18 Search Results

Abstract Ytterbium‐doped lanthanum titanate glasses were prepared by levitation melting for the detailed characterization of the spectroscopic properties in the rare‐earth titanate glass host. Low‐temperature fluorescence spectroscopy reveals distinct site‐selectivity in both static and lifetime fluorescence measurements suggesting an absence of clustering as well as significant variation of local ytterbium environments. Typical site‐selectivity behavior of a shrinking Stark manifold with lower excitation energy is observed. At 77 K, both the mean emission frequency and the fluorescence lifetime initially increase as the excitation energy decreases from about 11100 to 10750 and then slightly decrease at lower excitation energy. Temperature‐dependent lifetime measurementsmore » between 77 and 420 K show a decreasing lifetime with increasing temperature and are well described by a two‐level thermal activation model. The temperature‐dependent fluorescence spectroscopy coupled with a room temperature white light absorption measurement allow the determination of the Stark energy levels of in lanthanum titanate glass as well as the calculation of the laser cross‐sections.« less

Lanthanum titanate glasses (17 La₂O₃-83 TiO₂) are fabricated via aerodynamic levitation melting. Here, nonlinear refraction and absorption were measured with the Z scan technique using ps optical pulses at 532 nm wavelength with peak intensities in the range of 0.25–2.5 GW/cm². The two-photon absorption coefficient (β) and the effective nonlinear refraction coefficient (⁠$$n^{eff}_{2}$$) are found to be, respectively, 2.19 cm/GW and 152 × 10^-16 cm²/W (85 × 10^-13 esu). The Raman gain is measured to be 106 × 10^-11 cm/W. The nonlinearity strength is found to be nearly 60 times larger in lanthanum titanate glasses relative to silica.

Determining the structure-property relations of liquid and amorphous metal oxides is challenging, due to their variable short-range order and polyhedral connectivity. To predict chemically realistic structures, we have developed a Machine Learned, Gaussian Approximation Potential (GAP) for HfO₂, with a focus on enhanced sampling of the training database and accurate density functional theory calculations. By using training datasets for the GAP model at the level of Density Functional Theory-Strongly Constrained and Appropriately Normed (DFT-SCAN) level of theory, our results show that the topology of both the low viscosity liquid and the amorphous form are dominated by edge-shared chains and smallmore » corner-shared rings of polyhedra. This topology is shown to be consistent with the structure of other liquid and amorphous transition metal oxides of variable ion size, such as TiO₂ and ZrO₂. Current limitations of the ML-GAP modeling method for obtaining glass structures and future perspectives are also discussed.« less

This article reviews containerless methods in the context of measuring materials in-situ using x-ray techniques. Non-contact processing enables high chemical purity to be achieved even at temperatures above 2000 °C. The absence of extrinsic heterogenous nucleation allows supercooling and supersaturation of liquids and solutions. In many cases new types of glass can be made, particularly from fragile liquids. Current methods for containerless processing are briefly reviewed. The application of scattering, spectroscopic and emerging x-ray techniques is described. Examples including measurements on molten metal oxides, molten metallic alloys and organic molecules are presented. The work is discussed in the broader contextmore » of combining structure and property data to predict how to make new applied materials.« less

High-energy photon diffraction minimizes many of the corrections associated with laboratory x-ray diffractometers, and enables structure factor measurements to be made over a wide range of momentum transfers. The method edges us closer toward an ideal experiment, in which coordination numbers can be extracted without knowledge of the sample density. Three case studies are presented that demonstrate new hard x-ray methods for studying the structure of glassy and amorphous materials. First, the methodology and analysis of high-energy grazing incidence on thin films is discussed for the case of amorphous In₂O₃. Additionally, the connectivity of irregular InO₆ polyhedra are shown tomore » exist in face-, edge- and corner-shared configurations in the approximate ratio of 1:2:3. Secondly, the technique of high-energy small and wide angle scattering has been carried out on laser heated and aerodynamically levitated samples of silica-rich barium silicate (20BaO:80SiO₂), from the single phase melt at 1500 C-o to the phase separated glass at room temperature. Based on Ba-O coordination numbers of 6 to 7, it is argued that the although the potential of Ba is ionic, it is weak enough to cause the liquid-liquid immiscibility to become metastable. Lastly, high-energy small and wide angle scattering has also been applied to high water content (up to 12 wt.%) samples of hydrous SiO₂ glass quenched from 1500 C-o at 4 GPa. An increase of Si₁O₂ correlations at 4.3 angstrom is found to be consistent with an increase in the population of three-membered SiO₄ rings at the expense of larger rings.« less

