Optimization Of Thermoelectric And Mechanical Properties Of Te-substituted Skutterudites

As a kind of environment friendly materials, thermoelectric materials have great potential application in the fields of waste heat power generation and thermoelectric cooling, and lots of national attention has been focus on them as new functional materials since the late1990s. Skutterudite materials exhibit superior thermoelectric properties and robust structural stability, have been considered as one of most promising thermoelectric materials for application. Further improving the thermoelectric properties and mechanical properties of skutterudite material, is of great significance for the development of thermoelectric materials science and the application of the skutterudite thermoelectric materials.In this dissertation, optimizing the thermoelectric and mechanical properties of Te-substituted skutterudites are focused as research objects. Te-Se and Te-S codoped skutterudite compounds are fabricated by solid-state reaction and SPS technology, and the influences of codoping on microstructure and thermoelectric properties are investigated. Nano homogeneous particles and nano-TiN dispersed Te-doped skutterudite composites are prepared and the influences nano-particles on thermoelectric properties and mechnical properties are investigated. The main contents and results are listed as follows:Co-doping with Te and Se is the synergic combination of the beneficial effect of the two dopant atoms on skutterudite systems. Te-Se codoped skutterudite compounds, Co4Sb11.9-xTexSe0.1(x=0.4-0.6), are synthesized by the solid state reaction in view of their advantages in improving thermoelectric performance. It is found that Se doping results in decrease of the lattice parameter and refinement of the particle size compared with those of Se-free samples. The Te-Se codoped samples show a significant depression in the thermal conductivity and lattice thermal conductivity. The Se-free skutterudites have a total thermal conductivity ranging from5.4～4.9Wm-1K-1at300K, which decreases down to4.0～3.6Wm-1K-1when doped with Se. The Co4Sb11.3Te0.6Se0.1sample shows the lowest lattice thermal conductivity, which is lower by33%at300K and25%at800K than that of Co4Sb11.4Te0.6sample. The highest dimensionless figure of merit of1.09is achievable at800K for Co04Sb11.3Te0.6Se0.1compound. Double-substituted skutterudites Co4Sb11.3Te0.7-xSex (x=0.07,0.10,0.12,0.15,0.20,0.30) are synthesized for the sake of optimizing the thermoelectric performance of Te-Se codoped skutterudites. The results indicate that the lattice parameter, carrier concentration, electrical conductivity and thermal conductivity decrease with the increase of Se content. The lattice thermal conductivity of Co4Sb11.3Te0.7-xSex does not decrease monotonously with the Se content. Co4Sb11.3Te0.58Se0.12achieves the lowest lattice thermal conductivity of1.42Wm-1K-1at775K, which is very close to that of Co4Sb11.3Te0.6Se0.10sample. Then, the lattice thermal conductivity increases to a certain extent with the Se doping fraction. The ZT values of co-doped Co4Sb11.3Te0.7-xSex (x=0.07,0.10,0.12,0.15,0.20,0.30) compounds are higher than that of Se-free Co4Sb11.3Te0.7compound over the whole temperature range. Among the samples, ZT values of four samples Co4Sb11.3Te0.7-xSex (x=0.07,0.10,0.12,0.15) are over1.0in the temperature range of750-800K. Moreover, the largest ZT value reaches1.11at800K for Co4Sb11.3Te0.58Se0.12compound.We discussed the possibility and feasibility of improvement on the thermoelectric properties by codoping with Te and S. The XRD results together with the thermoelectric properties investigated indicate that S could not enter into the skutterudite lattice when when in the case of S single doping. The addition of S effectively decreases the thermal conductivity and lattice thermal conductivity of Co4Sb11.9-xTexS0.1. The lattice thermal conductivity decreases as Te fraction increases, and a minimum value of1.51Wm-1K-1(Co4Sb11.3Te0.6S0.1) is obtained at775K, which is relatively lower than the value of1.95Wm-1K-1(Co4Sb11.4Te0.6) at the same temperature. The highest ZT～1.05for Co4Sb11.4Te0.5S0.1and～1.08for Co4Sb11.3Te0.6S0.1are obtained at800K. Te-S codoped Co4Sb11.3Te0.7-xSx (x=0.07-0.20) compounds are fabricated for the sake of optimizing the thermoelectric performance of Te-S codoped skutterudites. The lattice parameter, carrier concentration, electrical conductivity, power factor and thermal conductivity decrease with the increasing S content. The lattice thermal conductivity changes unobviously, and Co4Sb11.3Te0.63Se0.07achieved the minimum value of2.83Wm-1K-1at300K and1.46Wm-1K-1at800K, respectively. Furthermore, all the ZT values of Te-S codoped samples exceed1.0at800K, thereinto, the Co4Sb11.3Te0.63Se0.07achieved the maximum value of1.1at800K. All the results indicate that S could enter into the skutterudite lattice codoping with Te, and small amount of S doping is effective in improving the thermoelectric performance of Te-based skutterudites.Homogeneous particles with different grain size dispersed Co4Sb11.5Te0.5nanocomposites are obtained by ultrasonic dispersion and ball mill processes before SPS. It is found that the nano particles grow apparently, most of the nanoparticle size are≥200nm in5%50h and10%50h, while100～200nm in3%100h and5%100h after SPS. A slight increase in the thermoelectric properties is achieved due to the few dispersion and grow up of nanoparticles. The flexural strength and fracture toughness of the material increases with the increase of nano skutterudite particles. Sample10%50h obtained the maximum flexural strength of141.9MPa, representing a22%increase compared with that of nano-free sample. Homogeneous nanoparticle dispersion has a relatively small impact on the fracture toughness of the material. The sample5%100h achieved the maximum of1.18MPam1/2, an11%increase compared with that of nano-free sample. The flexural strength and fracture toughness of sample5%100h are obviously greater than that of sample5%50h, and fracture toughness of sample5%100h is even larger than that of10%50h, which shows that differenet nanoparticle size have a certain effect on the mechanical properties of the nanocomposites, and the smaller the particle, the better the reinforcing and toughning effects may be.Nano-TiN dispersed skutterudite composites Co4Sb11.5Te0.5+x vol%TiN (x=0.0,0.3,0.6,1.0) are obtained by ultrasonic dispersion and ball mill processes before SPS. It is found that the TiN particles dispersed almost evenly and most were embedded in the matrix, and part of the nano-TiN embedded in the matrix is in the form of aggregation whose size is in the range of tens to a few hundreds of nanometers. The thermal conductivity and lattice thermal conductivity decrease gradually with the increasing TiN addition mainly due to the phonon scattering by nano-particles, resulting in an improvement in the thermoelectric performance especially at high temperature. The composite with1.0vol%TiN addition achieved the minimum thermal conductivity of3.99Wm-1K-1and lattice thermal conductivity of3.40Wm-1K-1at300K, representing15%and17%reduction compared with0.0vol%TiN samples. The composite with1.0vol%TiN addition achieved the maximum ZT value of1.0±0.1at800K, a10%improvement compared with the TiN-free sample. Compared with the TiN-free samples, flexural strength and fracture toughness of the composites are improved by30%and40%, respectively, upon an addition of just1.0vol%TiN. These nano-TiN particles embedded in the matrix can work as the barriers for crack propagation, and it is believed that the service reliability of the thermoelectric modules can be enhanced considerably even with a small volume addition.