Sylvain Tricot
email: sylvain.tricot@univ-rennes1.fr

Magdalena Nistor

Éric Millon

Chantal Boulmer-Leborgne

Bogdan Nicolae

Jacques Mandache

Wilfried Perriére

Seiler

E Millon

N B Mandache

J Perrière

W Seiler

Epitaxial ZnO thin films grown by pulsed electron beam deposition

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Pulsed electron beam deposition (PED) is a recent and simple method for deposition of thin films. In PED, a pulsed electron beam is used to ablate the surface of a target material and the emitted species are collected onto a substrate in front of the target [START_REF] Müller | Science and Technology of Thin Films[END_REF][START_REF] Nistor | [END_REF]. The PED setup is thus very similar to the pulsed laser deposition technique (PLD).

In a previous paper we showed that whatever the nature, composition and properties of the materials grown by PED, particulates of the order of tens-hundred of nm in diameter are present at the surface of the thin films, but by a careful optimization of the electron beam parameters it is possible to drastically reduce the density and size of particulates at the surface of the films [START_REF] Nistor | [END_REF]. This could be explained by a unique feature of the pulsed-beam which delivers electrons with a large energy distribution, with a dominant highenergy electron component at earlier times and a contribution of lower energy electrons increasing towards later times [3][4][5].

The investigation of the plasma plume produced by the pulsed electron beam with the target by fast imaging and optical emission spectroscopy [3,5] showed that the kinetic energies of the species emitted by the target is about 10-50 eV, thus enhancing the adatom mobility when arriving on the substrate surface and providing enough energy to enhance the film growth while avoiding surface droplets.

One interesting aspect of the PED method is its ability to ablate a wide range of target materials and especially wide band gap semiconductors that are more difficult to process by PLD since they are poorly absorbent to the laser light. Zinc oxide (ZnO) is an example of such materials and it has already been shown that ZnO thin films grown by PLD are of better quality when prepared using a laser wavelength shorter than the band gap of ZnO [6]. Thus, PED could be an alternative to PLD for large band gap materials.

Moreover, ZnO has very attractive properties. It is a natural n-type semiconductor with resistivity values ranging from 10 to 10 -2 Ω/cm [7] for undoped ZnO. Optical properties of ZnO also focus intense research due to its wide direct band gap of 3.3 eV which makes ZnO transparent to the visible light. Its properties makes therefore ZnO promising for applications in optoelectronics, piezoelectric devices, SAW filters, gas sensors, solar cell transparent electrodes, transparent electronics and spintronics [7][8][9]. Some of these applications require very well-crystallized ZnO thin films or even ZnO epitaxially grown on a substrate. However, the growth of ZnO films by PLD is now well optimized [10] but despite the large number of data already reported in the literature on the structural and physical properties of ZnO films grown under various experimental conditions, very few studies give an accurate description of ZnO films grown by PED [11].

In this work, we investigate the morphological, structural, optical and electrical properties of ZnO thin films prepared by PED. The aim is to compare these films with known results from the literature of PLD prepared ZnO films to know if PED could be a reliable alternative to PLD for wide band gap materials. To hold a comparison of both techniques and to be only sensitive to the deposition process, growths were performed on sapphire (000l) substrate held at 700 °C without any post-treatment. These conditionsclassically reported in PLDare known to give well crystallized and epitaxied films.

Experimental

ZnO thin films were grown by the PED method on c-cut sapphire (0001) single crystal substrates. The experimental set-up of PED, described in detail in previous papers [START_REF] Nistor | [END_REF]5] is sketched in Fig. 1. Basically, a pulsed-electron beam interacts with a solid ZnO target and induces a plasma. The ablated species are collected onto the substrate facing the target and a thin film is formed pulses after pulses much like the pulsed laser deposition technique. The produced electron beam pulse width is 100 ns (FWHM), the total current reaches 800 A and it is composed of electrons with energies ranging from a few eV to the energy corresponding to the applied high voltage (15 keV for an applied voltage of -15 kV for example). The beam diameter is in the 2-4 mm range leading to a fluence of about 2.5 J/cm². The electron beam source consists of a hollow cathode, a dielectric capillary tube (6 mm in diameter and 110 mm in length) and the vacuum chamber as grounded anode. At each pulse, a 16 nF external capacitor is discharged between the cathode and the anode. The discharge voltage was varied in the 14-16 kV range. The maximum pulse repetition rate of the device is 5Hz and was kept at 1Hz for these experiments. The target-substrate distance is set to 40 mm and the substrate was heated at 700 °C. Indeed growth temperatures higher than 300 °C is known to favor a better crystallinity of films [7]. During the film growth the argon pressure was kept at 1.4 × 10 -2 mbar inside the chamber. This argon pressure is needed for the electron beam operation. After the growth, films are cooled down in the ablation chamber under the same argon pressure.

Rutherford backscattering spectrometry, using 2 MeV energy 4 He + ion beams from the Van de Graaff accelerator of the Institut des NanoSciences de Paris (INSP-Université Paris 6) was used to measure the thickness and composition of the zinc oxide films. The accurate determination of the in-depth distribution of the elements in films was obtained by the RUMP simulation program [12]. Due to the low RBS yield on oxygen, the relative accuracy of this element in films was therefore close to 4%. The surface morphology of films was investigated by atomic force microscopy. The structural characterizations of the zinc oxide films were carried out by XRD analyses using a four circle diffractometer (Philips Xpert MRD of LIM laboratory-ENSAM-Paris) with the Cu Κα radiation (λ = 0.154 nm). The nature of the crystalline phases, their axis parameters and their preferred orientations with respect to the substrates were investigated by XRD experiments in the Bragg-Brentano geometry. The in-plane epitaxial relationships between films and single crystal substrates were studied by asymmetric diffraction with the measurements of pole figures of selected reflection peaks of the film and substrates. The optical absorption of films was measured in the 200-2500 nm range with a Cary 5000 spectrophotometer. To obtain a better understanding of the properties of excitons in ZnO thin films, photoluminescence measurements were carried out at room temperature using a xenon lamp as excitation source at λ = 350 nm. The transport properties (resistivity as a function of temperature from room temperature down to liquid He temperature) of the zinc oxide films were obtained by the classical four probe method. The carrier type and concentration as well as their mobility were determined with a MMR Hall measurement system in the Van der Pauw geometry at a magnetic field of 0.3 T.

Results and discussion

Film composition and morphology

Parameters of the pulsed-electron beam for the growth of zinc oxide films were optimized in order to obtain the formation of dense and smooth zinc oxide films free of particulates. The RBS spectra recorded on such zinc oxide films grown on c-cut sapphire substrate at 700 °C evidenced that no interdiffusion occurs between the film and the substrate and that the surface roughness is smooth. A uniform indepth distribution of Zn and O species was also observed in the films, leading to the conclusion that nearly stoichiometric ZnO films are formed in these conditions. Moreover, argon incorporation in the zinc oxide films was not evidenced despite the fact these films were grown under a relatively high argon partial pressure (1.4 × 10 -2 mbar). This point is one of the main specificity of the PED growth method with respect to other growth methods like conventional sputtering which lead to argon incorporation in the films during growth under similar argon pressure [13]. In PED, growth occurs in presence of an argon plasma whose species (Ar ions or atoms) have a low kinetic energy (maximum a few eV). Such species can be physically adsorbed at the surface or in the near surface region of the growing film. Meanwhile, species directly emitted by the target during and after the electron bombardment fly to the substrate with large kinetic energies (in the tens of eV range) [START_REF] Nistor | [END_REF]5]. In these conditions, the bombardment of the film surface with the energetic target species induces the desorption of any weakly adsorbed species like Ar which could be incorporated. Thus, the ZnO films grown by PED are free from argon. It is worth noticing that ZnO films prepared by PLD are usually grown under an oxygen partial pressure that can be varied in a wide range from 10 -5 to 10 -1 mbar. Without addition of oxygen during the growth, the resulting films are zinc enriched. Thanks to the optimization of the discharge parameters, we showed here that it is possible to obtain nearly stoichiometric ZnO thin film by PED without adding oxygen in the chamber.

