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Developing damage-resistant, sustainable glasses

CO2 emissions from cars and trucks, plastic waste in our seas and micro plastics polluting our environment and food – these are just some of the climate challenges the world is currently facing. Now, the Oxide Glass Chemistry Group at AAU may have made a breakthrough discovery that shows that an age-old material could turn out to be a significant part of the solution.

CO2 emissions from cars and trucks, plastic waste in our seas and micro plastics polluting our environment and food – these are just some of the climate challenges the world is currently facing. Now, the Oxide Glass Chemistry Group at AAU may have made a breakthrough discovery that shows that an age-old material could turn out to be a significant part of the solution.  

Whereas glass has been used by humans for thousands of years, the basic physical and chemical properties of the material are still to a large extent a mystery. Leader of the Oxide Glass Chemistry Group, Professor Morten Mattrup Smedskjær, works to understand the chemical composition, structure and properties of oxide glasses – and to utilize this knowledge to identify and develop new types of glasses that can be applied both in our everyday lives and, most importantly, limit the footprint of glass on our climate. In October 2019, he was awarded the prestigious Grundfos Prize for his groundbreaking work within amorphous materials, especially oxide glass.

When we think about materials, we see them as having three different states: solid, liquid and gaseous – in the case of H2O, these would be ice, water and vapour. However, in the case of glass, we perceive it as solid, but when we look closer at the atomic structure, this resembles what we find in liquid forms

Morten Mattrup Smedskjær

Because of this, the materials are described as “amorphous” or disordered. In solid ice, if you know the structure of the atoms in one section, you will know the structure of them in the entire piece of ice because the structure will repeat itself. This is not the case with glasses, and this means that it is very difficult to predict, first, what kind of structure a certain mix of chemical ingredients will result in when melted into glass, and, second, what properties this structure will have – a knowledge that is highly significant when considering what applications a certain type of glass may have in our everyday products and surroundings.

Using the latest technology to predict glass properties 

Traditionally, the connections between ingredients, structures and properties have been investigated and mapped using experiments – such as performing physical tests on specific glasses whose ingredients are known. For example, Morten Mattrup Smedskjær and his co-workers have developed a model for predicting hardness based on the chemical composition of the glass in question, and in the future, he hopes to be able to utilize the latest statistical and digital technologies such as artificial intelligence to accurately model which specific properties a certain glass will have.

- “The glass used for a smartphone is widely different from the glass fibre used in the wings of a wind turbine. Up until now, discovering new glasses has been a matter of trial-and-error, but I hope that by building computer models on the basis of physical and chemical data, we can markedly diminish the number of tests we need to perform. Instead of testing, say, 10,000 different glasses in order to find the properties we need, we can enter the data into a model and identify a specific group of glasses that will fit. Then we may only need to test maybe 1,000 or even 100 different types in order to reach the properties we need” Morten Mattrup Smedskjær explains.

Self-healing glass 

One such highly coveted property of glass is fracture toughness – a property that is crucial when manufacturing smartphone glass, windshields on cars or cockpit windows for airplanes. Usually, the limitation of low fracture toughness is overcome by post-production treatment to strengthen the surface of the glass. However, this processing is time-consuming, expensive and not very environmentally friendly. Therefore, the research group at AAU is working to develop a type of glass that is more damage-resistant of its own accord.

- “We have been investigating whether we could identify a type of glass in which the atom structures would rearrange rather than break in case of an impact. In the course of this work, we discovered that the surface of a specific type of glass changed over time when subjected to atmospheric air. When we tested it in our lab, we found that its crack-resistance increased, the longer it was left out in the air” Morten Mattrup Smedskjær says.

Subsequent tests showed that the change was due to the glass absorbing some of the water molecules from the air. In practice, the water molecules made the glass swell a little, and so what was a large dent or hole slowly grew smaller – in other words, the interaction with the air made the glass heal itself to a certain degree.

The researchers are now working on finding a way to utilize this new type of glass in relation to some of our everyday glass-based products.

We cannot expect to see this self-healing glass on our phones in just a year or two, since we still need to find out how to control the process. Otherwise we would end up with phones whose cover glass would frost in a very short time. But if we can make the glass absorb water at a certain level of humidity, this would give us an easy way to repair phones or windshields without needing to exchange the entire glass

Morten Mattrup Smedskjær

Preventing cracks from spreading 

The other aspect of the researcher’s work focuses on limiting the damage when a crack or dent does happens in a piece of glass. Morten Mattrup Smedskjær explains: “In this work, we utilize the fact that glass has that in-between solid and liquid form to create a kind of phase-separation. We know this type of separation from combining water and oil – they may separate as a layer of oil on top of the water or as small drops in the water.”

By using this technique, the researchers aim to create a kind of “glass drops in glass” separation of two different types of glass. “An impact on the glass would usually result in a crack, because the force of the impact makes the atoms break apart. By having a two-phase glass, the force could either be absorbed by the drops of glass – sort of like when you hit a pillow – or the force would have to move around them to break the glass. This would give the force a longer way to travel, so to speak, and as such its energy would be spent sooner and cause a smaller crack than would otherwise have been the case” he says.

Reducing fuel consumption, waste and plastic 

While both methods are still in the research phase, Morten Mattrup Smedskjær sees huge potential for their use – both for us as citizens, for industries and for the environment.  

- “Creating more damage-resistant and damage-tolerant glasses means that for instance the car industry can use glass that is much thinner than is the case today, but still have the same (or better) resistance to stone impacts. This in turn will lead to lighter cars and, as such, decreased fuel consumption, which in turn decreases CO2 emissions markedly” he says.

However, the new types of glass may also help industries that are not able to use glass as the key material today by providing them with more sustainable materials. “In colloquial terms, the definition of “glass” is that it is transparent, and it breaks easily. We of course need to maintain the transparency, but if we can create glass that is highly mechanically durable – and is fairly easy to manufacture – this will open up a range of new possibilities for various industries, from the building industry to manufacturers of various small products” Morten Mattrup Smedskjær says.

If we dare dream a little, maybe in the future we can start using glass instead of some plastic materials, or use glass as a load-bearing element of buildings. This will give us much more environmentally friendly products, because for instance plate glass is made from natural materials; it is highly stable, and the components will not be harmful to nature when the glass eventually dissolves. And of course there is the huge advantage that glass is typically easy to recycle, as it can be melted down and reused over and over

Morten Mattrup Smedskjær