I Yahyaoui

M Perrin

X J Gong

H Li

DAMAGE EVOLUTION IN WOOD UNDER TENSILE LOADING MONITORED BY ACOUSTIC EMISSION

Keywords: Wood, Acoustic Emission, Damage evolution, Failure mechanisms

Considerable progress can be observed within recent years in timber structures. But, the development of damage in these structures may cause catastrophic events. Therefore, frequent monitoring should be conducted to estimate the health condition of these structures. In this context, the acoustic emission (AE) is an efficient method for real time monitoring as well as damage growth in both structural components and laboratory specimens. In this study tensile tests in the axial direction were performed on standardized wood specimens. Three wood species were tested including Douglas fir, silver fir and poplar. Actually, these woods have close density, but their structures and their mechanical behaviors are quite different, so conduce to different damage process. The Douglas fir is softwood with a pronounced distribution of earlywood and latewood, their high strength among softwoods explains their important application for structural purposes. Silver fir is also softwood and it is abundant in the Pyrenean region in France, so interested to study for economical reason. Moreover its structure is quite different from that of Douglas fir. Finally poplar wood is a hardwood and it's well known as a homogenous wood. This work aims to investigate their damage process with help of AE sensor. During the test loading data were recorded simultaneously with the transient AE signals. And then we try to correlate the AE sources to the damage mechanism in the woods tested. According to results, the following conclusions can be stated: The AE data recorded from tensile tests indicate clearly that the scenario from damage initiation, accumulation up to final macroscopic failure is quite different from one wood to another one. It depends strongly on their distribution of early and final wood. The forms and the magnitudes of AE activities are directly related to the annual rings structure. It is shown that the wood with abrupt transition from earlywood to latewood are more emissive than wood with gradual transition. Also, acoustic emission monitoring allows the early detection of damage and the establishment of damage scenarios.

INTRODUCTION

Due to the increase of ecological awareness, the use of wood in civil engineering structures like timber bridges has been recently raised. Nevertheless, wood is an anisotropic material which is organized in a complex hierarchical structure distributed across multiple spatial scales, from microscopic to macroscopic scale. Hence, a thorough comprehension of the failure evolution in wood structures is required to avoid critical situations and to maintain safety standards. Until today, there is a lack of information in literature about tracking back the evolution of wood fracture zone to its origin. Such research should determine the temporal-spatial occurrence of damage mechanisms and their interactions at different length scales [START_REF] Ritschel | Nondestructive evolution of damage accumulation in tensile test specimens made from solid wood and layered wood materials[END_REF]. Regarding this issue, acoustic emission (AE) method should be suitable which is defined as a transient elastic wave generated by the rapid release of energy due to irreversible phenomenon within a material. It is shown that the acoustic emission (AE) method is appropriate for characterizing wood fracture process because it is sensitive to damage mechanisms at different length scales [START_REF] Ritschel | Nondestructive evolution of damage accumulation in tensile test specimens made from solid wood and layered wood materials[END_REF][START_REF] Varner | Acoustic emission during static bending of wood specimens[END_REF][START_REF] Wu | Acoustic emission characteristics and felicity effect of wood fracture perpendicular to the grain[END_REF]. This technique provides advantages of early detection of crack initiation [START_REF] Ansell | Acoustic emission from softwoods in tension[END_REF]. Besides, detailed monitoring of the damage evolution is allowed by the high time resolution of AE technique This paper analyses the damage process of three softwood species based on acoustic emission signal parameters. Douglas fir (Pseudotsuga Menziesii), silver fir (Abies pectinata) and poplar (Populus) were tested under tensile loading, the influence of the heterogeneity induced by the proportion of earlywood and latewood in annual rings on the acoustic emission response has been discussed.

EXPERIMENTAL DETAILS

Materials and specimens

The materials tested under tensile loading in this study are Douglas fir wood, silver fir wood and poplar wood. All of them have close density but different structure. Douglas fir has large well-defined growth rings and a sharp definition between the earlywood and latewood. For silver fir the transition between earlywood and latewood is more gradual. Only narrow bands of thicker latewood cells divide two regions of wide bands of earlywood. Silver fir structure is considered like uniform structure compared to Douglas fir structure. Poplar wood has different features compared to the two tested softwoods. Notable features for poplar wood are the diffuse pores in no specific arrangement, marginal parenchyma and nodded rays. The transition between earlywood and latewood is not distinct with naked eye so it is why poplar is considered as a homogenous wood.

Tensile specimens with dog-bone shape have been prepared from the three species of wood with nominal dimensions according to NF B51-017 (350 mm overall length, 100 mm length, 20 mm width and 20 mm thickness in grip section, 90 mm length, 20 mm width and 4 mm thickness in the calibrated section (Figure 1). All specimens have the length in longitudinal direction and the width in radial direction. To prevent crushing on the specimen by the grips, four tabs in steel of length 60 mm were glued to the ends of the specimen using bicomponent epoxy resin (ESK-48).

Test conditions

Five specimens were tested for each wood species. The total number was 5x3= 15 samples. They were conditioned and stored at 20°C and 65% relative humidity before testing. Tensile tests were performed by using universal test machine MTS© 20M with a load cell calibrated in the load range of 100kN. In all tests, load was applied parallel to the grain of wood sample and under displacement control with a constant cross-head speed 2.5mm/min until the specimen failed. Strain gauges are used to measure the strain along the longitudinal and the transversal axis for the purpose to determine Young's modulus and Poisson's coefficient of the materials.

Acoustic emission equipments

Wood is a very heterogeneous and a highly attenuative material. For these reasons, a preliminary study was done to select the suitable sensors for wood materials. In the literature the sensors used for monitoring damage in wood have resonant frequencies between 150 and 300 kHz [START_REF] Ritschel | Nondestructive evolution of damage accumulation in tensile test specimens made from solid wood and layered wood materials[END_REF][START_REF] Varner | Acoustic emission during static bending of wood specimens[END_REF][START_REF] Wu | Acoustic emission characteristics and felicity effect of wood fracture perpendicular to the grain[END_REF][START_REF] Ansell | Acoustic emission from softwoods in tension[END_REF][START_REF] Vautrin | Acoustic emission characterization of flexural loading damage in wood[END_REF]. But a recent study [START_REF] Varner | Acoustic emission during static bending of wood specimens[END_REF] indicated that in the cases of wood fracture process the maximum Power Spectral Density (PSD) defined as the distribution of energy over the frequency, resided in a low frequency area around 60 kHz. So in order to properly assess the frequency portion of the signal, resonant frequency of the sensor should be carefully taken into account. In this context, four types of piezoelectric sensors were tested in order to select the suitable one: three resonant sensors R15α, R6α and R3α with resonant frequency respectively 150 kHz, 60 kHz and 30 kHz, and one wideband sensor (WD) (frequency band-width between 100 kHz and 1 MHz) are tested. Results of wideband sensor showed that the frequency of damage in wood is situated between 30 kHz and 50 kHz (Figure 2). Consequently, R3α and R6α sensors are the most convenient for wood damage monitoring. In this research work, both of R3α and R6α sensors were used simultaneously. After analyses of the signals recorded by these sensors, it is found that the R6α sensors gives better results concerning localization of the events. Therefore we will limit our analyses on the results recorded by R6α sensor in the next parts of this paper. Figure 1 schematizes also the positions of AE sensors on the tested specimen. In order to locate AE events, two R6α sensors were placed on the front side of the tensile specimen one on the top and another on the bottom of the calibrated section. Sensors were fixed with clamps. In order to avoid any loss of acoustic signal at the transducer-sample interface, silicone-free vacuum grease was used to couple the sensors to the surface of specimens. The AE data are recorded with help of an Euro Physical Acoustics system composed by a PCI8 board. The acquisition is computed by using AEwin/SAMOS software. The analog filter frequency is set up between 20 and 400 kHz and the acquired signals are preamplified by 40 dB. The environmental noise was filtered using a threshold of 35 dB. The timing parameters, peak definition time (PDT), hit definition time (HDT) and hit lockout time (HLT) are set at 40, 200 and 300 μs, respectively [START_REF] Lamy | Acoustic emission technique for fracture analysis in wood materials[END_REF].These parameters are verified by Pencil-lead breaks (Hsu-Nielsen source) [START_REF] Nilson | Acoustic emission source based on pencil lead breaking[END_REF] before the beginning of tests. The same sources are used to calculate the wave velocities. These latter are given in table 1. Wave velocity is species dependent. In fact, it increases with the increasing of raw density of the material [START_REF] Bucur | Acoustics of wood[END_REF].