It is also important to note that the growth rate of the PED technique is in the 0.05-0.11 nm/pulse range which is more than 10 times greater than those classically obtained by PLD. This feature is an interesting characteristic of the PED process as the growth rate is known to be an experimental parameter limiting a broader use of the PLD technique for industrial trends.

According to atomic force microscopy experiments, the surface of ZnO films grown by PED is smooth with a mean roughness of 4.9 nm (RMS roughness of 7.9 nm) for a total thickness of 200 nm. The boundaries of the crystallites are also visible and their diameter can be estimated to 40 nm.

Film structure

The crystallization of the ZnO thin films on c-cut sapphire substrate was studied for the fixed experimental growth conditions leading to the growth of dense, smooth and stoichiometric films. The XRD diagram in the Bragg-Brentano geometry of ZnO thin films grown by PED on a c-cut sapphire substrate at 700 °C is shown in Fig. 2. A strong well-defined peak at 2θ = 34.375°as well as a weak peak around 72°are observed on this diagram. It corresponds respectively to the (0002) and the (0004) diffraction peaks of the ZnO hexagonal wurtzite structure, indicating that the ZnO thin films are grown with the c axis normal to the sapphire substrate plane. This c-axis orientation of crystallites is classically observed for ZnO films whatever the growth method. Indeed, textured films are generally obtained when the plane parallel to the substrate has the lowest surface energy density [14]. In the case of ZnO which is a polar crystal (alternative stacking of Zn planes and O planes), the surface free energy of the (000l) plane has not a finite value. To limit the surface of such plane, the growth rate along the c-axis is increased and thus a columnar growth of ZnO crystallites is evidenced. The value of the c axis lattice parameter deduced from the diffraction pattern is c = 0.52125 nm. This value nearly reaches the bulk ZnO value c bulk = 0.52066 nm. The estimated size of the ZnO crystallites through the Debye-Scherrer formula is 44 nm. The crystallite size in ZnO films grown by PLD is typically about 50 nm with the same columnar growth on sapphire [15].

The mosaic spread of this (0002) texture was studied by rocking curve measurements. The inset in Fig. 2 represents the (0002) rocking curve of the ZnO films on c-cut sapphire substrate. The FWHM of the rocking curve is only of 0.65°evidencing therefore a low mosaic spread of the crystallites according to the main axis growth and a high crystalline quality of the film similar to that of obtained in PLD films.

The (0002) texture observed in ZnO films suggests a possible epitaxial growth of the films. Pole figure measurements were performed to investigate the epitaxial relationships between the ZnO film and the sapphire substrate. Considering the sapphire substrate symmetry, only three poles should be observed for the {10 -13} family planes of ZnO. This has already been observed and explained by the fact that the sapphire surface exhibit steps which induce an unconventional number of poles (6 poles 60°apart) and a symmetry of order 6 instead of 3 [16].

The comparison of the position of the six poles of the {10 -13} ZnO planes, with the poles of the {01 -18} family planes of the sapphire substrate, evidences a rotation of 30º of the ZnO axis with respect to those of the c-cut sapphire substrate. The following orientation relationship can therefore be inferred:

ZnO 0001 ð Þ 2 - 1 - 10 Â Ã == Al 2 O 3 0001 ð Þ 1 - 100 Â Ã
Such an epitaxial growth is classically observed for the growth of ZnO thin films on c-cut sapphire substrates [17]. The 30°rotation between the 2 cells ensures the continuity of the hexagonal oxygen sublattice in the sapphire substrate and in the ZnO film. These orientations can be described in terms of "domain matching epitaxy" [18], which has been already used to describe the same epitaxial relationships evidenced in PLD ZnO films grown on sapphire at 700 °C [15].

In summary, regarding the main axis growth of the ZnO crystallites, their in-plane orientations with the sapphire substrate, and their estimated size in the several tens of nm range, the PED technique leads to ZnO films having a crystalline quality very similar to those obtained by PLD at the same substrate temperature.

Electrical measurements

The transport properties of the ZnO thin films grown by PED on ccut sapphire substrates were investigated by resistivity and Hall effect measurements. The normalized resistivity as a function of the temperature is reported in Fig. 4. The resistivity decreases while the temperature increases revealing the semiconducting behavior of the thin film in the whole temperature range as it is classically observed with PLD ZnO films.

Hall effect measurements were performed at room temperature. It is found that the ZnO film is n-type and has a high amount of charge carriers (n e = 3.4 × 10 19 cm -3 ). This is a very high value compared to undoped ZnO thin films prepared by PLD which are usually in the 10 16 cm -3 range [7]. However, the electron mobility is only of 11.53 cm²/Vs at room temperature. This value of μ e is far from the expected theoretical value of 300 cm²/Vs for the bulk single crystal [19]. Thanks to the high density of charge carriers, the resistivity at room temperature is as low as 1.6 × 10 -2 Ω cm. The good crystalline quality of the PED films and their stoichiometry (low concentration of oxygen vacancies) are in good agreement with a resistivity around 10 -2 Ω cm. This value is similar to that of measured on ZnO films grown by PLD. Nevertheless, the higher mobility (above 100 cm²/Vs) leading to a resistivity of 10 -2 Ω cm for PLD films can only be obtained for optimized PLD process such as a multi step growth, or buffer layers or even with substrates like ScAlMgO 4 to reduce the mismatch and increase the crystallite size [20].

The plot of ln(ρ) versus the reciprocal temperature for a ZnO thin film grown at 700 °C is presented in Fig. 5 and indicates an Arrhenius law versus temperature in the 230-300 K range. The activation energy (E a ) is determined from the slope of the linear region of the plot (inset in Fig. 5) and is 11.7 meV. From this activation energy, a donor level located at E D = 2E a = 0.024 eV below the bottom of the conduction band can be deduced.

A clear deviation from the Arrhenius law is observed when the temperature is below 230 K. This suggests that transport properties could not be anymore correctly described by thermally activated conduction. Classically, it is generally assumed that the transport properties of such films can be described at low temperature range by the variable range hopping model (VRH), in which electrons hop between deep donor states [21]. In this model, the resistivity follows the expression:

ρ = ρ 0 × exp T 0 T 1 = 4 ! ð1Þ
where ρ 0 is a function of T 1/2 and T 0 is constant. According to Eq. ( 1), it follows that ln(ρT -1/2 ) is proportional to T -1/4 . This relation is presented in Fig. 6. The data can be correctly adjusted by a linear variation in the low temperatures domain which confirms the VRH model at low temperature (below the Arrhenius activated regime) in the films, in the short range of 150-120 K. These transport properties of ZnO thin films are not PED specific since the transition between the classical thermally activated conduction to the variable range hopping mechanism has already been reported for ZnO films prepared by other methods and not fully explained. However, the temperature range in which the VRH model holds is quite reduced, and the temperature domain ranging from 150 to 230 K cannot be explained by the VRH model or the Arrhenius law. Such temperature dependent resistivity curve can be originated from the high density of charge carriers observed in these films linked to the oxygen vacancies and/or impurities which induces localized states. A metallic behavior of undoped ZnO PED film displaying a density of carriers in the 1 -3 × 10 20 cm -3 has been recently observed [22].

