M Gabard

Y Cherkaski

L Brissonneau

M C Steil

J Fouletier

Adel Mesbah

N Dacheux

Dr Nicolas Clavier
email: nicolas.clavier@icsm.fr

Preparation, characterization and sintering of yttriumdoped ThO 2 for oxygen sensors applications

Preparation, characterization and sintering of yttrium-doped ThO2 for oxygen sensors applications

Introduction

In the framework of the development of the fourth generation of nuclear reactors, a Sodium-cooled Fast Reactor (SFR) is planned to be operated in France in the 2020's. In the primary vessels, the oxygen content in the liquid sodium coolant must be kept under a specific value (usually few ppm), as it controls the corrosion rate of the stainless steel claddings [START_REF] Brissonneau | New considerations on the kinetics of mass transfer in sodium fast reactors: An attempt to consider irradiation effects and low temperature corrosion[END_REF].

The oxygen content in the sodium is fixed by a cold trap, a component where a part of the sodium flow is deviated and oxygen precipitates as Na2O when the saturation temperature is reached. A plugging indicator, associated to the cold trap, gives the total amount of impurity (susceptible to precipitate at low temperature) in the sodium. However, this device is not specific towards oxygen. In this context, the use of potentiometric sensors appears of great interest as it would bring a redundant, diversified and specific monitoring of the oxygen dissolved in sodium. In such sensors, the difference of oxygen activity in the sodium and in a reference induces a voltage drop according to Nernst's law. The reference and the sodium are separated by a solid electrolyte, which can be brazed on a metallic tube. The tightness between the outside and the inside of the sodium pipe is then easily performed on this metallic tube (the alternative being a long ceramic tube where tightness is performed via cooled sodium).

The quality of the electrolyte material is critical to insure the adequate operating of the sensors in the reactor environment [START_REF] Thompson | Experience of the production, performance and application of harwell oxygen meter tubes[END_REF]. It should be compatible with sodium at high temperature (about 400°C), mechanically resistant and a purely ionic conductor. In this context, the use of yttria-doped thoria (i.e. Th1-xYxO2-x/2) electrolyte was suggested ever since 1959 [START_REF] Hayes | Instrumentation for Liquid-Sodium in Nuclear-Reactors[END_REF] for potentiometric sensors to be operated in liquid sodium. The incorporation of Y 3+ in the fluorite-type structure of ThO2 produces oxygen vacancies which turns the ceramic into an ionic conductor. Thoria is much less sensitive to sodium corrosion compared to zirconia, which was also frequently used in oxygen sensors [START_REF] Taylor | Testing and Performance of Electrolytic Oxygen Meters for Use in Liquid-Sodium[END_REF]. Finally, one can achieve highly dense fine grains and high purity ceramic which is generally required to resist to sodium corrosion and mechanical stress [START_REF] Ganesan | Novel approach for the bulk synthesis of nanocrystalline yttria doped thoria powders via polymeric precursor routes[END_REF].

The first potentiometric sensors dedicated to oxygen monitoring in sodiul were developed in the 1960's [START_REF] Minushkin | Plugging meter and continuous oxygen meter[END_REF]. The electrolyte materials were generally based doped ThO2 or ZrO2, with metal/metal oxide or gas acting as reference electrode [START_REF] Mckee | Calibration Stability of Oxygen Meters for Lmfbr Sodium Systems[END_REF]. As instance, recent researches were conducted on Ca-doped [START_REF] Chandran | Performance testing of insodium sensors and simulated experiments in, sodium chemistry loop[END_REF] and Y-doped [START_REF] Nollet | Electrochemical Oxygen Sensor Development for Liquid Sodium[END_REF] zirconia because of the easier availability of the materials, but the best results were obtained with ThO2-based solid solutions [START_REF] Thompson | Experience of the production, performance and application of harwell oxygen meter tubes[END_REF][START_REF] Hayes | Instrumentation for Liquid-Sodium in Nuclear-Reactors[END_REF][START_REF] Ganesan | Novel approach for the bulk synthesis of nanocrystalline yttria doped thoria powders via polymeric precursor routes[END_REF][START_REF] Minushkin | Plugging meter and continuous oxygen meter[END_REF][START_REF] Mckee | Calibration Stability of Oxygen Meters for Lmfbr Sodium Systems[END_REF][START_REF] Hammou | Mécanismes de conduction électrique dans les solutions solides à base de dioxyde de thorium[END_REF][START_REF] Hobdell | Electrochemical Techniques for Monitoring Dissolved Carbon, Hydrogen and Oxygen in Liquid-Sodium[END_REF][START_REF] Jakes | Development of Electrochemical Oxygen Meter for Liquid-Sodium[END_REF][START_REF] Jung | Oxygen Determination in Liquid-Sodium with Industrial Electrochemical Measuring Probe[END_REF][START_REF] Kozlov | Electrochemical-Cell for Measuring the Activity of Oxygen in Sodium[END_REF][START_REF] Mason | The monitoring of oxygen, hydrogen and carbon in the sodium circuits of the PFR[END_REF][START_REF] Mckee | Sodium-Hydroxide Injections into Sodium[END_REF][START_REF] Osterhout | Operating experience with on-line meters at experimental breeder reactor II (EBR II)[END_REF][START_REF] Schouler | Complex Impedance of Electrochemical Cells Based on Yttria Doped Thoria[END_REF]. The majority of the probes produced up to now were designed in 1970's and 1980's, with some of them being tested in reactors [START_REF] Mason | The monitoring of oxygen, hydrogen and carbon in the sodium circuits of the PFR[END_REF][START_REF] Osterhout | Operating experience with on-line meters at experimental breeder reactor II (EBR II)[END_REF] and made commercially available. The yttrium incorporation ratio was usually in the 7.5-15 wt.% Y2O3 range, since the maximum ionic conductivity was reported for an yttrium doping of x = 0.15 [START_REF] Hammou | Mécanismes de conduction électrique dans les solutions solides à base de dioxyde de thorium[END_REF][START_REF] Schouler | Complex Impedance of Electrochemical Cells Based on Yttria Doped Thoria[END_REF]. However, if lifetimes up to 10 000 hours were reported [START_REF] Hobdell | Electrochemical Techniques for Monitoring Dissolved Carbon, Hydrogen and Oxygen in Liquid-Sodium[END_REF][START_REF] Jung | Oxygen Determination in Liquid-Sodium with Industrial Electrochemical Measuring Probe[END_REF] the probes generally suffered from their lack of robustness and reliability. Particularly, an important sensitivity to thermal and mechanical stress was observed and linked to a weakening of the probe both at the brazure level and within the ceramic electrolyte itself [START_REF] Kozlov | Electrochemical-Cell for Measuring the Activity of Oxygen in Sodium[END_REF][START_REF] Mckee | Sodium-Hydroxide Injections into Sodium[END_REF]. Also, several authors underlined the crucial effect of the electrolyte purity over the probe performances. Indeed, the presence of impurities in the ceramic, and particularly within grain boundaries, was systematically associated to an increase of the corrosion rate in liquid sodium [START_REF] Isaacs | Calibration of oxygen meters in sodium using uranium[END_REF], and sometimes to the formation of secondary phases such as Na2ThSiO5 or Na2SiO5 at the surface of the electrolyte [START_REF] Jakes | Development of Electrochemical Oxygen Meter for Liquid-Sodium[END_REF]. Nowadays, no more commercial oxygen sensors for sodium are available, and the sensor development is limited to few laboratories in India [START_REF] Chandran | Performance testing of insodium sensors and simulated experiments in, sodium chemistry loop[END_REF][START_REF] Ganesan | Development of Sensors for On-Line Monitoring of Nonmetallic Impurities in Liquid Sodium[END_REF], USA [START_REF] Nollet | Electrochemical Oxygen Sensor Development for Liquid Sodium[END_REF] and France.

New preparation routes for the Th1-xYxO2-x/2 electrolyte ceramic then have to be investigated in order to provide highly pure materials meeting the microstructural requirements detailed previously. For example, Cosentino et al. [START_REF] Cosentino | Powder synthesis and sintering of high density thoria-yttria ceramics[END_REF] chose a citrate technique, while Ganesan et al. [START_REF] Ganesan | Novel approach for the bulk synthesis of nanocrystalline yttria doped thoria powders via polymeric precursor routes[END_REF] tried different methods, including citrate technique as well as Pechini and PEG-assisted processes. As a result, both obtained very fine grains powders and dense pellets. In this framework, oxalic co-precipitation can provide a suitable method for the elaboration of reliable and robust sensors [START_REF] Thompson | Experience of the production, performance and application of harwell oxygen meter tubes[END_REF]. Indeed, this preparation route is well-known to provide rapid and quantitative precipitation of tetravalent actinides [START_REF] Abraham | Actinide oxalates, solid state structures and applications[END_REF][START_REF] Clavier | X-Ray Diffraction and µ-Raman Investigation of the Monoclinic-Orthorhombic Phase Transition in Th1-xUx(C2O4)2  2H2O Solid Solutions[END_REF], and, after heat treatment, to yield powders with a high sintering capability [START_REF] Ananthasivan | De-agglomeration of thorium oxalate-a method for the synthesis of sinteractive thoria[END_REF][START_REF] White | Optimization of Thorium Oxalate Precipitation Conditions Relative to Derived Oxide Sinterability[END_REF]. This paper is then dedicated to the synthesis of thorium(IV)-yttrium(III) mixed oxides through oxalic route, their complete physico-chemical characterization and the subsequent determination of sintering capability and evaluation of their electrical properties.

