Point Defect Relaxation Research Articles

Defect Concentration Relaxation During Interdiffusion Between MnO and FeO

AbstractFor the diffusional equilibration process between crystals of MnO and FeO it was investigated how far the usual assumption of local thermodynamic point defect equilibrium holds (given the intensive variables, T, P, and μo2 (gas)). To this end, accurate component concentration profiles which were normalized for the given linear diffusion geometry with t, have been determined. From the fact that these profiles do not coincide for different diffusion times it is concluded that deviations from the local point defect equilibrium occur. The largest deviations are found at the beginning of the interdiffusion process. The point defect relaxation (cation vacancy annihilation) takes place preferentially at the outer surfaces of the crystals, which have access to the oxygen gas atmosphere. — These cation vacancy processes can be studied directly with the help of a solid state galvanic cell, in which the evolving oxygen gas from the crystal surface is coulometrically titrated. The experimental data are in accordance with theoretical calculations which have been performed in order to understand under which conditions point defect concentration relaxation has an influence on chemical interdiffusion processes.

Point defect relaxation in silicon single crystals

Abstract Isothermal internal friction measurements reveal a relaxation peak in the temperature range 360–470 K for vibration frequencies between 10−3 and 10 Hz. The temperature range and the fact that the relaxation parameters are not sensitive to the structural state of the specimens indicate a point defect relaxation mechanism. Since the controlled defects in the bulk cannot explain the relatively high relaxation strength observed, we propose a mechanism based on stress-induced self-interstitial reorientation, this type of defect being the only one to be present in a sufficiently high concentration in our samples.

Local Defect Equilibria: The Conceptual Difficulties in Treating Solid State Reaction Kinetics

AbstractFor an AO — BO interdiffusion process a formal analysis is given. The conceptual difficulties in using either structural elements, building units or components in the framework of irreversible thermodynamics are discussed. The matrix of transport coefficients of a n‐component system is evaluated and is transformed into a matrix of diffusivities. The interdiffusion problem has been solved numerically, starting with a set of two coupled diffusion equations. Point defect relaxation during interdiffusion has been taken into account through a quasi‐homogeneous point defect relaxation term. The calculated supersaturation and undersaturation of point defects result in concentration profiles which cannot be normalized with √t. Experiments with the diffusion couple FeO — MnO demonstrate the theoretically predicted behavior and the influence of the cation vacancy relaxation at the sample surface on the concentration profiles.

Point defects in GaP single crystals investigated by mechanical damping

AbstractThe dependence of mechanical damping of LEC (liquid encapsulated Czochralski) crystals of GaP: S was measured in the temperature range 300… 600 K and at frequencies near 70 and 140 kHz, respectively. In samples of different stoichiometry two peaks could be observed: Peak A at 350 K and peak B at 450 K. Peak A is attributed to the relaxation of point defects near the surface, peak B is probably due to Ga vacancies.

The effects of Cr2O3 and Fe2O3 additions on the thermal conductivity of Al2O3

Thermal conductivity data for a series of Al2O3, Al2O3-Cr2O3, Al2O3-Fe2O3, and Al2O3-Cr2O3-Fe2O3 samples are reported. The results, which span the temperature range 300–360 K, show that grain size has an important effect on the thermal conductivity of Al2O3, and this effect is larger than would be anticipated from standard theories. Results for seven Al2O3-based solid solutions were examined in terms of a Callaway formula, and it was found that the point defect relaxation times were principally dependent on solute content and the mass difference between the solute and solvent. The data support a theory that suggests that a heavy atom in a light matrix scatters less effectively than a light atom in a heavy matrix. The data do not show that isolated point defects are more effective phonon scatterers than point defects in a concentrated solution.

Defect Structure and Transport Properties of Manganese Oxides: (III) Relaxation Kinetics of Manganosite (Mn1‐ΔO) in CO/CO2 Gas Mixtures

AbstractThe kinetics of point defect relaxation in manganosite after sudden changes of the oxygen activity in a surrounding CO/CO2 gas atmosphere have been studied by means of electrical conductivity measurements on single crystalline needle‐like samples at 1000°C. The results show that at very low oxygen activity the relaxation process is essentially controlled by diffusion processes in the bulk, but that at high oxygen activity the oxygen transfer at the manganosite/gas interface determines the kinetics.

Defects and Cation Diffusion in Magnetite (VI): Point Defect Relaxation and Correlation in Cation Tracer Diffusion

AbstractChemical diffusion coefficients have been derived from experiments on the point defect relaxation in magnetite. Based on data from preceding studies on the nonstoichiometry of magnetite and on the iron tracer diffusion of this oxide, the chemical diffusion coefficients are analyzed with regard to elementary diffusion processes and the correlated motion of tracer cations.

Dislocation bias and point-defect relaxation volumes

In the theory of irradiation swelling, a convenient measure of the preference for interstitial absorption at a dislocation is the bias, B, which depends on the energies of interaction of vacancies and interstitials with a dislocation. These energies in turn depend on the relaxation volume of each point defect Δυ. Furthermore, bias is a function of dislocation density. Using recently computed values for Δυ i and Δυ i, over the range of dislocation density from 10 9/cm 2 to 10 11/cm 2, the bias in copper ranges from 0.07 to 0.12 and that in nickel from 0.30 to 0.41. Criticism by Green and Weertman of a previous paper is shown to be invalid.

