Self-healing autonomous material comes to life

Now, in theJournal of Applied Physics, researchers at Arizona State University have created a material that may be able to not only sense damage instructural materials, such as cracking in a fiber-reinforced composite, but to even heal it. The aim of developing"autonomous adaptive structures"is to mimic the ability of biological systems such as bone to sense the presence of damage, halt its progression, and regenerate itself.

The novel autonomous material developed by Henry Sodano and colleagues uses"shape-memory"polymers with an embedded fiber-optic network that functions as both the damage detection sensor and thermal stimulus delivery system to produce a response that mimics the advanced sensory and healing traits shown inbiological systems. Aninfrared lasertransmits light through the fiber-optic system to locally heat the material, stimulating the toughening and healing mechanisms.

The material system is capable of increasing the toughness of a specimen by 11 times. After toughening the specimen, the crack can be closed using the shape-memory effect to recover an unprecedented 96 percent of the object's original strength. In fact, after the crack is closed, the new material is nearly five times as tough as the original specimen, even though it has been strained past its original failure strain point by a factor of four.

The material andhealing processcan be applied while the structure is in operation, which has not been possible with existing healing techniques.

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Certain doped-oxide ceramics resist Ohm's Law

Several of the slightly doped high-purity barium titanate (BT) ceramics his research group was investigating were not following the venerable Ohm's Law, which relateselectrical voltageto current and resistance. Applying or removing a voltage caused a gradual change in the materials'electrical resistance. The new effect was seen consistently regardless of the temperature or whether the experiments were conducted in vacuum, air, or in an oxygen atmosphere. The time to stabilize and the final, steady-state resistance were, however, both temperature-dependent.

"I was not immediately convinced myself about the non-Ohm's Law behavior,"said West, Professor of Electroceramics and Solid State Chemistry at the University of Sheffield in England."Interfacial effects are well known for their non-Ohmic behavior. We needed to be really convinced that our results were not influenced in some way by interfacial effects."

West's proposed mechanism for the non-Ohm behavior is also unconventional: the ionization of only one of the two extra electrons fromoxygen atomsthat are attached todopantatoms. This process leaves behind a positively charged"hole"that can move fairly readily in what is called a hole current. West and his colleagues at Sheffield and the Universidat Jaume 1 in Castellon, Spain, described their latest experiments with calcium-doped BT in the journalApplied Physics Letters, which is published by the American Institute of Physics. Similar results with zinc and magnesium dopants were published earlier this year in other technical journals. Calcium, zinc and magnesium are known as"acceptor"dopants, which can promote hole currents.

Undoped BT and"donor"-doped materials did not exhibit this unusual behavior. West believes that these results may ultimately lead to a better understanding of how ceramics used in electrical circuits degrade and may possibly even stimulate new insights into high-temperature superconductivity mechanisms in oxide ceramics.

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Materials research with antiparticles: Unravelling the secret of nanocrystalline materials

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Extremely hard but still easily malleable– the properties of the so-called nanocrystalline bulk metals give rise to many questions among physicists. Scientists at Graz University of Technology have finally managed to answer some of those questions through experiments.

The scientists set out to monitor the structural changes in the metals in real time. They were thus able to conclude that atomic defects are a central cause of the interesting physical material properties. Nanocrystalline metals consist of countless crystallites (grains), which are mostly smaller than one hundred nanometres– and the smaller the grain, the more solid is the metal. The structure of nanocrystalline metals is actually very regular: the atoms in the grains lie tightly packed in rank and file. However, when the metals are produced, atomic defects are involuntarily introduced which disturb the atomic order within the grains.

For example, certain layers are not located directly on top of each other: some atoms are missing or rows are misaligned. Austrian materials physicists have now produced the first experimental evidence of these effects, which are closely related to the mechanical properties. They have published their results in the journalPhysical Review Letters, where they describe how the combination of two special methods can be used to closely examine atomic defects.

Spy attacks on metals

As atomic defects on the nanoscale are difficult to spot, the scientists worked with so-called positrons. Dr. Wolfgang Sprengel from Graz University of Technology explains:"A positron is a subatomic particle that is almost identical to the electron, with one difference: it is positively charged. If a positron and an electron meet, they annihilate each other. In the places where atomic defects are present, there are fewer electrons and therefore fewer occurrences of annihilation. The positrons therefore serve as spies that deliver detailed information about the atomic defects. We have used this effect to analyse the fast processes of atomic defects in metals."To carry out the experiments, the scientists availed themselves of the FRM II research reactor at the Technische Universität München (TUM), where they deployed the positron beam with the highest intensity in the world.

