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High-resolution transmission electron microscopy of ion irradiated uranium oxide
ELSEVIER
Journal of Nuclear Materials 231 (1996) 155- 158
Letter to the Editors
High-resolution transmission electron microscopy of ion irradiated uranium oxide Hj. Matzke a, *, L.M. W a n g b Institute for Transuranium Elements, Joint Research Centre, European Commission, Postfach 2340, D-76125 Karlsruhe, Germany b Department of Earth and Planetary. Sciences, UnicersiO' of New Mexico, Albuquerque NM 87131, USA
Received 12 April 1996; accepted 23 April 1996
Abstract High resolution transmission electron microscopy was used to reveal, for the first time, clear pictures of edge dislocations in UO 2, using image filtering in the Fourier space. The same technique was applied to UO 2 irradiated with 0.5 MeV Xe-ions. Formation of subgrain boundaries associated with the ending of edge dislocations was observed. The small subgrains of nm size are suggested to act as nuclei for polygonization.
1. Introduction Transmission electron microscopy on UO 2 has been performed for more than 35 years [1,2]. The features investigated included fission gas bubbles, fission product precipitates in UO2, the microstructure of deformed UO2, and others. More recently, high burn-up UO 2 has been investigated in TEM, e.g., [3], and lattice plane resolution was achieved in ion implanted UO 2, however without revealing details of the UO 2 structure or of radiation damage [4]. A phenomenon occurring at extended burn-up in UO 2 fuel has recently gained much interest. It is generally termed 'rim-effect' [5] since it first occurs in the outer rim of operating fuel. In this outer shell of 150-200 txm thickness, the burn-up is increased by up to a factor of 2.5 above the cross-section averaged burn-up due to neutron resonance capture by U-238 forming fissile Pu-239. The consequence is a grain-subdivision process or polygonization: the grains of the as-sintered UO 2 of typically 10 ~ m diameter are subdivided into s o m e 104 new small subgrains of 0.2 to 0.3 p.m size. Simultaneously, a large
* Corresponding author. Tel.: +49-7247 951 273; fax: +497247 951 590.
porosity is formed causing the appearance of a 'cauliflower structure' [6]. At extended burn-up ( > about 70 GWd/t), this structure is also found deeper in the fuel. The term 'rim' is thus misleading. Therefore, the term 'polygonization' was suggested [7]. Polygonization of UO 2 could also be o b s e r v e d in a R u t h e r f o r d backscattering/channeling study of ion implanted UO 2 single crystals [8]. 300 keV Xe-ions were used in this RBS/channeling study, and polygonization occurred at rather well defined threshold fluences of 3 to 5 x 1016 i o n s / c m 2 [8]. Similar results could also be obtained with I-ions [9]. Ion irradiation-induced nano-scale polygonization has also been observed in other intermetallic and ceramic materials, e.g., Zr3AI, UaSi and the minerals olivine (Mgo.saFeo.12)2SiO 4 and neptunite Na2KLi(Fe,Mn)2Ti2(SiO3) 8 [10,11]. For the particular case of UO 2, polygonization is of large technological importance for its use as nuclear fuel. The restructured outer zone in the fuel pellets reduces the thermal conductivity, is of concern for reactivity increases and may be of direct relevance for the storage of spent fuel. It is therefore of significant interest to get additional information on the mechanisms of its formation with high resolution transmission electron microscopy (HRTEM). First results of such an activity are reported here, both on
0022-3115/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S 0 0 2 2 - 3 1 1 5 ( 9 6 ) 0 0 3 6 9 - 8
156
Hi. Mat:ke, L.M. Wan~ / Jottrmll qt Ntn'h,w Materials 231 (1996) 155 158 40
)
36
Electron_t_I diffraction
as
25dpa (EdU=60eV, E~°=3OeV)
T f o r
Ill
E
c
5.1015Xetcm2
30
"o
///
30
d
8
g
.(a
24
25
8
m
=~ 20 ._o
'\
\
× 15 g 10
_g s
18 ~5
12 F-N
X e distribution profile
,
(arbitrary units)
'\
6
i lHRTEM l I 0
50
a 100
150
200
0 250
Depth, nm
Fig. 1. Calculation with the TRIM-code [ 14] of range and damage profiles of 500 keV Xe-ions in UO,. The arrows indicate the approximate specimen thickness for HRTEM and electron diffraction, respectively.
as-prepared UO 2 and on UO 2 with radiation damage produced by Xe-ions of 0.5 MeV energy.
