Researchers develop high-performance bulk thermoelectrics

The same technology can be applied here on Earth to recover waste heat when fuel is burned."Cogeneration,"or the production of electricity as a by-product of a heat-generating process, already provides as much as 10 percent of Europe's electrical power. Systems for this purpose typically operate best at very high temperatures, are costly to build and operate, and suffer from substantial inefficiencies. That's why they can be found in spacecraft and power plants but not, say, in cars.

But recently, scientists have concocted a recipe for a thermoelectric material that might be able to operate off nothing more than the heat of a car's exhaust. In a paper published inNaturethis month, G. Jeffrey Snyder, faculty associate in applied physics and materials science at the California Institute of Technology (Caltech), and his colleagues reported on a compound that shows high efficiency at less extreme temperatures.

The heart of a thermoelectric generator is a flat array of semiconductor material. In operation, heat from an external source is directed against one side of the array, while the other side is kept cool. Like air molecules in a hot oven, the material within the array flows along the induced temperature gradient: away from the hot side and toward the cool side. But in the crystalline lattice of a semiconductor, there's only one"material"that isn't rigidly fixed: the charge carriers. Consequently, the only things that move in response to the thermal nonequilibrium are these charge carriers and the result is an electrical flow. Build up a circuit by laying out small semiconductor bricks side by side and wiring them together, and you've got a steady electric current.

The lead telluride (PbTe) family of compounds is commonly used in these applications, but regardless of the underlying technology, scientists designing new thermoelectric materials are continually constrained by structural issues at the most microscopic levels. Those moving charge carriers can run afoul of many complex effects, including electrical interactions, heat-induced vibrations (called phonons), and scattering caused by impurities and imperfections within the crystal structure.

The Caltech researchers began with lead telluride and then added a fractional amount of the element selenium, a concoction first proposed by Soviet scientists A. F. Ioffe and A. V. Ioffe in the 1950s. Because any semiconductor's properties are highly sensitive to the exact type and placement of each of its atoms, this small alteration in the formula produces important changes in the crystal's electronic structure.

Specifically, certain regions called"degenerate valleys"arrange themselves in such a way as to provide a more favorable pathway for charge carriers to follow, a trail of equal-energy stepping stones through the material. In addition, adding the selenium creates multiple regions called point defects."They're like air bubbles trapped in window glass,"says Snyder,"and they tend to scatter vibrations. The result is that heat dissipates more slowly through the material."

That dissipation is important, because in order for a material to be efficient, charge carriers should flow much more easily than heat. In other words, electrical resistance should be low, to maximize current, while thermal resistance should be high, to maintain the temperature gradient that causes the charge carriers to flow in the first place."It's a delicate tradeoff,"says Snyder."Something like trying to blow ice cream through a straw. If the straw's very narrow, the ice cream moves slowly. But if you widen it to help the ice cream move faster, you'll find that you also run out of air faster."

To make sense of these tradeoffs, scientists speak of a quantity known as the"thermoelectric figure of merit,"a dimensionless value that can be used to compare the relative efficiency of materials at specific temperatures. The temperature at which peak efficiency is seen depends on the material: each of the Voyager twins, for instance, produces enough juice to power a medium-sized refrigerator, but to do so it must draw heat from decaying radioisotopes."These new materials are roughly twice as effective as anything seen before, and they work well in a temperature range of around 400 to 900 degrees Kelvin,"says Snyder."Waste heat recovery from a car's engine falls well within that range."

In other words, the heat escaping out your car's tailpipe could be used to help power the vehicle's electrical components—and not just the radio, wipers, and headlights."You'll see applications wherever there's a solid-state advantage,"Snyder predicts."One example is the charging system. The electricity to keep your car's battery charged is generated by the alternator, a mechanical device driven by a rubber belt powered by the crankshaft. You've got friction, slippage, strain, internal resistance, wear and tear, and weight, in addition to the mechanical energy extracted to make the electricity. Just replacing that one subsystem with a thermoelectric solution could instantly improve a car's fuel efficiency by 10 percent."

As more automotive systems continue their gradual migration from mechanical or hydraulic to electrical—power steering and brakes, for instance, can both be made to run on electricity—the vehicle of the future will sport more than a passing commonality with the spacecraft of the 1970s."The future of automobiles is electric,"says Snyder."What we're doing now is looking at how to make it all more efficient."

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Spinning new materials in a thread for fiber-based electronics, photonics devices

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Fiber-drawing techniques are used to produce the optical fibers behind much of today’s broadband communications, but these techniques have been limited to materials that can partially melt and stretch like taffy at the temperatures being used for drawing the fibers. The new work demonstrates a way of synthesizing new materials during the fiber-making process, including materials whose melting points are far higher than the temperatures used to process the fibers. The simple proof-of-concept demonstration carried out by the MIT researchers could open the door to a wide array of sophisticated devices based on composite fibers, they say.

The findings, part of a doctoral research project in materials science by Nicholas Orf,have been publishedin the journalProceedings of the National Academy of Sciences. The paper was co-authored by Orf (now a postdoc at MIT); John Joannopoulos, the Francis Wright Davis Professor of Physics; Yoel Fink, the Thomas B. King Associate Professor of Materials Science; Marc Baldo, associate professor; Ofer Shapira, a research scientist in the Research Laboratory of Electronics; postdoc Fabien Sorin; and Sylvain Danto, who was a postdoc at the time. The work was carried out in Fink’s research group.

All previous work on fiber-drawing ended up with the same materials that were there to begin with, just in a different shape, Orf says, adding:“In this method, new materials are formed during the drawing process.”

Fiber drawing involves preparing a“preform” of materials, such as a large glass rod resembling an oversized model of the fiber to be produced. This preform is heated until it reaches a taffy-like consistency and then pulled into a thin fiber. The materials comprising the preform remain unchanged as its dimensions are drastically reduced.

In the current research, the preform contained selenium, sulfur, zinc and tin, arranged within a coating of polymer material. The drawing process, carried out at a temperature of just 260 degrees Celsius (500 degrees Fahrenheit), combined these materials to form fibers containing zinc selenide, even though that compound has a melting point of 1,530 degrees Celsius (2,786 degrees Fahrenheit).

The resulting fiber was a simple but functional semiconductor device called adiode— a sort of one-way valve for electrical current, allowing electrons to flow through it in only one direction. The diode, never before made by such a method, is a basic building block for electrical circuits.

“This shows that many more kinds of materials can be incorporated into fibers than ever before,” Orf says. Because the physical arrangements placed in the preform are preserved in the drawn fiber, it should ultimately be possible to incorporate more complex electronic circuits within the structure of the fiber itself.

Such fibers might find uses as sensors for light, temperature or other environmental conditions, Orf says. Or the fibers could then be woven, such as to make a solar-cell fabric, he says.

Fink says his research group has been working for more than a decade on expanding the kinds of materials and structures that can be incorporated into fibers. He says that despite the rapid progress made in the last few decades in various forms of electronics,“there has been little progress in advancing the overall functionality and sophistication of fibers and fabrics… one of the earliest forms of human expression.”

The group’s research, he says, has stemmed from the basic question,“How sophisticated can a fiber be?” Over the years they have incorporated more and more materials, structures and functions into fibers. But one of the biggest limitations has been the set of materials that could be incorporated into the fibers; this new work has greatly expanded that list. The work shows that it is possible, Fink says,“to use the fiber draw as a way to synthesizenew materials. It’s the first time this has been demonstrated anywhere.”

