SEARCH · Search NASA
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.
Time-resolved X-ray diffraction enabled mechanistic insight into the aluminothermic reduction of CeO 2 . The environmentally friendly process enables a direct route to Al–Ce alloy production and a high-value use for excess Ce from rare earth mining.
The visibility of pavement markings is one of the most critical factors for traffic safety, and a periodical assessment plan is crucial for maintaining this function. Traditional assessment methods, such as visual windshield surveys or manual testing using handheld devices, are unsafe, time-consuming, and labor-intensive. In recent years, transportation agencies have begun to adopt the use of mobile retroreflectivity units (MRUs) for condition assessment of pavement markings. MRUs, different from other manual methods, can be utilized to collect large-scale retroreflectivity data in an efficient manner. However, no relevant research has yet proposed a mathematical optimization model for arranging the evaluation schedule and paths of MRUs. This study aims to propose a MRU routing model, and an efficient solution methodology. A branch-and-price algorithm, including column generation and branch-and-bound, was implemented. Computational experiments have been conducted based on actual tasks from the Florida MRU program for validation. In conclusion, results show that the proposed solution methodology with a set partitioning model in this study not only finds the optimal solution for problems with tasks less than 60, but also effectively narrows the solution gap to be within 1.0% for problems with tasks less than 131.
In recent years, observations of highly ordered, hexagonal arrays of self-organized nanostructures on binary or impurity-laced targets under normal-incidence ion irradiation have excited interest in this phenomenon as a potential route to high-throughput, low-cost manufacture of nanoscale devices or nanostructured coatings. The currently prominent explanation for these structures is a morphological instability driven by ion erosion discovered by Bradley and Shipman; however, recent parameter estimates via molecular dynamics simulations suggest that this erosive instability may not be active for the representative GaSb system in which hexagonal structures were first observed. Motivated by recent experimental and numerical evidence suggesting the likely importance of phase separation during thin-film processing, we here generalize the Bradley-Shipman theory to include the effect of ion-assisted phase separation. The resulting system admits a chemically driven finite-wavelength instability that can explain the order of observed patterns even when the erosive Bradley-Shipman instability is inactive. In a relevant simplifying limit, it also provides an intuitive instability criterion similar to results in thin-film deposition, as well as predictions on pattern wavelengths that agree qualitatively with experimental observations. Finally, we identify a characteristic experimental signature that distinguishes the chemical and morphological instabilities and highlights the need for specific additional experimental data on the GaSb system.
The first-principles calculations are employed to provide a fundamental understanding of the structural features and relative stability, mechanical and electronic properties, and possible reactive route for chromium triboride. The predicted new phase of CrB{sub 3} belongs to the rhombohedral phase with R-3m symmetry and it transforms into a hexagonal phase with P-6m2 symmetry at 64 GPa. The mechanical and thermodynamic stabilities of CrB{sub 3} are verified by the calculated elastic constants and formation enthalpies. Also, the full phonon dispersion calculations confirm the dynamic stability of predicted CrB{sub 3}. Considering the role of metallic contributions, the calculated hardness values from our semiempirical method for rhombohedral and hexagonal phases are 23.8 GPa and 22.1 GPa, respectively. In addition, the large shear moduli, Young's moduli, low Poisson's ratios, and small B/G ratios indicate that they are potential hard materials. Relative enthalpy calculations with respect to possible constituents are also investigated to assess the prospects for phase formation and an attempt at high-pressure synthesis is suggested to obtain chromium triboride.
We have performed ab initio molecular dynamics simulations to study the alkali-metal dynamics in the Al-doped (KAl{sub 0.33}W{sub 1.67}O{sub 6} and RbAl{sub 0.33}W{sub 1.67}O{sub 6}) and undoped (KW{sub 2}O{sub 6} and RbW{sub 2}O{sub 6}) defect pyrochlore tungstates. The K atoms exhibit novel rattling dynamics in both the doped and undoped tungstates while the Rb atoms do not. The KAl{sub 0.33}W{sub 1.67}O{sub 6} experimental thermal conductivity curve shows an unusual depression between ∼50 K and ∼250 K, coinciding with two crossovers in the K dynamics: the first at ∼50 K, from oscillatory to diffusive, and the second at ∼250 K, from diffusive back to oscillatory. We found that the low-temperature crossover is a result of the system transitioning below the activation energy of the diffusive dynamics, whereas the high-temperature crossover is driven by a complex reconstruction of the local potential around the K atoms due to the cage dynamics. This leads to a hardening of the K potential with increasing temperature. This unusual reconstruction of the potential may have important implications for the interpretation of finite-temperature dynamics based on zero-temperature potentials in similar materials. The key result is that the novel K rattling, involving local diffusion, leads to a significant reduction in the thermal conductivity. We suggest that this may open a new route in the phonon engineering of cage compounds for thermoelectric materials, where the rattlers are specifically selected to reduce the lattice thermal conductivity by the mechanism of local diffusion.
