Metallic glass: A crystal at heart

ScienceDaily (June 16, 2011) — Glass, by definition, is amorphous; its atoms lack order and are arranged every which way. But when scientists squeezed tiny samples of a metallic glass under high pressure, they got a surprise: The atoms lined up in a regular pattern to form a single crystal.

It's the first time researchers have glimpsed this hidden property in a glass. The discovery, reported June 17th in Science, offers a new window into the atomic structure and behavior of metallic glasses, which have been used for decades in products such as anti-theft tags and power transformers but are still poorly understood. The more scientists learn about the structure of these commercially important materials, the more effectively they can design new metallic glasses and tinker with old ones to improve their performance.

"Maybe a lot of glasses have this underlying structure, but we just didn't know how to look for it," said paper co-author Wendy Mao, a mineral physicist at the Department of Energy's (DOE) SLAC National Accelerator Laboratory and Stanford University.

Daniel Miracle, a metallurgist at the Air Force Research Laboratory in Ohio who was not involved in the research, called the discovery "a really, really neat, important finding." Not only will it help researchers design better metallic glasses, he said, but it may help explain why these materials can be so tough: If each piece of glass is a single crystal at heart, it doesn't have any of the weak spots at the boundaries between crystals where fractures and corrosion tend to start.

Unlike familiar window glass, metallic glasses are alloys made of metals -- in this case cerium and aluminum. They resist wear and corrosion and they have useful magnetic properties. If you took apart the plastic anti-theft tag on a DVD case, you'd find a thin piece of metallic glass that looks like aluminum foil. When you rent or buy a DVD, the checkout clerk rubs it across a pad to demagnetize the metallic glass so it won't trigger an alarm when you leave.

Scientists have been investigating metallic glasses for half a century, and in 1982 turned up the surprising discovery that these glasses do have some atomic structure, forming patterns over distances spanning just a few atoms. But no long-range patterns were apparent.

"The structure of glass is still mysterious. We know little about it, even though we use glass a lot," said Qiaoshi (Charles) Zeng of Zhejiang University in China, who led a research team of scientists from SLAC, Stanford, the Carnegie Institution of Washington, George Mason University and China's Jilin University. "And it's not easy investigating the structure of glass by traditional methods."

Zeng, Mao and their colleagues were not looking for order when they squeezed samples of the metallic glass between the tips of two diamonds at Argonne National Laboratory's Advanced Photon Source, applying 250,000 bars of pressure (250,000 times the pressure of Earth's atmosphere at sea level). They were simply doing a series of experiments on how materials behave in extreme conditions.

All the samples were taken from a centimeter-long, extremely thin ribbon of the metallic glass. Under intense pressure, all of the samples "devitrified," abruptly switching out of their glassy state to form a face-centered cubic crystal -- one whose atoms are arranged like ping-pong balls packed into a box.

What's more, all the atoms in the crystallized samples lined up in the same direction -- an indication, the researchers wrote, that this underlying structure ran throughout the whole ribbon of glass, and was put there when the glass formed.

Zeng, who will be joining Mao's group at Stanford in July, said the high-pressure technique may offer a new approach for making single-crystal materials from glasses. In addition, he said, it provides a unified understanding of the atomic structures of materials by directly linking the two most extreme examples: highly ordered single crystals and highly disorganized glass.

This work was supported in part by DOE's Office of Science through the Center for Energy Frontier Research in Extreme Environments, a DOE Energy Frontier Research Center led by the Carnegie Institute of Washington.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/SLAC National Accelerator Laboratory.

Journal Reference:

Qiaoshi Zeng, Hongwei Sheng, Yang Ding, Lin Wang, Wenge Yang, Jian-Zhong Jiang, Wendy L. Mao, and Ho-Kwang Mao. Long-Range Topological Order in Metallic Glass. Science, 2011; 332 (6036): 1404-1406 DOI: 10.1126/science.1200324

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Metallic glass: A crystal at heart

ScienceDaily (June 16, 2011) — Glass, by definition, is amorphous; its atoms lack order and are arranged every which way. But when scientists squeezed tiny samples of a metallic glass under high pressure, they got a surprise: The atoms lined up in a regular pattern to form a single crystal.

It's the first time researchers have glimpsed this hidden property in a glass. The discovery, reported June 17th in Science, offers a new window into the atomic structure and behavior of metallic glasses, which have been used for decades in products such as anti-theft tags and power transformers but are still poorly understood. The more scientists learn about the structure of these commercially important materials, the more effectively they can design new metallic glasses and tinker with old ones to improve their performance.

"Maybe a lot of glasses have this underlying structure, but we just didn't know how to look for it," said paper co-author Wendy Mao, a mineral physicist at the Department of Energy's (DOE) SLAC National Accelerator Laboratory and Stanford University.

