Vincent Repellin

Rolland Mathieu

Guyonnet René

Keywords: Torrefaction, Grinding, Wood, Energy, Particle size distribution

The purpose of this study is to investigate the influence of torrefaction on wood grinding energy. Wood chips were torrefied at different temperatures and durations. The energy required to obtain fine powder was measured. Particle size analyses were carried out on each powder sample. It is showed that torrefaction decreases both grinding energy and particle size distribution. A criterion to compare grindability of natural and torrefied wood is proposed. It takes into account both grinding energy and particle size distribution. It accounts the energy required for grinding particles to sizes inferior to 200 μm, for given grinding conditions. Torrefaction is characterised by the anhydrous weight loss (AWL) of wood. For AWL inferior to around 8%, grinding energy decreases fast. Over 8%, grinding energy decreases at a slow rate. Particle size distribution decreases linearly as the AWL increases. Both for spruce and beech, the grinding criterion is decreased of 93% when the AWL is around 28%. K

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n n e e e r r r g g g y y y r r r e e e q q q u u u i i i r

I. Introduction

Facing fossil fuel resources decrease and green house gases emission concerns, it becomes crucial to enhance the use of biomass as a source of energy. Wood and biomass contain a lot of available energy. For example, the low heating value (LHV) of dry wood ranges between 4300 and 5400 kWh/t, depending on the species. Biomass can potentially supply 14% of the total world's energy requirement. For this aim, different ways are considered such as direct combustion, co-firing in power plants, production of biomass based motor fuel [START_REF] Demirbaş | Combustion characteristics of different biomass fuels[END_REF]. Different possibilities for making motor fuel from biomass can be developed such as agrochemical processes [START_REF] Demirbaş | Importance of biodiesel as transportation fuel[END_REF], biological processes and thermo-chemical processes [START_REF] Demirbaş | Biomass resource facilities and biomass conversion processing for fuels and chemicals[END_REF]. Among thermochemical treatments of biomass, gasification followed by Fischer-Tropsch synthesis is of particular interest [START_REF] Claudet | Produire du carburant par la transformation thermochimique de la biomasse[END_REF] and [START_REF] Demirbaş | Progress and recent trends in biofuels[END_REF]. It allows producing diesel of high quality, using ligno-cellulosic material from the entire plant. Different studies are carried out at the moment to improve the yield of the Fischer-Tropsch [START_REF] Schulz | Short history and present trends of Fischer-Tropsch synthesis[END_REF] and [START_REF] Prins | Exergetic optimisation of a production process of Fischer-Tropsch fuels from biomass[END_REF]. Such thermo-chemical treatments require preliminarily a fine grinding of wood in order to increase reaction rates and gas yield. Some authors have shown that 200 μm is the approximate size below which pyrolysis reactions are kinetically controlled [START_REF] Wei | Characteristics of fast pyrolysis of biomass in a free fall reactor[END_REF] and [START_REF] Simmons | Particle size limitations due to heat transfer in determining pyrolysis kinetics of biomass[END_REF]. Gasification reaction appears to behave similarly at the beginning so that we have kept the 200 μm specification. Nevertheless, biomass or wood has viscous-elastic and plastic behaviour. A lot of energy is thus dissipated in the material before failure. Consequently, grinding wood or biomass is very energy consuming. General ideas concerning wood grinding are that shearing, shredding and cutting are preferable to bending, compaction friction and tensile strength [START_REF] Schubert | Comminution of non brittle materials[END_REF]. Himmel et al. showed that for similar particles size reduction, energy requirement is higher with a hammer mill than with a knife mill [START_REF] Himmel | Comminution of biomass: hammer and knife mills[END_REF]. To obtain aspen wood powder with a d50 between 500 μm and 250 μm, these authors found typical net grinding energies values around 120 kWh/t. Straw required grinding energies less than 50 kWh/t. Other studies showed that higher energy consumption is necessary with attrition mill or explosion depressurisation than with knife mills [START_REF] Holtzapple | Energy requirements for the size reduction of poplar and aspen wood[END_REF]. For coarser grinding (d50 around 840 μm), energy requirements were 453 kWh/t for explosive depressurisation and 783 kWh/t for attrition mill. Fine grinding with attrition mill (d50 close to 105 μm) required 1900 kWh/t. Optimising grinding process with hammer mills, Esteban and Carrasco obtained 130 kWh/t for poplar and 170 kWh/t for pine, for a d50 approximately around 500 μm [START_REF] Esteban | Evaluation of different strategies for pulverisation of forest biomasses[END_REF]. By comparison typical coal grinding energy range between 7 kWh/t and 36 kWh/t. Other properties are also involved in the good progress of gasification or pyrolysis, in particular handling properties of wood powder, particles sizes and shapes, moisture content and energy density. Handling characteristics of wood powder is of primary importance for an easy injection in the thermo-chemical reactor. Natural ground wood particles have mainly needle shapes that confer a low flowability and a poor fluidisation behaviour to the power. Reina et al found that wood powder has a cohesive behaviour according to Geldart classification [START_REF] Reina | Predicting the minimum fluidisation velocity of polydisperse mixtures of scrap-wood particles[END_REF] and [START_REF] Geldart | Types of gas fluidization[END_REF]. Type of mill used to grind wood influences particles shape and powder flowability. The tendency to bridge is lower for powders ground in knife mills than for powders ground in hammer mils, owing to particles size and shape [START_REF] Paulrud | Particle and handling characteristics of wood fuel powder: effect of different mills[END_REF]. This bad flowability of wood powder is a drawback for conversion processing. To improve this property, new types of mills are being developed. For example, vibration mills decrease grinding energy requirements, and lead to round shaped particles [START_REF] Kobayashi | A new pulverized biomass utilization technology[END_REF].

Torrefaction is intended to modify wood or biomass properties, and to make easier its preparation for thermo-chemical treatments. In a more accurate way, torrefaction is a heat treatment of ligno-cellulosic material carried out at temperatures less than 300 °C [START_REF] Repellin | Modelling anhydrous weight loss of wood chips during torrefaction in a pilot kiln[END_REF]. From the structural point of view, wood is composed of three main structure constituents: cellulose (35-40 weight %), hemicelluloses (20-30 weight %) and lignin (20-30 weight %). It contains also 1-4 weight % of extractives compounds [START_REF] Fengel | Wood chemistry, ultrastructure, reactions[END_REF]. During torrefaction, slow pyrolysis predominates: wood is thermally decomposed at a slow rate [START_REF] Browne | Theories of the combustion of wood and its control[END_REF] and [START_REF] Degroot | First chemical events in pyrolysis of wood[END_REF]. At temperatures below 300 °C, the main products of wood decomposition are water, carbon dioxide, carbon monoxide, formic acid, acetic acid and furfural. Many studies have shown that these products correspond mainly to the hemicelluloses decomposition. Nevertheless, lignin and cellulose undergo also some depolymerisation and decomposition, in particular when the temperature goes over 250 °C [START_REF] Weiland | Analysis of controlled wood burning by combination of thermogravimetric analysis, differential scanning calorimetry and Fourier transform infrared spectroscopy[END_REF] and [START_REF] Elder | Pyrolysis of wood[END_REF].

In contrast to natural wood, torrefied wood has a brittle behaviour and a decreased strength. A lot of energy necessary for powdering wood may thus be saved [START_REF] Bergman | Torrefaction for biomass upgrading[END_REF]. Rapp et al. noticed the relationship between heat treatment severity and wood embrittlement [START_REF] Rapp | Interrelationship between the severity of heat treatments and sieve fractions after impact ball milling: a mechanical test for quality control of thermally modified wood[END_REF]. From studies concerning massive wood for decking applications, it is well known that mechanical strength of the material is decreased during heat treatment. Wood strength can be severely decreased according to the atmosphere, temperature and duration of the heat treatment [START_REF] Niemz | Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood[END_REF] and [START_REF] Repellin | Evaluation of heat treated beech by non destructive testing[END_REF]. Moreover torrefaction has other advantages: it increases the carbon content of wood as well as its energy content [START_REF] Bourgois | Characterisation and analysis of torrefied wood[END_REF] and [START_REF] Bourgois | Thermal treatment of wood: analysis of the obtain product[END_REF]. Material moisture content and hygroscopicity are decreased [START_REF] Repellin | Evaluation of heat-treated wood swelling by differential scanning calorimetry in relation to chemical composition[END_REF]. According to certain studies, torrefaction enhances wood powder flowability and fluidisation behaviour. The advantages provided by torrefaction may also be useful for cofiring, or pellets manufacturing [START_REF] Bergman | Torrefaction for biomass co-firing in existing coal-fired power stations[END_REF] and [START_REF] Bergman | Combined torrefaction and pelletisation: the TOP process[END_REF].

The purpose of this study is to investigate the influence of torrefaction on wood grinding energy. Wood chips were torrefied at different temperatures and durations. The energy required to obtain fine powder was measured. Particle size analyses were carried out on each powder sample. The influence of torrefaction temperature and duration on grinding energy and powder particle size was examined. A comparison criterion is proposed to compare the grindability of natural and torrefied wood.

