Low Power, High Sensitivity Gas Sensors for Detecting Propellant Aging

ABSTRACT Chemically robust, selective, low‐power sensors could aid in the monitoring of nitro‐based propellant compounds. The metal–organic framework (MOF) M‐MOF‐74 (M = Ni, Mg, Ni x Mg 1‐x ) offers exceptional sensitivity and selectivity toward trace amounts of NO 2 , a primary degradation product of nitro‐group‐containing molecules, such as propellants. MOF‐74‐based impedance sensors have been developed to detect nitro group off‐gassing due to propellant aging. Herein, additional studies were conducted to test the limits of sensor performance and investigate the long‐term stability of these sensors when exposed to elevated NO 2 concentrations indicative of propellant aging. Tested sensor materials consisted of either monometallic or mixed metal M‐MOF‐74 (M = Ni, Mg, Ni x Mg 1‐x ) films fabricated through drop‐cast or solvothermal synthesis. Repeated exposure studies were performed over a 24‐week period, with exposures to 5 ppm NO 2 in nitrogen occurring every 4 weeks at two different temperatures, with a reactivation step in between the change in temperature. The sensor performance was evaluated by analysis of the changing sensor impedance before and after repeated exposures. Both the MOF's metal center and fabrication method contributed to the sensor deactivation rate. Structural and chemical analysis of the MOF‐sensors indicated that the analyte, NO 2 , led to sensor degradation through a proposed mechanism involving NO 2 insertion and metal nitrite complexation. Understanding the abilities and limitations of these types of MOF‐based sensors under aggressive NO 2 conditions will further the development of devices specifically tailored for the relevant service environment conditions.