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

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.

Anatase TiO2 deposited at low temperature by pulsing an electron cyclotron wave resonance plasma source.

Photocatalytic surfaces have the potentiality to respond to many of nowadays societal concerns such as clean H2 generation, CO2 conversion, organic pollutant removal or virus inactivation. Despite its numerous superior properties, the wide development of TiO2 photocatalytic surfaces suffers from important drawbacks. Hence, the high temperature usually required (> 450 °C) for the synthesis of anatase TiO2 is still a challenge to outreach. In this article, we report the development and optimisation of an ECWR-PECVD process enabling the deposition of anatase TiO2 thin films at low substrate temperature. Scanning of experimental parameters such as RF power and deposition time was achieved in order to maximise photocatalytic activity. The careful selection of the deposition parameters (RF power, deposition time and plasma gas composition) enabled the synthesis of coatings exhibiting photocatalytic activity comparable to industrial references such as P25 Degussa and Pilkington Activ at a substrate temperature below 200 °C. In addition, to further decrease the substrate temperature, the interest of pulsing the plasma RF source was investigated. Using a duty cycle of 50%, it is thus possible to synthesise photocatalytic anatase TiO2 thin films at a substrate temperature below 115 °C with a deposition rate around 10 nm/min.

Scaling properties of force networks for compressed particulate systems.

We consider, computationally and experimentally, the scaling properties of force networks in the systems of circular particles exposed to compression in two spatial dimensions. The simulations consider polydisperse and monodisperse particles, both frictional and frictionless, and in experiments we use monodisperse and bidisperse frictional particles. While for some of the considered systems we observe consistent scaling exponents describing the behavior of the force networks, we find that this behavior is not universal. In particular, we find that frictionless systems, independently of whether they partially crystallize under compression or not, show scaling properties that are significantly different compared to the frictional disordered ones. The findings of nonuniversality are confirmed by explicitly computing fractal dimension for the considered systems. The results of the physical experiments are consistent with the results obtained in simulations of frictional disordered systems.

Percolation and jamming transitions in particulate systems with and without cohesion.

We consider percolation and jamming transitions for particulate systems exposed to compression. For the systems built of particles interacting by purely repulsive forces in addition to friction and viscous damping, it is found that these transitions are influenced by a number of effects, and in particular by the compression rate. In a quasistatic limit, we find that for the considered type of interaction between the particles, percolation and jamming transitions coincide. For cohesive systems, however, or for any system exposed to even slow dynamics, the differences between the considered transitions are found and quantified.

Persistence of force networks in compressed granular media.

We utilize the tools of persistent homology to analyze features of force networks in dense granular matter, modeled as a collection of circular, inelastic frictional particles. The proposed approach describes these networks in a precise and tractable manner, allowing us to identify features that are difficult or impossible to characterize by other means. In contrast to other techniques that consider each force threshold level separately, persistent homology allows us to consider all threshold levels at once to describe the force network in a complete and insightful manner. We consider continuously compressed system of particles characterized by varied polydispersity and friction in two spatial dimensions. We find significant differences between the force networks in these systems, suggesting that their mechanical response may differ considerably as well.