R Podor

V Trillaud

G I Nkou Bouala

N Dacheux

C Ricolleau

N Clavier

A multiscale in situ High Temperature High Resolution Transmission Electron Microscopy study of ThO2 sintering

Keywords: High temperature, HRTEM, sintering, neck formation, in situ, ThO2

Two-grain model systems formed by ThO2 nanospheres have been used to experimentally study for the first time the initial stage of sintering from room temperature to 1050°C using High Temperature High Resolution Transmission Electron Microscopy. In each grain, oriented attachment drove the reorganization and growth of the crystallites up to 300°C to form a pseudo single crystal. Crystallite size kept growing up to 950°C. At this temperature, a fast transformation probably corresponding to the elimination of stacking faults or dislocation walls led to the formation of single-crystals. The contact formed at room temperature between the two grains was stabilized during heat treatment by a slight reorientation of the crystallographic planes (T  400°C), leading the neck to be formed by numerous boundaries between the crystallites. At higher temperatures, the neck evolved and stabilized in the form of a plane of crystallographic orientation mismatch between the grains, which corresponds to the usual definition of the grain boundary. The growth of the neck by the addition of atomic columns was further observed in real time and quantified. At T = 950°C, the evolution of the macroscopic sintering parameter λ was obtained from HT-HRTEM images and indicated that the neck formation mostly processed through volume diffusion.

Introduction

Sintering is a process of materials elaboration by heating a shaped powder (sometimes under pressure) without reaching complete melting. Although modern approaches and recent advances in imaging techniques such as X-ray microtomography showed that the three-dimensional evolution of the microstructure of a powder compact is a deeply complex mechanism [1][2][3] , solid-state sintering is still frequently simplified as a three-step process starting from the formation and growth of necks between the grains. This initial stage is generally described through mathematical models based on geometrical and crystallographic simplifications, leading to consider two spherical and monocrystalline grains in contact, while real-life grains are mostly polycrystalline (i.e. formed by several elementary crystallites) and often present random shapes. The quantities of matter exchanged between the two grains were then computed using different methods including molecular dynamics 4 , Monte Carlo 5 , or Discrete Elements Methods 6 .

Conversely, only few experimental data showing this matter exchange are reported in the literature. They are mainly based on electron microscopy techniques that allow the direct observation of the morphological modifications associated to the heat treatment of powdered materials [7][8][9][10][11] . These works have experimentally evidenced nanometric / atomic scale processes that can drive the first stage of sintering. Most of the materials that have been studied are metallic 12,13 or CeO2 nanoparticles [14][15][16][17] .

The sintering behavior of large monocrystalline grains (several tens of nanometers) 18 , large polycrystalline aggregates 19,20 or groups of grains 21,22 has also been monitored by in situ High Temperature Transmission Electron Microscopy but remains limited to a few studies. Moreover, the authors generally pay attention to the general behavior of the grains / aggregates at high temperature, but only few information relative to atomic scale mechanisms occurring at high temperature are reported 18 .

When dealing with the solid-state sintering of polycrystalline materials, where grains can be viewed as aggregates constituted by nanoscale crystallites, oriented attachment (OA), first observed by Penn and Banfield (1998) 23 , is now considered as one of the main mechanisms responsible for the growth of the crystallites 8,24,25 . It is driven by the minimization of the surface energy through alignment of the crystallites along identical crystallographic orientations 26,27 . Some works have also reported the rotation of one grain relative to the others during the first stage of sintering even for micrometric metal particles 18,28,29 .

Although clearly evidenced by experimental data, these mechanisms are not yet considered in the mathematical models developed to describe the first stage of sintering. Nkou Bouala et al. 11 have shown that there was a good agreement between modelling and experiments when considering a system composed by two monocrystalline grains of CeO2. However, sintering was accelerated if the two grains were initially polycrystalline. Same authors then concluded that in real solids, the first stage of sintering was controlled not only by the behavior of the grains themselves, but also by crystallite rearrangements within the grains. Although they provide reliable qualitative descriptions, the classical two-grain models established in the 1960's 30 and developed since then 4,31 mostly fail to describe correctly the evolution of sintering rate for real materials, even when considering simple geometries.

Numerical models can be improved by taking into account new insights coming from microscopic observations and new models including polycrystalline grains are currently under development 31 .

However, many questions remain pending such as what happens at the very beginning of the first stage when the grains are in contact or what can be the role of polycrystallinity in the grains? It is also important to understand how the crystallites forming the grains rearrange themselves when heating the sample and how the grains rearrange one relative to each other. These missing pieces are essential to clearly understand which are the parameters controlling the formation of necks between the grains during the first stage of solid-state sintering.

In the present study, we propose to bring new insights into the very early stages of sintering of oxide materials through High Temperature -High Resolution TEM observations (HT-HRTEM) with a sub-second temporal resolution. ThO2 have been chosen as a model material, owing to its remarkable applications in the energy industry (nuclear fuel 32 , oxygen sensor 33 …), and the fact that its fluorite structure is adopted by many metal dioxides (YSZ, CeO2, UO2, PuO2…). Also, as Th(IV) is the unique stable oxidation state of thorium on the temperature and pressure range investigated, thorium oxide possesses a strict ThO2 stoichiometry that makes it insensitive to PO2, and avoids any bias due to the atmosphere surrounding the sample during the observations. Last, the first stage of sintering of 250 nm diameter ThO2 has been already studied experimentally by Nkou Bouala et al. 34

by High

Temperature -Environmental Scanning Electron Microscopy (HT-ESEM) which provides a microscopic point of comparison. The present work will be focused on the use of polycrystalline ThO2 nanospheres of 50 nm in diameter, which are transparent to the 200 kV electron beam. This size was chosen to be between that of nanoparticles (3-10 nm), which are generally monocrystalline and for which sintering behavior can be described by models, and real grains (from 0.5 to a few hundreds of micrometers), which are generally polycrystalline. We focused our attention on an experimental system constituted by two ThO2 nanospheres, regarding particularly i) the evolution of the neck between the grains at the grain scale and ii) the growth of the crystallites within the grains at the nanometer and atomic scale.

Particularly, the modification of the arrangement of the crystallites, their relative orientations, the neck size… will be monitored as a function of temperature.

Materials and methods

Preparation of ThO2 nanospheres

The synthesis of ThO2 nanospheres was performed through an hydrothermal route inspired from the protocol initially reported by Wang et al. 35 In this aim, 2 × 10 -4 mole of thorium nitrate pentahydrate (supplied by Sigma-Aldrich) was first dissolved in 5 mL of deionized water, along with 10 -4 mole of urea and 2.7 × 10 -3 mole of glycerol, here used as complexing and shaping agents, respectively. The starting mixture was aged for 10 minutes under magnetic stirring then transferred to a 23 mL autoclave (Parr). It was finally heated under hydrothermal conditions at 120°C for 10 hours.

The white precipitate collected by centrifugation at 4500 rpm was further washed twice with deionized water and ethanol, and then dried overnight at 60°C. X-ray diffraction characterization was already reported in our previous work 34 . It confirmed the formation ThO2 crystallizing in the cubic Fm-3m space group (fluorite-type structure), while Rietveld refinement of the XRD pattern allowed us to estimate the average crystallite size to be under 5 nm. Furthermore, image analysis of ESEM and TEM micrographs revealed that a monodisperse powder was obtained. It was composed by isolated nanospheres with an average diameter of 57 ± 9 nm (see Supplementary File S1). From these results, the nanospheres can be viewed as aggregates made of several elementary crystallites. For simplification purposes, these aggregates will be denominated as grains in the rest of the paper. Also, it is important to underline that the absence of two-grain systems already formed during the synthesis process was thoroughly checked. In this aim, TEM observations led us to conclude that all the grains were separated, meaning that no neck between the grains formed in solution was present at the beginning of our experiments.

