Enhancement in The Transport Critical Current Density J_c in YBa_2Cu_3O_(7-δ) Added with an Insulating Nano crystalline YBa_2HfO_(5.5) Perovskite

摘要

When a magnetic field is applied to type II superconductors, such as YBa_2Cu_30_(7-δ) (YBCO), the flux quanta penetrate the material as a regular array of vortices. However when transport currents are applied, they act to move these vortices, thus lowers the critical current density (J_c) as well as destroying superconductivity. The development of microstructures made of YBCO materials has enabled engineers to increase the critical current density, within Type II materials by introducing flux pinning centres into the material. The microstructure and flux pinning properties of YBa_2Cu_3O_(7-δ) system with varying levels (0-5 wt. %) of a nano perovskite ceramic insulator; YBa_2HfO_(5.5) addition was studied in detail. Orthorhombic YBa_2Cu_3O_(7-δ) powder was prepared through conventional solid state route and a modified combustion method was used for synthesizing nanocrystalline YBa_2HfO_(5.5). The structure and microstructure of the samples examined by X-ray diffraction and scanning electron microscopy showed that YBa_2HfO_(5.5) and YBCO remained unreacted even at higher processing temperature without deteriorating the superconducting properties. The scanning electron microscope image shows that YBa_2HfO_(5.5) forms an electrical-network between grains. These observations suggest that the YBa_2HfO_(5.5) addition to the Y-123-compounds improve the electrical connection between superconducting grains and substantial improvements in the relative electrical transport properties of the composites. The variation of sintering temperature, density, critical transition temperature (T_c) and magnetic field dependence of critical current density (J_c) of YBa_2Cu_3O_(7-δ) having different proportions of YBa_2HfO_(5.5) in the matrix were also studied in detail. It is found that the addition of these elements considerably enhances the flux pinning strength of the system, and there is also an increase of critical temperature (T_c) and critical current density (J_c) up to an optimum value of 8.76 x10~4 A/cm~2for a concentration of 2 wt % addition of nano YBa_2HfO_(5.5). The enhancement of the critical current density in the YBa_2Cu_3O_(7-δ) - YBa_2HfO_(5.5) samples is attributed to the sustained formation of the insulating and non-reacting YBa_2HfO_(5.5) phase acting as pinning center. The addition of YBa_2HfO_(5.5) in YBa_2Cu_3O_(7-δ) bulk superconductor also enhances the pinning force density from 3 x10~6 N/m~3 to 69.6 x 10~6N/m~3 at 77 K.