High Levitation Force and Trapped Field of Large Grain YBCO at 77K by Ga Doping
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High Levitation Force and Trapped Field of Large Grain YBCO at 77K by Ga Doping Yu X.Zhou*#, Wai Lo*, Tong B.Tang# and Kamel Salama* *Texas Center for Superconductivity and Department of Mechanical Engineering, University of Houston, Houston, TX 77204, USA # Department of Physics, HongKong Baptist University, Kowloon, HongKong ABSTRACT Levitation force and magnetic field trapping are the most important properties of YBCO large grain materials. They are dependent on the pinning force exerting on the magnetic flux lines by various crystal defects in YBCO. In this paper, Ga-doped bulk YBCO samples were fabricated by SmBCO seeded melt-growth process. The levitation force and trapped field were enhanced by the introduce of nanometer scale week superconducting regions which are formed by the Ga partially substituting for Cu on the Cu-O chain. INTRODUCTION Seeded melt-growth (SMG) process is known to be the most effective way to fabricate large grain high-temperature superconducting YBCO samples [1-11], which consist of superconducting YBa2Cu3O7 (123) and non-superconducting Y2BaCuO5 (211) inclusions. These large grains have significant potential for a variety of engineering applications such as magnetic bearings, fault current limiters and flywheel energy storage systems [12-15] by making use of the trapped magnetic fields and repulsive forces between the YBCO and permanent magnets. The magnitudes of both the trapped field and repulsive force are determined by the orientation, dimension as well as the critical current density (Jc) of grains [16]. To control the growth orientation and maximise the size of the grains, SmBCO or NdBCO seeds were placed on the top of YBCO either by cold or hot seeding during melt texturing. The Jc of the grains is enhanced by the introduction of crystal defects which tend to suppress local superconductivity in YBCO. These defects include oxygen vacancies, (110) twins, dislocations, 211 inclusions and columnar defects generated by high energy irradiation. The flux pinning force would be most effective when the size of the regions with suppressed superconductivity is close to the coherence length of YBCO (i.e. about 1.6 nm on ab-plane at 0 K [17]). In this case, it is energetically more favourable for the non-superconducting cores of the flux lines to overlap with the weak superconducting region, and hence gives rise to the force that prevents the flux lines from migrating. One of the possible methods of suppressing superconductivity on that length scale is the incorporation of cations onto the CuO2 planes or CuO chains of 123, which will affect the carrier concentration density. This has been achieved by doping YBCO using Zn [18-19]. In this case Zn resides preferentially on CuO2 planes, which suppress local superconductivity. However, strongly decreasing critical temperature Tc has been observed in YBCO powders and single crystals doped with Zn when the doping level exceeds 1% [20,21]. This paper presents the establishing of optimum processing conditions of Ga-YBCO large grains and the optimum
