Thermal diffusion of molybdenum in apatite

Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 646±650

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Thermal di€usion of molybdenum in apatite C. Gaillard a

a,*

, N. Chevarier a, N. Millard-Pinard a, P. Delichere b, Ph. Sainsot

c

Institut de Physique Nucl eaire de Lyon, IN2P3/CNRS, Universit e Claude Bernard, F-69622 Villeurbanne cedex, France b Institut de Recherche sur la Catalyse, F-69622 Villeurbanne cedex, France c Institut National des Sciences Appliqu ees, F-69621 Villeurbanne cedex, France

Abstract One of the solutions to store high level nuclear wastes (actinides and ®ssion products) is deep geological repository. Since some of these radionuclides have extremely long half-lives, they require an appropriate durable immobilisation system. Several materials, including apatites, are studied as potential inertial matrices. One of the most fed long life ®ssion isotopes, 99 Tc (half-life t1=2 ˆ 2 ´ 105 yr) is produced by the 99 Mo ® 99 Tc ®liation. Therefore, we focused on the migration study of molybdenum in hydroxyapatite Ca10 (PO4 )6 (OH)2 . Molybdenum was introduced by ion implantation, 40 nm deep, in pellets of hydroxyapatite. Then the samples were heated under air. The evolution of the molybdenum pro®les after each annealing step was followed by Rutherford backscattering spectroscopy (RBS), showing a volatilization of molybdenum. Moreover, XPS analysis made on molybdenum led us to think that the volatile species formed is MoO3 . Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.10.CB Keywords: Apatite; Molybdenum migration; Ion beam analysis

1. Introduction The crystal structure of apatite allows a wide range of anion and cation substitutions [1]. Apatites are widely studied as potential inertial matrices to actinides and some long lived ®ssion products con®nement. In this work, we focused on the di€usion of molybdenum in hydroxyapatite, which is spontaneously synthesized during conditioning of radioactive ashes in cement [2]. The

*

Corresponding author. Tel.: +33-4-72-43-10-63; fax: +334-72-44-80-04. E-mail address: [email protected] (C. Gaillard).

radioactive ashes, which originated from the nuclear fuel reprocessing, contain ®ssion products among which the widely fed molybdenum isotope 99 Mo, which leads by bÿ emission to the long lived 99 Tc …t1=2 ˆ 2  105 yr). Previous works [3,4] have shown that the hydroxyapatite structure is able to accommodate a variety of univalent, divalent, trivalent cation substitutions among which those of lanthanide and actinide ions. To our knowledge, no data are available on the behavior of molybdenum in hydroxyapatite but this element is known to precipitate under metallic form in UO2 [5]. The study of molybdenum migration in hydroxyapatite was performed using the comple-

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C. Gaillard et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 646±650

mentary data obtained by ion beam techniques and XPS. 2. Experimental This study is based on ion beam experimental methods which were performed at the ``Institut de Physique Nucleaire'' in Lyon. Ion implantation was used to introduce molybdenum in hydroxyapatite samples, which were obtained from synthetic microcrystalline hydroxyapatite (referenced as DNA grade biogel HP), stacked into pellets at 0.4 GPa. The density of the so-obtained samples, checked by the good agreement between the TRIM simulation and the RBS pro®les of the implanted hydroxyapatite pellets, is equal to 3.2 g cmÿ3 . The hydroxyapatite structure of the samples was con®rmed by X-ray di€raction. Moreover, the chemical composition of the pellets was obtained by X-ray ¯uorescence, giving the following formula: Ca9:69 Na0:25 (PO4 )5:63 (HPO4 )0:37 (OH)2 . We can notice that the water, initially included in the powder [6], is released when the material is stacked into pellets. The implantation of 98 Mo‡ ions was performed at 120 keV energy, the nominal dose was 1016 ions cmÿ2 corresponding to a maximum Mo concentration of 2 at.% at a 40 nm depth range. Annealings were performed under air in a furnace allowing a continuous temperature regulation and an air ¯ow control (60 ml minÿ1 ). Rutherford backscattering spectrometry (RBS) using 1.5 MeV alpha particles allows reliable measurements of sample evolution during annealing. The detection was performed by a silicon detector, set at 172° from the beam direction. The solid angle is 2 ´ 10ÿ3 sr, and the global energy resolution is 12 keV. The beam current was kept equal to 3 nA in order to minimize charging and pulse pile-up e€ects. The evolution of the oxidation state of molybdenum versus annealing conditions was studied by X-ray photoelectron spectroscopy (XPS) with a VG Escalab 200R spectrometer including a hemispherical analyzer adjusted at 50 eV pass energy. An Al anode ( Al Ka ˆ 1486.6 eV ) was used as X-ray source. The experiments were performed at Institut de Recherche sur la Catalyse (IRC) in

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Lyon. The measurements, on which we focused, were the two well-resolved spectral lines corresponding to the Mo 3d5=2 and Mo 3d3=2 spin orbit components. Charging e€ects were corrected using the C 1s line at 284.8 eV. All binding energies obtained in this study were precise to within ‹0.2 eV.

