Microfabrication methods to improve the kinetics of the yttria stabilized zirconia -- platinum -- oxygen electrode

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 183-194).Solid oxide fuel cells are a potential electrical power source that is silent, efficient, modular, and capable of operating on a wide variety of fuels. Unfortunately, current technologies are severely limited in that they provide sufficient power output only at very high temperatures (>800°C). One reason for this is because the electrodes have very poor (and poorly understood) kinetics. The work described in this dissertation involves the microfabrication of model systems with triple phase boundary lengths that varied over an order of magnitude to systematically quantify and ultimately improve the kinetics of platinum electrodes on the surface of yttria stabilized zirconia electrolytes. Platinum electrodes with well controlled geometry were sputtered onto the surface of bulk YSZ and onto sputtered YSZ thin films. An unexpected result was found whereby YSZ films of composition Y0.09Zr0.91O2-x had an ionic conductivity remarkably enhanced by a factor of 20-30. This is attributed to the films exhibiting nanometric grain sizes and thereby stabilizing the cubic morphology at considerably lower yttrium levels than is normally needed. This metastable cubic phase is suspected of having reduced defect ordering.(cont.) Grain boundary resistance, which in YSZ is normally due to impurities that segregate and block ionic transfer, was found to also be significantly reduced in YSZ films. The films had a specific grain boundary conductivity enhanced by a factor of 30-100 compared to the bulk polycrystalline sample. This was believed to be due to the very low impurity content of the film grain boundaries. Concerning the electrode polarization resistance, it was found that the electrodes placed on bulk standards and films deposited at high temperatures were on par with the best electrode conductance values from the literature. However, when the electrolyte surface was a film deposited at reduced temperature, the resistance decreased further by a factor of 300-500. The cause of this was revealed to be silicon contamination on the surfaces of the poorer-performing electrolytes. Triple phase boundary length-specific resistances as low as 3.7·104 O·cm at 378°C and 4.0·107 O·cm at 215°C were measured; these appear to be the lowest ever recorded. The measurements are possibly the first electrochemical characterization of nearly silicon-free YSZ surfaces. This study emphasizes the key role of chemical purity at the electrode-electrolyte interface.(cont.) Photolithography alone is unlikely to give technologically useful triple phase boundary lengths. In an attempt to achieve the triple phase boundary lengths needed for a practical device, reactive co-sputtering was used to produce composite Pt-YSZ thin films with a bi-continuous network morphology and grain sizes on the order of 30 nm. Such intimate mixing of the electronic and ionic conducting phases created an effective mixed ionic-electronic conductor with the entire surface of the film electrochemically active to the electrode reaction. The best processing conditions resulted in electrodes with an area specific polarization resistance less than 500 O·cm2 at 400°C and, by extrapolation, 10 O·cm2 at 511°C and 1 O·cm2 at 608°C. These films may enable operation of a micro-solid oxide fuel cell at intermediate temperatures (400-500°C), and perhaps even lower temperatures with further microstructural optimization.by Joshua L. Hertz.Ph.D

Genome sequence of the tsetse fly (Glossina morsitans):Vector of African trypanosomiasis

Tsetse flies are the sole vectors of human African trypanosomiasis throughout sub-Saharan Africa. Both sexes of adult tsetse feed exclusively on blood and contribute to disease transmission. Notable differences between tsetse and other disease vectors include obligate microbial symbioses, viviparous reproduction, and lactation. Here, we describe the sequence and annotation of the 366-megabase Glossina morsitans morsitans genome. Analysis of the genome and the 12,308 predicted protein-encoding genes led to multiple discoveries, including chromosomal integrations of bacterial (Wolbachia) genome sequences, a family of lactation-specific proteins, reduced complement of host pathogen recognition proteins, and reduced olfaction/chemosensory associated genes. These genome data provide a foundation for research into trypanosomiasis prevention and yield important insights with broad implications for multiple aspects of tsetse biology.IS

A tutorial introduction to Bayesian models of cognitive development

We present an introduction to Bayesian inference as it is used in probabilistic models of cognitive development. Our goal is to provide an intuitive and accessible guide to the what, the how, and the why of the Bayesian approach: what sorts of problems and data the framework is most relevant for, and how and why it may be useful for developmentalists. We emphasize a qualitative understanding of Bayesian inference, but also include information about additional resources for those interested in the cognitive science applications, mathematical foundations, or machine learning details in more depth. In addition, we discuss some important interpretation issues that often arise when evaluating Bayesian models in cognitive science.Amy Perfors, Joshua B. Tenenbaum, Thomas L. Griffiths, Fei X

Retrotransposons Are the Major Contributors to the Expansion of the Drosophila ananassae Muller F Element.

The discordance between genome size and the complexity of eukaryotes can partly be attributed to differences in repeat density. The Muller F element (∼5.2 Mb) is the smallest chromosome in Drosophila melanogaster, but it is substantially larger (>18.7 Mb) in D. ananassae To identify the major contributors to the expansion of the F element and to assess their impact, we improved the genome sequence and annotated the genes in a 1.4-Mb region of the D. ananassae F element, and a 1.7-Mb region from the D element for comparison. We find that transposons (particularly LTR and LINE retrotransposons) are major contributors to this expansion (78.6%), while Wolbachia sequences integrated into the D. ananassae genome are minor contributors (0.02%). Both D. melanogaster and D. ananassae F-element genes exhibit distinct characteristics compared to D-element genes (e.g., larger coding spans, larger introns, more coding exons, and lower codon bias), but these differences are exaggerated in D. ananassae Compared to D. melanogaster, the codon bias observed in D. ananassae F-element genes can primarily be attributed to mutational biases instead of selection. The 5' ends of F-element genes in both species are enriched in dimethylation of lysine 4 on histone 3 (H3K4me2), while the coding spans are enriched in H3K9me2. Despite differences in repeat density and gene characteristics, D. ananassae F-element genes show a similar range of expression levels compared to genes in euchromatic domains. This study improves our understanding of how transposons can affect genome size and how genes can function within highly repetitive domains

Retrotransposons Are the Major Contributors to the Expansion of the Drosophila ananassae Muller F Element

The discordance between genome size and the complexity of eukaryotes can partly be attributed to differences in repeat density. The Muller F element (∼5.2 Mb) is the smallest chromosome in Drosophila melanogaster, but it is substantially larger (>18.7 Mb) in D. ananassae. To identify the major contributors to the expansion of the F element and to assess their impact, we improved the genome sequence and annotated the genes in a 1.4-Mb region of the D. ananassae F element, and a 1.7-Mb region from the D element for comparison. We find that transposons (particularly LTR and LINE retrotransposons) are major contributors to this expansion (78.6%), while Wolbachia sequences integrated into the D. ananassae genome are minor contributors (0.02%). Both D. melanogaster and D. ananassae F-element genes exhibit distinct characteristics compared to D-element genes (e.g., larger coding spans, larger introns, more coding exons, and lower codon bias), but these differences are exaggerated in D. ananassae. Compared to D. melanogaster, the codon bias observed in D. ananassae F-element genes can primarily be attributed to mutational biases instead of selection. The 5′ ends of F-element genes in both species are enriched in dimethylation of lysine 4 on histone 3 (H3K4me2), while the coding spans are enriched in H3K9me2. Despite differences in repeat density and gene characteristics, D. ananassae F-element genes show a similar range of expression levels compared to genes in euchromatic domains. This study improves our understanding of how transposons can affect genome size and how genes can function within highly repetitive domains