New Massive Vapor‐Liquid‐Solid Deposition of α‐Al₂O₃ Fibers

Abstract

We have elsewhere [1,2] previously described a novel method for production of alumina whiskers from aluminum pieces and powdered silica in Ar atmospheres. In such procedure, at adequate temperatures, the Al evaporates and reacts with the SiO 2 , producing SiO (g) and AlO x(g) . The subsequent chain of reactions allows the deposition of thin alumina single crystals, or whiskers, with high aspect ratios. The incorporation [3] of different metal gasses (mainly Ni (g) ) to the furnace atmosphere allows the use of higher temperatures, thus increasing the yield of the whisker production. The growth of such alpha-alumina (corundum) whiskers was established to occur through Vapor-Liquid-Solid (VLS) deposition.Great benefits are expected with the use of VLS alumina whiskers as reinforcing elements in composites [4][5][6] and our experiments are presently corroborating it. Although the details of the analyses performed on our first demonstration material (a 6061 T6 aluminum matrix composite with a 10 % volume of VLS Al 2 O 3 whiskers) will be published as soon as possible, here we can anticipate that the improvement obtained in the mechanical performance of the composite is fairly good.Such excellence has forced a lot of research in the field of whiskers, although in the past their use has always been seriously restrained due to cost considerations. Therefore, new technologies that can produce affordable alumina whiskers are presently being demanded by industries interested in advanced composite materials. In our case (VLS alumina whiskers) our approach to such challenge was to investigate how the incorporation of controlled amounts of an oxidant agent affected the VLS deposition of crystals.Experiments were designed trying to maintain the system under such conditions that the VLS (instead of Vapor-Solid, VS) were still the dominant deposition mechanism. Therefore, all the advantages of the VLS c-axis alumina single crystals could be preserved, together with an increase in their production yield. Our experiments soon demonstrated that this increase was much higher than expected, in some cases more than a hundred-fold increase!). Surprisingly, a newly fashioned VLS mechanism was also found, which has not previously been described elsewhere. Here we will describe its main features together with the physical and chemical processes involved.In order to establish the reinforcing potential of the whiskers produced, we decided to employ atomic force microscopy (AFM). The results demonstrated that smooth-surfaced single crystal fibers were produced under this massive VLS deposition, although other microstructures have also been observed. ExperimentalOur predictions on the behavior of the system were grounded on the respective Al-Al 2 O 3 and Si-SiO 2 volatility diagrams [7,8] at 1550°C (Fig. 1). A fast oxidation of the Al wires (employed as raw materials in our experiments) was to be avoided, because this would restrain the production of Al(g), thus hindering the rest of the VLS process. Figure 1(b) reveals that below 10 -3 atm,...

Impact of microwave processing on porcelain microstructure

“…9a reveals an unusual mullite morphology growing inside a bubble in a MWS sample. Mullite fomred in the bubbles was believed to grow via a Vapour-Liquid-Solid (VLS) mechanism [31,32] . Mullite then continually grew in [001] direction in to the air which had nuclei randomly precipitated on the needles (Fig.…”

Growth mechanism of single-crystal α-Al2O3 nanofibers fabricated by electrospinning techniques

“…8 Later approaches have adopted a so-called "displacement reaction" by heating mixtures of powder aluminum (Al) and metal oxides, such as SiO 2 , 10-17 MoO 3 , 17 WO 3 , 17 or SiC 18 in an argon atmosphere to fabricate single-crystal fibers, whiskers, and nanowires. The growth of the fibers/whiskers was found to be mostly in the [0 0 1] direction through the VLS mechanism [10][11][12][13][14][15]18 or through a mechanism that is analogous to Ostwald ripening. 16,17 Both of these mechanisms are categorized as tip growth models, resulting in characteristic round droplets or slight sharp curvatures on the tips of the fibers/whiskers.…”

Preparation and Characterization of Alumina Nanofibers by Gas-Solid Reaction Method

“…Hence, the large scale synthesis of nanowires in bulk quantities is essential, not only to study the properties of this materials on nanoscale, but also for their used in various applications. Many researchers try their best to develop the preparation method of alumina nano fibers in large quantities [3][4][5][6][7][8].…”