RESULTS AND DISCUSSIONS

Mechanical properties from stress stress-strain graph

The fracture stress and the modulus of elasticity for the three woods determined from the tensile tests are shown in Figure 3. It is shown that the tensile strength of Douglas fir is 65% greater than that of silver fir, and 8% greater than that of popular, respectively. These results are compared with those obtained in reference [START_REF] Hearmon | The elasticity of wood and plywood[END_REF] (Figure 3). Good correlation can be pronounced for Douglas fir, but not for silver fir and popular wood. The failure stress for three woods in reference [START_REF] Hearmon | The elasticity of wood and plywood[END_REF] seems following the same variation of their density. But it is not really the case regarding the measurement on the woods tested in our study. Concerning the Young's modulus, it seems density dependent (Figure 3) since Douglas fir is stiffer than the two other species. Furthermore, silver fir and poplar fir have equivalent stiffness and close densities (Table 1). As an example we will focus on TD5 specimen to correlate the AE data with damage mechanisms. Figure 5 shows the evolution of the load and of AE parameters in term of cumulative AE hits as a function of the testing time. In general, the load/time curve could be decomposed in three stages based on cumulative AE hits. In the linear deformation part, some emissions have been observed. In their study [START_REF] Raczkowski | Acoustic emission in fracture machanics of wood[END_REF] suggested that the random AE signals observed in the elastic deformation stage are the result of the non uniform stress field resulting from nonhomogeneity of wood structure. Actually, reorientations in wood structure are observed, which lead to adaptation for new conditions of loading [START_REF] Raczkowski | Acoustic emission in fracture machanics of wood[END_REF]. When it entered into the nonlinear deformation part (part II), the rate of cumulated AE hits increased. This rapid increase could be attributed to inter-laminar shear in planes of weakness like earlywood/latewood interfaces [START_REF] Bucur | Acoustics of wood[END_REF]. In this stage parallel-to grain micro-cracks in EW/LW transition zones are developed. Then, the specimen entered to the fracture stage (part III) which leads to a sharp increase in cumulative AE hit. As damage in the specimen intensified, macro-cracking took place and the first visible crack was found at t= 64.3s, herein a sharp drop of the load can be observed (Figure 5-Point B). A transverse crack in the EW appeared, and then the crack propagated longitudinally in the EW/LW interfaces. Successively, a second, a third visible crack across EW occurred at t= 68.2s and at t=69.47s, respectively (Figure 5 -Point C and Point D), they are accompanied by the longitudinal crack growth or in EW tracheid, or at the EW/LW interface. Thereafter, the rate of cumulated AE hits increased as the damage continued to progress until the fourth and final fracture at t=78.1 s (Figure 5-Point E). The several drops of load indicate that the specimen still had high load-bearing capacity after the first visible fracture [START_REF] Liew | Fracture behavior of tropical hardwood under tensile load[END_REF]. Even though the damage process for each Douglas specimen could not be exactly the same, a typical damage process in Douglas specimen can be proposed. Under tensile loading, the different stiffness between earlywood (EW) and latewood (LW) generates a shear stress at the EW/LW interfaces, which is the weakest layer because the density of wood between EW and LW changes abruptly [START_REF] Kollmann | Principles of wood science and technology I: solid wood[END_REF]. Micro-cracks should initiate or at EW/LW interfaces or in EW, where the strength is much lower than that in LW. Their growth and coalescence conduce to longitudinal crack propagation. This process is sometimes accompanied by the fibre breaking in EW [START_REF] Sipolla | In situ longitudinal tensile tests of pine wood in ESEM[END_REF]. The transverse fracture in EW generates a tensile/flexion coupling effect, which accentuates even more the fibre breaking in EW. When the crack growth attains EW/LW interfaces levels, under shear stress (mode II) and peel stress (mode I), the longitudinal crack propagation (parallel-to-grain) along EW/LW interfaces will occur. And then, one of the latewood layers breaks, the rest of specimen cannot support the higher load and final fracture occurs [START_REF] Sipolla | In situ longitudinal tensile tests of pine wood in ESEM[END_REF]. On fracture surfaces, the longitudinal crack growth in EW and at EW/LW interfaces was evident, and the fiber breaking in EW is so shown stair-shaped.

Silver fir

Silver fir specimens under tensile loading failed usually in brittle manner (Figure 6). Much less crack propagation parallel-to-grain direction were observed along EW/LW interfaces or/and in the EW. The fracture surfaces appear more flat and demonstrate simultaneous transverse fracture in earlywood tracheid and in latewood tracheid. The representative specimen TS4 (Figure 6-b) displays transverse crack that extend entirely through the sample width across the EW and the LW. Some longitudinal crack propagations have been also seen along EW/LW interfaces. Figure 7 shows the evolution of the load and of cumulative AE hits as a function of the testing time for the specimen TS4. Under low load (Part I), silver fir generated few emissions. In the second part (Part II), the cumulative AE hits increased with the load rise. Emissions in this stage can be attributed to micro-cracking in cells of earlywood since the structure of wood with narrow rings consists principally of earlywood tracheid [START_REF] Johnson | Properties of white fir and their relation to the manufacture and uses of the wood[END_REF]. The brittle fracture across EW starts with a significant increase in the rate of cumulated AE hits (Part III). When the EW is sufficiently damaged, the LW can no longer withstand the applied loads, the final fracture occurred. Actually the latewood has to be more resistant as in silver fir as the walls of latewood cells are much thicker than those of earlywood cells. Moreover at the last part of the test, the tensile/flexion coupling due to the transverse crack promotes the longitudinal crack propagation at EW/LW interface.

Poplar

Poplar is a hardwood and considered as quasi homogeneous material, because no transition from earlywood (EW) and latewood (LW) can be distinguished. The specimens shown in Figure 8-a display combination of tension and shear fracture, because the fracture surface of all specimens runs partially across wood grain and partially inclined to the grain direction. The specimens TP3 and TP4 display a large part of the shear fracture where the fracture surface is inclined to the fibers. Moreover, more or less longitudinal macro-cracking was also observed in the specimens. For the representative specimen TP3 (Figure 8-b), the analysis of the AE cumulative hits during tensile test allows to divide the damage process in three parts (Figure 9). Part I is characterized by an absence of detectable acoustic emission. Poplar specimen (TP3) starts to generate acoustic signals at t=22.43s in part II. Then, the cumulative AE hit increased slightly with the load increase. At this stage the signals can come from the initiation and development of micro-cracking across wood cells since they are considered as the main spot of mechanical weakness in bulk wood specimens. The early stage in part III is characterized by a remarkable increase in cumulative AE hits, herein the damage mechanisms could be the growth of microcracks and the arrest of crack propagation at a vessel level, a mechanism specific to hardwood. At final fracture of the specimen, the load drops sharply and the cumulative AE hits suddenly rises when the visible cracks appeared at t=84.32s (Figure 9-Point B). Acoustic emission activity in this stage is mainly attributed to fiber breakage. Actually, in the final fracture process, under the effect of the stress concentration around the micro-cracks, the fibers of the wood could be broken, forming cross-grain macro-cracks. In the same time the growth and the coalescence of theses micro-cracks could also follow the direction parallel-to-grain due to the shear stress induced by wood structure at fiber/fiber interfaces. When a cross-grain crack Transverse fracture across wood grain

Parallel-tograin fracture inclined-tofiber fracture meets a parallel-to-grain crack, their coalescence results in a crack inclined to the grain direction. Consequently, the damage scenario of the tested poplar wood under tensile loading could be supposed as following: the damage process is started by micro-cracking in the wood cells. The growth of micro-cracks can be stopped at a vessel due to hardwood structure. Final fracture of the specimen occurs in a brittle manner. When a packet of fibers are broken simultaneously, instable crack propagation can be observed across-to-grain, parallel-to-grain as well as in the direction inclined to wood grain.

Comparison AE responses for species tested

The three wood species studied here show in general three stages of AE activity during tensile loading. But the total cumulative hits are quite different between species. Douglas fir generates higher total AE signals: more than twice of cumulated AE signals of silver fir and nine times more than cumulated AE signals of poplar (Figure 10). This investigation agrees well with the findings in reference [START_REF] Ansell | Acoustic emission from softwoods in tension[END_REF], which indicated that the higher is the contrast between early and latewood in annual ring, the higher AE cumulative counts should be expected during tensile parallel to grain loading. Moreover the important difference in AE responses between softwoods (Douglas fir and silver fir) and hardwood (poplar) can be also explained by the higher energy consumed when the crack propagate in softwood [START_REF] Reiterer | Mode I fracture and acoustic emission of softwood and hardwood[END_REF]. Furthermore, hardwood requires a high stress levels for macro-crack initiation and its structure holds crack arrest components. In fact, vessels in hardwood species can stop the crack propagation [START_REF] Ashby | Cellular solids: Structure and properties[END_REF]: the crack tends to open the vessel or deviates around the vessel and then stooped (Figure 11). Despite the crack arrest, the crack propagates in brittle manner in hardwood species [START_REF] Reiterer | Mode I fracture and acoustic emission of softwood and hardwood[END_REF]. These reasons explain why the poplar wood remained silent in early stage of loading and released less emission hits until the final fracture. Furthermore, wood structure contains radially oriented parenchyma called rays. They provide a mechanically strong structure which inhibits the crack propagation [START_REF] Reiterer | The radial reinforcement of the wood structure and its implication on mechanical and fracture mechanical properties-a comparison between two tree species[END_REF][START_REF] Bordner | Fracture initiation and progress in wood specimens stressed in tension: Part I: clear wood specimens stressed parallel to the grain[END_REF]. In fact, the volumetric proportion and the size of rays are different between soft and hardwood. The conifers wood contains around 5-10% of rays while it is around 10-32% in hardwood [START_REF] Ozden | Transverse fracture properties of green wood and the anatomy of six temperate tree species[END_REF]. Therefore, the rays could strengthen poplar wood to crack growths more than the two softwoods (Douglas fir and silver fir) which explains the small AE signals generated during poplar damage.

