Engineering Design Competition: "Extreme Recycling"

Congratulations to Chris Hallacy, Brad Saund, and Janet Chen for their victory March 8 in the 26th annual ME 72 engineering design competition. This year's theme: "Extreme Recycling." The mission: Design, build, and deploy two vehicles and traverse difficult terrain (water, sand, rocks, and wood chips, with one type of terrain in each of four different 6' x 10' boxes) to collect plastic water bottles, aluminum cans, and steel cans. During each five-minute round, the bots were to transport the recyclables and drop them (ideally, sorted by type, and—in the case of aluminum cans—crushed to less than half of their vertical height) into recycling bins, before scurrying back to the starting zone.

Twenty weeks earlier, at the start of ME 72—Caltech's undergraduate engineering design laboratory class—students were given a budget (ultimately $1200, of which up to $800 could be spent in the Caltech stockrooms) to purchase whatever they needed to build their bots. The ultimate designs followed a few basic themes: bots with scoopers and grippers, to grab the bottles and cans, and bots with baskets, to haul the loot. Other design features included ramps to wedge under opponent bots and trip them up.

Hallacy, Saund, and Chen—a.k.a. team "BRB"—bested five other teams without dropping a heat during the double-elimination contest. In the final round, against team Wall-E—headed by Keir Gonyea, Chris Pombrol, Allen Chen, and Gerardo Morabito—BRB scored first, delivering a plastic bottle to the recycling bin, and hung on (including by literally hanging on to one of the Wall-E bots) for the win.

Kathy Svitil
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Young Caltech Engineers Recognized for Innovative Work in Disease Diagnostic Technologies

$30,000 Lemelson-MIT Collegiate Student Prizes announced; four leading institutes celebrate winners

PASADENA, Calif.—California Institute of Technology (Caltech) graduate student Guoan Zheng is the recipient of the 2011 $30,000 Lemelson-MIT Caltech Student Prize

Zheng was among the four $30,000 Lemelson-MIT Collegiate Student Prize winners announced Wednesday, March 9. He was recognized for his innovative development of an on-chip, inexpensive microscopy imaging technology with many potential applications, including improved diagnostics for malaria and other blood-borne diseases in the developing world.

Zheng, a graduate student in electrical engineering working in the laboratory of Changhuei Yang, professor of electrical engineering and bioengineering, designed a simple, cost-effective, high-resolution on-chip microscope called a sub-pixel resolving optofluidic microscope (SROFM). The technology is suitable for biological research and enables more affordable clinical and field diagnostics. A prolific inventor, Zheng developed an additional low-cost 500-megapixel microscopy imaging system as well as a surface-wave-enabled darkfield aperture (SWEDA), a nanophotonic structure that can be used to boost the detection sensitivity of image sensors.

Zheng and two other finalists presented their inventions to a judging panel and the Caltech community on January 27. In his presentation, Zheng demonstrated his strong interest in the integration of complementary metal-oxide semiconductor (CMOS) technology with image processing, computer vision, microfluidics, and nanotechnology for the design of next-generation low-cost biomedical imaging and sensing devices. His three inventions are all aimed at improving disease diagnostics in the developing world.

According to Yang, "Guoan is a terrific engineer and researcher. His most significant invention to date is his development of SROFM, an original and highly practical approach for designing microscopes. On a different front, Guoan has also paved the way for the next generation of pixel design with his highly innovative work on SWEDA."

Zheng was born and raised in Canton, China. He received his undergraduate degree with honors in electrical engineering from Zhejiang University in Hangzhou, China, and his master's degree in electrical engineering from Caltech in 2008. Zheng is a coauthor on 13 peer-reviewed journal publications. Driven by what he sees as a need in the market for low-cost diagnostic tools, he plans to further develop his biomedical products with the goal of starting his own medical device company.

The Caltech selection committee also acknowledged finalist Wendian "Leo" Shi for the invention of the "μCyto," a portable lab-on-a-chip system for determining white blood cell counts for point-of-care diagnostics. Shi will receive a $10,000 award made possible through the support of Caltech alumnus Michael Hunkapiller (PhD '74). Also an electrical engineering graduate student, Shi works in the lab of Yu-Chong Tai, professor of electrical engineering and mechanical engineering.

