Physiea C 185-189 (1991) 811-812 North-Holland MECHANISM OF THE Tc ENHANCEMENt IN Bi2Sr2CaCu2Os+y BY SODIUM DOPING Shi-Xue DOU, Wei-Min WU, Hua-Kun LIU, and Charles C. SORRELL School of Materials Science and Engineering, University of New South Wales, P.O. Box I, Kensington, NSW 2033, Australia Superconductivity in Na-doped BifirzCaCuzOs.y has been investigated and a T~ of 94 K and T, of 90 K have been achieved through the use of a melting process. A careful X-ray diffraction study on the Na-doped samples revealed that, with increasing Na content, the a and c axes increased, the b axis decreased, and the unit cell volume increased, indicating that Na was introduced into the lattice. This was further confirmed by EDS and chemical analyses. Substitution of Na for divalent atoms will reduce the oxygen content due to the requirement of charge balance, giving tL~ to an effect similar to that from quenching or during heat treatment under low oxygen partial pressures, either of which is necessary to achieve a T~ of 90 K in undoped samples. Although a transition temperature of 90 K for BieSrzCaCu2Os÷y (2212) has been achieved through quenching samples from high temperatures or heat treating under low oxygen partial pressures (1), the T~ for 2212 is often found in the range 70 - 85 K (2,3). Kawai and co-workers (4) have reported a T~ greater than 90 K for 2212 through doping with Li. The present authors found that a T¢ of 92 K could be achieved in Li-, Na-, and K-doped 2212 by using a melt-processing technique (5). Lattice parameter measurements suggest that Na substitution and the enhancement in the T, are related. In the present work, experimental results in relation to the mechanism for the T¢ enhancement are presented. Mixtures of Bi203, SrCO3, CaCO3, CuO, and NazCO 3 were added together according to the formula Bi~.aSrl.gCal.osCu2j~_~Na~Os.y,where x = 0 - 0.7, ground with a mortar and pestle, calcined at 720°C for 10 h in air, reground, and calcined at 740~C for 10 h in air. The calcined powders were then pressed into peiiets and sintered at 760°C for 10 h in air. The pellets were loaded in silver boats and melt-processed at 875° - 890~C, depending on the Na content of the composition, for 5 minutes. They were then furnace-cooled to 850°C, soaked for 1 hour, and cooled to room temperature at 60°C per hour. Charaeterisation consisted of electrical resistivity 92.00 ,,.-.,,88.00 LLI 84.00 rY 80.00 a_ L~ 7600 72.00 • 0.40 0.80 1.20 t .60 × FIGURE 1 To versus X in Na-doped Bi-Sr-Ca-Cu-O system measurements by the standard four-p:~'~ d.c. technique, a.e. magnetic susceptibility measurements by a mutual inductance method, scanning electron microscopy (SEM), dispersive spectroscopy (EDS), and X-ray diffraction. Figure 1 shows the zero resistance temperature (To) versus Na content (x) for the No-doped samples. The To reached a maximum ef 91 K at x = 0.6. Figure 2 shows that the T~, as determined by a.c. susceptibility measurements, is in good agreement with the resistivity measurements. EDS analyses of Na-doped s;mgle 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.