Novel anion conductors - conductivity, thermodynamic stability and hydration of anion-substituted mayenite-type cage compounds C(12)A(7):X (X = O, OH, Cl, F, CN, S, N)

Mayenite (Ca12Al14O33) is a highly interesting functional material not only in view of its unique crystal structure as a cage compound but also for its variety of possible applications. Its ability to incorporate foreign ions into the cage structure opens the possibility to create new types of solid electrolytes and even electrides. Therefore, the conductivity of various anion substituted mayenites was measured as a function of temperature. Due to controversial reports on the stability of mayenite under specific thermodynamic conditions (dry, wet, reducing, and high temperature), a comprehensive study on the stability was performed. Mayenite is clearly not stable under dry conditions (ppm H2O < 100) at temperatures above 1050 degrees C, and thus, the mayenite phase vanishes from the calcium aluminate phase diagram below a minimum humidity. Two decomposition reactions were observed and are described in detail. To get further insight into the mechanism of hydration of mayenite, the conductivity was measured as a function of water vapour pressure in a range of -5 <= lg[pH(2)O/bar] <= -1.6 at temperatures ranging from 1000 degrees C <= theta <= 1200 degrees C. The hydration isotherms are described with high acuracy by the underlying point defect model, which is confirmed in a wide range of water vapour pressure

The model case of an oxygen storage catalyst - non-stoichiometry, point defects and electrical conductivity of single crystalline CeO2-ZrO2-Y2O3 solid solutions

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The ternary solid solution CeO2–ZrO2 is known for its superior performance as an oxygen storage catalyst in exhaust gas catalysis (e.g. TWC), although the defect chemical background of these outstanding properties is not fully understood quantitatively. Here, a comprehensive experimental study is reported regarding defects and defect-related transport properties of cubic stabilized single crystalline (CexZr1−x)0.8Y0.2O1.9−δ (0 ≤ x ≤ 1) solid solutions as a model system for CeO2–ZrO2. The constant fraction of yttria was chosen in order to fix a defined concentration of oxygen vacancies and to stabilize the cubic fluorite-type lattice for all Ce/Zr ratios. Measurements of the total electrical conductivity, the partial electronic conductivity, the ionic transference number and the non-stoichiometry (oxygen deficiency, oxygen storage capacity) were performed in the oxygen partial pressure range −25 < lg pO2/bar < 0 and for temperatures between 500 °C and 750 °C. The total conductivity at low pO2 is dominated by electronic transport. A strong deviation from the widely accepted ideal solution based point defect model was observed. An extended point defect model was developed using defect activities rather than concentrations in order to describe the point defect reactions in CeO2–ZrO2–Y2O3 properly. It served to obtain good quantitative agreement with the measured data. By a combination of values for non-stoichiometries and for electronic conductivities, the electron mobility could be calculated as a function of pO2, ranging between 10−2 cm2 V−1 s−1 and 10−5 cm2 V−1 s−1. Finally, the origin of the high oxygen storage capacity and superior catalytic promotion performance at a specific ratio of n(Ce)/n(Zr) ≈ 1 was attributed to two main factors: (1) a strongly enhanced electronic conductivity in the high and medium pO2 range qualifies the material to be a good mixed conductor, which is essential for a fast oxygen exchange and (2) the equilibrium constant for the reduction exhibits a maximum, which means that the reduction is thermodynamically most favoured just at this composition

Numerische Abbildung von Spaltprofilierprozessen und Bruchmechanische Beschreibung des Rissfortschrittsverhaltens von spaltprofilierten Strukturen

Im Rahmen der Forschungstätigkeiten des Sonderforschungsbereichs666 werden die Spaltprofilierverfahren weiterentwickelt. Innerhalb des ersten Teil dieses Teilprojektes steht die Abbildung der Verfahren mittels numerischer Methoden im Fokus. Die Finite Elemente Methode bietet hierfür ein sehr großes Potenzial und ist in der Industrie etabliert. Mit Hilfe der FEM lassen sich die Entwicklungszeiten reduzieren und so Kosten senken. Im ersten Teil dieses Beitrages wird eine neue Methode zur Verifikation von FE-Simulationen des Spaltprofilierprozessen vorgestellt. Im Anschluss wird eine Erweiterung der zuvor entwickelten Cut-Expand-Methode auf die Cut-FlexibleExpand-Methode beschrieben. Mit dieser lässt sich auch das flexible Spaltprofilieren effizient simulieren Im zweiten Teil des Beitrags geht es um die bruchmechanische Beschreibung des Rissfortschrittsverhaltens von spaltprofilierten Strukturen. Die Berücksichtigung der Restlebensdauer nach der Rissinitiierung bei der Schwingfestigkeitsanalyse kann die Abschätzung der Lebensdauer unter schwingender Belastung verbessern. Es wird ein bruchmechanischer Ansatz beschrieben, der das gefügeabhängige Rissfortschrittsverhalten und die fertigungsinduzierten Eigenspannungsfelder in der Rissfortschrittsanalyse berücksichtigt. In diesem Zusammenhang wird eine Methode zur experimentellen Bestimmung zyklischer bruchmechanischer Kennwerte in spaltprofilierten Strukturen entwickelt. Eine analytische Näherungslösung wird beschrieben, um den Spannungsintensitätsfaktor in der spaltprofilierten Struktur zu ermitteln. Die Ergebnisse werden mit den Ergebnissen aus FE-Berechnungen verglichen

