l686 Journal of the American Ceramic Sociery-Valcarcel et al Vol. 86. No: 10 100pm 100pm Fig. 2.(A)Fibers have lengths well above hundreds of micrometers, with cross-sections <10 um (B)Ribbons have very particular features, as described n the lexl. (C) Drops at the tip of fibers demonstrate that they grow via VLS deposition. (D) Schemes of a fiber. a ribbon, and the initiation of a fiber on The VLs process can be described as follows: When the aluminum evaporates, it reacts with the quartz powder in the crucible, and new gaseous species, such as aluminum and silicon oxides (i.e. AlO(g), Al,O(g), or Sio(g)) are generated Such gaseous species can react to produce Al,O,(allowing epitaxial growth to continue)and Si(l). This Si(/) forms drops in which gaseous species can dissolve, increasing their probability of reacting to produce Al,O The excess Si(l) diffuses back He-Si0, *Ni into the atmosphere. because of the Si( equilibrium vapor 9 si(h) is critical for drop formation. These drops at the fiber tips slowly dissolve gaseous phases rich in aluminum and silicon oxides, thus increasing their probability of reacting. In this way, a-Al,O, that can be deposited onto an adequate crystallization surface is produced. The fiber tip itself always acts as a crystallization surface, allowing epitaxial growth Ie continue with a constant cross section (fibers as long as -2 cm have been attained). Si(/) drops are the main factor responsible for the unidirectional growth of the c-Al,O, whiskers. 130013501400145015001550 The use of an argon-gas atmosphere with an initial oxygen Temp(C) content of about 2 ppm (Po.=10" atm) restrains aluminum oxidation and, therefore, its"passivation. "In addition, Al(g)reacts Fig 3. Effect of the addition of a 20%E of powder with oxygen, causing an even lower Po (<63m)to he SiO, at various temperatures. Below 1500 C, a sli provement can In summary, two phenomena are needed for VLS deposi be observed, but, above this temperature. the effect becomes considerable Deposition can progress as long as gaseous species are produced about 20 times more corundum fibers are obtained