In situ XRD study of nano-crystalline NiO formation on Ni-coated carbon fibers: Structural and optical properties

The study focused on nickel-coated carbon fibers, where nickel was partially substituted by carbon, forming a compound Ni_1-xC_x (with x  = 0 < x < 1). These modified fibers exhibit excellent surface properties, making them promising for nuclear applications, such as detectors and reflectors. The fibers were subjected to annealing treatments under vacuum and in open air at 500 °C. X-ray diffraction (XRD) analysis revealed that oxidation induces long-range carbon diffusion and results in the formation of a NiO layer on the fiber surfaces. Notably, cubic-type nanostructured NiO (NaCl type) was observed to begin forming at 350 °C. Scanning Electron Microscopy (SEM) showed that the coated fibers exhibit a granular morphology with nanometer-sized grains. Using a 375 nm excitation wavelength, photoluminescence measurements displayed two broad emission peaks. These peaks are likely attributed to electronic transitions involving the 3d electrons of Ni²⁺ ions. Overall, the work demonstrates that controlled oxidation can tailor the surface properties of these fibers, potentially enhancing their performance in nuclear applications by combining semiconducting and reflective characteristics.