Nouredine Oueldna

Alain Portavoce
email: alain.portavoce@im2np.fr

Maxime Bertoglio

Andréa Campos

Abelkhalek Kammouni

Khalid

Khalid Hoummada

Phase transitions in

Keywords:

come

Introduction

The increase of functionality of mobile technologies has led to an increase of energy consumption, limiting the autonomy of mobile devices. In this context, new technologies allowing for environmental energy harvesting (EH) are currently investigated [START_REF] Tritt | Thermoelectric phenomena, materials, and applications[END_REF][START_REF] Flage-Larsen | Thermoelectrics and its Energy Harvesting: Materials, Preparation, and Characterization in thermoelectrics[END_REF][START_REF] Liu | Materials for near-room temperatures[END_REF][START_REF] Tan | Rationally designing high-performance bulk thermoelectric materials[END_REF][START_REF] Twaha | A comprehensive review of thermoelectric technology: materials, applications, modelling and performance improvement[END_REF]. These energy-harvesting devices should enable the autonomy of mobile devices to be extended by either partially recharging the battery of the device or allowing some operations to be performed without using the energy stored in the battery.

In the case of technologies such as micro-sensors, microelectronics integrated circuits (IC) etc., for which part of the energy is thermally lost, the integration of thermoelectric devices appears as a potential EH solution [START_REF] Perez-Marín | Micropower thermoelectric generator from thin Si membranes[END_REF][START_REF] Moorthy | Thin Film Structures in Energy Applications[END_REF][START_REF] Pineda | Thermoelectric Energy Conversion: Basic Concepts and Device Applications[END_REF][START_REF] Bell | Cooling, heating, generating power, and recovering waste heat with thermoelectric systems[END_REF][START_REF] Li | Theoretical insight into magnetic and thermoelectric properties of Au doped ZnO compounds using density functional theory[END_REF][START_REF] Ahmoum | Structural, morphological and transport properties of Ni doped ZnO thin films deposited by thermal co-evaporation method[END_REF]. For this purpose, the integrated thermoelectric device should be based on nanostructures, such as thin films, elaborated with industrial processes compatible with the complementary metal oxide semiconductor (CMOS) technology [START_REF] Gambino | Silicides and ohmic contacts[END_REF][START_REF] Park | Energy harvesting using thermoelectricity for IoT (internet of things) and E-skin sensors[END_REF][START_REF] Feng | Flexible, High-power density, wearable thermoelectric nanogenerator and self-powered temperature sensor[END_REF][START_REF] Kim | Self-powered wearable electrocardiography using a wearable thermoelectric power generator[END_REF], and the material application domain should be around room temperature. Bismuth telluride based compounds are known as the best materials operating at room temperature and have attracted a wide attention [START_REF] Snyder | Complex thermoelectric materials[END_REF][START_REF] Heremans | Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states[END_REF][START_REF] Wright | Thermoelectric properties of bismuth telluride and its alloys[END_REF]. However, these materials are toxic and scarce [START_REF] Emsley | Nature's Building Blocks: Everything You Need to Know about the Elements[END_REF][START_REF] Lide | CRC Handbook of Chemistry and Physics[END_REF] as well as not CMOS-compatible. Hence, the development of alternative CMOS-compatible high-efficient thermoelectric materials made from non-toxic and earth-abundant elements is crucial.

Tremendous research studies on bulk MgAgSb-based materials have been conducted since 2012. MgAgSb was first described as a promising p-type thermoelectric material by Kirkham et al. and Zhao et al. [START_REF] Kirkham | Abinitio determination of crystal structures of the thermoelectric material MgAgSb[END_REF][START_REF] Zhao | High thermoelectric performance of MgAgSb-based materials[END_REF]. They studied the different crystal structures of MgAgSb and reported a high thermoelectric figure of merit ZT ~ 1 at room temperature (RT) for the phase α-MgAgSb, which can be increased by doping [START_REF] Zhao | High thermoelectric performance of MgAgSb-based materials[END_REF][START_REF] Liu | High thermoelectric performance of α-MgAgSb for power generation[END_REF][START_REF] Ying | High performance α-MgAgSb thermoelectric materials for low temperature power generation[END_REF][START_REF] Sui | Effect of Cu concentration on thermoelectric properties of nanostructured p-type MgAg 0.97-x Cu x Sb 0.99[END_REF][START_REF] Shuai | Study on thermoelectric performance by Na doping in nanostructured Mg 1-x Na x Ag 0.97 Sb 0.99[END_REF][START_REF] Liu | The microscopic origin of low thermal conductivity for enhanced thermoelectric performance of Yb doped MgAgSb[END_REF][START_REF] Zheng | Extraordinary thermoelectric performance in MgAgSb alloy with ultralow thermal conductivity[END_REF][START_REF] Zheng | Cost effective synthesis of p-type Zn-doped MgAgSb by planetary ball-milling with enhanced thermoelectric properties[END_REF]. Compared to the traditional Bi 2 Te 3 based solid solutions, the elements in the composition of α-MgAgSb are in extremely high abundance and have a low toxicity.

Bulk MgAgSb presents three different crystal structures as a function of temperature: the phase α at RT, the phase β at intermediate temperatures, and the phase γ at high temperatures [START_REF] Ren | Advanced Thermoelectrics Materials, Contacts, Devices, and Systems[END_REF]. α-MgAgSb has a tetragonal structure corresponding to the space group (I4c2). The RT lattice parameters are a = 9.1631 Å and c = 12.701 Å [START_REF] Mi | Elaborating the crystal structures of MgAgSb thermoelectric compound: polymorphs and atomic disorders[END_REF]. The Ag site-filling pattern for this phase forms one-dimensional chains of unfilled Mg-Sb pseudo-cubes in all three primary directions. The resulting unit cell is a superlattice with three different types of Ag atoms (Ag 1 , Ag 2 and Ag 3 ) depending on their position. β-MgAgSb also crystallizes in a tetragonal structure corresponding to the space group (P4/nmm). It is stable between 588 and 633 K. The structure is similar to the Cu 2 Sb structure with Mg and Ag atoms occupying one of the two Cu sites. γ-MgAgSb is a half-Heusler with a cubic structure corresponding to the space group (F4 3 m). It is stable at temperatures higher than 633 K, but was also observed at RT after cooling [START_REF] Kirkham | Abinitio determination of crystal structures of the thermoelectric material MgAgSb[END_REF]. Among these three phases, only α-MgAgSb presents interesting thermoelectric properties close to RT. However, achieving the fabrication of a homogeneous α phase remains a serious challenge, while its thermoelectric properties are considerably influenced by the presence of undesired phases such as Ag 3 Sb [START_REF] Kirkham | Abinitio determination of crystal structures of the thermoelectric material MgAgSb[END_REF][START_REF] Zhao | High thermoelectric performance of MgAgSb-based materials[END_REF][START_REF] Liu | High thermoelectric performance of α-MgAgSb for power generation[END_REF][START_REF] Ying | High performance α-MgAgSb thermoelectric materials for low temperature power generation[END_REF][START_REF] Liu | Effects of antimony content in MgAg 0.97 Sb x on output power and energy conversion efficiency[END_REF][START_REF] Liu | Lithium doping to enhance thermoelectric performance of MgAgSb with weak electron-phonon coupling[END_REF][START_REF] Lei | Enhancement of thermoelectric figure of merit by the insertion of multi-walled carbon nanotubes in α-MgAgSb[END_REF][START_REF] Liu | Understanding and manipulating the intrinsic point defect in α-MgAgSb for higher thermoelectric performance[END_REF][START_REF] Li | Atomic disorders induced by silver and magnesium ion migrations favor high thermoelectric performance in α-MgAgSb-based materials[END_REF][START_REF] Lei | Engineering electrical transport in α-MgAgSb to realize high performances near room temperature[END_REF]. Indeed, contrasting with other alloys [START_REF] Luo | Dopant-segregation to grain boundaries controls electrical conductivity of ntype NbCo(Pt)Sn half-Heusler alloy mediating thermoelectric performance[END_REF], Mg-Ag-Sb secondary phases are detrimental to α-MgAgSb thermoelectric properties. For example, a bulk α-MgAgSb matrix containing the secondary phases Ag 3 Sb and Sb shows a reduced ZT ~ 0.5 [START_REF] Kirkham | Abinitio determination of crystal structures of the thermoelectric material MgAgSb[END_REF]. The difficulty of obtaining a homogeneous α phase is generally attributed to a very small equilibrium window (temperature and composition) of this ternary phase. However, no data are currently available regarding the ternary phase diagram of Mg-Ag-Sb at low temperatures. Frost [START_REF] Frost | The system silver-magnesium-antimony, with reference to the theory of alloy formation[END_REF] gives the Mg-Ag-Sb ternary phase diagram only at 450 °C and 550 °C, and the stability limits of the phase α-MgAgSb at these temperatures are not in agreement with experimental data reported at RT [START_REF] Liu | High thermoelectric performance of α-MgAgSb for power generation[END_REF].

To date, MgAgSb has been only studied in its bulk state using elaboration methods incompatible with the CMOS technology. However, α-MgAgSb thin films produced by CMOS-compatible magnetron sputtering may be used for the integration of EH devices in microelectronics technology [START_REF] Perez-Marín | Micropower thermoelectric generator from thin Si membranes[END_REF][START_REF] Kittl | Silicides and germanides for nano-CMOS applications[END_REF][START_REF] Oueldna | Seebeck coefficient in multiphase thin films[END_REF][START_REF] Portavoce | Thermoelectric power factor of Ge 1-x Sn x thin films[END_REF]. Furthermore, α-MgAgSb nanocrystalline thin films may present improved thermoelectric properties due to nano-effects such as a higher Seebeck coefficients (charge carrier energy filtering [START_REF] Gayner | Energy filtering of charge carriers: current trends, challenges, and prospects for thermoelectric materials[END_REF][START_REF] Neophytou | Nanostructured potential well/barrier engineering for realizing unprecedentedly large thermoelectric power factors[END_REF][START_REF] Narducci | Silicon de novo: energy filtering and enhanced thermoelectric performances of nanocrystalline silicon and silicon alloys[END_REF]) or a lower thermal conductivity [START_REF] Zheng | Extraordinary thermoelectric performance in MgAgSb alloy with ultralow thermal conductivity[END_REF]. Thus, it is important to study the stability of α-MgAgSb in thin films as well as their properties for thermoelectric applications.

