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    EXC 3120 BlueMat - Research Topic A1: Hydrogen-Driven Nanoporous Materials
    We will develop a hydrogen-enabled acoustic metamaterial. The key parameter is the wavelength of sound, which needs to be (1) small (in the order of mm to cm) to enable miniaturization and (2) tunable to enable device switching. The wavelength is determined by the ratio of elastic modulus to mass density. As these parameters are usually strongly correlated, mate- rials with the required properties do not currently exist. A promising approach, demonstrated in preliminary research by the members of the RA A team, is np-Pd, which substantially changes its elastic compliance when alloyed with hydrogen. Thus, a metal/water hybrid nanomaterial achieves switching when hydrogen is relocated between its metal and water phases under the control of an electric potential bias. This new effect implies that one and the same metallic body can be reversibly switched between a hard-matter-like elastic response characteristic of metals and a supercompliant characteristic of soft matter, such as polymers or biological tissue. The key scientific questions are: How can the underlying materials design be shaped in terms of base metal composition and microstructure? How can the open-system elastic response in a nanoscale material with complex microstructural geometry be understood? How can processing routes for components that meet the requirements of Topic A4 be established?
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    EXC 3120 BlueMat - Research Topic A4: Bioinspired and Modeling-Guided Nanoporous Device Concepts
    3D hybrid nanomaterial systems developed in A1 to A3 exhibit a variety of water-driven functionalities, including electrochemo-mechanical coupling that enables an active control of acoustic properties (A1, A2), sensing and actuation (A2, A3), and transfer of large actuation energies in metal/polymer/water hybrid nanomaterials (A2, A3) that can be tuned via an external chemical or physical stimulus. By integrating these systems in A4, we aim to achieve applications with high flexibility and even free programmability. Inspiration from biological principles and advanced modeling approaches serve as sources for creativity and innovation, providing a theoretical proof of concept. Key scientific questions include: How can we translate concepts from biology, such as autonomous sensing and versatile shape change, into technological systems? How can we independently control defined material volumes within a material system via an external stimulus? How can we predict the macroscopic behavior of such multiscale systems to identify the most promising designs?
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    Mechanisms of nanostructure formation by dealloying
    Dealloying consists in the almost complete dissolution of only one component from a binary solid solution or an intermetallic compound, generating porosity at the scale of few nanometers from a homogeneous parent phase. Due to the outstanding definition, uniformity and reproducibility of their nanoscale microstructure, nanoporous metals produced in this way, and prototypically nanoporous gold, have become model systems for understanding size- and interface effects on the mechanical as well as functional behavior of nanoscale systems. At the state of the art, dealloying can be realistically modeled by atomistic numerical simulation. In other words, the key elementary processes at the atomic scale have been identified. However, open questions remain: what characteristic length scale is formed, what percentage of the sacrificial component remains in the structure, and how do these parameters depend on the experimental control parameters, in particular the composition of the initial phase and the driving force for dissolution? Answering these questions would enable further refinement of the process and thus the fabrication of homogeneous monolithic bodies of nanomaterials with structure sizes potentially < 5nm. Such materials could then be produced not only as pure components, but also as solid solutions. This would further expand the already highly interesting spectrum of mechanical properties and functional properties of nanoporous metals. The project aims at developing an empirical database, an understanding of the underlying mechanisms, and a materials law with predictive character. The work program in the project is based on an initial hypothesis for the interactions that determine the microstructural evolution. The hypothesis considers the dissolution of the less noble component, the passivation by enrichment of the more noble component at the surface, and the redistribution by curvature-driven diffusion. Closely matched experiments and atomistic simulations will be used to test the hypothesis and develop it further based on the observations. The experiment investigates the electrochemical dealloying of Ag-Au-Pt with different starting compositions, focusing on starting alloys diluted in the noble component, and alloyed with Pt to suppress coarsening by surface diffusion. Microstructural evolution during dealloying is monitored ex situ as well as in situ with small-angle X-ray scattering. Kinetic Monte Carlo simulations mimic the experiment under matched conditions. In addition, simple model scenarios - not accessible to the experiment - are investigated and the results compared with the expectations based on the initial hypothesis.
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