Gaohua Zhu

Enhanced Thermoelectric Figure-of-Merit in Nanostructured p-type Silicon Germanium Bulk Alloys

A dimensionless thermoelectric figure-of-merit (ZT) of 0.95 in p-type nanostructured bulk silicon germanium (SiGe) alloys is achieved, which is about 90% higher than what is currently used in space flight missions, and 50% higher than the reported record in p-type SiGe alloys. These nanostructured bulk materials were made by using a direct current-induced hot press of mechanically alloyed nanopowders that were initially synthesized by ball milling of commercial grade Si and Ge chunks with boron powder. The enhancement of ZT is due to a large reduction of thermal conductivity caused by the increased phonon scattering at the grain boundaries of the nanostructures combined with an increased power factor at high temperatures.

Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy

The dimensionless thermoelectric figure of merit (ZT) of the n-type silicon germanium (SiGe) bulk alloy at high temperature has remained at about one for a few decades. Here we report that by using a nanostructure approach, a peak ZT of about 1.3 at 900 °C in an n-type nanostructured SiGe bulk alloy has been achieved. The enhancement of ZT comes mainly from a significant reduction in the thermal conductivity caused by the enhanced phonon scattering off the increased density of nanograin boundaries. The enhanced ZT will make such materials attractive in many applications such as solar, thermal, and waste heat conversion into electricity.

Power Factor Enhancement by Modulation Doping in Bulk Nanocomposites

We introduce the concept of modulation doping in three-dimensional nanostructured bulk materials to increase the thermoelectric figure of merit. Modulation-doped samples are made of two types of nanograins (a two-phase composite), where dopants are incorporated only into one type. By band engineering, charge carriers could be separated from their parent grains and moved into undoped grains, which would result in enhanced mobility of the carriers in comparison to uniform doping due to a reduction of ionized impurity scattering. The electrical conductivity of the two-phase composite can exceed that of the individual components, leading to a higher power factor. We here demonstrate the concept via experiment using composites made of doped silicon nanograins and intrinsic silicon germanium grains.

Dramatic thermal conductivity reduction by nanostructures for large increase in thermoelectric figure-of-merit of FeSb2

In this report, thermal conductivity reduction by more than three orders of magnitude over its single crystal counterpart for the strongly correlated system FeSb2 through a nanostructure approach was presented, leading to a significant increase of thermoelectric figure-of-merit (ZT). For the samples processed with the optimal parameters, the thermal conductivity reached 0.34 Wm−1 K−1 at 50 K, leading to a ZT peak of about 0.013, compared to 0.005 for single crystal FeSb2, an increase of about 160%. This work suggests that nanostructure method is effective and can be possibly extended to other strongly correlated low temperature thermoelectric materials, paving the way for future cryogenic temperature cooling applications.

Theoretical studies on the thermoelectric figure of merit of nanograined bulk silicon

In this paper, we investigate the phonon transport in silicon nanocomposites using Monte Carlo simulations considering frequency-dependent phonon mean free paths, and combine the phonon modeling with electron modeling to predict the thermoelectric figure of merit (ZT) of silicon nanocomposites. The model shows that while grain interface scattering of phonons is negligible for large grain sizes around 200 nm, ZT can reach 1.0 at 1173 K if the grain size can be reduced to 10 nm. Our results show the potential of obtaining a high ZT in bulk silicon by the nanocomposite approach.

NANOCOMPOSITES TO ENHANCE ZT IN THERMOELECTRICS

The concept of using “self-assembled” and “force-engineered” nanostructures to enhance the thermoelectric figure of merit relative to bulk homogeneous and composite materials is presented in general terms. Specific application is made to the Si-Ge system for use in power generation at high temperature. The scientific advantages of the nanocomposite approach for the simultaneous increase in the power factor and decrease of the thermal conductivity are emphasized along with the practical advantages of having bulk samples for property measurements and a straightforward path to scale-up materials synthesis and integration of nanostructured materials into thermoelectric cooling and power generation devices.

Thermoelectric Transport in Silicon Germanium Nanocomposite

Direct energy conversion between heat and electrical energy based on thermoelectric effects is attractive for potential applications in waste heat recovery and environmentally-friendly refrigeration. The energy conversion efficiency depends on the dimensionless figure of merit of thermoelectric materials, ZT, which is proportional to the electrical conductivity, the square of the Seebeck coefficient, and the inverse of the thermal conductivity. Currently, the low ZT values of available materials restrict the applications of this technology. However, significant enhancements in ZT were recently reported in nanostructured materials such as superlattices mainly due to their low thermal conductivities. According to recent studies, the reduced thermal conductivity of nanostructures is attributed to the large number of interfaces at which phonons are scattered. Based on this idea, nanocomposites are expected to have a lower thermal conductivity than their bulk counterparts with low fabrication cost just by mixing nano sized particles. In this work, we will discuss mechanisms of thermoelectric transport via modeling and provide experimental evidence on the enhancement of thermoelectric figure of merit in SiGe-based nanocomposites.Copyright