J. K. Richard Weber

Dynamics of Oxidation of a Fe2+-Bearing Aluminosilicate (Basaltic) Melt

Rutherford backscattering spectroscopy (RBS) and microscopy demonstrate that the ∼1400°C oxidation of levitated droplets of a natural Fe2+-bearing aluminosilicate (basalt) melt occurs by chemical diffusion of Fe2+ and Ca2+ to the free surface of the droplet; internal oxidation of the melt results from the required counterflux of electron holes. Diffusion of an oxygen species is not required. Oxidation causes the droplets to go subsolidus; magnetite (Fe3O4) forms at the oxidation-solidification front with a morphology suggestive of a Liesegang-band nucleation process.

Glass fibres of pure and erbium- or neodymium-doped yttria–alumina compositions

Optical fibres doped with lanthanide or transition-metal elements can serve as in-line lasers and amplifiers for fibre-optic telecommunications systems. In general, most such fibre lasers use conventional silica-glass fibres doped with erbium or neodymium. But silicon dioxide absorbs strongly in the infrared for wavelengths of greater than 4 µm or so, limiting the infrared range over which such lasers can operate. Some other oxide materials do not absorb significantly until longer wavelengths—the absorption coefficient of crystalline silica at 4 µm is equal to that of yttrium oxide at 7.1 µm and of sapphire (a form of alumina) at 5.1 µm, for example. Glass fibres made from these materials would therefore expand the range of fibre lasers into the mid-infrared. But molten oxides that do not contain silica typically have a viscosity too low to support fibre-pulling processes. Here we demonstrate that containerless processing, in which a molten sample is levitated by a flow of inert gas, permits sufficient undercooling of molten yttrium aluminium garnet (YAG:Y3Al5O12) to access a viscosity range conducive to fibre-pulling. The process is particularly effective if the molten material of stoichiometric YAG composition is doped with Nd2O3 in place of Y2O3, or with excess Al2O3; and it should also work with other dopants, because molten oxides are good solvents. Fibres could be drawn from a melt doped with Er2O3 in the presence of excess Al2O3. These fibres have the potential to extend the operating range of oxide glass-fibre lasers.

Aero‐acoustic levitation: A method for containerless liquid‐phase processing at high temperatures

A method for containerless liquid‐phase processing was developed which has practical application in process and property research on virtually any material which is involatile at the melting point. It combines aerodynamic and acoustic forces to support and position the levitated material. The design provides forced convection control of the thermal boundary in the gas surrounding beam‐heated specimens, which stabilizes the acoustic forces and allows acoustic positioning necessary to stabilize the aerodynamic levitation forces on molten materials. Beam heating and melting at very high temperatures was achieved. Experiments were conducted on specimens with diameters in the range 0.25–0.4 cm, of density up to 9 g/cm3, at temperatures up to 2700 K, and in oxygen, air, or argon atmospheres. Unique liquid‐phase processing results included deep undercooling of aluminum oxide, glass formation at exceptionally small cooling rates, complete melting and undercooling of YBa2Cu3Ox superconductor materials, direct form...

Vitreous forsterite (Mg2SiO4): Synthesis, structure, and thermochemistry

Here we report the first synthesis of a forsterite (Mg2SiO4) composition glass as an essentially phase-pure bulk material. Under containerless conditions, with heterogeneous nucleation sites minimized, glass forms by cooling ca. 1 mm liquid Mg2SiO4 droplets in oxygen at 700 K/s. 29Si NMR spectroscopic data indicate that the SiO4 tetrahedra and MgO6 octahedra exist in a corner sharing arrangement in the glass, but upon crystallization the polyhedral units reorganize to form edge-sharing linkages. Transposed temperature drop calorimetry shows that the glass is 61.4±1.3 kJ/mol higher in enthalpy than the crystal.

Levitation apparatus for structural studies of high temperature liquids using synchrotron radiation

A new levitation apparatus coupled to a synchrotron-derived x-ray source has been developed to study the structure of liquids at temperatures up to 3000 K. The levitation apparatus employs conical nozzle levitation using aerodynamic forces to stably position solid and liquid specimens at high temperatures. A 270 W CO2 laser was used to heat the specimens to desired temperatures. Two optical pyrometers were used to record the specimen temperature, heating curves, and cooling curves. Three video cameras and a video recorder were employed to obtain and record specimen views in all three dimensions. The levitation assembly was supported on a three-axis translation stage to facilitate precise positioning of the specimen in the synchrotron radiation beam. The levitation system was enclosed in a vacuum chamber with Be windows, connections for vacuum and gas flow, ports for pyrometry, video, and pressure measurements. The vacuum system included automatic pressure control and multi-channel gas flow control. A phos...

