JOURNAL OF MATERIALS SCIENCE LETTERS 19 2 0 0 0 1063 – 1065 Growth of ultrafine whiskers from Bi-Sr-Ca-Cu-O microcrystals K. A. CHAUDHARY Department of Physics, Islamia University Bahawalpur, 63120-Pakistan A. MAQSOOD Department of Physics, Quaid-I-Azam University, Islamabad, Pakistan Generally high-T c ceramic superconductors are con- sidered to be rigid and brittle, thus posing serious diffi- culties and problems for wire applications. Many suc- cessful attempts have been made to fabricate flexible fibers, and tapes from the molten Bi-system precursors [1, 2]. Like normal bulk system, these flexible glassy fibers, after sintering become superconducting but these are polycrystalline in nature [3]. The fibers and wires thus obtained consist on large number of polycrystalline grains oriented in random directions. For such a sys- tem, during current flow the charge carriers encounter large number of grain boundaries. The grain boundaries are the main carrier scattering agents which reduce the critical current density, J c , in the bulk of the super- conductor. The intragrain current density is 10 3 times larger than the intergrain current density [4] thus con- firming that grain boundaries cause carrier loss and is the largest interfering factor in reducing the J c value. The whiskers being filamentary single crystals have no grain boundaries hence show high J c values. Since the discovery of Bi-Sr-Ca-Cu-O superconduct- ing whiskers growing from the sintered pellets and pow- der in 1988 [5], a lot of work has been reported on the growth and properties of these whiskers [6, 7]. Because of its application potential the whisker growth from Bi-based sintered pellets as well as from sintered glassy sheets have been a topic of paramount inter- est during the last decade. Remarkable superconduct- ing and elastic properties have been demonstrated by Matsubara et al., for these whiskers [8]. Previously all these whiskers had been grown from noncrystalline or glassy precursors. Herein we report the growth of ultra fine whiskers from Bi-Sr-Ca-Cu-O microcrystals. Al- though the length of the whiskers described here is small and may be of little practical importance in its present form yet it can be important in understanding and elucidating the whisker growth mechanism. The whiskers were grown by the scheme briefly de- scribed below: Reagent grade Bi 2 O 3 , SrCO 3 , CaCO 3 and CuO powders were mixed to give composition; Bi : Sr : Ca : Cu = 2.1 : 2 : 2.2 : 4. The powders were well mixed and melted in a muffle furnace at 1100 ◦ C in an alumina crucible for 30 min. The melt was then poured on a smooth stainless steel plate and quickly pressed by another steel plate to give about 0.5 mm thick black colored glassy sheets. The sheets were crushed in an agate mortal and pestle to give fine glassy pow- der. The powder was then sintered at 840 ◦ C for 72 h. This special scheme was adopted as it gives pore free and high density precursor material. The only drawback being that it is oxygen deficient and require long sinter- ing times, sometimes sintering in oxygen atmosphere is necessary. The powder containing microcrystals of varying shapes and sizes was thus obtained. The micro crystals with sizes 100 μm and above were separated from the powder with the help of a stainless steel mesh. The X-ray diffraction (XRD) measurements (Fig. 1) indicated that the microcrystalline powder predomi- nantly contained 2212 phase of Bi-Sr-Ca-Cu-O system with minute quantities of 2223 phase. The chemical composition ratio of a typical microcrystal obtained by EPMA is Bi : Sr : Ca : Cu = 2.0 : 2.07 : 0.97 : 2.1. These well characterized microcrystals were then used as pre- cursors for the growth of ultra fine whiskers descri- bed herein. The microcrystalline powder was dispersed onto an alumina substrate and given a heat treatment at 823 ◦ C for 24 h in air inside a tubular furnace. The heating rate was precisely controlled and kept at 2 ◦ C/min. Extremely fine and needle-like straight whiskers were observed shooting from irregular shaped Bi-Sr-Ca-Cu-O microcrystals with nominal dimen- sions as length = 700–1000 μm, thickness = 0.2–1 μm and width = 2–4 μm. The length to width or length to thickness ratio remained uniform within an accu- racy of 1% over the entire length of the whisker. It is quite contrary to the conventional whiskers, which are thicker at the base and thinner near the open end. The whiskers reported in this study retained uniformity from base at the microcrystal to the open end. Elec- tron photomicrograph of such a whisker along with the Figure 1 X-ray diffraction pattern of microcrystalline powder, used as a starting material in this study. Unmarked peaks correspond to 2212 phase while peaks with “O” correspond to 2223 phase. 0261–8028 C  2000 Kluwer Academic Publishers 1063