Q. Mei

Diffraction study of calcium aluminate glasses and melts: I. High energy x-ray and neutron diffraction on glasses around the eutectic composition

A series of four (CaO)x(Al2O3)(1−x) glasses over the narrow compositional range x = 57.1–66.7 have been studied using high energy x-ray and neutron diffraction. The coordination number of oxygen around aluminum was determined as 4 at all compositions. The coordination number of oxygen around calcium was extracted using a first-order difference method and found to be in the range of 5.2–5.5 up to a distance of 2.75 A for all compositions, with the possibility of additional correlations at higher distances. The results are in good agreement with two recent molecular dynamics simulations and confirm an asymmetric distribution of CaO correlations in the glass structure.

Diffraction study of calcium aluminate glasses and melts: II. High energy x-ray diffraction on melts

High energy x-ray diffraction measurements have been performed on CaO?Al2O3 liquids suspended in a flow of pure argon for six compositions containing 50?67?mol% CaO. The results indicate that AlO4 tetrahedra dominate the liquid structure. The radial distribution functions show a significant broadening of the Ca?O peak occurs in the liquid compared to the corresponding glass and, on average, each Ca is surrounded by approximately five oxygen atoms in the melt at a distance of 2.3??. It is also found that the structure for the eutectic (64%?CaO) liquid does not change measurably with temperature between 1600 and 1970??C.

Transitions in network and molecular glasses at high pressure.

The use of monochromatic, micro‐focused, high energy x‐ray beams have been used in combination with perforated diamond anvil cells to investigate transitions in several network glasses at high pressure. Of particular interest are glasses which readily form an open network structure e.g. SiO2, GeSe2, BeF2 and those which comprise of small molecular cages e.g. As4S3, As4O6. These materials have highly directional bonds and their normal pressure glassy forms tend to have a high degree of intermediate range order, signified by the existence of a first sharp diffraction peak. As the pressure is increased this peak decreases dramatically in intensity as the open regions of the network collapse. Simultaneously the (second) principal peak increases in intensity as the molecules in the glass move closer together resulting in increased extended range order. Gradual and abrupt amorphous‐amorphous transitions have been observed in these binary systems, associated with both topological and local coordination number ch...

In-situ X-ray structure measurements on aerodynamically levitated high temperature liquids

High energy, high flux X‐ray sources enable new measurements of liquid and amorphous materials in extreme conditions. Aerodynamic levitation in combination with laser beam heating can be used to access high purity and non‐equilibrium liquids at temperatures up to 3000 K. In this work, a small aerodynamic levitator was integrated with high energy beamline 11 ID‐C at the Advanced Photon Source. Scattered X‐rays were detected with a Mar345 image plate. The experiments investigated a series of binary in the CaO‐Al2O3, MgO‐SiO2, SiO2‐Al2O3 metal oxide compositions and pure SiO2. The results show that the liquids exhibit large changes in structure when the predominant network former is diluted. Measurements on glasses with the same compositions as the liquids suggest that significant structural rearrangement consistent with a fragile‐strong transition occurs in these reluctant glass forming liquids as they vitrify.