Collin R. Becker

In Situ Atomic Force Microscopy of Lithiation and Delithiation of Silicon Nanostructures for Lithium Ion Batteries

Using electron beam lithography, amorphous Si (a-Si) nanopillars were fabricated with a height of 100 nm and diameters of 100, 200, 300, 500, and 1000 nm. The nanopillars were electrochemically cycled in a 1 M lithium trifluoromethanesulfonate in propylene carbonate electrolyte. In situ atomic force microscopy (AFM) was used to qualitatively and quantitatively examine the morphology evolution of the nanopillars including volume and height changes versus voltage in real-time. In the first cycle, an obvious hysteresis of volume change versus voltage during lithiation and delithiation was measured. The pillars did not crack in the first cycle, but a permanent volume expansion was observed. During subsequent cycles the a-Si roughened and deformed from the initial geometry, and eventually pillars with diameters >200 nm fractured. Furthermore, a degradation of mechanical properties is suggested as the 100 and 200 nm pillars were mechanically eroded by the small contact forces under the AFM probe. Ex situ scanning electron microscopy (SEM) images, combined with analysis of the damage caused by in situ AFM imaging, demonstrate that during cycling, the silicon became porous and structurally unstable compared to as-fabricated pillars. This research highlights that even nanoscale a-Si suffers irreversible mechanical damage during cycling in organic electrolytes.

3-Axis acceleration switch for traumatic brain injury early warning

This paper reports on the design, fabrication, and testing of a 3-axis acceleration switch intended to serve as an early warning for traumatic brain injury (TBI). Mild TBI (colloquially termed “concussion”) resulting from rapid acceleration of the skull has been rising in the public consciousness with recently increasing awareness of the dangers and long-term health risks associated with it. The sensor described here is an array of acceleration switches designed to cover the range of acceleration associated with TBI, and to do so with no external power draw until an acceleration event within this range occurs.

Initiation of nanoporous energetic silicon by optically-triggered, residual stress powered microactuators

The integration of energetic materials with chip-scale MEMS fabrication processes, and in particular the development of nanoporous energetic silicon (NES), is a promising path to provide significant quantities of energy for certain microscale applications. Here we demonstrate the low-power wireless initiation of an on-chip energetic reaction, by absorbing optical energy, transmitting mechanical energy, and releasing a large amount of chemical energy, without the use of any external wires or batteries. A novel actuator powered by residual thin film stress absorbed 25 W/cm2 of optical power from a 532 nm visible laser, heated, and released up to 22 nJ of mechanical energy. The mechanical energy was sufficient to initiate 6.7 mg of NES and release up to 66 J of chemical energy.

A Novel Microdevice for In Situ Study of Mechano-Electrochemical Behavior with Controlled Temperature

Nanostructured electrodes have shown great potential in the development of Li-ion batteries with higher energy and power densities and longer cycle life. A fundamental understanding of the mechano-electrochemical behavior during charging/discharging cycles is essential for optimal and reliable design. Previous work has utilized in situ experimental techniques in an electron microscope to directly visualize material response during the reaction cycles. Unfortunately, the present in situ test methods are limited to room temperature and, as a result, the effect of temperature on charging/discharging cycles is not well understood. These electrochemical processes are intrinsically temperature sensitive, particularly for nanostructured electrodes. Here we present a novel microdevice that allows high resolution in situ observation of mechano-electrochemical response of nanomaterials in a scanning electron microscope with controlled temperature. The microdevice consists of built-in microcircuits for concurrent heating and temperature measurement during in situ experiments. To demonstrate these unique capabilities, we present the design, microfabrication and thermal characterization of this new class of microdevice.