A modified citrate gel route for the synthesis of phase pure Bi 2 Sr 2 CaCu 2 O 8 superconductor

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A simple method for the bulk synthesis of 80 K bismuth-based superconducting phase (2212) is described. It employs a modified citrate gel route giving rise to an auto-ignited combustion process. The precursor ash obtained after combustion is further calcined at 800 °C to produce the desired superconducting powder with excellent homogeneity and fineness, leading to good sinterability and fairly sharp transition temperature. For a particular citrate-nitrate ratio, the nature of combustion of the gels containing two different bases has been compared. The thermal decomposition characteristics of the gels and the plausible mechanism of formation of the superconducting phase are also described.

I. INTRODUCTION Synthesis of pure high Tc bismuth-based superconductors by chemical methods has attracted a great deal of attention and interest during the last several years. Many preparation techniques such as sol-gel process,1"4 co-precipitation method,5'6 polymeric precursor decomposition,7 precursor matrix method,8 and citrate gel process9"13 have been tried for the preparation of these oxide powders with varying degrees of success. Particular attention has been paid to homogeneous composition, monophase structure, uniform grain size, and good control of stoichiometry at relatively low temperatures. In the case of bismuth superconductors, it has always been difficult to prepare single-phase powders of either 2212 or 2223 phase. In one of our previous publications, a novel methodology for the preparation of bismuth 2223 superconductors using a modified citrate gel process has been described.14 Using this process, zero resistivity at a temperature of 125 K is observed for the first time in bismuth superconductors doped with lead. The method was also successfully used for the preparation of YBC superconductor.15 In this paper, we report the preparation of phase pure Bi2Sr2CaCu2O8 (2212) superconductor by a similar auto-ignited combustion process of citrate-nitrate gel. The process involves a low temperature initiated selfpropagating ignition process which is found to be very simple, rapid, and safe for the preparation of very fine and homogeneous powders of various complex oxides. II. EXPERIMENTAL PROCEDURE A. Preparation of the powder All chemicals used were of analytical reagent grade. Stock solutions of metallic nitrates were mixed in the mole ratio of Bi: Sr: Ca: Cu = 2 : 2 : 1 : 2 . An aqueous J. Mater. Res., Vol. 9, No. 6, Jun 1994 http://journals.cambridge.org

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solution of citric acid was also added to the mixed nitrate solution with stirring. The amount of citric acid used was that necessary to bind all the metal ions. Here, the ratio of citrate to nitrate was fixed at 0.45. Two different sets of experiments were carried out using two different bases for controlling the ultimate pH of the solution. This was done to investigate the effect of these bases on the nature of combustion of the redox mixture of citrate and nitrate. "Gel A" was prepared by adjusting the pH of the mixed solution to 6.0—6.

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