Observations of transient high temperature vortical microstructures in solids during adiabatic shear banding

By using a unique infrared high-speed camera especially constructed for recording highly transient temperature fields at the microscale, we are able to reveal the spatial and temporal microstructure within dynamically growing shear bands in metals. It is found that this structure is highly nonuniform and possesses a transient, short range periodicity in the direction of shear band growth in the form of an array of intense "hot spots" reminiscent of the well-known, shear-induced hydrodynamic instabilities in fluids. This is contrary to the prevailing classical view that describes the deformations and the temperatures within shear bands as being essentially one-dimensional fields. These observations are also reminiscent of the nonuniform structure of localized shear regions believed to exist, at an entirely different length scale, in the earth's lower crust and upper mantle

Real-time Measurement of Stress and Damage Evolution During Initial Lithiation of Crystalline Silicon

Crystalline to amorphous phase transformation during initial lithiation in (100) silicon-wafers is studied in an electrochemical cell with lithium metal as the counter and reference electrode. It is demonstrated that severe stress jumps across the phase boundary lead to fracture and damage, which is an essential consideration in designing silicon based anodes for lithium ion batteries. During initial lithiation, a moving phase boundary advances into the wafer starting from the surface facing the lithium electrode, transforming crystalline silicon into amorphous LixSi. The resulting biaxial compressive stress in the amorphous layer is measured in situ and it was observed to be ca. 0.5 GPa. HRTEM images reveal that the crystalline-amorphous phase boundary is very sharp, with a thickness of ~ 1 nm. Upon delithiation, the stress rapidly reverses, becomes tensile and the amorphous layer begins to deform plastically at around 0.5 GPa. With continued delithiation, the yield stress increases in magnitude, culminating in sudden fracture of the amorphous layer into micro-fragments and the cracks extend into the underlying crystalline silicon.Comment: 12 pages, 5 figure

Effect of loading rate on fracture morphology in a high strength ductile steel

Fracture experiments in a high-strength ductile steel (2.3Ni-1.3Cr-0.17C) were conducted under static and dynamic loading conditions in a three-point bend and a one-point bend configurations. A qualitative description of the influence of loading rate on the microscopic features of the fracture surfaces and their role in the fracture initiation process was considered. The fracture surfaces consist of tunneled region and shear lips. The size of the shear lips increases wit increasing loading rate and is characterized by microvoids and cell structures. The tunneled region consists of large voids and micro-voids that coalesce by impingement. At high loading rates, localized molten zones are observed at the tunnel-shear lip interface

On plastic deformation and fracture in Si films during electrochemical lithiation/delithiation cycling

An in situ study of deformation, fracture, and fatigue behavior of silicon as a lithium-ion battery electrode material is presented. Thin films (100-200 nm) of silicon are cycled in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1M lithium hexafluorophosphate in ethylene carbonate, diethylene carbonate, dimethyl carbonate solution (1:1:1, wt.%) as electrolyte. Stress evolution in the Si thin-film electrodes during electrochemical lithiation and delithiation is measured by monitoring the substrate curvature using the multi-beam optical sensing method. The stress measurements have been corrected for contributions from residual stress arising from sputter-deposition. An indirect method for estimating the potential errors due to formation of the solid-electrolyte-interphase layer and surface charge on the stress measurements was presented. The films undergo extensive inelastic deformation during lithiation and delithiation. The peak compressive stress during lithiation was 1.48 GPa. The stress data along with the electron microscopy observations are used to estimate an upper bound fracture resistance of lithiated Si, which is approximately 9-11 J/m^2. Fracture initiation and crack density evolution as a function of cycle number is also reported.Comment: 25 pages, 9 figure

Quantifying Capacity Loss due to Solid-Electrolyte-Interphase Layer Formation on Silicon Negative Electrodes in Lithium-ion Batteries

Charge lost per unit surface area of a silicon electrode due to the formation of solid-electrolyte-interphase (SEI) layer during initial lithiation was quantified, and the species that constitute this layer were identified. Coin cells made with Si thin-film electrodes were subjected to a combination of galvanostatic and potentiostatic lithiation and delithiation cycles to accurately measure the capacity lost to SEI-layer formation. While the planar geometry of amorphous thin films allows accurate calculation of surface area, creation of additional surface by cracking was prevented by minimizing the thickness of the Si film. The cycled electrodes were analyzed with X-ray photoelectron spectroscopy to characterize the composition of the SEI layer. The charge lost due to SEI formation measured from coin cell experiments was found to be in good agreement with the first-cycle capacity loss during the initial lithiation of a Si (100) crystal with planar geometry. The methodology presented in this work is expected to provide a useful practical tool for battery-material developers in estimating the expected capacity loss due to first cycle SEI-layer formation and in choosing an appropriate particle size distribution that balances mechanical integrity and the first cycle capacity loss in large volume expansion electrodes for lithium-ion batteries.Comment: 15 pages, 9 figures; Journal of Power Sources, 201

Stress Evolution in Composite Silicon Electrodes during Lithiation/Delithiation

We report real-time average stress measurements on composite silicon electrodes made with two different binders [Carboxymethyl cellulose (CMC), and polyvinylidene fluoride (PVDF)] during electrochemical lithiation and delithiation. During galvanostatic lithiation at very slow rates, the stress in a CMC-based electrode becomes compressive and increases to 70 MPa, where it reaches a plateau and increases slowly thereafter with capacity. The PVDF-based electrode exhibits similar behavior, although with lower peak compressive stress of about 12 MPa. These initial experiments indicate that the stress evolution in a Si composite electrode depends strongly on the mechanical properties of the binder. Stress data obtained from a series of lithiation/delithiation cycles suggests plasticity induced irreversible shape changes in contacting Si particles, and as a result, the stress response of the system during any given lithiation/delithiation cycle depends on the cycling history of the electrode. While these results constitute the first in-situ stress measurements on composite Si electrodes during electrochemical cycling, the diagnostic technique described herein can be used to assess the mechanical response of a composite electrode made with other active material/binder combinations.Comment: 22 pages, 8 figure

A. Venkert Effect of Loading Rate on Fracture Morphology in a High Strength Ductile Steel

Fracture experiments in a high-strength ductile stee