The evolution of toxic gases from heated polymers—III. Isocyanate products

Attempts have been made to measure the amount of toluene di-isocyanate evolved from flexible polyurethane foam heated at temperatures in the range from 200° to 470°. This study involved heating a sample of about 2 g in a stream of air in a furnace followed by titration of the isocyanate fumes trapped in an absorber. Further investigation concerning the identity of the isocyanate species was also made using chromatographic and spectroscopic techniques. Most of the products which are isocyanate-active appear to be oligomeric ureas having isocyanate end groups. The amount of titratable isocyanate varied from 0·7 to 1·6% w/w calculated as toluene di-isocyanate, and the free toluene di-isocyanate is considered not to exceed 0·02–0·04% (w/w).     

Measurement of the ratio of total and differential cross sections on neutrons and protons for charged-current neutrino events

Charged-current neutrino interactions have been analysed in a sample of pictures from BEBC equipped with a TST. Using a method independent of both the neutrino flux and nuclear interaction corrections, the ratio R= σ_n/σ_p has been measured. The result is R=1.98±0.19 for the ratio of total cross sections. Bjorken x distributions for proton and neutron targets and for u and d quarks are compared.     

A kinetic study of the initial oxidation of the Ni(110) surface by RHEED and X-ray emission

Nickel (110) surfaces, prepared by a combination of high temperature oxidation, argon ion bombardment and hydrogen reduction, were oxidized in pure O₂. The structural aspects of this interaction were studied by reflection high energy electron diffraction (RHEED) and the corresponding kinetics determined by electron excited X-ray emission spectroscopy. On exposure to oxygen the surface was observed to go through three ordered two-dimensional structures. These were a (2 × 1), a (3 × 1) and finally a (9 × 4) structure containing 0.015 microg/cm² of oxygen. From this surface NiO was produced in a (001) epitaxy which was compressed 4.5% in the plane of the surface. With oxidation beyond 0.060 microg/cm² the oxide strain was relieved and a complex oxide epitaxy developed which has been tentatively identified as a (117) oxide parallel to the nickel (110) surface. The kinetics have been explained on the basis of three distinct processes. (i) An initial chemisorption stage (0 to 0.015 microg/cm²) associated with the two-dimensional structures, (ii) Oxide nucleation and spreading to cover the surface, associated with the (001)-NiO epitaxy (0.015 to 0.06 microg/cm²), (iii) Logarithmic film thickening above 0.060 0.06 microg/cm² associated with the development of (117) epitaxy.     

Terahertz spectrum and normal-mode relaxation in pentaerythritol tetranitrate: effect of changes in bond-stretching force-field terms

Terahertz (THz) active normal-mode relaxation in crystalline pentaerythritol tetranitrate (PETN) was studied using classical molecular dynamics simulations for energy and density conditions corresponding to room temperature and atmospheric pressure. Two modifications to the fully flexible non-reactive force field due to Borodin et al. [J. Phys. Chem. B 112, 734 (2008)] used in a previous study of THz-active normal-mode relaxation in PETN [J. Chem. Phys. 134, 014513 (2011)] were considered to assess the sensitivity of the earlier predictions to details of the covalent bond-stretching terms in the force field. In the first modification the harmonic bond-stretching potential was replaced with the Morse potential to study the effect of bond anharmonicity on the THz-region mode relaxation. In the second modification the C-H and nitro-group N-O bond lengths were constrained to constant values to mimic lower quantum occupation numbers for those high-frequency modes. The results for relaxation times of the initially excited modes were found to be insensitive to either force-field modification. Overall time scales for energy transfer to other modes in the system were essentially unaffected by the force-field modifications, whereas the detailed pathways by which the energy transfer occurs are more complicated for the Morse potential than for the harmonic-bond and fixed-bond cases. Terahertz infrared absorption spectra constructed using calculated normal-mode frequencies, transition dipoles, and relaxation times for THz-active modes were compared to the spectra obtained from the Fourier transform of the dipole-dipole time autocorrelation function (DDACF). Results from the two approaches are in near agreement with each other and with experimental results in terms of main peak positions. Both theoretical methods yield narrower peaks than observed experimentally and in addition predict a weaker peak at ω ～ 50 cm(-1) that is weak or absent experimentally. Peaks obtained using the DDACF approach are broader than those obtained from the normal-mode relaxation method.     

Non-Fraunhofer interference pattern in inhomogeneous ferromagnetic Josephson junctions

Generic conditions are established for producing a non-Fraunhofer response of the critical supercurrent subject to an external magnetic field in ferromagnetic Josephson junctions. Employing the quasiclassical Keldysh-Usadel method, we demonstrate theoretically that an inhomogeneity in the magnitude of the energy scales in the system, including Thouless energy, exchange field and temperature gradient normal to the transport direction, influences drastically the standard Fraunhofer pattern. The exotic non-Fraunhofer response, similar to that observed in recent experiments, is described in terms of an intricate interplay between multiple "0-π" states and is related to the appearance of proximity vortices.     

Efficient quantification of non-gaussian spin distributions

We study theoretically and experimentally the quantification of non-gaussian distributions via nondestructive measurements. Using the theory of cumulants, their unbiased estimators, and the uncertainties of these estimators, we describe a quantification which is simultaneously efficient, unbiased by measurement noise, and suitable for hypothesis tests, e.g., to detect nonclassical states. The theory is applied to cold 87Rb spin ensembles prepared in non-gaussian states by optical pumping and measured by nondestructive Faraday rotation probing. We find an optimal use of measurement resources under realistic conditions, e.g., in atomic ensemble quantum memories.     

A multiscale material point method for impact simulation

To better simulate multi-phase interactions involving failure evolution, the material point method (MPM) has evolved for almost twenty years. Recently, a particle-based multiscale simulation procedure is being developed, within the framework of the MPM, to describe the detonation process of energetic nano-composites from molecular to continuum level so that a multiscale equation of state could be formulated. In this letter, a multiscale MPM is proposed via both hierarchical and concurrent schemes to simulate the impact response between two microrods with different nanostructures. Preliminary results are presented to illustrate that a transition region is not required between different spatial scales with the proposed approach.     

Novel half space radiation mode method for buried waveguide analysis

A new method is presented for the analysis of buried rectangular waveguides in the presence of a plane semiconductor–air interface. The method uses simple radiation modes for a half space to formulate closed form transcendental equations for the propagation constants in the scalar and polarized cases. Results are thereby obtained very efficiently and are shown to be highly accurate when compared to benchmark results obtained with the Finite Difference method.