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Retention of deuterium and tritium in Papyex graphite
Journal of Nuclear Materials North-Holland, Amsterdam
RETENTION
OF DEUTERIUM
R.A. CAUSEY Sandia
Nutional
Received
57
138 (1986) 57-64
IN PAPYEX
GRAPHITE
*
and K.L. WILSON
Lahorutones,
9 August
AND TRITIUM
Livermore,
1985; accepted
CA 94550,
31 October
USA
1985
The retention of 100 eV D-T ions in Papyex graphite over the temperature range 373 to 1573 K has been measured using both nuclear reaction analysis and tritium dissolution counting. Below 1000 K, the retention is characterized by saturation of the near surface region and atomic adsorption on internal porosity, both of which decrease with increasing temperature. Above 1000 K. a local maximum in the retention is seen near 1200 K due to intergranular diffusion and decoration of high energy traps.
1. Introduction In many ways, graphite is well suited for use in a fusion reactor. It is a low-Z material with a high melting point, low neutron absorption cross section, and a tolerable plasma erosion rate at lower temperatures. However, because of graphite’s high specific surface area and its affinity for hydrogen, tritium absorption in graphite must be considered as a potentially critical problem. TFTR has a graphite limiter and will run tritium plasmas in 1988, according to present schedules. Also, designs for tritium burning core machines such as TFCX generally call for the extensive use of graphite limiters and wall tiles. As a limiter material, graphite will be exposed not only to energetic tritium ions and charge exchange neutrals, but also to a background pressure of tritium gas during the burn. If retention by the graphite is high, tritium inventories may dictate that a material other than graphite should be chosen. Doyle et al. [l] measured the retention of 1.5 keV deuterons in pyrolytic graphite. Nuclear reaction analysis of the sample showed a well defined saturation deuterium retention of approximately 3 x 10” D/cm* at 303 K, with the saturation retention dropping monotonically with increasing temperature. All saturation concentrations were achieved at particle fluences of less that 5 X 10” D/cm*. Hucks et al. [2] performed similar measurements on pyrolytic graphite using a hydrogentritium mixture of 0.3 eV thermal atoms. Hydrogen retention in the entire sample was inferred from the * This work supported
by US Department
of Energy.
0022-3115/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
amount of tritium collected during the combustion of the sample. Because the range for these low energy particles is very small, the saturation region and the amount of hydrogen retained in that region should have been very small. In contrast to what might have been expected, a retention of almost 1 X 10” H/cm* was reported at 295 K with the retention still rising steadily as the particle fluence exceeded 5 X 1019 H/cm2. Also, a local maximum in the retention was reported to occur at a temperature near 700 K. In a similar study by Stangeby et al. [3] on the retention of sub-eV deuterium in Papyex graphite, the ‘He beam used in the nuclear reaction analysis was scanned from 0.5 to 2.3 MeV to determine the near surface deuterium concentration profile. The deuterium concentration was seen to decrease only slowly with distance into the sample, and measurable quantities were seen beyond 5 pm. This paper presents the results of an experimental study on the retention of tritium and deuterium by Papyex graphite during plasma and gas exposures. Measurements were performed for 100 eV ion fluences as high as 2.7 x 10 2o D/cm* over t he temperature range 373 K to 1573 K. Both nuclear reaction analysis and tritium tracer techniques were used to determine the hydrogen isotope content.
2. Experimental procedures Papyex is a high purity graphitized paper produced by the compression of exfoliated graphite layers [4]. It has a low density (1.1 g/cm3) and a high porosity that
B.V.
allows gas to penetrate easily to each of the layers. The thickness of the layers was not measured, but the quoted surface area of 20 m2/g would suggest an average exfoliated layer thickness of 0.1 pm. Papyex samples 0.15 mm thick were used for all experiments. For the lower temperatures (1273 K and below), samples with a 5 mm radius were heated by a small resistance heater. The temperatures was measured using a chromel-alumel thermocouple compressed between the sample and the heater. For temperatures above 1273 K, heating was supplied by flowing a current through sample strips 1 cm wide and 1.5 cm long held in place by molybdenum electrodes. Optical pyrometry was used to measure the higher temperatures. For the plasma exposures, a ceramic shield was placed over the sample holder. A hole in the end of the shield allowed the exposure of only the center of the Papyex sample to the plasma. All experiments were performed in the tritium plasma experiment (TPX) which has been described earlier [5]. Basically, it is a plasma discharge device utilizing RF heating and magnetic confinement. The energy of the hydrogen isotope ions striking the sample surface was regulated by applying a negative bias to the electrically isolated sample holder. Plasmas consisting of either pure deuterium or a mixture of 99% deuterium and 1% tritium were used in the different experiments. For a pure deuterium plasma, the ions incident on the sample consisted of approximately 80% DC, 10% D:, 10% D:, with an almost equal number of neutrals with energies of a few eV. In this paper, the listed fluxes and fluences will include all ions and neutrals. An ion energy of 100 eV was used for all experiments, and fluxes varied between 1 X 10lh and 1 X 10” D/(cm* s). The TRIM Code [6] was used to determine the ranges of 100 eV deuterons and tritons in Papyex. Because both maxima ranges are approximately 50 A, the tritium should be considered to be a good tracer for the behavior of deuterium and vice-versa. After a pure deuterium plasma exposure, the Papyex samples were analyzed for deuterium content using D(3He, p)4He nuclear reaction analysis (NRA). Maintaining the 3He beam at 800 keV allowed the absolute measurements of deuterium within approximately 1 pm of the external surface. The nuclear reaction measurements were frequently calibrated against known standards. Samples exposed to plasma or gas consisting of 99% deuterium and 1% tritium were analyzed for total tritium content by dissolving the samples in chromosulfuric acid [7]. The acid solution was then distilled to yield a tritiated water sample for liquid scintillation counting. The deuterium content was obtained by mul-
tiplying the tritium content by the ratios of the isotopes in the plasma. The process was absolutely calibrated by adding known amounts of a tritiated water standard using dissolution of uncontaminated Papyex samples. Because TPX operates with a 0.66 Pa background gas pressure, absorption of the gas competed with the plasma loading at the higher temperatures. To infer the effect of the plasma only. experiments without the plasma were performed to obtain the background effect of the gas. These experiments were also used at the highest temperatures to obtain results for comparison to predicted loadings using values for the solubility and diffusion of tritium in carbon available in the literature.
