Spin-glass dynamics in interacting nanoparticle system La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> obtained by mechanochemical milling

Spin dynamics in mechanochemically obtained nanoparticle manganite La_0.70Ca_0.30MnO₃ was investigated in this study by means of a series of AC and DC magnetic measurements. AC susceptibility indicates the presence of sizeable interparticle interactions, yielding collective magnetic behavior. The related properties were probed by experiments in weak DC field: memory effects were analyzed in both field-cooled (FC) and zero field-cooled (ZFC) regimes, while, after ZFC aging, magnetic relaxation was recorded. The system appears to be sensitive to magnetothermal history, in analogy with spin-glass-like compounds. The analysis of the data indicates the occurrence of slow dynamics in an ensemble of strongly interacting super spins.     

Numerical experiments on flow control of near wall turbulence

In this paper we demonstrate by means of direct numerical simulations (DNS) of channel flow with surface structuring or modified boundary conditions how turbulence can be controlled by influencing the anisotropy state of near wall turbulence. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of the spatial resolution on fine-scale features in DNS of turbulence generated by a single square grid

We focus in this paper on the effect of the resolution of direct numerical simulations (DNS) on the spatio-temporal development of the turbulence downstream of a single square grid. The aims of this study are to validate our numerical approach by comparing experimental and numerical one-point statistics downstream of a single square grid and then investigate how the resolution is impacting the dynamics of the flow. In particular, using the Q–R diagram, we focus on the interaction between the strain-rate and rotation tensors, the symmetric and skew-symmetric parts of the velocity gradient tensor, respectively. We first show good agreement between our simulations and hot-wire experiment for one-point statistics on the centreline of the single square grid. Then, by analysing the shape of the Q–R diagram for various streamwise locations, we evaluate the ability of under-resolved DNS to capture the main features of the turbulence downstream of the single square grid.     

Synthesis and characterization of silver—poly(methylmethacrylate) nanocomposites

The optical properties of silver nanoparticles embedded in poly(methylmethacrylate) (PMMA) was investigated as well as the influence of silver nanoparticles on the thermal properties of polymer matrix. The average size and particle size distribution of silver nanoparticles was determined using transmission electron microscopy. The obtained transparent nanocomposite films were optically characterized using UV-Vis and FTIR spectroscopy. Thermal stability of polymer matrix was improved upon incorporation of small amount of silver nanoparticles. Also, silver nanoparticles have pronounced effect on thermo-oxidative stability of PMMA matrix. The glass transition temperatures of nanocomposites are lower compared to the pure polymer.     

Impedance spectroscopy of reactive polymers. 3. Correlations between dielectric,spectroscopic, and rheological properties during cure of a trifunctional epoxy resin

An investigation was carried out of correlations between dielectric, vibrational spectroscopic, and rheological properties during cure of a thermoset formulation composed of trifunctional epoxy resin and tetrafunctional amine. Experimental techniques utilized include impedance spectroscopy, near-infrared spectroscopy, steady shear, and dynamic mechanical measurements. Reaction kinetics obtained from dielectric and spectroscopic results were in excellent agreement. Gelation and vitrification times determined by dielectric and rheological measurements were also found to agree very well, despite the empirical nature of such correlations. A characteristic pattern in plots of imaginary impedance as a function of reaction time was reported for the first time in the open literature, and it was suggested that it could be used to identify gelation and vitrification during the network formation. A realization of the full potential of dielectric impedance spectroscopy in monitoring the progress of chemophysical changes in reactive polymers, however, hinges upon a development of fundamental scientific correlations between dielectric and chemorheological phenomena during cure.     

Excitonic diffusion in thin molecular films

The analyses of exciton diffusion in thin molecular films have shown that the diagonal elements of the diffusion tensor, usually called diffusion coefficients, depend on the layer index labeling layers in the direction of disturbed symmetry. The particular analysis was done for a thin film having four layers. In this structure only two layers are occupied by optical excitations. It means that in the four layer film two films occur in which optical excitations can travel. The subfilm contains a boundary layer that noticeably differs from the subfilm with internal layers. If the subfilm contains the boundary layer, the diffusion coefficient of this layer differs from the diffusion coefficient of any internal layer. If the subfilm contains two internal layers, the diffusion coefficient of these layers are equal, expectably from the viewpoint of physics. The exciton diffusion is very low due to the high exciton energies. This work was supported by the Serbian Ministry of Science and Technology: Grant No 1895.     

