One-dimensional fluid and circuit simulation of an AC plasmadisplay cell

A one-dimensional multispecies model of a helium filled AC plasma display cell is described. The model includes a continuity equation for each species. Poisson's equation and a circuit equation. The fill gas is helium for which the cross sections for ionization and excitation for the various atomic states are well known. The reaction rates for the different species were parametrized as functions of E/N using a zero-dimensional spatial and two velocity multispecies Boltzmann kinetic code. Calculations are performed using address and sustain voltages operating at a frequency of 50 kHz to model the pulse behavior of a dielectric barrier discharge which occurs over a time scale on the order of 200 ns or less. The code simulates multipulse behavior and shows reasonably good agreement with experimental data. The simulations show that the circuit filters out the discharge dynamics and that the pulse width of the discharge current depends on the circuit RC time constant. Using a 123 V sustain voltage, the calculated discharge current is 0.4 A with a full width at half maximum (FWHM) of 28 ns. Experimental values using a 120 V sustain voltage are 0.32 A discharge current and 42 ns FWHM     

Thermogravimetric analysis of poly(alkyl methacrylates) and poly(methylmethacrylate-g-dimethyl siloxane) graft copolymers

The thermal stabilities of various poly(alkyl methacrylate) homopolymers and poly(methyl methacrylate-g-dimethyl siloxane) (PMMA-g-PSX) graft copolymers have been determined by thermogravimetric analysis (TGA). As expected, the thermal stabilities of poly(alkyl methacrylates) were a function of the ester alkyl group, and polymerization mechanism. In particular, thermally labile linkages, which result from termination during free radical or nonliving polymerization mechanisms, decrease the ultimate thermal stabilities of the polymers. However, graft copolymers, which were prepared by the macromonomer technique with free radical initiators, exhibited enhanced thermal stability compared to homopolymer controls. A more complex free radical polymerization mechanism for the macromonomer modified polymerization may account for this result. © 1994 John Wiley & Sons, Inc.     

Effect of acidification treatment and morphological stability of sulfonated poly(arylene ether sulfone) copolymer proton‐exchange membranes for fuel‐cell use above 100 °C

Directly copolymerized wholly aromatic sulfonated poly(arylene ether sulfone) copolymers derived from 4,4′‐biphenol, 4,4′‐dichlorodiphenyl sulfone, 3,3′‐disulfonated, and 4,4′‐dichlorodiphenyl sulfone (BPSH) were evaluated as proton‐exchange membranes for elevated temperature operation (100–140 °C). Acidification of the copolymer from the sulfonated form after the nucleophilic step (condensation) copolymerization involved either immersing the solvent‐cast membrane in sulfuric acid at 30 °C for 24 h and washing with water at 30 °C for 24 h (method 1) or immersion in sulfuric acid at 100 °C for 2 h followed by similar water treatment at 100 °C for 2 h (method 2). The fully hydrated BPSH membranes treated by method 2 exhibited higher proton conductivity, greater water absorption, and less temperature dependence on proton conductivity as compared with the membranes acidified at 30 °C. In contrast, the conductivity and water absorption of a control perfluorosulfonic acid copolymer (Nafion 1135) were invariant with treatment temperature; however, the conductivity of the Nafion membranes at elevated temperature was strongly dependent on heating rate or temperature. Tapping‐mode atomic force microscope results demonstrated that all of the membranes exposed to high‐temperature conditions underwent an irreversible change of the ionic domain microstructure, the extent of which depended on the concentration of sulfonic acid sites in the BPSH system. The effect of aging membranes based on BPSH and Nafion at elevated temperature on proton conductivity is also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2816–2828, 2003     

An SIS waveguide heterodyne receiver for 600 HGz-635 GHz

A waveguide SIS heterodyne receiver using a Nb/AlO_X/Nb junction has been built for astronomical observations of molecular transitions in the frequency range 600 GHz - 635 GHz, and has been successfully used at the Caltech Submillimeter Observatory (CSO). We report double sideband (DSB) receiver noise temperatures as low as 245 K at 600 GHz -610 GHz, and near 300 K over the rest of the bandwidth. These results confirm that SIS quasiparticle mixers work well at submillimeter-wave frequencies corresponding to photon energies of at least 90% of the superconductor energy gap. In addition, we have systematically investigated the effect on the receiver performance of the overlap between first-order and second-order photon steps of opposite sign at these frequencies. The receiver noise increases by as much as 40% in the region of overlap. We infer potential limitations for operating submillimeter-wave Nb/AlO_x/Nb mixers.     

Molecular weight characterization of soluble high performance polyimides. 1. Polymer-solvent-stationary phase interactions in size exclusion chromatography

Soluble, fully cyclized m-amino phenyl acetylene terminated polyimides based on several anhydride/diamine monomers were prepared in N-methylpyrrolidine (NMP) and cyclized by solution imidization to controlled molecular weight. The polyimides and a polyamic acid precursor were successfully analyzed by size exclusion chromatography (SEC) utilizing online parallel coupled refractive index and differential viscometer detectors. The calculated M _nvalues were varied from 3,000 to 20,000 daltons. N-methylpyrrolidone (NMP), tetrahydrofuran (THF), and chloroform served as mobile phases for the cross-linked polystyrene gel packings. Normal retention behavior of the polyimides was observed in chloroform, THF, and NMP containing LiBr, or in NMP stirred over P₂O₅ before use. Values of Mark-Houwink-Sakurada exponents for narrow distribution linear polystyrene indicate that pure NMP and NMP with 0.06 M LiBr are good solvents for polystyrene standards at 60°C. In contrast, SEC behavior of polyimides in pure NMP leads to splitting of the peaks with the major portion observed to pass through the columns at the exclusion limit. In contrast to strong polymeric chain expansion of the polyamic acid in dilute solution, presumably due to a polyelectrolyte effect, no increase of intrinsic viscosity of polyimide samples in pure NMP was observed. This exclusion effect of polyimides analyzed in NMP is discussed in terms of possible ion-exclusion from pores of the stationary phase. Differences in polystyrene calibration in NMP with or without additives and the temperature dependence of calibration curves in these mobile phases is discussed as well. ©1995 John Wiley & Sons, Inc.     

