Communications 1370 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,1998 0935-9648/98/1611-1370 $ 17.50+.50/0 Adv. Mater. 1998, 10, No. 16 Novel Ribbon-Shaped a-Al 2 O 3 Fibers** By Víctor Valcµrcel,* Antonio PØrez, Marek Cyrklaff , and Francisco Guitiµn* Owing to their brittleness, ceramics have been regarded as materials of modest performance, especially under ten- sion or bending conditions. In contrast to metals or poly- mers, however, ceramics' thermal stability above 700 C makes them suitable materials for high-temperature appli- cations. Microstructural design is critical in order to obtain reliable ceramic materials. Since the 1960s a wide range of oxide, non-oxide, and composite materials have been developed that have enhanced properties. Such materials are usually termed ªadvanced ceramicsº. Inorganic fibers are widely employed to improve fracture toughness in advanced ceramics. The use of Al 2 O 3 poly- crystalline fibers and whiskers (short single crystals) as strengtheners in high-temperature composites is of great interest owing to their high elastic modulus and their ther- mal and chemical stability. [1] The chemical composition of a-Al 2 O 3 makes alumina fibers preferable to non-oxide fi- bers, which alter their properties owing oxidation at high temperatures. Alumina whiskers and fibers are used in ad- vanced composites such as the so-called metal matrix com- posites (MMCs), which are mainly alumina included in molten aluminum. [2,3] An enormous effort is being made to obtain alumina and aluminosilicate fibers with high Al 2 O 3 content. Available methods for production of inorganic fibers have been re- viewed by Cooke, [4] although new techniques are continu- ally being developed. Al 2 O 3 whiskers and polycrystalline fibers have been obtained by procedures that are rather difficult and expensive. We have described [5] a novel method for production of a-Al 2 O 3 fibers using aluminum of commercial purity and powdered silica as raw materials. Small pieces of Al were placed over a shallow bed of SiO 2 in an inert furnace atmo- sphere of Ar at temperatures between 1300 and 1500 C. The fibers had hexagonal cross section and high aspect ra- tio (ratio of length to thickness). Such features make these fibers suitable as strengthening elements in composite ma- terials. These single a-Al 2 O 3 fibers were produced by a va- por±liquid±solid (VLS) [6] deposition mechanism. A common feature of inorganic fiber growth is the pres- ence of various slightly different crystals arising from the same procedure, [6,7] due to various mechanisms occurring simultaneously or to the same deposition mechanism tak- ing place under distinct local conditions at distinct furnace zones. Here we describe a novel type of a-Al 2 O 3 fiber, gen- erated in the same furnace zones as the previously de- scribed a-Al 2 O 3 fibers. Unlike the hexagonal fibers, the new fibers have two parallel flat faces and are very thin (Figs. 1a and b). We have termed them ribbons, reserving the term fibers for those with hexagonal cross section. We also report a procedure that allows ribbons to be obtained while the fiber growth is inhibited. Fig. 1. a) SEM micrograph of the a-Al 2 O 3 ribbons. Branches at fixed angles and triangular-shaped tips can be observed. Note the a-Al 2 O 3 whiskers, with hexagonal section, which we have termed fibers throughout the text, grow- ing on the ribbon's surface. b) Sporadic drops appear at ribbon endings. Details of the synthesis method are given in the Experi- mental section. Results from X-ray diffractometry (XRD), transmission electron microscopy (TEM), and scanning electron micros- copy (SEM) with microanalysis via energy dispersive spec- trometry (EDS) indicate that the chemical composition of the ribbons is a-Al 2 O 3 (corundum). Using TEM methods we have confirmed that ribbons crystallize on the same hexagonal system as the fibers, only the habit is different (Fig. 2c). When the piece of Al was not covered with a powder as described in the experimental section, after the tempera- ture processing a white cotton-wool-like mass was ob- tained, corresponding to a mixture of fibers with a small ± [*] V. Valcµrcel, A. PØrez,Prof. F. Guitiµn Instituto de Cerµmica Universidade de Santiago de Compostela (ICUS) Edificio Monte da Condesa, Campus Sur E-15706 Santiago de Compostela (Spain) Dr. M. Cyrklaff European Molecular Biology Laboratory (EMBL) Meyerhofstrasse 1, D-69117 Heidelberg (Germany) [**] This work was supported by CICYT under project MAT97-0728. The authors thank Dr. Salvador de Aza (ICV, Instituto de Cerµmica y Vidrio, Arganda, Madrid) for useful discussions.