Donald L. Anton

In-situ refractory intermetallic-based composites

Abstract With the ultimate objective of exploiting refractory intermetallics for high-temperature structural materials, several binary and ternary two-phase intermetallics/refractory-metal solid solutions were explored. The ductile solid solution is used to toughen the composite microstructure via in-situ phase separation. While the viability of ductile phase separation in solid state was briefly considered for systems such as Nb 3 Al/Nb, much of the work focused on processing eutectic systems such as Cr 2 Nb/Nb and (Nb,Mo) 5 Si 3 /Nb,Mo). This paper describes results obtained via containerless directional solidification of these high-melting eutectic alloys using an optical float-zone furnace. The observations are explained on the basis of solidification theory and parameters unique to the optical float-zone furnace. It is demonstrated that, by this technique, casting-defect- and macrosegregation-free material, with well-aligned microstructure, can be readily produced. Moreover, the potential to approach sub-micron laminate spacing at high growth rate in alloys with very high melting eutectics has also been established. Room-temperature bend test evaluation of directionally solidified material is discussed in light of prevailing theories of ductile phase toughening. The results of a preliminary exploration of the Nbue5f8Moue5f8Crue5f8Siue5f8Al multicomponent system are presented, showing the prevalence of eutectic phase separation and the potential for improving oxidation resistance.

High Density Hydrogen Storage System Demonstration Using NaAlH4 Based Complex Compound Hydrides

This final report describes the motivations, activities and results of the hydrogen storage independent project High Density Hydrogen Storage System Demonstration Using NaAlH4 Based Complex Compound Hydrides performed by the United Technologies Research Center under the Department of Energy Hydrogen Program, contract # DE-FC36-02AL67610. The objectives of the project were to identify and address the key systems technologies associated with applying complex hydride materials, particularly ones which differ from those for conventional metal hydride based storage. This involved the design, fabrication and testing of two prototype systems based on the hydrogen storage material NaAlH4. Safety testing, catalysis studies, heat exchanger optimization, reaction kinetics modeling, thermochemical finite element analysis, powder densification development and material neutralization were elements included in the effort.

Thermodynamic modeling of the sodium alanates and the Na–Al–H system

Abstract The thermodynamic properties of the Al–Na and Na–Al–H systems have been assessed by combining the “calculation of phase diagram” approach with first-principles predictions. The Gibbs energies of the individual phases were thermodynamically modeled, where the model parameters were obtained from best fit optimizations to combined experimental and first-principles predicted finite temperature data. The first-principles thermodynamic predictions were based upon density functional theory ground state minimizations and direct method lattice dynamics. The predictions proved to be important adjuncts to the assessments whenever experimental measurements were lacking or not feasible. It was shown that the phase stability conditions of sodium alanates, NaAlH4 and Na3AlH6, were well described with the present models.

Thermodynamic evaluation of the Al-H system

The thermodynamic properties of the Al-H system were assessed using models for the Gibbs energy of the individual phases, including the metastable hydride AlH3 phase. The model parameters were obtained through optimization by best fitting to selected experimental data. Particular attention was paid to hydrogen solubility in liquid and face-centered-cubic (fcc) Al. It was shown that the hydrogen can be treated as an ideal gas under normal conditions. The hydrogen solubility in liquid and fcc Al can be described very well with a regular solution model for liquid and fcc. The present calculations show satisfactory agreement with most experimental data for hydrogen solubility in fcc Al and selected data for hydrogen solubility in liquid Al, qualifying the extension of this binary model to higher-order Al-H-bearing systems.

