Supporting the Scientific Base for Competencies Essential to Sandia Missions
The DOE Office of Science (SC) is the single largest supporter of basic research in the physical sciences in the U.S., providing more than 40 percent of total funding in this area. Sandia has active research programs funded by:
Advances environmental and biomedical knowledge that promotes national security through improved energy production, development, and use; international scientific leadership that underpins the nation’s technological advances; and research that improves the quality of life for all Americans.
Supports world-class, high-performance computing and networking infrastructures as well as supporting fundamental research in mathematical and computational sciences to enable researchers in DOE scientific disciplines to analyze and predict complex phenomena for scientific discovery.
ARPA-E is an innovative and collaborative government agency that brings together America’s best and brightest scientists, engineers, and entrepreneurs.
The focus of Sandia’s ARPA-E program is to establish partnerships with universities, industry and other National Labs to create innovative energy solutions for the Nation through both maturation of industry capabilities and commercialization of our technologies.
Address Stationary and Transportation Energy pillars
Leverage differentiating facilities/capabilities and Research Foundations of Sandia Labs
The continued reliance of the global transportation energy sector on nonrenewable fossil fuels is a major challenge to sustainability, due to concerns related to carbon emissions and dependence on a finite resource. There is growing importance in using biobased feedstocks as advanced renewable resources for the production of liquid transportation fuels.
Transforming polysaccharides present in nonfood biomass feedstocks into fermentable sugars is one of the keys to the biochemical conversion of biomass into renewable fuels and chemicals. The critical challenges in converting biomass into drop-in fuels and chemicals are associated with the compact packing of polysaccharides and their interactions with lignins.
The planet’s most abundant plant polysaccharide, cellulose, exists in nature as microcrystalline cellulose (I) with two distinct crystalline forms (Iα and Iβ) that possess triclinic and monoclinic unit cells, respectively (see box). The cellulose chains are held together strongly by hydrogen bonding (H-bonding) and stacking of glucose units. These must be disrupted, usually through a pretreatment process, into individual chains in order to increase substrate accessibility to hydrolytic enzymes, thus generating high fermentable-sugar yields.
In recent years, biomass pretreatment with certain ionic liquids has received considerable attention due to their superior dissolution capability of lignocellulosic biomass, very low vapor pressure, and relatively low flammability. A fundamental understanding on how these ionic liquids, in aqueous environments, act on cellulose, particularly at lower ionic-liquid concentrations with water as a cosolvent, is essential for optimizing pretreatment efficiency, lowering pretreatment cost, and improving ionic liquid recyclability. The ionic liquid 1-ethyl-3-methylimidazolium acetate (
[C2C1Im][OAc]) is one of the most efficient cellulose solvents known, greatly altering cellulose structure for improved enzymatic saccharification.
Final structural snapshots of the simulated system for 100 ns at (A) 300 K & (B) 433 K.
Understanding cellulose dissolution and regeneration in aqueous ionic liquid provides knowledge on (1) efficient cellulose dissolution, (2) ionic liquid recycle and recovery, and (3) biomass solute separations—all of which are critical factors to the rational design of a cost-effective ionic liquid pretreatment process. Comparing the cellulose dissolution process under different conditions indicates that temperature has a dominant effect on the cellulose chain dissolution process in the presence of [C2C1Im][OAc] with cellulose bundle remaining intact at 300 K, whereas it is disrupted at 433 K in pure [C2C1Im][OAc].
The paper describes the research team’s investigation of the dissolution mechanism of microcrystalline cellulose in different water ratios at room (300 K) and pretreatment (433 K) temperatures using all atom molecular dynamics (MD) simulations. To understand the role of water as a cosolvent with [C2C1Im][OAc], The team investigated the dissolution mechanism of microcrystalline cellulose, type Iβ, in different [C2C1Im][OAc]:water ratios at room (300 K) and pretreatment (433 K) temperatures using all atom MD simulations. The MD simulations suggest that levels of 50% to 80% [C2C1Im][OAc] can effectively break the H-bonding present in cellulose. On the other hand, the presence of water at certain concentration increases the diffusivity of cellulose in the medium and aids in cellulose dissolution.
Diffusion of cellulose at different 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]):water ratios.
These MD simulations show that 80:20 [C2C1Im][OAc]):water ratios should be considered as “the tipping point” above which [C2C1Im][OAc]:water mixtures are equally effective on decrystallization of cellulose by disrupting the interchain hydrogen bonding interactions. Simulations also reveal that the resulting decrystallized cellulose from 100% [C2C1Im][OAc] begins to repack in the presence of water but into a less crystalline, or more amorphous, form.
The knowledge gained from this study provides a better understanding of the dual role played by the water (as a cosolvent/antisolvent) in dissolving cellulose. Evidence from this study provides possible clues for the targeted design of ionic liquid−water mixtures that are effective for pretreatment of biomass. Furthermore, this work presents a more general computational method for the selective identification of the mixtures of ionic liquid:water solvent systems that are necessary for dissolution of cellulose.
