Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “crystallization”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Enhanced absorption in two-dimensional materials via Fano-resonant photonic crystals

The use of two-dimensional (2D) materials in optoelectronics has attracted much attention due to their fascinating optical and electrical properties. However, the low optical absorption of 2D materials arising from their atomic thickness limits the maximum attainable external quantum efficiency. For example, in the visible and near-infrared regimes monolayer MoS{sub 2} and graphene absorb only ∼10% and 2.3% of incoming light, respectively. Here, we experimentally demonstrate the use of Fano-resonant photonic crystals to significantly boost absorption in atomically thin materials. Using graphene as a test bed, we demonstrate that absorption in the monolayer thick material can be enhanced to 77% within the telecommunications band, the highest value reported to date. We also show that the absorption in the Fano-resonant structure is non-local, with light propagating up to 16 μm within the structure. This property is particularly beneficial in harvesting light from large areas in field-effect-transistor based graphene photodetectors in which separation of photo-generated carriers only occurs ∼0.2 μm adjacent to the graphene/electrode interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Isoelectronic Oxygen Centers and Conductivity of CdS Crystals Compared with PbS Crystals

The influence of oxygen on the electrical properties of PbS are well known. The goal of this study is to compare and reveal this phenomenon in CdS(O) single crystals, which we have studied in detail previously. The experiments are performed for CdS single crystals with a known oxygen concentration, deviation from stoichiometry, and definite set of intrinsic point defects. They allow us to confirm the results described for PbS and clarify their nature. It is shown that the phenomenon is based on the capture of free charge carriers— electrons—by acceptor-like isoelectronic O{sub S} centers with the subsequent formation of associates having a complex structure. Previous conclusions on the dissolution mechanism of oxygen in CdS with deviations from stoichiometry are confirmed. Variations in the electrical properties in oxygen-activated PbS(O), similarly to CdS(O), showed that isoelectronic oxygen centers O{sub S} are present in lead sulfide.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Single‐Crystal to Single‐Crystal Transformations: Stepwise CO 2 Insertions into Bridging Hydrides of [(NHC)CuH] 2 Complexes

Abstract Mechanistic studies of substrate insertion into dimeric [(NHC)CuH] 2 (NHC=N‐heterocyclic carbene) complexes with two bridging hydrides have been shown to require dimer dissociation to generate transient, highly reactive (NHC)Cu−H monomers in solution. Using single‐crystal to single‐crystal (SC‐SC) transformations, we discovered a new pathway of stepwise insertion of CO 2 into [(NHC)CuH] 2 without complete dissociation of the dimer. The first CO 2 insertion into dimeric [(IPr*OMe)CuH] 2 (IPr*OMe=N,N′‐bis(2,6‐bis(diphenylmethyl)‐4‐methoxy‐phenyl)imidazole‐2‐ylidene) produced a dicopper formate hydride [(IPr*OMe)Cu] 2 (μ‐1,3‐O 2 CH)(μ‐H). A second CO 2 insertion produced a dicopper bis(formate), [(IPr*OMe)Cu] 2 (μ‐1,3‐O 2 CH)(μ‐1,1‐O 2 CH), containing two different bonding modes of the bridging formate. These dicopper formate complexes are inaccessible from solution reactions since the dicopper core cleanly ruptures to monomeric complexes when dissolved in a solvent.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Locally resonant phononic crystals band-gap analysis on a two dimensional phononic crystal with a square and a triangular lattice

Locally resonant phononic crystals (LRPC) are a new type of sound insulating material. Using the plane wave expansion method based on the Bloch theorem, we compute the band structure of two dimensional (2D) phononic crystals (PC) with square and triangular lattices. Such PC typically consists of infinitely long carbon rods coated with silicon rubber and embedded in an elastic background. Computational results show that gaps appear at the lower frequency range, which are lower than those expected from the Bragg mechanism. Those gaps are generated due to local resonances; the optimum gap is obtained by tuning the thickness ratio of the coating layer. The gap created by the LRPC depends on the filling fraction of the coating cylinders.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Estimation of Crystal Lattice Deformations with Depth of Proton-Exchanged Optical Waveguides on Lithium Niobate Single Crystal Substrates

