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Swelling and microstructure of A1N irradiated in a fast reactor
jrwrrnai of nuclear materials
Journal of Nuclear Materials 203 (1993) 249-254 North-Holland
Swelling and microstructure
of AlN irradiated
in a fast reactor
Toyohiko Yano a and Takayosh~ Iseki b *Research Laboratory for Nuckar Reactors, Tokyo Institute of
Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan ’ Department of inorganic Materials, Tokyo Institute of Technology, 2-12-1, 0-okayama, Meguro-ku, Tokyo 152, Japan
Received 6 January 1993; accepted 4 May 1993
Aluminum nitride (AlN) ceramics were neutron-irradiated in the fast-breeder reactor, JOYO, up to 9.1 X 10” n/m2 (E > 0.1 MeV) at about 500°C. The macroscopic length expanded 1.74%, whereas the lattice parameter of the c-axis expanded 2.11% compared with 0.05% that of the a-axis. Microstructural observation revealed the formations of high density interstitial-type dislocation loops in grains and microcracks at grain boundaries. The expansion of the c-axis should be related to the introduction of stacking faults by interstitial loop formation on the basal plane. The change in macroscopic volume calculated from macroscopic length became larger than that in a unit cell volume calculated from the lattice parameter. The discrepancy started at around 1% in volume change, corresponding to a fluence of 8X 1O24-2X f025 n/m’ at irradiation temperatures around 500°C. The bending strength of the specimen in this study reduced significantly, which was caused by the microcrack formation along the grain boundary due to the significant anisotropy of lattice expansion.
1. Introduction Insulating materials with high thermai conductive have been required for constructing fusion devices.
According to this stand point, neutron-irradiation effects of aluminum nitride (AlN) has been investigated by the present authors [l-5] and other researchers [6,7]. The swelling of the macroscopic length of AlN was less than that of Sic irradiated to 1.1 x lo*’ n/m* (E > 0.1 MeV) [l]. The lattice expansion of AlN showed an anisotropic manner, i.e., the expansion of the c-axis was greater than that of the u-axis after the irradiation beyond the level of N 1 x 1O24 n/m* [1,6]. After irradiation at 470°C up to 5.3 x 1O24 n/m2, the specimen swelled 0.4% in macroscopic length, while it kept a relatively high bending strength [1,4]. On the other hand, thermal diffusivity reduced significantly only after low-dose irradiation in the order of 10z3 n/m2 [1,7]. Microstructural observation showed the formation of small interstitial-type dislocation loops lying on the basal plane. The formation of the loops corresponded to the anisotropic lattice expansion [2,3]. The annealing behavior of Iength, lattice parameter, and some mechanical properties such as hardness and Young’s modulus of AlN were also reported [2,4,5]. Although some basic irradiation effects of AlN have been investigated, any report has not been published 0022-3115/93/$06.00
concerning irradiation effects of high fluence more than 2 x 10” n/m2. In this study, the changes of macroscopic length and corresponding lattice parameter of the specimen irradiated to 9.1 X lo*’ n/m* have been reported together with microstructural change.
2. Experimental
procedure
Sintered specimens manufactured by Tokuyama Soda Co., Japan, 1 X 2 X 10 mm in size, were neutronirradiated in a core region of the fast-breeder reactor, JOY0 operated by PNC (Power Reactor and Nuclear Fuel Development Co., Japan). The estimated neutron fluence and irradiation temperature were 9.1 x 10z n/m* (E > 0.1MeV) and 500°C respectively. The length of the specimen before and after irradiation was measured using a point-type micrometer at room temperature. The lattice parameter was determined by using powder X-ray diffractometory on a Philips PW-1700 diffractometer system, calibrated using Si as an internal standard. The condition was described before [2]. The volume change calculated from macroscopic length measurements is expressed as the macroscopic volume change and that was calculated from lattice parameter measurements as the unit cell volume change in this study.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
T.Yano, T. Iseki / Swelling and microstnccture of AlN
250
Three-point bending strength was measured using a custom-made fixture zig for miniature specimens. A lower span of 4 mm and a crosshead speed of 0.1 mm/min were used with an Instron-type testing machine. The number of specimen was 6 for irradiated and 8 for unirradiated specimens. The microstructure of the irradiated specimen was observed with a high-resolution transmission electron microscope operated at 300 kV (H-9000, Hitachi Ltd., Japan). The method to prepare TEM specimens was described elsewhere [3].
