Carlos A Castilla-Martinez

Dominique Granier

Christophe Charmette

Jim Cartier

Pascal G Yot

Umit B Demirci
email: umit.demirci@umontpellier.fr

Cesium hydrazinidoborane, the last of the alkali hydrazinidoboranes, studied as potential hydrogen storage material

Keywords: Hydrazine borane, Hydrazinidoborane, Hydrogen storage, Cesium

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

Introduction

Molecular hydrogen H2 is a renewable, non-toxic and clean fuel that releases water only when it is oxidized (burned). Furthermore, it possesses a high energy density (between 120 and 142 MJ kg -1 ) that represents more than three times the energy density of gasoline or diesel. It is an excellent alternative to carbonaceous fuels, in fact to fossil fuels [START_REF] Dunn | Hydrogen futures: Toward a sustainable energy system[END_REF]. However, the development of an economy of hydrogen as energy carrier (or even as energy source) has shown to be full of obstacles, one of them being storage [START_REF] Abe | Hydrogen energy, economy and storage: Review and recommendation[END_REF].

Currently, hydrogen can be stored in three ways: (i) at high pressure (200-800 bar); (ii) in liquid state (at -253 °C) in cryogenic tanks; and, (iii) in a material [START_REF] Barthelemy | Hydrogen storage: Recent improvements and industrial perspectives[END_REF]. The most mature technology is the first one, but the most attractive method, and surely the most promising in the near future, is the last one. Two types of materials have to be distinguished: porous materials (e.g. carbon nanotubes, metal organic frameworks, covalent organic frameworks, polymers) adsorbing and desorbing reversibly H2 at subzero temperatures (generally at -196 °C), and hydrides where H is chemically bonded to an element like magnesium, nickel, boron, and/or nitrogen [START_REF] Ren | Current research trends and perspectives on materials-based hydrogen storage solutions: A critical review[END_REF]. Belonging to the second type of materials, boron-and nitrogen-containing compounds have received considerable attention owing to several advantageous features: they have high hydrogen densities (e.g. 19.5 wt. % H for ammonia borane NH3BH3); they carry hydridic and protic hydrogens (H δ- and H δ+ ) through B-H and N-H bonds, rationalizing the solid state of these compounds at ambient conditions and favoring the H2 release at mild conditions (e.g. onset temperature of dehydrogenations between 50 and 100 °C); and, they are generally easy and safe to handle when kept under inert atmosphere [START_REF] Kumar | Solid-state hydrogen rich boron-nitrogen compounds for energy storage[END_REF].

An example of boron-and nitrogen-containing compounds is hydrazine borane N2H4BH3 (HB). It possesses a high gravimetric hydrogen density with 15.4 wt. % H, and it is stable at room temperature under inert atmosphere [START_REF] Hügle | Hydrazine borane: A promising hydrogen storage material[END_REF]. However, neat HB has shown to be unsuitable as hydrogen storage material because it decomposes under heating while producing undesired gaseous by-products (e.g. ammonia NH3, diborane B2H6, and hydrazine N2H4) and it generates a shock-sensitive solid product when it is heated above 300 °C [START_REF] Moury | Hydrazine borane: Synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF]. Despite these, HB, which has three H δ-versus four H δ+ , has been considered as precursor of derivatives carrying as much H δ- as H δ+ , thereby alkali hydrazinidoboranes MN2H3BH3 have been obtained by chemically modifying HB, namely by substituting one of the H δ+ of the middle N atom of NH2NH2BH3 by a metal cation M + .

The first alkali hydrazinidoborane that was reported is lithium hydrazinidoborane LiN2H3BH3 (LiHB; 11.7 wt. % H):

MH (s) + NH2NH2BH3 (s) → NH2NH(M)BH3 (s) + H2 (g) (1)
with M = Li and Na. LiHB presents two crystalline phases.

KH (s) + NH2NH2BH3 (sol) → NH2NH(K)BH3 (s) + H2 (g) (2)
It is the least stable of these hydrazinidoboranes as it is able to release H2 from ca. 50 °C.

Recently, we reported rubidium hydrazinidoborane RbN2H3BH3 (RbHB, 4.6 wt. % H; monoclinic, s.g. P21) [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. It was synthesized by reaction of metallic rubidium and HB dissolved in THF:

Rb (s) + NH2NH2BH3 (sol) → NH2NH(Rb)BH3 (s) + ½ H2 (g) (3)
It has better dehydrogenation properties than the parent HB and comparable to the ones of LiHB.

Unlike the aforementioned hydrazinidoboranes, RbHB is not stable when stored at room temperature and under argon atmosphere; it decomposes liberating H2 and forming boron-and nitrogen-containing polymeric compounds.

Cesium hydrazinidoborane (CsHB; 3.4 wt. % H) is the last derivative of the series. We focused on this derivative also, because it is essential to better understand boron and nitrogen-containing compounds through the discovery of new materials. Our other objectives were as follows:

widening the prospects for the hydrogen storage application; and, opening up new opportunities.

CsHB was synthesized by making metallic Cs react with HB in solution, under argon atmosphere:

Cs (s) + NH2NH2BH3 (sol) → NH2NH(Cs)BH3 (s) + ½ H2 (g) (4)
A crystalline material was recovered, characterized, and studied while focusing on its potential as hydrogen storage material. Herein CsHB is thus presented for the first time.

Material and methods

The experiments described hereafter were carried out in an inert atmosphere of argon, either in a glove box (MBraun M200B; O2 < 0.1 ppm and H2O < 0.1 ppm) or using a vacuum-argon line.

HB was synthesized according to a procedure we previously developed [START_REF] Moury | Hydrazine borane: Synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF]. Typically, 21.59 g of hydrazine hemisulfate N2H4·½H2SO4 (≥ 98%, Sigma-Aldrich) and 10.12 g of sodium borohydride NaBH4 (≥ 98%, Sigma-Aldrich) were weighted in the glove box. They were suspended, under stirring, in 150 mL of anhydrous 1,4-dioxane (≥ 99.8%, Sigma-Aldrich) in a round-bottom flask. The reaction was performed at 30°C for 48 h. After this time, the suspension (i.e. NaSO4) was filtered, and a solution (HB dissolved in 1,4-dioxane) was recovered. The solvent was removed under vacuum and HB was obtained (yield of 80%, and purity of 99.6%).

As it is the case with Rb [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF], HB readily reacts with metallic cesium. We therefore attempted to synthesize CsHB at subzero conditions, here at -20 °C; for this, a coolant and a cooling jacket were used. Under inert atmosphere, 120 mg of HB was dissolved in 20 mL of extra dry THF (99.5%, Acros Organics), and 300 mg of Cs (ingot, ≥ 99.95%, Sigma-Aldrich) was weighted and put into a round bottom flask inside the argon-filled glove box. The flasks were transferred outside the glove box. The Cs-containing flask was immersed in the cooling bath and the HB solution was cooled down in a freezer. Once the temperature of the flasks was stabilized, the HB solution was poured into the Cs-containing flask, and the reaction was left under stirring for 6 h.

After this time, the solid precipitate was washed two times with THF. It was dried for 24 h under vacuum, the remaining Cs was removed, and the solid product CsHB was recovered. In doing so, 340 mg of CsHB was recovered (yield of 85% with respect to Cs).

The molecular structure of CsHB was analyzed by solid-state 11 B magic angle spinning nuclear magnetic resonance (MAS NMR; Varian VNMR4000, 128.31 MHz; using zirconia rotors with a diameter of 3.2 mm), and Fourier Transformed Infrared (FTIR) spectroscopy (NEXUS instrument, ThermoFisher Scientific, equipped with an attenuated total reflection accessory from 600 to 4000 cm -1 wavelength). The FTIR analyses were performed under air, but the sample was kept under argon atmosphere in a vial until the last moment before the analysis.

Powder X-ray diffraction (PXRD) analysis was done in a PANalytical X'Pert Pro Multipurpose diffractometer equipped with an X'Celerator detector and using a Bragg Brentano θ-θ reflection geometry with Ni filtered Cu-Kα1/α2 radiation (λ = 1.5418 Å). The pattern was collected on a spinning glass sample holder protected with Parafilm®¸ and loaded into the glovebox to prevent any contact with O2 and H2O. The diffraction pattern was recorded in the 10-80° angle range in 2θ using a step size of 0.017, with a scan step time of ~450 seconds at room temperature. The indexation of the as-collected PRXD pattern was done by using the FoX software [START_REF] Favre-Nicolin | free objects for crystallography': A modular approach to ab initio structure determination from powder diffraction[END_REF]. The refinement (Rietveld method) was carried out in the JANA2006 package, and the determination of the position of the Cs atoms was done with the 'SuperFlip' algorithm included in the software [START_REF] Petrícek | Crystallographic computing system JANA2006: General features[END_REF]. After this, FoX was used to reach the position of the [N2H3BH3] -anions. The Rietveld refinement of the pattern was carried out using a Pseudo-Voight peak shape. The peaks asymmetry was corrected using Howard model (Boole's rule) and preferred orientation using March-Dollase model following (010) direction. The B-H and N-H distances were constrained to 1.20 and 1.04 Å respectively in accordance with the optimization realized by FoX program.

The uncertainties of the atomic coordinates for H atoms are not reported here because they were not freely refined. The isotropic thermal parameters (Uiso) were constrained to be identical for all atom types (Uiso = 0.0477(9) Å 2 for the Cs, B, and N atoms; Uiso = 1.2 Å 2 for the H atoms). The locations of the H atoms linked to the N and B atoms were determined by performing a geometry constrained optimization of the structure (Cs, B and N atoms fixed in their initial position) at the force-field level. The atoms of the system were represented as Lennard-Jones charged sites with LJ parameters taken from the UFF force field while the charges were calculated using the QeQ method.