II. Materials and methods

3 I II I. .1 1. . E Ex xp pe er ri im me en nt ta al l p pr ro oc ce ed du ur re e Figure 1 shows the experimental procedure followed for this work. At first, wood chips were torrefied at different temperatures and durations. Then, a pre-grinding was carried out in a first mill. Afterwards, pre-ground material was sieved. For each batch, the same sieve fraction (2-4 mm) was collected and employed for fine grinding. During fine grinding, energy measurements were done. The particles sizes of the resulting fine powder were analysed by laser diffraction. To or rr re ef fa ac ct ti io on ns s Torrefactions were carried out in a pilot kiln designed at the Ecole des Mines de Saint-Etienne, as previously described by Repellin et al. [START_REF] Repellin | Modelling anhydrous weight loss of wood chips during torrefaction in a pilot kiln[END_REF]. [Table 1] and [Table 2] show the torrefaction parameters and the corresponding anhydrous weight losses (AWL). Wood chips were of two species: spruce and beech. Each run is labelled according to three codes. The first one is the wood chips specie (S for spruce and B for beech), the second one is the torrefaction temperature and the third one is the duration. For example, a run labelled B-240-20, is a torrefaction of beech wood chips carried out at 240 °C during 20 min. In a first stage torrefaction temperature was investigated, for spruce and beech. For this first stage, torrefaction duration was kept equal to 5 min. The second stage investigated torrefaction duration until 60 min. This second stage was carried out only for beech wood, at torrefaction temperature of 220 °C, 240 °C and 260 °C. Pr re e--g gr ri in nd di in ng g Natural chips and torrefied chips of each batch were pre-ground in a knife mill Retsch SM1 equipped with an 8 mm grid. The material was collected and automatically sieved between 2 mm and 4 mm. The different sieve fractions were weighted in order to measure the influence of torrefaction on the particles size. This pre-grinding had two purposes. Firstly, it reduced the particle size so that they could be introduced in the fine grinding mill. Secondly, this procedure allowed filling the fine grinding mill always with particles of the same size (2-4 mm sieve fraction). This procedure leads to comparable energy and particle size measurements concerning fine grinding. I II I. .4 4. . F Fi in ne e g gr ri in nd di in ng g a an nd d g gr ri in nd di in ng g e en ne er rg gy y m me ea as su ur re em me en nt t For natural wood and torrefied wood, four samples of 30 g of well defined size particles were employed for fine grinding. They were finely ground in an ultra centrifugal mill (Retsch ZM 1). This type of mill can be equipped with grid of holes of different diameters. Simmons et al. and Wei et al. [START_REF] Wei | Characteristics of fast pyrolysis of biomass in a free fall reactor[END_REF] and [START_REF] Simmons | Particle size limitations due to heat transfer in determining pyrolysis kinetics of biomass[END_REF], showed that particles for thermo-chemical treatments should be inferior to 200 μm to avoid thermal transfer. Several grids were tested. We selected the grid that produced most particles less than 200 μm and the least grinding energy. The 500 μm grid was chosen for this aim. It allowed relevant observation of the influence of torrefaction on the volume fraction of particles inferior to 200 μm. The grinding mill nominal power was 400 W. The mill was fed with a vibrating feeder type Retsch D100. The feeding rate was adjusted according to the material to ensure a power of the mill motor close to its nominal value. A numerical watt meter ISW 8350 from IeS (Instruments and Systems) was employed to record the electrical power during grinding in the ZM1 mill. The power of this mill under no load conditions was measured at 280 W. However it was not subtracted to the power curve recorded during sample grinding. Thus, the total grinding energy (E) was evaluated by integration of the power curve over grinding duration. It counts both the energy required to the mill in no load conditions and the energy necessary to grind wood particles. I II I. .5 5. . P Pa ar rt ti ic cl le e s si iz ze e d de et te er rm mi in na at ti io on n Each powder sample was analysed by laser particle size analysis (Malvern Mastersizer 2000 instrument equipped with a sirocco 2000(A) cell). Disperser pressure was set to 3.5 bar, and feed rate was 60%. We define X <200μm as the volumetric percentage of particles of diameter inferior to 200 μm.

III. Results

I II II I. .1 1. . P

Pr re e--g gr ri in nd di in ng g Figure 2 shows the sieve fraction inferior to 2 mm in weight percent (WFI2), resulting from beech and spruce pre-grinding. As torrefaction temperature increases, the WFI2 increases, both for beech and spruce. The necessity to sieve and to employ the same sieve fraction in the ZM1 mill was thus confirmed. Variations of particles sizes happen in three steps. During the first step, WFI2 increases from natural spruce (18%) to torrefied spruce (S-160-5, 23%). Concerning beech, it increases in the same way from the natural state (18%) to B-180-5 (23%). The second step shows a constant value of WFI2. For Spruce, it is unchanged from S-160-5 to S-240-5. Concerning beech, the temperature range of this second step is small: from B-180-5 to B-200-5. The third step begins at temperatures from which WFI2 increases significantly. For spruce, it begins to increase for S-260-5 (26%). It represents respectively 32% for S-280-5 and 35% for S-300-5. Concerning beech, it is increased progressively from B-200-5 (24%) to B-280-5 (43%), with a rather large step present between B-200- 3 shows the energy required for grinding beech and spruce with a 500 μm grid. Fine grinding of natural wood requires a lot of energy: about one-sixth of its LHV. Natural spruce requires less grinding energy than natural beech (respectively 750 kWh/t and 850 kWh/t). Grinding energy is amazingly decreased by torrefaction. For example, it is possible to spare 90% of beech grinding energy at 280 °C. At high temperature, grinding energies of beech and spruce decrease to values inferior to 100 kWh/t. Grinding energy of spruce is decreased of 40% between natural state and S-160-5, whereas beech grinding energy is decreased only of 14% between natural state and B-180-5. When temperature increases over 200 °C for spruce and for beech; grinding energy decreases progressively. From 200 °C to 300 °C, this decrease is at a low and rather constant rate for spruce. For beech, the rate of energy decrease is high at temperatures from 180 °C to 240 °C, and rather low from 240 °C to 280 °C. 3] and [Table 4] summarise the main particle size properties: mode, average particle size and d50. For spruce wood, there is a decrease of particle sizes between natural state and S-160-5 (Table 3). Then, there is only a small decrease of particle sizes from S-160-5 to S-240-5. Above 240 °C, any temperature increase lead to a high decrease of the mode, average size particle, and d50, as it is visible both on Figure 4 and Table 3. Concerning beech, the particle size distribution is unchanged or slightly increased at 180 °C and 200 °C (Table 4). It is followed by a significant decrease visible for B-220-5. This decrease goes on as the torrefaction temperature increases (Figure 5). The two species behave differently. The effect of torrefaction on powder fineness is stronger for spruce than for beech at temperatures superior to 240 °C. 6, grinding energy of beech torrefied at 220 °C, 240 °C and 260 °C for 5 min to 60 min is presented. For each temperature, the AWL also increases during the first 20 min and is almost constant afterwards (Table 2). Correlatively, grinding energy decreases during 20 min of torrefaction. Then, it is almost constant until 60 min. Particle sizes decrease significantly for treatments of 5 and 20 min (Figure 7 and Table 4). When duration increases to 40 and 60 min particle sizes do not vary.

T Ta ab bl le e 3 3: : P Pr ro op pe er rt ti ie es s o of f p pa ar rt ti ic cl le e s si iz ze e d di is st tr ri ib bu ut ti io on n o of f spruce spruce w wo oo od d p po ow wd de er r, , o ob bt ta ai in ne ed d w wi it th h a a R Re et ts sc ch h Z ZM M1 1 u ul lt tr ra a c ce en nt tr ri if fu ug ga al l m mi il ll l e eq qu ui ip pp pe ed d w wi it th h a a 5 50 00 0 μ μm m g gr ri id d ( (A Av ve er ra ag ge e o of f 4 4 s sa am mp pl le es s, , s st ta an nd da ar rd d d de ev vi ia at ti io on n i in n b br ra ac ck ke et ts s. .) ). .

: : P Pa ar rt ti ic cl le e s si iz ze e d di is st tr ri ib bu ut ti io on n o of f f fi in ne e p po ow wd de er r o of f b be ee ec ch h w wo oo od d ( (n na at tu ur ra al l a an nd d t to or rr re ef fi ie ed d a at t d di if ff fe er re en nt t t te em mp pe er ra at tu ur re es s) ). .

IV. Discussion

These results may be explained by the physical and chemical phenomena involved in torrefaction. A mechanism in two steps is proposed. The first step is due to the transformations occurring during heat treatments at low temperature (S-160-5 and B-180-5). During this first step, no AWL happens. The second step is due to the progress of heat treatment and thermal decomposition of wood constituents, in relationship with an increase of AWL. I IV V. .1 1. . E Ef ff fe ec ct t o of f t te em mp pe er ra at tu ur re e o on n w wo oo od d g gr ri in nd di in ng g: : f fi ir rs st t s st te ep p Phenomena involved during this first step, may mainly be attributed to dehydration and lignin physical transformations. Dehydration induces a shrinking of the ligno-cellulosic material. This may create some stress in wood fibres that can favour cracks or defect creations. The structure shrinking induces porosity and density changes. During low temperature treatments, density varies in a different way depending on wood species. A previous study has shown that heat treatments at low temperature increase beech density, whereas they decrease maritime pine density [START_REF] Repellin | Optimisation des paramètres durée et température d'un traitement thermique du bois. Modification des propriétés d'usage du bois en relation avec les modifications physico-chimiques et ultrastructurales occasionnées par le traitement thermique[END_REF] Repellin V. Optimisation des paramètres durée et température d'un traitement thermique du bois. Modification des propriétés d'usage du bois en relation avec les modifications physicochimiques et ultrastructurales occasionnées par le traitement thermique. PhD Thesis Report, Ecole des Mines de saint Etienne, Saint-Etienne, France; 2006. p. 129. [START_REF] Repellin | Optimisation des paramètres durée et température d'un traitement thermique du bois. Modification des propriétés d'usage du bois en relation avec les modifications physico-chimiques et ultrastructurales occasionnées par le traitement thermique[END_REF]. Moreover, resin evaporation could occur for softwood and enhance the decrease of density [START_REF] Repellin | Optimisation des paramètres durée et température d'un traitement thermique du bois. Modification des propriétés d'usage du bois en relation avec les modifications physico-chimiques et ultrastructurales occasionnées par le traitement thermique[END_REF]. During this first step, lignin passes through its glass transition and softens. If cellulose is in a stressed state, tensions may be released by cracks or fiber/network defect creations. Moreover, after cooling lignin is in a tightened state. Tightening wood structure by low temperature heat treatments was thoroughly studied by Obataya and Tomita [START_REF] Obataya | Hygroscopicity of heat treated wood II -reversible reductions in the hygroscopicity of wood due to heating[END_REF]. In this stiff state, treated wood has a decreased plastic and viscoelastic behaviour by comparison to natural wood. In this state, a crack can propagate easily. Thus, crack creations, density decrease and material stiffening favour energy decrease and finer particles sizes. It is visible for spruce (S-160-5) on the WFI2 (Figure 2), on the grinding energy (Figure 3) and on the fine powder particles sizes. Concerning B-180-5, the trends are the same on WFI2 (Figure 2) and grinding energy, the grinding energy decrease being lower for beech than for spruce (Figure 3). However, the particle size of fine powder is not decreased, and even slightly increased (Table 4). A slight increase of density is observed for beech wood treated at low temperature [START_REF] Repellin | Optimisation des paramètres durée et température d'un traitement thermique du bois. Modification des propriétés d'usage du bois en relation avec les modifications physico-chimiques et ultrastructurales occasionnées par le traitement thermique[END_REF]. This density increase probably counterbalances the creation of cracks and material stiffening. I IV V. .2 2. . E Ef ff fe ec ct t o of f t te em mp pe er ra at tu ur re e o on n w wo oo od d g gr ri in nd di in ng g: : s se ec co on nd d s st ta ag ge e The second stage begins with thermal decomposition of wood. Thermal decomposition of wood results in a progressive and general embrittlement and degradation of wood cell walls. Consequently, the WFI2 increases (Figure 2), grinding energy (Figure 3) and fine powder particle sizes decrease, both for spruce and for beech ([Table 3] and [Table 4]). The WFI2 increases significantly between 200 °C and 220 °C for beech and between 240 °C and 260 °C for spruce (Figure 2). For fine grinding, significant decrease of particle size occurs at 260 °C for spruce and 240 °C for beech ([Table 3] and [ Table 4]). Effects of torrefaction on grinding energy, particle size distribution and WFI2 happen at lower temperature for beech than for spruce. As a result, grinding energy of beech becomes inferior to grinding energy of spruce at 220 °C (Figure 3). Chemical composition of spruce and beech could explain these differences. Wood thermal degradation begins by hemicelluloses degradation. Among hemicelluloses, xylans thermal decomposition occurs at lower temperature than other hemicelluloses [START_REF] Repellin | Modelling anhydrous weight loss of wood chips during torrefaction in a pilot kiln[END_REF], [START_REF] Degroot | First chemical events in pyrolysis of wood[END_REF], [START_REF] Weiland | Analysis of controlled wood burning by combination of thermogravimetric analysis, differential scanning calorimetry and Fourier transform infrared spectroscopy[END_REF] and [START_REF] Shimizu | Effect of thermal treatment on wood hemicelluloses IV: mechanism in early stage of xylan pyrolysis[END_REF]. Beech contains more xylanes (27.5%) than spruce (8.6%) [START_REF] Sjöström | Wood chemistry: fundamentals and applications[END_REF]. Consequently, thermal degradation occurs at lower temperatures for beech than for spruce. The temperature required to reach a given value of the grinding energy depends on the wood species. For optimising torrefaction and grinding processes, a particular care must be paid to the type of biomass employed. Fi in ne e g gr ri in nd di in ng g e en ne er rg gy y a an nd d p pa ar rt ti ic cl le e s si iz ze es s v vs s. . A AW WL L On one hand, the two species do not begin their thermal decomposition at the same temperature. On the other hand, thermal decomposition is characterised by the AWL of wood.

