A transient PEMFC model with CO poisoning and mitigation by O2 bleeding and Ru-containing catalyst

In this paper we present a transient, fully two-phase, non-isothermal model of carbon monoxide poisoning and oxygen bleeding in the membraneelectrode assembly of a polymer electrolyte fuel cell. The model includes a detailed description of mass, heat and charge transport, chemisorption,electrochemical oxidation and heterogeneous catalysis (when oxygen is introduced). Example simulation results demonstrate the ability of themodel to qualitatively capture the fundamental features of the poisoning process and the extent of poisoning with respect to channel temperatureand concentration. Further examples show how the multi-step kinetics can interact with other physical phenomena such as liquid-water flooding,particularly in the anode. Carbon monoxide pulsing is simulated to demonstrate that the complicated reaction kinetics of oxygen bleeding canbe captured and even predicted. It is shown that variations in the channel temperature have a convoluted effect on bleeding, and that trends inperformance on relatively short time scales can be the precise opposite of the trends observed at steady state. We incorporate a bi-functionalmechanism for carbon monoxide oxidation on platinum–ruthenium catalysts, demonstrating the marked reduction in the extent of poisoning, theeffect of variations in the platinum–ruthenium ratio and the influence of temperature. Finally, we discuss the implications of the results, extensionsto the model and possible avenues for experimental work

Formal Enforcement of Security Policies : An Algebraic Approach

La sécurité des systèmes d’information est l’une des préoccupations les plus importantes du domaine de la science informatique d’aujourd’hui. Les particuliers et les entreprises sont de plus en plus touchés par des failles de sécurité et des milliards de dollars ont été perdus en raison de cyberattaques. Cette thèse présente une approche formelle basée sur la réécriture de programmes permettant d’appliquer automatiquement des politiques de sécurité sur des programmes non sécuritaires. Pour un programme P et une politique de sécurité Q, nous générons un autre programme P’ qui respecte une politique de sécurité Q et qui se comporte comme P, sauf si la politique est sur le point d’être violée. L’approche présentée utilise l’algèbre [symbol] qui est une variante de [symbol] (Basic Process Algebra) étendue avec des variables, des environnements et des conditions pour formaliser et résoudre le problème. Le problème de trouver la version sécuritaire P’ à partir de P et de Q se transforme en un problème de résolution d’un système linéaire pour lequel nous savons déjà comment extraire la solution par un algorithme polynomial. Cette thèse présente progressivement notre approche en montrant comment la solution évolue lorsqu’on passe de l’algèbre de [symbol] à [symbol].The security of information systems is one of the most important preoccupations of today’s computer science field. Individuals and companies are more and more affected by security flaws and billions of dollars have been lost because of cyber-attacks. This thesis introduces a formal program-rewriting approach that can automatically enforce security policies on non-trusted programs. For a program P and a security policy Q, we generate another program P’ that respects the security policy Q and behaves like P except when the enforced security policy is about to be violated. The presented approach uses the [symbol] algebra that is a variant of the BPA (Basic Process Algebra) algebra extended with variables, environments and conditions to formalize and resolve the problem. The problem of computing the expected enforced program [symbol] is transformed to a problem of resolving a linear system for which we already know how to extract the solution by a polynomial algorithm. This thesis presents our approach progressively and shows how the solution evolves when we move from the [symbol] algebra to the [symbol] algebra

Statistical physics of isotropic-genesis nematic elastomers: I. Structure and correlations at high temperatures

Isotropic-genesis nematic elastomers (IGNEs) are liquid crystalline polymers (LCPs) that have been randomly, permanently cross-linked in the high-temperature state so as to form an equilibrium random solid. Thus, instead of being free to diffuse throughout the entire volume, as they would be in the liquid state, the constituent LCPs in an IGNE are mobile only over a finite length-scale controlled by the density of cross-links. We address the effects that such network-induced localization have on the liquid-crystalline characteristics of an IGNE, as probed via measurements made at high temperatures. In contrast with the case of uncross-linked LCPs, for IGNEs these characteristics are determined not only by thermal fluctuations but also by the quenched disorder associated with the cross-link constraints. To study IGNEs, we consider a microscopic model of dimer nematogens in which the dimers interact via orientation-dependent excluded volume forces. The dimers are, furthermore, randomly, permanently cross-linked via short Hookean springs, the statistics of which we model by means of a Deam-Edwards type of distribution. We show that at length-scales larger than the size of the nematogens this approach leads to a recently proposed phenomenological Landau theory of IGNEs [Lu et al., Phys. Rev. Lett. 108, 257803 (2012)], and hence predicts a regime of short-ranged oscillatory spatial correlations in the nematic alignment, of both thermal and glassy types. In addition, we consider two alternative microscopic models of IGNEs: (i) a wormlike chain model of IGNEs that are formed via the cross-linking of side-chain LCPs; and (ii) a jointed chain model of IGNEs that are formed via the cross-linking of main-chain LCPs. At large length-scales, both of these models give rise to liquid-crystalline characteristics that are qualitatively in line with those predicted by the dimer-and-springs model.Comment: 33 pages, 6 figures, 6 appendice

Time Dependent Saddle Node Bifurcation: Breaking Time and the Point of No Return in a Non-Autonomous Model of Critical Transitions

There is a growing awareness that catastrophic phenomena in biology and medicine can be mathematically represented in terms of saddle-node bifurcations. In particular, the term `tipping', or critical transition has in recent years entered the discourse of the general public in relation to ecology, medicine, and public health. The saddle-node bifurcation and its associated theory of catastrophe as put forth by Thom and Zeeman has seen applications in a wide range of fields including molecular biophysics, mesoscopic physics, and climate science. In this paper, we investigate a simple model of a non-autonomous system with a time-dependent parameter $p(\tau)$ and its corresponding `dynamic' (time-dependent) saddle-node bifurcation by the modern theory of non-autonomous dynamical systems. We show that the actual point of no return for a system undergoing tipping can be significantly delayed in comparison to the {\em breaking time} $\hat{\tau}$ at which the corresponding autonomous system with a time-independent parameter $p_{a}= p(\hat{\tau})$ undergoes a bifurcation. A dimensionless parameter $\alpha=\lambda p_0^3V^{-2}$ is introduced, in which $\lambda$ is the curvature of the autonomous saddle-node bifurcation according to parameter $p(\tau)$, which has an initial value of $p_{0}$ and a constant rate of change $V$. We find that the breaking time $\hat{\tau}$ is always less than the actual point of no return $\tau^*$ after which the critical transition is irreversible; specifically, the relation $\tau^*-\hat{\tau}\simeq 2.338(\lambda V)^{-\frac{1}{3}}$ is analytically obtained. For a system with a small $\lambda V$, there exists a significant window of opportunity $(\hat{\tau},\tau^*)$ during which rapid reversal of the environment can save the system from catastrophe