DECHEMA

About: DECHEMA is a nonprofit organization based out in Frankfurt am Main, Germany. It is known for research contribution in the topics: Corrosion & Oxide. The organization has 756 authors who have published 1307 publications receiving 25693 citations.

Beobachtungen über die Aufstickung hochhitzebeständiger austenitischer Werkstoffe in Luft und Verbrennungsgasen

Abstract: Schaden durch Aufstickung sind wiederholt an Strahlrohren aus hochhitzebestandigen Werkstoffen, die im Industrieofenbau Anwendung finden, beobachtet worden. Die Aufstickung erfolgt, obgleich die Werkstoffoberflache mit einer Oxidschicht bedeckt ist. Untersuchungen an langzeitig in Luft gegluhten Proben aus den Werkstoffen 1.4848 und 1.4840 zeigen, das Aufstickung nach einer von Temperatur und Probengeometrie abhangigen Gluhzeit einsetzt. Sie beginnt bei 900° C nach etwa 25.000 h, bei 1000° C nach etwa 1000 h und bei 1100° C nach bereits etwa 100 h bevorzugt an konvex geformten Oberflachenbereichen. Durch die Stickstoffaufnahme wird zunachst M23C6 in M2(C, N) umgeformt. Im weiteren Verlauf wachst dann M2(C, N) durch Chromentzug der Matrix. Das in diesen Werkstoffen auch auftretende M6C-Carbid der ungefahren Formel (Cr3Ni2Si)C wird durch die Aufstickung strukturell nicht verandert, jedoch wird ein Teil des C durch N substituiert. Ursache der Aufstickung ist wahrscheinlich ein wiederholtes Aufreisen und Ausheilen der Oxidschicht, sobald eine kritische Oxidschichtdicke uberschritten wird. Diese kritische Oxidschichtdicke wird zu etwa 20–30 mm abgeschatzt.

Synthesis of mesoporous carbon spheres via a soft-template route for catalyst supports in PEMFC cathodes

Abstract: Synthesis of carbon spheres via a soft-template route should be further improved for industrial applications especially in terms of time, cost and scalability. The present work reports on the relatively fast production of mesoporous carbon via an ammonia-catalyzed hydrothermal soft-template one-pot route denoted as CFAH with m-aminophenol as the carbon source and triblock copolymer Pluronic® F127 as the template. For comparison, an acidic route with resol as the carbon precursor (CFRH) was evaluated as well. The best results regarding particle size and pore distribution of the as-prepared CFRH and CFAH samples were obtained in 2 M HCl and 6 M NH4OH at 120 °C for 12 h and 700 °C pyrolysis temperature, respectively. GDE with CFRH and CFAH supported platinum showed excellent ECSA retention of about 60–70% during accelerated degradation testing under half-cell conditions compared to only 13% for GDE with Pt/CVulcan reference material.

Combined Al‐ plus F‐treatment of Ti‐alloys for improved behaviour at elevated temperatures

Abstract: Titanium is a widely used structural material because of its low specific weight, good mechanical properties and excellent corrosion resistance at ambient temperature. As a result of increased oxidation at elevated temperatures and environmental embrittlement the maximum operation temperature of standard Ti-alloys is only about 600 °C. The oxidation behaviour can be improved by different methods, e.g. coatings. This leads to an improvement which is, however, often limited. The combination of Al-enrichment in the sub surface zone, so that a TiAl-layer is formed, plus F-treatment gives impressively good results because a protective alumina scale is formed due to the fluorine effect. This alumina scale prevents oxygen inward diffusion which causes embrittlement and protects the material against environmental attack. The procedure is applied to alloys with a very low Al-content or even no Al at all. In the paper results of oxidation tests of α-Ti without any treatment, with Al-treatment and with a combination of Al- + F-treatment are presented. Aluminium was diffused into the samples by a powder pack process. Fluorine was applied by a liquid phase process. The formation of an alumina scale on treated samples was revealed by post experimental investigations. The results are discussed referring to the fluorine effect model for TiAl-alloys.

Bridging the bridge—Stabilization of CalB against H2O2 and its application in epoxidation reactions

Abstract: Chemo-enzymatic reactions combine the advantages of chemistry and biocatalysis. Chemical synthesis provides precursor molecules and low costs, whereas the application of enzymes provides selectivity. Additionally, the mild reaction conditions of enzymatic approaches allow the chemical application of intermediates and/or products that otherwise are not accessible. One example is the in situ production of peroxycarboxylic acids (peracids) by Candida antarctica lipase B (CalB). In contrast to the harsh conditions of chemical peracid synthesis, CalB catalyzed reactions run at low temperatures and without additives. Unfortunately, the enzyme is rapidly inactivated by the oxidative environment. Herein, we report on CalB stabilization by preventing disulfide cleavage after H2O2 exposure. Therefore, a bismaleimide functionalized linker was used to convert all the enzyme’s disulfide bridges to more stable thioether linkages. These bonds are still affected by hydrogen peroxide but will not open upon oxidation. A two- to fourfold excess of this linker was optimal to avoid enzyme oligomerization. At the same time, a 1.5 fold increase in half life time after exposure to hydrogen peroxide was achieved. To our knowledge, such an approach to intramolecular disulfide stabilization has never been reported before, but might be a general strategy for enzyme engineering. Furthermore, a carrier screening was performed, identifying different optimal carrier types for CalB immobilization. The combination of stabilization by disulfide conjugation and immobilization was expanded by the adoption of a thermostabilized double mutant. Finally, chemo-enzymatic epoxidations of alkenes were examined in batch experiments and under continuous process conditions. Activity loss was reduced by 50% and a 75% increase in long-term stability was achieved in comparison to the commercial preparation.

Reactive Element Effect Applied by Alloying and SiHfBCN Coating on the Oxidation of Pure Chromium

Abstract: To reduce scale spallation, scale volatilization, and nitrogen embrittlement in Cr-based systems, reactive elements such as Y, Zr, Hf, La, and Ce were introduced as oxide dispersions into pure chromium. In addition, Hf coating systems were investigated. One was a Hf sputter layer with varying thickness, and the other one a Hf-containing precursor ceramic, i.e., SiHfBCN, which may be considered as suitable material for environmental barrier coating applications. Oxidation tests at $$1050\,^{\circ }\hbox {C}$$ in synthetic air for 50 h were carried out using thermogravimetric analysis. The samples were analyzed via X-ray diffraction, optical microscope, electron microprobe analysis, and scanning electron microscope. All reactive elements led to a decrease in total mass gain after oxidation compared to pure Cr, with Y and Zr showing the strongest effect. Improvements in oxide attachment, oxide growth rate, volatilization rate as well as nitridation resistance were observed. Concerning these experiments, Y showed the most promising results. Concerning Hf, coating systems, especially SiHfBCN, showed a higher effect on improving the oxidation resistance. The reason for this outcome might be that not only Hf is active in the precursor ceramic layer. A more complex oxide layer has formed, which consisted of not only Cr2O3 but also of Hf and Si oxides. This layer prevents the material from any nitridation under the selected oxidation conditions.