Deformation of hierarchical and anisotropic porous solids by fluid adsorption

The goal of the proposed research is to enhance the fundamental understanding of adsorption-induced deformation in monolithic materials with hierarchical porosity. Macroporous networks consisting of struts of ordered mesoporous silica with microporous walls will be synthesized, while controlling pore size, pore volume-fraction, and particularly also pore anisotropy at all hierarchy levels. Adsorption-induced deformation will be investigated quantitatively at the level of the mesopore system as well as on the macroscopic level. The central motivation for the proposed research is to derive basic correlations between the physico-chemical parameters of adsorption- induced deformation at the nanometer scale, and the hierarchical and anisotropic network structure with resulting actuated mechanical behavior at the macroscopic level. This knowledge forms the basis for future applications of hierarchically organized porous systems in designed switchable components, for instance as actuators, thermal insulation, or acoustic-mechanical components. The results of the project will also provide a better understanding of the role of adsorption-induced deformation in degradation processes of such materials. The basic goals and innovative aspects of the proposed research are the following: i) New synthesis approaches will be developed to tailor the degree of anisotropy in monolithic silica with hierarchical porosity at different levels, while controlling fluid-wall interaction and microporosity. Synthesis is based on sol-gel routes under external force fields such as shear or uniaxial compression. Model carbon and silicon replica will be derived from these silica monoliths by hard templating and reactive conversion, respectively. ii) Different length scales and anisotropy will be explicitly considered experimentally in the investigation of adsorption-induced deformation. Structural investigations using sophisticated synchrotron radiation based techniques will allow to characterize structure at all scales. A unique combination of in-situ techniques applied during fluid adsorption (in-situ X-ray diffraction and in-situ dilatometry) will be used to quantitatively derive the influence of hierarchy on the macroscopic response to the fluid induced deformation at the nanometer level. Simple model fluids such as n-pentane and N2 will be employed to understand the basic underlying mechanisms. Moreover, water sorption will also be considered as model case for a more complex adsorptive towards applications. iii) Hierarchical mechanical models of adsorption-induced deformation will be developed and validated. They are based on analytical and/or numerical models for single mesopore deformation, coupled with finite element (FEM) calculations to derive the macroscopic strains. The project combines unique and complementary expertise of three experimental groups in Austria and Germany, supported by an international cooperation partner working on the theory of adsorption- induced deformation. The expected outcomes of this basic research project are believed to fertilize both, theoreticians and modeling groups working on a better understanding of sorption-induced deformation in general, as well as applied scientists developing devices for sensing and actuation, for energy related application, and for catalysis.

Hierarchically organized, porous structures exist in many biological materials where they perform different structural and/or functional tasks. Some of these materials are able to respond to changes in external conditions by mechanical deformation. This autonomous mechanical reaction to environmental influences based on material structure represents a promising fundamental concept for the design of artificial sensors or actuators. The aim of this project was to synthesize various hierarchically structured, porous materials and to investigate their respective mechanical and structural response to the adsorption of fluids. Monolithic samples with hierarchical and partly ordered pore structure were prepared, allowing the investigation of both, the macroscopic length-change and the deformation of the nanostructure as a response to fluid adsorption. The pore structure of the synthesized materials was varied by targeted modifications in order to evaluate their influence on adsorption-induced deformation. The project was divided into three work packages: In package 1 (Hüsing, University of Salzburg), hierarchically structured silica and carbons were synthesized by means of sol-gel processes and subsequently characterized in a comprehensive manner. In addition, anisotropic alignment of the macrostructure of the porous network was achieved by extrusion-based mechanical shearing during the gelation process. Package 2 (Paris, Montanuniversität Leoben) included the investigation of adsorption-induced deformation on the nanometer scale by means of in-situ scattering methods, while package 3 (AG Reichenauer, Bavarian Center for Applied Energy Research, Würzburg) covered the investigation of the deformation on the macroscale by means of in-situ dilatometry. For packages 2 and 3, specific experimental setups were designed in close cooperation of the Leoben- and Würzburg partners to be able to carry out these novel and complex measuring procedures, which were partly conducted at large-scale neutron and X-ray research facilities in Garching, Germany, and Grenoble, France. Theoretical models for adsorption induced deformation of anisotropic porous systems were set-up within a collaboration with two US research groups (Neimark, Rutgers University und Gor, New Jersey Institute of Technology), and validated against the experimental data. The models allowed also to extract absolute quantities characterizing the mechanical properties of the samples. In summary, the investigations in this project provided a very fundamental understanding of the correlation between material properties and adsorption-induced deformation as well as the mechanisms of this deformation on different scales. They constitute a solid basis for further targeted optimization of hierarchically structured and anisotropic porous materials. The work was broadly disseminated by 30 talks and posters at international conferences. Five peer-reviewed scientific papers were published so far, two more are under review and four more are in their final preparation phase.