J.-P Borra

N Jidenko

J Hou

A Weber

VAPORIZATION OF BULK METALS INTO SINGLE-DIGIT NANOPARTICLES BY NON-THERMAL PLASMA FILAMENTS IN ATMOSPHERIC PRESSURE DIELECTRIC BARRIER DISCHARGES

Keywords: Dielectric Barrier Discharge, plasma, nanoparticle, production, mechanism, nucleation, process

Vaporization of bulk metals into single-digit nanoparticles by non-thermal plasma filaments in atmospheric pressure dielectric barrier discharges

Introduction

Size-dependent properties are reported for particles smaller than 50 nm [START_REF] Kodas | Aerosol Processing of Materials[END_REF][START_REF] Schmid | Clusters in expanding Plasma[END_REF]. Hence, the ideal nano-material production process should control the size, the composition and the structure of primary nanoparticles as well as the coagulation enlarging the size distribution.

Nano-particles suspended in gases are formed by evaporation of liquid droplets or by condensation of vapors. In both cases, homogeneous nucleation occurs when the saturation concentration is overcome (saturation vapor pressure in gases and solute concentration for crystallization in liquids). Nucleation here refers to the bottom-up gas phase process producing nanoparticles from condensable vapors.

In reactive nucleation processes, condensable species are produced by chemical reactions between gaseous precursors and excited species from flames, plasmas or lasers. High production rates are reached but purity of the so-produced nanoparticles still is a critical limit that deserves to be addressed (e.g. in [START_REF] Pratsinis | Distinguishing between aggregates and agglomerates of flame-made TiO2 by high pressure dispersion[END_REF] for flames, in [START_REF] Vollath | The Microwave plasma process -a versatile process to synthesize nanoparticulate materials[END_REF] and in Kortschagen's chapter from Samukawa 2012 for Low pressure plasmas, as well as for ambient pressure processes by plasmas in [START_REF] Belmonte | Nanoscience with non-equilibrium plasmas at atmospheric pressure[END_REF][START_REF] Massines | Atmospheric pressure Low Temperature Direct Plasma Technology : Status, Challenges for thin film Deposition[END_REF], or by lasers in Sublemontier 2011).

To reach pure nanoparticle production, physical nucleation by cooling hot vapors evaporated from solid materials has been first developed for low pressure coating process. Solid vaporization is also performed at ambient pressure to produce particles with the same composition than the solid target. Continuous thermal arc or short successive and focused energy deposits on the material by lasers and sparks (first reported by [START_REF] Altgeld | Spark discharge particle generator for laser Doppler anemometry[END_REF]) are used. The interaction surface is referred to spot with related power densities above 10 10 W.m -2 and energy densities up to 10 6 J.m -2 above the threshold fluency of 10 3 J.m -2 for metal vaporization (e.g. for lasers in Gamali 2011). Then, high nucleation rates lead to concentrations of primary nanoparticles above 10 10 cm -3 , favorable to fast coagulation. The so-produced aerosol presents mean diameters up to a few hundred nm [START_REF] Kodas | Aerosol Processing of Materials[END_REF][START_REF] Horvath | A low-voltage spark generator for production of carbon particles[END_REF][START_REF] Byeon | Spark generation of monometallic and bimetallic aerosol nanoparticles[END_REF][START_REF] Buesser | Coagulation of highly concentrated aerosols[END_REF][START_REF] Bau | Properties of airborne nanoparticles produced by a commercial spark-discharge generator[END_REF]. Agglomeration has been limited in sparks by dilution [START_REF] Feng | Size Controlled Synthesis of Spherical Nanoparticles by Spark Discharge[END_REF], by quenching in pin-to-hole configurations [START_REF] Han | Study of Pin-to-Plate Type Spark Discharge Generator for Producing Unagglomerate Nanoaerosols[END_REF], and by unipolar charging [START_REF] Park | Anti-agglomeration of spark discharge-generated aerosols via unipolar air ions[END_REF]. Sparks in liquids also leads to a fraction of unagglomerated nanoparticles embedded in carbon (Hamdam et al. 2013). Besides, less energetic nonthermal plasma filaments (0.1-10 mJ prevented sparks and 10-100 µJ streamers) still lead to final size distributions with count mean diameters from 10 to 100 nm, arising from coagulation of primary nanoparticles [START_REF] Borra | Charging of aerosol and nucleation in atmospheric pressure electrical discharges Plasma[END_REF][START_REF] Borra | Atmospheric pressure plasmas for aerosols processes in materials and environment[END_REF][START_REF] Borra | Nano-droplet ejection and nucleation of materials submitted to non-thermal plasma filaments[END_REF].

The paper describes another method to produce even smaller primary metal particles from 1 to 5 nm and to control the agglomeration with final diameters still below 5nm. To do so, less energetic plasma filaments (from 1 to tens of µJ) are used to reduce the initial vapor flux. To compensate this low local vapor production per filament, filaments are repeated more than 10 6 s -1 for ms transit times in lab-scale DBD at 70 kHz.

In such streamer-like discharge filaments, the transition to thermal spark must be avoided by limiting the reduced electric field (E/N with N the gas density) and the related current. In that respect, DBD have been developed using an AC voltage with at least one dielectric material (barrier) between the electrodes. The electric field in the gap depends on the applied voltage and on charges first migrating in the gap and then deposited on dielectric surface during the development of filaments [START_REF] Gibalov | The development of dielectric barrier discharges in gas gaps and on surfaces[END_REF][START_REF] Gibalov | The development of dielectric barrier discharges in gas gaps and on surfaces[END_REF][START_REF] Eliasson | Modeling and applications of silent discharge plasmas[END_REF][START_REF] Eliasson | Modeling and applications of silent discharge plasmas[END_REF]. Surface polarization first amplifies the electric field to trigger economic plasmas filament development and then hinders the transition from non-thermal streamer to thermal spark. In air and nitrogen at atmospheric pressure, thin and transient discharge filaments, also called micro-discharges develop with diameters of ~10-100 µm and durations of ~20-500 ns. Here, the expression "discharge filament" includes all phases from Townsend pre-breakdown avalanches to streamer development towards the cathode, surface discharges and quenching.

A simple DBD design is presented with electro-thermal properties for the production of nanoparticles from the surface of solids. Nanoparticle formation and growth mechanisms are discussed from Selected Area Electron Diffraction (SAED) and Transmission Electron Microscopy (TEM) analysis.

Finally, production per unit energy (mol J -1 ), the final particle size and the mass production rate (g s -1 ) of nanoparticles are related to plasma parameters, to properties of materials for gold, silver and copper as well as to transit times in the plasma.

General set-up and Aerosol measurements

The experimental set-up is presented in Figure 1. Plasma reactors are fed with N2 at STP. Transit times in the gap lie from 1 to 20 milliseconds in lab-scale DBD.

Reactive nucleation may arise from gaseous impurities converted into condensable species by reactive non-thermal plasmas. It can be suppressed using filtered dry inert or non-reactive gases. Then whatever 99.99 to 99.999 % purities used, similar aerosol measurements are performed and detected aerosol can only be produced by physical nucleation of vapors emitted from spots of plasma filaments.

