Studies on magnetization and microwave properties of gamma-ray-irradiated Sm-Ba-Cu-O superconductors

Studies on the effect of gamma-ray irradiation on critical transition temperature, transport and magnetic critical current density (at 4.2 K), high-field magnetization (at 4.2 K), and microwave-induced d.c. voltage (inverse a.c. Josephson effect) have been performed on SmBa₂Cu₃O_7– ceramic superconducting samples prepared by the coprecipitation technique. Gamma-ray irradiation of the samples was carried out using a⁶⁰Co source of 10³ Ci strength for several hundreds of hours; the dose received by the samples was 80 K rad/h.-Ray irradiation was found to have no effect on its structural modification and on the critical transition temperature. However, transport and magnetic critical current density are increased. Irradiation also caused a significant increase in the high-field magnetic hysteresis, which is presumably connected with the creation of radiation-induced mobile defects. An appreciable decrease in the microwave-induced d.c. voltage at 77 K was also observed after irradiation, which suggests that the mobile defects are clustered at the major defect region and thus reduce the total number of weak links. Enhancement of transport and magnetic critical current density may be due to the stronger pinning of flux lines at the-ray-induced defect site in SBCO ceramic superconductors.     

Magnetic and microwave properties of Sm1?xBa2Cu3+xO7?δ superconductors at 4.2 K prepared by an organic carbonate coprecipitation route

Samples of Sm₁Ba₂Cu₃O_7– and Sm_0.95Ba₂Cu_3.05O_7– are prepared by a coprecipitation technique using organic carbonates in the presence of stable polymers. The electrical, magnetic, and microwave measurements have been carried out to characterize the materials. Both normal (Sm₁Ba₂Cu₃O_7– ) and copper-rich (Sm_0.95Ba₂Cu_3.05O_7– ) samples showed sharp superconducting transitions at 94 K. Lower and upper critical fields of the samples are estimated from magnetization measurements. The critical magnetization current density (J _mc ) and pinning force density (F _p ) of the samples are obtained from the magnetization hysteresis loop at 4.2 K for fields up to 5T. Superconducting parameters (H _c (O),K _GL,,) are obtained by using the values of lower and upper critical magnetic fields. Microwave-induced d.c. voltage measurements indicated a smaller number of weak links for copper-rich samples. The enhancement of critical current density and the reduction of total number of weak links for copper-rich samples may be due to the increase in hole concentration caused by the partial replacement of Sm by Cu.     

High-field magnetic and transport critical current density of Sm-Ba-Cu-O: Agx superconductors at low temperature

Variation of magnetic critical current density (J _mc) and transport critical current density (J _tc) in high magnetic fields at liquid helium temperature were investigated on Sm-Ba-Cu-O: Ag _x (x=0.0, 0.2, 0.4, and 0.6) superconducting samples prepared by the coprecipitation technique.T _c and crystallographic parameters remain unaffected by Ag addition. However, bothJ _mc andJ _tc were found to increase on Ag addition. The volume pinning force (F _p) also increased with increase in Ag concentration, resulting in stronger flux line sheer. Microwave-induced d.c. voltage measurement shows a significant reduction of the total number of weak links between the superconducting grains with increasing concentration of Ag, which creates stronger pinning potentials between the boundary of the superconducting intergrains. Hence, the volume pinning force is greatly enhanced by Ag addition, leading to flux line lattice movement which is responsible for highJ _mc andJ _tc.     

Low-field behavior of magnetic and transport properties of laser irradiated Nd-Ba-Cu-O ceramic superconductors

The low-field behavior of magnetic and transport properties of high-power laser-irradiated (Q-switched ruby laser, 694 nm, 30 ns) Nd-Ba-Cu-O (NBCO) ceramic superconductors prepared by the coprecipitation technique have been investigated. Laser-irradiated NBCO samples contribute significantly to the flux pinning for low applied magnetic fields. A well-defined field valueH _c was observed for different laser-irradiated samples which is characteristic of a cross-over between regions dominated by the inter- and intragranular effects. The value ofH _c for a laser-irradiated sample is increased with increasing number of laser pulses. Thermal modeling of laser heating reveals that on irradiation the surface melts up to a depth of 1m and laser-induced evaporation occurs at energy density 2.5 J/cm². SEM studies showed grain growth due to laser-induced surface sintering which improves the interconnectivity among the superconducting grain structure and thus attributed to physical densification and consequent reduction in the total number of weak links. The increase of flux trapping and transport critical current density after laser irradiation is determined by the rigidity of the flux line lattice which may be due to the presence of additional pinning centers caused by the creation of laser-induced mobile defects and their clusters.     

磁场扫描速度对测量高温超导体临界电流的影响

实验中使用的样品是熔融织构Yba2Cu3O7-δ高温超导体,用振动样品磁强计(VSM)测量了样品在温度T=52 K和不同的磁场扫描速率(β=800,80,8,0.8,0.08 G/s)下的磁滞回线.通过运用Bean模型和电磁学理论对此数据的分析,得到△M～β曲线及E～jc曲线,发现有近似于幂指数规律,而且末端有向下倾斜的趋势,我们认为这是由于样品的钉扎和磁通蠕动机制发生了变化.结果表明磁滞回线测量的临界电流(jc)与磁场扫描速度β有关,所测临界电流随着磁场速度的减小而减小.     

On dislocation-induced reentrant-like behavior in deoxygenated high-Tcsuperconductors

A possible scenario for dislocation-induced intragrain weak-link generation is proposed. The theory predicts a reentrant-like behavior of the critical current density vs. magnetic field due to the sample deoxygenation. A qualitative agreement with high-T _c single crystal anomalies is also discussed.     

Flux-Creep Mechanism of Critical Current Limitation in Tl-Based Thin Film Microstrips

Superconducting Tl-Ba-Ca-Cu-O thin film microstrips were prepared on MgO substrates by thallination of the lift-off patterned Ba-Ca-Cu-O precursor films.The highest zero-resistance critical temperare T_co of microstrips was 94 K. From the critical current vs. temperature dependence two various critical current limiting mechanisms were distinguished.     

Low-temperature magnetic and electromagnetic studies of Y-Ba-Cu-O: Agx Ceramic superconductor

Magnetization and magnetic relaxation measurements have been studied at 4.2 K on YBa₂Cu₃O_7–:Ag _x (x=0 to 1.2) superconducting samples prepared by a coprecipritation technique. Remanent magnetization and hysteresis loops were found to be increased on Ag addition. Transport current density (J _c) was enhanced with the addition of Ag concentration. Flux creep rate was also increased with increase in Ag addition resulting in stronger pinning potential. Microwave-induced d.c. voltage measurements show a reduction of weak links with increase in Ag concentration. The enhancement of transportT _c is attributed to the stronger Josephson current paths due to the reduction of the total number of weak links after Ag addition.     

Sb_2O_3掺杂对YBa_2Cu_3O_（7－δ）超导性质的影响

研究了Ｓｂ２Ｏ３掺杂的多晶ＹＢａ２Ｃｕ２Ｏ７－δ超导体的超导转变温度和输运临界电流密度Ｊｃ（Ｂ）在磁场中的行为。测量了超导性能对该掺杂浓度的依赖关系。适当浓度的掺杂可使Ｊｃ至少提高一个数量级。对掺杂样品进行了扫描电镜微结构观测和Ｘ射线结构分析。