This paper reports the cross-plane thermal conductivity of highly ordered cubic and hexagonal templated mesoporous amorphous silica thin films synthesized by evaporation-induced self-assembly process. Cubic and hexagonal films featured spherical and cylindrical pores and average porosity of 25% and 45%, respectively. The pore diameters ranged from 3 to 18 nm and film thickness from 80 to 540 nm while the average wall thickness varied from 3 to 12 nm. The thermal conductivity was measured at room temperature using the 3ω method. The experimental setup and the associated analysis were validated by comparing the thermal conductivity measurements with data reported in the literature for the silicon substrate and for high quality thermal oxide thin films with thicknesses ranging from 100 to 500 nm. The cross-plane thermal conductivity of the synthesized mesoporous silica thin films does not show strong dependence on pore size, wall thickness, or film thickness. This is due to the fact that heat is mainly carried by very localized non propagating vibrational modes. The average thermal conductivity for the cubic mesoporous silica films was 0.30 ± 0.02 W/m.K, while it was 0.20 ± 0.01 W/m.K for the hexagonal films. This corresponds to a reduction of 79% and 86% from bulk fused silica at room temperature.

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This paper expands our previous numerical studies predicting the optical properties of highly ordered mesoporous thin films from two-dimensional (2D) nanostructures with cylindrical pores to three-dimensional (3D) structures with spherical pores. Simple, face centered, and body centered cubic lattice of spherical pores and hexagonal lattice of cylindrical pores were considered along with various pore diameter and porosity. The transmittance and reflectance were numerically computed by solving 3D Maxwell's equations for transverse electric and transverse magnetic polarized waves normally incident on the mesoporous thin films. The effective optical properties of the films were determined by an inverse method. Reflectance of 3D cubic mesoporous thin films was found to be independent of polarization, pore diameter, and film morphology and depended only on film thickness and porosity. By contrast, reflectance of 2D hexagonal mesoporous films with cylindrical pores depended on pore shape and polarization. The unpolarized reflectance of 2D hexagonal mesoporous films with cylindrical pores was identical to that of 3D cubic mesoporous films with the same porosity and thickness. The effective refractive and absorption indices of 3D films show good agreement with predictions by the 3D Maxwell-Garnett and Nonsymmetric Bruggeman effective medium approximations, respectively.

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.

This paper reports the cross-plane thermal conductivity of highly ordered cubic and hexagonal templated mesoporous amorphous silica thin films synthesized by evaporation-induced self-assembly process. Cubic and hexagonal films featured spherical and cylindrical pores and average porosity of 25% and 45%, respectively. The pore diameters ranged from 3 to 18 nm and film thickness from 80 to 540 nm while the average wall thickness varied from 3 to 12 nm. The thermal conductivity was measured at room temperature using the 3ω method. The experimental setup and the associated analysis were validated by comparing the thermal conductivity measurements with data reported in the literature for the silicon substrate and for high quality thermal oxide thin films with thicknesses ranging from 100 to 500 nm. The cross-plane thermal conductivity of the synthesized mesoporous silica thin films does not show strong dependence on pore size, wall thickness, or film thickness. This is due to the fact that heat is mainly carried by very localized non propagating vibrational modes. The average thermal conductivity for the cubic mesoporous silica films was 0.30 ± 0.02 W/m.K, while it was 0.20 ± 0.01 W/m.K for the hexagonal films. This corresponds to a reduction of 79% and 86% from bulk fused silica at room temperature.

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This paper reports the room temperature cross-plane thermal conductivity of pure silica zeolite ͑PSZ͒ MEL and MFI thin films. 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 mass fraction of nanoparticles within the suspension varied from 16% to 55%. This was achieved by varying the crystallization time of the suspension. The thin films consisted of crystalline MEL nanoparticles embedded in a nonuniform and highly porous silica matrix. They featured porosity, relative crystallinity, and MEL nanoparticles size ranging from 40% to 59%, 23% to 47% and 55 nm to 80 nm, respectively. PSZ MFI thin films were made by in situ crystallization, were b-oriented, fully crystalline, and had a 33% porosity. Thermal conductivity of these PSZ thin films was measured at room temperature using the 3 method. The cross-plane thermal conductivity of the MEL thin films remained nearly unchanged around 1.02Ϯ 0.10 W m −1 K −1 despite increases in ͑i͒ relative crystallinity, ͑ii͒ MEL nanoparticle size, and ͑iii͒ yield caused by longer nanoparticle crystallization time. Indeed, the effects of these parameters on the thermal conductivity were compensated by the simultaneous increase in porosity. PSZ MFI thin films were found to have similar thermal conductivity as MEL thin films even though they had smaller porosity. Finally, the average thermal conductivity of the PSZ films was three to five times larger than that reported for amorphous sol-gel mesoporous silica thin films with similar porosity and dielectric constant.

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