• PDF / 240,967 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 78 Downloads / 208 Views

DOWNLOAD

REPORT

---

MICROSTRUCTURE OF SWIFT HEAVY ION IRRADIATED SiC, Si3N4 AND AlN S.J. ZINKLE*, J.W. JONES* AND V.A. SKURATOV** *Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6138 USA, [email protected] **Joint Institute for Nuclear Research, Flerov Lab, Center of Applied Physics, 141980 Dubna, Russia ABSTRACT Cross-section transmission electron microscopy was used to investigate the microstructure of single crystal silicon carbide and polycrystalline silicon nitride and aluminum nitride following room temperature irradiation with either 245 MeV Kr or 710 MeV Bi ions. The fluences ranged from 1x1012/cm2 (single track regime) to 1x1013/cm2. Ion track formation was observed in the Bi ion-irradiated Si 3N 4 specimen in regions where the electronic stopping power exceeded a critical value of ~15 keV/nm (depths 34 keV/nm. The high resistance of SiC and AlN to track formation may be due to their high thermal conductivity, but further study is needed to quantitatively evaluate the suitability of the various track formation models. INTRODUCTION Previous studies have shown that discontinuous track formation in ceramic insulators can be produced by swift heavy ions with electronic stopping powers (dE/dx)e above ~5 to 8 keV/nm, and continuous cylindrical tracks are typically observed for (dE/dx)e >15 to 20 keV/nm [1]. The physical mechanism(s) responsible for track formation in radiolysis-resistant solids are still under debate (e.g., thermal spike vs. Coulomb explosion models). Therefore, additional data on a wide range of materials are needed to investigate material parameters such as the effect of band gap energy and thermal conductivity on track radius, etc. As noted previously [2], it is possible that densely ionizing fission tracks (e.g. ~70 MeV I, Xe or Cs) can create irradiated microstructures that are not achievable with conventional (elastic collision) irradiations. Therefore, materials which exhibit good radiation resistance under elastic collision damage conditions (e.g., MgAl2O4) may not necessarily exhibit similar radiation damage resistance under fission track recoil conditions [2]. This observation needs to be considered during the screening of candidate inert matrix fuel hosts for fission reactors, due to the large number of fission track recoils in reactor fuels. Most previous swift heavy ion studies on ceramics have focussed on oxide insulators. Very little information is known about the behavior of carbides such as SiC [3-5], and even less is known about the behavior of nitride insulators such as AlN and Si3N4. EXPERIMENTAL PROCEDURE Single crystal SiC (Cree) and polycrystalline specimens of AlN (Tokuyama Shapal grade SH15) and Si3N 4 (Kyocera SN733, hot isostatically pressed) were irradiated at room temperature with either 245 MeV Kr or 710 MeV Bi ions at the Dubna U-400 cyclotron facility. The ion fluences ranged from 1x1012 ions/cm2 (isolated track regime) to 1x1013/cm2. The electronic stopping power (Se) and displacement damage profiles were calculated using the TRIM2000 program [6]. Figure 1 shows th