María Asenjo González

Effect of humidity on microwave dielectric losses of porous alumina

The microwave dielectric loss in highly porous alumina is measured in dry and moist atmospheres. Data are compared with those for sapphire and a fully densified alumina grade. Results indicate that the combination of humidity and porosity gives rise to a very high dielectric loss which is drastically reduced by replacing the moist atmosphere by dry gas. Measurements over a wide frequency range from 1 mHz to 100 MHz indicate that the origin of the microwave loss is due to the high frequency tail of a low frequency process. This low frequency loss peak shifts to lower frequencies with decreasing humidity, explaining the observed dependence of the microwave loss on humidity.

Dielectric spectroscopy of alumina ceramics over a wide frequency range

Abstract Aluminum oxide (either in single or polycrystalline form) is one of the insulator materials with broad application in high radiation environments. Despite this demanding use, very different values for the dielectric properties have been found in the literature before and after irradiation. In order to obtain a reliable database, the dielectric properties of several unirradiated commercial alumina grades have been measured over a wide frequency (1 kHz to 15 GHz) and temperature range (100 to 300 K). Loss tangent differences over three orders of magnitude have been observed. The possible explanations and the role of some impurities (mainly Mg and Fe) are analyzed.

Radiation resistant alternative substrates for ITER bolometers

Abstract The technical feasibility of producing thin substrates from commercially available sheets of alumina, aluminium nitride, silicon nitride, and silicon dioxide by mechanical grinding and polishing has been demonstrated. Thicknesses of about 50 μm with mean surface roughness down to 20 nm have been obtained. Some of these more radiation resistant alternatives to mica for ITER bolometer applications together with CVD diamond have been prepared with gold and platinum ‘U’ shaped 200 nm thick sputtered tracks, and characterized for their I–V behaviour as a function of temperature. Employing such alternatives reduces the problem of substrate lifetime due to radiation induced swelling of mica, as well as increasing the upper temperature limit. In addition used with platinum the transmutation problem associated with gold would be reduced, and the higher platinum resistivity would enable one to use a less delicate meander.

The role of C-impurities in alumina dielectrics

This work forms part of an extensive study on the effect of impurities on the structure and dielectric loss in alumina ceramics. In this case, the carbon impurity has been chosen since it can be an important contaminant in fusion reactors. Hot-Pressing at 1500 °C in a carbon rich atmosphere has been found to be a powerful method to introduce carbon in two different alumina powders. Due to the presence of C in the samples, they acquire a black colour. To check the ability of C to penetrate into alumina, microstructure (SEM,TEM) and chemical analyses (ICP-AES and EA) have been performed. The existence of pyrolitic carbon at the alumina grain boundaries in the as-sintered state has been observed. The dielectric properties (permittivity and loss tangent) as a function of frequency (from 0.1 Hz to 20 GHz) and DC conductivity of the sintered material were measured by means of several techniques. The most dramatic effect is that the DC conductivity is strongly enhanced when carbon reaches concentrations higher than about 0.5% wt. Oxidation of these samples by a treatment at 1500 °C in air reduces the C content and restores a white exterior colour. In general this treatment strongly reduces the dielectric loss. Changes between reduced and oxidised microstructures were analysed by SEM.

Development of sandwich flow channel inserts for an EU DEMO dual coolant blanket concept

Abstract The dual-coolant lead-lithium (DCLL) blanket concept, which is considered as a candidate for fusion power plants and possibly for a demonstration reactor (DEMO), is being investigated within the framework of the European Power Plant Physics and Technology (PPPT) study. One of major issues of the DCLL concept development is the design of the flow channel inserts (FCIs), which are essential for the reduction of magneto-hydrodynamic (MHD) pressure losses. Due to the tight schedule for the short-term PPPT DEMO, a low-temperature DCLL concept with a liquid metal outlet temperature below 500 °C has been proposed. This allows the use of a simpler type of FCI (taking into account the LM corrosion issues), e.g. Eurofer-Alumina-Eurofer sandwich FCI, instead of the SiCf/SiC version for high temperature case, the production thereof is challenging. This paper discusses the technological study on manufacturing of some FCI design variants and post-examination of the samples.

Overview of the HCPB Research Activities in EUROfusion

In the framework of the EUROfusion’s Power Plant Physics and Technology, the working package breeding blanket (BB) aims at investigating four different BB concepts for an EU demonstration fusion reactor (DEMO). One of these concepts is the helium-cooled pebble bed (HCPB) BB, which is based on the use of pebble beds of lithiated ternary compounds and Be or beryllides as tritium breeder and multiplier materials, respectively, EUROFER97 as structural steel and He as coolant. This paper aims at giving an overview of the EU HCPB BB Research and Development (R&D) being developed at KIT, in collaboration with Wigner-RCP, BUTE-INT, and CIEMAT. The paper gives an outline of the HCPB BB design evolution, state-of-the-art basic functionalities, requirements and performances, and the associated R&D activities in the areas of design, functional materials, manufacturing, and testing. In addition, attention is given also to the activities dedicated to the development of heat transfer augmentation techniques for the first wall and the corresponding testing. Due to their nature as design drivers, a brief overview in the R&D of key HCPB interfacing areas is given as well, namely, the tritium extraction and recovery system, the primary heat transfer and power conversion systems, and safety topics, as well as some specific activities regarding the integration of in-vessel systems through the BB. As concluding remarks, an outline of the standing challenges and future R&D plans is summarized.

Arguments politiques et culture urbaine. Dans la controverse entre Tolède et Grenade après 1492

Ce travail analyse deux lettres ecrites en reaction au declassement de la ville de Tolede au profit de Grenade apres la conquete de celle-ci en 1492. Elles sont expediees par la chancellerie des Rois Catholiques. Redigees par Alphonse Ortiz, chanoine de Tolede, ces deux missives defendent la preseance de la ville de Tolede sur deux plans complementaires : un plan strictement urbain et un plan religieux (la primatie du siege episcopal). La perspective envisagee, celle de la rhetorique urbaine, s’avere novatrice pour l’analyse du discours politique urbain en Castille a la fin du Moyen Âge. Cet episode de la reclamation de la preseance servit a mettre au point tous les ressorts argumentatifs en faveur de la cause de Tolede et en ce sens on peut le considerer comme un paradigme de la rhetorique argumentative de la ville, fondee sur la memoire et l’histoire pour defendre son identite face a une ville recemment conquise.