The thermal stability of CsHB was analyzed by thermogravimetric (TG) analysis. A TG analyzer (Netzsch STA 449 F1 Jupiter) coupled to a mass spectrometer (MS; Netzsch QMS 403 D Aëolos Quadro) was used. Differential scanning calorimetry (DSC) analysis (SDT Q600, TA Instruments) was also carried out. For both techniques, the samples preparation was done inside the glovebox, where the powder was enclosed in a Tzero hermetic aluminum crucible to protect it from air. The crucible was pierced with a needle right before the analysis to let release the evolving gases.

The thermal stability of CsHB was besides analyzed at isothermal conditions (90, 100, 110 and 120°C). Typically, 100 mg of CsHB was weighted in the glovebox, placed in a vial and put inside a stainless steel reactor. The reactor was connected to a pressure controller and immersed in an oil bath at the required temperature. The decomposition of CsHB was monitored by the change of pressure inside the reactor. Upon the completion of the reaction, the equivalents of gas (considered as ideal) liberated were calculated and plotted as a function of time. The gases released at these conditions were analyzed by a gas chromatograph (GC; PerkinElmer Clarus 400 equipped with a ShinCarbon ST column) coupled to a mass spectrometer (MS; PerkinElmer Clarus 600 T). The ShinCarbon ST column contains a treated carbon that allows the separation of hydrogen from the gas mixture, with the gas mixture being besides ionized by an electron beam in order to obtain the mass spectrum. Typically, after the reaction had finished and the reactor was cooled down at room temperature, the atmosphere of the reactor was collected with the help of a glass syringe and the sampled gases were injected into the GC column.

Results and discussion

Safety issues

All necessary precautions must be taken to work with Cs, CsHB, and the thermolytic solid residues. (i) Cs is highly reactive and violently reacts with water. (ii) HB must be dissolved in THF before putting into contact with Cs, which allows dissipating the heat released during the exothermic reaction between Cs and HB. This issue is comparable to what we noticed while working with RbHB [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. (iii) CsHB readily reacts with protic solvents like water, ethanol and methanol (occurrence of solvolysis with release of H2). (iv) CsHB has to be stored under inert atmosphere (preferably, inside a glove box). (v) Upon decomposition of CsHB, shock-sensitive products may form. This happened for a sample heated at 100°C. (vi) The solid residues forming upon decomposition of CsHB were found to ignite when put under air. Objectively, these cannot be seen as arguments in support of CsHB as hydrogen storage material.

To work under the best conditions of safety, the residues of CsHB must be always kept under inert atmosphere. To dispose of such unstable materials, the recommendation is to: put a small quantity (<100 mg) of the residue in a closed vial; add some mineral oil; slowly add isopropanol (few milliliters dropwise) to neutralize the sample by solvolysis (resulting in H2 release and formation of borates); repeat the process with a more reactive solvent like methanol; and, repeat with water. The as-obtained borates (basic compounds) can be finally and safely trashed with the aqueous bases.

Molecular structure of CsHB

CsHB is a white fine powder. It is insoluble in anhydrous organic solvents like dioxane, toluene, THF or dimethyl carbonate. Solubility in water, methanol and ethanol could not be assessed as CsHB readily reacts with these protic solvents (occurrence of hydrolysis or alcoholysis with release of H2). Molecular analyses based on solutions have thus been discarded.

CsHB was analyzed by FTIR, and the resulting spectrum was compared to the one of HB (Figure 1). The N-H stretching bands (3500-2600 cm -1 ) of CsHB are less complex. This indicates a weaker interaction between the H δ+ of the N2H3 group and the H δ-of the BH3 moiety, and thus a lesser stability in comparison to HB. The bands due to the B-N and BN-N bonds are more intense for CsHB, suggesting shorter bonds. For the B-H region, CsHB displays the bands at a lower wavelength than HB does. These observations are all due to the insertion of Cs + , which modifies the chemical environment and the electronic properties of the molecular entity

[N2H3BH3] - [START_REF] Kim | Rules and trends of metal cation driven hydride-transfer mechanisms in metal amidoboranes[END_REF][START_REF] Chua | Development of amidoboranes for hydrogen storage[END_REF][START_REF] Wang | Metal B-N-H hydrogen-storage compound: Development and perspectives[END_REF]. It is worth mentioning that the spectrum of CsHB is much comparable to the spectral fingerprint of the sister compound RbHB [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. CsHB was analyzed by 11 B MAS NMR (Figures 2 andS1). The spectrum is different from that of HB, and typical of an alkali derivative [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF][START_REF] Moury | Lithium hydrazinidoborane: A polymorphic material with potential for chemical hydrogen storage[END_REF][START_REF] Moury | Sodium hydrazinidoborane: A chemical hydrogen-storage material[END_REF][START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF][START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. CsHB presents a Gaussian-like signal at -16.3 ppm (versus a double-horn signal shape due to quadrupolar coupling [START_REF] Hansen | 11 B chemical shift anisotropies in borates from 11 B MAS, MQMAS, and single-crystal NMR spectroscopy[END_REF], signal centered at -24 ppm for HB), which is ascribed to its NBH3 environment. The change in the signal shape indicates a more isotropic environment around the B atom favored by the presence of Cs + . At least five other signals, of lower intensity, can be seen and have been tentatively assigned [START_REF] Kim | Synthesis and characterization of poly(aminoborane) as a new boron nitride precursor[END_REF][START_REF] Gervais | Chemically derived BN ceramics: Extensive 11 B and 15 N solid-state NMR study of a preceramic polyborazylene[END_REF][START_REF] Stowe | In situ solid state 11 B MAS-NMR studies of the thermal decomposition of ammonia borane: Mechanistic studies of the hydrogen release pathways from a solid state hydrogen storage material[END_REF][START_REF] Kobayashi | Mechanism of solid-state thermolysis of ammonia borane: A 15 N NMR study using fast magic-angle spinning and dynamic nuclear polarization[END_REF]. Three of them are at negative chemical shifts. The signals at -32 and -14 ppm are attributed to BH4 and BH2 environments respectively. The concomitance of the BH4 and BH2 suggests the possible formation of a dimeric compound, tentatively that of [CsN2H3BH2CsN2H3] + [BH4] - [START_REF] Shevlin | Dehydrogenation mechanisms and thermodynamics of MNH2BH3 (M = Li, Na) metal amidoboranes as predicted from first principles[END_REF]. The signal at -28.7 ppm is untypical of a BH3 environment. It is likely due to a boron hydride with B-B bonds (boron clusters) such as the anion B3H8 -. Such an attribution is consistent with the chemical shift of -29.8 ppm reported elsewhere for CsB3H8 [START_REF] Hill | Reduction of BH3•THF by alkali metal (K, Rb, Cs) and ytterbium mercury amalgams to form salts of a simple procedure for the synthesis of tetraborane(10)[END_REF] and with the formation of this salt [START_REF] Grinderslev | Potassium octahydridotriborate: Diverse polymorphism in a potential hydrogen storage material and potassium ion conductor[END_REF][START_REF] Chen | Facile synthesis of unsolvated alkali metal octahydrotriborate salts MB3H8 (M = K, Rb, and Cs), mechanisms of formation, and the crystal structure of KB3H8[END_REF]. The following reaction might be suggested based on the findings reported in refs.

[ [START_REF] Grinderslev | Potassium octahydridotriborate: Diverse polymorphism in a potential hydrogen storage material and potassium ion conductor[END_REF][START_REF] Chen | Facile synthesis of unsolvated alkali metal octahydrotriborate salts MB3H8 (M = K, Rb, and Cs), mechanisms of formation, and the crystal structure of KB3H8[END_REF]:

Cs (s) + 3 BH3 (sol) → CsB3H8 (s) + ½ H2 (g) (5

Stability of CsHB under inert atmosphere

After its synthesis, CsHB was stored for two months inside the glovebox to be further analyzed by FTIR and 11 B MAS NMR. The FTIR analysis shows that CsHB and aged CsHB (denoted CsHB*) present almost identical bands (Figure 1). For the latter, however, there is a slight broadening of the N-H stretching and bending regions. This could be due to the formation of a small amount of additional polymeric species.

The 11 B MAS NMR spectra of CsHB and CsHB* do not present significant differences (Figures 2 andS2). The BH3 environment at -16.3 ppm is the main peak in both spectra, showing the stability of CsHB. The signals at -32 and -14 ppm have disappeared for CsHB*. The spectrum of CsHB* shows an additional signal at +1.7 ppm. It can be due to a BN4 environment [START_REF] Kobayashi | Mechanism of solid-state thermolysis of ammonia borane: A 15 N NMR study using fast magic-angle spinning and dynamic nuclear polarization[END_REF].

These last two observations suggest a slight evolution of the sample, likely of the species

[CsN2H3BH2CsN2H3] + [BH4] -.
Based on these 11 B MAS NMR and FTIR analyses, it may be concluded that CsHB is rather stable when kept at ambient temperature under argon atmosphere. It is however less stable than HB, which is stable in the solid state [START_REF] Moury | Hydrazine borane: Synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF]. This suggests that the size of a big cation like Cs + has an important effect of destabilization on the [NH2NBH3] -anion. To avoid the evolution of CsHB, the compound can be stored at low temperature (subzero conditions). However, as mentioned before, there are safety issues in storing CsHB in an ambient where moisture is present. A watertight container must be used, carefully closed and stored in a freezer. CsHB is more stable than RbHB when they are kept at ambient temperature under argon atmosphere. The latter compound was found to significantly dehydropolymerize [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. RbHB is in fact synthesized at room temperature. In such conditions, the exothermic reaction between HB and Rb may lead to the formation of (unidentified) side-products that are instable and that react over time.

Crystal structure of CsHB

CsHB was analyzed by PXRD (Figure 3). The pattern was compared to that of HB, and the presence of a new phase was confirmed. The omitted Bragg peaks during the structure refinement were identified as coming from the Parafilm® film used to protect the solid.

The PXRD pattern of CsHB (Figure 3) was indexed and refined as described previously. The position of the Cs atoms and that of the [N2H3BH3] -anions were determined. In doing so, the crystallographic parameters of CsHB (Table 1) as well as the experimental structural parameters (Table S1) were compiled, and the calculated pattern was found to be in good fit with the experimental one (Figure S3).

