Thermoelctric Properties Optimization Of N-Type Bi-Te-Se Alloys

Thermoelectric material is a kind of new promising energy materials with the advantages of no pollution,no noise and small size etal.At present,the best room temperature thermoelectric materials are Bi₂Te₃ based solid solution,and the p-type alloy has higher ZT value than that of n-type material.However,the thermoelectric device will achieve the maximum conversion efficiency when the n-type material shows the same excellent performance as the p-type material.Therefore we choose n-type Bi₂Te₃ based thermoelectric materials as the research object in this work.The type and concentration of the intrinsic point defects of Bi₂Te₃-based alloys were modulated mainly by doping,hot deformation and other means,the electrical properties were optimized and the thermal conductivities of the alloys were reduced,resulting in the improvement of the n-type Bi₂Te₃-based bulk material thermoelectric properties.The main results were abtained as below.?1?By changing the technological conditions,the phases,microstruturess and their thermoelectric properties for the Bi1.95In0.05Te2.85Se0.15 samples at different SPS sintering temperatures?T = 300,350,400,450,500 ??and hot deformation temperatures?T = 350,400,450,500 ??were studied.The best temperature of spark plasma sintering is dertermined as 450?,and hot deformation temperature is 500 ?.?2?The investigation of Bi_2-xIn_xTe_2.85Se_0.15?x = 0,0.025,0.05,0.075,0.1,0.15,0.2?alloys shows that a single-phase solid solution alloy with R???m structure was successfully prepared by high temperature melting at 800? and SPS sintering.The texture of the alloys was enhanced by hot deformation.And all the alloys exhibited n-type conduction.The formation of the antisite defects in the alloy was increased with small amount of In doping,and the concentration of antisite defects of the alloy decreased,resulting in the inhibition of the donor-like effect during the grinding and SPS sintering process and the reduction of the electron concentration in the alloys.In this way the Seebeck coefficient was greatly improved.Moreover,the Seebeck coefficient of the material was further improved because of the electron concentration reduction by eliminating the donor defects in the process of hot deformation.In addition,the carrier mobility and the conductivity were increased during the hot deformation process leading the improvment of the power factor of the alloys.At the same time In dopant can enhance the scattering of phonons and reduce the thermal conductivity of the material.As a result,the figure of merit ZT in the alloys was enhanced.The maximum ZT of 1.0 at 400 K was obtained in the alloy with x = 0.025 after SPS sintering and hot deformation.For the samples with same composition,the HD samples have higher ZT value than the SPS samples.?3?The investigation of Bi_2-xSb_xTe_2.3Se_0.7?x = 0,0.05,0.1,0.15,0.2,0.25?shows that the single phase solid solution was obtained and no second phase was observed,which means that the crystal structure of the sample did not change after hot deformation.The deformation process enhanced the orientation texture of the samples.With small content of Sb dopant,the concentration of antisite defects increased because of the reduction of antisite defect formation in the alloys.Accordingly the donor-like effect during the milling and SPS process were enhanced,resulting in the increase of the electron concentration and the conductivity of the alloys.Moreover the carriers mobility inceased and the power factor were improved because of the enhancement of the orientation texture and recovery effect during the hot deformation process.Meanwhile the thermal conductivity of the alloys were reduced with Sb dopant.By a small amount of Sb doping and hot deformation,the highest ZT value of n-type Bi_1.95Sb_0.05Te_2.3Se_0.7 polycrystalline alloy at 400 K reached 1.3.