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Irradiation damage to pyrolytic graphites at very high temperatures
Carbon. 1973, Vol. 11, pp. 505-510.
Pergamon
Press.
Printed in Great Britain
IRRADIATION DAMAGE TO PYROLYTIC GRAPHITES AT VERY HlGH TEMPERATURES Department
D. D. BUBLEIGH and P. A. THROWER The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.
of Material Sciences,
(Received 17 April 1973) Six pyrolytic graphites of widely differing crystallite sizes (L, = l-120 pm) were irradiated at temperatures of 1020°C and 1475°C. The resulting damage was studied by transmission electron microscopy. Damage produced at 1020°C can be understood in terms of present nucleation ideas, assuming crystallite boundaries interfere with homogeneous nucleation when L, is smaller than the separation of homogeneously nucleated defects. At the higher temperature there is an increase in defect density rather than a decrease as observed over the temperature range 150-1200°C. It is proposed that this is caused by the nucleating cluster being larger than in the lower temperature range, and hence having a larger migration activation energy, approximately 2 eV as opposed to 1.2 eV.
Abstract-
The second modifying factor is the presence of impurity atoms which may also act as nuclei for defect growth. The latter effect was extensively studied by Mayer et al. [3] and is of no concern for this present work. The effect of crystallite size is evident in reactor graphites at temperatures as low as 300°C [2] and at much higher temperatures [< lOOO”C] some studies have been made with pyrolytic graphites[4] as well as with reactor graphites[5]. It is evident from these latter studies that defect migration may also occur perpendicular to the basal planes with the result that defects nucleate at basal plane twist boundaries. As a result of these investigations it was clear that more work would be necessary to elucidate the dependence of nucleation behavior on crystal structure. An obvious type of material for such studies is pyrolytic graphite which may be obtained with a wide range of crystallite sizes stretching from the small values found in polycrystalline reactor graphites to the high values usually found in single crystals. It is with this background that this study was initiated.
1. INTRODUCTION The analysis of neutron irradiation damage in graphite by electron microscopy has been the subject of many publications in the past twelve years. A summary of important results obtained using graphite single crystals has been given by Thrower [l] and present ideas deviate very little from those given by Reynolds and Thrower [2] in 1965. Their analysis indicated that in pure single crystals the nucleation behavior was controlled by a diffusion activation energy, E, of 1.2 eV. According to their model of nucleation and growth the mean defect separation (27-J in a basal plane is related to the irradiation temperature (7J and flux (4) by
Somewhat surprisingly the activation energy of 1.2 eV was applicable to the entire range of temperatures examined, viz. 150-120°C. The above considerations break down under two conditions. In polycrystalline graphites the nucleation separation may be larger than the actual crystallite size (L,), so that nuclei form at the crystallite boundaries. 505
506
D. D. BURLEIGH and P. A. THROWER 2. EXPERIMENTAL
2.1 Clystallite size determination The crystallite sizes of pyrolytic graphites are generally larger than may be readily measured by X-ray techniques. Consequently the oxygen etch technique to reveal crystallite boundaries was used [6]. The materials were etched in oxygen at 750°C for 15-20 min using a pressure of 30 Torr and then examined in an optical microscope. This technique reveals not only crystallite boundaries but the points of emergence of nonbasal defects. A wide range of materials was selected for irradiation and their crystallite sizes and non-basal defect densities are given in Table 1. The data were obtained directly from micrographs such as shown in Fig. 1. 2.2 Irradiation data. Samples of the materials of approximate dimensions 2 X 1 X 5 mm. were cut using a razor blade and irradiated in capsules machined from reactor graphite. A summary of irradiation details is given in
Table 2. Unfortunately the dose received at 1475°C was twenty times greater than at 1020°C which made comparison and evaluation of the results rather difficult. It should be pointed out that these studies are the first on materials irradiated at such a high temperature. 2.3 Specimen preparation and examination. Thin sections for electron microscopy were prepared by normal cleavage techniques although the amount of damage contained in the higher dose materials made cleavage difficult and sometimes very small areas were all that could be observed at any one time. Specimens were examined in a Philips EM300 electron microscope using the { lOi1) dark field viewing modes, except for defect identification when a (11%) reflection was used (I 2 2) [7]. Samples were examined both as-irradiated and after one hour anneals at various temperatures in a carbon tube furnace in a helium atmosphere.