0.101 I
I
100
110
CuKa 100 I
3. Results
I
110 I
I
120
130
28
I
120 1
I
130
b)
Macroscopic length changes (swelling) of 4 specimens were listed in table 1, together with the lattice parameter changes. The macroscopic length expanded 1.74% on average, which was much greater than those of lower-fluence specimens as discussed later. On the other hand, the c-axis parameter expanded 2.11% comparing that of u-axis 0.05%. The trace of powder X-ray diffraction is given in fig. 1 with that of the unirradiated specimen. All reflected peaks significantly broadened and decreased in intensity. Only the traces of (hki0) reflections show Ka,, Kol, peak-splitting. When the index 1 increased, the peak broadened more severely. This phenomena suggests a scatter of the lattice spacing parallel to the basal plane. Whereas the broadening of the reflections was significant, three peaks of (21501, (2131) and (3030) could be indexed, in the 28 range of 87-138” using
Table 1 Macroscopic length and lattice parameter of AIN before and after irradiation to 9.1 x 10” n/m* at 500°C Length (mm)
Lattice
parameter
(nm)
a-axis
c-axis
c-axis
(nm)
(nm)
-a-axis
Unirradiated
(a) 10.007 (b) 9.995 (c) 10.000 (d) 10.008
0.31115
0.49798
1.6005
Irradiated 9.1 x 1025
(a) (b) k) (d)
0.31130
0.50847
1.6334
(n/m’)
Change (%o)
10.180 10.169 10.176 10.179
+ 1.74 (average)
+ 0.05
+2.11
+ 2.06
0.101
I 100
I
I 110
I
I 120
I 130
CuKa
28 Fig. 1. Powder X-ray diffraction patterns of AlN (a) prior and (b) after irradiation up to 9.1 X 1O25 n/m*.
CuKa radiation, and the lattice parameter could be obtained, as shown above. Because the traces were obtained from the powdered specimen, the lattice parameter represented crystal lattice growth by the irradiation away from intergranular strain due to anisotropic growth of grains. A lower magnification electron micrograph of the irradiated specimen is shown in fig. 2. Microcracks are found along most of the grain boundaries. It was not observed for the unirradiated specimens and specimens irradiated to 5.3 X 1O24 n/m’ at 470°C [l]. Selected area electron diffraction patterns of the specimens prior and after irradiation to 9.1 x 1O25 n/m2 are presented in fig. 3. The streaks along the c*-direction is clearly observed only for the irradiated specimen. Fig. 4 is a bright field micrograph of the irradiated specimen taken along [ 1120]. There are many short line contrast parallel to the (0001) plane. The density of the line contrast is lower near the edge than at the thick part of the specimen. The distribution of the line contrast is seen as uniform but not periodic. The observed length of the line contrast is 5 to 1.5 nm and the estimated density is about 1 X 1024/m3.
T. Yano, T. Iseki / Swelling and microstnrcture of AIN
A high-resolution electron micrograph containing the line contrast in fig. 4 taken along the [ll?O] is presented in fig. 5. The photograph was taken from a very thin portion near the edge. In the micrograph, a single black dot corresponds to one AlN, tetrahedron [3]. A pair of layers stacked along the [OOOl]direction indicates a hexagonal wurtzite structure. One extra layer is inserted between two original layers, shown with arrows in the photograph. Thus, the loop is interstitial-type, and the Burgers vector of the loop is defined directly as c/2 [OOOl] from the photograph, as reported previously [3]. The bending strength of the irradiated and unirradiated specimens were obtained as 8.6 MPa (SD. 2.3 MPa) and 348 MPa (S.D. 49 MPa), respectively.