CONCLUSIONS

In this study, the evolution of damage of three wood species under tensile loading was monitored by AE. The following conclusions can be stated: -The magnitudes of AE activities as a function of time are directly related not only to the annual rings structure, but also to micro and meso-structures of woods. It is shown that the wood with well-defined growth rings is more emissive than wood with narrow rings and wood with homogeneous structure. The responses of AE are quite different between softwood and hardwood. More cumulative hits have been obtained during damage process in softwood than those in hardwood.

-AE is a promising technique for monitoring the health of timber structures and for detecting the onset of micro and macro cracking. In fact, the AE data of tensile tests on three different wood species indicate clearly different scenarios from damage initiation and accumulation up to final macroscopic failure. Therefore, it is possible to associate AE responses to the damage mechanisms and the damage level in wood structures, so as to predict service life or/and service safety for a timber structure. -Actually, different parameters, such as forms and the energy of AE activities are worthy to study in future works in order to correlate different damage mechanisms to AE events.

Figure 1 :

 1 Figure 1: Tensile specimen and sensors position

Figure 2 :

 2 Figure 2: Pic frequency vs amplitude of WD sensor showing the concentration of hits in a frequency range between 30 kHz and 50 kHz.

Figure 3 :

 3 Figure 3: Mechanical properties for the three tested wood species

Figure 4 :

 4 (a) Failure patterns observed for Douglas fir specimens (b) Failure modes in TD5 fracture surface

Figure 5 :

 5 Figure 5: Force vs time, cumulative hits vs time and damage scenario of TD5 specimen

Figure 6 :

 6 Figure 6: (a) Failure patterns observed for silver fir specimens (b) Failure modes in TS4 specimen

Figure 7 :

 7 Figure 7: Force vs time, cumulative hits vs time and damage scenario of TS4 specimenIn conclusion, silver fir has narrow growth rings, the specimens under tensile loading failed usually in brittle manner. The micro-cracking initiates firstly in the EW, the development of the micro-cracks leads to fiber breaking in the EW. When there are enough fibers broken in the EW, the transverse fracture in the LW occurs. The transverse crack across the section produces a tensile/flexion coupling effect, which promotes the longitudinal crack propagation at EW/LW interfaces, under mode I due to peel stress and mode II due to shear stress.

Figure 8 :

 8 Figure 8: Failure patterns observed for poplar specimens (b) Failure modes in TP3 specimen

Figure 9 :

 9 Figure 9: Force vs time, cumulative hits vs time and damage scenario of TP3 specimen

Figure 10 :

 10 Figure 10: Cumulated AE signals of tested wood species

Figure 11 :

 11 Figure11: Crack arrest at a vessel[START_REF] Ashby | Cellular solids: Structure and properties[END_REF]

Table 1 :

 1 Wave velocities and raw density of wood species

	Wood species	Wave velocities (m/s)	Raw density (Kg/m 3 )
	Douglas fir	4747 ± 560	501 ± 22.25
	Silver fir	4549 ± 164	446 ± 45.43
	Poplar	4345 ± 115	410 ± 11.35


innovation and R&D function have turned out to be a key strategic asset in the culture and media sectors and a key dimension for the competition and domination of their market. The sector illustrates the tension triggered by innovations supported by content, uses and services on the one hand, and by the technology on the other hand. It is particularly enlightening at a time of hot debates on net neutrality and balance between technological infrastructure investments made by telco, OTT platforms and content providers.

However; specific studies on the very nature of R&D strategies in the creative industries are scarce. This article gives an overview of the observable R&D expenditures, then scrutinizes more specifically the case of the cinema industry. Starting from the case of culture, it is a first step to better understand where and how R&D takes place in the content and services being offered and to track the various forms of innovation to be found in these sectors.

Observable R&D expenditures in the creative industries

The scarcity of specific studies on the management of R&D in the creative industries [START_REF] Green | Hidden innovation in the creative sectors[END_REF] echoes the fact that creative companies lack wide investment in R&D. One of the first reasons used to explain the poor level of investments in R&D is linked to the general size of these companies. Most of the firms in the creative industries are SMEs (BOUQUILLION & LE CORF, 2010) with related difficulties in financing R&D activities.

Notwithstanding, large creative companies are also known for underinvesting in R&D projects (Foundation for Research, Science and Technology, 2003). Therefore, other explanations have to be proposed to explain this: the second explanation could be that companies in the creative industries are considering that they already take so much commercial and aesthetic risk by creating new content that they cannot cope with additional technical risks and therefore prefer to build on the R&D outcomes originating from other industrial sectors instead of investing directly in R&D ventures.

Thirdly, each creative project turns out to be an innovative and prototype project. Companies consider "R&D" as being more related to investments in projects and contents rather than directly referring to the design and innovation of processes, infrastructure and devices. The innovative activity will stay hidden within the project production budget.

Observing company-level data of the Industrial Scoreboard (2013)2 , and leaving aside an (unfavourable) comparison with the data of the overall ICT sector, companies of the creative industries are allocating only a small share of their revenues to R&D: declining from around 3% (2007) to around 2% over the period 2008-2012. In fact, these expenditures were already standing below 2% over the period 2004-2008, a share most likely still overestimated as the data is biased in the Industrial Scoreboard towards large companies selected as top R&D investors. As stated above, this weak performance is usually explained as a result of the very structure of the sector characterized by the presence of small firms and the low propensity of firms to give importance to R&D activities.

Overall, in the Industrial Scoreboard, the low level of R&D expenditures seems to have been the standard case for media and content industries players. Comcast and Disney, the 1 st two media companies listed in the Forbes 2014 Global 2000 List, are not presented in the industrial Scoreboard, for the low level of their R&D expenditures.

Still various EU companies appear in the ranking. Vivendi (France) ranks 1 st (Table 1a) for its level of R&D in absolute terms3 but displays a low 2.5% ratio of R%D expenditures to revenues. Reed Elsevier (UK), a major of the publishing sector, ranks 2 nd for R&D expenditures with an R&D intensity of 4.14%. The company is technology oriented, investing US $ 500 million every year, claiming to be the fourth largest digital content provider in the world (Annual Report, 2013: 9). The firm pioneered the use of Big Data with its High Performance Computing Cluster Systems (HPCC), "one of the most advanced, fast-performing Big Data processing technologies available today" (Annual Report 2013: 10). Technicolor (France), following-up on the bankruptcy of Thomson, invested 4.5% of its revenues from sales in 2013.

The level of expenditures of broadcasters is equally low (below 2% of the sales), notwithstanding the technological orientation of the sector with its strong industrial components and complex network of players. This remains true even for "new" companies like Sky (UK) relying on more recent channels of distribution. In the multimedia sub-sector two leading non-European companies show major investments in R&D: Dolby and Sony (see Table 1a) 6 . Sony is ranking 24 th in the Industrial Scoreboard (2013). Both companies show significant R&D investments, In that context, Youview TV (UK), spotted by the Scoreboard, emerges as an exceptional case. The company is a young UK provider of digital TV which started offering services in July 2012. It introduced its own standard IPTV settop box, and its technical orientation may account for the amazing 83.9% ratio, after one year of operation.

Another example identified in the Industrial Scoreboard because of the volume of its R&D expenditures, is Rocksteady Studios, a UK video game developer based in Highgate, London, which shows a 56% R&D ratio! Founded in 2004, the studio was acquired by Time Warner in February 2010. The developer is renowned for its games "Batman: Arkham Asylum" and "Batman: Arkham City".

R&D expenditures in the cinema industries

In the cinema industries, one could have expected its more "technical industries" 7 to play a major role in R&D, considering them as the "labs" of the industry. However, these industries have even been described (in the French case) as a stagnating link in an expanding value chain (Imaginove, 2014: 9). Such a statement, focusing on the French cinema industry, seems to ignore the dire situation in those technical industries as witnessed by leading players such as Laboratoires Éclair, Technicolor (formerly Thomson) or innovative camera manufacturer Aaton, all on the verge of bankruptcy.

In the EU, Thomson used to be one of the companies competing with US and Japanese firms to produce equipment and provide audio-visual services. The firm was faced with serious challenges at the turn of the century and 6 On these data, see also [START_REF] Nepelski | The top world R&D-investing companies from the ICT sectora company-level analysis[END_REF].