Table 1. Materials used for irradiation studies
Designation MR8 GE36OOt 3100P HTA APG12 3600P
Material Treatment after deposition
Source Union Carbide General Electric High Temp Materials Penn State Univ. Union Carbide High Temp. Materials
Density of Crystallite non-basal defects size (pm) ( cmm2)
Hot pressed around 2800-3000°C Hot pressed at 3300-3600°C Heated to 3100°C Hot pressed at 2900°C (see Ref. [S]) Hot pressed above 3000°C Heated to around 3600°C
- 1 10 15 5-100 100 120
3x 2x 3x 8X 5x
107 lo8 lo6 105 104 lo4
tThis material was irradiated earlier at 1350°C; results are given in (4), where it is denoted 3600P. Table 2. Irradiation data Irradiation temperature (“C)
Neutron dose (ncm-“, E > 0.18 MeV)
Where performed
1020
2.4 x lO*O
2x 106
1475
5.2 x lo*’
8.4 x lo6
A.E.R.E. Harwell, U.K. PLUTO Reactor Idaho Falls, U.S.A. ETR Reactor
Fig. 1. Optical micrographs of basal plane surfaces of unirradiated pyro!ytic graphites etched in oxygen to reveal crystallite boundaries and non-basal defects. a. 31OOP Crystalhtes of uniform size (- 15 pm). b. APG12 Relatively perfect material, large crystallites (- 100 pm).
Fig. 2. Electron micrographs of graphites irradiated at 1020°C. Dark field micrographs taken using a { lOi1) reflection. a. APG‘LZ Few defects evenly distributed, b. MR8 Defects arranged on a linear network, c. 3600P Randomly distributed defects but higher density.
Fig,3. Electron micrographs of graphites irradiated at 1475°C. Dark field micrographs taken using a { 101 l} reflection. a. APG12 Damage is very dense and individual defects are barely discernable, b. MR8 Large interstitial loops are clearly visible.
Fig. 4. Electron micrographs of graphites irradiated at 1475°C and annealed for one hour at 2500°C. Dark field micrographs taken using a { 1121) reflection. a. APG12 Large remaining loops are in clusters and all loops in any cluster are of either vacancy or interstitial character, b. MR8 No remaining point defect clusters but an abnormally high dislocation density, compare with ‘c’, c. MR8 Unirradiated specimen for comparison with ‘6’.
IRRADIATION
DAMAGE
TO
3. RESULTS
3.1 IV%&atio?U at 1020°C These irradiations were the more easy to examine because of the much lower total neutron dose. The scale of defect nucleation varied quite markedly between materials and a summary of observations is given in Table 3, together with estimates of defect radii (r,). In samples MRS, 3lOOP and GE3600 defects are mainly nucleated along lines delineating areas corresponding approximately to the crystallite sizes given in Table 1. Diffraction analysis showed these lines to be crystallite boundaries. In contrast the damage was nucleated uniformly in materials APGlP and 36OOP, which had larger crystallite sizes. The HTA material exhibited a combination of both behaviors because of its wider range of crystallite sizes. Some typical examples of these effects are shown in Fig. 2. It should be noted that the familiar arrangement [2] of large interstitial loops decorated by smaller vacancy loops is present in all these observations. Three samples (MR8, HTA and 3100P) were examined after annealing at 2075°C. Table
3. Defects observed in materials after irradiation at 1020°C
Material
Defect characteristics
MR8
Circular defects (r, - 75 A); the larger number of which are on a linear network of the same size as the crystallites.
GE3600 3100P HTA
APG12
3600P
I
PYROLYTIC
507
The two materials with the smaller crystallite sizes contained few remaining defects while a good number of well defined circular loops remained in HTA. A few fainter twist boundary loops [4] were also observed. 3.2 Irradiations at 1475°C. The as-irradiated materials were very difficult to examine because of the complex damage they contained, however there is an obvious difference from irradiations at 1020°C. Most startling is the fact that APG12 is now the most highly damaged material whereas at 1020°C it was the least damaged. The least damaged material at 1475°C is GE3600 which contained a relatively complex damage structure at the lower temperature. A summary of all observations is given in Table 4 and some examples of observations are given in Fig. 3. Some samples were further examined after anneals at 2000°C and 2500°C. For the 2000°C anneal only the two samples with the clearest damage in the as-irradiated state (GE3600 and HTA) were examined. The damage was reasonably clear in GE3600 but still complex in HTA so that a higher annealing temperaTable
4.
