A Younes

G Montavon

C Alliot

M Mokili

F Haddad

D Deniaud

J Champion
email: champion@subatech.in2p3.fr

A route for polonium 210 production from alpha irradiated bismuth-209 target

Keywords: Polonium-210, bismuth, α irradiation, liquid-liquid extraction

A method is proposed for production and purification of polonium-210 via the 209 Bi (, 3n) 210 At nuclear reaction. Bombardment of a bismuth-209 target was performed with a 37 MeV alpha beam that conducts to the production of astatine-210 (8.1 hrs), which decays to polonium-210. It is further purified from the bismuth target matrix by employing liquid-liquid extraction method using tributyl phosphate (TBP) in para-xylene from 7 M hydrochloric acid.

Back extraction of polonium-210 was performed by 9 M nitric acid. This method allows to purify a tracer amount of Po-210 (2.6 10 -13 mol) from macroscopic amount of Bi (2.8 10 -2 mol).

Introduction

Although Pierre and Marie Curie made the discovery of the element polonium (Po) more than a century ago, physical and chemical properties of this element and its complexes are still barely known. This can be explained by two main reasons: first, polonium has forty-two known isotopes with a mass ranging from 188 to 220, all radioactive [START_REF] Fry | Discovery of the thallium, lead, bismuth, and polonium isotopes[END_REF]. Second, one of its isotopes, polonium-210 (Po-210) that occurs naturally in the uranium-238 decay is very rare in nature which impedes its studies. The equilibrium ratio of U to Po is 1. 19 10 10 , so that the Po concentration in uranium ores is less than 0.1 mg/ton [START_REF] Figgins | The radiochemistry of Polonium[END_REF]. Po-210 is a high-energy alpha emitter (E= 5.305 MeV) [START_REF] Lederer | TABLEOFISOTOPES[END_REF] with a radioactive half life of 138.376 days. It is known for being one of the most toxic radionuclides due to its high specific activity (1.66 10 14 Bq/g) [START_REF] Scott | Health risk evaluations for ingestion exposure of humans to polonium-210[END_REF] It presents an internal radiation hazard due to its short range of alpha particles in biological tissues (40-50 m) [START_REF] Harrison | Polonium-210 as a poison[END_REF]. With the advent of cyclotrons and nuclear reactors and their intense fluxes, production of milligrams of Po-210 has become feasible. Bombardment of stable bismuth-209 (Bi-209) with intense neutron fluxes has been studied in nuclear reactors [START_REF] Vogelsang | Polonium production in the Mars reactor[END_REF]. Bi-210 in its ground state Bi-210g (5.012 days) is produced by the Bi-209(n,)Bi-210 reactions and decays to 210 Po via -decay. The production cross-section of Bi-210 being about 15 mb, a Po-210 activity close to 4.10 3 GBq/kg of Bi-209 is obtained after 200 days of irradiation in a thermal neutron flux of 1.10 14 n/cm 2 .s [START_REF] Mikheev | Polonium.ChemikerZeitung[END_REF]. Po-210 can also be produced in cyclotrons via the direct reaction Bi-β0λ(α, t)Po-210 [START_REF] Hermanne | Experimental study of the cross-sections of alpha-particle induced reactions on 209Bi[END_REF]. Another possibility is to produce polonium from the decay of At-210 obtained via the reaction Bi-β0λ(α,γn)At-210 [START_REF] Kelly | Some excitation functions of bismuth[END_REF]. This reaction is used in the present work as an easy way to produce Po-210 for radiochemistry studies. For this application, the production of Po-210 with good radionuclide and chemical purities is required.

The separation of polonium includes several steps: (i) dissolution of the irradiated target, (ii) Bi-Po separation and (iii) reconditioning.

The bismuth target will not dissolve easily in hydrochloric acid solution alone. In other hand, bismuth could be dissolve in nitric acid and no special problems are presented. A mixture of hydrochloric acid and nitric acid in a proportion of 1:4 and 3:4 will dissolve bismuth metal [START_REF] Bhatki | Radiochemistry of bismuth[END_REF].

Many methods can be used for the separation of Po from Bi, such as electrodeposition, [START_REF] Meyer | [END_REF] solvent extraction and ion separation [START_REF] Reischmann | Chemistry at low concentrations: polonium at a level of 108 to 105 atoms[END_REF][START_REF] Vajda | A novel technique for the simultaneous determination of 210Pb and 210Po using a crown ether[END_REF]. For simplicity of setting up, liquid-liquid extraction has been chosen. Several extracting agents [START_REF] Martin | Routine analysis of naturally occurring radionuclides in environmental samples by alpha-particle spectrometry[END_REF][START_REF] Clayton | A cost-effective method for the determination of 210Po and 210Pb in environmental samples[END_REF][START_REF] Smithson | Radiochemical determination of lead-210 in environmental samples resulting from uranium mining-milling operations-Radiochemical procedures for determination of selected members of the uranium and thorium series[END_REF][START_REF] Wai | Extraction and separation of 210Pb, 210Bi and 210Po by diethyldithiocarbamate[END_REF][START_REF] Roseberry | Radiochemical analysis of 210Po in coal gasification samples[END_REF][START_REF] Sheppard | The distribution of bismuth (III) and polonium(IV) between trilaurylamine solution of xylene and hydrochloric and hydrobromic acid solution[END_REF][START_REF] Ibrahim | Plant accumulation and plant/soil concentration ratios of 210Pb and 210Po at various sites within a uranium mining and milling operation[END_REF][START_REF] Hataye | Solvent extraction study on the hydrolysis of tracer concentration of Po(IV) in perchlorate solutions[END_REF][START_REF] Suganuma | Solvent extraction study on the hydrolysis of tracer concentration of Po(IV) in chloride solutions[END_REF][START_REF] Hataye | Solvent extraction study on the hydrolysis of tracer concentration of Po(IV) in nitrate solutions[END_REF][START_REF] Johansson | Extraction of polonium from aqueous lactic acid solutions using dioctyl sulphide, Cyanex 272, Cyanex 301 or Cyanex 302 in toluene[END_REF][START_REF] Matsuura | Studies on extraction of polonium(IV) by hexone from acid Solution[END_REF][START_REF] Sheppard | The distribution of polonium-210 and bismuth-210 between linear aliphatic ethers and nitric acid solutions[END_REF][27][START_REF] Bagnell | Solvent extraction studies with polonium[END_REF][29][START_REF] Jia | Distribution coefficients of polonium between 5% TOPO in toluene and aqueous hydrochloric and nitric acids[END_REF][START_REF] Case | An improved sensitive assay for polonium-210 by use of a backgroundrejecting extractive liquid scintillation method[END_REF][START_REF] Jia | Distribution coefficients of polonium between 0.75 M HDEHP in cyclohexane and aqueous hydrochloric and nitric acids[END_REF][START_REF] Schulz | Dibutyl Carbitol Solvent Extraction of Polonium-210 from Nitric Acid Solutions of Irradiated Bismuth[END_REF] were used for the extraction of Po from different aqueous layers (HCl, HBr, HNO 3 , H 2 SO 4 , HClO 4 , H 3 PO 4 or lactic acid) into various organic layer (carbon tetrachloride, chloroform, xylene, toluene or cyclohexane) (Table .1). For all of the previously mentioned methods, the extraction yield of polonium is significant. However, some of the papers mention a poor extraction of Po/ Bi separation [START_REF] Wai | Extraction and separation of 210Pb, 210Bi and 210Po by diethyldithiocarbamate[END_REF] while others indicate a good separation but without quantitative data. In addition, most of the studies report the separation polonium-210 tracer from bismuth-210 tracer [27].

As it is mentioned before, a prerequisite in the approach is to dissolve Bi target before Po extraction. Therefore, an extracting agent efficient in strong acidic conditions (HCl and HNO 3 ) is required. Among the different molecules proposed, TBP has been chosen in this work. The challenge of this work was to separate radiotracer quantity of polonium-210 from macroscopic quantities of bismuth using TBP. In order to optimize this purification method, several parameters were studied (solvent nature, acidity, aqueous medium, extraction time, TBP dilution, organic solvent) for defining the best conditions for polonium separation.

Materials and method

Chemicals

All solutions were prepared using Milli-Q water and all experiments were conducted at room temperature (ββ±γ˚C). All the chemicals used were of analytical reagent grade.

Bismuth foils (chemical purity 99.999% content of Pb, Cu and Ag below 3, 2 and 2 ppm respectively, 25 x 25 mm 2 , thickness of 1 mm), kapton Polyimide Film (75 m , β5 x β5 mm 2 ), aluminium foil (chemical purity 99.999%, 25 x 25 mm 2 , thickness of 1 mm), silver foil (chemical purity 99.999%, 25 x 25 mm 2 , thickness of 0.25 mm ) were purchased from Goodfellow and used as received. Po-210 (2 M HCl) and Bi-207 (1 M HCl) tracers were purchased from Eckert Ziegler and CERCA LEA, AREVA, France, respectively. Both are used after reconditioning and diluting into the appropriate medium.

Irradiation

The targets are bismuth foil (Bi-209) placed behind an aluminium foil in a capsule. The ARRONAX cyclotron is designed to deliver a fixed 67.4 MeV alpha beam. In order to decrease the energy of the alpha beam, a degrader system was used. This system is built up and placed along the alpha beam line (Figure 1).The target was placed in air 13 cm away from the end of the beam line which is closed by a kapton foil. The alpha beam used for irradiation is extracted at 67.4 MeV. Typical irradiation stands one hour with an average beam current of 0.β A. According to literature cross section data of α-particle induced reactions on Bi-209, an energy of 37 MeV was selected for optimal production of At-210 (Figure 2). [START_REF] Hermanne | Experimental study of the cross-sections of alpha-particle induced reactions on 209Bi[END_REF][START_REF] Maiti | Theoretical approach to explore the production routes of astatine radionuclides[END_REF][START_REF] Lahiri | Recent developments in nuclear data measurements and chemical separation methods in accelerator production of astatine and technetium radionuclides[END_REF] An aluminum foil of 0.9 mm thick is used to degrade the incident energy .The target and its degrader foil are presented in Figure 1. The bismuth foil was ensured by an air cooling system during irradiation. After the end of irradiation, the target cell containing the aluminium and bismuth foils was left in a lead container for a week, allowing At-210 to decay into Po-210.

