E Flahaut

A Peigney

Ch Laurent

Double-Walled Carbon Nanotubes in Composite Powders

Keywords: Double-walled Carbon Nanotube, CCVD Synthesis, Methane, Solid Solution, Alumina, Iron, Nanocomposite

Double-walled carbon nanotubes (DWNTs) may be interesting in many applications since the outer wall would provide an interface with the rest of the system, without modifying the inner wall. CNT-Fe/Fe 3 C-Al 2 O 3 composite powders containing carbon nanotubes (CNTs) (65% of which are DWNTs) are prepared by reduction of an oxide solid solution in a H 2 -CH 4 gas mixture. The powders and CNTs are studied by both local and macroscopical techniques. The influence of the reducing atmosphere composition and of the dwell time at 1050 C is studied. There is a 6-fold increase in CNT content upon the increase in the CH 4 content from 3 to 30 mol.%, but the formation of undesirable carbon nanofibers can also be promoted. A CH 4 content of 12-18 mol.% is adapted for the particular iron content in these powders. Increasing the dwell time at 1050 C results in the formation of CNTs with more walls.

INTRODUCTION

Iijima's report 1 on carbon nanotubes (CNTs) prompted a major research effort notably driven by the very high potential of CNTs for technological applications. Most of the work has been focused on single-walled carbon nanotubes (SWNTs). However, interactions with its environment may dramatically change the properties of a SWNT. For example, SWNTs embedded in a matrix for applications such as a composite material may not exhibit the remarkable mechanical [2][3][4] and electrical 5 6 characteristics they display as individual objects. Thus, the use of double-walled carbon nanotubes (DWNTs) may be desirable since the outer wall would be available for functionalization or more generally integration into a system without modifying the characteristics of the inner wall too much. Some theoretical and experimental works specifically focused on DWNTs have been published. [7][8][9][10][11][12][13] The present authors have proposed an original catalytic chemical vapor deposition (CCVD) method based on the selective reduction of oxide solid solutions in a H 2 -CH 4 gas mixture. 14 The reduction of -Al 1 9 Fe 0 1 O 3 produces pristine Fe nanoparticles at a temperature high enough for them to catalyze the decomposition of CH 4 and the in situ formation of CNTs. The products obtained in this way are CNT-metal oxide composite powders, with a very homogeneous dispersion of the CNTs, which cannot be matched by methods involving a mechanical mixing. * Author to whom correspondence should be addressed.

Dense ceramic-matrix composites 15 16 prepared from such powders show some electrical conductivity due to the percolation of the CNT network. Aligned CNTs provide an anisotropic electrical conductivity. 17 To increase the overall selectivity of the reactions and the proportion of SWNTs and DWNTs with respect to other CNTs, several parameters related to the starting alumina-based solid solutions have been investigated, including the iron content, 18 the crystallographic form, 19 and the specific surface area. 20 Parameters related to the experimental conditions of the reduction, such as the composition of the H 2 -CH 4 atmosphere 21 and the temperature, 22 were also examined. The CNTs are mostly (90%) found as SWNTs and DWNTs with an average diameter close to 2.5 nm. It was shown that a second wall can grow inside the first one. 22 In the case of Co-MgO catalysts, a simple soaking in HCl allows separation of the SWNTs and DWNTs from the catalyst materials without damaging them. 23 The CNTs obtained in this way are individually dispersed rather than bundled and have a very high specific surface area (ca. 950 m 2 /g for carbon). 24 Dai et al. 25 DWNTs are also present, but their proportion is considered negligible. Bandow et al. 29 have reported the coalescence of C 60 pea pods 30 inside laser-vaporization SWNTs, thus forming a second tube inside the first one. Hutchison et al. 31 prepared DWNTs by a hydrogen arc discharge method.

The aim of the present paper is to build on a series of systematic experimental works on the preparation of nanocomposite powders containing well-defined CNTs, by studying the influence of the composition of the H 2 -CH 4 atmosphere and that of the dwell time at the reduction temperature. In a previous study, 21 , and the carbon quality is at maximum for the range 9-18 mol.%. In addition, experiments have been done with solid solutions containing twice more iron, applying the same temperature profile but changing the maximum temperature (800-1070 C). 22 It was found that using a higher reduction temperature increases the reduction yield, thus favoring the formation of surface metal nanoparticles and thereby producing more CNTs. Moreover, it was observed that the proportion of SWNTs decreased. One of the key points for getting a good selectivity in the formation of SWNTs or DWNTs is to ensure an appropriate ratio between the carbon source (CH 4 in the H 2 -CH 4 atmosphere) and the catalyst source (Fe 3+ ions in the oxide solid solution). Thus we are finetuning the experimental conditions to increase the selectivity toward DWNTs while increasing the total amount of CNTs in the powders. The formation mechanisms of the CNTs are briefly discussed.

EXPERIMENTAL DETAILS 2.1. Synthesis of Carbon Nanotubes

An -Al 1 8 Fe 0 2 O 3 powder was prepared by decomposition in air at 400 C and further calcination at 1000 C of the corresponding mixed oxalate. 32 As described in detail in a previous work, 22 the X-ray diffraction (XRD) pattern of the oxide powder shows the presence of a small proportion of -Fe 2 O 3 in addition to the peaks of the aluminahematite solid solution in the corundum form. This reveals that the crystallization from the metastable oxide solid solution (amorphous, then phases) takes place with some degree of phase partitioning, producing an intimate mixture of alumina-rich ( 1 , major phase) and hematite-rich ( 2 , minor phase) corundum forms. The powder is made up of 10-20-m agglomerates of submicronic or nanometric primary grains. Its specific surface area is equal to 8.5 (±0.3) m 2 /g. The oxide powder was divided into several parts, which were reduced in a flowing H 2 -CH 4 atmosphere (250 sccm), producing the CNT-Fe/Fe 3 C-Al 2 O 3 composite powders. The heating and cooling rates were set at 5 C/min, and the maximum temperature was 1050 C (Fig. 1). First, the CH 4 content was varied (3, 6, 9, 12, 18, 24, 30 mol.%), and no dwell was applied at 1050 C. For the sake of brevity, the powders obtained in this way will hereafter be noted as M3, M6, , M30. In a second set of experiments, a particular composition of the H 2 -CH 4 atmosphere was chosen (18 mol.% CH 4 , and a dwell time (60 or 120 min) was applied at 1050 C. The latter powders will hereafter be denoted as M18t60 and M18t120 and will be compared with M18 and other powders.

Characterization

The composite powders were studied by scanning electron microscopy (SEM) as well as high-resolution transmission electron microscopy (HRTEM). XRD patterns were acquired with the use of Co-K radiation ( = 0 17902 nm). In addition to the electron microscopy characterizations, a method based on carbon element analysis and specific surface area measurements 14 33 was used to characterize the composites at a macroscopic scale and thus produce quantitative data that are more efficient for the comparison of the different powders. The carbon content C n was determined by flash combustion with an accuracy of ±2%. Flash combustion is based on the complete and instantaneous oxidation of the sample; carbon is oxidized into CO 2 , which is detected by a thermal conductivity detector, giving an output signal proportional to the concentration in the sample. Two parameters representing the quantity of CNTs (noted S) and what is called the quality of the deposited carbon (noted S/C n are calculated from the specific surface area measurements. We assume that a higher quality parameter principally corresponds to more carbon in tubular form or CNTs with fewer walls or less bundled CNTs. 33 The specific surface area of the powders was measured by the BET method, using N 2 adsorption at liquid N 2 temperature in a Micromeritics FlowSorb II 2300 apparatus. This instrument gives a specific surface area value from one point (i.e., one adsorbate pressure) and requires calibration. The reproducibility of the results is ±3%. The oxidation of some composite powders was investigated by thermogravimetric analysis (TGA) in flowing air (heating rate 1 C/min). ). It is difficult to clearly separate the patterns of -Fe and Fe 3 C because the peaks are rather wide and overlap, but the trend is that the proportion of Fe 3 C increases when higher CH 4 contents are used. The absence of -Fe 2 O 3 peaks indicates that the complete reduction of the 2 phase into relatively large Fe particles (ca. 30 nm) was easy, in agreement with earlier studies. 22 32 34 A -Fe-C alloy is also present but cannot be resolved on the XRD patterns because the (111) diffraction peak (d 111 = 0 208 nm) is masked by the corundum (113) (d 113 = 0 209 nm) and the Fe 3 C (121) (d 121 = 0 210 nm) peaks. But since this phase does not participate in the formation of the CNTs, 22 it is not essential to follow its evolution. Indeed, the nature of the iron species (Fe 3+ , -Fe, Fe 3 C, -Fe-C) in the CNT-Fe/Fe 3 C-Al 2 O 3 powders is known 22 to be correlated with their location in the material (i.e., at the surface of, or within, the oxide matrix grains). The nanoparticles responsible for the formation of the CNTs come from the reduction of Fe 3+ ions in the 1 phase. The surface Fe 3+ ions are reduced first and then Fe 3+ ions located in the bulk of the solid solution grains diffuse toward the surface, forming more potentially active particles as the reduction proceeds. The particles are probably in the form of an Fe-C alloy at the time of CNT formation but are found as Fe 3 C by post-reaction Mössbauer analysis. However, it was also shown 35 that some of the surface particles that were inactive for the CNT formation also end up as Fe 3 C.

RESULTS AND DISCUSSION

Typical SEM images of carbon filaments at the surface of the matrix grains are shown in Figure 2. TEM studies on similar powders 14 18-22 have demonstrated that these filaments are mostly CNT bundles. The observations show that the quantity of bundles increases with increasing CH 4 content. For M3 (Fig. 2a,b), very few CNT bundles are observed. Their diameter is lower than ca. 20 nm. Some Fe/Fe 3 C nanoparticles with a diameter in the range of 10-40 nm are clearly visible at the surface of the matrix grains (arrow in Fig. 2b). Such large particles have not catalyzed the formation of CNTs. Many more bundles are observed for M12 (Fig. 2c,d). They appear to be a bit thicker (however, smaller than ca. 30 nm), possibly indicating that a bundle is made up of more CNTs than in M3. It is worth noting the presence of some carbon nanofibers (arrow in Fig. 2d) that are shorter (less than ca. 200 nm in length) and thicker (ca. 50 nm) than the CNT bundles. However, very few are observed in M12. For M30 (Fig. 2e-h), still more CNT bundles are observed (up to ca. 100 nm in diameter), but the undesirable nanofibers are much longer than for M12. It seems that they are preferentially localized in certain areas of the sample, in which they are quite numerous (Fig. 2g). It is difficult, however, to compare the quantity of nanofibers between M12 and M30. These nanofibers, which may be hollow, originate from the hematite-rich solid solution 2 particles. Because these latter particles are all formed early in the reduction process, the composition of the atmosphere (and its reducing power) should have little influence, and thus their number should not change from one powder to the other. It follows that the number of corresponding nanofibers is expected to be roughly the same in all samples. However, when the CH 4 content in the atmosphere is higher, more carbon is available for growth, and the nanofibers are longer.

