Flexible fluorinated graphite foils with high contents of the (C2F)n phase for slow neutron reflectors.

In order to prepare self-standing and flexible slow neutron reflectors made of graphite fluoride (GF) with high contents of (C2F)n structural phase, graphite foils of different thicknesses were used as starting materials for gas (F2)/solid fluorination. The maximal interlayer distance of GF was obtained with this phase thanks to the stacking sequence FCCF/FCCF; this is mandatory for the efficient reflection of slow neutrons. 71 and 77% of the (C2F)n phase were achieved for graphite foils with thicknesses of 1.0 and 0.1 mm, respectively. The interlayer distances were 8.6 Å as expected. The fluorination conditions (static mode, a long duration of 24 h, annealing in pure F2 gas for 24 h, and temperatures in the 390-460 °C range) were adapted to large pieces of graphite foils (7 × 7 cm2) in order to both avoid exfoliation and achieve a homogeneous dispersion of fluorine atoms. This process was also efficient for thinner (0.01 mm thick) graphitized graphene oxide foil. 56% of the (C2F)n phase and an interlayer of 8.6 Å were achieved for this foil when fluorination was performed at 430 °C. Whatever the nature and the thickness of the foil, their flexibilities are maintained.