Enhancing the Combustion of Magnesium Nanoparticles via Low-Temperature Plasma-Induced Hydrogenation

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.


Actinometry to Estimate Hydrogen Density:
An actinometry was employed to estimate the atomic hydrogen density in the plasma.By utilizing the intensity ratio of emission lines obtained from our optical emission spectroscopy (OES) measurements, a relationship summarized by Tatarova et   , transitions from the same excited states for the corresponding atomic species.It is necessary to achieve the accurate branching ratio.The transition probabilities used in this analysis are sourced from the NIST atomic spectra database. 2s quenching rate that takes into consideration the    nonradiative relaxation of the excited state for Ar.The variable is the density of the collision   partner, which is determined using the ideal gas law.( =Ar and H 2 ) For the specific numerical  values in this relation, we refer the reader to our previous report. 3nd represent the rates     of excitation by electron impact to the selected states of Ar and H.These rates were obtained through BOLSIG+ calculation, a commonly used software for the solution of the Boltzmann transport equation.4 The collision cross-section for Ar, H, and H 2 are acquired from the LXCat database.[5][6][7] Despite the dependence of excitation rates on the electron temperature, the derived atomic hydrogen density as function of plasma power shows almost same trend according to the electron temperature since emission line intensities are strongly influenced by the plasma power.Uner et al. actually measured electron temperature through quite similar plasma reactor with our reactor, and it was 4±1.5 eV. 8 Therefore, based on our previous estimation for electron temperature and reactor geometry that used in this study, we assumed that the electron temperature is 5 eV. 9

Details of the calculation of the hydrogen content
For the TPD measurement, we used 55 mg of h-Mg sample with 17.8 wt.% content according to Rietveld Refinement fitting method.Figure s5 shows the fitting of the XRD pattern.Profex software uses an algorithm to calculate cell parameters and peak positions of a space group.The peak intensities are then obtained from calculated structure factors from the atomic sites of a certain phase.The peak position on the 2theta axis is matched by optimizing the cell parameters while the intensities are matched by changing the scale factor.After the fitting, the weight fraction of a phase is calculated by the following equation: W is the weight percent of the phase, S is the Rietveld scale factor, Z is the number formula units per unit cell, M is the mass of the formula unit, and V is the volume of the unit cell. 10We used the program's fitting method through their algorithm to help quantify the relative phase composition of MgH 2 from the h-Mg sample.
The TPD measurements provided us the flow rate of H 2 according to time upon heating the 55 mg of h-Mg NPs in a furnace (Figure s1).This can be simply done because the argon flow rate through the furnace is well known.By integrating the area underneath the peak, we obtained the total volume of H 2 desorbed from the particles.The following equations helped estimate the mmol of H 2 from the weight percentage obtained by Rietveld Refinement and from the experimental measurement from TPD.We added this information to the supporting information document.

Figure s1 :
Figure s1: TPD of 55 mg h-Mg NPs with a heating rate of 10 o C min -1 .The integrated area

Figure s2 :
Figure s2: (a) Production rates of h-Mg NPs with RF power varied from 40 W to 100 W. (b) A

Figure s3 :
Figure s3: Particle size distributions of h-Mg NPs synthesized using (a) 80 A and (b) 90 A of
1l. is used to derive the atomic hydrogen density1: and argon at those specific wavelengths.The following emission lines were used to derive atomic hydrogen density: Ar line at 763.5 nm (transition 4p to 4s) and H line at 656.2 nm (transition from 3d 2 D to 2P 2 P 0 ).