碲掺杂方钴矿基材料热电性能及力学性能优化的研究
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摘要
热电材料是一种环境友好的新能源材料,在工业余热发电和热电制冷领域有广阔的应用前景,是我国和国际上自上个世纪九十年代后期以来高度重视发展的新型功能材料。方钴矿材料在中温领域具有优异的热电性能和结构稳定性,被认为是最具前景的热电材料之一。进一步提高方钴矿材料的热电性能、改善其力学性能,对发展热电材料科学并推动方钴矿热电材料的应用具有重要意义。
本论文围绕改善Te掺杂方钴矿基材料的热电性能和力学性能展开研究,采用固相反应结合放电等离子烧结技术,制备Te-Se共掺杂和Te-S共掺杂方钴矿材料,系统研究Te-Se及Te-S共掺杂对CoSb3基方钴矿材料的微结构和热电性能的影响规律,以期达到降低材料热导率和提高热电优值的目的。通过引入同质方钴矿纳米颗粒和异质TiN纳米颗粒,制备方钴矿基微纳复合材料,探索纳米颗粒的引入对材料热电性能和力学性能的影响及规律,以期达到在保证热电性能的前提下提高材料力学性能的目的。论文的主要研究内容和成果如下：
结合Te和Se两元素在改善CoSb3基方钴矿材料电传输性能和热传输性能方面各自的优点,采用固相反应法制备了Te-Se共同掺杂的Co4Sb11.9-xTexSe0.1(x=0.4-0.6)方钴矿材料。与Tex单掺杂材料相比,TexSe0.1共掺杂材料的晶格常数降低,晶粒尺寸减小且相对均匀。TexSe0.1共掺杂显著降低了材料的热导率和晶格热导率。300K时,Tex单掺杂样品的热导率为5.4～4.9Wm-1K-1(x=0.4-0.6), TexSe0.1共掺杂样品的热导率降为4.0～3.6Wm-1K-1(x=0.4-0.6).样品Co4Sb11.3Te0.6Se0.1在测试温度范围内表现出最小晶格热导率,300K时相比样品Co4Sb11.4Te0.6下降了33%,800K时下降了25%。TexSe0.1共掺杂样品的ZT值明显提高,Co4Sb11.3Te0.6Se0.1在800K时的ZT值达到1.09。为进一步优化Te-Se共掺杂材料的热电性能,将掺杂总量固定为0.7,制备了Co4Sb11.3Te0.7-xSex (x=0-0.30)化合物,结果表明：晶格常数、载流子浓度、电导率和热导率均随Se掺杂比例的增加而降低。晶格热导率随Se掺杂比例的增加先减小后增加,样品Co4Sb11.3Te0.58Se0.12在775K取得最小值为1.42Wm-1K-1。Te-Se共掺杂样品Co4Sb11.3Te0.7-xSex(x=007-0.30)的ZT值明显高于样品Co4Sb11.3Te0.7,且x=0.07-0.15四个样品的ZT值在750～800K范围内均超过了1.0,其中,样品Co4Sb11.3Te0.58Se0.12在800K时取得最大ZT值为1.11。
探索制备了Te-S共同掺杂的Co4Sb11.9-x(x=0.4-0.6)方钴矿材料。结合XRD和电热传输性能可以发现S0.1单独掺杂时不会取代Sb进入方钻矿晶格中。将TexS0.1共掺杂样品与Tex单掺杂样品相比发现,TexSo.1共掺杂样品的衍射峰峰位明显向高角度偏移,即晶胞收缩。TexSo.1共掺杂显著降低了材料的热导率和晶格热导率。晶格热导率随Te掺杂量的增加而降低,样品Co4Sb11.3Te0.6S0.1在775K取得最小值为1.51Wm-1K-1。样品Co4Sb11.4Te0.5S0.1和样品Co4Sb11.3Te0.6S0.1的ZT值在800K时分别达到1.05和1.08。为进一步探索和优化Te-S共掺杂材料的热电性能,制备了Te-S掺杂总量为0.7的Co4Sb11.3Te0.7-xSx (x=0.07-0.20)化合物。研究发现,晶格常数、载流子浓度、电导率、功率因子和热导率均随S掺杂比例的增加而降低。晶格热导率的变化相对较小,x=0.07的样品取得最小值,300K和800K时分别为2.83Wm-1K-1和1.46Wm-1K-1。Te-S共掺杂样品的ZT值在800K时均超过了1.0,其中x=0.07的样品在800K时取得最大热电优值ZT=1.1,说明Te-S共掺杂时,S可以取代Sb进入方钴矿晶格,并且微量S掺杂可以显著降低Te基方钴矿材料的热导率从而提高热电性能。
