Toward Reproducible Three-Dimensional Microstructure Analysis of Granular Materials and Complex Suspensions

Focused ion beam nanotomography (FIB-nt) is a novel method for high resolution three-dimensional (3D) imaging. In this investigation we assess the methodological parameters related to image acquisition and data processing that are critical for obtaining reproducible microstructural results from granular materials and from complex suspensions. For this purpose three case studies are performed: (1) The precision of FIB-nt is evaluated by analyzing a reference sample with nanospheres. Due to the implementation of an automated correction procedure, drift phenomena can be removed largely from the FIB data. However, at high magnifications remaining drift components can induce problems for 3D-shape reconstructions. (2) Correct object recognition from densely packed microstructures requires specific algorithms for splitting of agglomerated particles. To establish quantitative criteria for the correct degree of splitting, a parametric study with dry portland cement is performed. It is shown that splitting with a k-value of 0.6 leads to accurate results. (3) Finally, the reproducibility of the entire cryo-FIB analysis is investigated for high pressure frozen cement suspensions. Reproducible analyses can be obtained if the magnification is adapted to the particle size. At low magnifications the small particles and their surface area are underestimated. At high magnifications representativity is questioned because local inhomogeneities can become dominan

Characterization and modelling of structure and transport properties of porous ceramics

A general scheme of material design and virtual testing of porous ceramics is presented, which provides new strategies for efficient development and optimisation of such materials. The development cycle contains three stages, which can be repeated, until the final targeted properties are reached. The first stage is the preparation of porous ceramics with properties in the region of interest and thorough 3D-investigation and characterization of the porous microstructure. In the second step, microstructure data either directly from real materials or indirectly from a virtual, stochastic model are fed into a numerical model to calculate the physical property of interest, e.g. permeability, diffusivity, thermal conductivity or electrical conductivity of pore fluids. Finally, an evaluation of data representing microstructure characteristics as well as macroscopic properties for a wide range of fabrication scenarios yields as a third step a validated model, which leads to the definition of optimised design guidelines. This optimisation cycle is then closed with the production of a porous ceramic material with improved physical properties. The successful implementation of such scheme is demonstrated here with the development of porous zirconia membranes as electric liquid junction for pH-sensors

Application of the ESEM Technique in Wood Research: Part I. Optimization of Imaging Parameters and Working Conditions

A study using the ESEM (Environmental Scanning Electron Microscopy) technique was performed on wood objects in order to assess the particular advantages, possibilities, and limitations of this microscopic tool. In contrast to conventional high vacuum SEM, in ESEM specimens can be investigated in a gaseous atmosphere, usually of water vapor. This enables the observation of non-conductive, polymeric, composite, and porous materials (such as wood) in their natural state, without drying, evacuating, or sputtering them with a layer of carbon or metal. Further advantages include observations in a wide range of temperatures (-15° to 1000°C), conduction of dynamic processes such as condensation, freezing, and thawing of the specimen during observation, or mechanical testing.The imaging quality of ESEM for natural samples, however, is inferior to that of conventional SEM, and the specimens are liable to beam damage. The process of acquiring an image in ESEM is more complex than in SEM, demanding the optimization of a number of interacting parameters. These include the physical conditions of the specimen, conditions of the chamber environment, and electronic parameters of the formation and optimization of the image.The work on the ESEM can be performed through several operational modes that offer various sets of environmental and imaging conditions. This article presents guidance for assessment of influential operating parameters and their selection for the optimization of the ESEM work with wood

Advances in 3D focused ion beam tomography

This article summarizes recent technological improvements of focused ion beam tomography. New in-lens (in-column) detectors have a higher sensitivity for low energy electrons. In combination with energy filtering, this leads to better results for phase segmentation and quantitative analysis. The quality of the 3D reconstructions is also improved with a refined drift correction procedure. In addition, the new scanning strategies can increase the acquisition speed significantly. Furthermore, fast spectral and elemental mappings with silicon drift detectors open up new possibilities in chemical analysis. Examples of a porous superconductor and a solder with various precipitates are presented, which illustrate that combined analysis of two simultaneous detector signals (secondary and backscattered electrons) provides reliable segmentation results even for very complex 3D microstructures. In addition, high throughput elemental analysis is illustrated for a multi-phase Ni-Ti stainless steel. Overall, the improvements in resolution, contrast, stability, and throughput open new possibilities for 3D analysis of nanostructured material