The carbon content in the powders (C n , Table I) increases with the CH 4 content, with a 20-fold jump from M3 (0.65 wt%) to M30 (13.0 wt%). The parameter representing the quantity of CNT ( S, Table I) markedly increases from M3 (4 0 ± 0 5 m 2 /g) to M30 (24 6 ± 1 2 m 2 /g), showing that more CNTs are formed when the CH 4 content in the reduction atmosphere is increased and is in good agreement with the SEM observations. The parameter accounting for the carbon quality ( S/C n , Table I) decreases from M3 (609 ± 82 m 2 /g) to M12 (215±14 m 2 /g) and remains constant for higher CH 4 contents. These values are plotted in Figure 3 together with those previously obtained for powders prepared by reduction at 1050 C (60-min dwell) of an oxide powder with only half the iron content and a lower specific surface area (5 vs. 8.5 m 2 /g). 21 The carbon content (C n , Fig. 3a) is clearly higher in the present powders, notably from 9 mol.% CH 4 upward. S (Fig. 3b) is also higher and interestingly does not yet reach a plateau. Comparison of the carbon quality S/C n (Fig. 3c) is less straightforward. For the former powders there were three regions: S/C n increases below 9 mol.% CH 4 (region A), is constant in the range 9-24 mol.% (region B), and decreases above 24 mol.% (region C). In the present powders, the equivalent of region C is absent because there was no S plateau as noted above. Region B (a S/C n plateau) is slightly shifted to higher CH 4 contents (12-30 mol.%): the strong increase in tubular carbon is balanced by the formation of the nanofibers. Clearly, the main difference between the two series appears for the lower CH 4 contents. For powders in the former A region, the amount of deposited carbon is very low (less than 1%), and the quantity of CNTs is accordingly small. However, the moderate increase in S is reflected in the increase in S/C n , but starting from a very low value. For the present A region, the apparent decrease in quality could correspond to a bundling effect, 33 which would be more important than the net increase in CNT quantity because the carbon supply would be insufficient to activate all potential catalytic particles. Indeed, some particles will grow over the threshold size of about 3 nm 22 26 and become encapsulated. For higher CH 4 contents, the growth of such particles would be stopped earlier, but, as already noted, the resulting increase in quality would now be confronted to the formation of the nanofibers. From the above results, the powder prepared with 18 mol.% CH 4 (sample M18) was selected for the study of the influence of the dwell time at 1050 C.

Influence of the Dwell Time at 1050 C

XRD pattern analysis shows that there is a slight increase in the proportion of Fe 3 C compared with that of Fe with increasing dwell time. The carbon content (Table II) increases with t r , doubling from M18 (7.7 wt%) to M18t120 (16.4 wt%). However, there is only a small increase in S from M18 to M18t60 (14.9 and 19.5 m 2 /g, respectively), and then the quantity parameter remains constant for a longer dwell (19.6 m 2 /g for M18t120). Clearly the deposition of more carbon does not correspond principally to the formation of CNTs. Accordingly, the quality parameter S/C n steadily decreases with increasing dwell time. The values for M18t60 (Table II) can be compared to those (C n = 2 7 wt%, S = 6 02 m 2 /g, S/C n = 229 m 2 /g) obtained for a composite powder prepared in the same conditions (1050 C, 60 min, 18 mol.% CH 4 ), but starting from an oxide containing half as much iron. 21 One can see that the 5-fold increase in carbon content corresponds to only a 3-fold increase in the quantity of CNTs, because of the presence of nanofibers in the present powders. This could also have been explained by a difference in the bundling state of the CNTs in these different samples, but experimental observations by SEM clearly show that the presence of carbon nanofibers is more likely to be the correct explanation.

SEM observations reveal a web-like network of CNT bundles, and no difference in the quantity of CNTs can be detected for M18, M18t60, and M18t120. Typical images for M18 and M18t120 are shown in Figure 4. The CNT bundles are slightly thicker in the case of M18t120. Interestingly, it seems that no more nanofibers are observed in M18t120 compared with M18. Typical HRTEM images of the CNTs in powders M18 and M18t120 are shown in Figures 5 and6, respectively. Entangled CNTs and CNT bundles are shown in Figure 5a. Some catalyst nanoparticles (dark spots) are located at the surface of the oxide grains. A SWNT 3 nm in diameter and a DWNT (1.8 nm) are shown in Figure 5b andc, respectively. Figure 5d shows a triple-walled carbon nanotube (TWNT) (diameter 4 nm). Figure 5e shows a bundle with an individual CNT emerging. A higher magnification image of the same area (Fig. 5f) reveals that this CNT (2.8 nm) is a TWNT. For M18t120, Figure 6a shows a SWNT (2.5 nm) and two DWNTs (both 3.7 nm). A DWNT (2.5 nm) emerging from a bundle is shown in Figure 6b. Figure 6c shows a bundle and two CNTs crossing the focal plane, which allows their observation in cross section. A higher magnification image (Fig. 6d) reveals that they are CNTs with four and five walls. Figure 6e shows a TWNT (diameter 4 nm).

Figure 6f shows a thicker multiwalled CNT (eight walls, diameter 8.7 nm). It seems that the outer layer is not complete; it may have been damaged by electron irradiation during the observation. For both specimens, it is interesting to note that the surface of the CNTs appears to be free of amorphous carbon and that no pyrolytic thickening has taken place, even for M18t120. It is very difficult to distinguish between the different specimens based solely on the observation of such images.

To get more information from the HRTEM observations, the number of walls as well as the inner and outer diameters of more than 60 isolated CNTs were measured on HRTEM images similar to those shown in Figures 5 and6. For M18, the DWNTs form a large majority (ca. 65%) of the observed CNTs (Fig. 7a). For the construction of the histograms, diameter classes are defined as follows: class "1 nm" corresponds to diameters ranging from 0.5 to 1.49 nm; class "2 nm" corresponds to diameters ranging from 1.5 to 2.49 nm, and so on. The median inner and outer diameters (estimated from the whole population of CNTs) are equal to 2.30 nm and 3.13 nm, respectively (Fig. 7b). For M18t120, the DWNT still represent the majority (ca. 40%) of the observed CNTs (Fig. 7c), but CNTs with more walls are much more numerous than for M18. The inner and outer diameters are also higher (Fig. 7d), with median values equal to 2.85 nm and 3.78 nm, respectively. The higher inner diameter could indicate that slightly larger particles are activated during the dwell, probably early in the process, as indicated by the data given here ( S, Table II). The higher outer diameter could reflect the formation of CNTs with more walls from freshly formed catalyst particles (i.e., formed during the dwell). However, the results presented in the following section will make it possible to rule out the latter hypothesis.

The oxidation of powders M18, M18t60, and M18t120 was investigated by TGA. The TGA curves (Fig. 8a) and the corresponding differential thermogravimetric (DTG) curves (Fig. 8b) show that the oxidation of the composites occurs in several steps with both weight gains and weight losses. A previous study 19 during the dwell and consequently that the quantity of CNTs formed during the dwell is small, in good agreement with the macroscopic analysis ( S values). Interestingly, the position of the DTG peak accounting for the oxidation of the CNTs is shifted toward higher temperatures as the corresponding dwell time is increased (417, 475, and 488 C for M18, M18t60, and M18t120, respectively). This is in good qualitative agreement with the corresponding increase in the number of walls as revealed by HRTEM analysis. It is supposed that, in the present experimental conditions, the formation of CNTs occurs according to a mechanism close or similar to the so-called yarmulke mechanism. 37 This is a base-growth mechanism, with the notable feature that in the case of CNTs with several walls, the first wall is the outer wall. Thus, the diameter of the outer wall is that of the catalytic particle. A sketch describing the formation of CNTs by the yarmulke mechanism was made from the descriptions of Dai et al. 37 (Fig. 9). The simultaneous growth of all of the walls results from the impossibility of feeding the catalytic particle further with carbon species when the outer cap is completed. Graphitic caps would first all form stacked on top of the particle, and the growth of all of the walls would then start at the same time once the concentration of carbon within the particle reached a critical value. In these conditions, the catalytic particle diameter plays a key role because a larger particle will need more carbon to reach the saturation, leading first to a delayed CNT growth and second to a CNT with more walls. Obviously, very small particles (<2 nm) can only lead to CNTs with one or two walls, but slightly larger particles can produce a wider range of CNTs. It was even proposed that very small particles formed at a relatively high temperature could be more active for CH 4 decomposition and Fig. 9. Illustration of the yarmulke growth mechanism drawn from the descriptions of Dai et al. 37 thus form DWNTs rather than SWNTs. 22 The increase in the dwell time in the present study does not result in the formation of more catalytic particles, as shown above, but rather allows larger particles to be activated for the growth of CNTs. This thus favors the formation of CNTs with more walls, which appears very clearly in Figure 7. For a particle that has still not reached saturation during the dwell, encapsulation by graphitic layers occurs during cooling 38 because the solubility of carbon within the particle decreases with the temperature and the carbon is progressively expelled from the particle.

CONCLUSIONS

CNT-Fe/Fe 3 C-Al 2 O 3 composite powders in which the CNTs are mostly DWNTs (65%) have been prepared by reduction in a H 2 -CH 4 gas mixture of an oxide solid solution. The influence of the reducing atmosphere composition and of the dwell time at 1050 C has been investigated. Increasing the CH 4 content results in a strong increase in CNT content but can also lead to the formation of undesirable carbon nanofibers that are able to form because the iron content in the precursor powder is high enough to generate the formation of sufficiently large catalyst particles. It is found that a CH 4 content of ca. 12-18 mol.% is adapted for the particular iron content of these powders. Increasing the dwell time at the maximum temperature reached during the reduction process results in the formation of CNTs with more walls. It is proposed that the formation of the CNTs occurs by a base-growth mechanism and that locally matching the carbon source and the catalyst source is essential for the control of the number of walls.

Fig. 1 .

 1 Fig. 1. X-ray diffraction patterns of powders prepared at 1050 C (no dwell) with H 2 -CH 4 atmospheres of different composition.

3. 1 .

