In situ nuclear magnetic resonance study of defect dynamics during deformation of materials

Nuclear magnetic resonance techniques can be used to monitor in situ the dynamical behaviour of point and line defects in materials during deformation. These techniques are non-destructive and non-invasive. We report here the atomic transport, in particular the enhanced diffusion during deformation by evaluating the spin lattice relaxation time in the rotating frame, T-1p, in pure NaCl single crystals as a function of temperature (from ambient to about 900 K) and strain-rate (to approximate to 1.0s(-1)) in situ during deformation. The strain-induced excess vacancy concentration increased with the strain-rate while in situ annealing of these excess defects is noted at high temperatures. Contributions due to phonons or paramagnetic impurities dominated at lower temperatures in the undeformed material. During deformation, however, the dislocation contribution became predominant at these low temperatures. The dislocation jump distances were noted to decrease with increase in temperature leading to a reduced contribution to the overall spin relaxation as temperature is increased. Similar tests with an improved pulse sequence (CUT-sequence), performed on ultra-pure NaCl and NaF single crystals revealed slightly different results; however, strain-enhanced vacancy concentrations were observed. The applicability of these techniques to metallic systems will be outlined taking thin aluminium foils as an example

Hydrodynamic modeling of avalanche breakdown in a gate overvoltage protection structure

Abstract The breakdown of an overvoltage protection structure is analyzed in the temperature range from 298 to 523 K. The avalanche generation rates are modeled as a function of the carrier and lattice temperature. The generation rates are proportional to the carrier concentration. Careful attention is given to the pre-breakdown regime and to the breakdown process. The importance of various generation processes to the impact process is studied as well as the in¯uence on variations of the ionization threshold energy and of the energy loss during the impact process. It is shown that the carrier generation inside the junction causes adiabatic carrier cooling, which leads to dierent carrier heating eects at low and high lattice temperature. The behavior of carrier heating at room temperature is strongly aected by the asymmetric ®eld distribution inside the junction. The reason for this is the ®eld dependence of the used trap assisted band to band tunneling model and of the direct band to band tunneling model. It is shown that at room temperature, the onset of hole impact ionization plays an important role for the electron heating. This is dierent at a temperature of 523 K, where the electrons dominate the onset of impact ionization.

The role of metallic impurities in oxide semiconductors: first-principles calculations and PAC experiments

We report an ab-initio comparative study of the electric-field-gradient tensor (EFG) and structural relaxations introduced by acceptor (Cd) and donor (Ta) impurities when they replace cations in a series of binary oxides: TiO2, SnO2, and In2O3. Calculations were performed with the Full-Potential Linearized-Augmented Plane Waves method that allows us to treat the electronic structure and the atomic relaxations in a fully self-consistent way. We considered different charge states for each impurity and studied the dependence on these charge states of the electronic properties and the structural relaxations. Our results are compared with available data coming from PAC experiments and previous calculations, allowing us to obtain a new insight on the role that metal impurities play in oxide semiconductors. It is clear from our results that simple models can not describe the measured EFGs at impurities in oxides even approximately.Comment: 5 pages, 2 figures, and 1 table, published in Physica Status Solidi (b

In-Situ Nuclear Magnetic Resonance Investigation of Strain, Temperature, and Strain-Rate Variations of Deformation-Induced Vacancy Concentration in Aluminum

Critical strain to serrated flow in solid solution alloys exhibiting dynamic strain aging (DSA) or Portevin–LeChatelier effect is due to the strain-induced vacancy production. Nuclear magnetic resonance (NMR) techniques can be used to monitor in situ the dynamical behavior of point and line defects in materials during deformation, and these techniques are nondestructive and noninvasive. The new CUT-sequence pulse method allowed an accurate evaluation of the strain-enhanced vacancy diffusion and, thus, the excess vacancy concentration during deformation as a function of strain, strain rate, and temperature. Due to skin effect problems in metals at high frequencies, thin foils of Al were used and experimental results correlated with models based on vacancy production through mechanical work (vs thermal jogs), while in situ annealing of excess vacancies is noted at high temperatures. These correlations made it feasible to obtain explicit dependencies of the strain-induced vacancy concentration on test variables such as the strain, strain rate, and temperature. These studies clearly reveal the power and utility of these NMR techniques in the determination of deformation-induced vacancies in situ in a noninvasive fashion.

Dynamical <i>in situ</i> nuclear-magnetic-resonance tensile apparatus

A combination of a servohydraulic tensile machine and NMR pulse spectrometer is described enabling nuclear-spin relaxation rates to be recorded simultaneously with stress-strain data incorporating tension as well as compression of nonmetallic as well as of metallic samples. The data of the mechanical system are as follows: Maximum load: 5000 N; minimum deformation speed: 10 µm s–1, maximum deformation speed: 3×10^5 µm s–1; deformation stroke: digitally controlled between 1 and 8×10^3 µm; bandwidth: dc to 1 kHz; resolution: 2–4 µm; temperature conditions of the sample: from 80 to 570 K. The operation and performance of the system is described by means of experiments observing nuclear-spin relaxation rates which are induced by the movement of dislocations due to the finite deformation rate of the sample

Optimization of DMOS Transistors for Smart Power Technologies by Simulation and Response Surface Methods

Abstract DMOS transistors for smart power technologies were investigated by extensive use of process and device simulation. For the task of simultaneously optimizing a multitude of parameters. experimental designs and response surface methods were used

The problem of a metal impurity in an oxide: ab-initio study of electronic and structural properties of Cd in Rutile TiO2

In this work we undertake the problem of a transition metal impurity in an oxide. We present an ab-initio study of the relaxations introduced in TiO2 when a Cd impurity replaces substitutionally a Ti atom. Using the Full-Potential Linearized-Augmented-Plane-Wave method we obtain relaxed structures for different charge states of the impurity and computed the electric-field gradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, are dependent on the charge state of the impurity. This dependence is very remarkable in the case of the EFG and is explained analyzing the electronic structure of the studied system. We predict fairly anisotropic relaxations for the nearest oxygen neighbors of the Cd impurity. The experimental confirmation of this prediction and a brief report of these calculations have recently been presented [P.R.L. 89, 55503 (2002)]. Our results for relaxations and EFGs are in clear contradiction with previous studies of this system that assumed isotropic relaxations and point out that no simple model is viable to describe relaxations and the EFG at Cd in TiO2 even approximately.Comment: 11 pages, 8 figures, Revtex 4, published in Physical Review