CsHB was successfully indexed as a single phase in a monoclinic P21 (No. 4) unit cell (Figure 4), with Z = 2, a = 5.9156(5) Å, b = 7.0091(5) Å, c = 6.0023(5) Å, and β = 108.389(4)° (Table 1).

Except for β-LiHB (orthorhombic, s.g. Pbca) [START_REF] Moury | Lithium hydrazinidoborane: A polymorphic material with potential for chemical hydrogen storage[END_REF], all of the other hydrazinidoboranes crystallize in a monoclinic unit cell. α-LiHB does in a space group P21/c [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF] while NaHB does in a space group P21/n [START_REF] Moury | Sodium hydrazinidoborane: A chemical hydrogen-storage material[END_REF]. CsHB is isostructural with KHB [START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF] and RbHB [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. In comparison with HB [START_REF] Moury | Hydrazine borane: Synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF], CsHB has a bigger unit cell volume per formula (V/Z = 80.153 versus 118.083 Å 3 ). This is due to the big size of the cation Cs + . If CsHB is compared to its amidoborane analogous, i.e. cesium amidoborane (CsAB; orthorhombic, s.g. Pnam) [START_REF] Owarzany | Amidoboranes of rubidium and caesium: The last missing members of the alkali metal amidoborane family[END_REF], it has a bigger unit cell volume per formula (V/Z = 99.42 versus 118.083 Å 3 ). This is in good agreement with the bigger size of the [N2H3BH3] -anion of CsHB.

The coordination sphere of Cs + is featured by six surrounding [N2H3BH3] -entities (Figure S4).

The other hydrazinidoboranes have coordination numbers of four (LiHB) [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF][START_REF] Moury | Lithium hydrazinidoborane: A polymorphic material with potential for chemical hydrogen storage[END_REF], five (NaHB) [START_REF] Moury | Sodium hydrazinidoborane: A chemical hydrogen-storage material[END_REF],

four (KHB) [START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF], and six (RbHB) [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. RbHB and CsHB can coordinate more anions due to the bigger size of the cation. In comparison with the amidoboranes, CsAB is surrounded by seven

[NH2BH3] -anions [START_REF] Owarzany | Amidoboranes of rubidium and caesium: The last missing members of the alkali metal amidoborane family[END_REF], which is consistent with the smaller size of these anions (versus the

[N2H3BH3] -anions in CsHB). The positions of the H atoms were obtained by computational methods, allowing us to determine the interatomic distances (Figure S5). The shortest interatomic Cs⋅⋅⋅Cs distance is 4.326(4) Å.

The shortest Cs-N bond is 3.22 [START_REF] Moury | Hydrazine borane: Synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF] Å. Compared to the other hydrazinidoboranes, the M-N distance (M = Li, Na, K, Rb, Cs) shortens as the size of the alkali cation decreases: a bond distance of 3.13 Å was found for RbHB [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF], 2.96 Å for KHB [START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF], 2.41 Å for NaHB [START_REF] Moury | Sodium hydrazinidoborane: A chemical hydrogen-storage material[END_REF], and 2.11 Å for LiHB [START_REF] Moury | Lithium hydrazinidoborane: A polymorphic material with potential for chemical hydrogen storage[END_REF]. It is likely that the bond M-N has a higher ionicity for CsHB. The higher ionicity means that more electrons are attracted by the N atom from the Cs + cation in comparison with the H δ+ atom of HB; the Lewis basicity of the N atom and that of the H δ-in the B atom are increased, meaning a higher reactivity of CsHB in comparison with the other alkali hydrazinidoboranes and HB [START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF][START_REF] Wu | Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties[END_REF]. This represents an advantage of the Cs + cation over the H δ+ element in terms of destabilization. The B-N bond is shorter for CsHB (1.40(8) Å) in comparison with HB (1.59 Å) [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF], which is consistent with the changes observed in the 11 B MAS NMR and FTIR analyses.

Thermal analyses of CsHB

The thermal stability of CsHB was followed by TG analysis (heating rate of 5 °C min -1 ) and the evolving gaseous products were identified by MS (Figure 5). As shown by the DSC curve, there is a first event peaking at 56°C that is endothermic. A similar endothermic event was also observed for HB [START_REF] Moury | Hydrazine borane: Synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF], NaHB [START_REF] Moury | Sodium hydrazinidoborane: A chemical hydrogen-storage material[END_REF], KHB [START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF] and RbHB [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. They are associated to melting.

CsHB starts to decompose at ca. 75 °C, according a complex path as suggested by the DSC and MS results. The decomposition is featured by successive exothermic events. In the range 75-160 °C, CsHB loses 3.5 wt. % of its initial weight. H2 as main gaseous product, but also NH3 and B2H6, were detected. A second mass loss of ca. 1.4 wt. % occurs in the range of 160-190 °C.

There is a third one of ca. 2 wt % above 190 °C. The total weight loss between 75 and 250 °C is 6.9 wt. %. This is twice the 3.4 wt. % H carried by CsHB. This excess of weight loss is due to the release of NH3 and B2H6. The Li, Na and K hydrazinidoboranes release purer H2 [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF][START_REF] Moury | Lithium hydrazinidoborane: A polymorphic material with potential for chemical hydrogen storage[END_REF][START_REF] Moury | Sodium hydrazinidoborane: A chemical hydrogen-storage material[END_REF][START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF]. RbHB [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF] and CsHB tend to release more unwanted by-products. Big cations are thus less efficient in hindering decomposition of [N2H3BH3] -, in other words, in being selective towards dehydrogenation of the anions [N2H3BH3] -by dehydropolymerization.

Decomposition of CsHB in isothermal conditions

The thermal stability of CsHB was besides investigated by heating it at constant temperature, i.e. 90, 100, 110 and 120 °C (Figure 6). In these isothermal conditions, the decomposition of CsHB presents a two-step process preceded by an induction period that becomes shorter as the temperature increases. After the induction period, the sample starts to decompose slowly. This step is short, particularly at 110 and 120 °C (10 and 9 min respectively). It is then followed by a second step of decomposition, which is more important and much faster. This is typically the case at 120°C: the induction period, the first step and the second step are as short as 9, 2 and <1 min (versus e.g. 25, 10 and >30 min at 90 °C). Note that such a decomposition profile seems to be typical of hydrazinidoboranes; a similar behavior was reported for e.g. NaHB [START_REF] Moury | Sodium hydrazinidoborane: A chemical hydrogen-storage material[END_REF] and RbHB [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane : Synthesis, characterization and hydrogen release properties[END_REF]. The gases produced during these isothermal experiments were analyzed by GC and MS (Figures S6 to S8). As expected, H2 was detected. However, several unwanted gaseous by-products were also detected: namely, NH3, B2H6 and N2H4. With respect to the first two gases, the results are consistent with the TG-MS results. N2H4 was not observed by TG-MS but its formation cannot be discarded as, based on our experience, N2H4 is able to condensate on cold surfaces (between the TG and MS apparatuses), being thereby not seen by MS [START_REF] Moury | Hydrazine borane: Synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF][START_REF] Moury | Lithium hydrazinidoborane: A polymorphic material with potential for chemical hydrogen storage[END_REF][START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF].

Tentative reaction paths are proposed to explain the formation of the aforementioned unwanted products (Figure 7). As it has been reported, diammoniate of diborane (DADB) is an intermediate in the decomposition of ammonia borane [START_REF] Chatterjee | Development of a reaction mechanism for liquid-phase decomposition of ammonia borane[END_REF][START_REF] Rizzi | The onset of dehydrogenation in solid ammonia borane: an ab initio metadynamics study[END_REF]. Evidence of analogues compounds for alkali amidoboranes have also been reported [START_REF] Shevlin | Dehydrogenation mechanisms and thermodynamics of MNH2BH3 (M = Li, Na) metal amidoboranes as predicted from first principles[END_REF]. The formation of ammonia is explained by the formation of an analogous compound of DADB applied to CsHB (Figure 7a). The intermediate

[(NH2CsNH)2BH2] + [BH4]
-would form, and then, one of the H δ+ atoms would migrate to the end N atom (i.e. NH2 group), breaking the N-N bond and resulting in the release NH3 and H2 [START_REF] Banu | Dehydrogenation of lithium hydrazinidoborane: Insight from computational analysis[END_REF].

One of the possible paths to form B2H6 is as follows (Figure 7b): it is assumed that the same DADB analogous compound, i.e. [(NH2CsNH)2BH2] + [BH4] -, forms. The [BH4] -anion would react with another [BH4] -anion to liberate B2H6 and H2, with the subsequent growing of the polymer chain by dehydrocoupling. Such mechanisms could explain also the species detected in the TG-MS experiments. The exothermic processes seen in the DSC profile (Figure 5) can be due to the release of NH3 and H2, as these processes are known to be exothermic. The solid residues recovered after decomposition of CsHB could not be safely handled to analyze them by FTIR and PXRD (cf. sub-section 3.1 for more details about the safety issues). However, we were able to analyze them by 11 B MAS NMR. The NMR rotors were indeed filled in the glove box and thereby the samples were protected from air and moisture.