Material# MR8 GE3600 HTA
Nearly all defects in lines. r0 - 22Ow. In some crystallites nearly all defects in lines. In others defects are isolated and large (rO- 500 A) with r1 - 3 km. They also have some hexagonal character. Defects are somewhat hexagonal; pm. The damage r~-3ooi%,r,-3 here was most like that previously observed in single crystals at this temperature. Very similar to APCl2 but with much more retained damage; r0 250 A, r, - 0.9 pm.
GRAPHITES
APG12 36OOP
Defects observed in materials irradiation at 1475°C
after
Defect characteristicst Indications of loops of r0 to 750 A (4)“. The clearest damage. Loops with r0 to 10008, visible [5]. Possibly slightly less damaged than P200 but very similar to appearance
[31. The most diffuse images. Loop radii possibly up to 400 8, [ 11. High density of loops of r0 to 5OOw[2].
“Numbers in parentheses indicate the order of increasing defect clarity. tAl1 specimens were very highly damaged and the differences given here are mainly the observer’s impressions rather than the results of physical measurements. #Sample 31OOP was lost during handling after the irradiation.
508
D. D. BURLEIGH
and P. A. THROWER
ture was used. After an anneal at 2500°C most of the damage had been eliminated but in many cases had been replaced by an abnormally high basal plane dislocation density. A few loops remained in the 3600P material with a slightly higher density in both HTA and APG12. These loops were usually in clusters, each cluster being exclusively of either vacancy or interstitial type. A summary of the annealing results is given in Table 5 with corresponding micrographs in Fig. 4. 4.
DISCUSSION
Using the theory outlined in the Introduction and the irradiation data of Table 2 one can calculate the expected values of rl for the two irradiations assuming E remains constant at 1*2eV, and that the value of r, at 1200°C is 2.8 pm for 4 = 4.3 X 1013n. cme2 see-*[2]. One then finds that r, = 1.6 pm at 1020°C and 2.1 pm at 1475°C. If the above values were true two conclusions could be made. The nucleation behavior in any material for the two irradiation temperatures should be approximately the same, and grain boundary nucleation should
only occur in crystallites whose basal plane dimension is less than - 3 pm. Specimens irradiated at 1020°C contained defects nucleated on crystallite boundaries on a scale of - 1 pm, which is in fair agreement with the above prediction. The crystallite sizes measured by the etch technique were around four to five times greater, indicating, perhaps, that there are some boundaries not revealed by this technique. The only anomalous behavior exhibited here is that 3600P contains an order of magnitude more damage than APGl2, although both are apparently homogeneously nucleated. This difference is not understood but may be due to an impurity effect [3]. In specimens irradiated at 1475°C one would predict a similar nucleation scale but the defects to be much larger due to the higher dose. Assuming r0 to be proportional to the square root of dose p] one would expect a value of around 135OA. Examination shows that both rl and r0 are much smaller than predicted. According to nucleation theory a lower value of rl can only be accomplished by raising the value of E. Homo-
Table 5. Effect of annealing on samples irradiated at 1475°C Defect character Material
Annealed at 2000°C 1 hr
MR8
Not examined
GE3600
HTA
Only 20% of defects in HTA but same size distribution, some on lines r, - 200-15OOA; complex
APG12
Not examined
3600P
Not examined
Annealed at 2500°C 1 hr As if unirradiated but an order of magnitude higher dislocation density No distinct loops visible. Essentially as unirradiated Few remaining loops, r,, up to 5OOoA Few large loops remaining, both vacancy and interstitial, in clusters of same type. r, 500-3500 A. Relatively high dislocation density. Very few remaining signs of irradiation but fairly high dislocation density similar to APG12
IRRADIATION
DAMAGE
TO
nucleation would certainly be genous expected in APG12, and from several micrographs it is estimated that rl is around 0*50.8 pm in this material, corresponding to a range of E values 1.7-2.0 eV. The observed damage patterns may now be rationalized. In samples 3600P and APG12 the nucleation is homogeneous and similar because of the large crystallite sizes. Possible impurity effects in 3600P are now masked by the much lower rl value for homogeneous nucleation. Some nucleation even occurs within crystallites of MR8 now because 2r, -L, and in this case the value of r0 is approximately the same as for the most perfect materials. The defects in HTA are intermediate between these two extremes because of the wider spread in crystallite size. Surprisingly the clearest damage in