Nanocrystalline silicon: lattice dynamics and enhanced thermoelectric properties

Abstract

Silicon has several advantages when compared to other thermoelectric materials, but until recently it was not used for thermoelectric applications due to its high thermal conductivity, 156 W K(-1) m(-1) at room temperature. Nanostructuration as means to decrease thermal transport through enhanced phonon scattering has been a subject of many studies. In this work we have evaluated the effects of nanostructuration on the lattice dynamics of bulk nanocrystalline doped silicon. The samples were prepared by gas phase synthesis, followed by current and pressure assisted sintering. The heat capacity, density of phonons states, and elastic constants were measured, which all reveal a significant, ≈25%, reduction in the speed of sound. The samples present a significantly decreased lattice thermal conductivity, ≈25 W K(-1) m(-1), which, combined with a very high carrier mobility, results in a dimensionless figure of merit with a competitive value that peaks at ZT≈ 0.57 at 973 °C. Due to its easily scalable and extremely low-cost production process, nanocrystalline Si prepared by gas phase synthesis followed by sintering could become the material of choice for high temperature thermoelectric generators.

Crystalline–Amorphous Silicon Nanocomposites with Reduced Thermal Conductivity for Bulk Thermoelectrics

“…Figure 5 shows the comparison of the thermoelectric properties between this work and the previous works. 17,19,20,34 The electrical conductivity in this work is lower than those in previous works for the entire temperature range. Note that the difference between this work and Bux et al 17 comes from the difference in the carrier concentrations, and the mobility is almost the same.…”

Lattice dynamics and thermoelectric properties of nanocrystalline silicon–germanium alloys

“…As an alternative fabrication route, nanocrystalline Si–Ge can also be produced by a bottom‐up approach by a gas‐phase synthesis of nanopowder followed by current‐activated pressure‐assisted sintering (). This approach is easily upscalable for large‐scale synthesis, sometimes at the expense of slightly lower zT values for the scalable bottom‐up process (gas‐phase synthesis) compared to the top‐down approach (ball milling), and has recently been successfully applied in the preparation of nanostructured Si with an outstanding, for Si not alloyed with Ge, figure of merit at 900 °C (). In this work, we present a detailed study of the lattice dynamics and thermoelectric properties of nanocrystalline Si–Ge alloys produced by such a bottom‐up process.…”

Thermal transport in nanocrystalline Si and SiGe by ab initio based Monte Carlo simulation

“…Nanocrystalline materials, or polycrystals with nanoscale grain size, in particular have been demonstrated as efficient TE materials in bulk form46789101112. The thermoelectric efficiency is improved by the reduction of lattice thermal conductivity due to phonon grain boundary scattering when the phonon mean free paths are comparable to the grain size.…”