Yu Yang Huang

The 3d Electrons and Microdefects in Binary Ti-Al Alloys Studied by Positron Annihilation

The behavior of 3d electrons and microdefects in binary Ti-Al alloys with Al contents from 47at.% to 53at.% have been studied by coincidence Doppler broadening and positron lifetime techniques. It has been found that the 3d electron signals in the spectra of binary Ti-Al alloys increase with Ti content. In Al-rich Ti-Al alloys, on increasing Al content, the open volume of defect on grain boundary increases while the density of free electron decreases. On the contrary, in Ti-rich Ti-Al alloys, on increasing Ti content, the open volume of defect on grain boundary decreases, and the electron density of the grain boundary increases.

Defects in Carbon Allotropes Studied by Positron Annihilation

Slow positron beam technique has been employed to study the defects of graphite, nanophase C, as well as the structural changes in un-doped, B-doped and S-doped diamond films with annealing temperatures. The results show that the concentration of defects in nanophase C is higher than that in graphite. The vacancy concentration in the S-doped diamond film is higher than that in un-doped one. The addition of small amount of B atoms leads to the decrease of the vacancy concentration in the film. The vacancy concentration in un-doped diamond film would decrease after annealing at temperature below 600°C, while vacancy concentration will increase after annealing at temperatures above 900°C. The vacancy concentration in the 40 Ω cm B-doped diamond film decreases at annealing temperatures higher than 200°C.

Positron Annihilation on the Surfaces of Single Crystals of Si, SiO2, Graphite and Cu

Slow positron beam technique has been applied to measure the Doppler broadening spectra for single crystals of Cu, SiO2, graphite, virgin Si, and Si without oxide film. The results show that the Cu ratio curve shows a high peak due to Cu having 10 electrons in the 3d shells. The ratio curve of SiO2 is higher than that of graphite. For the single crystal of Cu, SiO2, graphite, and Si without oxide film, the S (W) parameters decrease (increase) as positron implantation energy increasing. Defects on the surface lead to higher S (lower W) value. For the virgin Si and the thermally grown SiO2-Si samples, the S (W) parameters increase (decrease) as positron implantation energy increasing. It can be due to Si atom at surface, with two dangling bonds, tend to form silicon oxide with O. The W parameter for the single crystal of Cu is relatively high as compared with that of the single crystals of SiO2 and graphite.

Influence of Sintering Temperature on Defects in Porous Silica Monoliths Studied by Positron Annihilation Techniques

Positron lifetime and coincidence Doppler broadening spectra for porous silica calcined at temperatures from 900 to 1500 oC have been measured. As the sintering temperatures increasing from 900 to 1000 oC, the height of the peaks of the ratio curves decrease, and the τ3 increases, which is due to the exclusion of the gas and the organic substance out of the cavities. As the sintering temperatures increasing from 1000 to 1250 oC, some of the defects were recovered, the grains of low quarts appearing and growing. This gives rise to the increase of the height of the peak of the ratio curve, and the decrease of lifetime (τ1,τ2 and τ3). As the sintering temperatures increasing from 1250 to 1500oC, the height of the peak of the ratio curve decreases, and the lifetime increases due to the phase transition of low quartz to cristobalite.

Studies of Microstructure and Electro-Magnetic Transport Property of La 0.67 Ca 0.33 MnO 3 Ceramic

The positron annihilation techniques and X-ray diffraction have been used to study the microstructure of the La0.67Ca0.33MnO3 ceramics prepared by the solid-state reaction method at different sintered temperatures (T=1573K, 1623K, 1673K, 1723K, 1773K, 1823K). And the electro-magnetic transport behavior of the samples was measured by VSM and Resistivity modular on PPMS. According to these results, all samples show a perovskite structure, the ferromagnetic-paramagnetic and metal-insulator transitions occur at the transition temperature Tc and TMI, respectively, which is almost the same. For La0.67Ca0.33MnO3 sintered at 1673K, the mean positron lifetime is the largest, the maximum value of the magnetization is achieved on the magnetization-temperature curve at H=0.2mT, while the transition temperature occurs at about 244K.

