From PNNL: “In Search of Stability: It’s All About Staying Super Cool”

PNNL Lab

March 2016
No writer credit found

While glass might be thought of in terms of holding wine or as a window, the stability of glass affects areas as diverse as nuclear waste storage, pharmaceuticals, and ice cream. Recently, chemical physicists at Pacific Northwest National Laboratory made a key discovery about how glass forms.

They discovered that the temperature at which glass-forming materials are deposited on a substrate affects the stability. Their findings, published in The Journal of Physical Chemistry Letters, show the ability of a technique called inert gas permeation to tell at what temperature a solid “melts.” Their work brings more understanding to the fundamental properties of glass.

“Glasses are metastable materials with the mechanical properties of a solid-you can touch and hold them, versus a gas,” said Dr. Scott Smith, a co-author on the paper. “But they are not like crystalline materials, which are in a perfect array. The molecules in glasses are arranged in a disordered pattern. In liquids the molecules are constantly moving, if you suddenly freeze a liquid, the molecules are randomly oriented and unstructured. In some sense, a glass can be thought of as a frozen liquid.”

Why It Matters: No matter how glass is made, understanding its properties is important. For example, the reason some medications have expiration dates is that their physical state changes from amorphous to crystalline. Once that happens, the medication doesn’t dissolve as readily when taken and is thus ineffective. Finding ways to increase its stability and effectiveness would extend its shelf life. Similarly, when nuclear waste is put into a glass matrix, the glass must remain stable to keep the radionuclides from being released. And as most ice cream lovers know, when you open a carton and see crystals have formed on the surface, it has lost much of its flavor.

Methods: “Our research is fundamental work that could be important for stable glass manufacture by adding to understanding of liquids and liquid behavior,” Smith said. Glasses depend on temperature for stability. At the correct temperature, a glass remains stable because its molecules stay put. At warmer temperatures, it transforms into a supercooled liquid and then crystallizes.

To create a glass, the materials must be cooled rapidly to a temperature low enough that the molecules don’t have enough time or energy to find the lowest energy configuration (a crystal). That temperature is called the glass transition temperature, or Tg, and it varies depending on the experimental conditions and the cooling rate.

Smith and colleagues Dr. Alan May and Dr. Bruce Kay took the glass-forming materials toluene and ethylbenzene and super cooled them by depositing them onto a surface at 30 K. When the materials hit the surface, they formed an amorphous solid—a glass. The researchers then heated the sample. A layer of krypton deposited between two layers of glassy material (a sandwich) remained trapped until the glass transformed into a supercooled liquid (see Figure). The onset of gas release revealed at what temperature the glass transformed into a supercooled liquid.

The researchers varied the material deposition temperature from 40 to 130 K. They observed that the stability of the glass depended on the deposition temperature. They found that for both toluene and ethylbenzene, deposition at a temperature a few degrees less than Tg, created the most stable glass-one that was the most resistant to turning into a supercooled liquid. These results are consistent with the calorimetric studies of Prof. Mark Ediger at the University of Wisconsin-Madison.

“We found we can control one variable: deposition temperature. Even a difference of one Kelvin can result in years of difference in material lifetime and stability,” said Smith.

Acknowledgments:

Sponsors: This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. The research was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at PNNL.

User Facility: EMSL

Research Team: Scott Smith and Bruce Kay, PNNL; Alan May, Intel Corporation.

Reference: Smith RS, RA May, and BD Kay. “Probing Toluene and Ethylbenzene Stable Glass Formation using Inert Gas Permeation.” Journal of Physical Chemistry Letters 6(18):3639-3644. DOI: 10.1021/acs.jpclett.5b01611

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Pacific Northwest National Laboratory (PNNL) is one of the United States Department of Energy National Laboratories, managed by the Department of Energy’s Office of Science. The main campus of the laboratory is in Richland, Washington.

PNNL scientists conduct basic and applied research and development to strengthen U.S. scientific foundations for fundamental research and innovation; prevent and counter acts of terrorism through applied research in information analysis, cyber security, and the nonproliferation of weapons of mass destruction; increase the U.S. energy capacity and reduce dependence on imported oil; and reduce the effects of human activity on the environment. PNNL has been operated by Battelle Memorial Institute since 1965.

From phys.org: “New technique could enable design of hybrid glasses and revolutionize gas storage”

phys.org

August 28, 2015
No Writer Credit

A new method of manufacturing glass could lead to the production of ‘designer glasses’ with applications in advanced photonics, whilst also facilitating industrial scale carbon capture and storage. An international team of researchers, writing today in the journal Nature Communications, report how they have managed to use a relatively new family of sponge-like porous materials to develop new hybrid glasses.

The work revolves around a family of compounds called metal-organic frameworks (MOFs), which are cage-like structures consisting of metal ions, linked by organic bonds. Their porous properties have led to proposed application in carbon capture, hydrogen storage and toxic gas separations, due to their ability to selectively adsorb and store pre-selected target molecules, much like a building a sieve which discriminates not only on size, but also chemical identity.

However, since their discovery a quarter of a century ago, their potential for large-scale industrial use has been limited due to difficulties in producing linings, thin films, fibrous or other ‘shaped’ structures from the powders produced by chemical synthesis. Such limitations arise from the relatively poor thermal and mechanical properties of MOFs compared to materials such as ceramics or metals, and have in the past resulted in structural collapse during post-processing techniques such as sintering or melt-casting.

