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Rüdiger Dieckmann - One of the best experts on this subject based on the ideXlab platform.
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Orientation, Oxygen Activity and temperature dependencies of the diffusion of manganese in manganese orthosilicate, Mn2SiO4
Solid State Ionics, 2012Co-Authors: Q. Tang, Rüdiger DieckmannAbstract:Abstract The diffusion of manganese in manganese silicate, Mn 2 SiO 4 , in equilibrium with MnSiO 3 was experimentally studied as a function of crystallographic orientation, Oxygen Activity and temperature using high purity synthetic single crystals of manganese silicate. The Oxygen Activity dependence of the manganese cation diffusion in Mn 2 SiO 4 along the three principle orientations at 1200 °C suggests that, at high Oxygen activities, manganese cations move via different types of vacancies, most likely isolated manganese vacancies, ( V Mn 2 + )″, and possibly neutral associates, {2(Mn Mn 2 + 3 + ) ⋅ ( V Mn 2 + )″} × , the latter being minority defects. At lower Oxygen activities, the Oxygen Activity dependence of the manganese tracer diffusion becomes smaller than that at higher Oxygen activities and eventually becomes negative at very low Oxygen activities. The latter is attributed to an increased concentration of manganese interstitials at lower Oxygen activities. The temperature dependence of the manganese tracer diffusion along the three principle orientations was experimentally investigated between 1150 and 1250 °C at log a O 2 = − 2.7 ( a O 2 = P O 2 / P O 2 o with P O 2 o = 1 atm) and between 1150 and 1300 °C at log a O 2 = − 9.8. The results obtained suggest that the orientation dependence of the diffusion of manganese in Mn 2 SiO 4 does not vary very significantly with temperature. When using the space group Pbnm to describe crystal orientations, the ratio found for the manganese tracer diffusion coefficients at log a O 2 = − 2.7 is approximately D Mn[001] ∗ :D Mn[010] ∗ :D Mn[100] ∗ = 5.3:2.2:1.
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Orientation, Oxygen Activity and temperature dependencies of the diffusion of cobalt in cobalt orthosilicate, Co2SiO4
Solid State Ionics, 2012Co-Authors: Q. Tang, Rüdiger DieckmannAbstract:Abstract To contribute to a better understanding of point defects and defect-related transport properties in cobalt orthosilicate, Co 2 SiO 4 , the orientation-dependent diffusion of cobalt in high purity, synthetic single crystals of Co 2 SiO 4 being in thermodynamic equilibrium with silica was experimentally studied as a function of crystallographic orientation, Oxygen Activity and temperature. The Oxygen Activity dependence of the diffusion of cobalt in Co 2 SiO 4 along the three principle orientations at 1300 °C is at high Oxygen activities compatible with cobalt vacancies and holes as majority defects. At lower Oxygen activities the Oxygen Activity dependence of the cobalt tracer diffusion coefficients becomes smaller than at high Oxygen activities. This is most likely related to an increase of the concentrations of cobalt interstitials at lower Oxygen activities and at such Oxygen activities also to an increased influence of the impurity ions being present in the samples used. The latter is due to smaller overall point defect concentrations at lower Oxygen activities compared to those found at higher Oxygen activities. The temperature dependence of the cobalt tracer diffusion along the orientations denoted above was investigated in the range between 1200 and 1300 °C at a O 2 = 1 (a O 2 = P O 2 /P° O 2 and P° O 2 = 1 atm). The results obtained suggest that the anisotropy of the diffusion of Co in Co 2 SiO 4 does not very significantly vary with the temperature. When using the space group Pbnm when assigning crystal orientations the ratio found for the cobalt tracer diffusion coefficients at a O 2 = 1 is approximately D ⁎ Co[001] :D ⁎ Co[010] :D ⁎ Co[100] = 30:3:1.
