Outmost Shell Research Articles

Molecular dynamics simulations on the melting, crystallization, and energetic reaction behaviors of Al/Cu core-shell nanoparticles

Using molecular dynamics simulations combined with the embedded atom method potential, we investigate the heating, cooling, and energetic reacting of core-shell structured Al-Cu nanoparticles. The thermodynamic properties and structure evolution during continuous heating and cooling processes are also investigated through the characterization of the total potential energy distribution, mean-square-distance and radial distribution function. Some behaviors related to nanometer scale Cu/Al functional particles are derived that two-way diffusion of Al and Cu atoms, glass phase formation for the fast cooling rate, and the crystal phase formation for the low cooling rate. Two-way atomic diffusion occurs first and causes the melting and alloying. In the final alloying structure, Cu and Al atoms mixed very well except for the outmost shell which has more Al atoms. For the investigation of the thermal stability and energetic reaction properties, our study show that a localized alloying reaction between the Al core and Cu shell is very slow when the initial temperature is lower than 600 K. But a two-stage reaction may occur when the initial temperature is 700 K. The reaction rate is determined by the solid-state diffusion of Al atoms in the Cu shell at the first stage, yet the reaction rate is much faster at the second stage, due to the alloying reaction between the liquid Al core and the Cu shell. At higher temperatures such as 800 K and 900 K, the alloying reaction occurs directly between the liquid Al core and the Cu shell.

Triple layered core–shell structure with surface fluorinated ZnO-carbon nanotube composites and its electron emission properties

Core-shelled structures such as zinc oxide (ZnO) on carbon nanotubes (CNTs) give rise to interesting material properties. In this work, a triple-layered core–shell–shell structure is presented where the effects of fluorine (F) incorporation on the outmost shell of the ZnO-CNT structure are studied. The samples prepared ranged from a short 2min to a 30min immersion in carbon tetraflouride (CF4) plasma. In addition, its effects on the electron emission properties also studied and it is shown that the plasma immersions create thinner field emitters with sharp tiny wrinkles giving rise to more electron emission sites and higher enhancement factor. In addition, X-ray photoelectron spectroscopy measurements showed that F ions replace O in ZnO coatings during immersion process, thus increasing the electrical conductivity and shifts the Fermi level of ZnO upwards. Both physical and electronic effects further contribute to a lower threshold field.

Hofmeister effects on the colloidal stability of poly(ethylene glycol)-decorated nanoparticles

The colloidal stability of poly(ethylene glycol)-decorated poly(methyl methacrylate), PMMA/Tween-20, particles was investigated by means of phase separation measurements, in the presence of sodium fluoride (NaF), sodium chloride, sodium bromide, sodium nitrate, or sodium thiocyanate (NaSCN) at 1.0 mol L−1. Following Hofmeister's series, the dispersions of PMMA/Tween-20 destabilized faster in the presence of NaF than with NaSCN. After the phase separation, the systems were homogenized and except for the dispersions in NaF, re-dispersed particles took longer to destabilize, indicating that anions adsorbed on the particles, creating a new surface. Except for F− ions, the adsorption of anions on the polar outmost shell was evidenced by means of tensiometry and small-angle X-ray scattering measurements. Fluoride ions induced the dehydration of the polar shell, without affecting the polar shell electron density, and the formation of very large aggregates. A model was proposed to explain the colloidal behavior in the presence of Hofmeister ions.