In his presentation, Shi described an innovative technology that provides a low-cost alternative to conventional blood counters. A five-part white blood cell (WBC) differential count is one of the most useful clinical tests performed in hospitals to directly evaluate how the immune system is functioning. Shi's low-cost, portable blood counter provides important diagnostic information for conditions such as leukemia, infections, allergies, and immunodeficiency, and can be used to monitor a patient's recovery during therapy. 

Shi's technology is the first successful demonstration of a miniaturized blood counter with a complete WBC five-part differential, and it opens up new possibilities for providing basic medical care to people living in remote rural areas where medical diagnostic tools are not readily accessible. According to Shi, the system can be easily expanded to incorporate the diagnosis of many more diseases by transferring test results wirelessly to doctors in central hospitals.

Shi was born in Zhejiang, China, and earned his BS in microelectronics from Peking University in Beijing. He moved to California in 2007 to complete his master's degree in electrical engineering and continue graduate work at Caltech. Inspired by his mother's career as a doctor, Shi plans to work in the medical device industry and would like to start his own company in the field.

"The innovative work of Zheng and Shi illustrates the impact engineers can have on addressing the greatest challenges faced by our society. They are two electrical engineering students who have chosen to focus their research on improving diagnostic tools for diseases such as malaria and leukemia," says Ares Rosakis, chair of Caltech's Division of Engineering and Applied Science and the Theodore von Kármán Professor of Aeronautics and professor of mechanical engineering.

"The Lemelson-MIT Collegiate Student Prize winners have shown their potential to invent broadly and bring new innovations into the world," says Joshua Schuler, executive director of the Lemelson-MIT Program. "These inventive achievements and the students' creativity, persistence, and overall collaboration must be celebrated at the collegiate level."

Lemelson-MIT Collegiate Student Prize Recipients

Prizes were also awarded to students at the University of Illinois at Urbana-Champaign, MIT, and Rensselaer Polytechnic Institute. The following winners of the annual Lemelson-MIT Collegiate Student Prize were announced March 9 at their respective universities:

-         2011 Lemelson-MIT Illinois Student Prize Winner

Lemelson-MIT Illinois Student Prize winner Scott Daigle developed a system that utilizes automatic gear shifting to reduce the efforts exerted by wheelchair operators. Daigle's company, IntelliWheels, Inc., has an entire suite of products to improve the everyday actions of wheelchair users.

-         2011 Lemelson-MIT Student Prize Winner

Lemelson-MIT Student Prize winner Alice A. Chen developed an assortment of innovations with promising drug development implications, including a humanized mouse with a tissue-engineered human liver designed to bridge a gap between laboratory animal studies and clinical trials.

-         2011 Lemelson-MIT Rensselaer Student Prize Winner

Lemelson-MIT Rensselaer Student Prize winner Benjamin Clough has demonstrated a new technique that employs sound waves to boost the distance from which researchers can use terahertz spectroscopy to remotely detect hidden explosives, chemicals, and other dangerous materials.


Celebrating innovation, inspiring youth

The Lemelson-MIT Program celebrates outstanding innovators and inspires young people to pursue creative lives and careers through invention.

Jerome H. Lemelson, one of U.S. history's most prolific inventors, and his wife Dorothy founded the Lemelson-MIT Program at the Massachusetts Institute of Technology in 1994. It is funded by the Lemelson Foundation and administered by the School of Engineering. The foundation sparks, sustains, and celebrates innovation and the inventive spirit. It supports projects in the United States and developing countries that nurture innovators and unleash invention to advance economic, social, and environmentally sustainable development. To date, the Lemelson Foundation has donated or committed more than $150 million in support of its mission.

About the Lemelson-MIT Caltech Student Prize: The $30,000 Lemelson-MIT Caltech Student Prize is funded through a partnership with the Lemelson-MIT Program, which has awarded the $30,000 Lemelson-MIT Student Prize to outstanding student inventors at MIT since 1995. 

Katie Neith
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Caltech Student Recognized as a "New Face" of Engineering

For Javad Lavaei, a PhD candidate in the Division of Engineering and Applied Science at Caltech, seeking a career in engineering came naturally.

"I was very interested in learning advanced mathematics when I was in high school," says Lavaei. "I had a craving to put my knowledge into practice and to design real-world systems."

With strong family support and motivational high-school teachers who encouraged his interest in engineering, Lavaei pursued and completed a bachelor's and master's degree in electrical engineering. Now, he has the chance to influence other young students to pursue engineering careers as part of the New Faces of Engineering program.

