Chen Zhi-Quan

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.

Precipitation Behavior of Al-Cu-Mg Alloy During Electric Field-Assisted Aging

In this article, we report the effect of electric field (EF) on the precipitation behavior of Al-Cu-Mg alloy. It is found that the applied field increases the hardness of Al-Cu-Mg alloy at initial aging stage. The peak of the S phase formation determined by differential scanning calorimetry also shifts to lower temperature with the applied EF. These are attributed to a lower decreasing rate of density of vacancies under field which is verified by positron annihilation measurements. The built-in field caused by the rearrangement of vacancy-solute atom complexes might lower the barrier for vacancy to escape from a complex. It leads to a longer survival time of vacancies, resulting in more possible nucleation sites for clusters.

Energy Variable Slow Positron Beam Study of Li+-Implantation-Induced Defects in ZnO

ZnO films grown on sapphire substrates are implanted with 100-keV Li ions up to a total dose of 1?1016?cm?2. Vacancy-type defects, mostly vacancy clusters, are observed by positron annihilation measurements after implantation. Upon annealing, they first have an agglomeration process which leads to the growth in the vacancy size. After annealing at about 500?C, vacancy clusters grow into microvoids, which is indicated by the positronium formation. With annealing temperature increases to above 500?C, the microvoids begin to recover, and finally all the implantation-induced vacancy defects are removed at 1000?C. No Li nanoclusters can be observed after Li+ implantation.

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.

An Experimental Study of Mg Aggregation in AA5754 Alloys by Positron Annihilation Spectroscopy

Defects in an AA5754 (Al-3.0%Mg) alloy are investigated by coincidence Doppler broadening spectroscopy and positron lifetime spectroscopy. The results indicate enhancement of positron trapping by Mg atoms in this Al-Mg alloy after quenching treatment at 623K, which may be due to the formation of vacancy-Mg complexes or the aggregation of Mg near the vacancy sites. It is speculated that the aggregation of Mg atoms in the moderate temperature range is responsible for cracking in spot welding of AA5754 alloys.

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.

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 Evolution of Defects in Deformed Cu-Ni-Si Alloys during Isochronal Annealing Studied by Positron Annihilation *

The effect of isochronal annealing on the deformation-induced defects in pure Cu and Cu-Ni-Si alloys is studied by positron annihilation spectroscopy. For the cold-rolled Cu, annealing up to 900°C causes a gradual recovery of the deformation-induced defects and monotonous decrease of the hardness. This indicates that its hardening is mainly related with defects such as dislocations. However, for the hot-rolled and quenched Cu-Ni-Si alloy, although there is a partial recovery of defects after annealing below 500°C, formation of additional defects is observed after annealing above 500°C. The hardness of Cu-Ni-Si alloy has a maximum value after annealing at 500°C, which suggests that the hardening of Cu-Ni-Si alloy is not due to defects, but primarily due to the precipitation formed during annealing. Further annealing of the Cu-Ni-Si alloy above 500°C results in over-aging effect and the precipitates lose coherence with the host matrix, which leads to positron trapping by vacancy clusters in the incoherent interface region.

Defect properties of GaAs by positron annihilation

Positron lifetime and Doppler Broadening spectra have been measured for three types of GaAs semiconductors. Direct evidence of native vacancy-type defects is found in the semi-insulating (SI-type) and n-type sample as its average lifetime {ie45-1} andS-parameter are larger than the bulk value. No positron trapping occurred in p-type GaAs. The lifetime spectrum of n-GaAs has also been measured as a function of temperature. The increase in average lifetime {ie45-2} from 226 ps to 234 ps at the temperature range 95–330 K was observed and was explained by the ionization of the vacancy. The slight increase in bulk lifetime {ie45-3} with the temperature was caused by the lattice expansion and expansion coefficient α=14×10−6K−1 was evaluated.