A new cellulose-based material offers hope for e-waste.
Image: Randen Pederson/Flickr
Good old wood. It's everywhere! And soon it may be in your computing devices as the newest, most cutting-edge semiconductor material.
Or at least the first semiconductor material that's fully adapted to the fact that the average consumer chews through portable electronics like they're toilet paper. Consumers, after all, demand not just the fastest and slimmest devices, they want—implicitly, at least—devices that can go in the trash.
From that perspective, wood seems pretty reasonable.
The wooden semiconductor is a real thing, thanks to researchers at the University of Wisconsin working in partnership with the US Department of Agriculture Forest Products Laboratory. The new device, which basically just acts like plant fertilizer in the environment, is described in the current issue of Nature Communications.
The UW researchers, led by electrical engineering professor Zhenqiang Ma, note that the average usage life of cell phones and portable electronics in general hovers around 18 months. The Electronics Takeback Coalition estimates that up to 50 million tons of electronic waste are generated each year worldwide, much, if not most, of it involving precious metals and dangerous contaminants like lead, cadmium, and beryllium. The same organisation estimates that some 142,000 computers are trashed every day.
Chips are mostly "waste" from the very start. "In a typical semiconductor electronic chip, the active region comprises the top thin layer and is only a small portion of the chip, whereas the bottom substrate that holds the chip consists of more than 99 percent of the semiconductor materials," Ma and his group write. "In microwave chips for wireless functions, besides the waste of the bottom substrate, only a tiny fraction of the lateral chip area is used for the required active transistors/diodes with the rest being used only for carrying other non-active components."
The new chip is constructed from a substrate layer of cellulose nanofibril (CNF), which, as one might guess, is composed of nanoscale cellulose fibers. Cellulose is the critical structural component underpinning the cell walls of most green plants and in its nano-ized form begins to take on properties most resembling plastic. Cellulose also happens to be the most abundant organic polymer on Earth.
Paper is produced from wood as it's broken down further and further until it's a pulp of micron-scale fibers. Essentially, cellulose nanofibril is just a continuation of the process until the resulting fibers are at the scale of nanometers. The result is a super-strong, transparent paper made up of CNF that also happens to be a well-suited electronics substrate that beats out previous attempts at bio-semiconductors in several respects, including performance relative to existing (synthetic) state-of-the-art electronics and resistance to water and other potential solvents.
"You don't want it to expand or shrink too much," offered study co-author Zhiyong Cai in a UW statement. "Wood is a natural hydroscopic material and could attract moisture from the air and expand. With an epoxy coating on the surface of the CNF, we solved both the surface smoothness and the moisture barrier."
As part of their experiments, the group demonstrated that their CNF material is indeed biodegradable—turns out that certain fungi have a fondness for it.
Finally, Ma and co. conclude, "Successful fabrication of key electrical components on the flexible cellulose nanofibril paper with comparable performance to their rigid counterparts and clear demonstration of fungal biodegradation of the cellulose-nanofibril-based electronics suggest that it is feasible to fabricate high-performance flexible electronics using eco-friendly materials."