Transmission electron microscopy study of barium hexaferrite formation from barium carbonate and hematite
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Transmission electron microscopy study of barium hexaferrite formation from barium carbonate and hematite Hanns P. Steier,a) Joaquin Requena, and Jose´ S. Moya Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid, Spain (Received 1 February 1999; accepted 24 May 1999)
The formation of barium hexaferrite from stoichiometric mixtures of BaCO3 and Fe2O3 has been investigated by means of differential thermal analysis and thermogravimetry, x-ray diffraction, and transmission electron microscopy–energy dispersive spectrometry. The first step, which implies decarbonatation and monoferrite formation, includes the formation of various intermediate compounds, which are formed at contact points between BaCO3 and Fe2O3 grains, and implies diffusion of both species. In the second step, barium hexaferrite is formed at interfaces between monoferrite and iron oxide mainly by diffusion of barium through the BF6 lattice into the hematite lattice. This exothermic reaction process leads to nonagglomerated pseudohexagonal platelets with an average particle size very close to the one of the starting powder mixture (∼1 m).
I. INTRODUCTION
Pure BaFe12O19 (BF6) is a material widely used as a permanent magnet.1,2 With small compositional modifications (i.e., Co, Ti), which reduce both the anisotropy constant K1 and the coercive field HC, barium hexaferrite is applied for magnetic information storage procedures, such as master tapes and magnetic cards.1,2 Recently, fine particles of BaFe12O19 have become most interesting as particulate perpendicular magnetic recording media because they offer many advantages such as high-density recording, reproducing, and oxidation and corrosion resistance.1 One of the main limitations of barium ferrite for its application as magneto-optical recording material has been its large uniaxial anisotropy, which can be overcome by ion substitution with Co2+ and Ti4+.3 The gyromagnetic properties of barium hexaferrite allow the possibility of applying this compound to microwave technologies such as radar absorption, filters, electromagnetic screening, microwave oven protection, and so forth. The operating frequency can be controlled in a broad range (5–70 GHz) by Ti/Co doping.2 The formation of barium hexaferrite by solid state reaction between barium carbonate and hematite has been investigated in many studies.4 –14 It is generally recognized that the overall reaction BaCO3 + 6Fe2O3 → BaFe12O19 + CO2 occurs in two main steps.15 First step (1) BaCO3 + Fe2O3 → BaFe2O4 + CO2
a)
Present address: Robert Bosch GmbH, Stuttgart, Germany J. Mater. Res., Vol. 14, No. 9, Sep 1999
http://journals.cambridge.org
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This first step takes place rapidly4 and starts between 600 and 750 °C, depending on the reactivity of the starting materials and homogenization or milling of the mixture.9,15 Decarbonation and formation of BaFe2O4 occur simultaneously, but this does not necessarily mean that this phase is formed in a single step. Inf
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