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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Uncovering phase transformation, morphological evolution, and nanoscale color heterogeneity in tungsten oxide electrochromic materials

Controlling the electrochemical interfacial processes that govern the durability of electrochromic devices represents a key challenge in developing sustainable and cost-effective smart windows. Here, we revisit the classical tungsten trioxide (WO 3 ) materials as a platform to uncover the previously unknown interrelationship between phase transformation, morphological evolution, and nanoscale color heterogeneity in these materials. Through synchrotron/electron spectroscopic and imaging analyses, we report that the WO 3 –electrolyte interface is influenced by the tungsten dissolution and redeposition during the electrochemical cycling. The tungsten redeposition provokes the in situ crystal growth, which ultimately leads to the phase transformation from the semicrystalline WO 3 to a nanoflake-shaped, proton-trapped tungsten trioxide dihydrate (H x WO 3 · 2 H 2 O). The multidimensional quantification of the electronic structure reveals that the tungsten reduction caused by the proton trapping is heterogeneous at the nanometric scale and is responsible for the nanoscale color heterogeneity. The interplay between phase transformation, morphological evolution, and nanoscale color heterogeneity results in degraded optical modulation, switching kinetics, coulombic efficiency, and bleached-state transparency. Our results highlight that the high interfacial reactivity near the electrode surface may be the underlying mechanism for undesired bulk structural changes of electrochromic materials, and calls for fundamental studies in probing and controlling the electrode–electrolyte interfacial processes in electrochromic devices.

36 MATERIALS SCIENCE↗
NASA NTRS1992

Determination of cleavage planes and fracture characterization of Ni-based single crystal superalloys

The room temperature fracture behavior of the Ge Rene N-4, CMSX-2, and CMSX-4C single crystal Ni-based superalloys was studied. All crystals were grown along the (001) direction and tensile tested in both helium and hydrogen atmospheres. A stereoscopic technique developed for use with a scanning electron microscope was applied to determine cleavage planes. Planar gamma(') morphologies also were examined to help determine cleavage planes. Helium charged specimens failed on a number of planes including the (111), (110), and (320). In most cases planes of the (111)-type initiated at the notch region and became smaller and smaller as they moved in radially. Tensile strengths in helium averaged 1000 MPa higher than that of the hydrogen charged specimens. Specimens tested in hydrogen generally failed on (100)-type planes originating from the notch region. This (100) region comprised 60 to 80 percent of the total fracture surface on most samples and appeared as large flat planes perpendicular to the growth direction of the crystal. The interior regions contained (100)-type planes as well as (321), (320), (210), and (111)-types. Hydrogen charged specimens also showed a high percentage of large cracks oriented at 90 deg to one another, indicative of the (100)-type fracture. The Ge Rene N4 and the CMSX-4C samples contained 3-5 percent gamma/gamma(') eutectic, while the CMSX-2 samples had little or no gamma/gamma(') eutectic. The relationship between gamma/gamma(') eutectic and the fracture surface has not been fully determined, but it is thought that the gamma/gamma(') eutectic may serve as a possible trapping site for hydrogen.

Merrill, John M.↗
DOE OSTI2022

The fate of liquid crystal topological defects on chemically patterned surfaces during phase transitions

Controlling topological defects in liquid crystals (LCs) is an essential element in the development of areas such as directed self-assembly and micropatterning materials. However, during the phase transition on confined patterned surfaces, how the morphologies in one liquid crystalline phase change from deformations or defects into another phase is much less known. Here, we examine the fate of defects in a LC confined on a patterned surface during smectic-A–nematic and nematic–isotropic phase transitions, using experiments and simulation analyses. Upon heating from smectic-A to nematic, a Toric focal conical domain (TFCD) melts into a +1 converging boojum defect, which then transitioned into a concentric configuration as temperature increases, attributed to a steeper decrease of the bend and twist modulus compared to splay modulus. During cooling, TFCDs are developed from two distinct pathways depending on the cooling rates. Our continuum simulation recapitulates these transformations and provides elastic constant-based explanations for the two pathways. Although the phase transition pathways of defects are independent of the geometry of the confined patterns, the arrangement of FCDs is highly dependent on the size and shape of the patterns. Finally, taken together, this simple approach offers promising opportunities for tuning the micro- or nano-patterning of topological defects in liquid crystals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1991