4. Discussion The macroscopic volume change (5.30%) and the unit cell volume change (2.20%) of the specimen irradiated to 9.1 x IO25 n/m2 does not coincide with each other. Fig. 6 summarizes the effect of the neutron fluence on the macroscopic volume and unit cell volume changes including a new result at an irradiation temperature of 500°C. From the figure, it is estimated that the discrepancy between macroscopic volume change and unit cell volume change starts at the level
251
of around l%, corresponding to the fluence of 8 X 1024-2 X 10z5 n/m2 (E > 0.1 MeV). This mismatch between the unit cell volume change and macroscopic volume change suggests the formation of bubbles or microcracks along grain boundary. The formation of grain boundary microcracks is confirmed by an electron micrograph, as shown in fig. 2. The observed crack width was about 0.05 to 0.1 urn, and the average grain size was about 5 urn. The ratio of crack width to average grain size is l-2% in length (3-6% in volume), which is consistent with the difference in the macroscopic volume change and unit cell volume change mentioned above. The ratio of c-axis length to a-axis length of AlN as a function of neutron fluence irradiated at about 500°C is shown in fig. 7. As compared with the previous results [1,6] shown in the figure, the degree of anisotropy of lattice growth increases significantly above 1O24 n/m2. From the microstructural observation, it is noted that the estimated density of the interstitial loop of the specimen in this study is about 1 x 10z4/m3, and is of the same order as that of the specimen irradiated to 1.1 X 10Z n/m2 [3] at 785°C. On the other hand, the average diameter of the loops increases markedly: 5-15 nm at 9.1 X lo*’ n/m2 compared with a few nanometer at 5.3 X 1O24 n/m’. The increment of loop diameter induces the increase in stacking fault area. The
Fig. 2. TEM photograph of AIN irradiated to 9.1 X 10z5 n/m’. Microcracks are found along grain boundaries as indicated with arrows.
diffuse streaks along the c*-direction in the electron diffraction pattern (fig. 3) and the broadening in X-ray peaks especially having large 1 index (fig. 1) are both caused by this stacking fault, which is preferentially aligned along the basal plane but those intervals are not periodic. The structure of interstitial loop is the same as reported previously [3]. From the study of AIN-related compounds [8,9], the existence of many long-period polytypes composed with the basic 2H wurtzite-type layer and the double layer sandwiched with stacking faults has been known. These two ~mponents are stacked along the c-direction. In those compounds, the c/n, i.e., the average interplaner spacing along the c-direction, where n is the number
Fig. 4. Bright-field electron micrograph of the specimen irradiated to 9.1 X 10” n/m2 taken along [ll?O]. Many short line contrast parallel to the basal plane is observed.
Fig. 3. Electron diffraction pattern of the specimens (a) prior and (b) after irradiation up to 9.1 x 10” n/m*. The streaks along the c*-direction is observed in the irradiated specimen.
of the Ramsdell notation and c the lattice parameter of the c-axis, increases with decrease in the ratio of M/X, where M and X represent the number of metal and nonmetal ions, respectively. The lower M/X ratio less than l/l suggests that the pofytype includes stacking faults. Next to 2H-AlN (M/X= l/l), the 39R (M/X= 13/14) polytype has been known [S]. The c/n is 0.260 nm in 39R polytype and 0.249 nm in 2%AlN. Furthermore, another polytype, of which the structure is basically 2H but containing randomly-distributed staking fault and denoted as 2HS, has been reported. The c/n of this polytype (0.265 nm) is also greater than that of 2H-AlN [8]. Consequently, the introduction of stacking faults is a cause of elongation of the c-axis in irradiated AIN as in the case of 2HS phase or long-period AlN-related compounds. Thus the formation of interstitial loops and the increase in the size of loops seem to be directly related to the anisotropic expansion of AIN.