To observe the influence of thermal decomposition on wood grindability, grinding energies and powders average particle sizes were plotted against anhydrous weight loss. On Figure 8, it is visible that during the first step (low temperature torrefaction -no anhydrous weight loss) wood grinding energy of the two species decreases by around 300 kWh/t. This first step has thus a strong influence on wood grindability. When the AWL increases, spruce and beech grinding energy values are very close and vary similarly in function of the AWL. Grinding energy decreases rapidly when the AWL increases from 0% to 8%. Over 8%, grinding energy decreases at a slow rate. Above 8% of AWL, any decrease of grinding energy requires high loss of material, and thus high loss of the energy initially contained in wood. Regardless a total energy balance of torrefaction and fine grinding, 8% of AWL may be considered as an optimum value to reduce wood grinding energy while maintaining a good energy yield. The average particle size decreases progressively and almost linearly as the AWL increases until 30%. The rate of decrease is globally similar for spruce and for beech. Average particle sizes are higher for beech than for spruce whatever the AWL. The difference between the average particle size of beech and spruce is almost constant (Figure 9). . G Gr ri in nd da ab bi il li it ty y c cr ri it te er ri io on n a at t 2 20 00 0 μ μm m To evaluate the real impact of torrefaction on wood grindability, both grinding energy and particle size distribution must be taken into account. In this study, it is intended to compare grindability of natural wood and wood torrefied at different temperatures. For this purpose, we propose a grindability criterion inspired from Sokolowski's works [START_REF] Sokolowski | Energy consumed in comminution -a new idea of a general law of comminution -new test stands and testing results[END_REF]. This criterion (G) is given by the ratio between grinding energy (E) and the volumetric fraction of particles inferior to 200 μm (X <200μm ). G is calculated from experimental data according to the following equation:

200 E G X µm < = (1)
Some authors defined and used G to compare grinding energy required to reduce wheat to flour (particles inferior to 200 μm). G was found 'easy to evaluate and very discriminatory' [START_REF] Pujol | Description of a micromill with instrumentation for measuring grinding characteristics of wheat grain[END_REF] and [START_REF] Greffeuille | Analysis of the milling reduction of bread wheat farina: physical and biochemical characterisation[END_REF]. G is determined for given grinding conditions: type of mill, size of the grinding grid holes. It gives comparable results only if the particle size distribution of the feeding materials is well defined and identical for each material. It is useful to compare different materials. Figure 10 presents the results of G in function of the AWL. Variations are very similar to those of grinding energy. As the particle size distribution is taken into account, differences between the two species are enhanced. At low AWL, G is superior for beech than for spruce. At AWL close to 5%, G of beech is reduced 4.5 times whereas it is reduced only 3.3 times for spruce. When the AWL increases over 5%, G has the same value for beech and for spruce. For AWL around 8%, G is 4.7 times lower than G of natural wood, both for spruce (S-260-5) and beech (B-240-5). G provides an evaluation of the amount of energy that can be saved owing to torrefaction. For AWL close to 28%, G is drastically reduced by 13-14 times in comparison with natural wood (S-300-5, B-280-5). G is a sensitive parameter to compare fine grinding energy of natural and torrefied wood.

V. Conclusions

An experimental protocol was established to measure fine grinding energy of wood. Wood grinding energy is high: about one sixth of natural wood LHV. Torrefaction decreases both grinding energy and particle size distribution. A two stage mechanism was observed. The first stage involves density variations, dehydration and lignin glass transition. It happens at low temperature. It decreases grinding energy, but has low impact on particle size distribution.

The second stage matches to thermal degradation of wood. It is characterised by the anhydrous weight loss of wood. For AWL inferior to around 8%, grinding energy decreases fast. Over 8%, grinding energy decreases at a slow rate. Particle size distribution decreases linearly as the anhydrous weight loss increases. A criterion to compare grindability of different material (natural wood and wood torrefied at different temperatures and durations) is proposed. It takes into account both grinding energy and particle size. It matches to the energy required for grinding particles to sizes inferior to 200 μm, for given grinding conditions. For AWL around 8%, G is 4.7 times lower than G of natural wood. Both for spruce and beech, the grinding criterion is decreased of 93% when the AWL is around 28%.

F

  Fi ig gu ur re e 1 1: : E Ex xp pe er ri im me en nt ta al l p pr ro oc ce ed du ur re e. . I II I. .2 2. . T

I

  II II I. .3 3. . F Fi in ne e p po ow wd de er r p pa ar rt ti ic cl le e s si iz ze e d di is st tr ri ib bu ut ti io on n [Figure 4] and [Figure 5] show cumulative particle size distribution of spruce and beech fine powders. [Table

5 F

 5 Fi ig gu ur re e 5 5


Basma Khadro

Philippe Namour

François Bessueille

Didier Leonard

Nicole Jaffrezic- Renault

A novel conductometric sensor based on a PVC membrane containing nonactin for ammonium determination

Keywords: Conductometric sensor, Ammonium, Nonactin, Polyvinylchloride, Natural water analyses

This work presents a new sensor for ammonium determination based on interdigitated conductometric thin-film planar electrodes. It was fabricated by including nonactin in a plasticized PVC membrane deposited on the sensitive area of the transducer. The effects of pH, buffer concentration, the interferences of Na + , K + and Ca 2+ ions, the effect of ionic strength and of temperature were investigated. The linear working range and sensitivity of the biosensor were also determined. The sensitivity of the ammonium detection was around 6.25µS/µM NH 4 + and the detection limit was 2 × 10 -5 M. The biosensor exhibits the advantages of a simple fabrication procedure, good sensor-to-sensor reproducibility, wide dynamic range, and good storage stability (12 weeks).

Introduction

Over the last years, the measurement of ammonium (NH 4 + ) for different purposes has become increasingly important. In environmental field, eutrophication, the excessive growth of biomass as a consequence of high nutrient inputs, leads to the perturbation of the ecological balance of a water body [1]. The growth and decay of biomass may cause oxygen depletion or hypoxia, resulting in problems such as increased fish mortality. On the other hand, ammonium is a preferred micronutrient for many plants and phytoplankton and thus makes it a key water quality parameter in studies of eutrophication [2,3]. Even low concentrations of ammonium can adversely affect aquatic life, such as reduction in hatching success and growth rate as well as injury to gill tissue, liver and kidneys [START_REF]Environmental Protection Agency, Quality Criteria for Water[END_REF]. In addition, ammonium can penetrate the cell membrane under alkaline conditions. In excess, it is harmful to aquatic life by accumulating in organism and causing alteration in metabolism and body pH [START_REF] Simpson | [END_REF]. Coma and even death may occur at extremely high levels [START_REF] Libes | An introduction to Marine Biogeochemistry[END_REF]. For human health, ingestion of ammonium-contaminated food may result in corrosion of mouth lining, esophagus and stomach [START_REF]Subcommittee on Ammonia[END_REF][START_REF] Felipo | [END_REF]. For water disinfection process, measurements of ammonium are important because it competitively reacts with the disinfection agent hypochlorous acid to form less effective disinfectant [START_REF]Ammonia[END_REF][START_REF] Köster | [END_REF]. Therefore, ammonium is an essential parameter in the assessment of potable water quality and industrial process. In river water, European directives have defined the maximum allowable concentration to be in the 0.04 to 0.2 mg/l NH 4 + range, depending on the sort of fish living in the river. So, ammonium sensors which could be placed in situ are needed to perform a direct monitoring.

Due to its importance, various methods have been developed for the estimation of ammonium concentration in different samples using different detection techniques such as spectrophotometry, chromatographic techniques [11], fluorescence [12,13], colorimetry [14].

Some enzymatic or chemical reactions involve either consumption or production of charged species and, therefore, lead to a global change in the ionic composition of the tested sample that can be detected with conductometric biosensors. These sensors present a number of advantages (a) thin-film electrodes are suitable for miniaturization and large-scale production

Experimental

Reagents

Nonactin from streptomyces griseus were provided by Sigma, diisononyl phtalate (DNP approx. 80%) and Hexamethyldisilazane purum ≥98.0% (GC) were purchased from Fluka and tetrahydrofuran (THF>99%) was purchased from Sigma. Ca(NO 3 ) 2 . 4H 2 O (>99.0%, Fluka), ammonium, sodium and potassium chloride were obtained from Fluka and were dissolved in ultra pure water (18 MΩ.cm). Polyvinylchloride (PVC) and EDTA (>99.0%, Fluka) was purchased from Fluka.

Solutions

Stock solutions of NH 4 + were prepared in a 20mM phosphate buffer solution, 1.0mM EDTA, pH being adjusted by addition of HCl or NaOH. Solutions containing other ions (Ca(NO 3 ) 2 , NaCl and KCl) were obtained by serial dilution with phosphate buffer.

Sensor design

Conductometric transducers, consisting of two identical pairs of gold interdigitated thin film electrodes, were obtained from Research Institute of Microdevices (Kiev, Ukraine). Two pairs of Au interdigitated electrodes (150nm thick) were made using the lift-off process on a ceramic substrate (5×30 mm).Both the digit width and interdigital distance were 20 µm, and their length was about 1.0 mm. As a result, the "sensitive" area of each electrode was about 1 mm 2 [17].

The lift-off process used is the following one:

The deposition is thermo vacuum sputtering of chromium (5 nm) and gold (150 nm) on ceramic substrates.

1. Photoresist is positive with thickness of 3 µm with resolution of 400 lines/mm.After deposition by centrifugation the photoresist is drying under air, after is heated at 60 °C, after is heated at 120 °C.

