The effects of neutron irradiation on the mechanical properties and microstructure of SiC and SiC-based fibers is a current focal point for the development of radiation damage resistant SiC/SiC composites. This report discusses the radiation effects on the Nippon Carbon Hi-Nicalon{trademark} fiber system and also discusses an erratum on earlier results published by the authors on this material. The radiation matrix currently under study is also summarized.
The effects of fast neutron irradiation on SiC and SiC composites have been studied. The materials used were chemical vapor deposition (CVD) SiC and SiC/SiC composites reinforced with either Hi-Nicalon{trademark} Type-S, Hi-Nicalon{trademark} or Sylramic{trademark} fibers fabricated by chemical vapor infiltration. Statistically significant numbers of flexural samples were irradiated up to 4.6 x 1025 n/m2 (E>0.1 MeV) at 300, 500 and 800 C in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Dimensions and weights of the flexural bars were measured before and after the neutron irradiation. Mechanical properties were evaluated by four point flexural testing. Volume increase was seen for all bend bars following neutron irradiation. Magnitude of swelling depended on irradiation temperature and material, while it was nearly independent of irradiation fluence over the fluence range studied. Flexural strength of CVD SiC increased following irradiation depending on irradiation temperature. Over the temperature range studied, no significant degradation in mechanical properties was seen for composites fabricated with Hi-Nicalon{trademark} Type-S, while composites reinforced with Hi-Nicalon{trademark} or Sylramic fibers showed significant degradation. The effects of irradiation on the Weibull failure statistics are also presented suggesting a reduction in the Weibull modulus upon irradiation. The cause of this potential reduction is not known.
The effects of neutron irradiation was investigated in both n- and p- type 4H silicon carbide. Photoluminescence (PL), deep level transient spectroscopy (DLTS), and Hall effect measurements where used to observe optical and electrical characteristics and identify changes in basic material properties. The material was irradiated using an open pool research reactor. Highly doped n- and p-type materials (ND-NA Æ 1.2E17 and NA-ND Æ 1.5E18 cm-3 respectively) were chosen to aid in device fabrication. The material demonstrated no measurable effect to 1 MeV neutrons at fluences of up to 1E14 n/ cm2 and devices were unable to be constructed when exposed to fluences greater then 1E16 n/cm2. The effective suppression of the near bandgap zero phonon PL luminescence lines was shown as a function of increasing neutron fluence, and attributed to the dislocation of neutral nitrogen donors. Deep level defects sites also developed and where shown to increase in density with increased neutron fluence. Hall measurements generally agreed with theoretical expectations but failed to yield conclusive results. Capacitance rollover was observed near 510 K beginning with fluences of around 5E15 n/cm2. Irradiated devices also showed unexpectedly permanent degradation after hour-long exposure to temperatures exceeding 600K during DLTS measurements.
A conclusion of this thesis is SiC detectors that are placed in the thermal neutron region of a graphite moderator-reflector reactor have a chance to survive at least one reactor refueling cycle, while their count rates are acceptably high.
