Oxidation behavior of high-purity nonstoichiometric Ti 2 AlC powders in flowing air
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The oxidation behavior of nonstoichiometric Ti2AlCx (x 5 0.69) powders synthesized by combustion synthesis was investigated in flowing air by means of simultaneous thermal gravimetric analysis-differential thermal analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscope/energy dispersive spectroscopy, with an effect of powder size. The oxidation of the fine Ti2AlC powders with the size of about 1 lm starts at 300 °C and completes at 980 °C, while with increasing the powder size around 10 lm the corresponding temperature increases to 400 and 1040 °C, respectively. The oxidation of nonstoichiometric Ti2AlCx (x 5 0.69) powders is controlled by surface reaction in 400–600 °C, and mainly diffusion in 600–900 °C, with the corresponding oxidation activation energy of 2.35 eV and 0.12 eV, respectively. In other words, the critical temperature of changing oxidation controlling step is around 600 °C. The oxidation products were mainly rutile-TiO2 and a-Al2O3. The tiny white flocculent particles of a-Al2O3 appeared on the surface of fine Ti2AlC powders and increased with increasing the oxidation temperature.
I. INTRODUCTION
Ti2AlC belongs to a family of Mn11AXn ternary compounds (MAX phases for short, where M represents early transition metal, A is group IIIA or IVA element, and X means C/or N, with n 5 1–3),1 which displays a unique combination of the advantageous attributes of both metals and ceramics. Like metals, Ti2AlC has good thermal conductivity [46 W/(m K)],2 high damage tolerance,1 microscale plastic deformation,3 and excellent thermal shock resistance.4 Like ceramics, it exhibits low density (4.11 g/cm3 ), 1 high Young’s modulus (277.6 GPa),5 and excellent high-temperature oxidation resistance.6 These unusual properties make Ti2AlC for potential structural and functional applications.7 Due to the great difficulty in the preparation of highpurity Ti2AlC, although early in 1963 the material was experimentally discovered by Jeitschko et al.,8 it has been ignored for a long time. Around 2000, Barsoum et al.9,10 fabricated polycrystalline high-purity Ti2AlC by reactive hot isostatic pressing and first identified its unusual mechanical and electrical properties. However, whether it is the long reaction time or high processing temperature will limit the future application of this technologically important material. To respond this problem, by the use of Contributing Editor: Yanchun Zhou a) Address all correspondence to this author. e-mail: [email protected], [email protected] DOI: 10.1557/jmr.2017.83
self-propagating high temperature combustion synthesis with pseudohot isostatic pressing (SHS/PHIP), Bai et al. have succeeded in the synthesis11,12 and characterization13,14 of high-purity Ti2AlC bulk, with the advantages of energy saving, short fabrication time, simple operation, and no external heat source. The resultant sample mainly contains typical plate-like nonstoichiometric Ti2 AlCx (x 5 0.69), which is rich in the lattice defects. Of importance, the fine microstructures with a mean g
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