Z B He

Jean-Luc Maurice
email: jean-luc.maurice@polytechnique.edu

C S Lee

A Gohier

P Legagneux

D Pribat

C S Cojocaru

J.-L Maurice

Etchant-induced

Introduction

Vertically aligned multi-walled carbon nanofibers (CNFs) or carbon nanotubes (CNTs) have been intensively studied over the last decade because of their numerous potential applications in field emission devices [START_REF] Heer | A carbon nanotube field-emission electron source[END_REF][START_REF] Fan | Self-oriented regular arrays of carbon nanotubes and their field emission properties[END_REF][START_REF] Tsai | Transfer of patterned vertically aligned carbon nanotubes onto plastic substrates for flexible electronics and field emission devices[END_REF][START_REF] Chhowalla | Growth process conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition[END_REF], including flat-panel displays and microwave diodes [START_REF] Teo | Carbon nanotubes as cold cathodes[END_REF]. Catalyst-assisted, direct-current, plasma-enhanced chemical vapour deposition (DC-PECVD) is so far the most popular technique employed to produce vertical CNTs and CNFs since it allows a precise control of alignment and density [START_REF] Meyyappan | Carbon nanotube growth by PECVD: a review[END_REF][START_REF] Melechko | Vertically aligned carbon nanofibers and related structures: Controlled synthesis and directed assembly[END_REF], and it is a relatively simple and low cost growth method. The nanotubes obtained with that technique grow with the catalyst particle on top, so that growth beneath the particle remains protected, while all tendencies of non-vertical growth are immediately etched away by incoming ions. Most 3d transition metals have been used to locally catalyse the decomposition of carbon-bearing radicals, thus releasing carbon atoms that fuel the growth of CNTs or CNFs. In the present paper we focus on nickel, which was the first, chronologically, to allow the DC-PECVD growth of vertically aligned nanofibres [START_REF] Chhowalla | Growth process conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition[END_REF][START_REF] Teo | Carbon nanotubes as cold cathodes[END_REF].

PECVD growth of CNTs or CNFs is usually performed in the 500-750°C temperature range. The mixture of gases used comprises a carbon precursor (typically a hydrocarbon gas) and an etchant gas, the latter preventing the parasitic deposition of amorphous carbon all over the sample and the poisoning of the catalysts [START_REF] Teo | Uniform patterned growth of carbon nanotubes without surface carbon[END_REF]. Although CNTs exhibit better transport and mechanical properties than CNFs, it is generally CNFs that are obtained when using nickel catalyst in the above-mentioned temperature range. Thus the problem is to obtain, at a given moderate temperature, a CNF structure as close as possible to that of a CNT. The primary difference between CNTs and CNFs is the cone angle () between the graphite basal planes and the tube axis ( = 0° for CNTs;   0° for CNFs) [START_REF] Melechko | Vertically aligned carbon nanofibers and related structures: Controlled synthesis and directed assembly[END_REF]. The cone angle of CNFs directly influences their conductivity as it determines the number of graphene planes that an electrical current will have to cross over a given distance. The cone angle and the diameter of CNFs prepared by (PE)CVD with nickel as catalyst depend on the size [START_REF] Kukovitsky | Correlation between metal catalyst particle size and carbon nanotube growth[END_REF] and shape of the Ni nanoparticles that drive growth; more specifically, the graphene cones are quasi-replicas of the conical tails of the catalyst nanoparticles [START_REF] Yang | Mechanism of carbon filament growth on metal catalysts[END_REF][START_REF] Tracz | High-resolution electron microscopy study of the carbon deposit morphology on nickel catalysts[END_REF][START_REF] Nolan | Hydrogen control of carbon deposit morphology[END_REF][START_REF] Kiselev | Carbon micro-and nanotubes synthesized by PE-CVD technique: Tube structure and catalytic particles crystallography[END_REF] (Figure 1). Therefore, in those processes that take place well below the melting temperature of the catalyst alloy, it is of crucial importance to study the shape and the crystallography of such catalyst particles. Catalytic growth is a dynamical process, during which the catalyst particle shape changes due to the varying compressive stress applied by the graphene planes adding one to the other in between the catalyst and the existing carbon planes [START_REF] Sun | Carbon nanotubes as high-pressure cylinders and nanoextruders[END_REF]. Thus in-situ TEM is certainly the best way to study the growth process [START_REF] Helveg | Atomic-scale imaging of carbon nanofibre growth[END_REF]. However, such experiments lack statistics, so that the in-situ experiments that have been performed so far mostly bring qualitative observations e.g. of the change of catalyst shape as the growth goes on [START_REF] Helveg | Atomic-scale imaging of carbon nanofibre growth[END_REF][START_REF] Hofmann | In situ observations of catalyst dynamics during surface-bound carbon nanotube nucleation[END_REF][START_REF] Lin | Dynamical observation of bamboo-like carbon nanotube growth[END_REF][START_REF] Kunadian | Growth kinetics of MWCNTs synthesized by a continuous-feed CVD method[END_REF][START_REF] Ichihashi | In situ observation of carbon-nanopillar tubulization caused by liquid like iron particles[END_REF][START_REF] Yoshida | Atomic-scale in-situ observation of carbon nanotube growth from solid state iron carbide nanoparticles[END_REF]. More quantitative studies have been carried out using ex-situ scanning electron microscopy (SEM) and conventional high-resolution transmission electron microscopy (TEM) [e.g. : 4, 7, 9, 11, 21]. The crystallography of nanoparticles has been specifically addressed by several authors [START_REF] Yang | Mechanism of carbon filament growth on metal catalysts[END_REF][START_REF] Audier | Crystallographic orientations of catalytic particles in filamentous carbon; Case of simple conical particles[END_REF][START_REF] Kuang | Catalytically active nickel {110} surfaces in growth of carbon tubular structures[END_REF], and the dynamical processes of catalysis have been analysed as a function of catalyst [START_REF] Esconjauregui | The reasons why metals catalyze the nucleation and growth of carbon nanotubes and other carbon nanomorphologies[END_REF]. Several reviews summarise the literature [START_REF] Melechko | Vertically aligned carbon nanofibers and related structures: Controlled synthesis and directed assembly[END_REF][START_REF] Dupuis | The catalyst in the CCVD of carbon nanotubes-a review[END_REF]. The dependence of catalyst shape as a function of growth conditions, and its influence on the form of the graphene conical sheets that make the CNFs, have been studied two decades ago by Tracz et al. [START_REF] Tracz | High-resolution electron microscopy study of the carbon deposit morphology on nickel catalysts[END_REF]. These authors have observed the alignment of the graphene layers on the facets of the nickel catalysts in all the conditions they use (H 2 O:C 4 H 10 , 2:1 to 8:1, 60 mbars, temperature range 500-600°C), and have showed the narrowing of the cone angle as a function of temperature [START_REF] Tracz | High-resolution electron microscopy study of the carbon deposit morphology on nickel catalysts[END_REF].

In other respects, the effects of etchants on the growth of CNFs have been poorly investigated. Ammonia is currently used as the main etchant in DC PE-CVD. However, in the case of standard CVD, adding a small amount of water vapour to the carrier gas mix (Ar or He with 40% H2 in that case) has been demonstrated to tremendously increase the growth rate in the case of Fe or Co catalysts [START_REF] Hata | Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes[END_REF]. Moreover, it has been shown to maintain the catalyst activity over a longer period of time, so that the maximum length in a given growth run could be increased by two orders of magnitude [START_REF] Hata | Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes[END_REF]. As those parameters are important for future industrial fabrication, it is of prior importance to understand and master the phenomena at the origin of these differences. Meanwhile, etchants have also been shown to influence particle shape: it has been noticed for instance that decreasing the amount of hydrogen in the growth atmosphere could diminish the cone angle of the catalyst particles [START_REF] Nolan | Hydrogen control of carbon deposit morphology[END_REF][START_REF] Murayama | A novel form of filamentous graphite[END_REF][START_REF] Nolan | Carbon deposition and hydrocarbon formation on group VIII metal catalysts[END_REF]. The explanation given in that case was that hydrogen would saturate carbon dangling bonds at the intersections with the tube surface of the conical graphene layers: its absence would force those layers to adopt a tubular shape, which limits the density of surface dangling bonds.

The effects of the carbon precursors have been investigated by Hoffmann et al. [START_REF] Hofmann | Low-temperature plasma enhanced chemical vapour deposition of carbon nanotubes[END_REF] who have shown that with ammonia (NH 3 ) as carrier gas, acetylene (C 2 H 2 ) was by far the most efficient in terms of growth rate, especially compared to methane (CH 4 ). In the following, we thus concentrate on acetylene.

In this study we compare the effects of using either ammonia or water vapour as etchant gases, on the vertical growth of CNFs with Ni catalyst on their tips, when growth is performed by direct current (dc) plasma-enhanced CVD (dc-PECVD). Compared to ammonia, water vapour appears to accelerate the growth by at least a factor of two. Using TEM observations, we show that, whatever the carbon precursor, the cone angle  is more than twice smaller when we use water. Those factors increase as growth temperature decreases.

We also show that correlatively, the top part of catalytic particles is about bare, clean and faceted with sharp edges, after growth in water vapour. On the other hand, the catalytic particles end up covered by a few layers of graphene and rounded in shape in the case of growth in ammonia. We discuss the implications of these differences in terms of carboncatalyst contact and its effects on the mobility of carbon atoms across the catalyst particle during growth.