Stability of Y-Ba-Cu-oxide phases in salt flux

The results of an investigation into the possibility of growing YBa2Cu3O7 (or Y123) single crystals from a NaCl-KCl flux (salt flux) are presented. They are compared with results for Y123 crystals grown from a Ba-Cu-oxide flux system. The chosen salt flux composition was 1:1 (NaCl:KCl). The preparation method of the solutes is presented, with particular attention given to the compounds surrounding the Y123 phase, such as Y2BaCuO6 (Y211). Optical microscopy was introduced to study the morphology of the sample; X-ray diffraction was used for phase identification; and a chemical analysis was effected. The Y123, Y211, and BaCuO2 phases were shown to be dissociated in the NaCl-KCl flux, and could not be recrystallized. A small amount of the flux could be added to create large-grain Y123 crystals, but single-crystal growth was not possible in a top-seeded NaCl-KCl solution. This is attributed to significant differences in solubility of the crystal elements, and phase separation caused by CuO crystal formation.

Tao, Y. K.↗
DOE OSTI2023

Diffusion growth mechanism of penta-twinned Ag nanocrystals from decahedral seeds

Crystals with penta-twinned structures can be produced from diverse fcc metals, but the mechanisms that control the final product shapes are still not well understood. By using the theory of absorbing Markov chains to account for the growth of penta-twinned decahedral seeds via atom deposition and surface diffusion, we predicted the formation of various types of products: decahedra, nanorods, and nanowires. We showed that the type of product depends on the morphology of the seed and that small differences between various seed morphologies can lead to significantly different products. For the case of uncapped decahedra seeds, we compared predictions from our model to nanowire morphologies obtained in two different experiments and obtained favorable agreement. Finally, possible extensions of our model are indicated.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Characterizing Self-Assembled Monolayer Breakdown in Area-Selective Atomic Layer Deposition

In order to enable area-selective atomic layer deposition (AS-ALD), self-assembled monolayers (SAMs) have been used as the surface inhibitor to block a variety of ALD processes. The integrity of the SAM throughout the ALD process is critical to AS-ALD. Despite the demonstrated effectiveness of inhibition by SAMs, nucleation during ALD eventually occurs on SAM-protected surfaces, but its impact on SAM structures is still not fully understood. In this study, we chose the octadecyltrichlorosilane (ODTS) SAM as a model system to investigate the evolution of crystallinity and structure of SAMs before and after ALD. The breakdown behavior of SAMs when exposed to ZnO and Al 2 O 3 ALD was systematically studied by combining synchrotron X-ray techniques and electron microscopy. We show that the crystallinity and structure of ODTS SAMs grown on Si substrates remain intact until a significant amount of material deposition takes place. In addition, the undesired ALD materials that grow on ODTS SAMs present contrasting morphologies: dispersed nanoparticles for ZnO while relatively continuous film for Al 2 O 3 . Lastly, substrate dependency was explored by comparing a Si substrate to single-crystal sapphire. Similar results in the evolution of SAM crystallinity and formation of ALD nuclei on top of SAM are observed in the ODTS–sapphire system. This study provides an in-depth view of the influence of ALD processes on the SAM structure and the nucleation behavior of ALD on SAM-protected surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

What we have learned about Mars from SNC meteorites

The SNC meteorites are thought to be igneous martian rocks, based on their young crystallization ages and a close match between the composition of gases implanted in them during shock and the atmosphere of Mars. A related meteorite, ALH84001, may be older and thus may represent ancient martian crust. These petrologically diverse basalts and ultramafic rocks are mostly cumulates, but their parent magmas share geochemical and radiogenic isotopic characteristics that suggest they may have formed by remelting the same mantle source region at different times. Information and inferences about martian geology drawn from these samples include the following: Planetary differentiation occured early at approximately 4.5 GA, probably concurrently with accretion. The martian mantle contains different abundances of moderately volatile and siderophile elements and is more Fe-rich than that of the Earth, which has implications for its mineralogy, density, and origin. The estimated core composition has a S abundance near the threshold value for inner core solidification. The former presence of a core dynamo may be suggested by remanent magnetization in Shergottite-Nakhlite-Chassignite (SNC) meteorites, although these rocks may have been magnetized during shock. The mineralogy of martian surface units, inferred from reflectance spectra, matches that of basaltic shergottites, but SNC lithologies thought to have crystallized in the subsurface are not presently recognized. The rheological properties of martian magmas are more accurately derived form these metorites than from observations of martian flow morphology, although the sampled range of magma compositions islimited. Estimates of planetary water abundance and the amount of outgassed water based on these meteorites are contridictory but overlap estimates based on geological observations and atmospheric measurements. Stable isotope measurements indicate that the martian hydrosphere experienced only limited exchange with the lithosphere, but it is in isotopic equilibrium with the atmosphere and has been since 1.3 Ga. The isotopically heavy atmosphere/hydrosphere composition deduced from these rocks reflects a loss process more severe than current atmospheric evolution models, and the occurence of carbonates in SNC meteorites suggest that they, rather than scapolite or hydrous carbonates, are the major crustal sink for CO2. Weathering products in SNC meteorites support the idea of limited alteration of the lithosphere by small volumes of saline, CO2-bearing water. Atmospheric composition and evolution are further constrained by noble gases in these meteorites, although Xe and Kr isotopes suggest different origins for the atmosphere. Planetary ejection of these rocks has promoted an advance in the understanding of impact physics, which has been accomplished by a model involving spallation during large cratering events. Ejection of all the SNC meteorites (except ALH84001) in one or two events may provide a plausible solution to most constraints imposed by chronology, geochemistry, and cosmic ray exposure, although problems remain with this scenario; ALH84001 may represent older martian crust sampled during a separate impact.

