A.C. Impedance and Transport Number Measurements of Ba 6 Ta 2 O 11
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A.C.
IMPEDANCE AND TRANSPORT NUMBER MEASUREMENTS OF Ba 6 Ta 2Ol1
TAKASHI GOTO AND A.R. WEST Chemistry Department, Scotland, U.K.
Aberdeen University,
Meston Walk,
Aberdeen AB9 2UE,
ABSTRACT Ba Ta 0 with an oxygen-deficient perovskite-related structure, was prepared through solid state reaction of BaCO3 and Ta 2 0 5 powders. The electrical conductivity of sintered Ba6 Ta 2 O11 pellets was measured by a.c. impedance spectroscopy in the temperature range between 200 and 8006C. The conductivity at 8009C was 3.7x10- 3 Q- 1 cm-'; the activation energy for bulk conduction was 0.93 eV at temperatures between 500 and 800"C and 1.22 eV below 5000C. The bulk conductivity was independent of atmosphere, while the grain boundary conductivity, with an activation energy of 1.08 eV, decreased as oxygen partial pressure increased. The oxygen ion transport number obtained from concentration cell measurements was 0.5 to 0.65 at temperatures from 450 to 8200C. INTRODUCTION Mixed conduction plays an important role in many processes, particularly reactions occurring at electrodes for solid oxide fuel cells and batteries. Development of non-noble metal electrodes compatible with electrolyte oxides, typically cubic ZrO2 stabilized by CaO or Y2 0 3 , has revolved around the possibility of finding oxide materials which exhibit high electronic and reasonably high ionic conductivity El]. It is known that the electrical properties of perovskite-related materials range widely from 1 electronic conduction with extremely high conductivity (-105 Q- cm at 250C) (2] to reasonably high ionic conduction [3]. Therefore, it is hoped to find new mixed conductors In these materials, possibly in oxygen-deficient perovskite-related materials since the oxygen vacancies may enable rapid diffusion of oxygen ions. Oxygen-deficient Ba 6 Ta 2O1 1 with a perovskite-related structure is a potential mixed conductor. Until now, however, little information on its electrical properties is available. Ittiq reported to have an electrical conductivity at 500"C of 5.8x105 'cm1 c and activation energy 1.13 eV [4]. Transport number measurements on Ba6 Ta 2O 1l to determine the nature of the current carriers have not been reported. In the present work, Ba 6 Ta 2 O1 1 pellets were prepared through solid state reaction of BaCO 3 and Ta 2 05 powders. The electrical conductivity of the Ba 6 Ta 2 O1 1 pellets was measured by a.c. impedance spectroscopy and the transport number determined using an oxygen concentration cell. EXPERIMENTAL The starting materials, BaCO3 (99.0%) and Ta 20 5 (99.9x), were weighed in the appropriate proportion, ana mixed in an agate mortar with a small amount of acetone. The powders were fired at 900"C in an Au-foil container for several hours to drive off CO2 , then ground and pelleted. The pellets were fired at 12506C for 24 hours in an MgO crucible. Pt and Al 0 crucibles were found to be unsuitable due to reaction between BaO and ?he crucibles. The pellets were reground, pressed again, then fired at 1250"C for 24 hours and at 1350"C for 24 hours.
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