Now, a team of researchers from Europe, China and Japan has discovered that careful MOF selection and heating under argon appears to raise their decomposition temperature just enough to allow melting, rather than the powders breaking down. The liquids formed have the potential to be shaped, cast and recrystallised, to enable solid structures with uses in gas separation and storage.

Dr Thomas Bennett from the Department of Materials Science and Metallurgy at the University of Cambridge says: “Traditional methods used in melt-casting of metals or sintering of ceramics cause the structural collapse of MOFs due to the structures thermally degrading at low temperatures. Through exploring the interface between melting, recrystallisation and thermal decomposition, we now should be able to manufacture a variety of shapes and structures that were previously impossible, making applications for MOFs more industrially relevant”.

Equally importantly, say the researchers, the glasses that can be produced by cooling the liquids quickly are themselves a new category of materials. Further tailoring of the chemical functionalities may be possible by utilising the ease with which different elements can be incorporated into MOFs before melting and cooling.

Professor Yuanzheng Yue from Aalborg University adds: “A second facet to the work is in the glasses themselves, which appear distinct from existing categories. The formation of glasses that contain highly interchangeable metal and organic components, in is highly unusual, as they are normally either purely organic, for example in solar cell conducting polymers, or entirely inorganic, such as oxide or metallic glasses. Understanding the mechanism of hybrid glass formation will also greatly contribute to our knowledge of glass formers in general.”

Using the advanced capabilities at the UK’s synchrotron, Diamond Light Source, the team were able to scrutinise the metal organic frameworks in atomic detail.

U.K. Diamond Light Source

Professor Trevor Rayment, Physical Science Director at Diamond, comments: “This work is an exciting example of how work with synchrotron radiation which deepens our fundamental understanding of the properties of glasses also produces tantalising prospects of practical applications of new materials. This work could have a lasting impact on both frontiers of knowledge.”

The researchers believe the new technique could open up the possibility of the production of ‘chemically designed’ glasses whereby different metals or organics are swapped into, or out of, the MOFs before melting.

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From U Chicago: “Molecular scientists unexpectedly produce new type of glass”

University of Chicago

August 13, 2015
Carla Reiter

UChicago researchers co-led experiments that produced glass with an organized molecular structure, a material previously thought to be entirely amorphous and random.

When Prof. Juan de Pablo and his collaborators set about to explain unusual peaks in what should have been featureless optical data, they thought there was a problem in their calculations. In fact, what they were seeing was real. The peaks were an indication of molecular order in a material thought to be entirely amorphous and random: Their experiments had produced a new kind of glass.

Their unforeseen discovery, reported in a paper published in the Proceedings of the National Academy of Sciences and chosen by Science as an editor’s choice paper in Materials Science, could offer a simple way to improve the efficiency of electronic devices such as light-emitting diodes, optical fibers and solar cells. It also could have important theoretical implications for understanding the still surprisingly mysterious materials called glasses.

“This is a big surprise,” de Pablo said. “Randomness is almost the defining feature of glasses. At least we used to think so. What we have done is to demonstrate that one can create glasses where there is some well-defined organization. And now that we understand the origin of such effects, we can try to control that organization by manipulating the way we prepare these glasses.”

De Pablo is a theorist and the Liew Family Professor in Molecular Engineering at the University of Chicago. He and Ivan Lyubimov, a postdoctoral fellow in his group, worked with a group of experimentalists led by Mark Ediger at the University of Wisconsin, doing computer simulations of their physical experiments.

The scientists grew the glass by vaporizing large organic molecules in a high vacuum and depositing them slowly, thin layer by thin layer, onto a substrate at a precisely controlled temperature. When the sample was thick enough, they analyzed it using spectroscopic ellipsometry—a technique that measures the way incident light or laser radiation interacts with the material being investigated.

“Our collaborators saw some intriguing peaks in these materials,” de Pablo said, “and those appear when you have some distinct molecular orientation in the material.” The researchers could not originally explain the origin of the peaks, or why their appearance depended on the temperature at which the glass was formed. However, when the group ran computer simulations of the experiments, the same signatures of orientation appeared. A significant fraction of the molecules in the glass were aligning themselves in concert. The question was, why?

The answer, the scientists discovered, lay in the way the material was created. In liquids—and glass is a type of liquid—the molecules at the surface interact with molecules in the air, sometimes causing them to pack together and line up differently than the randomly arrayed molecules in the bulk of the liquid. The vapor deposition process used in the experiments amounts to laying down one “surface” on top of another. The molecules in each layer get “trapped” in the orientation they had when they were truly, however briefly, on the surface.

In order for this to happen, the researchers discovered, the glass must be grown within the relatively narrow temperature range at which a liquid changes into a solid-like glass. Varying the temperature within that window allows the scientists to “tune” the degree of order. Once the deposition process is finished, the material is stable and changing temperatures within a wide range doesn’t affect it.

Only a small fraction of the molecules in the group’s samples are oriented in a different direction than the rest of the glass molecules. But that is enough to change the optical properties of these materials tremendously. The group will continue to investigate these new materials, trying different molecules and looking to find out if they can enhance the effect.

A theoretical investigation of these findings also awaits.

“Glasses are one of the least understood classes of materials,” de Pablo said. “They have the structure of a liquid—disorder—but they’re solids. And that’s a concept that has mystified people for many decades. So the fact that we can now control the orientation of these disordered materials is something that could have profound theoretical and technological implications. We don’t know what they are yet—this is a new field of research and a class of materials that didn’t exist before. So we’re just at the beginning.”

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