P Jana

E Palomo Del Barrio

M Dubois

M Duquesne

A Godin

C Vautrin-Ul

V Fierro

G Medjahdi

A Celzard
email: alain.celzard@univ-lorraine.fr

Hydrophobised carbon foams for improved longterm seasonal solar thermal energy storage

Keywords: Seasonal thermal heat storage, Sugar alcohols, Carbon matrix, Hydrophobisation, Subcooling, Heterogeneous nucleation

Composites carbon foams based on sucrose-based char matrix and graphite filler were prepared and characterised with the aim of hosting sugar alcohols as phase-change materials (PCMs) in the context of thermal energy storage (TES). Seasonal solar TES demands an excellent undercooling of the molten PCM infiltrated in the foam, so that the heat can be stored as long as possible. The present paper demonstrates how the surface of such composite foams, i.e., comprising two carbon phases of different reactivities, can be modified for promoting undercooling. For that purpose, 8 different hydrophobisation treatments were applied, and the results were compared with those of the non-treated foam, in which heterogeneous nucleation could not be avoided. We show that one kind of functionalisation was successful, i.e., it fully preserved the melting point and the enthalpy of melting of the hosted phase-change material and completely avoided the heterogeneous nucleation of the PCM, while maintaining the thermal conductivity in the range required for this kind of application.

Introduction

Despite a significant number of researches all over the past decades, storing efficiently solar thermal energy is still challenging. Most commercially available systems are indeed based on sensible heat storage technologies, which usually provide very low energy density, especially in applications where heat is stored over a narrow range of temperature. Latent heat storage based on phase-change materials (PCMs) offers the opportunity to overcome such drawback. Indeed, PCMs are able to absorb heat upon melting, and delivering it back upon crystallisation, at almost constant temperature and they provide energy density five to ten times higher than that of sensible heat storage [START_REF] Sharma | Developments in organic solid-liquid phase change materials and their applications in thermal energy storage[END_REF][START_REF] Pielichowska | Phase change materials for thermal energy storage[END_REF][START_REF] Kenisarin | Solar energy storage using phase change materials[END_REF][START_REF] Farid | A review on phase change energy storage: materials and applications[END_REF]. However, the thermal conductivity of PCMs is most of the time too low, thus leading to heat exchangers oversizing and reduction of the efficacy of the systems, especially when the temperature of the heat transfer fluid is close to the melting/crystallisation point of the PCM, as it is the case of solar thermal energy storage (STES) applied to the building sector [START_REF] Gracia | Phase change materials and thermal energy storage for buildings[END_REF][START_REF] Khudhair | A review on energy conservation in building applications with thermal storage by latent heat using phase change materials[END_REF][START_REF] Zalba | Review on thermal energy storage with phase change: materials, heat transfer analysis and applications[END_REF]. Besides using finned heat exchangers, heat transfer enhancement in PCM-based storage technologies can also be achieved by other ways [START_REF] Fan | Thermal conductivity enhancement of phase change materials for thermal energy storage: A review[END_REF][START_REF] Ji | Enhanced thermal conductivity of phase change materials with ultrathin-graphite foams for thermal energy storage[END_REF][START_REF] Khadiran | Encapsulation techniques for organic phase change materials as thermal energy storage medium: A review[END_REF][START_REF] Khodadadi | Thermal conductivity enhancement of nanostructure-based colloidal suspensions utilized as phase change materials for thermal energy storage: A review[END_REF], among them the encapsulation of PCMs, either in small spheres suspended in the heat transfer fluid, or in highly conducting porous matrices in close contact with the heat exchanger. The second option has been widely investigated, and various materials mainly based on graphitic porous structures have been developed [START_REF] Ji | Enhanced thermal conductivity of phase change materials with ultrathin-graphite foams for thermal energy storage[END_REF][START_REF] Canseco | Structural and mechanical characterization of graphite foam/phase change material composites[END_REF][START_REF] Acem | KNO 3 /NaNO 3 -Graphite materials for thermal energy storage at high temperature: Part I. -Elaboration methods and thermal properties[END_REF][START_REF] Zhong | Heat transfer enhancement of paraffin wax using compressed expanded natural graphite for thermal energy storage[END_REF][START_REF] Zhong | Heat transfer enhancement of paraffin wax using graphite foam for thermal energy storage[END_REF][START_REF] Lafdi | Graphite foams infiltrated with phase change materials as alternative materials for space and terrestrial thermal energy storage applications[END_REF][START_REF] Mesalhy | Carbon foam matrices saturated with PCM for thermal protection purposes[END_REF][START_REF] Kholmanov | Continuous carbon nanotube-ultrathin graphite hybrid foams for increased thermal conductivity and suppressed subcooling in composite phase change materials[END_REF].

The problem becomes more complex when seasonal STES is considered. Unlike most heat storage applications, for which the storage time is short (typically less than one day), seasonal storage near room temperature is expected to work over periods of several weeks to several months and therefore requires additional conditions for being efficient. PCMs are only useful for STES if it can be guaranteed that the PCM does not crystallise spontaneously during the storage period, which would lead to loss of the charge prior to use. This can be achieved by sizing the thermal insulation to prevent the temperature of the PCM from falling below its melting point. However, this solution entails unaffordable costs. An attractive alternative to oversized thermal insulation consists in using PCMs with very low probability of homogeneous nucleation. Obviously, heterogeneous nucleation must be avoided too; thus, solutions of heat transfer enhancement promoting such a phenomenon should be discarded. If these conditions are fulfilled, it is therefore expected that the heat can be stored for quite long times in undercooled PCMs, and might be recovered at the desired moment by any perturbation of the system such as mechanical or thermal shock [START_REF] Barret | Thermal energy storage in supersaturated salt solutions[END_REF][START_REF] Hirano | Long-term supercooled thermal energy storage[END_REF][START_REF] Sanders | Supercooling salt hydrates: stored enthalpy as a function of temperature[END_REF][START_REF] Seppälä | Factors affecting superccoling of liquids from a heat storage perspective[END_REF][START_REF] Godin | Bubble agitation as a new low-intrusive method to crystallize glass-forming materials[END_REF].

Seasonal STES based on the use of sugar alcohols as PCMs was shown to be particularly relevant, since the latter combine highly valuable properties such as low cost, availability and natural origin, absence of toxicity and of corrosive character, high energy density (130 -200 kWh m -3 ), rather low melting points (typically in the range 70 -120°C), and significant but stable undercooling [24,25 and refs. therein].

Whether used pure or as eutectic mixtures for getting PCMS, sugar alcohols such as xylitol, sorbitol, erythritol and mannitol, among others, are highly polar molecules. Therefore, their encapsulation in porous carbonaceous hosts of low surface energy should prevent the nucleation of crystallites and hence favour undercooling. Therefore, storing energy at temperatures far below the melting point should be possible for long periods, thus reducing thermal losses. In a recent work, composite carbon foams made of sucrose coke as matrix and natural graphite flakes as filler were produced and shown to meet all aforementioned prerequisites for seasonal STES after impregnation by sugar alcohols used as PCMs [START_REF] Jana | Design of carbon foams for seasonal solar thermal energy storage[END_REF]. Our group indeed demonstrated that, depending both on the requirements of the application (e.g. delivered power, temperature, operating conditions during heat delivery) and on the nature of the sugar alcohol, carbon foams having at the same time high open porosity (typically higher than 80%), moderate thermal conductivity (typically from 3 to 8 W m -1 K -1 ) and affordable cost (less than 6 €/kg) were needed. We showed in the same work how to fulfil such antagonistic properties using experimental design and related modelling (analysis of variance and response surface analysis) in order to find the optimum formulation. However, no experiments had been published proving the efficiency as such carbon foams as hosts for sugar alcohols for seasonal STES. These PCMs indeed have a low thermal conductivity in the solid state, typically 0.3 -1.3 W m -1 K -1 [START_REF] Del Barrio | Characterization of different sugar alcohols as phase change materials for thermal energy storage application[END_REF], and still the proof of concept was missing.

Furthermore, no surface treatment had been performed for making them more hydrophobic than they actually were, with the goal of observing a beneficial effect on long-term undercooling after heat storage.

The present paper thus aims at evaluating the aforementioned optimised carbon foams as conductive porous hosts for seasonal STES based on sugar alcohols. These carbon materials were tested as such as a reference and, after various hydrophobisation treatments, their efficiency in terms of delivered power upon heat discharge and undercooling was evaluated.

We show that grafting fluorine atoms or depositing silica onto a carbonaceous surface is an efficient route to increase its hydrophobic character and prevent heterogeneous PCM nucleation, thus promoting longer heat storage. The most promising sugar alcohols for seasonal STES, namely erythritol, xylitol and, more specifically, their eutectic mixture [START_REF] Del Barrio | New sugar alcohols mixtures for long-term thermal energy storage applications at temperatures between 70°C and 100°C[END_REF][START_REF] Del Barrio | Characterization of different sugar alcohols as phase change materials for thermal energy storage application[END_REF], are used in this work. Not only do they have a melting point suitable for seasonal STES and a high latent heat of melting, but also, unlike many other sugar alcohols, they have fairly good thermal stability [START_REF] Shukla | Thermal cycling test of few selected inorganic and organic phase change materials[END_REF][START_REF] Kaizawa | Thermophysical and heat transfer properties of phase change material candidate for waste heat transportation system[END_REF][START_REF] Neidermaier | Sugar alcohol-based materials for seasonal storage applications -Thermal stability, Final workshop and on-site demonstration of FP/ SAM.SSA project[END_REF].

