Combustion Behaviour of Single Silicon Particles in Different Oxidizing Environments
Silicon, despite its abundance and high energy density, remains underexplored as a carbon-free fuel, with limited data available on its combustion characteristics. In this work, the combustion behaviour of silicon particles is examined using an electrostatic levitator with laser ignition. Five oxidizing environments at atmospheric pressure are investigated: air, pure oxygen, and mixtures containing 40% oxygen (by mole) diluted with nitrogen, helium, or argon. The burning droplet peak temperature, measured by three-colour pyrometry, increases by 337 K from air to pure oxygen. The peak temperature of the silicon droplet in the 40%O2-60%He mixture is lower than that in the 40%O2-60%Ar mixture, in contradiction with thermodynamic predictions, due to a higher Lewis number of the helium-diluted mixture. Although oxygen diffusivity is higher in the helium-diluted mixture, a lower burning rate is observed, attributed to the lower combustion temperature. High-speed colour camera observations reveal that the square of the particle diameter decreases with time in each combustion run, following a strong linear relationship (R2 > 0.99) across all oxidizing environments. However, the combustion lifetime is proportional to the initial particle diameter to the power of n, with n ranging from 1.69 to 1.82. This deviation from the expected n = 2 appears to result from unavoidable measurement uncertainties and the limited particle size range, rather than differences in combustion physics. The decrease in silicon droplet size during combustion is attributed to the formation of gaseous SiO as an intermediate combustion product. The SiO species is observed using a UV camera, showing a UV intensity decay from the particle surface. High-speed imaging and LED absorption signals indicate that the final condensed product, SiO2 nanoparticles, are not optically visible, suggesting they possess very low emissivity.