Point-defect relaxations in dilute iron-nickel alloys after low-temperature neutron irradiation

Point-defect relaxations in dilute iron-nickel alloys after low-temperature neutron irradiation

Anelastic studies of intrinsic atomic defects

The use of the techniques of anelastic relaxation for the study of intrinsic point defects in metals is considered, with particular emphasis on defects produced by irradiation. The general principles of point defect relaxations, and the information about the defects which is obtainable from such studies, is first reviewed. Existing data, particularly for fcc and bcc metals, are then examined and interpreted. Relaxations attributable to close pairs and to isolated interstitials are identified and discussed. Other relaxations, which appear at higher temperatures, can be attributed to small interstitial clusters as well as to impurity-interstitial interactions. Finally, the author's assessment of fruitful directions for further investigation is presented.

Dielectric relaxation of point defects in α-quartz

A study of dielectric relaxation, primarily of synthetic crystals of α-quartz, in the temperature range from 10 to 200 °K shows four peaks. Two of the peaks which fall at 32 and 76 °K (frequency 2 kHz) are the same as those observed earlier by Stevels and Volger and attributed to Al3+Na+ substitutional-interstitial (s−i) pairs. The 76 °K peak was observed for X-cut as well as Z-cut samples, confirming a prediction of Nowick and Stanley. The other two peaks, which fall at 100 and 138 °K are apparently related to each other, but differ from any other peaks previously reported. They are believed to be due to a different s−i pair. A model is developed which provides a unified approach to the various peaks due to s−i pairs in α-quartz. Eine Untersuchung der dielektrischen Relaxation, hauptsächlich von synthetischen α-Quarz-Kristallen, im Temperaturbereich von 10 bis 200 °K ergibt vier Maxima. Zwei von diesen Maxima, die bei 32 und 76 °K (bei 2 kHz) liegen, sind dieselben, die früher von Stevels und Volger gefunden wurden, und von Al3+Na+-Substitutions-Zwischengitter-(s-i)-Paaren herrühren sollen. Das 76 °K-Maximum wurde sowohl für X-Schnitt- als auch für Z-Schnitt-Proben beobachtet, wie von Nowick and Stanley vorhergesagt. Die anderen zwei Maxima, die bei 100 und 138 °K liegen, sind miteinander verknüpft, sie sind jedoch verschieden von allen anderen Maxima, von denen früher berichtet wurde. Es wird vermutet, daß sie von anderen s−i-Paaren herrühren. Ein Modell wird vorgeschlagen, das ein einheitliches Bild über die verschiedenen Maxima gibt, die von s−i-Paaren in α-Quarzen herrühren.

Diffusional Relaxation of Crystal Lattice Defects in the Fields of Local Lattice Disturbances

AbstractDiffusional relaxation of mobile defects in crystals containing local disturbances is considered. It is shown that the distribution density of the inverse relaxation times in such a system calculated as the imaginary part of the trace of the diffusion equation Green's function may involve resonance maxima. Their positions are determined by the interaction potential of the mobile defect and the immobile local perturbation center. The relevant internal friction peaks when detected provide a possibility of investigating this potential. The distribution density of the inverse relaxation times is calculated for a number of particular systems which model the diffusional relaxation of point defects in the local fields of a point disturbance and of a plane defect and the kink relaxation in the presence of a disturbance. It is shown that several internal friction peaks can be associated with a single relaxation mechanism.

Internal friction maxima due to relaxation of point defects in neutron irradiated aluminium

The internal friction and shear modulus of pure recrystallized polycrystalline aluminium wires has been studied as a function of temperature by measurement of the logarithmic decrement of the free vibrations (64 and 200 c/s) in shear strain (ϵ ≈ 10 −5) after irradiation with fast neutrons at 4.6°K. The elementary processes related to these internal friction maxima are discussed in terms of the corresponding elastic dipole tensors and the annealing model for irradiated aluminium.

Sensitivity of point defect characteristics to the interatomic potential range

Abstract Point defect characteristics, including configuration and relaxation energies of vacancies and interstitials, migration energies of vacancies, and binding energies of divacancies, are determined by computer simulation using models representing copper and α-iron. The sensitivity of these characteristics to small variations in the range of the interatomic potential is studied using potentials of the Morse type modified to include third or fourth neighbors. Vacancy relaxation energies were found to increase and migration energies decrease with increased range. Divacancy binding energies were generally insensitive to range, but were appreciably higher than found with first and second neighbor potentials. The energies of interstitial configurations varied markedly with range, but retained their relative order with one minor exception. The lowest energy configuration for copper was the (100) split, in agreement with previous work. The lowest energy configurations for α-iron, however, were the crowdions, rather than the (110) split configuration found in previous work using shallower second neighbor potentials. The significance of arbitrary potential cutoffs and small discontinuities is discussed.