ScienceDaily (Mar. 17, 2010) — Metallic glasses are emerging as potentially useful materials at the frontier of materials science research. They combine the advantages and avoid many of the problems of normal metals and glasses, two classes of materials with a very wide range of applications. For example, metallic glasses are less brittle than ordinary glasses and more resilient than conventional metals.

Metallic glasses also have unique electronic behavior that scientists are just beginning to understand. In a new study, scientists at the Carnegie Institution used high pressure techniques to probe the connection between the density and electronic structure of a cerium-aluminum metallic glass, opening up new possibilities for developing metallic glasses for specific purposes.

"High pressure is an extremely powerful tool for understanding these materials," says Ho-kwang Mao of Carnegie's Geophysical Laboratory, a co-author of the study published in Physical Review Letters. "Pressure can cause changes in their properties, such as their volume or electronic behavior, which in turn tells us about their structure at the atomic scale. The more we know about the structure, the better we can predict their properties and more quickly we can develop new materials."

Unlike ordinary metallic materials, which have an ordered, crystalline structure, metallic glasses are disordered at the atomic scale. This disorder can actually improve some properties of the material, because boundaries between crystal grains are often sites of weakness, leading to breakage or corrosion. Metallic glasses can therefore have superior strength and durability as compared to other metals. The disordered structure also makes metallic glasses highly efficient magnets because it lacks the kinds of defects found in crystalline metals.

Density is a property that can be altered by subjecting a material such as glass to high pressure. But unlike other glasses, which reduce their volume under pressure by rearranging their atoms to take up less space, metallic glasses have a structure in which the atoms are already closely packed. For this reason, researchers previously thought that metallic glasses could not be converted into denser phases. But in 2007 two teams made the surprising discovery that cerium-rich metallic glasses did in fact become denser at high pressure. Theorists suggested that the volume collapse happens through changes in the electronic structure of the cerium atoms in which electrons bound to specific atoms under low pressure become "delocalized" (that is, free to move among the atoms) under high pressure. This causes the bond between atoms to shrink, allowing them to pack even more closely. Until now, however, there has been no direct experimental evidence for this transformation.

The research team, led by predoctoral fellow Qiaoshi Zeng of Carnegie's HPSynC (also a graduate student at Zhejiang University, China) with other co-workers from the Geophysical Laboratory, Zhejiang University, Stanford University and SLAC used a combination of in-situ high pressure synchrotron x-ray absorption spectroscopy and diffraction techniques to observe the electronic transformation in a cerium-aluminum metallic glass (Ce₇₅Al₂₅). The researchers used this glass because its high cerium content made the electronic transformation easier to detect. The experiments showed that at high pressures (between 1.5 and 5 gigapascals, equivalent to 100 to 360 tons per square inch) the volume of the glass decreased by close to 9%. At the same time, x-ray absorption spectra revealed that electrons in the cerium atoms known as 4f electrons did become delocalized, as predicted.

"This result confirms that the volume reduction is due to changes in electronic properties, and shows the key role cerium plays in the phase change." says Mao. "We may find similar transformations in other densely packed metallic glasses that contain cerium or similar rare earth metals. This is important because with the phase change the glass becomes a new material with new properties. It opens up possibilities for optimizing these materials and for fine-tuning their physical and electronic properties for a variety of applications."

Story Source:

Adapted from materials provided by Carnegie Institution.

Journal Reference:

1. Qiao-shi Zeng, Yang Ding, Wendy L. Mao, Wenge Yang, Stas. V. Sinogeikin, Jinfu Shu, Ho-kwang Mao, and J. Z. Jiang. Origin of Pressure-Induced Polyamorphism in Ce₇₅Al₂₅ Metallic Glass. Physical Review Letters, 2010; 104 (10): 105702 DOI: 10.1103/PhysRevLett.104.105702

ScienceDaily (Mar. 17, 2010) — Crash-test dummies could soon be facing vehicle collision tests in cars padded with cork rather than traditional materials such as polymer foams or porous aluminium metal, according to Portuguese engineers writing in the International Journal of Materials Engineering.

Synthetic cellular materials, polymeric and metallic foams, have been extensively used in energy-absorbing systems for decades. They are commonly lightweight, stiff, and can absorb energy well. However, they suffer from some drawbacks when compared to natural materials, namely cost and a lack of sustainability.

