Variable reflectance vertical cavity surface emitting lasers

Singlemode operation in 850 nm vertical cavity surface emitting lasers is reported. The singlemode behaviour results from a radially-varying number of periods in the top distributed Bragg reflector mirror. At the centre of the device there are six additional quarter-wave layers (three periods) as compared to the edge. This provides higher mirror reflectivity to the fundamental mode without adding loss. Thus the top mirror geometry promotes singlemode operation of the laser from threshold to rollover     

Glass transition phenomena in melt‐processed polystyrene/polypropylene blends

Blends of an amorphous and a semi‐crystalline polymer—polystyrene and polypropylene, respectively—were prepared by melt processing in an extruder at 220°C. These polymers are known to be immiscible and the composite morphologies were characterized by electron microscopy and thermal analysis. Fine micron‐scale morphologies, ranging from 0.5 to 20 microns were observed. Thermal analysis and dynamic mechanical analysis showed changes in both the polystyrene and polypropylene glass transition temperatures (T_g) over the composition range. The major effect was a sharp increase in polystyrene T_g with increasing polypropylene content in the blend. A T_g elevation of 5.5°C was observed at 85% polypropylene. The polypropylene T_g also increases with increasing polypropylene content, starting at a depressed value in discrete polypropylene domain environments and approaching the bulk polypropylene value after the phase inversion is crossed. Qualitative structural models are proposed based on spatial and mechanical interactions between the components. POLYM. ENG. SCI., 45:1187–1193, 2005. © 2005 Society of Plastics Engineers     

Vibrational control of nonlinear time lag systems with boundeddelay: averaging theory, stabilizability, and transient behavior

This paper develops the theory of vibrational control of nonlinear time lag systems with arbitrarily large but bounded delay. Averaging theory for fast oscillating, differential delay equations is presented and then applied to vibrational control. Conditions are given which ensure the existence of parametric vibrations that stabilize nonlinear time lag systems. Transient behavior is also discussed. Illustrative examples are given which show 1) the feasibility of the theory to important applications and 2) the differences in the theory presented and the existing known theory for vibrational control of ordinary differential equations     

Historical and recent biomass and food web relations of Limnocalanus in Lake Huron

Reported population biomass of Limnocalanus macrurus in Lake Huron has been underestimated by several-fold owing to application of an inappropriate length–weight regression model. During August and September the underestimates can exceed three-fold, suggesting that secondary production of the species in the lake is greater than previously thought. Increased representation of the species in the plankton community in recent years is associated with a decrease in its trophic level deduced through nitrogen stable isotope analysis of specimens from opportunistic samples obtained in 1993, 1995, and 2009. Limnocalanus¹⁵N enrichment relative to primary herbivores appears to have decreased by 1.5‰ over this time period. Similarly, Diaptomus¹⁵N enrichment relative to primary herbivores appeared to decrease by 1.1‰ from 1993 to 2009. Our data offer at least two possible explanations for this shift. (1) Limnocalanus has adopted a more herbivorous diet in response to the ecological changes in Lake Huron or (2) hypolimnetic Diaptomus have adopted a more herbivorous diet and Limnocalanus is relying in part on Diaptomus as a food source.     

Influence of Sn Grain Size and Orientation on the Thermomechanical Response and Reliability of Pb-free Solder Joints

The size and crystal orientation of Sn grains in Pb-free, near eutectic Sn-Ag-Cu solder joints were examined. A clear dependence of the thermomechanical fatigue response of these solder joints on Sn grain orientation was observed (Sn has a body centered tetragonal crystal structure). Fabricated joints tend to have three orientations in a cyclic twin relationship, but among the population of solder balls, this orientation triplet appears to be randomly oriented. In thermally cycled joints, solder balls with dominant Sn grains having the particular orientation with the c-axis nearly parallel to the plane of the substrate were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn in the basal plane (along the alpha-axis) is half the value along the c-axis; joints observed to be damaged had the maximum coefficient of thermal expansion mismatch between solder and substrate at the joint interface, as well as a tensile stress modes during the hot part of the thermal cycle. Localized recrystallization was observed in regions of maximum strain caused by differential expansion conditions, and its connection with crack nucleation is discussed.     