Electro-thermal characterization of DBD

Figure 2 shows the plane-to-plane DBD reactor design that has been used with millimeter gaps. This lab-scaleDBD is made of metal electrodes (Ag, Au or Cu with diameters of 4, 8 and 8 mm respectively), facing the dielectric material (Al2O3) polarized at 70 kHz by a 23cm² electrode. This asymetric planar DBD has been used for electrical characterisation. Indeed, the accurate electrical characterisation of current pulses related to properties of discharge filaments requires an electrode area smaller than 1 cm 2 to prevent from too numerous pulses occurring at the same time on larger electrodes [START_REF] Jidenko | Electrical characterization of microdischarges produced in dielectric barrier discharge in dry air at atmospheric pressure[END_REF]. Different gap lengths from 1 to 3.65 mm were fed with 1 to 10 lpm N2 to keep the gas velocity constant at 0.5 m s -1 , for all the results presented here. The experimental setup used to perform voltage and current measurements as well as calculations of the discharge power and the number of filaments during the transit time, are detailed in appendix 1.

For each operating condition, applied voltage, input power and individual current pulse characteristics such as the maximum current pulse, the duration, the charge per pulse have been measured in each half-period (e.g. for a 1 mm gap in Nitrogen, I0 ~ 20 mA, t ~ 100 ns, Q around nC and E/pulse from a few to tens of µJ). This allows one to calculate the number of pulses per square centimeter during a voltage period (Npulse/T of a few tens per square centimeter, for a 1 mm gap) and the energy per pulse (cf. appendix 1 and §-3.1 results).

The lab-scale DBD has been extensively studied in terms of electro-thermal properties, detailed in Jidenko 2006 and 2010. The temperatures of the electrode and of the gas have been measured with thermocouples.

Aerosol measurements

To evaluate the production of particles by DBD, losses and detection efficiencies versus the diameter of particles have to be taken into account to correct the post-discharge concentration measurements. To do so, all measurements presented here have been obtained at the quasi-stationary thermal equilibrium, i.e. 20 to 30 min after the lightning of the discharge, as depicted in the results (cf. §3.1).

Particle size distributions and concentration:

The aerosol size distribution, modal diameters and geometric standard deviations are measured by two methods: (i) just at the exit of the lab-scale DBD for particles with modal diameters smaller than 5 nm collected on TEM grids for image analysis of at least 500 particles for all size distribution presented in section 3 ; (ii) by in-line Scanning Mobility Particle Sizer (radial nano Differential Mobility Analyzer from Clausthal University) after coagulation for 60 s downstream the lab-scale DBD, as depicted in Figure 3.

Due to the small size of the primary nucleated particles as well as of agglomerates, respectively with count mean diameters below 3 nm and up to 5, only a fraction of particles is detected in the Condensation Particle Counter (Grimm 5.403), as depicted on Figure 3. Thus, to perform mass production rate evaluation discussed in section 3.4.5.1, the agglomerate number concentration (Nagglom.) was derived from the SMPS measurements or measured directly with the CPC for particles larger than 3 nm, formed in the post-discharge agglomeration chamber for 60 s. Finally, calculations detailed in appendix 2 have shown that post-DBD losses by thermophoresis and electrostatic repulsion can be neglected while diffusion losses in the DBD as well as in post-DBD have to be taken into consideration for particles smaller than 5 nm.

Collection for Transmission Electron Microscope (TEM) analysis:

The aerosol has been sampled at 0.3 lpm through a carbon coated TEM grid (lacey type) downstream the plasma aerosol generator. The time of collection depends on the aerosol concentration (e.g. 15 minutes for particle number concentration of 10 7 cm -3 ). The additional characteristics, such as the size and shape of primary nanoparticles and of agglomerates were investigated with a TEM. Then, the particle size distributions were determined from the TEM images including at least 500 particles. The count median diameter (dp) and the geometrical standard deviation (σg) were defined by fitting the size distribution with a log-normal distribution.

Atomic composition and crystalline structure were defined from Selected Area Electron Diffraction (SAED) in the same TEM (JEOL 2100) from diffraction pattern. From the electron diffraction pattern the crystal structure of the sample can be determined if the lattice constants are known. Besides, additional EDX analyses have also been performed on collected nano-particles.

Electro-thermal characterization and properties of plasma filament in DBD

Figure 4 depicts the spatial development of filaments integrated over a few periods for a ms in our labscale DBD. Brighter and longer filaments develop at the edges of the metal electrode than from the central part. This was expected from the geometrical field reinforcement on the edges. Besides, filaments are spread on more spots on the dielectric side than on the metal side. This is due to charge deposition by the filament and related surface polarization of the dielectric material. This prevents from the lightning of subsequent filaments at the same place on the dielectric side during the same half period.

On the metal side, the electric field is not retro-controlled by surface polarization and filaments can develop from the same spot on the edges of the metal electrode, as depicted in Figure 4b. The measured current is depicted in Figure 5a. All results presented here have been obtained after at least 30 min after the lightning of the discharge. Indeed, the surface and related gas temperatures evolve as presented in Figure 5b for a mean electrode temperature from 20 to 130 °C. During the positive half-period, the discharge filaments (D + M -) cross the gap towards the metal electrode with charge and energy per filament hereafter referred as Q+ (nC) and E+ (µJ). Reversely, during the following negative half-period, filaments impact the dielectric material, which is now acting as a cathode (M + D -) with charge and energy per filament, referred as Q-and E-.

Current measurement: the measured current is the sum of two discharge current components (conduction current due to the collection of charges on the metal electrode and displacement current of charges in the gas), as well as a third capacitive current component, shifted of T/4 from the voltage.

Once the capacitive current suppressed by a numerical method detailed in [START_REF] Petit | Electrical characterization of gas discharges using a numerical treatment. Application to DBD[END_REF], the measured current is quasi-exclusively composed of current pulses related to discharges filaments. Indeed, the pseudo-continuous current related to ion drift is not measurable. Besides, in nitrogen, electron attachment can be neglected. Then, the current is only related to electrons and positive much less mobile nitrogen ions. The measured current thus mainly corresponds to the conduction current related to the electron flux integrated on the surface of spots.

One important feature of these plasma filaments is the quasi-constant charge and related energy per filament in each polarity, whatever the applied voltage is for a 2.5 mm gap at 70 kHz (see Figure 6a).

The linear evolution of power with the applied voltage is depicted in Figure 6b. Indeed, the voltage

D + M -= Q+ M + D -= Q- T gas (

Dielectric

Metal controls the number of filament per half period at quasi-constant energy per filament in each polarity.

In the order of millions of filaments are so-produced per second in such lab-scale DBD with sub-square centimeter electrode surface. The voltage controls the input power and the directly related temperatures of surfaces and of gas, as already reported in Jidenko et al. 2010 (cf. Figure 5b). Moreover, the charge per filament is proportional to the gap (g, mm) as depicted in Figure 6c while the energy evolves faster up to a few tens µJ with the gap length, as depicted in Figure 6d.

Difference between metal-directed and dielectric-directed streamer-like discharge filaments

Both the charge and the energy of the metal-directed filaments are smaller than for dielectric-directed filament, as depicted in Figure 6 and already observed by Murata et al. 1995 andFalkenstein andCoogan 1997. This has been attributed to the dynamic of collection of charges on anodic and cathodic surfaces. The electric field in the gap is mainly controlled by the surface polarization. Hence, the anodic dielectric surface neutralization by electron during the positive ½ T (Q+) is faster than by positive ions during the negative one (Q-). This can account for a faster field reduction leading to smaller charge and energy per pulse during the positive ½ T than during the negative one.

