Modeling and development of a microwave heated pilot plant for the production of SiC-based ceramic matrix composites

This paper outlines the development of a microwave heated apparatus for the production of silicon carbide (SiC) based ceramic matrix composites via chemical vapor infiltration. An innovative pilot scale reactor was designed and built. A coupled thermal and electromagnetic model was developed in order to predict the temperature profile inside the reactor. The results obtained from the model demonstrated that the electric field inside the sample was constant. This fact is particularly important in order to prevent the thermal instabilities (run-aways) that are typical in the case of microwave heating. Therefore the heating was uniform with the aid of a mode stirrer that achieved a better distribution of the microwave power and then improved the process efficiency. The infiltration cycles were carried out on SiC fiber preforms and resulted in an excellent average weight increase with respect to the initial sample. By using microwave heating, the treatment times were considerably reduced with respect to the conventional process times reported in the literature. The microstructure of the SiC composites were observed by scanning the electron microscopy in order to evaluate the quality and the degree of densification which was achieved within the fiber tows. The SiC deposition inside of the sample was sufficiently homogeneous and compact, even if a certain degree of inter-tow porosity was still evident

A Determination of the Hubble Constant from Cepheid Distances and a Model of the Local Peculiar Velocity Field

We present a measurement of the Hubble Constant based on Cepheid distances to 27 galaxies within 20 Mpc. We take the Cepheid data from published measurements by the Hubble Telescope Key Project on the Distance Scale (H0KP). We calibrate the Cepheid Period-Luminosity (PL) relation with data from over 700 Cepheids in the LMC obtained by the OGLE collaboration; we assume an LMC distance modulus of 18.50 mag (d=50.1 kpc). Using this PL calibration we obtain new distances to the H0KP galaxies. We correct the redshifts of these galaxies for peculiar velocities using two distinct velocity field models: the phenomenological model of Tonry et al. and a model based on the IRAS density field and linear gravitational instability theory. We combine the Cepheid distances with the corrected redshifts for the 27 galaxies to derive H_0, the Hubble constant. The results are H_0 = 85 +/- 5 km/s/Mpc (random error) at 95% confidence when the IRAS model is used, and 92 +/- 5 km/s/Mpc when the phenomenological model is used. The IRAS model is a better fit to the data and the Hubble constant it returns is more reliable. Systematic error stems mainly from LMC distance uncertainty which is not directly addressed by this paper. Our value of H_0 is significantly larger than that quoted by the H0KP, H_0 = 71 +/- 6 km/s/Mpc. Cepheid recalibration explains ~30% of this difference, velocity field analysis accounts for ~70%. We discuss in detail possible reasons for this discrepancy and future study needed to resolve it.Comment: 33 pages, 8 embedded figures. New table, 5 new references, text revision

Oxygen- and carbon-rich variable red giant populations in the Magellanic Clouds from EROS, OGLE, MACHO, and 2MASS photometry

The carbon-to-oxygen (C/O) ratio of asymptotic giant branch (AGB) stars constitutes an important index of evolutionary and environment/metallicity factor. We develop a method for mass C/O classification of AGBs in photometric surveys without using periods. For this purpose we rely on the slopes in the tracks of individual stars in the colour-magnitude diagram. We demonstrate that our method enables the separation of C-rich and O-rich AGB stars with little confusion. For the Magellanic Clouds we demonstrate that this method works for several photometric surveys and filter combinations. As we rely on no period identification, our results are relatively insensitive to the phase coverage, aliasing, and time-sampling problems that plague period analyses. For a subsample of our stars, we verify our C/O classification against published C/O catalogues. With our method we are able to produce C/O maps of the entire Magellanic Clouds. Our purely photometric method for classification of C- and O-rich AGBs constitutes a method of choice for large, near-infrared photometric surveys. Because our method depends on the slope of colour-magnitude variation but not on magnitude zero point, it remains applicable to objects with unknown distances.Comment: 14 pages, 16 figures, 1 table, accepted for publication in Astronomy & Astrophysic

Feeding behaviour of larval European sea bass (Dicentrarchus labrax L.) in relation to temperature and prey density