Experimental details

Growth

Vertically aligned CNF arrays were grown from Ni catalysts originating from thermally evaporated thin films with different thicknesses. Si substrates were used, either bare or covered with a TiN diffusion barrier layer. Growth was performed by dc PECVD from 600 to 670 °C, in a gas mixture including either methane (CH 4 ), or acetylene (C 2 H 2 ) as carbon precursor gases and water (either H 2 O or H 2 :H 2 O, 2:1, by volume) or ammonia (either NH 3 or NH 3 :H 2 , 2:1) as etchant gases. The gas flows were precisely controlled to avoid the generation of amorphous carbon on the surface of catalysts. In order to obtain well aligned CNFs, the applied voltage between anode and cathode in the chamber was kept constant at 500 V, with a current intensity of 0.2 A. The extraction voltage and the current were slowly increased up to the optimum conditions at the beginning of the growth process. At the end of the growth process, the chamber was totally pumped out and cooled down. Cooling took around 60 minutes without any gas flowing in the growth chamber. The length of CNFs was controlled by the growth time and their average diameter by the thickness of the starting Ni film.

Starting from a continuous Ni film, it takes two stages to grow CNFs: (1) de-wetting of the film, in order to form Ni nanoparticles and initiate the nucleation of several graphene layers between those particles and the substrate, and (2) growth of the nanofibers from the Ni nanoparticles, fuelled by the decomposition of the carbon precursor on the exposed surface of those particles. As the two etchants yielded different particle sizes for a given thickness of the starting Ni thin film (larger with NH 3 , this will be published elsewhere), we have also prepared some samples with the same H 2 :H 2 O etchant during the de-wetting stage, and a switch (or not) to NH 3 only during the growth stage, so that the Ni-particle size be the same in the two cases. We also switched to NH 3 at various stages of growth, and could verify it was possible to benefit from the high growth speed of H 2 O, and from the protective coating of nickel provided by NH 3 .

Characterisation

The CNFs were observed using a Hitachi S-4800 FE-scanning electron microscope (SEM) with 25 kV accelerating voltage. The microstructure of the CNFs and Ni catalysts was studied by TEM using a Philips CM 30 electron microscope working at 300 kV, and a Topcon 002B microscope working at 120 kV or 160 kV. TEM samples were obtained by scratching deposits over holey amorphous carbon membranes. However, this substrate had the drawback of oxidising the samples under the beam (see below). Other details on the TEM procedure are given in the Supplementary Data. We have also prepared cross-sections made from sandwichtype samples, after sliding one part of the sandwich with respect to the other, so that the nanofibres lie parallel to the substrate and along the optical axis of the microscope. This allowed us to obtain cross sections of nanofibres perpendicular to their axes.

Electron beam damage

Electron beam irradiation is a significant issue when observing carbon nano-objects with TEM. While the knock-on threshold energy for a carbon atom in a nanotube is about 86 keV [START_REF] Smith | Electron irradiation effects in single wall carbon nanotubes[END_REF], observations remain possible at higher energies provided that the total dose (settings plus recordings) remains small. However, knock-on is not the only source of defects. Figure 2 shows that another source of damage is electron beam heating of the metal nanoparticle, which significantly modifies its surface, whatever the accelerating voltage or the microscope used. Except for those of Figure 2 d-f, all of the TEM images presented in this paper were recorded with nanofibres deposited on holey amorphous carbon TEM grids from Oxford Instruments. The Ni particles on such a substrate appeared to oxidize under the beam. This sensitivity had a consequence on characterisation: it was hardly possible to determine at the same time the microstructure of a surface and its orientation, as tilting the sample along a zone axis (convenient for surface characterisation) generated an irradiation that completely oxidised the surface. This is why our zone-axis images always exhibit surface oxide, while our images used to analyse the nature of surfaces, recorded as fast as possible, are not oriented along zone axes. This is why, in turn, and quite unfortunately, we present no high resolution images of the surfaces. It is interesting to note that the oxidation did not occur when using a thin silicon nitride membrane as substrate, on which we only observed the growth of a standard contamination film (Figure 2d-f). Silicon nitride had other drawbacks like charging and difficulty to deposit the CNFs on it, so that we did not use it except for this test. The oxygen thus comes from the amorphous carbon membrane. Selected area diffraction (Figure 2c) shows that the oxide obtained on the holey carbon grid has indeed the structure of fcc NiO.

Structure of catalyst particles

The crystallography of the Ni particles appears quite simple: they are all single crystals, and all of the standard fcc phase of Ni. We distinguish no effect of the growth atmosphere on crystallographic features, other than the surface faceting we discuss in sec.4. Characterising the crystallographic direction of growth is not a simple task because of projection artefacts (see Supplementary data, fig. S1, and ref. [START_REF] He | to appear in the Arabian Journal for Science and Engineering[END_REF]). When it can be determined, the growth axis exhibits low indices, quite generally <110>, confirming previous observations [START_REF] Audier | Crystallographic orientations of catalytic particles in filamentous carbon; Case of simple conical particles[END_REF][START_REF] Kuang | Catalytically active nickel {110} surfaces in growth of carbon tubular structures[END_REF], but sometimes <100>, <111>, <211>. It is interesting to note, however, that the present <110> oriented nanoparticles rarely have {110} top facets (see e.g. Figure 3, see a rare exception in Supplementary information, fig. S1), which will be discussed in the next section. Twinning may occur, with a frequency that seems to depend rather on the temperature and the carbon precursor used than on the etchant, so that we will not discuss it in the present work; the twin plane may lie parallel to the <110> growth direction, or at an angle with it. Let us add that it does not appear to influence the contact plane between particle and nanofibre, and only slightly the top surface faceting [START_REF] He | to appear in the Arabian Journal for Science and Engineering[END_REF].

Shape of catalyst particles

Preliminary observations

Figure 4 shows typical SEM images of CNFs obtained when starting with a 10-nm thick nickel film: they are well aligned and oriented perpendicular to the substrate. The CNFs in that case were grown on a patterned substrate designed for field emission (TiN/Si) [START_REF] Cojocaru | Synthesis of multi-walled carbon nanotubes by combining hot-wire and dc plasma-enhanced chemical vapor deposition[END_REF], at 4c) as etchant gases. Each CNF has a uniform diameter from bottom to top. As in other works, the present Ni-catalyst particles on the top of CNFs have inversed water drop shape, elongated along the CNF axis. They can be separated into two well-defined parts: a bulky top part facing the plasma, the "head", and a thinner bottom part surrounded by the beginning of the nanofibre, the "tail". In Figure 5, the head exhibits clearly defined facets in the case where water was used as etchant, and rounded shapes in the case of ammonia. We analyse those features in detail in the next section.

What is particularly remarkable in Figure 4 is the difference of size of the nanofibres in the two sets: during a growth treatment of the same duration, the use of water vapour leads to nanofibres that are about an order of magnitude longer than those made with ammonia.

Faceting of catalyst head

The most noticeable feature of the catalyst particle head is faceting, clearly visible and sharp in the case where hydrogen-water has been used as etchant, and non-apparent in the case of ammonia. However, a refined analysis reveals that the latter samples also have facets, but with smooth edges (see e.g. Figure 2 a, b), so that what differentiates the two kinds of samples is the sharpness (H 2 O) or roundness (NH 3 ) of edges rather than the presence or absence of facets. To analyse facet orientation, we restrained our study to the H 2 :H 2 O samples where it is easily visible. The results we present here are complementary to those we have published separately [START_REF] He | to appear in the Arabian Journal for Science and Engineering[END_REF]. On average, facets systematically correspond to low-index planes, mostly {111} and {100} (Figure 3, Figure 5 and Figure 6), and less often {110} (see Supplementary data, fig. S1). These correspond in turn to low surface energies, which will be discussed in sec. 5. cases except for the nature of the etchant and a slightly higher growth temperature in the case of H 2 O (675 °C instead of 650 °C). It should be noted that the present micrographs (Figure 6)

Capping of catalyst head

were taken after only a few tens of seconds of irradiation (see sec. 2.3), except Figure 6f which, recorded at a higher magnification, has undergone about 2 min of irradiation (in order to change the settings of the microscope), which started to break the surface film and oxidise the nickel. Still, one can see that the NH 3 -etched particle (Figure 6f) is covered by a ~2-nm (a) (b)

thick, partially crystallized, carbonaceous layer, while the H 2 O particle appears protected by at most a single monolayer of an undefined film.

Tail of catalyst particle

Using water vapour or ammonia has a dramatic effect on the shape of the conical tail of catalyst particles: we have measured that parameter for the two samples shown in Figure 4 and Figure 5, where all growth conditions were the same except for the etchant gas X: 600°C, 30 min, C 2 H 2 :H 2 :X. The measurements were performed on SEM and TEM micrographs and are summarised in Table 1. The cone angle appears three times smaller in the case of water ( ~13°) than in that of ammonia ( ~39°). We confirm its decrease with growth temperature [START_REF] Tracz | High-resolution electron microscopy study of the carbon deposit morphology on nickel catalysts[END_REF], so that the difference between the two etchants decreases as the temperature increases. A qualitative illustration of the smaller angle at 675°C for water and 650°C for ammonia is given by Figure 6a andd b, d-f) were used as etchants (same deposits as those shown in Figure 6).

Except at the limit between head and tail (b, e) where shoulders may exist, graphene planes stay parallel to the catalyst surface, whatever its shape or its density of steps. (f) Nanofibre embedded into glue, seen on end, in a horizontal cross-section. The interface between the tail of the catalyst particle and the nanofibre walls is smooth with no facet, except for a small planar {111} section.

