X-ray Tomography Coupled with Finite Elements, A Fast Method to Design Aerogel Composites and Prove Their Superinsulation Experimentally.

Composite aerogels can include fibers, opacifiers and binders but are rarely designed and optimized to achieve the best thermal/mechanical efficiency. This paper proposes a three-dimensional X-ray tomography-based method for designing composites. Two types of models are considered: classical and inexpensive homogenization models and more refined finite element models. XrFE is based on the material's real three-dimensional microstructure and/or its twin numerical microstructure, and calculates the effective conductivity of the material. First, the three-dimensional sample is meshed and labeled. Then, a finite element method is used to calculate the heat flow in the samples. The entire three-dimensional microstructure of a real or fictitious sample is thus associated with a heat flow and an effective conductivity. Parametric studies were performed to understand the relationship between microstructure and thermal efficiency. They highlighted how quickly a low volume fraction addition can improve or ruin thermal conductivity. A reduced set of three formulations was developed and fully characterized. The mechanical behavior was higher than 50 KPa, with thermal efficiencies ranging from 14 to 15 mW·m·K−1.

Quantification of the Local Topological Variations of Stripped and Plated Lithium Metal by X-ray Tomography.

Lithium (Li) metal is the most promising negative electrode to be implemented in batteries for stationary and electric vehicle applications. For years, its use and subsequent industrialization were hampered because of the inhomogeneous Li+ ion reduction upon recharge onto Li metal leading to dendrite growth. The use of solid polymer electrolyte is a solution to mitigate dendrite growth. Li reduction leads typically to dense Li deposits, but the Li stripping and plating process remain nonuniform with local current heterogeneities. A precise characterization of the behavior of these heterogeneities during cycling is then essential to move toward an optimized negative electrode. In this work, we have developed a characterization method based on X-ray tomography applied to model Li symmetric cells to quantify and spatially probe the Li stripping/plating processes. Ante- and post-mortem cells are recut in smaller cells to allow a 1 µm voxel size resolution in a conventional laboratory scanner. The reconstructed cell volume is postprocessed to numerically reflatten the Li electrodes, allowing us a subsequent precise measurement of the electrode and electrolyte thicknesses and revealing local interface modifications. This in-depth analysis brings information about the location of heterogeneities and their impact on the electrode microstructure at both the electrode grains and grain boundaries. We show that the plating process (reduction) induces more pronounced heterogeneities compared to the stripping (oxidation) one. The existence of crosstalking between the electrodes is also highlighted. In addition, this simple methodology permits to finely retrieve and then surface map the local current density at both electrodes based on the local thickness change during the redox process.

Insight into the Directional Thermal Transport of Hexagonal Boron Nitride Composites.

Ideal dielectric materials for microelectronic devices should have high directionally tailored thermoconductivity with low dielectric constant and loss. Hexagonal boron nitride (hBN) with excellent thermal and dielectric properties shows a promise for the fabrication of thermoconductive dielectric polymer composites. Herein, a simple method for the fabrication of lightweight polymer/hBN composites with high directionally tailored thermoconductivity and excellent dielectric properties is presented. The solid polymer/hBN composites are manufactured by melt-compounding and injection molding. The porous composites are successfully manufactured in an injection molding process through supercritical fluid (SCF) foaming. X-ray tomography provides direct visualization of the internal microstructure and hBN orientation, leading to an in-depth understanding of the directionally dependent thermoconductivity of the polymer/hBN composite. Shear-induced orientation of hBN platelets in the solid HDPE/hBN composites leads to a significant anisotropic thermal conductivity. The solid HDPE/23.2 vol % hBN composites show an in-plane thermoconductivity as high as 10.1 W m-1 K-1, whereas the through-plane thermoconductivity is limited to 0.28 W m-1 K-1. However, the generation of a porous structure via SCF foaming imparts in situ exfoliation, random orientation, and interconnectivity of hBN platelets within the polymer matrix. This results in highly isotropic thermoconductivity with higher bulk thermal conductivity in the lightweight porous composites as compared to their solid counterparts. Furthermore, the electrically insulating composites developed in this study exhibit low dielectric constant and ultralow dielectric loss. Thus, this study presents a simple fabrication method to develop lightweight dielectric materials with tailored thermal conductivity for modern electronics.

