Noboru Tsukuda

On the characterization of graphite

We have tried to make a new characterization of graphite from an aspect of lattice ordering in directions both parallel and perpendicular to the basal plane by means of laser Raman spectroscopy, X-ray diffraction, positron annihilation and transmission electron microscopy (TEM). The Raman technique is found to give us information on the defects both at intra- and inter-basal planes, and disordering of the basal planes. X-ray diffraction easily sees the disordering, but not the defects in the basal plane. And positron annihilation is very sensitive to the point defect in the basal plane which is not so easy to be detected even in high resolution TEM. The results obtained with the present techniques are compared to material properties like electrical conductivity and outgassing to extract useful correlations between them.

Positron annihilation lifetime measurement of irradiated stainless steels

Abstract Two types of austenitic stainless steels (316SS and JPCA) and two types of ferritic/martensitic stainless steels (HT-9 and JFMS) were irradiated by high energy electrons at 77 K and positron annihilation lifetime measurements were carried out to obtain the isochronal annealing behaviour above room temperature. The main decrease of the intensity of the second component, namely, the migration of vacancies to sinks was observed at 250°C for austenitic stainless steels and at 150°C for ferritic/martensitic stainless steels. By assuming the number of jumps to sinks as 103, the vacancy migration energy was obtained as 1.36 and 1.10 eV for austenitic and ferritic/martensitic stainless steels, respectively. This result was used to discuss the low swelling behaviour of the ferritic/martensitic stainless steels.

Simulation irradiation studies on iron

Abstract 4 MeV Ni ion irradiation was performed for pure Fe up to 50 dpa between 350°C and 500°C with and without simultaneous He ion injection to simulate and understand the fundamental irradiation behaviour of ferritic stainless steels in the fusion environment. Results were compared with those from electron (HVEM), neutron and proton irradiated pure Fe specimens. Voids were observed in Ni ion irradiated Fe at 450°C and 500°C for both single and dual beam irradiation, but no voids were observed at 350°C and 400°C. A number of small vacancy type dislocation loops were observed at 350°C and 400°C especially along dislocation lines. It is considered that these small loops are closely associated with cascade formation and contribute to suppression of void formation at low temperatures.

Stacking Disorder in KCuF_3

By X-ray diffraction method a stacking disorder in pseudo-perovskite KCuF 3 is investigated. The stacking disorder seems to be connected with the existence of two kinds of polytype structures. Five specimens are examined; the diffraction patterns of them are typical to the five structural groups of KCuF 3 crystals. In the cases of three specimens, diffuse scattering along the c * axis is observed. By a preliminary study including a leastsquares refinement of the atomic coordinate in the crystals, a model of the disorder is established; the disorder is characterized by the displacement (1/2) a in a stacking of the layer parallel to the (001). The diffuse scattering due to the disorder is analyzed by using the intensity equation in a matrix form. In the cases of Reichweite S =1, 2 and 3, the existence and continuing probabilities of the layers are directly determined from Fourier transforms of the observed intensities.

Damage correlation of high energy ion and D-T neutron irradiations in copper

Abstract Microstructural changes in copper are observed by TEM after irradiation with 100 MeV Ni ions, 30 MeV C ions, and 14 MeV neutrons to various fluences at room temperature. The majority of the observed defects are SFT (stacking fault tetrahedra). The correlation of these data is carried out by taking into account the primary recoil energy spectra and cascade structures. By a “Cut-off Calculation” with the MARLOWE code, the number of subcascades is estimated for primary recoils with energies of up to 1 MeV. The dpa is found to be a poor correlation parameter for the present results from three types of irradiations. Instead, the number of defect clusters observed for these cases has a close relation to the calculated number of 20 keV subcascades. Thus, “the number of subcascades” can be a reliable correlation parameter applicable to various kinds of irradiations with various collision cascade effects.

