Nanophononic metamaterials

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Abstract Nanophononic 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 nanophononic metamaterial structure is provided. The nanophononic 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 nanoscale locally resonant oscillator coupled to the at least partially crystalline base material. The at least one nanoscale locally resonant oscillator is configured to generate at least one vibration mode to interact with the plurality of phonons moving within the base material and slowing group velocities of at least a portion of the interacting phonons and reduce thermal conductivity through the base material. Field  The instant invention relates to reducing group velocities of phonons traveling within an at least partially crystalline base material. One purpose for group velocity reductions is to reduce thermal conductivity; another is to improve the thermoelectric energy conversion figure of merit. In particular implementations, the instant invention relates to reducing group velocities of phonons traveling within an at least partially crystalline base material by interacting one or more vibration modes generated by at least one locally resonant oscillator with one or more of the phonons.  Background  The thermoelectric effect refers to the ability to generate an electric current from a temperature difference between one side of a material and another. Conversely, applying an electric voltage to a thermoelectric material can cause one side of the material to heat while the other side stays cool, or, alternatively, one side to cool down while the other stays hot. Devices that incorporate thermoelectric materials have been used in both ways: to create electricity from a heat source or to provide cooling or heating by consuming electricity. To date, thermoelectric devices have been limited to niche or small-scale applications, such as providing power for the Mars Curiosity Rover or the cooling of precision instruments.  The widespread use of thermoelectric materials has been hindered by the problem that materials that are good electrical conductors also tend to be good conductors of heat. This means that at the same time a temperature difference creates an electric potential, the temperature difference itself begins to dissipate, thus weakening the current it created. Materials that have both high electrical conductivity, .sigma., and high thermal conductivity, .kappa., behave poorly in converting a temperature difference to an electric potential. In order for a material to perform well as a thermoelectric material, it should possess a high value of the figure of merit, ZT=(S.sup.2.sigma./.kappa.)T, where S is the Seebeck coefficient, and T is the temperature.