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    Open-system elasticity: experimental verification of the Larché-Cahn theory and application in functional materials with switcheable stiffness
    Many materials phenomena are governed by the coupling between chemistry and mechanics. The general theory of the action of that coupling in materials was exposed in the 1970s through the seminal work by Francis Larché and John W Cahn on the elasticity of open systems. As a central conclusion of that theory, the coupled chemo-mechanical equilibrium in the limit of small strain can be projected onto a classic problem of continuum mechanics, provided that the classic elastic constants for constant composition of Hooke’s law are replaced by new, open system elastic parameters. These parameters contain the information on the interaction between composition and stress at thermodynamic equilibrium and subject to constant chemical potential. Explicitly or implicitly, the Larché-Cahn theory is contained in many modern approaches at materials modeling. Yet, no direct experimental verification has been published. Specifically, the theory predictions for the elastic parameters of open systems have not been verified by experiment. These parameters can – according to the theory – be highly interesting because they can deviate strongly from the classic elastic constants at constant composition, because they are highly nonlinear in strain and concentration, and because there numerical values can diverge at critical points of the miscibility gap in alloy phase diagrams.With an eye on the above statements, the proposed research pursues the following twofold aim: Firstly, using a model alloy, we aim at verifying the theory of Larché and Cahn through experimental determination of the open system elastic parameters as the function of the composition. Secondly, we aim to demonstrate a materials design which exploits the results of the theory for achieving – for first time – operando tuneability of elastic stiffness in a wide interval. Ideally, it should be possible to reversibly switch the stiffness almost all the way to zero and back to the pure-metal value.As an original approach, we propose to use nanoporous palladium as the model material. External load induces bending moments on the nanoscale struts that define the microstructure. Hydrogen in the Pd crystal lattice redistributes within milliseconds between the tensile and compressive fiber of the struts. Dynamic mechanical analysis will be used for measuring the effective, macroscopic Young’s modulus, and electrochemical potential control affords fast, precise and reversible control over the hydrogen content.
      108
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    Discotic Liquid Crystals in Nanoporous Solids: From the Structure and Dynamics to Local Charge Transport
    Discotic liquid crystals embedded in nanoporous solids are very interesting hybrid materials with respect to fundamental questions in the physics and chemistry of confined molecular condensed matter. These systems find also increasing interest in organic electronics. Based on the results and expertise gained in our previous studies of these hybrid materials, the main objectives of this project are the exploration to what extent the competition between axial and radial order as well as the phase transitions in general can be influenced by varying the anchoring conditions for the molecules at the pore walls (surface grafting), the size of the discotic cores and/or by employing external magnetic fields. Moreover, we intend to explore how the structure, thermodynamics and molecular dynamics are affected by the resulting changed textures in the nanochannels, by the changed molecule/pore wall interaction and by a gradual filling of the nanochannels from the film-condensed state via the formation of capillary bridges to a complete filling of the channels. The latter will allow for a quantification of the thickness of the boundary layer, that is the layer in the pore wall proximity which is strongly affected by the interaction with the pore walls. To optimize the hybrid systems for real applications the considered pure discotic liquid crystals will be doped to obtain charge-transfer complexes by co-self-assembly. For the bulk systems this strategy has proven as very successful in order to optimize the electric transport properties. Thus, we intend to explore this approach also for the confined liquid crystalline state. Our objectives will be reached by a combination of structure sensitive methods (X-ray scattering, optical birefringence measurements, Differential Scanning Calorimetry etc.) with experiments which are related to the molecular mobility (dielectric spectroscopy, specific heat spectroscopy, in- and quasielastic neutron scattering). To relate the basic research to applications it will be tried to measure the local conductivity of the discotic liquid crystals directly in (isolated) cannels by an AFM-based approach. The project will highly benefit by the orthogonal expertise of both proposers. The successful realization of the project requires an intense cooperation between the two research groups because structure determines the molecular dynamics and vice versa. To warrant this intense cooperation joint project meetings and the exchange of the PhD students for complementary measurements as well as complementary training are plant (additionally to sample exchange). Besides the expected synergistic benefit on the scientific results both PhD students will significantly profit in their scientific education from the cooperative spirit of this research project.
