Improving the toughness of refractory compounds

Composition and processing studies were conducted on silicon nitride and silicon carbide materials. Charpy mode impact testing to 2415 F established the effectiveness of higher purity silicon nitride powder sources in reducing the scatter of measurements and in improving short time bend strengths as well as bend stress rupture properties. Stabilized zirconia additions in particular were observed to dramatically enhance low and high temperature bend strengths and stress rupture properties for all grades of silicon nitride powder. Silicon carbide samples showed relatively poor impact resistance, although the maxima in stress rupture behavior was exhibited by this material

Class of near-perfect coded apertures

Coded aperture imaging of gamma ray sources has long promised an improvement in the sensitivity of various detector systems. The promise has remained largely unfulfilled, however, for either one of two reasons. First, the encoding/decoding method produces artifacts, which even in the absence of quantum noise, restrict the quality of the reconstructed image. This is true of most correlation-type methods. Second, if the decoding procedure is of the deconvolution variety, small terms in the transfer function of the aperture can lead to excessive noise in the reconstructed image. It is proposed to circumvent both of these problems by use of a uniformly redundant array (URA) as the coded aperture in conjunction with a special correlation decoding method

Media Production and disruptive innovation: exploring the interrelations between children, tablets, teachers and texts in subject English settings

This article concerns Key Stage 3 boys’ and student teachers’ responses to literature through the production of moving image, sound and photography in a formal context. The researchers’ interests centre on media making with iPads and the ways in which young people articulate textual understanding through production. Under the guidance of their tutors, PGCE students and pupils engage with digital tools to explore new forms of cultural expression, inviting creative practice and critical thinking. The article incorporates multiple perspectives from: the children, the student teachers, the English and Drama teachers, the teacher trainers / lecturers in English and Drama, and a doctoral researcher interested in the distinctive nature of making with digital media and associated ontological issues. A sociocultural overview contextualises the study before an exploration of the ways in which pedagogies and epistemological understandings, often associated with non-formal media making, constructively challenge traditional Secondary English classroom practices

Localizing gravitational wave sources with optical telescopes and combining electromagnetic and gravitational wave data

Neutron star binaries, which are among the most promising sources for the direct detection of gravitational waves (GW) by ground based detectors, are also potential electromagnetic (EM) emitters. Gravitational waves will provide a new window to observe these events and hopefully give us glimpses of new astrophysics. In this paper, we discuss how EM information of these events can considerably improve GW parameter estimation both in terms of accuracy and computational power requirement. And then in return how GW sky localization can help EM astronomers in follow-up studies of sources which did not yield any prompt emission. We discuss how both EM source information and GW source localization can be used in a framework of multi-messenger astronomy. We illustrate how the large error regions in GW sky localizations can be handled in conducting optical astronomy in the advance detector era. We show some preliminary results in the context of an array of optical telescopes called BlackGEM, dedicated for optical follow-up of GW triggers, that is being constructed in La Silla, Chile and is expected to operate concurrent to the advanced GW detectors.Comment: 8 pages, 8 figures, Proceeding for Sant Cugat Forum for Astrophysic

Magnetic Fluctuations in a Charge Ordered State of the One-Dimensional Extended Hubbard Model with a Half-Filled Band

Magnetic properties in a charge ordered state are examined for the extended Hubbard model at half-filling. Magnetic excitations, magnetic susceptibilities and a nuclear spin relaxation rate are calculated with taking account of fluctuations around the mean-field solution. The relevance of the present results to the observation in the 1:1 organic conductors, (TTM-TTP)I$_3$, is discussed.Comment: 4 pages, 3 figures, to be published in J. Phys. Soc. Jpn. Vol.71 (2002) No.