High Temperature -High Resolution Transmission Electron Microscopy

A Jeol ARM microscope operating at 200 kV and equipped with a spherical aberration corrector was used to perform the experiments. A Protochips Inc. Atmosphere heating specimen holder [36][37][38] was attached to the microscope, allowing us to reach a maximum temperature of 1100°C through the circulation of a gas flow around the sample. The sample and the gas are isolated from the high vacuum TEM chamber by two 50 nm thick SixNy windows. the detailed heating procedure has been described by Chmielewski et al. 39 . Prior to the observations, about 5 mg of ThO2 powder were mixed with 2 mL of acetone and dispersed by ultrasonication. Then, a 1 µL drop was deposited on the SixNy window of the sample holder. The second window was broken in order to perform the experiment under high vacuum conditions in the microscope chamber with the aim to ensure a better image resolution. The sample was heated under vacuum with a 30°C/min heating rate from room temperature (RT) up to a maximum temperature of 1050°C. The microscope was associated with a OneView Gatan TM camera: the recording frequency was fixed to 7 images per second with a 2048 × 2048 pixels resolution (16 bits grey levels) during the heat treatment. The total duration for one experiment ranged from one to two hours.

During the experiments, the heating ramp was stopped several times and different isothermal plateaus were managed. When the temperature was stabilized, the blurring effect due to the heat treatment was avoided and images were recorded at several magnifications in order to observe sintering at different scales. Several systems composed of two nanospheres were observed to ensure the representativeness of the dataset.

Image analyses

During one typical experiment, approximately 22 000 images were recorded. All the blurred images -mainly caused by sample and/or sample holder deformation during heating -were first eliminated. This corresponds to approximatively 50% of the total number of images. Then, the stacks of images were aligned using Fiji 40 / Linear Stack Alignment with SIFT plugin 41 . Due to the highresolution mode used during image recording, the images were used in the direct space for the determination of interatomic distances. Then, Fast Fourier Transform (FFT) patterns of images from selected zones were computed to follow the evolution of the crystal structure and the corresponding inter-reticular distances.

A specific image analysis strategy was developed to characterize the crystallographic transformations occurring during the heat treatment using series of FFT patterns. A particular diffraction spot (arbitrarily set to the (200) planes) was first selected to monitor the variations of its shape and position as a function of temperature. In this aim, a polar projection of the FFT pattern series was achieved using polar_transformer plugin for imageJ [START_REF] Donnelly | Polar Transformer[END_REF] . In this representation, one line corresponds to a family of planes with the same orientation (i.e. (200)) at a given temperature. The variation of the positions was observed along the stack of images, i.e. as a function of temperature. This representation was used to describe the crystallographic transformations occurring within a grain during the first step of sintering.

Results

Observations of the as-deposited grains

Prior to any heat treatment, the as-deposited nanospheres (grains) were carefully characterized.

If Rietveld refinement of XRD patterns already shown that they were constituted by several crystallites, HRTEM observations revealed that these latter were not completely independent one from the other. Indeed, the FFT patterns (Figure 1a) determined from the images of each grain indicated that the crystallites were slightly misoriented with regards to a given orientation inside the grain. Each grain then can be described as a mosaic, formed by a large number of nanometric single crystals, randomly misoriented around the main lattice direction. This misorientation modifies the electron diffraction spots in the SEAD pattern (or in the pseudo spots of a FFT pattern) which becomes broadened, as misorientation induces a defocusing effect in the reflection plane. The misorientation angle was determined, as a first approach, as the half width at half maximum of the diffraction spot. In the present case, it was approximately 5° around the [200] and [220] directions at room temperature. This nearly perfect alignment of the crystallites in the grain was probably the consequence of a selfassembly process occurring during the synthesis. Indeed, such mechanism was frequently observed during the hydrothermal treatment of organics mixtures and tetravalent actinides in acidic solutions, leading nanometric An(IV) oxide/hydroxide crystallites to form spherical particles [START_REF] Manaud | [END_REF][44][45][46] . From HRTEM images, the average size of the crystallites in each grain was approximately equal to 1-3 nm (Figure 1b), in good agreement with the XRD data. High magnification images revealed that in the zone of contact, the crystallites were not organized and pores were observable. Thus, the neck between the grains was not formed before the heat treatment. However, as the grains were not perfectly spherical, the contact still cannot be described as a perfect single-point (Figure 1b).

Description of the systems obtained after heat treatment

Different two-grain systems were observed after heating, leading to two possible scenarios. First, the contact-point between the grains did not necessarily develop to form a neck. In this case, the grains were finally isolated after heating (Supplementary File S2). This clearly indicated that no neck was formed during the synthesis of nanospheres, i.e. prior to heat treatment. In this case, the sintering of crystallites within a single grain was faster than the formation of the neck between the grains. The average grain size decreased by about 5% in diameter and the contact between the grains was lost during the heat treatment. The competition between inner grains sintering and neck formation has already been discussed by Nkou Bouala et al. 11 . The authors showed that the crystallite growth within aggregates and the eventual elimination of residual porosity processed through oriented attachment then solid state diffusion, and generally occurred before the sintering of aggregates characterized by the formation of necks. Previous work already evidenced the existence of inner pores within the nanospheres 34 .

The general features observed when a neck was formed between the grains are illustrated in Figure 2a for a system heated during 15 minutes at 1025°C (see Supplementary File S3 for the complete movie). The grains exhibited a cubic shape typical of the materials crystallizing in the fluorite-type structure, and no pore was evidenced. The FFT pattern obtained from the whole image clearly showed that each grain was monocrystalline, but that the two grains did not share the same orientation (Figure 2b). This was also observable on the high magnification images of the neck (Figure 2a). The crystallographic networks of both grains are shown in the high resolution TEM images (inserts in Figure 2a). As the crystallographic orientation of the two grains differs, a planar grain boundary is formed in the neck zone. This disorientation led to the grain boundary to be a low-energy interface, as evidenced by its peculiar physico-chemical properties, such as chemical durability 47 . The FFT patterns obtained for the two grains are reported on Figure 2b where the red and green spots correspond respectively to the upper and lower grains. The same planes families ((200) and ( 220)) are identified on both crystals, with a 21° misorientation angle between both grains.

Figure 2. General observations by HRTEM of a two-grain system after heating at 1025°C in the TEM. a) Two-grain system of 50 nm in diameter with FFT patterns corresponding to each grain (size of the high magnification inserts is 5 × 6 nm²). b) Combined and indexed FFT patterns of both grains. Red = diffraction spots of the upper grain; Green = diffraction spots of the lower grain

Electron beam effect

The effect of the electron beam irradiation is an important parameter that could affect the validity of the measurements. Then, it is of primary importance to check if it modifies locally the sample during the observation. In this aim, the morphology of several nanospheres was only observed during isothermal plateaus (7 images were recorded during the heat treatment) and no significant morphological differences were evidenced when comparing to grains observed continuously (Figure 3a). Thus, the effect of the electron beam remained limited during the heating ramp up to 950°C. At 950°C, the morphology of the sample changed under the electron beam and the formation of a single crystal was observed. Conversely, neighbor grains did not undergo any transformation.

