We present an experimental and computational study of the thermal transport properties of nanoporous aluminosilicate crystalline materials. Our study is motivated by the increasing need for accurate characterization and prediction of structure-thermal property relationships in materials of technological significance [1]. Zeolite materials are being increasingly considered for applications as dielectrics in microelectronic devices[2], as micro-heat exchangers [3], as materials for adsorptive and adsorptive-thermoelectric cooling [4, 5], and as hosts for thermoelectric nanowires [6, 7]. Widely used theories for predicting thermal properties of simple crystals fail in these complex nanostructured materials. Furthermore, zeolite materials offer the ability to precisely tune the material structure and composition in order to potentially tune the thermal properties. This can be studied experimentally (using oriented thin films prepared by hydrothermal growth techniques), computationally (via molecular dynamics with high-quality interatomic force fields), and theoretically (via lattice dynamics and relaxation time models) [8]. Specifically, we are interested in studying the effects of composition (Si/Al ratio and presence of ionic guest species), crystal structure, and porosity on thermal properties − both macroscopic (thermal conductivity, specific heat) and microscopic (phonon velocities, relaxation times, scattering rates).

We chose the polycrystalline zeolite MFI as a well characterized model system for measuring and predicting thermal properties. MFI is a zeolite with ordered sinusoidal and straight channels of a nominal pore size ~ 0.55 nm, running along the a- and b- directions respectively. Polycrystalline oriented MFI films were synthesized by seeded hydrothermal growth, and their thermal conductivity was measured by 3w techniques. The 3w technique uses a metal line heater, which functions as both heater and temperature sensor, micro-fabricated onto the film surface. The metal line is heated by a sinusoidal current of frequency w, which results in joule heating and temperature oscillation at 2w frequency. These temperature oscillations are measured by voltage oscillations at the third harmonic 3w. We are able to measure the thermal properties of MFI thin films from 150K to 450K directly using this technique. Intrinsic values of the thermal properties can thus be obtained using the thin .film geometry rather than from powder samples.

We tuned the Si/Al ratio in the MFI thin films from 14 to infinity (pure SiO₂) by hydrothermal synthesis techniques, and then measured the thermal properties of these materials. We present an analysis of the effect of composition on the macroscopic thermal properties such as the thermal conductivity and specific heat. We also show that the detailed calculation of the phonon dispersion curves and the development of a full-Brillouin zone integration scheme allow accurate prediction of the specific heat as well as extraction of phonon relaxation times. Non-equilibrium ″fictitious force″ molecular dynamics (MD) simulations provide a potentially powerful tool for investigating thermal transport in these complex materials. Since accurate MD predictions may serve to limit the need for future experimentation, we also consider the validation of this computational tool versus our experimental data, by means of an MD simulation code developed in-house.

References

1. Cahill, D.G., et al., Nanoscale thermal transport. Journal of Applied Physics, 2003. 93(2): p. 793-818.

2. Wang, Z.B., et al., Pure silica zeolite films as low-k dielectrics by spin-on of nanoparticle suspensions. Advanced Materials, 2001. 13(19): p. 1463.

3. Wojcik, A.M.W., J.C. Jansen, and T. Maschmeyer, Regarding pressure in the adsorber of an adsorption heat pump with thin synthesized zeolite layers on heat exchangers. Microporous and Mesoporous Materials, 2001. 43(3): p. 313-317.

4. Dieng, A.O. and R.Z. Wang, Literature review on solar adsorption technologies for ice-making and air-conditioning purposes and recent developments in solar technology. Renewable & Sustainable Energy Reviews, 2001. 5(4): p. 313-342. 5. Gordon, J.M., et al., The electro-adsorption chiller: a miniaturized cooling cycle with applications to micro-electronics. International Journal of Refrigeration-Revue Internationale Du Froid, 2002. 25(8): p. 1025-1033.

6. Davis, M.E., Ordered porous materials for emerging applications. Nature, 2002. 417(6891): p. 813-821.

7. Tsapatsis, M., Molecular sieves in the nanotechnology era. AICHE Journal, 2002. 48(4): p. 654-660.

8. Greenstein, A., et al., Thermal Properties and Lattice Dynamics of Polycrystalline MFI Films. Microscale Thermophysical Engineering, 2005 (accepted).