Enhancement Of Thermoelectric Performance For Bismuth Telluride-based Composites Prepared By High Pressure Synthesis

The industrial process is accelerated with the rapid development of social economy,energy shortage and environmental protection problems followed.The increasing depletion of petroleum and fossil raw materials,the emission and inefficient utilization of industrial waste heat,and global warming make it urgent to seek green,environmentally friendly and sustainable energy sources.Thermoelectric(TE)materials can be used to produce thermoelectric generators or thermoelectric coolers,and can realize the direct and interactive transformation of electric energy and heat without rotating parts.It can realize the reuse of industrial waste heat,automobile exhaust,university waste heat,etc.,and provide energy while protecting the environment.Its non-toxic,noise-free,long service life and other advantages have become the best candidates for energy supply in special fields such as aerospace exploration,micro power supply,chip field,and biomedical field.However,the low energy conversion efficiency of thermoelectric systems seriously limits the participation of thermoelectric materials in industrial production and people's lives.Bismuth telluride-based semiconductor material has excellent thermoelectric properties around 293 K,and it is also the only material widely used in commercial.General preparation methods of Bi2Te3-based alloys include zone melting,powder metallurgy,hot pressing,spark plasma sintering and so on.These methods have long cycle,poor sample machining performance and high production cost.The high temperature and high pressure(HPHT)synthesis method has great advantages.Applying high pressure at high temperature can not only reduce the reaction temperature and reaction time,but also achieve results that are difficult to achieve under normal pressure.For example,the high-pressure quenching cooling method can also retain part of the material properties under high pressure to normal pressure,thereby optimizing the thermoelectric transport performance of the material.In addition,the current thermoelectric properties of n-type Bi2Te3-based thermoelectric materials are poor,resulting in that the conversion efficiency of thermoelectric devices composed of p-type and n-type thermoelectric devices has not been significantly improved.Therefore,it is particularly important to improve the thermoelectric properties of n-type Bi2Te3-based thermoelectric materials.In this paper,bismuth telluride-based composite materials were prepared by HPHT technology.The composite second-phase material is used to make pores in the matrix,and the effects of pore size and pore size distribution,synthesis pressure and temperature on ZT value and the grain morphology of the samples were also discussed in depth.The specific content is:1.The p-type(Bi,Sb)₂Te₃ and Na Cl multiphase high-density composite material was synthesized by HPHT technology.The size and distribution of pores in the matrix can be adjusted by changing the composite content of Na Cl.The cross-sectional morphology of the composite samples after high pressure synthesis was analyzed by scanning electron microscopy,and the porosity and pore size distribution in the samples were analyzed.The results indicates that the removal of Na Cl in the matrix by ultrasonic cleaning can construct micro-nano pores,and with the increase of Na Cl composite content,the size of the pores in the matrix gradually increases.The thermoelectric properties of the sample were tested,the maximum ZT of the sample with 25 % porosity is 1.05 at 493 K.Compared with the compact sample prepared under the same conditions,the ZT value of the porous sample increased by 33 %.2.The composites of n-type Bi₂(Se,Te)₃ and TiH1.924 with different composite ratios were prepared by high temperature and high pressure synthesis method.The effects of holding time,synthesis pressure and composite ratio on the thermoelectric properties of Bi₂(Se,Te)₃ were studied.The results show that the incorporation of Ti into the Bi₂(Se,Te)₃ lattice can control the carrier concentration of the material.When the synthesis pressure is 2.5 GPa and the compounding ratio is 10:1,the sample exhibits the best thermoelectric performance.The maximum ZT value of 0.94 is reached at373 K,which is about 1.6 times larger than the ZT value of 0.36 of the undoped sample at the same temperature.The ZT = 0.78 of this sample at 493 K is higher than that of the sample without Ti H1.924 at 493 K.A 32.2 % optimization over ZT = 0.59 at493 K for the sample without Ti H1.924 addition.