Shoji Kuma

Low thermal conductive Bi-2223 tapes sheathed with Ag-Au alloys

With the view of applying to power current leads for superconducting magnet systems and for other cryogenic power handling systems, low thermal conductive Bi-2223 superconducting tapes sheathed with Ag-Au alloy were fabricated and their thermal conductivities were measured from 12 to 260 K. The critical current density (overall-J/sub c/) was about 1700 A/cm/sup 2/ at 77 K, 0 T and remained nearly constant irrespective of Au concentration up to 11 at.%. The tape sheathed with Ag+11 at.%Au alloy, of which the superconductor cross-section ratio f/sub sc/ was 0.65, had a thermal conductivity value about 0.2 W/cmK at 77 K this value is as low as that of Cu-Zn. It was found that the thermal conductivity of the tape was close to the calculated one based on f/sub sc/ and the independently measured thermal conductivities of the Ag-Au alloy acid the Bi-2223 superconductor. The superconducting tapes sheathed with the alloy were confirmed to be suitable for the application as power current leads. >

CHARACTERISTICS OF AG-AU ALLOY SHEATHED BI-PB-SR-CA-CU-O SUPERCONDUCTING TAPES FOR CURRENT LEADS

Oxide superconductor tape with Ag‐Au sheath was studied for current leads. A 100‐cm‐long current lead of 1000 A with 0.2 W/kA heat leakage was designed using Ag‐11 at. % Au alloy sheath with low thermal conductivity. The overall current density of the superconducting tape is required to be 350–770 A/cm2 at 77 K when the superconductor cross‐section ratio in the tape is 0.35–0.7. Bi‐Pb‐Sr‐Ca‐Cu‐O superconducting tape with Ag‐11 at. % Au alloy sheath was fabricated and the critical current density Jc was measured. The overall‐Jc was 1700 A/cm2 at 77 K under 0 T. The possibility of the current lead of 1000 A under 0.28 T with 0.2 W/kA was made clear.

Observation of edge-facets in 〈100〉 InP crystals grown by LEC method

Abstract The growth of edge-facets on 〈100〉 InP crystals has been investigated using microscopy. The solid/liquid interface in the region of edge-facets was found to have a knife-edged shape, while that of a region without facets had a smooth shape. The irregular patterns of edge-facets point to instability in the growth at the periphery of a crystal, and this instability is found to be related to the generation of twins.

Synthesis of gallium nitride by ammonia injection into gallium melt

Abstract Gallium nitride (GaN) was synthesized by injecting ammonia gas into molten gallium at 900–980°C under atmospheric pressure. A large amount of GaN powder was reproducibly obtained using a simple apparatus. The synthesized powder was characterized by scanning electron microscopy, X-ray diffraction, photoluminescence and energy dispersive X-ray spectroscopy, and was found to consist of fine crystals of hexagonal GaN of good quality. The total of GaN obtained was far more than the amount calculated from expected saturation solubility in the Ga melt at that temperature. We speculate that the GaN crystals were largely formed by direct reaction between Ga and the gaseous N source at the surface of the NH 3 bubbles in the melt. GaN synthesized by this method may be useful as a starting material for bulk growth.

LEC growth of large GaAs single crystals

Abstract As the origin of polycrystallization in LEC grown GaAs crystals, accumulation of dislocations has been studied by microscopic observation of polycrystallized crystals. It has been found that dislocations have a tendency to propagate perpendicular to the solid⌜quid interface during growth. The shape of the interface near periphery of the growing crystal is usually concave towards the melt, this concave interface can lead to an accumulation of dislocations. This allocation causes polycrystallization. By control of the growing interface shape, polycrystalline growth has successfully been suppressed. Using this technology, GaAs single crystals of 75 mm diameter and 500 mm long, of 100 mm diameter and 320 mm long, and of 150 mm diameter and 170 mm long have been obtained with good stability.

