Shovit Bhattacharya

Improved thermoelectric performance of hot pressed nanostructured n-type SiGe bulk alloys

Silicon germanium alloys (Si80Ge20) have been used in thermoelectric generators for deep space missions to convert radioisotope heat into electricity. This work demonstrates the highest value of thermoelectric figure-of-merit (ZT) ∼1.84 at 1073 K for n-type SiGe nanostructured bulk alloys, which is 34% higher than the reported record value for n-type SiGe alloys. The optimized samples exhibit a Seebeck coefficient of ∼284 μV K−1, resistivity of ∼45 μΩ m and thermal conductivity of ∼0.93 W m−1 K−1 at 1073 K. The main contributing factor for the enhanced ZT is very low and almost temperature independent thermal conductivity, which overcomes the low power factor of the material. Significant reduction of the thermal conductivity is caused by the scattering of low, medium and high wavelength phonons by atomic size defects, dislocations, and grain boundaries that are present due to the formation of nanocrystalline grains in the bulk material.

High thermoelectric performance of (AgCrSe2)0.5(CuCrSe2)0.5 nano-composites having all-scale natural hierarchical architectures

Recent studies have shown that thermoelectric materials exhibit a high figure-of-merit if it consists of hierarchically organized microstructures that significantly lower the lattice thermal conductivity without any appreciable change in the power factor. Here, we report a new class of thermoelectric (AgCrSe2)0.5(CuCrSe2)0.5 nano-composites synthesized via the vacuum hot pressing of a mixture of the constituents, which naturally consists of phonon scattering centers in a multiscale hierarchical fashion, i.e. atomic scale disorder, nanoscale amorphous structure, natural grain boundaries due to layered structure and mesoscale grain boundaries/interfaces. The presence of a natural hierarchical architecture of different length scales in the composite samples is confirmed by scanning electron and transmission electron microscopy. Detailed characterization reveals that in the composite samples there is a slight migration of Cu into the Ag site. Composite samples exhibit extremely low thermal conductivity ∼2 mW cm−1 K−1 at 773 K, which is nearly one third of the pure AgCrSe2 and CuCrSe2. The composite samples exhibit a high ZT ∼ 1.4 at 773 K, which is attributed to the scattering of heat carrying phonons of all wavelengths via the natural hierarchical architecture of the material. The ease of synthesis of such high performance (AgCrSe2)0.5(CuCrSe2)0.5 nanocomposites with a natural hierarchical architecture offers a promise for replacing conventional tellurides.

CuCrSe2: a high performance phonon glass and electron crystal thermoelectric material

The efficient conversion of heat into electricity using a thermoelectric approach requires high performance materials with the thermoelectric figure of merit ZT ≥ 1. Here we report on bulk CuCrSe2, which exhibits a very high ZT ∼ 1 at 773 K. The titled compound exhibits an electrical resistivity of ∼2.8 mΩ cm, a Seebeck coefficient of ∼160 μV K−1, together with very low thermal conductivity ∼7 mW cm−1 K−1 at 773 K. The very low thermal conductivity of bulk CuCrSe2 is attributed to phonon scattering by various sources such as (i) superionic Cu ions between the CrSe2 layers, (ii) nanoscale precipitates in the bulk and (iii) natural grain boundaries due to the layered structure of the material. This unusual combination of thermoelectric properties for CuCrSe2 suggests that it is an ideal example of the phonon glass and electron crystal approach.

DNA-templated assemblies of nickel hexacyanoferrate crystals.

We report here the synthesis of nickel hexacyanoferrate (NiHCF) crystals using calf thymus DNA (CT-DNA) as a template. The double-stranded CT-DNA has been used as a template to self-assemble NiHCF crystals and to produce aggregates having different morphologies at different temperatures. The guided self-assembly behavior of DNA was studied at different temperatures by scanning electron microscopy. The cube-shaped crystals of NiHCF with an average diameter of 400 nm are observed along the DNA framework at room temperature; however, at higher temperatures, the morphology of NiHCF changed from open tubular to dendrimer. The intermediate temperatures show long chains (up to many micrometers) and spherical structures of NiHCF crystals. The micrometer long DNA template plays a key role in the formation of extended arrays of NiHCF crystals, suggesting that the templating action is retained even at the higher temperatures.