3. Results and discussion The total deuterium retention (based on the tritium tracer technique described in section 2) in Papyex as a function of temperature is shown in fig. 1. The dashed line is included only to guide the eye. All samples were exposed to a 100 eV flux of 5 X 1Or6 D/(cm* s) and a 0.66 Pa background gas pressure for 1.5 h. These data show a monotonically decreasing retention as temperature is increased to 1000 K, followed by a pronounced secondary retention peak at around 1200 K. The retention results are best described by dividing them in two sections. Section 3.1. given the results for 373 to 973 K where we conclude that hydrogen isotopes can migrate into graphite only by surface diffusion along internal porosity. Above 973 K, our results demonstrate that gas absorption and transgranular diffusion become important. These results are given in section 3.2. The implications of the measurements to D-T fusion reactors are discussed in section 3.3. 3.1. Retention at 373 to 973 K The near-surface deuterium retention (from the D(3He, p)4He measurements) and total deuterium retention (from the tritium tracer dissolution counting technique) are shown as a function of fluence at 673 K in fig. 2. Two points are immediately noticeable. First, saturation in both the near-surface and total retention is not approached until fluences exceed 1 X 10” D/cm*, in marked contrast to the higher energy ion implantation data of Doyle et al. [l] where fluences of approximately 5 X lo’* D/cm* appeared to saturate the surface. A second striking feature is that the total retention is significantly higher than the near-surface (- 1 pm depth) retention. This difference between total and near-surface retention is observed at high fluences for
20
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FLUENCE
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of fluence.
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TEMPERATURE Fig. 3. Comparison
of near surface
and total deuterium
retention
all temperatures below 1000 K, as shown in fig. 3. The high fluence required for the near-surface saturation is thought to be due at least partially to the energy mix of particles striking the surface in the TPX facility. Many measurements, such as those by Langley et al. [8] and Wampler et al. [9], have demonstrated that ion-implanted deuterium is apparently immobile in bulk graphite at low temperatures. These investigators also observed that the implant profiles broadened with increasing fluence, but only in the surface direction. Saturation is eventually reached, and the near-surface retention is dictated only by the saturation concentration and the range of the most energetic particles striking the surface. From the TPX ion distribution given in section 2 of this paper, it can be calculated that only 2% of the ions striking the surface during a deuterium plasma exposure were full energy 100 eV D: ions. The neutrals had energies of only a few electron volt, and the D: and D: dissociated upon hitting the surface with the individual deuterium atoms receiving i and + of the total ion energy. Therefore, a 1 x 10” D/cm2 total fluence in TPX corresponds to only 2 x lOI D/cm* fluence of the highest energy component of the
(K)
to that predicted
by local mixing model
flux. Hence saturation of the near-surface should not be expected until the total fluence exceeds 1 X 102” D/cm’. It is also possible that a surface roughness from physical sputtering could also have affected the fluence required for saturation because of the increased surface area. Electron microscopy showed samples irradiated at lower temperatures to have greatly enhanced surface roughness. Samples irradiated at the higher temperature did not have a comparable roughness, presumably due to the more uniform erosion processes of hydrocarbon formation and radiation enhanced sublimation that occur above 500 K [lo]. The difference between total and near-surface deuterium retention seen in figs. 2 and 3 is more perplexing. At these low temperatures, where transgranular migration of hydrogen is generally considered to be negligible [11,12,13], energetic deuterium should be uniformly distributed in a saturated layer extending from the surface to the end of the particle range. The local mixing model (LMM) [14] uses this concept of a saturated layer of immobile hydrogen to successfully model low temperature, near-surface retention of ion-implanted graphite. Fig. 3 shows_ LMM calculations of the near-surface
R.A. Causey, K. L. Wilson / Retention of deuterium and tritium rn Papyex graphite
