Glasses: from the frontiers of condensed matter physics to novel materials
Silica glass has been a feature of everyday life for 4000 years, so it is remarkable that after decades of research, deep questions remain over the nature of this essential material. From a fundamental perspective, understanding the liquid-to-glass transition is a scientific revolution in the sense epitomised by Thomas Kuhn in his landmark work “the structure of scientific revolutions”. Different, mutually incompatible, theories give equally good descriptions of the available data. Famously described as “the deepest problem in solid state physics” by Nobel Laureate Phillip Anderson some years ago, a full understanding of the glass transition continues to elude theory, experiment and computer simulation alike. Loosely speaking theories posit that the glass transition is driven by either dynamics (slowing down of the constituent molecules) or thermodynamics (a phase transition in the material). Much of the challenge in understanding the glass transition lies in obtaining data sufficiently closes to the glass transition (where the material becomes very viscous) so that theoretical predictions may be rigorously tested. One means to add support to a thermodynamic — rather than dynamic – interpretation is increasing dynamic and structural lengthscales, another is to identify a drop in configurational entropy.
Approaching the Ideal Glass
Beyond the challenges that glasses present to fundamental science, glasses continue to provide novel materials that make possible key technological developments: smart-phones for example are depend on super-tough “Gorilla Glass” developed by Corning Inc, and chalcogenide materials based around Germanium and Tellurium which have great potential for non-volatile hard drives.
Other examples include tableware, bottles, chemical storage etc. Many common glasses crucially contain large fractions of silicon (SiO2), Boron (B2O3) or Phosphorus (P2O5) oxides, the so-called network formers, that are necessary to avoid crystallization. Although technological glasses usually contain other metal oxides (aluminium, titanium, iron, calcium, lead …) they all necessitate significant quantities of one or more of these network formers to be stable.
Crystallisation in the world's favourite model glassformer.
The desirable properties of glasses include: durability, transparency over a wide wavelength range (current typical oxide glasses are not well suited for the infrared frequencies used in modern communication), high refractive indices (for lens and fibre coupling) and the ability to contain high quantities of optically active ions (as laser hosts).
The development of glasses with improved properties has been limited by the need to include significant quantities of the network forming oxides when they are made by traditional melt quenching methods. In our work we use levitation and laser heating techniques to form glasses from high temperature melts (in excess of 2000K). As the liquids are not in contact with a container we are able to deeply supercool (cool the liquid below its melting temperature) while avoiding crystallization caused by contact with the walls of a container or other impurities (this is known as heterogeneous crystal nucleation). Under these conditions and provided we can cool fast enough, the liquid will form a glass before crystals start to form spontaneously in the liquid (homogeneous nucleation). The cooling rate of the sample is primarily limited by blackbody radiation and so the smaller the sample the faster the quench rate obtained.
With these techniques we have demonstrated that we can form new glasses that don’t contain any of the conventional network formers described above. Examples include pure aluminate, titanate and gallate glasses. These are interesting glasses due to their high refractive indices and ability to contain significant quantities of rare-earth ions needed for laser host materials.
With aerodynamic levitation we are able to produce glass spheres with sizes between 1-3mm in diameter. An example of some rare-earth gallate glasses are show in the image.
Acoustic levitation has been used for some time and a recent development in Bristol is an acoustic levitator based on transducer arrays – TinyLev. We are currently developing a laser heating and acoustic levitation system based on this device that will be capable of producing glass spheres below 1mm down to 10s of microns in diameter. The ability to make small diameter glass spheres allows us to reach much higher cooling rates (to make new glasses) and also to produce glass spheres for applications in for example Whispering Gallery Mode lasers and resonators.
For more information about work within this field see Professor Paddy Royall's website