Publications

Journal of Vacuum Science and Technology A

Cardenas, R.E.; Stewart, Kenneth D.; Cowgill, D.F.

Abstract not provided.

Musculus, Mark P.; Zador, Judit Z.; Stewart, Kenneth D.; Li, Zheming L.; Cicone, Dave J.; Roberts, Greg R.

The high-level objective of this project is to solve national-s ecurity problems associated with petroleum use, cost, and environmental impacts by enabling more efficient use of natural-gas-fueled internal co mbustion engines. An improved sci ence-base on end-gas autoignition, or "knock," is re quired to support engineering of more efficient engine designs through predictive modeling. An existing optical diesel engine facility is retrofitted for natural gas fueling with laser-spark-ignition c ombustion to provide in- cylinder imaging and pressure data under knocking combustion. Z ero-dimensional chemical-kinetic modeling of aut oignition, adiabatically constr ained by the measured cylinder pressure, isolates the role of autoignition chemistry. OH* chemiluminescence imaging reveals six different c ategories of knock onset that de pend on proximity to engine surfaces and the in-cylinder deflagration. Modeling resu lts show excellent prediction regardless of the knoc k category, thereby validating state-of-the-art kinetic mechanisms. The results also provide guidance for future work t o build a science base on the factors that affect the deflagration rate.

Robinson, David R.; Luo, Weifang L.; Stewart, Kenneth D.

We have developed a new method to measure the composition of gaseous mixtures of any two hydrogen isotopes, as well as an inert gas component. When tritium is one of those hydrogen isotopes, there is usually some helium present, because the tritium decays to form helium at a rate of about 1% every 2 months. The usual way of measuring composition of these mixtures involves mass spectrometry, which involves bulky, energy-intensive, expensive instruments, including vacuum pumps that can quite undesirably disperse tritium. Our approach uses calorimetry of a small quantity of hydrogen-absorbing material to determine gas composition without consuming or dispersing the analytes. Our work was a proof of principle using a rather large and slow benchtop calorimeter. Incorporation of microfabricated calorimeters, such as those that have been developed in Sandia’s MicroChemLab program or that are now commercially available, would allow for faster measurements and a smaller instrument footprint.

Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films

Cárdenas, Rosa E.; Stewart, Kenneth D.; Cowgill, D.F.

In this study, the authors developed an approach for accurately quantifying the helium content in a gas mixture also containing hydrogen and methane using commercially available getters. The authors performed a systematic study to examine how both H2 and CH4 can be removed simultaneously from the mixture using two SAES St 172® getters operating at different temperatures. The remaining He within the gas mixture can then be measured directly using a capacitance manometer. The optimum combination involved operating one getter at 650 °C to decompose the methane, and the second at 110 °C to remove the hydrogen. This approach eliminated the need to reactivate the getters between measurements, thereby enabling multiple measurements to be made within a short time interval, with accuracy better than 1%. The authors anticipate that such an approach will be particularly useful for quantifying the He-3 in mixtures that include tritium, tritiated methane, and helium-3. The presence of tritiated methane, generated by tritium activity, often complicates such measurements.

Cowgill, D.F.; Stewart, Kenneth D.

Abstract not provided.

Cowgill, D.F.; Tewell, Craig R.; Stewart, Kenneth D.

Abstract not provided.

Cowgill, D.F.; Fares, Stephen J.; Ong, Markus D.; Arslan, Ilke A.; Tran, Kim T.; Sartor, George B.; Stewart, Kenneth D.; Cliff, Miles; Robinson, David R.; McCarty, Kevin F.; Luo, Weifang L.; Smugeresky, J.E.

Nano-structured palladium is examined as a tritium storage material with the potential to release beta-decay-generated helium at the generation rate, thereby mitigating the aging effects produced by enlarging He bubbles. Helium retention in proposed structures is modeled by adapting the Sandia Bubble Evolution model to nano-dimensional material. The model shows that even with ligament dimensions of 6-12 nm, elevated temperatures will be required for low He retention. Two nanomaterial synthesis pathways were explored: de-alloying and surfactant templating. For de-alloying, PdAg alloys with piranha etchants appeared likely to generate the desired morphology with some additional development effort. Nano-structured 50 nm Pd particles with 2-3 nm pores were successfully produced by surfactant templating using PdCl salts and an oligo(ethylene oxide) hexadecyl ether surfactant. Tests were performed on this material to investigate processes for removing residual pore fluids and to examine the thermal stability of pores. A tritium manifold was fabricated to measure the early He release behavior of this and Pd black material and is installed in the Tritium Science Station glove box at LLNL. Pressure-composition isotherms and particle sizes of a commercial Pd black were measured.

Journal of physical chemistry

Cowgill, D.F.; Causey, Rion A.; Stewart, Kenneth D.

Abstract not provided.

Ronnebro, Ewa R.; Wang, James C.; Stewart, Kenneth D.

Hydrogen energy may provide the means to an environmentally friendly future. One of the problems related to its application for transportation is 'on-board' storage. Hydrogen storage in solids has long been recognized as one of the most practical approaches for this purpose. The H-capacity in interstitial hydrides of most metals and alloys is limited to below 2.5% by weight and this is unsatisfactory for on-board transportation applications. Magnesium hydride is an exception with hydrogen capacity of -8.2 wt.%, however, its operating temperature, above 350 C, is too high for practical use. Sodium alanate (NaAlH{sub 4}) absorbs hydrogen up to 5.6 wt.% theoretically; however, its reaction kinetics and partial reversibility do not completely meet the new target for transportation application. Recently Chen et al. [1] reported that (Li{sub 3}N+2H{sub 2} {leftrightarrow} LiNH{sub 2} + 2LiH) provides a storage material with a possible high capacity, up to 11.5 wt.%, although this material is still too stable to meet the operating pressure/temperature requirement. Here we report a new approach to destabilize lithium imide system by partial substitution of lithium by magnesium in the (LiNH{sub 2} + LiH {leftrightarrow} Li{sub 2}NH + H{sub 2}) system with a minimal capacity loss. This Mg-substituted material can reversibly absorb 5.2 wt.% hydrogen at pressure of 30 bar at 200 C. This is a very promising material for on-board hydrogen storage applications. It is interesting to observe that the starting material (2LiNH{sub 2} + MgH{sub 2}) converts to (Mg(NH{sub 2}){sub 2} + 2LiH) after a desorption/re-absorption cycle.