Experimental

Preparation of the precursors

All the chemical used during the preparation of the powdered samples, including thorium nitrate Th(NO3)4.5H2O, were supplied by Sigma-Aldrich and were of analytical grade (purity ≥ 99%). Thorium-yttrium mixed oxalates were prepared following a protocol similar to that described by Horlait et al. [START_REF] Horlait | Synthesis and characterization of Th1-xLnxO2-x/2 mixed-oxides[END_REF], for 0.005 ≤ x ≤ 0.22, with x = Y/(Th+Y) is the cationic molar ratio. Thorium and yttrium nitrate salts were first dissolved in 1M HNO3 in order to reach a final concentration in cations close to 0.5M. The two solutions were then mixed together in the desired stoichiometry then poured quickly in a beaker containing a large excess of oxalic acid. The precipitate instantaneously formed was further separated by centrifugation at 4500 rpm, washed several times with deionized water then ethanol, and then dried overnight in an oven at 90°C. Precipitation yields were then determined through the analysis of the supernatant by ICP-AES measurements (Spectro Arcos EOP). Protocols for solution sampling and further ICP-AES analysis have been already described in one of our previous publication [START_REF] Costin | Preparation and characterization of synthetic Th0.5U0.5SiO4 uranothorite[END_REF]. The emission bands chosen were 401.913, 283.730 and 274.716 nm for thorium, and 371.030 and 324.2278 nm for yttrium concentration determination.

Characterization of the solids

The powders obtained through the precipitation route described above were first characterized in terms of morphology. SEM observations were directly conducted on powdered samples without any preparation such as metallization, using a FEI Quanta 200 electronic microscope, equipped with an Everhart-Thornley Detector (ETD), in high vacuum conditions with a very low accelerating voltage (2 -3.1 kV). These conditions were chosen in order to create a beam deceleration effect that led to high resolution images. For statistical X-EDS analyses, the powder was first embedded in an epoxy resin. The surface of the sample was then polished to reach an optical grade and covered by carbon coating. Experimental data were finally collected randomly from about 300 different locations of the sample surface, considering ThO2 and YPO4 as internal standards.

Powder X-Ray Diffraction (PXRD) patterns were obtained by the means of a Bruker D8 diffractometer equipped with a Lynx-eye detector adopting the parallel geometry (reflection mode) and using Cu K1,2 radiation ( = 1.54184 Å). PXRD patterns were recorded at room temperature in the 5° ≤ 2 ≤ 100° range, with a step size of (2) = 0.03° and a total counting time of about 1 hour. For in situ high temperature experiments, powders were placed in dedicated alumina boats and heated with a rate of 5°C.min -1 , under N2 atmosphere. HT-PXRD patterns were recorded each 30°C from 30 to 300°C then each 50°C from 300 to 500°C and finally with a 100°C step from 500°C to 1000°C. Temperature was held for 15 minutes before recording the pattern in order to ensure the thermodynamic equilibration of the sample.

All the PXRD patterns were refined by the Rietveld method using the Cox-Hastings pseudo-Voigt profile function [START_REF] Thompson | Rietveld refinement of Debye-Scherrer synchrotron X-ray data from Al2O3[END_REF] implemented in the Fullprof_suite program [START_REF] Frontera | FullProf as a new tool for flipping ratio analysis[END_REF]. During all the refinement, the conventional profile/structure parameters (zero shift, unit cell parameters, scale factors, global thermal displacement and asymmetric parameters) were allowed to vary. Moreover, for each phase, modelling of the intrinsic microstructure parameters was performed by applying an anisotropic size model. Specific surface area measurements were determined using adsorption/desorption of N2 at 77 K (BET method) by the means of a Micromeritics Tristar 3020 apparatus. Prior to the analyses, samples were outgassed at 90°C for 4 hours in order to ensure the release of water molecules adsorbed at the surface of the solids.

Thermal analyses and sintering

Dilatometric measurements were conducted on a Setaram Setsys Evolution apparatus.

Heat treatments were carried out under air flux up to 1600°C with a rate of 10°C.min -1 . Dwell time of 30 minutes was then considered before sample was cooled down to room temperature at 30°C.min -1 .

In order to perform the sintering of thorium-yttrium oxides, the powdered samples by conversion of the oxalate precursors were first shaped by uniaxial pressing (500 MPa) at room temperature thanks to a Specac Atlas manual hydraulic press. The pellets obtained were then placed in alumina boats and heated up to 1575°C under air flow in a tubular furnace (Nabertherm). Heating rate of 10°C.min -1 was applied on heating, while natural cooling was used after sintering.

Finally, density measurements of the sintered pellets were conducted through two complementary techniques. Helium pycnometry analyses (Micromeritics Accupyc 1340) led to the contribution of the closed porosity while geometrical measurements using a precision calliper were further used to estimate the global porosity of the samples.

Electrical characterizations

After sintering, both sides of the pellets were covered by a platinum layer (Metalor 6082). The oxygen partial pressure in the experimental cell was controlled and monitored by means of a zirconia oxygen pump-sensor device (GEN'AIR, Setnag). Three samples can be characterized simultaneously enabling to compare the properties of pellets in the same conditions of temperature and oxygen partial pressure. The temperature of the samples was controlled within ± 1°C in the 350-800°C range. The electrical characterizations were carried out by impedance spectroscopy, using a Solartron SI1260 impedancemeter. The numbers in the diagrams indicate the logarithm of the measuring frequency.

Results and discussion

Characterization and conversion of oxalate precursors

Following the synthesis of oxalate precursors of thorium-yttrium oxides, the precipitation yields were first determined from ICP-AES measurements of the supernatant.

Whatever the chemical composition considered, the precipitation of both cations appeared to be quantitative, with precipitation yields exceeding 99.8 % (Table 1), even if the concentration of yttrium in the supernatant always appeared to be higher than that of thorium.

Additionally, the x molar ratio in the solid precipitated was also checked after the complete dissolution of an aliquot of oxalate powder in a mixture of concentrated HNO3/HCl. Once again, the values obtained appeared in good agreement with the expected ones and confirmed the quantitative precipitation of the cations.

Table 1.

Precipitation yields of thorium and yttrium determined from ICP-AES measurements for different x = Y/(Th+Y) mole ratios.

x = 0.01 x = 0.08 x = 0.15 x = 0.22 Th Y Th Y Th Y Th Y Initial (mol.) 1.2×10 -2 1.2×10 -4 1.1×10 -2 9.6×10 -4 1.1×10 -2 1.8×10 -3 1.0×10 -2 2.8×10 -3
Soluble fraction (mol.) 4.3×10 -8 < L.D. 4.4×10 -8 2.0×10 -7 6.2×10 -8 1.4×10 -6 8.5×10 -8 4.9×10 -6 Precipitation yield (%) > 99.99 > 99.9 99.99 99.98 99.99 99.92 99.99 99.82 L.D. : limit of detection

The morphology of the oxalate precursors was then evaluated thanks to SEM observations (Figure 1). For all the chemical composition studied (i.e. typically 0.005 ≤ x ≤ 0.22), the powders appeared to be composed of rounded plate-like aggregates of about 1-2µm in length, themselves being composed by smaller submicrometric crystallites. This morphology is close to that already reported for thorium oxalate in the literature, frequently described as squareshaped agglomerates [START_REF] White | Optimization of Thorium Oxalate Precipitation Conditions Relative to Derived Oxide Sinterability[END_REF][START_REF] Hingant | Preparation, sintering and leaching of optimized uranium thorium dioxides[END_REF][START_REF] Heisbourg | The kinetics of dissolution of Th1-xUxO2 solid solutions in nitric media[END_REF]. Also, the amount of Y 3+ incorporated in the oxalate structure did not impact significantly the powders morphology since the grains size and the aggregation level appeared almost unchanged for all the samples studied. The powdered precursors prepared from oxalic co-precipitation of thorium and yttrium solutions were first investigated by the means of PXRD (Figure 2a). For yttrium amounts up to x = 0.15, the pattern recorded always appeared to fit with that already reported in the literature for thorium oxalate dihydrate (monoclinic, space group C2/c) [START_REF] Clavier | X-Ray Diffraction and µ-Raman Investigation of the Monoclinic-Orthorhombic Phase Transition in Th1-xUx(C2O4)2  2H2O Solid Solutions[END_REF]. The observation of a systematic and progressive shift of the PXRD lines toward the lower angles (as attested by the focus on the 15-18° range in the Figure 2a) accounts for the direct incorporation of yttrium within the thorium oxalate structure. Also, no evidence of Y2(C2O4)3.nH2O formation [START_REF] Balakrishnan | Studies on the synthesis of nanocrystalline yttria powder by oxalate deagglomeration and its sintering behaviour[END_REF] was noted on the PXRD patterns. On this basis, one can conclude that the precipitation step yielded to the formation of (Th,Y)(C2O4)2.2H2O solid solutions. Conversely, the sample prepared for x = 0.22 presented additional XRD lines assigned to the hexagonal structure (space group P63/mmc) already reported by Arab-Chapelet et al. for mixed An(IV)/Ln(III) or An(IV)/An(III) oxalates [START_REF] Arab-Chapelet | Synthesis and characterization of mixed An(IV)An(III) oxalates (An(IV) = Th, NP, U or Pu and An(III) = Pu or Am)[END_REF][START_REF] Brackx | Structural study of (N2H5,H)2.9U1.1Ce0.9(C2O4)5 . 10H2O from a conventional X-ray diffraction diagram obtained on a powder synthesized by a fast vortex process[END_REF]. Also, the position of the PXRD lines observed for this composition appeared to be closed to that noted for x = 0.15. The formation of a polyphase material can thus be associated to the existence of a limit of incorporation of Y 3+ in the structure of Th(C2O4)2.2H2O between 15 and 22 % in mole. This range of values appears in good agreement with the one previously evidenced by Horlait et al. during the study of (Th,Ln)(C2O4)2.2H2O compounds, as they ended up with polyphase systems for x  0.21 whatever the trivalent lanthanide studied [START_REF] Horlait | Synthesis and characterization of Th1-xLnxO2-x/2 mixed-oxides[END_REF]. The existence of such incorporation limit, and the subsequent formation of polyphase oxalate precursor, are thus likely to create heterogeneities in the distribution of yttrium in the solid.