Daniel Miracle, a metallurgist at the Air Force Research Laboratory in Ohio who was not involved in the research, called the discovery "a really, really neat, important finding." Not only will it help researchers design better metallic glasses, he said, but it may help explain why these materials can be so tough: If each piece of glass is a single crystal at heart, it doesn't have any of the weak spots at the boundaries between crystals where fractures and corrosion tend to start.

Unlike familiar window glass, metallic glasses are alloys made of metals -- in this case cerium and aluminum. They resist wear and corrosion and they have useful magnetic properties. If you took apart the plastic anti-theft tag on a DVD case, you'd find a thin piece of metallic glass that looks like aluminum foil. When you rent or buy a DVD, the checkout clerk rubs it across a pad to demagnetize the metallic glass so it won't trigger an alarm when you leave.

Scientists have been investigating metallic glasses for half a century, and in 1982 turned up the surprising discovery that these glasses do have some atomic structure, forming patterns over distances spanning just a few atoms. But no long-range patterns were apparent.

"The structure of glass is still mysterious. We know little about it, even though we use glass a lot," said Qiaoshi (Charles) Zeng of Zhejiang University in China, who led a research team of scientists from SLAC, Stanford, the Carnegie Institution of Washington, George Mason University and China's Jilin University. "And it's not easy investigating the structure of glass by traditional methods."

Zeng, Mao and their colleagues were not looking for order when they squeezed samples of the metallic glass between the tips of two diamonds at Argonne National Laboratory's Advanced Photon Source, applying 250,000 bars of pressure (250,000 times the pressure of Earth's atmosphere at sea level). They were simply doing a series of experiments on how materials behave in extreme conditions.

All the samples were taken from a centimeter-long, extremely thin ribbon of the metallic glass. Under intense pressure, all of the samples "devitrified," abruptly switching out of their glassy state to form a face-centered cubic crystal -- one whose atoms are arranged like ping-pong balls packed into a box.

What's more, all the atoms in the crystallized samples lined up in the same direction -- an indication, the researchers wrote, that this underlying structure ran throughout the whole ribbon of glass, and was put there when the glass formed.

Zeng, who will be joining Mao's group at Stanford in July, said the high-pressure technique may offer a new approach for making single-crystal materials from glasses. In addition, he said, it provides a unified understanding of the atomic structures of materials by directly linking the two most extreme examples: highly ordered single crystals and highly disorganized glass.

This work was supported in part by DOE's Office of Science through the Center for Energy Frontier Research in Extreme Environments, a DOE Energy Frontier Research Center led by the Carnegie Institute of Washington.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/SLAC National Accelerator Laboratory.

Journal Reference:

Qiaoshi Zeng, Hongwei Sheng, Yang Ding, Lin Wang, Wenge Yang, Jian-Zhong Jiang, Wendy L. Mao, and Ho-Kwang Mao. Long-Range Topological Order in Metallic Glass. Science, 2011; 332 (6036): 1404-1406 DOI: 10.1126/science.1200324

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Strong, tough and now cheap: New way to process metallic glass developed

ScienceDaily (May 13, 2011) — Stronger than steel or titanium -- and just as tough -- metallic glass is an ideal material for everything from cell-phone cases to aircraft parts. Now, researchers at the California Institute of Technology (Caltech) have developed a new technique that allows them to make metallic-glass parts utilizing the same inexpensive processes used to produce plastic parts. With this new method, they can heat a piece of metallic glass at a rate of a million degrees per second and then mold it into any shape in just a few milliseconds.

"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 in metallurgy." Johnson leads a team of researchers who are publishing their findings in the May 13 issue of the journal Science.

"We've taken the economics of plastic manufacturing and applied it to a metal with 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.

To make useful parts from a metallic glass, you need to heat the material until it reaches its glass-transition phase, at about 500-600 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.

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 500-600 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 novel metallic-glass alloys using this kind of plastic-forming technology.

The other authors on the Science paper, "Beating crystallization in glass-forming metals by millisecond heating and processing," are Caltech's Georg Kaltenboeck, Marios D. Demetriou, Joseph P. Schramm, Xiao Liu, Konrad Samwer (a visiting associate from the University of Gottingen, Germany), C. Paul Kim, and Douglas C. Hofmann. This research benefited from support by the II-VI Foundation.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by California Institute of Technology. The original article was written by Marcus Woo.

Journal Reference:

W. L. Johnson, G. Kaltenboeck, M. D. Demetriou, J. P. Schramm, X. Liu, K. Samwer, C. P. Kim, D. C. Hofmann. Beating Crystallization in Glass-Forming Metals by Millisecond Heating and Processing. Science, 2011; 332 (6031): 828 DOI: 10.1126/science.1201362

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Beetle bling: Researchers discover optical secrets of 'metallic' beetles

ScienceDaily (Apr. 25, 2011) — Costa Rica was once regarded as the poorest of all the colonies of the Spanish Empire, sadly deficient in the silver and gold so coveted by conquistadors. As it turns out, all of the glittering gold and silver those explorers could have ever wanted was there all along, in the country's tropical rainforests -- but in the form of two gloriously lustrous species of beetle.

