Eva Deronzier
email: eva.deronzier@unilim.fr

T Chartier

P.-M Geffroy

Symmetric and asymmetric membranes based on La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ perovskite with high oxygen semipermeation flux performances and identification of the rate-determining step of oxygen transport

Keywords: Oxygen semipermeation, LSFGa5573 perovskite membrane, BSF55 coating, Oxygen diffusion, Porous layer

This work focuses on identifying the rate-determining step of oxygen transport through La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ membranes with symmetric and asymmetric architectures. The best oxygen semipermeation fluxes are 3.4 10 -3 mol.m -2 .s -1 and 6.3 10 -3 mol.m -2 .s -1 at 900°C for the symmetric membrane and asymmetric membrane with the modified surface. The asymmetric membrane with a modified surface leads to an increase of approximately 7 times the oxygen flux compared to that obtained with the La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ dense membrane without surface modification. This work also shows that the oxygen flux is mainly governed by gaseous oxygen diffusion through the porous support of asymmetric La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ membranes.

Introduction

Mixed ionic and electronic conductors (MIECs) are used in numerous application areas, such as cathode materials for solid oxide fuel cells (SOFCs) or membrane materials for oxygen gas separation. However, dense ceramic membranes are subjected to critical conditions and have to respond to some criteria. They must present both chemical and dimensional stabilities and suitable mechanical properties at 900°C and under low oxygen partial pressure as well as high oxygen semipermeation fluxes [START_REF] Geffroy | Understanding and identifying the oxygen transport mechanisms through a mixed-conductor membrane[END_REF]. Currently, no mixed conductors fulfill all these requirements. For instance, strontium-doped lanthanum ferrites present suitable chemical and dimensional stabilities and mechanical resistance at 900°C [START_REF] Geffroy | Understanding and identifying the oxygen transport mechanisms through a mixed-conductor membrane[END_REF] [START_REF] Lipińska-Chwałek | Strength and elastic modulus of lanthanum strontium cobalt ferrite membrane materials[END_REF], but oxygen semipermeation fluxes through these materials are too low for potential industrial applications (<1 ml.cm -2 .min -1 or 7. 3 10 -3 mol.m -2 .s -1 ) [START_REF] Geffroy | Understanding and identifying the oxygen transport mechanisms through a mixed-conductor membrane[END_REF] [3] [4] [START_REF] Brisotto | High temperature stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ and La0.6Sr0.4Co1-yFeyO3-δ oxygen separation perovskite membranes[END_REF] [START_REF] Deronzier | Oxygen semi-permeation properties of La1-xSrxFeO3-δ perovskite membranes under high oxygen gradient[END_REF]. By contrast, cobalt and strontiumdoped barium ferrites possess low chemical and dimensional stabilities as well as poor mechanical resistance at 900°C [START_REF] Geffroy | Understanding and identifying the oxygen transport mechanisms through a mixed-conductor membrane[END_REF] [START_REF] Chanda | Micro-and macroindentation behaviour of Ba0.5Sr0.5Co0.8Fe0.2O3-d perovskite[END_REF], but the highest oxygen semipermeation fluxes are obtained for these materials (> 1 ml.cm -2 .min -1 or 7. 3 10 -3 mol.m -2 .s -1 ) [START_REF] Klande | Effect of microstructure on oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3-δ and SrCo0.8Fe0.2O3-δ membranes[END_REF] [START_REF] Wang | Investigation of phase structure, sintering, and permeability of perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes[END_REF] [5] [START_REF] Hong | Oxygen permeation of BSCF membrane with varying thickness and surface coating[END_REF] [START_REF] Wang | Oxygen permeation study in a tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ oxygen permeable membrane[END_REF].

The strategy developed is to associate different materials with satisfying all criteria in specific membrane architectures. Membranes with surface coatings (or symmetric membranes) consist of a thick dense membrane and thin porous coatings on both membrane surface to improve surface exchanges. Studies [START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF] [START_REF] Lee | Enhancement of oxygen permeation by La0.6Sr0.4CoO3-δ coating in La0.7Sr0.3Ga0.6Fe0.4O3-δ membrane[END_REF] reported fluxes obtained on symmetric membranes at 900°C ,2-3 times higher than those obtained on dense membrane without surface modification. Asymetric membranes consist [START_REF] Rachadel | Influence of porous structures on O2 flux of BSCF asymmetric membranes[END_REF] [15] [START_REF] Li | Oxygen permeability and phase stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ perovskite at intermediate temperatures[END_REF] [START_REF] Jin | Preparation of an asymmetric perovskite-type membrane and its oxygen permeability[END_REF] [START_REF] Betz | Supported Oxygen Transport Membranes for Oxyfuel Power Plants[END_REF] of a thin dense membrane with a thick porous support . Lee et al. [START_REF] Lee | Enhancement of oxygen permeation by La0.6Sr0.4CoO3-δ coating in La0.7Sr0.3Ga0.6Fe0.4O3-δ membrane[END_REF] studied a membrane architecture composed of a thick dense membrane and thin porous layers on both membrane sides and achieved an oxygen flux through the symmetric membrane 16 times higher than the dense membrane without porous layers at 900°C under an air/He gradient.

Beside, dual-phase membranes [START_REF] Cao | Asymmetric dual-phase membranes prepared via tape-casting and co-lamination for oxygen permeation[END_REF] [START_REF] Guo | Highly oxygen-permeable and CO2stable Ce0.8Sm0.2O2-δ-SrCo0.9Nb0.1O3-δ dual-phase membrane for oxygen separation[END_REF] [START_REF] Partovi | Enhanced oxygen permeability of novel Cu-containing CO2-tolerant dual-phase membranes[END_REF] are composed of two materials, an ionic conductor and an electronic or mixed conductor. The ionic conductor used has a fluorine-type structure, very stable material compared to perovskite materials with cobalt. The connected and uniform distribution of these phases is a prerequisite to get a high oxygen semipermeation flux through dual-phase membranes [START_REF] Bi | A novel CO2-resistant ceramic dual-phase hollow fiber membrane for oxygen separation[END_REF]. The microstructure allows two conducting paths for the ions and the electrons. However, dual-phase membranes have been specifically developed to be stable in oxyfuel operating condition where the flux has a lower priority relative to the phase stability [START_REF] Li | Rate determining step in SDC-SSAF dual-phase oxygen permeation membrane[END_REF]. Oxygen semipermeation flux of 1.54 ml.cm -2 .min -1 (1.13 10 -2 mol.m -2 .s -1 ) has been obtained through a 60SDC-40SCN (60wt% Ce 0.8 Sm 0.2 O 2-δ -40wt% SrCo 0.9 Nb 0.1 O 3-δ ) dual-phase membrane at 950°C under an air/He gradient (feed flow rate 120 ml/min, sweep flow rate 60 ml/min) [START_REF] Guo | Highly oxygen-permeable and CO2stable Ce0.8Sm0.2O2-δ-SrCo0.9Nb0.1O3-δ dual-phase membrane for oxygen separation[END_REF]. Some authors [START_REF] He | Oxygen permeation modeling for Zr0.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ asymmetric membrane made by phase-inversion[END_REF] [25] [START_REF] Li | Sandwich-like symmetric dual-phase composite membrane with an ultra-thin oxygen separation layer and excellent durability[END_REF] [START_REF] Cao | Asymmetric dual-phase membranes prepared via tape-casting and co-lamination for oxygen permeation[END_REF] combine two types of membrane architectures to get higher oxygen semipermeation fluxes. Zhang et al. [START_REF] Zhang | Oxygen permeation properties of supported planar Zr0.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ composite membranes[END_REF] investigated a 25µm-thick dense YSZ-LSCrF (Zr 0.84 Y 0.16 O 1.92 -La 0.8 Sr 0.2 Cr 0.5 Fe 0.5 O 3-δ ) dual-phase membrane supported by a porous support and reported a high flux of 1.58 ml.cm -2 .min -1 (1.2 10 -2 mol.m -2 .s -1 ) at 900°C under an air/CO gradient (feed flow rate 100 ml/min, sweep flow rate 35 ml/min). These authors also developed another asymmetric membrane with surface modification and obtained a higher oxygen semipermeation flux of 2.51 ml.cm -2 .min -1 (1.8 10 -2 mol.m -2 .s -1 ) in the same operating conditions. Li et al. [START_REF] Li | Sandwich-like symmetric dual-phase composite membrane with an ultra-thin oxygen separation layer and excellent durability[END_REF] also reported a high oxygen flux of 1.95 ml.cm -2 .min -1 (1.4 10 -2 mol.m -2 .s -1 ) through a sandwich-like symmetric YSZ-LSCrF dual-phase at 900°C under an air/CO gradient (feed flow rate 100 ml/min, sweep flow rate 40 ml/min).

In this work, three membrane designs based on the La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ (denoted LSFGa5573) material, which corresponds to the best compromise between high oxygen semipermeation fluxes and good chemical stability, were studied. A previous study [START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF] showed that surface exchange kinetics mainly govern oxygen semipermeation flux through dense LSFGa5573 membranes. Thus, the first configuration consists of a symmetric membrane with coatings added on both membrane surfaces to increase oxygen surface exchange kinetics. Two other architectures elaborated are asymmetric membranes consisting of a thin membrane layer reinforced by a porous support. The electrochemical performances of symmetric and asymmetric membranes have been studied thanks to a specific oxygen semipermeation method.

Although asymmetric membranes often show high semipermeation performances [START_REF] Gurauskis | Oxygen permeation in symmetric and asymmetric La0.2Sr0.8Fe0.8Ta0.2O3-δ membranes[END_REF] [START_REF] Serra | Oxygen permeation through tape-cast asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3-δ membranes[END_REF] [29] [START_REF] Van Der Haar | Mixed-conducting perovskite membranes for oxygen separation[END_REF], the rate-determining step of oxygen transport through the membrane is not clearly identified in the literature.

The novelty of this study is to accurately identify the rate-determining step of oxygen transport in relation with the membrane architecture based on stable cobalt-free materials in operating conditions, such as LSFGa5573 and Ba 0.5 Sr 0.5 FeO 3-δ (denoted BSF55) materials. The identification of this rate-determining step allows a better understanding of oxygen transport through the asymmetric membranes in relation with operating conditions and to define specific architectures of membranes to reach improved oxygen fluxes. This work is also based on the develop of novel asymmetric membranes based on LSFGa5573 material which shows high oxygen semi-permeation performance, good chemical stability and high mechanical properties.

Experimental

Figure 1 shows the three membrane architectures studied to improve the oxygen semipermeation fluxes. In addition, this work aims to identify the rate-determining step of oxygen transport concerning these three membrane architectures.

The symmetric membrane consists of a dense La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ membrane (1 mm thick) coated with Ba 0.5 Sr 0.5 FeO 3-δ perovskite on both sides (Figure 1

Elaboration of symmetric membranes

The elaboration of symmetric membranes (Figure 1 a) consists of elaborating dense La0.5Sr0.5Fe0.7Ga0.3O3-δ membranes Ba 0.5 Sr 0.5 FeO 3-δ coatings on both membrane surfaces.

