Identifying Composition-Process-Defect-Structure-Property Correlations in (La)-Co-X-Y-O Thin Film Libraries (X, Y: Fe, V, Mn, Al, Mo, Ni)

Research Data Management and AI-driven Knowledge Discovery

This project addresses the exploration of VO2-based thermochromic thin films in V-M1-M2-O systems (M1, M2: e.g. W, Mg, Er, Li) for future application in smart windows. Films to be developed will show high luminous transmittance (Tlum) and solar irradiation modulation (ΔTsol) upon a reversible phase transformation around room temperature. Addition of third elements can decrease the transition temperature (Tc) and improve the thermochromic performance (Tlum and ΔTsol) of VO2-based phases: e.g. W can effectively decrease Tc of VO2 (68°C), but Tlum and ΔTsol are deteriorated; alkaline and rare-earth metals improve Tlum but do not lower substantially Tc. Therefore, a multidimensional search space needs to be explored in order to identify a new multinary material which fulfils all necessary requirements. Thus, to achieve better performance of VO2-based thin films for smart windows, first, V-M1-O systems with unexplored elements for M1 will be investigated in search for elements which reduce Tc and improve Tlum. Then, quaternary systems (V-M1-M2-O) will be explored: synergistic effects of M1 and M2 on the thermochromic properties of VO2 are expected, to achieve lower Tc and higher Tlum. This needs a large experimental dataset which will be accomplished by synthesis of combinatorial materials libraries and their high-throughput characterization. The best identified systems will additionally be fabricated as nanoporous films by glancing angle deposition in order to further enhance Tlum, and a protective antireflective coating will be applied to further optimize Tlum.

This proposal represents the extension of the project “VO2-Based Thin Film Shape Memory Nanoactuators”. During the previous project, (V-M)O2 (M=3rd metals) thin-film materials libraries have been synthesized by combinatorial co-sputtering with their phase transformation properties and mechanical performances characterized by high-throughput methods. Furthermore, novel nanotechnology processes have been established to obtain VO2-based nanoactuators with dimensions down to 100 nm. A size effect in the electrical resistance change upon the phase transformation of the nanoactuator has been identified. Our research demonstrates the large potential of VO2-based devices for electro-thermo-mechanical as well as thermochromic switching and tuning at the nanoscale, which could meet the urgent need of compact multifunctional solutions in the rapidly evolving field of nanophotonics. The project extension addresses this challenge and goes well beyond the current state-of-the art combining two interlinked parts: Subproject I (RUB) will explore up to quaternary (V-M1-M2)O2 systems in order to improve the mechanical performance of the thin film (e.g., strain and stress change) and to tailor the phase transformation properties. The shape memory oxide ZrO2 will be combined with VO2 to the new system (V-Zr)O2 aiming at an extended stress change and working-temperature range of up to 200 °C. Based on the new material systems, subproject II (KIT) will develop novel nanoactuator designs with enhanced functionality and related fabrication processes for tailoring of out-of-plane curvature and bistable operation. The application of VO2-based films in the field of photonic waveguide switching will be exploited based on the electro-thermo-mechanical and the thermochromic effect as well as alternative electro-optical and all-optical concepts.

The project aims to accelerate catalyst discovery for CO2 electroreduction (CO2RR) by developing a new methodology for high-throughput testing of new electrocatalyst materials at industrially relevant current densities. We plan to bridge combinatorial co-sputter synthesis of thin-film materials libraries (MLs) with testing of catalyst materials at the gas diffusion electrode (GDE) level by using scanning electrochemical cell microscopy (SECCM) as a high-throughput screening technique to evaluate CO2RR electrocatalyst’s activity at high current densities. In the first part of the project, we aim to understand how SECCM measurements need to be conducted on thin-film electrocatalysts to obtain similar activities as in a GDE. Au and Cu will be used as model catalysts, and sputter techniques will be employed to generate thin-film catalyst layers on flat substrates or 3D-porous PTFE membranes. Improving the adhesion of the metal layer on the PTFE substrate during the sputtering process is targeted. In the second part of the project, we plan to synthesize and characterize MLs, obtained via co-sputtering of elements that strongly (Co, Ni) and weakly (Ag, Au) bind CO and H, aiming to discover new CO2RR to enable the selective synthesis of multicarbon products (C≥2) at industrially relevant current densities. SECCM will be used to evaluate the electrocatalytic activity of the different materials comprised in a ML to identify new catalysts that can reduce CO2 with a minimum current density of 100 mA cm-2, labeled as HITs. The identified HIT compositions will be transferred on GDEs by sputtering on membranes. The CO2RR selectivity of the catalysts embedded in a GDE will be evaluated in a flow-cell electrolyzer coupled with online gas-chromatography and high-liquid performance chromatography. Improving catalyst film stability in a GDE and tuning the catalyst microenvironment on the GDE to modulate selectivity will be realized by co-sputtering of binary and ternary catalysts with PTFE. The correlation of catalyst selectivity with catalyst and electrode structures will be derived, going thus beyond state of the art, where catalyst selectivity is often described without considering the complete system.