Optical properties

Optical properties of ZnO thin films grown by PED on c-cut sapphire substrate at 700 °C were studied by absorption measurements in the 190-900 nm wavelength range. Films are well transparent since the transmittance reaches 90% in the visible range for wavelength above 400 nm. A strong decrease of the transmittance is also observed around 370 nm which is consistent with good quality ZnO thin films [23]. This cut-off of the transmittance corresponds to band to band absorption. The energy gap was obtained by applying the Tauc model to absorption measurements [START_REF] Tauc | Optical properties of solids[END_REF]. In this model, the variation of the absorption coefficient α in the strong absorption range (α b 10 4 cm -1 ) is linked to the energy gap E g of the material by the following expression:

α = B × E-E g m E ð2Þ
In the above equation Ε is the photon energy, B is a constant and the allowed values for m are 1/2 for an allowed direct transition, 3/2 for a forbidden direct transition and 2 and 3 for an allowed and a forbidden indirect transition respectively. Since ZnO is a direct band gap semiconductor (m = 1/2), Fig. 7a shows (αE) 2 as a function of the photon energy E. The extrapolation of the linear region of the curve up to α = 0 gives the value of E g . From Fig. 7a, the optical band gap of ZnO thin films grown by PED is 3.28 eV. This value is in the same range than that of observed in ZnO films grown by PLD [START_REF] Zeng | [END_REF] or by other deposition techniques (sputtering, solgel [26]).

Fig. 7b shows the normalized photoluminescence spectrum recorded at room temperature. An intense near band edge emission is observed at 3.27 eV with a weak blue-green band (or deep level emission) at about 2.6 eV. The lower intensity of the latter indicates a high quality of the ZnO films grown by PED in Ar at 700 °C even without post-deposition thermal treatment under oxygen. The intensity ratio of the near band edge emission to deep-level emissions is about 7 at room temperature. The deep-level emission is usually related to structural defects but in the case of ZnO films grown on c-cut substrate the lattice mismatch plays an important role even for epitaxial thin films grown by PLD [27]. The long tail extending from the near band edge emission, which is usually caused by band structure deformation resulting from lattice deformation [17], is confirmed by the investigation of structural properties (cf. Section 3.2).

In relation to ZnO films grown on c-plane sapphire substrates by PLD [28] the as-deposited PED ZnO films show good optical properties. To improve them, a high temperature annealing treatment in oxygen is probably needed as it is classically performed on ZnO films grown by various techniques. Nevertheless it was not the aim of this work since the physical properties of the ZnO PED films are compared with those of as-deposited ZnO PLD films. These results therefore indicate that PED is a viable technique for producing high quality epitaxial thin films of ZnO for optical applications.

Finally, it has to be noticed that this band gap value is slightly lower than the one of bulk ZnO (3.3 eV). According to the Burstein-Moss effect, the high density of charge carriers is supposed to create donor states in the conduction band which limits thermal or optical excitation. As a consequence the band gap value would be supposed to increase (blue shift). As this effect is not observed for our films, the optical absorption edge may be modified by the change of interactions between donors and host crystal. This phenomenon induces a band gap shrinkage evidenced by a shift of the measured optical gap toward the lowest energy. The competitive effects of the Burnstein-Moss shift and the band gap shrinkage has already been observed in heavily defect-doped ZnO films [29].

Conclusions

In this work, we have demonstrated the possibility of growing dense, smooth, stoichiometric and crystalline zinc oxide films by the pulsed electron beam deposition method using a channel-spark discharge as the pulsed electron source. These ZnO films obtained by PED were found to grow epitaxially on c-cut sapphire substrates at 700 °C, with the classical epitaxial relationship also observed for ZnO films grown by the well-known PLD method under similar conditions. In the same way, the optical and electrical properties of PED and PLD ZnO films are very similar.

These results demonstrate the ability of PED to grow high crystalline quality oxide films showing the same functional properties than films grown by other methods. In addition, the PED technique highlights interesting advantages regarding the experimental process in term of growth rate (10 times higher than that of PLD) and the transfer of the target composition which does not require in the setup the incorporation of oxygen at a controlled pressure.

The PED ZnO films show a resistivity of 10 -2 Ω cm at room temperature. Such value classically observed for undoped ZnO films is due to charge carriers with a relatively high density (3.4 × 10 19 cm -3 ) but with a low mobility (11.53 cm²/Vs). This could be due to the fact that these films are composed of closely packed hexagonal columns separated by grain boundaries, which are usually thought to be detrimental for high carrier mobility. The temperature dependent resistivity curves of the PED films highlight a semiconducting behavior but cannot be explained by a simple law : at high temperature (above 230 K) the resistivity follow the Arrhenius law while the VRH model may be applied at low temperature (below 150 K). The optical band gap of PED ZnO films is close to the bulk value. Nevertheless, more detailed investigations have to be made to explain the shift of the optical edge observed on PED ZnO films according to the density of charge carriers and its consequence on the Burstein Moss effect and/or the band gap shrinkage. PED ZnO films grown at different temperatures appear therefore as good candidates to go deeper on this subject.

Fig. 1 .

 1 Fig. 1. Top view of the PED experimental setup. The dielectric tube defines an angle of 45°with the target.

Fig. 3

 3 reports pole figures recorded for the {10 -13} ZnO family planes (2θ = 62.86°). Six well defined poles at a declination angle Ψ = 32.5°are visible, indicating the existence of an epitaxial relationship between the film and the substrate. Moreover, three poles at a declination angle Ψ = 22.5°are also observed, corresponding to the poles of the {01 -18} planes of the c-cut sapphire substrate (2θ = 61.3°).

Fig. 2 .

 2 Fig. 2. θ-2θ XRD diagram of a ZnO thin film grown at 700 °C under 1.4 × 10 -2 mbar argon pressure on c-cut sapphire substrate. The inset shows the corresponding rocking curves of the (0002) reflection.

Fig. 3 .Fig. 4 .

 34 Fig. 3. Al 2 O 3 (01 -18) and ZnO (10 -13) pole figure of a zinc oxide film grown by PED at 700 °C on a c-cut sapphire substrate.

Fig. 5 .

 5 Fig. 5. Arrhenius plot of the resistivity of a ZnO thin film grown at 700 °C as a function of the reciprocal temperature. The inset shows the linear region (230-300 K) used for the determination of the activation energy.

Fig. 6 .

 6 Fig. 6. Variation of ln(ρT -1/2 ) versus T -1/4 for a ZnO film grown at 700 °C. The linear fit (solid line) indicates the temperature range where the VRH conduction applies.

Fig. 7 .

 7 Fig. 7. a) Tauc plot of the absorption coefficient of ZnO film grown on c-cut sapphire at 700 °C under 10 -2 mbar Ar. The dash line represents the extrapolation to zero of the linear part of the curve corresponding to the optical band gap of the film. b) Room temperature photoluminescence spectrum of a ZnO film grown on c-cut sapphire by PED at 700 °C.

Acknowledgements

The authors are grateful to S. Kilburger for the AFM experiments and to I. Enculescu for the photoluminescence measurements.


G Sutter
email: sutter@lpmm.univ-metz.fr

N Ranc

Flash temperature measurement during dry friction process at high sliding speed

Keywords:

An experimental method is presented in this paper to measure flash temperatures of sliding surfaces. High sliding velocities are reached by using a ballistic set-up equipped with a high speed camera. The temperature field on the friction surface was recorded during the process. Tests were conducted under dry sliding conditions by using an identical material for the rubbing bodies, which are of middle hard steel (C22). Experiments showed that the temperature distribution generated by frictional heating is made up of small hot spots that correspond to the friction of asperities located on the sliding surface during very short time. Deduced from observations, maximum local surface temperatures can exceed about 1100 • C around an area less than 100 m in diameter.

