Takaaki Koyanagi

Formation of Nickel Nanoparticles by Electroless Deposition Using NiO and Ni ( OH ) 2 Suspensions

Nickel particles ∼300 nm in diameter were fabricated by electroless deposition using hydrazine as a reducing agent in nickel hydroxide/ethylene glycol suspension at 353 K without any dispersing agent. The formation mechanism of nickel nanoparticles is discussed from the viewpoint of thermodynamics with in situ monitoring of nickel deposition and mixed potential. Specifically, in situ monitoring of mixed potential in combination with thermodynamic calculation is useful in discriminating whether or not nickel will be deposited in a reaction. The mixed potential drastically changed at the end point of the nickel deposition reaction, indicating that the cathodic reaction, which determined the mixed potential, switched from the nickel deposition reaction to hydrogen generation reaction.

Irradiation-induced β to α SiC transformation at low temperature

We observed that β-SiC, neutron irradiated to 9 dpa (displacements per atom) at ≈1440 °C, began transforming to α-SiC, with radiation-induced Frank dislocation loops serving as the apparent nucleation sites. 1440 °C is a far lower temperature than usual β → α phase transformations in SiC. SiC is considered for applications in advanced nuclear systems, as well as for electronic or spintronic applications requiring ion irradiation processing. β-SiC, preferred for nuclear applications, is metastable and undergoes a phase transformation at high temperatures (typically 2000 °C and above). Nuclear reactor concepts are not expected to reach the very high temperatures for thermal transformation. However, our results indicate incipient β → α phase transformation, in the form of small (~5–10 nm) pockets of α-SiC forming in the β matrix. In service transformation could degrade structural stability and fuel integrity for SiC-based materials operated in this regime. However, engineering this transformation deliberately using ion irradiation could enable new electronic applications.

Equilibrium shapes and surface selection of nanostructures in 6H-SiC

The equilibrium shape of 6H-SiC nanostructures and their surfaces were studied by analyzing nano-void (∼10 nm) shapes, which were introduced in monocrystalline 6H-SiC by high-temperature neutron irradiation, using transmission electron microscopy. The nano-voids were determined to be irregular icosahedrons truncated with six { 1 ¯ 100}, twelve { 1 ¯ 103}, one smaller top-basal, and one larger bottom-basal planes, which suggests that { 1 ¯ 100} and { 1 ¯ 103} are the next stable surface class after the basal planes. The relatively frequent absence of the { 1 ¯ 100} surface in the nano-voids indicated that the ( 1 ¯ 10 3 ¯ ) surface type is energetically rather stable. These non-basal surfaces were found not to be atomically flat due to the creation of nanofacets with half unit-cell height in the c-axis. The { 1 ¯ 100} and { 1 ¯ 103} surfaces were classified as two and four face types according to their possible nanofacets and surface termination, respectively. We also discuss the surface energy difference ...

Internal residual stress analysis of SiC/SiC composites following ion irradiation

A residual stress is one of the key parameters for determining the mechanical properties of SiC/SiC composites. The differential swelling between composite constituents is anticipated under irradiation, which likely modify the unirradiated residual stress. To clarify the magnitude of residual stress modification by irradiation and the effects on mechanical properties of SiC/SiC composites, the swelling in each component were evaluated. The volume expansion of the matrix and fibers in NITE SiC/SiC were measured at 1.9 and 1.4~1.7 %, respectively, following 5.1 MeV Si2+ ion irradiation at 280 °C, 3dpa-SiC. The calculation showed the decrease in the axial residual stress in matrix region, where the increasing in the proportional limit stress was implicated.

Atomic-Resolution Investigation of Irradiation-Induced Defects in Silicon Carbide

Silicon carbide (SiC) possesses excellent radiation tolerance, thus, SiC and its composites are promising materials for current and future nuclear systems [1]. However, the atomistic processes that underlie the irradiation response are not sufficiently understood, due largely to the limited spatial resolution of conventional analytical tools. Comparing aberration-corrected scanning transmission electron microscopy (AC-STEM) images with STEM image simulations of candidate defect structures is a method for analyzing individual defects in unprecedented detail. Conventional TEM methods can identify defects, such as dislocation loops and black dot damage (Figure 1; 10 dpa neutrons at 800°C), but cannot provide insight regarding the exact atomistic details of the defects present.

Development and Testing of Ion-Irradiation under the Applied Stress for Nuclear Ceramics

Simultaneous effects of the ion-irradiation and applied stress on the dimensional stability of SiC was studied up to 1 dpa at the irradiation temperature of 800°C. Tensioned surface of the curved SiC strips were irradiated using fixtures developed, and the irradiation creep strain of the irradiated samples was estimated from the irradiated curvatures. The irradiation creep rates were nearly proportional to the swelling rates. Linear relationship between the irradiation creep strain and stress was found below 1 dpa for the stress levels ranging 150-300 MPa.

M3FT-16OR020202112 - Report on viability of hydrothermal corrosion resistant SiC/SiC Joint development

Hydrothermal corrosion of four types of the silicon carbide (SiC) to SiC plate joints were investigated under PWR and BWR relevant chemical conditions without irradiation. The joints were formed by metal diffusion bonding using molybdenum or titanium interlayer, reaction sintering using Ti-Si-C system, and SiC nanopowder sintering. Most of the formed joints withstood the corrosion tests for five weeks. The recession of the SiC substrates was limited. Based on the recession rate of the bonding layers, it was concluded that all the joints except for the molybdenum diffusion bond are promising under the reducing activity environments. The SiC nanopowder sintered joint was the most corrosion tolerant under the oxidizing activity environment among the four joints.