Ranita Basu

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.

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 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.

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.

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.

Tailoring thermal conductivity in PbS by incorporation of copper for thermoelectric applications

Lead Chalcogenides (PbS, PbSe, PbTe) are the important class of materials and are the best thermoelectric materials in the mid temperature range (∼700K). In the present work, samples of CuxPb(1-x)S (x = 0, 0.2,0.4,0.6 and 0.8) were prepared in order to tailor its thermal conductivity (κ) for thermoelectric applications. Lowering the thermal conductivity is the most focused way to obtain high performance of thermoelectric material. The temperature dependence thermal conductivity of CuxPb(1-x)S (x = 0, 0.2,0.4,0.6 and 0.8) samples has been investigated in the temperature range of 300 K to 700 K. The reduction in thermal diffusivity (α) and thermal conductivity (κ) were observed for the CuxPb(1-x)S (x =0.2,0.4 and 0.6) samples compared to pure PbS sample in the entire temperature range of study. At room temperature the thermal diffusivity and thermal conductivity of the CuxPb(1-x)S (x = 0.2,0.4 and 0.6) samples were found to be ∼4.0 and 3.0 times lower than that of PbS respectively. The presence of natural h...

Improvement in thermoelectric power factor of mechanically alloyed p-type SiGe by incorporation of TiB2

Nearly 60% of the world’s useful energy is wasted as heat and recovering a fraction of this waste heat by converting it as useful electrical power is an important area of research[1]. Thermoelectric power generators (TEG) are solid state devices which converts heat into electricity. TEG consists of n and p-type thermoelements connected electrically in series and thermally in parallel[2]. Silicon germanium (SiGe) alloy is one of the conventional high temperature thermoelectric materials and is being used in radio-isotopes based thermoelectric power generators for deep space exploration programs.Temperature (T) dependence of thermoelectric (TE) properties of p-type SiGe and p-type SiGe-x wt.%TiB2 (x=6,8,10%) nanocomposite materials has been studied with in the temperature range of 300 K to 1100 K. It is observed that there is an improvement in the power factor (α2/ρ) of SiGe alloy on addition of TiB2 upto 8 wt.% that is mainly due to increase in the Seebeck coefficient (α) and electrical conductivity (σ) of...