retention based on the measurements of the temperature dependence of the saturation concentration at 1.5 keV [l], and on calculations of 100 eV and 1.5 keV ion ranges using the TRIM computer code [6]. Reasonably good agreement is seen between the present near-surface data and the LMM calculations. However, the tritium tracer measurements of total retention show significantly higher retention than expected from the LMM calculation alone. As discussed. in section 2, samples in TPX are exposed not only to energetic ions, but also to low energy neutral atoms and a rather high molecular gas pressure. To determine whether gas adsorption throughout the sample was causing the higher retention at these lower temperature, control samples were exposed to 0.66 Pa of the D-T mixture at temperatures from room temperature to 973 K. In every case, the retention due to gas adsorption was negligible compared to that for the plasma exposure. One sample was also expected to the D-T gas mixture at 673 K for one hour after a full deuterium plasma exposure. Again, tritium dissolution counting showed the gas adsorption to be negligibly small. This measurement eliminated the possibility that the enhanced surface roughness caused by the plasma exposure was creating active sites or producing new porosity for gas adsorption. The increase in total retention compared to nearsurface retention appears to be due to the plasma exposure and not simply due to gas exposure. The large bulk uptake is reminiscent of the tritium tracer measurements of graphite exposed to atomic protium by Hucks et al. [2] mentioned in section 1, although their observed temperature dependence was not reproduced in our experiments. The present TPX results suggest that the deuterium is being retained in the area that is beyond the implantation range as well as beyond the distance where it can be detected by nuclear reaction analysis. This agrees with the findings of Stangeby et al. [3] where deuterium was seen at great depths in the sample. Because this is happening at temperatures where transgranular diffusion is believed not to occur, we conclude that the bulk retention beyond the implantation range is due to adsorption of deuterium atoms on the massive internal surface area found in most graphites. Papyex, in particular, has a specific surface area of 20 m2/g that is completely connected to the external surface [4]. For temperatures below 1000 K, Papyex exposed only to deuterium gas showed negligible retention. Deuterium molecules were unable to absorb and dissociate on the sample surface. When the plasma was turned on, energetic ions began to strike the external surface. These particles penetrated to the end of range and eventually
61
saturated this near-surface region. Atoms released from this saturation region together with low energy neutrals from the plasma were able to diffuse along the internal porosity surfaces. When NRA was used, the atoms in the saturation region were detected along with those on internal surfaces within the first micron. Only when dissolution counting is used to determine the amount of tritium can the correct quality of hydrogen isotopes trapped in the sample be found. Because the amount of deuterium in the entire sample was considerably larger than that amount in the near-surface, the thermal behavior of the total retention was only partially affected by the amount in the saturation region. This overall behavior was more a function of the number of deuterium atoms able to remain trapped at the adsorption sites. The NRA results were also affected by the absorbed atoms, but not to the same extent as the dissolution results. 3.2. Retention
at 973 to 1573 K
NRA results for samples exposed to the plasma at these elevated temperatures showed only minimum near surface deuterium retention (< 1 X 10’” D/cm’). Fig. 4 repeats the higher temperature dissolution counting data from fig. 1 for temperatures above 1000 K. These results are for samples exposed to both plasma and background gas for 1.5 h. The dashed line is included only to guide the eye. Also shown in fig. 4 are results for samples exposed to only background gas for the same length of time. Comparison of the results of the plasma and gas exposure and the gas only exposure again showed the effect of the plasma to be small at these elevated temperatures. Because the surface adsorption sites have an activation energy of 1.5 eV or below [15,16], they should be depopulated at these elevated temperatures, and processes other than surface diffusion and adsorption must have controlled the tritium uptake. We postulate that the observed uptake above 1000 K is due to true bulk, transgranular diffusion, and the decoration of a uniform concentration of strong bulk trapping sites for hydrogen isotopes. Above approximately 1300 K the retention is observed to decrease because the traps begin to thermally depopulate. The DIFFUSE computer code [17] was used to model the observed hydrogen isotope retention properties of Papyex graphite. The detrapping energy was determined to be approximately 4 eV. a value close to that of the C-H bond [18]. The trapping density throughout the bulk of the sample was found to be around 1 X 10m4 atom fraction. The gas only exposure data from fig. 4 is repeated in
El
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of deuterium
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-----I 1700
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i I
- PLASMA& BACKGROUNDGAS
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R.A. Causey, K.L. Wilson / Retention of deuterium and triiium in Papyex graphiie
d
0 0
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Fig. 6. Time dependence of deuterium absorption in Papyex graphite at I’273 K and P = 0.66 Pa.
