Ion-selective electrodes based on a pyridyl-containing triazolophane: altering halide selectivity by combining dipole-promoted cooperativity with hydrogen bonding

Triazolophanes are ionophores, with preorganized cyclic cavities that have tunable selectivities for halides. The interaction with halides is based on hydrogen bonding between the eight CH hydrogen atoms of the cavity and the halide anion. The rigidity of the cavity in tetraphenylene triazolophane along with the hydrogen bonding favors planar 1:1 complexation of "snugly" encapsulated chloride and bromide. Manipulating the triazolophane's structure by introducing two pyridyl moieties into the cavity alters the receptor's binding mode. This change adds a dipole-promoted driving force that combines with hydrogen bonding to favor the formation of 2:1 sandwich complexes around halides. The potentiometric response of electrodes based on this new ionophore was evaluated for optimal halide selectivity. The new triazolophane-based electrode showed an anti-Hofmeister selectivity toward iodide with a submicromolar detection limit. The stoichiometry of complexation and the stability constants with different halides were evaluated using a segmented sandwich membranes method. The pyridyl-triazolophane demonstrated a response consistent with a 2:1 sandwich-type complex with iodide, in polyvinyl chloride (PVC) membranes.     

Distributed feedback lasers in chiral media

The combined effects of chirality and gain (or loss) on wave propagation and coupling in periodic structures is investigated here. The focus is on distributed feedback (DFB) lasers in a transversely unbounded periodic slab with spatially modulated electromagnetic parameters. The analysis uses a coupled-mode approach employing a canonical physical model of chiral materials to predict the effects of modulated chirality admittance on DFB lasers. Results for DFB laser behavior in chiral media are compared and contrasted to that in achiral media. It is found that, under certain circumstances, the electric and magnetic field coupling, which is characteristic of chiral materials, results in a lower threshold gain for DFB lasers in media with a given index of refraction and characteristic impedance. It is also found that chiral index-coupled or gain-coupled DFB lasers exhibit the same spectral mode properties as achiral DFB lasers     

EDFA-based DWDM lightwave transmission systems with end-to-end power and SNR equalization

An approximate analysis is presented which can be used to predict the performance of power and signal-to-noise ratio (SNR) equalization schemes when applied to dense wavelength-division multiplexing (DWDM) lightwave systems employing erbium-doped fiber amplifier (EDFA) cascades. Expressions are provided which relate the maximum number of amplifiers, EDFA gain imbalance, bit rate (R/sub b/), transmitter power, receiver dynamic range and number of channels. The relative advantages of these two equalization strategies are quantified by comparing the maximum number of amplifiers allowed by each scheme. It is shown that, while SNR equalization represents, on balance, the more desirable equalization strategy for future EDFA-based DWDM lightwave transmission systems, under certain conditions power equalization may be a better choice. When employing an APD receiver, for instance, power equalization can support 1.9 times more amplifiers than SNR equalization. However, when employing the more conventional preamplified PIN/FET receiver, SNR equalization can support 1.7 times more amplifiers than power equalization.     

Development of high‐strength fibers from aliphatic polyketones by melt spinning and drawing

We have investigated the formation of high‐strength, high‐modulus fibers from four aliphatic polyketone resins. One resin was a perfectly alternating copolymer of ethylene and carbon monoxide, while the other three were terpolymers containing up to 6 mol % propylene. The mechanical properties were measured as a function of processing conditions, and the structures of the filaments were characterized using birefringence, WAXS, SAXS, SEM, and thermal analysis. Fibers formed from all resins develop very high molecular orientations and a microfibrillar structure. Fibers having room temperature tenacities as high as 10 gpd (～1.1 GPa) were obtained. Tensile moduli reached values as high as 120 gpd (～13 GPa). The melting point of the fibers was primarily dependent on the composition of the resin, while the maximum strength and modulus were largely determined by the maximum draw ratio achieved. The maximum draw ratio achieved in the present experiments was greater for the terpolymers than for the copolymer. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1794–1815, 2001     

