Tag Archives: Lorna Gibson

Let’s make our turbine blades really big (greater than 75 metres) with new nanocomposite

The is a story about balsa wood, wind farms, turbine blades, and nanocomposites according to a June 25, 2014 news item on ScienceDaily,

In wind farms across North America and Europe, sleek turbines equipped with state-of-the-art technology convert wind energy into electric power. But tucked inside the blades of these feats of modern engineering is a decidedly low-tech core material: balsa wood.

Like other manufactured products that use sandwich panel construction to achieve a combination of light weight and strength, turbine blades contain carefully arrayed strips of balsa wood from Ecuador, which provides 95 percent of the world’s supply.

For centuries, the fast-growing balsa tree has been prized for its light weight and stiffness relative to density. But balsa wood is expensive and natural variations in the grain can be an impediment to achieving the increasingly precise performance requirements of turbine blades and other sophisticated applications.

As turbine makers produce ever-larger blades — the longest now measure 75 meters, almost matching the wingspan of an Airbus A380 jetliner — they must be engineered to operate virtually maintenance-free for decades. In order to meet more demanding specifications for precision, weight, and quality consistency, manufacturers are searching for new sandwich construction material options.

Now, using a cocktail of fiber-reinforced epoxy-based thermosetting resins and 3D extrusion printing techniques, materials scientists at the Harvard School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering have developed cellular composite materials of unprecedented light weight and stiffness.

A June 25, 2014 Harvard University news release (also on EurekAlert), which originated the news item, goes on to describe the new technology in more detail while throwing 3D printing into the mix,

Until now, 3D printing has been developed for thermo plastics and UV-curable resins—materials that are not typically considered as engineering solutions for structural applications. “By moving into new classes of materials like epoxies, we open up new avenues for using 3D printing to construct lightweight architectures,” says principal investigator Jennifer A. Lewis, the Hansjörg Wyss Professor of Biologically Inspired Engineering at Harvard SEAS. “Essentially, we are broadening the materials palate for 3D printing.”

“Balsa wood has a cellular architecture that minimizes its weight since most of the space is empty and only the cell walls carry the load. It therefore has a high specific stiffness and strength,” explains Lewis, who in addition to her role at Harvard SEAS is also a Core Faculty Member at the Wyss Institute. “We’ve borrowed this design concept and mimicked it in an engineered composite.”

Lewis and Brett G. Compton, a former postdoctoral fellow in her group, developed inks of epoxy resins, spiked with viscosity-enhancing nanoclay platelets and a compound called dimethyl methylphosphonate, and then added two types of fillers: tiny silicon carbide “whiskers” and discrete carbon fibers. Key to the versatility of the resulting fiber-filled inks is the ability to control the orientation of the fillers.

The direction that the fillers are deposited controls the strength of the materials (think of the ease of splitting a piece of firewood lengthwise versus the relative difficulty of chopping on the perpendicular against the grain).

Lewis and Compton have shown that their technique yields cellular composites that are as stiff as wood, 10 to 20 times stiffer than commercial 3D-printed polymers, and twice as strong as the best printed polymer composites. The ability to control the alignment of the fillers means that fabricators can digitally integrate the composition, stiffness, and toughness of an object with its design.

“This paper demonstrates, for the first time, 3D printing of honeycombs with fiber-reinforced cell walls,” said Lorna Gibson, a professor of materials science and mechanical engineering at the Massachusetts Institute of Technology and one of world’s leading experts in cellular composites, who was not involved in this research. “Of particular significance is the way that the fibers can be aligned, through control of the fiber aspect ratio—the length relative to the diameter—and the nozzle diameter. This marks an important step forward in designing engineering materials that mimic wood, long known for its remarkable mechanical properties for its weight.”

“As we gain additional levels of control in filler alignment and learn how to better integrate that orientation into component design, we can further optimize component design and improve materials efficiency,” adds Compton, who is now a staff scientist in additive manufacturing at Oak Ridge National Laboratory. “Eventually, we will be able to use 3D printing technology to change the degree of fiber filler alignment and local composition on the fly.”

The work could have applications in many fields, including the automotive industry where lighter materials hold the key to achieving aggressive government-mandated fuel economy standards. According to one estimate, shedding 110 pounds from each of the 1 billion cars on the road worldwide could produce $40 billion in annual fuel savings.

3D printing has the potential to radically change manufacturing in other ways too. Lewis says the next step will be to test the use of thermosetting resins to create different kinds of architectures, especially by exploiting the technique of blending fillers and precisely aligning them. This could lead to advances not only in structural materials, but also in conductive composites.

Previously, Lewis has conducted groundbreaking research in the 3D printing of tissue constructs with vasculature and lithium-ion microbatteries.

Here’s a link to and citation for the paper,

3D-Printing of Lightweight Cellular Composites by Brett G. Compton and Jennifer A. Lewis. Advanced Materials DOI: 10.1002/adma.201401804 Article first published online: 18 JUN 2014

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.

Batteries made of wood and the mechanical properties of plants

According to Ariel Schwartz in an Aug. 14, 2012 (?) article for Fast Company’s Co.Exist website, batteries made from wood waste may be in our future (Note: I have removed a link),

Researchers from Poznan University of Technology in Poland and Linköping University in Sweden have figured out how to combine lignin with polypyrrole (a conductive polymer) to create a battery cathode that could one day be used in energy storage. The lignin acts as an insulator, while the polypyrrole holds an electric charge.

