1,326 research outputs found
Liquid pair correlations in four spatial dimensions: Theory versus simulation
Using liquid integral equation theory, we calculate the pair correlations of
particles that interact via a smooth repulsive pair potential in d = 4 spatial
dimensions. We discuss the performance of different closures for the
Ornstein-Zernike equation, by comparing the results to computer simulation
data. Our results are of relevance to understand crystal and glass formation in
high-dimensional systems
Automatisch melken: risicofactoren voor de uiergezondheid = Automatic milking: risk factors for udder health
An epidemiological study is performed on 150 dairy farms milking with an automatic milking system to identify possible risk factors for udder healt
Heating and Trapping of Electrons in ECRIS from Scratch to Afterglow
Plasmas in Electron Cyclotron Resonance Ion Sources (ECRIS) are collisionless and can therefore be simulated by just following the motion of electrons in the confining static magnetic and oscillating microwave (MW) electric field of ECRIS. With a powerful algorithm the three-dimensional trajectories of 104 ECR-heated and confined electrons are calculated in a standard ECRIS with a deep minimum of |B| and a new ECRIS with a very flat minimum of |B|. The spatial electron (plasma) densities and electron energy densities deduced from these trajectories yield new and surprising insight in the performance of ECRIS. With computer animation we plan to present: The energy increase of certain electrons on extremely stable trajectories, the power dependence of the electron energy density up to the X-ray collapse, the time dependent build up of the electron density and energy density distributions, and the time evolution of these electron distributions under afterglow conditions
Successful modeling, design, and test of electron cyclotron resonance ion sources
Plasmas in electron cyclotron resonance ion sources (ECRIS) are collisionless and can therefore be simulated by only following the motion of electrons in the confining static magnetic and oscillating microwave field of ECRIS. The experimental performance of three different ECRIS is successfully compared to calculated spatial electron (plasma) and electron energy density as well as to the energy spectrum and the average energy of the electrons. Further simulations suggest a new and better design of an ECRIS, the good experimental performance of which corresponds to the predictions
Performance and modeling of superconducting ring resonators at millimeter-wave frequencies
Microstrip ring resonators operating at 35 GHz were fabricated from laser ablated YBCO thin films deposited on lanthanum aluminate substrates. They were measured over a range of temperatures and their performance compared to identical resonators made of evaporated gold. Below 60 Kelvin the superconducting strip performed better than the gold, reaching an unloaded Q approximately 1.5 times that of gold at 25 K. A shift in the resonant frequency follows the form predicted by the London equations. The Phenomenological Loss Equivalence Method is applied to the ring resonator and the theoretically calculated Q values are compared to the experimental results
Effects of interface morphology and geometry on the thermoelectric properties of artificially structured ZnO-based thin-films
Thermoelectricity may play a major role in waste heat recovery of fossil fuel consuming devices. Unfortunately thermoelectric generators to date only have poor conversion efficiencies (5 %). One way to improve the efficiency is to improve the performance of the active thermoelectric material. For this the figure of merit Z is given by
Z=(S^2 sigma)/kappa,
where S denotes the Seebeck coefficient, sigma the electrical conductivity, and kappa; the thermal conductivity. Z can be improved by either increasing the numerator S^2 sigma; (the so called power factor) or decreasing the denominator.
The typical and best understood thermoelectric materials so far are based on Te, such as Bi2Te3 or PbTe. Unfortunately, for a mass application of thermoelectric devices, estimations show that the tellurium resources will be consumed very quickly. Hence it is worth trying to develop novel thermoelectric materials which are more sustainable and green. Exemplarily the thermoelectric properties of ZnO as an ideal model system were investigated in the framework of this thesis.
Main goal of the work was to get a better understanding of the influence of effects on the microscopic length scale (e.g. due to thin-films, grain boundaries, artificial structuring) on the macroscopic behavior of the sample. In this context the following results were found:
Investigations of degenerately doped thin ZnO:Al films and subsequent annealing in air showed that at very high carrier concentrations, where the samples have metallic character, a sign reversal of S may occur. Although the sample is clearly n-type, small positive Seebeck coefficients can be measured, changing their sign with decreasing temperature. This is due to changes of the density of states at the Fermi-energy in a degenerately doped semiconductor.
The energy filtering effect due to grain boundaries, e.g. the increase of the power factor with increasing carrier concentration only works to a certain extend: If the carrier concentration n exceeds a certain value, screening effects diminish the barrier height and width leading to a decrease of the power factor.