Nevertheless, as soon as these grains were illuminated by the electron beam, the transformation begun. When the two-grain system was observed at T = 1025°C after heating in the TEM, the monocrystalline aspect of the grains was obtained out of the electron beam illumination field (Figure 2), which indicated that the sample evolved naturally towards this morphology at this temperature.

Thus, the electron beam can provide a supplementary amount of energy that can increase the temperatures within the nanoparticles (probably up to a few ten of °C). This additional heating then triggered mass transport leading to sintering phenomena and the evolution of the grains towards a single crystal.

Complete description of the first stage of sintering

Using 50 nm diameter ThO2 nanospheres, the first stage of sintering was continuously observed on a two isolated grains system from room temperature up to 950°C. The complete description of the different processes observed can be reported, from the grain scale to the atomic scale.

Observations at the grain scale

In the present study, HT-HRTEM images were recorded regularly from room temperature to 950°C, in order to describe the general behavior of a two-grain system (Figure 3). Between each temperature isotherm, the sample was heated with a 15°C/min heating rate. The experiment was stopped when the SiN1-x membrane began to recrystallize, at about 950°C under high vacuum. The general morphology of this system was directly compared to that reported by Nkou Bouala et al. 34 who recorded in situ HT-ESEM images series on 250 nm diameter grains systems to observe the morphological variations occurring during a heat treatment at 1275°C at the microscopic scale. This study evidenced that the number of crystallites present in the grains decreased during the heat treatment, while necks formed concomitantly and grew regularly with time. Meanwhile, an exaggerated growth of a crystallite located in the neck region was systematically observed at the end of the first stage of sintering.

In the case of the nanospheres investigated in the present study, the images in Figure 3 clearly demonstrate that two sintering processes occurred concomitantly in the two-grain systems. The first one took place within the grains and corresponds to the sintering and growth of the elementary crystallites (i.e. to the decrease of the crystallites number in each grain), while the other led to the formation and the development of a neck between the two grains. One can also observe the formation then the elimination of a pore between T = 800°C and 950°C (shown by the white arrow in Figure 3). This pore, which was observed as a white zone in the center of the upper grain, was present at 800°C and 900°C. Finally, when the temperature increased from 900°C to 950°C, the morphology of the grains turned from a spherical to a cubic shape, as already described before.

Even if the thermal treatments were not strictly identical, and despite the difference in the particles diameter (i.e. 50 nm vs 250 nm), the general behavior of the ThO2 particles appeared to be very similar when observed by HT-ESEM and HT-HRTEM. The nanoscale particles used in the present study then presented exactly the same behavior than their micrometric counterparts, and can be considered to be representative for real powders used in the fabrication of oxide ceramics.

Observations at the crystallite scale

When observing the sample at higher magnification, information was available down to the crystallite scale. The image series reported in Figure 4 is a magnification of the edge of the upper grain that is included in the white rectangle shown on the first image of Figure 3. This image series allowed a local and step-by-step description of sintering.

In this zone, the shape of four crystallites are underlined in the first image recorded at T = 55°C.

When temperature increased, the orientations of these crystallites relative to the electron beam direction and the crystallographic orientations of the grains were observed. As instance, the red crystallite was not observed at low temperature but became clearly visible when the temperature rose up to 77°C. Afterwards, its orientation remained stable up to 428°C. The blue crystallite was observable from time to time, with different crystallographic orientations, indicating that this latter was changing continuously during the heat treatment up to 328°C. This was also the case for the yellow and green crystallites. Some modifications in the orientations were observed up to 300°C and occurred very fast. Indeed, some of these rotations were found to happen between two consecutive images, i.e. within a 143 ms frame rate. Below 300°C, the edges of the crystallites remained unchanged. Only the plane orientation was modified between the two images. This process is very similar to the oriented attachment mechanism that was used to describe the organization of nanometric crystals in liquids 48 .

To our knowledge, if such a process was already reported to describe the reorganization of ceramic nanocrystallites at high temperature 10 , it is the first time that it is directly observed as it was already done for metallic nanoparticles [49][50][51] . Above 300°C, all the crystallites observed in this zone adopted a unique crystallographic orientation. However, point defects remained present in the grains as underlined in the image recorded at 428°C, where the white arrow (Figure 4) indicated the presence of an additional atomic column. Even if these local defects moved when the temperature increased, this nanostructure, made of oriented domains with walls containing point defects, remained observable up to 724°C.

In the zone of contact between the two grains (Figure 4b), crystallites can be hardly attributed to the upper or to the lower grain, and they can be considered to be shared between both grains, as already reported in Figure 1. At RT, the point-contact between the two grains was formed by numerous crystallites that have no chemical nor physical bonding. The misorientation between the crystallites that are observed in each grain is obvious (Figure 4b). When the temperature increased up to 300°C, the crystallites were re-oriented one in relation to each other as already described previously (see the crystallites outlined in green in Figure 4b). Surprisingly, the neck between the grains was already formed even for temperatures as low as 300°C. As crystallites located between the grains were already in contact at RT, they rotated to accommodate misorientation, and then formed the first physical link between the two grains. The reorientation of the crystallites yielded to crystallite growth when increasing the temperature. The grain boundary, which was clearly observed at 950°C, corresponded to a misorientation between the general orientations of the crystallites in each grain. As HRTEM micrographs were recorded, FFT patterns were calculated on limited areas of the images in order to determine the average crystallographic orientation of the crystallites in each grain, according to the image processing described in the Methods section and reported as Supplementary File S6. The FFT patterns obtained for the two-grain system at RT, 300°C, 450°C, 650°C, 800°C and 900°C are reported in Figure 5a, while the representation built from the series of FFT patterns is reported in Figure 5b. The FFT pattern obtained at RT presents the characteristic diffraction spots of a polycrystalline sample. The misorientation angle was evaluated to approximately 5° around the main [200] direction. Thus, at room temperature, the grains were formed of crystallites that were slightly tilted one with respect to the others inside each grain. Furthermore, the FFT pattern recorded at room temperature showed that both grains presented the same general orientations, even if they were misoriented. Furthermore, the 200 diffraction spot was observed for the upper grain while both 200 and 020 diffraction spots were observed for the lower grain. This second spot is underlined in green in Figure 5b. When the system was heated up to 200°C, the FFT pattern of the lower grain only presented the 200 diffraction spot (as the diffraction spot located at about 100° disappeared, Figure 5b, green frame), which clearly illustrated that a fast reorganization of crystallite orientations occurred at low temperature (typically in the 100-200°C temperature range). Such structural modifications were also observed in Figure 5b by looking at the signal centered at 210° (black frame). Here, the (200) planes tended to form 2 separated spots, each spot corresponding to one grain. This characterized the crystallites reorganization occurring inside each grain, as already observed in the HT-HRTEM images. This reorganization, i.e. orientation of crystallites in a common direction within each grain, can be directly observed on the FFT patterns obtained at 450°C and 650°C (Figure 5a), where the diffraction spots corresponding to the upper and the lower grains are represented in red and green, respectively.

