The nebulization of colloidal suspensions is analyzed by dynamic light scattering, scanning, and transmission electron microscopies. While primary agglomeration can be important for many nanoparticle-solvent couples, our results indicate that for TiO 2 nanoparticles dispersed in water, secondary agglomeration also occurs during nebulization. When nebulization is realized immediately after sustaining a plane-toplane dielectric barrier discharge at atmospheric pressure, the collection efficiency of TiO 2 nanoparticles increases due to the presence of a remanent electric field between the two electrodes. Finally, these findings are used to deposit SiO 2 -TiO 2 nanocomposite thin films by alternating the deposition of dense silicalike layers in a Townsend discharge and the collection of TiO 2 nanoparticles through nebulization of the nanocolloidal suspension.

This work examines the growth dynamics of TiO2-SiO2 nanocomposite coatings in plane-to-plane Dielectric Barrier Discharges (DBDs) at atmospheric pressure operated in a Townsend regime using nebulized TiO2 colloidal suspension in hexamethyldisiloxane as the growth precursors. For low-frequency (LF) sinusoidal voltages applied to the DBD cell, with voltage amplitudes lower than the one required for discharge breakdown, Scanning Electron Microscopy of silicon substrates placed on the bottom DBD electrode reveals significant deposition of TiO2 nanoparticles (NPs) close to the discharge entrance. On the other hand, at higher frequencies (HF), the number of TiO2 NPs deposited strongly decreases due to their “trapping” in the oscillating voltage and their transport along the gas flow lines. Based on these findings, a combined LF-HF voltage waveform is proposed and used to achieve significant and spatially uniform deposition of TiO2 NPs across the whole substrate surface. For higher voltage amplitudes, in the presence of hexamethyldisiloxane and nitrous oxide for plasma-enhanced chemical vapor deposition of inorganic layers, it is found that TiO2 NPs become fully embedded into a silica-like matrix. Similar Raman spectra are obtained for as-prepared TiO2 NPs and for nanocomposite TiO2-SiO2 coating, suggesting that plasma exposure does not significantly alter the crystalline structure of the TiO2 NPs injected into the discharge.

This work examines the functionalization of sugar maple (Acer saccharum) and black spruce (Picea mariana) wood surfaces using an atmospheric‐pressure dielectric barrier discharge in He and He/HMDSO (hexamethyldisiloxane) gas mixtures. Wood samples were placed on one of the electrodes and the plasma was sustained by applying a 3.5 kV peak‐to‐peak voltage at 12 kHz. Analysis of the discharge stability through current–voltage (I–V) characteristics revealed a filamentary behaviour, in sharp contrast with the homogeneous He discharge obtained with a glass sample. Optical emission spectroscopy performed near the wood vicinity revealed strong N2 and ${\rm N}_{{\rm 2}}^{{\rm + }} $ emissions, suggesting that wood outgassing plays an important role in the evolution of the discharge regime. Analysis of the surface wettability through water contact angle (WCA) measurements indicated that freshly sanded wood samples treated in He/HMDSO plasmas became more hydrophobic with WCAs in the 120°–140° range depending on treatment time and wood species. Attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy measurements on samples exposed to He/HMDSO plasmas revealed the deposition of hydrophobic Si(CH3)3‐O‐Si(CH3)2, Si(CH3)3 and Si(CH3)2 functional groups as well as an increase of the CH‐to‐OH band intensity ratio. For relatively thick coatings, the WCA following natural aging under uncontrolled conditions remained constant at 132° ± 3° which highlights the stability of the plasma‐deposited thin films, a very promising result for structural and decorative outdoor applications.

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Optical emission spectroscopy (OES) measurements coupled with a collisional-radiative model were used to characterize a plane-to-plane dielectric barrier discharge at atmospheric pressure operated in nominally pure helium. The model predicts the population densities for the n = 3 levels of He excited by electron impact processes from either ground or metastable states and takes into account excitation transfer processes between He n = 3 levels as well as all relevant radiative decays and quenching reactions. Time-resolved OES measurements indicate that line ratios from He n = 3 triplet states (for example, 587.5 nm-to-706.5 nm) and singlet states (for example, 667.8 nm-to-728.1 nm) first sharply rise as the discharge ignites and then slowly decrease as it extinguishes. Assuming that n = 3 levels are first populated only by electron impact on ground state He atoms and then only by electron impact on metastable He atoms as the discharge current and thus the metastable number density rise, triplet and singlet line ratios predicted by the model become in each opposite case solely dependent on the electron temperature T e (assuming Maxwellian electron energy distribution function). The values of T e deduced from the analysis of both ratios were relatively high early in the discharge cycle (around 1.0-1.4 eV) and then much lower near discharge extinction (around 0.15 eV). For analysis of time-integrated (or cycle-averaged) OES measurements, the electron temperatures were closer to the 0.15 eV values near the end of the discharge cycle, in good agreement with the values expected from theoretical predictions in the positive columns of He glow discharges at atmospheric pressure.

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Nanocomposite thin films can be obtained by polymerization of a colloidal solution in a dielectric barrier discharge (DBD) at atmospheric pressure. In such a process, the dispersion of nanoparticles into the matrix is driven by the charging, transport, and deposition dynamics of the atomized colloid. This work examines the interaction of atomized TiO2 nanoparticles with ac electric fields in a plane-to-plane dielectric barrier discharge reactor. Experiments are performed with the discharge off to examine transport and deposition phenomena over a wide range of experimental conditions with a fixed particle charge distribution. Scanning electron microscopy reveals that the size distribution of TiO2 nanoparticles collected at different locations along the substrate surface placed on the bottom electrode of the DBD reactor can judiciously be controlled by varying the amplitude and frequency of the ac electric field. These results are also compared to the predictions of a simple particle motion model accounting for the electrostatic force, the gravitational force, and the neutral drag force in the laminar flow. It is found that while the initial charge distribution of atomized particles strongly influences the total deposition yield, its maximal position on the substrate, and the width of the deposited area, the initial size distribution of the particles at the entrance of the reactor mostly changes the size distribution at each position along the substrate surface.

This work analyzes the effect of wood outgassing on the properties of He dielectric barrier discharges operated in a homogenous regime. Over the 60 min wood treatment investigated, the discharge current increased by almost a factor of two due to the release of air and humidity. Based on optical emission spectroscopy and collisional-radiative modeling, these effects further yield to an increase of the electron temperature and a significant decrease of the relative number density of He metastable atoms. Experiments were also performed in presence of trace amounts of N 2 , O 2 , and dry air to simulate wood outgassing. The amount of air released from the wood sample decreased with plasma treatment time, with a two-slope first-order decay behavior.