Fabrication and characterization of oxide-ion conducting thin films

Abstract

Metal-doped lanthanum molybdenum oxide and metal-doped ceria have been shown to conduct oxide ions efficiently. They are being considered as very promising solid electrolytes for solid oxide fuel cells (SOFC). In particular, their ability to provide reasonably high ionic conductivity at relatively low temperatures (500-600 oC) is an attractive feature making them useful in future low-temperature SOFCs targeted to operate at 500 oC or below. Thus utilizing these materials and employing the modern oxide thin film fabrication technology it is visualized that a miniaturized and low-temperature operating SOFC can be achieved. Consequently a new breed of efficient and much sought-after transferable power source for portable electronic devices and small scale electrical appliances is possible. In this project pulsed laser deposition was used to prepare some of these oxide electrolyte thin films. Structural and electrical characterizations of these films were carried out to evaluate their suitability for use in thin film SOFC. The laser ablation targets of Ce0.8Gd0.2O2, Ce0.8Sm0.2, Zr0.92Y0.08O2, La2Mo2O9, La1.9Ba0.1Mo2O9, La1.9Ca0.l Mo2O9, La0.85Sr0.15MnO3, La0.7Sr0.3MnO3, La0.85Sr0.15MnO3, La1-xSrxC0O3 (0.2<=x <=0.5) and SrVO3 were fabricated by the standard solid state reaction method. The SOFC oxide electrolyte and electrode thin films were fabricated by pulsed laser deposition on (100) LaAlO3, (100) Si, (100) MgO and A12O3 at various substrate temperatures spanning from 300 oC to 750 oC. The ambient oxygen pressure during film growth ranged from 100 mTorr to 300 mTorr. The crystal structure, crystallinity, thickness and lattice parameters of these as-deposited films were investigated by x-ray diffraction and transmission electron microscope. It has been shown that high quality epitaxial and polycrystalline films can be obtained at different growth conditions. A radiation heating tube furnace for electrical characterization were designed and made. It was primarily used for temperature dependant impedance measurements on films over the temperature range of 200 oC to 800 oC. Our results reveal good ionic conductivity in these films. In addition the observed ionic conductivities are closely related to the crystalline quality of the films. We have yielded strong experimental evidence that in utilizing these oxide electrolyte thin films, miniaturized SOFCs operating at below 550 oC are possible.