以Te掺杂方钴矿材料为基体,引入不同晶粒尺寸的同质纳米材料,结合超声分散和球磨技术,制备了纳米颗粒分布相对均匀的方钴矿复合材料。研究发现SPS后纳米颗粒长大较明显,5%50h和10%50h样品中纳米颗粒的尺寸大部分在200nm以上,而3%100h和5%100h样品中还存在较多尺寸在100～200nm的颗粒。由于纳米颗粒掺入量较少及纳米颗粒的长大,Co4Sb11.5Te0.5同质纳米复合材料的电热传输性能变化较小,热电性能略有提高。材料的抗弯强度和断裂韧性均随纳米方钴矿颗粒体积分数的增加而增加,但增加幅度不同。样品10%50h取得最大抗弯强度为141.9MPa,相比未掺入纳米颗粒样品提高了约22%。样品5%100h取得最大断裂韧性为1.18MPam1/2,相比未掺入纳米颗粒样品提高了约11%。样品5%100h的抗弯强度和断裂韧性均明显大于样品5%50h,并且样品5%100h的断裂韧性还略大于样品10%50h,说明不同尺寸的纳米颗粒对复合材料的力学性能影响显著,颗粒越小增强增韧效果越好。
以Te掺杂方钴矿材料为基体,引入TiN纳米材料,结合超声分散和球磨技术,制备了纳米TiN颗粒弥散分布的Co4Sb11.5Te0.5+x vol%TiN (x=0.0,0.3,0.6,1.0)复合材料。纳米TiN颗粒大多镶嵌在基体晶粒表面,且分散相对均匀,随体积分数的增加,部分纳米颗粒团聚,形成几十到上百纳米的团簇。随TiN含量的增加,热导率和晶格热导率均逐渐降低,300K时x=1.0的样品的热导率和晶格热导率分别为3.99Wm-1K-1和3.40Wm-1K-1,相对x=0.0的样品分别下降了15%和17%。随纳米TiN体积分数的增加ZT值增加,x=1.0的样品在800K时取得最大值为1.0,相对x=0.0的样品提高了约10%。微量纳米TiN的引入显著提高了材料抗弯强度和断裂韧性。相比x=0.0的样品,x=1.0的样品的抗弯强度增加了近30%,断裂韧性增加了约40%,这对提高方钴矿热电器件的机械稳定性和可靠性有重要意义。
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.
本论文围绕改善Te掺杂方钴矿基材料的热电性能和力学性能展开研究,采用固相反应结合放电等离子烧结技术,制备Te-Se共掺杂和Te-S共掺杂方钴矿材料,系统研究Te-Se及Te-S共掺杂对CoSb3基方钴矿材料的微结构和热电性能的影响规律,以期达到降低材料热导率和提高热电优值的目的。通过引入同质方钴矿纳米颗粒和异质TiN纳米颗粒,制备方钴矿基微纳复合材料,探索纳米颗粒的引入对材料热电性能和力学性能的影响及规律,以期达到在保证热电性能的前提下提高材料力学性能的目的。论文的主要研究内容和成果如下：
结合Te和Se两元素在改善CoSb3基方钴矿材料电传输性能和热传输性能方面各自的优点,采用固相反应法制备了Te-Se共同掺杂的Co4Sb11.9-xTexSe0.1(x=0.4-0.6)方钴矿材料。与Tex单掺杂材料相比,TexSe0.1共掺杂材料的晶格常数降低,晶粒尺寸减小且相对均匀。TexSe0.1共掺杂显著降低了材料的热导率和晶格热导率。300K时,Tex单掺杂样品的热导率为5.4～4.9Wm-1K-1(x=0.4-0.6), TexSe0.1共掺杂样品的热导率降为4.0～3.6Wm-1K-1(x=0.4-0.6).样品Co4Sb11.3Te0.6Se0.1在测试温度范围内表现出最小晶格热导率,300K时相比样品Co4Sb11.4Te0.6下降了33%,800K时下降了25%。TexSe0.1共掺杂样品的ZT值明显提高,Co4Sb11.3Te0.6Se0.1在800K时的ZT值达到1.09。为进一步优化Te-Se共掺杂材料的热电性能,将掺杂总量固定为0.7,制备了Co4Sb11.3Te0.7-xSex (x=0-0.30)化合物,结果表明：晶格常数、载流子浓度、电导率和热导率均随Se掺杂比例的增加而降低。晶格热导率随Se掺杂比例的增加先减小后增加,样品Co4Sb11.3Te0.58Se0.12在775K取得最小值为1.42Wm-1K-1。Te-Se共掺杂样品Co4Sb11.3Te0.7-xSex(x=007-0.30)的ZT值明显高于样品Co4Sb11.3Te0.7,且x=0.07-0.15四个样品的ZT值在750～800K范围内均超过了1.0,其中,样品Co4Sb11.3Te0.58Se0.12在800K时取得最大ZT值为1.11。
