Proportion Of Atoms Research Articles

Mineralogy texture and chemistry of the Ellicott meteorite using scanning electron microscope and energy disperse spectroscopy (SEM/EDS)

Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS) and Raman spectroscopy have been used to determine the chemistry and mineral components of Ellicott meteorite. Mineral phases and interstitial matrix in the meteorite slab were detected using backscattered electron mode (BSE) and scanning electron microprobe (SEM) and were assessed from atomic proportions of constituent elements, obtained by the EDS analysis. SEM/EDS and Raman shift analyses of mineral phases showed that Ellicott meteorite is characterized by the predominance of Fe-rich (Fe, Ni) alloy kamacite and Ni-Co-rich magnetite. The Iron-nickel phosphide minerals that are called schreibersite and rhabdites along with rare gold have been detected within the kamacite groundmass of meteorite. The presence of Neumann lines, which appear as fine parallel lines running in (up to) four directions, indicates impact with another body. These lines are caused by twinning due to the postconsolidation compression. The chemistry, particularly the lesser content of Ni and texture of unusual abundance ribbons of the iron-phosphide rhabdites and schreibersite where the Widmanstatten texture disappears after the etching by nitric acid, suggests that the Ellicott meteorite is an iron meteorite, hexahedrite type, IIG group.

Controlling Crystal Structure in Embedded Magnetic Nanoparticles

"Magnetic nanoparticles and nanocomposite materials have attracted much interest due to their novel magnetic behaviour, and their potential use in a range of applications. One of the main reasons for their novel magnetism, appreciated for some time, is the high proportion of under-coordinated atoms at the surface of nanoparticles. In the case of magnetic transition metals this leads to a narrowing of the 3d bands that are responsible for magnetism in these materials, leading to size-dependent nanoparticle properties. The atomic structure adopted by nanoparticles is also a key factor in determining their magnetism. Unlike in bulk materials atomic structure in nanoparticles can be changed more readily by, for example, embedding them in suitable matrix materials. Here we describe how a high level of control over crystal structure in nanoparticles can be achieved, using EXAFS to ""fingerprint"" their crystal structure, and show how this in turn leads to a high degree of control over nanoparticle magnetism. We describe a flexible co-deposition process based around a gas aggregation source, which enables a high degree of control over structure in transition metal nanoparticles embedded in various matrices. EXAFS experiments and analysis used to probe atomic structure in embedded Fe and Co nanoparticles are described [1]. Examples presented include the system of Fe nanoparticles embedded in a CuAu alloy matrix where we show that is not only the ability to change the atomic structure of embedded nanoparticles that is important but the ability to fine-tune their structure once changed [2]. In this case, this enables the atomic magnetic Fe moment to be fine-tuned to a value higher than in the bulk Fe structure, in agreement with theory. In some systems alloying at the particle/interface can be significant. We describe how this is the case for Fe nanoparticles in Pd [3], and how such alloying could be useful in forming magnetic nanocomposites with superior properties."

The origin of changes in the electronic structure of oriented multi-walled carbon nanotubes under the influence of pulsed ion radiation

On the basis of spectra obtained through the X-ray Auger-electron spectroscopy (XAES) of carbon (C KVV) and X-ray photoelectron spectroscopy (XPS) of the carbon valence band using the equipment of the Russian–German beam line of the synchrotron radiation facility BESSY II and a Kratos Axis Ultra DLD analytical system, the influence of pulsed ion radiation on the ratio of sp2/sp3-hybridized orbitals of carbon atoms in layers of oriented multi-walled carbon nanotubes (MWCNTs) is investigated. It is shown that when the MWCNTs are subjected to ten pulses, a substantial increase in the proportion of carbon atoms in the sp3 hybridization state occurs compared with MWCNTs subjected to a single pulse. This increase is associated with the formation of thin (<10nm) nanotubes and onion-like carbon, inside which masses of nanodiamond structures are observed in some cases.

