Study Of Defects In Metals Research Articles

The equilibrium measure for an anisotropic nonlocal energy

In this paper we characterise the minimisers of a one-parameter family of nonlocal and anisotropic energies I_alpha defined on probability measures in {mathbb {R}}^n, with nge 3. The energy I_alpha consists of a purely nonlocal term of convolution type, whose interaction kernel reduces to the Coulomb potential for alpha =0 and is anisotropic otherwise, and a quadratic confinement. The two-dimensional case arises in the study of defects in metals and has been solved by the authors by means of complex-analysis techniques. We prove that for alpha in (-1, n-2], the minimiser of I_alpha is unique and is the (normalised) characteristic function of a spheroid. This result is a paradigmatic example of the role of the anisotropy of the kernel on the shape of minimisers. In particular, the phenomenon of loss of dimensionality, observed in dimension n=2, does not occur in higher dimension at the value alpha =n-2 corresponding to the sign change of the Fourier transform of the interaction potential.

Defects in Ceramic Oxides

The topic that I am attempting to cover easily requires a whole book to do it justice. To deal with it in a short article, I will focus on a few basic principles that dominate the behavior of defects in oxides and then present some examples taken from well-studied and relatively well-understood materials.The study of defects in oxides differs from the study of defects in metals and alloys or in semiconductors in several important ways. First, oxides generally have very high melting points so that intrinsic point defect concentrations at temperatures below ~1500°C are usually negligibly small. Second, an oxide does not always maintain the composition at which it was fabricated, as alloys do, but can exchange oxygen with the ambient atmosphere, according to the ambient partial pressure of oxygen, P(O2). Thus, the stoichiometry, and accordingly the defect concentration, can be controlled by annealing in a specific P(O2) atmosphere. Third, and most important, oxides have a high degree of ionicity so that most point defects possess an effective charge. The consequences of this fact are far reaching; some of them are as follows:1. In a binary oxide, e.g. a metal oxide (MO), there are two sublattices. The diffusion of cations and of anions proceeds each on its own sublattice, involving different point defects. Accordingly, cation and anion diffusion rates can be very different and, in fact, usually are.

The Study of Defects in Metals Using High Resolution Transmission Electron Microscopy and Atomistic Calculations

AbstractThe coupling of HRTEM with atomistic calculations is described for the study of grain boundaries and dislocations in aluminum. HRTEM images of the Σ9 (221) [110] grain boundary are compared with molecular statics calculations using both the Embedded Atom Method (EAM) and two pair potentials. Comparison between observed and simulated images are shown to serve as a stringent test of the theoretical methods. Atomistic calculations can in turn provide threedimensional information about the defect structure. Using the EAM, it is also possible to account for the effects of thin foil geometries on the minimim energy configuration of defects. While these effects are found to be minimal for grain boundary structures, the influence of the thin-foil geometries on the core structure of the 60° dislocation in aluminum is discussed. These comparisons indicate that the EAM function provides a good description of grain boundary structures, but fails to reproduce the observed dislocation core structure due to a low predicted value of the intrinsic stacking fault energy (SFE) on the (111). In contrast, the pair potentials used in this study provide reasonable SFE values, but appear to be less accurate for the prediction of the Σ9 (221) [110] grain boundary structures.

Feasibilities of nuclear techniques for the study of molecular defects in metals and semiconductors

Nuclear techniques, like Mossbauer effect and perturbed γγ angular correlation spectroscopy (PAC), have proven themselves to be sensitive tools for labelling and identifying probe atom-defect complexes. In these experiments, the “molecular defect” is investigated via the nuclear hyperfine interaction, which is measured at the site of the radioactive probe atom. Here, we shall put the emphasis on the PAC spectroscopy, which often uses111In/111Cd as radioactive probe atom. In metals, based on the identification of simple probe atom-defect pairs, the agglomeration of defects after cold-working and the interaction of vacancies with He atoms will be discussed. In semiconductors, it will be focussed on the interaction between dopant atoms, which strongly determines the electrical properties of these materials.

