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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Emergent interface vibrational structure of oxide superlattices

As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices. However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity, are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries, the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate–calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Reorganization of Water at Aqueous Aluminum Chloride (AlCl 3 ) Interfaces: Vibrational Sum Frequency Generation and Molecular Dynamics Simulations

AlCl 3 hydration states and complexation are not well understood both in solutions and at the air–aqueous interface despite their potential significance in natural waters and their industrial and energy-related applications. Here, we investigated Al 3+ and Cl – ion behaviors in an AlCl 3 aqueous bulk solution and at the air–aqueous interface using interface-selective vibrational sum frequency generation (SFG), Raman and infrared spectroscopies, molecular dynamics (MD) simulation, as well as molecular-informed reduced modeling. Our reduced modeling reveals relatively long-range effects for Al 3+ as compared to monovalent ions such as Na + indicating that the interfacial depth of trivalent ions can be significantly larger than that of monovalent ions at the air–water interface. MD simulations reveal interfacial stratification and multiple layering of the ions. Compression of the Al 3+ and Cl – distributions with increasing concentrations from 0.5 to 2.5 m is also observed in the subsurface regions. Significant SSP- and PPP-polarized SFG OH spectral intensity increases are observed from 0.5 to 1.5 m and 0.5 to 2.5 m, respectively, indicative of interfacial depth increases and a change in average orientation above 1.5 m. Extensive evaluation of SFG spectra, Fresnel-corrected using several approaches, shows the same trends. The nonmonotonic trend points to a changing structure in surface and subsurface water orientation and hydrogen bonding environment generally consistent with the MD simulation of stratification and water orientation changes. Furthermore, solvent-shared ion pairing is implicated with MD simulation radial distribution analysis and consistent with infrared spectral identification of the hexaaqua aluminum ion in the solution phase. Spectral evidence of a strong Al 3+ hydration shell and the acidic behavior of the Al 3+ ions is obvious in the Raman and infrared spectra of the bulk solution. In conclusion, we show that the MD dipole potential is directly related to the MD second-order susceptibility of the interface, χ SFG–MD (2) , both of which correlate up to ∼35 Å with the spectral observations of increasing and then saturating intensities, suggesting that both ion stratification and interfacial depth determine the water orientations at an air–water interface of 1-3 electrolyte solutions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Competing mechanisms at vibrated interfaces of density-contrast fluids

Fluid-fluid interfacial instability and subsequent fluid mixing are ubiquitous in nature and engineering. The hydrodynamic instability of fluid interfaces has long centered on the pressure gradient-driven long-wavelength Rayleigh-Taylor instability and the resonance-induced short-wavelength Faraday instability. However, neither instability alone can explain the dynamics when both mechanisms are present. We identify a previously unseen multi-modal instability emerging from their coexistence. When the denser fluid is polydimethylsiloxane, the mixed region at a high density contrast (Atwood number = 0.9) spans a vibration amplitude range approximately twice the gravitational acceleration. Using Floquet stability analysis, we show how vibrations govern transitions between the RT and Faraday instabilities, leading to contention between these instabilities rather than resonant enhancement. Here, the initial transient growth is represented by the exponential modal growth of the most unstable Floquet exponent, along with its accompanying periodic behavior. Direct numerical simulations validate these findings and track interface breakup into the multiscale and nonlinear regimes. Specifically, we show that growing RT modes nonlinearly suppresses Faraday responses even when the initial growth rate of the Faraday instability is 3.63 times that of RT, so a bidirectional competition hinders their sustained coexistence.