T. Yano, T. Iseki / Swelling and
~.cr~t~ctureofAIN
253
Fig. 5. High-resolution electron micrograph of the specimen irradiated to 9.1 x lo*’ n/m* taken along [1120]. The interstitial loop having Burgers vector c/2 [OOOl]is directly observed. A single extra layer is inserted between the original layers as indicated with arrows.
As expected from the significant anisotropy of lattice expansion and consequent microcrack formation along grain boundaries, the bending strength of the irradiated specimen was very low, less than 10 MPa, which is much weaker than those of specimens unirradiated and irradiated up to 5.3 x 1O24 n/m2 in JMTR (both higher than 350 MPa) [4]. When the anisotropy of the lattice parameter exceeds a critical value, which corresponds to the intergranular strain equal to the grain boundary bonding strength, the microcrack between grains should be formed. The bending strength then decreased suddenly, as observed in noncubic compounds such as Be0 [lO,ll] and AlzO, [10,12,13]. The
Neutron fluence
(n/m*)
formation of microcracks in AIN induced by neutron irradiation could be detected by the discrepancy of macroscopic volume change and unit cell volume change.
5. Conclusions Aluminum nitride ceramics were neutron-irradiated in a fast breeder reactor up to 9.1 x 1O25n/m2 (E > 0.1 MeV) at about 500°C. Macroscopic length change and lattice parameter change by the irradiation were observed together with microstructural change. Compar-
(EzO. 1 MeV)
Fig. 6. Effect of neutron fluence on the macroscopic volume and unit cell volume changes of AIN. (2) 100°C (jrradiation temperature) Ill, (3, 4) < 660°C [6J, (5) 470°C [If, (6) 785°C [ll, (7) 500°C (this work).
Neutron fluence
(n/m2) (E>O.l MeV)
Fig. 7. Ratio of c-axis length/a-axis length of AIN as a function of neutron fluence. (1) Unirradiated, (2-7) the same as in fig. 6.
254
T. Yano, T. Iseki f ~weli~ng and mi~rost~&~e
ing the data with those of lower fluence examinations in literatures, the following conclusions were obtained. (1) The macroscopic volume change calculated from macroscopic length change exceeded the unit cell volume change calculated from lattice parameter change in the specimen observed in this study. The discrepancy of these two volume changes started at around 1% in volume change, corresponding to a fluence of 8 x 1O24-2 X 102’ n/m2 at irradiation temperatures around 5OW’C.The cause of the discrepancy could be attributed to the microcrack formation along the grain bounda~ due to the significant anisotropy of lattice expansion. (2) The lattice parameter of the c-axis increased significantly more than that of the u-axis. The expansion of the c-axis should be related to the introduction of stacking faults by interstitial loop formation on the basal plane. (3) The bending strength of the specimen in this study reduced significantly, which caused by microcrack formation.
This research was sponsored by the Ohkura Kazuchika Memorial Foundation and a Grant-in-Aid for Scientific Research from the Ministry of Education,
of AlN
Science and Culture, Japan. We would also like to thank the staff of 0-arai branch, Institute of Materials Research, Tohoku University, for supporting the irradiation experiment.