7 The segment covers: shooting (manufacturers equipment and film, shooting rentals, shooting studios/set, mobile production unit), post-production (labs photochemical/digital/video, image and sound post-production, dubbing and subtitling), diffusion (screening and broadcasting, duplications (prints/KDM), DVD), archiving, storage, and restoration. In 2012, all these activities reached 1,1 billion Euros in France according to the French Trade association FICAM (2014) for a panel of 109 companies: post-production accounted for 17%, immediately followed by dubbing/ subtitling and mobile production unit with 15%, then shooting studios and shooting rentals with 13% each. devised an ambitious plan to move into digital distribution, aiming at becoming the number one firm in the field, targeting first the US market. The plan failed. The company, now renamed Technicolor refocused on core activities, still manages an important portfolio of patents that generated 15% of its revenues in 2012. In 2013, Technicolor employed around 350 researchers and experts spread among four research centres: Rennes, Paris, Hanover, and Palo-Alto (company sites: 2014). The company invested 4.5% of its revenues from sales in 2013 (Scoreboard, 2013).

Éclair, another French company founded in 1907, active in all segments of the subsector (production/distribution/ archiving/storage and restoration) followed a similar pattern and almost went bankrupt in 2009: its revenues and workforce have been cut by two since 2009 [START_REF] Lucas | Pépites digitales en V. F[END_REF]. The company went through several "social plans" but seemed to re-emerge and to try taking the lead on the French market asaccording to the company website -"a pioneer of digital post-production". The company from Epinay (Paris area) introduced itself as "an innovator for more than a century" (company website). The company prides itself on being the only company with an R&D programme addressing all these segments. The company was agreed and labelled within the French government project "grand emprunt numérique" (146 men/month over two years). Laboratoires Éclair Laboratoires Eclairs highlights an in-house R&D team working with a US based company, Front Park Digital, specialised in "digital asset management solutions for migrating, managing and delivering media content" (company site). For distribution of copies, Éclair "created the first European Digital Cinema Package" (DCP) in 2005 (company website) and implemented a digital distribution hub in 2011. The two companies, after some downsizing, seem to have refocused on R&D8 . However the company went bankrupt in 2015 Part of the reason has to do with the fact that this sub segment is treated more as a plain supplier rather than a leading technical partner. A report for the French Centre National du Cinéma (CNC), [START_REF] Lepers | Avenir à 10 ans des industries techniques du cinéma et de l'audiovisuel en France[END_REF], commissioned to assess the economic strengths and weaknesses of this sub-sector, stressed that this technical part of the cinema industry is perceived as a "commodity" and not as the core value of the industry. The authors emphasized the negative aspects of this relationship (LEPERS & PORTUGAL, 2013: 25) and suggested some rebalancing and an improved consideration of R&D. Besides, the report underlined that this segment also suffered from serious economic challenges.

Potential U turns in the cinema industry: Windows of opportunity for new (R&D) players?

On the other hand, some technical industries are faring better than others (for instance UK post-production companies: LIVINGSTONE & HOPE, 2011). The entry of emerging new players may signal a change in the legacy relationships with these suppliers, between "creation" and technical provision of services, brought by digitization, pervasive computing and new distribution channels. Even in the case of the French industries, some start-ups managed to emerge recently.

For instance, Ymagis (France), founded in 2007 by executives from the world of cinema and high technologies, shows an amazing growth curve. The firm covers activities of the technical industries (production, post-production, storage) but focuses on digital distribution of digital copies. It claims to be an innovation company with half of its workforce being engineers and technicians, devoting 11% of its revenues to R&D in 2007 in its labs -Paris, Barcelona, Berlin (Ymagis, 2014).

Similarly, due to the growing role of software activities, some companies in the creative industries are reaching R&D intensity ratios that are much closer to those for regular technology-oriented or technology producing companies. This is also, for instance, the case of the digital content-creation solutions provider Avid spending on R&D up to 17% of its net revenues, (678 million US $ in 2011, 513 in 2012).9

Many companies are likely to escape usual investigation because of their sheer size even if some of them are publicly traded like Avid. This is all the more important to analyse as it may indicate the emergence of a new digital ecosystem with companies originating from the broadly defined content sector (like the Content Delivery Networks, Akamai). They provide IT services to all kinds of media customers. For a growing number of observers, these companies are now playing the rather recent role of "new middlemen" liaising between the different layers of a transformed industrial environment described alternatively under the notion of "new ICT ecosystem" (ARLANDIS et al., Source: Compiled by the authors from companies' website, internet sources and annual reports

These "new middlemen" are often the output of previous R&D investments, and/or spin-off from universities. To offer here an example, Akamai, a content delivery network originated from the MIT, from research on the dynamic routing of contents. Akamai provides services to content providers bringing their contents closer to the customer.

Table 2 documents R&D expenditures in a sample of selected companies involved in the cinema industry. Taking into account the wider "Telecom Media Technology" (TMT) ecosystem, it appears that IT MNEs have much larger R&D expenditures 11 , and are highly R&D intensive, while even the large multinational firms of the creative industries show rather small R&D efforts.

However, some of the evidence gathered shows the existence of smallersized companies' efforts in technology-related content and the possible emergence of larger companies (hence able to show on the industrial scoreboard). These smaller-sized companies, specialised technology suppliers as suggested, are "new middlemen" liaising between companies and layers of the new ecosystem, bringing and building mostly on their software expertise to enable further digitization of other players.

Conclusion

The actors of creative industries invest in R&D and in technological innovations, however they do in a way that appears concealed and not in areas where one would expect: digital technologies. Cinema is quite emblematic of this view. Its economic actors significantly invest in technology, but in the traditional processes of production and distribution, rather than in the most threatening disruptive digital mutations.

These developments are reflected as such by original new models, based on particular forms of specific collaboration formalized through the decisive role of new middlemen. These new actors are building their position by organizing the dialogue between their expertise in high tech skills and the creative dimension of cultural works. More broadly, the case of cinema thus makes it possible to understand how the wide spectrum of technologies 11 Even when the R&D are quickly identified in annual reports it remains difficult to see how much R&D is allocated by these IT firms to R&D for the creative industries.

Table 1a -Top ranking global media companies by R&D expenditures (2012)

 1a

	Rank	World Rank	Name	Country	R&D Expenditures (€ million)	Sales (€ million)	R&D Intensity (%)
	1	153	Vivendi	France	718.0	28 994.0	2.5
	2	296	Reed Elsevier	UK	312.2	7.523	4.14 (*)
	3	340	Dai Nippon Printing	Japan	269.9	12 668.0	2.1
	4	492	Technicolor	France	160.0	3 580.0	4.5
	5	609	British Sky Broadcasting UK	120.8	8 122.0	1.5
	6	675	Dolby Lab USA	USA	106.2	702.0	15.1
	7	801	Avid Technology	USA	88.3	513.1	17.2

(*) Computed by authors from two different sources: HERNANDEZ et al., Scoreboard for R&D; annual report 2013 for sales.

Table 1b -Top ranking EU media companies by R&D expenditures (2012)

 1b

	Rank EU	Name	Country R&D Expenditures	Sales	R&D Intensity
	Rank			(€ million)	(€ million)	(%)
	1	Vivendi	France	718.0	28 994.0	2.5
	2	Reed Elsevier	UK	312.2	7 523	4.14
	3	Technicolor	France	160.0	3 580.0	4.5
	4	British Sky Broadcasting UK	120.8	8 122.0	1.5
	6	IPSOS France Media	France	41.8	1 789.5	2.4
	7	Eniro 4	Sweden	39.3	466.0	8.4
	8	Youview TV	UK	32.8	39.1	83.9
	9	Euromonitor 5	UK	15.6	77.5	28.2
	10	Il Sole 24 Ore	Italy	8.6	430.9	2.0
	11	Rocksteady Studios	UK	7.4	13.3	56.0
	12	ITV	UK	7.2	2 626.4	0.3
	13	JC Decaux	France	6.9	2 622.8	0.3
	14	TF 1	France	5.4	2 620.6	0.2
			Source
	Authors, data from the Scoreboard 2013, World -2000 companies ranked by R&D, EU top 1000
	companies ranked by R&D. Annual reports. Major R&D intensities in bold, authors' emphasis.
	4 Eniro is a search company in the media industry, with operations in Sweden, Norway, Denmark,
	Finland and Poland. In 2013, 80% of the company's revenues derived from digital media
	(excluding directory assistance services). Source: company website.
	5 Euromonitor International Ltd is a London-based market intelligence firm, providing market
	research, business intelligence reports, and data to industry. The firm was founded in 1972.

The Industrial Scoreboard is a JRC-IPTS report. http://iri.jrc.ec.europa.eu/scoreboard13.html

One may suspect that the level of R&D expenditures may be linked to the video games subsidiaries (Activision Blizzard) on the media conglomerate. As, on October 11, 2013, Vivendi deconsolidated Activision Blizzard pursuant to the sale of 88% of its interest, it may appear in the 2014 financial data. Activision Blizzard Annual report for 2013 gives a 604 million US $ for "product development" amounting to 12% of total net revenues. Source: Annual Report 2013.

It is especially interesting to stress that since our report has been published, Éclair has just been acquired (July 2015) by Ymagis, the company we present below in this paper.

Source: Annual Report 2011: 20, Scoreboard data.

