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The initial interactions of beryllium with O2 and H2O vapor at elevated temperatures
Surface Science 601 (2007) 1326–1332 www.elsevier.com/locate/susc
The initial interactions of beryllium with O2 and H2O vapor at elevated temperatures S. Zalkind a, M. Polak b, N. Shamir
a,*
a b
Nuclear Research Center-Negev, P.O. Box 9001, Beer-Sheva 84190, Israel Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel Received 12 August 2006; accepted for publication 19 December 2006 Available online 22 December 2006
Abstract In the 310–790 K temperature range, the mechanism of initial oxidation by O2 is oxide island nucleation and growth. At the lower temperature range, oxygen is first chemisorbed and the oxide nucleates at coverage of 0.2. Increasing the temperature causes the oxide islands to nucleate at lower coverage and at 700 K and above, the oxide nucleates without any significant stage of chemisorbed oxygen. The temperature dependence shows that while the dissociation stage is not activated, the oxide nucleation and growth are thermally activated. Also, opposite to O2 adsorption, the initial H2O adsorption and oxidation rate was found to decrease with temperature. Opposite to the oxygen case, upon exposure to water vapor there is no noticeable stage of chemisorbed oxygen (or OH) and oxide is directly nucleated. Only after oxide coalescence, this tendency changes and the oxidation rate is increased with temperature. 2006 Elsevier B.V. All rights reserved. Keywords: Auger electron spectroscopy; Ion scattering spectroscopy; Adsorption kinetics; Oxidation; Beryllium
1. Introduction The interaction of beryllium with residual gases in UHV or plasma vessels is of grate interest due to its potential roll as a first wall material in fusion reactors. Comprehensive studies of the initial interaction of oxygen with beryllium surface were reported by Fowler and Blakely [1,2] and later by our group [3,4]. It has been found that oxygen first dissociatively chemisorbs and some of it probably penetrates into the surface. After a critical coverage is achieved, oxide islands nucleate and spread until coalescence and full coverage occur. Following coalescence, the oxidation rate is significantly reduced and the oxidation continues according to a logarithmic rate equation at high temperatures [2], or an inverse logarithmic one at room temperature [4]. Eylmore et al. [5] also found mostly logarithmic rate dependence in the temperature range of 500–650 C, and
*
Corresponding author. Tel.: +972 8 6568785; fax: +972 8 6568751. E-mail address: [email protected] (N. Shamir).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.12.060
deviations from that dependence after long exposure times. A parabolic rate during beryllium oxidation at relatively high pressure oxygen (660 mbar) at 350–600 C, was found by Roth et al. [6], with activation energy of 17 kcal/mol. It was also shown that oxidation occurs by permeation of Be from the metal/oxide interface to the outer surface. Former experiments on beryllium oxidation at relatively high pressures and high temperatures were based mainly on gravimetric and pressure change methods and showed also mainly parabolic growth until break-away occurs above 750 C [7–9]. Regarding water vapor interaction with beryllium, Petti et al. [10] found that up to 600 C the oxidation of dense beryllium in steam follows the parabolic rate equation, with activation energy of 11.8 kcal/mol. Andrell et al. [11] found similar results for water vapor interaction with Be flakes and Be powder. In previous publications we reported the initial interaction of H2O vapor with Be at room temperature (RT) [3] and below [12]. It was found that at RT, H2O fully dissociates and rapidly oxidizes the surface by oxide island nucleation and growth. Some hydrogen,
S. Zalkind et al. / Surface Science 601 (2007) 1326–1332
originated from water dissociation, was found to be trapped in the growing oxide layer. After coalescence, the oxidation continues at a significantly slower rate. At 150 K, H2O partially dissociates into H and OH, the later being adsorbed on the surface. Heating the OH adsorbed surface causes complete dissociation of the hydroxyls and oxidation of the Be surface. In the present work, we investigated the initial interaction of O2 and H2O with the beryllium surface, at the temperature range of 310–790 K.
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was pumped out. For high dose exposures, the AES measurements were performed during dosing at different exposure times. To avoid possible electron stimulated oxidation (ESO), which is pronounced especially for H2O exposure [19], relatively low electron current (0.5 lA) was used and the measurement point on the sample was frequently changed to avoid accumulation of the ESO. The average oxide thickness was calculated by using the ratio of the oxidized Be AES signal (at 95 eV) to that of the (attenuated) pure metal (at 105 eV), after mathematical separation [1–4].
