M.D. Taylor

The manufacture and characterisation of aligned fibres of the ferroxplana ferrites Co2Z, 0.67% CaO-doped Co2Z, Co2Y and Co2W

Abstract Gel fibres of Co 2 Z, 0.67% CaO-doped Co 2 Z, Co 2 Y and Co 2 W ferrite were blow spun from an aqueous inorganic sol and collected as an aligned tow blanket, with an alignment comparable to that found in commercial fibres. The fibres were then heated to produce the desired ceramic phases, characterised by various techniques and their ferromagnetic resonance spectra measured. Single phase Co 2 Z was found by X-ray diffraction to form at a relatively low temperature of between 1200°C and 1250°C, and the material exhibited the expected microwave properties. Furthermore, an addition of 0.67% CaO was found to promote the formation of Co 2 Z at an even lower temperature of below 1200°C and delay the exaggerated platy grain growth, which is normally encountered at the onset of formation of the Co 2 Z phase and which results in a mechanically weakened fibre.

The manufacture of yttrium aluminium garnet (YAG) fibres by blow spinning from a sol-gel precursor

Abstract Three different systems consisting of a yttria sol and alumina sols were investigated, and gel fibres were successfully spun which on subsequent heating produced ceramic fibres of yttrium aluminium garnet (YAG). The fibres were characterised by various techniques. The fibres were found to begin forming YAG between 700–750 °C and it was present as a single phase between 750–800 °C, the lowest reported temperature for the crystallisation of this material. The fibres had no discernible grain structure and the average crystallite size was calculated to be only 20 nm at this temperature; however, the fibres were estimated to remain 20% porous up to 1200 °C. The fibres were handleable after YAG crystallisation, but more investigation is required into sintering and the development of high temperature properties.

Novel aqueous sol–gel preparation and characterization of barium M ferrite, BaFe12O19 fibres

Gel fibres of barium M ferrite, BaFe12O19, were blow spun from an aqueous inorganic sol and calcined at temperatures up to 1200°C. The ceramic fibres were shown by X-ray diffraction to be single phase crystalline M ferrite at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a favourable grain structure of less than 0.1 μm at this temperature and maintained a small grain size of less than 4 μm even up to 1200°C, an important factor in the magnetic properties of this material.

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

Abstract The decomposition, shrinkage and evolution of aligned gel fibres with the stoichiometric compositions 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.

The manufacture of partially-stabilised and fully-stabilised zirconia fibres blow spun from an alkoxide derived aqueous sol–gel precursor

Aligned fibres of partially stabilised zirconia (PSZ, 4 mol% Y2O3) and fully stabilised zirconia (FSZ, 8 mol% Y2O3), 3–5 μm in diameter, were blow spun from a sol–gel precursor, and then fired to give the ceramic fibre. Various potential sol precursors were investigated and characterised, the optimum being an aqueous sol made from hydrolysed and peptised zirconium iso-propoxide. The resulting zirconia fibres were characterised and their evolution studied by XRD, DTA/TGA and SEM, and their mechanical properties assessed. It was found that both PSZ and FSZ fibres formed poorly crystalline cubic zirconia at only 200°C, which became fully crystalline after firing to 400°C. The cubic form was the only phase seen in the FSZ fibre, whereas the PSZ fibre formed the tetragonal phase after firing between 1000 and 1200°C/3 h, and all fibres were nanocrystalline (grain diameter<0.1 μm). Unusually the monoclinic form of zirconia was never observed in the PSZ fibres. After firing above 1200°C the fibres had a strain to break of 0.59%, and appeared to be well sintered from shrinkage data. They had superior creep resistance to Saffil zirconia fibres, creeping at temperatures 50°C higher.

Blow spun strontium zirconate fibres produced from a sol-gel precursor

A zirconium sol was produced from the hydrolysis and immediate peptization of a zirconium alkoxide, which was found to be stable with the addition of a large proportion of a strontium salt, even up to equimolar amounts. Get fibres were successfully blow spun from this doped sol, which on subsequent heating produced ceramic fibres of orthorhombic strontium zirconate. The fibres were characterized by various techniques. The fibres were found to begin forming SrZrO3 at 400°C and were present as single phase orthorhombic strontium zirconate at 800°C and above. The ceramic fibres had no discernible grain structure and the average crystallite size was calculated to be only 20 nm at this temperature, but the fibres contained small surface pores (< 0.2 μm) which continued to increase in size on subsequent heating, resulting in a weak and brittle fibre. Therefore, more investigation is required into sintering and the development of high temperature properties.

Aligned hexagonal ferrite fibres of Co2W, BaCo2Fe16O27 produced from an aqueous sol–gel process

Gel fibres of Co2W, BaCo2Fe16O27, were blow spun from an aqueous inorganic sol and collected as an aligned tow blanket with an alignment of over 90%. Upon calcination up to 1300°C, the ceramic fibres were shown by X-ray diffraction to form single-phase crystalline Co2W at 1200°C, an unusually low temperature for this compound to form as the major phase. The formation of Co2W was also accompanied by sudden grain growth, forming crystallites over 10 μm in length. While this reduced the mechanical strength of the fibres, they were still suitable for use in a composite material.

Magnetic Co2Y ferrite, Ba2Co2Fe12O22 fibres produced by a blow spun process

Gel fibres of Co2Y,Ba2Co2Fe12O22, 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 Co2Y 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 μm across the hexagonal plane and 0.1–0.3 μm thickness at 1000°C. This only increased to 3 μm in diameter and 1 μm 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.

The manufacture and characterisation of single phase magnetite and haematite aligned fibres from an aqueous sol-gel process

A stable iron(III)hydroxide sol was produced, spun as a gel fibre and collected as an aligned tow blanket. The alignment of the fibre was found to be 88.8% within ± 20° of the axis of alignment, comparable to that of some commercially developed ceramic fibres. Heating in air at a temperature of 250 °C for 1 hour yielded single phase haematite fibres, and these were characterised by X-ray diffraction and electron microscopy. Upon reduction of the gel fibre in 5% H2 in N2 at 350 °C for 1 hour, single phase magnetite was obtained and characterised by X-ray diffraction, infrared and Mössbauer spectroscopy. The morphology of the fibre was studied, and it was found that the transformation to magnetite was accomplished with no compromise of integrity of the fibre or its alignment, although there was an accompanying change in microstructure. The strain to break of the magnetite fibres was measured to be of a least 0.6%, and this compares well with some commercial fibres.

The comparison of alumina and zirconia fibres using simple thermal and mechanical techniques

The laboratory development of sol/gel ceramic fibres requires rapid objective means of assessing the mechanical and thermal properties of a product at the earliest stages of preparation. The merits of a simple tumbling test, leading to a fibre friability index, and the Bend Stress Relaxation test, which gives a high temperature creep rating, are demonstrated on commercial Saffil and Safimax alumina fibres and a development Saffil zirconia fibre, each in staple blanket form. Measurements on Altex continuous alumina/silica fibre and Nicalon are also presented.Standard and off specification alumina fibres are readily distinguished by their friability indices which correlate with the fibre strain to break.Saffil and Safimax alumina are comparable to Nicalonin terms of creep and superior to Altex. Saffil zirconia approaches alumina creep performance after post firing to 1250°C.