Benefits of mild wet milling of the intermediates for the synthesis of phase-pure Z-type hexaferrite
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Taisuke Masuko, Chie Ando, and Hiroshi Kishi Materials Research and Development Division, General Research and Development Laboratories, Taiyo Yuden Co., Ltd., Gunma 370-3347, Japan (Received 15 February 2005; accepted 9 May 2005)
A comparative study on the wet and dry milling of the intermediates for the Z-type hexagonal ferrite (Ba3Co2Fe24O41, Z phase) was performed. Phase-pure Z phase was synthesized by wet milling the intermediates comprising M and Y-type hexaferrites. The intermediates were obtained by calcining the stoichiometric powder mixture at 1080 °C. Subsequent wet milling by a planetary mill for 1 h increased the crystallization rate of Z phase upon subsequent heating at 1230 °C. In contrast, dry milling the intermediates resulted in the severe surface amorphization and led to heterogeneous crystalline states. The observed favorable effect of wet milling the intermediates was explained by the particle size reduction for the decrease of diffusion distance while preserving the basic layer units common to the related hexaferrites.
Z-type hexagonal barium ferrite (Z phase) is a promising candidate for inductor and electromagnetic wave absorber extending to the gigahertz region because of its resonance frequency and permeability are higher than those of spinel ferrites.1 However, its structure is far more complicated than those of the spinel ferrites. From the crystallographic point of view, Z-type ferrite is associated with relatively complex structure within the hexagonal family with its closest relatives such as M(BaFe12O19; M phase), Y- (Ba2Co2Fe12O22; Y phase), and W- (BaCo2Fe16O27; W phase) type ferrites. We may regard the Z phase as a sum of M and Y phases with its crystalline structure composed of 22 close-packed layers stacked along the hexagonal c axis.2 There are several reports on the synthesis of the unsubstituted pure Z phase. Most of them were started from chemically derived citrate precursor,2 stearic acid solgel,3 or aqueous inorganic sol-gel,4 with self-propagating methods, as well.5 Crystallization of the Z phase from chemically derived mixtures usually occurs around 1150–1250 °C. In a normal solid-state process, Z phase is synthesized in 2 steps. Raw mixtures are initially calcined at the temperature range between 980 and 1180 °C, ground and again sintered at about 1230–1300 °C.6–8 Between these a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0265 J. Mater. Res., Vol. 20, No. 8, Aug 2005
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two calcining steps, some samples were milled down to 10 m size.7 Milling can be performed in either a dry or wet state. In a dry state, mechanochemical reactions are often expected. In a wet state, in contrast, more homogeneous and fine particles can be obtained, increasing the specific surface area hence number of nucleation sites available. Simultaneously decreasing the diffusion path to the final phase upon subsequent heating is reasonably expected from the particle size reduction. All these c
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