the form of large resolvable loops is found in GE3600. The reason for this is not absolutely clear but is believed to be related to another structural feature. Examination of GE3600 irradiated at 1350”[4] revealed a high concentration of twist boundary interstitial loops, i.e. loops situated within twist boundaries. It is reasonable to assume that the influences present in this earlier experiment might be important here. The piping-off of defects to twist boundaries via non-basal defects would leave a smaller concentration of defects within the crystals which would therefore present a clearer picture. However, in spite of this the defects remaining for such a high dose are quite dense and would mask the preseIlce of the fainter twist boundary loops. The presence of such loops was confirmed by examining the sample after annealing for 1 hr at 2000°C when they were clearly visible. The higher value of E at 1475°C indicates that a larger defect cluster is responsible for nucleation at the higher temperature. This is hardly surprising. What is surprising is that E remains constant over the wide temperature range 150-1200°C. It has been suggested
PYROLYTIC
GRAPHITES
509
[2] that such a value corresponds to a group of three or four atoms which, when two come together, form a stable hexagon. No further migration then occurs. If this hypothesis is true then it would appear that at a temperature between 135o”C[4] and 1475°C a much larger cluster is required to form a stable nucleus. The effect of annealing is also simply related to crystallite size in each case. For a given annealing treatment, the percentage of defects annealed out of a material is larger, the smaller its crystallite size. This indicates that the majority of the defect annihilation occurs at crystallite boundaries where the point defect concentration is approximately zero. In the smaller crystallite materials this would produce a much higher point defect concentration gradient between the boundary and the emitting loop, so that the number of defects diffusing from the loop would be larger and hence the loop would anneal more rapidly. Given a situation in which defect nucleation may occur homogeneously at an irradiation temperature of 1475°C the process will be governed by an activation energy approaching 2 eV. Since equivalent nucleation patterns require the same values of E/T, where T is the absolute temperature, a temperature of 1475°C with E = 2 eV would be equivalent to 775°C with E = 1.2 eV. However, in real graphites of interest, i.e. nuclear graphites, L, is around 0.3 pm so that at 1475°C one would expect nucleation not to be homogeneous but to be more influenced by grain boundaries and non-basal defects. In addition any macroscopic effects would be influenced by interactions between neighboring misaligned crystallites. It would certainly be of fundamental interest to measure irradiation effects in well oriented pyrolytic graphites irradiated at these two temperatures to see whether there were in fact any similarities. Results on irradiation effects in nuclear should be more readily available in the near
510
D. D. BURLEIGH
future and it will be interesting to see whether there are any discontinuities in property changes with irradiation temperature at around 1400°C such as might be caused by a finer scale of nucleation, i.e., higher value of E.
5. CONCLUSIONS a. At the temperatures investigated there is a distinct dependence of nucleation behavior on the microstructure of the irradiated material. b. The homogeneous nucleation theory of Reynolds and Thrower[2] may be applied to temperatures up to 1475°C but a higher value of E becomes necessary somewhere above 1350°C. This value appears to be around 2 eV. c. The ease of defect annealing is related to crystallite size in that the smaller the crystallites the more rapid the annealing.
and P. A. THROWER Acknowled m&-We
wish to thank Mr. A. J. Perks of i .E _R.E. Harwell and Dr. W. J. Gray of the Pacific Northwest Laboratory, Hanford, Washington for arranging the irradiations, and to acknowledge the help of Mr. David Sorg in the crystallite size determinations. The financial support of The Atomic Energy Commission, Contract AT (30-l)-1710, is deeply appreciated.
REFERENCES 1. Thrower P. A., Chem+y and Physics of Carbon (Ed Walker P. L. Jr.), Vol. 5, p. 217. Dekker, New York (1969). 2. Reynolds W. N. and Thrower P. A., Phil. Mug. 14,573 (1965). 3. Mayer R. M., Brown L. M. and Kelly A., Phil. Mug. 19,701 (1969). 4. Thrower P. A., Carbon 6,687 (1968). 5. Thrower P. A., Carbon 9,265 (1971). 6. Roscoe C. and Baker J., J. Appl. Phys. 40, 1665
(1969). 7. Turnbull J. A. and Stagg M. 8, Phil. Mug. 14, 1049 (1966). 8. Roscoe C., Kline D. E. and Taylor R. E., Carbon 8,95 (1970).
