5.1 Spray Pyrolysis Deposition Modeling

Although much of the spray pyrolysis process is understood, many details regarding the deposition process are still not perfectly clear [222]. Many studies have claimed that the deposition process which occurs is much like a CVD deposition. This means that droplets released from the atomizer would vaporize in the vicinity of the substrate and the chemical would deposit on the substrate as a vapor [32], [157], [216]. Others, however, suggest a thin spreading of the liquid as it contacts the substrate as a more likely deposition method to generate the high quality dense films [172]. The spray pyrolysis mechanism analogous to CVD was suggested by Sears et al. [183]. However, the spreading of the droplets on the substrate was not considered until Perednis [171]. Beckel et al. [9] based their model on the precipitation of the precursor from the droplet before impact, followed by the impact and spreading of the droplet on the heated substrate and subsequent stacking of precipitates. They failed to mention, however, a description of the precursor solution decomposition and the subsequent film evolution. Perednis [171] and Gallage et al. [56] investigated the deposition mechanism as a combination of several processes occurring simultaneously. The full model in [56] includes the precursor atomization, droplet transport towards the heated substrate, the evaporation of the residual solvent, droplet hitting the substrate and spreading, the salt decomposition, and finally the nucleation and growth of oxide particles [222].

However, the [171] model was developed for the growth of YSZ thin films and [56] for the oxidation of Ce$^{3+}$ and Ce$^{4+}$ and both cannot be generalized to the formation of other oxide coatings. Recently, it has been shown that without an electrical force push, the droplets cannot reach the substrate before they vaporize. Therefore, it is suggested that for many pressure atomizers, the droplet will evaporate before reaching the substrate and a CVD-like reaction will occur. However, with an atomizer which provides an additional electrical driving force for the droplets, they are more likely to hit the substrate and deposit a thin film as the droplet spreads on the surface. The attempts to model the ESD spray pyrolysis process by Perednis [171] and the air blast atomization spray pyrolysis process from AIT [64] are described in this section.

In order to model the spray pyrolysis process in the LS environment, the MC method is employed to distribute particles which accelerate to the surface.