Principles of plasma-based high-energy accelerators are presented. Particles are accelerated by a high field gradient of plasma waves. Three concepts of accelerators will be discussed; the Beat Wave Accelerator (BWA), the Plasma Wake Field Accelerator (PWFA) and the Vp×B accelerator. BWA is characterized with a wave excitation by laser beat waves. The wave for PWFA is a wake of a high velocity bunch of electrons. The Vp×B accelerator has its uniqueness in the acceleration mechanism where Vp is the wave phase velocity and B is an external field applied perpendicularly to Vp. A brief discussion on needs of accelerators is also given.
Fluorescent perylene and donors (anilines or p-toluidines) are introduced into styrene monomer (St) and methyl methacrylate monomer (MMA) solvent. An exciplex emission peak of perylene with a donor of N, N-dimethyl aniline (DMA) was observed around at 557nm. The emission ratio of exciplex to intrinsic fluorescenct intensity increased with donor concentration. The emission peak wavelength depended on the kind of donors. The Stern-Volmer plots of fluorescence quenching of perylene by donors showed a linear relation in lower concentration region of donors. The association constant Kc was evaluated to be 19.6(mol/l)-1 from the Stern-Volmer relation of perylene with DMA in St. Interaction between perylene and donors were investigated using the solvent effect. Exciplex emission peak depended on the dielectric constant of solvents. The exciplex emission peak intensity and peak wavelength increased with the dielectric constant of solvent. The association constant Kc also depended on the dielectric constant. Intrinsic fluorescent decay curve of perylene in St at 474nm gave the life time of 3.8ns. Decay curve of perylene with DMA in St at 474nm showed two components in life times, 1.9ns as the intrinsic fluorescence and 24.0ns as the exciplex. The same analysis for exciplex emission from perylene with DMA in St at 557nm gave life time of exciplex as 25.9ns. The above results of life time measurements show that the intrinsic fluorescent life time decreases with addition of donor because of energy transfer to the exciplex which has longer life time than the excited perylene. Interaction between perylene and donors were analyzed using the electrochemical redox potencial. The electrochemical energy level of donor measured with by the potential was located near at that of perylene. It was shown that exciplex formation process of perylene could be clarified by the energy diagram obtained from the electrochemical potential measurements.
Electric discharge machining (EDM) is a very effective working process in die making. The surfaces formed by EDM were presented with an eroded layer which cause a large influences on the quality or accuracy for the hot die steel. The residual stress at the surface of hot die steel after EDM has been measured, using X-ray diffraction method. The results obtained can be summarized as follows; (1) The residual tensile stress decreased gradually with increasing in depth below the surface for the annealed SKD61 sample. (2) Stress distribution curves of the quenched and tempered SKD61 sample were different from those of the annealed SKD61 sample. The compressive stress was often found at the surface of the quenched and tempered SKD61 sample. The peaks of the tensile stress were found for below the surface. (3) It was shown that the stress affecting range increased with increasing in the energy supplying duration (τon) from 50 to 1200μs.
Luteolin-7-glucoside have been isolated from the fresh seedling of Safflower (Carthamus tinctrius L.). The chemical structure of Luteolin-7-glucoside as 3', 4', 5-trihydroxyflavon-7-O-β-D-glucoside is proved on the basis of chemical and spectroscopical evidences.
The application of superconductivity is a key to produce a number of breakthroughs in a vast area of science and technology covering new energy systems, computer electronics, medical diagnosis, high-speed transportations, high-energy particle accelerators, etc. The recent discovery of high Tc oxide superconductors may provide a more positive prospect for the future development of the superconductivity technology. In the area of metallic superconductors, the development of ultrafine filamentary wires for AC use, which may bring about a large impact in electrical engineering, is being carried out. The high-field performance of Nb3Sn wires has been significantly improved by the Ti-doping, and multifilamentary (Nb, Ti)3Sn wires capable of generating fields up to 17 T have become commercially available. The development of Nb3Al and other superconductors with high-field performance superior to that of (Nb, Ti)3Sn is also being proceeded using new fabrication technologies. The application of techniques elaborated the fabrication of metallic superconductors, e.g. powder process using metal sheath, solidification process, diffusion process and high-rate deposition process has been attempted for the processing of high-Tc oxide superconductors. Ag-sheathed BPSCCO tapes show a Jc (77K, OT) of 3×104/cm2, and keep nearly the same Jc at 25T and 4.2K. The solidification process has achieved an encouraging result for obtaining enough Jc in bulk high-Tc oxides. Superconducting tubes coated with a thick YBCO layer, promising for cavities and magnetic shieldings, have been fabricated by a plasma spraying process. Meanwhile, large Jc's at 77K and high magnetic fields have been obtained in thin oxide films prepared by different physical and chemical deposition processes. Various problems to be solved, e.g. homogenization, densification, crystal orientation and introduction of pinning centers, for improving Jc in bulk high-Tc oxides are also described in this article.