Another remarkable feature of the catalyst tail is its apparent absence of faceting regardless of the gases used for growth (Figure 7). To confirm this observation, we prepared a few TEM cross-sections with in-plane nanofibres embedded into glue, where some of the nanoparticles had their head cut off during sample thinning, allowing one to watch their tail surrounded by the nanofibre walls in a "horizontal" cross-section (Figure 7f). In such crosssections, the interface can be seen in another angle, confirming the quasi absence of faceting:

the catalyst tail is indeed, to a very good approximation, a truncated cone.

To summarise all our observations, the differences of catalyst shape between the two cases of etchants are thus striking. The catalyst tail has the shape of truncated non-faceted cone in both cases; but the cone angle drastically depends on the etchant used: ~39° at the growth temperature of 600°C in the case of ammonia, and ~13° in the case of water in the same conditions. The catalyst appears correlatively rounded and covered by a few layers of partially crystallised graphene layers in the case of NH 3 , while it looks sharply facetted and almost bare in the case of H 2 O.

Discussion: shape of Ni catalysts

Tail shape and graphene generation

Let us first discuss the epitaxial relationships between the Ni catalyst and the graphene layers. Yang and Chen showed in their pioneering study [START_REF] Yang | Mechanism of carbon filament growth on metal catalysts[END_REF] that the epitaxy of graphite should be favoured on the {111} and {311} facets of nickel. The {111} surface of Ni had already been singled out more than three decades ago by the observation that it promoted the epitaxial growth of graphene [START_REF] Eizenberg | Carbon monolayer phase condensation on Ni(111)[END_REF]. This specificity of the Ni{111} face regarding the epitaxy of graphene layers stems of course from the same six-fold symmetry, but also from the additional fact that the parameter mismatch then remains small: a/a = 1.2% when Ni <110> atomic rows are parallel to C <100>. However with the present observations of a conical growth surface, one sees that assuming given specific low-index planes of nickel to model epitaxy is an over-simplification. Although {111} facets may develop at the interface, they are certainly not representative of the general phenomena that are responsible for the observed growth (see Figure 7).

We thus find that the interface between Ni and the graphene layers is conical in such a way that nickel presents a high density of vertical as well as horizontal atomic steps. Quite remarkably, the graphene layers remain parallel to that surface, thus following its conical shape, which actually determines the conical shape of the carbon {002} planes in the nanofibre. Most Ni surface steps thus correspond to no additional graphene layer, as if the most general way of nucleation of the latter had been with a whole conical layer at a time (Figure 7a).

Existing models of graphene layer nucleation and growth have developed thanks to insitu TEM [START_REF] Sun | Carbon nanotubes as high-pressure cylinders and nanoextruders[END_REF][START_REF] Helveg | Atomic-scale imaging of carbon nanofibre growth[END_REF][START_REF] Lin | Dynamical observation of bamboo-like carbon nanotube growth[END_REF][START_REF] Hofmann | In situ observations of catalyst dynamics during surface-bound carbon nanotube nucleation[END_REF][START_REF] Rodriguez-Manzo | In situ nucleation of carbon nanotubes by the injection of carbon atoms into metal particles[END_REF]. They often involve growth of several graphite planes at a time, almost perpendicularly to a catalyst surface. Such a mechanism may well occur here when shoulders exist between catalyst head and tail (Figure 7b). But those shoulders are no general features of the present particles: they are not found in all particles in the case of growth in hydrogen-water, and they are even less frequent in the case of growth in ammonia. On the other hand Helveg et al. have also observed a step edge mechanism of generation of a graphene plane, by scanning of a Ni single surface step [START_REF] Helveg | Atomic-scale imaging of carbon nanofibre growth[END_REF][START_REF] Abild-Pedersen | Mechanisms for catalytic carbon nanofiber growth studied by ab initio density functional theory calculations[END_REF]. A similar mechanism probably takes place here, but with nickel "steps" which must be complicated objects that involve more than a monolayer of Ni, and that follow the rounded topology of the conical tail. However, it is important to note that the cone angle of the graphene sheets, , is radically smaller in the case of water than in that of ammonia: 13° vs. 39° at 600°C (Table I). This indicates that the same length of nanofibre includes a lower number of graphene sheets in the case of water than in that of ammonia, or else, that a same number of cone-shaped layers delivers a larger length of nanofibre in the former case than in the latter (Figure 8).

There is a direct relationship between those observations and the fact that the growth rate is indeed faster with the water etchant. But is it the high rate that forces the graphene layers to be more tubular, and in turn the Ni particle to get elongated, or is it the elongated shape of the particle that is the origin of the high growth rate? We propose in the following that it be the particle that would determine the change of shape, due to an increased rate of transfer of carbon atoms in the case of water compared to that of ammonia.

Faceting

Literature presents various results on surface structure and surface capping that reflect the variety of growth conditions used. In the case of the NH 3 etchant, the seminal work of Chhowalla et al. [START_REF] Chhowalla | Growth process conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition[END_REF] already showed that the top, free part of the catalyst particles, was covered by a carbon film, and that, if some facets could be visible, such features had rounded edges. Our observations are thus a confirmation of the smoothing effect of NH 3 on the shape of catalyst particles.

In the case of water, the faceting we observe seems to be specific of our growth conditions: PE-CVD, 600-670°C, large amounts of H 2 O in the gas mixture, as indeed, the faceting appearing after other growth conditions has quite different characteristics. Audier et al. were the first to study faceting in detail [START_REF] Audier | Crystallographic orientations of catalytic particles in filamentous carbon; Case of simple conical particles[END_REF]; they concluded that in the case of fcc metals, only {111} facets could be observed: their growth method was based on the catalytic disproportionation of CO at P = 1 atm and 450 °C < T < 650 °C [START_REF] Audier | Crystallographic orientations of catalytic particles in filamentous carbon; Case of simple conical particles[END_REF]. In contrast, Kuang et al., who worked at a pressure of 3 kPa (30 mbar) with a mix of N 2 :H 2 :CH 4 , found that Ni particles had <110> orientation and {110} top facets [START_REF] Kuang | Catalytically active nickel {110} surfaces in growth of carbon tubular structures[END_REF]. These authors used a hot-wire PE-CVD equipment, at T = 700 °C. Yang and Chen, who performed growth using a similar gas mix at a pressure of 1 atm [START_REF] Yang | Mechanism of carbon filament growth on metal catalysts[END_REF], reported that {110} faces were catalytically active for the dissociation of CH 4 , while {111} were not.

Here for the top part of the catalytic particles, we found only few {110} oriented facets, many {111}, and many {100}. Thus we cannot conclude like in previous works that a given surface orientation is dominating, but only that low-energy surfaces prevail, perhaps just indicating that the catalyst surfaces remain almost bare during catalysis, so that they adopt a configuration close to the equilibrium one, as in a Wulff polyhedron [START_REF] Wulff | Equilibrium nanoshape changes induced by epitaxial stress (generalised Wulf-Kaishew theorem)[END_REF]. The sharp edges sometimes visible (e.g. Figure 6) are faceted themselves at a lower scale (Figure 6b) which indicates that the top catalyst surfaces in the case of H 2 O etchant are close indeed to thermal equilibrium.

Capping of the head and carbon diffusion

The activation energies found for the growth rates are lower in the case of water (0.18 eV) [START_REF] Cojocaru | Water based PECVD process for the growth of oriented and patterned carbon nanotubes[END_REF], than in the case of ammonia (0.23 eV) [START_REF] Hofmann | Surface diffusion: the low activation energy path for nanotube growth[END_REF]. In both cases, the conclusion was that diffusion was a surface phenomenon. Thus, we are tempted to say that, for the present comparisons, the difference stems from the nickel catalytic activity, which would have been larger at the bare metal surfaces of the water case, than at the surfaces covered by a 2-nm carbon film of the ammonia case. But we must keep in mind that the present results were obtained after completed growth, and thus do not represent the state of the catalyst surface during growth. The surface graphene layers in the case of ammonia may also have built up after growth indeed; as carbon must have penetrated into nickel during growth up to its solubility limit at the growth temperature, and it must have precipitated at the surfaces after the decrease of solubility upon cooling [START_REF] Lander | Solubility and diffusion coefficient of carbon in nickel: reaction rates of nickel-carbon alloys with barium oxide[END_REF].

Reciprocally, graphene layers should have appeared on the surface upon cooling if the solubility was attained during growth. Thus, an absence of visible graphene layers as in the present case of water etchant would indicate that the carbon concentration has not reached the solubility limit during deposition. Thus diffusion would be a pure surface phenomenon in that case while it would include a bulk component in the ammonia case.

Finally, in-situ, environmental TEM [START_REF] Helveg | Atomic-scale imaging of carbon nanofibre growth[END_REF][START_REF] Giorgio | Environmental electron microscopy (ETEM) for catalysts with a closed E-cell with carbon windows[END_REF] would be the only way: to determine the time at which the surface layers have built up; and thus to allow one to conclude on the origin of the differences in the activation energy and rate of the growth.