Pergamon
Press.
Printed in Great Britain
IRRADIATION DAMAGE TO PYROLYTIC GRAPHITES AT VERY HlGH TEMPERATURES Department
D. D. BUBLEIGH and P. A. THROWER The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.
of Material Sciences,
(Received 17 April 1973) Six pyrolytic graphites of widely differing crystallite sizes (L, = l-120 pm) were irradiated at temperatures of 1020°C and 1475°C. The resulting damage was studied by transmission electron microscopy. Damage produced at 1020°C can be understood in terms of present nucleation ideas, assuming crystallite boundaries interfere with homogeneous nucleation when L, is smaller than the separation of homogeneously nucleated defects. At the higher temperature there is an increase in defect density rather than a decrease as observed over the temperature range 150-1200°C. It is proposed that this is caused by the nucleating cluster being larger than in the lower temperature range, and hence having a larger migration activation energy, approximately 2 eV as opposed to 1.2 eV.
Abstract-
The second modifying factor is the presence of impurity atoms which may also act as nuclei for defect growth. The latter effect was extensively studied by Mayer et al. [3] and is of no concern for this present work. The effect of crystallite size is evident in reactor graphites at temperatures as low as 300°C [2] and at much higher temperatures [< lOOO”C] some studies have been made with pyrolytic graphites[4] as well as with reactor graphites[5]. It is evident from these latter studies that defect migration may also occur perpendicular to the basal planes with the result that defects nucleate at basal plane twist boundaries. As a result of these investigations it was clear that more work would be necessary to elucidate the dependence of nucleation behavior on crystal structure. An obvious type of material for such studies is pyrolytic graphite which may be obtained with a wide range of crystallite sizes stretching from the small values found in polycrystalline reactor graphites to the high values usually found in single crystals. It is with this background that this study was initiated.
1. INTRODUCTION The analysis of neutron irradiation damage in graphite by electron microscopy has been the subject of many publications in the past twelve years. A summary of important results obtained using graphite single crystals has been given by Thrower [l] and present ideas deviate very little from those given by Reynolds and Thrower [2] in 1965. Their analysis indicated that in pure single crystals the nucleation behavior was controlled by a diffusion activation energy, E, of 1.2 eV. According to their model of nucleation and growth the mean defect separation (27-J in a basal plane is related to the irradiation temperature (7J and flux (4) by
Somewhat surprisingly the activation energy of 1.2 eV was applicable to the entire range of temperatures examined, viz. 150-120°C. The above considerations break down under two conditions. In polycrystalline graphites the nucleation separation may be larger than the actual crystallite size (L,), so that nuclei form at the crystallite boundaries. 505
506
D. D. BURLEIGH and P. A. THROWER 2. EXPERIMENTAL
2.1 Clystallite size determination The crystallite sizes of pyrolytic graphites are generally larger than may be readily measured by X-ray techniques. Consequently the oxygen etch technique to reveal crystallite boundaries was used [6]. The materials were etched in oxygen at 750°C for 15-20 min using a pressure of 30 Torr and then examined in an optical microscope. This technique reveals not only crystallite boundaries but the points of emergence of nonbasal defects. A wide range of materials was selected for irradiation and their crystallite sizes and non-basal defect densities are given in Table 1. The data were obtained directly from micrographs such as shown in Fig. 1. 2.2 Irradiation data. Samples of the materials of approximate dimensions 2 X 1 X 5 mm. were cut using a razor blade and irradiated in capsules machined from reactor graphite. A summary of irradiation details is given in
Table 2. Unfortunately the dose received at 1475°C was twenty times greater than at 1020°C which made comparison and evaluation of the results rather difficult. It should be pointed out that these studies are the first on materials irradiated at such a high temperature. 2.3 Specimen preparation and examination. Thin sections for electron microscopy were prepared by normal cleavage techniques although the amount of damage contained in the higher dose materials made cleavage difficult and sometimes very small areas were all that could be observed at any one time. Specimens were examined in a Philips EM300 electron microscope using the { lOi1) dark field viewing modes, except for defect identification when a (11%) reflection was used (I 2 2) [7]. Samples were examined both as-irradiated and after one hour anneals at various temperatures in a carbon tube furnace in a helium atmosphere.
