Takamasa Sugawara

Crystal Growth and Characterization in the System (Ga1-xInx)2O3

We have grown (Ga1-xInx)2O3 mixed crystals by the floating zone technique. The unit cell size of the mixed crystal increases with increasing In content according to Vegards law. For 50 mol% In2O3 we obtained single crystals of a structure different from the regular monoclinic one. Slightly doped (Ga1-xInx)2O3 crystals show n-type semiconduction. With increasing In content, the crystals became p-type semiconductors.

Helium and hydrogen trapping in W and Mo single-crystals irradiated by He ions

Abstract Retention of He and accumulation of H in the near surface layer of W and Mo single-crystals were studied during and after the implantation of He ions with 2–10 keV at 295 and 820 K. The He retention was saturated at a concentration of a He/metal ratio of about 0.25, depending on the implantation temperature. Subsequent He implantation caused H accumulation in the He saturated layer, up to a maximum concentration about equal to that of He. The initial H uptake rate just after the He irradiation was comparable to the impingement rate of the H2 or H2O molecule at the crystal surface from the residual gas. For the He irradiation at 820 K, blisters and exfoliation with large sizes were observed on the crystal surface, where impurities other than H and He were also enriched.

Solid solution range of boron, microhardness and oxidation resistance of the perovskite type RERh3Bx (RE=Gd, Y, Sc) compounds

Abstract Polycrystalline samples of RERh 3 B x (RE=Gd, Y, Sc), which consists of a reactive RE and high melting point Rh and B elements, are successfully synthesized by the arc melting method. RERh 3 B x compounds have the perovskite type cubic structure (space group Pm 3 m ). Perovskite type phases of GdRh 3 B x and YRh 3 B x exist in the range of ∼0.444≤ x ≤1.000. On the other hand, phase of ScRh 3 B x exists in the range of 0≤ x ≤1.000 (0 to 20 mol% B). ScRh 3 , in the case of x =0, has AuCu 3 type structure; its space group is also Pm 3 m . The lattice parameter a in RERh 3 B x depends on x and the value of a increases with increasing boron content x . The value of Vickers microhardness of the RERh 3 B x at room temperature ranges from 6 to 10 GPa. According to thermo-gravimetric and differential thermal analysis for RERh 3 B x (RE=Gd, Y) heating up to 1200°C in air, a mixed phase of REBO 3 and Rh is identified as an oxidized product. In the case of ScRh 3 B, the oxidized product consists of Rh, Sc 2 O 3 and ScBO 3 .

Giant magnetoresistance and microstructure in Cr-Fe and Cu-Co heterogeneous alloys

Abstract We have investigated the relationship between the giant magnetoresistance (GMR) and the microstructure in sputter-deposited CuCo heterogeneous alloy films. MR varies depending on Co concentration, x, and it shows a maximum around x = 20 at.%Co. The transmission electron microscopy and nuclear magnetic resonance measurements reveal that the size of Co particles and the compositional profile near the interfaces of Co particles do not depend on x. The magnetization measurements suggest that the particle density increases with increasing x. When the particle density is too high, the ferromagnetic coupling between Co particles is dominant, resulting in a reduction in MR. The particle density for x ~ 20 at.%Co is considered to be optimum for GMR.

Study of giant magnetoresistance behavior in sputter-deposited CrFe alloy films

Abstract We report the giant magnetoresistance (MR) behavior in CrFe alloy films and its variation with composition and substrate temperature in conjunction with microstructure observations. The MR observed as a function of Fe concentration shows a maximum around 20 at% Fe at any substrate temperatures. A large difference in the magnitude of MR is observed between the specimens deposited on cooled and heated substrates when the Fe concentrations are lower than 25 at%, while MR values do not depend on the substrate temperature for specimens containing Fe larger than 25 at%. The temperature dependence of the magnetization shows that the magnetization with a zero field cooling exhibits a maximum at a certain temperature ( T p ). T p increases gradually as a function of Fe concentration of the specimens, but it starts to increase drastically when the concentration of Fe exceeds 20 at%, where MR shows the maximum value.

