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Browsing by Subject "micropillar compression"

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    Development of Nanocomposites Based on Metal–Organic Frameworks and Three-Dimensionally Ordered Macroporous Materials as Potential Catalysts and Structural Materials
    (2021-05) Wang, Zhao
    Due to the presence of nanosized features, some properties of nanocomposites can be drastically improved compared to the individual components. As a result, nanocomposites are attractive in many fields, including energy storage, energy conversion, and catalysis. This dissertation focuses on the development of functional nanocomposites using metal–organic frameworks (MOFs) and three-dimensionally ordered macroporous (3DOM) materials. Three major topics are covered in this thesis research. This dissertation begins with an investigation of the influence of phase purity and pore reinforcement on the mechanical properties of NU-1000 MOF particles. The overall goal of this work is to develop methods of enhancing the mechanical strength of MOFs for practical applications, where densification is likely needed. By flat punch nanoindentation and finite element simulation, the elastic modulus of NU-1000 was found to increase by nearly an order of magnitude through maintaining phase purity and minimizing structural defects. Additionally, introduction of silica into the mesopores of NU-1000 significantly increased the load at failure of NU-1000 particles from 2000 μN to 3000−4000 μN. The results of this work suggest two potential MOF engineering pathways, namely, increasing the phase purity and introducing pore reinforcement to generate more robust MOFs. The second part of this dissertation describes new approaches of fabricating nanohybrids containing sub-nanosized clusters through nanocasting MOFs as potential catalysts. The purpose of this work is to preserve the catalytically active clusters in MOF materials at high temperatures by replacing the original organic framework with a more robust support, while the ordered and porous structures are maintained. In this context, two types of nanocomposites (a silica–oxocerium cluster nanohybrid and a carbon–oxozirconium cluster nanohybrid) were synthesized. Preservation of the clusters in both systems was proved by multiple characterization techniques, including XRD, TEM, Raman spectroscopy, and pair distribution function (PDF) analysis. In terms of the potential catalytic abilities, the silica–oxocerium cluster nanohybrid material has a remarkably high loading of cerium (51 wt%) and exhibits redox activity at 750 °C without aggregation, which are properties of interest for high-temperature redox catalysis. The carbon–oxozirconium cluster nanohybrid exhibits a relatively high surface area (266 m2/g), and moderate electrical conductivity (0.41 S/m), which are properties of interest for electrochemical catalysis. The last part of this dissertation focuses on the synthesis and mechanical properties of 3DOM tungsten (W) and a 3DOM tungsten–silicon oxycarbide (W–SiOC) nanocomposite. The purpose of this work is to investigate the possibility of applying architected materials as strong and low-density structural nanomaterials. Another goal is to fabricate a well-designed microstructure based on the 3DOM matrix. By micropillar compression, the yield strength of ligaments in 3DOM W was measured to be 6.1 GPa, ~6× yield strength of coarse-grained W. The 3DOM W was then used to prepare 3DOM W–SiOC, which exhibited an ordered, interpenetrating, and periodic 3D structure. The maximum stress that the 3DOM W–SiOC material could endure was 1.1 GPa at 30 °C, a 20-fold increase compared to the 3DOM W matrix. The results of this work demonstrate that metallic 3DOM structures could exhibit good strength and be used to fabricate other robust interpenetrating nanocomposites.