Thermal Conductivity of Ordered Mesoporous Nanocrystalline Silicon Thin Films Made from Magnesium Reduction of Polymer-Templated Silica

Abstract

This paper reports the cross-plane thermal conductivity of ordered mesoporous nanocrystalline silicon thin films between 25 and 315 K. The films were produced by evaporation-induced self-assembly of mesoporous silica followed by magnesium reduction. The periodic ordering of pores in mesoporous silicon was characterized by X-ray diffraction and direct SEM imaging. The average crystallite size, porosity, and film thickness were about 13 nm, 25−35%, and 140−340 nm, respectively. The pores were arranged in a face-centered cubic lattice. The cross-plane thermal conductivity of the mesoporous silicon thin films was measured using the 3ω method. It was between 3 and 5 orders of magnitude smaller than that of bulk single crystal silicon in the temperature range considered. The effects of temperature, film thickness, and copolymer template on the thermal conductivity were investigated. A model based on kinetic theory was used to accurately predict the measured thermal conductivity for all temperatures. On one hand, both the measured thermal conductivity and the model predictions showed a temperature dependence of k ∝ T 2 at low temperatures, typical of amorphous and strongly disordered materials. On the other hand, at high temperatures the thermal conductivity of mesoporous silicon films reached a maximum, indicating a crystalline-like behavior. These results will be useful in designing mesoporous silicon with desired thermal conductivity by tuning its morphology for various applications.

Thermal conduction in nano-porous silicon thin film

“…11 We can take advantage of the phonon scattering on boundaries to reduce the thermal conductivity for high performance of thermoelectric devices. Existing studies [12][13][14][15][16][17] suggested that nano-porous thin films can block heat transfer easily, and result in thin films with low thermal conductivity. However, the experimental study is often constrained by the nanofabrication due to the nanoscale, and high measurement accuracy of experimental system is often difficult to achieve and the cost is expensive as well.…”

Thermal properties of nanoporous silicon materials

“…To avoid the effect of the substrate upon experimental accuracy, self-standing PSi membranes have been used as specimens (Maccagnani et al ., 1999;Song and Chen, 2004;Bernini et al , 2005). Theoretical analyses have also been conducted on the basis of molecular dynamics simulations , nanodot array fabrication (Fang et al ., 2012), and the hydrodynamic approach (Alvarez et al ., 2010).…”

Mesoscopic description of boundary effects in nanoscale heat transport

“…The experimental results on pSi show that its thermal conductivity is strongly related to the pore size at a given total porosity [37][38][39]110]. Since an increasing porosity may deteriorate the electron transport properties [5,75], it would be advantageous if its effective thermal conductivity could be controlled by the volume fractions, as well as the characteristic size of the pores.…”