K. Shiiyama

Electrical properties of ceramics during reactor irradiation

Twelve different types of polycrystal and single crystal Al 2 O 3 (alumina and sapphire) specimens of varying grades of purity were irradiated for three reactor cycles in a removable beryllium position in a High Flux Isotopes Reactor (HFIR) at Oak Ridge National Laboratory at a temperature of 720-760 K up to a maximum dose of 3 dpa while a dc electric field of 200 V/mm was applied. The recently completed Temperature Regulated In Situ Test (TRIST) facility in the HFIR was used to perform in situ measurements of electrical conductivity. In addition, three Al 2 O 3 specimens were simultaneously irradiated without a continuously applied dc electric field. In situ electrical conductivity measurements were performed on the specimens before, during and following each irradiation cycle. Behavior of electrical conduction in Al 2 O 3 was studied, with special emphasis on detection of any long-term increase of the electrical conductivity.

In situ measurement of electrical conductivity of alumina under electron irradiation in a high voltage electron microscope

Radiation induced conductivity (RIC), thermally stimulated conductivity (TSC) and radiation induced electrical degradation (RIED) are major concerns of insulating ceramic materials under the effects of flux, electric field and temperature and may lessen their performance in fusion reactors. In situ measurements of the electrical conductivity of single crystal et-AlzO 3 (alumina) using the standard electric guarding technique has been performed under 1 MeV electron irradiation with an applied electric field of 93 kV/m at temperatures ranging from room temperature to 723 K. Experimental results imply that electronic excitation associated with radiation induced defects controls RIC of ct-A120 3 and show that TSC, especially the transient peak resulting from excess charges stored in defects, may affect the performance of ct-A120 3 in fusion reactors. A significant surface conductivity is confirmed from a 1 MeV electron dose of 3 x 1022 e/m 2 (3.3 x 10 -5 dpa) but no substantial bulk degradation is found under irradiation up to a dose of 7.1 x 1022 e/m 2 (7.7 × 10 -5 dpa) at 723 K. In conclusion, it is emphasized that RIC and RIED of et-A120 3 are not severe for insulators in the International Thermonuclear Experimental Reactor (ITER) but TSC could limit their applications even to ITER.

The electrical conductivity of zircaloy oxide films

Abstract The electrical conductivity of Zircaloy-2, improved Zircaloy-2 and Zircaloy-4 oxide films has been measured using gold, copper and zirconium electrodes over the temperature range of 296–723 K. No notable discrepancies among the temperature dependence of electrical conductivity in the oxide films obtained from all electrodes were found. Depending on the activation behavior it is thought that the conductivity corresponds to the thermal excitation of electrons. The current–voltage characteristics of all oxide films show non-Ohmic behavior where the bulk current (electronic in nature) is associated with the Schottky and/or the Poole–Frenkel processes. On the basis of these results, it is concluded that the electron motion dominates the electrical conductivity of zircaloy oxide films. Therefore the slow-diffusing negative oxygen ions control the oxidation process of zircaloys.

Electrical conductivity of Wesgo AL995 alumina under fast electron irradiation in a high voltage electron microscope

Electrical conductivity of a 295-μm-thick Wesgo AL995 alumina has been measured before and during 1 MeV electron irradiation in a dc electric field of 300 kV/m at temperatures up to 723 K. The difference between the activation energies before (0.49±0.02 eV) and during (0.19±0.06 eV) irradiation indicates a substantial impact of irradiation on the conductivity of Wesgo AL995 alumina. The electrical conductivity of Wesgo AL995 alumina is lower by approximately 2 orders of magnitude than its requirement for the magnetic coils in the international thermonuclear experimental reactor (ITER). Thermal disruption may not impact on Wesgo AL995 alumina insulating material in ITER because of the absence of thermally stimulated conductivity peaks in it. Although no substantial bulk degradation is observed under irradiation up to a fluence of 7.0×1022 e/m2 (7.97×10−5 dpa) at 723 K, surface degradation is detected that could limit the application of Wesgo AL995 in ITER as a potential insulator.

Measurements of electrical conductivity of ceramics under electron irradiation in a high voltage electron microscope

Abstract A specimen holder for in-situ measurements of electrical conductivity in a high voltage electron microscope (HVEM) was developed. The performance of newly developed holder was checked by measuring the temperature dependence of the electrical conductivity in ZrO 2 3mol%Y 2 O 3 (YSZ) and the radiation induced conductivity was subsequently measured in YSZ, α-Al 2 O 3 and MgAl 2 O 4 during irradiation with 1 MeV electrons. The electrical conductivity under irradiation, σ, depends on the flux as σ=σ 0 + δφ d where σ 0 is the conductivity in the absence of radiation, δ a constant, φ electron flux and d the exponent of the flux dependence. The values of d are 1.3±0.2 for YSZ and 1.0±0.1 for both α-Al 2 O 3 and MgAl 2 O 4 . The electrical conductivity in α-Al 2 O 3 was measured at 723 K during irradiation with a 1 MeV electron flux of 2.0 × 10 18 e/m 2 s with an applied electric field of 130 V/mm. No radiation induced electrical degradation was found in α-Al 2 O 3 up to a fluence of 8.0 × 10 22 e/m 2 .

