Structure Of Defects In Solids Research Articles

Bimodal Structures of Solids Obtained under Megaplastic Strain

The evolution of the defect structure of aluminum alloys of different compositions under megaplastic strain at a high quasi-hydrostatic pressure has been studied. The theoretical analysis and numerical calculation of the evolution of the defect structure of solids (metals) under this impact have been performed within the three-defect model of nonequilibrium evolution thermodynamics. The calculation of defect kinetics shows that the presence of coarse grains submerged into a matrix of fine grains at specified parameters and coefficients leads to the additional generation of dislocations and, as a consequence, to the greater dislocation strengthening of a material.

Computerized Data Reduction and Analysis in Positron Annihilation Coincidence Doppler Broadening Spectroscopy

Positron annihilation spectroscopy is a sensitive probe for studying the electronic structure of defects in solids. The high-momentum part of the Doppler-broadened spectra can be used to distinguish different elements at the annihilation site. This can be achieved by using a two-detector coincidence system, which reduces the peak to background ratio dramatically. The coincident events have to be extracted from a two-dimensional spectrum that is recorded by two high-purity germanium detectors. For this purpose the computer program MePASto was developed, which allows an automated data reduction from such Doppler-coincidence spectra, supplemented by a post-processing unit for data analysis. Additionally a case study of the identification of defect sites in an intermetallic compound is presented.

Chemical Identity of Atoms Using Core Electron Annihilations

Positron annihilation spectroscopy is a sensitive probe for studying the electronic structure of defects in solids. The high momentum part of the Doppler-broadened annihilation spectra can be used to distinguish different elements. This is achieved by using a new two-detector coincidence system and by imposing appropriate kinematic cuts to exclude background events. The new setup improves the peak to background ratio in the annihilation spectrum to {approximately}10{sup 5}. As a result, the line shape variations arising from different core electrons can be studied. The new approach adds elemental specificity to the Doppler broadening technique, and is useful in studying elemental variations around a defect site. Results from several case studies are reviewed.

Increased Elemental Specificity of Positron Annihilation Spectra.

Positron annihilation spectroscopy (PAS) is a sensitive probe for studying the electronic structure of defects in solids. We show that the high-momentum part of the Doppler-broadened annihilation spectra can be used to distinguish different elements. This is achieved by using a new two-detector coincidence system to examine the line shape variations originating from high-momentum core electrons. Because the core electrons retain their atomic character even when atoms form a solid, these results can be directly compared to simple theoretical predictions. The new approach adds increased elemental specificity to the PAS technique, and is useful in studying the elemental variations around a defect site.

Computational studies of hydrogen-related complexes in semiconductors

The two main computational techniques to calculate the microscopic electronic structure of localized defects in solids are reviewed. Cluster calculations using the Hartree-Fock method as well as density-functional methods have been widely used to determine the equilibrium structure of hydrogen and hydrogen-related complexes in semiconductors. Results for the structure of both acceptor-hydrogen and donor-hydrogen pairs in silicon obtained by various methods and at different levels of approximations are discussed. Comparisons are made among the cal­culated vibrational frequencies of hydrogen in these passivation complexes. The spin density distribution for bond-centred hydrogen or muonium in diamond is investigated. Owing to the light masses of the proton and the muon it is neces­sary to average the computed hyperfme parameters over the finite extension of the ground-state wave function.

Application of Positron Annihilation Techniques in the Field of Ceramics

The importance of positron annihilation techniques in investigating the defect structure of solids is now well established. The distinct advantage of these techniques in such studies arises due to the sensitivity of positrons to changes in the local electron density near the defect. Metals and alloys are the most extensively studied materials by these techniques. In recent years, however, the interest is steadily growing in the ceramic systems. The present paper highlights the potential of this new and powerful technique in understanding the lattice defect structure of ceramics.

Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2γ annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 A. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.

The Structure of Defects in Solids

MORE DIRE CT INVE STIGATIONS OF POINT DEFE CTS IN OXYGE N-DE FICIE NT RUTILE .. . .. .... .. ... .. .. . .... . . . . .... . . .. ... .. ..... .. . .. ... . . . . .. . . . . 69 Interstitial titanium ion..... . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 70 Reduction time as an experimental variable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 75 Di-interstitial of titanium ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 76 Oxygen ion vacancy and vacancy-impurity complexes. . . . . . . . . . . . . . . . . . . . . .. 79 Interstitial-impurity complexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 80 Plane defects . . . . .... ... . . .. ... : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 POINT DEFECT-DISLOCATION INTERACTIONS AS OBSE RVE D BY RELAXATIONAL INTE RNAL FRICTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Relaxational peaks due to dislocation unpinning. . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Theories of Hasiguti peaks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 85 Dislocation string theories ..... .... . ......... .. ..... .. . ... .. ... .. . .. . . 86 Peak temperature versus stress amplitude. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

The Structure of Defects in Solids

Gallium is a metal that literally melts in your hand. It has low toxicity, near-zero vapor pressure, and a viscosity similar to water. Despite possessing a surface tension larger than any other liquid (near room temperature), gallium can form nonspherical ...Read More