Elsevier

Acta Materialia

Volume 49, Issue 20, 3 December 2001, Pages 4241-4250
Acta Materialia

Decomposition, shrinkage and evolution with temperature of aligned hexagonal ferrite fibres

https://doi.org/10.1016/S1359-6454(01)00304-4Get rights and content

Abstract

The decomposition, shrinkage and evolution of aligned gel fibres with the stoichiometric compositions for BaM, SrM and Co2Z were investigated over a range of temperatures. SrM and BaM began to form by 750 and 800°C respectively, giving pure SrM and BaM phases at 900 and 1000°C. The Z phase formed at 1250°C, and only after the full crystallisation of the M and Y phases at 1000°C. Pure phase fibres of all ferrites were produced from stoichiometric mixes, unlike standard ceramic preparations. The fibres shrank by up to 22–23% at 1200°C, with no loss of alignment, and were appeared sintered (~97%) by this point. The formation of the M ferrite phases seemed to occur at unexpectedly high temperatures compared to previous work on bulk sol-gel M ferrites. Halides were retained in the fibre over 800°C, and ferrite formation was delayed until the halides were lost.

Introduction

The hexagonal ferrites are a group of magnetic compounds discovered by Philips between 1952 [1] and 1956 [2]. The hexagonal ferrites all have high resistivities, magnetocrystalline anisotropies and saturation magnetisations, low dielectric losses and are thermally stable well above their Curie temperatures [3], and M ferrites are the most commercially important permanent magnetic materials globally [4]. Strontium and barium M ferrites have the formulae Sr- or BaFe12O19, are uniaxial with the direction of magnetisation parallel to the C-axis [2] and are magnetically hard materials. Co2Z (Ba3Co2Fe24O41) is one of a group known as ferroxplana ferrites, so called because their preferred direction of magnetisation is at an angle to the c-axis [2]. Co2Z prefers the basal plane at room temperature following a transition to this from a cone of magnetisation at −53°C, but it then undergoes a further change to magnetisation parallel to the c-axis from 207°C to the Curie point at 400°C [3]. It is a soft magnetic material, but it has a high permeability and has applications in electronics, EM wave absorption and other niche applications.

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 [5]. We have previously reported the synthesis of a range of aligned hexagonal ferrite fibres, including BaM [6], SrM [7] and Co2Z [8] from aqueous inorganic sol-gel precursors. The fibres were blow spun using a proprietary blow spinning process [9], [10], yielding continuous fine fibres with diameters between 3 and 8 μm, and averaging 4–5 μm. They were manufactured in both random an aligned form, the latter being between 80% and 95% within ±20° of the axis of alignment. The composition and microstructure of the ferrite fibres were reported, and their microwave properties investigated [11]. This paper studies in more detail the decomposition and phase evolution of these fibres as they are heated from the dried gel to the final ceramic ferrite, with particular attention to their shrinkage, densification and compositional changes.

Section snippets

Fibre preparation and heating

The fibres were produced from an aqueous inorganic sol-gel precursor, and the process has been detailed and the sols produced characterised in our previous papers [6], [7], [8]. Iron(III)chloride was precipitated with a base, and then peptised with a mineral acid to produce an iron(III)oxy-hydroxide sol stabilised by halide counterions. This had an average particle size of around 5 nm, and could be concentrated up to around 36.5% Fe3+. The sol was then doped with stoichiometric amounts of

M ferrite fibres

The XRD patterns of the BaM fibres fired immediately after spinning, for samples heated to between 400 and 1200°C, are shown in Fig. 1. Polycrystalline hexagonal ferrite samples with oriented crystallites can be distinguished from those having randomly oriented crystallites by their different XRD patterns, giving totally different intensities of reflection from randomly oriented samples, as demonstrated by Jonker et al. [2] and illustrated in Smit and Wijn [3]. On this basis the XRD patterns of

Conclusions

The decomposition, shrinkage and evolution of gel fibres with the stoichiometric compositions for BaM, SrM and Co2Z were investigated over a range of temperatures. No cobalt halides or oxides were ever seen in the XRD patterns, and the only crystalline halide compounds observed were BaBr2 at 200°C, the rest of the fibre being amorphous below 250°C, and BaBrCl at 400–600°C. All fibres formed crystalline α-Fe2O3 at 250°C with a crystallite size of 17 nm, but the haematite fibres were porous with

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

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