The solid residues that formed after the heat treatments performed at isothermal conditions (90, 100, 110 and 120 °C) were thus recovered under argon atmosphere and analyzed by 11 B MAS NMR (Figure 8). They are denoted CsHB 90 , CsHB 100 , CsHB 110 , and CsHB 120 respectively. The spectra of the samples CsHB 90 , CsHB 100 and CsHB 110 are comparable. There are two signals at about -30 and -14 ppm. The former one is ascribed to a species comparable to the anion B3H8 - [START_REF] Hill | Reduction of BH3•THF by alkali metal (K, Rb, Cs) and ytterbium mercury amalgams to form salts of a simple procedure for the synthesis of tetraborane(10)[END_REF][START_REF] Grinderslev | Potassium octahydridotriborate: Diverse polymorphism in a potential hydrogen storage material and potassium ion conductor[END_REF][START_REF] Chen | Facile synthesis of unsolvated alkali metal octahydrotriborate salts MB3H8 (M = K, Rb, and Cs), mechanisms of formation, and the crystal structure of KB3H8[END_REF]. The latter is due to a BH2 environment [START_REF] Kim | Synthesis and characterization of poly(aminoborane) as a new boron nitride precursor[END_REF][START_REF] Stowe | In situ solid state 11 B MAS-NMR studies of the thermal decomposition of ammonia borane: Mechanistic studies of the hydrogen release pathways from a solid state hydrogen storage material[END_REF]. There is also a broad signal of high intensity at positive chemical shift. It is ascribed to BN3, and BN2H environments suggesting a polyborazylene-like solid [START_REF] Gervais | Chemically derived BN ceramics: Extensive 11 B and 15 N solid-state NMR study of a preceramic polyborazylene[END_REF][START_REF] Stowe | In situ solid state 11 B MAS-NMR studies of the thermal decomposition of ammonia borane: Mechanistic studies of the hydrogen release pathways from a solid state hydrogen storage material[END_REF]. There is also a signal of small intensity, particularly visible for CsHB 100 at 2 ppm, which may be due to a BN4 environment of a cross-linked polymeric compound [START_REF] Kobayashi | Mechanism of solid-state thermolysis of ammonia borane: A 15 N NMR study using fast magic-angle spinning and dynamic nuclear polarization[END_REF]. It forms within the temperature range 90-110 °C. This same signal disappears at higher temperatures, suggesting a low stability of this species and their further evolution at temperatures higher than 110 °C. With respect to the spectrum of CsHB 120 , the signals suggest only two types of compounds: a B3H8-like species (-32 ppm) and a cross-linked polyborazylenelike solid (other signals at > -2 ppm). These results are consistent with the decomposition extents measured at isothermal conditions (Figure 6): decomposed CsHB tends to form solid residues consisting of polyborazylene-like and a stable species likely to be CsB3H8, via the involvement of polymeric species with BH2 groups (i.e. polyaminoborane-like). To mechanistically explain the improved dehydrogenation properties of MHB (versus the parent HB), it has been suggested the occurrence of the so-called metal cation driven hydride-transfer [START_REF] Kim | Rules and trends of metal cation driven hydride-transfer mechanisms in metal amidoboranes[END_REF] where the key step is the in situ formation of the metal hydride M-H via H δ-transfer from

with m > n. Based on our observations, Cs + is not as efficient as e.g. Li + is for favoring the formation of pure H2 according to the stepwise reactions mentioned above (Eqs. 6 to 8). And, Cs + cannot prevent the decomposition of [N2H3BH3] -(Figure 7). In other words, CsHB in the form presented herein does not present suitable dehydrogenation properties to be considered as a potential hydrogen storage material. Further works are required to improve its properties and to better understand the formation of the unwanted CsB3H8, NH3, B2H6, and N2H4.

Conclusion

The last of the alkali derivatives of hydrazine borane N2H4BH3, cesium hydrazinidoborane

Figure 1 .

 1 Figure 1. FTIR spectra of CsHB, CsHB after two months (i.e. aged CsHB denoted CsHB*), and HB. The bands are assigned.

  )where BH3 would directly come from HB or from in situ forming B2H6 by decomposition of HB (as shown below B2H6 formed as by-product in our conditions). The other signals shown by CsHB are at positive chemical shifts and they are ascribed to BN3, and BN2H environments. They suggest that slight evolution of CsHB, or of its possible dimer, into a polyborazylene-like compound[START_REF] Kim | Synthesis and characterization of poly(aminoborane) as a new boron nitride precursor[END_REF] is likely to occur during synthesis, storage of the sample, and/or fast rotation of the sample during NMR analysis.

Figure 2 .

 2 Figure 2. 11 B MAS NMR spectra of CsHB, CsHB after two months (i.e. aged CsHB denoted CsHB*), and HB. The chemical shifts in ppm of the signals are indicated. The dashed line box focuses on the signals of very low intensity at positive chemical shift; cf. Figures S1 and S2 for zoomed versions of the spectra.

Figure 3 .

 3 Figure 3. PXRD patterns of CsHB and HB. The peaks indicated by the symbol * belong to Parafilm® foil.

Figure 4 .

 4 Figure 4. Crystal structure of CsHB along the [100], [010] and [001] directions. The H, B, N and Cs atoms are represented by pink, green, blue and turquoise spheres, respectively.

Figure 5 .

 5 Figure 5. Thermal analysis of CsHB: (a) TG analysis (5 °C min -1 ); (b) DSC analysis (5 °C min -1 ); (c,d,e) MS results with m/z = 2, 17, and 27 respectively). Weight losses at 160, 190 and 250 °C, and most of the peak temperatures (°C) are given.

Figure 6 .

 6 Figure 6. Time evolution of H2 release from CsHB under heating at isothermal conditions.

Figure 7 .

 7 Figure 7. (a) Reaction between 2 molecules of CsHB releasing NH3 and H2, resulting in compounds with sp 2 and sp 3 B centers. (b) CsHB molecules forming the intermediate [(CsN2H3)2BH2] + [BH4] -, and then releasing B2H6 and H2, leading to the formation of B-and N-based polymers. (c) Reaction of a polymer chain product of the thermolysis of CsHB and a molecule of CsHB, and this leads to a continued growth of the chain, forming CsH and releasing N2H4.

Figure 8 .

 8 Figure 8. 11 B MAS NMR spectra of the solid residues recovered after the isothermal treatment at 90, 100, 110 and 120°C, namely CsHB 90 , CsHB 100 , CsHB 110 , and CsHB 120 . The spectrum of CsHB is given for comparison.

[ 6 ) 7 )

 67 N2H3BH3] -to M + : MN2H3BH3 (s) → MH⋅⋅⋅H-N2H2BH2 (s) (Release of H2 is initiated from the intermediate species MH⋅⋅⋅H-N2H2BH2, such as: MH⋅⋅⋅H-N2H2BH2 (s) → MN2H2BH2 (s) + H2 (g) (The further steps are intermolecular reactions of MN2H2BH2 towards the formation of intermolecular N-B bonds leading to oligomeric/polymeric solids: n MN2H2BH2 (s) → [MBN2H<2]n (s) + m H2 (g)

CsN2H3BH3 (CsHB; 3 .

 3 3 wt. % H), was synthesized and characterized. The substitution of one H δ+ of the N2H4 moiety by Cs + has been demonstrated by FTIR and 11 B MAS NMR spectroscopy techniques. CsHB is a crystalline material. It is isostructural to KN2H3BH3 and RbN2H3BH3, with a monoclinic unit cell, a P21 space group (No. 4), and the following cell parameters: a = 5.9156(5) Å, b = 7.0091(5) Å, c = 6.0023(5) Å, and β = 108.389(4)°. The thermal behavior ofCsHB was studied. The destabilization effect of Cs + on the anionic entity [N2H4BH3] -has been confirmed: (i) under heating at 5 °C min -1 , CsHB starts to decompose at a lower temperature than HB does; (ii) at isothermal conditions, e.g. at 120 °C, CsHB releases more equivalents of gas and more rapidly than HB, LiHB and RbHB do. However, CsHB decomposes rather than it dehydrogenates. Together with H2, ammonia, hydrazine and diborane, for example, have been detected. In comparison to the other alkali hydrazinidoboranes, CsHB is definitely the least attractive in terms of H2 purity. As for the parent hydrazine borane, the thermolytic solid residues forming upon decomposition of CsHB were found to be unstable (shock-sensitive and highly reactive towards O2 and H2O). This may be due to unidentified Cs-based and/or nitrogencontaining products. Otherwise, the 11 B MAS NMR analyses of the solid residues are indicative of the formation of polyborazylene-like products and possibly of CsB3H8.

  The high temperature phase, denoted α-LiHB (monoclinic, s.g. P21/c), was first reported by Wu et al.[START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF]. The low temperature phase,

β-LiHB (orthorhombic, s.g. Pbca), was afterwards found by Moury et al.

[START_REF] Moury | Lithium hydrazinidoborane: A polymorphic material with potential for chemical hydrogen storage[END_REF]

. Under heating, β-LiHB transforms into α-LiHB that releases 2.6 equiv H2 and traces of NH3 below 150 °C.

Sodium hydrazinidoborane NaN2H3BH3 (NaHB; 8.9 wt. % H) was synthesized (Eq. 1) by Moury et al. by mechanosynthesis at ca. -30 °C

[START_REF] Moury | Sodium hydrazinidoborane: A chemical hydrogen-storage material[END_REF]

. The product obtained is a crystalline solid (monoclinic, s.g. P21/n) that starts the dehydrogenation process at ca. 60 °C, releasing H2 with traces of NH3 and N2. Potassium hydrazinidoborane KN2H3BH3 (KHB; 7.2 wt. % H; monoclinic, s.g. P21) was synthesized by Chua et al., in a stainless steel reactor containing dissolved HB because of the high reactivity of KH towards HB

[START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF]

:

Table 1 .

 1 Crystallographic parameters of CsHB.

	Formula	CsN2H3BH3
	Unit cell	Monoclinic
	s.g.	P21 (No. 4)
	Z	2
	a (Å)	5.9156(5)
	b (Å)	7.0091(5)
	c (Å)	6.0023(5)
	β (°) V (Å 3 )	108.389(4) 236.16(3)
	Density	2.5
	Temperature	20 °C
	R(all)/wR(all)	7.32/9.03
	R(obs) / wR(obs)	7.32/9.03
	GoF	5.08
	Rp	6.66
	wRp	10.17

Acknowledgments

The authors acknowledge the Mexican National Council for Science and Technology CONACyT for the scholarship to CACM. The authors, and especially PGY, acknowledge Prof. Guillaume MAURIN (University of Montpellier) for the fruitful discussions and his contribution during the determination of structural model of CsHB.