Mcsween, Harry Y., Jr.↗
NASA NTRS2015

Scaffold of Asymmetric Organic Compounds - Magnetite Plaquettes

Life on Earth shows preference towards the set of organics with particular spatial configurations, this 'selectivity' is a crucial criterion for life. With only rare exceptions, life prefers the left- (L-) form over the right- (D-) form of amino acids, resulting in an L-enantiomeric excess (L-ee). Recent studies have shown Lee for alpha-methyl amino acids in some chondrites. Since these amino acids have limited terrestrial occurrence, the origin of their stereoselectivity is nonbiological, and it seems appropriate to conclude that chiral asymmetry, the molecular characteristic that is common to all terrestrial life form, has an abiotic origin. A possible abiotic mechanism that can produce chiral asymmetry in meteoritic amino acids is their formation with the presence of asymmetric catalysts, as mineral crystallization can produce spatially asymmetric structures. Magnetite is shown to be an effective catalyst for the formation of amino acids that are commonly found in chondrites. Magnetite 'plaquettes' (or 'platelets'), first described by Jedwab, show an interesting morphology of barrel-shaped stacks of magnetite disks with an apparent dislocation-induced spiral growth that seem to be connected at the center. A recent study by Singh et al. has shown that magnetites can self-assemble into helical superstructures. Such molecular asymmetry could be inherited by adsorbed organic molecules. In order to understand the distribution of 'spiral' magnetites in different meteorite classes, as well as to investigate their apparent spiral configurations and possible correlation to molecular asymmetry, we observed polished sections of carbonaceous chondrites (CC) using scanning electron microscope (SEM) imaging. The sections were also studied by electron backscattered diffraction (EBSD) in order to reconstruct the crystal orientation along the stack of magnetite disks.

Chan, Q. H. S.↗
DOE OSTI2022

Not All Aggregates Are Made the Same: Distinct Structures of Solution Aggregates Drastically Modulate Assembly Pathways, Morphology, and Electronic Properties of Conjugated Polymers

Abstract Tuning structures of solution‐state aggregation and aggregation‐mediated assembly pathways of conjugated polymers is crucial for optimizing their solid‐state morphology and charge‐transport property. However, it remains challenging to unravel and control the exact structures of solution aggregates, let alone to modulate assembly pathways in a controlled fashion. Herein, aggregate structures of an isoindigo–bithiophene‐based polymer (PII‐2T) are modulated by tuning selectivity of the solvent toward the side chain versus the backbone, which leads to three distinct assembly pathways: direct crystallization from side‐chain‐associated amorphous aggregates, chiral liquid crystal (LC)‐mediated assembly from semicrystalline aggregates with side‐chain and backbone stacking, and random agglomeration from backbone‐stacked semicrystalline aggregates. Importantly, it is demonstrated that the amorphous solution aggregates, compared with semicrystalline ones, lead to significantly improved alignment and reduced paracrystalline disorder in the solid state due to direct crystallization during the meniscus‐guided coating process. Alignment quantified by the dichroic ratio is enhanced by up to 14‐fold, and the charge‐carrier mobility increases by a maximum of 20‐fold in films printed from amorphous aggregates compared to those from semicrystalline aggregates. This work shows that by tuning the precise structure of solution aggregates, the assembly pathways and the resulting thin‐film morphology and device properties can be drastically tuned.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Anion Exchange Ionomers: Impact of Chemistry on Thin-Film Properties

