Monte-Carlo simulations of the recombination dynamics in porous silicon

A simple lattice model describing the recombination dynamics in visible light emitting porous Silicon is presented. In the model, each occupied lattice site represents a Si crystal of nanometer size. The disordered structure of porous Silicon is modeled by modified random percolation networks in two and three dimensions. Both correlated (excitons) and uncorrelated electron-hole pairs have been studied. Radiative and non-radiative processes as well as hopping between nearest neighbor occupied sites are taken into account. By means of extensive Monte-Carlo simulations, we show that the recombination dynamics in porous Silicon is due to a dispersive diffusion of excitons in a disordered arrangement of interconnected Si quantum dots. The simulated luminescence decay for the excitons shows a stretched exponential lineshape while for uncorrelated electron-hole pairs a power law decay is suggested. Our results successfully account for the recombination dynamics recently observed in the experiments. The present model is a prototype for a larger class of models describing diffusion of particles in a complex disordered system.Comment: 33 pages, RevTeX, 19 figures available on request to [email protected]

Validation of a combined CFD/FEM methodology for the evaluation of thermal load acting on aluminum alloy pistons through hardness measurements in internal combustion engines

This work presents the results of amultidisciplinary research project, carried outin close collaboration with Ducati MotorHolding S.p.A., for the development of anintegrated methodology to design enginecomponents in aluminum alloy under highthermal loads. The results refer to the study ofan AA2618 (Al-Cu-Mg) alloy piston for highperformance motorcycle engines. The pistonhas been selected as the pilot component forthe development and validation of anadvanced Fluid Dynamics (CFD) and FiniteElement (FE) simulation methodology for theprediction of the inner thermal diffusion. Thesubsequent validation has been achievedthrough both the mechanical andmicrostructural characterization of thecomponent. The methodology here presentedconsists of close interaction between fluiddynamics(CFD) simulations of the combustionprocess and Finite Element (FEM) simulations ofthe thermal diffusion inside the components.Combustion is the main engine heat sourceand is simulated by means of a threedimensionalCFD code for reactive flows (FIREv2008-AVL), with the use of advancedcombustion (ECFM) and wall interactionmodels. The temperature map on the surfacesis based on the results of the iteration with FEMsimulation of thermal diffusion. The FEM modelused for the diffusion analysis receives theresults of combustion analysis as input. Twodifferent methods have been tested for thetransfer of the CFD thermal load to the FEMmodels: a) imposition on the piston crown of aspatial distribution of heat flux averaged overthe mean engine cycle; b) imposition on thepiston crown of both heat flux coefficients and..

A multi-sample microdialysis apparatus for proteins and nucleic acids.

A multiposition microdialysis system suitable for simultaneous microsample applications (between 10 microL and 500 microL), has been developed. Each sample, contained in a specially designed microfuge dialysis tube (mDT), is dialysed independently from the other samples. Each mDT has its own membrane, and this feature allows the use of different membranes and dialysis times for different samples. The microdialysis apparatus is kept at constant temperature by an external thermostat, avoiding the use of a cold box. The dialysis release time for small ions, a parameter used for quantitation of microdialysis efficiency, decreases from 22.9 min (for a 200 microL sample) to 7 min (for a 50 microL sample). The sample is efficiently recovered by centrifugation. Quantitative recoveries (90\%) of different proteins and DNA were achieved after microdialysis by mDT