Contact Resistance Evolution and Degradation of Highly Cycled Micro-Contacts

Reliable microelectromechanical systems (MEMS) switches are critical for developing high performance radio frequency circuits like phase shifters. Engineers have attempted to improve reliability and lifecycle performance using novel contact metals, unique mechanical designs and packaging. Various test fixtures including: MEMS devices, atomic force microscopes (AFM) and nanoindentors have been used to collect resistance and contact force data. AFM and nanoindentor test fixtures allow direct contact force measurements but are severely limited by low resonance sensors, and therefore low data collection rates. This thesis reports the contact resistance evolution results and fabrication of thin film micro-contacts dynamically tested up to 3kHz. The contacts consisted of a lower contact of evaporated Au and a thin film upper contact, consisting of sputtered Au, Ru or RuO2, with an Au electroplated structural layer. The fixed-fixed beam was designed with sufficient restoring force to overcome adhesion. The hemisphere-upper and planar-lower contacts are mated with a calibrated, external load resulting in approximately 200muN of contact force and are cycled in excess of 10 to the 7th power times or until failure. In addition, Au-Au contact pairs with a hemispherical upper an engineered lower contact were tested. These lower engineered contacts were constructed using gray-scale lithography. Contact resistance was measured, in situ, using Holm\u27s a cross-bar configuration and the entire apparatus was isolated from external vibration and housed in an enclosure to minimize contamination due to the ambient environment. Additionally, contact cycling and data collection are automated using a computer, integrated lab equipment and LabVIEW. Results include contact resistance measurements of Au, Ru and RuO2 samples and lifetime testing up to 323.6 million cycles

A critical analysis of skull osteology in Australian Agamidae with implications for the fossil record

The Australian agamid radiation was long noted for homogeneity of morphological characters and remains one of the most poorly studied major clades of squamates. Anatomical studies of the various lineages of endemic Australian agamids lag far behind genetic studies and would greatly enhance our knowledge of the current status and evolution of this complicated and dynamic clade. Australian agamids are an ideal group with which to test hypotheses of squamate speciation, diversity, and disparity because of their relatively recent diversification into Australia, about 30 Million Years Ago and geographical constraints. The two chapters of this thesis address two aspects of my research. In Chapter 1, I compile and evaluate morphological characters previously suggested to be useful for identifying Agamidae. I evaluate these characters for intraspecific variation, ontogenetic influence, and sexual dimorphism, for which sample sizes approached 20: Ctenophorus caudicinctus, Ctenophorus isolepis, and Ctenophorus reticulatus. Within the invariant characters, six were invariant only in Ctenophorus caudincinctus, five were invariant only for Ctenophorus isolepis, and two were invariant only in Ctenophorus reticulatus. Morphological characters that varied within taxa (thus, excluding all invariant morphological characters) were statistically tested for covariance with ontogeny (using skull length as a proxy) and sexual dimorphism. Sixteen of the 60 characters measured varied for at least one taxon with ontogeny and nine varied with sex. In Chapter 2, I use X-ray computed tomography (CT) of two specimens to describe the skull of the Australian agamid Cryptagama aurita, a species known only from only four alcohol-preserved specimens. Cryptagama aurita appears to share a great number of skull characters with other desert-dwelling Australian agamids. I conclude that the information currently available for Australian Agamidae is inadequate to interpret the fossil record of Australian Agamidae. Any identifications and phylogenetic analyses are likely to be inaccurate because most of the characters proposed by previous authors are not demonstrably apomorphic. Ontogenetic age is an important source of variation for the three species of Ctenophorous I examined. Morphological variation of extant taxa must be understood in order to compare extant and extinct species to study the recent radiation of Australian Agamidae.Geological Science

Podcasts Episode 02: Greg Stilson

At the time of this podcast, Mr. Greg Stilson was a Product Director at Humanware and since has become a Product Director at Aira. In this podcast, Mr. Stilson, who happens to be blind, reflects on the advances in orientation aids and the ways that these technologies complement older technologies, such as canes and tactile maps. Image credit: LinkedIn Greg Stilson profilehttps://pdxscholar.library.pdx.edu/sped_podcast/1001/thumbnail.jp

Fabrication of 3D surface structures using grayscale lithography

The ability to design and develop 3D microstructures is important for microelectromechanical systems (MEMS) fabrication. Previous techniques used to create 3D devices included tedious steps in direct writing and aligning patterns onto a substrate followed by multiple photolithography steps using expensive, customized equipment. Additionally, these techniques restricted batch processing and placed limits on achievable shapes. Gray-scale lithography enables the fabrication of a variety of shapes using a single photolithography step followed by reactive ion etching (RIE). Micromachining 3D silicon structures for MEMS can be accomplished using gray-scale lithography along with dry anisotropic etching. In this study, we investigated: using MATLAB for mask designs; feasibility of using 1 μm Heidelberg mask maker to direct write patterns onto photoresist; using RIE processing to etch patterns into a silicon substrate; and the ability to tailor etch selectivity for precise fabrication. To determine etch rates and to obtain desired etch selectivity, parameters such as gas mixture, gas flow, and electrode power were studied. This process successfully demonstrates the ability to use gray-scale lithography and RIE for use in the study of micro-contacts. These results were used to produce a known engineered non-planer surface for testing micro-contacts. Surface structures are between 5 μm and 20 μm wide with varying depths and slopes based on mask design and etch rate selectivity. The engineered surfaces will provide more insight into contact geometries and failure modes of fixed-fixed micro-contacts

A Study on the relationship between presidential term limits and the doctrine of separation of powers in Uganda

Thesis(Master) --KDI School:Master of Development Policy,2013masterpublishedStilson Muhwezi

Skeletal pathology and variable anatomy in elephant feet assessed using computed tomography

Foot problems are a major cause of morbidity and mortality in elephants, but are underreported due to difficulties in diagnosis, particularly of conditions affecting the bones and internal structures. Here we evaluate post-mortem computer tomographic (CT) scans of 52 feet from 21 elephants (seven African Loxodonta africana and 14 Asian Elephas maximus), describing both pathology and variant anatomy (including the appearance of phalangeal and sesamoid bones) that could be mistaken for disease. We found all the elephants in our study to have pathology of some type in at least one foot. The most common pathological changes observed were bone remodelling, enthesopathy, osseous cyst-like lesions, and osteoarthritis, with soft tissue mineralisation, osteitis, infectious osteoarthriti, subluxation, fracture and enostoses observed less frequently. Most feet had multiple categories of pathological change (81% with two or more diagnoses, versus 10% with a single diagnosis, and 9% without significant pathology). Much of the pathological change was focused over the middle/lateral digits, which bear most weight and experience high peak pressures during walking. We found remodelling and osteoarthritis to be correlated with increasing age, more enthesopathy in Asian elephants, and more cyst-like lesions in females. We also observed multipartite, missing and misshapen phalanges as common and apparently incidental findings. The proximal (paired) sesamoids can appear fused or absent, and the predigits (radial/tibial sesamoids) can be variably ossified, though are significantly more ossified in Asian elephants. Our study reinforces the need for regular examination and radiography of elephant feet to monitor for pathology and as a tool for improving welfare