The diffraction spots become punctual while temperature increases from 100 and 750°C (black rectangle in Figure 5b). This corresponds to an increase of the crystallites ordering, as all the crystallites tend to adopt a unique crystallographic orientation within a grain. This indicates that the misorientation of the crystallites decreases in each grain, which is consistent with previous findings from HT-HRTEM images. However, the two grains remained misoriented one with respect to the other by approximately 15° at T = 700°C Above 780°C, the angular positions of the 200 diffraction spots appeared to be quickly modified, indicating that there was a strong and fast modification of grains (Figure 5b). In this range of temperature, the 200 diffraction spots for each grain were located at different angular positions, which resulted from the rotation of one grain relative to the other one. The angular variation was approximately 10°. The rotation of the grains during the first stage of sintering has been already reported during studies devoted to the sintering of Cu metal spheres using X-ray tomography analyses 29, 52 during the study of ZrO2 monocrystalline grains sintering 18 . The authors indicated that the driving force for such a reorganization was the minimization of the surface energy in the system. The maximum rotation amplitude reported in these studies was 20°, which is in good agreement with the value determined in this work. This mechanism, which is important to explain the grain rearrangement inside the solids at high temperature, is herein observed directly for ceramic-type particles. The present study then confirm the observations made by Rankin 14 and proved that the grain reorientation also occurs in ceramic materials prior to grain growth. The driving force seemed to be related with the minimization of the contact energy between the two grains in the neck region. At T = 950°C, the FFT patterns became unusable as the recrystallization of the SiN1-x membrane induced the formation of additional diffraction spots superimposed to those of ThO2. However, as already described in the previous section, important transformations were observed at the grain scale, particularly in the upper grain and close to the neck. Indeed, even if the grains were already almost single crystals at 900°C when looking at the FFT pattern (Figure 5a), an important reorganization occurred in the upper grain at 950°C within a limited time (only 66 seconds from the beginning to the end of the transformation). From a morphological point of view, the transformed zone seemed to become a single crystal and the edge between the raw and transformed zone was easily identified, as there seems to be no crystallites in the transformed region (Figure 6). This transformation was associated to a very slight reorganization of the crystallites, leading to the elimination of stacking faults.

As the transformation seemed to begin within the neck zone to finally propagate in the grain itself, one can suggest that an important rearrangement took place at the boundary between the two grains.

The recovered energy was probably low, but sufficient enough to cause further significant changes step by step. In the present experiment, the transformation stopped before the grain turned completely into a single crystal. A supplementary energy barrier (maybe due to a high concentration of stacking faults that have been dragged during the transformation or to a high concentration of dislocations that forms walls or to dislocations that encounter point defects that stop their movement…) might have stopped the propagation of the transformation. However, the formation of a single crystal still remains the most stable configuration for the grain at higher temperature. Indeed, as reported in Figure 2, both grains became totally and perfectly monocrystalline after heating at 1025°C, even if they did not share the same crystallographic orientation at the end of the experiment. Such a transformation has already been described by Nkou Bouala et al. during the in situ study of the first stage of sintering of ThO2 by HT-ESEM 34 . Also, it is important to point out that this process has been observed three times during our experiment (at T = 950°C). Indeed, at the end of this experiment, the morphology of other two-grain systems that have never been exposed to the electron beam illumination, has been checked. In these systems, the sintering (formation and growth of a neck between the two grains) also proceeded although the formation of monocrystalline domain was not observed in the neck region. A few seconds after the sample irradiation by the electron beam, such a transformation was finally observed, as previously described.

Growth of the neck between the 2 grains At 950°C, as the grains are monocrystalline, the plane that forms the contact between the two grains is the grain boundary as defined by Coble 30 . Consequently, the classical laws describing the first stage of sintering, i.e. neck growth between two single crystalline particles, can be considered. The diameter of the neck increased from 17 (± 0.5) nm to 32 (± 0.5) nm in only 2 minutes. From the set of images, the evolution of the sintering degree, λ, defined as the ratio between the neck (x) and grain (r) diameters was followed at 950°C (Figure 7). It was fitted using the classical equation developed for a two monocrystalline grains system, according to Eq. 1:

λ = 𝑘. 𝑡 𝑟 (𝐸𝑞. 1. )
where λ = x/r denotes the sintering degree, k is the kinetic constant, t is the duration of heat treatment, and n and m are exponents characteristic of the diffusion process involved (i.e. mainly surface, volume or grain boundary diffusion).

As the grain diameter remained constant at 950°C (in a first approximation) and close to 50 nm, the r m term was considered to be almost unchanged, which led to the definition of an apparent kinetic constant k' such as : λ = 𝑘. 𝑡 𝑟 = (𝑘′. 𝑡) (2. ) As explained above, the n value was used to identify the main mechanism driving sintering, with n = 2 accounting for viscous flow; n = 3 for evaporation-condensation; n = 4 for volume diffusion; n = 6 for grain boundary diffusion and n = 7 for surface diffusion. In our study, fitting the experimental data obtained at 950°C led to n = 3.8 ± 0.2, thus indicating that the neck elaboration mostly processed through volume diffusion. This result complements the findings of Nkou Bouala et al. who reported a value of n = 6 from HT-ESEM observations performed at higher temperature (i.e. between 1175 and 1300°C) using bigger particles (typically around 250 nm in diameter) 34 . In our case, the process imaged corresponds to the early formation of a neck between the particles, which obviously cannot proceed through grain boundary diffusion, as this latter is not formed at t = 0. Once the neck initiated and partially developed, and once the grain boundary defined following the alignment of crystallites within both grains, grain boundary diffusion can be activated, which corresponds to the phenomena observed by Nkou Bouala et al. Also, it is important to note that both diffusion paths (i.e. volume and grainboundary diffusion) led to the sample densification and to close microstructural evolutions.

Observations at the atomic scale

To go further into the description of the first step of ThO2 sintering, HT-HRTEM images were recorded in the neck region at T = 950°C (Figure 8a). As described previously, the grain boundary between the two grains corresponds to a misorientation of both grains. When expanding the heat treatment at T = 950°C, atoms diffused to the neck surface and a fast reorganization of the atomic planes was observed within the same crystallographic orientation. The kinetic of the grain boundary displacement by atomic column additions was estimated by quantifying the motion of the atom front plane (Figure 8b). The motion of the front plane within a single crystal was found to be linear with time. The derived motion rate was close to 1 nm.s -1 , which corresponds to the addition of approx. 4 atomic columns per second to the neck. Another zone of the neck, corresponding to the opposite side of the upper grain was also studied. In this region, no fast transformation occurred during the experiment. One particular crystallite, where the (200) planes were observable remained misoriented compared to the others in the grain. Such a configuration in the upper grain might result from the fast transformation described before, for example from the formation of walls due to the concentration of dislocations. Snapshots of the 1/7 s movie recorded in this region (Supplementary File S9) are gathered in Figure 9. They show motions of atoms and atomic columns. The displacement of atom columns was evidenced by comparing the four images. It corresponded to a fast process, which occurred within a 0.15 s delay (the moving plane is indicated in Figure 9 by white and black arrows). However, the time between two displacements lasted up to a few seconds. During this delay, the structure was stable and no diffusion was observed at the atomic scale (even at 950°C). In this case, the motion rate of the grain border was lowered to 0.1 nm.s -1 , which was ten times slower than that determined for the atomic column motion within the same crystallographic orientation. Such a result suggests that misorientations between the crystallites were able to slow down or stop the recrystallization processes.

