| Abstract [eng] |
Solid oxide fuel cells (SOFCs) have become one of the most used and researched types of fuel cells as they can operate over a wide temperature range from 400 °C to 1000 °C. The electrolyte of SOFC as initial material has important requirements such as high density and good ionic conductivity. These requirements can be achieved by controlling the concentration of impurities in the sample, e.g., mol% of gadolinia in gadolinium-doped ceria (GDC) thin films and selecting the appropriate method for the deposition of thin films. Comparing to other methods of producing electrolytes, e-beam evaporation technique has large deposition area and high deposition rate. In this research, gadolinium-doped ceria (GDC) ceramic powders with the molar content of Gd2O3 10, 15, 20 mol% were synthesised by coprecipitation (CP) synthesis method. Pellets with diameter of 13 mm were formed from synthesized and calcined at 900 °C powders; and annealed at 1200 °C for 5 h in the air. Synthesised pellets were further used as targets to form thin films on Si (100) substrate using e-beam evaporation technique. Crystal structure and surface area were studied by using X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) methods. Structural properties and thermal decomposition of the samples were analysed using scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) technique. Electrical properties of 10GDC pellets were investigated by impedance spectroscopy in 200 - 800 °C temperature range, from 1 Hz to 1 MHz frequencies. In this research, it was aimed to synthesize and characterize target material, to form dense electrolyte thin films using EB-PVD technique. Due to the fact that the target may influence the chemical composition of the film, the chemical composition of evaporated thin films was estimated using XPS measurement. XRD analysis of synthesised GDC powders indicates that 10, 15, and 20 GDC have a cubic fluorite-type crystalline lattice (Fm▁3m space group) with dominating (111) crystallographic plane and were observed at a relatively low calcination temperature. SEM images revealed that the average grain sizes of GDC ceramics were about 201 and 317 nm, with no visible defects observed. Arrhenius plots of total ionic conductivities and activation energies for synthesized 10 GDC electrolytes were obtained from impedance plots, and showed that 10 GDC had the highest total ionic conductivity (0.011 S∙cm−1 at 600 °C) with the lowest activation energies both at LT and HT (0.85 and 0.65 eV). Complex impedance plots, obtained from the impedance spectroscopy showed four types of different processes occurring in the structure, which include grain, grain boundary at the low temperature range and polarisation of platinum electrodes at the higher temperature range. |
