6,538 research outputs found
Intrinsic point defects and volume swelling in ZrSiO4 under irradiation
The effects of high concentration of point defects in crystalline ZrSiO4 as
originated by exposure to radiation, have been simulated using first principles
density functional calculations. Structural relaxation and vibrational studies
were performed for a catalogue of intrinsic point defects, with different
charge states and concentrations. The experimental evidence of a large
anisotropic volume swelling in natural and artificially irradiated samples is
used to select the subset of defects that give similar lattice swelling for the
concentrations studied, namely interstitials of O and Si, and the anti-site
Zr(Si), Calculated vibrational spectra for the interstitials show additional
evidence for the presence of high concentrations of some of these defects in
irradiated zircon.Comment: 9 pages, 7 (color) figure
Phase separation in fluids exposed to spatially periodic external fields
We consider the liquid-vapor type phase transition for fluids confined within
spatially periodic external fields. For a fluid in d=3 dimensions, the periodic
field induces an additional phase, characterized by large density modulations
along the field direction. At the triple point, all three phases (modulated,
vapor, and liquid) coexist. At temperatures slightly above the triple point and
for low (high) values of the chemical potential, two-phase coexistence between
the modulated phase and the vapor (liquid) is observed. We study this
phenomenon using computer simulations and mean-field theory for the Ising
model. The theory shows that, in order for the modulated phase to arise, the
field wavelength must exceed a threshold value. We also find an extremely low
tension of the interface between the modulated phase and the vapor/liquid
phases. The tension is of the order 10^{-4} kB T per squared lattice spacing,
where kB is the Boltzmann constant, and T the temperature. In order to detect
such low tensions, a new simulation method is proposed. We also consider the
case of d=2 dimensions. The modulated phase then does not survive, leading to a
radically different phase diagram.Comment: 11 pages, 14 figure
Marsupial and Monotreme Enamel Structure
We present some recent developments in our understanding of two basic questions: the origin, extent, nature and course of marsupial enamel tubules; and the characterisation of monotreme enamel, more particularly, the prismatic nature of platypus enamel.
Methods used included SEM of methacrylate casts of marsupial enamel tubules, worn and cut surfaces of whole marsupial teeth, developing and erupted platypus teeth, and a well-developed molar of the newly discovered Miocene ornithorhynchid Obdurodon sp., and tandem scanning reflected light microscopy of intact marsupial teeth.
We conclude that there are significant species differences with respect to prism shape, row formation and tubule disposition in marsupials and, moreover, that these features change in a consistent way through the thickness of the enamel. Consideration of enamel prism course in incisor and molar enamel of Macropus eugenii, together with the tubule casts, enables us to conclude that there is a fundamental relationship of tubule to prism in the body of marsupial enamel. This and previously reported data put beyond dispute the essential relationship of the marsupial tubule to the formative ameloblast.
The enamel of Ornithorhynchus anatinus is shown to be prismatic only in part, with well-formed regular prisms not being a primary structural feature. The enamel of the fossil monotreme is prismatic and tubular and displays large areas of Pattern 2 prism packing. Monotreme enamel has been interpreted as representing a structural stage intermediate between that of known multituberculates and extant tribosphenid mammals
Thermodynamically consistent description of the hydrodynamics of free surfaces covered by insoluble surfactants of high concentration
In this paper we propose several models that describe the dynamics of liquid
films which are covered by a high concentration layer of insoluble surfactant.
First, we briefly review the 'classical' hydrodynamic form of the coupled
evolution equations for the film height and surfactant concentration that are
well established for small concentrations. Then we re-formulate the basic model
as a gradient dynamics based on an underlying free energy functional that
accounts for wettability and capillarity. Based on this re-formulation in the
framework of nonequilibrium thermodynamics, we propose extensions of the basic
hydrodynamic model that account for (i) nonlinear equations of state, (ii)
surfactant-dependent wettability, (iii) surfactant phase transitions, and (iv)
substrate-mediated condensation. In passing, we discuss important differences
to most of the models found in the literature.Comment: 31 pages, 2 figure
Enamel of Yalkaparidon Coheni: Representative of a Distinctive Order of Tertiary Zalambdodont Marsupials
The enamel of an incisor and a premolar of Yalkaparidon coheni was examined by scanning electron microscopy in fractured and in sectioned, polished surfaces. The enamel of both teeth demonstrated: complete, ovoid and horse-shoe shaped prisms in a Pattern 2 arrangement; a simple parallel prism course; and, enamel tubules in abundance in the premolar but restricted to the innermost enamel in the incisor. Overall, the enamel ultrastructure supports the marsupial affiliation proposed for Yalkaparidon coheni but does not unambiguously ally it with any other order of marsupials.