Advances in 3D focused ion beam tomography

This article summarizes recent technological improvements of focused ion beam tomography. New in-lens (in-column) detectors have a higher sensitivity for low energy electrons. In combination with energy filtering, this leads to better results for phase segmentation and quantitative analysis. The quality of the 3D reconstructions is also improved with a refined drift correction procedure. In addition, the new scanning strategies can increase the acquisition speed significantly. Furthermore, fast spectral and elemental mappings with silicon drift detectors open up new possibilities in chemical analysis. Examples of a porous superconductor and a solder with various precipitates are presented, which illustrate that combined analysis of two simultaneous detector signals (secondary and backscattered electrons) provides reliable segmentation results even for very complex 3D microstructures. In addition, high throughput elemental analysis is illustrated for a multi-phase Ni-Ti stainless steel. Overall, the improvements in resolution, contrast, stability, and throughput open new possibilities for 3D analysis of nanostructured materials

Estimating the Effective Elasticity Properties of a Diamond/β\beta-SiC Composite Thin Film by 3D Reconstruction and Numerical Homogenization

The main aim of the present work is to estimate the effective elastic stiffnesses of a two-phase diamond/$\beta$-SiC composite thin film that is fabricated by chemical vapor deposition. The parameters of linear elasticity are determined by numerical homogenization. The database is sparse since for the 3D volume of interest only two micrographs displaying the phase distributions in perpendicular planes are available; micrographs each of a cross-section and the surface of the thin film. A representative volume element (RVE) is reconstructed by an optimization software and by means of identified material symmetries in 2D of the specimen. The elastic homogenization results indicate that the two-phase diamond/$\beta$-SiC composite exhibits the behavior of transverse isotropy, for which the set of six independent material parameters is identified

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 > 10 nm) of the starting material was in the range of 4.3–4.6 vol.%, which was reduced to 10 nm), which yielded percolation thresholds with critical porosities ϕ in the range of 3–19 vol.%. 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 1 vol.%. 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

3D microstructure effects in Ni-YSZ anodes : prediction of effective transport properties and optimization of redox stability

This study investigates the influence of microstructure on the effective ionic and electrical conductivities of Ni-YSZ (yttria-stabilized zirconia) anodes. Fine, medium, and coarse microstructures are exposed to redox cycling at 950 ºC. FIB (focused ion beam)-tomography and image analysis are used to quantify the effective (connected) volume fraction (Φeff), constriction factor (β), and tortuosity (τ). The effective conductivity (σeff) is described as the product of intrinsic conductivity (σ0) and the so-called microstructure-factor (M): σeff = σ0 x M. Two different methods are used to evaluate the M-factor: (1) by prediction using a recently established relationship, Mpred = ε β^0.36/τ^5.17, and (2) by numerical simulation that provides conductivity, from which the simulated M-factor can be deduced (Msim). Both methods give complementary and consistent information about the effective transport properties and the redox degradation mechanism. The initial microstructure has a strong influence on effective conductivities and their degradation. Finer anodes have higher initial conductivities but undergo more intensive Ni coarsening. Coarser anodes have a more stable Ni phase but exhibit lower YSZ stability due to lower sintering activity. Consequently, in order to improve redox stability, it is proposed to use mixtures of fine and coarse powders in different proportions for functional anode and current collector layers

Multi-parameter improvement method for (micro-) structural properties of high performance ceramics

Many pH-measurement electrodes rely on porous diaphragms to create a liquid electrolyte junction between reference-electrolyte and the fluid to be measured. In field applications, the diaphragm is required to meet partly contradictory improvement criteria. To minimize measurement errors and to ensure durability of the measurement device, the diaphragm is supposed to maximize electrolyte conductivity and reference-electrolyte outflow velocity, while simultaneously minimizing reference electrolyte flow rate. The task of optimizing the overall performance of this small piece of ceramics has lead to the development of a novel multi-parameter improvement scheme for its (micro-) structural design. The method encompasses the consideration of microscopic material design parameters, such as porosity, pore-tortuosity and constrictivity, macroscopic material parameters such as diaphragm diameter and length, as well as process parameters like internal electrode pressure or the electrolyte viscosity and specific resistivity. Comprising sets of design parameters to dimensionless groups, concrete design guidelines as well as the introduction of a three-dimensional improvement space concept are proposed. The novel design space concept allows the improvement of each possible diaphragm-based measurement set-up, by considering the simultaneous, dimensionless interaction of all relevant design parameters