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Weber, J. K. R.

Publications and source records attributed to Weber, J. K. R..

17 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Boron coordination change in barium borate melts and glasses and its contribution to configurational heat capacity, entropy, and fragility

High-energy x-ray diffraction from molten and glassy BaB 2 O 4 and BaB 4 O 7 has been performed using aerodynamic levitation and laser heating over a wide range of temperatures. Remarkably, even in the presence of a heavy metal modifier dominating x-ray scattering, it was possible to extract accurate values for the tetrahedral, sp 3 , boron fraction, N 4 , which declines with increasing temperature, using bond valence-based mapping from the measured mean B–O bond lengths while accounting for vibrational thermal expansion. These are used within a boron-coordination-change model to extract enthalpies, ΔH, and entropies, ΔS, of isomerization between sp 2 and sp 3 boron. The results for BaB 4 O 7 , ΔH = 22(3) kJ mol -1 boron, ΔS = 19(2) J mol -1 boron K -1 , agree quantitatively with those found previously for Na 2 B 4 O 7 . Analytical expressions for N 4 (J, T) and associated configurational heat capacity, $C_{P}^{conf}$(J, T), and entropy, S conf (J, T), contributions are extended to cover a wide composition range 0 ≤ J = BaO/B 2 O 3 ≤ 3 using a model for ΔH(J) and ΔS(J) derived empirically for lithium borates. Maxima in the $C_{P}^{conf}$(J, T g ) and fragility index contributions are thereby predicted for J ≲ 1, higher than the maximum observed and predicted in N 4 (J, T g ) at J ≃ 0.6. We discuss the utility of the boron-coordination-change isomerization model in the context of borate liquids containing other modifiers and the prospect of neutron diffraction to aid in empirical determinations of modifier-dependent effects, illustrated by new neutron diffraction data on Ba 11 B 4 O 7 glass, its well-known α-polymorph, and lesser-known δ-phase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Extended range X-ray pair distribution functions

Here we describe a dual detector system for high-energy X-ray, simultaneous, small and wide-angle X-ray scattering (SAXS and WAXS), designed to extract extended-range pair distribution functions (ER-PDF) for disordered materials. The hardware and software provides continuous reciprocal space coverage over atomic to nanometer length-scales. Details of the varying resolution, splicing of data and normalization on an absolute scale are outlined. In addition, the combination of SAXS and WAXS theory is considered with a view to enabling a direct Fourier transformation of the structure factor spanning multiple length-scales into real space. Important distinctions between the ER-PDF and the pair distance distribution function (PDDF) representations are demonstrated. It is shown that when the SAXS intensity in the structure factor, S(Q), is similar to the WAXS intensity, the contributions to the ER-PDF are minimal. However, when the SAXS S(Q) intensities are substantially stronger than the WAXS, the ER-PDF can provide important structural information on the local, intermediate and nanometer length-scales. Notably, the ER-PDF method provides direct information on particle sizes and their density distributions, overcoming the limitations of PDDF analysis for densely packed systems.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Corium lavas: structure and properties of molten UO 2 -ZrO 2 under meltdown conditions

In the exceedingly rare event of nuclear reactor core meltdown, uranium dioxide fuel reacts with Zircaloy cladding to produce eutectic melts which can subsequently be oxidized by coolant/moderator water. Oxidized corium liquids in the xUO 2 ·(100 – x)ZrO 2 system were produced via laser melting of UO 2 -ZrO 2 mixtures to temperatures in excess of 3000 K. Contamination was avoided by floating the droplets on a gas stream within an aerodynamic levitator and in-situ high-energy x-ray diffraction experiments allowed structural details to be elucidated. Molecular dynamics simulations well reproduced diffraction and density data, and show less compositional variation in thermal expansion and viscosity than suggested by existing measurements. As such, corium liquids maintain their highly penetrating nature irrespective of the amount of oxidized cladding dissolved in the molten fuel. Metal-oxygen coordination numbers vary with both composition and temperature. The former is due to mismatch in native values, n UO (x = 100) ≈ 7 and n ZrO (x = 0) ≈ 6, and the requirement for oxygen site stabilization. Furthermore, the latter provides a thermal expansion mechanism.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2017

