INFLUENCE OF THE METHOD OF PARTICLES ORIENTATION ON THE MICROWAVE PROPERTIES OF M-TYPE HEXAFERRITES

摘要

Hexagonal ferrites are distinguished from cubic ferrites because they exhibit strong magnetocrystalline anisotropy. The strength and direction of the anisotropy can be in the direction of the hexagonal axis (c-direction) or in the basal plane, depending on the addition of cation substituents. We have studied Ba(Co,Ti)_xFe_(12-2x)O_(19) with x=1.2 for which the "easy" direction of magnetization is in the basal plane. This characteristic results in high values of magnetic permeability, but to derive the most benefit, all grains of the ceramic must be aligned with the c axis of the hexagonal cell perpendicular to the surface of the sample. The powders were synthesized by calcining at 1150℃ for 3 hours, a mixture of Fe_2O_3, BaCO_3, TiO_2 and Co_3O_4. Iron content was adjusted to take into account the iron supply due to the wear of iron mill balls. X-ray diffraction and electron scanning microscopy showed the major phase to be M-type hexaferrite platelets 1 μm in size. Test articles were formed by slip casting or tape casting. Densification of the materials was obtained by air or oxygen sintering between temperatures of 1250℃ and 1270℃. The comparison between tape cast samples dried with and without magnetic field showed that an alignment field is required for a high degree of alignment. The permeability of slip cast materials cast in the presence of a magnetic field is comparable to those that were tape cast and dried in a magnetic field. Higher values of permeability and density were obtained for materials sintered at the highest temperature of 1270℃.