Bertness, Kris A. | Search NASA

DOE OSTI2024

Nanomanufacturing of Nanophononic Devices: Ultra-high ZT Thermoelectrics for Efficient Conversion of Waste Heat

Fabrication and Characterization Summary: For this program, the team fabricated and tested silicon thermoelectric test structures that experimentally demonstrated record thermal conductivity reduction (> 85 %) without changes in electrical properties (conductivity and Seebeck coefficient). We also demonstrated the highest ever reported energy conversion efficiency for silicon thermoelectric devices. The team developed a design for 1 cm 2 die with thousands of nanophononic metamaterial structures in the form of nanowalled membranes but were unable to solve all fabrication challenges before the end of the program. Multiple aspects of the design were submitted for patent protection. Primary challenges were related to silicon-on-insulator material quality and high-aspect-ratio etching.

30 DIRECT ENERGY CONVERSION↗

DOE OSTI2023

Semiconductor Thermal and Electrical Properties Decoupled by Localized Phonon Resonances

Abstract Thermoelectric materials convert heat into electricity through thermally driven charge transport in solids or vice versa for cooling. To compete with conventional energy‐conversion technologies, a thermoelectric material must possess the properties of both an electrical conductor and a thermal insulator. However, these properties are normally mutually exclusive because of the interconnection between scattering mechanisms for charge carriers and phonons. Recent theoretical investigations on sub‐device scales have revealed that nanopillars attached to a membrane exhibit a multitude of local phonon resonances, spanning the full spectrum, that couple with the heat‐carrying phonons in the membrane and cause a reduction in the in‐plane thermal conductivity, with no expected change in the electrical properties because the nanopillars are outside the pathway of voltage generation and charge transport. Here this effect is demonstrated experimentally for the first time by investigating device‐scale suspended silicon membranes with GaN nanopillars grown on the surface. The nanopillars cause up to 21% reduction in the thermal conductivity while the power factor remains unaffected, thus demonstrating an unprecedented decoupling in the semiconductor's thermoelectric properties. The measured thermal conductivity behavior for coalesced nanopillars and corresponding lattice‐dynamics calculations provide evidence that the reductions are mechanistically tied to the phonon resonances. This finding paves the way for high‐efficiency solid‐state energy recovery and cooling.

Spann, Bryan T.↗