William De

Rezende Locatel

Chetna Mohabeer

Dorothée Laurenti

Yves Schuurman

Nolven Guilhaume
email: nolven.guilhaume@ircelyon.univ-lyon1.fr

Co-pyrolysis of beech wood and polyamide-6: Effect of HZSM-5 catalyst on the properties of pyrolysis oils

Keywords: Biomass pyrolysis, polyamide pyrolysis, co-pyrolysis, Bio-oils, Catalytic upgrading

Biomass residues and waste can be converted into bioliquids by pyrolysis, which can contribute to both the production of renewable fuels and the recycling of waste. In this paper, the pyrolysis of pure beech wood (BW) and of a mixture of BW + polyamide-6 (PA6), a nitrogen-containing plastic, was investigated to assess the effect of PA6 on the properties of bio-oils, obtained either by thermal pyrolysis or by pyrolysis with ex-situ catalytic treatment of the vapors. The presence of 20 wt.% PA6 in the blend affected the pyrolysis products yields and distribution (gases, liquids, chars), but also the composition of the bioliquids. In absence of a catalyst, the production of phenolic compounds (guaïacol, syringol…), sugars, furans decreased significantly with the BW+PA6 blend compared to pure BW, and N-containing compounds derived from PA6 were found in the bioliquids. When HZSM-5 catalyst was applied for the treatment of vapors, a beneficial effect of the catalyst on the reduction of the sugars and furans content in the bioliquids and on the production of deoxygenated compounds was observed with pure BW. This effect was mitigated with BW+PA6 blends and the formation of fully deoxygenated compounds was almost totally suppressed, suggesting that the strong catalyst acid sites were poisoned. Caprolactam was by far the main product of PA6 degradation found in the bioliquids, but the N-containing byproducts were different depending on the absence or presence of HZSM-5 catalyst. In particular, the formation of 5-cyano-1-pentene in a fairly large proportion was only observed in the presence of the catalyst.

Introduction

Plastics have assumed a crucial role in modern societies. Since the 1950s, the production and consumption of plastics has grown exponentially due to their multiple properties, such as versatility, durability, mechanical and chemical inertness, lightness, low cost etc. In 2019, the global plastic production reached 370 million tons [START_REF] Plasticseurope | Plastic -the Facts[END_REF]. Plastic materials can be employed in many sectors such as packaging, building and construction, automotive, electrical and electronics. Thus, it is expected that a significant part of generated plastic will end in municipal solid waste (MSW) to be disposed of with other waste materials or incinerated.

According to a report on solid waste management published in 2018 [START_REF] Kaza | What a Waste 2.0 : A Global Snapshot of Solid Waste Management to 2050[END_REF], the world waste production is expected to increase to 3.4 billion tons by 2050, while the plastic fraction present in MSW was estimated between 6 and 13 wt.% in 2016, depending on the countries considered. Indeed, Areeprasert et al. [START_REF] Areeprasert | Municipal Plastic Waste Composition Study at Transfer Station of Bangkok and Possibility of its Energy Recovery by Pyrolysis[END_REF] assembled data about the plastics present in MSW from different countries and the values fluctuated between 3 and 27%. Similarly, Patrício Silva and co-workers [START_REF] Silva | An urgent call to think globally and act locally on landfill disposable plastics under and after covid-19 pandemic: Pollution prevention and technological (Bio) remediation solutions[END_REF] reported plastic concentration between 2 and 20% for different countries. Additionally, these authors related the impact of the COVID-19 pandemic on the generation of waste plastics, due to the massive use of personal protective equipment (PPE), achieving a contribution of 3.5% of PPE in the plastic share of MSW that was sent to various landfill sites around the world in 2020.

The European Union fixed the targets of recycling at least 55% of municipal waste by 2025 and 65% by 2035 [5]. However, recycling the plastics present in this waste is difficult due to sample contamination and to the presence of blends of different polymers. In addition, recycling plastic materials is economically unprofitable due to their low production and processing costs [START_REF] Phakedi | Co-pyrolysis of polymer waste and carbon-based matter as an alternative for waste management in the developing world[END_REF].

Therefore, thermal conversion processes such as gasification, incineration and pyrolysis, are promising alternatives to deal with this complex mixture of solid wastes.

Pyrolysis is an interesting process because it can process different types of feedstocks and allows converting their organic fractions into bio-oils, which can then be integrated into conventional refinery processes (fluid catalytic cracking or hydroprocessing) to produce fuels, or be used as a source of high-value products [START_REF] Bridgwater | Fast pyrolysis processes for biomass[END_REF][START_REF] Ansari | Co-pyrolysis of biomass and plastic wastes: A review on reactants synergy, catalyst impact, process parameter, hydrocarbon fuel potential, COVID-19[END_REF]. Several studies [START_REF] Kumar | Catalytic Co-pyrolysis of Biomass and Plastics (Polypropylene and Polystyrene) Using Spent FCC Catalyst[END_REF][START_REF] Burra | Quantifying the sources of synergistic effects in co-pyrolysis of pinewood and polystyrene[END_REF][START_REF] Liu | Towards enhanced understanding of synergistic effects in co-pyrolysis of pinewood and polycarbonate[END_REF][START_REF] He | Enhancing hydrocarbon production via exsitu catalytic co-pyrolysis of biomass and high-density polyethylene: Study of synergistic effect and aromatics selectivity[END_REF][START_REF] Wang | Co-pyrolysis of waste plastic and solid biomass for synergistic production of biofuels and chemicals-A review[END_REF] and the recent review by Ansari et al. [START_REF] Ansari | Co-pyrolysis of biomass and plastic wastes: A review on reactants synergy, catalyst impact, process parameter, hydrocarbon fuel potential, COVID-19[END_REF] reported the beneficial effects of processing plastics and biomass together.

However, the co-pyrolysis of plastic and biomass can be affected by different parameters such as the biomass and plastic types, the biomass to plastic ratio, the process parameters (e.g. temperature, residence time, heating rate…) and the reactor configuration [START_REF] Lin | Effects of alkali and alkaline earth metals on the co-pyrolysis of cellulose and high density polyethylene using TGA and Py-GC/MS[END_REF].

Table 1 brings together some recent studies devoted to co-pyrolysis of biomass and plastics. The plastics most frequently studied recently were polyolefins, such as PE and PP, with plastic concentrations around 50 wt.% and the catalysts employed were essentially zeolites, particularly ZSM-5. Thus, co-pyrolysis with these main plastics has been exhaustively studied and its impacts on the pyrolysis process has already been well investigated. As general trends, an increase in the production of liquid aromatics and a decrease in the formation of light aromatics and chars were identified.

Studies involving plastics that contain heteroatoms, such as N atoms in polyamides, are scarce.

Polyamide-6 (PA6) is a whitish semi-crystalline thermoplastic polymer; its structure is depicted in Figure 1. It has a wide range of applications due to its properties of high strength, rigidity, chemical resistance and simple processing. Thus, PA6 is used in the automotive and construction industries, in electrical and electronic materials, in fibers applied for clothing and, more recently, in the confection of personal protective equipment (PPE), such as disposable surgical masks [23]. As a consequence of the COVID-19 pandemic, the concentration of this plastic in MSW has significantly increased. In this work, the co-pyrolysis beech wood (BW) chips, as lignocellulosic biomass, and PA6

was studied and coupled with catalytic upgrading of the pyrolysis vapors using a HZSM-5 catalyst.

For this purpose, the pyrolysis products (gases, liquids and solids) were characterized in depth using various analytical techniques, specifically optimized for this type of samples. This study aims at assessing the role of the catalyst on the products composition after catalytic treatment of the pyrolysis vapors and the effect of PA6 on the catalytic activity, by comparing experiments carried out in the absence or in the presence of the catalyst, using pure BW or a blend of BW + 20 wt.% PA6.

Experimental section 2.1. Feedstocks

The lignocellulosic biomass used consists of beech wood chips supplied by J. Rettenmaier & Söhne; they were sieved to obtain particle sizes between 0.75 and 1.7 mm. Polyamide-6 (BASF -Ultramid® B) was supplied as pellets, which were crushed and sieved before use to obtain particle sizes in the same range as the wood chips. BW chips were mechanically mixed with 5, 10 and 20

wt.% of PA6 (on dry basis). All feedstock samples were dried in an oven at 110°C for 1h before each pyrolysis experiment and stored in airtight flasks.

Catalyst

Zeolite HZSM-5 with a molar Si/Al ratio of 40 was supplied by Céramiques Techniques Industrielles (CTI) Salindres, France. The catalyst characterization has been described

previously [START_REF] De Rezende Locatel | Can Paints and Varnish Impair the Physicochemical Properties of Wood Pyrolysis Oils?[END_REF] and its main properties are summarized in Table 2.

Table 2 -Properties of HZSM-5 catalyst

Property Value

Particle size (µm) 100-300

Water content a (% wt.) 3.9

Si/Al atomic ratio b (SAR) 40.5

Total SSA c (m 2 g -1 )

Microporous surface (m 2 g -1 )

External surface (m

Pyrolysis set-up

Co-pyrolysis of beech wood and PA6 was carried-out in a lab-scale pyrolytic bench already described [START_REF] De Rezende Locatel | Ex-situ catalytic upgrading of pyrolysis vapors using mixed metal oxides[END_REF], with a few modifications to adapt the system to the mixed wood/plastic feedstocks (Figure 2). The jacketed reactor has an internal zone where the thermal pyrolysis is carried out and a bed of catalyst can be placed (or not) just below to treat the vapors which come out of the pyrolysis zone. The BW chips or BW+PA6 blends were previously dried at 110°C for 1.5 h before being placed in the gas-tight hopper and kept under N2 atmosphere.