2. Experimental The experiments were performed in an ultra-high-vacuum system (2 · 10 10 Torr base pressure), using Auger electron spectroscopy (AES) and direct recoil spectrometry (DRS). The DRS technique is based on grazing irradiation of the surface with a pulsed beam of 3 keV Ar+ ions, and the time of flight measurements of the surface elements (ions and neutrals) which are recoiled in a forward direction by a direct collision with the impinging primary ions. The technique and its capabilities in studying solid surfaces have been reviewed in Refs. [13–17] and in some of our previous publications. The main characteristics of this technique are topmost surface sensitivity and detection of light atomic masses, including hydrogen (sensitive to <1% of a monolayer). Due to the low primary ion dose required to acquire a typical spectrum (1011 ions/cm2) the technique is essentially non destructive. Combined measurements of DRS with electron spectroscopy methods, like AES, which probe a deeper range, can help to distinguish between processes occurring in the topmost layer and in the subsurface region [18]. The sample used in the present experiments was a 10 mm diameter sheet of fully dense polycrystalline beryllium, 0.2 mm thick, with a very strong (0 0 0 1) preferred orientation. The cleanliness found by chemical analysis was >99%, with the main impurity being BeO (0.8%). The sample was attached to a heating element and the temperature was monitored by a chromel–alumel thermocouple attached to a front edge of the sample. Sputtercleaning was preformed by 5 keV Ar+ until no impurities were found in the DRS spectra, followed by annealing (770 K, 5 min). The determination of the annealing parameters and the fully characterization of the surface are detailed in Ref. [4]. It has been found that the orientation of the sample (change of orientation of the grains) does not affect the results. The gases used for the experiments were research grade oxygen and distilled water after a few freeze-thaw cycles. The gases were introduced into the vacuum chamber via leak valves up to the desired pressures, as measured by a Bayard Alpert gauge (without correcting). The DRS measurements were performed during gas exposure. AES measurements for low dose exposures were performed after the sample was exposed to the desired dose and the chamber
3. Results and discussion 3.1. Initial oxidation by O2 The initial oxidation of the annealed beryllium surface was examined in the temperature range of 310–790 K. The O(DR) intensities versus O2 exposure dose in Langmuirs (1L = 10 6 Torr · 1 s) are shown in Fig. 1 for different sample temperatures. The curves were normalized to the saturation point, where full coverage on the surface is obtained. The data were fitted to the ‘‘clustering’’ or ‘‘island formation’’ model of O atoms on the surface during adsorption, which yields the kinetic equation [18]: ln(1 h) = KL, where h is the surface coverage fraction, L the exposure (in Langmuirs) and K a coefficient. Since DRS is sensitive mostly to the topmost layer, the increase in O(DR) intensity corresponds to the lateral oxygen (or oxide) coverage of the surface. It can be noticed that increasing the surface temperature causes a decrease in the oxygen coverage rate and full coverage is achieved at higher doses. Yet, O2 interaction with Be surface is by the clustering and oxide nucleation and growth mechanism for the whole temperature range. The average oxide thickness vs. oxygen exposure, as obtained from the AES measurements, is shown in Fig. 2. While from the DRS measurements, presented in Fig. 1, a reduction in oxidation rate with temperature may be
Fig. 1. Normalized O(DR) intensity vs. oxygen dose for different temperatures. The lines represent fits to the ‘‘clustering’’ model.
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Fig. 2. Average oxide thickness vs. oxygen dose for different temperatures. The spectra used for calculating the thickness are presented in the inset. For the full procedure see Ref. [3].
concluded, the AES results show a somewhat different picture. Although in the lower temperature range the initial oxidation rate is somewhat higher at 320 K than at 400 K and 500 K, at higher temperatures the oxidation rate slightly increases with temperature. In order to form the initial oxide nuclei, a certain amount of adsorbed oxygen is needed. Two contradicting processes seem to cause the oxidation to start at the highest exposure dose (30 L) for 400 K. On the one hand, desorption that increases with temperature dictates a delay in the formation of these adsorbed islands for temperatures higher than 320 K, thus delaying also the starting of initial oxidation. On the other hand, due to enhanced surface mobility, for higher temperatures the amount of adsorbed oxygen needed to start the oxidation process is reduced so much that it starts even at lower exposure doses. This is demonstrated in two different ways in Figs. 3 and 4. Fig. 3 displays the relation between the oxide thickness, derived from Fig. 2, and the surface coverage, derived from Fig. 1. For oxide islands growing
Fig. 3. Average oxide thickness vs. O coverage for oxygen exposure at different temperatures.
Fig. 4. The ratio of AES O(KLL) intensity, representing the total amount of oxygen on the surface, to the BeO AES intensity representing the oxygen turned to oxide during O2 and H2O exposure. The lines are guides to the eye. Inset: the AES peaks used for the calculation.
laterally with a constant thickness, the relation between the coverage and thickness should be linear, since both the DRS and AES sample proportional fractions of the growing oxide. For oxide islands also growing inwards, while spreading laterally, the rate of oxide thickness per coverage should rise and a concave curve will be obtained, as can be seen for the lower temperatures. At these lower temperatures the initial oxygen is chemisorbed, and oxide island nucleate (as can be observed from the Be(KLL) spectrum) only after a critical concentration of oxygen on the surface is obtained [4], at about a relative coverage of h approx 0.2 for room temperature. As temperature increases, especially above 500 K, the oxide islands nucleate at lower coverages, until at the high temperatures of 700–750 K, the oxide thickness starts to increase at a very early coverage. The linear relation obtained between the average oxide thickness and coverage for the high temperatures shows that at these temperatures the oxide islands spread after nucleation at a constant thickness of about 3 monolayers until coalescence is achieved. For the lower temperatures the oxide islands, at nucleation, are thinner and they thicken in the course of spreading. In a previous publication [4] it was shown for RT Be oxidation that the ratio between the O(KVV) intensity, which represents the total amount of oxygen on the surface
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(chemisorbed and oxidic), and the BeO intensity, which represents the oxygen turned oxidic, can distinguish between these two species. This O/BeO intensity ratio for Be exposure to oxygen and for comparison to H2O (which will be discussed later) is shown in Fig. 4. The steep increase in this ratio for oxygen exposure, especially at the lower temperatures, indicates that initially, chemisorbed oxygen dominates. Above a critical concentration, obtained at about 20 L, oxide nucleation starts and this ratio decreases, indicating the transformation of the chemisorbed oxygen to an oxidic one. Above 40 L, most of the oxygen is in the oxidic state. Increasing the temperature causes the O/BeO intensity ratio to decrease, until at high temperatures almost no chemisorbed oxygen can be observed and oxide islands nucleate from the beginning, in correlation with the results obtained from Figs. 2 and 3. 3.2. Initial oxidation by H2O It was found in a previous study [3], that exposure of beryllium to water vapor at room temperature causes the formation of an oxide layer that contains some trapped hydrogen (this was concluded mainly from the detection of hydrogen by the DRS and the lack of any shift in the O 1s XPS spectra to higher binding energy which can indicate the present of OH or H2O adsorbed species, that were detected beneath RT [12]). Some of the differences found between oxidation in water vapor and oxygen, like higher oxidation rates and widening of the O(KLL) signal were attributed to this hydrogen presence. In this section we report about the influence of temperature on H2O adsorption and initial oxidation. The O(DR) intensities, normalized to the saturation point (full coverage), are presented in Fig. 5. It can be seen that as temperature rises, the dose of H2O needed for complete coverage of the surface increases. The data also show good fittings to the clustering model, as was found for O2, but with a higher adsorption rate. Fig. 6 shows the average oxide thickness (as calculated from AES measurements) vs.
