Visiting The Super-Cool Land Of Glasses
“Although glasses are seemingly simple, they are yet to be fully understood.”
Did you know that according to Google Scholar searches regarding the number of articles on technologically important materials, glass tops the list with about 57 million hits as compared to steel, concrete, silicon, etc.?
Glasses exhibit fascinating properties and have been extensively used by the world, not just in the present times, but in prehistoric times as well. This is what makes them one of the longest and hottest areas of Materials research.
An Interesting Journey Through Time
About 1.5 million years ago, during the Stone Age, people utilized glasses as natural materials. Obsidian was formed when hot molten lava solidified with time, at ambient conditions. A natural volcanic glass, it was very strong and found its use in weapons. Similarly, Fulgurite, which was also used as a weapon, was created from natural instances — sand after being struck by the lightning formed this vitrified glass!
Fast forward to 3500 BC, there were several technological developments and people started processing glass to make jewelry and other decorative items. There is even a documented history of glass beads in Egypt and Mesopotamia.
Around the fifth to 15th century AD, during the Medieval Period, the real utilization of glasses was in designing various pieces of decorative items. These were considered as status symbols, such as the Roman cups and jars. An important point to mention here is that some of these glasses began to be shaped into spectacles that we know of today. Italy and France were pioneers in discovering the optical properties of glass! Another unmissable use of these glasses was in the exquisite architecture (like window panes) of churches and other buildings. These applications emphasized that one can not only control the processing of glass but also fine-tune how it appears by various chemical additions.
Talking about modern and contemporary research, the world has explored many more properties and processing conditions that can be employed to engineer glasses with far superior characteristics than was ever possible. The biggest example is the modern spectacles. Traditionally made of silica glasses, they have eventually progressed towards their plastic versions. Nowadays, they are photochromic, anti-glare, and scratch-resistant. In simple terms, the photochromic nature refers to the ability of a substance to undergo a reversible change in color/shade when exposed to the light of a particular frequency or intensity. Another popular example is the electronic displays of mobile phones and TVs. The high-strength Gorilla glass, an alkali-aluminosilicate glass manufactured by Corning more than 10 years ago, is used widely in mobile displays and is virtually indestructible. Glasses have also been used as shields in aircraft and automobile industries. All this shows that various technological advancements have greatly optimized glass structures to exhibit a diversity of high-strength properties.
Glasses Explained!
The nature of glass remains anything but clear — this title of a popular article published in The New York Times lays out all the challenges and advances in the field of glasses. A lot of research has been happening over the last couple of decades in this rapidly evolving field, but some of the concepts and physics of glass formation perplex the scientists even now! This brings us to a crucial question — what exactly is a glass?
The basic definition of glasses, according to Material Science, states that they are non-equilibrium, super-cooled, non-crystalline, condensed state of matter that exhibit glass transition. Glasses are formed by the rapid cooling of a liquid — one needs a reference state (melting point of the liquid) below which the liquid has to be cooled (supercooling). What happens next? This process essentially traps all the atoms in a metastable structure. Metastability here indicates that glasses are not in their true stable form, and given enough time they eventually tend to convert into a more stable structure.
In earlier times, people thought that glasses were basically made of very tiny crystals, arranged in a particular long-range order, invisible to the naked eye. It took a lot of time for them to realize that this order was short-ranged.
Hence, having an understanding of glasses is an ambiguous topic. On the one hand, some people call them solid because, for all practical purposes, they behave like solids. But, on the other, glasses resemble liquids, based on their structure. So, is glass a solid or a liquid? Well, it is both!
The Fascinating Phenomenon of Glass Transition
On heating a material above the melting point, it gets converted to a liquid. According to traditional thermodynamics, if one cools this liquid slowly, there is a sudden decrease in the volume (alternatively, increase in the density) at its transition point (melting point) and the liquid gets transformed into a more stable crystal (equilibrium structure). Now, instead of slow cooling if the liquid is subjected to rapid cooling below its melting point, one can preserve the liquid structure when it converts to solid. It forms this new metastable state known as glass (non-equilibrium structure).
The Glass Transition Temperature (Tg) is basically the temperature below which all the atoms are frozen. If a glass at a low temperature is heated gradually, there is a Tg above which the atoms have sufficient energy and they begin to exhibit rapidly evolving atomic motions. In other words, they transition into a liquid state. Therefore, Tg governs where the atomic motion is arrested. Similarly, if a liquid is cooled rapidly below Tg, all the molecular motion ceases and this leads to glass behaving like a solid! During this process, the viscosity (a measure of the resistance of a material to deformation or flow) increases drastically to about 10¹² Pascal seconds. This Glass Transition Temperature is highly material-specific. Another interesting aspect is that Tg is not well-defined — it is a function of how fast the liquid is cooled. The best part is that a used glass can actually recrystallize to form a crystal, as is sometimes seen in laboratory glassware!
Understanding the Glass-Forming Materials
Under the right conditions, any material can theoretically form a glass. There are traditional network glasses made of silica, borosilicate, pyrex, etc. One of their uses is in making laboratory equipment. Another category is of the polymers. These are long-chain organic molecules with some linking agents. One of their examples is polycarbonate, which can be utilized in many forms such as the UV heat shield materials, plastic glasses, and so on.