2. Experimental UO 2 specimens were prepared from commercial smtered fuel pellets (96% of theoretical density). Disks of 3 mm diam. were cut from these pellets. Following dim-
piing, they were electropolished in a Unithin double jet apparatus using Lenoir's solution at a polishing potential of 40 V. After perfl~ration, the samples are usually treated for 4 - 5 h in a rapid etching ion-milling apparatus (6 keV, 2.5 mA. 6 ° inclination) to clean the surfaces which are frequently covered with a thin contamination layer from the electropolishing. Some specimens were only ion-milled. Following this preparation procedure, the specimens were annealed at 1400°C in A r / H : to recover the polishing damage [12] and to adjust the O / U - r a t i o to the stoichiometric value of 2.0, since the polishing/dimpling procedure tends to oxidize the treated surfaces. The HRTEM was pertormed before and alter the Xe + irradiation using a JEM 2010 microscope with Gatan slow scan, Digital Micrograph:" image recording and processing systems. The microscope was operated at 200 keV. The 500 keV Xe + irradiation was conducted with in situ TEM using the HVEM-Tandem Facility at Argonne National Laboratory [13]. The facility consists of a high voltage electron microscope which is interfaced to a 2 MV tandem ion accelerator, so that the electron diffraction pattern and TEM image can be monitored during ion irradiation. A 300 keV electron beam was used for the in situ TEM observation. The Xe ~ irradiation was performed at room temperature with a dose rate of 3.4 x 10 ~2 l o n s / c m - s. The maximum sample temperature recorded was 170°C during the Xe ~ irradiation due to beam heating. According to a TRIM "95 [14] full cascade calculation, the peak damage depth is about 50 mn below the UO, surface (see Fig. I) and the peak damage value is around 25 dpa for a dose of 5 × 1015 i o n s / c m e. Included in Fig. 1 is the calculated range profile of the Xe-ions. Also, arrows are included showing the approximate specimen thickness
)5/z"iZ Fig. 2. (A) HRTEM micrograph of an unirradiated UO: crystal showing the presence of an edge dislocation m the framed region: (B) Fourier transform of the framed image in (A): (C) filtered image of l'ramed region in (A) (inverse Fourier transform using the two spots circled in (B)) showing the edge dislocation more clearly•
Hj. Matzke, L M. Wang~Journal of Nuclear Materials 231 (1996) 155-158
of the areas where either HRTEM was performed, or electron diffraction pattern were taken.
157
~ "~ ~
3. Results and discussion HRTEM of as-prepared UO 2 samples revealed no crystal defects except a very low density of edge dislocations on the (111) plane and a few small facetted pores. Fig. 2 shows a HRTEM micrograph of a UO 2 specimen before irradiation. Part A reveals an edge dislocation within the framed square. This dislocation is made more clearly visible after an image filtering process in the Fourier space. With an inverse Fourier transform using the two spots circled in B (the Fourier transform of A), a filtered image is obtained (part C) showing the dislocation very clearly. To the authors knowledge, this is the first time a dislocation has been clearly revealed in UO 2. After 5 x 10 ~s Xe + i o n s / c m 2, small arches around the original diffraction spots were observed with in situ TEM. These small arches are indications for the formation of small crystallites with a small misorientation. HRTEM of the sample indeed revealed many sub-grain boundaries across which crystals are slightly rotated ( 1 - 2 °) to each other. Also, the dislocation density in this irradiated sample is more than an order of magnitude higher than in the original sample. After a simple filtering in Fourier space, it was found that the sub-grain boundaries are associated with the ending of edge dislocations (Fig. 3). This indicates the starting of subgrain formation. This result has clearly demonstrated at least one of the mechanisms by which a large UO 2 grain or even crystal can be divided into many small subgrains. Some of these subgrains will eventually develop into individual grains with larger rotation angle due to increased strain fields caused by increased dislocation densities. This is also indicated by the fact, that ring patterns were obtained in electron diffraction from some areas of a sample irradiated to 1 x 10 t6 X e + i o n s / c m 2. When comparing this fact with the TEM observation, one has to keep in mind that HRTEM is performed on very thin areas, hence in material which has not experienced the maximum dpa-level. In contrast, for electron diffraction thicker areas are used which have been subjected to higher dpa-levels near the maximum level (see arrows in Fig. 1). Based on TEM observations of irradiation induced polygonisation in several intermetallic and ceramic materials (see Section 1), Wang et al. [10] suggested that randomly orientated nano-scale crystallites could either originate from the breakage of the original single crystal due to the formation of amorphous volumes in the irradiated material, or could be directly nucleated from the cascade cores when irradiation is performed near the critical amorphization temperature (above which irradiation could not fully amorphize the material). They concluded that the irradiation-induced polygonization is the result of competi-
J// t
Fig. 3. (A) HRTEM micrograph and electron diffraction from a UO 2 crystal irradiated with 500 keV Xe + at room temperature (the actual temperature was 170°C due to beam heating) to 5 x 10 t5 ions/cm 2 (ca. 25 dpa): (B) Fourier filtered image of (A) with the same filtering technique demonstrated in Fig. 2 showing that the crystal has been divided into subgrains with slightly different orientation (1-2 degrees).