A method for determining the crystal phase and estimating the crystal lattice deformations in H{sub x}Li{sub 1-x}NbO{sub 3} proton-exchanged optical waveguides from an analysis of the Raman light scattering spectra measured with high spatial resolution (the so-called method of micro-RS spectroscopy) is suggested.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Crystal structure of glycoside hydrolase family 127 β-L-arabinofuranosidase from Bifidobacterium longum

Graphical abstract: - Highlights: • HypBA1 β-L-arabinofuranosidase belongs to glycoside hydrolase family 127. • Crystal structure of HypBA1 was determined. • HypBA1 consists of a catalytic barrel and two additional β-sandwich domains. • The active site contains a Zn{sup 2+} coordinated by glutamate and three cysteines. • A possible reaction mechanism involving cysteine as the nucleophile is proposed. - Abstract: Enzymes acting on β-linked arabinofuranosides have been unknown until recently, in spite of wide distribution of β-L-arabinofuranosyl oligosaccharides in plant cells. Recently, a β-L-arabinofuranosidase from the glycoside hydrolase family 127 (HypBA1) was discovered in the newly characterized degradation system of hydroxyproline-linked β-L-arabinooligosaccharides in the bacterium Bifidobacterium longum. Here, we report the crystal structure of HypBA1 in the ligand-free and β-L-arabinofuranose complex forms. The structure of HypBA1 consists of a catalytic barrel domain and two additional β-sandwich domains, with one β-sandwich domain involved in the formation of a dimer. Interestingly, there is an unprecedented metal-binding motif with Zn{sup 2+} coordinated by glutamate and three cysteines in the active site. The glutamate residue is located far from the anomeric carbon of the β-L-arabinofuranose ligand, but one cysteine residue is appropriately located for nucleophilic attack for glycosidic bond cleavage. The residues around the active site are highly conserved among GH127 members. Based on biochemical experiments and quantum mechanical calculations, a possible reaction mechanism involving cysteine as the nucleophile is proposed.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

A dislocation-based crystal plasticity framework for dynamic ductile failure of single crystals

We developed a framework for dislocation-based viscoplasticity and dynamic ductile failure to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. Furthermore, an averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. In addition, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in [J. Wilkerson and K. Ramesh. A dynamic void growth model governed by dislocation kinetics. J. Mech. Phys. Solids, 70:262–280, 2014.], which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural connection between gallium crystals and near- T m liquids under ambient pressure

We study the short-range structural order in liquid gallium prepared by ab initio molecular dynamics simulations. We found that at 400 K, which is close to the melting point (Tm) of Ga, the dominant motif in the liquid phase is identical to that in the stable solid phase Ga-I. Strong directional bonding in this motif prevents it from nucleation in the liquid phase. Meanwhile, a newly identified motif, topologically distinct from yet related to those in β-Ga and Ga-III phases, is also abundant in Ga liquid. This new motif could serve as precursors for β-Ga or Ga-III during crystallization.

Zhang, F.↗
DOE OSTI2021

Crystal Structure and Properties of Layered Pnictides BaCuSi2Pn3 (Pn = P, As)

Two novel layered compounds BaCuSi 2 Pn 3 (Pn = P, As) adopting new structure types are reported. As revealed by single-crystal X-ray diffraction, both compounds are composed of unique Cu–Si–Pn layers featuring CuPn 3 and Si 2 Pn 6 structural motifs found in other archetypal pnictide materials. The stacking of the isostructural Cu–Si–Pn layers is different for phosphide and arsenide compounds. Synthesis from elements aided by in situ synchrotron powder X-ray diffraction resulted in the obtainment of bulk powders with a minimized amount of admixtures. Experimentally measured physical properties of BaCuSi 2 As 3 unexpectedly showed metal-like behavior at temperatures above 15 K, despite the fact that density functional theory calculations predict a small band gap of 0.4 eV. BaCuSi 2 As 3 exhibits ultralow thermal conductivity, which can be explained by the combination of a layered crystal structure with alternating covalent and ionic bonding, which feature rattling of Cu atoms similar to that in tetrahedrites.

