Wang Shao-Jie

Vacancy-Induced Ferromagnetism in SnO2 Nanocrystals: A Positron Annihilation Study

SnO2 nanopowders were pressed into pellets and annealed in air from 100 to 1400°C. Both XRD and Raman spectroscopy confirm that all annealed samples were single phase with a tetragonal rutile structure. Annealing induces an increase in the SnO2 grain size from 30 to 83 nm. Positron annihilation measurements reveal vacancy defects in the grain boundary region, and the interfacial defects remain stable after annealing below 400°C, then they are gradually recovered with increasing annealing temperature up to 1200°C. Room temperature ferromagnetism was observed for SnO2 nanocrystals annealed below 1200°C, and the magnetization decreases continuously with increasing annealing temperature. However, the ferromagnetism disappears at 1200°C annealing. This shows good coincidence with the recovery of interfacial defects in the nanocrystals, suggesting that the ferromagnetism is probably induced by vacancy defects in the interface region.

Temperature Dependence of Free Volume in Cross-linked Polyurethane Studied by Positrons

Positron annihilation lifetime spectroscopy has been used to study the free volume properties of the cross-linked polyurethane network as a function of temperature in the range from 15 K to 300 K. The glass transition of polyurethane was observed and the thermal expansion coefficient of the free volume was calculated. Analysis of the ortho-positronium (o-Ps) lifetime distribution shows that the distribution of the free volume in polyurethane is slightly broadened with the increasing temperature in the glassy state, and then significantly broadened with the increasing temperature above the glass transition. PACS: 78. 70. Bj, 64. 70. Pf, 65. 70. +y

Chemical Quenching of Positronium in CuO/Al2O3 Catalysts

CuO/Al2O3 catalysts were prepared by mixing CuO and γ-Al2O3 nanopowders. Microstructure and chemical environment of the catalysts are characterized by positron annihilation spectroscopy. The positron annihilation lifetime measurements reveal two long lifetime components τ3 and τ4, which correspond to ortho-positronium (o-Ps) annihilating in microvoids and large pores, respectively. With increasing CuO content from 0 to 40 wt%, both τ4 and its intensity I4 show significant decrease, which indicates quenching effect of o-Ps. The para-positronium (p-Ps) intensities derived from multi-Gaussian fitting of the coincidence Doppler broadening spectra also decreases gradually with increasing CuO content. This excludes the possibility of spin-conversion of positronium. Therefore, the chemical quenching by CuO is probably responsible for the decrease of o-Ps lifetime. Variation in the o-Ps annihilation rate λ4 (1/τ4) as a function of CuO content can be well fitted by a straight line, and the slope of the fitting line is (1.83 ± 0.05) × 10−7 s−1.

Identification of Defects in Undoped Semi-insulating InP by Positron Lifetime

Positron lifetime measurements, carried out over the temperature range of 10-300 K, have been used to investigate defects in two undoped semi-insulating InP samples. The positron lifetime spectra were analysed by both PATFIT and MELT techniques. The results at room temperature reveal a positron lifetime of around 273 ps, which is associated with indium vacancies VIn or VIn-hydrogen complexes. The positron average lifetime is temperature dependent and decreases with increasing temperature at the beginning (≤80 K and ≤120 K), and then remains unchanged, which is attributed to the influence of negative vacancies and detrapping of the positron from those negative ions of Mg, Zn, Ag and Ca with ionization level (1-).

Observation of Spin-Conversion of Positronium in Zeolite

We have measured the positron lifetime spectra of zeolite 13X as a function of exposed air pressure. Four lifetime components have been obtained and properly assigned. The longer lifetimes τ3 and τ4 are attributed to ortho-positronium annihilating in the β-cage and supercage in zeolite respectively. More interesting observation is that the positron annihilation parameters vary significantly with the exposed air pressure. It is interpreted in terms of the spin-conversion of positronium with paramagnetic oxygen.

Preparation and structural characterization of polystyrene-rectorite nanocomposites

The polystyrene/rectorite nanocomposites were prepared by free radical polymerization of styrene containing dispersed organophilic rectorite. The structures and thermal properties of these hybrids have been investigated by X-ray diffraction (XRD), fourier transform infrared (FT-IR), positron annihilation spectroscopy (PAS) and thermal gravimetric analysis (TGA) techniques. It was found that exfoliation of rectorite in polystyrene (PS) matrix was achieved. The average free-volume radius in the PS/clay nanocomposites is generally same as that in PS. Along with increment of rectorite contents, the interface between rectorite and polystyrene matrix increases, and the free-volume concentration decreases obviously. And the polystyrene nanocomposites have higher thermal decomposition temperature than pure PS.

Positron Annihilation Mechanism Correlated with Microstructure of C60-Contained Chitosan

Positron annihilation lifetime spectra measurements were performed on C60-containing chitosan as a function of temperature ranging from 290 to 380 K. Both the ortho-positronium annihilation lifetime τ3 and its intensity I3 increase with increasing temperature, however, the values of I3 in C60-containing chitosan are smaller than that in pure chitosan, and the slope of τ3 with temperature is also suppressed after C60 linked. The existing three models for positron annihilation in polymer have been compared to analyze the experiment result. It is realized that only the spur reaction model, combined with the free volume model, can explain it satisfactorily. The ortho-positronium lifetime and intensity were affected by the local physicochemical environment of material, i.e., free-volume structure, electron scavenging and trapping sites, etc. The linked C60 that play a significant chemical inhibition and quenching role on positronium formation and annihilation is first observed. The microstructure change brought by C60 group has also been discussed.

Observation of As-Grown Defects in Zn-Doped GaAs by Positron Lifetime Spectra

Positron lifetime spectra were measured for the Zn-doped p-type GaAs. In comparing the horizontal-Bridgman-method-grown and the floating-zone-method grown p-type GaAs with the liquid-encapsulation-Czochralski-grown p-type GaAs samples, positron trapping into vacancy type defects was observed in the former two grown p-type GaAs. Shallow positron traps were detected, and the dominant ones were attributed to acceptor the in p-type GaAs.

The surface structure and hydrophobic recovery of poly-dimethylsioxane insulator after ar plasma treatment

Feng Zheng1, Chunqing He1, Pengfei Fang1,∗, Zhe Chen2, Shaojie Wang1,∗, Jianguo Wang3, Xiangyang Peng4, and Zhihai Xu4 1Department of Physics and Hubei Nuclear-Solid Physics Key Laboratory, Wuhan University, Wuhan 430072, China 2Department of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China 3School of Electrical Engineering, Wuhan University, Wuhan 430072, China 4Guangdong Electric Power Research Institute, Guangzhou 510080, China

Investigation of Vacancy Defect in InP Crystal by Positron Lifetime

Abstract Positron lifetime measurements were carried out on liquid-encapsulated Czochralski-grown undoped InP samples sliced from middle of ingots over the temperature range of 10 ∼ 300 K. At 70 K, the spectra were measured in darkness, under illumination of infrared LED, and while illumination off respectively on one of samples. The measurements at low temperature revealed different concentration of hydrogen indium vacancy complex V In H 4 in these samples. A relatively higher concentration of V In H 4 existing in that grown from P-rich undoped InP melts could be shown. The increase of resistivity of these samples could be speculated when temperature was low enough.