Metal Organic Frameworks for Natural Gas Storage in Vehicles

With the increasing demand for alternative fuels the storage of natural gas (NG) in adsorbents like metal organic frameworks (MOFs) will become more important. In order to use MOFs as storage media in fuel delivery systems, the optimization of mass and energy transfer of the system is crucial. For rapid NG filling of a tank, molecules need to reach the adsorption sites within a reasonable time while the heat of adsorption should be dissipated to the environment. In this article, mass transfer in shaped bodies of MOFs was determined by permeability measurements and pulsed field gradient (PFG) NMR spectroscopy. The heat dissipation was also experimentally measured and both data sets were used to set up a theoretical density function theory model to predict the behavior of MOFs for NG storage.     

Design of LTCC with High Thermal Expansion

New applications of low-temperature co-fired ceramics (LTCC), such as pressure sensors or integrated functional layers, require materials that possess higher coefficients of thermal expansion (CTE). To fabricate LTCC with elevated CTE, two methods of material design are examined: firstly, glass ceramic composites (GCC), which consist of >50 vol% glass in the starting powder, and, secondly, glass-bonded ceramics (GBC), where glass is added as a sintering aid only. The CTE of GBC is mainly determined by the crystalline component. For GCC, the CTE can be well predicted, if CTE and elastic data of each phase in the microstructure are known. A nonlinear characteristic of the CTE versus phase composition was found with increasing E _crystals/ E _glass ratio and absolute CTE difference between the components. The glass composition and glass amount can be used to compensate the fixed properties of a crystalline material in a desired way. However, because the CTE and permittivity of a glass cannot be chosen independently, an optimum glass composition has to be found. For a given LTCC, it is possible to control the devitrification by shifting the glass composition. In this way, the resulting CTE values can be predicted more exactly and tailoring becomes possible. Different LTCC materials, based on the crystalline compounds Ba(La,Nd)₂Ti₄O₁₂, ZrO₂ (Y-TZP), SiO₂ (quartz), and specially developed glasses, possessing an elevated CTE of around 10 × 10⁻⁶ K⁻¹ while showing permittivity ɛ_r between 6 and 63, are introduced.     

Microsegregation: From Basic Concepts to Complexity in Liquid Crystal Self-Assembly

This review is focused on the basic concepts of microsegregation and a fundamental understanding of the formation of positionally ordered LC phases based on micro- and nanophases, interaction parameters and interfaces. Selected examples were chosen from the actual literature to illustrate the concepts. Microsegregation is the basis of classical LC phases and cybotaxis, and most importantly, it paves the way to a huge number of new LC phases. Beside the distinct modes of micellar packing motifs and liquid quasicrystals formed by self-assembly of dendritic molecules, attention is also focused on the specific effects of rigid anisometric units and polyphilicity. Honeycomb LC phases, vesicular LC phases and mesophases with 3D-lattices lead to enhanced complexity of LC self-assembly.     

Circuit and Decision Tree Complexity of Some Number Theoretic Problems

We extend the area of applications of the Abstract Harmonic Analysis to lower bounds on the circuit and decision tree complexity of Boolean functions related to some number theoretic problems. In particular, we prove that deciding if a given integer is square-free and testing co-primality of two integers by unbounded fan-in circuits of bounded depth requires superpolynomial size.     

Direct-Push-Technologien – Effiziente Untersuchungsmethoden für die Untergrunderkundung

About ten years ago, Direct Push technologies were introduced in Germany as a “new” tool for subsurface characterization. Ever since, they have been frequently used in various fields of site investigations. However, despite over ten years of experience, their application potential is often perceived with scepticism. Closer consideration of the range of applications of this method shows that the technology is indeed being increasingly used, even though it does not seem to be completely accepted in the “consulting world”. This is surprising as Direct Push is already used in Germany in its simplest form—i.e. by percussion coring—on a routine basis. Nonetheless, the full potential of Direct Push has not yet been exploited. This article aims to provide a comprehensive overview on the state of the art of this technology and its various application potentials and limitations.     

Ant Colony Optimization and the minimum spanning tree problem

Ant Colony Optimization (ACO) is a kind of metaheuristic that has become very popular for solving problems from combinatorial optimization. Solutions for a given problem are constructed by a random walk on a so-called construction graph. This random walk can be influenced by heuristic information about the problem. In contrast to many successful applications, the theoretical foundation of this kind of metaheuristic is rather weak. Theoretical investigations with respect to the runtime behavior of ACO algorithms have been started only recently for the optimization of pseudo-Boolean functions.We present the first comprehensive rigorous analysis of a simple ACO algorithm for a combinatorial optimization problem. In our investigations, we consider the minimum spanning tree (MST) problem and examine the effect of two construction graphs with respect to the runtime behavior. The choice of the construction graph in an ACO algorithm seems to be crucial for the success of such an algorithm. First, we take the input graph itself as the construction graph and analyze the use of a construction procedure that is similar to Broder’s algorithm for choosing a spanning tree uniformly at random. After that, a more incremental construction procedure is analyzed. It turns out that this procedure is superior to the Broder-based algorithm and produces additionally in a constant number of iterations an MST, if the influence of the heuristic information is large enough.     

On negative dispersion without absorption in bichromatically driven two-level systems

Abstract

The possibility of obtaining strong negative dispersion without absorption in the central resonance structure of the spectrum of a bichromatically driven two-level system is investigated. A nonlinear analysis is performed using the technique of continued fractions with respect to the dressedstate basis of a strong pumping laser. We discuss the transition between the two regimes of the monochromatic driving case with a weak probing laser and of nearly equally strong lasers in terms of the contributions from coherences and populations of these dressed states. In contrast with the strong dependence of the Rabi side bands on probe field intensity, we find a remarkable stability of the negative dispersion in the central resonance structure.     

Diurnal cortisol secretion and 2-year changes in older adults' physical symptoms: The moderating roles of negative affect and sleep.

Objective: This study examined whether the association between cortisol secretion and changes in physical health symptoms would depend on other factors in a person's life. The authors expected that physical health effects would emerge particularly when cortisol disturbances co-occur in the context of high levels of trait negative affect or poor sleep. Design: Physical symptoms, diurnal cortisol secretion, affective tendencies, and sleep efficiency were assessed in a 2-yr longitudinal study of 184 older adults. Main Outcome Measure: Two-year changes in physical symptoms. Results: High cortisol levels were associated with increases in physical symptoms, but only among participants who experienced high negative affect and poor sleep. Conclusion: Elevated levels of cortisol secretion contribute to older adults' physical symptoms if they co-occur in the context of other emotional and behavioral problems. By contrast, cortisol disturbances may not influence physical symptoms among people who are emotionally well or engage in efficient sleep behaviors. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

An impedance-based predictive control strategy for the state-of-health of PEM fuel cell stacks

This work presents a control strategy for PEM fuel cell systems based on simultaneous impedance measurements on single cells. This control strategy distinguishes between flooding and drying of the cells in a stack and helps to run the stack at an optimal operating point. In the presented experiments, it has been found that impedance measurements can detect flooding phenomena in single cells minutes before they can be seen in related polarisation curves. It is shown that impedance measurements at two specific frequencies, one high and one low frequency impedance, are sufficient to predict voltage drops caused by flooding and drying. In flooding mode, the imaginary part of the low frequency impedance changes while the high frequency impedance remains stable and vice versa in drying mode. This technique reduces measuring time compared to the measurement of whole impedance spectra, without losing important information for the control of the system.