Direct numerical simulations↗
DOE OSTI2024

Nonequivalent Atomic Vibrations at Interfaces in a Polar Superlattice

In heterostructures made from polar materials, e.g., AlN–GaN–AlN, the nonequivalence of the two interfaces is long recognized as a critical aspect of their electronic properties; in that, they host different 2D carrier gases. Interfaces play an important role in the vibrational properties of materials, where interface states enhance thermal conductivity and can generate unique infrared-optical activity. The nonequivalence of the corresponding interface atomic vibrations, however, is not investigated so far due to a lack of experimental techniques with both high spatial and high spectral resolution. Herein, the nonequivalence of AlN–(Al 0.65 Ga 0.35 )N and (Al 0.65 Ga 0.35 )N–AlN interface vibrations is experimentally demonstrated using monochromated electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and density-functional-theory (DFT) calculations are employed to gain insights in the physical origins of observations. It is demonstrated that STEM-EELS possesses sensitivity to the displacement vector of the vibrational modes as well as the frequency, which is as critical to understanding vibrations as polarization in optical spectroscopies. The combination enables direct mapping of the nonequivalent interface phonons between materials with different phonon polarizations. Furthermore, the results demonstrate the capacity to carefully assess the vibrational properties of complex heterostructures where interface states dominate the functional properties.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Probing specific ion effects at air-aqueous dibutyl phosphate interfaces using vibrational sum frequency generation spectroscopy

Molecular properties at air–liquid and liquid–liquid interface hold the key to many processes involving molecular transport across phase boundaries from aerosol formation to carbon cycling and material separation using solvent extraction techniques. Using dibutyl phosphate (DBP) as a representative for partially aqueous soluble surfactants, the specific ion effect (SIE) of the Hofmeister series cations Cs + , Na + , Li + , and Mg 2+ on the partition and interaction between surfactant molecules and water molecules in the air–aqueous interface are investigated using vibrational sum frequency generation spectroscopy and surface tension measurements. In the presence of 1 mM and 1M bulk aqueous phase ionic strength salt concentrations, fundamental qualitative relationships are observed for the salting out of DBP relative to bulk aqueous phase nitrate salt concentrations and the specific cations species. At 1 mM ionic strength, the interfacial charge and hence the interfacial potential modulates the electrostatic interactions; in particular, the counter cations partially screen the negatively charged interface induced by the DBP in a direct Hofmeister order. At 1M ionic strength, the electric field at the interface or interfacial potential is effectively neutralized, and the counter cations promote the partitioning of DBP to the interface depending on their specific interaction with the DBP head group and metal ion hydration properties. The present results lay a foundation to study SIEs of heavier metals on hydrophobic-aqueous DBP interfaces.

Louie, Christina [Washington State University, Pul↗
DOE OSTI2021

Probing the electrolyte/electrode interface with vibrational sum frequency generation spectroscopy: A review

Over the past decades, Lithium-ion batteries have seen extensive improvements, and as a result are now the primary choice in many applications for their power, energy, and durability. In recent years, battery cost has reduced by orders of magnitude through adoption of new materials and processes. Despite these advances, interfaces in these battery systems are yet to be fully understood. This is seen as a major limitation to further increase cycle life, calendar life, abuse tolerance, and performances. A major obstacle is a lack of comprehensive understanding of the complex dynamic chemical processes occurring at the electrolyte/electrode interface. In this context, vibrational sum frequency generation (vSFG) spectroscopy possesses the unique capability of probing a molecularly thin interfacial layer to obtain molecular-level information through nonlinear optical interaction. Probing the molecular level processes at the interfaces using such a versatile technique would be a game changer in the advancement of current battery research knowledge. This review article summarizes recent vSFG studies on the electrolyte/electrode interface of various electrode materials and nonaqueous electrolytes for LIBs and discusses future research perspectives. Finally, overall, this focused review highlights the advantages and versatility of vSFG that can be used to further advance present-day battery research.

