This study designed and tested a 30 L/min PM0.1 non-bouncing impactor (PM0.1 NBI) for ultrafine particle (UFP) mass concentration measurement. The PM0.1 NBI uses a wetted glass fiber filter with continuous water injection to maintain a clean surface, preventing particle loading and bounce effects. Lab tests showed the 30 L/min PM0.1 NBI outperformed the PM0.1 stage of NCTU micro-orifice cascade impactor (NMCI, non-rotating) with a silicon oil-coated aluminum foil substrate in collecting solid particles, as the particle collection efficiency of the latter decreased obviously with increasing particle aerodynamic diameter for particles larger than ca. 300 nm, an apparent particle bounce phenomenon. The ambient field tests showed very close UFP mass concentrations between the PM0.1 NBI sampler (the PM0.1 NBI preceded by 3 stages with 18, 10, and 2.5 & mu;m cutoff sizes) and the reference rotating 10-stages NMCI with the difference of & PLUSMN;0.5 & mu;g/m3 only. In addition, the 3 L/min PM0.1 NBI was combined with a condensation particle counter to measure the UFP number concentrations which were shown to be very close to those of the reference with the difference of & PLUSMN;5% only in the laboratory tests for loaded particle mass up to 300 & mu;g. Good agreement was also found in the field tests. Therefore, the current 3 L/min PM0.1 NBI and the future PM NBI with smaller flow rates (0.2-2.1 L/min) and larger cut-sizes (such as 0.4-0.9 & mu;m) can be used as viable UFP classifiers for improving UFP measurement accuracy both in mass concentrations and number concentrations and reducing the maintenance needs for the UFP classifiers which are important to long-term sampling and monitoring.

We have investigated the potentialities of dielectric barrier discharges (DBD) for aerosol electro-processing with a special focus on ion production and aerosol charging for deposition (filtration and materials) and aerosol measurement tools. The advantages and drawbacks of aerosol injection downstream or upstream of the discharge are compared according to the application. The main parameters that control ion production and transport are investigated based on post-discharge ion current measurements. Post-DBD aerosol chargers are compared for submicron particle size measurements based on aerosol bipolar charging and electrical mobility analysis. Finally, the principle of counterflow extraction of bipolar ion from an atmospheric pressure DBD is established from upstream measurements of positive and negative ion fluxes in air, without neither ozone (<0.1 ppmv) nor nanoparticles.

Supercritical Carbon Dioxide treatment of clean and soiled Filtering Face-Piece respirators is shown to meet mandatory requirements for safe reuse of FFP2. Preliminary tests enabled us to select optimal conditions for Supercritical Carbon Dioxide treatment of FFP2, for one hour at 343 K under 7.5 MPa, with a biocide solution. FFP2s from Paul Boye ' (R) were then tested before and after Supercritical Carbon Dioxide treatment with a soiling solution and a biological indicator (i.e. spores of GeoBacillus stearothermophilus) with respect to three objectives: (i) washing of organic deposits, (ii) sterilization (i.e. spores inactivation) and (iii) preservation of the filtration performances. The proposed Supercritical Carbon Dioxide treatment fulfils these three criterions, with effective decontamination of spores and promising washing preserving FFP2 requirements for filtration efficiency of aerosol ( 94%) with acceptable pressure drop (< 240 Pa). As a simple and low-cost one-step recycling method, Supercritical Carbon Dioxide fits to economical and sustainable development standpoints but also allows one to come back to good practices of face protective equipment use, lowering eventual shortage and tensions for most countries importing these respirators.