Global Telecom, Media and Technology (TMT) ecosystem.

(3) Estimated from indication in the company's annual report.

(4) Source: Manta, 2014. www.manta.com/c/mm2hh86/american-zoetrope mobilized in the digital age opens up strategies and allows differentiated positioning in the value chain.


M G Tsoutsouva

G Stokkan

G Regula

B Ryningen

T Riberi -Béridot

G Reinhart

N Mangelinck-Noël

Random angle grain boundary formation and evolution dynamics during Si directional solidification

Keywords: X-ray radiography, Bragg diffraction imaging (topography), crystalline defects, dislocations, grain growth, grain competition

The growth of multicrystalline silicon and the formation of a random angle grain boundary, as well as the dislocation generation and expansion is observed dynamically in situ, by Synchrotron X-ray imaging techniques. The focus is kept on a random angle grain boundary since its behavior is particularly important to better understand the HP mc-Si (High Performance Multi-crystalline Silicon) photovoltaic properties. Due to the process conditions and to the grain competition that occurs during the solidification, a facetted {111} /facetted {111} groove is formed by this random angle grain boundary at the solid/liquid interface. It is shown how the shape of the solid/liquid interface allows the change of the preferential {111} growth facet and affects the grain boundary propagation direction. In one of the groove configurations, the two adjacent {111}

Introduction

Unlike the traditional mc-Si process characterized by large grains and predominant electrically-inactive twin boundaries, the crystal growth of highperformance multi-crystalline silicon (HP mc-Si) by directional solidification is nucleated on uniform small grains, resulting in a material with a high fraction of random angle grain boundaries (RAGBs). The HP mc-Si process concept and the first results were presented in the 5th International Workshop on Crystal Growth Technology in June of 2011 by Professor C. W. Lan [START_REF] Lan | Grain control in directional solidification of photovoltaic silicon[END_REF]. The ingots solidified with this technique present not only a better uniformity and a higher production yield but also an improved material quality for photovoltaic applications. Since 2011, different approaches have been proposed to obtain such a grain structure [START_REF] Zhu | Seed-assisted growth of high-quality multi-crystalline silicon in directional solidification[END_REF][START_REF] Wong | The effect of silica nucleation layers on grain control of multi-crystalline silicon in directional solidification[END_REF][START_REF] Lan | Recent Progress of Crystal Growth Technology for Multi-Crystalline Silicon Solar Ingot[END_REF][START_REF] Zhang | Growth of multicrystalline silicon ingot with both enhanced quality and yield through quartz seeded method[END_REF][START_REF] Reimann | Influence of different seed materials on multicrystalline silicon ingot properties[END_REF]. As a result, the average efficiency

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3 of HP mc-Si solar cells in production increased from 16.6% in 2011 to 18.5% or beyond in 2016 [START_REF] Lan | The emergence of high-performance multi-crystalline silicon in photovoltaics, 18th[END_REF].

The lifetime of HP mc-Si is in general uniform and higher than what was achieved for the traditional mc-Si due to the limited influence of the harmful dislocation clusters.

Indeed, the areas with high dislocation density are confined in the small grains and the propagation of dislocation clusters from the bottom to the top seems to be blocked by the presence of RAGBs [START_REF] Lan | The emergence of high-performance multi-crystalline silicon in photovoltaics, 18th[END_REF]. Trying to understand the involved mechanism leading to the improved performance of the HP mc-Si, different theories have been proposed in relation to grain growth, dislocation generation, multiplication and interaction with grain boundaries.

It is believed that the uniform and small-grained structure reduces the stress concentration at the initial stage of solidification and thus the initial dislocation generation. It has also been suggested, however without clear evidence, that the presence of RAGBs can additionally relax stresses through a slip mechanism at high temperatures [START_REF] Reimann | Influence of different seed materials on multicrystalline silicon ingot properties[END_REF][START_REF] Yang | Development of high-performance multicrystalline silicon for photovoltaic industry[END_REF]. It is expected that since the global stress is reduced and that the dislocation sources are less abundant, the dislocation multiplication mechanisms are less active. Stokkan et al. [START_REF] Stokkan | Study of evolution of dislocation clusters in high performance multicrystalline silicon[END_REF] assumed that RAGBs contain heavily mis-reconstructed bonds so that when the predominantly vertical clusters of lattice dislocations interact with such boundaries the bonds can be more easily rearranged while the grain boundary energy

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4 changes very little. Hence, the boundaries correspond to more ductile areas (than the grain themselves) where the dislocations can be trapped. Whatever mechanism, due to the existence of a high length fraction of RAGBs in HP mc-Si, where dislocations are prevented to move across, the development of large dislocation clusters during growth is limited. Furthermore, from recent investigations, it was shown that the recombination activity of the grain boundaries in HP mc-Si wafers is much lower than in the conventional mc-Si but the reason of this observation still remains unknown [START_REF] Sio | Direct comparison of the electrical properties of multicrystalline silicon materials for solar cells: conventional p-type, n-type and high performance p-type[END_REF][START_REF] Castellanos | High-Performance and Traditional Multicrystalline Silicon: Comparing Gettering Responses and Lifetime-Limiting Defects[END_REF].

Apparently, apart from the grain size, the crystallographic relationship and the lattice misfit between two adjacent grains have a significant impact on the dislocation movement and thus, on the minority carrier lifetime in the area of the grain boundary [START_REF] Hoshikawa | Si multicrystals grown by the Czochralski method with multi-seeds[END_REF].

The detailed mechanisms of grain competition and selection that lead to grain boundary formation as well as dislocation generation, multiplication and interaction with grain boundaries (nucleation and/or termination) are not well understood. A better knowledge of these mechanisms is a prerequisite for controlling the final structure and is thus vital to be able to achieve the desired crystal properties. However, it is difficult to get an understanding of the phenomena that occur during the solidification process, by ex situ characterization of the cooled down crystal. There is no information left about

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the morphology of the solid/liquid interface and of the grain competition dynamics during growth.

In the present work, our approach to tackle this challenge is to observe dynamically the directional solidification of silicon, by a combination of X-ray diffraction imaging techniques: X-ray radiography and Bragg diffraction imaging (Topography). By X-ray radiography it is possible to follow the morphology of the solid/liquid interface which gives information about the growth kinetics, whereas Bragg diffraction imaging allows following in situ the evolution of the extended defect formation. The formation of a RAGB and its interaction with extended defects are studied in particular since RAGBs have been identified as one of the main features leading to the improved PV performance of the HP mc-Si ingots.

Experimental

The directional solidification experiment is carried out in a dedicated installation named GaTSBI (Growth at high Temperature observed by X-ray Synchrotron Beam Imaging), which is installed at the European Synchrotron Radiation Facility (ESRF).

GaTSBI is composed of a Bridgman-type high temperature furnace equipped with two resistive heating elements (top and bottom) and is coupled with in situ X-ray imaging techniques. X -ray radiography is performed using a specific CCD camera developed at size. An optic giving 5.8 µm pixel size and 11.9 × 11.9 mm 2 field of view is used.

Alternately during the same solidification experiment, X-ray Bragg diffracted images (of Von Laue patters), corresponding to different {hkl} diffracting planes, are regularly recorded on X -ray sensitive films (AGFA Structurix D3-SC, 17.6×12.5 cm 2 ) positioned at a distance of 300 mm from the sample. The Bragg diffraction imaging technique is sensitive to long range distortion fields and / or strain fields associated with crystal deformation. Indeed, the distortion fields affect the diffracted intensity, so that a contrast (non -homogeneous intensity distribution) is created in the recorded images.

As a consequence, defect-induced strain in general and in particular, dislocations are visible due to the deformation they cause to the crystalline network in their vicinity.

Due to the small beam divergence and to its large size, the whole width of the sample is illuminated and the recorded diffraction spots contain information concerning the level of crystal perfection and/or deformation including dislocations. It is worth mentioning that in the diffracted images, contrast variations can also be due to different Bragg angles (superimposition or separation) of two diffracted zones with a different orientation. This case is neither observed nor discussed in the present work. A more detailed description of the experimental conditions, the imaging techniques, image processing and the previously obtained results can be found elsewhere [START_REF] Tandjaoui | Real Time Observation of the Directional Solidification of Multicrystalline Silicon: X-ray Imaging Characterization[END_REF][START_REF] Riberi-Béridot | On the impact of twinning on the formation of the grain structure of multi-crystalline silicon for photovoltaic applications during directional solidification[END_REF][START_REF] Tsoutsouva | In situ investigation of the structural defect generation and evolution during the directional solidification of 〈110 〉 seeded growth Si[END_REF].

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In the present work, the initial monocrystalline Si sample is appropriately cut (38 mm × 5.8 mm × 0.3 mm) from a double side mechano -chemically polished intrinsic (resistivity beyond 5000 Ωcm) FZ wafer 50.8 mm, {110} in surface orientation provided by SIL'TRONIX Silicon Technologies. After the cutting process, the sample is chemically polished in order to smoothen the damaged areas due to the cutting step: first an HF bath followed by a CP4 polishing, with a newly fabricated solution at room temperature during 3 min. Solidification experiments are performed under dynamic vacuum (~10 -6 mbar) while the sample is housed in between tow pyrolytic BN plates, in a cavity with dimensions: length 40.0 mm, width 6.0 mm and depth 0.3 mm.

