J. C. Corelli

Radiation damage in silicon carbide and graphite for fusion reactor first wall applications

Silicon carbide and graphite materials were exposed to fast neutron fluences of 2 × 1023 to 2 × 1024n/m2 (E > 1 MeV) and a study was made of changes in fracture strength, Weibull modulus and electrical resistivity. Silicon carbide (Norton NC-430) exhibits a decrease in fracture strength (25%) at the higher fluence if the temperature is kept at 298 K, while at 1473 K the decrease in fracture strength is only 10% indicative of recovery due to thermal annealing. The fracture strength of the graphite (POCO AXF-5Q) tested at 298 K increases rapidly by ~20% after 2 × 1023n/m2 and remains constant at higher fluence. Analyses of the data using the Weibull weakest link model were given, in addition to annealing and swelling results.

Microstructure of neutron irradiation-induced defects in sintered and siliconized SiC

High voltage electron microscopy (HVEM) and X-ray diffractometry were used to examine the microstructure of boron sintered SiC and reaction bonded siliconized SiC doped with natural boron and with boron enriched in 10B and 11B to study the effect of helium gas from the 10B(n,α)7 Li reaction. X-ray studies show that both types of SiC exhibit a 1% swelling in the unit cell as contrasted with 3% macroscopic swelling. Analysis of the diffraction peaks produced strong evidence confirming the beta to alpha phase shift in the reaction bonded material. A Warren-Averbach analysis of the peak profiles of the irradiated 10B sintered material shows a crystallite size of 150 nm and a strain in the crystal of ~0.43%. Reaction bonded SiC is characterized by planar defects in the form of large loops (10–200 nm) and dislocation tangles for all irradiation levels regardless of dopant. Boron sintered SiC has many helium associated defects which appear as symmetric strain field structures of 40 to 90 nm dimensions. After annealing at 1773°K, the microstructure of reaction bonded SiC does not change significantly while the strain fields in the boron sintered SiC disappear.

Radiation response of reaction-bonded and sintered SiC: Effects of boron isotopes

Abstract The response of mechanical, thermal and microstructural properties of reaction-bonded SiC and sintered SiC were studied after reactor irradiation. The effects of 10 B(n,α) 7 Li reaction products were studied by doping the material from which samples were produced with enriched isotopes of 10 B, 11 B and natural boron. Silicon carbide doped with 10 B exhibits a pronounced effect on the fracture strength. The thermal diffusivity decrease can account for the lowering of the resistance against thermal shock of irradiated material.

Effects of hydrogen ion implantation on Al/Si Schottky diodes

Al/Si(p) and Al/Si(n) Schottky diodes were implanted with hydrogen ions such that the peak of the hydrogen distribution was localized at the metal‐semiconductor interface. Current‐voltage (I‐V) measurements indicated more ohmic behavior in the Al/Si(n) and more rectifying behavior in the Al/Si(p) diodes. For both cases, annealing at 200 °C for 30 min caused the I‐V curves to almost revert to the pre‐implantation characteristics. A similar behavior was observed using the capacitance‐voltage (C‐V) measurement technique. No significant change of the hydrogen concentration or redistribution of the concentration was observed after the 200 °C heat treatment. Correlation between the hydrogen depth profiling data and the electrical measurements indicated that, as far as I‐V and C‐V were concerned, the implanted hydrogens were electrically inactive.

Microstructure of polycrystalline SiC containing excess Si after neutron and ion irradiation

Abstract The microstructure of commercially available reaction bonded polycrystalline SiC containing 8–10 wt% excess silicon was studied utilizing transmission electron microscopy after irradiation by reactor neutrons, dual ion beams (silicon and alpha particles) and single ion beams (argon). For reactor irradiations performed with the sample temperature kept

Interaction of Li and O with Radiation‐Produced Defects in Si

The interaction of Li atoms with O impurities and with defects produced in Si by 1.5 MeV and 47 MeV electrons was studied using infrared spectroscopy. Li was diffused in low oxygen‐containing uncompensated Si with initial resistivity in the range 15–28 500 Ω·cm and in high oxygen‐containing Si with initial resistivity of 15–200 Ω·cm. The Li concentration in the samples varied from about 1016 to ≲1019 Li atoms/cm3. New radiation‐induced bands at 1.4 and 1.7 μ are observed. Divacancy production as measured by the 1.8 and 3.3 μ defect bands is markedly decreased in samples of high Li content with concomitant growth of the Li‐associated 1.4 and 1.7 μ bands. Annealing of the 1.4 and 1.7 μ bands strongly depends upon the Li concentration. Recovery occurs at a temperature of 150°C for the 1.4 μ band, and at 350°C for the 1.7 μ band, for samples containing ∼6×1017 Li/cm3. In samples containing ∼2×1016Li/cm3, the bands at 1.4 and 1.7 μ disappear after annealing at 330° and 470°C, respectively. Sharp Li‐ and O‐asso...

