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Thermal erosion of graphite by pulsed electron beam
176
Journal
of Nuclear
Materials
179-181
(1991) 1766179 North-Holland
Thermal
erosion of graphite
S. Nishikawa,
by pulsed electron
M. Sawada, Y. Marukawa,
beam
P. Son and M. Miyake
Depurtment of Nucleur Engineering. Fucu1t.v of Engineering. Osaka Unroersity, 2-I Yumadaoka,
Suitu, 565 Osuku, Japun
Electron beam heating tests were performed for several kinds of isotropic graphite to evaluate the dependence of thermal erosion behavior on the graphite species and on the heat load. The specimens were irradiated by electron beams with power densities of 25 to 45 MW/m2 and a pulse length of 2.0 s. A crater caused by erosion was found to be the major damage and preferential erosion was observed at the grain boundaries. In some kinds of graphite, a large weight loss caused by thermal erosion, which exceeded the expected weight loss due to sublimation, was observed. In these cases, some of the ejected bulk graphite particles that had collected on a MO sheet were observed with an optical microscope. These results suggest that some kinds of graphite will be eroded mainly by the “particle emission” mechanism under high heat loads.
1. Introduction High heat flux components such as the divertor, limiter and first wall are subjected to intense surface heat loads in the operation of fusion devices. Even during normal operation of experimental fusion devices, the heat loads of first wall and divertor are estimated to be 0.2-0.3 MW/m’ and 3-4 MW/m2, respectively [l]. Moreover, these components are subjected to higher heat fluxes during plasma disruptions: with present designs, the surface heat flux and the loaded duration time are supposed to be 1000 MW/m2 and 0.1-50 ms, respectively [I -31. Graphite is one of the candidate materials for these high heat flux components because of its low atomic number and its excellent high temperature capability and thermal shock resistance. In the present study, thermal erosion of graphite due to the irradiation of single-pulse electron beam heating was examined to evaluate the dependence of thermal erosion behavior on the graphite species and also on the heat load.
Prior to the heating tests, the graphite specimens, with a size of 9.5 X 9.5 X 1.0 mm3. were mechanically polished with Al,O, suspensions and cleaned in an ultrasonic bath of Ccl, and then degassed at 13OO’C for one hour in a vacuum below 10d4 Pa. The heating tests were performed by using 16 kV single-pulse electron beam irradiation at various heat loads (25-45 MW/m’) with a pulse length of 2.0 seconds. The irradiation area was adjusted using a graphite collimator with 5.0 mm 0 aperture. To minimize the temperature gradient in the graphite specimen, the specimen holder illustrated in fig. 1 was used in this experiment. Furthermore, a polished and cleaned MO sheet having a hole of 7.0 mm 0 was mounted on the graphite collimator to collect emitted particles from the specimens. After the heat load tests, the surface damage of the specimens was examined with an optical microscope, a SEM and a surface roughness gauge, and then weight losses of the specimens were evaluated from the volume reduction measured with the surface roughness gauge.
2. Experimental
3. Results and discussion
Heat load tests by electron beam were carried out for six kinds of isotropic graphite. The fundamental properties of the graphite specimens are shown in table 1.
3. I. Surface
Table 1 Fundamental properties thermal expansion Product name
k
IG-1lOU #781
116 116
G347S AX280K ISO-88OU T-6P
116 116 85 58
0022-3115/91/$03.50
of graphite
specimens.
k: thermal
damage
After the heat load test, a crater erosion
conductivity.
was
p: apparent
observed
density,
caused
of the
irradiated
c: specific
heat and a: coefficient
Ash content
Grain
;J/gW
;lO- 6,K)
(pm)
(pm)
1.77 1.60
0.71
4.6 5.0
2 loo0
14 _
1.80 1.77 1.90 1.91
0.84 0.75 0.71 0.80
5.4 4.8 6.2 6.5
5 10 2 20
i 60 15 5 1
(W/mK)
0 1991 - Elsevier Science Publishers
B.V. (North-Holland)
by thermal
in center
size
area.
of
S. Nishikawa
177
et al. / Thermal erosion of graphite by e-beam
collector
( MO)
graphite collimators
graphite collimator
spccIpc,n.J&/
( Mo)
graphite plate msu
/copper tube
m Fig. 1. Schematic
diagram
However, the size of the crater was quite different, depending on the kind of graphite. Typical SEM surface micrographs of center of the eroded regions are shown in fig. 2. In all specimens, the structure of the crystal
Fig.
