Engineering model of a passive planar air breathing fuel cell cathode

The behavior of an air breathing fuel cell (ABFC) operated on dry-hydrogen in dead-ended mode is studied using theoretical analysis. A one-dimensional, non-isothermal, combined heat and mass transport model is developed that captures the coupling between water generation, oxygen consumption, self-heating and natural convection at the air breathing cathode. The model is validated against planar ABFC experimental measurements over a range of ambient temperatures. The model confirms the strong effect of self-heating on the water balance within passive ABFCs. Model analysis provides several conclusions: (1) thermal runaway caused by inadequate heat rejection predominantly limits ABFC performance. (2) The natural convection boundary layer represents a significant barrier to cathode mass and heat transfer. (3) Because the mass and heat transport numbers associated with natural convection are small, even slight forced convection dramatically affects cell behavior. (4) Performance optimization requires maximizing heat rejection while minimizing flooding. Decoupling the latter two phenomena is challenging due to the exponential relationship between water vapor saturation and temperature.     

Analysis of Transmission Characteristics for Single and Double Disk Windows

The work reports about window design studies for both the JET EP ECRH project and the ASDEX-Upgrade ECRH system. Detailed calculations of the millimeter wave transmission characteristics for conventional single-disk windows, for frequency tunable double-disk windows and for ultra-broadband Brewster windows have been performed. The geometry of the window units has been optimized in order to obtain a suitable transmission characteristic, i.e. power reflection less than ?20 dB within a frequency bandwidth of about 1 GHz around the chosen frequencies. In particular the influence of mechanical tolerances on the transmission characteristic has been investigated in order to specify the mechanical dimensions of the CVD-diamond disks and the window unit. In case of a Brewster window, the thickness has been optimized to get low power reflection over a wide angles range around the Brewster angle.     

Marginal adaptation of resin-bonded light-cured glass ionomers in dentin cavities

PURPOSE: To investigate the marginal adaptation of resin-modified glass ionomer cements in dentin cavities placed with or without additional application of resin bonding systems. MATERIALS AND METHODS: Three resin-modified materials (Fuji II LC, Photac-Fil, Vitremer), one compomer (Dyract) and as reference an adhesively bonded resin composite system (Gluma CPS-Pekafill) were used. Flat peripheral dentin surfaces on human molar teeth were produced by wet grinding on SiC paper. Cylindrical cavities, 3.5 mm wide, were prepared in these dentin areas and restored with the individual materials. Sixty cavities were pretreated and restored as requested by the respective manufacturers. Following water storage of the specimens for 15 minutes or 24 hours, excess was gently removed by wet grinding for microscopic inspection of the marginal area. Additionally, in 30 cavities an experimental one-component adhesive resin system, a proprietary dimethacrylate and HEMA mixture dissolved in acetone, was combined with each of the restoratives for evaluation after 15-minutes water storage. Finally, in six cavities each, Dyract was combined with Prime and Bond 2.0, and Vitremer with Scotchbond Multi-Purpose Plus for assessment after 15 minutes. Maximum marginal gap widths (MGW) were measured. One-way ANOVA by ranks (Kruskal-Wallis-Test) followed by Wilcoxon's Two-Sample test were used to study the statistical difference of MGW among the treatment groups at a rejection level P = 0.05. RESULTS: Neither the conventionally placed material systems nor the restorations in combination with adhesives showed consistently gap-free margins after 15-minute water storage. After 24-hour storage with Vitremer 4 of the 6 restorations were gap-free, whereas with all other materials only perfect margins were registered. There was, however, no significant difference between the groups. Application of the experimental and/or the specific resin bonding agents had no effect on early MGW except for the Photac-Fil group, which was significantly reduced.     

An XPS investigation of polymer surface dynamics. I. A study of surface modified by CF4 and CF4/CH4 plasmas

For making the surface hydrophobic, poly(ethylene terephthalate) (PET) films were subjected to treatment by CF₄ and CF₄/CH₄ plasmas, and the PET fabrics, to impregnation (padding) with Oleophobol S for comparison. The electronic structure of the modified surface was differenentiated by XPS (ESCA). The surface dynamics upon heating and dipping into water was investigated by the angle-dependent XPS technique. Application of the XPS technique to these topics gave a wealth of information. © 1993 John Wiley & Sons, Inc.     

Oxidative autoaggression by phagocytes in human peritonitis

In 60 abdominal exudates from patients with the diagnosis of either acute or persistent (chronic) peritonitis, indicators of phagocyte-derived oxidative systems (myeloperoxidase, chemiluminescence) and proteases (elastase) were measured. These exudates reveal a picture of maximal inflammatory activation. Both types of exudates (30 each) showed a significant influx of inflammatory cells, with the mean leucocyte count being 73,000 microL and 32,000 microL-1 respectively. Local myeloperoxidase concentrations were approximately 1000-fold greater than that of normal plasma. Spontaneous and elicitable chemiluminescence--indicators of phagocyte respiratory burst activity--were dramatically increased. In addition, levels of extracellularly released elastase (from neutrophils) were found to be up to about 1000-fold that of normal plasma values. Although most of the elastase detected in the exudates was complexed with alpha 1-proteinase inhibitor (alpha 1 PI), enzymatically active elastase could be measured in approximately 60% of the samples being investigated. As there was an excess of immunoreactive alpha 1 PI in these exudates, the free elastase activity implies that much of the alpha 1 PI was inactive, presumably subjected to oxidative destruction. Moreover, a trypsin-inhibitory activity to antigen ratio below 1 (mean = 0.81) in 75% of the purulent exudates indicated also partial proteolytic degradation of alpha 1 PI. In contrast, 16 clear exudates (no bacteria, white cell count below 500 microL-1) taken from the non-infected peritoneal cavity of patients undergoing intra-abdominal surgery revealed a similar permeability increase of the peritoneum but did not show relevant oxidative and proteolytic activity or destruction of alpha 1 PI compared with purulent specimens. Thus, only the inflammatory process of peritonitis appears to result in an overwhelming local phagocytic activity that initiates and maintains protease inhibitor consumption and/or inactivation. The tremendous oxidative potential found in purulent exudates may cause destruction in a wide variety of defence systems.     

Beitrag zur Frage des Verhaltens von Turbinen- und Generatorwellen im Warmrundlaufversuch

Analyse des Eigenspannungszustandes zylindrischer Körper. Einfluß unterschiedlicher Abkühlungsbedingungen nach dem Anlassen, nach dem Spannungsarmglühen und während des Warmrundlaufversuches sowie des Verzuges während des Anlassens und einer einseitigen mechanischen Bearbeitung auf das Verhalten im Warmrundlaufversuch. Einfluß der Prüftemperatur auf die mechanischen Eigenschaften. Empfehlungen für neue Prüfrichtlinien.     

Zur röntgenspektrometrischen Analyse von Ferrosilizium und Kalziumsilizium

Verwendung von gepulvertem Probegut. Ermittlung der ausreichenden Mahldauer durch Messung der spezifischen Oberfläche. Arbeitsbedingungen für die Messung. Aussehen der Auswertekurven (Einfluß der Nebenbestandteile). Erreichte Reproduzierbarkeit der Prüfung.