Optimization Of Microstructures And Thermoelectric Properties For N-type Bismuth Telluride-Based Alloys

Thermoelectric materials,which can achieve the direct conversion between electrical energy and thermal energy,have received extensive attention recently for their great potential applications in power generation and thermoelectric cooling.Bismuth telluride（Bi₂Te₃）based alloys are known as the best commercially used thermoelectric materials at ambient temperature.Commercial Bi₂Te₃-based alloys are commonly prepared by Zone Melting or Bridgman techniques.However,the poor mechanical performance and low material utilization rate of products limit their further applications.To overcome the poor mechanical performance of Bi₂Te₃-based alloys and improve their thermoelectric properties by reducing lattice thermal conductivities,powder metallurgy technique has been widely investigated and applied.The ZT values of p-type sintered polycrystalline bulks have reached or even exceeded those of commercial Bi₂Te₃-based alloys,but the ZT values of n-type sintered bulks are still low,restricting the application of polycrystalline bulks.In this paper,starting from high pure elemental powders of Bi,Se and Te,n-type Bi2（Te,Se）3 bulks were prepared by Mechanical Alloying（MA）combined with the Microwave Activated Hot Pressing（MAHP）process or vacuum melting combined with the Spark Plasma Sintering（SPS）process,respectively.By changing the parameters of sintering process,the doping amount of Se elements and the powder particle size,the microstructures of sintered bulks were controlled and the thermoelectric properties were optimized.The following are the main conclusions:1.Bi₂Te_2.85Se_0.15 bulks with the relative densities of 99% or more were prepared by MA-MAHP process at relatively low sintering temperatures of 573⁶23K,their relative densities are 8¹2% higher than those of bulks prepared by microwave pressureless sintering.According to the FESEM results,there are lamellar structures without obvious orientation distributing in the bulks,and the grain size increases with increasing sintering temperature;Comparing with the performances of bulks prepared by traditional microwave sintering or Hot Pressing,the results indicate that the MAHP technique introduced here can obtain dense bulk materials at low sintering temperature,and be helpful to obtain fine grains and improve the thermoelectric properties of materials.2.The effects of sintering temperature and holding time of the MAHP process on the microstructures and thermoelectric properties of Bi₂Te_2.7Se_0.3 bulks were studied detailly.With the increase of sintering temperature or prolonging holding time,the room-temperature power factors for bulks increase gradually,indicating that higher sintering temperature or longer holding time is beneficial to the electrical transport properties.In the bulk sintered at 533 K,the crystallization is not complete,and there are a large number of small grains with size of less than ⁸5nm distributed between the lamellar structures.As a result,the electron and phonon scattering at grain boundaries are enhanced significantly,leading to the poor electrical conductivity and a minimum lattice thermal conductivity of 0.41Wm-1K-1.Due to the relatively low thermal conductivity and moderate power factor,the bulk sintered at 613 K for 0min obtains the maximum ZT value of 0.71.3.Bi₂Te₃–xSex（x=0.15,0.3,0.45,0.6）alloys were prepared by MA-MAHP process to explore the effects of Se content on microstructures and thermoelectric properties.All of bulks show a typical multi-scale structure and there are some irregular nano grains dispersing between the lamellar structures.With the increase of Se doping,the volatilizations of Se elements result in the increase of lattice vacancies and the decrease of electron concentration,leading to the enhancement of Seebeck coefficient;and the power factors increase first and then turn to decrease.The Bi₂Te_2.7Se_0.3 bulk achieve the maximum power factor of 1.75×10^-3Wm^-1K^-2.The doped Se increases the band gap of bulk,and also inhibites the intrinsic excitation effect,and the temperature points corresponding to the maximum ZT values are moved to the higher temperature range.The Bi2Te2.55Se0.45 bulk has obtained the maximum ZT value of 0.73 at 423 K.4.A series of n-type Bi₂Te₃–xSex（x=0.12,0.21,0.3,0.45,0.6,0.75,1）bulks were prepared by the combination of vacuum melting and SPS technique,Bi₂Te_2.4Se_0.6 bulk achieve the maximum power factor of 2.48×10^-3Wm^-1K^-2 at room temperature.With the decrease of particle size,the carrier concentrations of Bi₂Te_2.4Se_0.6 bulks increase,leading to the enhancement of electrical conductivity and the decrease of Seebeck coefficient,and the bulk with powder size of 200-300 mesh has the maximum power factor of 2.66×10^-3Wm^-1K^-2.The carrier concentration was adjusted by doping different content of Te I4,with the increase of TeI4 content,the electrical conductivities improve a lot,and the Seebeck coefficients are greatly reduced.For the sample doped a relatively appropriate TeI4 content of 0.02wt%,the maximum power factor of the bulk with powder size of 200-300 mesh has been increased to 2.82×10^-3Wm^-1K^-2.For the bulks from different particle sized powders,the electric conductivities decrease and the Seebeck coefficients increase after they were annealed,and the power factors also increase.The annealed bulk with powder size of 120-300 mesh has the maximum power factor of 3.01×10^-3Wm^-1K^-2 with an improvement of ²2%.