Identification of bimetallic electrocatalysts for ethanol and acetaldehyde oxidation: Probing C2-pathway and activity for hydrogen oxidation for indirect hydrogen fuel cells

Hydrogen, in the ethanol molecule, can be utilized in indirect hydrogen fuel cells. In this device, ethanol can be dehydrogenated producing H2 and acetaldehyde in an external fuel processor, and the H2 molecules are electro-oxidized in the anode. The anode electrocatalyst can, additionally, be active for the electro-oxidation of residual ethanol or acetaldehyde, but must catalyze the reaction via the C2-pathway (intact CC bond), in order to avoid the formation poisoning species. This work investigated potential materials that are active for H2 and catalyze the selective electro-oxidation of ethanol and acetaldehyde via the C2-pathway. The bimetallic electrocatalysts were formed by W, Ru and Sn-modified Pt nanoparticles. The reaction products were followed by on-line differential electrochemical mass spectrometry (DEMS) experiments. The results showed that Ru/Pt/C and Sn/Pt/C presented higher overall reaction rate when compared to the other studied materials. However, they were non-selective, even at different atomic proportions, and catalyzed the reaction in parallel pathways producing CO2 and acetaldehyde, with Ru/Pt/C presenting the highest average current efficiency for CO2 formation (16.6%). On the other hand, W/Pt/C with high W content was more selective to the C2 route, evidenced by the absence of the DEMS signals for molecules with one carbon atom such as CH4 and CO2. Additionally, this material was active and stable for H2 electro-oxidation, even in the presence of acetaldehyde in solution, contrarily to what was observed for Pt/C, and this was associated to its activity for H2 oxidation and its inability for the CC dissociation, as evidenced by the DEMS measurements. The high selectivity obtained for the W/Pt/C material to the C2-pathway, and its capability for hydrogen electro-oxidation, is an important novelty in this work, as it turns into a potential electrocatalyst for application in the anode of indirect hydrogen fuel cells powered by ethanol, mainly for those that operates as auxiliary power units of internal combustion engine cars.

Influence of Te doping on the dielectric and optical properties of InBi crystals grown by directional freezing

Stoichiometric pure and tellurium (Te) doped indium bismuthide (InBi) were grown using the directional freezing technique in a fabricated furnace. The X-ray diffraction profiles identified the crystallinity and phase composition. The surface topographical features were observed by scanning electron microscopy and atomic force microscopy. The energy dispersive analysis by X-rays was performed to identify the atomic proportion of elements. Studies on the temperature dependence of dielectric constant (ɛ), loss tangent (tanδ), and AC conductivity (σac) reveal the existence of a ferroelectric phase transition in the doped material at 403 K. When InBi is doped with tellurium (4.04 at%), a band gap of 0.20 eV can be achieved, and this is confirmed using Fourier transform infrared studies. The results thus show the conversion of semimetallic InBi to a semiconductor with the optical properties suitable for use in infrared detectors.

Grain-boundary-enhanced carrier collection in CdTe solar cells.

When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells.

PtRu Nanofilm Formation by Electrochemical Atomic Layer Deposition (E-ALD)

The high CO tolerance of PtRu electrocatalysis, compared with pure Pt and other Pt-based alloys, makes it interesting as an anode material in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). This report describes the formation of bimetallic PtRu nanofilms using the electrochemical form of atomic layer deposition (E-ALD). Metal nanofilm formation using E-ALD is facilitated by use of surface-limited redox replacement (SLRR), where an atomic layer (AL) of a sacrificial metal is first formed by UPD. The AL is then spontaneously exchanged for a more noble metal at the open-circuit potential (OCP). In the present study, PtRu nanofilms were formed using SLRR for Pt and Ru, and Pb UPD was used to form the sacrificial layers. The PtRu E-ALD cycle consisted of Pb UPD at -0.19 V, followed by replacement using Pt(IV) ions at OCP, rinsing with blank, then Pb UPD at -0.19 V, followed by replacement using Ru(III) ions at OCP. PtRu nanofilm thickness was controlled by the number of times the cycle was repeated. PtRu nanofilms with atomic proportions of 70/30, 82/18, and 50/50 Pt/Ru were formed on Au on glass slides using related E-ALD cycles. The charge for Pb UPD and changes in the OCP during replacement were monitored during the deposition process. The PtRu films were then characterized by CO adsorption and electrooxidation to determine their overpotentials. The 50/50 PtRu nanofilms displayed the lowest CO electrooxidation overpotentials as well as the highest currents, compared with the other alloy compositions, pure Pt, and pure Ru. In addition, CO electrooxidation studies of the terminating AL on the 50/50 PtRu nanostructured alloy were investigated by deposition of one or two SLRR of Pt, Ru, or PtRu on top.

