CERIA-DOPED ZIRCONIA GRAPHITE AS POSSIBLE REFRACTORY FOR TUNDISH NOZZLES IN STEELMAKING

In zirconia-graphite refractories, calcia-stabilized zirconia is sensitive to silica coming from oxidized additives (Si, SiC). Accordingly, ceria is considered here as an alternative cheap stabilizer. Ceria-doped zirconia has been prepared and the monoclinic/tetragonal distribution has been analysed. At 1500-degrees-C in oxygen, the solution of ceria in zirconia is sluggish; to obtain a pure tetragonal phase, a 16 mol% ceria concentration (equal to the solubility limit) is necessary. Such samples are stable below 1050-degrees-C, in spite of previous claims to the contrary. At 1300-degrees-C, silica does not destabilize these samples, even in reducing conditions. However, at low oxygen pressure, the pyrochlore Ce2Zr2O7 isformed, and the remaining zirconia transforms into the monoclinic modification. Nevertheless, below 1175-degrees-C, a metastable, purely tetragonal modification is obtained, because the activation energy needed for the formation of Ce2Zr2O7 is too high. In any case, the reduction does not destroy the polycrystalline texture. This fact, combined with the resistance to silica, favours the use of these materials in steelmaking.