Researchers at the Joint BioEnergy Institute (JBEI) are working to transform biomass into energy-rich fuel molecules.
Headquartered in Emeryville, California, the Joint BioEnergy Institute (JBEI) is now a member of the elite “100/ 500 Club,” having filed its 100th patent application and published its 500th scientific paper. (Photo by Roy Kaltschmidt)
“We are a basic-science research institute, but are focused on the particular problems of biomass-to-biofuels transformations,” says Jay Keasling, JBEI Principal Investigator and Chief Executive Officer, just after greeting a visitor to the lab. And he is very particular about which such problems he wants his team to tackle. “We work on risky future stuff—what no company in its right mind would take on. It’s not corn, yeast, and ethanol—those are not interesting.”
Breakdown: Intact plant cell walls (top) are degraded after 40 minutes of treatment with the ionic liquid 1-ethyl-3-methylimidazolium acetate (bottom). (Image credit: Lawrence Berkeley National Laboratory)
Instead, JBEI researchers are zeroing in on concepts that others have decided are too difficult or would take too long to prove. Can a plant’s cell walls be engineered to more readily release its sugar building blocks? Is there a new chemistry that can break cell walls down? Can we identify a new fuel
[biomass] candidate by its chemical structure and then engineer a microbe capable of producing it from sugar?
Sandia’s JBEI researchers have developed ionic liquids (molten salts that are liquid at room temp.) to attack cell walls and processes that harness their strong polarity to invade plant cell walls. They are pursuing several avenues to make ionic-liquid pretreatment commercially viable.
Thirty two Sandia researchers participate in JBEI’s Deconstruction and Technology divisions, including five JBEI directors and Blake Simmons as JBEI Chief Science & Technology Officer and Vice President of Deconstruction.
Researchers Joe Oefelein and Guilhem Lacaze (both in Sandia’s Reacting Flow Research Dept.) won the American Institute of Aeronautics and Astronautics’ (AIAA’s) best paper award for their work on scramjet engine simulations. The paper, “A Priori Analysis of Flamelet-Based Modeling for a Dual-Mode Scramjet Combustor,” was a collaboration with Jesse Quinlan and James McDaniel (Univ. of Virginia) and Tomasz Drozda (NASA Langley Research Center).
AIAA is the largest aerospace professional society in the world, serving a diverse range of more than 30,000 individual members from 88 countries and 95 corporate members. AIAA’s mission is to inspire and advance the future of aerospace for the benefit of humanity. Their award was presented by the AIAA High-Speed Air-Breathing Propulsion Technical Committee for accomplishment in the arts, sciences, and technology of air-breathing propulsion systems.
Sandia researchers Joe Oefelein (left) and Guilhem Lacaze, recently recognized for their combustion-simulation work with a best paper award, discuss their work on scramjet engine simulations. (Photo by Loren Stacks)
Their paper presents a detailed analysis of combustion regimes in a scramjet, an engine that operates at super- to hypersonic speed and will be used in the future for military, point-to-point transport, and access-to-space applications. “The results presented in the paper are an excellent example of how collaborative teams across institutions can combine their expertise to provide new knowledge supporting the development of predictive combustion models for these systems,” Oefelein said.
The transition of a scramjet engine from dual-mode operation to scram-mode operation occurs in flight as a vehicle accelerates along its flight trajectory. During this transition, the combustion changes from primarily subsonic to largely supersonic. Understanding and predicting the physics of this transition is important to maintaining robust engine operation. Experimental investigations of dual-mode transition are typically limited by the inability of current hypersonic test facilities to vary the flow Mach number in real time through the transition process.
In their research, they modeled JP-7 fuel combustion chemistry using a 22 species, 18-step chemical reaction mechanism. They compared simulation results to experimentally obtained, time-averaged, wall-pressure measurements to validate their simulation solutions. Their analysis of the flowpath’s dual-mode operation showed regions of predominately nonpremixed, high-Damköhler number*, combustion. Regions of premixed combustion were also present, but associated with only a small fraction of the flow’s total heat-release. Their results also suggest that for scram-mode flowpath operation, the primary injector flames exhibit mixed combustion modes, in which significant heat release was found in regions of both nonpremixed and premixed conditions and at both moderate (Da = 1) and high (Da >>1) Damköhler numbers. Detailed analysis of the premixed combustion data suggest thin and broken reaction zones.
The research team found the effects of compressibility and heat losses have a significant effect on combustion—suggesting that a suitable flamelet manifold’s defining parameters should be pressure and enthalpy. Their analysis of the temperature and pressure at theoretical flamelet boundaries further supported the necessity of including pressure and enthalpy as manifold dimensions and suggested that the standard practice of using a single set of flamelet boundary conditions is an approximation.
