A study of nanoscale thermoelectric oxides: From fabrication to characterization

Around 90% of the energy human beings used is generated by heat engines with typical efficiencies of 30∼40%. This means over 400 EJ (4e20 J) heat is dissipated into the environment every year. Thermoelectric materials, which can offer the most straightforward conversion between thermal and electrical energy is an ideal candidate to harvest these unclaimed energy.;The primary bottleneck of the wide application of thermoelectric materials is their relatively low conversion efficiency. Previous research shows their convert efficiency can be improved by reducing their dimension to nanoscale. Thermoelectric oxides are promising candidates for the applicable nanoscale thermoelectric materials because they do not have the oxidization problem which troubles traditional thermoelectric materials in nanoscale.;In this work, the benefit of reducing the size of thermoelectric oxides is studied, particularly the changing of Seebeck coefficient and the thermal conductivity of the thermoelectric oxides when their size are reduced to nanoscale. Firstly, the fabrication process of La0.95Sr0.05CoO 3 thermoelectric oxide nanofilm and nanofibers were developed. The fabricated samples were verified by XRD and SEM. Then a special MEMS device was developed and used to measure the Seebeck coefficient of the prepared nanofilm and nanofiber. The measured results are 350 µV/K and 650 µV/K respectively, which proved the potential of increasing the Seebeck coefficient of thermoelectric oxides by reducing their size to nanoscale.;In order to measure the thermal conductivity of these thermoelectric oxide nanostructures, another special MEMS device was developed. The thermal conductivity of a carbon nanofiber was measured and compared with previously reported data to verify this MEMS device, and a detailed error analysis was also offered. The analysis showed that the precision of the device was in a 17∼35% range depending on different test samples. This precision is high enough for the study of thermoelectric oxides' thermal conductivities in nanoscale.;Finally, a new procedure that can load the nanofibers prepared by electrospinning onto the tester was developed. La0.95Sr0.05CoO 3 nanofibers with different diameters were loaded onto the second MEMS device and their thermal conductivities were measured. The thermal conductivity of La0.95Sr0.05CoO3 nanofiber with the diameter of 105 nm was 27% of that of La0.95Sr0.05CoO3 nanofiber with the diameter of 290 nm. The decrease of the thermal conductivity of La0.95Sr0.05CoO3 nanofibers with the decrease of their diameters demonstrates that reducing the size of thermoelectric oxides to nanoscale can reduce their thermal conductivity as well.