Dense LSFGa5573 membrane

LSFGa5573 perovskite powder was synthesized by the solid-state reaction route from pure oxide or carbonate precursors, i.e., La 2 O 3 (99.99%, Alfa Aesar), Fe 2 O 3 (98%, Alfa Aesar), Ga 2 O 3 (99.999%, Alfa Aesar), and SrCO 3 (99.9%, Sigma-Aldrich). The synthesis was described in a previous study [START_REF] Guironnet | The surface roughness effect on electrochemical properties of La0.5Sr0.5Fe0.7Ga0.3O3-δ perovskite for oxygen transport membranes[END_REF].

The LSFGa5573 membranes were manufactured by the tape-casting process, published earlier [31] [32]. The green membranes were debinded and sintered at 1380°C for 4 h to obtain a relative density higher than 95% (Archimedes' method).

Electrophoretic coatings on LSFGa5573 dense membranes

The Ba 0.5 Sr 0.5 FeO 3-δ (BSF55) perovskite powder was synthesized by the citrate method from pure nitrate precursors, i.e., Ba(NO 3 ) 2 (99+%, Alfa Aesar), Sr(NO 3 ) 2 (99.0% min, Alfa Aesar) and Fe(NO 3 ) 3 .9H 2 O (98+%, Alfa Aesar). The synthesis was described in a previous study [START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF].

The preparation of the BSF55 suspension for electrophoretic coatings was detailed earlier [START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF]. The setup of electrophoretic deposition is shown in Figure SI in Supplementary information section. This setup is composed of a generator (Keithley 2400) and a beaker of 150 ml, which contains the suspension. Two electrodes were dropped in the suspension: a copper electrode and crocodile plier cathodic membrane. The distance between these two electrodes was fixed to 2 cm. A voltage of 20 V was applied, and the deposition time was 1 min. The coatings were carried out under magnetic stirring to prevent the eventual sedimentation of the suspension. After deposition, coatings were dried at room temperature for 48 h. Then, coated membranes were calcined at 1000°C for 1 h at a rate of 1°C/min to remove organic additives and consolidate the coating. Coated membranes were observed using scanning electron microscopy (JEOL IT 300 LV).

Elaboration of asymmetric membranes

Two architectures of asymmetric membranes were produced in this work, as reported in Figure 1 b) and c). The first type of asymmetric membrane architecture comprises thick porous support and a thin dense membrane (asymmetric membrane without thin porous layer: type 1). The second type of asymmetric membrane architecture comprises thick porous support in contact with the oxygen-rich side, a thin dense membrane, and a thin porous layer in contact with the oxygen lean side (asymmetric membrane with thin porous layer: type 2).

Asymmetric membranes were obtained by a tape-casting process. Two suspensions (including/excluding wood flour) used as pore-forming agents were prepared from LSFGa5573 powder synthesized by a solid-state reaction route. The procedure to obtain suspension and green tapes was published earlier [31] [33]. Porous and dense discs are stacked and laminated under a pressure of 50 MPa at 70°C. The green membranes were debinded and sintered at 1380°C for 4 h. Asymmetric membranes were observed using scanning electron microscopy (JEOL IT 300 LV).

Characterizations of powders and membranes

X-ray diffraction patterns, were carried out at room temperature under air on calcined powders with a D8 advance diffractometer with CuKα1. The patterns were recorded in the 10-70° 2θ range with a step of 0.02° and exposure time of 1.1 s per step.

After debinding and sintering, the membrane microstructures were observed using scanning electron microscopy (JEOL IT 300 LV). The tape-casting process allows to obtain a good surface finish, so the membranes surface has not been polished. The grain and pore size and the porosity of sintered membranes were evaluated from observations of membranes surface and cross-section by scanning electron microscopy (SEM).

The oxygen semipermeation flux was measured using a homemade setup described in previous studies [34] [1]. A membrane architecture (1 mm thick and 24 mm diameter after sintering) was sealed between two alumina tubes with gold O-rings to obtain an airtight system. The membrane architecture was submitted to an oxygen partial pressure gradient. The membrane architecture's feed side was flushed with an air (pO 2 =0,21 atm) flow of 100 ml/min, and the permeate side was flushed with an argon (pO 2 =10 -4 -10 -5 atm) flow of 200 ml/min. Measurements were performed between 400°C and 950°C.

Results and discussion

3.1 Symmetric membrane samples

DRX and SEM characterizations

The LSFGa5573 powder has a main perovskite phase and two secondary phases, La 2 O 3 and SrLaGa 3 O 7 . The BSF55 powder shows a pure perovskite phase. Diffraction patterns are presented in figure SII in Supplementary information.

The architecture obtained by tape casting and electrophoretic deposition given in Figure 2 shows, on a dense LSGa5573 membrane, a homogeneous BSF55 coating with a thickness of approximately 15 µm (Figure 2 c)) and composed of grains smaller than 2 µm (Figure 2 a) and b)). mol.m -2 .s -1 at 900°C, is four times higher than that obtained from the dense LSFGa5573 membrane (Table 1).

Furthermore, the oxygen flux obtained from the symmetric membrane in this work is only 1.3 times higher than that obtained by Guironnet et al. [START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF] for a symmetric LSFGa5573 membrane with a La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ coating and almost 3 times higher than that obtained by Reichmann et al. [START_REF] Reichmann | Impact of microstructure on oxygen semi-permeation performance of perovskite membranes: Understanding of oxygen transport mechanisms[END_REF] for a symmetric membrane with a La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ coating (Table 1). Indeed, the coefficient of surface exchange of coating materials for symmetric membranes has a low influence on the oxygen flux because the oxygen flux is primarily governed by oxygen diffusion through the dense layer in symmetric membranes. Lee et al. [START_REF] Lee | Enhancement of oxygen permeation by La0.6Sr0.4CoO3-δ coating in La0.7Sr0.3Ga0.6Fe0.4O3-δ membrane[END_REF] also reported that the flux through a symmetric La 0.7 Sr 0.3 Fe 0.6 Ga 0.4 O 3-δ membrane with a La 0.6 Sr 0.4 CoO 3-δ coating is three time higher than for the dense La 0.7 Sr 0.3 Fe 0.6 Ga 0.4 O 3-δ membrane.

In addition, Pan et al. [START_REF] Pan | Improvement of oxygen permeation in perovskite hollow fibre membranes by the enhanced surface exchange kinetics[END_REF] reported that the oxygen flux obtained through the membrane architecture with a 10 µm-thick LSFCo6482 coating is 2 times higher than that obtained from dense hollow fibre without coating and 3 times higher for a Ba 0.5 Sr 0.5 Fe 0.2 Co 0.8 O 3-δ (BSFCo5528) coating. Thus, barium-based materials allow the enhancement of even more kinetics of surface exchange than lanthanum-based materials. Figure 3 b) shows the Arrhenius plot of oxygen semipermeation flux versus 1/T (T: absolute temperature in K) for the symmetric membrane. The Arrhenius plot's slope gives the activation energy (Ea) corresponding to the rate-determining step (rds) of oxygen transport through the membrane. A variation in the slope of the Arrhenius plot is observed between 750°C and 800°C. Two activation energies of rds can be determined: one at high temperature (>800°C) and one at low temperature (<750°C). The lower activation energy at high temperature (>800°C) for the symmetric membrane in this work is close to that obtained from the membrane architecture reported by Guironnet et al. [START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF] (Table 1). At 900°C, the oxygen chemical potential profile through the LSFGa5573/BSF55 symmetric membrane (Figure 4) presents a large gradient of oxygen chemical potential in the membrane bulk, suggesting that the oxygen semipermeation flux is mainly governed by mechanisms of oxygen bulk diffusion, as expected.

At low temperatures (<750°C), the activation energy is slightly higher, corresponding to a slight evolution towards a mixed regimen when the temperature decreases. However, this value of activation energy at low temperatures is still close to that obtained at high temperatures, suggesting no significant evolution of the rate-determining step over the full temperature range studied in this work (550-950°C).

In conclusion, the oxygen flux through the dense LSFGa5573 membrane without coating is mainly governed by surface exchange mechanisms. The BSF coatings on the dense LSFGa5573 membrane led to a significant increase in oxygen surface exchange kinetics. Thus, the oxygen flux through the symmetric membrane becomes mainly governed by bulk oxygen diffusion through the membrane (Figure 4).

Asymmetric membrane samples

Oxygen semipermeation fluxes through the symmetric membrane architecture are mainly governed by oxygen bulk diffusion through the membrane, assuming the decrease in the dense membrane thickness leads to an increase in the oxygen fluxes. Thus, asymmetric membrane architectures have been produced from a thin dense layer and thick porous support (Figure 1 b) and c)) to ensure the membrane's mechanical resistance.

SEM characterizations

The porous layers of the two asymmetric membrane architectures were obtained using wood flour as a pore-forming agent, presenting acicular grains. The grain length varies between 10 and 300 µm (figure SIII in Supplementary information section). After debinding and sintering of the membranes, the thick porous support and the thin porous layer show pores of a few micrometers with an acicular shape and a porosity of approximately 25-30%, which is interconnected and homogeneous in the whole bulk of the thick support or the thin layer (Figure 5). Table 2 shows membrane characteristics: the thickness of porous and dense layers and the acronyms used in this work.

Electrochemical performances through asymmetric membranes

The porous support of asymmetric membranes of flux type 1 (without a thin porous layer) can contact the oxygen-rich or lean sides following the configuration of gas circulation in the setup. Figure 6 a shows the oxygen semipermeation fluxes through asymmetric membranes of type 1 as a function of the temperature and for both configurations (the porous support in contact with oxygen-rich or lean sides). In both cases, oxygen fluxes are similar at 900°C and approximately 2 times higher than those obtained through the dense LSFGa5573 membrane (1 mm thick) without porous layers (Table 3). Jeon et al. [START_REF] Jeon | Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3-δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3-δ layers[END_REF], Vente et al. [START_REF] Vente | Performance of functional perovskite membranes for oxygen production[END_REF] and Büchler et al. [START_REF] Büchler | Preparation and properties of thin La1-xSrxCo1-yFeyO3-δ perovskitic membranes supported on tailored ceramic substrates[END_REF] [START_REF] Vente | Performance of functional perovskite membranes for oxygen production[END_REF] also studied the oxygen semipermeation fluxes of asymmetric La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ membranes with a thin porous layer in contact with oxygen rich or lean side, and they found that fluxes are similar in the two cases, as shown in this work.

The thickness of the dense layer is different between the dense thick membrane and the asymmetric membrane. Some authors [START_REF] Li | Rate determining step in SDC-SSAF dual-phase oxygen permeation membrane[END_REF] [38] [START_REF] Zhang | Oxygen permeation properties of supported planar Zr0.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ composite membranes[END_REF] studied the influence of the dense layer thickness on oxygen semipermeation fluxes through the membrane. A characteristic thickness L c , was introduced by Bouwmeester et al. [START_REF] Bouwmeester | Dense ceramic membranes for methane conversion[END_REF], as a criteria to identify the rate-determining step of oxygen transport through the membrane material. If the dense membrane has a low thickness (10 L < L C ), the flux is considered governed only by surface exchanges, whereas if the dense membrane has a high thickness (L > 10 L C ), the flux is considered governed only by oxygen bulk diffusion.

