N. Gothard

Thermoelectric properties of La0.9CoFe3Sb12–CoSb3 skutterudite nanocomposites

A hydrothermal nanoparticle-plating technique has been employed in order to grow a layer of CoSb3 nanoparticles on the surface of La0.9CoFe3Sb12 bulk matrix grains. The nanoparticles have a typical size of 30–40 nm while the nano-layer can be up to several hundreds of nanometer thick. The nanoparticle layer, which resides at the grain boundary after hotpressing, provides an extra scattering channel for phonons, in addition to the “rattler” atoms (La), grain boundary scattering, and mass fluctuation mechanisms found within the bulk matrix grain. The electrical resistivity, thermopower, thermal conductivity, and Hall coefficient have been investigated as a function of temperature and the weight percentage (%) of nanoparticles. Largely due to the reduced lattice thermal conductivity, a ZT value of ∼0.5 is attained at 725 K on the sample with 5 wt % of nanoparticles showing a 15% improvement of the ZT from that of the sample without nanoparticles and comparable to the best value reported at this temperature.

Thermoelectric and transport properties of n-type Bi2Te3 nanocomposites

A series of Bi2Te3 nanocomposite samples have been prepared by incorporating nanoparticle concentrations of 5–50mol% into a bulk matrix via a mixing process and subsequently hot pressing into highly densified pellets. The electrical resistivity, thermopower, total thermal conductivity, carrier concentration, and Hall mobility have been measured for these composites, and the power factor, lattice thermal conductivity, and figure of merit (ZT) have been calculated. The results are discussed in light of the underlying electrical and thermal transport mechanisms.

Improved thermoelectric performance in polycrystalline p-type Bi2Te3 via an alkali metal salt hydrothermal nanocoating treatment approach

We report herein a proof-of-principle study of grain boundary engineering in the polycrystalline p-type Bi2Te3 system. Utilizing the recently developed hydrothermal nanocoating treatment technique, we fabricated an alkali-metal(s)-containing surface layer on the p-Bi2Te3 bulk grain, which in turn became part of the grain boundary upon hot pressing densification. Compared to the untreated bulk reference, the dimensionless figure of merit ZT has been improved by ∼30% in the Na-treated sample chiefly due to the reduced thermal conductivity, and ∼38% in the Rb-treated sample mainly owing to the improved power factor. The grain boundary phase provides a new avenue by which one can potentially decouple the otherwise inter-related electrical resistivity, Seebeck coefficient, and thermal conductivity within one thermoelectric material.

New Directions in Bulk Thermoelectric Materials Research: Synthesis of Nanoscale Precursors for “Bulk-Composite” Thermoelectric Materials

Over a decade ago it was predicted that nano-scaled thermoelectric (TE) materials might have superior properties to that of their bulk counterparts. Subsequently, a significant increase in the figure of merit, ZT (ZT > 2), has been reported for nano-scaled systems such as superlattice and quantum dot systems constituently based on those more commonly used bulk TE materials (e.g., Bi 2 Te 3 and PbTe). However, the challenge remains to achieve these higher performance results in bulk materials in order to more rapidly incorporate them into standard TE devices. Recent theoretical work on boundary scattering of phonons in amorphous materials indicates that micron and submicron grains could be very beneficial in order to lower the lattice thermal conductivity and yet not deteriorate the electron mobility. The focus in this paper will be to highlight some of our new directions in bulk thermoelectric materials research. Thermoelectric materials are inherently difficult to characterize and these difficulties are magnified at high temperatures. Specific materials will be discussed, especially those bulk materials that exhibit favorable properties for potential high temperature power generation capabilities. One potentially fruitful research direction is to explore whether hybrid TE materials possess possible enhanced TE properties. These “engineered” hybrids include materials that exhibit sizes from on the order of a few nanometers to hundreds of nanometers of the initial materials. These initial materials are then incorporated into a bulk structure. A discussion of some of the future research directions that we are pursuing is highlighted, including some bulk materials, which are based on nano-scaled or hybrid composites. The synthesis techniques and the synthesis results of many of these nano-scale precursor materials will be a primary focus of this paper.

Growth and characterization of Bi/sub 2/Te/sub 3/ nanostructures

Bulk Bi/sub 2/Te/sub 3/ is one of the best known thermoelectric materials with a ZT /spl sim/1 at room temperature. Theoretical studies have suggested that low-dimensional materials may exhibit ZT values that exceed 1. In this study, we used the pulsed laser deposition method to prepare Bi/sub 2/Te/sub 3/ nanostructures by ablating a rotating Bi/sub 2/Te/sub 3/ target in an inert atmosphere. Silicon or quartz substrates are pretreated with poly-1-lysine to form an adhesion layer for 10, 20, and 30 nm colloidal Au particles which serve as catalyst seed particles for the growth of the nanostructures. Alternatively, we have also prepared Bi/sub 2/Te/sub 3/ nanostructures by subliming Bi/sub 2/Te/sub 3/ powder in the presence of gold coated substrates. Results from electron microscopy and vibrational spectroscopic studies are presented.

CVD Growth of Nanostructures from Bi 2 Te 3

Research into thermoelectric materials has recently undergone a push into lower dimensional materials in the hopes that quantum confinement effects will enhance the performance of these structures. It has already been demonstrated that 2D superlattice materials show enhanced properties. More recently, materials known to have good thermoelectric properties, such as Bi 2 Te 3 or PbTe, have been grown in low dimensional morphologies. We investigate synthesis techniques for growing low dimensional structures of Bi-Te materials with the aim of incorporating them into a composite material alongside bulk Bi 2 Te 3 .