This work reports the fabrication of 300 nm-thick Mg-Ag-Sb films by magnetron sputtering using two different methods (alloyed target or co-sputtering), as well as the study of Mg-Ag-Sb phase stability up to 823 K in the thin films using in situ X-ray diffraction (XRD). Despite the significant control offered by the magnetron sputtering technique on the amount of deposited materials (controlled thickness below 3 nm), elaboration of homogeneous α-MgAgSb films is not trivial. The results show that the phase transitions α-to-β, β-to-γ, and α-to-γ are not allotropic, and that the phases α-MgAgSb and γ-MgAgSb are not stoichiometric compounds.

Experiments

300 nm-thick Mg-Ag-Sb films were deposited on the silicon native oxide of commercial Si(001) wafers using a commercial magnetron sputtering setup exhibiting a base pressure of 10 -8 Torr. The thin SiO 2 layer was used as a diffusion barrier, preventing any interactions between the film and Si. All the samples were deposited at RT using the same 99.9999% pure argon gas flow (3.10 -3 mbar). The samples were rotating at a rate of 5 rpm during deposition in order to improve thickness homogeneity.

Two different deposition conditions were used: for the first set of samples (#1), the Mg-Ag-Sb film was obtained from the 150 W DC sputtering of a single 99.99% pure Mg-Ag-Sb alloyed target exhibiting the averaged Mg:Ag:Sb stoichiometry 1/3:1/3:1/3 purchased from CODEX international. The Mg-Ag-Sb film growth rate was 1.41 nm s -1 . For the second set of samples (#2), the Mg-Ag-Sb film was obtained from the co-sputtering of three 99.999% pure Ag, Mg and Sb elementary targets supplied by CODEX international. The Mg and Sb targets were sputtered in the DC mode, while the Ag target was sputtered in the RF mode. The deposition flux of each target was calibrated thanks to the measurement by X-ray reflectivity of the thickness of pure Mg, Ag, or Sb films deposited at RT on the native SiO 2 layer of Si(001) substrates, considering a sticking coefficient of unity. The sputtering power of the Mg and Sb targets was set to 133 W, while the sputtering power of the Ag target was set to 150 W, corresponding to the stoichiometry Mg 1/3 Ag 1/3 Sb 1/3 and to a global growth rate of 5.17 nm s -1 . The composition of the as-deposited films was determined using Energy Dispersive X-ray Spectroscopy (EDS) measurements, performed with an EDAX Octane Silicon Drift Detector (129 eV energy resolution for Manganese) on a Zeiss Gemini 500 field emission gun scanning electron microscope (SEM) operating at 10 kV. In situ XRD measurements were performed in the Bragg-Brentano geometry (θ-2θ), using a Cu Kα source (λ Kα = 0.154 nm). The samples were loaded in an Anton Paar TTK 600 chamber under vacuum (10 -6 Torr) equipped with a heating stage, in a Philips X′pert MPD system equipped with an X′celerator PSD detector designed for high-speed data collection. The temperature (T) was increased from 303 to 823 K using 5 K min -1 steps and five minute-long XRD acquisitions, which correspond to an average temperature ramp of 0.5 K min -1 . The phase identification was performed using the X′Pert High Score software package for diffraction data analysis (files #96-154-3082 for α-MgAgSb, #96-154-3081 for β-MgAgSb, #96-154-3080 for γ-MgAgSb, #96-101-1093 for Ag 3 Sb, and #00-035-0732 for Sb).

Results and discussion

Phase transitions in thin films sputtered from the Mg 1/3 Ag 1/3 Sb 1/3 alloyed target

Fig. 1a shows the diffractograms acquired during in situ annealing of samples #1. The XRD intensity is presented as color gradients (from blue to red for low to high intensity) versus the Bragg-Brentano angle 2θ (20-65°) on the abscissa axis and the annealing temperature on the ordinate axis. The as-deposited film already presents five diffraction peaks at RT at 2θ = 34.42°, 37.21°, 39.39°, 51.72°, and 61.77°, respectively corresponding to the atomic planes (101), (020), ( 111), (022), and (103) of the same binary phase Ag 3 Sb (Fig. 1). The XRD peaks correspond to Ag 3 Sb nano-grains distributed in an amorphous matrix [START_REF] Oueldna | Seebeck coefficient in multiphase thin films[END_REF]. The intensity of these diffraction peaks increases during annealing due to the growth of the Ag 3 Sb nanograins, and at T ~ 433 K eleven additional diffraction peaks appear simultaneously (Fig. 1b). Eight diffraction peaks at 2θ = 24.2°, 26.14°, 31.68°, 41.12°, 44.75°, 53.37°, 58.1°, and 59.39° respectively belong to the atomic planes (202), (220), (114), (411), (422), (433), (008), and (514) of the phase α-MgAgSb; and three peaks at 2θ = 28.88°, 40.18°, and 42.12° respectively belong to the atomic planes (012), (104), and (110) of the Sb crystal. The diffraction peaks of the phase α-MgAgSb disappear at T ~ 713 K, concomitantly with the appearance of six peaks at 2θ = 23.09°, 38.01°, 44.93°, 54.85°, 60.25°, and 61.03° respectively corresponding to the atomic planes (111), (022), ( 222), (040), (133), and (042) of the phase γ-MgAgSb (Fig. 1a). The phase transitions observed during the annealing ramp correspond to the three steps: (RT) Ag 3 Sb → (433 K) Ag 3 Sb + Sb + α → (713 K) Ag 3 Sb + Sb + γ. According to bulk observations [START_REF] Kirkham | Abinitio determination of crystal structures of the thermoelectric material MgAgSb[END_REF], α-MgAgSb is expected to transform into β-MgAgSb at T ~ 588 K, and β-MgAgSb is expected to transform into γ-MgAgSb at T ~ 633 K. β-MgAgSb is not observed in the present case, and the stability domain of α-MgAgSb is extended from 588 K to 713 K (ΔT = 125 K). But as observed in bulk, the phase α coexists with the two undesired phases Ag 3 Sb and Sb. Fig. 1b shows the variations of the integrated diffraction peak intensities of each phase during the ramp annealing. The figure presents only the variations of the highest intensity peak of each phase. Indeed, during growth or consumption, the intensities of all the diffraction peaks of a same phase vary simultaneously in the same way and at the same rate. The presented measurements being normalized, the addition of the contribution of all the peaks gives the same results as that obtained from the diffraction peak with the highest intensity. As an example, Fig. 1c presents the integrated intensity variations of all the diffraction peaks of Ag 3 Sb normalized to their maximum during the ramp annealing. All the peak intensity variations are similar, only the peaks (020) and (103) present some fluctuations. Fig. 1d presents the same intensity variations normalized to the maximum intensity of the peak (111) that shows the highest diffraction intensity. It shows that the (020) and (103) peak fluctuations are actually due to detection limit, as the peaks (020) and (103) present the lowest intensities. Furthermore, considering the contribution of other peaks than that of the peak (111) does not significantly change the results. Fig. 1b shows that part of Ag 3 Sb is consumed during the formation of the phase α, while Ag and Sb are rejected during the α-to-γ transition, as the diffracted intensity (i.e. the volume) of Ag 3 Sb and Sb increases with the increase of the diffracted intensity of the phase γ. These observations lead to the conclusion that: i) the film stoichiometry does not correspond to the stoichiometry of the phase α in the investigated temperature range (above 433 K), the film contains an excess of Ag and Sb compared to Mg (no Mg phase observed); and ii) the phases α and γ do not possess the same stoichiometry, α is richer in both Ag and Sb.

Fig. 2a presents in situ XRD measurements acquired during the sample cooling under vacuum (heater switched off). The phase transition observed during cooling corresponds to the two steps: (823 K) Ag 3 Sb + Sb + γ → (513 K) Ag 3 Sb + Sb + γ + α, which is identical to the observations made in bulk [START_REF] Kirkham | Abinitio determination of crystal structures of the thermoelectric material MgAgSb[END_REF]. Atom transport is progressively reduced during cooling, and only allotropic phase transitions are generally observed. The phase α appears at T = 513 K during cooling and its diffraction intensity increase is proportional to the diffraction intensity decrease of the phase γ (Fig. 2b). However, only a small fraction of γ is changed to α, and the main fraction of γ is still observed back to RT. This is in agreement with a different stoichiometry between the phases α and γ, as in this case atomic transport is needed for the transformation, and is prevented due to the temperature decrease below 513 K. Indeed, Sb diffracted intensity is constant during cooling (Fig. 2b). However, the diffracted intensity of Ag 3 Sb increases during cooling from T ~ 650 K down to T ~ 450 K (Fig. 2b). The reason for this behavior is not obvious, but the increase of Ag 3 Sb XRD intensity appears to be independent of the simultaneous growth of α and respectively consumption of γ, as it starts at a temperature more than one hundred of Kelvin higher than the γ-to-α transition. One can note that the α-to-γ transition occurs at T = 693 K during heating, while the γ-to-α transition occurs at T = 513 K during cooling, corresponding to a ΔT ~ 200 K. The α-to-γ transition is not observed in bulk as the phase β is detected before the phase γ. Nevertheless, the γ-to-α transition temperature in the thin film is close to that in bulk (T ~ 473 K). The shift of the γ-MgAgSb(111) peak observed versus temperature in Fig. 2a is mainly related to thermal expansion. Fig. 3 between the planes (111) of γ-MgAgSb recorded during cooling. It is compared to the coefficient of thermal expansion calculated by Wang et al. [START_REF] Wang | Structural, elastic, electronic, and thermodynamic properties of MgAgSb investigated by density functional theory[END_REF]. The coefficient calculated by Wang et al. is in average about four times larger than the coefficient determined experimentally. Considering that thermal expansion may not be isotropic, our data corresponding to (111) planes nevertheless suggest that the shift of the γ-MgAgSb(111) diffraction peak is mainly related to thermal expansion.

