Papers by Darrick Williams
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Optical Materials, Feb 1, 2010
Ga 2S 3-Na 2S-CsCl (GNC) glasses were synthesized in open crucibles under inert atmosphere. The e... more Ga 2S 3-Na 2S-CsCl (GNC) glasses were synthesized in open crucibles under inert atmosphere. The evaporative loss of CsCl during glass melting was measured by energy-dispersive X-ray spectroscopy and corrected for by biasing the CsCl concentration in the mixture of starting materials to obtain glasses with accurately controlled stoichiometry. Glass-transition temperatures, refractive-index dispersions, visible and near-infrared transmittance, and band edge energies were measured for four GNC glasses with varying CsCl content, and the respective values were found to significantly improve over earlier studies that did not mitigate CsCl evaporative losses. Glass durability was assessed by a water immersion test at 74 °C. A respective weight loss rate of 39.2 ± 0.3 μg/(cm 2 h) was found for a GNC glass containing 14 mol% CsCl, indicating good glass durability despite the high CsCl content. The refractive-index dispersion measurements indicate that the Cs + and Cl - radii are 16% larger in GNC glass than in bulk crystalline CsCl. The band edge energy increases from 2.97 eV in Ga 2S 3-Na 2S glass to 3.32 eV in Ga 2S 3-Na 2S-CsCl glass containing 20 mol% CsCl as a result of introducing Cl - ions having a large optical electronegativity. The large bandgap of 3.32 eV, the low (450 cm -1) phonon energy, and the good chemical durability make GNC glass an attractive host material for rare-earth ions with radiative transitions in the near ultra-violet, visible, and near-infrared spectral regions.
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Simultaneous In Situ Neutron Diffraction Studies of the Anode and Cathode in a Lithium-Ion Cell. ... more Simultaneous In Situ Neutron Diffraction Studies of the Anode and Cathode in a Lithium-Ion Cell. [Electrochemical and Solid-State Letters 7, A8 (2004)]. Mark A. Rodriguez, David Ingersoll, Sven C. Vogel, Darrick J. Williams. Abstract. ...
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ABSTRACT
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The Journal of microwave power and electromagnetic energy: a publication of the International Microwave Power Institute
An experiment that seeks to investigate buoyancy driven mixing of miscible fluids by microwave vo... more An experiment that seeks to investigate buoyancy driven mixing of miscible fluids by microwave volumetric energy deposition is presented. The experiment involves the use of a light, non-polar fluid that initially rests on top of a heavier fluid which is more polar. Microwaves preferentially heat the polar fluid, and its density decreases due to thermal expansion. As the microwave heating continues, the density of the lower fluid eventually becomes less than that of the upper, and buoyancy driven Rayleigh-Taylor mixing ensues. The choice of fluids is crucial to the success of the experiment, and a description is given of numerous fluid combinations considered and characterized. After careful consideration, the miscible pair of toluene/tetrahydrofuran (THF) was determined as having the best potential for successful volumetric energy deposition buoyancy driven mixing. Various single fluid calibration experiments were performed to facilitate the development of a heating theory. Thereafter, results from two-fluid mixing experiments are presented that demonstrate the capability of this novel Rayleigh-Taylor driven experiment. Particular interest is paid to the onset of buoyancy driven mixing and unusual aspects of the experiment in the context of typical Rayleigh-Taylor driven mixing.
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ABSTRACT
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Neutron-diffraction measurements under hydrostatic pressure up to 10 kbar were performed on the C... more Neutron-diffraction measurements under hydrostatic pressure up to 10 kbar were performed on the Ce0.9-xLaxTh0.10 system to investigate the tricritical point at xc = 0.14. For x < xc, we observe first-order transitions with a pressure derivative of the transition temperature, dT/dP = 20 K/kbar. For x > xc we observe a continuous transition that is second order, which again demonstrates a tri-critical point in the pressure-temperature phase diagram. The results will be presented and discussed.
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ChemInform, 1997
ABSTRACT
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Optical Materials, 2012
ABSTRACT Oxyfluoride aluminosilicate glasses containing a high nominal concentration of 35 mol% L... more ABSTRACT Oxyfluoride aluminosilicate glasses containing a high nominal concentration of 35 mol% LaF3 and CeF3 were synthesized, and their structural, thermal, optical absorption, photoluminescence, radio-luminescence, and excited-state relaxation properties were investigated. The structural analysis confirms the amorphous nature of the glasses and finds up to 21 mol% of retained dissolved LaF3 and CeF3. Tetrahedral Si and Al units as well as Si–O–Si and Al–O functional groups are identified. Fluorine is found to replace bridging oxygens and to form both Si–F and Al–F bonds. Glass transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tm) were determined from the DSC thermograms. The CeF3-doped glasses show spectrally broad 5d → 4f luminescence in the violet-blue (centered around 402 nm) under both direct optical excitation of Ce3+ at 335 nm and X-ray excitation at 25 keV. The relative quantum yield was found to decrease with increasing CeF3 concentration due to energy transfer to residual Ce4+ ions or to Ce3+ coordinated to hydroxyl (OH) impurities followed by non-radiative relaxation. The studied glasses are promising starting materials for the fabrication of glass–ceramics with a high volume fraction of LaF3:Ce3+ nano-crystals.