Unlike the previous description [START_REF] De Rezende Locatel | Ex-situ catalytic upgrading of pyrolysis vapors using mixed metal oxides[END_REF], the new inner reactor (pyrolysis zone) was a straight quartz tube, in which the solid chips were fed semi-continuously from the top for 1 h, by small increments of ≈0.045 g every 10 s, and fell down by gravity onto a quartz porous frit at the bottom of this section, where the chars and solid residues were retained. The total mass of solid chips fed in the reactor was ≈16 g in 1 h of experiment. The temperature in the pyrolysis zone was 450-500°C along the tube and 480°C on the porous frit where the wood chips were stopped. The a trap filled with glass beads at -5°C (4) and a trap at ambient temperature filled with silica gel beads (5), that prevented any condensable compound from being introduced in the micro-GC for online gas phase analysis. The total mass of bio-oils formed in the experiments was assessed by mass difference, by weighing the different glassware parts before and after the experiments.

Prior to starting the reaction, the reactor (empty in thermal experiments, or loaded with 2 g of catalyst in ex-situ catalytic experiments) was heated to 500 °C under a N2 flow for 2 h to ensure fully stabilized temperatures in the whole system (reactor and cold traps) and to desorb the water adsorbed on the catalyst. Non-condensable products (hereinafter called "gas phase") were analyzed and quantified online using a micro-GC.

Products recovery

The fractions called "aqueous phase" and "organic phase" were recovered directly in the round bottom flask and simply separated by centrifugation; thus, they constituted the most representative share of the bio-oil produced in the process. Other fractions of bio-oils were also deposited in the downstream traps and remained stuck on the glassware parts. In order to obtain a detailed mass balance, these fractions were recovered by washing the glassware with methanol, followed by evaporation of the methanol at 50°C under a pressure of 400 mbar, yielding the "recovered organic phase". However, the evaporation step was found to modify the composition of this bio-oil fraction due to simultaneous evaporation of volatile components. In addition, 13 C-NMR analysis showed the presence of residual methanol in this fraction. Therefore, it was decided not to mix the "recovered organic phase" with the native "organic phase". Finally, a part of the bio-oil could not be recovered because it was adsorbed on the silicagel beads or lost during the different steps of recovering the other fractions. This fraction was called "organic not recovered" and was quantified by weighing the glassware parts before and after experiment.

The chars were recovered from the first quartz frit, weighed and are hereinafter called "solid phase".

Product characterization

Gaseous products were analyzed online using a micro gas chromatograph Inficon 3000 (SRA instruments) equipped with three analytical modules and TCD detectors, calibrated with appropriate compound mixtures. A MolSieve column equipped with a Plot U pre-column was used to analyze He, H2 and CO; a Plot U column was used to analyze CO2, C2H4 and C2H6; an alumina column was used to analyze the remaining hydrocarbons (C3-C5). The duration of each analysis was 6 minutes and the analyses followed each other for 90 minutes. Helium was used as internal standard to account for the variations of total gas flow rates at the reactor inlet and outlet.

Elemental compositions (CHNS) were obtained with a Flash2000 (Thermo Scientific) equipment. For each analysis, a known mass of sample (1-3 mg) was placed in a tin crucible and the experiment was repeated three times to determine the measurement uncertainty.

The HHV was calculated from the elemental analysis using the equation proposed by Channiwala and Parikh [START_REF] Channiwala | A unified correlation for estimating HHV of solid, liquid and gaseous fuels[END_REF].

HHV = 0.3491[C] + 1.1783[H] -0.1034[O] -0.0151[N]
In O: 0.00 -50.00%; N: 0.00 -5.60%.

The water content of bio-oils was determined by Karl-Fischer (KF) titration using a Metrohm Titrando 852 analyzer. The samples were diluted in THF at a concentration of 10 wt.% and the volumetric method was employed. To avoid side reactions of aldehydes and ketones with traditional KF reagents, which produce water, specific reagents for samples containing these organic compounds were chosen: Karl Fischer ROTI hydroquant C5K (5 mg of water/mL of solution) and Karl Fischer ROTI hydroquant working medium K. The calibration was carried out using a solution of 10 wt.% of H2O in THF.

Molar mass distributions were obtained by gel permeation chromatography (GPC) using an Agilent equipment (1200 series) equipped with two PL gel columns with different pore sizes (500 and 50 Å ) using THF as the mobile phase (1 mL/min). Two detectors were used, a refractive index detector (RID) and diode-array detector (DAD), collecting data each 0.4 s in the range of 200-450 nm with a resolution of 2 nm. The samples were prepared with concentration of 1 wt.% in THF and filtered before analysis (PTFE -0.45µm). The measurements were carried out at 35°C and the calibration was performed using hydrocarbons standards (molecular weight between 86 and 1000 g/mol). The spent catalyst (recovered after reaction) was analyzed by thermogravimetric analysis (TGA) under air (40 mL/min) using a TGA/DSC-1-Star e system (Mettler Toledo) equipped with a gas controller GC200. The temperature was initially held at 25°C for 5 minutes, then increased up to 900°C with a heating rate of 10°C/min.

Results and discussion

Characterization of pyrolysis and co-pyrolysis products

To assess the effect of ex-situ catalytic treatment of pyrolysis vapors, the results obtained with and without catalyst (HZSM-5) have been compared for experiments with pure beech wood (BW)

or a blend of BW + 20 wt. % PA6. The mass balance was close to 100 % in all experiments (98-103 %) and liquid products were the main fraction obtained, representing 57-67 wt.% of the total mass of products. Figure 3 displays the selectivity for solid, liquid and gas products, calculated as the mass of a given fraction over the total mass of products. In the absence of a catalyst, the mass of solid products corresponded to those collected on the first frit glass at the outlet of the pyrolysis zone, while in the presence of the catalyst it also included the mass of coke deposited on the spent catalyst, which was also quantified separately. For both feedstocks, the total mass of liquid products (aqueous + organic phases) decreased in the presence of HZSM-5, while the mass of gaseous products increased slightly. The selectivity towards the organic phase was higher with BW + PA6 than with pure BW, and with BW + PA6 the selectivity for the organic phase was higher in thermal than in catalytic pyrolysis.

Regarding the gas products, the increase in the formation of non-condensable gases in the presence of HZSM-5 is due to its strong acid sites that promote the cracking of pyrolysis vapors into small compounds [START_REF] Bridgwater | Review of fast pyrolysis of biomass and product upgrading[END_REF][START_REF] Luna-Murillo | Catalytic Fast Pyrolysis of Biomass: Catalyst Characterization Reveals the Feed-Dependent Deactivation of a Technical ZSM-5-Based Catalyst[END_REF]. This effect, however, was marginal when BW + PA6 was used.

Figure 4 shows the total mass of gases produced, normalized by the mass of feedstock. The experiment with catalyst and 20% of PA6 (▲) was repeated 3 times and the error bar is displayed on the figure. The increase in gas production in the presence of HZSM-5 was significant in experiments with BW. However, this effect was marginal when PA6 was added to BW: although in the first 20 min there were slightly more gases produced in the presence of HZSM-5 (▲), afterwards no more difference was noticeable compared to experiments without a catalyst (•) and the trend ended up being reversed for the last data point. This suggests a catalyst deactivation in the presence of PA6, which seems likely related to the adsorption of N-containing compounds on acid sites than to coke deposition, given that the two data sets were close at the reaction onset. The decomposition of PA6 resulted mainly in caprolactam, as will be discussed later, but it can also generate compounds such as pyrroles that may remain adsorbed even at high temperatures.

Figure 5 shows the detailed product distribution in the gas phase. representing almost 90% of the total mass of gaseous products. In the presence of HZSM-5, the CO production increased slightly, particularly when using BW, whereas the CO2 production did not change significantly. Indeed, in literature [START_REF] Corma | Processing biomass-derived oxygenates in the oil refinery: Catalytic cracking (FCC) reaction pathways and role of catalyst[END_REF][START_REF] Foster | Optimizing the aromatic yield and distribution from catalytic fast pyrolysis of biomass over ZSM-5[END_REF][START_REF] Wang | Catalytic pyrolysis of individual components of lignocellulosic biomass[END_REF][START_REF] Mohabeer | Investigating catalytic deoxygenation of cellulose, xylan and lignin bio-oils using HZSM-5 and Fe-HZSM-5[END_REF], the strong Brønsted acid sites present in HZSM-5 have been reported as being more efficient to remove oxygen from biomass through decarbonylation of carbohydrate-derived compounds rather than through decarboxylation reactions. Methane was also the main light hydrocarbon produced whatever the conditions used, Accumulated mass/Initial mass of feedstock (g/g) while the yields of light olefins (ethylene and propylene) increased strongly in the presence of HZSM-5 when pure BW was used, but less significantly with BW + PA6. Finally, the hydrogen production, which represented less than 0.1 wt.%, was similar in all experiments (the differences observed were within the margin of error).

The water content, the ultimate analysis and the HHV of the organic and aqueous phases are shown in Table 3. The effect of the catalyst was obvious considering the water content of the organic and aqueous phases: the organic phase obtained after ex situ catalytic treatment contained about half as much water, whereas the aqueous phase contained more water. Besides the decarbonylation and decarboxylation reactions mentioned previously, the oxygen from biomass-derived oligomers can be removed through dehydration of OH groups, which results in water formation [START_REF] Carlson | Catalytic fast pyrolysis of glucose with HZSM-5: The combined homogeneous and heterogeneous reactions[END_REF][START_REF] Liu | Catalytic fast pyrolysis of lignocellulosic biomass[END_REF][START_REF] Wang | Lignocellulosic biomass pyrolysis mechanism: A stateof-the-art review[END_REF]. Using BW, the total yield of water (sum of water content in organic and aqueous phases) changed from 19 without a catalyst to 21.5 % in the presence of HZSM-5. Changing the concentration of water in the organic and aqueous phases also modifies their polarity and consequently the products distribution between the two phases, depending on their more or less polar nature. Polar compounds such as acids and sugars will tend to remain in the aqueous phase during the phase separation process. Although the effect of the catalyst was still very clear with BW + PA6, it was less important, confirming a partial catalyst deactivation related to PA6.

