Takahiko Kato

Grain boundary segregation under electron irradiation in austenitic stainless steels modified with oversized elements

Abstract Effects of modification with oversized elements (Ti, Zr, Hf, V, Nb or Ta) on radiation-induced grain boundary segregation (RIS) in austenitic stainless steels were studied by means of electron irradiation in an HVEM and analytical electron microscopy. Solution-annealed 316L steels modified with the oversized elements were electron irradiated up to a dose of 10 dpa at temperatures of 673 to 773 K. Segregation behaviors of Cr and Ni near the grain boundary were strongly affected by the additional elements. Addition of Hf or Zr was particularly effective in preventing RIS. It was deduced from damaged microstructures that when the amount of irradiation-produced point defects decreased as a result of recombination and/or development of secondary defects in the matrix, segregation at the grain boundary was retarded. The change in net concentration of point defects in the matrix was associated with the trapping effect of point defects by the added elements, which depended on their size factor.

Post-irradiation annealing effects on microstructure and helium bubbles in neutron irradiated type 304 stainless steel

Abstract Helium bubble nucleation and growth processes were studied at elevated temperatures on type 304 stainless steel which had been neutron irradiated to 1.4 × 1026 n / m 2 ( E > 1 MeV) and annealed at 400, 550, 650 and 900°C for 1 h. After annealing, specimens for microstructural observation were prepared and observed (by transmission electron microscope (TEM).) Radiation defects were present at high density in the as-irradiated specimens and annealed out with increasing annealing temperature. Helium bubbles became visible where defects were annealed out (above 650°C). The bubbles grew preferentially at dislocations and grain boundaries. At the grain boundaries, most of the helium bubbles formed on grain boundary dislocations. It was concluded that during post-irradiation annealing, the dislocations which remained after annealing played an important role in helium bubble growth, even in the grain boundaries.

Tuning Optoelectrical Properties of ZnO Nanorods with Excitonic Defects via Submerged Illumination

When applied in optoelectronic devices, a ZnO semiconductor dominantly absorbs or emits ultraviolet light because of its direct electron transition through a wide energy bandgap. On the contrary, crystal defects and nanostructure morphology are the chief key factors for indirect, interband transitions of ZnO optoelectronic devices in the visible light range. By ultraviolet illumination in ultrapure water, we demonstrate here a conceptually unique approach to tune the shape of ZnO nanorods from tapered to capped-end via apical surface morphology control. We show that oxygen vacancy point defects activated by excitonic effects near the tip-edge of a nanorod serve as an optoelectrical hotspot for the light-driven formation and tunability of the optoelectrical properties. A double increase of electron energy absorption on near band edge energy of ZnO was observed near the tip-edge of the tapered nanorod. The optoelectrical hotspot explanation rivals that of conventional electrostatics, impurity control, and alkaline pH control-associated mechanisms. Thus, it highlights a new perspective to understanding light-driven nanorod formation in pure neutral water.

Subsystem optical interconnections using long-wavelength LD and singlemode-fiber arrays

Subsystem optical interconnections for lengths of over 100 m are proposed. Single-mode fiber (SMF) arrays and very-low-threshold-current long-wavelength laser diode (LD) arrays realize small-skew and error-free interconnections. The LD turn-on delay and receiver risetime and falltime (t/sub r//t/sub f/) skews are considered in addition to the electronic and fiber skews, and a total skew of less than 1.65 ns is obtained. It is shown that an SMF array with a skew of 2.1 ps/m is needed to obtain a small skew. The LD array can be coupled with an SMF array to give high signal-to-noise ratio and error-free transmission. Driving each LD at a modulation current of six times the threshold current results in an LD turn-on delay skew of less than 500 ps. Wideband preamplifiers with low feedback resistivity reduce receiver t/sub r//t/sub f/ skew. The total skew of 1.65 ns gives a transmission speed of less than 150 MB/s (150 Mb/s/ch*8 ch) through a 100-m fiber array.<<ETX>>

Fabrication of a Au/Si nanocomposite structure by nanosecond pulsed laser irradiation

A gold/silicon nanocomposite structure (NCS) was formed on a Si(100) surface by nanosecond pulsed laser irradiation. The Au/Si NCS contained both Au nanoparticles (NPs) and Au-Si alloy layers. We report that the use of laser irradiation to form Au NPs comprises two competing processes: a top-down effect involving decomposition into smaller NPs and a bottom-up effect involving self-assembly or self-organization into larger NPs. The formation of the periodic structure involved self-organization, i.e., the bottom-up effect, and was observed in situ using a pulsed-laser-equipped high-voltage electron microscope. The NCS formed by laser irradiation can be controlled by adjusting the laser energy density and the number of laser pulses.

Effects of Minor Elements in Cu Leadframe on Whisker Initiation From Electrodeposited Sn/Cu Coating

