Cecile Floer

Omar Elmazria

Natalya Naumenko

Florian Bartoli

Jaafar Ghanbaja

Thierry Aubert

Sami Hage-Ali

AlN/Pt/LN-Y128 packageless acoustic wave temperature sensor

Keywords: Packageless, guided acoustic wave, temperature

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. INTRODUCTION

urface acoustic wave (SAW) devices are widely used as filters, resonators or delay lines in electronic systems for a wide range of applications such as mobile communications (cell phones), TVs, radars or stable resonators for clock generation [START_REF] Campbell | Surface acoustic wave devices for mobile and wireless communications[END_REF]. More recently, thanks to the sensitivity of the resonance frequency and the signal transmission delay to the physical environmental parameters, the use of SAW devices has been expanded to measure gas concentrations, pressures, forces, temperatures, strains or radiations. In this regard, SAW sensors have a great added value, being passive (batteryless) and potentially remotely readable (wireless) [START_REF] Buff | Passive remote sensing for temperature and pressure using SAW resonator devices[END_REF][START_REF] Fu | Development of wireless, batteryfree gyroscope based on one-port SAW delay line and double resonant antenna[END_REF]. By combining the latter properties with a small size device, a low-cost radio system and small antennas (provided that it is a high frequency device), new and exciting perspectives for wireless measurement processes and ID-Tag applications have emerged. Moreover, by choosing the proper materials, those sensors can be used without embedded electronics in hostile environments, for example under high temperature, under ionizing radiations or under high pressure (up to several thousand bar), where conventional wireless sensors cannot survive. However, one current bottleneck of the SAW sensor technology in these harsh Experiments were carried out at MiNaLor clean-room platform which is partially supported by FEDER and Grand Est Region through the RaNGE project and at IJL TUBE-Davm equipment which is funded by the French PIA project "Lorraine Université d'Excellence" (ANR-15-IDEX-04-LUE). This work was supported by the Direction Générale de l'Armement (DGA), the ANR project "SAWGOOD" (ANR-18-CE42-0004-01), the CAPMAT project ("FEDER-FSE Lorraine et Massif Vosges 2014-2020" and ICEEL), the Ministry of Science and Higher Education of the Russian Federation (075-02-2020-1588), and the National University of Science and Technology MISIS (K2-2020-007).

environments is the lack of a reliable packaging solution [START_REF] François | High temperature packaging for surface acoustic wave transducers acting as passive wireless sensors[END_REF]. This technological barrier could be elegantly broken by the development of so-called "packageless" sensors. It has already been shown theoretically and experimentally that a packageless solution is achievable using Waveguiding Layer Acoustic Waves (WLAW) [START_REF] Bhattacharjee | Packageless SAW Devices with Isolated Layer Acoustic Waves (ILAW) and Waveguiding Layer Acoustic Waves (WLAW)[END_REF][START_REF] Elmazria | AlN/ZnO/diamond structure combining isolated and surface acoustic waves[END_REF][START_REF] Legrani | Packageless AlN/ZnO/Si Structure for SAW Devices Applications[END_REF][START_REF] Bartoli | AlN/GaN/Sapphire heterostructure for hightemperature packageless acoustic wave devices[END_REF]. In a WLAW structure, the wave propagates in a low velocity layer "sandwiched" between two high acoustic velocity layers [START_REF] Bhattacharjee | Packageless SAW Devices with Isolated Layer Acoustic Waves (ILAW) and Waveguiding Layer Acoustic Waves (WLAW)[END_REF]. A WLAW packageless temperature sensor was achieved on a Y+128° lithium niobate based AlN/ZnO/LN-Y128 trilayer [START_REF] Floer | AlN/ZnO/LiNbO3 Packageless Structure as a Low-Profile Sensor for Potential On-Body Applications[END_REF] with a Temperature Coefficient of Frequency (TCF) as high as -106 ppm/°C.

In a previous study, an AlN/LN-Y128 bilayer, coupled with heavy Pt electrodes [START_REF] Naumenko | AlN/Pt/LN structure for SAW sensors capable of operating at high temperature[END_REF], was proposed as an alternative packageless high temperature sensor and theoretically analyzed using SDA-FEM-SDA [START_REF] Naumenko | Advanced numerical technique for analysis of surface and bulk acoustic waves in resonators using periodic metal gratings[END_REF]. This bilayer structure is advantageous over the AlN/ZnO/LN-Y128 trilayer [START_REF] Floer | AlN/ZnO/LiNbO3 Packageless Structure as a Low-Profile Sensor for Potential On-Body Applications[END_REF] as only one additional layer has to be deposited over the LN-Y128 substrate and optimized. Moreover, ZnO has poor high temperature properties, whereas all the materials in AlN/IDT(Pt)/LN-Y128 can withstand high temperature operation at least up to 600°C [START_REF] Floer | SAW RFID Devices Using Connected IDTs as an Alternative to Conventional Reflectors for Harsh Environments[END_REF][START_REF] Streque | Stoichiometric Lithium Niobate Crystals: Towards Identifiable Wireless Surface Acoustic Wave Sensors Operable up to 600°C[END_REF][START_REF] Aubert | Investigations on AlN/sapphire piezoelectric bilayer structure for high-temperature SAW applications[END_REF].

In this paper, we aim at providing the experimental validation of the AlN/IDT(Pt)/LN-Y128 structure at frequencies over 2 GHz. Theoretical findings from [START_REF] Naumenko | AlN/Pt/LN structure for SAW sensors capable of operating at high temperature[END_REF] are first shortly described. Then the microfabrication, the AlN growth and the morphological and structural analyses are discussed. Electrical characterizations are compared with numerical analyses. Finally, the use as a packageless sensor and measurements up to 300°C are provided.

II. RESULTS AND DISCUSSION

A. Peculiarities of the AlN/IDT(Pt)/LN-Y128 structure

As a brief summary of [START_REF] Naumenko | AlN/Pt/LN structure for SAW sensors capable of operating at high temperature[END_REF], thanks to the use of thick and dense electrodes, Rayleigh-type acoustic waves can propagate along the AlN/LN-Y128 interface with a high velocity up to 4500 m/s, a high electromechanical coupling of 5.6% and negligible leakage into the LN-Y128 substrate. With optimized Pt electrodes and AlN thicknesses, the velocity of this mode can be engineered in the interval between 4029 m/s and 4644 m/s, i.e. between the slow and fast shear cut-off bulk acoustic wave (respectively written VB1-LN and VB2-LN). In this interval, the Rayleigh-type wave does not couple with the slow shear bulk acoustic wave and does not leak in the substrate, due to Cécile Floer, Omar Elmazria, Florian Bartoli, Jaafar Ghanbaja, and Sami Hage-Ali are with Université de Lorraine, CNRS, IJL, F-54000 Nancy, France (e-mail : cecile.floer@univ-lorraine.fr , sami.hage-ali@univ-lorraine.fr ) Natalya Naumenko is with National University of Science and Technology "MISIS", Moscow, Russia (e-mail : naumenko.natalya@gmail.com ) Thierry Aubert is with Université de Lorraine, CentraleSupélec, LMOPS, F-57000 Metz, France (e-mail : thierry.aubert@univ-lorraine.fr )

S

anomalous properties of the LN-Y128. Higher velocities (>VB2-LN = 4644 m/s) would result in strong leakage of the Rayleigh wave into the substrate, a phenomenon that is prevented in this structure by the use of optimized thick and dense electrodes that slow the wave. Assuming that a sufficient AlN thickness is used to protect the wave from environmental influence for packageless operation, i.e. about one wavelength 𝜆, the wave velocity is expected to be suitable when the Pt thickness is close to 0.1λ.

B. Microfabrication and structural analysis

In order to validate this structure experimentally, a single port resonator structure with a wavelength λ=1.7 µm was patterned on a LN-Y128 substrate by electron beam lithography and ion beam etching (IBE) methods on a ~170 nm (0.1λ) sputtered Pt layer. A top view of the platinum resonator and design information are shown in Fig. 1a. The bright color of the fingers' edge in the zoom image (Fig. 1b) indicates that the sides etching profile is not vertical. Cross sectional STEM images (Fig. 1c) show that the electrodes are of trapezoidal form, with titled sides with an angle of 45°, obtained repeatably due to masking effects during the IBE. The metallization ratio at the bottom 2*a1/λ is 78.1% and the bulk metallization ratio (a1+a2)/λthe important metric since it determines the mass loadingis 58.4 %.

Subsequently, (002) oriented AlN was sputtered on the resonator. Two types of deposition were investigated: one (deposition n°1) without intentional substrate heating in a commercial machine and the other (deposition n°2) at 480°C in a custom-built machine. The deposition parameters are summarized in Table I. Then, morphological and nanostructural characterizations were carried out with scanning and transmission electron microscope (SEM and TEM, respectively). STEM micrographs in Fig. 2 show that the AlN film growth is affected by the trapezoidal shape of the electrodes for both depositions. For the low temperature deposition (Fig. 2a,2b), the grains arise from the tilted electrodes' sides to join further on. For the second deposition conducted at 480°C, much less inhomogeneities can be observed (Fig. 2c,2d). The columns join each other much faster. Thus, the lower density areas do not appear over the entire thickness of the AlN film, highlighting a better crystallization. Moreover, an X-Ray diffraction rocking-curve analysis on the (002) orientation revealed a FWHM of 2.45° leading to the conclusion that a good AlN quality can be achieved even on trapezoidal electrodes.

C. Electrical characterizations & discussion

The devices were electrically characterized before and after the AlN deposition using a network analyzer (VNA Agilent-N5230A) and the acoustic waves velocities were extracted from the admittance measurements. Simulations were performed taking into account the trapezoidal geometry of the electrodes using the SDA-FEM-SDA software [START_REF] Naumenko | Advanced numerical technique for analysis of surface and bulk acoustic waves in resonators using periodic metal gratings[END_REF], with constants from [START_REF] Kushibiki | Accurate measurements of the acoustical physical constants of LiNbO3 and LiTaO3 single crystals[END_REF] for LN-Y128, and c11=348 GPa, c12=242 GPa and a density of 21500 kg/m 3 for Pt.

