Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2001, 40 (19), 3688 - 3690
Keeping in Shape
A new method for the easy production of components made of iron silicide
Compounds made of metals and the elements Boron, Carbon, and Silicon are interesting materials that are extraordinarily hard and melt only at very high temperatures. Producing these substances, called borides, carbides, and silicides, and forming them into the desired shapes is not so easy, however. Often, long drawn-out sintering processes at extremely high temperatures are necessary. Michael Binnewies, Arndt Meyer, and Mike Schütte have now found an unusual reaction by which components made of iron silicide can be produced quickly and easily in the desired shape.
Bringing a metal and a nonmetallic compound together in a reaction normally results in a powder - regardless of whether the metal was introduced as a powder, sheet, or wire. Binnewies and his co-workers were thus appropriately surprised by the results of their experiments: they exposed a coil of iron wire to silicon tetrachloride - a gas at high temperatures - for three hours at 1000 °C. The spiral form was maintained, although the iron was completely converted to iron silicide. "An object made of iron can thus be converted to an identical object made of iron silicide with no discernible change in shape," Binnewies explains the surprising observation. "This result contradicts nearly everything experience has taught us about reactions of solids with gases." Also unusual is the high speed at which the reaction proceeds.
How does such an atypical reaction occur? Binnewies thinks the formation of pores in the iron silicide is behind it: these tiny channels allow the transformation of the material to proceed. The silicon tetrachloride gas can move unhindered into the spiral. Aside from the iron silicide, the reaction also produces iron chloride, which is present as a gas under the conditions of the reaction. It can escape through the pores. Thus no dense outer layer of iron silicide is formed to hinder the further reaction. That the initial form is not lost is due to the stable iron core, which initially remains intact. This gives the individual granules of iron silicide on the surface of the wire time to grow together - instead of crumbling into a powder.
The production of iron silicide components by this method should be very simple: the iron is first formed into the desired shape, and then made to react.