The sample is heated by applying the same temperature at the bottom and top heating resistances of the furnace up to 1373 K. Then, a temperature gradient of 30 K/cm is imposed between the two heating resistances until partial directional melting from top to bottom is achieved. After the stabilization of the solid/liquid interface, a cooling rate of -1 K/min is applied on both heaters to start a directional solidification from bottom to top, while recording X-ray images of both types.

After solidification, the sample is analyzed by electron backscatter diffraction (EBSD) on a SEM JSM840 using 15 µm step size and on a FEG-SEM, Zeiss Ultra, 55 Limited Edition using 3 µm step size for selected regions. In order to depict the obtained crystallographic orientations of the grains in the sample, inverse pole figures (IPF) maps perpendicular to the sample surface (x), transverse (y) and along the growth direction (z). Coincidence site lattice maps (CSL) show CSL boundaries as defined by the Brandon criteria [START_REF] Brandon | The structure of high-angle grain boundaries[END_REF].

Experimental results

As already mentioned in the introduction, the purpose of the present work is the investigation of the growth kinetics and of the extended defect generation related to the formation and evolution of a random angle grain boundary (RAGB) during the solidification process. The CSL EBSD map, recorded at higher resolution (3 µm step size) in the area limited by the dotted rectangle (Figure 1b), reveals the details of the grain structure and of the grain boundary characters. In the following, the experimental results and discussion focus on the area where a RAGB is formed as the result of the grain competition between Grain 2/Grain 3 and Grain 2/Grain 4 (the respective grains are marked as G2, G3 and G4 in Figure 1). The complete EBSD information allows to conclude that grains G3 and G4 have exactly the same crystallographic orientation (they appear in purple color in Figure 1a) and are separated between them by the diagonal twin crystal G5 that can be hardly distinguished 1a, because of the lower resolution, but which is clearly revealed in Figure 1b.

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M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 10 in Figure
During growth, the grains G3 and G4 enter in competition with the grain G2 forming RAGBs that propagate along different growth directions. On the one hand, in the case of G2/G3, the RAGB formed is almost vertical and follows the growth direction. On the other hand, shortly after the intersection with the diagonal double twin G5, when G2/G4 meet each other, the RAGB growth direction changes remarkably (dashed circle in Figure 1b). Indeed, it turn towards the right with reference to the growth direction, looking along the x direction. In order to investigate this behavior, the facet growth kinetics as well as the competition mechanism between G2, G3 and G4 are further analyzed. For that purpose, the stereographic projections of the {111} poles corresponding to the plane view normal to the sample surface direction (axis x) are presented in Figure 2.

Figure 2: Stereographic projections of the {111} poles of grains G2, G3, G4 and G5, the pole axis (x) being along the direction perpendicular to the sample surface. The

M A N U S C R I P T A C C E P T E D facets that preferentially drive the G2 and G3/G4 growth during the solidification process respectively.

The morphology of the solid/liquid interface at the level of the RAGB formation between G2/G3 and G2/G4 is revealed in live by X-ray radiography and representative images are given in Figure 3a

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Figure 3: X-ray radiography zoomed image at the level of the facetted/facetted grain boundary groove formed at the solid/liquid interface, as the result of the grain competition between a) G2/G3 (t = t 0 + 75 s) and b) G2/G4 (t = t 0 + 194 s) respectively.

Graphical add -ins are superimposed on the images (ii) and (iii) for a better understanding of the discussion that follows. t o corresponds to the beginning of the solidification process.

The images are the result of a pixel by pixel division of two successive raw images. The image treatment is required to increase the contrast and reveal the solidification front and allows the study of the dynamical interface and facet evolution [START_REF] Tandjaoui | Real Time Observation of the Directional Solidification of Multicrystalline Silicon: X-ray Imaging Characterization[END_REF]. The solid appears in white (less dense) and the liquid in grey (denser) and since the image is a division of successive images, the white contrast means that a liquid layer in the previous image has solidified in the following image.

Both radiographs show that the encounter of grains G2/G3 and G2/G4 creates a facetted/facetted groove at the solid/liquid interface but of different shape. The traces of the {111} corresponding to the facets seen in projection at the solid/liquid interface are indicated in Figure 3a (ii), (iii), b (ii), (iii). The inclinations of the {111} facets are in agreement with the respective poles on the stereographic projections for both grains (Figure 2). The growth of G2 is driven by its 1 11 facet in both grain boundary grooves (G2/G3 and G2/G4). The 1 11 facet is perpendicular to the sample surface

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13 direction (edge zone) and 36° inclined by the growth direction. Even if G3 and G4 have exactly the same crystallographic orientation, their growth is driven by different {111} facets at the level of the grain boundary groove with G2. On the one hand, G3 grows along the 1 1 1 facet which is 66° inclined by the sample surface direction and 45° inclined by the growth direction (Figure 2). On the other hand, G4 grows along the 1 11 facet that is almost perpendicular to the growth direction and 56° inclined by the sample surface direction (34° by the edge zone). The trace of the bisector of the angle between the two facets is also shown with a dashed yellow arrow in the same radiographs. It must be noted that the actual bisector of the groove facets is slightly different from the apparent bisector of the traces in the figure due to the inclination of the G3 1 1 1 and G4 1 11 facets relatively to the plane of the figure. The actual bisector and its trace is found by the use of a Wulff plot on the two pole figures. In Figure 3, the black arrow shows the actual mesoscopic direction of the formed RAGB and the blue dotted line is an estimation of the isotherm shape.

The RAGB formed as the result of the grain competition between G2 and G3 (Figure 3a) follows the bisector of the angle between the two facets ( 1 11 and 1 1 1 ) as proposed by the model of Duffar et al. [START_REF] Duffar | On the twinning occurrence in bulk semiconductor crystal growth[END_REF], when both {111} facets of the facetted/facetted groove grow at the same rate. On the contrary, when G2 and G4 enter in competition, the formed RAGB does not follow the bisector of the angle between the To confirm this statement, the growth velocities normal to the facet planes in the case of G2/G3 and G2/G4 grain competition are measured from the radiography images recorded during the solidification. Figure 4 shows the displacement of the normal to the facets as a function of time as growth proceed and the slope of the trend line corresponds to the average facet growth velocities. As illustrated in Figure 4a, in the case of G2/G3 grain competition the average facet growth velocities of G2 1 11 and The respective curves for G2 1 11 and G4 1 11 are presented in Figure 4b.

For both grains, the measured points can be grouped in two zones, zone I and zone II, in which G2 as well as G4 accelerate. However, in both zones I and II, G2 grows faster than G4 implying that the growth kinetics of the two grains are not identical. The average measured growth velocity of the solid/liquid interface in this experiment is However, the intensity distribution is not homogeneous and some areas are significantly darker than others. As explained in the experimental section, in Bragg diffraction images, dark areas correspond to highly deformed/strained areas and the dislocation density is estimated to be ≥ 10 6 cm -2 . At the instant when G2 and G4 enter into contact, the formation of the inclined RAGB is accompanied with the generation and emission of dislocations in both adjacent grains that become more and more strained as growth proceeds (Figure 5 (iii) to (vi)). In general, there is a strong correlation between the highly strained areas and the grain boundary types (Figure 1b) formed due to the competition between G1 -G6 as will be discussed in further details in the following section.

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Facet growth kinetics and grain competition mechanism

Solidification started from the nucleation on numerous randomly oriented grains that can be seen in Figure 1. G1 -G6 grains resulted from the competition between the initially growing grains. Depending on the grain orientation and on the direction of the resulting grain boundary relatively to the growth direction, some of the grains expand during the solidification process while others lose the competition with the adjacent grains and disappear [START_REF] Nadri | Two-dimensional numerical modeling of grain structure in multi-crystalline silicon ingot[END_REF]. In the experiment described here, G3 and G5 lose the competition and as a consequence G2 and G4 enter into contact as growth proceeds (dotted circle in Figure 1b).

Under the present experimental conditions, independently from the macroscopic growth directions of G2, G3 and G4, the growth is bounded by the slowest growing planes and a clear {111} facet appearance at the solid/liquid interface can be seen for all grains in the radiography (Figure 3a,b) and Bragg diffraction images (Figure 5a,b).

