Pcw Fung

Optimum fabrication process and some relevant analysis for the vanadium-lead doubly substituted 2223 superconducting ceramics

Abstract In a previous investigation [2], we have fabricated the superconductor satisfying the nominal stoichiometric composition B2-(x+y)VxPbySr2Ca2Cu3Oq, where x, y cover wide ranges of combinations, based on a particular preheating and sintering procedure. We follow up such work by exploring other solid state sintering processes and have found an optimum method via which we can fabricate very good superconductors with Tcs = 118 K and Tc (R = 0) = 108 K. X-ray analysis indicates that more than 95% of the typical sample belongs to the 2223 structure. All our samples show a paramagnetic — diamagnetic transition within a very small temperature range which is greater than Tc (R = 0) and smaller than Tcs.

Vanadium-lead substituted 2223 Bi-Sr-Ca-Cu-O superconductors

Abstract Following up our previous work [24] on the fibrication of the Bi2-xVxSr2Ca2Cu3Oy and Bi2-xVxSr2CaCu2O8 superconductors with x ranging from a fraction to unity we report here the fabrication of double-substituted superconducting ceramics satisfying the nominal stoichiometric ratio of Bi2-(x+y)VxPbySr2Ca2Cu3Oq, where x, y run from small fractions to 0.7. For most of the samples, the lower Tc = 80 K phase disappears, leaving the higher Tc = 110 K phase. Since a wide range of combinations of x, y can lead to similar superconductors and that the pure Tc = 110 K phase samples are associated with larger values of (x + y), we believe that the VPb substitution is significantly different from the Pb-doped or PbSb-doped cases in the basic crystal structure. Our samples are found to pass through the paramagnetic to the diamagnetic state as the temperature is decreased across the critical temperature region.

Internal friction study of transformation dynamics in BaTiO3 ceramics

Abstract The internal friction (IF) and relative change of modulus of a doped semiconducting polycrystalline BaTiO 3 sample were measured by an inverted torsion pendulum with forced oscillation as the sample was heated in vacuum from −150°C to 100°C. Two IF peaks, P 1 and P 2 , and two modulus, minima C 3v / C 2v and C 2v / C 4v , were observed corresponding to two phase transformations. All the IF peaks exhibit the characteristics of the first-order phase transformations. Each peak temperature is independent of frequency and the peak height increases with an increasing value of ( T /ω) n . A coupling coefficient of the phase interface to the oscillating stress is found to be α ( ω )= α ′ ω −1 . For the two peaks P 1 and P 2 in the doped BaTiO 3 specimen, the values of n and l have been determined to be n 1 = 0.225, n 2 = 0.190, l 1 = 0.101, l 2 = 0.204. Our study indicates that the IF arises from the motion of phase interfaces during the process of the first-order phase transformation. A dynamic interaction model is proposed to explain the special features of IF during the two phase transformations.

Scaling behaviours in the thermoelastic martensitic transformation of Co

By performing the DSC measurements at different temperature scanning rates, the latent heat and the peak temperature are determined in the structural phase transformation of Co. It is found that the latent heat during the transformation increases with the increasing scanning rate, and complies with a general power law. The peak temperature does not follow the Kissinger relation, and can also be described by a scaling law. Our experimental observation gives a straight evidence for the influence of frictional work and stored elastic energy to the thermoelastic martensitic transformation.

A new method for quantitative investigation of dissipative flux behaviour

Abstract In order to study the pinning force F p of type II superconductors in a quantitative way, we first measured the internal friction (IF) during magnetization of Bi 0.8 Pb 0.2 SrCaCu 2 O y superconductor and found that a characteristic IF peak occurs only if the sample is vibrating and the magnetic field is changing in time. The experimental results and theoretical deductions indicate that a non-linear force exists which is proportional to the product of the oscillating force on the magnetic flux (during IF measurement) and ( F d − F p ), where F d is the driving force arising from the nonzero gradient of the flux density. Our analysis shows that if one can measure the absolute value of the d.c. magnetization against the external magnetic field, the pinning force for the sample can be determined quantitatively.

Certain mechanical behaviour of Y-123 specimens with pinning centres from different origins

Abstract Using the vibrating reed technique, we have carried out internal friction (I.F.) and flexural modulus (F.M.) measurements of a number of high Tc samples of the rare-earth class. The results of I.F. and F.M. measurements are related to the phenomenon of flux pinning in type II superconductors. In the first set of samples, the Y-123 specimen has been substituted partly and wholly by Gd which has a stronger intrinsic magnetic moment, and that is related to the consequential pinning behavior. The second sample is one being exposed to different fluences of neutron irradiation. To substantiate our result further, we used three samples with different mass density and repeat the I.F. and F.M. measurements. Our results indicate that these mechanical measurements provide good means of studying flux pinning.

Characteristics of current density and magnetic response of partially melted YBaCuO superconductors

Abstract In this investigation, we used a partial melting approach to treat a number of YBaCuO samples and studied the microscopic structure of the granules in the sample. Partial melting causes a decrease in granular sizes. We found that the current density increases by 26% when the sample suffers a shrinkage of 6% in volume upon melting. We also applied a time varying magnetic field to the samples and investigated their magnetic susceptibility and reluctance responses. Several interesting magnetic features were found. The lower critical magnetic field H c1 was estimated to be about 100 Oe. A comparison between a crude theoretical model and the experimental results is presented.

The superconducting 112 oxides

Abstract Many ceramic oxide superconductors can be viewed as stacking of certain simple substructures of metallic oxides. Some of these substructures are themselves intrinsically superconducting. One such substructure can be identified as the 112 structure. We report here the fabrication of the Ca1−xBixSrCu2O5 samples, as one class of this substructure, and X-ray diffraction analysis substantiates the discovery of this new 112 phase. We also present the idea of bridging the Tc of the sample and its structure in a quantified way. The crystal parameters for this new orthorhombic structure are a = 4.2022 A , b = 5.5445 A and c = 13.470 A . Due to oxygen deficiency in one Cu-O plane, c measures the length of a double half-unit cell.

Discrete bäcklund transformation

Abstract A discretized Backlund transformation is given for the discrete sine-Gordon equation, and the general soliton solution is obtained.