Introduction

Present in most engineering applications, friction influences mechanical efficiency and plays a major role in the performance and life of the machine. The friction is an inevitable phenomenon which is a consequence of heating the rubbing parts (pieces in motion) at the interface and governs the action between the mobile part and fixed part. To optimize mechanical performance or just the life of one component, it is necessary to improve knowledge of the overheating, the mean or maximum values of temperature and the temperature distribution generated at the interface. Indeed, the existence of high-localized temperature (flash temperatures) at the surface can strongly influence the mechanical behavior of concerned materials. Kennedy [START_REF] Kennedy | Thermal and thermomechanical effects in dry sliding[END_REF] reports that such temperature can be responsible for oxide formation, spot weld, thermoelastic instabilities, thermomechanical failure, and wear. Similarly in manufacturing processes, the high local temperature at the tool-workpiece interface on the rake face or the clearance face determines the tool's life and the geometrical characteristics of the manufacturing part. For example, during the cutting process, the local temperature induced by friction under the tool tip accelerates the tool wear and generates residual stresses at the workpiece surface. The knowledge of the heating effects associated with friction is all the more significant when sliding velocity increases (due to the increase of the cutting speed) and when the use of lubricant is reduced. Moreover, much of modeling of friction or wear mechanisms is based on the distribution of the surface temperature.

If it is well known that the temperature resulting from the frictional heating has very important consequences, it is also necessary to define an experimental measurement technique in order to measure the temperature field and to understand the mechanical and thermomechanical phenomena present during friction. The difficulty of the temperature measurement stands in the fact that the analyzed zone is difficult to probe without changing the friction conditions. In machining, the main method used for assessing temperature at the tool-chip interface is an embedded thermocouple [START_REF] El-Wardany | Cutting temperature of ceramic tools in high speed machining of difficult-to-cut materials[END_REF][START_REF] Stephenson | Tool-work thermocouple temperature measurementstheory and implementation issues[END_REF][START_REF] Kitagawa | Temperature and wear cutting tools in highspeed machining of Inconel 718 and Ti-6Al-6V-2Sn[END_REF]. Unfortunately, the introduction of the thermocouple in the cutting tool nearest to the interface alters the temperature distribution in the tool. In addition no direct measurement is obtained due to the distance between the thermocouple and the sliding surface. So it is possible to calculate only a mean temperature distribution at the tool-chip interface by using an analytical or numerical approach of the heat conduction problem. Moreover, only punctual measurements are possible and they have limited transient response due to their thermal inertia. With dynamic thermocouples [START_REF] Shaw | Metal Cutting Principles[END_REF], using just the difference of the temperature between the tool and the workpiece to generate electromotive force, only a mean temperature over the entire contact zone is obtained. For sliding surfaces, the thin film thermocouple (TFTC) used by Kennedy et al. [START_REF] Kennedy | Thin film thermocouple arrays for sliding surface temperature measurement[END_REF] seems to be a good compromise to reduce disruption of the process and to have a high response time (in a range of 1 s) and very small measuring junction. With a multi-junction sensor, Kennedy et al. observed temperature peaks (flash temperatures) which are much higher than the nominal value. Other numerous techniques have been developed to investigate temperature distribution of bodies in sliding contact. For example, during a manufacturing process, Wright et al. [START_REF] Wright | Metallographic methods of determining temperature gradients in cutting tools[END_REF] proposed a metallographic method to determine temperature gradients in cutting tools on which the chip has been rubbing. Yeo and Ong [START_REF] Yeo | Assessment of the thermal effect on chip surfaces[END_REF] estimated thermal effect on the chip surface by a chip color approach. However, these two methods are laborious and difficult to use, particularly on surfaces which are scraped during the process. To provide information on the entire temperature distribution field and to identify the maximum temperature area, which can move with sliding conditions, the pyrometry technique [START_REF] Boothroyd | Temperatures in orthogonal metal cutting[END_REF][START_REF] Dewes | Temperature measurement when high speed machining hardened mould/die steel[END_REF] seems to be actually the most adapted solution. This method has a very good spatial resolution and a short response time.

During the sliding motion, the friction energy is dissipated through a real area of contact which is generally different from the apparent area of contact. Depending on the manufacturing process, the surface roughness defines the profile of the contact surfaces. Although the surfaces are carefully prepared, they cannot be perfectly flat .The irregularities and asperities from surfaces that constitute the real area of contact can form interfacial junctions according to the compatibility of the sliding materials [START_REF] Rabinowicz | The determination of the compatibility of metals through static friction tests[END_REF]. Plastic strains appear with the breaking of adhesive junctions and with the ploughing of hard asperities through the surface of the softer material. From these different surface and volumetric contributions that are generating energy during friction, it appears that the choices of the friction materials and surface roughness are very important.

It is usually assumed that the most important part of friction energy is consumed through plastic strain in the near-surface layers [START_REF] Rigney | Plastic deformation and sliding friction of metals[END_REF]. The microscopic observation of the different regions of a cross section of a worn surface shows that the largest plastic strain is located in a thickness of few microns from the sliding surface [START_REF] Dautzenberg | Quantitative determination of deformation by sliding wear[END_REF]. To obtain results in accordance with initial conditions, the structure of the material near the friction surface did not have to change by superimposing several stages of friction. Results obtained with several successive rub passes for steel sliding over copper show that the depth of the severe deformation increases with the subsequent passes. However with single pass tests [START_REF] Kennedy | Single pass rub phenomena-analysis and experiment[END_REF] it was shown that the most important part of frictional energy (>90%) is dissipated as plastic work within 5 m of contact surfaces and is transformed into heat. In addition, the real contact is formed by several confined areas (asperities) which can constitute thermo-junction. The increase of temperature of the bodies dur-ing friction is mainly initiated in the immediate vicinity of the real contact area and is extended by conduction to all material according to the time duration of the process. So the whole difficulty of temperature measurements lies in taking a picture of the initial temperature field in the near surface during a short duration (less than 1 ms). Obtaining these experimental data is very important for a better understanding of thermal phenomena during friction.

The aim of this work is to propose an experimental approach to quantify the temperature fields generated at the interface of two contacting bodies in sliding contact, especially at high velocities. More precisely, the objective is to confirm and measure the magnitude of flash temperature in a sliding contact. However, if it is possible to estimate a mean value of the generated heat, a reliable measurement of temperature maps generally remains complex and difficult. In this study, in order to reach the contact zone, one of the sliding bodies is drilled. The temperature disturbance is limited by the use of a radiation technique. In addition, direct access to the friction zone allows to avoid interpolation calculations. A ballistic device is used in order to generate dry friction and produce a large range of sliding velocities [START_REF] Sutter | An experimental investigation of dry friction for a large range of sliding velocities[END_REF]. In this paper, the interpretation of temperature fields is realized at 35 m/s. To reduce the parameters which can interfere with the results, the rubbing surfaces are precision ground, with the same material, a middle hard steel C22, used for each of the two parts. Only one pass is realized by a specimen to obtain a continuously fresh rubbing surface.

Experimental device

High speed friction device

To reach high sliding velocities, a dynamic set-up used was developed to study processes of interaction during cutting under severe conditions [START_REF] Sutter | Chip geometries during high speed machining for orthogonal cutting conditions[END_REF]. An air gun is used to propel a projectile inside a launch tube designed to avoid any rotation. This tube, which is connected to an air gun, precisely drives the projectile and is long enough to assure a constant velocity after the launching. The velocity range can vary from 10 to 100 m/s according to the gun pressure. The mass of the projectile is chosen in order to have a kinetic energy larger than the mechanical work consumed during the friction process. The sliding velocity is thus nearly constant during the process and quasi-stationary conditions are realized. A second tube coaxial to the first one receives the projectile with a gradual slowing down after the test. The friction process, as shown in Fig. 1, is taking place at the entry of the receiving tube. Contrary to the earlier cutting study [START_REF] Sutter | Chip geometries during high speed machining for orthogonal cutting conditions[END_REF], the specimen is not fixed on the projectile but held in position before impact. A dynamometer ring, which ensures the compressive force during the friction, is fixed on a load sensor made up of a thin tube and strain gages. Two rectangular specimens M A (h = 24 mm × = 12 mm, e = 5 mm) are symmetrically fixed inside this dynamometer ring. During the friction process, the projectile launched by the air gun impacts the specimen M B (W = 10 mm × L = 60 mm, E = 10 mm). The sliding movement is realized after the impact of the projectile on the specimen M B . Dry friction of medium-low carbon steel, for which the chemical composition and the main mechanical characteristics are indicated in Table 1, is studied in this work. The surfaces are obtained by grinding operation following the sliding direction and the surface roughness is R a = 0.8 m. All the specimens are machined together to admit that the geometrical defects are negligible. To distinguish various parts of the device, the specimen M A will indicate the fixed specimen and M B the mobile specimen.