Phase stability in relaxed bulk at given temperature depends on composition. However, phase stability can be more complex in thin films, as it includes surface and interface energy minimization effects, as well as stress effects due to the difference of volume and/or thermal expansion between the coexistent phases. Furthermore, phase nucleation in thin films is influenced by surface and interfaces (surface segregation and hetero-nucleation). Even so, missing the phase β-MgAgSb in the phase formation sequence during heating suggests that the film composition is different from the Mg 1/3 Ag 1/ 3 Sb 1/3 stoichiometry. The composition of the as-deposited film was found to be 27 at% of Mg, 38 at% of Ag, and 35 at% of Sb using EDS measurements. This composition is different from the stoichiometry Mg 1/3 Ag 1/3 Sb 1/3 , but is still not too far off considering the usual EDS 2-3 at% error [START_REF] Portavoce | Ferromagnetic MnCoGe thin films produced via magnetron sputtering and non-diffusive reaction[END_REF]. The overall composition of the sputtered target corresponds to Mg 1/3 Ag 1/3 Sb 1/3 . However, the target is crystallized, and thus, contains different phases with probably an inhomogeneous distribution. Consequently, the sputtering rates of the elements Mg, Ag and Sb from the different phases in this target can be different and are not controlled. For example, the melting temperature of Mg (923 K) and Sb (904 K) being similar, their sputtering rates are expected to be comparable. However, their sputtering rate is expected to be significantly different from that of Ag, as the melting temperature of Ag (1235 K) is much higher. Thus, similar experiments are reported in the next section performed on samples #2. In this case, the Mg-Ag-Sb film exhibits the same thickness and was deposited using sputtering powers adjusted for each of the three elementary targets in order to independently control the elementary fluxes of Mg, Ag, and Sb. As expected, flux calibrations showed that the same sputtering power is used in order to get same Mg or Sb deposition fluxes, but a different sputtering power is needed for Ag. This way, the stoichiometry of samples #2 is expected to be Mg 1/3 Ag 1/3 Sb 1/3 with an error below 1%. 103), (004), (203), and (220) of the phase β-MgAgSb (Figs. 4 and5). At the same time, Sb XRD intensity is constant (Fig. 5). Finally, at T = 673 K the β XRD peaks disappear concurrently with the appearance of two peaks at 2θ = 23.07° and 2θ = 47.07°, corresponding to γ-MgAgSb(111) and γ-MgAgSb(222), respectively. Furthermore, the diffracted intensities of the phases Ag 3 Sb and Sb increase during the growth of the phase γ up to 720 K (Fig. 5). At higher temperature, the diffracted intensities of Sb and of the phase γ are almost constant, while the diffracted intensity of Ag 3 Sb decreases up to 823 K. In summary, the phase transitions observed during the annealing ramp correspond to the four steps: (RT) Ag 3 Sb → (423 K) Ag 3 Sb + Sb + α → (623 K) Sb + β → (673 K) Sb + Ag 3 Sb + γ. β-MgAgSb is now observed in the phase sequence. The phase transitions versus temperature are in better agreement with the transitions reported in the case of bulk material suggesting that the overall composition of the film is closer to that of the experiments reported on bulk material. The composition of the as-deposited film was found to be 33 at% of Mg, 30 at% of Ag, and 37 at% of Sb using EDS measurements, which is close to the stoichiometry Mg 1/3 Ag 1/3 Sb 1/3 considering EDS measurement error. However, three differences can be mentioned compared to bulk samples: i) the RT initial state of the asdeposited film before the formation of the phase α corresponds to an amorphous film containing Ag 3 Sb nanocrystals, while the bulk samples already contained the three phases Ag 3 Sb, Sb, and α-MgAgSb at RT after elaboration; ii) Ag 3 Sb vanishes during the β-to-γ transition in thin films, while it is always present during annealing of bulk samples; and iii) the α-to-β and β-to-γ transitions occur at higher temperatures in thin films: ΔT = 623 -588 = 35 K for α-to-β and ΔT = 673-633 = 40 K for β-to-γ.

Phase transitions in thin films sputtered from Mg, Ag, and Sb elementary targets

Integration of the diffraction peak intensities (Fig. 5) shows that part of Ag 3 Sb is consumed during the formation of the phases Sb and α, as observed during annealing of samples #1. Furthermore, Ag 3 Sb is fully consumed during the α-to-β transition, while the Sb volume slightly increases, meaning that the phase β incorporated the Ag atoms contained in the Ag 3 Sb phase, but not all the Sb coming from the same phase. However, Ag 3 Sb reappears and the Sb volume again increases during the β-to-γ transition, meaning that Ag and Sb are rejected from the phase γ. The consumption of the phase Ag 3 Sb observed for T > 720 K while the volumes of Sb and γ-MgAgSb are almost constant was also observed during the annealing of samples #1 (Fig. 1b). In the two cases (samples #1 and #2), the DRX intensity decrease is observed on all the detected peaks of Ag 3 Sb, and thus, cannot be explained by Ag 3 Sb grain growth competition. However, Ag 3 Sb consumption could be related to an increase of Ag "solubility" in the phase γ with temperature. These observations lead to the conclusion that: i) the stoichiometry of the films #2 correspond to the stoichiometry Mg 1/3 Ag 1/3 Sb 1/3 , but this stoichiometry may not correspond to the phase α in the investigated temperature range (above 433 K); ii) the phases α and β do not possess the same stoichiometry, β is significantly richer in Ag and richer in Sb; and iii) the phases β and γ do not possess the same stoichiometry, β is also significantly richer in Ag and Sb than γ, but the stoichiometry of γ may largely depends on temperature, the concentration of Ag and probably Sb increasing with temperature. the samples #1 as well as in bulk [START_REF] Kirkham | Abinitio determination of crystal structures of the thermoelectric material MgAgSb[END_REF]. However, the phase α appears at T = 473 K instead of 513 K for samples #1, which is the same temperature as in bulk. Similar to samples #1, the XRD intensity increase of the phase α is proportional to the XRD intensity decrease of the phase γ (Fig. 6b), and only a small fraction of γ is changed to α, the main fraction of γ being observed at RT. As mentioned before, the stability of γ during cooling could be explained by the difference of stoichiometry between the three phases γ, β, and α, and the atom mobility reduction due to cooling. Similar to the case of samples #1, the volume of the phase Sb is constant during cooling, while the volume of Ag 3 Sb increases and reaches a plateau as well as γ and α at T ~ 400 K, as atomic transport is probably stopped below this temperature. Fig. 3 shows that α d determined from samples #2 cooling is similar to that determined from samples #1.

Conclusion

Phase transitions in 300 nm-thick Mg 1/3 Ag 1/3 Sb 1/3 thin films deposited by magnetron sputtering have been studied using in situ XRD during an average temperature ramp of 0.5 K min -1 . Two types of samples were studied: samples #1 were deposited from a single alloyed target, while samples #2 were deposited from three elementary Mg, Ag, and Sb targets. The use of a single Mg 1/3 Ag 1/3 Sb 1/3 target (samples #1) does not allow a film with the initial target composition to be deposited, probably due to the difference of sputtering rate of the three elements Mg, Ag and Sb in the different phases present in the target. In this case, the phase β-MgAgSb is missing in the phase formation sequence compared to results obtained in bulk. Co-sputtering of three elementary Mg, Ag, and Sb targets (samples #2) allows the composition of the deposited films to be controlled. The binary phase Ag 3 Sb is present in the as-deposited film, as in the reported case of bulk Mg 1/3 Ag 1/3 Sb 1/3 samples, and the phase formation sequence is the same as in bulk materials, excepted that the transition temperatures of the α-to-β and β-to-γ transformations are shifted to higher temperatures (35 ≤ ΔT ≤ 40 K).

The comparison of the XRD intensity of the phases during the ramp annealing of the two samples leads to the conclusion that the phase transitions α-to-β, β-to-γ, and α-to-γ are not allotropic. The phases α, β and γ do not possess the same composition, and α and γ are not stoichiometric compounds. α is richer than γ in both Ag and Sb, and β is significantly richer in Ag and Sb than both α and γ, the composition of α and γ being closer than that of β. These results show that the thermoelectric phase α-MgAgSb should be obtained using a stoichiometry different from 1/3:1/3:1/3, which can vary with temperature. Furthermore, the phase α-MgAgSb accepting significant stoichiometry variations, significant variations of its Seebeck coefficient can be expected depending on elaboration conditions, in agreement with the variations already reported in the literature (74-240 µV K -1 ).

CRediT authorship contribution statement

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  presents the coefficient of thermal expansion α d = ΔT × d/Δd determined from the variations of the distance d

Fig. 1 .

 1 Fig. 1. a) X-ray diffractograms (20° ≤ 2θ ≤ 60°) acquired at different temperatures during in situ ramp annealing of samples #1 from 303 to 753 K; variations of integrated and normalized XRD peak intensities versus temperature measured during the same annealing: b) α-MgAgSb(220) (spheres), γ-MgAgSb(111) (open circles), Ag 3 Sb(101) (open triangles) and Sb(012) (open squares); c) Ag 3 Sb detected peak variations versus temperature normalized to their maximum; and d) Ag 3 Sb detected peak variations versus temperature normalized to the maximum of the peak Ag 3 Sb(111).

Fig. 4

 4 Fig. 4 shows the diffractograms recorded versus temperature during the ramp annealing of samples #2. Similar to samples #1, five diffraction peaks are detected at RT after deposition at 2θ = 34.42°, 37.21°, 39.39°, 51.72°, and 61.77°, corresponding to the atomic planes (101), (020), (111), (022), and (103) of the binary phase Ag 3 Sb. Thirteen additional peaks appear simultaneously at T = 423 K. Ten peaks at 2θ = 23.95°, 31.02°, 39.6°, 41.14°, 41.87°, 46.44°, 46.92°, 56.6°, 57.58°, and 58.8° respectively correspond to the atomic planes (202), (114), (224), (411), (330), (422), (206), (440), (008) and (514) of the phase α-MgAgSb, and three peaks at 2θ = 23.53°, 28.72°, and 48.21°, respectively correspond to the Sb atomic planes (003), (012), and (006). The XRD peaks of the phases Ag 3 Sb and α vanish at T = 623 K while nine additional peaks appear concomitantly at 2θ = 23.97°, 31.45°, 38.97°, 40.85°, 43.02°, 44.55°, 53.29°, 57.97° and 59.08°, respectively corresponding to the atomic planes (101), (111), (112), (200), (201), (103), (004), (203), and (220) of the phase β-MgAgSb (Figs. 4and 5). At the same time, Sb XRD intensity is constant (Fig.5). Finally, at T = 673 K the β XRD peaks disappear concurrently with the appearance of two peaks at 2θ = 23.07° and 2θ = 47.07°, corresponding to γ-MgAgSb(111) and γ-MgAgSb(222), respectively. Furthermore, the diffracted intensities of the phases Ag 3 Sb and Sb increase during the growth of the phase γ up to 720 K (Fig.5). At higher temperature, the diffracted intensities of Sb and of the phase γ are almost constant, while the diffracted intensity of Ag 3 Sb decreases up to 823 K. In summary, the phase transitions observed during the annealing ramp correspond to the four steps: (RT) Ag 3 Sb → (423 K) Ag 3 Sb + Sb + α → (623 K) Sb + β → (673 K) Sb + Ag 3 Sb + γ. β-MgAgSb is now observed in the phase sequence. The phase transitions versus temperature are in better agreement with the transitions reported in the case of bulk material suggesting that the overall composition of the film is closer to that of the experiments reported on bulk material. The composition of the as-deposited film was found to be 33 at% of Mg, 30 at% of Ag, and 37 at% of Sb using EDS

Fig. 3 .