By successfully synthesizing and studying cesium hydrazinidoborane, we have been able to complete the alkali family of the hydrazine borane derivatives. The present findings contribute to have a better knowledge of this kind of boron-and nitrogen-based materials. However, the thermal behavior of CsHB is not to its advantage: the released H2 is polluted by several unwanted gaseous by-products and the thermolytic solid residue poses safety issues. CsHB in the form presented herein is thus not suitable for hydrogen storage. Otherwise, it might be considered as energetic material.


Zakaria Ilyes Djamai
email: djamai@insa-toulouse.fr

Amir Si Larbi

Ferdinando Salvatore

Gaochuang Cai

A new PCM-TRC composite: a mechanical and physicochemical investigation

Keywords: PCM, TRC, composite material, mechanical behaviour, physicochemical investigation. 1-Introduction

The present paper presents a multiscale and multiphysics study that focuses on the development of an innovative composite material, which combines a cementitious matrix modified by a phasechange material (PCM) with textile reinforcement (PCM-TRC).

The development of this composite has been guided by the conjunction of several factors, among which it is appropriate to emphasise the lightweight of the composite due to the reduced covers in textile reinforced concrete (TRC) coupled to the high thermal inertia due to the addition of PCM microcapsules.

This research focuses on the effect of PCM on the mechanical and physicochemical properties of TRC. Special emphasis is placed on providing the phenomenological explanation for the resulting mechanical behaviour of TRC according to the rate and state of the PCM.

It has been found that the ductile mechanical behaviour of TRC composites is maintained in the presence of microencapsulated PCM; however, the mechanical performance of PCM-TRC decreases with the PCM rate. This is due to the disorder caused by PCM at the matrix and interface scales. Furthermore, it has been found that the PCM state (solid or liquid) can affect the mechanical performance of both the PCM-mortar matrix and PCM-TRC composite. This is due to PCM expansion during phase change from the solid to the liquid state, which can induce matrix microcracking.

2 panels, etc. [START_REF] Vervloet | Experimental investigation of the buckling behaviour of Textile Reinforced Cement sandwich panels with varying face thickness using Digital Image Correlation[END_REF][START_REF] Shams | An analytical model for sandwich panels made of textile reinforced concrete[END_REF][START_REF] Colombo | Bending behavior of textile reinforced concrete sandwich beams[END_REF][START_REF] Djamai | Analytical modeling of textile reinforced concrete (TRC) sandwich panels: Consideration of nonlinear behavior and failure modes[END_REF]. TRC can ensure fire resistance and constitutes an important alternative to fibre-reinforced polymer, which has limitations in terms of cost, fire resistance, toxicity, and sustainable-development features attributed to the polymer matrix [START_REF] Djamai | Analytical modeling of textile reinforced concrete (TRC) sandwich panels: Consideration of nonlinear behavior and failure modes[END_REF]. Despite these advantages, the light weight of TRC, which is due to the low thickness of the concrete covers, can lead to a considerable reduction of its thermal inertia.

Recent studies [START_REF] Pomnianowski | A new experimental method to determine specific heat capacity of inhomogeneous concrete material with incorporated microencapsulated -PCM[END_REF][START_REF] H W Min | Investigation on thermal and mechanical characteristics of concrete mixed with shape stabilized phase change material for mix design[END_REF][START_REF] Figueiredo | Mechanical and thermal characterization of concrete with incorporation of microencapsulated PCM for applications in thermally activated slabs in[END_REF] have focused on the search for intelligent materials that have the ability to regulate indoor temperature variations; these are phase-change materials (PCMs). PCMs change their phase from solid to liquid in warm periods by storing heat, the same heat that is released in cold periods, when the PCM returns to the solid state. This repetitive energy storage-release process can smooth out temperature variations inside buildings when PCMs are integrated in the materials constituting the building envelope.

The most promising association is probably the direct integration of PCMs in cement-type materials, which are the most commonly used in construction. Recent studies have identified several techniques for integrating PCMs into cementitious materials, such as direct incorporation of raw PCMs [START_REF] Cunha | Influence of adding phase change materials on the physical and mechanical properties of cement mortars[END_REF] and vacuum impregnation in cementitious materials [START_REF] Zhang | Development of thermal energy storage concrete[END_REF]. However, the technique that has received the most attention from the scientific community is the integration of PCMs after microencapsulation [START_REF] Djamai | Multiphysics analysis of effects of encapsulated phase change materials (PCMs) in cement mortars[END_REF][START_REF] Eddhahak-Ouni | Experimental and multi-scale analysis of the thermal properties of Portland cement concretes embedded with microencapsulated phase change materials (PCMs)[END_REF]. This method allows coating the raw PCM with a polymer film in the form of a capsule, which makes it possible to protect the PCM from a possible liquidphase flow and thus potential mixing with the cementitious components (whose effects are not controlled).

Recent investigations [START_REF] V D Cao | Microencapsulated phase change materials for enhancing the thermal performance of Portland cement concrete and geopolymer concrete for passive building applications[END_REF][START_REF] Thiele | Annual energy analysis of concrete containing phase change materials for building envelopes in[END_REF] have shown that the integration of microencapsulated PCM in cementitious materials can considerably improve their energy storage capacity; however, this is combined with an important degradation of their mechanical performance [START_REF] Aguayo | The influence of microencapsulated (PCM) characteristics on the microstructure and strength of cementitious composites: Experiments and finite element simulations[END_REF][START_REF] P K Dehdezi | Thermal, mechanical and microstructural analysis of concrete containing microencapsulated phase change materials[END_REF][START_REF] Cui | Study on functional and mechanical properties of cement mortar with graphite-modified microencapsulated phase-change materials[END_REF][START_REF] Lecompte | Mechanical and thermos-physical behaviour of concretes and mortars containing PCM[END_REF][START_REF] Hunger | The behavior of selfcompacting concrete containing micro-encapsulated phase change Materials[END_REF].

In this context, the present study aims to combine the advantages of PCM and TRC to propose a new PCM-modified TRC composite, PCM-TRC, which results from the association of a PCMmodified mortar matrix and an alkali-resistant glass fabric reinforcement. The main advantages of the new PCM-TRC composite material can be summarised as follows:

a) The possibility to produce lightweight cementitious structural elements (owing to TRC) with high mechanical performances (especially under tension solicitation) and high thermal inertia (owing to PCM).

b) It is a better match (even partial) to the criteria of sustainable development.

Few studies have focused on the effect of PCM on the mechanical performances of fibrous reinforced concrete and TRC. Savija et al. [START_REF] Savija | Development of ductile cementitious composites incorporating microencapsulated phase change materials[END_REF] studied the effect of PCM on the strength of PVA fibrous reinforced mortar. They concluded that the addition of small amounts of microencapsulated PCMs can be detrimental to compressive strength, although it has a small effect on flexural strength and deflection capacity.

For all these reasons, the main objective of this research is to investigate whether PCM affects the mechanical properties of TRC, especially regarding the tensile response of the composite. This study also focuses on the physicochemical explanation of the resulting changes in the mechanical strength of the composite due to the PCM rate. Furthermore, few studies [START_REF] Pilehvara | L Valentinie Mechanical properties and microscale changes of geopolymer concrete and Portland cement concrete containing micro-encapsulated phase change materials[END_REF] have focused on how the state of microencapsulated PCM (solid or liquid) influences the mechanical characteristics of PCM-cementitious materials. For this reason, this research analyses the effects of the temperature (and thus of the PCM state) on the mechanical performances of PCM-mortar matrices and its consequences on PCM-TRC composites. Phenomenological explanations of such changes due to the PCM state are also provided in the paper.

Recent studies [START_REF] De | Life cycle assessment of the inclusion of phase change materials in experimental building[END_REF] on the life cycle assessment (LCA) of PCM-concrete constructions, point out the need of research to reduce the environmental impact of using PCM in construction. In fact, these investigations indicate that the energy saving achieved during the operational phase of PCM-concrete constructions is balanced out with the manufacturing and end of life phases of raw PCM, which is generally derived from petroleum resources like paraffin. For this reason, the present research aims also, to compensate or at least reduce the negative effect of the manufacturing phase of PCM in LCA using a natural vegetal wax, which is a fatty ester, identified as methylexadecanoate in the PCM-TRC composites. In fact, recent researchers [START_REF] De | Life cycle assessment of the inclusion of phase change materials in experimental building[END_REF] pointed out that the use of hydrated salts and natural fatty esters can present a 75% lower manufacturing environmental impact than paraffin.

2-Material and methods

2-1 Materials

2-1-1 Mortar

A Portland cement mortar with fine aggregate (D<1.6 mm) was chosen to ensure the impregnation of the textile fabric for the 'PCM-TRC' composites.

2-1-2 PCM

The PCM used in the study is a non-paraffinic fatty ester vegetal wax chemically identified as methylexadecanoate; this PCM was selected to minimise the environmental impact. The PCM phase-change temperature ranges from 24 to 27°C with a phase-change peak at 25°C. The latent heat of the phase change of PCM is 160 kJ/kg. The differential scanning calorimetry (DSC) diagram for one endothermic cycle of the PCM is provided in [START_REF] Djamai | Multiphysics analysis of effects of encapsulated phase change materials (PCMs) in cement mortars[END_REF].

2-1-3 Textile fabric reinforcement

The textile fabric reinforcement used in the study is an alkali-resistant uncoated glass with a tensile yield stress and Young's modulus measured as 800±48 MPa and 53±2.5 GPa, respectively, and with a mesh width of 4 and 5 mm in the warp and weft directions, respectively.

2-2 Experimental methods

2-2-1 Casting procedure 2-2-1-1 Casting of the PCM-modified mortar matrix Microencapsulated PCM was added to different weight percentages of Portland cement finegrained mortar (0, 5, 10, and 15 wt%). A minimum of 5 wt% PCM is required to ensure a meaningful effect of the PCM in terms of thermal performance [START_REF] Castellón | Cabeza Experimental Study of PCM Inclusion in Different Building Envelopes[END_REF].