References [I] T. Yano and T. Iseki, J. Nucl. Mater. 179-W (1990) 387. 121T. Yano, M. Tezuka, H. Miyazaki and T. Iseki, J. Nuci. Mater. 191-194 (19921635. 131T. Yano and T. Iseki, Philos. Mag. Lett. 62 (1990) 83. [4] C.S. Kim, T. Iseki, T. Yano and M. Tezuka, J. At. Energy Sot. Japan 34 (1992) 335. [5] H. Suematsu, T.E. Mitchell, T. Iseki and T. Yano, Mater. Res. Sot. Proc. 235 (1992) 445. [6] M. Billy, J.-C. Labbe, Y.-M. Lee and G. Roult, Rev. Int. Hautes Temper. Refract. France 21 (1984) 19. [7] M. Rohde and B. Schulz, Thermal Conductivity 21 (1990) 509. [8] K.H. Jack, J. Mater. Sci. 11 (1976) 1135. [9] G. Van Tendeloo, K.T. Faber and G. Thomas, J. Mater. Sci. 18 (1983) 525. [IO] R.S. Wilks, J. Nucl. Mater. 26 (1968) 137. [ll] G.W. Keilholtz, J.E. Lee and R.E. Moore, Nucl. Sci. Eng. 26 (1966) 329. [12] F.W. Clinard, Jr., G.F. Hurley and L.W. Hobbs, J. Nucl. Mater. 108&109 (1982f 6.55. 1131 G.W. Keilholtz, R.E. Moore and H.E. Robertson, Nucl. Technol. 17 (1973) 234.
Journal of Nuclear Materials 203 (1993) 249-254 North-Holland
Swelling and microstructure
of AlN irradiated
in a fast reactor
Toyohiko Yano a and Takayosh~ Iseki b *Research Laboratory for Nuckar Reactors, Tokyo Institute of
Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan ’ Department of inorganic Materials, Tokyo Institute of Technology, 2-12-1, 0-okayama, Meguro-ku, Tokyo 152, Japan
Received 6 January 1993; accepted 4 May 1993
Aluminum nitride (AlN) ceramics were neutron-irradiated in the fast-breeder reactor, JOYO, up to 9.1 X 10” n/m2 (E > 0.1 MeV) at about 500°C. The macroscopic length expanded 1.74%, whereas the lattice parameter of the c-axis expanded 2.11% compared with 0.05% that of the a-axis. Microstructural observation revealed the formations of high density interstitial-type dislocation loops in grains and microcracks at grain boundaries. The expansion of the c-axis should be related to the introduction of stacking faults by interstitial loop formation on the basal plane. The change in macroscopic volume calculated from macroscopic length became larger than that in a unit cell volume calculated from the lattice parameter. The discrepancy started at around 1% in volume change, corresponding to a fluence of 8X 1O24-2X f025 n/m’ at irradiation temperatures around 500°C. The bending strength of the specimen in this study reduced significantly, which was caused by the microcrack formation along the grain boundary due to the significant anisotropy of lattice expansion.
1. Introduction Insulating materials with high thermai conductive have been required for constructing fusion devices.
According to this stand point, neutron-irradiation effects of aluminum nitride (AlN) has been investigated by the present authors [l-5] and other researchers [6,7]. The swelling of the macroscopic length of AlN was less than that of Sic irradiated to 1.1 x lo*’ n/m* (E > 0.1 MeV) [l]. The lattice expansion of AlN showed an anisotropic manner, i.e., the expansion of the c-axis was greater than that of the u-axis after the irradiation beyond the level of N 1 x 1O24 n/m* [1,6]. After irradiation at 470°C up to 5.3 x 1O24 n/m2, the specimen swelled 0.4% in macroscopic length, while it kept a relatively high bending strength [1,4]. On the other hand, thermal diffusivity reduced significantly only after low-dose irradiation in the order of 10z3 n/m2 [1,7]. Microstructural observation showed the formation of small interstitial-type dislocation loops lying on the basal plane. The formation of the loops corresponded to the anisotropic lattice expansion [2,3]. The annealing behavior of Iength, lattice parameter, and some mechanical properties such as hardness and Young’s modulus of AlN were also reported [2,4,5]. Although some basic irradiation effects of AlN have been investigated, any report has not been published 0022-3115/93/$06.00
concerning irradiation effects of high fluence more than 2 x 10” n/m2. In this study, the changes of macroscopic length and corresponding lattice parameter of the specimen irradiated to 9.1 X lo*’ n/m* have been reported together with microstructural change.