The polarization of the dielectric surface can also accounts for the higher dispersion of Q-related to dielectric-directed filaments than for Q+ related to metal-directed filament (see Figure 6a andc). At first, lower mobilities of positive ions than of electrons induces a faster surface extension of deposited electrons than of positive ions. Then, the retro-controlled total electric field is lowered for longer period and to a smaller value, just above the spots polarized by positive ions than by electron. Once polarized by positive ions of the first filament in the central part, subsequent filament during the same ½ period are repelled away on longer path to reach the less polarized periphery of the dielectric surface. Since the charge per pulse is proportional to the discharge length, shorter distances and homogeneous field in the central part of the electrode lead to smaller charge per filament in the central part than for longer filaments on the edges of the metal pin (field reinforcement).

Similar observations have been made for different gap lengths, as presented in Figure 6b as well as for different electrode material including gold, copper and silver.

The charge per filament and the number of filaments per unit surface and per period are quasiconstant from 1 to 100 kHz, in a DBD for any given voltage. Hence, in such DBD reactor, the number of filaments per second thus increases linearly with frequency and with the number of filaments per period that can be tuned with the voltage, the gap length and the surface of the electrodes.

Q+ Q- c D + M - M + D - D + M - M + D - D + M - M + D -

Physical Nucleation and agglomeration in expanding vapor jets

Micro-discharge  Surface vaporization and nucleation in expanding vapor jet

In lab-scale DBD, with transit times in the gap of a few milliseconds, the mean diameter of the produced particles are systematically smaller than 5 nm. A TEM image of gold particles produced by the Au-DBD is shown in Figure 7, with the corresponding size distribution. This confirms that we are dealing with nucleation, implying the production of saturated vapors. The size distributions presented in this paper integrate both populations of primary nucleated particles (< 5 nm) and of agglomerates (e.g. in Figure 7), distinguished from TEM pictures in Figure 8a.

Primary nucleated particles (< 5 nm): All individual particles sized from TEM observation lie in the range from 0.8 to 5 nm. Indeed, as depicted in Figure 7. The size distribution of primary nucleated particle is not a lognormal one since the smallest primary particle may not be visible due to enhanced losses by diffusion for smaller sizes. In that case, slightly more than half of the nanoparticles collected in post-DBD are unagglomerated isolated primary nucleated particles. Composition of nanoparticles: EDX analyses show only a few Al2O3 nanoparticles on the grid. The large excess of metal nanoparticles was expected since the enthalpy of melting required prior to vaporization is smaller and the saturation vapor pressures are higher for metals than for Al2O3.

Hence, the initial vapor flux is produced by local surface vaporization from the spots of interaction between plasma filaments and surfaces. Pure crystalline and amorphous nanoparticles (< 5nm) with the same composition than the metal electrode are so-formed by physical homogeneous nucleation in the mobile front of expanding vapor jets. 3.2.2 Growth of primary nucleated particles in the expanding vapor jetAgglomerates Rather spherical agglomerates with sizes from 3 to 10 nm made of primary nucleated particles (from 2 to 5 nm) are also observed in Figures 7 and8a. To check whether these agglomerates were produced by local agglomeration in the vapor jet and/or during the transit between the subsequent filaments in the DBD, the following plasma and material parameters have been varied.

Energy per filament

Figure 9b shows the particle size distributions produced by Au-DBD for different gap lengths at constant gas velocities. The number of filaments during the transit in the DBD was also kept constant by lowering the applied voltages in smaller gaps, as depicted in Table 1. From Figure 9a, it can be seen that the sizes of both primary nucleated particles (<5 nm) and the resulting aggregates (< 10 nm) increase with gap length and corresponding energy per filament. The count mean diameter of related size distribution integrating both populations, as well as the fraction of agglomerated particles increase with increasing gap (cf. Figure 9 and Table 1). This is typical of nucleation at higher vapor flux and was expected with more energetic filaments in larger gaps. Indeed, the resulting higher saturation leads to smaller critical diameters and subsequent longer nucleation, with related larger primary particles. Moreover, the nucleation frequency also increases with the local saturation. Then, higher concentrations of primary particles are produced by each filament, leading to faster coagulation and to larger agglomerates with more energetic filaments.

Electrode material

Nanoparticles have been produced by DBD with gold, silver and copper electrodes at constant energy per filament and number of filaments during the transit in the DBD. Similar isolated primary nucleated nanoparticles and related agglomerates are observed. Similar size distributions with modal diameters below 5 nm are produced, whatever the electrode material is, as depicted in Figure 10 and detailed in Table2. Higher amounts of materials are reported for silver, than for gold and then copper, as expected from the respective energy for melting required for vaporization and discussed in the last part.

Growth of nanoparticles during the post-production transit between the filaments

The effect of the number of filaments on particle growth during the transit in the gap, at constant energy per filament is depicted here. To do so, size distributions have been measured at different voltages with different number of similar filaments during a constant transit time, as well as with different transit times controlled by the flow-rate at constant voltage in the same DBD. Indeed, at 13.8 kV, a shoulder appears on the log normal size distribution as well as a new larger mode from 10 to a few tens of nanometers. This new mode for a few spherical particles observed with more volatile silver electrode at high voltage, is not detailed here but discussed in the last section.

The shoulder is related to the agglomeration of nanoparticles produced by similar filaments whatever the voltage is, but more numerous at higher voltages. Despite the apparent reduction of the modal diameter, the count mean diameter increase from 3.83 nm at 13 kV up to 4.1 nm at 13.8 kV, as well as the geometrical standard deviation from 1.4 to 1.53. This is typical of coagulation of more concentrated nanoparticles and this was expected from the increase number of filaments during the transit, producing more local injections in the gas of nanoparticles from the vaporized surface. The mean particle size and the standard deviation decrease from 4.1 to 2.78 nm and from 1.44 to 1.34, respectively, when increasing the gas flow rate from 2 to 10 lpm (cf. Figure 12). To sum up, both the voltage and the flow rate affect the post-production agglomeration during the transit in the DBD, as expected for self-preserving size distribution of agglomerates in nucleation processes for higher concentration or longer agglomeration of nanoparticles [START_REF] Buesser | Coagulation of highly concentrated aerosols[END_REF]. Hence, after the local nucleation and agglomeration in each vapor jet, presented above in §-3.2, the so-produced nanoparticles still grow during the post-production transit between the filaments. The first local agglomeration step can be limited at reduced energy per filament, while the second one also depends on both the number of filaments during the transit in the DBD and the transit time, as detailed in section 3.4.4. Indeed, in such lab-scale DBD, the final mean diameters can be controlled from 2 to 5 nm.

Discussion

This section aims to confirm that in DBD, non-thermal plasma filaments trigger surface vaporization. Here, only the heating and evaporation of the metal surface is considered by analogy to models developed for lasers and sparks. Sputtering by ions, has been considered for more energetic streamers and sparks [START_REF] Borra | Nucleation and aerosol processing in atmospheric pressure electrical discharges: powders production, coatings and filtration[END_REF]) and detailed in [START_REF] Warrier | Subroutines for some plasma surface interaction processes: physical sputtering, chemical erosion, radiation enhanced sublimation, backscattering and thermal evaporation[END_REF] and in [START_REF] Beilis | Handbook of vacuum arc science and technology[END_REF] for vacuum arcs, but can be neglected in atmospheric pressure DBD (Pitchord et al. 2006). The production of nanoparticles is then related to material properties and plasma parameters.

Energy deposition by discharge filaments

The electrical energy injected in the plasma filament is not entirely deposited on surfaces. The energy per filament is distributed over different processes: electron acceleration leading to avalanches and charge migration, then converted into chemical reactions, light emission and heating of both the gas and the surfaces [START_REF] Boeuf | Energy balance in a nonequilibrium weakly ionized nitrogen discharge[END_REF]. Moreover, the fraction of the energy per filament deposited in the spot probably varies with the gap and is not the same on both anode and cathode side of the filament.

