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Surface composition of high-nickel alloy after the impingement of atomic hydrogen at different temperatures
SURFACE COMPOSITION OF HIGH-NICKEL ALLOY AFI-ER THE IMPINGEMENT OF ATOMIC HYDROGEN AT DIFFERENT TEMPERATURES J.v. SEGGERN
and K.G. TSCHERSICH
Znstitut fir Grenzpiichenforschung und Vakuumphysik, D-5170 Jiilich, Fed. Rep. of Germany
Kemforschungsanlage
Jiilich GmbH,
Association EuratomlKFA,
Surface composition changes by incident atomic hydrogen have been investigated on Inconel samples. The samples have been exposed to thermal atomic hydrogen at temperatures of 1-C and have been analyzed using Auger electron spectroscopy. Carbon, oxygen, and sulphur are the main impurities. At sample temperatures from 100 to 3OOT, the carbon concentration is significantly reduced by incident atomic hydrogen. An apparent cross section of this process has been determined. It increases from 110 to 170°C by two orders of magnitude and is independent of temperature above 170°C. being 2 X lo-*’ cm’. At 400°C carbon migrates into the bulk. The disappearance of carbon is accompanied by the appearance of sulphur. We ascribe the appearance of sulphur at temperatures up to 300°C to chemisorption-induced segregation. A relatively slow removal of oxygen was generally observed.
1. Introduction Discharge cleaning with hydrogen is the most widespread method of preconditioning tokamak walls [l] and makes it feasible to reduce the impurity level in tokamak discharges from Z,S> 3 to Z,e= 1 [2]. The cleaning discharges are usually optimized by maximizing the reaction products of hydrogen with the surface impurities, oxygen and carbon, namely Hz0 and CH4. These compounds are determined by mass spectrometric analysis of the gas which is pumped out of the tokamak. This procedure was introduced by Taylor [2,3]. Dietz and, Waelbroeck [4] explain the chemical reactions producing Hz0 and CHI as being due to the chemical reactivity of the atomic hydrogen species (Ho, H+) impinging onto the tokamak wall. Therefore, we tested the cleaning efficiency of thermal atomic hydrogen using Auger electron spectroscopy (AES) for surface analysis. The material under test is Nicrofer 7216 [5], a material equivalent to Inconel 600 [6]. Inconel alloys are chosen as first wall materials for some present day tokamaks, e.g. TFR 600 [7], JET [8], and TEXTOR [9]. In a previous paper [lo] we reported the fast removal of graphitic carbon and the slow removal of oxygen by atomic hydrogen, as well as the segregation of the sulphur from the bulk. The
object of the present investigation was the dependence of the surface composition changes on sample temperature. The results may be important for JET and TEXTOR because liner temperatures as high as 500-6OO“C are envisaged for these tokamaks. 2. Experimental procedure The experimental device has been described in a previous publication [lo]. Briefly, in a preparation system atomic hydrogen is produced by thermal dissociation of molecular hydrogen on a hot tungsten ribbon (2OOOK) [ll]. The samples are exposed to a given atomic hydrogen dose and then transferred by vacuum locks to an AES analysis system. The procedure is repeated several times so as to measure the surface composition in dependence of increasing hydrogen dose. During exposure the samples are heated by the radiation from the hot tungsten ribbon and additionally by RF [lo]. 3. Results and discussion The samples were mechanically polished and degreased with acetone and methanol just before their installation into the vacuum system. Thereafter, the following surface composition, in
Journal of Nuclear Materials 93 & 94 (1980) 806-811 @ North-Holland R&lishing ContpMy
806
J.v. Seggem, K.G. Tschersich I Surfnce compositionchanges
