Why is glass transparent but not metal

"Glass can be deformed at room temperature"

The glasses used for window panes or drinking glasses are not flexible and break very easily. In their search for new materials, scientists have now produced a glass that is pliable even at room temperature. This makes it suitable, for example, for the foldable displays of future smartphones. Megan Cordill from the Austrian Academy of Sciences explains what glass is made of and how the properties of the material can be changed in an interview with Welt der Physik.

Megan Cordill

World of physics: what is glass?

Megan Cordill: Glass is characterized by the fact that it is a hard solid that lacks internal structure. In materials science we call this structurelessness amorphous. This can best be imagined in contrast to other solids: Many solids have a very regular internal structure - their atoms are always arranged in the same way or certain patterns regularly alternate. Glass has the external characteristics of a solid, but the internal structure of a liquid.

What is glass made of?

Glass can consist of very different compounds. The glasses that we know from everyday life are mainly made from silicon dioxide. This is basically molten sand. But metal compounds such as aluminum oxide, plastic or organic substances can also become glass. Glass therefore describes a structural property of a substance that is independent of its composition. Of course, not all materials can be made into glass equally well, may not remain stable in this state, or may not have any particularly interesting uses in the same.

How can glass be made?

When we cool a gaseous or liquid substance, it becomes increasingly viscous as the temperature falls. The substance often crystallizes - the molecules arrange themselves in regular structures. Under certain conditions, however, the internal disorder of the gas or liquid remains and merely solidifies. The glass is in its final state when it retains its state even with further cooling and normal household heating. The transition from the viscous to the vitreous state is called the glass transition. No matter what the glass is made of, this transformation process is always the same.

What are the properties of glass?

Glass is often transparent, heat-resistant and very brittle. However, this fragility, for example, is not a necessary property of glass. This is because it is caused by defects in the glass, on which regular structures, cavities or inclusions form when the material cools. These defects interrupt the actual disorder of the molecules and cannot be seen with the naked eye, so that a drinking glass, for example, is still completely transparent. The material is exposed to high stresses at the defects. If, for example, it falls on the floor or if a ball hits a window pane, it breaks at these points.

Megan Cordill at the microscope

What properties of glass are you concerned with?

We are investigating how these defects in the glass can be eliminated. Our colleagues in Finland make glass from aluminum oxide, which is usually as brittle as window glass. Their special manufacturing process prevents crystallization into regular structures in the glass. This special glass, which has no impurities, has new, interesting properties - it becomes malleable.

How is the deformable glass made?

A crystal of aluminum oxide is heated very strongly and very quickly with a laser in short pulses. In the crystalline state - i.e. with very regularly arranged molecules - we refer to the material as white sapphire. The heating creates a gas made of aluminum oxide. When this gas cools down, the molecules attach themselves to a substrate that itself does not have an ordered structure. This prevents the aluminum oxide from crystallizing again. Instead, the result is very pure glass that is very malleable at room temperature and, for example, can be stretched to twice its original length.

What are the possible uses for deformable glass?

The manufacture of electronic devices, such as cell phones and tablets, is a particularly large area of ​​application for flexible glass. If there are to be foldable cell phones in the near future, they also need displays that can withstand it. To do this, we need glass that can be severely deformed at room temperature without breaking. Since it is also lighter and at the same time harder than steel, there are numerous applications far beyond smartphone displays.

What is the next step in research into deformable glass?

At the moment, the samples that are produced in the laboratory are only a few micrometers in size - which is of course not enough to produce smartphone displays from them. So a way has to be found to produce the material in the same quality but in larger dimensions. In addition, such glass should also be made from other materials. Because we are interested in whether we can also produce a glass with similar purity and comparable properties from other raw materials.