The nitrogen content in both organic and aqueous phases was much higher when BW + PA6 was used, and also higher in the presence of the zeolite, which suggests that the catalyst also promoted the depolymerization of PA6, leading to small N-compounds that are condensed in the liquid fractions.

The Van Krevelen diagram obtained for the organic phases is shown in Figure 6. With both feedstocks, the effect of HZSM-5 on the deoxygenation of bio-oils was evidenced by the shift to the left of O/C ratios compared to un-catalyzed experiments. The H/C ratio remained close to that of the initial biomass. A similar figure with N/C ratios as a function of H/C ratios can be found in the supplementary information (Fig. S1). The molar mass distribution of organic fractions is presented in Figure 7. The refractive index detector (RID) signal (dw_logM) was normalized by the mass of dry bio-oil used in each analysis.

For experiments with pure BW, the catalyst caused the peak at ≈270 g/mol (in eq. HC) to decrease and the peak at ≈100 g/mol (in eq. HC) to increase, while the average molar mass decreased from 210 to 191 g/mol (in HC equivalent). This behavior can be associated to the promotion of biomass depolymerization in the presence of the catalyst. Conversely, when plastic was added to the feedstock, this difference was less pronounced and the average molar mass did not change in the presence of the catalyst (219 g/mol in HC eq.). C-NMR (Figure 8) was used to identify the different functional groups that can be found in the bio-oils. It allows a global analysis of all bio-oils components, independently of their solubility in solvents and without any limitation of molar mass range or volatility/thermal stability. The functional groups and their respective chemical shifts were reported in previous publications [START_REF] De Rezende Locatel | Can Paints and Varnish Impair the Physicochemical Properties of Wood Pyrolysis Oils?[END_REF][START_REF] De Rezende Locatel | Ex-situ catalytic upgrading of pyrolysis vapors using mixed metal oxides[END_REF]. The presence of the catalyst resulted in an increase in the production of aromatics (from 46 to 64% using BW only); when using BW + PA6, however, this increase was less pronounced.

Similarly, the decrease in sugars, alcohols and ethers production was less significant when PA6 was present in the feedstock. This corroborates the hypothesis of a partial deactivation of acid sites, which are responsible for the reactions of cracking, aromatization and deoxygenation [START_REF] Qian | Enhanced production of renewable aromatic hydrocarbons for jet-fuel from softwood biomass and plastic waste using hierarchical ZSM-5 modified with lignin-assisted re-assembly[END_REF][START_REF] Cheng | Production of targeted aromatics by using Diels-Alder classes of reactions with furans and olefins over ZSM-5[END_REF][START_REF] Lok | Promoted ZSM-5 catalysts for the production of bio-aromatics, a review[END_REF][START_REF] Nishu | A review on the catalytic pyrolysis of biomass for the bio-oil production with ZSM-5: Focus on structure[END_REF].

In addition, a significant increase in aliphatic C-C units was observed in co-pyrolysis experiments, which could be attributed to the production of compounds originating from the degradation of PA6.

GC-MS analyses provided a more detailed insight on the composition of bio-oils, in terms of compounds identification. Due to the large number of compounds present in the bio-oils, the calibration of each compound was not possible and the analysis was hence not quantitative, but the peak areas of similar compounds can be compared. The compounds that can be analyzed using GC-MS are also limited to molecules that can be vaporized in the injector without decomposition (which probably excludes some sugars) and elute at 250°C (maximum temperature that can be applied to the column). The main groups of compounds produced by pyrolysis of beech wood identified by GC-MS have been published previously [START_REF] De Rezende Locatel | Can Paints and Varnish Impair the Physicochemical Properties of Wood Pyrolysis Oils?[END_REF][START_REF] De Rezende Locatel | Ex-situ catalytic upgrading of pyrolysis vapors using mixed metal oxides[END_REF]. The detailed list of compounds As observed with 13 C-NMR, GC-MS analyses also showed that the presence of the catalyst resulted in an increase in aromatic compounds families such as phenols and aromatic ethers, but this increase was less significant for experiments making use of BW + PA6. Furthermore, the effect of the catalyst in reducing the amounts of furans and sugars was clearly diminished in the presence of PA6, in agreement with the reduced production of CO, CO2, confirming a partial catalyst deactivation [START_REF] Carlson | Catalytic fast pyrolysis of glucose with HZSM-5: The combined homogeneous and heterogeneous reactions[END_REF][START_REF] Hernández-Giménez | Effect of Mesoporosity, Acidity and Crystal Size of Zeolite ZSM-5 on Catalytic Performance during the Ex-situ Catalytic Fast Pyrolysis of Biomass[END_REF]. Finally, the formation of aromatic and polyaromatic deoxygenated compounds (PAH), which was observed only in the presence of HZSM-5 catalyst, was drastically reduced with BW + PA6, suggesting that the catalytic sites responsible for their formation were almost totally suppressed. Using pure BW, the HZSM-5 catalyst led to a strong increase in the concentration of acid/ester and phenol families in the aqueous phase. This trend was strongly mitigated using BW + PA6, which can also be associated to the catalyst deactivation. Organic acids, especially acetic acid, can be produced by depolymerization of the three biopolymers constitutive of lignocellulosic biomass (cellulose, hemicellulose and lignin), but hemicellulose mainly contributes to the formation of organic acids by cleavage of the acetyl groups linked to the polysaccharide structure [START_REF] Chu | The pyrolysis chemistry of a β-O-4 type oligomeric lignin model compound[END_REF][START_REF] Ben | Influence of Si/Al Ratio of ZSM-5 Zeolite on the Properties of Lignin Pyrolysis Products[END_REF][START_REF] Tan | Recent progress of catalytic pyrolysis of biomass by HZSM-5[END_REF][START_REF] Carrier | Quantitative Insights into the Fast Pyrolysis of Extracted Cellulose, Hemicelluloses, and Lignin[END_REF].

These reactions are promoted by acid sites and the poisoning of these sites might impair the production of such compounds.

GC-MS analyses also provided a specific insight on N-compounds. Bio-oils prepared from pure beech wood contained small amounts of nitrogen compounds, as shown in Table 3, from the presence in the wood of proteins involved in biological functions and plants metabolism. The bark In order to better understand the impact of the catalyst on the degradation PA6, Figure 11 presents the main N-containing products analyzed in the organic and aqueous phases. Over 100 compounds were identified, the complete list of which can be found in the supplementary information. However most of them are present only in very small amounts. With HZSM-5 -Aq. Phase also followed by cyclization [START_REF] Czernik | Catalytic pyrolysis of nylon-6 to recover caprolactam[END_REF][START_REF] Bockhorn | Kinetic study on the non-catalysed and catalysed degradation of polyamide 6 with isothermal and dynamic methods[END_REF] (Figure 12B). This pathway could be preferred in the presence of BW, the pyrolysis of which produces a lot of water. Bockhorn et al. found that the thermal pyrolysis of pure PA6 between 380 and 460°C led to caprolactam being formed with yields >92% [START_REF] Bockhorn | Kinetic study on the non-catalysed and catalysed degradation of polyamide 6 with isothermal and dynamic methods[END_REF]. This suggests that in co-pyrolysis of PW+PA6 blends, the presence of wood-derived products decreased the selectivity of PA6 degradation. The secondary products differed depending on the presence or not of the catalyst. Without a catalyst, the secondary products are also formed through cleavage of C-N bonds of PA6, but without cyclization, leading to linear compounds such as acexamic acid found in the organic phase, and N-butylidene-1-butanamine (BDBA) found in the aqueous phase. When ex situ catalytic treatment of vapors was applied, 5-cyano-1-pentene was the second most abundant product identified in both phases. This linear mono-nitrile can be formed by dehydration of caprolactam, catalyzed by Brønsted acid sites [START_REF] Kubatovics | Thermal decomposition of polyamide-6 in the presence of PVC[END_REF][START_REF] Ohtani | Studies on thermal degradation of aliphatic polyamides by pyrolysis-glass capillary chromatography[END_REF], which clearly exemplifies the effect of HZSM-5 catalyst in modifying the composition of bio-oils.

Figure 12: Possible reaction pathways for the formation of caprolactam from PA-6.

Coke deposition on the spent catalyst

The catalyst was heavily coked after pyrolysis: the coke content was 13.4 wt.% after pyrolysis of BW and 11.8 wt.% after co-pyrolysis of BW+PA6, which suggests that PA6 contributed to reducing the catalyst coking. Ultimate analysis of this coke, however, revealed that the N-content of this coke was 0.5 wt.% after pyrolysis of BW and 6.1 wt. % after pyrolysis of BW+PA6, a twelve-fold increase much greater than could be expected from the initial PA6 concentration in the feedstock (20 wt.%). This confirms that N-containing compounds derived from PA6 adsorb on the catalyst more strongly than compounds derived from BW and are probably responsible for the catalyst deactivation.

Conclusion

In this study, the co-pyrolysis of a N-containing plastic, polyamide-6 (PA6) and beech wood (BW) was investigated in the absence or presence of a HZSM-5 catalyst for ex-situ treatment of the pyrolysis vapors. The catalyst modified the composition of pyrolysis oils and products distribution in both liquid phases (aqueous and organic). Compared to the pyrolysis of pure BW, the effect of the catalyst on the composition of pyrolysis oils was less pronounced in the presence of the plastic, due to a partial catalyst deactivation, which essentially affected its deoxygenation properties. This deactivation can be assigned to the adsorption of N-containing compounds, produced by degradation of PA6, on strong acid sites of the zeolite that are responsible for the formation of deoxygenated compounds. Despite this deactivation, the catalyst remained capable of promoting the depolymerization of BW and PA6, which led to an increase in the concentration of N-containing products in the liquid phases, with caprolactam as the main product. The catalyst also modified the N-containing by-products found in bio-oils, notably the formation of 5-cyano-1-pentene was observed only in the presence of HZSM-5 catalyst.