Polymerization shrinkage of resin-based composites for dental restorations: A digital volume correlation study.

OBJECTIVE: Resin-based composites are widely used in dental restorations; however, their volumetric shrinkage during polymerization leads to several issues that reduce the restoration survival rates. For overcoming this problem, a deep study of shrinkage phenomena is necessary. METHODS: In this study, micro-tomography (µ-CT) is combined with digital volume correlation (DVC) to investigate the effect of several factors on the polymerization strain of dental composites in model cavities: the presence/absence of an adhesive, the use of transparent/blackened cavities, and irradiation times between 1 and 40s. RESULTS: The results indicate that the presence of an adhesive at the interface between the cavity and composite does not reduce the total strain but instead limits it to a preferential direction. In addition, regardless of the conditions, the main strain is generated along the axis parallel to the polymerization irradiation (the vertical axis). Finally, the total strain appears to occur in the first 5s of irradiation, with no further evolution observed for longer irradiation times. SIGNIFICANCE: This work provides new insight into resin-based composite shrinkage and demonstrates the benefit of coupling DVC and µ-CT to better understand the degradation mechanisms of these materials.

Role of Hydrogen-Charging on Nucleation and Growth of Ductile Damage in Austenitic Stainless Steels.

Hydrogen energy is a possible solution for storage in the future. The resistance of packaging materials such as stainless steels has to be guaranteed for a possible use of these materials as containers for highly pressurized hydrogen. The effect of hydrogen charging on the nucleation and growth of microdamage in two different austenitic stainless steels AISI316 and AISI316L was studied using in situ tensile tests in synchrotron X-ray tomography. Information about damage nucleation, void growth and void shape were obtained. AISI316 was found to be more sensitive to hydrogen compared to AISI316L in terms of ductility loss. It was measured that void nucleation and growth are not affected by hydrogen charging. The effect of hydrogen was however found to change the morphology of nucleated voids from spherical cavities to micro-cracks being oriented perpendicular to the tensile axis.

3-D printing of chitosan-calcium phosphate inks: rheology, interactions and characterization.

Bone substitute fabrication is of interest to meet the worldwide incidence of bone disorders. Physical chitosan hydrogels with intertwined apatite particles were chosen to meet the bio-physical and mechanical properties required by a potential bone substitute. A set up for 3-D printing by robocasting was found adequate to fabricate scaffolds. Inks consisted of suspensions of calcium phosphate particles in chitosan acidic aqueous solution. The inks are shear-thinning and consist of a suspension of dispersed platelet aggregates of dicalcium phosphate dihydrate in a continuous chitosan phase. The rheological properties of the inks were studied, including their shear-thinning characteristics and yield stress. Scaffolds were printed in basic water/ethanol baths to induce transformation of chitosan-calcium phosphates suspension into physical hydrogel of chitosan mineralized with apatite. Scaffolds consisted of a chitosan polymeric matrix intertwined with poorly crystalline apatite particles. Results indicate that ink rheological properties could be tuned by controlling ink composition: in particular, more printable inks are obtained with higher chitosan concentration (0.19 mol·L-1).

Two-Scale Tomography Based Finite Element Modeling of Plasticity and Damage in Aluminum Foams.

