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Gas desorption properties of low-activation ferritic steel as a blanket or a vacuum vessel material
Fusion Engineering and Design 39 – 40 (1998) 485 – 491
Gas desorption properties of low-activation ferritic steel as a blanket or a vacuum vessel material Yuko Hirohata *, Kaoru Suzuki, Tomoaki Hino Department of Nuclear Engineering, Hokkaido Uni6ersity, Kita-ku, Sapporo 060, Japan
Abstract The gas desorption properties of low-activation ferritic steel (F82H) were evaluated by thermal desorption spectroscopy (TDS), and compared with those of austenitic steels (316SS and 316LSS). The major desorbed gas species of F82H were CO, H2O, CO2 and H2. The fraction of oxygen-containing species such as CO, CO2 and H2O to the total outgassing amount was 98%. The gas desorption behavior was very similar to the case of 316SS. The gas desorption spectra for F82H show the required prebaking temperature becomes higher than 700 K. For the F82H sample exposed to atmosphere, the gas desorption properties were examined. As the time of exposure increase, desorption amounts of H2O and CO largely increased. This increase may be due to the corrosion in atmosphere. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction Since the vacuum vessel is exposed to neutrons in a fusion reactor, low-activation materials have to be used for such a component. Recently, the low-activated ferritic steel, F82H (Fe – 8Cr–2W), has been developed as a candidate material for a blanket of ITER, by Japan Atomic Energy Research Institute (A. Hishinuma et al., personal communication, 1996). This steel has good properties such as a low thermal expansion coefficient and high thermal conductivity. However, the chemical corrosion of this steel may become larger than that of austenitic stainless steel, due to * Corresponding author.
the high diffusion rate of oxygen and the low content of nickel. It is reported that the stain of steels is generally prevented by the addition of Ni [1]. Then, it is presumed that a large amount of gas is desorbed from the ferritic steel, F82H. In the present study, we investigated the gas desorption properties of F82H by using thermal desorption spectroscopy (TDS) [2], and then the results were compared with those of austenitic stainless steels (316SS and 316LSS). In order to examine the gas adsorption behavior, the outgassing was measured for the F82H sample which was exposed to the atmosphere for a long time period. The change of surface atomic composition of F82H during heating at 400, 600 or 800 K was also examined by Auger electron spectroscopy (AES).
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(97)00170-1
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
486
Table 1 Composition in wt.% of F82H, 316SS and 316LSS Sample
Chemical composition (wt.%) C
Si
Mn
P
S
Cu
Ni
F82H 316SS 316LSS
0.09 B0.08 B0.03
0.07 B1.00 B1.00
0.1 B2.00 B2.00
0.003 B0.045 B0.045
0.001 B0.03 B0.03
0.01 — —
0.02 10 – 14 12 – 15
Cr 7.84 16–18 16–18
Mo 0.003 2–3 2–3
V
Co 0.003 — —
Ti
F82H 316SS 316LSS
Ta 0.04 — —
W 1.98 — —
0.19 — —
0.004 — —
Fig. 1. Thermal desorption spectra of as-received sample (F82H).
2. Experimental Two kinds of F82H samples were used for evaluation of gas desorption. One was the sample supplied just after the preparation by the NKK Corporation. The gas desorption of this sample seems to be low because of the short time exposure to atmosphere. This sample is called the as-received sample, in the present paper. The other was the sample which was exposed to the atmosphere for 2 years. The surface color of this sample was darkened. This sample was mechanically polished to remove the dark surface layer. For the polishing, both Emery paper and alumina
powder were used. After that, the sample was rinsed by ethanol in an ultrasonic bath, and then dried in the atmosphere. The sample mechanically polished as above is called the sample exposed for 2 years. In order to examine the effect of gas adsorption in the atmosphere, the mechanically polished sample (sample exposed for 2 years) was degassed with a temperature of 960 K for 30 min. This sample was then exposed to the atmosphere for a period of 3 or 10 months. This sample is called the sample exposed for 3 or 10 months. The gas desorption properties of the above F82H samples were investigated. The gas desorption properties of mechanically polished 316SS or
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
487
Fig. 2. Thermal desorption spectra of 316SS (a) and 316LSS (b).
