Defect chemistry and transport properties of BaxCe0.85M0.15O3-d

The site-incorporation mechanism of M3+ dopants into A2+B4+O3 perovskites controls the overall defect chemistry and thus their transport properties. For charge-balance reasons, incorporation onto the A2+-site would require the creation of negatively charged point defects (such as cation vacancies), whereas incorporation onto the B4+-site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. We propose here, on the basis of x-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, and alternating current impedance spectroscopy, that nominally B-site doped barium cerate can exhibit dopant partitioning as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. Specific materials examined are BaxCe0.85M0.15O3-d (x = 0.85 - 1.20; M = Nd, Gd, Yb). The compositional limits for the maximum A-site incorporation are experimentally determined to be: (Ba0.919Nd0.081)(Ce0.919Nd0.081)O3, (Ba0.974Gd0.026)(Ce0.872Gd0.128)O2.875, and Ba(Ce0.85Yb0.15)O2.925. As a consequence of the greater ability of larger cations to exist on the Ba site, the H2O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs

The method of Gaussian weighted trajectories. V. On the 1GB procedure for polyatomic processes

In recent years, many chemical reactions have been studied by means of the quasi-classical trajectory (QCT) method within the Gaussian binning (GB) procedure. The latter consists in "quantizing" the final vibrational actions in Bohr spirit by putting strong emphasis on the trajectories reaching the products with vibrational actions close to integer values. A major drawback of this procedure is that if N is the number of product vibrational modes, the amount of trajectories necessary to converge the calculations is ~ 10^N larger than with the standard QCT method. Applying it to polyatomic processes is thus problematic. In a recent paper, however, Czako and Bowman propose to quantize the total vibrational energy instead of the vibrational actions [G. Czako and J. M. Bowman, J. Chem. Phys., 131, 244302 (2009)], a procedure called 1GB here. The calculations are then only ~ 10 times more time-consuming than with the standard QCT method, allowing thereby for considerable numerical saving. In this paper, we propose some theoretical arguments supporting the 1GB procedure and check its validity on model test cases as well as the prototype four-atom reaction OH+D_2 -> HOD+D

Physical processes leading to surface inhomogeneities: the case of rotation

In this lecture I discuss the bulk surface heterogeneity of rotating stars, namely gravity darkening. I especially detail the derivation of the omega-model of Espinosa Lara & Rieutord (2011), which gives the gravity darkening in early-type stars. I also discuss the problem of deriving gravity darkening in stars owning a convective envelope and in those that are members of a binary system.Comment: 23 pages, 11 figure, Lecture given to the school on the cartography of the Sun and the stars (May 2014 in Besan\c{c}on), to appear in LNP, Neiner and Rozelot edts V2: typos correcte

Outflows From Evolved Stars: The Rapidly Changing Fingers Of CRL 618

Our ultimate goal is to probe the nature of the collimator of the outflows in the pre-planetary nebula CRL 618. CRL 618 is uniquely suited for this purpose owing to its multiple, bright, and carefully studied finger-shaped outflows east and west of its nucleus. We compare new Hubble Space Telescope images to images in the same filters observed as much as 11 yr ago to uncover large proper motions and surface brightness changes in its multiple finger-shaped outflows. The expansion age of the ensemble of fingers is close to 100 yr. We find strong brightness variations at the fingertips during the past decade. Deep IR images reveal a multiple ring-like structure of the surrounding medium into which the outflows propagate and interact. Tightly constrained three-dimensional hydrodynamic models link the properties of the fingers to their possible formation histories. We incorporate previously published complementary information to discern whether each of the fingers of CRL 618 are the results of steady, collimated outflows or a brief ejection event that launched a set of bullets about a century ago. Finally, we argue on various physical grounds that fingers of CRL 618 are likely to be the result of a spray of clumps ejected at the nucleus of CRL 618 since any mechanism that form a sustained set of unaligned jets is unprecedented.HST GO 11580NASA through Space Telescope Science Institute GO11580NASA NAS5-26555Boeing ScholarshipOffice of Undergraduate Academic Affairs at the University of WashingtonSpanish MICINN CSD2009-00038NASA Office of Space Science NAG5-7584Astronom

Conformal mapping of ultrasonic crystals: confining ultrasound and cochlear-like wave guiding

Conformal mapping of a slab of a two-dimensional ultrasonic crystal generate a closed geometrical arrangement of ultrasonic scatterers with appealing acoustic properties. This acoustic shell is able to confine ultrasonic modes. Some of these internal resonances can be induced from an external wave source. The mapping of a linear defect produces a wave-guide that exhibits a spatial-frequency selection analogous to that characteristic of a synthetic "cochlea". Both, experimental and theoretical results are reported here.Comment: 4 pages, 4 figure

Condensation in an Economic Model with Brand Competition

We present a linear agent based model on brand competition. Each agent belongs to one of the two brands and interacts with its nearest neighbors. In the process the agent can decide to change to the other brand if the move is beneficial. The numerical simulations show that the systems always condenses into a state when all agents belong to a single brand. We study the condensation times for different parameters of the model and the influence of different mechanisms to avoid condensation, like anti monopoly rules and brand fidelity.Comment: Accepted in: International Journal of Modern Physics

The thermal operator representation for Matsubara sums

We prove in full generality the thermal operator representation for Matsubara sums in a relativistic field theory of scalar and fermionic particles. It states that the full result of performing the Matsubara sum associated to any given Feynman graph, in the imaginary-time formalism of finite-temperature field theory, can be directly obtained from its corresponding zero-temperature energy integral, by means of a simple linear operator, which is independent of the external Euclidean energies and whose form depends solely on the topology of the graph.Comment: 9 pages, 1 figure, RevTe