atomic %, was measured (mean value of all samples): Ni: 43% ; Cr: 4%; Fe: 4.5%; C: 36% ; 0: 9.5% ; and S: 1%. Furthermore N, Cl, and Ca were detected at around 1%. For comparison, the specified bulk composition is as follows [12]: Ni: ~70%; Cr: 1619%; Fe: 6-10%; Mn: 11% ; Cu: 10.5% ; Ti: ~0.4% ; Si: 11% ; C: ~0.4% ; S: 10.03% ; and P: sO.o6%. The volume composition as measured with AES after argon ion sputter etching was found to be within 1%) i.e. Ni: 71%; Cr: 19% ; and Fe: 10%. It has been observed that the first cleaning of a sample with atomic hydrogen was less efficient than the cleaning of a sample which was recontaminated, e.g. by exposure to air. We will concentrate on the initial cleaning. As stated above the main impurities are carbon, oxygen, and sulphur. Because they behave quite differently, we will discuss them separately. 3.1. Carbon Fig. 1 shows the initial surface concentrations of carbon, oxygen, and sulphur on an Inconel
807
sample and the concentrations measured after the following successive steps: heating to 150°C in vacuum, heating to 150°C in hydrogen, exposure to atomic hydrogen up to 1019 atoms cme2, exposure to air for 20 h, and finally a repetition of the first three steps. The carbon has been found to consist of graphitic carbon or a mixture of graphitic and carbidic carbon, which will be discussed in more detail below. The percentage of graphitic carbon is indicated along the carbon curve. Due to heating and exposure to molecular hydrogen there is only a small reduction of the carbon concentration, whereas the exposure to atomic hydrogen reduces the carbon concentration significantly. After recontamination the removal of carbon is qualitatively the same. During the removal of carbon by atomic hydrogen, the carbon changes from being highly graphitic to being predominantly carbidic. This has been found by analysing the carbon Auger peak shape by the following procedure. (A similar procedure has been applied for the analysis of the thermal decomposition of nickel carbide [ 141.)
I” u‘ b L”
T=150°C
.E n
0
5
10
15 90
Ho dose 11~‘atoms/crn2
95
I
Fig.1. Surfaceimpurityconcentrationof an Inconelsampleafter installation,after heatingin vacuumandin molecularhydrogen,and after stepwise exposure to atomichydrogen.The treatmentsare repeated after an exposureto air. During the exposure to molecular and atomichydrogen the hydrogen pressure was betweenps, = 7 X 10s4and 7 X low3mbar. All the treatments have been performed at 150°Capart from the exposureto air which was done at room temperature. The percentage of graphitic carbon is indicated along the carbon curve.
J.u. Seggem, KG. Tschersich 1 Surface compositionchanges
808
t
/
carbon but not the carbidic carbon. This characteristic feature of the carbon removal has been observed for sample temperatures of NO-300°C. In the same range of temperature the carbon concentration plotted versus the atomic hydrogen dose, cf. fig. 1, decreases exponentially at small doses. Therefore, the removal of graphitic carbon can be described by a cross section [lo]. This cross section is plotted in fig. 2 versus the sample temperature. It increases steeply from 110 to 170°C by more than two orders of magnitude and levels off at a value of about 2 x lo-” cm*. At sample temperatures between 300 and 400°C there is an initial removal of carbon, but the concentrations measured at hydrogen doses above 101’ atoms cme2 scatter heavily. This might be due to two opposing effects, i.e. carbon removal by atomic hydrogen and segregation of carbon from the bulk to the surface. In this situation our experimental procedure may not be adequate, because we heat, cool, and transfer the sample for every exposure and surface analysis. Fig. 3 shows the concentration of carbon, oxygen, and sulphur after treatments at 410°C. Obviously, the carbon concentration is reduced by the temperature treatment and there is no significant effect on the exposure to atomic hydrogen. We suppose that carbon migrates into the bulk. The disappearance of carbon from the Inconel surface has been observed by
4
I
1o-‘g1 / 100
200 sample
temperature
300 PC
1
Fig. 2. Apparent cross section for the removal of graphitic carbon by atomic hydrogen as a function of the sample temperature during exposure.