To date more than ten high temperature superconductors (HTSC) have been discovered, which are all oxides with perovskite related crystal structures. Starting from Tc=30K in La-Ba-Cu-O in 1986, Tc has risen every year along with the discovery of new materials. Y-Ba-Cu-O in 1987 was the first HTSC with Tc higher than the liquid nitrogen temperature and Tc exceeded 100K in 1988 by the discovery of Bi-Sr-Ca-Cu-O. These compounds are particularly important for technical applications. Although the rise of Tc seems to have taken a pause over a year, a representative HTSC has appeared also in 1989. This is Nd-Ce-Cu-O and the first N-type HTSC with electrons as charge carriers. From these discoveries it is now almost established that the two-dimensional CuO2 plane as well as doping carriers into it are essential for realizing high temperature superconductivity.
Structural features of high-Tc superconductors YBa2Cu3O7-x(YBCO) and Bi3Sr2Can-1CunO2n+4(BSCCO) are described. Various lattice deffects such as dislocations, modulations, tweed patterns, twiwns and grain boundaries were analysed by transmission electron microscopy. These defects differed extensively from those of simple metals reflecting the highly anisotropic nature off the crystals. The oriented nature may be taken advantage of the materials science and engineering of the still young materials.
Studies on factors affecting critical current density of high-Tc oxide superconductors of Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems are reviewed. Although the Ho2 of these materials are much higher than those of conventional metallic superconductors, it is highly anisotropic, as can be expected from an examination of their crystal structure. The superconducting current path in these materials is also limited in CuO2 layer which is parallel to the c-plane. In addition to this anisotropic nature, the transport current flow is suppressed at the boundaries which behave as barriers of Josephson junction (weak link). This causes a rapid decrease of Jc at low magnetic field. These problems of anisotropy and weak link can be overcome by the development of processes such as Ag-sheathed tape which produce grain allignmen. In magnetic fields above Ho1, pinning strength become dominent as a factor affecting Jc. Strong pinning force has been obtained in the melt processed Y-Ba-Cu-O. However, the pinning force at 77K in Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O is still much lower than those practical levels. The weak pinning also produces a problem of flux creep. Further developments in fabrication processes that can overcome various problems limitimg transport Jc are required.
Recent high pressure works revealed that there is a distinct difference between hole- and electron-doped oxide superconductors. Studies of insulator-metal transitions, antiferromagnetic transitions, carrier density and crystal structures in these oxides under high pressure are briefly reviewed and their correlations with high temperature superconductivity are discussed.
One of the most important properties for application of superconductors is critical current density(Jc). Jc of order 105 to 106A/cm2 is required often in the presence of large magnetic fields. However, Jc values of oxide superconductors remained low except thin films. It is also pointed out that large dissipation or flux creep becomes a serious problem for applications at higher temperatures. In fact, large flux creep is observed in thin films despite high Jc. In order to achieve high Jc values, we need to introduce effective pinning centers which can prevent the motion of flux against the Lorentz force (FL=JxB). In this paper, I first summarize how Jc is determined in type II superconductors. Then I present the methods to increase Jc values in oxide superconductors and show that the flux creep rate becomes small when the pinning force or Jc is increased. Finally I propose some possible applications for high Jc bulk oxide superconductors.
It is wellknown that Cu-O chains indicate superexchange interaction which is the origin of antiferromagnetism. YBCO perovskite has the chains which also originates high-Tc superconductivity. So we focused to the magnetism of superconducting samples at normal conductivity state and cleared the correlation between the antiferromagnetism and the high-Tc superconductivity To investigate the antiferromagnetism, the experiments were carried out by measuring the relative magnetic permeability(μs) at 77K and room tempreture. The samples which Cu molar ratio, X was changed from 2.5 to 3.5 in YBa Cu O7-δ were used to study Cu molar ratio dependence on μs in Cu-O chains. The samples which pelet forming pressure were varied from 400 to 800kg/cm2 were used in this study. Perfect diamagnetism was observed in each sample at 77K, except sample of X=2.5 which showed a value of about 1. The samples of 2.8<X<3.5 indicate antiferromagnetism at room temprature. The antiferromagnetism was the strongest at X=3.0 which was most suitable composition for high-Tc superconductors and this sample showed the highest Tc end and Jc.
In wire preparation, the prominant progress have been achieved mostly by powder-in-tube method. Ag-sheathed high Tc superconducting wires were prepared by this powder-in-tube method. Then Jc, Jc-B and microstructural analyses were evaluated. One of the most critical factors for Jc enhancement is grain alignment, which could be evaluated as the first approximate by degree of c-axis crystal orientation (F value) calculated from XRD. In YBCO system, Jc stayed in order of 103A/cm2 because of F value as low as 30%. In case of BSCCO system, F value was raised through the adjustment of process parameter near to 100%, resulting in Jc in order of 104A/cm2. However, this high Jc degradated rapidly under applied magnetic field over 0.1T, which was supposedly due to the weakness of intragrain pinning force and flux creep. TEM analysis of a cross section of Ag-sheathed BSCCO tape showed the presence of twist boundary.