Batch experiments

Tubes of polypropylene were used for liquid/liquid batch experiment, due to the adsorption of polonium on glass [START_REF] Matsuura | Studies on extraction of polonium(IV) by hexone from acid Solution[END_REF] which causes difficulty in the accurate determination of polonium activity. Before polonium addition, the organic layer was pre-equilibrated with the aqueous solution, i.e. the composition does not change in presence of the organic layer.

2 mL of organic and aqueous solution were brought in contact. After equilibrium of the biphasic system, an aliquot of polonium (300 Bq) and/or bismuth (100 Bq) was added and the tubes were shaken for 90 min. This time proved to be sufficient to achieve distribution equilibrium of polonium/ bismuth between the phases.

After phase separation, an aliquot of the aqueous and organic phases were withdrawn to derive the percentage of extraction, %E, corresponding to:

100 V V A A A %E org aq aq org org                    eq.(1)
where, A org and A aq define the polonium activities measured in the organic and aqueous phases, respectively. V org and V aq define the volume of organic and aqueous phases, respectively.

For the back-extraction, %BE is defined as:

100 V V A A A %BE aq org aq org aq                    eq.(2)

Analytical Tools

The radionuclide composition was checked by gamma spectrometry. A gamma spectrometer from ORTEC, using a high purity germanium detector with 1.82 keV resolution at 1.33MeV, was used to quantify the production of gamma rays-emitting radionuclides, and more particularly At-210 and At-211, after Bi-209 irradiation [START_REF] Hermanne | Experimental study of the cross-sections of alpha-particle induced reactions on 209Bi[END_REF][START_REF] Lambrecht | Cyclotron isotopes and radiopharmaceuticals-XXXV astatine-211[END_REF].

After the dissolution of the irradiated bismuth foil in 7 M HCl solution, an alpha spectrometer was used for the determination of polonium-210 activity. Sample for alpha spectrometry was prepared by spontaneous deposition of Po-210 on silver discs [START_REF] Flynn | The determination of low levels of polonium-210 in environmental materials[END_REF][38][START_REF] Feldman | Precision plating of polonium[END_REF]

Results and discussion

Production of Po-210

Using SRIM-2008 software [40], calculation shows that the bombardment of Bi-209 foil (1 mm) by an alpha particle beam with energy of 37.02 MeV is achieved by using a 0.9 mm aluminium foil. The energy of the delivered alpha beam arrived on the aluminium foil with a value of 64.5 MeV and an average current being of 0.2 µA for one hour. The stopping point inside the bismuth target was measured to be 0.314 mm at 37.02 MeV.

The produced activity can be determined numerically using the following formula:

     max min ) ( )) exp( 1 ( 0 E E a dE dx dE E t N A A    eq. (4)
Where,      a and are the incident particle number (particle/s/cm 2 ), target density, target number of mass, Avogadro number, radioactive constant (s -1 ) and production cross section, respectively.

dx dE is the energy loss calculated using the Bethe formulae: The analysis of the irradiated Bi-209 target by using gamma spectrometry (Figure 4), shows the production of At-210 and At-211 which decay to Bi-207 (t 1/2 =32.9 y) and Bi-206 (t 1/2 =6.423 d), respectively. No other ray emitting radionuclides were found. After bismuth target irradiation, the foil is kept 7 days in a lead container for the total decay of At-210 into Po-210.

        

Polonium / Bismuth batch experiments

The method for separation of polonium is essentially based on the work of Chen et al. [27],

i.e. the extraction is performed in HCl whereas the back extraction is realized in nitric acid.

However several optimization steps were studied in order to find the best conditions for The extraction of polonium from hydrochloric acid solution is strongly dependent on the acid concentration of the aqueous phase. Investigation of the extraction of Po and Bi as a function of hydrochloric concentration in 10 % TBP / para-xylene is presented in Figure 5. A good polonium extraction value of 89.0 ± 5.0 % was obtained at 0.17 M HCl. However, when the acidity increases, the extraction values decrease until it reaches a minimum value of 5.0 ± 0.3 % at 2 M HCl. After 2 M HCl, the extraction value of Po increases again and reaches its maximum value of 96.0 ± 4.7 % at 7 M HCl. For Bi, the extraction is weak in the acidic range studied with a maximum extraction value of 6.7 ± 0.5% for 0.17 M HCl. In conclusion, a good compromise (high extraction yield, good selectivity) would be to work at 0.1 M or 7 M HCl, with a preference for the latter condition.

Then, the influence of the concentration of tributyl phosphate in para-xylene from 7 M aqueous hydrochloric acid was performed (Figure 6). The extraction of Po is greater than 90 % whatever the concentration of TBP studied with a slight increase, as the TBP concentration increases. For Bi, the extraction is almost equal to zero for a TBP concentration below 1 M. Above 1 M, Bi extraction becomes significant with 25 % of Bi extracted at 3.5 M of TBP. To have a good compromise between extraction yields on selectivity, a concentration of 0.36 M in TBP (or 10 % in weight) was selected for the extraction step.

The following step is to back extract polonium to a new aqueous layer to be used for further studies. Investigation shows nitric acid to be the convenient aqueous layer for polonium back extraction [START_REF] Meinke | American Report AECD-2738[END_REF]. Back extraction of Po in para-xylene is studied as function of HNO 3 concentration (Figure 7). Back extraction of polonium increases as the acidity increases and reaches a maximum value of 96.0 ± 4.0 % for a solution of 9 M HNO 3 . This can be explained by the increasing competition between Po and HNO 3 for TBP [START_REF] Naganawa | Complex formation between tributyl phosphate and nitric acid and the hydration of the complexes in dodecane[END_REF]. The potential degradation of TBP can contribute to explain the important back-extraction. Indeed according to the literature, at high concentrations of nitric acid, TBP can be hydrolyzed like it occurs in reprocessing spent reactor fuels [START_REF] Naylor | TBP extraction systems-TBP and diluent degradation, KR-126[END_REF][START_REF] Richardson | The effect of high solvent radiation exposure on TBP processing of spent LMFBR fuels[END_REF]. To confirm the hydrolysis of TBP at high acidic nitric acid solution, the aqueous phase (8 M HNO 3 ) obtained after extraction was analyzed by mass spectrometer. Results show the presence of phosphoric acid, monobutyl phosphate and dibutyl phosphate which are the hydrolyzed products of tributyl phosphate. Thus, the concentration of 9 M nitric acid will be chosen for the purification process.

Finally, the idea was to test the protocol optimized for the polonium extraction and backextraction to see the behavior of bismuth using the whole procedure. The final step was to confirm the absence of cold bismuth in the back extracted solution. As the analysis using ICP-OES gives signals below the detection limit of ICP-OES (<1 ppb), absence of bismuth-209 can be guaranteed.

Target dissolution and Po/ Bi separation

In this part, we want to use the results previously presented in order to obtain the best conditions for the Po/ Bi separation from the irradiated target. The first step is the transfer all the polonium from the target into solution. The depth of penetration determined by SRIM-β008 software corresponded to 0.γ14 m. To recover all produced Po-210, a total dissolution of the irradiated foil (thickness of 1 mm) is required. The easiest way was to dissolve directly the target in 0.1 M or 7 M HCl (see previous part). However the solubility of metallic Bi is not important in HCl. It was then decided to follow the work of Bhatki et al. [START_REF] Bhatki | Radiochemistry of bismuth[END_REF] and to use 10 M HNO 3 . The next step is then the reconditioning of the extraction medium after evaporation. In order to remove the trace quantities of nitric and nitrous acids, two evaporation cycles in 7 M HCl using IR-lamp are done. IR-lamp was used for acidic solution evaporation to minimize the loss of polonium due to its high volatility [START_REF] Martin | The thermal volatilisation of caesium-137, polonium-210 and lead-210 from in vivo labelled samples[END_REF][START_REF] Cleary | Loss of polonium-210 on dry ashing rat tissues in a muffle furnace[END_REF][START_REF] Mabuchi | On the volatility of some polonium compounds[END_REF]. 8.0 ± 0.5 moles of nitric acid was used per mole of dissolved bismuth; the off gases appear to be nitric oxide and nitrogen dioxide [START_REF] Schulz | Dibutyl Carbitol Solvent Extraction of Polonium-210 from Nitric Acid Solutions of Irradiated Bismuth[END_REF].

Solubility tests showed that an HCl concentration lower than 3.5 M was sufficient for a complete dissolution of the residue. An attempt was done to decrease the HCl concentration to 0.1 M, after complete dissolution of the target, with the idea to precipitate bismuth while keeping polonium in solution. However, a strong decrease in polonium concentration was observed indicating a co-precipitation process in agreement. This was already evidenced by Holgye [START_REF] Holgye | Coprecipitation of polonium with bismuth phosphate[END_REF]. Therefore, it was decided to use 7 M HCl for the extraction step.

Alpha spectrometry analysis of the dissolved Bi foil shows only one peak at 5.3 MeV, which corresponds to the presence of Po-210 (Figure 3). Gamma spectrometry analysis indicates the presence of Bi-207 and Bi-206, produced from the decay of At-211 and At-210, respectively.

