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Eingeladener Vortrag
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ln den letzten Jahren wurden diverse Prüfverfahren und Methoden entwickelt, um die Leistungsfähigkeit von Beton bzw. Bindemittel hinsichtlich des Widerstands gegenüber betonaggressiven Einwirkungen beurteilen zu können. Ein wesentliches Ziel von Prüfverfahren zur Beurteilung der Dauerhaftigkeit von Baustoffen ist, die im Laufe der Nutzungsdauer zu erwartenden Einwirkungen auf den Baustoff realitätsnah abzubilden. Dazu sind die Prüfparameter auf die im jeweiligen Anwendungsbereich vorherrschenden Umgebungs- und Randbedingungen anzupassen. Daneben sind auch die Bewertungskriterien individuell unter Berücksichtigung der im Einzelfall geforderten Nutzungsdauer eines Bauwerks festzulegen. Eine systematische Zuordnung von adäquaten Prüfverfahren bzw. Prüfparametem und Bewertungskriterien für die verschiedenen Anwendungsbereiche im Betonbau, in denen betonaggressive Einwirkungen auftreten, ist mit Ausnahme von Einzelfällen bislang nicht vorhanden. Daneben stellt sich in der Praxis immer wieder das Problem, das Angriffspotenzial der Umgebung eines Bauwerks (prüftechnisch) zu erfassen und zu bewerten.
Der Hauptausschuss Baustofftechnik des DBV hat daher den Arbeitskreis „Chemischer Angriff auf Beton“ beauftragt, dieses Merkblatt zu erarbeiten, das auf Basis von einschlägigen Erfahrungen der Baupraxis und wissenschaftlichen Erkenntnissen Grundsätze und bewährte Methoden zur Beurteilung der Einwirkungs- und Widerstandsseite beim chemischen Angriff auf Beton aufzeigt und diesbezüglich Hinweise für verschiedene Anwendungsbereiche im Betonbau liefert.
Aiming at the overall negative surface charge of bacteria, a new strategy of antibacterial agents based on large polymer-modified graphene oxide (GO) sheets is assessed. The presented flexible, polycationic Sheets match the size and charge density of the Escherichia coli surface charge density (2 × 1014 cm−2). These matching parameters create an unspecific but very strong bacteria adsorber by multivalent, electrostatic attraction.
Their interaction with bacteria is visualized via atomic force and confocal microscopy and shows that they effectively bind and wrap around E. coli cells, and thereby immobilize them. The incubation of Gram-negative and -positive bacteria (E. coli and methicillin-resistant Staphylococcus aureus, MRSA) with these polycationic sheets leads to the inhibition of proliferation and a reduction of the colony forming bacteria over time.
This new type of antibacterial agent acts in a different mode of Action than classical biocides and could potentially be employed in medicinal, technical, or agriculture applications. The presented microsheets and their unspecific binding of cell interfaces could further be employed as adsorber material for bacterial filtration or immobilization for imaging, analysis, or sensor technologies.
Investigation of the orientation relationships of carlsbergite in the North Chile iron meteorite
(2005)
A naphthalimide-based fluorescent indicator monomer 1 for the integration into chromo- and fluorogenic molecularly imprinted polymers (MIPs) was synthesized and characterized. The monomer was equipped with a urea binding site to respond to carboxylate-containing guests with absorption and fluorescence changes, namely a bathochromic shift in absorption and fluorescence quenching. Detailed spectroscopic analyses of the title compound and various models revealed the signaling mechanism. Titration studies employing benzoate and Z-ʟ-phenylalanine (Z-ʟ-Phe) suggest that indicator monomers such as the title compound undergo a mixture of deprotonation and complex formation in the presence of benzoate but yield hydrogen-bonded complexes, which are desirable for the molecular imprinting process, with weakly basic guests like Z-ʟ-Phe. Compound 1 could be successfully employed in the synthesis of monolithic and thin-film MIPs against Z-ʟ-Phe, Z-L-glutamic acid, and penicillin G. Chromatographic assessment of the selectivity features of the monoliths revealed enantioselective discrimination and clear imprinting effects. Immobilized on glass coverslips, the thin-film MIPs of 1 displayed a clear signaling behavior with a pronounced enantioselective fluorescence quenching dependence and a promising discrimination against cross-analytes.