Zinc selenide, the specific compound formed in this drawing process, is an important material for both its electronic and its optical properties, Orf says. Such fibers might have uses in new photonic circuits, which use light beams to perform functions similar to those carried out by flowing electrons in electronic circuits.

While this experiment produced 15 individual diode devices in the fiber, each separate from the others, Fink says that through continuing research,“We think you could probably do hundreds” and even interconnect them to form electronic circuits.

Professor John Ballato, director of the Center for Optical Materials Science and Engineering Technologies at Clemson University, adds,“There has been growing international interest in semiconducting optical fibers over the past few years. Such fibers offer the potential to marry the optoelectronic benefits of semiconductors, {which} we know from the silicon photonics and integrated circuit worlds, with the light guidance and long path lengths of opticalfibers.” The new MIT work is particularly significant, he says, because of“the utilization of the fiber as a micro solid-state chemical reactor to realize materials that are not generally amenable to direct fiber fabrication.”

Ballato, who was not involved in this research, adds that a similar technique has been used to produce reactions using gases, but that to the best of his knowledge,“this is the first… to extend this concept to the solid state, where indeed a more bountiful opportunity exists to achieve a wider range of materials.” The process is so flexible and has the potential to be used with such a range ofmaterials, he says, that“it can be considered an important step to a‘fiber that does everything’— creates, propagates, senses and manipulates photons, electrons {and} phonons.”

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This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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Multiferroics could lead to low-power devices

Craig Fennie, assistant professor of applied and engineering physics, and research associate Nicole Benedek usedtheoretical calculationsto understand exactly why and how a particular crystalline ceramic, a layered perovskite, is multiferroic. Multiferroic materials are simultaneously ferroelectric (electrically polarized) and ferromagnetic (they exhibit a permanent magnetic field). Their results were published online March 7 inPhysical Review Letters, appearing later in print, and are also the subject of a"Viewpoint"in the journalPhysicsand a"News and Views"column in the journalNature Materials.

A lot of materials respond to electric fields; others to magnetic fields -- but a small subset of materials called multiferroics respond to both. This discovery decades ago caused excitement due to the potential implications for, for example, magnetic storage devices that barely require power.

The Cornell researchers'density functional theorycalculations revealed that octahedron rotations -- lattice distortions ubiquitous in complexcrystalline materialssuch as perovskite -- simultaneously induce and thereby couple ferroelectricity, magnetoelectricity and ferromagnetism.

This prediction is remarkable because octahedral rotations usually cannot produce a polarization. It also lends new insight into the problem of how to introduce multiferroic order into different materials and the possibility of discovering the best materials to make low-power electronics at room temperature.

Their study demonstrates the possibility of robust, controllable coupling of magnetization and ferroelectric polarization, as well as suggesting electric field switching of the magnetization.

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Nanocyrstalline diamond aerogel: New form of girl's best friend is lighter than ever

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Aerogels are a class of materials that exhibit the lowest density,thermal conductivity, refractive index and sound velocity of any bulk solid. Aerogels are among the most versatile materials available for technical applications due to their wide variety of exceptional properties. This material has chemists, physicists, astronomers, andmaterials scientistsutilizing its properties in myriad applications, from a water purifier for desalinizing seawater to installation on a NASA satellite as a meteorite particle collector.

In the new research appearing in the May 9-13 online edition of theProceedings of the National Academy of Sciences, a Livermore team created a diamond aerogel from a standard carbon-based aerogel precursor using a laser-heated diamond anvil cell.

Adiamond anvil cellconsists of two opposingdiamondswith the sample compressed between them. It can compress a piece of material small (tens of micrometers or smaller) toextreme pressures, which can exceed 3 million atmospheres. The device has been used to recreate the pressure existing deep inside planets, creating materials and phases not observed under normal conditions. Since diamonds are transparent, intense laser light also can be focused onto the sample to simultaneously heat it to thousands of degrees.

The new form of diamond has a verylow densityprobably similar to that of the precursor of around 40 milligrams per cubic centimeter, which is only about 40 times denser than air.

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The diamond anvil cell is small enough to fit in the palm of a hand, but it can compress a sample to extreme pressures -- up to about 3.6 million atmospheres at room temperature.

The diamond aerogel could have applications in antireflection coatings, a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. Less light is lost, improving the efficiency of the system. It can be applied to telescopes, binoculars, eyeglasses or any other device that may require a reflection reduction. It also has potential applications in enhanced or modified biocompatibility, chemical doping, thermal conduction and electrical field emission.

In creating diamond aergoels, lead researcher Peter Pauzauskie, a former Lawrence fellow now at the University of Washington, infused the pores of a standard, carbon-based aerogel with neon, preventing the entire aerogel from collapsing on itself.

At that point, the team subjected theaerogelsample to tremendous pressures and temperatures (above 200,000 atmospheres and in excess of 2,240 degrees Fahrenheit), forcing the carbon atoms within to shift their arrangement and create crystalline diamonds.

The success of this work also leads the team to speculate that additional novel forms of diamond may be obtained by exposing appropriate precursors to the right combination of high pressure and temperature.

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Strong, tough and now cheap: New way to process metallic glass developed (w/ video)

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"We've redefined how you process metals,"says William Johnson, the Ruben F. and Donna Mettler Professor of Engineering and Applied Science."This is a paradigm shift inmetallurgy."Johnson leads a team of researchers who are publishing their findings in the May 13 issue of the journalScience.

"We've taken the economics of plasticmanufacturingand applied it to ametalwith superior engineering properties,"he says."We end up with inexpensive, high-performance, precision net-shape parts made in the same way plastic parts are made—but made of a metal that's 20 times stronger and stiffer than plastic."A net-shape part is a part that has acquired its final shape.

Metallic glasses, which were first discovered at Caltech in 1960 and later produced in bulk form by Johnson's group in the early 1990s, are not transparent like window glass. Rather, they are metals with the disordered atomic structure of glass. While common glasses are generally strong, hard, and resistant to permanent deformation, they tend to easily crack or shatter. Metals tend to be tough materials that resist cracking and brittle fracture—but they have limited strength. Metallic glasses, Johnson says, have an exceptional combination of both the strength associated with glass and the toughness of metals.

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A piece of metallic glass is heated and smashed in just 10 milliseconds. Credit: Georg Kaltenboeck

To make useful parts from a metallic glass, you need to heat the material until it reaches its glass-transition phase, at about 500 degrees C. The material softens and becomes a thick liquid that can be molded and shaped. In this liquid state, the atoms tend to spontaneously arrange themselves to form crystals. Solid glass is formed when the molten material refreezes into place before its atoms have had enough time to form crystals. By avoiding crystallization, the material keeps its amorphous structure, which is what makes it strong.

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A piece of metallic glass being heated and squished in milliseconds, as seen in these infrared snapshots. Credit: Joseph P. Schramm

Common window glass and certain plastics take from minutes to hours—or longer—to crystallize in this molten state, providing ample time for them to be molded, shaped, cooled, and solidified. Metallic glasses, however, crystallize almost immediately once they are heated to the thick-liquid state. Avoiding this rapid crystallization is the main challenge in making metallic-glass parts.

Previously, metallic-glass parts were produced by heating the metal alloy above the melting point of the crystalline phase—typically over 1,000 degrees C. Then, the molten metal is cast into a steel mold, where it cools before crystallizing. But problems arise because the steel molds are usually designed to withstand temperatures of only around 600 degrees C. As a result, the molds have to be frequently replaced, making the process rather expensive. Furthermore, at 1,000 degrees C, the liquid is so fluid that it tends to splash and break up, creating parts with flow defects.