Table 1. Materials used for irradiation studies
Designation MR8 GE36OOt 3100P HTA APG12 3600P
Material Treatment after deposition
Source Union Carbide General Electric High Temp Materials Penn State Univ. Union Carbide High Temp. Materials
Density of Crystallite non-basal defects size (pm) ( cmm2)
Hot pressed around 2800-3000°C Hot pressed at 3300-3600°C Heated to 3100°C Hot pressed at 2900°C (see Ref. [S]) Hot pressed above 3000°C Heated to around 3600°C
- 1 10 15 5-100 100 120
3x 2x 3x 8X 5x
107 lo8 lo6 105 104 lo4
tThis material was irradiated earlier at 1350°C; results are given in (4), where it is denoted 3600P. Table 2. Irradiation data Irradiation temperature (“C)
Neutron dose (ncm-“, E > 0.18 MeV)
Where performed
1020
2.4 x lO*O
2x 106
1475
5.2 x lo*’
8.4 x lo6
A.E.R.E. Harwell, U.K. PLUTO Reactor Idaho Falls, U.S.A. ETR Reactor
Fig. 1. Optical micrographs of basal plane surfaces of unirradiated pyro!ytic graphites etched in oxygen to reveal crystallite boundaries and non-basal defects. a. 31OOP Crystalhtes of uniform size (- 15 pm). b. APG12 Relatively perfect material, large crystallites (- 100 pm).
Fig. 2. Electron micrographs of graphites irradiated at 1020°C. Dark field micrographs taken using a { lOi1) reflection. a. APG‘LZ Few defects evenly distributed, b. MR8 Defects arranged on a linear network, c. 3600P Randomly distributed defects but higher density.
Fig,3. Electron micrographs of graphites irradiated at 1475°C. Dark field micrographs taken using a { 101 l} reflection. a. APG12 Damage is very dense and individual defects are barely discernable, b. MR8 Large interstitial loops are clearly visible.
Fig. 4. Electron micrographs of graphites irradiated at 1475°C and annealed for one hour at 2500°C. Dark field micrographs taken using a { 1121) reflection. a. APG12 Large remaining loops are in clusters and all loops in any cluster are of either vacancy or interstitial character, b. MR8 No remaining point defect clusters but an abnormally high dislocation density, compare with ‘c’, c. MR8 Unirradiated specimen for comparison with ‘6’.
IRRADIATION
DAMAGE
TO
3. RESULTS
3.1 IV%&atio?U at 1020°C These irradiations were the more easy to examine because of the much lower total neutron dose. The scale of defect nucleation varied quite markedly between materials and a summary of observations is given in Table 3, together with estimates of defect radii (r,). In samples MRS, 3lOOP and GE3600 defects are mainly nucleated along lines delineating areas corresponding approximately to the crystallite sizes given in Table 1. Diffraction analysis showed these lines to be crystallite boundaries. In contrast the damage was nucleated uniformly in materials APGlP and 36OOP, which had larger crystallite sizes. The HTA material exhibited a combination of both behaviors because of its wider range of crystallite sizes. Some typical examples of these effects are shown in Fig. 2. It should be noted that the familiar arrangement [2] of large interstitial loops decorated by smaller vacancy loops is present in all these observations. Three samples (MR8, HTA and 3100P) were examined after annealing at 2075°C. Table
3. Defects observed in materials after irradiation at 1020°C
Material
Defect characteristics
MR8
Circular defects (r, - 75 A); the larger number of which are on a linear network of the same size as the crystallites.
GE3600 3100P HTA
APG12
3600P
I
PYROLYTIC
507
The two materials with the smaller crystallite sizes contained few remaining defects while a good number of well defined circular loops remained in HTA. A few fainter twist boundary loops [4] were also observed. 3.2 Irradiations at 1475°C. The as-irradiated materials were very difficult to examine because of the complex damage they contained, however there is an obvious difference from irradiations at 1020°C. Most startling is the fact that APG12 is now the most highly damaged material whereas at 1020°C it was the least damaged. The least damaged material at 1475°C is GE3600 which contained a relatively complex damage structure at the lower temperature. A summary of all observations is given in Table 4 and some examples of observations are given in Fig. 3. Some samples were further examined after anneals at 2000°C and 2500°C. For the 2000°C anneal only the two samples with the clearest damage in the as-irradiated state (GE3600 and HTA) were examined. The damage was reasonably clear in GE3600 but still complex in HTA so that a higher annealing temperaTable
4.