Fig. 3 shows the velocities of the Rayleigh and Shear Horizontal (SH) modes in LN-Y128 with platinum electrodes, first without the AlN film (Fig. 3a), then with a thin AlN film in-between the Pt electrodes (Fig. 3b) and on top of them (Fig. 3c). In both cases with AlN (Fig. 3b and3c) the Pt thickness is fixed at 0.1 λ.

In the first simulated case (Fig. 3a), the mass loading effect is highlighted by the SAW velocities decrease due to the increase of the Pt thickness. Moreover, the antiresonance velocities of Rayleigh and SH waves, determined experimentally from the resonators show a good agreement with the simulations (see the square symbols on Fig. 3a). They have been identified respectively at 2692 m/s and 2149 m/s when the Pt electrodes thickness is equal to 0.1 λ.

Then, the AlN elastic constants were fitted in order to match the experimental results from the 480°C AlN deposition (triangles Fig. 3c). The best agreement between simulations and experiments was obtained with the elastic stiffness constants c11=300 GPa and c44=86.5 GPa and a density of 3250 kg/m 3 .

These values where subsequently used to compute the velocities with increasing hAlN/λ (Fig. 3b and3c). The fast AlN layer explains the continuous increase of the velocities. The wave velocities hardly change as soon as the hAlN/λ relative thickness exceeds 1.0, meaning that the influence of surface perturbations is very small. In Fig. 3c, even if the hAlN/λ ratio is a bit different, we can observe significant experimental velocities difference between the sample with AlN deposited at low temperature (circles) and at 480°C (triangles). This can be attributed to the morphological and crystalline differences visible in Fig. 2. This also means that AlN deposition parameters can be used to engineer the fine tuning of velocities at constant Pt thicknesses.

It is also noteworthy that, even with the 480°C deposition, experimental velocities are quite far from theoretical predictions from [START_REF] Naumenko | AlN/Pt/LN structure for SAW sensors capable of operating at high temperature[END_REF] (that used c11=393 GPa and c44=130 GPa for AlN), at similar mass loading (i.e. 3751 m/s vs 4200 m/s, respectively, for the antiresonance of the Rayleigh type mode). From a design standpoint, this finding is actually highly beneficial, since thinner Pt electrodes can be used in order to reach velocities in the 4000-4500 m/s interval. Indeed, a thinner Pt electrode would lead to a lower velocity decline and thus to higher final velocities and frequencies, after adding the AlN fast layer. Fabrication would also be easier and viscoelastic diminished. Assuming no significant change of the AlN film quality when it is deposited on thinner electrodes, the optimal Pt thickness is estimated to be about 0.05 λ, with the same constants.

The wave confinement in the AlN(480°C)/IDT(Pt)/LN-Y128 bilayer structure has been tested by disturbing the surface of the structure with a very flexible elastomeric perturber named Solaris placed on top of it, like in [START_REF] Floer | AlN/ZnO/LiNbO3 Packageless Structure as a Low-Profile Sensor for Potential On-Body Applications[END_REF]. The comparison of the perturbed and unperturbed S11 frequency response is shown in Fig. 4. No influence of the elastomeric pertuber on the S11 coefficient is observed, proving that the wave is confined inside the bilayer structure and consequently that the structure is suitable to achieve packageless devices.

To investigate the potential of the structure as a high temperature sensor, the resonance frequency of the AlN/IDT(Pt)/LN resonator was measured when increasing the temperature from 30 to 300°C (Fig. 5a). The base level and impedance matching difference between Fig. 4 and Fig. 5a (case at 30°C) results from the probe position variability on our ball connection which is needed to protect the pad access during the AlN deposition. The repetition of measurements and the probes' thermal expansion also caused the connection balls to fail after a few iterations. It was thus not possible to test the device at higher temperatures. However, a high TCF value -138 ppm/°C is obtained for the Rayleigh type wave (Fig. 5b). Therefore, this bilayer structure is a good candidate for a packageless temperature sensor at high temperatures.

III. CONCLUSION

To summarize, the potential of the AlN/IDT(Pt)/LN-Y128 bilayer structure as a packageless sensor has been validated experimentally. The use of thick electrodes made from a dense metal such as platinum, placed on a LN-Y128 substrate and buried in a high velocity layer like AlN leads to a confined acoustic mode. A high sensitivity to the temperature was also observed making this bilayer structure promising for packageless high temperature sensing applications.

Future investigations will focus on the optimization of the Pt thickness and the AlN deposition parameters in order to reach optimal speeds, frequencies and loss mitigation. Other material choices are worth investigating [START_REF] Nicolay | Towards an AlScN-based packageless acoustic wave sensor with RFID capabilities, for applications above 350°C[END_REF]. This study could also be extended to reflective delay-lines (RDL). By combining a sensing and an identification functionality in a unique device, RDL become particularly interesting for the industry 4.0.

Fig. 1 .

 1 Fig. 1. (a) SEM micrograph and design of the platinum resonator. (b) Zoom on IDT fingers and the reflectors. (c) Cross-sectional STEM -Bright Field (BF) micrograph of the electrode geometry.

Fig. 2 .

 2 Fig. 2. Cross-sectional STEM -High Angle Annular Dark Field (HAADF) micrographs of the AlN film grown: (a),(b) without intentional heating and (c),(d) with a 480°C intentional heating.

Fig. 3 .

 3 Fig. 3. Simulated and experimental velocities of two propagating modes in the AlN/IDT(Pt)/LN-Y128 bilayer structure as a function: (a) of the Pt thickness, (b) of the AlN thickness placed in-between or (c) on top of the electrodes. For (b) and (c) the Pt thickness is fixed at 0.1 λ.

Fig. 4 .

 4 Fig. 4. S11 measurement of the AlN(480°C)/IDT(Pt)/LN-Y128 structure with and without a surface perturbation (Solaris elastomer) on top of the AlN layer.

Fig. 5 .

 5 Fig. 5. (a) S11 measurements of the AlN (480°C)/IDT(Pt)/LN-Y128 structure between 30°C and 300°C. (b) Relative frequency shifts with respect to the temperature of the AlN (480°C)/IDT(Pt)/LN-Y128 structure.

TABLE I :

 I ALN DEPOSITION PARAMETERS

		Deposition 1 (2.5µm)	Deposition 2 (3.2 µm)
	Sputtering	Alliance Concept DP650	Custom-built
	Target	Al (Ø 4-inch)	Al (Ø 2-inch)
	Gases	16 sccm N2	23.5 sccm N2
	Total pressure	8x10 -3 mbar	6.6x10 -3 mbar
	Temperature	No intentional heating	480°C
	Target power	200 W	200 W


Amélie Cavelan

Mohammed Boussafir

Claude Le Milbeau

Sandrine Delpeux

Olivier Rozenbaum

Fatima Laggoun-Défarge

Influence of organic matter properties on porosity of kimmeridge clay source rocks during laboratory thermal maturation

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INFLUENCE OF ORGANIC MATTER PROPERTIES ON POROSITY OF KIMMERIDGE CLAY SOURCE ROCKS DURING LABORATORY THERMAL MATURATION.

A. Cavelan 1 , M. Boussafir 1 , C. Le Milbeau 1 , S. Delpeux 2 , O. Rozenbaum 3 , F. Laggoun-Défarge 1 .

1 : Institut des Sciences de la Terre d'Orléans, France. 2 : ICMN, France 3 : CEMHTI, France Thermal maturation of organic matter (OM) and the ensuing generation of oil, gas and organic rich residues appear to be the major factor controlling the development, the size and the abundance of OM pores in marine organic-rich mudstones [START_REF] Katz | Organic porosity: A geochemist's view of the current state of understanding[END_REF][START_REF] Ko | Pore-types and pore-network evolution in Upper Devonian-Lower Mississippian Woodford and Mississippian Barnett mudstones: Insights from laboratory thermal maturation and organic petrology[END_REF]. However, different formations containing a similar type II OM can exhibit different OM pore evolution models indicating that other factors, such as maceral composition, may influence the evolution of OM porosity during burial [START_REF] Ko | Pore-types and pore-network evolution in Upper Devonian-Lower Mississippian Woodford and Mississippian Barnett mudstones: Insights from laboratory thermal maturation and organic petrology[END_REF]. To investigate how the chemical properties of the total particulate OM assemblage affect the evolution of porosity during thermal diagenesis of organic-rich marine mudstones, laboratory gold-tube anhydrous confined thermal maturation (325°C-470°C for 72h) was applied on immature Kimmeridge Clay (KCF) mudstones (Yorkshire, UK) which contain a type II oil-prone kerogen (HI: 418 to 617 mg HC/g TOC). The evolution of pore volume and pore size distribution was calculated using nitrogen adsorption measurement and evaluated as a function of OM properties: palynofacies analysis, Rock Eval 6 ® pyrolysis, the amount and the composition of oil and gas generated during maturation (GC-TCD and GC/MS analysis). The addressed question is to know how slight differences in the relative proportion of individual OM components throughout a same formation influence OM chemical transformations and pore development during thermal maturation of organic-rich marine mudstones.

