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Microstructural impacts on the electrical properties of copper and titanium substituted bismuth vanadates
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Microstructural impacts on the electrical properties of copper and titanium substituted bismuth vanadates
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Microstructural impacts on the electrical properties of copper and titanium substituted bismuth vanadates
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Dissertation
Microstructural impacts on the electrical properties of copper and titanium substituted bismuth vanadates
2015
Overview
In the search for a material that can exceed the performance of YSZ as an ionic oxide conductor at intermediate temperatures (300°C to 600°C) a group of Aurivillius phase ceramics dubbed the BIMEVOX (Bi 2V1-xMexO5.5-δ) family has garnered much attention over the past 20 years. Novel results regarding the influence of microstructure on electrical properties were obtained by non-conventional methods of fabrication and characterization. Approaches included: uniaxial, load assisted sintering, molten salt synthesis, templated grain growth, and the use of ion blocking electrodes to measure the partial electronic conductivity. Molten salt synthesis methods successfully produced high aspect ratio platelets of both BiCuVOx (Bi2V0.9Cu0.1O 5.5-δ) and BiCuTiVOx (Bi2V0.9Cu0.05 Ti0.05O5.5-δ), at a variety of temperatures and times. Uniaxial load assisted sintering (or “hot-forging”) when combined with templated grain growth produced high density (ρ>95% theoretical) samples of moderate texture (F(00l) up to 29%). Impedance spectroscopy measurements indicated that increased texture and grain size reduce the thermal stability of BiCuVOx below the critical γ-phase transition temperature. Measurements of total conductivity were made with changing oxygen partial pressure down to 10-4 atm of oxygen between temperatures of 400°C and 550°C. Under those conditions, total conductivity was invariant, confirming published results of operation within the ionic compensated regime. Partial electronic conductivity and electronic transference numbers were estimated by asymmetric DC polarization measurements down to 10-6 atm of oxygen between 500°C and 550°C. The results indicate that the partial pressure of oxygen in normal air is already below the intrinsic minimum of conductivity at 500°C and that electronic conductivity may become significant (te>0.01) no lower than 10-6 atm of oxygen. The culmination of research since its first publication poses uncertainty regarding the longevity of BIMEVOX compounds in long term and intermediate temperature electrochemical devices. The work performed on samples of BiCuVOx corroborates many of the conclusions found in the literature and does not support the use of these formulations as a continuous use electrolyte at temperatures greater than 500°C. Results also indicate that operation at temperatures below 500°C might be possible if the phase stability issues are addressed by tailoring the microstructure or use of dual substitutions, such as BiCuTiVOx.
