Spectroscopic Analysis of Pulsed Underwater Spark for Nanoparticles Synthesis Using Cu and Mo Electrodes

Pulsed underwater discharges are used for the synthesis of nanoparticles, through electrode tip vaporization and material condensation. The resulting colloid solution can have applications in biomedical (sterilization) or agriculture (plants fertilizing) field. The plasma formed from vaporized water and electrode material constitutes a key step in the process. This work is dedicated to studying plasma parameters as a function of current and used electrode material. Optical emission spectroscopic diagnostic is used to measure electronic temperature (based on copper or molybdenum lines) and density (based on $\mathrm{H}_{\alpha}$ and $\mathrm{H}_{\beta}$ lines). The setup allows spatially resolved study over the discharge cross-section. Results show that with molybdenum the core temperature is 30 to 90 % higher but the radial gradient is stronger when compared to copper. Electron density is also twice higher with molybdenum electrodes. High speed imaging confirms that with this metal the spark light intensity is much stronger and the formed cavitation bubble is larger. These results can be explained by the lower thermal conductivity of molybdenum electrodes, leading to a more energetic discharge.