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

Crystal and Magnetic Structures of the Ternary Ho 2 Ni 0.8 Si 1.2 and Ho 2 Ni 0.8 Ge 1.2 Compounds: An Example of Intermetallics Crystallizing with the Zr 2 Ni 1–x P Prototype

We report two new rare-earth (R) ternary intermetallic compounds—Ho 2 Ni 0.8 T 1.2 with T = Si and Ge—that correspond to the R 5 Ni 2 T 3 phase earlier reported to form in Dy–Ni–T and Ho–Ni–T ternary systems. The compounds crystallize in a filled version of the orthorhombic Zr 2 Ni 1–x P-type structure with x = 0.52; their stoichiometry, determined from both single-crystal and powder X-ray diffraction data, is centered on Ho 2 Ni 0.8 T 1.2 with a narrow solid solubility range for the silicide, while the germanide appears to be a line phase. In addition to R = Dy and Ho, R 2 Ni 0.8 T 1.2 compounds also form for R = Y and Tb, representing the first examples of rare-earth-based compounds adopting the Zr 2 Ni 1–x P structural prototype. Bulk magnetization data reveal the main transitions of the ferrimagnetic or ferromagnetic type at TC = 38 K for Ho 2 Ni 0.8 Si 1.2 and TC = 37 K for Ho 2 Ni 0.8 Ge 1.2 , which are followed by subsequent magnetic reordering at lower temperatures. Neutron diffraction shows complex magnetic structures below T C with both ferromagnetic and antiferromagnetic components and magnetic propagation vector κ 1 = [0, 0, 0]. Below T N ≅ 24 K (22 K) for the silicide (germanide), an additional antiferromagnetic coupling following an incommensurate magnetic propagation vector κ 2 = [κ x , 0, 0] appears to coexist with the first magnetic structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Controlled Placement of Microparticles at the Water–Liquid Crystal Elastomer Interface

Control of cells behavior through topography of substrates is an important theme in biomedical applications. Among many materials used as substrates, polymers show advantages since they can be tailored by chemical functionalization. Fabrication of polymer substrates with nano- and microscale topography requires processing by lithography, microprinting, etching, etc. In this work, we introduce a different approach based on anisotropic elastic properties of polymerized smectic A (SmA) liquid crystal elastomer (LCE). When the SmA liquid crystal coating is deposited onto a substrate with planar alignment of the molecules, it develops nanogrooves at its free surface. After photopolymerization, these nanogrooves show an excellent ability to align human dermal fibroblasts (hDFs) over large areas. The alignment quality is good for both bare SmA LCE substrates and for substrates coated with fibronectin. The SmA LCE nano-topographies show a high potential for tissue engineering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Contraction and Expansion of Stimuli-Responsive DNA Bonds in Flexible Colloidal Crystals

DNA surface ligands can be used as programmable “bonds” to control the arrangement of nanoparticles into crystalline superlattices. Here, we study the intrinsic responsiveness of these DNA bonds to changes in local dielectric constant (ε r ) as a new approach to dynamically modulate superlattice structure. Remarkably, ethanol (EtOH) addition can be used to controllably tune DNA bond length from 16 to 3 nm and to increase bond stability by >40 °C, while retaining long-range order and crystal habit. Interestingly, we find that these structural changes, which involve the expansion and contraction of crystals by up to 75% in volume, occur in a cooperative fashion once a critical percentage of EtOH is reached. These results provide a facile and robust approach to create stimuli-responsive lattices, to access high volume fractions, and to improve thermal stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

In situ microscopy across scales for the characterization of crystal growth mechanisms: The case of europium oxalate

A better understanding of how production pathway affects the final product is required in order to produce targeted syntheses, but many of the classical ex situ techniques used for studying nanoparticle growth are unsuitable as stand-alone methods for identifying and characterizing growth mechanisms. Using a combination of high resolution transmission electron microscopy (TEM), cryogenic TEM, and fluid cell scanning electron microscopy, we monitor europium oxalate growth over the range of nanometers to tens of micrometers and identify potential crystal growth pathways. We also compare the effects of stirring during crystal growth as an example of how changing processing conditions changes growth mechanisms.

in situ SEM, Cryo TEM, lanthanide, crystal growth↗