Junji Tabuchi

Fabrication of high-power electric double-layer capacitors

Abstract The electrochemical behavior of activated carbon/carbon (AC/C) composite electrodes was investigated for high-power electric doublelayer capacitors (EDLCs). It was found that high-rate charge/discharge characteristics are affected by the resistance of the electrolyte phase in the pores of the electrode. The charge/discharge characteristics were improved by optimizing the pore-size distribution of the electrodes. The size and total volume of the macro-pores in the electrodes were controlled by mixing and burning out polymer spheres. A high-power EDLC (15V, 470 F), which can discharge as much as 500 A, was fabricated by using improved AC/C composite electrodes.

Effect of oxygen deficiency on the crystal structure and superconducting properties of the Ba2YCu3Oy

Effect of oxygen deficiency on crystal structure and superconductivity of Ba2YCu3O7-δ was studied by annealing and quenching experiments. With increasing oxygen deficiency superconductivity was suppressed remarkably while axial ratio, a0/b0 in the orthorhombic phase approached to unity and c0 value increased. Structural transformation from orthorhombic to tetragonal lattice took place in the temperature range, 950~980°C and maximum oxygen deficiency, δ=0.7 was observed for the tetragonal phase.

Preparation of superconducting Y‐Ba‐Cu‐O thick films with preferred c‐axis orientation by a screen‐printing method

Superconducting Y‐Ba‐Cu‐O thick films with preferred c‐axis orientation were obtained by a screen‐printing method. Thick films with preferred c‐axis orientation appeared only when the substrate material was YSZ, the film thickness was around 10 μm, and the film was fired at 980 °C for 6 min. The preferred c‐axis orientation was caused by an interface reaction between the Y‐Ba‐Cu‐O thick film and the YSZ substrate. The superconducting characteristics of the thick film were T zeroc =89 K and Jc (77 K)=7 A/cm2 .

Determination of the Orthorhombic-Tetragonal YBa2Cu3O7-δ Phase Boundary in the δ-T Diagram

The orthorhombic-tetragonal YBa2Cu3O7-δ phase boundary in the δ-T diagram was determined by a detailed isobar TG measurement under the oxygen partial pressures of 1 to 0.01 atm. The phase boundary was found not to be located at δ = const., but to have a slope of dδ/dT~-4.7×10-4 K-1. The experimental results are discussed with a simple order-disorder transformation model of oxygen atoms on the Cu plane sandwiched by Ba planes.

All-Ceramics Josephson Junctions Operative up to 90 K

Ba-Y-Cu-O/Ba-Y-Cu-O Josephson junctions are made by creasing fissures in the bulk Ba-Y-Cu-O ceramic superconductors. The critical current of the junction fissured at room temperature decreases exponentially with the increase in the operating temperature, similar to that of an S/N/S junction. However, with the junction fissured at cryogenic temperature, critical current persists all the way up to about 90 K (Tc of Ba-Y-Cu-O compound). The a.c. Josephson effectis observed for these junctions in all temperature range up till 90 K. The onset of the formation of the macroscopic quantum state just below 90 K is implied.

Twinning of High-Tc Ba2YCu3O7-x Oxides

Orthorhombic crystallites of the high-Tc superconductor Ba2YCu3O7-x oxide prepared by the standard sintering technique are heavily laminated by twinning on (110) lattice planes. Upon heating these crystals under a vacuum in an electron microscope, the orthorhombic structure transformed irreversibly to a tetragonal one. Between these two forms, a transitional orthorhombic form was observed. This irreversible transformation seems to be caused by a lack of oxygen in the evacuated microscope.

The Orthorhombic-Tetragonal Phase Transformation and Oxygen Deficiency in LnBa2Cu3O7-δ

The orthorhombic-tetragonal phase transformation in LnBa2Cu3O7-δ (Ln=Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb) was studied using careful isobar TG measurements under an oxygen partial pressure of 1 atm. As the atomic number within the lanthanide series increases, the transformation temperature tends to increase as well. This tendency is strongly related to the spacing of the nearest-neighbor oxygen atoms and is successfully interpreted in terms of the repulsion energy between them on the basis of a simple order-disorder transformation model proposed in our previous paper.

Large capacitance electric double layer capacitor using activated carbon/carbon composite

An electric double-layer capacitor with large capacitance, 1000 F at 5.5 V, has been developed. It uses activated carbon/carbon (AC/C) composite as the polarizable electrodes and sulfuric acid as the electrolyte. The AC/C composite was synthesized through a simple process, namely, a cured mixture of activated carbon and phenol-formaldehyde resin was carbonized. The AC/C composite has a large surface area, 1300 mg, and low resistivity, 0.01 Omega -cm. Two kinds of pores exist in the AC/C composite: macropores with submicrometer diameters, and micropores with below 4-nm diameter. The micropores are impregnated with the sulfuric acid aqueous solution through the macropores open to the outer surface. The electric double-layer capacitance per unit volume for the AC/C composite in sulfuric acid is greater than that for activated carbon powder as a raw material. The capacitor is expected to be used as an energy storage device and a supplementary power source. >

Oxygen-ordering and ac-susceptibility in polycrystalline YBa2Cu3O7−δ (δ=0.15∼0.67)

A perfect shielding behavior has been observed by ac-susceptibility measurement in a wide oxygen-deficiency range of YBa 2 Cu 3 O 7−gd (δ=0.15∼0.67) and the measurement indicated the existence of a bulk superconducting phase (ortho.II) with T c ∼60K at δ∼0.5. The ortho.II superconductivity was very sensitive to the magnetic field. A detailed ac-susceptibility measurement was conducted on the quenched polycrystalline ortho.II samples with a range of measuring magnetic field. The ac-susceptibility of the ortho.II was found to be suppressed to a constant value; about 70% of the perfect one by the magnetic field of a few oersteds. Heat-treatments at low temperatures in an encapsulated condition did not alter the essential feature. The field sensitive nature of the ortho.II susceptibility was related to the weakly coupled Josephson link through ortho.II grain boundaries.