The S2 VLBI Correlator: A Correlator for Space VLBI and Geodetic Signal Processing

We describe the design of a correlator system for ground and space-based VLBI. The correlator contains unique signal processing functions: flexible LO frequency switching for bandwidth synthesis; 1 ms dump intervals, multi-rate digital signal-processing techniques to allow correlation of signals at different sample rates; and a digital filter for very high resolution cross-power spectra. It also includes autocorrelation, tone extraction, pulsar gating, signal-statistics accumulation.Comment: 44 pages, 13 figure

A Sr-Rich Star on the Main Sequence of Omega Centauri

Abundance ratios relative to iron for carbon, nitrogen, strontium and barium are presented for a metal-rich main sequence star ([Fe/H]=--0.74) in the globular cluster omega Centauri. This star, designated 2015448, shows depleted carbon and solar nitrogen, but more interestingly, shows an enhanced abundance ratio of strontium [Sr/Fe] ~ 1.6 dex, while the barium abundance ratio is [Ba/Fe]<0.6 dex. At this metallicity one usually sees strontium and barium abundance ratios that are roughly equal. Possible formation scenarios of this peculiar object are considered.Comment: 13 pages, 3 figures. Accepted to ApJ

AGC 226067: A possible interacting low-mass system

We present Arecibo, GBT, VLA and WIYN/pODI observations of the ALFALFA source AGC 226067. Originally identified as an ultra-compact high velocity cloud and candidate Local Group galaxy, AGC 226067 is spatially and kinematically coincident with the Virgo cluster, and the identification by multiple groups of an optical counterpart with no resolved stars supports the interpretation that this systems lies at the Virgo distance (D=17 Mpc). The combined observations reveal that the system consists of multiple components: a central HI source associated with the optical counterpart (AGC 226067), a smaller HI-only component (AGC 229490), a second optical component (AGC 229491), and extended low surface brightness HI. Only ~1/4 of the single-dish HI emission is associated with AGC 226067; as a result, we find M_HI/L_g ~ 6 Msun/Lsun, which is lower than previous work. At D=17 Mpc, AGC 226067 has an HI mass of 1.5 x 10^7 Msun and L_g = 2.4 x 10^6 Lsun, AGC 229490 (the HI-only component) has M_HI = 3.6 x 10^6 Msun, and AGC 229491 (the second optical component) has L_g = 3.6 x 10^5 Lsun. The nature of this system of three sources is uncertain: AGC 226067 and AGC 229490 may be connected by an HI bridge, and AGC 229490 and AGC 229491 are separated by only 0.5'. The current data do not resolve the HI in AGC 229490 and its origin is unclear. We discuss possible scenarios for this system of objects: an interacting system of dwarf galaxies, accretion of material onto AGC 226067, or stripping of material from AGC 226067.Comment: Accepted for publication in A&A. 6 pages, 4 figure

Abundances on the Main Sequence of Omega Centauri

Abundance ratios of carbon, nitrogen and strontium relative to iron, calculated using spectrum synthesis techniques, are given for a sample of main sequence and turnoff stars that belong to the globular cluster omega Centauri. The variations of carbon, nitrogen and/or strontium show several different abundance patterns as a function of [Fe/H]. The source of the enhancements/depletions in carbon, nitrogen and/or strontium may be enrichment from asymptotic giant branch stars of low (1--3 solar masses) and intermediate (3--8 solar masses) mass. Massive rotating stars which produce excess nitrogen without carbon and oxygen overabundances may also play a role. These abundances enable different contributors to be considered and incorporated into the evolutionary picture of omega Cen.Comment: 43 Pages, 13 Figures. Accepted for publication in Ap

Large deviations for ideal quantum systems

We consider a general d-dimensional quantum system of non-interacting particles, with suitable statistics, in a very large (formally infinite) container. We prove that, in equilibrium, the fluctuations in the density of particles in a subdomain of the container are described by a large deviation function related to the pressure of the system. That is, untypical densities occur with a probability exponentially small in the volume of the subdomain, with the coefficient in the exponent given by the appropriate thermodynamic potential. Furthermore, small fluctuations satisfy the central limit theorem.Comment: 28 pages, LaTeX 2