A Comprehensive Method for Monitoring the Key Functional Characteristics of the Self-Propagating High-Temperature Synthesis Reaction in Multilayer Thin-Film Structures

Introduction. Reactive multilayer foils undergoing self-propagating high-temperature synthesis (SHS) consist of alternating layers of two or more materials capable of participating in an exothermic reaction. These foils are typically fabricated using thin-film deposition techniques such as magnetron sputtering. Activation is achieved through external stimuli–electrical, thermal, laser, or mechanical–resulting in a rapid release of heat. Peak reaction temperatures can reach 1500 °C, and the energy released may be as high as 1200 J/g. The combustion front can propagate at speeds of 8–10 m/s. Due to the broad range of industrial applications for reactive multilayer foils, there is a need for a reliable method to monitor the critical parameters of the foil–both post-fabrication and during the optimization of fabrication processes. This study aims at developing and validating a comprehensive monitoring method for the SHS reaction in multilayer thin-film structures, allowing for simultaneous assessment of reaction characteristics using a single experiment on a single sample. The following objectives were posed in the study: to analyze existing monitoring methods and propose new or improved approaches suitable for integration into a unified methodology; to conduct experimental validation using sample foils tailored to the testing techniques employed within the comprehensive method; to develop and substantiate a unified monitoring approach based on experimental data, emphasizing the behavior of SHS in thin-film structures. Results. A calorimetric setup using copper plates and platinum sensing elements effectively measured the energy released during the SHS reaction. Thermal equilibrium was reached within 1–1.5 minutes. Preheating the system to +35 °C (above room temperature) accelerated cooling but had no effect on the final thermal equilibrium state. A method was proposed for determining the SHS reaction activation energy by gradually increasing the voltage across a capacitor, followed by direct contact between the capacitor electrodes and the foil surface. The activation energy was found to be independent of both pulse duration (capacitor capacitance) and amplitude (applied voltage). For example, activation occurred with a 300 μF capacitor at 12 V (yielding 23.8 mJ), as well as with a 1000 μF capacitor at 6.9 V, corresponding to the same energy level of 23.8 mJ. The propagation velocity of the SHS reaction front was successfully measured using two optical sensors (phototransistors). With a l = 20 mm distance between sensors, the required sensor response time and system measurement accuracy (error ≤ 0.1 ms) were established. The selected phototransistors demonstrated a response time of 15 μs.