Positron-lifetime study of electrically hydrogen charged Ni, austenitic stainless steel and Fe

Abstract Positron-lifetime study has been made for the electrically hydrogen charged Ni, austenitic stainless steel (316 stainless steel), and Fe in order to investigate the structural changes due to the presence of high concentration of hydrogen atoms in the bulk, such as hydrogen-induced defects, hydrogen-induced phase transformation and so on. Increase of mean lifetime was observed for Ni and 316 stainless steel, but almost no change was observed for Fe. The introduction of a long lifetime of about 150 ps to the Ni specimen was interpreted as the generation of vacancies by the hydrogen charging. But in the case of Fe, no vacancy generation was observed probably due to the low concentration of hydrogen atoms, even by the electrical hydrogen charging. In 316 stainless steel, both the phase transformation and the generation of vacancies were observed by the presence of high concentration of hydrogen atoms in the bulk. Elastic recoil detection (ERD) method showed that the hydrogen concentration reaches about 40% (H/atom=0.4) near the surface region of 316 stainless steel, electrically hydrogen charged.

In situ measurement of lattice parameter change on Pd and Ni during cathodic hydrogen charging

Abstract A device for in situ X-ray diffraction measurements during cathodic hydrogen charging has been developed. The lattice change on Pd before and after hydrogen charging has been clearly demonstrated. The results of Δa/a for Pd and Ni during increasing hydrogen concentration have been shown. The device is effective for understanding metal–hydrogen systems under various constrained hydrogen environments.

The effect of electrical hydrogen charging on the strength of 316 stainless steel

The effect of electrical hydrogen charging on the strength of 316 stainless steel specimens has been investigated in tensile tests at 223 K, and the increase of yield stress and the decrease of total elongation were observed. These tendencies increase with increasing hydrogen content of the specimens. This is considered to be due to hydogen-induced phase transformation from γ (fcc) to e (bcc), α (bcc), which was confirmed by X-ray diffraction method. Hydrogen concentration was determined by elastic recoil detection (ERD) method, the maximum of which reached 40% near the surface region. Positron annihilation lifetime was also measured after electrical hydrogen charging and a longer lifetime of about 300 ps was observed, which suggests the formation of microvoids in the specimens.

Positron lifetime calculation for defects and defect clusters in graphite

Calculations of positron lifetime have been made for vacancy type defects in graphite and compared with experimental results. Defect structures were obtained in a model graphite lattice after including relaxation of whole lattice as determined by the molecular dynamics method, where the interatomic potential given by Pablo Andribet, Dominguez-Vazguez, Mari Carmen Perez-Martin, Alonso, Jimenez-Rodriguez [Nucl. Instrum. and Meth. 115 (1996) 501] was used. For the defect structures obtained via lattice relaxation positron lifetime was calculated under the so-called atomic superposition method. Positron lifetimes 204 and 222 ps were obtained for the graphite matrix and a single vacancy, respectively, which can be compared with the experimental results 208 and 233 ps. For planar vacancy clusters, e.g., vacancy loops, lifetime calculation was also made and indicated that lifetime increases with the number of vacancies in a cluster. This is consistent with the experimental result in the region of higher annealing temperature (above 1200°C), where the increase of positron lifetime is seen, probably corresponding to the clustering of mobile vacancies.

Polarizing and Electron Microscope Studies of Structural Phase Transitions in CsVF4

The successive structural phase transitions of CsVF 4 have been studied by means of polarizing and electron microscopes. A new phase in the temperature range 240°C to 250°C has been found. It became clear that the transitions at 152°C, 240°C and 250°C have the character of the first ordering on account of the co-existence of two phases near the transitional point. Then, a notation of each phases is as follows: Phase I (≥250°C), II (240 ≤ ≤250°C), III (152 ≤ ≤240°C) and IV (≤152°C). From the results of electron diffraction, the satellite and the diffuse reflection, observed by electron diffraction, are interpreted in terms of an oscillation of the VF 6 octahedra about the principal axes of the idealized structure.