Two methods, one result

In addition to the positron-electron annihilation, macroscopic length-change measurements were made upon annihilation of the defects– by means of dilatometry. This combination of dilatometry and positron-electron annihilation is a first of its kind and it delivered the evidence that some of the seemingly mysterious physical properties of the nanocrystalline bulk metals can be attributed to these structural defects. The cause of the defects can be found in the production background of the metals. Nanocrystalline bulk metals are produced using very complex methods– such as high-pressure torsion (Erich Schmidt Institute Leoben)– which give rise to the atomic defects.

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How to soften a diamond

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It is the hardest material in the world, and yet it can not only be used to cut other materials, but can be machined itself. Already over 600 years ago firstdiamondswere cut and the same technique is still used to transform precious stones into exquisite jewelry and later into unrivaled industrial tools. Dr. Lars Pastewka's and Prof. Michael Moseler's team at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg/Germany can now reveal the secret of why it is that diamonds can be machined. The team published its findings in the current online issue ofNature Materials. This work represents major progress in tribology -the research of friction and wear. Despite the great significance for industry the scientific basics of tribology are poorly understood.

Diamonds have been ground by craftsmen for hundreds of years using cast iron wheels studded with fine diamond particles turning at around 30 meters per second at the outer rim. A highly tuned sense of sound and feeling enable an experienced diamond grinder to hold the rough diamond at just the right angle to achieve a smooth and polished surface. The fact that diamonds react directionally has been known for a long time, says Lars Pastewka. The physical phenomenon is known as anisotropy. Thecarbon atomsin the diamond lattice form lattice planes, some of which are easier to polish than others, depending on the angle at which the diamond is held.

For hundreds of years, researchers have been looking for a logical way of explaining this empirical phenomenon, and have so far been unsuccessful. Equally, no one has been able to explain why it is possible that the hardest material in the world can be machined. The scientists in Freiburg have answered both these questions with the help of a newly developed calculation method.

Michael Moseler explains the method in layman's terms:"The moment a diamond is ground, it is no longer a diamond."Due to the high-speed friction between the rough diamond and the diamond particles in the cast iron wheel, a completely different"glass-like carbon phase"is created on the surface of the precious stone in a mechanochemical process. The speed at which this material phase appears depends on the crystal orientation of the rough diamond."This is where anisotropy comes in,"explains Moseler.

The new material on the surface of the diamond, adds Moseler, is then"peeled off"in two ways: the ploughing effect of the sharp-edged diamond particles in the wheel repeatedly scratches off tiny carbon dust particles from the surface - this would not be possible in the original diamond state, which is too hard and in which the bond forces would be too great. The second, equally important impingement on the normally impenetrably hard crystal surface is due to oxygen (O) in the air. The O₂molecules bond with carbon atoms (C) within the instable, long carbon chains that have formed on the surface of the glassy phase to produce the atmospheric gas CO₂, carbon dioxide.

And how was it possible to determine when and which atoms would detach from the crystalline surface?»We looked extremely closely at the quantum mechanics of the bonds between the atoms at the surface of the rough diamond breaking. We had to analyze the force field between the atoms in detail«, explains Lars Pastewka.

If one understands these forces well enough, one can precisely describe - and model - how to make and break bonds."This provided the basis for investigations into the dynamics of the atoms at the friction surface between a diamond particle on the wheel and the rough diamond itself,"adds Pastewka. He and his colleague Moseler have calculated the paths of around 10,000 diamond atoms and followed them on screen. Their calculations paid off: their model is able to explain all the processes involved in the dusty and long misunderstood method of diamond grinding.

The newly developed model is not only a milestone in the field of diamond research:"It proves also that friction and wear processes can be described precisely with modern material simulation methods ranging from the atomic level to macroscopic objects,"emphasizes Prof. Peter Gumbsch, director of the institute. He considers this just as one example of the many questions on wear that industry needs answers to. These questions will be addressed in future by the Fraunhofer IWM within the newly founded MicroTribology CentreµTC under the motto"make tribology predictable".

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Scientists generate two energetic electronic states from one photon

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Under a collaborative research program sponsored by the U.S. Department of Energy, scientists at NREL and the University of Colorado, Boulder report the first designed molecular system that produces two triplet states from an excited singlet state of a molecule (1,3-diphenylisobenzofuran (DPIBF)) with essentially perfect efficiency. The process is called singletfission, and the experiments indicated a quantum yield at 77K of 200% for the creation of the two triplets in DPIBF. This means that it may be possible to generate two electron-hole pairs from absorbed singlephotonsinsolar cellsbased on singlet fission, and thus increase itsconversion efficiencyinto electricity or solar fuels by 35%.