The radiograph reveals that when G2 and G3 meet, a facetted/facetted groove is formed at the solid/ liquid interface at the level of their common RAGB. Such a configuration has been theoretically modelled by Duffar and Nadri [START_REF] Duffar | The grain-grain-liquid triple phase line during solidification of multi-crystalline silicon[END_REF] and experimentally shown previously using the GaTSBI device [START_REF] Tandjaoui | Investigation of grain boundary grooves at the solid-liquid interface during directional solidification of multi-crystalline silicon: in situ characterization by X-ray imaging[END_REF]. In the model case of a

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19 facetted/ facetted groove, the undercooling at the bottom of the groove is maximum and identical for both {111} facets. Therefore, both facets have the same growth velocity and as a consequence, the direction of the grain boundary formed follows the bisector of the angle between both facets [START_REF] Duffar | The grain-grain-liquid triple phase line during solidification of multi-crystalline silicon[END_REF]. As showed experimentally in ref. [START_REF] Tsoutsouva | In situ investigation of the structural defect generation and evolution during the directional solidification of 〈110 〉 seeded growth Si[END_REF], this is true when the two adjacent grains have no specific CSL crystallographic relation. If they do have, the grain boundary preferentially follows the common plane in order to minimize the energy of the formed grain boundary. Such a plane could be for example the {111} or {112} if the adjacent grains have a Σ3 CSL relation, the {122} or {114} if they have a Σ9 CSL relation. In the present experiment, this is the case of the two Σ3 {111} grain boundaries that are formed between both G3/G5 and G4/G5 when they compete (Figure 1b).

At first, G3 is in competition with both G2 and the double twin grain G5 in between G3 and G4. According to the radio images (Figure 3a) and trace of the random angle grain boundary (Figure 1b), G3 slowly expands at the detriment of G2 but ultimately loses competition with G5. For a very short time, a RAGB is established between G2 and G5. Finally, G5 loses competition against G2 and a new RAGB is formed at the encounter of G2 and G4. Even if G3 and G4 have the same crystallographic orientation, G4 is a grain that nucleated independently from G4 and its growth is driven by a different {111} facet as can be seen in both radiography (Figure 5b (ii) where G4 grows with the horizontal 1 11 facet at the level of the grain boundary groove. Thus, the groove configuration changes from the G3 to G4 interaction with G2 as illustrated in Figure 3a and Figure 3b. Such a growth {111} facet change probability for grains with the same crystallographic orientation has been previously theoretically proposed by Nadri et al. [START_REF] Nadri | Two-dimensional numerical modeling of grain structure in multi-crystalline silicon ingot[END_REF]. This event could be the result of the isotherm shape change with time according to Nadri's theory. As presented in Figure 3a (iii), b (iii), the isotherm shape is almost horizontal during the G2/G3 competition resulting in a flatter solid-liquid interface but the isotherm (derived from the average solid-liquid interface shape) is concave when G2 and G4 compete with each other. This isotherm change could be due to the sudden change (narrowing) of the sample lateral size as can be seen on the radiography image in Figure 3b. Thus G4 which has the same orientation as G3 cannot grow with the same diagonal 1 1 1 facet since it hardly faces the solid-liquid interface. Grain growth driven by facets, resulting the creation of a grain boundary that follows as close as possible the direction of the temperature gradient, is always favored. In the present case, this facet is the G4 1 11 which is the horizontal facet that guides the growth of G4 from the beginning of its nucleation (Figure 5b). As a consequence, the random angle grain boundary direction between G2

and G3 is different from the one between G2 and G4. A simplified 2D model that could

In addition the direction of the grain boundary formed deviates from the direction of the temperature gradient. Such a configuration is not favorable. iii) The isotherm shape is concave and another facet that faces the solid/liquid interface is selected to guide the growth. In the present case this is the horizontal 2 2 facet of G4. As a consequence the growth direction of the grain boundary is also different and parallel

to the temperature gradient.

During the concomitant growth of G2 and G4, the RAGB does not follow the bisector of the groove, which is illustrated with the yellow dotted line in Figure 3b.

Indeed, the actual direction of the RAGB is indicated with a black arrow in the same figure and it is inclined by ~ 17 ± 3° to the bisector of the groove angle. It is important to note that this is not in contradiction with the model of Duffar and Nadri [START_REF] Duffar | The grain-grain-liquid triple phase line during solidification of multi-crystalline silicon[END_REF] in which the starting hypothesis is that both facets grow at the same rate. In this particular case, both grain boundary groove {111} facets do not share the same growth rate, as can be seen in Figure 4. Two possible explanations for the fact that G4 grows faster that G3 are discussed in the following. The temperature distribution in the sample has a strong impact on the crystal growth velocity, the interface shape and the grain boundary formation during the crystal growth process. Similarly to what has been presented by Lian et. al [START_REF] Lian | Numerical Simulation of Multi-Crystalline Silicon Crystal Growth Using a Macro-Micro Coupled Method during the Directional Solidification Process[END_REF], in our experimental installation the shape of the crystal/melt interface at the level of G2/G4 grain competition is curved maybe due to the sample shape change. This leads to the creation of a radial temperature gradient that causes a curvature of the isotherm. Since the two facets meet at the triple junction of the two grains they should have the same undercooling temperature and thus grow with the same velocity. However, the inclination of the two {111} facets is not identical relatively to the isotherm shape as illustrated in Figure 3b (iii).Thus, it is most likely that the formed groove is not symmetric relative to the thermal gradient and as a consequence the thermal field in the groove can be asymmetric as well implying that locally the thermal undercooling is different for the two facets. As a result, the silicon grain G2 which is closer to the edge of the sample grows faster than the G4 which is closer to the center where the isotherm is flatter.

Dislocation density.

As mentioned earlier and it will be further discussed in section 4.2, the formation of the RAGB between G2 and G4 is associated with the generation and emission of

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dislocations in both grains (Figure 5 (iv, v, vi)). In one of our previous works, we showed that the {111} facet kinetics can be successfully locally described by the theoretical law proposed by Voronkov for {111} facets containing dislocations [START_REF] Stamelou | {111} facet growth laws and grain competition during silicon crystallization[END_REF]. Yet, the Voronkov law is related to a facet growth mechanism eased by the presence of dislocations. As illustrated in Figure 5a, b (iv), when G2 and G4 enter in competition, dislocation emission occurs in both grains. The dislocation density quantitative value cannot be given by the present experimental results. However, from the Bragg diffraction images, it is obvious that G2 is significantly darker than G4 denoting that the diffracting planes bear some bending, likely due to the strain field of numbered dislocations. For a facet having dislocation(s) the nucleation may not occur at the triple junction of the groove, but rather at the ledge-related dislocation emergence. So, dislocation density could also have an impact on the facet growth kinetics and could explain the difference in the growth velocities between these two grains. In addition, a closer look to the Figure 5b (iii) and (iv),

shows that the dislocation expansion does not occur exactly after the intersection of G2 and G4 but it is slightly delayed. The G2 and G4 grain growth acceleration observed in Figure 4b (zones I and II in Figure 4) could then be related to the higher density of dislocations they contain as growth proceeds.

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Lin and Lan [START_REF] Lin | Revisiting the twinning mechanism in directional solidification of multi-crystalline silicon sheet[END_REF] investigated macroscopically the orientation of the RAGBs relatively to the facet bisector. They found that the grain boundary would follow it when solidifying at high growth rates (6 mm/min) because of the existence of large facets growing at the same speed. However, it does not seem to be the case at lower growth rates (2 mm/min). They suggest that at low growth rates, the undercooling in the groove is small, and thus it is likely that a rough-rough junction might exist locally at the groove TJ (solid-solid-liquid) due to curvature effects. In these less frequent cases, they observed that the grain boundary does not follow the facet bisector. In the present experiment, the measured average growth velocity of the solid/liquid interface is 1.26 mm/min, which is considered as low growth rate when referring to [START_REF] Lin | Revisiting the twinning mechanism in directional solidification of multi-crystalline silicon sheet[END_REF]. Moreover, in the completion of G2/G4, a shallow groove is formed, so that the TJ could be rough according to the discussion in [START_REF] Lin | Revisiting the twinning mechanism in directional solidification of multi-crystalline silicon sheet[END_REF].

Unfortunately, we cannot confirm such a hypothesis since we do not have the required resolution to observe the possible existence of a curvature at the TJ. What is seen experimentally in the present work is that the two facets have a different growth rate which directly induces that the grain boundary does not follow the facet bisector.

The grain-boundary development from a facet-facet grove during the solidification of silicon has been recently reported by Hu et. al [START_REF] Hu | In situ observation of grain-boundary development from a facet-facet groove during solidification of silicon[END_REF]. They also showed that the direction of the RAGB is mainly determined by the velocities of the {111} facets forming the grain boundary groove at the crystal/melt interface. It is very interesting that similar observation are obtained from independent groups performing different experiments.

However, a quantitative comparison with the present work cannot be performed since

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the crystallographic orientation of the investigated grains is not the same and the experimental conditions (such as the applied temperature gradient, cooling rate etc.) are not known. In addition, the measured facet growth velocities in that work are at least 3 times higher than in the present experimental results.