Fig. 5. Normalized O(DR) intensity vs. H2O dose for different temperatures. The fits are to the ‘‘clustering’’ model.
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Fig. 6. Average oxide thickness vs. H2O dose at different temperatures.
exposure for different temperatures. A decrease in the oxide layer is noted with increasing temperature for the initial interactions. The relation between the oxygen coverage (from Fig. 5) and oxide thickness, shown in Fig. 7, at all temperatures is quite a linear relation, indicating the early formation of oxide islands that spread on the surface, with a relatively constant thickness. Support to this conclusion is obtained also from the relation between the O(KVV) and the BeO intensities, as shown in Fig. 4. In opposite to oxygen adsorption, where there is an initial steep increase in this ratio indicating chemisorbed oxygen, only a very moderate increase in this ratio is noticed at higher temperatures and at lower temperatures no increase in this ratio occurs for water adsorption (notice the difference in Y axis scale relative to oxygen), indicating the lack of chemisorbed oxygen specie during water adsorption and the immediate formation of oxide islands. This is in good agreement with previous results that showed that while below RT the dissociation of H2O is only partial, at RT and above the dissociation is complete and oxide nuclei rapidly form [12].
Fig. 7. Average oxide thickness vs. O coverage for H2O exposure at different temperatures. The inset presents a schematically lateral island growth.
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Fig. 8. H(DR) intensity vs. H2O dose for different temperatures.
The H(DR) intensity, as measured during exposure to water vapor at RT was previously reported and discussed in Ref. [3]. It shows an increase in hydrogen intensity and then a decrease to a constant value, as can be also seen in Fig. 8. The explanation given was that first hydrogen, originated from water dissociation, adsorbs on the Be metal between the oxide islands, and then it is expelled by the spreading oxide, reaching a value representative to the hydrogen incorporated in the oxide. Fig. 8 shows, clearly, a decrease in the H(DR) intensity with temperatures, in the initial value as well in the final one. Above 500 K no hydrogen is measured, indicating desorption of all the hydrogen from the oxide. In the previous report it was also suggested that trapped hydrogen cause divers surroundings of oxygen ions and may account for the increase in the O(KLL) peak width [3]. The O(KLL) widths, measured for high H2O exposures (see Section 3.3), in comparison with O2 exposures, are presented in Fig. 9, showing a decrease in width for H2O with temperature, in accordance with the relation in hydrogen decreasing content. 3.3. Beryllium oxidation under long exposures to O2 and H2O In order to gain some information on the oxidation process beyond the point of oxide island coalescence, the sample was continuously exposed to high doses at a pressure of 5 · 10 6 Torr. After the initial fast oxidation (nucleation and growth) there is a moderate increase in oxide thickness. The oxidation curves (after the initial stage) were best fitted for most temperatures, except of 320 and 400 K, to the logarithmic rate equation [20] (X = Kllog t + C, where X is the oxide thickness at time t, K is the rate constant and C a constant), as can be seen in Fig. 10. Good fits to the inverse logarithmic rate equation (1/X = C Kilog t) are obtained for the lower temperature data, 320 K and 400 K (fit not shown), as was also obtained in a previous publication for the RT case [4]. The logarithmic and inverse logarithmic models are usually correlated with a self induced elec-
Fig. 9. O(KLL) width vs. temperatures after long H2O exposures. The oxygen results are shown for comparison.
Fig. 10. Oxidation in O2 at different temperature plotted vs. logarithm of the exposure time.
tric field, formed across the thin growing oxide, which reduces the activation energy for ion movement through it [20–24]. The thickness of the oxide formed during H2O exposure is shown in Fig. 11. Similar to O2 oxidation, a sharp initial increase in oxide thickness is observed at low exposures, corresponding to the oxide island nucleation and growth stage, after where the oxidation rate is moderated. It can be noticed that while during the initial nucleation and growth stage the rate of the oxide formed is reduced with temperature (Fig. 6), after coalescence, the inward oxidation rate is increased with temperature, and was best fitted to the parabolic rate equation (X2 = 2Kt), indicating the possibility of a relatively simple diffusion rate control, or alternatively the influence of field transport [20]. Arrhenius fittings yielded a relatively low activation energy of 5.1 kcal/mol, pointing that the early oxidation by H2O is probably also enhanced by the formation of an electric field across the thin layer.
