用正电子湮没技术研究形变铁的恢复

本文介绍应用正电子湮没寿命和多普勒加宽方法研究60％形变铁等时退火各阶段中缺陷的恢复行为,实验结果说明正电子湮没平均寿命τ及多普勒加宽的S参数都是退火温度的函数,完整晶体中湮没寿命τ_f=111±lPS,缺陷中湮没寿命τ_d=162±lps,本文还根据捕获模型计算了正电子的捕获速率及τ₁,结果证明与理论预期的完全一致,经计算得出捕获正电于的相对缺陷浓度的范围为10^-7-10^-4数量级。 关键词：     

Fe掺杂YBCO体系结构变化与团簇效应的正电子实验研究

用固态反应法制备了YBa₂Cu_3-xFe_xO_y(x=0—0.5)一系列样品,反应在空气中进行.运用正电子湮没技术、扫描电子显微镜和X射线衍射进行了研究,并测定了氧含量.正电子湮没结果发现,x=0.12时短寿命分量τ₁和长寿命分量τ₂都存在一异常变化.X射线衍射和扫描电子显微镜研究分别表明,材料在x=0.12—0.15区间内发生正交四方相变,晶粒尺寸突然由小变大.通过对上述实验结果的分析可以得到,在该类材料中,正电子对结构相变十分敏感;另外,当Fe掺杂量增大到一定程度时,Fe原子由随机分布变为成簇分布 关键词： Fe掺杂YBCO 高温超导电性 正电子湮没 结构相变     

退火处理Fe43Co43Hf7B6Cu1非晶合金的正电子湮没研究

对熔体急冷法制备的Fe₄₃Co₄₃Hf₇B₆Cu₁非晶合金进行了200,300,400和500 ℃保温30 min的退火处理,用正电子湮没寿命谱、X射线衍射、穆斯堡尔谱等方法研究了退火后试样的结构及结构缺陷变化.结果表明,在非晶合金的制备态,正电子主要在非晶基体相空位尺寸的自由体积中湮没,湮没寿命τ₁为158.4 ps,强度I₁关键词： 43Co₄₃Hf₇B₆Cu₁非晶')" href="#">Fe₄₃Co₄₃Hf₇B₆Cu₁非晶 退火处理 正电子湮没寿命 结构与结构缺陷     

Hg1-xCdxTe晶体缺陷的正电子湮没寿命

利用正电子(e⁺)湮没寿命谱实验研究了Hg_1-xCd_xTe晶体样品的空位缺陷.碲溶剂法生长的样品，不论是n型导电还是p型导电都存在大量的Hg空位.经过合适的退火工艺，p型材料转为n型，同时对正电子的俘获效应减小，表现为正电子湮没平均寿命值减小14—17ps.若退火温度高于350℃，正电子湮没寿命值又增大，表明Hg空位浓度增加.得到HgCdTe中正电子的体寿命为τ_b=272ps.根据正电子湮没寿命和电参数的测量结果，得出 关键词：     

Gd替代YBa2Cu3O7－δ超导体的相结构与局域电子结构的研究

利用正电子湮没实验，结合x射线衍射(XRD)结构分析，研究了具有混合稀土特征的(Y_{1 -x}Gd_x)Ba₂Cu₃O_7-δ系列样品. XRD 实验结果表明，半径较大的Gd离子Y位替代使得样品晶胞参数和晶胞体积增大，但所有样品 仍保持与YBa₂Cu₃O_7-δ(YBCO)样品相同的单相正交结 构. 正电子湮没实验表明，正电子各寿命参数表现出很强的Gd替代依赖关系. 从正电子实验 结果出发，计算了Cu-O链区局域电子密度n_e的变化. 结果表明，局域电子密度n _e随Gd含量x的增加而减小，而超导转变温度T_c随局域电子密度ne的减小而增加，这种局域电子密度n_e与超导电性的关联是与铜位替代 完全不同的，且可能是近年来人们关于混合稀土铜氧化物体系具有较高临界电流密度的原因 之一. 该研究结果为铜氧化物超导体的应用和机理研究提供了相应的正电子实验资料. 关键词： 超导电性 正电子湮没 相结构 局域电子密度     

用正电子湮没技术研究USY沸石的“二次孔”结构

测量了NaY及USY沸石的正电子寿命谱．得到NaY沸石有四个寿命分量：两个短寿命分量和两个长寿命分量．两个长寿命分量中，τ₃，τ₄分别是o-Ps在β笼和超笼中的湮没．USY沸石有五个寿命分量：两个短寿命分量和三个长寿命分量，三个长寿命分量中，τ₃为o-Ps在笼中的湮没，τ₄，τ₅分别为o-Ps在“二次微孔”和“二次中孔”中的湮没．与NaY沸石相比，在真空中，USY的τ₄增加到 关键词：     

