WEB Determination of the graphite particle orientation and distribution in a carbon-fiber-reinforced epoxy resin by phase-contrast X-ray nanotomography (Nano-CT)
A novel approach to improve the transversal thermal and electrical conductivity while preserving a low material density and its outstanding mechanical properties is the development of a new fabrication process of carbon fiber- and particle-reinforced composite systems. This material features a matrix of epoxide resin, embedded layers of unidirectional carbon fibers, and up to 20 %Vol graphite platelets with an average size of 4 µm. The orientation of the embedded graphite particles, affected by the manufacturing process, strongly influence the electrical and mechanical properties .
Nano-CT is ideally suited for non-destructive 3D-imaging of this carbon-based material system. For this low Z material, the lab-based system ZEISS Xradia 810 Ultra includes beside the standard absorption contrast also phase-contrast imaging by implementing a Zernike phase-ring. While absorption contrast images materials containing regions and elements of sufficiently different atomic numbers Z, Zernike phase contrast can enhance the phase transition of composites and even exhibit the morphology of embedded particles with similar densities and materials, which are described as phase-objects (low Z). The high-resolution and flexible imaging modes of the instrument, in combination with Scanning Electron Microscopy (SEM), allows in-depth 3D-studies of the 3D-orientation and distribution of the graphite particles in a 3D-volume.
We recorded several tilt-series in phase-contrast mode (150 nm spatial resolution, 64 µm FOV) to determine the relative orientation of the graphite platelets relative to the carbon fibers. One of the challenges of the determination of the orientation is the segmentation of the small and often coinciding particles of different morphologies and sizes. The combination of eight slightly overlapping tilt series improves the uniformity of the segmentation and satistical significance. By combining the individual tilt series  and reconstructing the resulting datacube into a single 3D-reconstruction, an accurate and large scale analysis of the graphite particle size distribution in the 3D-volume is achieved. This approach conserves the spatial resolution using a novel method exploiting the wide-area, high transmittance, and edge enhanced X-ray imaging.
The resulting segmentation of the 3D-reconstruction and the orientation of the graphite particles can be further improved by correlation with corresponding SEM and Focused-Ion-Beam slice&view imaging.