Spark ablation is a highly effective and versatile method for producing nanoparticles from bulk conductive electrode materials. For a number of applications, however, the production throughput of the process needs to be increased with respect to the current state of the art. Here we show that this can be achieved by decreasing the diameter of the employed bulk-material electrodes from ca. 12 to 0.15 mm, corroborating previous observations, and demonstrate that the throughput is associated with the ablation efficiency (i.e., the energy spent to produce nanoparticles per total input energy) that respectively increases by a factor of 10. It is also shown that the commonly used theory for predicting the mass of nanoparticles produced by spark ablation cannot capture this effect, and thus we extend it to account for heat losses that affect the process when electrode diameter reduces below ca. 2 mm. Through this exercise we also show that reduced heat losses associated with thinner electrodes provide an effective recipe to increase the ablation efficiency, also referred as the nanoparticle production yield. The new extended theory for estimating spark ablation nanoparticle mass production throughput is also accompanied by an empirical equation predicting its dependence on electrode diameter.

The stability of nanoparticle (NP) production by atmospheric-pressure spark ablation was studied and found to depend on the composition of the electrodes and the carrier gas (here N₂ or Ar). For materials that do not react with N₂, such as Pd and Ni, NP production was rather stable regardless of the carrier gas employed. In contrast, for materials that can easily produce nitride species (e.g., Al and Mg), both the concentration and size of the resulting NPs exhibited noticeable fluctuations, when ablating them in N₂, which are more pronounced when the electrical energy input to the system is low. The variation in concentration and particle size is attributed to the formation of a metal-nitride region on the face of the electrodes where the sparks hit, as a result of its reaction with the carrier gas, altering the electrical and thermal conductivity, and consequently the ablatability of the electrode at that region. This explanation was corroborated by offline analysis of the face surface of the electrodes, showing two chemically distinct regions: one with high content of N and one without. In addition, the concentration of the Al and Mg NPs produced in N₂ decreased gradually over time until it reached a plateau after several hours. When using Ar, the fluctuation and decreasing trend in NP production, and consequently the formation of nitride compounds on the face surface of the electrodes, were negligible, providing an effective solution for stable ablation of materials that can easily react with N₂.

Nanoparticles (NPs) mixed at the atomic scale have been synthesized by atmospheric-pressure spark ablation using pairs of Pd and Hf electrodes. Gravimetric analysis of the electrodes showed that the fraction of each material in the resulting mixed NPs can be varied from ca. 15-85 at% to 85-15 at% by employing different combinations of electrode polarities and thicknesses. These results were also qualitatively corroborated by microscopy and elemental analysis of the produced NPs. When using pairs of electrodes having the same diameter, the material from the one at negative polarity was represented at a substantially higher fraction in the mixed NPs regardless of whether a pair of thin or thick electrodes were employed. This can be attributed to the higher ablation rate of the electrodes at the negative polarity, as already known from earlier experiments. When using electrodes of different diameters, the fraction of the element from the thinner electrode was always higher. This is because thinner electrodes are ablated more effectively due to, at least in part, the increased importance of the associated heat losses compared to its thicker counterpart. In those cases, the polarity of the electrodes had a significantly smaller effect. Overall, our results demonstrate, for the first time, that spark ablation can be used to control atomic scale mixing and thus produce alloyed NPs with compositions that can be tuned to a good extent by simply using different combinations of electrode diameters and polarities. This expands the capabilities of the technique for producing mixed nanoparticle building blocks of well-defined composition that are highly desired for a wide range of applications.