Analysis and applications of a group contribution sPC-SAFT equation of state

A group contribution (GC) method for estimating pure compound parameters for the molecular-based perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) is proposed in a previous work [A. Tihic, G.M. Kontogeorgis, N. von Solms, M.L. Michelsen, L. Constantinou, Ind. Eng. Chem. Res. 47 (2008) 5092–5101]. In this paper, an investigation of the predictive capability of the GC sPC-SAFT EoS through comparison of the method’s predictions for compounds with high molecular weights and several selected binary mixtures of industrial significance with experimental data such as thiols, sulphides and polynuclear aromatics is presented. Additionally, predictions of activity coefficient at infinite dilution for athermal systems are compared with the results using existing activity coefficient models. The results show that calculated pure compound parameters using the proposed GC method allow satisfactory representation of experimental data of investigated systems with the sPC-SAFT EoS. Moreover, the variety of functional groups in the available GC scheme ensures broad applications of the GC sPC-SAFT EoS.     

Measuring and tracking vitamin B12: A review of current methods with a focus on optical spectroscopy

Vitamin B12 deficiency has been associated with an increased risk of cognitive decline. This literature review explores the current methods available for measuring vitamin B12 in human blood, serum, and urine, and the need for a globally accepted reference range for vitamin B12. We present optical spectroscopy, including chemiluminescence measurements, absorption and fluorescence spectroscopy, surface plasmon resonance, and Raman spectroscopy, as a promising technique for detection and tracking of vitamin B12. Considerations for future research are highlighted, including enhancing the sensitivity of optical spectroscopy and prospective pathways to improve the reproducibility, selectivity, and speed of vitamin B12 detection.     

Evidence of holes in the Arnold tongues of flow past two oscillating cylinders

The wake of two oscillating cylinders in a tandem arrangement is a nonlinear system that displays Arnold tongues. We show by numerical simulations that their geometry depends on the phase difference theta between the two oscillating cylinders. At theta = 0 there may be holes inside these intraresonance regions unlike the solid Arnold tongues encountered in single-cylinder oscillations. This implies that, surprisingly, self-excitation of the system may be suppressed inside these holes, at conditions close to its natural frequency.     

Universal and approximate relations for the gravitational-wave damping timescale of <Emphasis Type="Italic">f</Emphasis>-modes in neutron stars

Existing estimates of the gravitational-wave damping timescale of the dominant quadrupole oscillation mode in the case of rapidly rotating stars are based on using a Newtonian estimate for the energy of the mode, in combination with the lowest-order post-Newtonian quadrupole formula for estimating the gravitational-wave luminosity. We investigate a number of other choices for estimating the gravitational-wave damping timescale in the nonrotating limit and construct a highly accurate, empirically corrected formula that has a maximum relative error of only 3% with respect to the perturbative result in full general relativity. The expressions involved are sufficiently general to be extended to the case of rapidly rotating stars. We also present a new higher-order empirical relation for the gravitational-wave damping timescale of quadrupole oscillations that is accurate in the whole range of expected values for the compactness of neutron stars, without the need for involving the moment of inertia.     

A torsional potential for graphene derived from fitting to DFT results

We present a simple torsional potential for graphene to accurately describe its out-of-plane deformations. The parameters of the potential are derived through appropriate fitting with suitable DFT calculations regarding the deformation energy of graphene sheets folded around two different folding axes, along an armchair or along a zig-zag direction. Removing the energetic contribution of bending angles, using a previously introduced angle bending potential, we isolate the purely torsional deformation energy, which is then fitted to simple torsional force fields. The presented out-of-plane torsional potential can accurately fit the deformation energy for relatively large torsional angles up to 0.5 rad. To test our proposed potential, we apply it to the problem of the vertical displacement of a single carbon atom out of the graphene plane and compare the obtained deformation energy with corresponding DFT calculations. The dependence of the deformation energy on the vertical displacement of the pulled carbon atom is indistinguishable in these two cases, for displacements up to about 0.5 Å. The presented potential is applicable to other sp² carbon structures.     

A systematic study of the electron mobility in V-shaped quantum wires at low temperatures

We present a systematic study of the electron mobility in V-shaped AlGaAs/GaAs quantum wires taking into account the impurity (background, remote and interface) and the acoustic-phonon scattering. The electron scattering rates are calculated for wires with electron concentrations up to 10⁶ cm⁻¹ and temperatures up to 40 K by using Fermi’s golden rule. The effects of the interface roughness scattering and the alloy scattering are also discussed. The energy eigenstates and eigenvalues of the system under study are calculated using a finite difference method. We analyze the importance of each scattering mechanism on the mobility of several quantum wires of different qualities as a function of the electron concentration and the temperature.     

An improved Benders decomposition algorithm for the logistics facility location problem with capacity expansions

We investigate a logistics facility location problem to determine whether the existing facilities remain open or not, what the expansion size of the open facilities should be and which potential facilities should be selected. The problem is formulated as a mixed integer linear programming model (MILP) with the objective to minimize the sum of the savings from closing the existing facilities, the expansion costs, the fixed setup costs, the facility operating costs and the transportation costs. The structure of the model motivates us to solve the problem using Benders decomposition algorithm. Three groups of valid inequalities are derived to improve the lower bounds obtained by the Benders master problem. By separating the primal Benders subproblem, different types of disaggregated cuts of the primal Benders cut are constructed in each iteration. A high density Pareto cut generation method is proposed to accelerate the convergence by lifting Pareto-optimal cuts. Computational experiments show that the combination of all the valid inequalities can improve the lower bounds significantly. By alternately applying the high density Pareto cut generation method based on the best disaggregated cuts, the improved Benders decomposition algorithm is advantageous in decreasing the total number of iterations and CPU time when compared to the standard Benders algorithm and optimization solver CPLEX, especially for large-scale instances.     

On asymptotic equivalence among the solutions of some defective renewal equations

We obtain some sufficient conditions for mutually asymptotic equivalence among solutions of some defective renewal equations, where the related distributions can be heavy-tailed but are not required to be subexponential. Particularly, the paper make clearer the asymptotically equivalent relations between the ruin probability and the function introduced by Gerber et al. [H.U. Gerber, M.J. Goovaerts, and R. Kaas, On the probability and severity of ruin, Astin Bull., 17:151–163, 1987] for the standard renewal risk model. The results we obtain modify, improve, and extend some existing results.     

General Distribution Function for Periods of Downtime of a Single System Exceeding a Limiting Downtime

A reparable two-state system whose components upon failure are replaced is considered. The time to failure and the time to repair of the components are a pair of renewal processes. The distribution of the random variable D(τ, t), which is defined as the random sum of those repair times of the system in the interval of time (0, t) that are greater than or equal to a constant time τ, is derived.     

线性磁性分子离子中由激光诱导的超快自旋转移

通过由激光诱导的Λ进程,很多分子磁性体系中涉及的退磁、 自旋翻转或自旋转移过程在理论上得以实现.本文针对具有线性构型的磁性分子离子, 应用量子化学从头计算与自编程序相结合的方法,实现了具有线性构型的双磁性中心分子体系 [Fe-O-Co]⁺中基于Λ进程的超快自旋转移,且电子占据的转移率达到90%以上. 本文的理论研究结果表明,采用改变磁场方向的方法来增强体系的磁晶各向异性, 可以避免为提高自旋转移能力而额外增加桥接原子在实际应用中的复杂性, 同时也可明显提高电子占据的转移率.