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pagetitle-crest Novel Glasses

LB Skinner, P Kidkunthod & AC Barnes

Collaborators: P Salmon (University of Bath) & D Holland (Warwick University)

pagetitle-crest Introduction

If crystal nucleation is avoided a liquid can be cooled below it's freezing temperature and remain liquid. This is known as supercooling, and is generally highly unstable. However if a material is cooled sufficiently fast nucleation can be avoided altogether and a glass is formed.

Glasses like liquids are structurally disordered materials and lack the long range periodicity of crystalline solids. However order still remains on inter-atomic length scales, where average nearest and next nearest neighbour distances are important characteristics of the glass structure.

We use both neutron and high energy x-ray diffraction techniques to measure the average local structure of liquids and glasses, at length scales up to tens of angstroms.

glass_freezing_rain

Freezing rain, supercooled liquid rain has flash frozen on contact with a solid surface.


pagetitle-crest Rare earth glasses

Although glass is an amorphous solid which can in principle be made from any liquid, people often refer to glass as if it was specific material or class of materials. This is probably because most common glass is based on silica, the archetypal glass former. Using aerodynamic levitation and laser heating we can vitrify materials that do not contain any traditional glass former (SiO2, GeO2, B2O3, P etc), and in several cases we have formed glass that has not been reported before (e.g. [1]).



glass_realo_samples

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.


One current study is the structure of rare earth aluminate glasses as a function of rare earth ionic radius. To measure structure we use both neutron and x-ray diffraction techniques. From this and comparison to more common glass formers we aim to learn about why these materials are reluctant glass formers, and how changing ion size mismatch affects structure, glass forming ability, and tendency for phase separation.

The rare earth content of these glasses also makes them interesting technologically as they are transparent up to 5 microns, and have potentially useful luminescence, lasing, and magneto-optical properties.


glass_ionic_radius

Minimum cooling rate for vitrification increases as rare earth size decreases.


pagetitle-crest Extended range ordering in glasses

There are also still many unanswered questions regarding traditional glass forming materials. The disordered nature of glasses makes obtaining reliable, detailed structural information is very difficult. Most studies are limited to nearest neighbour or next nearest neighbour distances. However recent work shows there is measureable order in several different glassy materials, namely GeO2, ZnCl and GeSe out to several tens of nearest neighbours [2,3].

glass_extended_range

Partial structure factors of GeO2 measured by neutron diffraction and isotopic substitution. Patterns are multiplied by r2 to emphasise high r correlations.


pagetitle-crest References

[1] L.B. Skinner, A.C. Barnes & W. Crichton, Novel behaviour and structure of new glasses of the type Ba-Al-O and Ba-Al-Ti-O produced by aerodynamic levitation and laser heating. J. Phys.: Condens. Matter, 18, L407-414 (2006).

[2] P.S. Salmon, A.C. Barnes, R.A. Martin & G.J. Cuello, Structure of glassy GeO2. J. Phys.: Condens. Matter, 19, 415110 (2007).

[3] P.S. Salmon, A.C. Barnes, R.A. Martin & G.J. Cuello, Glass fragility and atomic ordering on the intermediate and extended range. Phys. Rev. Lett., 96, 235502 (2006).