V Perrot Bernardet

C Dechaux

C Boutellier

A Cornier

Integrating environmental assessment in the assembly line design process

Keywords: design process, environmental assessment, assembly line

This study focuses on including the environmental assessment of a process in its design. Two improvement targets can be considered to optimize the environmental efficiency of a production phase: a cleaner process, and the optimization of the assembly of the different product parts. This work makes it possible to apply life cycle assessment to product industrialization, and specifically to assembly lines. The reduction of the environmental impacts (EI) of the production phase can be achieved by modeling the assembly line and/or optimizing equipment choices and organization. To accomplish this, we developed a tool. In this tool, equipment EI are calculated using two databases: i) an equipment database, listing equipment characteristics: lifetime, EI for individual functional units and use parameters; and ii) a consumption database, listing the consumables and energy EI. A comparison of the equipment EI is carried out for the same function, use conditions and functional unit. Equipment is then selected according to its EI. Finally, the tool determines the EI of the optimized assembly line.

Glossary consumption coefficient of equipment

i Eq : EI of equipment for a specific functional unit I Eq : EI equipment for the functional unit of assembly line I Input : Impact of input i Manu,Eq : EI of manufacturing phase of equipment for a specific functional unit I Manu,Eq : EI of manufacturing phase of equipment for the functional unit of assembly line i Use,Eq : EI of use phase of equipment for a specific functional unit I Use,Eq : EI of use phase of equipment for the functional unit of assembly line L AL : Lifetime of assembly line L Eq : Lifetime of equipment n Input : Amount of input n Eq : Amount of equipment V P : Average value of parameter "p"  V p,h : Value of parameter p for the processing of the product "h" % Prod,h : Ratio of production of the product "h"

Introduction

Standards ISO:14001 (for environmental management systems), and ISO:14040 and 14062 (for life cycle assessment and the eco-design approach) help manufacturers to reduce their environmental impacts (EI). Since 2011, standard ISO:14006 has defined the guidelines to link these three standards, involving the quality approach (ISO:9001). The current concern of manufacturers is to improve the performances of their manufacturing systems with respect to business objectives and environmental regulations. The environmental efficiency of a product's production phase can be improved through new, cleaner processes and through assembly optimization.

On one hand, life cycle assessment (LCA) [START_REF]International Organization for Standardization. ISO 14040 -Environmental management -Life cycle assessment -Principles and framework[END_REF][START_REF]Environmental management -Life cycle assessment -Requirements and guidelines[END_REF] is one of the most well-recognized environmental assessment methods. The LCA method focuses on the product (or process) and on the infrastructure required to make, maintain, and dispose of it. Nevertheless, LCA does not link the environmental consequences of design parameters. On the other hand, current design methods (e.g. discrete event simulation) do not measure the environmental performances of the manufacturing process.

In this context, Azapagic et al. [START_REF] Azapagic | Life cycle assessment and its application to process selection, design and optimisation[END_REF] have described how LCA can support decision-makers in process industries. Nevertheless, De Benedetto et al. [START_REF] Benedetto | The Environmental Performance Strategy Map: an integrated LCA approach to support the strategic decision-making process[END_REF] highlight LCA's limitations during strategic decisionmaking based on an environmental mapping strategy. In the past ten years, certain studies have combined environmental assessment with the design process in various fields. For example, Berlin et al. [START_REF] Berlin | A dynamic simulation tool for a productive and environmentally efficient food production[END_REF][START_REF] Berlin | A life cycle based method to minimise environmental impact of dairy production through product sequencing[END_REF] developed a method to test alternative dairy productions to minimize wasting milk. Bojarski [START_REF] Bojarski | Incorporating environmental impacts and regulations in a holistic supply chains modeling: An LCA approach[END_REF] studied incorporating the LCA approach into maleic anhydride production supply chain modeling. Other authors combine LCA with performance manufacturing assessment systems. They investigate the recycling strategies for plastic waste [START_REF] Rios | Plastics disassembly versus bulk recycling: engineering design for endof-life electronics resource recovery[END_REF] or predict the environmental impacts of various urban water management strategies [START_REF] Huang | Discrete event simulation for exploring strategies: an urban water management case[END_REF]. Wohlgemuth et al. [START_REF] Wohlgemuth | Combining discrete event simulation and material flow analysis in a component-based approach to industrial environmental protection[END_REF] and Reinhard et al. [START_REF] Reinhard | Material flow management in a motor fabrication: identifying saving potential in the mechanical production by modeling energy-and material flows as well as machine time using Umberto[END_REF] used a method combining discrete event simulation with ecological material flow analysis for motor fabrication. They include economic factors (bottleneck detection, maintenance planning and machine purchasing) and ecological factors (emissions, raw material and energy consumption). Other authors have developed a model focusing on only one impact category indicator, energy consumption, in two industrial sectors: manufacturing [START_REF] Dietmair | A generic energy consumption model for decision making and energy efficiency optimisation in manufacturing[END_REF], and vehicle assembly [START_REF] Galitsky | Energy Efficiency Improvement and Cost Saving Opportunities for the Vehicle Assembly Industry[END_REF].

In this framework, this article focuses on including environmental assessment in decision-making during design. Today, many state-of-the-art studies examine the LCA of built-in and operational processes. The specificity of our tool is that it enables assessment of the assembly line EI via LCA in the initial design phases. The first objective of the tool is to determine and compare the EI of equipment with the same function and used in the same conditions. The ultimate objective is to determine the optimum improvement strategies by selecting the best equipment alternative and the best equipment organization, taking environmental decisions into account with multiple, conflicting objectives (technical, economic, etc.). This article describes a method used to determine and compare the EI of assembly line equipment, and it does not describe equipment LCA in detail.

Scope of the LCA used in the tool

Assembly line LCA does not require detailed modeling: scheduling tasks, parallel tasks, prioritization of operations. Nevertheless, all of the assembly line and support activity equipment must be listed. This tool is focused on the assembly line manufacturing and use phases. Figure 1 illustrates the production process and the scope of the LCA study (black dotted line). The energy and consumables for production are taken into account. The products are eliminated from the study (raw materials, finished products, production scrap, etc.). For the assembly line, the distribution phase EI is considered to be negligible. Due to a lack of data, the assembly line end-of-life is not included in the scope.

Method to calculate assembly line environmental impacts

In the tool, only three phases are considered: raw material extraction, manufacturing and use. Nevertheless, the material modules in LCA software databases (e.g. the Ecoinvent database) include the transport between the first two phases. To assess EI, LCA studies of vehicle assembly lines take into account two indicator categories: global warming and non-renewable energy [START_REF] Geisler | Production of fine and speciality chemicals: procedure for the estimation of LCIs[END_REF][START_REF] Kobayashi | Car Life Cycle Inventory Assessment[END_REF][START_REF] Schuckert | Life Cycle Inventories -New Experiences to Save Environmental Loads and Costs[END_REF][START_REF] Boyd | Development of a Performance-based Industrial Energy Efficiency Indicator for Automobile Assembly Plants[END_REF]. In agreement with the Bojarski study [START_REF] Bojarski | Incorporating environmental impacts and regulations in a holistic supply chains modeling: An LCA approach[END_REF],

3
vehicle assembly line LCA [START_REF] Gazagne | Développement d'un outils d'écoconception[END_REF] highlights significant EI of two additional indicator categories: respiratory effects and terrestrial ecotoxicity. Therefore, our tool considers these four environmental indicators: global warming, non-renewable resources, respiratory effects and terrestrial ecotoxicity.

First, the tool determines the equipment EI for a specific functional unit. This functional unit takes into account the equipment function, the number of products processed, the equipment lifetime (e.g.: X product load for Y years). Users can study the effect of equipment and product line usage conditions on the EI of one type of equipment. Then, the tool calculates the equipment EI for the same functional unit as this one for the assembly line (for the assembly line lifetime, for the same number of products assembled, etc. e.g.: allowing the production of X cars for Y years). In this case, equipment with the same function can be compared.

To determine the equipment EI for a specific functional unit (i Eq ), the manufacturing phase EI and use phase EI are separated [START_REF]International Organization for Standardization. ISO 14040 -Environmental management -Life cycle assessment -Principles and framework[END_REF]. The manufacturing phase EI (i Manu,Eq ) are the sum of the raw material extraction and equipment manufacturing EI. The use phase EI are related to equipment inputs (consumables and energy): their impacts (I Input ) and their amount (n Input ) (2). The input amounts change according to use conditions. Therefore, i Use,Eq must be determined for each "equipment/use conditions" pair. i Eq = i Manu,Eq +i Use,Eq (1) i Use,Eq =∑(n Input . I Input )

(

Then, to compare and select the best equipment, the EI must be calculated for a same functional unit (I Eq ): the assembly line functional unit. As previously, the manufacturing phase EI (I Manu,Eq ) and use phase EI (I Use,Eq ) are separated. In this case, I Manu,Eq depends on i Manu,Eq and the number of equipment replacements (n Eq ) during the assembly line lifetime. n Eq is equal to the ratio of the assembly line lifetime (L AL ) over that of the equipment in use conditions (L Eq ) (3). In some cases, equipment lifetime is constant regardless of the use conditions. In other cases, the lifetime depends on use parameters (use time, number of uses). I Manu,Eq = i Manu,Eq .n Eq = i Manu,Eq .L AL / L Eq (3)

To determine I Use,Eq , the product profile and use conditions must be taken into account. The first query is the average value of parameter "p" (V P ) as a function of product profile (4): weighted by the product production rate h (% Prod,h ). The parameters are practical (e.g.: number of prints, number of screws, use time, etc.). V P,h is the value of parameter "p" for product "h".

V P  Prod,h (V p,h .% Prod,h ) (

Then the consumption coefficient () must be taken into account to determine the mean consumption value of the "equipment/use conditions" pair from the parameter value. Thus, the value of I Use,Eq is calculated using equation [START_REF] Berlin | A dynamic simulation tool for a productive and environmentally efficient food production[END_REF]. I Use,Eq = (V P . I Input . [START_REF] Berlin | A dynamic simulation tool for a productive and environmentally efficient food production[END_REF] Finally, the assembly line EI (I AL ) can be determined. Assembly line consumption can be expressed by the equipment consumption: energy or consumables. Thus, the assembly line EI can be related to the assembly line equipment impacts [START_REF] Berlin | A life cycle based method to minimise environmental impact of dairy production through product sequencing[END_REF]. I AL = ∑I Eq = ∑(I Manu,Eq +I Use,Eq ) (6)

Organization of the tool

The tool is built in Access software. In the tool, the assembly line is considered as a succession of workstations. Each workstation is characterized by certain functions and equipment (Figure 2). Five equipment rules are highlighted: transfer, assembly, loading/unloading, process control and maintaining working conditions (lighting, heating, safety equipment).