Figure 1 -

 1 Figure 1 -Structure of polyamide-6

  pyrolysis vapors (formed by thermal pyrolysis) were entrained downstream by a nitrogen flow of 260 smL/min and passed through the catalyst bed (in catalytic experiments) placed just below the pyrolysis zone on a second quartz frit. The temperature of the catalyst bed was 453 ± 2°C. In this external part of the reactor, an additional gas flow composed of 240 smL/min N2 and 20 smL/min He (internal standard) was added to prevent retro-diffusion of the vapors and to adjust the total gas flow rate. The residence time of the vapors in the pyrolysis zone was estimated at 41 s, while the contact time with the catalyst (when present) was around 2 s. The pressure in the reactor was continuously monitored by a pressure sensor connected to a safety device which cuts off the experiment automatically if the pressure exceeds 1.3 bars. The pressure measured during the experiments was 1.1-1.15 bar.

Figure 2 -

 2 Figure 2 -Pyrolysis setup scheme

  this equation the HHV is expressed in MJ/kg and [C], [H], [N] and [O] in mass percentages on dry basis. This equation is valid for the range of values: C: 0.00 -92.25 %; H: 0.43 -25.15%;

13 C

 13 nuclear magnetic resonance ( 13 C-NMR) spectra were recorded using a Bruker AVANCE 400 MHz spectrometer and the spectra were analyzed with the TopSpin 3.0 software. The analyses were carried out at room temperature with an accumulation of 4500 scans. Trioxane was used as internal standard, diluted in DMSO-D6 in a coaxial tube configuration that prevented the contact of the solvent with the bio-oil samples.Gas chromatography coupled with a mass spectrometer was performed with a GCMS-QP2010 SE apparatus (Shimadzu) equipped with a Zebron ZB Wax-Plus column (length 60 m, internal diameter 0.25 mm, film thickness 0.25 µm). The samples were diluted in methanol (20 wt.% for organic phase and 30 wt.% for aqueous phase) and filtered before the analysis (Cellulose acetate -0.45µm). The oven temperature program started at 70 °C (held for 10 min), then was heated at a rate of 5°C/min up to 150 °C. At this point the heating rate changed to 3°C/min up to 250 °C (held for 15 min). Two-dimensional gas chromatography coupled with mass spectrometry (GCxGC-MS) was carried out with an Agilent GC 6890N apparatus. The first separation was performed by a polar column VF-1701 MS (I.D 0.25 mm; film thickness 0.25 µm; length 30 m) followed by a second apolar column DB1 (I.D 0.25 mm; film thickness 0.25µm; length 1.5m) for the second dimension of separation. The modulation was performed by a cryogenic modulator (Zoex Corporation USA) each 12s. The compounds were identified by the coupled MS detector (5975B). The use of a first polar column followed by a less polar column favored the separation of oxygen and nitrogencontaining compounds, widely present in bio-oils[START_REF] Omais | Investigating comprehensive two-dimensional gas chromatography conditions to optimize the separation of oxygenated compounds in a direct coal liquefaction middle distillate[END_REF]. The temperature program was set as follows: first column from at 50°C to 300 °C with a ramp of 1.75 °C/min, then to 320°C with a ramp of 1.8 °C/min. The second column went from 50 °C to 320°C at 1.8 °C/min. The samples concentration was the same as described for GC-MS analyses.

Figure 3 -

 3 Figure 3 -Product selectivity in experiments with pure BW and BW+PA6 (20%) blends

Figure 4 -

 4 Figure 4 -Total mass flow of gaseous products normalized by the initial mass of feedstock

Figure 5 -

 5 Figure 5 -Cumulative mass of NCG normalized by the initial mass of feedstock

Figure 6 -

 6 Figure 6 -Van Krevelen diagram of organic phases (on dry basis).

Figure 7 -

 7 Figure 7 -GPC molar mass distribution of organic phases

13

 13

Figure 8 -

 8 Figure 8 -13 C-NMR functional group distribution in the organic phases

  identified individually using their mass fragmentation spectra can be found in the supporting information. The characterization of the organic fractions obtained in the present study are displayed in Figure9.

Figure 9 -

 9 Figure 9 -Relative areas of compounds families identified by GC-MS for the organic phases

AFigure 10

 10 Figure10shows the GC-MS analyses of the aqueous phase.

Figure 10 -

 10 Figure 10 -Relative areas of compounds families identified by GC-MS for aqueous phase

N i t r

  o g e n -c o n t a i n i n g A c i d / e s t e r A l d e h y d e / k e t o n e / e t h e r S u g a r s G u a i a c o l / s y r i n g o l U n i d e n t i f i e d p e a of beech trees also contains chlorophyll, of which the four pyrrole rings can decompose into Ncompounds. Three compounds were identified: 1-(2-propenyl)-piperidine (1.92 % of the total peak area), 5,6-dihydro-5-methyl-uracil (1.46 %) and 3-methylpyridazine (0.53 %).

Figure 11 -

 11 Figure 11 -N-containing products distribution in organic and aqueous phases obtained by pyrolysis of BW+PA6 without or with HZSM-5 catalyst

Table 1 -

 1 Summary of recent publications on co-pyrolysis of biomass and plastics 64

	Reference	Catalyst	Biomass	Catalyst/biomass ratio	Plastic	Plastic/biomass ratio	Conditions
	Yu, 2021[15]	HZSM-5	wallnut shell 1:8; 1:4; 1:2; 1:1	LDPE	1:1	450 -850°C
	He et al., 2021[12]	HZSM-5	corn stalk	1:1	HDPE	0:1; 4:1; 3:1; 2:1; 1:0 1:1; 2:1; 3:1; 4:1;	550 -800°C
	Qian et al., 2021[16]	Hierarchical ZSM-5	Douglas fir	0.75:1	LDPE	0; 0.15; 0.5; 0.85; 1	429 -571°C
	Dyer, Nahil, and Williams, 2021[17]	ZSM-5	waste wood pellets	2:1	HDPE, LDPE, PP, PS and PET	1:1 and 1:9; 1:4 (only for HDPE and PS)	500°C catalytic bed and temperature ramp of 10°C/min to 500°C
	Likun, Zhang, and Fan, 2021[18]	USY and ZSM-5	torrefied bagasse	1:1	HDPE, PS, EVA and PP	1:9; 3:7; 1:1; 7:3	600°C
	Praveen Kumar and Srinivas, 2020[9]	Spent FCC catalyst	groundnut shell	1:10	PP and PS	1:2; 1:1; 2:1	510°C
	Lin et al., 2020[19]	HZSM-5	corn stover	1:1	HDPE	1:1	550°C
	Razzaq et al., 2019[20]	(Fe, Ni, Co, Zn) ZSM-5	wet straw	2:1	PS	1:1	500 -650°C
	Y.-M. Kim et al., 2017[21]	HZSM-5 and Al-MCM-41	torrefied poplar yellow	10:1	HDPE	1:1	600°C
	B.-S. Kim et al., 2016[22]	ZSM-5 and HY	cellulose	10:1	PP and LLDPE	1:1	500 and 600°C

Table 3 -

 3 Water content, ultimate analysis and higher heating value of organic and aqueous phases

		%H 2 O	Elemental analysis (wt.% on dry basis) C H N O*	HHV (MJ/kg)
			Organic phase
			Beech wood
	no catalyst	18.7±1.0	65.4±1.0	6.0±0.4	0.6±0.1	28.0±1.5	27.0±1.0
	HZSM-5	11.9±1.8	70.6±1.2	6.4±0.6	0.5±0.6	22.4±0.6	30.0±0.2
			BW + 20% PA6
	no catalyst	21.2±0.3	66.1±1.0	7.2±0.2	5.1±0.4	21.6±0.7	29.3±0.6
	HZSM-5	13.8±1.4	71.6±2.4	7.5±0.5	6.1±0.4	14.8±2.2	32.2±0.7
			Aqueous phase
			Beech wood
	no catalyst	48.3±1.2	52.1±0.4	6.2±3.0	1.2±0.6	40.4±2.6	21.3±3.8
	HZSM-5	78.9±1.3	61.3±1.6	3.9±3.2	2.1±1.3	32.7±2.2	23.5±2.6
			BW + 20% PA6
	no catalyst	56.2±0.1	60.7±2.5	6.4±2.8	6.2±0.8	26.7±1.6	25.9±2.7
	HZSM-5	65.8±2.3	54.5±1.8	6.2±7.2	8.7±1.8	30.6±5.3	23.1±8.9
	*by difference

ACKNOWLEDGEMENTS

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement N° 818120 (Waste2Road

). The authors also thank Véronique Bounor-Légaré (CNRS and University of Lyon) for providing different samples of plastics.


Aissatou Diop

Danielle Ngoue

Amine Mahammou

Babacar Diallo

Béatrice Plujat

Angélique Bousquet

Thierry Sauvage

Sébastien Quoizola

Mireille Richard-Plouet

Jonathan Hamon

Audrey Soum-Glaude
email: audrey.soum-glaude@promes.cnrs.fr

Éric Tomasella

Laurent Thomas

Comprehensive study of WSiC:H coatings synthesized by microwave-assisted RF reactive sputtering

Keywords: Plasma processes, Optical Emission Spectroscopy, Thin films, Ion Beam Analysis

WSiC:H coatings were elaborated by reactive magnetron sputtering of a W target in an Ar/Si(CH3)4 plasma assisted by ECR microwave sources. Such coatings can be very interesting for optical or mechanical applications. The development of these coatings is based on the confrontation between insitu diagnostics of the reactive plasma phase and material characterization of the resulting coatings. The investigation by optical emission spectroscopy of the different phenomena occurring in the reactive plasma reveals that there is a precursor flow rate range where W particles can be favored in the reactive plasma, to promote the development of composite coatings based on a SiC:H matrix, in which Wcontaining particles or aggregates are present. Material characterization such as Scanning Electron Microscopy, Ion Beam Analysis and X-ray Photoelectron Spectroscopy are used to study the impact of the plasma phase on the microstructure and chemical composition of the deposited materials.

