Molecular conduction: Do time-dependent simulations tell you more than the Landauer approach?

A dynamical method for simulating steady-state conduction in atomic and molecular wires is presented which is both computationally and conceptually simple. The method is tested by calculating the current-voltage spectrum of a simple diatomic molecular junction, for which the static Landauer approach produces multiple steady-state solutions. The dynamical method quantitatively reproduces the static results and provides information on the stability of the different solutions. (c) 2006 American Institute of Physics.

Pattern formation of chevrons in the conduction regime in homeotropically aligned liquid crystals

We report on chevrons (herringbonelike patterns) observed in homeotropically aligned liquid crystals with high electric conductivity. We focus our attention on two types of chevrons observed in the conduction regime. The threshold voltage and the characteristic double periodicity of chevrons (i.e., the short wavelength lambda(1) of the striated rolls and the long wavelength lambda(2) Of the chevron bands) have been measured as functions of the applied electric frequency f. With the aid of a crossed polarizer set, we have, in addition, determined the director field which shows a periodic in-plane rotation for our chevrons (with a wavelength lambda(2)) We arrived at the types of chevrons after qualitatively different bifurcation sequences with increasing voltage. The frequency dependence of lambda(2) also shows a qualitatively different behavior with respect to the two types of chevrons. The experimental results are discussed in terms of recent theoretical investigations.

Transient Heat Conduction of Variable Stiffness Composite Laminate

Recent research on Variable Stiffness (VS) laminates, which are constructed by steering the fiber orientation as a spatial function of location, have shown to improve laminate performance under mechanical loads. Two distinct cases of stiffness variation can be achieved either by variation of the fiber orientation in the direction of the global x-axis, or perpendicular to it. In the present paper, thermal analysis of a VS laminate is performed to study the effect of steering fibers on transient heat conduction under uniform heat flux using finite element method. The goal of the present paper is a parametric study of the effect of variable stiffness properties on transient response including time to reach steady state and temperature profile. Also, stress resultants and maximum stress location are investigated under different boundary conditions. A FEM algorithm is applied to exactly incorporate the boundary conditions for stress resultant analysis.

Transient Heat Conduction of Variable Stiffness Composite Laminate

Recent research on Variable Stiffness (VS) laminates, which are constructed by steering the fiber orientation as a spatial function of location, have shown to improve laminate performance under mechanical loads. Two distinct cases of stiffness variation can be achieved either by variation of the fiber orientation in the direction of the global x-axis, or perpendicular to it. In the present paper, thermal analysis of VS laminate is performed to study the effect of steering fibers on transient heat conduction under uniform heat flux using finite element method. The goal of the present paper is a parametric study of the
effect of variable stiffness properties on transient response including time to reach steady state and temperature profile. Also, stress resultants and maximum stress location are investigated under different boundary conditions. A FEM algorithm is applied to exactly incorporate the boundary conditions.

Tailoring ionic liquid catalysts: structure, acidity and catalytic activity of protonic ionic liquids based on anionic clusters, [(HSO4)(H2SO4)x]− (x = 0, 1, or 2)

Aiming at inexpensive Brønsted-acidic ionic liquids, suitable for industrial-scale catalysis, a family of protonic ionic liquids based on nitrogen bases and sulfuric acid has been developed. Variation of the molar ratio of sulfuric acid, χH2SO4, was used to tune acidity. The liquid structure was studied using 1H NMR and IR spectroscopies, revealing the existence of hydrogen-bonded clusters, [(HSO4)(H2SO4)]−, for χH2SO4 > 0.50. Acidity, quantified by Gutmann Acceptor Number (AN), was found to be closely related to the liquid structure. The ionic liquids were employed as acid catalysts in a model reaction; Fischer esterification of acetic acid with 1-butanol. The reaction rate depended on two factors; for χH2SO4 > 0.50, the key parameter was acidity (expressed as AN value), while for χH2SO4 > 0.50 it was the mass transport (solubility of starting materials in the ionic liquid phase). Building on this insight, the ionic liquid catalyst and reaction conditions have been chosen. Conversion values of over 95% were achieved under exceptionally mild conditions, and using an inexpensive ionic liquid, which could be recycled up to eight times without diminution in conversion or selectivity. It has been demonstrated how structural studies can underpin rational design and development of an ionic liquid catalyst, and in turn lead to a both greener and economically viable process.

Theory of phonon dissipation in the conduction of stressed Au nanowires

Recent experiments on Au break junctions [Phys. Rev. Lett. 88 (2002) 216803] have characterized the nonlinear conductance of stretched short Au nanowires. They reveal in the voltage range 10-20 meV the signatures of dissipation effects, likely due to phonons in the nanowire, reducing the conductance below the quantized value of 2e(2)/h. We present here a theory, based on a model tight-binding Hamiltonian and on non-equilibrium Green's function techniques, which accounts for the main features of the experiment. The theory helps in revealing details of the experiment which need to be addressed with a more realistic, less idealized, theoretical framework. (C) 2004 Elsevier B.V. All rights reserved.

