A procedure to determine optimal partitions of weighted hypergraphs through a network-flow analogy

Many problems arising in computer science and applications are amenable to finding optimal partitions of weighted hypergraphs or, more simply, of ordinary weighted graphs. As recently noted by some authors, in many cases the construction of optimal or near-optimal partitions is made easier if a family of subsets of nodes of the given hypergraph, calledLS sets, is previously determined. Intiutively, andLS set is a subset of nodes of the hypergraph, which are more strongly connected to each-other than to the nodes in the complementary subset. This paper presents a polynomialbounded procedure to determine all theLS sets in a given weighted hypergraph. The proposed procedure, that is considerably faster than those previously known, is based upon a network-flow analogy and replaces the given hypergraph with a «flow equivalent» tree, into which a collection of subsets of nodes, which includes allLS sets, is easily determined.     

A system approach to information system: Design for a city planning agency

Information system design forms the most vital element in designing an organization. An information system for a city planning agency is a combination of people, computer hardware and software, a dynamic data base and a set of generalized interactive algorithms.City planning agencies require complete and current information in order to provide input into municipal government decision process. This paper presents a model for an information system based on the operational functions of various city departments. The emphasis of this paper is on system engineering considerations and there is no attempt to provide a detailed programming solution. Basic considerations include: operational requirements, planning requirements, political environment and level of computer expertise. The basic components of the model are an on-line interactive system with a data-base shared by the participating departments.     

Intercomparison of unfolding codes for benchmark gamma spectrometry

The unfolding codes FERDOR and RADAK were selected to compare gamma photon spectra from experimental measurements. The input data were obtained from penetration measurements of 6 MeV gamma photons from a 0.86 m diameter uniform isotropic disc source through 10 cm of steel and 5 cm and 10 cm of lead. Both codes produced similar spectra but RADAK allows adjustment to the response matrix elements within real constraints and has a realistic approach to error analysis. RADAK is recommended as a standard unfolding code to provide experimental benchmark data.     

Composite short-side-chain PFSA electrolyte membranes containing selectively modified halloysite nanotubes (HNTs)

Aquivion membrane displays improved properties as compared to Nafion membrane, partly due to shorter side chains. However, some improvements are still necessary for proton exchange membrane fuel cell to operate at low relative humidity. To overcome this drawback, the addition of clay nanoparticle into the Aquivion matrix can be considered. In this study, different composite membranes have been prepared mixing short-side-chain PFSA (perfluorosulfonic acid) Aquivion and selectively modified halloysite nanotubes for PEMFC low relative humidity operation. Halloysites were grafted with fluorinated groups, sulfonated groups, or perfluoro-sulfonated groups on inner or outer surface of the tubes. The obtained composite membranes showed improved properties, especially higher water uptake associated with reduced swelling and better mechanical strength compared to pristine Aquivion membrane and commercially available Nafion HP used as reference. The best performance in this study was obtained with Aquivion loaded with 5 wt% of pretreated perfluoro-sulfonated halloysite. The composite membrane, referred to as Aq/pHNT-SF5, displayed the largest water uptake and proton conductivity among the panel of membranes tested. The chemical stability was not affected by the presence of halloysite in the Aquivion matrix.

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Synthesis of esters: Development of the rate expression for the Dowex 50 Wx8-400 catalyzed esterification of propionic acid with 1-propanol

The kinetics of the esterification reaction of propionic acid with 1-propanol over the ion-exchange resin Dowex 50Wx8-400 has been studied in this investigation. Kinetic experiments were conducted using a 1 L Lab-Max system at a stirrer speed of 900 rpm over a temperature range of 303.15 -333.15 K. The catalyst loading was varied from 10 to 60 g dry cat/L and acid to alcohol molar ratios of 1:1, 1:2, 1:4, 2:1 and 4:1 were employed. The equilibrium constants for this reaction were determined in separate experiments at 303.15, 313.15 and 323.15 K. The values were equal to 33.18, 30.62 and 28.37, respectively, with a standard enthalpy change of reaction of 6.4 kJ/mol. These values show the reaction to be mildly exothermic. It was found that both external and internal diffusion limitations did not affect the overall reaction rate. The conversion of propionic acid increased with increasing temperature and catalyst loading and decreased with increasing initial mole fraction of acid. The increase in chain length of acid or alcohol or branching had a retarding effect on the conversion. Several kinetic models were tested to correlate the kinetic data, the pseudo-homogeneous (P-H) model, the Eley-Rideal (E-R) model, the Langmuir-Hinshelwood (L-H) model, the modified Eley-Rideal (M-E-R) model and the modified Langmuir-Hinshelwood (M-L-H) model. In all models, the activity coefficients were estimated using UNIFAC to account for the non-ideal thermodynamic behavior of reactants and products. A correction factor for the resin affinity for water (α) was used in both M-E-R and M-L-H models. The above models predicted the kinetic behavior of the studied system with an overall error ranging from 1.65% to 13.32%. Water was found to be more strongly adsorbed than other species present in the system. The M-E-R model between adsorbed 1-propanol and non-adsorbed propionic acid which assumes surface reaction as the rate controlling step, with α equal to 2, was found to be the best model with the least overall error (1.65%). The activation energy for the esterification was estimated to be 67.3 kJ/mol by this model.     