This article focuses on the mean charge per particle of monodisperse submicron aerosols charged by the diffusion of unipolar ions in post-corona discharge. It aims to confirm and discuss the limits of considering a single value ofN(i)center dot tto describe aerosol charging and then to present methods to control the size-charge relation. Three aerosol chargers with different mixings of ion and aerosol flows are investigated. Despite comparable ion sources with discharge currents of a few tens of mu A, the size-charge relations differ from one charger to another due to different ion-aerosol mixing conditions and subsequent differences in ion density along the particles' trajectories. Discrepancies are even more noticeable as the particle size increases. Discharge current, velocities of ion and aerosol flows and the electric field control post-discharge ion density at each point of the charging zone. The control of particle trajectory in expanding unipolar ion clouds leads to tuneable size-charge relations. Aerosol inertia and charging dynamics both depend on particle size, affect theN(i)center dot texperienced by the particle and thus the final charge of the particle. Operating conditions to reach a constant mean charge for particles larger than 200 nm are reported. Conclusions provide a basis to design aerosol chargers devoted to electric mobility selection for aerosol deposition, separation or electrical measurements, especially to overcome the limits of mobility-to-size data inversion due to the multiple charge ambiguity inherent to diffusion chargers. Copyright (c) 2020 American Association for Aerosol Research

Abstract The influence of aerosol concentration on the charge per particle is investigated in post-DC corona diffusion chargers for particle diameter between 10 nm and 1 µm. Particles are charged as they pass through the charger volume by collection of ions; the ion concentration decreases along the same path due to unipolar space charge repulsion and collection on walls. With crossflow of ions and aerosols, the final mean charge per particle decreases by up to one order of magnitude with increasing aerosol concentration from 109 to 1011 m−3. The evolution of the ion density profile along particle trajectories with aerosol concentration is shown to be due to the consumption of ions by aerosol charging and to ion dispersion by unipolar space charge repulsion including ions and charged particles. Both are investigated using a simplified 1.5 D model with axial symmetry and homogeneous mixing of ions and aerosols. It is shown that, for a given operating condition, the aerosol space charge is responsible for the modification of the spatial ion distribution and the related lower Ni × τ and charge per particle reported at higher aerosol concentration. From calculations, we define the maximal aerosol concentration to keep the mean charge per particle unaffected by aerosol space charge in the optimal case of homogeneous mixing of ion and aerosol. Finally, the comparison of different ion-aerosol mixing conditions highlights that an axial symmetry of the charger reduces the influence of aerosol space charge. This effect of aerosol concentration on the particle charge is critical for aerosol chargers devoted to electric mobility selection for size measurements or focused electro-deposition, as well as for concentration measurements as performed using emerging low cost sensors based on corona chargers. Copyright © 2021 American Association for Aerosol Research

A postplasma neutralizer for submicron particles size measurements by mobility analysis has been evaluated. Bipolar ion currents have been measured downstream a dielectric barrier discharge (DBD) to estimate the ion fluxes at the inlet of charging volume and the n(i).tau product that define the theoretical maximal concentration that can be neutralized. Charge distributions were measured versus DBD voltage, aerosol diameter and concentration for monodisperse aerosols. It is confirmed that the charge distribution of particles depends on the ratio of initial positive and negative ion currents controlled by the DBD voltage leading to a tuneable mean charge of aerosol in this post-DBD bipolar charger. As expected from Gunn's law, the mean charge and the variance are proportional to particle diameter above 50 nm and independent of the aerosol concentration. The size distributions measured with Kr-85 and post-DBD neutralizer present the same modal diameters and a maximal overestimation of the total concentration of 10%, for aerosol from 15 to 730 nm with concentrations up to 6 x 10(12) m(-3). This post-DBD bipolar charger can be used for submicron aerosol neutralization and thus for scanning mobility particle sizer size distribution measurements in air as well as in nitrogen to suppress ozone downstream DBD.

This paper deals with the electric and hydrodynamic confinement of negative ions in a point-to-plane corona discharge gap. Radial ion current density profiles have been measured on the earthed planar electrode, drilled in the axis of the point. The experimental setup is first validated by comparison with the Warburg's law without injected gas flow rate. The gas injected in the gap and blown from the discharge gap through the hole located at the centre of the plane affects neither the electric field close to the point nor the subsequent electric wind. However, it leads to the confinement of ions flux towards the central symmetry axis in the low electric field region up to a critical gas velocity, which for no more effect is measurable. Hence, electro hydro-dynamics confinement of ions can be achieved by limiting the outward radial expansion of ions to increase ion current densities on specific locations close to the low field planar electrode.