S. Zalkind et al. / Surface Science 601 (2007) 1326–1332
Fig. 11. Average oxide thickness as a function of H2O exposure at different temperatures. The lines represent fits to the ‘‘parabolic rate equation’’ model. The inward growth of the oxide, beyond the 2.5 monolayer thickness (dashed line) is schematically presented in the inset.
4. Summary and conclusions In the present study it is found that for the temperature range investigated, the initial oxidation by both, oxygen and water vapor, is by oxide islands nucleation and growth. At RT and above, oxygen is first chemisorbed and oxide start to nucleate at coverage of 0.2. Increasing the temperature causes the oxide islands to nucleate at lower coverage and at 700 K and above the oxide islands nucleate without any significant stage of chemisorbed oxygen. In addition, it seems that the initial oxide growth is governed by reduction in the sticking coefficient with temperature, which causes an initial reduction in oxide thickness (the initial thickness at 320 K is higher than at 400 K) and delays the attainment of full coverage. On the other hand, some increase in diffusion with temperature causes oxide thickness to increase. A similar phenomenon of reduction in initially adsorbed oxygen with temperature increase was also found in other systems, for example oxidation of iron in oxygen environment [25]. This temperature dependence shows that while the dissociation stage is not activated, the oxide nucleation and growth is thermally activated. Only after a critical concentration is formed on the surface, oxygen overcomes the activation barrier for oxide nucleation. It seems that increase of oxygen coverage reduces the activation barrier to oxide nucleation, as was also suggested for aluminum oxidation [26]. The initial H2O adsorption and oxidation was found to decrease with temperature. Full coverage is achieved at higher exposures and the thickness of the growing oxide islands is reduced. Opposite to oxygen, there is no noticeable stage of chemisorbed oxygen species and oxide is directly nucleated. Only after coalescence of an oxide layer, this tendency changes and the oxidation rate increases with temperature increase. The amount of hydrogen incorporated in the oxide is reduced with temperature, as mani-
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fested in the H(DR) intensity (Fig. 8) and its reduced influence on O(KLL) width. The oxidation rate of inward growth under H2O exposure, after oxide island coalescence, is best fitted to the parabolic rate equation, yielding activation energy of 5.1 kcal/ mol. A parabolic rate equation for Be oxidation in steam at a higher temperature range (and for thicker oxide) was previously found [10,11], with higher activation energy of 11.8 kcal/mol [10] and was attributed to thermally activated diffusion of Be2+ through the oxide. For oxides thicker than a few nm, the space charge across the layer diminished and therefore thermally activated diffusion takes place. As mentioned in Ref. [10], the oxidation kinetics was found to depend on the surface finish and Be form (i.e. dense, pebbles, etc.) and may also explain some of the difference found in the values of the activation energies. Oxidation in O2 atmosphere at 500 K and above, corresponds to a logarithmic rate equation. For lower temperatures, the oxidation data fits well the inverse logarithmic rate equation, as we previously found at RT for sputtered beryllium [4]. The logarithmic and the inverse logarithmic rate equations, found mostly for thin films, are usually applied when the diffusion of the metal cations through growing oxide is controlled by an electric field, formed across the oxide, accelerating the diffusion [20–24]. The results obtained here are generally in agreement with previous studies by Eylmore et al. [5] and by Fowler and Blakely [2] who found that beryllium oxidation by oxygen is best described by the logarithmic rate equation with similar activation energy of 5 kcal/mol for transport. The electric field across the oxide layer is formed by the relative fast electron flow from the metal to the surface to meet the incoming oxygen by tunneling and thermal emission. Graat et al. [25] found for Fe oxidation that tunneling dominates electrons flow at low temperatures and above 400 K the electron flow through the oxide is dominated by thermal emission. It seems, therefore, that a similar mechanism can control also the beryllium oxidation in oxygen atmosphere, and applying the coupled currents concept [24,25] to Be oxidation will be the subject of a further coming study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
D.E. Fowler, J.M. Blakely, J. Vac. Sci. Technol. 20 (1982) 930. D.E. Fowler, J.M. Blakely, Surf. Sci. 148 (1984) 265. S. Zalkind, M. Polak, N. Shamir, Surf. Sci. 385 (1997) 318. S. Zalkind, M. Polak, N. Shamir, Surf. Sci. 513 (2002) 501. D.W. Eylmore, S.J. Gregg, W.B. Jepson, J. Nucl. Mater. 3 (1961) 190. J. Roth, W.R. Wampler, W. Jacob, J. Nucl. Mater. 250 (1997) 23. D. Cubicciotti, J. Am. Chem. Soc. 72 (1950) 2084. M.M. Nakata, U.S. Atomic Energy Comm., NAA-SR-4737, 1960. E.A. Gulbransen, K. Andrew, J. Electrochem. Soc. 97 (1950) 383. D.A. Petti, G.R. Smolik, R.A. Andrel, J. Nucl. Mater. 283–287 (2000) 1390. R.A. Andrell, F. Scaffidi-Argentina, D. Davidov, R.J. Pawelko, G.R. Smolik, J. Nucl. Mater. 283–287 (2000) 1463. S. Zalkind, M. Polak, N. Shamir, Surf. Sci. 529 (2003) 189. J.A. Schultz, Y.S. Jo, J.W. Rabalais, Nucl. Instru. Meth. Phys. Res. B 10/11 (1985) 713. M.H. Mintz, J.A. Schultz, J. Less. Common Meta. 103 (1984) 349.