The results show that two organic rich KCF samples (TOC= 10.3 and 14.4 wt.% respectively), M1 and M2, with different particulate OM assemblage can exhibit different OM pore evolution during thermal maturation, especially in the gas window (Figure 1). M1 contain higher proportion of brown amorphous organic matter (AOM) derived from cell walls of microalgae. M2 contains mainly oil-prone phytoplanktonic OM preserved by natural sulfurization, which gives it slightly higher sterane, hopane and monoaromatic steroid concentrations and higher oil and gas generation potential (Figure 1A). M2 OM conversion has, indeed led to the production of higher amount of low viscous oil (S1, SAT+ARO, Figure 1A) and abundant gaseous hydrocarbon concentrations more favourable to the formation of abundant but small OM hosted micropores and mesopores (Figure 1B). Meanwhile, the secondary cracking of the heavier and more viscous oil produced by brown-AOM-rich OM (M1) led to formation of greater quantity of CO2 (Figure 1A) and an abundant organic-rich residue more favorable to the formation of fewer but larger mesopores (Figure 1B). In the dry gas zone, M2 micro and mesopores have coalesced in response of CH4 generation to form larger mesopores and macropores (Figure 1B). This phenomenon has surprisingly not conducted to the increase of M2 pore volume which remains relatively low contrary to M1 (Figure 1A). Despite their slightly higher oil-prone quality, samples containing higher amount of orange AOM seems less favorable to the formation of larges mesopores and important pore volume during thermal maturation (M2, Figure 1). Thus, a higher potential for oil and gas generation do not lead systematically to the formation of a greater pore volume in OM during maturation. This suggests that slight variations of the individual particulate OM assemblage of a similar type II kerogen throughout a same formation can significantly influence the composition and the amount of oil and gas 29 th International Meeting on Organic Geochemistry (IMOG)

1-6 September 2019, Gothenburg, Sweden generated during thermal maturation resulting in different OM pore development and evolution. Moreover, we can assume that the shorter duration and the higher temperature of our laboratory experiments may have limited the OM chemical transformations compared to natural systems. This may affect differently orange and brown AOM explaining some of the differences between samples such as the surprising low pore volumes and alkane concentrations of M2 (Figure 1A). Comparison of these results with longer duration laboratory experiments may lead to a better understanding of the influence of these processes on OM pore development.

Nevertheless, these results put forward the necessity to consider both the level of maturity and the composition of the original particulate OM assemblage to better predict the composition and the amount of oil and gas generated during maturation and the associated pore evolution model.

Figure 1

 1 Figure 1 A) Evolution of pore volume, S1, concentrations of monoaromatic steroids (MAS), hopanes and steranes (Hop+Ste), alkanes (including isoprenoids), gas (methane to pentane, CO2), relative proportion of aromatic (ARO) and saturated hydrocarbons (SAT) and B) pore size distribution during thermal maturation of two KCF mudstones.

th International Meeting on Organic Geochemistry (IMOG) 1-6 September 2019, Gothenburg, Sweden


Anaïs Loubat

Marianne Impéror-Clerc

Brigitte Pansu

Florian Meneau

Bertrand Raquet

Guillaume Viau

Lise-Marie Lacroix
email: lmlacroi@insa-toulouse.fr

Growth and Self Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in situ SAXS

Keywords: 1D growth, In situ SAXS, Metallic nanowires, Van der Waals forces

We report the self-assembly of gold nanowires into hexagonal superlattices in liquid phase followed by in-situ Small Angle X Ray Scattering and give new insights on their growth mechanism. The unprecedented large interwire distance of 8 nm strongly suggests the stabilization of the ultrathin gold nanowires by a ligand's double layer composed of oleylamine and oleylammonium chloride.

The one-dimensional growth is discussed, opening perspectives towards the control growth and self assemblies of metallic nanowires.

Introduction

In the last 20 years, improvement in liquid-phase synthesis opened the way towards controlled nanoparticles with adjustable size and very narrow size dispersion (σ < 10%). [START_REF] Kwon | Colloidal Chemical Synthesis and Formation Kinetics of Uniformly Sized Nanocrystals of Metals, Oxides, and Chalcogenides[END_REF] Though leading to a fine tuning of the physical properties, the shape control of the nanoparticles is still under investigation. Anisotropic metallic nanoparticles, which exhibit modified plasmonic resonance or stable magnetization compared to their spherical counterparts, [START_REF] Pérez-Justea | Gold nanorods: Synthesis, characterization and applications[END_REF][START_REF] Lu | Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application[END_REF] arisen from non isotropic growth processes. [START_REF] Sau | Nonspherical Noble Metal Nanoparticles: Colloid-Chemical Synthesis and Morphology Control[END_REF] Au nanorods, prepared in aqueous media, were one of the most studied systems, due to the inertness of the materials and the multiple potential applications ranging from catalysis to biomedical applications. [START_REF] Grzelczak | Shape control in gold nanoparticle synthesis[END_REF] However, several hypotheses are often invoked for such anisotropic growth: the nature of the seeds, [START_REF] Lofton | Mechanisms Controlling Crystal Habits of Gold and Silver Colloids[END_REF] the preferential adsorption of surfactant on peculiar crystallographic faces, [START_REF] Murphy | The Many Faces of Gold Nanorods[END_REF][START_REF] Katz-Boon | Three-Dimensional Morphology and Crystallography of Gold Nanorods[END_REF] or the existence of templates such as anisotropic micelles in which the growth occurred. [START_REF] Gao | Dependence of the Gold Nanorod Aspect Ratio on the Nature of the Directing Surfactant in Aqueous Solution[END_REF] Recently, ultrathin gold nanowires which exhibit a mean diameter in the range 1.5-2 nm and a micrometer length, attracted lots of interest due to their size homogeneity and their potential application as SERS probes, [START_REF] Feng | Simple and rapid synthesis of ultrathin gold nanowires, their self assembly and application in surface-enhanced Raman scattering[END_REF] lightweight foldable optoelectronics membranes, [START_REF] Chen | Mechanically strong, optically transparent, giant metal superlattice nanomembranes from ultrathin gold nanowires[END_REF][START_REF] Sanchez-Iglesias | Highly transparent and conductive Films of densely aligned ultrathin Au nanowire monolayers[END_REF] or elastic coiled springs. [START_REF] Xu | Mechanical nanosprings : induced coiling and uncoiling of ulltrathin Au nanowires[END_REF] Their unique 1D feature confers them remarkable conductivity properties, such as quantum phenomena at room temperature, opening innovative routes in nanoelectronic. [START_REF] Pud | Features of transport in ultrathin gold nanowire structures[END_REF][START_REF] Loubat | Cotunneling transport in ultra-narow gold nanowire bundles[END_REF][START_REF] Chandni | Tunability of Electronic States in Ultrathin Gold Nanowires[END_REF] Several strategies have been followed to synthesize these ultrathin nanowires. A gold salt (HAuCl4 or AuCl) is reduced in pure oleylamine (OY) [START_REF] Huo | Sub-Two Nanometer Single Crystal Au Nanowires[END_REF][START_REF] Lu | Ultrathin Gold Nanowires Can Be Obtained by Reducing Polymeric Strands of Oleylamine#AuCl complexes Formed by Aurophilic Interaction[END_REF][START_REF] Pazos-Pérez | Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media[END_REF] or in a solution containing OY and an additional reducing agent. [START_REF] Feng | Simple and rapid synthesis of ultrathin gold nanowires, their self assembly and application in surface-enhanced Raman scattering[END_REF][START_REF] Kang | Size-and shape-selective synthesis of metal nanocrystals and nanowires using CO as a reducing agent[END_REF][START_REF] Halder | Ultrafine Single-Crystalline Gold Nanowire Arrays by Oriented Attachment[END_REF] Self assembly of these ultrathin nanowires into very nice parallel arrays were observed on the TEM carbon grids, [START_REF] Feng | Simple and rapid synthesis of ultrathin gold nanowires, their self assembly and application in surface-enhanced Raman scattering[END_REF][START_REF] Huo | Sub-Two Nanometer Single Crystal Au Nanowires[END_REF] or at water interface, [START_REF] Sanchez-Iglesias | Highly transparent and conductive Films of densely aligned ultrathin Au nanowire monolayers[END_REF] opening perspectives for artificially designed metamaterials. [START_REF] Nie | Properties and emerging applications of self-assembled structures made from inorganic nanoparticles[END_REF] Though exiting, the reported synthesis did not univocally address the question of the growth mechanism, which is of prime interest to tune the size of the wires and thus, their physical properties. If the presence of OY was reported consensually as crucial for the 1D growth, Hadler et al. proposed an oriented attachment and ripening process of small seeds, [START_REF] Halder | Ultrafine Single-Crystalline Gold Nanowire Arrays by Oriented Attachment[END_REF] while Pazos-Perez et al. invoked a micellar growth. [START_REF] Pazos-Pérez | Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media[END_REF] Moreover, the driving force of the self assembly, which could be tuned to control the final metamaterial obtained, was not clearly evidenced. The potential pre assembly in solution, prior to solvent evaporation which is known as a strong driving force towards crystallization, [START_REF] Rabani | Drying-mediated self-assembly of nanoparticles[END_REF][START_REF] Park | Direct observation of nanoparticle superlattice formation by using liquid cell transmission electron microscopy[END_REF] requires in-situ characterizations based on dynamic light scattering with conventional DLS [START_REF] Miszta | Hierarchical self-assembly of suspended branched colloidal nanocrystals into superlattice structures[END_REF][START_REF] Pagès | Self-assembly of ZnO nanocrystals in colloidal solutions[END_REF] or Small Angle X-Ray Scattering (SAXS).

Real-time SAXS enables to address both the study of the growth mechanism of nanoparticles in liquid phase, [START_REF] Abécassis | Gold Nanoparticles Superlattice Crystallization Probed In Situ[END_REF] thanks to their specific scattering profile, and the in-situ crystallisation, [START_REF] Abécassis | Probing in situ the nucleation and growth of gold nanoparticles by small-angle X-Ray scaterring[END_REF] through additional Bragg peaks, similarly to conventional X-ray diffraction (XRD). Based on SAXS, we report here for the first time the formation of hexagonal superlattices of ultrathin gold nanowires in the liquid phase. The present in situ study gives new information about the driving force that originates the self assembly in solution. The unprecedented interwire distance of 8 nm suggests a bilayer stabilisation of Au nanowires leading to new perspective on their growth mechanism.

