A density functional study of the high-pressure chemistry of MSiN2(M = Be,Mg, Ca)

Normal pressure modifications and tentative high-pressure phases of the nitridosilicates MSiN2 with M = Be, Mg, or Ca have been thoroughly studied by density functional methods. At ambient pressure, BeSiN2 and MgSiN2 exhibit an ordered wurtzite variant derived from idealized filled β-cristobalite by a C1-type distortion. At ambient pressure, the structure of CaSiN2 can also be derived from idealized filled β-cristobalite by a different type of distortion (D1-type). Energy–volume calculations for all three compounds reveal transition into an NaCl superstructure under pressure, affording sixfold coordination for Si. At 76 GPa BeSiN2 forms an LiFeO2-type structure, corresponding to the stable ambient-pressure modification of LiFeO2, while MgSiN2 and CaSiN2 adopt an LiFeO2-type structure, corresponding to a metastable modification (24 and 60 GPa, respectively). For both BeSiN2 and CaSiN2 intermediate phases appear (for BeSiN2 a chalcopyrite-type structure and for CaSiN2 a CaGeN2-type structure). These two tetragonal intermediate structures are closely related, differing mainly in their c/a ratio. As a consequence, chalcopyrite-type structures exhibit tetrahedral coordination for both cations (M and Si), whereas in CaGeN2-type structures one cation is tetrahedrally (Si) and one bisdisphenoidally (M) coordinated. Both structure types, chalcopyrite and CaGeN2, can also be derived from idealized filled β-cristobalite through a B1-type distortion. The group–subgroup relation of the BeSiN2/MgSiN2, the CaSiN2, the chalcopyrite, the CaGeN2 and the idealized filled β-cristobalite structure is discussed and the displacive phase transformation pathways are illustrated. The zero-pressure bulk moduli were calculated for all phases and have been found to be comparable to compounds such as α- Si3N4, CaIrO3 and Al4C3. Furthermore, the thermodynamic stability of BeSiN2, MgSiN2 and CaSiN2 against phase agglomerates of the binary nitrides M3N2 and Si3N4 under pressure are examined

Density Functional Study of Calcium Nitride

The high-pressure behavior of Ca3N2 is studied up 100 GPa using density functional theory. Evaluation of many hypothetical polymorphs of composition A3X2 leads us to propose four high-pressure polymorphs for both α- and β-Ca3N2: (1) an anti-Rh2O3−II structure at 5 GPa, (2) an anti-B-sesquioxide structure at 10 GPa, (3) an anti-A-sesquioxide structure at 27 GPa, and (4) a hitherto unknown hexagonal structure (P63/mmc), derived from the post-perovskite structure of CaIrO3, at 38 GPa. The development of the density and bulk modulus under pressure has been examined

Contacts between wild birds and domestic poultry - a serious factor in transmission of avian influenza?

