Nanostructure design for drastic reduction of thermal conductivity while preserving high electrical conductivity

Abstract

The design and fabrication of nanostructured materials to control both thermal and electrical properties are demonstrated for high-performance thermoelectric conversion. We have focused on silicon (Si) because it is an environmentally friendly and ubiquitous element. High bulk thermal conductivity of Si limits its potential as a thermoelectric material. The thermal conductivity of Si has been reduced by introducing grains, or wires, yet a further reduction is required while retaining a high electrical conductivity. We have designed two different nanostructures for this purpose. One structure is connected Si nanodots (NDs) with the same crystal orientation. The phonons scattering at the interfaces of these NDs occurred and it depended on the ND size. As a result of phonon scattering, the thermal conductivity of this nanostructured material was below/close to the amorphous limit. The other structure is Si films containing epitaxially grown Ge NDs. The Si layer imparted high electrical conductivity, while the Ge NDs served as phonon scattering bodies reducing thermal conductivity drastically. This work gives a methodology for the independent control of electron and phonon transport using nanostructured materials. This can bring the realization of thermoelectric Si-based materials that are compatible with large scale integrated circuit processing technologies.

Tunable Thermal Transport Characteristics of Nanocomposites

“…Our predicted values of the room-temperature thermal conductivity for Si/Ge nanocomposites are in the same range as reported from experimental measurements on similar size PSLs [32][33][34][35] and NDSLs [36][37][38][39]. However, it should be clarified that a detailed comparison of our predicted results for Si/Ge nanocomposite structure with experimental measurements is not possible.…”

“…With regards to equal layer thickness Si(D/2)/Ge(D/2) PSL, the conductivity takes a minimum value when the period size D lies in the range 3-12 nm, depending of course on sample size L and interface quality. Reported experimental studies [32,33,[35][36][37][38][39] and the present systematic theoretical study point out that with the right choice of sample size, period size, volume insertion fraction, and short-range interface defects in a Si/Ge nanocomposite, it is possible to achieve room-temperature conductivity below the alloy and amorphous limit of around 4 Wm −1 K −1 . This positively points in the direction of the usefulness of nanocomposites for applications such as thermoelectricity.…”

Phonon and heat transport control using pillar-based phononic crystals

“…However, the experimentally observed 20% reduction in thermal conductivity is interesting to compare with the reduction obtained in other silicon-based nanostructures. On one hand, a stronger reduction was measured in silicon with pores (>50%) [9,18,19], nanodots (>70%) [13,68], polycrystalline grains (>80%) [15,69,70], dopants (>50%) [71,72] or germanium atoms (>70%) [14,71,73]. On the other hand, the 20% reduction by the pillars [63] is comparable to the reduction by holes (20–25%) [21,74] or slits (20–30%) [75] covering the same relative area.…”

Miniaturized planar Si-nanowire micro-thermoelectric generator using exuded thermal field for power generation

“…The reproducibility of such high ZT value has been proved difficult [ 12 ]. Nevertheless, SiNWs could potentially be used for thermoelectric applications owing to their low thermal conductivity and preserving electrical conductivity [ 13 ].…”