Fabrication of novel glasses and glass micro-spheres by acoustic levitation and laser heating.

Oxide glasses have been important materials for millennia. Their transparency at optical wavelengths makes them ubiquitous in windows for houses and cars and their use in lenses for microscopes and telescopes has been key to much scientific development. Today they remain key technological materials with additional applications in, for example, the hard glasses used in mobile phone screens, the fibre optics that underpin current high-speed communications and as laser host materials to name a few.

The process of making a typical oxide glass involves melting the material and allowing it to cool (quench) into a form in which the atoms have a disordered, non-crystalline arrangement. In practice, this non-crystalline form is difficult to achieve for most materials, apart from those that contain significant quantities of silicon dioxide, boron oxide or phosphorus oxide. In order to produce glasses with improved properties (e.g. refractive index, infrared transmission, rare-earth ion content ...) these components need to be avoided which has a significant impact on their glass forming ability. Hence, new methods for glass fabrication are required.

The ability of a material to form a glass reliably depends on how fast it can be cooled (the quench rate), the container it is in and the presence of any solid impurities (that promote crystallization). The rate at which a very hot material will cool freely in air by radiation depends on its size. Hence to improve the quench rate for a given material we need to make it as small as possible. To avoid crystallization due to a container we need to use either, a very smooth container, or no container at all. Hence to discover and produce new glassy materials it is ideal to work with small samples under containerless conditions.

In this project we will develop acoustic levitation methods to allow us to process materials at high temperatures without the need for a container. In this project we will exploit new techniques that have been developed recently in Bristol. In particular, we will develop further the 'TinyLev' device that allows routine levitation of materials with moderate density (up to 5 g. cm-3) and auto-tuning Langevin Horn based devices for use with high density materials (in excess of 12 g. cm-3).

To achieve high melt temperatures we will use an aligned carbon dioxide laser system to heat the samples to temperatures in excess of 2500K. The use of a laser heating system means that the samples may be heated and melted in a matter of seconds with small thermal gradients. As a heat source the lasers may be switched off instantaneously so that the sample will be free cooled at its maximum rate so that for a size of less than 1mm diameter, quench rates of the order of 10,000 Kelvin/second will be achieved. The system will be very suitable for rapid processing/prototyping of new glass materials.

The acoustic levitation and laser heating systems will be used to study the structure of novel silica-free glass forming systems, based on aluminium oxide, titanium oxide and gallium oxide, by X-ray and neutron diffraction. In particular, we will use the system to follow, in situ, the evolution of the liquid structure as it is rapidly cooled, to form either a glass or to observe the processes giving rise to crystal nucleation. The experiments will be coupled with state-of-the-art computer simulations to give new insight into the glass forming process.

There is increasing interest in the use of high quality glass spheres with sizes of the order 10-100 microns diameter for applications in Whispering Gallery Mode (WGM) devices such as biosensors, temperature sensors and lasers. This acoustic levitation and laser heating system will be ideal to produce these spheres and the final part of this project will be to explore and evaluate this method for producing WGM spheres for these applications.

The containerless processing system based on acoustic levitation proposed here has the potential to open up a new and wider range of glass compositions than can be developed for new applications of photonic, laser and solar voltaic glasses.
We have identified with Johnson Matthey PLC that these methods could be used, not only for fabrication of new materials but also as a method for rapidly surveying materials in order to reduce materials costs (only tiny samples are used) and the timescales of investigations compared to traditional methods.

We will continue to explore opportunities for exploitation of these methods as the project progresses by directly contacting and working with possible beneficiaries, such as NSG Group Limited (Pilkington), Corning Inc. via the Corning European Technology Centre, and Schott AG who offer custom-tailored components and innovative products based on advances in materials and processing.

Development and progress in the fabrication of whispering gallery mode applications from microspheres will be managed through our collaboration with Prof. Barker and Dr Li at UCL and their partners (DSTL and Harris Aerospace for optomechanical applications).

In future any relevant project outputs will be developed further in conjunction with industrial partners via case awards, collaborations and knowledge transfer partnerships through UKRI or the Innovate UK Knowledge Transfer Network, which has relevant interests in Energy, Materials, and Manufacturing.

Any work that leads directly to applications on the timescale of the award will be exploited through licence deals via the Research and Enterprise Development (RED) team at the University of Bristol (UOB) who are responsible for technology transfer, licensing and commercialization of research output.

Since its development we have used TinyLev for a large number of public demonstrations and open days. There is no doubt about the fascination and interest from the general public when they see it in operation. We will us this public facing side of our research to promote interest in science and technology and to encourage school pupils to consider pursuing careers in science and engineering. It will also be used to promote awareness of materials, our limited planetary resources and the importance for considering the complete life re-cycling (circular economy) of all materials.