Effects of vacuum heat treatment on the photoelectric work function and surface morphology of multilayered silver–metal electrical contacts

Contact materials used for electrical breakers are often made with silver alloys. Mechanical and thermodynamical properties as well as electron emission of such complicated alloys present a lack of reliable and accurate experimental data. This paper deals mainly with electron work function (EWF) measurements about silver–metal (Ag–Me) electrical contacts (Ag–Ni (60/40) and Ag–W (50/50)), before and after surface heat treatments at 513K–873K, under UHV conditions (residual gas pressure of 1.4×10−7mbar). The electron work function (EWF) of silver alloyed contacts was measured photoelectrically, using both Fowler's method of isothermal curves and linearized Fowler plots. An interesting fact brought to light by this investigation is that after vacuum heat treatments, the diffusion and/or evaporation phenomena, affecting the atomic composition of the alloy surface, somehow confine the EWF of the silver–nickel alloy, Φ(Ag–Ni), determined at room temperature in interval]Φ(Ag), Φ(Ni) [=] 4.26eV, 4.51eV[.Surface analysis of two specimens before and after heating showed a significant increase of tungsten atomic proportion on the contact surface for Ag–W contacts after VH treatments. A multilayer model, taking into account the strong intergranular and volume segregation gives a good interpretation of the obtained results.

High reflectance ta-C coatings in the extreme ultraviolet

The extreme ultraviolet (EUV) reflectance of amorphous tetrahedrally coordinated carbon films (ta-C) prepared by filtered cathodic vacuum arc was measured in the 30-188-nm range at near normal incidence. The measured reflectance of films grown with average ion energies in the ~70-140-eV range was significantly larger than the reflectance of a C film grown with average ion energy of ~20 eV and of C films deposited by sputtering or evaporation. The difference is attributed to a large proportion of sp3 atom bonding in the ta-C film. This high reflectance is obtained for films deposited onto room-temperature substrates. The reflectance of ta-C films is higher than the standard single-layer coating materials in the EUV spectral range below 130 nm. A self-consistent set of optical constants of ta-C films was obtained with the Kramers-Krönig analysis using ellipsometry measurements in the 190-950 nm range and the EUV reflectance measurements. These optical constants allowed calculating the EUV reflectance of ta-C films at grazing incidence for applications such as free electron laser mirrors.

An Excel spreadsheet to classify chemical analyses of amphiboles following the IMA 2012 recommendations

A Microsoft Excel spreadsheet has been programmed to assist with classification of chemical analyses of orthorhombic and monoclinic amphiboles following the 2012 nomenclature recommended by the International Mineralogical Association. The spreadsheet is intended for use only with compositional data (wt% oxides and halogens, rather than atomic proportions) and provides options for the estimation of Fe3+/ΣFe and Mn3+/ΣMn ratios and OH content. Various cation normalization schemes can be automatically or manually selected. For each analysis, the output includes the group, subgroup (or B-occupancy for the oxo-amphiboles), and species name including any mandatory chemical prefixes, along with a formula based on 24 anions. The formula results can be exported in a form suitable for the AMPH2012 program. Prefixes related to space groups (proto-) and suffixes (–P21/m) are not assigned in the spreadsheet. Large data sets (up to 200 analyses at a time) can be accommodated by the spreadsheet, which is accompanied by results calculated for more than 650 amphibole analyses taken from the literature.

The Origin of BC7 Sheet Metallicity and the Tuning of its Electronic Properties by Hydrogenation

Using first-principles calculations, we investigate the structural, electronic and hydrogenated properties of the hexagonal BC7 sheet. The computed energy bands and density of states indicate that the BC7 sheet is a metal, and its metallicity mainly originates from the non-bonding pz electrons of the diagonal carbon of the B atom. When these carbon atoms are fully passivated by H atoms, the BC7 sheet becomes a semiconductor with a band gap of 2.41 eV. Our studies demonstrate that changing both the proportion of the boron atoms in the boron carbon sheet and its hydrogenation can tune the electronic properties of boron carbon two-dimensional material.

The effect of doping on the combustion and reaction kinetics of silicon reactives

The influence of low levels of lattice doping in silicon on the combustion and reaction kinetics of silicon based reactives was investigated. Doped silicon powders were produced by ball milling intrinsic, As-, and B-doped silicon wafers. After morphological and physical characterization of the doped silicon powders, energetic composites consisting of silicon and polytetrafluoroethylene (PTFE) powder were produced and characterized by thermal analysis and combustion rate measurements. The apparent activation energy decreased from 283±3 to 243±25kJ/mol as dopant concentration was increased, and burning rates increased from 1.94±0.05mm/s to 3.04±0.10mm/s. This appears to be caused by inclusion of the dopant atoms in the Si lattice as no burning rate change was observed for Si reactive mixtures with the same dopant element added as a bulk powder at the same atomic proportions. The ability to modify combustion behavior through low levels of doping is an intriguing observation, with implications in general burning rate modification and tuning, and the emerging study of silicon based integrated nanoenergetics.