As in previous studies [START_REF] Berlin | A life cycle based method to minimise environmental impact of dairy production through product sequencing[END_REF][START_REF] Azapagic | The application of life cycle assessment to process optimization[END_REF], the EI values for equipment manufacturing or inputs are not calculated in the tool but taken from two environmental databases: "equipment database" and "consumption database". As previously explained, the EI of four indicator categories are recorded in these databases: respiratory effects, global warming, non-renewable resources and terrestrial ecotoxicity.

Figure 3 illustrates the organization of the data in the tool and their links. The "equipment database" lists all of the equipment and its characteristics (e.g.: picture, name, lifetime, and manufacturing EI). The comparison of equipment with the same function requires recording alternative types of equipment. The "consumption database" collects all the equipment inputs (several energy mixes, compressed air, sheets of paper, oil, etc.)

and their EI. To model the assembly line, the user must describe some characteristics i) of the assembly line (e.g.: lifetime, functional unit, product line), and ii) of the product line (e.g.: name, product use rate). To add equipment, the user must define its function and check the value of its use parameters (optional).

FIG. 2 -Organization of the tool

To determine equipment EI from use parameters, the tool links the characteristics of the equipment (lifetime, parameter values) and the use conditions (workstation, product line) (Figure 3). The user chooses the equipment, defines its function and its use parameters (workstation and product line). The tool determines the EI of the equipment and the EI of all alternative equipment with the same function (and for the same functional unit). Figure 5 illustrates the results of a comparison of two wrenches. In this case, the tool was used to model a section of a truck assembly line, composed of 77 elements of equipment. The assembly line functional unit is: processing 150 trucks per year for 25 years. i Eq was determined using Simapro software and the IMPACT2002+ method, and the Ecoinvent database was used.

Assembly Line description

The user can then compare the equipment EI and select the best alternative in view of the economic, technical and environmental constraints. To optimize assembly activities with respect to EI, the user can observe the effects of modifying the amount, use parameters or equipment distribution. Finally, the tool determines the assembly line EI. In addition, the tool indicates an EI ratio for a function, workstation, or equipment within the total value of each indicator category, as illustrated in Figure 6.a. As shown in Figure 6.b, the tool can separate the EI of the manufacturing phase and the use phase. The results in Figure 6

Conclusion

Many studies focus on the LCA of a process after its design. Contrary to these studies, this tool makes it possible to assess the assembly line EI in its initial design phase. The user can model the assembly line and determine its EI, and also modify equipment quantity and distribution. The user can thereby optimize assembly line EI by comparing and selecting equipment (and an organization) with less EI.

6

Thanks to the modeling results, this tool makes it possible to identify the workstation, function or equipment with higher consumption and/or EI. Users can then search for new equipment with the same function or a new process with less environmental impacts.

Currently this tool only concerns the manufacturing and use phases of an assembly line. To improve tool accuracy, the entire assembly line life cycle should be taken into account. Therefore, the "Equipment database" must be improved to allow recording of new data and new links. To improve tool performance, more equipment must be analyzed using LCA and recorded in the "Equipment database".

To go further, the EI could be converted into monetary values. For example, a carbon tax is used in certain countries, with the aim to limit and control emissions in air, water and soil. A spot market regulates the price of one ton of CO 2 produced. This tool should therefore be modified to take this parameter into account.

FIG. 1 -

 1 FIG. 1 -Diagram of the manufacturing process and scope of tool (black dotted line)

FIG. 3 -Figure 4

 34 FIG. 3 -Organization of data and links between characteristics

were 5 generatedFIG. 4 -FIG. 5 -FIG. 6 -

 5456 FIG. 4 -Selection of equipment according to its environmental properties


Numerical investigation of dense gas flows through transcritical multistage axial Organic Rankine Cycle turbines

L. Sciacovelli a , P. Cinnella a a. Laboratoire DynFluid, Arts et Métiers ParisTech, 151 Bd de l'Hôpital, 75013 Paris, France

Résumé :

Des études récentes suggèrent que les cycles de Rankine organiques supercritiques ont un grand potentiel pour les applications de récupération de chaleur à basse température, car ils permettent d'atteindre une meilleure rendement de récupération avec une architecture de cycle simplifiée. Dans ce travail, on étudie des écoulements de gaz denses à travers des turbines ORC axiales multi-étagées supercritiques, à l'aide d'un code numérique includant des lois d'état complexes et un schéma de discrétisation d'ordre élevé. Plusieurs fluides de travail sont pris en compte et les performances des turbines supercritiques sont comparées à celles de turbines subcritiques utilisant les mêmes fluides.

Abstract :

Many recent studies suggest that supercritical Organic Rankine Cycles have a great potential for lowtemperature heat recovery applications, since they allow better recovery efficiency for a simplified cycle architecture. In this work we investigate flows of dense gases through axial, multi-stage, supercritical ORC turbines, using a numerical code including advanced equations of state and a high-order discretization scheme. Several working fluids are considered, and performances of supercritical turbines are compared to those of subcritical ones using the same fluids.

Mots clefs : ORC turbine ; dense gas ; supercritical fluid 1 Introduction

Organic Rankine Cycles (ORCs) are Rankine cycles using an organic fluid instead of water working fluid. ORCs have been largely studied for their relatively high efficiency [START_REF] Schuster | Energetic and economic investigation of Organic Rankine Cycle applications[END_REF] at such unfavorable conditions. ORC technology has been applied in geothermal [START_REF] Shegjun | Performance comparison and parametric optimization of subcritical Organic Rankine Cycle (ORC) and transcritical power cycle system for low-temperature geothermal power generation[END_REF] and biomass [START_REF] Drescher | Fluid Selection for the organic Rankine Cycle (ORC) in biomass power and heat plants[END_REF] fired power plants, bottoming cycles for combined cycle power plants [START_REF] Invernizzi | Bottoming micro-Rankine cycles for micro-gas turbines[END_REF], solar reverse osmosis desalination plants, and others, allowing efficiency improvements, reduction of the cycle size and production in modular units. Many studies have demonstrated that supercritical ORCs, i.e., ORCs in which heat is supplied at a pressure greater than the liquid/vapor critical point pressure, have an even greater potential, since they allow better recovery efficiency for a simplified cycle architecture [START_REF] Karellas | Supercritical fluid parameters in organic Rankine cycle applications[END_REF]. The selection of the most suitable working fluid is of crucial importance in designing an ORC process, since it requires to take into account thermodynamic, environmental and safety aspects [START_REF] Liu | Effect of working fluids on organic Rankine cycle for waste heat recovery[END_REF].

In this work, we investigate supercritical flows of dense gases/vapors through axial, multi-stage, ORC turbines, using a numerical code with a high-order discretization scheme. Three working fluids are considered : the refrigerants R134a and R245fa, and carbon dioxide (CO 2 ). The two organic fluids are characterized by complex molecules and moderate to high molecular weights, and can be classified as dense gases according to Cramer and Kluwick [START_REF] Cramer | On the propagation of waves exhibiting both positive and negative nonlinearity[END_REF]. At this stage, we restrict our attention to inviscid flow effects related to the peculiar thermodynamic behavior of each fluid. In most cases, the thermodynamic working conditions of these gases are such that the ideal-gas approximation is no longer valid and real-gas effects become significant. For this reason, an advanced multiparameter equation of state is required to model the fluids. Hereafter, we first briefly recall the governing equations and thermodynamic models in use, then we describe the ORC turbines under investigation, compare the different flow behaviors obtained with different working fluids, and comment on entropy losses associated to shock wave formation as a function of fluid complexity.

Governing Equations and Flow solver

Since in this work we are mainly concerned with the impact of thermodynamic properties of fluids on their inviscid flow behavior, we restrict our attention to the Euler equations, written in integral form for a control volume Ω with boundary ∂Ω :

d dt ∫ Ω w dΩ + ∫ ∂Ω f • n dS = 0 (1)
where w = (ρ, ρv, ρE) T is the conservative variable vector, n is the outer normal to ∂Ω and f = (ρv, p Ī + ρvv, ρvH) T is the flux density ; v is the velocity vector, E the specific total energy, H = E +p/ρ the specific total enthalpy, p the pressure and Ī the unit tensor. These equations are completed by a thermal and a caloric equation of state :

p = p(ρ(w), T (w)) and e = e(ρ(w), T (w)) ( 2
)
where e is the specific internal energy and T the absolute temperature.

Dynamic behavior of dense gases is governed by the fundamental derivative of Gas Dynamics Γ [START_REF] Thompson | A Fundamental Derivative in Gas Dynamics[END_REF] :

Γ = 1 + ρ a ( ∂a ∂ρ ) s (3)
being ρ the density and a = [-v 2 (∂p/∂v) s ] 1 2 the speed of sound. Γ represents a measure of the rate of change of the sound speed. According to whether Γ is higher or lower than unity, the flow exhibits different sound-speed variation in isentropic perturbations : when Γ > 1, a drops in isentropic expansions and grows in isentropic compressions, which is what happens in common fluids ; an opposite behavior is obtained when Γ < 1. Thermodynamic regions with Γ lower than unity are observed for molecularly complex fluids at temperatures and pressures close to saturation conditions. The gas response is modeled through equations of state (EOS) based on Helmholtz free energy Φ, which represent the most accurate available models for the fluids of interest in thermodynamic regions close to the critical point and the saturation curves [START_REF] Lemmon | Short Fundamental equations of state for 20 industrial fluids[END_REF]. EOS are written in the form proposed by Setzmann and Wagner [START_REF] Setzmann | A new method for optimizing the structure of thermodynamic correlation equations[END_REF] : Φ(δ, τ ) = Φ 0 (δ, τ ) + Φ r (δ, τ ). They use a reduced form (variables normalized with critical-point properties) and are composed by an ideal-gas part, Φ 0 , function of the ideal-gas isobaric heat capacity, and by a residual term, Φ r taking into account real-gas corrections.

Φ r (δ, τ ) = M 1 ∑ m=1 a m δ i m τ j m + M 2 ∑ m=M 1 +1 a m δ i m τ j m exp(-δ k m ) + M 3 ∑ m=M 2 +1 a m δ i m τ j m exp[-α m (δ -ϵ m ) 2 -β m (τ -γ m ) 2 ] ( 4
)
where δ = ρ/ρ c is the reduced density and τ = T c /T is the inverse reduced temperature. The number of polynomial and exponential terms and the values of coefficients and exponents are calibrated from experimental data using an optimization algorithm described in [START_REF] Setzmann | A new method for optimizing the structure of thermodynamic correlation equations[END_REF] . A in-house flow solver has been equipped with reference EOS available for R134a [START_REF] Tillner-Roth | An international standard formulation for the thermodynamic properties of 1,1,1,2-tetrafluoroehtane (HFC-134a) for temperatures from 170 K to 455 K and pressures up to 70 MPa[END_REF] and CO 2 [START_REF] Span | A new equation of state for carbon dioxide covering the fluid region from the triple-point temperature to 1100 K at pressures up to 800 MPa[END_REF]. For R245fa, no reference EOS is available, thus we adopt the short technical multiparameter EOS proposed by Span and Wagner [START_REF] Lemmon | Short Fundamental equations of state for 20 industrial fluids[END_REF].