Lavaei was highlighted for his "interesting and unique work" by the program, which is organized by the National Engineers Week Foundation and recognizes engineers who have been in the workplace five years or less and have shown outstanding ability in projects that significantly impact public welfare or further professional development and growth.

"I am very happy that my work has been recognized by senior engineers and that they have considered me a successful engineer," says Lavaei, who is working toward a degree in control and dynamical systems. His current research focuses on techniques specialized for power systems that could potentially save energy, reduce costs, and improve reliability, such as the design of a smart grid. A smart grid utilizes digital technology to improve how electricity travels from power plants to consumers. In the past, he has worked in many different areas of electrical and computer engineering, including control, circuits, communications, and networks.

Lavaei encourages students to apply their enthusiasm for engineering to improving communications infrastructures, electrical power grids, and other areas that can benefit society. 

"I think it's very exciting to work on important engineering problems and find solutions for them that can, in principle, affect everyone's lives in the future," he says. "Students can be very successful in engineering if they have a passion for understanding and/or designing real-world systems."

The National Engineers Week Foundation is a coalition of more than 100 engineering societies, major corporations, and government agencies that seeks to increase public understanding and appreciation of engineers' contributions to society. For more information, visit

Katie Neith
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Daraio Awarded Sloan Fellowship

Caltech's Chiara Daraio is among this year's crop of Sloan Research Fellows. Daraio, who this year was promoted from assistant to full professor of aeronautics and applied physics, is one of 118 faculty from across the country to receive the two-year, $50,000 fellowship, given to early-career scientists and scholars in recognition of achievement and the potential to contribute substantially to their fields.

"It's a great honor for me to receive a Sloan Research Fellowship, a very competitive award," says Daraio, whose research focuses on the design and testing of new materials with "unprecedented" mechanical properties. "We design new materials by assembling fundamental building blocks that interact nonlinearly, and we can choose these nonlinear interactions by controlling, for example, the shape and material properties of the building blocks. The materials we design can have several practical applications, from acoustic imaging to shock absorption."

"I am particularly pleased because the fellowship is awarded primarily in the basic sciences—physics, in my case—and this means that our research is being recognized also for its contribution to the basic sciences, beyond its engineering origins," says Daraio. Her group will use the funds to support a new research area related to the study of strongly nonlinear mechanical phenomena at micro- and nanoscales.

Presented annually since 1955 by the Sloan Foundation, the fellowships are awarded in chemistry, computer science, economics, mathematics, evolutionary and computational molecular biology, neuroscience, and physics. Potential fellows must be nominated by their peers and are subsequently selected by an independent panel of senior scholars. Once named, Sloan Research Fellows are free to pursue whatever research most interests them, and they can use their fellowship funds in a wide variety of ways. Thirty-eight Sloan Research Fellows have gone on to win the Nobel Prize in their fields.

Kathy Svitil
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Caltech-Led Team Pinpoints Aggression Neurons in the Brain

Finding could lead to new treatments for impulsive violence

PASADENA, Calif.—Where does violence live in the brain? And where, precisely, does it lay down its biological roots? With the help of a new genetic tool that uses light to turn nerve cells on and off, a team led by researchers at the California Institute of Technology (Caltech) has tracked down the specific location of the neurons that elicit attack behaviors in mice, and defined the relationship of those cells to the brain circuits that play a key role in mating behaviors.

The researchers’ hope is that these insights might lead to treatments that can specifically address impulsive violence, a category of behavior that has been historically difficult to grapple with from a medical or psychological perspective.

In a study published in this week's issue of the journal Nature, the researchers were not only able to localize the neuronal circuits mediating attack behavior in mice, but were able to determine that these circuits are "intimately associated, deeply intertwined," with another basic social-behavioral drive—mating—according to David J. Anderson, the Benzer Professor of Biology at Caltech and a Howard Hughes Medical Institute investigator.

Indeed, the neurons for violence and mating live so close together, in a brain region known as the ventrolateral subdivision of the ventromedial hypothalamus (VMH), that "they are like a salt-and-pepper mixture," says Anderson.

And if you think of the brain as the world and the hypothalamus as a country, he adds, then the ventromedial hypothalamus is like a state and the ventrolateral subdivision is like a city within that state. "We've found that these 'mating' and 'fighting' neurons are not only located in the same city, but potentially in the same neighborhood," he says.

