Heart of glass

Renewed understanding of how glass is formed could form the basis for a new family of high strength materials. Julie Bieles reports

Scientists have made a breakthrough in the understanding of the nature of glass. The findings could lead to further development of metallic glass – a material much springier than normal metal, which is less likely to suffer metal fatigue and subsequent failure.
Glass is much more than just being something used to glaze windows, it is also a generic state of matter – distinct from liquid, solid or gas. Although it has been around for thousands of years – and we understand how to use it – the nature of glass is poorly understood.
Researchers from Bristol University, with colleagues from Canberra and Tokyo, say they have made a significant improvement in the understanding of this everyday material.
“We now have a better fundamental understanding of what glass is,” says Paddy Royall from Bristol University. “Until now it was one of the least understood things around in everyday life.”
The research could lead to cheaper production and more widespread use of metallic glass. Potential applications include airline and vehicle parts, or anything that needs to be strong, light, flexible, and not fail.
Despite its solid appearance glass is structurally similar to a liquid, and is actually a jammed state of matter that “moves very, very slowly”.
Royall says: “By very very slowly I am talking about the age of the universe. But because it flows, it is not a solid.”
The scientists have shown that glass fails to be a solid due to special atomic structures that form when it cools.
Royall says: “Some materials crystallise as they cool, arranging their atoms into a regular pattern called a lattice. But although glass ‘wants’ to be a crystal, as it cools the atoms become jammed in a nearly random arrangement, preventing it from forming a regular lattice.”
Back in the 1950s, Sir Charles Frank in the Physics Department at Bristol University suggested that the arrangement of the 'jam' should form what is known as an icosahedron – but at the time he was unable to provide experimental proof.
“We set out to see if he was right,” says Royall.
The researchers used a colloidal gel – in this case a colloidal suspension of 1 micron Perspex balls – to test the theory. Colloidal gels act in a similar way to glass but are easier to observe through a microscope. Royall found the gel these particles formed wanted to be a crystal, but failed to become one due to the formation of icosahedra-like structures, as Frank had predicted. The formation of the structures explains why a glass is a glass, not a liquid or a solid, according to the researchers.
The reason that metallic glass is less likely to undergo metal fatigue is down to its structure. Traditional metals normally crystallise when they cool. Stress builds up along the boundaries between crystals, which can lead to fatigue and subsequent failure. The liquid-like structure of metallic glasses means they do not have these boundaries – which in turn explains why they will not undergo fatigue.
Metallic glasses are also much springier than normal metals.
“This lack of failure and improved mechanical performance makes then suitable for use in structures where weight is important, such as aeroplanes – but also, in principle, cars,” says Royall.
Metallic glass is already used in some applications including sporting goods – such as golf clubs and tennis racquets – and medical devices. However it is expensive to produce.
Royall says: “If you could make the material cost-effective you could use it for anything where weight and strength are important.”

Glass is a generic state of matter – distinct from liquid, solid or gas

Stress in materials builds up along the boundaries between crystals. Because metallic glass – metal with a glass-like molecular structure – is non-crystalline, there is nowhere for the stress to build up

The materials are already used in sporting goods and medical devices. Cheaper production could help them move into aerospace or automotive

Tom Shelley

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