Wärmeübergang und Druckverlust wässeriger Tensidlösungen an einer querangeströmten berippten Rohrwendel

Zusammenfassung Durch Zugabe geeigneter Reibungsminderer in das Wasser von Fernw?rmenetzen kann der auftretende Druckverlust betr?chtlich gesenkt und somit die ben?tigte elektrische Antriebsenergie der F?rderpumpen reduziert werden. Neu entwickelte Tensidsysteme erfüllen die Anforderungen der Fernw?rmepraxis an die thermische und mechanische Belastbarkeit. Ein wichtiger Aspekt ist dabei die Beeinflussung des W?rmeüberganges in den für Fernw?rmenetze gebr?uchlichen W?rmeübertragern. In früheren Arbeiten wurden der W?rmeübergang und der Druckverlust w?sseriger Tensidl?sungen in geraden Rohren und in Rohrwendeln untersucht. Aufbauend auf diesen Erkenntnissen werden in dieser Arbeit der W?rmeübergang und der Druckverlust w?sseriger Tensidl?sungen bei umstr?mten Rippenrohren, wie sie bei Brauchwassererw?rmern auftreten, untersucht. Das Ziel ist die Darstellung und Beschreibung der auftretenden Effekte beim Einsatz solcher L?sungen, wobei aufgrund der komplexen Str?mungszust?nde bisher keine Berechnungsgleichungen entwickelt werden konnten.     

Evidence for the Aging of Wax Deposits in Crude Oils by Ostwald Ripening

The ageing of wax deposits is known to occur in paraffinic waxes forming in pipelines and storage tanks. It is characterized by a hardening of the deposit and an increase in paraffin content. A mechanism for the ageing of deposits, based on diffusion caused by temperature-concentration gradients within the wax, has been previously proposed. This work presents evidence indicating Ostwald Ripening to be another ageing mechanism of wax deposits. Rheology of paraffinic crudes, kept isothermally at temperatures in the neighborhood of the pour point, shows a kinetic of hardening of the oil samples. The X-ray diffraction and Cross Polar Microscopy indicate this phenomenon to be caused by an increase of the crystallites size with time. The DSC measurements support the idea that recrystallization takes place in the wax sample. The evidence gathered shows another ageing mechanism based on the recryztallization of the paraffins, such as Ostwald Ripening, to be responsible for the ageing of wax deposits besides the diffusion mechanism previously proposed     

Placing underground pressure pipes by machine

A machine system has been patented in the German Democratic Republic which links a digger and pipe placing unit for installing and connecting PVC pressure pipes underground with the minimum of soil displacement. The authors are engineers working at the Academy of Building, Institute for Civil Engineering, Leipzig.     

4-Methylimidazol in Caramel und caramelgef?rbten Lebensmitteln

Zusammenfassung Es wird eine spezifische und kochempfindliche Methode zur Bestimmung von 4-Methylimidazol (4-MeI) in Caramel und caramelgefärbten Lebensmitteln beschrieben. Nach der Extraktion wird 4-MeI gaschromatographisch mit einem stickstoffspezifischen elektrochemischen Detektor (Hall) bestimmt. Als weniger spezifische Alternative kann ein thermoionischer Detektor eingesetzt werden. Bei Zusätzen von 5–200 mg 4-MeI/kg Karamel beträgt die Wiederfindungsrate 90–94%.Die Nachweisgrenze der Methode liegt unterhalb 0,1 mg 4-MeI/kg Caramel; in karamelgefärbten Lebensmitteln können weniger als 0,01 mg/kg nachgewiesen werden.

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4-methylimidazole in caramel and caramel-colored foodsDetermination by gas-liquid-chromatography with nitrogen-specific detectors

Summary A specific and highly sensitive method for the determination of 4-methylimidazole (4-MeI) in caramel and caramel-colored foods is described. After extraction the amount of 4-MeI is determined by gas-liquid chromatography with a nitrogen-specific electrochemical detector (Hall). As a less specific alternative a thermoionic detector can be used. With additions in the range of 5–200 mg 4-MeI/kg caramel, recoveries of 90–94% are obtained.The limit of detection is <0.1 mg 4-Mel/kg caramel; in caramel-colored foods less than 0.01 mg/kg can be detected.

     

Molecule–Graphene Hybrid Materials with Tunable Mechanoresponse: Highly Sensitive Pressure Sensors for Health Monitoring

The development of pressure sensors is crucial for the implementation of electronic skins and for health monitoring integrated into novel wearable devices. Tremendous effort is devoted toward improving their sensitivity, e.g., by employing microstructured electrodes or active materials through cumbersome processes. Here, a radically new type of piezoresistive pressure sensor based on a millefeuille‐like architecture of reduced graphene oxide (rGO) intercalated by covalently tethered molecular pillars holding on‐demand mechanical properties are fabricated. By applying a tiny pressure to the multilayer structure, the electron tunnelling ruling the charge transport between successive rGO sheets yields a colossal decrease in the material's electrical resistance. Significantly, the intrinsic rigidity of the molecular pillars employed enables the fine‐tuning of the sensor's sensitivity, reaching sensitivities as high as 0.82 kPa^?1 in the low pressure region (0–0.6 kPa), with short response times (≈24 ms) and detection limit (7 Pa). The pressure sensors enable efficient heartbeat monitoring and can be easily transformed into a matrix capable of providing a 3D map of the pressure exerted by different objects.     

Photomodulation of Charge Transport in All-Semiconducting 2D–1D van der Waals Heterostructures with Suppressed Persistent Photoconductivity Effect

Van der Waals heterostructures (VDWHs), obtained via the controlled assembly of 2D atomically thin crystals, exhibit unique physicochemical properties, rendering them prototypical building blocks to explore new physics and for applications in optoelectronics. As the emerging alternatives to graphene, monolayer transition metal dichalcogenides and bottom-up synthesized graphene nanoribbons (GNRs) are promising candidates for overcoming the shortcomings of graphene, such as the absence of a bandgap in its electronic structure, which is essential in optoelectronics. Herein, VDWHs comprising GNRs onto monolayer MoS₂ are fabricated. Field-effect transistors (FETs) based on such VDWHs show an efficient suppression of the persistent photoconductivity typical of MoS₂, resulting from the interfacial charge transfer process. The MoS₂-GNR FETs exhibit drastically reduced hysteresis and more stable behavior in the transfer characteristics, which is a prerequisite for the further photomodulation of charge transport behavior within the MoS₂-GNR VDWHs. The physisorption of photochromic molecules onto the MoS₂-GNR VDWHs enables reversible light-driven control over charge transport. In particular, the drain current of the MoS₂-GNR FET can be photomodulated by 52%, without displaying significant fatigue over at least 10 cycles. Moreover, four distinguishable output current levels can be achieved, demonstrating the great potential of MoS₂-GNR VDWHs for multilevel memory devices.