Defect and Diffusion Forum Vols. 297-301

Abstract: Carbon nanotubes have shown the superior mechanical, electrical and thermal properties. These outstanding properties as well as a high aspect ratio and low density make carbon nanotubes an ideal reinforcement to develop superior nanocomposites. Mechanical tests indicated that the reinforcement role of carbon nanotubes is affected by the stiffness matrix. It is well known that the degree of cure of the epoxy has great influence on their mechanical properties. In this investigation, the matrix stiffness is controlled by changing the mixture ratio between the epoxy and hardener. Two different contents (1% wt. and 2% wt.) of the multi-wall carbon nanotubes in the epoxy are proposed in this work. Tensile tests are conducted to determine the mechanical properties of the nanocomposites, including the Young’s modulus, yield stress, tensile strength and fracture strain. Experimental results show that the mechanical properties are increasing with the increase of the addition of multi-wall carbon nanotubes. The reinforcement role of the multi-wall carbon nanotubes is decreasing while increasing the stiffness matrix.

Abstract: Numerical study of natural convection heat transfer inside a differentially heated square enclosure with adiabatic horizontal walls and vertical isothermal walls is investigated. Two insulated ribs are symmetrically located on horizontal walls. The governing non-linear equations are solved in a two-dimensional domain using a control volume method and the SIMPLER algorithm for the velocity–pressure coupling is employed. The results will be presented in forms of streamlines, isotherms and Nusselt number for Rayleigh number 106. It is shown that for small rib height the isotherms indicate the laminar boundary regime with high temperature gradient near the bottom of the hot surface and the top of cold one. However, as rib height increases this boundary layer is vanished. Also it is found that as the length and height of the ribs increase the mean Nusselt number decreases.

Abstract: Study of interdiffusion in the Co-Mo system is important to understand the performance of turbine blades in jet engine applications. Mo is added to superalloys to increase the solid solution strengthening and the creep resistance. In this study, the interdiffusion coefficient in the Co(Mo) solid solution and impurity diffusion coefficient of Mo in Co are determined. Further, the activation energy and pre-exponential factors are calculated, which provide an idea about the atomic mechanism of diffusion.

Abstract: Nb3Sn growth following the bronze technique, (i.e. by interdiffusion between Cu(Sn) alloy (bronze) and Nb) is one of the important methodologies to produce this superconductor. In this study, we have addressed the confusion over the growth rate of the Nb3Sn phase. Furthermore, a possible explanation for the corrugated layer in the multifilamentary structure is discussed. Kirkendall marker experiments were conducted to study the relative mobilities of the species, which also explained the reason for finding pores in the product phase layer. Based on the parabolic growth constant at different temperatures, the activation energy for the growth is determined. We have further explained the dramatic increase in the growth rate of the product phase by changing just one atomic percentage of Sn in the Cu-Sn bronze alloy.

Abstract: The modulated structure characterized by periodic concentration fluctuations is caused by the up-hill diffusion of solute atoms due to the spinodal decomposition, when the Gibbs free energy surface of the solid solution has a convex curve. This convex shape in the free energy surface will appear in multi-component systems, even though the energy in each binary system does not show the convex curve, when a binary system exhibits a strong attractive interaction, i.e., the Gibbs free energy shows a deep concave curve. This may be the case that steel containing an element having a strong tendency of carbide forming. The Fe-Ni-V-C system was selected in this study. V is a strong carbide forming element and forms VC carbide based on f.c.c. lattice. A series of experiments was carried out using Fe-25Ni-3V-3C (mol%) alloy after aging at elevated temperatures. A modulated structure was observed in specimens aged at temperature range of 550 °C to 650 °C, and the maximum temperature occurring the spinodal decomposition was estimated to be 715 °C in Fe-25Ni-3V-3C alloy. Furthermore, the coefficient of the gradient energy caused by composition fluctuations was estimated to be 2.8 x 10-15 J.m2/mol.

Abstract: There is a constant effort to understand the defect structure and diffusion behavior in intermetallic compounds with the A15 structure. Diffusion of elements in intermetallic compounds depends mainly on antisites and vacancies on different sublattices. In this article, we shall discuss the diffusion of elements in A3B compounds with the A15 structure.

Abstract: Hydrogen has attracted clean energy media as a high-quality and renewable energy source in recent years. On the one hand, a ceramic catalyst and/or catalyst support such as Al2O3 has often been a preferred choice due to their specific properties such as thermal stability and high surface area. Al2O3-CuO-ZnO was synthesized by using the sol-gel process with aluminum isopropoxide and primary distilled water as the precursor and solvent. In this synthesized process, metal oxides caused to precursors such as copper and zinc nitrate were added. To prepare membranes, cobalt metal powder was used to increase the strength and durability. The Al2O3-CuO-ZnO/Co composite membranes were produced using hot press sintering for consolidation of powders following reactively mechanical alloying process. The characterizations of membrane were measured XRD, FE-SEM and EDS. Hydrogen permeation evaluations were examined at a room temperature to 773 K under increasing pressure. Hydrogen permeation rate was obtained with 0.18 mol m-2 s-1 at 773 K result from complex diffusion mechanism such as Sievert’s type, Knudsen diffusion and/or molecular sieving.

Abstract: Over the last decades, numerous investigations have been conducted on Nitinol properties. However, the effects of alloying elements on Ni-rich NiTi alloys have been considered less. In this research, different effects of homogenization time and cooling rate on the behaviors of Ni-42.5wt%Ti-3wt%Cu alloy were evaluated. The mentioned alloy was fabricated by vacuum arc melting method. Three different homogenization times (half, one and two hours) and three cooling media (water, air and furnace) were selected. The microstructure and hardness were examined by means of optical and scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analysis and hardness test, respectively. According to the microscopic investigations, no significant changes were observed after half an hour. However, results indicate that increasing time of homogenization leads to finer precipitations and a uniform distribution of them. The various cooling environments result in the formation of two types of precipitation phases. It was seen that in the high cooling rate, the majority of precipitations consisted of Ti2(Ni,Cu) while by decreasing cooling rate NiTiCu precipitates appeared too, which affect the hardness.

Abstract: The great enhancement of the minority carrier recombination rate after optical activation of FeB pairs was used for investigation of interstitial Fe ions diffusion in p-type Czochralsky silicon by Surface Photo-Voltage Technique (SPV). Highly reproducible profiles of the Fe ions distribution were obtained after one-time thermodonors annealing process (650 °C, 30 min.) with Fe film evaporated on one side of silicon slab and successive layer by layer silicon slab etching. Evolution of Fe concentration distribution between both sides of initially inhomogeneously contaminated silicon slab in durable annealing processes was investigated. Gradual Fe concentration reduction near preliminary contaminated slab surface and Fe concentration increase near opposite surface were observed. Fe concentrations near both surfaces become equal after approximately 10-times annealing in agreement with diffusion rate obtained by the first method. Chaotic release and hiding of latent Fe were observed after 5-7 hours of annealing process. Two-exponential increase of other than Fe recombination centers concentration was observed during 650 °C annealing. Both exponential factors are found to be functions of interstitial oxygen concentration in the silicon ingot.

Abstract: Ion irradiation of surfaces leads to blistering (vacancy-gaseous bubbles formation into crystal lattice) and/or nano-scale islands of thin cover formation. Stochastic model of first order phase transition at fluctuation stage is presented by superposition of Wiener processes of nuclei clustering and it’s Brownian motion. Solution of Ito-Stratonovich stochastic differential equations allows studying the evolution of distribution functions versus clusters sizes and relative lattice locations of nuclei.