26 research outputs found
Million frames per second infrared imaging system
An infrared imaging system has been developed for measuring the temperature increase during the dynamic deformation of materials. The system consists of an 8×8 HgCdTe focal plane array, each with its own preamplifier. Outputs from the 64 detector/preamplifiers are digitized using a row-parallel scheme. In this approach, all 64 signals are simultaneously acquired and held using a bank of track and hold amplifiers. An array of eight 8:1 multiplexers then routes the signals to eight 10 MHz digitizers, acquiring data from each row of detectors in parallel. The maximum rate is one million frames per second. A fully reflective lens system was developed, consisting of two Schwarszchild objectives operating at infinite conjugation ratio. The ratio of the focal lengths of the objectives determines the lens magnification. The system has been used to image the distribution of temperature rise near the tip of a notch in a high strength steel sample (C-300) subjected to impact loading by a drop weight testing machine. The results show temperature rises at the crack tip up to around 70 K. Localization of temperature, and hence, of deformation into "U" shaped zones emanating from the notch tip is clearly seen, as is the onset of crack propagation
Orientation-dependent adhesion strength of a rigid cylinder in non-slipping contact with a transversely isotropic half-space
Recently, Chen and Gao [Chen, S., Gao, H., 2007. Bio-inspired mechanics of reversible adhesion: orientation-dependent adhesion strength for non-slipping adhesive contact with transversely isotropic elastic materials. J. Mech. Phys. solids 55, 1001–1015] studied the problem of a rigid cylinder in non-slipping adhesive contact with a transversely isotropic solid subjected to an inclined pulling force. An implicit assumption made in their study was that the contact region remains symmetric with respect to the center of the cylinder. This assumption is, however, not self-consistent because the resulting energy release rates at two contact edges, which are supposed to be identical, actually differ from each other. Here we revisit the original problem of Chen and Gao and derive the correct solution by removing this problematic assumption. The corrected solution provides a proper insight into the concept of orientation-dependent adhesion strength in anisotropic elastic solids
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