Future building materials: aerogels, nanocrystals, and smart windows

Bacteria bricks. Nanocrystals. Waste-to-energy thermoelectrics. Self-healing concrete. Aerogels. Thermo-bimetals. Nobel Prize–winning graphene.

That’s just a fraction of the innovation happening right now for future building materials. There’s a red-hot explosion of research in microlevel materials and alteration of basic molecular properties that underlie many things, including building materials.

But you have to wonder: What’s real and what’s fantasy?

“Aerogels, graphene, nanocrystals, thermoelectrics . . . these are real things,” says Dr. Christopher Spadaccini, director of the Center for Engineered Materials, Manufacturing, and Optimization at Lawrence Livermore National Laboratory. “However, these materials are in the very research-oriented, early phase. For example, graphene is carbon—single molecule sheets of carbon. We recently demonstrated that we can 3D-print graphene aerogel.

“Why would you care about that? Graphene has a very high surface area and has very interesting mechanical and electrical properties,” Spadaccini continues. “So it could be used in a supercapacitor or a battery. Generally, when you have materials with features at nanosize scales, you’re really affecting the physics of things like electrical and photonic properties. Even unique thermal properties can be achieved.”

When working at the molecular level, new terms must be created for what is essentially a new materials language. These are the elements that, once scaled up to a usable level, can be made available for everyday use.

A nanocrystal, for example, is a crystalline nanoparticle having at least one dimension smaller than 100 nanometers (a nanoparticle) and composed of atoms in either a single- or poly-crystalline arrangement. Nanocrystals are much smaller than larger crystals, so how could they be used in the here and now? One example is smart windows, with California’s Heliotrope Technologies leading the smart-window charge.

“The core technology my colleagues have developed are essentially nanocrystals that allow you to tune certain parts of the solar spectrum,” says Jason Holt, former staff scientist at Lawrence Livermore National Laboratory, and President of Heliotrope Technologies. “We can control heat—near infrared—transmission independently of visible light. The exciting opportunity is to have a window in which you don’t alter the appearance from the occupant’s or the outside observer’s point of view. You can allow as much light as you’d like, but you can also offer solar blocking.

“By applying voltage to the nanocrystalline film, you can tune its properties so that in the winter, you can allow some of that heat through for passive solar-heating purposes, but you can block it in the summer when you don’t want it,” Holt continues. “Much like charging a lithium ion battery, you in essence charge up your devices to control the state of visible or infrared transmission through the window.”

Holt explains the incredibly small scale of nanocrystals and their unique properties. “The nanocrystals we use are just a few nanometers in dimension. A human hair by comparison is 80 to 100 microns in diameter, so we’re talking about something that is almost 100,000-fold smaller than the thickness of a human hair. It’s the [nanocrystal’s] special optical properties on this small-length scale that offer the heat- and light-control characteristics of the products we’re developing.” Heliotrope’s product, a window sub-assembly, is a dynamic pane of glass that would comprise the outer pane in a double-pane window.

“In a commercial application, if dynamic windows enable you to eliminate blinds and downsize your HVAC system, the cost premium can be substantially offset,” Holt says. “We think our manufacturing-cost advantages can reduce the premium for dynamic windows to the point where you’re essentially cost neutral with high-end conventional windows.”

Yet another nanomaterial, aerogel, is an oddly familiar-sounding term that has nothing to do with hairspray and everything to do with supercapacitors.

“Aerogel is an extremely lightweight foam,” Spadaccini explains. “You can make it out of different materials. Aerogels could provide insulation; they have very low thermal conductivity because the structures within them are so thin. They like to call them ‘structural smoke’ because they are lighter than air. If you were to draw a box around it and calculate the density, it’s very low. But they’re essentially random foams that are extremely lightweight with very thin members and features.”

Spadaccini and his crew at Lawrence Livermore are researching many materials that he calls “architected or designed microstructures” and that he thinks of as a “one-unit cell, then repeat that unit cell in space to make a material. So you end up with a lattice of unit cells.”

Some of these materials have specific mechanical properties, such as very high stiffness and strength but very low weight, or other unique properties such as negative thermal expansion.

As scientists trade ideas on websites like Materials360 and ponder the future of all things nano, atom-like, and molecular, what of larger-scale materials?

“These things are so highly specialized, it is hard to predict what it will cost to make large quantities,” Spadaccini says. “Right now they are exorbitantly expensive because you have teams of PhDs developing them in small quantities. Whether that will scale to something that construction can handle economically remains to be seen. You may see low-volume, high-value applications before you see a commodity like a building material.”