Chemical erosion of graphite by oxygen

284

Journal

LETTER

of Nuclear Materials 136 (1985) 284-286 North-Holland, Amsterdam

TO THE EDITORS

CHEMICAL

EROSION OF GRAPHITE BY OXYGEN

The erosion of the first wall for thermonuclear fusion devices has been extensively studied from the viewpoint of impurity emission into the plasma [l]. Among a variety of the candidate materials, a great interest has been aroused to low-2 materials because of their high permissible concentration in the plasma 121. Pyrolytic graphite is one of the candidate materials because of its low-Z, refractory nature, and ease in fabrication. A number of investigators have studied the sputtering of graphite and revealed that the chemical sputtering, namely methane formation, is the predominant mechanism for the impurity emission from graphite [3]. We have studied the trapping states of implanted hydrogen isotopes in pyrolytic graphite and their thermal desorption to understand the mechanisms of fuel recycling/inventory and found that the graphite surface is modified due to formation of radiation damage; that is, the trapping states and desorption behavior change with accumulation of the radiation damage in the graphite [4]. It is expected that the modified graphite surface has greater chemical reactivity than the normal graphite surface. We measured the reaction probability of the damaged graphite surface against oxygen to form volatile products such as CO and CO, from the viewpoint of the impurity emission, because a fairly large amount of oxygen is escaped from the plasma formed in tokamak devices and strikes the first wall [5]. The sample graphite used in the present study was the pyrolytic graphite (PG-A) purchased from Nihon Carbon Co. The size and density were 10 X 10 X 0.3 mm and 2.2 g/cm3, respectively. An UHV system equipped with XPS-SIMS optics along with a conventional ion gun was used. The system was also equipped with a turbomolecular pump, sputter ion pump, and titanium getter pump. The residual pressure was routinely below 5 x 10-a Pa. The graphite sample was implanted with deuterium or helium ions with 5 keV at a current of 26 PA/cm* at room temperature. The incident angle of the ion beam was 30” with respect to the surface normal. The beam diameter was 3 mm. The XPS spectra of graphite were measured with use of the MB-K, line with 400 W power. The energy analyzer was a double pass cylindrical mirror analyzer, whose energy resolution was 1.0 0022-3115/85/$03.30 Q Elsevier Science Publishers (North-HolI~d Physics ~biishing Division)

eV (Full Width at Half Maximum, FWHM) for the of a given peak 4f,,, peak of Au. Th e reproducibility was within + 0.1 eV. The energy calibration was carried out using the 4f,,, peak of Au as 83.8 eV (61. The details of the system and procedures were described elsewhere 171. Fig. 1 shows changes in the Cls spectra of the pyrolytic graphite with the ion implantation, annealing. and oxygen exposure. The virgin graphite showed the I

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Binding Energy/eV Fig. 1. XPS spectra for graphite. (a) virgin; (b) implanted with He-ions

(1

X

10’X/cm2);

(c)

implanted

with

D-ions

(1

X

10tR/cmZ); (d) annealed at 9OO’C for 5 min after (c); (e) exposed to oxygen at 900°C for 30 min (P,: =1.3x 1O--2 Pa) after (d).

B.V.