When heating (Figure 2b), the typical PXRD pattern recorded at room temperature was conserved up to 120°C: even if a phase transition toward an orthorhombic variety was reported around 110°C [START_REF] Clavier | X-Ray Diffraction and µ-Raman Investigation of the Monoclinic-Orthorhombic Phase Transition in Th1-xUx(C2O4)2  2H2O Solid Solutions[END_REF], the conditions of acquisition used for HT measurements hinder the detection of the slight pattern modifications associated. From 150°C, the apparition of additional peaks marked the progressive dehydration of the initial compound, first leading to (Th,Y)(C2O4)2. H2O then to anhydrous (Th,Y)(C2O4)2. These latter were unambiguously identified by analogy with the homologous thorium based compounds, namely Th(C2O4)2. H2O and Th(C2O4)2 [START_REF] Dash | Temperature programmed decomposition of thorium oxalate hexahydrate[END_REF].

Above 240°C, the intensity of the PXRD lines decreased strongly, indicating the progressive amorphization of the sample. Such decrease of the crystallization state was correlated with the decomposition of the oxalate entities, as already demonstrated in several other thorium-based chemical systems, such as Th-U(IV) [START_REF] Hingant | Preparation, sintering and leaching of optimized uranium thorium dioxides[END_REF] or Th-Ce(IV) [START_REF] Claparede | Multiparametric Dissolution of Thorium-Cerium Dioxide Solid Solutions[END_REF] oxalates. For these types of compounds, the oxalate groups were found to be decomposed into CO and CO2

gases in the 300-400°C range. In good agreement with these values, the characteristic pattern of the m 3 Fm fluorite-type structure of ThO2 [START_REF] Clausen | Neutron scattering investigation of disorder in UO2 and ThO2 at high temperatures[END_REF] was detected as soon as 400°C, even if the wide PXRD lines collected evidenced the poorly crystallized and/or the nanosized nature of the sample. The increase of temperature up to 900°C was then correlated with the narrowing of the PXRD lines, thus to the growth of the coherence domains [START_REF] Claparede | Multiparametric Dissolution of Thorium-Cerium Dioxide Solid Solutions[END_REF]. In parallel to the HT-XRD analyses, the variation of the powders specific surface area was also monitored versus the temperature of conversion. As expected from previous studies [START_REF] Dollimore | The Thermal-Decomposition of Oxalates -a Review[END_REF], the decomposition of the oxalate groups was associated with a strong increase in the specific surface area, leading to a maximum value after heating at 500°C. Such a modification in the topology of the powders could result either from the breakaway of the biggest aggregates during the emission of the gaseous species formed during the decomposition [START_REF] Hingant | Preparation of morphology controlled Th1-xUxO2 sintered pellets from low-temperature precursors[END_REF] or from the formation of mesopores of a few nanometers in diameter [START_REF] Claparede | Multiparametric Dissolution of Thorium-Cerium Dioxide Solid Solutions[END_REF] due to the gas release from and through the aggregates. Whatever the chemical composition considered, both these phenomena resulted in the increase of the specific surface area by about one order of magnitude.

Conversely, above 500°C, the specific surface dropped down significantly for all the three samples investigated. As reported previously, this tendency both arose from the crystallization of the samples (increase of the average crystallite size) and from the apparition of sintering phenomena (coalescence of agglomerates and grain growth) [START_REF] Claparede | Influence of Crystallization State and Microstructure on the Chemical Durability of Cerium-Neodymium Mixed Oxides[END_REF]. Owing to this potential loss of reactivity and to the early formation of the oxide structure, the operating conditions for the conversion of the oxalate precursors were then set to a heat treatment of 6 hours at 500°C under air atmosphere. )

Conversion temperature (°C) Figure 3.

Variation of the specific surface area of Th1-xYx(C2O4)2.2H2O precursors versus temperature for x = 0.005 (), x = 0.01 () and x = 0.08 ().

Characterization of the oxide samples

Thorium-yttrium mixed dioxides prepared from the conversion of the oxalate precursors at 500°C were first characterized by PXRD. As already evidenced from the high temperature data presented in Figure 2b, all the investigated samples presented the characteristic PXRD lines of the fluorite type structure. However, the important FWHM value of the PXRD peaks could be associated to the nanosized character of the samples and/or to an heterogeneous distribution of yttrium in the ThO2 structure. As a matter of fact, Rietveld refinement of the PXRD pattern (Figure 4a) revealed the existence of two distinct fluoritetype structures exhibiting close unit cell parameters (see red and blue calculated patterns in Figure 4a and associated unit cell parameters gathered in Table 2). Indeed, the experimental data could only be fitted with the contribution of at least two populations of PXRD peaks. The peaks exhibiting the highest 2 values led to high values of unit volume, i.e. correspond to an yttrium-depleted phase owing to the respective ionic radii of Th 4+ (1.19 Å) and Y 3+ (1.159 Å) in the eight-fold coordination [START_REF] Shannon | Revised Effective Ionic-Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides[END_REF]. Such heterogeneity probably already existed within oxalate precursors and is likely to originate from differences in the precipitation kinetics of thorium(IV) and yttrium(III) in oxalic media. Additional heat treatments followed by PXRD measurements were further performed up to 1600°C for the Th0.92Y0.08O1.96 sample in order to monitor its homogenization. Indeed, the preparation of homogeneous samples at the submicrometric scale is a crucial point to ensure efficient ionic conductivity within the mixed oxides. Also, the determination of an homogenization temperature could act as a guideline to evaluate the sintering temperature to apply to prepare the final ceramic material. Up to 1200°C, the Rietveld refinement of the PXRD patterns systematically led to the determination of two distinct unit cell volumes (as illustrated by the shaded part of Figure 4b). However, their variation versus temperature revealed that they became closer and closer with increasing temperature. Such behaviour could be clearly assigned to the interdiffusion of cations in the system, leading to a progressive homogenization of the yttrium distribution in the structure. For the highest temperatures studied (i.e. 1400-1600°C), only one fluorite-type structure was pointed out from PXRD data refinement, thus corresponding to an homogeneous Th1-xYxO2-x/2 solid solution.

(a) The unit cell parameters collected from samples heated at 1400°C show a continuous decrease versus x, in good agreement with the smaller ionic radius of Y 3+ compared to Th 4+ (Table 2, Figure S1 supplied as supplementary material). This behaviour is in favour of the formation of a homogeneous solid solution at high temperature, even if the variation of the unit cell volume does not present a strictly linear trend with the yttrium content. Indeed, Horlait et al. already demonstrated that such a variation obeys to a second-order polynom, as a result of the antagonist effects of the M 3+ incorporation, on the one hand, and of the formation of oxygen vacancies that tend to decrease the O-O repulsion, on the other [START_REF] Horlait | Synthesis and characterization of Th1-xLnxO2-x/2 mixed-oxides[END_REF].

Also, it is to note that this result contrasts with that reported previously by Cosentino et al.

[42] who described the variation of the unit cell parameter versus x as linear (Figure S1).

However, they stated that a deviation from the standard Vegard law should occur and that the incorporation of yttrium in their samples might be incomplete. Also, the unit cell volumes determined in our study systematically appeared to be slightly higher than that reported by Cosentino et al. [START_REF] Cosentino | Lattice parameters of thoria-yttria solid solutions[END_REF]. Such deviation could be linked with the co-precipitation method used to prepare our samples. Indeed, even when conducted in air, the conversion of oxalate precursors systematically leads to small amounts of residual carbon in the final oxide compounds (typically below 0.1 wt.%). Even if the exact location of carbon within the oxide structure was never clearly assessed, it was systematically associated to a slight swelling of the crystal lattice [START_REF] Martinez | From uranium(IV) oxalate to sintered UO2: Consequences of the powders' thermal history on the microstructure[END_REF]. Nevertheless, the unit cell volumes determined in this work from the samples heated at 1400°C were considered as a reference and further used to determine the calculated density of the solids (Table 2).

The SEM observations performed on the oxide samples prepared after heating oxalate precursors at 500°C are gathered in Figure 1. From a general point of view, the morphological characteristics evidenced on the precursors were conserved on the oxide powders. Indeed, the grains conserved a general rounded-plates habit, even if their size slightly decreased. This modification, which is particularly noticeable on the grains thickness, could be directly linked with the dehydration then the decomposition of oxalate precursor.

Nevertheless, the comparison between SEM observations of raw precursors and oxide samples still argue for a pseudomorphic conversion. This conservation of the general morphology was already stated for other compounds [START_REF] Hingant | Preparation of morphology controlled Th1-xUxO2 sintered pellets from low-temperature precursors[END_REF] and thus appears to be a characteristic of oxalate samples whatever the metallic cations considered.

Additionally, statistical X-EDS analysis were performed on the Th0.85Y0.15O1.925 sample in order to quantify the heterogeneity of the yttrium distribution that was suggested from PXRD results. The statistical representation of the xexp./xcalc. ratio (Figure 5) confirmed a non-homogeneous distribution of the yttrium in the ThO2 structure. Moreover, it appears to follow a Gaussian law, with a Full Width at Half Maximum (FWHM) value of about 0.19. On this basis, and even if the average x ratio reached the expected value, the composition of the sample prepared for x = 0.15 was found to vary approximately between Th0.82Y0.18O1.91 and Th0.88Y0.12O1.94. Nevertheless, such heterogeneity remains far below that what can be observed when proceeding through dry chemistry methods, i.e. mixture of pure powders.

Indeed, no trace of pure ThO2 or Y2O3 was detected neither from XRD nor EDS analyses.

Also, from the XRD study, the samples were found to homogenize after heat treatment at high temperature (T  1400°C). In these conditions, the small composition variation observed in the starting oxide powder could be expected to slightly delay the sintering step due to cation interdiffusion [START_REF] Aybers | Analysis of the Initial-Stage of Oxide Sintering -(U0.8Th0.2)O2+x[END_REF], but will not affect the properties of the ceramic, especially concerning ionic conduction. Frequency (%)

x exp. /x calc.

