Processing and Structure of Disordered Pyrochlores for Solid Electrolyte Applications
Solid electrolytes based on fluoritic Y2O3-stablized ZrO2 (YSZ) have been used by companies like Westinghouse for many years in solid-oxide fuel cell designs, and NASA is currently studying them for their planar SOFC design. Oxide ion conductivity in pyrochlores occurs due to the presence of intrinsic anion disorder in the lattice. Ordered oxygen vacancies provide a low-energy pathway, but also cause low oxide mobility due to the discontinuous nature of the pathways. As the structure becomes more fluorite-like, disordered vacancies (Frenkel defects) provide more mobile oxide ions and a higher-energy continuous pathway for them; however, it is known that ordered pyrochlore phases can be better ionic conductors than the disordered fluorite phases of similar compositions, but a great deal of work remains to be done to establish structure-property relationships in this class of materials. Of key interest is the (dis)ordering mechanism of the anions and whether anion disorder can exist independently of cation disorder and what implications this possibility may have for the development of better ceramic electrolytes. Our goal will be to maximize the number of oxygen vacancies via aliovalent doping while still retaining the pyrochlore structure and avoiding low-mobility vacancy clusters. Pyrochlore stability is believed to exist for compounds whose ratio of A-site cation radius to B-site cation radius is between about 1.46 to 1.8. In this project, compounds based on Y2(ZryTi1 y)2O7 are being synthesized via an organic-inorganic steric-entrapment process, and the resulting structures will be characterized via electron- and x-ray diffraction. 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 Y2ZrTiO7. Analysis will help determine independently the contributions of both the rA/rB ratio and oxygen-vacancy concentration on structure and conductivity.