Identification of elastic, dielectric, and piezoelectric constants in piezoceramic disks

Three-dimensional modeling of piezoelectric devices requires a precise knowledge of piezoelectric material parameters. The commonly used piezoelectric materials belong to the 6mm symmetry class, which have ten independent constants. In this work, a methodology to obtain precise material constants over a wide frequency band through finite element analysis of a piezoceramic disk is presented. Given an experimental electrical impedance curve and a first estimate for the piezoelectric material properties, the objective is to find the material properties that minimize the difference between the electrical impedance calculated by the finite element method and that obtained experimentally by an electrical impedance analyzer. The methodology consists of four basic steps: experimental measurement, identification of vibration modes and their sensitivity to material constants, a preliminary identification algorithm, and final refinement of the material constants using an optimization algorithm. The application of the methodology is exemplified using a hard lead zirconate titanate piezoceramic. The same methodology is applied to a soft piezoceramic. The errors in the identification of each parameter are statistically estimated in both cases, and are less than 0.6% for elastic constants, and less than 6.3% for dielectric and piezoelectric constants.     

Size of web domains and interlinking behavior of higher education institutions in Europe

The aim of this paper is to empirically test whether interlinking patterns between higher education institutions (HEIs) conform to a document model, where links are motivated by webpage content, or a social relationship model, where they are markers of underlying social relationships between HEIs. To this aim, we analyzed a sample of approximately 400 European HEIs, using the number of pages on their web domains and the total number of links sent and received; in addition we test whether these two characteristics are associated with organizational size, reputation, and the volume of teaching and research activities. Our main findings are as follows: first, the number of webpages of HEI websites is strongly associated with their size, and to a lesser extent, with the volume of their educational activities, research orientation, and reputation; differences between European countries are rather limited, supporting the insight that the academic Web has reached a mature stage. Second, the distribution of connectivity (as measured by the total degree of HEI’s) follows a lognormal distribution typical of social networks between organizations, while counts of weblinks can be predicted with good precision from organizational characteristics. HEIs with larger websites tend to send and receive more links, but the effect is rather limited and does not fundamentally modify the resulting network structure. We conclude that aggregated counts of weblinks between pairs of HEIs are not significantly affected by the web policies of HEIs and thus can be considered as reasonably robust measures. Furthermore, interlinking should be considered as proxies of social relationships between HEIs rather than as reputational measures of the content published on their websites.     

FC2Q: exploiting fuzzy control in server consolidation for cloud applications with SLA constraints

Modern cloud data centers rely on server consolidation (the allocation of several virtual machines on the same physical host) to minimize their costs. Choosing the right consolidation level (how many and which virtual machines are assigned to a physical server) is a challenging problem, because contemporary multitier cloud applications must meet service level agreements in face of highly dynamic, nonstationary, and bursty workloads. In this paper, we deal with the problem of achieving the best consolidation level that can be attained without violating application service level agreements. We tackle this problem by devising fuzzy controller for consolidation and QoS (FC2Q), a resource management framework exploiting feedback fuzzy logic control, that is able to dynamically adapt the physical CPU capacity allocated to the tiers of an application in order to precisely match the needs induced by the intensity of its current workload. We implement FC2Q on a real testbed and use this implementation to demonstrate its ability of meeting the aforementioned goals by means of a thorough experimental evaluation, carried out with real‐world cloud applications and workloads. Furthermore, we compare the performance achieved by FC2Q against those attained by existing state‐of‐the‐art alternative solutions, and we show that FC2Q works better than them in all the considered experimental scenarios. Copyright © 2014 John Wiley & Sons, Ltd.     

Efficiency loss prevention in monolithically integrated thin film solar cells by improved front contact

For thin film solar cells, there is a large gap between the record efficiencies and panel power output. It was found that for a “typical industrial” CIGS cell efficiency of 15.5%, the efficiency drops to 11.7% when it is operating under the circumstances of a monolithically integrated solar panel. Part of this gap is due to limited conductivity and transmittance of the front contact. By application of a metallic grid, the conductivity can be improved by over two order of magnitude at a transmittance loss of only a few percent as was shown experimentally. In addition, modeling was used to quantify the impact of such approach on the power output of monolithically integrated solar panels. This model includes optical and resistive losses, as well as related losses caused by the inhomogeneity of the operating voltage over the surface. Both power output and the different types of losses are mapped out for various cell configurations. Optimization of transparent conductive oxide resistance, cell length, finger width, and finger spacing of grids was performed and led to an efficiency improvement from 11.7% to 13.8% when the front contact is upgraded with a metallic grid consisting of 20 µm wide parallel fingers positioned perpendicular to the interconnect. Further optimization for a wide variety of cell and grid configurations show that for a technically more feasible size of 100 µm wide fingers, the calculated efficiency is still 13.5%. Finally, the power output is mapped out for a large number of configurations as to create an overview and insight in the interdependencies of cell configuration and finger dimensions. Copyright © 2014 John Wiley & Sons, Ltd.     