Nevertheless, the oxygen flux is not inversely proportional to the dense membrane thickness, as expected from the usual Wagner model. The dense layer in the asymmetric membranes (type 1) is five times thinner than the dense membrane without surface modification. Conversely, the oxygen flux through the asymmetric membranes (type 1) is only approximately two times higher than that obtained through the dense LSFGa5573 membrane, suggesting the oxygen flux through asymmetric membranes (type 1) is not only governed by oxygen diffusion, but mainly by surface exchanges. Thus, the thickness of the thick dense membrane is most likely lower than the characteristic thickness L c defined by Bouwmeester et al. [START_REF] Bouwmeester | Dense ceramic membranes for methane conversion[END_REF].

Jeon et al. [START_REF] Jeon | Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3-δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3-δ layers[END_REF], Gurauskis et al. [START_REF] Gurauskis | Oxygen permeation in symmetric and asymmetric La0.2Sr0.8Fe0.8Ta0.2O3-δ membranes[END_REF], Serra et al. [START_REF] Serra | Oxygen permeation through tape-cast asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3-δ membranes[END_REF], Garcia-Torregrosa et al. [START_REF] García-Torregrosa | Development of CO2 Protective Layers by Spray Pyrolysis for Ceramic Oxygen Transport Membranes[END_REF], and Van der Haar [START_REF] Van Der Haar | Mixed-conducting perovskite membranes for oxygen separation[END_REF] reported similar conclusions when the porous support was in contact with the oxygen rich side. Indeed, these authors reported that asymmetric membranes (type 1) show an oxygen flux only 2 to 7 times higher than that obtained for the dense membrane without porous layers (Table 3), while the dense layer in the asymmetric membrane (type 1) is approximately 25 times thinner than the dense membrane without a modification surface.

Moreover, for the asymmetric membranes (type 1), two membranes with a dense layer of 170-230µm and 450-500 µm in thickness, have been characterized with the porous support in contact with air. However, the fluxes through these membranes are similar until 900°C and there is a slight discrepancy between oxygen fluxes above to 900°C (see figure S1). The dense layer thickness of the membrane has then not a significant impact on the oxygen flux through the asymmetric membrane of type 1. This result confirms that the oxygen semipermeation fluxes through dense LSFGa5573 membranes and asymmetric LSFGa5573 membranes of type 1 are mainly governed by oxygen surface exchanges.

) gradient, b) Arrhenius plots of oxygen fluxes versus 1/T for asymmetric membranes Table 3: Activation energies and oxygen semipermeation fluxes through asymmetric membranes of type 1 and dense membranes at 900°C

The activation energies (E a ) of the mechanisms limiting oxygen transport through the membrane architectures at high and low temperatures, determined from Arrhenius plots in Figure 6 b), are reported in Tables 3 and4. Two ranges of activation energies, one 3 and4). Indeed, if the oxygen flux is regulated by bulk oxygen diffusion through the membrane, then the activation energy range is between approximately 40 and 75 kJ.mol -1 ; if the oxygen flux is essentially directed by surface exchanges between the gas and the membrane, then the activation energy is between approximately 130 and 180 kJ.mol -1 . Thus, the membrane material's chemical composition does not significantly influence the activation energies of oxygen transport through the membrane, particularly when the oxygen flux is controlled by bulk oxygen diffusion.

In the case of the "PS-air" configuration, only one activation energy was found over this study's full temperature range, suggesting the same rate-determining step in this temperature range. The activation energy is close to that obtained for the dense LSFGa5573 membrane (Table 3) and corresponds to surface exchange mechanisms. Thus, when the porous support is in contact with the oxygen-rich side, the oxygen flux through our asymmetric membranes of type 1 is mainly governed by surface exchanges at the oxygen lean side. Data reported in the literature assume that a decrease in the dense membrane thickness leads to an increase in the oxygen flux and a decrease in the activation energy at high temperature associated with a progressive evolution of the rate-determining step from oxygen surface exchanges to oxygen bulk diffusion (Table 3).

For the "PS-Ar" configuration, two activation energies were observed from the Arrhenius plot, one at high temperature (>860°C) and another at low temperature (<860°C). However, the values of the two activation energies remain close. Thus, there is likely no change in the rate-determining step over the full temperature range in this study. The activation energy at a high temperature of approximately 80 kJ.mol-1 is lower than that obtained for the PS-air architecture and the dense LSFGa5573 membrane. Thus, the porous support allows for a slight increase in the kinetics of exchanges between the gas and the membrane surface, particularly at the oxygen lean side (which is often the limiting step of oxygen transport through the membrane). However, when the porous support is in contact with the oxygen lean side, the gaseous oxygen diffusion through the porous support could also correspond to the rate-determining step mainly for a sizeable porous support thickness. For instance, Guironnet et al. [START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF] show that a thick porous coating at the surface membrane (up to 100 µm) leads to decreased oxygen semipermeation flux through the membrane. A decrease in the porous layer thickness could then be beneficial to improve oxygen flux and gaseous oxygen diffusion through the porous layer.

In conclusion, when the porous support is in contact with the oxygen-rich side, the oxygen fluxes through the asymmetric membrane of type 1 are likely also governed by surface exchanges at the oxygen lean side (typically with high activation energy: 130-180 kJ.mol -1 ). However, when the porous support is in contact with the oxygen lean side, the oxygen fluxes through the asymmetric membrane (type 1) are governed by another kind of rate-determining step with lower activation energy, approximately 100 kJ.mol -1 (Figure 6 b) and Table 3).

Oxygen semipermeation fluxes through asymmetric membranes of type 1 are mainly governed by oxygen gaseous diffusion through the thick porous support when the support is in contact with the oxygen lean side. Thus, the porous layer thickness in contact with the oxygen lean side has to be decreased to improve the oxygen flux. The second type of asymmetric membrane architecture was then developed from a thick porous support, a thin dense membrane, and a thin porous layer (Figure 1 c). The characteristics of this type 2 asymmetric membrane with a thin porous layer is displayed in Table 2.

Two configurations are possible; either the porous support is in contact with the oxygen rich side and the thin porous layer with the oxygen lean side (configuration A), or the porous support is in contact with the oxygen lean side and the thin porous layer with the oxygen rich side (configuration B). Only the electrochemical performances of the configuration A have been presented in this work. Indeed, the configuration B leads to the rupture of the dense layer during the measurements. This is due to the large chemical expansion of the thin porous layer or the porous support, when it is in contact with the atmosphere with low oxygen partial pressure (or the oxygen lean side). Then, in case of the configuration A, there is not the rupture of thin dense layer solicited in compression. In the case of the configuration B, the thin dense layer is submitted to tensile stresses its fracture.

The oxygen semipermeation flux through the asymmetric membrane of type 2 is shown in Figure 6 a). The flux of 6.3 10 -3 mol.m -2 .s -1 reached at 900°C, is approximately three times higher than the one of the asymmetric membrane of type 1 and almost seven times higher than the one obtained from a dense LSFGa5573 membrane without porous layers (Table 4). However, the data in the literature do not show the same results. Gurauskis et al. [START_REF] Gurauskis | Oxygen permeation in symmetric and asymmetric La0.2Sr0.8Fe0.8Ta0.2O3-δ membranes[END_REF], Jeon et al. [START_REF] Jeon | Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3-δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3-δ layers[END_REF] and Watanabe et al. [START_REF] Watanabe | Preparation of oxygen evolution layer/La0.6Ca0.4CoO3 dense membrane/porous support asymmetric structure for high-performance oxygen permeation[END_REF] reported that an asymmetric membrane with a thin porous layer (type 2) shows an oxygen flux only slightly higher (approximately 1.2-2 times) than that obtained through the asymmetric membrane of type 1 for LSFTa2882, CaTi 0.8 Fe 0.2 O 3-δ and La 0.6 Ca 0.4 CoO 3-δ materials (Tables 3 and4). Figueiredo et al. [START_REF] Figueiredo | Surface enhanced oxygen permeation in CaTi1-xFexO3-δ ceramic membranes[END_REF] also observed that the oxygen flux through an asymmetric membrane of type 2 developed from the LSFCo1928 material is only two times higher than that obtained through the dense LSFCo1928 membrane without porous layers, suggesting the materials studied by these authors have high coefficients of surface exchange compared to the LSFGa5573 material. Furthermore, fluxes reported by Jeon et al. [START_REF] Jeon | Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3-δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3-δ layers[END_REF] are very high (>1.10 -2 mol.m -2 .s -1 ), which is surprising for this kind of material. 4). There is a change of limiting step, approximately 790°C. At high temperatures, the activation energy of the PS-PL architecture of 39 kJ.mol -1 is much lower than that of the dense LSFGa5573 membrane without porous layers and similar to the activation energy reported by Gurauskis et al. [START_REF] Gurauskis | Oxygen permeation in symmetric and asymmetric La0.2Sr0.8Fe0.8Ta0.2O3-δ membranes[END_REF] for a similar membrane architecture developed from the LSFTa2882 material.

Thus, a decrease in the porous layer's thickness in contact with the oxygen lean side allows a significant increase in oxygen gaseous diffusion through the porous layer.

Rate-determining step of oxygen transport through symmetric and asymmetric membranes

The first architecture of the membrane (symmetric membrane), composed of a dense LSFGa5573 membrane (1 mm) and thin BSF55 coatings on both sides, shows high oxygen semipermeation fluxes compared to the dense LSFGa5573 membrane without surface modification. Oxygen fluxes through the symmetric membrane are governed by oxygen bulk diffusion at high temperature (>860°C).

The second architecture (asymmetric membrane of type improve oxygen fluxes through the membrane if the membrane material has high oxygen bulk diffusion coefficients, such as the LSFGa5573 material. This trend also suggests that, in the case of an asymmetric membrane of type 1, gaseous oxygen diffusion through thick porous support in contact with the oxygen lean side can also be the rate-determining step of oxygen transport.

The last membrane architecture (asymmetric membrane of type 2) comprises thick porous support developed from wood flour in contact with the oxygen-rich side, a thin dense layer, and a thin porous layer produced from wood flour in contact with the oxygen lean side. This asymmetric membrane with a thin porous layer shows very high oxygen semipermeation fluxes compared to the dense LSFGa5573 membrane without surface modification. Oxygen fluxes through this asymmetric membrane are then governed by oxygen bulk diffusion at high temperature (>790°C). Thus, a thin porous layer in contact with the oxygen lean side allows significant enhancement of oxygen gaseous diffusion through the porous layer and kinetics of exchanges between the gas and the membrane surface if the membrane material shows low coefficients of surface exchanges, such as the LSFGa5573 material. Table 5 summarizes the main results of this work.

Figure 7 shows oxygen semipermeation fluxes through these different membrane architectures, which are compared with fluxes obtained through 1 mm-thick dense LSFGa5573 [START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF] [31] and Ba 0.5 Sr 0.5 Fe 0.2 Co 0.8 O 3-δ (BSFCo5528) membranes [START_REF] Reichmann | Impact of microstructure on oxygen semi-permeation performance of perovskite membranes: Understanding of oxygen transport mechanisms[END_REF]. The oxygen flux obtained for the asymmetric membrane of type 2 (with a thin porous layer) is close to that obtained for the dense BSFCo5528 membrane [START_REF] Reichmann | Impact of microstructure on oxygen semi-permeation performance of perovskite membranes: Understanding of oxygen transport mechanisms[END_REF] under the same working conditions and slightly lower than the oxygen flux through the 1 mm-thick dense BSFCo5528 membrane obtained in the literature at 900°C with a grain size of approximately 10-15 µm [START_REF] Klande | Effect of microstructure on oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3-δ and SrCo0.8Fe0.2O3-δ membranes[END_REF].

