1684 Journal of the American Ceramic Sociery-Valcarcel et al Vol 86. No. 10 work, the best conditions(e. g, temperatures and percentages) for The tensile strength of some of the so-obtained whiskers, one of fiber production have been established in relation to the metal the most important features for their reinforcing applications, was estimated. The measuring procedure was as follows: One hole was The aim of the present work was to attempt ve fiber drilled on a rectangular piece of autoadhesive tape. One fiber yield. To achieve this goal, one simple approach was to increase ribbon) then was pasted onto this hole, using two drops of an d sub epoxy-resin-based glue. Special care had to be taken to keep the more deposited Al-O,. However at >1500 C, the growth of sample parallel to the longitudinal edge of the plastic strip. AL,O, fibers via VLS is restrained. We hypothesized that this Carefully, the plastic with the fiber was placed on the appropriate growth restraint occurs because the liquid silicon (Si()drops test machine. Finally, the plastic strip was cut with a hot blade, needed for the VLS process become unstable at such high and, therefore, the fiber started to hold the force by itself. when the temperatures. Because of the increasing temperature, the equilib- force exceeded the tensile strength of the fiber, it fractured rium between Si(n formation and Si(n) evaporation is altered The fracture was then examined using SEM. Sometimes the causing the drops to disappear eventually and, thus, restraining the breakage resulted from debonding between the fiber and the glue. growth of c-AL,O, whisker in which case the recorded data were rejected. If the fiber was This theory prompted us to use different metals that could be actually broken, the cross section was measured using SEM, which melted (and would. eventually, form drops)at temperatures allowed us to calculate the tensile strength of the fiber. The tensile 1500C. We then started to grow a-Al, 0, fibers by adding strength was estimated dividing the force by the surface area of the certain selected metals to the SiO, powder. The resulting fibers were chemically characterized and their tensile properties esti mated. The c-Al,O, fibers and ribbons were analyzed using duction-coupled plasma(ICP), which showed that they had not llL. Results been contaminated by the metals used during their production described earlier, the present VLS method consisted of small pieces of aluminum over a shallow bed of quartz, inert furnace atmosphere of argon. at temperatures IL. Experimental Procedure between 1300 and 1500C. a-Al, O, crystals were readily Aluminum wires(diameter 4 mm, length 1 cm) of 99.9 wt% produced by this method and grew into a white, cottonlike mass purity were placed onto a powder bed, The powders in the bed of crystals around the aluminum piece(Fig. 1).Detailed SEM were composed of quartz sand, with the eventual addition of analysis revealed that, although these crystals showed varied various percentages of metals (or metal oxides), The metals used morphologies, two classes could be distinguished: fibers and ribbons(Fig. 2). Each fiber had a hexagonal section, drops at were in the form of either powders or small pieces (balls, etc. ) one of its tips. and one basal hexagonal pyramid. Strikingly, the Hm). The Co,O, powder used in our experiments actually con- fibers had aspect ratios of >10 and, sometimes, up to 10. The sisted of a mixture of cobalt ions coexisting under two oxidation -I cm, branch width-10 um thickness.5 um), triangular states(2+ and 3+) tips with eventual drops and branching out at fixed angles(60") A. The programmed temperature cycles consisted of an initial mperature- increase rate of 10 C/min to a plateau temperature to produce large single crystals with elaborate dendritic struc that was maintained for 2-4 h, after which the system was cooled tures, Dendrites usually appear in crystals developed inside liquids, contrary to the gaseous atmosphere in which ribbons to room temperature. at a rate of 10C/min, The plateau temper- develop. The drops at the tips of the fibers clearly showed that of the times, Furnaces with inert atmospheres of argon gas were the fibers had grown via VLS deposition. Although the growth mechanism of the ribbons remains unclear. our available data used, and a 0.2 L/min argon flux was maintained during the indicate that it was vapor-solid(VS) deposition. mperature-increase and the temperature-decrease steps. The furnace tube remained closed and airtight during the periods of c-axis growth. In contrast, the ribbons showed preferential growth constant temperature. -packed A series of assays was designed to investigate and monitor fiber basal plane. Thus. the fiber-growth direction was normal to the ield. Only one variable was changed at a time. Thus, the crucibles ribbon-growth direction This finding explains how fibers can use used were always of the same size and shape; the same amounts of ribbons as crystallization surfaces, with each ribbon resulting in a SiO, powder and aluminum wires were always used. and all the set of fibers and, thus, the use of other (expensive) materials other considered variables(temperature ramps, gas flux, etc. )were avoided. At>1500 C, the production of such crystals decreased held constant. The only difference between the two assays was ignificantly. Nevertheless, some additives can alter the situation either the addition of different percentages of powdered metals( romoting fiber development at higher temperatures. their oxides) or the plateau temperature used. The fibers were The first case study on the effect of metals addition was haracterized using optical microscopy. X-ray diffractometry performed using nickel powder. Several assays were conducted to (XRD), transmission electron microscopy(TEM), and scanning show the effect of adding different amounts of powdered nickel, at electron microscopy(SEM) with energy-dispersive X-ray spec- different temperatures. For a comparison of the results, diagran oscopy analysis(EDS were designed based on the ratio between the weight of the Al,O ICP analysis revealed that the purity of the fibers obtained using an additive and that of the fibers with no as of the fib metals added. Thus, improvement resulting from the additives SEM and EDS showed that the actual purity of the fibers could be read at a glanc have been higher. Three possibilities for this discrepancy were Figure 3 shows the effect of adding 20% powdered nickel (< the fibers: (ii) impurities may have been inside the VLS drops: and improvement was observed at <1500C. but the effect became (iii)impurities may have been present because of solid solutions of considerable at 1550"C:20 times more c-Al,O, tibers were uch elements into the c-Al, O. To distinguish between"clean obtained protocol was developed purities inside the fibers, a cleaning purities and im Figure 4(A) shows the effect of adding various percentages of nickel at 1550.C. Clearly, percentages >5% did not cause further Theoretically, impurities should be removable ng. improvement. We also tested the effects of adding NiO, cobalt, and because they are not truly"inside "the fibers. This was Co,O,(Figs. 4(B)(D)). ucted in a 35% HF-65% HNO, solution EDS microanalysis clearly showed that nickel and cobalt might immersion in HCl ppeared at the drops. whereas only aluminum was localized at th