The effect of synthesis technique on thermoelectric properties of nanocrystalline NaCo2O4 ceramics

Sodium cobalt oxide (NaCo2O4) nanocrystalline thermoelectric ceramic powders were synthesized using conventional polymerized complex sol-gel and electrospinning techniques and then consolidated into bulk ceramics. We have investigated the differences in the microstructure and thermoelectric properties of the samples prepared using electrospinning and sol-gel methods. The crystal structures of the powders and nanofibers were characterized using X-ray diffraction and scanning electron microscopy before and after calcination at different temperatures. Nanofibers prepared by electrospinning have a diameter of approximately 300 nm, while the diameter of the grains of sol-gel powders ranges from 1 to 5 mu m. Thermoelectric properties of the bulk ceramics were measured by physical properties measurement system in the temperature range of 15-300 K. The calculated values of dimensionless figure of merit at 300 K are 6 x 10(-7) and 4 x 10(-5) for sintered powders from sol-gel and nanofibers, respectively. While electrospinning production technique increase thermoelectric power approximately 15 % and it fold electrical conductivity average 20 times, it decreased thermal conductivity by 90 %.