Defect and Diffusion Forum Vols. 273-276

Abstract: Full formation of aluminum titanate compound at low temperatures was obtained by solgel method. With addition of various amount of Fe2O3 additive to tialite composition formation and decomposition of tialite phase have been investigated. X-ray diffraction (XRD), simultaneous thermal analysis (STA) and scanning electron microscopy (SEM) analysis showed that the formation of nano tialite at low temperatures (around 950oC) was improved at the presence of additive and decomposition rate of aluminum titanate to α-Al2O3 and TiO2 (Rutile) in sample with 5%wt Fe2O3 was controlled with formation of Al2TiO5-Fe2TiO5 solid solution.

Abstract: Aluminum titanate (Al2TiO5) is an engineering ceramic with low thermal expansion coefficient, excellent thermal shock resistance, good refractoriness and non-wetting with most metals. In this work the use of colloidal silica and nano boehmite mixture in proper stoichiometry of mullite in aluminum-titanate (tialite) has been studied. Analytical structural evaluations including XRD, TEM and SEM have been used to characterize the influence of these additives on phase transformation, sintering process and microstructure. The results show that the presence of nano size silica and boehmite enhanced the densification leads to fine microstructure, mullite phase formation and improve the tialite ceramics properties.

Abstract: The diffusion of molecules in liquids and dense gases is demonstrated to be nonclassical for long time intervals. This means that the time dependence of the mean-square displacement of molecules is nonlinear. This result was obtained by molecular dynamics simulations over a wide range of density of the medium. The problem of plateau values of the diffusion coefficient is discussed. Nonclassical diffusion equations are derived and discussed.

Abstract: The transport properties of nanofluids are investigated by the molecular dynamics method. It is shown that the force acting on a nanoparticle is nonstationary, in contrast to the Stokes force. In the initial stage of relaxation, the friction force is greater than the Stokes value. Subsequently, this force decreases and reaches an asymptotic value. This value is comparable to the Stokes force only for a massive particle. A correlation for determining the friction coefficient is constructed. It is established that the effective viscosity coefficient of nanofluids depends not only on the volume concentration of nanoparticles but also on the nanoparticle mass and radius.

Abstract: Ni-based superalloys are strengthed by refractory elements such as Re, Ru and W [1]. Thus, the information on the interdiffusion coefficient as well as the thermodynamic interaction between the refractory elements is important for the future alloy design. In this study, interdiffusion coefficients of the refractory elements in Ni-X-Y (X, Y=Co, Re, Ru, W) ternary systems were estimated by a series of experiments. In the all systems studied in the present works, the main interdiffusion coefficients were much larger than the cross interdiffusion coefficients. In some systems, two cross interdiffuion coefficients had opposite signs each other. For example, in Ni-Co-Ru system, the main interdiffusion coefficients are 2.7 10 14 ~ Ru = × − CoCo D and 15 6.8 10 ~ Ru = × − RuRu D , while the cross interdiffusion coefficients are 16 6.6 10 ~ Ru = − × − CoRu D and 16 8.9 10 ~ Ru = × − RuCo D at 1523K. In Ni-Co-Ru and Ni-Re-Co systems, the activation energies and frequency factors for two main interdiffusion coefficients. For example, Q (kJ ) Co = 268 , 5 ( 2 1 ) 0( ) 4.4 10 D = × − m s − Co , 3 ( 2 1 ) 0( ) 2.9 10 D = × − m s − Ru in Ni-Co-Ru system.

