Thermal Conductivity of Mesoporous Titania Films Made From Nanocrystalline Building Blocks and Sol-Gel Reagents
Published Web Locationhttp://pubs.acs.org/doi/abs/10.1021/jp203400t
This paper reports the cross-plane thermal conductivity of amorphous and crystalline mesoporous titania thin films synthesized by evaporation-induced self-assembly. Both sol-gel and nanocrystal-based mesoporous films were investigated, with average porosities of 30% and 35%, respectively. The pore diameter ranged from 7 to 30 nm and film thickness from 60 to 370nm while the average wall thickness varied from 3 to 50 nm. The crystalline domain sizes in sol-gel films varied from 12 to 13 nm while the nanocrystal-based films consisted of monodisperse nanocrystals 9 nm in diameter. The cross-plane thermal conductivity was measured at room temperature using the 3w method. The average thermal conductivity of the amorphous sol-gel mesoporous titania films was 0.37 ± 0.05 W/m.K. It did not show strong dependence on pore diameter, wall thickness, and film thickness for sol-gel amorphous mesoporous titania thinfilms. This result can be attributed to the fact that heat is carried, in the amorphous matrix, by localized non-propagating vibrational modes. The thermal conductivity of crystalline sol-gelmesoporous titania thin films was significantly larger at 1.06 ± 0.04 W/m.K and depended on the organic template used to make the films. The thermal conductivity of nanocrystal-based thin films was 0.48 ± 0.05 W/m.K and significantly lower than that of the crystalline sol-gel mesoporous thin films. This was due to the fact that the nanocrystals were not as well interconnected as the crystalline domains in the crystalline sol-gel films. These results suggest that both connectivity and size of the nanocrys-tals or the crystalline domains can provide control over thermal conductivity in addition to porosity.