Preparation of Grain-oriented Ba₂(Zn, Fe²⁺)₂Fe₁₂O₂₂ Ceramics by Unidirectional Solidification of Their Melts

Abstract

A batch mixture of the nominal composition 2BaO⋅1.4ZnO⋅0.6FeO⋅6Fe₂O₃+0.3(2BaO⋅4ZnO⋅4B₂O₃) (Table 1) was melted in a Pt 10 Rh crucible at 1470°C for 2h. During melting, the melt was stirred with a Pt spatula every 30min. The crucible was then taken out of thefurnace and allowed to cool naturally. The polycrstalline ingot obtained in the crucible was remelted, except its lowest part 5mm thick, in the temperature gradient furnace by keeping the temperatures of its top and bottom at 1450°C and 1300°C, respectively (Fig. 1). After 10h, the both temperatures were lowered at the same rate of 5°C/h to solidify the melt upwards at a rate of 1mm/h. The lowest part of the ingot left unmolten acted as seed crystals. The thermal gradient in the melt was kept at 50°C/cm during solidification. The resultant ingot was composed of many Ba₂(Zn, Fe²⁺)₂Fe₁₂O₂₂ columnar crystals 1-3mm in diameter elongated with its ‹100› direction parallel to its solidification direction (Figs. 2 and 4, Table 2). A thin glassy film formed at the grain boundaries of the columnar crystals relieved effectively by its viscous flow the thermal stresses caused by a large difference in thermal expansion between a- and c-axis, thus suppressing the formation of microcracks in the ingot (Figs. 5 and 6). The apparent porosity of the ingot achieved was 0.3% (Table 3). The permeability of the ingot in its solidification direction was higher than that of a sintered ceramic of the same composition, but lower than that of Ba₂Zn_1.98Fe_0.02Fe₁₂O₂₂ single crystal in its c-plane (ref. 7). Attempt to prepare the ingots of the compositions 2BaO⋅2ZnO⋅6Fe₂O₃ and 2BaO⋅1.4ZnO⋅0.6FeO⋅6Fe₂O₃ by the same method as described above proved abortive, because many microcracks were formed in their resultant ingots (Tables 2 and 3).