With relatively low energies per filament in DBD (up to tens µJ) compared to laser and spark, the duration and the area of interaction with the surface (< 10 ns and dspot ~ few µm) lead to energy density (Eper fil/Sspot) and energy flux (also referred as surface power density = Eper fil/(Sspot.tspot)) up to 10 6 J m -2 and 10 14 W m -2 , far above the one required for heating, melting and vaporization, detailed below. The melted area of the spot depends on the deposited heat flux, defined as a fraction of the total energy of the filament, minus radiative and heat conduction losses, as introduced by Llewellyn Jones (1950).

A large fraction of the power injected in the DBD is devoted to electrode heating (e.g. up to 50% in [START_REF] Jidenko | Temperature profiles in filamentary dielectric barrier discharges at atmospheric pressure[END_REF]. The mean surface temperature varies here from ambient to 130 °C at 70 kHz in the lab-scale DBD. This macroscopic temperature does not only result from the heat flux in the spot, but includes other processes like surface reactions subsequent to the filament. Hence, the fraction of the total energy per filament related to the initial heat flux in the spot of filaments still has to be defined in DBD. However, while the plasma and the mean surface temperatures are far below the melting point of metals, the spots reach higher local temperatures since craters formation with melted edges is reported with Si and glass surfaces [START_REF] Yeo | Electro-thermal modelling of anode and cathode in micro-EDM[END_REF]Wang et al. 2010a andb). Besides, with such short energy injection on so small surfaces, the spot temperature may increase above the melting point, as already modeled in sparks by [START_REF] Gleizes | Thermal plasma modelling[END_REF]. Hereafter, we refer to the total energy per filament rather than to an undefined heat flux in the spot, since the last one is far above the one required for heating, melting and vaporization.

Vapor emission, transport and nucleation

Vaporization of solid is usually modeled versus the deposited energy flux (W m -2 ) to account for dynamics of heat transfers in the electrode or versus the energy density (J m -2 ) to deal with the related production of vapors. Despite the short duration of the filament-surface interactions in the order of a few ns, heat conduction occurs in the volume of the material since the characteristic times of propagation in metals are shorter (<10 -10 s). Then, longer characteristic times for melting and vaporization have been calculated for sparks (e.g. from 1.08 µs at 10 11 W.m -2 down to 35 ns at 10 12 W m -2 [START_REF] Rossignol | Numerical modelling of thermal ablation phenomena due to cathodic spot[END_REF]. At atmospheric pressure, the subsequent adiabatic expansion induces vapor jets up to a fraction of millimeters from the spot within a few hundred ns with sparks (Smirnov 1993, Itina and Voloshko 2013) as well as with lasers (at ~ 10 9 W.m -2 in [START_REF] Farid | Emission features and expansion dynamics of nanosecond laser ablation plumes at different ambient pressures[END_REF][START_REF] Chen | Effect of ambient pressure on laser ablation and plume expansion dynamics: A numerical simulation[END_REF][START_REF] Autrique | The role of mass removal mechanisms in the onset of ns-laser induced plasma formation[END_REF]). Hence, with energy densities above the threshold fluencies for metal vaporization and energy fluxes in the same order of magnitude as in lasers and sparks, surface vaporization of solids, vapor jet formation and expansion are also expected in non-thermal DBD.

In different models of surface vaporization by lasers and sparks, the amount of melted material is first related to the inverse of the energy required for melting (∆𝐻 = (∫ 𝐶 𝑝 .

𝑇 𝑚𝑒𝑙𝑡𝑖𝑛𝑔 𝑇 0 𝑑𝑇 + 𝐿 𝑚𝑒𝑙𝑡𝑖𝑛𝑔 ), i.e. the sum of the energy to heat up the material to the melting point and of the melting enthalpy L in J mol -1 , where Cp is the heat capacitance in J K -1 mol -1 ). Then, the vapor production per spot is calculated by integration of the vapor flux over the spot diameter and over the duration of existence of melted metal at the surface of the spot. The vapor flux (mol m -2 s -1 or kg m -2 s -1 ) can be calculated with two approaches detailed in Lasagni 2004 and Murphy 2010 for sparks, versus the saturation vapor pressure or versus the vaporization enthalpy.

In the "energy flux" approach, the vapor flux is related to the heat flux (J.m -2 .s -1 ) deposited by the plasma divided by the required energy for heating, melting and surface vaporization with Jvap = input heat flux /H heating + melting + vaporization.

In the "vapor pressure" approach, the melted surface is defined by the input heat flux divided by the energy required for melting, with Jvap = psat. (mM / 2kBTmelting) 1/2 from Langmuir 1913, generally assumed to be at the melting temperature.

Finally, the saturation of the hot metal vapors triggers nucleation in each vapor plume, as depicted in models of fume formation in atmospheric pressure (in [START_REF] Tashiro | Numerical analysis of fume formation mechanism in arc welding[END_REF] for welding arc in quasistationary vapor profile and in [START_REF] Wen | Experimental and theoretical studies of particle generation after laser ablation of copper with a background gas at atmospheric pressure[END_REF] for expanding vapor plume around repetitive laser).

Before calculations discussed in the next section, surface vaporization has been first confirmed here in DBD by the size distributions depicted above. Such small primary particles (<5nm) can only be produced by physical nucleation of hot vapors quenched by the carrier gas in the mobile front of expanding jets. It has been shown that in DBD as well, the local vapor flux from surfaces depends on both the energy per filament and the material properties in Figures 9 and 10 respectively.

The large excess of metal nanoparticles compare to alumina nanoparticles can be related to higher energy fluxes and temperatures in the metal spots than in the ones on alumina, as depicted in section 3.1. More vapors is thus produced from hotter metal spots confined on the same positions on the edges of the metal electrode, further intensified by the smaller enthalpy required for heating and melting and the higher saturation vapor pressures for metals than for Al2O3.

Larger mean particle diameters are produced with more filaments during the transit in the gap, at higher voltages and/or lower flow rates. This can only arise from larger amount of material vaporized during the transit in the DBD (see Figures 11 and12).

Therefore, the results presented above confirm that the solid material is vaporized by discharge filaments. Pure crystalline and amorphous nanoparticles (< 5nm) with the same composition as the solid target are formed by physical nucleation. Each discharge filament is a local and transient source of energy with local production of vapors and nanoparticles. DBD can thus be considered as a generator of nanoparticles locally produced from similar sources.

Agglomeration in the DBD  controlled size distribution and morphology

A fraction of the nanoparticles are coagulated into rather spherical agglomerates with sizes up to 10 nm made of a few nucleated particles (from 2 to 5 nm). As depicted above, agglomeration occurs at the end of the vapor jet expansion and during the transit between subsequent filaments in the DBD when previously formed particles cross expanding jets of vapors and particles. The final particle size of agglomerates has been shown to increase with the energy per filament in section 3.2.2.1 and with both the number of filaments during the transit and the transit time in the DBD in section 3.3. Both the energy per filament and the number of filaments during the transit in the DBD control the vapor and nanoparticles fluxes in a unit gas volume during the transit. Neglecting the evolution of particle concentration as well as vapor and particle losses on the wall along the DBD, the nanoparticles flux divided by the flow rate is related to a mean particle concentration. The final particle size of agglomerates then increases with the mean particle concentration on the one hand and with the transit time on the other hand, both controlling the agglomeration.