Effect of composition on electrical response to humidity of TiO2:ZnO sensors investigated by impedance spectroscopy

For impedance type sensors based on semiconducting metal oxides, the overall conduction mechanisms have a large influence on the magnitude and on the direction of the sensor signal variation. For humidity in particular, the diverse electronic/ionic charge transfer phenomena that take place at the semiconductor surface and porous structure, can be used to monitor and control this vital environmental characteristic. Various mechanisms have been proposed to explain the variations of electrical response to humidity. With the study of composite materials we look for a better sensitivity of these sensors, comparing to the ones made out of only one metal oxide. This could be due to the fact that some of the positions initially occupied by the atoms of one of the metals are now occupied by atoms of the other metal: if a single covalent/ionic adsorption is decisive in the observed changes in the materials conductivity, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity. In this paper, Ti, Zn oxide bulk sensors, using different proportions of Ti and Zn atoms, were prepared by a conventional sintering method, and the dependence of their complex impedance spectra, measured in the range 100Hz to 10MHz, on the relative humidity (RH), operating temperature and on the measuring frequency is shown and explained.

Coupling of Disilane and Trisilane Segments Through Zero, One, Two, and Three Disilanyl Bridges in Cyclic and Bicyclic Saturated Carbosilanes

Several six-membered cyclic and [2.2.2]bicyclic organosilanes with varying proportions of silicon atoms in the bridges have been prepared following a stepwise approach that exploits dianionic polysilanes. Focus in our analysis was placed on the bicyclic compounds which all have silicon atoms at the bridgehead positions. Quantum chemical calculations of these compounds revealed the possibility to enhance the coupling through a single cisoid tetrasilane cage segment by replacing one or two of the other −SiMe2SiMe2– bridges with −CH2CH2– bridges. UV absorption spectroscopy revealed a red shift in the lowest visible transitions when going from a bicyclo[2.2.2]octane with three −SiMe2SiMe2– bridges to those with two or one such bridge. However, these red shifts are deceptive, as the lowest vertically excited singlet states, which are dark according to TD-DFT calculations, do not display the same trend. Still, since these compounds have (i) excellent structural rigidity, (ii) provide potentials for functionalization through their exocyclic trimethylsilyl groups, and (iii) display electronic structure variations with the number of −SiMe2SiMe2– bridges, they could be interesting for further studies: e.g., in single-molecule electronics.

Effect of Ni Addition on Formation of Nanocrystalline Phase during Mechanical Alloying of (Fe Al)-40(60) at.% Ni and (Fe Al&lt;sub&gt;3&lt;/sub&gt;)-10(30) at.% Ni Powders

Al-Fe-Ni ternary powder mixtures containing (FeAl)-40(60) at.% Ni and (FeAl3)-10(30) at.% Ni were mechanically alloyed by a high-energry planetary ball mill. The structural evolution of the powders during milling was studied by X-ray diffraction technique (XRD). During milling of (FeAl)-40(60) at.% Ni system, Al and Fe solid solutions formed at the early stage change to FeAl, AlNi3 and FeNi3 intermetallic compounds. However, the Al and Fe solid solutions observed at the early stage transform into Al3Ni2, AlFe3 and AlFe0.23Ni0.77 intermetallic compound at last. The experimental results showed that the last milling products were decided by the proportion of atom between Al and Fe in the powder and the Ni content in the power had not affected to the last products.

Influence of dimensionality on phase transition in VO2 nanocrystals

Hydrothermally synthesized one-dimensional and two-dimensional nanocrystals of VO2 undergo phase transition around 65?C, where temperature and mechanism of phase transition are dependent on dimensionality of nanocrystals. Both nanocrystalline samples exhibit depression of phase transition temperature compared to the bulk material, the magnitude of which depends on the dimensionality of the nanocrystal. One-dimensional nanoribbons exhibit lower phase transition temperature and higher values of apparent activation energy than two-dimensional nanosheets. The phase transition exhibits as a complex process with somewhat lower value of enthalpy than the phase transition in the bulk, probably due to higher proportion of surface atoms in the nanocrystals. High values of apparent activation energy indicate that individual steps of the phase transition involve simultaneous movement of large groups of atoms, as expected for single-domain nanocrystalline materials.