Defects in metals

Recent study of defects in metals using Mossbauer spectroscopy is presented. Vacancy and interstitial atoms are focussed as defects in metals. General remarks for the study of defects are presented first and then, as examples of the study,119Sb and57Co Mossbauer measurements to monitor the trapping and detrapping process of quenched-in vacancies in Au are discussed with the aid of the complementary techniques which are resistivity measurement, computer simulation and positron lifetime measurement. Detailed results from these complementary techniques are described. Recent theoretical calculations for defect-associated states are also presented briefly.

The Study of Point Defects in Metals by Perturbed Angular Correlation and the Mössbauer Effect

The Study of Point Defects in Metals by Perturbed Angular Correlation and the Mössbauer Effect

The positive muon and the positron as probes of defects

A brief but hopefully general comparison has been made between muons and positrons as probes for the study of defects in metals. Since muon experiments are not only more demanding in manpower, cost and availability than positron experiments, they should be carefully designed in light of the knowledge that the muon is extremely sensitive to both intrinsic and extrinsic defects. Initial experiments should provide estimates of the muon diffusion coefficients as a function of sample temperature. High temperature hydrogen diffusion measurements do provide quidelines, although so far most of the observations of muon trapping have been made at low sample temperatures where hydrogen diffusion data do not exist. Given that the diffusion constant is known as a function of temperature, high-purity Fe26 after low temperature electron irradiation is therefore a good candidate to study with muons. Since the defect type and concentration can be controlled in electron irradiated samples, such investigations could confirm the stated values of the diffusion constants in Fe thereby providing a new method for evaluating diffusion coefficients below Stage III in different metals.

Positron annihilation — a localized probe of lattice defects in metals

It is evident that positron annihilation spectroscopy has become a valuable technique for the study of metals. Owing to the ability of the positron to localize in a variety of metal defects where its subsequent annihilation yields information regarding its local electronic environment, we now have a new and unique tool available with which to probe the nature of lattice defects in metals, which supplements previously available methods. Many new PAS experimental and theoretical techniques and additional applications to problems concerning defects and microstructures in metals are evolving in this rapidly developing area. It seems clear that in the future, the usefulness of PAS for the study of defects in metals will continue to increase.

On the study of defects in metals with positive muons

The technique of positive muon spin depolarization is reviewed, and its sensitivity to the presence of defects in metals is discussed. Results are presented of an experimental survey on copper, aluminum, indium and beryllium. There are indications of μ+ trapping at defects in neutron-irradiated and deformed copper, in indium near its melting point, and in beryllium. The results are discussed in the context of muon diffusion.

A computer-controlled time-of-flight atom-probe field-ion microscope for the study of defects in metals

A time-of-flight (TOF) atom-probe field-ion microscope (FIM) specifically designed for the study of defects in metals is described. This atom probe features (i) a variable-magnification internal image-intensification system based on a channel electron multiplier array for viewing the FIM image; (ii) a liquid-helium-cooled goniometer stage which allows the specimen to be maintained at a temperature anywhere in the range 13-450K; (iii) a low-energy (<or=3 keV) gas ion gun for in situ irradiations; (iv) an ultra-high-vacuum (about 4*10-8 Pa) chamber to minimize specimen contamination; (v) a high-vacuum (about 1.33*10-4 Pa) specimen-exchange device; (vi) a Chevron ion detector; and (vii) an eight-channel digital timer with +or-10 ns resolution for measuring the TOFS of the pulsed-field-evaporated ions. The entire process of applying the evaporation pulse to the specimen, measuring the DC and pulse voltages and analysing the TOF data is controlled by a Nova 1220 computer.

A time-of-flight atom-probe field-ion microscope for the study of defects in metals

An ultra-high vacuum time-of-flight (TOF) atom-probe field-ion microscope (FIM) specifically designed for the study of defects in metals is described. Performance experiments show that this instrument can clearly resolve the seven stable isotopes of molybdenum, the five stable isotopes of tungsten, and the two stable isotopes of rhenium in a tungsten-25at.% rhenium alloy. The entire process of applying the evaporation pulse to the FIM specimen, measuring the dc and the pulse voltages, and analyzing the TOF data is controlled by a Nova 1220 computer. With this automated system we can presently record and analyze 600 TOF events min −1.

Applications of positron-lifetime measurements to the study of defects in metals

Applications of positron-lifetime measurements to the study of defects in metals