2. SEM
ator
of the specimen
holder
grains became relatively dominant after the heating. It can be considered that the substances in the grain boundaries, such as binder and impurities, are preferentially eroded.
surface micrographs of the eroded region of graphite specimen: (a) IG-1lOU (34.8 MW/m’), (37.4 MW/m’), (c) AX280K (37.2 MW/m2) and (d) G347S (39.5 MW/m’). Pulse length: 2.0 s.
(b)
ISO-880U
178
S. Nishikawa et al. / Thermal erosion
ofgraphite by e-beam
MW/m*. In this specimen, microcracks occurred from the eroded region to outer region. These microcracks propagated along the pore structure in the specimen, and thus the crystal grains were clearly observed in the central part of the eroded region, i.e. the left side of micrograph (a). The second micrograph (b) shows the edge of the eroded region, and microcrack propagation along the pores can be observed. The third micrograph (c) shows a grain cluster near the edge of the eroded region. This grain cluster is supposed to be formed by the further erosion along the pore, as seen in (b). Many crystal grains can be seen in the grain cluster, some of which are relatively large and elongated. In a high heat load experiment for various types of graphite using of a hydrogen ion beam, Bolt et al. [4] have reported that materials with a high coefficient of thermal expansion and a fine homogeneous microstructure are prone to the propagation of microcracks, and in materials with a low thermal expansion coefficients and coarse inhomogeneous microstructure it is hard for microcracks to occur. Therefore, by the fact that microcracks at the grain boundaries accelerate the local erosion of graphite, it is expected that the erosion of graphite at high heat loads depends greatly on the character of the pores, grain boundaries and the grain themselves. 3.2. Weight loss
Fig. 3. SEM surface micrographs of the eroded region of specimen IG-11OU (36.9 MW/m’, 2.0 s): (a) microcrack, (b)
edge of eroded region and (c) grain cluster.
The specimens, ISO-880U and T-6P (not shown in fig. 2) have small grains, and AX280K and #781 (not shown in fig. 2) have relatively large grains which are consistent with the catalogue value. In the case of IG-1lOU and G347S, the grain sizes could not be clearly observed; they may be smaller than the catalogue values. The SEM surface micrographs of the boundary of the eroded region are shown in fig. 3. This specimen, IG-llOU, was heated with a power density of 36.9
After the heat load tests by electron beam, the weight losses of the specimens were evaluated from the volume reductions caused by thermal erosion and from the apparent densities of the specimens. The volume reductions were measured with a surface roughness gauge. The dependence of weight loss on heat load presents a different aspect according to the kind of graphite, as shown in fig. 4. The hatched regions in fig. 4 indicate the expected weight losses due to sublimation, which were estimated from the surface temperature of a graphite specimen. The surface temperature of graphite and its variation with time were also calculated based upon a finite element method in the standard heat equation in three dimensions. The graphite AX280K and G347S show a small weight loss, the amount of which can be explained well by the sublimation of the graphite. The graphite #781, through not shown in fig. 4, showed similar behavior to these graphites. In contrast, the graphites IG-11OU and ISO-880U show large weight losses, much larger than that expected due to sublimation, although there is a large scatter of the values of weight loss. The graphite T-6P showed similar behavior to these graphites. These results suggest that the erosion of graphite under high heat loads arises not only by sublimation, but also by a particle emission mechanism.
S. ~i~hjkuwa el al. / Thermal erosion of gf~pb~te by e-beam 1
I
,
r
0
0 0
0 0
s, 30 5
0
0
35
LO
45
0.05
3
0 25
30
Power
35
density
45
( MW/m2 f
Fig 4. Weight loss of graphite specimens after electron beam heating (pulse length: 2.0 s).