Cork, the bark of the cork oak tree, Quercus suber, is one such cellular natural material. It can be compacted to form a micro-agglomerated material that rivals aluminium foam for its ability to absorb the energy of an impact. Now, Mariana Paulino of the University of Aveiro, and colleagues there and at the University of Coimbra have pitted cork against metal foams, polymer padding and a novel polymer foam material from Dow Automative, known as IMPAXX 300, to see which might make the optimal vehicle safety material.

The results obtained in energy-absorption tests indicate that polyurethane foam performs the worst of all the materials tested, despite its widespread use as an impact safety material in vehicles. Aluminium foam can absorb the most energy, marginally beating cork.

As an impact protection material for car bumpers, doors, headliners, knee bolsters and door pillars, cork even outperforms the novel material IMPAXX 300 in terms of the value of impact acceleration peak. Indeed, at higher energies, which would equate to a high-speed collision, cork has the best acceleration peak value.

The researchers also investigated the extent to which the different materials tested would intrude into the vechicle occupants' space in a collision. From a global point of view, aluminium foam showed the lowest displacement, followed by cork and then IMPAXX. Polyurethane foam was again the least suitable material in this test.

Aside from its well-known application as a bottle stopper material, cork is already widely used as a thermal and sound insulator and in various energy-absorbing applications including packaging and footwear. It is often used as damping pads under the keys in wind instruments such as clarinets and saxophones. However, its potential as a safety material for vehicles is only now emerging.

The researchers conclude that while aluminium foam marginally performs better than micro-agglommerated cork, cork could be a much better choice for future vehicle design as it is less costly and much easier to process than metal foam.

Story Source:

Adapted from materials provided by Inderscience, via AlphaGalileo.

Journal Reference:

1. Paulino et al. Hyperelastic and dynamical behaviour of cork and its performance in energy absorption devices and crashworthiness applications. International Journal of Materials Engineering Innovation, 2009; 1 (2): 197 DOI: 10.1504/IJMATEI.2009.029364

ScienceDaily (Mar. 17, 2010) — Since its development in China thousands of years ago, silk from silkworms, spiders and other insects has been used for high-end, luxury fabrics as well as for parachutes and medical sutures. Now, National Science Foundation-supported researchers are untangling some of its most closely guarded secrets, and explaining why silk is so super strong.

Researchers at the Massachusetts Institute of Technology's Center for Materials Science and Engineering say the key to silk's pound-for-pound toughness, which exceeds that of steel, is its beta-sheet crystals, the nano-sized cross-linking domains that hold the material together.

Markus Buehler, the Esther and Harold E. Edgerton Associate Professor in MIT's department of civil and environmental engineering, and his team recently used computer models to simulate exactly how the components of beta sheet crystals move and interact with each other. They found that an unusual arrangement of hydrogen bonds--the "glue" that stabilizes the beta-sheet crystals--play an important role in defining the strength of silk.

They found that hydrogen bonds, which are among the weakest types of chemical bonds, gain strength when confined to spaces on the order of a few nanometers in size. Once in close proximity, the hydrogen bonds work together and become extremely strong. Moreover, if a hydrogen bond breaks, there are still many hydrogen bonds left that can contribute to the material's overall strength, due to their ability to "self-heal" the beta-sheet crystals.

The researchers conclude that silk's strength and ductility--its ability to bend or stretch without breaking--results from this peculiar arrangement of atomic bonds. They say controlling the size of the area in which hydrogen or other chemical bonds act can lead to significantly enhanced properties for future materials, even when the initial chemical bonds are very weak.

The journal Nature Materials reported the findings online March 14.

Story Source:

Adapted from materials provided by National Science Foundation.

Journal Reference:

1. Sinan Keten, Zhiping Xu, Britni Ihle & Markus J. Buehler. Nanoconfinement controls stiffness, strength and mechanical toughness of β-sheet crystals in silk. Nature Materials, 14 March 2010 DOI: 10.1038/nmat2704

ScienceDaily (Mar. 16, 2010) — Researchers at the University of Rochester's Institute of Optics have discovered a way to make liquid flow vertically upward along a silicon surface, overcoming the pull of gravity, without pumps or other mechanical devices.

In a paper in the journal Optics Express, professor Chunlei Guo and his assistant Anatoliy Vorobyev demonstrate that by carving intricate patterns in silicon with extremely short, high-powered laser bursts, they can get liquid to climb to the top of a silicon chip like it was being sucked through a straw.

Unlike a straw, though, there is no outside pressure pushing the liquid up; it rises on its own accord. By creating nanometer-scale structures in silicon, Guo greatly increases the attraction that water molecules feel toward it. The attraction, or hydrophile, of the silicon becomes so great, in fact, that it overcomes the strong bond that water molecules feel for other water molecules.