The synthesis and characterization of cobalt doped barium titanate; BaTi{sub 0.96}Co{sub 0.04}O{sub 3} was investigated with a view to understand its structural and dielectric properties. A finest possible sample of Cobalt doped micro particles of BaTiO{sub 3} (BTO) with possible cubic structure via a solid-state route was prepared. Prepared samples were structural characterized by X-ray diffraction (XRD). The dielectric constant measurements of the sample above and below the Curie temperature were carried out at various frequencies. The Transition temperature is found shifted towards lower side from that of pure BaTiO{sub 3}.
Core-shell CdS/CdSe nanostructures have been synthesized by new microwave assisted ultrasonic sol gel route. The solution was obtained by dissolving cadmium acetate and Thiourea in the molar ratio 1:1 in Triethlioamine. The solution was Ultrasonically irradiated by Ultrasonic crystal at 40 Hz for 3 hours at 70°C. The sol was kept for another 24 hours for gel formation. Selenium dioxide was used as a selenium source and added separately. The gel was spin coated on Quartz and Glass slides followed by microwave heat treatment. The samples were characterized by structural morphological and optical characterization. XRD studies confirm the zinc blende phase of the CdS nanoparticles. The mean nanocrystal sizes calculated using Scherrer equation is ∼1.2nm. Optical studies show the strong blue shift in the spectra due to very small size of the nanocrystals. TEM and HRTEM confirm the formation of core shell structures.
Liesegang bands are formed when solutions of co-precipitate ions interdiffuse in a 1D gel matrix. In a recent study [R. F. Sultan, Acta. Mech. Sin. 27, 119 (2011)], Liesegang patterns have been characterized as fractal structures. In addition to experimentally obtained patterns, geometric Liesegang patterns were constructed in conformity with the well-known empirical laws. Both mathematical fractal dimensions and box count dimensions for images of PbF{sub 2} and PbI{sub 2} Liesegang patterns have been calculated. Liesegang patterns can also be described by the entropy state function, and categorized as more or less ordered structures. We revisit the relation between entropy and fractal dimension, and apply it to simulated geometrical Liesegang patterns. We have resort to three different routes for the estimation of the entropy of a Liesegang pattern. The HarFA software enabled the calculation of the Hausdorff dimension and the topological entropy, then the information dimension and the Shannon entropy. In a third pathway, analytical calculations were carried out by estimating the probability of occurrence of a fractal element or coverage. The product of Shannon entropy and Boltzmann constant yields the thermodynamic entropy. The values for PbF{sub 2} and PbI{sub 2} Liesegang patterns attained the order of magnitude of the reported Third Law entropies, but yet remained lower, in conformity with the more ordered Liesegang structures.
Ferroelectric nanowires are promising candidates for miniaturized ferroelectric devices. Some potential nanoscale applications of the nanowires, such as ultra high density ferroelectric memory, utilize their reversible polarization. To meet the ever increasing demand for low energy consumption, it is extremely desirable to reduce the operational fields associated with polarization reversal. In this Letter, we use first-principles-based simulations to explore an unusual route to polarization reversal that utilizes a combination of relatively low bias field and THz pulsed radiation. Such an approach allows for lower operational fields and may lead to other potential applications such as THz radiation sensing and remote switches.
