Ajay Dhar

Optical properties of reduced lithium niobate single crystals

The optical transmission of LiNbO3 single crystals has been measured in the wavelength range 200–900 nm, for different degrees of reduction, to study the effect of reduction on the optical characteristics of LiNbO3 near the fundamental absorption edge. The optical transitions in LiNbO3 were found to be indirect and the band gap decreased with increasing degree of reduction. The band observed at 2.48 eV in the absorption spectrum in heavily reduced samples has been attributed to the formation of polarons, and the theoretical model of Reik and Heese [J. Chem. Solids 28, 581 (1967)] for small polarons is used to correlate the optical and electrical properties.

Enhanced thermoelectric figure-of-merit in spark plasma sintered nanostructured n-type SiGe alloys

We report a significant enhancement in the thermoelectric figure-of-merit of phosphorous doped nanostructured n-type Si80Ge20 alloys, which were synthesized employing high energy ball milling followed by rapid-heating using spark plasma sintering. The rapid-heating rates, used in spark plasma sintering, allow the achievement of near-theoretical density in the sintered alloys, while retaining the nanostructural features introduced by ball-milling. The nanostructured alloys display a low thermal conductivity (2.3 W/mK) and a high value of Seebeck coefficient (−290 μV/K) resulting in a significant enhancement in ZT to about 1.5 at 900 °C, which is so far the highest reported value for n-type Si80Ge20 alloys.

Mg3Sb2-based Zintl compound: a non-toxic, inexpensive and abundant thermoelectric material for power generation

The deployment of thermoelectric materials for deriving an enhanced figure of merit (ZT) for power generation in inexpensive, non-toxic and relatively abundant bulk homogeneous solid relies on the extent of achieving the “phonon-glass electron crystal” (PGEC) characteristics. Here, a proof of principal has been established experimentally in the present work for a Zintl compound of Mg3Sb2 and its derivative of isoelectronically Bi doped Bi; Mg3Sb2−xBix (0 ≤ x ≤ 0.4) alloys in Mg3Sb2. Single phase p-type Mg3Sb2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg3Sb2. However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are toxic. We note that the system becomes metal with carrier density exceeding 15 × 1020/cm3 for x ≥0.25. The substantial increase in ZT in Mg3Sb2−xBix (0 ≤ x ≤ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg3Sb2−xBix (0 ≤ x ≤ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved in this class of materials with appropriate doping. Further, relative abundance of the material ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation.

Implications of nanostructuring on the thermoelectric properties in half-Heusler alloys

High energy ball milled Zr0.25Hf0.75NiSn alloys subjected to spark plasma sintering show an enhanced thermoelectric figure of merit in comparison with its normal bulk material synthesized by arc-melting process. The enhancement is due to increase in Seebeck coefficient with simultaneous decrease in thermal conductivity which follows due to increase in the cell volume. Theoretical calculations find that volume expansion facilitates band narrowing effects leading to high Seebeck coefficient and that decreasing orbital overlap which results in weak bonding leads to dampening the phonon propagation in addition to the interface scattering of phonons from phase boundaries.

Thermoelectric properties of Cu3SbSe3 with intrinsically ultralow lattice thermal conductivity

We report the synthesis, characterization and evaluation of the thermoelectric properties of Cu3SbSe3 with a view to explore its utility as an useful thermoelectric material due to its intrinsically low thermal conductivity. Cu3SbSe3 was synthesized employing a solid state reaction process followed by spark plasma sintering, and the synthesized material was extensively characterized for its phase, composition and structure, which suggested formation of a single-phase. The measured electrical transport properties of Cu3SbSe3 indicated p-type conduction in this material. The electrical transport behavior agrees well with that predicted theoretically using first-principle density-functional theory calculations, employing generalized gradient approximation. The measured thermal conductivity was found to be 0.26 W m−1 K−1 at 550 K, which is the lowest reported thus far for Cu3SbSe3 and is among the lowest for state-of-the-art thermoelectric materials. Despite its ultralow thermal conductivity coupled with a moderate Seebeck coefficient, the calculated value of its thermoelectric figure-of-merit was found to be exceptionally low (<0.1), which was primarily attributed to its low electrical conductivity. Nevertheless, it is argued that Cu3SbSe3, due its environmentally-friendly constituent elements, ultralow thermal conductivity and moderate thermopower, could be a potentially useful thermoelectric material as the power factor can be favorably tailored by tuning the carrier concentration using suitable metallic dopants.