The raw material for the formation of polymeric glasses is not just restricted to long molecules, rather it can also consist of small ones such as sorbitol (a simple sugar). Researchers, in the present times, are proposing the idea of using glasses made from such sources to deliver drugs (medications) inside the body — something that is not possible with crystalline substances. There are metallic glasses as well.
A fun fact — there are many amorphous glasses that can be formed by cooling water under different extreme conditions of temperature and pressure! Interesting, isn’t it?
The Rules of Glass-Formation:
Atoms are the smallest unit of a substance. Interactions between them play a crucial role in determining how easy it is to process a certain material into a glass. The next rule is, more the number of elements (different types and sizes of atoms), easier is the glass formation. Since this process is material-specific, there are different critical cooling rates for different substances for them to be converted into glass.
Exploring Some Intriguing Properties
From the perspective of Material Science, there are several factors that govern how a particular material is eventually used. The structure, properties, processing, and performance of any material are interdependent — altering or influencing one of these parameters will impact the others.
Glasses exhibit a wide variety of properties that lead to a huge diversity in their applications. Starting from their simple use for decorative purposes, they have evolved with time to find applications in all spheres of life.
The first property, viscosity, is the clearest physical indicator of a glass structure. This trait is material-specific and highly sensitive to temperature. On cooling a specific material, the viscosity shows either very strong (strong glasses) or very weak (fragile glasses) scaling.
Mechanical properties are the second trait that is critical from a performance point of view. Unprocessed glasses are very brittle — they do not show a large amount of extension (strain) on the application of stress, failing catastrophically in the process which leads to their breakage. Why does this happen? The reason behind it is their structure. Glasses have small regions (called shear transformation zones) that undergo some deformations and form small displacements at a very local scale. These expand, communicate with their neighbors, grow further in size (forming shear bands) and eventually result in the breaking of glass. In simple terms, a pre-existing defect in glass can grow and finally break it. Even though they possess very high strength, glasses are not tough and therefore, cannot withstand a large amount of stress. To combat this drawback, a research group led by Duwez and coworkers at Caltech in the 1960s familiarized us with the bulk metallic glasses. These are made by adding many metallic elements — the more complex the entities that make the structure, the easier it is to form this glass. In short, this category is an amalgamation of the strength of metals and glasses and has found its uses in medical equipment, sports goods, etc.
The third property is the nature of dynamics in the glassy state. Even though glass is frozen below Tg, it shows small atomic perturbations (at various length and time scales, which also depends on the temperature). However, these events still happen at scales that are hard to visualize with naked eyes. There are several important properties and processing capabilities that depend on these dynamics shown by glasses.
How are Glasses Processed?
Over the course of many years, people have come up with several protocols to enhance the structure of glasses to equip them with superior properties. It is well known that the traditional method for glass synthesis involves a routine melting and subsequent cooling of the raw materials in a controlled manner. Casts are used to give them specific shapes, sizes, and designs.
A technologically crucial technique is the float glass process. In this, raw materials are heated up in a furnace and then passed through a molten tin bath. Tin is very essential because it forms an immiscible layer over which the glass can be passed, and eventually processed into flat sheets. It generates large volumes of sheet-glass that can then be used in construction.
There were a lot of research and technological innovations that went into forming the modern glasses which have applications in almost all the available industries today. Processing boosts the functionality of these materials. Many techniques have been developed to achieve the strengthening of glasses. One of them is by inducing compressive stresses. Basically, any deformation imposed on the glass is resisted by these stresses, and the glass does not break! It yields glasses that are several times stronger than the unprocessed ones. Several other technologies such as the vapor deposition, in-situ liquid polymerization reaction, and cold compression/ shock/ irradiation/ intense grinding of crystals can be utilized for obtaining hi-tech glasses.
Walking Through the Advancements
Glass Science and Technology have seen several advancements through all these years and there have been tremendous improvements over the last few decades. We can now characterize glasses at an unprecedented scale by various advanced experimental techniques such as the transmission electron microscope. Apart from this, several sophisticated synthesis and processing techniques have been developed to achieve a diversity of cooling rates under different conditions. All these procedures have allowed us to make very specialized glasses which are more efficient than ever. Moreover, significant research on optimizations in terms of the chemical modifications, heat, and surface treatments of these materials has opened up a lot of possibilities. Lastly, the amount of theory and simulations that have gone behind understanding glasses is unparalleled. These try to answer some of the most interesting questions like — what happens at the nanoscale, what’s the difference between the arrangement of atoms in normal and supercooled liquids, what’s the relaxation time of glass, and so on. Atomistic simulations that model the behavior and dynamics of glasses at the atomic length and time scale, can answer several fundamental questions that were unimaginable a decade ago.
The world has come a very long way in terms of using glasses. These, indeed, are the original supercool materials, both in literal as well as figurative sense!
What do you think?
— — — — — — — — — — — — — — — — — — — — — — — — — —
This article is based on one of the sessions (delivered by Raghavan Ranganathan, faculty in the Material Science and Engineering discipline) of the Virtual Seminar Series by IIT Gandhinagar. It is an online program started by the Institute in the wake of the current pandemic as a means to engage the people so that they can learn about a diversity of topics from the comfort of their homes, in an interesting manner. (The 3rd article of this series can be found here. The 5th article is available here.)