Abstract: Evaporation in restricted domains, e.g. in capillaries, is of industrial importance but is poorly understood. Where the evaporating liquid is a binary mixture, preferential evaporation of the more volatile component occurs initially and the evaporation rate is not constant, indeed it appears to occur in stages. Experiments of evaporation from the entrance of a capillary were performed for various binary mixtures of acetone and water and for pure liquids for comparison. Measurements of mass were taken over time for a range of capillary diameters from 0.6 mm to 2 mm. For simplicity, the experiments were performed with the meniscus “stationary” at the entrance of the tube, rather than allowing the meniscus to recede. The data were analysed and showed that, for the binary mixtures, the evaporation process had two distinct stages for the mixtures. The second stage always had a lower slope than the first, indicating a slower evaporation (similar multistage evaporation processes have been observed for sessile drops of binary mixtures). There are many phenomena at work in this process: surface evaporation; diffusion (or natural convective mass transfer) in the air beyond the capillary; diffusion in the binary mixture; circulation in the liquid; thermal effects of evaporative cooling. These are investigated, comparisons made and further studies are proposed.

Abstract: Chronoamperometry is carried out with a Au disc electrode in alkaline sodium borohydride solutions of varying concentrations and temperatures. By selecting conditions for which the borohydride oxidation is controlled by diffusion, it was possible to deduce diffusion coefficients and activation energies for borohydride concentrations ranging from 0.03 M to 0.12 M, and temperatures ranging from 25 to 65 °C. The estimated parameters were compared with other values reported in the open literature.

Abstract: Anodic polarisation curves of treated and untreated MmNi3.6Mn0.4Al0.3Co0.7 (Mm = mischmetal) electrodes were measured under the conditions of various initial concentrations of absorbed hydrogen (H/M), rates of potential sweep and temperatures. The alloy powders of the electrodes were treated with 6 M NaOH solution containing y M NaBH4 (y = 0.0, 0.005, 0.01, 0.02, 0.03 and 0.05). The initial concentration of absorbed hydrogen in the electrodes varied between 0 and 0.06 H/M, and the working temperature range was 15 °C to 50 °C. Anodic peak current (Ip) at the MmNi3.6Mn0.4Al0.3Co0.7 electrodes increased with an increase in the y value. In addition, the Ip value depended linearly on initial hydrogen concentration and square root of potential rate irrespective of y value. Furthermore, the activation energy for hydrogen diffusion decreased with an increase in y value. From these results, it is considered that the hydrogen diffusion in the MmNi3.6Mn0.4Al0.3Co0.7 alloys and the electrocatalytic activity at their surfaces influence the Ip value and that they are important factors for improving the charge-discharge performance of the negative hydride electrode. Its low temperature (15-30 °C) performances are also improved by the simple powder treatment with the 6 M NaOH solution containing NaBH4 as reducing agent.

Abstract: The reaction of a metal or alloy with an oxidising environment to form a scale often involves a diffusion process as the rate limiting step. The most protective oxide scales are slow growing, adherent to the substrate, and free of cracks or pores. The growth of these scales is typically by solid state diffusion of metal or oxygen ions that move via point defects in the oxide lattice. In 1933, C. Wagner established a scientific basis for oxidation processes controlled by solid state diffusion, with his celebrated derivation of the parabolic rate constant, which connects scaling rates, diffusion coefficients, point defect concentrations, point defect types, and effect of external parameters, e.g. pO2. These aspects are discussed in this paper. The importance of the Wagnerian theory is to provide a relatively simple model upon which more comprehensive models may be built. For many applications, the rate of degradation of the metal or alloy, owing to oxidation by lattice diffusion would be quite acceptable. Several examples of oxidation processes controlled by vacancy and/or interstitial diffusion will be discussed.

Abstract: Ultramicroelectrodes offer several unique characteristics which enable new types of electrochemical measurements. These include: 1) small size; 2) minimisation of iR effects; 3) rapid response; and 4) steady-state response at moderate times. These features enable experiments as diverse as in vivo electrochemistry, electrochemistry in pharmacology, nanoelectrochemistry, electrochemistry in solvents such as benzene, microsecond electrochemistry, and flow-rate independent electrochemistry. Thus, it is apparent that the use of ultramicroelectrodes has become a rapidly growing area of interest. In this paper, the attributes of ultramicroelectrodes, its construction, the equations of diffusion, and key applications of electrochemistry at ultramicroelectrodes, are analysed.