Thermal and Optical Properties of Highly Ordered Mesoporous Thin Films for Energy Applications

Mesoporous materials are of great interest for a wide range of applications such as low-k dielectrics, energy conversion and storage, photocatalysis, sensing, or optical and thermal coatings, to name a few. Their properties and surface area can be tuned by varying the matrix material and its crystallinity, as well as the porosity and the pore size and shape. The present study aims to measure and accurately model the effects of these various parameters on the effective thermal and optical properties of highly ordered mesoporous thin films.;First, highly ordered, surfactant-templated, mesoporous thin films with monodisperse pore size, and controlled porosity, pore shape and pore spatial arrangement were synthesized and fully characterized. These films were made of silica, titania and zirconia as well as pure silica zeolite (PSZ) MEL and MFI. They were synthesized (i) by sol-gel process, (ii) from preformed nanoparticle suspensions, or (iii) by zeolite in-situ crystallization. They featured cubic or hexagonal arrangements of spherical and cylindrical pores with porosity, pore size and film thickness ranging from 20 to 60%, 0.5 to 25 nm and 80 to 540 nm, respectively. The phase of the solid matrix was amorphous or crystalline.;Moreover, the thermal conductivity of the various mesoporous thin films was experimentally measured at room temperature using the 3w method. It was shown to depend strongly on film porosity. It depended, to a lesser extent, on the crystallinity of the matrix as well as the connectivity of the crystalline domains within the dense phase. Moreover, when the matrix is crystalline and/or the pores are extremely small, such as with PSZ MFI films, the effect of pore size was found to be important. Finally, the thermal conductivity of amorphous sol-gel mesoporous silica thin films was numerically modeled using molecular dynamics simulations. Results were compared with experimental measurements.;Finally, the study focused on the numerical modeling of the reflectance and effective refraction and absorption indices of mesoporous thin films. Finite element analysis was used to solve the three-dimensonal Maxwell's equations faithfully accounting for the morphology of the mesoporous films. The variations in effective optical properties of mesoporous materials with respect to morphology, pore size, pore shape, polarization and porosity were investigated in details. Optical properties of sol-gel mesoporous thin films with cubic arrangements of spherical pores were found to depend only on porosity. For films featuring spherical pores, the effective refractive and absorption indices showed good agreement with the 3D Maxwell-Garnett and nonsymmetric Bruggeman effective medium approximations, respectively. Finally, the numerical tool was validated by comparing experimentally measured reflectance spectra of mesoporous silica films with those predicted numerically for identical morphologies. Both dehydrated and fully hydrated films were considered. Excellent agreement was found between experimental results and numerical predictions.;Results of the present study, can be used to guide the synthesis of novel nanostructured thin film materials with desired thermal and optical properties and to design devices and systems for numerous applications.