Structure and Liquid Fragility in Sodium Carbonate

In this paper, the relationship between local structure and dynamics is explored for molten sodium carbonate. A flexible fluctuating-charge model, which allows for changes in the shape and charge distribution of the carbonate molecular anion, is developed. The system shows the evolution of highly temperature-dependent complex low-dimensional structures which control the dynamics (and hence the liquid fragility). By varying the molecular anion charge distribution, the key interactions responsible for the formation of these structures can be identified and rationalized. Finally, an increase in the mean charge separation within the carbonate ions increases the connectivity of the emerging structures and leads to an increase in the system fragility.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Characterization of the acoustic field generated by a horn shaped ultrasonic transducer

A horn shaped Langevin ultrasonic transducer used in a single axis levitator was characterized to better understand the role of the acoustic profile in establishing stable traps. The method of characterization included acoustic beam profiling performed by raster scanning an ultrasonic microphone as well as finite element analysis of the horn and its interface with the surrounding air volume. The results of the model are in good agreement with measurements and demonstrate the validity of the approach for both near and far field analyses. Our results show that this style of transducer produces a strong acoustic beam with a total divergence angle of 10 degree, a near-field point close to the transducer surface and a virtual sound source. These are desirable characteristics for a sound source used for acoustic trapping experiments

acoustic, horn, levitation, acoustic simulation↗
DOE OSTI2017

The structure of liquid alkali nitrates and nitrites

State of the art high energy X-ray diffraction experiments and simulation models (employing a description of charge transfer) are applied to pure molten alkali nitrates and nitrites and uncover significant emerging structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

The structure of liquid UO 2-x in reducing gas atmospheres

High energy X-ray diffraction experiments performed on hypostoichiometric UO 2-x liquids in reducing gas mixtures of 95%Ar:5%CO and 95%Ar:5%H 2 are compared to that conducted in a pure Ar atmosphere [Skinner et al., Science 346, 984 (2014)]. The measurements are pertinent to severe accident scenarios at nuclear reactors, where core melts can encounter reducing conditions and further shed light on the oxide chemistry of the low valence states of uranium, particularly U(III), which become stable only at very high temperatures and low oxygen potentials. The radioactive samples were melted by floating small spheres of material using an aerodynamic levitator and heating with a laser beam. In the more reducing environments, a 1.7% shift to lower Q-values is observed in the position of the principal peak of the measured X-ray structure factors, compared to the more oxidizing Ar environment. This corresponds to an equivalent elongation in the U-U nearest neighbor distances and the U-U periodicity. The U-O peak (modal) bond-length, as measured from the real-space total correlation functions, is also observed to expand by 0.9–1.6% under reducing conditions, consistent with the presence of 15–27% U 3+ cations, assuming constant U-O coordination number. The slightly larger U-U elongation, as compared to the U-O elongation, is interpreted as a slight increase in U-O-U bond angles. Difficulties concerning the determination of the hypostoichiometry, x, are discussed, along with the future directions for related research.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Iron K-edge X-ray absorption near-edge structure spectroscopy of aerodynamically levitated silicate melts and glasses

Here, the local structure about Fe(II) and Fe(III) in silicate melts was investigated in-situ using iron K-edge X-ray absorption near-edge structure (XANES) spectroscopy. An aerodynamic levitation and laser heating system was used to allow access to high temperatures without contamination, and was combined with a chamber and gas mixing system to allow the iron oxidation state, Fe 3+ /ΣFe, to be varied by systematic control of the atmospheric oxygen fugacity. Eleven alkali-free, mostly iron-rich and depolymerized base compositions were chosen for the experiments, including pure oxide FeO, olivines (Fe,Mg) 2 SiO 4 , pyroxenes (Fe,Mg)SiO 3 , calcic FeO-CaSiO 3 , and a calcium aluminosilicate composition, where total iron content is denoted by FeO for convenience. Melt temperatures varied between 1410 and 2160 K and oxygen fugacities between FMQ – 2.3(3) to FMQ + 9.1(3) log units (uncertainties in parentheses) relative to the fayalite-magnetite-β-quartz (FMQ) buffer.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Thermal expansion in UO 2 determined by high-energy X-ray diffraction

Here we present crystallographic analyses of high-energy X-ray diffraction data on polycrystalline UO2 up to the melting temperature. The Rietveld refinements of our X-ray data are in agreement with previous measurements, but are systematically located around the upper bound of their uncertainty, indicating a slightly steeper trend of thermal expansion compared to established values. This observation is consistent with recent first principles calculations.

