Novel Glasses

Glasses containing rare earth ions have many applications in laser and other optical technologies. The solubility of these active ions in good glass formers such as silica and germania is however limited. The solubility may be increased by, for example, the addition of alumina to silica but is accompanied by a decrease in glass forming ability. In this work we are exploiting aerodynamic levitation and laser heating to explore the glass forming ability of a wider range of materials including aluminates, gallates and titanates. The advantage of this technique is that melting of these very high melting point (> ~ 2300K) and pure materials may be achieved quickly and efficiently under containerless conditions such that contamination of the materials by the crucible is avoided. In addition, by abruptly removing the laser power the samples may be quenched more quickly than by conventional techniques. In addition the containerless conditions minimise the likelihood of heterogeneous crystal nucleation and favours vitrification. We are studying these glasses by neutron and X-ray diffraction and spectroscopic techniques in order to understand the relation between their structure and the optical activity of the rare earth ions.

Rare earth aluminate glass spheres produced by aerodynamic levitation and laser heating. left to right rare earth content is La, Pr, Nd, Eu, Gd, Tb.

Research highlights

The ability to control the quench rate and environmental conditions of the liquids has enabled us to establish and control various properties of these materials. For example:

  • The production of the Praesodymium glasses in oxidising and reducing atmospheres has enabled us to control the oxidation state of the ion to reveal emerald green glasses (containing only Pr3+ ions) and dark purple glasses in which the Pr occurs in a mixed 3+/4+ state.
  • The production of Europium aluminate glasses reveals that the optical activity of the ions as evidenced by the UV induced luminescence is strongly dependent and controllable by the quench rate and conditions.
  • Titanate glasses obtained through mixing with alumina or barium oxide are able to form glasses for all the rare-earths. These are again notable for their sensitivity to the production method whereby rapdily quenched glasses are dark and opaque whereas those produced at the limiting quench rate are transparent with enhanced optical activity.
  • The glass forming region in (Y2O3)-(Al2O3) glasses is unusual and interesting. Contrary to recent reports we find no evidence of a liquid-liquid transition in the system. There is a good glass forming region centred around 30% Yttria. On the Yttria poor side of this region at about 25% Yttria we find the rapidly cooled liquids favour the production of nano-crystalline Yttrium Aluminium Perovskite (50% Yttria)where as the Yttrium rich compositions around 37.5% (Yttria - Yttrium Aluminium Garnet (YAG)) favour the nucleation of micron size inclusions of YAG in a glass matrix.

Colloidal Dispersions

Colloidal dispersions allow us to tackle some of the most challenging and fundamental unsolved physical problems that surround us in everyday life. How do solids melt? How do liquids freeze? Why, when we cool silicon dioxide (or a host of other materials) does it form glass, not quartz? Perhaps amazingly these problems remain unsolved. Why? The answer in a nutshell is that atoms or molecules are too small to be seen, and that, in order to answer these questions, we need to be able to see them. So how is this resolved? Enter colloidal dispersions: we take micron-sized particles, which, crucially, are big enough to resolve in an optical microscope, yet small enough to exhibit thermal Brownian motion, as shown in the movie above. What this leads to that colloids obey the same laws of statistical mechanics that atoms and molecules do, and so, like atoms and molecules, they for gases, liquids and solids. Unlike atoms we can see them easily in a microcope, and thus, by looking at colloids, and understanding the local phenomena which control their freezing, melting and vitrification, we are simultaneously answering the same questions about atoms and molecules.

nucleation.

The birth of a nucleus in a metastable colloidal liquid

simulation.

A close up of a colloidal gel, showing the local structures which minimise the potential and lead to dynamical arrest

Working in this area

The following people are involved in this research:

Schematic of an aerodynamic levitator (click to enlarge).

Latest publications