Sunday, January 13, 2008

Smart Foam

A spongelike shape-memory alloy could find use in communications, robotics, and aerospace.

Memory foam: A foam of nickel-manganese-gallium has the alloy’s shape-memory properties but is lighter and cheaper to make than other forms of the material.
Credit: P. Mullner, M. Chmielus, and S. Donovan, Boise State University, and D. C. Dunand and Y. Boonyongmaneerat, Northwestern University

Researchers have made a lighter and potentially cheaper kind of shape-memory alloy: materials that change shape in response to a magnetic field but remember their original shape. The new material, a porous foam made from a nickel-manganese-gallium alloy, stretches slightly when exposed to a magnetic field. It retains its new form when the field is turned off, but it goes back to its original shape when the field is rotated 90 degrees.

Most shape-memory alloys are driven by temperature changes. Magnetically driven alloys, however, respond faster than those that respond to temperature. Another important advantage of materials that change shape under a magnetic field is that they can be activated from a distance, says Robert O'Handley, a materials-science and engineering researcher at MIT. Because magnetic shape-memory materials can be remotely changed, he says that they are particularly promising for biomedical applications. "You could make a stent, where you apply a magnetic field to it from outside the body and gradually open up an artery," he says.

But magnetic shape-memory alloys have been difficult and expensive to make. The new alloy could be cheaper and easier to synthesize.

And it could be useful in devices that need very precise, repeatable, and rapid positioning, says David Dunand, a materials-science and engineering professor at Northwestern University, who led the work on the new alloy. These devices include microscopes, tiny mirrors used in optical communication, and robots used in medicine. Because the foam is light, it could lead to aerospace applications, such as airplane wings that morph to become more aerodynamic.

The alloy that Dunand and his colleagues used is not new. Single crystals of nickel-manganese-gallium are known to stretch by 10 percent when exposed to a magnetic field. But single crystals, in which all the atoms are packed in a regular, repeating pattern, are expensive and time consuming to make.

Normally, the problem is that in polycrystalline metals, the individual crystals have random orientations. In the presence of a magnetic field, they stretch along different directions, pushing against each other and canceling out each other's motion, Dunand says. "The dream is to make a polycrystal but somehow give space to [the individual crystals] so they can move and not cancel each other's motions." This is precisely what happens in the foam because of the pores. The tiny crystals in the alloy get room to stretch, and the foam changes shape. The change is tiny right now--only 0.12 percent--but it's a start, Dunand says.

Making the foam is cheap and easy. The researchers pour molten alloy into a porous piece of sodium aluminate salt. After the alloy cools, the researchers dissolve the salt using acid, leaving behind a spongelike structure of the alloy. "The foam is a quite promising preparation route--significantly more efficient compared to the growth of single crystals," says Sebastian Fahler, who studies shape-memory alloys at the Leibniz Institute for Solid State and Materials Research, in Dresden, Germany. But the shape change will have to be much higher than 0.12 percent to have practical applications, he says.

Dunand and his colleagues have a plan for increasing the foam's shape change. Just like a sponge, the foam has struts connected at nodes, he explains. Each strut right now contains multiple tiny crystals. These crystals are still canceling out each other's motion to some extent, which is why the overall change in the foam is only 0.12 percent.

To get a larger shape change, Dunand says, the trick will be to make each strut behave like a single crystal, so that the foam on the whole will be more like a single crystal. That means the researchers would have to make individual crystals span each of the struts in the foam.

The material will still face competition. Nickel-titanium shape-memory alloys, which are suitable for use inside the body and are driven by temperature, are already employed to make stents.

For micropositioning applications, says O'Handley, the material will have to compete with piezoelectric materials such as quartz and lead titanate, which deform in response to electric current. But because the process to make the foam is easy and cheap, he says that it brings nickel-manganese-gallium closer to being cost competitive with piezoelectric materials.

Super-Charging Lithium Batteries

Nanowire electrodes could improve the performance of electric vehicles.