Effects of thermal history during LEC growth on behavior of excess arsenic in semi-insulating GaAs

Abstract The effects of the thermal history during the LEC growth of a semi-insulating GaAs crystal are discussed from the viewpoint of changes of the behavior of the excess arsenic dissolved in the GaAs crystal. Near the melting point, solid-solution of excess arsenic is observed in the crystal. In the B2O3 layer, both the formation of EL2 and the generation of scattering centers occur, and the two phenomena are competitive. In the ambient gas, the homogeneous nucleation of arsenic occurs.

Investigation of changes of As precipitates in semi-insulating GaAs crystals at several temperatures by infrared light scattering tomography

Abstract Thermal behavior of arsenic precipitates in undoped semi-insulating GaAs crystals has been investigated. Crystals heat-treated at a temperature between 500 and 1150°C were observed by infrared light scattering tomography, which is powerful technique capable of detecting the arsenic precipitates. The arsenic particles on the dislocations became large increased in scattering intensity with temperatures from 800 to 1000°C. A dramatic change occurred above 1100°C: the images disappeared over the whole area. This disappearance means that the arsenic precipitates dissolves into the GaAs crystal. Using the samples in which the excess arsenic was solid-soluted, the same experiments were repeated. At 800–1000°C, the scattering images of the dislocations again appeared. Below 600°C, new images, consisting of misty zones of scattering, were observed in the dislocation-free areas. This suggests that the excess arsenic condenses like mist over the limit of saturation solubility.

Influence of Ag‐Au and Ag‐Cu alloys on Bi2Sr2CaCu2Ox superconductor

The influence of Ag‐Au and Ag‐Cu alloy substrates on the microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi‐2212) was studied. The critical current density (Jc) of Bi‐2212/(Ag‐Au) was not degraded by increasing the Au content of the Ag‐Au alloy, while the Jc of Bi‐2212/(Ag‐Cu) was drastically degraded by increasing the Cu content of the Ag‐Cu alloy. The microstructure of Bi‐2212 on the Ag‐Au alloy was uniform and no precipitates were observed. The Jc degradation of Bi‐2212 in contact with the Ag‐Cu alloy was understood by the fact that Cu absorption of Bi‐2212 from the Ag‐Cu alloys changed the composition of the oxide layer. Ag‐Au alloys are superior substrate materials for Bi‐2212 for power current leads for superconducting magnets, taking into account their low thermal conductivity.

Characteristics of strengthened Ag substrates for Bi2Sr2CaCu2Ox doctor‐bladed tapes

The characteristics of Ag‐(Mg,Ni) and Ag‐Mn alloys as substrate materials for Bi2Sr2CaCu2Ox (Bi‐2212) tapes and their influence on the superconducting properties were studied. Alloying with Mg+Ni or Mn drastically decreased the grain size of Ag after heat treatment. The additives in Ag‐(Mg,Ni) and Ag‐Mn alloys were oxidized during heat treatment in air. Both the fine grain structure and the internal oxidation of the additives increased the mechanical strength of the Ag alloys. Both alloys were 3–6 times stronger than pure Ag. The superconducting properties of Bi‐2212 prepared on these Ag alloys were slightly degraded; however the critical current densities at 4.2 K in 8 T were still well over 104 A/cm2.

Quasi-quantitative estimation of solidus curve on As-rich side of Ga-As system

Abstract A quantitative investigation of the solidus curve on the As-rich side of the Ga-As system was done by estimating the amount of excess As in crystals grown from As-rich melts. Excess As atoms in a GaAs crystal are usually subjected to two forms: precipitation and dissolution. To obtain the solidus curve, it was assumed that the EL2 concentration gives the composition of the crystal when almost all the dissolved excess As form EL2. The maximum EL2 concentration was produced in crystals by a new thermal treatment that dissolves As precipitates and is assumed to change almost all the dissolved As into EL2. The maximum EL2 concentration showed a strong linear correlation with the melt composition. A solidus curve has been drawn that has a parabolic shape similar to the liquids curve. The congruent point is shifted from stoichiometry toward the As-rich side by an amount equivalent to an EL2 concentration of 10 16 cm -3 .