Hindered lithium dialkylamide initiators for the living anionic polymerization of methacrylic esters

Lithium diisopropylamide is an efficient initiator for the anionic polymerization of methyl methacrylate in polar solvents at –78°C. Predictable molecular weights based on the initiator concentration were obtained, yields were quantitative, and molecular weight distributions were relatively narrow (<1.30). The presence of an amino end group and confirmation of the initiation mechanism were confirmed by potentiometric titration using HClO₄. Number-average molecular weights determined by titration agreed well with values determined by SEC. Other lithium dialkylamides that contain larger alkyl groups such as lithium diisobutylamide were less efficient initiators. This was attributed to the more nucleophilic anion due to less steric hindrance near the nitrogen atom. Molecular weight distributions were significantly broadened (>2.0), and molecular weight control was not achieved. However, polymer yields were quantitative. In all cases, PMMA stereochemistry was indicative of a solvent-separated lithium counterion, and triad compositions were identical to organolithium initiated homopolymers, i.e., 78% syndiotactic, 20% heterotactic, and 2% isotactic. © 1994 John Wiley & Sons, Inc.     

ACUTE EFFECTS OF LOW-FLUENCE ULTRAVIOLET LIGHT ON HUMAN T-LYMPHOCYTE SUBSETS

Abstract-The acute effects of low-fluence ultraviolet light, primarily between wavelengths 280 and 320 nm (ultraviolet B) on T-lymphocyte subsets were assessed in virro and in vivo by quantitative cytofluorometric analysis. In virro, 90J/m² of ultraviolet irradiation of human mononuclear cells produced a significant increase in the OKT4 (helper-inducer)/OKT8 (suppressor-cytotoxic) ratio (P>0.008), due primarily to a decrease in the OKT8 subset. In vivo, 24 h after whole-body UV irradiation of human subjects with one half of a minimal erythemal dose of ultraviolet B the OKT4/OKT8 ratios again increased (P > 0.002), again with somewhat greater effects on the OKT8 subset. These results indicate that suberythemal exposure to UV light increases OKT4/OKT8 ratios in normal human subjects because of differential effects on lymphocyte subsets. They suggest that casual exposure to sunlight mediates immunologic changes in normal human subjects.     

A subjective Bayesian approach to the theory of queues I — Modeling

This is the first of an expository two-part paper which outlines a point of view different from that currently used in queueing theory. In both parts, the focus is on concepts. Here we adopt a personal probability point of view to all sources of uncertainty in the theory of queues and explore the consequences of our approach by comparing our results to those that are currently available. For ease of exposition, we confine attention to the M/M/1/ and the M/M/1/K queues. In Part I we outline the general strategy and focus on model development. In Part II we address the problem of inference in queues within the subjective Bayesian paradigm and introduce a use of Shannon's measure of information for assessing the amount of information conveyed by the various types of data from queues.     

The synthesis and characterization of polyimide homopolymers based on 5(6)‐amino‐1‐(4‐aminophenyl)‐1,3,3‐trimethylindane

The isomeric diamine monomer 5(6)‐amino‐1‐(4‐aminophenyl)‐1,3,3‐trimethylindane (DAPI) was successfully synthesized via the dimerization of α‐methylstyrene followed by nitration and reduction. High molecular weight, soluble polyimides were synthesized via ester–acid solution imidization techniques and had glass‐transition temperature values ranging from 247 to 369 °C. The polymers were soluble in common organic solvents because of the asymmetric and nonplanar nature of DAPI and displayed good short‐term thermal stability by thermogravimetric analysis, as shown by their 5% weight‐loss values above 500 °C in air. The DAPI/(3,4‐dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) polyimide also showed 2‐h thermal stability at 400 °C under nitrogen, despite the partial aliphatic character. Refractive index values as low as 1.571 were observed for DAPI/6FDA, which allows an estimated dielectric constant of 2.47 to be derived. The permeation of O₂ and N₂ was conducted on thin dense films. The bulky, bent, and isomeric nature of DAPI imparted film‐forming membranes that permitted high O₂ permeability. In combination with 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride (BTDA), DAPI had a good combination of O₂ permeability and O₂/N₂ selectivity values of 2.8 Ba and 7.3, respectively. The polymerization method utilized to facilitate the cyclization of DAPI/BTDA to the polyimide affected the final thermal properties of the resulting polymer. The chemical imidization of DAPI/BTDA generated a polyimide with a glass‐transition temperature value of 311 °C and a 5% weight‐loss value in air of 457 °C. However, thermal and ester–acid imidization routes yielded an increase in the thermal properties. The ester–acid solution imidization of DAPI/BTDA produced a polymer glass‐transition temperature value of 333 °C and a 5% weight‐loss value of 525 °C in air. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2840–2854, 2000