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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