Alumina microtubes prepared via template-directed pulsed chemical vapor deposition (pulsed CVD)
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Bundles of alumina microtubes were prepared by depositing alumina onto bundles of “endless” carbon fibers via pulsed chemical vapor deposition and subsequent removal of the fibers. Thin alumina films were deposited onto “endless” carbon fibers at 77 °C by gas phase exposures to sequential pulses of trimethylaluminum and water vapor, respectively. The carbon fibers were selectively removed using thermal oxidation in air at temperatures exceeding 550 °C. The length of the tubes was primarily limited by the dimension of the used furnace. The longest tubes thus had a length of 30 cm. Scanning electron microscopic (SEM) images of the microtubes revealed that each individual tube was separated from its neighbors and that the tubes had an almost uniform wall thickness. SEM and transmission electron microscopic (TEM) images indicate that the inner side of the wall has the same morphology as the fiber template. As deposited, the alumina films have a predominantly amorphous structure; this is transformed into a polycrystalline structure during thermal oxidation. At low thermal oxidation temperatures, such as 550 °C, the alumina microtubes still comprise a substantial fraction of amorphous structure, at higher oxidation temperatures, 900 °C or above, a dominating polycrystalline structure (with bigger grains) is formed. This transformation gives rise to grain boundaries. These grain boundaries might facilitate oxygen diffusion and thus oxidative removal of the fiber templates.
KeywordsCarbon Fiber High Resolution Transmission Electron Microscopic Thermal Oxidation Atomic Layer Deposition Coated Fiber
We are grateful to Harry Rose, Micromanufacturing Technology, Chemnitz University of Technology, for designing the control software of the ALD reactor, Frank Diener and Frank Sternkopf from the metal-workshop, Chemnitz University of Technology, for building components of ALD reactor. We are thankful to Steffen Schulze from Solid Surfaces Analysis Group, Chemnitz University of Technology, for TEM analysis and to Thomas Mäder, Institute of Material Science and Engineering, for SEM images.
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