A mathematical model for mechanically-induced deterioration of the binder in lithium-ion electrodes

This study is concerned with modeling detrimental deformations of the binder phase within lithium-ion batteries that occur during cell assembly and usage. A two-dimensional poroviscoelastic model for the mechanical behavior of porous electrodes is formulated and posed on a geometry corresponding to a thin rectangular electrode, with a regular square array of microscopic circular electrode particles, stuck to a rigid base formed by the current collector. Deformation is forced both by (i) electrolyte absorption driven binder swelling, and; (ii) cyclic growth and shrinkage of electrode particles as the battery is charged and discharged. The governing equations are upscaled in order to obtain macroscopic effective-medium equations. A solution to these equations is obtained, in the asymptotic limit that the height of the rectangular electrode is much smaller than its width, that shows the macroscopic deformation is one-dimensional. The confinement of macroscopic deformations to one dimension is used to obtain boundary conditions on the microscopic problem for the deformations in a 'unit cell' centered on a single electrode particle. The resulting microscale problem is solved using numerical (finite element) techniques. The two different forcing mechanisms are found to cause distinctly different patterns of deformation within the microstructure. Swelling of the binder induces stresses that tend to lead to binder delamination from the electrode particle surfaces in a direction parallel to the current collector, whilst cycling causes stresses that tend to lead to delamination orthogonal to that caused by swelling. The differences between the cycling-induced damage in both: (i) anodes and cathodes, and; (ii) fast and slow cycling are discussed. Finally, the model predictions are compared to microscopy images of nickel manganese cobalt oxide cathodes and a qualitative agreement is found.Comment: 25 pages, 11 figure

Letters to the Editor

Letter responding to an article predicting the phaseout of government information librarians in a survey of Association of Research Library (ARL) Directors. We criticize this conclusion maintaining that the multifaceted and interdisciplinary nature of government information makes it more critical that libraries users needing government information resources need to have highly trained and specializing librarians with extensive knowledge of these resources assisting them

Gas Concentration Measurements in Underground Waste Storage Tanks

Currently over 100 underground tanks at the Hanford facility in eastern Washington state are being used to store high-level radioactive waste. With plans for a long-term nuclear-waste repository in Nevada in place (though not yet approved), one promising use for these underground storage tanks is as a temporary waystation for waste destined for the Nevada repository. However, without a reasonable understanding of the chemical reactions going on within the tanks, transporting waste in and out of the tanks has been deemed to be unsafe. One hazard associated with such storage mechanisms is explosion of flammable gases produced within the tank. Within many of the storage tanks is a sludge layer. This layer, which is a mixture of liquid and solids, contains most of the radioactive material. Radioactive decay and its associated heat can produce several flammable materials within this layer. Two components of particular concern are hydrogen (H2) and nitrous oxide (N2O), since they are highly volatile in the gaseous phase. Though the tanks have either forced or natural convection systems to vent these gases, the possibility of an explosion still exists. Measurements of these gases are taken in several ways. Continuous measurements are taken in the headspace, which is the layer between the tank ceiling and the liquid (supernatant) or sludge layer below. In tanks where a supernatant layer sits atop the sludge layer, there are often rollovers or gas release events (GREs), where a large chunk of sludge, after attaining a certain void fraction, becomes buoyant, rising through the supernatant and releasing its associated gas composition to the headspace. Such changes trigger a sensor, and thus measurements are also taken at that time. Lastly, a retained gas sample (RGS) can be taken from either the supernatant or sludge layer. Such a core sample is quite expensive, but can yield crucial data about the way gases are being produced in the sludge and convected through the supernatant. Unfortunately, the measurements from these three populations do not seem to match. In particular, the ratio r = [N2O]/[H2] varies from population to population. r also varies from tank to tank, but this can more readily be explained in terms of the waste composition of each tank. Since H2 is more volatile than N2O (and since there are more sources of oxygen in the headspace), lower values of r correspond to more hazardous situations. This variance in r is troubling, since we need to be able to explain why certain values of r are lower (and hence more dangerous) in certain areas of the tank. In this report we examine the data from three tanks. We first verify that the differences in r among populations is significant. We then postulate several mechanisms which could explain such a difference

Derivation of a macroscopic model for Brownian hard needles

We study the role of anisotropic steric interactions in a system of hard Brownian needles. Despite having no volume, non-overlapping needles exclude a volume in configuration space that influences the macroscopic evolution of the system. Starting from the stochastic particle system, we use the method of matched asymptotic expansions and conformal mapping to systematically derive a nonlinear nonlocal partial differential equation for the evolution of the population density in position and orientation. We consider the regime of high rotational diffusion, resulting in an equation for the spatial density that allows us to compare the effective excluded volume of a hard-needles system with that of a hard-spheres system. We further consider spatially homogeneous solutions and find an isotropic to nematic transition as density increases, consistent with Onsager's theory

Evaluation of the CRACK approach for the control of drench resistance in sheep worms

The Department of Agriculture launched the CRACK approach to worm control in September 1985 in response to finding that anthelmintic (drench) resistant worms were present on 68 per cent of a random selection of Western Australian sheep farms. The high cost of internal parasites in terms of lost production and drench costs, together with the high prevalence of resistant worms of farms, confirmed that changes were needed to existing parasite control measures in sheep.

FIRST GRADERS’ PREFERENCES FOR NARRATIVE AND/OR INFORMATION BOOKS AND PERCEPTIONS OF OTHER BOYS’ AND GIRLS’ BOOK PREFERENCES

In this article, we report on grade‐one children’s preferences for narrative and/or information books, and their perceptions of what boys and girls like to read. Data include responses on two book preference tasks by 40 children in four schools. Children chose books and explained the reasons for their choices. One task was a closed, force‐choice task, the other, an open‐ended task. Boys and girls had similar interests, either preferring stories or liking information books and stories to the same degree. Yet boys and girls perceived that boys prefer information texts and girls prefer narratives. The children’s perceptions reflect gendered stereotypes. Key words: literacy, reading, motivation, genre, gender Dans cet article, les auteurs signalent que les élèves de 1re année préfèrent les livres qui racontent des histoires ou donnent de l’information et présentent ce que, selon de ces élèves, les garçons et les filles aiment lire. Les données comprennent les réponses de 40 enfants dans quatre écoles à deux questionnaires, l’un à réponses libres et l’autre à choix multiples, sur les préférences en matière de livres. Les enfants ont choisi des livres et donné les raisons de leur choix. Les garçons et les filles avaient des intérêts similaires, préférant soit les histoires, soit les livres d’information et les histoires au même degré. Et pourtant, les garçons comme les filles avaient l’impression que les garçons aimaient mieux les livres d’information et les filles, les histoires. Les perceptions des enfants reflètent les stéréotypes marqués par le sexe. Mots clés : littératie, lecture, motivation, genre