Conclusions

Vertically aligned CNFs were grown by the tip-growth mechanism, using dc PECVD in the 600-670°C temperature range and using different gas precursors. We compared the effects of etchants based on H 2 O and NH 3 on the shape of the catalytic Ni particles. The water-based mixtures appeared to provide faster growth with lower cone angle. The top catalyst surfaces were in that case bare and sharply faceted, which contrasted with the rounded surfaces of the NH 3 -treated samples which were capped with a 2-nm thick, partially crystallised, graphenelike film. The exposed surfaces of the Ni particles at the growth front were a combination of low index facets such as {111} and {100}, as in a Wulff polyhedron, rather than a unique type of facet such as {111} [START_REF] Audier | Crystallographic orientations of catalytic particles in filamentous carbon; Case of simple conical particles[END_REF] or {110} [START_REF] Kuang | Catalytically active nickel {110} surfaces in growth of carbon tubular structures[END_REF]. We associate the lower cone angle and higher growth rate of water-etched nanofibres with faster diffusion of carbon atoms across their bare top surfaces.

competence center of Paris Region, supported by CNRS, CEA, MESR and Region Ile-de-France.

Figure 1 .

 1 Figure 1. (a) High-resolution TEM image of the head of a nanofibre grown at 650°C in C 2 H 2 :

Figure 2 .

 2 Figure 2. Comparison of the effects of electron irradiation when nanofibres are deposited on

Figure 3 .

 3 Figure 3. TEM images of a CNF grown at 675°C with H 2 :H 2 O:C 2 H 2 after it has been carefully tilted to

  30 min, using acetylene as carbon precursor, and either hydrogen-water C 2 H 2 :H 2 :H 2 O (5:10:5) (Figure 4a,b) or hydrogen-ammonia C 2 H 2 :H 2 :NH 3 (8:10:18) (ratios in sccm) (Figure

Figure 4 .Figure 5 .

 45 Figure 4. SEM images of vertically aligned top-type CNFs grown for 30 min at 600°C in

Figure 6

 6 Figure 6 compares a nanofibre grown in H 2 :H 2 O with one obtained in NH 3 , after de-

Figure 6 .

 6 Figure 6. TEM images of CNFs with Ni catalyst particles on top, grown with a H 2 :H 2 O

Figure 7 .

 7 Figure 7. (a-d) Longitudinal views of catalysts in the case where ammonia (a, c) and

Figure 8 .

 8 Figure 8. Schematic of the typical catalyst shape in the case of water (a) and in the case of

Table 1 :

 1 Cone angle in the case of growth in C 2 H 2 :H 2 :X, where X= H 2 O or NH 3 , at 600°C for 30 min.

		Number of Ni	Average cone angle 	Standard deviation
		particles measured	(degrees)	(degrees)
	H 2 O	88	12.6	2.6
	NH 3	40	39.0	12.1

ACKNOWLEDGEMENTS ZBH thanks Dr. G. Rizza and F. Attouchi at Laboratoire des Solides Irradiés, Ecole Polytechnique, France for useful discussions. JLM is grateful to Dr. G. Garry and Dr. S. Enouz-Vedrenne (Thales Research and Technology France) for access to the Topcon 002B. DP would like to acknowledge partial support from WCU program, through the NRF of Korea, funded by MEST (R31-2008-000-10029-0). This work has been supported by the Region Ile-de-France in the framework of C'Nano IdF. C'Nano IdF is the nanoscience


I. INTRODUCTION

Only a fraction of the global ocean energy resource is to be found in sites which are economically feasible to explore with available technology. However, this fraction could still make a considerable contribution to electricity supply. This is the reason why the marine renewable sector is currently the focus of much industrial and academic research around the world [START_REF] Johnstone | EC FPVI co-ordinated action on ocean energy: A European platform for sharing technical information and research outcomes in wave and tidal energy systems[END_REF]. Sites with attractive wave climate and intense tidal currents are abundant in the vicinity of the European coastline. It has been shown that 48% of the European tidal resource is in the UK, 42% in France, and 8% in Ireland. Three examples in France are shown in Fig. 1. The Raz Blanchard situated in Cap de la Hague experiences extreme tidal currents exceeding 8 knots and leading to a large amount of kinetic energy flux.

There are basically two ways of generating electricity from marine and tidal currents: by building a tidal barrage across an estuary or a bay in high tide areas, or by extracting energy from free flowing water (tidal kinetic energy). Within the last few decades, developers have shifted towards technologies that capture tidally-driven coastal currents or tidal stream [START_REF] Ben Elghali | Marine tidal current electric power generation technology: State of the art and current status[END_REF]. The astronomic nature of this resource makes it predictable, to within 98% accuracy for decades, and independent of prevailing weather conditions. This predictability is critical to a successful integration of renewable energy in the electrical grid [START_REF]Harnessing the untapped energy potential of the oceans: Tidal, wave, currents and OTEC[END_REF].

In this context, it is obvious that there is a need to quantify the potential to generating electricity from these various sites [START_REF] Myers | Simulated electrical power potential harnessed by marine current turbine arrays in the Alderney Race[END_REF]. This paper deals then with the modeling and the control of a variable speed DFIGbased marine current turbine with and without tidal current speed sensor. The proposed MPPT control strategy is tested using tidal current data from the Raz de Sein (Brittany, France) for a 7.5-kW prototype over various flow regimes. It should be noted that when scanning the literature, one will find very few papers in this topic [START_REF] Park | Wide speed operation of a doubly-fed induction generator for tidal current energy[END_REF].

II. MARINE CURRENT TURBINE MODELING

The global scheme for a grid-connected marine current turbine is given by Fig. 2.

A. The Resource Model

1) Resource Potential. The total kinetic power in a marine current turbine has a similar dependence to that of a wind turbine and is governed by the following equation [START_REF] Couch | Tidal current energy extraction: Hydrodynamic resource characteristics[END_REF][START_REF] Bahaj | Fundamentals applicable to the utilisation of marine current turbines for energy production[END_REF]. However, a marine energy turbine can only harness a fraction of this power due to losses and (1) is modified as follows. For marine turbines, C p is estimated to be in the range 0.35-0.5 [START_REF] Myers | Power output performance characteristics of a horizontal axis marine current turbine[END_REF].

2) Resource Model. Tidal current data are given by Hydrographic and Oceanographic Services as for example the French Navy Hydrographic and Oceanographic Service (SHOM) and is available for various locations in chart form. The available charts give, for a specific site, the current velocities for spring and neap tides. These values are given at hourly intervals starting at 6 hours before high waters and ending 6 hours after. Therefore, knowing tides coefficient, it is easy to derive by interpolation a simple and practical model for tidal current speeds V tide .

   

45 95 45

st nt tide nt C V V VV    (3)
Where C is the tide coefficient which characterize each tidal cycle (95 and 45 are respectively the spring and neap tide medium coefficient). This coefficient is determined by astronomic calculation of earth and moon positions. V st and V nt are respectively the spring and neap tide current velocities for hourly intervals starting at 6 hours before high waters and ending 6 hours after. For example, 3 hours after the high tide in Brest, V st = 1.8 knots and V nt = 0.9 knots. Therefore, for a tide coefficient C = 80, V tide = 1.53 knots.

This first-order model is then used to calculate the tidal velocity each hour. The implemented model will allow the user to compute tidal velocities in a predefined time range. Figure 3 shows the model output for a month (March 2007) and for a year (2007). This adopted resource model has several advantages including its modularity not to mention its simplicity. Indeed, the marine turbine site can be changed, the useful current velocity can be adapted, and the time range taken into account can also be adapted from one month to one year.

B. The Turbine Rotor Model

The harnessing of the energy in a tidal flow requires the conversion of kinetic energy in a moving fluid, in this case water, into the motion of a mechanical system, which can then drive a generator. It is not too surprising, therefore, that many developers suggest using technology that mirrors that which has been successfully utilized to harness the wind, which is also a moving fluid [START_REF] Ben Elghali | Marine tidal current electric power generation technology: State of the art and current status[END_REF]. Moreover, much of the technology is based upon the use of horizontal axis turbines. Compared to the largest wind turbines operating today, the output power and the size of a classical tidal turbine are extremely promising. Indeed, this is due to the sea water huge density and to the current velocity. For illustration, Fig. 4 shows a tidal turbine against an offshore wind turbine of the same power rating. Therefore, much can be transferred from the modeling and operation of wind turbines [START_REF] Bossanyi | Wind Energy Handbook[END_REF]. There are, however, a number of fundamental differences in the design and operation of marine turbines.

Particular differences entail changes in force loadings, immersion depth, different stall characteristics, and the possible occurrence of cavitation [START_REF] Batten | Experimentally validated numerical method for the hydrodynamic design of horizontal axis tidal turbines[END_REF][START_REF] Batten | Hydrodynamics of marine current turbines[END_REF].

Turbine rotor aerodynamics refers to the interaction of the wind turbine rotor with the incoming wind.

The treatment of rotor aerodynamics in all current design codes is based on Glauert well-known, and well established BEM theory [START_REF] Glauert | The Elements of Airfoil and Airscrew Theory[END_REF]. The BEM method has therefore been used for the marine turbine rotor modeling. It allows an iterative calculation of drag and lift coefficient for each blade section and the calculation of the global performance of a turbine. Indeed, it is widely used in the industry as a computational tool to predict aerodynamic loads and power of turbine rotors. Details of the model adaptation to marine turbines are given in the authors' previous work [START_REF] Ben Elghali | A simulation model for the evaluation of the electrical power potential harnessed by a marine current turbine[END_REF]. For a given turbine geometry (number of blades, geometry of the blades, pitch, etc.), this model allows to calculate the mechanical power and torque for each fluid speed and turbine rotational speed. This model is relatively simple and computationally fast meeting the requirements of accuracy and control loop computational speed.

Examples of the obtained results are given in section IV.A.

