Abstract:
High anisotropy in thermal conductivity (κ) coupled with anisotropy in Seebeck coefficient (S) has resulted in peak zT value of 0.85 at 150 °C in the textured Sb1.5Bi0.5Te3 nanomaterial in perpendicular to the preferential ab-plane direction. Further, strong and opposite temperature dependences in electrical conductivity (σ) and thermal conductivity, in the ranges of 1.6–1.8 and 1.4–1.8 respectively, have resulted in better average zT value of 0.72 in the 50°C–250 °C temperature range along this direction, where zT = (S2σ/κ)T. The high anisotropy in Seebeck coefficient in the range of 0.82–0.84 is peculiar, which may be attributed to differential scattering for holes and electrons by oxide interface on the ab-planes. Sample temperature and laser power dependent Raman spectroscopy have revealed that Eg(2) mode is dominant phonon transfer mode in this material and therefore, scattering of Eg(2) mode phonons may be critical in obtaining thermal conductivity reduction. The nanomaterial has been synthesized by a facile bottom-up physical synthesis process and consolidated by direct current hot pressing. Our synthesis process requires controlled melting of ingredient metals at just above their melting points-rocking and air quenching. The synthesized nanomaterial hardly experiences its melting point in this process. This energy efficient process does not require any kind of milling and produces single phase nanomaterial in unique plate-like morphology, which easily results in high texturing. This texturing has been studied by XRD analysis as well as SEM images and is also correlated with texture factor in the thermoelectric measurements. HRTEM image has shown high grain boundary density within the plates, but these have not been able to scatter phonons for thermal conductivity reduction within ab-plane. This is possibly due to smaller than optimum size of these randomly oriented grains. Such features also offer possibility of zT enhancement along preferential ab-plane direction in the textured specimen. Further, the study of anisotropy in power output density and thereby, engineered power factor under practical temperature gradients is also presented. The process also offers simplicity in obtaining further thermal conductivity reduction in perpendicular to the preferential ab-plane direction by use of tellurium or semiconductor interface layer or by use of metallic cluster in the ab-plane direction.