Materials and methods

Preparation of carbon foams

The synthesis and properties of carbon foams having both porous structure and thermal conductivity optimised for being used as hosts for sugar alcohols have been thoroughly described elsewhere [START_REF] Jana | Design of carbon foams for seasonal solar thermal energy storage[END_REF]. However, it is useful to recall here that such materials were prepared by dissolving sucrose and suspending natural graphite flakes in an aqueous solution of nickel nitrate. After homogenisation, the resultant blend was heated for 2 days in a ventilated oven at 120°C. During this step, polycondensation of sucrose catalysed by the metal salt occurred, the result of which was the release of water vapour and the production of rigid foam. After stabilisation of the latter, pyrolysis was carried out at 900°C in nitrogen flow, leading to a composite (sucrose char / graphite particles) carbon foam. It was shown that nickel behaved as a catalyst for both foaming and subsequent partial graphitisation. Catalytic graphitisation was required for further improving the thermal conductivity, which was not possible by adding more graphite filler since the latter reduced the final porosity. Finding the delicate compromise leading to the optimal amount of each compound therefore required the use of experimental design and the statistical analysis of the results. Fig. 1 shows a few samples having open porosity and thermal conductivity of 85.8% and 3.1 W m -1 K -1 , respectively. Fig. 1. Typical composite (sucrose char / graphite particles) carbon foams for seasonal STES based on sugar alcohols. Each sample is about 1.5 cm thick and 6 -7 cm long, on average.

Hydrophobisation of carbon foams

Various kinds of surface treatment were considered here for making sucrose-based carbon foams more hydrophobic: direct fluorination with molecular fluorine, silanisation, chemical grafting of perfluoroalkyl chains, and electrochemical grafting of fluorinated moieties.

Direct fluorination by molecular F 2

Direct fluorination by F 2 is the easiest method for covalent grafting of fluorine atoms onto carbon materials and guarantees a very homogeneous treatment when the treatment conditions are optimised. However, because the present materials were composite foams, i.e., containing two kinds of carbon (sucrose char and graphite) with very different reactivities, many trials had to be carried out to find the best way of fluorinating them. All the corresponding details are given in section I of the Supplementary Information, leading to Figs. S1 -S6. A lot of information about the reactivity of carbon with fluorine is also given there, supporting the need of multiple preliminary trials for getting a material of relevant composition for the intended application.

Silanisation

Sucrose-based composite carbon foams were covered by silica using a sol-gel process described elsewhere and already successfully carried out for hydrophobising activated carbons [START_REF] Cosnier | Hydrophobisation of active carbon surface and effect on the adsorption of water[END_REF] and, more recently, organic foams for thermal insulation [START_REF] Delgado-Sánchez | Hydrophobisation of tannin-based foams by covalent grafting of silanes[END_REF]. In order to promote the grafting of silane, the carbon surface was first oxidised to create as much surface functions as possible. Then, decarboxylation was carried out by heating the material, and the remaining oxygenated surface functions were selectively reduced into hydroxyls. Finally, vinyltrimethoxysilane (vtmos) was grafted by refluxing in dry toluene. The scheme of the process is given in Fig. S7, and the equipment used for the last step is shown in Fig. S8 of the Supplementary Information. The resultant sample was named CF900_vtmos.

Grafting of perfluoroalkyl chains

Grafting long perfluoroalkyl chains was carried out in a very similar way as before, using a method inspired from [START_REF] Wang | Super-hydrophobic ordered mesoporous carbon monolith[END_REF]. Composite carbon foam (1.82 g) was again first oxidised in nitric acid, washed, then heat-treated at 450°C in N 2 atmosphere, and reduced by NaBH 4 (0.5 g in 100 mL of dry ethanol). Then, the resultant carbon foam was soaked in 100 g of methanol containing 1 g of perfluorodecyltrimethoxysilane (pfdtms). The resultant mixture was stirred (300 rpm) for 3 h (see Fig. S9 of the Electronic Supplementary Information). The grafted sample was finally thermally treated at 140°C in N 2 atmosphere, and was called CF900_pfdtms.

Electrochemical grafting of fluorinated moieties

Since carbon is an electrically conducting material, grafting fluorinated functions has been possible by electrochemical reduction of diazonium salts according to a procedure described elsewhere [START_REF] Chaussé | The Electrochemical Reduction of Diazonium Salts on Iron Electrodes. The Formation of Covalently Bonded Organic Layers and Their Effect on Corrosion[END_REF]. All experimental details are given in the Supplementary Information, especially the general scheme of the process in Fig. S11. The experimental device and typical voltammograms are shown in Fig. S12.

Four samples of grafted carbon foam were first prepared, in aqueous sulphuric acid (0.1 mol L -1 ) or in acetonitrile containing NBu 4 BF 4 (0.1 mol L -1 ) as electrolyte, and from two amines as diazonium precursors: trifluoro-methyl-aniline and pentafluoro-aniline, leading to the functionalisation of carbon surface by -C 6 H 5 CF 3 and -C 6 F 5 groups, respectively. The preparation conditions are summarised in Table 1. 2.

Given the hydrophobic nature of studied carbon foams, vacuum-assisted infiltration is necessary to prepare the carbon foamsugar alcohol composites. It is well known that a porous medium with interconnected (open) porosity can be infiltrated with a non-wetting fluid by establishing a pressure difference between the inside and the outside (ΔP), which forces the intrusion of fluid into the porous structure. The minimum ΔP required for intrusion to occur is determined by the Young-Laplace equation [START_REF] Washburn | The dynamics of capillary flow[END_REF]:

, where and are, respectively, the surface tension and the contact angle. is the pore diameter and is the geometry coefficient of the pores ( for cylindrical pores, for spherical pores). The carbon foams studied in this work are characterised by large globular pores of millimetre size [START_REF] Jana | Design of carbon foams for seasonal solar thermal energy storage[END_REF]. As shown in Table 7, the observed contact angles range from 96° to 155°. The surface tension of sugar alcohols is of the order of 60 mJ/m 2 [START_REF] Zhang | Experimental and in silico characterization of xylitol as seasonal heat storage material. Fluid Phase Equilibria[END_REF]. Therefore, considering , , and , we obtain , which means that using primary vacuum is largely sufficient to achieve the infiltration of the foam with the molten sugar alcohol.

Sucrose-based carbongraphite composite foam samples (5 -7 cm 3 ) were first ultrasonically cleaned with water and next dried in a ventilated oven at 120°C for 12 hours.

The carbon foams were then impregnated with the xylitol-erythritol (64 / 36 %) blend in a vacuum oven heated at 120°C for 6 hours. After the MASA was molten and infiltrated, the whole was cooled down to room temperature under vacuum. Under these conditions, it took around 12 hours for the blend to solidify. It should be noted that in DSC experiments, which are rather short, unlike the present case, it is generally not possible to observe the crystallisation of the undercooled eutectic mixture. After solidification, the excess blend was removed from the foam by cutting it with a knife, the carbon-MASA hybrid materials were weighed, and their physical characteristics and properties were measured. The small-volume DSC allows sample volumes up to 0.3 mL to be tested, whereas the maximum volume that is allowed by the large-volume DSC is 6.5 mL. The testing conditions used are detailed in Fig. 2 and Table 3. To minimise the thermal gradients within the samples, especially during the cooling of the subcooled melts, the applied heating/cooling rates were 1°C min -1 and 0.5°C min -1 in small and large-volume DSC, respectively. The duration of the isotherm preceding the cooling stage was 15 min in the small-volume DSC but was higher (1 h) in the large-volume DSC to achieve isothermal conditions within the sample before starting the cooling stage. The erythritol content in large samples of hybrid materials varied between 1300 mg and 4470 mg, depending on the sample. Therefore, in order to make comparisons, 3 for details.

different
The same DSC tests were used to analyse whether or not the confinement of the sugar alcohol into the carbon foam or the applied hydrophobisation treatments modified the melting point or the latent heat of the phase-change material. The melting temperature was determined by the onset temperature of the endothermic peak in the DSC-thermograms, whereas the latent heat was calculated by integration of the latter, assuming a linear baseline. The DSCs were calibrated for heat flow and temperature using high-purity (> 99.99 %) reference materials: indium, tin, zinc and aluminum. The accuracy in the determination of melting temperatures was about ±1 K, whereas that of the enthalpies of phase transition was ±5 %.

Results and discussion

Characteristics of hydrophobised carbon foams

Structural and thermal properties of the pristine carbon foam were investigated in our former work [START_REF] Jana | Design of carbon foams for seasonal solar thermal energy storage[END_REF]. Hence, the study now mainly focuses on the analysis of surface chemistry and roughness, which are the main parameters that can influence the wettability properties of the foams and, therefore, the subcooling phenomenon in the encapsulated sugar alcohol. The main features of the different hydrophobised carbon foams are presented in sections 3.1.1 to 3.1.3, whereas wettability properties of all of them are discussed in section 3.1.4.