316LSS were also examined under the same conditions, in order to compare with the case of the as-received sample (F82H). Table 1 shows the composition in wt.% of F82H, 316SS and 316LSS. The desorption properties of these samples were measured by means of thermal desorp-
tion spectroscopy (TDS) with a constant ramp rate. The sample shape was a slab with a size of 15× 15× 0.2 mm. In the TDS analysis, the sample was located on a heater made by Mo. The Mo heater was resistively heated up to 1273 K. In order to obtain the apparent activation energies
488
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Table 2 Gas desorption amounts of as-received sample (F82H), 316SS and 316LSS Sample
As-received sample (F82H) 316SS 316LSS
Gas species: Desorption amount (×1020 molecules m−2) [fraction: %] H2
H2O
CO
CO2
Total
0.11 [2] 2.22 [38] 16.9 [81]
1.08 [21] 1.30 [22] 0.98 [5]
3.06 [59] 1.98 [34] 2.53 [12]
0.95 [18] 0.34 [6] 0.39 [2]
5.20 5.84 20.8
of desorbed gases without the assumption of the order of reaction, the ramp rate, b, was changed from 0.08 to 0.83 K s − 1. The temperature which has the maximum desorption rate, Tp, shifts to a higher temperature with the increase of b. The apparent activation energies, Ed, may be obtained from the slope of a straight line between natural logarithms of T 2p/b and a reciprocal of Tp [2]. The gas desorption spectra of samples exposed for 3 months, 10 months and 2 years were employed to examine the effect of gas adsorption in the atmosphere. The surface atomic composition and depth profile of the sample exposed for 2 years (F82H), during heating from room temperature (RT) to 820 K, was analyzed by Auger electron spectroscopy (AES) with Ar ion etching.
3. Results and discussions Fig. 1 shows typical desorption spectra of the as-received sample (F82H). Here, the ramp rate and the final temperature were 0.17 K s − 1 and 1200 K, respectively. The major desorbed gas species were H2O, CO, CO2 and H2. The desorption rates of all gases increased in the temperature range above 400 K. H2O was desorbed in a wide range up to 1200 K. The largest desorption rate of CO2 was observed at 700 K. Around 900 K, the desorption rates of H2, CO and CO2 increased. A large desorption of CO occurred at the temperature above 1100 K. This desorption rate of CO was one order of magnitude larger than other peaks. The total desorption amount of all gases due to the heating up to 1200 K was 5.2× 1020
molecules m − 2. The fraction of desorption amount for oxygen-containing species, such as H2O, CO and CO2, was very large, approximately 98% of the total desorption amount. Fig. 2 shows the thermal desorption spectra of 316SS (a) and 316LSS (b) [2]. The gas desorption properties of 316SS, such as gas species, fractions of desorbed gas amount, desorption peaks and a total desorption amount, were similar to those of the as-received sample (F82H). On the contrary, the major desorbed gas species of 316LSS was H2, which was 80% of the total desorption amount. In the H2 spectrum of 316LSS, one shoulder and one peak were observed. The large desorption of CO at around 1100 K was observed in both 316SS and 316LSS. However, the desorption rate of CO in 316LSS was very small, perhaps due to the the small content of carbon in bulk, compared with the case of the as-received sample (F82H) or 316SS (A. Hishinuma et al., personal communication, 1996; Ref. [1]). Table 2 shows the desorption amounts of major gas species, and the total desorption amounts from the three steels. Here, the desorption amounts were obtained by integration of the desorption rate up to 1200 K. The desorption amounts of CO and CO2 of the as-received sample (F82H) were larger than those of the austenitic stainless steels. However, the total desorption amount of the as-received sample (F82H) was almost the same as that of 316SS. Fig. 3 shows the TDS spectra of H2O (a), CO (b) and CO2 (c) for the three exposed samples, and the as-received sample as a reference. Here, the samples exposed for 3 or 10 months were
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489
Fig. 3. Thermal desorption spectra of H2O (a), CO (b) and CO2 (c) for exposed samples. Table 3 Gas desorption amounts of three exposed samples and as-received sample Sample name
As-received sample Exposed sample for 3 months Exposed sample for 10 months Exposed sample for 2 years
Gas species: Desorption amount (×1020 molecules m−2) H2
H 2O
CO
CO2
Total
0.15 0.70 0.96 2.38
0.73 0.23 2.04 4.90
1.38 0.51 0.56 5.4
0.96 0.55 0.50 1.20
3.2 2.1 4.2 14.0
degassed before exposure to the atmosphere. The gas desorption amounts were obtained by the integration of the desorption rate until 960 K, and the results are shown in Table 3. For the three exposed samples, the desorption rate of H2O increased with the exposure time in a high tempera-
ture range. In the case of the sample exposed for 2 years, a very large desorption of H2O was observed in the temperature range 400–800 K. This result suggests that the chemical corrosion of F82H takes place due to water, by the exposure to atmosphere. The desorption amount of CO was
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Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
Fig. 4. Change of surface atomic composition of exposed sample for 2 years (F82H) during the heating.