 3 Fig. 3. Coefficient of thermal expansion α d of γ-MgAgSb determined from de variations of the interatomic distance of γ-MgAgSb(111) planes during cooling. The experimental data are compared with calculations from Ref. [46].

Fig. 2 .

 2 Fig. 2. a) X-ray diffractograms (20° ≤ 2θ ≤ 60°) acquired at different temperatures during in situ thermal cooling of samples #1 from 753 to 303 K, b) Variations of integrated and normalized XRD peak intensities versus temperature measured during the cooling: α-MgAgSb(220) (spheres), γ-MgAgSb(111) (open circles), Ag 3 Sb(101) (open triangles) and Sb (012) (open squares).

Fig. 6

 6 presents in situ XRD measurements acquired during the sample cooling under vacuum. The phase transition observed during cooling corresponds to the two steps: (823 K) Ag 3 Sb + Sb + γ → (473 K) Ag 3 Sb + Sb + γ + α. It is identical to the observations made on

Fig. 4 .

 4 Fig. 4. a) X-ray diffractograms (20° ≤ 2θ ≤ 60°) acquired at different temperatures during in situ ramp annealing of samples #2 from 303 to 823 K, b) the same X-ray diffractograms for 21° ≤ 2θ ≤ 26°.

Fig. 5 .

 5 Fig. 5. Variations of integrated and normalized XRD peak intensities versus temperature measured during in situ ramp annealing of samples #2 from 303 to 823 K: α-MgAgSb(220) (spheres), β-MgAgSb(101) (open diamonds), γ-MgAgSb(111) (open circles), Ag 3 Sb(101) (open triangles) and Sb(006) (open squares).

Fig. 6 .

 6 Fig. 6. a) X-ray diffractograms (20° ≤ 2θ ≤ 60°) acquired at different temperatures during in situ cooling of samples #2 from 773 to 313 K, b) Variations of integrated and normalized XRD peak intensities versus temperature measured during the cooling: α-MgAgSb(202) (spheres), γ-MgAgSb(111) (open circles), Ag 3 Sb(101) (open triangles) and Sb (003) (open squares).

Nouredine Oueldna :

 Oueldna Investigation, Writing -original draft, Visualization. Alain Portavoce: Conceptualization, Validation, Writing -original draft, Supervision, Project administration. Maxime Bertoglio: Validation, Resources. Andréa Campos: Investigation. Abelkhalek Kammouni: Validation, Supervision, Funding acquisition. Khalid Hoummada: Conceptualization, Writing -review & editing, Project administration, Funding acquisition.

Journal of Alloys and Compounds 900 (2022) 163534

Acknowledgement

The authors would like to thank Prof. Dr. Dominique Mangelinck for XRD analysis software.

This work was financially supported by CAMPUS FRANCE-PHC TOUBKAL 2018 (French-Morocco bilateral program) Grant number: TBK/18/65.


domain: chim sde

text:
Asier Goñi-Urtiaga

Denis Courtier-Murias

Giuseppe Picca

Juan L Valentin

César Plaza

Marco Panettieri
email: marco.panettieri@csic.es

Response of water-biochar interactions to physical and biochemical aging

Keywords: Biochar, Aging, Co-composting, NMR Relaxometry, Water interactions, Porosity

Biochar aging may affect the interactions of biochar with water and thus its performance as soil amendment; yet the specific mechanisms underlying these effects are poorly understood.

By means of FTIR, N2 adsorption, Hg intrusion porosimetry, thermogravimetric analysis, 13 C solid state nuclear magnetic resonance (NMR) and 1 H NMR relaxometry, we investigated changes in the chemistry and structure of biochar as well as its interaction with water after biochar aging, both physical (simulated by ball-milling) and biochemical (simulated by cocomposting). Three different porosities of biochar were examined: <5 nm, 1 µm and 10 µm diameter sizes. Physical aging caused the disappearance of the porosity at 10 m. With biochemical aging, biochar underwent an enrichment of oxygenated functional groups either as a result of surface functionalisation processes or by the deposition of fresh organic matter layers on the surface and pores of biochar. 1 H NMR relaxometry revealed that the proportion of water strongly interacting with biochar increased with both physical and biochemical aging.

Although biochemical aging significantly altered the composition of biochar surface and modulates its interaction with water, 1 H NMR relaxometry proved that physical aging had a relatively stronger influence on water mobility and dynamics in biochar, lowering both T1 and T2 relaxation times in the initial contact times of biochar and water.

Introduction

In the last decades, the interest in the use of biochar as soil amendment has greatly increased.

Biochar is a carbonaceous material derived from the pyrolysis of biomass with great potential for carbon sequestration while improving soil fertility [START_REF] Joseph | How biochar works, and when it doesn't: A review of mechanisms controlling soil and plant responses to biochar[END_REF][START_REF] Schimmelpfennig | One Step Forward toward Characterization: Some Important Material Properties to Distinguish Biochars[END_REF]. During the pyrolysis process to produce biochar, the biomass feedstock loses most of its labile organic matter leading to a chemically stable and porous material with a considerably high degree of condensed aromatic rings. Biochar has an estimated residence time of hundreds of years when applied to soils [START_REF] Lehman | A hand full of carbon[END_REF] but during degradation suffers chemical and physical alterations, a process known as biochar [START_REF] Wang | Biochar Aging: Mechanisms, Physicochemical Changes, Assessment, And Implications for Field Applications[END_REF][START_REF] Kuzyakov | Biochar stability in soil: Decomposition during eight years and transformation as assessed by compound-specific 14 C analysis[END_REF][START_REF] Gross | Soil Organic Carbon Sequestration after Biochar Application: A Global Meta-Analysis[END_REF]. The characteristics of biochar, its interactions with plants, water and soils, and its persistence within soils depend on the feedstock of biomass and on the temperature and residence time of the pyrolysis process [START_REF] Agegnehu | The role of biochar and biochar-compost in improving soil quality and crop performance: A review[END_REF][START_REF] Fellet | Changes in physical-chemical biochar properties following addition to soil[END_REF].

In recent years, the combination of compost and biochar has been proposed to obtain a fertile mixture for soil amendment. The addition of biochar to compost improves the performance of the composting process, increasing the temperature and duration of the thermophilic phase by the enhancement of microbial activity and capturing potentially toxic metals and organic compounds [START_REF] Kamman | Plant growth improvement mediated by nitrate capture in co-composted biochar[END_REF][START_REF] Wiedner | Acceleration of Biochar Surface Oxidation during Composting[END_REF]. During co-composting, biochar undergoes surface modifications by the addition of surface functional groups which increases its cationic exchange capacity (CEC) and interaction with other chemical compounds present in soil, facilitating nutrient retention as well as acting as a pH buffer [START_REF] Joseph | Microstructural and associated chemical changes during the composting of a high temperature biochar: mechanisms for nitrate, phosphate and other nutrient retention and release[END_REF]. Besides, the high porosity of biochar provides large specific surface area which can greatly improve water holding capacity in soils or plant-growing media [START_REF] Nocentini | Recycling pyrolyzed organic waste from plant nurseries, rice production and shrimp industry as peat substitute in potting substrates[END_REF], maintaining desired moisture and promoting the aforementioned nutrient retention. Some biochar modifications during co-composting are produced by biochemical processes driven by microbial decomposition of organic matter. As reported by [START_REF] Wang | Biochar Aging: Mechanisms, Physicochemical Changes, Assessment, And Implications for Field Applications[END_REF], this can produce an enrichment of oxygenated functional groups on its surface. Besides, they consider that biochar aging within composting piles and within soil after application should be driven by similar biochemical reactions leading to similar aged biochars, but on a different timescale considering co-composting process as a way to accelerate biochar aging [START_REF] Wang | Biochar Aging: Mechanisms, Physicochemical Changes, Assessment, And Implications for Field Applications[END_REF][START_REF] Fischer | Synergisms between Compost and Biochar for Sustainable Soil Amelioration[END_REF]. Some authors proposed that, during this biochemical aging taking place in biochar, a layer of fresh organic matter is formed on the surface of biochar particles and pores, which creates a chemically reactive and nutrient-rich material with beneficial properties for soil fertilisation and plant growth [START_REF] Hagemann | Organic coating on biochar explains its nutrient retention and stimulation of soil fertility[END_REF][START_REF] Baiamonte | Structure alteration of a sandy-clay soil by biochar amendments[END_REF]. To comprehend the mechanism concerning the benefits of aged biochar and the effect on plant-biochar interactions modulated by water holding capacity and nutrient release to the rhizosphere, the role of water interactions in the biochar particles must be understood.

Physical fragmentation is another aging mechanism that may affect biochar porosity, particle size and accessibility to inner porosity, which are key to water holding capacity and water interactions in biochar. This physical aging (or disruption) is caused by soil weathering, freeze/thaw temperature cycles and by tillage with agricultural machinery [START_REF] Wang | Biochar Aging: Mechanisms, Physicochemical Changes, Assessment, And Implications for Field Applications[END_REF]. However, it occurs during long time spans, thus for this study we used ball milling of biochar to reduce particle size, which roughly led to a comparable effect as physical weathering (aging) of biochar in the field. Several authors stated that water retention is mainly dependent on the porosity and particle size of biochar [START_REF] Conte | Nature of water-biochar interface interactions[END_REF][START_REF] Conte | Water dynamics in different biochar fractions[END_REF], proposing different mechanisms where interaggregate porosity (between particles), intra-aggregate porosity (structural pores), zeta potential or poly-condensed carbon ring interaction by H-bonds play the main role in water retention. Thus, a number of studies of literature show that both biochemical and physical aging are key parameters on the modulation of water holding capacity of biochar but their relative importance is still unknown [START_REF] Batista | Effect of surface and porosity of biochar on water holding capacity aiming indirectly at preservation of the Amazon biome[END_REF]. Most of these studies are based on biochars produced from different feedstock materials or pyrolysis conditions, which also affect physical and chemical properties of the final product and, thus, water retention [START_REF] Huang | Effects of pyrolysis temperature, feedstock type and compaction on water retention of biochar amended soil[END_REF]. Moreover, the information on water interactions concerning biochar aged in composted piles or in soils, where the interaction between water and functionalised surface of the material should change compared to that of pristine biochar, mainly deals with quantification or macro-scale processes, while processes at smaller scales are unaddressed [START_REF] Wang | Biochar Aging: Mechanisms, Physicochemical Changes, Assessment, And Implications for Field Applications[END_REF]. In the present investigation, the effects of physical and biochemical aging on the chemical properties of biochar and its interactions with water were investigated. FTIR, porosimetry analysis, 13 C solid state nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA) were used to examine the chemical modification of biochar after physical aging (simulated by ball-milling) and during biochemical aging (simulated by co-composting). This information was used in combination with 1D 1 H NMR relaxometry and, for the first time, with 2D 1 H NMR relaxometry to study water dynamics (i.e. molecular tumbling) in the different pores of biochar at the molecular scale.