In this study, time-resolved high-energy synchrotron x-ray total scattering measurements on supercooled molten lithium metaborate (LiBO₂) reveal an isomerization reaction involving conversion of trigonal sp² boron to tetrahedral sp³ boron during quenching and glass formation. Van't Hoff analysis yields an accurate enthalpy change, ΔH = 21(1) kJ mol^–1 boron, from which we develop an analytical model for the sp³ isomer fraction and its contribution to configurational heat capacity (C_p^conf) and entropy as a function of temperature and composition. Isomerization constitutes 40% of the total calorimetric C_p^conf at the glass transition for LiBO₂ and directly contributes to the observed rise inmore » liquid fragility with the lithium content.« less

An aerodynamic conical nozzle levitator (CNL) has been used to measure the melting temperatures of refractory ceramics. The well-established method of cooling traces has been adapted to allow measurements for air-sensitive samples using a controlled atmosphere, where the oxygen partial pressure can be adjusted and monitored in real time. In this study, we explain the details of a system that we developed for use at the Spallation Neutron Source (Oak Ridge National Laboratory). Measurements of the melting behavior of (Fe,Cr,Al)3O4 spinels were made between log (PO2 (atm)) of -0.7 and -5. The melting temperature of hercynite is found to bemore » oxygen potential dependent, ranging between 1692 and 1767°C at log (PO2) values from -4.42 to -0.70.« less

Extreme temperatures and non-equilibrium materials are both central to Department of Energy’s (DOE) mission of addressing grand challenges in emerging energy and advanced materials technologies. The goal of this project is to develop and perfect a system for studying materials in extreme and non-equilibrium conditions using neutron scattering. Neutron scattering has many advantages in the study of materials structure including the ability to use isotopic substitution to determine partial structure factors. When used at a pulsed neutron source, the potential for event-based detection can enable a stroboscopic technique to interrogate very short lived metastable states in supercooled liquids. The workmore » performed in this Phase I project included: (i) literature reviews and discussions with beamline scientists to establish key requirements for the instrument, (ii) laboratory-based research at Materials Development, Inc. (MDI) to test design concepts, modify control software and model designs, (iii) feasibility experiments and measurements on a prototype sample at the Spallation Neutron Source, and (iv) analysis of the results and design of the proposed Phase II instrument. The results of the work show that the proposed approach is viable and provides a strong basis for the Phase II instrument design and Phase III commercialization. MDI participated in the DOE Commercialization Assistance Program with the Larta Institute (Larta) to accelerate commercial sales of the instruments. The market research identified a served available market of ~13M including sales to beamlines, university research laboratories, and industrial glass and materials businesses. Over the ca. 10 year product life cycle, MDI expects to sell instruments with a cumulative net present value profit of approximately 5M.« less

The modeling of a loss-of-coolant-accident scenario involving nuclear fuels with FeCrAl cladding materials in consideration to replace a Zircaloy requires knowledge of the thermodynamics of oxidized structures. At temperatures higher than 1500 °C, oxidation of FeCrAl alloys forms (Fe,Cr,Al)3O4 spinels. In situ high-energy X-ray diffraction in a conical nozzle levitator installed at beamline 6-ID-D of the APS was used to study the structural evolution of the oxides as a function of the temperature. Single-phase (spinel) and multiphase (spinel–corundum–FeAlO3) regions are mapped as a function of the temperature for three different compositions of FeCrAl oxidation products. The thermal expansion coefficients andmore » cation distribution in the spinel structure have been refined. The temperature at which complete melting of the fuel cladding is expected has been determined by the liquidus temperatures of the oxidized products to be between 1657 and 1834 °C in a 20% O2/Ar atmosphere using the cooling trace method. The liquidus temperature increases with increasing Al and Cr content in the spinel phase.« less

The structure of rare–earth titanate melts and glasses of composition 17RE₂O₃.83TiO₂ have been investigated in situ by aerodynamic levitation with laser heating. Ti K–edge X–ray absorption near–edge structure (XANES) spectroscopy reveals an effect of RE cation size on mean Ti–O coordination numbers (n_TiO), which increase from ~4.8(2) in glass–forming La titanate to ~5.1(2) in non–glass–forming Sc titanate liquids. We suggest that the associated increase in OTi₃ triclusters in melts bearing smaller RE cations tends to inhibit glass formation. Both XANES and high–energy X–ray diffraction indicate increases in n_TiO as the liquids supercool and vitrify. Results are discussed in the contextmore » of alkali and alkaline–earth titanate glasses, extending the observed dependence of n_TiO on structural basicity (modifier content divided by potential) to trivalent modifiers and the molten state. We suggest that the most stable titanate glasses form close to compositions where, on average, two oxygen anions bond to each titanium, allowing a continuous, disordered Ti–O network of bridging oxygen (OTi₂), or with equal numbers of OTi₃ triclusters and OTi₁ non–bridging oxygen in charge–balance. Here, we report on new glasses formed from praseodymium, europium, and gadolinium titanate melts, the latter being the smallest rare–earth for which binary titanate glasses have been obtained.« less