Dynamic mechanical study on multilayer core–shell latex for damping applications

Multilayer core–shell poly (styrene-butyl acrylate) latex particles were synthesized via semi-continuous emulsion polymerization, and the process was monitored by a dynamic laser scattering (DLS). The layers of the latex particles were designed to have progressively decreasing glass transition temperatures (Tg) from the core (layer 1) to the outmost shell (layer 4), which was achieved by varying the mass ratio of butyl acrylate (BA) to styrene (St) in the synthesis of each layer. Divinylbenzene (DVB) was added as the crosslinking agent in each layer except for the outmost layer in order to ensure that a continuous film can be formed at room temperature. The damping properties of the formed films as well as the influences of synthesis variables, including the content of DVB added in the internal layers (i.e., layers 1, 2, and 3), the total mass ratio and sequence between layers 3 and 4, and the Tg of each layer were studied by dynamic mechanical analysis (DMA). The results showed that four-layer core–shell latex particles with proper DVB content in each layer exhibited the best damping properties, with a broad effective damping range (tanδ>0.3) ranging from −12.0°C to 97.2°C. The widening of the damping peak can be explained by the formation of a gradient IPN structure in latex particles. Furthermore, the morphology of the formed films was studied by AFM in tapping mode.

Single-electron detachment for Cu−, Ag− and Au− in collision with Ar

Single-electron-detachment (SED) cross sections for 5–30 keV Cu−, Ag− and Au− in collision with Ar are measured. The SED cross sections increase as the incident velocity increases. These data are compared with other available experimental data of Li−, Na−, K−, and H− in collision with the same target, argon. The scaling law of ne/E2 is obtained where ne is the number of the participating electrons for anions and E is the affinity energy of anions. For transition elements, the participating electrons in the electron detachment process are not only from the outmost shell.

Ab‐initio studies of magnetic properties of small‐sized and cuboctahedral aluminum nanoclusters

AbstractMagnetic properties of Aln clusters are investigated using ab initio density functional calculations based on plane‐wave and the all electron projector augmented‐wave (PAW) method. We present stable structures of small‐sized Aln (n = 2–17) clusters obtained from optimization with binding energy, magnetization, and compared our results with experiment data. The size dependence of binding energy and magnetization are clarified. For large clusters, we discuss the stability of cuboctahedral Aln clusters with up to 923 atoms based on the size dependent binding energy. The coefficients corresponding to surface and edge energies for cuboctahedral Aln clusters have been estimated. We have obtained the cohesive energy of the bulk system to be Ec = 3.418 eV, close to the experimental value (3.39 eV) through the fitting function of size dependent binding energy. We discuss the magnetization of cuboctahedral Aln clusters in relation to shell structure. When the size of the cluster is enlarged, the distribution of magnetization shows that the spin density is largest at the outmost shell. From Al309 to Al923 clusters, electron and spin density in the center and innermost shell behave similar to the bulk. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Estimates of Europa’s ice shell thickness and strain rate from flanking cracks and bulge along Ridge R

Europa, the second Galilean satellite outward from Jupiter, has an outer layer of water of about 100 km thick and an outmost ice shell. The thickness of the ice shell is very important in understanding Europa’s habitability and thermal history, but estimates from different studies are very inconsistent, ranging from 0.2 to 30 km. Here we obtain an estimate of the ice shell thickness from locations of flanking crack and forebulge along Ridge R. Considering the water’s heating process to nearby ice shell in the crack, a flexure model is applied and it suggests the thickness of an ice shell to be 500–1500 m without a convective layer. Compared with previous studies using the same method but ignoring the water’s heating process, the rationality and accuracy have been improved dramatically in our results. We also get some constraints on the strain rate \(\dot \varepsilon \) and the characteristic temperature T c, which defines the base of the elastic layer.

Semiconductor “Nano-Onions” with Multifold Alternating CdS/CdSe or CdSe/CdS Structure

Nano-onions with multifold alternating CdS/CdSe or CdSe/CdS structure have been synthesized via a two-phase approach. The influences of shell on photoluminescence (PL) quantum yields (QYs) and PL lifetimes are investigated and discussed. It is found that the outmost shell plays an important role in the PL QYs and PL lifetimes of the multishells onion-like nanocrystals. The PL QYs and PL lifetimes fluctuate regularly with CdSe and CdS shells. The PL QY increases when the nanocrystals have an outmost CdS shell; however, it decreases dramatically with the outmost CdSe shell. The trend of the change of PL lifetimes is consistent with that of the QYs. The crystal structure and composition of the novel nano-onions are characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectra techniques.

