The Study On Cold Sintered Telluride Based Materials And Their Thermoelectric Properties

With the acceleration of global industrialization,environmental pollution and energy shortage have become a problem that cannot be ignored.Thermoelectric materials,as a green functional material that can achieve direct heat-to-electricity energy conversion,have attracted widespread attention from domestic and foreign researchers.However,the conversion efficiency of thermoelectric devices based on thermoelectric materials is relatively low,which limits the practical application.Sintering the medium-temperature and low-temperature thermoelectric materials into a whole to prepare segmented gradient thermoelectric devices will effectively improve the thermoelectric conversion efficiency.Nevertheless,sintering different operating temperature zones of thermoelectric material into a whole at the same time will lead to the deterioration of thermoelectric performance due to the incompatibility of sintering temperatures.Therefore,it is of great significance to find a new sintering technology to reduce the sintering temperature of medium-temperature materials and make it consistent with that of low-temperature thermoelectric materials.Cold sintering technology is a new type of liquid-phase assisted sintering technology,which can greatly reduce the sintering temperature of materials and is expected to solve the above problems.Pb Te-based thermoelectric materials and Bi₂Te₃-based thermoelectric materials are one of the best medium and low temperature thermoelectric materials respectively.In this paper,Pb Te_0.94Te_0.06and Bi₂Te_2.7Se_0.3 thermoelectric materials are taken as research objects,and the effects of different cold sintering process parameters on their density,microstructure and thermoelectric properties are studied,and Pb Te_0.94Te_0.06 and Bi₂Te_2.7Se_0.3 thermoelectric materials with good thermoelectric properties are prepared.The main research contents and results are as follows:First,the cold sintering temperature is controlled at 300?,and the influences of holding time and pressure on the density,microstructure and thermoelectric properties of Pb Te_0.94Te_0.06 bulk were investigated.It is found that the density and grain size of the material can be effectively improved by properly prolonging the cold sintering holding time and the increasing the pressure,thus improving the electrical conductivity.In addition,excessive sintering pressure will inhibit the density and grain growth of Pb Te_0.94Te_0.06,which will lead to the decrease of electrical conductivity,thus lowering the thermoelectric performance.The sample with 30 min cold holding time and 160MPa pressure has the best thermoelectric performance.Second,select the best sintering temperature and pressure explored in the first step,study the influence of adding different contents of hydrothermal solution(HS)in cold sintering process on the phase,density,microstructure and thermoelectric properties of Pb Te_0.94Te_0.06 bulk,and then compare it with the sample prepared by SPS.Compared with SPS sample,the temperature of cold sintered samples is 150?lower,which inhibits the precipitation of Pb and maintains a higher carrier concentration.There is no obvious difference in the density and grain size of the samples cold sintered with different contents(0 wt%,5 wt%and 10 wt%)of HS,but adding 5 wt%and 10wt%of HS in cold sintering process can make the grains bond more tightly,greatly improve the Hall mobility,thus significantly improve the electrical conductivity and ultimately improve the ZT value.This indicates that HS plays a very important role in the cold sintering of Pb Te_0.94Te_0.06.At the cold sintering temperature of 300?,the ZT value of the sample cold sintered with 10 wt%HS reaches 0.96 at 700 K,which is 140%and 52%higher than that of the sample cold sintered with 0wt%HS and sample prepared by SPS.Third,the cold sintering temperature is controlled at 300?,and the influences of cold sintering holding time and pressure on the density,microstructure and thermoelectric properties of Bi₂Te_2.7Se_0.3 bulk samples were investigated.It is found that the density of Bi₂Te_2.7Se_0.3 bulk smaple increases gradually with the prolonging of holding time,but the grain size does not change obviously,and the density growth of the sample holding for more than 30 min gradually stagnates.The density,grain size and thermoelectric properties of cold sintering samples are only slightly different under different pressures.The sample with 30 min cold holding time and 160 MPa pressure has the best thermoelectric performance.Forth,select the best sintering temperature and pressure explored in the third step,study the influence on the phase,density,microstructure and thermoelectric properties of Bi₂Te_2.7Se_0.3 bulk sample cold sinterd wiht different type of liquid phase.XRD pattern indicates that all the samples are not oxidized.Compared with the sample sintered with 0 wt%liquid phase,the density,mobility and conductivity of the sample sintered with 10 wt%H₂O are significantly improved.The density of the sample cold sintered with 10 wt%Na OH(aq)does not increase,but the mobility and conductivity are further improved,which suggests that the aid liquid phase plays an important role in the cold sintering of Bi₂Te_2.7Se_0.3.Eventually,the sample cold sintered with 10 wt%Na OH(aq)has the highest ZT value,reaching 0.90 at 375 K.