Properties of magnetocaloric La(Fe,Co,Si)13 produced by powder metallurgy

We present a comprehensive study of the magnetocaloric materials series La(Fe_1−xCo_x)_11.9Si_1.1 with 0.055<x<0.122. The ferromagnetic samples were manufactured using a novel powder metallurgy process by which industrial scale production is feasible. This new production method makes the materials more attractive as magnetic refrigerants for room temperature magnetic refrigeration. The Curie temperature of the compounds can be easily tuned by altering the Co content and all samples have little magnetic anisotropy and present a second-order magnetic transition so that thermal and magnetic hysteresis is absent. For all seven samples, we have calculated the magnetic entropy change, ΔS_M, from initial curve measurements and measured the adiabatic temperature change, ΔT_ad, directly. In addition, for two of the samples, we determined the heat capacity as a function of applied magnetic field and the thermal conductivity. Where relevant, the results are compared with those of Gd, the benchmark material for room temperature magnetic refrigeration.     

Birefringence-induced splitting of the zero-dispersion wavelength in nonlinear photonic crystal fibers

We investigate how the strongly wavelength-dependent birefringence in nonlinear photonic crystal fibers leads to a splitting in the zero-dispersion wavelength for the two polarizations. We translate the requirements for the maximum splitting of the zero-dispersion wavelength to requirements for transverse structural uniformity by adopting a simple effective-index approach in which the birefringence is calculated in a step-index fiber with an elliptical core. We find that to reduce the splitting to less than 1 nm the birefringence should be less than 2 x 10(-5), resulting in a transverse uniformity requirement of 1-3%, depending on the index step from the core to the cladding.     

Variable neighborhood search for harmonic means clustering

Harmonic means clustering is a variant of minimum sum of squares clustering (which is sometimes called K-means clustering), designed to alleviate the dependance of the results on the choice of the initial solution. In the harmonic means clustering problem, the sum of harmonic averages of the distances from the data points to all cluster centroids is minimized. In this paper, we propose a variable neighborhood search heuristic for solving it. This heuristic has been tested on numerous datasets from the literature. It appears that our results compare favorably with recent ones from tabu search and simulated annealing heuristics.     

An electrocapillary study to test the applicability of the Flory-Huggins isotherm

The Flory-Huggins isotherm with interaction parameter has been tested using accurate electrocapillary data for four aliphatic compounds at the mercury-electrolytic solution interface. When surface coverages derived by differentiation of surface pressure-activity data were tested in the Flory-Huggins isotherm and when all parameters were freely optimized, the equation fit the data very well in all cases, but the size ratio parameter thus obtained was about 0.5. When parameters of the Flory-Huggins isotherm were fixed by limiting properties of surface pressure-activity data directly (with no differentiations performed), the size parameter was about 1.0. The size parameter expected physically is about 3.0. The Flory-Huggins isotherm with realistic size parameters did not fit the data as well as the simpler Frumkin equation in any of the cases considered. The physical basis underlying both models is weak for interaction parameters as large as encountered in aqueous-organic solute systems.     

Probing fuel-specific reaction intermediates from laminar premixed flames fueled by two C5 ketones and model interpretations

The flame chemistry was explored for two C₅ ketones with distinct structural features, cyclopentanone (CPO) and diethyl ketone (DEK). Quantitative information for numerous species, including some reactive intermediates, was probed from fuel-rich (?= 1.5) laminar premixed flames fueled by the ketones with a photoionization molecular-beam mass spectrometer (PI-MBMS). Furthermore, a new kinetic model was proposed aimed at interpreting the high-temperature combustion chemistry for both ketones, which could satisfactorily predict the current flame speciation measurements. Experimental observations in combination with modeling analyses were used to reveal the similarities and differences between the compositions of the species pools of the two flames, with emphasis on the effects of the carbonyl functionality on pollutants formations. Besides some primary species which preserve fuel-specific features produced from initial steps of fuel consumptions, basic C₁C₄ intermediates also differ much between the two flames. More abundant intermediates were observed in the CPO flame because the cyclic fuel structure enables ring-opening processes followed by formations of C₃ and C₄ hydrocarbons which cannot be easily produced from the two isolated ethyl moieties in DEK under flame conditions. The consumptions of C₃C₄ hydrocarbons in the CPO flame further lead to larger C₅C₆ species which were under the detection limit in the DEK flame. In both flames, the tightly bonded carbonyl groups in the fuels tend to be preserved, leading to carbon monoxide through α-scissions of fuel-related acyl radicals. The carbonyl moieties in most detected C₁C₃ aldehydes and ketones form through oxidations of hydrocarbon species rather than directly originating from the fuels.     

Multifractality in elastic percolation

We present numerical results on the distribution of forces in the central-force percolation model at threshold in two dimensions. We conjecture a relation between the multifractal spectrum of scalar and vector percolation that we test for central-foce percolation. This relation is in excellent agreement with our numerical data.     

Nuclear structure at the drip lines

This paper gives examples of gross nuclear structure and properties across the nuclear chart and especially near the drip lines for protons and neutrons. New measurements of spins and transition multipolarities in beta decay could greatly enhance our understanding in a number of ways, for example by providing a more quantitative understanding of isospin mixing. The neutron-rich nuclei show novel structural features near the neutron drip line, where the combined effect of low neutron binding and the short range of the nuclear forces leads to the appearance of a neutron distribution extending far beyond the nuclear core (a “halo”). With this is associated a soft electric dipole mode that leads to large cross-sections for Coulomb dissociation and to short E` lifetimes.     

Mean field kinetic theory of a classical electron gas in a periodic potential. III. The high-temperature limit in two dimensions

The Vlasov-like mean field kinetic equation for a classical electron plasma in the periodic field of an ionic lattice is solved in the high-temperature limit in two dimensions. The predictions for the one-electron density, the static structure factor, and the long-wavelength charge fluctuation spectrum are compared to the results of extensive molecular dynamics simulations. The predicted shift and damping of the plasma oscillation mode are in reasonable agreement with the simulation data at intermediate couplings (=e ²/k _B T1), but the agreement deteriorates as the temperature or the density is lowered, because mean field theory does not lead to the expected Kosterlitz-Thouless transition to the dielectric phase where electrons are localized.