Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Tri-isotripic (TRISO) coated fuel particles are being developed for use in high temperature gascooled reactors. TRISO coated particles consist of an oxide kernel ~500 um in diameter surrounded by four layers: a buffer, which consists of low density carbon; an inner pyrolytic carbon (IPyC) layer; silicon carbide (SiC); and an outer pyrolytic carbon (OPyC) layer [1]. A backscattered SEM image of a TRISO particle cross section is shown in Fig. 1. The SiC layer is of particular interest as a pressure vessel for containing fission products within each particle. A full understanding of the SiC mechanical and structural properties in as-deposited and irradiated is essential in order to qualify this material for high burnup applications. In this work, Raman spectroscopy and electron backscattered diffraction (EBSD) were used to characterize the residual strain of the SiC layer in asdeposited and after indentation states.Raman spectroscopy was performed on the SiC layer using a confocal -Raman system. A characteristic Raman spectrum of the SiC layer is shown in Fig. 1. The peaks at 796 cm -1 and 972 cm -1 correspond to -SiC [2]. Using a combination of Raman spectroscopy and FIB-prepared EBSD surfaces, the relative compressive and tensile strains in the SiC lattice were quantified by measuring the shift of the characteristic peaks for SiC or through variations in the calculated d-spacings of the SiC. Close analysis of the Raman spectra indicated that only the cubic polymorph of as-deposited SiC ( -SiC) was present. This polymorph is desired in nuclear applications because of resistance to irradiation swelling [3]. The residual strain in the SiC layer, which results from the CVD process is related to deposition temperature and carrier gas, and may contribute to the mechanical failure of the layer, and thus the release of fission products. As-deposited residual strains were significantly in tension, as determined by Raman and EBSD. These residual strains may contribute to premature failure of the SiC and subsequent release of fission products.Further mechanical response of the SiC layer was probed by placing indentations over a range of lengths scales. Quantification of the Raman shift in the x, y, and z directions as well as EBSD patterns around the indents illustrated changes in strain from a tensile state to either a compressive state beneath the indent or a completely relaxed state on the edges of the indent. EBSD also illustrated crack propagation from the indents via transgranular mechanisms as shown in Fig. 2. Future investigations of post-irradiated samples are planned and will be compared directly with the as-deposited states.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Tip geometry can have a large impact on image quality, resolution, and, depending on the sensing mode, image interpretation of scanned probe microscopy data [1]. Until recently, SPM investigations have lacked detailed characterization of tip geometries. For instance, an in-depth analysis of the tip geometry could lead to standards-based tip imaging via envelope functions [2]. Similar to SPM, atom probe tomography and field ion microscopy could also benefit from a better understanding of tip geometries for improved reconstruction accuracy, evaporation potential determination, and trajectory aberration correction. Successful atomically precise manufacturing (APM) using STM tips depends heavily on the ability to develop and characterize tips with near atomic surface precision [3].During development of tips for APM, several processing techniques for extremely low radius of curvature W tips were developed. In order to fully characterize the tip geometry, hardware was developed that allows for cross sectional tomographic imaging of W wire shaped tips in a modern TEM or STEM, along with FIM, Atom Probe Tomography, and imaging in a UHV STM [4]. During characterization of the tip geometries using SEM, TEM, FIM, and atom probe, it was found that the tip radius was several orders of magnitude larger than expected (Fig 1). Characterization of the tips using STEM-EELS, -EDS and atom probe tomography revealed the presence of a thick (>50 nm), low density WO x . The sub-oxide layer was significantly thicker and less dense than expected for simple reactions with air.Subsequent changes in the experimental methods for cross-examination with all of the above techniques revealed that the oxide formation was actually due to electrostatic discharge (ESD). It is worth noting that most ESD events that result in tip morphology changes are not observable visually or via human touch. Using good ESD tip handling protocol such as continuous grounding of specimen holders and tweezers during transfer had a profound effect on the ability to image in STM mode with nearly atomically sharp tips (Fig 2). Correlating the tip geometry with STM operation has also been illustrated in this work.