113 research outputs found
Object-oriented modelling of thermal building behaviour
Zur Entwicklung und Planung energiesparender Gebäude, zum Entwurf geeigneter Regelungsalgorithmen benötigt man detailliertes Wissen über das thermische und energetische Verhalten eines Gebäudes, das in Wechselwirkung mit seiner Umgebung und seinen Bewohnern steht. Dies leistet ein mathematisches Modell. Die Beschreibung großer, komplexer technischer Systeme führt zu hoch komplexen, umfangreichen mathematischen Modellen, die - zur Simulation implementiert - große Softwaresysteme ergeben. Es liegt daher nahe, Konzepte der Informatik auch in der mathematischen Modellbildung zu nutzen. Neben der Dekomposition in Teilsysteme, den Strukturierungskonzepten zur Beherrschung der Komplexität ist hier ein aktueller Forschungsgegenstand der Informatik von besonderem Interesse. Es handelt sich um die Nutzung der Wiederverwendung als methodisches Element des Softwareentwicklungsprozesses großer Systeme. Es wurde eine Modellbibliothek zur Simulation thermischen Gebäudeverhaltens in Modelica erstellt. Sie untergliedert sich in die Abschnitte Gebäude-, Thermohydraulik-, Umgebungs- und Algorithmenbibliothek. Die objektorientiert implementierten, nicht berechnungskausalen Modellkomponenten sind hierarchisch strukturiert. Ihre Implementierung orientiert sich am intuitiven physikalischen Verständnis des zu beschreibenden technischen Prozesses. So aggregiert ein Gebäude einzelne Räume, Fenster, Wände und diese wiederum einzelne Wandschichten.For the development and construction of energy-saving buildings, their HVAC system, for the design of adequate control algorithms detailed knowledge about its thermal and energetic behaviour is needed. The external influences of the outdoor climate, user behaviour and internal loads have to be taken into account. An adequate mathematical description of the processes requires a wide spectrum of mathematical models from different physical and engineering disciplines. Explicit usage of reuse as design-method enhances the quality and efficiency of mathematical model development. Crucial for the reusability of the model components is besides the development of an adequate library structure the usage of modern concepts like object-oriented and non causal mathematical modelling. According to these principles an interdisciplinary model library for simulation of thermal building behaviour has been developed in Modelica. The model library is divided into four sublibraries (building, weather, heating, controller). Building models have been validated in exemplary configurations with the building simulation system TRNSYS
Achieving Ultra-Low Friction with Diamond/Metal Systems in Extreme Environments
In the search for achieving ultra-low friction for applications in extreme environments, we evaluate the interfacial processes of diamond/tungsten sliding contacts using an on-line macro-tribometer and a micro-tribometer in an ultra-high vacuum. The coefficient of friction for the tests with the on-line tribometer remained considerably low for unlubricated sliding of tungsten, which correlated well with the relatively low wear rates and low roughness on the wear track throughout the sliding. Ex situ analysis was performed by means of XPS and SEM-FIB in order to better understand the underlying mechanisms of low friction and low-wear sliding. The analysis did not reveal any evidence of tribofilm or transferfilm formation on the counterface, indicating the absence of significant bonding between the diamond and tungsten surfaces, which correlated well with the low-friction values. The minimal adhesive interaction and material transfer can possibly be explained by the low initial roughness values as well as high cohesive bonding energies of the two materials. The appearance of the wear track as well as the relatively higher roughness perpendicular to the sliding indicated that abrasion was the main wear mechanism. In order to elucidate the low friction of this tribocouple, we performed micro-tribological experiments in ultra-high vacuum conditions. The results show that the friction coefficient was reduced significantly in UHV. In addition, subsequently to baking the chamber, the coefficient of friction approached ultra-low values. Based on the results obtained in this study, the diamond/tungsten tribocouple seems promising for tribological interfaces in spacecraft systems, which can improve the durability of the components
An in-depth evaluation of sample and measurement induced influences on static contact angle measurements
Static contact angle measurements are one of the most popular methods to analyze the wetting
behavior of materials of any kind. Although this method is readily applicable without the need
of sophisticated machinery, the results obtained for the very same material may vary strongly.