Today, the brilliant gold- (Chrysina aurigans) and silver-colored (Chrysina limbata) beetles have given optics researchers new insights into the way biology can recreate the appearance of some of nature's most precious metals, which in turn may allow researchers to produce new materials based on the natural properties found in the beetles' coloring.

A team of researchers at the University of Costa Rica has found that the beetles' metallic appearance is created by the unique structural arrangements of many dozens of layers of exo-skeletal chitin in the elytron, a hardened forewing that protects the delicate hindwings that are folded underneath. A paper about the discovery appears in the first issue of the Optical Society's (OSA) newest open access journal, Optical Materials Express, which launched this month.

The beetles were captured in the University of Costa Rica's Alberto Brenes Mesén Biological Reserve, a tropical rainforest environment. "The metallic appearance of these beetles may allow them to be unnoticed, something that helps them against potential predators," says physicist and study leader William E. Vargas. The surface of their elytra "reflects light in a way that they look as bright spots seen from any direction," he explains. "In a tropical rainforest, there are many drops of water suspended from the leaves of trees at ground level, along with wet leaves, and these drops and wet leaves redirect light by refraction and reflection respectively, in different directions. Thus, metallic beetles manage to blend with the environment."

To interpret the cause of this metallic look, Vargas and his team assumed that a sequence of layers of chitin appears through the cuticle, with successive layers having slightly different refractive indices.. In these beetles, the cuticle, which is just 10 millionths of a meter deep, has some 70 separate layers of chitin -- a nitrogen-containing complex sugar that creates the hard outer skeletons of insects, crabs, shrimps, and lobsters. The chitin layers become progressively thinner with depth, forming a so-called "chirped" structure.

"Because the layers have different refractive indices," Vargas says, "light propagates through them at different speeds. The light is refracted through -- and reflected by -- each interface giving, in particular, phase differences in the emerging reflected rays. For several wavelengths in the visible range, there are many reflected rays whose phase differences allow for constructive interference. This leads to the metallic appearance of the beetles."

This is similar to the way in which a prism breaks white light into the colors of the rainbow by refraction, but in the case of these beetles, different wavelengths, or colors of light are reflected back more strongly by different layers of chitin. This creates the initial palette of colors that enable the beetles to produce their distinctive hues. The mystery the researchers still needed to understand in more detail, however, was how the beetles could so perfectly create the structure causing the brilliant metallic tones of silver and gold.

Using a device they specially designed to measure the reflection of light when it strikes the curved surface of the beetles' elytra, Vargas and his colleagues found that as light strikes the interface between each successive layer (the first interface being the boundary between the outside air and the top chitin layer), some of its energy is reflected and some is transmitted down to the next interface.

"This happens through the complete sequence of interfaces," Vargas says.

Because a portion of the light is reflected, it combines with light of the exact same wavelength as it passes back through layer upon layer of chitin, becoming brighter and more intense. Ocean waves can exhibit the same behavior, combining to produce rare but powerful rogue waves. In the case of the beetles, this "perfect storm" of light amplification produces not only the same colors but also the striking sheen and glimmer that we normally associate with fine jewelry.

In the two beetle species, interference patterns are produced by slightly different wavelengths of light, thus producing either silver or gold colors. "For the golden-like beetle, the constructive interference is found for wavelengths larger than 515 nm, the red part of the visible wavelength range," Vargas says, "while for the silver-like beetle it happens for wavelengths larger than 400 nm -- that is, for the entire visible wavelength range."

"The detailed understanding of the mechanism used by the beetles to produce this metallic appearance opens the possibility to replicate the structure used to achieve it," Vargas says, "and thus produce materials that, for example, might look like gold or silver but are actually synthesized from organic media."

This potentially could lead to new products or consumer electronics that can perfectly mimic the appearance of precious metals. Other products could be developed for architectural applications that require coatings with a metallic appearance. Vargas notes that in the solar industry, for example, chirped multilayer reflectors could be used as back layers supporting the active or light-absorbing medium, to improve the absorption of the back-reflected light.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Optical Society of America.

Journal Reference:

Cristian Campos-Fernández, Daniel E. Azofeifa, Marcela Hernández-Jiménez, Adams Ruiz-Ruiz, William E. Vargas. Visible light reflection spectra from cuticle layered materials. Optical Materials Express, 2011; 1 (1): 85 DOI: 10.1364/OME.1.000085

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here