fig. 5 along with the results from the DIFFUSE calculations. The parameters used in the calculations are given in table 1. These same parameters were also used to model the time dependence of the deuterium uptake at 1273 K. Fig. 6 shows a comparison of these results to experimental values determined by tritium dissolution counting of samples exposed to 0.66 Pa of the D/T gas mixture for varying times at 1273 K. As an additional measurement of the deuterium uptake in graphite, samples were also loaded to saturation at a gas pressure of 66 Pa. At 1273 K there was no effective increase in the saturation retention at 66 Pa compared to the 0.66 Pa case shown in fig. 6. This result further confirms that the bulk saturation behavior obTable 1 Parameter used in DIFFUSE code Trap concentration Trap energy Diffusion coefficient Solubility
1.5 X 10m4 atom fraction 4.1 eV 2.0 X 10W6exp[( - 1.0 eV)/kT] 2.0 X 10m6exp[( -0.1 eV)/kT]
served above 1000 K is due to the decoration of a fixed concentration of traps and not due to an intrinsic lattice solubility (which would have scaled with the square root of pressure). Although the parameters given in table 1 do give a good fit to the experimental data, they are certainly not unique. Several other parameter combinations, including diffusivity activation energies as high as 2.5 eV, were seen to give fits that were also close to the experimental data. Our values for the diffusion activation energy are considerably smaller than the 3 to 5 eV range typically reported in the literature [ll-131. Most of these previous studies were conducted with low tritium lattice concentrations, and were hence sensitive to trapping effects. In more recent work, Saeki [19] reported diffusion activation energies between 1.0 and 2.7 eV for pyrolytic carbon, in good agreement with our present results. 3.3. Implications for fusion reactors These measurements have several implications for the use of graphite in fusion reactors. While the Papyex
microstructure is not typical of those found in current fusion devices, its fine grain size and high internal surface area should provide an effective upper limit for the uptake of hydrogen isotopes in graphite (at least in the absence of neutron damage). The high retention at low temperatures, followed by a continuous decrease in the retention with increasing temperature emphasized a potential recycling problem for graphites. Hydrogen isotopes adsorbed at the start of a discharge could be subsequently released as the limiter temperature rises during the later stage of the discharge. This rapid rise in fueling rate could raise the plasma density beyond the disruption limit. Tritium inventory could also be a serious problem of graphite limiters operated at low temperatures where surface adsorption dominates the retention, or at elevated temperatures where bulk migration and trapping occur. However, the tritium retention behavior shown in fig. 1 also suggests an optimum limiter operating temperature range for reduced tritium uptake. At temperatures of around 1000 K, the near surface saturation and surface adsorption on internal porosity are significantly reduced compared to lower temperatures. In addition, 1000 K is not hot enough to allow significant transgranular diffusion of tritium into the bulk graphite. It should be noted that the 1000 K operating temperature fortuitously coincides with a region of reduced hydrogen ion erosion of graphite between the lower temperature hydrocarbon formation peak and the higher temperature onset of radiation damage enhanced sublimation [lo].
4. Summary The retention of 100 eV deuterium or tritium ions in Papyex is characterized by a near surface saturation concentration that decreases with increasing temperature, by adsorption on internal surfaces that also decreased with increasing temperature, and by decoration of high energy traps at temperatures where transgranular diffusion is possible. The NRA results of this study, when adjusted for energy, agree with the results presented by Doyle et al. [l] for near-surface retention. The total deuterium retention results determined by tritium dissolution counting are in fairly good agreement with
similar measurements by Hucks et al. [2]. If the graphite used in fusion reactors has retention properties similar to that for Papyex, it may retain significant quantities of hydrogen isotopes unless the temperature is maintained at approximately 1000 K or a coating is applied to prevent free access to internal porosity.
Acknowledgments The authors would like to thank Wayne Chrisman for his technical assistance and Michael Baskes for his helpful discussions.
References and D.K. Brice, J. Nucl. [II B.L. Doyle. W.R. Wampler Mater. 103 & 104 (1981) 513. PI P. Hucks, K. Flaskamp and E. Vietzke, J. Nucl. Mater. 93 & 94 (1980) 558. [31 P.C. Stangeby. 0. Auciello, A.A. Haasz and B.L. Doyle. J. Nucl. Mater. 122 & 123 (1984) 1592. [41 C. Bockel, J.P. Coulomb and N. DuPont-Pavlovsky. Surf. Sci. 116 (1982) 369. [51 R.A. Kerst, J. Nucl. Mater. 103 & 104 (1981) 439. [61 J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth.
149 (1980) 594. 171 J. Engelhard, USAEC Report. JUL-752-RG (1971). PI R.A. Langley and R.S. Blewer, J. Nucl. Mater. 76 & 77
(1978) 313. D.K. Brice and C.W. Magee, J. Nucl. [91 W.R. Wampler, Mater. 102 (1981) 304. UOI J. Roth, J.B. Roberto and K.L. Wilson, J. Nucl. Mater. 122 & 123 (1984) 1447. [Ill R.A. Causey. T.S. Elleman and K. Verghese. Carbon 17 (1979) 323. u21 M. Saeki, J. Appl. Radiat. Isot. 34 (1983) 739. [I31 H.D. Rohring, P.G. Fischer and R. Hecker. J. Am. Ceram. Sot. 59 (1976) 316.
J. Nucl. [I41 D.K. Brice, B.L. Doyle and W.R. Wampler, Mater. 111 & 112 (1982) 598. 1151 R.M. Barrer, Proc. Sot. London 149A (1935) 253. [161 R.C. Bansal, F.J. Vastola and P.L. Walker. Carbon 9 (1971) 185. u71 M.I. Baskes, DIFFUSE 83, SAND83-8231. 1181 L. Pauling, The Nature of the Chemical Bond, Second Edition (McGraw-Hill, New York, 1940) p. 522ff. 1191 M. Saeki, J. Nucl. Mater. 131 (1985) 32.