      101
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    FOR 2213: Subproject - Microstructure evolution and impact of mechanics for catalysis
    Processes at materials surfaces in heterogeneous catalysis depend on the state of the surface through local chemical composition, density of defects such as step edges or kinks, and the presence of stable adsorbed species such as oxygen. Chemical composition is a particularly important issue, since previous work on the catalysis of nanoporous gold made by alloy corrosion has emphasized the impact of the re-sidual content of the sacrificial element in the corrosion process, Ag, for catalysis. The first of the two superordinate aims of this project in the ongoing first funding period is to synthesize and characterize nanoporous metal samples in which variations in composition, adsorbate coverage, and structure size are explored. These samples are being supplied to all experimental subprojects (SPs) for studies of catalytic activity (SP1–SP4) and for characterization of surface composition (SP4) and structure (SP6). In the requested second funding period we propose to continue preparing samples via the established protocols as benchmarks for the established sample quality. The research focus in SP3 will shift to investi-gating the microstructure evolution during dealloying and during annealing (a key step for tuning the lig-ament size) as well as in operando during catalysis. This investigation will be based on atomistic computer simulation studies, conducted so as to account for the environmental variables such as time-temperature profiles and the chemical environment at the surface. To this end we shall use a Kinetic Monte Carlo computer code that is already established in our team. The simulation will use materials parameters from DFT (SP7, SP8, SP8(new)) and will be matched to experimental studies of surface composition by x-ray photoelectron spectroscopy (SP4) and of solute distribution by transmission electron microscopy (SP6).Recent theory and experiment advertise a decisive role of elastic strain for heterogeneous catalysis. Na-noporous metals are inherently strained. Exploring and understanding the relevance of mechanics for the catalytic performance of nanoporous gold is the second superordinate aim of this project. The lattice strain is being experimentally quantified for all sample qualities under study, partly in cooperation with SP6. Reference measurements on planar electrodes explore modulated electrocatalytic reaction rates during fast cyclic straining of planar electrode surfaces. The observations will be connected to the exper-imental studies of catalytic or electro-catalytic activity (in SP1–SP4) and of atomic-scale structure (in SP6), and they will be conducted in close feedback with the theory projects supplying in data for adsorption enthalpies (SP7) and surface composition profiles (SP8).
      103
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    Mechanical-chemical coupling at surfaces: prediction and experiment
    While the partial atomic volume of foreign atoms in solid solutions or of constituents in intermetallic compounds, and hence the coupling between the stress state and the chemical potential, have been studied already for many decades by materials scientists, up to now the analogous processes at surfaces have not received much attention. The key goal of this project is the investigation of the coupling between the mechanics of surfaces and adsorption. The basic phenomenon is that a capillary force, the surface stress, is coupled to changes of state at the materials surface. Thus, the adsorption of atoms from gas or ions from solution, or the enrichment/depletion of electronic charge at the surface prompts deformation of the material. This phenomenon has recently found application in modern biosensors, whose miniaturized cantilevers deform measurably upon adsorption of atoms or molecules. Furthermore, it has been shown that the controlled reversible adsorption/desorption of atoms or ions can prompt large strain in nanoporous solids that promise application as metallic actuators with large amplitude and strain energy density. Understanding of the microscopic mechanisms behind the coupling of changes of state at materials surfaces and the stress state is still rudimentary at best. A theory that could predict the coupling strength for a given materials surface and adsorbate is still missing. Moreover, the surface stress changes upon reversible adsorption and desorption of light elements, which are particularly relevant for applications in actuation and sensing, remain poorly studied. In this context, the present proposal aims at 1.) providing for the first time quantitative values for the coupling between capillary force and adsorption from both experiment and theory and 2.) insight into the underlying microscopic mechanims. As model processes we plan to study the adsorption of H and O on Au, Pt and Ir surfaces. The theoretical method for calculation of the coupling strength is density functional theory. For efficient and exhaustive screening of the configuration space we additionally employ cluster expansion methods. In experiment, changes of the surface stress upon adsorption from gas and electrosorption are determined from cantilever bending. Complementary measurements of the variation of the electrode potential as a function of strain during different electrosorption processes are carried out with Dynamic Electro-Chemo-Mechanical Analysis, a technique developed by one of the applicants.
      87
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    SFB 986: Subproject B02 - Hybrid materials based on nanoporous metal - hierachy and function
    The strength of nanosize objects increases with decreasing size and can approximate the theoretical shear strength. We use dealloying for creating nanoporous materials that are networks of metallic nanoscale "ligaments". These materials can be made with macroscopic dimensions and can, in principle, be shaped into engineering components comprising in the order of 1014 ligaments per mm3. Nanoporous metals also offer new opportunities for mate-rials design towards functionality. Throughout the pore space, the state of the metal surface can be reversibly modulated under control of electrical or chemical signals. This affords changes of the capillary forces and of the optical or chemical properties in the interfacial regions, strongly affecting the effective macroscopic materials behavior. So far we have addressed size-dependent strength and elasticity, the impact of the surfaces for the mechanical behavior, and the role of the network topology. In the second funding period we aim to demonstrate materials that simultaneously bring interesting mechanical behavior and novel functionality. As one instance, we have shown that nanoporous gold-based hybrid nanomaterials can exhibit an effective piezoelectric response. This is remarkable since piezoelectricity is classically restricted to ceramics.
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    SFB 986: Subproject B04 - Micromechanical behavior of hierarchical materials
    Das Teilprojekt (TP) B4 befasst sich mit der Untersuchung der mechanischen Eigenschaften von hierarchischen Materialien mit dem Fokus auf die Mikroskala. Der Einfluss von Größeneffekten und Grenzflächeneigenschaften steht dabei im Vordergrund. Die in TP B4 entwickelten inversen Methoden zur Bestimmung des nichtlinearen mechanischen Verhaltens einzelner Phasen und Grenzflächen sowie die gewonnenen Erkenntnisse leisten einen wichtigen Beitrag zum Verständnis, wie die Mikromechanik das makroskopische Verformungs-, Versagens- und Bruchverhalten kontrolliert. Das TP B4 unterstützt mit diesen Erkenntnissen die Entwicklung der hierarchischen Materialien im SFB im Hinblick auf optimale makroskopische Eigenschaften.
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