This paper aims to study the oxidation kinetics of the nanocrystalline Al ultrathin films. The influence of structure and composition evolution during thermal oxidation will be observed. The reason for the change in the oxidation activation energy on increasing the oxidation temperature will be discussed.
Al thin films are deposited on the silicon wafers as substrates by vacuumed thermal evaporation under the base pressure of 2 × 10−4 Pa, where the substrates are not heated. A crystalline quartz sensor is used to monitor the film thickness. The film thickness varies in the range from 30 to 100 nm. To keep the silicon substrate from oxidation during thermal oxidation of the Al film, a 50-nm gold film was deposited on the back side of silicon substrate. Isothermal oxidation studies of the Al film were carried out in air to assess the oxidation kinetics at 400-600°C.
The activation energy is positive and low for the low temperature oxidation, but it becomes apparently negative at higher temperatures. The oxide grains are nano-sized, and γ-Al2O3 crystals are formed at above 500°C. In light of the model by Davies, the grain boundary diffusion is believed to be the reason for the logarithmic oxidation rate rule. The negative activation energy at higher temperatures is apparent, which comes from the decline of diffusion paths due to the formation of the γ-Al2O3 crystals.
It is found that the oxidation kinetics of nanocrystalline Al thin films in air at 400-600°C follows the logarithmic law, and this logarithmic oxidation rate law is related to the grain boundary diffusion. The negative activation energies in the higher temperature range can be attributed to the formation of γ-Al2O3 crystal.
This work was supported by the Key Scientific and Technological Research and Development Project of Jilin Province (20180201080GX), the National Natural Science Foundation of China (No. 61674069 and No. 51631004), the Project of First-Class Universities and First-Class Disciplines, the Fundamental Research Funds for the Central Universities, the Program for Innovative Research Team (in Science and Technology) in University of Jilin Province and the Program for JLU Science and Technology Innovative Research Team (2017TD-09). This research received no external funding.
Luo, J., Zhang, L., Yang, H., Zhang, N., Zhu, Y., Liu, X. and Jiang, Q. (2019), "Oxidation kinetics of nanocrystalline Al thin films", Anti-Corrosion Methods and Materials, Vol. 66 No. 5, pp. 638-643. https://doi.org/10.1108/ACMM-11-2018-2037
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