RETENTION
OF DEUTERIUM
R.A. CAUSEY Sandia
Nutional
Received
57
138 (1986) 57-64
IN PAPYEX
GRAPHITE
*
and K.L. WILSON
Lahorutones,
9 August
AND TRITIUM
Livermore,
1985; accepted
CA 94550,
31 October
USA
1985
The retention of 100 eV D-T ions in Papyex graphite over the temperature range 373 to 1573 K has been measured using both nuclear reaction analysis and tritium dissolution counting. Below 1000 K, the retention is characterized by saturation of the near surface region and atomic adsorption on internal porosity, both of which decrease with increasing temperature. Above 1000 K. a local maximum in the retention is seen near 1200 K due to intergranular diffusion and decoration of high energy traps.
1. Introduction In many ways, graphite is well suited for use in a fusion reactor. It is a low-Z material with a high melting point, low neutron absorption cross section, and a tolerable plasma erosion rate at lower temperatures. However, because of graphite’s high specific surface area and its affinity for hydrogen, tritium absorption in graphite must be considered as a potentially critical problem. TFTR has a graphite limiter and will run tritium plasmas in 1988, according to present schedules. Also, designs for tritium burning core machines such as TFCX generally call for the extensive use of graphite limiters and wall tiles. As a limiter material, graphite will be exposed not only to energetic tritium ions and charge exchange neutrals, but also to a background pressure of tritium gas during the burn. If retention by the graphite is high, tritium inventories may dictate that a material other than graphite should be chosen. Doyle et al. [l] measured the retention of 1.5 keV deuterons in pyrolytic graphite. Nuclear reaction analysis of the sample showed a well defined saturation deuterium retention of approximately 3 x 10” D/cm* at 303 K, with the saturation retention dropping monotonically with increasing temperature. All saturation concentrations were achieved at particle fluences of less that 5 X 10” D/cm*. Hucks et al. [2] performed similar measurements on pyrolytic graphite using a hydrogentritium mixture of 0.3 eV thermal atoms. Hydrogen retention in the entire sample was inferred from the * This work supported
by US Department
of Energy.
0022-3115/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
amount of tritium collected during the combustion of the sample. Because the range for these low energy particles is very small, the saturation region and the amount of hydrogen retained in that region should have been very small. In contrast to what might have been expected, a retention of almost 1 X 10” H/cm* was reported at 295 K with the retention still rising steadily as the particle fluence exceeded 5 X 1019 H/cm2. Also, a local maximum in the retention was reported to occur at a temperature near 700 K. In a similar study by Stangeby et al. [3] on the retention of sub-eV deuterium in Papyex graphite, the ‘He beam used in the nuclear reaction analysis was scanned from 0.5 to 2.3 MeV to determine the near surface deuterium concentration profile. The deuterium concentration was seen to decrease only slowly with distance into the sample, and measurable quantities were seen beyond 5 pm. This paper presents the results of an experimental study on the retention of tritium and deuterium by Papyex graphite during plasma and gas exposures. Measurements were performed for 100 eV ion fluences as high as 2.7 x 10 2o D/cm* over t he temperature range 373 K to 1573 K. Both nuclear reaction analysis and tritium tracer techniques were used to determine the hydrogen isotope content.
2. Experimental procedures Papyex is a high purity graphitized paper produced by the compression of exfoliated graphite layers [4]. It has a low density (1.1 g/cm3) and a high porosity that
B.V.
allows gas to penetrate easily to each of the layers. The thickness of the layers was not measured, but the quoted surface area of 20 m2/g would suggest an average exfoliated layer thickness of 0.1 pm. Papyex samples 0.15 mm thick were used for all experiments. For the lower temperatures (1273 K and below), samples with a 5 mm radius were heated by a small resistance heater. The temperatures was measured using a chromel-alumel thermocouple compressed between the sample and the heater. For temperatures above 1273 K, heating was supplied by flowing a current through sample strips 1 cm wide and 1.5 cm long held in place by molybdenum electrodes. Optical pyrometry was used to measure the higher temperatures. For the plasma exposures, a ceramic shield was placed over the sample holder. A hole in the end of the shield allowed the exposure of only the center of the Papyex sample to the plasma. All experiments were performed in the tritium plasma experiment (TPX) which has been described earlier [5]. Basically, it is a plasma discharge device utilizing RF heating and magnetic confinement. The energy of the hydrogen isotope ions striking the sample surface was regulated by applying a negative bias to the electrically isolated sample holder. Plasmas consisting of either pure deuterium or a mixture of 99% deuterium and 1% tritium were used in the different experiments. For a pure deuterium plasma, the ions incident on the sample consisted of approximately 80% DC, 10% D:, 10% D:, with an almost equal number of neutrals with energies of a few eV. In this paper, the listed fluxes and fluences will include all ions and neutrals. An ion energy of 100 eV was used for all experiments, and fluxes varied between 1 X 10lh and 1 X 10” D/(cm* s). The TRIM Code [6] was used to determine the ranges of 100 eV deuterons and tritons in Papyex. Because both maxima ranges are approximately 50 A, the tritium should be considered to be a good tracer for the behavior of deuterium and vice-versa. After a pure deuterium plasma exposure, the Papyex samples were analyzed for deuterium content using D(3He, p)4He nuclear reaction analysis (NRA). Maintaining the 3He beam at 800 keV allowed the absolute measurements of deuterium within approximately 1 pm of the external surface. The nuclear reaction measurements were frequently calibrated against known standards. Samples exposed to plasma or gas consisting of 99% deuterium and 1% tritium were analyzed for total tritium content by dissolving the samples in chromosulfuric acid [7]. The acid solution was then distilled to yield a tritiated water sample for liquid scintillation counting. The deuterium content was obtained by mul-
tiplying the tritium content by the ratios of the isotopes in the plasma. The process was absolutely calibrated by adding known amounts of a tritiated water standard using dissolution of uncontaminated Papyex samples. Because TPX operates with a 0.66 Pa background gas pressure, absorption of the gas competed with the plasma loading at the higher temperatures. To infer the effect of the plasma only. experiments without the plasma were performed to obtain the background effect of the gas. These experiments were also used at the highest temperatures to obtain results for comparison to predicted loadings using values for the solubility and diffusion of tritium in carbon available in the literature.