The specific nature of the encapsulated PCM [START_REF] Cunha | A Garbacz Mortars with phase change materials -Part I: Physical and mechanical characterization[END_REF][START_REF] Felix | Study of a cement mortar with incorporation of PCM microcapsules in proceeding of the 6 th symposium of polymers in concrete[END_REF][START_REF] Djamai | Multiphysics analysis of effects of encapsulated phase change materials (PCMs) in cement mortars[END_REF][START_REF] Lecompte | Mechanical and thermos-physical behaviour of concretes and mortars containing PCM[END_REF] induce two main phenomena. The first is water absoption in the internal porosity of PCM (the voids between microcapsules) due to its very high porosity and high surface specific area (~1000 m /kg). The second is water adsorption due to the existence of -OH groups on the polymer shell used for encapsulation, which lead to chemical water adsorption by creation of hydrogen bonds. The high hydrophilic nature of the encapsulated PCM require an adjustment of the water-to-binder ratio for each PCM-mortar matrix to fulfil the requirement of equivalent workability that ensures equivalent conditions for the impregnation of the alkali-resistant (AR) glass textile fabrics in each PCM-mortar matrix.

The amount of additional water required for each PCM-mortar matrix was evaluated using a water absorption test on PCM powder according to EN1097-6 [START_REF]Tests for mechanical and physical properties of aggregates Part 6: Determination of particle density and water absorption[END_REF]. The results are shown in Table 1. The in situ hand lay-up technique (Figure 1) was applied to produce the PCM-TRC composites.

Mass

A suitable amount of each PCM-mortar matrix (0, 5, 10, 15 and 20wt% PCM-mortar, as described above) was spread on the bottom of a mould (600 mm-length, 400 mm-width, 10 mmthickness). Then, the AR glass fabric was impregnated by the matrix with a roller so that PCMmortar matrix penetrated inside the textile fabric meshes. The procedure was repeated so that the PCM-TRC composites were reinforced with two layers of textile fabric. It should be emphasised that special care was taken to avoid damage to the PCM during the described process.

After casting, the samples were cut into 500 mm-length, 70 mm-wide, and 10 mm-thick specimens (making sure to eliminate curbs to avoid border effects) for mechanical evaluation under uniaxial tensile solicitation. The specimens tested at 40°C were stored for 6 h in an oven at 45°C to induce PCM microcapsules phase change from the solid to the liquid state. Compression tests, three-points bending tests and stiffness analysis were performed 3 minutes after removal from the cold room.

Five specimens (of each PCM content) whose temperature was raised to 40°C were then cooled at 15°C for 6 hours in a cold room to ensure PCM re-solidification (sample surface temperature = 20°C). Compression tests, three-points bending tests and stiffness analysis were performed 3 minutes after removal from the cold room.

2-2-2-2 Matrix scale: explanation of the effect of temperature on the mechanical behaviour of PCM-mortar matrix.

The PCM state may affect the mechanical behaviour of both the PCM-mortar matrices and the PCM-TRC composites (see section 3-1).

The authors explored the hypothesis that a change occurs in the volume of the hardened PCMmortar mixes, which is controlled by the PCM state (solid or liquid). Indeed, when PCM melts, the PCM capsule may expand, which can induce localised strains in PCM-mortar.

To evaluate the veracity of this hypothesis hardened PCM-mortar samples (40 mm × 40 mm × 160 mm) were first cooled to 15°C in a cold chamber to ensure solidification of all PCM in the hardened mortar. The samples containing 0, 5, 10, 15, and 20 wt% PCM were then instrumented before being placed in an oven at 45°C for 80 min (Figure 2).

The instrumentation of the samples in the oven (Figure 2) consisted of a strain gage (30 mm in length) placed on the front face of a PCM-mortar sample and a surface temperature sensor placed on a different face of the sample to correlate the strain evolution with the surface temperature of the sample. The two sensors (strain gage and surface temperature sensors) were connected to a data acquisition system. The PCM-mortar samples were then placed in a 45°C oven for approximately 80 min while the data acquisition system recorded the evolution of the axial strain and the surface temperature of the samples. (which means that PCM was in the solid state), according to the recommendation of the Rilem committee [START_REF]TDT: test methods and design of TRC Materials and structures[END_REF].

The tests were performed on an INSTRON electromechanical press with a capacity of 100 kN.

The PCM-TRC specimens were clamped by applying a pressure of approximately 3 MPa to their upper and lower ends, while thin rubber plates (~2 mm thick) were glued on the grips to avoid any local compression crushing due to clamping. The clamping length was set to be 125 mm to avoid any local pull of the textile fabrics during the test.

During the test, the PCM-TRC specimens with 0, 5, 10, 15 and 20wt% PCM were instrumented with two LVDT sensors, one on each side of the sample (see Figure 3). The LVDT sensors measured the longitudinal displacement of the 200 mm central zone of the specimens. To analyse the effect of the temperature (and thus the PCM state) on the mechanical performance of PCM-TRC, the mechanical strength of the 10wt% PCM-TRC composite was evaluated in two different configurations (three specimens for each configuration):

-A-10wt% PCM-TRC test specimens (500 mm × 70 mm × 10 mm) were maintained at a temperature of 20 °C and were then tested under tension at the same temperature.

-B-10wt% PCM-TRC test specimens (500 mm × 70 mm × 10 mm) were initially kept at 20 °C and then placed in an oven at 40 °C for 6 h to induce phase change of the PCM (from solid to liquid). Three minutes after their removal from the oven, the specimens were tested under tension.

2-2-3 Physico-chemical characterisation of the PCM-TRC composites 2-2-3-1 SEM analysis

To better understand the effect of PCM on the mechanical behaviour of the PCM-TRC composites, SEM analyses of the matrix/textile interface were carried out on control TRC composites (without PCM) and on PCM-TRC composites with different PCM rates.

A TESCAN Mira3 FEG microscope with an electron voltage of 10 kV and a high-vacuum chamber with a pressure of 10 Pa were used for the SEM investigations. A backscattered electron detector (BSE) was used to identify the different phases existing in the composite.

2-2-3-2 DSC analysis

The specific heat capacity of PCM-TRC composites cured for 28 days was evaluated using a 3D CALVET scanning calorimeter, which contains an array of thermocouple sensors that totally surround the sample in the three directions and thus allow the introduction of approximately 2 g of hardened PCM-TRC composite (of each PCM rate) without crushing the material.

The heating range used for the endothermic test was 5-40°C with a heating rate of 2°C/min under low-vacuum pressure and a nitrogen purge (20 mL/min).

3-Results and discussion

3-1 mechanical characterisation

3-1-1 Mechanical characterisation of PCM-mortar matrices

Figure 4 (a, b andc) shows compression strength, bending strength and stiffness, respectively of PCM-mortar samples at 28days. The tests were performed on three types of samples: samples with temperatures maintained at 20°C; samples with temperatures raised to 40°C; and samples that were cooled to 20°C after having been heated to 40°C.

-a-(a) A decrease in the mechanical strength (under compression and flexion) and stiffness of the mortar was found for increasing PCM content in the three cases defined above (samples maintained at 20°C, samples heated to 40°C, and samples cooled to 20°C after heating at 40°C) (b) A decrease in the mechanical performance of PCM-mortars was observed when the PCM state changed from solid to liquid when the sample was placed in the oven. The degree of strength degradation due to the change of the PCM state increased with increasing PCM content.

(c) Mechanical tests performed on PCM-mortar samples re-cooled to 20°C after having been heated in a 40°C oven for 6 h indicate that the mechanical strengths of the PCM-mortar samples were not totally recoverable.

(d) The degree of strength recovery of re-cooled PCM-mortar samples (samples cooled to 20 °C after heating at 40°C) decreased with the increase of PCM content.

(e) Bending strength exhibits a lower rate of recovery compared to compression strength.

(f) In the same trend of the compression and flexural behaviour, the stiffness of the PCM-mortar is not totally recoverable.

3-1-2 Explanation of the effect of temperature on the mechanical behaviour of the PCM-mortar matrix.

Figure 5 shows the evolution of the longitudinal strain and surface temperature versus time in the oven for the 15 and 20wt% PCM-mortar samples.

The 15 and 20wt% PCM-mortar samples exhibited an increase in the strain, which began in the range 24-25°C; this corresponds to the phase change of PCM from the solid state to the liquid state. No strain variation was observed in the range 15-23°C (all PCM in the mortar is in the solid state) or in the range 34-40°C (all PCM in the mortar has melted and is in the liquid state). In the temperature range 25-34°C, the PCM melts gradually from the surface to the internal volume of the sample.

Correspondingly, analysis of the change of the samples surface temperature as a function of time spent in the oven shows a constant rate of temperature increase until 24-25°C, where a slowdown in the temperature increase is observed. This is explained by the fact that at 24-25°C, PCM absorbs heat from the oven (in the form of latent heat) to undergo phase change, which causes a decrease in the rate of temperature increase. After the phase change, the rate of temperature increase raises in the 15 and 20wt% PCM-mortar samples (Figure 5 a andb).

-a-15wt% PCM-mortar -b-20wt% PCM-mortar Figure 6 shows the evolution of the longitudinal strains of all the PCM-mortar samples as a function of the surface temperature. It can be seen that the induced strains increase with increasing PCM content in the PCM-mortar matrices. The internal strain ε caused by the PCM expansion and the yield strain under direct tensile loading (experimentally evaluated for each PCM-mortar matrix) are summarised in Table 3. It can be concluded from Table 3 that the volume change of the PCM during melting can induce multicracking in the mortar (induced strain due to PCM expansion > yield strain), which explains the significant degradation of the mechanical performances of the matrix during the tests at temperatures above 40°C (see Section 3-1). The number of formed cracks increases with increasing PCM content; therefore, the degradation of the mechanical performance of matrices becomes more significant with increasing PCM content. The partial recovery of the initial mechanical properties of the PCM-mortar matrices when the PCM re-solidifies is attributed to the partial closure of crack lips (caused by volume contraction) and a strength gain due to PCM solidification. The bending strength being more controlled by the presence of microcracking in the matrices exhibits a rate of recovery less important than the compressive strength (see Figure 4).

