Phononic metamaterials and methods for reducing thermal conductivity in at least partially crystalline base material are provided, such as for thermoelectric energy conversion. In one implementation, a method for reducing thermal conductivity through an at least partially crystalline base material is provided. In another implementation, a phononic metamaterial structure is provided. The phononic metamaterial structure in this implementation includes: an at least partially crystalline base material configured to allow a plurality of phonons to move to provide thermal conduction through the base material; and at least one disordered (e.g., amorphous) material coupled (e.g., as an inclusion or layer) to the at least partially crystalline base material. The at least one disordered material is configured to generate at least one vibration mode to interact with the plurality of phonons moving within the base material and slow group velocities of at least a portion of the interacting phonons and reduce thermal conductivity through the base material.

A thermal interface material (TIM) using high thermal conductivity nano-particles, particularly ones with large aspect ratios, for enhancing thermal transport across boundary or interfacial layers that exist at bulk material interfaces is disclosed. At least one of the interfacial layers is a vertically aligned metal nanowire array. The nanoparticles do not need to be used in a fluid carrier or as filler material within a bonding adhesive to enhance thermal transport, but simply in a dry solid state. The nanoparticles may be equiaxed or acicular in shape with large aspect ratios like nanorods and nanowires.