Das verhalten von copolymeren aus styrol und acenaphthylen bei der einwirkung von licht,I

A series of styrene and acenaphthylene copolymers were prepared by thermal polymerization at 100°C. Films of copolymers with various contents of acenaphthylene were exposed to light in an apparatus (Xenotest) under the atmosphere of normal air and in some cases under nitrogen. The degradation of copolymers was followed by measurements of solution viscosity. Copolymers with small contents of acenaphthylene (4,2 mol-%) show slower degradation than polystyrene up to exposure times of about 80 h, but then a stronger degradation takes place with increased content of acenaphthylene. In the presence of oxygen the formation of benzene-insoluble particles besides degradation was observed. The amount of these “benzene-insoluble” particles, which have a specific absorption in the infrared at 1730 cm ^–1, depends upon the exposure time and the content of acenaphthylene. No such benzene-insoluble products were found if the films were exposed to light in the same apparatus under nitrogen atmosphere.     

FM noise and power-dependent linewidth of GaAs/AlGaAsGRINSCH-SQW-MCRW flared waveguide lasers

The power-dependent part of the linewidth of GaAs/AlGaAs GRINSCH-SQW flared waveguide lasers has been determined from FM noise measurements. For the linewidth-power product a low value of 4 MHz mW has been found, which can be related to the small spontaneous emission coefficient β_sp≃(3.7±1)×10^-6 and a linewidth enhancement factor α≃1.2±0.5     

High field transport in PbTe

The drift velocity and Hall coefficient of tn and p-type PbTe at 77K were measured for various crystallographic directions of current up to field strengths of 1.5 kV/cm. Both the conductivity and the Hall effect are anisotropic with the latter depending also strongly on the magnetic field. These effects are attributed to nonuniform heating of the equivalent conduction band L-valleys of PbTe and equivalent intervalley transfer. For field strengths beyond 1 kV/cm oscillations of the current and the potential distribution occur. The Hall coefficient decreases sharply above this threshold indicating avalanche breakdown. At still higher field strengths a sign reversal of the Hall coefficient is observed. Probing the potential distribution, it could be shown that this instability is caused by high field domains, which travel along the sample with the drift velocity of the carriers. Possible mechanisms for the formation of high field domains are discussed.     

Überprüfung von Verfahren zur Extrapolation der Zeitstandfestigkeit warmfester Stähle

Übersicht über Verfahren zur Extrapolation der Zeitstandfestigkeit. Einteilung der Verfahren nach der Vorgehensweise (graphisch, graphisch-numerisch und numerisch) und nach dem Umfang der benutzten Angaben. Anforderungen an Bereich und Umfang der zugrunde gelegten Ergebnisse von Zeitstandversuchen. Bedeutung des Abkürzungsverfahrens für die Überprüfung und Bewertung von Extrapolationsverfahren. Hinweis auf die Parameter einer Extrapolation, insbesondere das Extrapolationszeitverhältnis. Neue numerische Verfahren, Mittelwertbildung von Voraussagen. Neue Zeitstandversuche als Grundlage für die Entwicklung von Extrapolationsverfahren.     

Patient exposure in medical X-ray imaging in Europe

Patients are exposed to X rays when undergoing medical examinations in diagnostic radiology. Exposure data acquired and assessed in Germany for the year 1997 resulted in a mean annual effective dose of 2 +/- 0.5 mSv per head of the population, thereby reaching or exceeding the average level of environmental radiation in many cases. The underlying frequency of medical X-ray examinations was approximately 136 million, i.e. approximately 1.7 examinations annually per head of the population. For comparison, corresponding data of other countries were extracted from the UNSCEAR 2000 report or originate from the literature. Data analysis shows significant differences in national radiological practices and a very uneven distribution of patient doses amongst the world population. The mean annual effective dose per head of the population varies by up to a factor of 60 between health care level I and IV countries, and still by a factor of approximately 6 within health care level I countries. While projection radiography has succeeded in reducing dose consumption, computed tomography and radiological interventions have given rise to a significant growth of patient exposure, and interventional radiology can even exceed thresholds for deterministic radiation effects. Patient exposure is further shown to result from misadministration and retakes of X-ray examinations, usually not registered, as well as from technical failures of X-ray facilities, which can cause significantly enhanced exposure times. Corresponding data are presented and comments are made on the international situation of non-harmonised data collection on patient exposure as well as of parameters affecting the assessment of exposure and risk.     

Rasterelektronenmikroskopische Direktbeobachtung metallurgischer Reaktionen bis zu Schmelztemperaturen

Strukturveränderungen bei der Reduktion von Eisenoxiden, ermittelt aus der direkten Beobachtung der metallurgischen Reaktionen im Rasterelektronenmikroskop. Wachstumsmechanismus nadelförmiger Eisenausscheidungen. Volumenänderung bei der Reduktion. Sinterprozesse nach Abschluß der Reduktion. Diskussion der Auswirkungen der Strukturveränderungen auf den Hochofenbetrieb. Hochofentemperatur- und Schmelzversuche im Zusatzgerät zum Rasterelektronenmikroskop.     

Microwave transmission through ferromagnetic metals

Transmission of microwave radiation through a metal ferromagnet is enhanced at the field corresponding to ferromagnetic antiresonance. This phenomenon can be used to measure the magnetic parameters of a specimen. It is a particularly useful and sensitive tool for studying the damping processes in metals. This technique is compared with that of ferromagnetic resonance absorption (FMR). It is pointed out that the FMAR transmission method is complimentary to the FMR method when non-local (i.e. wave-number dependent) effects are important, because the spatial variation of the RF magnetization and fields is much greater at FMR than at FMAR. This point is illustrated using experiments on nickel for which the damping parameter measured using FMAR is larger than that measured using FMR at temperatures less than 300K.     

Der Einfluß von Temperatur und Verformungsgeschwindigkeit auf die Konstanten der Ludwik-Gleichung

Beschreibung der Spannung-Dehnung-Kurve ferritischer und ferritisch-perlitischer Stähle mit der Ludwik-Gleichung. Abhängigkeit des Verfestigungsexponenten und des Vorfaktors der Ludwik-Gleichung von Prüftemperatur und Verformungsgeschwindigkeit bei 1-, 2- und 3-n-Verhalten.