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991.
We observe that the reverse current under illumination in solar cells containing C60 and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current–voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C60 layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C60 absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power‐generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
992.
Median nerve stimulation is a commonly used technique in the clinical setting to determine areas of neuronal function in the brain. Neuronal activity of repeated median nerve stimulation is well studied. The cerebral hemodynamic response of the stimulation, on the other hand, is not very clear. In this study, we investigate how cerebral hemodynamics behave over time using the same repeated median nerve stimulation. Ten subjects received constant repeated electrical stimulation to the right median nerve. Each subject had functional magnetic resonance imaging scans while receiving said stimulations for seven runs. Our results show that the blood oxygen level-dependent (BOLD) signal significantly decreases across each run. Significant BOLD signal decreases can also be seen within runs. These results are consistent with studies that have studied the hemodynamic habituation effect with other forms of stimulation. However, the results do not completely agree with the findings of studies where evoked potentials were examined. Thus, further inquiry of how evoked potentials and cerebral hemodynamics are coupled when using constant stimulations is needed. 相似文献
993.
Tamerlan A. Saidov Rosa M. Espinosa-Marzal Leo Pel George W. Scherer 《Journal of Crystal Growth》2012,338(1):166-169
Porous building materials are very susceptible to the aggressive action of salts, sodium sulfate being one of the most damaging. Laboratory and field experiments show that the intensity of damage depends on the structure of the porous material, as well as the type and amount of salt. In this paper we investigate the nucleation process of sodium sulfate on two kinds of substrates: calcite and quartz particles, the main components of calcitic and quartzitic stones, respectively, to assess the influence of the substrate chemistry on the crystalline phase being formed and on the nucleation process. Nuclear Magnetic Resonance (NMR) is used to investigate the cooling-induced crystallization of sodium sulfate in mixtures of solution with different amounts of substrate particles. Temperature and concentration are measured by NMR and used to calculate the crystallization rate. The phase that precipitates is the metastable heptahydrate. Our experiments show that nucleation is energetically more favorable on a calcitic substrate; that is, the required supersaturation is lower than on the quartzitic substrate. Under isothermal conditions, the precipitation rate is faster in the presence of calcite than quartz. 相似文献
994.
Leo Liberti 《Mathematical Programming》2012,131(1-2):273-304
If a mathematical program has many symmetric optima, solving it via Branch-and-Bound techniques often yields search trees of disproportionate sizes; thus, finding and exploiting symmetries is an important task. We propose a method for automatically finding the formulation group of any given Mixed-Integer Nonlinear Program, and for reformulating the problem by means of static symmetry breaking constraints. The reformulated problem—which is likely to have fewer symmetric optima—can then be solved via standard Branch-and-Bound codes such as CPLEX (for linear programs) and Couenne (for nonlinear programs). Our computational results include formulation group tables for the MIPLib3, MIPLib2003, GlobalLib and MINLPLib instance libraries and solution tables for some instances in the aforementioned libraries. 相似文献
995.
In this paper conditions on the commutative ring R (with identity) and the commutative semigroup ring S (with identity) are found which characterize those semigroup rings R[S] which are reduced or have weak global dimension at most one. Likewise, those semigroup rings R[S] which are semihereditary are completely determined in terms of R and S. 相似文献
996.
Carlile Lavor Leo Liberti Nelson Maculan Antonio Mucherino 《Computational Optimization and Applications》2012,52(1):115-146
Given a simple weighted undirected graph G=(V,E,d) with d:E???+, the Molecular Distance Geometry Problem (MDGP) consists in finding an embedding x:V???3 such that ??x u ?x v ??=d uv for each {u,v}??E. We show that under a few assumptions usually satisfied in proteins, the MDGP can be formulated as a search in a discrete space. We call this MDGP subclass the Discretizable MDGP (DMDGP). We show that the DMDGP is NP-hard and we propose a solution algorithm called Branch-and-Prune (BP). The BP algorithm performs remarkably well in practice in terms of speed and solution accuracy, and can be easily modified to find all incongruent solutions to a given DMDGP instance. We show computational results on several artificial and real-life instances. 相似文献
997.
Carlile Lavor Jon Lee Audrey Lee-St. John Leo Liberti Antonio Mucherino Maxim Sviridenko 《Optimization Letters》2012,6(4):783-796
Given a weighted, undirected simple graph G = (V, E, d) (where \({d:E\to\mathbb{R}_+}\)), the distance geometry problem (DGP) is to determine an embedding \({x:V\to\mathbb{R}^K}\) such that \({\forall \{i,j\} \in E\;\|x_i-x_j\|=d_{ij}}\) . Although, in general, the DGP is solved using continuous methods, under certain conditions the search is reduced to a discrete set of points. We give one such condition as a particular order on V. We formalize the decision problem of determining whether such an order exists for a given graph and show that this problem is NP-complete in general and polynomial for fixed dimension K. We present results of computational experiments on a set of protein backbones whose natural atomic order does not satisfy the order requirements and compare our approach with some available continuous space searches. 相似文献
998.
M. Ferraris S. Ferraris M. Miola S. Perero C. Balagna E. Vernè G. Gautier Ch. Manfredotti A. Battiato E. Vittone G. Speranza I. Bogdanovic 《Journal of nanoparticle research》2012,14(12):1-19
Silver nanocluster/silica composite coatings were deposited on both soda-lime and silica glasses by radio frequency (RF) co-sputtering. The effect of thermal treatments on the microstructure in the range of 150?C450?°C were examined by UV?Cvisible spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Time of Flight-Elastic Recoil Detection Analysis. Sodium/silver ionic exchange was evidenced for coatings sputtered on soda-lime substrates after heating at 450?°C; presence of silver ions and/or silver nanoclusters, nanocluster size and their position inside the sputtered layers will be discussed for as-deposited and heated coatings on both substrates. The antibacterial activity of all coatings was determined against Staphylococcus aureus and Candida albicans by disk diffusion method and colonies forming units count; in agreement with microstructural results, the antibacterial activity present on all coatings was slightly reduced after heating at 450?°C. All coatings have been submitted to humidity plus UV ageing and sterilization by autoclave, gamma ray and ethylene oxide gas. Tape resistance (ASTM D3359-97) tests have been done on each coating before and after ageing and sterilizations, revealing a good adhesion on soda-lime substrates, except for those aged in humidity plus UV and sterilized by autoclave. Scratch tests and nanoindentation tests have been done on each coating, as-deposited and after heating at 450?°C. The coating hardness was improved by heating only when coatings were deposited on silica. The heating of coatings deposited on soda-lime substrates gave opposite effect on their hardness. 相似文献
999.
1000.
We study the problem of packing equal circles in a square from the mathematical programming point of view. We discuss different formulations, we analyze formulation symmetries, we propose some symmetry breaking constraints and show that not only do they tighten the convex relaxation bound, but they also ease the task of local NLP solution algorithms in finding feasible solutions. We solve the problem by means of a standard spatial Branch-and-Bound implementation, and show that our formulation improvements allow the algorithm to find very good solutions at the root node. 相似文献