 1 Influence of the Composition of the H 2 -CH 4 Atmosphere Analysis of the XRD patterns (Fig. 1) of the composite powders reveals the presence of -Fe and Fe 3 C besides a corundum phase accounting for -Al 2 O 3 or a partially reduced -Al 2-2y Fe 2y O 3 y < 0 1 solid solution (corresponding to the evolution of the above-mentioned 1 phase

Fig. 2 .

 2 Fig. 2. SEM images of powders prepared at 1050 C (no dwell) with H 2 -CH 4 atmospheres of different composition: 3 mol.% CH 4 (a, b), 12 mol.% CH 4 (c, d), 30 mol.% CH 4 (e-h).

Fig. 3 .

 3 Fig. 3. Evolution of the carbon content C n (a), the quantity of CNTs S (b), and the carbon quality S/C n (c) versus the composition of the H 2 -CH 4 . , This work; , previous study 21 (see text for details). The error bars have been omitted for the sake of clarity.

6 Fig. 4 .

 64 Fig. 4. SEM images of the powders prepared in a H 2 -CH 4 atmosphere (18 mol.% CH 4 with a dwell time of 0 min (a, b) and 120 min (c, d) at 1050 C.

Fig. 5 .

 5 Fig. 5. HRTEM images of CNTs in the powders prepared in a H 2 -CH 4 atmosphere (18 mol.% CH 4 with a dwell time of 0 min (sample M18) at 1050 C.

Fig. 6 .

 6 Fig. 6. HRTEM images of CNTs in the powders prepared in a H 2 -CH 4 atmosphere (18 mol.% CH 4 with a dwell time of 120 min (sample M18t120) at 1050 C.

Fig. 7 .

 7 Fig. 7. Distribution of the number of walls and of the inner and outer diameters of the CNTs in the powders prepared in a H 2 -CH 4 atmosphere (18 mol.% CH 4 with a dwell time of 0 min (a, b) and 120 min (c, d) at 1050 C.

Fig. 8 .

 8 Fig. 8. TGA curves (a) and the corresponding derivative curves (b) of the powders prepared in a H 2 -CH 4 atmosphere (18 mol.% CH 4 with a dwell time of 0, 60, and 120 min at 1050 C.

  reported the formation of SWNTs together with a small amount of DWNTs by disproportionation of CO at 1200 C on a Mo-Al 2 O 3 catalyst. Hafner et al. 26 synthesized a mixture of SWNTs and DWNTs by catalytic decomposition of C 2 H 4 at 700-850 C over Moand Fe/Mo-Al 2 O 3 catalysts. They report that the DWNT proportion increases from 30% at 700 C to 70% at 850 C. Ivanov et al. 27 reduced a zeolite-supported Co catalyst in N 2 -C 2 H 2 and obtained a small amount of CNTs doi:10.1166/jnn.2003.177 with two or three walls together with much thicker tubes. Cassell et al. 28 claimed to have prepared gram quantities of SWNTs by CCVD of CH 4 at 900 C on an Fe/Mo catalyst supported on a new Al 2 O 3 /SiO 2 hybrid material.

  CNT-Fe/ Fe 3 C-Al 2 O 3 nanocomposite powders have been prepared by selective reduction (1050 C, 60 min) of a monophase -Al 1 9 Fe 0 1 O 3 solid solution in pure H 2 and 12 different H 2 -CH 4 gas mixtures (1.5-45 mol.% CH 4 . More carbon is deposited when the CH 4 concentration is increased, but the CNT quantity reaches a maximum plateau for 24-45 mol.% CH 4

Table I .

 I Carbon content C n and parameters representing the quantity of CNT ( S) and the carbon quality ( S/C n in the powders prepared in different H 2 -CH 4 atmospheres at 1050 C (no dwell).

	CH 4 content	C n	S	S/C n
	(mol.%)	(wt%)	(m 2 /g)	(m 2 /g)
	3	0 65 ± 0 01	4 0 ± 0 5	609 ± 82
	6	1 56 ± 0 03	6 8 ± 0 6	435 ± 40
	9	2 77 ± 0 06	8 3 ± 0 7	299 ± 24
	12	5 3 ± 0 1	11 5 ± 0 7	215 ± 14
	18	7 7 ± 0 2	14 9 ± 1 0	192 ± 13
	24	10 7 ± 0 2	20 2 ± 1 2	188 ± 11
	30	13 0 ± 0 3	24 6 ± 1 2	189 ± 9

  checked by monitoring the CO 2 release during the TGA run). It is partly superimposed on the first weight gain, so that the weight loss is smaller than the actual carbon content C n as determined by flash combustion. The high-temperature (>800 C) weight gains correspond to the oxidation of the intragranular Fe/ -Fe-C nanoparticles. Although it is known that TGA of such composite powders cannot give quantitative results for the reduction yield of the starting oxide, a combination of the present C n and TGA data leads to the estimation that ca. 20% of the total iron remains as Fe 3+ ions in the corundum lattice, whatever the dwell time at 1050 C. This is in excellent agreement with a study36 of the reduction of such powders in pure H 2 (rather than in H 2 -CH4 showing that the reduction yield does not change appreciably for dwell times up to 300 min and that the reduction is still not complete for 1800 min at 1050 C. Moreover, extrapolation of the Mössbauer spectroscopy data for powders reduced in a H 2 -CH 4 atmosphere 22 is also in agreement with the present figure (20% unreduced iron). This clearly shows that almost no new catalytic nanoparticles are formed

Acknowledgments: The authors thank Mr. L. Datas for his assistance with the HRTEM observations, which were performed at the Service Commun de Microscopie Electronique à Transmission-Université Paul-Sabatier.


Nalini Rakotonandraina

Loïc Sauvée
email: loic.sauvee@lasalle-beauvais.fr

Mark Wever

Nel Wognum

Broadening the scope of chains and networks Alignment between governance designs and quality management systems in European pork meat chains: A comparison between IKB (The Netherlands) and VPF (France)

Keywords: alignment, governance, quality management, pork meat

The objective of the communication is to explore the question of alignment between quality management systems (hereafter QMS) and governance designs in EU pork meat supply chains, based on a cross country comparison between VPF ('French Pig Meat', France) and IKB ('Integrated Chain Control', The Netherlands). According to the literature review, we propose an analytical grid based upon an original dimensionalization of QMS and governance designs. This allows us to extend the notion of alignment in considering simultaneously in two concrete case studies the interplay of strategic and organizational aspects with the specific roles assigned to QMSs by chain actors.

1-Introduction

The rise and complexity of quality management systems (hereafter QMS) in agrifood chains constitute one of the key phenomena of the recent past. The reasons of this trend are well known: search of efficiency, competition between products at the consumer level, between chains at the industry level, global rise of standards, food crises. Nevertheless the impacts and consequences of the proliferation of QMS for the organization of chains are rarely addressed by researchers in the literature. This link between QMS and chain organization is identified as the alignment principle: a 'proper' alignment being the way to reach efficiency, in the sense of the transaction cost theory [START_REF] Williamson | The Economic Institutions of Capitalism[END_REF][START_REF] Williamson | Comparative Economic Organization: The Analysis of Discrete Structural Alternatives[END_REF][START_REF] Williamson | The Mechanisms of Governance[END_REF]. Previous research on that subject has shown the importance of that question, for both managerial and theoretical reasons. Indeed for managers the question is of relevance as the competition progressively shifts from inter-firms to inter-systems competition, the system being the quality system as a whole, the complete agrifood chain network or a combination of both. This paper considers this question in the specific context of European pork chains [START_REF] Trienekens | European pork chains. Diversity and quality challenges in consumer-oriented production and distribution[END_REF] in proposing an original conception of alignment between governance structures and quality management systems. To do so, we develop, based mainly on previous works on the definition of alignment, an analytical grid applied to two QMSs developed in two countries, The Netherlands and France. This paper is organized as follows. In a first part we develop our main theoretical backgrounds, i. e. the question of alignment and its relation with the topic of chain organization (2). Then in a second part we propose a synthesis of our findings in a form of an analytical framework which puts the stress on the relevant criteria (3). In a third part the grid is applied to the situation of two case studies in France and the Netherlands respectively: VPF and IKB, including a global synthesis (4); followed by concluding comments (5).

2-Theoretical backgrounds: the core question of alignment revisited Concerning alignment, as suggested by Nickerson et al. (2001), a preliminary and central question appears with the notion of dimensionalization. For [START_REF] Williamson | Comparative Economic Organization: The Analysis of Discrete Structural Alternatives[END_REF] the dimensionalization is the "identification of key attributes with respect to which governance structures differ" (Williamson, 1991:277). In other words, the dimensionalization answers to the question "what are the factors that are responsible for the aforementioned differential costs and competencies" (Williamson, 1991:277). This step is necessary to reach a concrete comparison of real cases. In a first part we propose a grid to dimensionalize governance structures (2-1), and then we propose the same for the quality management systems (2-2). Finally, built on these assumptions, we propose an extension of the alignment principle in the context of agrifood networks (2-3).

2-1. Governance of vertical transactions: how to dimensionalize them?

