Correlative Zernike phase contrast X-ray nanotomography to determine the distribution and orientation of graphite particles in a carbon fiber reinforced epoxy resin for improved thermal conductivity
Simon Carl, Silvan Englisch, Janis Wirth, Benjamin Apeleo Zubiri, Simon Bard, Volker Altstädt, Erdmann Spiecker
2021
Microscopy and Microanalysis
X-ray nanotomography (Nano-CT) is ideally suited for non-destructive three-dimensional (3D) imaging of carbon-based material systems on the nanoscale. The lab-based ZEISS Xradia 810 Ultra X-ray microscope enables standard absorption contrast imaging as well as edge-enhancing phase contrast imaging for low-Z materials by inserting a Zernike phase ring into the beam path behind the sample and Fresnel zone plate. While absorption contrast is usually employed for materials containing regions of
more »
... iciently different atomic numbers, Zernike phase contrast allows to distinguish between different phases in composite materials and accentuates the morphology of embedded particles of similar density. The high-resolution and flexible imaging modes of the Nano-CT instrument, combined with scanning electron microscopy (SEM), allow for a detailed 3D study of the orientation and distribution of graphite particles in a representative 3D volume. Light and resilient materials play an essential role in the manufacturing of novel electric turbines for airplanes. Likewise, thermal conductivity is one of the crucial properties of the turbine's shell that must be optimized before operational use. We investigated a promising material for the shell: carbon fiber and graphite particle reinforced composites. Besides the mechanical properties, the thermal conductivity is significantly improved by incorporating graphite platelets in between layers of unidirectional carbon fibers [1] . To achieve this, the carbon fibers are embedded in a graphite platelet enriched epoxide resin during a rolling manufacturing process. The platelet size averages at 4 µm and the volume ratio composes up to 15 vol% [1]. In particular, the local distribution and orientation of the graphite particles impact their interconnectivity and therefore influence the thermal conductivity. We acquired several Nano-CT tilt-series in large-fieldof-view (LFOV) phase contrast mode (150 nm spatial resolution, 64 µm field of view (FOV)) to determine the 3D distribution and relative orientation of the graphite platelets towards the carbon fibers in detail. The corresponding pillar sample with a diameter of 120 µm was prepared by site-specific laser ablation using a 3D-Micromac microPREP PRO system. The combination of eight slightly overlapping LFOV tilt series (Figure 1b ) enlarges the reconstructable volume and especially improves its statistical significance concerning the local variations in particle orientation and distribution. A smaller pillar of 10 µm in diameter was site-specifically prepared from the larger pillar by focus ion beam (FIB) milling in a Dual-Beam FEI Helios NanoLab 660 to examine the particle interconnectivity of close-packed particles at a higher resolution. The particle size and morphology were examined on the milled surface by SEM imaging and in the pillar volume by high-resolution (HRES) phase contrast Nano-CT with 50 nm spatial resolution (Figure 1c inset) . These measurements reveal that the particles indeed have a flat platelet morphology and already indicate, on the small scale, that there is a certain alignment of the the platelets along the carbon https://www.cambridge.org/core/terms. https://doi.
doi:10.1017/s1431927621003615
fatcat:gq2c44oayvb7xbo46mf2wb4p4m