Microstructure And Thermoelectric Properties Of Al₂O₃ Doped N-type Pseudoternary Semiconductor Materials

Semiconductor thermoelectric materials can realize the direct mutual conversion of heat energy and electric energy,and are often used for thermoelectric power generation and thermoelectric cooling.Thermoelectric conversion devices made of semiconductor materials have no pollution to the environment.Bi₂Te₃based semiconductor thermoelectric materials have good thermoelectric characteristics at room temperature and have a very broad application prospect.However,at present,the thermoelectric conversion efficiency of Bi₂Te₃-based semiconductor thermoelectric materials is low,far from reaching the theoretical limit,so the current research mainly focuses on how to improve the conversion efficiency.In this paper,Al₂O₃-doped N-type pseudo-ternary semiconductor thermoelectric materials were prepared by mechanical alloying hot pressing sintering method.Based on X-ray diffraction analysis,scanning electron microscopic analysis and the Hall effects,the micro structures of analysis were studied.Through the thermoelectric performance test,the influence law of preparation process parameters such as doping concentration and sintering temperature on the microstructure and thermoelectric parameters of the material were established.And the optimal thermoelectric properties of the material were revealed.The intrinsic physical mechanism aims to provide basis and guidance for further improving the performance of thermoelectric materials.Firstly,N-type pseudo-ternary semiconductor mechanical alloying powder was prepared by mechanical alloying method.In order to make the powder material easy to be hot pressed,the powder was sintered at 500℃to release the internal stress generated during the mechanical alloying process.Al₂O₃was mixed with different concentrations（0 wt%,0.5 wt%,1 wt%,1.5 wt%,2 wt%）.Then,the pseudo-ternary powders with different Al₂O₃concentrations were hot-pressed at 550 MPa and 200℃.Al₂O₃-doped N-type pseudo-ternary semiconductor mechanical alloying sintered hot-pressed block materials were obtained.Al₂O₃doped N-type pseudo-ternary semiconductor mechanical alloying sintered hot pressing block materials with concentration of 0.5 wt%were obtained after sintering at 240℃,280℃,320℃,340℃and 360℃respectively.X-ray diffraction analysis shows that the half-width of the diffraction peak of the materials becomes narrower and the peak value increases after sintering at 500℃,indicating that sintering at high temperature can enhance the degree of crystallization.The diffraction peaks of N-type pseudo-ternary semiconductor mechanical alloying sintered hot pressing block material did not shift after adding Al₂O₃,indicating that the crystal plane spacing did not be changed adding Al₂O₃.Compared with other crystal planes,the（0001）plane of the prepared material exhibits a growth lag phenomenon at a sintering temperature of 240℃,and with the further increase of the sintering temperature,the corresponding peaks of all crystal planes continue to increase,and the crystallization degree of the material continues to rise.The results of scanning electron microanalysis show that the grain size increases,aggregation occurs,and the area of the layered structure increase with the increase of sintering temperature.Hall effects analysis shows that the carriers’concentration of the material does not change by adding Al₂O₃.For the influence of Al₂O₃concentration on the materials,it is found that the absolute value of Seebeck coefficient of the Al₂O₃-doped materials increases first and then decreases with the increase of Al₂O₃concentration,and reaching the maximum value at 1 wt%,while the electrical conductivity and thermal conductivity decrease continuously.The change of Seebeck coefficient is mainly caused by the enhancement of carriers’scattering effect and the increase of carriers’effective mass caused by the addition of Al₂O₃.The continuous decrease of conductivity and thermal conductivity is mainly caused by the continuous enhancement of carriers’scattering effect caused by the increase of Al₂O₃incorporation concentration.The change of thermal conductivity mainly depends on the change of carriers’thermal conductivity.The addition of Al₂O₃has no effect on the lattice thermal conductivity.The results of the influence of sintering temperature on materials show that the absolute value of Seebeck coefficient decreases continuously and the electrical conductivity increases significantly with the increase of sintering temperature,while the thermal conductivity decreases first and then increases,and reached the lowest value around 280℃.The absolute value of Seebeck coefficient and the conductivity are mainly caused by the enhancement of crystallization degree and the weakening of carriers’scattering effect of the materials.The thermal conductivity is mainly determined by the lattice thermal conductivity and the carriers’thermal conductivity with sintering temperature.With the increase of sintering temperature,the thermal conductivity is mainly determined by the change of lattice thermal conductivity when the temperature is lower than 280℃,and the change of electron thermal conductivity when the temperature is higher than 280℃.The Z value is determined by Seebeck coefficient,electrical conductivity and thermal conductivity.The Z value of Al₂O₃doped N-type pseudo-ternary semiconductor mechanical alloying hot pressing sintering block material increase first and then decrease,reaches the maximum value of 1.66×10^-3K^-1at 340℃.The research results in this paper reveal the internal physical mechanism of the effect of preparation process parameters on the microstructure and thermoelectric properties of the materials,which can provide theoretical guidance and experimental basis for further improving the thermoelectric properties of existing thermoelectric materials,and have certain academic and engineering practical application value.