The oxygen concentration in Y-Ba-Cu-O is one of the dominant factors to control the superconducting properties. Since oxygen concentration is controlled by the cooling rate, the cooling rate affects the superconducting properties. The lattice constant of the c-axis is related to oxygen concentration. Thus, we undertook to obtain the change of superconducting properties (Tc and Jc) and the lattice constant of the c-axis with the cooling rate in Y-Ba-Cu-O. The cooling curves were dominated Newton analysis. Based on the relaxation theory, we showed the existence of an approximately linear relationship between the lattice constant of the c-axis and the cooling rate, between Tc and the cooling rate and between Jc at 15K and the cooling rate. we conclude that the relaxation theory can be explained by oxygen concentration change in oxygen-deficient perovskite of Y-Ba-Cu-O. The abnormal Jc change is found from 45 to 65K for slow cooled specimen. The probable explanation is discussed by two phases. The slow cooled specimen has the ortho-I (matrix) and the ortho-II, the ortho-II can be the pinning site and then causes the abnormal Jc change.
A relationship between X-ray powder diffraction pattern and superconducting characteristics has been systematically investigated with respect to Bi-Pb-Sr-Ca-Cu superconducting oxides. The starting composition ratio was changed to obtain an optimum high-Tc single phase. At a composition ratio of Bi1.85Pb0.35Sr1.9Ca2Cu3.2, a pure high-Tc phase was obtained in the X-ray diffraction pattern and ac susceptibility measurement, although the temperature-resistivity (T-R) characteristics was a mixing of two phases with the low-Tc and high-Tc. On the contrary, at a composition of Bi1.85Pb0.35Sr2.2Ca2 Cux(X=3.3, 3.4), a simple single phase pattern with Tc (end) at 107k was observed in its T-R characteristics, despite the fact that this composition give a mixed phase in its X-ray diffraction and ac susceptibility measurement. The zero resistance transition temperature is more sensitive to Sr molar ratio than to Cu molar ratio. Thus, under the solid-state reaction conditions of the present study, ther molar ratio of Sr play an important role for forming a large quantity of high-Tc crystal phase grain with such thin grain boundaries as to connect the high-Tc phase grains electrically.
High Tc superconductor/metal Ag composite thick film has been fabricated on PSZ substrate. By controlling the fabrication conditions, the critical current density at 77K was elevated to 1300A/cm2. The c-axis of Y1Ba2Cu3O7-x plate in Y1Ba2Cu3O7-x/metal Ag composite thick film was preferably oriented perpendicular to the substrate, though the non-conducting phases of Bal Cu1O2, Y2Ba1 Cu1O5 etc were 40% in volume.
For artificial construction of new high Tc superconducting structures, the preparation of compositionally modulated films has been studied by using such processes as the laser ablation 1), multi-targets sputtering 2), and plasma-controlled sputtering 3). Here, we report on the syntheses of Bi-Sr-Ca-Cu-O and Y-Ba-Cu-O multilayered films by plasma controlled sputtering. The apparatus used for the film preparation has two solenoid coils which can work for varying the shape of the plasma above the targets. The sputtered area can be controlled by changing the coil current. Binary targets consisting of a smaller Ca1Cu1.5Ox(φ40mm) and a larger Bi1.6Sr1Oy(φ70mm) disks were used for the preparation of Bi-Sr-Ca-Cu-O modulated film. In case of the synthesis of Y-Ba-Cu-O modulated film, a smaller Y1Cu1.25Ox(φ50mm) and a larger Ba1Cu1Oy(φ70mm) disks were employed. The substrates, MgO(100), were. eated from 450°C to 550°C. In case of the preparation of compositionally modulated Bi-Sr-Ca-Cu-O film, the periodicity of 20A was designed in view of c-axis length of 18A between two Bi2O2 layers in the Bi2Sr2Ca2Cu3Ox structure. By the low angle X-ray diffraction, peaks attributed to planar spacing of 19.9A appeared. Annealing the as-grown amorphous film in a PbO atmosphere made it superconductor whose Tconset of 113K and Tczero of 103K. Auger electron spectroscopy revealed the existence of Pb in the film. X-ray diffraction patterns of Y2Cu2O5 and BaCuO2 were observed for the films prepared from Y-Cu-O and Ba-Cu-O targets, respectively. When we designed the multilayered film with periodicity of 12A, peaks identified to (003) and (006) planes of Ba2YCu3O7-δ were observed for the as-grown film prepared at the substrate temperature of 500°C. Increase of substrate temperature up to 550°C made their intensities high. The as-grown film deposited at the substrate temperature of 450°C was annealed to be strongly c-axis oriented Ba2YCu3O7-δfilm. Planar-like crystals texture was observed in the annealed film by SEM measurement. Its Tconset and Tczero were 65K and 49K, respectively.