Markus D. Ong

Influence of Physico-Chemical Changes on Enzymatic Digestibility of Ionic Liquid and AFEX pretreated Corn Stover

Ionic liquid (IL) and ammonia fiber expansion (AFEX) pretreatments were studied to develop the first direct side-by-side comparative assessment on their respective impacts on biomass structure, composition, process mass balance, and enzymatic saccharification efficiency. AFEX pretreatment completely preserves plant carbohydrates, whereas IL pretreatment extracts 76% of hemicellulose. In contrast to AFEX, the native crystal structure of the recovered corn stover from IL pretreatment was significantly disrupted. For both techniques, more than 70% of the theoretical sugar yield was attained after 48 h of hydrolysis using commercial enzyme cocktails. IL pretreatment requires less enzyme loading and a shorter hydrolysis time to reach 90% yields. Hemicellulase addition led to significant improvements in the yields of glucose and xylose for AFEX pretreated corn stover, but not for IL pretreated stover. These results provide new insights into the mechanisms of IL and AFEX pretreatment, as well as the advantages and disadvantages of each.

Three-dimensional pore evolution of nanoporous metal particles for energy storage.

A well characterized and predictable aging pattern is necessary for practical energy storage applications of nanoporous particles that facilitate rapid transport of ions or redox species. Here we use STEM tomography with segmentation to show that surface diffusion and grain boundary diffusion are responsible for pore evolution at intermediate and higher temperatures, respectively. This unprecedented three dimensional understanding of pore behavior as a function of temperature suggests routes for optimizing pore stability in future energy storage materials.

Effect of electrolyte and adsorbates on charging rates in mesoporous gold electrodes.

The classical model for porous electrodes reported by De Levie several decades ago (and expanded upon since then) was developed mainly to describe pores with micrometer-scale diameters. Presumably it will break down as pore diameters approach atomic dimensions. Mesoporous gold formed by dealloying is a valuable test platform for this because its 10 nm pores are on the boundary of this expected breakdown and because the electrochemical and surface properties of gold are relatively well understood. The De Levie model works for these electrodes at high salt concentrations, but under dilute conditions, there is not enough salt locally to charge the interface, increasing real impedance on intermediate time scales. Specific adsorption on pore walls can cause a similar increase and also cause an effective mobility decrease, tunable through electrolyte choice and the use of alkanethiol monolayers. These effects are not expected in micrometer-scale pores and are important considerations when designing devices with nanoporous electrodes.

Solution chemistry effects on cracking and damage evolution during chemical-mechanical planarization

We describe progress in understanding the effect of simulated chemical-mechanical planarization (CMP) slurry chemistry on the evolution of defects and formation of damage that occurs during CMP processing. Specifically, we demonstrate the significant effect of aqueous solution chemistry on accelerating crack growth in porous methylsilsesquioxane (MSSQ) films. In addition, we show that the same aqueous solutions can diffuse rapidly into the highly hydrophobic nanoporous MSSQ films containing interconnected porosity. Such diffusion has deleterious effects on both dielectric properties and the acceleration of defect growth rates. Crack propagation rates were measured in several CMP solutions, and the resulting crack growth behavior was used to qualitatively predict the extent of damage during CMP. These predictions are compared with damage formed during actual CMP processes in identical chemistries. We discuss the effects of both the high and low crack growth rate regimes, including the presence of a crack growth threshold, on the predicted CMP damage. Finally, implications for improved CMP processing were considered.

Fracture Properties of Porous MSSQ Films: Impact of Porogen Loading and Burnout

This work investigates the effect of porogen loading on the fracture properties of methylsilsesquioxane (MSSQ) both before and after the porogen burnout process. The fracture behavior of the hybrid porogen/matrix materials differed significantly from that of the post-burnout materials. The most notable differences were alternative fracture paths and a trend of increasing fracture energy with increasing porogen loading. Characterization of the fracture surfaces indicate increasing amounts of carbon at the fracture interface corresponding to the increases in fracture energy and suggest bridging porogen molecules may be responsible for the increase in adhesion for the hybrid materials.

Quantitative EDS of Surface Modified Pd Powders for Hydrogen Storage.