tion between amorphization and crystalline recovery in the materials studied by them. The polygonization seen in irradiated UO 2 is apparently the result of a different mechanism. Amorphization of UO 2 has not been observed even after high dose irradiation at very low temperatures (5 and 20 K), e.g., [15]. The image in Fig. 3(B) clearly demonstrates the association of possible subgrain boundaries with the high density dislocations induced by irradiation as a first step for subgrain formation. A more detailed study is needed for the determination of the exact nature of these dislocations, i.e., to define their Burgers vector and to deduce whether the dislocations are due to an extra oxygen or an extra uranium plane, or both. The final rim structure in irradiated UO 2 shows well developed subgrains in the size range of 150 -t- 50 nm [16]. However, smaller subgrains of 20-30 nm in size as nuclei
158
Hj. Matzke, L.M. Wang/Journal of Nuclear Materials 231 (1996) 155-158
for polygonization were also observed [17]. The present results are thus thought to represent the very first step of polygonization. Note that this step is not due to the presence of large concentrations of fission gases and high pressure fission gas bubbles. These can also act as nuclei for polygonization as shown in previous ion implantation work [8].
Acknowledgements The authors thank the HVEM-Tandem Facility staff at Argonne National Laboratory for assistance during the ion irradiation. The TEM was completed, in part, at the Electron Microbeam Analysis Facility of the Department of Earth and Planetary Sciences at the University of New Mexico. Thanks are also due to T. Wiss (ITU) for preparing the thin foils for TEM analysis and to Professor R.C. Ewing for his interest in this investigation. This work was supported partially by the D O E / B E S under Contract DEFG03-93ER45498.
References [1] A.D. Whapham and B.E. Sheldon, J. Nucl. Mater. 10 (1962) 157. [2] Hj. Matzke, Nucl. Appl. 2 (1968) 131.
[3] I.L.F. Ray, H. Thiele and Hj. Matzke, J. Nucl. Mater. 188 (1992) 95. [4] Hj. Matzke, in: Fundamental Aspects of Inert Gases in Solids, eds. S.E. Donelly and J.H. Evans (Plenum, New York, 199t) p. 401. [5] Hj. Matzke, J. Nucl. Mater. 189 (1992) 141. [6] Hj. Matzke, H. Blank, M. Coquerelle, K. Lassmann, I.L.F. Ray, C. Ronchi and C.T. Walker, J. Nucl. Mater. 166 (1989) 165. [7] Hj. Matzke, in: Ceramics: Charting the Future, ed. P. Vincenzini, Advances in Science and Technology 3D (Techna Srl., 1995) p. 2913. [8] Hj. Matzke, A. Turos and G. Linker, Nucl. Instr. Meth. Phys. Res. B91 (1994) 294. [9] Hj. Matzke, A. Turos and J.C. Dran, in preparation. [10] L.M. Wang, R.C. Birtcber and R.C. Ewing, Nucl. Instr. Meth. Phys. Res. B80&81 (1993) 1109. [l 1] R.C. Birtcher and LM. Wang, Nucl. Instr. Meth. Phys. Res. B59&60 (1991) 966. [12] Hi. Matzke and A. Turos, J. Nucl. Mater. 114 (1983) 349. [t3] C.W. Allen, L.L. Funk., E.A. Ryan and S.T. Ockers, Nucl. Instr. Metb. B40&41 (1989) 553. [ 14] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985). [15] Hj. Matzke, O. Meyer and A. Turos, Rad. Eft. Def. Solids 119-121 (1991) 885. [16] I.L.F. Ray, Hj. Matzke, H. Thiele and M. Kinoshita, in: CEC Report EUR 15154 EN (1992) p. 17: J. Nucl. Mater., submitted for publication. [17] K. Nogita and K. Une, J. Nucl. Mater. 226 (1995) 302.
