Author Correction: Poisson's ratio and modern materials

In the version of this Review Article originally published, parentheses were misplaced and the longitudinal and transverse speeds were inverted in two expressions for Poisson's ratio in Box 2; the expressions should have read, respectively, ν = (3B/G - 2)/(6B/G + 2) and ν = [½(V_l/V_t)² - 1]/[(V_l/V_t)² - 1].

Aerodynamic levitator furnace for measuring thermophysical properties of refractory liquids

The development of novel contactless aerodynamic laser heated levitation techniques is reported that enable thermophysical properties of refractory liquids to be measured in situ in the solid, liquid, and supercooled liquid state and demonstrated here for alumina. Starting with polished crystalline ruby spheres, we show how, by accurately measuring the changing radius, the known density in the solid state can be reproduced from room temperature to the melting point at 2323 K. Once molten, by coupling the floating liquid drop to acoustic oscillations via the levitating gas, the mechanical resonance and damping of the liquid can be measured precisely with high-speed high-resolution shadow cast imaging. The resonance frequency relates to the surface tension, the decay constant to the viscosity, and the ellipsoidal size and shape of the levitating drop to the density. This unique instrumentation enables these related thermophysical properties to be recorded in situ over the entire liquid and supercooled range of alumina, from the boiling point at 3240 K, until spontaneous crystallization occurs around 1860 K, almost 500 below the melting point. We believe that the utility that this unique instrumentation provides will be applicable to studying these important properties in many other high temperature liquids.

Poisson's ratio and modern materials

In comparing a material's resistance to distort under mechanical load rather than to alter in volume, Poisson's ratio offers the fundamental metric by which to compare the performance of any material when strained elastically. The numerical limits are set by ½ and -1, between which all stable isotropic materials are found. With new experiments, computational methods and routes to materials synthesis, we assess what Poisson's ratio means in the contemporary understanding of the mechanical characteristics of modern materials. Central to these recent advances, we emphasize the significance of relationships outside the elastic limit between Poisson's ratio and densification, connectivity, ductility and the toughness of solids; and their association with the dynamic properties of the liquids from which they were condensed and into which they melt.

Elucidating Structural Mechanisms for Cordierite Ceramic Formation using Synchrotron Radiation

Cr2O3 is a common nucleating agent useful for forming ceramics from oxide glasses. In this study we have used a variety of synchrotron radiation techniques to examine the atomic structure, crystallinity and microstructure of a magnesium alumino-silicate glass ceramic whose composition is close to that of the mineral cordierite, Mg2Al4Si5O13. X-ray Absorption spectra on the Cr K-edge have been performed with samples that were heat treated at different temperatures and times to examine the metamorphosis of the nucleating site. This study reveals that Cr3+ is always octahedrally coordinated with oxygen. In addition, the second nearest neighbour environment changes from an amorphous, single shell of Al/Si, but upon crystallisation, develops into a well ordered Al/Mg shell indicative of the formation of a dilute Cr spinel phase. Powder X-ray Diffraction (XRD) patterns, however, reveal that the major phase initially precipitated (˜900°C) is a stuffed quartz and from the lattice parameters that the composition is SiO2 -rich. With prolonged heat treatment a small quantity of a spinel phase emerges whose composition from its lattice parameter is close to MgAl2O4. Combined Small Angle X-ray Scattering (SAXS) and XRD establish that devitrification at these temperatures is associated with particles about 250 Å in size, on average. Energy dispersive powder diffraction patterns were collected in situ to observe changes in crystallinity with temperature and time. From these measurements the stuffed quartz phase identified at 900°C is found to be intermediate, being eventually replaced at higher temperatures by cordierite with a small quantity of spinel.

Two million hours of science

After over a quarter of a century, the doors of the world's first synchrotron radiation source have closed. Its contribution to materials science in the past and the future should not be underestimated.

Structure of molten yttrium aluminates: a neutron diffraction study

We used the aerodynamic levitation technique combined with CO₂ laser heating to study the structure of liquid yttrium aluminates above their melting point with neutron diffraction. For various yttria contents, we determined the structure factors and corresponding pair correlation functions describing the short-range order in the liquids. In particular, we derived Al-O and Y-O bond distances and coordination numbers. Experimental data are compared with ab initio molecular dynamics, carried out using the VASP code where the interatomic forces are obtained from density functional theory. In particular, partial pair correlation functions have been calculated and are in relatively good agreement with the experimental observations.

Development of structural order during supercooling of a fragile oxide melt

The authors have studied the structural evolution of the fragile glass-forming liquid CaAl2O4 during supercooling from the stable liquid phase to the cold glass below Tg. The evolution is characterized by a sharpening of the first diffraction peak and a shortening of the average nearest-neighbor bond length around 1.25Tg, indicating an increase in the degree of both intermediate-range and short-range orders occurring close to the dynamical crossover temperature. The cooling curve developed a kink at this temperature, indicating a simultaneous change in thermodynamic properties.