We use multiple-tip Langmuir probes and fast imaging to unambiguously identify and study the dynamics of underlying instabilities during the controlled route to fully-developed plasma turbulence in a linear magnetized helicon plasma device. Langmuir probes measure radial profiles of electron temperature, plasma density and potential; from which we compute linear growth rates of instabilities, cross-phase between density and potential fluctuations, Reynold's stress, particle flux, vorticity, time-delay estimated velocity, etc. Fast imaging complements the 1D probe measurements by providing temporally and spatially resolved 2D details of plasma structures associated with the instabilities. We find that three radially separated plasma instabilities exist simultaneously. Density gradient driven resistive drift waves propagating in the electron diamagnetic drift direction separate the plasma into an edge region dominated by strong, velocity shear driven Kelvin-Helmholtz instabilities and a central core region which shows coherent Rayleigh-Taylor modes propagating in the ion diamagnetic drift direction. The simultaneous, complementary use of both probes and camera was crucial to identify the instabilities and understand the details of the very rich plasma dynamics.
We explored optical excitation pathways in the multivalley semiconductor Ge in an attempt to expedite selective electron injection into the indirect L-band-edge. An off-peak resonant excitation route was developed, which offers the pumping efficiency outperforming the phonon-assisted near-indirect-edge absorption by more than six orders of magnitude. The valley selectivity results from the intra-valley relaxation that separates electrons and holes in momentum space following excitation. Fortuitously, the widely used green laser, 532 nm, is found to be nearly ideally suited to the efficient L-valley-selective excitation in Ge. Such valley-specific pumping may help clarify the otherwise complicated electron dynamics involving intervalley processes.
In view of their technological impact in materials chemistry, a simplified and more efficient synthetic route to mesoporous films is highly sought. We report, herein, a smart UV-mediated approach coupling in a one-stage process sol-gel photopolymerization and photoinduced template decomposition/ablation to making mesoporous silica films. Performed at room temperature with a solvent-free solution of silicate precursor and amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer, the synthesis relies on photoacid generation to induce the fast formation (≈10 min) of mesostructured silica/surfactant domains. Continuation of UV exposure for three additional hours enables subsequent and complete photodegradation of the polyether copolymer, resulting in ordered or disordered mesoporous silica film. One of the most attractive features is that the one-step procedure relies on a continuous illumination provided by the same conventional medium-pressure Hg-Xe arc lamp equipped with a 254 nm reflector to enhance the emission of energetic photons <300 nm. In addition to X-ray diffraction and transmission electron microscopy, time-resolved Fourier transform infrared spectroscopy has proved to be a powerful in situ technique to probe the different chemical transformations accompanying irradiation. Photocalcination strengthens the inorganic network, while allowing to preserve a higher fraction of residual silanol groups compared with thermal calcination. A polyether chain degradation mechanism based on oxygen reactive species-mediated photo-oxidation is proposed.
In this paper, we present a facile method for production stable aqueous dispersion of ferrocene (FO) nanoparticles. Ferrocene compounds were employed to achieve stable nanodispersions, stabilized with three different biopolymers namely, alginate, CM-dextran and chitosan. The nanoparticles produce are spherical, less than 10 nm in mean diameter and highly stable without any sedimentation. Fourier infrared transform (FTIR) and X-ray diffraction (XRD) studies confirmed the purity of ferrocene nanoparticles there is no modifications occur during the preparation route. FTIR spectra results were consistent with the presence of absorption band of cyclopentadienyl ring (C{sub 5}H{sub 5}{sup −} ion) which assigned to ν(C-C) vibrations (1409 cm-1), δ(C-H) stretching at 1001 cm{sup −1} and π(C-H) vibrations at 812 cm{sup −1}. Furthermore, all functional group for biopolymers such as C O from carboxyl group of CM-dextran and sodium alginate appears at 1712 cm{sup −1} and 1709 cm{sup −1} respectively, indicating there are steric repulsion interactions for particles stabilization. Powder X-ray diffraction patterns of sedimented samples of the biopolymers-stabilized ferrocene (FO) showed all reflections which were indexed respectively to the (−110), (001), (−201), (−111), (200), (−211), (210), (120) and (111) according to the monoclinic phase ferrocene. This confirmed that the products obtained were of high purity of Fe and EDAX analysis also suggests that the presence of the Fe element in the colloidal dispersion.
