Synthesis, structure, and application of oriented mesoporous films and microporous titanosilicate molecular sieves
Unlike most other porous materials, which have broad pore size distributions and poorly defined pores, microporous and mesoporous molecular sieve materials are frameworks with pores of well-defined size and geometry. The pore diameters are tunable and are of molecular dimensions allowing the materials to exclude molecules based on size and shape. As a result, powders of both microporous and mesoporous molecular sieves have found applications in catalysis and selective adsorption. Many new and potentially beneficial applications would be possible if thin films of these materials could be synthesized with a desired orientation of the pore structure.
Previously, mesoporous silica films had been grown only with the pores parallel to the substrate. In order to achieve directional control of the pores, we synthesized mesoporous silica films in externally applied fields. Growth in shear flow fields gave the best results. However, this technique is limited to imparting directional orientation to channels, which remain in the plane of the substrate. We have utilized this method of channel orientation in an attempt to form mesoporous membranes by depositing mesostructured precursors in the pores of a macroporous substrate.
Also, we have researched the structure of a new microporous titanosilicate molecular sieve, designated ETS-4. Thermally treated samples of this material have been shown to be nitrogen selective for methane/nitrogen mixtures. By using the methods of Fourier synthesis recycling and Rietveld refinement of x-ray and neutron diffraction data, we have revealed the shifting of cation positions and a lattice contraction of the structure during heating.
Microporous and mesoporous materials are formed from the bottom-up and have pores that are of dimensions unattainable with topdown lithographic techniques. As a result, the pores of these materials give us the potential to template the growth of materials on the nanometer length scale. In order to evaluate their potential application in thermoelectric devices, we have modeled the thermoelectric transport properties of composite films using the semi-classical theory of electron transport. Our results suggest that oriented microporous and mesoporous films would make excellent hosts for nanowires and may yield thermoelectric devices with efficiencies a factor of 4 higher than current materials.
0794: Materials science