1,597 research outputs found
Theory of site-disordered magnets
In realistic spinglasses, such as CuMn, AuFe and EuSrS, magnetic atoms are
located at random positions. Their couplings are determined by their relative
positions. For such systems a field theory is formulated. In certain limits it
reduces to the Hopfield model, the Sherrington-Kirkpatrick model, and the
Viana-Bray model. The model has a percolation transition, while for RKKY
couplings the ``concentration scaling'' T_g proportional to c occurs. Within
the Gaussian approximation the Ginzburg-Landau expansion is considered in the
clusterglass phase, that is to say, for not too small concentrations. Near
special points, the prefactor of the cubic term, or the one of the
replica-symmetry- breaking quartic term, may go through zero. Around such
points new spin glass phases are found.Comment: 26 pages Revtex, 6 figure
Cloning and characterization of the human DNA-excision repair gene ERCC-1
It is the aim of the work described in this thesis to isolate and characterize
human genes involved DNA excision repair. This will facilitate the understanding
of the mechanism of this repair process whereas it also provides an
important step to better understand the relationship betwe
High-pressure melting curve of hydrogen
The melting curve of hydrogen was computed for pressures up to 200 GPa, using molecular dynamics. The inter- and intramolecular interactions were described by the reactive force field (ReaxFF) model. The model describes the pressure-volume equation of state solid hydrogen in good agreement with experiment up to pressures over 150 GPa, however the corresponding equation of state for liquid deviates considerably from density functional theory calculations. Due to this, the computed melting curve, although shares most of the known features, yields considerably lower melting temperatures compared to extrapolations of the available diamond anvil cell data. This failure of the ReaxFF model, which can reproduce many physical and chemical properties (including chemical reactions in hydrocarbons) of solid hydrogen, hints at an important change in the mechanism of interaction of hydrogen molecules in the liquid state
Thermal decomposition of RDX from reactive molecular dynamics
We use the recently developed reactive force field ReaxFF with molecular dynamics to study thermal induced chemistry in RDX [cyclic-[CH2N(NO2)]3] at various temperatures and densities. We find that the time evolution of the potential energy can be described reasonably well with a single exponential function from which we obtain an overall characteristic time of decomposition that increases with decreasing density and shows an Arrhenius temperature dependence. These characteristic timescales are in reasonable quantitative agreement with experimental measurements in a similar energetic material, HMX [cyclic-[CH2N(NO2)]4]. Our simulations show that the equilibrium population of CO and CO2 (as well as their time evolution) depend strongly of density: at low density almost all carbon atoms form CO molecules; as the density increases larger aggregates of carbon appear leading to a C deficient gas phase and the appearance of CO2 molecules. The equilibrium populations of N2 and H2O are more insensitive with respect to density and form in the early stages of the decomposition process with similar timescales
Beware of the Tyrannosaurus rex!
Nederland raakte recent in de ban van de dinosaurussen. Spielberg scoorde na E.T. opnieuw een internationale klapper. Toch is dat geen goede reden voor een column in dit Bestuurskundig lijfblad. Er is tenslotte nog geen reden te veronderstellen dat de dinosaurussen onze wereld opnieuw zullen veroveren en wij als leden van het Crisis Onderzoek Team weer voldoende werk krijgen om deze crisis is nauwgezet te onderzoeken. Een rampenplan hoeft nog niet gemaakt te worden. Toch aandacht voor Steven Spielberg of beter Michael Crichtons boek 'Jurassic Par
Thermal properties of fluorinated graphene
Large scale atomistic simulations using the reactive force field approach
(ReaxFF) are implemented to investigate the thermomechanical properties of
fluorinated graphene (FG). A new set of parameters for the reactive force field
potential (ReaxFF) optimized to reproduce key quantum mechanical properties of
relevant carbon-fluor cluster systems are presented. Molecular dynamics (MD)
simulations are used to investigate the thermal rippling behavior of FG and its
mechanical properties and compare them with graphene (GE), graphane (GA) and a
sheet of BN. The mean square value of the height fluctuations and the
height-height correlation function for different system sizes and
temperatures show that FG is an un-rippled system in contrast to the thermal
rippling behavior of graphene (GE). The effective Young's modulus of a flake of
fluorinated graphene is obtained to be 273 N/m and 250 N/m for a flake of FG
under uniaxial strain along arm-chair and zig-zag direction, respectively.Comment: To appear in Phys. Rev.
Secondary structure of the ribosome binding site determines translational efficiency: a quantitative analysis.
Sustainable EPM rubber compounds
Two important aspects that should be considered when designing new, sustainable rubber products are the bio-based character of the rubber compound ingredients and the recyclability of the vulcanized rubber product. In this work, both are addressed by compounding a thermoreversible cross-linked EPM rubber with pyrolysis carbon black and squalane as sustainable filler and plasticizer, respectively. The resulting rubber product is fully reprocessable in the melt and it displays material properties comparable to those of compounds with conventional additives with high retention of the material properties upon reprocessing
Chemical and morphological evolution of PA-6/Epm/Epm-g-MA blends in a twin screw extruder
Chemical conversion and morphological evolution of PA-6/EPM/EPM-g-MA blends along a twin screw extruder were monitored by quickly collecting small samples from the melt at specific barrel locations. The results show that the MA content of all blends decreases drastically in the first zone of the extruder, i.e., upon melting of the blend components. Significant changes in morphology are also observed at this stage. A correlation between chemistry and morphology could thus be established.- (undefined
Practical application of thermoreversibly Cross-linked rubber products
Currently, rubber products cannot simply be reprocessed after their product life, due to the irreversible cross-linking methods traditionally applied. The purpose of this work is to investigate how thermoreversible cross-linking of rubbers via Diels Alder chemistry can be used for the development of recyclable rubber products. Unfortunately, the applicability of the thermoreversible EPM-g-furan/BM system appears to be limited to room temperature applications, because of the rapid deterioration of the compression set at elevated temperatures compared to irreversibly cross-linked EPM. However, the use of EPM rubber modified with thiophene or cyclopentadiene moieties may extend the temperature application range and results in rubber products with acceptable properties. Finally, rubber products generally comprise fillers such as silica, carbon black or fibers. In this context, the reinforcing effect of short cut aramid fibers on the material properties of the newly developed thermoreversibly cross-linked EPM rubbers was also studied. The material properties of the resulting products were found to be comparable to those of a fiber reinforced, peroxide cured reference sample
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