The synthesis and characterisation of hexagonal ferrite fibres
Hexagonal ferrites are a family of magnetic oxides which include M ferrite, the most commonly used magnetic material in the world today. They also have more specialist potential applications, such as microwave absorption and catalysis. They are highly complex crystal structures which usuaUy require high temperatures to form a well sintered product, at the expense of grain growth, which in tum affects the magnetic properties of the material. They all exhibit magneto crystalline anisotropy; depending upon the ferrite they can have a preferred direction of magnetisation along the c axis of the hexagonal crystal (uniaxial), or in the hexagonal plane or in a cone at an angle to the plane (ferroxplana). A material made in a fibrous form has many properties; firstly it can be incorporated into a matrix to form a composite material, imparting any special properties into the whole composite. Provided their microstructure is sufficiently fme, fibres tend to be stronger than the bulk ceramic form, and if fme enough they can even be woven into a cloth. It has also been suggested that some properties, such as magnetisation, will be enhanced when the material is an aligned fibre, although this specific case has never been proven previously. This thesis presents the manufacture of a range of hexagonal ferrite fme fibres from an inorganic aqueous sol-gel precursor, and their subsequent characterisation both physically and magnetically. The fibres were blow spun from a viscous solution using a modified commercial process, and they were collected in both random and aligned forms. The sols were made from the peptisation of a precipitated iron(III)hydroxide with a mineral acid, and various combinations of iron (III) salt and acid were investigated using PCS. The best results were found to come from iron(III)chloride precipitated by ammonia and peptised with HBr, with totally nitrate and chloride based systems proving inferior. Chloride was found to add structuring to the sol, aiding spinning but resulting in a thixotropic gel if too much was present. The stoichiometric ferrite sols were made by doping the iron (I II) sol with solution of metal salts, and these were found to be critical to the stability of the sol, with barium causing particular problems, and titania was added as a second colloidal species. Stoichiometric sols of BaM, SrM, BaCoxTixFel12-12xO19, SrCoxTixFe12 2x019, CozY, COzZ, COzW, CozX and CozU ferrites were made, among others, and aU were successfully spun as fibres. They typically had diameters of 4 - 6 um and an alignment of over 90% within +/. 20° of the axis of alignment, comparable to commercially developed fibres. The sol-gel process typically produces ceramic materials at lower temperatures and with smaller grain sizes than standard ceramic techniques, and this proved to be the case. SrM and BaM began to form at 800°C and had become the pure phase product by 1000 °C, being 98% sintered at this stage and with a submicron grain size. CozY ferrite was also the sole phase at a low temperature of 1000 °C, although it consisted of hexagonal platelets up to 3 um wide and 0.2 um thick. Substituted M fcrrites also formed the M phase at similar temperatures, but were more porous and did not have the greatly reduced grain size reported previously. None of these ferrites underwent the sudden, exaggerated grain growth seen in many hexagonal ferrites, even at 1200 °C. However, both the CozZ and CozW ferrite fibres underwent this process concurrently with the formation of these phases, and it appears to be an unavoidable step under normal conditions, although steam firing or doping with CaO enabled the formation of a majority-Z product without this growth. It was found that halide retained in the fibre inhibited the formation of the hexagonal ferrite phase, as it was retained up to 1000 °c. When the fibres were treated to remove halides or made from a halide free precursor they formed M ferrites -200°C lower, and with a reduced grain size below 0.1 !lm. The fibres were characterised by XRD, XRF, XPS and SEM. The samples were characterised magnetically by VSM, and while the M ferrites proved to have typical Ms values and high coercivities for polycrystalline samples, the ferroxplana ferrites all had Ms equal to or better than the previously reported maximum values. It was also found that there were defmite alignment effects on Ms in all fibres except those which had .. under gone the exaggerated grain growth, with an enhancement in Ms of up to +40% along the ilxis of alignment when parallel to the applied field compared to values for random fibre. This is the first'time that the effect of fibre alignment upon magnetic properties has been demonstrated.