The research has been published in theJournal of the American Chemical Societyin the online ASAP version prior to final publication; the manuscript is entitled“High triplet yield from singlet fission in a thin film of 1,3-diphenylisobenzofuran.” The NREL authors are Justin C. Johnson, Sr. Scientist in the Center for Chemical and Materials Science, and Arthur J. Nozik, Sr. Research Fellow; the author at the University of Colorado is Professor Josef Michl, who heads a team of graduate students, post docs, and several other collaborators working on singlet fission.

Singlet fission is the molecular analogue of Multiple Exciton Generation (MEG) in inorganic semiconductor quantum dots; MEG is another process under investigation at NREL whereby two electron-hole pairs, existing as excitons in quantum dots, can be generated from single photons. Thermodynamic modeling indicates that a relatively simple bilayer solar cell incorporating singlet fission (or MEG) could increase the photoconversion yield of an ideal solar cell by more than one-third.

Both the molecular design principles and broader concepts of coupling chromophores into functional networks were key advances that produced 2 electrons per absorbed photon for future use in cells for photovoltaics and solar fuels. The NREL research on singlet fission was sponsored by the Hydrogen Fuel Initiative within DOE’s Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences. Professor Michl’s research was funded by the Photovoltaics Program at the DOE Office of Energy Efficiency and Renewable Energy.

Until this recent advance, singlet fission had been known as a somewhat obscure phenomenon occurring primarily in single crystals of tetracene, and most of the investigations occurred more than three decades ago. Recent notions of utilizing singlet fission in light harvesting schemes were developed by Hanna and Nozik (Hanna, M. C.; Nozik, A. J.J. Appl. Phys.2006, 100, 074510/1), leading to a much more extensive search for appropriate candidate molecules. However, in the absence of detailed mechanistic information about the process itself it was unclear about how to proceed.

The compound DPIBF was identified as a promising candidate by using theory to direct a search for chromophores likely to have a desirable ratio of singlet and triplet excitation energies (Paci, I. et alJ. Amer. Chem. Soc.2006, 128, 16546), the latter of which can be difficult to measure experimentally. Fundamental characterization of the DPIBF molecules isolated in solution (Schwerin, A. et alJ. Phys. Chem. A2010, 114, 1457-1473) was followed up by considerations of how best to connect two DPIBF molecules together (Greyson et al, J. Phys. Chem B ASAP), a necessary step in order to have two triplets form after photoexcitation. Initial attempts to couple the molecules covalently in a chain-like fashion did not produce high singlet fission yields (Michl et alProc. SPIE2007, 6656, 66560E1), but more recent theory (Smith et al.Chem. Rev.in press) has led to the discovery that the natural geometry that the molecules adopt while packing into crystals, a sandwich type orientation, might be optimal.

Due to the fact that single bulk crystals can be disadvantageous for optical spectroscopy, thin polycrystalline films of DPIBF were fabricated by subliming the compound in vacuum onto prepared substrates. The details of the process of forming these films are extremely important to the ultimate result because the nanoscale molecular geometry depends on the nature of the microcrystals that form. When done correctly, the films showed strong spectroscopic signatures indicating a high yield of triplets. In order to quantify the yield, ultrafast laser experiments were performed to accurately measure the initial populations of photoexcited singlet states and their time evolution intotripletson a picosecond time scale.

A careful analysis of the data led to yields approaching 200% and an increase in the triplet formation rate of more than ten thousand compared with the isolated DPIBF molecule. Both of these observations support the notion that efficient singlet fission is occurring and that the design criteria set forth may be quite general.

Further work on DPIBF and compounds that absorb a larger fraction of the solar spectrum could lead to a significant future role for singlet fission chromophores in various types of ultraefficient light harvesting devices.

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Hard X-rays probe model fuel-cell catalyst

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"People have tried to use synchrotron radiation and X-ray spectroscopic techniques over many years—decades—to measure what's happening to the platinum,"Nilsson said. But insufficient resolution and sensitivity made those measurements hard to interpret."And so nobody had been able to understand exactly what they see. I think what this particular study has done is improve the understanding of what we see."

Fuel cells show potential for generating electricity in a clean, renewable fashion. Like batteries, fuel cells provide energy using a chemical process divided into two"half-reactions"that take place at separate positive and negative electrodes. Unlike batteries, a fuel cell can run continuously so long as the negative electrode—or cathode—is supplied with oxygen and the positive electrode—or anode—is supplied with a fuel. One promising type of fuel cell is the polymer electrolyte membrane fuel cell, which uses hydrogen as fuel and creates water as its only by-product.