To take into account the diversity of governance modes combined with the multiple stage nature of agrifood chains, two dimensions are in first analysis necessary: the one is the number of stages, and the second one is the governance mode. According to [START_REF] Trienekens | European pork chains. Diversity and quality challenges in consumer-oriented production and distribution[END_REF], in European pork chains, the stages usually consist of breeding, farrowing, finishing, slaughtering, processing, retail (Trienekens and Wognum, 2009:26). These stages are not necessarily performed by independent actors. But in a transaction cost approach, any mode of organizing vertical transactions, and even vertical integration, is seen as an alternative choice. The question of governance mode is more complex. The classical typology of [START_REF] Williamson | Comparative Economic Organization: The Analysis of Discrete Structural Alternatives[END_REF] who distinguishes market, hybrid form, and hierarchy has been extended by [START_REF] Ménard | The Economics of Hybrid Organizations[END_REF] especially for the category of hybrid form. Ménard notes the fact that "when it comes to intermediate arrangements, the terrain is a shifting one. The vocabulary itself is not stabilized. Besides hybrids, one can read papers about clusters, networks, symbiotic arrangements, supply-chain systems, administered channels, nonstandard contracts, and so forth" (Ménard, 2005:3). The objective is then to emphasize regularities "in the traits exhibited by the abundant literature on these forms" (Ménard, 2005:3). Based on a large analytical comparison of case studies, this author has shown that there is more, in hybrid form, than a global and uniform category. Indeed the logic of hybrid forms is to be found in a balance between flexibility and coordination. For Ménard the main proposition is that "the more specific mutual investments are, the higher are the risks of opportunistic behaviour, and the tighter are the form of control implemented" (Ménard, 2005:10-11). Several mechanisms are available to reach actors' objectives. In the vein of [START_REF] Raynaud | Alignment between Quality Enforcement Devices and Governance Structures in the Agro-food Vertical Chains[END_REF], we propose a categorization in six governance modes, including market and hierarchy: spot market contract, relational (or implicit) contract, relational contract with approved partners, formal written contract, equity-based contract, vertical integration. Nevertheless, and following the work of [START_REF] Provan | Modes of Network Governance: Structure, Management, and Effectiveness[END_REF] on network governance, we suggest that "networks are forms of social organization, which are more than the sum of actors and their links and which deserve to be studied in their own right" (Provan and Kenis, 2007:233). These authors consider that networks are a kind of 'variable': different configurations are possible and each of them will have distinct global properties. Thus, according to these authors, it is possible to identify "different network-level effects can a rationale for developing network-level theories be established" (Provan and Kenis, 2007:233). Provan and Kenis propose a categorization along two dimensions. Network governance may or may not be brokered. At one extreme, "every organization in the chain would interact with every other organization to govern the network, resulting in a dense and highly decentralized form. This is what we call shared governance" [START_REF] Provan | Modes of Network Governance: Structure, Management, and Effectiveness[END_REF]. Then "at the other extreme, the network may be highly brokered, with few direct-to-direct organization interactions, except regarding operational issues". In this situation, a single organization would act as a highly centralized broker or lead organization. Of course intermediate positions are possible, where for instance actors in the chain or network may divide governance responsibilities. A second axis of tremendous importance for our case is the distinction between participant-governed or externally governed. In participant-governed network, participants are, at one extreme, governed collectively by the members themselves, or at the other extreme, by a single actor. The table gives a synthesis of their approach adapted to the context of agrifood chains [START_REF] Omta | Chain and network science: a research framework[END_REF][START_REF] Schiefer | From enterprise activity quality management to sector initiative quality assurance: development, situation and perspectives[END_REF]. Three distinct forms of network governance are at the end defined: shared governance, lead organization, network administrative organization (NAO). Our proposal is thus to coin the term of governance design: to the classical description of institutional structure in the TCE tradition (with a multiple-stage perspective in agrifood chains) we add a qualitative characteristic. We could summarize the governance design as, in the vein of [START_REF] Provan | Modes of Network Governance: Structure, Management, and Effectiveness[END_REF], the form taken by collective actions in supply chains. Thus the approach proposed by Provan and Kenis add a complementary perspectives on governance in the TCE tradition. Its interest is patent when there are complex and multilevel relationships as this is the case in most of agrifood chains. In other words the logic and rationale of chain organization is to be found beyond the pure TCE principle of transactioncost minimization and their grid provides an attempt to overpass the difficulty. In spite of their diversity, QMS can be categorized along three dimensions [START_REF] Wever | Alignment between chain quality management and chain governance in EU pork supply chains: A Transaction-Cost-Economics perspective[END_REF]: the owner of (parts of) the system, the scope of the system, and the scale of the system. Let us summarize quickly the content of these dimensions, based on [START_REF] Wever | Alignment between chain quality management and chain governance in EU pork supply chains: A Transaction-Cost-Economics perspective[END_REF]. The owner is distinguished on the basis of its status, private or public. The scope refers to the extent to which the system is adopted along the stages, from a company-to-company system to a whole supply chain covering all transactions. The scale dimension refers to the market penetration of the system, from small-scale (a few participants have adopted the system) to wide scale (all actors in all stages have adopted the system). These descriptive dimensions are necessary steps to identify the diversity of QMS. But an interesting controversy about the role of QMS among social science researchers has raised the relevance of other criteria to characterize QMS. Let us summarize the points of view. The classical view is to consider quality standards purely as "natural market lubricants", in the words of Hanataka, Bain and Busch (2006:39). In that view the main roles of QMS are thus to increase trust, diminish transaction costs, develop transparency within the chains, and in that sense emphasizing the standardization side of QMSs.

At the same time, other researchers have challenged this conception: the set up of quality standards is a way to differentiate products, to reduce market access, or to create entry barriers, thus emphasizing the strategic and differentiation aspects of quality standards and QMS. The role of retail companies in this trend have been particularly stressed [START_REF] Reardon | The retail-led transformation of agrifood systems and its implications for development policies[END_REF].

For Hanataka, Bain and Busch (2006) this dichotomy is fallacious. Indeed it is necessary to consider both conceptions as equally important. For Busch et al. (2006:41-42), standardization and differentiation are not opposing tendencies, but dual outcomes of the globalization of food and agriculture. In this way, "standardization and differentiation are actually aspects of the same phenomenon, each proceeding inside of the other". They continue: "the strategic use of standards and third party certification (hereafter TPC) by different actors is producing both standardization and differentiation simultaneously in the global agrifood system". For instance the authors show that when developing HACCP standards, the retailers also use quality or size to differentiate their products, turning the standards into a competitive advantage. But, as indicated by the authors, the reverse is also true: when non-governmental organizations promote fair-trade products, in order to differentiate their products, they at the same time develop different methods of standardization to improve their efficiency and transparency.

Following these authors, we will adopt a conception where quality standards and TPC are not strategically neutral but instead socially mediated. The situation of TPC will be added in our analytical grid to grasp this strategic role. [START_REF] Nadvi | Making sense of global standards, in Local Enterprises in the Global Economy[END_REF] have shown the wide diversity of QMSs, and it is important to have an extended conception of them in order to "make sense" of their uses and objectives. These authors have identified several criteria: scope (process, product standards), geographical reach (regional, national, international), function (social, labour, environmental, policy, safety, ethical), key drivers (public, private i.e. business or not-for-profit association), forms (management standards, company codes, labels), coverage (generic, sector specific, firm/value chain specific), regulatory implications (legally mandatory, necessary for competition, voluntary).

In European pork chains, the diversity of QMSs is also patent, although the situation can be synthesized around a few key features (Ellebrecht et al., 2009). First of all, the General Food Law has set up a new framework of legal requirements known as the EU hygiene package. But, at the same time, several nation-based systems have been developed, generally oriented towards the improvement of transparency and traceability. In 2002 an initiative has been launched by the EMA (European Meat Association, a partnership between retailers and quality assurance systems) to reduce the number of schemes (Ellebrecht et al., 2009:77). Following this literature review and considering the concrete situation of the European pork meat chain, our proposal is to extend the dimensionalization proposed by [START_REF] Wever | Alignment between chain quality management and chain governance in EU pork supply chains: A Transaction-Cost-Economics perspective[END_REF] based on ownership, scope (in a sense of vertical penetration) and scale, to the status and role of third party certification and resources. The role of resources in QMSs has been acknowledged mainly in the business marketing literature (Håkansson et al., 2004;[START_REF] Coulibaly | Value creation in brand alliances: a dynamic conceptualization[END_REF] and in some case studies in the pork meat chains [START_REF] Rakotonandraina | Cochon de Bretagne': a Brand and Producers' Organization[END_REF]). The idiosyncratic nature, characteristics of resources mobilized in QMS as well as their embeddedness in networks of actors and institutions are also to be considered in the question of alignment.

2-3.Extending the alignment principle in real agrifood contexts

The basic proposal of the alignment principle in TCE (transaction-cost economics) is found in [START_REF] Williamson | Comparative Economic Organization: The Analysis of Discrete Structural Alternatives[END_REF], a principle according to which "transactions, which differ in their attributes, are aligned with governance structures, which differ in their costs and competencies". Central to problems of economic organization, the notion has already been considerably extended by agricultural economists and strategic management researchers (see for instance [START_REF] Gonzalez-Diaz | Quality Assurance Mechanisms in Agrofood: the Case of the Spanish Fresh Meat Sector Case[END_REF][START_REF] Lazzarini | Integrating Supply Chain and Network Analyses: the Study of Netchains[END_REF][START_REF] Klein | Organizational Issues in the Agrifood Sector: Toward a Comparative Approach[END_REF][START_REF] Hendrikse | Governance of chains and networks: a research agenda[END_REF][START_REF] Neves | Marketing and network contracts (Agreements)[END_REF][START_REF] Zylbersztajn | 'Alimentation Animale (Nationale syndicate of animal feed industries) SYNCOPAC: Syndicat National des Coopératives de Production et d'Alimentation Animal (National syndicate of production and animal feed cooperatives) FNP: Fédération Nationale Porcine (French Pork National Federation) FNCBV: Fédération Nationale de la Coopération Bétail et Viande (National federation of cattle and meat) FNEAP: Fédération Nationale des Exploitants d'Abattoirs Prestataires de Services (National federation of slaughterhouses) FNICGV: Fédération Nationale des Industriels et Commerçants en Gros des Viandes (national federation of meat industries and wholsalers) SNCP: Syndicat National du Commerce du Porc (National syndicate of pork marketing) FICT: Fédération Française[END_REF]. One of the most interesting extensions of this original TCE framework has been proposed by Nickerson et al. (2001). These authors suggest that "market position, resources, and governance are interdependent, which means each must be chosen with respect to the other" (Nickerson, Hamilton and Wada, 2001:252). In bringing together elements that are usually separated, these authors extend the logic of alignment closer to real situations, where complex decisions have to be made regarding investments in specific resources and strategic orientations. For them, a resource profile is a "set and type (i. e. the degree of idiosyncrasy) of resources and capabilities employed in the constellation of activities in a vertical chain" (Nickerson et al., 2001:252). No doubt that when it comes to QMS, this question of resources is crucial [START_REF] Ghosh | Governance value analysis and marketing strategy[END_REF][START_REF] Leiblein | An empirical examination of transaction-and firm-level influences on the vertical boundaries of the firm[END_REF][START_REF] Coulibaly | Value creation in brand alliances: a dynamic conceptualization[END_REF].

But the main difficulties of this approach are the questions of dimensionalization and the question of the unit of analysis for the analysis of alignment. The question of dimensionalization has been developed previously, the second one remains. Is it the transactions, the set of interdependent transactions or the whole system constituted by the actors and their relevant resources that is to be considered? Two concrete situations in agrifood chains have been widely acknowledged by researchers: the rise of QMS and the scale and scope of chains where several stages are interdependent.

As showed in previous work (see for instance [START_REF] Raynaud | Aligning Branding Strategies with the Governance of Vertical Transactions[END_REF], a complete story necessitates taking into account the whole set of interdependent transactions, which will be our unit of analysis. Indeed, actors in chains and networks behave under a global constraint of remediableness. This notion of remediableness helps to understand not how but why actors modify governance design and QMS. For Williamson, the remediableness criterion "holds that an extant practice or mode of organization for which (1) no feasible superior alternative can be described and (2) implemented with expected net gains is (3) presumed to be efficient." (Williamson, 2009:153). As explained by Williamson, all of these three propositions are necessary to explain the choice of organizational forms. For him, the "remediableness criterion thus both disallows pronouncements of inefficiency that rest on a comparison of an actual (hence flawed) practice with a hypothetical (ideal) alternative and asks the public policy analyst to be more respectful of the political process." (Williamson, 2009:153). Thus we will consider this criterion as the explaining variable of alignment, the decisions of actors affecting simultaneously several transactional levels throughout the supply chain.