Nanoparticles are employed in a wide variety of applications including catalysis and functional coatings. However, the inherent instability of small metal nanoparticles at elevated temperatures limits their range of application or their processing, e.g. in the calcination of particle layers to the substrate to obtain robust coatings. While the thermal stability may be improved by a silica coating of the nanoparticles, a hampering of the accessibility of the active particle surface, e.g. for heterogeneous catalysis, has to be avoided at the same time. So far these two opposing requirements have been addressed by permeable silica coatings as a consequence of cracking under heat treatment [1] or the use of pore builders [2]. However, not all materials can withstand higher treatment temperatures and the liquid route for porous silica coatings is rather tedious and time consuming [2]. The aim of this contribution is to present a simple method to apply a defined silica coating on arbitrary core nanoparticles by mixing the aerosol particles with TEOS vapour and reactive species from a DBD plasma. Due to the reaction of the TEOS vapour with the ozone (and probably some nitric oxides) formed in the DBD plasma an intermediate species is formed which condenses on the nanoparticles. This liquid coating may reach a thickness of several 10 nm, quite independent of the core particle size (cf. Fig.1).

The plasma-based aerosol process developed for the direct coating of particles in gases with silicon oxide in a continuous chemical vapor deposition (CVD) process is presented. It is shown that non-thermal plasma filaments induced in a dielectric barrier discharge (DBD) at atmospheric pressure trigger post-DBD gas phase reactions. DBD operating conditions are first scanned to produce ozone and dinitrogen pentoxide. In the selected conditions, these plasma species react with gaseous tetraethyl orthosilicate (TEOS) precursor downstream of the DBD. The gaseous intermediates then condense on the surface of nanoparticles and self-reactions lead to homogeneous solid SiOx coatings, with thickness from nanometer to micrometer. This confirms the interest of post-DBD injection of the organo-silicon precursor to achieve stable production of actives species with subsequent controlled thickness of SiOx coatings. SiOx coatings of spherical and agglomerated metal and metal oxide nanoparticles (Pt, CuO, TiO2) are achieved. In the selected DBD operating conditions, the thickness of homogeneous nanometer sized coatings of spherical nanoparticles depends on the reaction duration and on the precursor concentration. For agglomerates, operating conditions can be tuned to cover preferentially the interparticle contact zones between primary particles, shifting the sintering of platinum agglomerates to much higher temperatures than the usual sintering temperature. Potential applications for enhanced thermal stability and tunable photoactivity of coated agglomerates are presented.

A compact, inexpensive and simple dielectric barrier discharge (DBD) design is presented with related electro-thermal properties for the production of metal nanoparticles.Nanoparticle formation and growth mechanisms are depicted from size distributions and chemical analyses of particles collected just after the 70 kHz DBD in nitrogen. At first, it is confirmed that the initial local vapor flux is produced from the spots of interaction between plasma filaments and different metal electrodes (Au, Ag, and Cu). Amorphous and crystalline pure metal primary nanoparticles with diameters below 5 nm are then produced by physical nucleation in expanding vapors jets. Finally, some small agglomerates with diameters still below 5 nm are also formed by ballistic agglomeration of a fraction of these primary particles. This happens at the end of the vapor jet expansion, as well as after the production during the transit between subsequent filaments in the DBD. The first local agglomeration step can be limited at reduced energy per filament by lowering the initial vapor flux in smaller gaps, while the second growth step depends on the transit time in the DBD. Hence, such "low" energy plasma filaments (up to a few tens of mu J) lower the initial vapor flux to control the agglomeration. DBD were thus successfully tested for the production of tailored nanoparticles with tunable size, controlled morphology of spherical agglomerates and the same composition as the metal electrode.The production per unit energy (mol J(-1)) is related to both plasma and material properties. Besides, neglecting vapor and nanoparticles losses, the mass production rate (g s(-1)) depends on the input power related to the product of the energy controlling the production per filament times the number of filaments per second, for any given material.This non-thermal plasma process presents great potentialities for nano-technologies since it is performed at atmospheric pressure and can be used to reach size-dependent properties of nano-materials, without any gaseous precursor or solvent. (C) 2014 Elsevier Ltd. All rights reserved.