Figure 5.

Statistical distribution of the Y/(Th+Y) mole ratio determined from EDS analysis of Th0.85Y0.15O1.925.

Sintering of oxide samples

The sintering of the oxide samples obtained by firing oxalate precursors at 500°C was further investigated. In this aim, the powders were shaped by uniaxial pressing (500 MPa) at room temperature which resulted in pellets exhibiting a green density of about 50% whatever the yttrium content considered (Table 3). Uncertainty attached to density measurements : ± 1%.

dcalc. values are given in Table 2.

Dilatometric tests were then undertaken to determine the optimal conditions in terms of heating time and temperature required to reach the full densification of oxide samples. For all the yttrium contents investigated the relative shrinkage appeared to present similar variation versus temperature (Figure 6a). The densification proceeded through very progressive shrinkage with an onset between 500 and 600°C. Such temperature is obviously too low to be associated to sintering processes since full densification of ThO2 is usually reported in the 1500-1700°C range in the literature [START_REF] Cosentino | Powder synthesis and sintering of high density thoria-yttria ceramics[END_REF][START_REF] Ananthasivan | De-agglomeration of thorium oxalate-a method for the synthesis of sinteractive thoria[END_REF][START_REF] Jorgense | Final Stage Sintering of ThO2[END_REF][START_REF] Kutty | Development of a master sintering curve for ThO2[END_REF]. More likely, it could be assigned to the crystallite growth within the square-shaped agglomerates constituting the powder and/or to the elimination of residual carbon coming from the conversion of oxalate precursor. Indeed, Martinez et al. recently demonstrated that even small amounts of amorphous carbon (i.e. below 0.5 wt.%) could impact significantly the shrinkage of oxide powders prepared through oxalic co-precipitation method [START_REF] Martinez | From uranium(IV) oxalate to sintered UO2: Consequences of the powders' thermal history on the microstructure[END_REF].

The shrinkage rate then appeared to increase when reaching higher temperatures, typically above 1300-1400°C. Even though, the relative linear shrinkage kept decreasing up to 1575°C, indicating that the sintering was incomplete at this latter temperature. On this basis, complementary experiments were run during an isotherm dwell of 8 hours at 1575°C (Figure 6b). For the three compositions studied, an additional shrinkage of about 2% was measured within the 6 first hours of heating. Based on the hypothesis of an isotropic shrinkage, such additional shrinkage should correspond to an increase in the densification of about 8% of the calculated density. In these conditions, a plateau of several hours at 1575°C appears to be compulsory in order to meet the requirements generally reported for electrolyte materials in oxygen sensors (i.e. dgeom./dcalc. ≥ 97%). The pellets collected after dilatometry tests were further analysed through geometrical and He-pycnometry measurements to estimate their relative density (Table 3). For all the samples, the density was found to be very high, i.e. above 95%. Such values thus confirmed that the densification process was almost fully achieved after heating 8 hours at 1575°C.

Moreover, the comparison between geometrical and pycnometric data clearly showed that the remaining porosity was mainly constituted by closed pores, which is characteristic of the final step of the sintering. Nevertheless, due to the number of samples considered and to the uncertainty attached to the density measurements, it appeared impossible to conclude about the possible impact of the yttrium content on the final density of the pellets.

The good densification stated for all the samples was also confirmed by complementary SEM observations (Figure 7). Indeed, the micrograph recorded at the sample surface did not present any trace of porosity. Conversely, few pores were detected during the observation of a polished cross-section. They were found to be located in inter-granular position and then could probably be eliminated by prolonging the heat treatment. Also, a statistical analysis of the grain size was performed from several micrographs collected either from the surface or the bulk material. From this measurement, the grain size appeared to be typically distributed from 0.1 to 1.2 µm for the Th0.92Y0.08O1.96 sample. The average grain size reached 0.45 µm and appeared to be homogeneous in the pellet, evidencing the absence of exaggerated grain growth at the surface. However, and as for the relative density values, it was not possible to establish a clear correlation between the average grain size and the yttrium content.

Evaluation of the conducting properties

An example of impedance diagram obtained for the Th0.095Y0.08O1.96 sample is presented in the Figure 8 for experimental conditions corresponding to pure ionic conductor behaviour, i.e. at low oxygen partial pressure (10 -6 bar). Three arcs were observed in the Nyquist representation, corresponding, in decreasing frequencies, to the bulk, the grain boundaries (referred to as G.B. in Figure 8), and the electrode contributions, respectively.

The impedance diagrams were fitted with the ZView program. The bulk resistance (Rb) was determined from the intercept with the real axis of the high-frequency simulated arcs (frequencies higher than ca. 10 3 -10 4 Hz according to temperature). In this paper, only the bulk contribution has been taken into account for the calculation of the electrical conductivity of the materials.

It has been shown previously that the thoria-doped oxides are mixed ionic-electronic conductor at high oxygen partial pressures with an additional p-type conductivity [START_REF] Hammou | Mécanismes de conduction électrique dans les solutions solides à base de dioxyde de thorium[END_REF][START_REF] Schouler | Complex Impedance of Electrochemical Cells Based on Yttria Doped Thoria[END_REF].

However, for sufficiently doped materials, yttria-doped thoria exhibit a pure ionic conductivity under oxygen partial pressures lower than 10 -5 bar. The aim of the preliminary study presented in this paper is to show the influence of the yttria content on the ionic conductivity. The variation of the electrical conductivity as a function of the oxygen partial pressure will be presented in a forthcoming publication. Figure 8 shows the variation of the conductivity as a function of the yttrium concentration, at 400°C (Figure 9a) and 800°C (Figure 9b). These results are compared to Hammou's work [START_REF] Hammou | Mécanismes de conduction électrique dans les solutions solides à base de dioxyde de thorium[END_REF][START_REF] Schouler | Complex Impedance of Electrochemical Cells Based on Yttria Doped Thoria[END_REF]. The ionic conductivity varies linearly with the yttrium incorporation rate between x = 0 and x = 0.15. This phenomenon has been previously observed [START_REF] Hammou | Mécanismes de conduction électrique dans les solutions solides à base de dioxyde de thorium[END_REF]. It should be pointed out that the lower conductivity observed by Hammou can be ascribed to differences in the purity of the oxide pellets used for the experiments and to some difficulties in the evaluation of the grain boundaries contribution.

Arrhenius plot of the bulk conductivity for three dopant levels, between 350 and 800°C, is reported in Figure 10. The activation energy of the electrical conductivity was 1.3 eV for Th0.995Y0 .005O1.9975 and 1.1 eV for Th0.92Y0.08O1.96 and Th0.85Y0.15O1.925. The activation energy decreases as a function of the dopant concentration. This is consistent with the results reported in literature, showing that the activation energy exhibits a minimum value for a composition close to x = 0.15 [START_REF] Hammou | Mécanismes de conduction électrique dans les solutions solides à base de dioxyde de thorium[END_REF].

Figure 1 .

 1 Figure 1.SEM observations of thorium-yttrium oxalates and corresponding mixed oxides prepared from the conversion of oxalate precursors at 500°C.

Figure 2 .

 2 Figure 2. (a) PXRD patterns of (Th,Y)(C2O4)2.nH2O precursors and comparison to Th(C2O4)2.2H2O, with detailed focus on the 15-18 ° range [23]. The star indicates additional phase for x = 0.22. (b) HT-PXRD monitoring of the conversion into dioxide for x = 0.08.

  Figure 4. (a) Rietveld refinement of the PXRD data collected from Th0.92Y0.08O1.96 after heating oxalate precursor at 500°C : black line stand for experimental data, while red and blue dashed lines account for calculated patterns. (b) Variation of the unit cell volumes versus heat temperature.

Figure 6 .

 6 Figure 6. Variation of the relative linear shrinkage of Th1-xYxO2-x/2 solid solutions pellets versus temperature (a) and heating time at 1575°C (b).

Figure 7 .

 7 Figure 7. SEM observation of Th0.92Y0.08O1.96 pellet after sintering at 1575°C during 8 hours : raw surface (left) and polished cross section (right).

Figure 8 .

 8 Figure 8. Typical impedance diagram obtained for Th0.95Y0.08O1.96 sample (10 -6 bar of oxygen, T = 500°C). The numbers in the diagram indicate the logarithm of the measuring frequency.

Figure 9 .

 9 Figure 9.Variation of the electrical conductivity of Th1-xYxO2-x/2 compounds as a function of x, under 10 -6 bar of oxygen, at 400°C (a) and 800°C (b) comparison with Hammou's works[START_REF] Hammou | Mécanismes de conduction électrique dans les solutions solides à base de dioxyde de thorium[END_REF][START_REF] Schouler | Complex Impedance of Electrochemical Cells Based on Yttria Doped Thoria[END_REF].

Figure 10 .

 10 Figure 10.Arrhenius plot of the bulk conductivity of Th1-xYxO2-x/2 samples, for various dopant levels, under 10 -6 bar of oxygen.

Table 2 .

 2 Data determined from Rietveld refinement of the XRD patterns of thoriumyttrium dioxides prepared after conversion of oxalate precursors at 600°C or 1400°C.

	x	0.005	0.01	0.08	0.15	0.22
				Tconversion = 600°C
	a1 (Å)	5.5810(5)	5.5816(4)	5.5663(5)	5.5082(5)	5.5543(6)
	a2 (Å)	5.6128(4)	5.6145(3)	5.6046(4)	5.5761(3)	5.5964(6)
				Tconversion = 1400°C
	a (Å)	5.5964(1)	5.5959(1)	5.5911(1)	5.5788(1)	5.5690(1)
	dcalc. (g.cm -3 ) 9.98	9.95	9.58	9.24	8.88

Table 3 .