Journal of Nuclear Materials 231 (1996) 155- 158
Letter to the Editors
High-resolution transmission electron microscopy of ion irradiated uranium oxide Hj. Matzke a, *, L.M. W a n g b Institute for Transuranium Elements, Joint Research Centre, European Commission, Postfach 2340, D-76125 Karlsruhe, Germany b Department of Earth and Planetary. Sciences, UnicersiO' of New Mexico, Albuquerque NM 87131, USA
Received 12 April 1996; accepted 23 April 1996
Abstract High resolution transmission electron microscopy was used to reveal, for the first time, clear pictures of edge dislocations in UO 2, using image filtering in the Fourier space. The same technique was applied to UO 2 irradiated with 0.5 MeV Xe-ions. Formation of subgrain boundaries associated with the ending of edge dislocations was observed. The small subgrains of nm size are suggested to act as nuclei for polygonization.
1. Introduction Transmission electron microscopy on UO 2 has been performed for more than 35 years [1,2]. The features investigated included fission gas bubbles, fission product precipitates in UO2, the microstructure of deformed UO2, and others. More recently, high burn-up UO 2 has been investigated in TEM, e.g., [3], and lattice plane resolution was achieved in ion implanted UO 2, however without revealing details of the UO 2 structure or of radiation damage [4]. A phenomenon occurring at extended burn-up in UO 2 fuel has recently gained much interest. It is generally termed 'rim-effect' [5] since it first occurs in the outer rim of operating fuel. In this outer shell of 150-200 txm thickness, the burn-up is increased by up to a factor of 2.5 above the cross-section averaged burn-up due to neutron resonance capture by U-238 forming fissile Pu-239. The consequence is a grain-subdivision process or polygonization: the grains of the as-sintered UO 2 of typically 10 ~ m diameter are subdivided into s o m e 104 new small subgrains of 0.2 to 0.3 p.m size. Simultaneously, a large
* Corresponding author. Tel.: +49-7247 951 273; fax: +497247 951 590.
porosity is formed causing the appearance of a 'cauliflower structure' [6]. At extended burn-up ( > about 70 GWd/t), this structure is also found deeper in the fuel. The term 'rim' is thus misleading. Therefore, the term 'polygonization' was suggested [7]. Polygonization of UO 2 could also be o b s e r v e d in a R u t h e r f o r d backscattering/channeling study of ion implanted UO 2 single crystals [8]. 300 keV Xe-ions were used in this RBS/channeling study, and polygonization occurred at rather well defined threshold fluences of 3 to 5 x 1016 i o n s / c m 2 [8]. Similar results could also be obtained with I-ions [9]. Ion irradiation-induced nano-scale polygonization has also been observed in other intermetallic and ceramic materials, e.g., Zr3AI, UaSi and the minerals olivine (Mgo.saFeo.12)2SiO 4 and neptunite Na2KLi(Fe,Mn)2Ti2(SiO3) 8 [10,11]. For the particular case of UO 2, polygonization is of large technological importance for its use as nuclear fuel. The restructured outer zone in the fuel pellets reduces the thermal conductivity, is of concern for reactivity increases and may be of direct relevance for the storage of spent fuel. It is therefore of significant interest to get additional information on the mechanisms of its formation with high resolution transmission electron microscopy (HRTEM). First results of such an activity are reported here, both on
0022-3115/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S 0 0 2 2 - 3 1 1 5 ( 9 6 ) 0 0 3 6 9 - 8
156
Hi. Mat:ke, L.M. Wan~ / Jottrmll qt Ntn'h,w Materials 231 (1996) 155 158 40
)
36
Electron_t_I diffraction
as
25dpa (EdU=60eV, E~°=3OeV)
T f o r
Ill
E
c
5.1015Xetcm2
30
"o
///
30
d
8
g
.(a
24
25
8
m
=~ 20 ._o
'\
\
× 15 g 10
_g s
18 ~5
12 F-N
X e distribution profile
,
(arbitrary units)
'\
6
i lHRTEM l I 0
50
a 100
150
200
0 250
Depth, nm
Fig. 1. Calculation with the TRIM-code [ 14] of range and damage profiles of 500 keV Xe-ions in UO,. The arrows indicate the approximate specimen thickness for HRTEM and electron diffraction, respectively.
as-prepared UO 2 and on UO 2 with radiation damage produced by Xe-ions of 0.5 MeV energy.