Effects of the electric field on the aggregation of point defects in ion-irradiated α-Al2O3

We have investigated the effects of an electric field on the formation process of interstitial-type dislocation loops in f -Al 2 O 3 irradiated at 760 K with 100 keV He + ions to a damage level of 0.5 displacements per atom. An electric field of 100 kV m m 1 depresses the density of dislocation loops and enhances their growth process, compared with the microstructure irradiated without an electric field. In addition, a higher fraction of interstitials escapes to surface sinks in a wedge-shaped thin-foil specimen when irradiated in the presence of an electric field. The kinetic behaviour of interstitials is discussed in terms of diffusion processes driven by the electric field and the concentration gradient. This is the first transmission electron microscopy observation to demonstrate the effects of an electric field on the aggregation of radiation-induced defects in f -Al 2 O 3 .

Ion-beam induced defect formation in α-alumina with applied electric field

An attempt is made to understand the kinetic behavior of radiation-induced defects in a-Al2O3 irradiated with applied electric fields. To this end, a special device is developed for ion-beam irradiation on insulating ceramics at temperatures up to 920 K and with applied electric fields to 300 kV/m. We have found that electric fields of 100 and 300 kV/m influence the nucleation-and-growth process of defect clusters in a-Al2O3 irradiated with 100 keV He þ ions. The electric fields reduce the formation of interstitial-type dislocation loops at 760 K and retard the formation of defect clusters (probably vacancy-type clusters) at 870 K. Results are discussed in terms of the directed migration of interstitials and the recombination rate of vacancies and interstitials. 2003 Elsevier Science B.V. All rights reserved.

Significance of sample thickness and surface segregation on the electrical conductivity of Wesgo AL995 alumina under ITER environments

Abstract The electrical conductivity of 158, 295 and 610 μm thick Wesgo AL995 alumina was measured under 1 MeV electron irradiation with an electric field of 300 kV/m at temperatures up to 723 K. A significant increase in the conductivity with increasing the sample thickness is confirmed, but no substantial bulk degradation is found under irradiation up to a dose of 7.0×10 22 e / m 2 (7.97×10 −5 dpa ) at 723 K. However, surface breakdown is found only in 295 and 610 μm thick specimens. The non-existence of the surface breakdown in 158 μm thick specimen is thought to be due to the sinks effect of point defects at the surface. The X-ray analysis of the virgin and degraded specimens through scanning electron microscopy (SEM) reveals the segregation of impurities along the grain boundaries on the degraded surface. The segregation of impurities assists leaking of surface current along the grain boundaries.

In situ measurement of electrical conductivity of Zircaloy oxides and their formation mechanism under electron irradiation

Abstract Insitu electrical conductivity of Zircaloy-2, improved Zircaloy-2 and Zircaloy-4 oxide films has been measured with 1 MeV electron irradiation using gold, copper and zirconium electrodes during beam-on and -off in the temperature range of 296–680 K in order to find the rate controlling factor of oxidation of the alloy. Current–voltage characteristics of all oxides during irradiation show almost ohmic behavior for Zircaloy-2 and Zircaloy-4 oxides, and non-ohmic for improved Zircaloy-2 oxide where the current is electronic in nature and may be associated with ohmic and Schottky and/or Poole–Frenkel processes, respectively. The electrical conductivity of such films under irradiation, called radiation induced conductivity (RIC), is proportional to the irradiation flux and it is predominantly due to electronic excitation. Subsequent subjection of the specimens under irradiation with beam-on and -off shows RIC by electron excitation from the valence to conduction band and annealing of the conductivity at temperature, respectively. It is concluded that the electron motion dominates the electrical conductivity of Zircaloy oxides. Therefore, the slow-diffusing negative oxygen ions control the oxidation process of Zircaloys.

Development of specimen holders for measuring electrical properties of oxides under electron irradiation in HVEM

Abstract Two types of specimen holders have been developed for in situ measurements of electrical properties during microstructural observation in a high voltage electron microscope (HVEM). Each specimen holder contains an electrode cell consisting of electrodes, a heater, a thermocouple and a base made from MACOR insulating ceramic. The temperature dependence of the electrical conductivity in ZrO2-3mol% Y2O3 (YSZ) and the dielectric loss in MgO have been investigated without irradiation for evaluating the reliability and the limitation of the specimen holders. YSZ specimens were irradiated with 1 MeV electrons at temperatures from 450 K to 870 K in the HVEM. The radiation induced electrical conductivity was detected by in situ measurements. The dielectric loss in MgO was also measured and found to increase under electron irradiation.