There has been a growing interest in thermal management materials due to the prevailing energy challenges and unfulfilled needs for thermal insulation applications. We demonstrate the exceptional thermal management capabilities of a large-scale, hierarchal alignment of cellulose nanofibrils directly fabricated from wood, hereafter referred to as nanowood. Nanowood exhibits anisotropic thermal properties with an extremely low thermal conductivity of 0.03 W/m·K in the transverse direction (perpendicular to the nanofibrils) and approximately two times higher thermal conductivity of 0.06 W/m·K in the axial direction due to the hierarchically aligned nanofibrils within the highly porous backbone. The anisotropy of the thermal conductivity enables efficient thermal dissipation along the axial direction, thereby preventing local overheating on the illuminated side while yielding improved thermal insulation along the backside that cannot be obtained with isotropic thermal insulators. The nanowood also shows a low emissivity of less than 5% over the solar spectrum with the ability to effectively reflect solar thermal energy. Moreover, the nanowood is lightweight yet strong, owing to the effective bonding between the aligned cellulose nanofibrils with a high compressive strength of 13 MPa in the axial direction and 20 MPa in the transverse direction at 75% strain, which exceeds other thermal insulation materials, such as silica and polymer aerogels, Styrofoam, and wool. The excellent thermal management, abundance, biodegradability, high mechanical strength, low mass density, and manufacturing scalability of the nanowood make this material highly attractive for practical thermal insulation applications.
The wood materials are being marketed through a UMD spinout startup called InventWood. They also have transparent wood.
Transparent wood provides better thermal insulation and lets in nearly as much light as glass, while eliminating glare and providing uniform and consistent indoor lighting. Hu noted that when the transparent wood is installed as a daylight-harvesting roof, the natural aligned wood channels inside help guide the sunlight into the house without relying of the sun’s angle. Transparent wood is also sturdier than traditional wood, and can be used in place of less environmentally friendly materials, such as plastics.
Wood “conducts” heat along the channels that were used when the tree was alive to shuttle water and nutrients from roots to leaves. However, heat trying to cross the wood grain is blocked. With the wood oriented in the right direction, heat could be blocked or transmitted as the designer desires.
To test how much heat was insulated, they measured the temperature on one side of the nanowood, while on the other side of the wood, shining a light in one test and applying heat in another test. The nanowood insulated better in both cases. The wood blocked at least 10 degrees more heat than styrofoam or silica aerogel, which had been awarded the Guinness World Record for ‘best insulator’. The nanowood, which is white, also effectively reflects sunlight.
They also tried to crush it and found that, in one direction, the nanowood was 30 times stronger than commercially used thermal insulation materials such as Styrofoam, aerogel or other foams made of cellulose.
Nanowood’s tiny fibers don’t cause allergic reactions or irritate lung tissues, unlike glass or wool insulators.
The secret to the nanowood is the removal of lignin, the part that makes it brown and rigid. The team also removed some of the short fibers that tangle themselves in with the cellulose fibers that make up the scaffolding-like base structure of the wood. The aligned cellulose fibers then bond with each other and results in a high mechanical strength.
Solar devices made from wood
Inspired by the process by which water is carried through trees from roots to small pores on the underside of leaves, the UMD research team created several new ways in which water can be transported through wood, purifying it for safe use. Energy from the sun and a block of wood smaller than an adult’s hand are the only components needed to heat water to its steaming point in these devices.
The global crisis of water scarcity is a pressing global challenge, and the situation is far worse in developing countries, where safe water is difficult to secure for 1 billion people.
“Cost and manufacturing are key challenges in using the solar-steam technology for seawater desalination and for the first time, wood-based structures can potentially provide solutions,” said Liangbing Hu, UMD associate professor of materials science and engineering and the leader of the projects. Hu is interested in scaling up these devices for commercial use, which includes designing ways to easily manufacture the devices and bring down their cost. The team is racing other research groups to invent a successful solar steam generation device that is cost efficient and easy to use. He is also a member of the University of Maryland Energy Research Center and the Maryland NanoCenter, where the devices were studied closely.
The team is trying out a few twists on the basic idea of using a darkened surface on the wood to heat the water, then pulling it through the wood’s natural porous structures.
Picture a bowl of unpurified water sitting in a sunny spot. On top of it floats a small block of wood about two inches by two inches. The side of the block facing up is darkened, to catch the sun’s rays. As the sun heats the wood, the water below is drawn up through the wood’s natural channels. The hot dark surface evaporates the water, which can be condensed and distilled off. The salt or other contaminants are too heavy to evaporate, so they stay below.
One design, as published in the journal Advanced Materials, uses carbon nanotubes — tiny, naturally dark structures grown in a lab — to coat one side of the wood and heat the water inside. Another, described in the journal Advanced Energy Materials, uses metal nanoparticles to achieve the same results. Both of these designs are very efficient, but come with a higher cost to produce.
Another innovative design involves carbonizing — essentially, burning — the top layer of wood to create a dark surface. The team tried this with the natural wood’s channels oriented up-and-down, just as they would be inside the tree (described in another paper, published today in Advanced Materials).
By the same measure used to test solar cells’ efficiency, the team measured how efficient the solar steam generation devices are. The most efficient device was the burned-top wood, with 87% efficiency at ten suns of light. It was also the least expensive to produce, coming in at only $1 per square meter.