Pongun Son

Self-Diffusion of Carbon in Uranium Monocarbide

The diffusion of C in UC has been measured using radioactive tracer and sectioning technique. Self-diffusion coefficients of C in UC are described well by the equation over the temperature range of 1,400°–2,000°C.

Self-diffusion of carbon in hyperstoichiometric uranium monocarbide

Abstract The self-diffusion of C in hyperstoichiometric UC has been measured in the single-phase region of UC, using a radioactive tracer and sectioning technique. The diffusion equations as a function of temperature are: 1. (i) for UC1.38, D = 0.10 exp (−(53.5 ± 10.6)/RTkcal/mole) cm2/sec for the temperature range 2100–2230°C; 2. (ii) for UC1.52, D = 0.11 exp (−(54.6 ± 4.6)/RTkcal/mole) cm2/sec for the temperature range 1950–2150°C. The dependence of the diffusion coefficients and the activation energy is described as follows, 1. (i) when x (C/U molar ratio) ≲ 1.11, the diffusion coefficients increase with x, while the activation energy decreases, 2. (ii) when ≳, the diffusion coefficients no longer increase with x but remain constant, as does the activation energy, r.e., E is about 54 kcal/mole. The mechanism for C diffusion in hyperstoichiometric UC is considered to be an interstitial or interstitialcy.

ESR spectra in graphite irradiated with He+ ions

The irradiation damage was studied by means of ESR measurement for highly oriented pyrolytic graphite irradiated by 20 keV He+ ions. In addition to the line which was due to the charge carriers, a symmetric line was observed in the ESR spectrum after irradiation. This line was considered to be due to the localized spins produced in damaged region. The intensity, the line width and the g value of this line depended on irradiation dose. The temperature dependences of intensity and line width also changed with irradiation dose up to about 5 × 1026He+/cm2, but above this dose they were independent of irradiation dose. These changes in ESR spectra were correlated with those in Raman spectra and discussed by an exchange interaction between localized and delocalized spins.

Thermal cycling test of TiC-graphite☆

Thermal cycling tests were performed on the TiC-Poco graphite prepared by diffusion annealing. The TiC coating survived quite well after 1000 thermal cycles between 500 and 1100°C. However, at the maximum temperature of 1200°C, the development of grain boundary steps and the localized microcracking were found to occur after 1000 cycles. The surface damage and erosion due to blistering and exfoliation could be observed on the polished TiC coating irradiated with 40 keV He+ to a dose of 1.8 × 1017 ions/cm2 whereas the surface of the as-prepared coating showed no damage up to a dose of 1.4 × 1018 ions/cm2. The results for thermal cycling tests of the post-irradiated TiC coating indicated that in the case of the coating irradiated below the critical dose for blister formation the gaseous release of the implanted atoms caused the serious surface damage.

Ion irradiation and thermal cycling tests of TiC coatings

Abstract Ion irradiation of TiC coatings prepared by diffusion annealing was performed with 20–40 keV He + ions for different doses at room temperature. The polished TiC 0.99 coatings irradiated with 40 keV He + ions showed the surface damage and erosion due to blistering and exfoliation above a dose of 1.8 × 10 17 ions/cm 2 , whereas no change in the surface mophology could be detected for the as-prepared coatings up to a dose of 1.4 × 10 18 ions/cm 2 . The results suggested that surface erosion due to blistering can be effectively reduced on the rough surface of the as-prepared TiC coating. The average blister diameter in the polished TiC 0.99 coating increased with increasing projectile energy. For the 40 keV He + ion irradiation of the polished TiC 0.5 coatings, general features in blisters were similar to those observed for the TiC 0.99 coatings, but the critical dose for blistering shifted to a higher value in comparison with the polished TiC 0.99 coating. Thermal cycling between 500 and 1200°C caused serious surface damage for the TiC 0.99 coating irradiated with 40 keV He + ions below the critical dose for blistering, while the coating with surface damage due to blistering showed no significant change in the surface topography after thermal cycling.

Thermal erosion of graphite by pulsed electron beam

Abstract Electron beam heating tests were performed for several kinds of Isotropie graphite to evaluate the dependence of thermal erosion behavior on the graphite species and on the heat load. The specimens were irradiated by electron beams with power densities of 25 to 45 MW/m2 and a pulse length of 2.0 s. A crater caused by erosion was found to be the major damage and preferential erosion was observed at the grain boundaries. In some kinds of graphite, a large weight loss caused by thermal erosion, which exceeded the expected weight loss due to sublimation, was observed. In these cases, some of the ejected bulk graphite particles that had collected on a Mo sheet were observed with an optical microscope. These results suggest that some kinds of graphite will be eroded mainly by the “particle emission” mechanism under high heat loads.

Thermal cycling test of titanium in deuterium atmosphere

Abstract Thermal cycling tests of titanium in deuterium atmosphere were carried out with an infrared image furnace in a constant volume system. No change in the surface microstructure of the titanium specimen was observed after thermal cycles between 800 and 600°C whereas thermal cycling between 800 and 250°C caused microcracking along the grain boundaries of α-titanium on the surface of the specimen. From metallographic observations of the cycled specimens, it was found that the morphology of deuteride precipitation in the specimen strongly depended upon temperature range of cycles. The number of cycles was found to affect the distribution and orientation of deuteride precipitations. Decrease in the heating and cooling rates resulted in more oriented deuteride precipitates. The isothermal desorption behavior of the cycled titanium specimen was also studied by comparing with that of non-cycled titanium. Above the desorption temperature of 600°C, no marked difference in the desorption behavior could be observed for either cycled or non-cycled specimen. However, at relatively lower temperature, the desorption rate for the cycled specimen was considerably lower than that for the non-cycled specimen.