Enhanced Thermoelectric Properties of Selenium-Deficient Layered TiSe2–x: A Charge-Density-Wave Material

In the present work, we report on the investigation of low-temperature (300-5 K) thermoelectric properties of hot-pressed TiSe2, a charge-density-wave (CDW) material. We demonstrate that, with increasing hot-pressing temperature, the density of TiSe2 increases and becomes nonstoichiometric owing to the loss of selenium. X-ray diffraction, scanning electron microscopy, and transimission electron microscopy results show that the material consists of a layered microstructure with several defects. Increasing the hot-press temperature in nonstoichiometric TiSe2 leads to a reduction of the resistivity and enhancement of the Seebeck coefficient in concomitent with suppression of CDW. Samples hot-pressed at 850 °C exhibited a minimum thermal conductivity (κ) of 1.5 W/m·K at 300 K that, in turn, resulted in a figure-of-merit (ZT) value of 0.14. This value is higher by 6 orders of magnitude compared to 1.49 × 10(-7) obtained for cold-pressed samples annealed at 850 °C. The enhancement of ZT in hot-pressed samples is attributed to (i) a reduced thermal conductivity owing to enhanced phonon scattering and (ii) improved power factor (α(2)σ).

Development of Angle Resolved Photoelectron Spectroscopy Beamline at Indus-1 Synchrotron Radiation Source

A beamline for angle resolved photoelectron spectroscopy (ARPES) using Indus-1 synchrotron source has been recently commissioned at the Centre for Advanced Technology, Indore India. The optical system for the beamline consists of a prefocussing toroidal mirror, 1.4 meter toroidal grating monochromator (TGM-1400) and a post-focussing toroidal mirror to cover the spectral region of 40Å to 1000Å. The beamline is coupled with an ARPES experimental station which houses the indigenously built electron analyser, sample manipulator, LEED/Auger probes, sputter etch Ar ion gun, sample load and lock facility. In this paper we are presenting the photoemission spectra of some solid samples which have been recorded through this beam line.

Synthesis of optically transparent ceramic of CaF2 doped with Mn and Ce for thermoluminescent dosimetry

Nano-particles of CaF2: Mn were synthesized by a co-precipitation method. Optically transparent ceramics were obtained by vacuum hot-pressing at 1000°C under 20 MPa pressure for 2 h. The duration of pressure and dehydration of the initial powder was found important to achieve the transparency. 50% transparency was observed for a polished disc of 1 mm thickness. SEM micrographs revealed the absence of voids in hot pressed samples. These samples were found to be highly sensitive and linear for TLD and can measure doses from mGy to kGy.

Thermoelectric performance of layered SrxTiSe2 above 300 K

In this paper we report the thermoelectric performance of Sr intercalated TiSe(2) above 300 K. Refined x-ray diffraction, high resolution transmission electron microscopy and scanning electron microscopy images show well oriented polycrystalline grains along a (0 0 l) direction and layered growth of the sample. Intercalation of Sr in TiSe(2) shows an improved Seebeck coefficient (α) value without altering the polarity of the majority charge carrier. A drastic reduction in the thermal conductivity (κ) from 3.8 W m K(-1) to 1.2 W m K(-1) (at 650 K) was observed which is ascribed to the: (i) scattering of the phonon by natural layer interfaces, grain boundaries and lattice defects and (ii) rattling of intercalated Sr atoms among weakly bound TiSe(2) layers. This led to the maximum ZT of ~0.08 at 650 K for Sr(x)TiSe(2) (x > 0.1) which is almost twice as high as the parent TiSe(2).

H[sub 2]S sensing properties of RF sputtered SnO[sub 2] films

RF sputtered SnO2 films were investigated for their response for various toxic gases (CO, NO, NH3, H2S, CO2, Cl2). These films were found to be highly sensitive and selective towards H2S. H2S gas sensing characteristics of these films have been studied as a function of temperature and concentration. These films were found to show maximum sensitivity (S = 54 for 10 ppm H2S) at a temperature of 150°C. Further, at this temperature, sensor response is found to vary linearly with concentration over a wide range from 500 ppb to 500 ppm.

H2Ssensing properties of RF sputtered SnO2films

RF sputtered SnO2 films were investigated for their response for various toxic gases (CO, NO, NH3, H2S, CO2, Cl2). These films were found to be highly sensitive and selective towards H2S. H2S gas sensing characteristics of these films have been studied as a function of temperature and concentration. These films were found to show maximum sensitivity (S = 54 for 10 ppm H2S) at a temperature of 150°C. Further, at this temperature, sensor response is found to vary linearly with concentration over a wide range from 500 ppb to 500 ppm.