After the extraction of polonium in the presence of 10% TBP in para-xylene, the trace quantities of bismuth extracted in the organic phase were removed by washing two times with a new equilibrated layer of 7 M HCl. The affinity of Bi at this acid concentration is largely favorable for the aqueous phase. The extraction yield for Po was 96.0 ± 5.0 %.

The following step was to back extract polonium from the previously extracted organic phase to 9 M HNO 3 .The percentage of polonium back extracted was 96.0± 4.0 % and the percentage of the total recovery yield of polonium in this methodology was 85.2 ± 4.5%. Bismuth was confirmed to be under the detection limit.

The final step was to evaporate the nitric acid and to recondition in the appropriate medium such as HClO 4 or HCl for the radiochemical studies. The phosphoric acid coming from the degradation of TBP (see previous part) was not observed in the reconditioned solution (Ionic chromatography, DL = 50 ppb). The complete methodology is depicted in Figure 8.

Conclusion

The bombardment of bismuth-209 target with a 37 MeV alpha particle beam conducts to production of astatine-210 (8.1 hrs) which decays to polonium-210. The produced radiotracer of polonium is purified from macroscopic quantities of bismuth by a wet methodology using liquid-liquid extraction. The procedure is presented in Figure 8. Results confirm that polonium can be extracted (96.0 ± 4.7%) to an organic phase containing 10% TBP in paraxylene from 7 M HCl while the bismuth nuclides remain in the aqueous phase. Moreover, the back-extraction of polonium from the organic phase can be performed by using a 9 M HNO 3 phase (96.0 ± 4.0%). The optimized process leads to a solution of Po-210 with a global recovery yield of 85.2 ± 4.5% with good radionuclide and chemical purities.

  in a water bath (80˚C) for two hours. The detector consists of a 24 mm ruggedized surface barrier detector connected to a 7401 VR Canberra spectrometer. Spectra are obtained by a TRUMP 8 K multichannel buffer card on PC. Efficiency of the detector is determined by a multi-alpha surface source with geometry identical to the Po-silver sample disks. (Figure 3) For batch experiments, activity of the radionuclides (Po-210 or Bi-207) was determined by liquid scintillation counting using a Packard 2550 TR/AB Liquid Scintillation analyzer. The samples were prepared by mixing an aliquot of 0.2 mL of the solution to measure with 2 mL of 1 M HCl and 2.2 mL of ultimate gold AB scintillation cocktail. The measuring time was fixed at 1 hour. The quenching arising from the organic solvent was taken into account according to the following relation: A = A m (-9.10 -5 × Tsie 3 + 673 10 -4 ×Tsie 2 -17.26×Tsie + 1565.8)eq.(3)A m being the activity measured by liquid scintillation and Tsie, an independent parameter from polonium analyses defined by the apparatus to determine the quenching parameter.Determination of stable bismuth-209 concentration was performed by Inductively CoupledPlasma Atomic Emission Spectroscopy (ICP-OES, Thermo Scientific iCAP 6500) with a plasma power of 1150 W. These analyses were done with a rate of 0.50 L/min auxiliary gas, 1β.00 L/min gas cooling and 0.45 L/min for gas nebulizer. The wavelength ( ) used for bismuth analysis was 223.061 -306.770 nm. Mass spectrometer -Electrospray (MS-ESI) was used to analyze the hydrolysis of tributyl phosphate in 8 M nitric acid solution. The analysis of phosphate ions in the final purified polonium solution was done by Ion-exchange chromatography ICS2500 with a conductivity detector. These analyses were done using AS18 (4 x 250 mm) column type with a 50 L injection loop, 1 mL/ min flow and 120 mA current suppression.

  e , Z p , Z, I, and  correspond to a constant (equal to 0.307 MeV.cm 2 ), electron mass, projectile charge, target charge, ionization potential and relativistic coefficients associated to the projectile, respectively. The theoretical activity calculated based on the irradiation parameters (0.2 µA, 1 hr) amounts to 6.34 MBq of At-210. The experimental results shows the production of 4.11 MBq of At-210 which corresponds to the production of almost 10 kBq of Po-210.

  achieving a high extraction yield of polonium-210 and a high selectivity for polonium extraction over bismuth. These experiments were done with synthetic solutions containing either Po-210 or Bi-207. The influence of the solvent nature containing TBP (10%) was first studied from 6.9 M aqueous hydrochloric acid. Hexane, para-xylene, toluene, ortho-xylene, and chloroform were studied. Results show no effect of the investigated solvent on the extraction of Po with an approximate extraction percentage of 83.0 ± 6.0 %, except for chloroform for which a negligible extraction value is found. Para-xylene was chosen for the further studies.

Figure 1 :

 1 Figure 1: Schematic diagram for the irradiation setup of bismuth-209 foil by a 37 MeV

Figure 2 :

 2 Figure 2: Direct production cross-section for At-211, At-210, and Po-210 by nuclear reaction of Bi-9 α, n At-211, Bi-9 α, n At-210 and Bi-9 α, t Po-210,

Figure 3 :

 3 Figure 3: Alpha spectrum of the irradiated bismuth target after dissolution in 2 M HCl

Figure 4 :

 4 Figure 4: Gamma spectrum obtained for a Bi-209 foil after irradiation with a 37.0 MeV alpha beam with a current of 0.2 μA for hour.

Figure 5 :

 5 Figure 5: Effect of the nature and acidity of aqueous layer on the extraction of polonium-

Figure 6 :

 6 Figure 6: Effect of the concentration of tributyl phosphate on the extraction of

Figure 7 :

 7 Figure 7: Effect of the concentration of nitric acid on the back-extraction of polonium-

Figure 8

 8 Figure 8 Methodology followed to recover Po-210 (2.6 10 -13 mol) from the irradiated bismuth target (2.8 10 -2 mol).

Acknowledgement

The authors thank the National Center of scientific research (CNRS) and the Region Pays de

Loire for the grant allocated to Ali Younes to perform his thesis. We thanks Veronique Baty for the ionic chromatography analysis.

Table and Figures


Remi Ponche
email: remi.ponche@irccyn.ec-nantes.fr

Olivier Kerbrat
email: olivier.kerbrat@irccyn.ec-nantes.fr

Pascal Mognol
email: pascal.mognol@irccyn.ec-nantes.fr

Jean-Yves Hascoet
email: jean-yves.hascoet@irccyn.ec-nantes.fr

A novel methodology of design for Additive Manufacturing applied to Additive Laser Manufacturing process

Keywords: Additive Manufacturing, Numerical chain, DFAM, Manufacturing path, Numerical modelling, Additive Laser Manufacturing (ALM)

Nowadays, due to rapid prototyping processes improvements, a functional metal part can be built directly by Additive Manufacturing. It is now accepted that these new processes can increase productivity while enabling a mass and cost reduction and an increase of the parts functionality. However, the physical phenomena that occur during these processes have a strong impact on the quality of the produced parts. Especially, because the man-

ufacturing paths used to produce the parts lead these physical phenomena, it is essential to considerate them right from the parts design stage.In this context, a new numerical chain based on a new design for Additive Manufacturing (DFAM) methodology is proposed in this paper, the new DFAM methodology being detailed; both design requirements and manufacturing

Introduction

Additive Manufacturing is a new way to produce metal parts compared to the classical manufacturing processes such as milling or casting. They allow new design perspectives in terms of material [START_REF] Muller | Modeling and control of a direct laser powder deposition process for Functionally Graded Materials (FGM) parts manufacturing[END_REF], shape [START_REF] Tomlin | Topology optimization of an additive layer manufactured ( ALM ) aerospace part[END_REF] and internal structure [START_REF] Yadroitsev | Strategy of manufacturing components with designed internal structure by selective laser melting of metallic powder[END_REF]. Indeed, inter alia because these manufacturing processes eliminate the need of tooling, many of the current restrictions of design for manufacturing and assembly are no longer valid. However, they have also their own characteristics and specificities which have to be considered during the design stage to ensure the manufactured parts quality. In this paper a new global numerical chain for metallic Additive Manufacturing is introduced based on a global design for Additive Manufacturing (DFAM) methodology. It is applied to the Additive Laser Manufacturing (ALM) technology to design and manufacture thin-walled metal parts.

In the last years, ALM has become an attractive research topic [START_REF] Dubourg | Technological and scientific landscape of laser cladding process in 2007[END_REF], it allows production of functional metal parts. In this process, a five-axis deposition head injects metal powder onto an underlying substrate locally melted with a laser beam to form a deposition bead. Parts are thus manufactured continuously with the deposition head motion along the designed manufac-turing paths. This technology has broad applications in die mould industry, aviation industry and aerospace industry, however, the fabrication of thinwalled metal parts could be one of the important research topic for ALM. This type of structure is especially used in aviation and aerospace industry, where the most common features are rib structures with large pockets in order to strongly lighten the aircraft without damage the desired mechanical strength. Usually, this type of part is manufactured by milling with a long production sequence and a lot of wasted material. They are all the more difficult to machine in the case of a high ratio length/diameter of the tool [START_REF] Kerbrat | A new DFM approach to combine machining and additive manufacturing[END_REF] whereas with the ALM process, only the useful material is used and high depth-to-diameter ratios can be easily fabricated. Nevertheless, for the use of the ALM processes to become a industrial reality, the control of the deposit geometry is a critical issue since it impacts the manufactured parts quality.

