M. Kutsuwada

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.

A new method for evaluating stress-strain properties of metals using ultra-microhardness technique

Abstract A new method for evaluating the yield stress σ0.2 and the strain-hardening exponent n of metals is proposed through ultra-microhardness (UMH) technique with an application to fcc metals. UMH measurements with a triangular-base pyramidal indenter have been conducted on copper and copper-aluminum alloys under cold-rolled and annealed conditions at loads ranging from 0.05 g (4.9 × 10−2mN) to 50 g (49 mN), and their results are compared with those of the Vickers hardness (Hv) at 200 g (1.96 N) and of tensile tests. The method is based on Cahoon et al.s relation that σ0.2(MPa) = 3.27Hv (0.1)n and is composed of (i) the determination of the relationship between Hv and ultra-microhardness (Hum), (ii) the evaluation of n through analyzing the load dependence of Hum and (iii) the evaluation of σ0.2 from Cahoon et al.s relation together with the results on (i) and (ii). The value of n can be extracted through the UMH test, and the value of σ0.2 evaluated from the present method, shows good coincidence with that obtained by tensile tests.

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.

Structure and kinetics of cascades in copper under ion and/or electron irradiation

Abstract An in-situ observation system of radiation damage under ion and/or electron irradiation, which consists of a 1250 keV transmission electron microscope, an ion accelerator and an imaging system, is outlined. The microstructural evolution of cascades during dual beam irradiation with 30 keV Xe+ ions and 250 keV or 1000 keV electrons has been examined in copper at room temperature using the system. The density of cascades shows a linear increase followed by a square root dependence on irradiation time under irradiation with ions and 250 keV electrons. On the contrary, dual beam irradiation with 30 keV Xe+ ions and 1000 keV electrons gives a linear nucleation rate followed by a much slower nucleation rate eventually leading to saturation. The saturation value is lower at the higher 1000 keV electron flux. A kinetic equation for describing the microstructural evolution of cascades, which includes the effect of free interstitials produced by 1000 keV electrons, is proposed.

The inhomogeneity of stress and plastic deformation in neutron-irradiated copper single crystals

Abstract The plastic deformation in stage I of neutron-irradiated copper single crystals in a uniaxial tensile test is studied at 77 K and room temperature from the view point of stress-inhomogeneity due to the constraint of grips. Primary slip bands appearing in clusters are examined on the two surfaces where the slip step height of a slip band is the maximum by using optical microscopy. Main results are as follows: 1. (i) in the specimen deformed at 77 K, the morphology of primary slip bands and the width of individual clusters of them on one surface are not the same as those on the opposite surface except around the middle part of the specimen; 2. (ii) in the specimen deformed at room temeprature, the difference mentioned in (i) is not found; 3. (iii) in both specimens, the growth of the clusters in width in the upper half of the specimen is larger in the direction of the upper grip than in the direction of the lower grip. This situation is reversed in the lower half. It is concluded that the heterogeneous plastic deformation of neutron-irradiated copper single crystals in a uniaxial tensile test is related with the stress-inhomogeneity caused not only by the constraint of the grips but also by the clustering of primary slip bands.

Nucleation and growth process of defect clusters in copper during helium ion irradiation

Abstract The formation process of interstitial dislocation loops in copper under He + ion irradiation has been studied by using transmission electron microscopy. Wedge shaped specimens of pure copper are subjected to irradiation with 5 to 100 keV He + ions at irradiation temperatures between 300 to 570 K. The minimum thickness were interstitial loops are formed increases with increasing He + ion energy and irradiation temperature, although this thickness scarcely depends on He + ion dose rate. The density of loops shows a linear increase against dose for irradiation with 20 and 30 keV He + ions but has a higher order dependence with respect to doses greater than 2 under irradiation with 5 and 10 keV He + ions. On the basis of the results induced by He + ion irradiation, we have concluded that a low fraction of high energy PKAs play an important role for the nucleation of interstitial loops for energies higher than about 20 keV.

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 100 keV He-ion irradiation on early stage of plastic deformation of copper polycrystals at 77 K

The effects of 100 keV He-ion irradiation on the early stage of plastic deformation of copper polycrystals were studied by examining dose-dependence of the yield stress at 77 K and slip behavior at 90 K, and ultra-micro hardness measurements on thin surface damaged layers were also carried out at room temperature. The yield stress increased slightly at a dose of 2 × 1021ions/m2, but did not increased at 2 × 1019ions/m2. The hardness increased markedly at 2 × 1021ions/m2 and slightly at 2 × 1019ions/m2. The slip behavior was markedly different between unirradiated surfaces and irradiated ones at a higher dose, and slightly at a lower dose. These facts show that the very thin surface damaged layer affects the mechanical properties of fcc bulk materials. The possibility of the use of ultra-micro hardness testing in He irradiation effects is also briefly discussed.

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 .