Discussion

From a polycrystalline to a single-crystal grain

From the observations reported above, a multi-step mechanism describing the reorganization of the crystallites within a grain during a heat treatment at high temperature was proposed. First, the characterization of the starting material showed that the crystallites forming the nanospheres were not randomly oriented. They all shared a common orientation, yet with a small angle of misorientation.

Second, the reorganization of the crystallites within a single grain occurred between RT and 300°C.

This yielded the disorientation angle to decrease between the crystallites when temperature was higher than 100°C. The underlying driving force is probably the minimization of the surface free energy of the nanoparticles in the grain 53,54 . This reorganization was achieved by a slight rotation of nanocrystallites one with respect to the others, so that a coherent combination of their crystalline planes could occur. This mechanism is close to the oriented attachment mechanism described for the growth of crystallites in solution 27 or at high temperature 8 . With the 50 nm diameter ThO2 nanospheres made of 3-5 nm crystallites used in the present study, this reorganization was observed up to 300°C. Nevertheless, it was not reported during the sintering of UO2 (3-5 nm diameter) or ThO2 (19 nm size) nanoparticles 55 that were initially isolated 56 . In the case of 3-5 nm CeO2 nanoparticles sintering, Cordeiro and Leite 22 have shown that even if the sintering onset was delayed up to 870°C

(due to the presence of organics at the surface of the nanoparticles that turned into an amorphous carbon layer at high temperature and stabilize the NPs), nanoparticle rotations were observed. This yielded to the reorientation of the crystallographic planes and to a fast growth mechanism of the nanoparticles close to the mechanism observed in this study. These authors noted that a neck between the nanoparticles was effectively formed after this reorientation.

Above 300°C, the crystallites were mainly reorganized and aligned along a main crystallographic orientation in the grain. The walls of the domains were made by concentrated point defects that migrated with increasing temperature. Their concentration resulted in the formation of a pore in the upper grain observed at 800°C. The elimination of this pore then occurred between 900 and 950°C, probably through a volume diffusion mechanism driving the matter from the grain center to the neck.

Simultaneously, the size of the crystallites still increased up to 950°C.

Finally, for T ≥ 950°C, the crystallites present in the grain had the same crystalline orientation.

However, boundaries probably remained present between the crystallites in the form of high defects concentration zones. A slight temperature increase (or illumination by the electron beam) brought sufficient energy to eliminate the last point defects or dislocation walls present between the crystallites. This process was fast and yielded to the formation of a single crystal. The 50 nm diameter grain then became monocrystalline and finally adopted a cubic shape.

Formation of the neck

On the selected systems, a point of contact was observed as soon as the two grains were deposited on the SiN1-x layer, and appeared to be formed by the outer crystallites present in both grains. However, this contact was not consolidated at room temperature. When temperature increased, the neck was consolidated by the oriented attachment mechanism that yielded to the growth of the crystallites present in the region of the neck 57 . Even if the crystallites exhibited similar crystallographic orientations in the grain, those located in the neck region were not specifically oriented. Both grains could then detach as the neck was not sufficiently consolidated, i.e. up to 300-400°C.

Above 400°C, the crystallites in the region of the neck were reoriented with respect to the main crystallographic planes in one grain. In these conditions, the neck was physically formed and consolidated. The grain boundary corresponded to the plane of crystallographic disorientation between the two grains. The large number of crystallites that are present in the zone of the neck was expected to increase the growth rate of the neck 31 . However, up to 950°C, only the growth of crystallites was visible on the HT-HRTEM images (Figure 4b) and the growth of the neck size was not obvious. Thus, a temperature lower than 950°C is not high enough to induce the growth process of the neck.

At 950°C, i.e. after the grains turned to single crystals, the grain boundary was clearly observed in the images (Figure 6). It separated both grains that had two distinct crystallographic orientations. The disorientation between the grains was not eliminated while pursuing the heat treatment and the grain boundary (which is the neck) remained stable with time. The growth of the neck was mainly associated to matter transport from the bulk of the grain to the neck and was clearly associated to the grain transformation into a single crystal (Figure 8). The presence of misoriented crystallites in the grain decreased the neck growth rate. This finding is in good agreement with the results reported by Cordeiro et al. 22 . If one grain rotated relative to the other in order to recover the same crystallographic orientation, the grain boundary was eliminated and the neck grew faster. This mechanism, which was observed for small nanoparticles 8 was probably too energetically consuming for grains as large as those used in the present study (50 nm diameter).

Conclusions

High Temperature and High Resolution TEM observations were used for the first time to image the early processes leading to the formation of necks between the grains during the sintering of oxide ceramics, herein ThO2 polycrystalline particles. High spatial and time resolutions offered by HT-HRTEM led us to distinguish several successive steps giving birth to grain boundaries that rely on morphological modifications from the atomic scale to the grain scale.

First, observations of 50 nm-diameter ThO2 spherical particles prepared by wet chemistry methods revealed that the crystallites constituting the grains were not randomly oriented, but shared a common orientation following self-assembly in solution. As such, the starting material still differed from the single crystals generally considered in numerical models, but cannot be described as a fully disordered polycrystalline assembly.

Upon heating, rearrangement of crystallites first occurred within each grain. This process was mainly driven by oriented attachment at low temperature and led to the growth of crystallites. At high temperature (typically above 900°C), the elimination of the remaining defects yielded the formation of single crystals. This modification of the inner microstructure of the grains was also found to impact the nature of the contact between the grains, which pre-exists at room temperature then progressively evolve to form a neck. At low temperature, it is constituted by numerous crystallites that can be considered to be shared between both grains, and then significantly differs from the usual representation of the grain boundary reported in the literature and used in the numerical models. Such a state-of-the-art grain boundary was only obtained following the reorientation of the crystallites at high temperature, and corresponds to the plane of disorientation between the two grains. The growth of the neck finally proceeds by atomic displacements. These latter were faster (and seemed favored) when the grains were monocrystalline.

All these findings showed that the initial stage of sintering, dedicated to the formation and the growth of necks between the grains, is strongly governed by the behavior of elementary crystallites.

Monitoring their size/morphology and self-assembly processes during the synthesis then appears as a key challenge to improve the sintering ability of ceramic powders. Also, their prevailing role in the first instants of the densification process underlines the need to consider polycrystalline materials in numerical models. As such, advanced microscopic techniques allowing in situ observations with high time and space resolution should be considered as a tool of choice to collect new information and quantitative data for the update or development of future sintering models.

Figure 1 .

 1 Figure 1. a) General view of an as-prepared two-grain system (50 nm in diameter) with FFT patterns corresponding to each grain obtained from the HRTEM image. b) High resolution image of the point-contact region between the two grains. White squares correspond to the zones presented in inserts for high magnification observations (10 × 10 nm² in each grain).

Figure 3 .

 3 Figure 3. HT-TEM images recorded at different temperatures during in situ heating of a two ThO2 grains system (50 nm in diameter) at low magnification (scale bar = 20 nm). The white arrow reported on the image recorded at T=900°C shows the formation of a pore in the upper grain.

Figure 4 .

 4 Figure 4. a) HT-HRTEM images recorded at different temperatures and at high magnification during in situ heating of a two grains system of 50 nm in diameter showing the morphological modifications occurring at the edge of the upper grain. This zone corresponds to the white rectangle reported on the first image of Figure 3. b) Lower magnification images showing the evolution of the neck during the heat treatment (scale bars = 2 nm). See Supplementary Files S4 and S5 for the complete movies.

