This week I'm trying something new: A quick roundup of some relevant R&D stories that have come my way. For this installment I'll be discussing silicon fatigue, self-healing composites, self-healing transistors, and an amazing waterproof coating.
Until last month, when researchers at NIST demonstrated otherwise, the prevailing view was that silicon didn't experience mechanical fatigue. In part, this is because if you subject bulk silicon crystals to the typical fatigue test (pulling on a notched sample and checking to see if a crack springs into being at the notch) it passes with flying colors. The researchers reasoned that MEMS devices are subjected to rather more complicated stresses, especially since examination has shown that their tiny mechanical features do exhibit stress-induced cracks. To understand why this occurs, they pushed down on the silicon surface using small tungsten carbide balls and cycled the pressure. The result? Gradually increasing surface damage (indicating mechanical fatigue) around the impact point. The current challenge is to see if the same mechanisms prevail at the submicrometer level.
The research is published in Applied Physics Letters 91, 201902, "Bulk silicon is susceptible to fatigue," authors S. Bhowmick, J.J. Meléndez-Martínez, and B.R. Lawn.
Mystery mechanism heals high-tech composite, a recent story in New Scientist, concerns University of Illinois at Urbana-Champaign research into self-healing polymers. By combining microcapsules of healing agent and a catalytic chemical trigger in an epoxy matrix, the resulting material can heal itself. A growing crack ruptures the microcapsule, releasing the healing agent into the crack plane where it reacts with the catalyst to polymerize and heal the crack. The researchers report that in their experiments they've been able to recover more than 90% of the material's original toughness. Unfortunately, the solvent used is toxic, so they are trying to find alternative chemistries.
Pentacene is an organic semiconductor material which shows promise in organic electronics. While it's possible to mold a thin-film pentacene transistor easily, the resulting devices frequently have defects, which slow them down. Wolfgang Kalb and his team at the Swiss Federal Institute of Technology have discovered that if you make the transistors and leave them at room temperature for a week (in vacuum), the defects disappear, resulting in a doubling of the parts' speed.
John Simpson, a researcher at Oak Ridge National Labs, has developed a water-repellent coating that outperforms natural ones. The superhydrophobic glass power he produced is both easier and cheaper to produce than previous materials and can be used in smaller quantities to coat a given surface. The nanostructured material maintains a microscopic layer of air on surfaces even when those surfaces are submerged in water. The powder's nanoscale structure amplifies the effect of surface tension and makes the powder unwettable. It's also a great thermal and electrical insulator. As Simpson says in the press release, "Staying dry in a rainstorm may only have a small personal value, but reducing the energy required to transport products by boat or barge or extending the life of bridges or buildings would have a great value to society and individuals alike."