84 search hits
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Assessment of the environmental acceptability of refrigerants by discrete mathematics: cluster analysis and Hasse diagram technique
(2008)
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Guillermo Restrepo
- The recognition of the adverse environmental impact of chlorofluorocarbons (CFCs), mainly used as refrigerants, has lead to look for environmentally acceptable CFC replacements. Main environmental concern CFCs face is their ability to deplete the stratospheric ozone layer, quantified by the ozone depletion potential (ODP). Some of the first replacements mooted were hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), which contribute to the global warming, quantified by the global warming potential (GWP). ODP and GWP are related to the atmospheric lifetime (ALT), a third indicator. Hence, the environmental impact of a refrigerant may be characterised by a triple of ODP, GWP and ALT values. In this respect, an acceptable refrigerant is a chemical with low ODP, GWP and ALT values. One of the first steps to assess the environmental acceptability of refrigerants is to classify them to find substances with common features. Hence, a supervised and unsupervised classification was performed over 40 refrigerants. First one was a classification based upon elemental composition and functional groups present in refrigerant molecules which leads to: CFCs, HCFCs, HFCs, hydrocarbons (HCs), hydrofluoro ethers (HFEs), chloromethanes (CMs), carbon dioxide, trifluoroiodomethane, dimethyl ether and ammonia. The unsupervised classification was performed using hierarchical cluster analysis. In this case, refrigerants were characterised by three kinds of descriptors: Environmental properties (ODP, GWP, ALT), thermodynamic features related to their refrigeration performance and molecular descriptors derived from their molecular structure. Eight clustering methodologies were applied to each kind of refrigerant descriptors. To assess the stability of these classifications, the cluster index, a method for quantifying classification similarities was developed and further applied to refrigerant results. It was found that environmental descriptors are the only case in which refrigerant classes are stable when varying the classification method. The chemotopological procedure, a method for studying similarity relationships, was applied to the environmental classification of refrigerants. It was found that CFCs are similar to themselves and also to 1,1,1,3,3,3-hexafluoropropane, a HFC. The most similar substances to all CFCs considered were trichlorofluoromethane and 1,1,2-trichloro-1,2,2-trifluoroethane. The other refrigerant families were found to be similar to many other substances, therefore there is no clear affiliation of refrigerants of one family to one certain class. It was found a disagreement between the supervised classification leading to refrigerant families and the three unsupervised classifications (environmental, thermodynamic and molecular ones). Therefore, refrigerant classification into families does not imply same classification based upon environmental properties, thermodynamic features and molecular descriptors. A different refrigerant classification was performed, i.e. the one based upon order relationships of refrigerant environmental properties. In this case the Hasse diagram technique, a method based on partial order theory, was applied to the 40 refrigerants characterised by environmental properties. A parameter free procedure for ordering classes based upon order relationships of their elements was developed. For that purpose, the dominance and separability degrees were introduced, first one indicates the extent to which members of one class hold higher descriptor values than the members of another class; while separability degree quantifies the lack of order relationships between two classes. Dominance and separability degrees were related by a theorem. By the application of dominance and separability degrees to refrigerant families three main classes were detected: problematic substances, gathering CFCs, octafluorocyclobutane and bromochlorodifluoromethane; least problematic ones, collecting HCs, CMs, carbon dioxide, trifluoroiodomethane, dimethyl ether and ammonia; and moderately problematic refrigerants, made from HCFCs, HFCs and HFEs. It was found that some HFEs are not dominated by CFCs, which raises the question on the applicability of these substances as environmentally acceptable replacements. METEOR (Method of evaluation by order theory), a procedure for prioritising descriptors and studying its effect on the order relationships of the objects considered was discussed. When applied to the refrigerants, the effect of prioritising ODP, GWP and ALT in the order relationships of these substances was studied. It was found that pentafluorodimethyl ether, a HFE, is one of the most problematic refrigerants under a large range of priorities of the environmental properties considered. Due to the mathematical generality of the methods here introduced, they are not restricted to the analysis of refrigerants but can be used to the study of different sets whose elements are characterised by various attributes.
