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UC Riverside Electronic Theses and Dissertations (2024)

Non-thermal plasmas are non-equilibrium plasmas in which the temperature of the electrons is substantially hotter than ions and neutral species. With sufficient kinetic energy, collisions of free electrons accelerated by an external electric field ionize gas molecules to create reactive precursors, valuable reactants for surface treatment and modification of thin films and nanoparticles. Plasma modification of materials is particularly useful for engineering energetic materials because the contingent surface chemical reactions directly impact the ignition properties and kinetics. Magnesium (Mg) is a desirable fuel for energetics because of its high reactivity and high energy density. However, the boiling point of its native oxide layer is greater than Mg, forcing ignition to proceed via a diffusion-limited mechanism. This dissertation focuses on plasma-induced surface engineering of Mg nanoparticles to improve their reactivity by manipulating the surface chemistry relevant for ignition. Part 1 of this dissertation describes a silicon (Si) coating applied to Mg to prevent oxide layer formation and passivate the material. In addition to surface passivation, the Si shell reacts with the Mg core upon heating to form an intermetallic alloy and provide supplemental heat release, accelerating ignition kinetics. Partial oxidation of the Si shell results in a nanothermite interface, which further increases the reactivity and supplemental heat release of the Mg nanoparticles. Part 2 will discuss how magnesium hydride (MgH2) is formed by subjecting Mg nanoparticles to hydrogen plasma treatment. MgH2 releases hydrogen gas upon heating which generates pressure to push nanoparticles apart and prevent agglomeration and sintering, but the released hydrogen can also serve as a gaseous fuel and release heat. Thus, hydrogenating the surface of Mg alters its ignition mechanism while accelerating the ignition kinetics via supplemental heating. Overall, this dissertation highlights the potential of low-temperature plasmas to engineer reactive nanoparticle surfaces in-flight for energetic applications.

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UC Riverside Previously Published Works (2023)
The hydrogenation of metal nanoparticles provides a pathway toward tuning their combustion characteristics. Metal hydrides have been employed as solid-fuel additives for rocket propellants, pyrotechnics, and explosives. Gas generation during combustion is beneficial to prevent aggregation and sintering of particles, enabling a more complete fuel utilization. Here, we discuss a novel approach for the synthesis of magnesium hydride nanoparticles based on a two-step aerosol process. Mg particles are first nucleated and grown via thermal evaporation, followed immediately by in-flight exposure to a hydrogen-rich low-temperature plasma. During the second step, atomic hydrogen generated by the plasma rapidly diffuses into the Mg lattice, forming particles with a significant fraction of MgH2. We find that hydrogenated Mg nanoparticles have an ignition temperature that is reduced by ∼200 °C when combusted with potassium perchlorate as an oxidizer, compared to the non-hydrogenated Mg material. This is due to the release of hydrogen from the fuel, jumpstarting its combustion. In addition, characterization of the plasma processes suggests that a careful balance between the dissociation of molecular hydrogen and heating of the nanoparticles must be achieved to avoid hydrogen desorption during production and achieve a significant degree of hydrogenation.
    Cover page: Enhancing the Combustion of Magnesium Nanoparticles via Low-Temperature Plasma-Induced Hydrogenation.Creative Commons 'BY' version 4.0 license
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