This was expected since the requirements for ballistic agglomeration are largely fulfilled at the end of the vapor jet expansion as well as during the transit between subsequent filaments in the DBD, close to STP. Indeed, with nanoparticles smaller than the gas mean free path, collisions between particles occurs on ballistic trajectories and leads to compact agglomerates [START_REF] Weber | In situ characterization and structure modification of agglomerated aerosol particles[END_REF]. Hence, plasma filaments in DBD allow one to design the particle characteristics such as the size of particles below 5 nm and the morphology of spherical agglomerates while preventing dendritic agglomeration in DBD.

In principle, electrostatic coagulation could also contribute to agglomeration in the DBD. However, the small nucleated particles below 5 nm are only charged by diffusion and partial charging would only lead to a fraction of charged nanoparticles (<10%). Hence, although electrostatic coagulation cannot be excluded, it is safe to say that this is not the main agglomeration mechanism.

At this point, it has to be mentioned that some larger spherical particles up to a few tens of nm are observed with the more volatile silver electrode, when higher amount of vapors are emitted from the surface during the transit in DBD. Indeed, they appear more numerous when the number of similar filaments during the transit in the DBD is increased for higher voltages and/or for longer transit times from 4 to 20 ms. EDX analyses show that these larger spherical particles are made of silver. Hence, these larger spherical particles are not formed in the initial expanding vapor plume.

Figure 13: TEM pictures of silver nanoparticles (smaller primary nucleated nanoparticles up to 5 nm and agglomerates up to 20 nm, as well as perfectly spherical particles up to a few tens of nanometers

These larger spherical particles are thus probably formed by heterogeneous nucleation on primary particles and/or resulting agglomerates, called coalescence [START_REF] Teleki | Distinguishing between aggregates and agglomerates of flame-made TiO2 by high pressure dispersion[END_REF]. Indeed, the sintering of melted agglomerates can be excluded since the cooling of expanding vapor jet leads to temperatures below the melting and sintering ones during the post-production transit in the DBD.

3.4.5 Production of nanoparticles versus material properties and plasma parameters 3.4.5.1 Evaluation of mass production rates of nanoparticles The number concentration of primary nanoparticles produced by the DBD is derived from the concentration of agglomerates measured by SMPS after 60 seconds in post-DBD and corrected for the detection efficiency of the CPC and for losses in and after the DBD, depicted in §2.2 and appendix 2.

To do so, we used the work of [START_REF] Weber | In situ characterization and structure modification of agglomerated aerosol particles[END_REF], describing the shape of agglomerates in the free molecular regime for ballistic and diffusion limited agglomeration. The number of primary particles per agglomerate with diameter d50, agglom., is given by Npp/agglom= A(d50, agglom. /dpp) Df , with dpp the primary particle diameter, a filling parameter A = 0.5 and the fractal-like dimension Df = 3, for ballistic agglomeration. The total number of primary particles per unit volume is defined as the product of Npp/agglom time the number concentration of agglomerates. The mass production rate (kg s -1 ) is calculated as the product of the mass concentration of primary particle times the volume gas flow, then divided by the input power and the molar mass to define the production per unit energy (mol J -1 ).

3.4.5.2 Production per unit energy (mol J -1 ) and energy yield (%) versus the material properties The production per unit energy is compared versus material properties for gold, silver and copper to account for the molecular vapor flux rather than for the mass flux depending on the molecular weight. Thus, the production per unit energy (in mol J -1 rather than in g J -1 ) is presented here versus the inverse latent heat of melting (mol J -1 ). As discussed in §3.4.2, the temperature of the metal spot submitted to successive filaments during the same half period, can be assumed to be close to the melting temperature. Therefore, the heating enthalpy up to the melting temperature is not taken into account here. This is done for different gap lengths controlling the energy per filament in Figure 14a. The number of filaments during the transit in the DBD was kept constant by lowering the voltage in smaller gap, as depicted in Table 1, implying higher power in larger gaps. Mass production rate versus the mean charge per filament in the sub-cm 2 lab-scale DBD at 70 kHz Despite rough assumptions neglecting the energy required for vaporization, the production per unit energy tends to be proportional to the inverse of the melting enthalpy, in Figure 14a for different gaps.

Hence, as a rule of thumb, the melting enthalpy can be used at first approximation to estimate the production rate of nanoparticles by DBD on different metals.

These results are coherent neither with the "energy flux" nor with "vapor pressure" approaches. However, attempts to correlate the production rate and the vapor flux with the "energy flux" and the "vapor pressure" approaches, all failed. Indeed, the vapor flux per spot is not evaluated properly since the melted surface is probably controlled by heat exchanges, neglected in both cases. At first, neither the energy density nor the flux in the spot is accurately defined as detailed in 3.4.1. Then, neither the area of the melted metal surface producing vapor nor the evolution of the temperature profile, nor the duration of existence of melted metal in the spot is defined as detailed in 3.4.2. Moreover, local recollection of vapors and nucleated nanoparticles in the DBD is not included to correct the production rates. Different spot diameters and heat exchanges are both expected for each metal and may be amplified with our different shapes of the edges of the electrodes. Hence, this relation still deserves to be confirmed with other metals and without edge effects, for the evaluation of the amount of vapor produced per spot, from such measurements of nano-particles. Finally, Figure 14a also shows that the production per unit energy of nanoparticles production derived from post-DBD aerosol measurements, decreases in larger gaps with more energetic filaments. Since the production per filament has been shown to increase with the energy per filament in section 3.2.2.1, this implies that the nanoparticles production per filament derived from post-DBD aerosol measurements increases less than the energy per filament. Either the vapor production increases less than the energy per filament or more probably, the lost fraction of vapors and nanoparticles recollected increases with more energetic filament. Indeed, the local recollection of vapors and nanoparticles around the hot spot, from the vapor jet to colder surfaces between spots probably increases with the energy per filament. As a result, in these 70 kHz lab-scale DBD with ms transit times, the energy yield (defined as the ratio of the energetic cost for vaporization from melting temperature of the amount of material produced per second on the input power) lies between 10 -7 to 10 -9 %. However, the production per unit energy and the related energy yield can be improved with other DBD configurations reducing vapor and aerosol recollection as well as heat losses in the electrode.

Production rate versus plasma parameter

At first approximation, neglecting vapor and nanoparticles losses, the mass production rate (g s -1 ) depends on the input power i.e. on the product of the number of filaments per second related to plasma parameters (voltage, gap, frequency and surface, as defined in section 3.1), times the production per filament. The production per filament i.e. the local vapor flux from surfaces depends on the energy per filament for any given material, as shown in section 3.2.2.1 and discussed in section 3.4.3.

Finally, to control the process, the nanoparticles production rate (g s -1 ) is related to an easily measured electrical characteristic of DBD. The mass production rate is plotted on Figure 14b versus the charge per filament for different materials at constant number of filaments per second, in the 70 kHz lab-scale DBD. The mass production rates for all metals are correlated to the charge per pulse i.e. to the total transferred charge by all filaments during the transit in DBD. Hence, the slope corresponding to the mass of nanoparticles per unit charge (g C -1 ) is constant for each material in a given DBD, whereas the mass of nanoparticles per unit energy (g J -1 ) evolves with the total energy injected per filament. Indeed, the charge per filament is proportional to the maximum current of the pulse related to the temperature of the plasma filament and probably to the energy flux in the spot (Marode, 1999), controlling the local vapor flux.

