Concentration Of Atomic Defects Research Articles

In-situ solid solution of Mo or Cr in WB2: Densification, microstructure, and properties

Pure tungsten diboride (WB2) ceramic prepared by the boro/carbothermal reduction combined with spark plasma sintering (SPS) of WB2 can reach a relative density >90%. However, the final sintered density still needs further improvement to meet the application requirement. In this study, the WB2 ceramics doped with Mo or Cr (W0.95Mo0.05B2 and W0.95Cr0.05B2) were prepared and the effects of Mo or Cr doping on powder characters, sintering behavior, microstructure, mechanical and physical properties of WB2 were systematically investigated. The results showed that the average particle size of the WB2 powders was increased significantly after adding either dopant, and the relative density of WB2 ceramics was improved from ∼93% to ∼95% and 96% with Cr and Mo addition, respectively. However, the Vickers hardness of WB2 with either dopant was reduced. On the other hand, the W0.95Mo0.05B2 solid solution exhibited slight improvement on both of the thermal and electrical conductivity as compared with WB2. But, both of the thermal and electrical conductivity of W0.95Cr0.05B2 solid solution decreased significantly to about 50% of the values obtained for WB2. The decrease in thermal and electrical conductivity of W0.95Cr0.05B2 solid solution was mainly attributed to the formation of high concentration of atomic defects, i.e. dislocations.

Intercalation of Ca into a Highly Defective Manganese Oxide at Room Temperature

The utilization of oxide frameworks as intercalation cathodes for nonaqueous Ca-ion batteries potentially unlocks a new energy-storage system that delivers high energy density. However, the slow kinetics of Ca2+ in oxide electrodes strongly handicaps their activity and reversibility at room temperature. Herein, nanocrystals of layered MnOx containing a high concentration of atomic defects and lattice water are shown to have remarkable electrochemical activity toward Ca2+, amounting to a capacity of ∼130 mAh/g at room temperature. Multimodal characterization revealed the notable degree of intercalation by probing the structural, compositional, and redox changes undertaken by the defective MnOx nanocrystals. The results suggest that the existence of atomic defects and lattice water played a role in improving Ca2+ diffusivity in the oxide. These outcomes reaffirm the prospects for functional Ca-ion batteries using oxide cathodes under moderate conditions.

Plane wave propagation in transversely isotropic laser-excited solids

This paper concentrates on the study of body harmonic wave propagation in an anisotropic laser-excited solid in the context of the model based on coupled equations for the displacement and atomic defect concentration fields. The complex secular equations for transversely isotropic solids are derived and discussed. It is found, in general, four types of dispersive waves, namely a quasi-longitudinal (QL), two quasi-transverse (QT) and a quasi-defect wave (N-mode) can propagate in these types of crystals. The different characteristics of waves like phase velocity, attenuation coefficient are obtained and presented graphically. It is demonstrated that there is an appreciable variation in case of the QL mode as compared with QT and N-modes. Some particular cases have also been discussed.

Small-Scale Effect on Longitudinal Wave Propagation in Laser-Excited Plates

Longitudinal wave propagation in an elastic isotopic laser-excited solid plate with atomic defect (vacancies, interstitials) generation is studied by the nonlocal continuum model. The nonlocal differential constitutive equations of Eringen are used in the formulations. The coupled governing equations for the dynamic of elastic displacement and atomic defect concentration fields are obtained. The frequency equations for the symmetrical and antisymmetrical motions of the plate are found and discussed. Explicit expressions for different characteristics of waves like phase velocity and attenuation (amplification) coefficients are derived. It is shown that coupling between the displacement and defect concentration fields affects the wave dispersion characteristics in the nonlocal elasticity. The dispersion curves of the elastic-diffusion instability are investigated for different pump parameters and larger wave numbers.

Defect disorder in UO2

A defect disorder model has been developed to determine equilibrium off-stoichiometry and its spatial variations in UO2 crystals. The model gives the concentrations of atomic defects and electronic carriers as functions of oxygen partial pressure and temperature in the bulk and near crystal surfaces subject to an oxygen environment. Energetic parameters from published density functional theory calculations have been integrated into the defect disorder model for an accurate determination of the defect density and off-stoichiometry. The ionosorption theory has been used to couple the oxygen environment with the defect state in the crystal as we solved for the defect disorder near crystal surfaces. Contrary to the common belief that hyper-stoichiometry of UO2 is dominated by oxygen interstitials, the current model predicts that this regime is rather dominated by uranium vacancies. The model predictions also show that, in the presence of surfaces, the point defect concentrations vary by orders of magnitude in the subsurface region relative to the bulk region.

Reducing Thermal Conductivity of Crystalline Solids at High Temperature Using Embedded Nanostructures

Thermal conductivity of a crystalline solid at high temperature is dominated by the Umklapp process because the number of high frequency phonons increases with temperature. It is challenging to reduce the thermal conductivity of crystalline solids at high temperature although it is widely known that, by increasing the atomic defect concentration, thermal conductivity of crystalline solids can be reduced at low temperature. By increasing the concentration of ErAs nanoparticles in In 0.53Ga 0.47As up to 6 atom %, we demonstrate a thermal conductivity reduction by almost a factor of 3 below that of In 0.53Ga 0.47As at high temperature. A theoretical model suggests that the mean free path of the low frequency phonons is suppressed by increasing the ErAs nanoparticle concentration.