Experimental section 2.1 Au NWs synthesis

The ultrathin gold nanowires (NWs) were prepared following a synthesis adapted from B. Xing and co-workers. [START_REF] Feng | Simple and rapid synthesis of ultrathin gold nanowires, their self assembly and application in surface-enhanced Raman scattering[END_REF] Typically 10 mM solution of gold was prepared: 40 mg of HAuCl4,3H2O were dissolved in a solution containing 1.32 mL oleylamine (OY, 400 mM, OY/Au = 40) in 6.60 mL of hexane. 2.05 mL of triisopropylsilane (TIPS, 1M, TIPS/Au = 100) were added to initiate the gold reduction. Though the exact mechanism still unclear, TIPS acts as a strong reducing agent, fastening the reaction from several days to few hours. The solution was then kept undisturbed at 40°C for few hours. Magnetic stirring did not drastically change the morphology of the final nanowires as shown in figure S1.

The effect of OY concentration on the growth of Au NWs was studied following the same procedure. OY was varied from 20 mM to 2000 mM, corresponding to 66 µL and 6.60mL respectively. The volume of hexane was adjusted from 7.85 mL to 0 mL in order to keep a gold concentration of 10 mM.

Characterization

A kinetic study of the growth mechanism of ultrathin gold nanowires coupling ex situ (TEM) and in situ (SAXS) was performed. The 10 mL solution was splitted into a 15 mL-glass vial and a sealed glass capillary tube for the SAXS analysis. Both conditioners were placed at 40 °C during 30 h. Aliquots were taken regularly with time to perform an ex situ TEM study of the evolution of the nanoparticles. To limit artefacts, aliquots were directly deposited, without any dilution or purification, on copper grids. TEM images were obtained with a JEOL-1011F microscope, operating at 100kV.

In situ SAXS measurements were performed on the same solution at 40 °C every 5 minutes for the first 3 hours and every 30 minutes from 3 to 30 h. The experiments were performed at the SWING beamline at the SOLEIL synchrotron using a monochromatic X-ray beam (10 keV). The reaction solution was introduced inside a glass capillary (1.5 mm in diameter) and the SAXS intensity was recorded on a CCD bi-dimensional detector placed inside a vacuum tube. The accessible q-range was between 6×10 -3 and 0.6 Å -1 . For background subtraction, the capillary was measured first filled with the pure solvent (hexane) before introducing the reaction solution (inside the same capillary) to perform the in situ experiment. The SAXS intensities were experimentally normalized in absolute units (cm -1 ) using water as a standard. This allowed quantifying the volume fractions in solution of the spheres and the nanowires from the modeling results (supporting information for details), assuming a known value for the scattering contrast between gold and the solvent of 4420 ± 20 e/nm 3 .

The final Au NWs were characterized by X-ray photoelectron spectroscopy (XPS) and solid state nuclear magnetic resonance (NMR) after a purification process. Briefly, 20 mL of absolute ethanol was added (Technical grade, VWR). Centrifugation at 4000 rpm during 5 min was performed. Au NWs were then redispersed in 5mL hexane and the precipitation repeated twice. X-ray photoelectron spectra were recorded using a Kratos Analytical Limited Axis ultra limited system fitted with a microfocused monochromatic Al Kα X-ray source (1486,6 eV, 12kV , 120W). The pass energy was set at 160 eV and 20 eV for the survey and the regions spectra respectively. Solid state NMR experiment of 1 H at magic angle spinning (MAS) was recorded on a Bruker Advance 400 spectrometer. Sample was packed into a 2.5 mm zirconia rotor and was spun at 24 kHz at room temperature.

Results and Discussion

Synthesis and characterization of Au NWs

We first tried to optimize the experimental condition for the synthesis of ultrathin Au NWs adapted from Xing et al. [START_REF] Feng | Simple and rapid synthesis of ultrathin gold nanowires, their self assembly and application in surface-enhanced Raman scattering[END_REF] We evidenced the strong influence of the OY concentration over the shape and size of the gold NPs. Solutions with [Au] = 10 mM, TIPS/Au = 100 and OY concentration varying from 20 to 2000 mM (OY/Au = 2 to 200) were prepared following the protocol previously described and placed at 40 °C for 24 h. Aliquots were taken after 5 h (Figure S2) and 24 h (Figure 1) directly deposited, without any dilution or purification, on copper grids for TEM characterization. For [OY] = 20 mM, only small spheres exhibiting a diameter ~ 1.5 nm were observed. These spheres self organize on TEM grids, exhibiting an interparticle distance of ~ 2nm.

For [OY] = 50 mM, very short rods were observed after 5 h but evolved towards spheres after 24 h (Figure 1b). Long wires were obtained and remained stable for oleylamine concentration in the range 100-1000 mM. At 2000 mM, the reaction was drastically slow down and after 12 h a white precipitate was observed. This precipitate could be identified as a lamellar phase composed of Au-Cl-oleylamine complexes, as previously described by Yang et coworkers. [START_REF] Huo | Sub-Two Nanometer Single Crystal Au Nanowires[END_REF] After 24 h, Au nanowires were finally obtained concomitantly with ~ 8 nm spheres. Since the increase of oleylamine concentration favours the stabilization of Au NWs but decreases the reaction kinetics, a compromise should be found. In the following experiments, the oleylamine concentration was kept constant at 400 mM, in agreement with Xing et al. [START_REF] Feng | Simple and rapid synthesis of ultrathin gold nanowires, their self assembly and application in surface-enhanced Raman scattering[END_REF] We then follow the evolution of ultrathin gold nanowires with time for [OY] = 400 mM. After 15 min, the TEM images showed two populations of spheres with mean diameter of 1.5 nm and 5 nm, respectively. Short wires exhibiting a mean diameter of ~1.5 nm and a length of ~100 nm were also observed in a smaller proportion (Figure 2a). After 45 min, the proportion of nanowires increased, their diameter remained constant at ~1.5 nm while their length increased to ~500 nm (Figure 2b).

No sign of size evolution could be detected for the spheres. After 3 h of reaction, parallel assemblies of micrometric-long (l ≥ 2 µm) nanowires were mostly observed (Figure 2c). After 27 h of reaction (Figure 2d andS3), micrometric wires were still observed in coexistence with a reduced amount of the two populations of spheres (1.5 nm and 5 nm).

Final gold nanowires, precipitated from the hexane solution by addition of absolute ethanol, were characterized by XPS and solid state NMR. The main peaks observed in the XPS survey scan were Au 4f, C 1s, N 1s and Cl 2p peaks centered at ca. 84, 285, 400 and 198 eV, respectively (Figure S4). The Au 4f7/2 and 4f5/2 high resolution spectra were fitted with only one component corresponding to Au(0) peaks (Figure S5a). The N 1s high resolution spectrum revealed two peaks centered at 399 and 401 eV (Figure S5b) corresponding to the binding energy of N 1s of nitrogen atom in the oleylamine (OY) and in the oleylammonium cation (referred to as OY + ), respectively. [START_REF] Kumar | Investigation into the interaction between surface-bound alkylamines and gold nanoparticles[END_REF] The molar ratio N(OY + )/Cl, determined from the integrated peak areas, was found very close to 1 as expected for an ammonium chloride. The molar ratio N(OY)/N(OY + ) was also close to 1, indicating that the stabilization of purified Au nanowires was insured fairly equally by long chain amine and ammonium. The important presence of ammonium at the surface of Au NWs was confirmed by [START_REF] Kwon | Colloidal Chemical Synthesis and Formation Kinetics of Uniformly Sized Nanocrystals of Metals, Oxides, and Chalcogenides[END_REF] H MAS NMR spectra of pure Au NWs (Figure S6). A signal at 7.7 ppm typical of NH3 + group of OY + could be observed. Thus, the ammonium chloride, resulting from the protonation of oleylamine by the gold precursor HAuCl4, is strongly attached to the gold NWs.

SAXS growth study

We performed a kinetic study of the Au NWs growth obtained at 40°C from a 10mM gold solution with [OY] = 400 mM thanks to in-situ SAXS. The variation of the SAXS intensity during the first 150 minutes is reported in Figure 2a. The successive scattering curves were analyzed using the form factors of a mixture of polydisperse spheres and nanowires, in agreement with the simultaneous exsitu TEM study (Figure 2). The SAXS profiles were nicely fitted (Inset Figure 3a) with the following adjustable parameters: diameter, polydispersity and volume fraction of gold spheres, diameter, polydispersity on the diameter and volume fraction of gold nanowires, and a small constant background contribution (details in supporting information).

At t = 0, the scattering profile fitting indicates only the presence of spheres, with a diameter of 1.7 nm and a large polydispersity (>30%). The larger spheres (5-10 nm) that appeared on the TEM grid (Figure 2a) were not detected with SAXS, revealing that their volume fraction in solution is very low compared to the ones of the small spheres. During the reaction, an additional scattering profile is observed, its intensity increased up to 80 min, following a q -1 law in the low-q region (Figure 3a), indicating the presence of more and more nanowires in solution. The diameter of the wires was found constant at 1.7 nm with a polydispersity on the diameter distribution of only 2%, in good agreement with the TEM observations. The length of the gold nanowires was always too large to be estimated from the SAXS signals during the kinetics, and was fixed to an effective value of 100 nm for all fits, corresponding to the size resolution of the SAXS experimental setup.

The variation of the volume fraction of the 1.7 nm spheres and of the nanowires with time is plotted in Figure 3b. While only few variation of the sphere volume fraction was observed with time, the nanowires were actually detected by SAXS after 20 minutes in good agreement with the TEM observations. At 30 min, the sphere volume fraction reached a maximum and concomitantly, the rate of growth of the nanowires was the highest. Between 80 and 150 min the SAXS intensity hardly varied and was still fitted by a mixture of small spheres and ultrathin nanowires with nearly constant volume fractions. At 150 min these fractions were 5.3×10 -5 and 1.8×10 -5 for the wires and the spheres, respectively. The total volume fraction of 7.1×10 -5 derived from SAXS at 150 min is in good agreement with the 10 -4 value expected from a 10 mmol.L -1 solution, taking into account the overall precision of the calibration procedure of the SAXS intensity. Thus, we can conclude that after only 80 min of reaction i) essentially all Au atoms were contained either in the small spheres or in the nanowires and ii) that the proportion of the wires was large, corresponding to 73% of the initial Au introduced.