The graphitic carbon peak was measured on glassy carbon and the carbidic carbon peak on a tungsten carbide single crystal. The peak shapes were very similar to those given by Davis et al. [13] for graphite and tantalum carbide, respectively. By linear superposition of the measured peaks, we calculated the peaks of graphitelcarbide mixtures and used them for assessing the mixtures corresponding to the carbon peaks as measured on Inconel. Although this determination of the portions of graphitic and carbidic carbon is a rather crude one, we conclude that atomic hydrogen incident on Inconel predominantly removes the graphitic
0.5
Ii @dose
20
[ 10’8atoms/cm2
LO
1
Fig. 3. Surface impurity concentration of an Inconel sample after installation, after heating in vacuum and in molecular hydrogen, and after stepwise exposure to atomic hydrogen. During the exposure to molecular and atomic hydrogen the hydrogen was between PH2= 7 X lo4 and 7 X 10e3mbar. All the treatments have been performed at 410°C.
pressure
J.u. Seggem, K.G. Tschcrsich I Surface compositionchanges
Mathewson [15] at temperatures above 600°C. For nickel single crystals the diffusion of carbon into the bulk has been reported to start at around 4OO“C[16,17]. The mechanism of carbon removal by incident atomic hydrogen proceeds presumably by hydrogenation of carbon to methane. This can be deduced from the literature, which we now discuss. Balooch and Olander [I81 reported that during the impingement of a thermal atomic hydrogen beam onto pyrolytic graphite at temperatures below 600°C only methane was produced. At temperatures from 150 to 300°C the apparent reaction probability was independent of the temperature. They fit their experimental results in this temperature range quite well by a surface reaction model which assumes the sequential addition of adsorbed hydrogen atoms to CH, (n = O-3) species. Rye [19] reported the formation of methane, some ethane, and small amounts of cyclic compounds from the reaction of atomic hydrogen with evaporated carbon films. The production rates increase exponentially from 60 to 300°C by a factor of 30 (activation energy 21 kJ/mol). We may also refer to measurements of the catalytic reaction of CO/H, gas mixtures (Fischer-Tropsch synthesis). There is convincing evidence that exclusively surface carbon (C,) is hydrogenated [20,21], presumably after the disproportionation of CO according to 2CO+ C, + CO* [20,22]. Hydrogenation of surface carbon to methane has been found for different catalysts, e.g. nickel ribbon [23], evaporated nickel [20] and iron foil [21], and is reported to be highly selective [21,23]. Hydrogen enters the reaction atomically after dissociative adsorption on the surface [21]. It is interesting to note that on iron the carbidic carbon is active and the graphitic carbon is passive with respect to hydrogenation [21]. This is contrary to the results we found on Inconel. Finally, it has been demonstrated that hydrogen diffusing from the bulk to the surface also reacts with carbon to form methane [24,25]. Erents et al. [24] conclude from results obtained for the system hydrogen/pyrolytic graphite that
80!3
the methane production rate is determined primarily by the equilibrium concentration of atomic hydrogen on the surface. We must be aware that a certain surface concentration of atomic hydrogen will turn up when we expose the samples to molecular hydrogen. Referring to the discussion above we expect the formation of methane and thus the removal of carbon by molecular hydrogen as well. Indeed, an exposure to molecular hydrogen for 16 h at 200°C revealed a decrease of the carbon concentration. However, the apparent cross section for this removal is six orders of magnitude lower than that of atomic hydrogen. 3.2. Oxygen The changes in the surface concentration of oxygen are generally less pronounced than those of carbon. Heating in vacua reduces the oxygen concentration which might be ascribed to the outgassing of water. Exposure to atomic hydrogen usually results in a further decrease of the oxygen concentration, typically from 6 to 2% at a dose of the order 1019atoms crnT2. However, some samples did not behave that way, e.g. the sample of fig. 1. Fig. 1 shows the recontamination of a sample with oxygen by exposure to air. It is interesting to note that this amount of oxygen is as easily removed as carbon. Taylor [3] reported that he checked the efficiency of discharge cleaning by introducing known amounts of oxygen into the tokamak and by determining the time for its removal. Obviously, the cleaning efficiency determined this way may not be generalized.
