A novel aqueous route for the synthesis of mullite fibres

Abstract

There can be significant advantages in the prepara- tion of ceramic materials as fibres rather than as bulk solids. In a fibrous form, brittle materials can develop higher strengths and moduli (1), while fine fibres are reasonably flexible and can be manipulated in large-scale fabrication processes without risk of breakage. Although the electronic properties of certain materials are usually utilized in thin films or as small components, there is potential for the use of fibrous forms. Mechanical performance can become an issue in larger devices with linear dimensions greater than about 2 cm, and then the introduction of a ceramic component in a fibrous form is an obvious solution. However, in composites the best electronic properties can often be achieved when the ceramic is in the form of a fibre and the geometry is controlled to take advantage of the high aspect ratio (2, 3). This work on mullite is part of a programme to demonstrate the preparation of a number of refrac- tory and electronic effect fibres by entirely aqueous sol-gel routes. The preparation of zirconium titanate fibres (4) is an example of such a route producing a ceramic fibrous material with potential electronic properties. Mullite is a simple example of a refractory although its preparation in fibrous form is not novel. A number of manufacturers, for example Denka, made mullite fibres as refractory insulation blanket (5) by aqueous sol-gel routes, while on a smaller scale a strong textile fibre (6) has been produced in development quantities for mech- anical applications. This letter describes the preparation of a mullite spinning dope by an unconventional aqueous sol-gel route. The dope is converted to gel fibres of unusual configuration, which are readily converted to highly crystalline mullite fibres below 950 °C. A 2.6M aqueous solution was prepared from reagent grade aluminium nitrate. The solution was then extracted with three equal successive batches of anion extracting solvent to give a transparent sol where NO3/A1 = 0.9. The solution after separation was uncovered and dried to a solid gel in an oven at 60-70 °C. The alumina precursor could be stored in this form for later use. The mullite precursor (A1203/ SiO2 = 1.5) was prepared by dispersing the alumina gel in water and mixing rapidly with the stoichio- metric amount of Syton 30 silica sol. The solution was filtered through a Whatman GF/B glass fibre filter, then mixed with a 2% solution of polyethylene oxide spinning aid (Union Carbide Polyox WSR-N- 1654 12K), which had been adjusted to pH 3 with nitric acid. The aid was used at a level of 2% on the mullite ceramic. The clear solution was refiltered and concentrated to spinning viscosity (-3 poise at 1000 s -1 shear rate; -3 dynecm -2 s) by vacuum evaporation in a Buchi evaporator. The solution was spun in a proprietary blow spinning process (7), in which the spinning dope is extruded through a row of five holes, on either side of which are slits directing impinging jets of humidified air at near ambient temperature. The fibres are drawn by the high velocity primary slit jets and dried by in mixing of hot dry secondary air; in this case 105 °C was needed to set the fibres. The fibre-air mixture was passed down a short diverging section into a parallel duct which discharged into a