The methodology’s critical component, apart from the details of the model itself, is the use of the a priori data in building a suitable flamelet table. After determining the applicability of flamelet models for a given flow field, a priori data regarding the flames must be elicited from a prior simulation or available experimental data. These data plots show the PDFs of static temperature and pressure (f[sub]T and f[sub]P, respectively) for fuel (Z > 0.99) and oxidizer (Z < 0.01) conditions for cases D584A (top) and S800A (bottom). Regions from which the data are sampled are shown above the respective plots, with the leftmost representing the primary injector flames and the rightmost representing the secondary injector flames.
This work fits into the philosophy of Sandia’s Combustion Research Facility where simulations complement experiments and bring key insights to improve real engines. “Because of the extreme velocities, experiments are rare and limited. That’s why we simulate those systems, to better understand how to optimize them,” Lacaze said. “To perform those simulations, we need to use models that accurately represent the flame, and our paper shows which approach is the most relevant and why.”
The study will help define the best simulation techniques needed to optimize future scramjets. Improved numerical accuracy at lower cost should help designers explore the key design attributes required for supersonic engine breakthroughs. The work has also helped establish new funding for Sandia through an award from the Defense Advanced Research Projects Agency (DARPA) involving uncertainty quantification of scramjet combustion.
The theme of the Nanomechanics and Nanometallurgy of Boundaries project at Sandia National Labs is to understand the connection between mechanical deformation and grain boundary instability in nanocrystalline metals. Our prior observations have led to the hypothesis that fatigue-induced abnormal grain growth is a precursor to crack initiation in nanocrystalline metals
[Padilla et al, Metall. Mater. Trans. A, 2011]. The development of this new technique is an instrumental step forward in connecting mechanical performance to rare microstructural events. Capturing the abnormal grain growth process associated with nanocrystalline fatigue was particularly problematic because it had only ever been observed by post-mortem fractography where the final fracture was needed to locate a ‘needle in the haystack’. While we have been developing a parallel effort to observe grain boundary mechanisms during high-cycle fatigue in the TEM, that effort will always be impeded somewhat by the limited interrogation volume of the TEM and hence will require a pre-existing starter notch/crack to localize the failure process.
While the experiments described in this paper were performed as a user at SSRL with a beamline limited to a ~100 μm monochromatic beam, we are currently exploring the extension of this technique to white beam experiments at the ALS or CHESS with either a microbeam or a broad ~1 mm scale beam, respectively. Both new sources also offer much faster area detectors that could greatly improve the techniques temporal ability to capture fatigue-induced (or otherwise thermomechancially induced) grain boundary instability. The technique could also be extended to cryogenic experiments where the kinetics for conventional boundary motion mechanisms are glacial. Finally, it is worth noting that this technique could apply to other materials science studies where there is a strongly bimodal grain size distribution, such as electrical steels (so-called Goss texture) or heterogeneous recrystallization phenomena.
In parallel with these in-situ characterization techniques, we are also exploring various metallurgical schemes to control grain boundary instability. While the current study is focused on a nanocrystalline Ni-based alloy, this technique could also be applied to emerging ‘thermodynamically stabilized’ alloys where grain boundary segregation is thought to eliminate the driving force for boundary motion [Abdeljawad and Foiles, Acta Materialia, 2015]. Such a stabilization scheme may one day offer nanocrystalline metals that are impervious or highly resistant to fatigue crack initation.
Recent experimental and theoretical results suggest that nanocrystalline metals can be thermodynamically stabilized by the free energy gain associated with the segregation of solute atoms to the grain boundary which will reduce grain boundary energy and so the driving force for nanocrystalline coarsening. A detail understanding of this proposed mechanism requires detangling the thermodynamics of the coupled bulk and interfacial regions of the metal. We have developed a novel diffuse-interface model of grain boundary segregation in binary metallic alloys that is capable of accounting for both bulk and interfacial energetics and the interaction of alloying elements with the grain boundary. The model presented here extends prior treatments by independently treating solute-solute interactions within both the bulk grains and the grain boundary regions. This alloys for the simultaneous treatment of both grain boundary segregation and phase separation processes.
Through analytic analysis of 1-D systems, the dependence of grain boundary energy on the alloy segregation energy parameters is examined and the results demonstrate that in order to obtain substantial reduction in the grain boundary energy and so thermodynamic stabilization of nanocrystalline metals that high concentrations of solute atoms is required. These analysis also show that this model can recover classic segregation isotherms such as the McLean isotherm and the Fowler-Guggenheim isotherm. Simulation of 2-D polycrystalline systems demonstrate that substantial retardation of grain growth can be obtained for appropriate alloy and segregation energetics.
In broader terms, this modeling approach provides an avenue to explore gran boundary solute segregation and its competition with phase separation in the understanding and design of hopefully stable nanocrystalline metals.
Future work will extend this model in three critical ways: (1) the influence of solute segregation on the grain boundary mobility, (2) a system with heterogeneous grain boundary properties will be incorporated, (3) the addition of a stress term in the energy functional to account for mechanically-induced instability. These model predictions could help guide alloy development for the purpose of boundary stabilization and aid in the mechanistic interpretation of binary alloy boundary evolution observations.