largely increased by the exposure to the atmosphere for 2 years. The desorption amounts of CO2 for exposed samples also increased with the exposure time. The desorption peak was shifted to a lower temperature range by the exposure. If the sample with degassing treatment is exposed to the atmosphere for a short time period, the desorption rate of gases may be smaller than those of the as-received sample (Table 3, Fig. 3). Therefore, in order to use F82H as an in-vessel component, degassing with a high temperature (\ 700 K) is required before the installation. Fig. 4 shows surface atomic composition of the sample exposed for 2 years, when the sample temperature was increased from RT to 820 K. When the sample was heated from RT to 420 K, the oxygen concentration slightly decreased due to a large desorption of H2O (Fig. 3a). At 620 K, large desorptions of H2O and CO2 were observed in the TDS spectra (Fig. 3a and c). The oxygen concentration again slightly decreased due to
these desorptions, but the carbon concentration gradually increased with heating time. At 820 K, the carbon concentration largely increased due to the segregation of carbon. The atomic composition of carbon changed from 45 to 70 at.% when the temperature was raised from RT to 820 K. It is assumed that a large desorption of CO corresponds to the recombination between oxygen, and carbon diffused from bulk. In addition, chromium was observed on the top surface. A similar segregation of chromium was observed in 316LSS [4]. The apparent activation energies of desorption, Ed, for F82H and 316LSS [3], are shown in Table 4. Here, the activation energy for second peak of CO in F82H (at around 1100 K) was not obtained due to the limit of the heating temperature. The values of H2 and CO2 for F82H were similar to those for 316LSS.
4. Conclusion The gas desorption properties of the as-received sample (F82H) were examined, and compared with those of 316SS and 316LSS. The major desorbed gas species of F82H were H2O, CO, CO2 and H2. The fraction of oxygen-containing species, such as CO, CO2 and H2O, to the total outgassing amount was 98%. The gas desorption behavior was very similar to the case of 316SS. The gas desorption spectra for F82H show the required prebaking temperature becomes higher than 700 K. For the sample exposed to the atmosphere, the gas desorption behavior was considerably changed. As the time of exposure increased, desorption amounts of H2O and CO largely in-
Table 4 Apparent activation energies (Ed) of desorbed gases for as-received sample (F82H) and 316LSS [2] Gas species
H2 H2O CO CO2
F82H (eV)
316LSS (eV)
Ed (1st peak)
Ed (2nd peak)
Ed (1st peak)
Ed (2nd peak)
0.61 0.2–0.3 0.77 1.5
0.67 0.5–0.8 Not obtained 1.77
0.84 Not obtained No low temperature peak 1.23
0.62 Not obtained 3.47 1.17
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
creased. This increase may be due to corrosion in the atmosphere.
Acknowledgements We appreciate Drs A. Hishinuma and K. Shiba, JAERI, for the supply of F82H samples.
.
491
References [1] Jpn. Soc. Met., Kinzoku-binran, Maruzen, Tokyo, 1982, p. 824. [2] P. Redhead, Vacuum 12 (1962) 203 – 211. [3] Y. Hirohata, A. Mutoh, T. Hino, T. Yamashina, T. Kikuchi, N. Ohsako, Vacuum 47 (1996) 728 – 731. [4] M. Miyauchi, A. Toyama, Y. Hirohata, T. Yamashina, J. Vac. Soc. Jpn. 35 (1992) 169 – 172.
