Photolysis of saturated bicyclic peroxides

Under direct photolysis, [n.2.1]-peroxides (n = 3–5) isomerise chiefly to epoxyaldehydes, [n.2.2]-peroxides (n = 2–4) mainly undergo dehydrogenation to cycloalkane-1,4-diones, and 2,3-dioxabicyclo[2.2.1]heptane exhibits both types of behaviour; photo-isomerisation of 6,7- into 6,8-dioxabicyclo[3.2.1]octane occurs when benzophenone is present as sensitizer.     

Dislocation injection, reconstruction, and atomic transport on {001} Au terraces

High-resolution electron microscopy investigations of Au films show that adatoms on (100) surfaces insert into the underlying terrace to form surface dislocations. This injection readily occurs when the number of adatoms on a terrace is approximately 20 atoms or less. The surface dislocation glides along the terrace, but is repelled from the edges. The dislocation escapes by squeezing out in the dislocation line direction (not gliding out the terrace edge). Atomistic simulations confirm the dislocation stability, easy glide along the terrace and trapping at the terrace edge. These results have profound implications for film growth.     

Superlubricity of graphite

Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resolution down to 15 pN, we have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring atomic-scale friction as a function of the rotational angle between two contacting bodies, we show that the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of "superlubricity" [Phys. Rev. B 41, 11 837 (1990)]].     

Microbial adhesion to poly(ethylene oxide) brushes: influence of polymer chain length and temperature

Glass surfaces were modified by end-grafting poly(ethylene oxide) (PEO) chains having molecular weights of 526, 2000, or 9800 Da. Characterization using water contact angles, ellipsometry, and X-ray photoelectron spectroscopy confirmed the presence of the PEO brushes on the surface with estimated lengths in water of 2.8-, 7.5-, and 23.7-nm, respectively. Adhesion of two bacterial (Staphylococcus epidermidis and Pseudomonas aeruginosa) and two yeast (Candida albicans and Candida tropicalis) strains to these brushes was studied and compared to their adhesion to bare glass. For the bacterium P. aeruginosa and the yeast C. tropicalis, adhesion to the 2.8-nm brush was comparable to their adhesion on bare glass, whereas adhesion to the 7.5- and 23.7-nm brushes was greatly reduced. For S. epidermidis, adhesion was only slightly higher to the 2.8-nm brush than that to the longer brushes. Adhesion of the yeast C. albicans to the PEO brushes was lower than that to glass, but no differences in adhesion were found between the three brush lengths. After passage of an air bubble, nearly all microorganisms adhering to a brush were removed, irrespective of brush length, whereas retention of the adhering organisms on glass was much higher. No significant differences were found in adhesion nor retention between experiments conducted at 20 and those conducted at 37 degrees C.     

Effects of ultrahigh vacuum on the response characteristics of strain gages

Strain gages are extensively used in spacesimulation research; yet, little if any information has been reported about the environmental effects on the gage installations themselves. Since ultrahigh vacuum affects the physical and chemical properties of materials, one may expect that the response characteristics of strain-gage installations will also be affected. A study was initiated to determine the behavior of a number of strain-gage installations subjected to ultrahigh-vacuum environments. This paper presents the results of the first phase of the program. The gages and adhesives were selected to provide optimum chance for failure in order to establish a time parameter for following tests and, more importantly, to verify the extent and nature of failure possible under the environmental test conditions. Data were obtained on a number of strain-gage-per-formance characteristics. The performances of the gage installations varied widely being dependent in part upon gage and adhesive composition, whether a gage was used as an active or inactive device, and the level of strain to which a gage was subjected. Detailed pre- and post-test examinations showed that there was little permanent damage to any of the installations.     

Properties and performance of BaxSr1−xCo0.8Fe0.2O3−δ materials for oxygen transport membranes

The present paper discusses the oxygen transport properties, oxygen stoichiometry, phase stability, and chemical and mechanical stability of the perovskites (BSCF) and (SCF) for air separation applications. The low oxygen conductive brownmillerite phase in SCF is characterized using in-situ neutron diffraction, thermographic analysis and temperature programmed desorption but this phase is not present for BSCF under the conditions studied. Although both materials show oxygen fluxes well above 10 ml/cm²·min at T=1,273 K and pO₂=1 bar for self-supporting, 200 μm-thick membranes, BSCF is preferred as a membrane material due to its phase stability. However, BSCF’s long-term stable performance remains to be confirmed. The deviation from ideal oxygen stoichiometry for both materials is high: δ>0.6. The thermal expansion coefficients of BSCF and SCF are 24×10⁻⁶ and 30×10⁻⁶ K⁻¹, respectively, as determined from neutron diffraction data. The phenomenon of kinetic demixing has been observed at pO₂<10⁻⁵ bar, resulting in roughening of the surface and enrichment with alkaline earth metals. Stress–strain curves were determined and indicated creep behavior that induces undesired ductility at T=1,073 K for SCF. Remedies for mechanical and chemical instabilities are discussed.     

Flexible hiring in a make to order system with parallel processing units

In this paper, we study a make-to-order production system with parallel, identical processing units. Each order needs to be satisfied on a single processing unit that is run by a crew. The inter-arrival time and the service time for each order are random variables. The system operates under a lead time performance constraint, which demands the completion of each order within a pre-determined lead time with a certain probability. The minimum number of processing units needed to satisfy this constraint is determined at the tactical level. Our research focuses on the cost savings that can be realized with the use of flexible crews via contractual hiring agreements with an External Labor Supply Agency (ELSA). The ELSA can periodically provide an agreed number of crews. The cost incurred for a flexible crew is higher than that for a permanent crew, and is decreasing in the period length. We model and analyze this system using the transient behavior analysis of multi-server queues and propose several empirically testable functions for the cost of flexible crews. In our computational study, we demonstrate possible cost savings of 2-level, threshold type hiring policies, relative to the fixed capacity system, under 9 scenarios with three demand-to-processing rate ratios and three lead time performance constraints, each of which reflects a different level of ambition. We observe that the maximum savings occur when the cost of a flexible crew is same as that of a permanent crew, and range from 29.38% to 50.56%. However, as the flexible crews become more expensive, the system may choose to employ permanent crews only. We observe that cost savings consist of two parts: savings due to the cancellation of the sclerosis of capacity discreteness, and savings due to the use of workload information in hiring actions. The latter part is higher for more ambitious lead time performance constraints, and for higher mean processing times. Finally, when there is an additional cost for transacting an agreement with the ELSA, we observe that the capacity flexibility option loses its charm, especially if the transaction cost is higher than the cost of a permanent crew.