There are several examples of 1 H NMR relaxometry (i.e. study of NMR relaxation) applications in porous media in general such as wood, concrete, clay, etc [START_REF] Faure | An MRI-SPI and NMR relaxation study of drying-hydration coupling effect on microstructure of cement-based materials at early age[END_REF][START_REF] Faure | Water transfers within Hemp Lime Concrete followed by NMR[END_REF][START_REF] Faure | Proton NMR relaxation as a probe for setting cement pastes[END_REF]. Concerning wood/lignocellulosic samples, the technique has already been used for some decades [START_REF] Labbé | Moisture content and extractive materials in Maritime Pine wood by low field 1 H NMR[END_REF] but it is gaining popularity in recent years due to the development of low-cost (and -maintenance) low-field NMR spectrometers. These instruments allow to perform time domain (TD)-NMR experiments to obtain NMR relaxation information but can also obtain 1D MRI profiles [START_REF] Penvern | How Bound Water Regulates Wood Drying[END_REF] and diffusion information [START_REF] Telkki | Accelerating Restricted Diffusion NMR Studies with Time-Resolved and Ultrafast Methods[END_REF]. NMR relaxometry allows to study wood materials under different conditions (non-invasively) to quantify water or organic molecules and to obtain information regarding their local environment (i.e. pore size, affinity with molecules/surfaces they are bonded to). The major part of this research focused on studying water adsorbed on ry. For instance, Sharp et al. [START_REF] Sharp | Determination of moisture content of wood by pulsed nuclear magnetic resonance[END_REF] moisture content (MC) and other related studies quantified bound water and free water contents by spin-spin (T2) NMR relaxation time measurements [28 32]. In addition, by means of T2 NMR measurements [START_REF] Araujo | Behaviour of solid wood and bound water as a function of moisture content: a proton magnetic resonance study[END_REF] focused on the separation of hydration and solution water [START_REF] Araujo | Behaviour of solid wood and bound water as a function of moisture content: a proton magnetic resonance study[END_REF]. Recently, [START_REF] Cox | A study of water exchange in wood by means of 2D NMR relaxation correlation and Exchange[END_REF] and [START_REF] Bonnet | NMR determination of sorption isotherms in earlywood and latewood of Douglas fir. Identification of bound water components related to their local environment[END_REF] have used 2D NMR relaxometry experiments via spin lattice (T1)-T2 correlation [START_REF] Cox | A study of water exchange in wood by means of 2D NMR relaxation correlation and Exchange[END_REF][START_REF] Bonnet | NMR determination of sorption isotherms in earlywood and latewood of Douglas fir. Identification of bound water components related to their local environment[END_REF]. This better separate water components detecting two types of bound water in wood cell-wall, which were not differentiated in 1D experiments. Cox et al. also studied liquid water in the lumen, which was identified as the peak with the highest T1 and T2 values and being close to the T1=T2 line. This technique has also been used to study biomass recalcitrance in the presence of liquid water [START_REF] Jeoh | Two-Dimensional 1 H-Nuclear Magnetic Resonance Relaxometry for Understanding Biomass Recalcitrance[END_REF]. Based on these previous works, the current study thus provides a characterization of water-biochar interactions under aging simulated conditions, to optimize the use of biochar for fertility enhancement and carbon sequestration under environmental conditions.

Materials and methods

Biochar, biochemical and physical aging, and sampling

Biochar produced from the pyrolysis of pine (Pinus Halepensis) prune using a low-tech Kon-tiki oven with a high heating temperature (HTT) of 650-750 °C and a residence time of 3-3.5 h was obtained from Carbón Vivo, SCCL (Barcelona, Spain). The pyrolysis conditions (i.e. the absence of oxygen) are assured by the fire curtains created on top of the material in the Kon-tiki oven and quenching was carried out watering the oven from the top using approximately twice the volume of water than the oven capacity. Biochemical aging of biochar was achieved cocomposting a mixture of prune, spent coffee grounds and biochar (1:1:1 v/v) in a 200 L composter during 30 days under monitored temperature (Figure SI-3) and 40-60% humidity conditions. A composite sample of about 500 g of the mixture was collected after 0, 9 and 30 days from five random points ( 100 g per point) within the composting pile and mixed together to assure the homogenization of the sample. Biochar particles were separated by hand from the composite samples using tweezers and selecting 2 mm diameter composted particles. A portion of the samples were ground with a ball mill for 10 min at a frequency of 25 s -1 to simulate physical aging and for physicochemical characterization. Samples were labelled as unaged biochar (BC), physically aged biochar (pa-BC), biochemically aged biochar (ba-BC), and physically and biochemically aged biochar (pba-BC).

Physicochemical characterisation

Particle size was measured using a Mastersizer 3000 laser diffraction particle size analyser (Malvern Panalytical). Fourier transformed infrared spectroscopy (FTIR) was performed in a FTIR-8400 Shimadzu spectrometer [START_REF] Liu | Thermogravimetry-Infrared Spectroscopy Analysis of the Pyrolysis of Willow Leaves, Stems and branches[END_REF]; samples were prepared by grinding 1 mg of biochar materials (previously dried at 105 °C, 24 h) with 200 mg KBr and pressing into a pellet. Infrared spectra were recorded in a wavenumber range of 450 4000 cm -1 . TGA was carried out in a Setaram SETSYS 16/18 TGA-ATD analyser [START_REF] Carballo | Characterization of different compost extracts using Fourier-transform infrared spectroscopy (FTIR) and thermal analysis[END_REF] under He and synthetic air atmospheres in a temperature range of 25 900 °C and 10 K•min -1 heating ramp; 20 mg of the sample was weighted in an alumina crucible for the measurement. Porosity was measured using two different methods targeting micro and macro porosity using N2 adsorption and Hg intrusion, respectively. N2 adsorption porosimetry was determined using a Quantachrome Autosorb AS-1 instrument [START_REF] Sing | The use of nitrogen adsorption for the characterisation of porous Materials[END_REF]. Briefly, biochar samples were degasificated at 150 °C and biochemically aged biochar samples at 105 °C (to avoid thermal decomposition of labile matter), both for 20 h in vacuum conditions. Surface area and pore size distribution were obtained using Brunauer-Emmett-Teller (BET) and Barrett Joyner Halenda (BJH) methods, respectively, using AS1-Win software. Hg intrusion porosimetry was carried out with a AutoPore IV 9510 Micromeritics equipment with a pore size resolution between 0.1 and 360 µm [START_REF] Giesche | Mercury Porosimetry: A General (Practical) Overview[END_REF]. 13 C solid state NMR measurements were performed using a cross-polarization sequence at the magic angle spinning (CP-MAS) [START_REF] Courtier-Murias | Cross polarization-single pulse/magic angle spinning (CPSP/MAS): A robust technique for routine soil analysis by solid-state NMR[END_REF] in a 400 MHz Bruker Advance III spectrometer, using a 4 mm zirconia rotor, 13 kHz spinning speed. A ramped 1 H-pulse from 100 to 50% of the full power was used during a contact time of 1 ms to reduce Hartmann-Hahn mismatches and 500 ms recycle delay between scans. A contact time of 1 ms is frequently used to obtain an efficient coupling between protons and 13 C atoms in environmental samples rich in aromatic rings, such as biochar, as a compromise for a good polarization for the different organic components [START_REF] Courtier-Murias | Cross polarization-single pulse/magic angle spinning (CPSP/MAS): A robust technique for routine soil analysis by solid-state NMR[END_REF][START_REF] Knicker | 2D Solid-State HETCOR 1 H-13 C NMR Experiments with Variable Cross Polarization Times as a Tool for a Better Understanding of the Chemistry of Cellulose-Based Pyrochars A Tutorial[END_REF]. The obtained spectra were divided in six regions contributing to the total measured 13 C intensity and corresponding to the carbon atoms in the following functional groups: alkyl (0-45 ppm), N-alkyl/mehoxyl (45-60 ppm), O-alkyl (60-90 ppm), anomeric (90-110 ppm), aryl (110-140 ppm), heteroaromatic (140-160 ppm) and carboyl/amide (160-220 ppm) [START_REF] Knicker | Soil organic N -An under-rated player for C sequestration in soils?[END_REF][START_REF] Ludemann | Carbon-13 nuclear magnetic resonance spectra of lignins[END_REF]. FTIR, TGA-DSC and particle size analysis were performed on triplicate samples. For 13 C NMR and porosimetry, to reduce experimental time and costs, analyses were performed on one composite sample per treatment.

Measurements of water-biochar interactions using 1 H NMR relaxometry

Three different levels of water content were set up for all biochar samples. Firstly, dry samples were wetted with an excess of deuterated water (D2O) to increase the mobility of structural hydrogen atoms and hence to increase their relaxation time signal without adding hydrogens from water. This way, it is possible to evaluate the structural proton NMR signal from biochar at the experiment conditions and its effect on the subsequent measurements with water (H2O). Secondly, samples were placed in different desiccators for 5 days in specific relative humidity (RH) controlled by a saturated solution of salt; Mg(NO3)2 (RH=55%), NaCl (RH=75%) and pure water (RH=100%), to prepare samples with adsorbed but not free water. Finally, all samples were mixed with deionized water in a 1:4 mass ratio for liquid water analysis. H relaxometry measurements were carried out in a low field (0.5 Tesla) Bruker Minispec mq20 spectrometer. About 80 mg of sample was weighted for each sample and placed in 8 mm diameter glass tube. Temperature was controlled by a BVT3000 temperature controller with air flow of 1200 L h -1 at 30 °C for all measurements. Samples were maintained at these conditions for 30 min before each measurement to ensure temperature control. Transverse NMR relaxation times (1D T2) were estimated using the Carr-Purcell-Meiboom-Gil (CPMG) sequence [START_REF] Carr | Effects of Diffusion on Free Precession in Nuclear Magnetic Resonance Experiments[END_REF]46], using the following test variables (i) for controlled % RH samples: 0.08 ms echo time repeated 625 times, 1 s recycle delay and 96 scans and (ii) for samples with liquid water: 0.5 ms echo time repeated 10000 times, 10 s recycle delay and 96 scans. The obtained data was treated with CONTIN-based home-made method, as reported in previous work [START_REF] Faure | Proton NMR relaxation as a probe for setting cement pastes[END_REF] to obtain 1D T2 distributions.