C. The Generator Model

The generator chosen for the marine current system was the DFIG [START_REF] Müller | Doubly fed induction generator systems[END_REF]. DFIG-based marine turbines, as for wind turbines, will offer several advantages including variable speed operation, and four-quadrant active and reactive power capabilities. Such system also results in lower converter costs and lower power losses compared to a system based on a fully fed synchronous generator with full-rated converter.

Moreover, the generator is robust and requires little maintenance [START_REF] Park | Wide speed operation of a doubly-fed induction generator for tidal current energy[END_REF], [START_REF] Carrasco | Power-electronic systems for the grid integration of renewable energy sources: A survey[END_REF][START_REF] Tapia | Proportional-integral regulator-based approach to wind farm reactive power management for secondary voltage control[END_REF][START_REF] Brekken | Control of a doubly fed induction wind generator under unbalanced grid voltage conditions[END_REF][START_REF] Tapia | Modeling and control of a wind turbine driven doubly fed induction generator[END_REF]. A schematic diagram of a DFIG-based generation system is shown in Fig. 5.

The control system is usually defined in the synchronous d-q frame fixed to either the stator voltage or the stator flux [START_REF] Müller | Doubly fed induction generator systems[END_REF]. For the proposed control strategy, the generator dynamic model written in a synchronously rotating frame d-q is given by (4).

d V R I dt d V R I dt d V R I dt d V R I dt L I M I L I M I L I M I L I M I T pM I I I I d J T T f dt                                                                        (4)

III. CONTROL OF THE DFIG-BASED MARINE CURRENT TURBINE

The DFIG stator-side reactive power is given by

  3 2 s sq sd sd sq Q V I V I  (5)
For a decoupled control, a d-q reference frame attached to the stator flux was used. Therefore, assuming the per phase stator resistance negligible and setting the stator flux vector aligned with the daxis, the reactive power can be expressed as [START_REF] Müller | Doubly fed induction generator systems[END_REF]  

3 2 s s s rd s V Q M I L    (6)
Since the DFIG control objective is to generate the maximum power, the speed reference is given by an MPPT strategy while setting the reactive power to zero [START_REF] Koutroulis | Design of a maximum power tracking system for wind-energyconversion applications[END_REF]. Therefore, the reference of the rotational speed control loop is adjusted so that the turbine will operate around the maximum power for the current tidal speed. In order to determine whether this reference must be either increased or decreased, it is necessary to estimate the operating point current position regarding the P MCT () curves maximum (Fig. 6).

As the rotor turbine model is well-known [START_REF] Ben Elghali | A simulation model for the evaluation of the electrical power potential harnessed by a marine current turbine[END_REF], the optimal value of the tip speed ratio λ opt , and P max = P(λ opt ) can be easily deduced. Therefore, the MCT will operate at the optimal operating point if

() opt t    (7)
which supposes that the shaft rotational speed is closed-loop controlled so as to reach its optimal value ( ) ( )

opt ref tide t V t R   (8)
Here R is the marine turbine rotor radius.

The above proposed control strategy of the DFIG-based MCT is illustrated by the block diagram in Fig. 7.

MPPT MPPT Strategy Strategy

T m V tide

 ref Resource  Vtides Cp  Vtides Cp Hydrodynamic Torque C p V tide V tide  rot T m T m  DFIG DFIG T DFIG I s,r Gearbox Grid V s  V r V r V s  rot  rot

MCT Control MCT Control

MPPT MPPT Strategy Strategy

T m V tide

 ref Resource Resource Resource  Vtides Cp  Vtides Cp Hydrodynamic Torque C p V tide V tide V tide  rot T m T m T m  DFIG DFIG DFIG DFIG T DFIG I s,r Gearbox Grid V s V s  V r V r V s  rot  rot

MCT Control MCT Control

Speed sensor Fig. 7. The proposed control structure.

IV. VALIDATION RESULTS

A. Validation Data and Parameters

In this work, the Raz de Sein site was chosen above several others listed in the European Commission report EUR16683 [START_REF]The exploitation of tidal marine currents[END_REF] due to the presence of high speed current coupled with appropriate depths suitable for marine turbine. Moreover, the marine current speed distribution for most of the time is greater than the minimum, estimated to be 1 m/sec, required for economic deployment of marine current turbine [START_REF] Myers | Simulated electrical power potential harnessed by marine current turbine arrays in the Alderney Race[END_REF].

The turbine rotor model was validated through the comparison of the simulation model with experimental data from the available literature [START_REF] Ben Elghali | A simulation model for the evaluation of the electrical power potential harnessed by a marine current turbine[END_REF], [START_REF] Bahaj | Power and thrust measurements of marine current turbines under various hydrodynamic flow conditions in a cavitation tunnel and a towing tank[END_REF]. Indeed, the simulated marine turbine corresponds to the one tested in [START_REF] Bahaj | Power and thrust measurements of marine current turbines under various hydrodynamic flow conditions in a cavitation tunnel and a towing tank[END_REF] (Fig. 8). The obtained power coefficient C p is shown in Fig. 9: For a 0.8 m model of an MCT, a current velocity V tide = 1.5 m/sec, and a pitch angle  = 25°; a C p of about 0.32 was found in comparison to 0.36 in the tank test. The adopted marine current turbine is of 1.44 m diameter and 7.5-kW.

For the given turbine geometry the power for each rotor speed and a tidal current speed is determined by the BEM hydrodynamic model. In this context, the obtained power coefficient C p and the extractable power curves are shown in Fig. 10. The 7.5-kW DFIG parameters are given in the appendix. Fig. 8. The tested marine current turbine [START_REF] Ben Elghali | A simulation model for the evaluation of the electrical power potential harnessed by a marine current turbine[END_REF].

B. Validation Results for a Filtered Resource

In this first case, the marine current turbine is simulated considering a resource first-order model (3).

The resource is assumed to be non disturbed by sea-surface effects and by other turbulences (filtered resource).

The DFIG-based MCT control performances are shown in Figs. 11 and 12 respectively illustrating the rotor speed tracking performance and the generated active power. The main merit of the DFIG is its capability to deliver constant voltage and frequency output for ±30% speed variation around conventional synchronous speed for a reduced power rotor AC/AC converter. Therefore, the rotor speed reference generated by the MPPT strategy was limited at ±30% of the conventional synchronous speed.

The obtained results show good tracking performances of the DFIG rotor speed. However, the active power exhibits some tracking errors.

The main difficulty to set the MCT multiphysic model was to include a variety of sub-models with different timescale such as the hydrodynamic loads, the turbine, the generator, etc. The built MCT model includes all the dynamic effects from very fast effects to very slow hydrodynamic loads. In our case and for efficient analysis, the tidal velocity timescale was changed from hours to seconds (a period of 24-h is represented by 100-sec). It should be noticed that the DFIG dynamic response is still faster than the hydrodynamic loads and the generated power is not affected by this change.

C. Validation Results for a Turbulent Resource

To evaluate the resource model and its impact on the generated power, three types of resources taking into account external disturbance have been simulated in different cases summarized in Table 1. In the considered cases, simulations are carried out during 90 sec. This simulation time has been chosen to evaluate the influence of swell and sea surface effect disturbances which have time constants or periods of few seconds. In these conditions, the predicted average value of the tidal speed (3) is assumed to be constant and equal to 2 m/sec (4 knots). It should be noted that the carried out simulations will also allow the evaluation of the predictability degree of the kinetic energy.

2.821

In the simulated cases, we consider several waveform cases for the tidal speed (V real ) with and without disturbances. The three taken resource values are composed by the kinetic speed, the swell disturbance, and a random signal b(t) that represents all the other disturbances. Several tidal speed references (V ref )

taken as a reference for the control block (MPPT) are considered. In particular, the DFIG-based MCT performances are evaluated when a perturbed resource is associated to a constant reference tidal speed (cases 1, 3, and 4). These cases allow the use of the tidal predicted average speed as a reference for the MPPT calculations and therefore allow the DFIG-based MCT control without a tidal speed sensor.

To take into account the swell effect, which is considered as the most disturbing one for our resource model, a Stokes model have been used. This model is a very classical first order model used to predict the swell influence in the sea water column. For given swell amplitude H, a period T, a swell length L and ground sea depth d as shown in Fig. 13, the speed potential  can be calculated for each depth z. The water speed created by the swell effect can be deduced by spatial derivation of this potential [START_REF] Wise | Wave modelling -The state of the art[END_REF]. This speed disturbance, calculated for typical intense swell specifications, is added to the predicted tidal current speed to roughly estimate how the swell can disturb the tidal current values in the turbine disk (case 2 to 5). It is also possible to add a white noise b(t) to the tidal current value which allows taking into account turbulence phenomena (case 4 and 5).

2 sin 2 2 2 tid es V grad zd ch H L t x L d T T L sh L                            (
Figure 14 shows the DFIG rotor speeds and the corresponding generated active powers for Table 1 cases. It should be noted that Table 1 also gives the predicted active power for the different cases. The obtained results prove that the adopted first order model that predicts the resource tidal speed is quite efficient as a reference for the MPPT control block and can provide up to 95% -power ratio between cases 4 and 5 in Table 1-of the extractable power (in average) even if the resource is heavily disturbed. It can be noticed that if the real tidal current speed is used as a reference for the MPPT (cases 2 and 5); it leads to high variations of DFIG-based MCT speed and power that will mechanically stress the MCT. However, if the predicted average speed is used as a reference (cases 1, 3 and 4) these phenomena are strongly reduced without significantly affecting the MCT global extracted power. This leads to the conclusion that a control strategy using the predicted tidal current average speed for the MPPT will not overload the marine current turbine even with a heavily perturbed resource.

