Thin films of these glasses can have density values even higher than crystal.
Using vapor deposition, the researchers can create very dense thin-film glasses, corresponding to the packing of this new liquid phase, with a density much higher than was predicted to be possible without applying immense amounts of pressure. "There are a lot of interesting properties that came out of nowhere, and nobody had thought that in thin films you would be able to see these phases. "The two liquids have distinct structures, akin to graphene and diamond, which are both solids made of carbon but exist in very different solid forms," says Fakhraai. A phase transition is when a material changes from one state (gas, liquid or solid) into another. Jin spent several years conducting detailed experiments, from changing the glass substrate, properties and deposition rates to ensuring that all of the equipment was thoroughly cleaned to rule out contamination or experimental error.Īfter running all of the control experiments needed, the researchers were surprised to find that when using vapor deposition they could access a different type of liquid, which underwent a phase transition to the typical bulk liquid upon heating. "Yi kept discovering different properties, none of the data made sense, and so we dug deeper until we had enough data to put a picture together," says Fakhraai.
While this method has allowed researchers to create denser types of bulk glasses, it was initially thought that thin glass films made using this method would still have the same liquid-like properties that can lead to degradation and instability.īut Yi Jin, a recent PhD graduate who worked in the lab of Zahra Fakhraai, ran experiments and found that this was not actually the case. In vapor deposition, a material is changed from a gas into a solid directly. To make better glasses, researchers have tried using vapor deposition instead of cooling a liquid. The resulting material can be prone to droplet formation or crystallization, which limits the size of the smallest features that are possible. But when these types of glasses are made into thin films, they behave more like a liquid, even at cold temperatures. Glasses that are made into ultrathin, nanometer-scale films are widely used in applications such as OLED displays and optical fibers. The structure of a glass closely resembles the liquid phase, but its properties are similar to solids, akin to a crystal. Glass is typically created through the solidification, or falling out of equilibrium, of a liquid when it is cooled to a temperature where its motion arrests. Their results demonstrate how these glasses and other similar materials can be fabricated to be denser and more stable, providing a framework for developing new applications and devices through better design. In a paper in the Proceedings of the National Academy of Sciences, researchers in the University of Pennsylvania's Department of Chemistry report a new type of liquid in thin films that can form a high-density glass. Shivajee Govind (left), a PhD student in the lab of Zahra Fakhraai, and Yi Jin (right), lead author and recent PhD graduate, stand by the ultra-high vacuum chamber that the researchers use to deposit glass thin films.
These results support the hypothesis that the extrapolated properties of supercooled ethylbenzene are correct to within just a few Kelvin of T K, consistent with the existence of a phase transition to an ideal glass state at T K. The highest density glass is within 0.15% of the density expected for the ideal glass. Down to 103 K, glasses prepared in the limit of low deposition rate have densities consistent with the extrapolated supercooled liquid. Ethylbenzene glasses were vapor-deposited at substrate temperatures between 100 (∼0.86 T g ) and 116 K (∼ T g ), using deposition rates of 0.02–2.1 nm/s. For this system, previous calorimetric experiments have established that a transition to the ideal glass state is expected to occur near 101 K (the Kauzmann temperature, T K ) if the low-temperature supercooled liquid has the properties expected based upon extrapolation from above T g. Spectroscopic ellipsometry was used to characterize vapor-deposited glasses of ethylbenzene ( T g = 115.7 K). Vapor-Deposited Ethylbenzene Glasses Approach “Ideal Glass” Density
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