In Situ Measurements of Strains in Composite Battery Electrodes during Electrochemical Cycling

The cyclic stress in lithium-ion battery electrodes induced by repeated charge and discharge cycles causes electrode degradation and fracture, resulting in reduced battery performance and lifetime. To investigate electrode mechanics as a function of electrochemical cycling, we utilize digital image correlation (DIC) to measure the strains that develop in lithium-ion battery electrodes during lithiation and delithiation processes. A composite graphite electrode is cycled galvanostatically (with constant current) in a custom battery cell while optical images of the electrode surface are captured in situ. The strain in the electrode is computed using an in-house DIC code. On average, an unconstrained composite graphite electrode expands 1.41 % during lithiation and contracts 1.33 % during delithiation. These strain values compare favorably with predictions based on the elastic properties of the composite electrode and the expansion of graphite-lithium intercalation compounds (G-LICs). The establishment of this experimental protocol will enable future studies of the relationship between electrode mechanics and battery performance.     

Micro-interferometry for measurement of thermal displacements at fiber/matrix interfaces

A micro-interferometric technique for measuring out-of-plane thermal displacements on a scale commensurate with the dimensions of the fiber/matrix unit cell is described. A scanning micro-interferometer is used to image surface displacements of samples containing a single-pitch-based carbon fiber embedded in an epoxy matrix. The interferometer design gives the necessary resolution to detect small changes in thermal displacements in the fiber/matrix interface region. The samples were heated electrically through the fiber to create radially symmetric temperature and displacement fields. Repeatable displacement measurements were obtained on a radial line across the interface region with an accuracy of ±25 Å. A sharp expansion of the matrix surrounding the fiber was observed with each heating. Overall, the experiments demonstrate the utility of micro-interferometry for measuring submicron displacements.     

Local Strain Concentrations in a Microvascular Network

Strain concentrations associated with the presence of a microvascular network in a polymer matrix are measured using fluorescent digital image correlation (FDIC). The accuracy of the measurement technique is established for a specimen containing only a single microchannel. The influence of localized particle reinforcement around the channel is also investigated using this simplified geometry. Three-dimensional network specimens with different structural designs were fabricated and loaded in uniaxial tension. The resulting strain concentrations are compared as a function of channel spacing and location. As expected, decreasing channel spacing leads to increased strain local to the channels. In addition, the three-dimensional nature of the channel architecture influences the resulting strain. The results provide insight into the mechanical behavior of microvascular networks and demonstrate the utility of FDIC as a characterization tool at these length scales.     

Micro- and Nanoscale Deformation Measurement of Surface and Internal Planes via Digital Image Correlation

The digital image correlation (DIC) technique is successfully applied across multiple length scales through the generation of a suitable speckle pattern at each size scale. For microscale measurements, a random speckle pattern of paint is created with a fine point airbrush. Nanoscale displacement resolution is achieved with a speckle pattern formed by solution deposition of fluorescent silica nanoparticles. When excited, the particles fluoresce and form a speckle pattern that can be imaged with an optical microscope. Displacements are measured on the surface and on an interior plane of transparent polymer samples with the different speckle patterns. Rigid body translation calibrations and uniaxial tension experiments establish a surface displacement resolution of 1 μm over a 5×6 mm scale field of view for the airbrushed samples and 17 nm over a 100×100 μm scale field of view for samples with the fluorescent nanoparticle speckle. To demonstrate the capabilities of the method, we characterize the internal deformation fields generated around silica microspheres embedded in an elastomer under tensile loading. The DIC technique enables measurement of complex deformation fields with nanoscale precision over relatively large areas, making it of particular relevance to materials that possess multiple length scales.     

A parametric study of laser induced thin film spallation

We report parametric studies of elastic wave generation by a pulsed laser and associated spalling of thin surface films by the corresponding high stresses. Two different substrate materials, single crystal Si (100) and fused silica, are considered. Spallation behavior of Al thin films is investigated as a function of substrate thickness, film thickness, laser energy, and various parameters governing the source. Surface displacement due to the stress wave is measured by Michaelson interferometry and used to infer the stresses on the film interface. Consistent with previous studies, the maximum stress in the substrate and at the film/substrate interface increases with increasing laser fluence. For many of the conditions tested, the substrate stress is large enough to damage the Si. Moreover, the maximum interface stress is found to increase with increasing film thickness, but decrease with increasing substrate thickness due to geometric attenuation. Of particular significance is the development of a decompression shock in the fused sillica substrates, which results in very high tensile stresses at the interface. This shock enhances the failure of thin film interfaces, especially in thicker samples.     

Characterization of Microvascular-Based Self-healing Coatings

A protocol is described to assess self-healing of crack damage in a polymer coating deposited on a substrate containing a microvascular network. The bio-inspired coating/substrate design delivers healing agent to cracks in the coating via a three-dimensional microvascular network embedded in the substrate. Through capillary action, monomer flows from the network channels into the crack plane where it is polymerized by a catalyst embedded in the coating. The healing efficiency of this materials system is assessed by the recovery of coating fracture toughness in a four-point beam bending experiment. Healing results for the microvascular networks are compared to data for a coating containing microencapsulated healing agents. A single crack in a brittle epoxy coating is healed as many as seven times in the microvascular systems, whereas microcapsule-based healing occurs for only one cycle. The ability to heal continuously with the microvascular networks is limited by the availability of catalyst in the coating.     

Mixed-mode interfacial adhesive strength of a thin film on an anisotropic substrate

The mixed-mode interfacial adhesion strength between a gold (Au) thin film and an anisotropic passivated silicon (Si) substrate is measured using laser-induced stress wave loading. Test specimens are prepared by bonding a fused silica (FS) prism to the back side of a 〈1 0 0〉 Si substrate with a thin silicon nitride (Si_xN_y) passivation layer deposited on the top surface. A high-amplitude stress wave is developed by pulsed laser ablation of a sacrificial absorbing layer on one of the lateral surfaces of the FS prism. Due to the negative non-linear elastic properties of the FS, the compressive stress wave evolves into a decompression shock with fast fall time. Careful selection of the incident angle between the pulse and the FS/Si interface generates a mode-converted shear wave in refraction, subjecting the Si_xN_y/Au thin film interface to dynamic mixed-mode loading, sufficient to cause interfacial fracture. A detailed analysis of the anisotropic wave propagation combined with interferometric measurements of surface displacements enables calculation of the interfacial stresses developed under mixed-mode loading. The mixed-mode interfacial strength is compared to the interfacial strength measured under purely tensile loading.     

Digital Image Correlation for Improved Detection of Basal Cell Carcinoma

Border detection is a critical aspect during removal of a basal cell carcinoma tumor. Since the tumor is only 3% to 50% as stiff as the healthy skin surrounding it, strain concentrates in the tumor during deformation. Here we develop a digital image correlation (DIC) technique for improved lateral border detection based upon the strain concentrations associated with the stiffness difference of healthy and cancerous skin. Gelatin skin phantoms and pigskin specimens are prepared with compliant inclusions of varying shapes, sizes, and stiffnesses. The specimens with inclusions as well as several control specimens are loaded under tension, and the full-field strain and displacement fields measured by DIC. Significant strain concentrations develop around the compliant inclusions in gelatin skin phantoms, enabling detection of the tumor border to within 2% of the actual border. At a lower magnification, the lateral border between a pigskin/inclusion interface is determined within 23% of the border. Strain concentrations are identified by DIC measurements and associated with the lateral edges of the compliant inclusions. The experimental DIC protocol developed for model specimens has potential as a tool to aid in more accurate detection of basal cell carcinoma borders.