3-1-3 Mechanical characterisation of PCM-TRC composites 3-1-3-1 Mechanicals tests at variable PCM rate and constant temperature

The results obtained during the tensile tests on the PCM-TRC composites at different PCM rates (0, 5, 10, 15, and 20wt%) were analysed to determine their average macroscopic constitutive law in terms of stress versus strain, as presented in Figure 7. The global behaviour of the PCM-TRC composite (for all PCM rates) can be summarised as follows:

Phase I (see Figure 7) corresponds to a perfectly linear evolution governed by the mechanical properties of the matrix up to the initiation of the first macrocrack. During phase II-a (see Figure 7), the elastic limit of the matrix is reached progressively along the length of the composite, and each time a crack is created, the force is transmitted progressively from the matrix towards the textile at the location of the crack. In phase II-b (see Figure 7) the textile recovers almost all the tractive effort and the stiffness modulus of the composite ( ; see Table 4) is governed by the elastic modulus of the reinforcing textile and its rate of work.

The first finding obtained by analysing the stress versus strain curves is that the strain hardening behaviour (ductile behaviour) and the matrix/textile charge transfer kinetics are maintained in the presence of microencapsulated PCM in TRC. However, it can be seen that the tensile strength of the PCM-TRC composites decreases when increasing the PCM rate. By analysing in more detail the stress versus strain curves, one can note that:

In phase I, the stress at the onset of the initial cracking, which corresponds to the elastic limit of the PCM-mortar matrix, decreases with increasing PCM rate. This is explained by the fact that the addition of PCM in the mortar weakens its mechanical performance. Similarly, the elastic modulus in phase I decreases with decreasing PCM content owing to the weakening of the rigidity of the matrix.

The multicracking phase (phase II-a) is clearly visible in the majority of PCM-TRC composites; however no particular conclusion can be drawn as to the effect of the PCM rate on this phase.

During phase II-b, the load is almost entirely transmitted to the textile reinforcement. It can be seen after calculation (see Table 4) that the stiffness modulus of the composite ( , during phase II-b, see Figure 7) decreases with increasing PCM content. The rigidity modulus of the PCM-TRC composite in phase II-b is a direct indicator of the textile efficiency (textile rate of inside the It can also be seen in phase II-b that the strain at failure of the composites is almost constant up to the level of 15% PCM, where an increase of the strain at rupture is observed. This is explained by the fact that for the 0, 5, and 10wt% PCM-TRC samples, the mode of failure was rupture of the textile under tension, whereas for the 15 and 20wt% PCM-TRC samples, rupture occurred by delamination of the textile layers due to the weakening of the shear strength of the interlaminar layer by the addition of PCM (see Figure 8). Indeed, the LVDT sensors positioned on the faces of the 15 and 20wt% PCM-TRC samples measured a parasitic displacement due to interlaminar slip upon the occurrence of shear delamination, which explains the large increases of the strain at failure for the 15 and 20% PCM-TRC samples in comparison with the 0, 5, and 10wt% PCM-TRC composites.

Figure 8 Failure modes of PCM-TRC composites

It should also be emphasised that even though a similar textile rupture mode occurred up to a PCM rate of 10wt%, a clear distinction exists between the failure mode of the reference composite (without PCM), where a simultaneous failure of the textile yarns occurred, and the 5 and 10wt% PCM-TRC composites, where localised filament breaks (which initiate the failure of the textile fabric) are observed (see Figure 8 above). This is explained by the heterogeneous distribution of stresses in the textile yarns due to PCM addition in the 5 and 10wt% PCM-TRC composites. This phenomenon is explained in Section 3-2-1-2.

3-1-3-2 Mechanical tests at variable temperature and constant PCM rate

The average curves (stress versus strain for three samples) of the tensile behaviour of the 10wt% PCM-TRC composite held at 20°C and tested at the same temperature and for the 10wt% PCM-TRC composite heated at 40°C for 6 h (to induce PCM phase change) are presented in Figure 9. In phase I, the 10wt% PCM-TRC composite heated at 40°C exhibits a decrease of 25% in stress at the onset of cracking ( and a decrease of 33% in the elastic modulus of the matrix (E ) in comparison with the composite maintained at 20 °C (Figure 9). This is explained by the degradation of the mechanical performance of the matrix by the creation of internal microcracks at temperatures higher than the melting temperature of PCM (see Section 3-1-1).

In phase II-b, the stiffness modulus E of the 10wt% PCM-TRC composite heated at 40 °C shows a decrease of ~30% compared with the composite maintained at 20°C (see Figure 9). This is explained by a degradation of the intensity of the matrix/textile interaction due to the creation of internal microcracks in the interface. In fact, the creation of microcracks at the matrix/textile interface can reduce the effective contribution of the textile owing to the degradation of the kinetics of charge transfer from the matrix to the cracked interface to the textile. This trend is confirmed by analysing the failure mode of the 10wt% PCM-TRC sample heated to 40°C. In fact, the rupture of that sample occurred by delamination between layers of reinforcement (see Figure 10) due to the degradation of the shear strength of the interlaminar layer by cracking during placement in the oven. Meanwhile, we can observe that the rupture of the 10wt% PCM-TRC composite maintained at 20°C occurred by textile breaking under tension (see Figure 10). The increase of the strain at failure for the 10wt% PCM-TRC composite heated at 40°C is due to its characteristic failure mode by delamination. Indeed, an interlaminar slip was detected by the displacement transducers, which explains the increase of the strain at failure while the transmitted stress is almost constant. Figure 11 shows an SEM image representative of a longitudinal cross-section at the "matrix /textile" interface in a reference TRC composite (without PCM).

It can be seen from Figure 11 that a very low porosity remains at the transition zone between the reference mortar matrix (without PCM) and the textile; therefore, high stiffness can be achieved in the characteristic zone II-b of the stress-strain plot of TRC under tension (see Figure 7, reference TRC) in the reference TRC composite (without PCM). It can be seen that the broken PCM particles contains small nodules, which represents the pure PCM wax. BSE detector distinguishes PCM wax (small nodules) from undamaged PCM capsules owing to the higher density of the former. From Figure 13, it can be seen that the long chain of PCM capsules surrounding the anhydrous cement grains in the matrix/textile interface zone can limit or even prevent their hydration by restricting their access to water and thus their dissolution. This reduces the amount of hydrated products from the cement in the matrix/textile interface zone and therefore limits the contribution of the textile reinforcement to recover the tractive effort. This trend of PCM to inhibit the hydration of cement was confirmed by semi-adiabatic calorimetry realised by the authors in [START_REF] Djamai | Multiphysics analysis of effects of encapsulated phase change materials (PCMs) in cement mortars[END_REF] on PCM-mortar matrix with different PCM rates.

3-2-1-2-c Effect of PCM damage in the vicinity of the interface

Figure 14 shows another SEM image presenting a longitudinal cross-section at the matrix/textile interface in a 10wt% PCM-TRC composite. The purpose of this figure is to highlight the effect of the deposition of pure PCM wax in the matrix/textile interface zone. The deposition of a large quantity of pure PCM nodules in the direct vicinity of the matrix/textile interface shown in Figure 14 is most likely due to the damage of PCM microcapsules during the application of the hand lay-up technique to the PCM-TRC composite and/or during the mixing procedure. The immiscibility of pure PCM wax in water also inhibits the hydration of the cement and promotes the formation of porosity in the vicinity of the interface. It can clearly be seen in Figure 14-A that the progression of the CSH gel growth (CSH gel is visible in legend 2 of Figure 14) is inhibited in the deposition zone of the leaked pure PCM oil in the direct vicinity of the matrix/textile interface.

3-2-1-2-d Effect of macrospores in the vicinity of the interface

Figure 15 shows another SEM image of a 15wt% PCM-TRC composite. The figure shows the existence of macrospores (cavities, macro air bubble) near the 'matrix/textile' interface. These macro cavities are attributed to the presence of PCM agglomerate, which is favoured by the presence of pure PCM wax (PCM nodules) acting as an adhesive between PCM capsules, increasing the viscosity of the pore media and inducing porosity. These macro air bubbles contribute to the global degradation of the 'matrix/textile' interaction efficiency. b, c andd) show also the variability of the PCM effects at the interface level that contributes in different ways to the alteration of the matrix/textile interaction intensity. This induces a disparity between the stress distributions of the textile yarns as the PCM level increases in the PCM-TRC composites. This phenomenon explains the localised filament failures in the 5 and 10wt% PCM-TRC samples, while a more homogeneous and almost simultaneous rupture of the textile yarns was found for the reference (without PCM) TRC composite (see Section 3-1-3-1).

Figure 16 illustrates a qualitative scheme that explains in a simplified manner the distribution of normal stresses at the matrix/textile interface before and after the integration of PCM into the cement matrix. The figure shows the distribution disparity in the normal stresses on the circumferential filaments (external filaments) of the yarn due to the effect of PCM. The evolution of the specific heat measured by the CALVET calorimeter for the 0, 5, 10, 15, and 20wt% PCM-TRC composites as a function of the temperature is presented in Figure 18.

Figure 18 Evolution of PCM-TRC heat capacity during an endothermic cycle

The DSC analysis demonstrates that PCM significantly increases the thermal capacity of PCM-TRC.

The evaluated performance factor T (equation 1), defined as the ratio of the heat stored during an endothermic cycle and the theoretical storable heat Q , is given in Table 5.

Where Q is evaluated by the homogenisation law:

Q = " [ η :% ;< = Cp :% + η #@A BA Cp #@A BA ] (2)
η :% and η #@A BA are the massic rates of the PCM and mortar, respectively. Table 5 Thermal performance factor of PCM-TRC composites

From Table 5 it can be seen that the performance factors are close to 100% with a slight uncertainty due to the relative error in the PCM rates. It can therefore be concluded that the thermal performances of the PCM-TRC composites are roughly conserved despite the damage observed in some PCM microcapsules.

3-Conclusion

This paper focuses on the mechanical and physicochemical characterisation of new PCM-TRC composites derived from the association of a non-paraffinic encapsulated PCM modified cementitious matrix and textile reinforcement.

