A halide free route to the manufacture of microstructurally improved M ferrite (BaFe12O19 and SrFe12O19) fibres

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Abstract

In hexagonal ferrite precursor fibres halide was found to be retained up to high temperatures, delaying the formation of the BaM (BaFe12O19) and SrM (SrFe12O19) phases, and resulting in a product with a grain size of 0.5–1 μm. This paper reports a halide free route for these fibres, using a nitrate stabilised sol precursor, from which the M phases formed at lower temperatures with smaller grain sizes. This sol was characterised and compared to the halide based sol, the evolution of the crystalline phases was studied and the hysteresis loops of the M ferrite fibres assessed by VSM. It was shown that retained halide inhibited the formation of the ferrite phase, as both SrM and BaM formed at a temperature 200 °C lower than in the halide containing precursor fibres. The grain sizes were accordingly lower, the SrM grains being below 100 nm, although the crystallite sizes were only slightly smaller than those reported for the halide containing fibres fired to 1000 °C, and the magnetic properties were also similar. The BaM fibres had Ms=58.4 kA m2 g−1 and Hc=401 kA m−1, while the SrM fibres had Ms=65.0 kA m2 g−1 and Hc=440 kA m−1, comparable to small grained polycrystalline powdered samples. The M ferrite phase was formed directly from haematite, with no BaFe2O4 observed as an intermediate, and the pure ferrites were made from stoichiometric precursors.

Introduction

Globally the hexagonal M ferrites are the most commercially important magnetic materials, with a range of applications from simple permanent magnets and electric motors to memories and niche electromagnetic applications. They are magnetically hard, with high coercivities and magnetic permeabilities, and contain a high magnetocrystalline anisotropy along the c-axis of the hexagonal structure. It has been predicted that properties such as thermal and electrical conductivity, and magnetic, electrical and optical behaviour could be enhanced in material in fibrous form.1 We have previously reported the synthesis of a range of aligned hexagonal ferrite fibres, including BaM (BaFe12O19)2 and SrM (SrFe12O19)3, blow spun from an aqueous inorganic sol–gel precursor stabilised by halide ions, and these were characterised physically and magnetically. These sols contained both chloride ions, retained from the iron(III)chloride precursor material despite several washes, and bromide ions from the HBr peptising agent. The precise compositions and decomposition of these fibres have been investigated and reported previously.4

However it was found that a relatively high temperature of 900–1000 °C was required to form the pure ferrite phase, resulting in a well-sintered product but with grains as large as 1 μm in diameter. Whilst these temperatures and grain sizes were low compared to standard ceramic methods, they were surprisingly high for a sol–gel derived product. It was suspected that the formation of the ferrite phases was restricted by the retention of halide (both Cl and Br) ions in the fibre up to 1000 °C,4 and so it was decided to produce fibres from a nitrate stabilised analogue of the halide based sol. It was hoped that these would form the ferrite phase at a lower temperature, with a corresponding reduction of grain size resulting in a mechanically superior fibre product.5

BaM and SrM have high saturation magnetisations (Ms) of 72 kA m2 g−1 1 and at least 74.3 kA m2 g−1 respectively.6 They also have high magnetocrystalline anisotropies along the c-axis,1 this uniaxial character producing large theoretical maximum coercivities (Hc) of up to 594 kA m−1, although polycrystalline samples rarely approach these high values.7 The M ferrite fibres made from halide containing routes had high values for both Ms and Hc, despite their large grain sizes which approached the domain size of 1 μm.8 Therefore the effect of reducing grain size upon the magnetic properties of the M ferrite fibres was also investigated.

Section snippets

Sol preparation and spinning

The precursor sol was made in a method similar to the of the previously reported halide stabilised iron(III)oxyhydroxide (FeO1−xOH1+2x) sols.2, 3 A solution of iron(III)nitrate nonahydrate was precipitated with 4% ammonia solution until the mixture had reached a pH of 5. The resulting thick brown precipitate was filtered, washed and then peptised over 24 h with nitric acid, after which time there was no further significant decrease in the average sol particle size. The resulting sol had a

Sol characterisation and stability

When spinning gel fibres from an inorganic sol, the existence of any large species, even in small numbers, inhibit spinning, block spinnerets and cause shot to form, resulting in a poor quality fibre product. Also very important for spinning is the degree to which the sol can be concentrated without flocculating or gelling, the minimum and optimum levels varying with each sol, but generally requiring at least 10 wt.% metal ions.

It has been found that the ferrite precursor sols usually require a

Conclusions

A nitrate stabilised, halide free iron(III)hydroxide sol was synthesised, characterised, and compared to the halide stabilised sol reported previously, and was shown to have a 30% larger volume average particle size of 7.8 nm, with an upper limit 50% higher than the halide stabilised sol. While this would not be a problem in normal sol-gel work, for blow spinning an average size of over 50 nm has been found to be detrimental to spinning. Upon addition of stoichiometric amounts of Sr(NO3)2, the

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1

Present address: Centre for Physical Electronics and Materials, School of Electrical, Electronic and Information Engineering, South Bank University, 103 Borough Road, London SE1 0AA, UK.

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