Elsevier

Ceramics International

Volume 36, Issue 4, May 2010, Pages 1423-1427
Ceramics International

Effects of trace of Bi2O3 addition on the morphology of strontium ferrite particles

https://doi.org/10.1016/j.ceramint.2010.02.003Get rights and content

Abstract

A strontium ferrite powder added with trace of Bi2O3 was prepared by the conventional high-temperature solid phase reaction. The effects of Bi2O3 addition on the morphology of Sr-ferrite particles fired at different temperatures and times were investigated. The results show that a small quantity of Bi2O3 addition accelerates the reaction of SrO and Fe2O3 to form SrM ferrite and obviously improves the morphology and size of the strontium ferrite particles. A possible mechanism was proposed to interpret the influence of trace of Bi2O3 addition on the morphology of strontium ferrite particles.

Introduction

M-type strontium hexaferrite SrFe12O19 is one of the most important hard magnetic materials, widely used for permanent magnets and magnetic-recording media, due to its fairly large magnetocrystalline anisotropy, high Curie temperature, relatively large magnetization, as well as excellent chemical stability and corrosion resistivity [1], [2].Currently, strontium ferrites were prepared by traditional ceramic process with SrCO3 and Fe2O3 as the main raw material in the industry. Besides the chemical composition [3] and sintering process [3], [4], [5], [6], [7], the additive is also important here, because it influences the microstructure and magnetic properties of SrM ferrites. Bi2O3 used in preparing soft magnets as one sintering additive has been widely studied. Liu et al. [8] found that Bi2O3 is able to promote the densification LiZn ferrite. High Bs and lower Hc can be achieved due to the liquid-phase sintering under the action of Bi2O3. Yu et al. [9] also revealed that Bi2O3 enriched in the grain boundary area promotes the densification of MnZn ferrite during the sintering. In the field of injection molding hard magnets, the shape of nearly spherical ferrite particles are needed in order to meet the requirement of good flowability. The action mechanism of trace of Bi2O3 on the morphology of Sr-ferrite particles is rarely reported though it has attracted great interest to improve the magnetic properties of the sintered ferrites [10], [11], [12], [13], [14]. In this paper, the strontium ferrite powder was prepared by conventional solid-phase synthesis process with trace of Bi2O3 (≤1 wt%) and a probable mechanism was proposed to explain the effects of Bi2O3 on the morphology of the strontium ferrite particles.

Section snippets

Experimental procedure

The starting materials used were SrCO3, Fe2O3, and Bi2O3. SrM ferrite of composition SrFe2O19 + xBi2O3 (x = 0, 0.5, 1, 1.5 wt%) were prepared by a solid-state reaction method (defined x = 0 as pure sample). Stoichiometric mixtures of oxides were thoroughly ground in a porcelain mortar, then precalcined for 0–2 h in air within the temperature range of 900–1150 °C in corundum crucibles.

SEM (Hitachi S-4700) was used to observe the particle morphology of the as-received powders. The contents of Bi and Sr in

Results and discussion

Fig. 1 illustrates the typical SEM photographs of the sintered specimens. It can be seen from this figure that the sizes of SrM ferrite particles obviously increase as the Bi2O3 contents increase from 0 to 0.5 wt%, which implies that trace of Bi2O3 is able to stimulate the particle growth of SrM ferrites. However, the particle sizes are not obviously becoming larger with the Bi2O3 contents continuing to rise except that the particles get together and display a sintered morphology.

Fig. 2 shows

Conclusions

  • (1)

    The addition of trace of Bi2O3 improves the particle size and morphology in preparing the Sr-ferrite powders for inject molding. When the calcination temperature is at 1050 °C, the particles of samples with Bi2O3 addition are more homogeneous and display a near spherical shape with the size of about 1.2 μm. In the same firing process, the particles are irregular and have a size of about 0.8 μm in the sample without Bi2O3 addition.

  • (2)

    The action mechanism of trace of Bi2O3 on the morphology of ferrite

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