Thermoelectric Properties Of Nano-Bi₂Te₃Sintered By High Pressure And High Temperature

Thermoelectric material is a functional material which can be directly converted heatenergy to electrical energy or reversely. This material has a great future in powergeneration and thermoelectric cooling. As the world energy shortage crisis andenvironmental pollution is increasingly serious, people are more and more attentions thismaterial. The figure of merit ZT is an important factor to characterize the thermoelectricconversion efficiency. ZT can be expressed as:In this formula, S is the Seebeck coefficient, σ is the conductivity, κ is the thermalconductivity, T is absolute temperature, S2σ is called power factor of thermoelectricmaterial. From this formula we can know there are two kinds of methods to improve thevalues of ZT. The first method is to improve the Seebeck coefficient and electricalconductivity; those can increase the power factor. The second method is to lower thermalconductivity. S, σ, and κ are interrelated constraints, how to balance the relationship ofpower factor and thermal conductivity has become the key to improve the value of ZT.Conversion efficiency is dependent on the value of ZT.Bi₂Te₃is the optimum thermoelectric materials which can use in room temperature.In this paper, Bi₂Te₃nanopowders were synthesized by the hydrothermal method. Thephase structures of product were investigated by X-ray diffraction （XRD）; themicrostructures of product were investigated by scanning electron microscopy （SEM） andtransmission electron microscopy （TEM）. Bi₂Te₃nanopowders were synthesized indifferent reaction time and temperature by the hydrothermal method. These conditions onthe phase structures and microstructures of product also are discussed. Then, Bi₂Te₃powders were sintered by high pressure and high temperature （HPHT）. The effects oftemperature and pressure on thermoelectric properties were investigated in roomtemperature. We got those following findings.Bi（NO3）3and Te powders as the precursors，N2H4as reductant, when we chose12hours as reaction time, there are some Te powders remain. When we extend reaction timeto24hours, Bi₂Te₃nanopowders were synthesized at150°C and180°C. It was foundthat the Bi₂Te₃nanopowders are n-type and the particle size is50nm. The particle size isincreased by the reaction temperature. We also found that the optimum condition is24hours and180°C. Bi₂Te₃nanopowders were sintered by HPHT at different sintering temperature andsintering pressure. The effects of different sintering temperature on thermoelectricproperties were investigated in room temperature. As the sintering temperature increased,the room temperature of power factor increased first and then decreased the overallshowed an upward trend. The room temperature of power factor reaches the maximumwhen sintered at753K. The sintering temperature has little effect on the thermalconductivity. So, the optimum sintering temperature is753K in this experiment. Theeffects of different sintering pressure on thermoelectric properties also were investigatedin room temperature. As the sintering pressure increased, the room temperature of powerfactor increased first and then decreased the overall showed a downward trend. The roomtemperature of power factor reaches the maximum when sintered at0.94GPa. Theconductivity decreased by the increase of sintering pressure. After sintered at0.94GPaand753K, the room temperature of power factor reached to16μw·cm-1·k-2, the roomtemperature of ZT reached to0.31. Compared whit the sample which synthesized by theHPHT method, the power factor increased by60%, the value of ZT increased2times.Those shows use HPHT sintered Bi₂Te₃nano-powders; temperature and pressure canprecipitation Te and Bi. A little nano second phase of Te and Bi can effectively improvethe thermoelectric properties of the material.