A similar kinetic study, coupling TEM (Figure S7) and SAXS (Figure S8), was performed at 25 °C.

A growth following the same trend than at 40 °C was observed. Spheres were already present at t = 0 and their volume fraction remained fairly constant with time. Wires growth was slow down compared to the reaction at 40 °C. After 5h, the reaction was just completed as evidenced by the evolution of their respective volume fraction (Figure S9). Thus, as expected, reaction temperature has a drastic effect on the kinetic of the nanowires growth.

Self-assembly of Au NWs in liquid phase

Whereas no significant difference was detected on the TEM images between 3 and 28 h (Figure 2cd), the SAXS profiles revealed four well resolved Bragg peaks, showing the presence of a longrange positional order between the nanowires after a few hours (Figure 4). The peak positions in the reciprocal space relative to the position of the first peak were 1 : 3 : 4 : 7 characterizing a bidimensional hexagonal positional order in the plane normal to the wires. The lattice parameter of this hexagonal phase inferred from the peak positions was a = 9.7 nm ± 0.1 nm. The estimated domain size for the hexagonal phase derived from the width of the Bragg peaks was of 64 nm after 5 hours and 73 nm after 10 hours. Along with the increase in the intensity at small angles (characterized by a small oscillation near q = 0.015 Å -1 ), this indicated that the hexagonal phase could grow in a morphology of elongated fibers, with a typical diameter of 70 nm. The isotropic SAXS signal on the 2D detector shows that these NW hexagonal organizations are randomly oriented in the capillary, evidencing an homogeneous nucleation of the self-assembly in liquid phase and not at the surface of the capillary glass tube. The experimental SAXS spectra could be completely fitted (for q values greater than 0.02 Å -1 ) by a model curve including the hexagonal phase of nanowires (Bragg peaks) in coexistence with free nanowires in solution (details in supporting information). Interestingly, the form factor of the nanowires was equal for both contributions, with a diameter of d = 1.7 ± 0.05 nm, constant with time, indicating that the same type of nanowires was present in solution and inside the hexagonal phase. Quantitatively, the volume fraction of the free nanowires is found equal to 3.0×10 -5 after 10 h, corresponding to a proportion of 60 % of nanowires, the other 40% being embedded within the hexagonal phase. Since the diameter d of the nanowires in the hexagonal phase was characterized, the interwire distance in the plane normal to the wires could be calculated as (a -d) = 8 nm (Figure 5). The oleylamine chain length being 2.1 nm, [START_REF] Mourdikoudis | Oleylamine in Nanoparticle Synthesis[END_REF] the interwire distance is then approximately 4 times the oleylamine length.

The hexagonal phase could thus be described as a parallel assembly of 1.7 nm gold NWs coated with an oleylamine/oleylammonium bilayer (Figure 5). The hypothesis of an hexagonal phase composed of Au NWs coated with a monolayer of ligands, and the vacancies filled with solvent, was at the present time discarded due to the large interwire distance inferred from TEM (from 5.2 to 6.1 nm, Figure S10), of course this assumption would only be validated by further characterizations, such as Small Angle Neutron Scattering (SANS). [START_REF] Gomez-Grana | Surfactant (Bi)layers on gold nanorods[END_REF] After solvent evaporation, Au NWs coated by a monolayer would indeed lead to interwire distance in the range 2 -4 nm, due to the partial interdigitation of alkyl chain upon solvent evaporation. [START_REF] Ge | Fast surface diffusion of large disk-shaped nanocrystals aggregates[END_REF] The same kinetic study was performed by SAXS at 25°C (Figure S8). The hexagonal phase appeared after 6 h with a very similar lattice parameter, 9.9 ± 0.1 nm, taken into account the modification of the alkyl chain configuration with temperature.

Hexagonal phases of elongated objects in solution like DNA, [START_REF] Livolant | The Highly Concentrated Liquid-Crystalline Phase of DNA is Columnar Hexagonal[END_REF] carbon nanotubes [START_REF] Thess | Crystalline Ropes of Metallic Carbon Nanotubes[END_REF] or viruses, [START_REF] Grelet | Hexagonal Order in Crystalline and Columnar Phases of Hard Rods[END_REF] and self assemblies of inorganic rods and wires were already characterized by SAXS [START_REF] Hamon | Three-dimensional self-assembling of gold nanorods with controlled macroscopic shape and local smectic B order[END_REF] or on by TEM [START_REF] Kang | Size-and shape-selective synthesis of metal nanocrystals and nanowires using CO as a reducing agent[END_REF] but, to our knowledge, this is the first report of a well organized hexagonal phase in solution of metallic nanowires. Interestingly, our in situ study reveals that the nucleation of the hexagonal phase required an incubation time of nearly 3h after the formation of the NWs. In contrast, only a few seconds were needed for the self-assembly into super-lattices of 11.6 nm spherical Au nanoparticles at a similar volume fraction. [START_REF] Abécassis | Gold Nanoparticles Superlattice Crystallization Probed In Situ[END_REF] Such discrepancy can be explained by diffusion and orientation requirements prior to the nanowire organization and thus, a slower dynamics of the nucleation/growth in solution. The formation of nanoparticle super-lattices results from the delicate balance of attractive and repulsive forces. [START_REF] Henzie | Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices[END_REF][START_REF] Goubet | Which forces control supercrystal nucleation in organic media?[END_REF] In most cases, the driving force towards self-assembly is a strong Van der Waals attraction between the particles, which increases with nanoparticle polarizability and diameter. In such superlattices, interdigited ligands, leading to short interparticle distances are generally found. [START_REF] Jiang | Capturing the crystalline phase of two-dimensional nanocrystal superlattices in action[END_REF][START_REF] Lee | Comparison of structural behaviour of nanocrystals in randomly packed films and long-range ordered superlattices by time-resolved small angle X-ray scattering[END_REF] In the case of water suspensions of charged anisotropic nanoparticles with strong electrostatic repulsion, entropic effects are inducing parallel organizations with crystal liquid behaviour (Onsager theory). [START_REF] Lekkerkerker | Liquid crystal phase transitions in suspensions of mineral colloids: new life from old roots[END_REF] Depletion attraction through excess of surfactants can also be at the origin of nanoparticles organizations. [START_REF] Henzie | Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices[END_REF] In the present case, the small diameter and the very large interwire distance insured by the ligand bilayer around each nanowire raise the question of the driving force of such self-assembly.

Considering that the wire radius (R = 0.85 nm) is much smaller than the interaxial distance (a = 9.7 nm) and negligible compared to the wire length (L = 1000 nm), the Van der Waals energy between two parallel Au nanowires can be estimated according to Equation 1 (approximate expression valid for R<<a<<L). [START_REF] Parsegian | Van der Waals forces[END_REF][START_REF] Russel | Colloidal dispersions[END_REF] ( )

4 5 3 8 VdW R A E L a π π = - (Eq. 1)
Due to comparable Hamaker constant of hexane and oleylamine, [START_REF] Israelachvili | Intermolecular and surface forces[END_REF][START_REF] Jia | Couples of colloidal semiconductor nanorods formed by self-limited assembly[END_REF] the classical value of 75 kT usually considered for the Hamaker constant of gold nanoparticules in hexane, [START_REF] Bargeman | Van der Waals forces between immersed particles[END_REF][START_REF] Pansu | Insertion of Gold Nanoparticles in Fluid Mesophases: Size Filtering and Control of Interactions[END_REF] is certainly a good approximation in our system, leading to EVdW = -5.3 kBT. This value has the same order of magnitude than in the case of spherical gold particle superlattices already reported. [START_REF] Goubet | Which forces control supercrystal nucleation in organic media?[END_REF] The steric repulsion appears with ligands interdigitation, thus for interwire distances lower than the full extend of two oleylamine bilayers, i.e. below 4 × 2.1 nm = 8.4 nm. This steric repulsion insured by the bilayer at the surface of Au NWs could be responsible for the stabilisation of the hexagonal superlattices with an interwire distance as large as 8 nm.

Nanowire growth mechanism

Regarding the growth mechanism, we observed that the wire growth, at 40 °C or 25 °C, did not induce a diminution of the number of spheres, as revealed by the time evolution of the volume fraction of each type of particle (Figures 3b andS9). Such observation is in contradiction with a growth mechanism based on oriented attachment of small spheres, where the wire growth would consume the initial 1.7 nm spheres. These small spheres appear mainly as side-product, but the slight decrease of their volume fraction after 30 min at 40 °C, concomitant with the highest nanowire growth rate, may stress their role as seeds. [START_REF] Azulai | Seed concentration control of metal nanowire diameter[END_REF] The unprecedented large interwire distance in the hexagonal superlattice suggests a bilayer stabilisation of the Au nanowires. One can infer that this bilayer stabilisation may be also present at the surface of the free NWs, and not only inside the hexagonal superlattice. The XPS and NMR analysis showed that the stabilisation of purified Au nanowires is insured fairly equally by oleylamine and oleylammonium chloride. Such charged stabilisation reminds the CTAB bilayer, recently characterized by SANS, [START_REF] Gomez-Grana | Surfactant (Bi)layers on gold nanorods[END_REF] at the surface of Au nanorods in water. We have also observed that the shape of the Au nanoparticles was strongly dependent of the oleylamine concentration initially introduced. This concentration effect reminds the phase diagram of surfactants like CTAB in aqueous media [START_REF] Israelachvili | Intermolecular and surface forces[END_REF] and the critical micellar concentration associated with the transition from spherical to cylindrical micelles. All these observations suggest that a micellar growth mechanism drives the one-dimensional growth.

