Toshio Usui

CRYSTAL GROWTH OF YBA2CU3O7-X AT THE MGO (100) SURFACE STEPS

Abstract We have investigated the crystal growth of YBa2Cu3O7−x thin films on MgO(100) with particular interest on the growth at substrate surface steps. Films were deposited by the activated reactive co-evaporation method at a substrate temperature of 550°C. Cross-sectional transmission electron microscopy was used to study the microstructure. The film is composed primarily of grains oriented with the c-axis normal to the substrate surface and a small volume fraction of grains with the a-axis normal to the substrate surface. This a-axis oriented grain grows at the substrate surface steps. Furthermore, a very small fraction of off-axis grain grows at the substrate surface steps or microcracks. At both sides of the grain at the edge of an 8 A step, cationic stacking is matched above the step. Many microstructure differences are observed at the substrate surface steps.

Gas-polarographic multifunctional sensor: oxygen-humidity sensor

Abstract A gas-polarographic oxygen-humidity sensor using the electrochemical oxygen pump of a zirconia electrolyte and an artificial gas-diffusion over-potential has been investigated to clarify its sensing mechanism from the theoretical viewpoint. The sensor shows artificial limiting current plateaux in O 2 -inert gas, H 2 O-inert gas and O 2 H 2 O-inert gas mixtures, analogous to the polarographic process. It is capable of continuous measurement of oxygen and humidity with high sensitivity. This is because the rate-determining step for oxygen and humidity sensing is the diffusion of oxygen gas and water vapour through the gas-diffusion hole of the sensor. The electrical output of the sensor exhibits neither drift nor hysteresis. Thus, this sensor can be used for accurate and reliable oxygen and humidity control.

Output Characteristics of a Gas-Polarographic Oxygen Sensor Using a Zirconia Electrolyte in the Knudsen Diffusion Region

Output characteristics of a gas-polarographic oxygen sensor using a zirconia electrolyte have been investigated at 450°C and low pressures (~1 mmHg) in O2-N2 gas mixtures with the aim of clarifying those in the Knudsen diffusion region. The limiting current obtained at 1 mmHg of the total pressure increased linearly with the oxygen concentration up to 100% in the ambient atmosphere as expected from the theoretical consideration on the output characteristics of the sensor in the Knudsen diffusion region.

In Situ X-Ray Chemical Analysis of Y1Ba2Cu3O7-x Films by Reflection-High-Energy-Electron-Diffraction Total-Reflection-Angle X-Ray Spectroscopy

Both the surface crystalline structure and chemical composition of Y-Ba-Cu-O film were examined in situ using reflection-high-energy-electron-diffraction and total-reflection-angle X-ray spectroscopy (RHEED-TRAXS). The chemical composition determined by TRAXS agreed well with that determined by inductively coupled plasma spectroscopy. The compositional deviation from Y1Ba2Cu3O7-x affected the surface crystalline structure of films, as was detected by RHEED and confirmed by scanning electron microscopy. RHEED-TRAXS is a powerful tool for in situ examination of the crystal growth of Y-Ba-Cu-O films.

In situ observation of Ba oxide layer at the interface between Y1Ba2Cu3O7−x film and MgO substrate

The very first stage of the growth of c‐axis oriented Y1Ba2Cu3O7−x films prepared by coevaporation of metallic constituents on MgO(100) was examined in situ by new surface sensitive x‐ray spectroscopy. Using reflection high‐energy electron diffraction and total reflection‐angle x‐ray spectroscopy, the Ba oxide layer of Y1Ba2Cu3O7−x was found to grow at the interface between MgO(100). It was further observed that the coverage and the flatness of this Ba–oxide layer was higher than that of Y and Cu–oxide layers.

Humidity‐Sensing Characteristics in Wet Air of a Gas Polarographic Oxygen Sensor Using a Zirconia Electrolyte

This paper reports on humidity-sensing characteristics in wet air of a gas polarographic oxygen sensor using a zirconia electrolyte that have been investigated. This oxygen sensor showed two limiting current plateaus in wet air by analogy with the polarographic process, compared to one limiting current plateau in dry air. The first limiting current was approximately proportional to the oxygen concentration in the atmosphere. On the contrary, the second limiting current was related to the fact the oxygen ions, produced by the electrochemical decomposition of water vapor at the cathode of the sensor, passed through the zirconia electrolyte. As a result, the difference between the second and the first limiting currents increased with water vapor concentration in the ambient atmosphere. Thus, this oxygen sensor can be also used for detecting humidity in wet air.

Gas Polarographic Oxygen Sensor Using the Pores of the Pt Cathode as a Gas Diffusion Barrier

A gas polarographic oxygen sensor using the pores of the Pt cathode as an oxygen diffusion barrier has been investigated in O2–N2 gas mixtures at 600°C. As expected, the pores of the cathode functioned as a gas diffusion barrier restricting the access of gaseous oxygen to the interface of the Pt cathode and a zirconia electrolyte; consequently, the limiting current was realized. The resulting current increased linearly with the oxygen concentration in the range of 3–25% in the ambient atmosphere when the voltage applied to the cell was held constant within a range of 0.6–2.4 V. In addition, the output current of the sensor was interpreted to be a mixed current of both an ohmic current proportional to the applied voltage and a current based on the diffusion restriction of gaseous oxygen.

N2O-Gas Sensing Using a Gas Polarographic Oxygen Sensor

N2O-gas sensing in N2O-O2 and N2O-N2 gas mixtures was investigated using a gas polarographic oxygen sensor which was operated at 500°C. The sensor detected the gaseous oxygen formed by the thermal decomposition of N2O; consequently, the output of the sensor agreed well with that calculated on the basis of the assumption that N2O was fully decomposed into O2 and N2 gases.

Preparation of High-Tc Tl-Ba-Ca-Cu-O Superconductors

Bulk superconductors of the Tl-Ba-Ca-Cu-O system have been investigated to clarify their superconducting properties; these were found to be significantly sensitive to the sintering temperature and the composition of the oxides. Zero-resistance was achieved at 118 K for Tl2Ba2Ca2Cu3Ox sintered at 880°C for 1 h in O2. In addition, an ac susceptibility result also supports its superconductivity with high critical temperature.

X-ray chemical analysis of YBa2Cu3Ox thin film by total-reflection-angle X-ray spectroscopy

Abstract Scanning electron microscopy and total-reflection-angle X-ray spectroscopy (SEM-TRAXS) was applied to X-ray chemical analysis of a 800 A thick YBa 2 Cu 3 O x (YBCO) film on an MgO substrate. In this method, the emission efficiency of characeti X-rays, excited by the electron beam of a SEM, from elements composing a thin film is drastically enhanced, as compared to that in the conventional SEM-EDX method. As a result, the energy dispersive X-ray spectrum obtained from an 800 A thick YBCO on MgO substrate was quite clear and almost the same as that of a thick YBCO film or a bulk YBCO crystal from the viewpoint of X-ray intensity because the intensity of the characteristic X-rays emitted from the MgO substrate was reduced below a detectable intensity.