A very large protein with diverse functional motifs is deficient in rjs (runty, jerky, sterile) mice

Three radiation-induced alleles of the mouse p locus, p6H, p25H, and pbs, cause defects in growth, coordination, fertility, and maternal behavior in addition to p gene-related hypopigmentation. These alleles are associated with disruption of the p gene plus an adjacent gene involved in the disorders listed. We have identified this adjacent gene, previously named rjs (runty jerky sterile), by positional cloning. The rjs cDNA is very large, covering 15,264 nucleotides. The predicted rjs-encoded protein (4,836 amino acids) contains several sequence motifs, including three RCC1 repeats, a structural motif in common with cytochrome b5, and a HECT domain in common with E6-AP ubiquitin ligase. On the basis of sequence homology and conserved synteny, the rjs gene is the single mouse homolog of a previously described five- or six-member human gene family. This family is represented by at least two genes, HSC7541 and KIAA0393, from human chromosome 15q11-q13. HSC7541 and KIAA0393 lie close to, or within, a region commonly deleted in most Prader-Willi syndrome patients. Previous work has suggested that the multiple phenotypes in rjs mice might be due to a common neuroendocrine defect. In addition to this proposed mode of action, alternative functions of the rjs gene are evaluated in light of its known protein homologies.     

Isothermal Fatigue Behavior of the Near-Eutectic Sn-Ag-Cu Alloy between −25°C and 125°C

When near-eutectic Sn-Ag-Cu (SAC) alloys are used to make soldered ball-grid-array (BGA) assemblies, the grain size of the joints is very large. During thermomechanical cycling, the solder joint fatigue process is often initiated with recrystallization of the Sn grains, resulting in a smaller grain size in the deformed areas. Grain boundary sliding and increased grain boundary diffusion then results in intergranular crack nucleation and propagation along the recrystallized Sn grain boundaries. In this work, fatigue tests were used to study the initial stages of cyclic deformation damage in Sn-Ag-Cu alloy samples. To separate the solder properties from the constraints introduced by the substrate, the tests were done with free-standing solder specimens, instead of solder joints. The test samples were cast dog-bone specimens that have a cross-sectional diameter of 1 mm, which corresponds to a typical solder joint diameter in BGAs. Mechanical cycling was performed isothermally at several temperatures, from −25°C to 125°C. Typical test conditions were ± 1.5% strain and 15-minute hold at tensile peak stress to allow stress relaxation to take place. Optical microscopy, scanning electron microscopy, and electron backscattering diffraction were used to study the microstructures of the samples before and after fatigue testing in order to obtain insight into the nucleation and growth mechanisms of fatigue damage.     

Learning considered within a cultural context: Confucian and Socratic approaches.

A Confiician-Socratic framework is used to analyze culture's influence on academic learning. Socrates, a Western exemplar, valued private and public questioning of widely accepted knowledge and expected students to evaluate others' beliefs and to generate and express their own hypotheses. Confucius, an Eastern exemplar, valued effortful, respectful, and pragmatic acquisition of essential knowledge as well as behavioral reform. Expressions of these approaches in modern postsecondary contexts are discussed, as are the effects these approaches may have for students who either fit or do not fit the cultural ideal. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Single-wall carbon nanotube coating on a pyroelectric detector

Carbon single-wall nanotubes (SWNTs) are studied as the thermal-absorption coating on a large area pyroelectric detector. The SWNTs were produced by a laser vaporization method and dispersed onto the detector surface by use of a simple airbrush technique. The detector was based on a 1-cm-diameter, 60-microm-thick lithium tantalate disk having nickel electrodes. We report the spectral responsivity of the detector ranging from 600 to 1800 nm, as well as the spatial and directional uniformity at 850 nm. Using Drude and Lorentzian dielectric functions and an effective medium approximation to obtain the indices of refraction of semiconductor and metallic SWNTs, we compared the expected theoretical relative responsivity for the two types of tube with the measured relative responsivity of the detector. Values of thermal conductivity, specific heat, and damage threshold obtained from the literature are compared with properties of alternatives for thermal coatings such as gold-black and carbon-based paint.