Supercritical antisolvent precipitation: A new technique for preparing submicronic yttrium powders to improve YBCO super
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Supercritical antisolvent precipitation: A new technique for preparing submicronic yttrium powders to improve YBCO superconductors E. Reverchon, C. Celano, and G. Della Porta Dipartimento di Ingegneria Chimica e Alimentare, Universit`a di Salerno, Via Ponte Don Melillo, 84084 Fisciano (SA), Italy
A. Di Trolio and S. Pace INFM e Dipartimento di Fisica, Universit`a di Salerno, Via S. Allende, 84081 Baronissi (SA), Italy (Received 24 February 1997; accepted 8 May 1997)
The solvent, supercritical antisolvent technique (SAS) has been used to produce submicronic particles of yttrium acetate for the synthesis of YBCO superconductors. For this purpose, in a continuous SAS apparatus dimethylsulfoxide (DMSO) as yttrium acetate solvent and supercritical carbon dioxide as antisolvent have been adopted. Experiments have been performed in the pressure range between 70 and 160 bar and for temperatures between 40 and 70 ±C. Different concentrations of yttrium acetate in DMSO have also been tested. Various morphologies of yttrium acetate particles have been obtained, having mean particle diameters from 0.1 to 7 mm. At 40 ±C and pressures larger than 120 bar, submicronic spherical particles of yttrium acetate of about 0.1 mm diameter and with a narrow particle size distribution have been achieved.
I. INTRODUCTION
Superconducting materials show non-negligible energy losses as soon as the critical current density is exceeded. This critical value decreases as the temperature or the magnetic field increases. For this reason, one of the challenges in producing High Temperature Superconductors (HTS) for electrotechnical applications is to overcome the effect of magnetic fields (B . 4 T) which are present in the ordinary operation. Indeed, a sufficiently strong magnetic field penetrates the superconductor in the form of bundles of flux lines, so that, in the presence of an electric current, the flux lines move and cause energy losses. Therefore, to control this problem, pinning centers are necessary to avoid the flux motion. In the HTS one possibility is represented by the precipitation of a secondary phase that introduces fine nonsuperconducting microstructural defects. These structural inhomogeneities are expected to pin the flux lines in their respective positions and limit their disruptive tendency. In the case of yttrium, barium, copper (YBCO) HTS, YBa2 Cu3 O72d (the “123” phase) can contain dispersed inclusions of unconnected Y2 BaCuO5 particles (the “211” phase). It has been suggested that a reduced 211 particle size can directly enhance the flux-pinning properties of YBCO.1 Thus, the control of the 211 particle size in the precursor powder can be important to optimize the transport properties of this ceramic material. Several techniques have been proposed in the literature to process the YBCO precursor powders starting 284
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J. Mater. Res., Vol. 13, No. 2, Feb 1998
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from acetates, nitrates, and oxalates. Among these are plasma sp
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