Palladium and its alloys are known to be useful for applications such as catalysis [1], electrocatalysis [2], and hydrogen isotope storage and separation [3, 4]. Atomic-scale surface and subsurface layers of other metals have surface hydride energetics that may enhance the kinetics of hydrogen absorption and desorption [5]. In addition, increasing the surface area/volume ratio by making Pd alloys nanoporous enhances surface-limited reaction rates. When Pd is alloyed with a higher melting temperature metal such as Rh, the temperature range over which the nanoporous structure remains morphologically stable is extended [6]. We use a variety of techniques to fabricate surface modified Pd powders at different length scales, including, the consolidation of dendrimer-encapsulated nanoparticles [7], surfactant templates [8], and atomic layer electroless deposition (ALED) [9]. It is crucial to understand the spatial distribution of alloying elements in these structures because of the large effect it has on thermal stability, hydrogen storage properties, and the kinetics of hydrogen uptake and release.

Material synthesis and hydrogen storage of palladium-rhodium alloy.

Pd and Pd alloys are candidate material systems for Tr or H storage. We have actively engaged in material synthesis and studied the material science of hydrogen storage for Pd-Rh alloys. In collaboration with UC Davis, we successfully developed/optimized a supersonic gas atomization system, including its processing parameters, for Pd-Rh-based alloy powders. This optimized system and processing enable us to produce {le} 50-{mu}m powders with suitable metallurgical properties for H-storage R&D. In addition, we studied hydrogen absorption-desorption pressure-composition-temperature (PCT) behavior using these gas-atomized Pd-Rh alloy powders. The study shows that the pressure-composition-temperature (PCT) behavior of Pd-Rh alloys is strongly influenced by its metallurgy. The plateau pressure, slope, and H/metal capacity are highly dependent on alloy composition and its chemical distribution. For the gas-atomized Pd-10 wt% Rh, the absorption plateau pressure is relatively high and consistent. However, the absorption-desorption PCT exhibits a significant hysteresis loop that is not seen from the 30-nm nanopowders produced by chemical precipitation. In addition, we observed that the presence of hydrogen introduces strong lattice strain, plastic deformation, and dislocation networking that lead to material hardening, lattice distortions, and volume expansion. The above observations suggest that the H-induced dislocation networking is responsible for the hysteresis loop seen in the current atomized Pd-10 wt% Rh powders. This conclusion is consistent with the hypothesis suggested by Flanagan and others (Ref 1) that plastic deformation or dislocations control the hysteresis loop.

Nanostructures from Hydrogen and Helium Implantation of Aluminum

This study investigates a pathway to nanoporous structures created by hydrogen and helium 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 implantations of hydrogen and helium ions at 25 keV in aluminum. The hydrogen and helium systems result in remarkably different nanostructures of aluminum at the surface. Scanning electron microscopy, focused ion beam, and transmission electron microscopy show that both implantations result in porosity that persists approximately 200 nm deep. However, hydrogen implantations tend to produce larger and more irregular voids that preferentially reside at defects. Implantations of helium at a fluence of 10{sup 18} cm{sup -2} produce much smaller porosity on the order of 10 nm that is regular and creates a bicontinuous structure in the porous region. The primary difference driving the formation of the contrasting structures is likely the relatively high mobility of hydrogen and the ability of hydrogen to form alanes that are capable of desorbing and etching Al (111) faces.

Nanostructures from hydrogen implantation of metals.

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.

A Novel Bonding Technique Using Metal-Induced Crystallization of Amorphous Silicon

This study investigates the used of aluminum-induced crystallization of amorphous silicon is a potential bonding mechanism for a sandwich stack of films between two silicon substrates. Similar procedures using copper diffusion bonds have been in use, but these require temperatures as high as 400°C. Using the crystallization of amorphous silicon as the bonding mechanism has allowed the bonding temperature to be lowered by more than 100 K. Fracture experiments for a low-k material were conducted, and the results using amorphous silicon bonding was compared to epoxy bonding control experiments. Essentially identical results were obtained for the two bonding mechanisms. Low-temperature bonding techniques are of great interest to future progress in the microelectronics industry, and these results are promising advances.