Studies of Microdefects, Thermal Expansion and Magnetic Properties of Fe-Ni-Co Invar Alloys

The microdefects, the thermal expansion coefficients and the magnetization -temperature curves of the Fe64Ni36-xCox (x=1~10) were characterized by means of positron lifetime, X-ray diffraction, Michelsons interferometer and VSM modular on PPMS, respectively. The Fe64Ni30Co6 alloy is a mixture of BCC and FCC structures. With the Co content increasing in Fe64Ni36-xCox alloys, the BCC phase increases, while the FCC phase decreases. In comparison with other Fe64Ni36-xCox alloys, the Fe64Ni31Co5 alloy has a rather high magnetization at temperature lower than Tc, a relatively large change of the magnetization with the temperature near Tc, and a rather low thermal expansion coefficient.

The Microdefects, Magnetic Properties and Electron Characters of Fe-Ga Alloys

The microdefects, magnetic magnetization curves and electric characters of Fe100-xGax (x=16, 17, 18, 19, 20) were investigated by the means of X-ray diffraction, VSM modular on PPMS, positron lifetime spectroscopy and electric paramagnetic resonance spectroscopy. The X – ray diffraction patterns show that the Fe100-xGax alloys is similar to that of α – Fe, but the peaks move to low angle slightly. The result indicates Ga entered the iron lattice and the larger radium lead to the changing of the patterns. Positron lifetime spectra show that the intensity of the defects of the Fe100-xGax alloys varies obviously with different Ga content. The magnetizing curves were measured to check the saturation magnetization at room temperature. The variations of saturation magnetization do not monotonic decrease as expect with the increasing of Ga. When x=18, the magnetization is maximum; the minimum is x=20 because of the lowest iron content. The electron paramagnetic resonance spectrum shows that the peaks become narrower when the x decreases from 20 to 16. These results indicate that the addition of Ga has effect on the 3d electrons of Fe.

Effect of TiO 2 Doping on Microdefects and Electrical Properties of ZnO-Based Varistors

Positron lifetime spectrum and electrical property measurements were performed on ZnO-based ceramics doped with different contents of TiO2. For ZnO-based ceramics with TiO2 content less than 1.8 mol%, the mean positron lifetime of the ZnO-based ceramic decreases with increasing in TiO2 content, and reaches a minimum value at 1.8 mol% TiO2. As the TiO2 content higher than 1.8 mol%, the mean positron lifetime increases with TiO2 content. The ZnO-based varistor with 1.8 mol% TiO2 exhibites an optimized varistor property; it has a relatively low leakage current IL, a relatively low breakdown voltage VB, and a relatively high nonlinear coefficient α. The effects of TiO2 doping on microdefects and electrical properties of ZnO-based varistors were discussed.

Positron Annihilation Studies on La1-xSrxMnO3 Manganites

Positron lifetime and coincidence Doppler broadening spectra of La1-xSrxMnO3 and LaMnO3 manganites were measured. The effects of Sr contents on the microdefects of La1-xSrxMnO3 were investigated by means of positron lifetime results. The signals of 3d electrons of La1-xSrxMnO3 were extracted from the coincidence Doppler broadening spectra. The results show that the electron density of La1-xSrxMnO3 is larger than that of LaMnO3. The open volume of the defect in La1-xSrxMnO3 increases with the Sr contents. The signals of 3d electrons of ferromagnetic La1-xSrxMnO3 is larger than that of antiferromagnetic LaMnO3, and it reaches the maximum at x=0.33, that is, the signals of 3d electrons of the La0.67Sr0.33MnO3 is the highest in all testing samples, which corresponds to a lowest resistivity and a higher Curie temperature of the manganite. The influence of 3d electrons on the ferromagnetic double-exchange coupling and anti-ferromagnetic coupling of La1-xSrxMnO3 were discussed.

Studies of 3d Electrons and Hyperfine Interactions in Ordered and Disordered Fe50Al50 Alloys

The positron annihilation techniques and Mössbauer spectroscopy have been used to study the signals of 3d electrons and the hyperfine interactions in ordered and disorder Fe50Al50 alloys. The results show that the signals of 3d electrons of ordered Fe50Al50 alloy is very low. Mössbauer spectrum of ordered Fe50Al50 alloy shows a singlet, this alloy is paramagnetic. However, Mössbauer spectrum of Fe50Al50 powder being ball-milled for 70 h shows a discrete sextet, indicating a decrease in the long-range order within the Fe50Al50 powder being ball-milled for a long time.