Acta mater. 49 (2001) 4241–4250 www.elsevier.com/locate/actamat DECOMPOSITION, SHRINKAGE AND EVOLUTION WITH TEMPERATURE OF ALIGNED HEXAGONAL FERRITE FIBRES R. C. PULLAR†‡, M. H. STACEY, M. D. TAYLOR and A. K. BHATTACHARYA Warwick Process Technology Group, School of Engineering, University of Warwick, Coventry CV4 7AL, UK ( Received 3 April 2000; received in revised form 12 July 2001; accepted 8 August 2001 ) Abstract—The decomposition, shrinkage and evolution of aligned gel fibres with the stoichiometric compo- sitions for BaM, SrM and Co 2 Z 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.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PACS codes: 75.50.Ggi; 81.20.Fw; 83.20. Hn Keywords: Sol-gel; Sintering; Fibres; Magnetic; Microstructure 1. INTRODUCTION The hexagonal ferrites are a group of magnetic com- pounds discovered by Philips between 1952 [1] and 1956 [2]. The hexagonal ferrites all have high resis- tivities, magnetocrystalline anisotropies and satu- ration 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 BaFe 12 O 19 , are uniaxial with the direction of magnetisation parallel to the C-axis [2] and are mag- netically hard materials. Co 2 Z (Ba 3 Co 2 Fe 24 O 41 ) 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]. Co 2 Z 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 par- allel 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 † To whom all correspondence should be addressed. Tel.: +44-207-815-7540; fax: +44-207-815-7599. E-mail address: r.c.pullar@sbu.ac.uk (R. C. Pullar) ‡ Present address: School of Electrical, Electronic and Information Engineering, South Bank University, 103 Bor- ough Road, London SE1 0AA, UK. 1359-6454/01/$20.00  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(01)00304-4 high permeability and has applications in electronics, EM wave absorption and other niche applications. It has been predicted that properties such as ther- mal and electrical conductivity, and magnetic, electri- cal 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 Co 2 Z [8] from aqueous inorganic sol-gel precursors. The fibres were blow spun using a proprietary blow spin- ning 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. 2. EXPERIMENTAL 2.1. Fibre preparation and heating The fibres were produced from an aqueous inor- ganic sol-gel precursor, and the process has been