Low thermal conductivity of Al-doped ZnO with layered and correlated grains

Bulk Al-doped zinc oxide (ZnO) with a novel self-assembled layered and correlated grain structure is found to exhibit sharp reduction in thermal conductivity. The microstructure consists of a two-dimensional layered network of oriented grains, which interconnect in the third dimension through inter-planar contacts, and grains are embedded with nano-precipitates. The contact represents anisotropic connectivity of voids trapped between the grain layers. The effects of the synthesis atmosphere and Al doping concentration upon the formation of porous correlated grains are explained by taking into account the contributions of the vapor transport mechanism for grain growth under vacuum. The inhomogeneous density distribution with spring-back effect due to the uniaxial compaction leads to the anisotropic grain growth. Compared with traditional dense ZnO, Al-doped ZnO with layered microstructure exhibits a 52% decrease in the thermal conductivity across layers (3.0 W m^-1 K ^-1 at 573 K) while maintaining the magnitude of electrical conductivity (1000 S cm^-1). The resultant power factor 4.78 × 10^-4 W m^-1 K^-2 at 423 K and figure of merit of 0.14 × 10^-3 K^-1 at 572 K is higher in comparison to the normal grain structured material.