From this statement, without changing the usual numerical chain for Additive Manufacturing (Fig. 1), several studies about the improvement of the ALM technology have been reported by proposing classical closed-loop controls on some manufacturing parameters to control the deposit height [START_REF] Tang | Layer-to-layer height Control for laser metal deposition Process[END_REF], the molten pool dimensions [START_REF] Salehi | Melt pool temperature control using LabVIEW in Nd:YAG laser blown powder cladding process[END_REF] or its temperature [START_REF] Bi | Development and qualification of a novel laser-cladding head with integrated sensors[END_REF]. However, these works are usually applied to simple wall and never to complex parts. Despite improving the geometry precision of the deposit and therefore of the thinwalled parts, a loop control is not always enough to ensure the desired quality. Indeed, in ALM, a large number of manufacturing parameters govern the physical phenomena that occur during the manufacturing process [START_REF] Boddu | Empirical modeling and vision based control for laser aided metal deposition process[END_REF]. These physical phenomena are sensitive to the environmental variations and interact with each other. It is therefore very difficult to ensure the quality of a complex part by controlling only few parameters. The process control is limited by its high complexity. In order to ensure the complex parts quality it is therefore also needed to choose the deposition head path regarding the physical phenomena that occur during the manufacturing process.

Theoretical CAD model

Manufacturing paths generation

Manufactured part

Manufacturing process It is proved that, for all the metallic Additive Manufacturing processes, the shapes of the manufacturing paths have a strong impact on the manufactured part quality in terms of micro-structure [START_REF] Alimardani | On the delamination and crack formation in a thin wall fabricated using laser solid freeform fabrication process: An experimental-numerical investigation[END_REF] and of mechanical behaviour [START_REF] Foroozmehr | Effect of path planning on the laser powder deposition process: thermal and structural evaluation[END_REF]. Moreover, they have a direct impact on the part geometry [START_REF] Kao | Optimal motion planning for deposition in layered 32 manufacturing[END_REF]. It is all the more true in the particular case of the thin-walled structures, where the parts thickness is close to one deposit width. A modification of the manufacturing path cannot be thus done without modifying the part geometry. Because unidirectional (without any possible feed-back from the floor-shop [START_REF] Bonnard | Integration of Rapid Manufacturing processes in a high-level numerical chain[END_REF]), the usual numerical chain for Additive Manufacturing presented in figure 1 cannot considerate this impact on the CAD model and is thus no longer suitable .

Not controled geometric deviations

From this statement, a new numerical chain is proposed, based on a global DFAM methodology. It allows to determine an optimized process planning regarding the process characteristics and constraints directly from the functional specifications of a part. The manufacturing program used to control the manufacturing process is then defined in parallel with the corresponding CAD model which is therefore as realistic as possible (Fig. 2).

As a result, the manufactured part is not only close to the CAD model but it is also ensured to fulfil the initial functional specifications.

DFAM methodology

Manufactured part

Functional specifications This paper's purpose is to present the proposed DFAM methodology and the associate numerical tools in the particular case of thin-walled parts manufactured by an ALM process. The global methodology is firstly presented in section 2, then in the section 3, each step is detailed in the case of using ALM process to manufacture thin-walled parts. An example is finally proposed in section 4.

Realistic

A new general DFAM methodology

Organized into three main steps [START_REF] Ponche | Consideration of the manufacturing trajectories in a global design for additive manufacturing methodology[END_REF][START_REF] Ponche | A new global approach to design for additive manufacturing[END_REF], the proposed DFAM methodology (Fig. 3) allows to balance the requirements from both the functional specifications and the chosen Additive Manufacturing process specificities.

Part orientation

The first step consists of determining the functional surfaces position of the studied design problem into the manufacturing machine to determine the "design area", that is to say the volume merging the functional surfaces which is also accessible by the manufacturing process. It has to be done according to the functional surfaces and the global process characteristics which are: the dimensions of the machine work area, the kinematics and the required accessibility.

The orientation problem is, in the case of the Additive Manufacturing processes, a multi-criteria problem since both the fabrication cost and quality for a given orientation needs to be assessed. In order to take into account the both, decision support tools have been developed in recent years, they provide estimations of surface roughness, accuracy, time, and cost in order to help the operator in the estimation of the quality-cost trade-offs in orientation selection [START_REF] Byun | Determination of optimal build direction in rapid prototyping with variable slicing[END_REF][START_REF] Canellidis | Genetic-algorithm-based multi-objective optimization of the build orientation in stereolithography[END_REF]. These numerical tools have been developed to help the manufacturing operator in the pre-processing phase and are exclusively applied to the CAD model of the parts. In this paper, similar methods are applied but directly to the functional surfaces to help the designer define the design space of the study and thus the future CAD model.

Functional optimization

From the design area, the second step is to define the optimal part geometry regarding the functional objectives of the design problem. This geometry is going to be the "initial part geometry" locally adapted to the chosen Additive Manufacturing process in the next methodology step.

To ensure to get a real functional optimized geometry and to avoid the psychological inertia phenomena which may prevent the designers from innovating [START_REF] Li | Design creativity in product innovation[END_REF][START_REF] Liao | Relationships between knowledge inertia , organizational learning and organization innovation[END_REF], a numerical optimization approach is ideal. Among the different optimization methods, the topological optimization [START_REF] Bendsoe | Topology optimization: theory, methods, and applications[END_REF] seems to be the most suitable one for a the Additive Manufacturing. Indeed, in contrary to the others, it is not dependent on the initial geometry of the design area and thus allows a real optimization without any initial idea of the part geometry. The only known quantities in this case are the applied loads, the possible support conditions, the volume where there can be material and possibly some additional design restrictions such as the location and sizes of prescribed holes and solid area.

Many commercial topological optimization softwares are available, in this paper TOPOSTRUCT R [START_REF]Topostruct[END_REF] is used.

Manufacturing paths optimization

The last step of the methodology allows from the initial part geometry to determine the optimized manufacturing paths regarding the local manufacturing process characteristics. From these manufacturing paths, the manufacturing program, used to control the machine is generated in parallel with the final part CAD model. The proposed optimization method (Fig. 4) is general and can be applied to all the metallic AM processes.

The manufacturing paths are firstly modelized. The point here is to set the manufacturing paths topology from the initial geometry according to the physics of the process and thus to the process technology.

The model parameters are then adapted to the process thanks to the consideration of global manufacturing rules which come basically from manufacturing experiences. These rules allow, with a high abstraction level, to define manufacturing path topologies (rather, spiral,...) suitable for the manufacturing process. The purpose is that the manufacturing paths topology is determined from, first of all, the manufacturing process constraints and just guided by the initial parts geometry. The result is the definition of the different set of the model parameters which are suitable for the manufacturing process and in accordance with the design area.

Once the different adapted manufacturing paths topology are determined, their geometries must be accurately defined. It is done thanks to the consideration of local manufacturing rules. The abstraction level is, at this point, much lower. Indeed, the local rules concern the local geometric characteris- tics of the manufacturing paths as for example the radius of curvature or the space between two adjacent paths. The result is a series of manufacturing paths which have to be analysed and ranked against one another.

Thus, for each one of them, the corresponding geometry is estimated by a numerical simulation. This geometry is then assessed in terms of functional-

Manufacturing paths optimization: case of the thin-walled parts produce by ALM

ALM is a recent process for rapidly forming complex metal components.

A gas jet containing metal powder is directed via a moving nozzle, through the path of the laser beam (Fig. 5), which is focused above the workpiece, to

give a spot at the surface. This laser/powder stream is traversed across the workpiece. The laser forms a small melt pool on the workpiece into which the powder falls and melts, resulting in a new metal layer on cooling after the laser beam has moved on. The part is build continuously with the nozzle motion through the manufacturing paths.

Modelling of the manufacturing paths

Principle

This manufacturing principle involves that the geometry of the manufacturing paths is constrained by the initial part geometry. It is all the more true in the case of a thin-walled part because the part thickness is very close to the minimal dimension manufacturable by the process. There is then only one possible manufacturing path geometry to build the part. In this case, the manufacturing paths modelling can thus be done directly from the initial part geometry and a graph modelling of the manufacturing path is appropriate (Fig. 6). Indeed, if the graph nodes symbolize the intersection points of the manufacturing path and the graph edge the portion of path between these intersection points, the manufacturing path topology is modelled and each way of scan it can be described as a series of edges and nodes. This modelling is particularly suitable to take into consideration the ALM global manufacturing rules to adapt the manufacturing path in terms of topology.

Indeed, the latter is directly defined by the edges and nodes of the graph and its adjacency matrix (the mathematical graph description).

Once the different suitable topologies are determined, the different ways to scan them are defined and one of them is randomly selected. The local manufacturing rules are then applied to precisely generate the shapes of the corresponding manufacturing path.

Thin-walled part V 1 E 1 E1 E 2 E 5 E 4 E 3 E 6 V 3 V 2 V 4 Initial graph model V 1 E 1 E1 E 2 E 5 E 3 E 6 V 3 V 4

Consideration of the global rules

Consideration of the local rules toplogical skeleton

Random selection of an adapted path

V 1 E 1 E1 E 2 E 5 E 3 E 6 V 3 V 4 0 2 1 0 2 0 0 1 1 0 0 2 0 1 2 0 0 1 1 1 1 0 0 1 1 0 0 2 1 1 2 0 -1 1 0 0 0 0 1 0 1 0 0 0 0 1 0 0 -1 1 0 0 -1 1 -1 1 V 2 V 2 E 4 E 4
Real Virtual

Implementation tools

In practice, the transition from the geometry of the initial part to the graph model of the topological skeleton from the part section is numerically done using MATLAB R . Once the nodes and edges are determined, the adjacency matrix is then automatically generated.

The ALM process constraints

In the case of the ALM process, parts are manufactured layer by layer.

A first layer is deposited onto a substrate then the next ones are successively added ones upon the others. As a result, the geometry of a given layer is directly linked to the above one. The variation of the deposit height in a same layer has a particularly strong impact on the physical quality and geometry accuracy of the upper layer and consequently on the final product quality (fig. 7). Indeed, it is proved that the distance between the nozzle and top surface of the part (noted ∆(t i )) is a very influential manufacturing parameter because it controls both the energy and the material repartition added to the part during the manufacturing process [START_REF] Cheikh | Analysis and prediction of single laser tracks geometrical characteristics in coaxial laser cladding process[END_REF]. It involves that if the height variation is too high, the part can no longer be manufacturable without a face milling which provides a scallop-free nascent surface [START_REF] Karunakaran | Techno economic analysis of hybrid layered manufacturing[END_REF].

