Thermal stability of the cubic phase in Ba0.5Sr0.5Co0.8Fe0.2O3 - δ (BSCF)1

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 - δ (BSCF) is a material with excellent oxygen ionic and electronic transport properties reported by many research groups. In its cubic phase, this mixed ionic-electronic conducting (MIEC) perovskite is a promising candidate for oxygen permeation membranes. For this application, its long-term stability under operating conditions (especially temperature and oxygen partial pressure) is of crucial importance.The present work is focused on the thermal stability of the BSCF cubic phase in the targeted temperature range for applications (700…900 °C) in light of previous studies in literature reporting a reversible transition to a hexagonal phase somewhere below 900 °C.To this end, single phase cubic BSCF powders were annealed at different temperatures over varying periods of time. Phase composition was subsequently analysed by X-ray diffractometry (XRD) in order to determine both the temperature limit and the time-scale for the formation of the hexagonal phase. Additionally, the long-term behaviour of the electrical conductivity was examined on bulk samples at 700 °C, 800 °C and 900 °C over several hundreds of hours, showing a prolonged decrease at 800 °C. The decrease in electrical conductivity at this temperature was also examined on bulk samples with different grain sizes, showing a more pronounced decrease the smaller the average grain size.Coexistence of both phases (cubic and hexagonal) could also be shown for 700 °C, however with a different phase equilibrium than at 800 °C.     

Model reduction of porous-media problems using proper orthogonal decomposition

In the context of finite-element simulations of porous media, computing time and numerical effort is an important issue because the number of degrees of freedom of such coupled problems can become very large. Following this, model reduction plays an important role. A broad variety of materials exhibit a porous microstructure. In order to evaluate the overall response of these materials, a macroscopic continuum-mechanical modelling approach is used. Therefore, the complex inner structure of porous media is regarded in a multi-phasic and multi-component manner by means of the well-founded Theory of Porous Media (TPM). The mechanical behaviour of porous media is solved using the Finite-Element Method (FEM). The basic idea of model reduction is to transform a high dimensional system, in terms of the system's degrees of freedom, to a low dimensional subspace to minimise the computational effort while maintaining the accuracy of the solution. The method of proper orthogonal decomposition (POD) can be seen as a method to approximate a given data set with a low dimensional subspace. Furthermore, the POD method is independent of the type of the model and can be used for nonlinear systems as well as for systems of second order. In several applications, such as consolidation problems of partially saturated soils, commonly occurring motion sequences can be found, which can be used as typical “snapshots” of the system. Therefore, the application of the POD method to the simulation of porous media is discussed in the present contribution. Investigated computations of a biphasic standard problem show that the POD method reduces the numerical effort to solve the linearised system of equations in each iteration step. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Eigenvalues of a statistical mechanics matrix

We consider the problem of finding the spectrum of an n × n matrix which arises in the study of a certain model of long-range interactions in a one-dimensional statistical mechanics system. Our analysis exhibits a curious resemblance of the suitably normalized distribution of eigenvalues to the Marčenko–Pastur law in the limit n → ∞.     

Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report

Gas sensors based on semiconducting materials have become of great interest to both sensor users and researchers. In this context, a huge number of publications have appeared in the literature which deal with metal oxide gas sensors, in general, and with the prototype material SnO₂, in particular. The amount of data published grows continuously and has led to a situation in which even experts in this field tend to lose an overview. The present review describes the reasons for this complexity and outlines unifying concepts to understand the huge amount of published, mostly empirical data. This leads to a comprehension of gas-sensing phenomena in both the application and research domains. Received: 7 December 1998 / Revised: 6 May 1999 / Accepted: 19 May 1999     

Exclusively Heteronuclear NMR Experiments to Obtain Structural and Dynamic Information on Proteins

Provided that ¹³C‐detected NMR experiments are either preferable or complementary to ¹H detection, we report here tools to determine C^α? C′, C′? N, and C^α? H^α residual dipolar couplings on the basis of the CON experiment. The coupling constants determined on ubiquitin are consistent with the subset measured with the ¹H‐detected HNCO sequences. Since the utilization of residual dipolar couplings may depend on the mobility of the involved nuclei, we also provide tools to measure longitudinal and transverse relaxation rates of N and C′. This new set of experiments is a further development of a whole strategy based on ¹³C direct‐detection NMR spectroscopy for the study of biological macromolecules.     

Elementare Eigenschaften lokaler–Polynom–Schätzer: ein Streifzug durch die lineare Algebra

Zusammenfassung. Der lokale–Polynom–Sch?tzer besteht darin, an eine Sequenz von aufeinanderfolgenden Datenpunkten lokal ein Polynom anzupassen und damit zuf?llige Schwankungen in den Daten wegzugl?tten. Dieser Ansatz gewinnt in der nichtparametrischen Kurvensch?tzung st?ndig an Bedeutung. Es wird gezeigt, wie die Herleitung einiger elementarer Eigenschaften dieses Handwerkszeuges des Datenanalytikers einen Streifzug durch die lineare Algebra erforderlich macht. Eingegangen am 27.03.1998 / Angenommen am 15.06.1998