Stoichiometry, nanotexture and surface chemistry after direct fluorination

The compositions of all carbon foams obtained after treatment in pure F 2 in the various dynamic conditions detailed in section I of the Electronic Supplementary Information are given in Table 4. The resultant stoichiometry formula was calculated from the weight uptake of the sample during the fluorination process. As expected, under the same fluorination conditions, the composite (graphite / sucrose char) foams having less reactivity due to their graphite content led to less F-rich compounds than pure sucrose carbon foams [START_REF] Zhang | Effect of graphitization on fluorination of carbon nanocones and nanodiscs[END_REF]. Besides, the fluorination increased for all the foams when increasing the temperature of the F 2 stream. The nanotexture of these materials has been investigated by Raman spectroscopy, 19 F NMR was used to study the surface chemistry, and XRD was employed to analyse whether direct fluorination may have modified the carbon structure. Nitrogen and carbon dioxide adsorption studies were not carried out on these materials because of the risk of desorbing highly corrosive fluorine compounds during the degassing step. The Raman spectra of pure sucrose-based carbon foams and composite (graphite/sucrose char) foams are shown in Fig. 3. Pure sucrose-based carbon foam has, as expected, a rather disordered carbon structure since the intensity of the G band is only slightly higher than that of the D-band. Fluorination in increasingly harsh conditions did not really change this characteristic, suggesting that the additional disorder that has been introduced by the chemical treatment is negligible. Fluorine indeed first reacts with the more reactive carbon areas, i.e., with the most disordered ones (edges of carbon layers and other structural defects), reducing their extent and letting the most organised parts of the material almost unchanged, at least at the beginning of the reaction. However, a shift of D-and G-bands towards lower wavenumbers can be seen when the severity of the fluorination increased, and corresponds to an increasing number of sp 3 carbons. The D-band is thus shifted towards the expected position of diamond. Furthermore, it is known that the D-band shift is correlated linearly to the F/C atomic ratio of the compound [START_REF] Giraudet | Study of the postfluorination of (C 2.5 F) n fluorine-GIC[END_REF]. Rather high amounts of CF 2 groups are evidenced in fluorinated, pure sucrose-based, carbon foam. These F-rich moieties originate from the reaction of carbon layers edges and structural defects, initially present in higher amounts. Higher reaction temperature made the number of CF 2 and CF 3 groups increase, the next step being the burning of the foam into CF 4 and C 2 F 6 . Finally, the spectra of CF 0.07 is noisier than that of CF 0.23 only because of the lower amount of F, which decreased the signal/noise ratio.

As the covalent grafting of fluorine atoms by F 2 has been proved by 19 F NMR, XPS was not performed with these samples.

XRD studies have been carried out to see whether direct fluorination may have modified the carbon structure. The corresponding XRD patterns are given in Fig. 5, where the effect of grafting fluorinated functions is also shown for comparison. The intense, narrow bands in the pristine composite carbon foams correspond to the graphite filler, whereas the diffusion background is mainly due to the highly disordered sucrose-based carbon. The comparison with the fluorinated material suggests that the chemical treatment was mostly superficial, since no significant change could be noticed. As shown in the Electronic Supplementary Information (Fig. S13), modelling the XRD pattern at Bragg angles ranging from 40 to 50°

indicates that the fraction of rhombohedral phase of graphite in the composite carbon foam after fluorination by molecular F 2 (7.2 %) is decreased compared to that found for non-treated composite foam (9.7 %). The higher reactivity of rhombohedral graphite compared to hexagonal graphite explains that the volume fraction of the former decreased upon fluorination. All N 2 isotherms are combinations of types I and IV, according to the IUPAC classification [START_REF] Sing | Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity[END_REF], characteristic of micro-mesoporous materials. The adsorbed amounts are low, which indicates that the corresponding pore volumes are also low, unlike the macroporosity that can never be evidenced by adsorption techniques. Surface treatments led to lower adsorbed volumes with respect to the reference sample CF900 with a corresponding decrease of the plateau height, which indicates that a significant amount of microporosity (pores < 2 nm) has been lost. CO 2 isotherms evidenced the opposite trend, i.e., an increase of microporosity induced by surface treatment. The corresponding data are given in Table 5. Based on the value of surface area, the amounts of reactants required for the chemical grafting of vtmos and pfdtms could be calculated as explained in the Supplementary Information.

Table 5. Pore texture parameters derived from Fig. 6.

Sample

A BET (m 2 g -1 )

V 0.99 (cm 3 g -1 )

V µ (N 2 ) (cm 3 g -1 ) V µ (CO 2 ) (cm 3 g -1 )
V m (cm 3 g - It is useful to recall here that CO 2 allows probing narrower pores than N 2 . Thus, finding almost similar values of V µ (CO 2 ) and V µ (N 2 ) for the non-treated carbon foam indicates that no extremely narrow micropores exist in this material. In contrast, the chemical treatment blocked the wider micropores, but created new narrow micropores inside the grafted layer.

In N 2 isotherms, all the hysteresis cycles at high relative pressure are broad and of H2 type.

Type H2 corresponds to porous channels with a pore mouth smaller than the pore body (this is the case of ink-bottle-shaped pores). The surface treatments decreased the width of the cycles, suggesting a slight loss of mesoporosity (2 nm < pores < 50 nm), see again Table 5.

The pore size distributions calculated by application of Density Functional Theory (DFT) to the isotherms are presented in Fig. 6(c). They clearly confirm the decrease of the amount of wide micropores (black curves) and the increase of the amount of narrow micropores (red curves) after surface treatment. As expected, CO 2 allowed to complete and to extend the pore size distribution in the narrow pore range of the plot.

Surface chemistry after electrochemical grafting of fluorinated moieties

X-ray photoelectron spectroscopy (XPS) studies were performed on samples treated by electrochemical diazonium salt reduction, thanks to which the surface composition could be estimated. Representative examples of the corresponding spectra are given in Fig. S14 of the Supplementary Information, and the results for all samples are reported in Table 6. As the covalent grafting of fluorine atoms by F 2 has been proved by 19 F NMR, XPS was not performed with those samples.

The quantification of the amount of fluorine at the surface is given by the at.% of F from the C1s bands. The amount of F in C-F bonds only is given by the at.% of F from the F1s also induced an increase of oxygen at the surface, and the O content of H 2 SO 4 -CF5 was also the highest (9.9 at.%). Therefore, it is possible that the positive effect of -C 6 F 5 groups with respect to hydrophobisation can be reduced by an enhanced hydrophilicity due to a higher number of oxygenated moieties. These results also show the significant impact of the grafting conditions, since a same molecule (see again The samples hydrophobised by electro-reduction of diazonium salts were not investigated by adsorption methods, neither by XRD. On the one hand, the amount of material produced is too low for getting a meaningful information of their porous texture by absorption methods.

On the other hand, electrochemical grafting involves a surface modification only (such as grafting perfluorodecyltrimethoxysilane above) and, therefore, it is not expected that it changes anything to the bulk of the carbon structure.

Wettability properties

The water contact angles, obtained by fitting the (circular) shape of the water droplet deposited on the surfaces of the carbon foams, themselves fitted by a linear baseline, are

presented in Table 7. The values show that, whatever the treatment, an increase of contact of angle is observed. Whereas the non-treated foam was already spontaneously (although moderately) hydrophobic, an enhanced hydrophobicity was noticed, but the effects are very different from each other. The highest hydrophobic character was obtained by direct fluorination at the highest amount of F incorporated in the carbon structure (CF 0.07 ), but the chemical grafting of perfluoroalkyl chains (pfdtmos) also had a significant effect. Silanisation by vtmos also proved to be as effective as the latter treatment. In comparison, the electrochemical grafting of fluorinated surface groups was rather disappointing and, unexpectedly, some treatments of this kind (H 2 SO 4 -CF3, ACN-CF3, ACN-CF5) were even almost ineffective with respect to the raw carbon foam.

Overall, the values presented in Table 7 are in good agreement with former data reported in the literature. The presence of fluorine atoms on the carbonaceous surface through covalent C-F bonds indeed allows an increase of the hydrophobic character of, for instance, carbon fibers [START_REF] Zayat | Fluorination of carbon fibers by halogen fluorides[END_REF][START_REF] Ho | Bismarck Surface and bulk properties of severely fluorinated carbon fibres[END_REF], even at low fluorine content for which the advancing water contact angle increases up to 115° (102° for CF 0.02 composition) [START_REF] Ho | Fluorination of carbon fibres in atmospheric plasma[END_REF]. In comparison, flat polytetrafluoroethylene (PTFE) exhibits a contact angle of 110°. The grafting of pfdtmos on organic foams has been shown to lead to contact angles ranging from 137 to 143° [START_REF] Delgado-Sánchez | Hydrophobisation of tannin-based foams by covalent grafting of silanes[END_REF],

whereas the same applied to ordered mesoporous carbons gave 150° [START_REF] Wang | Super-hydrophobic ordered mesoporous carbon monolith[END_REF]. As for the deposition of silica, a rather broad range of contact angles has been reported, typically from less than 60° to 120° [START_REF] Fuji | Structure and wettability of various silica surfaces: evaluation on the nano and macro levels[END_REF], and even up to 165° [START_REF] Park | Nanotextured Silica Surfaces with Robust Superhydrophobicity and Omnidirectional Broadband Supertransmissivity[END_REF] due to different final surface roughness. a assuming a blend density of 1.5 g cm -3 b defined as the ratio: Infiltrated MASA volume (cm -3 ) / Initial pore volume (cm 9.

The second part of Table 9 shows that the efficiency of the impregnation dramatically decreased with respect to non-hydrophobised foams, since the impregnation yield was significantly lower than what was found for non-treated samples. Only 54 -72 % of the total porosity was indeed filled. Hydrophobisation, although not changing the macroscopic 9. They show that the impregnation ratio increased significantly, but remained lower than typical values of non-hydrophobised foams.

Table 9. Main characteristics of hydrophobised carbon foam samples before and after MASA impregnation.