INTRODUCTION

Ceramic-metal composites or cermets are known for their advantages such as resistance to corrosion and oxidation [START_REF] Gant | The evaluation of tribo-corrosion synergy for WC-Co hardmetals in low stress abrasion[END_REF][START_REF] Singh | Cavitation erosion resistant nickelbased cermet coatings for monel K-500[END_REF][START_REF] Li | Improvement mechanism of the oxidation resistance of Al-doped Ti(C,N)-based cermets: Influence of enriched Al in the subsurface oxide layer[END_REF] but also for their disadvantages such as brittleness and the random nature of their cracking. This cracking depends essentially on the nature of the metallic phases inserted in the ceramic matrix. [START_REF] Jie | Effect of metallic content on mechanical property of Ni/(10NiO-NiFe204) cermets®[END_REF][START_REF] Zhang | Effect of metallic phase content on mechanical properties of (85Cu-15Ni)/(10NiO-NiFe2O4) cermet inert anode for aluminum electrolysis[END_REF] They have also attracted a lot of interest in the thermodynamic conversion of solar energy because of their suitable optical properties. Indeed, inserted in a multilayer stack with a metallic lower layer and an antireflective top layer, their solar absorptance can reach 0.94 and their thermal emittance 0.07 as shown by Sathiaraj et al. [START_REF] Sathiaraj | Ni-AI20 3 SELECTIVE CERMET[END_REF]. However, cermets also have low thermal resistance, especially in air, especially when the nanometric particles included in the matrix are noble metals [START_REF] Barshilia | Structure and optical properties of Ag-Al2O3 nanocermet solar selective coatings prepared using unbalanced magnetron sputtering[END_REF] as opposed to metals such as molybdenum and cobalt. [START_REF] Nyberg | Surface roughness and thermal stability of graded cermet photothermal absorber coatings with very high absorptivities[END_REF][START_REF] Niklasson | Optical properties and solar selectivity of coevaporated Co Al 2 O 3 composite films[END_REF]. On the subject of thermal stability, Pt -Al2O3 cermet was found to be stable up to 600°C. [START_REF] Nyberg | Surface roughness and thermal stability of graded cermet photothermal absorber coatings with very high absorptivities[END_REF][START_REF] Lafait | Pt Al2O3 selective absorber coatings for photothermal conversion up to 600[END_REF] Antonaia et al also showed that W -Al2O3 cermet was thermally stable after annealing at 580°C. [START_REF] Antonaia | Stability of W-Al2O3 cermet based solar coating for receiver tube operating at high temperature[END_REF] In this perspective, nanocomposite cermets of the W-SiCH type could be one of the solutions to this problem since the materials used are known to be resistant and stable at high temperatures. [START_REF] Ngoue | Ceramics for concentrated solar power (CSP): From thermophysical properties to solar absorbers[END_REF][START_REF] Rebouta | Solar selective absorbers based on Al2O3:W cermets and AlSiN/AlSiON layers[END_REF] Indeed, silicon carbide is an ultra-refractory ceramic due to its thermal stability which can go beyond 2000°C [START_REF] Wuchina | UHTCs: Ultra-High Temperature Ceramic Materials for Extreme Environment Applications[END_REF] and has good oxidation and corrosion resistance after the formation of a silica (SiOx) passivation layer on its surface. Also, Tungsten is a refractory transition metal with good heat and electrical conductivity. A protective oxide layer is formed on its surface when exposed to air above 400°C. [START_REF]The degree of oxidation of tungsten. Tungsten in nature., Promen. (n.d.)[END_REF][START_REF]Britannica, tungsten | Uses, Properties, & Facts[END_REF], Tungsten (melting point of 3420°C) is resistant to corrosion and other various chemical attacks to which it may be exposed.

There are many possible ways to elaborate cermet coatings, depending on the application for with these materials are going to be used. [START_REF] Henini | Handbook of Thin-Film Deposition Processes and Techniques[END_REF][START_REF] Chesnaud | Densification par Spark Plasma Sintering (SPS) de matériaux d'électrolytes, difficilement densifiables, pour piles à combustible[END_REF][START_REF] Braval | Sol-gel prepared Ni-alumina composite materials[END_REF][START_REF] Gonzlez-Gutirrez | Powder Injection Molding of Metal and Ceramic Parts[END_REF] In this work, we investigate the possibility of synthesizing WSiC:H coatings by reactive radio frequency (RF) magnetron sputtering [START_REF] Jyothi | Design and fabrication of spectrally selective TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO tandem absorber for hightemperature solar thermal power applications[END_REF][START_REF] Du | Microstructure and thermal stability of Al/Ti0.5Al0.5N/Ti0.25Al0.75N/AlN solar selective coating[END_REF][START_REF] Doland | Particulate nature of solar absorbing films: Gold black[END_REF] of a tungsten target in an Ar plasma, in the presence of TetraMethylSilane (TMS: Si(CH3)4) gaseous precursor. The dissociation of the TMS precursor is assisted by microwave Electron Cyclotron Resonance (ECR) sources. The particularity of this process, which is nevertheless versatile [START_REF] Celis | Hybrid processes -a versatile technique to match process requirements and coating needs[END_REF], resides in the presence of the reactive plasma phase during the sputtering of the metallic target, leading to the formation of metallic inclusions in a ceramic matrix. This works aims at a better understanding of the links between the plasma deposition process and the microstructure of the resulting films, to better control the latter. For this purpose, nonintrusive optical emission spectroscopy (OES) was implemented to identify the different emitting species occurring in the plasma phase, in order to optimize the coatings deposition process by correlating the species responsible for their growth with the chemical properties of the resulting films. To do so, the physicochemical properties of the latter were thoroughly studied thanks to SEM (Scanning Electron Microscopy), EDS (Energy Dispersive X-ray Spectrometry), IBA (Ion Beam Analysis) techniques including RBS (Rutherford Back Scattering Spectroscopy), NRA (Nuclear Reaction Analysis) and ERDA (Elastic Recoil Detection Analysis) and X-ray Photoelectron Spectroscopy (XPS).

EXPERIMENTAL DETAILS

Coating synthesis

The materials were developed in the IDEFIX vacuum reactor designed at PROMES-CNRS laboratory [START_REF] Soum-Glaude | Reactor for depositing layers and associated deposition process[END_REF]. It consists in a cylindrical enclosure on which are placed four coaxial microwave sources of dipolar type (with magnet) which are maintained by water-cooled flanges and connected to a SAIREM GMP16KTS microwave generator operating at 2.45 GHz, which supplies them with a maximum power of 250 W each. It also comprises a heated substrate holder (up to 600°C), a magnetron sputtering cathode (connected to a Caesar RF generator supplied by Advanced Energy operating at a frequency of 13.56 MHz) and is connected to a pumping system (a primary roots pump from Alcatel Adixen to achieve a pressure of around 10 -2 mbar and secondary turbomolecular pump from Edwards to achieve a vacuum of down to 10 -6 mbar). The flanges allow the reactor to be used in two different configurations: a cylindrical configuration for deposition on cylindrical substrates and a planar configuration, which was used in this study to deposit on planar samples.

A W underlayer was deposited by RF magnetron sputtering (referred to as Physical Vapor Deposition, PVD in the following) of a solid W target in 20 sccm of Ar gas during 4.5 min. Depending on the final application, this underlayer can be used as an infrared reflector, as a substrate protector or simply as a layer to avoid the effect of the substrate [START_REF] Ngoue | Ceramics for concentrated solar power (CSP): From thermophysical properties to solar absorbers[END_REF]. The distance between the substrate and the target was set at 8.5 cm. For the WSiC:H coatings, a dual process was used: the SiC:H ceramic phase was provided by the plasma decomposition of the gaseous precursor TetraMethylSilane TMS (Si(CH3)4) diluted in Ar (Plasma Enhanced Chemical Vapor Deposition, PECVD), while the W metallic phase was provided by RF magnetron sputtering. This dual process is assisted by a microwave power of 850 W supplied by the ECR sources distributed coaxially at mid-height of the reactor. The total gas flow rate in the reactor for each deposition is 25 sccm (Ar + TMS) corresponding to a working pressure of 0.1 Pa (10 -3 mbar). The deposition duration is fixed at 10 min. The two layers of the coatings were deposited consecutively in the same run, without exposure to air.

For more flexibility in the characterizations, especially for those that require slicing the material, the coatings were deposited on silicon wafers (diameter 50.9 +/-0.3 mm and thickness 280 +/-25 µm, Pdoped, N-type, resistivity 1-10 Ωcm, both sides polished) and T91 steel wafers (diameter 1 and 2 inches, polished on one side). The substrates were in-situ cleaned by an argon plasma generated by a LF-5 RFPP low frequency generator with an Ar flow rate of 50 sccm, and then heated at 350°C for 30 min

Characterization techniques

Optical emission spectroscopy (OES)

OES is a plasma characterization technique that allows the detection of different emissive species present in the plasma and their quantification without disturbing the plasma. This can be done in the plasma volume but also close to the surface of the growing film, which can give indications on the species sputtered or re-emitted by the surface.

To carry out the spectroscopic measurements, a quartz window (wide spectral range of optical transmission) was installed on the reactor. The experimental set-up for the acquisition consisted of three main parts:

•

The optical fiber that guides the light from the source to the spectrophotometer. It is fitted at the end with a lens and a diaphragm for optical sighting, which is itself fixed on a plate. In this study, the aiming point of the fiber is located close to the surface of the substrate and at equidistance from the microwave sources;

• The AvaSpec spectrophotometer [START_REF] Avantes | AvaSpec-ULS2048XL-EVO[END_REF] from Avantes, which separates the lines brought back by the fiber as a function of wavelength thanks to a mirror and diffraction grating system. It can acquire spectra in the 200-1100 nm range;

• The acquisition system: the spectrophotometer is connected to a computer which allows the acquisition and processing of data using the Avasoft software. [START_REF] Avantes | AvaSoft-Basic[END_REF] To determine the relative (or normalized) intensities (𝐼𝐼𝑟𝑟(𝑋𝑋)) of the monitored species, we use a normalization technique called actinometry demonstrated by Equation [START_REF] Gant | The evaluation of tribo-corrosion synergy for WC-Co hardmetals in low stress abrasion[END_REF]. It consists in the ratio between the raw emission intensity (𝐼𝐼(𝑋𝑋)) of each peak of the spectra acquired and the product of the integration time 𝑡𝑡 of the system and the emission intensity of the argon line (𝐼𝐼𝐴𝐴𝑟𝑟) at 696 nm called actinometer.