Wasservögel gelten als Hauptreservoir für viele Subtypen niedrig pathogener Influenza A Viren. Auch auf eine mögliche Verbreitung hoch pathogener Formen durch Wasservögel gibt es Hinweise. Daher wird die Übertragung von Geflügelpest-Erregern von Wildvögeln auf Hausgeflügel und umgekehrt bei Kontakten zwischen beiden Gruppen (z.B. in Freilandhaltungen) als Risikofaktor für die Ausbreitung der Seuche angesehen und Aufstallungsgebote zählen zum Standardrepertoire der Reaktion auf eine mögliche Geflügelpest-Gefahr. Daten zum tatsächlichen Ausmaß solcher Wildvogel-Geflügel-Kontakte waren jedoch bislang nur in Form einzelner Anekdoten verfügbar. Im Rahmen des Projektes „Constanze“ zur Untersuchung des Geflügelpest-Risikos am Beispiel des Bodenseeraumes wurden von Oktober 2007 bis einschließlich Januar 2008 im Rahmen einer Diplomarbeit schweizerische Geflügel-Freilandhaltungen auf Kontakte von Wildvögeln mit Hausgeflügel untersucht. Ziel dabei war, das Ausmaß dieser Kontakte hinsichtlich eines möglichen Übertragungsrisikos von Geflügelpest-Erregern abzuschätzen. Die Beobachtungen wurden an 21 Geflügelhaltungen mit Freilauf in den Kantonen Thurgau und Sankt Gallen durchgeführt. Während der Gesamtbeobachtungszeit von 65 Stunden wurde abgesehen von Besuchen von einmal drei und einmal einer Lachmöwe kein Fall eines Auftretens von Wasservögeln in den Geflügelhaltungen festgestellt. Es gab lediglich Kontakte zu Landvögeln urbaner Lebensräume wie Haussperling, Buchfink oder Rabenkrähe und indirekte Kontakte zu bzw. Anwesenheit von Greifvögeln, Lachmöwen, Tauben und einigen weiteren Arten in der näheren Umgebung. Die beobachteten Vogelarten sind nach derzeitigem Kenntnisstand nicht an der Übertragung der Vogelgrippe beteiligt. Obwohl die Beobachtungen im Winter zu einer Zeit stattfanden, als mehrere zehntausend Entenvögel am Bodensee anwesend waren, wurde kein einziger Kontakt mit dieser für das Vogelgrippegeschehen relevanten Gruppe beobachtet. Insgesamt kann geschlossen werden, dass das Risiko einer Übertragung der Geflügelpest von einem Wildvogel auf Hausgeflügel oder umgekehrt durch die direkten Kontakte aufgrund von deren Seltenheit im Bodenseeraum eher vernachlässigbar ist.Waterbirds are seen to be the main reservoir for many subtypes of low pathogenic avian influenza A virus. There is also evidence for a potential spread of high pathogenic subtypes through waterbirds. Transmission of avian influenza virus from wild birds into poultry and vice versa therefore is seen as a risk factor for the spread of the disease and a prohibition of keeping poultry outdoors is among the standard repertoire in veterinary disease defense. Data about the extent of contacts between wild birds and poultry until now are only available through single anecdotes. Within the framework of project „Constanze“ (exploration of the bird flu risk at the example of Lake Constance) between October 2007 and January 2008 Swiss poultry farms with open air enclosures were searched for contacts between poultry and wild birds. It was aimed to estimate the extent of these contacts in the light of a potential transmission risk of influenza virus. Observations were made on 21 poultry holdings with open air enclosures in the districts of Thurgau and St. Gallen. During 65 hrs. of observation no visit of a waterbird was registered, except for registration of three and one Black headed Gulls. Only contacts with land birds of typical species of urban areas like House Sparrow, Chaffinch or Carrion Crow were registered. In addition, birds of prey, Black-headed Gulls and pigeons have been observed in the vicinity of the enclosures. According to current knowledge the bird species observed are not involved in the transmission of avian influenza. Despite several tens of thousands of anatids being present in the Lake Constance area at time of the study not a single contact between this relevant group for avian influenza and poultry has been seen. We conclude that - due to it’s infrequency - the transmission risk of avian influenza between wild birds and poultry through direct contacts in the Lake Constance area is of minor relevance

Investigation of optimal bipedal walking gaits subject to different energy-based objective functions

Optimal bipedal walking gaits subject to different energy-based objective functions are investigated using a simple planar rigid body model of a bipedal robot with upper body, thighs and shanks. The robot’s segments are connected by revolute joints actuated by electric motors. The actuators’ torques are generated by a trajectory tracking controller to produce periodic walking gaits. A numerical optimization routine is used to find optimal reference trajectories for average speeds in the range of 0.3 – 2.3 m/s to investigate the influence of different objective functions

High-pressure phases and transitions of the layered alkaline earth nitridosilicates SrSiN2 and BaSiN2

