Critical Current Capacity Of High Temperature Superconductor example essay topic

Large scale applications of high transition temperature, Tc superconductor require critical current density (Jc) of at least 105 A / cm 2 at liquid nitrogen temperature. Due to their low flux pinning energy, the critical current decreases rapidly with increasing temperature and applied magnetic field. Among the cuprate high Tc materials, the Bi (Pb) -Sr-Ca-Cu-O is considered to be the most promising superconducting materials for application due their high Tc and Jc. The bismuth based cuprate superconductor is made up of three phases. The general formula of Bi 2 Sr 2 Can-1 Cu nO 2 n+4+f^O with n = 1, 2 and 3 have Tc values of 20, 95 and 110 K, respectively.

Extensive research revealed that Jc in polycrystalline Bi-2212 is limited by the weak links and weak flux pinning [1]. Although partial melt processing can partially solve the weak link problem in Bi-2212, the limitation from flux pinning which determines the intrinsic critical current density remains the most serious challenge for the application of Bi-2212 at elevated temperatures [2] Significant progress has been achieved in improving the critical current capacity of high temperature superconductor. The most interesting ideas were: i. Amorphous tracks about 10 nm diameter and 1-10 f'Ym length created by heavy ion bombardment were shown to increase pinning substantially in single crystal of Y 123 and Bi-2212 [3]. ii. Proton irradiation of nuclei created amorphous tracks in Bi 2 Sr 2 Ca 2 Cu 2 Ox similar to heavy ion tracks although of shorter range.. Neutron irradiation of H gBa 2 C aCu 2 O 6+x can increase the magnetization hysteresis and to lift the irreversibility line. iv.

Single crystal like Ya 2 Cu 3 O 7 (Y 123) with Y 211 precipitates of 0.1 f'Ym diameter with improved magnetic irreversibility behavior. Although these technique is scientifically interesting, there are several severe practical difficulties. In the first case, an industrial scale up to bulk application is nearly impossible. In the second case, extreme requirements in term of available accelerator energies are major obstacle. For the case of neutron irradiation the long half-life of Ag isotope makes it impractical due to safety requirements.

An alterna tif approach is by inclusion of nanoparticles or atomic constituents into precursor powder followed by solid state reactions. By this technique a number of advantages can be achieved. Firstly, industrial scale up would be possible. Secondly, the anisotropy of magnetic properties could be eliminated. and high critical current could be achieved for a wide range of applications. Flux line network and magnetic texture can interact effectively if their characteristic scales have the same order of magnitude. The characteristic scales for flux line network is the coherence length f'e, which varies in range of 1 to 100 nm.

Another characteristic distance is London penetration depth f"U which in the range of 60 to 1000 nm. In a magnetic system with a characteristic length f'e L f"U, a strong interaction between flux line network and magnetic subsystem can be expected [5]. As the coherence length in high temperature superconductor is as short as 1 nm, introducing nanoparticles can enhance Jc. The addition of nano size MgO was found to increase the critical current when the maximum heat treatment temperature was 910 oC [4]. In another work no significant enhancement of Jc was evidenced after nano metric SnO 2 powder was added into YB CO [6]. In this work magnetic nanoparticle maghemite (fx-Fe 2 O 3) with needle-like shape was used to act as artificial defect in bulk Bi-2223 superconductor.

EXPERIMENTAL PROCEDURE Samples with nominal composition (Bi, Pb) Sr 2 Ca 2 Cu 3 O 10 were prepared from analytical grade powders of Bi 2 O 3, PbO, SrO, CaO and CuO of at least 99.9% purity. These powders were grinded using mortar and pestle. These samples were calcined at 800 oC for 24 hours. Further calcination was done at 830 oC for 24 hours after grinding. Finally the powders of this nominal composition were pressed into disk shape with 1.3 cm diameter and 2 mm thickness before heated for 150 hours at 850 oC.

After the sintering process, the pellets were grinded for 1 hour followed by addition of fx-Fe 2 O 3 with (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10- (fx-Fe 2 O 3) x with x = 0.00, 0.01, 0.03, 0.04, 0.05 and 0.1. The powder were mixed in an agate mortar, pressed into pellets and calcined at 840 oC for 48 hours. The standard four point probe method was use to measure the transition temperature. Transport critical current density, Jc was determined from the I-V characteristic at 77 K using 1 f'YV / cm criterion. A Philips XL-30 scanning electron microscope was used to observe the microstructure of the samples.