To ascertain the contribution of the particle emission mechanism in the erosion of graphite, we tried to detect the bulk graphite particles ejected from the samples, as Bohdansky et al. [5] had done. For this purpose, a MO sheet collector was mounted on collimator, as previously described. We detected some of the bulk graphite particles ejected from the samples. Ejected particles observed with an optical microscope are shown in fig. 5 for the graphite IG-IlOU. Thus, we may conclude that particle emission is the dominant process in the erosion of graphite under high heat loads where the weight loss of graphite greatly exceeds the value expected from sublimation.
179
4. Summary Thermal erosion tests were performed by single-pulse electron beam for six kinds of isotropic graphite. The specimens were heated at various heat loads (25-45 MW/‘m*) a pulse length of 2.0 s. After the heat load test, the surface damage of the specimens was examined and their weight losses were evaluated from their volume reduction. A crater caused by erosion was the major damage and preferential erosion was observed at the grain boundaries. Sublimation weight losses were also estimated from the calculated surface temperatures of graphite and compared with the experimental results. The graphites AX280K, G347S and #781 showed small weight losses caused by thermal erosion and the amounts of weight loss corresponded well to the values expected by sublimation. The graphites IG-IlOU, ISO-8XOU and T-6P showed larger weight losses than the values expected from sublimation. In these cases, some of the ejected bulk particles of graphite that had collected on a MO sheet were observed with an optical microscope. These results suggest that some kinds of graphite would be mainly eroded by the “particle emission” mechanism under high heat loads.
References
K. Miya, M. Seki and M. Araki, J. Atomic Energy Sot. Jpn. 29 (1987) 855. F. Engelmann, M. Chazalon, M.F.A. Harrison, E.S. Hotston, F. Moons and G. Vieider, J. Nucl. Mater. 145-147 (1987) 154. J.M. Dupouy, M.F. Harrison, G. Vieder and C.H. Wu, J. Nucl. Mater. 141 -143 (1986) 19. H. Bolt, C.D. Croessmann, A. Miyahara, T. Kuroda and Y. Oka, Fusion Eng. Des. 6 (19888) 167. J. Bohdansky et al.. Nucl. Instr. and Meth. B23 (1987) 527.
Fig. 5. Emitted graphite particles collected on a MO sheet: (a) IG-1IOU (37.0 MW/m’, 2.0 s) and (b) IG-IlOU (44.1 MW/n?,
2.0 s).
Journal
of Nuclear
Materials
179-181
(1991) 1766179 North-Holland
Thermal
erosion of graphite
S. Nishikawa,
by pulsed electron
M. Sawada, Y. Marukawa,
beam
P. Son and M. Miyake
Depurtment of Nucleur Engineering. Fucu1t.v of Engineering. Osaka Unroersity, 2-I Yumadaoka,
Suitu, 565 Osuku, Japun
Electron beam heating tests were performed for several kinds of isotropic graphite to evaluate the dependence of thermal erosion behavior on the graphite species and on the heat load. The specimens were irradiated by electron beams with power densities of 25 to 45 MW/m2 and a pulse length of 2.0 s. A crater caused by erosion was found to be the major damage and preferential erosion was observed at the grain boundaries. In some kinds of graphite, a large weight loss caused by thermal erosion, which exceeded the expected weight loss due to sublimation, was observed. In these cases, some of the ejected bulk graphite particles that had collected on a MO sheet were observed with an optical microscope. These results suggest that some kinds of graphite will be eroded mainly by the “particle emission” mechanism under high heat loads.
1. Introduction High heat flux components such as the divertor, limiter and first wall are subjected to intense surface heat loads in the operation of fusion devices. Even during normal operation of experimental fusion devices, the heat loads of first wall and divertor are estimated to be 0.2-0.3 MW/m’ and 3-4 MW/m2, respectively [l]. Moreover, these components are subjected to higher heat fluxes during plasma disruptions: with present designs, the surface heat flux and the loaded duration time are supposed to be 1000 MW/m2 and 0.1-50 ms, respectively [I -31. Graphite is one of the candidate materials for these high heat flux components because of its low atomic number and its excellent high temperature capability and thermal shock resistance. In the present study, thermal erosion of graphite due to the irradiation of single-pulse electron beam heating was examined to evaluate the dependence of thermal erosion behavior on the graphite species and also on the heat load.