Conclusion

In DBD, the initial local vapor fluxes from spots of plasma-surface interaction is reduced compared to the ones from laser and sparks. Indeed, mean diameters of isolated primary particles below 3nm and agglomerates thereof below 5 nm are produced for ms transit times in lab-scale DBD, with upper diameters below 5 and 10 nm, respectively. It has been shown that the energy deposited in the spot of filaments controls the local vapor flux leading to single digit nanoparticles by a purely physical transformation of bulk metals. Up to 50% of isolated unagglomerated amorphous and crystalline particles are so-formed by physical homogeneous nucleation in mobile fronts of expanding vapor jets. Hence, each filament can be considered as a local and transient source of energy with related pulsed local production of vapors and nanoparticles. Then, some rather spherical agglomerates are also formed by ballistic agglomeration at the end of the jet expansion and during the transit between subsequent filaments. The first agglomeration step can be limited at reduced energy per filament, while the second one depends on the transit time.

The final particle size of non-agglomerated primary particles increases with the energy per filament. While the final particle size of agglomerates evolves on the one hand with energy per filament and with the number of filaments during the transit in the DBD. Both control the vapor and nanoparticles fluxes in a unit gas volume during the transit, leading to a mean particle concentration when divided by the flow rate. The final particle size of agglomerates also evolves on the other hand with the transit time controlling the agglomeration.

The production per unit energy can be roughly related to the inverse of melting enthalpies of the metal electrodes. However, this relation deserves to be refined with other metals and without edge effects, to account for subsequent vaporization and nucleation on both cathode and anode sides of the filament.

The mass production rate (g s -1 ), neglecting vapor and nanoparticles losses, depend on the input power related to the product of the number of filaments per second times the energy controlling the production per filament, for any given material. Moderate production rates in these lab-scale DBD with ms transit times powered with a few Watt have been increased with larger DBD at 50 W. However, as in other physical nucleation processes like laser and spark, increasing the input power leads to larger agglomerates. If this is suitable for the production of nanopowders with large geometric standard deviation, it cannot be used directly for nanomaterials production processes. Indeed, nanomaterials exhibit tunable size-dependent properties mainly below 30 nm, which implies to control the enlargement of the size distribution by coagulation. Hence, the main advantage of DBD is to avoid expensive postprocessing. In fact, to reach similar single digit metal nanoparticles with very high added value for nanotechnologies directly at atmospheric pressure, other nucleation processes such as spark, laser and flames require size selection and/or thermal sintering of agglomerates. As a consequence, this one step process still is interesting for industrial applications despite the low energy yields (< 10 -6 %) of these lab-scale DBD. This is especially true since great improvements are expected with other DBD configurations reducing vapor and nanoparticles recollection as well as heat losses in the electrode.

A new application of DBD is highlighted as gas phase process for the production of tailored metal particles with tunable size. This non-thermal plasma process presents great potentialities for nanotechnologies since compact, inexpensive and simple DBD is performed at atmospheric pressure and can be used to reach size-dependant properties of nano-materials, without any gaseous precursor nor solvent. Indeed, agglomeration has been successfully limited by short transit times of a few milliseconds in DBD with "low" energy plasma filaments (up to tens of µJ) and can still be optimized to produce only single digit unagglomerated metal nanoparticles. Indeed, with more homogeneous discharge filament properties, without edge effects, such as those described in the asymmetric lab-scale planar DBD, even less dispersed size distributions of metal nanoparticles are expected.

Finally, it has to be underlined that this nanoparticles production process by DBD has been proved to be versatile. Indeed, single digit nanometer sized, carbon, polymer, metal oxides and silicon nanoparticles have also been produced in this lab-scale DBD.

APPENDIX 1: ELECTRICAL CHARACTERIZATION of DBD A short overview of critical points for the electrical characterization of discharge filaments induced in DBD are framed. For each functioning condition, applied voltage, input power and individual current pulse characteristics (I0, Q, t) have been measured. Figure 15 shows the setup used to perform voltage and current measurements as well as discharge power calculations. Coaxial cable and 50 ohm resistors prevent from electrical noise, current pulse distortion and echo. The measured current depicted on Figure 5, has to be corrected by the parameter , because of the attenuation of the current amplitude due to the presence of dielectric materials. The correction parameter for current amplitude () has to be calculated and evaluated (from the first lightning voltage, Vth) for each DBD reactor.

Relation between charge displacements in the gap and the measured current in a DBD

The real discharge current (idis) can be evaluated by:

                 dt t du C t i C C i t i m d g ind dis ) ( ) ( . 1 . ) ( 
where C, Cd, Cg the capacitances of the cell, of the dielectric materials and of the gap, im and iind are respectively the measured current and the current in the external circuit induced by the displacements of charge in the gap, and u is the applied voltage. The parameter  can be calculated from measurements:

t lightning first V V  
with Vfirst lightning the value of the instantaneous applied voltage when the first filamentary discharge occurs when the dielectric surfaces are not polarized (acquired with a single sequence when the dielectric surfaces are depolarized after a few days without functioning) and Vt the threshold voltage of self sustained discharge. For further explanation see [START_REF] Jidenko | Electrical characterization of microdischarges produced in dielectric barrier discharge in dry air at atmospheric pressure[END_REF].

Then, the following electrical characteristics of pulses can be defined from the corrected current for a 1 mm gap.

 Electrical charge per pulse Q/pulse (about nC) measured by integrating the instantaneous discharge current (without the capacitive current) during a pulse. (Q/pulse is proportional to d.r/e, with d the gap length, e the thickness and r the relative permittivity of dielectric material.

 Maximum current pulse (I0 ~ 20 mA)  Duration of the pulse (t about 20 to 100 ns in air and nitrogen)  Npulse/T: Number of pulses per square centimetre during a voltage period (few 10's for 1 mm gap)  Energy per pulse (E/pulse in J) Two methods are used for the evaluation of the energy per pulse:

-The real energy per pulse is ∫(duration f the pulse) idis.vgapdt. Unfortunately the vgap (voltage in the gap on the axis of the discharge filament) cannot be measured in a DBD. A rough estimation consists in assuming that the voltage is nearly constant equal to the threshold voltage of self-sustained discharge (Vth~10.5 kV for 2.5 mm in N2). Thus, the energy per pulse is the product of the charge per pulse by the threshold voltage. En/pulse~Q/pulse.Vt -Assuming that all filament are identical, the mean energy per pulse can also be estimated by the ratio of the energy per period (input power divided by the frequency) by the number of pulse per period (En/pulse~EnT/ Npulse/T).

The input power can be calculated either by ui product integration over an integer number of periods, as detailed on Figure 15.a (P =1/nT ∫(nT) u(t)i(t)dt), or by the Lissajou's method, depicted on Figure 15.b. Here, the 50 ohm resistor is replaced by a capacitance. The input power (P =F.∫ udq, with q=Cm.v , Cm the capacitance and v the voltage across the capacitance) is the area of the parallelogram, which represents the input energy per period (EnT in J) times the frequency. The number of filaments per period or per second is measured at low voltage when the number of filaments per period is small so that most of filaments start at different times. At higher voltages, a great fraction of current pulses related to filaments are superimposed. In that case, the number of filaments is calculated as the ratio of the energy per period or of the input power (derived from Lissajou's figures) on the energy per pulse calculated as the product of the charge per pulse by the threshold voltage. En/pulse~Q/pulse.Vt

Post DBD losses

Thermophoresis : In DBD, the energy deposited on surfaces by filament heats the electrode as well as the dielectric material, which then heat the gas flowing between in the gap. Then, losses of particles by thermophoresis may occur from the gas towards colder surfaces of post-discharge tubes. An evaluation of post-discharge losses by thermophoresis has been performed [START_REF] Housiadas | Thermophoretic Deposition in Tube Flow Aerosol[END_REF]. Penetrations higher than 50 % have been calculated in the worst conditions for 2 nm particles at Tgas ~ 80 °C, encountered in post-DBD (see Figure 5b). Hence, these losses can be neglected.

