Reordering and Crystallization of Silicon Carbide Amorphized by Neutron Irradiation
- PDF / 128,268 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 89 Downloads / 207 Views
Reordering and Crystallization of Silicon Carbide Amorphized by Neutron Irradiation Lance L. Snead Metals and Ceramics Division, ORNL Building 4508 Oak Ridge, TN 37831-6087, USA
Martin Balden Max-Planck-Institut für Plasmaphysik Boltzmannstrasse 2 D-85748 Garching bei Munchen, Germany
ABSTRACT Densification and crystallization kinetics of bulk SiC amorphized by neutron irradiation is studied. The temperature of crystallization onset of this highly pure, fully amorphous bulk SiC was found to be between 875-885°C and crystallization is nearly complete by 950°C. In-situ TEM imaging confirms the onset of crystallization, though thin-film effects apparently alter the kinetics of crystallization above this temperature. It requires >1125°C for complete crystallization of the TEM foil. Annealing at temperatures between the irradiation and crystallization onset temperature is seen to cause significant densification attributed to a relaxation, or reordering, of the as-amorphized structure. INTRODUCTION A number of studies exist on the properties and crystallization kinetics of amorphous SiC as discussed in recent reviews.[1-3] Typically, such studies have utilized ion-beams at low irradiation temperatures to form either a buried layer or amorphize completely the specimen within the range of the ion. There are obvious challenges and possible intrinsic errors associated with inferring bulk properties from ion-irradiated materials. These include the implanted-ion effects, stress fields associated with non-uniform damage, and the presence of crystallites at the (beginning and) end of range. For the case of physical-property measurements, the need for very small-scale measurements, contributed to a large variation in published data. In previous studies[4,5], fission neutron amorphization of SiC was demonstrated. By using uniformly amorphized, high-purity materials, precise measurements of properties such as density, hardness, elastic modulus, and thermal conductivity were possible. This purpose of this study was to better understand the kinetics and physical properties which occur during reordering and crystallization of implanted-ion-free amorphous SiC. EXPERIMENTAL PROCEDURE Materials for this study were Cree single-crystal 6-H SiC and Morton CVD SiC. These 25 2 materials were irradiated to 2.6 and 4.0 x 10 n/m (E>0.1 MeV) in the target region of the High Flux Isotope Reactor at ORNL. These fluence levels are taken to be 2.6 and 4 dpa, respectively. However, it is noted that there will be greater displacements on the carbon sublattice as compared to the silicon sublattice. The irradiation temperature was ~70°C. Annealing of each sample was performed in an air furnace for a 30-min.period followed by a 30min.cleaning in hydrofluoric acid to remove any surface silica formation. Density was measured using a density-gradient-column technique. Microscopy was carried out with a Philips CM-12 microscope and a Gatan in-situ annealing stage. TEM samples were prepared by conventional
R5.1.1
thinning and ion millling at 6 keV, 15° usin
Data Loading...
