J. K. R. Weber

Relationship between topological order and glass forming ability in densely packed enstatite and forsterite composition glasses

The atomic structures of magnesium silicate melts are key to understanding processes related to the evolution of the Earth’s mantle and represent precursors to the formation of most igneous rocks. Magnesium silicate compositions also represent a major component of many glass ceramics, and depending on their composition can span the entire fragility range of glass formation. The silica rich enstatite (MgSiO3) composition is a good glass former, whereas the forsterite (Mg2SiO4) composition is at the limit of glass formation. Here, the structure of MgSiO3 and Mg2SiO4 composition glasses obtained from levitated liquids have been modeled using Reverse Monte Carlo fits to diffraction data and by density functional theory. A ring statistics analysis suggests that the lower glass forming ability of the Mg2SiO4 glass is associated with a topologically ordered and very narrow ring distribution. The MgOx polyhedra have a variety of irregular shapes in MgSiO3 and Mg2SiO4 glasses and a cavity analysis demonstrates that both glasses have almost no free volume due to a large contribution from edge sharing of MgOx-MgOx polyhedra. It is found that while the atomic volume of Mg cations in the glasses increases compared to that of the crystalline phases, the number of Mg-O contacts is reduced, although the effective chemical interaction of Mg2+ remains similar. This unusual structure-property relation of Mg2SiO4 glass demonstrates that by using containerless processing it may be possible to synthesize new families of dense glasses and glass ceramics with zero porosity.

Acoustic levitator for structure measurements on low temperature liquid droplets

A single-axis acoustic levitator was constructed and used to levitate liquid and solid drops of 1-3 mm in diameter at temperatures in the range -40 to +40 degrees C. The levitator comprised (i) two acoustic transducers mounted on a rigid vertical support that was bolted to an optical breadboard, (ii) an acoustic power supply that controlled acoustic intensity, relative phase of the drive to the transducers, and could modulate the acoustic forces at frequencies up to 1 kHz, (iii) a video camera, and (iv) a system for providing a stream of controlled temperature gas flow over the sample. The acoustic transducers were operated at their resonant frequency of approximately 22 kHz and could produce sound pressure levels of up to 160 dB. The force applied by the acoustic field could be modulated to excite oscillations in the sample. Sample temperature was controlled using a modified Cryostream Plus and measured using thermocouples and an infrared thermal imager. The levitator was installed at x-ray beamline 11 ID-C at the Advanced Photon Source and used to investigate the structure of supercooled liquids.

More accurate X-ray scattering data of deeply supercooled bulk liquid water

Deeply supercooled water droplets held containerless in an acoustic levitator are investigated with high-energy X-ray scattering. The temperature dependence of the X-ray structure function is found to be nonlinear. Comparison with two popular computer models reveals that structural changes are predicted too abrupt by the TIP5P-E model, while the rate of change predicted by TIP4P-Ew is in much better agreement with experiment. The abrupt structural changes, predicted by the TIP5P-E model to occur in the temperature range between 260 and 240 K as water approaches the homogeneous nucleation limit, are unrealistic. Both models underestimate the distance between neighbouring oxygen atoms and overestimate the sharpness of the OO distance distribution.

Molten uranium dioxide structure and dynamics

Uranium dioxide (UO2) is the major nuclear fuel component of fission power reactors. A key concern during severe accidents is the melting and leakage of radioactive UO2 as it corrodes through its zirconium cladding and steel containment. Yet, the very high temperatures (>3140 kelvin) and chemical reactivity of molten UO2 have prevented structural studies. In this work, we combine laser heating, sample levitation, and synchrotron x-rays to obtain pair distribution function measurements of hot solid and molten UO2. The hot solid shows a substantial increase in oxygen disorder around the lambda transition (2670 K) but negligible U-O coordination change. On melting, the average U-O coordination drops from 8 to 6.7 ± 0.5. Molecular dynamics models refined to this structure predict higher U-U mobility than 8-coordinated melts. Levitation of molten uranium dioxide allowed structural determination of the solid and melt at high temperature. [Also see Perspective by Navrotsky] Containing the nuclear elephants foot Molten nuclear fuel composed of large amounts of uranium dioxide is extremely dangerous. Liquid UO2 has a high melting temperature and is very reactive, making it difficult to find a suitable sample container within which to study it. Skinner et al. bypassed the container and used instead a laser to heat beads of UO2 levitated in a synchrotron x-ray beam with inert gas. They found an unexpected increase in the fluidity of molten nuclear fuel caused by a fall in the number of oxygen atoms surrounding each uranium cation. These findings are important when considering how to contain nuclear fuel during an accident. Science, this issue p. 984

Structure of molten CaSiO3: neutron diffraction isotope substitution with aerodynamic levitation and molecular dynamics study.