This project builds up on the results of its predecessor project “Exploring Multinary Nanoparticles by Combinatorial Sputtering into Ionic Liquids and Advanced Transmission Electron Microscopy” (DFG projects LU1175/23-1 and SCHE 634/21-1) with the aim to further enhance the knowledge about multinary nanoparticles synthesized by (co-)sputtering into ionic liquids in comparison to thin films grown on solid substrates employing the same sputter deposition chamber and parameters. Special emphasis is laid on multinary intermetallic (Fe-Co-Ni)(Pt,Pd) systems in their equilibrium ordered crystal structure in comparison to their non-equilibrium solid solution counterparts with the same chemical composition as this offers unique opportunities to understand the influence of order/disorder on electrochemical performance and stability. By the application of versatile sputter techniques like high-power impulse magnetron sputtering new capabilities of influencing the nanoparticle formation and their phase constitution, crystal structures and composition might become possible in comparison to direct current sputtering used in the predecessor project which appeared to be limited to bulk-miscible elements. We also want to explore whether we can obtain the same crystal structure and composition of the nanoparticles and thin films which, with direct current sputtering, was not possible. The nanoparticles typically adopted the equilibrium crystal structure and composition according to the bulk phase diagram while the thin films grew with a non-equilibrium structure and required annealing to reach the equilibrium state. The advantage of achieving comparable structure and composition for nanoparticles and thin films is immense as screening via material libraries is easier for thin films. The main focus is laid on the synthesis of multinary (Fe-Co-Ni)(Pt,Pd) intermetallic systems with high catalytic activity for the oxygen reduction reaction or hydrogen evolution reaction and stability during electrochemical load. Besides the composition, the crystal structure (solid solution versus ordered phases) and the crystallinity itself (amorphous versus short range order versus crystalline) are most likely tuneable when applying high-power impulse magnetron sputtering and by annealing treatments. The most active systems will be investigated in depth using advanced aberration corrected high-resolution scanning transmission electron microscopy, including various high-resolution imaging techniques, tomography, energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. In addition, the most interesting nanoparticles will be extracted from the ionic liquid and be studied at identical location with scanning transmission electron microscopy before and after several thousand cyclic voltammetry measurements to investigate their (in-)stability and identify at the atomic scale reasons for example for dissolution.

The demand for green energy has constantly risen with increasing energy consumption and the impending climate change. This is exacerbated by unabated fossil fuel depletion whereas 60% of primary energy is wasted as unused heat. Thermoelectric (TE) materials are a viable green energy alternative as they convert heat directly into electricity. However, limited performance but also lack of mechanical robustness and usage of expensive, scarce, and toxic elements in to-date TE materials critically limit their application. Half-Heusler (HH) materials, equiatomic ternary compounds with high mechanical strength, have shown promise for TE applications and offer a huge compositional space with cheap, abundant, and environmentally benign elements that allows to vary their TE performance. The proposal targets at evaluating the full potential of the HH compositional space including iso- and alioelectronic substitution towards highly efficient TE materials by a combination of artificial intelligence tools and high-throughput synthesis and characterization. Machine learning (ML) can facilitate this exploration by combinatorial search, collation and application of small experimental datasets. However, deciphering the structure–property relationships from ‘black-box’ ML models, Density Functional Theory (DFT) validation of ML-screened substitutional compounds, but also experimental techniques for fabrication and high-throughput characterization of material libraries, involving microstructure, to validate ML and DFT results are major challenges here. A multi-throng approach involving ML, DFT, and experiments shall address these. It covers development of physically interpretable ML models using small-dataset compliant symbolic regression- and symbolic distillation-based ML techniques in an active learning framework for predicting the TE properties of the entire HH chemical space. TE properties predicted by the ML high-throughput screening will be validated by advanced DFT calculations on phase stability, electronic and thermal properties. To fully exploit the compositional space irrespective of the dopant-bound carrier concentration of the TE material, a novel concept using the material quality factor instead of the TE figure of merit zT as a guiding target parameter for ML screening will be rated. Experimental screening of promising compositions by thin-film and bulk material libraries and high-throughput characterization as well as synthesis of selected compositions as homogeneous bulk samples along with microstructural analysis will feed back to the ML training and shall resolve discrepancies between ML, DFT, and experimental results. This approach will not only facilitate the exhaustive combinatorial development of efficient HH compounds but will also allow for future inverse design of TE materials due to gene features that shall be identified from physically interpretable ML descriptors.