Electrical conduction and rheological behaviour of composites of poly(epsilon-caprolactone) and MWCNTs

Two mechanisms of conduction were identified from temperature dependent (120 K-340 K) DC electrical resistivity measurements of composites of poly(c-caprolactone) (PCL) and multi-walled carbon nanotubes (MWCNTs). Activation of variable range hopping (VRH) occurred at lower temperatures than that for temperature fluctuation induced tunneling (TFIT). Experimental data was in good agreement with the VRH model in contrast to the TFIT model, where broadening of tunnel junctions and increasing electrical resistivity at T > T-g is a consequence of a large difference in the coefficients of thermal expansion of PCL and MWCNTs. A numerical model was developed to explain this behavior accounting for a thermal expansion effect by supposing the large increase in electrical resistivity corresponds to the larger relative deformation due to thermal expansion associated with disintegration of the conductive MWCNT network. MWCNTs had a significant nucleating effect on PCL resulting in increased PCL crystallinity and an electrically insulating layer between MWCNTs. The onset of rheological percolation at similar to 0.18 vol% MWCNTs was clearly evident as storage modulus, G' and complex viscosity, vertical bar eta*vertical bar increased by several orders of magnitude. From Cole-Cole and Van Gurp-Palmen plots, and extraction of crossover points (G(c)) from overlaying plots of G' and G '' as a function of frequency, the onset of rheological percolation at 0.18 vol% MWCNTs was confirmed, a similar MWCNT loading to that determined for electrical percolation. 

Hydrogen-permeation characteristics of a SrCeO3-based ceramic separation membrane:Thermal, ageing and surface-modification effects

Dense ceramics with mixed protonic-electronic conductivity are of considerable interest for the separation and purification of hydrogen and as electrochemical reactors. In this work, the hydrogen permeability of a Sr_0.97Ce_0.9Yb_0.1O_{3 - δ} (SCYb) membrane with a porous Pt catalytic layer on the hydrogen feed-exposed side has been studied over the temperature range 500-804 °C employing Ar as the permeate sweep gas. A SiO₂-B₂O₃-BaO-MgO-ZnO-based glass-ceramic sealant was successfully employed to seal the membrane to the dual-chamber reactor. After 14 h of exposure to 10% H₂:90% N₂ at 804 °C, the H₂ flux reached a maximum of 33 nmol cm^{- 2} s^{- 1}, over an order of magnitude higher than that obtained on membranes of similar thickness without surface modification. The permeation rate then decreased slowly and moderately on annealing at 804 °C over a further 130 h. Thereafter, the flux was both reproducible and stable on thermal cycling in the range 600-804 °C. The results indicate an important role of superficial activation processes in the flux rate and suggest that hydrogen fluxes can be further optimised in cerate-based perovskites. © 2009 Elsevier B.V. All rights reserved.

Wireless electrochemical modification of catalytic activity on a mixed protonic-electronic conductor

A novel approach to electrochemical modification of catalytic activity using a wireless configuration has been undertaken. This paper presents preliminary results on the modification of a platinum catalyst film supported on a pellet of Sr_0.97Ce_0.9Yb_0.1O_3-δ (SCYb), considered to be a mixed protonic-electronic conductor under reducing conditions. The wireless configuration utilises the mixed ionic and electronic conductivity of the supporting membrane to supply an ionic promoting species to the catalyst surface. Control of the flux of this species is achieved by adjusting the effective hydrogen chemical potential difference across the membrane in a dual-chamber reactor with one chamber acting as the "reaction side" and the other as the "sweep side". The reaction rate can be promoted by up to a factor of 1.6, for temperatures around 500 °C and low reactant concentrations, when hydrogen is introduced on the sweep side of the membrane reactor. The use of helium, moist helium and oxygen in helium as sweep gases did not modify the reaction rate. © 2007 Elsevier B.V. All rights reserved.

Combined Conduction-Convection-Radiation Heat Transfer of Slip Flow inside a Micro-channel Filled with a Cellular Porous Material

Combined conduction–convection–radiation heat transfer is investigated numerically in a micro-channel filled with a saturated cellular porous medium, with the channel walls held at a constant heat flux. Invoking the velocity slip and temperature jump, the thermal behaviour of the porous–fluid system are studied by considering hydrodynamically fully developed flow and applying the Darcy–Brinkman flow model. One energy equation model based on the local thermal equilibrium condition is adopted to evaluate the temperature field within the porous medium. Combined conduction and radiation heat transfer is treated as an effective conduction process with a temperature-dependent effective thermal conductivity. Results are reported in terms of the average Nusselt number and dimensionless temperature distribution, as a function of velocity slip coefficient, temperature jump coefficient, porous medium shape parameter and radiation parameters. Results show that increasing the radiation parameter (Tr)(Tr) and the temperature jump coefficient flattens the dimensionless temperature profile. The Nusselt numbers are more sensitive to the variation in the temperature jump coefficient rather than to the velocity slip coefficient. Such that for high porous medium shape parameter, the Nusselt number is found to be independent of velocity slip. Furthermore, it is found that as the temperature jump coefficient increases, the Nusselt number decrease. In addition, for high temperature jump coefficients, the Nusselt number is found to be insensitive to the radiation parameters and porous medium shape parameter. It is also concluded that compared with the conventional macro-channels, wherein using a porous material enhances the rate of heat transfer (up to about 40 % compared to the clear channel), insertion of a porous material inside a micro-channel in slip regime does not effectively enhance the rate of heat transfer that is about 2 %.