Combustion synthesis and properties of fine particle Li-Zn ferrites

Abstract

Lithium ferrites are attractive for microwave devices as a low-cost replacement for ferrimagnetic garnets [1, 2]. A serious problem with Li-Zn ferrite used in millimetre wave devices lies in poor densification. The pressure of porosity in Li ferrite samples enhances the insertion loss of the microwave device with increasing humidity. The porosity of ferrite manufactured by a conventional ceramic technique (with compositional inhomogeneity) is about 5-10% [3]. In order to attain better homogeneity and control over purity and particle size distribution, chemical routes involving metal formates [4, 5] have been used. Sintering aids such as $Bi_20 _3$ [6] or HIP have been studied for the preparation of high-den- sity materials. With HIP, under appropriate condi-tions the density of the bead material approaches X-ray density [7]. The density of magnetic ceramics depends on various factors, of which the reactivity and homogeneity of the ferrite powders are of special importance. The difficulty encountered when sintering Li ferrites is the volatilation of Li and Zn at temperatures above 1100°C. Thus, technological applications necessitate the preparation of highly reactive, pure, homogeneous Li ferrite powders which can be sintered to a fully dense ceramic body at 1000 °C with controlled grain growth and the desired magnetic properties. In continuation of our studies on spinel ferrites by a precursor method [8, 9], we recently reported a rapid combustion synthesis of spinel ferrites [10-12]. In this letter, we report the preparation and properties of $Li_{0.5}Zn_x$ $Fe_{2.5-x}O_4$ (where x = 0 - 1 . 0 ) and $Li_{0.325}Zn_{0.275}$ $Co_{0.075}$$Fe_{2.325}0_4$ by the combustion of lithium nitrate, zinc nitrate, iron (III) nitrate and oxalyl dihydrazide (ODH) at 350 °C in les than 5 min.