3. Experimental results 3.1. XPS spectra The results are presented in Fig. 1 in the case of 300°C annealing. We observed a continuous evolution towards the higher binding energies depending on increasing annealing times. Such behavior is characteristic of the molybdenum oxidation. Moreover, the peaks, which in the case of the as-implanted sample were quite broad, became sharper with annealing time.

Fig. 1. XPS spectra of the Mo 3d doublet after 300°C annealing: (a) as-implanted sample, (b) 15 min annealing, (c) 45 min annealing, (d) 60 min annealing, (e) 3 h 40 min annealing and (f) 4 h 30 min annealing.

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C. Gaillard et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 646±650

The distribution of oxidation states for Mo was estimated by the deconvolution of Mo 3d spectra (Fig. 2(a) and (b)). The Mo 3d5=2 ±Mo 3d3=2 doublet was ®tted so that each peak had the same Gaussian/Lorentzian line shape and width. On the as-implanted sample (Fig. 2(a)), two oxidation states can be observed. The Mo 3d5=2 binding energy of 229.6 eV was assigned to Mo (IV),while the results of the deconvolution led to a component standing between the (IV) and (VI) Mo state. The Mo (IV) oxidation state corresponds to a 36% ratio. After 4.5 h annealing, two oxidation states were still observed (Fig. 2(b)) but the main oxidation state was attributed to the Mo (VI) at the 232.4 eV binding energy. The Mo (VI) component corresponded to a 81% ratio, the remaining 19% left can be assigned to an oxidation state standing between (IV) and (VI).

Fig. 2. Deconvolution of the Mo 3d3/2-Mo 3d5/2 doublet: (a) as-implanted sample and (b) after 4 h 30 min annealing.

The displacement of the Mo 3d5=2 binding energy value deduced after deconvolution increases linearly with annealing time, such evolution is representative of the Mo oxidation rate from the (IV) to the (VI) state [7]. 3.2. RBS spectra The evolution of the molybdenum RBS pro®les is given in Fig. 3(a) in the case of 550°C annealing temperature. Within the experimental resolution, no pro®le broadening could be observed but a decrease in molybdenum concentration was demonstrated. This molybdenum volatilization is observable in Fig. 3(b) by a decrease of the total molybdenum quantity A with the annealing time. This variation is slower and slower as the temperature decreases, corresponding to a decrease of the volatilization rate with the annealing temper-

Fig. 3. Molybdenum migration data obtained by RBS: (a) evolution of molybdenum pro®les after 550°C annealing, the di€usion ®ts are represented by lines and (b) molybdenum concentration evolution versus annealing time.

C. Gaillard et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 646±650

ature. At 300°C, this rate is too slow to allow a molybdenum release observation, even after 27 h annealing. As the most volatile molybdenum compound is MoO3 oxide (Tsublimation ˆ 550°C), the correlation between XPS and RBS data assesses the formation of the volatile MoO3 , while the intermediate oxidation states observed between Mo (IV) and Mo (VI) could be related to mixed oxides. 4. Analysis 4.1. Di€usion coecient extraction from pro®le evolution In order to simulate our experimental results, we have considered the di€usion law (Fick's second law) with an additional term P representative of the molybdenum loss. Indeed, we assumed that ambient oxygen di€used inside the specimens, combining with Mo to form the volatile MoO3 compound, oC…x; t† o2 C…x; t† ˆD ÿ P: ot ox2

…1†

As P is related to the molybdenum oxidation, it has to be proportional to molybdenum and oxygen concentrations P ˆ k C…x; t† CO …x; t†: CO (x,t) is the oxygen concentration, C(x,t) the Gaussian shape pro®le of the di€using species, D the di€usion coecient of molybdenum and k is an adjustable parameter. Assuming that the oxygen concentration outside and at the surface of the sample is constant, the oxygen di€usion pro®le in the sample depends on    x 1 ÿ erf p ; 2 DO t where DO is the oxygen di€usion coecient [8]. The initial molybdenum concentration is de®ned by the as-implanted molybdenum Gaussian shape ! …x ÿ Rp †2 ; C…x; t ˆ 0† ˆ Cm exp ÿ 2r2