25 ENERGY STORAGE↗
DOE OSTI2022

Recognition competes with hydration in anion-triggered monolayer formation of cyanostar supra-amphiphiles at aqueous interfaces

The triggered self-assembly of surfactants into organized layers at aqueous interfaces is important for creating adaptive nanosystems and understanding selective ion extraction. While these transformations require molecular recognition, the underlying driving forces are modified by the local environment in ways that are not well understood. Herein, we investigate the role of ion binding and ion hydration using cyanosurf, which is composed of the cyanostar macrocycle, and its binding to anions that are either size-matched or mis-matched and either weakly or highly hydrated. We utilize the supra-amphiphile concept where anion binding converts cyanosurf into a charged and amphiphilic complex triggering its self-organization into monolayers at the air–water interface. Initially, cyanosurf forms aggregates at the surface of a pure water solution. When the weakly hydrated and size-matched hexafluorophosphate (PF 6 - ) and perchlorate (ClO 4 - ) anions are added, the macrocycles form distinct monolayer architectures. Surface-pressure isotherms reveal significant reorganization of the surface-active molecules upon anion binding while infrared reflection absorption spectroscopy show the ion-bound complexes are well ordered at the interface. Vibrational sum frequency generation spectroscopy shows the water molecules in the interfacial region are highly ordered in response to the charged monolayer of cyanosurf complexes. Consistent with the importance of recognition, we find the smaller mis-matched chloride does not trigger the transformation. However, the size-matched phosphate (H 2 PO 4 - ) also does not trigger monolayer formation indicating hydration inhibits its interfacial binding. These studies reveal how anion-selective recognition and hydration both control the binding and thus the switching of a responsive molecular interface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Probing Heterogeneous Charge Distributions at the α-Al 2 O 3 (0001)/H 2 O Interface

Unlike metal or semiconductor electrodes, the surface charge resulting from the protonation or deprotonation of insulating mineral oxides is highly localized and heterogeneous in nature. In this work the Stark active C≡N stretch of potassium thiocyanate is used as a molecular probe of the heterogeneity of the interfacial electrostatic potential at the α-Al 2 O 3 (0001)/H 2 O interface. Vibrational sum frequency generation (vSFG) measurements performed in the OH stretching region suggest that thiocyanate species organize interfacial water similarly to halide ions. Changes in the electrostatic potential are then tracked via Stark shifts of the vibrational frequency of the thiocyanate stretch. Our vSFG measurements show that we can simultaneously measure the vSFG response of SCN – ions experiencing charged and neutral surface sites. We assign local potentials of +308 and -154 mV to positively and negatively charged aluminol groups that are present at pH = 4 and pH = 10, respectively. Thiocyanate anions at positively charged surface sites and negatively charged surface sites and those participating in contact ion pairing adopt similar orientations and are oppositely oriented relative to thiocyanate ions near neutral surface sites. All four species followed Langmuir adsorption isotherms. Density functional theory–molecular dynamics (DFT-MD) simulations of SCN – near the neutral α-Al 2 O 3 (0001)/H 2 O interface show that the vSFG response in the C≡N stretch region originates from a SCN–H–O–Al complex, suggesting the surface site specificity of these experiments. To our knowledge this is the first spectroscopic measurement of local potentials associated with a heterogeneously charged surface. The ability to probe the evolution of local charges in situ could provide vital insight into many industrial, electrochemical, and geochemically relevant interfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1975

Heat flow and convection experiments aboard Apollo 17

Experiments conducted aboard Apollo 17 showed that in uncovered liquids, convection driven by surface tension can occur at lower temperature gradients in low gravity than in 1 g. In completely confined fluids (no liquid-gas interface), vibrations caused by spacecraft and astronaut movements increased the heat transfer considerably over the pure conduction case.

Grodzka, P. G.↗
DOE OSTI2020

Competitive Adsorption of Ionic Liquids Versus Friction Modifier and Anti-Wear Additive at Solid/Lubricant Interface—Speciation with Vibrational Sum Frequency Generation Spectroscopy

