Ranu Bhatt

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.

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).

Thermoelectric performance of Cu intercalated layered TiSe2 above 300 K

High temperature (300–650 K) thermoelectric performance of Cu intercalated Cu x TiSe2 (x = 0−0.11) material has been investigated. Cu intercalation transforms the p-type TiSe2 to n-type Cu xTiSe2 with significant Seebeck coefficient value saturating to ∼−90 μV/K (x ≥ 0.06) at 650 K. Spanning the entire range of Cu xTiSe2 samples, very low thermal conductivity has been observed which is attributed to the layered growth structure and rattling effect of weakly bound Cu atoms in the van der Waals gaps of TiSe2 layers. Figure of merit (ZT) value of 0.1 and 0.15 is achieved for pure TiSe2 and Cu 0.11TiSe2 samples, respectively, at 650 K. The 4-element thermoelectric generator fabricated using a combination of p-type TiSe2 and n-type Cu 0.11TiSe2 is designed, which gives an output power of 0.64 mW at 650 K.

Thermoelectric properties of Ag added Ca0.98La0.02MnO3

Temperature (T) dependence of thermoelectric (TE) properties of Ca0.98La0.02MnO3 has been studied with in the temperature range of 300 K to 900 K. It is observed that the Seebeck coefficient (S) increases with increasing temperature. The negative sign of S indicates that CMO is an n-type TE material. Resistivity (ρ) shows a metal-insulator type transition. The addition of Silver to these samples caused a reduction in the electrical resistivity of the samples, where as the Seebeck coefficient remained almost constant, desirable for thermoelectric materials.

Swift heavy ions for controlled modification of soft magnetic properties of Fe0.85N0.15 thin film

It is shown that the soft magnetic properties of amorphous Fe₀.₈₅N₀.₁₅ film can be modified in a controlled manner by irradiation with 120 MeV Ag ions. Irradiation causes relaxation of short range as well as long range stresses, resulting in an improvement in the soft magnetic properties. Nuclear resonance reflectivity has been used to measure the diffusion length of Fe atoms as a function of irradiation fluence. It is found that the atomic motion associated with irradiation is not expected to modify significantly the interfaces with substrate or other layers.

Study of thermal stability of Cu2Se thermoelectric material

Sustainability of thermoelectric parameter in operating temperature range is a key consideration factor for fabricating thermoelectric generator or cooler. In present work, we have studied the stability of thermoelectric parameter of Cu2Se within the temperature range of 50-800°C. Temperature dependent Seebeck coefficients and electrical resistivity measurement are performed under three continuous thermal cycles. X-ray diffraction pattern shows the presence of mixed cubic-monoclinic Cu2Se phase in bare pellet which transforms to pure α-Cu2Se phase with repeating thermal cycle. Significant enhancement in Seebeck coefficient and electrical resistivity is observed which may be attributed to (i) Se loss observed in EDS and (ii) the phase transformation from mixed cubic-monoclinic structure to pure monoclinic α-Cu2Se phase.

Prussian blue based molecular magnet K0.3Mn2.85[Cr(CN)6]2⋅nH2O with ferrimagnetic ordering temperature of 60 K

Molecular magnet based on Prussian blue analogues, K0.3Mn2.85[Cr(CN)6]2⋅nH2O, has been synthesized using co-precipitation method, and its structural and magnetic properties are investigated using X-ray diffraction (XRD), infra red spectroscopy (IR) and dc magnetization measurement techniques. The XRD study confirms the crystalline nature of the compound with a face centered cubic (fcc) structure (space group Fm3m). The positions of the absorption bands in the IR spectrum (∼1900-2300 cm−1), confirm the formation of Prussian blue analogue. The magnetization measurement shows a soft ferrimagnetic nature of the compound with negligible coercive field. The Curie temperature (TC) and magnetization at 5 K are found to be ∼60 K and ∼ 8.2 μB/f.u. respectively.

Optimisation of electrical contact resistance in Bi0.5Sb1.5Te3 for development of thermoelectric generators

p-type Bi0.5Sb1.5Te3 material were synthesised by mechanical alloying in ball-mill for 24 hours. We achieved best ZT of 0.65 at 500 K which is attributed to the extremely low thermal conductivity and high power factor. Contact resistance in the p-type thermoelement has been optimised by varying the combination of interfacial layer. Ni coated p-type thermoelement shows minimum change in the pellet resistance. Open circuit voltage of unicouple module fabricated using Ni coated thermoelement was measured. The maximum power output of 70.89 mW at hot end temperature of 200°C is achieved.

Synthesis & tailoring the thermal conductivity of Sr doped Bi2Se3 thermoelectric material

We have investigated the thermal transport properties of SrxBi2-xSe3 (x=0, 0.05, 0.2). The samples were synthesized by melt route method followed by vacuum hot press. The structural and morphological information of sample has been retrieved using x-ray diffraction (XRD) and scanning electron microscopy (SEM). The thermal transport measurement were performed in the temperature range of 300-550 K. It is found that with increasing Sr content the total thermal conductivity of the material decreases which is attributed to the enhance phonon scattering due to natural grown layered structure and defect induced by Sr doping.