85 research outputs found
Sintering—Pressure- and Temperature-Dependent Contact Models
No full textSintering granular materials involves the application of pressure and temperature to make the particulate material a permanent solid. In order to better understand this complex process, the pressure-, temperature-, and time-dependent contact behaviour of micron-sized particles has been studied in close collaboration by the groups of Luding, Staedler and Kappl within the DFG SPP PiKo. This chapter summarises the modelling advances made during the project, with direct links given to the experimental results. Two aspects have been studied: (a) the dependence of the elastic as well as frictional contact forces and torques on an applied normal pressure; and (b) the formation and evolution of adhesive bonds between particles during heat-sintering. Both contact models have been experimentally calibrated and validated, using advanced techniques such as nanoindentation and AFM. As materials, borosilicate particles were used to study the pressure-dependency, while polystyrene particles were chosen due to their low glass transition temperature to study the temperature-dependency near the transition. Combining both aspects provides a multi-purpose contact model that allows the simulations of a wide range of sinter and agglomeration processes for a variety of practically relevant materials
Ecology and genetics of interspecific hybridization in Daphnia
No full textA central question in evolutionary biology concerns the ecological and genetic processes by which species hybridize. We analyze the evolutionary consequences of hybridization, biogeographic patterns and fitness comparisons in the crustacean Daphnia (Branchiopoda) within the conceptual framework of theories on interspecific hybridization. In contrast to most cases of hybridization among animals, Daphnia species and hybrids do not form interpopulational transition zones (hybrid zones), but rather patchy distributions of hybrids and parentals have been found. In addition, due to ameiotic parthenogenetic reproduction, hybrid breakdown can be avoided and hybrids can reach higher abundances than parental species. Hybrids within the D. galeata complex occur across broad geographic ranges, and lakes vary significantly in species and hybrid composition over time. Species differ in traits related to predator avoidance, such as size at maturity, intrinsic rate of increase (r), behaviour, and induction of helmets and spines. Hybrids tend to exhibit a combination of characters which enables them to persist in ecological niches which arise seasonally due to changes in predation and food regimes. Hybrids seem to be of recent and multiple origin and seem to form backcrosses occasionally, but no mitochondrial and only low levels of nuclear introgression have been detected. Interspecific matings are non-random, which leads to directionality of hybridization and introgression. Environmental settings seem to faciliate hybridization (temporal hybrid superiority model), whereas evolutionary consequences may arise from repeated backcrossing, which in some cases results in paterns of reticulate evolution
Mechanische und tribologische Charakterisierung dünner Schichten mit Hilfe rastersondenbasierter Verfahren
No full textIn dieser Arbeit werden vornehmlich dünne amorphe Kohlenwasserstoffschichten wie auch Kohlenstoffnitritschichten mit Schichtdicken im Bereich zwischen einig en Nanometern bis hin zu einigen hundert Nanometern mit Hilfe einer Kombination aus Rastersondenmikroskop und Nanoindentor mechanisch und tribologisch charakterisiert. Es kann an verschiedenen Beispielen gezeigt werden, daß das Substrat eines Schicht-Substrat-Systems nicht nur im Falle der nanomechanischen Charakterisierung sondern insbesondere auch bei der nanotribologischen Charakterisierung einen deutlichen Einfluß auf das Verhalten des Systems hat. Im Falle eines solchen Systems läßt sich der Reibungskoeffizient während eines Kratz-Tests in vier Phasen einteilen, die die Kontaktcharakteristik (elastisch/plastisch) wie auch den Anteil des Substrates an der Antwort des Systems wiedergeben. Des weiteren wird der Einfluß des Substrates auf den Verschleiß - sowie auf das Ermüdungsverhalten verschiedener Schicht-Substrat-Systeme untersucht und modelliert
Impairment of sleep homeostasis in cervical dystonia patients
No full textAlterations in brain plasticity seem to play a role in the pathophysiology of cervical dystonia (CD). Since evidences indicate that sleep regulates brain plasticity, we hypothesized that an alteration in sleep homeostatic mechanisms may be involved in the pathogenesis of CD. We explored sleep in control subjects (CTL) and CD patients before (T(pre-BoNT)) and after (T(post-BoNT)) botulinum toxin (BoNT) treatment. A physiological slow wave activity (SWA) power decrease throughout the night was observed in CTL but not in CD at T(pre-BoNT). BoNT restored the physiological SWA decrease in CD at T(post-BoNT). Furthermore, in the first part of the night, CD at T(post-BNT) showed a frontal increase and parietal decrease in SWA power compared to CD at T(pre-BoNT), with a SWA distribution comparable to that observed in CTL. Our data highlighted a pathophysiological relationship between SWA during sleep and CD and provided novel insight into the transient central plastic effect of BoNT
Correlation of nanomechanical and nanotribological behavior of thin DLC coatings on different substrates
No full textInfluence of tipradius and substrate on the nanotribological characterization of thin DLC coatings
No full textS.Q9.3.1-Q9.3.6In this work diamond-like carbon films deposited via plasma enhanced chemical vapor deposition with a thickness of 250nm on different substrates are characterized by nanoindentation and microscratching in combination with scanning probe microscopy. The substrates (SU8 photoresist, quartz glass, Si(100) and AlTiC ceramic) provide a wide range of combinations between film and substrate properties. The experimental configuration allows the testing of samples with loads between 1μN and 10mN and a high local resolution. The tribological response of the various film/substrate systems is analysed in two different ways. A single scratch test with linear increasing load and an oscillating wear tests at constant loads is used. The tip radii of the conical diamond tips utilized vary between 0.6 and 6μm. Both, the substrates as well as the tip radii, show a significant influence on the tribological behavior of the systems
Micromechanical and microtribological properties of thin CNx and DLC coatings
No full textS.333-337Diamond-like carbon (DLC) can be used as a tribological coatings in mechanical engineering to reduce friction and wear. As well as the amorphous carbon nitride system (CNx), it is also interesting for various applications in the hard-disk and microsystems industries. Here, examples of the influence of film thickness and substrate on the mechanical and tribological properties of thin CNx and DLC films in the low-load regime are analyzed and discussed.3Nr.
Mikrotribologische und mikromechanische Eigenschaften dünner CNx- und DLC-Schichten
No full textS.794-79631Nr.
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