Swelling nanowires: Upon charging with lithium ions, these silicon nanowires swell from 89 nanometers wide (top) to 141 nanometers wide and elongate (bottom); they can accommodate 10 times more lithium ions than conventional graphite electrodes can. As a result, the nanowires could more than triple the energy capacity of lithium batteries.
Credit: Yi Cui

Existing lithium batteries can enable battery-powered electrical vehicles to travel hundreds of miles on a charge, prompting a race among major automakers to demonstrate that the batteries are safe and durable enough for mass marketing. Battery developers, meanwhile, continue to push lithium performance. Last month, Stanford University materials scientists unveiled a nanowire electrode that could more than triple lithium batteries' energy storage capacity and improve their safety.

The development, reported in the scientific journal Nature Materials, stems from the labs of nanowire innovator Yi Cui and battery expert Robert Huggins at Stanford's Materials Science and Engineering Department. The researchers show that nanowires of silicon just a few atoms across can function as high-capacity electrodes, absorbing and releasing about 10 times more lithium ions than the graphite electrodes that are commonly used today.

Charging a lithium battery usually means moving lithium ions from the battery's positive electrode or cathode into its negative electrode or anode. Silicon has the right electrochemical affinity for lithium ions to make it a promising material for anodes. In fact, until now, it has been a bit too promising. Silicon anodes absorb too much lithium. Upon charging, the silicon anodes swell to four times their previous volume, fracturing the material. After just a few charging cycles, the anodes are finished.

Nanowires, in contrast, take the swelling in stride. The Stanford collaborators' silicon nanowires swell when charged from 89 nanometers wide to 141 nanometers wide and simultaneously elongate, thereby releasing the strain. They show no signs of mechanical failure after more than 20 cycles.

Nor, according to Cui, do the silicon nanowires appear as susceptible as graphite to typical failure mechanisms that cause safety problems (including fires that prompted new rules from the U.S. Department of Transportation this week limiting lithium batteries in checked luggage). "Potentially, silicon is going to be much safer than carbon," says Cui, who points out that improved safety could be key to lithium's future acceptance in vehicles. "It only takes an accident or two to destroy a technology." He says that testing over many more cycles is under way to confirm the silicon-nanowire anode's enhanced durability and safety.

The downside is that the nanowire growth process that Cui uses, which feeds gaseous silicon to a liquid gold catalyst to make the solid electrode, is a high-temperature (600 to 900 °C) process that could be costly to scale up. Cui believes that scale-up of the vapor-liquid-solid process is nevertheless feasible, but he acknowledges that he is also "exploring another approach."

Ohio State University chemist Yiying Wu, who also works on nanowire electrodes, calls the Stanford work "definitely very important." But Wu and other materials scientists caution that additional advances will be required before lithium batteries with nanowire electrodes deliver major increases in performance of electric-vehicle batteries. Not least is the need to scale up the process of making nanowires, which have yet to be mass-produced for commercial application.

Another limitation is that while Cui's silicon nanowires make great anodes, lithium-battery technology has greater need for improved cathodes. In a given battery, substituting an anode that stores more lithium ions has no impact without a corresponding cathode that can supply more charge.

Both Cui and Wu (who reported his own lithium anode development last month with a high-capacity cobalt-oxide nanowire) say that their labs are working on novel materials for cathodes. "That's the holy grail for this business," says Wu. "Anyone who can generate much higher cathode capacity will bring a huge breakthrough for the lithium battery."

The Year in Nanotech

Better batteries and super-sticky glues are becoming possible because of nanomaterials.

Nano power: This year nanostructures, such as the nanowires seen here, were shown to be useful for generating electricity.
Credit: Zhong Lin Wang

Nano Power
Nanowires and carbon nanotubes are proving valuable for generating and storing energy. Researchers have shown that nanowires can convert vibrations into electricity. (See "Nanogenerator Fueled by Vibrations" and "A New Nanogenerator.") Other nanowires can generate power from light. (See "Tiny Solar Cells.") Carbon nanotubes could be useful for extracting more power from cheap solar-cell materials. (See "Cheap Nano Solar Cells.")