𝐼𝐼 𝑟𝑟(𝑋𝑋) = 𝐼𝐼 (𝑋𝑋) (𝑡𝑡 * 𝐼𝐼 𝐴𝐴𝑟𝑟 ) ⁄ (1)
OES characterization also allowed estimation of the electron temperature (𝑇𝑇𝑒𝑒) which corresponds to the mean energy of the electrons present in the plasma phase expressed in eV and given by equation (2

Where 𝑔𝑔 and 𝐴𝐴 are respectively the static weights of the energy levels and the probabilities of transitions, h is Planck's constant, k is Boltzmann's constant and c is the speed of light. We use the values of theses parameters mentioned in Table 1. This temperature actually corresponds to the average temperature of the electrons in the case of a distribution of their energy according to a Maxwell-type distribution function.

[29] Light speed (c) (𝑚𝑚. 𝑠𝑠 -1 ) 300000000

Physicochemical characterization

Scanning Electron Microscopy

The morphology of the deposited layers was observed by SEM using a Hitachi model S-4500 SEM with a resolution of 1.5 nm at a voltage of 15 kV. The coupling of the SEM with a Kevex EDS analysis system allowed the quantification of the species present in the deposited coatings, providing a semi-quantitative analysis of the materials.

Ion Beam Analysis

For a more in-depth analysis, we used a coupling of Ion Beam Analysis ) 13 C nuclear reactions with 900 keV deuterons at the same detection angle. ERDA was applied with a 2.8 MeV 4 He + beam at 30° angle to determine the hydrogen depth profile through the nuclear reaction 1 H( 4 He, 1 H) 4 He. The obtained depth resolution for hydrogen oxygen and carbon close to the sample surface was found to be about 300-400.10 15 at/cm². The depth profiles were extracted from IBA spectra by using SIMNRA software [START_REF] Mayer | SIMNRA, a simulation program for the analysis of NRA, RBS and ERDA[END_REF] with SigmaCalc cross section [START_REF] Gurbich | SigmaCalc recent development and present status of the evaluated crosssections for IBA[END_REF] for NRA and ERDA. These IBA techniques were carried out using the Pelletron accelerator of CEMHTI laboratory.

[32] From these chemical analyses, it was possible to determine the atomic flux of each species present in the material, knowing their atomic concentration. This quantity noted ΦX corresponds to the atomic flux of the species X incorporated in the material during deposition. It is given by equation 7 where t is the deposition time (min), d the number of atoms per unit area (at/cm²) and [X]

the atomic concentration of X, determined by IBA techniques.

Φ 𝑋𝑋 = [𝑋𝑋] * 𝑑𝑑 𝑡𝑡 (3)

X-Ray Photoelectron Spectroscopy

In the same perspective, XPS was used to qualitatively support our chemical analyses. It gave information on the nature and content in chemical elements to obtain a semi-quantitative analysis, and on the nature of the various chemical bonds. The XPS measurements were carried out using a Kratos Axis Nova operating at 1486.6 eV (Al source). After recording wide spectra at a low resolution with pass energy equal to 160 eV to check for any spurious elements, the pass energy and the step scan to measure the region of interest was fixed to 40 eV and 0.1 eV, respectively. The probed depth was 10 nm over a 700×300 µm 2 zone. The dwell times and number of sweeps were chosen such as the highest XPS peak of the selected zone exhibits, at least, 10000 counts. Charge neutralization was used when acquiring the spectra. The energy calibration was set from C 1s signal allotted to carbon linked to W fixed at 283.6 eV since no adventitious carbon (C-C bonds) is detected after etching. Using CasaXPS software [START_REF] Fairley | Systematic and collaborative approach to problem solving using X-ray photoelectron spectroscopy[END_REF], after removal of a Tougaard-type background, a Lorentzian Asymmetric lineshape was used for peak fitting.

The latter allows numerical convolution of a Lorentzian with a Gaussian to produce a traditional Voigt profile but also an asymmetric Voigt like lineshape. The latter was required to account for the W-C environment expected in the samples. In order to remove the surface pollution, etching using conventional Ar + ion gun (2 kV, 70 µA as extracting current) was undertaken according to 3×3 mm 2 raster for 30 s only, in order to avoid beam damages and differential sputtering effect between the elements that could modify chemical information.

RESULTS AND DISCUSSIONS

Process diagnostics

The objective of this study is to identify and monitor by OES the different species present in the gaseous phase. These species are chemical bond carriers in the direction of the growing layer on the substrate holder immersed in the radiofrequency and microwave plasma discharges. This study allows us to understand the growth phenomena resulting in WSiC:H coatings.

Optical emission spectra

Figure 1 superimposes the optical emission spectra measured in: 1) an argon/TMS microwave (MW) plasma in SiC:H PECVD deposition mode, without any source of W; 2) a RF argon plasma during the sputtering of the tungsten target (pure PVD mode); 3) an argon/TMS reactive plasma during the sputtering of the tungsten target, with microwave assistance (reactive MW PVD mode). For this third case, the TMS precursor flow rate was varied from 1% to 20% (0.25 sccm to 5 sccm) of the total gas flow rate (Ar + TMS) maintained constant at 25 sccm, corresponding to a working pressure between 10 -3 mbar and 10 -2 mbar. The curves obtained for the different percentages of TMS show the same peaks at the same wavelengths. To lighten the representation, we present only the results for 5 sccm of TMS in Figure 1. This comparison allowed us to identify the main species at play during coating deposition (Table 2). Regarding the SiC:H phase, some of these species result from the direct dissociation of the Si(CH3)4 precursor, such as CH (431 nm) and 𝐻𝐻𝛼𝛼 (656 nm) which are already present in the precursor, others result from the recombination of these species, such as 𝐶𝐶2 (516 nm), 𝐻𝐻2 (601 nm) and 𝑆𝑆𝑆𝑆𝐻𝐻 (415 nm). The emission line at 404 nm is attributed to atomic W, since it is present in both pure PVD mode (W sputtering) and reactive MW PVD mode, but not in SiC:H PECVD deposition mode (Figure 1).

Atomic W detected in reactive MW PVD mode is the source for the W incorporation in the growing SiC:H matrix.

Evolution of electron temperature with precursor flow rate

We have also characterized the reactive MW PVD plasma by studying the electron temperature of the medium. This temperature is related to the emission intensities of species such as argon, through the electron density. It can be useful for a first approach on the behavior of species in the plasma (density evolution, nature of the chemical reactions, etc.).

The values of the electron temperature (Figure 2) in the Ar + W + TMS RF reactive MW PVD plasma are found to be in the range of [0.9 eV; 1.6 eV], which is consistent with the literature for microwave plasmas. [START_REF] Latrasse | Conception, caractérisation et applications des plasmas micro-onde en configuration matricielle[END_REF] For this type of ultra-high frequency plasma, electron densities can be high (up to few 10 12 electrons/cm 3 ). This is because electrons, thanks to their low mass, can oscillate at high frequency so as to easily acquire sufficient energy to create ionization cascades that maintain the discharge through collisions. When undergoing these collisions, they lose their energy very quickly, which corresponds to an electron mean temperature of the order of those found in Figure 2. 𝑇𝑇𝑒𝑒 is here an indicator for the dissociation of the TMS precursor. Indeed, this dissociation is ensured by the collisions of energetic electrons from the plasma phase with the precursor molecules. It could be expected that 𝑇𝑇𝑒𝑒 decreases when injecting a higher quantity of precursor to be dissociated via collisions with the energetic electrons, thus consuming the energy of the latter. Since in our case 𝑇𝑇𝑒𝑒 tends to increase with increasing precursor content (Figure 2), it means that here the dissociation of TMS mainly consumes the low and medium energy electrons of the electron energy distribution function (EEDF), thus leaving mostly the very high energy electrons. [START_REF] Chen | Measurement of electron temperatures and electron energy distribution functions in dual frequency capacitively coupled CF4/O2 plasmas using trace rare gases optical emission spectroscopy[END_REF] As more and more of the lower energy electrons are consumed for precursor dissociation when the TMS flow rate is increased, the mean electron temperature rises due to the higher rate of high energy electrons. Nevertheless, we note that there is a still unexplained and reproducible phenomenon at 8% TMS involving a non-linearity of the 𝑇𝑇𝑒𝑒 curve.

Evolution of plasma chemistry with precursor flow rate (actinometry)

We also studied the Ar + W + TMS reactive PVD microwave plasma chemistry as a function of the percentage of TMS injected in the reactor. The emission intensities of each species monitored were normalized according to the principle of actinometry [START_REF] Coburn | Optical emission spectroscopy of reactive plasmas: A method for correlating emission intensities to reactive particle density[END_REF], as an indicator of their concentration in the plasma phase. The spectra were acquired during the deposition of the layer, at different times after plasma stabilization (0 min being the first acquisition, then 5 min after the first acquisition and 10 min after the first acquisition).

Species resulting from precursor dissociation

For the species directly resulting from the dissociation of TMS (Si(CH3)4), 𝐻𝐻𝛼𝛼 among others which are optical tracers representative of the dissociation of the precursor (Figure 3), there is an increase in concentration with the increase of TMS content. This is expected in that the more TMS is injected into the reactor, the more it dissociates. This also means that the electrons from the discharge are able to dissociate efficiently the TMS molecules whatever the percentage injected in the reactor in the range [1%-20%]. Furthermore, we observe that overall, at a fixed percentage of TMS, whatever the duration of the deposition, the plasma chemistry remains unchanged regarding the dissociation of the TMS precursor. This stability with time is very useful, since it allows us to have a margin of error on the deposition duration and to adapt the deposited layer thicknesses depending on the application. To study if this phenomenon occurs in our case, we followed the evolution of atomic W emission, i.e., the species resulting from the sputtering of the W target, with the percentage of TMS precursor injected in the gas phase. Figure 4 shows the evolutions of W relative intensity versus time and %TMS in the discharge mixture. The W emission globally tends to increase with the increase in the percentage of TMS.