The observation of a significant ultrastructural difference between the anterior and posterior teeth of a marsupial emphasizes the need to sample both if available. In pursuing this, we report here also the lack of tubules in the anterior teeth of the extant Tarsipes rostratus. This together with a similar absence of typical marsupial tubules from the incisor of the extinct Yalkaparidon coheni, would suggest that the wombat is not the only surviving marsupial to have experimented so extensively with this particular structural feature. It is likely that further study will demonstrate an unexpected and relative lack of tubules in the incisor enamel of other fossil Australian marsupials
Revision of basal macropodids from the Riversleigh World Heritage Area with descriptions of new material of Ganguroo bilamina Cooke, 1997 and a new species
The relationship of basal macropodids (Marsupialia: Macropodoidea) from the Oligo-Miocene of Australia have been unclear. Here, we describe a new species from the Bitesantennary Site within the Riversleigh's World Heritage Area (WHA), Ganguroo bites n. sp., new cranial and dental material of G. bilamina, and reassess material previously described as Bulungamaya delicata and 'Nowidgee matrix'. We performed a metric analysis of dental measurements on species of Thylogale which we then used, in combination with morphological features, to determine species boundaries in the fossils. We also performed a phylogenetic analysis to clarify the relationships of basal macropodid species within Macropodoidea. Our results support the distinction of G. bilamina, G. bites and B. delicata, but 'Nowidgee matrix' appears to be a synonym of B. delicata. The results of our phylogenetic analysis are inconclusive, but dental and cranial features suggest a close affinity between G. bilamina and macropodids. Finally, we revise the current understanding of basal macropodid diversity in Oligocene and Miocene sites at Riversleigh WHA
Phase separation dynamics in a two-dimensional magnetic mixture
Based on classical density functional theory (DFT), we investigate the
demixing phase transition of a two-dimensional, binary Heisenberg fluid
mixture. The particles in the mixture are modeled as Gaussian soft spheres,
where one component is characterized by an additional classical spin-spin
interaction of Heisenberg type. Within the DFT we treat the particle
interactions using a mean-field approximation. For certain magnetic coupling
strengths we calculate phase diagrams in the density-concentration plane. For
sufficiently large coupling strengths and densities, we find a demixing phase
transition driven by the ferromagnetic interactions of the magnetic species. We
also provide a microscopic description (i.e., density profiles) of the
resulting non-magnetic/magnetic fluid-fluid interface. Finally, we investigate
the phase separation using dynamical density functional theory (DDFT),
considering both nucleation processes and spinodal demixing.Comment: 15 pages, 10 figure
The oldest fossil record of bandicoots (Marsupialia; Peramelemorphia) from the late Oligocene of Australia
Two new late Oligocene representatives of the marsupial order Peramelemorphia (bandicoots and bilbies) from the Etadunna Formation of South Australia are described here. Bulungu muirheadae sp. nov., from Zone B (Ditjimanka Local Fauna [LF]), is represented by several dentaries and isolated upper and lower molars. Bulungu campbelli sp. nov., from Zone C (Ngapakaldi LF), is represented by a single dentary and maxilla. Together, they represent the oldest fossil bandicoots described to date. Both are small (estimated body mass o
SWIRL AND COMBUSTION EFFECTS ON FLOW DYNAMICS IN LEAN DIRECT INJECTION GAS TURBINE COMBUSTION
Abstract The effect of swirl and combustion are presented for a Lean Direct Injection (LDI) configuration in gas turbine combustion. Specifically, the effect of radial distribution of combustion air and swirl in a burner are examined under nonburning and burning conditions using propane as the fuel. The present study explores single swirler interaction with the use of an experimental double concentric swirl burner that simulates one swirler of a practical gas turbine combustor. Flowfield data has been obtained using Particle Image Velocimetry (PIV) for varying swirl distributions. The flow characteristics of the resulting flowfields have been examined under lean direct injection (LDI) conditions. The affects of coand counter-swirl have also been carried out. Results showed that both swirl and combustion has significant effect on the characteristics of the internal and external recirculation zones. Combustion provides greater axial velocities than their counter non-combustion conditions. The counter-swirl combination resulted in smaller and more well defined internal recirculation regions. The results provide the role of swirl and combustion on the mean and turbulence characteristics of flows over a range of swirl and shear conditions between the inner and outer flow of the burner. This data provides important insights on the flow dynamics in addition to providing data for model validation and model development. Introduction Hydrocarbon fuels continue to be the major source of energy in all propulsion and power systems throughout the world in the foreseeable future. A major choice for converting chemical energy to thermal energy is via the combustion of hydrocarbon fuels. Therefore the world today and in the near future will continue to rely heavily on the combustion of different kinds fuel for energy. Paramount to improving combustion is to enhance efficiency, reduce the emission of pollutants, and minimize degradation of the atmosphere. Detailed insight through sufficient understanding of a combustion process can provide an insight to improve the combustion process. Many alternatives to the traditional gas turbine are being developed and investigated. One such alternative is th
Examining the effectiveness of technology use in classrooms: A tertiary meta-analysis
Identifying effective literacy instruction programs has been a focal point for governments, educators and parents over the last few decades (Ontario Ministry of Education, 2004, 2006; Council of Ontario Directors of Education, 2011). Given the increasing use of computer technologies in the classroom and in the home, a variety of information communication technology (ICT) interventions for learning have been introduced. Meta-analyses comparing the impact of these programs on learning, however, have yielded inconsistent findings (Andrews, Freeman, Hou, McGuinn, Robinson, & Zhu, 2007; Slavin, Cheung, Groff, & Lake, 2008; Slavin, Lake, Chambers, Cheung, & Davis, 2009; Torgerson & Zhu, 2003). The present tertiary meta-analytic review re-assesses outcomes presented in three previous meta-analyses. Four moderator variables assessed the impact of the systematic review from which they were retrieved, training and support, implementation fidelity and who delivered the intervention (teacher versus researcher). Significant results were found when training and support was entered as a moderator variable with the small overall effectiveness of the ICTs (ES = 0.18), similar to those found in previous research, increasing significantly (ES = 0.57). These findings indicate the importance of including implementation factors such as training and support, when considering the relative effectiveness of ICT interventions
- …