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Recovery of c-axis spacing in pyrolytic graphite after neutron irradiation at 5 K
Volume 53A, number 4
PHYSICS LETTERS
30 June 1975
RECOVERY OF c-AXIS SPACING IN PYROLYTIC GRAPHITE AFTER NEUTRON IRRADIATION AT 5 K H. MAETA, T. IWATA and S. OKUDA Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan
Received 23 April 1975 Recovery of the c-axis spacing expansion in pyrolytic graphite after neutron irradiation at 5 K was measured in the range of 5 K ~ 900 K by the X-ray diffraction method. An unexpectedly large recovery of the spacing was found around 100 K. Many works have been done on the low-temperature annealing of lattice defects in irradiated graphite (see, for example, [1,2]). The measurements of electrical resistivities in electron-irradiated pyrolytic graphite show a small recovery from almost liquid helium temperature to 85 K, and then a marked reverse annealing in 85 to 180 K. The identification of defects in these annealing stages, however, is not yet clear and new information is needed. It has usually been assumed in graphite that, because of the anisotropic bonding nature of the crystal, interstitial atoms can expand markedly the c-axis spacing by straining the layer planes around them, while vacancies have almost no effect on the spacing [1 ]. Therefore, the measurement of the c-axis spacing used here is a very useful and almost unique method in graphite to pick up directly the contribution of interstitial atoms only. Austerman [3] found a steady recovery of the c-axis spacing in neutron-irradiated graphite in the range from about 115 K to room temperatures. However, no measurement has been made below 115 K. For the identification of defects, it is desirable to measure its recovery in the temperature range down to liquid helium temperature. The present experiment was undertaken to measure the isochronal thermal annealing of the c-axis spacing expansion of pyrolytic graphite in the range from 5 K to 900 K after neutron irradiation at 5 K. Specimens were well-crystallized, monochrometergrade pyrolytic graphite obtained from Union Carbide Corp.. The c-axis misalignment of crystallites was less than 0.4 °. ne fast neutron irradiation was performed at 5 K in the LHTL (Liquid Helium Temperature Loop) of JRR-3 reactor at the Japan Atomic
Energy Research Institute. The fast neutrons have nearly the fission neutron spectrum and the flux is about 1.1 X 1012 n/cm2/sec (E > 0.1 MeV). After the irradiation at 5 K, specimens were transferred to the X-ray measuring cryostat [4] without any intermediate warm-up. All measurements of the c-axis spacing were carried out at 4.2 K using the monochromated CuKt~1 and the (00-8)' reflection. A N a I scintillation counter with a pulse height analyzer was used as the detector of X-rays. Fig. 1. shows the result of isochronal pulse annealing on the specimen irradiated with fast neutron to the fluence of 2.8 X 1017 n/cm2 ; this irradiation increased the c-axis spacing (Co) by Aco/c 0 = 1.0 × 10 -3. The isochronal annealing time was 6 min at each temperature. The characteristic features of the recovery of c-axis spacing are as follows. A small recovery is seen around 40 K. A large recovery occurs around 100 K, where about 30 per cent of the total expansion recovered. Above 120 K, a recovery occurs steadily and continuously up to about 400 K in agreement with the result found by Austerman [3], and then a smaller recovery follows continuously up to 900 K. The large recovery around 100 K is discussed here. The recovery during annealing of the c-axis spacing expansion is caused by the relaxation of strains through either (1) the annihilation of interstitials at vacancies and/or crystallite boundaries, or (2) the clustering of interstitials. The reverse annealing of electrical properties, such as the mobility and the density of charge carriers, in this temperature range [2,6] may deny the possibility of the process (1). In addition, the stored energy release measured during this recovery 295
Volume 53A, number 4 I00
PtlYSICS LETTERS °e6e
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ANNEALING TEMPERATURE
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(°K)
lig. I. l'ractional isochronal annealing curve and its differential curve of the ~-axis spacing in pyr, Uytic graphite neutron-irradiated at5K. which is less than 1/I 5 of the total stored energy [5,6], is m u c h smaller than that expected if about 30 per cent of interstitials annihilate in the process (1). The most probable process (2) may be divided into two cases, that is, (2a) the formation of ('2 carbon molecules from interstitial atoms in the same interlayer spaces [8], and (2b) the formation of clusters from interstitial atoms over the adjacent interlayer spaces [2]. A great reduction in the strain of layer planes can be expected by the formation of C 2 molecules [8]. Then, if" a b o u t halt of interstitial atoms form C 2 molecules by the process (2a) and the other half fibrin clusters by the process (2b), the recovery of the c-axis spacing obtained here and the release or the stored energy may be explicable consistently. By the way, the present result on the c-axis spacing recovery is inconsistent with the model proposed by l l e n n i g and ltove [71 and developed by others I1 I where the recovery around 100 K is due Iv the dissociation of C 2 molecules or the dispersion of close Frenkel pairs. A detailed account of the presenl work will bc published elsewhere.