  ; V st (V nt ) = Spring (neap) tide current speed; λ = Tip speed ratio; s, (r) = Stator (rotor) index (superscripts); d, q = Synchronous reference frame index; V (I) = Voltage (Current); P (Q) = Active (Reactive) power;  = Flux; T em (T m ) = Electromagnetic torque (Mechanical torque); R = Resistance; L (M) = Inductance (Mutual inductance);  = Total leakage coefficient,  = 1 -M 2 /L s L r ; θ r = Rotor position; ω (ω s ) = Rotor electrical speed (electrical synchronous speed); ω r = Rotor current frequency (ω r = ω s -ω);  = DFIG speed ( = ω/p); Navy Hydrographic and Oceanographic Service; BEM = Blade Element Momentum.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Raz Blanchard, Fromveur, and Raz de Sein sites in the French western coast.

Fig. 3 .

 3 Fig. 3. Tidal velocity in the Raz de Sein for the year 2007 and March 2007.

Fig. 4 .

 4 Fig. 4. Tidal turbine against an offshore wind turbine [© MCT].

Fig. 5 .

 5 Fig. 5. Schematic diagram of a DFIG-based generation system.

Fig. 6 .

 6 Fig. 6. Power curves for different tidal current speed.

Fig. 9 .

 9 Fig. 9. (a) Model [13] and (b) experimental C p curves [21].

Fig. 11 .

 11 Fig. 11. The DFIG rotor speed and its reference.

Fig. 12 .

 12 Fig. 12. The DFIG active power.

9 )T

 9 Fig. 13. Swell characteristics.

Fig. 14 .

 14 Fig. 14. The DFIG rotor speed and generated active power for a turbulent resource.

Table 1 .

 1 The simulated cases.

	Cases			Simulation Conditions / Tidal speed (m/sec)	Power (kW)
	1	V real = 2 V ref = 2								2.535
	2	V	real	 2 0.3252 cos 0.4189  	t			 0.2749 cos 0.6283	t		2.694
		V	ref	 2 0.3252 cos 0.4189  	t				 0.2749 cos 0.6283	t	
	3	V	real	 2 0.3252 cos 0.4189  		t				 0.2749 cos 0.6283	t		2.612
		V ref = 2
		V	real	 2 0.3252 cos 0.4189  	t					 0.2749 cos 0.6283	t	   b t 
	4	V ref = 2								2.691
		V	real	 2 0.3252 cos 0.4189  	t					 0.2749 cos 0.6283	t	   b t 
	5	V	real	 2 0.3252 cos 0.4189  		t				 0.2749 cos 0.6283	t	

V. CONCLUSIONS

This paper dealt with the modeling and the control of a variable speed doubly-fed induction generator based marine current turbine. An MPPT-based control strategy was proposed for variable speed control. It relies on the resource and the marine turbine models that were validated by experimental data. Tidal current data from the Raz de Sein (Brittany, France) have been used to run simulations of a 7.5-kW prototype over various flow regimes. Simulation results show that the proposed control strategy is effective in terms of speed tracking. However, the active power exhibits some tracking errors of a maximum of 10%. These results prove the need for direct active power control.

Moreover, the sensitivity of the proposed control strategy was analyzed regarding a disturbed resource including the swell effect as it is considered as the most disturbing one.

The obtained results are very encouraging. Indeed, they prove that the adopted first order model that predicts the resource tidal speed is quite efficient as a reference for the MPPT control block and can provide up to 95% of the extractable power even if the resource is heavily disturbed. Furthermore, simulation results also show that a control without a tidal current speed sensor can be used for a marine current turbine system thanks to the predictability of the resource.

APPENDIX PARAMETERS OF THE USED DFIG

R s = 0.455 Ω, L s = 0.084 H, R r = 0.62 Ω, L r = 0.081 H, M = 0.078 H, J = 0.3125 kg.m 2 , f = 6.73 10 -3 Nms -1


Ophélie Robineau
email: robineauophelie@gmail.com

Martin Châtelet
email: martin.chatelet@gmail.com

Isabelle Michel-Dounias
email: isabelle.michel@supagro.inra.fr

Christophe-Toussaint Soulard
email: soulard@supagro.inra.fr

Joshua Posner
email: jlposner@wisc.edu

Carlos Tapia

Adriana Vasquez

Tatiana Menjura

Maria Isabel Vieira

Maria Antonia Espitia

Diana Duque Marin

Maria Paula

Vereda Firita

Peña Arriba

Campo Elias

Ana Olga

WHAT ARE FARMERS' DEVELOPMENT OPTIONS IN THE CONTEXT OF PÁRAMO CONSERVATION ? A CASE STUDY FROM COLOMBIA

Keywords: páramo, integration agriculture-environment, farming system, typology, logic of land use, geo-agronomy

Although the Colombian environmental policy is unfavorable to farming in the páramo ecosystem, it is believed that an integrated management of the páramo is possible, and that farming activities can be part of the solution at this agriculture-environment problem. This issue has become particularly acute on the Rabanal páramo (western cordillera) where the projected expansion of a core protection zone will soon include major areas used for agriculture and settlements. In this paper, we show how a multi-scale approach combining agronomy, geography and management sciences, can help to identify areas of flexibility within the páramo farms for the introduction of technical and organizational changes that combine conservation and sustainable livelihoods. A study of local agrarian history and aerial photos indicates that this zone has been a dynamic part of local farming systems since at least the 1950's and that the páramo has evolved from a subsistence farming area to a zone that now includes intensive potato production conducted by external entrepreneurs. The presence of entrepreneurs is mainly due to the need of family farmers for a solution to maintain the productivity of their pastures. Through a process of interviewing participants in the debate, as well as mapping land use in the contested area, perspectives for action were developed that would result in more sustainable farming practices.

Résumé -Quelles capacités d'adaptation des agriculteurs dans un contexte de conservation d'un páramo ? Bien que les politiques environnementales colombiennes soient contre la présence d'activités agricoles sur les páramos, la présente étude considère qu'une gestion intégrée de ces écosystèmes est possible, et que les activités agricoles sont porteuses de solutions dans ce problème d'intégration agriculture-environnement. Cet enjeu est devenu central sur le páramo Rabanal (cordillère orientale), où l'expansion de l'aire protégée centrale inclura bientôt des zones cultivées majeures. Dans cet article, l'intérêt d'une approche multi-scalaire combinant agronomie, géographie et sciences de gestion, afin d'identifier des aires de flexibilité pour l'introduction de changements techniques et organisationnels dans les fermes permettant de combiner conservation et développement local durable, est mis en avant. Une étude de l'histoire agraire locale et de photographies aériennes souligne que l'évolution des activités agricoles dans cette zone fut dynamique depuis les années 50 et que le páramo a évolué d'une zone d'agriculture de subsistance à une zone où se met désormais en place une culture intensive de pomme de terre, produite par des entrepreneurs agricoles. Leur présence est principalement due à la nécessité des agriculteurs familiaux de maintenir la productivité de leurs pâturages. A travers un processus d'entretiens avec les acteurs locaux et la cartographie de l'utilisation du territoire depuis l'échelle parcellaire à celle du páramo, des perspectives d'actions pour la mise en place de pratiques plus favorables à la conservation du páramo furent identifiées.

Mots clés : páramo, intégration agriculture-environnement, systèmes de production, typologie, logique d'utilisation du territoire, géo-agronomie

PÁRAMO CONSERVATION, A CENTRAL ISSUE IN THE ANDES

Protected areas such as national parks and reserves now cover more than 12% of the world's land area (Chape et al 2003). However, as the United Nations convened Rio conference (1992) highlighted, due to the failures of strict protection policies, it is crucial that there remain close links between environment and development policy (IUCN/UNEP/WWF 1991). Nevertheless, the viewpoint "protection versus development" remains a contentious element in the analysis of protected areas (Héritier 2007). This is perhaps not surprising as integrated landscape planning, making specific choices about land use, wildlife protection, and resource development that are acceptable to communities and conservation authorities remains a difficult task (Slocombe 1993).

In this context, the Andean páramo, a unique mountain grassland ecosystem located between 3,000 and 4,200 m.a.s.l, is at a crossroads (Figure 1). While simultaneously its role in biodiversity conservation, water regulation, and carbon storage is growing more valuable (Van der Hammen 2002), there is an increasing demand for potatoes and milk production. For decades, on most of Colombian páramo, potato and dual-purpose cattle production have been the main stay of family farms. More recently, there has been an expansion of an alternative system where the land is rented for a year for capital-intensive potato production followed by several years of grazing. However, this newer system is in direct confrontation with national environmental policy. Increasingly, policy is unfavorable to farming in this ecosystem and is centered on the creation of protected areas free from productive activities.