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Does the valence state of an ion affect its diffusivity? Part I: Oxygen Activity dependence of the diffusion of iron in alumina-doped MgO
Solid State Sciences, 2008Co-Authors: E. Chen, T.-l. Tsai, Rüdiger DieckmannAbstract:Abstract To investigate whether a change in the valence state of tracer ions affects their diffusivity or not, the iron tracer diffusion in Al2O3-doped MgO, in which 0.5% of the cations were Al3+ ions, has been studied experimentally. Samples were prepared from high purity aluminum and magnesium nitrates using a chemical solution method and from powders of high purity Al2O3 and MgO. Because the concentration of the Al3+ dopant ions present in the samples was much larger than that of all other impurities, the concentration of the majority point defects, cation vacancies, was determined by the Al3+ concentration. Therefore, when changing the Oxygen Activity, the diffusivity of iron tracer ions can only be altered by changes in their valence state. Measurements of iron tracer diffusion coefficients were performed as a function of the Oxygen Activity at 1100 and 1200 °C. The experimental results indicate that the mean diffusivity of iron ions in Al2O3-doped MgO increases with increasing Oxygen Activity at both temperatures, suggesting that Fe3+ ions diffuse in Al2O3-doped MgO faster than Fe2+ ions.
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Influence of impurities on the Oxygen Activity-dependent variation of the Oxygen content of a commercial, CaO-doped ZrO2
Solid State Ionics, 2004Co-Authors: Janet Hill Tinkler, Rüdiger DieckmannAbstract:Abstract A thermogravimetric study has been performed on the variation of the Oxygen content of (partially stabilized) zirconia, doped with 10 mol% CaO and with Fe 2 O 3 and TiO 2 in varying concentrations, and of a commercial, CaO-doped zirconia, ZR23. The variation of the Oxygen content was investigated as a function of the Oxygen Activity at 1300 °C and in the case of the commercial material also at 1200 and 1350 °C. The Oxygen Activity range considered was approximately between about 10 −12 and 0.5. In all cases, it was observed that the Oxygen content increased with increasing Oxygen Activity, in (Ca 0.1 Zr 0.9 )O 1.9 more than in pure zirconia, in TiO 2 - or Fe 2 O 3 -doped (Ca 0.1 Zr 0.9 )O 1.9 more than in pure (Ca 0.1 Zr 0.9 )O 1.9 and in the commercial, CaO-doped zirconia similar as in (Ca 0.1 Zr 0.9 )O 1.9 with concentrations of TiO 2 and Fe 2 O 3 very similar to those present in the commercial material. The results obtained indicate that the largest contribution to the variation of the Oxygen content in the commercial, CaO-doped zirconia investigated is from the oxidation of Fe 2+ to Fe 3+ ions. A smaller, but significant contribution to the variation of the Oxygen content is related to valence state changes of titanium ions.
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Deviation from stoichiometry and point defects in (ZnxMn1−xFe2)1−δ/3O4
Solid State Ionics, 2003Co-Authors: Jörg Töpfer, Rüdiger DieckmannAbstract:Abstract The deviation from stoichiometry, δ, in (ZnxMn1−xFe2)1−δ/3O4 was measured thermogravimetrically at 1200 °C as a function of Oxygen Activity, aO2, and cationic composition, x. Point defect thermodynamic modeling of the Oxygen Activity dependence of δ suggests that cation vacancies are the majority ionic defects at high Oxygen Activity and cation interstitials dominate at lower activities.
Janusz Nowotny - One of the best experts on this subject based on the ideXlab platform.
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Chromium segregation in Cr-doped TiO 2 (rutile): impact of Oxygen Activity
Ionics, 2019Co-Authors: Kazi A. Rahman, N. Sharma, Armand J. Atanacio, Eric D. Wachsman, Michelle C. Moffitt, Janusz NowotnyAbstract:This work considers the effect of chromium surface segregation for polycrystalline Cr-doped TiO2 on surface vs. bulk defect disorder. It is shown that annealing of Cr-doped TiO2 (0.04 at% Cr) in the gas phase of variable Oxygen Activity at 1273 K results in a gradual transition in the valence of chromium at the surface from predominantly Cr3+ species in reduced conditions, p(O2) = 10−12 Pa, to comparable concentrations of both Cr3+ and Cr6+ species in oxidising conditions, p(O2) = 105 Pa. The reported data is considered in terms of defect equilibria leading to the formation of positively and negatively charged chromium in both the cation sub-lattice and interstitial sites. The derived theoretical models represent the effect of Oxygen Activity on the surface charge and the resulting electric field leading to migration mechanism of charged chromium species.