The governing equations are discretized using a cell-centered finite volume scheme for structured multiblock meshes of third-order accuracy. The equations are then integrated in time using a four-stage Runge-Kutta scheme. Local time stepping, implicit residual smoothing and multi-grid acceleration are used in order to drive the solution to the steady state. The accuracy of the numerical solver, already demonstrated in previous works [START_REF] Cinnella | Numerical solver for dense gas flows[END_REF][START_REF] Cinnella | Aerodynamic performance of transonic Bethe-Zel'dovich-Thompson flows past an airfoil[END_REF], will be not investigated further.

The geometrical configurations under investigations are axial reaction turbines with a variable number of stages according to the working fluid in use. The reaction degree is fixed to 0.5. The rotational speed is fixed at 3000 RPM to allow direct coupling with the alternator. The number of stages and their relative average radius are calculated in the pre-design phase by considering the operating conditions (inlet total conditions, mass flow and pressure ratio), as well as cost and system complexity considerations. A three-stage turbine working with refrigerant R245fa is designed to work at subcritical nominal conditions, and four-stage turbines, designed for supercritical nominal conditions, are used with fluids R134a and CO 2 . The blades use the same airfoils for both stator and rotor wheels. Both the subcritical and supercritical turbines were tested by using both R134a and R245fa as the working fluid, to check the impact of different working fluids on the overall performance of a given turbine.

The different turbine configurations under investigation are described in Tab. 1. Blade vanes are discretized by C-meshes made of 272x32 cells. Total temperature and pressure are imposed at inlet, static pressure at the outlet, periodicity at upper and lower boundaries, and no-slip at the wall. A mixing plane condition is used at stator/rotor interfaces.

Numerical simulation results

For each turbine configuration and each stage, we computed the isentropic efficiency defined as the real-to-ideal static enthalpy jump ratio. In the present 2D inviscid calculations, only losses due to shock waves can be taken into account. Numerical results show that shock waves are generated, at upper surfaces of rotor blades, for some turbine configurations and operating conditions. However, these shocks remain relatively weak, and the associated entropy losses small. Even in the case the flow field is entirely shock-free, the computed efficiencies are slightly lower than unity, because of errors introduced by the numerical approximation of governing equations and boundary conditions.

Fig. 1 shows distributions of relative Mach number, Fundamental Derivative of Gas Dynamics Γ, and sound speed for case SUPR134a. Weak shock waves are created on the suction side of the rotor blades for all stages. Going from the first to the last stage, Γ becomes lower than unity, and the sound speed increases as the expansion proceeds. Since flow velocity grows more slowly than the sound speed, stronger shocks are generated in the first stages where Mach is higher ; this is reflected by stage isentropic efficiencies, which increase as shocks weaken (see Tab. 2). For case SUPR245fa shown in Fig. 2, a similar behavior is observed. Nevertheless, entropy losses are higher than for R134a. In this case, Γ is lower than unity through the entire turbine : as a consequence, the speed of sound increases during the expansion. However, its value in the upstream stages is much lower than in SUPR134a, whereas flow speed is almost the same, so that Mach number is higher and shocks stronger. This results in efficiencies roughly 5% lower than in the SUPR134a case.

For subcritical configurations SUBR134a and SUBR245fa (not shown for brevity), the overall performance is lower. Lower isentropic efficiencies are due to the fact that the computed Γ value is nearly constant and close to unity : the sound speed growth is negligible with respect to the flow speed one, so that Mach number and shock strength increase moving downstream. Performance loss is partly due to the fact that the subcritical turbine is designed for 3 stages instead of 4 ; as a consequence, each stage processes a higher pressure ratio with respect to the supercritical turbine (see Tab. 1). Subcritical turbine efficiencies about 1.5% lower w.r.t. supercritical configurations.

Finally, case SUPCO 2 is shown in Fig. 3. In this case the flow remains always subsonic and no shocks are formed, since carbon dioxide has a sound speed about double than R134a and R245fa. Thus, even if Γ is higher than unity and the speed of sound decreases with fluid expansion, it is still high enough to prevent the flow from becoming supersonic. In the absence of shocks and viscous effects, isentropic efficiencies differ from unity only because of numerical errors. According to these numerical results, CO 2 seems to be the working fluid that offers the highest adiabatic efficiency. Nevertheless, the choice of an optimal working fluid for ORC turbines also involve other kinds of consideration, like economical and safety considerations. For instance, supercritical CO 2 cycles involve pressures of the order of 80 bar (the inlet pressure being of about 150 bar in this case), which increases fabrication and installation costs. In addition, the high pressure drop per each stage, greater than 20 bar, could lead to significant leakage flows with respect to the organic fluid turbines.

Conclusions

In this work, dense gas flows through supercritical multistage axial ORC turbines were analyzed by means of a in-house dense gas numerical solver equipped with high-accurate multiparameter equations of state based on Helmholtz free energy and using a high-order finite volume scheme. Calculations were carried out for 3 different supercritical turbine configurations. Two subcritical ORC turbines were also studied for comparison. Steady, inviscid, two-dimensional numerical simulations were carried out in order to evaluate entropy losses associated to shock wave formation in the different cases. Different behaviors were observed according to the working fluid and turbine configuration considered. The Fundamental Derivative of Gas Dynamics Γ was studied to understand and explain differences in the computed results. Shock-wave formation has a crucial impact on the overall performance : carbon dioxide provides an optimal behavior since, because of the high values of the speed of sound in this fluid, the flow field is completely subsonic and no shocks are created. The use of R134a ensures satisfactory adiabatic efficiencies, both for the subcritical and the supercritical configuration, despite the presence of weak shocks at the suction sides of rotor blades, whereas R245fa develops, for the turbine configuration studied, stronger shocks leading to more significant losses. For both fluids, the use of supercritical inlet conditions tends to increase turbine isentropic efficiency for a given pressure ratio since, at high pressures, their thermodynamic behavior significantly deviates from that of a perfect gas, slowing down the increase of the Mach number during turbine expansion, and leading to weaker shocks. In the next future, we plan to take into account viscous and unsteady effects. Also, 3D calculations are planned as future work.

( a )

 a Speed of sound. (b) Mach number. (c) Fundamental Derivative of Gas Dynamics.

Figure 1 -

 1 Figure 1 -Mach, sound speed and Γ contours for case SUPR134a. M = 1 and Γ = 1 : ---.

( a )

 a Speed of sound. (b) Mach number (c) Fundamental Derivative of Gas Dynamics.

Figure 2 -

 2 Figure 2 -Mach, sound speed and Γ contours for case SUPR245fa. M = 1 : ---.

Table 1 -

 1 Summary of the considered turbine configurations ; β i are stage pressure ratios, β t ot the total one.

	Parameters SUBR134a SUBR245fa SUPR134a SUPR245fa SUPCO 2
	p0 (bar)	10.4	9.5	47.1	46.9	150.5
	T0 (K)	315.51	370.15	396.57	450.43	416.21
	Stages	3	3	4	4	4
	β 1	1.832	1.840	1.703	1.706	1.214
	β 2	1.819	1.823	1.630	1.652	1.229
	β 3	1.836	1.838	1.596	1.605	1.242
	β 4	-	-	1.586	1.593	1.258
	β tot	6.118	6.165	7.026	7.208	2.331

Table 2 -

 2 Adiabatic efficiencies for each stage.

	Stage SUBR134a SUBR245fa SUPR134a SUPR245fa SUPCO 2
	1	95.07	92.55	94.63	91.12	98.72
	2	94.03	89.59	95.80	91.99	98.27
	3	92.94	88.36	95.87	92.45	99.86
	4	-	-	98.41	93.62	99.11


Alaedine Kossi

Jacques Persello

LPMC Bernard Cabane

Acetylacetone stimulus effect on electrorheological properties of TiO 2 aggregated nanoparticles

Acetylacetone stimulus effect on electrorheological

properties of TiO2 aggregated nanoparticles.

INTRODUCTION

Electrorheological (ER) fluids are made by suspending particles in a liquid dielectric constant and conductivity of which are mismatched to create dipolar particle interactions in the presence of an ac or dc electric field [START_REF] Winslow | Induced fibration of suspensions[END_REF]. The ER phenomenon is related to changes in rheology of dispersions. Indeed, in response to an applied electric field, ER fluids increase their resistance to flow, and in some cases, they manifest in the conversion of fluid into solid behavior [START_REF] Block | Electro-rheology[END_REF]. The main cause inducing this phenomenon is the polarization of particles which can be aligned in the direction of an applied electric field. For this reason, the ER effect, besides the nature of dispersed particles and insulating liquid, is sensitive to the electric field and its frequency and all additives concentration that can affect the dielectric properties of the fluid. Several authors have shown that the ER response decreases with the increasing frequency of the electric field beyond a frequency of 100 Hz [START_REF] Ma | Frequency dependent electrorheological properties: origin and bounds[END_REF]. Other workers have studied the effects of additives such as water, urea, or other polar molecules [START_REF] Ikazaki | Mechanisms of electrorheology: the effect of the dielectric property[END_REF][START_REF] Wen | The giant electrorheological effect in suspensions of nanoparticles[END_REF][6][START_REF] Cho | Enhanced electrorheology of conducting polyaniline confined in MCM-41 channels[END_REF][START_REF] Ying | Electrorheological fluids: smart soft matter and characteristics[END_REF][START_REF] Ying | Silica nanoparticle decorated polyaniline nanofiber and its electrorheological response[END_REF] on performance of an ER fluid. These studies have shown that these additives improve the ER response of ER fluids in terms of shear stress or elastic modulus.

In this study, our aim is to investigate the influences of adsorbed acetylacetone (Acac), as polar molecules, and electric field frequency, on the dielectric properties and their impacts on the ER espo se of tita iu dio ide ER fluid, i te s of elasti odulus G' a d loss odulus G". The hoi e of Acac is motivated by its dielectric and polarizing properties (Acac has a high dielectric constant r ≈ . a d a dipole o e t µ ≈ . D (1D = 3.33564 10 -30 C.m)), and because its good affinity with TiO 2 surface, we expect that the adsorbed Acac can enhance the ER response of the TiO 2 ER fluids.

