Kouichi Hamada

Fission-fusion correlation by fission reactor irradiation with improved control

Abstract The necessity for the elimination of exposure to neutrons at lower temperatures during start-up and shut-down of the reactor is confirmed by experiments which compare the result of irradiation of metals with conventional and improved temperature control in JMTR. Only several percent of exposure to neutrons at lower temperatures is found to result in a one hundred percent difference of radiation induced microstructures in some cases. All differences can be understood from the microstructural development mechanisms, i.e. from the temperature dependence of the stability of point defect clusters and from the relationship of the transient temperature to the temperature for nucleation and growth. Fission neutron irradiation data with improved control are compared with fusion neutron irradiation data from RTNS-II. The differences of vacancy and interstitial clusters formed directly from cascades, observed in samples irradiated as thin foils, are understood when the difference in the primary recoil energy spectrum and the thermal stability of the clusters are taken into consideration. The defect structures which are developed and/or modified by the reactions of freely migrating point defects, such as vacancy clusters interstitial type dislocation structures and voids, observed in samples irradiated as bulk, are remarkably different in the two cases. Factors to be applied to the fission neutron irradiation dose to introduce microstructures equivalent to those by fusion neutrons are found to range widely, depending on the kind of microstructure, materials and irradiation temperature, from a value less than one up to 30 in the scaling of damage energy per atom.

Transmission electron microscopy of La0.7Ca0.3MnO3 thin films

La 0.7 Ca 0.3 MnO 3 (LCMO) films grown on (001) surfaces of various (pseudo-) cubic substrates, MgO, SrTiO 3 , LaSrGaO 4 and YAlO 3 , were studied by means of transmission electron microscopy. In all cases, the (001) plane of LCMO perovskite primitive cell is parallel to the substrate surface. The film on SrTiO 3 showed the single-variant structure with many twin boundaries while the films on other substrates had the three-variant structure where three [0 1 0] axes of the orthorhombic unit cell are perpendicular to each other. The LCMO cell on SrTiO 3 was expanded by about 1% in the film plane and shrank by about 1% out of the plane. Many planar defects and intergrown grains were observed in the film on MgO. They originated at the substrate surface.

Fission reactor irradiation of materials with improved control of neutron flux-temperature history

Abstract Eliminating the deficiency in the conventional temperature control, irradiation of materials with the Japan Material Testing Reactor is performed with an newly-designed in-core irradiation ring with which the sample temperature can be maintained regardless of the reactor power. The defect microstructures in various materials are compared with those introduced by irradiation with conventional control. The strong influence of the transient lower temperature irradiation during the start-up of the reactor is obvious for the samples irradiated with conventional control. The influence is fully understood from the temperature dependence of the microstructure evolution mechanism. The necessity of improved control is reconfirmed.

I-V measurement of NiO nanoregion during observation by transmission electron microscopy

Conduction measurements with simultaneous observations by transmission electron microscopy (TEM) were performed on a thin NiO film, which is a candidate material for resistance random access memories (ReRAMs). To conduct nanoscale experiments, a piezo-controlled TEM holder was used, where a fixed NiO sample and a movable Pt-Ir counter electrode were placed. After the counter electrode was moved to make contact with NiO, I-V measurements were carried out from any selected nanoregions. By applying a voltage of 2 V, the insulating NiO film was converted to a low resistance film. This phenomenon may be the “forming process” required to initialize ReRAMs. The corresponding TEM image indicated a structural change in the NiO layer generating a conductive bridge with a width of 30–40 nm. This finding supports the “breakdown” type forming in the so-called “filament model” of operation by ReRAMs. The inhomogeneity of resistance in the NiO film was also investigated.

Microstructure evolution by neutron irradiation during cyclic temperature variation

Abstract Utilizing a technique to control the temperature which is not influenced by the operation mode of a reactor, an irradiation during which the temperature was alternatively changed several times between two temperatures ( T -cycle) has been performed. Some defect structures are understood as combinations of the defect processes at lower and higher temperatures, and some others are understood if the defect processes during the transient between the two temperatures are taken into consideration. However, the most remarkable characteristic of defect processes associated with the temperature variation is the reaction of point defect clusters induced by lower-temperature irradiation at the higher temperature. During lower-temperature irradiation, there is a greater accumulation of vacancy clusters as stacking fault tetrahedra in fcc metals than that of interstitial clusters as dislocation loops. Vacancies evaporated from the vacancy clusters at higher temperature can eliminate interstitial clusters completely, and the repetition of these processes leads to unexpectedly slow defect structure development by T -cycle irradiation.