Complex Hydride Compounds with Enhanced Hydrogen Storage Capacity

The United Technologies Research Center (UTRC), in collaboration with major partners Albemarle Corporation (Albemarle) and the Savannah River National Laboratory (SRNL), conducted research to discover new hydride materials for the storage of hydrogen having on-board reversibility and a target gravimetric capacity of ≥ 7.5 weight percent (wt %). When integrated into a system with a reasonable efficiency of 60% (mass of hydride / total mass), this target material would produce a system gravimetric capacity of ≥ 4.5 wt %, consistent with the DOE 2007 target. The approach established for the project combined first principles modeling (FPM - UTRC) with multiple synthesis methods: Solid State Processing (SSP - UTRC), Solution Based Processing (SBP - Albemarle) and Molten State Processing (MSP - SRNL). In the search for novel compounds, each of these methods has advantages and disadvantages; by combining them, the potential for success was increased. During the project, UTRC refined its FPM framework which includes ground state (0 Kelvin) structural determinations, elevated temperature thermodynamic predictions and thermodynamic / phase diagram calculations. This modeling was used both to precede synthesis in a virtual search for new compounds and after initial synthesis to examine reaction details and options for modifications including co-reactant additions.morexa0» The SSP synthesis method involved high energy ball milling which was simple, efficient for small batches and has proven effective for other storage material compositions. The SBP method produced very homogeneous chemical reactions, some of which cannot be performed via solid state routes, and would be the preferred approach for large scale production. The MSP technique is similar to the SSP method, but involves higher temperature and hydrogen pressure conditions to achieve greater species mobility. During the initial phases of the project, the focus was on higher order alanate complexes in the phase space between alkaline metal hydrides (AmH), Alkaline earth metal hydrides (AeH2), alane (AlH3), transition metal (Tm) hydrides (TmHz, where z=1-3) and molecular hydrogen (H2). The effort started first with variations of known alanates and subsequently extended the search to unknown compounds. In this stage, the FPM techniques were developed and validated on known alanate materials such as NaAlH4 and Na2LiAlH6. The coupled predictive methodologies were used to survey over 200 proposed phases in six quaternary spaces, formed from various combinations of Na, Li Mg and/or Ti with Al and H. A wide range of alanate compounds was examined using SSP having additions of Ti, Cr, Co, Ni and Fe. A number of compositions and reaction paths were identified having H weight fractions up to 5.6 wt %, but none meeting the 7.5 wt%H reversible goal. Similarly, MSP of alanates produced a number of interesting compounds and general conclusions regarding reaction behavior of mixtures during processing, but no alanate based candidates meeting the 7.5 wt% goal. A novel alanate, LiMg(AlH4)3, was synthesized using SBP that demonstrated a 7.0 wt% capacity with a desorption temperature of 150°C. The deuteride form was synthesized and characterized by the Institute for Energy (IFE) in Norway to determine its crystalline structure for related FPM studies. However, the reaction exhibited exothermicity and therefore was not reversible under acceptable hydrogen gas pressures for on-board recharging. After the extensive studies of alanates, the material class of emphasis was shifted to borohydrides. Through SBP, several ligand-stabilized Mg(BH4)2 complexes were synthesized. The Mg(BH4)2*2NH3 complex was found to change behavior with slightly different synthesis conditions and/or aging. One of the two mechanisms was an amine-borane (NH3BH3) like dissociation reaction which released up to 16 wt %H and more conservatively 9 wt%H when not including H2 released from the NH3. From FPM, the stability of the Mg(BH4)2*2NH3 compound was found to increase with the inclusion of NH3 groups in the inner-Mg coordination sphere, which in turn correlated with lowering the dimensionality of the Mg(BH4)2 network. Development of various Ak Tm-B-H compounds using SSP produced up to 12 wt% of H2 desorbed at temperatures of 400°C. However, the most active material can only be partially recharged to 2 wt% H2 at 220-300°C and 195 bar H2 pressure due to stable product formation. While gravimetric & volumetric targets are feasible, reversibility remains a persistent challenge.«xa0less

Development and Characterization of Novel Complex Hydrides Synthesized via Molten State Processing

This study developed novel hydrides for hydrogen storage through a novel synthesis technique utilizing high hydrogen overpressure at elevated temperatures denoted as Molten State Processing, MSP. The ultimate goal is to produce materials that have high hydrogen capacity, are stable after cycling and possess favorable thermodynamic and kinetic characteristics compatible with onboard hydrogen storage for automotive applications. In order to achieve these goals the MSP Process was developed and used to modify and form new complex hydride compounds with desired characteristics. This synthesis technique holds the potential of fusing different known complex hydrides at elevated temperatures and pressures to form new complexes having different sorption and thermodynamic properties. The new complex hydrides produced by this method were identified through structural determination and thermodynamic characterization in order to achieve a more fundamental understanding of their formation and dissociation mechanisms.