285

K. Ashida / Chemical erosion of graphite by oxygen

peak at 284.5 eV with the FWHM of 0.9 eV as spectrum (a) in fig. 1. The implantation with the He-ions (1 X lO’“/cm*) gave rise to peak shift toward the lower binding energy side (spectrum(b): 284.2 eV with FWHM of 1.4 eV), indicating that electronic charge on carbon decreased with the implantation [8]. This shift has been attributed to the formation of radiation damage in the graphite [7,9]. It will be denoted as the damage shift in the present paper. The damage shift could not be annealed out by vacuum heating at 900°C. On the other hand, the D-ion implantation amounting to 1 x lO”/cm* caused the peak shift toward the higher binding energy side (spectrum (c): 284.8 eV with FWHM of 1.7 eV), indicating that electronic charge on carbon increased with the D-ion implantation. This shift is attributed to the formation of chemical bond between the implanted deuterium and carbon atoms in the normal graphite lattice [7]. It will be denoted, therefore, as chemical shift in the present paper. When the graphite implanted with the D-ions was heated in uucuo up to 9OO”C, the trapped deuterium was desorbed from the graphite [lo] and the Cls peak moved toward the lower binding energy side (spectrum (d): 284.2 eV with FWHM of 1.4 eV). The peak center and FWHM for this surface were exactly the same as those observed for the surface implanted with He-ions as seen in fig. 1. Namely, the chemical shift disappeared by the vacuum heating, whereas the damage shift remained. Active carbon is more reactive than pyrolytic graphite against hydrogen, oxygen and other gases. This fact suggests that the modified (damaged) graphite surface is more reactive than the virgin graphite surface. To see the chemical reactivity of the damaged graphite surface against oxygen, which is known as one of the main impurity in the actual plasma formed in tokamak devices (51, the reaction probability to form volatile gases such as CO and CO, was measured in a flow of oxygen gas. An example of the mass spectra observed for the modified graphite surface is shown in fig. 2. After the graphite was implanted with D-ions amounting to 1 x 10’X/cm2, the implanted deuterium was desorbed by vacuum heating at 900°C for 5 min. Subsequently, the graphite whose surface state corresponded to spectrum (d) in fig. 1 was exposed to the oxygen flow at the constant pressure of 6.7 X 10m3 Pa at various temperatures. Although a certain amount of CO and CO, was observed at 20°C almost the same amounts of these gases were observed in the absence of the graphite sample. Therefore, the CO and CO, observed in the mass spectrum for the exposure at 20°C is due to the residual gas in the system or impurities in oxygen gas.

In addition, the virgin graphite did not cause any detectable increase in CO and CO, pressures even at 900°C indicating that the virgin graphite surface was inactive to the oxidation reaction. On the other hand, in the case of the modified graphite, it is clearly seen in fig. 2 that the intensities of CO and CO, peaks increased with elevation of the temperature, indicating that the oxidation reaction of carbon took place. Since the graphite was exposed to oxygen at a constant pressure in the gas flow system, the rate of CO and/or CO, formation is described as v,o = ScoAPco

(1)

vco, = ScozAPco*

(2)

10’

10:

ElO’ : x .c: z c( l(

1 M/e Fig. 2. Changes in the mass spectra for the modified graphite due to oxygen exposure (Po2= 6.7~10~~ Pa). The triangle symbols are also valid for the residual gas and/or impurity level of the oxygen gas. and for the virgin graphite up to 9OO’C under the same exposure conditions as the modified graphite.

286

K. Ashrda

/ Chemical

where S is the pumping speed of the system for a given gas, which was determined as S,,, = 60 and Sc.o, = 55 l/s by means of the constant volume method and AP is the increase in the pressure of CO or CO, due to the oxidation reaction. The increase in the pressure of each product was easily determined from the mass spectra because the mass spectrometer had been calibrated for both CO and CO,. The pressure increase of CO was 8.0 x lo-’ Pa at 200°C and 2.3 x lo-’ Pa at 900°C. As for CO,. the pressure increase was 4 x 10mh and 9 x lo- ’ Pa at 200 and 9OO”C, respectively. The rate at which a molecule strikes the surface, r, is simply calculated form the kinetic theory of gases. In the present case, the rate is 1.8 x 10’h/cm2. sec. The reaction probability, R, is calculated from R = V/TA, where A is the area of the damaged region. The area was determined as 0.9 cm’ from the damage shift by moving the sample against the analyzer. The reaction probability were determined as Rco = 0.008 (200°C) and 0.022 (900°C), and RC.()? = 0.0004 (200°C) and 0.0008 (900°C). After exposure at 900°C in higher pressure (ca. 1.3 X IO-* Pa) for 30 minutes, the surface state changed to the one as shown by spectrum (e) in fig. 1. The peak and FWHM were 284.5 eV and 1.2 eV, respectively. This spectrum is quite similar to that observed for the virgin graphite. This is due to that the oxidation reaction removed the damaged region to cause undamaged carbon layers to appear on the surface. In addition. it was observed that this surface did not produce any detectable level of CO and CO,. It means that the reactivity of carbon against oxygen is significantly enhanced in the presence of the radiation damage. The reaction probability to make CO was on the order of 0.01 in the temperature range from 200 to 900°C. It is believed that the chemical sputtering making methane plays the predominant role for the impurity emission from graphite. The methane formation yield has been reported as 0.07 CH,/ion at maximum (1 keV, 525’C) [ll]. To avoid such high erosion yield, the wall temperature will be reduced. When the wall temperature is at 350°C the methane yield will decrease to the level of 0.01 CH,/ion [ll]. It is expected that the methane yield will be further reduced by lowering the wall temperature and incident ion energy. However, the chemical erosion by oxygen becomes important in this