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APL MATERIALS, 2014
ABSTRACT We report a method for creating hybrid organic-inorganic &quot;nanoflowers&quot;... more ABSTRACT We report a method for creating hybrid organic-inorganic &quot;nanoflowers&quot; using calcium or copper ions as the inorganic component and a recombinantly expressed elastin-like polypeptide (ELP) as the organic component. Polypeptides provide binding sites for the dynamic coordination with metal ions, and then such noncovalent complexes become nucleation sites for primary crystals of metal phosphates. We have shown that the interaction between the stimuli-responsive ELP and Ca2+ or Cu2+, in the presence of phosphate, leads to the growth of micrometer-sized particles featuring nanoscale patterns shaped like flower petals. The morphology of these flower-like composite structures is dependent upon the temperature of growth and has been characterized by scanning electron microscopy. The composition of nanoflowers has also been analyzed by energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The temperature-dependent morphologies of these hybrid nanostructures, which arise from the controllable phase transition of ELPs, hold potential for morphological control of biomaterials in emerging applications such as tissue engineering and biocatalysis.
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Scripta Materialia, 2006
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Optical Materials, 2010
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Metallurgical and Materials Transactions A, 2007
ABSTRACT Texture changes during the α(bcc)-γ(fcc) phase transformation in ultra-low-carbon (ULC) ... more ABSTRACT Texture changes during the α(bcc)-γ(fcc) phase transformation in ultra-low-carbon (ULC) steel were investigated in situ by neutron diffraction, making use of a vacuum furnace. The initial texture is a typical bcc rolling texture. Upon heating above 500°C, it recrystallizes, and above 900°C, it transforms into an fcc texture with (111) fcc pole figures resembling (110) bcc and (110) fcc resembling (111) bcc. Upon cooling, the reverse transformation produces a texture that is close to the initial one, documenting a texture memory. Modeling the texture changes with the Kurdjumov–Sachs (KS), Nishiyama–Wassermann (NW), and Bain relationships reproduces the correct texture patterns but considerably weaker texture strengths, indicating that in the experimental case, variant selection occurs during both the forward and reverse transformations.
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Journal of the American Chemical Society, 2009
Colloidal nanocrystal quantum dots (NQDs) of narrow band gap materials are of substantial general... more Colloidal nanocrystal quantum dots (NQDs) of narrow band gap materials are of substantial general interest because of their unparalleled potential as infrared fluorophores. While PbSe NQDs are a promising class of infrared-active nanocrystals due to high emission quantum yields and a wide useful spectral range, typical synthetic methods are sensitive to a variety of factors, including the influence of solvent/ligand impurities that render reproducibility difficult. In this work, we specifically examine the effects of diphenylphosphine and 1,2-hexadecanediol, as surrogates for putative trioctylphosphine-based reducing impurities, on the synthesis of PbSe NQDs. Specifically, we compare their influence on NQD size, chemical yield, and photoluminescence quantum yield. While both additives substantially increase the chemical yield of the synthesis, they demonstrate markedly different effects on emission quantum yield of the product NQDs. We further examine the effects of reaction temperature and oleic acid concentration on the diol-assisted synthesis. Increased oleic acid concentration led to somewhat higher growth rates and larger NQDs but at the expense of lower chemical yield. Temperature was found to have an even greater effect on growth rate and NQD size. Neither temperature nor oleic acid concentration was found to have noticeable effects on NQD emission quantum yield. Finally, we use numerical simulations to support the conjecture that the increased yield is likely a result of faster monomer formation, consistent with the activation of an additional reaction pathway by the reducing species.
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Journal of the American Chemical Society, 1997
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Journal of the American Chemical Society, 2000
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Journal of Solid State Chemistry, 1997
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Journal of Solid State Chemistry, 2001
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Journal of Luminescence, 2013
ABSTRACT The excited-state relaxation dynamics of Tb3+, Sm3+, and Eu3+ doped into a 50SiO2–20Al2O... more ABSTRACT The excited-state relaxation dynamics of Tb3+, Sm3+, and Eu3+ doped into a 50SiO2–20Al2O3–10Na2O–20LaF3 (mol%) oxyfluoride glass are studied. Multiphonon relaxation of the primary emitting states in Tb3+ (5D3 and 5D4), Sm3+ (4G5/2), and Eu3+ (5D0) was found to be negligible in the present host. The relaxation of Tb3+ (5D4) and Eu3+ (5D0) is dominated by radiative decay. For Tb3+ (5D3) and Sm3+ (4G5/2) in contrast, radiative relaxation is in competition with several non-radiative cross-relaxation processes. This competition was found to be particularly pronounced for the 5D3 excited state in Tb3+, where a 124-fold decrease of the (5D3→7F5)/(5D4→7F5) emission intensity ratio and a ∼10-fold shortening of the 5D3 lifetime was observed upon increasing the Tb3+ concentration from 0.01% to 1%. The Tb3+ concentration dependence of 5D3 also points to some degree of ion aggregation in the “as quenched” glasses. A Judd–Ofelt intensity analysis was performed for Sm3+ and used to estimate the relative magnitude of 4G5/2 cross-relaxation processes. Four cross-relaxation processes in particular were identified to account for 92% of the total 4G5/2 non-radiative decay, and a 11% quantum efficiency was estimated for the 4G5/2 excited state. Non-exponentiality in the 5D0 decay of Eu3+ is evidence for several Eu3+ coordination environments in the glass host that manifest in different 5D0 decay constants because of the hypersensitivity of the 5D0→7F2 transition.
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Inorganic Chemistry, 1998
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Papers by Darrick Williams