ENVIRONMENTAL REACTIVITY OF SOLID STATE HYDRIDE MATERIALS: MODELING AND TESTING FOR AIR AND WATER EXPOSURE

To make commercially acceptable condensed phase hydrogen storage systems, it is important to understand quantitatively the risks involved in using these materials. A rigorous set of environmental reactivity tests have been developed based on modified testing procedures codified by the United Nations for the transportation of dangerous goods. Potential hydrogen storage material, 2LiBH4{center_dot}MgH2 and NH3BH3, have been tested using these modified procedures to evaluate the relative risks of these materials coming in contact with the environment in hypothetical accident scenarios. It is apparent that an ignition event will only occur if both a flammable concentration of hydrogen and sufficient thermal energy were available to ignite the hydrogen gas mixture. In order to predict hydride behavior for hypothesized accident scenarios, an idealized finite element model was developed for dispersed hydride from a breached system. Empirical thermodynamic calculations based on precise calorimetric experiments were performed in order to quantify the energy and hydrogen release rates and to quantify the reaction products resulting from water and air exposure. Both thermal and compositional predictions were made with identification of potential ignition event scenarios.

Transport Process Study in Sodium Alanate Hydrogen Storage System During Desorption

Transport processes in a sodium alanate hydrogen storage system during desorption are presented. The mathematical model, which considers heat conduction and convection, hydrogen flow governed by Blake-Kozeny law and the chemical kinetics, is solved using the COMSOL Multiphysics® finite element software. The numerical simulation is used to present the time-space evolutions of the temperature, pressure and hydride concentration. The results are discussed for two cases: a finned storage system and a finless one. It is shown that the whole process occurring in the bed is governed and controlled by heat transfer from the heating fluid to the storage media and strengthened by axial heat transfer through the fins. The importance of the hydride bed thermal conductivity has also been evaluated. It was observed that the hydrogen discharge rate in a finless system can be improved if we find ways of increasing the thermal conductivity of the storage media. On the other hand, for a reservoir with fins, heat transfer is good enough that the discharge rate is limited by the kinetics.Copyright

Beyond Weight Percent - The Influence of Material Characteristics on Hydrogen Storage System Performance

The attribute of solid state hydrogen storage materials that is most commonly the focus of evaluations is reversible hydrogen weight percent. Other material characteristics, including density, charging pressure, enthalpy and conductivity can influence the weight of storage system components and hence the overall hydrogen weight percent that is ultimately of interest. However, accounting for these effects involves some level of storage system representation that typically is not undertaken when making material assessments and comparisons. The current paper will present a simple model that represents system elements and trade-offs on a high level so that overall system performance can be estimated without the burden of detailed design studies. The model should be useful to evaluate novel materials in a more complete manner for a better assessment of their potential when implemented in a storage system. While the model has been derived based on the design of a particular NaAlH 4 system, the key attributes are sufficiently general to be applicable to a range of system designs. Using this approach, the properties of materials can be related more precisely to goals for overall system performance with modest additional effort.

Practical Sorption Kinetics of TiCl 3 Catalyzed NaAlH 4

Sodium alanate has been studied as a promising candidate material for reversible hydrogen storage due to its intermediate temperature range and relatively high storage capacity. Its rates of desorption and absorption of hydrogen have been shown to be enhanced by the addition of Ti in various compounds. To date, the sorption kinetics, especially absorption kinetics, is not well understood. In this study, a practical sorption kinetics model for TiCl 3 catalyzed NaAlH 4 has been developed to assist in the engineering design and evaluation of a prototype hydrogen storage system.