Dynamical modeling of water transport in polymer electrolyte membrane fuel cell (PEMFC) design

A two-dimensional finite element computational fluid dynamics (CFD) model, including coupled partial differential equations of mass, momentum and charge conservation inside a membrane electrode assembly of a polymer electrolyte membrane fuel cell (PEMFC) are developed. The CFD model is solved for PEMFCs with conventional and interdigitated gas flow fields. For the PEMFC with interdigitated flow fields both coflow and counterflow designs are studied. Furthermore a dynamic lumped model based on the formulation of Pukrushpan et al. (2003) is developed with the addition of membrane's transient water transport. Models are validated by comparing the polarization curves with the experimental data of Ticianelli et al. (1988) for MEAs with conventional gas distributors and He et al. (2000) for MEAs with counterflow interdigitated gas distributors. The results of the lumped model and the CFD model for conventional design are shown to be comparable and lumped model proves to be a good substitute of CFD model for control studies. For the interdigitated case, coflow is found to be superior to counterflow in the performance of the cell. Transient and steady-state responses of the fuel cell system to changes in cell voltage, air pressure and relative humidity of air are investigated for each design. The effect of transient water transport is emphasized and it is observed that it plays a critical role in the operation of a PEMFC for both designs

Raman and TEM studies of Ge nanocrystal formation in SiO x:Ge/SiOx multilayers

Alternating germanosilicate-siliconoxide layers of 10-30 nm thickness were grown on Si substrates by plasma enhanced chemically vapor deposition (PECVD). The compositions of the grown films were determined by X-ray photoelectron spectroscopy measurements. The films were annealed at temperatures varying from 670 to 1000°C for 5 to 45 minutes under nitrogen atmosphere. High resolution cross section TEM images, electron diffraction and electron energy-loss spectroscopy as well as energy-dispersive X-ray analysis (EDAX) data confirm presence of Ge nanocrystals in each layer. The effect of annealing on the Ge nanocrystal formation in multilayers was investigated by Raman spectroscopy and Transmission Electron Microscopy (TEM). As the annealing temperature is raised to 850°C, single layer of Ge nanocrystals observed at lower annealing temperatures is transformed into a double layer with the smaller sized nanocrystals closer to the substrate SiO2 interface. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA

Formation of silicon nanocrystals in sapphire by ion implantation and the origin of visible photoluminescence

Cataloged from PDF version of article.Silicon nanocrystals, average sizes ranging between 3 and 7 nm, were formed in sapphire matrix by ion implantation and subsequent annealing. Evolution of the nanocrystals was detected by Raman spectroscopy and x-ray diffraction (XRD). Raman spectra display that clusters in the matrix start to form nanocrystalline structures at annealing temperatures as low as 800 degrees C in samples with high dose Si implantation. The onset temperature of crystallization increases with decreasing dose. Raman spectroscopy and XRD reveal gradual transformation of Si clusters into crystalline form. Visible photoluminescence band appears following implantation and its intensity increases with subsequent annealing process. While the center of the peak does not shift, the intensity of the peak decreases with increasing dose. The origin of the observed photoluminescence is discussed in terms of radiation induced defects in the sapphire matrix. (c) 2006 American Institute of Physics

An Open-Source Library for the Numerical Modeling of Mass-Transfer in Solid-Oxide Fuel Cells

The generation of direct current electricity using Solid Oxidize Fuel Cells (SOFCs) involves several interplaying transport phenomena. Their simulation is crucial for the design and optimization of reliable and competitive equipment, and for the eventual market deployment of this technology. An open-source library for the computational modeling of mass-transport phenomena in SOFCs is presented in this article. It includes several multicomponent mass-transport models (ie Fickian, Stefan-Maxwell and Dusty Gas Model), which can be applied both within porous media and in porosity-free domains, and several diffusivity models for gases. The library has been developed for its use with OpenFOAM(R), a widespread open-source code for fluid and continuum mechanics. The library can be used to model any fluid flow configuration involving multi-component transport phenomena and it is validated in this paper against the analytical solution of one-dimensional test cases. In addition, it is applied for the simulation of a real SOFC and further validated using experimental dat

Spectroscopic investigation of quantum confinement effects in ion implanted silicon-on-sapphire films