The feeding behaviour of larval European sea bass (Dicentrarchus labrax, L.) was analysed in relation to temperature and prey density under controlled laboratory conditions with the aim to assess the ability of larval fish to change the feeding tactic as a response to environmental changes. Larvae were acclimated for 20 days at three different temperatures (19, 22 and 26°C), and their feeding behaviour was then video-recorded in experimental trials, at two prey densities, consisting of swarms of 400/l and 1440/l Artemia nauplii. Results showed that there was a significant effect of the interaction between temperature and prey density on the proportion of swimming activity that was reduced at the high temperature-high prey density combination. This suggested a switching in the larval feeding behaviour from an active to an ambush tactic, when the temperature reached 26°C and the prey density was 1440 /l Artemia nauplii. These results are consistent with the current literature on fish larval behaviour in showing that the foraging tactic can be modulated by the interaction of different abiotic and biotic factors characterising the rearing environment

Fingerprinting ash deposits of small scale eruptions by their physical and textural features

Correlation of distal ash deposits with their proximal counterparts mainly relies on chemical and mineralogical characterization of bulk rock and matrix glasses. However, the study of juvenile fragments often reveals the heterogeneity in terms of clast shape, external surface, groundmass texture and composition. This is particularly evident in small-scale eruptions, characterized by a strong variability in texture and relative abundance of juvenile fragments. This heterogeneity introduces an inherent uncertainty, that makes the compositional data alone inadequate to unequivocally characterize the tephra bed. Pyroclast characteristics, if described and quantified, can represent an additional clue for the correct identification of the tephra. The paper presents morphological, textural and compositional data on the products of an ash eruption from Middle Age activity of Vesuvius, to demonstrate the information that can be extracted from the proposed type of analysis. Juvenile fragments from five ash layers throughout the studied products were randomly hand-picked, and fully characterized in terms of external morphology, particle outline parameterization, groundmass texture and glass composition. Statistical analysis of shape parameters characterized groups of fragments that can be compared with the other textural and physical parameters. The main result is that the data do not show important cross-correlations, so suggesting that all of these parameters, together with accurate field data are needed for the complete fingerprinting of a tephra bed. We suggest that this approach is especially important for characterizing the products of small scale, compositionally undistinguishable, eruptions and represents the necessary step to deal with before going into more detailed compositional analyses

Architectures and Key Technical Challenges for 5G Systems Incorporating Satellites

Satellite Communication systems are a promising solution to extend and complement terrestrial networks in unserved or under-served areas. This aspect is reflected by recent commercial and standardisation endeavours. In particular, 3GPP recently initiated a Study Item for New Radio-based, i.e., 5G, Non-Terrestrial Networks aimed at deploying satellite systems either as a stand-alone solution or as an integration to terrestrial networks in mobile broadband and machine-type communication scenarios. However, typical satellite channel impairments, as large path losses, delays, and Doppler shifts, pose severe challenges to the realisation of a satellite-based NR network. In this paper, based on the architecture options currently being discussed in the standardisation fora, we discuss and assess the impact of the satellite channel characteristics on the physical and Medium Access Control layers, both in terms of transmitted waveforms and procedures for enhanced Mobile BroadBand (eMBB) and NarrowBand-Internet of Things (NB-IoT) applications. The proposed analysis shows that the main technical challenges are related to the PHY/MAC procedures, in particular Random Access (RA), Timing Advance (TA), and Hybrid Automatic Repeat reQuest (HARQ) and, depending on the considered service and architecture, different solutions are proposed.Comment: Submitted to Transactions on Vehicular Technologies, April 201

Pulsating stars in the VMC survey

The VISTA survey of the Magellanic Clouds system (VMC) began observations in 2009 and since then, it has collected multi-epoch data at Ks and in addition multi-band data in Y and J for a wide range of stellar populations across the Magellanic system. Among them are pulsating variable stars: Cepheids, RR Lyrae, and asymptotic giant branch stars that represent useful tracers of the host system geometry.Comment: 8 pages, 7 figures, proceeding contribution of invited presentation at "Wide-field variability surveys: a 21st-century perspective", San Pedro de Atacama (Chile