G.K Williamson

Bio: G.K Williamson is an academic researcher from University of Birmingham. The author has contributed to research in topics: Yield (engineering) & Solid solution. The author has an hindex of 3, co-authored 3 publications receiving 6821 citations.

III. Dislocation densities in some annealed and cold-worked metals from measurements on the X-ray debye-scherrer spectrum

Abstract: Two basic equations are derived for deducing the dislocation density in powdered materials from the particle size and strain breadth measured from the Debye-Schemer spectrum. In the particle size estimate, it is assumed that the material has a block structure similar to that found in microbeam studies and that the dislocations lie along the block surfaces. The number of dislocations along each face, n, is not known. In the strain broadening estimate the x-ray line broadening from a dislocation array is calculated in terms of the broadening due to an isolated dislocation and a strain energy factor F, which allows for the effect of dislocation arrangement. Both methods involve an unknown quantity but by equating the two results it is possible in most cases to get both a narrow bracket for the dislocation density and considerable information on the dislocation arrangement. In annealed metals the values of p range from 2 × 107 cm of dislocation line per cm3 for aluminium to 3 × 108 for tungsten and m...

Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics

Abstract: We report on reversible, light-induced transformations in (CH3NH3)Pb(BrxI1−x)3. Photoluminescence (PL) spectra of these perovskites develop a new, red-shifted peak at 1.68 eV that grows in intensity under constant, 1-sun illumination in less than a minute. This is accompanied by an increase in sub-bandgap absorption at ∼1.7 eV, indicating the formation of luminescent trap states. Light soaking causes a splitting of X-ray diffraction (XRD) peaks, suggesting segregation into two crystalline phases. Surprisingly, these photo-induced changes are fully reversible; the XRD patterns and the PL and absorption spectra revert to their initial states after the materials are left for a few minutes in the dark. We speculate that photoexcitation may cause halide segregation into iodide-rich minority and bromide-enriched majority domains, the former acting as a recombination center trap. This instability may limit achievable voltages from some mixed-halide perovskite solar cells and could have implications for the photostability of halide perovskites used in optoelectronics.