On the basis of the experimental investigations presented above, the following conclusions can be drawn:

A/ At the matrix scale 1. The addition of encapsulated PCM to the mortar results in a significant reduction in its resistance.

2. A decrease in the mechanical performance of the PCM-mortars is observed when the PCM state changes from the solid to the liquid state. The inability of PCM-mortar matrices to recover all of their initial mechanical strengths upon re-solidification of the PCM is due to microcracks induced by the expansion of the microcapsules during the phase change.

B/ At the composite scale 1. The ductile mechanical behaviour and the matrix/textile charge transfer kinetics in the TRC composites are maintained in the presence of microencapsulated PCM.

2. The mechanical performances of the PCM-TRC composites degrade with increasing PCM content; furthermore, a critical failure mode by delamination between layers of reinforcement is detected for PCM content around 15wt%.

3. A decrease of the textile rate of work is observed when increasing the PCM content in the PCM-TRC composites. This is due to the disorder caused by the PCM in the matrix/textile interface area, as revealed by SEM observations. Specifically:

The agglomeration of PCM capsules surrounding the anhydrous cement grains in the matrix/textile interface zone. This agglomeration limits or even prevents the hydration of cement grains by restricting their access to water and thus their dissolution. This reduces the amount of hydrated products from the cement in the matrix/textile interface zone and consequently limits the contribution of the textile reinforcement to the recovery of tractive efforts.

The deposition of pure PCM wax in the direct vicinity of the matrix/textile interface zone due to the damage of the PCM microcapsules during the hand lay-up and/or mixing procedures, which inhibits the progression of hydrates and thereby promotes pore formation in the interface zone. The formation of macrospores (cavities) due to the presence of PCM agglomerates in the interface zone, which contributes to the degradation of the matrix/textile interaction efficiency.

4. The temperature has an effect on the mechanical behaviour of the PCM-TRC composites. A drop of tensile strength and a decrease of the textile efficiency work are observed for the 10wt% PCM-TRC composite heated at 40°C in comparison with the composite maintained at 20°C. This is due to the degradation of the matrix/textile interface caused by multicracking, which is due to PCM expansion during its phase change from solid to liquid.

5. A proportion of the PCM microcapsules break during the mixing process and/or the hand layup process of the composites. However, DSC analysis reveal that the thermal energy storage of PCM-TRC composites is not affected.

Figure 1

 1 Figure 1 In-situ hand lay-up technique for PCM-TRC composites 2-2-2 Mechanical characterisation 2-2-2-1 Matrix scale: mechanical characterisation of the PCM-mortar matrix Compression test, three-point bending test and stiffness analysis were realised on 40 mm × 40 mm × 160 mm specimens of the PCM-mortar matrices containing 0, 5, 10, and 15 wt% PCM at temperatures of 20°C and 40°C and an age of 28 days (five samples for each PCM content and at each temperature).

Figure 2

 2 Figure 2 Experimental setup for investigating the effect of temperature 2-2-2-3 Composite scale: mechanical characterisation of PCM-TRC composites. 2-2-2-3-a Mechanical tests at variable PCM rate and constant temperature Thin PCM-TRC specimens (500 mm-length, 70 mm-width, and 10 mm-thick) consisting of PCM-mortar matrix (0, 5, 10, 15 and 20wt% PCM) reinforced with two layers of AR glass fabric (which corresponds to a volume rate of reinforcement of 2.66% and 2.1 % in the warp and weft directions, respectively and a total fiber fraction volume of 4.76%) were tested under unidirectional tensile solicitation in the warp direction of the textile reinforcement (three

Figure 3

 3 Figure 3 Tensile tests on PCM-TRC composites 2-2-2-3-b Mechanical tests at variable temperature and constant PCM rate

Figure 5

 5 Figure 5 Evolution of strain and temperature for (a) 15wt % and (b) 20wt %PCM-mortars

Figure 6 Table 3

 63 Figure 6 Evolution of longitudinal strain versus temperature for PCM-mortar samples

Figure 7

 7 Figure 7 Stress versus strain behaviour of PCM-TRC composites during tensile test at T=20°C

Figure 9

 9 Figure 9 Effect of PCM phase change on the mechanical behaviour of 10% PCM-TRC

Figure 10

 10 Figure 10 Effect of PCM phase change on the failure mode of the 10wt% PCM-TRC sample

3- 2

 2 Physico-chemical characterisation of the PCM-TRC composites 3-2-1 SEM observations 3-2-1-1 SEM observations in the control TRC composite (without PCM)

Figure 11 Figure 12 ,

 1112 Figure 11 SEM image of the reference TRC composite (without PCM) The very low porosity at the interface also allows a homogeneous distribution of stresses between the external textile filaments, which are in direct contact with the matrix. This explains the simultaneous failure of the textile yarns during the tensile test on the reference TRC specimens (see Section 3-1-3-1) 3-2-1-2 SEM observations in the PCM-TRC composites 3-2-1-2-a PCM-particles identification Figure 12, which was obtained by the BSE detector, shows the microstructure of a small amount of PCM powder sheared between two pieces of glass. Manual shearing allows distinguishing intact PCM particles from damaged ones.

Figure 12

 12 Figure 12 SEM observation of PCM powder with damaged and undamaged capsules

3- 2

 2 Figure[START_REF] Eddhahak-Ouni | Experimental and multi-scale analysis of the thermal properties of Portland cement concretes embedded with microencapsulated phase change materials (PCMs)[END_REF] presents an SEM image of a 15wt% PCM-TRC composite. The objective here is to determine the effect of PCM on the hydration of cement near the matrix/textile interface.

Figure 13

 13 Figure 13 SEM image illustrating the effect of PCM on cement hydration near the interface

Figure 14

 14 Figure 14 SEM image illustrating the effect of PCM wax leakage near the interface Figure 14 clearly shows the existence of large quantities of pure PCM wax nodules resulting from the damage of PCM microcapsules (previously identified in the SEM analysis of the PCM microcapsules and visible in the legend 3 of Figure 14) in the matrix/textile interface zone. The presence of PCM wax is confirmed by the energy-dispersive X-ray spectroscopy analysis in the red highlighted zone in Figure 14-B. In fact, the intense carbon peak and the less pronounced oxygen peak are attributed to the pure PCM oil, which has a semi-developed chemical formulation of CH -CH (14)-C(O)-O-CH .

Figure 15

 15 Figure 15 SEM observations illustrating the existence of macrospores near the interface The phenomena discussed above (Section 3-2-1-2, b, c and d) show that the loss of bond between mortar and textiles due to the reduced interface surface (that can be a assessed in the different SEM images Section 3-2-1-2, b, c and d) is one of the main reasons for the decrease of the textile rate of work when increasing PCM content. The phenomena discussed above (Section 3-2-1-2,b, c and d) show also the variability of the PCM effects at the interface level that contributes in different ways to the alteration of the matrix/textile interaction intensity. This induces a disparity between the stress distributions of the textile yarns as the PCM level increases in the PCM-TRC composites. This phenomenon explains the localised filament failures in the 5 and 10wt% PCM-TRC samples, while a more homogeneous and almost simultaneous rupture of the textile yarns was found for the reference (without PCM) TRC composite (see Section 3-1-3-1).

Figure 16 2 Figure 17

 16217 Figure[START_REF] Aguayo | The influence of microencapsulated (PCM) characteristics on the microstructure and strength of cementitious composites: Experiments and finite element simulations[END_REF] Distribution of stresses in a textile yarn section before and after integration of PCM

see table 2)].

	c		0.48	0.48	0.48	0.48
	slump (mm)	182	185	178	179
		Table 2 PCM-mortar mixture compositions
	2-2-1-2 casting of the 'PCM-TRC' composites
	of surface dried (24h in	Mass of saturated PCM powder	Water absorption
	100°C oven) PCM powder M (g)	after 24h immersion M (g)	coefficient (	) in %
	100		154		54
		Table 1 Test results of water absorption capacity of PCM powder
	The mix composition of the PCM-mortar matrix are presented Table 2 [ is the total water to
	cement ratio,	is the ratio of water not absorbed by the PCM to cement, which is maintained
	constant in all the mix composition and leads to a quasi-constant slump (equivalent workability)
	in all the mix compositions (The 40 mm × 40 mm × 160 mm specimens used for the mechanical tests on the PCM-mortar
	matrix at an age of 28 days were cast and cured under constant laboratory conditions (20°C).
	PCM-mortars	Reference	5wt%	10wt%	15wt%
	PCM (kg)	0	0.3	0.6	0.9
	Cement (kg)	1.65	1.65	1.65	1.65
	Fine aggregate(kg	4.35	4.35	4.35	4.35
	Water (L)	0.80	0.96	1.12	1.29
			0.48	0.58	0.68	0.78
	c

Table 4

 4 presents the evolution of main PCM-TRC characteristics according to PCM rate.

	PCM-TRC	(GPa)	(MPa)	(MPa)	(MPa)
	Reference	31.2 ±1.2	4.15±0.18	516±23	10.85±0.5
	5wt% PCM-TRC	20.1 ±0.8	2.92±0.13	448±27	8.64± 0.51
	10wt% PCM-TRC	12 ±0.5	2.0±0.09	348±21	6.54± 0.4
	15wt% PCM-TRC	5 ±0.2	1.05±0.05	233±19	5.17± 0.41
	20wt% PCM-TRC	2.1±0.1	0.52±0.02	30±3	2.02± 0.21
	: modulus of PCM-mortar matrix, : stiffness modulus of PCM-TRC in phase II-b
	: stress at the initiation of multicracking,	: stress at composite failure

Table 4

 4 Evolution of PCM-TRC characteristics during the tensile test

Acknowledgement

The authors thanks KAST company (Sonthofen, Germany) for providing the textile reinforcement used in the study

Funding:

The Academic research community (ARC ENERGIES and SCUSI) of the Rhone-Alpes and Auvergne FRANCE region funded this study The authors declare that they have no conflicts of interests.

The main results that can be derived from Figure 4 can be summarised as follows: