Defects and Diffusion in Ceramics X

Paper Title Page

Authors: M. El-Hofy, A. Salama
Abstract: ZnO-BaO particles (4-10) nm have been synthesized by applying 40 KHz ultrasonic wave simultaneously with the co-precipitation process of Zn and Ba oxalate at 50 0C. DTA curves of the co-precipitate were carried out and the decomposition temperature was determined. Mixed oxide obtained from thermal decomposition of Zn2 Ba (C2O4)2.2H2O in air has been investigated by TEM, XRD, IR and UV-Vis measurements. The results revealed that the obtained particle consists of a core of highly deformed Ba contaminated ZnO nano-crystals. The ZnO crystalline core is coated by Ba rich amorphous layer which prevents its growth. Decreasing concentration of ZnCl2 in the precipitation solution from 0.1M to .05 M leads to decrease the size of the particle from (10-20) nm to(4-10) nm, in average, increases UV cut-off, the cut-off tail and rounded of the shape of the particle.
Authors: Sergey I. Sidorenko, Yu.N. Makogon, S.M. Voloshko, O.P. Pavlova, I.E. Kotenko, A.V. Mogilatenko, G. Beddies
Abstract: Thermally stimulated solid state reactions in the Ni(10 nm)/Si(001) film system that occur under the annealing in the nitrogen ambient were researched by methods of сross-sectional transmission electron microscopy and scanning electron microscope. It was established that NiSi2 formation consists of several steps: a formation of the NiSi polycrystalline silicide thickness of which twice higher initial thickness of Ni layer; prevailed diffusion of Ni atoms out of NiSi into Si substrate according with lattice mechanism and appearing of exceeding vacancies at grain boundaries; a formation of epitaxial NiSi2 nuclei at separate spots of NiSi/Si(001) interface; regular growth of NiSi2 phase inclusions at the expense of NiSi layer “diffusion dissolution”; a formation of NiSi2 spherical inclusions in the lattice of Si matrix and their coalescence.
Authors: Li Hua Wei, Shao Yi Wu, Yue Xia Hu, Xue Feng Wang
Abstract: The EPR g factors g// and g and the hyperfine structure constants for one substitutional Cu2+ center in CdS are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion under trigonally distorted tetrahedra. In view of the significant covalency of the [CuS4]6 cluster, the ligand contributions are taken into account on the basis of the cluster approach. According to the calculations, the impurity Cu2+ is found not to occupy exactly the ideal Cd2+ site but to suffer a small inward displacement 0.12 Å toward the ligand triangle along C3 axis. The theoretical results by considering the above impurity displacement show reasonable agreement with the experimental data. The defect structure of this Cu2+ center is also discussed.
Authors: Girjesh Singh, S.B. Shrivastava, M.H. Rathore
Abstract: The mechanism of slow positron annihilation in ion-implanted Si has been discussed in terms of the Diffusion-Trapping model (DTM). The trapping of positron has been considered in native vacancies (monovacancies) and ion induced vacancies i.e. vacancy clusters. The model has been used to calculate the Doppler broadening line shape parameter (S-parameter) as a function of incident positron energy for different ion-implanted Si. It has been found that at lower energies the monovacancies and vacancy clusters both contribute to the S-parameter while, with the increase in positron energy the vacancy clusters are reduced. The S-parameter is found to be dependent on the fluency of the implanted ions.