In this study, finite element (FE) modeling of open-cell aluminum foams in tension was performed based on laboratory X-ray tomography scans of the materials at two different scales. High-resolution stitching tomography of the initial state allowed local intermetallic particles to be distinguished from internal defects in the solid phase of the foam. Lower-resolution scans were used to monitor the deformation and fracture in situ during loading. 3D image-based FE models of the foams were built to simulate the tensile behavior using a new microstructure-informed Gurson⁻Tvergaard⁻Needleman model. The new model allows quantitative consideration of the local presence of brittle intermetallic particles in the prediction of damage. It performs well in the discrimination of potential fracture zones in the foam, and can be easily adapted to any type of architectured material where both the global architecture and local microstructural details should be taken into account in the prediction of damage behavior.

Revealing the Effect of Local Connectivity of Rigid Phases during Deformation at High Temperature of Cast AlSi12Cu4Ni(2,3)Mg Alloys.

The 3D microstructure and its effect on damage formation and accumulation during tensile deformation at 300 °C for cast, near eutectic AlSi12Cu4Ni2Mg and AlSi12Cu4Ni3Mg alloys has been investigated using in-situ synchrotron micro-tomography, complemented by conventional 2D characterization methods. An increase of Ni from 2 to 3 wt.% leads to a higher local connectivity, quantified by the Euler number χ, at constant global interconnectivity of rigid 3D networks formed by primary and eutectic Si and intermetallics owing to the formation of the plate-like Al-Ni-Cu-rich δ-phase. Damage initiates as micro-cracks through primary Si particles agglomerated in clusters and as voids at matrix/rigid phase interfaces. Coalescence of voids leads to final fracture with the main crack propagating along damaged rigid particles as well as through the matrix. The lower local connectivity of the rigid 3D network in the alloy with 2 wt.% Ni permits localized plastification of the matrix and helps accommodating more damage resulting in an increase of ductility with respect to AlSi12Cu4Ni3Mg. A simple load partition approach that considers the evolution of local connectivity of rigid networks as a function of strain is proposed based on in-situ experimental data.

Climate-Dependent Heat-Triggered Opening Mechanism of Banksia Seed Pods.

Heat-triggered fruit opening and delayed release of mature seeds are widespread among plants in fire-prone ecosystems. Here, the material characteristics of the seed-containing follicles of Banksia attenuata (Proteaceae), which open in response to heat frequently caused by fire, are investigated. Material analysis reveals that long-term dimensional stability and opening temperatures of follicles collected across an environmental gradient increase as habitats become drier, hotter, and more fire prone. A gradual increase in the biaxial curvature of the hygroscopic valves provides the follicles in the driest region with the highest flexural rigidity. The irreversible deformation of the valves for opening is enabled via a temperature-dependent reduction of the elastic modulus of the innermost tissue layer, which then allows releasing the stresses previously generated by shrinkage of the fiber bundles in the adjacent layer during follicle drying. These findings illustrate the level of sophistication by which this species optimizes its fruit opening mechanism over a large distribution range with varying environmental conditions, and may not only have great relevance for developing biomimetic actuators, but also for elucidating the species' capacity to cope with climatic changes.

Tensile rupture of medial arterial tissue studied by X-ray micro-tomography on stained samples.

Detailed characterization of damage and rupture mechanics of arteries is one the current challenges in vascular biomechanics, which requires developing suitable experimental approaches. This paper introduces an approach using in situ tensile tests in an X-ray micro-tomography setup to observe mechanisms of damage initiation and progression in medial layers of porcine aortic samples. The technique requires the use of sodium polytungstate as a contrast agent, of which the conditions for use are detailed in this paper. Immersion of the samples during 24h in a 15g/L concentrated solution provided the best compromise for viewing musculo-elastic units in this tissue. The process of damage initiation, delamination and rupture of medial tissue under tensile loading was observed and can be described as an elementary process repeating several times until complete failure. This elementary process initiates with a sudden mode I fracture of a group of musculo-elastic units, followed by an elastic recoil of these units, causing mode II separation of these, hence a delamination plane. The presented experimental approach constitutes a basis for observation of other constituents, or for investigations on other tissues and damage mechanisms.

Corrigendum: Strong, tough and stiff bioinspired ceramics from brittle constituents.