Diffusion have been estimated using Gormley et Kennedy equations [START_REF] Gormley | Diffusion from a stream following through a cylindrical tube[END_REF], which have been validated for particles larger than 2 nm, for laminar flow [START_REF] Alonso | Penetration of nanometer-sized aerosol particles through wire screen and laminar flow tube[END_REF]. This formula takes into account the effect of the gas temperature after the DBD, measured from 20 et 200 °C on the diffusion coefficient of particles [START_REF] Rudyak V Ya | Measurements of the temperature dependent diffusion coefficient of nanoparticles in the range of 295-600 K at atmospheric pressure[END_REF]. For smaller particles than 2 nm, the diffusion coefficient is multiplied by a correction factor to ensure the continuity with the diffusion coefficient obtained from the gas kinetic theory [START_REF] Ramamurthi | On improving the validity of wire screen unattached fraction radon daughter measurements[END_REF]. These calculations show that losses by diffusion are lower than 50% for particles larger than 1.5 and 2.5 nm at 20°C and 200°C respectively for 60s in post-DBD (see Figure 16). Then, in this work at Tgas ~ 80 °C, losses by diffusion are around 55% at 2 nm for 60s in post-DBD (see Figure 16b) and around 45% for 0.13 sec in the labscale DBD without the post-DBD coagulation volume (see Figure 16a). For the more numerous particles between 2 to 5 nm from the lab-scale DBD, lower losses have been included for correction of the measured concentration, required for the calculation of the mass production rates.

Besides, the effect of the gas temperature from 20 to 200 °C is negligible for diffusion losses above 2nm. However, losses by diffusion at 1 nm increase up to 70% for short post-DBD on Figure 16a and up to 80 % for 60sec in the coagulation volume in post-DBD (see Figure 17b).

Coulombian losses by electrostatic repulsion have also been considered since particles may be charged by diffusion during their transit in bipolar ions in as well as after the DBD [START_REF] Bourgeois | Nucléation et croissance de nanoparticules formées par décharge à barrières diélectriques[END_REF], for ion densities higher than 10 6-7 cm -3 , as described in [START_REF] Kasper | Electrostatic dispersion of homopolar charged Aerosols[END_REF]. However, for particles smaller than 10 nm, only a small fraction of particles are charged at the Boltzmann equilibrium (0.6 % at 10 nm, in Hinds, 1982). Losses by electrocollection of charged particle after the DBD can thus be neglected, as confirmed by tests with or without an electrostatic precipitator before the measurements.

As a result, it is first shown that post-DBD losses by thermophoresis and electrostatic repulsion can be neglected while diffusion losses have to be taken into consideration for particles smaller than 2 nm.

Losses in the lab-scale DBD

With transit times in the order of ms, diffusion and electrostatic losses of nanoparticles can be neglected in the DBD. Electro-collection has been proved to be inefficient for such small particles in DBD at 60 kHz [START_REF] Jidenko | Kinematics of charged nanometric particles in silent discharge[END_REF]. Indeed, such small nucleated particles below 5 nm are only charged by diffusion and partial charging would only lead to a fraction of charged nucleated nanoparticles (<10%). Hence, at first approximation, initial losses of vapors and nanoparticles are neglected in the DBD despite probable thermal diffusion and thermophoresis inducing local recollection of vapors around the hot spot, from the expanding vapor jet to the colder surfaces between spots.

Figure 1 :

 1 Figure 1: General set-up for aerosol production studies versus electro-thermal properties of plasma filaments

Figure 2 :

 2 Figure 2: Set-up and picture of discharge filaments in a 2.5 mm gap at Upp =13 kV in the lab-scale DBD

Figure 3 :

 3 Figure 3: Counting efficiency of CPC and size distributions of primary particles (TEM) and of agglomerated particles produced by Cu-DBD (Upp = 13 kV, g = 2.45 mm, QN2 = 5 lpm)

  Figure 4: photos of filaments during 1.1 ms aperture in the DBD at 13 kV, 70 kHz, g = 2.45 mm, VN2 = 0.5 m s -1 with a) less luminous filaments in the central volume of the gap in lower electric field than on the edges and b) more spots on the dielectric polarized material than on the metal

Figure 5

 5 Figure 5 : a) Instantaneous discharge current and applied voltage and zoom on current pulses ; b) Evolution of gas temperatures in post-DBD at 70 kHz and 12 kV, fed with 5 lpm nitrogen.

Figure 6 :

 6 Figure 6 : Influence of the applied voltage (a) on the charge per filament and (b) on the discharge power in Au-DBD at 2.45 mm; Influence of the gap (g in mm) (c) on the charge and (d) on the energy per filament at 15 kV.

Figure 7 :

 7 Figure 7: Overview of gold particles and corresponding size distribution (Upp = 13 kV, g = 2.5 mm, transit time = 4 ms in Au-DBD with 1506 particles)

  TEM micrograph of gold particles and corresponding SAED figure of Au fcc crystalline structure (Upp = 11 kV, g = 1.65 mm, Au-DBD) Crystalline structure of nanoparticles: Parallel lattice planes are visible on crystalline nanoparticles as depicted in Figure 8a. From the concentric circles derived from SAED on crystalline nanoparticles, shown in Figure 8b, the lattice constants with the Miller indices can be determined. Particles show lattice constants of 2.355 Å corresponding to the (111) reflection plane and 2.039 Å for the (002) reflection plane of the lattice face-centered cubic (fcc) crystal structure of gold.From all primary nucleated particles, about 20-30% is mono-crystalline versus an amorphous fraction of about 70-80%. This confirms that the nucleated particles are formed in different thermal conditions, as expected in expanding nucleation fronts depicted by Wang 2010 a and b, Autrique 2013 and Farid 2014 for lasers and by Smirnov 1993, Itina and Voloshko 2013 for sparks.

Figure 9 :

 9 Figure 9: (a) TEM pictures of gold nanoparticles produced in Au-DBD and (b) related size distribution, versus gap lengths (g in mm)

Figure 10 :

 10 Figure 10: TEM pictures and corresponding size distributions of the produced nanoparticles from different materials in DBD (g = 2.45 mm, Upp = 13 kV, VN2 = 0.5 m s -1 , QN2 = 5 l/min)

  Figure 11: (a) Influence of the applied voltage on the particle size distribution (g = 2.45 mm) and (b) TEM pictures of silver nanoparticles produced at 13 kV.

Figure 12 :Flowrate

 12 Figure 12: Influence of the flow rate on particle size distributions (Au-DBD, g = 2.45 mm, Upp = 13 kV)

Figure 14 :

 14 Figure 14: (a) Production per unit energy versus the inverse of melting enthalpy for different gap lengths and (b)Mass production rate versus the mean charge per filament in the sub-cm 2 lab-scale DBD at 70 kHz

Figure 15 :

 15 Figure 15: Setup (a) for current and voltage measurements and (b) for power measurements by Lissajou's method

Figure 16 :

 16 Figure 16: Aerosol Penetration versus particle diameter at 20 and 200°C before measurements. (a) for 0.13s in the DBD and in post-DBD before collection on TEM grids and (b) for 60s in the post-DBD coagulation volume (300 cm 3 at 0.3 lpm) before SMPS measurements.