The influence of Al content on the relaxation peaks in quenched Fe–Al alloys

The influence of the Al content on the two relaxation peaks (P1 and P2) in quenched Fe–Al alloys has been investigated using a multifunctional internal friction apparatus. It has been shown that the Al content has a significant effect on the two peaks and their relaxation strength varies non-monotonically with the Al content. The variation of the relaxation strength results from the changes of the properties and the concentrations of atomic defects with the Al content. The peak P1 originates from the reorientation of di-vacancies on nearest neighbor Fe-sublattice sites. The peak P2 is associated not only with vacancies but also with antisite atoms. The relaxation strength of P1 has a maximum in the vicinity of 23 at.%Al and it declines rapidly when the Al content is over 23 at.% because of gradual reduction of the concentration of di-vacancies with increasing the Al content. The peak P2 does not appear in alloys with less than 23 at.%Al. Its relaxation strength keeps roughly constant in the range of Al contents from 23 to 38 at.%, in which antisite atoms can be formed because of ordering and the number of vacancies at a site (Fe sublatices in B2 structure for Fe–Al alloys) does not change very much.

Measurement of self-diffusion coefficients in Li ionic conductors by using the short-lived radiotracer of 8Li

For the effective use of short-lived radioactive beams, soon to be available at the Tokai Radioactive Ion Accelerator Complex, the authors have developed a radiotracer method for diffusion studies in solids. The experimental test was performed by the measurement of the diffusion coefficients of Li in a sample of the compound βLiAl using an α-emitting radiotracer of 8Li (T1/2=0.84 s). It was found that the time-dependent yields of the α particles from the diffusing 8Li that was initially implanted in the sample could be used as a measure of the diffusivity of the tracer in a nondestructive way. The method was applied to measure the self-diffusion coefficients of Li in βLiGa, and for investigating how the Li diffusion in the Li ionic conductors is affected by the concentration of atomic defects (i.e., the existence of the atomic vacancies of Li and the defects in Ga sites that are replaced by Li).

Master Equations for the Concentrations of Atomic Defects in B2 Compounds

It is shown that the equations for the concentrations of atomic defects in B2 compounds obtained by the established grand-canonical approach are equivalent to the master equations derived recently by Ren and Otsuka within the framework of the empirical nearest-neighbour pair interaction model. Because the grand-canonical approach is not bound to any specific calculational method for the determination of the defect energetics, these master equations are valid beyond the pair interaction model originally used for their derivation.

Concentration of Atomic Defects in Ordered Compounds: Canonical and Grandcanonical Formalism

A canonical and a grandcanonical formalism for the calculation of concentrations of atomic defects in ordered compounds are considered. It is shown explicitly that the two formalisms yield identical results. The implications of the results for the interpretation of experimental data are discussed, and warnings against possible misinterpretations are given.

Ambipolar diffusion phenomena in BaTiO3 and SrTiO3

The electrical conductivity of resistive oxygen sensors based on semiconducting titanates (BaTiO3, SrTiO3) is determined by the amount and the ratio of the different concentrations of atomic defects in the sensor material. At sufficiently high temperatures the sensor exchanges oxygen with the surrounding gas atmosphere resulting in variations of the concentration of defects. The kinetics of these oxygen exchange processes are determined by the diffusion of the defects in the solid. In this paper the diffusion coefficients of the defects, which are decisive for the electrical conductivity, are determined by measurement of the conductivity during the diffusion processes. The evaluation of these results by an analytical model which considers the interaction between all simultaneously diffusing types of defects, allows a survey of the effects influencing the diffusion rate. With this knowledge, it is possible to estimate the influence of grain boundaries, acceptors, temperature and oxygen pressure on the response time of the sensor.

Analysis of thermally-activated flow in iron single crystals

New data obtained in an investigation of thermally-activated flow in iron single crystals agree quantitatively with the predictions of the empirical relations of Smidt (B) and with the Dorn-Rajnak lattice-hardening theory (3) at temperatures below about 200°K. Although the data could also be fitted to the solid-solution hardening theories of Fleischer (4,5) and Frank, (6,7) the predicted values for the effective stress at 0°K and the atomic defect concentration did not agree with the experimental results. Therefore, the experimental data for the orientation and temperature dependence of the critical resolved shear stress and the strain-rate sensitivity of the shear stress in iron single crystals support a lattice-hardening mechanism as controlling the low-temperature deformation of iron.

Electrical conductivity and thermogravimetric studies of single crystalline cobaltous oxide

Electrical conductivity and thermogravimetric measurements on single crystalline cobaltous oxide indicate that the electronic and atomic defect concentration is a function of the 1 4 power of the oxygen partial pressure over the temperature range of 900°–1200°C and oxygen partial pressure of 10°−10 −4atm. These results are in agreement with singly ionized cation vacancies as the predominant defect. The enthalpy of defect formation, Δ H f 0, was obtained from data of conductivity, of vacancy concentration and also determinations of constant weight for the same defect concentration. The results were 13·2, 13·8 and 13·4 kcal/mole, respectively. Activities of oxygen and cobalt in CoO relative to CoO 1.002 have been determined as well as the entropies of activation for electrical conduction and formation of nonstoichiometric disorder. Measurements of the electrical conductivity while at thermodynamie equilibrium at constant composition confirm that it is possible to quench in a constant concentration of disorder. Mobilities for electron holes at fixed compositions for these data were also determined.