3. Results and discussion The total deuterium retention (based on the tritium tracer technique described in section 2) in Papyex as a function of temperature is shown in fig. 1. The dashed line is included only to guide the eye. All samples were exposed to a 100 eV flux of 5 X 1Or6 D/(cm* s) and a 0.66 Pa background gas pressure for 1.5 h. These data show a monotonically decreasing retention as temperature is increased to 1000 K, followed by a pronounced secondary retention peak at around 1200 K. The retention results are best described by dividing them in two sections. Section 3.1. given the results for 373 to 973 K where we conclude that hydrogen isotopes can migrate into graphite only by surface diffusion along internal porosity. Above 973 K, our results demonstrate that gas absorption and transgranular diffusion become important. These results are given in section 3.2. The implications of the measurements to D-T fusion reactors are discussed in section 3.3. 3.1. Retention at 373 to 973 K The near-surface deuterium retention (from the D(3He, p)4He measurements) and total deuterium retention (from the tritium tracer dissolution counting technique) are shown as a function of fluence at 673 K in fig. 2. Two points are immediately noticeable. First, saturation in both the near-surface and total retention is not approached until fluences exceed 1 X 10” D/cm*, in marked contrast to the higher energy ion implantation data of Doyle et al. [l] where fluences of approximately 5 X lo’* D/cm* appeared to saturate the surface. A second striking feature is that the total retention is significantly higher than the near-surface (- 1 pm depth) retention. This difference between total and near-surface retention is observed at high fluences for
20
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59
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TEMPERATURE Fig. 1. Deuterium 10.00
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DEUTERIUM retention
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FLUENCE
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I 2.00
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K. L. Wilson / Retention of deuterium und tritrum rn Prrp,~.~ Xruphlte
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-
LOCALMIXINGMODEL
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;
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TEMPERATURE Fig. 3. Comparison
of near surface
and total deuterium
retention
all temperatures below 1000 K, as shown in fig. 3. The high fluence required for the near-surface saturation is thought to be due at least partially to the energy mix of particles striking the surface in the TPX facility. Many measurements, such as those by Langley et al. [8] and Wampler et al. [9], have demonstrated that ion-implanted deuterium is apparently immobile in bulk graphite at low temperatures. These investigators also observed that the implant profiles broadened with increasing fluence, but only in the surface direction. Saturation is eventually reached, and the near-surface retention is dictated only by the saturation concentration and the range of the most energetic particles striking the surface. From the TPX ion distribution given in section 2 of this paper, it can be calculated that only 2% of the ions striking the surface during a deuterium plasma exposure were full energy 100 eV D: ions. The neutrals had energies of only a few electron volt, and the D: and D: dissociated upon hitting the surface with the individual deuterium atoms receiving i and + of the total ion energy. Therefore, a 1 x 10” D/cm2 total fluence in TPX corresponds to only 2 x lOI D/cm* fluence of the highest energy component of the
(K)
to that predicted
by local mixing model
flux. Hence saturation of the near-surface should not be expected until the total fluence exceeds 1 X 102” D/cm’. It is also possible that a surface roughness from physical sputtering could also have affected the fluence required for saturation because of the increased surface area. Electron microscopy showed samples irradiated at lower temperatures to have greatly enhanced surface roughness. Samples irradiated at the higher temperature did not have a comparable roughness, presumably due to the more uniform erosion processes of hydrocarbon formation and radiation enhanced sublimation that occur above 500 K [lo]. The difference between total and near-surface deuterium retention seen in figs. 2 and 3 is more perplexing. At these low temperatures, where transgranular migration of hydrogen is generally considered to be negligible [11,12,13], energetic deuterium should be uniformly distributed in a saturated layer extending from the surface to the end of the particle range. The local mixing model (LMM) [14] uses this concept of a saturated layer of immobile hydrogen to successfully model low temperature, near-surface retention of ion-implanted graphite. Fig. 3 shows_ LMM calculations of the near-surface
R.A. Causey, K. L. Wilson / Retention of deuterium and tritium rn Papyex graphite