3-Proposal for an analytical framework of alignment between GS and QMS In order to address the question of alignment in agrifood chains and networks, we propose an analytical framework composed of two basic constituting blocks: the dimensionalization of QMS and governance designs (3-1) and the targeted position of the pairing QMS/governance designs in terms of standardization and/or differentiation (3-2).

3-1 The dimensionalization Based upon our previous comments and findings on dimensionalization, we propose in the table 2 the key factors for both governance design and quality management system. Three factors have been defined for the description of the governance designs: the number of stages, the institutional structure, and the network governance form. The quality management system is described through four factors: scale and scope, ownership, resources, organization of control (table 2). [START_REF] Raynaud | Alignment between Quality Enforcement Devices and Governance Structures in the Agro-food Vertical Chains[END_REF][START_REF] Raynaud | Aligning Branding Strategies with the Governance of Vertical Transactions[END_REF]; [START_REF] Wever | Alignment between chain quality management and chain governance in EU pork supply chains: A Transaction-Cost-Economics perspective[END_REF].

3-2 The conceptualization of alignment

Adapted from Nickerson et al. (2001) we propose the following chart (figure 1) to conceptualize the concept of alignment. In the Williamsonian tradition, attributes of transactions are in a sense the 'medium' of alignment: this is because these attributes are simultaneously linked to governance structures and QMSs that these components are on the same wavelength. In Nickerson's conception, the concept is extended. For Nickerson et al., "each target position and corresponding resource profile/organization pairing represents a strategy. (…) From a normative perspective, firms prefer the strategy with the greatest profitability. Heterogeneity in firm strategies reflects that firm occupy different feasible resource profile/organization pairings" (Nickerson et al., 2001:254). In the context of governance design/QMS alignment and in line of reasoning with Hanataka et al. (2006), this targeted position can be oriented towards different modes of standardization or differentiation or a combination of the two. The combination is seen here as situations where firms have made efforts to standardize element of differentiation of products or processes, or on the contrary to differentiate (mainly) products that are already standardized on the market (figure 1). The abbreviation VPF of the French pork meat is an 'interprofession' (professional body), a collective brand using a private charter (www.civ-viande.org

Governance structure

The forms of coordination in the French pork sector are diversified and can go to integration. The coordination relations between actors evolve towards investment in slaughterhouses by the producer groups or in other links of the chain. Besides, partnership contract, and fusion strategy between producer groups (e.g. Cooperl and Arca) may appear to assure the perpetuity of companies. Downstream pork chain companies can have also financial participation in those of upstream to answer quality criteria corresponding to the consumers' expectations (Rieu, 1998). It is done by means of the research and development in genetics (improvement of races) and animal feed, and/or by pig production, via the producer groups. During the last 2 decades, one of the major facts, in the animal sector, is the increase in importance of large retailers. Large retailers sell about 75% of fresh pig products to French households (MPB, 2007). Besides, the main part is shared by five main retailers which monopolize an important part of the margins. INAPORC plays a central role in the economic organization and development by promoting the pork and the export of the French pork chain. A financial contract for VPF programme has been concluded between the retailer and slaughterhouse. Indeed, retailers paid 0.16 euro higher per kilo to the slaughterhouses subdivided as: 0.10 euro as added value for the producers and 0.1euro for the promotion of the programme (LSA, 2001).

Quality management system VPF specifications, as created in 2000, are mainly for producers and respected at a level of 90% of the production ground according to VPF data in 2004. The objectives are to guarantee the French origin of pork products and to assure good practices relatively to the sanitary conditions and pig production in order to offer to consumers a level of quality and optimal safety of pork meat (CNA, 2002). VPF specifications are a voluntary commitment of producers or producer groups (ITP, 2004). The membership requires signature of commitment and an initial audit. At the production level, pig producers must respect:  origin of pig (Pig born and raised in France),  traceability,  animal feed plan without animal flour and antibiotic components, besides animal feed formula must be preserved during three years,  sanitary and veterinarian treatments regulations for breeding, farrowing, finishing and preparation of pig for slaughter. Pig producers must stock up with referenced suppliers. Beyond production, the VPF programme obliges the respect of traceability system which allows finding the origin of raw materials. Thus, traceability of pig meat must be respected between the different stages of the chain (slaughtering-cutting, processing, packaging) with a numeral identification. VPF is subjected to controls exercised by authorities and also by third certification body, according to the request of the professionals' members (ITP, 2004). Objective of monitoring is to follow the product as regard to the VPF control plan and to verify good respect of traceability system imposed by the programme. A monitoring report is provided at the end which may contain various points audited with non conformity. Thus, an internal audit one time per year is realised by a technician from the producer group. This control is completed by an external audit of 5 to 10% of producers, insured by a thirdparty certification body accredited by COFRAC or French Accreditation Committee, an association created in 1994 and governed by the 1901 law (www.cofrac.fr). A control is organised in the retail stores about two or three times per year (Lirot, 2009). 1980's and 1990's (MAF, 1999)). IKB has been implemented by 95% of all parties in the pork chain in the Netherlands (Wognum and al., 2007). In 2005, about 98% of all pig farmers in the Netherlands were 'IKB Varken (pig)' certified (EMA, 2008 c). The objective of the programme is to support and guarantee pork quality and traceability (IKB Varken, 2008a). IKB is a private quality management system introduced by PVV (Productschap Vee en Vlees), which serves as the product board for livestock and meat in the Netherlands (EMA, 2008c). The slaughterhouse plays a central role in creating IKB production chain for primary production (European pork chains, 2009& Beckmann, 1998).

There are two competing IKB systems are used in the Dutch pork sector (Brouwer et al., 2004):  IKB Varken, set up by PVE (Brouwer et al., 2004).  IKB 2004, which is owned by the Dutch farmers Association 5NVV) (Brouwer et al., 2004;IKB, 2004IKB, , 2008a)). Thus, according to the type of IKB system we have two private parties signal owners: PVE who transferred responsibility to a certifying agency (CBD/CBS) in the first part, and in the second part the NVV or Dutch Farmers Association. PVE, the Product Boards for Livestock, Meat and Eggs is a vertically organized interest group, including a board with representatives from horizontally organized groups like farmers associations and meat producer organizations (Wever, 2010). PVE prepares policy and executes it on behalf the two boards of PVV (Product board for livestock and meat) and the PPE (Commodity Poultry and eggs), which are autonomous organizations with their own administrative responsibility. The role of PVE/PVV is now to support activities if initially it made drafts of IKB standards, and initiating changes to IKB (Wever, 2010). For IKB Netherlands pigs, the management structure includes a college of expert with a president, a secretary and a board composed with various players in the upstream link (pork producer, pig farmer, slaughterhouse, and veterinarian, IKBNV, 2007).

Governance structure According to Wever and Wognum (2008), different governance structures can be found at different stages in the supply chain (contracts exist in breeding stage of chain, market transactions can be found in farmer-slaughterhouse relations). Overall, contract relations are relatively rare in the chain: even though most relations are long-term, the relations are often not formalized into written contracts. IKB has various characteristics' which affect the GS. It allows for more market types of GSs by setting chain-wide quality standards (facilitates in achieving a coordinated response by the various chain actors in meeting consumer demands, communicating quality requirements), standardizing quality across the industry by means of its large scale adoption rate, and increasing quality standards without requiring large investments in quality management resources (Wever, 2010). Monitoring mechanism A compliance monitoring (audits and sanctions) with the IKB programme is done by third certification body control (Wognum and al., 2007). This inspection is annual. Chain actors wanting to participate in IKB can select between three certifying institutions. The third party certifying institutions are monitored by the Dutch accreditation council. In addition, two public inspection agencies AID (General inspection service) and VWA (Food Consumer product Safety Authority) monitored compliance with legislative requirements (Wever, 2010). Besides, chain-actors might inspect their suppliers for monitoring compliance with their own standard (Wever, 2010). According to [START_REF] Wever | Alignment between chain quality management and chain governance in EU pork supply chains: A Transaction-Cost-Economics perspective[END_REF], the comparison synthesis of the two signals IKB-VPF focuses mainly on three aspects: type of signal owners, aspect of segmentation strategy used and the characteristics of QMSs and GSs. Firstly, similarities are found between the signal owners because both programme involved collective chain actors for administration (college, committee and board) and decisions concerning the signal, and investment. However, some difference appears about the importance role of Certification Institution in the organization of IKB schemes. Secondly, concerning the differentiation strategy, as similar, food safety, traceability, animal welfare constitutes differentiation aspect for VPF and IKB. Differences for VPF are the environmental (impact) management included in the topic list of specification. In addition, the French origin of end product (pig born, raised, slaughtered and processing in France) is a key element for VPF differentiation. Considering the characteristics of QMSs, QMSs of VPF as IKB are quite similar. VPF and IKB are two private quality management systems based on public regulation and legislation regarding traceability and identification from production to retailer and completed by some private specifications. Both monitoring of compliance is done by third-party certification. High membership is also observed in the both programme. IKB as VPF cover each over the whole supply chain. As regards to the governance design, both VPF and IKB systems participation are voluntary for companies. For VPF there is no specific formal contract and trust: only market relations and verbal agreements between the chain actors. Nevertheless, there is a written agreement for membership toward the VPF association mainly for producers. About the price coordination mechanism, there is a price agreement between the retailer and the slaughterhouse for VPF: a premium price is paid by the retailer to slaughterhouse subdivided as in one part, added value for producer and association contribution in other part. For IKB, the premium price is also paid by slaughterhouse to pig producers. For IKB slaughterhouse plays an important role in the IKB production chain but no formal contracts exist in most parts of the chain. In addition, the slaughterhouse is responsible for auditing all aspects of the chain supply (breeder, fattener, and the feed supplier, MAF, 1999). Veterinarians are heavily involved in the programme and are required to give advice to participating farmers on animal health issues as well as providing some audits of breeders and fatteners (MAF, 1999).

