A Comprehensive Review of Thermal Anisotropy in Inorganic Functional Materials: Characterization Techniques and Applications

Thermal transport has become a critical design parameter in inorganic functional materials, especially as modern electronic, photonic, and energy devices operate at higher power densities and smaller dimensions. A key feature of many advanced inorganic systems is thermal anisotropy, meaning that heat conduction differs significantly depending on crystallographic direction. Such anisotropy arises from directional bonding, lattice symmetry, phonon dispersion, microstructural alignment, and interface-dominated transport. Thermal anisotropy can be beneficial because it enables directional heat spreading and selective thermal insulation, but it can also create thermal gradients and reliability issues if not properly understood. This review summarizes the theoretical foundations of anisotropic thermal transport, the primary inorganic material classes exhibiting thermal anisotropy, and the most widely used experimental characterization approaches. A major emphasis is placed on thermal measurement strategies, interpretation challenges, and common sources of uncertainty when quantifying in-plane and cross-plane thermal conductivity. Finally, the review discusses emerging applications, including thermal management in electronics, thermoelectric energy conversion, phase-change and neuromorphic devices, solid-state batteries, and extreme-environment systems. The paper concludes by highlighting current challenges, knowledge gaps, and future opportunities for engineering thermal anisotropy through defect control, strain, heterostructure design, and microstructural orientation control.