Akira Nomoto

Curie Temperature, Biaxial Elastic Modulus, and Thermal Expansion Coefficient of (K,Na)NbO3 Piezoelectric Thin Films

We investigated fundamental parameters such as the Curie temperature, biaxial elastic modulus, and thermal expansion coefficient of (K,Na)NbO3 (KNN) films on Pt/Ti/SiO2/Si substrates. From the temperature dependence of the dielectric constant of the KNN films, their Curie temperature was approximately 360 °C; this value was confirmed from critical changes in the lattice parameters of the films and the stress induced in the films at approximately this temperature. By using the optical lever method, the biaxial elastic modulus and thermal expansion coefficient of the KNN films were found to be 92 GPa and 8.0×10-6 (1/°C), respectively. The fundamental properties of the KNN films were similar to those of widely used Pb(Zr,Ti)O3 films, indicating that KNN films are potential candidates for lead-free piezoelectric thin films in micro-electro-mechanical system (MEMS) applications.

Improvement of Piezoelectric Properties of (K,Na)NbO3 Films Deposited by Sputtering

(K,Na)NbO3 (KNN) films with high transverse piezoelectric coefficients were successfully deposited onto Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structures with preferential orientation. To improve their piezoelectric properties, we investigated the effects of annealing after the deposition and the Na/(K+ Na) ratio of the films. Annealing in air at 750 °C led to a decrease in the residual strain in the KNN crystal and the disappearance of openings at the grain boundary, thereby improving the transverse piezoelectric coefficient and leakage current properties. We also investigated the transverse piezoelectric coefficient and dielectric constant as a function of the Na/(K+ Na) ratio; both had maximum values at a ratio of approximately 0.55. For the KNN films, e31* ranged between -10.0 and -14.4 C/m2; thus, it was superior to previously reported values for lead-free piezoelectric films and was comparable to the best commercially available Pb(Zr,Ti)O3 films.

Flux Pinning Characteristics in Tl Series Superconductors

Magnetic hysteresis and flux creep characteristics were measured with partially-melted Tl 1223 and solid-state-reacted Tl 2223 superconductors at 77 K. The Tl 1223 sample shows large magnetic hysteresis which linearly depends on sample thickness, and less time decay of magnetization, though the sample has many grain boundaries.

Crystalline Structure of Highly Piezoelectric (K,Na)NbO3 Films Deposited by RF Magnetron Sputtering

We have previously reported (K,Na)NbO3 (KNN) films, whose piezoelectric properties are the highest reported thus far. In this study, we investigate the detailed crystalline structures of these KNN films after deposition on Pt/MgO and Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. The KNN film on Pt/MgO was epitaxially grown on the Pt lower electrode with a perfect orientation in the perovskite structure. The KNN film on Pt/Ti/SiO2/Si was polycrystalline with a preferential orientation in the perovskite structure having dense columnar grains. X-ray diffraction measurements revealed that the KNN films grown on Pt/MgO and Pt/Ti/SiO2/Si were tetragonal; the lattice parameters c and a were related as c/a > 1. The KNN film on Pt/MgO had a higher c/a value than the KNN film on Pt/Ti/SiO2/Si, indicating that the former had more compressed strain. We conclude that this difference in compressed strain may contribute to the difference in piezoelectric properties of the KNN films on Pt/MgO and Pt/Ti/SiO2/Si.

Effect of Lattice Strain and Improvement of the Piezoelectric Properties of (K,Na)NbO3 Lead-Free Film

(K,Na)NbO3 (KNN) films with very high transverse piezoelectric coefficient d31, which attained values comparable to those of Pb(Zr,Ti)O3 (PZT) films for the first time, were successfully deposited on Pt/Ti/SiO2/Si 4-in. substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structure with a preferred orientation. Furthermore, we focused on the effect of lattice strain on d31 of KNN to clarify the relationship between the piezoelectric properties and structural parameters apart from orientation. We found that -d31 increases with decreasing lattice strain c/a ratio of KNN when the in-plane lattice parameter a increases and the out-of-plane lattice parameter c decreases. By controlling the lattice strain c/a ratio strictly and maintaining a homogeneous strain, we achieved a high d31 (~-100 pm/V) that can be uniformized on 4-in. substrates and with a standard deviation that decreases to about 4 pm/V.

