Interfacial Reactions and Energy Transfer in Sputter Deposited Mg/CuO Thermite Reactive Nanolaminates

Multilayered stacks of reactive nanolaminates (RNLs) with well-defined interfaces consisting of a metal fuel and oxidizer offer a streamlined process for observing energy transduction and chemical reactions between thermite reactants. This presentation explores oxygen transfer in magnetron sputtered Mg/CuO RNLs as a function of bilayer thickness and metal-oxide layering sequence. Energy production in stoichiometric Mg-CuO films with bilayer thicknesses between 170-1020 nm is examined via differential scanning calorimetry (DSC), while Kissinger analysis methods are implemented to determine effective activation energies (E_a). To elucidate structural evolution in the Mg/CuO system, select samples are analyzed via in-situ high temperature x-ray diffraction (HTXRD). Calorimetry data show exothermic maxima near critical temperatures reported in the Mg-Cu phase diagram, along with a decrease in reaction temperature and effective E_a values as bilayer spacing increases. The 1 BL Mg-CuO RNL demonstrates an onset temperature near the Mg melting point of 650 °C and has the highest effective E_a of 600 kJ/mol. The 2-4 BL Mg-CuO films exhibit effective E_a values of 138-162 kJ/mol and reaction temperatures between 511-548 °C, which is within the Mg₂Cu/liquid region of the binary phase diagram. At 5-6 BL, a solid-state reaction process emerges with samples showing exothermic maxima between 388-421 °C and effective E_a values of 102-106 kJ/mol. These data are further corroborated by HTXRD data on 1, 3 and 5 BL Mg-CuO samples, with Cu peaks forming at 640 °C, 530 °C, and 415 °C, respectively. Further film characterization and preliminary thermal analysis also suggests that the epitaxial relationship (or lack thereof) between the substrate and initial RNL monolayer impacts film crystallinity, grain growth and reaction onset temperatures. Mg films deposited on c-sapphire are highly crystalline and strongly (000<i>l</i>) textured, while depositing Mg on CuO results in a decrease in overall crystallinity and a less textured Mg film. Conversely, CuO deposited on the substrate results in poorly crystalline films, but CuO crystallinity increases when CuO is deposited on Mg. Preliminary DSC analysis on Mg/CuO samples with different layering sequences also indicate that bilayer films with CuO deposited first have lower reaction temperatures and effective activation energies. These results are supported by HTXRD results for 3 BL Mg/CuO samples with opposing stacking sequences – when CuO is deposited first Cu peaks appear at ~ 475 °C, while a Cu peak does not appear until 540 °C when Mg is deposited on the substrate. It is proposed that the highly textured Mg layer has a lower defect density in comparison to more disordered layers, resulting in decreased diffusion and increased reaction temperatures. The above findings indicate that reaction properties such as energy release and onset temperature are not easily estimated by solid state reaction dynamics and enthalpies of formation alone. Factors such as bilayer thickness, oxide passivation and intermetallic formation significantly influence observed reaction pathways, while seemingly innocuous deposition decisions such as RNL stacking sequence effect crystallinity and grain growth, subsequently impacting mass transport and reaction onset temperatures.