Conclusion

Owing to a in situ SAXS study, we were able to describe precisely for the first time the self assembly of ultrathin Au NWs into a hexagonal super-lattice in apolar solvent. This super-lattice exhibits a very large lattice parameter corresponding to an interwire distance of 8 nm. Such large value can be explained by the presence of a bilayer at the surface of each nanowires, composed of oleylamine and oleylammonium, as revealed by NMR and XPS. Along with the strong influence of the oleylamine concentration on the final shape of the Au nanoparticles, all these results suggest a micellar growth mechanism for the NWs. To have a deeper understanding of the supramolecular organisation of OY and OY + /Cl -, both at the surface of the nanowires and in the second layer, and their impact on the growth of Au nanoparticles, coupled SANS and SAXS studies will be performed, while NMR, periodic-DFT and Monte-Carlo theoretical studies should give new insight on the chemical organisation of these micelles. The unprecedented hexagonal self assembly of micrometric long Au nanowires opens new perspectives for dedicated applications such as catalysis or electronic nanocontact.

  Figures

  Figure 3

Figure 1 :

 1 Figure 1: TEM image of Au NPs obtained after 24h at 40°C for TIPS/Au = 100 and [OY] = a) 20 ; b) 50 ; c) 100 and d) 2000 mM.

Figure 2 :Figure 3 :

 23 Figure 2: TEM images of Au nanoparticles obtained after a) 15 min, b) 45 min, c) 3 h and d) 27 h of reaction at 40 °C with [Au] 10 mM and [OY] = 400 mM Figure 3: (a) Small angle X-ray scattering results measured at 40 °C every 5 min. Inset:Comparison with the experimental data at 150 min and the fitting curve as a sum of nanowires and spheres contributions. The q -1 slope in dashed line is a guide to the eyes; (b) Volume fraction calculated from the SAXS modelling at different times.

Figure 4 :

 4 Figure 4: SAXS data of the hexagonal phase of nanowires at 5h, the fitting curve is a sum of the signal of the hexagonal phase (Bragg peaks) and of free nanowires remaining in solution. Inset: SAXS profile at different times.

Figure 5 :

 5 Figure5: Scheme of the hexagonal phase unit cell (arrows), we considered a 1.7 nm AuNW with an hexagonal cross-section, as suggested by A. Roy et al.[START_REF] Roy | Single crystalline ultrathin gold nanowires: Promising nanoscale interconnects[END_REF] . The double layer around each nanowire is represented with a dashed circle and a possible arrangement of the four OY and/or OY + molecules is shown in the enlarged view. For simplification, only one type of isomers, corresponding to the fully extended molecules, is shown.

Acknowledgments

Patrick Davidson (LPS, Orsay), Bruno Chaudret and Romuald Poteau (LPCNO, Toulouse) are warmly thanked for fruitful discussions. We thank Y. Coppel (LCC, Toulouse) for RMN measurements and G. Antorrena (INA, Saragossa) for XPS measurements. A. Robert is thanks for his help in the experimental section. Jan Skov Pedersen (University of Aahrus, Denmark) is specially thanked for his advices concerning the SAXS modelling. The authors acknowledge the financial support of the Midi-Pyrénées region, of the university of Toulouse, PRES, of the Labex NEXT, N° 11 LABX 075, of the French GDR Or-Nano and of the French network METSA.

Model for the SAXS intensity, XPS survey and high resolution scans and 1 H-MAS NMR spectrum of purified Au NWs. TEM and SAXS characterization of the kinetic study at 25 °C and the corresponding volume fraction evolution with time. This information is available free of charge via the Internet at http://pubs.acs.org/.


MD Agnès Dupret-Bories
email: dupretbories.agnes@iuctoncopole.fr

MD Mathieu Roumiguie

MD Guillaume De Bonnecaze

MD Jérome Sarini

Benjamin Va

MD Sébastien Vergez

Bastien Benbassat

Carres Ponde Nce Agnès

MD Dupret Bories

The super thin external pudendal artery (STEPA) free flap for oropharyngeal reconstruction -A case report

The radial forearm flap is one of the most used micro-anastomotic flaps in cervicofacial reconstruction in a carcinological context. This flap is an ideal in terms of reliability and fineness; it has, however, some disadvantages in terms of the functional and aesthetic complications of its donor site. ln alternative to a radial forearm free flap, we report the

use of the free super thin extemal pudendal artery flap (STEPA flap) for an oropharyn geal reconstruction. The aim was to decrease the donor site morbidity. A 71-years-old man with a T2NOMO oropharyngeal squamous cell carcinoma has undergone surgical treatment. A left STEPA free flap was performed to reconstruct a defect about 8 x 6 cm 2 . This flap was designed as a half-scrotal free flap sized 9 x 7 cm 2 and was inset after tunneling of the pedicle at the floor of the mouth. A surgical revision was needed on the 15th day postoperative for disunion. There was no skin flap failure.

After 12 month of follow-up, no complication was observed at the donor site and no erectile dysfunction was recorded. lts characteristics in terms of fineness, flexibility, ease of conformation, and pedicle length are similar to those of the radial forearm flap with less aesthetic and functional sequelae of the donor site. The STEPA flap may be a promising free flap in oropharyngeal or oral cavity reconstruction.

Oropharyngeal reconstruction requires the use of a reliable free flap with tissue-like qualities to be reconstructed: fineness and flexibility for an ease of struct ure at the receiving site and a compact spatial design.

scarring, the scrotal free flap named super thin extemal pudendal artery (STEPA) flap was recently described and use d for the recon struction of critical areas such as limbs or joint areas at the shoulder or knee [START_REF] Phoon | The super thin extemal pudendal artery (STEPA) flap[END_REF]. The STEPA flap was used for the first time in free form for the reconstruction of a foot defect in 2017 [START_REF] Kiranantawat | The scrotal free flap: First suc cessful clinical application of a free super-th in external Pudendal artery (STEPA) flap for reconstruction of a foot defect[END_REF]. ln this article, we report the use of the STEPA flap for an orophar yngeal reconstruction. Since its description in 1981 by Yang et al. [START_REF] Loeffelbein | Reduction of donor site morbidity of free radial forearm flaps: What level of evi dence is availab le? Ep/asty[END_REF], the free radial forearm flap or Chinese flap (Benateau, Laraba, Alix, & Compere, 2002;[START_REF] Loeffelbein | Reduction of donor site morbidity of free radial forearm flaps: What level of evi dence is availab le? Ep/asty[END_REF] is the most used in cervicofacial reconstruction in a carcinological context due to its fine ness and plasticity. The primar y criticisms are the sacrifice of one of the two major vascular axes of the hand and the functional and aes thetic morbidity caused at the donor site [START_REF] Orlik | Long-term functional donor site morbidity of the free radial forearm flap in head and neck cancer survivors[END_REF]. With equivalent features in terms of fineness and flexibility for minimal The installation was standard for the resection of an oropharyn geal tumor in cervicofacial surgery, with the left lower limb entirely in the operative field so as to be able to position it in slight abduction and release the scrotum to perform the removal of the flap. An arciform incision was performed on the inguinal ligament then the dis section began in the subfascial layer (Figure 1). The first stage con sisted of locating the suprapubic artery at its cross-over with the spermatie cord (Figure 2 • FI GU RE 5 Scarring of the donor site at 6 month. Donor site with a good quality scar which is hidden Problems with localized sensitivity at the donor site and in the sensory area of the radial nerve occur in 7.7 (Meland, Core, Hoverman, Dunet, & Leyder, 1993)-54% [START_REF] Lutz | Donor site morbidity after suprafascial elevation of the radial forearm flap: A prospective study in 95 consecutive cases[END_REF] of cases and up to 82% at 3 months postoperative [START_REF] Chen | Complications of free radial forearm flap transfers for head and neck reconstruction[END_REF]. Functional compli cations such as stiffness of the wrist with decreased joint range or grip strength may occur, respectively, in more than 26.7% of cases [START_REF] Lutz | Donor site morbidity after suprafascial elevation of the radial forearm flap: A prospective study in 95 consecutive cases[END_REF] and in 11-40% of cases [START_REF] Huang | Comparison of the radial forearm flap and the thinned anterolateral thigh cutane ous flap for reconstruction of tongue defects: An evaluation of donor site morbidity[END_REF][START_REF] Timmons | Complications of radial forearm flap donor sites[END_REF]. Patients with a radial forearm flap may also have other complications at the donor site such as wrist or hand edema or an intolerance to cold. Regarding cosmetic results, two studies report patient satisfaction in 94.3% [START_REF] Toschka | Aesthetic and functional results of harvesting radial forearm flap, especially with regard to hand function[END_REF] and 98% [START_REF] Lutz | Donor site morbidity after suprafascial elevation of the radial forearm flap: A prospective study in 95 consecutive cases[END_REF] of cases, others are more mixed with 28.4% [START_REF] Richardson | Radial forearm flap donor-site complications and morbidity: A pro spective study[END_REF], 24% [START_REF] Bootz | Reduction of complication rate at radia l forearm flap donor sites[END_REF], and 16.7% [START_REF] Swanson | The radial forearm flap: Reconstructive applications and donor-site defects in 35 consecutive patients[END_REF] of patients being somewhat dissatisfied to dissatisfied. ln cervicofacial reconstruction, the execution of a radial forearm flap can be carried out simultaneously with the excision of the operative specimen but the proximity of the two sites may constrain surgeons during the ipsilateral lymph node dissection. Another advantage of the STEPA Flap is allowing for more comfortable dou ble team work.

Various studies were carried out in order to simplify closure of the donor site and decrease its morbidity when radial forearm free flap was the procedure of choice. But these flaps designs may involves extensive scars or, as the radial forearm snake free flap, may limits flap length pedicle or skin paddle size [START_REF] Garg | The radial forearm snake flap: A novel approach to oral cavity and oropharyngeal reconstruction that reduces forearm donor site morbidity[END_REF]. The direct closure of the donor site without tension with a hidden scar is one of the main advantages of the STE PA flap.

