Kinetics and Mechanism of Formation of Barium Zirconate from Barium Carbonate and Zirconia Powders.pdfVIP

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Kinetics and Mechanism of Formation of Barium Zirconate from Barium Carbonate and Zirconia Powders.pdf

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Kinetics and Mechanism of Formation of Barium Zirconate from Barium Carbonate and Zirconia Powders

Kinetics and Mechanism of Formation of Barium Zirconate from Barium Carbonate and Zirconia Powders Alberto Ubaldini, ? Vincenzo Buscaglia,*,? Claudio Uliana,§ Giorgio Costa,? and Maurizio Ferretti ? INFM, Dipartimento di Chimica e Chimica Industriale, Universita? di Genova, I-16146, Genoa, Italy Istituto di Chimica Fisica Applicata dei Materiali, Consiglio Nazionale delle Ricerche, I-16149 Genoa, Italy Dipartimento di Chimica e Chimica Industriale, Universita? di Genova, I-16146, Genoa, Italy The formation of BaZrO 3 from very fine (70–90 nm) ZrO 2 powders and coarser (1 m) BaCO 3 powders has been studied in dry and humid air up to 1300°C using TGA/DTA, XRD, SEM, TEM, and EDS microanalysis. In the temperature range 900°–1100°C, barium is rapidly transported at the surface of the ZrO 2 particles and reacts, forming BaZrO 3 . The compound grows as a concentric layer with gradual consump- tion of the central ZrO 2 particle. The overall formation kinetics of BaZrO 3 is well described by a diminishing core model, and the most likely rate-determining step is a phase- boundary process at the ZrO 2 –BaZrO 3 moving interface. The size and shape of the final particles is generally determined by the morphology of the starting ZrO 2 particles and not by that of the BaCO 3 . The reaction is faster in humid air than in dry air, and the activation energy decreases from 294 kJmol1 (dry air) to 220 kJmol1 (humid air). When the fraction reacted is 80–90 mol%, the reaction rate rapidly decreases. I. Introduction B ARIUM ZIRCONATE (BaZrO 3 ) is different from other metallic and ceramic materials, 1 because it is virtually inert with respect to high-temperature superconductors of the YBa 2 Cu 3 O 7 (YBCO) family, 2–4 and, therefore, it is particularly suitable for their processing. In particular, BaZrO 3 allows for the growth of high- purity single crystals of YBCO from the BaO–CuO flux 5–8 and can be a good substrate for the deposition of thin and thick superconductor films. 9,10