Prior to the heating tests, the graphite specimens, with a size of 9.5 X 9.5 X 1.0 mm3. were mechanically polished with Al,O, suspensions and cleaned in an ultrasonic bath of Ccl, and then degassed at 13OO’C for one hour in a vacuum below 10d4 Pa. The heating tests were performed by using 16 kV single-pulse electron beam irradiation at various heat loads (25-45 MW/m’) with a pulse length of 2.0 seconds. The irradiation area was adjusted using a graphite collimator with 5.0 mm 0 aperture. To minimize the temperature gradient in the graphite specimen, the specimen holder illustrated in fig. 1 was used in this experiment. Furthermore, a polished and cleaned MO sheet having a hole of 7.0 mm 0 was mounted on the graphite collimator to collect emitted particles from the specimens. After the heat load tests, the surface damage of the specimens was examined with an optical microscope, a SEM and a surface roughness gauge, and then weight losses of the specimens were evaluated from the volume reduction measured with the surface roughness gauge.
2. Experimental
3. Results and discussion
Heat load tests by electron beam were carried out for six kinds of isotropic graphite. The fundamental properties of the graphite specimens are shown in table 1.
3. I. Surface
Table 1 Fundamental properties thermal expansion Product name
k
IG-1lOU #781
116 116
G347S AX280K ISO-88OU T-6P
116 116 85 58
0022-3115/91/$03.50
of graphite
specimens.
k: thermal
damage
After the heat load test, a crater erosion
conductivity.
was
p: apparent
observed
density,
caused
of the
irradiated
c: specific
heat and a: coefficient
Ash content
Grain
;J/gW
;lO- 6,K)
(pm)
(pm)
1.77 1.60
0.71
4.6 5.0
2 loo0
14 _
1.80 1.77 1.90 1.91
0.84 0.75 0.71 0.80
5.4 4.8 6.2 6.5
5 10 2 20
i 60 15 5 1
(W/mK)
0 1991 - Elsevier Science Publishers
B.V. (North-Holland)
by thermal
in center
size
area.
of
S. Nishikawa
177
et al. / Thermal erosion of graphite by e-beam
collector
( MO)
graphite collimators
graphite collimator
spccIpc,n.J&/
( Mo)
graphite plate msu
/copper tube
m Fig. 1. Schematic
diagram
However, the size of the crater was quite different, depending on the kind of graphite. Typical SEM surface micrographs of center of the eroded regions are shown in fig. 2. In all specimens, the structure of the crystal
Fig.
2. SEM
ator
of the specimen
holder
grains became relatively dominant after the heating. It can be considered that the substances in the grain boundaries, such as binder and impurities, are preferentially eroded.
surface micrographs of the eroded region of graphite specimen: (a) IG-1lOU (34.8 MW/m’), (37.4 MW/m’), (c) AX280K (37.2 MW/m2) and (d) G347S (39.5 MW/m’). Pulse length: 2.0 s.
(b)
ISO-880U
178
S. Nishikawa et al. / Thermal erosion
ofgraphite by e-beam
MW/m*. In this specimen, microcracks occurred from the eroded region to outer region. These microcracks propagated along the pore structure in the specimen, and thus the crystal grains were clearly observed in the central part of the eroded region, i.e. the left side of micrograph (a). The second micrograph (b) shows the edge of the eroded region, and microcrack propagation along the pores can be observed. The third micrograph (c) shows a grain cluster near the edge of the eroded region. This grain cluster is supposed to be formed by the further erosion along the pore, as seen in (b). Many crystal grains can be seen in the grain cluster, some of which are relatively large and elongated. In a high heat load experiment for various types of graphite using of a hydrogen ion beam, Bolt et al. [4] have reported that materials with a high coefficient of thermal expansion and a fine homogeneous microstructure are prone to the propagation of microcracks, and in materials with a low thermal expansion coefficients and coarse inhomogeneous microstructure it is hard for microcracks to occur. Therefore, by the fact that microcracks at the grain boundaries accelerate the local erosion of graphite, it is expected that the erosion of graphite at high heat loads depends greatly on the character of the pores, grain boundaries and the grain themselves. 3.2. Weight loss
Fig. 3. SEM surface micrographs of the eroded region of specimen IG-11OU (36.9 MW/m’, 2.0 s): (a) microcrack, (b)
edge of eroded region and (c) grain cluster.