Concerning the investigation of interfaces first measurements on a multilayer sample series of alternating ZnO/ZnS layers in in-plane geometry gave hints for the formation of interface layers of very high electrical conductivity between ZnO and ZnS, dominating the transport behaviour at large layer thicknesses (d > 100 nm). At smaller d, where d becomes comparable to the typical fluctuation length of the interface roughness, the transport path and hence the thermoelectric properties are strongly determined by the surface fluctuations. These results could be approved qualitatively by simulations within a Network Model (NeMo).
Stronger impact on the thermoelectric parameters, especially on the thermal conductivity, were found in cross plane direction, i.e. perpendicular to the interfaces. Unfortunately measurements of multilayers in cross-plane direction are very difficult to perform. To overcome this problem lateral structuring of thin-films offers attractive possibilities. To realize bar structures of alternating materials the method of self-aligned pattern transfer was developed and employed.
Measurements perpendicular to the interfaces show that the number of interfaces as well as their shape (i.e. length) and morphology has a strong influence on the power factor. Supported by numerous NeMo simulations the results indicated that the thermoelectric properties across the sample are dominated by the shortest path of electrical conductance. The transport path is strongly influenced by assuming space-charge regions of different width and conductivity. Best agreement between experiment and simulations has been achieved by replacing a certain fraction of the lowly conducting material with a highly conducting space-charge region. However, the origin of this highly conducting surface region requires further clarifications.
The findings of this work suggest that due to its high Seebeck coefficients and the possibility to tune the electrical conductivity by doping, ZnO is a promising candidate for an environmentally friendly and sustainable n-type thermoelectric material. The fact that its thermal conductivity is quite high may be overcome by a combination with ZnS. However this back door shown by theory still needs to be approved by experiment.Thermoelektrizität kann eine wichtige Rolle bei der Nutzung der bei der Verbrennung fossiler Rohstoffe entstehenden Abwärme spielen. Leider weisen thermoelektrische Generatoren bisher nur geringe Wirkungsgrade (5%) auf. Eine Möglichkeit, die Effizienz zu verbessern, ist die Leistung des thermoelektrisch aktiven Materials zu verbessern. Kennzahl dafür ist der Gütefaktor Z
Z=(S^2 sigma)/kappa,
wobei S den Seebeck-Koeffizienten, sigma die elektrische Leitfähigkeit und kappa die thermische Leitfähigkeit bezeichnen. Z kann entweder durch Erhöhen des Zählers S^2 sigma (der sog. Leistungsfaktor) oder Verringern des Nenners verbessert werden.
Die zurzeit typischen und am besten verstandenen thermoelektrischen Materialien basieren auf Tellur (Te), wie Bi2Te3 oder PbTe. Für eine breite Anwendung thermoelektrischer Bauteile zeigen allerdings Abschätzungen, dass die Tellurvorkommen schnell aufgebraucht sein werden. Somit macht es Sinn, neue nachhaltige und grüne Materialien zu untersuchen. Beispielhaft wurden dafür innerhalb dieser Arbeit die thermoelektrischen Eigenschaften des idealen Modellsystems ZnO untersucht.
Hauptziel dabei war es, die Auswirkungen der Effekte auf mikroskopischer Ebene (z. B. durch Dünnschichten, Korngrenzen, künstliche Strukturierung) auf das makroskopische Verhalten der Probe besser zu verstehen. In diesem Zusammenhang wurden folgende Ergebnisse gefunden:
Untersuchungen an entartet dotierten - und anschließend an Luft getemperten ZnO:Al Schichten zeigen, dass bei sehr hohen Ladungsträgerkonzentrationen, bei denen die Proben metallischen Charakter aufweisen, ein Vorzeichenwechsel von S stattfindet. Obwohl die Proben klar n-Typ sind, konnten kleine positive Seebeck-Koeffizienten gemessen werden, die mit abnehmender Temperatur das Vorzeichen wechselten. Dies kann Änderungen in der Zustandsdichte am Ferminiveau dieses entarteten Halbleiters zugeschrieben werden.
Der Energie-Filter Effekt bedingt durch Korngrenzen, d. h. das Ansteigen des Leistungsfaktors mit steigender Ladungsträgerkonzentration, konnte nur bis zu einem gewissen Grad beobachtet werden: Falls nämlich die Ladungsträgerkonzentration einen bestimmten Wert übersteigt, verringern sogenannte Abschirmungseffekte die Barrieren Höhe und - Breite, was wiederum zu einer Verkleinerung des Leistungsfaktors führt.
