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Cross-section SEM images showing the morphological evolution of the films as a function of the oxidation time: (a, b) 3 min, (c, d) 15 min and (e, f) 30 min. For all the samples the electrical power was fixed at 100 W and the pressure was fixed at 3 Pa. The SEM cross-section in panel (e) was prepared using FIB to verify whether the formed microspheres were hollow or not. Scale bar: 1 µm.
