JOURNAL OF MATERIALS SCIENCE 32 (1997) 365 — 368 Magnetic Co 2 Y ferrite, Ba 2 Co 2 Fe 12 O 22 fibres produced by a blow spun process R. C. PULLAR, M. D. TAYLOR, A. K. BHATTACHARYA Centre for Catalytic Systems and Materials Engineering, Department of Engineering, University of Warwick, Coventry CV4 7AL, UK Gel fibres of Co 2 Y, Ba 2 Co 2 Fe 12 O 22 , were blow spun from an aqueous inorganic sol and calcined at temperatures of up to 1200°C. The ceramic fibres were shown by X-ray diffraction to form crystalline Co 2 Y at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a small grain size of 1—3 m across the hexagonal plane and 0.1—0.3 m thickness at 1000 °C. This only increased to 3 m in diameter and 1 m thickness even at temperature up to 1200 °C. The fibrous nature combined with the improved microstructures could be an important factor in improving the magnetic properties of this material. 1. Introduction A new class of planar hexagonal ferromagnetic mixed oxides, related to BaFe  O  (M ferrite), were dis- covered in 1956 at the Philips Research Laboratory [1], and they were named the ferroxplanar com- pounds. The complex chemical formulae and crystal structures reported [2] included Ba  Co  Fe  O  , or Co  Y, which unlike M ferrite has the direction of magnetism perpendicular to the c-axis. This results in an even larger crystalline anisotropy of 2222.8 Am (M ferrite " 1352.8 Am) and a high magnetic per- meability [3], and led to the development of a group of new soft ferromagnetic materials with low losses at high frequencies coupled with a very low conductivity [1]. These properties, combined with the rather low saturation magnetization meant that Co  Y was ideally suited to a variety of uses in microwave devices rather than as a permanent magnet [4]. General methods for producing ferroxplanar com- pounds are very similar to those used for M ferrites [5], but processes also exist to produce crystal- oriented Co  Y from sintering [6] and topotactic reac- tions [7], and near-perfect single crystals useful for some microwave devices can be grown by the flux method [8]. However, even greater care must be taken to get the stoichiometry [9], sintering conditions [10] and homogeneity and particle size [11] of the precur- sor correct due to the more complex nature of the chemical compositions. This work on Co  Y ferrite fibres is part of a pro- gramme to demonstrate how a number of refractory and effect fibres can be made by an aqueous sol—gel route. The fibrous form of a ceramic material can be made stronger and often stiffer than the bulk ceramic [12], and this would be advantageous if the Co  Y were used in resin composites. Hale [13] has reviewed the effects of composite phase geometry on material properties and it is apparent that the incorporation of a magnetic material in fibrous form would have a much greater impact. The practical consequences have been demonstrated by Goldberg [14] who showed that in special cases, short fibres of 50 : 1 aspect ratio give a 50 fold advantage in magnetic permeability over the same volume of material in non-fibrous form. Sol—gel routes to inorganic fibre forms bring ad- vantages in processing. Sol—gel provides a means for the fine scale mixing of multiple components at low temperatures, resulting in a more homogenous precur- sor. Consequently improved sintering rates at lower temperatures can be expected, leading to improved microstructure, and the inconvenient increased shrinkage between gel and ceramic product is more acceptable in an inorganic fibre due to its virtually one-dimensional nature. Therefore, given the potential advantages of a sol—gel based route for spinning Co  Y ferrite fibres, such a process has been investigated. 2. Experimental procedure 2.1. Sample preparation An acid-peptized halogen-stabilized iron(III)hydrox- ide sol (Fe : anion"3 : 2) was doped with stoichiomet- ric amounts of cobalt(II) and barium salts, which had been previously dissolved into a solution with an organic chelating agent. Spinnability was conferred by the addition of a small amount of polyethylene oxide as a spinning aid, and the fibres were produced using a proprietary blow spinning process [15]. The result- ing gel fibres were collected as random staple and stored in a circulating air oven at 110 °C. The gel fibres were heat treated in a muffle furnace, firstly being pre-fired to 400 °C at 200 °C per h to remove water and any residual organic compounds. The samples were then further heat treated at 200 °C per h to 600, 800, 1000, 1100 and 1200°C in a re-crystallized alumina vessel for 3 h. 0022—2461  1997 Chapman & Hall 365