Defect generation and expansion during growth

At the initial stage of solidification, grains G2 and G4 (Figure 5) grow as high quality crystals, free from dislocations. This is confirmed by the presence of equalthickness fringes, related to Pendellösung fringes, at the crystal edges and at the solid/liquid interface. As described above and as shown in Figure 5a (i), the growth of G2 is driven by its 1 11 facet which is almost perpendicular to the sample main surface and 36° inclined by the growth direction (z). G4 grows with a 1 11 facet almost horizontal relatively to the solid/liquid interface and inclined by 56° from the surface direction (x). A simple calculation assuming that the facet extends towards the whole thickness of the sample (300 µm) with its 56° inclination gives the height of the facet: 445 µm. This height corresponds well to the height of the area with the fringes in When G2 and G4 meet each other (dotted circle in Figure 1b), dislocations are generated and emitted inside both grains. The generation of dislocations is an efficient way to release the stresses (mainly thermal stresses) induced in silicon during the solidification and cooling down process [START_REF] Srinivasan | Numerical modelling on stress and dislocation generation in multi-crystalline silicon during directional solidification for PV applications[END_REF][START_REF] Jiptner | Thermal stress induced dislocation distribution in directional solidification of Si for PV application[END_REF][START_REF] Chen | Three-dimensional global analysis of thermal stress and dislocations in a silicon ingot during a unidirectional solidification process with a square crucible[END_REF]. In the case of G4, the dislocation expansion likely occurs along the 1 1 1 slip planes. The strain field is evidenced by dark areas in the diffraction images, given in Figure 5a (iv) and b (iv), as well as by some asterism. The dislocation emission is related to the presence of the RAGB between those two grains. Emission of dislocations is also observed at the lower part of G2 likely due to the formation of another RAGB between G2 and G6. The same reasoning holds for G3 that appears to be strained (dark in Figure 5(vi)), since multiple

RAGBs are formed at its left side. However, even if high density of dislocations are accumulated in G3, they mainly do not expand inside G4. It is believed that this is due to the presence of the Σ3 diagonal double twin grain boundary formed between G3/G5 and G4/G5 where dislocations can direct slip along the {111} common plane or cross slip and move out of the sample.

From the presented experimental results, it is shown that RAGBs formation induces a local stress able to nucleate and propagate dislocations. At the same time it is

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well known that HP mc-Si contains a higher proportion and density of RAGBs than what was common before, and this is linked to the observed lower dislocation density in this material through mechanisms of cluster annihilation [START_REF] Stokkan | Study of evolution of dislocation clusters in high performance multicrystalline silicon[END_REF] and relaxation of thermal stress at RAGBs [START_REF] Yang | Development of high-performance multicrystalline silicon for photovoltaic industry[END_REF]. Furthermore, RAGBs are very seldom involved in the generation of vertical columnar dislocation clusters dominating multi-crystalline silicon produced on an industrial scale [START_REF] Ryningen | Growth of dislocation clusters during directional solidification of multicrystalline silicon ingots[END_REF]. There is however no contradiction between these observations. When the thermal stress is large enough, which is likely to be the case in this setup, RAGBs can induce plastic deformation in the crystal and thus dislocations. It is also interesting to note that the sample contains, at this stage of growth, no other CSL boundaries except the low energy Σ3 twins, i.e. none of the more potent dislocation sources, such as the Σ27 boundaries often observed in connection with dislocation generation [START_REF] Tsoutsouva | In situ investigation of the structural defect generation and evolution during the directional solidification of 〈110 〉 seeded growth Si[END_REF][START_REF] Ryningen | Growth of dislocation clusters during directional solidification of multicrystalline silicon ingots[END_REF][START_REF] Autruffe | Dislocation generation at near-coincidence site lattice grain boundaries during silicon directional solidification[END_REF][START_REF] Ervik | Dislocation formation at ∑=27a boundaries in multicrystalline silicon for solar cells[END_REF]. This could contribute to building up of higher stress at this level and to the activation of a less potent grain boundary source such as a RAGBs. Note that the presence of a Σ27 boundary further up in the crystal is also observed to correlate with a highly strained and dislocated area (upper left side of G2 Figure 1b) and Figure 5a (iii-v)), although it is not possible to establish clearly from which source the dislocations in this strained region originate. In summary, this experiment demonstrates how a RAGB may, under special circumstances generate dislocations, but it is not possible to infer any conclusions about the benefits/drawbacks of these boundaries in the growth of HP mc-Si under industrial conditions. Importantly, the generated strain

Conclusions

A better understanding of the facet growth kinetics and defect dynamics, associated with the propagation of a RAGB in silicon crystals, is provided from the in situ observation of the directional solidification process by X-ray diffraction and radiography imaging techniques. The main observations of this work can be summarized in the following points:

• The RAGB is at the origin of a facetted/facetted grain boundary groove at the melt front. Its orientation is determined by the {111} facet growth velocities of the adjacent grains that form the grain boundary grooves at the crystal/melt interface.

• The selection of the growth of the {111} facet in the grain boundary groove is determined by two main parameters; the direction of the normal to the facet with regards to the growth direction (normal to the isotherm) and by the competition with adjacent grains.

• The RAGB observed in this experiment is clearly a source of local dislocations which could be due to the fact that, at this stage of growth, no other potent grain boundary dislocation source was present, so that under thermal stress of a less potent grain boundary source such as RAGBs could be activated.

• On the one hand, the Σ3 {111} GB formation does not emit dislocations but favors their propagation through either direct or cross slips. On the other hand, the • Thanks to the in situ experiments, evidence are provided to show that the dislocation generation and propagation is mainly a growth phenomenon that evolves during the solidification process and grain competition.

Aknowlegments

The

  FReLoN (Fast Readout Low-Noise) with 2048×2048 image pixel

  respect to the three main directions relatively to the sample:

Figure

  Figure 1a presents the inverse pole figure map (IPF) along the direction

Figure 1 :

 1 Figure 1: EBSD measurements (Gi are the names of different grains, 1<Gi<6); a)

  Figure 3b corresponds to the instant t = t o + 194s, after the position indicated by the

  M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 14 two facets (Figure 3b) denoting that the 1 11 and 1 11 facets of the groove grow with different rates.

G3 1 1 1 15 Figure 4 :

 1154 Figure 4: Displacement of the normal to the a) G2/G3 and b) G2/G4 {111} facets as a

) = 2 .

 2 10 ± 0.05 × 10 * m/s (measured from the dynamic radiography images) and is always higher than the {111} facet growth rate as expected due to the slower kinetics of {111} facets. The local temperature gradient G is calculated with the equation -= ./ ) where . = 1.66 × 10 K/s is the cooling rate applied to both resistive heaters to start solidification during the experiment: -= 7.9 ± 0.4 × 10 K/m[START_REF] Stamelou | {111} facet growth laws and grain competition during silicon crystallization[END_REF].White beam X-ray Bragg diffraction images are recorded in situ during the same solidification experiment and representative images of the G2, G3 and G4 grain growth evolution are shown in Figure5a and b. Figure5a(ii) and b (ii) show the diffraction images of the grains when G2 grows in competition with its neighbor G3, while FigureM A N U S C R I P T A C C E P T E D5a (iii) and b (iii) correspond to an instant just after G2 enters in contact with G4, instead of G3 (position of the dotted circle in Figure1b). G3 and G4 diffract together in the same diffraction spot (Figure5b) because they have the same crystallographic orientation[START_REF] Tandjaoui | Twinning occurrence and grain competition in multi-crystalline silicon during solidification[END_REF]. Equal-thickness fringes, related to Pendellösung fringes are clearly observed in the crystal. This contrast is attributed to the dynamical diffraction effect inside the crystal matrix and indicates a high-level of crystalline quality (less than 10 2 cm -2 dislocation density[START_REF] Burns | HPHT growth and x-ray characterization of high-quality type IIa diamond[END_REF]). Thus, as long as these fringes are observed, G2 and G4 grow as high quality crystals.

Figure 5 :

 5 Figure 5: White beam X-ray Bragg diffraction images taken during the solidification

  ) and Bragg diffraction images in Figure5b (i), where G3 grows with the diagonal 1 1 1 and in Figure

  growth facet change, during the solidification of two grains that are in competition, due to the change of the isotherm shape is presented in Figure6. Moreover, it is also possible that the presence of the double diagonal twin grain G5 contributes to the selection of the {111} growth facet of G4 at the RAGB groove since G4 and G5 are in competition prior to the encounter of G4 and G2.

Figure 6 :

 6 Figure 6: A simplified 2D model that illustrates and explains the growth facet {111}

  radial temperature gradient causing an asymmetric thermal field in the grain boundary groove.

Figure 5b (

 5b Figure 5b (ii) which is a projected image of the facet. Indeed, the Bragg diffraction

  localized close to the RAGB and cannot contribute to longrange dislocation propagation due to the absence of compatible glide planes nor crossslip possibilities.

  the RAGB remain localized and cannot contribute to long range dislocation propagation.

  work is supported by the Research Council of Norway's ENERGIX program in the frame of the INSIDES project and by the ANR within the CrySaLID Project (Nº ANR-14-CE05-0046-01). The ESRF (European Synchrotron Radiation Facility) BM05 team is acknowledged for support during the Xray imaging experiments. Professor Thierry Duffar is greatelly aknowleged for the fruitfull scientific discussions.

& = . ! ± . × % "/#

Proc., Hamburg, Germany, 2011: pp. 1895-1899. doi:10.4229/26thEUPVSEC2011-2BV.4.31.