Material# MR8 GE3600 HTA
Nearly all defects in lines. r0 - 22Ow. In some crystallites nearly all defects in lines. In others defects are isolated and large (rO- 500 A) with r1 - 3 km. They also have some hexagonal character. Defects are somewhat hexagonal; pm. The damage r~-3ooi%,r,-3 here was most like that previously observed in single crystals at this temperature. Very similar to APCl2 but with much more retained damage; r0 250 A, r, - 0.9 pm.
GRAPHITES
APG12 36OOP
Defects observed in materials irradiation at 1475°C
after
Defect characteristicst Indications of loops of r0 to 750 A (4)“. The clearest damage. Loops with r0 to 10008, visible [5]. Possibly slightly less damaged than P200 but very similar to appearance
[31. The most diffuse images. Loop radii possibly up to 400 8, [ 11. High density of loops of r0 to 5OOw[2].
“Numbers in parentheses indicate the order of increasing defect clarity. tAl1 specimens were very highly damaged and the differences given here are mainly the observer’s impressions rather than the results of physical measurements. #Sample 31OOP was lost during handling after the irradiation.
508
D. D. BURLEIGH
and P. A. THROWER
ture was used. After an anneal at 2500°C most of the damage had been eliminated but in many cases had been replaced by an abnormally high basal plane dislocation density. A few loops remained in the 3600P material with a slightly higher density in both HTA and APG12. These loops were usually in clusters, each cluster being exclusively of either vacancy or interstitial type. A summary of the annealing results is given in Table 5 with corresponding micrographs in Fig. 4. 4.
DISCUSSION
Using the theory outlined in the Introduction and the irradiation data of Table 2 one can calculate the expected values of rl for the two irradiations assuming E remains constant at 1*2eV, and that the value of r, at 1200°C is 2.8 pm for 4 = 4.3 X 1013n. cme2 see-*[2]. One then finds that r, = 1.6 pm at 1020°C and 2.1 pm at 1475°C. If the above values were true two conclusions could be made. The nucleation behavior in any material for the two irradiation temperatures should be approximately the same, and grain boundary nucleation should
only occur in crystallites whose basal plane dimension is less than - 3 pm. Specimens irradiated at 1020°C contained defects nucleated on crystallite boundaries on a scale of - 1 pm, which is in fair agreement with the above prediction. The crystallite sizes measured by the etch technique were around four to five times greater, indicating, perhaps, that there are some boundaries not revealed by this technique. The only anomalous behavior exhibited here is that 3600P contains an order of magnitude more damage than APGl2, although both are apparently homogeneously nucleated. This difference is not understood but may be due to an impurity effect [3]. In specimens irradiated at 1475°C one would predict a similar nucleation scale but the defects to be much larger due to the higher dose. Assuming r0 to be proportional to the square root of dose p] one would expect a value of around 135OA. Examination shows that both rl and r0 are much smaller than predicted. According to nucleation theory a lower value of rl can only be accomplished by raising the value of E. Homo-
Table 5. Effect of annealing on samples irradiated at 1475°C Defect character Material
Annealed at 2000°C 1 hr
MR8
Not examined
GE3600
HTA
Only 20% of defects in HTA but same size distribution, some on lines r, - 200-15OOA; complex
APG12
Not examined
3600P
Not examined
Annealed at 2500°C 1 hr As if unirradiated but an order of magnitude higher dislocation density No distinct loops visible. Essentially as unirradiated Few remaining loops, r,, up to 5OOoA Few large loops remaining, both vacancy and interstitial, in clusters of same type. r, 500-3500 A. Relatively high dislocation density. Very few remaining signs of irradiation but fairly high dislocation density similar to APG12
IRRADIATION
DAMAGE
TO
nucleation would certainly be genous expected in APG12, and from several micrographs it is estimated that rl is around 0*50.8 pm in this material, corresponding to a range of E values 1.7-2.0 eV. The observed damage patterns may now be rationalized. In samples 3600P and APG12 the nucleation is homogeneous and similar because of the large crystallite sizes. Possible impurity effects in 3600P are now masked by the much lower rl value for homogeneous nucleation. Some nucleation even occurs within crystallites of MR8 now because 2r, -L, and in this case the value of r0 is approximately the same as for the most perfect materials. The defects in HTA are intermediate between these two extremes because of the wider spread in crystallite size. Surprisingly the clearest damage in the form of large