Cryogenic/high temperature structural adhesives

Results are described of the work performed to develop a structural adhesive system which possesses useful properties over a 20K (-423 F) to 589 K (600 F) temperature range. Adhesives systems based on polyimide, polyphenylquinoxaline polyquinoxaline, polybenzothiazole and polybenzimidazole polymers first were screened for suitability. Detailed evaluation of two polyimide adhesive sytems, Br34/FM34 and P4/A5F or P4A/A5FA, and one polyphenylquinoxaline adhesive system, PPQ II (IMW), then was performed. Property information was generated over the full temperature range for shear strength, stressed and unstressed thermal aging, thermal shock and coefficient of thermal expansion. Both polyimide adhesive systems were identified as being capable of providing structural adhesive joints for cryogenic/high temperature service

Scanning Optical Microscopy of Semiconductor Materials and Devices

In the scanning optical microscope a focused light spot is used to illuminate the object and some property monitored as the spot is scanned relative to the object to build up an image. By monitoring different properties it is thus possible to use the scanning optical microscope in a wide range of imaging modes, which can be used to give much information concerning the structure and properties of semiconductor materials and devices. In the optical-beam induced current method the focused light spot generates electronic carriers in a semiconductor specimen, and the resultant current monitored. The technique can be used to study defects in semiconducting materials and to measure electronic properties. If instead the reflected light is monitored we can obtain images in which resolution, contrast and depth of focus are all improved relative to conventional optical microscopy. Using the confocal imaging mode surface topography of thick structures can be investigated. In the scanning optical microscope we gain all these advantages whilst avoiding the disadvantageous effects of an electron beam and the necessity for a vacuum environment

Atoll rim expansion or erosion in Diego Garcia Atoll, Indian Ocean? : comment on Hamylton, S., East, H. A geospatial appraisal of ecological and geomorphic change on Diego Garcia Atoll, Chagos Islands (British Indian Ocean Territory). Remote Sens. 2012, 4, 3444–3461

Hamylton and East [1] reported a remarkably substantial expansion and accretion of land since the 1960s at numerous sites along the island of Diego Garcia atoll. This atoll has a near continuous rim encircling its lagoon, and they reported that the width of the island rim has expanded by several tens of metres in many places. Those results contrast markedly with my own near-annual observations on the ground where erosion and increased seawater inundation, rather than land accretion, is evident in many places. Hamylton and East have never visited the atoll but their results have been picked up for various reasons. Therefore I note possible reasons for the substantial discrepancy between their measurements and my own observations, and propose a resolution

Development of autoclavable addition type polyimides

Two highly promising approaches to yield autoclavable addition-type polyimides were identified and evaluated in the program. Conditions were established for autoclave preparation of Hercules HMS graphite fiber reinforced composites in the temperature range of 473 K to 505 K under an applied pressure of 0.7 MN/m2 (100 psi) for time durations up to four hours. Upon oven postcure in air at 589 K, composite samples demonstrated high mechanical property retention at 561 K after isothermal aging in air for 1000 hours. Promise was shown for shorter term mechanical property retention at 589 K upon exposure in air at this temperature

Development of autoclavable polyimides

A poly(Diels-Alder) (PDA) resin approach was investigated as a means to achieve autoclavability of high temperature resistant resin/fiber composites under mild fabrication procedures. Low void content Type A-S graphite reinforced composites were autoclave fabricated from a PDA resin/fiber prepared from an acetone:methanol:dioxane varnish. Autoclave conditions were 477K (400F) and 0.7 MN/sq m (100 psi) for up to two hours duration. After postcure at temperatures up to 589K (600F), the composites demonstrated high initial mechanical properties at temperatures up to 561K (550F). The results from isothermal aging studies in air for 1000 hours indicated potential for long-term ( 1000 hours) use at 533K (500F) and shorter-term (up to 1000 hours) at 561K (550F)

Improved expressions for performance parameters for complex filters

Improved expressions are given for the performance parameters for transverse and axial gains for complex pupil filters. These expressions can be used to predict the behavior of filters that give a small axial shift in the focal intensity maximum and also predict the changes in gain for different observation planes

Lorentz Beams

A new kind of tridimensional scalar optical beams is introduced. These beams are called Lorentz beams because the form of their transverse pattern in the source plane is the product of two independent Lorentz functions. Closed-form expression of free-space propagation under paraxial limit is derived and pseudo non-diffracting features pointed out. Moreover, as the slowly varying part of these fields fulfils the scalar paraxial wave equation, it follows that there exist also Lorentz-Gauss beams, i.e. beams obtained by multipying the original Lorentz beam to a Gaussian apodization function. Although the existence of Lorentz-Gauss beams can be shown by using two different and independent ways obtained recently from Kiselev [Opt. Spectr. 96, 4 (2004)] and Gutierrez-Vega et al. [JOSA A 22, 289-298, (2005)], here we have followed a third different approach, which makes use of Lie's group theory, and which possesses the merit to put into evidence the symmetries present in paraxial Optics.Comment: 11 pages, 1 figure, submitted to Journal of Optics

Are infestations of Cymomelanodactylus killing Acropora cytherea in the Chagos archipelago?

Associations between branching corals and infaunal crabs are well known, mostly due to the beneficial effects of Trapezia and Tetralia crabs in protecting host corals from crown-of-thorns starfish (e.g., Pratchett et al. 2000) and/or sedimentation (Stewart et al. 2006). These crabs are obligate associates of live corals and highly prevalent across suitable coral hosts, with 1–2 individuals per colony (Patton 1994). Cymo melanodactylus (Fig. 1) are also prevalent in branching corals, mostly Acropora, and are known to feed on live coral tissue, but are generally found in low abundance (<3 per colony) and do not significantly affect their host corals (e.g., Patton 1994). In the Chagos archipelago, however, infestations of Cymo melanodactylus were found on recently dead and dying colonies of Acropora cytherea