Defect Chemistry of Oxide Nanomaterials with High Surface Area: Ordered Mesoporous Thin Films of the Oxygen Storage Catalyst CeO₂–ZrO₂

Herein we report the electrical transport properties of a series of ordered mesoporous ceria–zirconia (Ce_<i>x</i>Zr_1–<i>x</i>O₂, referred to as mp-CZO) thin films with both a cubic structure of (17 ± 2) nm diameter pores and nanocrystalline walls. Samples over the whole range of composition, including bare CeO₂ and ZrO₂, were fabricated by templating strategies using the large diblock copolymer KLE as the structure-directing agent. Both the nanoscale structure and the chemical composition of the mesoporous materials were analyzed by a combination of scanning and transmission electron microscopy, grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The total conductivity as a function of the film composition, temperature, and oxygen partial pressure was measured using impedance spectroscopy. The mesoporous solid solutions of CeO₂–ZrO₂ prepared in this work showed a higher stability against thermal ripening than both binary oxides, making them ideal model systems to study both the charge transport properties and the oxygen storage at elevated temperatures. We find that the redox properties of nanocrystalline mp-CZO thin films differ significantly from those of bulk CZO materials reported in the literature and, therefore, propose a defect chemical model of surface regions

Chlorine ion mobility in Cl-mayenite (Ca12Al14O32Cl2): An investigation combining high-temperature neutron powder diffraction, impedance spectroscopy and quantum-chemical calculations

The crystal structure of Cl-mayenite (Ca12Al14O32Cl2) is very similar to that of the well-known oxygen ion conductor O-mayenite (Ca12Al14O33), showing zeolite-type cages with partial occupancy by oxygen anions. In Cl-mayenite chlorine ions occupy the cage centers instead of oxygen ions, and it is of interest whether these chlorine ions are mobile and whether Cl-mayenite is a chlorine ion conductor. The answer for these questions is the focus of the paper. High temperature neutron powder experiments and impedance spectroscopy measurements were performed. For information on possible chloride migration pathways and activation energies, quantum-chemical calculations based on density-functional theory were carried out. The behavior of Cl− is in clear contrast to O2− in O-mayenite: even at high temperatures it only shows normal harmonic thermal displacement without indications for long range diffusion between the cages. The total ionic conductivity was found to be very low with a value of σ ≈ 10−6 S cm−1 at 1073 K. Quantum-chemical calculations result in a very high activation barrier for Cl− migration of 3.07 eV

Chlorine ion mobility in Cl-mayenite (Ca12Al14O32Cl2): An investigation combining high-temperature neutron powder diffraction, impedance spectroscopy and quantum-chemical calculations

The crystal structure of Cl-mayenite (Ca12Al14O32Cl2) is very similar to that of the well-known oxygen ion conductor O-mayenite (Ca12Al14O33), showing zeolite-type cages with partial occupancy by oxygen anions. In Cl-mayenite chlorine ions occupy the cage centers instead of oxygen ions, and it is of interest whether these chlorine ions are mobile and whether Cl-mayenite is a chlorine ion conductor. The answer for these questions is the focus of the paper. High temperature neutron powder experiments and impedance spectroscopy measurements were performed. For information on possible chloride migration pathways and activation energies, quantum-chemical calculations based on density-functional theory were carried out. The behavior of Cl− is in clear contrast to O2− in O-mayenite: even at high temperatures it only shows normal harmonic thermal displacement without indications for long range diffusion between the cages. The total ionic conductivity was found to be very low with a\u3cbr/\u3evalue of σ ≈ 10−6 S cm−1 at 1073 K. Quantum-chemical calculations result in a very high activation barrier for Cl− migration of 3.07 eV

CN-mayenite Ca12Al14O32(CN)2 ions:replacing mobile oxygen ions by cyanide

CN-mayenite (Ca12Al14O32(CN)2) is a promising candidate for high-temperature CN- conductivity in a solid due to its special structural features. It was prepared by a solidegas reaction. The transport processes were investigated by means of in situ high-temperature neutron diffraction, impedance spectroscopy and additional HebbeWagner measurements. Diffusion pathways and activation energies were determined additionally by quantum-chemical calculations.\u3cbr/\u3eCa12Al14O32(CN)2 shows a surprisingly high ionic conductivity of s = 1.4∙10^-3 S/cm at 1173 K (Ea =4.3 eV) and can be considered as first example of a new kind of solid anion conductor with a mobile molecular anion