This additional manufacturing operation increases manufacturing time and can also distort the part, therefore, controlling this parameter to minimize its variation is crucial.

To this end, an experimentation is proposed to determine, in terms of manufacturing path, the different variation sources of the deposit height.

The results of this experimentation will allow to define the manufacturing rules used to select the different possibilities of manufacturing paths (Fig. 4).

Furthermore, a first manufacturing indicator, allowing to quantify the maximal variation of the deposit height in a layer, is also defined to classify the candidate manufacturing paths in terms of manufacturability:

I manuf = M AX(|z(x, y) -z(x, y)|) z(x, y) (1)
Where z(x, y) is the deposit height or build height and z(x, y), its moving average in the overall studied layer. Here, the lower I manuf is, the lower the height variability there is and thus the fewer face milling operation are needed. In other words, for a given manufacturing path, the lower I manuf is, the better its manufacturability is.

Δ(t 0 ) Δ(t n ) Δ(t m )

Design of experimentation

In order to analyse the different sources of geometrical variation, a benchmark part is defined (Fig. 8). It contains the different features that can be found in a complex thin-walled part, which means in terms of manufacturing trajectory:

• Two edge points: with and without an adjacent path.

• Three points of discontinuity: with an acute, an obtuse and a right angle.

• Different curvatures with four different radius from 0.4 mm to 10 mm.

• Two area of adjacency, with two different curvature radius.

The benchmark part is manufactured on a 3 mm thickness substrate plate with the CLAD R machine, using the process parameters given in table 1.

The deposit geometry is measured with the InfiniteFocus R , an optical 3D micro coordinate system for form and roughness measurement. In order to take into account the variability of the CLAD R process, this experimentation has been repeated three times.

Parameters Symbol Value

Laser power (W) P 253

Scanning velocity (mm.s -1 ) v 12

Powder feeding rate (g.min -1 ) ṁp 5

Gaz feeding rate (L.min -1 ) ṁg 5

Substrate material Low carbon steel

Powder material 316 L

Table 1: Basic process parameters for the fabrication of thin-walled parts.

The deposit height of the first manufactured part is detailed along the manufacturing path (in red) in the figure 9. Its high frequency variability (with a amplitude of ± 50 µm) is mainly due to metal powder grains which are, on the upper surface of the deposit, not perfectly melted. Experience shows that it does not impact on the manufacturing process, it is thus smoothed with a moving average (in blue). However, these measurements allow to highlight eight zones of the manufacturing path where there is a particularly significant variation of the deposit height: at the path ends (zones 1 and 8), at the discontinuous points (zones 4, 5 and 6) and at the small radius (zones 2, 3 and 7). These height zones appear in the three manufactured benchmark parts. Moreover, the variation of I manuf between the three different manufactured benchmark parts is relatively low (Tab. 2).

part 1 part 2 part 3 To complete this analysis, a fourth benchmark part has been produced reversing the order of the travel path. The results are broadly the same as the first ones, the corresponding I manuf is equal to 1.18.

I manuf 0.99 1.21 1.13

Manufacturing rules

In order to ensure the physical quality and the geometrical accuracy of the final products, it has been proved in the section 3.2 that the deposit height variation in a same layer has to be minimized.

As it is shown by the experimentation (Fig. 9), the manufacturing path has a significant impact on the deposit height. It is therefore needed to define rules in order to select suitable paths. From the experimentation, three rules can be easily defined:

1. To minimize the number of path ends 2. To maximize the curvature radius 3. To minimize the number of points of discontinuity

The rule 1 is a global one and impacts on the manufacturing paths topology and thus on the graph model of the manufacturing paths. It corresponds in terms of graph to have a trail that visits each edge of the graph once. This is possible when the studied graph is an Eulerian Graph or a Semi-Eulerian graph [START_REF] Diestel | Graph Theory[END_REF], which involves that the graph model of the manufacturing path must have zero or two nodes of odd degree.

The rules 2 and 3 are local rules and allow to define precisely the manufacturing paths geometry.

Modelling of the ALM process

During the manufacturing process, the local deposit geometry is controlled by the powder which melts in the small melt pool formed on the workpiece by the laser beam. The thermal distribution is thus firstly determined then, secondly, the deposit geometry is determined.

Thermal model

To calculate the temperature distribution, and thus to determinate the melt pool geometry, the finite element method was used to numerically solve the following heat transfer equation:

∂ ∂x (k ∂T ∂x ) + ∂ ∂y (k ∂T ∂y ) + ∂ ∂z (k ∂T ∂z ) + Q = ∂ρc p T ∂t (2)

Geometric model

The manufacturing path is discretised in n points C i (x i , y i ). The length between two points C i and C i-1 is given by:

dl i = v(t i ).dt (6)
Where v(t i ) is the manufacturing head travel speed at t = t i determined from maximal jerk, acceleration and speed of the machine used, dt is chosen equal to the time step of the thermal simulation.

The powder flow is considered as an uniform distribution, the thickness dh i of the added material on the substrate during a period dt can be thus mathematically modelled by the following expression [START_REF] Alimardani | A 3D dynamic numerical approach for temperature and thermal stress distributions in multilayer laser solid freeform fabrication process[END_REF]:

dh i (x, y) = dt. ṁp ρ.S powder if (x, y) ∈ S melt ∩ S powder (7)
= 0 if (x, y) / ∈ S melt powder [START_REF] Bi | Development and qualification of a novel laser-cladding head with integrated sensors[END_REF] Where (x, y) are the elements coordinates of the workpiece top surface, S melt and S powder (mm 2 ) are the intersections of the melt pool and the powder stream projection with the workpiece top surface, ρ (g.mm -3 ) is the powder density and ṁp (kg.s -1 )is the powder feed rate.

The additive process can be then modelled at each point M (x, y) of the wokpiece surface as:

for i from 1 to t total /dt h i (x, y) = h i-1 (x, y) + dh i (x, y) (9)
where t total is the total manufacturing time.

The initial condition is defined by: ∀(x, y), h 0 (x, y) = 0 (10)

Implementation tools

To calculate the temperature distribution in the workpiece, Morfeo R

[26] is employed to solve the governing equations and their corresponding boundary conditions introduced in the previous section. To implement the proposed method for simulating the additive process and finally the local deposit geometry, a code is developed using MATLAB R .

Application to the benchmark part

The ALM process modelling has been applied to estimate the geometry of the benchmark part, the model parameters used have been previously determined experimentally in the case of the CLAD R machine, they are detailed in table 3.

Parameters Symbol Value

Absorption (%) ν 30

Laser beam radius (mm) R l 0.4

Powder stream projection (mm 2 ) S powder 3

Initial temperature (K) T 0 293

Maximal Jerk (m.s -3 ) J max 50

Maximal acceleration (m.s -2 ) A max 100

Maximal speed (m.s -1 ) V max 10

Table 3: Simulation parameters.

The results are showed in figure 10. Along the manufacturing path, the simulated deposit height (in green) is compared with the experimental measurement (in red). The manufacturing indicator I manuf is here equal to 1.09.

Figure 11 shows the manufactured part and allows to show the gaps between the measurements and the simulation in three different points of the manufacturing path. Table 4 gives the values of these gaps.

Although the variable gaps between the simulated and experimental results (between 0% and 29% for the deposit height), the eight particular zones with a significant variation of the deposit height are detected. Moreover, the manufacturing indicators of the manufacturing and the simulated geometries are relatively close (with a maximal gap of 15.6%).

The proposed model can thus be used to compare different manufacturing paths and to select the one which minimize the deposit height variations. The measured height The simulated height

Example

The proposed methodology has been applied to the design of a turbine blade (Fig. 12(a)) in stainless steel which is manufactured with the CLAD R machine, using the manufacturing parameters given in table 3 directly onto the blade root previously obtained by milling. Since the studied blade geometry is symmetric, the design problem is implied as a 2D study which the mechanical boundary conditions are described in figure 12(b), the maximal value of pressure (p) being 0.5 MPa. The functional objective of the study is to minimize the blade mass while ensuring a minimal mechanical strength. That is to say to get mechanical stresses lower than R, the 316L Yield stress and a maximal displacement lower than D max =0.5 mm. A functional indicator linked is thus defined, to optimize the blade geometry regarding its mass:

I f unct = M i M f ull (11)
Where M i is the mass of the blade geometry corresponding to the i th candidate manufacturing path and M f ull is the mass of the full blade which is entirely manufactured by milling. Finally, the manufacturing paths used to obtain the optimal turbine blade are going to be optimized regarding I f unct , defined in equation 11 and I manuf defined in equation 1.

Part orientation

The manufacturing direction of the blade is defined normal to the upper surface of the blade root (Fig. 13) to minimize the support volume needed to obtain the functional surface and therefore to minimize also the final part mass. With this manufacturing direction, the design area is the entire volume inside the functional surface of the blade (in green in figure 13) .

Functional optimization

From the design area, the functional optimization is done with TOPOSTRUCT R .

In order to both minimize the blade mass and to ensure its mechanical strength, the optimisation is done in several steps. The objective volume fraction is firstly chosen at 50%, then it is reduced as long as the mechanical stress is lower than R and the maximal displacement is lower than D max . In order to carry out the optimization, the 316L properties are assumed isotropic. They are given in table 5.

Properties Value

Yield tensile (M P a) 260

Poisson's ratio 0.31

Young modulus (GP a) 192 Density (kg.m -3 ) 8000

Table 5: The 316L stainless steel properties.