Temperature field measurement on the sliding surface

The experimental technique used to measure the temperature during the sliding process is based on the visible pyrometry technique developed previously for the measurement of the temperature during the cutting process [START_REF] Sutter | Temperature fields in a chip during high speed orthogonal cutting -an experimental investigation[END_REF][START_REF] Ranc | Temperature measurement by visible pyrometry-orthogonal cutting application[END_REF]. The pyrometry technique is non intrusive and in order to have a weak sensitivity of the emissivity of the surface on the determination of the temperature, the visible pyrometry technique is used. This measurement is carried out with an intensified CCD Camera whose spectral range is between 0.4 m and 0.8 m. The aperture time of the camera can be 40 s, 300 s and 500 s. An objective with a focal length of 80 mm allows to obtain a spatial resolution of 4 m.

To reach the zone of friction, a hole is drilled through the ring and through the fixed specimen. The diameter of the hole is 2 mm and remains small compared to the size of the specimen (Fig. 2). The temperature measurement device is calibrated on a black body (Pyrox: PY24, temperature range: 400-1600 • C, temperature uncertainty: 1 • C). For example, the curves in Fig. 3 show the evolution of camera level according to the black body temperature for the aperture times of 40 s and 300 s.

Results and discussion

A series of tests was carried out for aperture time of the camera of 40 s, 300 s and 500 s. Table 2 sums up the different experimental conditions (each test is reproduced twice to confirm the recording and the mean values are notified). The delay is the time between the beginning of friction and the beginning of the aperture of the camera. For a sliding speed of 34 m/s, the length of the specimen corresponds to a friction duration of about 1700 s. Fig. 4 represents the evolution of the tangential force according to time for the test 3. The average value of this force during friction is calculated by the usual relation F T = 1/(t f -t i ).

t f t i
f T dt where f T is the instantaneous tangential force, t f and t i respectively are the initial and the final friction time corresponding to a constant normal stress. Indeed, as shown in Fig. 1, the specimen M B has a specific design to assure its guiding through the receiving tube after the impact [START_REF] Sutter | An experimental investigation of dry friction for a large range of sliding velocities[END_REF]. This design imposes a variation in rigidity, which results in a decreasing normal stress (see Fig. 4) after a sliding distance (L-E-). In the case of test 3, a constant normal loading is considered for a distance of 38 mm which corresponds to an average tangential force equal to 1.35 kN. In Fig. 4, the delay and the aperture time of the camera are specified. It must be noted, that the low frequency oscillations observed on the recorded signal, are produced by the eigenmode of the sensor and are filtered by considering the average value of this signal. The moving time in the hole of a material point on the mobile specimen is approximately 70 s. Normal pressure is deduced from the dimensions of samples measured at the beginning of each test and the stiffness of the dynamometer ring.

Two thermographs which correspond to the tests 3 and 5 are represented respectively in Figs. 5 and6. These two thermographs are representative of all thermographs obtained during these tests. The temperatures are calculated by using the respective calibration curves given in Fig. 3. Thus, this temperature determination method implies that those temperatures remain constant during the aperture time of the camera. This assumption will be justified later in this document. X and Y axes, with origins in the center of the hole, are directed respectively along the sliding direction and orthogonal to the sliding direction.

Table 3

Location and value of the different hot spots.

Streak reference

Maximal temperature ( • C) Position along X axis (mm) These two thermographs are similar in spite of a very different aperture time. They both present a certain number of hot spots emerging from the right-hand side of the hole in the mobile specimen and forming streaks towards the left of the hole with a decreasing temperature. During tests 3 and 5, respectively eight and nine streaks can be counted. The maximum temperatures vary between 839 • C and 1004 • C for test 3 and between 780 • C and 918 • C for test 5; Table 3 gives the maximum temperatures measured and their location along X axis. Thermographs, presented in Figs. 5 and6, highlight the same temperature levels even though measured with very different aperture times and thus show that during 300 s the temperature seems to be stationary.

Fig. 7a presents the temperature profile along the axis X around the hot spot 5 (test 3) where the maximum temperature is 991 • C. The origin of distances in Fig. 7a is centered on the hot spots in order to make the reading easier. Due to the difficulty to define the hot zone width, this size will be chosen in an arbitrary way like the width corresponding to a temperature higher than 90% of the maximum temperature. This choice is a compromise between sufficiently weak variation in temperature in the hot source (10% of the maximum temperature in our case) and enough variation compared to the thermal sensitivity of our measurement device. With this definition, the width of the hot spot ( 5) is estimated to be 130 m, corresponding to a temperature higher than 864 • C. Fig. 7b gives the temperature evolution along the Y axis for the hot spots (5), ( 7) and ( 2). The origin of the distances of each hot spot in this figure is placed on the edge of the hole and directed along the Y axis. A temporal scale obtained by dividing the distance from the edge of the hole by the sliding velocity, is plotted at the top of the graph to ensure a better comparison of results. For the hot spots 7 and 5 (test 3), whose temperatures at the friction zone exit are almost identical, a very good reproducibility of the temperature evolution can be noticed. For the hot point (5), the temperature decreases from 1000 • C to 800 • C in approximately 25 s whereas for hot point (2) the time to decrease from 900 • C to 700 • C is slightly longer (approximately 30 s). Due to the similar rubbing surfaces (same roughness, same materials) used during the tests, it can be supposed that the asperities located on the specimens M A and M B are submitted to the same heating and also generate the same temperatures.

Two mechanisms could explain these temperature results. In the first one, an asperity which is located on the mobile specimen is considered. Before emerging in the hole, it warms up during friction with the fixed specimen (configuration schematized in Fig. 8). The camera observes this hot asperity, which emerges from the right of the hole and which moves along the Y axis with a decreasing temperature due to the conduction through the mobile specimen and to the convection with the air located in the hole. The passage time of the asperity in the hole is approximately 60 s. According to the triggering of the camera which occurred during the friction process, hot streaks which are in the middle of the hole and not only on the edge could have been observed. However, this type of streak is not observed in our experimental results.

In the second possible mechanism, an asperity which is now located on the fixed specimen, warms up by friction. This hot asperity will also heat the mobile specimen by contact (as shown in Fig. 9). On this part, as a function of the number of asperities in contact, hot streaks are initiated from the contact points. The temperature of these streaks decreases along the Y axis because of the heat transfer phenomena. In this case, the camera also records hot streaks where temperatures are stationary during this time. Moreover streaks always seem to emerge from the right-hand side of the hole and the maximum temperature primarily depends on the distance between the edge of the hole and the location of the asperity on the fixed specimen. Constant temperatures were observed throughout all the aperture times of the camera (40 s or 300 s); this allows to conclude that this parameter has a negligible effect on the temperature measurement.

In accordance with thermographic observations and previous analysis, this second mechanism seems to correspond to the effective process.