Gas desorption properties of low-activation ferritic steel as a blanket or a vacuum vessel material Yuko Hirohata *, Kaoru Suzuki, Tomoaki Hino Department of Nuclear Engineering, Hokkaido Uni6ersity, Kita-ku, Sapporo 060, Japan
Abstract The gas desorption properties of low-activation ferritic steel (F82H) were evaluated by thermal desorption spectroscopy (TDS), and compared with those of austenitic steels (316SS and 316LSS). The major desorbed gas species of F82H were CO, H2O, CO2 and H2. The fraction of oxygen-containing species such as CO, CO2 and H2O to the total outgassing amount was 98%. The gas desorption behavior was very similar to the case of 316SS. The gas desorption spectra for F82H show the required prebaking temperature becomes higher than 700 K. For the F82H sample exposed to atmosphere, the gas desorption properties were examined. As the time of exposure increase, desorption amounts of H2O and CO largely increased. This increase may be due to the corrosion in atmosphere. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction Since the vacuum vessel is exposed to neutrons in a fusion reactor, low-activation materials have to be used for such a component. Recently, the low-activated ferritic steel, F82H (Fe – 8Cr–2W), has been developed as a candidate material for a blanket of ITER, by Japan Atomic Energy Research Institute (A. Hishinuma et al., personal communication, 1996). This steel has good properties such as a low thermal expansion coefficient and high thermal conductivity. However, the chemical corrosion of this steel may become larger than that of austenitic stainless steel, due to * Corresponding author.
the high diffusion rate of oxygen and the low content of nickel. It is reported that the stain of steels is generally prevented by the addition of Ni [1]. Then, it is presumed that a large amount of gas is desorbed from the ferritic steel, F82H. In the present study, we investigated the gas desorption properties of F82H by using thermal desorption spectroscopy (TDS) [2], and then the results were compared with those of austenitic stainless steels (316SS and 316LSS). In order to examine the gas adsorption behavior, the outgassing was measured for the F82H sample which was exposed to the atmosphere for a long time period. The change of surface atomic composition of F82H during heating at 400, 600 or 800 K was also examined by Auger electron spectroscopy (AES).
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(97)00170-1
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
486
Table 1 Composition in wt.% of F82H, 316SS and 316LSS Sample
Chemical composition (wt.%) C
Si
Mn
P
S
Cu
Ni
F82H 316SS 316LSS
0.09 B0.08 B0.03
0.07 B1.00 B1.00
0.1 B2.00 B2.00
0.003 B0.045 B0.045
0.001 B0.03 B0.03
0.01 — —
0.02 10 – 14 12 – 15
Cr 7.84 16–18 16–18
Mo 0.003 2–3 2–3
V
Co 0.003 — —
Ti
F82H 316SS 316LSS
Ta 0.04 — —
W 1.98 — —
0.19 — —
0.004 — —
Fig. 1. Thermal desorption spectra of as-received sample (F82H).
2. Experimental Two kinds of F82H samples were used for evaluation of gas desorption. One was the sample supplied just after the preparation by the NKK Corporation. The gas desorption of this sample seems to be low because of the short time exposure to atmosphere. This sample is called the as-received sample, in the present paper. The other was the sample which was exposed to the atmosphere for 2 years. The surface color of this sample was darkened. This sample was mechanically polished to remove the dark surface layer. For the polishing, both Emery paper and alumina
powder were used. After that, the sample was rinsed by ethanol in an ultrasonic bath, and then dried in the atmosphere. The sample mechanically polished as above is called the sample exposed for 2 years. In order to examine the effect of gas adsorption in the atmosphere, the mechanically polished sample (sample exposed for 2 years) was degassed with a temperature of 960 K for 30 min. This sample was then exposed to the atmosphere for a period of 3 or 10 months. This sample is called the sample exposed for 3 or 10 months. The gas desorption properties of the above F82H samples were investigated. The gas desorption properties of mechanically polished 316SS or
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
487
Fig. 2. Thermal desorption spectra of 316SS (a) and 316LSS (b).
