Handling Nanoparticle Content in Nanocomposite Thin Films Deposited by Misty Plasma Processes through Controlled Flash Boiling Atomization

Abstract

Misty plasma processes based on colloidal solutions sprayed into low-pressure plasmas have recently shown great potential for multifunctional thin film deposition. In such processes, nanoparticle accumulation in ring-shaped structures remains the main obstacle to the synthesis of high-quality coatings containing abundant, small-scale, and evenly dispersed nanoparticles. These local buildups appear after a colloidal droplet evaporates from a substrate. Accordingly, controlling the droplets’ size in the spray is of key importance to ensure a uniform nanoparticle content in the plasma-deposited nanocomposite film. In this work, it is shown that the use of more volatile solvents produces finer droplets on the substrate, thereby improving nanoparticle dispersion in the matrix. A one-dimensional evaporation model is further developed and used to show that, contrary to what one might expect, this result cannot be attributed to faster evaporation during droplet transport in the low-pressure plasma. Instead, a so-called “flash” boiling atomization mechanism is discussed to support the experimental findings.

Time-resolved analysis of Ar metastable and electron populations in low-pressure misty plasma processes using optical emission spectroscopy

“…When PC is heated, the increase in pressure is almost twice as pronounced as for the unheated injection, and the decline is faster (figure 5(a)). These changes are consistent with the recently identified spray-formation mechanism, involving socalled flash-boiling atomization [39]. In short, heating the PC increases its superheat degree in the vacuum of the reactor, which produces smaller droplets.…”

“…The injection time was set to 1 ms, resulting in volume flows of about 1.1 µl per injection. With an injector-sample distance of 40 cm, the droplet transport time in the plasma (determined by v = q/A, with q the liquid flow and A the surface area of the injector's opening) is about 15 ms [39]. An Avantes AvaSpec-3648-2 spectrometer was used to carry out time-resolved OES measurements with a spectral resolution of 0.07 nm and an integration time of 210 ms.…”

“…The greater amplitude in the case of heated methanol is once more a consequence of the improved vaporization stemming from the elevated superheat degree. However, as opposed to PC, methanol is already in a highly superheated state when injected at room temperature [39]. This leads to comparable kinetics in the case of heated and unheated methanol, whereas room-temperature PC injection is limited by the sluggish evaporation of non-superheated PC.…”