Hexagonal SrFe12O19 ferrites: Hydrothermal synthesis and their sintering properties

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Abstract

Hexagonal M type SrFe12O19 (SrM) ferrites were synthesized by the hydrothermal method, and the effects of molar ratio of OH/NO3 (RO/N), the atomic ratio of Fe/Sr (RF/S) and sintering on the phase composition and magnetic properties of as-synthesized specimens are studied. It is found that the RO/N and RF/S in starting materials affect the phase composition and magnetic properties greatly. Fe2O3 is found as impurity in some specimens, while in all as-synthesized specimens, SrCO3 forms inevitably during the synthesis process. After sintering at 1100 °C, the perovskite type SrFeO3−x ferrite forms as the product of SrCO3 and Fe2O3 at high temperature, and the specimens exhibit the Perminvar magnetic hysteresis loops which show two-phase magnetic behaviors. However, when sintered at 1200 °C, SrFeO3−x further reacts with Fe2O3 to produce SrFe12O19, and the specimens with RF/S=8, 9 and 10 exhibit the single-phase hexagonal structure and magnetic behavior. It is also found that the saturation magnetization and coercivity are both enhanced after sintering at high temperatures due to the elimination of impurities.

Highlights

► SrM ferrites are hydrothermally synthesized under a pressure less than 1 MPa. ► SrCO3 forms inevitably in the hydrothermal process. ► Perovskite SrFeO3−x forms in specimens after sintering at 1100 °C. ► Pure SrM ferrite forms after sintering at 1200 °C. ► The magnetic properties are enhanced after sintering at high temperatures.

Introduction

Hexagonal M-type SrFe12O19 (SrM) ferrite plays an important role in hard magnetic materials especially for its large magnetic anisotropy, high performance-to-cost ratio and good chemical stability. Usually, SrM ferrite is produced by a conventional ceramic method which involves calcination at high temperatures [1], [2], [3], [4]. In order to improve the material properties, some chemical methods, such as the coprecipitation method [5], [6], the sol–gel method [7], [8] and the hydrothermal method [9], [10], [11], have also been used to synthesize SrM ferrite. As one of the wet chemical methods, the powder prepared via the hydrothermal method is homogeneous, fine and of high-purity [12], [13]. Moreover, it can synthesize hexagonal ferrites without subsequent sintering at high temperatures. However, in current reports, SrM ferrite is usually hydrothermally synthesized at a very high pressure. Wang et al. [9], [14], [15], [16] and Ataie et al. [11] have synthesized a series of SrM ferrites at about 2.5 MPa (25 atm) and 68.5 MPa (685 atm), respectively. Recently, we have tried to hydrothermally synthesize SrM ferrites in a common Teflon liner in which the pressure is surely not more than 1 MPa (10 atm), and some new results are obtained, which is important for the production of SrM ferrite.

Section snippets

Synthesis of specimens

All the reagents used are analytically pure. First, aqueous solutions of Fe(NO3)3 and Sr(NO3)2 were coprecipitated by NaOH. In order to study the synthesis conditions, the molar ratio of OH/NO3 (RO/N) and the atomic ratio of Fe/Sr (RF/S) were set to be from 1 to 5 and from 2 to 11 with steps of 1, respectively. Then, the precipitates and aqueous solution were hydrothermally reacted in a Teflon liner at 220 °C for 5 h. Finally, in order to study the effects of sintering on the properties of

Effects of different RO/N

According to Ref. [16], we first chose RF/S=8 to study the effect of RO/N on the phase composition. Fig. 1 shows the XRD patterns of specimens hydrothermally synthesized with different RO/N. Seen from Fig. 1(a), when RO/N=1, only Fe2O3 and several weak SrCO3 diffraction peaks are found in the XRD pattern, which shows a large amount of Sr2+ ions lost in the solutions and no hexagonal SrM ferrite was synthesized in the specimen. However, there are typical peaks from hexagonal SrM ferrite as well

Conclusions

Hexagonal M type SrFe12O19 ferrites were synthesized by the hydrothermal method. For as-synthesized powder specimens, the molar ratio of OH-/NO3, RO/N, and the atomic ratio of Fe/Sr, RF/S, in starting materials affect their phase composition and magnetic properties greatly. Fe2O3 is found as impurity in some specimens, while SrCO3 forms inevitably during the synthesis process in all as-synthesized specimens. After sintering at 1100 °C, SrCO3 vanishes in all the specimens, but a new impurity

Acknowledgments

This work was supported by the National Natural Science Foundation of China under Grant nos. 11204003 and 21071003.

References (19)

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