Boron carbide, B 4 C, is widely used as a neutron absorber in nuclear reactors. Since neutron absorption by 10 B leads to 4 He production, it appears necessary to study He behavior and its possible effects on the B 4 C ceramic. In this study, the diffusion characteristics of ion-implanted He in B 4 C (500 keV, fluences from 1 x 10 +13 to 2 x 10 +15 He.cm-2) were investigated mainly by Thermo-Desorption Spectroscopy (TDS) from 600°C up to B 4 C melting point. The experiments were done on dense B 4 C samples having large grains (30-60 µm) to render grain boundaries effects on He outward diffusion ineffective and thus to access intragranular He diffusion kinetics. From controlled temperature ramp experiments, it was notably observed that He release was realized in two main stages. A first He population was able to exit the material at moderate temperatures by interstitial diffusion. Then a second population was quantitatively released only over 1150°C. This was attributed to He atoms that, in their initial interstitial diffusion course at moderate temperatures from their implantation sites, got trapped in defect aggregates and/or He bubbles. As the nucleations of both these traps are expected to be related to helium and irradiation defect concentrations, the ratio of the two He populations was indeed found to be correlated with the implantation fluence. From the obtained He release curves, the apparent activation energies (E a) of He intragranular diffusion in B 4 C was determined (2.6-3.1 eV) in the 800-1100°C temperature range. This value appears slightly higher than the one determined at lower temperatures, hinting that a change in diffusion mechanism may occur around 800°C. The apparent E a of He detrapping from He bubbles (~2.5 eV) and from defect aggregates (~4 eV) were also determined for temperatures within 1200-1500°C.

Boron carbide samples exhibiting nanometric and submicronic microstructure were sintered by Spark Plasma Sintering to investigate the effect of grain size on mechanical properties. The mechanical properties of sintered monoliths were characterized at the grain and macroscopic scales. Although nanostructured material exhibits finer grains than the submicronic material (i.e. mean diameter of 82 vs. 474 nm), its apparent rigidity and hardness are found to be reduced by 6.8 % and 8.4 % respectively. This contradiction with the Hall-Petch law is linked to the chemical compositions of both materials, which show significant difference in terms of B/C ratio and higher structural oxygen content especially for nanostructured material.

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