ABSTRACT
Previously we showed that the thermoelectric (TE) performance of bulk n-type Bi2Te2.7Se0.3 can be enhanced by subjecting it to a combined process of chemical or mechanical exfoliation (C/ME) followed by a rapid densification and restacking of the exfoliated layers via the spark-plasma-sintering technique (SPS). Here, we present a systematic micro-Raman study of two-dimensional flakes of n-type Bi2Te2.7Se0.3 produced by the C/ME process, as a function of the flake thickness. We found Raman evidence for flakes with: (i) integer number of quintuples which exhibited a strong electron-phonon coupling, and (ii) non-integer number of quintuples, or sub-quintuples which exhibited the forbidden IR active mode due to symmetry lowering. Detailed atomic force microscopy was used to confirm the number of quintuples in all flakes examined in this study. The restacking and densification of these flakes by SPS promoted the formation of charged grain boundaries, which led to the enhanced TE properties via the energy filtering process.
ABSTRACT
Over the past two decades several nano-structuring methods have helped improve the figure of merit (ZT) in the state-of-the art bulk thermoelectric materials. While these methods could enhance the thermoelectric performance of p-type Bi2Te3, it was frustrating to researchers that they proved ineffective for n-type Bi2Te3 due to the inevitable deterioration of its thermoelectric properties in the basal plane. Here, we describe a novel chemical-exfoliation spark-plasma-sintering (CE-SPS) nano-structuring process, which transforms the microstructure of n-type Bi2Te3 in an extraordinary manner without compromising its basal plane properties. The CE-SPS processing leads to preferential scattering of electrons at charged grain boundaries, and thereby increases the electrical conductivity despite the presence of numerous grain boundaries, and mitigates the bipolar effect via band occupancy optimization leading to an upshift (by ~ 100 K) and stabilization of the ZT peak over a broad temperature range of ~ 150 K.
ABSTRACT
Interface modification in transport properties of single elemental polycrystalline Bi via spark plasma sintering results in 'double-decoupling' (simultaneous decoupling of thermopower, electrical, and thermal conductivity) of otherwise coupled entities. In spark plasma sintering, the DC pulse current helps in controlling the nature and extent of surfaces of ball-milled Bi and hence results in six-fold improvement in the dimensionless figure of merit (ZT) relative to as-purchased samples.
ABSTRACT
We find that the electrical and thermal connectivity in multiwalled carbon nanotube buckypaper can be tuned using a spark plasma sintering (SPS) technique. Elevated SPS temperatures promote the formation of inter-tube connections and consequently impact the electrical resistivity, thermoelectric power and thermal conductivity of the buckypaper. In particular, the electrical resistivity as a function of SPS temperature exhibits a percolation-type behavior while the low temperature lattice thermal conductivity shows a crossover behavior in the sample dimensionality. The results are discussed in terms of the quasi-one-dimensional metallic nature of multiwalled carbon nanotubes, the packing density and the electron-phonon coupling.