The crystallographic orientation of carlsbergite (CrN) in the north Chile meteorite (hexahedrite) was investigated using electron backscatter diffraction and transmission electron microscopy. These studies examined the CrN crystals in the rhabdites (idiomorphic schreibersite) and in kamacite. It was found that the CrN crystals embedded in rhabdite show a number of different orientation relationships with the host crystals. These orientations can be explained based on the lattice dimensions of both coexisting crystalline materials. It was also found that both carlsbergite and kamacite are characterized by a high dislocation density (109 cm2) while rhabdite is free of dislocations. It is supposed that in spite of the deformed metallic matrix, a general connection between the orientation relation of all the phases involved exists.
Integration of a polymerisable fluorescent urea-nitrobenzoxadiazole monomer 1 into a few-nanometre thin molecularly imprinted polymer (MIP) shell coated onto a 300 nm silica core with the tetrabutylammonium salt of N-carbobenzyloxy-L-phenylalanine (Cbz- or Z-ʟ-Phe) as the imprinted template yields core/shell MIP nanoparticles that respond to the designated analyte with a strong fluorescence increase in the visible spectral range. The system shows promising imprinting and enantioselectivity factors of 3.6 and 1.9 when benzylmethylacrylate (BMA) is used as co-monomer and ethylene glycol dimethacrylate (EDMA) as cross-linker. A second, naphthalimide-based fluorescent urea 2 is employed in the preparation of MIP thin-films and monolithic polymers for separation techniques. Spectroscopic and chromatographic studies of the MIPs with the analytes Z-ʟ-Phe, Z-ʟ-glutamic acid (Z-ʟ-Glu) and penicilin G (PenG) revealed the (enantio)selective discrimination behaviour of these materials.
Iron-based catalysts are employed in CO2-FTS due to their ability to convert CO2 into CO in a first step and their selectivity towards higher hydrocarbons in a second CO hydrogenation step. According to the literature, iron carbides represent the active phase for hydrocarbon formation and are claimed to emerge in the presence of CO. We propose nanostructured FeOx films as model systems to assess information about the complex phase transformations during CO2-FTS. Mesoporous hematite, ferrihydrite, maghemite, maghemite/magnetite films were exposed to CO2-FTS atmospheres at 20 bar and 300°C. Up to three distinct phases were observed depending on the timeon-stream (TOS): a sintered maghemite/magnetite phase, a carbidic core-shell structure, and a low-crystalline, needle-type oxide phase. Our findings indicate that the formation of an intermediary maghemite/magnetite phase, predominant after short TOS (30 h), precedes the evolution of the carbide phase.
Yet, even after prolonged TOS (185 h), no full conversion into a bulk carbide is observed.
Biofilm formation and microbially influenced corrosion of the iron-reducing microorganism Shewanella putrefaciens were investigated on stainless steel surfaces preconditioned in the absence and presence of flavin molecules by means of XANES (X-ray absorption near-edge structure) analysis and electrochemical methods. The results indicate that biofilm formation was promoted on samples preconditioned in electrolytes containing minute amounts of flavins. On the basis of the XANES results, the corrosion processes are controlled by the iron-rich outer layer of the passive film. Biofilm formation resulted in a cathodic shift of the open circuit potential and a protective effect in terms of pitting corrosion. The samples preconditioned in the absence of flavins have shown delayed pitting and the samples preconditioned in the presence of flavins did not show any pitting in a window of −0.3- to +0.0-V overpotential in the bacterial medium. The results indicate that changes in the passive film chemistry induced by the presence of minute amounts of flavins during a mild anodic polarization can change the susceptibility of stainless steel surfaces to microbially influenced corrosion.