2. Experimental UO 2 specimens were prepared from commercial smtered fuel pellets (96% of theoretical density). Disks of 3 mm diam. were cut from these pellets. Following dim-
piing, they were electropolished in a Unithin double jet apparatus using Lenoir's solution at a polishing potential of 40 V. After perfl~ration, the samples are usually treated for 4 - 5 h in a rapid etching ion-milling apparatus (6 keV, 2.5 mA. 6 ° inclination) to clean the surfaces which are frequently covered with a thin contamination layer from the electropolishing. Some specimens were only ion-milled. Following this preparation procedure, the specimens were annealed at 1400°C in A r / H : to recover the polishing damage [12] and to adjust the O / U - r a t i o to the stoichiometric value of 2.0, since the polishing/dimpling procedure tends to oxidize the treated surfaces. The HRTEM was pertormed before and alter the Xe + irradiation using a JEM 2010 microscope with Gatan slow scan, Digital Micrograph:" image recording and processing systems. The microscope was operated at 200 keV. The 500 keV Xe + irradiation was conducted with in situ TEM using the HVEM-Tandem Facility at Argonne National Laboratory [13]. The facility consists of a high voltage electron microscope which is interfaced to a 2 MV tandem ion accelerator, so that the electron diffraction pattern and TEM image can be monitored during ion irradiation. A 300 keV electron beam was used for the in situ TEM observation. The Xe ~ irradiation was performed at room temperature with a dose rate of 3.4 x 10 ~2 l o n s / c m - s. The maximum sample temperature recorded was 170°C during the Xe ~ irradiation due to beam heating. According to a TRIM "95 [14] full cascade calculation, the peak damage depth is about 50 mn below the UO, surface (see Fig. I) and the peak damage value is around 25 dpa for a dose of 5 × 1015 i o n s / c m e. Included in Fig. 1 is the calculated range profile of the Xe-ions. Also, arrows are included showing the approximate specimen thickness
)5/z"iZ Fig. 2. (A) HRTEM micrograph of an unirradiated UO: crystal showing the presence of an edge dislocation m the framed region: (B) Fourier transform of the framed image in (A): (C) filtered image of l'ramed region in (A) (inverse Fourier transform using the two spots circled in (B)) showing the edge dislocation more clearly•
Hj. Matzke, L M. Wang~Journal of Nuclear Materials 231 (1996) 155-158
of the areas where either HRTEM was performed, or electron diffraction pattern were taken.
157
~ "~ ~
3. Results and discussion HRTEM of as-prepared UO 2 samples revealed no crystal defects except a very low density of edge dislocations on the (111) plane and a few small facetted pores. Fig. 2 shows a HRTEM micrograph of a UO 2 specimen before irradiation. Part A reveals an edge dislocation within the framed square. This dislocation is made more clearly visible after an image filtering process in the Fourier space. With an inverse Fourier transform using the two spots circled in B (the Fourier transform of A), a filtered image is obtained (part C) showing the dislocation very clearly. To the authors knowledge, this is the first time a dislocation has been clearly revealed in UO 2. After 5 x 10 ~s Xe + i o n s / c m 2, small arches around the original diffraction spots were observed with in situ TEM. These small arches are indications for the formation of small crystallites with a small misorientation. HRTEM of the sample indeed revealed many sub-grain boundaries across which crystals are slightly rotated ( 1 - 2 °) to each other. Also, the dislocation density in this irradiated sample is more than an order of magnitude higher than in the original sample. After a simple filtering in Fourier space, it was found that the sub-grain boundaries are associated with the ending of edge dislocations (Fig. 3). This indicates the starting of subgrain formation. This result has clearly demonstrated at least one of the mechanisms by which a large UO 2 grain or even crystal can be divided into many small subgrains. Some of these subgrains will eventually develop into individual grains with larger rotation angle due to increased strain fields caused by increased dislocation densities. This is also indicated by the fact, that ring patterns were obtained in electron diffraction from some areas of a sample irradiated to 1 x 10 t6 X e + i o n s / c m 2. When comparing this fact with the TEM observation, one has to keep in mind that HRTEM is performed on very thin areas, hence in material which has not experienced the maximum dpa-level. In contrast, for electron diffraction thicker areas are used which have been subjected to higher dpa-levels near the maximum level (see arrows in Fig. 1). Based on TEM observations of irradiation induced polygonisation in several intermetallic and ceramic materials (see Section 1), Wang et al. [10] suggested that randomly orientated nano-scale crystallites could either originate from the breakage of the original single crystal due to the formation of amorphous volumes in the irradiated material, or could be directly nucleated from the cascade cores when irradiation is performed near the critical amorphization temperature (above which irradiation could not fully amorphize the material). They concluded that the irradiation-induced polygonization is the result of competi-
J// t
Fig. 3. (A) HRTEM micrograph and electron diffraction from a UO 2 crystal irradiated with 500 keV Xe + at room temperature (the actual temperature was 170°C due to beam heating) to 5 x 10 t5 ions/cm 2 (ca. 25 dpa): (B) Fourier filtered image of (A) with the same filtering technique demonstrated in Fig. 2 showing that the crystal has been divided into subgrains with slightly different orientation (1-2 degrees).