However, one major obstacle stands in the way of widespread use of fuel cells: cost. Large quantities of platinum are needed to speed up the conversion of oxygen to water on the cathode side. What's more, over time a process Nilsson terms"the second biggest issue"for fuel cells takes place; the platinumcatalystdegrades, requiring even more of the precious metal to keep a fuel cell working. Researchers do not fully understand how this degradation happens, and observing the microscopic details of the various processes taking place at the cathode has proven to be difficult, Friebel said.

"Monitoring only the surface of a fuel-cell catalyst under realistic conditions is a challenge,"Friebel said."We needed a probe that could penetrate a relatively dense liquid environment that surrounded the catalyst, so that’s why we use hardX-rays."At the same time, Friebel's group wanted to examine anÅngström-thin slice at the catalyst surface where the fuel-cell reaction takes place. Here, the same penetrating ability that let the X-rays cut through the liquid surrounding their sample worked against them."Their ability to pierce through the liquid let them also enter the bulk of the catalyst,"Friebel said.

Previous experiments generally looked at platinum nanoparticles—tiny bits of pure platinum with dimensions measured in nanometers, or billionths of a meter. However, even a particle that small still has the majority of its atoms sitting on the inside, and their response to the X-rays diluted the data from surface catalytic activity.

To get around this problem, the researchers coated a single crystal of rhodium with one layer of platinum atoms, in essence creating a platinum catalyst that was"all surface."The unique sample design allowed Friebel and Stanford graduate student Daniel Miller to observe how the catalyst surface interacted with the type of acid–water environment typical of fuel cells.

"A major part of the study was conducted using a relatively new type of spectroscopy"called high-energy resolution fluorescence detection, said SLAC senior scientist Uwe Bergmann, a collaborator on the project, who had built the spectrometer located at SSRL's Beamline 6-2 where Friebel's group ran their experiment. The instrument enabled the researchers to identify how oxygen is bound to the platinum surface under different conditions. These oxygen-platinum interactions ranged from merely placing oxygen atoms onto an intact metallic surface to forming a surface oxide, which was very difficult to remove. According to Miller, this surface oxide could play an important role in degrading the performance of fuel cells.

"{Platinum oxide} could be involved in many things,"Miller said."It could be one reason why the reaction on the oxygen side of thefuel cellis inefficient, but it could also be involved in the degradation of the catalyst."

These findings were made possible because the group was able to put together"three key ingredients,"Nilsson explained:"a well-defined model catalyst, a high-resolution spectrometer that is unique in the United States, and theoretical calculations using a sophisticated structure model that reflects the quality of the sample in the experiment."

In previous studies, Nilsson and his group have proposed methods to reduce the cost of fuel cells by reducing the amount of platinum needed. The new findings could nicely complement their previous efforts toward both improving the efficiency and extending the lifetime of theplatinumcatalyst.

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Researchers find corn starch solution can help shape solid materials

Their work is described in the journalPhysical Review Letters.

Manufacturers use a variety of methods for shapingsolid materials, ranging from laser cutting to high-speed jets of water. While altering the shape of such materials, such as glass, metal, or stone, is relatively straightforward, doing so with precision often proves challenging.

With this in mind, the NYU researchers sought to create an alternative, but rudimentary, method to shape solid materials in a precise fashion. To do so, they considered a process involving a corn starch solution.

Similar solutions have proved valuable in creating body armor—but for different reasons. The molecules in these fluids—also called shear-thickening fluids—are closely packed, but loosely arranged. Under most conditions, they flow like most liquids. However, when met with pressure from an object or other force, its particles interlock and the fluid acts like a solid. Body armor comprised of shear-thickening fluids, when met with bullets, become hard and deflect incoming projectiles.

The NYU researchers sought to apply these principles in a different manner. Instead of using the solution to deflect objects, they aimed to use it as part of a process to shape solid materials—in this case, a wall of modeling clay.

To do this, they submerged a motor-powered, plastic sphere through the cornstarch solution toward a containing wall made of modeling clay, stopping just short of the wall. Using the force of the sphere to harden the cornstarch solution, the researchers were able to make indentations in the wall of modeling clay. In addition, they were able to do so with a degree of precision by taking into account speed, force, and geometry. By moving the sphere at fast speeds through the solution, they created large depressions in the clay; by slowing it down, they created smaller depressions.

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