Sol-gel synthesis and properties of YBa2(Cu1-xMx)(4)O-y (M=Co, Ni) and effects of additional replacement of yttrium by calcium

In this work the influence of replacing Cu by Ni or Co on the structural an d superconducting properties of YBa2Cu4O8 is studied by means of XRD, TGA, SEM, EDX and resistivity measurements. The samples are prepared by the acet ate-tartrate sol-gel method, which has proved to be very appropriate for th e synthesis of Y-124 at 1 arm with even very small amounts of dopants. It i s demonstrated that the transition temperature drastically decreases upon C o or Ni substitution, and that superconductivity is lost at 3% Ni and 6% Co doping. XRD measurements show that Ni and Co produce different structural effects to the orthorhombic YBa2Cu4O8 phase, possibly because they are intr oduced in different copper positions. Interesting results are obtained by a dditional substitution of Y by Ca in the Co- and Ni-doped samples: It is sh own that the superconducting properties are enhanced in the samples with le ss than 3% Ni or 6% Co, and that superconductivity is even recovered in sam ples with more than 3% Ni or 6% Co. (C) 1999 Elsevier Science Ltd. All righ ts reserved.