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where Cm is the molybdenum maximum concentration, Rp the range and r is the straggling of the distribution. With dimensionless variables, x x ˆ ; r

Cˆ

C ; Cm

t tˆ ; s

where t is the annealing time and s ˆ r2 =D, we obtain  r x D CO ˆ 1 ÿ erf 2 DO t and o2 C oC ; 2 ÿ P ˆ ot ox

with P ˆ

Ps : Cm

The boundary conditions corresponding to experimental considerations are C…1; t† ˆ 0 and C…0; t† ˆ 0. The ratio DO /D was set equal to 106 , which was estimated from a comparison between the oxygen di€usion coecient in ¯uoroapatite [9] and the order of magnitude of di€usivity of high mass elements such as strontium, lead and rare earth [10]. Therefore the oxygen concentration CO …x; t† is nearly constant within the analysed depth. The integration was performed using the ®nite di€erence method. Starting from the initial implanted distribution, the time t, necessary to ®t the distribution pro®le correctly after an annealing time t, allowed us to deduce the di€usion coecient D, since D ˆ tr2 =t. The ®ts obtained are represented in Fig. 3(a) by solid lines. It shows clearly that the experimental data are quite well reproduced by the simulation. The mean apparent di€usion coecients are deduced from the slope of the D t ˆ f …t† line graph at the di€erent temperatures. In the range of 400± 500°C, the D values so deduced stood around 10ÿ18 cm2 sÿ1 . Taking our experimental resolution into account, such small values have no physical meaning. Therefore, the di€usion term could be neglected and the observed phenomena could be correlated to the molybdenum oxidation reaction kinetics, oC…x; t† ˆ ÿP ˆ ÿk C…x; t† CO …x; t†: ot

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C. Gaillard et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 646±650

Since CO (x, t) can be considered as constant, then oC…x; t† ˆ K C…x; t†: ot Let us consider A, the molybdenum quantity obtained by pro®les integration for each annealing time t, oA ˆ ÿK A: ot In Fig. 3(b) is represented the variation of A versus the annealing time. The best ®t for these experimental data, at each temperature, is an exponential ®t, which is in good agreement with the model. The ®ts allowed us to obtain the K values, for each annealing temperature. The activation energy deduced from the Arrhenius plot is 0.8 eV at.ÿ1 . This value has been deduced from high temperature experiments (from 400°C to 550°C). In this range, the RBS pro®le evolution (no broadening, no shift towards the surface) proved that the volatile species are released very promptly, which corroborates the very low activation energy. 5. Conclusion The correlation between XPS and ion beam analysis data allowed to model the observed mo-

lybdenum release in the 400±550°C annealing temperature range. This approach is based on the molybdenum oxidation kinetics induced by oxygen di€usion in an apatite lattice during annealing. In case of lower temperature, namely 300°C, XPS measurements showed that almost all the molybdenum is converted in MoO3 , but no release was observed. The question which arises is to know if, at this temperature, the oxidized species are incorporated in the hydroxyapatite lattice? Luminescence and EXAFS measurements are in progress to answer this question. References [1] E. Fleet, Y. Pan, J. Solid State Chem. 112 (1994) 78. [2] E. Revertegat, G. Moine, Treatment and Conditioning of Radioactive Incinerator Ashes, Elsevier, London, 1991. [3] J. Carpena, J.L. Lacout, LÕactualite chimique 2 (1997) 3. [4] P. Martin, G. Carlot, A. Chevarier, C. Den Auwer, G. Panczer, J. Nucl. Mater., to be published. [5] S. Nicoll, H. Matzke, R.W. Grimes, C.R.A. Catlow, J. Nucl. Mater. 240 (1997) 240. [6] J. Jeanjean, U. Vincent, M. Fedoro€, J. Solid State Chem. 108 (1994) 68. [7] J.G. Choi, L.T. Thompson, Appl. Surf. Sci. 93 (1996) 143. [8] J. Philibert, Di€usion et transport de matiere dans les solides, Monographies de physique, 1985. [9] J.R. Farver, B.J. Giletti, Geochim. Cosmichim. Acta 53 (1989) 1621. [10] D.J. Cherniak, W.A. Lanford, F.J. Reyrson, Nucl. Instr. and Meth. B 54 (1990) 230.