 3 Relative density values determined for green and sintered Th1-xYxO2-x/2 pellets (T = 1575°C, t = 8 hours) from geometrical (dgeom./dcalc.) and He-pycnometry (dHe/dcalc.) measurements with associated porosity distribution.

	x	Green density (%)	dgeom./dcalc. (%)	dHe./dcalc. (%)	Open porosity (%)	Closed porosity (%)
	0.005	55	95	95	0	5
	0.08	49	99	99	0	1
	0.15	48	97	98	1	2

Acknowledgements

Authors would like to thank Johann Ravaux (ICSM/L2ME) for performing SEM observations. They are also grateful to the TECNA project of CEA for its continuous financial support.

Abstract :

The preparation of Th1-xYxO2-x/2 ceramics, to be used as electrolyte in oxygen sensors for sodium-cooled nuclear reactors, was successfully undertaken from oxalate precursors. Such method was found to provide quantitative precipitation of the cations into Th1-xYx(C2O4)2.2H2O solid solutions up to x = 0.15 while a polyphase system was obtained for x = 0.22. The corresponding oxides were obtained through heat treatment in air at 500°C and characterized by the means of PXRD, SEM and statistical X-EDS measurements. The conditions for the densification of Th1-xYxO2-x/2 ceramics were further determined by dilatometry (T = 1575°C, t = 8 hours) resulting in densification rates up to 99%. Finally, a first estimation of the electric properties of the solids was undertaken by impedance spectroscopy. Electrical conductivity was found to increase linearly with the incorporation of Y 3+ content while the associated values of activation energy decreased, with a minimum value of 1.1 eV for Th0.85Y0.15O1.925.

Keywords : yttria-doped thoria, synthesis, oxalate route, characterizations

Conclusion

Pure Th1-xYxO2-x/2 ceramics, aimed to be used as electrolyte material in oxygen sensors, were successfully obtained from the conversion of oxalate precursors at 500°C. Such wet-chemistry route ensured the rapid and quantitative precipitation of the cations into Th1-xYx(C2O4)2.2H2O solid solutions. Nevertheless, a statistical EDS study coupled to XRD measurements revealed that the distribution of the cations in the samples remained slightly heterogeneous, probably as a consequence of differences in the precipitation kinetics of thorium(IV) and yttrium(III).

The conditions leading to the full densification of Th1-xYxO2-x/2 ceramics were thereafter determined by dilatometry and set to a heat treatment of 8 hours at 1575°C. Such operating conditions led to relative densities up to 99%, associated to a fine-grains microstructure, in good agreement with the specifications generally required for electrolyte material in oxygen sensors to be used in liquid sodium.

Finally, a very first estimation of the electric properties of the samples was undertaken by impedance spectroscopy. Electrical conductivity was found to increase linearly up to x = 0.15 with the incorporation of trivalent yttrium in ThO2, in good agreement with the formation of oxygen vacancies. Accordingly, a decrease in the associated values of the activation energy was observed.

Additional experiments are now under progress to get a better understanding on both sintering behavior and electrical properties. On the one hand, the influence of the yttrium content on the final microstructure of the pellets will be evidenced, based on the establishment of sintering maps. On the other hand, complementary impedance spectroscopy measurements should allow to point out the relative contributions of grains and grains boundaries in the global electrical conductivity of the samples. Variation of the unit cell volume of Th1-xYxO2-x/2 solid solutions versus x, and comparison with the results reported by Cosentino et al. [START_REF] Cosentino | Lattice parameters of thoria-yttria solid solutions[END_REF]. XRD measurements were conducted at room temperature on samples previously heated for 6 hours at 1400°C (this study) or 2 hours at 1550°C [START_REF] Cosentino | Lattice parameters of thoria-yttria solid solutions[END_REF].

Supplementary data


Dr Nicolas Clavier
email: nicolas.clavier@icsm.fr

Yanis Cherkaski

Julien Martinez

Sophie Costis

Théo Cordara

F Audubert

Laurent Brissonneau

Nicolas Dacheux

Synthesis and Direct Sintering of Nanosized (MIV,MIII)O2-x Hydrated Oxides as Electrolyte Ceramics

Keywords:

come   L'archive ouverte pluridisciplinaire

Introduction

Due to the rarefaction of natural resources and to the considerable increase of the world global demand, the cost of energy, and particularly that of electricity, is planned to rise during the next decades [1] . The development of innovative production sources then appears as one of the XXI th century top challenges, with the aim to provide massive energy supply and still assuming a sustainable politics for resources, notably through the choice of closed-cycles that integrate the recycling of the fuels and various materials involved. In this framework, France already possesses a long-term experience in two main sectors thought to provide promising answers to the energetic issues of the next decades.

First, the historical knowledge acquired in the domain of electronuclear energy is still pursued through the development of new concepts of reactors, called as Generation IV reactors [START_REF] Pioro | Handbook of Generation IV Nuclear Reactors[END_REF] . The Sodium-cooled Fast Reactor (SFR) concept was more particularly selected [START_REF] Kelly | [END_REF] to provide a sustainable politics regarding uranium resources, for example through the use of depleted uranium as fuel, with improve safety, reliability and resistance to proliferation. In this context, one of the correlated challenges in the material sciences domain is to assess the peculiar chemistry of liquid sodium. Particularly, the oxygen content, which controls the corrosion rate of the stainless steel claddings [4] , must be kept under a specific value (usually few ppm), and then monitored through the use of potentiometric sensors. Such systems generally contain a ceramic electrolyte which must be compatible with sodium at high temperature (≈400°C), mechanically resistant and purely ionic conductor [5] . In this aim, several studies already suggested the use of yttrium-doped thoria (i.e. Th 1-x Y x O 2-x/2 ) [6] . Indeed, the incorporation of Y 3+ in the fluorite-type structure of ThO 2 (space group Fm-3m) produces oxygen vacancies which turns the ceramic into an ionic conductor [7] while doped thoria was generally reported to be much less sensitive to sodium corrosion compared to zirconia [8] . It is thus interesting to study new ways of preparation for these mixed oxides, but also to evaluate their ability to sinter in order to optimize their processing as a ceramic material.

On the other hand, the development of fuel cells constitutes from many years a major research effort. This interest has even grown recently due to the possible application of these technologies in the automotive industry and in portable devices. Among the numerous concepts envisaged, Solid Oxide Fuel Cells (SOFC) offer important potentialities due to their high electrical efficiency and their fuel flexibility. They also provide high electrical power (up to 2MW) [9] but remained associated to high temperatures of operation that can reach 800-1000°C [10] . To lower this temperature, electrolyte materials, which can be seen as the heart of the SOFC unit, are strongly investigated. One can particularly retain fluorite-type materials such as Bi 2 O 3 , ZrO 2 or CeO 2 [11] , which are usually doped with aliovalent cations in order to create oxygen vacancies : a good example is the substitution of the host Ce 4+ by acceptor cations such as M 3+ (M = Ln, Y) [12] . Once again, it then appears essential to develop original ways of synthesis that offer the possibility to prepare homogeneous mixed oxides and then to proceed to their densification with the aim to control their final microstructure.

Strong similarities then arise between the (M IV ,M III )O 2-x electrolyte ceramic materials used either in the field of nuclear industry or for SOFC units. An interesting point of comparison lies in the preparation methods reported in the literature for such compounds.

Indeed, powder metallurgy processes, which are based on the mechanical mixture of parent oxide compounds and subsequent heat treatment are, so far, the most frequently used [13] .

Although, the homogeneity of the cation distribution in the fluorite structure remains an important parameter to consider if one wants to optimize the performances of the materials within their life-cycle. As a matter of example, the presence of Ln 3+ -enriched phases in doped-ceria electrolytes can induce the formation of complex defects that tend to lower the ionic conductivity [14] . Thus, even if the use of wet chemistry methods could ensure the coprecipitation of the various cations considered in homogenous precursor systems, it remains poorly investigated so far. For actinide-based systems, crystallized precursors precipitated at room temperature or in mild hydrothermal conditions were particularly focused on the use of oxalic acid as a complexing agent [15] . Similarly, attempts to prepare homogenous doped-ceria electrolytes mainly concerned sol-gel methods [16] . The development of new ways of preparation based on the direct preparation of mixed-oxides is thus an open field for investigation.

On this basis, the preparation of Th solid solutions (with Ln = Nd, Sm, Gd) through an hydroxide wet chemistry route was considered in this work [17] . On the one hand, several compositions of yttrium-doped thoria were explored, both below and above the reference value of x = 0.15 corresponding to the optimal ionic conduction reported in the literature [5,18] . On the other hand ceria doped with 20 mol% in trivalent lanthanides were considered as this peculiar composition was frequently investigated for electrode materials [19] . The high tendency of tetravalent lanthanides and actinides towards hydroxylation, which was early reported to result in the formation of colloids in solution, further yielding nanoparticles [10,11] , was used in this work to finally obtain carbon-free and highly reactive oxide solid solutions. After the description of the preparation method, this paper reports the characterization through various physico-chemical techniques of the precursors at room temperature as well as during heat treatment leading to oxides. Finally, the densification of the samples through direct sintering of raw powders was investigated through dilatometry and SEM observations.

Experimental section

Synthesis of powdered samples.

All the chemicals used, including Th(NO 3 ) was finally determined by a colorimetric titration method already detailed in our previous works [20] .

The precipitation of hydroxide precursors was based on the protocol already reported for the preparation of UO 2 .nH 2 O by Martinez et al. [21] . In the case of Ce 0.8 Ln 0.2 O 1.9 samples (Ln = Nd, Sm, Gd), the precipitation of the hydroxide precursors was performed by mixing trivalent cerium and chosen lanthanide cation in nitric acid, in the desired stoichiometric ratio x = Ln/(Ce + Ln). The mixture of cations was then poured into a large excess (400%) of 2M NH 4 OH solution at room temperature, leading to the instantaneous formation of a yellowish precipitate. This latter was further maintained under magnetic stirring for one hour.

Afterwards, the powders were centrifuged at 4500 rpm, then washed twice with deionized water then once with ethanol in order to get rid of any traces of NH 4 OH.

