Focused Ion Beam Nanotomography Research Articles

Nanoscale imaging of Fe-rich inclusions in single-crystal zircon using X-ray ptycho-tomography

We apply X-ray ptycho-tomography to perform high-resolution, non-destructive, three-dimensional (3D) imaging of Fe-rich inclusions in paleomagnetically relevant materials (zircon single crystals from the Bishop Tuff ignimbrite). Correlative imaging using quantum diamond magnetic microscopy combined with X-ray fluorescence mapping was used to locate regions containing potential ferromagnetic remanence carriers. Ptycho-tomographic reconstructions with voxel sizes 85 nm and 21 nm were achievable across a field-of-view > 80 µm; voxel sizes as small as 5 nm were achievable over a limited field-of-view using local ptycho-tomography. Fe-rich inclusions 300 nm in size were clearly resolved. We estimate that particles as small as 100 nm—approaching single-domain threshold for magnetite—could be resolvable using this “dual-mode” methodology. Fe-rich inclusions (likely magnetite) are closely associated with apatite inclusions that have no visible connection to the exterior surface of the zircon (e.g., via intersecting cracks). There is no evidence of radiation damage, alteration, recrystallisation or deformation in the host zircon or apatite that could provide alternative pathways for Fe infiltration, indicating that magnetite and apatite grew separately as primary phases in the magma, that magnetite adhered to the surfaces of the apatite, and that the magnetite-coated apatite was then encapsulated as primary inclusions within the growing zircon. Rarer examples of Fe-rich inclusions entirely encapsulated by zircon are also observed. These observations support the presence of primary inclusions in relatively young and pristine zircon crystals. Combining magnetic and tomography results we deduce the presence of magnetic carriers that are in the optimal size range for carrying strong and stable paleomagnetic signals but that remain below the detection limits of even the highest-resolution X-ray tomography reconstructions. We recommend the use of focused ion beam nanotomography and/or correlative transmission electron microscopy to directly confirm the presence of primary magnetite in the sub 300 nm range as a necessary step in targeted paleomagnetic workflows.

Authigenic clays as precursors to carbonate precipitation in saline lakes of Salar de Llamara, Northern Chile

The Salar de Llamara, Atacama Desert is host to saline lakes known as Puquios, which are possible analogs for lakes on early Earth or Mars. Here we examine associations between microbial communities, Mg-clay minerals, and carbonates in microbial mat samples from the Puquios using scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam nanotomography, and transmission electron microscopy. We observe micrometer-scale aggregates of nanocrystalline Mg-clay around cyanobacterial cell sheaths and throughout alveolar networks of microbial extracellular polymeric substances. High-Mn calcite occupies pore spaces and surfaces within the clay matrix. We propose a sequence whereby extracellular polymeric substance networks serve as surfaces for precipitation of Mg-clay, which permineralizes the original microorganisms. Formation of the clay reduces kinetic barriers and provides a suitable substrate for the formation of carbonate minerals. We therefore suggest that authigenic clays are important in the preservation of microbial communities and the precipitation of carbonates in non-marine settings such as the saline lakes of the Atacama Desert.

3D microstructural study of selachimorph enameloid evolution

Enameloid, the hyper-mineralized tissue covering shark teeth is a complex structure resulting from both ameloblast and odontoblast activity. The way these two types of cells interact to set up this tissue is not fully understood and results in the formation of subunits in the enameloid: the Single Crystallite Enameloid (SCE) and the Bundled Crystallite Enameloid (BCE). Using the Focused Ion Beam Nanotomography (FIB-nt), 3D images were produced to assess the relationship between the SCE and BCE of one fossil and one recent neoselachian shark teeth. 3D analysis of crystallite bundles reveals a strong connection between the crystallites forming the SCE and those forming the bundles of the Radial Bundle Enameloid (RBE), a component of the BCE, although it has been suggested that SCE and BCE have a different origin: epithelial for the SCE and mesenchymal for the BCE. Another significant result of the use of FIB-nt is the visualization of frequent branching among the radial bundles forming the RBE, including horizontal link between adjacent bundles. FIB-nt demonstrates therefore a strong potential to decipher the complex evolution of hyper-mineralised tissue in shark teeth, and, therefore, to better understand the evolution of tooth structure among basal Gnathostomes.