Conclusion

Three membrane architectures were developed from LSFGa5573 and BSF55 materials and characterized by a semipermeation method. LSFGa5573 material was used for the dense layer concerning symmetric membranes because this material has high oxygen bulk diffusion coefficients and high mechanical properties. The BSF55 material was used for coatings because it shows high oxygen surface exchange coefficients but low mechanical properties. Concerning asymmetric membranes, the thin dense LSFGa5573 layer was supported by a thick porous layer of the same material. The other surface of the thin dense LSFGa5573 layer was coated or not by a thin porous LSFGa5573 layer. The rate-determining step of oxygen transport depends strongly on the membrane architecture and membrane materials, and the dense layer's thickness. In addition, in the case of asymmetric membranes of type 1 (without a thin porous layer), gaseous oxygen diffusion through the thick porous support corresponds to the potential ratedetermining step of oxygen transport, which is not clearly identified in the literature. The asymmetric membrane of type 2 (with a thin porous layer) is the optimal architecture corresponding to the best compromise between high mechanical properties and high oxygen semipermeation performance. This architecture shows an oxygen flux of 6.3 10 -3 mol.m -2 .s -1 at 900°C, almost 7 times higher than that obtained from a dense LSFGa5573 membrane without porous layers.

  a). The asymmetric membranes consist of a dense La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ membrane (approximately 0.15 mm thick) supported by a thick porous La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ layer (approximately 0.85 mm thick), without (Figure 1 b) or with a thin porous La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ layer on the opposite membrane surface (Figure 1 c).

Figure 1 :

 1 Figure 1: The three studied membrane architectures: symmetric membrane (a), asymmetric membranes without (b), and with thin porous layer (c)

Figure 2 : 3 . 1 . 2

 2312 Figure 2: Microstructure of the symmetric membrane with porous layer a), b) on the surface top and the cross-section c)

Figure 3 :

 3 Figure 3: a) Temperature dependence of the oxygen semipermeation fluxes (JO 2 ) through the symmetric LSFGa5573/BSF55 membrane and the dense LSFGa5573 membrane under an air (100 ml.min -1 )/argon (200 ml.min -1 ) gradient, b) Arrhenius plot of oxygen flux versus 1/T of the symmetric LSFGa5573/BSF55 membrane

Figure 4 :

 4 Figure 4: Profiles of the oxygen chemical potential (µO 2 ) through dense LSFGa5573 membrane and symmetric LSFGa5573/BSF55 membrane at 900°C

Figure 5 :

 5 Figure 5: Cross-section of asymmetric membrane a) and thick porous layer b) and asymmetric membrane with modified surface c)

Figure 6 :

 6 Figure 6: a) Temperature dependence of the oxygen semipermeation fluxes (JO 2 ) through asymmetric membranes under an air (100 ml.min -1 )/argon (200 ml.min -1 ) gradient, b) Arrhenius plots of oxygen fluxes versus 1/T for asymmetric membranes

Figure 6 b

 6 Figure 6 b) shows the Arrhenius plot of oxygen semipermeation flux versus 1/T (T: absolute temperature in K) for asymmetric membranes with thin porous layers (type 2). Two activation energies of mechanisms limiting oxygen transport can be determined from the Arrhenius plot, one at high temperature (>790°C) corresponding to mechanisms of oxygen bulk diffusion and another at low temperature (<790°C) likely cor responding to a mixed regimen (Table4). There is a change of limiting step, approximately 790°C. At high temperatures, the activation energy of the PS-PL architecture of 39 kJ.mol -1 is much lower than that of the dense LSFGa5573 membrane without porous layers and similar to the activation energy reported by Gurauskis et al.[START_REF] Gurauskis | Oxygen permeation in symmetric and asymmetric La0.2Sr0.8Fe0.8Ta0.2O3-δ membranes[END_REF] for a similar membrane architecture developed from the LSFTa2882 material.

Figure 7 :

 7 Figure 7: Oxygen semipermeation fluxes (JO 2 ) through membrane architectures studied in this work and through the 1 mmthick dense LSFGa5573 [12][START_REF] Guironnet | The surface roughness effect on electrochemical properties of La0.5Sr0.5Fe0.7Ga0.3O3-δ perovskite for oxygen transport membranes[END_REF] and BSFCo5528 membranes[START_REF] Reichmann | Impact of microstructure on oxygen semi-permeation performance of perovskite membranes: Understanding of oxygen transport mechanisms[END_REF] at 900°C under an air (100 ml.min -1 )/argon (200 ml.min -1 ) gradient

Figure 1 : 3 Figure 2 : 7 Figure 4 : 8 Figure 5 : 11 Figure 7 :

 1327485117 Figure 1: The three studied membrane architectures: symmetric membrane (a), asymmetric membranes without (b), and with thin porous layer (c) ............................................................. 3 Figure 2: Microstructure of the symmetric membrane with porous layer a), b) on the surface top and the cross-section c) ........................................................................................................ 6 Figure 3: a) Temperature dependence of the oxygen semipermeation fluxes (JO 2 ) through the symmetric LSFGa5573/BSF55 membrane and the dense LSFGa5573 membrane under an air (100 ml.min -1 )/argon (200 ml.min -1 ) gradient, b) Arrhenius plot of oxygen flux versus 1/T of the symmetric LSFGa5573/BSF55 membrane .......................................................................... 7 Figure 4: Profiles of the oxygen chemical potential (µO 2 ) through dense LSFGa5573 membrane and symmetric LSFGa5573/BSF55 membrane at 900°C ......................................... 8 Figure 5: Cross-section of asymmetric membrane a) and thick porous layer b) and asymmetric membrane with modified surface c) ........................................................................................... 9 Figure 6: a) Temperature dependence of the oxygen semipermeation fluxes (JO 2 ) through asymmetric membranes under an air (100 ml.min -1 )/argon (200 ml.min -1 ) gradient, b) Arrhenius plots of oxygen fluxes versus 1/T for asymmetric membranes ............................... 11 Figure7: Oxygen semipermeation fluxes (JO 2 ) through membrane architectures studied in this work and through the 1 mm-thick dense LSFGa5573[START_REF] Guironnet | La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes[END_REF] [31] and BSFCo5528 membranes[START_REF] Reichmann | Impact of microstructure on oxygen semi-permeation performance of perovskite membranes: Understanding of oxygen transport mechanisms[END_REF] at 900°C under an air (100 ml.min -1 )/argon (200 ml.min -1 ) gradient ............................... 17

Table 1 :

 1

	Dense membrane material/ Thickness	Coating material/ Process/ Thickness	Reference	pO 2 in oxygen rich/lean chamber (atm.)	Oxygen semi-permeation flux (mol.

Activation energies and oxygen semipermeation fluxes through symmetric membranes and dense membranes without coatings at 900°C

m -2 .s -1 ) Activation energy at high and low temperatures (kJ.mol -1 ) La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ / 1 mm

	Ba 0.5 Sr 0.5 FeO 3-δ /
	Electrophoretic method/	This work	0.21/6.9 10 -3	3.4 10 -3	57 (760-960°C) 75 (600-760°C)
	15 µm

La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ / 1 mm

	La 0.6 Sr 0.4 Fe 0.8 Co 0.2
	O 3-δ / Electrophoretic 115 µm method/	et al. [12] Guironnet	Air/Argon	2.6 10 -3	51 (800-960°C) 60 (620-800°C)

La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ / 1 mm

		-		Air/Argon	8.5-9.5 10 -4	120 (820-970°C) 155 (640-820°C)
		La 0.5 Sr 0.5 Fe 0.7 Ga 0.3
	La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ / 1 mm	O 3-δ / Dip-coating method/	Reichmann et al. [34]	Air/Argon	1.2 10 -3	-
		-
		La 0.6 Sr 0.4 CoO 3-δ /
	La 0.7 Sr 0.3 Fe 0.6 Ga 0.4 O 3-δ / 1.7 mm	Screen-printing method/	Lee et al.	Air/He	5.5 10 -4	-
		2-3 µm	[13]
	La 0.7 Sr 0.3 Fe 0.6 Ga 0.4 O 3-δ / 1.7 mm	-		Air/He	1.83 10 -4	-

Table 2 :

 2 Characteristics of asymmetric membranes developed in this work

	Thickness of the dense membrane	Thickness of support/thin layer	Atmosphere in support contact with the thick porous	Type of architecture Notation
				Asymmetric
	170-230 µm	750-800 µm/0 µm	Air	membrane of type 1 (without thin porous	PS-air *
				layer)
				Asymmetric
	170-230 µm	750-800 µm/0 µm	Argon	membrane of type 1 (without thin porous	PS-Ar **
				layer)
				Asymmetric
	170-230 µm	500-550 µm/ 150-200 µm	Air (configuration A)	membrane of type 2 (with thin porous	PS-PL ***
				layer)

* PS-air: Porous support (PS) in contact with air side ** PS-Ar: Porous support (PS) in contact with Argon side *** PS-PL: Porous support (PS) in contact with the air side and porous layer (PL) in contact with the argon side

  also reported that oxygen fluxes through asymmetric membranes are only about 2 times higher than for La 0.1 Sr 0.9 Fe 0.2 Co 0.8 O 3-δ , La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ and La 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ materials. Moreover, Vente et al.

