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.

No abstract

No abstract

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.

The effect of the Fe2O3dopant on the electronic densities and the electrical properties has been studied in a ZnO-Bi2O3-SnO2-Co2O3-MnO2-TiO2-Ni2O3-Fe2O3system by the measurements of positron lifetime spectra, coincidence Doppler broadening spectra and current-voltage characteristics. The results show that the 3d electron signal in the spectrum of the varistor increases with the Fe2O3content. The addition of small amount of Fe2O3into the ZnO-based varistor leads to an increase in the donor concentration in the bulk and the defects in the varistor. As the Fe2O3content increased, the threshold voltage (VT) and the nonlinear coefficient (α) of the varistor decreased monotonously.