316LSS were also examined under the same conditions, in order to compare with the case of the as-received sample (F82H). Table 1 shows the composition in wt.% of F82H, 316SS and 316LSS. The desorption properties of these samples were measured by means of thermal desorp-
tion spectroscopy (TDS) with a constant ramp rate. The sample shape was a slab with a size of 15× 15× 0.2 mm. In the TDS analysis, the sample was located on a heater made by Mo. The Mo heater was resistively heated up to 1273 K. In order to obtain the apparent activation energies
488
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
Table 2 Gas desorption amounts of as-received sample (F82H), 316SS and 316LSS Sample
As-received sample (F82H) 316SS 316LSS
Gas species: Desorption amount (×1020 molecules m−2) [fraction: %] H2
H2O
CO
CO2
Total
0.11 [2] 2.22 [38] 16.9 [81]
1.08 [21] 1.30 [22] 0.98 [5]
3.06 [59] 1.98 [34] 2.53 [12]
0.95 [18] 0.34 [6] 0.39 [2]
5.20 5.84 20.8
of desorbed gases without the assumption of the order of reaction, the ramp rate, b, was changed from 0.08 to 0.83 K s − 1. The temperature which has the maximum desorption rate, Tp, shifts to a higher temperature with the increase of b. The apparent activation energies, Ed, may be obtained from the slope of a straight line between natural logarithms of T 2p/b and a reciprocal of Tp [2]. The gas desorption spectra of samples exposed for 3 months, 10 months and 2 years were employed to examine the effect of gas adsorption in the atmosphere. The surface atomic composition and depth profile of the sample exposed for 2 years (F82H), during heating from room temperature (RT) to 820 K, was analyzed by Auger electron spectroscopy (AES) with Ar ion etching.
3. Results and discussions Fig. 1 shows typical desorption spectra of the as-received sample (F82H). Here, the ramp rate and the final temperature were 0.17 K s − 1 and 1200 K, respectively. The major desorbed gas species were H2O, CO, CO2 and H2. The desorption rates of all gases increased in the temperature range above 400 K. H2O was desorbed in a wide range up to 1200 K. The largest desorption rate of CO2 was observed at 700 K. Around 900 K, the desorption rates of H2, CO and CO2 increased. A large desorption of CO occurred at the temperature above 1100 K. This desorption rate of CO was one order of magnitude larger than other peaks. The total desorption amount of all gases due to the heating up to 1200 K was 5.2× 1020
molecules m − 2. The fraction of desorption amount for oxygen-containing species, such as H2O, CO and CO2, was very large, approximately 98% of the total desorption amount. Fig. 2 shows the thermal desorption spectra of 316SS (a) and 316LSS (b) [2]. The gas desorption properties of 316SS, such as gas species, fractions of desorbed gas amount, desorption peaks and a total desorption amount, were similar to those of the as-received sample (F82H). On the contrary, the major desorbed gas species of 316LSS was H2, which was 80% of the total desorption amount. In the H2 spectrum of 316LSS, one shoulder and one peak were observed. The large desorption of CO at around 1100 K was observed in both 316SS and 316LSS. However, the desorption rate of CO in 316LSS was very small, perhaps due to the the small content of carbon in bulk, compared with the case of the as-received sample (F82H) or 316SS (A. Hishinuma et al., personal communication, 1996; Ref. [1]). Table 2 shows the desorption amounts of major gas species, and the total desorption amounts from the three steels. Here, the desorption amounts were obtained by integration of the desorption rate up to 1200 K. The desorption amounts of CO and CO2 of the as-received sample (F82H) were larger than those of the austenitic stainless steels. However, the total desorption amount of the as-received sample (F82H) was almost the same as that of 316SS. Fig. 3 shows the TDS spectra of H2O (a), CO (b) and CO2 (c) for the three exposed samples, and the as-received sample as a reference. Here, the samples exposed for 3 or 10 months were
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
489
Fig. 3. Thermal desorption spectra of H2O (a), CO (b) and CO2 (c) for exposed samples. Table 3 Gas desorption amounts of three exposed samples and as-received sample Sample name
As-received sample Exposed sample for 3 months Exposed sample for 10 months Exposed sample for 2 years
Gas species: Desorption amount (×1020 molecules m−2) H2
H 2O
CO
CO2
Total
0.15 0.70 0.96 2.38
0.73 0.23 2.04 4.90
1.38 0.51 0.56 5.4
0.96 0.55 0.50 1.20
3.2 2.1 4.2 14.0
degassed before exposure to the atmosphere. The gas desorption amounts were obtained by the integration of the desorption rate until 960 K, and the results are shown in Table 3. For the three exposed samples, the desorption rate of H2O increased with the exposure time in a high tempera-
ture range. In the case of the sample exposed for 2 years, a very large desorption of H2O was observed in the temperature range 400–800 K. This result suggests that the chemical corrosion of F82H takes place due to water, by the exposure to atmosphere. The desorption amount of CO was
490
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
Fig. 4. Change of surface atomic composition of exposed sample for 2 years (F82H) during the heating.