Nanotechnology could also greatly improve batteries. MIT researchers made fibers out of viruses coated with functional materials. The fibers could lead to textiles that collect energy from the sun, convert it into electricity, and store it until it's needed. (See "Virus-Built Electronics.") At the end of the year, Stanford researchers published research showing that silicon nanowires can significantly increase the storage capacity of battery electrodes.

Making Objects Invisible
Theorists have predicted a new class of materials that could render objects invisible. The materials work because they interact with light in unusual ways. Now a number of researchers are beginning to put those theories into practice, making rudimentary invisibility cloaks by controlling the micro- and nanostructure of materials. In addition to making things disappear, such materials could be useful for patterning tiny features for computer chips or for novel antennae for communications. (See "Invisible Revolution," "Superlenses and Smaller Computer Chips," "How to Make an Object Invisible," and "Invisibility Made Easier.")

Materials That Stick to Nothing--or Anything
Teflon pans are easy to clean. But a new super self-cleaning material actually causes oil to bounce off. (See "No More Thumbprints.") Another material--this one transparent--could be used to keep windows fog and oil free. (See "Self-Cleaning, Fog-Free Windows.")

Other researchers are developing supersticky materials. They have made structures out of carbon nanotubes that are like the structures on geckos' feet that allow the lizards to climb walls. They've also made glues similar to the proteins that allow mussels to stick to nearly anything, even underwater. (See "Climbing Walls with Nanotubes," "Nanoglue Sticks Underwater," and "Glue That Sticks to Nearly Everything.")

Flexible Electronics Coming to Market
Electronics patterned on flexible substrates that could be used for roll-up displays have previously been demonstrated in the lab. Now products are on the way. In February we described the plans of two companies to manufacture flexible electronics. (See "Plastic Electronics Head for Market.") One of the companies has now actually started production on a flexible-display device.

Meanwhile, researchers are developing methods for making flexible electronics with higher performance. (See "Printing Cheap Chips" and "Expandable Silicon.")

Tiny Memory
Novel approaches to storing data could lead to memory chips as much as a hundred times more compact than today's devices. These include materials that change structure (see "Novel Nanowires for Faster Memory") and ones that grow atoms-thick wires (see "Terabyte Storage for Cell Phones") in response to tiny electronic signals. Researchers at IBM are developing memory chips that exploit newly understood physical mechanisms to provide a cheap and fast alternative to hard drives and flash memory. (See "IBM Attempts to Reinvent Memory.")

A Better Battery for Laptops

Boston-Power ramps up production of its long-lasting battery.

Staying power: These battery cells are capable of recharging up to 80 percent of their capacity in 30 minutes, and they retain 80 percent of their strength after three years. The image on bottom shows a side-by-side comparison of the heat given off by two batteries generating the same level of energy: on the left is a battery from a current market leader, and on the right, Boston-Power’s Sonata battery. The green colors represent cooler temperatures. High temperatures can lead to explosive battery malfunction. (The brightly colored section outlined in black represents the batteries. The remaining area shows heat emitted by the laptop.)
Credit: Boston-Power

says that it's poised to enter the market for portable power, with a notebook battery the company claims is safer, lasts longer, and can be charged faster. The Westborough, MA, startup recently announced that it is more than tripling production of its high-performance battery, called the Sonata, after receiving $45 million in a third round of venture financing. The move puts the company in a position to mass-produce and commercialize its next-generation lithium-ion battery within months.

"In partnership with GP Batteries, one of Asia's largest battery manufacturers, we now have our second factory up and running in the greater China region," says Christina Lampe-Onnerud, the company's founder and CEO. In 2002, Technology Review named Lampe-Onnerud one of its top innovators under the age of 35 for her efforts to develop better-performing lithium-ion batteries with less volatile substances. Based on that research, she founded Boston-Power in 2005. Now, after raising $68 million in total, she says that her company will be able to manufacture a million battery cells per month by the end of 2008.