2. Experimental
procedure
Sintered specimens manufactured by Tokuyama Soda Co., Japan, 1 X 2 X 10 mm in size, were neutronirradiated in a core region of the fast-breeder reactor, JOY0 operated by PNC (Power Reactor and Nuclear Fuel Development Co., Japan). The estimated neutron fluence and irradiation temperature were 9.1 x 10z n/m* (E > 0.1MeV) and 500°C respectively. The length of the specimen before and after irradiation was measured using a point-type micrometer at room temperature. The lattice parameter was determined by using powder X-ray diffractometory on a Philips PW-1700 diffractometer system, calibrated using Si as an internal standard. The condition was described before [2]. The volume change calculated from macroscopic length measurements is expressed as the macroscopic volume change and that was calculated from lattice parameter measurements as the unit cell volume change in this study.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
T.Yano, T. Iseki / Swelling and microstnccture of AlN
250
Three-point bending strength was measured using a custom-made fixture zig for miniature specimens. A lower span of 4 mm and a crosshead speed of 0.1 mm/min were used with an Instron-type testing machine. The number of specimen was 6 for irradiated and 8 for unirradiated specimens. The microstructure of the irradiated specimen was observed with a high-resolution transmission electron microscope operated at 300 kV (H-9000, Hitachi Ltd., Japan). The method to prepare TEM specimens was described elsewhere [3].
0.101 I
I
100
110
CuKa 100 I
3. Results
I
110 I
I
120
130
28
I
120 1
I
130
b)
Macroscopic length changes (swelling) of 4 specimens were listed in table 1, together with the lattice parameter changes. The macroscopic length expanded 1.74% on average, which was much greater than those of lower-fluence specimens as discussed later. On the other hand, the c-axis parameter expanded 2.11% comparing that of u-axis 0.05%. The trace of powder X-ray diffraction is given in fig. 1 with that of the unirradiated specimen. All reflected peaks significantly broadened and decreased in intensity. Only the traces of (hki0) reflections show Ka,, Kol, peak-splitting. When the index 1 increased, the peak broadened more severely. This phenomena suggests a scatter of the lattice spacing parallel to the basal plane. Whereas the broadening of the reflections was significant, three peaks of (21501, (2131) and (3030) could be indexed, in the 28 range of 87-138” using
Table 1 Macroscopic length and lattice parameter of AIN before and after irradiation to 9.1 x 10” n/m* at 500°C Length (mm)
Lattice
parameter
(nm)
a-axis
c-axis
c-axis
(nm)
(nm)
-a-axis
Unirradiated
(a) 10.007 (b) 9.995 (c) 10.000 (d) 10.008
0.31115
0.49798
1.6005
Irradiated 9.1 x 1025
(a) (b) k) (d)
0.31130
0.50847
1.6334
(n/m’)
Change (%o)
10.180 10.169 10.176 10.179
+ 1.74 (average)
+ 0.05
+2.11
+ 2.06
0.101
I 100
I
I 110
I
I 120
I 130
CuKa
28 Fig. 1. Powder X-ray diffraction patterns of AlN (a) prior and (b) after irradiation up to 9.1 X 1O25 n/m*.
CuKa radiation, and the lattice parameter could be obtained, as shown above. Because the traces were obtained from the powdered specimen, the lattice parameter represented crystal lattice growth by the irradiation away from intergranular strain due to anisotropic growth of grains. A lower magnification electron micrograph of the irradiated specimen is shown in fig. 2. Microcracks are found along most of the grain boundaries. It was not observed for the unirradiated specimens and specimens irradiated to 5.3 X 1O24 n/m’ at 470°C [l]. Selected area electron diffraction patterns of the specimens prior and after irradiation to 9.1 x 1O25 n/m2 are presented in fig. 3. The streaks along the c*-direction is clearly observed only for the irradiated specimen. Fig. 4 is a bright field micrograph of the irradiated specimen taken along [ 1120]. There are many short line contrast parallel to the (0001) plane. The density of the line contrast is lower near the edge than at the thick part of the specimen. The distribution of the line contrast is seen as uniform but not periodic. The observed length of the line contrast is 5 to 1.5 nm and the estimated density is about 1 X 1024/m3.
