A method for making alumina ®bres by co-extrusion of an alumina and starch paste R. Greenwood a, *, K. Kendall a ,O.Bellon b a School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b Birchall Centre, Keele University, Staordshire ST5 5BG, UK Received 8 June 2000; received in revised form 8 August 2000; accepted 9 August 2000 Abstract A method of producing alumina ®bres has been developed. The process used a water-based alumina and rice starch paste. An initial feed-rod was made from a central cylindrical rod of alumina paste surrounded by a square shaped section of starch paste. Thisfeedrodwassubsequentlyextrudedsothatitisreducedinsizebyadeterminedratio.Theextrudatewascutintoshortlengths and re-assembled to the exact dimensions of the initial feed rod so that it contained many smaller alumina rods. This latter speci- men was re-extruded and the steps repeated as many times as required to obtain a large amount of small diameter ®bres. By selecting the die diameter and the number of extrusions the diameter of the ®bres was easily controlled. Since all the ®bres were in one easily handleable bunch, the bottleneck associated with sintering could be avoided. Hence an economic method of ®bre production can be envisaged. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Al 2 O 3 ;Al 2 O 3 -®bres; Extrusion; Starch 1. Introduction The use of ®bres to reinforce bodies has been exten- sively practicised over the years and reviewed in numerous articles, e.g. Ruys and co-workers, 1 Donald and McMillian, 2 Karpman and Clark, 3 Schioler and Stiglich 4 and Philips. 5 Availability of economic ®bres is stillthemajorproblemforalumina,zirconiaandsimilar refractory materials. Existing commercial ®bres are mainly manufactured from glass, e.g. Owens Corning S2 ®bre, alumina, e.g. ICI Sal, zirconia alumina, e.g. 3M Nextel, Silicon Carbide or Carbon. Most of these ®bres have diameters in the range 1±20 microns. The methods of ®bre production are numerous and varied and have been reviewed previously reviewed by Birch- all, 6,7 Tucker et al., 8 Abe and co-workers, 9 but we now brie¯y describe the various production methods and their advantages and disadvantages. In melt spinning, a material is heated and drawn out through a spinneret. This is a relatively inexpensive method, but is unsuitable for materials with high melting points such as alumina or materials with low viscosities on melting. The technique is used for the manufacture of ®bre glass and aluminosilicate ®bres. In chemical vapour deposition, a solid ®bre is formed by depositionofdecomposedorreducedgaseousmolecules on a heated substrate. Again, this method is expensive due to the long fabrication time. Fibres can also be manufactured using sol±gel pro- cesses. Extrusion of the sol through a spinneret is used or centrifugal force is applied to form ®bres when a sol is spun from a disc. Advantages include good homo- geneity, high purity and low sintering temperatures. However, there is a large degree of shrinkage on ®ring, long processing times and diculty in burning out the organics.Nevertheless,thetechniqueiswidespreadand used for several ®bre materials. The relic process involves soaking an organic textile ®bre in an inorganic salt solution and burning o the organics to leave the sintered ®bre. Extrusion has also been widely used to manufacture ®bres, e.g. Kumar et al. 10 and Kellet et al. 11 A con- centratedpasteisforcedthroughanarrowapertureand collected onto a spool. A ®bre of diameter 50 microns can easily be produced, but smaller sizes are more dicult. Advantages of extrusion include the ability to add sintering aids, low shrinkage and ¯aws can be eliminated by colloidal processing of the paste. The 0955-2219/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00234-X Journal of the European Ceramic Society 21 (2001) 507±513 www.elsevier.com/locate/jeurceramsoc * corresponding author. Tel.: +44-121-414-5303; fax: +44-121- 414- 5324. E-mail address: r.w.greenwood@bham.ac.uk (R. Greenwood).