Thermal Conductivity of Ordered Mesoporous Titania Films Made from Nanocrystalline Building Blocks and Sol−Gel Reagents

Abstract

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 370 nm, 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 3ω 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 thin films. This result can be attributed to the fact that heat is carried, in the amorphous matrix, by localized nonpropagating vibrational modes. The thermal conductivity of crystalline sol−gel mesoporous 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 nanocrystals or the crystalline domains can provide control over thermal conductivity in addition to porosity.