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[15] J.W. Rabalias, CRC Crit. Rev. Solid State Mater. Sci. 14 (1988) 319. [16] M.S. Hammond, J.A. Schultz, A.R. Krauss, J. Vac. Sci. Technol. A 13 (1995) 1136. [17] M. Tassatto, P.R. Watson, Surf. Sci. 464 (2000) 251. [18] E. Swissa, J. Bloch, U. Atzmony, M.H. Mintz, Surf. Sci. 21 (1989) 323. [19] S. Zalkind, M. Polak, N. Shamir, Phys. Rev. B 71 (2005) 125413. [20] K. Hauffa, Oxidation of Metals, Plenum Press, 1965.
[21] N. Cabrera and N.F. Mott, Rep. Progr. Phys. 12 (1948–49) 163. [22] F.P. Fehlner, Philosophical Mag. B 52 (1985) 729. [23] A.T. Fromhold, Theory of metal oxidation, Fundamentals, vol. 1, North Holland, 1976. [24] M.M.E. Fromm, J. Alloys Comp. 258 (1997) 7. [25] P.C.J. Graat, M.A.J. Somers, A.M. Vredenberg, E.J. Mittemeijer, J. Appl. Phys. 82 (1997) 1416. [26] V. Zhukov, I. Popova, J.T. Yates Jr., Surf. Sci. 441 (1999) 251.
The initial interactions of beryllium with O2 and H2O vapor at elevated temperatures S. Zalkind a, M. Polak b, N. Shamir
a,*
a b
Nuclear Research Center-Negev, P.O. Box 9001, Beer-Sheva 84190, Israel Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel Received 12 August 2006; accepted for publication 19 December 2006 Available online 22 December 2006
Abstract In the 310–790 K temperature range, the mechanism of initial oxidation by O2 is oxide island nucleation and growth. At the lower temperature range, oxygen is first chemisorbed and the oxide nucleates at coverage of 0.2. Increasing the temperature causes the oxide islands to nucleate at lower coverage and at 700 K and above, the oxide nucleates without any significant stage of chemisorbed oxygen. The temperature dependence shows that while the dissociation stage is not activated, the oxide nucleation and growth are thermally activated. Also, opposite to O2 adsorption, the initial H2O adsorption and oxidation rate was found to decrease with temperature. Opposite to the oxygen case, upon exposure to water vapor there is no noticeable stage of chemisorbed oxygen (or OH) and oxide is directly nucleated. Only after oxide coalescence, this tendency changes and the oxidation rate is increased with temperature. 2006 Elsevier B.V. All rights reserved. Keywords: Auger electron spectroscopy; Ion scattering spectroscopy; Adsorption kinetics; Oxidation; Beryllium
1. Introduction The interaction of beryllium with residual gases in UHV or plasma vessels is of grate interest due to its potential roll as a first wall material in fusion reactors. Comprehensive studies of the initial interaction of oxygen with beryllium surface were reported by Fowler and Blakely [1,2] and later by our group [3,4]. It has been found that oxygen first dissociatively chemisorbs and some of it probably penetrates into the surface. After a critical coverage is achieved, oxide islands nucleate and spread until coalescence and full coverage occur. Following coalescence, the oxidation rate is significantly reduced and the oxidation continues according to a logarithmic rate equation at high temperatures [2], or an inverse logarithmic one at room temperature [4]. Eylmore et al. [5] also found mostly logarithmic rate dependence in the temperature range of 500–650 C, and
*
Corresponding author. Tel.: +972 8 6568785; fax: +972 8 6568751. E-mail address: [email protected] (N. Shamir).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.12.060
deviations from that dependence after long exposure times. A parabolic rate during beryllium oxidation at relatively high pressure oxygen (660 mbar) at 350–600 C, was found by Roth et al. [6], with activation energy of 17 kcal/mol. It was also shown that oxidation occurs by permeation of Be from the metal/oxide interface to the outer surface. Former experiments on beryllium oxidation at relatively high pressures and high temperatures were based mainly on gravimetric and pressure change methods and showed also mainly parabolic growth until break-away occurs above 750 C [7–9]. Regarding water vapor interaction with beryllium, Petti et al. [10] found that up to 600 C the oxidation of dense beryllium in steam follows the parabolic rate equation, with activation energy of 11.8 kcal/mol. Andrell et al. [11] found similar results for water vapor interaction with Be flakes and Be powder. In previous publications we reported the initial interaction of H2O vapor with Be at room temperature (RT) [3] and below [12]. It was found that at RT, H2O fully dissociates and rapidly oxidizes the surface by oxide island nucleation and growth. Some hydrogen,
S. Zalkind et al. / Surface Science 601 (2007) 1326–1332
originated from water dissociation, was found to be trapped in the growing oxide layer. After coalescence, the oxidation continues at a significantly slower rate. At 150 K, H2O partially dissociates into H and OH, the later being adsorbed on the surface. Heating the OH adsorbed surface causes complete dissociation of the hydroxyls and oxidation of the Be surface. In the present work, we investigated the initial interaction of O2 and H2O with the beryllium surface, at the temperature range of 310–790 K.