If the solid metallic glass is heated to about 500 degrees C, it reaches the same fluidity that liquid plastic needs to have when it's processed. But it takes time for heat to spread through a metallic glass, and by the time the material reaches the proper temperature throughout, it has already crystallized.

So the researchers tried a new strategy: to heat and process the metallic glass extremely quickly. Johnson's team discovered that, if they were fast enough, they could heat the metallic glass to a liquid state that's fluid enough to be injected into a mold and allowed to freeze—all before it could crystallize.

To heat the material uniformly and rapidly, they used a technique called ohmic heating. The researchers fired a short and intense pulse of electrical current to deliver an energy surpassing 1,000 joules in about 1 millisecond—about one megawatt of power—to heat a small rod of the metallic glass.

The current pulse heats the entire rod—which was 4 millimeters in diameter and 2 centimeters long—at a rate of a million degrees per second."We uniformly heat the glass at least a thousand times faster than anyone has before,"Johnson says. Taking only about half a millisecond to reach the right temperature, the now-softened glass could be injected into a mold and cooled—all in milliseconds. To demonstrate the new method, the researchers heated a metallic-glass rod to about 550 degrees C and then shaped it into a toroid in less than 40 milliseconds. Despite being formed in open air, the molded toroid is free of flow defects and oxidation.

In addition, this process allows researchers to study these materials in their molten states, which was never before possible. For example, by heating the material before it can crystallize, researchers can examine the crystallization process itself on millisecond time scales. The new technique, called rapid discharge forming, has been patented and is being developed for commercialization, Johnson says. In 2010, he and his colleagues started a company, Glassimetal Technology, to commercialize novelmetallic-glassalloys using this kind of plastic-forming technology.

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Shaking down frozen helium: In a 'supersolid' state, it has liquid-like characteristics

Why is this important? Understandingsupersolidhelium brings us closer to understanding its close cousins superconductivity and superfluidity.

Physicists had long thought that the unusual behavior of torsion oscillators containing solid helium meant that chilling helium down to temperatures nearabsolute zeroprompts its transformation into a supersolid. It is certainly solid, but in this physical quest, there was a nagging question: Is it a true supersolid?

To gain new perspectives on solid helium, new research tools were needed."Think of this analogy: when Galileo first peered through a telescope, he saw ears onSaturn. With improved technology, humanity began to understand those ears were actually rings around the planet. And with better technology, we saw the differences in the rings. To further understand solid helium, science had to invent new approaches,"says Séamus Davis, Cornell professor of physics."Helium is a pure material. We're gaining a new understanding of the fundamental issues of how nature works, of how the universe works."

In fact, in this paper, the researchers show instead a more prosaic explanation: There are moving defects in the solid helium crystals, and their relaxation time falls with rising temperatures. This is more consistent with the torsional oscillation (shaking) experiments conducted at Cornell.

The researchers learned that the unusual properties of solidheliumdo not reflect a clunky transition between the solid state and a supersolid state. It behaves like a dimmer switch and presents a smooth transition near absolute zero.

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Water waves exhibit negative gravity near a periodic array of buoys

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The researchers, Xinhua Hu from Fudan University in Shanghai, China, and coauthors from China and the US, have published their study in a recent issue ofPhysical Review Letters.

Throughmathematical calculationsandnumerical simulations, the researchers have shown that, when a periodic array of vertical bottom-mounted split tubes resonates near a certain low frequency, the array strongly reflects approaching water waves. They found that such a strong reflection can dramatically modify theabsorptionefficiency of the waves.

“It is a surprising result that a periodic structure can block long-wavelength water waves (namely, with wavelength longer than the periodic length) because conventional periodic structures such as a periodic array of bottom-mounted cylinders cannot block long-wavelength water waves,” Hu toldPhysOrg.com.“In order to block long-wavelength water waves, the building block of the structure should have a low resonant frequency or a long resonantwavelength. Bottom-mounted split tubes or heaving buoys can present such a low-frequency resonance.”

As the researchers explain, because the water waves cannot pass through the periodic array of resonators, it’s as if the water has negative effectivegravity.

“The gravity is usually positive or pointed to the center of the Earth,” Hu explained.“Effective gravity is a parameter in our effective medium theory for long-wavelength water waves propagating through a periodic structure. The effective gravity is also usually positive for conventional periodic structures such as a periodic array of bottom-mounted cylinders.”

Although the researchers’ simulations involved the split tubes as resonators, they predict that other resonators such as damping buoys would have the same effect.Buoysthat can block water waves could be used to extract ocean wave energy, and play a key role in future ocean wave power plants.

“Although current researches focus on improving the efficiency of a single resonator, an array of damping resonators is regarded as a key part of future ocean wave power plants,” Hu said.“Our work reveals that the absorption spectrum of an array of damping resonator (two absorption peaks) is quite different from that of a single damping resonator (one absorption peak). Such a modification is not expected by engineers on ocean wave energy extraction. Knowing such a modification is important for the future design of the resonator in ocean wave power plants.”

Hu added that the research group has experimentally verified the predicted results, which will be published in an upcoming paper.

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Atomic-level crystal gazing: Revelation of crystallization mechanism enables fast writing of data to DVDs

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Rewritable memory, such as the random-access memory found in computers or on DVDs, is based on a phase change in specific types of materials in which the atoms change from one stable arrangement to another. Pulses of laser light can induce a phase change, a process known as‘writing,’ and the material’s phase can be identified by‘reading’ its signature optical properties.

To provide the first full understanding of the atomic structure of one such phase-change material, AgInSbTe (AIST)—often used in rewritable DVDs—Takata and his colleagues combined state-of-the-art materials-analysis techniques and theoretical modeling. A pulse of light can change AIST from an amorphous state, in which the atoms are disordered, into a crystalline phase in which the atoms are form an ordered-lattice structure. This process of crystallization happens in just a few tens of nanoseconds: the faster the crystallization, the faster data can be written and erased. No-one understood, however, why phase changes in AIST were so fast.

The teams’ analyses and modeling showed that AIST crystallizes in a different way to other commercially available phase-change materials. They found that crystallization of AIST is a simple process: the laser light excites the bonding electrons and causes them to move. A central atom of antimony (Sb) switches between one long (amorphous) and one short (crystalline) bond without any bond breaking (Fig. 1).“We hope to verify this bond-interchange model in the near future,” says Takata.“Crystallization is the storage-rate-limiting process in all phase-change materials, and an atomistic understanding of it is essential.”

The researchers also discovered that the absence of cavities within the crystal structure contributes to the faster writing speeds on AIST. This contrasts starkly with the alternative material germanium antimony telluride in which 10% of lattice sites in are empty.

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Thermoelectrics generating electricity from waste heat is a step closer

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A thermoelectric material consists of alternating n-type and p-typesemiconductorsthat together convert heat into electricity. In theory the heat could be sourced from any process that generates heat, but at present the materials are too inefficient to provide a commercially feasible way ofgenerating electricityfromwaste heat, such as that produced in car exhausts.

The most common thermoelectric p-type material in use is based on lead telluride (PbTe) and devices based on this material have been used in satellites, with heat sourced fromradioisotopes, and in niche markets on Earth, where the heat is generated by burning fuels such as gas.