3D imaging techniques for characterising microcracks in cement-based materials

Concrete inherently contains pores and microcracks that can adversely impact its mechanical properties and long-term durability. However, characterising microcracks is difficult due to their complex, multiscale and three-dimensional (3D) nature. This paper presents an evaluation of 3D imaging techniques for characterising microcracks induced by different mechanisms. Seven cement pastes, mortars and concretes subjected to drying shrinkage, autogenous shrinkage and freeze-thaw cycles were investigated using focused ion beam nanotomography (FIB-nt), broad ion beam serial section tomography (BIB-SST), laser scanning confocal microscopy (LSCM) combined with serial sectioning and X-ray microtomography (μCT). The study shows that the characteristics of microcracks vary significantly depending on exposure conditions. Yet there is no single technique that can capture the entire size range of microcracks from sub to tens of μm within a sufficiently representative sampling volume. The achievable image volume and resolution, and the advantages and disadvantages of each technique are compared and discussed.

Hysteresis of Natural Magnetite Ensembles: Micromagnetics of Silicate‐Hosted Magnetite Inclusions Based on Focused‐Ion‐Beam Nanotomography

AbstractThree‐dimensional geometries of silicate‐hosted magnetic inclusions from the Harcus intrusion, South Australia have been determined using focused‐ion‐beam nanotomography (FIB‐nt). By developing an effective workflow, the geometries were reconstructed for magnetic particles in a plagioclase (162) and a pyroxene (282), respectively. For each inclusion, micromagnetic modeling using Micromagnetic Earth Related Rapid Interpreted Language Laboratory provided averaged hysteresis loops and backfield remanence curves of 20 equidistributed field directions together with average Ms, Mrs, Hc, and Hcr. The micromagnetic structures within each silicate are single‐domain (SD), single‐vortex (SV), multivortex (MV) and multidomain states. They have been analyzed using domain‐state diagnostic plots, such as the Day plot and the Néel plot. SD particles can be subdivided into groups with dominant uniaxial anisotropy (Mrs/Ms ∼0.5 and 10 < Hc < 100 mT) and mixed uniaxial/multiaxial anisotropy (Mrs/Ms ∼0.7 and 10 < Hc < 30 mT). Most SV particles lie on a trend with 0 < Mrs/Ms < 0.1 and 0 < Hc < 10 mT, while others display a broad range of intermediate Mrs/Ms and Hc values. SV and MV states do not plot on systematic grain‐size trends. Instead, the multicomponent mixture of domain states within each silicate spans the entire range of natural variability seen in bulk samples. This questions the interpretation of bulk average hysteresis parameters in terms of grain size alone. FIB‐nt combined with large‐scale micromagnetic simulations provides a more complete characterization of silicate‐hosted carriers of stable magnetic remanence. This approach will improve the understanding of single‐crystal paleomagnetism and enable primary paleomagnetic data to be extracted from ancient rocks.

Development of novel 2D and 3D correlative microscopy to characterise the composition and multiscale structure of suspended sediment aggregates

Suspended cohesive sediments form aggregates or ‘flocs’ and are often closely associated with carbon, nutrients, pathogens and pollutants, which makes understanding their composition, transport and fate highly desirable. Accurate prediction of floc behaviour requires the quantification of 3-dimensional (3D) properties (size, shape and internal structure) that span several scales (i.e. nanometre [nm] to millimetre [mm]-scale). Traditional techniques (optical cameras and electron microscopy [EM]), however, can only provide 2-dimensional (2D) simplifications of 3D floc geometries. Additionally, the existence of a resolution gap between conventional optical microscopy (COM) and transmission EM (TEM) prevents an understanding of how floc nm-scale constituents and internal structure influence mm-scale floc properties. Here, we develop a novel correlative imaging workflow combining 3D X-ray micro-computed tomography (μCT), 3D focused ion beam nanotomography (FIB-nt) and 2D scanning EM (SEM) and TEM (STEM) which allows us to stabilise, visualise and quantify the composition and multi-scale structure of sediment flocs for the first time. This new technique allowed the quantification of 3D floc geometries, the identification of individual floc components (e.g., clays, non-clay minerals and bacteria), and characterisation of particle-particle and structural associations across scales. This novel dataset demonstrates the truly complex structure of natural flocs at multiple scales. The integration of multi scale, state-of-the-art instrumentation/techniques offers the potential to generate fundamental new understanding of floc composition, structure and behaviour.