2. Photomask is positive, photolithography is contact optical positive. After exposition under UV is development in 0.5% KOH and washing-off the photoresist from exposed area.

3. Wet etching of gold in nitro/hydrochloric acid, etching of chromium in special solution.

4. Washing-off photoresist by dimethylformamide, washing by distilled water and drying.

Measurements

With each pair of interdigitated electrodes, (sensor) conductometric measurements were performed by applying a small-amplitude alternating voltage 10mV peak-to-peak of 100 kHz generated by a low-frequency wave-form generator (Schlumberger type 4431). The differential output signal between working and reference sensor was recorded using a Stanford Research Systems lock-in amplifier SR510 and the responses of the sensor were recorded as a function of substrate concentration. At frequencies higher than 10 kHz, the impedance of diffusion can be neglected and the total impedance of the system consisted mainly of the resistance of the electrolyte solution in series with, the double layer capacitance and the polarization resistance in parallel to each other [17]. The in phase signal from both sensors were compared only in terms of the resistance contribution and its variation was followed.

Measurements were carried out in daylight at room temperature in a 5ml glass cell filled with 1.0mM phosphate buffer pH 7.5 strongly agitated. After the stabilization of the signal, the substrate was added to the phosphate buffer.

Preparation of the electrode

For the preparation of the membrane, a typical procedure was applied using PVC and DNP (7mg PVC and 3mg of DNP ) in a ratio of 7:3 (w/w), dissolved in 1mL THF, with 3% of nonactin as ionophore ; NH 4 Cl was added to the resulting solution in order to obtain identical concentrations of NH 4 + and nonactin. For the reference sensor the membrane composition was PVC and DNP (7mg PVC and 3mg of DNP) in a ratio of 7:3 (w/w) diluted in 1mL THF.

Before deposition of the PVC membranes, a surface treatment was performed; the electrode surface was treated with HMDS (hexamethyldisilazane) vapor for 20 min, in order to functionalize surface sites (hydroxyl groups). Moreover, the hydrophobic methyl groups at the surface insured a good adhesion of the PVC membrane. Then, the plasticized membranes were formed by depositing 0.2µl with a micro syringe of each of the previous solutions on the sensitive area of the sensor and the solvent was evaporated under nitrogen flow. The sensor was then dried at room temperature for 30 minutes. The electrodes were stored at 4°C in 20mM phosphate buffer (pH 7.5) containing 1.0mM EDTA.

Results and discussion

Effect of buffer concentration

To examine the effect of buffer concentration on the response of the conductometric sensor, the slopes of the ammonium sensor were measured at different buffer concentrations varying from 0.25 to 20mM. The responses for the ammonium sensor for different buffer concentration are presented in Figure 1. The sensitivity of the sensor was very low at buffer concentration higher than 1.0mM. Buffer concentration lower than 0.25mM was not investigated due to poor buffering capacity of the solution. Thus, optimum buffer concentration 1.0mM was then used for further studies.

Effect of pH

The influence of pH on the response of the ammonium sensor was studied. The effect of pH on the sensor response was determined in 1.0mM phosphate buffer at different pH from 5 to 12, Figure 2. The sensor sensitivity is higher and stable between pH 7 and 8. We then use a buffer solution at pH 7.5 throughout the experiments to obtain the highest sensitivity.

The sensitivity and selectivity of the ammonium conductometric sensor

After optimization of the working conditions of the sensor in1.0mM phosphate buffer, pH 7.5

we obtained a calibration curve as presented in Figure 3.The sensor response depends on NH + 4 concentration between 4 x 10 -4 and 35 x 10 -4 M with a slope 6.25µS/µM and r 2 = 0.99.

The detection limit in about 2x10 -5 M NH 4 + , calculated from the lower measurable variation of conductivity.

The selectivity of ammonium sensor

For the determination of ammonium in water or wastewater samples it was necessary to take into account interferences by Na + , K + and Ca 2+ ions abundantly present in such samples. The selectivity has been determined by fixing concentration of the interfering ions (0.1mM) diluted in 1.0mM phosphate buffer, pH 7.5 with varying concentrations of NH 4 + , the results are presented in Figure 4. The definition of the selectivity coefficient was extrapolated to our conductometric ammonium sensor. The value of selectivity coefficient is around 1 for the three ions. Nevertheless, it appears in Figure 4 that K + ions showed the higher interfering effect compared to that of Na + and Ca 2+ . The strong interference of potassium was probably because of the similar size between both ions, it was found that this antibiotic exhibits only a slight preference for NH 4 + over K + . The interference of calcium is probably due to the double positive charge [18, 19 and 20]. These results explained, the increase of 15% in conductivity in the calibration curve for natural water sample compared to the calibration curve for standard solution of ammonium, because of the presence of ions in the sample water, including K + and Ca 2+ (cf. Figure 5).

Effect of temperature

The effect of temperature on the sensitivity of this ammonium sensor was investigated by measurements at different temperatures varying from 5 to 60°C as presented in Figure 6. The sensor sensitivities remained almost constant between 20 and 40°C and therefore it was thought that ammonium determinations can be carried out at room temperature (20-25°C) by using this sensor. It can be seen that slopes decrease when temperature is higher than 45 °C, but it can be noticed that the slopes decreased when temperatures was (≤ 15°C), which was very important for its use on site (change of water temperature according to the season).

Reproducibility

The sensor was tested for its reproducibility by plotting the calibration curves for five different concentrations of NH 4 + (2, 4, 6, 8 and 10 x 10 -4 M) in 1.0mM phosphate buffer, pH 7.5 and repeated measurements five times for each concentration within 1 day with the same sensor (cf. Figure 7). The variation coefficient for the sensor (C.V %) was calculated as 2.5%.

The acceptable linear regression was obtained in both cases in the measurement range.

Storage stability

Long-term storage stability is one of the key factors of a sensor performance. The variation of the sensitivity of the sensor as a function of storage time, the sensor being stored in 20mM phosphate buffer (pH 7.5) at 4°C, was presented in Figure 8. During a first period an increase of the sensitivity was observed due to the membrane conditioning. Then the sensitivity of the sensor remained stable for more than 12 weeks. After 12 weeks, a sudden decrease of the sensitivity occurs due to deterioration of the polymeric membrane.

Conclusion

In this paper a new sensor for ammonium determination based on the interdigitated conductometric thin-film planar electrodes, was fabricated by including nonactin in a plasticized PVC membrane deposited on the sensitive area of the transducer. Ammonium conductometric sensor prepared by using PVC membrane had more advantages: high stability, short time of stabilization of the signal (5 min), good reproducibility 2.5%, high lifetime 12 weeks, easy for sensor construction. The linear working range and sensitivity of the sensor were also determined. The sensitivity of the ammonium detection was around 6.25µS/µM NH 4 + and the detection limit 2 × 10 -5 M, and linear range from 4 to 35 × 10 -4 M in 1.0mM phosphate buffer, pH 7.5, this sensitivity remaining constant between 20 to 40°C.

Furthermore, we investigated whether this sensor can be used for the determination of ammonium in natural water or wastewater samples. Sensitivity, µS/µM Buffer concentration (mM)

Figure captions

Figure 1 .

 1 Figure 1. Dependence of sensor response on phosphate buffer concentration. Measurements were conducted with 10 -3 M of NH 4+ in phosphate buffer pH7.5.

Figure 2 .

 2 Figure 2. Dependence of sensor response on pH. Measurements were conducted with 10 -3 M of NH 4 + in 1.0mM phosphate buffer.

Figure 3 .

 3 Figure 3. Dependence of sensor response (µS) on NH 4 + concentration. Measurements were conducted in 1.0mM phosphate buffer, pH 7.5.

Figure 4 .

 4 Figure 4. Calibration curves of the ammonium sensors in presence of 0.1mM of interfering cations Na + , K + and Ca 2+ . Measurements were conducted in 1.0mM phosphate buffer, pH 7.5.

Figure 5 . 4 +.

 54 Figure 5. Comparison of the calibration curve of sensor response for the standard ammonium solution to the calibration curve for natural water samples, with different concentrations of NH 4 + . Measurements were conducted in 1.0mM phosphate buffer, pH 7.5.

Figure 6 .

 6 Figure 6. Dependence of sensor response on temperature. Measurements were conducted with 10 -3 M of NH 4 + in 1.0mM phosphate buffer, pH 7.5.

Figure 7 .

 7 Figure 7. Calibration curve of the sensor response for increasing NH 4 + concentration in natural water (study of the response conductivity). Measurements were conducted in 1.0mM phosphate buffer, pH 7.5.

Figure 8 .

 8 Figure 8. Study of the sensor stability. Measurements were conducted in 1.0mM phosphate buffer, pH 7.5 for 10 -3 M NH 4 + concentration.

  Figure 2

Acknowledgements

This work was financially supported by CEMAGREF of Lyon.


Keywords: anode, Nickel, single chamber, SOFC, solid oxide fuel cells, catalytic study, C 3 H 8 oxidation, Ce 0.9 Gd 0.1 O1.95, oxygen, propane, electrical conductivity measurements

Single chamber solid oxide fuel cells (SCFC) are an alternative concept to traditional SOFC's. Conventional anodes, which consist of a cermet of metallic Ni and ionic conductor, have to work under a mixture of hydrocarbon and oxygen. This paper presents a catalytic study of C 3 H 8 oxidation by Ce 0.9 Gd 0.1 O 1.95 and nickel. The influence of temperature and oxygen to propane ratio on the nickel oxidation state are clarified thanks to catalytic tests and electrical conductivity measurements. The results show that the temperature must be higher than a critical value in order to maintain the nickel particles in the metallic state. Furthermore, at high temperature, the conversion of the initial mixture leads to the water-gas-shift equilibrium. K Ke ey yw

I. Introduction

Fifteen years ago, Hibino [1] has shown the feasibility of a fuel cell consisting of only one chamber, with a mixture of reducing and oxidizing gases (generally hydrocarbon and oxygen). This concept allows avoiding the sealing procedure of traditional fuel cell, and a better control of gases. Single Chamber Fuel Cells (SCFC's) have quickly shown good results, close to traditional SOFC's power densities [2][3][4][5][6][7]. Some research groups nevertheless reported some drawbacks such as voltage oscillation and nickel-based anode instability [8]. They attributed these oscillations to oxidation/reduction cycles of the metal. However, to our knowledge, very few studies have been devoted to the anode properties in the working conditions of SCFC's. Traditional anodes consist of a cermet, based on a solid ionic conductor (for example GDC: Ce 0.9 Gd 0.1 O 1.95 ) mixed with an electronic conductor, such as metallic nickel. They are generally used in hydrogen, which is easily dissociated in presence of metallic nickel. Because of the problems involved by the direct use of H 2 , an alternative way is to use the catalytic partial oxidation of hydrocarbons, leading to a mixture of CO and H 2 . An example is given by the partial oxidation of propane [1] (ΔH° (850K) = -206 kJ.mol -1 ):

3 C H O 3CO 4H 3 8 2 2 2 + → + (1)
Nickel is known to have a good catalytic activity for this reaction [9] ; the exothermicity of the reaction helps to maintain the temperature of the cell, and the oxygen partial pressure limits the carbon deposition. But, using propane with oxygen may induce an undesirable reaction (2) (ΔH° (850K) = -2043 kJ.mol -1 ) which consists of the direct total oxidation:

2 C H 5 O 3CO 4H O 3 8 2 2 + → + (2)
The conditions of stability of metallic nickel versus its oxidation into NiO, and competition effects between reactions (1) and ( 2) have to be clarified for achieving a good SCFC efficiency.