Before MASA impregnation

Thermal and mechanical properties

The impact of the surface treatment on the thermal conductivity of the composite carbon foam designed to serve as porous host for enhancing the thermal transfer towards phasechange materials was checked. The conductivity of the pristine composite carbon foams was indeed 3.1 W m -1 K -1 (see again Table 8) [START_REF] Jana | Design of carbon foams for seasonal solar thermal energy storage[END_REF], and this value should ideally not decrease due to some possible thermal resistance created by the surface treatment. Whatever the kind of hydrophobisation, no change of thermal conductivity outside of the typical uncertainty for this kind of measurement could ever be observed, as expected from so thin and superficial chemical modification. For instance, as seen in Table 9, the samples hydrophobised by pfdtms and by molecular F 2 have thermal conductivities of 3.1 and 3.2 W m -1 K -1 , respectively. The treated carbon foams can therefore be considered as hydrophobised without any drawback with respect to their initial requirements, i.e., as high porosity and as high thermal conductivity as possible at the same time.

In contrast, replacing the air filling the porosity of the carbon foams by solid MASA was expected to have a significant impact. Thermal conductivity studies of the blend alone and of the blend-carbon foam hybrid materials were then carried out at room temperature. The measured thermal conductivity of the solid MASA alone was found to be 0.65 W m -1 K -1 .

After impregnation, the thermal conductivity of the MASA-carbon foam hybrid materials reached 3.6 W m -1 K -1 (see again Table 8).

Such 16% increase with respect to the non-impregnated composite carbon foam is readily explained by the higher thermal conductivity of the MASA compared to that of air (0.0257 W m -1 K -1 ) at room temperature. Indeed, the thermal conductivity of open-cell foams,  (W m -1 K -1 ), is expected to follow the well-known equation of Gibson and Ashby [START_REF] Gibson | Cellular Solids: Structure and Properties[END_REF], which reads (neglecting the contribution of radiative transfer, quite low at room temperature):

   (1)
where (dimensionless) is the porosity,  s (W m -1 K -1 ) is the solid thermal conductivity (i.e., the thermal conductivity excluding the sample porosity), and  g (W m -1 K -1 ) is the thermal conductivity of the gas saturating the porosity. In our former work,  s was found to be 65 W m -1 K -1 [START_REF] Jana | Design of carbon foams for seasonal solar thermal energy storage[END_REF]. Filling the porosity of the foam with a MASA having a thermal conductivity

 MASA (W m -1 K -1
) and with an infiltration yield Y (dimensionless) leads to Eq. ( 2):

    (2)
Using the average infiltration yield Y = 0.85 taken from Table 8 and the measured value  MASA = 0.65 W m -1 K -1 , one finds that the thermal conductivity of the MASAcarbon foam hybrid material is 3.55 W m -1 K -1 , in excellent agreement with the measurements. This is a very positive finding for the foreseen (seasonal STES) application that the thermal conductivity of MASA-carbon foams hybrid materials is always higher than that of each individual component.

After hydrophobisation, and as already shown in the second part of Table 9, the yield of the first infiltration was lower than without surface treatment. Consequently, the hybrid materials based on hydrophobised foams after a first MASA impregnation had a slightly lower thermal conductivity than that of materials prepared from non-treated carbon foam.

After a second impregnation, however, the thermal conductivity of samples hydrophobised by pfdtms did not change and remained equal to 3.3 W m -1 K -1 . All these values are lower than what can be calculated from Eq. ( 2) using the data of infiltration yields given in Table 9. In contrast, Eq. ( 2) led to excellent predictions when non-treated carbon foams were used. This finding suggests that a non-negligible thermal resistance exists between the hydrophobised surface of carbon foam and the MASA, which might favour undercooling, and which is the desired effect once the MASA has been molten for storing heat. This point will be addressed in the next subsection.

Finally, mechanical tests have been carried out on the hybrid material based on xylitolerythritol (64 / 36) blend infiltrated in composite carbon foam. Fig. 9 compares the stressstrain compression curves of the composite carbon foam before and after MASA impregnation. The general behaviour did not change after impregnation, since the usual three distinct regions: linear elastic, collapse and densification are observed. The corresponding compressive strengths and elastic moduli calculated from Fig. 9 are listed in Table 10.

Impregnation by MASA, which is a hard material after solidification, made the compressive strength and the elastic modulus increase from 3.0 to 9.5 MPa and from 107.5 to 195.9 MPa, respectively. Table 10: Mechanical properties of composite carbon foam before and after impregnation.

Samples

Composite carbon foam 3.0 107.5

Impregnated carbon composite foam 9.5 195.9

Effect of hydrophobisation on SA subcooling

The presence of interfaces can modify the nucleation process through various means, such as favourable molecular interactions with the crystallising molecule and a lattice match between the substrate and the compound to be crystallised. In addition, surface morphology, especially in porous structures, is also known to play an important role in controlling nucleation kinetics [START_REF] Page | Heterogeneous nucleation in and out of pores[END_REF][START_REF] Frenkel | Seeds of phase change[END_REF][START_REF] Diao | Surface design for controlled crystallization: The role of surface chemistry and nanoscale pores in heterogeneous nucleation[END_REF].

The heterogeneous nucleation rate, or the number of nuclei surpassing the nucleation barrier per unit area per unit time [START_REF] Kashchiev | Nucleation: basic theory with applications[END_REF], is a direct measure of the nucleation activity of a surface with regard to the liquid of interest. Due to the stochastic nature of nucleation events, determining nucleation rates requires statistical measurements performed under exactly the same conditions, involving too long experimental time. However, the objective of this paper was not to perform rigorous analysis of nucleation kinetics but to qualify the ability of hydrophobised carbon foams to prevent heterogeneous nucleation of sugar alcohols and to discard inefficient hydrophobisation methods. To this end, DSC testing as described in section 2.4.2 was used. Indeed, DSC thermograms (temperature vs. compensation heat flux) easily allow identifying the onset of crystallisation and the corresponding temperature. The latter is a good indicator of the SA nucleation activity of a surface, because it can increase dramatically when the presence of this surface lowers the kinetic barriers to nucleation.

Pure erythritol was tested in both small-volume DSC (1 sample of 30.28 mg) and largevolume DSC (3 samples with different masses, see Table 11). In all cases, comparable values for the melting point (118 ± 0.5°C) and the enthalpy of melting (340 ± 17 J.g -1 ) were obtained. The most relevant information is, however, the temperature of the sample at the onset of the crystallisation (shortly, crystallisation onset). The results achieved are also reported in Table 11, which indicates the crystallisation onset range observed over the five cycles of melting/crystallisation applied, namely where represents the onset of the crystallisation during the k th cycle. The maximum and minimum subcooling observed, calculated as the difference between the melting temperature and the onset of crystallisation is also given. It can be seen that deep subcooling (> 68 °C) is observed in all cases. Moreover, the crystallisation onset range shifts progressively towards higher temperatures as the volume of the sample is increased. This reveals that the nucleation is affected by impurities in the erythritol melt, which act as potential sites for heterogeneous nucleation. It is indeed well known that, for a fixed value of impurity content in a product, small samples are less likely to contain these foreign agents than large samples, which justifies the increase in subcooling observed with the reduction of sample volume. DSC testing for CF900 materials (non-hydrophobised carbon foams) infiltrated with erythritol was also performed for small-and large-volume samples. Fig. 10(a) shows the DSC thermograms (heat flux vs. temperature) obtained for a large-volume sample subjected to five heating and cooling cycles. During heating, all the endothermic (melting) peaks are superimposed except the first one, which, as usual, behaves differently due to thermal resistances of the powders before the first melting. During cooling, the crystallisation of the sample (exothermic peaks) occurs at a much lower temperature than melting. It can be seen that the crystallisation peaks have a loop, which means that the DSC apparatus is not able to dissipate/compensate for the heat flux delivered during sample crystallisation so that the sample temperature rises. This is typically the case for samples with a high degree of undercooling, where crystallisation is a kind of explosive phenomenon involving a large amount of heat delivered in a very short time that the DSC cannot accommodate. The crystallisation temperature is determined as the onset temperature of the thermal loop (crystallisation peak). It can be seen that, in contrast to melting, the onset of crystallisation is sharp and, therefore, the uncertainty in determining the crystallisation temperature is lower (less than ±0.5 K approximately) than in melting (±1 K approximately, according to the calibration performed). The crystallisation onset ranges are reported in Table 11 and are also displayed in Fig. 10(b).