Multimode equivalent network representation for junctions between coaxial and circular waveguides

In this article we present two contributions to the study of coaxial waveguide structures. First, we present an efficient and accurate technique to evaluate the modes in the coaxial waveguides; second, we analyze junctions involving coaxial and circular waveguides using the multimode equivalent network representation. Numerical and experimental results are presented, showing good agreement. © 1997 John Wiley & Sons, Inc. Int J Microwave Millimeter-Wave CAE 7: 180–194, 1997.     

A strategy for the efficient full-wave description of complex waveguide networks

The mode-matching analysis of rectangular waveguide networks requires a complex segmentation of the structure into simple elements. The way these elements are connected has a strong influence on the computing performance of the code. Moreover, for complex structures the size of the coefficient matrix of the main system can easily reach unaffordable dimensions, so that the use of an efficient solving strategy becomes indispensable. A new approach to the optimum automatic choice of the interconnections is presented, leading to a highly stable and outperforming method of analysis. A Butler 4 × 4 matrix, with six branch guide couplers and four phase shifters, containing 336 uniform waveguide sections and bifurcations, has been studied, and the presented strategy has allowed a performance increase up to two orders of magnitude. © 1996 John Wiley & Sons, Inc.     

Deep Resequencing of 9 Candidate Genes Identifies a Role for ARAP1 and IGF2BP2 in Modulating Insulin Secretion Adjusted for Insulin Resistance in Obese Southern Europeans

Type 2 diabetes is characterized by impairment in insulin secretion, with an established genetic contribution. We aimed to evaluate common and low-frequency (1–5%) variants in nine genes strongly associated with insulin secretion by targeted sequencing in subjects selected from the extremes of insulin release measured by the disposition index. Collapsing data by gene and/or function, the association between disposition index and nonsense variants were significant, also after adjustment for confounding factors (OR = 0.25, 95% CI = 0.11–0.59, p = 0.001). Evaluating variants individually, three novel variants in ARAP1, IGF2BP2 and GCK, out of eight reaching significance singularly, remained associated after adjustment. Constructing a genetic risk model combining the effects of the three variants, only carriers of the ARAP1 and IGF2BP2 variants were significantly associated with a reduced probability to be in the lower, worst, extreme of insulin secretion (OR = 0.223, 95% CI = 0.105–0.473, p < 0.001). Observing a high number of normal glucose tolerance between carriers, a regression posthoc analysis was performed. Carriers of genetic risk model variants had higher probability to be normoglycemic, also after adjustment (OR = 2.411, 95% CI = 1.136–5.116, p = 0.022). Thus, in our southern European cohort, nonsense variants in all nine candidate genes showed association with better insulin secretion adjusted for insulin resistance, and we established the role of ARAP1 and IGF2BP2 in modulating insulin secretion.     

Plastification Behaviour of Different Native Starches

Based on physical and thermal conservation laws a quantitative variable is introduced, which allows evaluation of the thermomechanical starch plastification much more independent of the transformation procedure. By that method, a comparison of plastification behaviour during extrusion of native potato, maize and wheat starch without any additives is realized for the first time. The complete energetical balance of the extrusion process supplemented by microscopical investigations leads to an essential better understanding of thermomechanical proceedings during extrusion of starch or compounding of other polymers. After optimization of extruder and screw configuration the resulting data for the energy needed for the plastification of starch, i.e. for the transformation of native starch granules into a thermoplastic melt by extrusion, have been measured under constant conditions. On these adequat extrusion conditions the native, laminated starch granules of all three used starches have been destroyed completely during its plastification by extrusion. The specific energy input needed for the plastification of potato starch was about 650 kJ/kg, for maize 380 and for wheat starch 435 kJ/kg. In comparison of these results the energy input for melting of polyethylene high density was about 585 kJ/kg under same conditions.