Polyanion Conduction Mechanism in Solid Scandium Tungstate

Scandium tungstate is investigated as a model material for solid electrolytes in which polyatomic anions, here WO42–, are mobile in the solid state. Simulations using structures with artificially induced WO42–vacancy, Frenkel defect and Schottky defects produced lower activation energy compared to the initially defect-free model. Simulations with Frenkel defect structures show low activation energy but the interstitial WO42–has initially a strong preference to return to the vacant tungstate site. The vacancy defect model reproduces the activation energy to the experimental conductivity studies more closely. Qualitative considerations support the idea that vacancies formed during the sample preparation are the most abundant mobile defect among the investigated cases. Nonstoichiometric samples with varying initial Sc2O3:WO3ratios Sc2O3- x WO3, where x = 2.9, 3.0 and 3.1, are synthesized and characterized by XRD and impedance measurements, but a significant influence on the conductivity could not be confirmed experimentally.

1/f NOISE DUE TO THE RETURN STATISTICS OF MOBILE POINT DEFECTS IN EXTRINSIC SEMICONDUCTOR MATERIALS

We investigate fixed dopants in the presence of mobile point defects like foreign atoms or vacancies. A mobile defect entering the first Bohr radius RB of a dopant will modulate the generation-recombination process. The times a defect walks inside or outside RB are shown to be power-law distributed giving rise to 1 / fb noise. The predicted Hooge coefficient αdef depends on RB, on the normalized fluctuations of charge carriers and on the number of charge carriers compared to lattice sites; our model suggests that the magnitude of 1/ f noise can be decreased at will by increasing the ionization of dopants.

Kelvin Probe Microscopy and Cathodoluminescence Microanalysis of the Irradiation Induced Modification of Insulating Materials.

ABSTRACTA combination of Kelvin Probe Microscopy (KPM) and Cathodoluminescence (CL) microanalysis has been used to characterize ultra pure silicon dioxide (SiO2) exposed to electron irradiation in a Scanning Electron Microscope. Charged beam irradiation of poorly conducting materials results in the trapping of charge at pre-existing or irradiation induced defects thereby inducing a localized electric field within the irradiated micro-volume of specimen. The residual surface potentials associated with the localized electric field have been mapped using KPM. Evidence of electro-diffusion and defect micro-segregation in charged beam irradiated SiO2 is observed. The associated mobile defect species are identified using CL microanalysis techniques. The high correlation between KPM and CL images confirms the significant influence of localized potentials on the microstructure of technologically important SiO2.

Atomistic Simulation of Mobile Defect Clusters in Metals

The structure, stability and thermally-activated motion of interstitial and vacancy clusters in Fe and Cu have been studied using atomic scale computer simulation. All studied interstitial clusters and perfect interstitial loops (PILs) in Fe are mobile whereas their mobility in Cu can be suppressed at large sizes (bigger than 49–61 self-interstitials depending on the temperature) due to dissociation. A comparative study of relaxed configurations has shown that the structure of small perfect dislocation loops of vacancy and self-interstitial nature is very similar. Molecular dynamics simulation has demonstrated that small perfect vacancy loops (PVLs) in Fe consisting of more than 37 vacancies are stable over a wide temperature range and produce atomic displacements by a thermally-activated movement in the direction of the Burgers vector. The mechanism is qualitatively similar to that of SIA clusters studied earlier. Motion of vacancy loops in Cu does not occur because they transform into sessile configurations similar to stacking fault tetrahedra. These results point to the possibly important contribution of vacancy loop mobility to the difference in radiation damage between bcc and fcc metals, and between fcc metals with different stacking fault energy.

Atomistic Simulation of Mobile Defect Clusters in Metals

The structure, stability and thermally-activated motion of interstitial and vacancy clusters in Fe and Cu have been studied using atomic scale computer simulation. All studied interstitial clusters and perfect interstitial loops (PILs) in Fe are mobile whereas their mobility in Cu can be suppressed at large sizes (bigger than 49-61 self-interstitials depending on the temperature) due to dissociation. A comparative study of relaxed configurations has shown that the structure of small perfect dislocation loops of vacancy and self-interstitial nature is very similar. Molecular dynamics simulation has demonstrated that small perfect vacancy loops (PVLs) in Fe consisting of more than 37 vacancies are stable over a wide temperature range and produce atomic displacements by a thermally-activated movement in the direction of the Burgers vector. The mechanism is qualitatively similar to that of SIA clusters studied earlier. Motion of vacancy loops in Cu does not occur because they transform into sessile configurations similar to stacking fault tetrahedra. These results point to the possibly important contribution of vacancy loop mobility to the difference in radiation damage between bcc and fcc metals, and between fcc metals with different stacking fault energy.