Compared to pure erythritol, it can be seen that: (i) the effect of the volume of the sample is much lower this time (Fig. 10(b)), with comparable subcooling values for small-volume and large-volume samples (Table 11); and (ii) the crystallisation onset ranges are shifted towards much higher temperatures. These two observations indicate that the nucleation of erythritol mainly takes place on the surface of the carbon foam (heterogeneous nucleation). On the one hand, heterogeneous nucleation is determined by the nucleation activity of the carbon foam surface and, therefore, it depends on the surface density of active sites for nucleation and on the specific surface area of the foam and not on the volume of erythritol. On the other hand, it is well known that a surface rich in nucleation sites decreases significantly the nucleation barriers and, therefore, reduces subcooling effects. 12. In Fig. 11(a), the crystallisation onset range of each carbon-SA hybrid material is represented by a vertical thick bar, whereas the onset crystallisation range of pure erythritol is displayed by horizontal dashed lines. It can be seen that all hydrophobised carbon-SA materials perform better than CF900, but only CF 0.07 seems able to avoid completely heterogeneous nucleation. Indeed, the crystallisation onset range for CF 0.07 lies within that of pure erythritol. In order to rank hydrophobisation methods by their ability to prevent heterogeneous nucleation, the following efficiency indicator has been defined: ]

where , and represent the upper limits of the crystallisation onset ranges of CF900, of pure erythritol and of hydrophobised hybrid materials, respectively. The choice of the upper limits of the crystallisation interval in the definition of (instead of the lower limits) is due to the fact that, for the application under consideration, it does not matter how deep the subcooling may be, but rather what is the minimum temperature that guarantees the absence of spontaneous crystallisation. According to Eq. ( 3), the efficiency indicator is when , which means that the hydrophobisation method does not work, the performance obtained being equal to ( , or worse than ( ), that of CF900. Values such that correspond to : the higher the value of , the closer the behaviour of the hybrid material is to that of erythritol ( for ). The best possible result corresponds to hydrophobised foams without nucleation sites, so that the hybrid material behaves like pure erythritol. In such a case, and the efficiency indicator is . It should be noted that values below can only be observed due to the random nature of the nucleation process as is the case with sample CF 0.02 (Fig. 11a). In other words, values above 1 have no physical meaning. For this reason, the maximum value of the efficiency indicator was set to 1. Despite its widespread occurrence, mechanistic understanding of the role of a surface in heterogeneous nucleation still is limited. Nevertheless, to provide preliminary understanding of the results above, it is worth recalling the role of several factors, such as the surface chemistry and the surface morphology (especially porous structures: pore size and pore morphology), which might have great relevance in our context. In a first attempt to separate the contribution of these two factors to the nucleation kinetics, Daio et al. [START_REF] Diao | Surface design for controlled crystallization: The role of surface chemistry and nanoscale pores in heterogeneous nucleation[END_REF] proved that the surface chemistry influences the surface nucleation activity through interfacial free energies only, and they also revealed the importance of surface polarity on the nucleation activity of polar substances. The studies of Page and Sear [START_REF] Page | Heterogeneous nucleation in and out of pores[END_REF][START_REF] Page | Crystallization controlled by the geometry of a surface[END_REF] clearly demonstrated the important role of surface geometry on the nucleation behaviour. They found by Monte Carlo simulation of Lennard-Jones molecules that nucleation in nanoporous surfaces is orders of magnitude faster than on smooth surfaces, provided that the pores have a size close to that of the crystallites. They also showed that the nucleation in grooves formed by two intersecting planes is many orders of magnitude faster than on a flat surface. Moreover, when the wedge angle is the same as an intrinsic angle of the crystal, the nucleation rate reaches a maximum.

The latter findings provide theoretical explanation for a common observation; that is, the beginning of crystallisation on edges, kinks and structural defects.

The nucleation results of the carbon-SA materials based on hydrophobised foams are next discussed considering the preceding remarks. As pointed out in section 3.1.1, regarding the direct fluorination method, fluorine first reacts with the most disordered carbon areas, such as edges of carbon layers and other structural defects, which are typically also the most active sites for nucleation. The nucleation activity of the foam surface is therefore reduced. This can explain the better ability to prevent nucleation shown by CF 0.02 and CF 0.07 compared to CF900. In addition, CF 0.07 , with higher fluorine content, appears to provide greater reduction of structural defects and shows excellent nucleation prevention efficiency.

The vtmos sample also performs quite well, probably because (i) its specific surface area is significantly reduced compared to CF900 (Table 5); and (ii) the pore size distribution changes in the right way. Regarding this latter factor, it must be highlighted that samples CF900 and pfdtms present a significant number of pores narrower than 0.4 nm, whereas such small pores are not present in vtmos sample (Fig. 6). Likely, the pore size maximising the nucleation rate is less than 0.4 nm. The pfdtms sample behaves slightly better than CF900 but much worse than vtmos. The raison for this is probably that it has a lower surface area than CF900 but higher than vtmos in addition to pores narrower than 0.4 nm. The combined effect of micropores and low surface energy has indeed also been suggested for explaining the remarkable hydrophobicity of fluorinated activated carbon fibres [START_REF] Touhara | Property control of carbon materials by fluorination[END_REF].

For samples obtained by electrochemical grafting of fluorinated moieties, there is a quite good correlation between the amount of fluorine at the surface of the foam (Table 6, 3 rd column) and the nucleation results (Table 12), which indicates that the ability of the carbon foam in preventing the nucleation of erythritol on its surface decreases as its fluorine content is increased. The best hydrophobisation methods that emerge from the initial screening carried out are, therefore, direct fluorination and silica deposition through silanisation.

Finally, yet importantly, it must be noticed that in none of the cases, neither the melting temperature nor the latent heat of erythritol were significantly modified by the presence of carbon surfaces (see Table 13). The latent heat of the erythritol confined into the carbon foam has been calculated as Q/m ery , where m ery is the mass of erythritol within the carbon foam (see Tables 11 and12) and Q represents the energy of the peak of melting calculated from the DSC thermogram. The uncertainty values in Table 13 have been estimated from all experimental results available for each material (one or two samples, depending on the material, and 5 cycles of heating and cooling per sample). It can be seen that both the melting point and the latent heat of pure erythritol measured by DSC (118℃ and 350 J g -1 , respectively) are always within the estimated uncertainty range.

Conclusion

Hydrophobisation of sucrose-based carbongraphite filler composite foams was carried out by four different techniques: (i) direct fluorination under a flow of hot molecular F 2 ; (ii) silanisation followed by heat treatment; (iii) chemical grafting of perfluoroalkyl chains; (iv) electrochemical grafting of trifluoro-methyl-benzene and pentafluoro-benzene. Whereas not all characterisation techniques could be systematically used, the enhanced hydrophobic character of treated samples could be observed, although to very different extents.

Direct fluorination was highly efficient from that point of view, but rather difficult to control because the resulting stoichiometry is hardly predictable. Silanisation was easier and the resultant hydrophobicity was only a little less than with the carbon foam fluorinated at the highest F stoichiometry. Grafting long perfluoroalkyl chains was also easy and quite efficient.

Finally, electrochemical grafting led to very different results, depending on the nature of the diazonium salt and the solvent in which the synthesis was done. Overall, the final hydrophobicity was the combination of surface chemistry and roughness / microporosity.

Whatever the treatment, all carbon foams could be successfully impregnated with molten sugar alcohols, and the structure of the latter, once crystallised, was not influenced by the presence of the carbon host. The mechanical properties and the thermal conductivity of the hybrid materials based on sugar alcohol infiltrated in carbon foam were significantly higher than the pristine carbon foams. A very important point is that no surface treatment induced losses of either macroscopic porosity or thermal conductivity. The hydrophobised materials thus fully retained their initial characteristics, making them relevant for solar thermal energy storage.

Moreover, DSC experiments clearly stated that neither the melting point nor the enthalpy of melting of the hosted phase-change material was modified by the presence of carbon surfaces, whether hydrophobised or not. Most importantly, the most hydrophobic carbon foams were able to induce a significant subcooling of the hosted phase-change material, which is an excellent feature for long-term, i.e., seasonal, heat storage. The carbon foam with stoichiometry CF 0.07 was the only material able to avoid completely heterogeneous nucleation of the sugar alcohol. A demonstrator is presently about to be built to prove the interest of hybrid materials based on carbon foam infiltrated by sugar alcohols for the seasonal storage of solar thermal energy. Several options for activating the crystallisation of the sugar alcohol on demand are being investigated and the most efficient one will be implemented in the demonstrator. In parallel, a thorough analysis of the thermal stability of the sugar alcohol is being carried out.

Table 1 . 3

 13 Conditions of electrochemical reduction of diazonium salts for grafting aromatic fluorinated moieties (amine concentration 2 × 10 -3 mol L -1 , tBuNO 2 or NaNO 2 6 × 10 -3 mol L -1 Infiltration of carbon foams with molten sugar alcohols A eutectic xylitol-erythritol blend, prepared by mixing 64 parts of xylitol and 36 parts of erythritol, was used. In the following, such intimate blend will be referred to as Molecular Alloy based on Sugar Alcohols (MASA). The physical properties of xylitol, erythritol and MASA are shown in Table

  samples of pure erythritol were tested within this range of mass. In all cases, the DSC experiments were carried out in open crucibles continuously flushed by high-purity nitrogen. The combination of small-and big-volume-DSC with applied operation conditions thus allowed the effect of key external parameters on subcooling, such as sample volume and cooling rate, to be studied.

Fig. 2 .

 2 Fig. 2. Temperature cycle used in DSC experiments, see also Table3for details.

Fig. 3 .

 3 Fig. 3. Raman spectra of carbon foams before and after direct fluorination in a stream of hot

Fig. 4 .

 4 Fig. 4. MAS 19 F NMR spectra (30 kHz) of several carbon materials submitted to direct

Fig. 5 Fig. 5 .Fig. 6 :

 556 Fig. 5. XRD patterns of composite carbon foam before fluorination (CF900) and after

Fig. 7 :Fig. 8 .

 78 Fig. 7: Photographs of composite carbon foam: (a) before MASA impregnation; (b) after one

  the vacuum impregnation much less efficient. For samples treated with pfdtms, a second impregnation was thus tried in the same conditions as for the first one. The results are also given in Table

  Just like before, XRD studies have been carried out to check any possible effect of the hydrophobised carbon surface towards the crystallisation of the xylitolerythritol blend. Fig. 8(b) compares the XRD patterns of the MASA-carbon foam hybrid materials: (i) based on pristine composite carbon foam, and (ii) based on composite carbon foam hydrophobised by pfdtms. It can be seen that the two patterns are again identical, and that the peaks are also the same as those of the pure xylitolerythritol blend. It can therefore be concluded that no change of crystalline structure, possibly induced by the hydrophobisation of the porous carbon structure, happened.