Finally, a mechanic treatment was applied to the samples with the aim to avoid aggregation of the powders that can further decrease their sintering capability. Samples were then mixed with 50 mL of ethanol, well-known solvent used in de-agglomeration processes [22] , and poured into a flask. The suspension was then stirred mechanically at 40°C under vacuum until the solvent was fully evaporated. A similar protocol was followed for the Th 1-

x Y x O 2-x/2 samples series, starting with a mixture of Th 4+ and Y 3+ in nitric acid media.

The precise chemical composition of the powdered samples was further evaluated by two different methods (Table 1). For the Th 1-x Y x O 2-x/2 series, a small amount of powder was first dissolved in aqua regia, then diluted in 1% HNO 3 before ICP-AES (Spectro Arcos EOP) measurements (protocols for solution sampling and further analysis have been already described in our previous publication [23] ). The emission bands chosen were 401.913, 283.730 and 274.716 nm for thorium, and 371.030 and 324.2278 nm for yttrium concentration determination. For the Ce 0.8 Ln 0.2 O 1.9 series, the compositions were determined from Energy Dispersive Spectrometry (EDS) analyses coupled with the SEM device, using a Bruker AXS X-Flash 5010 detector. Each powdered sample was embedded in an epoxy resin, polished to the 1 µm grid and carbon coated prior analyses. EDS results were then obtained from around 10 different analyses. At least 400000 counts spectra were collected by the EDS detector using routinely an optimal working distance of 11.4 mm and an acceleration voltage of 15 kV.

Whatever the sample under consideration, weight percentages of cations and associated molar ratios were then determined and found in very good agreement with the values expected. One can then assume that the precipitation of both tetravalent and trivalent cations was quantitative using the hydroxide route for the two systems investigated.

Table 1.

Weight percentages of the cations and associated molar ratios in the assynthesized Th

Characterization.

Powder X-Ray Diffraction (PXRD) diagrams were recorded with a Bruker D8 apparatus at Cu Kα 1,2 radiation (λ = 1.54184 Å). Patterns were collected in the 5° ≤ 2θ ≤ 120° domain, with a step size of ∆(2θ) = 0.03° and a total counting time of about 2.5 hours.

Rietveld refinement were further performed with the Cox-Hastings pseudo-Voigt profile function [24] of the Fullprof_suite program [25] .

Scanning Electron Microscope (SEM) observations of powders were performed directly on raw samples using a FEI Quanta 200 apparatus. High vacuum conditions and low accelerating voltage (2 -3.1 kV) were selected to obtain high resolution images. In parallel, TEM observations and EDS analyses were performed with a JEM-2200FS microscope coupled with a JEOL SDD 30 mm 2 HyperNine (138 eV) spectrometer. Sample preparation consisted in the deposition of a drop of powder suspension in ethanol on a carbon copper grid.

Finally, specific surface area of the samples was measured by the BET method using a Micromeritics Tristar 3020. Samples were first outgassed at 90°C under vacuum for 4 hours before measurement.

Thermal analyses and sintering.

Thermogravimetric analyses and dilatometric measurements were conducted on Setaram Setsys Evolution apparatus. For TGA, about 20 mg were placed into an alumina crucible, then weight loss was recorded between room temperature and 1000°C (heating rate :

10°C.min -1 ) under air. For dilatometry, heat treatment was maintained up to 1600°C then a dwell time of 30 minutes was applied. In each case, samples were cooled down to room temperature at 30°C.min -1 .

Finally, the densification state of the pellets was estimated through geometrical measurements. The average of at least ten values of pellets thickness and diameter, obtained thanks to a precision calliper, was considered to calculate the relative density, which is correlated to the global porosity of the samples.

Results and discussion

Characterization of the powdered samples.

Powder samples prepared from the co-precipitation through the hydroxide route of thorium and yttrium, on the one hand, and of cerium and trivalent lanthanides, on the other hand, were first characterized by PXRD analyses (Figure 1). For both samples series, the patterns exhibited all the PXRD lines of the fluorite-type structure (cubic, Fm-3m space group) which is characteristic of ThO 2 [26] and CeO 2 [12] compounds. The synthesis process then did not end up with the formation of Th-or Ce-based hydroxides but with the precipitation of hydrated oxides. In the case of thorium, and more generally for all tetravalent actinides, similar results were already reported in the literature [27] , and described as the initial formation of hydroxide colloids [28] , followed by an aging step leading to hydrated oxide compounds as follows, eventually through the formation of AnO(OH) 2 .nH 2 O as an intermediate [29] :

An 4+ + 4 OH -↔ An(OH) 4 ↔ AnO 2 . xH 2 O + (2-x) H 2 O ( 1.
)

When dealing with Ce-based samples, several authors also mentioned the formation of hydrated cerium oxides, for example from the thermal hydrolysis of Ce 4+ in aqueous solution.

Quite similarly to tetravalent actinides, intermediate complexes with the form [Ce(OH) x (H 2 O) y ] (4-x)+ are formed and further give rise to CeO 2 .nH 2 O [30] . However, it is important to note that this process is much more favourable for Ce 4+ ions compared to Ce 3+

(due to the more important tendency of the former one to hydrolyse considering its higher charge and smaller radius size). On this basis, it is more likely that, in our system, the reaction first proceeded through the oxidization of Ce 3+ into Ce 4+ , then through the hydrolysis of this latter. Such oxidization was already reported to occur for trivalent cerium in aqueous solution, in presence of various complexing and/or shaping agents such ammonia, urea or hexamethylenetetramine [31] .

For all the prepared samples, the PXRD lines exhibited a large width, associated to a poor crystallization state and/or to a particle size in the nanometer range. Indeed, Rietveld refinement of the PXRD patterns generally led to an average crystallite size (i.e. coherent length domains), of about 1-2 nm. Also, it is important to note that no additional PXRD lines were detected in the patterns, such as those of Ln 2 O 3 sesquioxides [32] , indicating that the trivalent elements considered in this work (i.e. Y 3+ or Ln 3+ ) were co-precipitated along with Th 4+ and Ce 4+ then did not segregate during the aging of the hydroxide colloids.

(a) The morphology of the (Ce,Ln III ) and (Th,Y) hydrated oxides was then explored by both scanning (SEM) and transmission (TEM) electron microscopies. For the two sample series considered, SEM micrographs (Figure 2) exhibit a similar habit, with the powders being composed of very small particles. Due to the resolution limit of the SEM apparatus in the observations conditions, it was not possible to precisely estimate their size, which obviously In spite of this partial aggregation, the very high reactivity of the samples was confirmed by specific area values which all ranged between 130 and 180 m 2 .g -1 for the Ce 0.8 Ln 0.2 O 1.9 •nH 2 O compounds, while they could reach up to 230 m 2 .g -1 for the Th-based samples. However, it appeared difficult to establish a clear trend between the chemical composition of the samples and their specific surface area. Most probably, the surface is essentially dependent on the drying process performed under vacuum, and can vary sensibly from one batch to another. Nevertheless, all the studied samples presented a surface significantly higher than 100 m 2 .g -1 , which can be used to improve the sintering properties of these compounds and control their final microstructure, typically to obtain dense and finegrain microstructures [33] . Finally, additional TEM micrographs were recorded in order to precisely estimate the size of the elementary particles constituting the samples and their chemical composition (Figure 3). All the images collected accounted for nanosized powders in the two series of samples prepared. Indeed, the crystallites generally reached 2 to 5 nm and further aggregate to form agglomerates of about 10-20 nm. This result can be considered to be in good agreement with those obtained through PXRD Rietveld refinement, owing to the uncertainties of both techniques.

Qualitative EDS analyses were also performed on the crystallites, and confirmed the simultaneous presence of tetravalent (i.e. Th 4+ or Ce 4+ ) and trivalent (Y 3+ or Gd 3+ ) elements.

This result confirmed that the protocol developed for the preparation of the powders led to the co-precipitation of the cations as hydrated based oxides solid solutions without any segregation of the trivalent cations. One can then expect the formation of very homogenous compounds after sintering, which in some cases cannot be obtained by other wet chemistry routes. As a matter of example, the preparation of Th 1-x Y x O 2-x/2 oxides was frequently reported through an initial step of oxalic precipitation [34] . However, for x > 0.15, polyphase oxalate systems can be obtained and further led to heterogeneities in the cation distribution after conversion at high temperature [35] . The use of the so-called hydroxide route could then be considered as an easy and direct path towards highly reactive and homogeneous oxide

Thermal behaviour.

The second part of this work was devoted to the conversion of hydrated precursors into anhydrous Th 1-x Y x O 2-x/2 and Ce 0.8 Ln 0.2 O 1.9 oxides through heat treatment at high temperature. In this aim, all the prepared samples were first analyzed by TGA in order to estimate their initial hydration state and to assess the dehydration process. Whatever the type of sample studied and its chemical composition, the general aspect of the thermograms remained very similar (Figure 4). Indeed, the weight loss systematically appeared to be very progressive up to 500-600°C, where the anhydrous oxide form is obtained, even if a more rapid dehydration was observed at the onset of the heat treatment, i.e. from room temperature to 200°C. Such behavior is consistent with previous studies performed on MO 2 .nH 2 O compounds with M = U, Th or Ce, which also exhibited progressive dehydration rather than sharp transition from the hydrated oxide precursor to the final anhydrous form [36] . The total weight loss recorded for the various samples studied also appeared to be in the same range of magnitude, i.e. between 13 and 20%. For the Ce 0.8 Ln 0.2 O 1.9 •nH 2 O series, it corresponds to a value of n = 3 -3.5, which do not seem to be strongly dependent on the trivalent dopant. Conversely, the hydration state of the Th 1-x Y x O 2-x/2 •nH 2 O oxides was found to slightly vary with the amount of yttrium incorporated; the n value increasing from 3.5 for x = 0.08 to 4.4 for x = 0.22. Nevertheless, it is to note that the total amount of water measured in the hydrated oxides exceeds from far what could be expected from the aging of an initial hydroxide compound. Indeed, as described in equation ( 1), the maximum value of n should be 2, while the various works reported on tetravalent actinide oxides frequently reported samples with the general formula An IV O 2 .H 2 O or An IV O 2 .2H 2 O [37] . On this basis, two types of water

Sm

molecules could be present in our samples. On the one hand, adsorbed water is expected to be present due to the very high specific surface area of the samples, and is probably eliminated in the first part of the heat treatment, i.e. below 200°C. On the other hand, constitutional water, which comes from the aging process of the initial hydroxide, is probably eliminated at higher temperature, typically between 200 and 600°C. However, as the weight loss was found to be very progressive, it was not possible to discriminate and to quantify these two kinds of water content in this work.