Table 1 :

 1 Results from the analysis of TEM images in Au-DBD for different gap lengths

	Gap length (g in mm)

Table 2 :

 2 Results from TEM analysis for Ag, Au, Cu

	1.65 mm	2.45	mm	3.1	mm


David Prangishvili
email: davidprangishvili@pasteur.fr

Ying Liu

Mart Krupovic
email: martkrupovic@pasteur.fr

ICTV Virus Taxonomy Profile: Portogloboviridae

Keywords: ICTV, taxonomy, Portogloboviridae. 001605

Portogloboviridae is a family of viruses with circular, double-stranded DNA genomes of about 20 kbp. Their icosahedral virions have a diameter of 87 nm, and consist of an outer protein shell, an inner lipid layer and a nucleoprotein core wound up into a spherical coil. Portogloboviruses infect hyperthermophilic archaea of the genus Saccharolobus, order Sulfolobales and are presumably nonlytic. Portogloboviruses encode mini-CRISPR arrays which they use to compete against other co-infecting viruses. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Portogloboviridae, which is available at ictv. global/ report/ portogloboviridae.

VIRION

Virions are icosahedral, with a diameter of about 87 nm from vertex to vertex and 83 nm from facet to facet (Table 1, Fig. 1a) and consist of three structural units: (a) the outer icosahedral protein shell, (b) the subcapsomer proteins and lipid membrane and (c) a circular nucleoprotein wrapped into a spherical core (Fig. 1b) [START_REF] Liu | A novel type of polyhedral viruses infecting hyperthermophilic archaea[END_REF][START_REF] Wang | A packing for A-form DNA in an icosahedral virus[END_REF]. The outer shell carries capsid proteins VP4 and VP10, both displaying a single jelly-roll fold. VP10 and VP4 form pentameric and hexameric capsomers, respectively, which assemble into a T=43 icosahedral shell. The fivefold vertices of the capsid are decorated with the vertex complexes extending above the capsid surface and potentially involved in receptor binding [START_REF] Wang | A packing for A-form DNA in an icosahedral virus[END_REF]. The internal membrane of the virus also displays an icosahedral shape, presumably due to the presence of an intricate array of subcapsomer proteins that span the space between the membrane and the external icosahedral capsid [START_REF] Wang | A packing for A-form DNA in an icosahedral virus[END_REF][START_REF] Baquero | Structure and assembly of archaeal viruses[END_REF]. Lipids are selectively acquired from the pool of host lipids, with the lipid compositions in the viral and host membranes being quantitatively very different [START_REF] Wang | A packing for A-form DNA in an icosahedral virus[END_REF]. The nucleoprotein filament, formed by multiple copies of the nucleocapsid protein VP1 and circular dsDNA, is condensed into a globular core with a characteristic appearance of concentric rings, seen in the axial view, and striations, seen in the side view (Fig. 1a) [START_REF] Liu | A novel type of polyhedral viruses infecting hyperthermophilic archaea[END_REF][START_REF] Wang | A packing for A-form DNA in an icosahedral virus[END_REF]. The viral dsDNA is stored in the capsid in the A-form (11.0 bp/ turn) [START_REF] Wang | A packing for A-form DNA in an icosahedral virus[END_REF], as previously observed for filamentous archaeal viruses of the realm Adnaviria [START_REF] Baquero | Structure and assembly of archaeal viruses[END_REF][START_REF] Wang | Structures of filamentous viruses infecting hyperthermophilic archaea explain DNA stabilization in extreme environments[END_REF].

GENOME

The circular dsDNA genome of Sulfolobus polyhedral virus 1 is 20 222 bp long and contains 45 open reading frames (ORFs) [START_REF] Liu | A novel type of polyhedral viruses infecting hyperthermophilic archaea[END_REF][START_REF] Liu | New archaeal viruses discovered by metagenomic analysis of viral communities in enrichment cultures[END_REF]. ORFs are generally short (median 103 codons) and are tightly arranged, occupying 89.1 % of the genome (Fig. 2). Sequence analyses allow the assignment of putative functions to nine ORFs, of which seven encode putative DNA-binding proteins and the other two encode the glycosyltransferase and the methyltransferase (Fig. 2) [START_REF] Liu | A novel type of polyhedral viruses infecting hyperthermophilic archaea[END_REF].

REPLICATION

Viral replication occurs by chronic infection without an apparent lytic cycle [START_REF] Liu | A novel type of polyhedral viruses infecting hyperthermophilic archaea[END_REF]. No information is available on entry or egress. The viruses do not encode identifiable DNA and RNA polymerases or other recognizable genome replication initiation proteins [START_REF] Krupovic | Viruses of archaea: structural, functional, environmental and evolutionary genomics[END_REF] and, as in the case of most other viruses that infect members of the Sulfolobales, apparently depend on the host machinery for genome replication and transcription [START_REF] Prangishvili | The enigmatic archaeal virosphere[END_REF]. Portogloboviruses carry mini-CRISPR arrays containing spacers targeting each other as well as other viruses, exemplifying a mechanism promoting inter-viral conflicts and superinfection exclusion in extreme environments [START_REF] Medvedeva | Virus-borne mini-CRISPR arrays are involved in interviral conflicts[END_REF].

TAXONOMY

Current taxonomy: www. ictv. global/ taxonomy. The family Portogloboviridae comprises a single genus, Alphaportoglobovirus, with two species. Portogloboviruses infect members of the hyperthermophilic archaeal genus Saccharolobus, order Sulfolobales.

RESOURCES

Full ICTV Report on the family Portogloboviridae: www. ictv. global/ report/ portogloboviridae.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Electron micrograph of Sulfolobus polyhedral virus 1 virions. (a) Sample embedded in vitreous ice. Open arrowhead -projection in axial view, filled arrowhead -projection in side view. Scale bars, 100 nm [from [1], copyright American Society for Microbiology]. (b) Overall cryo-EM density map of Sulfolobus polyhedral virus 1, rendered to display the outer capsid (left half) and a central slice showing the interior of the virus (right half). The virus is coloured by radius. The red rectangle, triangle and pentagon denote the twofold, threefold and fivefold axes of symmetry, respectively. The grey numbered hexagons outline the hexameric VP4 capsomers from one asymmetric unit of the icosahedral capsid [2].

Table 1 .

 1 Characteristics of members of the family Portogloboviridae

	Genome	Circular, double-stranded DNA genome of about 20 kbp
	Replication	Viral replication occurs without observable cell lysis
	Translation	Not characterized
	Host range	Hyperthermophilic archaea of the genus Saccharolobus, order Sulfolobales
	Taxonomy	One genus, two species

Example:

Sulfolobus polyhedral virus 1 (KY780159), species Sulfolobus alphaportoglobovirus 1, genus Alphaportoglobovirus Virion Icosahedral virion, 87 nm in diameter, consisting of an outer protein shell, an inner lipid layer and a spherical nucleoprotein core

Acknowledgements

Members of the ICTV Report Consortium are Stuart G. Siddell, Elliot J. Lefkowitz, Sead Sabanadzovic, Peter Simmonds, F. Murilo Zerbini, Donald B. Smith, Richard J. Orton and Mart Krupovic.

Funding information

Production of this summary, the online chapter, and associated resources was funded by a grant from the Wellcome Trust (WT108418AIA).

Conflicts of interest

The authors declare that there are no conflicts of interest