3.3. Sulphur The changes in the sulphur concentration are always opposite to those of carbon. This holds during the removal of carbon by atomic hydrogen (T I 300°C) as well as during heating to 410°C. Presumably, sulphur segregates from the bulk during heating to 41O”C, although Mathewson [15] reported segregation of sulphur to start at 600°C. The accumulation of sulphur at temperatures
810
J.u. Seggem, K.G. Tschersich I Swface composition changes
below 300°C is due to the exposure to hydrogen and/or the removal of carbon. We suppose a chemisorption-induced segregation due the presence of hydrogen, especially of atomic hydrogen. Sxymerska and Lipski [26,27] investigated chemisorption-induced segregation of sulphur on single crystals of Pd, Pt, and Cu in the presence of molecular hydrogen at sample temperatures as low as room temperature. They explain the chemisorption-induced segregation by the formation of the strong H-S bond on the surface. To the presence of sulphur we ascribe the stability of hydrogen cleaned surfaces, which do not change under UQV conditions. On the contrary, a sputter etched sample is recontaminated with carbon and oxygen in UHV. Sulphur poisons carbon monoxide adsorption on Ni [28]. If this holds also for Inconel, a hydrogen cleaned surface covered with sulphur does not adsorb carbon monoxide, whereas a sputter etched sample does. The adsorbed carbon monoxide can dissociate due to structural defects of the surface [17] if the sample is heated or due to electron bombardment [16]. The present observations may explain why Staib and Staudenmaier [29] detect S in TFR 600, when they expose a probe (C evaporated onto Si) to cleaning discharges. We must assume from our measurements that the Inconel walls of TFR 600 are partially covered with sulphur. During dicharges sulphur may enter the plasma due to the plasma-wall interaction and may be redeposited on the wall and the probe in the quenching phase of the discharge. 4. Summary The most remarkable effect of atomic hydrogen incident on Inconel is the removal of graphitic carbon at temperatures of 10&3oo”C. From a literature search we conclude that it proceeds via the formation of volatile methane by adsorbed atomic hydrogen. The removal of carbon is accompanied by the segregation of sulphur from the bulk which might be due to the formation of HS bonds on the surface. At temperatures around 400°C there is no
signil%ant effect of the exposure to atomic hydrogen. However, due to heating to this temperature, carbon migrates into the bulk and sulphur segregates. Acknowledgements We gratefully acknowledge the skillful technical assistance of H.P. Fleischhauer and wish to thank Prof. G. Comsa for valuable comments on the manuscript. References t11 G.M. McCracken and P.E. Stott, Nucl. Fusion 19 (1979) 889.
