排序方式: 共有54条查询结果,搜索用时 437 毫秒
31.
在143~373K用正电子湮没寿命谱(PALS)方法研究了高密度聚乙烯(HDPE)中的自由体积与温度 的关系.对实验谱分别进行了三寿命成分和四寿命成分分析,并对实验拟合结果作了微分的尝试.发现经这种方 法处理后的结果对温度变化十分灵敏,并且从四寿命分析的微分曲线可以完整地观察到HDPE在该温度区间的 三重α弛豫和表观双玻璃化转变过程,而用三寿命分析的微分曲线将难以解释这些转变.这说明对部分结晶的 聚合物而言,用四寿命拟合比三寿命拟合更符合实际的物理过程;讨论了正电子在HDPE中的湮没机制,证实正 电子在HDPE的结晶区和非晶区都可能形成电子偶素(Ps),而且正电子所处的空间大小对Ps产额有影响;最后 根据PALS实验的结果直接估算了HDPE在不同温度下的热膨胀系数. 相似文献
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在密度函数理论的基础上,采用中性原子叠加模型和有限差分方法(SNA-FD)计算了石墨,金刚石和C60这三种碳的同素异形体中的正电子分布和湮没情况. 计算表明,在片层结构的石墨晶体中,正电子主要在石墨层间的空隙中湮没,计算出的石墨中的正电子寿命为208ps,与文献中的实验结果210ps符合很好. 在金刚石单晶中,正电子主要在碳原子之间的空隙中存在并发生湮没,计算出的金刚石中的正电子寿命为1159ps,与文献中的实验结果110ps相符合;在面心立方结构的C60晶体中,正电子主要在C60分子球壳内外侧及分子之
关键词:
石墨
金刚石
C60
正电子寿命 相似文献
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A new simple digital positron lifetime spectrometer has been developed. It includes a DRS4 waveform digitizing board and two scintillation detectors based on the XP2020Q photomultiplier tubes and LaBr3 scintillators. The DRS4 waveform digitizing can handle small pulses, down to few tens of millivolts, and its time scale linearity and stability are very good. The new system has reached a 206 ps time resolution, which is better than the conventional analog apparatus using the same detectors. These improvements make this spectrometer more simple and convenient in comparison with other spectrometers, and it can be applied to the other scintillation timing measurements with picosecond accuracy. 相似文献
36.
Dependence of Intrinsic Defects in ZnO Films on Oxygen Fraction Studied by Positron Annihilation
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Defects in ZnO films grown by radio-frequency reactive magnetron sputtering under variable ratios between oxygen and argon gas have been investigated by using the monoenergetie positron beam technique. The dominate intrinsic defects in these ZnO samples are O vacancies (Vo) and Zn interstitials (Zni) when the oxygen fraction in the O2/Ar feed gas does not exceed 70% in the processing chamber. On the other hand, zinc vacancies are preponderant in the ZnO films fabricated in richer oxygen environment. The concentration of zinc vacancies increases with the increasing O2 fraction. For the oxygen fraction 85%, the number of zinc vacancies that could trap positrons will be smaller. It is speculated that some unknown defects could shield zinc vacancies. The concentration of zinc vacancies in the ZnO films varies with the oxygen fraction in the growth chamber, which is in agreement with the results of photolurninescence spectra. 相似文献
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首次用正电子湮灭寿命谱仪(PALS)测量SmFeAsO多晶样品常温下的寿命谱,得到两个寿命成分1516 ps和2903 ps,根据捕获模型得到正电子在SmFeAsO中湮灭的体寿命为1870 ps,与理论计算(广义梯度近似)得到的SmFeAsO单晶中的正电子体寿命173 ps符合较好.基于中性原子叠加模型-有限差分方法(SNA-FD)的理论计算得到正电子与单晶SmFeAsO中各个原子价电子的总湮灭率是其与各个原子核心内层电子总湮灭率的106倍,正电子与Fe,As,Sm,O原子的电子湮灭的概率之比是1∶13∶12∶1. 相似文献
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用单能慢正电子束,测量了不同氧分压下生长的La0.7Sr0.3MnO3外延膜的S参数与入射正电子能量E的关系.结果发现La0.7Sr0.3MnO3外延膜中S参数与氧分压是非单调变化的;这与沉积氧分压的两种作用相关联的.在氧分压较高的LSMO薄膜中, 空位浓度的增加主要是由沉积原子(离子)与氧原子碰撞几率增大,使其缺乏足够的动能去填补空位引起的;在低氧分压的LSMO薄膜中, 空位浓度的增大则主要是提供成膜所需要的氧原子缺乏,从而导致氧空位及其相关缺陷增加. 相似文献
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Based on the atomic superposition approximation(ATSUP) and first-principles pseudopotential plane-wave methods,the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Boron’ski-Nieminen positron-annihilation-rate interpolation formula respectively.The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results.The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide. 相似文献
40.
Identification of the pressure-induced phase transition of ZnSe with the positron annihilation method
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This paper studies the pressure-induced phase transition between zincblende (B3) and NaCl (B1) structure ZnSe by using the hydrostatic pressure first-principles pseudopotential plane wave method. The energy-volume and enthalpy-pressure curves are employed to estimate the transition pressure. It is found that ZnSe undergoes a first-order phase transition from the B3 structure to the B1 structure at approximately 15 GPa derived from the energy-volume relation and 14 GPa based on deduction from enthalpy-pressure data. The pressure-related positron bulk lifetimes of the two ZnSe structures are calculated with the atomic superposition approximation method. In comparison with the 13.4% reduction in volume of ZnSe at the transition pressure, the positron bulk lifetime decreases more significantly and the relative value declines up to 22.3%. The results show that positron annihilation is an effective technique to identify and characterize the first-order phase transition and can give valuable information about changes in micro-scale, such as volume shrinkage and compressibility. 相似文献