Thermoelectric Performance Enhancement by Surrounding Crystalline Semiconductors with Metallic Nanoparticles

Abstract

Direct conversion of thermal energy to electricity by thermoelectric (TE) devices may play a key role in future energy production and utilization. However, relatively poor performance of current TE materials has slowed development of new energy conversion applications. Recent reports have shown that the dimensionless Figure of Merit, ZT, for TE devices can be increased beyond the state-of-the-art level by nanoscale structuring of materials to reduce their thermal conductivity. New morphologically designed TE materials have been fabricated at the NASA Langley Research Center, and their characterization is underway. These newly designed materials are based on semiconductor crystal grains whose surfaces are surrounded by metallic nanoparticles. The nanoscale particles are used to tailor the thermal and electrical conduction properties for TE applications by altering the phonon and electron transport pathways. A sample of bismuth telluride decorated with metallic nanoparticles showed less thermal conductivity and twice the electrical conductivity at room temperature as compared to pure Bi 2 Te 3 . Apparently, electrons cross easily between semiconductor crystal grains via the intervening metallic nanoparticle bridges, but phonons are scattered at the interfacing gaps. Hence, if the interfacing gap is larger than the mean free path of the phonon, thermal energy transmission from one grain to others is reduced. Here we describe the design and analysis of these new materials that offer substantial improvements in thermoelectric performance.

A High Performance Ag Alloyed Nano-scale n-type Bi2Te3 Based Thermoelectric Material

“…The nanostructured materials effectively scattered the phonons at the boundary, decreased lattice thermal conductivity, and consequently increased the ZT value [2]. The ZT value of the n-type Bi 2 Te 3 bulk TE materials can also be increased to 1.8 at the room temperature by mixing Ag nanoparticles and Bi 2 Te 3 nanopowders [5]. Although the performance of TE materials has been improved by utilizing nanotechnology, the productivity of nanostructured materials is still a bottleneck for industry application due to the brittleness of Bi 2 Te 3 based materials.…”

Enhancement in thermoelectric properties due to Ag nanoparticles incorporated in Bi2Te3 matrix

“…Thus the electron-energy filtering, in which high-energy electrons remain unaffected, strongly enhances Seebeck coefficient. [ 20 – 21 29 ]. In our samples, one can clearly observe the increased Seebeck coefficient for 5% Ag at the lower annealing temperature, which decreases again for higher Ag content because of the smaller nanoparticles (<30 nm) in the matrix.…”