Observation of rotationally mediated focused inelastic resonances in D2 scattering from Cu(001)

6 págs.; 4 figs.; 1 tab.Rotationally mediated focused inelastic resonances (RMFIR's) in the angular distributions of D2 scattered from Cu(001) are observed. The FIR effect involves a phonon-assisted focusing of an incident beam of arbitrary energy and direction into a final channel of one single well-defined energy and direction. Surprisingly for an incident energy Ei = 27 meV the RMFIR conditions for the scattered beam coincide with the kinematic conditions required for a further elastic selective adsorption mechanism called the rotationally mediated critical kinematic (RMCK) effect. By taking advantage of the RMFIR and elastic RMCK effects, three effective bound states of energy ¿n,J = -21.5 meV, -12.4 meV, and -10.3 meV are determined. They are attributed to the lowest bound states ¿0 = -28.9 meV and ¿1 = -19.8 meV combined with the rotational excitation energy for J = 1 to be BrotJ(J + 1) = 7.41 meV, respectively, and ¿3 = -10.3 meV combined with the rotational ground state (J = 0). While the ¿1 and ¿3 states appear as maxima in the angular distribution at RMFIR conditions, the ¿0 yields a striking minimum which represents the first evidence of what we call an anti-FIR feature. Theoretical arguments to explain the different FIR signatures observed are provided. A fit of a phenomenological interaction potential to the experimental bound-state values yields a value for the well depth D = 32.5 meV which is somewhat deeper than that found previously. © 1997 The American Physical SocietyS.M.A. acknowledges the Alexander von Humboldt Stiftung for support. G.B. was supported by Max-Planck Grant No. V-3.MPF-1028848. M.F.B. was supported by the ‘‘Training and Mobility of Researchers’’ Program of the European Union ~Grant No. ERB4001GT952431!. This work was started within the framework of Acciones Integradas Hispano-Alemanas Grant No. 165 A.Peer Reviewe

Observation of Large Differences in the Diffraction of Normal- and Para-H2 from LiF(001)

Theoretical Chemistr

Exchange Bias and Vertical Shift in CoFe2O4 nanoparticles

Magnetic properties of core-shell cobalt ferrite nanoparticles 15 to 48nm prepared by a sol-gel route have been studied. It is shown that the coercivity follows non-monotonic size dependence varying as 1/d above the maximum (d is the particle size). Field cooled magnetization exhibited both horizontal (exchange bias) and vertical shifts. The exchange bias is understood as originating at the interface between a surface region with structural and spin disorder and a core ferrimagnetic region. The dependence of the exchange bias and vertical shifts on the particle sizes and cooling fields are found to have significant differences and the differences are explained in the light of recent results which suggest that both weakly and strongly pinned spins are present at the interface. It is suggested that the exchange bias is dominated by the weakly pinned spins while the vertical shift is affected by the strongly pinned ones.Comment: 2

Freeze-drying method as a new approach to the synthesis of polyurea aerogels from isocyanate and water

Polyurea (PUA) aerogels were prepared as the polycondensation product of triisocyanate and water. PUA aerogels were formed through supercritical drying (SC) and, for comparison, by freeze-drying (FD). The effect of isocyanate concentration, catalyst concentration, and drying method on properties like density, specific surface area, pore volume, compressive strength, and morphology was investigated. The properties of aerogels strongly depend on the concentration of the isocyanate in the parent solution and, to a lesser extent, on catalyst concentration. For example, aerogels with higher isocyanate concentration exhibited a higher surface area and modulus. Depending on the formulation, the materials had a density between 0.128 and 0.220 g/cm3, surface area between 140 and 210 m2/g, pore volume between 0.593 and 1.500 cm3/g, compressive modulus between 6.5 and 25 MPa, and thermal conductivity between about 0.028 and 0.033 W/m K. Most importantly, the drying method did not strongly affect the properties of the materials. These were within ~15% for SC and FD. The results indicate that freeze-drying can be successfully used to fabricate aerogels in a cost-effective way. Open image

Density and shrinkage as guiding criteria for the optimization of the thermal conductivity of poly(urethane)-class aerogels

We investigated the effect of gelation solvent, monomer type, and monomer concentration on the physical properties of freeze-dried poly(urethane)-poly(isocyanurate) (PUR-PIR) aerogels, with particular emphasis on their thermal conductivity. It was found that the gelation solvent considerably affects aerogel morphology and physical properties. Aerogels with the lowest thermal conductivity were obtained using a mixture of tetrahydrofuran (THF) and acetonitrile, in a 50% volume ratio. The influence on thermal conductivity of polyol and isocyanate structure and of their concentration was also investigated. Rigid precursors, phloroglucinol (POL), and an aromatic polyisocyanate based on toluene diisocyanate (Desmodur RC) yielded the lowest thermal conductivity. Our results were compared with recent work reporting on parameters that could be used as predictors of thermal conductivity and other physical properties of organic aerogels. None of these parameters were found to be satisfactory predictors of aerogel properties. For example, no systematic correlation between solvent solubility parameters and aerogel properties was observed. We also examined the role of the K-index. This index, defined as the ratio between porosity and contact angle, was shown recently to be a good predictor of the properties of polyurea aerogels. While the thermal conductivity scaled with the K-index, the scaling was different for each of the isocyanate monomers considered in our experiments. Thermal conductivity, instead, scaled well with the product of density and shrinkage of aerogels, independent of monomer type. The reasons of this dependence on shrinkage and density are discussed, and the use of these parameters to guide experimentation on other systems is discussed. Physical properties such as static and dynamic compression modulus and thermal stability of the most promising formulations were also examined

Linseed oil as a natural modifier of rigid polyurethane foams

Fast growing awareness of environmental and economic problems associated with global climate warming and anticipated future depletion of petroleum resources is driving the fast growing demand for new sustainable biomaterials. A scale of new developments of versatile materials made from abundant and inexpensive natural sources is growing worldwide. Following a wide volume of research studies focused on application of bio-based components for production of plastic foams, composites, coatings, sealants or adhesives, this paper reports the development of a novel type of rigid polyurethane foams (RPUFs), containing linseed oil (LO) as a natural modifier of the rigid polyurethane foams. The use of natural bio-oils for polyurethane composites, broadens the range of functional properties and reduces the costs of production. The bio-composites are more ecofriendly and creates the opportunity to utilize linseed oil, a raw material available in many countries in large quantities. Our research shows that polyurethane foams containing linseed oil demonstrate variety of favorable properties, including the improvement of mechanical strength characteristics. The aim of this work was to determine the influence of the linseed oil on the foam morphology and their physical properties. Our major goal was to optimize the foam formulation, including the content of linseed oil in the reaction mixture

Specifying and reasoning about dynamic access-control policies

Abstract. Access-control policies have grown from simple matrices to non-trivial specifications written in sophisticated languages. The increasing complexity of these policies demands correspondingly strong automated reasoning techniques for understanding and debugging them. The need for these techniques is even more pressing given the rich and dynamic nature of the environments in which these policies evaluate. We define a framework to represent the behavior of accesscontrol policies in a dynamic environment. We then specify several interesting, decidable analyses using first-order temporal logic. Our work illustrates the subtle interplay between logical and state-based methods, particularly in the presence of three-valued policies. We also define a notion of policy equivalence that is especially useful for modular reasoning.