The manufacture, characterisation and microwave properties of aligned M ferrite fibres

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Abstract

Gel fibres of strontium and barium M ferrite were blow spun from an aqueous inorganic sol and collected as aligned tow blankets. Both were then calcined to 1000°C and characterised using a variety of techniques. The ceramic fibres were shown to be the respective single phase crystalline M ferrites at 1000°C by X-ray diffraction, and compared to standard commercially available M ferrites at this temperature they demonstrated a favourable grain structure of less than 1 μm. Measurement of the microwave permeability spectra showed both materials exhibiting ferromagnetic resonance frequencies consistent with those reported in the literature.

Introduction

The M-type hexaferrites are among a group of useful magnetic compounds discovered by Philips between 1952 [1]and 1956 [2]. Strontium and barium M ferrites have the formulae Sr- or BaFe12O19, and the hexagonal magnetoplumbite structure [3]. They are uniaxial with the direction of magnetisation parallel to the c-axis [2] and are magnetically hard materials, barium M ferrite being one of the most commercially important permanent magnetic materials [4]. The M ferrites also have high resistivities [5], magnetocrystalline anisotropies [6]and saturisation magnetisations [7], low dielectric losses [8]and are thermally stable well above their Curie temperatures [9].

These properties make M ferrites potentially ideal for use in non-reciprocal microwave devices provided the resonance line-width can be reduced to single crystal values, by exploiting the effect of gyromagnetic resonance that appears when the material is subjected to a low power microwave field perpendicular to the axis of magnetisation.

This investigation into the formation, characterisation and microwave properties of aligned M ferrite fibres is part of an ongoing programme into the development of refractory fibres, manufactured from aqueous sol–gel routes. The many advantages of sol–gel processes, fibrous versus bulk materials and fibre composites have been discussed in previous publications 10, 15.

Section snippets

Sample preparation

An acid-peptised, halogen-stabilised iron(III)hydroxide sol (Fe : anion=3 : 2) was doped with a stoichiometric amount of a strontium salt, which had been previously dissolved into a solution with an organic liganding agent. Spinnability was bestowed by the addition of a small amount of polyethylene oxide as a spinning aid, and the fibres produced on a proprietary blow spinning process [11]. The resulting gel fibres were collected as an aligned tow blanket and stored in a circulating air oven at

Sol characterisation and stability

The stability and the resulting size of the sol particles is sensitive to the preparative techniques and conditions employed, and PCS enabled us to measure and control the properties of the strontium doped iron (III) sol to a certain extent. The PCS data indicates that the average particle size of the strontium doped iron (III) sol was 6.4 nm, with a polydispersity of 0.82 and an average particle weight of 4.2×104 amu. By volume distribution, the mean size was found to be 7.0 nm, with an upper

Conclusions

A stable, strontium doped iron (III) sol was produced, and from it gel-fibres were successfully spun and collected in the form of an aligned tow blanket. The gel fibres were calcined to 1000°C and then characterised by various techniques. They were found to be pure phase SrM ferrite at this temperature whilst maintaining their fibrous nature, and with an improved microstructure compared to conventionally prepared specimens. BaM fibres were also spun from a doped sol, and collected as an aligned

Acknowledgements

R.C. Pullar wishes to thank the Centre for Catalytic Systems and Materials Engineering and the DERA for providing funding for his research associateship. Our thanks to D. Croci for surface area measurements and R.C. Reynolds for the XPS and XRD characterisation (both at the Centre for Catalytic Systems and Materials Engineering, Department of Engineering, University of Warwick) and R. Burton for the XRF analysis (Materials Research Institute, Sheffield Hallam University). The microwave data

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    Resonance frequency of (Ba0.7Bi0.2)4(Co1-xNix)2Fe36O60 is around 11.3 GHz [204]. Type -M ferrites, such as BaFe12O19 and SrFe12O19, are important and permanent materials of the wave absorbers including a resonant frequency of ca. 40.0 GHz [205–207]. With M = CoTi the microwave absorption of BaFe12-2xMxO19 is transmitted in a range of 26.0 to 40.0 GHz [208].

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