The variation of the crystal structure was further monitored versus temperature. In this aim, a heat treatment of 6 hours was performed at various temperatures on the Th 0.92 Y 0.08 O 1.96 sample (Figure 5). As already described in the first section, the PXRD pattern recorded at room temperature only presented broad peaks, still characteristic of the ThO 2 based solid solutions. After heating at 500°C, i.e. after the complete dehydration of the sample, the same PXRD peaks were observed, although the departure of constitutional water is likely to provoke a slight shift in their position. Also, a significant decrease of their FWHM, indicating an increase of the average crystallite size (coherent domain length) was observed. Indeed, growth phenomena can proceed even at low temperature, for example from the mechanical rearrangement of adjacent crystallites, frequently referred to as 'oriented attachment' (OA) [38] . Oriented attachment is further probably still responsible for the narrowing of PXRD lines observed up to 1000°C.

RT

For higher temperatures, the decrease of the FWHM was even more pronounced and was once again correlated to an increase of the average crystallite size due to solid state diffusion processes. Indeed, OA and diffusion mechanisms were recently reported to occur successively during the sintering of CeO 2 and ThO 2 microspheres [39] and probably act similarly during the heat treatment of Th 1-x Y x O 2-x/2 and Ce 0.8 Ln 0.2 O 1.9 solid solutions [40] .

The growth of the crystallites within the powder samples during heat treatment was also correlated to the variation of the specific surface area, evaluated by the BET method after firing at various temperatures (Figure 6). One can note that specific surface drastically decreased between room temperature and 500°C, while the PXRD pattern only exhibited moderate narrowing of the FWHM. Hence, the decrease of the specific surface area in this temperature range is surely linked to a strong agglomeration of nanometric particles under dehydration. The growth of the particles, which occurs at higher temperatures due to oriented attachment and diffusion mechanisms, then accounts for a smaller decrease of the specific surface area, typically from about 30 to less than 1 m 2 .g -1 between 500 and 1500°C.

Sintering capability.

The sintering of the samples was finally investigated in a last step, in order to prepare dense ceramic materials with fine grain microstructure. Indeed, densities above 97% are generally required to ensure good electrical properties (i.e. to avoid undesired resistivity due to closed porosity) while submicrometric grains are frequently correlated to good mechanical properties. Owing to the direct precipitation of the samples as hydrated mixed oxides, on the one hand, and to the loss of reactivity observed upon heating, particularly through the drastic drop of the specific surface area, the sintering of the powders was performed without any preliminary heat treatment. Raw powders were then shaped as cylindrical pellets (Ø = 5-8 mm) by uniaxial pressure (600 MPa), leading to green density of about 40 to 50 % of the theoretical value.

Dilatometric analyses were then performed in order to estimate the sintering temperature of the pellets prepared (Figure 7). Whatever the chemical composition of the sample considered, both within the Ce IV 1-x Ln III x O 2-x/2 or the Th 1-x Y x O 2-x/2 series, the linear shrinkage systemically exhibited two different steps. The first one, starting from the beginning of the heat treatment to about 500°C, was correlated to the dehydration of the powders. It resulted in a linear shrinkage of around 10-15%, corresponding to a significant volume shrinkage which might lead to the formation of cracks within the pellet. Hence, the heating profile must be thoroughly adapted to ensure the complete dehydration of the oxide samples without compromising the mechanical integrity of the pellet. The second shrinkage step was found to be very progressive and started slightly above 1000°C. One can then assume that it corresponds both to the densification of the pellet and to grain growth processes which are enhanced due to the nanosized character of the starting powder. Also, one can note that between 1200 and 1500°C, the densification of Ce-based solid solutions proceeded faster than for thoria-based materials, which can be assigned to the higher sintering temperature generally reported for ThO 2 (i.e. in the 1500-1600°C range [41] ) compared to CeO 2 (around 1400°C [42] ). However, the densification of both samples was not completed at 1600°C, meaning that higher temperature of heat treatment and/or longer holding time are required to obtain highly dense samples.

On this basis, the sintering conditions leading to the full densification of the pellets, as well as the associated microstructure in term of grain size, were more specifically investigated for the Th 1-x Y x O 2-x/2 samples series. In this aim, pellets were heated at 1500°C or 1600°C, considering holding times ranging between 1 and 8 hours. Moreover, heating rate of 5°C.min -1 as well as a 2 hours dwell at 300°C were considered to prevent the formation of cracks in the pellets. As expected from the dilatometric study, the relative density of the samples was found to increase with the heating time for the two temperatures considered (Figure 8). Nevertheless, heat treatment at 1600°C rapidly led to highly densified samples, i.e.

typically with d > 95% TD, even though if the highest yttrium content studied was initially associated to significantly lower density values, probably due to solute drag effects slowing down the motion of grain boundaries [43] . This result can be compared with those recently reported for the sintering of Th 1-x Y x O 2-x/2 powders initially prepared through oxalic co-precipitation and subsequent thermal conversion [43] . In the present study, the initial important reactivity of the hydrated oxide powder did not lead to a significant decrease of the sintering temperature. Nevertheless, the density achieved after only 1 hour of heat treatment at 1600°C already reached more than 95%TD while it was below 90% for the powders prepared from oxalic precipitation. On this basis, the very high specific surface area offered by the hydroxide route led to the strong increase of the densification kinetics during the first hours of heat treatment at high temperature.

The SEM micrographs recorded on polished sections of samples sintered at 1600°C for 4 and 8 hours confirmed the very good densification state of the pellets (Figure 9). Indeed, no significant porosity was observed at the surface of the samples, neither in intra-nor in intergranular position. Moreover, the grain size was estimated by image analysis of all the sintered samples and was typically found between 100 and 500 nm whatever the chemical composition. It did not seem to significantly evolve when increasing the heating time at 1600°C. Once again, these values appear to be in the same order of magnitude than those reported when working with oxalate precursors [43] . This observation thus confirms that the sintering process is rapidly marked by a strong grain growth step, which probably proceeds concomitantly to the densification. Once the grains big enough (i.e. tens of nanometers), their growth kinetics probably slowed down and became comparable to that observed on similar systems prepared through alternative chemical routes.

Also, it is to note that the initial nanosize character of the grain did not generate an enhanced migration of the trivalent elements to the grain boundaries. Indeed, this typical behavior was frequently reported during heat treatment of M IV 1-x M III x O 2-x/2 (with M IV = Ce, Th and M III = La-Yb) fluorite-type solid solutions [44] , and resulted in solute drag effect that can slow down, if not hinder, the sintering and grain growth processes. In our study, the important occurrence of grain boundaries in the starting powdered compact did not result in such an effect since a full densification of the pellets was stated after heating at 1600°C. On this basis, the initial co-precipitation of tri-and tetravalent lanthanide and actinide cations through the hydroxide route successfully allowed the direct sintering of the as-synthesized powders and ended up with ceramics exhibiting microstructural features of interest for their application as electrolyte materials, i.e. high density associated to submicrometric grain size.

Figure 1 .

 1 Figure 1. PXRD patterns of the as-synthesized samples in the Th 1-x Y x O 2-x/2 (a) and Ce 0.8 Ln 0.2 O 1.9 (b) series (* symbols mark the PXRD lines of the sample holder).

9 *

 9 remained lower than 10 nm. Moreover, despite the drying step performed under vacuum, one can still observe the formation of large aggregates of about 10-20 µm. Nevertheless, they could arise from the deposition of the powders on carbon sticks prior to observation and/or from the high vacuum conditions used during image recording.

Figure 2 .

 2 Figure 2. SEM micrographs of as-synthesized powders in the Th 1-x Y x O 2-x/2 and Ce 1-x Ln x O 2-x/2 series.

Figure 3 .

 3 Figure 3. TEM micrographs of Th 0.92 Y 0.08 O 1.96 •nH 2 O (a) and Ce 0.8 Gd 0.2 O 1.9 •nH 2 O (b) samples.

Figure 4 .

 4 Figure 4. TG analyses recorded for Th 1-x Y x O 2-x/2 •nH 2 O (a) and Ce 0.8 Ln 0.2 O 1.9 •nH 2 O (b) samples.

Figure 5 .

 5 Figure 5. Variation of the PXRD pattern of Th 0.92 Y 0.08 O 1.96 •nH 2 O versus the heating temperature.

Figure 6 .

 6 Figure 6. Variation of the specific surface area versus heating temperature obtained for Th 0.92 Y 0.08 O 1.96 •nH 2 O .

Figure 7 .

 7 Figure 7. Dilatometric curves obtained for Ce 0.8 Nd 0.2 O 1.9 (green) and Th 0.85 Y 0.15 O 1.925 (black) samples.

Figure 8 .

 8 Figure 8. Evolution of the relative density of Th 0.85 Y 0.15 O 1.925 (black) and Th 0.78 Y 0.22 O 1.89 (red) samples during their sintering at 1500°C (open symbols) and 1600°C (full symbols).

Figure 9 .