The system consists of aggregated nanoparticles of titanium dioxide dispersed in silicone oil. In order to control the polarization forces, the dielectric constants of the titanium dioxide particles are modified by adsorbing different Acac concentrations on titanium dioxide surface. We use a Scanning Electron Microscopy, X-Ray diffraction and small-angle neutrons scattering to characterize titanium dioxide morphology, and UV spectroscopy to determine the adsorption isotherm of Acac on titanium dioxide surface. The dielectric properties of titanium dioxide dispersed in silicone oil are measured using impedance spectroscopy. We analyze the ER response under ac electric field depending in adsorbed Acac amount onto TiO 2 surface and in electric field frequency.

EXPERIMENTAL SECTION

Synthesis of nanoporous TiO 2 powder

Nanosized TiO 2 powder was synthesized via a sol-gel method [START_REF] Music | Chemical and microstructural properties of TiO 2 synthesized by solgel procedure[END_REF], using titanium tetraisopropoxide (Sigma-Aldrich, 98%, solution), 2-Propanol (Sigma-Ald i h, ACS eage t, . % , aluminum isopropoxide (Aldrich, 99.99%), ammonium hydroxide (Sigma-Aldrich,28% NH 3 in H 2 O), Acac (Sigma-Ald i h, Reage tPlus®, % , it i a id Sig a-Aldrich, ACS reagent, 70%), acetic acid (Sigma-Ald i h, ACS eage t, . % a d ult apu e ate M .cm) as starting materials.

In a typical procedure, titanium (IV) tetraisopropoxide (0.388 mol), was rapidly added to ultrapure water (640 ml) and then stirred for 30 min. A white precipitate formed immediately upon addition of the titanium (IV) isopropoxide. The resultant colloid was recovered by centrifugation (10,000 x g for 30 min). The centrifugation cake was added into a jacketed three-necked flask reactor equipped with a mechanical stirring containing 750 ml of an aqueous solution, of molar composition ; 0.36 M nitric acid, 1 M acetic acid and 0.05 M aluminium isopropoxide. The pH of the colloidal solution after addition of the cake was measured to be between 1 and 2.

Peptization occurred after heating the product at 80 °C for 1 h under medium stirring, whereupon the slurry became a stable sol. Then, the sol was cooled down to room temperature, and 17 M ammonium hydroxide solution was added drop wise into the TiO 2 sol under mild stirring to form a white gel. The pH of the resultant colloidal suspension was measured to be close to 7. The gel was then collected by centrifugation (10,000 x g for 30 min) and dried at 110°C over night. The powder was crushed and ground into fine powder using a mortar and pestle and was further calcined in air at 500°C for 4 h.

Characterization of the nano-porous TiO 2 particles

Electron Microscopy was performed on the CCMA EM Core Facility (University of Nice Sophia Antipolis) using a Scanning Electron Microscopy apparatus (JEOL 6700F) to characterize the TiO 2 obtained after heat treatment at 500°C. The crystal structure of the obtained TiO 2 powder was analyzed using an X-ray diffractometer (CPS120/Cu) (XRD K α Cu, λ= , .

Morphological characteristics of aggregated particles can be deduced from the small angle neutron scattering [11] spectra realized on a TiO 2 heat-treated at 500°C and dispersed in silicone oil.

The SANS experiments of a concentrated TiO 2 /PDMS suspension are realized at the ILL institute using the D11 instrument. In a typical signal of concentrated dispersions where isotropic interactions between identical particles are considered, the average distance between the nearest neighbors is given by = / 0 where q 0 corresponds to the maximum in scattered intensity I max (q 0 ). In the case of two-phase system, at large q vectors, the interface between particles and solvent is observed at small scale. The Porod limit can be used for a sharp interface (in the normal direction but smooth in any parallel direction) where the scattered intensity decreases as q -4 does at large q. the Porod limit in this case reaches a constant at large q values and is related to the specific area of particles (independently of the shape of the two phase) as [START_REF] Marchal | Small-angle neutron scattering by highly oriented hybrid bilayer membranes confined in anisotropic porous alumina[END_REF]:

V S cste I q q q / ) ( 2 lim 2 4
(1) Whe e Δ 2 , given in m -4 , is a constant related to the difference in contrast between the TiO 2 and PDMS [START_REF] Schweizer | Neutrons et matériaux: introduction[END_REF], and S/V is the surface per volume unit ratio. The specific area denoted S S (given in m 2 /g) is connected to S/V through the relation:

V S d S TiO s . 1 0 2 (2)
where d 0 TiO2 is the TiO 2 density. q q I q 4 lim (q.a>10) is determined experimentally by plotting q 4 Iq versus q.

For porous systems (aggregates formed from nano-particles assembling), in some cases, the two length scales: nanopores (or nano-particles) of few nanometer on one hand, and micronic grains on the other are largely separated. A typical signal may be obtained where three different regimes a e o se ed o all sa ples: I a Po od's egi e at lo alues < 1 ) due to the envelope of the g ai s, II a t a sitio egi e i et ee , a d III a se o d Po od's egi e at high alues > 2 )

due to the inner pores of the grains [START_REF] Girard | Time resolved alteration process of oxide glasses[END_REF]. Solvent PDMS is present in the inner pores and intergrain porosity. Introducing V e as the total volume of the envelopes of the grains, being the total inner surface of the solid grains, and S e as the total surface of the envelopes of the grains, one gets:

from high-Po od's la :

2 2 4 2 1 ) ( 2 ) ( ) ( q I q m V e (3)
from low-Po od's la :

2 1 4 1 1 ) ( 2 ) ( ) ( q I q m V S e e (4)
Fi all , o i i g the t o Po od's la e o tai :

e S q I q q I q ) ( ) ( 1 4 1 2 4 2
(5)

Preparation of ER fluids

In a typical procedure, several solutions of various amounts of Acac [START_REF] Buemi | An AM1 study of their molecular structures and keto-enol tautomerism[END_REF] in PDMS oil silicone (Dow Corning Corporation XIAMETER ® PMX-200 silicone fluid, viscosity 10 cSt at 25°C) were prepared (in the range of 0 -3.215 wt%). The final ER fluids were obtained by dispersing 5 g of TiO 2 powder in 5 g of Acac silicone oil solutions by means of a Sonifier Vibra-cell (75041) ultrasonic apparatus operating at 30% amplitude for 4 min, to have a volume fraction of 0.2 of TiO 2 in PDMS. Then the ER fluid was out-gassed at room temperature and at 10 -3 Pa for 1 h. Afterward, dry N 2 was introduced into the bowl up to a pressure of 1 atm to avoid any contamination by the humidity of the air. The ER fluids were stored in desiccators before use.

Adsorption isotherms of Acac onto TiO 2 dispersed in PDMS

Various amounts of Acac were added to TiO 2 -PDMS dispersions, and the part of Acac that had reacted with TiO 2 in each sample was determined through the depletion method: after 48 h of equilibration, the TiO 2 particles were separated through centrifugation, and the concentration of Acac in the supernatants was measured through UV spectroscopy after filtration of the samples through 0.22 µm filters.

We defi e su fa e o e age, θ, as the fraction of the adsorption sites of TiO 2 occupied by Acac molecules; at equilibrium we have:

2 2 . . TiO TiO Acac a Acac ads Acac S m A N M m (6)
where, A°A cac is the area occupied by one molecule of Acac, M Acac is the Acac molar mass, N a is the Avogadro number, m TiO2 is the TiO 2 weight and S TiO2 is the TiO 2 specific area.

The experimental surface coverage can be compared with θ being calculated from the Langmuir equation [START_REF] Langmuir | The constitution and fundamental properties of solids and liquids[END_REF][START_REF] Liu | Study of interfacial charge-transfer complex on TiO 2 particles in aqueous suspension by second-harmonic generation[END_REF], it is given as follows:

équil Acac équil Acac Langmuir b b 1 (7)
where, equil Acac is the volume fraction of unadsorbed Acac in equilibrium with the dispersion. b is adsorption/desorption equilibrium constant of Acac on TiO 2 , it is given by following:

Electrorheological measurements

The dynamic properties of electrorheological fluids were measured by means of a Rheometrics Dynamic Analyzer II (RDA II, Rheometrics, Inc., NJ) in time sweeps mode. These measures consist of applying an alternating electric field (Trek Model 609E-5 High Voltage amplifier and Hameg function generator HM 8030-5 on a sample placed between two parallel plane-plane geometry electrodes and measuring moduli as a function of time until a stationary state is reached. The diameter of plane electrode was 25 mm, and the gap between electrodes was 1 mm. The moduli are measured at low shear strains, where dynamic properties are independent of shear strain amplitude. The mobile ele t ode uppe ele t ode applies a os illato shea st ai ith m =0.2% of amplitude, at constant te pe atu e of °C a d ith a a gula elo it of ω= ad/s. A a ete TENMA -7735

Multimeter) is used to measure the electrical current passing through the sample during rheological measurements under applied electric field.

Complex relative permittivity measurement

We use an HP 4192A LF impedance analyzer to measure the dielectric properties of the prepared ER fluids. The frequency range of the applied alternating field was between 5Hz and 1MHz. The dielectric cell consists of parallel stainless steel electrodes separated by a distance l that we can change. The electric circuit can be represented by a parallel equivalent schema (Fig. 1) where we consider the contribution, in total measured impedance (G, C), an impedance (G k , C k ) due to the sample polarization, a stray impedance (G e , C e ) due to electrode polarization, and a residual capacitance (C r ) due to the field lines that are not located in the sample. We can determine G e and C e by measuring the total impedances at different electrode distances for a given frequency. To calculate C r , we measure capacitance C of an empty cell as a function of the inverse of distance between electrodes, C is given by:

l S C C R 0 (9)
he e 0 is the vacuum permittivity, S * is the electrode effective surface, and l is the distance between parallel electrodes.

Henceforth, once these corrections are made, we can determine the samples impedance (G k , C k ) [18,[START_REF] Wu | An enhanced polarization mechanism for the metal cations modified amorphous TiO 2 based electrorheological materials[END_REF] that can be related to the complex relative permittivity by: [START_REF] Music | Chemical and microstructural properties of TiO 2 synthesized by solgel procedure[END_REF] where C 0 is the vacuum capacitance, a d ω is the a ele t i field a gula f e ue .

Experimental measurements of (ε rk ') and (ε rk ") obtained from Eq. ( 10) are fitted by using the Maxwell-Wagner equations [START_REF] Dudley | Low frequency impedance behavior of montmorillonite suspensions: Polarization mechanisms in the low frequency domain[END_REF], yielding ε rk ' and ε rk " as: [START_REF] Marchal | Small-angle neutron scattering by highly oriented hybrid bilayer membranes confined in anisotropic porous alumina[END_REF] where, (ε rk ') s and (ε rk ') ∞ are, respectively, the stati alue he ω te ds to a d the opti alue he ω te ds to ∞ of elati e pe itti it , σ k ' is the sample conductivity, a d is the ela atio time of the dielectric function.