The efficiency of the thermoelectric material is expressed as a“thermoelectric figure of merit,” ZT, which is a dimensionless figure derived from several factors including theelectrical conductivityand thermal conductivity. The figure of merit needs to be over 1.5 for the material to be capable of generating useful amounts of electricity in commercial applications. PbTe thermoelectric materials are capable of withstanding high temperatures, but their figures of merit are around 0.8, which makes them suitable only for niche markets such as satellites.

Now physicists from the California Institute of Technology and the Chinese Academy of Sciences have modified the amount of tellurium in the PbTe alloy and added selenium and sodium to produce a material with a figure of merit of 1.8 at 850K, which lead author Dr. Jeffrey Snyder described as“extraordinary.”

In previous research Snyder and colleagues had achieved a ZT of 1.5 by doping PbTe with thallium and 1.4 by using sodium. Adding selenium to the mix improved the electrical conductivity while also reducing the thermal conductivity. The selenium increases the number of“degenerate valleys” in the electronic band structure of the material, and this boosts the electrical conductivity and raises the ZT figure. Known thermoelectrics have a typical valley degeneracy of less than six, but the number for the new material is 12 or greater.

Dr. Snyder said he thought a figure of merit of 1.8 was the highest ever to be reproduced in independent laboratories. He also suggested that doping other thermoelectrics in the same way should improve their performance.

Dr. Snyder said the team is now working on creating a promising n-type material and in improving the p-type material’s effectiveness at higher temperatures. The paper is published inNature.

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'Swiss cheese' design enables thin film silicon solar cells with potential for higher efficiencies

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One long-term option for low-cost, high-yield industrial production of solar panels from abundant raw materials can be found in amorphous siliconsolar cellsand microcrystalline silicon tandem cells (a.k.a. Micromorph)—providing an energy payback within a year.

A drawback to these cells, however, is that the stable panel efficiency is less than the efficiency of presently dominate crystalline wafer-based silicon, explains Milan Vanecek, who heads the photovoltaic group at the Institute of Physics in Prague.

"To make amorphous and microcrystalline silicon cells more stable they're required to be very thin because of tight spacing between electrical contacts, and the resulting optical absorption isn't sufficient,"he notes."They're basically planar devices. Amorphous silicon has a thickness of 200 to 300 nanometers, while microcrystalline silicon is thicker than 1 micrometer."

The team's new design focuses on optically thick cells that are strongly absorbing, while the distance between the electrodes remains very tight. They describe their design in the American Institute of Physics' journalApplied Physics Letters.

"Our new 3D design of solar cells relies on the mature, robust absorber deposition technology of plasma-enhanced chemical vapor deposition, which is a technology already used for amorphous silicon-based electronics produced for liquid crystal displays. We just added a new nanostructured substrate for the deposition of the solar cell,"Vanecek says.

This nanostructured substrate consists of an array of zinc oxide (ZnO) nanocolumns or, alternatively, from a"Swiss cheese"honeycomb array of micro-holes or nano-holes etched into the transparent conductive oxide layer (ZnO) (See Figure).

"This latter approach proved successful for solar cell deposition,"Vanecek elaborates."The potential of these efficiencies is estimated within the range of present multicrystalline wafer solar cells, which dominate solar cell industrial production. And the significantly lower cost of Micromorph panels, with the same panel efficiency as multicrystallinesiliconpanels (12 to 16 percent), could boost its industrial-scale production."

The next step is a further optimization to continue improving efficiency.

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Diamond center defect helps scientists measure electrical fields

Electrical charges use varied ways to control almost 100 % of all physical, chemical or biological processes. A case in point is deoxyribonucleic acid (DNA) and the exact distribution of electrons on it. This distribution is critical for the precise transmission of genetic information, and modern electric circuits trigger electric currents up to single electrons.

Experts say that measuring minor electronic fields linked to the charge is no easy task. Enter the Stuttgart team that devised a new sensor consisting of just one single atom. Thisnitrogen atomis an impurity captured in diamond, they say.

The team points out that the diamond lattice 'fixes' the atom and enables a laser to address the nuclear vacancy center."The interaction of the atom with the measured field can be determined by the light emitted by the impurity and, therefore, electrical fields can be measured which are just a fracture of theelectrical fieldof an elementary charge in 0.1 um distance,"the scientists explain.

Because the sensor is about the size of an atom, scientists can measure electrical fields with the same spatial precision. The sensor-generated optical readout enables it to be placed in any geometry. The process also attains its sensitivity and resolution at room temperature and ambient conditions.

While researchers have succeeded in demonstrating the existence of small magnetic fields, this latest finding of combining both measurement techniques permits the measurement of electrical and magnetic fields in a single place without changing the sensor, the team points out.

Thanks to this latest development, novel applications can and will emerge. Measuring the magnetic moments' distribution of the chemical compounds' nuclei at the same time is an example, they say, adding that the structure of a substance and its chemical reactivity can be measured simultaneously.

"The ability to sensitively detect individual charges under ambient conditions would benefit a wide range of applications across disciplines,"the authors write."However, most current techniques are limited to low-temperature methods such as single-electron transistors, single-electron electrostatic force microscopy and scanning tunnelling microscopy. Here we introduce a quantum-metrology technique demonstrating precision three-dimensional electric-field measurement using a single nitrogen-vacancy defect centre spin in diamond."

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Germanium-tellurium alloy could form basis for reconfigurable electronic switches

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The researchers focused on a group of chalcogenide materials with anatomic structurethat changes from amorphous to crystalline in response to an applied voltage. The atomically disordered amorphous state is highly resistive, whereas the orderedcrystalline stateis highly conductive. This change in resistivity can be exploited as the basis for switch-like behavior, and as both states are energetically stable, the on or off state can be maintained without needing to continuously apply a voltage. Such‘phase-change’ switches have been studied for some time. To date, however, the ratio of the resistances in the on and off states of phase-change switches has been too low for important classes of applications like radiofrequency electronics.

Chua and his co-workers addressed this issue by building electrical phase-change switches using a binary alloy ofgermaniumandtellurium(GeTe), which had previously been used in phase-change memory. By carefully adjusting fabrication parameters including temperature, heating rate, gaseous flow rate, sputtering power and annealing time, they were able to fashion a thin film with a resistivity in the amorphous state over ten million times that in the crystalline state. After constructing the switch (pictured) by the attachment of copper electrodes, which are needed in order to interface the device with external electronics, this resistance ratio was 1.6 million—orders of magnitude greater than previous phase-change switches of this kind and sufficient for radiofrequency devices.

With usage, the on/off ratio of the switch was found to deteriorate gradually, which the researchers attribute to the incomplete recrystallization of the GeTe upon repeated switching. This is an issue that Chua’s team plans to address with further device optimization.“The switches may find use in electronics that are reconfigurable on the fly, with particularly promising applications in communications electronics,” says Chua.“By making components such as inductors reconfigurable, radiofrequency circuits could operate at multiple frequencies to accommodate different wireless standards with the use of a single physical structure. This could allow such circuits to be made even smaller.”

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Nature of bonding determines thermal conductivity

Phase change materials are among the favourite candidates for developing a"universal memory", which is as fast as DRAM (dynamic RAM), has highstorage density, is always ready for use and does not lose data even when inadvertently turned off. The data is stored in tiny areas of differentelectrical resistance, which are written to by heating with the aid of electric pulses. In doing so, the atomic ordering of the material and its electrical resistance is changed.

