Electrochemistry Research Group, Membrane Science and Technology, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands.
Phys Chem Chem Phys. 2019 Oct 9;21(39):21824-21835. doi: 10.1039/c9cp04911h.
Calcium manganite-based perovskite-type oxides hold promise for application in chemical looping combustion processes and oxygen transport membranes. In this study, we have investigated the structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1-x-yTixFeyO3-δ. Distinct from previous work, data of high-temperature X-ray diffraction (HT-XRD) in the temperature range 600-1000 °C (with intervals of 25 °C) demonstrates that CaMnO3-δ (CM) transforms from orthorhombic to a mixture of orthorhombic and tetragonal phases between 875 °C and 900 °C. Rietveld refinements show the formation of a pure tetragonal phase at 975 °C and of a pure cubic phase at 1000 °C. Partial substitution of manganese by iron and/or titanium to yield CaMn0.875Ti0.125O3-δ (CMT), CaMn0.85Fe0.15O3-δ (CMF) or CaMn0.725Ti0.125Fe0.15O3-δ (CMTF) leads to different phase behaviours. While CMT remains orthorhombic up to the highest temperature covered by the HT-XRD experiments, CMF and CMTF undergo an orthorhombic → tetragonal → cubic sequence of phase transitions. Electrical conductivity relaxation measurements are conducted to determine the chemical diffusion coefficient (Dchem) and the surface exchange coefficient (kchem) of the materials. The results demonstrate that oxygen transport is hindered in the tetragonal phase, when occurring, which is attributed to a possible ordering of oxygen vacancies. The small polaron electrical conductivity of CM in the cited temperature range is lowered upon partial manganese substitution, by about 10% for CMF and up to half an order of magnitude for CMT and CMTF.
钙锰矿型钙钛矿氧化物有望应用于化学链燃烧过程和氧气传输膜。在这项研究中，我们研究了钙钛矿型氧化物 CaMn1-x-yTixFeyO3-δ 的结构、电导率和氧气传输性能。与以前的工作不同，高温 X 射线衍射（HT-XRD）的数据表明，CaMnO3-δ（CM）在 875°C 至 900°C 之间从正交相转变为正交相与四方相的混合物。Rietveld 精修表明，在 975°C 时形成纯四方相，在 1000°C 时形成纯立方相。部分用铁和/或钛替代锰，得到 CaMn0.875Ti0.125O3-δ（CMT）、CaMn0.85Fe0.15O3-δ（CMF）或 CaMn0.725Ti0.125Fe0.15O3-δ（CMTF），导致不同的相行为。虽然 CMT 一直保持正交相，直到 HT-XRD 实验覆盖的最高温度，但 CMF 和 CMTF 经历了正交相→四方相→立方相的相变顺序。电导率弛豫测量用于确定材料的化学扩散系数（Dchem）和表面交换系数（kchem）。结果表明，在四方相中，氧传输受到阻碍，这归因于氧空位的可能有序化。在引用的温度范围内，CM 的小极化子电导率在部分锰取代后降低，对于 CMF 降低约 10%，对于 CMT 和 CMTF 降低约半个数量级。