In this paper, we present first the catalytic activity and stability of GDC and Ni powders in presence of propane and oxygen mixtures. Then, the variation of the electrical conductivity of Ni-GDC anodes is reported as a function of temperature in presence of a mixture of propane and oxygen. The results are interpreted on the basis of the oxidation state of nickel together with its catalytic properties.

II. Experimental

I II I. .1 1. . S Sa am mp pl le es s GDC (Ce 0.9 Gd 0.1 O 1.95 ) was supplied by Fuel Cell Materials TM (Nextech TM division). Its specific surface area, measured using BET method with nitrogen (Micromeritics ASAP 2000) was 8m².g -1 . The average particle size corresponding to this area surface is 0.1µm, assuming spherical particles of a same size. The powder was then characterized by laser granulometry (Malvern Mastersizer 2000), as presented in Figure 1. The particle size distribution of GDC powder as supplied showed three maxima at 1, 5 and 110 µm. A sieving procedure at 100µm was able to eliminate the largest agglomerates. NiO was supplied by Sigma-Aldrich TM . Its BET specific surface area was equal to 5 m².g -1 , corresponding to a particle size of 0.13µm, which is very close to that of GDC. Laser Granulometry analyses see Figure 1 revealed a very broad particule size distribution with various maxima, the most important being at 4 and 500 µm. To lower the NiO average particule size, a grinding procedure has been applied using a planetary ball mill (stainless steel balls) at 180 rpm for 30 min. XPS analyses after treatment did not show any pollution of the surface due to grinding. Laser granulometry analyses show that all the agglomerates have been broken into aggregates smaller than 100 µm. Sieved GDC or ground and sieved NiO will be refered as "treated" powders in the following. I II I. .2 2. . C Ca at ta al ly yt ti ic c a ac ct ti iv vi it ty y The treated powders were mixed with alpha-alumina powder from Alpha-Aesar TM to obtain samples of 10 (or 20%wt) of NiO or GDC. Alpha-alumina was used in order to minimize over temperature effects due to the exothermicity of reactions (1) and (2). This powder was also inert with respect to the gaseous mixtures. The catalytic activity was measured in a catalytic reactor equipped with a tubular 10 mm diameter quartz tube. Quartz wool was placed in the middle of the tube to support the sample, allowing a fixed bed reactor. A thermocouple protected by an alumina tube to avoid catalytic artefacts was fixed near the powder. The tube was introduced inside a vertical furnace whose temperature was regulated by a thermocouple placed near to the sample bed. A condenser at the outlet of the tube allowed to trap water before gas analyses. The composition of the gaseous mixture was then determined with a micro-gas chromatograph coupled with a mass spectrometer (SRA instruments™). The different partial pressures of the system were calculated using calibrated gases. It allows to determine partials pressures of around 10 -2 atm with a precision of ± 10 -4 atm. Prior to the catalytic activity measurements, the GDC / α-alumina mixtures were heated at 580°C and maintained during 1h at this temperature in order to eliminate adsorbed species from the surface. In case of NiO / α-alumina mixtures, a reduction step treatment in hydrogen (2% H 2 in Ar at 10 l.h -1 ) was first done at 580°C during 3 hours in order to transform NiO particles into metallic nickel. Then, once the desired temperature was established under flowing argon. O 2 / C 3 H 8 / Ar was introduced in the reactor ( for example 0.6 / 0.4 / 9 l.h -1 respectively) and the catalytic activity was measured after a stabilization period of one hour, during which the reactor reaches a constant value after an initial increase due to the exothermicity of the reactions. Temperature tests were done by decreasing temperature after the stabilization time at a fixed ratio, with a constant ramping rate of 1°C.min -1 . Gas analyses were done every ten minutes, corresponding to a measure every ten degrees. In all the experiments, the amount of propane and oxygen never exceeded 10% of the gaseous phase for two reasons. First, it ensured that whatever the ratio O 2 / C 3 H 8 could be, the reactor gaseous atmosphere was far from exploding conditions ; second at low partial pressures, the volume expansion of the gas due to the reactions is low and thereby the errors of outlet quantitative analysis. The various O 2 : C 3 H 8 volumic ratios were comprised between 1:9 and 9:1 in argon and the total flow rate was fixed to 10 l.h -1 , for all experiments. In the following the notation x / 10-x will be used for the oxygen to propane ratio, which means x % of oxygen and (10-x) % of propane in the gaseous mixture. I II I. .3 3. . E El le ec ct tr ri ic ca al l c co on nd du uc ct ti iv vi it ty y Four points measurements of the electrical conductivity of Ni -GDC cermet supported on GDC pellets were done in order to follow the oxidation state of nickel as a function of temperature in the O 2 / C 3 H 8 / Ar mixture. First, a GDC support was prepared from the treated powder by sintering at 1200°C for 6 hours, then at 1350°C for 4 hours a pellet previously pressed at 100MPa. The initial and final diameter of the pellet was 22mm and 19mm respectively; the thickness of the sintered pellet was 1.5mm. The Ni-GDC cermet was deposited onto the GDC sintered pellet using screenprinting technology. First, treated powders were mixed using 60%wt of NiO and 40%wt of GDC, then a commercial binder and a solvent (supplied by ESL Europe) were added. The final ink was shaken using an UltraTurrax™ for 20 min. After deposition due to an Aurel TM device, we obtained a homogeneous layer whose thickness was 25µm. Four platinum contacts located on the edges of the cermet, allowed to monitor the electrical conductivity according to the Van der Pauw method, with a current source of I= ±50mA (Keithley Sourcemeter 2600C). The sample was placed in a tubular furnace equipped with a quartz tube of 3 cm diameter. The NiO-GDC as prepared cermet anode was treated with the same conditions as for the catalytic powder reactor in order to achieve the reduction of NiO into metallic Ni. Then the electrical conductivity of the sample was measured for various temperatures under a flowing 6 / 4 ratio at 20 l.h -1 . X-ray diffraction (Siemens D5000) has been used to characterize the samples after the catalytic activity and electrical measurements. The X-ray emission has been done with a copper anticathode. The detector is a scintillator with a graphite monochromator.

III. Results and discussions

I II II I. .1 1. . C
Ca at ta al ly yt ti ic c a ac ct ti iv vi it ty y III. 1

.1. GDC results

The results concerning GDC catalytic activity at 590°C (sample mass: 15 mg of GDC mixed with 85mg of α-Al 2 O 3 ) are presented in Figure 2 as a function of the O 2 / C 3 H 8 ratio. It can be seen that the maximum catalytic activity is reached at 6-7% of O 2 (and respectively 4 and 3% of C 3 H 8 ), leading to a high production of carbon dioxide. This maximum suggests that the major reaction taking place in this case is the total oxidation of propane, producing CO 2 and H 2 O according to reaction (2). Effectively, the stoechiometric proportions of this reaction would correspond to an O 2 / C 3 H 8 ratio equal to 8.33 / 1.67 which is near to the experimental observed ratio. In addition to this reaction, it can be observed a small quantity of CO and H 2 corresponding to the propane partial oxidation, according to reaction (1). The maximum amount for these gases is observed around 6 / 4 oxygen to propane ratio, which is in agreement with the stoechiometric proportions of reaction (1). Finally, some C 2 H 4 and CH 4 could be observed, especially at low O 2 / C 3 H 8 ratios. Both products were also identified during a blank test (without any powder in the reactor) showing that propane cracking takes place in the gas phase, according to reaction (3).

C 3 H 8 → CH 4 + C 2 H 4 (3)
CH 4 was not detected for most of the ratios, probably because it is readily oxidized.

III.1.2. Ni/ NiO results

As for GDC, the reduced nickel samples have been exposed to various O 2 /C 3 H 8 ratios at 580°C. Figure 4 shows the results of catalytic activity measurements obtained as a function of O 2 / C 3 H 8 ratio. The produced gases correspond to the reactions (1) and ( 2). For mixtures rich in propane, namely from 1/9 to 4/6 ratios, carbon dioxide is the major product. The amount of carbon monoxide and hydrogen increase when the O 2 /C 3 H 8 ratio increases. The maximum observed in CO and H 2 filled in the range 6/4 -7/3. The O 2 conversion was close to 100%. For this last ratio, for such high conversions, over-temperatures of 15°C could be observed, consequently to the strong exothermic reactions taking place. For ratios of 8/2 and 9/1, the catalytic activity decreased drastically, leading to practically carbon dioxide production. After measurement of the catalytic activity at 580°C for 2 hours, (with 6/4, 7/3, 8/2 and 9/1 oxygen to propane ratio), the samples were analysed by X-Ray diffraction. Figure 5 shows the various diffraction patterns in which NiO peak at 63° can be easily notices for the highest O 2 / C 3 H 8 ratio (9/1), and suspected for the ratio equal to 8/2. This means that in oxygen rich mixtures, metallic nickel is not stable and is oxidized into NiO. This corresponds to the drop of the catalytic activity, especially in CO and H 2 (see Figure 4). As for GDC, temperature experiments have been done at various oxygen to propane ratios. :6 shows the outlet gas composition as a function of temperature. The curves can be decomposed into three parts: first, below 535°C, only the total oxidation was took place leading to carbon dioxide; between 535°C and 565°C, catalyst started to promote the partial oxidation, forming CO and H 2 . Finally, above 565°C, all the oxygen was consumed, and the composition did not vary too much with temperature. The reproducibility is very good for low temperatures below 535°C. Above this temperature, it is still acceptable. Such a behavior can be explained by the ability of nickel to promote the water gas shift reaction (3):

CO + H 2 O CO 2 + H 2 (3)
If this equilibrium occurs and reach an equilibrium state at high temperature, the values of K exp defined as: In our experiments, the amount of water was unknown, but from the mass balance equation upon oxygen, it could be estimated and the values of K exp could be calculated at each temperature. Figure 7 displays the results obtained for K exp compared to the thermodynamic prediction. It can be seen that above 570°C, the values deduced from gas analyses and mass balance are very close to those of the equilibrium constant, and follow a similar variation versus temperature, whereas it is not the case at lower temperatures. Consequently, taking into account the imprecision on the estimation of the water amount, it appears that the water gas shift equilibrium controls the gas composition above 570°C. co on ns si id de er ri in ng g w wa at te er r g ga as s--s sh hi if ft t r re ea ac ct ti io on n. .

