R. Vila

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.

Wide frequency dielectric properties of CVD diamond

Abstract The dielectric properties of CVD diamond are studied in a wide frequency range (1–10 10 Hz) at room temperature and in a wide temperature range at microwave frequencies. The results obtained at room temperature can be described by a Hill conduction process using previously published parameters. It is concluded that CVD diamond is a strong candidate to be considered in the design of high-power micro- and millimetre-wave devices.

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.

On the cation vacancy distribution in MgAl2O4 spinels

Abstract Calculations by Dupree, Lewis and Smith (1986) on the ratio of aluminium in tetrahedral to octahedral sites in non-stoichiometric MgAl2O4 have been revised taking into account the cation disorder ever present in synthetic crystals. Comparison with the experimental results also obtained by these authors shows that not only octahedrally, but also tetrahedrally coordinated, cation vacancies may be introduced for charge compensation.

Thermally stimulated depolarization of ellipsoidal particles in an insulating medium

The equations governing thermally stimulated depolarization (TSD) processes in a suspension of ellipsoidal particles in an insulating matrix, which exhibits Maxwell-Wagner type polarization, have been obtained from the Sillars-Van Beek formalism. A first-order TSD process is obtained when all particles are identical and arranged with one of their principal axes parallel to the electric field. For a random distribution of particle shapes, sizes and orientations, the TSD peak is wider than a first-order one. The dependence of the depolarization processes on the physical properties of the matrix and the embedded particles is shown for different particle size distributions.

Optical and dielectric properties of neutron irradiated MgAl2O4 spinels

Abstract The radiation effects on the optical and electrical properties of stoichiometric MgAl2O4 spinel specimens irradiated in FFTF—MOTA at temperatures between 385 and 750°C to fluence ranging from 5.3 to 24.9 × 1026 n m−2 (E ≫ 0.1 McV) are measured. In the optical properties a strong absorption in the ultraviolet range is observed together with a small band around 20 000 cm−1 (510 nm). Two strong luminescence emissions are also observed around 700 nm, with excitation spectra in the ultraviolet region. In the electrical properties a strong decrease of conductivity is observed in the temperature range from 0 to 500°C. Other techniques (like dielectric spectroscopy and EPR) have been used.

Technological exploitation of Deuterium-Tritium operations at JET in support of ITER design, operation and safety

Abstract Within the framework of the EUROfusion programme, a work-package of technology projects (WPJET3) is being carried out in conjunction with the planned Deuterium–Tritium experiment on JET (DTE2) with the objective of maximising the scientific and technological return of DT operations at JET in support of ITER. This paper presents the progress since the start of the project in 2014 in the preparatory experiments, analyses and studies in the areas of neutronics, neutron induced activation and damage in ITER materials, nuclear safety, tritium retention, permeation and outgassing, and waste production in preparation of DTE2.

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.

Radiation effects on dielectric losses of Au-doped silicon

Effects of electron and neutron irradiation on dielectric properties of Au-doped silicon are examined as a function of the frequency between 1 kHz and 150 GHz. The studies compare the Au-doped Si with a high resisitivity (HR) pure Si in the as-received state and after electron irradiation. The obtained data for both materials show that electron irradiation and neutron irradiation do not cause degradation of the dielectric loss behaviour, but even improve it. This beneficial effect already observed earlier in pure silicon is also observed in Au-doped silicon. Loss data obtained in-beam under electron irradiation are also reported.

In-beam dielectric properties of alumina at low frequencies

Abstract Loss tangent data for three commercial alumina grades measured in the range 0.1–300 kHz together with the effect of radiation are presented. Very different dielectric behaviour has been found for the three grades irradiated at 150°C and 250°C with 1.8 MeV electrons at dose rates of up to 3.5 kGy/s. In the case of Ceraten C9999, an extremely pure alumina, the increase in the loss tangent with dose rate and its dependence on frequency is due to the dc radiation induced conductivity (RIC). For Wesgo AL995 and Morgan Matroc Vitox 999 the behaviour is more complex. The influence of an applied electric field has also been examined. The importance of material selection is noted, as is the dose-dependent degradation for Wesgo AL995.