Im Hinblick auf die Charakterisierung von Grenzflächen wurden erste Messungen an Übergittern aus alternierenden ZnO/ZnS Schichten in in-plane Geometrie durchgeführt. Die Ergebnisse ließen auf die Ausbildung elektrisch hochleitender Grenzschichten zwischen ZnO und ZnS schließen, welche das Transportverhalten bei hohen Schichtdicken (d > 100 nm) dominieren. Zu geringeren Schichtdicken hin, wo d mit der typischen Oberflächenrauigkeit vergleichbar wird, sind die Transportpfade und damit auch die thermoelektrischen Eigenschaften stark durch Oberflächenfluktuationen bestimmt. Diese Ergebnisse konnten auch qualitativ durch Simulationen innerhalb eines Netzwerkmodells (NeMo) bestätigt werden.
Ein stärkerer Einfluss auf die thermoelektrischen Parameter, insbesondere auf die Wärmeleitfähigkeit, wurde in der Literatur in cross-plane Geometrie, d. h. senkrecht zur Grenze, gefunden. Unglücklicherweise sind Messungen an Übergittern in dieser Geometrie sehr schwer durchzuführen. Um dieses Problem zu umgehen bietet die laterale Strukturierung dünner Schichten attraktive Möglichkeiten. Zur Realisierung einer Stegstruktur aus abwechselnden Materialien wurde die Methode der selbstausrichtenden Strukturübertragung im Rahmen dieser Arbeit entwickelt und angewendet.
Messungen senkrecht zu den Grenzen zeigen, dass die Anzahl der Grenzen sowie deren Gestalt (d. h. Länge) und Morphologie einen erheblichen Einfluss auf den Leistungsfaktor nehmen. Unterstützt von zahlreichen NeMo Simulationen zeigten die Ergebnisse, dass die thermoelektrischen Eigenschaften über die strukturierte Probe hinweg vom elektrisch kürzesten Transportpfad dominiert werden. Dieser wiederum hängt stark von der Annahme sogenannter Grenzflächenregionen verschiedener Breite und Leitfähigkeit ab. Beste Übereinstimmung zwischen Experiment und Simulationen wurde unter der Annahme erreicht, dass ein bestimmter Teil des schlecht leitenden Materials durch eine hochleitende Grenzflächenregion ersetzt wird. Der Ursprung dieser hochleitenden Region konnte jedoch noch nicht geklärt werden.
Die Ergebnisse dieser Arbeit zeigen, dass aufgrund seiner hohen Seebeck-Koeffizienten und der Möglichkeiten durch Dotieren die elektrische Leitfähigkeit einzustellen, ZnO ein geeignetes Materialsystem für umweltfreundliche und nachhaltige thermoelektrische Anwendungen ist. Das Problem, dass es eine hohe Wärmeleitfähigkeit aufweist, könnte durch eine geeignete Kombination mit ZnS gelöst werden. Dieses von der Theorie gezeigte Hintertürchen konnte bislang jedoch noch nicht experimentell bestätigt werden
Suspended animation inducer hydrogen sulfide is protective in an in vivo model of ventilator-induced lung injury
Acute lung injury is characterized by an exaggerated inflammatory response and a high metabolic demand. Mechanical ventilation can contribute to lung injury, resulting in ventilator-induced lung injury (VILI). A suspended-animation-like state induced by hydrogen sulfide (H2S) protects against hypoxia-induced organ injury. We hypothesized that suspended animation is protective in VILI by reducing metabolism and thereby CO2 production, allowing for a lower respiratory rate while maintaining adequate gas exchange. Alternatively, H2S may reduce inflammation in VILI. In mechanically ventilated rats, VILI was created by application of 25 cmH(2)O positive inspiratory pressure (PIP) and zero positive end-expiratory pressure (PEEP). Controls were lung-protective mechanically ventilated (13 cmH(2)O PIP, 5 cmH(2)O PEEP). H2S donor NaHS was infused continuously; controls received saline. In separate control groups, hypothermia was induced to reproduce the H2S-induced fall in temperature. In VILI groups, respiratory rate was adjusted to maintain normo-pH. NaHS dose-dependently and reversibly reduced body temperature, heart rate, and exhaled amount of CO2. In VILI, NaHS reduced markers of pulmonary inflammation and improved oxygenation, an effect which was not observed after induction of deep hypothermia that paralleled the NaHS-induced fall in temperature. Both NaHS and hypothermia allowed for lower respiratory rates while maintaining gas exchange. NaHS reversibly induced a hypometabolic state in anesthetized rats and protected from VILI by reducing pulmonary inflammation, an effect that was in part independent of body temperatur
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