Portable electronics—typically made of non-renewable, non-biodegradable and potentially toxic materials—are discarded at an alarming rate as consumers pursue the next best electronic gadget yielding lots of environmentally taxing waste.
However, in an effort to alleviate the environmental burden of these electronic devices, a team of University of Wisconsin-Madison researchers has collaborated with researchers in the Madison-based U.S. Department of Agriculture Forest Products Laboratory (FPL) to develop a surprising solution: a semiconductor chip made almost entirely of wood.
The research team, led by Zhenqiang “Jack” Ma, the Lynn H. Matthias Professor and Vilas Distinguished Achievement Professor of electrical and computer engineering at UW-Madison, described the new device in a paper published May 26, 2015, in the journal Nature Communications. The paper demonstrates the feasibility of replacing the substrate, or support layer, of a computer chip, with cellulose nanofibril (CNF), a flexible, biodegradable material made from wood.
“The majority of material in a chip is support—we only use less than a couple of micrometers for everything else,” says Ma. “Now the chips are so safe you can put them in the forest and fungus will degrade it. They become as safe as fertilizer.”
Zhiyong Cai, a project leader of engineering composite science research group at FPL, has been developing sustainable nanomaterials since 2009. “If you take a big tree and cut it down to the individual fiber, the most common product is paper. The dimension of the fiber is in the micron stage,” Cai says. “But what if we could break it down further to the nano scale? At that scale you can make this material, very strong and transparent CNF paper.”
Working with Shaoqin “Sarah” Gong, a professor of biomedical engineering and Wisconsin Institute for Discovery at UW-Madison, Cai’s group addressed two key barriers to using wood-derived materials in an electronics setting: surface smoothness and thermal expansion. “You don’t want it to expand or shrink too much. Wood is a natural hygroscopic material and could attract moisture from the air and expand,” Cai says. “With an epoxy coating on the surface of the CNF, we solved both the surface smoothness and the moisture barrier.”
Gong and her students also have been studying bio-based polymers for more than a decade. She says that, combined with Ma’s background in advanced and flexible electronics, CNF offers many benefits over current chip substrates. “The advantage of CNF over other polymers is that it’s a bio-based material and most other polymers are petroleum-based polymers. Bio-based materials are sustainable, bio-compatible and biodegradable,” she says. “And, compared to other polymers, CNF actually has a relatively low thermal expansion coefficient.”
The Nature Communications paper also demonstrates a more environmentally friendly process that showed similar performance to existing chips. The majority of today’s wireless devices use gallium arsenide-based microwave chips due to their superior high-frequency operation and power handling capabilities. However, gallium arsenide can be environmentally toxic, particularly in the massive quantities of discarded wireless electronics.
Yei Hwan Jung, a graduate student in electrical and computer engineering and a co-author of the paper, says the new process greatly reduces the use of such expensive and potentially toxic material. “I’ve made 1,500 gallium arsenide transistors in a 5-by-6 millimeter chip. Typically for a microwave chip that size, there are only eight to 40 transistors. The rest of the area is just wasted,” he says. “We take our design and put it on CNF using deterministic assembly technique, then we can put it wherever we want and make a completely functional circuit with performance comparable to existing chips.”
While the biodegradability of these materials will have a positive impact on the environment, Ma says the flexibility of the technology can lead to widespread adoption of these electronic chips. “Mass-producing current semiconductor chips is so cheap, and it may take time for the industry to adapt to our design,” he says. “But flexible electronics are the future, and we think we’re going to be well ahead of the curve.”
Source: University of Wisconsin College of Engineering, Cleantech Concepts