Structure and Chemical Characterization at the Atomic Level of Reactions in Al/CuO Multilayers

Abstract

Sputter-deposited Al/CuO multilayers exhibit fast combustion reactions in which an exothermic chemical reaction wavecontrolled by the migration of oxygen atoms from the oxide matrix toward the aluminum layers through interfacial layersmoves throughout the multilayer at subsonic rates (meters per second to tens of meters per second). We directly observed the structural and chemical evolution of Al/CuO/Al multilayers upon heating to 700 °C using high-magnification transmission electron microscopy (TEM) and scanning TEM, providing simultaneous subnanometrer imaging resolution and detailed chemical analysis. Interestingly, as deposited, the trilayer is characterized by two distinct interfacial layers: 4.1 ± 0.2 nm thick amorphous alumina and a 15 ± 5 nm thick mixture of AlO x and Cu x Al y O z , at the bottom interface and top interface, respectively. Upon heating, we accurately characterized the evolving nature and structure of these interfaces, which are rapidly replaced by the reaction terminal oxide (Al 2 O 3 ). For the first time, we unraveled the release of gaseous O from the sparse columnar and defective CuO well below reaction onset (at ∼200 °C) which accumulates at interfaces and contributes to initiate the Al oxidation process at the vicinity of native interfaces. The oxidation process is demonstrated to be accompanied by a continuous densification and modification of the CuO layer. Between 300 and 350 °C, we observed a brutal shrinkage of the CuO layer (14% loss of its initial thickness) leading to the mechanical fracture in the top alumina growing layer. Consequently, this latter becomes highly permeable to oxygens leading to a brutal enhancement of the oxidation rate (×4). We also characterized stressed-induced interfacial delamination at 500 °C pointing clearly to the mechanical fragility of the top interface after the CuO transformation. Altogether, these results permit one to establish a multistep reaction scenario in Al/CuO sputter-deposited films supporting to an unprecedented level a mechanistic assignation of differential scanning calorimetry peaks. This study offers potential benefits for the development of aging models enabling the virtual prediction of the calorimetric response of exothermic Al/CuO thin-film reactions.

Engineering of Al/CuO Reactive Multilayer Thin Films for Tunable Initiation and Actuation

“…The RMS surface roughness is found to be 5.2 AE 0.2 nm in a 1 mm 3 1 mm scanning area. This roughness is substantially larger than that of the initial SiO 2 surface (RMS~0.3 nm) [22].…”

“…High magnification TEM micrographs ( Figure 4) and Atomic Force Microscopy (AFM) maps ( Figure 3) obtained after CuO deposition on an oxidized Silicon wafer show the columnar growth by the pillar-like structure [22]. Grain sizes in the CuO film grown under sputtering parameters are reported in Table 2 and vary between 10 and 20 nm.…”

“…We can distinguish the two distinct interfaces on the TEM images of Figure 4 top interface (TI CuO/Al ) interface created upon the deposition of Al (Al top ) onto CuO is overall ill-defined (see dark-field image in Figure 4 [25].…”

“…EDX profile confirms that this top interface is much thicker and less abrupt than the bottom one. Figure 6 also reveals a slight oxygen intensity along the aluminum bottom layer, but in the EELS spectra, no presence of alumina was observed along the layer and no aluminum is detected in the CuO [25]. Further analysis (not reported in this paper) of the oxygen elemental distribution image by EELS indicated that the oxygen apparent in the Al layer is primarily a result of surface oxidation after TEM lamella preparation.…”

“…AFM maps of 200 nm thick CuO after sputter deposition (substrate is oxidized silicon) and X-ray diffraction scan[22].…”