A modern lubricant contains various additives with different functionalities and the interactions or reactions between these additives could induce synergistic or antagonistic effects in tribological performance. In this study, sum frequency generation (SFG) spectroscopy was used to investigate competitive adsorption of lubricant additives at a solid/base oil interface. A silica substrate was used as a model solid surface. The lubricant additives studied here included two oil-soluble ionic liquids (ILs, [N888H][DEHP] and [P8888][DEHP]), an antiwear additive (secondary ZDDP), an organic friction modifier (OFM), and a dispersant (PIBSI). Our results showed that for mixtures of ZDDP and IL in a base oil (PAO4), the silica surface is dominated by the IL molecules. In the cases of base oils containing OFM and IL, the silica/lubricant interface is dominated by OFM over [N888H][DEHP], while it is preferentially occupied by [P8888][DEHP] over OFM. The presence of PIBSI in the mixture of PAO4 and IL leads to the formation of a mixed surface layer at the silica surface with PIBSI as a major component. The SFG results in this investigation provide fundamental insights that are helpful to design the formulation of new lubricant additives of desired properties.

25 ENERGY STORAGE↗
NASA NTRS1994

Hardware interface for isolation of vibrations in flexible manipulators: Development and applications

NASA's Langley Research Center (LaRC) is addressing the problem of isolating the vibrations of the Shuttle remote manipulator system (RMS) from its end-effector and/or payload by modeling an RMS flat-floor simulator with a dynamic payload. Analysis of the model can lead to control techniques that will improve the speed, accuracy, and safety of the RMS in capturing satellites and eventually facilitate berthing with the space station. Rockwell International Corporation, also involved in vibration isolation, has developed a hardware interface unit to isolate the end-effector from the vibrations of an arm on a Shuttle robotic tile processing system (RTPS). To apply the RTPS isolation techniques to long-reach arms like the RMS, engineers have modeled the dynamics of the hardware interface unit with simulation software. By integrating the Rockwell interface model with the NASA LaRC RMS simulator model, investigators can study the use of a hardware interface to isolate dynamic payloads from the RMS. The interface unit uses both active and passive compliance and damping for vibration isolation. Thus equipped, the RMS could be used as a telemanipulator with control characteristics for capture and berthing operations. The hardware interface also has applications in industry.

Manouchehri, Davoud↗
DOE OSTI2021

Two-dimensional electronic–vibrational sum frequency spectroscopy for interactions of electronic and nuclear motions at interfaces

Significance Electronic–vibrational couplings are pivotal in many photo-induced processes at interfaces and surfaces. Two-dimensional electronic–vibrational sum frequency generation spectroscopy is a unique tool to decipher electronic–vibrational couplings for excited states of molecules at interfaces and surfaces, enabling the correlation of photo-induced electronic–vibrational manifolds with their subsequent time evolution of high-frequency vibrational modes. This technique is especially powerful to monitor ultrafast processes such as charge transfer, energy transfer, proton transfer, and proton-coupled charge transfer at interfaces and surfaces on a real-time basis.

Deng, Gang-Hua↗
DOE OSTI2023

Vibrational Probe at the Electrochemical Interface: Dependence on Plasmon Coupling and Potential of the Lineshape in Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared spectroscopy of vibrational probes at an electrode surface shows promise for studying the structural dynamics at an active electrochemical interface. This interface is a complex environment where the solution structures in response to the applied potential. A strategy for achieving the necessary monolayer sensitivity is to use a plasmonically active electrode, which enhances the electromagnetic fields that produce the spectroscopic response. Here, in this study, we show how the coupling between the plasmon and the vibrations of the molecular monolayer impacts the FTIR and 2D IR spectroscopy, with an emphasis on the electrochemical potential difference spectra. We show how mixing between the vibrational and plasmonic states gives rise to the distortions that are observed in these measurements. This provides an important step toward 2D IR measurements of vibrational probes at the electrochemical interface as a tool for probing the structural dynamics in the double layer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1987

Impact of space station appendage vibrations on the pointing performance of gimballed payloads

A study of the interface problems between the Space Station Structure (vibrations) and the Payload Pointing Control System was undertaken. A major goal of the study was to identify any bounding factors that might limit the achievement of required pointing accuracies. A major result is that the space station will have a disturbance-rich environment and the background levels will be large enough to impact the pointing of some of the payloads. The need for an interface vibration specification between the structure and the payloads was identified.

Hughes, Robert O.↗