 9 Figure 9. SEM micrographs of Th 0.85 Y 0.15 O 1.925 and Th 0.78 Y 0.22 O 1.89 sintered samples.

  1-x Y x O 2-x/2 (0.01 ≤ x ≤ 0.22) and Ce 0.8 Ln 0.2 O 1.9

AES Th 1-x Y x O 2-x/2 x calc. Th (wt.%) Y (wt.%) x exp.

	0.08	84.6 ± 0.9	2.9 ± 0.2	0.08 ± 0.01
	0.15	82.1 ± 2.2	5.9 ± 0.6	0.14 ± 0.02
	X-EDS

1-x Y x O 2-x/2 (ICP-AES measurement) and Ce 1-x Ln x O 2-x/2 (X-EDS) sample series.

ICP-

Ce 0.8 Ln 0.2 O 1.9 Ln Ce (wt.%) Ln (wt.%) x exp.

	Nd	62.2 ± 0.3	20.1 ± 0.3	0.23 ± 0.01
	Sm	65.9 ± 0.5	19.0 ± 1.5	0.21 ± 0.02
	Gd	63.4 ± 1.8	19.1 ± 2.5	0.21 ± 0.03

Acknowledgements

The authors would like to thank Johann Ravaux and Renaud Podor for their help during SEM examinations of the samples, as well as Xavier Le Goff for his support during TEM observations. Y. Cherkaski and J. Martinez are grateful to CEA for its continuous financial support and for funding their PhD thesis.

Conclusion

Highly reactive and nanosized Th 1-x Y x O 2-x/2 or Ce 0.8 Ln 0.2 O 1.9 mixed oxides were prepared through the initial precipitation of hydroxide precursors which were further dried under vacuum. Whatever the chemical system investigated, i.e. Th 1-x Y x O 2-x/2 or Ce 0.8 Ln 0.2 O 1.9 , the characterization of the powdered samples evidenced a rapid aging process leading to hydrated oxides. Moreover, the precipitation appeared to be quantitative and no segregation of the trivalent cations was pointed out, either by PXRD or by EDS analyses conducted during TEM observations. The thermal behavior of such hydrated oxides was further explored and first showed a two-step dehydration process, with the successive departure of adsorbed water up to 200°C, followed by that of constitutive water coming from the aging of initial hydroxide colloids, both yielding a drastic drop of the powders reactivity (i.e. decrease of the specific surface area).

Sintering of the samples was then undertaken by starting from raw powders in order to preserve their reactivity. Such a process, composed by the direct shaping of the samples followed by their heat treatment at high temperature, then eliminates potential preliminary steps, such as firing or milling, which can led to safety or pollution issues when handling radioactive materials. Dilatometry experiments coupled to SEM observations and density measurements then pointed out the very rapid densification kinetics. Indeed, if the sintering temperature (1600°C) was not significantly modified compared to other wet-chemistry routes, highly densified pellets above 95%TD were obtained after only 1 hour of heat treatment. The initial preparation of nanosized powders also led to a fine grain microstructure, which is frequently linked to improved physico-chemical properties in use of the ceramics. This easy and versatile precipitation process that can be followed by direct densification of the powders, then appears as a promising option for the elaboration of homogenous ceramic electrolytes.

The associated electrical properties are then currently under investigation to assess their potential utilization in SOFC units or in oxygen sensors.


domain: sdv sde shs sdv.bv

text:
Jean-Marc Blazy

François Causeret

Pierre Chopin

Dominique Denon

Loic Guinde

David Hammouya

Jacky Paul

Mirza Publicol

Jorge J Sierra

Building sustainable biomass-based energy system for Caribbean islands

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

BUILDING SUSTAINABLE BIOMASS-BASED ENERGY SYSTEM FOR CARIBBEAN ISLANDS

Jean-Marc Blazy, François Causeret, Pierre Chopin, Dominique Denon, Loïc Guindé, David Hammouya, Jacky Paul, Mirza Publicol and Jorge Sierra, INRA, UR1321 ASTRO Agrosystèmes tropicaux, F-97170 Petit-Bourg (Guadeloupe), France Email: jeanmarc.blazy@antilles.inra.fr Abstract: Interest in renewable energy from agricultural biomass has increased in recent years. Among renewable energy sources, biomass is considered indeed as an attractive option for sustainable energy production. This is particularly true in the case of small tropical island states, where energy demand is increasing and the energy mix currently heavily relies on fossil imported resources. Diversifying and orientating the energy mix toward locally grown energy crops could therefore be a promising sustainable option. However, the introduction of such bioenergy systems under existing conditions and current agricultural models is still rather uncertain and requires achieving several sustainable goals. In order to build new sustainable energy systems, modelling the biomass supply is a critical step prior to investment. In this paper we propose an original modelling framework in order to tackle this issue in an integrated approach. Our framework consists in the linkage of three types of models: 1) a biophysical crop model to simulate agronomic options (including choice of genotypes, crop and soil management), 2) a farm scale model aimed at simulating farmer's decisions to adopt growing biomass and 3) a spatially explicit model whose aim is to simulate at the regional scale the biomass supply and sustainability indicators. The framework is applied to the building of scenarios of fibercane-based energy systems at territorial scale in Guadeloupe, a small tropical island in the Caribbean. Through an optimisation module, it is possible to identify agroeconomic scenarios of biomass systems satisfying multiple objectives and constraints in a sustainable way. We finally discuss about the genericity of our approach and on factors affecting the success of biomass systems implementation in small tropical island states.

  Keywords: energy crops, sustainability, farm model, landscape scale

Proceedings of the Caribbean Food Crops Society. 50:131. 2014




  To compare FOUR TYPES OF SWARDS grazed by dairy cows on SEASONAL and ANNUAL grazing days and milk production per ha. Session 1.5.4. Interest of multi-species for pasture-based milk production systems 5.4. Interest of multi-species for pasture-based milk production systems Mineral N fertilisation = 75 kg N/ha/year for each treatment GRAZING RULES Same grazing calendar. Similar pasture allowance (22 kg DM/cow/day > 3 cm). Extra non experimental cows to adjust grazing pressure. 5.4. Interest of multi-species for pasture-based milk production systems Grazing days/ha  Milk production  per cow/day  per ha/year  Pasture DM intake (ytterbium in 5 grazing cycles) SWARD CHARACTERISTICS Session 1.5.4. Interest of multi-species for pasture-based milk production systems Sward species: T~20%, C~30%, F~10% -No treatment effect on pasture mass -Greater pasture quality on MSS: NDF (-56 g/kg DM) pasture DM digestibility (+ 24 g/kg DM) 5.4. Interest of multi-species for pasture-based milk production systems + 13% NO INTERACTION WITH SEASON Session 1.5.4. Interest of multi-species for pasture-based milk production systems Good weather conditions favourable for ryegrass growth Grazing days/ha Milk production/ha


domain: sdv shs sde

text:
est destinée au dépôt

Impact of soil and canopy management practices on pesticide use in viticulture in french regions

To face the negative impacts of pesticides on human health and biodiversity, France adopted in 2008 an action plan aiming at decreasing pesticide use by 50% within 10 years. This is a major challenge, particularly for viticulture, which consumes a very large quantity of pesticides, as vineyard is very sensitive to pests and diseases. Different methods (eg IPM) have been proposed for a more integrated production. Among them, prophylactic practices such as fertilization management, soil surface management and pruning management (shoot thinning, de-budding, leaf thinning, green harvesting,..), aim at regulating production (especially for wine quality), but also contribute to limit pests and diseases development. Thus these practices can be considered as levers for reducing pesticide use. We used a statistical analysis to explore the relationships between the use of these practices and the level of pesticide use. The analysis was based on two surveys carried out by the French Ministry of Agriculture on the cropping practices in vineyards in 2006 (5217 fields) and 2010 (6007 fields) throughout the country. Several indicators were used to evaluate the correlations between these a priori prophylactic practices and the intensity of pesticides use. Indicators on the use of pesticides (Treatment Frequency Index) and fertilizers were calculated, and indicators on canopy and soil surface management, were designed based on variables available in the database. The analysis was conducted for ten regions (NUTS2) and for the two available years, which corresponded to medium vintages in terms of climate and pest pressure. Results showed that the nature and the number of assumed prophylactic practices varied between and within regions. We found a significant reduction of herbicide use between 2006 and 2010 at the national scale, which was not observed for all regions. At the regional scale, this decrease was correlated with a change in the type and proportion of soil management practices (chemical weeding, permanent grass cover, tillage…). However, combinations of other practices commonly used to reduce vine vegetative vigour (reduced fertilization, green pruning) did not lead to a reduction of pesticide use at the regional scale. These relationships should be further studied, as some information were missing in the survey, such as biophysical field characteristics (soil type, vine vegetative vigour …) or farmers' protection strategies. To this end, individual surveys will be performed to explore in detail these relationships.

  Florine F. Mailly, Jean Marc J. M. Barbier, Marie Thiollet-Scholtus, Laure Hossard, Christian C. Gary To cite this version: Florine F. Mailly, Jean Marc J. M. Barbier, Marie Thiollet-Scholtus, Laure Hossard, Christian C. Gary. Impact of soil and canopy management practices on pesticide use in viticulture in french regions. 2. Stakeholders Congress IPM innovation in Europe, Jan 2015, Poznan, Poland. 174 p., 2015, IPM Innovation in Europe: book of abstracts. hal-02743717 iMpacT Of sOil and canOpy ManagEMEnT pracTicEs 71 ON PESTICIDE USE IN vITICULTURE IN FRENCH REGIONS Florine Mailly 1,2 , Jean-Marc Barbier 1 , Marie Thiollet-Scholtus 3 , Laure Hossard 1 , Christian Gary 4 1 INRA, UMR 951 Innovation, F-34060 Montpellier, France 2 ADEME, F-49004 Angers cedex 01, France 3 INRA, UE1117 Vigne et Vin, UMT Viniterra, F-49071 Beaucouzé cedex 01, France 4 INRA, UMR 1230 System, F-34060 Montpellier cedex 01, France florine.mailly@supagro.inra.fr