Received

17 May 1985; accepted

29 July 1985

erosion

of graphite

b_v o.xygen

case, because the reaction probability against oxygen becomes comparable to or surpass the methane yield. It should be mentioned here that the presence of small amount of the catalysts like iron (which is one of the common impurities in the tokamak devices) greatly enhances the catalytic oxidation of graphite [12]. It should be also taken into account that the oxygen striking the first wall must be atomic. ionic, and/or other excited states. They are more reactive than oxygen molecules in the ground state [ 131. Therefore, the oxidation reaction will also play an important role for the impurity emission from graphite first wall. In addition, the recycling coefficient measured by using pure hydrogen may become invalid for the graphite first wall of tokamak devices when the chemical erosion with oxygen plays a role. This is because the deuterium or tritium trapped in the graphite first wall is released due to the oxidation of carbon atoms to form volatile gases. References [I] Proc. 4th Intern. Conf. on Plasma Surface Interactions

[13]

in Controlled Fusion Devices. Garmisch, Germany (1980). J.L. Cecchi, J. Nucl. Mater. 93&94 (1980) 28. R. Yamada. K. Nakamura and M. Saidoh. J. Nucl. Mater. 98 (1981) 167. K. Ashida. K. Ichimura, M. Matsuyama and K. Watanabe. J. Nucl. Mater. 128&129 (1984) 792. E. Taglauer. B.M.U. Scherzer. P. Verma. R. Behrlsch. Chen Ceng Kai and ASDEX-TEAM. J. Nucl. Mater. 111&112 (1982) 142. V.I. Nefedov. Ya.V. Salyn. G. Leonhardt and R. Scheibe. J. Electron Spectrosc. Relat. Phenom. 10 (1977) 121. K. Ashida, K. Ichimura and K. Watanabe. J. Vat. SCI. Technol. Al (1983) 1465. K. Siegbahn et al.. ESCA: Atomic. Molecular and Solid Structure Studied by means of Electron Spectroscopy. (Almquivist and Wilkes, Stockholm, 1967) p. 139. Y. Gotoh and 0. Okada, J. Nucl. Sci. Technol. 21 (1984) 205. K. Ashida, K. Ichimura, M. Matsuyama, M. Miyake and K. Watanabe, J. Nucl. Mater Ill&l12 (1982) 769. R. Yamada, K. Nakamura, K. Sone and M. Saidoh. J. Nucl. Mater. 95 (1980) 278. K.W. Sykes and J.M. Thomas. J. Chim. Phys. 58 (1961) 70. D.E. Rosner and H.D. Allendorf, Carbon 3 (1965) 153.

K.

Ashida

[2] [3] [4] [S]

(61 [7] [8]

[9] [lo] [Ill [12]

*,

K.

Kanamori,

M. Matsuyama

and

K.

Watanabe Tr~tu.m~ Research * Department

Center

of Chemistry,

rity, GoJuku 3190

Toyama

Fucul~v of Science 930, Japan

Tovama

Unrcrer-