At lower temperatures than 570°C, the gas composition was found to change significantly with temperature. Figure 8 which represents the Arrhenius plot for carbon dioxide production, indicates the existence of two distinct values of the apparent activation energy (70 and 140 kJ.mol -1 ) on both sides of a limiting temperature of 520°C which is approximately the temperature at which began the production of CO and H 2 according to reaction (2) (partial oxidation). Since CO 2 can only be produced by reaction (1) (total oxidation), a change in the catalyst surface must be suspected. The oxidation of nickel has already been mentioned for a temperature equal to 580°C and oxygen rich O 2 / C 3 H 8 ratios. Here the O 2 / C 3 H 8 ratio was fixed equal to 6/4, so the oxidation of nickel may be expected at a different temperature. Since XRD is not able to detect very small amounts of phases, or superficial oxidation in case of very thin layers, electrical measurements were performed on a Ni-GDC cermet. I II II I. .2 2. . E El le ec ct tr ri ic ca al l m me ea as su ur re em me en nt ts s A 25µm thickness Ni-GDC anode, supported on GDC has been placed under a O 2 /C 3 H 8 ratio of 6/4, in order to follow the electrical conductivity change with temperature. Figure 9 shows the total cermet conductivity and temperature, as a function of time. At the beginning of the experiment, the temperature was equal to 580°C, and metallic nickel was present due to the H 2 reduction treatment: value of the conductivity was as high as 800 S.cm -1 . The O 2 / C 3 H 8 mixture was introduced, the conductivity decrease during a short period, probably due to the time necessary to reach a homogeneous mixture, and then returned to a high value of 900 S.cm -1 . Temperature was set up to 590°C and then decreased stepwise down to 520°C. The conductivity remained at a high value except when the temperature was changed from 530 to 520°C where a sharp decrease was monitored. When the temperature decreases, the conductivity is quiet stable except some small oscillations. They are probably due to a carbon deposition to the anode (black appearance of the cermet after measurements), which is a good electronic conductor. Such a behaviour can be explained by the oxidation of nickel into NiO. The limiting temperature corresponds to that of CO and H 2 production and to the different slopes in the Arrhenius plot for CO 2 production. As a consequence, as far as Ni is maintained in the metallic state, CO and H 2 are produced according to reaction (2). Below a limiting temperature, which depends of the O 2 / C 3 H 8 ratio (cf Figure 5), Ni-GDC cermet is not suitable for SCFC's operation. I II II I. .3 3. . T Th he er rm mo od dy yn na am mi ic c c ca al lc cu ul la at ti io on ns s In order to better understand the formation of NiO below the limiting temperature, we have looked to the thermodynamics of the Ni / NiO system in a gaseous atmosphere containing oxidising (O 2 , H 2 O, CO 2 ) and reducing (C 3 H 8 , CO, H 2 ) gases. Five independent reactions may be proposed:

H 2 + NiO → Ni + H 2 O (5) CO + NiO → Ni + CO 2 (6) 3 8 2 1 4 C H NiO 6CO H Ni 3 3 + → + + (7) 2 1 Ni O NiO 2 + → (8) 3 8 2 2 1 3 C H NiO Ni CO H O 10 10 5 + → + + 2 (9)
For each of these reactions, the reaction Gibbs energy has been calculated considering the composition of the real gas very close to 530°C, namely 535°C. As previously done, the amount of water was estimated according to the mass balance equation for oxygen. Hence, ΔG°(T) could be obtained according to the general equation ( 10):

( ) ( ) ( ) ln ( tan ) i j r P products G T G T RT P reac ts ν ν ⎛ ⎞ Δ =Δ °+ ⎜ ⎜ ⎝ ⎠ ∏ ∏ ⎟ ⎟ (10)
where ΔG°(T) is the standard free enthalpy, R the ideal gas constant, T the temperature, P the partial pressures of the gaseous products and reactants with ν i and ν j the stoechiometric coefficients, respectively. The results are presented in Figure 10 which shows the variations of Δ r G°(T) as a function of temperature. Considering minimum Δ r G°(T) Gibbs Energy, two reactions are expected to control the chemical state of nickel: the oxidation of Ni by oxygen (8) and the reduction of NiO by propane (7) It can be seen that below 520°C (as indicated by the arrow), reaction (8) is the most favored reaction, whereas above 520°C, reaction (7) is the most favored. This is in very good agreement with the observed phenomenon since metallic Ni was found to oxidize at temperatures lower than 530°C.

IV. Conclusions

To better understand the behaviour of a Ni-GDC cermet anode working in a single chamber fuel cell under O 2 / C 3 H 8 mixtures, the catalytic properties of GDC and nickel powders were studied separately as a function of the oxygen to propane ratio and of temperature.

The GDC powder was found to catalyze the total oxidation of propane [2] in the range 400-600°C. However, even at high temperatures, the catalytic activity was not very high compared to that of nickel in similar conditions. Nickel was found to catalyze also partial oxidation of propane [1]. The maximal amounts in CO and H 2 were obtained for O 2 / C 3 H 8 ratios close to the stoechiometric proportions. The catalytic activity versus temperature showed three domains which temperature limits way depend on the O 2 / C 3 H 8 ratio. For a ratio equal to 6/4: above 565°C, the water gas shift equilibrium controls the gaseous composition, between 535°C and 565°C, simultaneous total and partial oxidation occurred, below 535°C, only total oxidation took place due to oxidation of nickel into NiO in agreement with thermodynamic prediction. This study not only clarifies the nature of the reactions involved in the SCFC's operated with O 2 / C 3 H 8 , but also points out the existence of a limiting temperature. Above this limiting temperature, metallic nickel is stable and the anode is able to provide CO and H 2 at sufficient rates. However, the temperature must not exceed too high values since once the water gas shift equilibrium is reached, the gas composition does not vary too much. Studying the catalytic activity of nickel for CO and H 2 production allows to determine the best compromise with respect to temperature.

F

  Fi ig gu ur re e 1

  to those deduced from thermodynamic data (K ref ).

  om mp pa ar ri is so on n b be et tw we ee en n t th he er rm mo od dy yn na am mi ic c c co on ns st ta an nt t K K o ob bt ta ai in ne ed d b by y c ca al lc cu ul la at ti io on n( (⎯ ⎯) ), , a an nd d t th he e a ap pp pa ar re en nt t c co on ns st ta an nt t o ob bt ta ai in ne ed d b by y e ex xp pe er ri im me en nt ta at ti io on n( (• •) ), , c

2 E

 2 app = 70 kJ.mol -1 E app = 140 kJ.mol-1

F

  Fi ig gu ur re e 1 10 0: : T Th he er rm mo od dy yn na am mi ic c c co on ns si id de er ra at ti io on ns s : : f fr re ee e e en nt th ha al lp py y v va ar ri ia at ti io on n a as s a a f fu un nc ct ti io on n o of f t te em mp pe er ra at tu ur re e f fo or r t th he e d di if ff fe er re en nt t r re ea ac ct ti io on ns s i in nv vo ol lv vi in ng g n ni ic ck ke el l

  Figure3presents the catalytic activity of GDC as a function of temperature obtained for a 6 / 4 oxygen to propane ratio. It can be seen that the catalytic activity increases with the temperature increase. The maximal conversion for propane and oxygen was 9 and 22 % respectively, the major reaction product being CO 2 for all temperatures. Arrhenius plot deduced from the CO 2 catalytic production leads to a value of the apparent activation energy equal to 75 kJ.mol -1 . Other temperature tests have shown good reproducibility, by starting measurements at low temperature, and applying a increasing ramping rate of 1°C.min -1 .

		0.3				10
						9
	Catalytic activity (mol.g -1 .h ) -1	0.1 0.15 0.2 0.25	CO2 CO 2 CO CO H2 H 2 C2H4 C 2 H 4 Série1 propane conv.			8 3 4 5 6 7	Porpane conversion (%)
		0.05				2
						1
		0				0
		400	450	500	550	600
			Temperature (°C)
	F Fi ig gu ur re e 3 3: : C Ca at ta al ly yt ti ic c a ac ct ti iv vi it ty y o of f G GD DC C ( (1 15 5 m mg g; ; 1 10 0% %w wt t i in n a al lu um mi in na a) ) a as s a a f fu un nc ct ti io on n o of f t te em mp pe er ra at tu ur re e, , w wi it th h a an n O O2 2 / / C C3 3H H8 8
	r ra at ti io o e eq qu ua al l t to o 6 6/ /4 4. .
			r(CO2) CO 2
			r(CO) CO
			r(H2) H 2
			r(C2H4) C 2 H 4
			Série5 propan
			e Conv.
	F Fi ig gu ur re e 2 2: : C Ca at ta al ly yt ti ic c a ac ct ti iv vi it ty y o of f G GD DC C ( (1 15 5 m mg g; ; 1 10 0% %w wt t i in n a al lu um mi in na a) ) a at t 5 59 90 0°°C C, , a as s a a f fu un nc ct ti io on n o of f t th he e O O2 2/ /C C3 3H H8 8 r ra at ti io o. .

  The catalytic activity of O 2 and C 3 H 8 is much higher with Ni than with GDC, especially at high temperature. Due to this comparative study, it appears that the contribution of GDC to the catalytic activity of Ni-GDC cermets for the conversion of O 2 and C 3 H 8 mixtures into CO 2 is practically negligible.

	1.8								100
	1.6	Série1 CO 2						90
	-1 )	Série2 CO						80
	Catalytic activity (mol.g -1 .h	Série3 Série4 H 2 C 2 H 4 Série5 propane conv.					20 30 40 50 60 70	Propane conversion (%)
									10
									0
	1/9	2/8	3/7	4/6	5/5	6/4	7/3	8/2	9/1
				Oxygen to propane ratio
	F Fi ig gu ur re e 4 4: : C Ca at ta al ly yt ti ic c a ac ct ti iv vi it ty y o of f N Ni i ( (8 8 m mg g; ; 1 10 0% %w wt t i in n a al lu um mi in na a) ) a at t 5 58 80 0°°C C, , a as s a a f fu un nc ct ti io on n o of f t th he e O O2 2/ /C C3 3H H8 8 r ra at ti io o. .


Jean-François Clément

Véronique Zehnlé

Jean Claude Garreau

Pascal Szriftgiser
email: pascal.szriftgiser@univ-lille1.fr

Atomic Dipole Traps with Amplified Spontaneous Emission: A Proposal

Keywords: 37.10.Gh Atom traps and guides Amplified spontaneous emission, cold atoms, optical traps, heating rate

We propose what we believe to be a novel type of optical source for ultracold atomic Far Off-Resonance optical-dipole Traps (FORTs). The source is based on an Erbium Amplified Spontaneous Emission (ASE) source that seeds a high power Erbium Doped Fiber Amplifier (EDFA). The main interest of this source is its very low coherence length, thus allowing an incoherent superposition of several trapping beams without any optical interference. The behavior of the superimposed beams is then a scalar sum greatly simplifying complex configurations. As an illustration, we report an estimation of the intensity noise of this source and an estimation of the atomic excess heating rate for an evaporative cooling experiment application. They are both found to be suitable for cold atoms experiments.