Finally, the optimal geometry (Fig. 14), corresponding to an objective volume fraction of 8%, is a complex thin wall structure which the thickness varies from 0.23 mm to 0.8 mm. Its corresponding graph model is thus determined.

Manufacturing path optimization

The third step of the methodology is described in figure 15. Because the initial graph model is not Eulerian nor Semi-Eulerian, it has to be modified (rule 1). Moreover, to ensure the minimal required mechanical strength, the different analysed Eulerian possibilities are determined by only adding edges to connect the nodes of odd degree (the removal of edge is forbidden). Once all the Eulerian solutions are determined, each one of their Eulerian trail are translated in manufacturing paths using the two local manufacturing rules (rules 2 & 3) defined in section 3.2.2. In order to achieve it, the local manufacturing paths geometry is defined using C-splines curves. Concerning the discontinuity, in each layer, the only points of discontinuity are, finally, the paths ends.

The corresponding geometry of each manufacturing path is then simulated and quantified in terms of manufacturability (for a same graphe, just I manuf significantly varies). In order to minimize the simulation time, here only one layer of the blade is simulated. The best manufacturing path of each Eulerian solution can be then classified regarding the both functional and manufacturing indicators I f unct and I manuf .

Final result

At the end of the methodology, the best manufacturing path, with I f unct = 0.138 and I manuf = 1.27 is selected. Assuming that all the layers have the same behaviour as the first one, the overall optimized blade geometry, is presented in figure 16, it is an optimized blade for both functionality and manufacturability. Indeed, firstly, it has a mass 85% lower than a fully blade which would be manufactured by milling and secondly it allows to minimize the need of face milling operations. Moreover, as only the solutions which added material to the initial geometry (Fig. 14) had been analysed, its mechanical strength is thus necessarily ensured. Figure 16 shows the blade geometry after a last face milling.

The obtained geometry is not only optimized but also realistic regarding the manufacturing process. Indeed, because the geometry comes from the simulation of the CLAD R process, the obtained CAD model includes nonfunctional characteristics directly linked to the physical phenomena that occur during the process. The figure 16 shows in particularly the area of the starting point of the manufacturing trajectory.

Conclusion

Today, Additive Manufacturing processes and in particularity ALM can be used to manufacture very complex metal parts. However to benefit from this new design possibilities and meet at the best the functional specifications of a part, it is needed to help designers. Firstly to avoid the psychological inertia phenomena and find the optimal geometry in terms of functionality and secondly to considerate these manufacturing processes characteristics. The proposed methodology as well as the corresponding numerical tools as been detailed then illustrated for the designing and manufacturing of thinwalled metal parts manufactured by an ALM process. However, this work is still limited to extruded parts and 2.5 axis manufacturing paths. It has thus to be continued.

Future work

Our future works should focus on two main points. Firstly, the methodology has to be extended to the design of not only extruded parts. It involves generating 5 axis manufacturing paths. Moreover, it would be interesting to generate continuous paths. Indeed, it would allow to further reduce the deposit height variation minimizing the paths ends number no more in only each layer but in the whole parts.

The second issue concerns the manufacturing process simulation. It has to be improved taking into account, in particularly, the thermomechanical phenomena which could involve parts distortions and thus involving gaps between the geometrical model and the manufactured geometry of the parts.

Figure 1 :

 1 Figure 1: The usual numerical chain for Additive Manufacturing.

Figure 2 :

 2 Figure 2: Concept of the proposed numerical chain for Additive Manufacturing.

Figure 3 :

 3 Figure 3: The proposed design for Additive Manufacturing (DFAM) methodology.

Figure 4 :

 4 Figure 4: Optimization method of the manufacturing paths.

  ity and manufacturability thanks to functional indicators (I f unct )linked to the study's functional objectives and manufacturing indicators (I manuf ) linked to the Additive Manufacturing process used. These indicators are used to compare and rank the different candidate manufacturing paths. Finally, at the end, the CAD model and the manufacturing program corresponding to the best manufacturing path are simultaneously generated.

Figure 5 :

 5 Figure 5: Illustration of the ALM process.

Figure 6 :

 6 Figure 6: Principle of the graph modelling.

Figure 7 :

 7 Figure 7: Illustration of manufacturing issues which may appear because of the deposit height variation.

Figure 8 :

 8 Figure 8: Definition of the benchmark part.

Figure 9 :

 9 Figure 9: The measured deposit height along path.

Figure 10 :

 10 Figure 10: Comparison between the simulated and measured deposit height along the manufacturing path.

Figure 11 :

 11 Figure 11: Comparison between the simulated and measured deposit section in 3 different points of the manufacturing path.

  The mechanical boundary conditions (MPa).

Figure 12 :

 12 Figure 12: Functional specification of the analysed blade.

Figure 13 :

 13 Figure 13: Illustration of the first methodology step.

Figure 14 :

 14 Figure 14: Illustration of the second step of the methodology.

Figure 15 :

 15 Figure 15: Illustration of the third methodology step.

Figure 16 :

 16 Figure 16: 3D view of the optimized blade geometry.

  These characteristics are, like all the manufacturing process dictated by the physical phenomena involved which have a direct and strong impact on the final quality of the manufactured part. In this paper a new global numerical chain for the metallic Additive Manufacturing processes has been introduced. It is based on a global DFAM methodology which allows to optimize the parts geometry regarding the specificities of the Additive Manufacturing processes in parallel with the initial functional specifications. This work purpose is to propose a methodology which allows to minimize the gap between a CAD model and the corresponding manufactured part. Based on the simulation of the physical phenomena that occur during the manufacturing process, the methodology allow to optimize the manufacturing paths to finally propose a realistic CAD model in parallel with the corresponding manufacturing program.

Table 2 :

 2 Comparison of the three manufactured benchmark parts.

Table 4 :

 4 Comparison of the deposit height and width in the 3 analysed points.

	measured width(mm) 0.485 0.457 0.492
	simulated width(mm) 0.52 0.52 0.52
	gap(%)	7.2	13.7	5.6
	measured height (mm) 0.199 0.176 0.182
	simulated height(mm) 0.164 0.164 0.164
	gap(%)	18	6.8	9.8

Where T (K) is temperature, t (s) is time, c p (J.kg -1 .K -1 ) is specific heat capacity, ρ (kg.m -3 ) is density, k (kg.m -1 .K -1 ) is thermal conductivity and Q (W.m -3 ) is the power generated per volume within the workpiece.

The boundary conditions are:

Where n is the normal vector surface , ε R is emissivity,

is the heat convection coefficient, σ R is the StefanBoltzman constant (5.6710 8 W.m -2 .W -4 ), S laser (mm 2 ) is the area of the laser beam on the workpiece, S deposit (mm 2 )

is the surface of the previous deposits and I(x, y, z, t) (W.m -2 ) is the laser power distribution on the workpiece which is considered as a Gaussian. It can be computed according to the formula:

Where R l (mm) is laser beam radius, β (%) is the abortion factor, P (W )

is the laser power and r (mm) is the distance from the point which is located inside the beam to the center of beam.

The initial conditions are: Where T 0 is the ambient temperature.


Amalia Todirascu
email: todirasc@loria.fr

Laurent Romary
email: romary@loria.fr

Dalila Bekhouche
email: bekhouche@loria.fr

Combining Syntax & Ontologies for Information Extraction

This paper presents an information extraction system, dedicated to message filtering for a specific domain (security systems). The paper focuses on a method for identifying domain-specific ontology elements (terms and concepts), using syntactic information and an existing domain ontology. The domain ontology is represented using description logics. The system uses description logics inference mechanisms to validate the candidate concepts.

Introduction

Information Extraction applications identify relevant entities in texts, for a given domain. They use shallow natural language processing techniques for identifying relevant data, and, in some cases, domain-specific knowledge to validate these entities. Most IE systems lack portability due to language-dependent linguistic resources and domain-dependent knowledge.

The IE systems' portability depends on the domain model (ontology) portability. Predefined or fixed ontologies are often incomplete and the costs of adapting them to another domain or application are enormous. Existing ontologies, like WordNet [START_REF] Miller | Introduction to WordNet: An On-Line Lexical Database[END_REF] and Corelex [START_REF] Buitelaar | CORELEX: Systematic Polysemy and Underspecification[END_REF], are useful for IE application on free texts but they do not contain domain-specific senses.

IE systems' portability might be increased using semiautomatically extracted ontologies. Several methods of extending existing ontologies have been proposed: statistical methods [START_REF] Daille | Study and Implementation of Combined Techniques for Automatic Extraction of Terminology[END_REF], as well as methods using logical inferences [START_REF] Vilain | Inferential Information Extraction[END_REF]. We used an inference-based method for extending ontologies to improve our system's portability.

Due to large amounts of processed texts, most IE systems propose candidate concepts among the noun phrases, noun-noun collocations [START_REF] Heid | A linguistic bootstrapping approach to the extraction of term candidates from German text[END_REF], applying shallow Natural Language Processing (NLP) techniques: finite-state methods [START_REF] Chanod | Natural Language Processing and Digital Libraries[END_REF] and simple pattern matching [START_REF] Daille | Study and Implementation of Combined Techniques for Automatic Extraction of Terminology[END_REF][START_REF] Riloff | Extraction-based Text Categorization Generating Domain-Specific Role Relationships Automatically[END_REF]. Some systems use domain-specific patterns to build a semantic representation from the syntactic structures [START_REF] Riloff | A Corpus-Based Approach for Building Semantic Lexicons[END_REF]. Other methods extract domain-specific morphemes to filter the noun-to-noun and noun-to-adjective collocations [START_REF] Heid | A linguistic bootstrapping approach to the extraction of term candidates from German text[END_REF]. Special named entity recognisers have been proposed [START_REF] Cunningham | New Methods, Current Trends and Software Infrastructure for NLP[END_REF].

