In-Flight Coating of Magnesium Nanoparticles via Non-Thermal Plasma for Energetics

Nano-sized energetic materials allow for better combustion performance compared to micron-sized materials. Among them, magnesium nanoparticles have shown promise for energetic applications due to its low ignition temperature, light weight, and high theoretical energy density [1]. A significant disadvantage of magnesium as a fuel is its moisture and air sensitivity resulting in formation of an oxide layer hindering combustion performance [2]. For thermite reactions utilizing all solid-state fuel and oxidizer materials, the oxide layer surrounding the fuel impedes the reaction by slowing the diffusion kinetics [3]. Direct coating of the magnesium surface with the oxidizer is expected to enhance the combustion performance by enabling faster diffusion kinetics. In this work, we use a non-thermal plasma approach to coat magnesium particles in-flight, in the gas-phase with a thin silicon layer, with the goal of both preventing magnesium oxide formation and achieving a direct contact between fuel (magnesium) and oxidizer (silicon oxide).<br/>Magnesium core and silicon shell nanoparticles were synthesized via a two-step process involving resistive thermal evaporation of commercial magnesium followed by a radiofrequency non-thermal silane plasma. Ex-situ characterization of nanoparticle crystallinity, morphology, and chemical composition was accomplished by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive x-ray spectroscopy (EDS). Processing parameters were manipulated in an attempt to prevent secondary nucleation of silicon nanoparticles and alloying of magnesium with silicon. The silicon shell was treated with oxygen to convert it to silicon oxide, resulting in a nano-thermite consisting of a nano-sized magnesium core and a silicon oxide shell.<br/>The combustion performance of the synthesized nano-thermites was tested against non-coated magnesium nanoparticles mixed with oxidizer. Nano-thermites were ignited using a resistively heated nichrome wire, and constant-volume combustion cell measurements were performed to obtain the peak pressure, pressurization rate, burn time, and flame temperature. Ignition temperature and combustion products were characterized via temperature-jump time-of-flight mass spectrometry, wherein a platinum wire was coated with sample and heated until ignition. Our findings suggest that the reaction onset of coated magnesium is faster due to the close proximity of the fuel and oxidizer during ignition. In addition, the ignition temperature and ignition delay time decreased in the coated samples compared to non-coated magnesium.<br/><b>References:</b><br/>[1] Dilip Sundaram, Vigor Yang, Richard A. Yetter, Metal-based nanoenergetic materials: Synthesis, properties, and applications, Progress in Energy and Combustion Science, 61, 293-365 (2017).<br/>[2] Stanislaw Cudzilo, Waldemar A. Trzcinski, Jozef Paszula, Mateusz Szala, Zbigniew Chylek, Performance of Magnesium, Mg-Al Alloy and Silicon in Thermobaric Explosives – A Comparison to Aluminum, Propellants, Explosives, Pyrotechnics, 45, 1691-1697 (2020).<br/>[3] Kelsea K. Miller, Jennifer L. Gottfried, Scott D. Walck, Michelle L. Pantoya, Chi-Chin Wu, Plasma surface treatment of aluminum nanoparticles for energetic material applications, Combustion and Flame, 206, 211-213 (2019).