Introduction

Far off-resonance optical-dipole traps (FORTs) are at the heart of a constantly increasing number of cold atom experiments [START_REF] Miller | Far-off-resonance optical trapping of atoms[END_REF][START_REF] Adams | Evaporative Cooling in a Crossed Dipole Trap[END_REF][START_REF] Grimm | Optical dipole traps for neutral atoms[END_REF]. They are widely used as a convenient way to manipulate cold atoms or molecules while minimizing decoherence processes such as spontaneous emission. As a very incomplete list of examples, this includes: atomic interferometers, evaporative cooling to produce Bose-Einstein Condensates (BECs), time dynamics and momentum exchange experiments, [START_REF] Kasevich | Atomic interferometry using stimulated Raman transitions[END_REF][START_REF] Barrett | All-Optical Formation of an Atomic Bose-Einstein Condensate[END_REF][START_REF] Clément | All-optical runaway evaporation to Bose-Einstein condensation[END_REF][START_REF] Chabé | Experimental Observation of the Anderson Metal-Insulator Transition with Atomic Matter Waves[END_REF][START_REF] Lignier | Reversible Destruction of Dynamical Localization[END_REF]… In the present paper we focus on the possibility of improving the all-optical BEC evaporative cooling setup with trapping beams in the optical telecommunication window, around 1550 nm. Indeed, roughly two general categories of experiments can be made. Firstly, the user needs to generate complex potentials such as a grating in one, two or three dimensions. In this case, the laser that generates the potential must have a high coherence level to preserve a good interference contrast at the crossing of the beams. This is generally not a problem. Nowadays lasers currently have linewidths of the order of the MHz, or below. These linewidths corresponds to a coherence length of a few hundreds of meters or better; a value that will be enough, by far, for most applications. On the other hand, one might be interested by the potential resulting from of the scalar sum of several laser beam intensity. The interferometric feature of light is then unwanted. This is generally the case in the preliminary phases of the experiment where the atomic cloud needs to be hold for an additional preparation procedure as, e.g., a transfer from one trap to another, or for a further cooling stage. The question is then how to get rid of light interferences? The first idea is to use as many laser with different frequencies as the number of the required beams. If the frequency difference between each laser is greater than the characteristic frequencies of the atomic system, their beat note will not affect this system. However, this solution is both complex and expensive compared to a single laser source realization. The second possibility is to use a single laser source, and to cross at right angle the polarizations of its multiple beams in the overlapping area. If this is feasible, this solution has several drawbacks. The number of available beams is typically limited to two or three, but especially, it is very difficult to check that the polarizations have the required orientation in an ultra-high vacuum cell. This is even more difficult, if not impossible, with high numerical aperture optics and short Rayleigh lengths used, for instance, in single atom experiments [START_REF] Sortais | Diffraction-limited optics for single-atom manipulation[END_REF].

The simplest solution is probably to employ a spectrally broad light source with a very short coherence length, so that a small difference in the optical paths will be enough to insure a complete loss of coherence. This solution is especially attractive since there is a very intense activity in the field of both fiber lasers and amplifiers. These devices are widely available with all the advantages of fiber devices such as: low cost, low weight, high compactness, insensitivity to acoustic noise, easy beam delivery… Furthermore, they can provide very high power with a continuous operating mode. For instance, up to 100 W are presently available with Erbium Doped Fiber Amplifiers (EDFA) in the optical telecommunication C-band (from ~1530 to ~1570 nm). The C-band is of particular interest. Indeed, an optical-dipole trap to achieve Bose-Einstein condensation has recently been demonstrated with an Erbium doped fiber laser [START_REF] Clément | All-optical runaway evaporation to Bose-Einstein condensation[END_REF]. This recent work showed demonstrated the feasibility of an efficient production of a 87 Rb BEC in a 1565 nm optical-dipole trap. In this work, tThe loading stage of the experimental sequence needs a two-beam configuration. The with two beams crossing each other at roughly right angle, and one of them is frequency shifted with respect to the other one with by an acousto-optical modulator.

However, this configuration is not optimal. In fact, the key point of the loading efficiency is based on the common trap volume of the beams which could be improved if these two beams were aligned. With an ASE-based FORT, one could propose a two-beam geometry based on a single retro-reflected beam without any acousto-optical modulator. A first wide beam is then dedicated to the loading process, and can be re-circulated to provide a tightly confining beam confinement, making possible an efficient evaporative cooling sequence. The ASE-based FORT is notably very well suited for this multi-beam configuration without paying attention to interference effects and optical power losses, i.e. using wave plates for each beam or frequency shifting one of them. It would thus allows to create FORTs for almost all the alkali-metal atoms used in cold-atoms experiments (Li, Na, K, Rb and Cs) with only one powerful laser source, this with a high efficient loading rate and an improved evaporation stage for many different beam-geometries: dimple, Misaligned Crossed-Beam FORT [START_REF] Clément | All-optical runaway evaporation to Bose-Einstein condensation[END_REF][START_REF] Weber | Bose-Einstein Condensation of Cesium[END_REF]

…
In the C-band, both fiber lasers and amplifiers are available. High power fiber lasers have spectrally broad characteristics capabilities. Typically, when they are not optimized to be spectrally narrow, their width ranges from a few tens of GHz to a few hundred [START_REF] Miniscalco | Erbium-doped glasses for fiber amplifiers at 1500 nm[END_REF]. At first sight, they could be good candidates for our present requirements. Nevertheless, they are highly multimode. Their spectrum is composed by a multitude of longitudinal modes with a frequency spacing determined by their free spectral range. Individually, each of these modes is spectrally narrow (~1 MHz). These individual modes have a very long coherence length. Although very attractive, this kind of lasers cannot be used as an incoherent source. To both overcome this difficulty and take advantage of the optical telecommunication technology, we propose to construct a spectrally broad optical source starting from the spontaneous emission generated by an erbium doped fiber. These sources are usually pumped at 980 nm by a multimode laser diode. They are commercially available, but can also easily be implemented. They might also simply be obtained from a low power EDFA without seeding it. Amplified Spontaneous Emission (ASE) sources have a very broad spectral width. They roughly cover the whole C-band (~3 THz). This corresponds to a very short 1 μm coherence length, an ideal value for our present requirement. Furthermore, one should notice that an ASE source is not a laser, a critical feature. Indeed, in a general manner, the problem with broad multimode lasers including for instance high-power multimode laser diodesis not restricted to the high coherence level of their individual modes. Notably, because of Kerr effect, four wave-mixing, hole-burning, each of these mode strongly interacts with each other. This leads to a complex chaotic dynamic of their spectrum, rendering them unusable for cold particles handling. As mentioned above, an ASE source is not a laser. It displays a continuous spectrum with a spectral density too low to induce observable non linear effects. These sources are thus very stable (they are noisy, but their spectra are stable).

Cold atom experiments need very low intensity noise light. Intensity noise will induce additional heating rate and possible excess atom losses [START_REF] Savard | Laser-noise-induced heating in far-off resonance optical traps[END_REF]. As an ASE source is a 100% noisy light source, this proposition could appear paradoxical at first sight. However, the noise of an erbium ASE source is distributed over a very wide frequency range (~3 THz), the fraction of its spectrum overlapping the atomic system resonances (in the kHz range) is very low. In the following, we show that for well chosen parameters, this source can be seen as a low noise source with features comparable to common lasers used in every day cold atom laboratories.

In section 2, a possible setup is presented and discussed. An experimental estimation of the noise level for a low power ASE source is also reported. Section 3 is devoted to a numerical estimation of the heating rate with parameters adapted for two practical applications: evaporative cooling of Rubidium atoms, and single atoms trapping.

Experimental setup

Before discussing the ASE source, it is interesting to mention that broad band sources have been successfully used or proposed for light-matter interaction experiments. For instance, white light is of potential interest for optical tweezers application for cold atoms [START_REF] Sackett | A white-light trap for Bose-Einstein condensates[END_REF], or small biological items [START_REF] Neuman | Optical trapping[END_REF]. The proposition to use an ASE source is new to the best of our knowledge. A scheme for a realization is proposed figure 1. The starting point is a commercial low power Erbium ASE source. The simplest configuration is to directly amplify this source with a high power EDFA. Typically, 1 mW is sufficient to seed a high power EDFA, a value easily reach by most ASE sources. The grey insets indicate possible optional components to adapt the device for particular requirements. The use of a high power EDFA is a very convenient way to obtain a high power source with potential complex features. It is simpler to start by building a low power light source with the desired features, then, amplify it.

The first proposed option is the polarization beam splitter (PBS). In its basic configuration, an ASE source is not polarized. It is composed of a statistics mixture of the two polarization states without any correlations between them. This is a very convenient feature, and one can take advantage of it. As the source is depolarized, there is no need to perform any polarization alignment. This is not only potentially the case for the final dipole trap beams, but also for the fiber part of the device. This implied that the fibered system will be stable even if realized without polarization maintaining (PM) fibers and components; an interesting point as the cost a non PM EDFA is greatly reduced compared to a PM one. On the other hand, if a polarized source is necessary, a simple PBS will achieve this purpose. The second option is a filter stage. It could be useful if for instance, the detuning of the source must be accurately defined. In this case, an additional EDFA stage will be necessary to compensate the filter insertion loss. The filter can easily be realized with a Bragg grating, and slightly tuned by applying strain on it. We would like to stress that the proposed solution with an ASE source is very flexible and can easily be adapted to many cases.

Before implementing this solution in a cold atom experiment, it is important to estimate its noise level. The output of a 4 mW commercial erbium ASE source has been directly connectedwithout any optical filtering stageto a 2 GHz band-pass photodetector. The photo-current is monitored with a high-speed real time 6 GHz oscilloscope. The sampling rate is set to F S = 20 GS/s. A set of 2 10 7 samples have been logged. The first one thousand samples of this raw signal are displayed Fig. 2(a). The mean value have been subtracted. As expected, this signal is noisy. It presents (for the entire set of data) a |~3 mV rms (root mean square) intensity noise level for a 168 mV mean value. This corresponds to a 57 SNR (Signalto-Noise Ratio), or a ~2 % noise level. To take into account the fact that an atomic system will be sensitive to fluctuations typically below the MHz, the signal has been numerically filtered. Fig. 3 displays the SNR as a function of the cutoff frequency. From the right to the left, each point is obtained from the precedent one after a filtering stage, followed by a downsampling stage of the previous filtered signal.

