Interface Chemistry in Al-Based Metal/Metal Oxide Reactive Nanolaminates




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It has been reported that the interface layer plays an important role in the performance of Al-based metal/metal oxide reactive nanolaminates. The naturally formed interfacial layer is one of the major obstacles preventing the implementation of nanolaminates with bilayer thicknesses less than 50 nm. Therefore, it is crucial to study the chemistry of the naturally formed interfacial layers. CuO, the most commonly used oxide in reactive nanolaminates, has a very rough surface. Therefore, atomic layer deposition grown ZnO films are chosen to build a model oxide instead of CuO for the study on Al-based metal/metal oxide energetic nanolaminates. This dissertation mainly focuses on understanding the reaction mechanism during Al metal deposition on ZnO films. Data shows that in ZnO/Al stacks, the interface composition is a mixture of Zn, Al and O. Liberated Zn is produced on the surface because of the reaction between Al and ZnO and those surface Zn clusters desorb at 150 ºC. With an increasing temperature, O diffuses into Al and the complete mixture is achieved at 600 ºC. A 2-nm ALD Al2O3 layer deposited between ZnO and Al is able to stop the destruction of ZnO by Al. Furthermore, the oxidation of the metal layer requires higher temperature for ZnO/Al2O3/Al stacks. Unfortunately, considering that our Al source is not normal to the sample surface, the roughness of ALD ZnO film still brings about difficulties in having homogeneous reactions between Al and ZnO. Hence, theoretical calculations are launched to compare with our experimental results.
Our DFT calculations indicate that Al can destroy the ZnO lattice and push Zn atoms above the surface. Meanwhile, Zn is the least favorable surface for Al atoms. The least favorable surface sites for a Zn atom is Zn surface and this is the reason why Zn desorption from the surface is observed at 150 ºC. The activation barrier for Zn interstitials diffusing into ZnO lattice is large. Also, no surface Zn atoms are observed to diffuse into ZnO. In summary, the overall conclusions derived from the experimental results are totally consistent with the theoretical calculation results.
The experimental data also indicate that ZnO is a promising candidate to replace the naturally formed interface in CuO/Al stacks. Because of the formation of ZnAl2O4, the reduction of CuO by Al at low temperature is avoided and only one exothermic process is observed for CuO/ZnO/Al stacks. Also, the heat delivery efficiency is enhanced 32% compared to the ZnO free CuO/Al stacks.



Aluminum oxide, Nanocomposites (Materials), Interfaces (Physical sciences), Surface chemistry


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