According to these conclusions the position of the asperity on the fixed specimen can be determined from the maximum temperature of one streak (at the edge of the hole) and from the temperature decrease of the streak [START_REF] Wright | Metallographic methods of determining temperature gradients in cutting tools[END_REF] given in Fig. 7b. In the case of test 3, it is supposed that the streak no. 7, which corresponds to the maximum temperature, is located very close to the edge. In Table 4, the estimations of the distances between the asperities and the edge of the hole are reported for each streak. The minimal measurable temperature being above 650 • C, beyond a distance of approximately 2 mm, the thermal effects associated with an asperity are also not visible in the hole.

Conclusions

This experimental investigation has been carried out in order to understand mechanism of the heat generation during the friction. The temperature field at the interface of sliding surfaces is measured by visible pyrometry using an intensified CCD camera. Tests are conducted under dry sliding conditions with contacting bodies made from medium-low carbon steel and for a sliding velocity of 34 m/s. The originality of this device lies in the fact that no transparent materials are necessary to reach the contact surface. The temperature recorded during the process confirms the non-uniformity of the contact temperature field and shows the existence of very small hot spots. Deduced from the experimental process, the flash temperature can exceed 1100 • C over an area of about 100 m in diameter. These high-localized temperatures resulted from contact between very small asperities localized on the mobile specimen and the stationary slider.

Due to the fact that similar maximum temperature are reached during the tests at the same sliding velocity, the shape and the spacing between the contacts seem to have limited effect at high sliding velocities. For similar surface roughness, main parameters governing the temperature magnitude remain speed, normal pressure and thermal properties of the contacting materials. The performances of this equipment allow at the moment to consider a more systematic study of these various parameters.

Fig. 1 .

 1 Fig. 1. Schematic of the experiment set-up used to measure heat during dry friction.

Fig. 3 .

 3 Fig. 3. Calibration curve of the intensified CCD camera. (a) Aperture time of 40 s (b) Aperture time of 300 s.

Fig. 4 .

 4 Fig. 4. Recorded signal by the load sensor according to time during test 3.

Fig. 5 .

 5 Fig. 5. Thermography of the sliding surface for a sliding velocity of 33.9 m/s and a 40 s aperture time of the camera (test 3).

Fig. 6 .Fig. 7 .

 67 Fig.6. Thermography of the sliding surface for a sliding velocity of 34.5 m/s and a 300 s aperture time of the camera (test 5).

Fig. 8 .

 8 Fig. 8. Thermal effect associated with an asperity located on the mobile specimen on plane A-A.

Fig. 9 .

 9 Fig. 9. Thermal effect of an asperity supposed located on the fixed specimen in the plane A-A.

Table 1

 1 Chemical composition, main mechanical and thermal properties of the medium-low carbon steel C22.

	Mechanical properties	Hardness (Brinell)	Yield stress (MPa)	Ultimate tensile strength (MPa)	Specific heat capacity (J/kg K)	Thermal conductivity (W/m K)
	C22	120	340	405	≈480	≈52
	Elements × 10 -3 wt%	C		Mn	S	P
	C22	140-200	300-600	max 5	max 4

Fig. 2. Geometric characteristics of friction generated on the present experimental set-up.

Table 2

 2 Experimental conditions for the different tests.

	Test reference Sliding velocity	Normal	Delay (s) Aperture time
		(m/s)	pressure (MPa)		(s)
	1	33.4	90.7	101	40
	2	35.8	91.2	463	40
	3	33.9	89.3	593	40
	4	34.2	88.4	640	40
	5	34.5	84.9	325	300
	6	34.2	84.9	381	300
	7	33.8.	84	573	300
	8	33.4	92.8	261	500

Table 4

 4 Estimation of the distance between the asperities and the edge of the hole for the test 3.

	Streak reference	Distance
		Asperity-edge of the hole (mm)
	1	0.83
	2	0.35
	3	0.21
	4	0.16
	5	0
	6	0.16
	7	0
	8	0


domain: info spi

text:
T Delval

A Jolibois

S Carré

S Aguinaga

A Mailhac

A Brachet

J Soula

Building/City Information Model for Simulation and Data Management

This publication is related to a research partnership between the CSTB (Centre Scientifique et Technique du Bâtiment) and the territorial administrative authority of Paris La Défense, regarding multiscale model management and interoperability processes. The project included several steps, starting with the semantization of building data but also information for infrastructures and street furniture. Then, a standard BIM urban model (or City Information Model) has been structured and enriched to address a set of simulations carried out on a pilot urban area. The research challenges were to define data dictionaries, pivotal models and replicable simulation methodologies using the CityGML format and then to integrate them through a multi-scale collaborative format into a database system. In that sense, the objective was to study the potential of BIM as a vector of simulations data for the urban scale.

SCOPE OF WORKS: MODEL STRUCTURATION AND EXPERIMENTATION

Acquisition process

An acquisition process starting from photogrammetry data has enabled the implementation of a BIM model on the entire territory through an interoperable format adapted to the urban area: the CityGML. The experiment focused on a spatial planning area extracted from the overall model, where urban challenges were expected by local authorities.

Semantization process

Our process aimed at automatizing the generation of the BIM model, without manual operations made from operators using CAD tools, which would be in any case almost impossible considering the mass of data to be processed on the territory. It consisted in translating topographical plan data into volumes and objects in the CityGML standard format. The achieved semantization allowed to assign a function to each part of the territory, as well as to cluster acquired meshes and forms into infrastructure objects such as roads, railways, green spaces and buildings and to structure them into the CityGML format (see in Figure 1). During the mesh acquisition process, infrastructure object groups integrated relevant information from the corresponding PostGIS spatial database. The first sketch, produced by the CSTB, has been stored in database using 3DCityDB solution. In this way, Paris La Défense was able to enrich it semantically (in particular adding generics attributes) and geometrically by usual SQL queries and GIS tools (QGis) before exporting it. The urban model has thus been enriched with structured information that can be requested and manipulated in the same way as the spatial GIS data from which it is derived (using for instance query or filtering), adding 3D volume properties to the BIM semantics.

Implementation and experimentation results

As stated above, the main objective of our research was to define an integrated BIM process (see in Figure 2) taking as an input a section of the territory, performing a set of simulations and returning the results within the same standard format. This process could allow storage of information (for Data Management through 3DCityDB) and could be used as an input for further simulations. The first step was to structure and enrich the BIM model to address many simulations from interoperable formats. Our transversal research team -which included experts from several technical domains -carried out wind, acoustic and visual comfort simulations (including shadow and solar reflection simulation) in exterior spaces as well as LCA-based environmental evaluation of buildings. Creating and managing a "simulationready" model suited to all performed simulations represented a very challenging task, both from a scientific and technical standpoint. To achieve this, the CityGML format has been extended using extension mechanisms (ADE) as shown in Figure 3. Each extension was built thanks to a tight collaboration between a BIM expert and a simulation expert for each technical domain so as to be able to acquire and structure all required data, and to get the relevant geometrical description, this for each technical domain. One should point that our purpose is to be able to carry out the results within the OGC consortium to be exploited in future version of the standard. We now give some details about the way each simulation has been set up, run and integrated back into the CityGML database. To do so, the commercial software MithraSIG v5 [START_REF] Geomod | Release of MithraSIG v5 software[END_REF] -Topography (the ground), here defined as a triangulated complex surface part of a CityGML file -Road traffic (road sections, traffic flow per section, rolling speed and percentage of heavy vehicles), defined as a set of polylines containing traffic flow attributes such as the Average Annual Daily Traffic (AADT) per section, here defined in a GIS (SHP) file -Railway traffic (railway sections, number of passbys per section, rolling speed and type of train), defined as a set of polylines for the train tracks, but in which typical number of pass-bys and type of trains were added as attributes.

