Defect and Diffusion Forum Vols. 283-286

Abstract: This work presents a theoretical and experimental study of nickel deposition on iron samples at relatively high pressure using a pulsed DC glow discharge. The deposition process was conducted in conditions similar to that used for plasma sintering, using the confined anode-cathode configuration. The cathode was made from nickel commercially pure and the samples were made from interstitial free steel and sintered pure iron. The samples were characterized by mass weight gain, scanning electron microscopy and energy-dispersive X-ray microanalysis. The deposition process was mathematically modeled and the model was numerically solved using a conservative finite-volume method. The experiments demonstrated that the deposition occurs at a constant rate, with the mass flux changing linearly with the cathode voltage in the range of parameters considered. The results obtained from the diffusion model applied to the sample presented good agreement with the experimental values. Concerning the gas phase, the proposed model helped us to clarify some phenomenological aspects of the process. However, further studies, principally in the area of electrical discharges, are needed to permit a complete comprehension of this process.

Abstract: We have recently developed an experimental protocol to study surface self-diffusion using Pulsed Field Gradient (PFG) Nuclear Magnetic Resonance (NMR) techniques. While the study of diffusion phenomena with PFG NMR techniques evolved in the 1960s and is nowadays part of the standard NMR toolbox, our novel approach allows us the simultaneous quantification of diffusion coefficients of molecules in a surface layer and the bulk pore liquid of a porous medium. This is of relevance when attempting to explain solvent effects in catalysis. Solvent effects are known to have a significant impact on catalyst performance. For example, water has been reported to influence the reaction rates during carbonyl hydrogenations. In this work we have assessed the effect of two solvents – water and isopropyl alcohol (IPA) - on the bulk pore and surface diffusivity of methyl ethyl ketone (MEK) in porous Ru/SiO2 catalyst pellets. We find that when MEK is dissolved in water, it appears to interact more closely with the surface than when it is dissolved in IPA.

Abstract: The kinetics of oxidation of liquid Cu-0.5mass%Ti alloy at 1473K in CO2-CO gas mixture has been investigated by varying the oxygen potential. The pO2 is set to be enough to oxidize Ti in liquid Cu-Ti alloy but not for the oxidation of liquid Cu. Alloy samples were taken with a quartz tube during the experiments and Ti and O contents in the samples were analyzed by inductively coupled plasma method, and infrared absorption method. The phases of the formed titanium oxide layers were consisted of several different titanium oxides and were identified by using EBSD. The oxidation rate was evaluated based on the change of the Ti content in the Cu-Ti melts. The apparent overall oxidation rate is reasonably represented by the parabolic rate law. This oxidation behaviour was well explained based on the multi-layered oxidation theory.

Abstract: By bombarding a target with stable ion beams it is possible to produce nuclear radioactive species. The EXCYT facility at the Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali del Sud (INFN-LNS), in Catania, is based on a K-800 superconducting cyclotron injecting high intensity beam (up to 500 W) in a Target Ion Source (TIS) assembly, and on a 15 MV Tandem for post-accelerating the Radioactive Ion Beams (RIB). Radioactive species are produced in a solid porous target where, by diffusion, they will reach the target surface. After effusion through the target container, radioactive atoms are ionized, extracted at low energy, and changed in charge (if needed). Two stages of isobaric mass separators will clean the resulting radioactive beam from contaminants, thus the RIB is injected into the Tandem for the post-acceleration. TIS is operating at high temperature (up to 2700 K) under vacuum (10-4 mbar). The production of 8Li radioactive beam was performed by injecting a 13C4+ primary beam of 45 MeV/u on a porous graphite target. In this work we present a study to optimize the target geometry in order to increase the production of the RIB. Our first RIB was 8Li which mean life time is 1.212 s. The generation density of 8Li inside the solid target is calculated by EPAX II code. A simple diffusion model which takes into account either the nuclei generation and their decay, was computer simulated to estimate the efficacious diffusion coefficient of 8Li in porous graphite which resulted to be in the range of 10-6 - 10-5 cm2/s. Since the porous nature of the target, many diffusion mechanisms are active, our assumption, supported by experimental evidence, is that at such high temperature multi-grain diffusion prevails. Diffusion length inside the target of 8Li, before its decay, resulted to be sensible smaller than the target size, therefore we modified its geometry enhancing the RIB production by a factor of 3-4.

