Masafumi Akiyoshi

A new type of defect on {1120} planes in ß-Si3N4 produced by neutron irradiation

A high-resolution electron microscopy study has been carried out on a sintered β-Si 3 N 4 specimen which had been heavily neutron irradiated in an experimental fast reactor. A new type of planar defect lying on {1120} planes has been observed. The defect can be described as an interstitial dislocation loop with Burgers vector of approximately 1/18 (1/6 ), corresponding to an extra layer of SiN 4 tetrahedra inserted into the {1120} planes. Based on image simulations, a structural model for the observed defect is proposed.

Thermal Diffusivity of Ceramics During Neutron Irradiation

Several structural ceramic materials possess many superior properties for nuclear applications, such as blanket of future fusion reactor or core material of high-temperature gas cooling fission reactor for hydrogen generation, where they would be exposed to high fluence of neutrons at temperatures up to 1400K [1–3]. High fluence of neutron irradiation introduces various changes in physical properties of the materials, especially in thermal diffusivity. Thermal diffusivity is one of the most important factors for the plant efficiency. Thermal conductivity K(W/K·m) is obtained as K = αCpρ, where Cp (J/K·kg) is specific heat at constant pressure and ρ (kg/m3) is density. Change in density after an irradiation is measured by linear or volume swelling, but the amount is not so large ( < 2% ), and the specific heat keep almost same after an irradiation [4]. So a change in thermal diffusivity almost represents a change in thermal conductivity. In ceramic materials, unlike metals, heat is mainly carried by phonon. Phonon transportation is obstructed by two factors. Even in unirradiated ceramics, phonons are scattered among each other, and this phonon-phonon scattering increases with measurement temperature arise. In addition, phonon is scattered by lattice defects. This phonon-lattice scattering is decided by grain size or concentration of impurity in the case of unirradiated ceramics, but neutron irradiated material contains many lattice defects, especially vacancy scatters phonon severely. Hence, it has also been reported that neutron-irradiated specimens show severe degradation in the thermal diffusivity [5–12]. These post irradiation measurements were performed at room temperature to specimens after the irradiation. Nevertheless, it is very important to estimate the thermal diffusivity during the irradiation. But it is very difficult to measure thermal diffusivity during neutron irradiation directly. So it is required to estimate that from the post irradiation measurement, with consideration of both phonon-phonon scattering and phonon-lattice scattering. Thermal diffusivity α (m2/s) of neutron-irradiated ceramics depend onmeasured temperature T (K) as α = k/Tn, where k is the constant that is related to the absolute value, and n is the constant that represents the state of induced defects. The n constant is usually 1 for unirradiated ceramics, but it decreased with the irradiation, so it is not easy to 3

Irradiation effects on thermal diffusivity and positron annihilation lifetime in ceramics induced by neutron and 30 MeV electron

Measurements of thermal diffusivity and positron annihilation lifetime were performed on neutron- and 30 MeV electron-irradiated ceramics. Thermal diffusivity of heavily neutron-irradiated ceramics decreased to a very low level and the difference between specimens was small. Positron annihilation lifetime of these specimens also showed a distinct change; however, the difference between specimens was also small. Accordingly, in this study, electron irradiation was performed to 0.01–0.02 dpa via the 30 MeV KURRI-Linac. The electron-irradiated specimens also showed obvious degradation in thermal diffusivity and an increment in positron annihilation lifetime, and it was clarified that positron annihilation lifetime showed some correlation with thermal diffusivity.

Evaluation of radiation tolerance of silicon dioxide layer for field emitter arrays

Current-voltage characteristics of silicon dioxide layer prepared by chemical vapor deposition with TEOS, which is used as an insulting layer for field emitter arrays, were investigated with and without a MeV ion irradiation, in order to demonstrate high radiation tolerance. Sandwich structures with niobium electrodes and silicon dioxide layers were irradiated by a 2 MeV helium ion beam with the diameter of 1 mm to the ion dose of 0.05 mC. It was found that no significant deterioration of insulating properties of the samples.

In Situ Observation of Damage Evolution in Polycarbonate under Ion Irradiation with Positrons

We report β–γ coincidence positron annihilation lifetime spectroscopy of in situ observation of ion damage in polycarbonate under irradiation by MeV-energy H ions. Ion damage was investigated from changes in the relative intensity of the long-lived ortho-positronium pick-off annihilation lifetime component measured under irradiation and non-irradiation conditions. It was found that at fluences of less than 1015 ions·cm−2 the relative intensity of this component during irradiation was significantly reduced compared to that after irradiation. This reduction disappears at fluences higher than 1015 ions·cm−2. Results suggest that at fluences up to 1015 ions·cm−2, transient damage structures are formed under irradiation.