121P. Ginot, J. Nucl. Mater. 76/77 (1978) 30. 131 R.J. Taylor, J. Nucl. Mater. 76/77 (1978) 41. [41 K.J. Dietz and F. Waelbroeck, in: Proc. Intern. Symp. on Plasma Wall Interaction, Jiilich, 1976, EUR5782e (1972) p. 445. 151 Delivered by Vereinigte Deutsche Metallwerke, Altena, Germany. Kd Trade name of H. Wiggin Ltd., Hereford, UK. [71 TFR Group, J. Nucl. Mater. 76/77 (1978) 587. PI M. Bernardini, D. Eckhartt and M. Snykers, in: Proc. Intern. Symp. on Plasma Wall Interaction. Jiiiich, 1976, EUR-5782e (1976) p. 679. PI H. Conrads, 10th Symp. on Fusion Technology, Padova, 1978, EUR-6215 (1978) p. 25. WI J.v. Seggern and K.G. Tschersich, J. Nucl. Mater. 76/77 (1978) 600. WI T.W. Hickmott, J. Chem. Phys. 32 (1960) 810. WI Nicrofer (Vereinigte Deutsche Metallwerke, Altena, Germany). [13] L.E. Davis, W.C. McDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy, 2nd ed. (Physical Electronics, Eden Prairie, 1976). [14] M.A. Smith, S. Sinharoy and L.L. Levenson, J. Vacuum Sci. Technol. 16 (1979) 462. [15] A.G. Mathewson, in: Proc. Intern. Symp. Plasma Wall Interaction, Jiilich, 1976, EUR-5782e (1976) p. 517. [16] K. Christmann, 0. Schobcr and G. Ertl, J. Chem. Phys. 60 (1974) 4719. [17] W. Erley and H. Wagner, Surface Sci. 74 (1978) 333. [18] M. Balooch and D.R. Olander, J. Chem. Phys. 63 (1975) 4772. [19] R. Rye, Surface Sci. 69 (1977) 653. [20] M. Araki and V. Ponec, J. Catal. 44 (1976) 439. [21] H.J. Krebs and H.P. Bonzel, Surface. Sci., submitted. [22] S. Andersson, B.I. Lundquist and J.K. Nlbrskov, in: Proc. 7th Intern. Vacuum Congr., Vienna, 1977, p. 815. [23] R.D. Keiley, T.E. Madey, K. Revesz and J.T. Yates, Appl. Surface Sci. 1 (1978) 264%
J.v. Seggem, K.G. Tschersich 1 Surface composition changes [24] S.K. Erents,
C.M. Braganza and G.M. McCracken, J. Nucl. Mater. 63 (1976) 399. [2_5]K. Flaskamp, H.R. Ihle, G. Stiicklin, E. Vietzke, K. Vogelbruch and C.H. Wu, in: Proc. Intern. Symp. on Plasma Wall Interaction, Jiilich, 1976, EUR-5782e (1976) p. 285.
[26] [27] [28] [29]
811
I. Szymerska and M. Lipski, J. Catal. 41 (1976) 197. I. Szymerska and M. Lipski, J. Catal. 47 (1977) 144. W. Erley and H. Wagner, J. Catal. 53 (1978) 287. P. Staib and G. Staudenmaier, J. Nucl. Mater. 76/77 (1978) 78.
and K.G. TSCHERSICH
Znstitut fir Grenzpiichenforschung und Vakuumphysik, D-5170 Jiilich, Fed. Rep. of Germany
Kemforschungsanlage
Jiilich GmbH,
Association EuratomlKFA,
Surface composition changes by incident atomic hydrogen have been investigated on Inconel samples. The samples have been exposed to thermal atomic hydrogen at temperatures of 1-C and have been analyzed using Auger electron spectroscopy. Carbon, oxygen, and sulphur are the main impurities. At sample temperatures from 100 to 3OOT, the carbon concentration is significantly reduced by incident atomic hydrogen. An apparent cross section of this process has been determined. It increases from 110 to 170°C by two orders of magnitude and is independent of temperature above 170°C. being 2 X lo-*’ cm’. At 400°C carbon migrates into the bulk. The disappearance of carbon is accompanied by the appearance of sulphur. We ascribe the appearance of sulphur at temperatures up to 300°C to chemisorption-induced segregation. A relatively slow removal of oxygen was generally observed.