Nondestructive 3D Imaging and Quantification of Hydrated Biofilm-Sediment Aggregates Using X-ray Microcomputed Tomography.

Biofilm-sediment aggregate (BSA) contains a high water content, either within internal pores and channels or bound by extracellular polymeric substances (EPS) forming a highly hydrated biofilm matrix. Desiccation of BSAs alters the biofilm morphology and thus the physical characteristics of porous media, such as the binding matrix within BSA and internal pore geometry. Observing BSAs in their naturally hydrated form is essential but hampered due to the lack of techniques for imaging and discerning hydrated materials. Generally, imagery techniques (scanning electron microscopy (SEM), transmission electron microscopy (TEM), and focused ion beam nanotomography (FIB-nt)) involve the desiccation of BSAs (freeze-drying or acetone dehydration) or prevent differentiation between BSA components such as inorganic particles and pore water (confocal laser scanning microscopic (CLSM)). Here, we propose a novel methodology that simultaneously achieves the 3D visualization and quantification of BSAs and their components in their hydrated form at a submicron resolution using X-ray microcomputed tomography (μ-CT). It enables the high-resolution detection of comparable morphology of multiphase components within a hydrated aggregate: each single inorganic particle and the hydrated biofilm matrix. This allows the estimation of aggregate density and the illustration of biofilm-sediment binding matrix. This information provides valuable insights into investigations of the transport of BSAs and aggregate-associated sediment particles, contaminants (such as microplastics), organic carbon, and their impacts on aquatic biogeochemical cycling.

The Vortex State in Geologic Materials: A Micromagnetic Perspective

AbstractA wide variety of Earth and planetary materials are very good recorders of paleomagnetic information. However, most magnetic grains in these materials are not in the stable single domain grain size range but are larger and in nonuniform vortex magnetization states. We provide a detailed account of vortex phenomena in geologic materials by simulating first‐order reversal curves (FORCs) via finite‐element micromagnetic modeling of magnetite nanoparticles with realistic morphologies. The particles have been reconstructed from focused ion beam nanotomography of magnetite‐bearing obsidian and accommodate single and multiple vortex structures. Single vortex (SV) grains have fingerprints with contributions to both the transient and transient‐free zones of FORC diagrams. A fundamental feature of the SV fingerprint is a central ridge, representing a distribution of negative saturation vortex annihilation fields. SV irreversible events at multiple field values along different FORC branches determine the asymmetry in the upper and lower lobes of generic bulk FORC diagrams of natural materials with grains predominantly in the vortex state. Multivortex (MV) FORC signatures are modeled here for the first time. MV grains contribute mostly to the transient‐free zone of a FORC diagram, averaging out to create a broad central peak. The intensity of the central peak is higher than that of the lobes, implying that MV particles are more abundant than SV particles in geologic materials with vortex state fingerprints. The abundance of MV particles, as well as their single domain‐like properties point to MV grains being the main natural remanent magnetization carriers in geologic materials.

Nanoscale Origins of the Size Effect in the Compression Response of Single Crystal Ni-Base Superalloy Micro-Pillars.

Nickel superalloys play a pivotal role in enabling power-generation devices on land, sea, and in the air. They derive their strength from coherent cuboidal precipitates of the ordered γ’ phase that is different from the γ matrix in composition, structure and properties. In order to reveal the correlation between elemental distribution, dislocation glide and the plastic deformation of micro- and nano-sized volumes of a nickel superalloy, a combined in situ nanoindentation compression study was carried out with a scanning electron microscope (SEM) on micro- and nano-pillars fabricated by focused ion beam (FIB) milling of Ni-base superalloy CMSX4. The observed mechanical response (hardening followed by softening) was correlated with the progression of crystal slip that was revealed using FIB nano-tomography and energy-dispersive spectroscopy (EDS) elemental mapping. A hypothesis was put forward that the dependence of material strength on the size of the sample (micropillar diameter) is correlated with the characteristic dimension of the structural units (γ’ precipitates). By proposing two new dislocation-based models, the results were found to be described well by a new parameter-free Hall–Petch equation.

Quantifying the Structure and Composition of Flocculated Suspended Particulate Matter Using Focused Ion Beam Nanotomography.