) ' ( 1 )

RESULTS AND DISCUSSION

Morphology and structure of TiO 2 aggregated nanoparticles SEM performed on TiO 2 after heat treatment at 500 °C (Fig. 2) shows nanosized particles (between 10 and 20nm) highly aggregated forming a porous system. The x-ray spectra (Fig. 3) show that the obtained TiO 2 powder had an anatase crystalline structure. The density of TiO 2 was determined using a standard flask pycnometer of 5 cm 3 , and we found a value of 3.84g/cm 3 that is close to the TiO 2 anatase density (3.89g/cm 3 ).

The SANS curve giving the scattered intensity Iq versus wave vector q is shown in (Fig. 4a). A a i u of the s atte ed i te sit is o se ed fo a i te ediate alue of a e e to I a ≈ 3.56.10 -1 nm -1 , hi h ha a te izes the a e age i te pa ti le dista e = / max , in our case (adhesive particles heat treated at 500°C) we can confuse r with the average size of the nanoparticles. We obtain the average radius of the TiO 2 a opa ti les a out a ≈ . hi h is i the sa e a ge as that observed in the SEM picture. From the Porod plot (Fig. 4b) at very high value of q (where Iq is proportional to q -4 and q.a>10), we can use the Porod limit (Eqs. ( 1) and ( 2)) to calculate the specific area of TiO 2 particles. We obtain a specific area S S ≈ 2 /g. (Fig. 4b) shows also two porod's regimes: the first for q < q 1 = 0.09 nm -1 which characterizes the interface of the envelope of the grains S e , the second for q > q 2 = 0.53 nm -1 which can be attributed to the surface of inner porous structure σ. Usi g Eq. ( 5), a high atio σ/ S e ) = 263 was obtained indicating that the porosity of the system is dominated by inner nanoporous structure. ) as a function of wave vector q (A°-1 )

Influences of electric field and its frequency on electrorheological properties

The elastic modulus G' a d loss odulus G" fo TiO 2 -Acac/PDMS ER fluid measured in the applied electric field range 0-4 kV/mm versus electric field frequency are shown in (Fig. 5a,b).

We ote that G' a d G" de ease ith the increasing electric field frequency, and this occurs whatever be the applied electric field supportable by the sample. We also note that the increase in electric field frequency is accompanied by a decrease in the maximum of electric field E max supportable by the apparatus. This limitation of applied electric field is due to the appearance of an electric current density which is insignificant at low frequency, but becomes increasingly important as soon as the frequency increases. (Figure 6) illustrates this situation where at lo f e ue = Hz E max can be up to 4kV/mm, and the electric current density in the sample is very low (j (Emax) =0.01µA/mm 2 ), while at high f e ue = kHz E max is limited to 1.5kV/mm (j (Emax) ≈ . µA/ 2 ); beyond this limit, the measurement is interrupted because of the high conduction in the sample.

Dielectric properties of TiO 2 -Acac/PDMS ER fluid

To understand the link between the ER behavior depending on the frequency and the dielectric properties, we have made measurements of the complex impedance versus frequency for TiO 2 -Acac/PDMS ER fluid. Figure 7 shows the experimental and the calculated results (using the Maxwell-Wagner equations) of the real part of elati e pe itti it r ' Fig. 7a) and imaginary part r " of elati e pe itti it Fig.

e sus f e ue ν fo TiO 2 -Acac/PDMS ER fluid.

We observe that at low frequency we have a macroscopic polarization due essentially to an interfacial polarization where its contribution to dielectric constant is maximal because the motions of the charge carriers that are accumulated at the interface between TiO 2 and insulating medium. At high frequency, the electric field alternates so fast such that the interfacial polarization cannot increase, and there is no contribution to the dielectric constant. However, between these two limits (low and high frequency), at intermediate frequencies, the interfacial polarization begins to lag behind the electric field causing dissipative losses [21]. This energy dissipation is maximal when the loss ta ge t tg = r "/ r ' ea hes a a i u alue at a a gula f e ue ω m that can be related to the ha a te isti ela atio ti e as follo s: The influence of frequency on the polarization in terms of dielectric losses caused by the e e g dissipatio i the sa ple a e plai the ER eha io he e the elasti odulus G' de eases as the electric field frequency increases between 10 Hz and 10 kHz. The ER results and dielectric characterization clearly show that ER effect is optimal at low frequency. The following ER experiences will be investigated with a fixed applied electric field E = kV/ a d ith a f e ue = Hz to ha e a ER effect satisfactory without the occurrence of a current strong enough, energy consumer and disruptive of ER measurement.

Influence of the added Acac on electrorheological properties

To quantify the role of Acac on the TiO 2 /PDMS ER properties, the Acac onto TiO 2 surface adsorption isotherm is shown in (Fig.

. The e pe i e tal esults gi i g the su fa e o e age θ as a function of the volume fraction of free Acac in equilibrium with the dispersion are compared to those calculated from Langmuir equation. The best fit between experiment and Langmuir equation was o tai ed usi g a i te a tio e e g =-8.85 10 -20 J (-21.5kT) and an adsorption/desorption equilibrium constant b=6.88 10 3 where the adsorption of Acac onto TiO 2 surface reaches a plateau for θ ≈ . 0 corresponding to a maximum adsorption about 2Acac/nm 2 . However, a discrepancy was observed between the calculated and the e pe i e tal esults fo θ > . suggesti g that A a is adsorbed onto TiO 2 surface in a multilayer manner. The results show also that the Acac adsorption onto TiO 2 su fa e is i tuall total fo θ . , while beyond this limit, the Acac remaining free in PDMS e o es sig ifi a t a d i eases ith su fa e o e age θ. Figure 9 shows that the intrinsic ER properties of TiO 2 ER fluid ithout addi g A a θ= a e e o e t a d ea h a elasti odulus G' of a out kPa a d G" of a out kPa ith a applied ac electric field of 1kV/mm and 10Hz of frequency. It also shows that, for the same electric field and dynamic shear rheology conditions, the increase of adsorbed Acac onto TiO 2 surface leads to a i ease of G' u til it reaches a a i u G' max ≈ , MPa hi h is . ti es greater than that for pure TiO 2 /PDMS ER fluid. This maximum corresponds to the su fa e o e age θ≈ .45 (about 1Acac/nm 2 . Be o d this li it G' decreases. As observed in (Fig. 8), we can attribute this behavior to the TiO 2 /Acac/PDMS adsorption equilibrium that may affect the dielectric properties of ER fluid. I deed fo θ . the a ou t of Acac, remaining free in the dispersing medium is negligible. Hence, the increase in surface coverage, from 0 to 0.45, results in an increase of interaction energy between particles due to the improvement of the interfacial polarization of the particles, by increasing the number of charge carriers that move to the interface between the TiO 2 particles and the insulating ediu . While fo θ > . , the is at h o diele t i o sta t a d o du ti it et ee dispe sed phase (TiO 2 ) and insulating medium (PDMS) begins to fall as θ i eases due to the increase of Acac dipoles remaining free in PDMS. Therefore, the part of the interaction energy that is dissipated through the insulating medium becomes significant leadi g to a de ease i G'. Figu e shows also, that loss modulus G" e ai s irtually constant over the entire θ a ge. This ea s that adso i g Acac dipoles on TiO 2 su fa e, o l the elasti it depe de e of G' o the i te a tio e e g as influenced by adding Acac, a d i this d a i os illatio o ditio s = . % a d ω =100rad/s) there are no restructuring (no change on viscosity) and the deformation of the aligned structures developed by the applied electric field that still remains unchanged.

To validate the TiO 2 -Acac/PDMS ER performance, we carried out measurements of elastic modulus (Fig.

he e the applied a ele t i field = Hz s it hes et ee a d kV/ su essi el . We fi d that the elasti odulus G' i eases d a ati all ith the ele t i field and returns to its initial state as soon as the electric field is removed, and this is reproducible over several cycles where E switches between 0 and 1kV/mm successively. This result confirm that at low f e ue = Hz the studied s ste s a e e e si le. between TiO 2 particles and the insulating medium. For θ = . , G' ea hes . MPa hi h is . times greater than that of pure TiO 2 /PDMS ER fluid. Ho e e , G' de eases i easi g θ e o d 0.45; in such cases, the mismatch on dielectric constant and conductivity between dispersed phase (TiO 2 a d i sulati g ediu PDMS egi s to fall as θ i eases because of the increase of Acac dipoles remaining free in PDMS. The performance of TiO 2 -Acac/PDMS ER fluid at low frequency was alidated a e e si ilit test he e elasti odulus G' i eases d a ati all a d etu s to its initial state as soon as several cycles of a switching electric field were applied between 0 and 1 kV/mm successively.



  is the mass of one Acac molecule, h is the Pla k o sta t, k is the Boltz a o sta t, T≈ K is the te pe atu e a d is the i te a tio energy between TiO 2 and Acac.

Fig. 1 :

 1 Fig. 1: Schematic representation of the equivalent circuit for measuring dielectric properties.

Fig. 2 :

 2 Fig. 2: SEM picture of TiO 2 particles calcined at 500°C/4h.

Fig. 3 :

 3 Fig. 3: X-Ray spectra (XRD K α Cu, λ= , pe fo ed o TiO 2 particles after calcination at 500°C/4h, black line for synthesized TiO 2 calcined at 500°C/4h and red lines for pure TiO 2 anatase I41amd.

Fig. 4 : 1 )

 41 Fig. 4: The SANS spectra of the heat treated TiO 2 dispersed in PDMS: (a) giving the scattered intensity Iq (cm -1 ) versus wave vector q (A°-1 ); (b) giving the Porod representation q -4 Iq (cm -5

Fig. 5 :

 5 Fig. 5: Elasti odulus G' (a), a d loss odulus G" (b) versus electric field frequency at various applied electric fields of TiO 2 -Acac/PDMS ER fluid.

Fig. 6 :

 6 Fig. 6: Variation of the current density with respect to the applied ac electric field at various frequencies.

For

  TiO 2 -A a /PDMS ER fluid, e fou d ≈ . -5 s, o espo di g to m about 2.3 kHz. The f e ue elo m determines the range in which charge carriers are mobile on long distances. At f e ue a o e m , the charge carriers are confined to potential wells being mobile on short distance.

Figure 7 :

 7 Figure 7: TiO 2 -Acac/PDMS ER fluid dielectric properties, (a) eal pa t of elati e pe itti ity ε r ' e sus f e ue y; (b) i agi a y pa t of elati e pe itti ity ε r " e sus f e ue y o pa ed to the al ulated ε r ' a d ε r " usi g Maxwell-Wagner equations (M-W).

