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AC killers: These acrylic microcapsules are filled with a paraffin wax that can absorb heat from buildings. Credit: Peter Schossig

The "phase-change" materials inside the BASF capsules keep a room cool in much the same way that ice cubes chill a drink: by absorbing heat as they melt. Each polymer capsule contains paraffin waxes that melt at around room temperature, enabling them to keep the temperature of a room constant throughout the day. The waxes work best in climates that cool down at night, allowing the materials inside the capsules to solidify and release the heat they've stored during the day.

In some southern European climates, for example, the materials absorb enough heat during the day to save 20 percent of the electricity needed for air-conditioning. In northern Europe, where nighttime temperatures are cooler, a building incorporating the materials may not need an air conditioner at all, says Peter Schossig, an engineer at the Fraunhofer Institute in Munich, Germany, whose research group worked with BASF to develop the capsules.

The work is part of a push in the construction industry toward greener building materials that help maintain comfortable temperatures without using electricity. According to the U.S. Energy Information Administration, buildings consume more than 70 percent of the electricity generated in America, and about 8 percent of that is used for air-conditioning in homes and offices. Widely used lightweight construction materials including wooden framing and drywall enable contractors to put up buildings rapidly, but they don't store much heat, so temperatures inside fluctuate throughout the day.

Phase-change materials offer a way to add thermal mass to lightweight building materials, says Leon Glicksman, professor of building technology and mechanical engineering at MIT. Since the 1950s, several companies have tried to develop passive cooling systems that take advantage of phase-change materials. But they had limited success because it's difficult to incorporate these new materials into existing building substances.

BASF makes the microcapsules by rapidly beating melted wax into hot water. Since wax and water repel one another, the wax forms small droplets. When the researchers add acrylic precursors to the mix, the repulsion between wax and water drives them to coat the droplets' surface. Finally, they add a catalyst to form an acrylic polymer shell around the wax. The resulting wet mixture can then be added to the powder that's used to make drywall or dried out and incorporated into other construction materials, including concrete and plasters.

Chemical giant DuPont also makes encapsulated phase-change materials and has incorporated them into heat-absorbing panels that it markets in Europe. BASF's strategy is a little different: the company sells the capsules to other companies to incorporate into a range of building materials, including ceiling panels, aerated concrete blocks, and drywall. Based on German electricity prices and climate conditions, a study conducted by BASF estimated that a family home made with plaster that incorporated 360 grams of the phase-change material (at a cost of $4,883) would save enough electricity to recoup the cost within five years.

National Gypsum is working with the National Renewable Energy Laboratory and California's Emerging Technologies Coordinating Council to field-test its heat-absorbing panels, which will be marketed under the name Thermalcore. The panels are made by mixing the BASF microcapsule-water solution with gypsum, the mineral used to make drywall. The paraffins used by BASF can be tailored to melt at different temperatures; those in National Gypsum's panels liquefy at 22.8 ºC (about 73 ºF). According to the company, the panels can store 22 British thermal units per square foot.

National Gypsum will take at least a year to test the panels' performance through all four seasons before bringing them to market. A spokesperson says the company may reformulate the panels to include more or less of the capsules, depending on how they perform in the climate of the western United States. Data from the field trials will also be used to model how much the panels help reduce energy consumption.

So far, the heat-absorbing capsules have only been tested in passive systems. But they could also be used in active systems in warmer climates, says MIT's Glicksman. "In commercial buildings, you could run the air conditioner at night when electricity is cheaper and use the phase-change materials to maintain lower temperatures during the day," he says. Schossig says his research group and BASF are gathering data from experimental active systems.

Copyright Technology Review 2010.

Print out: This plastic material is used as the backing for Phicot's amorphous silicon electronics. Credit: Phicot

Now a company called Phicot has adapted a technique for printing amorphous silicon electronics onto plastic that could finally make such displays practical. The manufacturing technique, already used to make cheap solar cells, involves depositing chemicals on long sheets of plastic as they are fed through a series of rollers. Phicot is a subsidiary of PowerFilm of Ames, IA, which already makes amorphous silicon solar cells using this roll-to-roll process.

"The basic technology of roll-to-roll can bring the price down and make plastic an excellent option for the back half of the display," says Frank Jeffrey, cofounder and CEO of PowerFilm.

Most modern displays rely on transistors made of polysilicon--a semiconductor that allows electrons to move fast enough for video. The problem with polysilicon is that it needs to be deposited at high temperatures that melt plastic. So Phicot turned to amorphous silicon, which can be deposited at low temperatures and yet is still fast enough to control the pixels of electrophoretic displays such as E-Ink, and eventually those in an organic light-emitting diode (OLED) display.

At Phicot's facility, layers of amorphous silicon and insulating materials are deposited onto plastic. These rolls of plastic are then sent to a facility at HP Labs, where engineers use a novel kind of lithography, called self-aligned imprint lithography (SAIL), to etch transistors onto the plastic's surface.

Once the transistors have been deposited, the screen itself must be added. HP has tested its transistors using E-Ink and with its own reflective display technology, capable of showing color and video. According to Carl Taussig, director of HP's information surfaces labs, the amorphous silicon transistors could, with slight changes to their chemical makeup, also drive OLEDs.

Phicot isn't the only company trying to make plastic-based displays. Polymer Vision, a spin-off of Philips, and Plastic Logic are both promising products in the near future. However, these devices will rely on transistors made of organic materials, which are easy to deposit on plastic at low temperatures, but operate more slowly than those made of amorphous silicon. While organic transistors are good enough to power electrophoretic displays, they are incompatible with OLEDs. Another company, Kovio, is aiming to print silicon on plastic using technology that resembles an ink-jet printer; the main applications at this point are RFID tags.
 

Another advantage of amorphous silicon over organic materials is that its electronic performance is well known. "Organics are less well developed," says John Rogers, professor of materials science at the University of Illinois, Urbana-Champaign. "[Organic components] have uncertain stability and limited long-term reliability; they also have not been used in large-scale applications."

The transition from thin-film solar to printing display transistors was relatively simple, says PowerFilm's Jeffrey. The major difference is that the layers are deposited in a different order. Some layers also need to be doped--adding impurities to change a layer's electrical properties--in a different way. "We used our solar-cell machines to do the first development work," he says. "It was very straightforward."

Phicot is planning to move to a facility in Silicon Valley by the end of the year to take HP's SAIL technology to a large space.

But the process is far from perfect. Jeffrey says that the engineers are quickly solving many of the defect challenges--the major obstacle to mass production at this point. For instance, the process must be fine-tuned so that the plastic isn't stressed in a way that could, down the line, result in defective circuitry.

Another challenge is making sure that the plastic is as defect-free as possible to start with. "Your plastic has to be smooth," says Alberto Salleo, professor of materials science and engineering at Stanford. "And as it goes through the rolls it shouldn't expand or crack."

Jeffrey says there may be ways to adapt the electronics to flaws in the plastic. "We have a possible option of designing our circuitry to tolerate those" he says.

Copyright Technology Review 2010.