2-Aminopropionitrile (APN), a probable candidate as a chiral astrophysical molecule, is a precursor to amino-acid alanine. Stereochemical pathways in 2-APN are explored using Global Reaction Route Mapping (GRRM) method employing high-level quantum-mechanical computations. Besides predicting the conventional mechanism for chiral inversion that proceeds through an achiral intermediate, a counterintuitive flipping mechanism is revealed for 2-APN through chiral intermediates explored using the GRRM. The feasibility of the proposed stereochemical pathways, in terms of the Gibbs free-energy change, is analyzed at the temperature conditions akin to the interstellar medium. Notably, the stereoinversion in 2-APN is observed to be more feasible than the dissociation of 2-APN and intermediates involved along the stereochemical pathways, and the flipping barrier is observed to be as low as 3.68 kJ/mol along one of the pathways. The pathways proposed for the inversion of chirality in 2-APN may provide significant insight into the extraterrestrial origin of life.
The structural and optical behavior of ZnO and ZnO doped with K synthesized by solution combustion route is reported in this article. The XRD patterns confirm the substitution of K atoms into ZnO lattice. There are no secondary peaks observed in the XRD patterns. The band gap of the K doped samples show a red shift on comparison with the bandgap of ZnO. The photoluminescence spectral study discloses the quenching behavior of UV emission and the aggrandizing blue emission when K content increases. The rich presence of defects is confirmed from the optical analysis and a practical mechanism, involving Zn{sub i} for the origin of the blue emission in ZnO is discussed here.
The controlled synthesis of epitaxial thin films offers opportunities for tuning their functional properties via enabling or suppressing strain relaxation. Examining differences in the epitaxial crystallization of amorphous oxide films, we report on an alternate, low-temperature route for strain engineering. Thin films of amorphous Bi–Fe–O were grown on (001)SrTiO{sub 3} and (001)LaAlO{sub 3} substrates via atomic layer deposition. In situ X-ray diffraction and X-ray photoelectron spectroscopy studies of the crystallization of the amorphous films into the epitaxial (001)BiFeO{sub 3} phase reveal distinct evolution profiles of crystallinity with temperature. While growth on (001)SrTiO{sub 3} results in a coherently strained film, the same films obtained on (001)LaAlO{sub 3} showed an unstrained, dislocation-rich interface, with an even lower temperature onset of the perovskite phase crystallization than in the case of (001)SrTiO{sub 3}. Our results demonstrate how the strain control in an epitaxial film can be accomplished via its crystallization from the amorphous state.
In this study, with the aid of Raman measurements, we have observed transformations in small (∼3 nm and ∼10 nm) free-standing Ge nanoparticles under laser light exposure. The nanoparticles were obtained by the chemical stain etching of a monocrystalline Ge wafer and of Ge powder and by colloidal synthesis route. We found that the transformation path depends on laser power and exposure time. At relatively low values of the laser power (2 mW) over a period of 100 min, the Raman signal indicates transformation of the sample from a nanocrystaline to bulk-like state, followed by partial oxidation and finally a conversion of the entire sample into alpha-quartz type GeO{sub 2}. However, when the laser power is set at 60 mW, we observed a heat release during an explosive crystallization of the nanocrystalline material into bulk Ge without noticeable signs of oxidation. Together with the transmission electron microscopy measurements, these results suggest that the chemical stain etching method for the preparation of porous Ge may not be a top-down process as has been widely considered, but a bottom up one. Systematic studies of the laser exposure on Ge nanoparticles prepared by colloidal synthesis results in the fact that the explosive crystallisation is common for H-terminated and partially disordered Ge nanoparticles regardless of its particle size. We suggest possible bio-medical applications for the observed phenomena.
With advancement in time, researchers has drawn great attention in the synthesis and characterization of mono dispersed alloyed nanocomposites of II-VI compounds. Ternary semiconductor alloyed ZnCdS quantum dots (QD’s) exhibit properties intermediate between those of ZnS and CdS. It shows high absorption coefficients, a composition tunable and size tunable band gap. Moreover, ZnCdS alloyed NC’s display unique composition dependent properties distinct from those of their bulk counterparts. The most striking feature of the alloyed NC’s nanocrystals is their unusual long time stability in emission wavelength. ZnCdS alloyed QD’s at room temperature has been synthesized using chemical precipitation method. Undoped and Mn{sup 2+} doped ZnCdS QDs have been synthesized and studied. UV-visible absorption spectrum shows absorbance in the visible region and photoluminescence (PL) emission spectra of the doped ZnCdS QD’s shows orange emission in comparison to weak blue emission from undoped QDs. The crystallite size is calculated from the XRD patterns. The experimental results indicate that this easy synthesis route would prove a versatile approach for the preparation of doped and undoped ZnCdS QD’s.