The direct injection of liquid droplets into low pressure plasmas

Abstract

A much greater number of useful precursors for plasma-enhanced chemical vapor deposition (PECVD) can be dispersed in high vapor pressure solvents than can be put into the vapor phase directly. In order to enable the use of such precursors, the authors investigated a method by which one can directly inject these liquids as microdroplets into low pressure PECVD environments. The solvent evaporates first leaving behind the desired precursor in the gas/plasma. The plasma dissociates the vapor and causes the deposition of a composite film (from precursor, solvent, and plasma gas). The authors made preliminary tests using Fe nanoparticles in hexane and were able to incorporate over 4% Fe in the resulting thin films. In addition, the authors simulated the process. The time required for a droplet to fully evaporate is a function of the background pressure, initial liquid temperature, droplet-vapor interactions, and initial droplet size. A typical evaporation time for a 50μm diameter droplet of hexane is ∼3s without plasma at 100mTorr. The presence of plasma can decrease the evaporation time by more than an order of magnitude. In addition, the model predicts that the temperature of the injected droplet first decreases by evaporative cooling (to ∼180K for hexane); however, once the solvent has fully evaporated/sublimated, the plasma heats any remaining solute. As a result the solute temperature can first fall to 180K, then rise to nearly 750K in less than 1s.

TiO2–SiO2 nanocomposite thin films deposited by direct liquid injection of colloidal solution in an O2/HMDSO low-pressure plasma

“…In particular, at low pressure, the nature of the flow is mainly molecular flow whereas it is laminar or even turbulent at atmospheric pressure. [12,58,59] It could also be related to the low injection rates of the colloidal solution. Using a direct-liquid injection in a CVD system at low pressure, Varvaele et al [11] have identified that low colloidal solution injection rates (as in this study) promotes the dispersion of the NPs whereas at higher rates, they tend to coalesce such that the mean size distribution shifts towards higher values.…”

“…To the best of our knowledge, these results are unique compared to the reported approaches. [12,15,22,46,56,60] Due to the set of experimental conditions used, the combination of adjusted volatility of solvents, remarkable stability of the colloidal solution and condition of spraying in a low-pressure plasma reactor resulted in an efficient transport of the droplets containing the non-agglomerated NPs towards the substrate. The optimization of the injection sequence permitted to keep the NPs protected inside the liquid droplets down to the substrate, thus preventing them to aggregate following their interaction with the plasma species.…”

Plasma–liquid interactions: a review and roadmap

“…Most often the aerosol contains a precursor for plasma deposition [91][92][93][94][95][96]. Evidence of droplets surviving in non-equilibrium low-pressure [95] and atmospheric pressure [97] plasmas have been discussed and the impact of the aerosol on the plasma properties have been quantified [82,96]. However, the impact of the plasma on the physical and chemical state of the droplets is still poorly understood [88].…”

Hybrid approaches coupling sol–gel and plasma for the deposition of oxide-based nanocomposite thin films: a review

“…This is linked to the novel character of these approaches and the complex phenomena induced when liquid and more specifically an organic solvent is Fig. 9 A schematic diagram of the experimental apparatus used in [127][128][129]. Reproduced from [129], Open Access IOP Publishing Fig.…”

“…Three years later, in 2009, Ogawa et al [127] reported a complete experimental study of directly introducing the liquid inside the low-pressure Ar capacitively coupled plasma (CCP) powered by an RF power supply (Fig. 9).…”