The sensitivity of the measurement against environmental conditions, sample preparation
and measurement conduction is a main factor for inconsistent results. Since often no detailed
measurement protocols exist alongside published data, contact angle values as well as elaborated
wetting studies do not allow for any comparison. This paper therefore aims to discuss possible
infuences on static contact angle measurements and to experimentally demonstrate the extent of
these efects. Sample storage conditions, cleaning procedures, droplet volume, water grade and
droplet application as well as the infuence of evaporation on the static contact angle are investigated
in detail. Especially sample storage led to diferences in the contact angle up to 60%. Depending
on the wetting state, evaporation can reduce the contact angle by 30–50% within 10 min in dry
atmospheres. Therefore, this paper reviews an existing approach for a climate chamber and introduces
a new measuring setup based on these results. It allows for the observation of the wetting behavior for
several minutes by successfully suppressing evaporation without negatively afecting the surface prior
to measurement by exposure to high humidity environments
Atomistic insights into lubricated tungsten/diamond sliding contacts
The reinforcement of coatings with diamond particles results in superior tribological performance for automotive applications. In addition to improving the coating’s bulk properties, sliding of diamond on metallic counter bodies contributes to improved tribological performance. Therefore, in order to design better diamond reinforced coatings, it is imperative to understand the atomistic mechanisms at sliding metal/diamond interfaces. Here, we investigate the interfacial tribo-chemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts by combining reactive atomistic simulations with on-line tribometry experiments linked to chemical analysis. Reactive classical molecular dynamics simulations reveal the dehydrogenation of hexadecane lubricant molecules between tungsten/diamond contacts by proton transfer from the hexadecane to octahedral sites of the tungsten surface. Subsequent chemisorption of the radicalized hexadecane on dangling C-bond sites of the diamond surface leads to the formation of low-density hydrocarbon films, which significantly lower frictional resistance in the tribo-contact. Quasi-static density functional theory calculations confirm the classical molecular dynamics results and reveal that radicalized hydrocarbon molecules can also bond via C-O bonds on a WO3 layer covering the tungsten counter surface. The on-line tribometry experiments confirm the reduction of friction under hexadecane lubrication and ex situ chemical analysis by means of XPS, AES and EELS provide evidence of the formation of a carbon-rich tribofilm on the diamond and tungsten-oxide surfaces as predicted by the atomistic simulations
The high-pressure phase diagram of BaNiAs: unconventional charge-density-waves and structural phase transitions
Structural phase transitions accompanied by incommensurate and commensurate
charge density wave (CDW) modulations of unconventional nature have been
reported in BaNiAs, a nonmagnetic cousin of the parent compound of
Fe-based superconductors, BaFeAs. The strong dependence of the
structural and CDW transitions of BaNiAs on isoelectronic substitutions
alongside original dynamical lattice effects suggests strong tunability of the
electronic phase of the system through structural effects. Here, we present a
comprehensive synchrotron x-ray diffraction and first-principles calculation
study of the evolution of the crystal structure and lattice instabilities of
BaNiAs as a function of temperature and hydrostatic pressure (up to 12
GPa). We report a cascade of pressure-induced structural phase transitions and
electronic instabilities up to 10 GPa, above which all CDW superstructures
disappear. We reveal that the stable high-pressure phase consists of planar Ni
zigzag chains, from which the surrounding As atoms have been pushed away. This
yields a strong reduction of the interlayer As-As distance (along the original
c axis), akin to what is observed in the collapsed tetragonal structure of
other pnictides, albeit here with a monoclinic structure. The discovery of
polymorphs in the pressure-temperature phase diagram of BaNiAs
emphasizes the importance of the relative Ni-Ni and Ni-As bond lengths in
controlling the electronic ground state of this compound and increases our
understanding of viable electronic phases under extreme conditions.Comment: 10 pages, 5 figures, 1 table, published versio
Fabrication and Characterization of Single-Crystal Diamond Membranes for Quantum Photonics with Tunable Microcavities
The development of quantum technologies is one of the big challenges in modern research. Acrucial component for many applications is an efficient, coherent spin–photon interface, and coupling single-color centers in thin diamond membranes to a microcavity is a promising approach. To structure such micrometer thin single-crystal diamond (SCD) membranes with a good quality, it is important to minimize defects originating from polishing or etching procedures. Here, we report on the fabrication of SCD membranes, with various diameters, exhibiting a low surface roughness down to 0.4 nm on a small area scale, by etching through a diamond bulk mask with angled holes. A significant reduction in pits induced by micromasking and polishing damages was accomplished by the application of
alternating Ar/Cl2 + O2 dry etching steps. By a variation of etching parameters regarding the Ar/Cl2 step, an enhanced planarization of the surface was obtained, in particular, for surfaces with a higher initial surface roughness of several nanometers. Furthermore, we present the successful bonding of
an SCD membrane via van der Waals forces on a cavity mirror and perform finesse measurements which yielded values between 500 and 5000, depending on the position and hence on the membranethickness. Our results are promising for, e.g., an efficient spin–photon interface
Colossal c-axis response and lack of rotational symmetry breaking within the kagome plane of the CsVSb superconductor
The kagome materials AV4Sb (A = K, Rb, Cs) host an intriguing
interplay between unconventional superconductivity and charge-density-waves.
Here, we investigate CsVSb by combining high-resolution
thermal-expansion, heat-capacity and electrical resistance under strain
measurements. We directly unveil that the superconducting and charge-ordered
states strongly compete, and that this competition is dramatically influenced
by tuning the crystallographic c-axis. In addition, we report the absence of
additional bulk phase transitions within the charge-ordered state, notably
associated with rotational symmetry-breaking within the kagome planes. This
suggests that any breaking of the C invariance occurs via different
stacking of C-symmetric kagome patterns. Finally, we find that the
charge-density-wave phase exhibits an enhanced A-symmetric
elastoresistance coefficient, whose large increase at low temperature is driven
by electronic degrees of freedom
Dynamics of collective modes in an unconventional charge density wave system BaNi2As2
BaNi 2As 2 is a non-magnetic analogue of BaFe2 As2 , the parent compound of a prototype
pnictide high-temperature superconductor, displaying superconductivity already at ambient
pressure. Recent diffraction studies demonstrated the existence of two types of periodic
lattice distortions above and below the triclinic phase transition, suggesting the existence of
an unconventional charge-density-wave (CDW) order. The suppression of CDW order upon
doping results in a sixfold increase in the superconducting transition temperature and
enhanced nematic fluctuations, suggesting CDW is competing with superconductivity. Here,
we apply time-resolved optical spectroscopy to investigate collective dynamics in BaNi 2 As 2.
We demonstrate the existence of several CDW amplitude modes. Their smooth evolution
through the structural phase transition implies the commensurate CDW order in the triclinic
phase evolves from the high-temperature unidirectional incommensurate CDW, and may
indeed trigger the structural phase transition. Excitation density dependence reveals excep-
tional resilience of CDW against perturbation, implying an unconventional origin of the
underlying electronic instability
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