Papers by Keyword: Microdefect

Abstract: Positron lifetime and coincidence Doppler broadening spectra of La_1-xSr_xMnO₃ and LaMnO₃ manganites were measured. The effects of Sr contents on the microdefects of La_1-xSr_xMnO₃ were investigated by means of positron lifetime results. The signals of 3d electrons of La_1-xSr_xMnO₃ were extracted from the coincidence Doppler broadening spectra. The results show that the electron density of La_1-xSr_xMnO₃ is larger than that of LaMnO₃. The open volume of the defect in La_1-xSr_xMnO₃ increases with the Sr contents. The signals of 3d electrons of ferromagnetic La_1-xSr_xMnO₃ 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 La_0.67Sr_0.33MnO₃ 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 La_1-xSr_xMnO₃ were discussed.

Abstract: The microdefects, magnetic magnetization curves and electric characters of Fe_100-xGa_x (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 Fe_100-xGa_x 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 Fe_100-xGa_x 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.

Abstract: 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 I_L, a relatively low breakdown voltage V_B, and a relatively high nonlinear coefficient α. The effects of TiO₂ doping on microdefects and electrical properties of ZnO-based varistors were discussed.

Abstract: The microdefects, the thermal expansion coefficients and the magnetization -temperature curves of the Fe₆₄Ni_36-xCo_x (x=1~10) were characterized by means of positron lifetime, X-ray diffraction, Michelson's interferometer and VSM modular on PPMS, respectively. The Fe₆₄Ni₃₀Co₆ alloy is a mixture of BCC and FCC structures. With the Co content increasing in Fe₆₄Ni_36-xCo_x alloys, the BCC phase increases, while the FCC phase decreases. In comparison with other Fe₆₄Ni_36-xCo_x alloys, the Fe₆₄Ni₃₁Co₅ alloy has a rather high magnetization at temperature lower than T_c, a relatively large change of the magnetization with the temperature near T_c, and a rather low thermal expansion coefficient.