Fig. 8 :

 8 Fig. 8: A a /TiO i PDMS adso ptio isothe gi i g su fa e o e age θ e sus the f ee A a olu e f a tioat equilibrium in PDMS compared to that calculated trough Langmuir equation.

Fig. 9 :

 9 Fig. 9: Elasti odulus G' a d loss odulus G" e sus A a /TiO 2 surface coverage θ. The ha a te isti s of the applied electric field and the shear strain are, respectively: E=1kV/ ν E =10Hz γ m = . % a d ω γ = 100 rad/s. According to the Acac onto TiO 2 surface adsorption isotherm, four ER fluids samples were investigated where different situations are represented: pure TiO 2 /PDMS ER fluid θ= , total A a adsorption onto TiO 2 su fa e θ= . , and two samples with partial Acac adsorption onto TiO 2 surface, respectively: θ= . a d θ= . . I all sa ples, Φ V (TiO 2 /PDMS) = 0.2. We carried out easu e e ts of elasti odulus G' a d loss odulus G" o the four samples prepared above.

Figure 9

 9 Figure 9 ep ese ts the e pe i e tal esults of G' a d G" o tai ed f o the a ious sa ples prepared according to surface coverage ate, θ, of A a adso ed o to TiO 2 .

Fig. 10 :

 10 Fig. 10: Reversibility test; Elasti odulus G' e sus the s it hi g ele t i field et ee a d kV/ υ= Hz successively, for different amount of Acac adsorbed on TiO 2 surface.

Acknowledgments

The authors wish to thank Ralph Schweins from the Institut Laue-Langevin (ILL) for technical assistance in SANS experiments. The authors also gratefully acknowledge the financial support from the ANR CNRS.


Fatima Zahra Benamar

Cédric Demonceaux

Sanaa El Fkihi

Mustapha El

Driss Mouaddib

Aboutajdine

F Benamar
email: benamarfz@gmail.com

E Mouaddib

D Aboutajdine

Time to Contact Estimation on Paracatadioptric Cameras

Introduction

The theory of time to contact (TTC) was first introduced by Lee and Young [START_REF] Lee | A theory of visual control of braking based on information about time-to-collision[END_REF]. The goal of time to contact for robot navigation is to estimate the free space before the robot and, thus, to decide if the robot has to turn or to stop when the collision is imminent. This problem has been studied by many authors with a perspective camera such as [START_REF] Lee | A theory of visual control of braking based on information about time-to-collision[END_REF], [START_REF] Arnspang | Estimating time to contact with curves, avoiding calibration and aperture problem[END_REF]. If we consider that such camera moves along the optical axis Z, the time to contact is defined as :

τ = - Z Ż ( 1
)
where Z is the distance of the object which is approaching the camera, and Ż its velocity.

Let us consider a perspective camera of focal length f , and let us note m(x, y, f ) the projection of a 3D point M (X, Y, Z) on the camera plane. Thus:

x = f X Z and y = f Y Z (2)
Differentiating the equation of perspective projection [START_REF] Camus | Calculating time-to-contact using real-time quantized optical flow[END_REF], the TTC formula on perspective image is:

τ = x ẋ = y ẏ (3)
where ( ẋ, ẏ) is the motion field in the camera plane. The existing methods can be decomposed to three classes: 1)Optical flow based time-to-contact: [START_REF] Camus | Calculating time-to-contact using real-time quantized optical flow[END_REF].

2) Gradient-based time-to-contact: [START_REF] Horn | Hierarchical framework for direct gradient-based time-to-contact estimation[END_REF].

3) Time-to-contact from closed contours: using Green's theorem [START_REF] Marco | A set theoretic approach for time-to-contact estimation in dynamic vision[END_REF].

In this paper, we propose to estimate the TTC thanks to a catadioptric camera. A lot of works deal with the problem of TTC estimation with classical perspective camera, whereas few methods have been developed for catadioptric cameras. If we consider these sensors as spherical cameras, some works [START_REF] Mccarthy | Real time biologically-inspired depth maps from spherical flow[END_REF] can be used. In 1991, Tistarelli and Sandini [START_REF] Tistarelli | On the advantages of polar and log-polar mapping for direct estimation of time-to-impact from optical flow[END_REF] have described an estimation of time to contact using particular sensors based on polar and log polar representations. This method consists in using optical flow as a continuous function. Kazuaki Kondo et al. [START_REF] Kondo | Non-isotropic omnidirectional imaging system for an autonomous mobile robot[END_REF] employ this function using HBP mirror, but this method does not propose any adaptation of TTC in a paracatadioptric image. Thus, in order to compute TTC with a catadioptric camera, the aforementioned authors use the spherical equivalence of this type of image. We discuss in this paper the problem of estimating TTC maps, we propose to work directly in the image plane by using optical flow. We will see that due to the introduction of a mirror, perspective projection (2) is not appropriate to project 3D points to the image plan. So, equation (3) is not valid for the paracatadioptric camera. Firstly, we have to define a new formulation of TTC for omnidirectional images which takes into account the introduction of the mirror. In this formulation, we have to compute the optical flow, but for the same reasons, the classical optical flow methods are not valid. Thus, we will see how to estimate the op-tical flow in order to take into account the distortions introduced by the mirror. Finally, we will show some results with simulated and real image sequences.

TTC and paracatadioptric cameras

Let P be a fixed obstacle of coordinates (X, Y, Z) in the environment space R 3 . The image point is obtained thanks to a double projection. First, a 3D point P (X, Y, Z) is projected on the mirror on m p (x p , y p , z p ).

m p :      x p = h X √ X 2 +Y 2 +Z 2 -Z y p = h Y √ X 2 +Y 2 +Z 2 -Z z p = h Z √ X 2 +Y 2 +Z 2 -Z (4)
Then, as the projection between the mirror and the image plane is orthographic (see Fig. 1), we obtain the image point m(x, y):

m : x = α x x p + x 0 y = α y y p + y 0 (5)
Where (x 0 , y 0 ) are coordinates of a principal point and α x (respectively α y ) is a scale conversion mm to pixel in X direction (respectively Y direction).

Figure 1. Paracatadioptric camera

Nevertheless, due to this double projection, (3) becomes invalid. So, we need to find a new equation which connects the TTC with m(x, y). To this aim, let us consider a robot which is moving in a straight direction. Without loss of generality, we can suppose that this direction corresponds to X axis. If not, we can estimate the direction using [START_REF] Stratmann | Omnidirectional imaging and optical flow[END_REF] and rotate the camera axis relatively to this direction. Thus, the TTC equation is defined by:

τ = - X Ẋ (6)
Thanks to (4), we can write:

x p y p = X Y (7)
Then, by derivation, we obtain:

ẋp = Y (ypX)-(ypX) Ẏ Y 2 = X Y ( ẏp + yp X Ẋ - yp Y Ẏ ) (8)
Such as the robot is moving along X direction, we have Ẏ = 0 and in substituting xp yp by X Y , (8) becomes:

ẋp = x p y p ( ẏp -y p τ -1 ) (9)
Thus, in mirror frame, the TTC is defined by the following equation:

τ -1 = ẏp y p - ẋp x p . ( 10
)
Finally by using ( 10) and ( 5), we obtain the TTC expression in the image plane:

τ -1 = ẏ y -y 0 - ẋ x -x 0 . (11)
Equation ( 11) is an adaptation of the TTC equation in a paracatadioptric sensor. Let us note that the formulation is independent of h, α x and α y . Equation ( 11) gives an expression for calculating TTC in a paracatadioptric image at each pixel. Consequently, to compute the TTC on a point m(x, y), we have to estimate the apparent motion ( ẋ, ẏ) of this current point. Once again, while a lot of methods exist in a perspective camera for optical flow estimation, due to the introduction of a mirror, these methods must be revisited. Thus, we have chosen to use an adapted framework.

Optical flow

Many optical flow methods have been developed in the literature. The Lucas-Kanade's method is still one of the most popular solutions for motion estimation. Radgui et al. [START_REF] Radgui | An adapted lucas-kanade's method for optical flow estimation in catadioptric images[END_REF] have shown that this method is not valid when we deal with omnidirectional image. Indeed, due to the distortion introduced by the mirror, the motion field cannot be considered constant around a neighborhood. This is why, they have proposed to adapt the Lucas-Kanade's method by replacing the constant model by the following form:

u=a(x -x 0 ) 2 + a(y -y 0 ) 2 + c v=b(x -x 0 ) 2 + b(y -y 0 ) 2 + d (12)
where (x 0 , y 0 ) is the image center and (a, b, c, d) are constants. Moreover, this neighborhood cannot be fixed in all the image as in a perspective camera. [START_REF] Demonceaux | Markov random fields for catadioptric image processing[END_REF] has proposed to adapt the neighborhood in order to take into account the distortion of these atypical images. In this paper, we propose a similar neighborhood.

Experimental results

In order to validate our method, we have tested the proposed algorithm on synthetic and real sequences. Synthetic sequences. Our synthetic sequence is generated by Pov-ray 1 software. The sequence corresponds to a virtual robot which is moving with a uniform unidirectional motion towards X-axis. For all sequence, the intrinsic parameters 2 are fixed on α = 40, h = 2.3 and (x 0 , y 0 ) = (100, 100). The main advantage is that ground truth here can be exactly known. Three of the 100 stimulus frames corresponding to the time t = 2, t = 50 and t = 74 are shown in the top row of Fig. ?? in columns 1, 2 and 3. So, to improve the accuracy of TTC algorithm, we are applied [START_REF] Tistarelli | On the advantages of polar and log-polar mapping for direct estimation of time-to-impact from optical flow[END_REF] with two different techniques based on optical flow estimation: Lucas-Kanade optical flow and adapted Lucas-Kanade optical flow. We note, that, the purpose of this paper, is not a comparison of different problem solutions of optical flow in paracatadioptric images, but rather to show how estimation of optical flow can affect accuracy of calculating of TTC. The classical neighborhood windows 1 www.povray.org 2 We need h and α only to adapt optical flow estimation. To simplify this modelisation, we consider that αx and αy will be equal αx = αy = α. size used in Lucas-Kanade method is 15 * 15 and equivalently the adapted neighborhood windows [START_REF] Demonceaux | Markov random fields for catadioptric image processing[END_REF] ϑ is fixed by dθ = pi 25 and dϕ = pi 50 . The algorithm computes TTC at every pixel. To reduce effect of outliers, we use median filter at the neighborhood of the current pixel of size 6 * 6. Comparing TTC of Lucas-Kanade (TLK) with TTC adapted of Lucas-Kanade (TALK), we can see that TALK is able to estimate TTC correctly (see Fig. 2), in progress in time, results shown that TTC dropped linearly, near of end of sequence (t > 80), the results become unreliable (see Fig. 3). However, the main problem near the end of the sequence is temporal aliasing (i.e. missed value of TTC where objects move through each other due to discrete sampling). The behavior of the TTC expression is clearly shown in Fig. 3, we applied [START_REF] Tistarelli | On the advantages of polar and log-polar mapping for direct estimation of time-to-impact from optical flow[END_REF] in two pixels in the image, the first pixel is chosen away from the FOE (the points on the motion field where the flow vectors seem to be emerging [START_REF] Stratmann | Omnidirectional imaging and optical flow[END_REF] (see Fig. 4)) that the TTC is illustarted in Fig. 3, second pixel is chosen near of FOE that TTC is illustrated in Fig. 3. TLK was a bias in the estimate towards larger values than the true TTC (i.e the optical flow was overestimated) with pixel near of FOE. Thus, in the situation of translation discribed here, an other common problem to enhance estimation of TTC appears, it is the problem of expansion of optical flow. Thus, around the FOE, TTC is not defined for two reasons: first, because of the dividing on term xx 0 in [START_REF] Tistarelli | On the advantages of polar and log-polar mapping for direct estimation of time-to-impact from optical flow[END_REF], that it becomes zero in the expansion axis. Second, in the FOE point, optical flow is bad estimated. Video sequence. We have also used sequences of real omnidirectionnal images. The platform used for the experiment was a Pioneer-AT robot where we have embedded a paracatadioptric camera. The mobile robot was moving along the X-axis towards a subset of posters pinned on a wall (see Fig. 2 in the top row in column 4,5 and 6). The calibration of the camera was conducted with the toolbox Hyscac 3 . Unfortunately in this case, the real TTC is not exactly known, but the speed is approximately constant (= 12s) and distance is equal to 3.74m. Three of the 190 real frames corresponding to the time t = 52, t = 100 and t = 180 are shown in the top row of Fig. 2. So, in the same way of the previous section, we applied [START_REF] Tistarelli | On the advantages of polar and log-polar mapping for direct estimation of time-to-impact from optical flow[END_REF]