Figure 5 .

 5 Figure 5. Processing of the FFT patterns determined from the HT-HRTEM images recorded on the 50nm two grains system shown in Figure 3. The red and green dots correspond to the diffraction spots of the upper and lower grain respectively. a) FFT patterns obtained on the complete system at 22°C, 300°C, 450°C, 650°C, 800°C and 900°C. b) Diagram showing the positions and the intensities of the 200 diffraction spots as a function of temperature. Several domains are identified for discussion.

Figure 6 .

 6 Figure 6. HT-TEM image series recorded at T = 950°C on a two grains system showing the morphological modifications occurring on the upper grain and on the neck region (see Supplementary File S7 for the complete movie).

Figure 7 .

 7 Figure 7. Evolution of the sintering parameter  obtained at T = 950°C from HT-HRTEM images on ThO2 nanospheres of 50 nm in diameter.

Figure 8 .

 8 Figure 8.a) HT-TEM Image series recorded at 950°C on a two polycrystalline spheres system (see Supplementary File S8 for the complete movie). The delay between two images was 2/7 second (Scale bar = 2 nm). The image series must be read from left to right and from top to bottom. b) Variation of the position of the front plane with time.

Figure 9 .

 9 Figure 9. HT-HRTEM images of the grain boundary showing the motion of atomic columns. Interplanar spacing in the crystallite of interest and misorientation between to neighboring crystallite are shown on the first image. The black and white arrows indicate the moving plane (see Supplementary File S9 for the complete movie).

Acknowledgments

The authors acknowledge financial support from the CNRS-CEA "METSA" French network (FR CNRS 3507) for the TEM experiments conducted on the MPQ (Université de Paris) platform.

Conflict of interest

are no conflicts to declare.


O Coindreau

G Vignoles

P Cloetens

DIRECT 3D MICROSCALE IMAGING OF CARBON-CARBON COMPOSITES WITH COMPUTED HOLOTOMOGRAPHY

Keywords: Tomography, C/C composites, phase contrast, holography PACS numbers : 81.70.Fy, 61.10.Yh, 81.05.Rm, 81.15.Gh

As part of the modelling of CVI (Chemical Vapor Infiltration), one of the processing techniques of Carbon -Carbon composites, a better understanding of the relation between fiber architecture and transport properties is required. An excellent starting point for the porescale modeling of heat and gas transport is the acquisition of 3D images of the porous media through Computed Micro Tomography at various densification stages. Due to the low X-ray absorption rate of light materials such as carbon, a poor image contrast is obtained with absorption tomography. On the other hand, phase contrast imaging is readily feasible using the coherence properties of modern synchrotron beams. Holotomography has been performed on these materials and it provides quantitative density images where fibers and pyrocarbon matrix deposit are easily distinguishable. Such images are appropriate for the pore-scale computation of many effective transport properties.

Introduction

Carbon-carbon composites are produced, among other processes, by chemical vapor infiltration (CVI) : it consists in densifying a heated fibrous preform by the cracking of a vapor precursor of the matrix material [START_REF] Naslain | Materials Chemistry at High Temperatures[END_REF]. In addition to processing conditions (such as vapor precursor concentration, temperature and pressure), structural and transport properties of the preform determine final characteristics of C/C composites and especially their density. As part of CVI modeling, the evolution of the structural and transport properties of the C/C composite during the densification is required to optimize its final porosity and homogeneity [START_REF] Starr | [END_REF]. X-ray tomography appears as an ideal technique for nondestructive characterization at the different pore scales of 3D microstructure. Absorption tomography has already been used successfully to characterize the structure of SiC fiber cloth layup preforms at macro-porosity scale (pixel size of 15.6 µm) [3,4]. In this method, the image contrast results from the difference between the X-ray attenuation coefficient of the phases (porosity, fiber and deposit). For C/C composites, however, the difference between these coefficients is small and the contrast is faint. Therefore, we have used a new imaging method called holotomography [5,6] to characterize at microporosity scale carbon-carbon composites.

Principle of microtomography and holotomography

The interaction of X-rays with a medium can be described by its complex refractive index n=1-δ+iβ [START_REF] Cloetens | X-ray tomography in material science[END_REF]. The form of a plane monochromatic wave propagating along the Z-axis in this material is :

The imaginary part β determines the attenuation of the wave and the refractive index decrement δ determines the phase variation compared to propagation in vacuum. For inhomogeneous samples the transmission function T(x,y) is :

2 2 exp[ z ] exp[ i( 1 ) z ] π π β δ λ λ - - (1)
(2)

0 2 2 T( x, y ) exp[ ( x, y,z )dz ] exp[ i( ( x, y,z )dz )] π π β ϕ δ λ λ = - - ò ò
In classical absorption tomography, the detector, set directly behind the sample, measures the attenuation, which allows to calculate the integral of β along the considered path. Repeating this measurement for a large number of angular positions of the sample permits to reconstruct the β(x,y,z) map through the use of a tomographic reconstruction algorithm [START_REF] Robert-Coutant | X-ray tomography in material science[END_REF].

Holotomography, on the other hand, allows to reconstruct the δ(x,y,z) map thanks to the knowledge of the phase distribution for each angular setting of the sample. The phase map is extracted from a series of images (three or four) recorded at different distances from the sample taking benefit of the coherence properties of third-generation synchrotron radiation beams (holographic reconstruction). Then the δ map is reconstructed with the same algorithm as in classical tomography (tomographic reconstruction). Contrast obtained with this technique is supposed to be enhanced since the ratio δ/β is much higher than unity for hard Xrays.

Experimental method

Experiments have been carried out on a preform made by stacking carbon fiber satin-weave (X-Y plane) held together by needlings (Z direction). The volume fraction of fiber is about 27%. This preform had been densified by isothermal-isobaric CVI and its core was consequently more porous than its borders. A cylindrical specimen was extracted from this bulky preform with its axis parallel to Z. Pieces at three different depths were sampled from it.

A sample was then cut in each piece which had been previously embedded in organic resin.

The diameter of those cylindrical samples is 1.3 mm and their length is 3 mm. Their macroscopic densities are 0.77 (sample 1), 1.23 (sample 2) and 1.52 (sample 3) g/cm 3 .

Holotomographic experiments were performed on the ID19 line at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The synchrotron X-ray beam on this line, devoted to high-resolution imaging, shows a high lateral coherence due to the small source size and its large distance to the experimental hutch. The beam energy was set to 15 keV (λ=0.8339 Å).

The sample was positioned with a precise goniometer (fine rotation and translation) and 2D radiographs were recorded every 0.12° of rotation along the vertical axis (up to 180°). Four sets of 1500 radiographs were recorded on a 2048×2048 CCD based detector [START_REF] Labiche | [END_REF] with sampledetector distances of 12, 50 ,150 and 500 mm respectively. The effective pixel size of the high resolution detector was 0.7×0.7 µm 2 and the investigated volume of material 0.9×0.9×0.7 mm 3 . The average exposure time was 1 s and a complete acquisition lasted about two hours.