The Dynamics of Spreading of Oils on Hydrophilic and Hydrophobic Surfaces Revisited Using Computational Fluid Dynamics

The dynamics of drop spreading on horizontal smooth surfaces of different wettabilities is revisited using computational fluid dynamics （CFD）. For this purpose, a recently developed CFD model, based on the volume of fluid technique （VOF）, with piecewise linear interface calculations method （PLIC） for interface reconstruction, is generalized and applied to simulate the time evolution of spreading drops on solid surfaces （drop base radius and dynamic contact angle）. The CFD simulations are quantitatively compared with previously published experimental results from other research groups. The influence of different factors, such as oils nature （silicone, mineral, peanut and coconut）, viscosity （0.02-1 Pa.s）, drop volume （0.3-38 μL） and type of surfaces （hydrophilic glass, stainless steel and hydrophobic glass） on the temporal evolution of the drop base radius and contact angle is investigated. For hydrophilic surfaces, the predictions of the CFD model agree remarkably well with the measurements. For hydrophobic surfaces, a small deviation between calculated and experimental results occurs because the model does not consider the partial slippage which can take place on hydrophobic materials. Despite neglecting this aspect, the simulations are found to capture the key features of drop spreading on hydrophobic surface. The fact that we obtain a good agreement between the proposed theory and the experimental results for a large range of oils and surfaces over five decades of time is a strong argument in favor of the model. The accuracy of the model demonstrates also that the influence of the surface wettabity （partial wetting and complete wetting） can be successfully simulated. The numerical results reproduce perfectly the spreading regimes which occur during the time course of the drop. The succession of two different regimes takes place in the following order： a hydrodynamic regime followed by a gravity regime.     

Tuning the optical absorption of potential blue emitters

The quest for more efficient blue emitters to be applied in organic light-emitting diodes is one of the challenging tasks of contemporary nanotechnologies. An approach to enhance substantially the intrinsic efficiency of luminescent organic molecules is the so-called thermally activated delayed fluorescence. A prerequisite for its occurrence is a vanishing energy separation between the first singlet and triplet excited states. A series of donor–acceptor molecules is investigated theoretically within this study in order to validate a molecular model for design of efficient organic blue emitters with closely spaced singlet and triplet excited states. The model is based on meta-linkage of the donor and acceptor residues to a spacer ensuring frontier molecular orbitals partitioning. The optimal geometries of the molecules are obtained with density functional theory (B3LYP/6-31G^*) and the singlet and triplet absorption spectra are simulated within the time-dependent density functional framework. The excited singlet-triplet energy gap is estimated and correlated to structural and energetic characteristics of the donors and acceptors. Several requirements for achieving high-energy triplet states at the molecular level in such donor–acceptor systems are outlined, the main being disjoint character of the molecular orbitals on the spacer and sufficient energy separation of the two topmost occupied orbitals. It is shown that by variation of the acceptor moiety the optical absorption transitions of the compounds can be fine-tuned in a systematic fashion. Molecules with degenerate singlet and triplet first excited states are proposed, combining bisdimethylaminotriphenylamine or phenoxazine as donors with diphenyloxadiazole or diphenyl-2,2′-bipyridine as acceptors. Bipolar molecules derived from this model could be used as prospective building blocks for efficient emissive materials in blue organic light-emitting diodes.     

Stereometric Analysis of Effects of Heat Stressing on Micromorphology of Si Single Crystals

Silicon - The purpose of this work is study of silicon single crystal wafer thermal stability in correlation with three-dimensional (3D) surface characterization using atomic force microscopy...