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was pumped out. For high dose exposures, the AES measurements were performed during dosing at different exposure times. To avoid possible electron stimulated oxidation (ESO), which is pronounced especially for H2O exposure [19], relatively low electron current (0.5 lA) was used and the measurement point on the sample was frequently changed to avoid accumulation of the ESO. The average oxide thickness was calculated by using the ratio of the oxidized Be AES signal (at 95 eV) to that of the (attenuated) pure metal (at 105 eV), after mathematical separation [1–4].
2. Experimental The experiments were performed in an ultra-high-vacuum system (2 · 10 10 Torr base pressure), using Auger electron spectroscopy (AES) and direct recoil spectrometry (DRS). The DRS technique is based on grazing irradiation of the surface with a pulsed beam of 3 keV Ar+ ions, and the time of flight measurements of the surface elements (ions and neutrals) which are recoiled in a forward direction by a direct collision with the impinging primary ions. The technique and its capabilities in studying solid surfaces have been reviewed in Refs. [13–17] and in some of our previous publications. The main characteristics of this technique are topmost surface sensitivity and detection of light atomic masses, including hydrogen (sensitive to <1% of a monolayer). Due to the low primary ion dose required to acquire a typical spectrum (1011 ions/cm2) the technique is essentially non destructive. Combined measurements of DRS with electron spectroscopy methods, like AES, which probe a deeper range, can help to distinguish between processes occurring in the topmost layer and in the subsurface region [18]. The sample used in the present experiments was a 10 mm diameter sheet of fully dense polycrystalline beryllium, 0.2 mm thick, with a very strong (0 0 0 1) preferred orientation. The cleanliness found by chemical analysis was >99%, with the main impurity being BeO (0.8%). The sample was attached to a heating element and the temperature was monitored by a chromel–alumel thermocouple attached to a front edge of the sample. Sputtercleaning was preformed by 5 keV Ar+ until no impurities were found in the DRS spectra, followed by annealing (770 K, 5 min). The determination of the annealing parameters and the fully characterization of the surface are detailed in Ref. [4]. It has been found that the orientation of the sample (change of orientation of the grains) does not affect the results. The gases used for the experiments were research grade oxygen and distilled water after a few freeze-thaw cycles. The gases were introduced into the vacuum chamber via leak valves up to the desired pressures, as measured by a Bayard Alpert gauge (without correcting). The DRS measurements were performed during gas exposure. AES measurements for low dose exposures were performed after the sample was exposed to the desired dose and the chamber
3. Results and discussion 3.1. Initial oxidation by O2 The initial oxidation of the annealed beryllium surface was examined in the temperature range of 310–790 K. The O(DR) intensities versus O2 exposure dose in Langmuirs (1L = 10 6 Torr · 1 s) are shown in Fig. 1 for different sample temperatures. The curves were normalized to the saturation point, where full coverage on the surface is obtained. The data were fitted to the ‘‘clustering’’ or ‘‘island formation’’ model of O atoms on the surface during adsorption, which yields the kinetic equation [18]: ln(1 h) = KL, where h is the surface coverage fraction, L the exposure (in Langmuirs) and K a coefficient. Since DRS is sensitive mostly to the topmost layer, the increase in O(DR) intensity corresponds to the lateral oxygen (or oxide) coverage of the surface. It can be noticed that increasing the surface temperature causes a decrease in the oxygen coverage rate and full coverage is achieved at higher doses. Yet, O2 interaction with Be surface is by the clustering and oxide nucleation and growth mechanism for the whole temperature range. The average oxide thickness vs. oxygen exposure, as obtained from the AES measurements, is shown in Fig. 2. While from the DRS measurements, presented in Fig. 1, a reduction in oxidation rate with temperature may be
Fig. 1. Normalized O(DR) intensity vs. oxygen dose for different temperatures. The lines represent fits to the ‘‘clustering’’ model.
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Fig. 2. Average oxide thickness vs. oxygen dose for different temperatures. The spectra used for calculating the thickness are presented in the inset. For the full procedure see Ref. [3].
concluded, the AES results show a somewhat different picture. Although in the lower temperature range the initial oxidation rate is somewhat higher at 320 K than at 400 K and 500 K, at higher temperatures the oxidation rate slightly increases with temperature. In order to form the initial oxide nuclei, a certain amount of adsorbed oxygen is needed. Two contradicting processes seem to cause the oxidation to start at the highest exposure dose (30 L) for 400 K. On the one hand, desorption that increases with temperature dictates a delay in the formation of these adsorbed islands for temperatures higher than 320 K, thus delaying also the starting of initial oxidation. On the other hand, due to enhanced surface mobility, for higher temperatures the amount of adsorbed oxygen needed to start the oxidation process is reduced so much that it starts even at lower exposure doses. This is demonstrated in two different ways in Figs. 3 and 4. Fig. 3 displays the relation between the oxide thickness, derived from Fig. 2, and the surface coverage, derived from Fig. 1. For oxide islands growing
Fig. 3. Average oxide thickness vs. O coverage for oxygen exposure at different temperatures.
