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.     

Thickness Dependence of Piezoelectric Properties for PZT Thin Films with Regard to MEMS Applications

In this work we report on the investigation of the piezoelectric and ferroelectric properties of Pb(Zr,Ti)O₃ (PZT) thin films with a composition of 45% zirconium and 55% titanium prepared by chemical solution deposition (CSD) regarding the major properties required in MEMS applications. Therefore we have measured the polarization and the piezoelectric coefficient d ₃₃ and strain using a high-resolution double beam laser interferometer. We found that d ₃₃ alike the strain is nearly independent of the sample thickness. To estimate the lifetime of MEMS we stressed the samples by applying switching and non-switching voltage pulses. In the non-switching case, no change of the material properties was observed whereas the use of switching voltage pulses resulted in a strong decrease of the piezoelectric properties and the polarization. Here we found a linear behavior between the material properties in the fatigued state and the sample thickness.     

The Reactivity of Difluorocarbene with Hydroxylamines: Synthesis of Carbamoyl Fluorides

Carbamoyl fluorides are formed in reactions of hydroxylamines with difluorocarbene generated from sodium bromodifluoroacetate as readily available and non‐toxic carbene precursor. The process shows a high functional group tolerance, and the reaction path has been rationalized by computational calculations.

     

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.     

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.     

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.     

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.     

Process Deviations and Spot Defects: Two Aspects of Test and Test Development for Mixed-Signal Circuits

By their nature, mixed-signal circuits have to be tested for both structural integrity and parametric performance. For the example of data converters we review test pattern selection strategies geared towards structural and performance testing. We introduce a novel test pattern selection strategy that merges both objectives, and by that we achieve a significant reduction in the size of the set of test patterns applied on the production line.