Thermal properties of neutron-irradiated SiC; effects of boron doping

The temperature dependence (25°C to 1000°C) of thermal conductivity for siliconized (reaction bonded) SiC and alpha phase (sintered) SiC irradiated to neutron fluences of 4 to 8 × 1024n/m2 (E>1 MeV) were studied utilizing the heat pulse technique. The fluences are equivalent to 0.8 and 1.6 dpa and the sample temperature during irradiation was ~ 140°C. Silicon carbide exhibits a significant decrease in thermal conductivity after irradiation, specifically a factor of ~ 5 decrease is observed for siliconized SiC. Comparisons were made with SiC samples doped with 10B, 11B, and natural boron to investigate the effects of impurity doping. It was found that the presence of natural boron and 11B have no significant effect on the thermal conductivity of irradiated SiC, whereas SiC doped with 10B exhibits a slightly larger decrease in thermal conductivity due to the enhanced radiation damage (e.g., helium production) through the 10B (n, α)7Li reaction. The lowering of the thermal conductivity after irradiation can explain the decreased resistance against thermal shcok of irradiated SiC. The decrease in thermal conductivity is due to enhanced phonon scattering by radiation-induced vacancies and dislocations. Results on annealing effects and comparison with mechanical properties are presented.

Study of atomic disorder produced by fast neutrons in silicon using infrared spectroscopy

A study has been made of the absorption bands of silicon, irradiated with high doses of fast neutrons (E≳1 MeV), by infrared spectroscopy in the region between 200 and 4000 cm−1 (50–2.5 μ). The relationship between several effects [band tailing (0.2–0.5. eV), divacancy‐associated band (3.45 μm), higher‐order bands (700–1400 cm−1), and single‐phonon bands (observed at 488, 410, and 332 cm−1)] and different irradiation doses up to 1019 fast neutrons/cm2 has been investigated. From the results of the absorption due to band tailing it can be assumed that the crystal is not amorphous. However, the observed vanishing of the higher‐order bands at the highest doses indicates an important change in the damaged lattice. The annealing behavior up to 600 °C, at the measuring temperatures of 78 and 300 °K, showed a slower annealing out of the single‐phonon bands for the high dose. From the stressing experiments, performed for single‐phonon bands, a constant dichroic ratio of 1.12 was measured. From this an estimation ...

Infrared stress birefringence in KBr, KCl, LiF, and ZnSe

The application of a variation of intensity method using static stress and polarized light to measure stress birefringence in the infrared wavelength region (∼4−11 μm) is described. We have used this method to measure the stress optic coefficient Cλ for the alkali halide cubic crystals KBr, KCl, LiF, and polycrystalline ZnSe. A detailed discussion of the definition of Cλ in terms of piezo−optic coefficients is given in the paper. Our results indicate that no wavelength dependence is observed for Cλ in the 4−11−μm region for KCl and KBr which have typical measured Cλ values of +3.3 and +3.4 Brewsters, respectively. For LiF at 8 μm we get Cλ=−3.08 Brewsters and for ZnSe we obtain Cλ values of −13.2 Brewsters at 9.4 μm and −13.0 Brewsters at 11 μm.

Infrared properties of 40-60 mev electron- irradiated germanium

The infrared properties of germanium containing low oxygen concentration ( 1017 cm−3) have been studied after irradiation at T≤25°C by 40‐ to 60‐MeV electrons. One cold‐temperature irradiation at 125°K was also performed on a high oxygen concentration germanium sample. The germanium samples that were n type initially exhibit p‐type infrared properties in the 2‐ to 8‐μ wavelength range after being converted to p type by the irradiation. Germanium containing high oxygen concentration exhibits new vibrational bands in the 9‐ to 16‐μ wavelength range which are associated with oxygen‐defect complexes. No radiation‐induced bands from 9 to 16 μ are observed in oxygen‐free germanium. Detailed isochronal annealing experiments (up to ∼700°K) were performed on all samples. Strong evidence is given which shows that one of the oxygen vibrational bands induced in germanium (at 618 cm−1) appears to be a single oxygen atom plus a vacancy defect which corresponds to the comparabl...