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Towards sustainable energy. Photocatalysis of Cr-doped TiO 2 : 3. Effect of Oxygen Activity
Ionics, 2018Co-Authors: Kazi A. Rahman, Armand J. Atanacio, Mihail Ionescu, Janusz NowotnyAbstract:The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO2-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including electronic structure, photocatalytic Activity, intrinsic defect disorder, electrochemical coupling and surface vs. bulk properties. The present work reports the effect of Oxygen Activity in the oxide lattice on photocatalytic Activity of pure and Cr-doped TiO2 (0.04 at% Cr). Processing of specimens included annealing at 1273 K in the gas phase of controlled Oxygen Activity in the range 10−12 Pa < p(O2) < 105 Pa. We show that the increase of Oxygen Activity results initially in a decrease of photocatalytic Activity, minimum around the n-p transition point, and then increase assuming maximum at p(O2) = 105 Pa. The obtained results are considered in terms of a theoretical model that explains the effect of defect disorder on the reActivity of TiO2 with water. The minimum of the photocatalytic Activity corresponds to the n-p transition point. The maximum of performance at high p(O2) is determined by increased concentration of titanium vacancies forming surface active sites.
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Photocatalytic properties of TiO2: Effect of niobium and Oxygen Activity on partial water oxidation
Applied Catalysis B-environmental, 2016Co-Authors: Wenxian Li, Armand J. Atanacio, Janusz NowotnyAbstract:Abstract This work reports the effect of niobium (0–1 at%) on photocatalytic Activity and the band gap of TiO2 with controlled Oxygen Activity in the range 10−12 Pa
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Effect of Oxygen Activity on the n–p Transition for Pure and Cr-Doped TiO2
Journal of Physical Chemistry C, 2016Co-Authors: Janusz Nowotny, Eric D. Wachsman, Wojciech Macyk, Kazi A. RahmanAbstract:Titanium dioxide, TiO2, is commonly considered as an n-type semiconductor. The present work shows that TiO2 exhibits both n- and p-type semiconducting properties. It is shown that p-type properties may be imposed by either increasing of Oxygen Activity or incorporation of acceptor-type ions, such as chromium. This work reports the n–p transition for both pure and Cr-doped TiO2 (0.05 at % Cr) single crystals at elevated temperatures (1023–1323 K) in the gas phase of controlled Oxygen Activity. The n–p transition was determined by the measurements of thermoelectric power as a function of Oxygen Activity in the range 10 to 105 Pa. It is shown that the n–p transition line for Cr-doped TiO2 exhibits a sharp change at 1173 K that is related to the conversion in chromium oxidation state from trivalent below 1173 K to six-valent above. This effect, which is reflective of a dual mechanism of chromium incorporation into the rutile structure of TiO2, can be used for imposition of desired semiconducting properties th...
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Effect of Oxygen Activity on semiconducting properties of TiO2 (rutile)
Ionics, 2014Co-Authors: Janusz Nowotny, Wenxian LiAbstract:Intensive research aims to reduce the band gap of TiO2-based semiconductors by the incorporation of a range of cations and anions in order to enhance their performance in solar energy conversion. In this work, we show that the band gap, as well as the related semiconducting properties, may be modified by impositions of variable Oxygen Activity in the oxide lattice instead of doping with extrinsic ions. It is shown that the band gap may be decreased when TiO2 is annealed in strongly reducing conditions [p(O2) ∼10−12 Pa]. The reported data indicate that Oxygen Activity in the oxide lattice, including TiO2, is the key quantity which must be taken into account in considering compatibility and reproducibility of a range of properties, such as band gap and charge transport.
M. Backhaus-ricoult - One of the best experts on this subject based on the ideXlab platform.
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Modelling of the Gibbs adsorption at transition-metal–oxide interfaces: effect of the Oxygen chemical potential on interfacial bonding, interfacial energy and equilibrium precipitate shape
Philosophical Magazine, 2001Co-Authors: M. Backhaus-ricoultAbstract:Abstract Non-reactive ternary metal–oxide interfaces are thermodynamically stable over extended ranges of Oxygen activities and temperatures. At each condition within this range, the interface adopts a different equilibrium structure and chemistry. A continuum model of the Gibbs adsorption–desorption at transition-metal–oxide interfaces is developed, which predicts interfacial chemistry and modifications in the specific free interfacial energy as a function of Oxygen Activity. Three Oxygen Activity domains can be distinguished according to this model: the upper part of the metal–oxide coexistence range characterized by an enrichment in interfacial Oxygen, established by adsorption of Oxygen at structural vacancies in the case of polar interfaces or by desorption of the less noble metal in the case of mixed interfaces; an intermediate-Oxygen-Activity range with the interface remaining free of adsorption; a lower-Oxygen-Activity range, where the interface is enriched in less noble metal by adsorption of exc...