Thus, instead of sticking to each other, the water molecules climb over one another for a chance to be next to the silicon. (This might seem like getting energy for free, but even though the water rises, thus gaining potential energy, the chemical bonds holding the water to the silicon require a lower energy than the ones holding the water molecules to other water molecules.) The water rushes up the surface at speeds of 3.5 cm per second.

Yet the laser incisions are so precise and nondestructive that the surface feels smooth and unaltered to the touch.

In a paper a few months ago in the journal Applied Physics Letters, the same researchers proved that the phenomenon was possible with metal, but extending it to silicon could have some important implications. For instance, Guo said, this work could pave the way for novel cooling systems for computers that operate much more effectively, elegantly, and efficiently than currently available options.

"Heat is definitely the number one problem deterring the design of faster conventional processors," said Michael Scott, a professor of computer science at the University, who is not involved in this research.

Computer chips are essentially wafers of silicon covered with billions of microscopic transistors that communicate by sending electrical signals through metal wires that connect them. As technological innovations make it possible to pack astounding numbers of transistors on small pieces of silicon, computer processing speeds could increase substantially; however, the electrical current constantly surging through the chips creates a lot of heat, Scott said. If left unchecked, the heat can melt or otherwise destroy the chip components.

Most computers these days are cooled with fans. Essentially, the air around the circuit components absorbs the heat that is generated and the fan blows that hot air away from the components. The disadvantages of this method are that cold air cannot absorb very much heat before becoming hot, making fans ineffective for faster processors, and fans are noisy.

For these reasons, many companies have been eager to investigate the possibility of using liquid as a coolant instead of air. Liquids can absorb far more heat, and transmit heat much more effectively than air. So far, designers have not created liquid cooling systems that are cost-effective and energy efficient enough to become widely used in economical personal computers. Although Guo's discovery has not yet been incorporated into a prototype, he thinks that silicon that can pump its own coolant has the potential to contribute greatly to the design of future cooling systems.

Story Source:

Adapted from materials provided by University of Rochester.

ScienceDaily (Mar. 16, 2010) — Superconductors, materials in which current flows without resistance, have tantalizing applications. But even the highest-temperature superconductors require extreme cooling before the effect kicks in, so researchers want to know when and how superconductivity comes about in order to coax it into existence at room temperature. Now a team has shown that, in a copper-based superconductor, tiny areas of weak superconductivity hold up at higher temperatures when surrounded by regions of strong superconductivity.

The experiment is reported in current issue of Physical Review Letters and highlighted with a Viewpoint in Physics by Jenny Hoffman of Harvard University.

Researchers have long known that both superconducting and normal currents can leak back and forth between adjacent layers of superconducting material and metal. In copper-based ceramic superconductors, made up of many different elements, superconductivity varies within nanometers depending on which atoms are nearby. These tiny regions can influence each other in much the same way that thin layers of metal and superconductor interact.

Now a collaboration of researchers from Princeton University, Brookhaven National Laboratory, and the Central Research Institute of Electric Power Industry in Japan has used Scanning Tunneling Microscopy to investigate for the first time how this happens on the nanoscale. As they warmed a superconducting sample, they saw that superconductivity died out at different temperatures in regions just a few nanometers apart. Superconductivity didn't just depend on the characteristics of the local region, but on what was going on nearby. Regions of stronger superconductivity seemed to help regions of weaker superconductivity survive at higher temperatures.

Researchers might exploit this interplay by micromanaging a superconductor's structure, so that regions of strong superconductivity have the maximum benefit to weak regions, potentially resulting in a new material that's superconducting at a higher overall temperature than is possible with randomly arranged ceramic superconductors.

Story Source:

Adapted from materials provided by American Physical Society, via EurekAlert!, a service of AAAS.

Journal References:

1. Colin V. Parker, Aakash Pushp, Abhay N. Pasupathy, Kenjiro K. Gomes, Jinsheng Wen, Zhijun Xu, Shimpei Ono, Genda Gu, and Ali Yazdani. Nanoscale Proximity Effect in the High-Temperature Superconductor Bi₂Sr₂CaCu₂O_8+δ Using a Scanning Tunneling Microscope. Phys. Rev. Lett., 2010; 104: 117001 DOI: 10.1103/PhysRevLett.104.117001
2. Jennifer E. Hoffman. Proximity to understanding the cuprates. Physics, 2010; 3 (23) DOI: 10.1103/Physics.3.23