We have performed neutron diffraction isotopic substitution experiments on aerodynamically levitated droplets of CaSiO(3), to directly extract intermediate and local structural information on the Ca environment. The results show a substantial broadening of the first Ca-O peak in the pair distribution function of the melt compared to the glass, which comprises primarily of 6- and 7-fold coordinated Ca-polyhedra. The broadening can be explained by a redistribution of Ca-O bond lengths, especially toward longer distances in the liquid. The first order neutron difference function provides a test of recent molecular dynamics simulations and supports the MD model which contains short chains or channels of edge shared Ca-octahedra in the liquid state. It is suggested that the polymerization of Ca-polyhedra is responsible for the fragile viscosity behavior of the melt and the glass forming ability in CaSiO(3).

A neutron and x-ray diffraction study of calcium aluminate glasses

Spallation neutron diffraction and high-energy x-ray diffraction methods have been used to study CaO:Al2O3 glasses at the 64:36 mol% eutectic and 50:50 mol% compositions. The samples were produced by the containerless cooling of liquid droplets heated by a laser beam and suspended in an aerodynamic levitator. The results show aluminium on average to be surrounded by 4.0(1) oxygen atoms at a distance of 1.76(1) A in CaAl2O4, which increases to 4.8(1) at the eutectic composition. The two techniques have also been combined to reveal the local structure of the calcium atoms in the glass. In CaAl2O4 the calcium is found to be surrounded, on average, by 5.6(2) oxygen atoms at a distance of 2.38 A. The Ca coordination decreases to the unusually low value of 3.9(2) oxygen atoms at a distance of 2.40 A at the eutectic composition. No additional Ca–O correlations are observed up to 2.7 A, but longer bonds cannot be ruled out. The higher-r correlations are shown to be similar in the two glasses, suggesting that both Al and Ca may act as network formers. The results are compared to previous studies on splat-quenched glasses with compositions near the eutectic.

Changes in the local environment surrounding magnesium ions in fragile MgO-SiO2 liquids

X-ray diffraction data have been collected in situ for a series of magnesium silicate liquids between and including the two mineral compositions enstatite and forsterite (MgSiO3 and Mg2SiO4). The pair distribution functions obtained from these data show that the local oxygen environment around the magnesium ions contrasts with that found in the equivalent glasses. We identify a transition window between 42 and 38 mole %SiO2 where the average magnesium coordination number changes on cooling from a Mg-O coordination number of ~5.1±0.2 in the liquid state to a value of 4.5±0.1 in the glass. This change in structure reflects a competition between a sparsely polymerised silicate network and a disordered magnesium framework in the liquid, while the silicate network dominates in the glass.

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.

Aerodynamic levitator for in situ x-ray structure measurements on high temperature and molten nuclear fuel materials

An aerodynamic levitator with carbon dioxide laser beam heating was integrated with a hermetically sealed controlled atmosphere chamber and sample handling mechanism. The system enabled containment of radioactive samples and control of the process atmosphere chemistry. The chamber was typically operated at a pressure of approximately 0.9 bars to ensure containment of the materials being processed. Samples 2.5-3 mm in diameter were levitated in flowing gas to achieve containerless conditions. Levitated samples were heated to temperatures of up to 3500 °C with a partially focused carbon dioxide laser beam. Sample temperature was measured using an optical pyrometer. The sample environment was integrated with a high energy (100 keV) x-ray synchrotron beamline to enable in situ structure measurements to be made on levitated samples as they were heated, melted, and supercooled. The system was controlled from outside the x-ray beamline hutch by using a LabVIEW program. Measurements have been made on hot solid and molten uranium dioxide and binary uranium dioxide-zirconium dioxide compositions.

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.