1. Introduction Discharge cleaning with hydrogen is the most widespread method of preconditioning tokamak walls [l] and makes it feasible to reduce the impurity level in tokamak discharges from Z,S> 3 to Z,e= 1 [2]. The cleaning discharges are usually optimized by maximizing the reaction products of hydrogen with the surface impurities, oxygen and carbon, namely Hz0 and CH4. These compounds are determined by mass spectrometric analysis of the gas which is pumped out of the tokamak. This procedure was introduced by Taylor [2,3]. Dietz and, Waelbroeck [4] explain the chemical reactions producing Hz0 and CHI as being due to the chemical reactivity of the atomic hydrogen species (Ho, H+) impinging onto the tokamak wall. Therefore, we tested the cleaning efficiency of thermal atomic hydrogen using Auger electron spectroscopy (AES) for surface analysis. The material under test is Nicrofer 7216 [5], a material equivalent to Inconel 600 [6]. Inconel alloys are chosen as first wall materials for some present day tokamaks, e.g. TFR 600 [7], JET [8], and TEXTOR [9]. In a previous paper [lo] we reported the fast removal of graphitic carbon and the slow removal of oxygen by atomic hydrogen, as well as the segregation of the sulphur from the bulk. The
object of the present investigation was the dependence of the surface composition changes on sample temperature. The results may be important for JET and TEXTOR because liner temperatures as high as 500-6OO“C are envisaged for these tokamaks. 2. Experimental procedure The experimental device has been described in a previous publication [lo]. Briefly, in a preparation system atomic hydrogen is produced by thermal dissociation of molecular hydrogen on a hot tungsten ribbon (2OOOK) [ll]. The samples are exposed to a given atomic hydrogen dose and then transferred by vacuum locks to an AES analysis system. The procedure is repeated several times so as to measure the surface composition in dependence of increasing hydrogen dose. During exposure the samples are heated by the radiation from the hot tungsten ribbon and additionally by RF [lo]. 3. Results and discussion The samples were mechanically polished and degreased with acetone and methanol just before their installation into the vacuum system. Thereafter, the following surface composition, in
Journal of Nuclear Materials 93 & 94 (1980) 806-811 @ North-Holland R&lishing ContpMy
806
J.v. Seggem, K.G. Tschersich I Surfnce compositionchanges
atomic %, was measured (mean value of all samples): Ni: 43% ; Cr: 4%; Fe: 4.5%; C: 36% ; 0: 9.5% ; and S: 1%. Furthermore N, Cl, and Ca were detected at around 1%. For comparison, the specified bulk composition is as follows [12]: Ni: ~70%; Cr: 1619%; Fe: 6-10%; Mn: 11% ; Cu: 10.5% ; Ti: ~0.4% ; Si: 11% ; C: ~0.4% ; S: 10.03% ; and P: sO.o6%. The volume composition as measured with AES after argon ion sputter etching was found to be within 1%) i.e. Ni: 71%; Cr: 19% ; and Fe: 10%. It has been observed that the first cleaning of a sample with atomic hydrogen was less efficient than the cleaning of a sample which was recontaminated, e.g. by exposure to air. We will concentrate on the initial cleaning. As stated above the main impurities are carbon, oxygen, and sulphur. Because they behave quite differently, we will discuss them separately. 3.1. Carbon Fig. 1 shows the initial surface concentrations of carbon, oxygen, and sulphur on an Inconel
807
sample and the concentrations measured after the following successive steps: heating to 150°C in vacuum, heating to 150°C in hydrogen, exposure to atomic hydrogen up to 1019 atoms cme2, exposure to air for 20 h, and finally a repetition of the first three steps. The carbon has been found to consist of graphitic carbon or a mixture of graphitic and carbidic carbon, which will be discussed in more detail below. The percentage of graphitic carbon is indicated along the carbon curve. Due to heating and exposure to molecular hydrogen there is only a small reduction of the carbon concentration, whereas the exposure to atomic hydrogen reduces the carbon concentration significantly. After recontamination the removal of carbon is qualitatively the same. During the removal of carbon by atomic hydrogen, the carbon changes from being highly graphitic to being predominantly carbidic. This has been found by analysing the carbon Auger peak shape by the following procedure. (A similar procedure has been applied for the analysis of the thermal decomposition of nickel carbide [ 141.)
I” u‘ b L”
T=150°C
.E n
0
5
10
15 90
Ho dose 11~‘atoms/crn2
95
I
Fig.1. Surfaceimpurityconcentrationof an Inconelsampleafter installation,after heatingin vacuumandin molecularhydrogen,and after stepwise exposure to atomichydrogen.The treatmentsare repeated after an exposureto air. During the exposure to molecular and atomichydrogen the hydrogen pressure was betweenps, = 7 X 10s4and 7 X low3mbar. All the treatments have been performed at 150°Capart from the exposureto air which was done at room temperature. The percentage of graphitic carbon is indicated along the carbon curve.