Ionomer thin-films (i.e., 20–100 nm) on supports serve as model systems to understand ionomer-catalyst interfacial behavior as well as the confinement-driven deviation in properties from bulk membranes. While ionomer thin-films have been examined for proton exchange ionomers, the thin-film properties of anion exchange ionomers (AEIs) remain largely unexplored. More importantly, delineating the convoluted impact of chemistry and confinement on thin-film morphology and hydration is of interest to advancing the field on functional ionic interfaces. In this work, these aspects are studied by using AEIs of different backbones (perfluorinated, aliphatic, and aromatic) and side chains (various lengths, and single versus dual functional groups). Quartz-crystal microbalance and spectroscopic ellipsometry are used to analyze density and coupled with calculated free volume fraction of thin-films to provide insights on their gas transport properties. AEI side-chain's chemical character plays a key role in how confinement modulates hydration (in thin-film versus bulk). Overall, the results elucidate the effects of backbone, side-chain chemistry versus anion/cation type in the confinement-driven changes in thin-film morphology and swelling. This study also provides new insights for tuning AEI transport functionalities at interfaces via chemistry, which can benefit the design and development of electrode-ionomers for alkaline membrane-based energy systems.

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

Crystals of Benzamide, the First Polymorphous Molecular Compound, Are Helicoidal

Abstract The growth of spontaneously twisted crystals is a common but poorly understood phenomenon. An analysis of the formation of twisted crystals of a metastable benzamide polymorph (form II ) crystallizing from highly supersaturated aqueous and ethanol solutions is given here. Benzamide, the first polymorphic molecular crystal reported (1832), would have been the first helicoidal crystal observed had the original authors undertaken an analysis by light microscopy. Polymorphism and twisting frequently concur as they are both associated with high thermodynamic driving forces for crystallization. Optical and electron microscopies as well as electron and powder X‐ray diffraction reveal a complex lamellar structure of benzamide form II needle‐like crystals. The internal stress produced by the overgrowth of lamellae is shown to be able to create a twist moment that is responsible for the observed non‐classical morphologies.

Shtukenberg, Alexander G.↗
DOE OSTI2020

Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo-BiVO 4 Thin Films

The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO 4 . By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V 2 O 5 at grain boundaries of Mo-BiVO 4 thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO 4 . After photo-electrochemical operation, the degraded Mo-BiVO 4 films show similar grain center and grain boundary spectra indicating V 2 O 5 dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synthesis and Characterization of MoNb Films Superconducting at 100-200 mK

We have developed a new transition-edge sensor material with critical temperature (T-c) in the range 100-200 mK. The new material is a solid solution of two superconducting components, MoxNb1-x, co-sputtered from two high-purity single-component targets (Mo and Nb). The T-c has a minimum (dT(c)/dx = 0) at an intermediate concentration of the components. We have optimized the deposition parameters and composition to provide films with a sharp superconducting transition at ~ 150 mK. We investigated structural features of the films and surface morphology using X-ray diffraction (XRD) and scanning electron microscopy. The XRD measurements indicate that the grown films are polycrystalline, with a preferred orientation along the (110) crystal direction and a clear correlation between superconducting properties and film microstructure.

superconducting alloys↗
DOE OSTI2021

The Dynamic of Complex Two-Phase Mixtures During Coarsening: From Dendritic to Bicontinuous Mixtures

Dendrites are tree-like crystals that frequently form during the solidification of castings. In systems with nearly isotropic interfacial energy, these dendrites possess secondary and sometimes even tertiary arms, and lead to morphologically complex two-phase mixtures. Dealloying, in which one component is selectively leached, leads to nanoporous structures that have proven useful for many technological applications. These bicontinuous two-phase mixtures are topologically and morphologically complex with spatially varying mean and Gaussian curvatures. The evolution of these structures during coarsening is of considerable technological importance since the morphology of the dendrites and nanoporous structure sets the properties of the resulting material. During coarsening, the total surface area per volume of the mixture decreases, thus resulting in an increase in the size-scale of the two-phase mixture. The evolution of interfacial morphology of these interconnected two-phase mixtures is being studied using large-scale computer simulations. The simulations are used to develop a theory of coarsening in these systems that will elucidate the impact of the complicated morphology found in dendritic and nanoporous systems on the coarsening process. The experiments employ four-dimensional X-ray microtomography, which enables the morphology of the two-phase mixture to be followed in three dimensions and time (an additional dimension). The experiments also serve as a test of simulations and theory of the coarsening process, as well as to provide initial conditions for the simulations. Through these experiments and theory, we aim to develop a comprehensive description of coarsening in these complex and technologically important two-phase mixtures.