retention based on the measurements of the temperature dependence of the saturation concentration at 1.5 keV [l], and on calculations of 100 eV and 1.5 keV ion ranges using the TRIM computer code [6]. Reasonably good agreement is seen between the present near-surface data and the LMM calculations. However, the tritium tracer measurements of total retention show significantly higher retention than expected from the LMM calculation alone. As discussed. in section 2, samples in TPX are exposed not only to energetic ions, but also to low energy neutral atoms and a rather high molecular gas pressure. To determine whether gas adsorption throughout the sample was causing the higher retention at these lower temperature, control samples were exposed to 0.66 Pa of the D-T mixture at temperatures from room temperature to 973 K. In every case, the retention due to gas adsorption was negligible compared to that for the plasma exposure. One sample was also expected to the D-T gas mixture at 673 K for one hour after a full deuterium plasma exposure. Again, tritium dissolution counting showed the gas adsorption to be negligibly small. This measurement eliminated the possibility that the enhanced surface roughness caused by the plasma exposure was creating active sites or producing new porosity for gas adsorption. The increase in total retention compared to nearsurface retention appears to be due to the plasma exposure and not simply due to gas exposure. The large bulk uptake is reminiscent of the tritium tracer measurements of graphite exposed to atomic protium by Hucks et al. [2] mentioned in section 1, although their observed temperature dependence was not reproduced in our experiments. The present TPX results suggest that the deuterium is being retained in the area that is beyond the implantation range as well as beyond the distance where it can be detected by nuclear reaction analysis. This agrees with the findings of Stangeby et al. [3] where deuterium was seen at great depths in the sample. Because this is happening at temperatures where transgranular diffusion is believed not to occur, we conclude that the bulk retention beyond the implantation range is due to adsorption of deuterium atoms on the massive internal surface area found in most graphites. Papyex, in particular, has a specific surface area of 20 m2/g that is completely connected to the external surface [4]. For temperatures below 1000 K, Papyex exposed only to deuterium gas showed negligible retention. Deuterium molecules were unable to absorb and dissociate on the sample surface. When the plasma was turned on, energetic ions began to strike the external surface. These particles penetrated to the end of range and eventually
61
saturated this near-surface region. Atoms released from this saturation region together with low energy neutrals from the plasma were able to diffuse along the internal porosity surfaces. When NRA was used, the atoms in the saturation region were detected along with those on internal surfaces within the first micron. Only when dissolution counting is used to determine the amount of tritium can the correct quality of hydrogen isotopes trapped in the sample be found. Because the amount of deuterium in the entire sample was considerably larger than that amount in the near-surface, the thermal behavior of the total retention was only partially affected by the amount in the saturation region. This overall behavior was more a function of the number of deuterium atoms able to remain trapped at the adsorption sites. The NRA results were also affected by the absorbed atoms, but not to the same extent as the dissolution results. 3.2. Retention
at 973 to 1573 K
NRA results for samples exposed to the plasma at these elevated temperatures showed only minimum near surface deuterium retention (< 1 X 10’” D/cm’). Fig. 4 repeats the higher temperature dissolution counting data from fig. 1 for temperatures above 1000 K. These results are for samples exposed to both plasma and background gas for 1.5 h. The dashed line is included only to guide the eye. Also shown in fig. 4 are results for samples exposed to only background gas for the same length of time. Comparison of the results of the plasma and gas exposure and the gas only exposure again showed the effect of the plasma to be small at these elevated temperatures. Because the surface adsorption sites have an activation energy of 1.5 eV or below [15,16], they should be depopulated at these elevated temperatures, and processes other than surface diffusion and adsorption must have controlled the tritium uptake. We postulate that the observed uptake above 1000 K is due to true bulk, transgranular diffusion, and the decoration of a uniform concentration of strong bulk trapping sites for hydrogen isotopes. Above approximately 1300 K the retention is observed to decrease because the traps begin to thermally depopulate. The DIFFUSE computer code [17] was used to model the observed hydrogen isotope retention properties of Papyex graphite. The detrapping energy was determined to be approximately 4 eV. a value close to that of the C-H bond [18]. The trapping density throughout the bulk of the sample was found to be around 1 X 10m4 atom fraction. The gas only exposure data from fig. 4 is repeated in
El
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1500 I
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TEMPERATURE
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-
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- BACKGROUNDGAS ONLY -
DIFFUSECODE RESULTS
TEMPERATURE Fig. 5. DIFFUSE
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results.
-----I 1700
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,
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i I
- PLASMA& BACKGROUNDGAS
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R.A. Causey, K.L. Wilson / Retention of deuterium and triiium in Papyex graphiie
d
0 0
______L_.._L--__
1
20
10 fi
6
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63
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Fig. 6. Time dependence of deuterium absorption in Papyex graphite at I’273 K and P = 0.66 Pa.