To determine whether aggression and mating involve the same—or, rather, distinct but intermingled—neurons, Dayu Lin, a former postdoctoral fellow in Anderson's lab and now an assistant professor at New York University, carried out a series of challenging electrophysiological recording experiments in the VMH. Because the VMH is located deep within the brain, it is exceedingly difficult to target accurately. But by inserting a bundle of 16 wire electrodes into this region, Lin was able to get recordings from multiple neurons during repeated episodes of fighting or mating. "It's the first time in which it's been possible to record electrical activity from deep-brain hypothalamic structures in animals while they are engaging in aggressive and mating behaviors," says Anderson.

Using software developed by Allen E. Puckett Professor of Electrical Engineering Pietro Perona and senior postdoctoral scholar Piotr Dollar—and with the help of several Caltech undergraduates—the researchers annotated behavioral changes on a frame-by-frame basis from video taken at the same time as the electrical recordings were performed. This annotation allowed them to make correlations between neuronal activity and behavior with a temporal resolution of approximately 30 milliseconds.

These experiments indicated that while there is some overlap between "mating" neurons and "fighting" neurons, the majority of these cells are distinct, despite their close proximity. Perhaps most surprising, Anderson notes, is the way that the neurons responsible for aggression and mating communicate—or, rather, how they shut each other up. Sex and violence, it seems, are actually at odds: a neuron that is turned on during aggressive behavior will turn off during mating, and vice versa. "We found that they talk to each other in an inhibitory way," he says.

But a correlation between neuronal activity and fighting behavior doesn't indicate whether the activity causes the behavior or the behavior causes the activity. And so Lin, Anderson, and colleagues carried out experiments to activate or inhibit VMH neurons, to distinguish between those alternatives and to determine the effect of such manipulations on behavior.

In order to activate neurons in the VMH, they used a technique known as optogenetic stimulation. Using a disabled virus as a kind of "disposable molecular syringe," Lin injected VMH neurons with DNA that carries the code for channelrhodopsin-2, a protein from blue-green algae that increases neuronal activity in response to blue light. The sensitized neurons could then be turned on or off with the literal flip of a light switch, allowing the scientists to watch what happens to the behavior of an individual mouse.

Remarkably, says Anderson, for mice in which the injection was targeted to the correct location, blue light induced an attack—even toward an inanimate object such as an inflated latex glove. Conversely, using a technique developed by Caltech’s Bren Professor of Biology Henry A. Lester that allowed the scientists to genetically inhibit neuronal activity, Lin and colleagues were able to show that neuronal activity in the VMH was necessary for normal aggressive behavior, as well.

"This answers an important, long-standing question in the field," says Anderson. "Are regions of the brain that can evoke aggression when artificially stimulated actually necessary for normal aggressive behavior? In this case, the answer is clearly 'yes.'"

The researchers also found that stimulating a male to be aggressive toward a female became more difficult as a mating encounter progressed to its consummatory phase. This result was consistent with the observation that neurons activated during fighting appear to become inhibited in the presence of a female. "The question," says Anderson, "is how that inhibition is achieved."

The answer may lead to new areas of research—and, perhaps, to new treatments for impulsive, violent behaviors. Specifically, notes Anderson, scientists can begin thinking about treatments that target violence-begetting neurons while sparing those involved in normal sexual behavior.

"For the last 500 years, we've really had no viable treatments for pathological violence other than execution or imprisonment," says Anderson. "And part of the reason is that we haven't understood enough about the basic neurobiology of aggression. The new studies are an important step in that direction."

In addition, he says, "mapping out the brain circuitry of aggression will provide a framework for understanding where and how in the brain genetic and environmental influences—nature vs. nurture—exert their influences on aggressive behavior."

The other authors on the Nature paper, "Functional identification of an aggression locus in the mouse hypothalamus," in addition to Anderson, Lin, Dollar, and Perona, are Caltech postdoctoral scholar Hyosang Lee and Maureen Boyle and Ed Lein from the Allen Institute for Brain Science in Seattle.

Their work was funded by the Weston-Havens Foundation, the Jane Coffin Childs Foundation, and the Howard Hughes Medical Institute.


Lori Oliwenstein

Two Caltech Professors Named to National Academy of Engineering

Two prominent Caltech faculty have been named members of the National Academy of Engineering (NAE). Election to the NAE is among the highest professional distinctions accorded to an engineer, says NAE President Charles M. Vest.