Figure 1 :

 1 Figure 1: Alignment between governance structure and quality management system: a conceptualization

Figure

  Figure 3: Monitoring organization

Figure 4 :

 4 Figure 4: Composition of IKB´s administrator (PVE)

Figure 5 :

 5 Figure 5: Governance structures

Figure 6 :

 6 Figure 6: Public and private monitoring in the Dutch fresh pork meat chain

Table 1

 1

		Network governance forms: a synthesis
	Governance forms	Number of participants	Goal consensus
	Shared governance	Few	High
	Lead organization	Moderate number	Moderately low
	Network	administrative	Moderate to many	Moderately high
	organization (NAO)
			Source: adapted from Provan and Kenis (2007:237)
	2-2.The diversity of quality systems: how to dimensionalize them?
	Following Wever et al. (2009) we will consider that QMSs consist in three basic components,
	i. e. quality signals, quality standards, and quality monitoring mechanisms. Quality signals are
	usually seen as a mean for companies to communicate towards customers and/or consumers,
	in order to differentiate the product. The implementation of quality signals necessitates in turn
	more or less specific investments as well as a monitoring system that could be a firm or a
	collective actor within or outside the chain.

Table 2 :

 2 Dimensionalization of governance design and quality management systems

	Governance design	Quality management system
	 Number of stages	 Scale and scope
	• 1 to 4	• scale: + to +++
		• scope: + to +++
	 Institutional structure
	• Spot market	 Ownership
	• Verbal agreement	• Private
	• Formal contract	• Public
	• Equity-based contract
	• Vertical integration	 Resources
		• Nature and characteristics
	 Network governance form	• Idiosyncrasy
	• Shared governance
	• Lead organization	 Organization of control
	• NAO (network administrative organization)	• Nature of third party certifiers
		• Degree of specificity
	Source: adapted from Provan et al. (2007);

  This body is in charge of the technological part and marketing of the programme. The VPF programme is particularly managed by the VPF association which is one of the various committees ofINAPORC (Lirot, 2009). Indeed, INAPORC includes six working committees for collective interest respectively(Interbev & INAPORC, 2006): Communication committee: for promotion of pork products;  Export committee: in charge of promotion of pork products in foreign countries;  Risk management committee: which assure some control to prevent possible risks like antibiotic, bird flu, etc.;  Committee for identification, traceability and quality: in charge of traceability and quality programme (VPF, LR, CCP, IGP, Organic farming, etc.);  Price committee which is an observatory of the prices indicators of market;  Research and development committee. Members paid contribution. For the retailer (Retailer Auchan as example), this financial participation is comprised into the pork cost price directly(personal communication, Lirot, 2009).

	Figure 2: VPF organization
		INAPORC
	Governance Design	Attributes of transaction: -frequency 5 COLLEGES
		-uncertainty
	FEEDING	2 PROFESSIONNAL -idiosyncrasy FEDERATIONS :	GENERAL ASSEMBLY
		SNIA, SYNCOPAC
	PIG PRODUCTION 20000 pig producers producing 25 000 000 pigs/year And 70 producer groups (cooperatives) holding 96% of French pig production	Targeted position of Governance Design/QMS alignment: BOARD 2 PROFESSIONNAL FEDERATIONS: FNCBV, FNP OFFICE
	Quality Management SLAUGHTERING-CUTTING 218 Slaughterhouses with 2 100 000 tons equivalent carcasses of pig meat System	-standardization 4 PROFESSIONNAL -differentiation FEDERATIONS : -combination of the two FNCBV, FNEAP, FNICGV, 6 COMMITTEES SNCP Identification & traceability
			Communication
	PROCESSING 25% fresh pig meat and 75% charcuteries	1 PROFESSIONNAL FEDERATION: FICT	Export Research &development Risk management
		4 PROFESSIONNAL	Price observatory
	RETAILING-CATERING	FEDERATIONS:
		CFBCT, CNCT, FCD/GMS,
		CCC
	). VPF guarantees information about the origin: pig born, raised, slaughtered and processed in France (Ministry of CONSUMPTION 2 189 000 tons equivalent carcasses of pig meat
	agriculture and fishery, 2001). A VPF chain relates to the identification and traceability for
	pork meat in France. More than 90 % of the operators of the French pork sector are referenced
	in the VPF approach (www.leporc.com). VPF concerns particularly fresh pig meat. Its
	distribution network concerns craft butcher shops, hypermarkets and supermarkets. Regarding
	consumers, VPF is considered like a quality signal and a guarantee (Lirot, 2009).
	The label VPF is a private programme initiated by the professionals of the French pork chain

or INAPORC-French pork national council. Indeed, INAPORC groups together professional organizations of the French pork sector: animal feed manufacturers, pork production, slaughtering-cutting, processing and distribution

(Interbev & INAPORC, 2006)

.

Table 3 :

 3 QMS, a comparison of VPF and IKB

	VPF	IKB

5-Concluding comments

The objective of the communication is to explore the question of alignment, applied to the pork meat chain and its network context. Stemming from a conception of alignment in the TCE paradigm, extended by strategic management scholars, we have proposed an original analytical grid where the notion of dimensionalization appears to be crucial. Indeed for heuristic motives -i. e. disentangle the complexity of pork meat supply chains -we have suggested reducing the number of components, and we have identified the singularities and differences between the two systems around a limited number of criteria. The VPF and IKB systems have many similarities in their scale and scope of adoption, ownership and organization of control. The resources seem to be quite similar. The governance designs have shown some differences, mainly in the respective roles of professional bodies and leading firms.

The positioning of the pairing governance design/QMS has a main difference. VPF is at the same time company and consumer-oriented, thus aiming at some forms of differentiation. IKB clearly positioned itself as a company-to-company system without communication towards the final consumers. Does this difference between the two systems clearly affect the way the alignment is done? Further investigation on that point will be necessary. This question of alignment is important both for policy makers and managers. A misalignment may have negative consequences either at the firm level (sub optimality of resources) and at the chain level (discrepancies between actors, conflicts of objectives etc.).

The sub optimality could also be found for marketing and standardization reasons: the differentiation could be either irrelevant or too costly, and the standardization could lack of efficiency in its procedures and implementation. So it is necessary to acknowledge the fact that the alignment principle has consequences for economic reasons as well as organizational and strategic reasons. In spite of limitations, we think that our research on alignment shows the interest of an often neglected yet important topic.


M Nedjar

Y Mecheri

A Lamure

M Aufray

C Drouet

Effect of moisture on breakdown voltage of polyesterimide used in electrical machines

This paper deals with the influence of humidity on breakdown voltage of polyesterimide. The specimens were submitted to an hydrolytic aging, under 90% humidity and 30°C temperature, until 3000 h. The breakdown tests were executed under AC and DC voltage. A statistical analysis of the breakdown voltage data was carried out using Weibull model. The 90% confidence bounds were calculated and plotted. The study shows that breakdown voltage varies versus the aging time. Its increase is allotted to the crosslinking of the material structure. Whereas its decrease is ascribed to the plastificization of the polymer. The breakdown voltage under DC ramp is higher than that obtained under AC ramp. The DC breakdown voltage depends on the polarity.

I. INTRODUCTION

The utilization of polymers in electrical equipments should not missed nowadays thanks to their electrical and mechanical properties as well their availability. However, due to the different stresses to which they are subjected in service, these insulating materials can be degraded and lose their good properties.

A wide variety of solid dielectrics as polyesterimide, polyamide imide and polyimide are usually employed in electrical machines. Their choice is dictated by many factors: working environment, applied voltage, rotating speed, and operating temperature.

Polyesterimide is one of the most polymers used in electrical devices as motors, generators,…etc. It has a greater resistance to abrasion and an excellent resistance to high temperature and good electrical and chemical properties [START_REF] Ueda | Progress of magnet wire technology in Japan[END_REF].

It is generally agreed that degradation of polymers will be speeded up by the presence of moisture. Kamal and Saxon [START_REF] Kamal | Recent developments in the analysis and prediction of the weather ability of plastics[END_REF] point out that water can have three kinds of effects. One is chemical: hydrolysis of the ester or amide bonds. The second is physical: loss of the bond between the vehicle and a substrate or pigment. The third is photochemical: generation of hydroxyl radicals or other chemical species.

Another influence could be the facilitation of ionization and the mobility of ionic entities, an important aspect of corrosion chemistry [START_REF] Kamal | Recent developments in the analysis and prediction of the weather ability of plastics[END_REF].

It was reported that water content in XLPE and TR-XLPE cable increases as a function of aging time and it was shown a decrease in AC dielectric strength versus the water quantity [START_REF] Nikolajevic | Accelerated aging of cross-linked polyethylene (XLPE and TRXLPE) cable insulations in wet conditions[END_REF]. C. Chauvet et al pointed out that the presence of water in XLPE cable slices leads to a development of water tree; the water amount varies from 50 ppm before aging to 400 ppm after aging [START_REF] Chauvet | Influence of wet ageing on the electrical breakdown strength of XLPE cable slices[END_REF]. Many studies were done on the action of water on polymers: concentrations of oxidation or macroscopic characteristics like lifetime for mechanical failure [START_REF] Henry | The kinetics of polyolefin oxidation in aqueous media[END_REF], tensile strain [START_REF] Dörner | Influence of various stabilizer systems on the ageing behavior of PE-MD-I. Hot-water ageing of compression molded plaques[END_REF] or cristallinity percentage [START_REF] Amato | Degradation studies of crosslinked polyethylene[END_REF].

In the polymers, hydrolysis can be summarized as follows [START_REF] David | Aging of polymers[END_REF]: functional groups such as esters, amides, and acetals can be attacked by water to form the parent acid or aldehyde group and the corresponding alcohols or amines. When the hydrolysis-sensitive bonds are part of the polymer backbone chain links, hydrolysis will result in depolymerization and release of the hydrolyzed monomers from the polymer. When the functional groups are attached to the side of the backbone chain, the polymer remains intact but small molecules are released. Penetration of water into many polymers is surprisingly great, but in many cases hydrolysis (like oxidation) may be restricted to surface regions.

This work reports the effect of hydrolytic aging on breakdown voltage of polyesterimide.

II. EXPERIMENTAL METHOD

A. Preparation of samples

The specimens were elaborated according to IEC Standard Publication 172 [9]. The samples are constituted by twists of copper wire covered with a 35 ± 1 μm polyesterimide layer of class H (180°C). The diameter of the wire and the length of the twist are 0.75 mm and 125 mm, respectively. In order to avoid any presence of microscopic cracks which can be the seat of partial discharges, the samples are checked under a microscope before the tests.

B. Hydrolytic aging

The samples were put in an enclosure and exposed to 90% relative humidity (RH) and 30°C temperature. The aging was executed until 3000 h. After a given aging time, a population of 90 specimens was taken and submitted to electrical breakdown.