T. Yano, T. Iseki / Swelling and microstnrcture of AIN
A high-resolution electron micrograph containing the line contrast in fig. 4 taken along the [ll?O] is presented in fig. 5. The photograph was taken from a very thin portion near the edge. In the micrograph, a single black dot corresponds to one AlN, tetrahedron [3]. A pair of layers stacked along the [OOOl]direction indicates a hexagonal wurtzite structure. One extra layer is inserted between two original layers, shown with arrows in the photograph. Thus, the loop is interstitial-type, and the Burgers vector of the loop is defined directly as c/2 [OOOl] from the photograph, as reported previously [3]. The bending strength of the irradiated and unirradiated specimens were obtained as 8.6 MPa (SD. 2.3 MPa) and 348 MPa (S.D. 49 MPa), respectively.
4. Discussion The macroscopic volume change (5.30%) and the unit cell volume change (2.20%) of the specimen irradiated to 9.1 x IO25 n/m2 does not coincide with each other. Fig. 6 summarizes the effect of the neutron fluence on the macroscopic volume and unit cell volume changes including a new result at an irradiation temperature of 500°C. From the figure, it is estimated that the discrepancy between macroscopic volume change and unit cell volume change starts at the level
251
of around l%, corresponding to the fluence of 8 X 1024-2 X 10z5 n/m2 (E > 0.1 MeV). This mismatch between the unit cell volume change and macroscopic volume change suggests the formation of bubbles or microcracks along grain boundary. The formation of grain boundary microcracks is confirmed by an electron micrograph, as shown in fig. 2. The observed crack width was about 0.05 to 0.1 urn, and the average grain size was about 5 urn. The ratio of crack width to average grain size is l-2% in length (3-6% in volume), which is consistent with the difference in the macroscopic volume change and unit cell volume change mentioned above. The ratio of c-axis length to a-axis length of AlN as a function of neutron fluence irradiated at about 500°C is shown in fig. 7. As compared with the previous results [1,6] shown in the figure, the degree of anisotropy of lattice growth increases significantly above 1O24 n/m2. From the microstructural observation, it is noted that the estimated density of the interstitial loop of the specimen in this study is about 1 x 10z4/m3, and is of the same order as that of the specimen irradiated to 1.1 X 10Z n/m2 [3] at 785°C. On the other hand, the average diameter of the loops increases markedly: 5-15 nm at 9.1 X lo*’ n/m2 compared with a few nanometer at 5.3 X 1O24 n/m’. The increment of loop diameter induces the increase in stacking fault area. The
Fig. 2. TEM photograph of AIN irradiated to 9.1 X 10z5 n/m’. Microcracks are found along grain boundaries as indicated with arrows.
diffuse streaks along the c*-direction in the electron diffraction pattern (fig. 3) and the broadening in X-ray peaks especially having large 1 index (fig. 1) are both caused by this stacking fault, which is preferentially aligned along the basal plane but those intervals are not periodic. The structure of interstitial loop is the same as reported previously [3]. From the study of AIN-related compounds [8,9], the existence of many long-period polytypes composed with the basic 2H wurtzite-type layer and the double layer sandwiched with stacking faults has been known. These two ~mponents are stacked along the c-direction. In those compounds, the c/n, i.e., the average interplaner spacing along the c-direction, where n is the number
Fig. 4. Bright-field electron micrograph of the specimen irradiated to 9.1 X 10” n/m2 taken along [ll?O]. Many short line contrast parallel to the basal plane is observed.
