Synthesis of silicon carbide ceramics by novel microwave methods

The work described in this thesis was carried out on the synthesis of silicon carbide using microwave processing and aimed to develop procedures to reduce processing complexity and cut processing times. Si-C/SiO2-C systems were first studied due to the ready availability at reasonable cost of the starting powders and the excellent microwave absorption properties of carbon. Silicon carbide was synthesised from silicon or silica combined with activated carbon or graphite via microwave heating over timescales from minutes to seconds without the need for inert atmospheres or subsequent purification. In the reactions performed in a MMC, graphite was found fundamental not only as a microwave susceptor, but also as a reductant, preventing the oxidation of silicon carbide. Another important beneficial factor was water, used as a binder in the pellet making process, it minimised the intergrain void space between particles and possibly acted as a polar liquid microwave susceptor. It was found the carbide morphology and phase purity can be controlled by the microwave cavity used, the power applied and hence by the heating rate. Short irradiation times (ca. 5 minutes) in a multimode cavity using silicon and activated carbon powders produced single phase β-SiC nanofibres as small as 5 nm in diameter while large crystallites of β-SiC can obtained in ~1 minute using high power, single mode cavity microwave techniques. Furthermore, similar microwave cavity systems shown that the removal of the susceptor, starting from silica and carbon mixtures, is possible and the successful conversion to silicon carbide can be performed using loose powders. This represented a major step with respect to designing a flow process and reducing carbon contamination. Studies of microwave processing of silicon carbide were then extended to x-aerogels, to probe whether the produced silicon carbide would mimic the porous microstructures offered by the precursor. This indeed resulted in the production of porous silicon carbide (in 15 minutes) and also sintered crystallites of micrometre sizes (after 3.5 minutes) whether MMC or SMC systems were employed

Microstructural Characterization of Mg-Al Spinel Powders

Mg-Al spinel powders have been prepared by thermal decomposition of a mixture of: A-aluminium and magnesium nitrates; B-aluminium and magnesium hydroxides; and C-aluminium hydroxide and magnesium oxalate. The initial and the final powders were both characterized by specific surface area measurements, mercury intrusion porosimetry, X-ray diffraction, and scanning electron microscopy. The results showed that the preparation process sharply influences the final microstructure of the spinel powders. In particular while the shape and particle dimensions of the samples prepared by mixture of aluminium and magnesium nitrates are mainly influenced by crushing process, the preparation via mixed magnesium and aluminium hydroxides precipitation permits use of spinel formation temperatures as low as 350°C. Characteristically this powder is very uniform and consists of small particle sizes (0.1 micrometers)

Laue Lens Development for Hard X-rays (>60 keV)

Results of reflectivity measurements of mosaic crystal samples of Cu (111) are reported. These tests were performed in the context of a feasibility study of a hard X-ray focusing telescope for space astronomy with energy passband from 60 to 600 keV. The technique envisaged is that of using mosaic crystals in transmission configuration that diffract X-rays for Bragg diffraction (Laue lens). The Laue lens assumed has a spherical shape with focal length $f$. It is made of flat mosaic crystal tiles suitably positioned in the lens. The samples were grown and worked for this project at the Institute Laue-Langevin (ILL) in Grenoble (France), while the reflectivity tests were performed at the X-ray facility of the Physics Department of the University of Ferrara.Comment: 6 pages, 12 figures, accepted for publication in IEEE Transactions on Nuclear Scienc

Hard x-ray broad band Laue lenses (80 - 600 keV): building methods and performances

We present the status of the laue project devoted to develop a technology for building a 20 meter long focal length Laue lens for hard x-/soft gamma-ray astronomy (80 - 600 keV). The Laue lens is composed of bent crystals of Gallium Arsenide (GaAs, 220) and Germanium (Ge, 111), and, for the first time, the focusing property of bent crystals has been exploited for this field of applications. We show the preliminary results concerning the adhesive employed to fix the crystal tiles over the lens support, the positioning accuracy obtained and possible further improvements. The Laue lens petal that will be completed in a few months has a pass band of 80 - 300 keV and is a fraction of an entire Laue lens capable of focusing X-rays up to 600 keV, possibly extendable down to 20 - 30 keV with suitable low absorption crystal materials and focal length. The final goal is to develop a focusing optics that can improve the sensitivity over current telescopes in this energy band by 2 orders of magnitude

Almanacco SBN. News sui servizi CILEA per i Poli SBN: luglio-settembre 2008

In the quarter just ended, a lot of activities were carried out by CILEA team providing support to SBN poles. In particular, for the Polo USM, that is moving to Sebina OL, CILEA has made the first and important activity essential to the success of the process. Other important activities were carried out for the pole Unix client / server (two UNIMARC imports, both completed successfully) and the pole LO2 (customization of the OPAC for the Library of the Regional Council of Lombardia).Nel trimestre appena trascorso, diverse attività sono state svolte dai team CILEA che forniscono servizi ai poli SBN. In particolare, per il Polo USM (in procinto di passare a Sebina OL) il CILEA ha effettuato le prime operazioni indispensabili per il buon esito del passaggio. Altre attività importanti sono state svolte per i poli Unix client/server (due importazioni UNIMARC, entrambe conclusesi con successo) e per il polo LO2 (la personalizzazione dell’OPAC per la biblioteca del Consiglio regionale della Lombardia)

The OLYMPUS Internal Hydrogen Target

An internal hydrogen target system was developed for the OLYMPUS experiment at DESY, in Hamburg, Germany. The target consisted of a long, thin-walled, tubular cell within an aluminum scattering chamber. Hydrogen entered at the center of the cell and exited through the ends, where it was removed from the beamline by a multistage pumping system. A cryogenic coldhead cooled the target cell to counteract heating from the beam and increase the density of hydrogen in the target. A fixed collimator protected the cell from synchrotron radiation and the beam halo. A series of wakefield suppressors reduced heating from beam wakefields. The target system was installed within the DORIS storage ring and was successfully operated during the course of the OLYMPUS experiment in 2012. Information on the design, fabrication, and performance of the target system is reported.Comment: 9 pages, 13 figure

New results on focusing of gamma-rays with Laue lenses

We report on new results on the development activity of broad band Laue lenses for hard X-/gamma-ray astronomy (70/100-600 keV). After the development of a first prototype, whose performance was presented at the SPIE conference on Astronomical Telescopes held last year in Marseille (Frontera et al. 2008), we have improved the lens assembling technology. We present the development status of the new lens prototype that is on the way to be assembled.Comment: 8 pages, 11 figures, to be Published in SPIE Proceedings, vol.7437-19, 200