La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 6 µm

	Dense membrane material/ Thickness	Support material/Rate of porosity/ Thickness	Reference	pO 2 in oxygen rich/lean chamber (atm.)	Oxygen semi-permeation flux (mol.m -2 .s -1 )	Activation energy at high and low temperatures (kJ.mol -1 )
	PS-air PS-Ar	La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ / 25-30%/750-800 µm	This work	0.21/3.9 10 -3 0.21/4.6 10 -3	2.0 10 -3 2.3 10 -3	138 (675-960°C) 83 (860-960°C) 107 (675-860°C)
	La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ / 1 mm	-	Guironnet et al. [12] [31]	Air/Argon 8.5-9.5 10 -4 120 (820-970°C) 155 (640-820°C)
	La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3-δ / 20 µm	La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3-δ / 34%/3 mm	Gurauskis	O 2 /0.01	2.7 10 -2	34 (900-1000°C) 105 (800-900°C)
	La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3-δ / 500 µm	-	et al. [27]	O 2 /0.01	3.7 10 -3	77 (800-1000°C)
	La 0.1 Sr 0.9 Fe 0.2 Co 0.8 O 3-δ / 12 µm La 0.1 Sr 0.9 Fe 0.2 Co 0.8 O 3-δ / 270 µm	La 0.1 Sr 0.9 Fe 0.2 Co 0.8 O 3-δ / 28,52%/1 mm -	Jeon et al. [29]	0.21/0.01 0.21/0.01	2.3 10 -2 1.3 10 -2	--
	La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 30 µm	La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 39%/630 µm	Serra et al. [28]	Air/Argon	1.8 10 -2	72 (850-1000°C) 123 (750-850°C)
	La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 800 µm	-	Garcia-Torregrosa et al. [40]	Air/Argon	2.6 10 -3	89 (850-1000°C) 119 (750-850°C)
	La 0.5 Sr 0.5 CoO 3-δ / 20 µm	La 0.5 Sr 0.5 CoO 3-δ / 30%/500 µm	Van der	Air/He	2.2 10 -2	-
	La 0.5 Sr 0.5 CoO 3-δ / 440 µm	-	Haar [30]	Air/He	4.5 10 -3	-
		La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 12%/200 µm		Air/He	1.1 10 -2	-
			Vente et al.
			[36]

La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 200 µm

	La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 35%/10 µm	Air/He	6.1 10 -3	-

La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 200 µm

   kJ.mol -1 and the other approximately 130-180 kJ.mol -1 , can be associated with the nature of the rate-determining step (Tables

		-		Air/He	5.8 10 -3	-
		La 0.58 Sr 0.4 Fe 0.8 Co 0.
	La 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 10 µm	2 O 3-δ -Ce 0.8 Gd 0.2 O 1.9 /	Büchler et	Air/He	5.5 10 -4	133 (750-900°C)
		36%/ 1.1 mm	al. [37]
	La 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ / 1.2 mm	-		Air/He	2.8 10 -4	238 (750-900°C)
	La 0.6 Ca 0.4 CoO 3-δ / 10 µm	La 0.6 Ca 0.4 CoO 3-δ /-/ 2 mm	Watanabe	Air/He	1.0 10 -2	-
	La 0.6 Ca 0.4 CoO 3-δ / 1.2 mm	-	et al. [41]	Air/He	2.9 10 -3	-

approximately

Table 4 :

 4 Activation energies and oxygen semipermeation fluxes through asymmetric membranes of type 2 and dense membranes at 900°C

	Dense membrane material/ Thickness	Porous layer material/ Porosity rate/Thickness	Reference	pO 2 in oxygen rich/lean chamber (atm.) Temperature	Oxygen semi-permeation flux (mol.m -2 .s -1 )	Activation energy at high and low temperatures (kJ.mol -1 )
	PS-PL	La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ / 25-30%/150-	This work	0.21/1.3 10 -2 900°C	6.3 10 -3	39 (790-970°C) 71 (600-790°C)

  1) is composed of a thin dense LSFGa5573 layer (150-200 µm) supported by thick porous support developed from wood flour. This work shows that the decrease in the dense layer thickness does not significantly

		200 µm
	La 0.5 Sr 0.5 Fe 0.7 Ga 0.3 O 3-δ / 1 mm	-	Guironnet et al. [12] [31]	Air/Argon 900°C	8.5-9.5 10 -4	120 (820-970°C) 155 (640-820°C)
	CaTi 0.8 Fe 0.2 O 3-δ / 80 µm CaTi 0.8 Fe 0.2 O 3-δ / 1 mm	CaTi 0.8 Fe 0.2 O 3-δ /20-45%/25-30 µm -	Figueiredo et al. [42]	0.21/0.021 950°C 0.21/0.021 950°C	7.1 10 -4 4.2 10 -4	--
	La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3-δ / 20 µm	La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3-δ /-/-	Gurauskis	O 2 /0.01 900°C	3.1 10 -2	37 (800-1000°C)
	La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3-δ / 500 µm	-	et al. [27]	O 2 /0.01 900°C	3.7 10 -3	77 (800-1000°C)
	La 0.1 Sr 0.9 Fe 0.2 Co 0.8 O 3-δ /12 µm La 0.1 Sr 0.9 Fe 0.2 Co 0.8 O 3-δ /270 µm	La 0.1 Sr 0.9 Fe 0.2 Co 0.8 O 3-δ /20.45%/20 µm -	Jeon et al. [29]	0.21/0.01 900°C 0.21/0.01 900°C	2.7 10 -2 1.3 10 -2	--
	La 0.6 Ca 0.4 CoO 3-δ / 10 µm	La 0.6 Ca 0.4 CoO 3-δ /-/ 10 µm	Watanabe	Air/He	2.3 10 -2	-
	La 0.6 Ca 0.4 CoO 3-δ / 1.2 mm	-	et al. [41]	Air/He	2.9 10 -3	-

Table 5 :

 5 Characteristics of membrane architectures studied in this work and byGuironnet et al. [12] [START_REF] Guironnet | The surface roughness effect on electrochemical properties of La0.5Sr0.5Fe0.7Ga0.3O3-δ perovskite for oxygen transport membranes[END_REF]

Membrane architecture Oxygen semi- permeation flux (mol.m -2 .s -1 ) Activation energy at high and low temperatures (kJ.mol -1 ) Rate-determining step

	Dense LSFGa5573 membrane	8.5-9.5 10 -4	120 (820-970°C) 155 (640-820°C)	Oxygen surface exchanges
	Symmetric LSFGa5573/BSF55 membrane	3.4 10 -3	57 (760-960°C) 75 (600-760°C)	Oxygen bulk diffusion
	Asymmetric membrane of type 1 (without thin porous layer)	2.3 10 -3	83 (860-960°C) 107 (675-860°C)	Oxygen gaseous diffusion
	Asymmetric membrane of type 2 (with thin porous layer)	6.3 10 -3	39 (790-970°C) 71 (600-790°C)	Oxygen bulk diffusion

Table captions Table 1 :

 captions1 Activation energies and oxygen semipermeation fluxes through symmetric membranes and dense membranes without coatings at 900°C .................................................. 7

Table 2 :

 2 Characteristics of asymmetric membranes developed in this work ........................... 10

Table 3 :

 3 Activation energies and oxygen semipermeation fluxes through asymmetric membranes of type 1 and dense membranes at 900°C ............................................................. 12

Table 4 :

 4 Activation energies and oxygen semipermeation fluxes through asymmetric membranes of type 2 and dense membranes at 900°C ............................................................. 14

Table 5 :

 5 Characteristics of membrane architectures studied in this work and by Guironnet et al. [12] [31] ............................................................................................................................... 16


Michele Portolan

Emanuele Valea

Paolo Maistri

Giorgio Di

Flexible and Portable Management of Secure Scan Implementations Exploiting P1687.1 Extensions

Keywords: IEEE 1687, IEEE P1687.1, Reconfigurable Scan Networks, Secure Access, Encryption, Automated Test Environment

Current Design-for-Test solutions for scan-based testing such as IEEE 1687 offer rich and powerful access to embedded resources. While this is extremely helpful for testing purposes, it also raises serious security concerns as malicious users could exploit it to gain control of the chip or leak sensitive information. Current solutions, such as Scan Encryption, are all hardware based and incompatible with the Automated Test Flow, seriously limiting their applicability, portability and reuse. In this paper, we show how the features of the upcoming IEEE P1687.1 can be leveraged and extended to not only include Scan Encryption as a native feature but also as a portable Embedded IP block. The experimental results show that thanks to the new Standard, a fully Secure Automated Test Flow can be integrated inside existing tools without custom software modifications.

INTRODUCTION

The emergence of new standards derived from classical JTAG [1], like IEEE 1500 [START_REF]IEEE std 1500[END_REF] and most notably IEEE 1687 [START_REF]IEEE Standard for Access and Control of Instrumentation Embedded within a Semiconductor Device[END_REF], opens new and exciting opportunities not only for test and debug, but also for Design for Test (DfT). Indeed, the new software infrastructure is now able to support IP-Based approaches also in DfT. Third-party ATPG vectors or algorithms defined over instruments and cores can now be described thanks to Instrument Connectivity Language (ICL) and Procedure Description Language (PDL) and be retargeted to the chip-edge through dynamic topologies. However, these opportunities are not without threats: easy and detailed access to the DfT infrastructure can be used to leak sensitive information, and malicious IPs could jeopardize top-level security strategies.

Several approaches have been proposed to provide security within the test and debug infrastructures, while remaining compliant to the hardware of the over-mentioned standards, but little or no effort has been made over the integration within the EDA software flow. Most of the time the vectors issued from the standard retargeting step need to undergo custom post-processing, which limits their applicability, portability and most of all scalability. This is not due to limitations in the approaches themselves, but rather on the difficulty in describing them in a Standard manner. One such example is Scan Encryption: all data exchanged with the System Under Test is encrypted to protect it from tampering. Even though hardware-wise the system remains standard compliant, the coding and decoding operations cannot be described into standard constructs and therefore have to be processed by specific modules outside of the EDA Tool Flow. As a result, this solution can work only on top-level pre-retargeted vectors, limiting reusability and making this solution close to impossible in interactive setups where the behavior of the system is not known beforehand.

Among the techniques introduced to guarantee the security of the test infrastructures, scan encryption has proven to be easily integrable in the hardware test flow, and to assure confidentiality of test data. Nevertheless, its usage is still to be proven. In this paper, we will show how scan encryption, can be efficiently added inside the Standard Automated Test Flow, by exploiting and extending the features of the upcoming P1687.1 Standard [4]. The paper is structured as follows: after introducing the principles of Scan Encryption and P1687.1 in Section 2, we will show in Section 3 how these approaches can not only cooperate, but also extended to propose a novel management of security solutions. In particular, we will show how Scan Encryption can be used also at the IP level to divide the system in Trusted and Untrusted zones, while still remaining inside the standard EDA flow. Section 4 is dedicated to the experimental validation of the solution by an extensive co-simulation campaign, while lastly Section 5 will draw conclusions and point out evolution perspectives.

STATE OF THE ART

Security Actors in Electronics Design

Security is peculiar because it implies a contention: an Attacker is trying to access a system, while a Defender is trying to prevent it. The Design, and Test of an electronics system requires the intervention of a big number of actors: Designers, Implementers, Test Engineers, Users. Etc…, each of whom could potentially be either a Defender or an Attacker, often regrouped as Trusted and Untrusted Users respectively. This setup is widely used in EDA for confidentiality management: for instance, a Founder will provide detailed information about his technology to a Synthesis or Place&Route Trusted Tool. A generic Untrusted User (for instance, a third-party designer) will use the Tool as a Black Box, while a Trusted User (ex:, a Verification engineer) will have access to all the confidential information. The assumption of Testing is to allow access, so complete locking of the system is not feasible. The application of Security implies a clear identification of Trusted and Untrusted Users and of the methods to allow/deny access to information.

Scan Encryption

The scan encryption technique sits on the cryptographic foundation of the symmetric encryption schemes. These primitives provide confidentiality to the communication between two parties: in the test scenario, the two communicating parties are the tester and the device under test. Both of them share a secret key, which the circuit manufacturer properly deploys to authorized testers and, at the same time, stores it inside the target device in a secure memory. All data that is exchanged between the tester and the device is encrypted/decrypted thanks to cryptographic modules that are placed at the TDI/TDO interfaces of the device.