296
We wish to thank Dr. Y. Obata for continual encouragement during the course of this work, and also thank the members of LHTL and JRR-3 for invaluable help.
References [ 1 ] J.tf.W. Sinmlon~: Radiation damage in graphite (Pergam~m. 1965! I21 T. Iw:Jt:, |. Nihira arid If. Matsuo: J. Phys. Soeo Japan 33 {I9721 J()60:36 (19741 t23. [31 S.B. Auslerman: NAA-SR-2457 (1988). [41 it. Maeta, J. Kato and S. Okuda: to be published. [5] 1,. Bochirol and E. Bonjour: Carbon 6 (1968) 661. [6[ f. Iwata and T. Nihira: quoted by R. Nagasaki et aL in Peacclul uses of ah)mic energy, Vol. 10 (United Nations, 1972) p. 367. i 7 ] (;.R. Ilennig and .I.1. tlove: Proc. Intern., Conf. on Peaceful uses of atomic energy Vol. 7 (United Nations, 19561 p. 666. 18J T. Iwata and tt. Suzuki: Radiation damage in reach)r materials H.A.I'.A., Vienna, 19631 p. 565.
PHYSICS LETTERS
30 June 1975
RECOVERY OF c-AXIS SPACING IN PYROLYTIC GRAPHITE AFTER NEUTRON IRRADIATION AT 5 K H. MAETA, T. IWATA and S. OKUDA Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan
Received 23 April 1975 Recovery of the c-axis spacing expansion in pyrolytic graphite after neutron irradiation at 5 K was measured in the range of 5 K ~ 900 K by the X-ray diffraction method. An unexpectedly large recovery of the spacing was found around 100 K. Many works have been done on the low-temperature annealing of lattice defects in irradiated graphite (see, for example, [1,2]). The measurements of electrical resistivities in electron-irradiated pyrolytic graphite show a small recovery from almost liquid helium temperature to 85 K, and then a marked reverse annealing in 85 to 180 K. The identification of defects in these annealing stages, however, is not yet clear and new information is needed. It has usually been assumed in graphite that, because of the anisotropic bonding nature of the crystal, interstitial atoms can expand markedly the c-axis spacing by straining the layer planes around them, while vacancies have almost no effect on the spacing [1 ]. Therefore, the measurement of the c-axis spacing used here is a very useful and almost unique method in graphite to pick up directly the contribution of interstitial atoms only. Austerman [3] found a steady recovery of the c-axis spacing in neutron-irradiated graphite in the range from about 115 K to room temperatures. However, no measurement has been made below 115 K. For the identification of defects, it is desirable to measure its recovery in the temperature range down to liquid helium temperature. The present experiment was undertaken to measure the isochronal thermal annealing of the c-axis spacing expansion of pyrolytic graphite in the range from 5 K to 900 K after neutron irradiation at 5 K. Specimens were well-crystallized, monochrometergrade pyrolytic graphite obtained from Union Carbide Corp.. The c-axis misalignment of crystallites was less than 0.4 °. ne fast neutron irradiation was performed at 5 K in the LHTL (Liquid Helium Temperature Loop) of JRR-3 reactor at the Japan Atomic
Energy Research Institute. The fast neutrons have nearly the fission neutron spectrum and the flux is about 1.1 X 1012 n/cm2/sec (E > 0.1 MeV). After the irradiation at 5 K, specimens were transferred to the X-ray measuring cryostat [4] without any intermediate warm-up. All measurements of the c-axis spacing were carried out at 4.2 K using the monochromated CuKt~1 and the (00-8)' reflection. A N a I scintillation counter with a pulse height analyzer was used as the detector of X-rays. Fig. 1. shows the result of isochronal pulse annealing on the specimen irradiated with