In the tropical Andes, many researchers have focused on the ecology and on human-induced ecological effects on these high-altitude tropical grasslands [START_REF] Etter | Andean Forests and Farming Systems in part of the Eastern Cordillera (Colombia)[END_REF]Cardenas and Vargas 2008). [START_REF] Hofstede Rgm | Effects of livestock farming and recommendations for management and conservation of páramo grasslands (Colombia)[END_REF] has shown that the combination of grazing and burning reduces soil organic matter levels and increases soil bulk density, markedly reducing soil water retention capacity. More recently, other studies have been conducted to characterize the agricultural and settlement frontiers focusing on socio-economic aspects, hence providing new inputs for an integrated management of this fragile ecosystem. Lopez [START_REF] Sandoval M F | Agricultural and Settlement Frontiers in the Tropical Andes: the Páramo Belt of Northern Ecuador, 1960-1990[END_REF] working in the Ecuadorian páramo highlighted the complexity of the reasons for expanding or stagnating agricultural and settlement pioneer zones. [START_REF] Llambi Ld | Participatory Planning for Biodiversity Conservation in the High Tropical Andes: Are Farmers Interested[END_REF], working in the Venezuelan páramo, reported on the necessary implementation of a participatory planning to improve the conservation of the páramo by incorporating local communities in the land planning exercise. This implies a deep understanding of the páramo What are farmers' development options in the context of páramo conservation ? (Robineau O, Châtelet M, Michel-Dounias I, Soulard C, Posner J) users' objectives, particularly farmers', put in parallel with the environmental objectives. However, little information is yet available on that topic in Colombia.

The objective of the field research presented in this article is to provide inputs to facilitate the negotiation between local communities and environmental authorities on the Rabanal páramo. It relies on the hypothesis that to enhance a dialogue between farmers and institutions, it is necessary to consider farmers' objectives and strategies. An analysis of the local farming systems was undertaken to highlight the drivers of land use changes, and to identify options for allowing farmers to remain in the páramo while simultaneously meeting environmental objectives.

CASE STUDY OF THE RABANAL PÁRAMO IN COLOMBIA

The Rabanal páramo is a pilot study area for the Global Environmental Fund (GEF) Andean Páramo Project. This project is lead by the Consortium for the Sustainable Development of the Andean Ecoregion (CONDESAN) and in Colombia, the lead agency is the Institute Alexander von Humboldt. The Rabanal páramo is located 100 km north of Bogotá (5°24'00 N latitude, 73°36'00 W longitude). This páramo was ch osen for inclusion in the project in part because the core protected area had been expanded from 2681 to 8600 ha in 2008 [START_REF] Iavh | [END_REF], and although this legislation had been largely ignored, current emphasis on enforcement would markedly affect páramo land management in the six villages that surround the zone (Figure 2). 2) The páramo area also used for agriculture (about 560 ha, above 3,200 m.a.s.l), and home of 10% of the population. This is the focal area of the study (Figure 3).

MATERIAL AND METHODS

A three step approach combining agronomy, geography and management sciences was followed to identify areas of flexibility for the introduction of technical and organizational changes that would promote combining the conservation of páramo biodiversity and water resources with sustainable livelihoods. The core of the research was a spatio-temporal analysis of farming activities in order to understand the logic of land use: that is to say, why and how farmers use the páramo land. Spatial and temporal scales were considered, from the páramo to the cultivated parcel, and from the daily management of farming activities to the life cycle of a farm. the first step consisted of key informant interviews with the objective of understanding the forces driving land use changes on the Rabanal Páramo from the parcel scale to the whole páramo area used for agriculture. A focus was made on the agrarian history of the study area. In parallel, geographical data in the form of aerial photos (Instituto Geográfico Agustin Codazzi, 1:24,000 scale, 1955, 1985[START_REF] Heritier S | Les parcs nationaux entre conservation durable et développement local[END_REF] was gathered to identify the changes that occurred in the páramo landscape; the second step was devoted to in-depth interviews with páramo farmers to understand the range of farm diversity and dynamics. Elements of the farming systems, notably the decision-making process for potato production, herd and pasture management, and economic data, were collected using semi-structured interviews. A typology of farmers was built, based on the strategies and objectives of the páramo agricultural users; the third step was the identification of farmers' margins of maneuver for implementing more environmentally friendly farming practices. With the aid of a Global Positioning System (GPS), páramo fields were identified and mapped. A typology of parcels, based on the logic of land use, was built, which corresponds to a theoretical model of land occupation. Then propositions for modified land use that fit both the typology of parcels and farmers' strategies were developed. Of the 35 users of the contested 506 hectares of páramo, intensive interviewing was conducted with 23 (each interview lasted for 2-4 hours, most were re-interviewed up to 3 times). It took a team of two people a six month period (May to October 2009) to collect the necessary information.

RESULTS

A brief history of agricultural use of the páramo

In the 1950s, the páramo landscape of Firita Peña Arriba was a mosaic of parcels within the native páramo vegetation. As opposed to the advance of a well delineated altitudinal frontier, as Lopez Sandoval ( 2004) observed in her study in Ecuador, it does not appear to have been an altitudinal pioneer frontier but rather that over time the cultivated and grazed area within the already colonized páramo expanded (Figure 4).

The farm territories were divided into two sites, in the upper and lower part of the vereda. The cultivated area did not exceed 1 ha on the farm. Family farmers were mainly growing tubers on plots tilled by animal traction, with the help of their children who would eventually inherit the farm.

This traditional scheme was modified in the 1960s with the departure of many of the younger generation to work in the newly opened mines in the lower part of the vereda, and to the cities. Today there are 133 coal mines and 772 coke ovens in the region (IAvH 2008). As a result of this employment opportunity and migration to urban centers, nearly 40 % of the páramo land is in the hands of absentee owners and of miners. Some rent their land to the remaining family farmers and others developed low labor-intensive animal production to complement their non-farm income.

As a partial response to this loss of workforce, family farmers started to put more emphasis on dairy production and purchased Normandy dual purpose cattle in the 1970s. This activity requires less labor than potato production -the cattle are managed year round on meadows -and has progressively become the main source of income. However, the dairy cows in the herd require higher quality feed that is primarily found in new-growth pasture following potato production. This shift also coincided with increasing use of inputs (especially fertilizer) on potatoes enabling farmers to maintain, or even to increase, their tuber production despite the reduction of the area cultivated.

The maintenance of the forage quality of the pastures gradually became problematic. With now up to 5 milking cows and their progeny, and smaller and smaller areas sown to potatoes (due to the high costs of production), family farmers did not have enough higher quality pasture for their milk animals. However, in the 1980s a new solution presented itself. With the arrival of the Guatemalan potato moth (Tecia solanivora) to Colombia, agricultural entrepreneurs from the plains began looking for cooler, lower pest pressure páramo land to rent in order to produce potatoes on a large-scale. Páramo farmers started to rent-out parcels to these new arrivals as a way to solve their forage quality problems. Initially they benefited from the entrepreneurs' tractors to clear the rest of páramo vegetation on their farm to create more meadows, and then on a more regular basis, their pastures benefit from the tillage and residual effect of fertilizer inputs associated with this intensive large-scale potato production.

With the law 99 of 1993 (National decree 1713 of 2002) however, due to an increased focus on the role the Rabanal páramo played in potable and irrigation water provisioning, local environmental authorities tripled the size of the core protected area and threatened páramo farmers with fines for the land they had cleared. This strategy backfired and some farmers, afraid of losing their farm land still covered by páramo vegetation, quickly cleared them to affirm their property rights. More recently however, environmental authorities have become more determined to halt agricultural production in the expanded core area.

Family farmers: central actors to the management of the páramo

Based on the surveys and farm mapping, three general types of páramo agricultural users were identified having different short and long term objectives that generate distinct impacts on the natural resource base: 1) active páramo family farmers, 2) miners and absentee land owners, 3) entrepreneurs from the plains. It is estimated that active family farmers manage approximately 60% of the páramo due to the land they own and rent on a long-term basis from absentee owners. In addition, about 75% of the 80 to 120 ha rented-out yearly to the entrepreneurs come from the family farms. For that reason family farmers are strategic actors for the management of the páramo.

Understanding land-use systems: 1) local farming practices

Family farmers live exclusively from farming activities and have farms of about 20 ha. Based on household budgets of 10 families, we estimate that their income of about 150 U$/month, well below the Colombian minimum wage (230 US$/month, Decree 4965 of 27-12-2007).

The main economic activities of family farmers are dual-purpose cattle and potato production which contribute respectively to 60% and 25 % to their income. The remaining 15% of their income comes from marketing subsistence crops (maize, Andean tubers and broad beans) and land rental. This data suggests that the more important component of the land rental is not the income from the rent itself, but the positive impact the associated tillage and fertilizer additions have on subsequent forage productivity and quality.

4.3.1. Livestock farming system: core activity connected to the large scale potato production

Livestock production remains the primary activity for family farmers on the páramo. They generally keep 10 to 12 heads of cattle, of which 5 are milking cows, and up to two-thirds of the 20 ha of the farm are devoted to grazing each year. The stocking rate does not exceed 1 animal unit per hectare. The main constraint páramo farmers face is the difficulty of maintaining forage productivity. They find that pastures need to be renovated every 3 to 4 years. But because of the small surface that farmers can cultivate in annual crops each year, they are not able to renew the pastures as rapidly as they feel is necessary. Hence, most of the potatoes grown on farmers' land are planted by commercial potato growers, to whom 5 to 7 ha is rented out yearly. This explains how small-scale livestock production is tightly linked with the large-scale potato production, which constitutes the main activity targeted by the environmental authorities.

Almost all their plots are part of the potato-pastures crop rotation: 1 year of potato (2 cycles) followed by 3 years of pastures. The logic of this rotation is: 1) to benefit from the positive carry-over affect of the potato production on pasture productivity and, 2) to maintain a period of time between replanting potatoes on the same plot in order to lower the pressure of soilborne pests and diseases.