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Wetting anisotropy and Oxygen Activity dependency for oxides by liquid transition metals studied through shape changes of liquid Cu inclusions within MgO
Acta Materialia, 2001Co-Authors: M. Backhaus-ricoultAbstract:Abstract The equilibrium shape of liquid metal inclusions in oxides is used to study anisotropy and Oxygen partial pressure dependency of the metal–oxide specific interfacial free energy. Liquid copper inclusions are formed within single crystalline magnesia by internal reduction of mixed oxide (Mg,Cu)O. Their equilibrium shape is studied by TEM for various Oxygen chemical potential. Precipitates always adopt cubo-octahedral morphology, but relative facet size strongly depends on the Oxygen chemical potential. The shape evolution with Oxygen chemical potential reveals important and anisotropic Gibbs' adsorption of excess Oxygen to the interfaces at high Oxygen activities. Gibbs' adsorption to crystallographically different MgO–liquid Cu interfaces is discussed in terms of a point defect model recently developed by the author (Backhaus-Ricoult, M., Phil. Mag., 2001, in press), where Oxygen Activity-dependent formation of interfacial charge transfer clusters between liquid metal and excess Oxygen or magnesium is considered, while the relevant site and charge conservation laws in the two-phase system are respected and matter exchange with the surrounding gas atmosphere is allowed. Present experimental results are interpreted in terms of this interfacial defect model and compared with interpretation by other wetting models from the literature.
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A model of Oxygen-Activity-dependent adsorption (desorption) to metal–oxide interfaces
Acta Materialia, 2000Co-Authors: M. Backhaus-ricoultAbstract:Abstract Non-reactive metal–oxide interfaces are thermodynamically stable over an extended range of Oxygen Activity and temperature. It is supposed that, at each point of this range, the interface adopts a (slightly) different equilibrium structure and chemistry due to variations in the local composition of the interface by Gibbs' adsorption. The quality and quantity of adsorbed species at the interface, chemical bonding across the metal–oxide interface and related properties, such as adhesion for example, should then vary with the external parameters, temperature and chemical potential of the components. A physico-chemical continuum model is developed that applies point defect chemistry to transition metal–oxide interfaces. It predicts specific interfacial energies through Gibbs' adsorption isotherms as a function of component chemical potentials. Experimental results obtained for MgO–Cu interfaces are reviewed and compared with the model's results.
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Optimisation of Adhesion at Transition Metal-Oxide Interfaces by Processing at Well-Chosen Oxygen Activity
MRS Proceedings, 1999Co-Authors: M. Backhaus-ricoultAbstract:Transition metal-oxide interfaces suffer within their thermodynamic stability range Gibbs' adsorption and show important changes in chemical composition with Oxygen Activity. As a consequence, specific free interfacial energy and adhesion energy also vary with Oxygen Activity. Adhesion at a given non-reactive transition metal-oxide interface can then be optimised by establishing the proper Oxygen Activity during processing or by a post-treatment at the interface. In the present work, the approach of Gibbs' adsorption is extended to crystalline, anisotropic (special) transition metal-oxide interfaces. It is demonstrated that interfacial energy varies with Oxygen Activity. The variation in energy is studied for different adsorption energies, temperatures and interfacial planes.
Ying Shirley Meng - One of the best experts on this subject based on the ideXlab platform.
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Exploring Oxygen Activity in the High Energy P2-Type Na0.78Ni0.23Mn0.69O2 Cathode Material for Na-Ion Batteries
Journal of the American Chemical Society, 2017Co-Authors: Chuze Ma, Judith Alvarado, Raphaële J. Clément, Moses Kodur, Wei Tong, Clare P. Grey, Jing Xu, Ying Shirley MengAbstract:Large-scale electric energy storage is fundamental to the use of renewable energy. Recently, research and development efforts on room-temperature sodium-ion batteries (NIBs) have been revitalized, as NIBs are considered promising, low-cost alternatives to the current Li-ion battery technology for large-scale applications. Herein, we introduce a novel layered oxide cathode material, Na0.78Ni0.23Mn0.69O2. This new compound provides a high reversible capacity of 138 mAh g–1 and an average potential of 3.25 V vs Na+/Na with a single smooth voltage profile. Its remarkable rate and cycling performances are attributed to the elimination of the P2–O2 phase transition upon cycling to 4.5 V. The first charge process yields an abnormally excess capacity, which has yet to be observed in other P2 layered oxides. Metal K-edge XANES results show that the major charge compensation at the metal site during Na-ion deintercalation is achieved via the oxidation of nickel (Ni2+) ions, whereas, to a large extent, manganese (Mn...