The specimens, ISO-880U and T-6P (not shown in fig. 2) have small grains, and AX280K and #781 (not shown in fig. 2) have relatively large grains which are consistent with the catalogue value. In the case of IG-1lOU and G347S, the grain sizes could not be clearly observed; they may be smaller than the catalogue values. The SEM surface micrographs of the boundary of the eroded region are shown in fig. 3. This specimen, IG-llOU, was heated with a power density of 36.9
After the heat load tests by electron beam, the weight losses of the specimens were evaluated from the volume reductions caused by thermal erosion and from the apparent densities of the specimens. The volume reductions were measured with a surface roughness gauge. The dependence of weight loss on heat load presents a different aspect according to the kind of graphite, as shown in fig. 4. The hatched regions in fig. 4 indicate the expected weight losses due to sublimation, which were estimated from the surface temperature of a graphite specimen. The surface temperature of graphite and its variation with time were also calculated based upon a finite element method in the standard heat equation in three dimensions. The graphite AX280K and G347S show a small weight loss, the amount of which can be explained well by the sublimation of the graphite. The graphite #781, through not shown in fig. 4, showed similar behavior to these graphites. In contrast, the graphites IG-11OU and ISO-880U show large weight losses, much larger than that expected due to sublimation, although there is a large scatter of the values of weight loss. The graphite T-6P showed similar behavior to these graphites. These results suggest that the erosion of graphite under high heat loads arises not only by sublimation, but also by a particle emission mechanism.
S. ~i~hjkuwa el al. / Thermal erosion of gf~pb~te by e-beam 1
I
,
r
0
0 0
0 0
s, 30 5
0
0
35
LO
45
0.05
3
0 25
30
Power
35
density
45
( MW/m2 f
Fig 4. Weight loss of graphite specimens after electron beam heating (pulse length: 2.0 s).
To ascertain the contribution of the particle emission mechanism in the erosion of graphite, we tried to detect the bulk graphite particles ejected from the samples, as Bohdansky et al. [5] had done. For this purpose, a MO sheet collector was mounted on collimator, as previously described. We detected some of the bulk graphite particles ejected from the samples. Ejected particles observed with an optical microscope are shown in fig. 5 for the graphite IG-IlOU. Thus, we may conclude that particle emission is the dominant process in the erosion of graphite under high heat loads where the weight loss of graphite greatly exceeds the value expected from sublimation.
179
4. Summary Thermal erosion tests were performed by single-pulse electron beam for six kinds of isotropic graphite. The specimens were heated at various heat loads (25-45 MW/‘m*) a pulse length of 2.0 s. After the heat load test, the surface damage of the specimens was examined and their weight losses were evaluated from their volume reduction. A crater caused by erosion was the major damage and preferential erosion was observed at the grain boundaries. Sublimation weight losses were also estimated from the calculated surface temperatures of graphite and compared with the experimental results. The graphites AX280K, G347S and #781 showed small weight losses caused by thermal erosion and the amounts of weight loss corresponded well to the values expected by sublimation. The graphites IG-IlOU, ISO-8XOU and T-6P showed larger weight losses than the values expected from sublimation. In these cases, some of the ejected bulk particles of graphite that had collected on a MO sheet were observed with an optical microscope. These results suggest that some kinds of graphite would be mainly eroded by the “particle emission” mechanism under high heat loads.
References
K. Miya, M. Seki and M. Araki, J. Atomic Energy Sot. Jpn. 29 (1987) 855. F. Engelmann, M. Chazalon, M.F.A. Harrison, E.S. Hotston, F. Moons and G. Vieider, J. Nucl. Mater. 145-147 (1987) 154. J.M. Dupouy, M.F. Harrison, G. Vieder and C.H. Wu, J. Nucl. Mater. 141 -143 (1986) 19. H. Bolt, C.D. Croessmann, A. Miyahara, T. Kuroda and Y. Oka, Fusion Eng. Des. 6 (19888) 167. J. Bohdansky et al.. Nucl. Instr. and Meth. B23 (1987) 527.
Fig. 5. Emitted graphite particles collected on a MO sheet: (a) IG-1IOU (37.0 MW/m’, 2.0 s) and (b) IG-IlOU (44.1 MW/n?,
2.0 s).