Guthrie, M.↗
NASA NTRS1994

Containerless liquid-phase processing of ceramic materials

Containerless melting and solidification provides a powerful tool for investigation and synthesis of ceramic and glass materials. The work described in this article explored and extended the limits of ground-based experimentation by using aero-acoustic and aerodynamic levitation in combination with laser beam heating and melting to investigate ceramic and glass processing. Results of liquid-phase processing experiments on calcia-gallia and calcia-gallia-silica glass-forming mixtures, aluminum oxide, and ceramic superconductors are summarized. The work is discussed in the context of low gravity experimental and materials synthesis requirements and opportunities.

Weber, J. K. R.↗
NASA NTRS1993

Optical properties and spectral emissivities at 632.8 nm in the titanium-aluminum system

Optical properties (including spectral emissivity, refractive index, and extinction coefficient) of liquid Ti-Al alloys were measured as functions of temperature (including temperatures up to 350 K below the equilibrium liquidus temperatures), using laser ellipsometry at 632.8 nm. The experiments were conducted under containerless conditions, using electromagnetic levitation and heating supplemented by CO2 laser beam heating. It is shown that the emissivities of liquid Ti-Al alloy vary with temperature in a manner that can be understood by the same theories which are applicable to solid metals, for regions near to and above the liquidus temperature. At temperatures below the liquidus temperature, the optical properties of highly undercooled liquid Ti-Al alloys are not dependent on temperature.

Krishnan, S.↗
NASA NTRS1991

The use of containerless processing in researching reactive materials

It has recently become possible to perform containerless, high-temperature liquid-phase processing of many nonvolatile materials without resort to orbital microgravity, thereby facilitating the conduct of materials research in conjunction with noncontact diagnostic instruments. The melt-levitation techniques are electromagnetic, aerodynamic, acoustic, aeroacoustic, and electrostatic; nonorbital microgravity conditions are obtainable aboard NASA's KC-135 aircraft on parabolic flight paths, as well as in drop tubes and towers. Applications encompass the purification of metals and the creation of nonequilibrium and metastable structures. Process control and property measurements include optical pyrometry and emissivity, laser polarimetry, and drop calorimetry.

Weber, J. K. R.↗
NASA NTRS1991

Non-contact temperature measurement

Three methods for noncontact temperature measurement are presented. Ideal gas thermometry is 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 is investigated by measuring the spatially resolved intensities of polarized light from a hot tungsten sphere. Laser polarimetry is used to measure the optical properties, emissivity, and, in combination with optical pyrometry, the temperature of electromagnetically levitated liquid aluminum. The precision of temperature measurements based on the ideal gas law is + or - 2.6 percent at 1500-2000 K. The polarized emission technique is found to have the capability to determine optical properties and/or spectral emissivities of specimens over a wide range of wavelengths with quite simple instruments.

Nordine, Paul C.↗
NASA NTRS1991

Optical and thermodynamic property measurements of liquid metals and alloys

Optical properties and spectral emissivities of liquid silicon, titanium, niobium, and zirconium were investigated by HeNe laser polarimetry at 632.8 nm. The metals were of a high purity and, except for zirconium, clean. The more demanding environmental requirements for eliminating oxide or nitride phases from zirconium were not met. Containerless conditions were achieved by electromagnetic levitation and heating. CO2 laser beam heating was also used to extend the temperature range for stable levitation and to heat solid silicon to form the metallic liquid phase. Corrections to previously reported calorimetric measurements of the heat capacity of liquid niobium were derived from the measured temperature dependence of its spectral emissivity. Property measurements were obtained for supercooled liquid silicon and supercooling of liquid zirconium was accomplished. The purification of liquid metals and the extension of this work on liquids to the measurement of thermodynamic properties and phase equilibria are discussed.

Weber, J. K. R.↗
NASA NTRS1991

Stabilised electromagnetic levitation at 2-13 MHz

SEL, the Stabilised Electromagnetic Levitator, has been developed to exploit the unique design opportunities available in containerless microgravity experiments. Efficiency and versatility are obtained with multiple coils driven by individual broadband amplifiers whose phase and frequency are controlled. The heating and positioning fields are decoupled. Specimen translation, spin, and for liquids, shape may be adjusted. An open coil structure provides access for optical and diagnostic probes. Results of experiments with a prototype device are discussed. Levitating and heating materials on earth were demonstrated at frequencies up to 13 MHz.

Danley, T. J.↗