To develop a “real time” aerosol sizer, a postcorona charger is studied. Aerosol charging depends mainly on N i.τ product (Ni is the ion density and τ is the charging time) and on the aerosol diameter. The time devoted to charging is shorter than 0.1 s. To compensate this short time, mean ion densities of about 10 8 cm are required. At first, geometrical and electrical parameters of the corona discharge are chosen to achieve a stable ion production and quasi-stationary ion density profiles in the charger. Then, the ion extraction to reach such post-discharge ion density is optimized. Since the mixing of ions with aerosol is critical for post-discharge ion distribution and so for N i.τ product “seen” by the aerosol, two mixing arrangements are compared. Indeed, it is confirmed that the final number of elementary charge collected by the aerosol depends on the mixing.

This study aims to develop a post-Dielectric Barrier Discharge (DBD) neutralizer for aerosol (solid or liquid particles suspended in a gas). Indeed, DBD in air, at atmospheric pressure are known to produce bipolar ions required to this application. This work focuses on ion production, extraction and losses from DBD based on measured post-discharge ions currents. It is shown that post-DBD ion currents depend on discharge electrical properties defining ion production as well as losses in the discharge and in post-discharge related to an electro-hydrodynamic competition. In a plane-to-plane DBD arrangement with wire electrodes, all the filaments have the same charge and energy within 30%. Due to losses, the post-DBD ion currents of both polarities are not proportional to the charge production, related to the time integrated charge per filament. At last, post-DBD ions densities reach the same order of magnitude than those in radioactive neutralizers proving the potentialities of post-DBD aerosol neutralization.

Two lab-scale self-cleaning filters based on dielectric barrier discharges in air at atmospheric pressure have been developed and tested. Experimental results on aerosol removal by charging and electro-collection are presented versus plasma and hydrodynamic parameters for monodisperse aerosol from 20 nm to 1.2 μm. For classical atmospheric aerosol, the average mass and number filtration efficiencies exceed 95% and 87%, respectively in the most penetrating size range (100-700 nm). The frequency of the applied voltage controls the amplitude of the oscillation of charged particle and can be adjusted to favour either filtration or cleaning. Low frequency (1 kHz) is suitable for electro-collection, while high frequency (60 kHz) is favourable for filter cleaning. Electrical characterization and filter efficiency are two indicators of the filter loading. The durations of both filtration step at maximal efficiency and cleaning step depends on the deposited mass, the surface input power and subsequent dielectric surface temperature.

This paper focuses on the production of metal nanoparticles in a dielectric barrier discharge (DBD) reactor at atmospheric pressure. In the developed reactor only one electrode is provided with a dielectric (alumina), while the point-shaped counter electrode is not covered and is accessible to the filamentary discharges (asymmetrical DBD). The metallic particles are formed from the material of the open counter electrode, on which the filamentary discharges lead to the formation of craters. A high number of larger nanoparticles (with diameter of some tens of nanometers) are observed in the cold reactor at the beginning of the discharges. After the achievement of the thermal equilibrium in the reactor mainly very small metallic nanoparticles are produced with constant size and number concentration. The produced metallic particles in thermal equilibrium are crystalline and not agglomerated. The mean diameter of the produced particles is about 4 nm. For given operating conditions of the reactor (fixed electrode gap and gas flow rate) the electrode material has almost no influence on the size of the produced particles, but it determines the number concentration.

Methods to induce non-thermal atmospheric pressure plasma filaments are presented with related properties for micro, streamer and prevented spark discharges, respectively, induced in planar Dielectric Barrier Discharges with one electrode covered by dielectric material (mono-DBD) or point-to-plane Corona. Two mechanisms of nano-particles formation are depicted from aerosol size distributions and TEM analysis. 0.1-10 mJ prevented spark discharges produce 10-100 nm droplets ejected from melted craters as well as nucleated primary particles and subsequent 10-100 nm agglomerates, by nucleation and coagulation in expanding vapor jets. With smaller energy per filament, 0.1-10 mu J micro-discharges and 0.1-100 mu J streamers, the initial local vapor fluxes emitted from spots of interaction between plasma filaments and electrodes are reduced. Subsequent smaller primary particle density limits the local coagulation in the vapor plume since 2-10 nm non-agglomerated crystalline metal nano-particles are produced in mono-DBD with Au, Ag and Cu electrode. Besides, the evolution of the aerosol size from primary nano-particles to agglomerates with transit time suggests slow coagulation of these primary metal particles in mono-DBD. Aerosol properties depend on the energy per filament and on the electrode. The final size is controlled by plasma parameters and transit time in and after the plasma. The aim is to underline emerging applications of atmospheric pressure plasmas for the production of tailored particles with tunable size, composition and structure with non-thermal plasma filaments to control the resulting properties of nano-powders and materials. Production rates and related energetic yields are compared.