Identifying Vibrations That Destabilize Crystals and Characterize the Glassy State

High-resolution inelastic neutron scattering was used to identify major sources of low-frequency vibrations in zeolite crystals. Dispersed and nondispersed modes were found, both of which are prominent in the early stages of compressive amorphization but decline dramatically in strength once a glass of conventional density is created. By identifying the dispersed modes with the characteristic vibrations of the various secondary building units of zeolitic structures, the Boson peak, a characteristic of the glassy state, can be attributed to vibrations within connected rings of many different sizes. The nondispersed phonon features in zeolites, retained in the amorphized glass, were also replicated in silica. These modes are librational in origin and are responsible for destabilizing the microporous crystalline structure, for converting the resulting glass from a low- to a high-density phase, and for the associated changes in network topology that affect the Boson peak.

The new materials processing beamline at the SRS Daresbury, MPW6.2

A new beamline (MPW6.2) has been designed and built for the study of materials during processing where three synchrotron techniques, SAXS, WAXS and XAS, are available simultaneously. It has been demonstrated that Rietveld refinable data can be collected from silicon SRM 640b over a 60 degrees range in a time scale of 1 s. The data have been refined to a chi(2) of 2.4, the peaks fitting best to a Pearson VII function or with fundamental parameters. The peak halfwidths have been found to be approximately constant at 0.06 degrees over a 120 degrees angular range indicating that the instrumental resolution function has matched its design specification. A quantitative comparison of data sets collected on the same isotactic polypropylene system on MPW6.2 and DUBBLE at the ESRF shows a 17% improvement in angular resolution and a 1.8 improvement in peak-to-background ratio with the RAPID2 system; the ESRF data vary more smoothly across detector channels. The time-dependent wide-angle XRD was tested by comparing a hydration reaction of gypsum-bassanite-anhydrite with energy-dispersive data collected on the same system on the same time scale. Three sample data sets from the reaction were selected for analysis and gave an average chi(2) of 3.8. The Rietveld-refined lattice parameters are a good match with published values and the corresponding errors show a mean value of 3.3 x 10(-4). The data have also been analysed by the Pawley decomposition phase-modelling technique demonstrating the ability of the station to quickly and accurately identify new phases. The combined SAXS/WAXS capability of the station was tested with the crystallization and spinodal decomposition of a very dilute polymer system. Our measurements show that the crystallization of a high-density co-polymer (E76B38) as low as 0.5% by weight can be observed in solution in hexane. The WAXS and SAXS data sets were collected on the same time scale. The SAXS detector was calibrated using a collagen sample that gave 30 orders of diffraction in 1 s of data collection. The combined XRD and XAS measurement capability of the station was tested by observing the collapse and re-crystallization of zinc-exchanged zeolite A (zeolite Zn/Na-A). Previous studies of this material on station 9.3 at the SRS were compared with those from the new station. A time improvement of 38 was observed with better quality counting statistics. The improved angular resolution from the WAXS detector enabled new peaks to be identified.

The rheology of collapsing zeolites amorphized by temperature and pressure

Low-density zeolites collapse to the rigid amorphous state at temperatures that are well below the melting points of crystals of the same composition but of conventional density. Here we show, by using a range of experimental techniques, how the phenomenon of amorphization is time dependent, and how the dynamics of order-disorder transitions in zeolites under temperature and pressure are equivalent. As a result, thermobaric regions of instability can be charted, which are indicative of polyamorphism. Moreover, the boundaries of these zones depend on the rate at which temperature or pressure is ramped. By directly comparing the rheology of collapse with structural relaxation in equivalent melts, we conclude that zeolites amorphize like very strong liquids and, if compression occurs slowly, this is likely to lead to the synthesis of perfect glasses.

A SAXS/WAXS XAFS study of crystallisation in cordierite glass

New Cr X-ray absorption fine structure (XAFS) data have been combined with the results of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) experiments to probe in detail the crystallisation mechanism in cordierite (Mg2Al4Si5O18) glass doped with 0.34 mol% Cr2O3. By direct comparison with chromo-aluminate spinels (MgCr2xAl2(1 - x)O4) Cr XAFS is used to determine the composition of the devitrified Cr species. This is identified as MgCr(0.18)Al(1.82)O4, which can be directly related to the Cr content in the starting glass and as a result the total crystalline volume in the fully developed ceramic is predicted to be 4%. In situ WAXS not only reveals the presence of the spinel phase but also a silica-rich stuffed quartz phase. This grows independently of the spinel and is probably nucleated from the glass surface. From our knowledge of the compositions of both crystalline phases we are able to deduce that the SAXS contrast between the surrounding glass and the spinel crystallites is 30 times greater than that between the quartz crystallites and the glass matrix, and therefore dominates the measured scattered intensity and the SAXS invariant that is derived from it. As a consequence we are able to show that the spinel crystalline volume fraction inherent in the SAXS is in close agreement with the 4% value obtained from the Cr XAFS. Furthermore in situ SAXS reveals the gradual development of the spinel particle size and shape during heat treatment. This is conducted in the super-cooled region just above the glass transition temperature, Tg. By employing a two-step annealing process nucleation can be separated from growth and from time-resolved SAXS measurements the alumino-chromate nanocrystals are found to be closely monodispersed. Over a total time course of 600 min they grow from rough crystallites to smooth spherical particles of radius 21 +/- 2 nm, with a final density of (1.2 +/- 0.4) x 10(21) m(-3). As the process of ceramic formation takes place in the viscous melt, growth is indeed found to be limited by diffusion and is complete when all the Cr is exhausted. We use this comprehensive in situ study of crystallisation in cordierite glass to demonstrate the advantages of combining SAXS, WAXS and XAFS for probing the time-resolved chemistry, the microstructure and its development from nucleation sites, that underpins the processing of nanoparticle ceramics.