Oak Investment Partners, based in Westport, CT, provided this latest infusion of capital, building upon earlier investments by Venrock Associates, Granite Global Ventures, and Gabriel Venture Partners.

Although the Sonata will not offer greater energy capacity per use--with a four-hour run time, its performance will be average for the market--the company hopes that the battery's three-year life span, innovative safeguards, and ability to recharge quickly will help it gain a foothold in the battery market. As opposed to existing notebook batteries, which can take an hour to recharge to 80 percent capacity, the Sonata can reach that same level in just 30 minutes, according to Boston-Power. And whereas current batteries degrade very quickly, permanently losing up to 50 percent of their capacity within months, the Sonata retains up to 80 percent of its capacity over three years. In fact, since the typical laptop battery tends to degrade very rapidly, the Sonata will have a greater per-use capacity in the long run.

To make the cell retain its capacity over its lifetime, Boston-Power found it necessary to change the current lithium-ion design. The company identified a combination of new chemistry mixtures and electrode compositions, and it created a new shape--all of which enables a consistent performance over the cell's lifetime. The different shape made it possible for the company to increase the volume of the cell and more efficiently use the space within a battery pack, allowing it to reach energy-storage levels competitive with current conventional batteries.

In the past, it has been very difficult to make lithium-ion cells larger, since a larger energy density creates a potential for greater catastrophic malfunctioning. Conventional lithium-ion batteries use cobalt oxides, but the substance has been partly responsible for some of the more dramatic laptop explosions in recent years. So instead of using cobalt, which also tends to degrade quickly, the company incorporated manganese. Boston-Power isn't the only company using manganese; other companies, such as Compact Power, are also trying to take advantage of its stability. Boston-Power is incorporating the element into a larger than average cell.

The company has also made the battery safer by separating several conventional safety measures and by inventing new ones. In existing notebook batteries, the current interrupt device and the thermal fuse are packaged on top of each other in the cell's lid. But by separating these elements from each other, the company has built an extra layer of redundancy into the system. These elements are able to control and cut off the current flow, should the battery begin to overcharge. The company has also devised a new ventilation system to alleviate the pressure and heat before they build to catastrophic levels. With aluminum in its canister, rather than carbon steel or nickel, as is common, the Sonata's shell softens much sooner at high temperatures and then self-destructs with a hiss. More-durable elements like carbon steel, which melts at even higher temperatures than aluminum, exacerbate explosions by letting extraordinary pressure and heat build inside the cell until its breaking point. (This is why conventional laptops emit loud booming cracks when they burn.)

"There is a lot of progress being made in battery technology with different chemistries," says Robert Kanode, president and CEO of Valence Technology, an Austin, TX, startup that manufactures phosphate lithium-ion batteries. His company is a competitor with Boston-Power, but Kanode adds, "We know we will not be standing alone: this will be a huge market with many viable players in it."

Lampe-Onnerud says that Boston-Power is in discussions with most of the world's top-tier notebook makers, including Hewlett-Packard, which over the past two years has worked closely with the company, helping it design battery packs that can be dropped into existing notebooks.

"The Sonata opens up a whole new business model for notebook manufacturers that hasn't been available in the past," says Ifty Ahmed, a general partner with Oak Investment Partners, who worked on the deal. Although notebook makers can presently offer a three-year warranty for a computer, they can't make the same offer on a battery, a component that can cost about 10 percent of a laptop's total value. "The market for warranties is extremely profitable," Ahmed says. "So if you can sell a warranty on the battery for three years, you have a very exciting idea."

Boston-Power says that it is focused on commercializing the Sonata, but it also believes that its patented safety features could eventually be used in lithium-ion batteries for smaller consumer-electronics devices as well as for hybrid electric vehicles.