ACOUSTIC SIMULATION

Preparing simulation from the BIM Model

-Buildings (building ground coverages as polygons and building height as an attribute), which usually come from a GIS database, but here directly integrated from the LOD1 BIM model. -Urban features, including vegetal areas (polygons, here defined as part of the ground CityGML file) and noise barriers (polylines with a height attribute, here defined in a GIS SHP file). One should point out that for the calculation of the Lden, one needs to know how traffic conditions vary between the three periods of the day (day: 6h-18h; evening: 18h-22h; night: 22h -6h). Therefore, from average traffic indicators (such as the AADT), a typical distribution of traffic was applied. Gathering all these data, a complete acoustic simulation model was generated (see Figure 5).

Results computing

MithraSIG can generate several kinds of results, including: horizontal color maps (each color corresponding to a noise level interval), façade color maps, and calculation of a set of predefined points. To generate maps, MithraSIG runs a calculation on a mesh of points generated on the map.

To run the calculation, each road section or train track are divided into equivalent point sources, and ray propagation paths are calculated between each source and each receiver using a ray-tracing algorithm. Then, acoustic propagation effects (reflection, diffraction, geometric attenuation, etc.) for each path are calculated using the standard French method for noise mapping (NMPB'08). Finally, contributions of all paths to a given point are summed to get the level.

The acoustic simulation was first run over an entire neighborhood of Paris La Défense as a horizontal map (at height 1.5m above the ground) with the following calculation parameters: 20m receiver mesh size (yielding about 11400 calculation points); 2 reflections per path; 500m maximum propagation distance; homogenous atmosphere. An example of generated Lden noise map in dB(A) is shown in Figure 6. Other simulations were run in order to calculate noise exposure on building facades (see in Figure 7, and on a set of predefined points (see in Figure 9) which was used in coordination with other simulations (see in section 6). MithraSIG can export its calculation results in many different formats, including GIS type formats (SHP, etc.), PDF for graphical rendering, but also table-like formats (Excel, CSV, etc.) containing a list of objects with their identifiers and attributes values, such as calculated noise level. SHP files can contain polygons defining each colored area. Since one of the objectives was to re-integrate results into the BIM model, transformations were applied on color maps SHP files to integrate them into the BIM model, both for horizontal maps (see in Figure 9) and for buildings façades.

SOLAR COMFORT SIMULATIONS

Regarding lighting issues, it was decided to focus the analysis on three subjects:

-shading analysis to qualify the impact of a development project on the nearby environment. It is therefore necessary to quantify the importance of solar losses / gains for public or private spaces.

-importance of solar reflections by glazed facades, which can induce significant glare risks.

-public lighting performance

Preparing simulation from the BIM Model

Expertise on artificial or natural lighting (sunshine) essentially requires 3D models of building surfaces and the environment as well as the knowledge of the photometric properties of relevant elements: artificial light sources, surface reflection factors, finer descriptions, spectral reflectance, brightness. Urban BIM models are therefore generally well adapted for this purpose, although it is necessary to choose the level of detail most suited to the required complexity (see Figure 10).

Results computing

For all lighting calculations, the software Phanie, developed by CSTB, has been used. It implements a path tracing method using adaptive records [START_REF] Ribardière | Adaptive Records for Irradiance Caching[END_REF]).

The solar shadows analyses are performed on a set of areas of interest. These can be defined based on a list of geo-referenced coordinates or by exploiting various identifiers such as unique IDs of CityGML objects, CityGML types ("road", "wall", ...), or object groups associated with the same material. Then, the most synthetic mode of representation of the results is a map of the number of hours in the shade (or in the sun). This mapping is also available for each time step of the year, which allows for finer analyzes (generation of animations for a typical day for example).

The solar reflections analyses are also performed on a set of areas of interest that can be defined in the same way as for the analysis of solar shadows.

The most synthetic mode of representation is a mapping of the number of hours in solar indirect lighting situation via specular elements. This mapping is also available for each time step of the year.

Finally, artificial lighting analyses are also performed on a set of areas of interest that can be defined in the same way as for the analysis of solar shadows. This time, the simplest representation is a map of lighting levels, calculated at a typical human height (see Figure 11). of urban spaces (excluding roads). The definition of computing grids goes further because it meets normative requirements. For example, it makes it possible to calculate the longitudinal and transverse uniformities of the artificial lighting of the road. Its operation is, however, limited to small areas, which are supposed to be representative of an entire facility. More specific ADEs would be necessary to extend artificial lighting analyses.

ENVIRONMENTAL SIMULATIONS

Preparing simulation from the BIM Model

Two types of environmental simulations have been conducted at district scale: 1) Life-cycle assessment (LCA) and 2) calculation of a biodiversity potential: the biotope area factor (BAF). Life-cycle assessment (LCA) is a method to assess environmental impacts associated with all the stages of a product's or service's life from raw material extraction through materials processing, manufacturing, distribution, use, repair and maintenance, and disposal or recycling. In the construction sector, LCA has been used since the 1990s to evaluate construction products and building and for few years its application has extended to urban scale, and is more and more used to assess urban precincts and building stocks at large scale [START_REF] Mastrucci | Life Cycle Assessment of building stocks from urban to transnational scales: A review[END_REF].

In this study, ELODIE, the building LCA software developed by CSTB, has been used (ELODIE 2018). ELODIE is a collaborative tool for quantifying the environmental impacts over the entire life cycle of a building, or an urban object (infrastructure or street furniture).

Performing LCA at building and urban scale requires to collect large amount of informationdataon the foreground system to complete the life cycle inventory (LCI). The integration of BIM to building and urban LCA can reduce efforts during data acquisition, as well as allowing feedback of LCA results into BIM. As mentioned in section 1.3, the CityGML format has been extended using extension mechanisms (ADE) to provide "simulation-ready" model suited to all performed simulations. Regarding environmental simulations, propositions have been made to extend CityGML and ADE Energy standards for exchanging information for LCA simulation at urban scale. Detailed information is given in [START_REF] Mailhac | A proposition to extend CityGML and ADE Energy standards for exchanging information for LCA simulation at urban scale[END_REF].

The biotope area factor (BAF) describe the proportion of surfaces favoring the biodiversity into an urban area. In a context of massive extinction, the integration of this indicator is a good way to use the BIM model to take into account the biodiversity conservation. In the remainder of this section, main data requirements for both LCA and CBS calculation are listed. To perform LCA at urban scale, urban objects within the physical boundaries of the urban project must be identified. Depending on the objective of the LCA study, and the data availability, a screening, simplified or complete/ detailed LCA approach can be applied [START_REF] Wittstock | Burden of disease from environmental noise[END_REF]. In this study, the environmental assessment focuses on the elements listed above and consider a 50-year period for the district lifetime. All buildings within the project area are considered to be new construction (i.e. impacts related to their construction, use phase and end of life are taken into account). Data related to each object under study are collected (general information, materials and products used, quantitative data…) and stored in the BIM files.

The BAF expresses the ratio of the ecologically effective surface area to the total land area:

𝐵𝐴𝐹 = Ecologically-effective surface areas Total land area (1)

To calculate the BAF, the different surface's types of the urban area are identified and their areas are calculated thanks to information contained in BIM files. Then, each surface type is weighted according to its ecological value given by the (ADEME 2018), see Table 1.

Results computing

Table 1. LCA results on the indicator Global warming potential (kg eq CO2), for the entire district "les Groues"

Once foreground information on the district are collected in the BIM files, environmental impacts are calculated using Elodie and information from environmental databases that can be generic; such as Ecoinvent database (Ecoinvent 2018); or specific to the construction sector; such as the INIES database (French national reference database of environmental declarations for products, equipment, and services in the construction sector) (INIES 2017). LCA results are a set of environmental indicators that reflect the environmental burden of the urban project. The results can be aggregated or presented for each life cycle phase of the project, or for each object. For convenience reasons, it has been decided to present in this paper results obtained only for the indicator « Global warming potential », in kg eq. CO2 (see and Table 2). However, six other environmental categories have been calculated: hazardous waste disposed, nonhazardous waste disposed, total use of primary energy resources, net use of fresh water, photochemical ozone creation, and total use of nonrenewable primary energy resources (primary energy and primary energy resources used as raw materials).