This can mean that the percentage of TMS is low enough so that the process remains in a pure target sputtering regime whatever the TMS flow rate, and not a compound sputtering regime (in other words, the target is not polluted by the deposition of a metal/ceramic compound from the reactive plasma phase).

This phenomenon can be explained by the increase in W emission intensity with %TMS which could also be related to the increase in the electron temperature with increasing TMS flow rate (Figure 2), as high-energy electrons are available for the excitation of W while low-energy electrons are consumed for precursor dissociation. relative intensity is time dependent. Meanwhile, powders have been collected in the reactor after measurements (Figure 5). SEM/EDS analyses conclude that these powders are constituted of aggregates of nanoparticles with 42% of W, 56% of C, 1% of O and 1% of Si. This formation of W containing particles in the discharge is consistent with OES measurements, as the growth of W powders in the plasma phase during deposition can lead to the observed decrease of emitting W atoms in time at intermediate %TMS (Figure 4), the latter being "consumed" to form powders in the discharge, that cannot be detected by OES.

Conclusions on process diagnostics

All in all, it appears that for this reactive PVD deposition process, there is a domain of precursor flow rate (%TMS) where W particles can be favored inside the reactive plasma process, to promote the elaboration of composite coatings based on a SiC:H matrix containing metallic W particles or aggregates.

Such information is useful to determine the potential of the process to deposit a WSiC:H compound in a specific experimental domain. Thanks to the previous observations, we can deduce that in the medium range of %TMS (from 6% to 18% of TMS precursor in the injected gas mixture with total flow rate of 25 sccm) it is possible with this process to obtain W particles in the gas phase, as the target surface is not completely saturated so that a pure sputtering mode is maintained. Such a process could then be used to obtain cermet-like materials, by incorporating these W particles in the SiC:H matrix.

In view of these results, the impact of the changes in the gas phase on the physicochemical properties of the deposited materials is also studied, to correlate the phenomena in the gas phase and the growth mechanisms of the coatings, as well as the corresponding microstructure of the materials.

at. % of W : 42

WSiC:H coatings characterization

Series of W/WSiC:H coatings were deposited and characterized. Depending on the sample, the TMS precursor flow rate in the Ar/TMS gas mixture during WSiC:H deposition ranges from 0.25 sccm to 5 sccm (from 1% to 20% of the total gas flow rate of 25 sccm in the reactor) and the deposition duration is adapted to have 180 nm of thickness for each WSiC:H layer.

Morphology (SEM)

As an illustration, Figure 6 presents the cross-section SEM images for samples synthesized with 1%, 8% and 20% of TMS (i.e., 0.25 sccm, 2 sccm and 5 sccm of TMS in the plasma Ar/TMS mixture). In Figure 6, the samples present two layers that grow differently due to the different deposition processes (RF PVD for the W sublayer and reactive PVD with microwave source assistance for the top layer), with a columnar growth for the W underlayer. At lower TMS flow rates (from 1% to 4%, represented by Figure 6 (a)), the interface between the two layers is not very clear and the whole coating seems homogeneous. Between 5% and 18% of TMS, the interface between layers is much clearer, with the WSiCH layer having a granular appearance while the W sublayer is columnar (as seen in Figure 6 (b) for 8% TMS). At 20%, the layers are again not clearly separated (Figure 6 (c)).

Chemical composition (EDS, IBA, XPS)

EDS measurements (Figure 7) give information on the overall chemical composition of the samples. The layers are mostly formed of carbon, whose atomic content rises between 50 at.% and 60 at.%, much more than silicon content. This can be explained by the fact that there is more four times carbon than silicon in the gas precursor. The absolute Si and W atomic contents cannot be considered here due to the Si nature of the substrate and the presence of the W sublayer. The latter are indeed detected within the interaction bulb of the SEM used for these analyses (# 1 µm in depth, much larger than the coating thickness). Nevertheless, since the thickness are identical we can consider that the incorporation of Si in the coatings starts at least from 10% of TMS in the plasma, coinciding with the decrease of the detected W content (Figure 4). This hypothesis can be supported by the observations made in OES where the drastic increase of 𝑇𝑇𝑒𝑒 from 10% refers to a consumption of low energy electrons and thus to a more important dissociation of the TMS. To go deeper in the coatings analysis, we referred to NRA, ERDA and RBS methods to find out their indepth chemical composition. These analyses allowed us to take a different look at the materials. For the analysis of the results, all the samples were considered to be constituted of four layers, L1 to L4, as shown in Figure 8. Even if the substrate was cleaned by Ar plasma before deposition, we notice a SiO2 noted L1 underlayer which corresponds to the natural oxidation of the Si substrate surface before W layer deposition. It is systematically present on all samples with a number of atoms per unit area close to 20.10 15 at/cm². The composition profiles of the two stacked thin films (supposedly W and WSiC:H) are represented by three layers (L2 to L4). The L3 layer is the main SiCW material, L2 is the first layer deposited on the substrate (supposedly W) and L4 the layer that we supposed to be a part of L3 which is oxidized by the exposition to air. Composition of the L2 "W:C" layer (Figure 9)

We find for this layer an average number of atoms per unit area of 593 ±13 x 10. 15 at/cm² with 95.8 ±

1.5 at.% W. This sublayer is intended to be composed of 100% W as it corresponds to the pure PVD mode (sputtering of W target in Ar plasma). However, we detected a low amount of carbon (5.2 ± 1.5 at.%) in this layer in all samples. This carbon could either come from residues present in the reactor before the deposition (even if it is cleaned before each deposition) or from the SiCW layer deposited above, due to atomic diffusion during the synthesis at 350°C. A study of the chemical composition of the W layer as a separate layer would be appropriate to obtain more information about the carbon present in this layer. For all samples, we detected and measured hydrogen and oxygen onto the surface, with a number of atoms per unit area of 230 to 290 x 10 15 at/cm². The concentration of hydrogen varies between 6 and 8 at.% and that of oxygen between 3 and 7 at.%. This estimated atomic amount is of the order of magnitude of the depth resolution of the NRA and ERDA techniques. It is likely that this hydroxide layer is actually thinner, with higher O and H concentrations as observed by XPS analysis (see below). Its formation is believed to be due to a slight adsorption of water and/or hydroxyl after deposition.

As suspected from EDS measurements (Figure 7), the presence of silicon is only observed starting from a significant amount of precursor in the discharge, here 8% TMS. Si content in this layer then increases as expected with the increase of TMS flow rate. For the main elements in L4, C and W, a plateau is observed between 3 and 14% TMS, with average values of 51.9 ± 2.2 at.% for W and 33.9 ± 2.9 at.% for carbon. Above 14% TMS, we notice an increase of C due to the increase of the TMS injected in the reactor and a decrease of W, with nevertheless a convergence in the atomic content of these two elements at 20% of TMS.

XPS analyses

To confirm the amount of oxygen in L4 layer detected by RBS, XPS analyses were performed in order to get information about the chemical bonds present at the surface of coatings elaborated with 5% and 20% of TMS, respectively, thus investigating a TMS percentage in each of the abovementioned groups.

The measurements were carried out on the pristine sample to get information on L4 layer (as defined in in agreement with [START_REF] Greczynski | Reference binding energies of transition metal carbides by core-level x-ray photoelectron spectroscopy free from Ar+ etching artefacts[END_REF] and [START_REF] Katrib | The multisurface strucure and catalytic properties of partially reduced WO3, WO2 and WC[END_REF], we mostly find W-C bonds at 31.8 and 34.0 eV (for W 4f7/2 and 4f5/2 respectively) after removal of the surface contamination and a very low amount of Si compared to previous analysis. This is consistent with the results obtained in RBS for this category of %TMS, with a high W content (Figure 10). For the sample elaborated with 20% of TMS, we find at the surface the same elements as for the 5%TMS, with in addition Si bound to C at 101.1 eV after the decomposition of Si 2p

signal. The attribution of the C1s component detected at 284.4 eV after etching needs complementary study. However, since TMS encompasses 4 methyl group per Si atom we cannot exclude the deposition of carbon-rich zones whose nature needs to be clarified. After removal of the surface contamination, we again have W-C environment, accompanied by Si bound to C and O. This confirms the incorporation of silicon in this coating elaborated with 20% of TMS and is also coherent with IBA results (Figure 10). The analysis of the main components in the samples before and after etching showed the surface stoichiometries (chemical formulae) gathered in Table 3. The W 4f peaks attributed to carbide bonds (at 31.8 and 34.0 eV) were used as reference for the W-C contribution, as they are detected in all cases. The numbers before brackets are molar ratios, obtained by dividing the atomic percentage of each element by the sum of the two atomic percentages of W and C in W-C bonds in the W 4f peak. To conclude on this part, at high percentage of TMS precursor, W and Si are each bound to C. This means that Si in the films certainly comes from the direct plasma dissociation of TMS and not from the sputtering of a target poisoned with a WSiCH compound, or otherwise we would detect Si-W bonds as well. However, the W target seems to be poisoned by carbon-containing species which are logically more numerous in the gas phase due to the fact that there is four times more carbon than silicon in the precursor (Si(CH3)4), creating a WC film on its surface which is sputtered towards the substrate. This explains the WC bonds observed in XPS for this layer.

