EFTEM measurements of the sp2/sp3 ratio on SiC/SiC pyrolytic carbon interphases before and after irradiation

Abstract

We investigated the behaviour of a specific component of silicon carbide based ceramic matrix composites (SiC/SiC) under irradiation, namely the pyrolytic carbon (PyC) interphase linking the matrix and fibres together. SiC/SiC is studied for its potential as a new nuclear fuel cladding to replace the zirconium based alloys currently used. The hybridization of the carbon atoms found in this interface layer is studied using energy filtered TEM (EFTEM) to determine the ratio between sp 2 and sp 3 bonding states (R‐ratio), before and after irradiation with ions and neutrons. Indeed, the initial state of the interphase is graphite‐like PyC, in which the electrons occupy the π bonding states of the sp 2 orbitals. The ratio between the π and σ occupied levels is different in each carbon allotropes, be it amorphous, graphite or diamond. Radiation damage is known to amorphise and open micro‐cracks in nuclear graphite [1], [2]. In cases where the graphitic interphase would become completely amorphous, both the mechanical and thermal properties of the composite would be impacted. In this work, TEM lamellae prepared from both pristine and neutron irradiated SiC/SiC have been examined. These samples are extracted from actual prototype cladding sections produced by General Atomics. In addition to this, the samples prepared from pristine material have been subsequently irradiated at the in‐situ TEM JANNuS facility in Orsay, France [3]. Using the unique capabilities of this apparatus, self‐ion irradiations with both Si + and C + were performed. Damage levels approaching one displacement per atom (dpa) were reached whilst observing the effects of said ions at the same time with a TECNAI G 2 20 Twin TEM. The EFTEM measurements were carried out at EPFL, Lausanne, using a Schottky FEG JEOL 2200FS with an in‐column omega filter. All of these measurements were carried out as close as technically possible to magic angle conditions [4]. The ratio between the peaks corresponding to the π * and σ * anti‐bonding states can be quantified with different techniques, two of which are used in the present work: the two window (TWM) [5], whereby the absorption spectrum is numerically integrated over two energy ranges or windows are centered on the aforementioned peaks and spectrum fitting using Gaussian functions [6] is also used to ensure the robustness of the measurement. The maps displayed in Figure 1 and 2 show an overall decrease of the R‐ratio after irradiation of the samples, meaning that even sub‐dpa damage levels are already inducing an amorphisation of the PyC interlayer. Additionally, the overall layer is homogeneously amorphised, as further shown with the histograms displayed in Figure 3. The authors would like to thank Westinghouse and General Atomics for providing the prototype cladding tubes within the CARAT research program as well as JANNuS‐Orsay part of the CSNSM, Orsay, France which is part of the EMIR French accelerators network, where the in‐situ experiments were carried out.