.It was found that the stable n-type materials over the full measured temperature range can only be obtained with ethylenediamine as a stabilizer.Series of special microstructures can be observed in the crystal,including a lath-like structure,small-angle grain boundaries,dislocations,and nano precipitates.The doping mechanism was analyzed,which incorporating the adsorption of Bi in liquid phase and the diffusion of Bi under high temperature and high pressure during sintering.The thermoelectric properties were studied.The maximum power factor is ~0.44 μW cm-1 K-2 at the optimum doping amount x = 0.03.The effect of different doping precursors on the thermoelectric properties of SnSe-based materials was explored.It is difficult to prepare stable n-type SnSe by using BiCl3 or Bi as doping precursors.As a result,Bismuth(Ⅲ)2-ethyhexanoate is the best doping precursor. |
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