Fig. 9 :

 9 Fig. 9: Stress-strain characteristics of sucrose-based carbongraphite composite foam before

Figure 10 .

 10 Figure 10. (a) DSC-thermograms (5 cycles) obtained for CF900-erythritol hybrid material

  Since the aforementioned results show that the sample volume has little influence on the nucleation outcomes, the tests on carbon-SA hybrid materials with hydrophobised carbon foams were carried out only in small-volume or large-volume DSC, depending on the volume of available samples. The crystallisation onset ranges observed during DSC testing are reported in Table

Figure 11 .

 11 Figure 11. (a) Crystallisation onset ranges of carbon-SA hybrid materials; (b) Efficiency

Table 2 .

 2 Physical properties of sugar alcohols xylitol, erythritol and MASA[START_REF] Del Barrio | Characterization of different sugar alcohols as phase change materials for thermal energy storage application[END_REF]. The values of density and thermal conductivity are those of the solid phase at room temperature.

	Property	Sugar alcohol	Xylitol	Erythritol	MASA
	Density (g cm -3 )		1.52	1.45	1.50 (*)
	Melting point (°C)		92-96	121	82
	Latent heat of fusion (J g -1 )		267	340	270
	Thermal conductivity (W m -1 K -1 )	1.32	0.76	0.38
	(*) The density of MASA (xylitol-erythritol 64/36 %) was calculated from density data of
	xylitol and erythritol as 1.52  64/100 + 1.45  36/100  1.5 g cm -3 .
	Sucrose-based carbon -graphite composite foam samples infiltrated with molten erythritol
	were also prepared (called carbon-SA hybrid materials hereafter). They were only used to
	study the efficacy of hydrophobisation by Differential Scanning Calorimetry (DSC). Although
	xylitol-erythritol eutectic mixture has more interest for seasonal STES application than
	erythritol, the latter is preferred for DSC testing because the eutectic presents a very low
	nucleation rate. This characteristic, which is searched for long-term TES, is however a
	drawback for DSC testing purpose. On the one hand, it implies too long testing times because
	of the very long nucleation induction time. On the other hand, it reduces significantly the

Table 3 .

 3 Parameters of the temperature cycle shown in Fig.2.

		Small volume-DSC	Big volume-DSC
	β (°C min -1 )	1	0.5
	t 1 (min)	15	30
	t 2 (min)	15	60
	Nature of spans	Al	Al
	Diameter (mm)	6.17	14
	Height (mm)	11.3	45
	Mass of erythritol (mg)	ca. 30	1300 -4770
	Total mass (mg)	34 -37	1750 -6500

Table 4 .

 4 Stoichiometry of fluorinated carbons obtained by direct fluorination with F 2 under

	dynamic conditions.
	Samples	Dynamic F 2 (1 h, 300°C)	Dynamic F 2 (1 h, 280°C)	Dynamic F 2 (1 h, 180°C)
	Pure sucrose-based carbon foam		CF 0.23	CF 0.09
	Graphite / sucrose char composite carbon foam	CF 0.21	CF 0.07	CF 0.02

  band. The difference between C1s and F1s thus corresponds to the at.% of F involved in CF 2 and CF 3 groups. The sample H 2 SO 4 -CF5, i.e., prepared in 0.1 mol L -1 H 2 SO 4 with pentafluoro-aniline, presents 19.75 at.% of F at its surface. The fluorine content is thus the highest in this material, followed by ACN-CF3 (8.29 at.%), H 2 SO 4 -CF3 (6.70 at.%) and

ACN-CF5 (1.13 at.%) for which the grafting was poor. But electrochemical grafting in H 2

SO 4

Table 1 )

 1 can produce very different fluorine concentrations at the carbon surface. This is especially the case with pentafluoro-aniline, which produced the highest atomic F fraction when grafted in H 2 SO 4 , but the lowest one in acetonitrile.

Table 6

 6

		C1s bands	F1s bands	O1s
		BE (eV)	at. %	BE (eV)	at. %	BE (eV)	at. %
	Non-treated foam						2.72
	H 2 SO 4 -CF3	286 -293	6.70	686 -689	5.10	532 -534	4.02
	H 2 SO 4 -CF5	286 -293 19.75 686 -689	15.28	532 -534	9.90
	ACN-CF3	286 -293	8.29	686 -689	6.87	532 -534	3.13
	ACN-CF5	286 -293	1.13	686 -689	0.38	532 -534	3.56

. XPS results in terms of binding energy (BE, expressed in eV) of elements C (C1s of CF, CF 2 and CF 3 groups), F (F1s of CF bonds) and O (O1s), and corresponding abundance (expressed in atomic %).

Table 7 .

 7 Contact angles measured for non-treated and hydrophobised carbon foams.

	Hydrophobisation treatment	Contact angle (°)
	None	96.1 ± 4.3
	vtmos	133.9 ± 2.5°
	pfdtmos	128.7 ± 4.5°
	F 2 (280°C, 1 h): CF 0.07	155.2 ± 1.4°
	F 2 (180°C, 1 h): CF 0.02	123.1 ± 3.8°
	H 2 SO 4 -CF3	99.2 ± 2.7°
	H 2 SO 4 -CF5	110.9 ± 3.1°
	ACN-CF3	101.8 ± 3.0°
	ACN-CF5	98.0 ± 1.5°

Indeed, surface roughness and possible nano/micro-texturation can account for improved hydrophobicity. Only the Cassie-Baxter equation

[START_REF] Cassie | Wettability of porous surfaces[END_REF]

, contrary to Wenzel's one

[START_REF] Wenzel | Resistance of solid surfaces to wetting by water[END_REF]

, can predict extremely high contact angles, which indicate the presence of air between the droplet and the surface. The Cassie-Baxter equation can be written as cos θ = r f f cos θ S + f -1, where θ S is the contact angle measured on a smooth surface, r f is the roughness ratio of the substrate wetted by the liquid, f the solid fraction and thus (1f) is the fraction of air. This equation

Table 8 ,

 8 were tested for calculating the yield of impregnation by a molten blend of xylitol and erythritol. After impregnation, their characteristics changed according to the values presented in the same Table8. It can be seen from the latter that 85 -86% of the total porosity of the carbon foams has been filled. Pictures of sample #1 before and after impregnation are shown in Fig.7. A second impregnation carried out in the same conditions only increased the yield up

to 91%. Of course, the resultant hybrid materials cannot retain the PCM by capillary action once it has melted within the porous support structure. Indeed, due to the hydrophobic characteristics of the carbon foams, spontaneous extrusion of the liquid from the pores may occur. From a practical point of view, this means that the proposed hybrid materials must be properly enclosed in the storage vessel to prevent leakage.

Table 8 .

 8 Main characteristics of composite carbon foams before and after impregnation.

	Impregnation	Sample #	Mass (g)	Volume (cm 3 )	Density (g cm -3 )	Porosity (%)	Initial pore (cm 3 ) volume	Infiltrated MASA (cm 3 ) a volume	Infiltration yield (%) b	Thermal conductivity (W m -1 K -1 )
	Before	1 2	8.13 7.92	25.4 24.7	0.32 0.32	85.8 85.8	21.79 21.19	--	--	3.1 3.1
	After	1 2	36.5 34.8	25.4 24.7	1.44 1.41	--	21.79 21.19	18.91 17.92	86.8 84.6	3.6 3.6

Table 11 .

 11 Results of DSC-testing of pure erythritol and carbon-SA materials with non-

	hydrophobised foams.
		Mass of	Onset of	Maximum	Minimum
	Sample name	erythritol (mg)	crystallisation	subcooling	subcooling
			range (°C)	observed (°C)	observed (°C)
		4771.73	{46, 50}	72	68
		2707.89	{41, 46}	77	72
	Pure erythritol
		1795.40	{39, 43}	79	75
		30.28	{32, 45}	86	73
		4771.18	{56, 63}	62	55
	CF900 -	1305.53	{53, 58}	65	60
	eryrhritol	30.28	{53, 57}	65	61
		29.10	{53, 58.5}	65	59.5

Table 12 .

 12 Results of DSC-testing of carbon-SA materials with hydrophobised carbon foams.

			Mass of	Onset of
	Hydrophobisation method	Sample Name	Erythritol	crystallisation
			(mg)	range (°C)
		CF 0.02	1514.53	{48.5, 56.5}
	Direct fluorination by molecular F 2
		CF 0.07	1163.31	{44, 46}
	Grafting of perfluoroalkyl chains	pfdtms	2708.71	{48, 58}
	Silanisation by grafting vinyltimethoxysilane	vtmos	1795.05	{48, 52}
		ACN-CF 3	29.77	{53, 58}
	Electochemical grafting of	ACN-CF 5	29.30	{58, 60}
	fluorinated moieties	H 2 SO 4 -CF 3	29.53	{55, 58.5}
		H 2 SO 4 -CF 5	29.92	{50, 56}

Table 13 .