Ares J. Rosakis, the Theodore von Karman Professor of Aeronautics, professor of mechanical engineering, and chair of the Division of Engineering and Applied Science, was elected for his "discovery of intersonic rupture, contributions to understanding dynamic failure, and methods to determine stresses in thin-film structures," according to the NAE.

Michael Hoffmann, the James Irvine Professor of Environmental Science, was elected for his work on "oxidative treatment technologies for the removal of organic and inorganic contaminants from water."

Lori Oliwenstein
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Plasmonic Metamaterials: From Microscopes to Invisibility Cloaks

A new class of artificial materials called metamaterials—which derive their properties from carefully engineered, nanostructured building blocks rather than from their chemical composition—may one day be used to create ultrapowerful microscopes, advanced sensors, improved solar cells, computers that use light instead of electronic signals to process information, and even an invisibility cloak.

In a Perspectives piece in this week's issue of the journal Science, Caltech's Harry Atwater and Purdue University colleague Alexandra Boltasseva describe advances in a particular subtype of these materials—plasmonic metamaterials. They also describe two of the major limitations in the field: the loss of light or, rather, its absorption by metals such as silver and gold, which are contained in the metamaterial; and difficulties in precisely tuning the materials so they bend incoming light to the required index of refraction.

In their article, Atwater and Boltasseva suggest new approaches to overcoming these obstacles by replacing the silver and gold in the metamaterials with semiconductors made more metallic by the addition of metallic impurities, or by adding non-metallic elements to metals, making them less metallic. Examples of these "intermetallic materials" include aluminum oxides and titanium nitride.

Some of the new metamaterials, the researchers say, are showing promise in uses involving near-infrared light, the range of the spectrum critical for telecommunications and fiber optics. Other materials—such as the negative-index metamaterial developed by Atwater and Caltech graduate student Stanley Burgos and described in an April 2010 Nature Materials article—might even work with light in the visible range of the spectrum.

Future photonics technologies will revolve around new types of optical transistors, switches, and data processors, Atwater and Boltasseva note. Indeed, as they point out in the article's abstract, "these materials can be tailored for almost any application because of their extraordinary response to electromagnetic, acoustic, and thermal waves that transcends the properties of natural materials."

Lori Oliwenstein

New Reactor Paves the Way for Efficiently Producing Fuel from Sunlight

PASADENA, Calif.—Using a common metal most famously found in self-cleaning ovens, Sossina Haile hopes to change our energy future. The metal is cerium oxide—or ceria—and it is the centerpiece of a promising new technology developed by Haile and her colleagues that concentrates solar energy and uses it to efficiently convert carbon dioxide and water into fuels.

Solar energy has long been touted as the solution to our energy woes, but while it is plentiful and free, it can't be bottled up and transported from sunny locations to the drearier—but more energy-hungry—parts of the world. The process developed by Haile—a professor of materials science and chemical engineering at the California Institute of Technology (Caltech)—and her colleagues could make that possible. 

The researchers designed and built a two-foot-tall prototype reactor that has a quartz window and a cavity that absorbs concentrated sunlight. The concentrator works "like the magnifying glass you used as a kid" to focus the sun's rays, says Haile.

At the heart of the reactor is a cylindrical lining of ceria. Ceria—a metal oxide that is commonly embedded in the walls of self-cleaning ovens, where it catalyzes reactions that decompose food and other stuck-on gunk—propels the solar-driven reactions. The reactor takes advantage of ceria's ability to "exhale" oxygen from its crystalline framework at very high temperatures and then "inhale" oxygen back in at lower temperatures.

"What is special about the material is that it doesn't release all of the oxygen. That helps to leave the framework of the material intact as oxygen leaves," Haile explains. "When we cool it back down, the material's thermodynamically preferred state is to pull oxygen back into the structure."

The ETH-Caltech solar reactor for producing H2 and CO from H2O and CO2 via the two-step thermochemical cycle with ceria redox reactions.

Specifically, the inhaled oxygen is stripped off of carbon dioxide (CO2) and/or water (H2O) gas molecules that are pumped into the reactor, producing carbon monoxide (CO) and/or hydrogen gas (H2). H2 can be used to fuel hydrogen fuel cells; CO, combined with H2, can be used to create synthetic gas, or "syngas," which is the precursor to liquid hydrocarbon fuels. Adding other catalysts to the gas mixture, meanwhile, produces methane. And once the ceria is oxygenated to full capacity, it can be heated back up again, and the cycle can begin anew.