探索制备了Te-S共同掺杂的Co4Sb11.9-x(x=0.4-0.6)方钴矿材料。结合XRD和电热传输性能可以发现S0.1单独掺杂时不会取代Sb进入方钻矿晶格中。将TexS0.1共掺杂样品与Tex单掺杂样品相比发现,TexSo.1共掺杂样品的衍射峰峰位明显向高角度偏移,即晶胞收缩。TexSo.1共掺杂显著降低了材料的热导率和晶格热导率。晶格热导率随Te掺杂量的增加而降低,样品Co4Sb11.3Te0.6S0.1在775K取得最小值为1.51Wm-1K-1。样品Co4Sb11.4Te0.5S0.1和样品Co4Sb11.3Te0.6S0.1的ZT值在800K时分别达到1.05和1.08。为进一步探索和优化Te-S共掺杂材料的热电性能,制备了Te-S掺杂总量为0.7的Co4Sb11.3Te0.7-xSx (x=0.07-0.20)化合物。研究发现,晶格常数、载流子浓度、电导率、功率因子和热导率均随S掺杂比例的增加而降低。晶格热导率的变化相对较小,x=0.07的样品取得最小值,300K和800K时分别为2.83Wm-1K-1和1.46Wm-1K-1。Te-S共掺杂样品的ZT值在800K时均超过了1.0,其中x=0.07的样品在800K时取得最大热电优值ZT=1.1,说明Te-S共掺杂时,S可以取代Sb进入方钴矿晶格,并且微量S掺杂可以显著降低Te基方钴矿材料的热导率从而提高热电性能。
以Te掺杂方钴矿材料为基体,引入不同晶粒尺寸的同质纳米材料,结合超声分散和球磨技术,制备了纳米颗粒分布相对均匀的方钴矿复合材料。研究发现SPS后纳米颗粒长大较明显,5%50h和10%50h样品中纳米颗粒的尺寸大部分在200nm以上,而3%100h和5%100h样品中还存在较多尺寸在100～200nm的颗粒。由于纳米颗粒掺入量较少及纳米颗粒的长大,Co4Sb11.5Te0.5同质纳米复合材料的电热传输性能变化较小,热电性能略有提高。材料的抗弯强度和断裂韧性均随纳米方钴矿颗粒体积分数的增加而增加,但增加幅度不同。样品10%50h取得最大抗弯强度为141.9MPa,相比未掺入纳米颗粒样品提高了约22%。样品5%100h取得最大断裂韧性为1.18MPam1/2,相比未掺入纳米颗粒样品提高了约11%。样品5%100h的抗弯强度和断裂韧性均明显大于样品5%50h,并且样品5%100h的断裂韧性还略大于样品10%50h,说明不同尺寸的纳米颗粒对复合材料的力学性能影响显著,颗粒越小增强增韧效果越好。
以Te掺杂方钴矿材料为基体,引入TiN纳米材料,结合超声分散和球磨技术,制备了纳米TiN颗粒弥散分布的Co4Sb11.5Te0.5+x vol%TiN (x=0.0,0.3,0.6,1.0)复合材料。纳米TiN颗粒大多镶嵌在基体晶粒表面,且分散相对均匀,随体积分数的增加,部分纳米颗粒团聚,形成几十到上百纳米的团簇。随TiN含量的增加,热导率和晶格热导率均逐渐降低,300K时x=1.0的样品的热导率和晶格热导率分别为3.99Wm-1K-1和3.40Wm-1K-1,相对x=0.0的样品分别下降了15%和17%。随纳米TiN体积分数的增加ZT值增加,x=1.0的样品在800K时取得最大值为1.0,相对x=0.0的样品提高了约10%。微量纳米TiN的引入显著提高了材料抗弯强度和断裂韧性。相比x=0.0的样品,x=1.0的样品的抗弯强度增加了近30%,断裂韧性增加了约40%,这对提高方钴矿热电器件的机械稳定性和可靠性有重要意义。
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.
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