Microstructural features and superconducting properties of Mn-doped Indu Dhingra Division of Materials, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-110012, India S.C. Kashyap Department of Physics, I.I.T, Hauz Khas, New Delhi-110012, India B.K. Das Division of Materials, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-110012, India (Received 1 March 1994; accepted 28 July 1994) Small additions of MnO2 up to 15 at. % in YBa2Cu307_5 in place of Cu have been carried out to study their influence on the superconducting properties. It has been found that transition temperature and microstructure of Mn-doped samples are highly sensitive to the sintering temperature and Mn content. The normal state resistance increases with Mn content. Microstructure shows plate-like grain growth for low Mn contents (up to x = 0.05) at higher sintering temperature. XRD and microstructure reveals the presence of additional Y2 BaCu0 5 and Ba 3Mn2O8 phases with an increase in Mn content. Also, reduction in the diamagnetic signal and broadening of transition is observed beyond certain Mn content. A detailed discussion of microstructure, XRD, normalized resistance, and magnetic properties has been carried out in correlation with sintering temperature and Mn content. I. INTRODUCTION Several intensive investigations have been carried out to study the effect of substituents, in particular the 3-d metals,1"6 on the properties of high Tc ceramic super- conducting systems. All these studies mainly emphasize the changes induced by these dopants on the microstruc- tural features and the transport properties. The prime interest of the above efforts has been to improve the transport properties through densification, better inter- grain connectivity, and aligned grain growth.7-8 Some of the substituents (like silver9"11) have been found to help in partially achieving the above objectives, with little or no deleterious effects. Of all the 3-d transition metals1"3 substituted for Cu in YBCO studied, Mn is found to be the best,2 as in this case, up to a certain percentage, there is no marked change in the Tc. Compared to YBCO substituted by Fe, Co, Ni, and Zn, the work on the substitution by Mn is rare in the literature. There is an indication only that superconductivity of Mn substitution in YBCO is a function of sintering temperature.12-13 Furthermore, no results on transport measurements in Mn-doped YBCO have been reported so far. Here we report the detailed investigation of Mn substitution in YBa2Cu3O7_s (YBCO) in place of Cu in terms of microstructural, electrical, and magnetic properties in correlation with the sintering temperature and Mn concentration. II. EXPERIMENTAL PROCEDURES The samples of YBa2(Cui_xMn^)3O7_5 were synthe- sized by the solid state reaction method for different values of x (0.0 =s x *s 0.15). To study the effect of sintering temperature on superconducting properties of Mn-doped YBCO, these samples were finally sintered at four different temperatures (TS) of 930, 950, 970, and 1000 °C (referred to as SI, S2, S3, and S4 series, respectively) for 15 h in oxygen. The characterization of the samples was carried out for microstructure (SEM), phase identification (XRD), normalized resistance (dc four probe method), and ac susceptibility (mutual in- duction method at 73 Hz and rms field of 2 Oe). For resistance measurements, the current density used was in the range of 25-50 mA/cm2 , and the temperature was measured by using a calibrated PRT (Pt-100) sensor. The Tc reported here corresponds to zero resistance state in the R-T method or the onset of diamagnetic transition in ac susceptibility method. The critical current density of the samples was measured at 77 K under zero magnetic field using the 1 fjuV/cm criterion. J. Mater. Res., Vol. 9, No. 11, Nov 1994 © 1994 Materials Research Society 2771