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Structural Variants of Perovskite Oxides
Published in Gibin George, Sivasankara Rao Ede, Zhiping Luo, Fundamentals of Perovskite Oxides, 2020
Gibin George, Sivasankara Rao Ede, Zhiping Luo
Anion-deficient perovskite with the general formula AnBnO3n−1 adopt numerous different structures; a few common ones are shown in Figure 4.13. Figure 4.13a shows a typical structure adopted by compounds with Mn3+ or Cu2+ ions in the B-site, which adopts an ordered structure of tetragonal pyramids, which is formed by the removal of apical oxygen ions from J-T active ions (e.g. A2Mn2O5 (A = Ca, Sr)) (Abakumov et al. 2008). Figure 4.13b shows a brownmillerite structure, which is the structure of a naturally existing mineral brownmillerite with the formula Ca2FeAlO5. The perovskites with brownmillerite structures are composed of alternating BO6 octahedra and BO4 tetrahedra layers. BO4 tetrahedra are formed due to the missing oxygen atom in the equatorial position (Colville and Geller 1971). The brownmillerite-structured perovskites are the most common anion-deficient perovskite structures, which can be viewed as an ideal perovskite with oxygen vacancies ordered along the [101] direction in alternate layers. Thus, the unit cell structure is enlarged along a- and c-axes as compared to the ideal perovskite. In the structure of brownmillerite perovskites, the apex-linked chains of tetrahedra are not regular; therefore, with respect to the orientation of chains, they adopt different symmetries with the same crystal structure, as listed in Table 4.9.
Survey of Types of Solid Electrolytes
Published in P.J. Gellings, H.J.M. Bouwmeester, Electrochemistry, 2019
The crystal structure of brownmillerite (Ca2AlFeO5) is closely related to that of perovskite, being an oxygen-deficient perovskite structure with vacancies ordered along the [101] direction. Some compounds with this structure have been investigated as an oxide ion conductor. However, they do not show a high conductivity until they experience a transition from a vacancy ordered state to a statistically disordered state. For example, Ba2In2O5 shows a conductivity jump of about 2 orders of magnitude at the transition temperature (930°C).14 The ionic conductivity after transition is as large as 0.1 S cm-1. On the other hand, the conductivity of Sr2Fe2O5 at 1000°C is as low as that of stoichiometric perovskite compounds, because its oxygen sublattice is not completely disordered up to this temperature.7 It is interesting to note that BaBi4Ti3MO14.5 (M = Ga, In, and Sc) compounds that consist of intergrowths between the Aurivillius phase (Bi4Ti3O12) and brown millerite layers of BaMO2.5 show a high conductivity (10−2 to 10−1 S cm−1 at 900°C) after a similar order–disorder transition.15
Composition of renders and plasters of award-winning buildings in Lisbon (Portugal): A contribution to the knowledge of binders used in the 20th Century
Published in International Journal of Architectural Heritage, 2023
Luís Almeida, A. Santos Silva, Rosário Veiga, José Mirão
The qualitative mineralogical composition determined by XRD (Table 3) in the binder-rich fraction showed that calcite is the main mineral in almost every analysed sample. Calcite is present mainly due to the carbonation of lime since scarce or none carbonate aggregates were found, except for the Marmorite mortars. The occurrence of unslaked lime in the form of nodules (lime lumps) marks a trail of air lime used as a binder, often detectable macroscopically. Non-hydrated hydraulic phases (e.g. alite-C3S, belite-C2S or brownmillerite-C4AF), typically from Portland cement clinker (Lea 1988), were detected in several samples, notably from the IRF (1938) case study onward (Table 3, Figures 5 and 6), as well as ettringite, hydrocalumite, monocarboaluminate and portlandite.
Stabilization of carbon dioxide and chromium slag via carbonation
Published in Environmental Technology, 2017
Xingxing Wu, Binbin Yu, Wei Xu, Zheng Fan, Zucheng Wu, Huimin Zhang
Powdered X-ray diffraction (XRD) of pre- and post-carbonated chromium slags was conducted on a sample taken from setup using sample 1 and pure CO2. As shown in Figure 4(a), the main mineralogical composition of the pre-chromium slag are periclase (MgO), portlandite (Ca(OH)2) and some calcium salt, including larnite (Ca2SiO4), calcium manganese oxide (CaMnO3), chromatite (CaCrO4) and brownmillerite (Ca2(Al, Fe+3)2O5). Chromium is present in the form of eskolaite (Cr2O3), sodium chromium oxide (NaCrO2) and chromatite (CaCrO4). Compared with pre-carbonated chromium slag, there are some changes in the powdered XRDs of post-carbonated chromium slags (Figure 4(b)). Although calcite is the only carbonate species in the dynamic leaching procedure, the presence of calcite indicates that the sequestration of CO2 in chromium slag is feasible and the absence of portlandite may be responsible for it. No chromatite or other species of Cr(VI) were identified on post-carbonated sample, one reason of which is that Cr(VI), with strong oxidation in acidic conditions, reacted with other mineral except the part in the leakage.
Corrosion of mild steel under insulation – the effect of dissolved metal ions
Published in Corrosion Engineering, Science and Technology, 2020
Q. Cao, M. Esmaily, R. L. Liu, N. Birbilis, S. Thomas
Dissolved metal ions from mild steel corrosion and metallic ions leaching from insulation, influence the kinetics of CUI. This arises from an increase in ionic conductivity of moist insulation. The corrosion products formed upon MS during CUI were characterised in an attempt to validate the effects of the dissolved metal ions on CUI. Findings from the present study include: Long-term mass loss testing revealed that increasing moisture content within the insulation could lead to an increase in the kinetics of CUI.Dissolved metal ions either from the steel substrate corrosion or leaching from the insulation can increase the kinetics of CUI by increasing the electrolyte conductivity. The major ions (cations) that leached from the insulation were analysed using ICP-OES, and were found to be Fe3+, Ca2+, Mg2+ and K+.The packing of insulation around the steel substrate may introduce the formation of differential aeration cells. The rate of steel corrosion when exposed to insulation containing a significant degree of moisture, therefore, could be greater than that of steel exposed to a solution (with no insulation).The crystalline corrosion products formed upon MS specimens during CUI include silicon dioxide, magnetite, wollastonite, dolomite, mayenite and brownmillerite.CUI may be a pH-dependent process, as the resistivity of moist insulation can be affected by pH; with the rate of metallic ions leaching or dissolving into the moist insulation dependent on the pH. Correspondingly, in the pH range 3–5, more severe CUI was observed in this study.