Fig. 4. The ratio of AES O(KLL) intensity, representing the total amount of oxygen on the surface, to the BeO AES intensity representing the oxygen turned to oxide during O2 and H2O exposure. The lines are guides to the eye. Inset: the AES peaks used for the calculation.
laterally with a constant thickness, the relation between the coverage and thickness should be linear, since both the DRS and AES sample proportional fractions of the growing oxide. For oxide islands also growing inwards, while spreading laterally, the rate of oxide thickness per coverage should rise and a concave curve will be obtained, as can be seen for the lower temperatures. At these lower temperatures the initial oxygen is chemisorbed, and oxide island nucleate (as can be observed from the Be(KLL) spectrum) only after a critical concentration of oxygen on the surface is obtained [4], at about a relative coverage of h approx 0.2 for room temperature. As temperature increases, especially above 500 K, the oxide islands nucleate at lower coverages, until at the high temperatures of 700–750 K, the oxide thickness starts to increase at a very early coverage. The linear relation obtained between the average oxide thickness and coverage for the high temperatures shows that at these temperatures the oxide islands spread after nucleation at a constant thickness of about 3 monolayers until coalescence is achieved. For the lower temperatures the oxide islands, at nucleation, are thinner and they thicken in the course of spreading. In a previous publication [4] it was shown for RT Be oxidation that the ratio between the O(KVV) intensity, which represents the total amount of oxygen on the surface
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(chemisorbed and oxidic), and the BeO intensity, which represents the oxygen turned oxidic, can distinguish between these two species. This O/BeO intensity ratio for Be exposure to oxygen and for comparison to H2O (which will be discussed later) is shown in Fig. 4. The steep increase in this ratio for oxygen exposure, especially at the lower temperatures, indicates that initially, chemisorbed oxygen dominates. Above a critical concentration, obtained at about 20 L, oxide nucleation starts and this ratio decreases, indicating the transformation of the chemisorbed oxygen to an oxidic one. Above 40 L, most of the oxygen is in the oxidic state. Increasing the temperature causes the O/BeO intensity ratio to decrease, until at high temperatures almost no chemisorbed oxygen can be observed and oxide islands nucleate from the beginning, in correlation with the results obtained from Figs. 2 and 3. 3.2. Initial oxidation by H2O It was found in a previous study [3], that exposure of beryllium to water vapor at room temperature causes the formation of an oxide layer that contains some trapped hydrogen (this was concluded mainly from the detection of hydrogen by the DRS and the lack of any shift in the O 1s XPS spectra to higher binding energy which can indicate the present of OH or H2O adsorbed species, that were detected beneath RT [12]). Some of the differences found between oxidation in water vapor and oxygen, like higher oxidation rates and widening of the O(KLL) signal were attributed to this hydrogen presence. In this section we report about the influence of temperature on H2O adsorption and initial oxidation. The O(DR) intensities, normalized to the saturation point (full coverage), are presented in Fig. 5. It can be seen that as temperature rises, the dose of H2O needed for complete coverage of the surface increases. The data also show good fittings to the clustering model, as was found for O2, but with a higher adsorption rate. Fig. 6 shows the average oxide thickness (as calculated from AES measurements) vs.
Fig. 5. Normalized O(DR) intensity vs. H2O dose for different temperatures. The fits are to the ‘‘clustering’’ model.
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Fig. 6. Average oxide thickness vs. H2O dose at different temperatures.
exposure for different temperatures. A decrease in the oxide layer is noted with increasing temperature for the initial interactions. The relation between the oxygen coverage (from Fig. 5) and oxide thickness, shown in Fig. 7, at all temperatures is quite a linear relation, indicating the early formation of oxide islands that spread on the surface, with a relatively constant thickness. Support to this conclusion is obtained also from the relation between the O(KVV) and the BeO intensities, as shown in Fig. 4. In opposite to oxygen adsorption, where there is an initial steep increase in this ratio indicating chemisorbed oxygen, only a very moderate increase in this ratio is noticed at higher temperatures and at lower temperatures no increase in this ratio occurs for water adsorption (notice the difference in Y axis scale relative to oxygen), indicating the lack of chemisorbed oxygen specie during water adsorption and the immediate formation of oxide islands. This is in good agreement with previous results that showed that while below RT the dissociation of H2O is only partial, at RT and above the dissociation is complete and oxide nuclei rapidly form [12].
Fig. 7. Average oxide thickness vs. O coverage for H2O exposure at different temperatures. The inset presents a schematically lateral island growth.
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Fig. 8. H(DR) intensity vs. H2O dose for different temperatures.
The H(DR) intensity, as measured during exposure to water vapor at RT was previously reported and discussed in Ref. [3]. It shows an increase in hydrogen intensity and then a decrease to a constant value, as can be also seen in Fig. 8. The explanation given was that first hydrogen, originated from water dissociation, adsorbs on the Be metal between the oxide islands, and then it is expelled by the spreading oxide, reaching a value representative to the hydrogen incorporated in the oxide. Fig. 8 shows, clearly, a decrease in the H(DR) intensity with temperatures, in the initial value as well in the final one. Above 500 K no hydrogen is measured, indicating desorption of all the hydrogen from the oxide. In the previous report it was also suggested that trapped hydrogen cause divers surroundings of oxygen ions and may account for the increase in the O(KLL) peak width [3]. The O(KLL) widths, measured for high H2O exposures (see Section 3.3), in comparison with O2 exposures, are presented in Fig. 9, showing a decrease in width for H2O with temperature, in accordance with the relation in hydrogen decreasing content. 3.3. Beryllium oxidation under long exposures to O2 and H2O In order to gain some information on the oxidation process beyond the point of oxide island coalescence, the sample was continuously exposed to high doses at a pressure of 5 · 10 6 Torr. After the initial fast oxidation (nucleation and growth) there is a moderate increase in oxide thickness. The oxidation curves (after the initial stage) were best fitted for most temperatures, except of 320 and 400 K, to the logarithmic rate equation [20] (X = Kllog t + C, where X is the oxide thickness at time t, K is the rate constant and C a constant), as can be seen in Fig. 10. Good fits to the inverse logarithmic rate equation (1/X = C Kilog t) are obtained for the lower temperature data, 320 K and 400 K (fit not shown), as was also obtained in a previous publication for the RT case [4]. The logarithmic and inverse logarithmic models are usually correlated with a self induced elec-
Fig. 9. O(KLL) width vs. temperatures after long H2O exposures. The oxygen results are shown for comparison.