J.u. Seggem, KG. Tschersich 1 Surface compositionchanges
808
t
/
carbon but not the carbidic carbon. This characteristic feature of the carbon removal has been observed for sample temperatures of NO-300°C. In the same range of temperature the carbon concentration plotted versus the atomic hydrogen dose, cf. fig. 1, decreases exponentially at small doses. Therefore, the removal of graphitic carbon can be described by a cross section [lo]. This cross section is plotted in fig. 2 versus the sample temperature. It increases steeply from 110 to 170°C by more than two orders of magnitude and levels off at a value of about 2 x lo-” cm*. At sample temperatures between 300 and 400°C there is an initial removal of carbon, but the concentrations measured at hydrogen doses above 101’ atoms cme2 scatter heavily. This might be due to two opposing effects, i.e. carbon removal by atomic hydrogen and segregation of carbon from the bulk to the surface. In this situation our experimental procedure may not be adequate, because we heat, cool, and transfer the sample for every exposure and surface analysis. Fig. 3 shows the concentration of carbon, oxygen, and sulphur after treatments at 410°C. Obviously, the carbon concentration is reduced by the temperature treatment and there is no significant effect on the exposure to atomic hydrogen. We suppose that carbon migrates into the bulk. The disappearance of carbon from the Inconel surface has been observed by
4
I
1o-‘g1 / 100
200 sample
temperature
300 PC
1
Fig. 2. Apparent cross section for the removal of graphitic carbon by atomic hydrogen as a function of the sample temperature during exposure.
The graphitic carbon peak was measured on glassy carbon and the carbidic carbon peak on a tungsten carbide single crystal. The peak shapes were very similar to those given by Davis et al. [13] for graphite and tantalum carbide, respectively. By linear superposition of the measured peaks, we calculated the peaks of graphitelcarbide mixtures and used them for assessing the mixtures corresponding to the carbon peaks as measured on Inconel. Although this determination of the portions of graphitic and carbidic carbon is a rather crude one, we conclude that atomic hydrogen incident on Inconel predominantly removes the graphitic
0.5
Ii @dose
20
[ 10’8atoms/cm2
LO
1
Fig. 3. Surface impurity concentration of an Inconel sample after installation, after heating in vacuum and in molecular hydrogen, and after stepwise exposure to atomic hydrogen. During the exposure to molecular and atomic hydrogen the hydrogen was between PH2= 7 X lo4 and 7 X 10e3mbar. All the treatments have been performed at 410°C.
pressure
J.u. Seggem, K.G. Tschcrsich I Surface compositionchanges
Mathewson [15] at temperatures above 600°C. For nickel single crystals the diffusion of carbon into the bulk has been reported to start at around 4OO“C[16,17]. The mechanism of carbon removal by incident atomic hydrogen proceeds presumably by hydrogenation of carbon to methane. This can be deduced from the literature, which we now discuss. Balooch and Olander [I81 reported that during the impingement of a thermal atomic hydrogen beam onto pyrolytic graphite at temperatures below 600°C only methane was produced. At temperatures from 150 to 300°C the apparent reaction probability was independent of the temperature. They fit their experimental results in this temperature range quite well by a surface reaction model which assumes the sequential addition of adsorbed hydrogen atoms to CH, (n = O-3) species. Rye [19] reported the formation of methane, some ethane, and small amounts of cyclic compounds from the reaction of atomic hydrogen with evaporated carbon films. The production rates increase exponentially from 60 to 300°C by a factor of 30 (activation energy 21 kJ/mol). We may also refer to measurements of the catalytic reaction of CO/H, gas mixtures (Fischer-Tropsch synthesis). There is convincing evidence that exclusively surface carbon (C,) is hydrogenated [20,21], presumably after the disproportionation of CO according to 2CO+ C, + CO* [20,22]. Hydrogenation of surface carbon to methane has been found for different catalysts, e.g. nickel ribbon [23], evaporated nickel [20] and iron foil [21], and is reported to be highly selective [21,23]. Hydrogen enters the reaction atomically after dissociative adsorption on the surface [21]. It is interesting to note that on iron the carbidic carbon is active and the graphitic carbon is passive with respect to hydrogenation [21]. This is contrary to the results we found on Inconel. Finally, it has been demonstrated that hydrogen diffusing from the bulk to the surface also reacts with carbon to form methane [24,25]. Erents et al. [24] conclude from results obtained for the system hydrogen/pyrolytic graphite that
