The power of the chemical potential – Beyond textbook wisdom

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Volume changes caused by alkali ions in borate, germanate and silicate glasses and their relation to cation mobility

It can be shown that volume changes in silicate, borate and germanate glasses can be simply described by the ionic volumes of the corresponding cation addition, if these changes are related to the total number of oxygen atoms. Changes in the volume per mole oxygen are also caused by a varying coordination number as in the case of borate and germanate glasses. Molar volumes defined this way obey simple superposition rules for binary and ternary glasses. In addition, the volume per oxygen atom (or mole), a measure of the average distance of oxygen atoms, provides a quantity which is closely related to the diffusion of gas atoms and alkali ions. Thus a decreasing (increasing) volume per mole oxygen gives rise to a decrease (increase) of the diffusion coefficient of cations or gas atoms. Examples will be presented where this relation is fulfilled qualitatively or quantitatively even. For the quantitative description of diffusion the volume changes caused by external hydrostatic pressure and their known effect on diffusion will be used, in order to model the effect of volume changes caused by a changing composition

The effect of high current densities on iron-carbon alloy thin films

The recently discovered flash sintering method for preparing high quality oxide materials can be applied to the preparation of high performance nanocrystalline metals as well. Just as for the oxide materials, it is possible to use electric fields and currents to enhance densification of metal powders while limiting grain growth, however, the exact mechanism is still under discussion. The goal of our study is to understand how electric currents effect impurity redistribution and grain growth in fine grained metals. Please click Additional Files below to see the full abstract

In-Situ TEM study of the effect of hydrogen on crack propagation in steel

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Chemical Composition of Advanced Materials as Obtained by the 3-Dimensional Atom Probe

The 3-dimensional atom probe is based on field ion microscopy where the screen is replaced by a 2-dimensional, position sensitive detector. Atoms were removed from a conducting sample by a high voltage pulse. The time of flight reveals their chemical nature, and continuous stripping allows lateral and in-depth analysis. The spatial resolution permits a chemical analysis on the sub-nanometer scale. Results of this new technique are presented for (i) the initial stages of interdiffusion at the boundary between two metals, (ii) P-segregation at grain boundaries in nanocrystalline Ni-P alloys, (iii) initial stages of nucleation and growth in various alloys, (iv) composition of thin oxide films in TMR-structures, and (v) distribution of hydrogen in metallic multilayers. It will be also shown that the new technique not only allowed a characterization on the atomic scale but verified and/or falsified existing models for the examples given before