Physico-chemical properties of atmospheric pressure filamentary dielectric barrier discharges (f-DBD) depend on coupled electrical characteristics and thermal profiles. In this paper, a method for studying thermal and electrical effects is developed. Therefore, thermal profiles of f-DBD are studied for well-defined electrical characteristics of quasi-identical filaments with controlled distribution in time and space. The temperatures of gas, dielectric surface and plasma depend on the surface density and on the temporal frequency of filaments, defining the input power, and can be tuned by controlling heat transfers. Different methods to control these temperatures are depicted. Moreover, heat transfer through conduction and convection from dielectric surface is shown to be the dominant heating mechanism of the flowing gas in the reactor. Finally, experimental results show that the local temperature gradient around each filament can be controlled by the frequency of the applied voltage. Actually, the temperature difference between the filament and the surrounding gas is constant below 10 kHz but increases linearly with the frequency above 10 kHz. At high frequency, the time between two successive filaments occurring at the same position becomes smaller than the relaxation time constant of thermal exchanges (∼0.1 ms). Hence, this rise in local temperature can be attributed to time-limited heat transfers from the filament axis.

The paper highlights applications of some atmospheric pressure plasmas (dc-corona, streamer and spark and ac-Dielectric Barrier Discharges) to aerosol processes for Materials and Environment (filtration, diagnostics). The production of vapor i.e. condensable gaseous species, leads to nano-sized particles by physical and chemical routes of nucleation in these AP plasmas: (i) when dc streamer and spark filamentary discharges as well as ac filamentary dielectric barrier discharges interact with metal or dielectric surfaces, and (ii) when discharges induce reactions with gaseous precursors in volume. It is shown how composition, size and structure of primary nano-particles are related to plasma parameters (energy, number per unit surface and time and thermal gradients). Then the growth by coagulation controls the final size of agglomerates versus plasma parameters and transit time in and after the plasma. Charging and electro-thermal collection are depicted to account for the related potential applications of controlled kinematics of charged aerosol.

Aerosol particle charging is involved in many scientific and industrial applications such as electrostatic precipitation, coating, post-production particle assembly, particle self-repulsion to preserve high interfacial areas, neutralization to prevent dust explosion, coulombian agglomeration of bipolar aerosols and measurements based on electrostatic techniques. Depending on the application, high charge level, or high particles penetration through the charger, control charge distribution, a mix of these criteria is required. Non-thermal Atmospheric Pressure Electrical Discharges are an efficient way to charge particles (Borra 2005). Charging mechanisms by collection of ions and related charging laws are defined and have been validated in quasi-stationary ions densities, especially in DC corona discharges (Fuchs 1963). Dielectric Barrier Discharges (DBD) are constituted of two electrodes that are separated by a gas gap spacing with at least one dielectric material in the gap. The dielectric barrier prevents arc formation but involves an alternative polarization of the system (typically a few kV at 50 Hz–1 MHz). In air, at atmospheric pressure, DBD occur as thin and brief Filaments Discharge (a few 10’s of μm, a few 10’s of ns) homogeneously distributed over the dielectric surface (Petit et al. 2002). Thus, DBD produce high transient charging conditions (bipolar ions densities and electric field) in which particles charging is investigated. Post discharge ions densities and mobilities as well as particle charge distributions and losses are investigated versus plasma and hydrodynamics parameters with monodisperse aerosol from 20 nm to 900 nm injected in the DBD.