When heated, phase change materials switch from the unordered (amorphous) to the ordered (crystalline) state, which leads to a change in their physical properties. This feature has been exploited by industry for many years in optical data carriers such as DVDs, Blu-rays and CD-RWs. By means of a laser, theatomic structureand thus theoptical propertiesare changed in minute areas of the discs. This allows bits to be written to the disc and be read out again by a laser.

"In order to produce energy-saving and tightly packed electronic memories, it is important when the data are written to the disc that the electrical resistance is significantly changed but that the energy remains as localized as possible,"explains Dr. Raphaël Hermann from the Jülich Centre for Neutron Science, who is also currently a visiting professor at the University of Liège."Phase change materials are very well suited because they are poor conductors of heat not only in the unordered but also in the ordered state, in contrast to semiconductors, for example,"adds Prof. Matthias Wuttig from RWTH Aachen University. As part of an international research team, Hermann and Wuttig are investigating the reasons for this surprising material behaviour on alloys of germanium, antimony and tellurium. With the aid of sophisticated scattering experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble, they demonstrated that the bonding conditions between the atoms in the crystalline state as well as deviations from the perfect lattice structure influence the transmission of these vibrations through the material and thus reduce itsthermal conductivity.

"The starting point for our investigations was the observation by our Japanese colleagues that the amorphous material is harder than the crystalline,"says Hermann."This contradicted all assumptions, but the measured stronger bonding forces between the atoms in the amorphous state fitted the picture."The Jülich scientists investigated how the atoms in the specimens vibrate– both locally in the atomic range and also over longer distances."In the crystalline material, we found harder vibrations for the long-range order and better conductivity for sound than in amorphous material. This is normal and is related to an increase in the order. However, we were surprised by the results for short-range vibrations in the crystal. They were softer. The short-range order in crystalline material is therefore lower than in amorphous material. This is very unusual."

On the basis of all the experimental results, the Aachen research group headed by Wuttig developed a model to explain the apparent contradictions."Normally, the propagation of sound waves in material correlates with the thermal conductivity. However, this is not the case withphase changematerials. This is due to the fact that in the crystalline state atoms experience resonance bonding– in other words, the bonding electrons are shared between several atomic pairs. In contrast, in amorphous material the atoms are covalently, that is more strongly and more locally, bonded. The crystalline material is therefore softer and the atoms vibrate more gently. In addition, there is more disorder in the local range. Both of these aspects impair the conductivity for heat carriers, which are partially of short wavelength, but not for the long-wavelength sound waves."The researchers assume that their findings will facilitate a targeted search for materials with the desired properties.

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Toward a more efficient use of solar energy

Even thoughhydrogen productionfrom water andsunlightby means of oxide powders has been studied extensively for several decades, the basic physical and chemical mechanisms of the processes involved cannot yet be described in a satisfactory way. Together with colleagues from the universities of St. Andrews (Scotland) and Bochum and Helmholtz-Forschungszentrum Berlin, scientists at KIT's Institute for Functional Interfaces, headed by Professor Christof Wöll, have succeeded in gathering new findings on the fundamental mechanisms ofphotochemistryontitanium dioxide(TiO₂).

Titanium dioxide, or titania, is a photoactive material occurring in nature in the rutile and anatase modifications, the latter of which being characterized by a ten times higher photochemical activity. When the white TiO₂powder, which is also used as a pigment in paints and sunscreens, is exposed to light, electrons are excited and can, for example, split water into its components oxygen and hydrogen. The hydrogen produced in that way is a"clean"energy source: No climate-killing greenhouse gases are generated but only water is produced during combustion. Titanium dioxide is also used to manufacture self-cleaning surfaces from which unwanted films are removed through photochemical processes triggered by incident sunlight. In hospitals, this effect is used for sterilizing specially coated instruments by means of UV irradiation.

So far, the physical mechanisms of these photochemical reactions on titania surfaces and especially the reason for the much higher activity of anatase could not be explained. The powder particles used in photoreactors are as tiny as a few nanometers only and are thus too small for use in studies by means of the powerful methods of surface analysis. By using instead mm-sized single-crystal substrates, the researchers were for the first time able to precisely study photochemical processes on the surface of titanium dioxide by means of a novel infrared spectrometer.

Using a laser-based technique, the scientists, in addition, determined the lifetime of light-induced electronic excitations inside the TiO₂crystals. According to Professor Christof Wöll, Head of the IFG, exact information about these processes is of great importance:"A short lifetime means that the excited electrons fall back again at once: We witness some kind of an internal short circuit. In the case of a long lifetime, the electrons remain in the excited state long enough to be able to reach the surface of the crystal and to induce chemical processes."Anatase is particularly well suited for the latter purpose because it is provided with a special electronic structure that prevents"internal short circuits". Knowledge of this feature will allow the researchers to further optimize shape, size, and doping of anatase particles used inside photoreactors. The objective is to develop photoactive materials with higher efficiencies and longer lifetimes:"The results obtained by Professor Wöll and his co-workers are of great importance regarding the generation of electrical andchemical energyfrom sunlight, and especially regarding the optimization of photoreactors,"says Professor Olaf Deutschmann, spokesman of the Helmholtz Research Training Group on"Energy-related Catalysis".

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Understanding how glasses 'relax' provides some relief for manufacturers

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Manufacturers who design newmaterialsoften struggle to understand viscous liquids at a molecular scale. Many substances including polymers andbiological materialschange upon cooling from a watery state at elevated temperatures to a tar-like consistency at intermediate temperatures, then become a solid"glass"similar to hard candy at lower temperatures. Scientists have long sought a molecular-level description of this theoretically mysterious, yet common,"glass transition"process as an alternative to expensive and time-consuming trial-and-error material discovery methods. Such a description might permit the better design of plastics and containers that could lengthen the shelf life of food and drugs.

A fundamental question is why many materials behave differently when temperature changes. In some"fragile"glass-forming materials, a modest variation in temperature can make the material change from highly fluid to extremely viscous, while in"strong"fluids this change in viscosity is much more gradual. This effect influences how long a manufacturer has to work with a material as it cools."For decades, material scientists have heavily relied on empirical rules of thumb to characterize these materials,"says NIST theoretician Jack Douglas."But if you want to design a material that does precisely what you want, you need a molecular understanding of the underlying physical processes involved."

According to Douglas, the increasingly viscous nature of glass-forming liquids is related to molecules that move together in long strings around other atoms that are almost frozen in their motion. The growth of these snake-like structures leads to an increase in the viscosity of the liquid: the lower the temperature, the longer the chains, and the more viscous the fluid. The team found that the rate at which these spontaneously organizing snake-like strings grow in size as the material cools is quantitatively related mathematically to the fluid fragility—confirming intuitive arguments made nearly half a century ago by physicists G. Adams and J.H. Gibbs, but now bolstering them with a firm computational underpinning.

Douglas and his collaborator Francis Starr of Wesleyan University achieved a large variation of fluid fragility through use of a computer model, which mimics a polymer fluid that includes tiny nanometer-sized particles. Portraying the addition of various amounts of nanoparticles and varying their interaction with the polymers, Starr says, gave the team a sort of"knob to tweak"to reveal how the fluidity changed with temperature and how the motion of the clusters was quantitatively related to changes in the fluid's properties. This tuning of cooperative motion in glass-forming liquids and fragility should be crucial in material design. Douglas says.

