Concentration Dependence Of Activation Energy Research Articles

Change in Concentration-Dependent Diffusion Activation Energy and Frequency Factor with Time: Identified by Numerical Analyses of Experimental Data

A newly developed fully explicit numerical diffusion model coupled with a forward simulation technique, to avoid flaws in conventional methods such as the Boltzmann–Matano, Sauer–Freise and Hall procedures, is used to compute the concentration-dependent activation energy and frequency factor for diffusion in two different alloy systems. In contrast to what has been commonly reported in the literature, the results show that the concentration dependence of activation energy and frequency factor can significantly change with time. In this situation, the use of concentration-dependent activation energy and frequency factor that does not vary with time to predict diffusion effect can lead to considerable error. Furthermore, when there is a time variation in the dependence of activation energy and frequency factor on concentration, a common expectation that diffusion-controlled kinetics would increase with an increase in temperature may not always apply for all time durations.

The kinematic viscosity of liquid copper-aluminum alloys

The method of damped torsional vibrations of crucible with liquid under investigation is used for measuring the temperature dependences of kinematic viscosity ν of Cu100 − xAlx melts (x = 0–100 at.%) in the range of temperatures t between the liquidus and 1100–1450°C. Unlike the majority of such experiments, the measurements are performed both in the process of heating the sample after its melting and in the course of subsequent cooling. The branching of ν(t) curves corresponding to the foregoing modes is revealed below certain temperatures dependent on the composition of sample (viscosity hysteresis). The curves obtained under cooling have a simple exponential shape predicted by activation theories of viscous flow. The obtained temperature dependences are used for constructing isotherms of kinematic viscosity, on which maxima are observed in the vicinity of stoichiometric concentration of CuAl3 and composition of Cu-30 at.% Al. For the same compositions, extrema are observed on the concentration dependence of activation energy of viscous flow.

Effect of Off-Stoichiometry on the Creep Behavior of Binary and Ternary Ni3Al

ABSTRACTCompressive creep behavior is investigated In polycrystalline Ni3Al intermetallic compounds in which the focus is put on the effect of offstoichiometry and ternary additions of Ti, Ta, V and Co on the creep resistance of the compounds. In all cases, the steady-state creep rate has the power-law type dependence on the applied stress. The stress exponent thereby obtained leads us to regard the creep behavior of the compound to be of the Class I type. It is shown that the creep resistance Increases with increasing Ni concentration on both sides of the stoichiometric Ni3Al composition and a discontinuity exists in the variation at stoichiometry. The gap at the discontinuity becomes clearer by the addition of ternary element in most cases as compared to the binary Ni3Al. Although the exact reason for the phenomenon is not clearly understood at present, it is shown that it could be interpreted by the concentration dependence of activation energy in the power-law type creep equation.

Diffusion of Iron and Nickel in Magnesium Oxide Single Crystals

Diffusion kinetics and mechanisms were studied in the FexO‐MgO (vacuum), NiO‐MgO (vacuum and air), and Fe203‐MgO (air) systems. In the FexO‐MgO system, Fe entered MgO by a redox reaction; the diffusivity and activation energy depended on concentration. In the NiO‐MgO system in air the diffusivity depended on concentration and the activation energy did not; in vacuum both the diffusivity and activation energy were concentration‐independent. In the Fe2O3‐MgO system in the MgO phase the activation energy and diffusivity did not depend on concentration. Because of impurities, the diffusion results were for the extrinsic region. Formation of trivalent ions and consequent chemically created vacancies in the FexO‐MgO and NiO‐MgO (air) systems resulted in the concentration dependence of diffusivity. Concentration dependence of activation energy in the FexO‐MgO system is associated with structural changes due to a change with concentration of the Fe3+ octahedraI/Fe3+ tetrahedral ratio. In the Fe2O3‐MgO system structural changes do not occur during diffusion because this ratio remains constant.

Viscosity of moderately concentrated aqueous solutions of polyvinyl alcohol

For a number of polyvinyl alcohol (PVA) samples polymerized at 60°C., viscosities of their concentrated aqueous solutions were measured over the ranges of concentration from 1% to 25% ( w w ) and of temperature from 30°C. to 80°C. It was found that: ( 1) The relative viscosity may be expressed in terms of a single function of ( C 2 5 M w 3.4) over the range of C 2 5 M w 3.4 from 1.5 × 10 11 to 1.6 × 10 13 at 30°C.; this range is temperature dependent. ( 2) The critical value of V 2 Z w , which is interpreted as the onset of a continuous network with ξ = 2, changes from 200 at 30°C. to 240 at 80°C., where V 2 is the volume fraction of polymer in solution, Z w is the weight-average chain length, and ξ is the average number of entanglement points per molecule. Ferry's empirical law, η r ∝ C 2 5 M w 3.4, appears to hold in the range 2 < ξ < 5 for PVA and many other polymers (polystyrene, polymethyl methacrylate, and cellulose derivatives), except for polyisobutylene, for which the glass transition temperature is very low compared with that of other polymers. ( 3) The concentration dependence of activation energy for viscous flow of PVA is similar to that of cellulose derivatives. ( 4) In the low molecular weight region ( M < M crit), the molecular weight dependence of viscosity varies with temperature and concentration.