Nuclear Instruments and Methods in Physics Research B 268 (2010) 1880–1883

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Study of xenon thermal migration in sintered titanium nitride using nuclear micro-probe R. Bes a,*, N. Millard-Pinard a, S. Gavarini a, S. Cardinal b, V. Garnier b, H. Khodja c, A. Malchère b, P. Martin d, C. Peaucelle a a

Université de Lyon, Université Lyon 1, CNRS/IN2P3, UMR5822, Institut de Physique Nucléaire de Lyon (IPNL), F-69622 Villeurbanne, France INSA de Lyon, CNRS, UMR5510, MATEIS, Lyon, France Laboratoire Pierre Süe, CEA-CNRS, UMR9956, CEA Saclay, Gif sur Yvette, France d CEA Cadarache, DEN/DEC/SESCC/LLCC, 13108 Saint Paul lez Durance cedex, France b c

a r t i c l e

i n f o

Article history: Available online 25 February 2010 Keywords: Titanium nitride Fission product Xenon Thermal migration Inert matrix GFR Oxidation Grain boundary Grain size Dislocations

a b s t r a c t Micro-Rutherford backscattering spectrometry experiments were performed on a set of sintered titanium nitride samples implanted with xenon to a depth of about 150 nm. Implanted samples were annealed at 1500 °C during 5 h. Xe depth profile and its lateral distribution on the surface were measured. Surface morphology was observed using scanning electron microscopy. The results reveal that the microstructure plays an important role on xenon release. Moreover, the crystalline orientation of each grain could be a key parameter to explain the heterogeneous evolution of the surface during thermal treatments as well as Xe release from surface. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Titanium nitride is foreseen to surround advanced nuclear fuels [1] in gas cooled concepts of generation IV reactors. Its interesting mechanical, chemical and thermal properties [2], combined with the relative ease of reprocessing of nitrides [3], make TiN relevant to play the role of an inert matrix. Its aim would be to prevent the release of fission products. However, the retention properties of this material with respect to the most volatile fission products created in nuclear fuel (i.e. Cs, Xe, and I) still has to be explored [4]. Among these fission products, xenon is known to be one of the most abundant [5]. Its ability to aggregate and to form gas bubbles at high fluences may cause swelling and crack propagation within the fuel assembly [5]. Recent studies carried out by Gavarini et al. [6,7] have shown that xenon species, implanted at room temperature in polycrystalline TiN (grain size about 18 lm), was transported ‘‘as a unit” towards the surface during subsequent thermal treatments at

* Corresponding author. Address: Université de Lyon, F-69622, Lyon, France. Tel.: +33 4 72448359. E-mail address: [email protected] (R. Bes). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.02.018

temperatures ranging from 1400 to 1650 °C. In this previous study, xenon depth profiles were determined by Rutherford Backscattering Spectrometry using a millimetric He2+ ion beam and the measured rates of transport have to be interpreted as mean values for a large number of probed grains (including grain boundaries). The aim of the present study is to better understand the dependence of Xe thermal migration on microstructure. In this purpose, polycrystalline TiN was synthesised by hot pressing (HP) using suitable conditions to obtain coarse grains (i.e. 36 ± 13 lm). Coarse grains were used in order to minimize grain boundary effects on Xe migration and also to facilitate the distinctness between intra-granular and inter-granular regions. Samples were polished to micron scale and then implanted with 129 Xe2+ ions (conditions similar to that of references [6,7]). After thermal treatments at a temperature of 1500 °C for several durations, the samples were analyzed using nuclear micro-probe (lRBS) and the lateral distribution of the implanted species was determined as well as the depth profiles for each considered region. The results are discussed with emphasis on the dependence of xenon mobility on microstructure and surface morphology evolution.

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2. Experimental procedure

3. Results

2.1. Sintering process

3.1. Surface morphology

The starting material is commercial titanium nitride powder1 with initial grain size of 0.8–1.2 lm. The main impurities are O (about 1.5 cg gÿ1), C (about 0.2 cg gÿ1) and Fe (about 2500 lg gÿ1). The hot pressing process was carried out under vacuum up to 600 °C (10 °C minÿ1) and under argon atmosphere until the dwell temperature (i.e. 2000 °C) which was maintained for 1 h [8]. During the whole process, a pressure of 50 MPa was applied through graphite punches. Hot-pressed bodies density was measured by the Archimedes’s method [8] and the densification was found to be about 98% (theoretical density: 5.39 g cmÿ3). The mean grain size after sintering was determined using secondary electrons images and an imaging software. It was found to be 36 ± 13 lm.