Results and discussion

Quantitative density images

The holotomographic reconstruction was performed for the three samples using the first three ρ is in g/cm 3 and λ in Å. 3D reconstruction of δ allows to draw the density map provided that the approximation of K is accurate enough. For materials containing mostly carbon (fiber and deposit), K is about 1.34×10 -6 and for materials containing carbon and hydrogen (organic

(3) 2 K δ ρλ =
resin), K is higher, about 1.42×10 -6 for polycarbonate. Fig. 1c shows a profile along the line segment shown in Fig. 1b. The grey level is actually proportional to the reconstructed value of -(2π / λ)δ (see equation 2). The value in the region outside the sample is taken as reference.

The value of δ is 1.28×10 -6 in the region that contains resin, 1.85×10 -6 in deposit and 1.57×10 -6 in fiber corresponding to densities of 1.3 ± 0.2, 2 ± 0.1 and 1.7 ± 0.15 respectively. This is in agreement with expected values [START_REF] Sauder | Relation microstructure / propriétés à haute température dans les fibres et matrices de carbone[END_REF]. The profile along a line intersecting an isolated fiber has also been performed for the three samples to compare the thickness of the carbon deposit. These profiles are shown in Fig. 2. For sample 1, the deposit around the fibers is too thin to be detected on the image. Its thickness is about 7 µm in sample 2 and 9.4 µm in sample 3.

Segmented images to compute effective properties

Images of C/C composites using phase contrast at a single propagation distance had been obtained previously [START_REF] Vignoles | [END_REF]. In this « edge detection regime », a dark/light fringe appears at density jumps. The interior of both phases (porosity and carbon) displays an indistinguishable mean grey level, while their interface appears as a double black-white layer. An image processing procedure has been designed to assign the color of its border to the interior of each phase. However, this treatment is lengthy and results are qualitative (quantitative information on density is lost). Images obtained with holotomography are expected to be segmented more easily. An extract from the reconstructed volume of sample 1 has been taken. Fig. 3 shows the histogram of grey levels. Two grey levels are easily distinguishable and correspond to the porosity and the carbon respectively. A 3D visualization of the binary image is shown in fig. 4. Another volume has been extracted from the sample 3. Its histogram in fig. 5 shows 3 grey levels corresponding respectively to carbon deposit, carbon fiber and resin. It is not easy to separate the two carbon phases since their mean grey levels are quite close and their distributions spread out. However the mean grey levels of all forms of carbon on one hand and resin on the other hand are different enough to enable a direct segmentation of this image.

However, two problems persist for the fully automated processing of the complete reconstructed data sets :

1. the image background level is not constant, so that density measurements and segmentation can only be performed locally, 2. fibers lying in a plane perpendicular to the rotation axis are prone to yield strong artifacts after reconstruction. This is probably due to the very large optical phase introduced by the fibers when beam and fiber direction are parallel.

Conclusions

Partially densified C/C composites have been imaged with holographic microtomography.

Preliminary results validate holotomography as a quantitative tool for the determination of the density of carbon-based material parts, sensitive enough to distinguish between two types of carbon. Moreover, it provides readily segmented images of the porosity, which may then be used for the computation of effective gas transport properties [START_REF] Coindreau | Etude 3D de composites C/C par microtomography X synchrotron[END_REF].

Reference number of Manuscript : I -XI.5

List of Figure Captions

  distances. This results in the 3D distribution of the refractive index decrement δ or equivalently of the electron density for each of them. Fig 1a shows a slice of the reconstructed distribution in sample 3 and fig. 1b an extract from it. The refractive index decrement δ has a much simpler dependency on the energy and the material characteristics (essentially on the electron density) than the absorption index β. For many atomic species, a good approximation of δ is [7] :

Fig. 1 :

 1 Fig. 1 : A slice (1925×1925 pix) of the reconstructed volume in sample 3 (a), an extract from

Fig. 2 :

 2 Fig. 2 : Profiles along 3 lines intesecting isolated fibers in sample 1, 2 and 3 respectively.

Fig. 3 :

 3 Fig. 3 : Histogram of a volume (160×200×154 voxels) extracted from sample 1.

Fig. 4 :

 4 Fig. 4 : 3D rendering of a volume extracted from sample 1.

Fig. 5 :

 5 Fig. 5 : Histogram of a volume (200×200×190 voxels) extracted from sample 3.

Fig. 1

 1 Fig. 1

Fig. 1a

  Fig. 1a

  Fig. 1b

  Fig. 1c reconstructed value of δ

Acknowledgments

The authors are indebted to Snecma for sample supply and thank the ESRF for providing beam time and support.


Théo Marchal

Noha Gamal Said
email: noha.gamalsaid@eng.asu.edu.eg

NOHA GAMAL SAID Theo Marchal
email: theo.marchal@grenoble.archi.fr

Parametric tool to

published or not. The documents may come

Esquis'Sons! Design support tools for sustainable sound environments

Esquis'Sons! is a parametric tool that constitutes the main focus of this paper. Its name couples two French terms: esquisse and son. Esquisse in French is equivalent to "Sketch" in English; it signifies an architectural tool for elaborating the main design concept and corresponds to the early design phase as well; while son means sounds. This combination 'Esquis'Sons!' highlights the objective behind the creation of this tool, which involves primarily introducing the sound dimension as a main design material to be manipulated in the early stages of the design process. Esquis'Sons! is therefore a sound sketch tool built as a parametric interface using the Grasshopper plug-in in order to link Rhinoceros (3D modeling) and Max MSP (3D sound creation). The tool allows for the possibility to hear sound scenes generated from a digital model of space in 3D. It also allows for assigning acoustic characteristics, such as the level of porosity of volumes and faces, to spatial contexts, in addition to positioning points of listening anywhere in the 3D modeled space. The auralization module is informed by the geometrical characteristics of the spatial model and vice versa. We then adjust the qualities and the origin of the sound en snifi 0.00 re W* n BT /F1 10.98 Tf 1 0 0 1 393.1 380.84 Tm 0 g 0 G [(Th)-3(e )-239(au)4000886 841.98 re W* n Q ist5800

D1P1S2 EMERGENT MODES OF LEARNING I

158 | ASCAAD 2021 tool status applicable in any urban context [START_REF] Marchal | Esquis'Sons ! : Outil d'esquisse de l'environnement sonore[END_REF]. The next step was to test the validity of the Esquis'Sons tool in an educational exercise.

We have chosen the form of an intensive one-week thematic school on soundscapes that took place in Grenoble in January 2017 during the week of sound-A European event on Sounds. Managed by the Cresson research team at the Grenoble School of Architecture, the objective of this thematic winter school was to introduce the sound dimension in contemporary architectural issues by coupling work on sound environments and dense urban forms. Sound, which is today an important element to enhance the quality of life of the built space, is particularly important in the increasingly dense urban environment.

In fact, dense urban forms propose a specific architectural language in which aspects such as building heights, shared spaces or more generally the question of interfaces play an important role in constituting ordinary listening and changing sound practices. Providing a dynamic relation to the surrounding, varying from ground to sky, from near to far or from intimacy to exposure, the dense habitat brings out remarkable listening situations, including suspension, immersion, nesting, panoramic sound, dual listening, exposure, or acoustic retreat (Gamal Said, Marchal, 2017).

Based on a space-sound pairing concept, the winter school focused on two archetypal dense urban forms: towers and courtyards, therefore defining a language of protrusions and voids (Figure 1). In this urban configuration, the interfaces located at the articulation between private and public spaces, such as balconies, loggias, corridors and landings, form shared sound spaces, which constitute in this winter school the main architectural object of study.