Liquid alumina: detailed atomic coordination determined from neutron diffraction data using empirical potential structure refinement

The neutron scattering structure factor S(N)(Q) for a 40 mg drop of molten alumina (Al2O3) held at 2500 K, using a laser-heated aerodynamic levitation furnace, is measured for the first time. A 1700 atom model of liquid alumina is generated from these data using the technique of empirical potential structural refinement. About 62% of the aluminum sites are 4-fold coordinated, matching the mostly triply coordinated oxygen sites, but some 24% of the aluminum sites are 5-fold coordinated. The octahedral aluminum sites found in crystalline alpha-Al2O3 occur only at the 2% level in liquid alumina.

A New High-Flux Chemical and Materials Crystallography Station at the SRS Daresbury. 1. Design, Construction and Test Results

A new single-crystal diffraction facility has been constructed on beamline 9 of the SRS at Daresbury Laboratory for the study of structural problems in chemistry and materials science. The station utilizes up to 3.8 mrad horizontally from the 5 T wiggler magnet which can be focused horizontally and vertically. The horizontal focusing is provided by a choice of gallium-cooled triangular bent Si (111) or Si (220) monochromators, giving a wavelength range from 0.3 to 1.5 A. Focusing in the vertical plane is achieved by a cylindrically bent zerodur mirror with a 300 mum-thick palladium coating. The station is equipped with a modified Enraf-Nonius CAD-4 four-circle diffractometer and a Siemens SMART CCD area-detector system. High- and low-temperature facilities are available to cover the temperature range from about 80 to 1000 K. Early results on test compounds without optimization of the beam optics demonstrate that excellent refined structures can be obtained from samples giving diffraction patterns too weak to be measured with conventional laboratory X-ray sources, fulfilling a major objective of the project.

A New High-Flux Chemical and Materials Crystallography Station at the SRS Daresbury. 1. Design, Construction and Test Results

A new single-crystal diffraction facility has been constructed on beamline 9 of the SRS at Daresbury Laboratory for the study of structural problems in chemistry and materials science. The station utilizes up to 3.8 mrad horizontally from the 5 T wiggler magnet which can be focused horizontally and vertically. The horizontal focusing is provided by a choice of gallium-cooled triangular bent Si (111) or Si (220) monochromators, giving a wavelength range from 0.3 to 1.5 A. Focusing in the vertical plane is achieved by a cylindrically bent zerodur mirror with a 300 mum-thick palladium coating. The station is equipped with a modified Enraf-Nonius CAD-4 four-circle diffractometer and a Siemens SMART CCD area-detector system. High- and low-temperature facilities are available to cover the temperature range from about 80 to 1000 K. Early results on test compounds without optimization of the beam optics demonstrate that excellent refined structures can be obtained from samples giving diffraction patterns too weak to be measured with conventional laboratory X-ray sources, fulfilling a major objective of the project.

Examination of the mixed-alkali effect in (Li,Na) disilicate glasses by nuclear magnetic resonance and conductivity measurements

Results from 29Si, 23Na and 7Li magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, 7Li NMR relaxation and electrical conductivity in a series of [Li(1-x).Nax]2O.2SiO2 (disilicate) glasses are used to investigate the mixed-alkali effect. From the 29Si NMR spectra there is relatively little change of the network with alkali composition. 23Na and 7Li NMR linewidths and shifts change continuously as a function of composition, indicating that the alkali ions are intimately and uniformly mixed rather than separated into lithium and sodium-rich domains. The activation energy from electrical conductivity shows a distinct maximum at the central composition (x = 0.5), whereas the local activation energy for lithium motion determined from NMR shows only a smaller but monotonic increase as the lithium-content decreases.

Biotransformation of intracellular minerals by zinc ions in vivo and in vitro

Inorganic granules of the general type CaMgP2O7 are found within cells of a wide range of species. The granules are amorphous to X-rays and occur in membrane-bound compartments. They accumulate a variety of cations from the body fluids of these animals. The interaction of zinc ions with pyrophosphate granules from cells of the snail Helix aspersa have been studied in vivo and in vitro by X-ray absorption spectroscopy and X-ray diffraction. The in vivo deposits incorporate zinc and remain amorphous although the pyrophosphate is hydrolysed to orthophosphate. In this in vitro reaction the mineral becomes crystalline but there is little change in the form of the The results clearly indicate a role for the cellular system in maintaining the amorphous state and in the hydrolysis of pyrophosphate to orthophosphate. As such they provide a model system for some of the changes observed in precursors of bone mineral and in pathological changes in the calcification of cartilage.