largely increased by the exposure to the atmosphere for 2 years. The desorption amounts of CO2 for exposed samples also increased with the exposure time. The desorption peak was shifted to a lower temperature range by the exposure. If the sample with degassing treatment is exposed to the atmosphere for a short time period, the desorption rate of gases may be smaller than those of the as-received sample (Table 3, Fig. 3). Therefore, in order to use F82H as an in-vessel component, degassing with a high temperature (\ 700 K) is required before the installation. Fig. 4 shows surface atomic composition of the sample exposed for 2 years, when the sample temperature was increased from RT to 820 K. When the sample was heated from RT to 420 K, the oxygen concentration slightly decreased due to a large desorption of H2O (Fig. 3a). At 620 K, large desorptions of H2O and CO2 were observed in the TDS spectra (Fig. 3a and c). The oxygen concentration again slightly decreased due to
these desorptions, but the carbon concentration gradually increased with heating time. At 820 K, the carbon concentration largely increased due to the segregation of carbon. The atomic composition of carbon changed from 45 to 70 at.% when the temperature was raised from RT to 820 K. It is assumed that a large desorption of CO corresponds to the recombination between oxygen, and carbon diffused from bulk. In addition, chromium was observed on the top surface. A similar segregation of chromium was observed in 316LSS [4]. The apparent activation energies of desorption, Ed, for F82H and 316LSS [3], are shown in Table 4. Here, the activation energy for second peak of CO in F82H (at around 1100 K) was not obtained due to the limit of the heating temperature. The values of H2 and CO2 for F82H were similar to those for 316LSS.
4. Conclusion The gas desorption properties of the as-received sample (F82H) were examined, and compared with those of 316SS and 316LSS. The major desorbed gas species of F82H were H2O, CO, CO2 and H2. The fraction of oxygen-containing species, such as CO, CO2 and H2O, to the total outgassing amount was 98%. The gas desorption behavior was very similar to the case of 316SS. The gas desorption spectra for F82H show the required prebaking temperature becomes higher than 700 K. For the sample exposed to the atmosphere, the gas desorption behavior was considerably changed. As the time of exposure increased, desorption amounts of H2O and CO largely in-
Table 4 Apparent activation energies (Ed) of desorbed gases for as-received sample (F82H) and 316LSS [2] Gas species
H2 H2O CO CO2
F82H (eV)
316LSS (eV)
Ed (1st peak)
Ed (2nd peak)
Ed (1st peak)
Ed (2nd peak)
0.61 0.2–0.3 0.77 1.5
0.67 0.5–0.8 Not obtained 1.77
0.84 Not obtained No low temperature peak 1.23
0.62 Not obtained 3.47 1.17
Y. Hirohata et al. / Fusion Engineering and Design 39–40 (1998) 485–491
creased. This increase may be due to corrosion in the atmosphere.
Acknowledgements We appreciate Drs A. Hishinuma and K. Shiba, JAERI, for the supply of F82H samples.
.
491
References [1] Jpn. Soc. Met., Kinzoku-binran, Maruzen, Tokyo, 1982, p. 824. [2] P. Redhead, Vacuum 12 (1962) 203 – 211. [3] Y. Hirohata, A. Mutoh, T. Hino, T. Yamashina, T. Kikuchi, N. Ohsako, Vacuum 47 (1996) 728 – 731. [4] M. Miyauchi, A. Toyama, Y. Hirohata, T. Yamashina, J. Vac. Soc. Jpn. 35 (1992) 169 – 172.