Lukas Grünewald

Bio: Lukas Grünewald is an academic researcher from Karlsruhe Institute of Technology. The author has contributed to research in topics: Thin film & Pulsed laser deposition. The author has an hindex of 3, co-authored 9 publications receiving 33 citations.

The impact of grain size, A/B-cation ratio, and Y-doping on secondary phase formation in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

Abstract: The application of mixed ionic–electronic conducting (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ (BSCF) as gas separation membrane is up to now hampered by secondary phase formation which impairs the excellent oxygen permeation properties of this material. In this work, we have studied the impact of grain size and A/B-cation ratio on secondary phase formation in BSCF and Y-doped (Ba0.5Sr0.5)(Co0.8Fe0.2)0.9Y0.1O3−δ (BSCF10Y) by electron microscopic techniques before and after long-term thermal exposure at an application-relevant temperature (~760 °C). A large content of secondary phases is found in samples with small grain sizes because grain boundaries provide nucleation sites for secondary phases. Higher sintering temperatures increase the grain sizes and substantially reduce the content of secondary phases. Variations of the A/B-cation ratio between (Ba0.5Sr0.5)0.95(Co0.8Fe0.2)O3−δ and (Ba0.5Sr0.5)1.05(Co0.8Fe0.2)O3−δ do not lead to a change of the composition of the cubic BSCF phase but changes the volume fraction of Co3O4 precipitates which are already formed during sintering. BSCF with an excess of A-site cations contains the smallest overall amount of secondary phases in undoped BSCF due to the minimization of Co3O4 precipitation during sintering and the reduction of nucleation sites for other secondary phases at application-relevant temperatures. Secondary phase formation in BSCF10Y can be almost completely suppressed due to the stabilization of the cubic BSCF phase by Y-doping and large grain sizes after high-temperature sintering.

Structural and chemical properties of superconducting Co-doped BaFe2As2 thin films grown on CaF2

Abstract: Thin films of Co-doped BaFe$_2$As$_2$ of similar thickness (~40 nm) were grown with different growth rates (0.4 A s$^{-1}$ and 0.9 A s$^{-1}$) by pulsed laser deposition on CaF$_2$(001) substrates. Analytical transmission electron microscopy (TEM) was applied to analyze the microstructure and secondary phases. The formation of BaF$_2$ and a high concentration of planar defects (mainly stacking faults) are observed for the sample grown at a low rate. A higher growth rate results in high-quality epitaxial films with only few antiphase boundaries. A higher $T_\text{c}$ was measured for the sample grown at a low growth rate, which is attributed to the difference in strain state induced by the high concentration of defects. Large crystalline Fe precipitates are observed in both samples. Chemical analysis shows a pronounced O and slight F content at the planar defects which highlights the role of O in defect formation. Electron-beam-induced irradiation damage during TEM measurements is observed and discussed.

Fabrication of phase masks from amorphous carbon thin films for electron-beam shaping

Abstract: Background: Electron-beam shaping opens up the possibility for novel imaging techniques in scanning (transmission) electron microscopy (S(T)EM). Phase-modulating thin-film devices (phase masks) made of amorphous silicon nitride are commonly used to generate a wide range of different beam shapes. An additional conductive layer on such a device is required to avoid charging under electron-beam irradiation, which induces unwanted scattering events. Results: Phase masks of conductive amorphous carbon (aC) were successfully fabricated with optical lithography and focused ion beam milling. Analysis by TEM shows the successful generation of Bessel and vortex beams. No charging or degradation of the aC phase masks was observed. Conclusion: Amorphous carbon can be used as an alternative to silicon nitride for phase masks at the expense of a more complex fabrication process. The quality of arbitrary beam shapes could benefit from the application of phase masks made of amorphous C.

Material Discovery and Design Principles for Stable, High Activity Perovskite Cathodes for Solid Oxide Fuel Cells

Abstract: Critical to the development of improved solid oxide fuel cell (SOFC) technology are novel compounds with high oxygen reduction reaction (ORR) catalytic activity and robust stability under cathode operating conditions. Approximately 2145 distinct perovskite compositions are screened for potential use as high activity, stable SOFC cathodes, and it is verified that the screening methodology qualitatively reproduces the experimental activity, stability, and conduction properties of well-studied cathode materials. The calculated oxygen p-band center is used as a first principle-based descriptor of the surface exchange coefficient (k*), which in turn correlates with cathode ORR activity. Convex hull analysis is used under operating conditions in the presence of oxygen, hydrogen, and water vapor to determine thermodynamic stability. This search has yielded 52 potential cathode materials with good predicted stability in typical SOFC operating conditions and predicted k* on par with leading ORR perovskite catalysts. The established trends in predicted k* and stability are used to suggest methods of improving the performance of known promising compounds. The material design strategies and new materials discovered in the computational search help enable the development of high activity, stable compounds for use in future solid oxide fuel cells and related applications.

Surface Segregation in Solid Oxide Cell Oxygen Electrodes: Phenomena, Mitigation Strategies and Electrochemical Properties

Abstract: Solid oxide cells (SOCs) are highly efficient and environmentally benign devices that can be used to store renewable electrical energy in the form of fuels such as hydrogen in the solid oxide electrolysis cell mode and regenerate electrical power using stored fuels in the solid oxide fuel cell mode. Despite this, insufficient long-term durability over 5–10 years in terms of lifespan remains a critical issue in the development of reliable SOC technologies in which the surface segregation of cations, particularly strontium (Sr) on oxygen electrodes, plays a critical role in the surface chemistry of oxygen electrodes and is integral to the overall performance and durability of SOCs. Due to this, this review will provide a critical overview of the surface segregation phenomenon, including influential factors, driving forces, reactivity with volatile impurities such as chromium, boron, sulphur and carbon dioxide, interactions at electrode/electrolyte interfaces and influences on the electrochemical performance and stability of SOCs with an emphasis on Sr segregation in widely investigated (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3−δ. In addition, this review will present strategies for the mitigation of Sr surface segregation.

Thermal scanning probe lithography—a review

Abstract: Fundamental aspects and state-of-the-art results of thermal scanning probe lithography (t-SPL) are reviewed here. t-SPL is an emerging direct-write nanolithography method with many unique properties which enable original or improved nano-patterning in application fields ranging from quantum technologies to material science. In particular, ultrafast and highly localized thermal processing of surfaces can be achieved through the sharp heated tip in t-SPL to generate high-resolution patterns. We investigate t-SPL as a means of generating three types of material interaction: removal, conversion, and addition. Each of these categories is illustrated with process parameters and application examples, as well as their respective opportunities and challenges. Our intention is to provide a knowledge base of t-SPL capabilities and current limitations and to guide nanoengineers to the best-fitting approach of t-SPL for their challenges in nanofabrication or material science. Many potential applications of nanoscale modifications with thermal probes still wait to be explored, in particular when one can utilize the inherently ultrahigh heating and cooling rates. Thermal scanning probe lithography is reviewed in the context of material removal, conversion and deposition. Scanning probe lithography has long been a promising technique for direct-write nanoscale patterning on surfaces. However, while the technique is widely used in research labs, the slow write speed has limited its use in industrial settings. Instead, thermal scanning probe lithography has emerged, in which a heated tip is used to induce localized changes in the material, enabling write speeds limited by the speed of movement of the tip itself. A team from Ecole Polytechnique Federale de Lausanne led by Juergen Brugger now reviews the current state of play for thermal scanning probe lithography, focusing on whether material is removed, changed or deposited by the heated tip, and the types of materials that have been studied.