Papers by Keyword: Vacancies

Abstract: By the method of thermal oxidation of n-type CuInSe2 crystals, n - n+ structures with a maximum absolute current photosensitivity of up to 10 mA / W were obtained at low rectification and no-load photovoltage. The used modes of thermal oxidation led to the formation of n-type layers on the surface of the n-CuInSe2 plates, the resistivity of which is 2-2.5 times higher concerning the initial substance. Measurements of the stationary current-voltage characteristics have shown that the structures obtained have a slight rectification K. All the structures obtained exhibit photosensitivity, which dominates when illuminated from the side of the layer in the spectral region of about 1 eV. The optimization of the process can reveal the technological possibilities of a significant improvement in the rectifying properties of isotypic structures based on CuInSe2.

Abstract: The analysis of the strength characteristics and electrical conductivity of annealed Cu-1.0wt.%Cr-0.1wt%Zr alloy subjected to subsequent deformation under various conditions was carried out by analytical modeling methods. The contributions of the regions with nanotwins, as well as such defects of the crystal structure as vacancies, alloying atoms, dislocations, and particles of the secondary phase to the strength and electrical conductivity of the material were estimated.

Abstract: Polycrystalline Bi_1-xCuSeO (0 ≤ x ≤ 0.05) ceramics were prepared by self-propagating high-temperature synthesis followed by spark plasma sintering method. All the samples correspond with single BiCuSeO phase and high vacancies sample had higher density. The highest power factor of 4.71×10^-4 W.m^-1.K^-2 was obtained by 5% Bi vacancies at 873K, which is about 32% higher than that of the pristine sample. Along with slight reduction of thermal conductivity, the maximum ZT reached 0.68. The results show that vacancy engineering is a promising method for thermoelectric applications of BiCuSeO and related ceramics.

Abstract: Natural aging during storage of Al-Mg-Si alloys at room temperature can significantly reduce the maximum strengthening potential (T6) during artificial aging and, therefore, is a key topic in aluminium research and industry. Many different strategies to understand and reduce the negative effect of natural aging have been investigated during the last decades, including analysis of different thermal pre-treatments and considering the effect of different microalloying elements. From these investigations, the vacancy evolution and the formation of clusters containing Mg and Si were found to be the governing aging mechanisms behind natural aging. In this work, we present a model to simulate and predict the behavior of these alloys when subjected to room temperature aging after solutionizing and demonstrate the effects of different thermal routes and chemical composition variations. In the implemented model, the evolution of excess quenched-in vacancies and the effect of solute vacancy traps are considered. Special emphasis is placed on co-cluster formation and its contribution to strengthening. The thermokinetic software MatCalc is used for the simulations and the results of the simulations are validated by experimental investigation.

Abstract: To verify the assumption that the anelastic relaxation effect observed in Ni₃Al is due to stress-induced reorientation of antisite Al atoms [Numakura and Nishi, Mater. Sci. Eng. A 442 (2006) 59-62], the magnitudes of the anisotropic distortion produced by the intrinsic point defects have been evaluated by ab initio calculations. The anisotropy of the λ tensor (the strain per unit concentration of a particular defect) for the two candidate defect species, namely a Ni vacancy and an antisite Al atom, has been computed by full structure optimization of a supercell containing a single point defect: the difference in the principal values is +0.46 and −1.12, respectively. The relaxation strength estimated for antisite Al atoms agrees fairly well with experiment, while that for Ni vacancies is far too small because of their much lower concentration. The relaxation is, therefore, conclusively attributed to antisite Al atoms.

Abstract: As a result of the work of Frank-Read dislocation source the shear zone is formed. It is filled with deformation defects forming as a consequence of the dynamic features of the motion of dislocation loops and due to the interaction of shear forming dislocations with dislocations of non-coplanar slip systems. The accumulation of jogs on screw segments leads to the fact that the edge segments are moving faster than the screw segments so the shear zone is swept out generally by screw segments. The expressions of the intensities of the deformation defects accumulation in shear zones are given in the article. The point defects plays special role in the formation and evolution of misorientation substructures into deformed monocrystals, polycrystals and nanocrystals.

Abstract: Elastic fields, generating by defects of the structure, influence the diffusion processes. It leads to the alteration of the phase transformation kinetic. One of the chief aims of our work is to obtain general equations for the diffusion fluxes under strain that give the possibility for using these equations at low temperatures, as in this case the strain influence on the diffusion fluxes is manifested in maximal degree. Our approach takes into consideration, that the strains can alter the surrounding atom configuration near the jumping one and consequently the local magnitude of the activation barrier and a rate of atom jump. The rates of atom jumps in different directions define the flux density of the vacancies. Now we take into account, that strain values are different in the saddle point and in the rest atom position, in differ from our consideration that was done by us earlier. As a result in the development of our approach the general equations for the vacancy fluxes are obtained for fcc and bcc metals. In our paper we discuss the main features of the theory of diffusion under stress and its applications. In particular we examine how elastic stress, arising from nanovoids, influence the diffusion vacancy fluxes and the growth rate of voids in metals.

Abstract: Vacancies are the simplest type of lattice defect. However, they play a major role in the kinetics of diffusional processes, such as solid-state precipitation, where mass transport is directly proportional to the concentration of vacancies. We present a physical modelling framework, where we simulate the evolution of excess vacancies on the example of Al-alloys during simplified time-temperature treatments. Interaction energies between solute atoms and vacancies are evaluated by first-principle analysis. Assuming that the escape of vacancies from existing traps is dependent on temperature and binding energies, we explore the life-time of non-equilibrium vacancies and the natural and artificial aging response of Al alloys. The predictions of the model are finally compared to experimental data.

Abstract: In this paper the role of vacancies in the aging of Al-Mg-Si alloys is examined and novel concepts to improve their aging behavior are presented. It has been proposed that the technologically favored fast nucleation of the major hardening phase during artificial aging requires quenched-in vacancy assisted diffusion. The well-known interdependence of natural aging and subsequent artificial in Al-Mg-Si alloys can be understood in terms of quenched-in vacancy trapping in Mg/Si-clusters formed during natural aging. Diffusion during artificial aging is then determined by the dissolution of these vacancy-containing Mg/Si-clusters. This simple concept can guide the development of strategies to avoid the negative effect of natural aging. It is shown that the aging behavior of Al-Mg-Si alloys can be improved not only by processing related measures, but also by compositional interventions, which apply the following recipe: (i) avoid the trapping of vacancies in Mg/Si-clusters, (ii) prevent the vacancy annihilation during RT, and (iii) make them available for diffusion during artificial aging. It is shown that this strategy can be executed in Al-Mg-Si alloys by adding defined trace amounts of elements with an attractive binding energy to vacancies and sufficient solubility in the aluminum matrix.