Finally, 2D NMR correlation spectra of longitudinal (T1) and transverse (T2) relaxation times were obtained using a coupled sequence of Inversion Recovery and the CPMG. For the Inversion Recovery experiments, the delay was increased logarithmically in 48 steps from 0.01 to 3000 ms. The CPMG block utilised an echo time of 0.5 ms, among which 300 echoes where recorded following a logarithmic series from 0.5 to 1000 ms. For this 2D sequence, 4 scans were accumulated with a recycle delay of 5 s. T1-T2 correlation spectra were calculated using an in-house software, reproducing 2D-inverse Laplace transform (ILT) algorithm [START_REF] Song | T1 T2 Correlation Spectra Obtained Using a Fast Two-Dimensional Laplace Inversion[END_REF]. For further information, refer to Bonnet et al. 2017 [START_REF] Bonnet | NMR determination of sorption isotherms in earlywood and latewood of Douglas fir. Identification of bound water components related to their local environment[END_REF] and Rostom et al. 2019 [START_REF] Rostom | Investigation of the effect of aging on wood hygroscopicity by 2D 1 H NMR relaxometry[END_REF].

In general, 1D T2 NMR relaxometry is used to distinguish bound and free water (being the main advantage a short experimental time lasting few minutes) and 2D T1T2 correlation NMR relaxometry to improve resolution and obtain physical and chemical information on the samples [START_REF] Rostom | Analysis of water content in wood material through 1D and 2D 1 H NMR relaxometry: Application to the determination of the dry mass of wood[END_REF]. Concerning data interpretation, the area under each peak of either 1D or 2D spectra is proportional to the amount of H atoms and thus water molecules. 1D spectra will allow to quantify the amount of water in different environments. For 2D spectra, the T1 and T2 NMR values of each peak give unambiguous access to T1-T2 ratios. This is characteristic of H atoms mobility and confinement and greatly helps spectrum interpretation. Depending on the value of these ratios, water can be identified as unconfined water, mobile free water, mobile adsorbed water, and immobile water molecules bonded to solid macromolecules. Ratios lower than one are physically not permitted [START_REF] Abragam | The Principles of Nuclear Magnetism[END_REF].

For readers interested on this technique, let us briefly explain more about the influence of water mobility and environment on the relaxation times T1 and T2 but cited references should be consulted for detailed information. T1 and T2 NMR values depend on the local environment of the measured hydrogen atoms (H). In this case this will correspond to water molecules and their NMR relaxation values will depend on the size of the molecules they are bound to, affinity with molecules interacting and pore size. T2 NMR relaxation times decreases as pore size is reduced (increase of surface-to-volume ratio) and/or when molecular tumbling (internal mobility) decreases. They will also decrease if H atoms are in molecules with restricted mobility due to viscosity, or the interactions being in macromolecules. For T1 NMR relaxation times, first they will decrease when molecular tumbling decreases (from liquid phase) but it increases when molecular tumbling is low (in gel and solid like molecules) [START_REF] Rostom | Investigation of the effect of aging on wood hygroscopicity by 2D 1 H NMR relaxometry[END_REF]. Here is where we can see the importance of T1-T2 ratios, which are characteristic of the mobility of H atoms and will provide physical information of water. Values close to 1, 2 6, and over 10 have been assigned to (i) unconfined water, (ii) mobile water at free or adsorbed state and (iii) bonded immobile water molecule and solid macromolecules, respectively [START_REF] Bonnet | NMR determination of sorption isotherms in earlywood and latewood of Douglas fir. Identification of bound water components related to their local environment[END_REF]. These values may vary depending on the system studied but the trend in the increase of T1-T2 should be valid for any material.

Results

Physicochemical properties of biochar

The FTIR spectrum of unaged biochar (BC) exhibits weak bands at 3400 cm -1 (mainly due to O-H stretching) and at 2900 cm -1 (due to aliphatic C-H stretching). Relatively stronger bands were found at 1600 cm -1 (attributed to C=C and C=O stretching of conjugated ketones and quinones), at 1050 cm -1 (corresponding to C-O stretching likely from non-decomposed lignocellulosic materials), and at 1400, 885 and 775 cm -1 (corresponding to C-H2, aromatic C-H and aromatic out-of-plane C-H stretching, respectively), characteristic of samples with a high degree of C condensation (Figure 1). The absorption at 3400 cm -1 increases with the time of biochemical aging (co-composting), indicating an increased presence of hydroxyl and carboxyl groups. Similarly, the intensity of the signal at 2900 cm -1 increases with time of biochemical aging, suggesting an increased contribution of signals from microbial lipids and/or persistent vegetal lipids.

According to solid state NMR data (Figures 2 andSI-1), unaged biochar exhibits a high content of aromatics (55.0% of the total C intensity measured) relative to other types of carbon. After 9-and 30 days of biochemical aging, the contribution of O-alkyl and alkyl groups increases (from 5.5 to 11.0% and from 7.0 to 10.2%, respectively). The intensity of the aromatic carbon signal is similar for unaged and biochemically aged biochar, indicating that the separation process of biochar particles from the compost mixture, as referred in Materials and Methods, is highly efficient.

All samples exhibit an initial mass loss of 4% with heating below 110 °C corresponding to adsorbed atmospheric humidity (Figure 3). In inert atmosphere, thermal decomposition of BC starts at 350 °C, with a 6% mass loss below 680 °C and a 5% mass loss at temperatures above 680 °C. An additional mass loss of 11% is appreciated in ba-BC above 250 °C, corresponding to the modification of biochar with a labile organic fraction or to surface functionalities during cocomposting. In the presence of air atmosphere, mass loss above 250 °C is similar, and ba-BC underwent a 9% loss from oxidation. Both samples exhibit a high final residue of 30% corresponding to ash and remnants of high recalcitrance. BET surface area of biochar remains almost unaltered after the biochemical aging (obtained from P/P0 region between 0.05-0.30 of the adsorption isotherms, type I) but it increases by an order of magnitude when physical aging is applied (Table 1 and Figure 4a). Consistently with these results, micropore volume and pore area are also an order of magnitude higher in physically aged samples which shows, in addition, a deep reduction in particle size. 1 and Figure 4b) provide information about macroporosity in the studied samples. Biochar before and after biochemical aging exhibits two different macroporous characterized by approximately 1 and 10 µm pore diameter, respectively. After physical aging, the porosity at 10 µm disappears, leading to a sharp porosity distribution at 1 µm. The total Hg intrusion volume and total area is higher in physically unaged samples, because of the disappearance of the porosity at 10 µm after samples were ball-milled. To avoid ambiguous and field-specific expressions in terms of pore size ranges, such as macro-, meso-and microporosity, the authors defined the ranges meaningful for this study as small, medium and large pore-size as as defined in Table 2. Biochar interactions with water 1 H NMR relaxometry measurements were carried out to evaluate biochar-water interactions using the T1 and T2 NMR observables, as it was previously described. Results were collected from samples with adsorbed water from controlled RH atmospheres and from samples in liquid water.

Results of Hg intrusion porosimetry (Table

In samples with adsorbed water, the proton intensity of the peaks of T2 NMR relaxation distribution increases as expected with relative humidity, as there is more water adsorbed in the sample (Figure 5). At this point it is important to remark that for all dry samples, in absence of adsorbed water (0% RH), proton signal was not detectable, suggesting that structural protons of biochar before and after aging are not visible with the parameters used in this work. To verify this statement in presence of water (i.e., the actual conditions for the set of studied samples) and check if water interactions could augment the mobility of structural protons (increasing their T2 NMR values and making them visible in the NMR experiments), dry samples were wetted with D2O to establish water-biochar interaction without providing additional hydrogen atoms from water. No changes in NMR signal was either appreciated, demonstrating that structural protons have no effects on the reported NMR results. The small signal measured (Supplementary Information, Figure SI-2) is attributed to a minor replacement of deuterium ions by hydrogen ions from atmospheric humidity.

The maxima of the broad and asymmetrical distribution of T2 NMR relaxation times in presence of adsorbed water in biochar before and after biochemical aging (BC and ba-BC) appear at 3-4 ms whereas it is slightly shifted to lower times (around 1 ms) in physically aged samples (pa-BC and pba-BC). These results allow us characterize the T2 NMR relaxation times of water interacting with biochar at RH < 100 %. T2 NMR relaxation time distributions of samples obtained with biochar and liquid water (using the same 1:4 mass ratio for all samples) are shown in Figure 6. After 30 minutes, the added water is able to distribute similarly in BC and ba-BC, whereas the effect of physical aging is evident in pa-BC and pba-BC, as the amount of water with short T2 values (small pores) has greatly increased. In BC and ba-BC water protons show a complex behaviour, being possible to differentiate four different T2 values with maxima at approximately 5, 50, 500 and 2000 ms.

The main signal at 2000 ms is attributed to the relaxation time of water outside the sample [START_REF] Courtier-Murias | Potential and Application of Magnetic Resonance Imaging and relaxometry in Environmental Research[END_REF], representing more than 50% of the total intensity at t=30 min. After 7 and 30 days this signal at 2000 ms disappears and its intensity is redistributed to the intermediate water signals characterized by a relaxation time at 500 ms for BC and at 200 ms for ba-BC, becoming the main signal for water in these samples. The faster T2 relaxation times remain at 5-10 ms and around 50 ms and their intensity does not increase with time.

On the other hand, physically aged samples exhibit a small ( 10% for pa-BC) or absent (pba-BC) contribution of water outside the sample (characterized by a relaxation time around 2000 ms) after a short treatment time (e.g., t=30 min), indicating that most of the water interacts almost immediately with biochar. This rapid interaction is observed as the NMR value of water with the long T2 decrease during the first 30 minutes and then stay constant over 30 days.