This corrects the article DOI: 10.1038/nmat3915.

Publisher's Note: Imaging grain boundary grooves in hard-sphere colloidal bicrystals [Phys. Rev. E 94, 042604 (2016)].

This corrects the article DOI: 10.1103/PhysRevE.94.042604.

Imaging grain boundary grooves in hard-sphere colloidal bicrystals.

Colloidal particles were sedimented onto patterned glass slides to grow three-dimensional bicrystals with a controlled structure. Three types of symmetric tilt grain boundaries between close-packed face-centered-cubic crystals were produced: Σ5(100),Σ17(100), and Σ3(110). The structure of the crystals and their defects were visualized by confocal microscopy, and characterized by simple geometric measurements, including image difference, thresholding, and reprojection. This provided a quick and straightforward way to detect the regions in which the atoms are mobile. This atomic mobility was higher at the grain boundaries and close to the solid-liquid interface. This method was compared to the more conventional analysis based on the calculation of the local order parameter of the individual particles to identify the interface. This was used in turn to identify the presence of grooves at the grain-boundary-liquid triple junction for every type of grain boundary, except for the twin [Σ3(110)], for which no groove could be detected. Images of these grooves were processed, and the angle linking the grain boundary energy to the solid-liquid interfacial energy was measured. The resulting values of the grain boundary energy were compared to estimates based on the density deficit in the boundary.

Fast virtual histology using X-ray in-line phase tomography: application to the 3D anatomy of maize developing seeds.

BACKGROUND: Despite increasing demand, imaging the internal structure of plant organs or tissues without the use of transgenic lines expressing fluorescent proteins remains a challenge. Techniques such as magnetic resonance imaging, optical projection tomography or X-ray absorption tomography have been used with various success, depending on the size and physical properties of the biological material. RESULTS: X-ray in-line phase tomography was applied for the imaging of internal structures of maize seeds at early stages of development, when the cells are metabolically fully active and water is the main cell content. This 3D imaging technique with histology-like spatial resolution is demonstrated to reveal the anatomy of seed compartments with unequalled contrast by comparison with X-ray absorption tomography. An associated image processing pipeline allowed to quantitatively segment in 3D the four compartments of the seed (embryo, endosperm, nucellus and pericarp) from 7 to 21 days after pollination. CONCLUSION: This work constitutes an innovative quantitative use of X-ray in-line phase tomography as a non-destructive fast method to perform virtual histology and extends the developmental stages accessible by this technique which had previously been applied in seed biology to more mature samples.

Strong, tough and stiff bioinspired ceramics from brittle constituents.

High strength and high toughness are usually mutually exclusive in engineering materials. In ceramics, improving toughness usually relies on the introduction of a metallic or polymeric ductile phase, but this decreases the material's strength and stiffness as well as its high-temperature stability. Although natural materials that are both strong and tough rely on a combination of mechanisms operating at different length scales, the relevant structures have been extremely difficult to replicate. Here, we report a bioinspired approach based on widespread ceramic processing techniques for the fabrication of bulk ceramics without a ductile phase and with a unique combination of high strength (470 MPa), high toughness (22 MPa m(1/2)), and high stiffness (290 GPa). Because only mineral constituents are needed, these ceramics retain their mechanical properties at high temperatures (600 °C). Our bioinspired, material-independent approach should find uses in the design and processing of materials for structural, transportation and energy-related applications.

Self-assembly of faceted particles triggered by a moving ice front.

The possibility to align and organize faceted particles in the bulk offers intriguing possibilities for the design and discovery of materials and architectures exhibiting novel functional properties. The growth of ice crystals can be used to trigger the self-assembly of large, anisotropic particles and consequently to obtain three-dimensional porous materials of large dimensions in a limited amount of time. These mechanisms have not been explored so far due to the difficulty to experimentally investigate these systems. Here we elucidate the self-assembly mechanisms of faceted particles driven by ice growth by a combination of X-ray holotomography and discrete element modeling, providing insights into both the dynamics of self-assembly and their final packing. The encapsulation of particles is the result of a delicate balance between the force exerted by the percolating network of concentrated particles and the force exerted by the moving interface. We illustrate the benefits of such self-assembly for thermal management composite materials.

