Open AccessPositron annihilation studied defects and their influence on thermal conductivity of chemically synthesized Bi2Te3 nanocrystal
Author(s) -
He Hui-Fang,
Zhiquan Chen
Publication year - 2015
Publication title -
wuli xuebao
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.199
H-Index - 47
ISSN - 1000-3290
DOI - 10.7498/aps.64.207804
Subject(s) - materials science , spark plasma sintering , sintering , thermoelectric effect , nanocrystal , microstructure , bismuth telluride , grain size , scanning electron microscope , positron annihilation spectroscopy , analytical chemistry (journal) , positron , thermal conductivity , thermoelectric materials , composite material , nanotechnology , positron annihilation , electron , thermodynamics , chemistry , physics , chromatography , quantum mechanics
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 m 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 (2). 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 Wm-1K-1 to about 2.4 Wm-1K-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.