The discovery is a potential boon for the renewable energy world. As the researchers explain in the journal Science, “Widespread application of electrical power storage may require more abundant materials than those available in inorganics (which often require rare metals), and at a lower cost. Materials for charge storage are desired from easily accessible and renewable sources. Combining cellulose materials and conjugated polymers for charge storage has … attracted attention.”

For anyone (like me) who’s heard the word lignin but doesn’t know the precise meaning, here’s a definition from a Wikipedia essay (Note: I have removed links and footnotes),

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose, employing 30% of non-fossil organic carbon, and constituting from a quarter to a third of the dry mass of wood.

This next item also mentions lignin but in reference to mechanical properties that engineers are observing in plant cells.  From the Aug. 14, 2012 news item on Nanowerk,

From an engineer’s perspective, plants such as palm trees, bamboo, maples and even potatoes are examples of precise engineering on a microscopic scale. Like wooden beams reinforcing a house, cell walls make up the structural supports of all plants. Depending on how the cell walls are arranged, and what they are made of, a plant can be as flimsy as a reed, or as sturdy as an oak.

An MIT researcher has compiled data on the microstructures of a number of different plants, from apples and potatoes to willow and spruce trees, and has found that plants exhibit an enormous range of mechanical properties, depending on the arrangement of a cell wall’s four main building blocks: cellulose, hemicellulose, lignin and pectin.

The news item was originated at the Massachusetts Institute of Technology (MIT) by Jennifer Chu’s Aug. 14, 2012 news release,

Lorna Gibson, the [researcher] at MIT, says understanding plants’ microscopic organization may help engineers design new, bio-inspired materials.

“If you look at engineering materials, we have lots of different types, thousands of materials that have more or less the same range of properties as plants,” Gibson says. “But here the plants are, doing it arranging just four basic constituents. So maybe there’s something you can learn about the design of engineered materials.”

A paper detailing Gibson’s findings has been published this month [freely accessible] in the Journal of the Royal Society Interface.

To Gibson, a cell wall’s components bear a close resemblance to certain manmade materials. For example, cellulose, hemicellulose and lignin can be as stiff and strong as manufactured polymers. A plant’s cellular arrangement can also have engineering parallels: cells in woods, for instance, are aligned, similar to engineering honeycombs, while polyhedral cell configurations, such as those found in apples, resemble some industrial foams.

To explore plants’ natural mechanics, Gibson focused on three main plant materials: woods, such as cedar and oak; parenchyma cells, which are found in fruits and root vegetables; and arborescent palm stems, such as coconut trees. She compiled data from her own and other groups’ experiments and analyzed two main mechanical properties in each plant: stiffness and strength.

Among all plants, Gibson observed wide variety in both properties. Fruits and vegetables such as apples and potatoes were the least stiff, while the densest palms were 100,000 times stiffer. Likewise, apples and potatoes fell on the lower end of the strength scale, while palms were 1,000 times stronger.

“There are plants with properties over that whole range,” Gibson says. “So it’s not like potatoes are down here, and wood is over there, and there’s nothing in between. There are plants with properties spanning that whole huge range. And it’s interesting how the plants do that.”

Since I’m always interested in trees, from Chu’s news release,

In trees such as maples and oaks, cells grow and multiply in the cambium layer, just below the bark, increasing the diameter of the trees. The cell walls in wood are composed of a primary layer with cellulose fibers randomly spread throughout it. Three secondary layers lie underneath, each with varying compositions of lignin and cellulose that wind helically through each layer.

Taken together, the cell walls occupy a large portion of a cell, providing structural support. The cells in woods are organized in a honeycomb pattern — a geometric arrangement that gives wood its stiffness and strength.

Parenchyma cells, found in fruits and root vegetables, are much less stiff and strong than wood. The cell walls of apples, potatoes and carrots are much thinner than in wood cells, and made up of only one layer. Cellulose fibers run randomly throughout this layer, reinforcing a matrix of hemicellulose and pectin. Parenchyma cells have no lignin; combined with their thin walls and the random arrangement of their cellulose fibers, Gibson says, this may explain their cell walls’ low stiffness. The cells in each plant are densely packed together, similar to industrial foams used in mattresses and packaging.

Unlike woody trees that grow in diameter over time, the stems of arborescent palms such as coconut trees maintain similar diameters throughout their lifetimes. Instead, as the stem grows taller, palms support this extra weight by increasing the thickness of their cell walls. A cell wall’s thickness depends on where it is along a given palm stem: Cell walls are thicker at the base and periphery of stems, where bending stresses are greatest.

There’s even a nanotechnology slant to this story, from Chu’s news release,

Gibson sees plant mechanics as a valuable resource for engineers designing new materials. For instance, she says, researchers have developed a wide array of materials, from soft elastomers to stiff, strong alloys. Carbon nanotubes have been used to reinforce composite materials, and engineers have made honeycomb-patterned materials with cells as small as a few millimeters wide. But researchers have been unable to fabricate cellular composite materials with the level of control that plants have perfected.

“Plants are multifunctional,” Gibson says. “They have to satisfy a number of requirements: mechanical ones, but also growth, surface area for sunlight and transport of fluids. The microstructures plants have developed satisfy all these requirements. With the development of nanotechnology, I think there is potential to develop multifunctional engineering materials inspired by plant microstructures.”

Given the problems with the forestry sector, these developments (wooden batteries and engineering materials inspired by plant cell walls) should excite some interest.