Synthesis and stimuli-responsive properties of chitosan/poly(acrylic acid) hollow nanospheres

We report here a synthetic study on the formation process of hollow polymeric nanospheres based on a simple, core-template-free route, and the effects of polymerization concentration, shell cross-linking, pH, salt concentration and temperature on the size and stability of hollow polymeric nanospheres. The hollow structure of polymeric nanospheres is spontaneously formed by polymerization of acrylic acid monomers inside the chitosan–acrylic acid assemblies. It is found that (i) the hollow structure of nanospheres is stabilized by both physical cross-linking in the inner shell and chemical cross-linking in the outmost shell; (ii) the size of the hollow spheres can be adjusted over the range of 77–500 nm by controlling the concentration of chitosan–acrylic acid assemblies in the reaction system; (iii) the synthesized nanospheres are stimuli-responsive. The size of the hollow nanospheres can be manipulated by changing pH, salt concentration and temperature. Furthermore, with heating and cooling the variation in size of hollow nanospheres is completely reversible and reproducible; (iv) the surface of the hollow nanospheres obtained is chemically active, which provides the functional sites with chemical groups for subsequent chemical reactions at the surface.

Ab initiostudy of transport properties of multiwalled carbon nanotubes

We present ab initio calculations of transport properties of the multiwalled carbon nanotubes (MWNTs). The tube-electrode contact mode is found to have crucial effects on the transport behavior of MWNTs. It is the outmost-shell contact between MWNT and electrodes that leads to the extensively observed outmost-shell-conducting characteristic in MWNTs transport. In multishell contact mode, however, the outmost shell has no priority to be the route of carrier transport and the quantum conductance is mainly determined by the number of metallic shells which directly contact with electrodes. Our calculations indicate that the dangling bonds usually behave like a carrier scattering site. When the two electrodes are connected to the two different shells of the MWNT, the hopping between adjacent shells would play a decisive role in carrier transport.

Preparation of polymer/silica composite nanoparticles bearing carboxyl groups on the surface via emulsifier‐free emulsion copolymerization

AbstractPolymer/silica organic/inorganic composite nanoparticles bearing carboxyl groups on the surface were prepared via the emulsifier‐free emulsion copolymerization of methyl methacrylate and sodium methacrylate (NaMA). Carboxyl groups were generated by the addition of hydrochloric acid at the end of the copolymerization. Two methods of NaMA addition were studied: batch and two‐stage procedures. The batch procedure allowed only a limited number of carboxyl groups to effectively bond to the composite nanoparticles. In contrast, the number of carboxyl groups could be altered over a wide range with the two‐stage procedure. Fourier transform infrared spectroscopy and chemical titration were independently used to quantify the number of carboxyl groups, giving values close to each other and to the feed. A kinetic study indicated that the copolymerization followed a mechanism different than that found earlier. The average size of the composite nanoparticles was approximately 40 nm, as measured by both transmission electron microscopy (TEM) and laser scattering, and their polydispersity index was close to 1, indicating a fairly narrow size distribution. TEM photographs of the composite nanoparticles showed a multilayered core–shell structure with one silica bead as the core and with poly(methacrylate acid) as the outmost shell. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2826–2835, 2005

Periodic Mesoporous Dendrisilicas

We report the synthesis of a new class of materials called periodic mesoporous dendrisilicas. They are prepared from dendrimers with trialkoxysilyl groups at the outmost shell. Dendrimers of various cores and numbers of shells are used. The dendrisilica synthesis is based on acid- or base-catalyzed hydrolysis of the trialkoxysilyl groups and subsequent template-directed condensation of the dendrimers into an ordered template-dendrisilica nanocomposite. The template can be removed with organic solvents, producing a periodic mesoporous dendrisilica whose pore walls consist of interconnected dendrimer building blocks.