36 MATERIALS SCIENCE↗
NASA NTRS2000

Phase Shift Interferometer and Growth Set Up to Step Pattern Formation During Growth From Solutions. Influence of the Oscillatory solution Flow on Stability

We have assembled an experimental setup based on Michelson interferometry with the growing crystal surface as one of the reflective surfaces. The crystallization part of the device allows optical monitoring of a face of a crystal growing at temperature stable within 0.05 C in a flow of solution of controlled direction and speed. The reference arm of the interferometer contains a liquid crystal element that allows controlled shifts of the phase of the interferograms. We employ an image-processing algorithm, which combines five images with a pi/2 phase difference between each pair of images. The images are transferred to a computer by a camera capable of capturing 60 frames per second. The device allows data collection on surface morphology and kinetics during the face layers growth over a relatively large area (approximately 4 sq. mm) in situ and in real time during growth. The estimated depth resolution of the phase shifting interferometry is approximately 50 Angstroms. The data will be analyzed in order to reveal and monitor step bunching during the growth process. The crystal chosen as a model for study in this work is KH2PO4 (KDP). This optically non-linear material is widely used in frequency doubling applications. There have been a number of studies of the kinetics of KDP crystallization that can serve as a benchmark for our investigations. However, so far, systematic quantitative characteristics of step interaction and bunching are missing. We intend to present our first quantitative results on the onset, initial stages and development of instabilities in moving step trains on vicinal crystal surfaces at varying supersaturation, flow rate, and flow direction. Behavior of a vicinal face growing from solution flowing normal to the steps and periodically changing its direction in time was considered theoretically. It was found that this oscillating flow reduces both stabilization and destabilization effects resulted from the unidirectional solution flow directed up the step stream and down the step stream. This reduction of stabilization and destabilization comes from effective mixing which entangles the phase shifts between the spatially periodic interface perturbation and the concentration wave induced by this perturbation. Numerical results and simplified mixing criterion will be discussed.

Chernov, Alex A.↗
DOE OSTI2020

A Mechanistic Understanding of Nonclassical Crystal Growth in Hydrothermally Synthesized Sodium Yttrium Fluoride Nanowires

Sodium yttrium fluoride (NaYF 4 ) is an important upconverting material with many potential uses in chemistry, materials science, and biology, which can be synthesized hydrothermally in both cubic (α) and hexagonal (β) crystallographic polymorphs. Understanding the mechanisms underlying the phase conversion between the cubic and hexagonal polymorphs is of great interest to help inform future synthetic efforts, for example in the design of atomically precise quantum materials with well-defined sizes and morphologies. In this work, we use a combination of analytical transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), powder X-ray diffraction (XRD), in situ liquid cell TEM, atom probe tomography (APT), and extended X-ray absorption fine structure (EXAFS) measurements to show that the hexagonal NaYF 4 nanowires form through a nonclassical crystal growth mechanism involving the formation and subsequent oriented attachment of mesocrystals consisting of cubic (α) phase units. Additionally, EXAFS measurements suggest that substitutional Yb 3+ point defects within NaYF 4 are distributed evenly throughout the crystal lattice without clustering and also that they may exhibit selective substitution into one of the two possible trivalent yttrium sites in the unit cell for hydrothermally synthesized β-NaYF 4 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Characteristics of Non-Fullerene Acceptor-Based Organic Photovoltaic Active Layers Using X-ray Scattering and Solid-State NMR

With the recent development of non-fullerene acceptors, power conversion efficiencies of bulk-heterojunction organic solar cells have exceeded 18%. The morphology of the BHJ active layer, including packing, ordering, orientation, and phase behavior of the donor(s) and acceptor(s), plays critical roles in determining the device performance. We characterized the morphology of active layers consisting of mixtures of a PTB7/PTB7-Th (donor) and ITIC (acceptor) using grazing incidence wide-angle X-ray scattering, resonant soft X-ray scattering, and solid-state nuclear magnetic resonance (ssNMR) to correlate the morphology with device performance. PTB7-Th/ITIC shows a better device performance than that of PTB7/ITIC due to a smaller π-πstacking distance and smaller domain size of the phase separated morphology. One of PTB7/ITIC samples, processed from mixed solvents (chlorobenzene/benzene = 1:1 by volume), shows a possible partial miscibility and smaller domain size as revealed by ssNMR T 1ρ relaxation times, which is detrimental to device performance. ssNMR results showed that ITIC could crystallize into different forms depending on processing conditions, which may have implications on the manufacturing of devices using it as an ingredient, as well as its long-term stability in service.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