Processing of Bi 2.1 Sr 1.8 Ca 1.1 Cu 2 O 8 source material for float-zone fiber growth
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L. V. Moulton and Z. Lu Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-2205 (Received 19 February 1991; accepted 24 June 1991)
Bi2.iSr 18 Cai.iCu 2 0 8 fibers having excellent superconducting properties can be grown by a laser-heated float zone process. In order to maintain stable growth conditions and thereby obtain fibers free of diameter fluctuations and voids, dense ceramic starting material containing only the 2212 phase is required. In this study various processing parameters, including calcining and sintering temperatures and times, grain size of the powders used, and pressing pressures were optimized to yield dense, chemically homogeneous starting material. It was found that under most conditions there was no increase in the density on sintering. Retrograde densification was the usual situation except at higher pressures and was found to depend on pressing pressure, calcination history, and sintering temperature. Cold-pressing at higher pressures (100 000 psi) yielded denser but chemically inhomogeneous material. Ceramic samples sintered for long times (>48 h) yielded source rods that produced instabilities during fiber growth, presumably due to preferential loss of mass during sintering.
I. INTRODUCTION A laser-heated miniaturized float zone technique has been shown to be a useful technique for growing superconducting B i - S r - C a - C u - 0 fibers.1'2 This technique produces grains aligned along the fiber axis and with the c-axis oriented normal to the growth direction. The C u - 0 planes therefore are ideally oriented for electrical conduction along the fiber. One of our primary goals has been the growth of long lengths of small, constant diameter (~30 /im) fibers of Bi 2 .iSr 18 Cai. 1 Cu2O 8 , at high growth rates. In order to prepare such fibers, starting material with compositional and dimensional uniformity is needed. Recent work by Ono 3 suggested that single phase 2212 ceramic samples can be prepared only from compositions containing bismuth in excess of the stoichiometric amount. This extra Bi presumably goes onto the alkaline earth sites replacing strontium or calcium [Bi2(Bi, Sr, Ca) 3 Cu 2 0 8 ]. The single phase region is also affected by the Ca/Sr ratio. Based on Ono's work, Moulton et al.4 were able to produce more uniform source material and thereby fibers with more uniform diameter and composition than those grown from the stoichiometric 2212 starting composition. When using small diameter source rods, as required for the growth of small diameter fibers, major growth instabilities can occur due to the presence of a nonuniform distribution of second phase particles and gaseous inclusions (porosity).4 This paper discusses the effect of various processing parameters on the composition, density, and micro2280
J. Mater. Res., Vol. 6, No. 11, Nov 1991
structure of Bi 2 .iSr 18 Cai.iCu 2 0 8 starting material to be used for float-zone fiber growth. The goal was to determine the optimum processing conditions (including calcining and sinterin
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