Fig. 3. Electron diffraction pattern of the specimens (a) prior and (b) after irradiation up to 9.1 x 10” n/m*. The streaks along the c*-direction is observed in the irradiated specimen.
of the Ramsdell notation and c the lattice parameter of the c-axis, increases with decrease in the ratio of M/X, where M and X represent the number of metal and nonmetal ions, respectively. The lower M/X ratio less than l/l suggests that the pofytype includes stacking faults. Next to 2H-AlN (M/X= l/l), the 39R (M/X= 13/14) polytype has been known [S]. The c/n is 0.260 nm in 39R polytype and 0.249 nm in 2%AlN. Furthermore, another polytype, of which the structure is basically 2H but containing randomly-distributed staking fault and denoted as 2HS, has been reported. The c/n of this polytype (0.265 nm) is also greater than that of 2H-AlN [8]. Consequently, the introduction of stacking faults is a cause of elongation of the c-axis in irradiated AIN as in the case of 2HS phase or long-period AlN-related compounds. Thus the formation of interstitial loops and the increase in the size of loops seem to be directly related to the anisotropic expansion of AIN.
T. Yano, T. Iseki / Swelling and
~.cr~t~ctureofAIN
253
Fig. 5. High-resolution electron micrograph of the specimen irradiated to 9.1 x lo*’ n/m* taken along [1120]. The interstitial loop having Burgers vector c/2 [OOOl]is directly observed. A single extra layer is inserted between the original layers as indicated with arrows.
As expected from the significant anisotropy of lattice expansion and consequent microcrack formation along grain boundaries, the bending strength of the irradiated specimen was very low, less than 10 MPa, which is much weaker than those of specimens unirradiated and irradiated up to 5.3 x 1O24 n/m2 in JMTR (both higher than 350 MPa) [4]. When the anisotropy of the lattice parameter exceeds a critical value, which corresponds to the intergranular strain equal to the grain boundary bonding strength, the microcrack between grains should be formed. The bending strength then decreased suddenly, as observed in noncubic compounds such as Be0 [lO,ll] and AlzO, [10,12,13]. The
Neutron fluence
(n/m*)
formation of microcracks in AIN induced by neutron irradiation could be detected by the discrepancy of macroscopic volume change and unit cell volume change.
5. Conclusions Aluminum nitride ceramics were neutron-irradiated in a fast breeder reactor up to 9.1 x 1O25n/m2 (E > 0.1 MeV) at about 500°C. Macroscopic length change and lattice parameter change by the irradiation were observed together with microstructural change. Compar-
(EzO. 1 MeV)
Fig. 6. Effect of neutron fluence on the macroscopic volume and unit cell volume changes of AIN. (2) 100°C (jrradiation temperature) Ill, (3, 4) < 660°C [6J, (5) 470°C [If, (6) 785°C [ll, (7) 500°C (this work).
Neutron fluence
(n/m2) (E>O.l MeV)
Fig. 7. Ratio of c-axis length/a-axis length of AIN as a function of neutron fluence. (1) Unirradiated, (2-7) the same as in fig. 6.
254
T. Yano, T. Iseki f ~weli~ng and mi~rost~&~e
ing the data with those of lower fluence examinations in literatures, the following conclusions were obtained. (1) The macroscopic volume change calculated from macroscopic length change exceeded the unit cell volume change calculated from lattice parameter change in the specimen observed in this study. The discrepancy of these two volume changes started at around 1% in volume change, corresponding to a fluence of 8 x 1O24-2 X 102’ n/m2 at irradiation temperatures around 5OW’C.The cause of the discrepancy could be attributed to the microcrack formation along the grain bounda~ due to the significant anisotropy of lattice expansion. (2) The lattice parameter of the c-axis increased significantly more than that of the u-axis. The expansion of the c-axis should be related to the introduction of stacking faults by interstitial loop formation on the basal plane. (3) The bending strength of the specimen in this study reduced significantly, which caused by microcrack formation.
This research was sponsored by the Ohkura Kazuchika Memorial Foundation and a Grant-in-Aid for Scientific Research from the Ministry of Education,
of AlN
Science and Culture, Japan. We would also like to thank the staff of 0-arai branch, Institute of Materials Research, Tohoku University, for supporting the irradiation experiment.
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