Fig. 10. Oxidation in O2 at different temperature plotted vs. logarithm of the exposure time.
tric field, formed across the thin growing oxide, which reduces the activation energy for ion movement through it [20–24]. The thickness of the oxide formed during H2O exposure is shown in Fig. 11. Similar to O2 oxidation, a sharp initial increase in oxide thickness is observed at low exposures, corresponding to the oxide island nucleation and growth stage, after where the oxidation rate is moderated. It can be noticed that while during the initial nucleation and growth stage the rate of the oxide formed is reduced with temperature (Fig. 6), after coalescence, the inward oxidation rate is increased with temperature, and was best fitted to the parabolic rate equation (X2 = 2Kt), indicating the possibility of a relatively simple diffusion rate control, or alternatively the influence of field transport [20]. Arrhenius fittings yielded a relatively low activation energy of 5.1 kcal/mol, pointing that the early oxidation by H2O is probably also enhanced by the formation of an electric field across the thin layer.
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Fig. 11. Average oxide thickness as a function of H2O exposure at different temperatures. The lines represent fits to the ‘‘parabolic rate equation’’ model. The inward growth of the oxide, beyond the 2.5 monolayer thickness (dashed line) is schematically presented in the inset.
4. Summary and conclusions In the present study it is found that for the temperature range investigated, the initial oxidation by both, oxygen and water vapor, is by oxide islands nucleation and growth. At RT and above, oxygen is first chemisorbed and oxide start to nucleate at coverage of 0.2. Increasing the temperature causes the oxide islands to nucleate at lower coverage and at 700 K and above the oxide islands nucleate without any significant stage of chemisorbed oxygen. In addition, it seems that the initial oxide growth is governed by reduction in the sticking coefficient with temperature, which causes an initial reduction in oxide thickness (the initial thickness at 320 K is higher than at 400 K) and delays the attainment of full coverage. On the other hand, some increase in diffusion with temperature causes oxide thickness to increase. A similar phenomenon of reduction in initially adsorbed oxygen with temperature increase was also found in other systems, for example oxidation of iron in oxygen environment [25]. This temperature dependence shows that while the dissociation stage is not activated, the oxide nucleation and growth is thermally activated. Only after a critical concentration is formed on the surface, oxygen overcomes the activation barrier for oxide nucleation. It seems that increase of oxygen coverage reduces the activation barrier to oxide nucleation, as was also suggested for aluminum oxidation [26]. The initial H2O adsorption and oxidation was found to decrease with temperature. Full coverage is achieved at higher exposures and the thickness of the growing oxide islands is reduced. Opposite to oxygen, there is no noticeable stage of chemisorbed oxygen species and oxide is directly nucleated. Only after coalescence of an oxide layer, this tendency changes and the oxidation rate increases with temperature increase. The amount of hydrogen incorporated in the oxide is reduced with temperature, as mani-
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fested in the H(DR) intensity (Fig. 8) and its reduced influence on O(KLL) width. The oxidation rate of inward growth under H2O exposure, after oxide island coalescence, is best fitted to the parabolic rate equation, yielding activation energy of 5.1 kcal/ mol. A parabolic rate equation for Be oxidation in steam at a higher temperature range (and for thicker oxide) was previously found [10,11], with higher activation energy of 11.8 kcal/mol [10] and was attributed to thermally activated diffusion of Be2+ through the oxide. For oxides thicker than a few nm, the space charge across the layer diminished and therefore thermally activated diffusion takes place. As mentioned in Ref. [10], the oxidation kinetics was found to depend on the surface finish and Be form (i.e. dense, pebbles, etc.) and may also explain some of the difference found in the values of the activation energies. Oxidation in O2 atmosphere at 500 K and above, corresponds to a logarithmic rate equation. For lower temperatures, the oxidation data fits well the inverse logarithmic rate equation, as we previously found at RT for sputtered beryllium [4]. The logarithmic and the inverse logarithmic rate equations, found mostly for thin films, are usually applied when the diffusion of the metal cations through growing oxide is controlled by an electric field, formed across the oxide, accelerating the diffusion [20–24]. The results obtained here are generally in agreement with previous studies by Eylmore et al. [5] and by Fowler and Blakely [2] who found that beryllium oxidation by oxygen is best described by the logarithmic rate equation with similar activation energy of 5 kcal/mol for transport. The electric field across the oxide layer is formed by the relative fast electron flow from the metal to the surface to meet the incoming oxygen by tunneling and thermal emission. Graat et al. [25] found for Fe oxidation that tunneling dominates electrons flow at low temperatures and above 400 K the electron flow through the oxide is dominated by thermal emission. It seems, therefore, that a similar mechanism can control also the beryllium oxidation in oxygen atmosphere, and applying the coupled currents concept [24,25] to Be oxidation will be the subject of a further coming study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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