In table 1, results are presented for each object category and for the entire district considering a 50-year period of analysis. Buildings account for more than 90% of the district's impact on global warming. However, one should remind that 1) results are based on hypotheses taken and 2) main objective of the simulation is to prove the feasibility of an environmental computation using BIM files. Regarding biodiversity potential of the district, the BAF result is a single score reflecting the ability of the urban project to welcome the biodiversity. The closer the factor is to 1, the more favorable the site is to biodiversity. "Les Groues" district's BAF amount is 0.16 (intermediate results are presented in the Table 2). For this kind of area (central business facilities) a BAF at 0.30 should be reach as recommended in the city of Berlin (Senate Department for the Environment, 2018).

AERAULIC SIMULATION

Preparing simulation from the BIM Model

The purpose of the aeraulic simulations is to quantify the pedestrian comfort regarding the wind blowing between the buildings. To do so, it has been chosen to perform CFD (Computational Fluid Dynamics) simulations. They solve the Navier-Stokes equations on a discretized mesh representing the volume of air surrounding the buildings. The quality of the results of the simulations in very dependent of the quality of this mesh. The challenge is then to be able to extract a boundary surface from the CityGML suitable for CFD simulations as shown in the process Figure 12. This surface has indeed to be watertight and with a Level Of Detail adapted to the flow features at urban scales. As part of this work, an algorithm has been developed in order to simplify the geometry extracted from the CityGML, but a lot of "manual" operations remain to obtain a geometry compliant with CFD simulations. The LOD2 model which has been chosen was a good starting point regarding flow features, however some details are yet to be removed from the geometry to limit the number of cells in the mesh. Finally, the most difficult task is to connect the building geometry with the ground surface to make a single watertight boundary. Once the "cleaned" geometry has been obtained, it must be meshed. As the pedestrian comfort is monitored at 1.5m from the ground, it is important to have a so-called "boundary layer" mesh which allows a good resolution of the flow near the ground surface (see in Figure 13).

Figure 13. View of a vertical cut of the CFD mesh with a focus on the boundary layer mesh.

Results computing

Simulations have been carried out using the so called steady RANS (Reynolds Average Navier Stokes) approach. It solves the averaged flow and uses turbulence models which mimic the impact of the unsteady part of the flow on the mean flow. A k-epsilon realizable model is used. The RANS approach proposes the most efficient ratio between confidence in the results and computational time. Simulations are ran using the open source CFD code OpenFOAM. The flow is solved for 18 wind directions (from 0° to 360° with 20° steps), an example of the wind flows for directions 40° and 240° is shown Figure 14. A boundary layer profile is prescribed at the inlet of the simulations. This profile is relevant of the density of the surrounding urban area according to the Eurocode regulation. Special boundary conditions on the ground (rough walls) are used in order to maintain this boundary layer profile all over the simulations. For each wind directions the wind at pedestrian level (1.5m) is measured and compared to the wind speed prescribed at the inlet of the simulation, which is also relevant of the wind speed which is measured at a local weather station. It is then possible to calculate a "transfer function" between the wind speed measured at the weather station and the wind speed all over the urban area. The wind statistics of the weather station can be then extrapolated to every single point of the simulation.

A quantitative comfort criterion is then defined. It is based on the probability of the wind speed exceeding 3.6m/s. This "discomfort frequency" shall not exceed a given value depending of the usage of the area. For example, for a restaurant terrace, it shall not exceed 2%, for a strolling area 10%.

Integration into the BIM model

The results of the simulations provide a map of this comfort criterion all over the pedestrian area. This data is then helpful to assess the impact of modifications of the urban area on the pedestrian wind comfort. Those maps are then directly inserted inside the BIM model (see Figure 15). It is also possible to insert some ribbons drawn from the CFD results which picture the flow behavior between the buildings. These ribbons are then helpful to understand the origin of comfort issues and to design corrective devices. For example, in dense areas with high rise buildings, it is not uncommon to see comfort issues induced by streams of wind collected at the top of the tower and sent toward the ground. This can be solved using wind barriers or tall vegetation. Although directly displaying simulation results (like noise or pressure maps) allows some form of analysis, the issue of storage and cross-analysis is still a key issue which our work tried to address. The proposed approach was to store all results as BIM data in the same CityGML format as the input data. Multi-disciplinary analysis was then made possible thanks to the development of a generic colorization tool in order to display any type of CityGML object attributes in agreement with attribute values.

To apply this idea, a set of "sensor" points were generated (from a CFD meshing tool) and given as input for all simulations. This set constituted a common basis on which all simulations above were performed (see in sections 2 to 5). Once all simulations results were generated, the sensor points were integrated into the BIM model. They were structured in the form of City-Furniture objects of implicit geometry, which means that each point geometry was an instanced parallelepiped. The geometry is thus stored only once and then replicated when displayed on all points. Files containing results are thus very light since they only contain point coordinates and values coming from the different simulations (see in Figure 16).

Future Works

The next objective of our research is to design a decision support tool for the analysis of public spaces overall quality. The main feature of this tool would be to combine the results of physical simulations, statistical data and socio-urban approaches into a set of key performance indicators.

Besides, our social scientists and the administrative authority in charge of urban strategy are working on innovative user-centric approaches for urban planning and on ways of translating urban quality criteria into BIM standards.

Figure 1 .

 1 Figure 1. Partial view of the generated BIM model in CityGML format including CFD simulation results.

Figure 2 .

 2 Figure 2. Schematic of the BIM simulation integrated process

Figure 3 :

 3 Figure 3: Example of Transportation Complex UML diagram developed for use of simulation.

  As noise exposure is still one of the main concerns for people annoyance and a well-documented cause for adverse health effects (World Health Organization 2011), one of the decided indicator to be calculated was average noise level. Several indicators are typically used in European and French noise regulations, including Lday (day-noise indicator), Lnight (nightnoise indicator) and Lden (day-evening-night noise indicator). For consistency with the European directive 2002/49/EC, we chose as an example to calculate the Lden, which is the specified indicator to assess overall annoyance.

Figure 4 .

 4 Figure 4. Application of the CityGML LOD2 to LOD1 transformation algorithm to buildings objects.

Figure 5 .

 5 Figure 5. View of the MithraSIG acoustic simulation model one all relevant data were integrated.

Figure 6 :

 6 Figure 6: Generated Lden horizontal noise map. dB values vary between below 45 (deep green) to above 75 (deep purple).

Figure 7 :

 7 Figure 7: Example of building facade noise level calculation (same color code as above)

Figure 8 :

 8 Figure 8: Noise level calculated on a set of predefined set of points (same color code as above).

Figure 9 .

 9 Figure 9. Integration of a horizontal noise map exported from MithraSIG as a SHP file into the BIM model.

Figure 10 .

 10 Figure 10. Example of a BIM model imported by Phanie (used lighting simulation software).

Figure 11 .

 11 Figure 11. Map of lighting levels on a set of geo-referenced points

Figure 12 .

 12 Figure 12. Illustration of the process to generate a correct CFD mesh from the CityGML model

Figure 14 :

 14 Figure 14: Examples of flow features for two wind directions

Figure 15 :

 15 Figure 15: Pedestrian wind comfort map inserted in the BIM model

Figure 16 .

 16 Figure 16. Aggregation of simulation results into data points defined as City Furniture Objects into the CityGML model.

Table 2 .

 2 Intermediate results for the BAF