For all of this to work, the temperatures in the reactor have to be very high—nearly 3,000 degrees Fahrenheit. At Caltech, Haile and her students achieved such temperatures using electrical furnaces. But for a real-world test, she says, "we needed to use photons, so we went to Switzerland." At the Paul Scherrer Institute's High-Flux Solar Simulator, the researchers and their collaborators—led by Aldo Steinfeld of the institute's Solar Technology Laboratory—installed the reactor on a large solar simulator capable of delivering the heat of 1,500 suns.

In experiments conducted last spring, Haile and her colleagues achieved the best rates for CO2 dissociation ever achieved, "by orders of magnitude," she says. The efficiency of the reactor was uncommonly high for CO2 splitting, in part, she says, "because we're using the whole solar spectrum, and not just particular wavelengths." And unlike in electrolysis, the rate is not limited by the low solubility of CO2 in water. Furthermore, Haile says, the high operating temperatures of the reactor mean that fast catalysis is possible, without the need for expensive and rare metal catalysts (cerium, in fact, is the most common of the rare earth metals—about as abundant as copper).

In the short term, Haile and her colleagues plan to tinker with the ceria formulation so that the reaction temperature can be lowered, and to re-engineer the reactor, to improve its efficiency. Currently, the system harnesses less than 1% of the solar energy it receives, with most of the energy lost as heat through the reactor's walls or by re-radiation through the quartz window. "When we designed the reactor, we didn't do much to control these losses," says Haile. Thermodynamic modeling by lead author and former Caltech graduate student William Chueh suggests that efficiencies of 15% or higher are possible.

Ultimately, Haile says, the process could be adopted in large-scale energy plants, allowing solar-derived power to be reliably available during the day and night. The CO2 emitted by vehicles could be collected and converted to fuel, "but that is difficult," she says. A more realistic scenario might be to take the CO2 emissions from coal-powered electric plants and convert them to transportation fuels. "You'd effectively be using the carbon twice," Haile explains. Alternatively, she says, the reactor could be used in a "zero CO2 emissions" cycle: H2O and CO2 would be converted to methane, would fuel electricity-producing power plants that generate more CO2 and H2O, to keep the process going.

A paper about the work, "High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria," was published in the December 23 issue of Science. The work was funded by the National Science Foundation, the State of Minnesota Initiative for Renewable Energy and the Environment, and the Swiss National Science Foundation.

Kathy Svitil
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Caltech-Led Team Creates Damage-Tolerant Metallic Glass

Amorphous palladium-based alloy demonstrates unprecedented level of combined toughness and strength; could be of use in biomedical implants

PASADENA, Calif.—Glass is inherently strong, but when it cracks or otherwise fails, it proves brittle, shattering almost immediately. Steel and other metal alloys tend to be tough—they resist shattering—but are also relatively weak; they permanently deform and fail easily.

The ideal material, says Marios Demetriou, a senior research fellow at the California Institute of Technology (Caltech), has the advantage of being both strong and tough—a combination called damage tolerance, which is more difficult to come by than the layperson might think. "Strength and toughness are actually very different, almost mutually exclusive," he explains. "Generally, materials that are tough are also weak; those that are strong, are brittle."

And yet, Demetriou—along with William Johnson, Caltech's Ruben F. and Donna Mettler Professor of Engineering and Applied Science, and their colleagues—report in a recent issue of the journal Nature Materials that they have developed just such a material. Their new alloy—a combination of the noble metal palladium, a small fraction of silver, and a mixture of other metalloids—has shown itself in tests to have a combination of strength and toughness at a level that has not previously been seen in any other material.

"Our study demonstrates for the first time that this class of materials, the metallic glasses, has the capacity to become the toughest and strongest ever known," Demetriou says. Indeed, the researchers write in their paper, these materials allow for "pushing the envelope of damage tolerance accessible to a structural metal."

What gives metallic glasses their unusual qualities is the fact that they are made of metals—with the inherent toughness that comes with that class of material—but have the internal structure of glass, and thus its strength and hardness. (Despite its name, it is this internal structure that is the only glasslike thing about metallic glass: the material is not transparent, Demetriou notes, and is both optically and electronically like metal.)