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Elasticity measurements at extreme conditions: application to FeO and FeNi-alloy
(2008)
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Anastasia Kantor
- The picture of Earth’s deep interior is rapidly improving from the seismic tomography data and indicates more complexity than previously thought. The presence of Earth’s seismic anisotropy requires the knowledge of fully anisotropic elasticity data for mineral phases. The single-crystal elastic constants of minerals, Cij, are elements of the fourth-rank elasticity tensor, which relates stress to strain. The fact that elastic strain also defines seismic wave propagation, the elastic tensor of minerals can be applied to interpret the bulk mineralogy of the interior from seismological observation. Knowledge of the elasticity of crystalline materials as a function of pressures and temperature is also of primary interest for solid state physics because elastic tensors reveal the nature of interatomic interactions. In order to determine the full elastic tensor of minerals under high pressure and temperature, several techniques are available, including ultrasonic interferometry and inelastic x-ray scattering methods. One of the most accurate techniques is high-frequency acoustic interferometry, which is capable for measuring sound wave velocities in very small samples under high pressures. The ultrasonic interferometry system operating at 0.5-2.0 gigahertz (GHz) frequencies was developed in the Bavarian Geoinstitut of the University of Bayreuth for in situ high pressure and temperature experiments. Here, GHz-ultrasonic interferometry has been used to study the elastic properties of monoxide minerals such as FeO, liquids and nanocrystalline samples, each with particular importance to Earth or material sciences. FexO, wüstite, is the end-member phase of the (Mg,Fe)O solid solution, thought to be the most abundant non-silicate oxide in the mantle. The full elastic tensor of wüstite is determined by three elastic constants (C11, C12, C44), which have been probed at high-pressures. At about 17-20 GPa, FeO is known to undergo a displacive cubic-to-rhomobhedral phase transformation. Prior to this transformation, we observe a pressure-induced mode softening of the C44 elastic constant. In addition, previously undetected discontinuities in the pressure derivatives of C11 and C12 at 4.7  0.2 GPa were observed. This pressure is consistent with that of the magnetic ordering commencement, as was observed by high-pressure Mössbauer spectroscopy in a 57Fe-enriched sample of FeO. The results indicate that an intermediate, partially magnetic but still cubic phase of FeO probably exists at room temperature and in pressure range from ~5 GPa to ~17 GPa. In order to provide deeper knowledge of the magneto-elastic coupling in the material, neutron diffraction experiments were performed under ambient pressure and low temperatures. The results indicate that the magnetically ordered cubic phase of FexO that was observed at high pressures also exists at ambient pressure at temperatures between 160 and 201 K. Combined inelastic x-ray scattering and x-ray diffraction studies on a single crystal of Fe0.95O were performed up to 20 GPa at room temperature. The results show strong anelastic behaviour of wüstite, which should be accounted for at high pressure. Transition-metal oxides, non-stoichiometric compounds, and materials with complex mesostructure have some internal degree of freedom, and could therefore experience internal relaxation and show deviations from normal elastic behaviour. A methodology to measure inelastic x-ray scattering in externally heated diamond anvil cells have also been developed. This technique was used to study polycrystalline fcc-Fe0.78Ni0.22 alloy at high pressures (up to 72 GPa) and temperature (up to 715 K). The bulk elasticity and its P and T derivatives were obtained for the material. No significant deviation of the elastic properties from those of pure iron was observed and furthermore no deviation from Birch’s law. Although the bulk elasticity of fcc Fe-Ni alloy and Fe seem to be very similar, the elastic anisotropy of hexagonal and cubic phases should be quite different. If the metal phase in the inner core is not hexagonal, but cubic (or a mixture of the two phases exists), seismic anisotropy may provide a better way to discriminate between them two.