Phase contrast synchrotron microtomography: improving noninvasive investigations of fossil embryos in ovo.

Fossil embryos are paramount for our understanding of the development of extinct species. However, although thousands of fossil amniote eggs are known, very few embryos in ovo have been described. First reports of fossil embryos were based on broken eggs, where the embryonic remains were already exposed, because destructive methods on complete eggs were avoided. Investigations of complete eggs therefore required nondestructive approaches, such as X-ray microtomography (µCT). However, due to the general low density contrast between fossilized bones and infilling matrix, only a few specimens have been reported using these techniques. Using propagation phase contrast X-ray synchrotron microtomography (PPC-SR-µCT), we report here the discovery of three well-preserved embryos in Early Cretaceous eggs from Thailand. By scanning these eggs using different imaging techniques, we show that vastly different interpretations can be made regarding the preservation state and/or the developmental stage of these embryos. PPC-SR-µCT also revealed differential contrast between bone categories, presumably reflecting the ossification pattern of these embryos. Applying such an approach to large-scale studies of fossil eggs could lead to more discoveries and detailed studies of fossil embryos, providing important developmental and phylogenetic information on extinct and extant amniotes.

Ice shaping properties, similar to that of antifreeze proteins, of a zirconium acetate complex.

The control of the growth morphologies of ice crystals is a critical issue in fields as diverse as biomineralization, medicine, biology, civil or food engineering. Such control can be achieved through the ice-shaping properties of specific compounds. The development of synthetic ice-shaping compounds is inspired by the natural occurrence of such properties exhibited by antifreeze proteins. We reveal how a particular zirconium acetate complex is exhibiting ice-shaping properties very similar to that of antifreeze proteins, albeit being a radically different compound. We use these properties as a bioinspired approach to template unique faceted pores in cellular materials. These results suggest that ice-structuring properties are not exclusive to long organic molecules and should broaden the field of investigations and applications of such substances.

Metastable and unstable cellular solidification of colloidal suspensions.

Colloidal particles are often seen as big atoms that can be directly observed in real space. They are therefore becoming increasingly important as model systems to study processes of interest in condensed-matter physics such as melting, freezing and glass transitions. The solidification of colloidal suspensions has long been a puzzling phenomenon with many unexplained features. Here, we demonstrate and rationalize the existence of instability and metastability domains in cellular solidification of colloidal suspensions, by direct in situ high-resolution X-ray radiography and tomography observations. We explain such interface instabilities by a partial Brownian diffusion of the particles leading to constitutional supercooling situations. Processing under unstable conditions leads to localized and global kinetic instabilities of the solid/liquid interface, affecting the crystal morphology and particle redistribution behaviour.

X-ray tomography and three-dimensional image analysis of epoxy-glass syntactic foams.

Syntactic foams (glass hollow spheres embedded in an epoxy matrix) were produced on purpose to be used as test materials for the present study. Two kinds of spheres (MS1 and MS2) were adjoined to a same polymer matrix, MS1 with a volume fraction of 55 and MS2 with 30 and 55%. The samples were analysed by X-ray tomography using synchrotron radiation. The three-dimensional images were used to observe the qualitative differences between the three samples. Three-dimensional image processing was then carried out to quantify the differences. The images were used to retrieve the fraction of the different phases which was in fairly good agreement with the expected values. The external and internal diameter of the spheres and their thickness were also measured. The MS1 spheres are smaller, thicker and their size distribution is less homogeneous compared to the MS2. The size distribution of the spheres before blowing was retrieved and evidenced to be similar for the two kinds of spheres. The thickness depends only weakly on the diameter of the spheres.