J. Molla

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.

Alumina ceramics for heating systems

Abstract Data on the dielectric properties of several high purity (> 99.7%) commercial alumina grades and sapphire are presented in wide frequency (1 kHz-145 GHz) and temperature (100 K–500 K) ranges. Loss tangent values show differences over more than three orders of magnitude. For fusion applications and in the frequency range of ICRH and LH, the loss tangent effect is consistent with relaxation processes and it is found that values as low as 10 −5 are available. On the other side, in the frequency range of ECRH the loss tangent can be dominated either by relaxation processes or by the interaction with intrinsic lattice vibrations.

Dielectric losses of self-supporting chemically vapour deposited diamond materials

Abstract The dielectric properties of microwave-assisted plasma chemical vapour deposition diamond are investigated over a wide frequency range. The dielectric losses of several grades of diamond from three different origins are measured from 1xa0kHz to 15xa0GHz by resonant methods. Very low losses were measured in some samples, but high losses were also found in similar grades. This gives rise to a great dispersion in the results, with dielectric loss values ranging from 10 −5 to 10 −1 . This shows that the dielectric loss is still an uncontrolled parameter in the commercially available diamond grades. A characterisation of the samples by X-ray photelectron spectroscopy and radioluminescence is also reported. The measured dielectric spectrum does not show a clear relationship with any of the parameters analysed.

Effect of Mg doping on dielectric properties of alumina

Alumina samples with different magnesia concentrations between 0.02 and 1 wt% have been sintered and characterized by electron microscopy. Their dielectric properties between 1 kHz and 15 GHz are studied. Different dielectric behaviors are found for different magnesia concentrations. A model based mainly on dipolar relaxation processes is proposed to explain the results obtained. The effect of magnesia on dielectric properties of alumina shown in the present work should be considered in the engineering designs for heating systems of future fusion machines and other applications.

Electron-irradiated silicon: An optimized material for gyrotron windows

Abstract The special potential of high resistivity (HR) silicon for electron cyclotron wave transmission components for nuclear fusion is related to the dominant contribution of free charge carriers to the mm-wave absorption. It is demonstrated by dielectric property measurements that electron irradiation can practically overcome the problems of special concern for applications, such as the sensitivity of dielectric loss (tan δ) to ionizing radiation and the complex temperature dependence of tan δ. Also the loss levels are lowered at room temperature. The improvements are correlated with the total dose of electron irradiations and are of long term stability. It is inferred that new structural defects are formed, which act on the charge carriers as centers of trapping, recombination or scattering. As a consequence, tan δ values are measured at 145 GHz that fall below those of sapphire in the range of 120–350 K.

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.

Insulator dielectric properties during irradiation

Abstract The use of insulators for fusion applications is subject to considerable uncertainty due to the lack of a reliable database about the dielectric properties of insulator materials during irradiation at different frequencies. A set-up for the room-temperature measurement of these properties during electron irradiation at 15 GHz is described. Sapphire, spinel, beryllia and several alumina grades have been measured. Some very complex flux and fluence effects are observed. Several sapphire samples degraded by the RIED effect at 250 and 450°C are examined.