Electrical Properties And Photocatalytic Effects Study On Functional Materials By Positron Annihilation Technique

The microstructure of functional materials is the internal factors to determine its functional properties and functional parameters. And the machining process parameters of functional materials directly affect the microstructure of functional materials. Positron annihilation spectroscopy is an advanced non-destructive detection technology for studying material microstructure defects. It has been widely applied in the fields of solid state physics, materials science, surface and interface system, as well as biological and medical research. Positron annihilation spectroscopy is based on the detection of electron-positron annihilation gamma photons of time, energy, momentum information to analyze the changes of material structure and defects. We can get the material microstructure of electron density, electron momentum distributions and defect state distribution as well as other structural information. Positron annihilation spectroscopy methods mainly contain Positron annihilation lifetime spectra (PALS), Doppler broadening spectra (DBS), slow positron beam technology and so on. In this thesis, the variety of experimental positron annihilation technique, combined with conventional testing methods were used to study the microstructure defects evolutionary mechanisms of several functional materials, and to discuss microstructure defects on the physical effect mechanisms of the macro performance of functional materials.The major research achievements of this doctoral dissertation are as follows:1) The positron annihilation technique (positron annihilation lifetime spectroscopy and coincident doppler broadening spectroscopy), combined with X-ray diffraction and scanning electron microscopy was used to investigate a new type non-ferroelectric piezoelectricity SrTi03-Bi12Ti02o (ST-BT) composite ceramics, which were prepared by conventional solid-state reaction route in Shandong University. In addition, the dielectric properties and piezoelectric properties were measured to explore the link between the ST-BT composite ceramic structural defects and macroscopic electrical properties. The result of this definite link provides convincing evidence that the flexoelectric effect is responsible for non-ferroelectric piezoelectricity.The experimental results indicate that the structural characteristic of ST-BT composite ceramics shows good stability with the sintering temperature between920 to940℃and the sintering time between3to5hours. Divacancy complex defects are formed by the association of the single-vacancy defects in the sintering temperature of960℃or longer sintering time. However, these divacancy complex defects are the foundation stone of contributions to the high dielectric constant and stable dielectric frequency characteristics. The piezoelectric effect appeared concurrently with a high concentration single-vacancy defects in the ceramics, suggesting that it may be closely related to the microstructure. This further proves that the flexoelectric effect is the origin of non-ferroelectric piezoelectricity.2) Slow positron beam technique was used to investigate the defect mechanism of Fe doped TiO2(Fe-TiO2) films. The effect of the defect mechanism on the photocatalytic activity of Fe-TiO2films was discussed in the dissertation. The experimental results provide the theoretical basis for improving photocatalysis efficiency of Fe-TiO2films.Fe3+doping into TiO2leads to a significant enhancement on photocatalysis as well as complex defects. A series of Fe doped TiO2(Fe-TiO2) films with different ratios of Fe to Ti were prepared by the sol-gel dip and the effects caused by defects on photocatalytic activity were studied by X-ray diffraction (XRD), UV-vis absorbance spectra, positron annihilation spectroscopy (PAS) and the degradation experiment of methyl orange. For different Fe3+doping concentrations, it was found that the evolution of S parameter, which was obtained from PAS and reflects the comprehensive effects from various defects, matches well with the evolution of photocatalytic activity. While the S-W plot well reveals the defects mechanism in Fe doped TiO2, it was found that both the bulk defects and the surface defects in Fe-TiO2films play very important roles in photocatalysis. These results for the first time experimentally prove that defects are the essential origin of photocatalysis enhancement.3) Positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy based on the slow positron beam have been used to investigate three kinds of CuInSe2solar cell thin films. The films were grown on Mo-coated soda lime glass substrates by the electrochemical deposition processing technique. As-grown samples are found to contain large concentration of vacancy defects. The selenium (Se) atmosphere and sulfur (S) atmosphere annealing of as-grown samples at800K can dramatically reduce the number of vacancy defects and the film becomes crystalline. In addition, a defect layer of about50nm thickness was observed for the first time at the surface of the CuInSe2thin film. This layer results from the electrochemical deposition method, but the defect concentration in defects layer can be greatly reduced by annealing in selenium atmosphere. The Doppler broadening line shape parameter correlation plot provided evidence that the positron trapping defect states where in three samples. These discussions provide theoretical foundation for improving light absorption efficiency of CuInSe2thin film.4) The positron annihilation technique was applied to study the microscopic structure of the polycrystalline ceramic barium titanate with different sinter process. The average lifetime of positron in the barium titanate ceramics matches to the dielectric properties of those samples. It reflects the dielectric properties are determined by space-charge-limited current. In addition, as a function of Sn doping concentration x, dielectric properties and X-ray diffraction measurements were carried out on BaTi1-xSnxO3(BTSx) ceramics. Positron annihilation lifetime spectroscopy and coincident Doppler-broadening spectroscopy were also measured for the evaluation of defects in the BTSx ceramics. This change of permittivity is found to agree well with the relative defect concentration estimated using two positron annihilation techniques. The variation correlations between defects and dielectric properties further proves that BTSx ceramics with the higher relative defect concentration present a lower permittivity.These results indicate that positron annihilation technique is a sensitive prob to study material microstructure defects. The positron annihilation technique, combined with measurements of macroscopic electrical, magnetic and optical performance can clarify the microscopic mechanism of the changes in macroscopic properties. Therefore, they can provide important information for preparation of new functional materials with superior performance.