SOLID-STATE SYNTHESIS OF YBA2CU3O7-X - MECHANISMS OF BACUO2 FORMATION

Different chemical reactions leading to BaCuO2, a key precursor of YBa2Cu3O7-x, were studied on powder mixtures using isothermal kinetic and thermogravimetric measurements, X-ray diffraction and optical and scanning electron microscopy observations. BaCuO2 formation from BaCO3 + CuO at 860-degrees-C follows the same simple (diffusion driven) mechanism previously found using pellet/single crystal couples. At higher temperatures, the reaction is more complex and complete conversion is obtained only with several thermal cycles under pure oxygen. The reaction 2BaO2 + 2CuO --> 2BaCuO2 + O2 is faster, starts at a lower temperature and does not require thermal cycles for complete conversion. The combustion synthesis BaO2 + Cu --> BaCuO2 produces a highly porous and highly reactive material with exact stoichiometry. The combustion is characterized by a low propagation rate (approximately 1 mm s-1) and a very low temperature (approximately 500-degrees-C).