Authors: Valentin A. Tatarenko, S.M. Bokoch, V.M. Nadutov, Taras M. Radchenko, Yong Bum Park
Abstract: Within the framework of the lattice-statics and static fluctuation-waves’ methods, the available energies of strain-induced interaction of interstitial–interstitial, interstitial–substitutional and substitutional–substitutional impurity atomic pairs are collected and analysed for f.c.c.-(Ni,Fe)–C solutions allowing for discrete atomic structure of the host-crystal lattice. The lattice spacings, elasticity moduli and/or quasi-elastic force parameters of the host-crystal lattice, and concentration coefficients of the dilatation of solid-solution lattice due to the respective solutes are selected as the input numerical experimental data used. The above-mentioned interaction energies prove to have non-monotonically decreasing (‘quasi-oscillating’) and anisotropic dependences on discrete intera-tomic radius-vector, and themselves are strong and long-range. In all f.c.c.-(Ni,Fe)-base solutions, there is strain-induced attraction in many co-ordination shells. In general, the strain-induced interaction between impurity atoms in γ-Fe is weaker than in α-Ni (but in some solid solutions, it may prove to be of the same order). The verification of applicability of the approximation of strain-induced interaction of impurities for f.c.c.-(Ni,Fe)–C alloys (by means of analysis of thermodynamic C activity and ‘short-range order’ parameters of C-atoms’ distribution revealed by Mössbauer spectroscopy) showed that it must be supplemented with additional short-range (‘electrochemical’) repulsion in the first co-ordination shell. Nevertheless, in any case, the strain-induced interaction of impurity atoms must be taken into account for analysis of structure and properties of f.c.c.-(Ni,Fe)-base solutions. The Monte Carlo simulation procedures applied for constitution of a nanoscale Fe–C-austenite crystallite and based on analysis of the dependences of numbers of the different atomic configurations on C–C interatomic-interaction energies reveal correlation between the potential energy of such a modelling system and the numbers of iterations as well as Monte Carlo steps for the approach to constrained equilibrium. As shown by the example of austenite, for adequate representation of the experimental data on thermodynamic C activity, one needs to take into account for computations the ‘electrochemical’ (direct) and strain-induced (indirect) contributions to C–C interaction. The estimated sets of energies of such a total interaction within the first several interstitial co-ordination shells with rated radii are presented, and optimal set is selected, which optimally corresponds to experimental concentration and temperature dependences of C activity and the Mössbauer-spectroscopy data on the nearest neighbourhoods of Fe atoms with octahedral C interstitials. The ‘equilibrium’ relative parts of the different atomic Fe–C and C–C configurations (depending on C–C interaction energies) are determined. The Khachaturyan–Cook microscopic approach is considered to relate the time dependence of the long-range order (LRO) or short-range order to atomic diffusion. It enables to use the data of measurements of time dependence of radiation diffraction or diffuse-scattering intensity for a Ni–Fe solid solution for calculation of both probabilities of elementary atomic-migration jumps to different lattice sites per unit time and ‘exchange’ or vacancy-controlled diffusion coefficients, respectively. By the use of quantitative experimental information about the Curie temperatures, TC, and neutron diffuse-scattering intensities for disordered f.c.c.-Ni–Fe alloys, it is possible to evaluate the Fourier components, w~tot(k), of effective Ni–Fe atomic ‘mixing’ energies (inclusive the competing exchange interactions of respective permanent magnetic moments) taking into account both long-range ‘paramagnetic’ (‘electrochemical’ + ‘strain-induced’) and Ising-type magnetic contributions that drive the long-range ordering. Magnetism and ‘chemical’ (atomic) LRO in f.c.c.-Ni1cFeFecFe alloys are analysed within the self-consistent mean-field approximation, in which the statistical thermodynamics of the non-stoichiometric L12(Ni3Fe)-type permalloy (as well as L10(NiFe)-type elinvar) is determined by several energy parameters {w~tot(k)}. There is a revealed interplay of magnetism and long-range atomic ordering with the order–disorder transformation temperatures, TK, (below TC) appreciably different from the corresponding isolated TK values (above TC). The interplay of these two phenomena is examined along two lines, i.e. through the estimation of both temperature–cFe dependence of spatial LRO parameter and magnetisations of Ni and Fe subsystems. As shown, not only the temperature-dependent phase states of such binary f.c.c. alloys can be reproduced, but also the dependence of TK vs. cFe, including the observed asymmetry of phase-diagram curves due to the T- and cFe-dependent magnetic contribution to the effective interatomic interactions etc. As revealed for f.c.c.-Ni–Fe alloy with the use of single relaxation-time kinetics approximation for calculation of equilibrium intensity values, the magnetic contribution to the ‘mixing’ energy of atoms (in low-spin states) of this alloy facilitates its atomic ordering, and the presence of atoms with essentially different spins may cause the virtually abrupt phase transition from paramagnetic state into magnetic one. The optimal sets of exchange-interaction energy parameters for f.c.c. Ni–Fe alloy are selected. As shown, the doping of small amounts of interstitial C impurities most likely increases ferromagnetic component of bond of Ni spins with Fe spins, reduces ferromagnetic component of bond of Ni spins with Ni spins, and in-creases antiferromagnetic component of bond of Fe spins with Fe spins in an f.c.c.-Ni–Fe alloy.
