Growth kinetics and texture formation of pyroxene reaction rims in thin films settings

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The temperature driven diffusion controlled growth of reaction layers is an established method for laboratory measurements of reactive diffusion in polycrystals. The problem arising when applying the experimental results to diffusion in the earth crust due to different timescales will be solved by miniaturising the settings within thin films. The proposed work focuses on texture and residual strain in spinel reaction layers between corundum and periclase as well as orthopyroxene layers between olivine and quartz. We will investigate two different kinds of settings: a 2-layer system consisting of substrate and seed layer and a 3-layer system consisting of substrate, seed layer and reactant layer. The proposed project is designed as a complement both in experimental and in analytical aspects to project 6, that aims at the effect of differential stress on texture formation. In this FOR 741 D-A-CH Proposal “Nanoscale Processes and Geomaterials Properties” - Project No. 2 project stress will not be externally applied to the reacting samples, but we will detect and quantify the lattice strain that is induced by internally derived stress. The trade mark of this project is the use of use in-situ methods to observe strain and texture not only in their final states but also the way they emerge. Because the crystallinity of the layers (substrate, seed layer as well as reactant) changes during the annealing process we propose to perform in-situ energy dispersive synchrotron X-ray diffraction (EDXRD) experiments to follow the development of the reaction rim during the growth process. For this experiment the thin film setting will be annealed within a special reaction chamber, which can be installed at the EDDI beam line of the Berlin synchrotron facility Bessy. This special experimental setup enables a real time monitoring of microstructure and texture evolution and will raise the project to an outstanding analytical level. To investigate the microstructure, texture and residual strain of the reaction layers ex-situ, the project bears strongly on up-to-date X-ray diffraction methods. These are in principle bulk methods as opposed to high spatial resolution methods. However due to the specialised applications for thin film investigations that we intend to apply (grazing incidence X-ray diffraction) as well as the high resolution of synchrotron beams we actually address structural features in reaction rims that are on a scale of 10s to 100s of nanometers, that is the range between micro- and nanoscale that is sometimes referred to as mesoscale. This region is of special interest for the understanding of texture and microstructure evolution, because typically here the energy budget of net-transfer reactions becomes clearly dominated by the contributions of strain and interface energy.