They are as much configured by the sound qualities of the street as they themselves configure the sound qualities of the latter. It is in this reciprocal exchange that we shall consider these liminal spaces, in the way that they redraw new acoustic arenas and negotiate their intimacy with public spaces.

In dialogue with the existing environment, we sketch sound atmospheres based on the Esquis'Sons tool.

In order to auralize in real time the different audio-spatial intervention scenarios, participants engaged in in situ sound recordings, representations, manipulations and sketches, where the intention was to explore sounds in order to create an atmosphere (Figure 2). Participants of the winter school were invited to pursue the following steps:

The first step is to define a spatial context, which was to be modeled in Rhinoceros3D via Grasshopper. The second step is to set listening points by associating a point in the model with the position of the listening point from which auralization is to be calculated in real-time. In the third step, they build a sound scene by defining sources associated with points « source_location » in the Rhinoceros / Grasshopper 3D model. The last step involves auralization in real-time by connecting these different objects to the main sketch motor through which we obtain a sound result [START_REF] Remy | Esquis'sons ! Outils d'aide à la conception d'environnements sonores durables[END_REF].

Sound Diagnosis in Two Urban Districts in Grenoble

Two districts in Grenoble, France, Village Olympique and Vigny Musset were chosen as case studies for conducting the sound diagnosis and modification (Figure 03). The urban configurations of the two districts are composed of pedestrian spaces which are qualified in terms of sound as impermeable in certain spaces and porous in others due to the presence of sound doors opening to the outside, or completely open, exposed to urban traffic. Despite the contrast in urban and architectural forms of the two districts, their geographical juxtaposition has allowed participants to compare and articulate them by means of transversal listening. Two sound walks were conducted, one was free and silent during the day and the other was nocturnal and guided by Pascal Amphoux. These listening exercises have allowed participants to identify certain sono-architectural typologies and to distinguish the sound markers and the singularity of the soundscapes of both districts. Beyond a simple diagnosis, the main objective of this phase is to spot specific characteristics in order to steer prospective transformation.

Sound Intervention Scenarios using Esquis'sons tool

The main objective of the thematic winter school is to use the Esquis'sons tool to design and project new sound environments for both districts. In The main idea of this project is to remove or open the gates and the grid surrounding the courtyard. This simple intervention changes the status of the courtyard as a private entity to a public space. This modification helps accommodating other usage patterns such as traversing, encounters, etc. The sonic sketch helped incorporating by sounds the increase in the degree of permeability between the interior ambience of the courtyard and the outer one of the street. The soundtrack permits to better understand the transformation in the sound environment of the courtyard (Figure 4).

Conclusion

The Esquis'Sons tool has allowed participants to propose real sound projects that constitute a solid material to listen and to debate. Thanks to this tool, it is possible to « sonorize » a 3D virtual environment and them to hear the consequences of architectural choices. In other words, this application allows you to sketch a space while listening to it. The projection of new sound scenarios results from combining the use of digital design tools (Rhinoceros / Grasshopper / Esquis'Sons!) with fieldwork by means of in situ sound recording and brief interviews with the inhabitants. The hybridization of the different working methods over three half-days has made it possible to bridge the theoretical presentations given during the week on the methods and fundamentals of the Cressonian approach to understand, modify, and create sound spaces and to apply them in an empirical exercise.

Figure. 1 .

 1 Figure. 1. The two urban fabrics: courtyards and towers and their acoustic behavior

Figure 2 .

 2 Figure 2. The in-situ sound recordings and the studios for elaborating the sound scenarios

Figure 3 .

 3 Figure 3. The urban form of Vigny-Musset to left composed of courtyards and Village Olympic to the right composed of linear pedestrian streets with residential towers.

Figure 4 .

 4 Figure 4. The location of the intervention scenario in Vigny-Musset with a Rhino and Grasshopper sketch.

Figure 6 .

 6 Figure 6. A sketch of Variation in suspension in Vigny-Musset using Rhino and Grasshopper

D1P1S2 EMERGENT MODES OF LEARNING I

	عمرانية أحياء علي تطبيقية دراسة خالل من وذلك البارامترية األداه هذه كفاءه مدي إختبار
	والعمران العمارة تشمل والتي التخصصات مختلف من عمل فريق بها قام جرونوبل بمدينة
	التغير	دراسة هو	العمل	ورشة من	الهدف	كان	ماع.	اإلجت وعلم الموقع وتنسيق والصوتيات
	والذي للمكان والمعماري العمراني الشكل في تغيير	إقتراح	عند العمرانية األصوات في
	االحياء في الصوتية الحسية الخبرة تحسين أجل من العمل ورشة في المشتركين يقترحه
									الدراسة. تحت العمرانية

D1P1S2 EMERGENT MODES OF LEARNING I

162 | ASCAAD 2021

THE FLYING BENCH, LE BANC VOLANT

Scale of intervention: the tower as building

The main idea is to work the relationship between building heights as a main characteristic of the dense urban form and sound perception. By modifying the presence of a sound source (a ball game), the Esquis'sons tool enabled a continuous reconfiguration of the listening experience as one climbs the stairs of a tower located in the Olympic Village. The building height underlines the change in sound perception through alternating between near and faraway sounds.

InTERREaction

Scale of intervention: the street and the courtyard This project proposes a sound prospective for the pedestrian alley "Allée des Romentiques" -mainly in front of Marie Reynoard School in Vigny-Musset.

It is a green linear space or promenade traversing the district and delimits enclosed spaces (courtyard and school). The proposed sound sketch seeks to make the urban configuration more porous, create spaces for sociability, and make the school entrance more secure by prohibiting vehicular passage.

Participants propose rural imprints by transforming certain green spaces into community gardens. The change in land covering (gravel and soil) creates a sonorous ground that marks the thresholds between public and private space (Figure 5).

Variations in suspension, Variations en suspension

Scale of intervention: the district

The sound-walk along the two districts -Vigny Musset and the Olympic Village -led the group to compare the two sound environments. The objective of this project is to import the different sono-spatial typologies composing the soundscape of the Olympic Village (pedestrian bridges, heights of the towers, and sound door) into the district of Vingy-Musset. In this optic, the team proposes a traversing sonorous bridge. The chosen path for this bridge flies over different urban elements such as the courtyard, street, pedestrian street, and main garden, giving rise to a new listening experience. The sonic sketch offers new porosities between private and public spaces: the street, school yard, buildings, courtyards, gardens and avenues. The variation in height along the bridge provides a more distant perception of the district's soundscape.

4.6

From the fountain to the market, De la fontaine au marché

Scale of intervention: the neighborhood

This project is about designing a new soundwalk that is available to hear on smart phones. After capturing the existing different sounds along the chosen path, the team develops a more exotic soundscape of Vingy-Musset! In the gridiron pattern of Vigny-Musset, the group wants to create sonic poles. It proposes to insert charismatic sources and to polarize public life in order to give back to the courtyards their main role as spaces of intimacy. A market, D1P1S2 EMERGENT MODES OF LEARNING I

Elaborating on a variety of sound sketches using Esquis'Sons! adds another dimension in understanding architectural and urban configurations. It opens new perspectives in fabricating and transforming city spaces. Putting the soundscape into debate has enriched and consolidated different sound projections. The proposed intensive one-week exercise helped elaborate rich prospective sound scenarios both in terms of content and presentation. In brief, it was a fruitful exchange!