用正电子湮没研究纳米碲化铋的缺陷及其对热导率的影响

利用水热法合成了Bi₂Te₃纳米粉末, 并在300–500 ℃的温度范围内对其进行等离子烧结. X射线衍射测试表明制得的Bi₂Te₃粉末是单相的. 对于300–500 ℃范围内烧结的样品, 扫描电子显微镜观察发现随着烧结温度的升高样品颗粒明显增大, 但是根据X射线衍射峰的宽度计算得到的样品晶粒大小并没有明显的变化. 正电子湮没寿命测试结果表明, 所有的样品中均存在空位型缺陷, 而这些缺陷很可能存在于晶界处. 正电子平均寿命随着烧结温度的升高而单调下降, 说明较高的烧结温度导致了空位型缺陷浓度的降低. 另外, 随着烧结温度从300 ℃升高到500 ℃, 样品的热导率从0.3 W·m^-1·K^-1升高到了2.4 W·m^-1·K^-1, 这表明在纳米Bi₂Te₃中, 空位型缺陷和热导率之间存在着密切的联系.

Positron annihilation studied defects and their influence on thermal conductivity of chemically synthesized Bi2Te3 nanocrystal

Bismuth telluride (Bi₂Te₃) and its alloys are regarded as the best thermoelectric materials available nowadays at room temperature and can be well prepared by using existing technology. In this paper, Bi₂Te₃ nanocrystals are prepared by hydrothermal method and then treated by a spark plasma sintering (SPS) process at five temperatures of 300, 350, 400, 450 and 500 ℃ each for 5 min under a pressure of 20 MPa. X-ray diffraction (XRD) and positron annihilation spectroscopy are used to study the microstructures of the samples after SPS treatment at different temperatures. According to the XRD patterns, the diffraction peaks of the as-grown powder are consistent with those indicated in the standard card for Bi₂Te₃, which confirms successful synthesis of Bi₂Te₃ powders. Scanning electron microscope images show that the particles of all the samples take on flake-like structures, and the particle sizes increase from about 100 nm to a few μm with the sintering temperature increasing from 350 to 500 ℃. This suggests significant reorganization of nanograins in sintering process, and some grains are agglomerated into larger particles. However, the grain sizes estimated from the X-ray diffraction peaks show little change in all the samples sintered at temperatures between 300-500 ℃. And most of the grains have sizes around 30 nm. Positron lifetime spectra are measured for Bi₂Te₃ samples sintered at different temperatures. The measurements reveal vacancy defects existing in all the sintered samples. With the increase of sintering temperature, there appears no significant change in trapped positron lifetime (τ₂). This suggests that the defect size has no change during sintering. However, intensity I₂ decreases monotonically with increasing sintering temperature, which indicates the lowering of vacancy concentration. The average positron lifetime shows a monotonous decrease with increasing sintering temperature, which indicates the recovery of vacancy defects at higher sintering temperatures. The thermal conductivity of the sample increases from 0.3 W·m^-1·K^-1 to about 2.4 W·m^-1·K^-1 with the sintering temperature increasing from 300 to 500 ℃. Since the lattice thermal conductivity dominates the total thermal conductivity, it can be inferred that sintering at higher temperature leads to the increase of lattice thermal conductivity. According to the positron annihilation lifetime result, the vacancy defects in the interface region gradually recover after sintering at higher temperatures. This shows good correlation with the increase of lattice thermal conductivity, indicating that vacancy-type defects are effective phonon scattering centers for Bi₂Te₃.