Laser hearth melt processing of ceramic materials

A new technique for synthesizing small batches of oxide‐based ceramic and glass materials from high purity powders is described. The method uses continuous wave CO2 laser beam heating of material held on a water‐cooled copper hearth. Contamination which would normally result during crucible melting is eliminated. Details of the technique are presented, and its operation and use are illustrated by results obtained in melting experiments with a‐aluminum oxide, Y–Ba–Cu–O superconductor material, and the mixtures, Al2O3–SiO2, Bi2O3–B2O3, Bi2O3–CuO. Specimen masses were 0.05–1.5 g.

Non-contact temperature measurement

Abstract Investigations of three methods for non-contact temperature measurement are presented. Ideal gas thermometry was realized by using laser-induced fluorescence to measure the concentration of mercury atoms in a Hg-Ar mixture in the vicinity of hot specimens. Emission polarimetry was investigated by measuring the spatially resolved intensities of polarized light from a hot tungsten sphere. Laser polarimetry was used to measure the optical properties, emissivity, and in combination with optical pyrometry, the temperature of electromagnetically levitated liquid aluminum. The ideal gas method is difficult to carry out on earth. It would be applicable to temperature measurements on any material in the quiescent gas environment that may be achieved in space-based containerless experiments, if a convenient method for more accurate and precise density measurements by laser scattering were available. The precision of temperature measurements based on the ideal gas law was ± 2.6% at 1500 – 2300K. The polarized emission technique has the capability to determine optical properties and/or spectral emissivities of specimens over a wide range of wavelengths with quite simple instruments. Its accurate use requires a high degree of specimen stability, position control, and modelling of the experimental measurements. These requirements are avoided by the laser polarimetric method for emissivity measurements which is well developed, uses quite simple instruments, provides nearly instantaneous results, and has been used to measure optical properties and emissivities for a variety of liquids and solids at high temperatures. Further development of the laser polarimetric method is in progress for non-contact temperature measurements in containerless space-based experiments, by combining polarimetric measurements of emissivity with spectral radiation pyrometry.

Properties of high-temperature melts using levitation

Containerless conditions allow well-controlled investigation of liquids at high temperatures. Levitation methods used for this purpose are reviewed, and their application is illustrated by discussion of the properties and behavior of deeply undercooled yttrium-aluminum-oxide melts.

Network topology for the formation of solvated electrons in binary CaO–Al2O3 composition glasses

Glass formation in the CaO–Al2O3 system represents an important phenomenon because it does not contain typical network-forming cations. We have produced structural models of CaO–Al2O3 glasses using combined density functional theory–reverse Monte Carlo simulations and obtained structures that reproduce experiments (X-ray and neutron diffraction, extended X-ray absorption fine structure) and result in cohesive energies close to the crystalline ground states. The O–Ca and O–Al coordination numbers are similar in the eutectic 64 mol % CaO (64CaO) glass [comparable to 12CaO·7Al2O3 (C12A7)], and the glass structure comprises a topologically disordered cage network with large-sized rings. This topologically disordered network is the signature of the high glass-forming ability of 64CaO glass and high viscosity in the melt. Analysis of the electronic structure reveals that the atomic charges for Al are comparable to those for Ca, and the bond strength of Al–O is stronger than that of Ca–O, indicating that oxygen is more weakly bound by cations in CaO-rich glass. The analysis shows that the lowest unoccupied molecular orbitals occurs in cavity sites, suggesting that the C12A7 electride glass [Kim SW, Shimoyama T, Hosono H (2011) Science 333(6038):71–74] synthesized from a strongly reduced high-temperature melt can host solvated electrons and bipolarons. Calculations of 64CaO glass structures with few subtracted oxygen atoms (additional electrons) confirm this observation. The comparable atomic charges and coordination of the cations promote more efficient elemental mixing, and this is the origin of the extended cage structure and hosted solvated (trapped) electrons in the C12A7 glass.

Spectralemissivity and optical properties at λ = 632.8 nm for liquid uranium and zirconium at high temperatures

Abstract The spectral emissivities, refractive indices, and extinction coefficients of pure liquid uranium and zirconium were measured versus temperature by He-Ne laser polarimetry at a wavelength of 632.8 nm. The experiments were conducted under containerless conditions using electromagnetic levitation and heating supplemented by CO2 laser beam heating. Clean liquid metal surfaces were achieved by heating the specimens to high temperatures at which oxides evaporated and nitrides decomposed. Results were obtained for liquid uranium and zirconium in the temperature ranges 2000–2800 K and 2000–2600 K, respectively and included data for liquid zirconium undercooled by 125 K. The spectral emissivity of zirconium was equal to 0.345 and was independent of temperature. The spectral emissivity of uranium increased with temperature from 0.272 at 2000 K to 0.294 at 2800 K. The melting temperature of zirconium was determined from its emissivity and apparent melting temperature to be 2125±11 K , in good agreement with values in the literature.