Tanguy Rouxel analyses and reinforces the resistance of glass

Tanguy Rouxel is a professor at the Université de Rennes 1 and a researcher at the Institute of Physics of Rennes. He is a specialist in glassy materials. In 2013, his project for the research of damage-resistant glasses won nearly two million euros in grants from the European Research Council (ERC "advanced grand" of 1.8 million euros over 5 years).
Tanguy Rouxel - Photo UR1/Dircom/JLB

Glassy materials are brittle and sensitive to surface mechanical aggression (indentations, scratches). In this context, the DAMREG project (for DAMage REsistant Glasses) was organised in two phases:

  1. Exploration: what are the factors responsible for the brittleness of glass, and how can they be countered?
  2. Development: to develop materials that neutralise these brittleness factors and divert the mechanical stress to innovative functions.

"Once the news of our selection was known, we asked to postpone the start of the project for six months. During each week of this period, my colleagues and I discussed the exact lines of research that we were going to retain, in order to identify the most promising themes and to seek a positioning that was as original as possible and in line with the ambitious nature of the call for proposals. We chose two lines of research".

Mechano-optical coupling
Is it possible, by placing the glass under mechanical stress, to generate an optical field (make it emit light)? Conversely, by illuminating a glass, can we affect its mechanical behaviour? In Rennes, Tanguy Rouxel and his colleagues have already succeeded in making a material more fluid in this way: this is called photo-fluidity.

Mechanical-electrical coupling
Can an electric field be generated by mechanically stressing a glassy material? And vice versa: by applying an electric field, do we obtain a mechanical response? This is one of the project's recent discoveries (see "Flash effect" below).

"These axes aim to bring about breakthroughs in fundamental research, and in this respect they respect the framework set by the European Research Council. But I never lose sight of the possibility of application, the improvement of existing industrial technologies. I believe that the project must also be able to respond to a specific need in society, or at least aim to do so."

The use of ERC funding

"With the doctoral students whose theses are financed by the project's European credits, we have begun work on phase 2. But three years is a short time, and I'm looking to continue beyond that," says Tanguy Rouxel.

The ERC grant allocated to Tanguy Rouxel for his DAMREG project made it possible to finance a partial release from teaching obligations for two professors and two university lecturers at the Rennes Institute of Physics. For each phase of the project, two thesis students were recruited, supported by a young researcher on a postdoctoral contract. As it was not easy to find a competent person in the field of the project, which combines highly specialised skills in physics, chemistry and engineering, a fifth doctoral student was even recruited while the post-doctoral researcher was identified.

As for the equipment, it is not the largest item in the budget, representing around 300,000 euros, mainly in the form of an experimental stage equipped with an ultrasensitive camera.

The flash effect, a discovery made possible by the DAMREG project

Take a glass coaster from your kitchen drawer and press it down: although it is imperceptible, it will deform and then return to its original size when you release the pressure. Glass is elastic at room temperature. Above a certain temperature (500°C for window glass), subjected to the same pressure, your object would crush permanently: this is called flow.

Let's try an experiment: let's place a small block of glass a few millimetres square in an oven, on a plate. Heat it to over 500°C and put a load of about 40-50 kg on the glass. The material deforms slowly but noticeably over several days.

If we now apply a large electric field (equivalent to a few hundred volts per centimetre), the viscosity of the glass drops sharply and the flow is 50 times faster, and this is not the result of Joule heating. At this critical electric field level, the glass starts to light up inside the furnace, not an electric arc: this is the flash effect!

This effect was described by Pierre Mézeix, in a thesis funded by the DAMREG project.

"We still don't fully understand what the cause is. How does the composition of the glass play a role? We think that there is a link with the conduction of electrons, and that charge displacements are facilitated by the nature of the cations (glasses with high dielectric constants). We are going to verify this thanks to a young doctor who is currently an ATER in the team. This is an exotic activity, which emerged while studying mechano-electric couplings with the ERC project, in connection with the laboratory's work on the design of glasses with specific mechanical properties. And it is typically a discovery that can be of industrial interest: it can facilitate the shaping of objects by making it faster, through the simple application of an electric field."

In addition to the scope of his work among his peers and major European or Japanese industrialists, Tanguy Rouxel is also careful to make his approach known to the general public: the "Ruptures" exhibition held in 2014 at the Palais de la Découverte in Paris, was based on the experience gained from studying the mechanical behaviour of materials and structures, and more recently on the research developed by DAMREG.