Suspended particulate matter (SPM) is present in the natural aquatic environment as loosely bound aggregates or "flocs" and is responsible for the transport and fate of sediment, carbon, nutrients, pollutants, pathogens and manufactured nanoparticles from catchment to coast. Accurate prediction of SPM hydrodynamics requires the quantification of 3D floc properties (size, shape, density and porosity) that span several spatial scales. Yet, current techniques (video camera systems, optical microscopy and transmission electron microscopy, TEM) can only provide 2D simplifications of size and shape with a spatial resolution gap between the "gross" (>100s μm) and nanoscale (<1 μm). Here, we translate 3D-microscopy techniques (focused ion beam nanotomography, FIB-nt) typically used in the biomedical sciences to the study of natural flocculated SPM filling both this spatial and dimensional gap. Fragile 3D floc samples were successfully captured and stabilized, identifying five basic organic and inorganic floc components and quantifying porosity and bacteria numbers. This provides new 3D floc geometric data sets at the nanoscale that will be critical in the development of cohesive sediment transport models. Detailed compositional and structural information could provide novel insights into the association of pathogens and pollutants with SPM and their impact on aquatic life.

Porosity Connectivity Within the Spinel Layer of the 9Cr Steel Grade 91 when Exposed to High-Temperature Steam

Focused ion beam nanotomography has been employed in the study of pore morphology in spinel oxide formed on Grade 91 steel exposed to high-temperature steam for 1, 24 and 100 h under laboratory settings and at 91 kh in-service. The pores, also termed voids, exhibited a higher degree of connectivity than was expected and are likely to make a significant contribution to the prevailing mass transport mechanism across the scale. A mechanism for the formation of these pores and their relationship to other microstructural features is discussed.

Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites

Dusty olivine (olivine containing multiple sub-micrometer inclusions of metallic iron) in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence, capable of maintaining a faithful record of pre-accretionary magnetization acquired during chondrule formation. Here we show how the magnetic architecture of a single dusty olivine grain from the Semarkona LL3.0 ordinary chondrite meteorite can be fully characterized in three dimensions, using a combination of focused ion beam nanotomography (FIB-nT), electron tomography, and finite-element micromagnetic modeling. We present a three-dimensional (3D) volume reconstruction of a dusty olivine grain, obtained by selective milling through a region of interest in a series of sequential 20 nm slices, which are then imaged using scanning electron microscopy. The data provide a quantitative description of the iron particle ensemble, including the distribution of particle sizes, shapes, interparticle spacings and orientations. Iron particles are predominantly oblate ellipsoids with average radii 242 ± 94 × 199 ± 80 × 123 ± 58 nm. Using analytical TEM we observe that the particles nucleate on sub-grain boundaries and are loosely arranged in a series of sheets parallel to (001) of the olivine host. This is in agreement with the orientation data collected using the FIB-nT and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host. The shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets. Individual particle geometries are converted to a finite-element mesh and used to perform micromagnetic simulations. The majority of particles adopt a single vortex state, with “bulk” spins that rotate around a central vortex core. We observed no particles that are in a true single domain state. The results of the micromagnetic simulations challenge some preconceived ideas about the remanence-carrying properties of vortex states. There is often not a simple predictive relationship between the major, intermediate, and minor axes of the particles and the remanence vector imparted in different fields. Although the orientation of the vortex core is determined largely by the ellipsoidal geometry (i.e., parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids), the core and remanence vectors can sometimes lie at very large (tens of degrees) angles to the principal axes. The subtle details of the morphology can control the overall remanence state, leading in some cases to a dominant contribution from the bulk spins to the net remanence, with profound implications for predicting the anisotropy of the sample. The particles have very high switching fields (several hundred millitesla), demonstrating their high stability and suitability for paleointensity studies.