Let's further detail this procedure. For a F S sampling frequency, F N = F S / 2 is the Nyquist frequency, the highest frequency that can be recovered. The 3 dB cutoff frequency F C of a low pass filter can be normalized with respect to F N . F C ranges from 0 to 1. Without a downsampling stage, each point of Fig. 3 would require the design of a particular filter with each time a different cutoff frequency in order to process the whole set of logged data. Designing numerical filters for extremely low cutoff frequencies is delicate. For instance, F C = 10 -3 requires a 4300 order Finite Impulse Response (FIR) filter. Beyond, our algorithm failed to design a 10 -4 cutoff frequency. The technique in numerical signal processing is then the following. Let's consider a downsampling factor D = 2 (one sample is kept every D samples), and the corresponding F C = 1 / D= 0.5 filter. For this medium value, there is no particular problem, and the 30 order FIR filter that we have used will give accurate enough results. Once the filter has been applied, there are no significant informations anymore in the signal for frequencies ranging from 0.5 to 1. Downsampling the filtered signal by D = 2 will reduce the Nyquist frequency by the same factor without losing informations. Once the signal is downsampled, there is both a new Nyquist frequency which 1/D the previous one, and a new associated frequency space. The same filter might then been applied to the resulting signal, which can then downsampled, and so on... With this procedure there is no limit to the lower cutoff frequency (excepted of course the initial sample number). Furthermore, always the same well controlled filter is applied. The final obtained cutoff frequency is the cumulative product of the inverse of all the applied downsampling factors. Excepted for the first calculated point, all the point reported Fig. 3 are computed with D = 2 and F C = 0.5. For the first point the parameters are D = 10 and F C = 10 -1 . Since for the raw data, F S = 20 GS/s, this give a 1 GHz cutoff frequency for this first point, i.e. half the photodetector bandwidth.

According Fig. 3, for the ~1 MHz cutoff frequency, the SNR reaches ~2300, a 0.05 % noise level. This value is low enough for most applications (see the next section). The corresponding intensity signal is displayed Fig. 2(b). We have also checked that the choice of the filter is not critical. As an illustration, the filter might be applied twice at each stage, instead of once. This effectively doubles the filter order and its attenuation, i.e. 6 dB at F C instead of 3. As the filter is stronger, we expect a higher calculated SNR. However, the effect is weak. For instance, still at ~1 MHz, the SNR would be increased by only 1%, and by 7% at 60 kHz. The data reported Fig. 3 are thus good guidelines to estimate whether an experiment is feasible or not. An additional remark might be done: the SNR curve is well fitted with a F - 1/2 function. Indeed, the spectrum of an ASE source is flat on the GHz scale. Applying a F C = 1/2 filter destroys half of the energy of the intensity fluctuation signal. The energy is proportional to the square of the amplitude of a signal, or to be more precise, to the square of its rms amplitude. Reducing the energy by a factor 2 reduces the rms value by 2 , and as consequence, increases the SNR by 2 . This explain the ~F-1/2 behavior of the SNR. It is also worthwhile to mention that if such a behavior is assumed to be valid for the high frequency range, the value SNR = 1 is reached for a ~6 THz cutoff frequency. Considering the simplicity of the reasoning, this is a rather good estimation of the ASE spectral width.

One should notice that if a lower noise level (or a wider bandwidth) is needed; it is possible to go further, as recently demonstrated for a Fiber Optical Parametric Amplifier (FOPA) [START_REF] Mussot | All-optical SBS reduction in fiber optic parametric amplifiers[END_REF]. Thanks to Kerr effect, the intensity noise of an ASE source can be transferred as phase noise to a low intensity DFB laser diode; at the cost however, of a more complex setup. The next section studies the effect of the estimated noise level for a BEC cooling experiment.

Noise level and heating rate

A potential problem of the proposed scheme is the heating of atoms (and the associated losses) induced by the intensity fluctuations of the beams. Heating in a dipole trap has been modeled in [START_REF] Savard | Laser-noise-induced heating in far-off resonance optical traps[END_REF] by considering an atom trapped in a fluctuating harmonic potential

2 / ) ( ) ( 2 x t k t V  : 2 2 ) ( 2 1 2 x t k M p H   (1) with   ) ( 1 ) ( 0 t k t k    , ) (t 
being a stochastic function with a coherence time 0  . As

I dx V d k   2 2 / , 1 ) (  t 
corresponds to the fractional fluctuation of the radiation intensity I . Writing

2 0 0 ) ( 2 1 x t k H H    (2)
reduces the problem to the calculus of a quadratic perturbation of a harmonic oscillator. As shown in [START_REF] Savard | Laser-noise-induced heating in far-off resonance optical traps[END_REF], this perturbation only couples eigenstates n (of eigenenergy

M k n E n / 0   0   n 
) and m such that  diminishes, the amplitude of the noise diminishes, but the width of the spectrum increases, including more harmonics of 0 2 . These new harmonics included in the spectrum tend to increase heating; on the other hand they contribute only in higher orders of perturbation theory. It is thus reasonable to expect that using a large spectrum of small amplitudes (with constant total power) lead to an overall reduction of heating and losses. In order to verify that quantitatively, we performed a numerical simulation of the dynamics of the heating process.

Writing the general solution of the Schrödinger equation associated with Eq. ( 2) in the form

n e t a t t i n n 0 ) ( ) (     
, one founds, after a simple algebra, a system of coupled ordinary differential equations for the coefficients ) (t a n , which can be easily solved numerically by e.g. a Runge-Kutta routine. For any time t one can then calculate the average system energy . For high enough excitation, the optical potential departs from the harmonic approximation. However, for a typical laser beam with a Gaussian transverse profile, atoms having a high enough energy will escape from the potential. In most cases, one is interested only in low energy atoms (which are in the harmonic range), so, in our simulation, we simply considered that atoms arriving at a high enough level . The loss rate is small even for higher of 0  , becoming important only when the amplitude of the noise is high enough that higher order couplings come into play. This simple simulation thus confirms quantitatively that using a large bandwidth optical source with a low spectral density has very favorable properties with respect to heating and losses of atoms.

For a chosen filter cutoff frequency, and from Fig. 4, one may determine the corresponding heating rate to determine whether this value is acceptable for a particular experiment. Let us now consider a real application case: a 1560 nm optical-dipole trap for 87 Rb BEC production made by a tightly confining beam, as used in [START_REF] Clément | All-optical runaway evaporation to Bose-Einstein condensation[END_REF]. The heating rate due to intensity fluctuations of the beam can be estimated, according to our model (Fig 4 .). First, we must pay attention to the waist value and the optical power. An initial collision rate (~ 10 4 s -1 for 3 millions of 87 Rb atoms) and an initial potential depth U (U/k B ~ 900 µK) are required for an efficient evaporative cooling process towards Bose-Einstein condensation. We thus estimate the heating rate in the ASE-based optical trap made by one focused light beam: a 28 micrometer waist and a 6 Watt power give transverse and longitudinal trapping frequencies of respectively 3.35 kHz and 42 Hz. In the case of an ASE-based optical-dipole trap with a ~1 MHz bandwidth as described above, we obtain a heating rate for each frequency which is well below 10 -10 ħω 0 2 (and as a consequence out of the diagram Fig 4 .). Since 10 -10 ħω 0 2 corresponds to 3 pK/s (pico-Kelvin per second), the heating rate would thus be completely negligible for a real experimental realization compared to the potential depth and to others losses mechanism such as laser-beam-pointing noise, collisions with residual gas... For instance, the heating rate induced by spontaneous photon scattering is 30 nK/s at the trap center, i.e. a rate larger by four orders of magnitude. Furthermore, the chosen cutoff frequency is well above the trapping frequencies giving a high level of confidence to this result. One can also consider a single-beam optical trap at the same wavelength dedicated to single atom manipulation. Optical trap parameters, such as waist and optical power, are chosen to give a sufficient potential depth for single-atom manipulation and a tiny trap volume where only one atom can be loaded [START_REF] Schlosser | Collisional Blockade in Microscopic Optical Dipole Traps[END_REF]. With a 3 micrometers waist and a 100 mW optical power, the corresponding frequencies for each direction are respectively: 37.6 kHz, 37.6 kHz and 4.4 kHz. Similarly, the heating rate due to intensity fluctuations is negligible with a value below 10 -10 2 0   , i.e. 300 pK/s. Once again, this is a negligible value compared with the heating rate related to the spontaneous scattering of photons for the same trap parameters, i.e. 43 nK/s.

Conclusion

In summary, we have proposed an optical-dipole trap technique based on an Amplified Spontaneous Emission source. This novel type of application for BEC production can be easily implemented in a typical cold-atom apparatus. The main advantage of this trapping scheme is its low coherence length. It should easily allow the creation of multi-beam geometry configurations in order to optimize both loading and evaporation stages. Furthermore this source as a seed is very stable and flexible. It has all the advantages of the fibered optical telecommunications technology and can simply be adapted to many particular experimental requirements. Although this source is naturally very noisy, the heating rate estimated for real world potential applications is found to be completely negligible in the range of the pK/s to a few hundred. Furthermore, the heating rate related to the light intensity fluctuations of our filtered ASE source is much lower than the heating rate related to spontaneous photon scattering by a few orders of magnitude for BEC all-optical production and single-atom optical trap. Finally, this method can be extended to numerous situations where optical trapping is needed: cold molecules, single atom experiments, nanoparticles manipulations, and optical tweezers for biology.

PBS

  n max = 12 in the simulations) are lost. For a trap frequency see below), for example, n max = 12 corresponds to atoms at a temperature ~1.8 µK. In e.g. evaporative cooling in a condensation experiment, atoms are expelled from the trap well below such temperatures. Thus, this simple procedure allows us to obtain a reasonable evaluation of the loss rate on the process. Fig.4displays typical curves of heating and losses rates in a log-log plot. It clearly shows that the heath rate significantly for

Fig. 1 .

 1 Fig. 1. Schematic view of the proposed device. ASE: erbium amplified spontaneous emission source. PBS: Polarization beam splitter. BPF: band-pass filter. EDFA 1: low power amplifier. EDFA 2: High power erbium doped fiber amplifier. The grey insets show optional components.

Fig. 2 .

 2 Fig. 2. (a) ASE Intensity signal trace as a time function of time (1000 samples). The mean value (~168 mV) has been subtracted. The acquisitioned is performed with a 2 GHz photodetector. Noise level: ~2 %. (b) Filtered and downsampled signal with a ~1 MHz cutoff frequency at ~3 dB (1000 samples). Noise level: ~0.05 %.

Fig. 3 .Fig. 4 .

 34 Fig. 3. Red solid line: estimation of the signal-to-noise ratio (SNR) of the source intensity as a function of the cutoff frequency. Black dashed line: fit of the SNR curve with a F -1/2 function.

Acknowledgements

Jean-François Clément is supported by the project "MICPAF", ANR-07-BLAN-0137-01 and by a CNRS grant. This work is partially being performed under the auspices of Région Nord-Pas-de-Calais and European Union (FEDER).