Papers by Author: Ola Hunderi

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Authors: Ola Hunderi, Knut Marthinsen, Nils Ryum
Abstract: The kinetics of grain growth in real systems is influenced by several unknown factors, making a theoretical treatment very difficult. Idealized grain growth, assuming all grain boundaries to have the same energy and mobility (mobility M = k/ρ, where k is a constant and ρ is grain boundary curvature) can be treated theoretically, but the results obtained can only be compared to numerical grain growth simulations, as ideal grain growth scarcely exists in nature. The validity of the simulation techniques thus becomes of great importance. In the present investigation computer simulations of grain growth in two dimensions using Monte Carlo simulations and the grain boundary tracking technique have been investigated and compared in small grain systems, making it possible to follow the evolution of each grain in the system.
Authors: N. Giskeødegård, O. Blajiev, A. Hubin, H. Terryn, Ola Hunderi, Kemal Nisancioglu
Abstract: The properties of oxide formed on pure aluminium in acetate buffers under controlled potential, temperature, and pH are characterized by in situ electrochemical techniques and spectroscopic visual ellipsometry (VISSE). Monitoring the current density (cd) during transition between different applied potentials gives direct information about Faradaic changes in the oxide properties, while the ellipsometer monitors the changes in the actual thickness and refractive index of the film. Electrochemical impedance spectroscopy (EIS) reveals, in addition to film properties, information about the charge transfer processes and adsorption at steady-state conditions. VISSE and ex situ transmission electron microscopy (TEM) indicate the presence of a two layered oxide structure, consisting of the barrier and a porous hydrated layer. The thickness and density of these two layers are monitored in situ during growth and dissolution. The thickness of the barrier layer depends reversibly on the applied potential, while the thickness of the hydrated layer is less affected by the potential. The absorption sensitive parameter is correlated with the buffer concentration and therefore believed to be related to the acetate.
Authors: M.W. Nordbakke, N. Ryum, Ola Hunderi
Abstract: Computer simulations of 2D normal grain growth have shown that size correlations between adjacent grains exist in 2D grain structures. These correlations prevail during the coarsening process and influence on the kinetics of the process and on the grain size distribution. Hillert’s analysis starts with the assumption that all grains in the structure have the same environment. Since computer simulations contradict this assumption, the mean-field theory for normal grain growth needs to be modified. A first attempt was made by Hunderi and Ryum, who modified Hillert’s growth law to include the effect of spatial grain size correlations. In the 1D case the distributions derived by means of the modified growth law agreed well with simulation data. However, the distribution derived for 2D grain growth retained unwanted properties of the Hillert distribution. We review some recent progress in developing a mean-field statistical theory. A paradox related to curvilinear polygons is shown to support the expectation that the grain size distribution has a finite cutoff.
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