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Timid and shy or bold and welcoming, water behaves in unexpected ways on surfaces

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Controlling the interactions between water and surfaces could alter a great deal of how we design technologies and solve the nation's energy problems. Water's interactions with surfaces are important to the next generation of fuel cells, converting biomass to fuels, and understanding cloud formation for weather patterns and global climate change.

Before these applications can be realized, water must be understood. The theories of how water works must agree with the experimental data, and vice versa. Two studies by scientists at PNNL have overturned decades of conventional wisdom about water. Drawing upon expertise in different scientific disciplines and new instruments, the researchers built computational models that captured their understanding for others to use.

You build models of the world every day. You use them to predict the choices someone will make. For example, you want to ask that cute barista on a date. You consider a lot of variables that would influence the outcome. For example, are they more likely to say"yes"if you ask in person or send a text message? Would sending yellow roses be romantic or over the top? Should you suggest dinner at the fancy new bistro or a simple picnic in the park? You codify your observations and experiences to predict outcomes. That's modeling.

Scientists do the same with water and surfaces. But, because liquid water and steam are still too difficult to predict with today's instruments, scientists work with frozen water. It is simply easier to study. The ice has the same structure as water in other forms. And, when working at the molecular level, ice isn't stationary. It moves and flows, even at very low temperatures.

"If you want to have a theory and an understanding of how water interacts with surfaces, you must have a model case,"said Dr. Greg Kimmel, an American Physical Society Fellow at PNNL who has led key water studies."The model is the test bed for sharpening our understanding of the underlying physics of what happens. With it, we can test—in detail—the structures. But, if you can't even do ice, how are you going to have confidence in the structures you define for fuel cells?"

As an example, in the 1980s and 1990s, scientists believed that one of the best ways to grow a perfectly smooth, very thin layer of ice was to start with platinum. Scientists created clean, well-characterized surfaces and added water. However, the experimental results didn't quite match the predictions; they didn't contradict them either.

Flash forward to 2005 and a team of scientists at PNNL—they wanted to understand conflicting studies about water. They were especially interested in an intriguing study from Stanford University. So, the PNNL team created a smooth platinum wafer and added water. They examined the surface using a technique called rare gas physisorption. This technique propels atoms of the rare gas krypton at the ice layers and records how they respond. The results, however, did not match their predictions.

So, they went back to the basics and re-examined their assumptions about water's behavior. They determined that the first thin layer, or monolayer, of water spread across the platinum surface as they had expected. But, the subsequentwater moleculesclumped together, afraid to expand across the surface and none too thrilled about the water layer below them.

"In other words, the first layer of water is hydrophobic,"said Kimmel.

The V-shaped water molecules preferred to bind to the platinum. By binding to the platinum, the first water layer became very unattractive to the incoming water molecules. So, the next layer of water beaded up, forming icy patches. As more layers of water were added, the islands slowly grew and eventually covered the surface.

"When you look at it, the standard model that had been around for 20 to 30 years is just not a good model for how water grows,"said Kimmel.

The discovery of self-loathing water led the team to ask what would happen if layers of water were grown on top of a truly hydrophobic surface, one that has absolutely no interest in water.

So, the scientists looked to graphene, a thin slice of carbon.

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(a) Top view. The two-layer ice has hexagonal symmetry. (b) Side view. Two flat layers of molecules with hydrogen bonds connecting the layers. (c) Side view of normal, puckered hexagonal ice.

In 2009, a team at PNNL, including scientists from the 2005 study, placed graphene on top of the platinum. The surface does not interact strongly with the water, merely providing a playground for the water molecules to meet in two dimensions. Then, they introduced a small amount of water vapor onto the surface at very low temperatures, around 125 K or the approximate temperature of an evening on the moon.

Next, the team used rare gas physisorption in combination with low-energy diffraction, a technique that sends waves of slow-moving electrons at the surface. How those electrons bounce off the surface tells the researchers about the structure of the material. They expected the water to form icy patches that did not cover the surface.

Surprisingly, they found a smooth, two-layer-thick film of ice had grown on the graphene. The water stretched out evenly across the surface and bonded with its neighboring compatriots. In contrast to platinum, the ice was flat and two layers thick. The angles between the atoms in the water molecules were stretched or compressed compared to normal ice."This makes for stressed ice,"said Kimmel.

The team subjected the ice to infrared spectroscopy and determined that the water molecules were in unusual configurations. With these results, PNNL theoreticians Dr. Christopher Mundy and Dr. Marcel Baer used EMSL's supercomputer to determine that in each layer of the ice, the water molecules formed slightly larger rings than normal. These six-sided rings stacked on top of each other. They also calculated that each water molecule formed four hydrogen bonds-three with other molecules in the same layer, and one with water in the layer on top.

These studies have changed the fundamental concept of water, explaining disparate results and confounding theories. In addition, this work has become part of a larger movement. Scientists at major institutions are making great advances in understanding water to control it for industrial processes, chemical reactions, and biochemical systems.

But, by no means is this the conclusion to water's story. At PNNL, scientists are exploring the reactions of water on a common catalyst, titanium dioxide. While the structure of water and the surface are well known and fairly straightforward, they aren't expecting the interactions to be simple.

"In a lot of cases,watergets pretty complicated pretty fast,"said Kimmel."That's what keeps this research so interesting."

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New method found for controlling conductivity

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“It’s a new way of changing and controlling the properties” ofmaterials— in this case a class called percolated compositematerials— by controlling their temperature, says Gang Chen, MIT’s Carl Richard Soderberg Professor of Power Engineering and director of the Pappalardo Micro and Nano Engineering Laboratories. Chen is the senior author of a paper describing the process that was published online on April 19 and will appear in a forthcoming issue ofNature Communications. The paper’s lead authors are former MIT visiting scholars Ruiting Zheng of Beijing Normal University and Jinwei Gao of South China Normal University, along with current MIT graduate student Jianjian Wang. The research was partly supported by grants from the National Science Foundation.

The system Chen and his colleagues developed could be applied to many different materials for either thermal or electrical applications. The finding is so novel, Chen says, that the researchers hope some of their peers will respond with an immediate,“I have a use for that!”

One potential use of the new system, Chen explains, is for a fuse to protect electronic circuitry. In that application, the material would conduct electricity with little resistance under normal, room-temperature conditions. But if the circuit begins to heat up, that heat would increase the material’s resistance, until at some threshold temperature it essentially blocks the flow, acting like a blown fuse. But then, instead of needing to be reset, as the circuit cools down the resistance decreases and the circuit automatically resumes its function.

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Graduate student Jianjian Wang holds a flask containing the suspension of graphite flakes in hexadecane, as Gang Chen looks on. Photo: Melanie Gonick

Another possible application is for storing heat, such as from a solar thermal collector system, later using it to heat water or homes or to generate electricity. The system’s much-improvedthermal conductivityin the solid state helps it transfer heat.

Essentially, what the researchers did was suspend tiny flakes of one material in a liquid that, like water, forms crystals as it solidifies. For their initial experiments, they used flakes of graphite suspended in liquid hexadecane, but they showed the generality of their process by demonstrating the control of conductivity in other combinations of materials as well. The liquid used in this research has a melting point close to room temperature— advantageous for operations near ambient conditions— but the principle should be applicable for high-temperature use as well.