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Photochemische Abbauprozesse von polybromierten Diphenylethern in homogenen und heterogenen Systemen
(2008)
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Ana Maria Geller
- Ziel dieser Arbeit war es, die photochemischen Abbauprozesse von technischen Produkten zu untersuchen, die als Flammschutzmittel verwendet werden: die polybromierten Diphenylether (PBDEs) Deca-, Octa- und PentaBDE. Die Bestrahlung der kommerziellen Mischungen wurde in einer Karussell-Bestrahlungsapparatur und/oder mittels Gesichtsbräuner durchgeführt. Für den photolytischen Abbau unter umweltrelevanten Bedingungen ist ein sonnenähnliches Spektrum notwendig. Dieses wurde durch die eingesetzte Xenon-Lampe (XOP-7, 500 W) mit einem Filter aus Duranglas (1,5 mm Wandstärken) und durch 4 oder 6 UV-Leuchtstoffröhren (TL29D16, 16 W) erreicht. Der Verlauf der Photolyse wurde mit HPLC-DAD und/oder mit GC-MS verfolgt. Von den PentaBDEs sind BDE 99 und BDE 100 am häufigsten in der Umweltprobe aufgetreten und haben zugleich den größten Gewichtsanteil in der kommerziellen Mischung DE-71. Während der Photolyse von BDE 100, 99 und 85 wird hauptsächlich TetraBDE 47 gebildet. Die Geschwindigkeitskonstanten für die Debromierung von BDE 85 und BDE 99 waren mit beiden Lampen deutlich größer als bei BDE 100. Die Quantenausbeuten der PentaBDEs 85, 99 und 100 in THF/Hexan (2/1 v/v) betrugen 0,22, 0,17 und 0,13. Die größere Quantenausbeute für BDE 85 verursacht den schnelleren Photoabbau dieses Penta-Kongeners im Vergleich zu BDE 99 und BDE 100. BDE 100 wurde als das photostabilste der drei Penta-Kongeneren der kommerziellen Mischung DE-71 identifiziert. Der Verlauf der Photolyse der kommerziellen Mischung DE-79 zeigt einen schnellen Abbau von DecaBDE 209 und NonaBDE 207+208. Nachdem DecaBDE 209 mehr als zur Hälfte abgebaut ist, beginnt die Debromierung des NonaBDE 206 mit einer Geschwindigkeit, die ca. der Hälfte der Geschwindigkeit der Summe BDE 207+208 entspricht. Auch beim Abbau der OctaBDEs 203, 197 und 196 und von HeptaBDE 180 in der kommerziellen Mischung DE-79 handelt es sich um Folgereaktionen mit fortschreitender Debromierung. Die Quantenausbeuten in THF der DecaBDE 209 und NonaBDE 207 waren 0,20 bzw. 0,14. Die Photodebromierung von HeptaBDE 183 als einzelnes Kongener verläuft über die Bildung von 7 Hexa-Kongeneren, hauptsächlich bei BDE 153. Eine Quantenausbeute von 0,11 wurde für den Abbau von BDE 183 in THF/Hexan (2/1 v/v) bestimmt. DecaBDE 209 wurde in THF-Lösung und auf anorganischem Trägermaterial photolysiert. Die Lebensdauer von DecaBDE 209 in Lösung ist ca. 1,7 Stunden. Als Octa-Produkte aus dem Abbau von DecaBDE 209 bzw. NonaBDEs traten (in Reihenfolge der Häufigkeit) BDE 196 > BDE 203 > BDE 201 > BDE 197 auf. Es wurden keine PBDEs mit Bromierungsgraden < 5 bei der Photolyse von DecaBDE auf den anorganischen Trägermaterialien Quarzglaspulver (Aerosil 380), Mikro-Glashohlkugeln und Kieselgel gefunden. Eine schnellere Debromierung von DecaBDE wurde auf Kieselgel und Aerosil 380 im Vergleich zu den Mikro-Glashohlkugeln beobachtet. Die unterschiedlichen Trägermaterialen haben einen Einfluss auf die Abbaugeschwindigkeit von DecaBDE, aber das Photolyseprodukt-Muster bleibt gleich. Bei der Photolyse der kommerziellen Mischung DE-79 und von DecaBDE 209 in Lösung wurden gleichzeitig mit der Debromierung der PBDEs verschiedene PBDFs und sogar das Muttermolekül Dibenzofuran gebildet. Die Bildung von polybromierten Dibenzofuranen spielt eine wichtige Rolle in dem Abbauweg bei der Photolyse von PBDEs mit höherem Bromierungsgrad. In der kommerziellen Mischung DE-79 nach 48 h Bestrahlung liegt der Gewichtsanteil von PBDFs bei. ca. 17 %. Während beim DecaBDE in Lösung nach 46 h Bestrahlung der Gewichtsanteil von PBDFs bei ca. 13 % liegt. Die PBDFs wurden bei der Photolyse von DecaBDE auf anorganischem Trägermaterial hauptsächlich bei Versuchen mit dem Kieselgel beobachtet. Bei DecaBDE in Lösung wurden polybromierte Dibenzofurane mit kleinerem Bromierungsgrad als bei der gesättigten Lösung mit Bodensatz und auf anorganischem Trägermaterial gebildet. Die Bildung der toxischen 2,3,7,8-bromierten Dibenzofurane konnte in allen Versuchen ausgeschlossen werden.
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A Scalability Analysis of Grid allocation Mechanisms
(2008)
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Michael Reinicke
Werner Streitberger
Torsten Eymann
- This article examines the broker´s behavior with regard to a varying number of participating nodes and shows that incremental losses have to be accepted in central resource allocation when introducing new nodes.
