共查询到19条相似文献,搜索用时 187 毫秒
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用固态反应法制备了YBa2Cu3-xFexOy(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 相似文献
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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 相似文献
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利用正电子(e+)湮没寿命谱实验研究了Hg1-xCdxTe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τb=272ps.根据正电子湮没寿命和电参数的测量结果,得出
关键词: 相似文献
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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(Y1 -xGdx)Ba2Cu3O7-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YBa2Cu3O7-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ne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Tc随局域电子密度ne的减小而增加,这种局域电子密度ne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 相似文献
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以三嵌段共聚物聚环氧乙烷-聚环氧丙烷-聚环氧乙烷(P123)为模板,硅酸四乙酯(TEOS,C8H20O4Si)为硅源合成了比表面积高达712.5 m2/g,孔体积为2.44 cm3/g的新型介孔氧化硅载体(MCF).通过正电子湮没寿命谱(PALS)以及常规表征手段;例如N2吸附脱附、透射电子显微镜、热重分析、傅里叶变换红外光谱等系统研究了聚乙烯亚胺(PEI)改性MCF对纳米尺度孔结构的影响.结果表明,合成的MCF具有明显的无序介孔结构,孔与孔之间通过窗口相互连接形成了一个连续的、具有良好热稳定性的多孔通道网络,同时可以直观看到有机胺PEI已被成功引入到MCF通道中.为了更全面地评估材料孔径的变化情况,通过高灵敏度、可探测亚纳米量级的正电子湮没技术研究正电子在PEI负载MCF中的湮没机制,发现存在τ3和τ4个长寿命分量,表明样品中存在微孔和介孔.同时由于PEI分子的引入,导致τ3和τ... 相似文献
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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 相似文献
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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. 相似文献
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B. Staśkiewicz I. Turowska-Tyrk J. Baran Cz. Górecki Z. Czapla 《Journal of Physics and Chemistry of Solids》2014
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. 相似文献
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F. H. Hsu W. C. Mallard J. H. Hadley Jr. 《Applied Physics A: Materials Science & Processing》1974,4(1):83-86
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. 相似文献
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《光谱学快报》2012,45(10):633-641
AbstractPositron 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. 相似文献
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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. 相似文献
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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. 相似文献
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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 (τav), 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. 相似文献
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S. Abhaya 《Applied Surface Science》2008,255(1):142-144
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. 相似文献