Fig. 1 shows surface morphology evolution of implanted samples as observed by SEM (secondary electrons). After annealing at 1000 °C during 10 h (Fig. 1a), grain boundaries and grain textures are revealed as well as very sparse porosity holes which were already present to a lower extend before thermal treatment. Moreover, some grains are covered with micrometric crystallites (Fig. 1a). The number and the size of these crystallites increase after annealing at 1500 °C (Fig. 1b and c). Their presence seems to be correlated with a global enrichment of the surface in oxygen as indicated by using the 16O(a, a0 )16O nuclear reaction [14] with 7.5 MeV a-particles and a millimetric beam size. On the other hand, backscattered electrons (BE) did not reveal any contrast between these crystallites and the subjacent matrix. This may indicate either that the elements contained in these phases present masses very close to major component (i.e. Ti and N) or that these phases are too superficial to be characterized using BE. It could be for example an intermediate phase containing Ti, N and O (written TiN1 ÿ xOx, with 0 6 x 6 1). Note that low angle X-ray diffraction (XRD) did not reveal clearly the presence of any other crystalline phase, distinct from titanium nitride. However, the presence of a TiN1 ÿ xOx cannot be excluded at this stage as the position of X-rays corresponding to TiO is known to be very close to that of TiN [15]. High resolution XRD analysis is under progress to highlight this point. As it can be seen in Fig. 1d, crystallites size seems to vary on each grain. Grain periphery is generally crystallite-free and crystallite size globally increases with heating duration.

2.2. Sample preparation and thermal treatments Sample pellets of size 15  15  2 mm3 were polished down to micron scale with diamond powders. The heating procedure consisted in several steps beginning with a pre-implantation thermal treatment during 10 h at 1000 °C, under secondary vacuum (65  10ÿ6 mbar), in order to relax most of the constrains induced by polishing near the surface [9]. The same treatment was also achieved just after the ion implantation to heal irradiation damages. These thermal conditions were chosen because the order/disorder transition temperatures generally given for TiN are below 1000 °C [9]. However, all the defects could not be healed by this treatment (especially dislocations) as it will be specified further. As a last step, implanted samples were annealed under a vacuum of about 5  10ÿ6 mbar, at 1500 °C for 1 and 5 h using a 12 kW induction heating system [6]. 2.3. Ion implantation and l-RBS analyses Samples were implanted at ambient temperature under normal incidence, using the 400 kV ion accelerator of the Nuclear Physics Institute of Lyon (IPNL). The implantation energy was chosen to be 800 keV (maximum energy with doubly charged particle) and the targeted fluence was 5  1015 ion cmÿ2. In these conditions, the experimental projected range was found to be 150 ± 8 nm (in agreement with SRIM 2008 code [10]), the maximum concentration was about 0.39 at.% and the full width at half maximum (FWHM) was 94 ± 8 nm. Depth profiles after implantation were measured by using nuclear micro-probe. Micro-Rutherford Backscattering Spectrometry was performed at Pierre Süe laboratory of Saclay [11] in order to follow local Xe depth profile evolution. Scanning mode was also performed to determine Xe lateral distribution. Beam size was 5  5 lm2 what made possible to discern areas much smaller than a grain. Other experimental conditions were as follows: 2.5 MeV 4 He+ ions, detection angle of 170° and a micro-beam intensity of 500 pA. SIMNRA 6.04 simulation software [12] was used to deduce Xe depth profiles from experimental spectra. Note that the depth resolution of RBS technique is about 15 nm at a depth of 300 nm (from RESOLNRA 1.0 program [12]). Xe lateral distributions were deduced with RISMIN 4.1 software [13] from the experimental spectra. The evolution of surface morphology was also followed using scanning electron microscopy (SEM). 1

H.C. Starck, grade C powder.