When scan encryption is implemented, the typical test procedure is executed through the following steps: 1. All data that are produced by the tester are fully encrypted with a secret key that is known being associated to the device under test; 2. Encrypted data are shifted through the TDI interface of the device, where they are decrypted through a decryption module that is located at the interface with the rest of the test infrastructure; 3. All results of the test procedure, which are sent through the TDO interface of the device, are first encrypted by a cryptographic module; 4. Encrypted results are retrieved by the tester, decrypted, and analyzed. Scan encryption can be classified according to the kind of cipher that is employed, namely stream ciphers or block ciphers. Scan encryption based on block ciphers, or block-based scan encryption, has been introduced in [START_REF] Silva | Scan chain encryption for the test, diagnosis and debug of secure circuits[END_REF], where the PRESENT block cipher has been employed. This work, and its extension in [START_REF] Silva Et_Al | Preventing Scan Attacks on Secure Circuits Through Scan Chain Encryption[END_REF], proved that scan encryption based on block ciphers induces a negligible area overhead (less than 0.5%) when implemented on complex SoCs. However, block ciphers need to process test data in blocks of a fixed size (usually 128 bits), thus requiring a careful parsing of test data and their subsequent padding in order to fit a multiple of the block size. Even if it does not cause a significant overhead on the test time, it complicates the interface between the serial test interface and the encryption/decryption modules. Scan encryption based on stream ciphers, or stream-based scan encryption, has been proposed several times [START_REF] Rosenfeld | Defenses for JTAG[END_REF]: encryption is performed serially when processing the data. This property conveniently fits with the serial structure of test infrastructure interfaces, leading to perfectly transparent encryption/decryption operations that do not impact on the performance of the test procedure. Moreover, the capability of stream ciphers to adapt inherently to any input length makes them ideal candidates for IEEE 1687. In this work, we chose the stream-based scan encryption based on the TRIVIUM cipher for implementing a representative automated test flow that is able to both benefit of the flexibility of the P1687.1 standard and guarantee full confidentiality of the involved test data. Indeed, TRIVIUM is often exploited as reference stream cipher implementation because it is a "trade-off soft spot" which provides good security properties for a very limited hardware overhead [8]. Integration into the Test Flow While its lightweight hardware implementation is undoubtedly Scan Encryption strongest point, its great Achilles's heel is its software part. Coding and decoding of vectors need to happen outside of the traditional Test Flow, demanding had-hoc postprocessing and imposing constraints on the design. For instance in [START_REF] Thiemann | On Integrating Lightweight Encryption in Reconfigurable Scan Networks[END_REF] the authors need to impose a specific hardware architecture ( PUK for every protected segment) and an extremely heavy software infrastructure that needs to modify both the input PDL/ICL files and post-process the retargeted vector, completely outside any standardized EDA flow (ex: by manually modifying the shift cycle count). These choices limits its applicability and limits both scalability and portability, putting it outside standard flows.

P1687.1: Interfaces and Callbacks

The IEEE P1687.1 Working Group aims at extending the IEEE 1687 standard by supporting access to the DfT infrastructure through other interfaces than JTAG [4]. Even though the draft has not been finalized yet, the current proposed solution is based on the concept of Transformation Callbacks: as the data (going to or coming from the System Under Testthe SUT) goes through the different interfaces, the EDA Tool needs to "transform" and adapt it to the new interface. So, for instance, SDR or SIR operations might be transformed in a series of I2C transactions.

Because of the high variability of possible interfaces and the difficulty of extracting their behavior and usage from languages such as ICL or even RTL, the WG is rather proposing to encapsulate these transformations into external functions, the Callbacks, called through a standardized API. The MAST tool [START_REF] Portolan | A Novel Test Generation and Application Flow for Functional Access to IEEE 1687 instruments[END_REF][11] implements a working solution of such a solution, and it is being used as an example for WG discussions.

3 SCAN ENCRYPTION AS P1687.1

3.1

Top-Level Scan Encoding As explained in Section 2.2, the principle of Scan Encryption is to encrypt/decrypt all data coming in and out of the JTAG TAP exploiting a Key obtained from a Stream Cypher. Functionally speaking, this is exactly the role of a P1687.1 Translator, obtaining the setup of Figure 1.

Figure 1: P1687.1 Model of Scan Encryption

The Upper half of the figure depicts the SUT, with a Stream Cypher module before the TAP. In the Lower half, the EDA Tool constructs a mirror of the SUT for the ICL file. In this file, some language extensions (not standardized yet, hence the star) will allow the EDA tool to identify and include the Cypher callback code to handle decoding/encoding of the Vectors (in the Middle) as a P1687.1 transformation. The implementation itself of the callback is extremely simple, as depicted in Figure 2: in the "to SUT" direction, the Translator receives the Plaintext vectors obtained from the retargeting: as their size is known, the Cypher Callback simply needs to generate a Keystream of the correct length and apply it to the Plaintext: most of the times this is obtained through an XOR masking. The same happens in the "from SUT" direction: the plaintext is obtained by applying the same keystream to the encoded data. This approach is compatible with the "Relocatable Vector Format" (RVF) [4] [START_REF]The Automated Test Flow, the Present and the Future[END_REF] currently under discussion in the WG, which encapsulates binary data into specific packets.

The advantage of this setup over legacy approach is that the Encryption is directly handled inside the EDA tool, without the need of external post-processing, therefore allowing the tool to exploit 1687 features both for Dynamic Topology configuration (which directly impacts vector length) and for interactive behavior, as we will point out in the Experimental Section. Inline Scan Encoding Top-Level protection, as implemented in the previous Paragraph, is effective and simple to set up, but security-wise it is far from optimal. In reference to Figure 1, only the external exchanges (i.e. the Encoded Vectors) are protected, while data inside of the SUT is in plaintext format, therefore vulnerable to simple attacks like snooping or Trojans. Stream ciphers work on a bit-by-bit basis, so from a hardware point of view they can be easily inserted in the middle of the scan chain, as depicted in Figure 3-a. However, this setup is extremely difficult to handle software-wise through traditional approaches. In a classical top-level setup as presented in the State of the Art and implemented in Figure 1, the system is symmetrical: each bit going into or coming out of the SUT is masked exactly once, so coding/decoding is extremely simple when the keystream is known. This is not true anymore in the inline setup: the scan chain segments Before and After the Stream Cipher break the symmetry. While nothing changes for the Protected segment itself, in the "to SUT" direction the bits intended for the non-protected segment AFTER need to go through two encryptions (with different key bits) before reaching their destination. Similarly, "from SUT" bits coming from the non-protected BEFORE section will be encoded twice while being shifted through the Inline Stream cipher. Legacy solutions like [START_REF] Thiemann | On Integrating Lightweight Encryption in Reconfigurable Scan Networks[END_REF] impose specific hardware constraints to avoid this behavior, reducing flexibility and augmenting overhead costs.

The only way to solve this problem is to know exactly the position of the Stream Cypher in the Scan Chain, as well as the length of each Before, Protected and After section. However, each one of them is potentially a full 1687 sub-network comprising SIBs or ScanMuxes, therefore having variable length. This information is close to impossible to obtain from a set of pre-computed vectors and EDA tools typically do not provide them, making these approaches practically unusable or require heavy post-processing steps as in [START_REF] Thiemann | On Integrating Lightweight Encryption in Reconfigurable Scan Networks[END_REF]

Figure 3: Asymmetric Masking for an Inline Streamer

To solve this impasse, we propose to extend the P1687.1 approach presented in the previous section by defining a new 1687 component called the "Streamer", which can be freely instantiated inside a scan chain and declared to the EDA tool thanks to ICL extensions, replicating the setup of Figure 1. From this information, the Retargeter can extract the exact position of the Streamer inside the active scan chain and the size of the Before, Protected and After segments, as depicted in Figure 3-a In the "from SUT" direction, depicted in Figure 3-b, the Streamer needs to encode the "N-bits" Plaintext data coming from the Protected segment before shifting it out, 0 being the bit closest to TDI and therefore the last to be shifted The shift operation will generate a N-bit Keystream: the first 'protected_size' bits, labelled as KEYSTREAM_PRO are used to encrypt the Protected segment, as usual. However, the asymmetrical setup implies that the data coming from the Before segment will go through both the masking Input operator at the beginning of Protected segment and the Output operator, being encrypted twice at different positions in the Keystream: the two masks are labelled KEY_IN_Before and KEY_OUT and are depicted in Figure 3-b in their position relative the whole Keystream.,. The same happens for the "to SUT" data but in the reverse direction, as depicted in Figure 3-c.

As a P1687.1 extension, we therefore propose a Streamer Callback that can provide a Keystream for a given length, so that the Retargeter can use it to compute the masks detailed above.

EXPERIMENTAL RESULTS

4.1

Co-Simulation Setup We implemented the two approaches described in Section 3 with our MAST tool [START_REF] Portolan | A Novel Test Generation and Application Flow for Functional Access to IEEE 1687 instruments[END_REF], which implements P1687.1 Callbacks and RVF messages as C++ classes. As a Proof of Concept, we established the an Interactive Co-Simulation Environment of Figure 4, also based on P1687.1. To implement this use case, we chose the Trivium Stream cipher, available in VHDL for the hardware-and in Python for the software-side implementation. The SUT is composed by a 12-bit register driven by a TAP, before which is instantiated a Trivium cipher. The PDL-1 test program makes 5 successive iWrites to the Register and displays on the screen the value obtained through an iGet. It is extremely simple algorithmically, but it requires a full interactive setup to work. The P1687.1 Callback exploits C++ to call and execute the Python Trivium implementation and takes care of encoding/decoding the RVF packets.

Figure 4: Top-Level Scan Encryption Test Case

The SIT file gives a straightforward hierarchical description of the SUT: The interesting part is line 3: the P1687.1 Translator of type Trivium is instantiated and initialized with two hexadecimal values (respectively, the Secret Key and the Initialization Vector). Thanks to this information, MAST is able to identify and load the Trivium plugin and use it for its Transformations [4]. The Exchange files shows that the data was encrypted: On the other hand, the execution output proves that encoding/decoding was done correctly in both MAST and the SUT: As per 1687 specification, the first iGet returns a random number, while in the other cycles the Register maintained the value written in the previous iApply cycle. A direct inspection at Modelsim simulation further proves the correctness.

4.2

Top-Level Scan Encoding and Authentication This Test Case proves the functionality of the P1687.1 implementation, but its behavior is static: similar results could be obtained through post-processing of the generated vectors. To obtain an unpredictable Test Case we leveraged the Dynamic Authentication setup of [START_REF] Portolan | Dynamic Authentication-Based Secure Access to Test Infrastructure[END_REF] where the target Register is behind Secure SIB (S2IB).

The Authentication is based on a random Challenge value, to which a Response must be computed based on the secret key shared between the tester and the device [START_REF] Portolan | Dynamic Authentication-Based Secure Access to Test Infrastructure[END_REF]. This implies dynamic decisions made inside MAST, making external vector post-processing for Scan Encryption extremely difficult and cumbersome. On the other hand, thanks to the P1687.1 abstraction, MAST is directly able to use both approaches together by simply describing them in SIT, as follows: )

For brevity's sake, we omitted in line 8 the detailed description of the Authentication infrastructure: the key getaway is the completely seamless integration of Encryption and Authentication, as it can be seen from the instantiation of the Trivium Scan cipher at line 3, which is not modified.

The execution provides exactly the same results as the first Test Case, proving both the complete transparency of the P1687.1 setup and its capability to react to unpredictable data.

