Bicontinuous minimal surface nanostructures for polymer blend solar cells

This paper presents the first examination of the potential for bicontinuous structures such as the gyroid structure to produce high efficiency solar cells based on conjugated polymers. The solar cell characteristics are predicted by a simulation model that shows how the morphology influences device performance through integration of all the processes occurring in organic photocells in a specified morphology. In bicontinuous phases, the surface de. ning the interface between the electron and hole transporting phases divides the volume into two disjoint subvolumes. Exciton loss is reduced because the interface at which charge separation occurs permeates the device so excitons have only a short distance to reach the interface. As each of the component phases is connected, charges will be able to reach the electrodes more easily. In simulations of the current-voltage characteristics of organic cells with gyroid, disordered blend and vertical rod (rods normal to the electrodes) morphologies, we find that gyroids have a lower than anticipated performance advantage over disordered blends, and that vertical rods are superior. These results are explored thoroughly, with geminate recombination, i.e. recombination of charges originating from the same exciton, identified as the primary source of loss. Thus, if an appropriate materials choice could reduce geminate recombination, gyroids show great promise for future research and applications

Transport properties of heterogeneous materials derived from Gaussian random fields: Bounds and Simulation

We investigate the effective conductivity ($\sigma_e$) of a class of amorphous media defined by the level-cut of a Gaussian random field. The three point solid-solid correlation function is derived and utilised in the evaluation of the Beran-Milton bounds. Simulations are used to calculate $\sigma_e$ for a variety of fields and volume fractions at several different conductivity contrasts. Relatively large differences in $\sigma_e$ are observed between the Gaussian media and the identical overlapping sphere model used previously as a `model' amorphous medium. In contrast $\sigma_e$ shows little variability between different Gaussian media.Comment: 15 pages, 14 figure

Structure-property correlations in model composite materials

We investigate the effective properties (conductivity, diffusivity and elastic moduli) of model random composite media derived from Gaussian random fields and overlapping hollow spheres. The morphologies generated in the models exhibit low percolation thresholds and give a realistic representation of the complex microstructure observed in many classes of composites. The statistical correlation functions of the models are derived and used to evaluate rigorous bounds on each property. Simulation of the effective conductivity is used to demonstrate the applicability of the bounds. The key morphological features which effect composite properties are discussed

Comparative assessment of continuum-scale models of bimolecular reactive transport in porous media under pre-asymptotic conditions

We compare the ability of various continuum-scale models to reproduce the key features of a transport setting associated with a bimolecular reaction taking place in the fluid phase and numerically simulated at the pore-scale level in a disordered porous medium. We start by considering a continuum-scale formulation which results from formal upscaling of this reactive transport process by means of volume averaging. The resulting (upscaled) continuum-scale system of equations includes nonlocal integro-differential terms and the effective parameters embedded in the model are quantified directly through computed pore-scale fluid velocity and pore space geometry attributes. The results obtained through this predictive model formulation are then compared against those provided by available effective continuum models which require calibration through parameter estimation. Our analysis considers two models recently proposed in the literature which are designed to embed incomplete mixing arising from the presence of fast reactions under advection-dominated transport conditions. We show that best estimates of the parameters of these two models heavily depend on the type of data employed for model calibration. Our upscaled nonlocal formulation enables us to reproduce most of the critical features observed through pore-scale simulation without any model calibration. As such, our results clearly show that embedding into a continuum-scale model the information content associated with pore-scale geometrical features and fluid velocity yields improved interpretation of typically available continuum-scale transport observations

Combustion en milieu poreux. Simulations numériques 3D à l'échelle des pores

Colloque avec actes et comité de lecture. Internationale.International audienceUne extension d'un modèle numérique pour la simulation à la microéchelle de la combustion en milieu poreux (Debenest et al., 2005a) est présentée. L'enrichissement porte sur le modèle chimique, qui fait notamment intervenir quatre espèces réactives (C, O2, CO et CO2), et quatre réactions dont trois à la surface des grains et une homogène en phase gazeuse. Un premier ensemble de résultats est discuté qui met en évidence différents régimes possibles

Retest change at the questionnaire item "Like to solve complex problems"

A sample Ω108 of 108 French adults (mean age = 36.69, SD = 15.41, 54 males) took part in a survey which consisted of answering questionnaire items twice. The time lapse between the test and the retest ranged from three to five weeks. The questionnaire item "J’aime résoudre des problèmes complexes" (Like to solve complex problems) from the International Personality Item Pool (IPIP, Goldberg et al., 2006) was used along with a 5-point Likert scale from "Very Inaccurate" (coded 1) to "Very Accur..

On the influence of boundary conditions when determining transport coefficients from digital images of porous media

International audienc

Microscale simulation and numerical upscaling of a reactive flow in a plane channel

International audienceA bimolecular homogeneous irreversible reaction of the kind A + B -> C is simulated in a plane channel as a base example of reactive transport processes taking place at the microscale within porous and/or fractured media. The numerical study explores the way microscale processes embedded in dimensionless quantities such as Peclet (Pe) and Damkohler (Da) numbers propagate to upscaled coefficients describing effective system dynamics. The microscale evolution of the reactant concentrations is obtained through a particle-based numerical method which has been specifically tailored to the considered problem. Key results include a complete documentation of the process evolution for a wide range of Pe and Da, in terms of the global reaction rate, space-time distribution of reactants, and local mixing features leading to characterization of effective reaction and dispersion coefficients governing a section-averaged upscaled model of the system. The robustness of previously presented theoretical analyses concerning closures of volume-averaged (upscaled) formulations is assessed. The work elucidates the dependence of the effective dispersion and reactive parameters on the microscale mixing and reactive species evolution. Our results identify the role played by Da and Pe on the occurrence of incomplete mixing of reactants, which affects the features of the reactive transport scenario

THERMOCHEMICAL CHARACTERIZATION OF A COMBUSTION FRONT PROPAGATION IN REACTIVE POROUS MEDIUM: A NEW EXPERIMENTAL DEVICE.

Abstract. We describe here a new experimental device developed for the study of a combustion front propagation in a reactive medium: crushed oil shale (500 to 1000 µm). We are interested in the propagation of a front supplied by air in co-current. The cell is a vertical cylinder of 90mm internal diameter and 300mm height, made up of an insulating material. To start the combustion and to obtain a plane flame front, it was set up a sophisticated device of ignition, namely the cone calorimeter. The reactor is finely instrumented. A whole of thermocouples makes it possible to measure the temperature along the axis for the cell and over a horizontal cross section (at middle height) of the packed bed. The pressure drop and the total mass of the particle bed are continuously recording. A new device was developed and set up to improve the physicochemical investigation of the structure of the flame front: micro-sampling. Its originality lies in the possibility of micro sampling gas at a fined point in the cell before, during and after the passage of the combustion front. Lastly, the oil “pushed ” by the flame front is collected in a reservoir placed at the botto