Pore geometry effects on elastic properties of Opalinus Clay

Regarding the storage of nuclear waste within clay rock formations requires fundamental understanding of elastic properties of this rock type with regard to the risk evaluation process. The influence of the pore geometry on elastic properties of Opalinus Clay is studied on the basis of realistic pore microstructure, which is reconstructed from image data acquired by focused ion beam nanotomography. These microstructures are used as input pore geometries for linear elastic finite-element modeling to determine Thomsen’s [Formula: see text], [Formula: see text], and [Formula: see text] anisotropy parameters and the effective elastic moduli related to the porous material. The presence of fully drained intergranular pores substantially increases the values of [Formula: see text] and [Formula: see text]. For the investigated sample with an expected porosity of approximately 10 vol.%, the anisotropic pore space contributes similarly to the anisotropy parameters when compared with the contribution related to the preferred orientation of minerals. On the other hand, if the pore space is undrained, the effect of pores is smaller and the anisotropy is largely controlled by the preferred orientation of minerals. It is revealed that the value of [Formula: see text] is most sensitive to changes in water saturation. In case water is drained from the pores, the vertical Young’s modulus [Formula: see text] reduces significantly more when compared with the horizontal modulus [Formula: see text]. Presuming that the drainable porosity corresponds to a volume fraction of 10 vol.%, [Formula: see text] reduces by approximately 15%–20%. The effect of drainage is even more pronounced for the Poisson’s ratios, whereas the shear moduli are not much affected by drainage.

Three-Dimensional Analysis of the Porosity in MgB2Wires Using FIB Nanotomography

Porosity is one of several current limiting mechanisms in MgB2 wires. We have compared the microstructural homogeneity and the porosity distribution in different ex situ and in situ MgB2 powder-in-tube wires. The submicrometer structure was determined using focused ion beam nanotomography. The ex situ wires exhibit an isotropic microstructure, which has been quantified in terms of an identical tortuosity in transverse and longitudinal filament direction. The very homogenous microstructure in the new ex situ wire generation is probably one reason for its strongly improved critical current density. The in situ wire has an anisotropic microstructure with a lower tortuosity in the axial direction. The microstructural inhomogeneity of the in situ filaments makes microstructural characterization and the comparison between materials and superconducting properties particularly challenging.

A FIB-nanotomography method for accurate 3D reconstruction of open nanoporous structures

We present an automated focused ion beam nanotomography method for nanoporous microstructures with open porosity, and apply it to reconstruct nanoporous gold (np-Au) structures with ligament sizes on the order of a few tens of nanometers. This method uses serial sectioning of a well-defined wedge-shaped geometry to determine the thickness of individual slices from the changes in the sample width in successive cross-sectional images. The pore space of a selected region of the np-Au is infiltrated with ion-beam-deposited Pt composite before serial sectioning. The cross-sectional images are binarized and stacked according to the individual slice thicknesses, and then processed using standard reconstruction methods. For the image conditions and sample geometry used here, we are able to determine the thickness of individual slices with an accuracy much smaller than a pixel. The accuracy of the new method based on actual slice thickness is assessed by comparing it with (i) a reconstruction using the same cross-sectional images but assuming a constant slice thickness, and (ii) a reconstruction using traditional FIB-tomography method employing constant slice thickness. The morphology and topology of the structures are characterized using ligament and pore size distributions, interface shape distribution functions, interface normal distributions, and genus. The results suggest that the morphology and topology of the final reconstructions are significantly influenced when a constant slice thickness is assumed. The study reveals grain-to-grain variations in the morphology and topology of np-Au.

Focused Ion Beam Nanotomography of Chondritic Meteorites: Closing the Mesoscale Length Gap in Paleomagnetic Studies

Focused Ion Beam Nanotomography of Chondritic Meteorites: Closing the Mesoscale Length Gap in Paleomagnetic Studies

Static and Dynamic Aspects of Black Silicon Formation

We present a combination of experimental data and modeling that explains some of the important characteristics of black silicon (BSi) developed in cryogenic reactive ion etching (RIE) processes, including static properties (dependence of resulting topography on process parameters) and dynamic aspects (evolution of topography with process time). We generate a phase diagram predicting the RIE parameter combinations giving rise to different BSi geometries and show that the topographic details of BSi explain the metamaterial characteristics that are responsible for its low reflectivity. In particular, the unique combination of needle and hole features of various heights and depths, which is captured by our model and confirmed by focused ion beam nanotomography, creates a uniquely smooth transition in refractive index. The model also correctly describes dynamical characteristics, such as the dependence of aspect ratio on process time, and the prediction of new etching fronts appearing at topographical saddle points during the incipient stages of BSi development--a phenomenon reported here for the first time.

