Temperature Dependent Thermal Conductivity of Pure Silica MEL and MFI Zeolite Thin Films

Abstract

This paper reports the temperature dependent cross-plane thermal conductivity of pure silica zeolite (PSZ) MEL and MFI thin films between 30 and 315 K. PSZ MFI thin films were made by in situ crystallization. They were b-oriented, fully crystalline and had a 33% porosity. PSZ MEL thin films were prepared by spin coating a suspension of MEL nanoparticles in 1-butanol solution onto silicon substrates followed by calcination and vapor-phase silylation with trimethylchlorosilane. The MEL films consisted of MEL nanoparticles embedded in a non-uniform and highly porous silica matrix. They featured porosity, relative crystallinity, and MEL nanoparticles size ranging from 40% to 59%, 23% to 47% and 55 to 80 nm, respectively. The cross-plane thermal conductivity of these PSZ thin films was measured using the 3ω method. Although the PSZ MFI thin film was fully crystalline and had smaller porosity than the MEL films, it was found to have thermal conductivity only slightly larger than that of MEL thin films for all temperatures. This was due to the fact that the MFI thin film had large microporosity and micropore surface area. This, in turn, greatly enhanced phonon-pore scattering and reduced thermal conductivity. In addition, for PSZ MEL films, the effect of increases in the relative crystallinity and nanoparticle size on the thermal conductivity was compensated by the simultaneous increase in porosity. Finally, all measured thermal conductivity was linearly proportional to Tn with n varying from 2 to 2.6 for temperature T < 60 K. This can be attributed to the fact that the PSZ thin films featured crystalline nanostructures which were highly disordered due to the large surface area of pores and nanocrystals.