Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Reactive rare-earth / transition metal multilayers exhibit a variety of complex reaction behaviors depending on surrounding gaseous environment and material design. Small period (< 100 nm bilayer), 5 gm-thick Sc/Ag multilayers undergo self-sustained formation reactions when ignited in air or in vacuum. High-speed videography reveals unstable reaction waves in these samples, characterized by the repeated, transverse passage of narrow, spin bands. Intermediate Sc/Ag designs — with multilayer period between 100 and 200 nm — only react in air. These multilayers exhibit propagating reactions with alternating unstable and stable characteristics. Narrow, spin bands advance the reaction front stepwise. Soon after the passage of a transverse band, a trailing oxidation wave encroaches on the intermetallic reaction front temporarily pushing the stalled wave forward in a uniform manner. Viewed in full, these events repeat giving rise to a new oscillatory behavior. Sc/Ag multilayers having a large period (> 200 nm bilayer) also react exclusively in air but exhibit a different propagating mode. The oxidation of Sc combined with the exothermic reaction of metal species results in continually-stable waves characterized by a smooth wavefront morphology and uniform velocity. The flame temperatures associated with propagating waves are estimated using measured heats of reaction and enthalpy-temperature relationships in order to provide insight into the possible phase transformations that occur during these different exothermic reactions.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The thickening behavior of aluminum scandium nitride (Al_0.88Sc_0.12N) films grown on Si(111) substrates has been investigated experimentally using X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy, and residual stress measurement. Al_0.88Sc_0.12N films were grown with thicknesses spanning 14 nm to 1.1 um. TEM analysis shows that the argon sputter etch used to remove the native oxide prior to deposition produced an amorphous, oxygen-rich surface, preventing epitaxial growth. XRD analysis of the films show that the A1ScN(002) orientation improves as the films thicken and the XRD A1ScN(002) rocking curve full width half maximum decreases to 1.34 q for the 1.1 pm thick film. XRD analysis shows that the unit cell is expanded in both the a- and c-axes by Sc doping; the a-axis lattice parameter was measured to be 3.172 ± 0.007 A and the c-axis lattice parameter was measured to be 5.000 ± 0.001 A, representing 1.96% and 0.44% expansions over aluminum nitride lattice parameters, respectively. The grain size and roughness increase as the film thickness increases. A stress gradient forms through the film; the residual stress grows more tensile as the film thickens, from -1.24 GPa to +8.5MPa.

Various versions of deep borehole nuclear waste disposal have been proposed in the past in which effective sealing of a borehole after waste emplacement is generally required. In a high temperature disposal mode, the sealing function will be fulfilled by melting the ambient granitic rock with waste decay heat or an external heating source, creating a melt that will encapsulate waste containers or plug a portion of the borehole above a stack of the containers. However, there are certain drawbacks associated with natural materials, such as high melting temperatures, inefficient consolidation, slow crystallization kinetics, the resulting sealing materials generally being porous with low mechanical strength, insufficient adhesion to waste container surface, and lack of flexibility for engineering controls. In this study, we showed that natural granitic materials can be purposefully engineered through chemical modifications to enhance the sealing capability of the materials for deep borehole disposal. The present work systematically explores the effect of chemical modification and crystallinity (amorphous vs. crystalline) on the melting and crystallization processes of a granitic rock system. The approach can be applied to modify granites excavated from different geological sites. Several engineered granitic materials have been explored which possess significantly lower processing and densification temperatures than natural granites. Those new materials consolidate more efficiently by viscous flow and accelerated recrystallization without compromising their mechanical integrity and properties.

We have investigated cubic zirconium tungstate (ZrW2O8) using density functional perturbation theory (DFPT), along with experimental characterization to assess and validate computational results. Cubic zirconium tungstate is among the few known materials exhibiting isotropic negative thermal expansion (NTE) over a broad temperature range, including room temperature where it occurs metastably. Isotropic NTE materials are important for technological applications requiring thermal-expansion compensators in composites designed to have overall zero or adjustable thermal expansion. While cubic zirconium tungstate has attracted considerable attention experimentally, a very few computational studies have been dedicated to this well-known NTE material. Therefore, spectroscopic, mechanical and thermodynamic properties have been derived from DFPT calculations. A systematic comparison of the calculated infrared, Raman, and phonon density-of-state spectra has been made with Fourier transform far-/mid-infrared and Raman data collected in this study, as well as with available inelastic neutron scattering measurements. The thermal evolution of the lattice parameter computed within the quasi-harmonic approximation exhibits negative values below the Debye temperature, consistent with the observed negative thermal expansion characteristics of cubic zirconium tungstate, α-ZrW2O8. These results show that this DFPT approach can be used for studying the spectroscopic, mechanical and thermodynamic properties of prospective NTE ceramic waste forms for encapsulation of radionuclides produced during the nuclear fuel cycle.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

