The effect of mechanical milling on the solid state reactions in the barium oxalate–iron(III) oxide system

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Abstract

The formation of barium hexaferrite, BaFe12O19, from a 1:6 molar ratio mixture of barium oxalate and iron oxide has been investigated. Thermogravimetry (TGA), high temperature X-ray powder diffraction (HT-XRPD), differential scanning calorimetry (DSC) and micro-Raman spectroscopy have been used to determine the effect of mechanical activation on the solid state reactions occurring during heating. The resulting magnetic properties were investigated measuring hysteresis loops. For the activated mixtures, the mass loss is over at ≈600 °C i.e. well below the temperature where BaCO3 spontaneous decomposition is complete (T>850 °C). Such a noticeable temperature lowering is a consequence of the high energy milling enhancing the formation of BaFe2O4. After heating the milled mixture to 850 °C, BaFe12O19 was rapidly formed from the BaFe2O4 and residual Fe2O3. Starting from an unmilled mixture, only minor amounts of BaFe12O19 were formed by heating to 850 °C. The favourable formation of barium hexaferrite, when starting from milled powders, has been confirmed by micro-Raman spectroscopy. The powder from the activated sample was shown to have far better magnetic properties than the unactivated sample.

Introduction

Barium ferrites are widely employed for various applications due to their good chemical stability and magnetic properties. In particular, BaFe12O19 (barium hexaferrite) is extensively used as a permanent magnet in microwave devices and in magnetic recording media.1 Among the different synthetic routes2 leading to barium ferrites, mechanical milling3 is an economical one, since the starting materials are usually commercially available and inexpensive. Production of barium ferrites is traditionally achieved by calcining stoichiometric mixtures of hæmatite (Fe2O3) and witherite (BaCO3) at temperatures above 1000 °C. Such high temperatures result in BaFe12O19 with low coercivity (Hc), although this can be improved by producing particles smaller than the single domain size (≈1 μm), e.g. by mechanically activating coprecipitated precursors of barium and iron(III) hydroxides that were subsequently annealed at 800 °C.4, 5 Indeed, it is well known that mechanical activation enhances homogeneity in the precursors and lowers the reaction temperature.6 In a recent work Steier et al.7 investigated the formation of barium hexaferrite starting from mechanically activated, stoichiometric mixtures of BaCO3–Fe2O3. They found that BaFe2O4 formation was rapid and started at 600–750 °C while BaFe12O19 formation was slower and occurred at 720–900 °C. The kinetics of BaFe2O4 formation from BaO2 and α-Fe2O3 has been studied by Watanabe8 who found evidence of BaFe2O4 at 500 °C.

The aim of this investigation is to study the effect of mechanical activation on the solid state reactions occurring in the BaC2O4–6 Fe2O3 (for BaFe12O19) system. Such a study was performed by high resolution thermogravimetric analysis (Hres-TGA), differential scanning calorimetry (DSC) and high temperature X-ray powder diffraction (HT-XRD). Micro Raman spectroscopy was used to show the phase composition and homogeneity in the system. Finally, the magnetic properties and the microstructure of the hexaferrite from both milled and unmilled powders were compared.

Section snippets

Experimental

The starting chemicals were BaC2O4 (Sigma Aldrich, Italy, 98%) and α-Fe2O3 (Sigma Aldrich, Italy, 99.9%). A physical mixture (i.e. not mechanically activated) with a molar ratio 1:6 (BaC2O4:Fe2O3) was prepared by weighing the appropriate amounts of the two starting materials and mixing them in an agate mortar for about 10 min. The powders were then suspended in acetone, stirred for 3 h and the solvent evaporated at 60 °C.

About 1 g of the physical mixture was mechanically activated by high

Pure BaC2O4

To assess the effect of mechanical activation, if any, on the decomposition of barium oxalate, high resolution TGA measurements were performed both on commercial samples of BaC2O4 and on samples ball milled for different times (4, 27 and 46 h). Fig. 1 reports the TGA thermogram of the sample milled for 46 h. Two mass loss stages are completed during the heating ramp while a third one, though starting at ≈700 °C, mostly takes place during the isotherm at 850 °C. The first stage up to 250 °C is

Conclusions

The effect of mechanical milling on the formation of barium hexaferrite, BaFe12O19 from barium oxalate and hæmatite has been investigated. The main points can be listed as follows:

  • Milling of the pure oxalate resulted in an increase in the hydration from 0.165 to ∼0.5 water molecules. In the presence of hæmatite the hydration increased to 1.7.

  • Thermogravimetric analysis shows that mechanical activation of BaC2O4–6Fe2O3 mixtures results in the mass loss being complete at ≈600 °C i.e. well below

References (12)

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