C. Dielectric breakdown test

The dielectric breakdown test was performed, in air and at ambient temperature, with high voltage generator under alternating voltage, 50 Hz (AC) and direct voltage (DC) of positive and negative polarity. The voltage is uniformly increased with a rate of 4 kV/s.

III. RESULTS AND DISCUSSION

A. Statistical analysis of breakdown voltage

The cumulative probability P(v), generally used to treat values of breakdown voltage, is the two-parameter Weibull distribution. It is given by the expression [START_REF] Stone | Parameter estimation for the Weibull distribution[END_REF]:

⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ - - = β α v v P exp 1 ) ( (1)
with: α: scale parameter corresponding to the breakdown voltage for a probability of 63.2%. β: shape parameter representing the slope of the straight line of Weibull plot.

The experimental data plot must be a straight line whose slope is β in the coordinate system:

V X log = (2) and ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ - = P Y 1 1 ln log (3)
The cumulative breakdown probability P i is calculated for each breakdown voltage v i using the relationship [START_REF]IEEE Guide for the statistical analysis of electrical insulation voltage endurance data[END_REF]:

% 100 1 + = N i P i (4)
with: N: total number of samples tested. i: value rank of the breakdown voltage v i .

On the graph, the best linear fit of Weibull plot was determined by an estimate based on the maximum likelihood method. The scale parameter (α) and the shape parameter (β) were deduced. The 90% confidence bounds were calculated by the method described by J.F. Lawless for a Weibull distribution [START_REF] Lawless | Confidence interval estimation for the Weibull and extreme-value distribution[END_REF]. A computation was used to carry out the various calculations: α, β and confidence bounds. Only the Weibull plots before aging and after 3000 h were presented.

B. Variation of the breakdown voltage versus the aging time

The evolution of breakdown voltage in function of aging time (t) is presented in Figure 7. The results can be summarized as follows:

-At the beginning of aging, breakdown voltage under AC ramp (V ac ) decreases from 14.7 kV to 13.1 kV, then increases and reaches a maximum of 14.8 kV after 510 h. Beyond this time, V ac decreases slowly until 14.7 kV and grows up to 15.9 kV after 910 h. After this peak, Vac lowers abruptly till 12.3 kV and raises up to 13.9 kV for t = 1620 h. At the end of aging, V ac decreases to 13.2 kV after an aging time of 3000 h. -At the first, the breakdown voltage under DC ramp of positive polarity (V dc+ ) decreases from 20.1 kV to 19.4 kV, then raises and reaches a peak of 20.0 kV for t = 500 h. After this time, V dc+ diminishes slowly until 18.8 kV corresponding to t = 910 h. Beyond this time, it raises up to a maximum of 19.8 kV, then lowers to 18. Breakdown voltage (kV)

Aging time (h)

C. Discussion

1. The results show that the breakdown voltage of polyesterimide, under AC and DC electric field, varies as a function of aging time. Its increase is attributed to the crosslinking and the rearrangement of the material structure which lower mean free path of charge carriers and leads to a decrease of their mobility. Whereas its decrease is ascribed to the plasticization of the material expressing a weakening of the molecular bonds and an increase in the free volume.

The breakdown voltage under AC electric field decays

between 1000 h and 1250 h. Beyond this time, it increases and reaches a maximum corresponding to 1500 h. Its reduction is due to the increase of moisture amount within the material which facilitates the initiation of water trees. This latter leads to shorten breakdown voltage. While, between 1250 h and 1500 h, breakdown voltage grows resulting to a decrease of humidity ratio. 3. The breakdown voltage under AC ramp is 1.37 orders of magnitude smaller than that under DC ramp. The phenomenon is attributed partly to the different dissipated energy under the two types of field and partly due to the fatigue induced by the alternating stress [START_REF] Crine | AC and DC electrical aging of polyethylene[END_REF]. 4. It is clear that the breakdown voltage under DC ramp depends on the polarity. The maximum variation is of 4.6%. This phenomenon can be explained by the fact that the amplitude and the repartition of space charge between the electrodes vary with the polarity.

IV. CONCLUSION

The study shows that the breakdown voltage of polyesterimide vary as a function of hydrolytic aging time. Its increase is allotted to the crosslinking and the rearrangement of the material structure. Its decrease is ascribed to the plasticization of the material expressing a weakening of the molecular bonds and an increase in the free volume. The breakdown voltage under AC ramp is 1.37 orders of magnitude smaller than that under DC ramp. This phenomenon is due partly to de different dissipated energy under AC and DC voltage and partly due to the fatigue induced by the alternating stress. Under DC ramp, the breakdown voltage depends on the polarity. The variation is equal to 4.6%. This phenomenon is attributed to the fact that the amplitude and the repartition of space charge between the electrodes depend on the polarity of the applied voltage.

Figure 7 :

 7 Figure 7: Variation of the breakdown voltage versus the aging time.

  Figures1-6show the Weibull graphs with 90% confidence intervals corresponding to AC and DC applied voltage (positive and negative polarity). The width of the tolerance intervals is more important for the lower probabilities. The values of the shape parameter vary from a distribution to another one.

		Breakdown voltage (kV)
	0.8 -2 -1 0 6.3 1 Before aging DC voltage (-) Figure 5: Weibull plot of the breakdown voltage with 90% confidence 1.0 1.2 1.4 25.1 15.9 10.0 99.99 63.2 9.54 0.99 Weibull cumulative probability (%) logLn(1/(1-P)) logV intervals under DC ramp of negative polarity before aging. 19.193<v=19.798< 20.413 6.220<α=7.599< 8.857 Y= 7.599x -9.853.	logLn(1/(1-P))	-2 -1 0 1	4.0 t = 3000 h 2.5 AC voltage	10.0 Breakdown voltage (kV) 6.3	15.9	25.1	99.99 63.2 9.54 0.99	Weibull cumulative probability (%)
								0.4	0.6	0.8	1.0	1.2	1.4
											logV
		Breakdown voltage (kV)
	0.8 -2 -1 0 6.3 1 t = 3000 h DC voltage (-) Figure 6: Weibull plot of the breakdown voltage with 90% confidence 1.0 1.2 1.4 25.1 15.9 10.0 99.99 63.2 9.54 0.99 Weibull cumulative probability (%) logLn(1/(1-P) logV intervals under DC ramp of negative polarity for an aging time of 3000 h. 18.131<v=18.843< 19.572 4.890<α=5.946<6.914	Figure 2: Weibull plot of the breakdown voltage with 90% confidence intervals under AC ramp for an aging time of 3000 h. 12.570<v=13.214< 13.883 3.618<α=4.360< 5.044 Y = 4.360 x -4.888. 0.8 1.0 1.2 1.4 1.6 -1 0 1 Before aging DC voltage (+) 39.8 25.1 15.9 10.0 6.3 99.99 63.2 9.54 0.99 Breakdown voltage (kV) logLn(1/(1-P)) Weibull cumulative probability (%) -2
			Y= 5.946x -7.5954.							logV
							Figure 3: Weibull plot of the breakdown voltage with 90% confidence
								intervals under DC ramp of positive polarity before aging.
									19.346<v=20.091< 20.884	5.073<α=6.269< 7.357
											Y= 6.269x -8.169.
		Breakdown voltage (kV)							Breakdown voltage (kV)
	logLn(1/(1-P))	0 . 6 -2 -1 0 1 4.0 Before aging 0 . 8 6.3 AC voltage	1 . 0 10	1 . 2 15.9	1 . 4 25.1 99.99 63.2 9.54 0.99	Weibull cumulative probability (%)	logLn(1/(1-P))	0.8 -2 -1 0 6.3 1 t = 3000 h DC voltage (+)	1.0 10.0	1.2 15.9	1.4 25.1 99.99 63.2 9.54 0.99	Weibull cumulative probability (%)
			log V								logV
	Figure 1: Weibull plot of the breakdown voltage with 90% confidence intervals under AC ramp before aging. 14.140<v =14.817< 15.517 3.953<α= 4.789< 5.556. Y = 4.789 x -5.606.	Figure 4: Weibull plot of the breakdown voltage with 90% confidence intervals under DC ramp of positive polarity for an aging time of 3000 h. 18.102<v=18.816< 19.549 4.776<α=5.786< 6.712 Y = 5.786 x -7.374.

  2 kV for t = 1170 h. At the end, V dc+ increases slowly until 19.0 kV and decreases up to 18.8 kV after an aging time of 3000 h. -During the first hundred hours of aging, the breakdown voltage under DC ramp of negative polarity (V dc-) increases from 19.8 kV to 20.1 kV, then decreases slowly until 18.7 kV corresponding to t = 780 h. Beyond, it grows and reaches a maximum of 20.4 kV for t = 1410 h. After this peak, V dc-lowers until 17.6 kV and grows slowly up to 18.8 kV after 3000 h.

	21
	18
		AC Voltage
		DC Voltage (+)
	15	DC Voltage (-)
	12
	0	500 1000 1500 2000 2500 3000


Naïm Naouar

Emmanuelle Vidal-Sallé

Philippe Boisse

Naim Naouar
email: naim.naouar@insa-lyon.fr

Emmanuelle Vidal-Salle

Meso F.E. Simulation of Composite Reinforcement Deformation based on X-ray Computed Tomography

Meso FE simulation of composite reinforcement deformation based on X-ray computed tomography

INTRODUCTION

The present paper aims to perform 3D F.E. analyses of the 3D reinforcement based on a model obtained from X-ray tomography in order to be as close as possible of the real geometry of the 3D reinforcement. The quality of a meso FE analysis strongly depends on the F.E. model, its geometry and the associated data, most important of which being the fibre directions. The geometry can be obtained with textile geometrical simulators such as TexGen or WiseTex [START_REF] Long | Composite reinforcements for optimum performance[END_REF][START_REF] Lomov | Composite reinforcements for optimum performance[END_REF]. Nevertheless, they provide an idealized geometry of the reinforcements which cannot take into account the geometrical imperfections or specificities of the material being analysed. Moreover, reinforcement architectures are diversified and complex for 3D fabrics and all the geometrical possibilities cannot be described by these simulators. In some cases, when the textile reinforcement is thick, and complex, the internal geometry is not completely known. Lastly, interpenetration can occur between the yarns defined by the simulators in some configurations. In the present study, the initial geometry of the meso-FE model of a 3D composite reinforcement is directly obtained from X-ray Micro Tomography (XRMT) also called micro computed tomography (µCT) [START_REF] Baruchel | X-Ray Tomography in Material Science[END_REF]. This fairly recent technique allows for detailed, accurate and non-destructive 3D observations inside a material especially for composites and composite reinforcements [START_REF] Naouar | [END_REF]5]. It distinguishes yarns and even fibres and defines the anisotropic directions of the material. In the present paper, a methodology is used to build automatically finite element models from X-ray micro CT images of 3D composite reinforcements. These models take into account the specificities of the analysed material's geometry. They can be obtained for any reinforcement's weaving style or architecture. A meso FE modelling of a 3D orthogonal non-crimp woven fabric deformation is taken as an example. A new segmentation method is used. It is based on the analysis of image texture. For 2D fabrics, the separation of the warp and weft yarns can be obtained by using the structural tensor of the fibre's direction [START_REF] Naouar | [END_REF]. This is not possible for 3D reinforcements because the fibres flow in threedimensional space. An approach based on texture analysis for images is used for the 3D composite reinforcement. A method is then submitted to build a prismatic mesh of the yarns. Meso F.E. analysis of the 3D RUC's deformation uses a hyperelastic law for finite strains of fibrous yarns [6]. After a compaction, the internal geometry of the computed and of the experimental 3D reinforcement are compared.

