Micromechanics-Based Homogenization of the Effective Physical Properties of Composites With an Anisotropic Matrix and Interfacial Imperfections

Micromechanics-based homogenization has been employed extensively to predict the effective properties of technologically important composites. In this review article, we address its application to various physical phenomena, including elasticity, thermal and electrical conduction, electric, and magnetic polarization, as well as multi-physics phenomena governed by coupled equations such as piezoelectricity and thermoelectricity. Especially, for this special issue, we introduce several research works published recently from our research group that consider the anisotropy of the matrix and interfacial imperfections in obtaining various effective physical properties. We begin with a brief review of the concept of the Eshelby tensor with regard to the elasticity and mean-field homogenization of the effective stiffness tensor of a composite with a perfect interface between the matrix and inclusions. We then discuss the extension of the theory in two aspects. First, we discuss the mathematical analogy among steady-state equations describing the aforementioned physical phenomena and explain how the Eshelby tensor can be used to obtain various effective properties. Afterwards, we describe how the anisotropy of the matrix and interfacial imperfections, which exist in actual composites, can be accounted for. In the last section, we provide a summary and outlook considering future challenges

Stress Induced Structural Transformations in Au Nanocrystals

Nanocrystals can exist in multiply twinned structures like the icosahedron, or single crystalline structures like the cuboctahedron or Wulff-polyhedron. Structural transformation between these polymorphic structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high temperature diffusion mediated processes. Thus, there is limited experimental evidence of displacive motion mediated structural transformations. Here, we report the high-pressure structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell that is driven by displacive motion. In-situ X-ray diffraction and transmission electron microscopy were used to detect the transformation of multiply twinned nanocrystals into single crystalline nanocrystals. High-pressure single crystalline nanocrystals were recovered after unloading, however, the nanocrystals quickly reverted back to multiply twinned state after redispersion in toluene solvent. The dynamics of recovery was captured using transmission electron microscopy which showed that the recovery was governed by surface recrystallization and rapid twin boundary motion. We show that this transformation is energetically favorable by calculating the pressure-induced change in strain energy. Molecular dynamics simulations showed that defects nucleated from a region of high stress region in the interior of the nanocrystal, which make twin boundaries unstable. Deviatoric stress driven Mackay transformation and dislocation/disclination mediated detwinning are hypothesized as possible mechanisms of high-pressure structural transformation.Comment: 32 pages, 14 figures, and 1 movie (please open pdf with Adobe Acrobat Reader to see the embedded movie

Nucleation of Dislocations in 3.9 nm Nanocrystals at High Pressure

As circuitry approaches single nanometer length scales, it is important to predict the stability of metals at these scales. The behavior of metals at larger scales can be predicted based on the behavior of dislocations, but it is unclear if dislocations can form and be sustained at single nanometer dimensions. Here, we report the formation of dislocations within individual 3.9 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell. We used a combination of x-ray diffraction, optical absorbance spectroscopy, and molecular dynamics simulation to characterize the defects that are formed, which were found to be surface-nucleated partial dislocations. These results indicate that dislocations are still active at single nanometer length scales and can lead to permanent plasticity.Comment: 33 pages, 12 figure

Metallic and complex hydride-based electrochemical storage of energy

The development of efficient storage systems is one of the keys to the success of the energy transition. There are many ways to store energy, but among them, electrochemical storage is particularly valuable because it can store electrons produced by renewable energies with a very good efficiency. However, the solutions currently available on the market remain unsuitable in terms of storage capacity, recharging kinetics, durability, and cost. Technological breakthroughs are therefore expected to meet the growing need for energy storage. Within the framework of the Hydrogen Technology Collaboration Program—H2TCP Task-40, IEA\u27s expert researchers have developed innovative materials based on hydrides (metallic or complex) offering new solutions in the field of solid electrolytes and anodes for alkaline and ionic batteries. This review presents the state of the art of research in this field, from the most fundamental aspects to the applications in battery prototypes

Metallic and complex hydride-based electrochemical storage of energy

The development of efficient storage systems is one of the keys to the success of the energy transition. There are many ways to store energy, but among them, electrochemical storage is particularly valuable because it can store electrons produced by renewable energies with a very good efficiency. However, the solutions currently available on the market remain unsuitable in terms of storage capacity, recharging kinetics, durability, and cost. Technological breakthroughs are therefore expected to meet the growing need for energy storage. Within the framework of the Hydrogen Technology Collaboration Program - H2TCP Task-40, IEA's expert researchers have developed innovative materials based on hydrides (metallic or complex) offering new solutions in the field of solid electrolytes and anodes for alkaline and ionic batteries. This review presents the state of the art of research in this field, from the most fundamental aspects to the applications in battery prototypes

Analysis of Velocity Potential around Pulsating Bubble near Free or Rigid Surfaces Based on Image Method

An analytical method for predicting the velocity potential around a pulsating bubble close to a free or rigid wall was established using an image method. Because the velocity potential should satisfy two boundary conditions at the bubble surface and rigid wall, we investigated the velocity in the normal direction at the two boundaries by adding the image bubbles. The potential was analyzed by decomposing the bubble motion as two independent motions, pulsation and translation, and we found that when the number of image bubbles was greater than ten, the two boundary conditions were satisfied for the translation term. By adding many image bubbles after the approximation of the pulsation term, we also confirmed that the boundary condition at the wall was satisfied

Computational analysis of metallic nanowire-elastomer nanocomposite based strain sensors

Possessing a strong piezoresistivity, nanocomposites of metal nanowires and elastomer have been studied extensively for its use in highly flexible, stretchable, and sensitive sensors. In this work, we analyze the working mechanism and performance of a nanocomposite based stretchable strain sensor by calculating the conductivity of the nanowire percolation network as a function of strain. We reveal that the nonlinear piezoresistivity is attributed to the topological change of percolation network, which leads to a bottleneck in the electric path. We find that, due to enhanced percolation, the linearity of the sensor improves with increasing aspect ratio or volume fraction of the nanowires at the expense of decreasing gauge factor. In addition, we show that a wide range of gauge factors (from negative to positive) can be obtained by changing the orientation distribution of nanowires. Our study suggests a way to intelligently design nanocomposite-based piezoresistive sensors for flexible and wearable devices