Richard Weber

Acoustic levitation: recent developments and emerging opportunities in biomaterials research

Containerless sample environments (levitation) are useful for study of nucleation, supercooling, and vitrification and for synthesis of new materials, often with non-equilibrium structures. Elimination of extrinsic nucleation by container walls extends access to supercooled and supersaturated liquids under high-purity conditions. Acoustic levitation is well suited to the study of liquids including aqueous solutions, organics, soft materials, polymers, and pharmaceuticals at around room temperature. This article briefly reviews recent developments and applications of acoustic levitation in materials R&D. Examples of experiments yielding amorphous pharmaceutical materials are presented. The implementation and results of experiments on supercooled and supersaturated liquids using an acoustic levitator at a high-energy X-ray beamline are described.

Structure and bonding in single- and two-phase alumina-based glasses

Pulsed neutron and high energy X-ray diffraction experiments on single- and two-phase Al2O3–Y2O3 and single-phase Al2O3–La2O3 glasses suggest that a polyamorphic phase transition occurs via changes in the Y–O coordination and connectivity. The transformation from a high temperature to a low temperature amorphous phase is mainly associated with the formation of YO8 polyhedra at the expense of YO7 polyhedra. The transition produces a more ordered network structure with increased oxygen connectivity, while maintaining the overall metal∶oxygen stoichiometry. It is estimated that for the two-phase sample studied ∼12% of the Y ions had transformed to the higher coordinate form in the supercooled liquid as the onset of the glass transition was reached.

Er3+ fluorescence in rare-earth aluminate glass

Er3+ ion fluorescence was excited with a 980-nm pump laser in Er-doped rare-earth aluminate (REAl) glasses with Er-dopant concentrations from 0.5–30mol% (oxides basis). The spectral and decay characteristics were measured at ∼1550nm from Er3+I13∕24 and at ∼2750nm from Er3+I11∕24. Red and green light emissions were also observed, from Er3+F9∕24 and S3∕24+H11∕22, respectively. The fluorescence decay rates are described by a model that yields an accurate fit of results at Er concentrations from 0.5to7mol%. The radiative lifetime of Er3+I13∕24 in Er:REAl glass is 6.12±0.26ms. Hydroxyl ion quenching occurs at a rate given by 9.88×10−20 aOHnEr Hz, where aOH is the glass absorption coefficient (in cm−1) at a wavelength of 2950nm and nEr is the total Er ion concentration. The I13∕24 upconversion rate constant increases with the Er concentration to 1.35+0.05×10−18cm3∕s at and above 7-mol% Er2O3. Er3+I11∕24 fluorescence decays primarily by multiphonon quenching to I13∕24, at 7700±800Hz, a rate that is slightly less...

Levitating water droplets formed by mist particles in an acoustic field

Understanding the physics behind levitation and the flow field around suspended liquid droplets is key to enhancing the drying process of pharmaceuticals and food products. Here an acoustic levitator has been successfully integrated at the Advanced Photon Source for In-situ high-energy x-ray diffraction measurements on particles suspended in an acoustic field. It is demonstrated that acoustic levitation can be utilized to mimic the spray drying amorphization process under controlled conditions. Investigating the velocity field around levitating droplets is also important to understand the forces acting upon the droplets during the levitation process. This paper presents experimental results of the flow field in an acoustic field using Particle Imaging Velocimetry and high-speed imaging and using 3D finite element analysis. The finite element analysis was employed to evaluate the required experimental conditions. The finite element results of acoustically levitated droplets from an ultrasonic wave operating at 22 KHz are compared and discussed. The finite element simulation results are in good agreement with the experimental observations.

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.

New infrared transparent oxide glasses

Glass materials based on rare earth oxides and aluminum oxide can provide a combination of infrared transparency, strength, hardness, and environmental stability in a formable material. This article describes a new family of rare earth oxide-aluminum oxide glass materials that can be made by casting from melts formed in platinum crucibles. The glasses transmit light in the wavelength range from 0.3 to 5 μm in sections of ~0.3 cm, they have a Vickers hardness of 800-1000, and exhibit excellent environmental stability typical of refractory oxide materials. The composition of the glass can be adjusted to achieve refractive indices in the range 1.7-1.8 and Abbe numbers of 30-60. The materials are promising candidates for passive optical elements or as a host for optically active ions such as Yb or Nd that provide laser action or absorb at laser line wavelengths.

Device materials based on Er-, Ho-, Tm-, and Yb-doped rare earth aluminum oxide (REAl) glass

Glasses based on rare earth oxide-aluminum oxide and containing high concentrations of Er2O3, Tm2O3, Yb2O3, or Ho2O3 were synthesized. The host glass is strong, hard, highly resistant to chemical attack, and stable to temperatures ~1000°C. Addition of up to 20 mole % silica markedly increased glass formability while maintaining infrared transmission to ~ 5000 nm in sections up to a few mm. The fluorescence lifetime of excited states in the dopant ions was measured as a function of dopant concentration, pump power and host composition. The absorption cross section and fluorescence line shape were measured for selected compositions. We present details of the glass synthesis and properties, and results of the optical measurements in the context of developing glass-based optical devices.

Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO2 and HfO2

Structure and thermodynamics of pure cubic ZrO2 and HfO2 were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automated ab initio molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO2 and 0.87 ± 0.08% for HfO2), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO2 and 61 ± 10 kJ/mol for HfO2, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO2 and HfO2 are similar and reach (4 ± 1)·10−5/K from experiment and (5 ± 1)·10−5/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO2 and ZrO2, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO2 is an excellent oxygen conductor, perhaps even better than YSZ.

Probing disorder in pyrochlore oxides using in situ synchrotron diffraction from levitated solids–A thermodynamic perspective

Pyrochlore, an ordered derivative of the defect fluorite structure, shows complex disordering behavior as a function of composition, temperature, pressure, and radiation damage. We propose a thermodynamic model to calculate the disordering enthalpies for several RE2Zr2O7 (RE = Sm, Eu, Gd) pyrochlores from experimental site distribution data obtained by in situ high temperature synchrotron X-ray diffraction. Site occupancies show a gradual increase in disorder on both cation and anion sublattices with increasing temperature and even greater disorder is achieved close to the phase transition to defect fluorite. The enthalpy associated with cation disorder depends on the radius of the rare earth ion, while the enthalpy of oxygen disordering is relatively constant for different compositions. The experimental data support trends predicted by ab initio calculations, but the obtained enthalpies of disordering are less endothermic than the predicted values. Thermal expansion coefficients are in the range (8.6–10.8) × 10−6 K−1. These new experimental determinations of defect formation energies are important for understanding the stability of pyrochlore oxides and their disordering mechanisms, which are essential in the context of their potential applications in nuclear waste management and other technologies.