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International audience; We present the results of extensive fully atomistic molecular dynamics (MD) simulations of tetracosane (C24H50) bilayer and trilayer systems adsorbed onto the basal plane of graphite. At low temperature, both layers of the bilayer exist in well-defined solid phases. With increasing temperature, the system exhibits separated smectic phases that eventually lead to melting. During this process, we observed a strong interlayer translational correlation and mobility between layers; however, the upper layer presents more intra- (chain) and intermolecular disorder because of a lack of confinement and a greater distance to the graphite substrate. Simulations of the perpendicular trilayer patch show that gauche defects provide the main mechanism for spreading of the bottom and outer perimeter of the patch in the solid, leading to the ultimate collapse of the patch with increasing temperature and formation of a flat (parallel) trilayer that melts at a higher temperature than the bilayer structure. The wide variety of structural order parameters, thermodynamic functions, and probability distributions we employed provide a clear picture of the roles of gauche defects, confinement, and interlayer correlation in the phases and phase transitions exhibited by these confined organic layers.

A constant-temperature, constant density molecular-dynamics (MD) method is utilized to study the behavior of bond-orientational, translational and thermodynamic parameters in the solid, melting transition and fluid regimes of argon physisorbed onto graphite (Ar/gr) at complete coverage. For Ar/gr, completion occurs near r=1.26 in units of 0.0636 atoms/Au2, where r=1 corresponds to the complete E3×E3 R30 ° commensurate adlayer. Several bondorientational and newly introduced pair-distribution order parameters and e ffective densities prove especially useful in helping evaluate the temperature behavior of the system’s structural order. A broad melting transition accompanied by vacancy formation through large vertical fluctuations in the adsorbate atoms is observed at T m =135 ±5 K, spanning a temperature range of roughly 40 K. The melting transition accompanies a rapid formation of a lowdensity second layer. Substantial bond-orientational order persists into the high-temperature fluid ( T#300 K), which is not hexatic in nature and which exhibits infrequent desorption from the first layer. The behavior and interaction of the neighbor shells are compared and contrasted to those observed in Kr/gr. © 1999 Elsevier Science B.V. All rights reserved.

International audience; The efficient storage and transportation of natural gas is one of the most important enabling technologies for use in energy applications. Adsorption in porous systems, which will allow the transportation of high-density fuel under low pressure, is one of the possible solutions. We present and discuss extensive grand canonical Monte Carlo (GCMC) simulation results of the adsorption of methane into slit-shaped graphitic pores of various widths (between 7 angstrom and 50 angstrom), and at pressures P between 0 bar and 360 bar. Our results shed light on the dependence of film structure on pore width and pressure. For large widths, we observe multi-layer adsorption at supercritical conditions, with excess amounts even at large distances from the pore walls originating from the attractive interaction exerted by a very high-density film in the first layer. We are also able to successfully model the experimental adsorption isotherms of heterogeneous activated carbon samples by means of an ensemble average of the pore widths, based exclusively on the pore-size distributions (PSD) calculated from subcritical nitrogen adsorption isotherms. Finally, we propose a new formula, based on the PSD ensemble averages, to calculate the isosteric heat of adsorption of heterogeneous systems from singlepore-width calculations. The methods proposed here will contribute to the rational design and optimization of future adsorption-based storage tanks.

A review is presented of ATR (automatic target recognition), and some of the highlights of neural network technology developments that have the potential for making a significant impact on ATR are presented. In particular, neural network technology developments in the areas of collective computation, learning algorithms, expert systems, and neurocomputer hardware could provide crucial tools for developing improved algorithms and computational hardware for ATR. The discussion covers previous ATR system efforts. ATR issues and needs, early vision and collective computation, learning and adaptation for ATR, feature extraction, higher vision and expert systems, and neurocomputer hardware. >

It is shown that feed-forward and graded response Hopfield neural networks can implement an optimum postdetection target track receiver. For the Hopfield net the spurious states correspond to the important case of multiple track detection. Simulations are presented that show that substantial signal-to-noise gain can be achieved. The advantage of the Hopfield network is that the desired final output cells are just time-evolved input cells, so that no additional cells are required for output display. Although a feed-forward network may relegate display to an external device and compute only cells corresponding to the presence of particular patterns, additional cells are still required, over and above the input cells, for the detection of each pattern. If a large number of patterns is required, then the advantage of the Hopfield network, which needs only the input cells, becomes evident. >