Interaction of tungsten films with glassy carbon

Abstract

The demand for dry casks for extended storage of radioactive nuclear waste is driven by dearth of permanent repositories. Most of the dry cask storage system currently in use are made up of stainless steel which could become vulnerable to corrosion over a period of time. The service lifetime of these steel-based canisters can be improved by electrolytic treatment of its outer surface with layers of glassy carbon and tungsten. This will ensure sufficient protection against extended long term corrosion and chemical attacks. In this study, the focus has been on the solid state interaction between the W films and glassy carbon substrates. W films were sputtered on the glassy carbon substrates to form diffusion couples. The stability of the diffusion couples under the heat treatment, the interface interaction and carbide phases formed have been studied. To gain more insight on the interface mixing re- gions due to annealing, quantitative measurements of the solid state reactions between the deposited W films and glassy carbon substrates were carried out. The as-deposited samples were sequentially annealed isothermally un- der vacuum at temperatures ranging from 400 to 1000 ◦C in steps of 100 ◦C. The microstructural changes due to thermal annealing were monitored by Rutherford backscattering spectrometry (RBS) and grazing incidence X-ray diffraction (GIXRD). RUMP software was used to simulate the RBS spectra. The thickness of W thin film deposited, atomic composition of deposited layer and the intermixed layer growth were deduced from the RUMP simulation results. The RBS and GIXRD analysis showed that carbide formation was first observed at an annealing temperature of 900 ◦C. The kinetics of the solid-state interaction was found to be diffusion controlled at the interface between W and C. The activation energy for the diffusion of C in W was estimated as 2.23 eV. The XRD results showed that the average crystallite size of the glassy carbon was estimated as 2.57 nm, while that of the as-deposited W film was 9.77 nm. This value for W film increased with annealing temperature up to 18.05 nm at 1000 ◦C. The first carbide phase observed was W2C in the sample annealed at 900 ◦C, while WC was the dominant carbide phase at 1000 ◦C. The surface morphology of the deposited W films was characterized by scanning electron microscopy (SEM). The SEM micro-graphs showed that the as-deposited films were smooth and homogeneous. SEM images of the annealed sample showed complete absence of delamination even at 1000 ◦C. This showed that the W films were firmly adhered to the glassy carbon substrate, indicating that appropriate sputtering parameters were used. Furthermore, the microstructural changes of GC under the influence of heat treatments and highly charged ion (HCI) irradiation were monitored by XRD and Raman techniques. Raman results for the heat treated glassy carbon samples showed that the graphitic domains experienced a growth in size upon annealing. On the other hand, under the influence of HCI irradia- tion, Raman results showed that glassy carbon experienced microstructural disorder. The crystallite size of the glassy carbon irradiated with fluence of 1.0×1011 ions/cm2 at kinetic energy of 60 keV was estimated as 1.61 nm. This value decreased to 1.54 nm in the sample bombarded with the highest kinetic energy 460 keV with intermediate fluence of 5.0×1011 ions/cm2. The atomic force microscopy (AFM) analysis of these irradiated samples showed that the induced surface roughness increased with both fluence and kinetic energy of the HCI. In conclusion, the XRD study of the microstructure of glassy carbon by method of intensity calculation showed that it is composed of both amorphous and crystalline carbon materials. The evaluated percentage amorphous and crystalline contents are 25% and 75%, respectively.