The problem with trying to increase strength in ordinary metals is that their atoms are organized in a crystal lattice, Demetriou explains. "And whenever you try to make something as perfect as a crystal, inevitably you will create defects," he says. Those defects, under stress, become mobile, and other atoms move easily around them, producing permanent deformations. While this rearrangement around defects results in an ability to block or cap off an advancing crack, producing toughness, it also limits the strength of the material.

A notched, glassy palladium sample does not shatter after severe bending, despite the generation of multiple cracks.
Credit: LBNL/Maximilien E. Launey

On the other hand, glass has an amorphous structure, its atoms scattered about without a specific discernible pattern. In metallic glasses—also called amorphous metals because of their structure—this results in an absence of the extended defects found in crystalline metals. The actual defects in glasses are generally much smaller in size and only become active when exposed to much higher stresses, resulting in higher strengths. However, this also means that the strategy used in ordinary metals to stop a crack from growing ever longer—the easy and rapid rearrangement of the atoms around defects into a sort of cap at the leading edge of a crack—is not available.

"When defects in the amorphous structure become active under stress, they coalesce into slim bands, called shear bands, that rapidly extend and propagate through the material," says Demetriou. "And when these shear bands evolve into cracks, the material shatters."

It was this tendency to shatter that was thought to be one of the limiting factors of metallic glasses, which were first developed in the 1960s at Caltech. The assumption was that, despite their many benefits, they could never match or exceed the toughness of the toughest steels.

But what the Caltech scientists found, much to their surprise, was that creating more of a problem could actually solve the problem.

In the new palladium alloy, so many shear bands form when the material is put under stress that it "actually leads to higher toughness, because the bands interact and form networks that block crack propagation," Demetriou explains. In other words, the number of shear bands that form, intersect, and multiply at the tip of an evolved crack is so high that the crack is blocked and cannot travel very far. In essence, then, the shear bands act as a shield, preventing shattering. Thus, the palladium glass acts very much like the toughest of steels, using an analogous blocking mechanism of arresting cracks.

"And," Demetriou adds, "this high toughness does not come at the expense of strength. This material has both strength and toughness, which is why it falls so far outside what's previously been possible. That's why this material is so special."

The palladium alloy described in the paper could soon be of use in biomedical implants, says Demetriou. "One example is dental implants," Demetriou says. "Many noble-metal alloys, including palladium, are currently used in dentistry due to their chemical inertness and resistance to oxidation, tarnish, and corrosion. Owing to its superior damage tolerance, the present palladium glass can be thought of as a superior alternative to conventional palladium dental alloys. Plus, the absence of any elements considered toxic or allergenic—nickel, copper, aluminum—from the composition of this alloy will likely promote good biological compatibility."

The class of such tough metallic glasses potentially could be used in other structural applications like automotive and aerospace components, the team says. But this particular alloy is unlikely to be part of any large-scale manufacturing process. "It's prohibitively expensive," says Demetriou. "The cost is much too high for any large-scale, widespread use."

Still, he notes, the fact that it was created at all, with these particular properties, tells scientists that this level of toughness and strength is well within reach. Now it's just a matter of figuring out specifically what gives this alloy its unique damage tolerance, and how that can be replicated with an alloy containing less-expensive, less-precious metals. 

In addition to Demetriou and Johnson, the other authors on the Nature Materials paper, "A Damage-Tolerant Glass," are Caltech graduate student Glenn Garrett, visitor in applied physics and materials science Joseph Schramm, and lecturer in applied physics and materials science Douglas Hofmann; Robert Ritchie from the Lawrence Berkeley National Laboratory (LBNL) and UC Berkeley; and Maximilien Launey, formerly of LBNL and now at the Cordis Corporation. Their work was supported by the National Science Foundation and the U.S. Department of Energy.

Lori Oliwenstein

John Dabiri Named to EBONY "Power List"

Congratulations to Caltech's John Dabiri, who has been named to EBONY magazine's annual Power 100 List. Among his companions there: President Barack Obama, Wyclef Jean, Michael Jordan, and Will Smith.

"I'm honored to be included in a list with so many distinguished leaders from all walks of life," Dabiri says. "I hope that my inclusion in the list encourages the next generation of young people that they too can use science and engineering to positively impact their communities. And I especially hope that we will start to see them in greater numbers here at Caltech."

Kathy Svitil
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