Synthesis of superconductors from soluble metal oxo alkoxide precursors
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Synthesis of superconductors from soluble metal oxo alkoxide precursors M.W. Rupich,a) Y.P. Liu, J. Ibechem, and J.P. Hachey EIC Laboratories, Inc., Norwood, Massachusetts 02062 (Received 17 August 1992; accepted 8 March 1993)
Soluble metal oxo alkoxide precursors are useful for the production of homogeneous, micron-sized YBa 2Cu3O 7-^ powders and the fabrication of superconducting YBa2Cu3O7_^ films on large area and nonplanar substrates. This paper describes the synthesis and chemistry of a series of soluble metal oxo alkoxide precursors for the YBa2Cu3O7_x superconducting material and the effect of the thermal processing conditions on the formation of the superconducting phase. We have found that the direct conversion of the metal oxo alkoxide precursors to an amorphous or crystalline YBa2 Cu 3 0 6 .5 phase without the intermediate formation of the individual metal oxides or carbonates allows the formation of the superconducting phase at temperatures as low as 720 °C. The prevention of the intermediate formation of the metal oxide and carbonate phases is achieved by the controlled thermolysis of the metal oxo alkoxide precursors in an inert atmosphere. The precursors have potential use in the low-temperature deposition of films on large area or nonplanar surfaces and for the production of homogeneous powders.
I. INTRODUCTION There is currently a significant interest in the development of chemical approaches for the synthesis of the high-temperature superconductors for the preparation of homogeneous powders and the inexpensive deposition of large area and nonplanar films. Casting of films from soluble precursors and the preparation for powders by freeze-drying or spray pyrolysis of aerosols requires the availability of soluble precursors that undergo a facile conversion to the superconducting phase. Numerous solution techniques utilizing metal nitrates, acetates, citrates, alkoxides, or metal organics have been described for the preparation of the hightemperature superconducting materials.1"9 The primary goal in all the solution processes is the preparation of a homogeneous solution from which a solid precursor can be isolated with an atomic level homogeneity. In general, the precursors are pyrolyzed in an oxidizing atmosphere to a mixture of the individual metal oxides and carbonates and then converted, by solid-state reactions at 850 to 950 °C, to the superconducting phase. However, these approaches have been only marginally successful due to the limited availability of soluble copper precursors, difficulties in controlling the homogeneity of precipitated precursors, the inability to control the gel or precursor structure, phase segregation during the pyrolysis, and the intermediate formation of BaCC>3.
a
'Current address: American Superconductor Corporation, Watertown, Massachusetts 02172. J. Mater. Res., Vol. 8, No. 7, Jul 1993
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In order to yield superconducting powders or films after thermal processing, the precursors must be homogeneous and undergo
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