Composition of the L3 "SiCW" layer (Figure 12)

The main layer contains only the elements Si, C and W with a majority for the last two and its number of atoms per unit area is in between 1120 to 1890 x 10 15 at/cm². Again, the presence of Si is only observed above 8% TMS, then the Si content increases with increasing TMS flow rate, up to a maximum of 10 at.% for 20% TMS in the plasma phase. For W, a plateau is observed between 3 and 14% TMS with an average concentration of 58.3 ± 0.7 at.%. The value of carbon concentration remains relatively the same at 40.0 ± 2.3 at.% for the whole series. Above 14% TMS, the W content then naturally decreases as Si content tends to increase. To go further and understand these microstructures of these materials, we have plotted the estimated atomic flux (Figure 13) of W and Si, i.e., the number of atoms of these species incorporated in the materials as a function of the deposition duration. This quantity is the product between the atomic percentages of these elements, the layer atomic amount (at/cm²) and the deposition duration (min) for the L3 SiCW layer and L4 top layer.

At a very low percentage of TMS, i.e., between 1% and 8%, the content of W is rather constant. This confirms the previous hypothesis that in this flow range the W target is not polluted by a metal/ceramic compound deriving from TMS dissociation. From 10% TMS onwards, the flux of W decreases markedly and that of silicon increases. This drop of W incorporation, while the sputtering power is constant, could indicate the target poisoning in this range.

Considering the content of W and Si in the top layers of the coatings, we can identify two groups of TMS percentages: the group with a percentage below 10%TMS for which mostly W is incorporated in the film, and the group above 10%TMS for which we start to incorporate Si.

Conclusions

WSiCH coatings were elaborated by the reactive RF sputtering of a W target in an Ar/Si(CH3)4 plasma assisted by ECR microwave sources. The points discussed in this first paper show the complexity of these materials, both in terms of the plasma chemistry leading to its elaboration and its resulting chemical composition due to the simultaneous sputtering of the target and dissociation of the reactive gas, but allowed us to estimate the conditions for which the development of these cermets is feasible.

Process diagnostics performed by optical emission spectroscopy showed the ability to form powders in the gas phase. Investigations showed a poisoning of the W target by C forming W-C aggregates towards the growing layer. These results are supported by the chemical analyses performed on the samples from this gas phase. Indeed, the IBA analyses showed very high amounts of W and C compared to that of Silicon. The XPS measurements carried out on the top layer (SiCWOH) also highlighted this formation of W-C bonds.

The correlation between the process diagnostics and the material characterizations allowed us to estimate the conditions in terms of percentage of TMS for which the development of tungsten-containing nanoparticles incorporated in a SiC matrix is feasible, insofar as silicon is effectively detected from 10% TMS. In future work, the formation of such nanocomposites will be investigated by TEM.

Such cermets could be considered for spectrally selective absorber materials used to improve the thermooptical performance of concentrating solar power plant receivers. [START_REF] Antonaia | Stability of W-Al2O3 cermet based solar coating for receiver tube operating at high temperature[END_REF][START_REF] Price | Advances in Parabolic Trough Solar Power Technology[END_REF][START_REF] Bermel | SELECTIVE SOLAR ABSORBERS[END_REF][START_REF] Kennedy | Review of Mid-to High-Temperature Solar Selective Absorber Materials[END_REF] Indeed, the inclusion of metallic nanoparticles can lead to the appearance of evanescent waves resulting from the plasmonic effect [START_REF] Barshilia | Structure and optical properties of Ag-Al2O3 nanocermet solar selective coatings prepared using unbalanced magnetron sputtering[END_REF][START_REF] Boriskina | Plasmonic materials for energy: From physics to applications[END_REF][START_REF] Kuzma | Plasmonic properties of Au-Ag nanoparticles: Distinctiveness of metal arrangements by optical study[END_REF][START_REF] Coulibaly | From colloidal precursors to metal carbides nanocomposites MC (M=Ti, Zr, Hf and Si): Synthesis, characterization and optical spectral selectivity studies[END_REF] within the material when it is illuminated, increasing its absorbing power. In the continuity of this work, we will in fact consider in our next paper their insertion in a multilayer coating architecture with a top antireflective layer of TaOxNy, [START_REF] Bousquet | Structural and ellipsometric study on tailored optical properties of tantalum oxynitride films deposited by reactive sputtering[END_REF][START_REF] Zoubian | Potential of TaOxNy thin films deposited by reactive sputtering as antireflective coatings: composition and optical properties[END_REF] for which first optical simulations and optical measurements proved promising performance for the receivers of concentrated solar power plants.

  techniques. RBS was used to determine the depth profile of chemical elements in the material, by bombarding the sample placed in a vacuum chamber with an alpha beam MeV energy. The backscattered alpha particles were detected with a 25 mm² Passivated Implanted Planar Silicon (PIPS) detector located at a 166° angle with respect to the beam axis. The Si and W depth resolution was close to 130.10 15 at/cm². RBS analyses were completed by NRA and ERDA analyses to get more precise information on the chemical composition of the materials, especially in light elements contents. The oxygen and carbon concentrations were measured by the 16 O(d,α0) 14 N and 12 C(d,p0

Figure 1 :

 1 Figure 1: Comparison of Ar + W (pure PVD mode), Ar + TMS (5 sccm, 850 W) (PECVD mode) and Ar + W + TMS (5 sccm, 850 W) (reactive MW PVD mode) plasma emission spectra

Figure 2 :

 2 Figure 2: Evolution of the electron temperature of an Ar + W + TMS RF reactive MW PVD plasma with TMS precursor content

Figure 3 :

 3 Figure 3: Evolution of the concentration of 𝐻𝐻𝛼𝛼 species representing the dissociation of TMS precursor (Si(CH3)4) in the Ar + W + TMS reactive PVD microwave plasma, vs. TMS content, for different deposition durations

Figure 4 :

 4 Figure 4: W intensity as a function of TMS percentage at different times in Ar + W + TMS plasma

Figure 5 :

 5 Figure 5: SEM images of powders collected in the reactor after reactive PVD deposition with 5 sccm of TMS and 850 W of microwave power

Figure 6 :

 6 Figure 6: Cross-section SEM images of samples deposited with (a) 1%, (b) 8% and (c) 20% of TMS in the Ar/TMS plasma for the WSiC:H top layer

Figure 7 :

 7 Figure 7: Chemical composition of the deposited coatings determined by EDS analysis as a function of the percentage of TMS injected in the reactor during reactive MW PVD deposition

Figure 8 :

 8 Figure 8: Sample layout considered for the interpretation of IBA simulations

Figure 9 :

 9 Figure 9: Chemical composition of "W:C" L2 layer measured by IBA

Figure 10 :

 10 Figure 10: Chemical composition of L4 "SiCWOH" layer measured by IBA (closed circle) and by XPS (open circle)

Figure 8 )

 8 Figure 8) and after etching for 30 seconds to minimize the surface contamination due to storage conditions. The results presented in Figure 11 show the spectra obtained for the levels O 1s, C 1s, Si 2p and W 4f whose decomposition gives us the different environments contained in the materials and their stoichiometry. For the 5%TMS sample, O 1s can be decomposed into O,OH-Si (from SiOx) at 532.4 eV and O,OH-W (from WOx species) at 531.3 eV. The binding energies of 285.1 eV and 283.6 eV can respectively be assigned to C-C/C-H and C-W bonds [37]. Environments typical of C-O and C=O are also identified at 287.0 eV and the peak at 289.6 eV may arise from the formation of carbonate complex due to interaction with atmospheric CO2. Si-O bond is characterized by its 2p1/2 and 2p3/2 peaks located at 103.2 eV and 102.6 eV, respectively, on the surface of this sample. The decomposition of W 4f7/2 peak shows W-O bond at 36.5 eV and W linked to C at 32.2 eV (W 4f5/2 at 38.6 and 34.3 eV, respectively). After etching,

Figure 11 :

 11 Figure 11: Experimental XPS spectra of the topmost L4 layer (squares) and after erosion for 30 s (circles) of samples elaborated with 5 % and 20% of TMS and fitted components (grey lines) with the total fitted envelop before (red line) and after etching (blue line).

Figure 12 :

 12 Figure 12: Chemical composition of L3 "SiCW" layer measured by IBA

Figure 13 :

 13 Figure 13: Atomic flux of W and Si as a function of the percentage of TMS precursor during reactive PVD MW deposition, estimated from IBA measurements

Table 1 :

 1 Values of the constants used for the calculation of the electron temperature

	Constants	m  s (Ar transition) p  n (Ar transition)
	Wavelength (𝜆𝜆) (nm)	702.19	706.16
	Frequency (ν) (𝑠𝑠 -1 )	4.27503 * 10 14	4.25098 * 10 14
	Einstein's coefficients (A) (𝑠𝑠 -1 )	2670000	3800000
	Energie (E) (𝑚𝑚 -1 )	11968303	10728970
	Static weight (g)	5
	Planck's constant (h) (J.s)	6.62 * 10 -34
	Boltzmann's constant (k) (J. 𝐾𝐾 -1 )	1.38 * 10 -23

Table 2 :

 2 Identification of emitting species detected by OES in the gaseous phase

	Species	W	SiH	CH	𝑪𝑪 𝟐𝟐	𝑯𝑯 𝟐𝟐	𝑯𝑯 𝜶𝜶
	Wavelength (nm)	404	415	431	516	601	656

Table 3 :

 3 Composition of samples elaborated with 5% and 20% of TMS according to XPS characterization. The W 4f peaks attributed to W-C bonds were used as reference.

	Sample	Before etching (surface of L4 layer)	After etching for 30 sec (L4 layer)
	5 %	1.34[W-Ox]/[W-C0.5]/0.9[Si-Ox]	[W-C0.5]
	20 %	0.74[W-Ox]/[W-C0.9]/0.5[Si-C1.6]/1.34[Si-Ox] [W-Cx]/0.19[Si-C0.5]/0.19[Si-Oy]

Acknowledgments

This work was financed by the French National Research Agency (ANR), as part of the NANOPLAST project (nanoplast-project.cnrs.fr) under contract ANR-19-CE08-0019, and the "Investments for the future" programme under contract ANR-10-LABX-22-01-SOLSTICE. The authors thank C. Escape, Y.

Gorand, E. Hernandez, H. Glénat, O. Wendling, A. Bellamy and all the technical partners for their involvement.