Most IE tools are highly language-dependent. It is difficult to adapt them for another language or domain. For this reason, we use linguistic tools and resources which are highly modular and which could be parameterized (the Lexical Tree Adjoining Grammar parser [START_REF] Lopez | Robust Parsing with Lexicalized Tree Adjoining Grammars[END_REF]), for identifying candidate concepts.

Goal of the paper

The main goal of the paper is to present a methodology for extending a domain ontology with new concepts extracted from text. The paper focuses on the interface between syntax and semantics, in the context of a specific domain.

The method we propose uses partial syntactic structures provided by a Lexical Tree Adjoining Grammars (LTAG) parser to identify candidate concepts, as well as logical inferences from Description Logics, for knowledge representation. The method is tested with a specific application for filtering electronic messages about computer security problems. The corpus contains various errors (spelling, syntactic), wrong segmentations and computer commands. Due to these properties, a concept instance identification method based on partial parsing results is required for this corpus.

The architecture

The methodology proposed includes several steps in order to identify the key elements of the ontology: the concept instances, the relations between the concepts and the relations between the instances and the concepts. Concepts instances are identified among the results of partial parsing, while relations between concepts and between instances and the concepts requires domainspecific resources.

The system includes several modules for identifying domain concepts: the LTAG parser, the module handling the domain ontology and a module linking the syntax and semantics. The semantic representations, built from the syntactic structures and from some domain-specific resources (the semantic lexicon), will be validated by the domain ontology.

For our specific application, we developed modules for extracting lexicons or other resources from reference corpora (see figure 1). The reference corpus is used, together with the LTAG grammar and lexicon, to create a domain-specific lexicon (containing syntactic information). The corpus is also used by the human expert for designing the ontology.

We also built a module assigning concepts to each word, as well as assigning conceptual descriptions to grammar trees. The domain-specific lexicon is used by the LTAG parser to identify potential concept instances, among simple noun phrases and verbs.

Ontologies

Domain knowledge is a key element in an information extraction system. It is used to validate the entities identified in the texts. Existing generic ontologies (WordNet [START_REF] Miller | Introduction to WordNet: An On-Line Lexical Database[END_REF], Corelex [START_REF] Buitelaar | CORELEX: Systematic Polysemy and Underspecification[END_REF])) are useful for providing synonyms if the application is designed for free texts. Domain-specific ontologies often contain specific senses that would not be found in a general-purpose ontology.

To avoid the main drawback of domain-specific ontology, the low portability, statistical-based and inference-based methods for automatically extracting ontologies have been proposed. The methods identify candidate terms in texts, create classes of similar terms and identify relations between these classes.

Statistical methods group terms with similar contexts into classes associated with a unique concept [START_REF] Assadi | Analyse syntaxique et statistique pour la construction d'ontologies à partir des textes[END_REF], [START_REF] Daille | Study and Implementation of Combined Techniques for Automatic Extraction of Terminology[END_REF]. Relations between various classes are identified by interpreting syntactic structures [START_REF] Capponi | Interprétation de classes de termes par généralisation de structures prédicatargument[END_REF] or using statistical information [START_REF] Bouaud | Regroupements issus de dépendances syntaxiques sur un corpus de spécialité: catégorisation et confrontation à deux conceptualisations du domaine[END_REF]. Statistical methods require large, stable, training corpora to extract the classes, as well as human experts for validating and interpreting the results.

Figure 1. System architecture Inference-based methods use semi-automatic methods to check the validity of the existing knowledge. New concepts, deduced by inference rules, are added to the domain hierarchy if they are coherent with the existing knowledge [START_REF] Todirascu | Semantic Indexing for Information Retrieval Systems[END_REF]. Relations are identified using syntactic knowledge, as subcategorisation [START_REF] Capponi | Interprétation de classes de termes par généralisation de structures prédicatargument[END_REF]. Concept overgeneration and the cost of checking knowledge incoherence and inconsistency are the main drawbacks of these approaches. Several formalisms have been proposed to avoid these problems, and among them description logics are very promising for representing domain knowledge.

Description Logics

Description Logics are knowledge representation formalisms derived from semantic nets, that provide welldefined syntax and semantics. They also have features of object-oriented systems, frame-based systems and modal logics.

DLs provide a hierarchical organization of knowledge structuring it on a conceptual level (the T-Box), describing abstract classes of objects relevant for domain modeling, and an assertional level (the A-Box), containing the instances of the classes. The classes of objects (concepts) are described by their relations (named roles) with other concepts and their attributes (roles with atomic values).

Syntax and Semantics

The DL operators are inspired by first order logics: Table 1: The DL operators Using all these operators, or only a part of them, we define our knowledge items: the definition of concepts and roles ALC (Attributive Language with Complements) (using atomic concept names and roles, the SOME, ALL, AND, OR, NOT operators, concept axioms), the possibility of handling transitive roles (R+), of inverse roles (I), of role hierarchy (H), of attributes (f) or numeric constraints.

DL

A few examples of the DL commands are given below. CN is a concept name, C is a conceptual description (any combination of AND, SOME, NOT, ALL operators). The DL commands are KRSS-like [START_REF] Patel-Schneider | Description logic specification from the KRSS effort[END_REF], [START_REF] Baader | A Terminological Knowledge Representation Systems with Complete Inference Algorithms[END_REF]

Input text

Entities Conceptual representations

3) (implies C1 C2) -introduces a new concept axiom, defining necessary conditions C1 for the conceptual description C2; DLs are deterministic fragments of first order logics. They provide decidable algorithms for coherence and consistency checking. DLs propose logical mechanisms to identify concept subsumption, instance retrieval and role paths relating concepts. Classification is a partial ordering of the concept hierarchy due to the subsumption relation.

Some example of commands (using KRSS-like (Baader & all, 1991)

Description Logics for IE Systems

The role of domain knowledge in an IE system is to validate the semantic representation of the potentially relevant entities identified in the text using NLP techniques. These entities could be used for adding new concepts to the existing ontology. Most IE systems use shallow NLP techniques and some candidate entities might not have a valid semantic interpretation. IE systems might use some implicit knowledge, the case of hyponymy/hyperonymy relations being just an example.

Unlike frame-based systems, DLs deal with semistructured or incomplete data. There is no requirement to explicitly define some values as concept instances. Unlike frame-based systems, default values are not used by DLs. Some role fillers are left unspecified as in the following example.

(define-concept Administrator (and Person (some hasHandleType OSystem) (some hasName Name))) (define-primitive-concept Name) (define-primitive-concept OSystem) (instance harry1 (and Administrator (some hasHandleType Linux)))

In this example, we illustrate that implicit definitions are supported by DLs (Linux is not defined explicitly as an instance or a subconcept of the concept OSystem). The filler of the role hasName is not provided.

These properties are interesting for our application, while errors are possible, and the domain knowledge is incomplete.

Hyperonymy or hyponymy are handled by subsumptions between domain concepts. For example, if a candidate concept is identified in the text as (instance x (and PC (some hasOperatingSystem Linux))) (define-concept PCcomputer (and computer (some hasType PC)))

x is also an instance of the concept computer.

(instance y (and File (some hasOwner Root))) (define-concept File (and Object (some hasName Name) (some hasPartOf OSystem) (some hasOwner User))) (define-concept Root (and User (some hasRight ilimited)))

y is an instance of the concept File and it is related to the concept Root, which is also a User.

Semantic networks or frame-based systems provide hyponymy/hyperonymy handling, but the possibility of expressing negations is an argument in the favor of using description logics as knowledge representation. However, the interpretation of negation as the complement of the concept is not always very useful in a NLP application.

Functionalities

To choose the best DL classifier for our application, we identify precisely the functionalities that we require for representing knowledge.

We need to define concepts, roles and attributes to describe domain ontology. This is a feature, as well as axiom definitions, provided by all existing DL systems. Transitive roles are necessary in computing path roles between various concepts. Inverse roles must be avoided due to cyclical definitions which makes computing processes undecidable.

The input text contains proper nouns, data, person and organization names, which are represented in Description Logics as instances. Reasoning with instances is a necessary feature. We chose RACER [START_REF] Haarslev | Description of the RACER System and its Applications[END_REF], which is one of the few classifiers featuring such reasoning. It also has an XML-like representation of the ontology. Our aim is to build portable ontologies, and we intend to adopt a standard format for ontology representationthe Ontology Inference Layer (OIL) [START_REF] Fensel | OIL in a nutshell[END_REF].

The Ontology

The ontology is designed by a human expert. From the reference corpus, the human expert extracts a list of the most frequent words, except the functional words (the determiners, the pronouns, the auxiliaries). The list of nouns and verbs (considered as main candidates to identify primitive concepts) are evaluated by the human expert who assigns to each word a number (5-very relevant, 1-not very relevant) representing relevance of the word to the security domain. Some examples of words and their relevance: 3: Verbs and their relevance to domain From these words, the experts created a set of 35 primitive concepts and a set of concept axioms and roles. The concept candidates are added to the ontology if the ontology coherence is not violated.

Verb

The human expert also selects concept instances (the terms which are associated with each primitive concept). The expert asks the Racer classifier to test the coherence of the knowledge base.

NLP tools and resources

This section presents a short description of NLP resources and tools, as well as of the small ontology used for our application. The system is still under development. We developed the various NLP tools and resources, the modules extracting the lexicons and we built the initial ontology.

The Resources

The Reference corpus

The reference corpus used for generating the lexicon and for creating the ontology is a collection of phrases extracted from e-mail messages. It contains about 50,000 tokens (4039 word forms). The list of the most significant words, used also to design the domain concept hierarchy, was extracted from this reference corpus.

The corpus contains a lot of abbreviations, organization, system, program and function names (UNIX functions, constants, variable names), patterns introducing the content of a dialogue ("X wrote:"), DOS or UNIX commands, which require special preprocessing modules using finite state automata designed for entity recognition.