80!3
the methane production rate is determined primarily by the equilibrium concentration of atomic hydrogen on the surface. We must be aware that a certain surface concentration of atomic hydrogen will turn up when we expose the samples to molecular hydrogen. Referring to the discussion above we expect the formation of methane and thus the removal of carbon by molecular hydrogen as well. Indeed, an exposure to molecular hydrogen for 16 h at 200°C revealed a decrease of the carbon concentration. However, the apparent cross section for this removal is six orders of magnitude lower than that of atomic hydrogen. 3.2. Oxygen The changes in the surface concentration of oxygen are generally less pronounced than those of carbon. Heating in vacua reduces the oxygen concentration which might be ascribed to the outgassing of water. Exposure to atomic hydrogen usually results in a further decrease of the oxygen concentration, typically from 6 to 2% at a dose of the order 1019atoms crnT2. However, some samples did not behave that way, e.g. the sample of fig. 1. Fig. 1 shows the recontamination of a sample with oxygen by exposure to air. It is interesting to note that this amount of oxygen is as easily removed as carbon. Taylor [3] reported that he checked the efficiency of discharge cleaning by introducing known amounts of oxygen into the tokamak and by determining the time for its removal. Obviously, the cleaning efficiency determined this way may not be generalized.
3.3. Sulphur The changes in the sulphur concentration are always opposite to those of carbon. This holds during the removal of carbon by atomic hydrogen (T I 300°C) as well as during heating to 410°C. Presumably, sulphur segregates from the bulk during heating to 41O”C, although Mathewson [15] reported segregation of sulphur to start at 600°C. The accumulation of sulphur at temperatures
810
J.u. Seggem, K.G. Tschersich I Swface composition changes
below 300°C is due to the exposure to hydrogen and/or the removal of carbon. We suppose a chemisorption-induced segregation due the presence of hydrogen, especially of atomic hydrogen. Sxymerska and Lipski [26,27] investigated chemisorption-induced segregation of sulphur on single crystals of Pd, Pt, and Cu in the presence of molecular hydrogen at sample temperatures as low as room temperature. They explain the chemisorption-induced segregation by the formation of the strong H-S bond on the surface. To the presence of sulphur we ascribe the stability of hydrogen cleaned surfaces, which do not change under UQV conditions. On the contrary, a sputter etched sample is recontaminated with carbon and oxygen in UHV. Sulphur poisons carbon monoxide adsorption on Ni [28]. If this holds also for Inconel, a hydrogen cleaned surface covered with sulphur does not adsorb carbon monoxide, whereas a sputter etched sample does. The adsorbed carbon monoxide can dissociate due to structural defects of the surface [17] if the sample is heated or due to electron bombardment [16]. The present observations may explain why Staib and Staudenmaier [29] detect S in TFR 600, when they expose a probe (C evaporated onto Si) to cleaning discharges. We must assume from our measurements that the Inconel walls of TFR 600 are partially covered with sulphur. During dicharges sulphur may enter the plasma due to the plasma-wall interaction and may be redeposited on the wall and the probe in the quenching phase of the discharge. 4. Summary The most remarkable effect of atomic hydrogen incident on Inconel is the removal of graphitic carbon at temperatures of 10&3oo”C. From a literature search we conclude that it proceeds via the formation of volatile methane by adsorbed atomic hydrogen. The removal of carbon is accompanied by the segregation of sulphur from the bulk which might be due to the formation of HS bonds on the surface. At temperatures around 400°C there is no
signil%ant effect of the exposure to atomic hydrogen. However, due to heating to this temperature, carbon migrates into the bulk and sulphur segregates. Acknowledgements We gratefully acknowledge the skillful technical assistance of H.P. Fleischhauer and wish to thank Prof. G. Comsa for valuable comments on the manuscript. References t11 G.M. McCracken and P.E. Stott, Nucl. Fusion 19 (1979) 889.
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