Wei Chongde

Ultrasound anomalies in single phase (Bi1-xPbx)2Sr2CanCun+1O2n+6+y superconducting ceramics

Ultrasonic anomalies near 200 K in single-phase (Bi1-xPbx)2Sr2CanCun+1O2n+6+y (x=0-0.2, n=1, 2) superconducting samples were observed by velocity and attenuation measurements of both longitudinal and transverse sound waves. The remarkable isothermal-like anomalous changes near 200 K indicate that there are possible structural changes or phase transitions in the samples. On comparison with YBa2Cu3O7-y, the different behaviours in ultrasonic anomalies are attributed to the differences in structure. It is suggested, however, that the lattice instability appearing near T approximately=2Tc may be a common feature of the existing superconducting perovskites and may have some relation to their high Tc.

Superconductivity of LaBa2-xCaxCu3Oy system

The high Tc superconductor LaBa2-xCaxCu3Oy has been studied and the composition dependence of Tc examined. For x=1, Tconset =94K, TcR=0 =80K. The structure of this system was refined utilizing the x-ray powder diffraction data, and it was found that the material is isostructural with YBa2Cu3Oy.

Local Magnetic Relaxation in High-Temperature Superconductors

The local magnetic relaxation in superconducting slab is investigated numerically for Anderson-Kim model, U(j)=Uc(1-j/jc), and periodic pinning potential. The relaxation of B(x,t) depends on the starting profile and the position x. It is important to note that the relaxation of B(x,t) exhibits approximate scaling law for some initial condition, showing that the relaxation rate is roughly uniform over the sample. For long time, B(x,t) is towards an asymptotic behavior independent of x.

Peak Effects in EuBa2Cu3Oy Crystal Prepared by Melt-Textured Growth Method

The magnetization curves of EuBa2Cu3Oy crystal prepared by melt-textured growth method were measured from 1.5 to 87 K up to field 7.5 T for H || C and H ⊥ C. Peak effects were observed at 60, 70 and 73 K for H || C, and 70, 73 and 77 K for H ⊥ C. It is shown that the magnetic field Hp at which the magnetization or critical current reach their extremum is approximately independent of temperature and the peak effects observed can be explained by matching effects.

FABRICATION AND PROPERTIES OF HIGH Jc Y—Ba—Cu—O SUPERCONDUCTOR BY (MTG) PROCESS

Superconducting Y—Ba—Cu—O bulk materials were fabricated by melt—textured growth (MTG) method, where cooling slowly under temperature gradient produces an aligned structure exhibiting high Jc value. The critical current density was measured by continuous dc four—probe method with criterion of 1μV/cm. Jc value has reached 23, 800 A/cm 2 at 77K under 2T magnetic field perpendicular to the current. The magnetic field dependences of Jc from 1 to 7 Tesla show the anisotropy for two orientations (H┴I, H┴a—b plane and H┴I, H//a—b plane). The X—ray diffraction patterns show the crystals preferentially aligned. The SEM and TEM observations of the MTG sample reveal that the sample has dense layered—structure with regular alignment of plate—shaped crystals. Twinning planes of fairly high density exist. A great quantity of dislocations, dislocation loops and stacking faults exist around 211 phase. These crystal defects may be main flux pinning centers. We have measured the zero—field cooled (ZFC) and the field cooled (FC) magnetizations as functions of temperature at various external dc magnetic field. The dependence of irreversibility point T * on 2/3 power of field H for MTG sample was demonstrated. The flux jumping was found in the hysteresis loop below 10K.

Superconductivity of (Tl1-xAlx)2Ba2Ca2Cu3Oy

The structure, magnetic, and superconducting properties of the (Tl1-xAlx)2Ba2Ca2Cu3Oy with x = 0.05, 0.1, 0.15, 0.2 were studied. The TcR = 0 depends on whether the sample has a 2223 phase structure or has a 2212 phase structure. The magnetic properties in low field were measured and discussed.

Superconductivity of Tl2Ba2(Ca1-xMgx)2Cu3 Oy

The structural, magnetic and superconducting properties of the Tl2Ba2 (Ca1-xMgx)2Cu3Oy system with x=0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5 were studied. Replacement of calcium by magnesium depresses Tc from 116.3K (x=0) to 93.3K (x=0.5). The critical current also decreases with increasing x.