3.2. l-RBS 3.2.1. Xenon lateral distribution cartography Xenon lateral distribution is homogeneous just after the implantation and thermal treatment at 1000 °C for 10 h (not shown here). After annealing at 1500 °C during 5 h (Fig. 2), xenon concentration varies from low concentration near the grain boundary (about 0.05 at.%) to high concentration in the center of grain 2 (about 0.2 at.% against 0.4 at.% just after implantation). Xe concentration in both adjacent grains (i.e. grain 1 and 3 in Fig. 2a) seems homogeneous and corresponds to the as-implanted concentration. Note that the three adjacent grains analyzed here present similar sizes which indicates that grain size is not the only controlling parameter for Xe depletion inside a given grain. On the other hand, grains 1 and 3 do not exhibit as visible crystallites as for grain 2 which may suggest a possible correlation between the presence of coarse crystallites and xenon depletion. Note that this correlation was also observed elsewhere on the surface. 3.2.2. Xenon depth distribution Xe depth profiles of the three grains represented in Fig. 2a are displayed in Fig. 2c. As it can be seen, xenon species present a tendency to be transported ‘‘as a unit” to the surface during thermal treatment (concordant with reference [6]). Grain 2 is divided into three zones: zone a (left side), zone b (center) and zone c (right side). Thus, xenon depletion in grain 2 corresponds to a fast transport and a subsequent release resulting in profile truncation. In the center of grain 2, xenon seems to be slightly accumulated on the surface but it is still to be confirmed. At the same time, the shape of the profiles remain almost unchanged for grains 1 and 3 compared to the as-implanted profile, with only a slight shift towards the surface.

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Fig. 1. Surface morphology evolution during thermal treatments for implanted samples as observed by SEM (a) after 10 h at 1000 °C, and (b) annealed at 1500 °C during 1 h and (c) and (d) after 5 h at 1500 °C.

4. Discussion The growth of crystallites on samples surface after thermal treatments is probably correlated with oxidation (perhaps an oxinitride phase). The heterogeneous distribution and size of these crystallites could indicate that the crystalline orientation of each TiN grain may play a role on their formation. This hypothesis has to be confirmed but it could also be used to explain the strong dependence of xenon profile on the considered grain. More precisely, xenon transport rate seems higher in grains that are strongly oxidized (coarse crystallites on the surface). It could mean that both xenon mobility and oxidation are enhanced for given

orientations of the lattice. Ponctual and linear defects formation, depending on the crystalline orientation of each TiN grain, could also influence the xenon migration properties. Moreover, the high transport rate reported in the periphery of some grains is surprising as the width of the zone concerned by this depletion is of a few lm which is much larger than the awaited width of a grain boundary (generally a tenth of nanometers). An hypothesis to explain this heterogeneous depletion could be that typical modification of the crystalline structure in the vicinity of a given grain boundary plays a role on Xe mobility. Indeed, geometrically necessary dislocations near grain boundaries due to grain orientation mismatch as well as residual thermal stresses

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annealing at 1000 °C during 10 h may not be sufficient to heal completely the dislocation network [18] developed during ion implantation, as the temperature range reported by Hultman et al. [19] for TiN thin films’ re-crystallization is 900–1300 °C, depending on synthesis process. The mechanism responsible for the particular behaviour of regions located near grain boundaries is not clear at this stage, even if a correlation with the dislocation network is strongly suspected. Complementary Electron BackScattered Diffraction (EBSD) experiments are in progress to highlight this point and to confirm the role played by lattice orientation on intra-granular mobility of xenon in TiN. However, if this hypothesis is confirmed, the behaviour described here may also concern other polycrystalline materials and should thus be taken into account in models to predict more precisely the release of many fission products. 5. Conclusion A heterogeneous crystallite formation was observed on TiN samples surfaces after thermal treatment. Their growth seems to be correlated with oxidation. The nature of these crystallites still has to be clearly determined but it could be an oxinitride phase. The formation of crystallites and xenon mobility could both depend on the crystalline orientation of each considered grain. Moreover, for some grains, a clear xenon depletion was observed near grain boundaries on a width of a few micrometers. This depletion could possibly be due to particular lattice’s modification (dislocation or other structural defects for example) near grain boundaries. Acknowledgements The authors thank A. Perrat-Mabilon, A. Gardon, Y. Champelovier and R. Fillol from the ‘‘accelerator group” of the IPNL for their grateful contributions. They thank also the technical team of the Pierre Süe laboratory for their help during micro-probe experiments. References

Fig. 2. (a) Xenon concentration map (in at.%) measured on a sample annealed at 1500 °C for 5 h. (b) Xe concentration as a function of x-position. The black full line symbolizes the grain boundaries. The gray dotted line is the arbitrary limit between the central region of grain 2 (with coarse crystallites) and the crystallites free region near boundaries. (c) Xenon depth distribution measured on sample annealed at 1500 °C during 5 h.

have been used by many authors [16,17] to explain the distinctness between periphery and center of grains. In our case, the

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