Thermoelectric and mechanical properties of spark plasma sintered Cu3SbSe3 and Cu3SbSe4: Promising thermoelectric materials

We report the synthesis of thermoelectric compounds, Cu3SbSe3 and Cu3SbSe4, employing the conventional fusion method followed by spark plasma sintering. Their thermoelectric properties indicated that despite its higher thermal conductivity, Cu3SbSe4 exhibited a much larger value of thermoelectric figure-of-merit as compared to Cu3SbSe3, which is primarily due to its higher electrical conductivity. The thermoelectric compatibility factor of Cu3SbSe4 was found to be ∼1.2 as compared to 0.2 V−1 for Cu3SbSe3 at 550 K. The results of the mechanical properties of these two compounds indicated that their microhardness and fracture toughness values were far superior to the other competing state-of-the-art thermoelectric materials.

Microstructure and mechanical properties of thermoelectric nanostructured n-type silicon-germanium alloys synthesized employing spark plasma sintering

Owing to their high thermoelectric (TE) figure-of-merit, nanostructured Si80Ge20 alloys are evolving as a potential replacement for their bulk counterparts in designing efficient radio-isotope TE generators. However, as the mechanical properties of these alloys are equally important in order to avoid in-service catastrophic failure of their TE modules, we report the strength, hardness, fracture toughness, and thermal shock resistance of nanostructured n-type Si80Ge20 alloys synthesized employing spark plasma sintering of mechanically alloyed nanopowders of its constituent elements. These mechanical properties show a significant enhancement, which has been correlated with the microstructural features at nano-scale, delineated by transmission electron microscopy.

EM shielding effectiveness of Pd-CNT-Cu nanocomposite buckypaper

We report the synthesis of a nanocomposite consisting of Pd doped multiwall carbon nanotubes decorated with Cu nanoparticles, as a lightweight and flexible microwave absorbing material, using an electroless technique. The synthesised nanocomposite was extensively characterized by employing X-ray diffraction, Raman spectroscopy, FESEM, and HRTEM and their results were correlated with the high electromagnetic interference (EMI) shielding observed in the present study. The optimum dielectric properties coupled with good electrical conductivity of this nanocomposite contribute to designing this absorption-based microwave shield, which exhibited a good EMI shielding effectiveness (EMI SE) of ∼35 dB at a thickness of 200 μm, resulting in a high specific EMI SE of ∼108 dB cm3 g−1 in the Ku-band.

Polyaromatic-hydrocarbon-based carbon copper composites for the suppression of electromagnetic pollution

A facile method of developing carbon–copper (C–Cu) nanocomposites by coating nanocrystalline Cu on heat-treated polyaromatic hydrocarbons (HTPAHs) has been reported. These synthesized nanocomposites have been extensively characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), a scanning electron microscope (SEM), and transmission electron microscopy (TEM). The synthesized HTPAHs-based C–Cu nanocomposites exhibit improved mechanical and electrical properties, which could be tailored by varying the Cu nanoparticle loading. The highest electromagnetic interference shielding effectiveness (EMI SE) due to absorption and reflection at 12.4 GHz is 46.1 dB and 12.5 dB, respectively, for a 2 mm thick sample resulting in a total shielding effectiveness of 58.7 dB. This observed shielding effectiveness in these C–Cu nanocomposites is far above the threshold shielding effectiveness required for techno-commercial applications, especially in the Ku band of RF.

Conducting grain boundaries enhancing thermoelectric performance in doped Mg2Si

The thermoelectric properties of Pb doped Mg2Si, synthesized using reactive sintering employing spark plasma sintering, are investigated and are compared with other dopants reported in literature. While a moderate decrease in Seebeck coefficient and thermal conductivity is observed for 2 at. % of Pb doping in Mg2Si, a substantial enhancement in the materials thermoelectric figure-of-merit is observed, which is due to an enormous increase in its electrical conductivity. A brick-layer model is proposed to explain these results, wherein the inter-granular electronic conductivity is facilitated by Pb (or Mg2Pb) phases at grain boundaries, which is supported by microstructural evidences.