Processing and Structure of Disordered Pyrochlores for Solid Electrolyte Applications
Solid-oxide fuel cells (SOFCs) are high-efficiency devices that can convert a wide variety of fuels (e.g., hydrogen, methane, butane, gasoline, etc.) into electrical energy. The main components of fuel cells are the anode, electrolyte, cathode, and interconnect. This research includes synthesis and characterization of ceramic compositions for potential electrolyte materials. The project objective is to determine the structure-property relationships of specific ceramics (pyrochlores), which are ordered forms of the fluorites most commonly used as the solid electrolyte. Pyrochlore stability is believed to exist over a certain range of cation radius ratios, ~1.4 ≤ rA/rB ≤ 1.8, at atmospheric pressure. This study involves both pure and doped Y2Zr2O7. Of key interest is the ordering mechanism of the anions, whether anion disorder can exist independently of cation disorder, and what implication this possibility may have for the development of better ceramic electrolytes. The goal is to maximize the number of oxygen vacancies, which are the charge carriers, by appropriate doping while still retaining the pyrochlore structure (i.e., cation ordering) and avoiding low-mobility vacancy clusters. Pure Y2Zr2O7, with rA/rB = 1.4153 and no stoichiometric anion vacancies, has been prepared as a benchmark. In addition, two doped compositions have been prepared. The first was a singly-doped composition in the (Y2-xCax)Zr2O7-x/2 system (x = 0.1505) with rA/rB = 1.4258 and 7.525% oxygen vacancies on the 8b site. Next, a co-doped composition in the (Y2-200x/99CaxLi101x/99)Zr2O7-301x/198 system (x = 0.0495) was synthesized to achieve the same vacancy concentration but with a lower rA/rB ratio, equal to that for pure Y2Zr2O7. Analysis will help determine independently the contributions of both the rA/rB ratio and oxygen-vacancy concentration on structure and conductivity. The best densities for Y2Zr2O7 (~95% rth) were achieved by sintering at 1700°C for 6 hours. Future work involving electron and neutron diffraction is needed to determine the structure of these materials. Also, an apparatus remains to be built to determine the ionic conductivity of the samples at high temperatures.