The process works because when the liquid freezes, the pressure of its forming crystal structure pushes the floating particles into closer contact, increasing their electrical and thermal conductance. When it melts, that pressure is relieved and the conductivity goes down. In their experiments, the researchers used a suspension that contained just 0.2 percent graphite flakes by volume. Such suspensions are remarkably stable: Particles remain suspended indefinitely in the liquid, as was shown by examining a container of the mixture three months after mixing.

By selecting different fluids and different materials suspended within that liquid, the critical temperature at which the change takes place can be adjusted at will, Chen says.

“Using phase change to control theconductivityof nanocomposites is a very clever idea,” says Li Shi, a professor of mechanical engineering at the University of Texas at Austin. Shi adds that as far as he knows“this is the first report of this novel approach” to producing such a reversible system.

“I think this is a very crucial result,” says Joseph Heremans, professor of physics and of mechanical and aerospace engineering at Ohio State University.“Heat switches exist,” but involve separate parts made of different materials, whereas“here we have a system with no macroscopic moving parts,” he says.“This is excellent work.”

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Physicists isolate bound states in graphene-superconductor junctions

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Led by University of Illinois physics professor Nadya Mason, the group published its findings in the journalNature Physics.

When a current is applied to a normal conductor, such as metal or graphene, it flows through the material as a stream of single electrons. By contrast, electrons travel in pairs in superconductors. Yet when a normal material is sandwiched between superconductors, the normal metal can carry the supercurrent.

Normal metals can adopt superconducting capacity because the pairedelectronsfrom the superconductor are translated to special electron-hole pairs in the normal metal, called Andreev bound states (ABS).

"If you have two superconductors with a normal metal between, you can actually transport thesuperconductivityacross the normal material via these bound states, even though the normal material doesn't have the electron pairing that thesuperconductorsdo,"Mason said.

ABS are extremely difficult to measure or to observe directly. Researchers can measure conduction and overall magnitude of a current, but have not been able to study individual ABS to understand the fundamental physics contributing to these unique states.

Mason's group developed a method of isolating individual ABS by connecting superconducting probes to tiny, nanoscale flakes of graphene called quantum dots. This confined the ABS to discrete energy levels within the quantum dot, allowing the researchers to measure the superconducting ABS individually and even to manipulate them.

"Before this, it wasn't really possible to understand the fundamentals of what is transporting the current,"Mason said."Watching an individual bound state allows you to change one parameter and see how one mode changes. You can really get at a systematic understanding. It also allows you to manipulate ABS to use them for different things that just couldn't be done before."

Superconductor junctions have been proposed for use as superconducting transistors or bits for quantum computers, called qubits. Greater understanding of ABS may enable other applications as well. In addition, it may be possible to use the superconducting graphenequantum dotsthemselves as solid-state qubits.

"This is a unique case where we found something that we couldn't have discovered without using all of these different elements– without thegraphene, or the superconductor, or the quantum dot, it wouldn't have worked. All of these are really necessary to see this unusual state,"Mason said.

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Plasmonic metamaterials: From microscopes to invisibility cloaks

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In a Perspectives piece in this week's issue of the journalScience, 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 newmetamaterials, 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 anddescribed in an April 2010Nature Materialsarticle—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."

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How to tame hammering droplets

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MIT’s Kripa Varanasi, co-author of a report on the new findingpublished this weekin the journalPhysical Review Letters, says the phenomenon could help engineers design more durable condensing surfaces, which are used in desalination plants and steam-based power plants. Other co-authors include MIT mechanical-engineering graduate students Hyuk-Min Kwon and Adam Paxson, and associate professor Neelesh Patankar of Northwestern University.

Varanasi, the d’Arbeloff Assistant Professor of Mechanical Engineering, says the effect explains why blades used in power-plant turbines tend to degrade so rapidly and need to be replaced frequently, and could lead to the design of more durable turbines. Since about half of all electricity generated in the world comes from steam turbines— whether heated by coal, nuclear fuel, natural gas or petroleum— improving their longevity and efficiency could reduce the down time and increase the overall output for these plants, and thus help curb the world emissions of greenhouse gases.

You need Flash installed to watch this video

This video from a high-speed camera shows a droplet being deposited on a superhydrophobic surface, just before it separates from the dropper. At the moment of separation, ripples move down through the droplet, showing the deceleration caused by impact with the surface, which causes a brief burst of high pressure. Credit: Kripa Varanasi

There has been widespread interest in the development of superhydrophobic (water-repelling) surfaces, Varanasi says, which in some cases mimic textured surfaces found in nature, such as lotus leaves and the skin of geckos. But most research conducted so far on how such surfaces behave have been static tests: To see the way droplets of different sizes spread out on such surfaces (called wetting) or how they bead up to form larger droplets, the typical method is to add or subtract water slowly in a stationary droplet. But this is not a realistic simulation of how droplets react on surfaces, Varanasi says.

“In any real application, things are dynamic,” he says. And Varanasi’s research shows the dynamics of moving droplets hitting a surface are quite different from droplets formed in place.

Specifically, such droplets undergo a rapid internal deceleration that produces strong pressures— a small-scale version of the water-hammer effect. It is this tiny but intense burst of pressure that accounts for the pitting and erosion found on power-plant turbine blades, he says, which limits their useful lifetime.

“This is one of the biggest unsolved problems” in power-plant design, he says. In addition to damaging the blades, the formation and growth of water droplets mixed with the flow of steam saps much of the power, accounting for up to 30 percent of the system losses in such plants. Since some steam-based power plants, such as natural-gas combined-cycle plants, can already have efficiencies of up to 85 percent in converting the fuel’s energy to electricity, if these droplet losses could be eliminated it could provide almost a 5-percent boost in power.

“This is a new finding, indeed,” says David Quéré, director of research at the laboratory of physics and mechanics of heterogeneous materials at ESPCI, Paris. He explains that“Superhydrophobic materials, on which water can glide and roll in a unique fashion, have interesting properties, provided water stays at the tops of the decorations we find on them. (I like to call that the fakir effect, since water then sits at the tops of a bed of micro-nails.)”

This research, Quéré says, explains why droplets often fail to stay on top and instead get impaled on the“nails,” and so the new findings are“interesting in the context of superhydrophobic materials, because it helps to design materials able to resist this kind of detrimental effect.”

Small-scale texturing of surfaces can prevent the droplets from wetting the surfaces of turbine blades or other devices, but the spacing and sizes of the surface patterns need to be studied dynamically, using techniques such as those developed by Varanasi and his co-authors, he says. Regularly spaced bumps or pillars on the surface can produce a water-shedding effect, but only if the size and spacing of these features is just right. This research showed that there seems to be a critical scale of texturing that is effective, while sizes either larger or smaller than that fail to produce the water-repelling effect. The analysis developed by this team should make it possible to determine the most effective sizes and shapes of patterning for producing superhydrophic surfaces on turbine blades and other devices.

The work is related to Varanasi’s research on how to prevent ice formation on airplane wings, also using nano-texturing of surfaces, but the potential applications of this latest research are much broader. In addition to power-plant turbines, this could also affect the design of condensers in desalination plants, and even the design of inkjet printers, whose operation is based on depositingdropletsof ink on a surface.

This work was funded by the MIT Energy Initiative, the National Science Foundation, the Dupont-MIT Alliance, and the Initiative for Sustainability and Energy at Northwestern. MIT’s Edgerton Center also provided high-speed video equipment.