Preparation of Carboxyl‐Functionalized Polystyrene/Silica Composite Nanoparticles

AbstractSummary: This paper describes the synthesis of carboxyl‐functionalized polystyrene/silica (PS/SiO2) composite nanoparticles with various contents of poly(methacrylic acid) (PMAA) on the surface by post‐addition of methacrylic acid (MAA) via emulsion polymerization. High yields and binding efficiencies (around 90%) are achieved by an optimal procedure mainly involving the appropriate addition of MAA and amounts of surfactant and silica. The kinetics investigated indicates that the polymerization follows a mechanism different than that found in some earlier studies. The amount of grafted PMAA was determined by titration and FT‐IR, and altered in a wide range (1–40 wt.‐% to PS). Transmission electron microscopy (TEM) photographs show that the composite nanoparticles are about 60 nm in spherical shape and have a multi‐layered core‐shell structure with a cluster of primary silica beads as the core and PMAA as the outmost shell. There are approximately 4 to 21 primary silica beads in one composite nanoparticle, depending on the amount of silica added.Preparation of carboxyl‐functionalized polystyrene(PS)/silica composite nanoparticles.magnified imagePreparation of carboxyl‐functionalized polystyrene(PS)/silica composite nanoparticles.

Removal of shells of multi-wall carbon nanotubes by repeatedly scanning bias voltage

Carbon nanotubes can either be metallic or semiconducting, depending on their chirality. IBM converted multi-wall nanotubes into either a metallic or a semiconducting conductor by selectively removing the shells of the MWNTs. This was realized by applying a constant voltage to the tubes in air. Here we report a new method to remove the shells of a single MWNT just by repeatedly scanning the bias voltage in vacuum. Both the direct contact of the outmost shell with the electrodes and the high anisotropy of thermal conductivity help to remove the shells one by one.

High-field response and direct identification of metallic and semiconducting multiwalled carbon nanotubes

The current through a multiwalled carbon nanotube (MWCNT) was measured as a function of time at high bias $(>3\mathrm{Volts}),$ and the current curve was found to show both up and down steps. The phenomenon was explained as resulting from current induced shell-by-shell break down of the MWCNT following Collins et al. [Science $292,$ 706 (2001)], and the nature of the outmost shell of the MWCNT was directly identified by monitoring the current curve.

Extreme nitrogen enrichment in the asymmetrical planetary nebulae M1-75

view Abstract Citations (20) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Extreme Nitrogen Enrichment in the Asymmetrical Planetary Nebula M1-75 Guerrero, Martin A. ; Stranghellini, Letizia ; Manchado, Arturo Abstract In the framework of the morphological survey of northern sky planetary nebulae, in progress at the IAC we investigate in detail those nebulae that show peculiarities or that have not been studied before. By means of narrowband imaging and accurate two-dimensional spectroscopy we analyzed the peculiar nebula M1-75. Its morphology is very complex, consisting of two contiguous shells, surrounded by two previously undetected fainter lobes. Detailed plasma analysis showed an extreme nitrogen enrichment in the outmost shell (N/O = 2.85), as well as high helium abundance (He/H = 0.18), making this object a good candidate for peculiar stellar evolutionary paths. The detected abundances for heavy elements are among the highest ever detected in a galactic planetary nebulae. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/187857 Bibcode: 1995ApJ...444L..49G Keywords: Abundance; Helium; Line Spectra; Nitrogen; Planetary Nebulae; Sky Surveys (Astronomy); Stellar Evolution; Brightness Distribution; Extinction; Flux Density; Spectral Energy Distribution; Spectrum Analysis; Astronomy; ISM: PLANETARY NEBULAE: INDIVIDUAL ALPHANUMERIC: M1-; 5 full text sources ADS | data products SIMBAD (9)