The role of alkylamine in the stabilization of CuO nanoparticles as a determinant of the Al/CuO redox reaction

We report on a new strategy to synthesize Al/CuO nanothermites from commercial Al and ultra-small chemically synthesized CuO nanoparticles coated with alkylamine ligands. These usual ligands stabilize the CuO nanoparticles and prevent them from aggregating, with the goal to enhance the interfacial contact between Al and CuO particles. Using a variety of characterization techniques, including microscopy, spectroscopy, mass spectrometry and calorimetry (ATG/DSC), the structural and chemical evolution of CuO nanoparticles stabilized with alkylamine ligands is analyzed upon heating. This enables us to describe the main decomposition processes taking place on the CuO surface at low temperature (<500 degrees C): the ligands fragment into organic species accompanied with H2O and CO2 release, which promotes CuO reduction into Cu2O and further Cu. We quantitatively discuss these chemical processes highlighting for the first time the crucial importance of the synthesis conditions that control the chemical purity of the organic ligands (octylamine molecules and derivatives such as carbamate and ammonium ions) in the nanothermite performance. From these findings, an effective method to overcome the ligand-induced CuO degradation at low temperature is proposed and the Al/CuO nanothermite reaction is analyzed, in terms of onset temperature and energy released. We produce original structures composed of aluminium nanoparticles embedded in CuO grainy matrices exhibiting an onset temperature similar to 200 degrees C below the usual Al/CuO onset temperatures, having specific combustion profiles depending on the synthesis conditions, while preserving the total amount of energy released.