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

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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.