MoO3/Al2O3催化剂中Mo分散的正电子研究

用浸渍法制备了一系列不同Mo含量的MoO₃/Al₂O₃催化剂.测量了这些样品的正电子湮没寿命谱(PALS)与符合多普勒展宽(CDB)谱,以研究其孔洞结构以及Mo分散.正电子寿命测量结果表明,Al₂O₃载体中存在两种不同尺寸的孔洞.掺入MoO₃之后,Mo原子主要进入Al₂O₃的大孔中,使孔洞体积减小.符合多普勒展宽谱结果表明,当MoO 关键词： 3/Al₂O₃催化剂')" href="#">MoO₃/Al₂O₃催化剂 正电子湮没寿命谱 符合多普勒展宽 Mo 分散     

MgB2超导体的正电子湮没研究

本文利用正电子湮没技术(PAT)对不同密度的MgB2样品分别进行了测定,结果发现正电子在MgB2中自由态的湮没时间(本征寿命τ1)和在捕获态的湮没时间(缺陷寿命τ2)比其他高温超导体都明显要高,这反映了MgB2的体电子浓度相对于其它超导体要低,可能归因于MgB2超导材料本身的晶体结构和化学组成,MgB2超导材料的正电子寿命不仅与晶体的结构缺陷,而且与样品的密度或孔隙度有关.     

钠米材料Fe2O3微结构的正电子寿命谱学研究

用Ｘ射线衍射确定了纳米材料Fe₂O₃的平均粒径。测量了正电子的寿命。给出了平均粒径随退火温度的变化和短寿命成分τ₁、中等寿命成分τ₂以及它们的强度比Ｉ₂／Ｉ₁随平均粒径的变化。讨论了纳米材料Fe₂O₃的界面结构、平均正电子寿命τ与多普勒展宽谱Ｓ参效间的对应关系。 关键词：     

掺镧PbWO4闪烁晶体的缺陷研究

利用正电子湮没寿命谱(PAT)和X射线电子能谱(XPS)研究了掺镧所引起的PbWO₄ 晶体缺陷的变化.结果表明：掺镧后,PbWO₄晶体中的正电子捕获中心铅空位(V< sub>Pb)浓度增加,并进一步诱导低价氧浓度的增加.讨论了掺La的作用机制,认为掺 La将抑制晶体中的氧空位,增加铅空位浓度. 关键词： 掺镧钨酸铅晶体 正电子湮没寿命谱 X射线电子能谱 缺陷     

Studies on the microstructure of phase transitions in two kinds of liquid crystals by PAT and DSC

The positron lifetime and DSC measerments for EBBA and DOBAMBC have been made with heating and cooling clycles. The experimental results show that a shorter lifetime (₁) is essentially independent of temperature while the longer lifetime (₂) and the intensity (I ₂) change abruptly double or triple with temperature. Consequently, the EBBA has only nematic phases while the DOBAMBC has two liquid-crystalline phases (smectic C* and smectic A) with transition temperatures as follows: for EBBA, solid nematic (304.5 K), nematic isotropic (356.5 K), isotropic nematic (356.5 K), nematic solid (301 K); and for DOBAMBC, solid smectic C* (346 K), smectic C* smectic A (357.5 K), smectic A isotropic (389 K). These critical temperatures are in accordance with the transition temperatures measured by DSC. In addition, the difference in the solid-nematic transition temperature in the heating and cooling cycles was also observed. A discussion about the correlation of the observed changes in lifetime (₂) with the changes in molecular orientational order (S) and dielectric anisotropy () is presented.     

Structural characterization,thermal, vibrational properties and molecular motions in perovskite-type diaminopropanetetrachlorocadmate NH3(CH2)3NH3CdCl4 crystal

The work presents a detailed analysis of the sequencing of the structural phase transitions in NH₃(CH₂)₃NH₃CdCl₄ crystal by differential scanning calorimetry (DSC), X-ray, infrared, far infrared and Raman spectroscopy. DSC studies have shown that in analyzed crystal occurring one reversible continuous phase transition at 375/374 K (on heating/cooling). Observed in Nujol and Fluorolube mulls in the wide temperature range between 296 K and 413 K spectral changes through the structural phase transition can be attributed to an onset of motion of cations. An assignment of some bands due to internal modes has been also proposed.     