4.3

Inline Scan Encoding As stated in Section 2.2, the hardware implementation of an inline stream cipher is straightforward: we simply need to instantiate the Trivium block inside the scan chain rather than before the TAP. Please note that the Callback is not the same as in the top level Test Cases, but follows the P1687.1 Extension we defined in Section3.2, even though we were able to reuse most of the cipherrelated code. Similarly, the instantiation in the SIT file is almost the same, even though we defined a new "STREAMER" keyword to identify it, as can be seen in line 8:

Figure 2 :

 2 Figure 2: Implementation of the P1687.1 Callback

ACKNOWLEDGMENTS

This work has been partly funded by the French Government under the framework of the PENTA HADES ("Hierarchy-Aware and secure embedded test infrastructure for Dependability and performance Enhancement of integrated Systems") European project. Trusted Zone Trusted Zone UnTrusted Zone Trusted Tool UnTrusted Zone

The console output is still the same, proving the portability of the approach, while the values in the Exchange files are encoded as specified in Figure 3.

To further prove the flexibility of the approach, we replicated several times the Streamer block in different configurations: daisychained, behind SIBs, nested, etc… In those scenarios, multiple masking steps are needed at different and unpredictable points of the topology, a situation almost impossible to handle by classical vector post-processing. The results were conclusive: in all cases MAST was able to handle coding/encoding correctly with only the information extracted from the SIT descriptions.

4.4

Trusted and Untrusted Zone From a Security point of view, thanks to our setup the system can be easily split in Trusted and Untrusted zones, as highlighted in Figure 5 for an Inline Streamer: the Trusted Tool (MAST in this example, in the orange-dotted rectangle) in the middle verifies the Credentials of the User (i.e., the cryptographic key in the SIT/ICL description), allowing him to access the Trusted Hardware zone behind the Streamer (in the Green dashed rectangle). Everything else is freely accessible, as part of the Untrusted Zone (in the Red boxes).

This scheme is completely transparent from the user's point of view and allows plug-and-play packaging and re-use of Secure IP as part of the Standard 1687/P1687.1 flow, a major advantage over current ad-hoc implementations. Applicability and Scalability If compared to literature approaches, our solution is completely transparent from the User's point of view: everything is included inside the standard P1687.1 flow, without any pre-or post-processing steps as in [START_REF] Thiemann | On Integrating Lightweight Encryption in Reconfigurable Scan Networks[END_REF]. Moreover, our experiments demonstrate the support of interactive bi-directional execution on the SUT, which to the author's best knowledge is a unique feature while both literature and EDA focus on offline generation. Last but not least, the MAST tool has already been proven to be able to both scale up to extremely complex systems and to be portable on most Test Execution platforms (Desktop, ATE, Embedded) [START_REF]The Automated Test Flow, the Present and the Future[END_REF], providing an immediate transfer path for the proposed approach.

CONCLUSIONS AND PERSPECTIVES

In this paper, we first showed how Scan Encryption can be added to Automated Test Flow thanks the features of the upcoming IEEE P1687.1 standard, and then extended the approach to cover multiple inline scan encryption, therefore obtaining a flexible and truly plug-and-play solution that can be used to add security to IP-based design. To our knowledge, this is the first time that seamless integration of vector encryption exists in an Automated Test Flow. All solutions were proven by hardware co-simulation and even though we used the MAST tool for this step, they could be easily replicated in any 1687/P1687.1 compliant setup equipped with any symmetric cryptographic primitive.


Anna Kravchenko
email: anna.kravchenko@loria.fr

Malika Smaïl-Tabbone

Malika Smail-Tabbone

Isaure Chauvot De Beauchene

Jacob De Vries

New strategy for

published or not. The documents may come

Computational prediction of a 3D structure of a molecular complex, also known as docking, is essential in modern biological research. It can complement MD, provide working directions to experimentalists, etc. We are invested in fragment-based docking, specifically for the single-stranded RNA-protein complexes. Why ssRNA specifically? Generally speaking, molecular flexibility is a scourge of docking: it increases its complexity and decreases results' reliability. High flexibility leads to a near-infinite number of docking models, the processing of which is too expensive computationally. Hence, highly flexible ssRNA is a challenging target to work on.

A fragment-based docking approach was developed to tackle this high flexibility issue [START_REF] Chauvot De Beauchene | Fragment-based modelling of single stranded RNA bound to RNA recognition motif containing proteins[END_REF]. Its core idea is to split the ligand into overlapping fragments, and dock them onto the rigid receptor separately, assembling the fragments back into the whole ligand afterwards. For the full procedure to succeed, each fragment must return at least one correct pose (so-called near-native): this is the sampling problem. The poses are obtained by minimisation using a differentiable energy function. Then, before assembling, docked fragments must be filtered, keeping a high percentage of near-natives. Otherwise, the assembly task once again becomes too expensive computationally: this is the scoring problem. The filtration is done using a scoring function. We are working with the ATTRACT docking engine, where the same function is used both for sampling and scoring. It has the shape of a Lennard-Jones potential, and 2 parameters per atom type pair (1054 in total). The current parameters were obtained in 2010 by extraction of the statistical potentials from RNA-protein crystal structures and were optimized by a random Monte Carlo-like strategy [START_REF] Setny | A coarse-grained force field for Protein-RNA docking[END_REF]. These parameters were not initially tailored to ssRNA and their performance is not flawless. Our goal is to optimize docking parameters, improving both sampling and scoring performance. To achieve it, we created an up-to-date dataset of ssRNA-protein complexes and set up a novel histogram-based approach. For each pair of interacting atom types, we (a) convert the current energy function into a log-odds histogram of the expected occurrences of atom-atom distances (discretized into bins) in native/non-native poses, using the Boltzmann equation; (b) obtain the corresponding histogram on a benchmark-wide docking test, which corresponds to the residual error of the energy function; (c) sum the predicted and real histograms; (d) analytically fit the energy parameters to the resulting histogram. Repeat until convergence -until the residual histogram is flat.

Our newly created dataset of ssRNA-protein complexes is expected to be sufficient for optimization. The proposed approach for the ssRNA-protein fragment-based docking parameter optimization is expected to be more robust in terms of the fitness of the initial parameters compared to the previous approach. It has potential to benefit both the sampling and the scoring problems.

Acknowledgements

This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 813239.


Benjamin Darses

Pascale Maldivi

Christian Philouze

Philippe Dauban

Jean-François Poisson

Asymmetric intramolecular Buchner reaction: from high stereoselectivity to coexistence of norcaradiene, cycloheptatriene and an intermediate form in the solid state

Bicyclic compounds bearing a quaternary stereogenic center have been obtained using asymmetric intramolecular Buchner reaction with excellent yields and level of enantioselectivity. X-ray crystallography determination of the absolute configuration of one product has led to the serendipitous observation of an unusual behavior within the crystal structure, with equilibrating norcaradiene and cycloheptatriene valence isomers at the solid state, as well as an even more unexpected intermediate form. DFT calculations were performed to support these observations.

Polycyclic structures containing seven-membered carbocycles are widely found in natural products. Among them, terpenes (daphnanes, tiglianes, ingenanes…) or alkaloids (colchicine, aconitine, cortistatins…) possess original architectures (Figure 1). However, the cycloheptane ring is not as common as the five-and six-membered rings within the commercially available building blocks.

Figure 1. Seven-membered carbocycle-containing natural products.

Hence, seven-membered ring carbocycles must be built using synthetic methods such as ionic cyclizations or ring-closing metathesis, and also through elegant cycloadditions requiring the design of complex precursors. 1 Alternatively, enlargement of broadly accessible smaller rings has also been considered. 1a Within the latter, the Buchner reaction is unique in its ability to induce a neutral expansive dearomatization, directly affording valuable functionalized cycloheptatrienes from widely available aromatic precursors. 2 This original transformation consists in an unusual cyclopropanation of an aromatic ring by a carbene, delivering a norcaradiene, in equilibrium through an electrocyclic rearrangement with the generally more stable cycloheptatriene (Scheme 1). [START_REF] Mcnamara | The norcaradiene-cycloheptatriene equilibrium[END_REF] Scheme 1. The Buchner reaction.

MeO

The intramolecular Buchner reaction is particularly efficient to rapidly access bicyclic sevenmembered ring-containing frameworks, while providing higher selectivities for the addition of the reactive carbenes, more prone to competitive C-H insertions in intermolecular processes. While the generation of carbenes by thermal or photochemical decomposition of diazo precursors generally leads to mixtures of products, metal-catalyzed denitrogenation of diazo compounds can master the reactivity of carbene, 4 allowing much more selective aromatic cyclopropanation and providing opportunities for synthetic developments. Though copper complexes were initially used, 4a dirhodium(II) complexes rapidly showed greater catalytic activities, 4b,c and still remain the most widely used catalysts to efficiently perform the Buchner reaction.

Despite a great synthetic potential, very few examples of efficient asymmetric synthesis of cycloheptatrienes have been reported using the intramolecular Buchner reaction. 5,[START_REF] Fleming | Regioselective and Enantioselective Intermolecular Buchner Ring Expansions in Flow[END_REF] Moderate to poor enantioselectivities are generally reported for Buchner reactions involving benzylic a-diazoesters or a-diazoamides. Moreover, these intramolecular processes often raise the issue of chemoselectivity that translates to the formation of mixtures of products and, thus, the isolation of the expected cycloheptatrienes in only moderate yields. 5d Very recently, the use of a cationic chiral Ru-complex has led to a great improvement in the enantiocontrol of the reaction for the generation of tertiary stereogenic centers. [START_REF] Phan | Highly stereoselective intramolecular Buchner reaction of diazoacetamides catalyzed by a Ru(II)-Pheox complex[END_REF] Inspired by our recent studies on rhodium-catalyzed asymmetric amination, 8 we have decided to investigate the use of chiral dirhodium(II) complexes in the intramolecular Buchner reaction for the formation of more challenging quaternary stereogenic centers. In this communication, we report an efficient synthesis of enantioenriched cycloheptatrienes through catalytic asymmetric intramolecular Buchner reaction using cyano substituted diazo derivatives. 9,[START_REF] Smith | Catalytic, Asymmetric Dearomative Synthesis of Complex Cyclohexanes via a Highly Regio-and Stereoselective Arene Cyclopropanation Using α-Cyanodiazoacetates[END_REF] Our investigations started by reacting benzyl b-cyano a-diazoacetamide 1a in the presence of 1 mol% of the commercially available chiral dirhodium(II) complex (tetrakis[N-phthaloyl-(S)-tertleucinate] (Rh 2 [S-pttl] 4 ), in dichloromethane at 0 °C (Table 1, entry 1). We were delighted to observe the selective formation of the expected Buchner product 2a in 95% yield and with a promising 87:13 enantiomeric ratio. Lowering the temperature to -30 °C did not impede the efficiency of the reaction while improving the enantiomeric ratio to 90.5:9.5 (Table 1, entry 2). A screening was thus undertaken at this temperature to find the most efficient chiral catalyst.