Crystalline Silicon-on-Sapphire (SOS) films were implanted with boron (B$^+$) and phosphorous (P$^+$) ions. Different samples, prepared by varying the ion dose in the range $10^{14}$ to 5 x $10^{15}$ and ion energy in the range 150-350 keV, were investigated by the Raman spectroscopy, photoluminescence (PL) spectroscopy and glancing angle x-ray diffraction (GAXRD). The Raman results from dose dependent B$^+$ implanted samples show red-shifted and asymmetrically broadened Raman line-shape for B$^+$ dose greater than $10^{14}$ ions cm$^{-2}$. The asymmetry and red shift in the Raman line-shape is explained in terms of quantum confinement of phonons in silicon nanostructures formed as a result of ion implantation. PL spectra shows size dependent visible luminescence at $\sim$ 1.9 eV at room temperature, which confirms the presence of silicon nanostructures. Raman studies on P$^+$ implanted samples were also done as a function of ion energy. The Raman results show an amorphous top SOS surface for sample implanted with 150 keV P$^+$ ions of dose 5 x $10^{15}$ ions cm$^{-2}$. The nanostructures are formed when the P$^+$ energy is increased to 350 keV by keeping the ion dose fixed. The GAXRD results show consistency with the Raman results.Comment: 9 Pages, 6 Figures and 1 Table, \LaTex format To appear in SILICON(SPRINGER

Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4

An extensive theoretical study is performed for wide bandgap crystalline oxides and nitrides, namely, SiO_{2}, GeO_{2}, Al_{2}O_{3}, Si_{3}N_{4}, and Ge_{3}N_{4}. Their important polymorphs are considered which are for SiO_{2}: $\alpha$-quartz, $\alpha$- and $\beta$-cristobalite and stishovite, for GeO_{2}: $\alpha$-quartz, and rutile, for Al_{2}O_{3}: $\alpha$-phase, for Si_{3}N_{4} and Ge_{3}N_{4}: $\alpha$- and $\beta$-phases. This work constitutes a comprehensive account of both electronic structure and the elastic properties of these important insulating oxides and nitrides obtained with high accuracy based on density functional theory within the local density approximation. Two different norm-conserving \textit{ab initio} pseudopotentials have been tested which agree in all respects with the only exception arising for the elastic properties of rutile GeO_{2}. The agreement with experimental values, when available, are seen to be highly satisfactory. The uniformity and the well convergence of this approach enables an unbiased assessment of important physical parameters within each material and among different insulating oxide and nitrides. The computed static electric susceptibilities are observed to display a strong correlation with their mass densities. There is a marked discrepancy between the considered oxides and nitrides with the latter having sudden increase of density of states away from the respective band edges. This is expected to give rise to excessive carrier scattering which can practically preclude bulk impact ionization process in Si_{3}N_{4} and Ge_{3}N_{4}.Comment: Published version, 10 pages, 8 figure

Enhanced performance of counter flow SOFC with partial internal reformation

We study a counter-flow solid oxide fuel cell system and consider the challenges faced in minimizing thermal variations from the nominal operating conditions for a reasonable range of power tracking. Blower dynamics, reformer transport delays, spatial distribution of the heat generated and the resulting thermal response are among the issues considered. A novel approach, relying on partial internal reformation of the feedstock is proposed as a remedy to maintain a strong level of power tracking with minimal thermal stress to the fuel cell

Sio2 içinde yarıiletken nanokristallerin iyon ekme tekniği ile oluşturulması

In this study, we used ion implantation technique to synthesize semiconductor (Ge, Si) nanocrystals in SiO2 matrix. Ge and Si nanocrystals have been successfully formed by Ge and Si implantation and post annealing. Implanted samples were examined by characterization techniques such as TEM, XPS, EDS, SAD, SIMS, PL, Raman and FTIR spectroscopy and the presence of Ge and Si nanocrystals in the SiO2 matrix has been evidenced by these measurements. It was shown that implantation dose, implantation energy, annealing temperature, annealing time and annealing ambient are important parameters for the formation and evolution of semiconductor nanocrystals embedded in SiO2 matrix. The size and size distribution of Ge and Si nanocrystals were estimated successfully by fitting Raman and PL spectra obtained from Ge and Si implanted samples, respectively. It was demonstrated that Si implanted and post annealed samples exhibit two broad PL peaks at ~ 625 and 850 nm, even at room temperature. Origin of these peaks was investigated by temperature, excitation power and excitation wavelength dependence of PL spectrum and etch-measure experiments and it was shown that the peak observed at ~ 625 nm is related with defects (clusters or chain of Si located near the surface) while the other is related to the Si nanocrystals. As an expected effect of quantum size phenomenon, the peak observed at ~ 850 nm was found to depend on the nanocrystal size. Finally, the formation and evolution of Ge and Si nanocrystals were monitored by FTIR spectroscopy and it was shown that the deformation in SiO2 matrix caused by ion implantation tends to recover itself much quicker in the case of the Ge implantation. This is a result of effective segregation of Ge atoms at relatively low temperatures.Ph.D. - Doctoral Progra