Effect ofV K centers on positron annihilation in KCl and Ag-doped KCl

Positron lifetime spectra have been measured at 77 K for KCl and Ag-doped KCl before and after x-irradiation at 77 K and after annealing at room temperature. Radiation at 77 K reduces the intensity of the intermediate lifetime (τ₂) component. Radiation-induced defects were monitored optically and by ESR. The experiment shows that the changes observed in the positron decay are associated with the presence ofV _K centers.     

High Pressure effect on fluorescence lifetime τ for magnetic dipole 5D0→5F1 transitions in YAG:Eu3+

The fluorescence lifetime for magnetic dipole ⁵D₀→⁷F₁ transition in yttrium aluminum garnet doped with Eu³⁺ (YAG:Eu³⁺) crystal was studied under the pressure of up to 10.4?GPa at room temperature. The fluorescence lifetime τ (⁵D₀→⁷F₁ transition) slowly decreased with pressure. The pressure effect on τ (⁵D₀→⁷F₁ transition) was explained with a model which considered pressure effect on line position: inter-ionic distance, ion volume, molecular volume, ion polarizability, molecular polarizability, sample refractive index, and surrounding hydrostatic medium refractive index. The fluorescence lifetime τ calculated by the presented model was in close correspondence with the experimental values.     

Defect characterization of slow-cooled and quenched samples of calcium-copper-titanate through positron annihilation spectroscopy

Abstract

Positron annihilation lifetime and Doppler-broadened gamma-ray spectra have been analyzed for slow-cooled and thermally quenched polycrystalline samples of calcium-copper-titanate. Two positron lifetimes revealing the characteristic defects in the respective samples were carefully analyzed to compare and contrast the significance of their origin and implication. A third component arising from positronium formation at the powdered particle surfaces has been considered in the analysis although its significance is lost in its very small intensity (～ 1.1–1.2%). In the quenched sample, the defect-specific long positron lifetime (τ₂) is found to larger and the mean lifetime smaller while its intensity I₂ is found drastically smaller and the concentration of defects less by an order of magnitude as compared to the slow-cooled sample. The observed changes in electrical parameters of slow-cooled and quenched samples were found to have correlations with the positron annihilation lifetime and Doppler-broadened lineshape parameters.     

Phase transition and structure of (C3N2H5)2SbF5 single crystal

New crystal of the formula (C₃N₂H₅)₂SbF₅ was obtained and characterized with DSC, DTA, TGA, structural and dielectric studies. DSC and dielectric studies revealed a structural phase transition of the first order at 216 K on cooling and 220 K on heating. The entropy of the transition ΔS equal to 11.5 J/mol·K gives evidence that the phase transition is order-disorder type. X-ray studies showed that transition undergoes from orthorhombic phase I with a space group of Pmmn to monoclinic phase II with a space group P2₁/m. The phase transition is proposed to be ferroelastic type. The molecular mechanism of the phase transition is related to ordering of imidazolium cations in phase II that are disordered in phase I.     

Thermal stability of grain boundaries in nanocrystalline Zn studied by positron lifetime spectroscopy

Nanocrystalline Zn prepared by compacting nanoparticles with mean grain size about 55 nm at 15 MPa has been studied by positron lifetime spectroscopy. For the bulk Zn sample, the vacancy defect is annealed out at about 350 °C, but for the nanocrystalline Zn sample, the vacancy cluster in grain boundaries is quite difficult to be annealed out even at very high temperature (410 °C). In the grain boundaries of nanocrystalline Zn, the small free volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ₁). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τ_av), which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. This stabilization is very important for the nanocrystalline materials using as radiation resistant materials.     

聚丁二烯中的等待时间效应

测量了高聚物材料聚丁二烯的比热,发现在温度T_g=178K出现玻璃转变且转变点附近的比热与降温过程有关。在降温过程中,若控制样品在某一温度T_wg等待时间t_w,则升温比热测量表明,T_g处的比热跃变△c_p存在明显的等待时间效应,即△c_p随t_w的增大而增大。在T_w=169K条件下,△c_p(t< 关键词：     

Experimental and theoretical positron annihilation studies on bulk nickel silicides

Bulk nickel silicides (NiSi and NiSi₂) have been studied using the experimental positron lifetime and depth-resolved positron beam measurements. Ab-initio calculations of positron lifetime for the silicides have also been carried out using the atomic superposition method. For NiSi phase, it is found that the theoretically computed positron lifetime compares favourably with the experimentally deduced value indicating that NiSi is defect free. However, for NiSi₂, the experimental positron lifetime does not compare well with the theoretical value suggesting that NiSi₂ contains vacancy defects. This is further supported by the positron diffusion lengths deduced from the VEPFIT analysis of the positron beam results.