fig. 5 along with the results from the DIFFUSE calculations. The parameters used in the calculations are given in table 1. These same parameters were also used to model the time dependence of the deuterium uptake at 1273 K. Fig. 6 shows a comparison of these results to experimental values determined by tritium dissolution counting of samples exposed to 0.66 Pa of the D/T gas mixture for varying times at 1273 K. As an additional measurement of the deuterium uptake in graphite, samples were also loaded to saturation at a gas pressure of 66 Pa. At 1273 K there was no effective increase in the saturation retention at 66 Pa compared to the 0.66 Pa case shown in fig. 6. This result further confirms that the bulk saturation behavior obTable 1 Parameter used in DIFFUSE code Trap concentration Trap energy Diffusion coefficient Solubility
1.5 X 10m4 atom fraction 4.1 eV 2.0 X 10W6exp[( - 1.0 eV)/kT] 2.0 X 10m6exp[( -0.1 eV)/kT]
served above 1000 K is due to the decoration of a fixed concentration of traps and not due to an intrinsic lattice solubility (which would have scaled with the square root of pressure). Although the parameters given in table 1 do give a good fit to the experimental data, they are certainly not unique. Several other parameter combinations, including diffusivity activation energies as high as 2.5 eV, were seen to give fits that were also close to the experimental data. Our values for the diffusion activation energy are considerably smaller than the 3 to 5 eV range typically reported in the literature [ll-131. Most of these previous studies were conducted with low tritium lattice concentrations, and were hence sensitive to trapping effects. In more recent work, Saeki [19] reported diffusion activation energies between 1.0 and 2.7 eV for pyrolytic carbon, in good agreement with our present results. 3.3. Implications for fusion reactors These measurements have several implications for the use of graphite in fusion reactors. While the Papyex
microstructure is not typical of those found in current fusion devices, its fine grain size and high internal surface area should provide an effective upper limit for the uptake of hydrogen isotopes in graphite (at least in the absence of neutron damage). The high retention at low temperatures, followed by a continuous decrease in the retention with increasing temperature emphasized a potential recycling problem for graphites. Hydrogen isotopes adsorbed at the start of a discharge could be subsequently released as the limiter temperature rises during the later stage of the discharge. This rapid rise in fueling rate could raise the plasma density beyond the disruption limit. Tritium inventory could also be a serious problem of graphite limiters operated at low temperatures where surface adsorption dominates the retention, or at elevated temperatures where bulk migration and trapping occur. However, the tritium retention behavior shown in fig. 1 also suggests an optimum limiter operating temperature range for reduced tritium uptake. At temperatures of around 1000 K, the near surface saturation and surface adsorption on internal porosity are significantly reduced compared to lower temperatures. In addition, 1000 K is not hot enough to allow significant transgranular diffusion of tritium into the bulk graphite. It should be noted that the 1000 K operating temperature fortuitously coincides with a region of reduced hydrogen ion erosion of graphite between the lower temperature hydrocarbon formation peak and the higher temperature onset of radiation damage enhanced sublimation [lo].
4. Summary The retention of 100 eV deuterium or tritium ions in Papyex is characterized by a near surface saturation concentration that decreases with increasing temperature, by adsorption on internal surfaces that also decreased with increasing temperature, and by decoration of high energy traps at temperatures where transgranular diffusion is possible. The NRA results of this study, when adjusted for energy, agree with the results presented by Doyle et al. [l] for near-surface retention. The total deuterium retention results determined by tritium dissolution counting are in fairly good agreement with
similar measurements by Hucks et al. [2]. If the graphite used in fusion reactors has retention properties similar to that for Papyex, it may retain significant quantities of hydrogen isotopes unless the temperature is maintained at approximately 1000 K or a coating is applied to prevent free access to internal porosity.
Acknowledgments The authors would like to thank Wayne Chrisman for his technical assistance and Michael Baskes for his helpful discussions.
References and D.K. Brice, J. Nucl. [II B.L. Doyle. W.R. Wampler Mater. 103 & 104 (1981) 513. PI P. Hucks, K. Flaskamp and E. Vietzke, J. Nucl. Mater. 93 & 94 (1980) 558. [31 P.C. Stangeby. 0. Auciello, A.A. Haasz and B.L. Doyle. J. Nucl. Mater. 122 & 123 (1984) 1592. [41 C. Bockel, J.P. Coulomb and N. DuPont-Pavlovsky. Surf. Sci. 116 (1982) 369. [51 R.A. Kerst, J. Nucl. Mater. 103 & 104 (1981) 439. [61 J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth.
149 (1980) 594. 171 J. Engelhard, USAEC Report. JUL-752-RG (1971). PI R.A. Langley and R.S. Blewer, J. Nucl. Mater. 76 & 77
(1978) 313. D.K. Brice and C.W. Magee, J. Nucl. [91 W.R. Wampler, Mater. 102 (1981) 304. UOI J. Roth, J.B. Roberto and K.L. Wilson, J. Nucl. Mater. 122 & 123 (1984) 1447. [Ill R.A. Causey. T.S. Elleman and K. Verghese. Carbon 17 (1979) 323. u21 M. Saeki, J. Appl. Radiat. Isot. 34 (1983) 739. [I31 H.D. Rohring, P.G. Fischer and R. Hecker. J. Am. Ceram. Sot. 59 (1976) 316.
J. Nucl. [I41 D.K. Brice, B.L. Doyle and W.R. Wampler, Mater. 111 & 112 (1982) 598. 1151 R.M. Barrer, Proc. Sot. London 149A (1935) 253. [161 R.C. Bansal, F.J. Vastola and P.L. Walker. Carbon 9 (1971) 185. u71 M.I. Baskes, DIFFUSE 83, SAND83-8231. 1181 L. Pauling, The Nature of the Chemical Bond, Second Edition (McGraw-Hill, New York, 1940) p. 522ff. 1191 M. Saeki, J. Nucl. Mater. 131 (1985) 32.