Publications

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The counter-rotating-ring receiver/reactor/recuperator (CR5) solar thermochemical heat engine is a new concept for production of hydrogen that allows for thermal recuperation between solids in an efficient counter-current arrangement. At the heart of the CR5 system are annular rings of a reactive solid ferrite that are thermally and chemically cycled to produce oxygen and hydrogen from water in separate and isolated steps. This design is very demanding from a materials point of view. The ferrite rings must maintain structural integrity and high reactivity after months of thermal cycling and exposure to temperatures in excess of 1100°C. In addition, the design of the rings must have high geometric surface area for gas-solid contact and for adsorption of incident solar radiation. After performing a series of initial screenings, we chose Co0.67Fe2.33O4 as our baseline working material for a planned demonstration of CR5 and have begun additional characterization and development of this material. Our results to date with powders are consistent with the expectation that small particle sizes and the application of a support to inhibit ferrite sintering and enhance the chemistry are critical considerations for a practical operating device. Concurrent with the powder studies, we are using Robocasting, a Sandia-developed technique for free form processing of ceramics, to manufacture monolithic structures with complex three-dimensional geometries for chemical, physical, and mechanical evaluation. We have demonstrated that ferrite/zirconia mixtures can be fabricated into small three-dimensional monolithic lattice structures that give reproducible hydrogen yields over multiple cycles. Copyright © 2006 by ASME.

Atmospheric pressure chemical vapor deposition (APCVD) of tin oxide is a very important manufacturing technique used in the production of low-emissivity glass. It is also the primary method used to provide wear-resistant coatings on glass containers. The complexity of these systems, which involve chemical reactions in both the gas phase and on the deposition surface, as well as complex fluid dynamics, makes process optimization and design of new coating reactors a very difficult task. In 2001 the U.S. Dept. of Energy Industrial Technologies Program Glass Industry of the Future Team funded a project to address the need for more accurate data concerning the tin oxide APCVD process. This report presents a case study of on-line APCVD using organometallic precursors, which are the primary reactants used in industrial coating processes. Research staff at Sandia National Laboratories in Livermore, CA, and the PPG Industries Glass Technology Center in Pittsburgh, PA collaborated to produce this work. In this report, we describe a detailed investigation of the factors controlling the growth of tin oxide films. The report begins with a discussion of the basic elements of the deposition chemistry, including gas-phase thermochemistry of tin species and mechanisms of chemical reactions involved in the decomposition of tin precursors. These results provide the basis for experimental investigations in which tin oxide growth rates were measured as a function of all major process variables. The experiments focused on growth from monobutyltintrichloride (MBTC) since this is one of the two primary precursors used industrially. There are almost no reliable growth-rate data available for this precursor. Robust models describing the growth rate as a function of these variables are derived from modeling of these data. Finally, the results are used to conduct computational fluid dynamic simulations of both pilot- and full-scale coating reactors. As a result, general conclusions are reached concerning the factors affecting the growth rate in on-line APCVD reactors. In addition, a substantial body of data was generated that can be used to model many different industrial tin oxide coating processes. These data include the most extensive compilation of thermochemistry for gas-phase tin-containing species as well as kinetic expressions describing tin oxide growth rates over a wide range of temperatures, pressures, and reactant concentrations.

Calibrated by both experimental data and high-level coupled-cluster calculations, the BAC-MP4 methodology was applied to 51 SbL n (L = H, CH 3, C 2H 5, Cl, and OH, n = 1-5) molecules, providing calculated heats of formation and associated thermodynamic parameters. These data identify a linear variation in heats of formation with ligand substitution, trends in bond dissociation energies (BDEs) with ligand identity [BDE(Sb-C 2H 5) < BDE(Sb-CH 3) < BDE(Sb-H) < BDE(Sb-Cl) < BDE(Sb-OH)], and a monotonie decrease in BDE upon successive ligand elimination. The linear variation in BDE is consistent with the behavior of other group V elements, in contrast to the characteristic high-low-high trend of adjacent group III (In) and group IV (Sn) elements. Additionally, these data complement those of previous studies of metal-organic species and provide a foundation of thermochemical data that can aid in the selection of CVD precursors and deposition conditions for the growth of antimony-containing materials. © 2006 American Chemical Society.

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This report summarizes the results of a five-year effort to understand the mechanisms and develop models that predict the corrosion of refractories in oxygen-fuel glass-melting furnaces. Thermodynamic data for the Si-O-(Na or K) and Al-O-(Na or K) systems are reported, allowing equilibrium calculations to be performed to evaluate corrosion of silica- and alumina-based refractories under typical furnace operating conditions. A detailed analysis of processes contributing to corrosion is also presented. Using this analysis, a model of the corrosion process was developed and used to predict corrosion rates in an actual industrial glass furnace. The rate-limiting process is most likely the transport of NaOH(gas) through the mass-transport boundary layer from the furnace atmosphere to the crown surface. Corrosion rates predicted on this basis are in better agreement with observation than those produced by any other mechanism, although the absolute values are highly sensitive to the crown temperature and the NaOH(gas) concentration at equilibrium and at the edge of the boundary layer. Finally, the project explored the development of excimer laser induced fragmentation (ELIF) fluorescence spectroscopy for the detection of gas-phase alkali hydroxides (e.g., NaOH) that are predicted to be the key species causing accelerated corrosion in these furnaces. The development of ELIF and the construction of field-portable instrumentation for glass furnace applications are reported and the method is shown to be effective in industrial settings.

The thermochemistry of the transition-metal fluorides and chlorides MF{sub n} and MCl{sub n} (M = Cr, Mn, Fe; n = 1, 2) has been characterized by high-level ab initio electronic structure methods. Geometries and harmonic vibrational frequencies were computed at the B3LYP level of theory using triple-{zeta} basis sets including diffuse and polarization functions. Heats of formation were computed from isogyric reaction energies at the CCSD(T) level using high-quality basis sets, including corrections for core-valence correlation and scalar relativistic effects. To investigate the possible linearity of the ground states of CrCl{sub 2} and CrF{sub 2}, we performed geometry optimizations for these species at the CCSD(T) level using large basis sets. In both cases, a bent ({sup 5}B{sub 2}) minimum structure was located, but the bent structure is only slightly below the linear form, which was found to be a transition state. For all of the investigated halides, polynomial fits were carried out for the heat capacity and the standard enthalpy and entropy in the 300-3000 K temperature range.

A self-consistent set of thermochemical data for 55 molecules in the Al-H-C-O-F-Cl system are obtained from ab initio quantum-chemistry calculations using the BAC-G2 method. Calculations were performed for both stable and radical species. Good agreement is found between the calculations and experimental heats of formation in most cases where data are available for comparison. Electronic energies, molecular geometries, moments of inertia, and vibrational frequencies are provided in the Supporting Information, as are polynomial fits of the thermodynamic data (heat of formation, entropy, and heat capacity) over the 300--3000 K temperature range.