ABSTRACT
Recent advances in high performance thermoelectric materials have garnered unprecedented attention owing to their capability of direct transformation of heat energy to useful electricity. Copper Telluride (Cu2Te), a member of the chalcogenide family has emerged as a state-of-the-art thermoelectric material with low thermal conductivity and high thermoelectric (TE) performance, however, this material exhibits exceptional transport properties only at very high temperatures. In this study, we have investigated the synergistic effects of Ga doping on the TE performance by first principles calculations along with experimental validations. The DFT (Density Functional Theory) calculations predicted that Ga doping, within considerable limits enhanced the electrical conductivity and Seebeck coefficients in Cu2Te. This proof of concept was validated by experimental synthesis of Ga doped Cu2Te by simple direct annealing for shorter durations of 48 hours at 1120 ºC (~1/4th) than in previous work and subsequent thermoelectric characterization. The enhanced electrical conductivity, thermopower, and moderate thermal conductivities led to the optimized TE performance in 3 atomic % Ga doping (Cu1.97Ga0.03Te), exhibiting a ZT value of 0.46 at 600 K, almost three times that of pristine Cu2Te in this temperature range. This comprehensive study provides the platform for developing new low-cost and energy efficient TE materials with enhanced ZT performance in medium temperature applications.
ABSTRACT
The potential of thermoelectric materials to generate electricity from the waste heat can play a key role in achieving a global sustainable energy future. In order to proceed in this direction, it is essential to have thermoelectric materials that are environmentally friendly and exhibit high figure of merit, ZT. Oxide thermoelectric materials are considered ideal for such applications. High thermoelectric performance has been reported in single crystals of Ca3Co4O9. However, for large scale applications single crystals are not suitable and it is essential to develop high-performance polycrystalline thermoelectric materials. In polycrystalline form, Ca3Co4O9 is known to exhibit much weaker thermoelectric response than in single crystal form. Here, we report the observation of enhanced thermoelectric response in polycrystalline Ca3Co4O9 on doping Tb ions in the material. Polycrystalline Ca3-xTbxCo4O9 (x = 0.0-0.7) samples were prepared by a solid-state reaction technique. Samples were thoroughly characterized using several state of the art techniques including XRD, TEM, SEM and XPS. Temperature dependent Seebeck coefficient, electrical resistivity and thermal conductivity measurements were performed. A record ZT of 0.74 at 800 K was observed for Tb doped Ca3Co4O9 which is the highest value observed till date in any polycrystalline sample of this system.
ABSTRACT
A dramatic drop of ≈5 orders of magnitude in the resistance (R) of La(0.175)Pr(0.45)Ca(0.375)MnO(3) epitaxial films upon exposure to optical photons derived from both continuous and pulsed lasers, as well as broad-band sources at temperatures (T) < 30 K is reported. The strength of change is a sensitive function of both the incident photon flux and temperature. Under isothermal conditions the photo-generated low resistance state persists eternally after removal of light. This non-equilibrium state is metallic, as revealed by the positive dR/dT for T ≤ T(p) (≈120 K). This electrically conducting state is presumably ferromagnetic as T(p) coincides with the temperature where a weak ferromagnetism sets in on cooling the insulating film from room temperature. To rule out the possibility of photon-induced local heating of the sample as a mechanism of the observed effects, photo-illumination experiments were performed under identical conditions on thin films of two non-charge-ordered manganites deposited on substrates of similar thermal conductivity. Our model for the observed transition encompasses a global charge-ordered state in which ferromagnetic metallic clusters of fraction p much less than the critical fraction p(c) for percolation exists at low temperatures. Photo-induced melting of the charge-ordered state increases this fraction beyond p(c) in a cumulative manner as successive pulses of light fall on the sample.
