He Yusheng

Elastic constants of single-phase Bi2Sr2CaCu2O8+y superconductor

The elastic constants of Bi2Sr2CaCu2O8+y ceramic have been determined for the first time from ultrasonic velocity measurements. Near 200 K both longitudinal and transverse wave velocities show noticeable anomalous behaviour, indicating a possible structural change or a phase transition. The strong hysteresis and the bulk modulus softening are also remarkable. A Debye temperature theta D= 250 K is estimated. The derived elastic constants of this material are substantially smaller than those of YBa2Cu3O7-x and BaTiO3, which is attributed to the weakly interacting multilayers along the very large c axis of Bi2Sr2CaCu2O8+y. However, the Poisson ratios of these perovskite ceramics are quite close, suggesting similar interatomic bonding.

Fabrication, characterization and welding of Bi(Pb)-Sr-Ca-Cu-O superconducting fibres

Superconducting fibres of Bi(Pb)-Sr-Ca-Cu-O high Tc superconducting materials have been prepared by means of the laser-heated pedestal growth (LHPG) method. The highest zero resistance temperature Tco reached is 114 K, and the highest critical current density Jc (77 K, 0 T) is greater than 5000 A cm-2. As-grown superconducting fibres were successfully fabricated without post-growth heat treatment. Amorphous materials were used for the first time to make high quality fibres. The strong textured structure and highly aligned morphology of the fibres were studied. The influence of growth conditions, thermal treatment and the composition of the fibres was also discussed.

Experimental study of the pre-crystallization process in the amorphous fast ionic conductor 0.60AgI0.25Ag2O0.15P2O5

Abstract Systematic investigations on the pre-crystallization process in the amorphous ionic conductor 0.60AgI0.25Ag 2 O0.15P 2 O 5 have been carried out by means of ultrasonic measurements, thermal analysis, electron microscopy (SEM and TEM) and ionic conductivity experiments. The temperature dependence of the longitudinal ultrasound attenuation and velocity showed anomalous attenuation peaks and sudden unusual relative sound velocity changes near 330 and 341 K, indicating that possible structural changes and softening of interatomic bonds occurred. These are consistent with the anomalous changes of ionic conductivity of the same samples. Thermal analysis revealed that such anomalous changes are directly related to the glass transition and the microcrystalline formation, respectively. Moreover, direct evidence for the processes of crystallizations above 344 K were given from our SEM and TEM study. Therefore, it is suggested that the observed anomalous enhancements near 326 and 344 K of ionic conductivity of this amorphous fast ionic conductor is related to the destruction of the amorphous structure and the crystallization of the amorphous state.

Microstructure of electroless Co alloy films for perpendicular magnetic recording

The mechanism of columnar crystallite orientation in electroless Co-Ni-Mn-P films for perpendicular magnetic recording media was investigated using SEM, TEM and other methods. Micro-region analysis revealed that the Ni and P in the film is mainly distributed in the grain boundary. So it is suggested that the film has a segregated microstructure. The segregation of Ni-P alloy is the main cause which makes the c axis of alpha -Co orient along the film normal and the columnar grain become fine. Magnetic measurements also showed that the films had a good perpendicular property.

Lattice readjustment of YBa2Cu3O7-y between 150 and 300 K: a transmission-microscope study

The lattice instability of the high-Tc superconductor YBa2Cu3O7-y has been studied at temperatures between 150 and 300 K by transmission electron microscopy. Anomalous changes in the lattice constant c were observed near 170 and 250 K. The axial ratio b/a was found to vary in a range from 1.014 to 1.018 (error: +or-0.1%), and a tendency towards a reversible orthorhombic-tetragonal phase transition was observed between 170 and 200 K. Possible explanations are proposed.

Bi-based superconducting fibers prepared from amorphous materials

Bi-based superconducting fibers have been prepared from amorphous materials by the LHPG method. The onset transition temperature reached is 102 K, and the zero resistance temperature reached is 86 K. The critical current density Jc (77 K, O T) is greater than 5000 A/cm2. Structure analysis indicates that the superconducting fibers have highly aligned morphology with a-axis parallel to the growth direction.

Ultrasonic investigations of the layered perovskite ceramic superconducting systems

Ultrasonic measurements have been carried out on three type high T c perovskite ceramic superconductors La-Sr-Cu-O, Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O. Anomalies in both sound velocity and attenuation of the above three systems were observed. The elastic constants and the Debye temperatures were also obtained and discussed.

Experimental evidence for the lattice instability of Bi-based superconducting systems

Abstract Ultrasonic measurements, specific heat and thermal analysis experiments, X-ray diffraction study and infrared investigation revealed that there are anomalous structural changes or lattice instabilities near 200 K in single 2212 or 2223 phase samples of Bi(Pb)-Sr-Ca-Cu-O system. Detailed study showed that anomalous changes or lattice instabilities are isothermal-like processes and have the characteristics of a structural phase transition, accompanying with increases in lattice constants. Possible mechanism for this lattice instability is discussed.

Ultrasonic anomalies in n-type superconducting Nd2−xCexCuO4−y compounds

Ultrasonic measurements were carried out on a series samples of Nd2−xCexCuO4−y with x ranged from 0 to 0.2. The anomalous changes in both attenuation and velocity around 190 to 200 K were found for the superconducting samples (x=0.15) during cooling, whereas no anomalies can be found in this region for non-superconducting samples. The implications of this phenomenon is discussed.

A study of electroless cobalt alloy films for magnetic recording media

Abstract The perpendicularly oriented crystal structure conditions of electroless plated films with polyimide and glass substrates were investigated by Co(002) plane rocking curves and scanning electron microscopy observation. It is concluded that not only the MnSO4 concentration in the bath but also the substrate has a remarkable effect on the film microstructures.