We investigate the high-pressure phase diagram of SrSiN2 and BaSiN2 with density-functional calculation. Searching a manifold of possible candidate structures, we propose new structural modifications of SrSiN2 and BaSiN2 attainable in high-pressure experiments. The monoclinic ground state of SrSiN2 transforms at 3 GPa into an orthorhombic BaSiN2 type. At 14 GPa a CaSiN2-type structure becomes the most stable configuration of SrSiN2. A hitherto unknown Pbcm modification is adopted at 85 GPa and, finally, at 131 GPa a LiFeO2-type structure. The higher homologue BaSiN2 transforms to a CaSiN2 type at 41 GPa and further to a Pbcm modification at 105 GPa. Both systems follow the pressure-coordination rule: the coordination environment of Si increases from tetrahedral through trigonal bipyramidal to octahedral. Some high-pressure phases are related in structure through simple group–subgroup mechanisms, indicating displacive phase transformations with low activation barriers

The normal parameterization and its application to collision detection

Collision detection is a central task in the simulation of multibody systems. Depending on the description of the geometry, there are many efficient algorithms to address this need. A widespread approach is the common normal concept: potential contact points on opposing surfaces have antiparallel normal vectors. However, this approach leads to implicit equations that require iterative solutions when the geometries are described by implicit functions or the common parameterizations. We introduce the normal parameterization to describe the boundary of a strictly convex object as a function of the orientation of its normal vector. This parameterization depends on a scalar function, the so-called generating potential from which all properties are derived: points on the boundary, continuity/differentiability of the boundary, curvature, offset curves or surfaces. An explicit solution for collisions with a planar counterpart is derived and four iterative algorithms for collision detection between two arbitrary objects with the normal parametrization are compared. The application of this approach for collision detection in multibody models is illustrated in a case study with two ellipsoids and several planes

Explicit analytical solutions for two-dimensional contact detection problems between almost arbitrary geometries and straight or circular counterparts

Contact between complex bodies and simple counterparts like straight lines or circles occur in many two-dimensional mechanical models. The corresponding contact detection problems are complicated and thus far, no explicit formulas have been available. In this paper, we address the contact detection problem between two planar bodies: one being either a straight line or a circle and the other an almost arbitrary geometry—the only requirement is a unique contact point for all possible contact situations. To solve this general problem, a novel procedure is applied which provides necessary conditions for the description of the geometry based on the special case of a rolling contact. This results in a parameterization of the geometry which gives the potential contact point depending on the relative orientation between the two bodies. Although the derivation is based on a rolling contact, the result is valid in general and can also be used for efficient contact detection when the bodies are separated. The derived equations are simple and easy to implement, which is demonstrated for two examples: a foot-ground contact model and a cam-follower mechanism

Optimal elastic coupling in form of one mechanical spring to improve energy efficiency of walking bipedal robots

This paper presents a method to optimize the energy efficiency of walking bipedal robots by more than 80% in a speed range from 0.3 to 2.3 m/s using elastic couplings – mechanical springs with movement speed independent parameters. The considered planar robot consists of a trunk, two two-segmented legs, two actuators in the hip joints, two actuators in the knee joints and an elastic coupling between the shanks. It is modeled as underactuated system to make use of its natural dynamics and feedback controlled via input-output linearization. A numerical optimization of the joint angle trajectories as well as the elastic couplings is performed to minimize the average energy expenditure over the whole speed range. The elastic couplings increase the swing leg motion’s natural frequency thus making smaller steps more efficient which reduce the impact loss at the touchdown of the swing leg. The process of energy turnover is investigated in detail for the robot with and without elastic coupling between the shanks. Furthermore, the influences of the elastic couplings’ topology and of joint friction are analyzed. It is shown that the optimization of the robot’s motion and elastic coupling towards energy efficiency leads to a slightly slower convergence rate of the controller, yet no loss of stability but a lower sensitivity with respect to disturbances. The optimal elastic coupling discovered via numerical optimization is a linear torsion spring with transmissions between the shanks. A design proposal for this elastic coupling – which does not affect the robot’s trunk and parallel shank motion and can be used to enhance an existing robot – is given for planar as well as spatial robots