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- ItemHow to grow single-crystalline and epitaxial NiTi films in (100)- and (111)-orientation(Bristol : IOP Publishing, 2023) Lünser, Klara; Undisz, Andreas; Nielsch, Kornelius; Fähler, SebastianUnderstanding the martensitic microstructure in nickel-titanium (NiTi) thin films helps to optimize their properties for applications in microsystems. Epitaxial and single-crystalline films can serve as model systems to understand the microstructure, as well as to exploit the anisotropic mechanical properties of NiTi. Here, we analyze the growth of NiTi on single-crystalline MgO(100) and Al2O3(0001) substrates and optimize film and buffer deposition conditions to achieve epitaxial films in (100)- and (111)-orientation. On MgO(100), we compare the transformation behavior and crystal quality of (100)-oriented NiTi films on different buffer layers. We demonstrate that a vanadium buffer layer helps to decrease the low-angle grain boundary density in the NiTi film, which inhibits undesired growth twins and leads to higher transformation temperatures. On Al2O3(0001), we analyze the orientation of a chromium buffer layer and find that it grows (111)-oriented only in a narrow temperature range around 500 ∘C. By depositing the Cr buffer below the NiTi film, we can prepare (111)-oriented, epitaxial films with transformation temperatures above room temperature. Transmission electron microscopy confirms a martensitic microstructure with Guinier Preston-zone precipitates at room temperature. We identify the deposition conditions to approach the ideal single crystalline state, which is beneficial for the analysis of the martensitic microstructure and anisotropic mechanical properties in different film orientations.
- ItemSolving the puzzle of hierarchical martensitic microstructures in NiTi by (111)-oriented epitaxial films(Amsterdam : Elsevier, 2023) Lünser, Klara; Undisz, Andreas; Wagner, Martin F.-X.; Nielsch, Kornelius; Fähler, SebastianThe martensitic microstructure decides on the functional properties of shape memory alloys. However, for the most commonly used alloy, NiTi, it is still unclear how its microstructure is built up because the analysis is hampered by grain boundaries of polycrystalline samples. Here, we eliminate grain boundaries by using epitaxially grown films in (111)B2 orientation. By combining scale-bridging microscopy with integral inverse pole figures, we solve the puzzle of the hierarchical martensitic microstructure. We identify two martensite clusters as building blocks and three kinds of twin boundaries. Nesting them at different length scales explains why habit plane variants with ⟨011⟩B19' twin boundaries and {942} habit planes are dominant; but also some incompatible interfaces occur. Though the observed hierarchical microstructure agrees with the phenomenological theory of martensite, the transformation path decides which microstructure forms. The combination of local and global measurements with theory allows solving the scale bridging 3D puzzle of the martensitic microstructure in NiTi exemplarily for epitaxial films.
- ItemElectrochemical growth mechanism of nanoporous platinum layers([London] : Macmillan Publishers Limited, part of Springer Nature, 2021) Stanca, Sarmiza-Elena; Vogt, Oliver; Zieger, Gabriel; Ihring, Andreas; Dellith, Jan; Undisz, Andreas; Rettenmayr, Markus; Schmidt, HeidemariePorous platinum is a frequently used catalyst material in electrosynthesis and a robust broadband absorber in thermoelectrics. Pore size distribution and localization determine its properties by a large extent. However, the pore formation mechanism during the growth of the material remains unclear. In this work we elucidate the mechanism underlying electrochemical growth of nanoporous platinum layers and its control by ionic concentration and current density during electrolysis. The electrode kinetics and reduction steps of PtCl4 on platinum electrodes are investigated by cyclic voltammetry and impedance measurements. Cyclic voltammograms show three reduction steps: two steps relate to the platinum cation reduction, and one step relates to the hydrogen reduction. Hydrogen is not involved in the reduction of PtCl4, however it enables the formation of nanopores in the layers. These findings contribute to the understanding of electrochemical growth of nanoporous platinum layers in isopropanol with thickness of 100 nm to 500 nm.