Power Current Leads Using Ag-Sheathed High-Tc Superconducting Tapes

To investigate a feasibility of Power current lead using High-Tc superconducting tapes, the assembled conductors stacking Ag-sheathed High-Tc superconducting tapes were fabricated and V-I characteristics were measured. The fabricated conductors had the electrically stable against aging and over current test, whose properties corresponded to that of tape itself considering self-magnetic field resulting from transport current. Calculated results as a function of Current Leads indicate that these conductors can efficiently reduce loss of Liquid He during operation of superconducting facilities.

E-J characteristics of dozens of meters-long Ag sheathed Bi-2223 wire

(Bi,Pb)2Sr2Ca2Cu3Ox Ag sheathed wire were fabricated by a powder-in-tube method. For short length Bi-2223/Ag sheathed superconducting tape measured ρ-J characteristics at 77K, OT corresponds with the relation ρ∝exp[-Jt/J], particularly perfect agreement for Ic=47A(Jc=26000A/cm2) sample. Higher-Ic sample is suggested less Jc distribution in the wire of series superconductors. The sample of 11.3m-long superconducting wire was evaluated Ic=17.OA at 77K, OT on the criterion of 1μV/cm. The local characteristics are distributed at the Ic from 10A to more than 44A. It is suggested that the region of 99 percent length of the 11.3m-length have the Ic>30A.

Properties of Silver-Sheathed Tl-Series Oxide Superconducting Tapes and Coils

As a preliminary study on the development of Tl-series oxide superconducting coils operated in liquid nitrogen, a twelve-layered pancake coil made of silver-sheathed Tl-2223 tapes was fabricated and the characteristics were measured. This multi-layered coil generated a maximum magnetic field of 510 Gauss at 77K and 1410 Gauss at 4.2K, respectively. To improve characteristics at high magnetic fields over IT, the properties of the new composition Tl-1223 oxide superconducting bulk and wires were examined. The Tl-1223 bulk has large ΔM in M-H characteristics even over 1T at 77K. Also the Jc value of Tl-1223 wire remains about 500 A/cm2 in high magnetic fields over 5T at 77K. It was confirmed that Tl-1223 wire has a potential to produce a high Jc in high magnetic fields at 77K.

10M Long Silver-Sheathed Tl-Ba/Sr-Ca-Cu-O Superconducting Tapes

Silver-sheathed Tl-Ba/Sr-Ca-Cu-O tapes, 10m long, were fabricated by a newly developed continuous pressing technique instead of the conventional rolling or short sample pressing technique. This continuous pressing technique provides highly densified polycrystalline oxide cores in the 10m superconducting tapes, similar to those in the pressed high-quality short samples. Critical current densities at several sections of the 10m long continuous pressed tape reached 104 A/cm2 at 77K without external magnetic fields, and deviations of the Jc values were within +2% throughout the whole tape length. Furthermore, the 10m long tape was wound into a pancake type coil of 30mm bore diameter. This coil generated a maximum magnetic field of 103 gauss and also a magnetic field of 47 gauss at the center of the coil.

Fabrication and Characteristics of Multi-core Tl-Ba(Sr)-Ca-Cu Oxide Superconducting Tapes

The Tl-Ba(Sr)-Ca-Cu oxide superconducting material (TBSCCO) was formed into single- or multi-core (36- or 1332-core) tapes by means of the Ag-sheathed powder method (the rolling or pressing). The maximum critical transport current density Jc of 5800 A/cm2 was achieved at 77 K and 0 T in a 36 multi-core rolled tape with 0.07 mm thickness. The Ic values of the pressed tapes with single- and 36-core reached 21 and 17 A at 77 K, respectively. It was found that the pressing method improved the Jc of the multi-core tapes in 0-2 T magnetic fields and that there were no indications of difference of Jc values in any direction of the magnetic field.