The primary disadvantages of the STEPA flap are the hairiness of the cutaneous palette, which can hinder the patient during hair growth, as well as the risk of inguinal lymphedernas, estimated at 10% [START_REF] Phoon | The super thin extemal pudendal artery (STEPA) flap[END_REF].

The scrotal flap was widely used in urology reconstruction includ ing hypospadias reconstruction or reconstruction of penile after injury. The rate of scrotal surgery complications was well-described in the literature with reported rate of hematoma and infections in 5% and 3.6% of case [START_REF] Swartz | Complications of scrotal surgery for benign conditions[END_REF]. As in this case, no impairment of erectile function and no impact on testicular func tion is described at this point in the literature [START_REF] Phoon | The super thin extemal pudendal artery (STEPA) flap[END_REF][START_REF] Zhao | The super thin external pudendal artery (STEP A) free flap for oropharyngeal reconstruction -A case report Microsurgery[END_REF]. Although no psychological impact of moving scrotal skin in the mouth was recorded, it needs to be taken into account. Whether hyper-pigmentation could be a problem in facial reconstruction, it is not a disadvantage to be considered in the context of oral cavity reconstruction.

The STEPA flap presents the plastic qualities suitable to cer vicofacial reconstruction. The authors believe it may therefore be an alternative to the radial forearm flap in the consideration of recon struction in a carcinological context, particularly for the oropharynx and oral cavity. However, this is the first STEPA flap used in head and neck surgery. Additional cases are still required to determine the reliability of this flap compared to the radial forearm free flap in particula r.

FINANCIAL DISCLOSURE STATEMENT

None.

ORCID

Agnès Dupret-Bories G https://orcid.org/0000-000 2-7068-3500

2 1 FIGURE 1

 11 FIGURE 1 Initial skin markings. Arciform incision made on the inguinal ligament continuing on the lateral base of penis

  FIGURE 3 Scrotal free flap harvested, sized 9 x 7 cm 2 • Super thin extemal pudendal artery flap (STEPA) free flap after vascular ligature and before tunneling of the pedicule at the floor of the mouth

FIGURE 4

 4 FIGURE 4 Postoperative result. Super thin external pudendal artery flap (STE PA) flap inset in the defect after sutures


Sylvain Chataigner
email: sylvain.chataigner@ifsttar.fr

Laurent Gaillet

Yannick Falaise

Jean-François David

Richard Michel

Christophe Aubagnac

Adrien Houel

Didier Germain

Jean-Philippe Maherault

Acoustic monitoring of a prestressed concrete beam reinforced by adhesively bonded composite

The use of adhesively bonded composite reinforcement is relatively widely used for concrete structures. Yet, some questions remain regarding its use in the case of prestressed concrete structures especially in relation with the influence of existing cracking and the verification of the encountered damage phenomena at real scale. French National Organism CEREMA with the help of French motorway bridge owners association ASFA and French National Research Organism IFSTTAR realized several real size experimental investigations of an old prestressed concrete beam coming from a deconstructed bridge to answer these questions (Project CLERVAL). Both flexure and shear tests up to failure were carried out and several measurement methods were used to understand the role of the composite reinforcement on the behavior of the structure and the damage scenario. Acoustic emission was one of these methods and two different systems were investigated. The proposed communication will first describe the two used acoustic systems and their dedication (localized acoustic emission and overall acoustic survey). A specific development will then be presented aiming at optimizing the obtained acoustic phenomena localization taking into account the anisotropy of the prestressed concrete beam. Finally, main results will then be presented for both flexure and shear tests.

General introduction on "CLERVAL" project

In France, many precast prestressed concrete beam bridges were built in the 1970s. They may suffer from the corrosion of the prestressed cables and from a lack of passive reinforcement that may induce brittle failure. In addition, the traffic increase may require their strengthening. For all these situations, the use of adhesively bonded composite reinforcement represents a good alternative to realize the required reinforcement or to ensure structural safety [START_REF] Afgc | Réparation et renforcement des structures en béton au moyen des matériaux compositesrecommandations provisoires[END_REF][START_REF] Benzarti | Preface for special issue on composite materials and adhesive bonding technology[END_REF] Though many studies have been carried out to check the adequacy of such reinforcement methods, there are still some issues regarding reinforcement of damaged prestressed concrete beams with existing cracks. There are also very few real scale tests, more particularly on aged beams and it was consequently decided to carry out such a test in a project called "CLERVAL". This project was led by CEREMA (National centre for studies and expertise on risks, environment, mobility and urban and country planning) in collaboration with ASFA (French motorway owners association) and IFSTTAR (French Institute of Science and Technology for Transport, Development and Networks.

Presentation of the tested beam

The used prestressed concrete beam comes from a viaduct over the river Doubs at Clerval that was erected between 1952 and 1954 and demolished in 2002 (Figure 1). This beam was 30 m long and 1.3 m high and it had two initial bend cracks with a maximum opening of 0.2 mm (sections S2 and S4). Prestressing cables are not straight and their position is not symmetric (Figure 2).

Fig. 1. Photo of the studied beam

The prestressing system is STUP 12 Phi5. Several investigations took place on the beam before the realization of the test program to identify existing defects, characterize the concrete, check the exact reinforcement and prestressing cables position and amount, assess residual prestress level and prestressing ducts injection quality [START_REF] Houel | Experimental breaking of a prestressed concrete bridge beam reinforced with bonded FRP, Implementing innovative ideas in structural engineering and project management[END_REF]. In order to obtain a maximum of information from the realized investigations, the test program was divided in several steps. A specific test platform was developed so that all the steps may be carried out on the same location (Figure 3). Second steps were realized on third of the beam and dedicated to the study of shear reinforcement in 3-point bending configuration (span: 9.7 m, and force applied at 1.5 m from support). The reinforcement implied the application of adhesively bonded carbon sheets: five 150mm-wide U-sheets with a spacing around 300 mm, and one specific horizontal edge reinforcement for the support area (105 GPa). Due to lack of space and to the geometry, the use of localized meshing for the anchorage was adopted (Figure 5). The experimental program included:  First loading (up to 1100 kN): This phase allowed identifying and localizing crack activation along the beam.  Second loading after additional reinforcement up to failure (up to 1970 kN): this allowed identifying failure process and its localization. Many different monitoring systems were settled on the beam during the different test steps in order to be able to compare modeling or design theories with the actual beam behavior with or without composite reinforcement. More traditional sensors included rotation, temperature, resistive strain, crack opening, and displacement sensors. Less usual systems were also applied as curvature measurement, optical fiber strain measurement, and acoustic techniques. The present article focuses on the results obtained with this last method. Two different acoustic systems were used and are described in the next part.

Description of the used acoustic systems

Two different acoustic systems were used during the campaign. The first is called « overall acoustic survey » and aims at investigating the overall beam behavior. The second one is called « localized acoustic emission » and aims at studying specific areas on the studied structure.

Overall acoustic survey

The overall acoustic system was applied for each test up to failure and aimed at investigating the overall failure process focusing on high energy release phenomena detection. The system is currently used for cable-stayed bridges survey in France [START_REF] Le Cam | A smart and wireless sensors network for cable health monitoring[END_REF] and is commercialized by A3IP (CASC system). It relies on the use of accelerometer sensors with a natural frequency of 22 kHz, and a specific synchronizing system Pegase. The system allows obtaining 1D localization of high energy phenomena mostly encountered at steel wire failures. The detection threshold was fixed at 2G and it was decided that three sensors should detect the phenomenon before obtaining an acoustic event. The maximum distance between sensors was fixed on site at 3.5 m based on preliminary investigations. Seven sensors were used in the case of the flexural test on a single line (Figure 6 a). This allows 1D localization of acoustic events. Ten sensors were used in the case of the shear test along two lines (Figure 6 b). In this last case, an additional work was carried out to dispose of 2D localization of the events as many events occurred close to the edge where prestressing cables are curved (Figure 2). All the sensors were fixed to the beam using a low melting point material.

Localized acoustic emission

The localized acoustic emission system used during this test is a commercialized by MISTRAS and relies on the use of piezoelectric sensors having a natural frequency of 150 kHz. The system allows the synchronization of up to 16 sensors and 2D localization of the events. It is also possible to synchronize the data with an external signal. This was carried out using the applied load in our case. A threshold of 35 dB was used and the speed of acoustic wave was calibrated before each test campaign. Other acoustic parameters were chosen according to experience obtained from previous experimental campaigns in [START_REF] Rossi | Basic creep behavior of concretes investigation of the physical mechanisms by using acoustic emission[END_REF] and [START_REF] Ramadan | Detection of stress corrosion cracking of highstrength steel used in prestressed concrete structures by acoustic emission technique[END_REF]. In the case of the flexure test, three locations were more particularly studied: sections S2 and S4 above areas where cracks were identified prior to the experiment, and section S3/S31 which is located in the center of the beam (Figure 7 a).

In the case of the shear test, the edge zone reinforced with composite and between the applied force and the support was more particularly studied positioning the sensors between the CFRP sheets on one face of the beam and on the bottom flange right under the applied load (Figure 7 b). Similarly to the first system, the sensors were all fixed to the beam using a low melting point material.

Main results obtained during flexure tests

It will not be possible to present here all the results from the flexural test campaign. Yet, several main results from both systems will be highlighted to address the potentiality of these methods and their comparison with more traditional monitoring systems.

