Thermal Conductivity of 2/1‐Mullite Single Crystals

The thermal diffusivities D th of mullite single crystals with about 2/1‐composition parallel to the crystallographic a, b, and c axes (i.e., parallel to [100], [010], and [001]) and of monophase, dense mullite ceramics were measured between room temperature and 1200°C using the laser flash method. The semi‐transparent mullite disks were covered by thin layers of argon‐sputtered platinum and sprayed colloidal graphite in order to minimize heat radiation transfer effects. The phonon‐produced thermal conductivity of mullite was calculated according to K = D th C p ρ using experimentally measured values for thermal diffusivity, D th ( T ), and specific heat, C p ( T ), and thermal expansion coefficients, α i , for the determination of temperature‐dependent density ρ( T ) and sample thickness d ( T ). The anisotropy of thermal diffusivity and conductivity is evident with highest values parallel to the crystallographic c axis (i.e., [001]; e.g., at T =100°C K [001] =6.862 W/mK), corresponding to the direction of strongest bonds and highest elastic stiffness in the mullite crystal structure. Perpendicular to the c axis, thermal diffusivities and conductivities are smaller, with the values parallel to the a axis (i.e., [100]) being slightly higher than parallel to b (i.e., [010]; e.g., at T =100°C K [100] =4.563 W/mK and K [010] =4.400 W/mK), indicating lower bond strength and elastic stiffness in these crystallographic directions. Conductivity anisotropy factors are around 1.33 for K [001] / K average , 0.90 for K [100] / K average , and 0.85 for K [010] / K average with little variation between room temperature and 1200°C. The thermal diffusivity and conductivity of dense, monophase mullite ceramics display similar values as the averaged single crystal data, thus providing a master curve for the temperature‐dependent thermal conductivity.