Synthesis and transport properties in La_2−xA_xMo₂O_9−δ (A = Ca²⁺, Sr²⁺, Ba²⁺, K⁺) series

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• 作者：D. Marrero-Ló ; pez ; D. Pé ; rez-Coll ; J.C. Ruiz-Morales ; J. Canales-Vá ; zquez ; M.C. Martí ; n-Sedeñ ; o ; P. N&#xfa ; ñ ; ez
• 关键词：La₂Mo₂O₉ ; Nanocrystalline powder ; Oxide conductor ; SOFC ; Transport numbers
• 刊名：Electrochimica Acta
• 出版年：2007
• 期刊代码：30_00134686
• 类别：ch
• 出版时间：30 April 2007
• 卷：52
• 期：16
• 页码：5219-5231
• 文件大小：1609 K

摘要

The La_2−xA_xMo₂O_9−δ (A = Ca²⁺, Sr²⁺, Ba²⁺ and K⁺) series has been synthesised as nanocrystalline materials via a modification of the freeze-drying method. The resulting materials have been characterised by X-ray diffraction (XRD), thermal analysis (TG/DTA, DSC), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The high-temperature β-polymorph is stabilised for dopant content x > 0.01. The nanocrystalline powders were used to obtain dense ceramic materials with optimised microstructure and relative density >95%. The overall conductivity determined by impedance spectroscopy depends on both the ionic radius and dopant content. The conductivity decreases slightly as the dopant content increases in addition a maximum conductivity value was found for Sr²⁺ substitution, which show an ionic radii slightly higher than La³⁺ (e.g. 0.08 S cm⁻¹ for La₂Mo₂O₉ and 0.06 S cm⁻¹ for La_1.9Sr_0.1Mo₂O_9−δ at 973 K). The creation of extrinsic vacancies upon substitution results in a wider stability range under reducing conditions and prevents amorphisation, although the stability is not enhanced significantly when compared to samples with higher tungsten content. These materials present high thermal expansion coefficients in the range of (13–16) × 10⁻⁶ K⁻¹ between room temperature and 753 K and (18–20) × 10⁻⁶ K⁻¹ above 823 K. The ionic transport numbers determined by a modified emf method remain above 0.98 under an oxygen partial pressure gradient of O₂/air and decreases substantially under wet 5%H₂–Ar/air when approaching to the degradation temperature above 973 K due to an increase of the electronic contribution to the overall conductivity.