Tomoya Aoba

Generation of Sr2Ca(n-1)CunOy phases (n = 5-7) by high pressure synthesis

We prepared Sr2Ca(n-1)CunOy (n = 3-7) phases using a high-pressure apparatus. In the Sr-Ca-Cu-O system, the n = 5-7 members of a homologous series, Sr2Ca(n-1)CunOy, were prepared as mixed phases for the first time. The c-axis length increases linearly with increasing n; c = 6.46(n-1) + 14.3 A. Half of this slope gives reasonable value of 3.2 A as the thickness of a Ca and CuO2 plane. It is likely that a 0(Sr)2(n-1)n phase is formed from a 0(Sr)2(n-2)(n-1) phase to intercalate CaCuO2 into its infinite layer because the number of CuO2 planes increases with increasing sintering temperature and synthetic time. Too high temperature and too long synthesis time result in the formation of the infinite layer phase.

Synthesis of CxSr2Ca2Cu3Oz Superconductor from Sr2Ca2Cu3Oy Precursor

A novel compound was derived by annealing a high-pressure-synthesized sample of Sr2Ca2Cu3Oy at 350 °C for 30 min in air. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy indicated that the composition of Sr, Ca, and Cu was not changed from the original material, and the C-K edge energy-loss near-edge structure was similar to that of CaCO3. Structure analysis revealed that this compound has a primitive tetragonal unit cell with a=3.88(3) and c=14.13(2) A. It is likely that carbonate is incorporated either from air or secondary-phase grains into the pristine structure and forms the CxSr2Ca2Cu3Oz compound during annealing. Magnetization measurements revealed superconductivity with Tconset=89 K.

Observation of Morphology Changes of Fine Eutectic Si Phase in Al-10%Si Cast Alloy during Heat Treatment by Synchrotron Radiation Nanotomography

A series of three-dimensional morphology changes of fine eutectic Si-particles during heat treatment have been investigated in Self-modified and Sr-modified Al-10%Si cast alloys by means of synchrotron radiation nanotomography utilizing a Fresnel zone plate and a Zernike phase plate in this study. The coral-like shape particles observed in Sr-modified cast alloy fragmented at branch and neck during heat treatment at 773 K. The fragmentation occurred up to 900 s. After that, the fragmented particles grew and spheroidized by Ostwald ripening. On the other hand, rod-like shaped eutectic Si-particles observed in self-modified cast alloy were larger in size compared with the particle size in Sr-modified cast alloy. Separation of eutectic Si-particles in Self-modified cast alloy occurred up to approximately 900 s, which was similar tendency to that in Sr-modified cast alloy. However, it was found that the morphology change behavior was very complex in rod-like shape Si-particles. The three-dimensional morphology changes of fine eutectic Si-particles in both cast alloys, specifically fragmentation and spheroidizing, can be connected to changes in mechanical properties.

Synthesis of CxSr2Ca(n?1)CunOz superconductors using high-pressure-synthesized Sr2Ca(n?1)CunOy precursors (n = 2, 4)

Novel compounds of CxSr2CaCu2Oz and CxSr2Ca3Cu4Oz were derived by annealing high pressure-synthesized samples of Sr2Ca(n−1)CunOy (n = 2, 4) at 350 °C for 30 min in air. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy indicated the composition of Sr, Ca, and Cu was not changed from the original materials and the C-K edge energy loss near edge structure was similar to that of CaCO3. It is likely that carbonate groups are incorporated either from air or secondary-phase grains into the pristine 0(Sr)2(n − 1)n structure to form the CxSr2Ca(n−1)CunOz compounds during the annealing. Magnetization measurements revealed superconductivity with Tc,onset = 89 K for CxSr2CaCu2Oz and 65 K for CxSr2Ca3Cu4Oz.

Transition temperatures and irreversibility-field properties of (Cu1−xCx)Sr2CaCu2O7−δ superconductors with precisely controlled oxygen contents

(Cu1?xCx)Sr2CaCu2O7?? samples with precisely controlled oxygen contents (? = 0.10?0.35) were successfully synthesized under a pressure of 5.5 GPa and followed by annealing in He gas at various temperatures. The oxygen content of an as-synthesized sample was determined by iodometric titration, and the changes of the oxygen content were estimated by measuring weight-loss during the post-annealing. Superconducting transition temperatures (Tc) decreased from 93.1 to 83.1 K with decreasing the oxygen content from 6.90 to 6.65. Hole-doping level was tuned to be underdoped so that the slope of the irreversibility field (Hirr) increased.

High-Pressure Synthesis of CuBa2Ca3Cu4O10+δ Superconductor from Precursors Prepared by a Polymerized Complex Method

Samples of a CuBa2Ca3Cu4O10+δ superconductor were synthesized under a high pressure of 5 GPa at 1100–1200 °C for 30 min using precursors produced by solid-state reaction and polymerized complex methods. Compared with the precursors prepared by the solid-state reaction method, the precursors produced by the polymerized complex method have low grain sizes. The superconductive transition temperature of the samples prepared using precursors made by the polymerized complex method was found to be 113 K. The volume fractions of the superconducting phase in the samples prepared using precursors made by the solid-state reaction and polymerized complex methods were 49 and 36%, respectively. From these results, precursors made by the polymerized complex method can be used in the high-pressure synthesis of superconductors similarly to those made by the solid-state reaction method.