Electrocatalytic Dioxygen Reduction by Carbon Electrodes Noncovalently Modified with Iron Porphyrin Complexes: Enhancements from a Single Proton Relay

Oxygen reduction in acidic aqueous solution mediated by a series of asymmetric iron (III)‐tetra(aryl)porphyrins adsorbed to basal‐ and edge‐ plane graphite electrodes is investigated. The asymmetric iron porphyrin systems bear phenyl groups at three meso positions and either a 2‐pyridyl, a 2‐benzoic acid, or a 2‐hydroxyphenyl group at the remaining meso position. The presence of the three unmodified phenyl groups makes the compounds insoluble in water, enabling catalyst retention during electrochemical experiments. Resonance Raman data demonstrate that catalyst layers are maintained, but can undergo modification after prolonged catalysis in the presence of O₂. The introduction of a single proton relay group at the fourth meso position makes the asymmetric iron porphyrins markedly more robust catalysts; these molecules support higher sustained current densities than the parent iron tetraphenylporphyrin. Iron porphyrins bearing a 2‐pyridyl group are the most active catalysts and operate at stable current densities ≥1 mA cm^?2 for over 5 h. Comparative analysis of the catalysts with different proton relays also is reported.     

Effect of matrix on IR frequencies of acetylene and acetylene-methanol complex: infrared matrix isolation and ab initio study

Effect of nitrogen and argon matrices on the C-H asymmetric stretching and bending infrared frequencies of the acetylene molecule, C(2)H(2), has been studied by matrix isolation experiments as well as by calculations at MP2 level of theory. The complexes of C(2)H(2) in nitrogen and argon matrices, viz., C(2)H(2)(N(2))(m) (with m=2-8) and C(2)H(2)(Ar)(n) (with n=2-10) are theoretically explored. The computed acetylenic C-H asymmetric stretch in C(2)H(2)-nitrogen complexes shows a redshift of 3.0 to 11.9 cm(-1) compared with the frequencies of the free acetylene molecule, and a corresponding blueshift of 7.4 to 26.2 cm(-1) when C(2)H(2) is complexed with argon atoms. The trends in the computed shifts are in good agreement with the experiments. The molecular electrostatic potential minimum of C(2)H(2) becomes more negative when complexed with nitrogen than on complexation with argon. This observation implies a greater basic character for C(2)H(2) in the nitrogen matrix, favoring the formation of H-pi(C(2)H(2)-MeOH) complex as compared to that in the Ar matrix. Experimentally the preferential formation of H-pi(C(2)H(2)-MeOH) complex in the N(2) matrix has indeed been observed.     

Characterization and Optimization of a Resonant Cavity Enhanced P-i-N Photodiode Response

This paper presents linear pulse response of a Resonant Cavity Enhanced (RCE) P-i-N fotodiode. The RCE P-i-N photodiode designed for high-speed aplication is analysed for various submicron thicknesses of absorption layer, bias voltages, active areas and incident pulse optical excitations. The results are obtained by numerical simulation of the complete phenomenological model for two valley semiconductor. Great enhancement of the quantum efficiency and the product bandwidth-quantum efficiency, is obvious from obtained results for this photodiode type.     

Charpit System of Partial Differential Equations with Algebraic Constraints

In this paper the Charpit system of partial differential equations with algebraic constraints is considered. So, first the compatibility conditions of a system of algebraic equations and also of the Charpit system of partial differential equations are separately considered. For the combined system of equations of both types sufficient conditions for the existence of a solution are found. They lead to an algorithm for reducing the combined system to a Charpit system of partial differential equations of dimension less than the initial system and without algebraic constraints. Moreover, it is proved that this system identically satisfies the compatibility conditions if so does the initial system.