Pooja Puneet

Preferential Scattering by Interfacial Charged Defects for Enhanced Thermoelectric Performance in Few-layered n-type Bi2Te3

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.

Tuning electrical and thermal connectivity in multiwalled carbon nanotube buckypaper

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.

High temperature thermoelectric properties of p-type skutterudites BaxYbyCo4-zFezSb12

Several polycrystalline p-type skutterudites with compositions BaxYbyCo4−zFezSb12, with varying filler concentrations x and y, and z = 1 to 2, were synthesized by reacting the constituents and subsequent solid state annealing, followed by densification by hot-pressing. Their thermoelectric properties were evaluated from 300 to 820 K. The Yb filling fraction increased with Fe content while the amount of Fe substitution had little influence on the Ba filling fraction. High purity specimens were obtained when the Fe content was low. Bipolar conduction contributed to the thermal conductivity at elevated temperatures. A maximum ZT value of 0.7 was obtained at 750 K for the specimen with the highest Fe content and filling fraction. The potential for thermoelectric applications is also discussed.

Thermoelectric properties and Kondo behavior in indium incorporated p-type Ce0.9Fe3.5Ni0.5Sb12 skutterudites

Herein, we report the effects of indium (In) incorporation upon the thermoelectric and magnetic properties of Ni doped Fe-based filled skutterudites (InxCe0.9Fe3.5Ni0.5Sb12 with x = 0, 0.1, and 0.5). We find that secondary phases (such as InSb) can be formed upon surpassing the filling fraction limit and these in turn result in improved thermoelectric properties. A maximum dimensionless figure of merit, ZT ≈ 0.9 at ∼650 K was obtained for the sample with the nominal composition In0.1Ce0.9Fe3.5Ni0.5Sb12. Interestingly, we also observe Kondo-like behavior and evidence of the crystal field effect in these samples. The low-temperature (T < 100 K) thermopower and electrical resistivity of our samples exhibit Kondo-like behavior while their corresponding magnetic susceptibility suggests that the Ce3+ ions are influenced by the cubic crystal symmetry of the skutterudites class of materials, thus resulting in the crystal field effect. Lastly, the magnetic susceptibility data can be interpreted in the context of a...

A micro-Raman study of exfoliated few-layered n -type Bi 2 Te 2.7 Se 0.3

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.

Synthesis and superconductivity in spark plasma sintered pristine and graphene-doped FeSe0.5Te0.5

Abstract Here, we present a new ball-milling and spark plasma sintering (SPS)-based technique for the facile synthesis of FeSe0.5Te0.5 superconductors without the need for pre-alloying. This method is advantageous since it is quick and flexible for incorporating other dopants such as graphene for vortex pinning. We observed that FeSe0.5Te0.5 exhibits a coexistence of ferromagnetic (FM) and superconductivity signature plausibly arising from a FM core-superconducting shell structure. More importantly, the Hc2 values observed from resistivity data are higher than 7 T, indicating that SPS process synthesized FeSe0.5Te0.5 samples could lead to next-generation superconducting wires and cables.