Detection of the role of free point defects from the variation of defect structures near permanent sinks in neutron irradiated metals

Abstract The role of freely migrating point defects during neutron irradiation in the defect structure development with the presence of permanent sinks is studied using rate equations. Three specific cases are examined as a function of the distance from the surface: (1) only interstitial cluster existing system, (2) interstitial cluster and vacancy cluster coexisting system, and (3) only vacancy cluster existing system. The obtained results are as follows. In case (1): easy formation of interstitial clusters in thin specimen, and near the surface and the grain boundary of thick specimen; no clusters in the deeper region of thick specimen. In case (2): easy formation of interstitial clusters in thin specimen, and near the surface and the grain boundary of thick specimen just the same as those in case (1); the growth of both types of clusters in the deeper region of thick specimen. In case (3): easy formation of vacancy clusters near the surface of the specimen. These results are compared with the experimental results.

I–V hysteresis of Pr0.7Ca0.3MnO3 during TEM observation

Conduction measurements with simultaneous transmission electron microscopy (TEM) were performed on thin Pr0.7Ca0.3MnO3 (PCMO) films deposited on tip-shaped Pt-Ir electrodes. A movable counter electrode was used to choose nanoscale region with contact areas smaller than 180 nm2 for investigation. The corresponding I-V data from this region showed a hysteresis curve which is characteristic to the material usable in resistance random access memories. In TEM images of PCMO before and after the measurement, no remarkable changes inside PCMO such as the formation of conducting path were recognized.

Role of solute atoms on microstructural evolution in neutron irradiated nickel

Abstract Fission reactor irradiation of nickel alloys with solutes of widely varying volume size factors 2 at% of Si, Cu, Ge and Sn, was performed for a wide range of irradiation temperature. At a lower temperature 473 K, a high density of small vacancy clusters are mixed with interstitial-type dislocation loops in all the alloys. At higher temperatures, in the alloys with solutes of negative volume size factor, Si, and extremely large positive factor, Sn, the unfaulting of interstitial-type dislocation loops during irradiation is suppressed, and the development of dislocation structures is moderate. In the alloys with solutes of positive but not large volume size factor, Cu and Ge, including pure Ni, unfaulting of loops strongly promotes the development of dislocation structures. As a consequence of the difference in these dislocation structure developments, void formation is almost completely suppressed in the first category and is enhanced strongly in the second.

In-situ transmission electron microscopy of conductive filaments in NiO resistance random access memory and its analysis

We used thermal oxidization at various temperatures to prepare NiO/Pr-Ir for use in resistance random access memory (ReRAM) samples. In-situ transmission electron microscopy (TEM) was used to investigate the forming process of these ReRAM samples, where a needle-shaped top electrode of Pt-Ir was attached to the NiO/Pt-Ir ReRAM layer. The forming voltage initializing the NiO layer increased at an oxidization temperature of between 200 and 400 °C. In this process, conductive bridges, which are thought to be conductive filaments of a ReRAM, appeared, and their sizes showed a correlation with the injection power. It was as small as about 300 nm2 when the injection power was 10−6 W. Energy dispersive X-ray spectroscopy was used to analyze the bridge, and it was experimentally confirmed that the oxygen content of the bridge was lower than that of the initial NiO layer. However, these bridges in the low resistance state did not show further ReRAM switching to the high resistance state inside of a TEM instrument....

Defect structure development in a pure iron and dilute iron alloys irradiated with neutrons and electrons

Abstract The defect structure and mechanical property changes were observed for pure iron of 99.99% purity and a series of Fe–(0.1% and 0.4%) Cr and Fe–(0.1% and 0.4%) Mn dilute alloys irradiated with neutrons. From the comparison of the defect structures with yield strength change, a large contribution of the invisible defect clusters to the irradiation hardening was expected in the specimens irradiated in Japan Materials Test Reactor (JMTR), whereas these clusters are not found after irradiation in the Fast Flux Test Facility (FFTF). The electron irradiation experiments showed that addition of chromium and manganese to 0.1% in pure iron, development of large dislocation loops is suppressed, and frequent nucleation of small loops at the early stage of the electron irradiation is observed, similar to that in ultra-high purity iron of 99.9999% purity. The mechanisms of dislocation loop development in the early stage of irradiation for Fe–Cr and Fe–Mn are considered to be different.