Publications

Abstract not provided.

Abstract not provided.

Nanoporous metallic particles are of great interest for a range of applications including catalysis, gas storage, and electrical energy storage. In particular, recent work has shown that bulk powders of porous palladium can be synthesized in a scalable fashion. This material has pore sizes in the 2-5 nm range and has promise for use in hydrogen storage applications. However, because of the small pore size such materials are very susceptible to morphological evolution during aging, especially at elevated temperatures, leading to degradation of their storage properties. To better understand and predict the phenomena at work in nanoporous metal aging, we have developed a kinetic Monte Carlo (kMC) model for the simulation of atomic diffusion in a Pd lattice. The model is implemented in Sandia's parallelized kMC code SPPARKS. SPPARKS utilizes a spatial decomposition parallelization scheme, allowing large-scale simulations including millions of atoms. The diffusion model includes single-atom hops as well as Schwoebel barrier events that mimic concerted atom motions involving multiple lattice sites. Our simulations show that for statistically homogeneous nanoporous networks, coarsening at elevated temperature as measured by the surface area can be described by a scaling law that closely follows the L {approx} {sup 1/4} scaling predicted by continuum surface diffusion theory. This scaling holds despite the presence of surface faceting due to our simulations being run at temperatures below the roughening temperature of the material. Sensitivities of the rate of coarsening, the scaling exponent, and the amount of surface faceting to model parameters including temperature and event activation rates are explored. Because of the large spatial scales attainable in our computations, we are able to simulate nanoporous particle geometries similar to those synthesized in the laboratory, and compare directly to material aging experiments including porosimetry measurements and TEM images of particles.

This study investigates a pathway to nanoporous structures created by hydrogen implantation in aluminum. Previous experiments for fusion applications have indicated that hydrogen and helium ion implantations are capable of producing bicontinuous nanoporous structures in a variety of metals. This study focuses specifically on hydrogen and helium implantations of aluminum, including complementary experimental results and computational modeling of this system. Experimental results show the evolution of the surface morphology as the hydrogen ion fluence increases from 10{sup 17} cm{sup -2} to 10{sup 18} cm{sup -2}. Implantations of helium at a fluence of 10{sup 18} cm{sup -2} produce porosity on the order of 10 nm. Computational modeling demonstrates the formation of alanes, their desorption, and the resulting etching of aluminum surfaces that likely drives the nanostructures that form in the presence of hydrogen.

Nanoporous materials have maximum practical surface areas for electrical charge storage; every point in an electrode is within a few atoms of an interface at which charge can be stored. Metal-electrolyte interfaces make best use of surface area in porous materials. However, ion transport through long, narrow pores is slow. We seek to understand and optimize the tradeoff between capacity and transport. Modeling and measurements of nanoporous gold electrodes has allowed us to determine design principles, including the fact that these materials can deplete salt from the electrolyte, increasing resistance. We have developed fabrication techniques to demonstrate architectures inspired by these principles that may overcome identified obstacles. A key concept is that electrodes should be as close together as possible; this is likely to involve an interpenetrating pore structure. However, this may prove extremely challenging to fabricate at the finest scales; a hierarchically porous structure can be a worthy compromise.

Abstract not provided.

Nanoporous palladium powders are synthesized on milligram to gram scales by chemical reduction of tetrachloro complexes by ascorbate in a concentrated aqueous surfactant at temperatures between -20 and 30 C. Particle diameters are approximately 50 nm, and each particle is perforated by 3 nm pores, as determined by electron tomography. These materials are of potential value for storage of hydrogen isotopes and electrical charge; producing them at large scales in a safe and efficient manner will help realize this. A slightly modified procedure also results in nanoporous platinum.

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.