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. 2017 Sep;267(3):384-396.
doi: 10.1111/jmi.12577. Epub 2017 May 15.

Laser-preparation of geometrically optimised samples for X-ray nano-CT

J J Bailey  1 , T M M Heenan  1 , D P Finegan  1 , X Lu  1 , S R Daemi  1 , F Iacoviello  1 , N R Backeberg  2 , O O Taiwo  1 , D J L Brett  1 , A Atkinson  3 , P R Shearing  1
Affiliations

Laser-preparation of geometrically optimised samples for X-ray nano-CT

J J Bailey et al. J Microsc. 2017 Sep.

Abstract
in English, Portuguese

A robust and versatile sample preparation technique for the fabrication of cylindrical pillars for imaging by X-ray nano-computed tomography (nano-CT) is presented. The procedure employs simple, cost-effective laser micro-machining coupled with focused-ion beam (FIB) milling, when required, to yield mechanically robust samples at the micrometre length-scale to match the field-of-view (FOV) for nano-CT imaging. A variety of energy and geological materials are exhibited as case studies, demonstrating the procedure can be applied to a variety of materials to provide geometrically optimised samples whose size and shape are tailored to the attenuation coefficients of the constituent phases. The procedure can be implemented for the bespoke preparation of pillars for both lab- and synchrotron-based X-ray nano-CT investigations of a wide range of samples.

A novel way of making samples so that they can be successfully imaged with X‐rays has been developed. This process involves using highly focused lasers to mill away excess material, to leave cylindrical samples ready to be placed in the X‐ray beam. The X‐ray procedure investigated is known as X‐ray computed tomography and is the materials science equivalent of medical CT scanners found in most hospitals. The technique involves rotating the small pillar in the path between a laboratory X‐ray source and a detector, producing a number of images, each similar to a classical bone scan. Using a sophisticated mathematical procedure, these images are reconstructed into a three‐dimensional volume, giving information about the complex microstructure at the nanoscale. This has been applied to materials used for energy generation and a geological sample to illustrate the versatility and robustness of the preparation route.

Keywords: X-ray tomography; laser micro-machining; lithium-ion batteries; sample preparation; shale; solid oxide fuel cells.

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Figures

Figure 1
Figure 1
Schematic of the imaging set‐up used in typical lab‐based quasi‐monochromatic X‐ray nano‐computed tomography instrumentation.
Figure 2
Figure 2
Workflow for sample preparation route and corresponding optical images for (A) base tier of structure and (B) fine pillar tier.
Figure 3
Figure 3
(A) Photograph of 36° Al stub with a dowel attached, SEM micrographs of (B) tiered structure at low magnification, (C) pillar aligned before milling and (D) pillar after FIB milling (clockwise from top‐left).
Figure 4
Figure 4
(A) Optical image of traditional mechanical preparation where the red square highlights the location of the sample material. (B) Corresponding mosaic of radiographs of the sample material. (C) Single radiograph with a FOV equal to that used for CT. (D) Single raw orthoslice from reconstructed tomogram. (E) Tomographic vertical slice from 60 μm pillar, magnified features at (F) 901 projections, binning 2, (G) 1601 projections, binning 1 and (H) 1601 projections, binning 2.
Figure 5
Figure 5
(A) Optical image of the new laser‐prepared pillar where the red square highlights the region of interest at the top of the pillar. (B) Corresponding radiograph of the top of the pillar. (C) Reconstructed vertical slice directly after laser micromachining. (D) Reconstructed vertical slice after further FIB milling. (E) Raw slice from usable volume after FIB milling. (F) Corresponding segmented image showing the three phases. (G) 3D reconstruction of full volume (white = YSZ, grey = nickel, transparent = pore volume), (H) 3D TPB map.
Figure 6
Figure 6
Plot displaying representative volume element analysis data for pore volume fraction and directional tortuosity factor (all three orthogonal directions).
Figure 7
Figure 7
(A) Optical image of traditional mechanical preparation. (B) Corresponding radiograph. (C) Optical image of new laser‐prepared pillar. (D) Corresponding radiograph. (E) Raw orthoslice showing NMC electrode particles. (F) 3D surface rendering of electrode particles (blue) and current collector (black). (G) RVE Analysis of tortuosity calculated in the through‐plane direction, with error inset.
Figure 8
Figure 8
Vertical plane radiograph of the side of Li‐NMC electrode pillar, laser‐affected zone (LAZ) highlighted in red.
Figure 9
Figure 9
(A) Optical image of traditional mechanical preparation of Li‐ion graphite anode where the red square captures a region that appears to be sufficiently far from the edge of the sample to avoid severe distortion of the microstructure. Note the large amount of material that would be outside the FOV during imaging. (B) Corresponding radiograph of the tip of the sample where the microstructure is distorted from the mechanical preparation technique. (C) Optical image of new laser‐prepared pillar of the graphite anode with copper current collector still attached, (D) Corresponding radiograph. (E), (F) Absorption contrast tomographic slices from the horizontal and vertical planes, respectively. (G), (H) Phase contrast tomographic slices from the horizontal and vertical planes, respectively.
Figure 10
Figure 10
(A) Raw orthoslice from the vertical plane showing the entire depth of the graphite anode where the current collector has been cropped out. (B) Corresponding segmented orthoslice. (C) 3D volume rendering of graphite electrode (grey) on Cu current collector (yellow). (D) RVE Analysis of tortuosity calculated in the through‐plane direction (direction of Li‐ion percolation), with error inset.
Figure 11
Figure 11
(A) Optical image of coarse pillar for micro‐CT. (B) Corresponding radiograph. (C) Raw orthoslice displaying four phases. (D) 3D reconstruction showing the extra segmentable phase. (E) Optical image of fine pillar for nano‐CT. (F) Corresponding radiograph. (G) Particulate microstructure evident from raw slice. (H) 3D reconstruction of clay particles at high resolution.
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