Opposite to the behaviour described for BC and ba-BC, after 30 minutes of treatment the higher fraction of water signal relaxes at 10 ms and its intensity remains almost constant for physically aged samples. In addition, there is a decrease of T2 NMR values for this peak (this is clearer for pba-BC but the same tendency is observed for pa-BC). Compared to physically unaged samples, another signal was found at 100-200 ms which follows different patterns for pa-BC and pba-BC. A low intensity and constant signal with time for pa-BC and a sharp increase with time for pba-BC. In T1-T2 1 H NMR relaxometry spectra of physically unaged samples (Figures 7a and7b), most water signal is placed close to the T1=T2 line, indicating different degrees of confinement caused by the size of the pores (Table 2) [START_REF] Wang | Review: A quantitative understanding of the role of co-composted biochar in plant growth using meta-analysis[END_REF]. The highest intensity signal, in the 100-1000 ms range and falling straight in the T1=T2 line, corresponds to water in larger size pores, while the signal exhibiting lower T1-T2 ratios (10-100 and 1-10 ms) is placed slightly below the T1=T2 line indicating lower water dynamics. Three regions with different T1-T2 ratios were observed in BC (1.68, 2.52 and 4.50) and ba-BC (1.30, 2.12 and 3.78), while two regions were differentiated in pa-BC (3.00 and 4.25) and pba-BC (1.12 and 8.50). For physically aged biochar samples (Figures 7c and7d), the region with high T1 and T2 values (100-1000 ms), which is close to the T1=T2 line, shows a higher T1-T2 ratio for Figure 7c and the region with low relaxation time values (1-50 ms) shows a higher T1-T2 ratio (Figure 7d) in comparison to the other 3 spectra.

Discussion

Chemical and physical alteration of biochar during aging

The properties of unaged biochar vary depending on the nature of the feedstock and pyrolysis conditions of the process, in terms of chemical composition and stability, reactivity and porosity [START_REF] Huang | Effects of pyrolysis temperature, feedstock type and compaction on water retention of biochar amended soil[END_REF]. Besides, it has been described that biochar undergoes several physicochemical changes in contact with soil and during co-composting processes [START_REF] Joseph | Microstructural and associated chemical changes during the composting of a high temperature biochar: mechanisms for nitrate, phosphate and other nutrient retention and release[END_REF][START_REF] Wang | Review: A quantitative understanding of the role of co-composted biochar in plant growth using meta-analysis[END_REF][START_REF] Godlewska | Biochar for composting improvement and contaminants reduction. A review[END_REF][START_REF] Haider | Mineral nitrogen captured in field-aged biochar is plant-available[END_REF]. This aging process could be related to biotic and/or abiotic processes. Abiotic aging could be mainly attributed to the physical disruption of biochar coarse particles, exposing internal surfaces of the biochar skeleton and thus changing its porosity and reactivity. Biotic aging is mediated by microbial communities which could directly feed on biochar reactive surfaces edges [START_REF] Wiedner | Acceleration of Biochar Surface Oxidation during Composting[END_REF] or use biochar pore space as habitat [START_REF] De La Rosa | Soil-borne fungi challenge the concept of longterm biochemical recalcitrance of pyrochar[END_REF].

In this study, two different factors of biochar aging were studied to understand the changes undergoing in the material. On one hand, we considered the effect of 30-day co-composting process of prune and coffee on wood biochar (biochemical or biotic aging) and, on the other hand, we evaluated the physical disruption of the biochar particle (physical or abiotic aging) and the combined effect of both processes.

According to the data obtained by FTIR analysis, pristine biochar spectrum exhibits bands corresponding to stable carbon forms, characteristic of pyrolyzed organic matter with a clear aromatic ring input and a series of bands in the 650-1800 cm -1 range that is attributed to nonpyrolyzed lignocellulosic remains [START_REF] Liu | Thermogravimetry-Infrared Spectroscopy Analysis of the Pyrolysis of Willow Leaves, Stems and branches[END_REF]. The presence of oxygenated functionalities is more evident after 9 and 30-days co-composting process, where the spectrum exhibits a higher signal corresponding to functional groups, indicating the presence of fresh organic matter or minerals obtained in contact with compost and microbial activity [START_REF] Carballo | Characterization of different compost extracts using Fourier-transform infrared spectroscopy (FTIR) and thermal analysis[END_REF]. This is consistent with the bands attributed to aliphatic C-H stretching, suggesting the presence of plant lipids (i.e., suberines, cutines, surface wax) or microbial lipids (i.e., membrane phospholipids) which appear after co-composting. These results are also in agreement with the data obtained by 13 C solid state NMR, where an increase in the signals of O-alkyl and alkyl groups is appreciated.

The continuous increase of signal during co-composting on the areas of O-alkyl and alkyl groups (in comparison to aromatic signal) indicates a process of surface functionalisation or strong adsorption of labile organic matter on biochar, rather than a physical mixture of biochar and compost. However, direct functionalization of the aromatic rings cannot be assessed due to the overlapping of the heteroaromatic peak with the large aromatic one. The combination of both techniques shows that biochar undergoes surface functionalisation with acidic groups in non-aromatic chains, coming from the interaction with compost and by microbial activity.

These results could also be explained by the presence of fresh organic matter coating on the surface and within pores of biochar created during the co-composting process, as reported by several authors [START_REF] Kamman | Plant growth improvement mediated by nitrate capture in co-composted biochar[END_REF][START_REF] Joseph | Microstructural and associated chemical changes during the composting of a high temperature biochar: mechanisms for nitrate, phosphate and other nutrient retention and release[END_REF][START_REF] Hagemann | Organic coating on biochar explains its nutrient retention and stimulation of soil fertility[END_REF]. This biochemical aging of biochar during co-composting is also appreciated in thermogravimetric analysis. Mass loss difference in the 250-350 °C range between both materials (in He and air atmospheres) indicates the presence of labile organic compounds coming from functional groups generated in the co-composting process by the interaction of biochar and compost, or from the decomposition of biochar by microbial activity. Both samples exhibit rather high final residue of approximately 30% m/m corresponding to ash and remnants of high recalcitrance.

Aging process also affects the physical properties of biochar, as seen in the results by porosimetric analysis by Hg intrusion and N2 adsorption. The porosity in the large pore-size range (>0.5 µm) measured by Hg intrusion porosimetry described how physically unaged samples exhibit two well defined porosity regimes at 1 and 10 µm pore diameter. After ballmilling (simulating physical aging) the porosity at 10 µm is lost, leading to a single well-defined porosity with a mean value of pore size distribution 1 µm. No reduction of pore size diameter was appreciated after biochemical aging, which indicates that if a coating layer is formed on the small size pore walls, it has no effect in the pore diameter at this scale. Regarding N2 adsorption measurements, it is worth noting that all measured samples exhibit hysteresis between N2 adsorption and desorption isotherms in all the range of P/P0 values. This is not a type IV isotherm hysteresis caused by the capillarity in mesopores as it appears in the whole P/P0 range. This hysteresis is attributed to the entrapment of N2 gas in the pores which is an effect sometimes appreciated in carbon materials [START_REF] Braida | Sorption Hysteresis of Benzene in Charcoal Particles[END_REF] and biochars [START_REF] Gargiulo | Assessing the Potential of Biochars prepared by Steam-Assisted Slow Pyrolysis for CO2 Adsorption and Separation[END_REF]. In terms of BET surface area results, there is an expected difference of an order of magnitude for physically aged samples because of the reduction of particle size after the grinding process. Milled samples (both after and before biochemical aging) exhibit porosity in the small size pore range

(1-5 nm), characteristic of this kind of carbon materials. In this sense, bulk samples are characterized by a particle size distribution at 2 mm diameter size, whereas ground samples exhibit a bimodal distribution with maxima at 7 and 75 µm after physically aging and a single mean value of 9 µm for the biochemically aged.

Water distribution and interactions with biochar as affected by aging 1 H NMR relaxometry measurements were carried out to obtain the T2 NMR relaxation distributions to estimate the distribution of water on different biochar porosities for unaged and aged samples in contact with adsorbed and liquid water. This also allowed understanding the effect of biochemical aging during co-composting process, which are reported to be comparable to aging within the soil matrix [START_REF] Wang | Biochar Aging: Mechanisms, Physicochemical Changes, Assessment, And Implications for Field Applications[END_REF]. Similarly, ball-milled samples were analysed and compared to bulk samples to address the effect of physical aging, weathering, or physical disruption that biochar undergoes with time after its application to soils.

Regarding the obtained results, it is important to note that there is not observable NMR signal from protons coming from dry samples, demonstrating that structural hydrogen atoms from biochar after and before aging have faster relaxation times than the spectrometer dead time.

To verify if structural hydrogen atoms could increase their relaxation times in presence of water and, thus, contribute to the total measured intensities, dry samples were mixed with deuterated water (D2O) producing water-biochar interactions but without providing protons from water. Results exhibited no increase in the signal (Figure SI-2), indicating that structural hydrogen atoms of biochar do not provide relevant proton NMR signal for our experimental condition and, thus, all incoming NMR signals for these samples will correspond only to adsorbed water.

The relatively short transverse relaxation times obtained for biochar at RH > 50% is assigned to water adsorbed to the surface of the particle (and pores), probably attached to remaining functional groups and structural defects, describing strong interactions that do not allow fast proton dynamics. However, as the amount of adsorbed water is relatively high (Water Retention Capacity (WRC) data showed in supplementary information, Table SI-1) and similar to what we can observe in wood samples (where the amount of functional groups is more important) [START_REF] Bonnet | NMR determination of sorption isotherms in earlywood and latewood of Douglas fir. Identification of bound water components related to their local environment[END_REF], thus other mechanisms could contribute to adsorption of water in biochar.

Relaxation time is shorter for ball-milled (simulating physical aging) samples (1 ms) than for bulk samples (3-4 ms) probably because of a higher surface interaction due to the increase on surface area or to the accessibility of water to smaller and inner pores after ball-milling.

T2 NMR relaxation time results of liquid water in contact with physically unaged biochar before and after biochemical aging (BC and ba-BC) (Figure 6a and 6b) indicate that after 30 minutes of contact between water and the biochar, the liquid distributes similarly in both materials. In these samples water shows a complex transverse relaxation behaviour with at least four quite different relaxation times measured at around 2000, 500, 50 and 5 ms, respectively. The signal at the largest T2 time, e.g. 2000 ms, is attributed to the relaxation time of free water outside the sample [START_REF] Zhou | Magnetic resonance imaging evidences of the impact of water sorption on hardwood capillary imbibition dynamics[END_REF], indicating that after 30 minutes, more than 50% of the added water remains without any interaction with the biochar. Results after 7 and 30 days show the progressive disappearance of the signal at 2000 ms and an increment in the intensity at intermediate relaxation times (500 ms for BC and 200 ms for ba-BC), indicating that water permeates into the large pore size system with time. The intensities of the shorter T2 NMR peaks (at 50 and 5 ms) remain constant with time, indicating that the small (and accessible) porosity regimes are filled with a water fraction at the first stages of imbibition, remaining unaltered with the time.