 13 Melting temperature and estimated values of the latent heat of erythritol confined within the composite carbon foams.

	Hydrophobisation method	Sample Name	Melting point (℃)	Latent heat of melting (J g -1 )
		CF900	117.5 ± 1.5	348 ± 17
	None
		CF 0.02	118.2 ± 1.0	343 ± 17
	Direct fluorination
	by molecular F 2	CF 0.07	117.6 ± 1.0	351 ± 15
	Grafting of	pfdtms	118.2 ± 1.0	341 ± 19
	perfluoroalkyl chains
	Silanisation by grafting	vtmos	117.3 ± 1.0	345 ± 13
	vinyltimethoxysilane
		ACN-CF 3	118.1 ± 0.5	352 ± 10
	Electochemical	ACN-CF 5	117.6 ± 0.5	347 ± 13

Acknowledgements

The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement 296006. The authors also gratefully acknowledge the financial support of the CPER 2007-2013 "Structuration du Pôle de Compétitivité Fibres Grand'Est" (Competitiveness Fibre Cluster), through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (ERDF) funds, as well as the funding support received from the "Agencia Estatal de Investigación" of Spain (ref.: RTI2018-099557-B-C21).


HAL is

However, archival research sometimes leads to unexpected discoveries. An unpublished document concerning the family of Joan of Arc had remained hidden in the darkness of the thick registers of the French Royal Chancery. It is true that the richness of the documents in this collection, which is kept in the French National Archives, has prevented a precise inventory from being made until now. But it would have been necessary to combine this catalogue with a precise knowledge of Joan of Arc's family.

The document in question is a letter of pardon in favour of Jean de Vouthon dated January 1449. But no one had paid attention to this letter, lost among so many others, and above all no one had yet identified this "Jean de Vouthon" as a member of Pucelle's family.

It should be pointed out that the Maleyssie collection, amassed over the centuries by the heirs of Joan, was marred by a transcription error which prevented any comparison between the so-called receiver, Jean de Vouthon, and the poor Jean de Vouthon, a common roofer, prosecuted by the justice of Charles VII. We shall see that the position of the person concerned also played a role in confusing the issue. The maze of archives and the investigation that is the job of the historian thus offer surprising archival treasures to those who know how to efficiently cross-reference their sources But first, let us return to the context of the 15th century in the confines of the kingdom, and then focus on the character himself, as well as his tragic fate, which reflects the ordeals suffered by the civilian population during the Hundred Years' War.

I-War and disorder on the borders of the kingdom of France

The decade from 1435 to 1444 saw innumerable excesses being perpetrated by employed soldiers, whom some sources refer to as "Écorcheurs". No Distinguishing between friend and ennemy, these warriors plundered and pillaged because they did not receive regular payment for their armed service. They set fires and occasionally raped with almost total impunity. Most of them were soldiers who had served, and still served, under French captains. As the historiographer of the Constable of France, Guillaume Gruel, reports: 'L'an 1439, noz gens d'armes estoient allez vivre en Champaigne pour ce qu'ilz n'estoient point poyez. Beauce, Brie, Hainaut, Île-de-France and Champagne were particularly affected by these movements. Gruel continues: "... les gens de monseigneur de Bourbon qui estoient au boys de Viesaine et à Corbail faisoient autant de maulx que les Angloys.

Et estoit la pillerie par toute Champaigne et Brie et la Beausse en telle maniere que homme n'y povoit mettre remede. Et le roy et tous les seigneurs en son endroit soustenoit ces pilleries..." The reproach is severe without being unjustified. The excesses of the soldiers gave rise to numerous complaints and the king was asked to find a remedy. Charles VII did not yet have the necessary means to stop these oppressions, but he began to legislate in this direction from 1438 onwards, no doubt at the instigation of his constable Arthur de Richemont. Richemont, as constable, was naturally given the task of punishing captains who were guilty of the worst excesses, as well as those committed by their men. He thus delivered Bouzon de Fages, the former bailiff of Montargis, who had been accused of numerous crimes, to the dreaded justice of the provost marshal, who executed him in Troyes at the end of 1438.

However, neither the royal legislation nor this exemplary sentence brought calm to the countryside. The year 1439 was undoubtedly one of the worst years of the Écorcherie, a scourge combined with the famine and plague that were ravaging the kingdom of France. The situation was particularly difficult in the eastern regions of the kingdom where the population never ceased to deplore the looting and exactions of all kinds associated armed troops. It was at this time that the mercenaries of the Sire de Commercy, Robert de Sarrebrück, crossed the Champagne region on their way back from an expedition on the border of the Duchy of Luxembourg. In 1439, the fortress of Chauvency, which belonged to Robert de Sarrebrück, was heavily besieged by Évrard de la Marck, a formidable warrior who was supported against all odds by the King of France's Constable, Richemont, and who intended to take revenge on Sarrebrück, despite his loyalty to Charles VII. Sides were switched in this game of war, Saarbrück allying itself with the crown's enemies, Vaudémont and Jean de Luxembourg.

Old personal quarrels disrupted the game of traditional alliances. Invoking the label "desobeyssants au roy Charles de France" the Constable of France had decided to use armed force against Saarbrück. The Herald Berry reports: "Si assembla ledit seigneur de Commersy des gens ce qu'il en peut finer pour lever le siège de devant sa place de Chavancy ou estoit ledit demyseau Everart ad ce qu'il povoit finer de gens avecques partie de ceulx de mondit seigneur le connestable, qui estoient au siege." In order to win the place of Chauvency, the chronicler, Enguerrand de Monstrelet, reports that: "si bailla ledit connestable pluiseurs de ses capitaines avec leurs gens audit damoiseau Evrard, qui alerent assegier ladicte ville de Chavensy, environ huit jours aprés Pasques. Et là firent une grande et forte bastille, où se logerent environ quatorze cens combatans, avec grans nombre des communes, des bonnes villes et du plat pays, qui aloient et venoient."

To face such a coalition, Robert of Sarrebrück reinforced the garrison of the fortress with two hundred men. Two of his lieutenants were charged with coordinating the defence: Inglebert de Dale and Girard de Marescot. According to Monstrelet, the siege lasted four months, during which the damoiseau de Commercy "se tenoit tousjours sur sa garde et bien garny de gens d'armes". There is no indication that the dreaded Sire de Commercy lived there. He did, however, muster his men to rescue the place. As reported by the Hérald Berry, who mentions the flight of the Constable's troops: "Quant ilz sceurent la venue du seigneur de Commersi, du conte de Vaudesmons et des gens messire Jehan de Lucembourc, qui venoient à l'aide dudit seigneur de Commersy, si se leverent incontinant ; et ainsi perdit ledit de La Marche ses souldoiers qui le planterent [là]. Et s'en vindrent les gens de mondit seigneur le connestable raffraichir en Champaigne ; et les aucuns allerent au siege de Meaulx où estoit allé mondit seigneur le connetable". After this failure, the Constable and his men did indeed go to the siege of Meaux, where fate was more favourable to them. The mercenaries in the employ of Sire de Commercy also left the place in search of new expeditions. But when the band of soldiers burst into the small village of Champagne, in Togny, no one intervened. It was there that Jean Vouthon sold his labour as a simple wheat thresher after the harvest, perhaps to supplement his income as a roofer? He worked on the land of Jean Jossier, the mayor of a neighbouring village. No doubt the soldiers had taken supplies, and then rested for a few days in one of the village cottages.

II-Joan of

But they were forced to leave one of their number there, who had fallen ill. The Herald Berry quite rightly describes the deteriorating state of these unpaid soldiers. He gives the example of the troops wandering in Beaujolais at the same time: "les gens d'armes dessusdiz, qui estoient plusieurs malades, à pié et desarméz tellement que se estoit grant hydeur de les veoir".

After terrorising and holding the villagers of Togny to ransom, the gang moved on, leaving a man called Cornille in the house of Jean Jossier, Jean de Vouthon's employer. For ten days, the man tormented the occupants of the house with the story of his bloody misdeeds. On the tenth day, once he had recovered, he asked his host for a horse and a valet to take him to a village called Mesnil, where he thought he would find his fellow soldiers. It was at this precise moment that he asked Jean de Vouthon to accompany him. No one obviously dared to refuse.

The search soon proved useless; Cornille could not find the rest of the gang. The two men walked for days. The poor roofer began to lose patience. After having travelled more than two hundred kilometers, they arrived in Champignelles, at the gates of Burgundy. This was too much! Jean de Vouthon announced that he was stopping here and that he would take the horse back to its owner. Cornille got angry. Voices were raised. Cornille insulted the unfortunate man and scorned the honour of his family by promising to "chevaucher sa mère /(ride (i.e. fuck) his mother".

Then Vouthon, who feared for his life, seized a stick and struck his persecutor on the head. The bribe-taker fell down dead. Jean de Vouthon took off his victim's clothes and hurriedly buried him in a stone quarry. But the soldiers of the Cornille's band, who were prowling around, in turn came upon the roofer and stripped him of his clothes, leaving him only his shirt. The exact circumstances of the discovery of the crime are unknown but Vouthon, suspected of being the culprit, was brought before Vauchelin de La Tour, the royal bailiff of Vitry. The latter threw him in prison to begin his investigation. Jean de Vouthon managed to escape although the details of how that came about are not known. He then resumed his profession, putting the incident behind him: "es prisons dudit lieu desquelles il s'est yssi et party, et a tousjours depuis demeré au pays en vivant de son mestier et labour au mieulx qu'il a peu".

Ten years later, perhaps because Jean Cornille's family had come forward in the meantime, Vouthon was once again arrested by the same provost of Vitry. When questioned, he confessed to his crime. But the accused was claimed as a "homme de corps" (serf) by the Treasury of the Church of Châlons. He was therefore entrusted to the jurisdiction of the Church, to which he was subject. While awaiting a decision on his fate, he was placed in the prisons of the bishop of Châlons. His family and "amis charnels" mobilised to obtain the king's pardon. They argued that the man was an honest worker and that he had a family to support.