Intergranular pore space evolution in MX80 bentonite during a long-term experiment

Focused ion beam nanotomography (FIB-nt) was applied to MX80 bentonite samples from the long-term Alternative Buffer Material (ABM) experiment in order to study the evolution of the intergranular pore space under similar condition that is supposed to prevail in repositories of nuclear waste. The applied high-resolution imaging method revealed the presence of two different types of pore filler. The first type is related to corrosion of iron and is represented by newly formed heavy minerals. Extensive formation of heavy minerals occurred only near the iron parts of the experimental set up. Based on comparison with other studies, the second filler type was interpreted as clay-gel that was likely formed during water uptake and swelling. A large fraction of the initial pore space was filled with such a clay gel. By attributing filled pores to the present open porosity, the initial intergranular porosity (radii>10nm) of the starting material was in the range of 4.3–4.6vol.%, which was reduced to <1vol.% during the experiment. A finite scaling approach was applied to the initial pore space (i.e. pores with radii>10nm), which yielded percolation thresholds with critical porosities ϕ in the range of 3–19vol.%. Thus, the residual open porosity was far below the percolation threshold.The initial porosity of one sample was above the percolation threshold, but also in this material percolation was restricted to one spatial direction. This indicated anisotropy with respect to percolation. The formation of a clay-gel and heavy minerals led to a decrease in intergranular porosity, which in turn affected connectivity of the pore network. Using results from pore-network modelling in combination with percolation theory illustrates that a minor reduction of porosity led to a substantial decrease in pore connectivity. Depending on water saturation within the observed intergranular pore space, air permeability decreases exponentially over three to four orders of magnitude within a narrow porosity range of about 1vol.%.Based on observations and calculations, gas transport along the intergranular pore space of MX80 bentonite from the ABM experiment is not considered as a possible scenario and can reasonably be excluded.

Three-Dimensional Organization of Surface-Bound Silicone Nanofilaments Revealed by Focused Ion Beam Nanotomography

One-dimensional (1D) nanostructures have been identified as key technology for future devices and integrated into surface-bound materials. The roughness of surface-bound 1D silicone nanofilaments (SNFs) has been used extensively to create surfaces with extreme wetting properties and as carrier material. Electron microscopy has shown that this material is made of individual filaments with diameters spanning tens of nanometers and a length of several micrometers which arrange into a highly entangled quasi-porous network. However, a comprehensive analysis of the three-dimensional (3D) superstructure has remained elusive so far. In this study, focused ion beam nanotomography (FIB-nt) is used to quantify the otherwise hardly accessible structural parameters roughness (12.68) and volume fraction (2.80). The volume fraction is anisotropic, and two major species of SNFs are quantified to contribute equally to the overall surface area. Spatial statistics reveals a self-avoiding growth pattern of SNFs over the substrate, and a 3D model of the data is rendered. The presented analysis therefore significantly advances the understanding of SNF surface coatings with regard to their structure at the nano- and microscale. Finally, the described procedure may serve as a useful tool to analyze other surface-bound 1D nanostructures of similar complex arrangement.

On the Potential of Tomographic Methods when Applied to Compacted Crushed Rock Salt

Compacted crushed rock salt is considered as potential backfill material in repositories for nuclear waste. To evaluate the sealing properties of this material knowledge concerning the nature of the pore space is of eminent interest. Here, the pore microstructures of crushed rock salt samples with different compaction states were investigated by X-ray (XCT) computed tomography and Focused Ion Beam nanotomography (FIB-nt). Based on these methods the pore microstructures were reconstructed and quantitatively analyzed with respect to porosity, connectivity and percolation properties. Regarding pores with radii $$> 4\,\upmu \hbox {m}$$ , porosity differs substantially in the two analyzed samples ( $$\phi = 0.01$$ and 0.10). The pore microstructures are considered isotropic in connectivity and percolation threshold. Using two finite-scaling schemes we found percolation thresholds with critical porosities $$\phi _{c} > 0.05$$ . Based on statistical considerations, the millimeter size samples that can be analyzed by XCT are large enough to provide a meaningful picture of the pore geometry related to macroporosity. The samples contain also a small fraction (i.e. $$< 0.01$$ ) of pores with radii $$< 1\,\upmu \hbox {m}$$ , which were resolved by FIB-nt. Often these pores can be found along grain boundaries. These pores are granular shaped and are not connected to each other. Typical samples size that can be analyzed by FIB-nt is on the order of tens of microns, which turned out to be too small to provide representative geometric information unless an effort is made that involves several FIB-nt realizations per sample.