It demonstrates that the number of small and accessible pores does not increase with the exposure time. Each of these relaxation times correspond to water confined in a certain porosity regime, which has also been defined as transmission like (>50 µm), storage-like (50-5 µm) and residual-like (<5 µm) pores in soil samples [START_REF] Greenland | Soil damage by intensive arable cultivation: temporary or permanent?[END_REF] and used as well for biochar studies [START_REF] Conte | Water dynamics in different biochar fractions[END_REF]. Following this classification, the intermediate T2 signal measured at steady state could be attributed to water in transmission and storage like pores where a relatively high mobility of water is allowed. The shorter T2 NMR values could be attributed to storage and residual-like pores where water loses mobility because of a higher degree of confinement and, arguably, due to the interactions with the surface of the pore walls. It is worth noting that the shortest T2 values are rather similar to that measured for adsorbed water (1-3 ms), which may be understood as a shared contribution of small pore size and surface adsorption, which would decrease the overall mobility of water molecules, provoking a faster transverse NMR relaxation of water molecules (lower T2 values) because of increasing dipolar interactions [START_REF] Conte | Nature of water-biochar interface interactions[END_REF].

Water added to physically aged samples (Figures 6c and6d) exhibit significantly shorter T2 relaxation times after 30 minutes of treatment when they are compared with bulk samples. It indicates that interaction of water with the biochar is strongly affected by the variation in the particle size and pore structure caused by the ball-milling process. On the one hand, the reduction in the particle size promotes a faster interaction of the added water with biochar, showing a deep decrease or even the disappearance of free water (characterized by a relaxation time T2 around 2000 ms) in pba-BC after 30minutes. On the other hand, the water fraction with restricted mobility, i.e., those characterized by a shorter relaxation time [START_REF] Zhou | Magnetic resonance imaging evidences of the impact of water sorption on hardwood capillary imbibition dynamics[END_REF],

increases after ball-milling. This behaviour could be explained by the combination of different factors, such as the increase in surface area and an improved access to the small size pores volume as well as the reduction in the inter-aggregate space between biochar particles, that increases the overall fraction of interacting water with shorter relaxation times. Nevertheless, the interaction of water at the surface, in the small size pores or at the inter-aggregate space appears to be different, which can be the cause of the observed differences on water mobility and hence, in the corresponding NMR relaxation time [START_REF] Bonnet | NMR determination of sorption isotherms in earlywood and latewood of Douglas fir. Identification of bound water components related to their local environment[END_REF]. In pa-BC, however, there is a T2 signal at 2000 ms which is attributed to an excess of water that remains outside the sample after all the inter-aggregate spaces of the system were saturated, which could increase because of particle sedimentation over time.

T1-T2 spectra at steady state for physically unaged samples (BC and ba-BC) show three different regions. The strongest signal, at long T1 and T2 values, corresponds to free water with high mobility which falls straight in the T1=T2 line, presumably water that permeated in the large pore size system of the material. The two regions at lower T1-T2 ratios would correspond to water in a higher surface-to-volume ratio system, this is, smaller porosity regime. As they appear slightly below the T1=T2 line, lower dynamics (molecular tumbling) could be assumed by a certain degree of adsorption or interactions with the material reactive surfaces. In T1-T2 spectra at steady state of ball-milled samples (pa-BC and pba-BC) the high-mobility free water, with the most intense peak in the 100-1000 ms range it is clearly differentiated from water placed in the smaller porosity regime [START_REF] Rostom | Analysis of water content in wood material through 1D and 2D 1 H NMR relaxometry: Application to the determination of the dry mass of wood[END_REF]. After biochemical aging, T1-T2 ratios for the three different regions are similar to that of BC sample, indicating the higher degree of chemical functionalisation originated from the composting process is not increasing water adsorption. In addition, after physical aging we do not see an increase of T1-T2 ratio for the water in the smaller porosity (which is likely to be the water that interacts strongly with biochar) but we see an increase of the T1-T2 ratio for the water in the larger porosity. However, it seems that this effect should be important as for the pba-BC sample we can see and important increase on the T1-T2 for the water in smaller porosities (from about 4 to 8).

The capacity of biochar to retain water in their structure is crucial to understand the mobility and availability of nutrients for plants. The water placed in the inter-aggregate space and in large size pores (i.e. transmission-like) can move and permeate easily between biochar and soil being easily available for the roots of the plant. As a counterpart, water in this state can also be more easily lost by evaporation or diffusion. Actually, we can see that some of this water transfer from the inter-aggregate state to a more mobile state over the 30 days but the amount is relatively low, showing the capability of aged biochar to keep water for long periods of time. In addition, water in medium size pores (i.e. storage-like) exhibit stronger interactions microorganisms. Finally, water in small size pores (i.e. residual-like) appears to be strongly bonded to the material and would require certain negative pressure by root suction to be available [START_REF] Greenland | Soil damage by intensive arable cultivation: temporary or permanent?[END_REF]. To allow the assignment of each of the T2 signals to a specific porosity regime further studies are needed with deep analysis in 1 H NMR relaxometry and porosimetry techniques.

Conclusions

This study indicates that a surface modification in chemical composition of biochar takes place during co-composting process, with an enrichment of oxygen functionalities. The mobility and plant availability of soil cations such as nutrients and contaminants will be dependent of water mobility throughout the soil matrix and its and its porosity, making biochar/water interactions a key issue in the understanding of the role of this material as soil amendment. 1 H NMR relaxometry results indicate that water interacts in a more intimate manner with biochar, exhibiting shorter T2 NMR relaxation times and slightly higher T1-T2 ratios, after the biochemical aging process, probably due to a higher wettability and stronger interactions with the functionalised surface. Particle size reduction has a clear effect in the interaction with water, making stronger and faster interactions when particle size is reduced as proved by 1 H NMR relaxometry. Water placed in the inter-aggregate porosity is the main contributor to lower T2 times. Physical and biochemical aging of biochar is therefore critical to understand the enhancement on the agronomic performance of biochar with time when used as amendment in soils, not only because of the enrichment in nutrients, but also because of water dynamics and retention capacity in biochar particles.

1

 1

Figure 1 .

 1 Figure 1. FTIR spectra of (a) BC and ba-BC after (b) 9 and (c) 30 days of co-composting.

Figure 2 .

 2 Figure2.13 C-NMR spectra of (a) BC and ba-BC after (b) 9 and (c) 30 days co-composting. Spectral regions in which carbon atoms resonate were assigned according to Knicker (2011)[START_REF] Knicker | Solid state CPMAS 13 C and 15 N NMR spectroscopy in organic geochemistry and how spin dynamics can either aggravate or improve spectra interpretation[END_REF]. Note that two peaks correspond to the spinning sidebands (SSB) of the aromatic one.

Figure 3 .

 3 Figure 3. Thermogravimetric analysis of BC (solid line) and ba-BC (dashed line) in He (black) and in air (red).

Figure 4 .

 4 Figure 4. (a) BJH porosimetric analysis of N2 adsorption isotherms and (b) Hg intrusion porosimetry analysis of BC (black solid line) and ba-BC (black dashed line), pa-BC (red solid line) and pba-BC (red dashed line).

Figure 5 . 1 H

 51 Figure 5. 1 H NMR relaxometry of (a) BC, (b) ba-BC, (c) pa-BC and (d) pba-BC at 0, 55, 75 and 100% of relative humidity.

Figure 6 . 1 H

 61 Figure 6. 1 H NMR relaxometry of (a) BC, (b) ba-BC, (c) pa-BC and (d) pba-BC with liquid water after 30 minutes, 7 days, and 30 days.

Figure 7 .

 7 Figure 7. T1-T2 1 H NMR relaxometry correlation spectra of (a) BC, (b) ba-BC, (c) pa-BC and (d) pba-BC with liquid water.

Table 1 .

 1 Particle size distribution, N2 adsorption and Hg intrusion porosimetry results of unaged (BC), physically aged (pa-BC), biochemically aged (ba-BC) and physically and biochemically aged biochar (pba-BC).

		Particle size distribution	N2 adsorption porosimetry	Hg intrusion porosimetry
		Mean	BET surface area	Micropore volume	Pore area	Total intrusion volume	Total area
		(µm)	(m 2 g -1 )	(cm 3 g -1 )	(m 2 g -1 )	(mL g -1 )	(m 2 g -1 )
	BC	2 x 10 3	22.15	0.015	4.36	1.41	14.76
	ba-BC	2 x 10 3	20.50	0.011	2.91	1.35	13.39
		7 / 75
	pa-BC	*(bimodal	176.01	0.121	22.48	0.85	7.55
		distribution)
	pba-BC	9	311.19	0.201	20.91	1.03	7.96

Table 2 .

 2 Definition of porosity ranges

	Porosity range	Pore size
	Small-size pores	1 5 nm
	Medium-size pores	5 500 nm
	Large-size pores	0.5 -50 µm

Acknowledgements

This study was financially supported by the Comunidad de Madrid and the Spanish National Research Council (CSIC) research grant Atracción de Talento [grant number 2019T1/AMB14503]. The authors acknowledge the funding received from the Spanish National Research Council and the French National Centre for Scientific Research through the cooperation project PICS [grant number 018FR0044] and IEA (ex-PICS) grant number 203026, respectively. This work was also partially funded by the Spanish Ministerio de Ciencia e Innovación (PTA2018-015282-I and PID2020-119047RB-I00) and Comunidad de Madrid (PEJ-2018-TL/IND-11075). The authors want to thank the analytical services for the 1 H relaxometry NMR at ICTP-CSIC, for the solid state 13 C NMR at IRNAS-CSIC and for FT-IR, TGA and porosimetry analyses at ICA-CSIC. Technical support provided by Mr. Héctor Fritis, Ms. Cristina Gómez Ruano, Mr. David Mabilais, Mr. Fernando Martín Salamanca and Dr. Rebeca Herrero is gratefully acknowledged. Credit Author Statement Asier Goñi-Urtiaga: Investigation, Methodology, Writing-Original Draft. Denis Courtier-Murias: Conceptualization, Data curation, Methodology, Writing-review & editing, Validation. Giuseppe Picca: Investigation. Juan López-Valentín: Methodology, Reviewing. César Plaza: Reviewing, Validation. Marco Panettieri: Supervision, Conceptualization, Funding acquisition, Writing-review & editing, Validation.