Japanese encephalitis on Saipan: a survey of suspected mosquito vectors

An outbreak of Japanese encephalitis (JE) occurred on Saipan, Commonwealth of Northern Mariana Islands, in October 1990. Adult and larval mosquitoes were collected during September-October 1991 to retrospectively determine the probable mosquito vector(s). Virus was not isolated from 119 mosquito pools composed of 7,250 adult specimens as follows: Aedes vexans nocturnis (14%), Culex tritaeniorhynchus (39%), Cx. sitiens group (11%), Culex (Culex) species (35%), and < 1% each of Ae. albopictus, Ae. oakleyi, Aedes saipanensis, Cx. annulirostris marianae, and Cx. fuscanus. Three additional species were collected only as larvae: Anopheles indefinitus, Ae. neopandani, and Cx. quinquefasciatus. Among the vectors of JE incriminated in other areas, Cx. tritaeniorhynchus was the predominant species in our collections and the principal species feeding on swine. This is the first published record of the occurrence of this species on Saipan. Culex tritaeniorhynchus is abundant and widely distributed on the southern half of Saipan where human JE cases occurred in 1990, and where swine seroconversions were detected. Although the identity of the mosquito vector(s) responsible for the 1990 outbreak cannot be established with certainty, our results suggest that Cx. tritaeniorhychus was probably involved.     

METHODOLOGY CHOICE,COMPLEXITY TEXTURE,SELF-REFLECTION

Substantial efforts have been dedicated toward the development of systems methodologies for “problem solving,” with various research groups concentrating on particular approaches. Recently, however, there have been moves toward reconciling these varying approaches within one overall framework, thus cutting out “isolationism” and inevitable ensuing sniping. This paper provides an overview of three attempts to promote such openness and conciliation (in terms of theory, methodology, and ideology), draws them together, and then demonstrates the practical importance of a critical approach through a series of contrasting applications and by reflecting on ideas from the three approaches.     

Advanced space solar cells

This paper will present a brief overview of the status of research and development of advanced space solar cells from a variety of materials. Most of the investigations at present are focused on binary, ternary and quaternary III–V semiconducting compounds such as InP, GaInAs and GaInP₂. Growth techniques used for producing laboratory cells include liquid-phase epitaxy (LPE), organometallic vapor phase epitaxy (OMVPE), molecular beam epitaxy (MBE) or any of a number of variations of these techniques, such as atomic layer epitaxy (ALE), etc. Gallium arsenide is at present the only commercially available III–V compound solar cell. Compound III–V multiple bandgap cells are now under development in a jointly sponsored NASA/Air Force manufacturing technology demonstration program. In general, the decision to use a particular cell technology in space is determined by several factors, emphasis on any particular one depending on the mission environment: some are related to the properties of the photovoltaic material itself, such as efficiency and resistance to radiation damage, and some are related to details of the cell structure and associated materials, such as survivability under repeated thermal cycling and resistance to atomic oxygen erosion. The impact of these requirements on cell material selection and structural design is briefly discussed. This paper was produced under the auspices of the US Government and is therefore not subject to copyright in the USA     

Influence of the dislocation density on the performance ofheteroepitaxial indium phosphide solar cells

The authors point out that heteroepitaxial indium phosphide solar cells developed to date have low efficiency due to misfit dislocations. Dislocations act as recombination centers and strongly influence the solar cell performance. Calculations have been made to study the dependence of heteroepitaxial InP solar cell efficiency on dislocation density. The effects of surface recombination velocity and cell emitter thickness are also considered. Calculated results are compared with the available experimental results on representative InP solar cells. It is shown that heteroepitaxial InP cells with over 20% AM0 efficiency could be fabricated if dislocation density can be reduced to <10⁵ cm^-2 and the surface recombination velocity reduced to <10⁵ cm/s     

Genre, rhetorical interpretation, and the open case: teaching theanalytical report

Analytical reports, being one of the most difficult genres to teach in a technical writing course, are best taught through the “open case” method. Open cases take advantage of the fact that students are already situated in a workplace environment, the college campus. Engineering students can use the genre to impose order on this chaotic environment, conducting various forms of research on engineering-related campus issues. A process for developing open case assignments is provided