Anomalous reduction of thermal conductivity in coherent nanocrystal architecture for silicon thermoelectric material

Abstract

Reduction of thermal conductivity κ while preserving high electrical conductivity σ in materials continues to be a vital goal in thermoelectric study for the reuse of exhaust heat energy. In the use of an eco-friendly and ubiquitous element, Si as thermoelectric material, high κ value in bulk Si is the essential bottleneck to achieve high dimensionless figure of merit. This is a motivation for many recent studies on reducing κ in Si, by nanostructuring, e.g., using grains/wires with size smaller than the phonon mean free path. However, κ reduction that can be achieved tends to be saturated presumably due to an amorphous limit. Here, we present a nanoarchitecture for defeating the κ amorphous limit while preserving bulk-like σ. This new nanoarchitecture is an assembly of Si nanocrystals with oriented crystals separated by a 1-monolayer amorphous layer with well-controlled nanoscale shaped interfaces. At these interfaces, novel phonon scattering occurs resulting in κ reduction below the amorphous limit. Preservation of bulk-like σ results from the coherency of the carrier wavefunctions among the oriented nanocrystals separated by the ultrathin amorphous layer.

Enhanced Thermoelectric Properties of Silicon via Nanostructuring

“…Furthermore, the smallest ¬ value reported to date, which was approximately 1/100 of that of bulk Si, was achieved by inhibiting the deterioration of · via the coupling of Sinanodot crystals with aligned crystallization orientations. 14) However, it is true that the estimated standard required for the practical implementation of TE power generation (ZT = 1) has not been achieved with a high degree of reproducibility when using bulk bodies. On the other hand, recent attempts have been made to evaluate the performance of TE materials in a comprehensive manner by considering the toxicity and cost of the raw materials, as well as the ease of manufacture, and not merely focusing on ZT.…”

Thermoelectric Properties of (100) Oriented Silicon and Nickel Silicide Nanocomposite Films Grown on Si on Insulator and Si on Quartz Glass Substrates

“…The κ values of (Ni-Si NC lm)/SOI were also measured by TCN-2ω methods (ULVAC-RIKO Corp.) 11) . Vertical heat transport of lm samples are measured by the TCN-2ω method, however the method was used to measure thermal conductivities of nanocomposite lms in literatures 6,7,12,13) , because the lms had isotropic thermal transport properties owing to the uniformly-distributed phonon scattering center in the lms. In the TCN-2ω method, the measurement specimen consists of four layers, an Au thin lm, the Ni-Si NC lm, and the box SiO 2 layer and Si substrate.…”

“…−1 using the TCN-2ω method 6,7,[11][12][13] . Nakamura et al used this method to measure the thermal conductance of Si NC lms which have similar nanostructures with our Ni-Si NC lms 12) .…”

“…Nakamura et al used this method to measure the thermal conductance of Si NC lms which have similar nanostructures with our Ni-Si NC lms 12) . We observed good correspondence between the κ avg and κ value obtained from two different measurement methods of the thermal conductivity for orientational grown Ni-Si NC lms.…”

Control of Phonon Transport by Phononic Crystals and Application to Thermoelectric Materials

“…In nanostructures, since the size of the system is equivalent to the mean free path (MFP) of the carrier, in order to more clearly understand the phenomenon of thermal conductivity we must consider the ballistic transport of phonons without describing their behavior as a diffusion process. Ballistic phonon transport, as well as characteristic phonon transport due to scattering caused by the structure, has been reported in various structures including atomic scale carbon nanotubes 1) , graphene 2) , semiconductor super lattices 3,4) , structures containing nano particles 5,6) , porous structures 7,8) , nanowires 9) and phononic crystal (PnC) structures [10][11][12][13][14][15][16] . Since the thermal conductivity in nanostructures is system-dependent, using certain materials it is possible to fabricate monolithic devices with various thermal conductivities.…”