Changing the amino acid side chain or introducing fluorine atoms on the phthaloyl moiety of the ligands proved deleterious to the asymmetric induction (Table 1, entries 3-5), while a tert-butyl group in position 4 of the phthaloyl group slightly improved the enantioselectivity (Table 1,entry 6). [START_REF] Adly | Design and Synthesis of Novel Chiral Dirhodium(II) Carboxylate Complexes for Asymmetric Cyclopropanation Reactions[END_REF] The best result was obtained with the S-bpttl ligand, [START_REF] Kitagaki | Enantiocontrol in Tandem Carbonyl Ylide Formation and Intermolecular 1,3-Dipolar Cycloaddition of α-Diazo Ketones Mediated by Chiral Dirhodium(II) Carboxylate Catalyst[END_REF] where the phenyl is replaced by a 2,3naphthoyl motif (Table 1,entry 7). However, other isomeric naphthalene derivatives led to lower enantiomeric ratio (Table 1, entries 8-9). The dosp or the btpcp ligands offered moderate to poor enantioselectivities in this transformation (Table 1, entries 10-11). 13 Variation of the solvent also led to lower enantioselectivities (see Table S1, SI). Carrying the reaction at -50 °C with the S-bpttl ligand allowed both an improvement of the enantiomeric ratio to 96:4 and the yield to 95% (Table 1,entry 12). Similar results were obtained using of 0.5 mol% of rhodium catalyst (Table 1,entry 13). 14

R H

norcaradiene cycloheptatriene 1. Screening chiral ligands and temperature. [a] Entry L Yield of 2a

1 2 3 4 5 6 7 1 6 7 R X X R X Table
[%] [b] e.r. [c] 1 [d] S 12 [e] S-bpttl 95 96:4

13 [e][f] S-bpttl 95 96:4

[a] Reaction conditions: to a solution of 1a (0.300 mmol, 1.00 eq.) in CH 2 Cl 2 (6 mL) at -30 °C, under an argon atmosphere, was added Rh 2 L 4 (3.0 1 mol%). The mixture was stirred at this temperature for 2 h. [b] Isolated yields after flash chromatography. [c] Determined by chiral HPLC. [d] Reaction carried at 0 °C. [e] Reaction carried at -50 °C for 24 h. [f] 0.5 mol% of Rh 2 L 4 .

The optimized reaction conditions have then been applied to a variety of substituted aromatic derivatives (Scheme 2). Substitution of the aromatic ring in para position allowed the preparation of the corresponding Buchner products in excellent yields and enantioselectivities.

N O O O O N O O O O N O O O O N O O O O F F F F N O O O O N O O O O N O O O O N O O O O N S O O O O C 12 H 25 O O Ph Ph Br Rh 2 L 4 1 mol% CH 2 Cl 2, -30 °C t-BuN O N t-Bu N O N 2 N 1a 2a
The presence of an alkyl or a silyl group (2b-c) maintained a similar selectivity and halogens induced even greater enantiomeric ratio (2d-f). Nitrogen-containing functional groups such as an azide or a phthalimide also afforded the expected products both in high yields and selectivities (2g-h). The best enantiomeric ratios were obtained with aromatic bearing a substituent extending the p-conjugation such as a phenyl or a phenylacetylene moiety (2i-j).

The asymmetric formation of cycloheptatrienes is therefore very efficient, both in terms of yield and enantioselectivity, with moderately electron donating or withdrawing group at the para position of the aromatic ring. However, with more electron donating group, such as a methoxy (2k), the racemic product was obtained. This could be due to an equilibrating process involving an achiral spiro intermediate. 15 This phenomenon has previously been postulated to explain such interconversion, and the presence of the nitrile substituent is probably favoring the push-pull racemization process. 16 The reaction is not limited to N-tert-butyl derivatives as a N-benzyl amide (2l) also afforded the cycloheptatriene in good yield though in moderate enantioselectivity. It was also possible to introduce an additional methylene between the carbene and the aromatic ring with only a slight decrease of efficiency of the reaction. The 6,7bicyclic derivative 2m was thus isolated in 74% yield and 90.5:9.5 e.r.

Scheme 2. Reaction Scope.

Reactions performed on 0.300 mmol of substrate for 24 h. Isolated yields, and e.r. determined by chiral HPLC.

[a] 7:1 mixture of regioisomers. [b] For the shown major regioisomer.

An ester in the meta position proved very interesting as it leads to the isolation of a single regioisomer, notably as the norcaradiene product (2n) in 73% yield and an excellent enantioselectivity of 98.5:1.5. This product corresponds to the addition of the carbene to the most hindered of the two accessible positions of the aromatic ring. Similarly, a meta phenyl group afforded the same chemical reactivity, but in this case, the isolated product 2o is a 7:1 mixture of norcaradiene regioisomers, in favor of the most hindered one.

R'N O N Rh 2 [S-bpttl] 4 0.5 mol% N O N 2 N CH 2 Cl 2, -50 °C 1,2 R R R' 1 2 1,2 t-BuN O N t-BuN O N t-BuN O N t-BuN O N t-BuN O N t-BuN O N t-BuN O N t-BuN O N t-BuN O N t-BuN O N 2a
We decided to test the reliability of the transformation on a synthetically relevant scale. Applying the same reaction conditions on 5.00 mmol (1.28 g) of 1a delivered more than one gram of the expected product 2a with exactly the same yield and enantiomeric ratio. On 5.00 mmol scale, the catalyst loading could also be notably lowered to 0.05 mol% (i.e. 3.6 mg), affording 2a in 93% yield and an unchanged stereoselectivity (Scheme 3). Scheme 3. Gram-scale reaction.

As aforementioned (Scheme 2), the substitution of the products can sometimes shift the cycloheptatriene/norcaradiene equilibrium in favor of the latter. It is therefore important to evaluate the position of the equilibrium between the two forms: observing the proton and carbon NMR chemical shifts of C 6 -H provides useful information. 17 Indeed, they are known to be located at a weighted value reflecting the ratio between the two fast-equilibrating isomers. 2 The structural isomers 2i and 2o, deriving respectively from a 4-phenyl or a 3-phenyl-substituted substrate, provide good comparison data. Whereas the C 6 -H 1 H and 13 C NMR chemical shifts respectively appears at 4.92 and 98.8 ppm for 2i, showing a vinylic character, those of compound 2o at 3.74 and 62.1 ppm are more consistent with a cyclopropane ring (Figure 2). However, the unprecise position of the equilibrium frequently leads to contradictions. For example, (±)-2m was described as a norcaradiene by Reisman, 9a whereas Xu reattributed the structure of the same product to a cycloheptatriene. 9b Observing the chemical shifts corresponding to C 6 -H which appear at 4.56 and 81.9 ppm by 1 H and 13 C NMR, it is possible to evaluate that this compounds may better be represented as a cycloheptatriene rather than a norcaradiene.

In order to determine the absolute configuration of the prepared bicycles, we also decided to use the bromine anomalous diffusion at Mo Ka wavelength, on compound 2e. [START_REF]Measurements were undertaken at room temperature on an orthorhombic single-crystal grown from compound 2e. The crystal has the following cell: a = 9[END_REF] We first examined the NMR spectral data of this compound as we did for 2i, 2m and 2o. In this case, the chemical shifts of 1 H and 13 C NMR for C 6 (d = 5.13 and 111.7 ppm respectively; CDCl 3 , r.t.) clearly account for an even more pronounced sp 2 character and therefore a highly prevalent cycloheptatriene form in solution. The absolute configuration of 2e was then determined to be R by single-crystal X-ray crystallography. Interestingly, the asymmetrical unit displays two types of molecular forms in sharp contrast with the NMR observations in solution (Figure 3). The form A appears as a statistical disorder of the two valence isomers of 2e: the norcaradiene A-NCD and the cycloheptatriene A-CHT, in a 0.512/0.488 ratio. On the other hand, the form B is a slightly thermally agitated structure, that seems to be structurally laying at the intermediacy of the two extreme valence isomers. The C 1 -C 6 distance of 1.853(9) Å is indeed between those of A-NCD and A-CHT (1.56(4) Å and 2.475 Å respectively). Similarly, the C 6 -C 7 -C 1 angle has a value of 77.2(5)° for B, far from the 109.5° expected for a perfect tetrahedron or the 111(2)° angle observed for A-CHT, but slightly closer to the 62(2)° angle observed for A-NCD. Consequently, B could be seen as an artificially observable transition state between the two valence isomers A-NCD and A-CHT, stabilized within the crystalline matrix. To the best of our knowledge, this behavior has never been reported for this type of compounds. Moreover, in the solid state, the undeniable presence of a large amount of norcaradiene seems to indicate a network stabilization of this isomer. The observation of both forms of 2e in the solid state, including an intermediate structure, as well as its different behavior in solution, revealed by NMR, indicate an equivalent or close stability of the two isomers and a fast exchange between both. Although the equilibrium is a known phenomenon, the easy balance between the two isomers, by physical state modification, may lead to contradictory product descriptions, as well as an unexpected reaction of the misidentified isomer upon condition modifications. We thus decided to model the thermodynamics profile of the isomers of 2e at 298K using Density Functional Theory (DFT) calculations to evaluate the Gibbs energies of the norcaradiene and cycloheptatriene isomers (see SI for details). The optimized geometries thus showed a weak stabilization of -3.6 kcal mol -1 in favor of the cycloheptatriene form (in accordance with the NMR observation). Moreover, the activation free energy for the interconversion, computed at 6.7 kcal mol -1 , is low and consistent with a possible fast exchange between the two valence isomers.

It is worth noting that optimized geometrical parameters are also in very good agreement with the experimental crystal data (see Table S2, SI). In particular, the calculated transition state (TS) structure is close to the observed form B found in the crystal of 2e (Figure 3): a C 1 -C 6 distance of 1.91 Å and a C 6 -C 7 -C 1 angle of 79.5°: the stabilization of this unconventional form B within the crystalline matrix is quite puzzling, and to the best of our knowledge unprecedented.

In conclusion, we have reported an efficient strategy to prepare enantioenriched sevenmembered carbocycle-containing bicyclic skeletons. The asymmetric intramolecular Buchner reaction allowed the highly stereoselective installation of a quaternary stereogenic center, delivering a library of diversely substituted products. The unexpected observation of a compound present as the cycloheptatriene form in solution, but as a mixture of cycloheptatriene and noracardiene in the solid state prompted us to undergo some DFT calculation to rationalize this observation. Further investigations on this striking valence isomerization in solution or in the crystalline structure are currently under progress, so is the calculation of the enantiodetermining transition state with the chiral catalyst to rationalize the origin of the enantioselectivity.

Figure 2

 2 Figure 2. 1 H and 13 C NMR chemical shifts of C 6 -H for different products 2.

Figure 3 .

 3 Figure 3. X-Ray structures of 2e: structures A and B present in the crystal, and decomposition of A into its valence isomers.

ACKNOWLEDGMENT

We wish to thank the Labex ARCANE and CBH-EUR-GS (ANR-17-EURE-0003) for their financial support. The NanoBio-ICMG platforms (FR 2607), the Département de Chimie Moléculaire and the Institut de Chimie des Subtances Naturelles are acknowledged for their technical and/or financial support.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, characterization data, computational details and crystallographic data (PDF)

AUTHOR INFORMATION Corresponding Author

Benjamin Darses -Département de Chimie Moléculaire (DCM), team SeRCO, CNRS -Université Grenoble Alpes UMR-5250 301 rue de la chimie, 38058 Grenoble CEDEX 9, France; orcid.org/0000-0002-2284-5931; Email: benjamin.darses@univ-grenoble-alpes.fr

Authors

Notes

The authors declare no competing financial interest.