fast neutron to the fluence of 2.8 X 1017 n/cm2 ; this irradiation increased the c-axis spacing (Co) by Aco/c 0 = 1.0 × 10 -3. The isochronal annealing time was 6 min at each temperature. The characteristic features of the recovery of c-axis spacing are as follows. A small recovery is seen around 40 K. A large recovery occurs around 100 K, where about 30 per cent of the total expansion recovered. Above 120 K, a recovery occurs steadily and continuously up to about 400 K in agreement with the result found by Austerman [3], and then a smaller recovery follows continuously up to 900 K. The large recovery around 100 K is discussed here. The recovery during annealing of the c-axis spacing expansion is caused by the relaxation of strains through either (1) the annihilation of interstitials at vacancies and/or crystallite boundaries, or (2) the clustering of interstitials. The reverse annealing of electrical properties, such as the mobility and the density of charge carriers, in this temperature range [2,6] may deny the possibility of the process (1). In addition, the stored energy release measured during this recovery 295
Volume 53A, number 4 I00
PtlYSICS LETTERS °e6e
o
,
~
3(1 ,hme 1975 r
r ~ "
r
o
-----T~
. . . . .
~
r
-
-
-
-
-
-
-
?
PYROLYTIC GRAPHITE e
4'f : 2Bx10tr n/cr~
75
eeeoe °eeeeo eeoe
50
zod eeoee
i
25
10
~ e
I
0 [-o"°%/"~° 0
' ~00
~°,~-%A__v.,~%,-o~,'~ ,o" ',~,/-,200
,
m
gOO'K~
-
300
400
ANNEALING TEMPERATURE
L) <1 ,o
5OO
6OO
(°K)
lig. I. l'ractional isochronal annealing curve and its differential curve of the ~-axis spacing in pyr, Uytic graphite neutron-irradiated at5K. which is less than 1/I 5 of the total stored energy [5,6], is m u c h smaller than that expected if about 30 per cent of interstitials annihilate in the process (1). The most probable process (2) may be divided into two cases, that is, (2a) the formation of ('2 carbon molecules from interstitial atoms in the same interlayer spaces [8], and (2b) the formation of clusters from interstitial atoms over the adjacent interlayer spaces [2]. A great reduction in the strain of layer planes can be expected by the formation of C 2 molecules [8]. Then, if" a b o u t halt of interstitial atoms form C 2 molecules by the process (2a) and the other half fibrin clusters by the process (2b), the recovery of the c-axis spacing obtained here and the release or the stored energy may be explicable consistently. By the way, the present result on the c-axis spacing recovery is inconsistent with the model proposed by l l e n n i g and ltove [71 and developed by others I1 I where the recovery around 100 K is due Iv the dissociation of C 2 molecules or the dispersion of close Frenkel pairs. A detailed account of the presenl work will bc published elsewhere.
296
We wish to thank Dr. Y. Obata for continual encouragement during the course of this work, and also thank the members of LHTL and JRR-3 for invaluable help.
References [ 1 ] J.tf.W. Sinmlon~: Radiation damage in graphite (Pergam~m. 1965! I21 T. Iw:Jt:, |. Nihira arid If. Matsuo: J. Phys. Soeo Japan 33 {I9721 J()60:36 (19741 t23. [31 S.B. Auslerman: NAA-SR-2457 (1988). [41 it. Maeta, J. Kato and S. Okuda: to be published. [5] 1,. Bochirol and E. Bonjour: Carbon 6 (1968) 661. [6[ f. Iwata and T. Nihira: quoted by R. Nagasaki et aL in Peacclul uses of ah)mic energy, Vol. 10 (United Nations, 1972) p. 367. i 7 ] (;.R. Ilennig and .I.1. tlove: Proc. Intern., Conf. on Peaceful uses of atomic energy Vol. 7 (United Nations, 19561 p. 666. 18J T. Iwata and tt. Suzuki: Radiation damage in reach)r materials H.A.I'.A., Vienna, 19631 p. 565.