Effect of HPT processing on the structure, thermoelectric and mechanical properties of Sr0.07Ba0.07Yb0.07Co4Sb12
Effect of HPT processing on the structure, thermoelectric and mechanical properties of Sr0.07Ba0.07Yb0.07Co4Sb12
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DOI：
10.1016/j.jallcom.2012.05.011
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发表时间：
2012-10
影响因子：
6.2
通讯作者：
G. Rogl;Z. Aabdin;E. Schafler;J. Horky;D. Setman;M. Zehetbauer;M. Kriegisch;O. Eibl;A. Grytsiv-A.-Gr
中科院分区：
文献类型：
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作者：
G. Rogl;Z. Aabdin;E. Schafler;J. Horky;D. Setman;M. Zehetbauer;M. Kriegisch;O. Eibl;A. Grytsiv-A.-Gr
在800K温度下，采用高压扭转（HPT）技术对ZT=1.4的n型方钨矿sr0.07 ba0.07 yb0.07 co4sb12进行了加工，得到了具有许多变形引起的位错和空位等晶格缺陷的纳米晶材料。如前所述，经过HPT处理后，达到ZT ~ 1.8主要是由于导热系数显著降低（晶格导热系数几乎达到理论计算的最小值），尽管电阻率更高。本文研究了高温热处理后导致如此高ZT值的显微组织变化。HPT前后x射线谱分析（XPA）发现，HPT处理后的晶体尺寸较小，缺陷（位错和空位）数量较多，导致电阻率增加，但导热系数明显降低。晶粒尺寸的减小也可以被认为是显微硬度提高的原因，这意味着霍尔-佩奇强化适用。此外，还首次采用能量过滤透射电子显微镜（TEM）对HPT处理后的方突石进行了研究。可以直接观察到两种类型（极化偶极子壁和极化倾斜壁）的位错和晶界，证实了迄今为止的假设。此外，首次在室温以下和室温以上测量了热膨胀，并与HPT前的结果进行了比较，结果显示热膨胀系数略有降低，德拜温度相同，但爱因斯坦温度仅为HPT前的一半，后者表明HPT处理后填充原子的频率较低。此外，在热膨胀测量中，电阻率达到最大值后的下降与样品的收缩平行，证明了微裂纹的退火和关闭是造成这种行为的原因。
N-type skutterudite Sr0.07Ba0.07Yb0.07Co4Sb12with ZT=1.4 at 800K was processed by high pressure torsion (HPT), a technique of severe plastic deformation (SPD) to produce a nanocrystalline material with many deformation induced lattice defects like dislocations and vacancies. As already shown previously, after HPT processing ZT∼1.8 was reached mainly due to a significantly reduced thermal conductivity (the lattice thermal conductivity reached almost the theoretical calculated minimum) although the electrical resistivity was higher. In this paper, the microstructural changes after HPT leading to such high ZT values were investigated. X-ray line profile analysis (XPA) before and after HPT was used to detect a smaller crystallite size and a high number of defects (dislocations and vacancies) resulting in an increase of the electrical resistivity but a significant decrease of the thermal conductivity after HPT processing. The decrease of the crystallite size could also be identified as the reason for enhanced microhardness, which means that Hall–Petch strengthening applies. In addition, for the first time, energy filtered transmission electron microscopy (TEM) was employed for the investigation of HPT processed skutterudites. Dislocations as well as grain boundaries of two types (polarised dipole walls and polarised tilt walls) could be directly observed, confirming what so far was assumed. Also for the first time thermal expansion was measured below and above room temperature and compared with the results before HPT revealing a slightly lower thermal expansion coefficient, the same Debye temperature but an Einstein temperature only half of that before HPT, the latter indicating lower frequencies of the filler atoms after HPT processing. Furthermore it could be shown that the decrease of the electrical resistivity after reaching a maximum runs parallel with a shrinking of the sample during thermal expansion measurements, proving that annealing out and closing of microcracks are responsible for this behaviour.