Realization of Bulk Multicrystalline Silicon with Controlled Grain Boundaries by Utilizing Spontaneous Modification of Grain Boundary Configuration

We succeeded in the realization of bulk multicrystalline silicon (mc-Si) with electrically inactive grain boundaries. A group of single-crystal Si wafers was used as an artificial multicrystalline seed with random grain boundaries. The crystal growth was carried out by the floating zone technique under ultrahigh vacuum at a growth rate of 1.0 mm/min. Under these conditions, most of the grain boundaries were spontaneously modified to Σ3. In contrast, random grain boundaries remained even after 40-mm growth when the growth rate was decreased to 0.2 mm/min. From these results, we suggest that the control of both the initial grain boundary configuration and the growth conditions is important to realize mc-Si with electrically inactive grain boundaries, which is a promising material for solar cell applications.

Ion beam analysis of helium and its irradiation effect on hydrogen trapping in W single crystals

Abstract Retention of He implanted into W single crystals and the He irradiation effects on H behavior were studied by ion beam analysis techniques. During implantation of 4 He+ with 2–10 keV at 295 K, an accumulation of H started in the He implanted layer when the retained He concentration saturated. For the crystal irradiated by 10 keV He+ at 820 K, a remarkable increase of H was found in the He saturated layer, after stopping the implantation and cooling down the crystal below 400 K. Though blisters and exfoliation were observed for the surface irradiated at 820 K, less lattice disorder was found in the implanted layer and the thermal release of H occurred at lower temperature, in comparison with the crystal implanted at 295 K.

Boron-carbon atomic ratio dependence on the hardness and oxidation resistance of solid solutions of perovskite-type borocarbide YRh3BxC1-x (0 ≤ x ≤ 1)

Solid solutions of the quaternary borocarbide system YRh3BxC1-x have been synthesized and studied from the viewpoints of their microhardness and oxidation resistance. Perovskite-type YRh3B and YRh3C form a continuous solid solution in the range of 0?x?1 with a cubic structure (space group: Pm3m, Z=1). The microhardness of YRh3BxC1-x increases with increasing boron content. The oxidation onset temperature also increases with boron content. Thus, it is indicated that the chemical stability of solid-solution YRh3BxC1-x is strongly dependent on the boron content.

Displaced-T Site Occupancy of Hydrogen in Nb Alloyed with a High Concentration of Mo

The site occupancy of hydrogen in 52 at. % Nb–48 at. % Mo alloys is investigated at room temperature for hydrogen concentrations 0.028 and 0.055 in the hydrogen–metal-atom ratio ( C H =[H]/[M]) by the channelling method utilizing a nuclear reaction 1 H( 11 B,α)αα with a 11 B beam. A large portion of H atoms are located at sites displaced from tetrahedral ( T ) sites towards their nearest neighbour octahedral ( O ) sites by about 0.3 A (displaced- T sites; d - T ), which are similar to the sites observed in V under uniaxial stress and completely different from the sites observed in the Nb alloys containing undersized Mo atoms up to 26 at. %. For C H =0.028 and 0.055, 30–40% and 15–25% of H atoms are located at T sites, respectively.

Site Change of Hydrogen in Niobium on Alloying with Oversized Ta Atoms

In order to clarify a difference in hydrogen interaction with oversized solute atoms and with undersized solute atoms in bcc metals in the low solute concentration region, the site occupancy of hydrogen in Nb alloyed with 5 at. % of oversized Ta atoms has been studied at room temperature for hydrogen concentrations of 0.018 and 0.025 at the hydrogen-to-metal-atom ratio ( C H =[H]/[M]) by the channelling method utilizing a nuclear reaction 1 H( 11 B,α)αα with a 11 B beam of an energy of 2.03 MeV. Clearly different from the result on hydrogen in Nb alloyed with undersized Mo atoms, in both specimens H atoms are distributed over tetrahedral ( T ) sites and the displaced- T sites ( d - T sites) which are displaced from T sites by about 0.25 A towards their nearest neighbour octahedral ( O ) sites. The T site is more favourable for hydrogen occupancy, but the number of available T sites is limited, and excess H atoms occupy the d - T sites. Therefore, in contrast to a strong attractive interaction between hydr...