In this study, the scaling of polarization and pyroelectric response across a thickness series (5–20 nm) of Hf_0.58Zr_0.42O₂ films with TaN electrodes was characterized. Reduction in thickness from 20 nm to 5 nm resulted in a decreased remanent polarization from 17 to 2.8 μC cm^-2. Accompanying the decreased remanent polarization was an increased absolute pyroelectric coefficient, from 30 to 58 μC m^-2 K^-1. The pyroelectric response of the 5 nm film was unstable and decreased logarithmically with time, while that of 10 nm and thicker films was stable over a time scale of >300 h at room temperature. Finally, the sign of the pyroelectric response was irreversible with differing polarity of poling bias for the 5 nm thick film, indicating that the enhanced pyroelectric response was of electret origins, whereas the pyroelectric response in thicker films was consistent with a crystallographic origin.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The different rate-limiting processes underlying ignition and self-propagating reactions in Al/Pt multilayers are examined through experiments and analytical modeling. Freestanding, ∼1.6 μm-thick Al/Pt multilayers of varied stoichiometries and nanometer-scale layer thicknesses ignite at temperatures below the melting point of both reactants (and eutectics) demonstrating that initiation occurs via solid-state mixing. Equimolar multilayers exhibit the lowest ignition temperatures when comparing structures having a specific bilayer thickness. An activation energy of 76.6 kJ/mol at. associated with solid state mass transport is determined from the model analysis of ignition. High speed videography shows that equimolar Al/Pt multilayers undergo the most rapid self-sustained reactions with wavefront speeds as large as 73 m/s. Al- and Pt-rich multilayers react at reduced rates (as low as 0.3 m/s), consistent with reduced heat of reaction and lower adiabatic temperatures. An analytical model that accounts for key thermodynamic properties, preliminary mixing along interfaces, thermal transport, and mass diffusion is used to predict the wavefront speed dependencies on bilayer thickness. Good fits to experimental data provide estimates for activation energy (51 kJ/mol at.) associated with mass transport subject to high heating rates and thermal diffusion coefficient of premixed interfacial volumes (2.8 × 10-6 m2/s). Pt dissolution into molten Al is identified as a rate-limiting step underlying high temperature propagating reactions in Al/Pt multilayers.

Abstract not provided.

Abstract not provided.

Cubic zirconium tungstate (α-ZrW2O8), a well-known negative thermal expansion material, has been investigated within the framework of density functional perturbation theory (DFPT), combined with experimental characterization to assess and validate computational results. Using combined Fourier transform infrared measurements and DFPT calculations, new and extensive assignments were made for the far-infrared (<400 cm−1) spectrum of α-ZrW2O8. A systematic comparison of DFPT-simulated infrared, Raman, and phonon density-of-state spectra with Fourier transform far-/mid-infrared and Raman data collected in this study, as well as with available inelastic neutron scattering measurements, shows the superior accuracy of the PBEsol exchange-correlation functional over standard PBE calculations for studying the spectroscopic properties of this material.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The powder-bed laser additive manufacturing (AM) process is widely used in the fabrication of three-dimensional metallic parts with intricate structures, where kinetically controlled diffusion and microstructure ripening can be hindered by fast melting and rapid solidification. Therefore, the microstructure and physical properties of parts made by this process will be significantly different from their counterparts produced by conventional methods. This work investigates the microstructure evolution for an AM fabricated AlSi10Mg part from its nonequilibrium state toward equilibrium state. Special attention is placed on silicon dissolution, precipitate formation, collapsing of a divorced eutectic cellular structure, and microstructure ripening in the thermal annealing process. These events alter the size, morphology, length scale, and distribution of the beta silicon phase in the primary aluminum, and changes associated with elastic properties and microhardness are reported. The relationship between residual stress and silicon dissolution due to changes in lattice spacing is also investigated and discussed.

Abstract not provided.