Jean de Vouthon, it was explained, had always behaved well. After four trying months of imprisonment, they obtained a royal pardon for Jean. The letter of remission came in January 1449. The episode reveals the links of violence between men of war and civilian population. In particular, it shows the many manifestations of self-defence that individuals resorted to in order to protect themselves or to fight against the excesses of the soldiers during the Hundred Years' War. Like Jean de Vouthon, other peasants fought back and easily obtained a pardon from the king in the name of legitimate defence. Forgiveness was all the easier to grant when the applicant presented himself as a man of good reputation against a perjured and cruel brigand. The pardon was also justifiable insofar as the king used legal means to condemn these excesses even within the ranks of his captains, as many examples show. But this document, which has remained unpublished to this day, that has made it possible to reconstruct the facts of this unfortunate incident, could have been a simple example of the misfortunes of war, if the main protagonist was not Joan of Arc's own uncle.

Indeed, this letter of remission also sheds new light on Joan of Arc's family environment. The document uses the nickname Jeanssonnet ("little John"), i.e. the son of John, which is consistent with the fact that his father (Joan's grandfather) bore this name. This Jean de Vouthon is also mentioned by other sources, in particular testimonies. A survey carried out in 1476 with a view to enabling the descendants of La Pucelle to benefit from the ennoblement granted to them by Charles VII (in December 1429) provides elements for comparison. Fourteen people were questioned about Jeanne's family, and all were able to name Jean de Vouthon. Thus the testimony of the carpenter Henry de Vouthon who mentions his grandfather Jean de Vouthon: "dit et deppose en sa loyaulté et conscience qu'il est natif dudit Sermaize, fils de feu Perrinet de Voulthon, lequel Perrinet son pere estoit filz de Jehan de Voulton, lequel se disoit estre frere de Ysabelot, mere de feue Jehanne la Pucelle. Lequel Jehan de Voulton, son grand pere, vint à demourance audit Sermaize du pays de Barrois ou de Lorraine, mesmement d'un village appelé Voulton, distant d'une lieue ou environ d'un autre village nommé Dompremy, scitué et assiz sur la riviere de Meuze".

En 1476, the introductory petition was filed by the descendants of a man named Pierre de Perthes, husband of Mangotte, daughter of Jean de Vouthon, more than forty years after ennoblement was granted. This lapse of time is surprising, especially as the benefit of this recognition was primarily fiscal. It can probably be explained by the distant nature of the relationship between this cousin and Joan of Arc?

It is necessary here to return to the genealogy of Jeanne. Joan of Arc's mother, Isabelle, who was probably born around 1385-1390, had three brothers -Henri, Mangin and Jean -as well as a sister named Aveline. The boys were named "Vouthon", which was the village from which the family came and where they retained ties since Isabelle's eldest son, Jacques, settled there as an adult.

Isabelle's father was named Jean de Vouthon. He was mentioned in a register of the legal proceedings of the provostry of Gondrecourt in 1385. Jean de Vouthon and his wife thus gave birth to two daughters and three sons, including Jean who was named after his father and was to be one of Joan of Arc's uncles. As Joan of Arc pointed out at her trial, the women were named after their mothers and the boys after their fathers. Logically, according to Lorraine custom, she should have been named Jeanne Romée.

It is Jean de Vouthon, Joan of Arc's uncle, who interests us here. Cross-checking all the testimonies collected during the 1476 investigation allows us to draw a fairly accurate portrait. Originally from Vouthon, he settled in Sermaize (about 80 kilometers away), in Champagne, with his wife, Marguerite Colnel, and their three children, probably around 1406. He was then a roofer. One event provides us with a precious chronological marker. Jean de Vouthon's daughter, Mangotte had just married a man named Colot Turlot when the latter was killed in 1423 by a bombardment, by "a fire stick", during the siege of the fortified church of Sermaize led by the Count of Salm on behalf of the Duke of Lorraine. The place was then held by La Hire, captain of Vitry. He fortified his position from where he launched raids throughout the duchy of Bar. The young widow was remarried the following year to Pierre de Perthes, whose son, Colin, was at the origin of the 1476 survey. Some witnesses claimed that Mangotte was born in Sermaize, around 1407, after the couple had settled, but this seems impossible. If Jean de Vouthon had children at a very young age, the indication is that he was born around the year 1385. The letter of remission indicates that the man was 44 years old at the time of the events (in 1439), which would mean he had been born in 1395. This is a gap of about ten years. This inconsistency in the letter of remission is not surprising: it is known that records of age, often indicative, include many variants. Jean de Vouthon was probably ten years older at the time of the events so that he could have had a daughter born around 1407-1408 since she is supposed to have been married in 1423. The other three children of Jean de Vouthon were named Perrinet, Paresson (or Pierresson) and Nicolas. They must have already died by 1476 as none of them was called to testify. However, all the witnesses called during the investigation knew them, the two eldest sons having remained in Sermaize. Henri de Vouthon, one of the grandsons of Jean de Vouthon, said he was 52 years old, which means he had been born in 1424. He was the son of Perrinet de Vouthon.

Henri remembered meeting Jeanne as a child one day when her parents came to visit their family in Domrémy. He was therefore no more than five years old. If he was born around 1424, his father would have been born around 1405, which confirms the date of birth estimated for his grandfather (around 1385). As the letter of remission was issued in 1449, Jean de Vouthon must have been a little over sixty years old. At the time of the 1476 survey, he was obviously dead. Jean Le Montigneue, one of the witnesses, who said he was about seventy years old and had always lived in Sermaize, stated that he had known Jean de Vouthon well. The latter had lived in the village for about thirty years until his death, which occurred shortly after his release, perhaps as a result of the second imprisonment. The families of Vouthon and d'Arc, despite the distance, maintained close relations. Thus we see Jeanne's brothers, who had meanwhile become knights of the Lys, visiting their cousins in Vouthon. Jean du Lys had become provost of Vaucouleurs.

The two brothers, Perrinet and Perreson de Vouthon, were even involved in the episode of the "false Joan of Arc", Claude des Armoises having invited herself to their home, where they made "bonne chere ensemble plusieurs foys". Jean de Vouthon's crime occurred between the execution of his niece (1431) and her rehabilitation (1456). This is probably why there is no mention of his relationship with La Pucelle. Jean de Vouthon was judged as "homme de corps" (i.e. of servile condition), whereas he could have benefited from the ennoblement of Joan of Arc's family. No one in his family seems to have understood the extent of this decision for he himself never asked for this recongnition, nor did his sons. It was not until one of his grandsons initiated the procedure in 1476, and thanks to this move, that it was possible to find a filiation with La Pucelle. Colin de Perthes, who had started the procedure, argued for a female lineage. But in the mid-16th century, several royal edicts attempted

1"

  figures of the medieval period.

  like a police investigator. It was while conducting my research on Robert de Sarrebück, Lord of Commercy and Lorraine warlord in the service of Charles VII, that I came across Jean de Vouthon, Joan of Arc's maternal uncle. The man, who hailed from Sermaize in Lorraine, was accused of having killed one of the mercenaries in the employ of the Lord of Commercy.

  Arc's uncle and the murder of Jean Cornille Moving southwards, the armed band that swept through the village of Togny was clearly a remnant of a company that Sarrebrück had engaged. It joined other conflict zones, crossing the Champagne region, perhaps towards the fortress of Châteauvillain. As Jean de Vouthon's letter of remission underlines: "Plusieurs routiers et gens de guerre venans du siege de Chavency pour le seigneur de Comercy arriverent ou païs de Champaigne, faisant plusieurs maulx, ranssonnemens et pilleries oudit Tougny et ailleurs, et pour ce que lesdis compaignons de guerre n'estoient pas lors sur la frontiere des ennemis du roy et ne venoient aucunement de son service, ilz furent habandonnez par nous et par cry publique." This provision was taken up again by the ordinance of 2 November 1439: "Et en outre le roi abandonne tous capitaines et gens de guerre qui feront contre cette presente loi et ordonnance ; et veut et ordonne que chacun par voye de fait à assemblée de gens et force d'armes leur resiste, et donne le roi à un chacun congé, auctorité et licence de ce faire." The king also authorised anyone to resist these warriors, even by force.

to prohibit this use by Joan's descendants (for the female lineage). At the same time, the same rules required that the applicants had to "vivre noblement". This is perhaps what differentiated Colin de Perthes from his uncles and cousins, for the former, who had embraced a military career, was an equerry and a man-at-arms under the king's orders. One of his descendants, Jules-César Le Besgue, was a councillor and advocate of the king at the bailliage and presidial of Vitry. He had his nobility confirmed by the same presidial in 1585. He had married the daughter of a Superintendent of Hunting in the bailliages of Vitry and Châlons.

Jean de Vouthon, on the other hand, remained in servitude. In his time, many inhabitants of the Barrois were still subject to this constraining legal status, which was disappearing in many other regions of the kingdom of France. In contrast, all the families living in the French part of the parish of Domrémy, like the parish of Joan of Arc, was under the jurisdiction of the châtellenie de Vaucouleurs, were free. Those who had been in servitude until then were freed when this territory was attached to the kingdom of France. In the other part of the village, which came under the jurisdiction of the Barrois mouvant and the seigneury of Bourlemont, a certain number of inhabitants remained in servitude. The confession and list of 1398 gives a precise inventory of the "non-free".

The mainmortables or serfs of the seigneury of Bourlemont thus represented thirty-five families. At the end of the Middle Ages, serfdom was still the last vestige of ancient times in these outlying territories of Lorraine and Champagne.