Pocket Printer

Polaroid and Zink Imaging announce a miniature photo printer for cell phones and cameras.

Quick pix: This Polaroid printer is about the size of a deck of cards and prints photos on two-inch-by-three-inch sheets of paper, without using ink or toner. Instead, it uses a novel type of thermal-printing technology developed by startup Zink Imaging. Zink developed special paper (bottom) that contains layers of crystals that release pigment when heated.
Credit: Zink

Polaroid, the company famous for cameras that print instant pictures, unveiled an ultrasmall photo printer today at the Consumer Electronics Show in Las Vegas. The company's new handheld printers produce color photos using novel thermal-printing technology developed at Polaroid spinoff Zink Imaging and first demonstrated earlier this year. (See "Printing without Ink.") John Pollock, the vice president and general manager of digital imaging at Polaroid, says that the printers will be available to consumers by the summer, and they will be priced at less than $150.

Pollock calls the device, preliminarily dubbed the "digital instant mobile photo printer," the ultimate in mobile printing. "When you talk about most portable printers today, you're dealing with lunchbox-sized printers," he says. "What Zink allows us to do is to get ultrasmall form factors."

The printer is about the size of a deck of cards. A user who takes a picture on a cell phone or camera can wirelessly send the file to the printer using Bluetooth, a common short-range wireless technology used in cell phones, or PictBridge, a wireless technology found in a number of cameras. The result is a two-inch-by-three-inch photo printed on paper engineered by Zink.

The printing technology is similar to that of a common thermal printer, says Steve Herchen, chief technology officer at Zink. Inside the printer is a dense collection of tiny heaters--300 per square inch. Zink's paper--which looks and feels like normal photo paper--consists of a white plastic sheet covered with three thin layers of dye molecules. Initially, these molecules have an orderly crystalline structure that renders them transparent. But when heat is applied, the molecules change orientation to become an amorphous glass, reflecting either yellow, magenta, or cyan light. By precisely controlling the temperature and duration of the heat emitted by each heater, the printer can blend the three colors into any combination.

Since Zink's technology eliminates the need for printer cartridges, Herchen says, it has led to the smallest printers on the market, and it could eventually be integrated into cell phones and cameras. It would also dispense with the inconvenience of ink cartridges that unexpectedly begin to run out of ink, and which have to be replaced. "When you go to replace an ink-jet cartridge today, it's in the $40 range," Herchen says. With Zink, a person pays only by the print. Polaroid expects to sell the photo paper for $0.30 a page. "You know exactly how many prints you're paying for," Herchen says.

While the technology may be clever, some analysts aren't sure that it has a very large market. "It's going to be a hard sell, in my opinion," says Ron Glaz, program director for digital-capture devices and photofinishing research at IDC, a technology research firm. One reason, he says, is that people are accustomed to e-mailing pictures to each other or sending them to each other's phones, and they probably won't want to carry around another gadget just to print pictures on the spot. "All these pocket technologies are supposed to simplify the process, but it's creating a new problem of finding a way to carry all these things with us," Glaz says. "Other than niche users who might need to print something out on the spot, the use of this type of technology is just limited."

Polaroid's Pollock says that his company's market research shows that teenagers, in particular, like the convenience of being able to print and share digital photos instantly, and 68 percent of teens who tested the device expressed the intent to buy it. Other demographic groups found the device less appealing, Pollock says.

Another reason that Polaroid's printer might not become widely adopted is more technical. While the Bluetooth communications protocol is common, manufacturers often prevent nonproprietary devices from linking wirelessly with their phones. "Right now we're working on making sure compatibility with cell phones is as high as it can possibly be," says Pollock. "The vast majority of cell phones work, but there are instances where the carrier blocks down the Bluetooth interface." Currently, the iPhone will not work with Polaroid's printer. Pollock says one of the company's priorities is to make sure that it will in the future.