(27d) Analysis Of Phonon Scattering Mechanisms And Compositional Effects In Thermal Transport Properties Of Zeolite Thin Films

Zeolites are nanoporous mixed-oxide crystals with complex structures formed by corner-sharing oxide tetrahedra (TO4; T = Si, Al, etc). The capability of altering the composition of a given zeolite (e.g., by lattice atom substitution or by introducing metal cations and organic molecules into the pores) while maintaining the same crystal structure, or conversely the ability to synthesize different crystal structures with the same composition, makes them extraordinarily versatile materials with novel structure-function relationships as well as many important applications. Newly emerging applications of zeolites include low-k dielectric films for computer chips, materials for adsorption cooling devices, and hosts for nanotube/wire arrays.

Despite the significance of thermal transport properties inherent in their emerging applications, current knowledge of thermal transport in zeolites is relatively limited. This is due both to experimental difficulties as well as theoretical challenges in describing thermal transport in complex materials with large unit cells. Thermal conductivity of zeolites is usually measured by compacting zeolite powders into disks. Computational approaches have relied on classical molecular dynamics (MD) simulations, which can provide reasonable estimates but do not efficiently describe several phonon scattering mechanisms nor the quantum statistical aspects of the phonon physics. Here we report the first study that combines systematic variation of zeolite composition, measurements of thermal conductivity using well-intergrown polycrystalline zeolite films, and modeling of thermal transport incorporating detailed input from atomistic lattice dynamics calculations (made with a high-quality interatomic potential). This approach allows separation and analysis of the structural and dynamical factors that influence the conductivity.

We have chosen the important zeolite MFI as a well-characterized model system. MFI has ordered sinusoidal and straight channels of pore size ~0.55 nm running along [100] and [010] directions respectively. We prepared oriented MFI films of ~10 micron thickness with various Si/Al ratios and constant out-of-plane orientations on polished ?Ñ-alumina substrates by secondary (seeded) growth from a spin-coated seed layer of MFI nanoparticles. The films were polished to sub-10-nm surface roughness, and were characterized by X-ray diffraction, scanning probe microscopy, scanning electron microscopy, and energy dispersive spectroscopy to determine the crystal structure, out-of-plane crystal orientation of the film, surface roughness, and Si/Al ratio. Thermal conductivity measurements were then performed by a 3-omega method after deposition of two-layer metallic line heaters. The experimental results showed that the thermal conductivity was a monotonically increasing function of temperature (in the 100-450K range) and a decreasing function of aluminum content. However, previous theoretical approaches failed to predict the thermal properties accurately.

In our computational approach, we first optimized the crystal structures of MFI materials with various Si/Al ratios and then calculated their phonon dispersions across the entire Brillouin zone using lattice dynamics calculation with an accurate force field. To model phonon scattering processes, we considered three types of scattering: umklapp phonon-phonon scattering, boundary scattering (which includes grain boundaries, stacking faults, lattice strain planes), and point defect scattering (which includes zero-dimensional defect or lattice point substitutions of Si with Al). The model was tested by fitting to the entire set of experimental data with a minimal number of fitted physical parameters from each of the phonon scattering contributions.

The most significant finding from the modeling effort is the fact that the boundary scattering term limits the thermal conductivity and has a much stronger influence than the other scattering mechanisms. In particular, the effective domain size is about 5 nm. This is much smaller than the nanocrystalline domain sizes (25-100 nm) reported in zeolite materials. Therefore, the obtained effective domain size is more reflective of the pore structure of the zeolite (whose periodicity is about 2 nm), which is the main scatterer of lattice vibrations. Below the limit defined by boundary scattering, we found that the thermal conductivity can be effectively suppressed by increasing Al content. We show that this is due both to increased defect scattering as well as phonon slowing. The umklapp scattering model used here, while being the most rigorous of the previously proposed models, is based on a number of assumptions that may not be perfectly valid for MFI. We discuss the development of more generalized umklapp scattering models that would allow a more quantitatively correct estimate of the the relative strength of boundary and umklapp scattering. It is important to emphasize that the present approach, although approximate in the handling of phonon scattering, still represents a considerable advance in measuring and modeling the thermal conductivity of zeolite materials. The synthesis of polycrystalline thin films with controlled compositional variation, thermal property measurements on these films, and the current modeling approach together have potential as a robust platform for understanding thermal transport in complex crystals and separating the contributions from different scattering processes.