tion between amorphization and crystalline recovery in the materials studied by them. The polygonization seen in irradiated UO 2 is apparently the result of a different mechanism. Amorphization of UO 2 has not been observed even after high dose irradiation at very low temperatures (5 and 20 K), e.g., [15]. The image in Fig. 3(B) clearly demonstrates the association of possible subgrain boundaries with the high density dislocations induced by irradiation as a first step for subgrain formation. A more detailed study is needed for the determination of the exact nature of these dislocations, i.e., to define their Burgers vector and to deduce whether the dislocations are due to an extra oxygen or an extra uranium plane, or both. The final rim structure in irradiated UO 2 shows well developed subgrains in the size range of 150 -t- 50 nm [16]. However, smaller subgrains of 20-30 nm in size as nuclei
158
Hj. Matzke, L.M. Wang/Journal of Nuclear Materials 231 (1996) 155-158
for polygonization were also observed [17]. The present results are thus thought to represent the very first step of polygonization. Note that this step is not due to the presence of large concentrations of fission gases and high pressure fission gas bubbles. These can also act as nuclei for polygonization as shown in previous ion implantation work [8].
Acknowledgements The authors thank the HVEM-Tandem Facility staff at Argonne National Laboratory for assistance during the ion irradiation. The TEM was completed, in part, at the Electron Microbeam Analysis Facility of the Department of Earth and Planetary Sciences at the University of New Mexico. Thanks are also due to T. Wiss (ITU) for preparing the thin foils for TEM analysis and to Professor R.C. Ewing for his interest in this investigation. This work was supported partially by the D O E / B E S under Contract DEFG03-93ER45498.
References [1] A.D. Whapham and B.E. Sheldon, J. Nucl. Mater. 10 (1962) 157. [2] Hj. Matzke, Nucl. Appl. 2 (1968) 131.
[3] I.L.F. Ray, H. Thiele and Hj. Matzke, J. Nucl. Mater. 188 (1992) 95. [4] Hj. Matzke, in: Fundamental Aspects of Inert Gases in Solids, eds. S.E. Donelly and J.H. Evans (Plenum, New York, 199t) p. 401. [5] Hj. Matzke, J. Nucl. Mater. 189 (1992) 141. [6] Hj. Matzke, H. Blank, M. Coquerelle, K. Lassmann, I.L.F. Ray, C. Ronchi and C.T. Walker, J. Nucl. Mater. 166 (1989) 165. [7] Hj. Matzke, in: Ceramics: Charting the Future, ed. P. Vincenzini, Advances in Science and Technology 3D (Techna Srl., 1995) p. 2913. [8] Hj. Matzke, A. Turos and G. Linker, Nucl. Instr. Meth. Phys. Res. B91 (1994) 294. [9] Hj. Matzke, A. Turos and J.C. Dran, in preparation. [10] L.M. Wang, R.C. Birtcber and R.C. Ewing, Nucl. Instr. Meth. Phys. Res. B80&81 (1993) 1109. [l 1] R.C. Birtcher and LM. Wang, Nucl. Instr. Meth. Phys. Res. B59&60 (1991) 966. [12] Hi. Matzke and A. Turos, J. Nucl. Mater. 114 (1983) 349. [t3] C.W. Allen, L.L. Funk., E.A. Ryan and S.T. Ockers, Nucl. Instr. Metb. B40&41 (1989) 553. [ 14] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985). [15] Hj. Matzke, O. Meyer and A. Turos, Rad. Eft. Def. Solids 119-121 (1991) 885. [16] I.L.F. Ray, Hj. Matzke, H. Thiele and M. Kinoshita, in: CEC Report EUR 15154 EN (1992) p. 17: J. Nucl. Mater., submitted for publication. [17] K. Nogita and K. Une, J. Nucl. Mater. 226 (1995) 302.