Electrical field freezing effect on laser floating zone (LFZ)-grown Bi_2Sr_2Ca_2Cu_4O_(11) superconducting fibres

摘要

The electrical assisted laser floating zone (EALFZ) solidification process makes the tailoring of fibre microstructures possible. The application of a dc electrical current of approx 8 A cm~(-2) during the solidification process of Bi_2Sr_2Ca_2Cu_4O_(11) nominal composition fibres strongly modified phase development, crystal shapes and effective distribution coefficients. Growth conditions with the solidification interface positively polarized deviate the system from metastability, leading to the development of the equilibrium cuprate (Sr_xCa_(1-x))_(14)Cu_(24)O_(41) (14/24) as primary phase dendrites. Compared to the morphology of the Sr_xCa_(1-x)CuO_2 (1/1) primary crystals of the conventional LFZ process, the 14/24 crystals are aligned higher along the fibre axis, with half the thickness and twice the extension. One of the major effects of EALFZ is the control of the effective distribution coefficients, kappa. At equal values of the fibre pulling rate, R, the copper partition between the liquid and the solid is the most affected, the kappa_(Cu) increasing from 1 to 1.22 due to the ionic drift from the zone melt to the negative polarized feed rod. Bismuth and calcium effective distribution coefficients present the lowest values (kappa_(Bi) = 0.69 and kappa_(Ca)=1.13) in these conditions, according to the field-modified BPS theory. When the reverse current is applied, the dendritic morphology disappears and a globular structure of completely new phases develops.