Conclusion

A new formulation of TTC using paracatadioptric images has been described in [START_REF] Tistarelli | On the advantages of polar and log-polar mapping for direct estimation of time-to-impact from optical flow[END_REF] and demonstrated 3 www.hyscas.com on both synthetic and real sequences. We have presented a new method which provides dense time-tocontact maps. This method reformulates the TTC for paracadioptric in order to take into account the distortion introduced by the mirror. The results show a good behavior of our method with an adapted optical flow. As a short time perspective, we will reduce the temporal aliasing introduced by the optical flow estimation by taking into account temporal filtering of the TTC.

Figure 2 .

 2 Figure 2. Estimation of TTC on synthetic and real sequence:The top row shows a progression of synthetic and real images at time. The second row shows the results of TLK and the third row shows results of TALK. Dark shading indicates short TTC, light shading shows long TTC, white shading representing the outliers value of TTC and black shading shows T T C = 0.

Figure 3 .

 3 Figure 3. Left column: TTC of pixel away from FOE. Right:TTC of pixel near of FOE.

Figure 4 .

 4 Figure 4. Example of an optical flow field with focus of expansion.

  with two different techniques based on optical flow estimation: Lucas-Kanade optical flow and adapted Lucas-Kanade optical flow. Overall, the results in Fig 2 show that the TTC algorithm is more appropriate with the adapted optical flow, but severals problem can reduce the robustness of the TTC estimation, when the camera is very close to the obstacle; the main problem is due to temporal aliasing (t > 180).


X Liu

J-M Mérolla

S Guérandel

E De Clercq

R Boudot

Ramsey spectroscopy of high-contrast CPT resonances with push-pull optical pumping in Cs vapor

We report on the detection of high-contrast and narrow Coherent Population Trapping (CPT) Ramsey fringes in a Cs vapor cell using a simple-architecture laser system. The latter allows the combination of push-pull optical pumping (PPOP) and a temporal Ramsey-like pulsed interrogation. An originality of the optics package is the use of a single Mach-Zehnder electro-optic modulator (MZ EOM) both for optical sidebands generation and light switch for pulsed interaction. Typical Ramsey fringes with a linewidth of 166 Hz and a contrast of 33 % are detected in a cm-scale buffer-gas filled Cs vapor cell. This technique could be interesting for the development of high-performance and low power consumption compact vapor cell clocks based on CPT.

Introduction

The development of CPT atomic clocks [START_REF] Vanier | Atomic clocks based on coherent population trapping: a review[END_REF][START_REF] Knappe | Emerging topics: MEMS Atomic Clocks[END_REF] of high frequency stability requires to detect a narrow atomic resonance with the highest signal to noise ratio. In traditional CPT clocks, atoms interact with circularly polarized laser light. Numerous atoms are then trapped in extreme Zeeman sublevels where they do not contribute to the magnetic field insensitive 0-0 clock transition. This technique limits to a few percent the contrast, defined as the signal height to background ratio, of the detected resonance. To circumvent this issue, different optimized double-lambda CPT pumping schemes such as push-pull optical pumping (PPOP) [START_REF] Jau | Pull-pull optical pumping of pure superposition states[END_REF], the use of polarized counterpropagating waves of σ + -σ -field configuration in small cells [4], lin-per-lin [5] or lin-par-lin [START_REF] Taichenachev | On the unique possibility of significantly increasing the contrast of dark resonances on the D 1 line of 87 Rb[END_REF] were proposed to maximize the number of atoms in the clock levels. More recently, Yudin et al. proposed a non-standard spectroscopic technique using a feedback control of the input probe field parameters to detect atomic resonances with high contrast [START_REF] Yudin | Feedback spectroscopy of atomic resonances[END_REF].

The detection of high contrast resonances with optimized CPT interaction schemes is known to be improved with high operating laser intensities [START_REF] Jau | Pull-pull optical pumping of pure superposition states[END_REF][START_REF] Zanon | High Contrast Ramsey Fringes with Coherent-Population-Trapping Pulses in a Double Lambda Atomic System[END_REF][START_REF] Zibrov | Coherent-population-trapping resonances with linearly polarized light for all-optical miniature atomic clocks[END_REF]. Nevertheless, in continuous regime, high intensities broaden the CPT resonance line, limiting the clock short-term frequency stability and increasing the light shift. To get rid of these limitations, a well-known technique is the Ramsey interference scheme using two separated oscillating fields [START_REF] Ramsey | A Molecular Beam Resonance Method with Separated Oscillating Fields[END_REF]. The first Ramsey fringes on CPT resonance were observed by Thomas et al. [START_REF] Thomas | Observation of Ramsey Fringes Using a Stimulated, Resonance Raman Transition in a Sodium Atomic Beam[END_REF]. Godone et al. used this method to develop a pulsed rubidium CPT maser exhibiting a frequency stability of 1.2 × 10 -12 τ -1/2 for measurement times up to τ ≃ 10 5 s [START_REF] Godone | Physics characterization and frequency stability of the pulsed rubidium maser[END_REF]. Zanon et al. proposed to combine the lin per lin interaction scheme with a temporal Ramsey-like pulsed interrogation [START_REF] Zanon | High Contrast Ramsey Fringes with Coherent-Population-Trapping Pulses in a Double Lambda Atomic System[END_REF]. The detection of high-contrast CPT Ramsey fringes with greatly reduced sensitivity to light shift was demonstrated [START_REF] Castagna | Investigations on continuous and pulsed interrogation for a CPT atomic clock[END_REF] leading to a clock short-term frequency stability of 7 × 10 -13 at 1 s [START_REF] Boudot | Current status of a pulsed CPT Cs cell clock[END_REF]. Esnault et al. reported CPT Ramsey fringes using a lin-par-lin scheme on cold rubidium atoms. A frequency stability of 4 × 10 -11 τ -1/2 was obtained [14]. Nevertheless, these clocks remain complex and voluminous systems.

Recently, interesting studies were proposed to implement advanced CPT clock architectures with original and simple setups [START_REF] Yim | Polarization orthogonalizer for a pair of laser beams with nearly equal frequencies[END_REF][START_REF] Yun | High coherent bi-chromatic laser with gigahertz splitting produced by the high diffraction orders of acousto-optic modulator used for coherent population trapping experiments[END_REF][START_REF] Zhang | Spin-polarized dark state free CPT state preparation with co-propagating left and right circularly polarized lasers[END_REF][START_REF] Yun | Multipulse Ramsey-CPT interference fringes for the 87 Rb clock transition[END_REF]. Up to date, to our knowledge, no frequency stability performances were measured with these setups. However, we consider that such investigations are interesting to make compact CPT atomic clocks suitable for industrial transfer that requires low power consumption and a reduced number of components.

In a recent study, we investigated CPT resonances in Cs vapor cells with PPOP [START_REF] Liu | Coherent population trapping resonances in buffer gas-filled Cs vapor cells with push-pull optical pumping[END_REF]. Resonance contrasts as high as 78 % were demonstrated in continuous regime in cm-scale cells. However, such contrasts were obtained for high laser intensities that induce CPT line broadening. We propose here an original simple-architecture laser system combining for the first time PPOP and Ramsey spectroscopy for the detection of high-contrast and narrow Ramsey fringes. While light switching for the Ramsey pulsed interaction is currently realized in vapor cell clocks by an additional high-power consuming and polarization-sensitive acousto-optical modulator (AOM), our system uses a single MZ EOM both to generate optical sidebands and to switch on-off the light. This strategy allows to reduce the number of components and is compatible with the development of a compact and high-performance atomic clock.

Experimental set-up

Description

Figure 1 shows the experimental set-up. nm [START_REF] Liu | A distributed-feedback diode laser frequency stabilized on Doppler-free Cs D 1 line[END_REF] and shifted from Cs atom resonance by 4.596 GHz. The laser light is injected into a polarization maintaining pigtailed MZ EOM (Photline NIR-MX800-LN-10). The latter allows both the generation of optical sidebands and the on-off light switch for the Ramsey regime. The EOM is driven at 4.596 GHz with a stabilized RF power of 22 dBm by a low noise microwave oscillator to generate first-order optical sidebands frequency-separated by 9.192 GHz.

