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  • 1
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present an extended set of incoherent neutron scattering measurements on the van der Waals liquido-terphenyl, obtained by time-of-flight and backscattering spectroscopy. In the supercooled liquid regime, data from three instruments are combined and analysed in terms of the selfcorrelationS(Q, t). In the time range 1...100 ps, the crossover from α-to β-relaxation is well described by the masterfunction of mode coupling theory, and fitted parameters are consistent with the previously established critical temperatureT c [Z. Phys. B83, 175 (1991)]. In the glassy regime, vibrations are harmonic and can be described by a density of states. Deviations at lowQ are quantitatively explained by a multiple scattering simulation. Throughout the article, experimental difficulties are discussed in some detail.
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  • 2
    ISSN: 1435-1536
    Keywords: Microgels ; thermal diffusion ; self diffusion ; forced Rayleigh scattering
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Thermal diffusion in solutions of polystyrene micro-network spheres (microgels) in toluene has been studied by the holographic scattering technique of thermal diffusion forced Rayleigh scattering (TDFRS) and by photon correlation spectroscopy (PCS). Size distributions of microgels of different crosslink ratios are obtained from TDFRS measurements on dilute solutions at very lowq-values around 4000cm−1. At low concentrations a single diffusive mode is observed and the diffusion coefficient increases with concentration. It is attributed to the collective diffusion of the microgels and the solvent. At high concentrations an additional slow mode appears whose diffusion coefficient decreases with increasing concentration. Both diffusive modes are observed with PCS and TDFRS. Contrary to PCS, heterodyne TDFRS-measurements reveal a negative amplitude of the slow mode. We attribute the slow mode to self-diffusion of the microgels, made visible by the polydispersity of their size distribution. It is discussed in terms of a fast coupled thermal diffusion with subsequent decoupling of the individual microgels and relaxation into a new Soret equilibrium by self-diffusion of the microgels.
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  • 3
    ISSN: 1435-1536
    Keywords: Photon correlation spectroscopy ; nonergodic systems ; ensemble averaging
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract We present a modification of the conventional dynamic light scattering set-up which allows to monitor the intensity fluctuations of many independent spatial Fourier components of the density fluctuations, i.e. “speckles”, simultaneously by using a charge-coupled device (CCD) camera as area detector. By averaging over the intensity autocorrelation function the final 10–20% decay of the intermediate scattering function in very dense colloidal dispersions is obtained with much higher accuracy. At the same time this multi-speckle autocorrelation spectroscopy provides an alternative route for constructing ensemble-averaged intermediate scattering functions in nonergodic media by replacing the average over many independent sample volumes by an average over independent spatial Fourier components of the density fluctuations. We will survey the methods proposed so far to generate ensemble averages in nonergodic media and discuss their merits and limits. We then demonstrate the advantages of the new technique, taking as an example a colloidal dispersion where in the glassy state long-lived density fluctuations superimpose on the frozen ones. Finally, we make a direct comparison with another “speckle-averaging” technique, the “interleaved sampling” method, which has been proposed and applied to the same system recently [J. Müller, T. Palberg, Progr. Coll. Polym. Sci. (1996) 100:121–126].
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  • 4
    ISSN: 0044-2313
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Oxydative Addition of Benzyl Halides to Bis(triphenylphosphine) Ethylene Nickel(0)By the oxydative addition of benzyl chloride to (Ph3P)2Ni(C2H4) at -20°C (Ph3P)2Ni(CH2Ph)Cl (II) is formed. Under similar conditions benzyl bromide gives (Ph3P)2Ni(CH2Ph)Br, but in an impure state. It is shown, that the oxydative addition to (Ph3P)2Ni(C2H4) may involve both SN2 displacement and one electron mechanismens.In benzene solution an equilibrium between (Ph3P)2Ni(σ-CH2Ph)Cl and the blue violet (Ph3P)Ni(π-CH2Ph)Cl exists. The benzyl ligands of II are split off by the reaction with ethereal hydrochloric acid, forming toluene and dibenzyl, with mercury(II) chloride, forming PhCH2HgCl, or with triphenylphosphine, forming dibenzyl and (Ph3P)3NiCl.
    Notes: Durch oxydative Addition von Benzylchlorid an (Ph3P)2Ni(C2H4) bei -20°C entsteht (Ph3P)2Ni(CH2Ph)Cl(II). Mit Benzylbromid wird unter vergleichbaren Bedingungen (Ph3P)2Ni(CH2Ph)Br erhalten, das aber mit (Ph3P)2NiBr2 verunreinigt ist. Es wird nachgewiesen, daß die oxydative Addition an (Ph3P)2Ni(C2H4) sowohl nach einem SN2- als auch nach einem Einelektronen-Mechanismus verlaufen kann.(Ph3P)2Ni(σ-CH2Ph)Cl liegt in benzolischer Lösung im Gleichgewicht mit dem blauvioletten (Ph3P)Ni(π-CH2Ph)Cl vor. Die Benzylliganden von II können mit ätherischer Salzsäure (Bildung von Toluol und Dibenzyl) oder mit Quecksilber(II)-Chlorid (Bildung von Benzylquecksilberchlorid) quantitativ abgespalten werden. Mit Triphenylphosphin reagiert II unter Bildung von (Ph3P)3NiCl und Dibenzyl.