Identification of the crack activation using localized acoustic emission

In the case of flexural tests, the localized acoustic emission system was mainly aimed at identifying the crack propagation in pre-determined cracked areas (sections S2 and S4 on figure 7a). Though, from calculation, during flexural test before reinforcement, crack propagation should have occurred first in section S2, it was proved that it actually started in section S4. This was detected both from acoustic emission, crack opening sensor and strain gage measurements. It was checked that the localization of acoustic events (Figure Besides, for the three flexural phases, acoustic emission data was compared with crack opening measures made at the bottom flange (Figure 9). It can be seen that both are not always correlated. Acoustic emission allows following crack propagation, though crack opening measure may not induce crack propagation. Consequently, the slope modification point of crack opening does not always occur at the same point than the start of acoustic emission. Both measurements may thus give complementary information on the state of the structure. It can also be seen that the reinforcement seems to have slightly delayed the onset of crack propagation. Acoustic emission technique is thus a good indicator to check the activity of a crack on-site and to detect the crack tip localization. It may also be used to assess the quality of reinforcement or repair.

Identification of the failure process using overall acoustic survey

The overall acoustic survey with CASC system was carried out during the final test up to failure (Figure 10). It allows detecting main acoustic events and proposing corresponding failure phenomenon depending on the measured parameters (number of sensors detecting the event, wave speed, maximum amplitude). To do this, it was decided to rely on the methodology proposed in [START_REF] Robert | La surveillance acoustique appliquée au Pont Neuf à Foix (Ariège)[END_REF] to determine events corresponding to one prestressing steel wire failure, several prestressing steel wires failure and a bunch of prestressing steel wire failures. Most important events are given in Table 1. The average measured acceleration value is obtained from the average of the maximum amplitude of the three first sensors that detect the event. The average wave speed is determined from the position of these three sensors and the time when the threshold is exceeded. All the 1D localized events (Table 1) with the corresponding energy have been reported on the beam profile on figure 11: -First events were detected from 400 kN to 480 kN in section S4 and in a section where corrosion was detected before the realization of the test. It seems that the encountered failures of prestressing steel wires induced a strong change in the behavior of the beam. This was correlated with crack measurements made in section S4.

-Then, several prestressing steel wires broke before the final failure that occurred at 615 kN closed to section S2 (Figure 10 b) (composite peeling-off). This was correlated with strain measurements made on one of the prestressing cables.

-From this moment, the applied force continued decreasing while the applied mid-span displacement increased. Numerous acoustic events occurred during this last step.

Acoustic survey allowed with the other monitoring devices to determine the failure process. All the detected acoustic events seem in that case to be attributed to prestressing steel wires failures.

Main results obtained during shear tests

It is important to note that the shear test was carried out on one third of the previous beam that had already been submitted to flexural test up to failure. Yet, the test has been realized on a part at the edge that has been little damaged and where no major acoustic event had been detected.

Identification of the damage in the edge zone using localized acoustic emission

The localized acoustic emission system was settled in the reinforced area between the load application and the support. It allowed localizing the acoustic events during the shear test in this area during the whole test up to failure (Figure 12 and 13).

Three main periods were determined studying acoustic emission localization (Figure 14):

-During the first period up to 1700 kN, acoustic events mainly occurred under the load application demonstrating flexure damage rather shear damage. At the end of this period, flexural composite plates peeled-off (Figure 14 a); -From 1700 kN to 1900 kN, most of acoustic events were detected in the corner close to the edge. This corresponds to a crack that was visually detected at the end of the test in this zone (Figure 14 b);

-From 1900 kN to the ultimate capacity (1970 kN), the acoustic emission was again localized under the applied load (Figure 14 c). At failure, a main inclined crack was observed from the applied load to the bottom flange out of the monitored zone using the localized device (Figure 12 and 13). Though, some investigations are still under progress, acoustic emission allowed in that case identifying different failure steps during the test. The results were compared with visual inspection led during and after the test and good match was obtained. No particular acoustic emission could be revealed under the bonded carbon sheets though high mechanical stresses have been measured on them using strain gages. Unexpectedly, the final crack occurred outside of the monitored area using the localized acoustic emission system (Figure 12). In the case of the shear test, two lines of five sensors have been disposed along the beam (Figure 6 b). The system being settled to carry out 1D localization, it was possible to localize the detected events along the beam. Yet, in the case of the shear test, as main failure occurs at the edges where most of prestressing cables are not linear (Figure 2), it was decided to develop the existing system to realize a 2D localization of the events. This was done in [START_REF] Thobie | Traitement des données issues de capteurs acoustiques lors d'un essai à rupture sur une poutre de pont en béton précontraint[END_REF] and relied on a hyperbolic triangulation taking into account the anisotropy of the structure caused by the inclination of prestressing steel wires. The precision of the method was assessed through pre-test calibration investigations to be around 16 cm. Some acoustic events were detected during first shear loading phase. It actually corresponded to a concrete crack propagation induced by a poor mechanical link between the top flange and the rest of the beam. Such a detection seems surprising as it is often too low energy to be detected using such a system. Additional shear reinforcement was applied before the realization of the final shear test to avoid this unexpected damage mode.

The detected events during the final shear test after CFRP reinforcement have been localized on the beam in 2D in Figure 14. A similar work to the one carried out in the case of the flexure test was done here for each of the detected events (Table 2) and adopting the proposed methodology in [START_REF] Robert | La surveillance acoustique appliquée au Pont Neuf à Foix (Ariège)[END_REF] based on the measured parameters (number of sensors detecting the event, wave speed, maximum amplitude).. The events 1, 2, 3, 4 and 7 proved to have too low propagation speed to be attributed to prestressing steel wire failure. They may be attributed to flexural composite peeling-off in flexure or high energy fretting. Events 6, 8, 9 and 10 may be attributed to prestressing steel wire failure. They are all localized in the same area (cable 19 closed to the final crack: Figures 12 and15

Conclusions

Though additional investigations are currently still under progress to validate the made observations or to compare the results obtained from different measurement systems, this article aimed at highlighting the main information gathered from acoustic monitoring during the different mechanical tests that were realized on a real-scale prestressed concrete beam coming from a real bridge and therefore in realistic ageing conditions. Acoustic techniques proved to be complementary with other monitoring techniques. Two different systems were used to get interest in specific locations (localized acoustic emission) or on the overall beam behavior during the test (CASC system).

The localized acoustic emission allowed checking the activation of existing cracks during the different phases of the flexural test and is one of the most adapted techniques for such an issue. It also allowed studying the edge zone during the shear test to propose a damage scheme of the beam.

The CASC system allowed to localize in the case of flexure test in 1D the most energetic events and a specific methodology was used to match the event with a physical phenomenon (mainly prestressing steel wire failure). It allowed proposing a damage process in flexure that matched well with the other measurement. In shear, a 2D localization methodology was developed and allowed identifying the cables where failures occurred. The obtained results are still under study and it was observed that the acoustic events may not be only attributed in that case to prestressing steel wire failures. Some additional investigations are under progress taking into account additional exploitation methodologies proposed in [START_REF] Grosse | Acoustic emission testing[END_REF] and [START_REF] Nair | Acoustic emission monitoring of bridges: review and case studies[END_REF]. In addition, the results have been compared with modeling, and with the results of an autopsy of the beam led by hydro demolition. Good correlation has been determined though more failures were observed during the autopsy as the beam was led up to large damage.

The results may allow determining acoustic parameters that should be used during structure survey. They also allow checking some of the literature criteria regarding damage identification.

Fig. 2 .

 2 Fig. 2. Scheme of the prestressing cables position on one side of the beam

Fig. 3 .

 3 Fig. 3. Test platform used for the whole program

Fig. 4 .

 4 Fig. 4. Scheme of the flexural reinforcement applied on the beam flange

Fig. 5 .

 5 Fig. 5. Scheme of the shear reinforcement applied closed to the loaded end of the beam

Fig. 6 .

 6 Fig. 6. Disposal of CASC sensors along the beam for flexure test (a) and shear test (b)

Fig. 7 .

 7 Fig. 7. Disposal of localized acoustic emission sensors for flexure test (a) and shear test (b)

Fig. 8 .Fig. 9 .

 89 Fig. 8. Comparison of 2D events-localization (a) with visual crack identification at S4 section during initial tests (b)

Fig. 10 .

 10 Fig. 10. Photos of the beam after the flexure test (overall beam: a; detail on failure closed to the load application: b)

Fig. 11 .

 11 Fig. 11. 1D acoustic events along the beam during flexure test

Fig. 12 .

 12 Fig. 12. Photo of the beam after the shear test up to failure

Fig. 14 .

 14 Fig. 14. Localized acoustic emission events during shear test up to failure (carbon sheets and prestressing cables are visualized) before 1 700 kN (a); between 1 700 and 1 900 kN (b) and from 1 900 kN to 1 970 kN corresponding to the ultimate load (c)

Fig. 15 .

 15 Fig. 15. 2D localization of acoustic events during shear test up to failure (whole beam with final crack: a; zoom on the events: b)

Table 1 .

 1 List of most important detected acoustic events using the overall acoustic survey system during the flexure test up to ultimate load

	Event	Load, in kN	Average acceleration value, in g	Average wave speed, in m/s
	1	400	10.7	4033
	2	450	13	3675
	4	460	12.9	3596
	6	470	7.8	5751
	7	-	11.7	5024
	8	480	15.6	5400
	9	-	13.8	5720
	10	-	20	3846
	11	-	7.5	4573
	12	490	9	3519
	13	500	9.7	3596
	14	-	11.5	5688
	15	510	16.6	3503
	16	520	6.4	2154
	17	530	7.6	2325
	19	560	24.3	4454
	20	580	7.5	5740
	21	590	16.3	3895
	22	610	9.3	5140
	23	610	17.9	5339

Table 2 .

 2 List of most important detected acoustic events using the overall acoustic survey system during the shear test

	Event	Load, in kN	Average acceleration value, in g	Average wave speed, in m/s
	1	1570	12	1346
	2	1805	11.3	1949
	3	1791	17.3	1435
	4	1870	6.7	1949
	6	1765	25.5	3085
	7	1725	13.3	2049
	8	1657	19.2	3896
	9	1626	10.3	3806
	10	1597	13.1	6756

The authors wish to acknowledge the support of ASFA (French motorway owners association).