Drastically different tendencies of whisker initiation have been observed from the same Sn/Cu coating electrodeposited on two different Cu leadframe materials, CUFE and CUCR. After long-term storage at room temperature, no whisker initiation was observed on the coating on CUCR, whereas long whiskers with a maximum length of more than 200 mum were formed on the coating on CUFE. FE-STEM and FE-TEM microstructural observations of vertical sections of each Cu leadframe with the coating showed that the cross-sectional morphologies of Cu-Sn intermetallic compounds (IMCs) formed at the interface between the coating and the leadframe were as follows: a wedge-shaped structure for the Sn/Cu- CUFE sample and a comb-tooth structure for the Sn/Cu-CUCR sample. Energy dispersive X-ray (EDX) analysis results confirmed that the formation morphologies of the Cu-Sn IMC were affected by minor elements in the Cu leadframes. Fe particles with a large diameter of mainly 50-200 nm precipitated nonuniformly in the CUFE leadframe, which had Fe, Zn, and P as minor elements. Conversely, very small Cr-rich particles, 10-20 nm in diameter, were precipitated in the CUCR leadframe, which had Cr, Sn, and Zn as minor elements. The Fe particles suppressed the growth of the Cu-Sn IMC by acting as obstacles to IMC formation, whereas grain boundaries in the Sn/Cu coating acted as preferential IMC formation sites. The combination of these two effects is thought to produce the wedge-shaped cross-sectional structure of the IMC. On the other hand, the Cr-rich particles did not have such a prominent suppression effect on Cu-Sn IMC growth: only the grain boundaries in the Sn/Cu coating affected the IMC formation site. As a result, IMC with a comb-tooth structure was formed in the Sn/Cu- CUCR sample. The difference in whisker initiation tendency was inferred to be due to the difference in compressive stress on the basis of the Cu-Sn IMC formation morphology in the Sn/Cu coating. Therefore, the stresses were finally measured by X-ray diffraction to determine the correlation between whisker initiation tendency and the morphology of Cu-Sn IMC formation. Based on the results, a model for controlling whisker initiation in Sn/Cu-coated Cu leadframe systems is presented.

Correlation Between Whisker Initiation and Compressive Stress in Electrodeposited Tin–Copper Coating on Copper Leadframes

To evaluate the contribution of coating stress to whisker initiation from IC package leads, the stress distribution in the coating was investigated by finite-element analysis (FEA). Two different leadframe samples, which were composed of the same tin-copper coating on two different copper-leadframe materials, namely, copper-iron (hereafter, CUFE; corresponding to CDA number C19400) and copper-chromium (CUCR; CDA number C18045), were used to examine the whisker-initiation behavior on the coating surfaces. The two samples showed significantly different tendencies of whisker initiation from the coating. That is, after long-term storage at room temperature, no whisker initiation was observed on the coating on the CUCR sample, whereas long whiskers (with a maximum length of more than 200 μm) were formed from the coating on the CUFE sample. The FEA calculation on the leadframe samples revealed that the coatings had a two-directional stress gradient, namely, one gradient toward the surface and another toward the base leadframe material. It also indicated a difference between the stress distributions in the two samples. The gradient of normal stress on the coatings grain boundaries (GBs), toward the surface of the CUFE sample, was found to be larger than that in the CUCR sample. This result implies that the tin-atom flux along a GB in the coating on the CUFE sample was larger than that on the CUCR sample because the atom flux along the GB was proportional to the stress gradient. It agrees with the above-mentioned whisker-initiation behaviors in the samples. We thus conclude that in the CUFE sample, a whisker initiates either from a surface grain immediately on top of a GB or from surface grains located on both sides of the same GB. To confirm this conclusion, the correlation between the tin-diffusion sites and whisker formation sites was investigated. Simulation of atom diffusion by molecular dynamics indicated that the dominant tin-diffusion site is a GB when compressive stress is applied in the direction normal to the GB. Investigation of the correlation between the whisker roots and coating microstructures of the CUFE sample showed that the whisker roots were located on top of GB intersections in the coating. These results indicate that whisker-initiation sites are correlated with dominant tin-diffusion sites and that each whisker initiates either from a surface grain located immediately on top of a GB or from surface grains located on both sides of the same GB.

The effect of solute content on grain boundary segregation in electron-irradiated Fe-Cr-Mn alloys

Solute distribution and microstructural development in the vicinity of grain boundaries in ferritic Fe-10Cr-xMn-3Al (x = 5, 10 or 15) alloys were studied during electron irradiation to 10 dpa at 723 K. In addition, X-ray diffraction analysis was performed for determination of the volume size factor of solutes. The oversized solute atoms, manganese and aluminum, were depleted at grain boundaries, whereas the concentration of the oversized chromium rose sharply at the boundary. The amount of segregation of the solutes decreased with increasing atomic volume depending on manganese content. Segregation of aluminum, which had the greater volume size factor relative to that of manganese, was higher than that of manganese. The amount of radiation-induced segregation of manganese and aluminum at the grain boundary is consistent with arguments based on the atomic size effect, but the enrichment of chromium at the grain boundary is not and seems to related primarily to the formation of chromium-rich precipitates at the boundary.

Phase Transition in YBa2Cu3O7-x through Hydrogen Ion Implantation

Phase transition in YBa2Cu3O7-x through hydrogen ion implantation was investigated by means of X-ray diffraction. It was found that the orthorhombic-to-tetragonal phase transition was induced by the implantations in the ion fluences of 7.6×1016 H+/cm2 and above. At the 2.3×1017 H+/cm2 implantation, the extra phases Y2BaCuO5 and CuO were also formed. These phenomena suggested that the implanted hydrogen atoms deoxidized the YBa2Cu3O7-x to produce H2O which reacted with YBa2Cu3O7-x.

Detection of intergranular cracking susceptibility due to hydrogen in irradiated austenitic stainless steel with a superconducting quantum interference device (SQUID) sensor

To investigate the detection method of intergranular (IG) cracking susceptibility by hydrogen in irradiated austenitic stainless steel (SS), magnetic and mechanical properties were examined after two repeats of hydrogen charging and discharging (hydrogen treatment) in Type 304 SS which had been irradiated during use in different reactor cores. The residual magnetic flux density (Br) was measured with a superconducting quantum interference device sensor and Br increased with increased neutron fluence and repeated hydrogen treatments. Elongation decreased with an increase of Br and IG cracking appeared above Br of 2×10−5 T for this measuring method after repeated hydrogen treatments. These phenomena would be caused by hydrogen-induced martensite phase being formed on grain boundaries. It was thought the appearance of IG cracking susceptibility due to hydrogen in irradiated SS could be predicted by measuring the Br of the steel.