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  • 5
    Electronic Resource
    Electronic Resource
    Weinheim [u.a.] : Wiley-Blackwell
    ISSN: 0947-5117
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: The inhibitory effect of sorbic acidSorbic acid is utilized as a fungicidal and antimicrobial preservative. The structure of the sorbic acid molecule containing two double bonds together with a carboxyl group suggests an inhibitory effect on the dissolution of metals in acids.In view of the use of sorbic acid in the packing industry, its effect on the acid corrosion of aluminium, iron and tin was examined. Its inhibitory action was proven by means of dissolution tests and potentiokinetic measurements. Concentrations of 1 to 2 g/kg, applied in conservation, cause predominantly inhibition of metal corrosion in hydrochloric acid, sulfuric acid and acetic acid. Lower concentrations may stimulate the dissolution of the metals.
    Notes: Sorbinsäure wird als fungistatischer und antimikrobieller Konservierungsstoff eingesetzt. Der strukturelle Aufbau des Sorbinsäuremoleküles mit zwei Doppelbindungen in Kombination mit der Carboxylgruppe läßt eine inhibierende Wirkung bei der Metallauflösung in Säuren erwarten.Im Hinblick auf die Verwendung in der Konservenindustrie wurde die Wirkung der Sorbinsäure auf die Säurekorrosion der Metalle Aluminium, Eisen und Zinn untersucht. Löseversuche, ergänzt durch potentiokinetische Messungen, bestätigten die Erwartung. In den Korrosionsmedien Salzsäure, Schwefelsäure und Essigsäure wirken die bei der Konservierung angewendeten Konzentrationen von 1 bis 2 g/kg vorwiegend inhibierend auf die Metallkorrosion, während niederere Konzentrationen die Metallauflösung stimulieren können.
    Additional Material: 4 Ill.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents a light scattering study of the dynamics of concentrated solutions of nearly monodisperse (σ≈0.16) spherical micronetwork particles consisting of highly cross-linked polystyrene dissolved in carbon disulfide, i.e., a "good'' solvent. Above volume fractions of cursive-phi=0.50 the intermediate scattering function, measured over a time window of 10−7 to 103 s using the ALV5000 correlator, decays in two steps and shows indications of nonergodic behavior for cursive-phi≥0.64. Such behavior is typical for glass forming systems and has recently been found close to the glass transition of a hard sphere colloidal system [W. van Megen and P. N. Pusey, Phys. Rev. A 43, 5429 (1991)]. Thus the introduced system can be used for modeling the glass transition of atoms on a mesoscopic scale. The traditional analysis of structural relaxation in terms of a Kohlrausch–Williams–Watts distribution yields a mean relaxation time which follows the empirical Mooney equation as a function of concentration and thus corresponds to Vogel–Fulcher–Tammann behavior. However, the necessity to add an unspecified "intermediate'' process between the short and long time KWW decays demonstrates the limitations of this "pragmatic'' approach. The mode coupling theory of the glass transition interprets the intermediate scattering function consistently over nearly seven decades in time, the intermediate region corresponding to the crossover from β to α relaxation (von Schweidler law). The critical volume fraction of 0.636 derived by this analysis corresponds to a value of 0.59 for an ideal monodisperse system which is well in accord with other experimental and computer simulation studies of the glass transition of atomic systems.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 3743-3756 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We studied the glass transition dynamics of polystyrene micronetwork colloids with an average cross-link density of 1:50 (inverse number of monomer units between cross-links) and a hydrodynamic radius of about 100 nm by dynamic light scattering. Special emphasis was put on extracting correct intermediate scattering functions in a system that might be termed as partially nonergodic. By using a charge-coupled device camera as a detector and averaging the intensity autocorrelation functions of 50 simultaneously monitored speckles the duration of the experiment could be significantly reduced as compared to the conventional "brute force'' ensemble averaging. Despite some striking similarities to the behavior of hard sphere colloids the glass transition scenario in our system differs in several respects when analyzing the dynamics in the glass transition regime within the framework of mode coupling theory. Besides the existence of structural relaxation processes above cursive-phic we find indications that additional dynamic processes modify the β relaxation in the glassy phase. Our findings cannot be explained by the occurrence of hopping processes, but are rationalized via an increase of the particle compressibility and the surface friction on decreasing the cross-link density from its hard sphere limit. © 1997 American Institute of Physics.
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  • 8
    ISSN: 0009-2614
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 0921-4526
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
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  • 10
    ISSN: 0378-4371
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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