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gas solid interfacial modification of Oxygen Activity in layered oxide cathodes for lithium ion batteries
Nature Communications, 2016Co-Authors: Minghao Zhang, Lijun Wu, Jun Wang, Danna Qian, Sunny Hy, Yan Chen, Ke An, Ying Shirley MengAbstract:Lattice Oxygen can play an intriguing role in electrochemical processes, not only maintaining structural stability, but also influencing electron and ion transport properties in high-capacity oxide cathode materials for Li-ion batteries. Here, we report the design of a gas-solid interface reaction to achieve delicate control of Oxygen Activity through uniformly creating Oxygen vacancies without affecting structural integrity of Li-rich layered oxides. Theoretical calculations and experimental characterizations demonstrate that Oxygen vacancies provide a favourable ionic diffusion environment in the bulk and significantly suppress gas release from the surface. The target material is achievable in delivering a discharge capacity as high as 301 mAh g(-1) with initial Coulombic efficiency of 93.2%. After 100 cycles, a reversible capacity of 300 mAh g(-1) still remains without any obvious decay in voltage. This study sheds light on the comprehensive design and control of Oxygen Activity in transition-metal-oxide systems for next-generation Li-ion batteries.
Jacques Fouletier - One of the best experts on this subject based on the ideXlab platform.
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METAL/OXIDE INTERFACES: CHEMISTRY, WETTING, ADHESION, AND Oxygen Activity
Ceramic Microstructures, 1998Co-Authors: Dominique Chatain, V. Ghetta, Jacques FouletierAbstract:This paper is a review of recent work on the effect of Oxygen on liquid metal/oxide interfaces and metal surfaces. The system considered consists of a two-phase ternary at fixed temperature. The metallic phase and the oxide are in equilibrium when the Oxygen Activity ranges between a lower limit, where the oxide is reduced, and an upper limit, where the metal is oxidized. In between, small concentrations of Oxygen are in solution in the metallic phase, and a large amount of Oxygen may segregate at the surface and at the interface of the metallic phase, thus inducing drastic changes in the wetting and the adhesion in metal/oxide systems.
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New Experimental Setup for Wettability Characterization under Monitored Oxygen Activity: II, Wettability of Sapphire by Silver‐Oxygen Melts
Journal of the American Ceramic Society, 1994Co-Authors: Dominique Chatain, Françoise Chabert, V. Ghetta, Jacques FouletierAbstract:Wetting of silver on sapphire and surface tension of silver are both determined as a function of Oxygen Activity in a range corresponding to Oxygen content in silver varying from 10 −12 to 10 −2 . The contact angle is shown to decrease at high Oxygen Activity when simultaneous decreases in surface tension of silver and interfacial energy between silver and alumina occur. This double Oxygen adsorption is thermodynamically analyzed and some relevant parameters inducing Oxygen segregation at the silver interfaces are proposed
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New Experimental Setup for Wettability Characterization under Monitored Oxygen Activity: I, Role of Oxidation State and Defect Concentration on Oxide Wettability by Gold
Journal of the American Ceramic Society, 1993Co-Authors: Dominique Chatain, Françoise Chabert, V. Ghetta, Jacques FouletierAbstract:A sessile drop setup has been combined with an Oxygen sensor, an electrochemical pump, and a renewable Oxygen getter. The Oxygen Activity of a metal–oxide sample can be monitored in the 10−9–1 range, in a flowing gas, that enables the control of the defect concentration in the oxide and of the Oxygen content in the metal. The stoichiometry of the substrate can also be modified by electrochemical reduction, allowing the Oxygen Activity of the sample to be monitored down to values as low as 10−20. The dependence on Oxygen Activity of the wetting and the thermodynamic adhesion of gold on sapphire, titanium oxides, and yttria-stabilized zirconia is presented to illustrate the versatility of the equipment.