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This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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Nanotechnology benefits from volcanoes in the outer solar system

Neutron scattering has discovered that methanol crystals that may be found in outer solar system‘ice lavas’ have unusual expansion properties. The unexpected finding by a British planetary geologist usingneutronsat the Institut Laue-Langevin and the ISIS neutron source will interest developers of‘nano-switches’– single atom thick valves used in‘micro-electronics’ at the nano scale.

Dr Dominic Fortes, UCL (University College London) made the discovery whilst investigating the internal structure of icy moons, such asNeptune’s Triton, to explain the icy eruptions seen by passing space-craft. By studying the behaviour of methanol monohydrate, a known constituent of outer solar system ice, under conditions like those within the moons’ interiors Fortes hoped to understand its role in volcanism.

Fortes measured structural changes in methanolcrystalsover a range of temperatures and pressures. He found that when heated at room pressure they would expand enormously in one direction whilst shrinking in the other two dimensions. However when heated under an even pressure they expanded in two directions, whilst compressing in the third. This unexpected expansion (elongating and thinning) under uniform pressure is known as negative linear compressibility (NLC).

Whilst these results form the next step towards understanding outer solar system volcanic activity, Fortes’ discovery is of significant interest for material scientists developing nanotechnology. The predictable expansion of NLC materials in a particular direction under pressure makes them a good candidate for nano-switches where their shape-shifting properties can be used like a microscopic, pressure-controlled valve directing the flow of electricity.

NLC materials are extremely rare with only around 15 known examples. What causes this property is still relatively unknown. Scientists hope better understanding of the phenomenon can bring forward potential technological application.

“Currently the use of NLC materials in technologies such as nano-switches is purely theoretical and limited by our lack of understanding of the underlying physics”, says Prof. Reinhard Neder chairman of the ILL crystallographic committee who approved Dr Fortes beam-time at the world’s flagship centre for neutron science.“However, the simple structure ofmethanolmonohydrate gives us a good chance to understand the source of this property and how to look for it in other more commercially viable materials.”

“It was certainly unexpected,” explains Dr Fortes.“As a planetary geologist my focus is understanding the mechanisms behind volcanic eruptions in theouter solar system. If my results open doors for more applied science back on Earth, that’s a bonus.”

Professor Richard Wagner, Director at the Institut Laue Langevin added“This research is a good example of how even basic academic studies can have completely unpredictable benefits in other areas of science and technology. It’s because of discoveries like this that the ILL strives to maintain our delivery of world leading neutron science in both‘fundamental’ and‘applied’ fields.”

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Delving into manganite conductivity

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At ambient conditions, manganites have insulating properties, meaning they do not conduct electric charges. When pressure of about 340,000 atmospheres is applied, these compounds change from an insulating state to a metallic state, which easily conducts charges. Scientists have long debated about the trigger for this change inconductivity.

The research team's new evidence, to be published online byPhysical Review Letterson Friday, shows that for the manganite LaMnO₃, this insulator-to-metal transition is strongly linked to a phenomenon called the Jahn-Teller effect. This effect actually causes a unique distortion of the compound's structure. The team's measurements were carried out at the Geophysical Laboratory.

Counter to expectations, the Jahn-Teller distortion is observed until LaMnO3 is in a non-conductive insulating state. Therefore, it is reasonable to believe that the switch from insulator to metal occurs when the distortion is suppressed, settling a longstanding debate about the nature of manganite insulating state. The formation of inhomogeneous domains—some with and some without distortion—was also observed. This evidence suggests that the manganite becomes metallic when the breakdown of undistorted to distorted molecules hits a critical threshold in favor of the undistorted.

"Separation into domains may be a ubiquitous phenomenon at high pressure and opens up the possibility of inducing colossal magnetoresistance by applyingpressure"said Baldini, who was with Stanford at the time the research was conducted, but has now joined Carnegie as a research scientist.

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Discovery of mini 'water hammer' effect could lead to materials that water really hates

In piping systems, the water hammer occurs when fluid is forced to stop abruptly, causing huge pressure spikes that can rupture pipe walls. Now, for the first time, the researchers have observed this force on the scale of microns: such pressure spikes can move through a water droplet, causing it to be impaled on textured superhydrophobic surfaces, even when deposited gently.

This insight of how droplets get stuck on surfaces could lead to the design of more effective superhydrophobic, or highly water-repellant, surfaces for condensers in desalination and steam power plants, de-icing for aircraft engines andwind turbine blades, low-drag surfaces in pipes and even raincoats. In certain cases, improved surfaces could improveenergy efficiencyon many orders of magnitude. (About half of all electricity generated in the world comes from steam turbines.)

The research is published by the journalPhysical Review Letters.

"We want to designsurfacetextures that will cause the water to really hate those surfaces,"said Neelesh A. Patankar, associate professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science."Improving current hydrophobic materials could result in a 60 percent drag reduction in some applications, for example."

Patankar collaborated with Kripa K. Varanasi, the d'Arbeloff Assistant Professor of Mechanical Engineering at MIT. The two are co-corresponding authors of the paper. Patankar initiated this study in which he and Varanasi led the analytical work, and the experiments were conducted at MIT in Varanasi's lab. Other co-authors are MIT mechanical engineering graduate students Hyuk-Min Kwon and Adam Paxson.

In designing superhydrophobic surfaces, one goal is to produce surfaces much like the natural lotus leaf. Water droplets on these leaves bead up and roll off easily, taking any dirt with them. Contrary to what one might think, the surface of the leaves is rough, not smooth. The droplets sit on microscopic bumps, as if resting on a bed of nails.

"If a water droplet impales the grooves of this bumpy texture, it becomes stuck instead of rolling off,"Patankar said."Such transitions are well known for small static droplets. Our study shows that the impalement of water is very sensitive to the dynamic 'water hammer' effect, which was not expected."

To show this, the researchers imaged millimeter-scalewater dropletsgently deposited onto rough superhydrophobic surfaces. (The surfaces were made of silicon posts, with spacing from post edge to post edge ranging from 40 to 100 microns, depending on the experiment.) Since these drops were on the millimeter scale and being deposited gently, prior understanding was to assume that gravitational force is not strong enough to push the water into the roughness grooves. The Northwestern and MIT researchers are the first to show this is not true.

They observed that as a droplet settles down on the surface (due to the drop's own weight) there is a rapid deceleration in the drop that produces a brief burst of high pressure, sending a wave through the droplet. The droplet is consequently pinned on the rough surface. That's the powerful mini water hammer effect at work.

By understanding the underlying physics of this transition, the study reveals that there is actually a window of droplet sizes that avoid impalement. Although focused on drop deposition, this idea is quite general and applies to any scenario where the water velocity is changing on a short (less than a millisecond) time scale. This insight can lead to the design of more robust superhydrophobic surfaces that can resist water impalement even under the dynamic conditions typical in industrial setups.

"One way to reduce impalement is to design a surface texture that results in a surface that sustains extremely high pressures,"Patankar said."It is the length scale of the roughness that is important."To resist impalement, the height of a bump and the distance between bumps need to be just right. Hundreds of nanometer scale roughness can lead to robust surfaces.

"Our ultimate goal,"he added,"is the invention of textured surfaces such that a liquid in contact with it will, at least partially, vaporize next to the surface -- or sustain air pockets -- and self-lubricate. This is similar to how an ice skater glides on ice due to a cushion of thin lubricating liquid film between the skates and the ice. A critical step is to learn how to resist impalement of water on the roughness. Our work on water hammer-induced impalement is a crucial advance toward that goal of ultra-slippery vapor stabilizing surfaces."

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