Hydrothermal synthesis and sintering of nickel and manganese-zinc ferrites
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Hydrothermal synthesis and sintering of nickel and manganese-zinc ferrites Anderson Diasa) and Vicente Tadeu Lopes Buono Departamento de Engenharia Metal´urgica e de Materiais, Escola de Engenharia-UFMG, Rua Esp´ırito Santo, 35/sala 206, 30160-030, Belo Horizonte, MG, Brazil (Received 16 December 1996; accepted 17 March 1997)
The influence of the starting materials on the crystalline phases observed after hydrothermal synthesis of nickel and manganese-zinc ferrites was investigated. The combination of sulfates and sodium hydroxide showed the best results for the conditions studied. The morphological parameters of MnZn ferrites produced at different hydrothermal conditions (110–190 ±C, 4–30 h) were analyzed. Changes in lattice parameter, particle size, density, size and total volume of pores, and in the surface area of the particles were analyzed as a function of temperature and processing time. The sintering process was employed in order to verify the reactivity of the hydrothermal powders at controlled atmosphere. High density and surface homogeneous ceramic bodies were obtained, without zinc volatilization. Lattice parameter variations were associated with changes in the cations distribution of the spinel during sintering.
I. INTRODUCTION
Ferrimagnetic ceramic materials based on the nickeland manganese-zinc ferrites are commercially important electronic materials.1 The conventional way of synthesizing these materials is through the solid-state reaction of a mixture of oxides, carbonates, and hydroxides of Ni, Mn, and Fe at elevated temperatures. Hydrothermal synthesis has been demonstrated on a laboratory scale to be a potentially superior method for low-cost production of advanced ceramic powders. This method is a wetchemical technique for directly forming complex oxide powders.2 The major difference between hydrothermal processing and the other technologies is that there is no need for high-temperature calcination. This, in turn, eliminates the need for milling. Powders are formed directly from solution by taking advantage of the complex reactions taking place in high-temperature water. By controlling these reactions, it is possible to produce anhydrous crystalline powders with controlled particle size, controlled stoichiometry and, in some cases, controlled particle shape. Powders produced by this method have been demonstrated to be highly reactive toward sintering.3 The effects of reaction conditions on the formation of NiZn and MnZn ferrites in hydrothermal synthesis are more complex than that of simple oxides, such as NiFe2 O4 . To our knowledge, the reaction mechanisms involved in the formation of ferrites under hydrothermal conditions have not yet been systematically investigated. a)
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http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 12, Dec 1997
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The objective of the present work is to characterize the influence of the starting materials on the crystalline phases ob
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