Approaching extremely low thermal conductivity by crystal structure engineering in Mg_2Al_4Si_5O_(18)

摘要

One of the challenges in developing a low thermal conductivity material addresses on searching lightweight ceramic without heavy or rare-earth (RE) elements. Mg_2Al_4Si_50i8 interests us for its very low density and complex crystal structure. The first-principle calculations were performed to predict mechanical and lattice thermal conductivity of hexagonal and orthorhombic phases of Mg_2Al_4Si_5O_(18). According to Debye approximation and the Slack model, the lattice thermal conductivity varies with temperature in 804.6/r and 719.7/T, yielding 2.95 and 2.64 W/(mK) at room temperature, respectively. The high temperature limits of thermal conductivities are as low as 1.33 and 1.29 W/(m·K). The thermal conductivities of both polymorphs of Mg_2Al_4Si_5O_(18) are lower than most of RE-containing silicates and zirconates. The present work suggests that Mg_2Al_4Si_5O_(18) is a promising lightweight ceramic with extremely low thermal conductivity. We also highlight that enhancing complexity of the crystal structure rather than incorporating heavy RE elements may be an alternative wisdom to explore lightweight thermal insulators.