Development and Characterization of Novel Bicomposite Mixed Ionic and Electronic Conductor Metal Oxide Thin Films Fabricated by Ultrasonic Spray Pyrolysis
By mixing a previously known electronic conductor, strontium-doped lanthanum manganite (LSM) with one of two ionic conductors, strontium-doped lanthanum ferrite (LSF) or copper-doped bismuth vanadate (BiCuVOx), the fabrication of a mixed ionic and electronic conductor (MIEC) was investigated. The thin films of the materials were produced by the ultrasonic spray pyrolysis (USP) technique. Unlike traditional USP techniques, we have employed the USP source to spray precursor solutions to grow the films directly on the substrate and attempted to grow complex bicomposite films consisting of crystallites of two kinds of materials. Two different spraying methods were undertaken for binary film fabrication. One approach involved spraying a mixed solution of two precursor solutions, and the resulting films were compared with those formed when the two precursor solutions were sprayed independently and sequentially from two independent ultrasonic sources. We expected to observe the mixed solutions producing a solid solution of uniform stoichiometry of the mixed metal ions, while the two-source approach was anticipated to produce films containing crystallites of the two independent materials. Spectroscopic, microscopic, and diffraction techniques were used to characterize these films. The binary films of LSM and LSF resulted in a solid solution of LSMF regardless of the spraying methods. The binary film of LSM and BiCuVOx produced by the two-source spraying method produced a bicomposite film. Films were originally deposited on quartz substrates. However, thermal expansion coefficient (TEC) mismatch between the films and the quartz substrate led to significant interfacial stress resulting in substantial film cracking. Film growth was switched to yttria-stabilized zirconia substrates which had a better TEC match with the films, resulting in improved film integrity. The conductivity of the films was also measured and showed small-polaron like conductivity with changing temperatures. Our conductivity measurements were performed under ambient atmospheric conditions and as such characterized the total ionic and electronic conductivity. Experimental limitations prevented us from characterizing the two conduction mechanisms independently but measurements on the LSM showed that our conductivity was comparable to that found in several literature references (1-2 S cm-1), though it was not equal to some of the best (~100 S cm-1).