SEGMENTATION

A finite element model is built from tomography images. The pictures extracted from the CT need to undergo a segmentation process in order to differentiate the wrap, weft and binder yarns. This phase of the process is critical because it impacts the quality of the model significantly. Several segmentation methods are available, one of them, based on the images structural tensor, seems interesting [START_REF] Naouar | [END_REF]. This tensor defines the orientation of fibres in a yarn. This method is efficient if the picture display stringy yarns. It is generally the case for 2D reinforcements. In certain situations, the segmentation based on the orientation tensor cannot be used because the results from the scanning process leads to pictures without stringy yarns but with a grainy aspect. This occurs in 3D reinforcements for images that are perpendicular to or cut the yarns. The present work is based on a Texture Analysis for Image segmentation that is outlined below. This method differentiates 3D reinforcement yarns well.

Grey Level Co-Occurrence Matrix

Texture Analysis for Image Segmentation is based on the calculation of Grey Level Co-occurrence Matrix (GLC Matrix) [7] . This method highlights pixel pairs spatially separated by a translation.

The GLC Matrix calculates how often a pixel with grey-level (greyscale intensity) value i occurs horizontally adjacent to a pixel with the value j. Each element (i, j) in the GLC Matrix specifies the number of times that the pixel with value i occurs horizontally adjacent to a pixel with value j in the input image. Figure 1 shows on the left a matrix that represents an input image of size4x4andcontains five grey levels (0-4). Figure 1 shows on the right the calculated GLC Matrix using the position operator of one horizontally pixel. In the GLC Matrix, element [START_REF] Long | Composite reinforcements for optimum performance[END_REF][START_REF] Naouar | [END_REF] contains the value 3 because there is three instances in the input image where two horizontally adjacent pixels have the values 0 and 3, respectively. Similarly, element (4,5) contains the value 1 because there is only one instance where two horizontally adjacent pixels have the values 3 and 4. The Grey Level co-occurrence Matrix contains an important amount of data. However, it cannot be used directly. Hence, fourteen parameters have been defined by Haralick [7] so that textural features can be characterized using the GLC Matrix. The most widely adopted parameters are the contrast, correlation, energy and homogeneity. Once the GLC Matrix is calculated, it is normalized, so that the sum of its components (called p(i, j)) is equal to 1. Then, the four statistical parameters (contrast, correlation, energy and homogeneity) are extracted.

Image Processing

In order to distinctly segment warp and weft yarns, a threshold using the most pertinent parameter is performed on 3D orthogonal non-crimp woven fabric. This should result in a concentration of information inside the yarn sections and as few as possible disruptive pixels outside of them. Homogeneity is the best parameter for extracting yarn sections (Fig. 2d). Contrast (Fig. 2a) and energy (Fig. 2c) separate the yarns as well, however, there is less information inside the sections when compared with the homogeneity method.

Moreover, applying a smoothing filter on the parameter can facilitate the threshold. By using a 3D Gaussian filter or a 3D Median filter, it is possible to homogenize the image information: the yarn sections appear more complete. Furthermore, the yarns' cross sections are often not full enough. Additional mathematical morphology operations (closing, convex hull) are needed to attain acceptable yarn cross sections and to achieve segmented volume (Fig. 3).

MESH GENERATION

In meso-scale textile composite modelling, the yarn is considered as a 3D domain. Meshes of the yarn have been generated with hexahedral elements and tetrahedral elements. Nevertheless, neither of these element shapes is completely satisfactory. The hexahedral elements are numerically more efficient and they are well adapted to describe the yarn in the fibre direction [6,8,9]. However, it is difficult to mesh the transverse section of the yarn in a suitable manner. Most of the composite yarns have a lenticular shape and, therefore, both extremities are difficult to mesh. On the other hand, the tetrahedral elements can mesh any volume and they efficiently describe the transverse section of the yarn. However, a mesh based on tetrahedrons needs a large number of elements to be sufficiently accurate. In the present work, a prismatic element mesh generator is used [10]. The section is meshed with the triangle edge of the prisms and the quadrangle edges are in the fibre's direction.

CONSTITUTIVE LAW OF THE YARN

A hyperelastic constitutive law is used to describe the mechanical behaviour of fibre bundles of woven composite reinforcements at finite strain [6].

Hyperelastic Constitutive Equation

The potential energy can be written as a function of the right Cauchy-Green strain tensor C:

w = w C with C = F T . F (1)
The symmetry group of a transversely isotropic material is characterised, in the initial configuration, by a unit vector M which represents the preferred direction. This allows the definition of a structural tensor M = M ⊗ M. Then, the strain energy potential becomes:

w = w (I 1 , I 2 , I 3 , I 4 , I 5 ) ( 2
)
where I 1 , I 2 , I 3 are the invariants of C defined by:

I 1 = Tr C I 2 = 1 2 Tr C 2 -Tr C 2 I 3 = Det C (3)
and where I 4 and I 5 are mixed invariants defined from the structural tensor M

I 4 = C : M I 5 = C 2 : M (4)
The contribution of each deformation mode is considered as independent from the others; the second PiolaKirchhoff stress tensor is then defined as:

S = 2 ∂w ∂C = ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ ∂w ∂I 1 ∂I 1 ∂C + ∂w ∂I 2 ∂I 2 ∂C + ∂w ∂I 3 ∂I 3 ∂C + ∂w ∂I 4 ∂I 4 ∂C + ∂w ∂I 5 ∂I 5 ∂C ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ (5)

Physically Based Invariants

The behaviour law is based on the physics of the tow deformation [6]. Four deformation modes can be identified: elongation in the fibre direction, compaction in the transverse section of the yarn, distortion (shear) in the transverse section and shear along the fibre direction. The last mode (longitudinal shear) mainly controls the bending rigidity of the yarn. It is a compound of two equivalent deformation modes corresponding to the two directions of shear. The following set of invariants, corresponding to these four deformation modes, is defined in function of the classical invariants I 1 to I 5 defined above:

I elong = 1 2 ln (I 4 ) , I comp = 1 4 ln I 3 I 4 , I dist = 1 2 ln ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ I 1 I 4 -I 5 2 √ I 3 I 4 + I 1 I 4 -I 5 2 √ I 3 I 4 2 -1 ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ , I sh = I 5 I 2 4 -1 (6)
These invariants are used to define the strain energy functions associated to each deformation mode:

w C = w I elong , I comp , I dist , I sh (7)
Finally, the whole constitutive equation is the summation of all contributions discussed above:

S = 2 ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ ∂w elong ∂I elong ∂I elong ∂C + ∂w comp ∂I comp ∂I comp ∂C + ∂w dist ∂I dist ∂I dist ∂C + ∂w sh ∂I sh ∂I sh ∂C ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ = S elong + S comp + S dist + S sh (8)
The constitutive law depends on eight parameters: four corresponding to longitudinal elongation, two to yarn compaction, one to yarn distortion and one to longitudinal shear. This constitutive equation has been implemented as a user material subroutine VUMAT in the ABAQUS/Explicit finite element code.

MESO-SCALE F.E SIMULATIONS

Meso-scale F.E. simulations of transverse compaction are performed on the 3D orthogonal non-crimp woven fabric. The material parameters are displayed in [10]. The initial section of the yarn,S 0 , is determined by mesoscale observations of the fabric, using X-ray tomography for instance. The three other parameters are identified with a tensile test on a single yarn. Since the yarn may have been damaged during weaving, this test must be performed on a yarn extracted from the fabric rather than before weaving. The others parameters come from [6]. Figure 4 shows geometries obtained by tomography and simulation of the 3D reinforcement submitted to a transverse compaction. The reinforcement was crushed to 22 % of its original length between two plexiglass plates linked by four screws. The crushing of the binder yarn obtained in the simulation is consistent with tomography. The computed deformed geometry is in agreement with the geometry obtained from tomography. Figure 5 shows the comparison between the experimental and computed compaction loads (loads on the compression machine). These loads are coherent.

CONCLUSION

X-ray tomography is well adapted to the development of 3D textile composite meso-FE models directly from reinforcement specimens. The complexity of their geometry and of the yarn interlacing can be taken into account with µCT images. A segmentation method based on image texture has been explored to separate the warp, weft and binder yarns in the µCT images. This approach is better adapted to the study than a structure tensor approach commonly used for 2D fabrics because the yarns are oriented in all directions of space. The yarns are meshed by prism finite elements. This shape is well suited to fibrous yarn geometry. The triangle edges mesh the transverse section of the tows. Meso-F.E. analyses have been performed using a hyperelastic model developed previously for fibre bundles. This constitutive law is well adapted to these simulations. The comparisons between experimental trials and the computed internal deformed geometry have shown coherent results. This method must now be applied in other cases in order to check its robustness. Moreover, the meso-FE is used in the computation of 3D reinforcement permeability. The composite reinforcement is compacted and/or sheared before the resin injection phase of the R.T.M process. The internal geometry after compaction or shearing can be used to perform flow analyses and determine the permeability properties.

FIGURE 1 .

 1 FIGURE 1. Example of calculation of the GLC Matrix using a position operator of one horizontally pixel from an input image with five grey levels.

FIGURE 2 .

 2 FIGURE 2. GLCM's statistical parameters applied to 3D reinforcement.

FIGURE 3 .

 3 FIGURE 3. 3D orthogonal non-crimp segmented volume.

FIGURE 4 .

 4 FIGURE 4. Comparison between the compacted tomography and the 3D orthogonal non-crimp woven simulation.

FIGURE 5 .

 5 FIGURE 5. Comparison between simulated and tomographied 3D orthogonal non-crimp woven compacted to 22%.

ACKNOWLEDGMENTS

The authors thank the support of INSIS-CNRS for this study.