Abstract: The isothermal oxidation behavior of Zr58Nb3Cu16Ni13Al10 bulk metallic glass (BMG) under dry air in the glassy state and the supercooled liquid state (SLS) was studied by the thermogravimetric method. The oxidation rate and thickness growth speed in the SLS were both hugely higher than in the glassy state. The oxidation kinetics of BMG in both states for 1.5 hours was different, the parabolic law was followed in the glassy state at 300°C and 350°C, contrarily the linear law was followed in SLS at 400 °C. After the oxidation for 126 hours in SLS, the oxidation kinetics possessed two stages, the linear stage and the parabolic stage. The diffusion of the Cu2+ ion and CuZr intermetallic alloys were detected by GIXRD.

Abstract: When developing new types of materials with special properties for the automotive industry, steels with high content of manganese (up to 25 wt. % Mn) were studied. These steels embody increased failure resistance, high level of deformation hardening, plastic response and high value of energy absorption in crash situations [1,2]. The aim of our work was to study an interaction of a FeMn alloy with pure iron from the view of diffusion processes. For that purpose, diffusion joints Fe/FeMn-steel were created and long-term heated at the temperatures of 800, 900 and 1050 °C. Dependences of diffusivities of Mn on its concentration were calculated from concentration profiles of manganese after diffusion heating applying three analytical methods. In the first method, constant values of diffusivities Di, i = 1,2 for both Fe and steel were taken into account. In the second case a modified Matano-Boltzmann method utilizing polynomials was applied, which simplifies solution in numerous cases. The third method performs the solution of the non-linear equation of diffusion on the assumption of a linear dependence of diffusivity on concentration applying a procedure based on perturbation calculus. The results of the solution will be graphically documented in details together with the description of experiments.

Abstract: A lot of food industries are producing legumes such as bean and chickpea. Soaking which is diffusion of water in them is usually one of the processes in the production line before cooking. In this study, it is experimentally investigated the water diffusion on red beans as a function of time and temperature, and then the influence of this diffusion on the cooking time and the final volume of cooked legumes in different temperatures. Thus, the energy management and final volume of the production could be optimized with respect to the cooking time, cooking temperature and the duration of water diffusion and temperature condition of soaking before the cooking process. Also in the present study, volumetric water diffusion is defined for legumes, that is a function of the diffused water, the initial volume (before soaking) and final volume of the productions (after cooking). The results confirm that the final volume of the product and the cooking time are a function of the conditions of the water diffusion before cooking.

Abstract: The hydrogen diffusion coefficients are investigated during the hydrogen permeation through Nb-based hydrogen permeable membranes at high temperature. It is found that the hydrogen diffusion coefficient for pure niobium under practical conditions is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd-Ag alloy with fcc crystal structure than in pure niobium with bcc crystal structure at 773K during the hydrogen permeation. It is also found that the addition of Ru or W into niobium increases the hydrogen diffusion coefficient under the practical conditions.

Abstract: The diffusion of hydrogen has been investigated in the AB2 Laves phase Compounds TiCr1.78 and TiCr1.4Mn0.4, by absorption techniques. It has been found that H at temperatures higher than 700 K diffuses through the classical over-barrier mechanism, while at low temperature (around 100 K) the diffusion is governed by phonon-assisted tunnelling. The activation energy for classical hopping is rather high and increases with the substitution of Mn for Cr. In the range of H contents nH investigated (nH=H/Me0.03) the chemical diffusion coefficient does not depend on the H concentration.

Abstract: Properties of diffusive-hardening pastes based on liquid gallium have been investigated. The optimal conditions were found for producing the paste (solder) with best technological characteristics. The liquid-solid ratio, powder alloy composition and dispersity, dust fractions percentage, exposition time and temperature are the main factors which determine the rheological properties of the gallium paste.