Porous materials under chemical and mechanical stimuli

An apocryphal quote attributed to Michelangelo regarding how he made his most famous sculptures is: “It is easy – just remove everything that isn’t David.” This idea can also be useful in materials science in design. In gas sensing, engineering the environment around an active site to have a more favorable environment for an analyte with metal-organic frameworks is becoming an attractive method of achieving high selectivity. In cements, the amount of porosity controls the way that the matrix will deform under a structural load and the way that heat will propagate through the cement matrix – and when porosity decreases below a certain level, the grains themselves start to yield. In this thesis, I will discuss my work with porous materials and how tuned porosity engenders favorable properties. In the first section, porosity was tuned towards a desired chemical adsorption property, using metal organic frameworks (MOFs) as chemical sensing material. MOFs are porous, crystalline materials comprising metal nodes joined by organic linkers. The porosity of the MOFs means that if the MOF’s substrate is also reactive to the analyte of interest then the surface should be made inert, which I showed how to do with self-assembled monolayers. I measured how the work function response of a MOF changed as molecules adsorbed to the interior surfaces. Work function is a measure of the electrical potential in a structure and is highly MOF – analyte specific. I determined the mechanism for work function responses by MOFs on a chemical-sensitive field-effect transistor (CS-FET). In comparing the response to adsorption isotherms and to spectroscopic studies, I found that adsorption at metal sites tended to generate larger changes in work function than adsorption at the linker. I showed that MOFs could also be used as a selective membrane over another active sensing material, such as PdO, yielding improved sensing performance for hydrogen. In the second section, porosity affects the thermal and mechanical properties of the main binding phase in Portland cement, calcium-silicate-hydrates (C-S-H). C-S-H has a nanocrystalline, lamellar structure of 2-D calcium oxide sheets decorated with 1-D silicate chains. The distance between the sheets is controlled by the chemistry of the material. The porosity makes comparing the properties of different structures challenging, because the measured property is a function of the porosity. Using high-pressure Raman spectroscopy, I measured the relative rate of thermal scattering, and found that structures with more spacing between the CaO sheets are more thermally insulating. The technique could also be used to refine atomistic simulations of C-S-H; I found that the current dominant force field CSH-FF underestimates the thermal conductivity of cross-linked silicate chains, and that the force field could be improved by including a covalent term to the Si – O interactions. I also studied C-S-H under deviatoric stress Raman spectroscopy and found the plastic deformation mechanism depends on the structure. In cross-linked silicate structures the grains slide on their calcium oxide sheets, but in non-cross-linked structures, the grains deform by sliding between the silicate chains. X-ray diffraction experiments also showed sliding defects induced by a deviatoric stress. X-ray diffraction experiments also showed a segregation effect, where in high-porosity C-S-H, the lowest-porosity aspect ratio (defined as thickness divided by diameter) grains condense to an ordered phase first. The number of sliding defects is inversely proportional to the volume fraction of the ordered phase and the thickness of water between calcium oxide sheets. My experiments informed the mechanism for hydrated cement creep at the grain level to improve models to inform engineers about the effect of new concrete formulations, such as the inclusion of graphitic waste from methane decarbonization.

Advisor: Maboudian, Roya