Authors: Amitava Ghorai
Abstract: Using pseudopotential approach, vacancy formation energy , different non-split interstitial formation energies and binding energy for the vacancy-impurity pair and that for interstitial impurity over host have been calculated in some cubic fcc metal systems, viz. copper, silver, gold and lead using Ashcroft's potential and Taylor's exchange and correlation function with standard (AT) and fitted to (ATF) and also Heine-Abarenkov’s model potential and same exchange and correlations (HAT). It is difficult to have a universal value for all types of atomic property calculations. The results show that ATF and HAT combinations are better in comparison to AT. Also, the substitutional impurity adjacent to a vacancy is found to be more loosely bound than the interstitial impurity in fcc metals.
Authors: Christoph Veyhl, Rolf Winkler, Markus Merkel, Andreas Öchsner
Abstract: The paper investigates the mechanical properties of sintered hollow sphere struc- tures. First of all, the Young's modulus, plastic modulus, initial flow stress, initial flow strain, plateau stress and densifcation strain are derived from compressive tests. In addition, the material behaviour during the compressive testing is described. Furthermore, the influence of different specimen shapes on the results is discussed.
Authors: Virginie Taillebot, S. Divinsky, Christian Lexcellent, Jean Bernardini, Dezső L. Beke
Abstract: Classically a master curve as Dorn's equation is applied for elucidating stationary creep behaviour within high temperature range (T > 0.6 Tm). As the diffusion of both 63Ni and 44Ti have been measured in an equiatomic NiTi, an effective choice of creep-relevant diffusion coefficient D may be possible. Moreover, creep measurements in the same temperature range performed can be found in the literature. The correlation does not permit to establish precisely what D coefficient to integrate in the Dorn's equation.
Authors: Christoph Veyhl, Rolf Winkler, Markus Merkel, Andreas Öchsner
Abstract: This paper investigates the structural properties of sintered hollow sphere struc- tures. First of all, the packing density is analysed using three different methods: namely, liquid infiltration, Archimedes' principle, and image processing of micrographs. In addition, the pore fraction in the metallic sphere shells is characterised based on micrographs and the density of the structure and the base material is determined. In the final part, the geometrical characteristics of the sphere structure and the material composition of the base material are analysed.
Authors: M.A. Abdel-Rahman, M.S. Abdallah, N.A. Kamel, Emad A. Badawi
Abstract: Recovery behavior of 20% plastically deformation of casting AlSi11.35Mg0.23 in various stages of isochronal annealing has been investigated by positron Lifetime (LT). The experimental results show that the positron mean lifetime is a function of annealing temperature. Lifetime of the positron annihilating in perfect lattice is 187.3ps and in 20% deformed is 229.8 ps. There are two regions in the isochronal annealing, one of them relating to the point defect and the other to the dislocation. The activation enthalpy for the dislocation is calculated from the isothermal study in the dislocation region from (575-675) K by slow and fast cooling as 0.16±0.02 and 0.53±0.06 eV respectively.

Showing 1 to 10 of 10 Paper Titles