Qingbin Hao

Optimization of Bi-2212 high temperature superconductors by potassium substitution

Polycrystalline bulks of Bi2Sr2Ca1−xKxCu2.0O8+δ (Bi-2212) with x = 0, 0.05, 0.10 and 0.15 were fabricated by the spark plasma sintering technique. The influences of K doping on the microstructures, electronic structures, as well as the related superconducting properties, were systematically investigated. The XRD analyses confirmed that K+ ions have successfully substituted into the matrix of Bi-2212, and lead to a systematical change of lattice parameters. Due to the change of thermodynamic properties, bulks with higher density, larger grain size and better texture structures were obtained after doping. Therefore, ac susceptibility measurement revealed the optimization of intergrain connections, which lead to the optimization of both self- and in-field critical current density, Jc of this system. The optimization of microstructures also caused the enhancement of surface pinning. Based on the enhancements of both intergrain connections and flux pinning properties, an obvious improvement of critical current density was obtained with the optimal doping content of K = 0.05. Meanwhile, Bi-2212 single filament tapes with K doping content of 0 and 0.05 were also fabricated by the powder-in-tube process. The XRD patterns also proved the successful doping of K ions in the Bi-2212 matrix. The critical current density Jc, measured by the transport method under the magnetic field from 0 to 20 T at 4.2 K, proved the effectiveness of K doping on the enhancement of flux pinning properties of Bi-2212.

Phase evolution study and optimization of the heat treatment process for high current capacity Bi-2223 tapes

Powder in tube process (PIT) was adopted for the fabrication of single filament Bi-2223 tapes, and a heat treatment process including the first heat treatment (HT1), intermediate rolling (IR), and second heat treatment (HT2) was performed. The phase evolution mechanism and microstructure changes during these heat treatment processes were systematically discussed. The influences of HT1 parameters on the phase evolution process of Bi-2223 tapes were discussed. With the optimized HT1 process, a proper Bi-2223 content of about 90% was achieved. HT2 process was also optimized by adding a post annealing process. An obvious increase of current capacity was obtained due to the enhancement of intergrain connections. Single filament Bi-2223 tapes with the critical current of Ic-90 A were fabricated with the optimized sintering process.

Optimization of intergrain connection in high-temperature superconductor Bi2Sr2CaCu2Ox*

A modified spark plasma sintering (SPS) technique was developed for the fabrication of Bi2Sr2CaCu2Ox (Bi-2212) superconducting bulks with better intergrain connections. The influences of the modified SPS process on the microstructures, intergrain connections, and related superconducting properties were systematically analyzed. The modified SPS process can not only increase the final density of the bulk samples but also enhance the texture structures. Clean grain boundaries were obtained instead of the intergrain amorphous layers. Therefore the intergranular properties were obviously improved. Due to the better intergrain connections and the stronger flux pinning properties, the critical current densities of the Bi-2212 bulks obtained via the modified SPS process were greatly increased. The obtained improvements imply the possibility for the modified SPS technique to be used for enhancing the superconducting properties of the Bi-2212 tapes.