An intensive potato production

The land cultivated in potatoes is primarily on fields in the páramo area, rather than the lower village due to the larger fields and flatter terrain. In the lower site parcels are slopping and rocky, which makes cultivation and pasture renovation difficult: hence, a reallocation of activities to the lower part of the vereda (outside the limits of the future regional park), seems quite difficult. Currently for 100 kg of tubers planted, which corresponds to 1000 square meters, 200 kg of chemical fertilizer, 200 kg of lime and 500 kg of poultry droppings are spread. Farmers consider it is essential to bring those quantities to ensure a minimum economic yield (25T/ha on commercial fields and 20 T/ha in farmers' fields). They have a planning of systematic treatments to which they stick. Since growing potatoes is costly and risky, they do not want to take the risk of losing their production. Somewhat surprisingly, family farmers in their small potato plot (rarely more than 0.2 ha, due to the high costs of production, and near the farmstead), follow nearly the same input schedule and the same cropping calendar. As the entrepreneurs, they do the soil tilling by tractor, with a disk plow.

What are farmers' development options in the context of páramo conservation ? (Robineau O, Châtelet M, Michel-Dounias I, Soulard C, Posner J) However, the real difference between these two kinds of productions lies in the production scale: while familial potato production occurs on less than 5% of the expanded core area, up to 25% of this expanded area is cultivated annually by the entrepreneurs.

Clearly, dual-purpose cattle and potato production are two complementary sources of income and essential to balancing the family budget: while the regular sale of livestock product provides for the basic daily needs, family potato production brings more money at once and allows making investments.

4.4.

Understanding land-use systems: 2) spatial organization, from the farm to the páramo scale

All páramo parcels are engaged in potato-pastures rotation but both the rotation's duration and the surface planted in potato vary. The underlying logic of land use is related to the parcels characteristics, to the spatial organization of the farm territory and to the system of land tenure. A classification of fields was constructed based on farmers' logic of land use, to approach spatially the environmental impact of agriculture. Mapping this typology is a good basis to settle a discussion between the stakeholders, formulate specific problems to be tackled and to start a negotiation. Three kinds of parcels were identified at the farm scale (Figure 5). It was thought that this would be an important site specific input to the process of designing new practices resulting from a compromise between the current constraints encountered by the agricultural users, and the expectations from environmental authorities.

Figure 5 Example of the territorial organization of a family farm in Firita Peña Arriba

Conception and design : Robineau O, Châtelet M 1) Large (3 to 8 ha) relatively flat parcels that are used for grazing and then rented-out to the potato entrepreneurs (more than 85% of the area). From the family farmers' point of view, a key issue in reducing large-scale potato production on these parcels, would be finding alternatives to renovate their pastures. The stocking rate suggests that there are possibilities of land intensification in the livestock system; 2) Parcels around the farmsteads where the renovation of pastures follows familial potato production (10% of the area). On this relatively small area best management practices for potato production would be promoted;

3) Parcels inaccessible to tractors, where the pastures are not renovated (only 1% of the area). On these areas left as permanent grazing, a compromise could be found between ecological restorations and controlled grazing.

The three kinds of parcels were mapped at the páramo scale (Figure 6), which allows us to consider the potential role of farming practices in supplying negative environmental services, according to the pattern of these three field types at the landscape level. Then, one can give priority to actions according to specific sites identified as more sensitive (e.g. sector Quebrada Baricha, where mostly parcels of type 3 are found; a third of this site is exclusively under intensive potato production each year).

FUTURE PROSPECTS AND DISCUSSION

One can imagine three scenarios over the next decade: 1) environmental legislation is strictly enforced, which would eventually lead to the disappearance of family farming in the area since a reallocation of activities to the lower part of the vereda is not possible due to the small holding and rough terrain, 2) environmental legislation is not enforced, so the current dynamics would go on, resulting in a greater importance of absentee land owners and entrepreneurs on the páramo, since only a few new family farmers would settle there, or 3) family farming currently within the core area is supported with the objective of enhancing their incomes and improving their integration with the páramo environment. To see the third scenario happen and building a consensus about protecting the páramo, negotiation between the stakeholders, support to village organizations and research is essential.

The geographical approach to farming practices with its spatial configuration of farming activities highlights the logic of land use in a space and time perspective, evaluating the impact of farming activities on the environment according to the location of farming activities. On the other hand, it allows one to link land use to the farm level which is where these decisions are taken (Soulard 2005). The methodology used in this research provides valuable inputs to stimulate the dialogue between the different stakeholders of the páramo's management and to initiate a negotiation process. The results would enable the environmental authorities to better identify the constraints that lead farmers to adopt unsustainable practices, and thus to consider the root problem to be tackled, rather than adopt a strict position where any activity is prohibited on the páramo. Results from our case were discussed with farmers in the neighboring hamlets who generally received them positively. For an integrated management of the páramo, similar studies should be conducted in the other hamlets included in the extension of the protection area. Then multi-stakeholders platforms could be implemented with the different actors concerned by the land-use planning, both at the hamlet and at the páramo scale. Until now, farmers have not been involved in the conservation planning but rather excluded. Opening discussion, promoting farm visits with farmers and environmental authorities to exchange on conservation issues, would better a mutual understanding of each one's objectives, expectations and constraints. Then, the inclusive design of solutions that suit farmers' strategies and help them to live better from farming activities might enhance a change in farmers' behaviors.

However, for this work to be complete and having all the necessary inputs to go to the negotiation table, information is needed on the hydrology of the three micro-watershed (Quebradas) well outlined in Figure 5, as well as the distribution of key páramo plant communities. Then, with the environmental and farming systems information available, the actors (especially farmers, entrepreneurs, environmental authorities and research institutes) could be at the table to discuss a micromanagement plan for the different zones. Some of the elements of the compromise will be:

-developing improved pasture management techniques. Although it remains a hard task to improve the pasture management in an ecologically and economically wise way (Hofstede 1995), alternative methods of pasture restoration without frequent tillage and heavy fertilization would have to be identified as well as grazing management patterns that protect particularly fragile landscapes and maintain páramo biodiversity; -using less chemical inputs. There exists discrepancies between actual and recommended practices for potato cultivation (it has been estimated that farmers apply three times more nitrogen that recommended by the Secretary of Agricultural and Rural Development of Cundimamarca). Along with efforts to apply best management practices on family farmers' potato plots, hydrologic monitoring could take place to estimate the impact of farming practices on surface water quality, by comparing the three sectors of Figure 5 (Quebrada El Moral, Seca and Baricha). This would provide a basis to determine the level of contamination of the streams and the source of contamination, to target the farming practices involved, and to identify margins of progress that could be reached to lower the impact of farming activities on water quality; -setting aside key landscape areas (for biodiversity or hydrology), and establish buffer strips between pastures and potato fields and stream channels: they would protect the streams from both animal excrements and agrochemical residues and would enable keeping ecological corridors within and between the farms, thus participating in the maintenance of páramo biodiversity.

The case studied here illustrates a situation found elsewhere in the Andes, where high input potato production is taking place on the páramo, thus impacting on the maintenance of the páramo's biodiversity and on water quality. Moreover, with a warming climate, it is likely that this type of farming will be possible at higher elevations. In other Colombian and Venezuelan pilot sites of the Andean Páramo Project, the link between potato and livestock production has been observed, leading to the same environmental issues.

The Venezuelan agents of the Andean Páramo Project highlighted the importance of building a consensus among the different participants included in the establishment of a protected area [START_REF] Yerena | Building Consensus on Biological Corridors in the Venezuelan Andes[END_REF], in order to involve them in the participatory process, from the project presentation to the technical evaluation of the action plan designed. Moreover, an integration of spatial scales and disciplines, to highlight the links between farmers' strategies, environmental and agricultural policies, and biophysical and socio-economic environment, allows an improvement in the integrated management of natural resources [START_REF] Castella | Une méthode d'analyse multi-échelle des transitions agraires. Application aux zones de montagnes dans le Nord Viêt-Nam. Espace géographique[END_REF].

Figure 1

 1 Figure 1 a. small-scale farmers planting potatoes; b. spraying a potatoe field; c. watering a calf; d. entrepreneur planting potatoes; e. herbaceous páramo

Figure 2

 2 Figure 2 Localization of the hamlet Firita Peña Arriba: among the four hamlets having major cultivated areas within the future regional park

Figure 3

 3 Figure 3 Organization of the hamlet Firita Peña Arriba, Ráquira

  part of the hamlet (< 3,200 m.a.s.l) Páramo used for agriculture (3200-3400 m.a.s.l) are farmers' development options in the context of páramo conservation ?(Robineau O, Châtelet M, Michel-Dounias I, Soulard C, Posner J)

Figure 4

 4 Figure 4 Evolution of the agricultural frontier over time in Firita Peña Arriba

  Location of the farm in the veredaWhat are farmers' development options in the context of páramo conservation ?(Robineau O, Châtelet M, Michel-Dounias I, Soulard C, Posner J)

Figure 6

 6 Figure 6 Extension of the typology of parcels to the páramo scale

  with little accessibilityWhat are farmers' development options in the context of páramo conservation ?(Robineau O, Châtelet M, Michel-Dounias I, Soulard C, Posner J)

Acknowledgment

The results presented in this article have been published in the international, peer-review open access journal Mountain Research and Development (Volume 30 No 3, August 2010).

The research was conducted thanks to grants from the Proyecto Páramo Andino and from the Institut des Régions Chaudes of Montpellier SupAgro. It was supported locally by the Colombian Research Institute Alexander von Humboldt (IAvH).