Structural properties of silicon dioxide thin films densified by medium-energy particles

Classical molecular-dynamics simulations have been carried out to investigate densification mechanisms in silicon dioxide thin films deposited on an amorphous silica surface, according to a simplified ion-beam assisted deposition (IBAD) scenario. We compare the structures resulting from the deposition of near-thermal (1 eV) SiO$_{2}$ particles to those obtained with increasing fraction of 30 eV SiO$_{2}$ particles. Our results show that there is an energy interval - between 12 and 15 eV per condensing SiO$_2$ unit on average - for which the growth leads to a dense, low-stress amorphous structure, in satisfactory agreement with the results of low-energy ion-beam experiments. We also find that the crossover between low- and high-density films is associated with a tensile to compressive stress transition, and a simultaneous healing of structural defects of the {\em a-}SiO$_2$ network, namely three- and four-fold rings. It is observed, finally, that densification proceeds through significant changes at intermediate length scales (4--10 \AA), leaving essentially unchanged the ``building blocks'' of the network, viz. the Si(O$_{1/2}$)$_{4}$ tetrahedra. This latter result is in qualitative agreement with the mechanism proposed to explain the irreversible densification of amorphous silica recovered from high pressures ($\sim$ 15--20 GPa).Comment: 12 pages including 10 postscript figures; submitted to Phys. Rev. B; related publications can be found on web site http://www.centrcn.umontreal.ca/~lewi

Seeing motion and apparent motion

In apparent motion experiments, participants are presented with what is in fact a succession of two brief stationary stimuli at two different locations, but they report an impression of movement. Philosophers have recently debated whether apparent motion provides evidence in favour of a particular account of the nature of temporal experience. I argue that the existing discussion in this area is premised on a mistaken view of the phenomenology of apparent motion and, as a result, the space of possible philosophical positions has not yet been fully explored. In particular, I argue that the existence of apparent motion is compatible with an account of the nature of temporal experience that involves a version of direct realism. In doing so, I also argue against two other claims often made about apparent motion, viz. that apparent motion is the psychological phenomenon that underlies motion experience in the cinema, and that apparent motion is subjectively indistinguishable from real motion

Are Proxima and Alpha Centauri Gravitationally Bound?

Using the most recent kinematic and radial velocity data in the literature, we calculate the binding energy of Proxima Centauri relative to the center of mass of the Alpha Centauri system. When we adopt the centroids of the observed data, we find that the three stars constitute a bound system, albeit with a semi-major axis that is on order the same size as Alpha Centauri AB's Hill radius in the galactic potential. We carry out a Monte Carlo simulation under the assumption that the errors in the observed quantities are uncorrelated. In this simulation, 44% of the trial systems are bound, and systems on the 1-3 sigma tail of the radial velocity distribution can have Proxima currently located near the apastron position of its orbit. Our analysis shows that a further, very significant improvement in the characterization of the system can be gained by obtaining a more accurate measurement of the radial velocity of Proxima Centauri.Comment: 10 pages total, 4 pages of text, 1 page of references, 3 figures, and 2 tables This article will be published in The Astronomical Journa

The Changing Role of Pharmacists as Evidenced by the Strong Vocational Interest Blank

The Strong Vocational Interest Blank/Strong Campbell Interest Inventory-Merged Form, or simply Strong Vocational Interest Blank (SVIB), is widely used to determine the suitability of a student\u27s interests to a particular field. In the process of norming the instrument, data concerning the satisfaction of practitioners of a field and their responses on the instrument are gathered. Such data lend themselves to analysis of trends in the characteristics of those practitioners both over time and over the satisfaction spectrum. Hence changes in the field and characteristics lending themselves to satisfaction can be ascertained

Community Partnerships: An Innovative Model of Social Work Education and Practice

Community challenges force human service agencies to collaborate in providing services. Such collaborations require practitioners to have skills not found in mainstream social work curricula. This paper explores how a new MSW program evolved through dialog with community leaders and resulted in a curriculum with a sole concentration of community partnerships

VUV Photodeposition of Thiol-Terminated Films: A Wavelength-Dependent Study

Photoinitiated chemical vapor deposition (PICVD) has become attractive for selective and specific surface functionalization, because it relies on a single energy source, the photons, to carry out (photo-) chemistry. In the present wavelength (λ)-dependent study, thiol (SH)-terminated thin film deposits have been prepared from gas mixtures of acetylene (C2H2) and hydrogen sulfide (H2S) via PICVD using four different vacuum-ultraviolet (VUV) sources, namely, KrL (λpeak = 123.6 nm), XeL (λpeak = 147.0 nm), XeE (λpeak = 172.0 nm), and Hg (λ = 184.9 nm) lamps. Different λ influence the deposition kinetics and film composition, reflecting that photolytic reactions are governed by the gases’ absorption coefficients, k(λ). Thiol concentrations, [SH], up to ∼7.7%, were obtained with the XeL source, the highest reported in the literature so far. Furthermore, all films showed islandlike surface morphology, regardless of λ

G 112-29 (=NLTT 18149), a Very Wide Companion to GJ 282 AB with a Common Proper Motion, Common Parallax, Common Radial Velocity and Common Age

We have made a search for common proper motion (CPM) companions to the wide binaries in the solar vicinity. We found that the binary GJ 282AB has a very distant CPM companion (NLTT 18149) at a separation s=1.09 \arcdeg. Improved spectral types and radial velocities are obtained, and ages determined for the three components. The Hipparcos trigonometric parallaxes and the new radial velocities and ages turn out to be very similar for the three stars, and provide strong evidence that they form a physical system. At a projected separation of 55733AU from GJ 282AB, NLTT 18149 ranks among the widest physical companions known.Comment: 13 pages, 3 figures, submmited to Ap

O processo de produção de vitrais sob a ótica da tradição

It's impossible to make a discussion about stained glass without bringing up the concepts of tradition. The manual process of stained glass manufacture is based on the knowledge transmitted from generation to generation, and this process has changed just a little until today. The overlaps of glasses colored or not in the plumb gutter define the production of stained glass and this manufacture process is a well-known process since the middle age. This kind of artisanal technique would not remain to date if there were no memory and tradition to use this practice. Such practice originates from the hierarchic relationship of masters and apprentices. Concentrating on the vision that the memory and tradition are fonts of a cultural and patrimonial identity, this work studied the historical, artisanal and technological process of stained glass manufacture and their close link with the tradition and the knowledge. We also reflected about innovation and continuity of the process of stained glass production.Não se pode falar em vitral sem responder a aspectos de conceitos de tradição. O processo de trabalho manual que envolve a produção vitral baseia-se na memória contida no conhecimento transmitido de geração a geração, que a pratica de atelier pouco mudou, visto que a fabricação do vitral está basicamente vinculada ao seu processo de obtenção. A justaposição de vidros, coloridos ou não, e uma calha de chumbo é que define a produção do vitral e esse processo se encontra amadurecido desde a Idade Média. Esta arte não chegaria aos dias de hoje se não houvesse a memória e a tradição desta prática artesanal, originária de uma hierarquia formada por mestres e aprendizes. Focado na visão de que a memória e a tradição são fontes de identidade cultural e patrimonial, no presente trabalho, fez-se um estudo sobre o processo histórico, artesanal e tecnológico de produção de vitrais e seu estreito vínculo com o saber e a tradição. Refletiu-se, também, sobre a tradição e os aspectos de inovação e continuidade do processo de produção de vitrais