Positron Annihilation And Atom Probe Study Of Phase Transition And Defects For Functional Materials

Since the functional materials have various structures and advanced properties in many fields, they have attracted more attentions, and become a valuable and meaningful research topic. In order to make new system materials and enlarge their applications, it is very important and necessary to understand their microstructure and the relation between their structure and property. Positron annihilation technique (PAT) is one technology which applies nuclear physics and detection technique to physics, materials science, chemistry, biological medicine etc. Due to its high sensitivity to the phase transition and defects, it becomes one of indispensable detection techniques to study the phase transition, defects and Feimi structure etc. Three dimensional atom probe (3D AP) is one of the newest and advanced technology which can provide three dimensional analytical mapping of materials with atomic-scale resolution. In addition,3D AP is widely used to study the precipitated clusters, interface of different phases, chemical composition and atomic structure of interested area for many materials.This thesis mainly applies PAT and3D AP to study the microstructure (phase transition and defects) and their effects on properties of several functional materials, such as Mg-Zn-Y alloys, Fe-Cu alloys, Fe3O4nanomaterials, N2-xCexCuO4superconductor and ZnO/Ag nano films etc. The main research results obtained are concluded as follows.First, both PAT and3D AP are utilized to study the formation, microstucture, intrinsic defects and chemical composition of long period stacking ordered (LPSO) phases in Mg-Zn-Y alloys. Positron annihilation lifetime (PAL) results show all Mg-Zn-Y alloys with various LPSO phases have single positron lifetime, quite similar to the value for pure Mg bulk:222ps. Coincidence Doppler broadening (CDB) results show that the CDB ratio curves for all Mg-Zn-Y alloys to Mg is flat and there is no obvious Zn or Y annihilation peak, which further indicates almost all the positrons are annihilated with electrons of Mg in LPSO phases. Then two possibilities are given to explain why positrons do not detect the vacancy-size open volumes in experimental as expected by the first-principles calculations. The stacking ordered structures and chemical composition of Zn/Y enriched layers for14H and18R-LPSO phases are obtained by3D AP. The results from3D AP are found to be constant well with that from first-principles calculations, and much better than that from Energy Dispersive X-ray spectroscopy (EDS).Second, both PAT and3D AP are used to measure the effect of aging on precipitated Cu clusters for Fe-Cu alloys, in order to study the positron trapping regularity in Cu clusters as a function of temperature and aging. Based on above PAT and3D AP results, the positron trapping rates in Cu clusters for various Fe-Cu alloys are drawn as a function of temperature and aging. In addition, positron trapping models,such as difffusion-limited regime,propagation regime and combined regime are used to fit the experimental results. Then the positron trapping in precipitated particles is compared to that in vacancies, clusters and bubbles. At last, it is found that they have the same trapping regularity that positron trapping is limited by propogation at low temperature region and limited by diffusion at high temperature region.Third, PAT, X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS) etc. are used to study the effect of pressure, annealing environment and annealing temperature on defects, phase transition and electron distribution for Fe3O4. PAT results find that there are mainly two types of defects in nano FesO4, which are open volumes on surface and micro-voids on interface. Although annealing is proceeded in N2/Ar atmosphere, the oxidation is occurred to Fe3O4through XRD and XPS measurements. The reason may be due to the air taken to sample when compressing the sample or the adsorption of open volumes in nano-materials. Furthermore, all the results show the phase transition region from Fe3O4to γ-Fe2O3is below350℃, the transition region from y-Fe2O3to α-Fe203is between450℃and500℃. Due to change of crystal structure, increase of crystallite and decrease of the defects after the phase transition, the fraction of positrons annihilated at surface and interface is decreased but increased within crystallite, thus the positron lifetime and S parameter is shown to decrease, but W parameter is increased.Fourth, both temperature dependence of resistivity and PAT are utilized to measure the effet of Ce doping on the superconduct transition temperature (Tc), defects and electron distribution of Nd2-xCexCuO4superconductor. The temperature dependence of resistivity for Nd2-xCexCuO4shows that the highest Tc value is25K when x=0.15. PAL results reveal the main defects type in Nd2-xCexCuO4is Cu-vacancy and there is no difference in positron lifetime as a function of Ce doping for stationary state. However, the positron lifetime is lower in superconductivity state than that in stationary state for Nd1.85Ce0.15CuO4, which maybe caused by the decrease in Cu vacancy and increase in electron density. CDB results show after the transition from stationary to superconductivity state for Nd1.85Ce0.15CuO4, electron distribution at low momentum decrease, but increase at high momentum, which indicate the decrease in Cu vacancy and increase in electron density at CuO2plane causing appearance of superconductivity.Fifth, both PAT and photoluminescence (PL) spectrum are used to study the effect of Ag doping on quality of crystal, PL properties and defects of ZnO/Ag nano-films. Both the PAT and PL results show that the best doping thickness of Ag film is10nm. When doping thickness is high than10nm, the crystal structure and PL intensity will be worsen and decreased with increase of Ag films, since Ag films will condense to nano-clusters and diffuse in the films during annealing, which introduce some voids and deteriorate the quality of crystal. However, when doping thickness is smaller than10nm, the PL of ZnO/Ag nano-films will be enhanced with the increase of Ag films due to the enhancement of local surface plasmon polaritons (LSPPs) of Ag nano-particles.