resolvable loops is found in GE3600. The reason for this is not absolutely clear but is believed to be related to another structural feature. Examination of GE3600 irradiated at 1350”[4] revealed a high concentration of twist boundary interstitial loops, i.e. loops situated within twist boundaries. It is reasonable to assume that the influences present in this earlier experiment might be important here. The piping-off of defects to twist boundaries via non-basal defects would leave a smaller concentration of defects within the crystals which would therefore present a clearer picture. However, in spite of this the defects remaining for such a high dose are quite dense and would mask the preseIlce of the fainter twist boundary loops. The presence of such loops was confirmed by examining the sample after annealing for 1 hr at 2000°C when they were clearly visible. The higher value of E at 1475°C indicates that a larger defect cluster is responsible for nucleation at the higher temperature. This is hardly surprising. What is surprising is that E remains constant over the wide temperature range 150-1200°C. It has been suggested
PYROLYTIC
GRAPHITES
509
[2] that such a value corresponds to a group of three or four atoms which, when two come together, form a stable hexagon. No further migration then occurs. If this hypothesis is true then it would appear that at a temperature between 135o”C[4] and 1475°C a much larger cluster is required to form a stable nucleus. The effect of annealing is also simply related to crystallite size in each case. For a given annealing treatment, the percentage of defects annealed out of a material is larger, the smaller its crystallite size. This indicates that the majority of the defect annihilation occurs at crystallite boundaries where the point defect concentration is approximately zero. In the smaller crystallite materials this would produce a much higher point defect concentration gradient between the boundary and the emitting loop, so that the number of defects diffusing from the loop would be larger and hence the loop would anneal more rapidly. Given a situation in which defect nucleation may occur homogeneously at an irradiation temperature of 1475°C the process will be governed by an activation energy approaching 2 eV. Since equivalent nucleation patterns require the same values of E/T, where T is the absolute temperature, a temperature of 1475°C with E = 2 eV would be equivalent to 775°C with E = 1.2 eV. However, in real graphites of interest, i.e. nuclear graphites, L, is around 0.3 pm so that at 1475°C one would expect nucleation not to be homogeneous but to be more influenced by grain boundaries and non-basal defects. In addition any macroscopic effects would be influenced by interactions between neighboring misaligned crystallites. It would certainly be of fundamental interest to measure irradiation effects in well oriented pyrolytic graphites irradiated at these two temperatures to see whether there were in fact any similarities. Results on irradiation effects in nuclear should be more readily available in the near
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future and it will be interesting to see whether there are any discontinuities in property changes with irradiation temperature at around 1400°C such as might be caused by a finer scale of nucleation, i.e., higher value of E.
5. CONCLUSIONS a. At the temperatures investigated there is a distinct dependence of nucleation behavior on the microstructure of the irradiated material. b. The homogeneous nucleation theory of Reynolds and Thrower[2] may be applied to temperatures up to 1475°C but a higher value of E becomes necessary somewhere above 1350°C. This value appears to be around 2 eV. c. The ease of defect annealing is related to crystallite size in that the smaller the crystallites the more rapid the annealing.
and P. A. THROWER Acknowled m&-We
wish to thank Mr. A. J. Perks of i .E _R.E. Harwell and Dr. W. J. Gray of the Pacific Northwest Laboratory, Hanford, Washington for arranging the irradiations, and to acknowledge the help of Mr. David Sorg in the crystallite size determinations. The financial support of The Atomic Energy Commission, Contract AT (30-l)-1710, is deeply appreciated.
REFERENCES 1. Thrower P. A., Chem+y and Physics of Carbon (Ed Walker P. L. Jr.), Vol. 5, p. 217. Dekker, New York (1969). 2. Reynolds W. N. and Thrower P. A., Phil. Mug. 14,573 (1965). 3. Mayer R. M., Brown L. M. and Kelly A., Phil. Mug. 19,701 (1969). 4. Thrower P. A., Carbon 6,687 (1968). 5. Thrower P. A., Carbon 9,265 (1971). 6. Roscoe C. and Baker J., J. Appl. Phys. 40, 1665
(1969). 7. Turnbull J. A. and Stagg M. 8, Phil. Mug. 14, 1049 (1966). 8. Roscoe C., Kline D. E. and Taylor R. E., Carbon 8,95 (1970).