Processing of the low workability Fe-Co-1.5V (Hiperco® equivalent) alloy is demonstrated using the Laser Engineered Net Shaping (LENS) metals additive manufacturing technique. As an innovative and highly localized solidification process, LENS is shown to overcome workability issues that arise during conventional thermomechanical processing, enabling the production of bulk, near net-shape forms of the Fe-Co alloy. Bulk LENS structures appeared to be ductile with no significant macroscopic defects. Atomic ordering was evaluated and significantly reduced in as-built LENS specimens relative to an annealed condition, tailorable through selection of processing parameters. Fine equiaxed grain structures were observed in as-built specimens following solidification, which then evolved toward a highly heterogeneous bimodal grain structure after annealing. The microstructure evolution in Fe-Co is discussed in the context of classical solidification theory and selective grain boundary pinning processes. Magnetic properties were also assessed and shown to fall within the extremes of conventionally processed Hiperco® alloys. Hiperco® is a registered trademark of Carpenter Technologies, Readings, PA.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Cubic zirconium tungstate (α-ZrW₂O₈), a notorious negative thermal expansion (NTE) material, has been investigated within the framework of density functional perturbation theory (DFPT), combined with experimental characterization to assess and validate computational results. Spectroscopic, mechanical and thermodynamic properties have been derived from DFPT calculations. A systematic comparison of DFPT-simulated infrared, Raman, and phonon density-of-state spectra with Fourier transform far-/mid-infrared and Raman data collected in this study, as well as with available inelastic neutron scattering measurements, shows the supe-rior accuracy of the PBEsol exchange-correlation functional over standard PBE calculations. The thermal evolution of the Grüneisen parameter computed within the quasi-harmonic approximation exhibits negative values below the Debye temperature, consistent with the observed NTE characteristics of α-ZrW₂O₈. The standard molar heat capacity is predicted to be C$0\atop{P}$=193.8 and 192.2 J.mol^-1.K^-1 with PBE and PBEsol, respectively, ca. 7% lower than calorimetric data. In conclusion, these results demonstrate the accuracy of the DFPT/PBEsol approach for studying the spectroscopic, mechanical and thermodynamic properties of materials with anomalous thermal expansion.

Abstract not provided.

Abstract not provided.

Herein, we describe a novel multifunctional metal-organic framework (MOF) materials platform that displays both porosity and tunable emission properties as a function of the metal identity (Eu, Nd, and tuned compositions of Nd/Yb). Their emission collectively spans the deep red to near-infrared (NIR) spectral region (∼614-1350 nm), which is highly relevant for in vivo bioimaging. These new materials meet important prerequisites as relevant to biological processes: they are minimally toxic to living cells and retain structural integrity in water and phosphate-buffered saline. To assess their viability as optical bioimaging agents, we successfully synthesized the nanoscale Eu analog as a proof-of-concept system in this series. In vitro studies show that it is cell-permeable in individual RAW 264.7 mouse macrophage and HeLa human cervical cancer tissue culture cells. The efficient discrimination between the Eu emission and cell autofluorescence was achieved with hyperspectral confocal fluorescence microscopy, used here for the first time to characterize MOF materials. Importantly, this is the first report that documents the long-term conservation of the intrinsic emission in live cells of a fluorophore-based MOF to date (up to 48 h). This finding, in conjunction with the materials' very low toxicity, validates the biocompatibility in these systems and qualifies them as promising for use in long-term tracking and biodistribution studies.

The impact on the final morphology of yttria (Y2O3) nanoparticles from different ratios (100/0, 90/10, 65/35, and 50/50) of oleylamine (ON) and oleic acid (OA) via a solution precipitation route has been determined. In all instances, powder X-ray diffraction indicated that the cubic Y2O3 phase (PDF #00-025-1200) with the space group I-3a (206) had been formed. Analysis of the collected FTIR data revealed the presence of stretches and bends consistent with ON and OA, for all ratios investigated, except the 100/0. Transmission electron microscopy images revealed regular and elongated hexagons were produced for the ON (100/0) sample. As OA was added, the nanoparticle morphology changed to lamellar pillars (90/10), then irregular particles (65/35), and finally plates (50/50). The formation of the hexagonal-shaped nanoparticles was determined to be due to the preferential adsorption of ON onto the {101} planes. As OA was added to the reaction mixture, it was found that the {111} planes were preferentially coated, replacing ON from the surface, resulting in the various morphologies noted. The roles of the ratio of ON/OA in the synthesis of the nanocrystals were elucidated in the formation of the various Y2O3 morphologies, as well as a possible growth mechanism based on the experimental data.