The EOM bias electrode dc voltage V is actively controlled using a microwave synchronous detector technique [START_REF] Liu | Coherent population trapping resonances in buffer gas-filled Cs vapor cells with push-pull optical pumping[END_REF] to the so-called dark point to stabilize the carrier suppression. This carrier suppression is inherently limited by the EOM extinction ratio parameter measured to be 24 dB with our device. Both first-order sidebands exhibit equal power. The power of second-order harmonics is in correct agreement with theoretical calculations [START_REF] Liao | Modulation efficiency of a LiNbO 3 waveguide electro-optic intensity modulator operating at high microwave frequency[END_REF], about 30 dB lower than the power of first-order sidebands at the dark point. The EOM is temperature-controlled at 48 • C at the mK level for improved laser power stabilization at the output of the EOM [START_REF] Liu | Long-term and wideband laser intensity stabilization with an electro-optic amplitude modulator[END_REF]. At the output of the EOM, the laser is split into two directions. In the first direction, the laser light can be analyzed using a Fabry-Perot interferometer (FPI) or a fast photodiode and a spectrum analyzer. In the second direction, the modulated light of fixed linear polarization is sent through a Michelson interferometer ensemble in order to time-delay and to rotate the polarization of one half of the beam. A differential optical path ΔL = λ 0 /4 of about 8.1 mm, with λ 0 the clock tran- sition wavelength, is adjusted between both arms of the interferometer. Both beams are then respectively right and left circularly polarized by a quarter-wave plate to produce PPOP [START_REF] Jau | Pull-pull optical pumping of pure superposition states[END_REF]. The collimated laser beam diameter is expanded to 2 cm with a combination of two convergent lenses. Laser light is then sent through a 5 cm-long and 2 cm-diameter pyrex vapor cell filled with Cs atoms and a mixture of buffer gases, Ar and N 2 in the pressure ratio P Ar /P N 2 = 0.4, with a total pressure P of 15 Torr. The cell is temperature stabilized at the mK level at 38 • C and located in a static magnetic field parallel to the laser beam to raise the Zeeman degeneracy.

The cell is isolated from external electromagnetic perturbations with a double layer mu-metal shield. The transmitted optical power through the cell is detected by a low noise photodiode. The resulting signal is analyzed by a computer that drives a microwave switch and the synthesizer output frequency. CPT spectroscopy is realized by scanning the local oscillator frequency around 4.596 GHz.

Pulsed interaction with a MZ EOM

Figure 2 displays the optical harmonics power (height of the peaks at the output of the FPI) and the CPT signal (in continuous regime) at the output of the MZ EOM versus the bias voltage V when the RF power is continuously activated. The CPT signal height is found to be maximized for the so-called dark point V = V d = 2.6 V where the optical carrier is optimally rejected and the power in first-order optical sidebands is maximized. At the opposite case, the CPT signal vanishes for V = V c = 5 V where the optical carrier is maximized and first-order optical sidebands power minimized. We checked that when no RF modulation is applied, the output optical power is minimized (carrier suppression point) for V = V d . According to the transfer functions reported above, we implemented the Ramsey sequence described in Fig. 3 to make the atoms interact with a sequence of optical pulse trains. This pulsed interaction scheme is inspired from [START_REF] Zanon | High Contrast Ramsey Fringes with Coherent-Population-Trapping Pulses in a Double Lambda Atomic System[END_REF].

A first pulse allows to pump the atoms in the CPT state during a time τ p . For this purpose, the RF power is switched on using a microwave switch (Mini-Circuits ZASW-2-50-DR+) and the EOM bias voltage is adjusted to V d to reject the optical carrier. Then, the RF power is turned off while the EOM bias voltage is kept to V d to switch off the light. During this period, the atoms evolve freely in the dark for a time T . The next pulse is used both for the atomic signal detection and pumping again the atoms in the CPT state. The EOM-based light switching time was measured to be < 10 µs. The signal is typically detected a time τ d = 10 µs after the pulse trigger and averaged on a measurement time t m =2 5µs. The Ramsey time T is ultimately limited by the microwave coherence relaxation time T 2 in the CPT cell.

Experimental results

Detection of CPT Ramsey fringes

Fig. 4 reports, on a large span of 30 kHz, Ramsey fringes detected with τ p = 2 ms and T =3ms for a total laser input power of 3.8 mW. The central fringe exhibits a linewidth Δν =1/(2T )= 166 Hz with a contrast C of 33 %. In order to highlight the efficiency of the PPOP technique, we compare on Fig. 5 Ramsey fringes obtained with standard circular polarization and pushpull optical pumping for identical experimental conditions. The contrast of the central fringe is found to be 31 % with push-pull interaction scheme whereas it is only 2.8 % with circular polarization.

Impact on the fringe signal of some experimental parameters

Figures 6(a) and 6(b) show the contrast and the contrast/linewidth ratio of the central Ramsey fringe versus the Ramsey time T . We note that contrasts as high as 50 % and 24.2 % are obtained for T = 1 ms and T = 3 ms respectively. For T = 10 ms (fringe linewidth = 50 Hz), the contrast of the Ramsey fringe is still significant (3 %). Experimental data on Fig. 6(a) are well fitted by an exponential decay function with a time constant of about 3 ms. This value can be interpreted as an experimental estimation of the CPT hyperfine coherence lifetime T 2 . The ratio contrast/linewidth is maximized for T = T 2 = 3 ms. This result indicates that a Ramsey time T = T 2 is a good trade-off between a large signal and a narrow fringe for atomic clocks applications, as already observed in [START_REF] Micalizio | Metrological characterization of the pulsed Rb clock with optical detection[END_REF] and [START_REF] Guérandel | Raman-Ramsey interaction for coherent population trapping Cs clock[END_REF]. Figure 7 plots the contrast of the central fringe versus the laser power P L for T =5msandτ p = 2 ms. The contrast of the resonance is increased with increased laser power until P L ≃ 3 mW before reaching a plateau. For P L =1 mW, the contrast is found to be about 10 %. For comparison, in [START_REF] Boudot | Current status of a pulsed CPT Cs cell clock[END_REF], a contrast of 4.6 % was obtained at T = 5 ms for a laser intensity 2 times lower in a similar cell. Contrary to the CW regime where the CPT linewidth broadens with increased laser power, we measured in the pulsed case that the fringe linewidth does not depend on the laser power and narrows as 1/(2T ). A possible drawback of the used MZ EOM is to exhibit a relatively low extinction ratio of only 24dB. In order to estimate its effect (see Fig. 8), we degraded intentionally the light extinction by changing the EOM bias voltage V during the free evolution time T . The laser power during the pumping time is 3.6 mW. Maximum and minimum optical power seen by the atoms in the dark time are respectively 5.46 mW and 25.6 µW, limited by the EOM extinction ratio. Note that MZ EOMs with extinction ratio of 60 dB are commercially available. Such devices could allow to reduce greatly the minimum optical power to a level of a few nW during the time T . The CPT fringe signal is measured to decrease rapidly when the laser power in the time T is The signal is found to vanish completely for P L > 1 mW. Interestingly, for a residual light power < 100 µW, a plateau seems to be reached indicating that a better light extinction does not bring a relevant improvement on the clock signal. Nevertheless, we expect that total light during time is of great importance and to be preferred in order to minimize light shift effects for atomic clocks applications.

Discussion and summary

The frequency stability of atomic clocks is currently expressed by the Allan deviation σ y (τ). Assuming the photon shot noise as the only noise source, the shot-noise limit of the clock relative frequency stability can be estimated by:

σ y (τ) ≃ Δν ν 0 1 C hν P o T c t m τ , (1)
where Δν is the fringe width, ν 0 is the clock frequency, C is the contrast of the resonance, hν is the energy of a single photon, P 0 is the laser power at the output of the cell, T c = T + τ p is the clock cycle time, t m is the length of the detection window and τ is the averaging time of the measurement. From the clock signal shown in Fig. 4, Eq. (1) yields σ y (τ) ≃ 9 × 10 -15 at 1 s averaging time. This value is about one order of magnitude better than shot-noise estimations in the CW regime [START_REF] Liu | Coherent population trapping resonances in buffer gas-filled Cs vapor cells with push-pull optical pumping[END_REF]. In real life, this result will be degraded by various noise mechanisms such as the detector noise, the laser intensity and frequency noise or the oscillator phase noise through Dick effect [START_REF] Dick | Local oscillator induced instabilities in trapped ion frequency standards[END_REF]. In a first step, we estimate that demonstrating an experimental frequency stability better than 10 -12 at 1 s integration time with our system is a reasonable objective.

In summary, a simple-architecture MZ EOM-based laser system was developed to detect for the first time high-contrast CPT Ramsey fringes by combination of PPOP and Ramsey interrogation.The MZ EOM is used to operate simultaneously as an optical sidebands generator and a fast optical switch. Ramsey fringes with a linewidth in the 50-200 Hz range with contrasts of 3 to 50 % were detected. The impact of different experimental parameters on the clock fringe was reported. This system could be of great interest for the development of a compact and highperformance CPT-based vapor cell frequency standard.

Fig. 1 .

 1 Fig. 1. Experimental setup. DFB: Distributed-Feedback diode laser, HWP: half-wave plate, EOM: electro-optic modulator, MS: microwave synthesizer, FPI: Fabry-Perot interferometer, BPF: bandpass filter, S.A: microwave spectrum analyzer, PD: photodiode, FPD: fast photodiode, Cs+BG: Cs + buffer gas, PBS: polarizing beam splitter, QWP: quarter-wave plate, M1 and M2: mirrors, LNA: low-noise amplifier. The inset shows the energy-level diagram involved in Cs vapor with PPOP. The clock transition is the ground-state 0-0 transition.The laser source is a distributed-feedback (DFB) diode laser tuned on the Cs D 1 line at 894.[START_REF] Taichenachev | On the unique possibility of significantly increasing the contrast of dark resonances on the D 1 line of 87 Rb[END_REF]

Fig. 2 .

 2 Fig. 2. Optical power in the carrier (squares), first-order sidebands (circles) at the output of the MZ EOM and height of the CPT signal (triangles) versus the EOM bias voltage V .

Fig. 3 .

 3 Fig. 3. Pulsed sequence to scan and detect the CPT Ramsey fringes. The CPT pumping time τ p is typically 1-2 ms. The Ramsey time T is of a few ms.

Fig. 4 .Fig. 5 .

 45 Fig. 4. 166-Hz width Ramsey fringes with a contrast of 33 %. Experimental conditions: T = 3 ms, τ = 2 ms, P L = 3.8 mW.

Fig. 6 .

 6 Fig. 6. Contrast (a) and ratio (b) of the central Ramsey fringe versus the dark time T . The solid line on (a) is an exponential decay function fit to experimental data. Experimental conditions are: τ = 2 ms, P L =3mW .

Fig. 7 .

 7 Fig. 7. Contrast of the central fringe versus the laser power P L . Experimental conditions are: T = 5 ms, τ p = 2 ms.

Fig. 8 .

 8 Fig.8. CPT fringe peak-peak signal versus laser power seen by the atoms in the Ramsey time T . The CPT signal for the lowest power point is normalized to 1. The inset shows a zoom on first points of the graph.

Acknowledgments

This work was supported by Agence Nationale de la Recherche (ANR) and Délégation Générale de l'Armement in the frame of ISIMAC project (ANR-11-ASTR-0004). X. Liu PhD thesis is funded by Région de Franche-Comté.