The corpus contains syntax or spelling errors, so we applied robust, fault-tolerant, shallow NLP techniques for identifying potential concept instances. Another major problem of the corpus is that the sentences are not very well delimited, which is one of the sources of tagging errors.

The lexicon

We used the reference corpus for building a lexicon. The lexicon is encoded using a subset of XML dedicated to represent TAG grammars: TAGML (Tree Adjoining Grammar Markup Language) [START_REF] Bonhomme | TAGML: XML encoding of Resources for Lexicalized Tree Adjoining Grammars[END_REF]. The English LTAG lexicon (511 lexicon entries), available in TAGML format, was incomplete for our security domain. We built a module that semiautomatically created the lexicon from the reference corpus and from the existing English lexicon. For this, we used the TreeTagger POS tagger [START_REF] Schmid | Probabilistic Part-of-Speech Tagging Using Decision Trees[END_REF], which annotates words with their lexical category and extracts the lemmas. Noun and adjective entries are generated automatically from the existing lexicon. Verbs must be added manually.

The list of POS tags provided by the TreeTagger is compatible with the lexical categories encoded into the lexicon.

The POS-tagging results must be validated by a human expert. We detected a set of errors including: spelling errors, POS tagging errors due to bad sentence construction and Unix commands (which must be preprocessed by a separate module) which must be deleted from the list of entries: The resulting lexicon has 3000 entries, distributed as follows: 1400 nouns, 787 adjectives, 300 verbs, 105 prepositions and 379 adverbs. The lexicon also contains a set of syntagms ("one of", "kind of", "number of").

The grammar

The initial English LTAG grammar [START_REF] Joshi | An Introduction to Tree Adjoining Grammars[END_REF]) contains about 420 elementary trees. The grammar is represented, as the lexicon is, in TAGML format. We concentrate at this stage on simple noun phrases (without relative clauses) and simple verb phrases, which will be the candidate concepts.

LTAG grammars have an interesting property, making them suitable for our application: the extended domain locality. It creates a context of each word, formed by a set of elementary trees. This property is used to avoid some candidate trees and to select a small subgrammar.

The local grammar built for this application consists of the noun phrase trees (trees associated with nouns, nouns and adjectives that might modify nouns, past participle verbs playing the role of the modifiers, comparative adverbs and adjectives), proper nouns and prepositions relating two nouns. We do not yet include verb trees handling long-distance dependency trees or relative clauses in the local grammar.

The parser

The parser we use is a current version of Lopez's parser (1999) that is modular and supports TAGML-like input and output. We chose this parser due to its ability to produce partial parsing results, and due to the existing TAG grammars (being available in TAGML format for French and for English). It is able to identify candidate terms, even if an error (e.g. syntactic or spelling) occurs. It is implemented in Java. The parser loads only a part of the lexicon and of the grammar (the entries associated with the current sentence).

TAG parsers combine the set of elementary trees associated with input words, using two operations: adjunction and substitution. TAG parsers produce as output a set of derived trees (which represent the syntactic structures identified in the text) as well as a set of derivation trees. Derivation trees are usually used to generate a dependency tree: a semantic representation. TAG parsing provides a large set of derived and derivation trees. The derivation trees are used to create a semantic representation and this representation is validated by the domain ontology.

Linking syntax and domain ontology

This section presents in detail the relations between ontology concepts and roles to lexicons, grammars and parsing output. The interface between syntax and semantics consists of a list of pairs (lemma, concept), a set of conceptual descriptions built for each Elementary tree and an algorithm building conceptual descriptions from derivation trees.

The Semantic Lexicon

The LTAG lexicon entries contain references to lemmas associated with the current word, and each lemma is associated with a set of elementary trees and coanchors. While each lemma might be associated with a different sense, we should obviously associate a different concept with each lemma.

Nouns are associated with one or several domain concepts. They are also associated with an entry describing the situation of a noun playing the role of a modifier. In this case the semantic description is a DL formula as (some hasMod Concept). Adjectives might be modifiers, but could also be predicative (and the DL representation is similar to verbal entries).

The verbs are associated with the concept and have some constraints on their arguments' type.

Example.

-for the noun 'system': <sem concept="system" lemma="system"/> -for the adjective 'main': <sem concept="(some hasMod main)" lemma="main"/> -for the verb 'connect': <sem concept="connect" lemma="connect"> <constr arg0="Substitution" address = "1"/> <constr arg1="Substitution" address = "3"/> </sem> Some lexicon entries are highly ambiguous: prepositions might represent several domain-specific relations or general relations (type, possession). For example, the preposition 'of' could be represented as: <sem concept="(some hasType)" lemma="of"/> <sem concept="(some hasPoss)" lemma="of"/> If we define a function Sem taking as argument a lemma and returning a DL concept as a value:

Sem(lemma)=Concept  Constraint1  …  ConstraintJ
The concepts will be associated with words using a method similar to (Riloff et Sheperd, 1997), starting from a set of seed words and concepts (words extracted from the list of the most relevant words).

Elementary Trees

The elementary trees are related to the lemmas associated with each word. The semantic representation associated with each lemma is a primary concept and a set of constraints. We define an algorithm building a DL conceptual description for each elementary tree and the lemma associated to it.

Sem(ElementaryTree) = Sem(lemma), if no substitution or adjunction is found in the tree;

Sem(ElementaryTree) = (and Sem(lemma) (some hasSubstitution A))  (implies (some hasSubstition A) (some argi A)), if a substitution is found in the tree and A is a generic concept.

Sem(ElementaryTree) = (some hasAdjunction Sem(lemma)).

Derivation Trees

The derivation tree encodes all the operations (substitution, adjunction) applied for building a tree from elementary trees and other derivation trees. We define an algorithm for extracting complex representations from derivation trees. where Sem(A), Sem(B) and Sem(C) are defined recursively, if B or C are derivation trees, and as in the previous subsection if A is elementary tree. (constraints B) represents the information related to the arguments.

Some Examples

The phrase "the root passwords" is parsed by the LTAG parser, and the results are two derivation trees (figure 2

  R C x (xRC) there is at least one instance of C related by the relation R D = ALL R C x(xRC) restricts the co-domain of the relation least n objects of the concept C in relation R with D

  ):1) (define-concept CN C) -defines a new concept as a conceptual description;2) (instance IN C) -defines an instance of a given concept;

  syntax) are: (concept-subsumes? C1 C2) tests if C1 subsumes C2 (concept-parents C) retrieves the direct ancestors of the concept C (concept-children C) retrieves the children of C (classify-tbox) computes all the subsumption relations between all the concepts defined in the T-Box (concept-instances C) retrieves the instances of the concept C

  Figure 1. A derivation tree where A, B and C are also elementary or derivation tree The algorithm building the DL representation for the tree from the fig. 1: Sem(Tree) = (and Sem(A) (Some hasSubstitution Sem(B)) Sem(C))  (constraints B)

  Fig. 2 Derivation trees for "the root password"

Table 2 :

 2 The results of POS tagging and tagging errors

Acknowledgements. Amalia Todirascu worked on this project as a post-doctoral fellowship in LORIA, sponsored by the European Research Consortium for Informatics and Mathematics (ERCIM) group.

Sem(1) = (and Password (some hasMod Root)(some hasDef definite))

Sem(2) = (and Password (some hasMod (and Root (some hasDef definite))))

The phrase "The hacker connected to the server" results into the following derivation trees (fig. 3).

The conceptual descriptions associated with each lemma are:

Sem(connected_to) = Connect  (implies (some hasSubst A) (some arg0 A))  (implies (some hasSubst B))

Sem(hacker) = Hacker Sem(server) = Server Sem(the) = (some hasDefine Defined)

Fig. 3 Derivation trees for "the hacker connected to the server"

The representations of the trees substituted to the elementary tree anchored by 'connected' are:

(and Hacker (some hasDefine Defined)) (and Server (some hasDefine Defined))

The representation associated with the elementary tree anchored by 'connected' is the set of axioms:

(and Connect (some hasSubst A)(some hasSubst B)) (implies (some hasSubst A)(some arg0 A)) (implies (some hasSubst B)(some arg1 B))

In the derivation tree we found the values for A and B:

(implies A (and hacker (some hasDefine Defined))) (implies B (and server (some hasDefine Defined)))

The concept associated with the derivation tree:

(and Connect (some arg0 (and hacker (some hasDefine Defined))) (some arg1 (and server (some hasDefine Defined))))

is satisfiable by the existing knowledge.

Extending the ontology

We use the results of partial parsing to identify a set of relevant entities. Some of them might be added to the existing ontology, after a human expert validates them.

The corpus contains a lot of errors and a complete parsing will never result in a correct syntactic structure.

Example from corpus:

Remains the question how are you going to run DOS programs if the first thing that the computer does after the password protected BIOS is coming up with a lilo prompt for a password ?

The entities identified in the text are : run, DOS, the first thing, the computer, the password protected BIOS, a lilo prompt for a password.

We obtained the following conceptual descriptions:

(and run (some has arg1 (and program (some hasType DOS)))) (and computer (some hasdef Definite)) (and entity (some hasname BIOS) (some hasProp (and password (some hasType protected) (some hasdef definite))) (and prompt (some hasType lilo) (some hasdef indefinite) (some hasPurpose (and password (some hasdef indefinite))))

These representation are validated by the existing ontology. We might add a new concept run with an argument restricted to the type program.

Conclusion and further work

The paper presents a methodology for extracting concepts from texts, using partial syntactic analysis and domain knowledge. It focuses on the relation between the syntax and the domain-specific knowledge, in order to build candidate concepts for the domain ontology. We intend to use a meta-grammar for generating automatically the elementary trees. We will extend this meta-grammar to generate conceptual descriptions together with the elementary trees. We will also develop a method for generating verbs lexicon entries automatically.