Our previous work identified that additional to LiAlO2, Li-manganates (e.g. LiMnO2 or Li2MnO3) are promising EnAM phases, that crystallize in synthetic slags with high purity. The formation of Li-manganates is expected to be adjustable by controlling the Mn oxidation state, namely by preventing the formation of Mn-containing spinels by stabilizing Mn4+ rather than Mn2+/3+ states with processing conditions. In addition, Li-manganates are interesting for direct reuse as cathode material in Li-ion batteries. The continuation of our joint project will therefore focus on exploring the possibilities of Li-manganates as new EnAM phases, and how their formation correlates with LiAlO2 formation. Combinatorial thin film materials libraries of the ternary and quaternary subsystems Li-M1-M2-O, with M1, 2 = Mn, Al, Mg, Fe, will be prepared by sputter deposition and investigated with high-throughput methods to study the phase formation of the systems, in particular to evaluate all possible Li-manganates in dependence of the Mn speciation. The formation of the compounds identified as potentially suitable EnAM phases (presumably Li-manganates) will then be studied in synthetic slags comprising additional elements like Si and Ca. Synthetic slag samples will be prepared at various O2 partial pressures and investigated by X-ray diffraction, electron probe microanalysis, nanoscale mass spectrometry and thermoanalysis. In conjunction with thermodynamic modeling, these experimental results will provide insight into the solidification behavior of Li-manganates in Ca-silicate slag and in presence of limited amounts of common minor elements (contaminants) like Mg and Al. Additionally this results will help to optimize cooling curves for maximum scavenging efficiency and favorable grain morphology and size. In addition to Ca-silicate slags, fayalitic slags (Fe2SiO4-dominated) are investigated, as they maximize the available oxygen in the melt due to their potential to significantly increase oxygen transport into the melt, which should lead to better oxidation of the Mn in the melt (e.g. stabilization of Mn4+). To assess the suitability of the Li-manganates formed in the synthetic slag and thin film samples for direct reuse as cathode materials in Li-ion batteries, the nanostructure of selected samples will be additionally investigated using aberration-corrected transmission electron microscopy and atom probe tomography. All results combined will provide a better insight into the stability of Li-manganates, their properties and their recoverability from lithium-containing slags.

This proposal represents the renewal of the joint project “Cooperative shape memory actuator systems for nanomechanics and nanophotonics”. During the first funding period we demonstrated that bi-directional actuation by Joule heating above room temperature is possible for TiNiHf shape memory alloy (SMA) films with thicknesses down to 220 nm. However, trimorph PMMA/TiNiHf/Si structures envisioned for bistable actuation could only be investigated by coupled simulations so far revealing the need for thick polymer layers, which introduces issues in manufacturing and in thermal actuation. Therefore, in this project extension, we will (1) mitigate these issues by introducing and investigating novel concepts for bistable SMA actuation in-plane and out-of-plane, (2) explore and numerically describe downscaling and size effects, and (3) develop cooperative multistable in-plane and out-of-plane microactuators for Si micromechanics and Si nanophotonics applications, respectively. Bistable SMA microactuation will be based on coupled pre-strained SMA bridge microactuators. Their functionality will depend on the stress from thermal treatment and intrinsic stress from the thin film growth process. A computationally-aided iterative design process will help to identify robust and energy-efficient actuator system layouts with high precision and downsizing potential (S. Wulfinghoff). Accurately controlled material properties, including stress engineering (A. Ludwig) will be combined with state-of-the-art micromachining for the co-integration of SMA/Si microactuators with Si micromechanical and/or nanophotonic waveguide structures (M. Kohl). Based on a combined experimental and simulation-based assessment of the multistable microactuator systems, undesired cross-sensitivities will be minimized and synergies will be enhanced.

Semi-autonomous combinatorial sputter system for the synthesis of multinary material libraries

xemX:
Exploration of high-dimensional compositional spaces for the accelerated development of electrochemical catalysts

DIMENSION: Determining materials for energy conversion - Establishing a fast track towards processing and evaluation