Annealing of magnetic nanoparticles for their encapsulation into microcarriers guided by vascular magnetic resonance navigation

Iron, cobalt and iron–cobalt nanoparticle properties, such as diameter, saturation magnetization (Ms), crystal structure, surface composition and stability in physiological solutions, were investigated according to the annealing temperature used prior to their encapsulation into poly(d, l-lactic-co-glycolic acid) (PLGA) microcarriers. These new 60-μm microparticles should exhibit an Ms around 70 emu g−1 to be guided in real time from their intravascular injection site to a tumor with a magnetic resonance imaging scanner. The challenge in the preparation of the nanoparticles consisted in limiting Ms loss by oxidation and the release of metallic ions. It was found that when the annealing temperature reached 650 °C, Fe nanoparticles coalesced, the mean diameter reached (O) 361 ± 138 nm and Ms increased to 171 emu g−1. These nanoparticles exhibited a core of α-Fe and a shell of Fe3O4. On the opposite, Co nanoparticle properties were not affected by the annealing temperature: O and Ms were around 120 nm and 140 emu g−1, respectively. FeCo (60:40, atomic percent) nanoparticles coalesced at an annealing temperature >550 °C, O and Ms reached 217 nm and 213 emu g−1, respectively. Co and FeCo nanoparticles with a Co atomic proportion >15 % were coated with a graphite shell when the temperature was set to 550 °C. In physiological solution, Fe and Co nanoparticles significantly released more ions than FeCo nanoparticles. After the preparation steps prior to their encapsulation, the Ms of Fe and FeCo nanoparticles decreased by 25 and 3 %, respectively. FeCo–PLGA microparticles possessed a relatively high Ms (73 emu g−1) while that of Fe–PLGA microparticle (20 emu g−1) was too low for efficient targeting. The graphite shell was efficient to preserve Ms during the encapsulation.

Shape-Controlled Synthesis of Pd Nanocrystals and Their Catalytic Applications

Palladium is a marvelous catalyst for a rich variety of reactions in industrial processes and commercial devices. Most Pd-catalyzed reactions exhibit structure sensitivity, meaning that the activity or selectivity depends on the arrangement of atoms on the surface. Previously, such reactions could only be studied in ultrahigh vacuum using Pd single crystals cut with a specific crystallographic plane. However, these model catalysts are far different from real catalytic systems owing to the absence of atoms at corners and edges and the extremely small specific surface areas for the model systems. Indeed, enhancing the performance of a Pd-based catalyst, in part to reduce the amount needed of this precious and rare metal for a given reaction, requires the use of Pd with the highest possible specific surface area. Recent advances in nanocrystal synthesis are offering a great opportunity to investigate and quantify the structural sensitivity of catalysts based on Pd and other metals. For a structure-sensitive reaction, the catalytic properties of Pd nanocrystals are strongly dependent on both the size and shape. The shape plays a more significant role in controlling activity and selectivity, because the shape controls not only the facets but also the proportions of surface atoms at corners, edges, and planes, which affect the outcomes of possible reactions. We expect catalysts based on Pd nanocrystals with optimized shapes to meet the increasing demands of industrial applications at reduced loadings and costs. In this Account, we discuss recent advances in the synthesis of Pd nanocrystals with controlled shapes and their resulting performance as catalysts for a large number of reactions. First, we review various synthetic strategies based on oxidative etching, surface capping, and kinetic control that have been used to direct the shapes of nanocrystals. When crystal growth is under thermodynamic control, the capping agent plays a pivotal role in determining the shape of a product by altering the order of surface energies for different facets through selective adsorption; the resulting product has the lowest possible total surface energy. In contrast, the product of a kinetically controlled synthesis often deviates from the thermodynamically favored structure, with notable examples including nanocrystals enclosed by high-index facets or concave surfaces. We then discuss the key parameters that control the nucleation and growth of Pd nanocrystals to decipher potential growth mechanisms and build a connection between the experimental conditions and the pathways to different shapes. Finally, we present a number of examples to highlight the use of these Pd nanocrystals as catalysts or electrocatalysts for various applications with structure-sensitive properties. We believe that a deep understanding of the shape-dependent catalytic properties, together with an ability to experimentally maneuver the shape of metal nanocrystals, will eventually lead to rational design of advanced catalysts with substantially enhanced performance.

The thermal conductivity of SiGe heterostructure nanowires with different cores and shells

Non-equilibrium molecular dynamics (NEMD) simulation is performed to investigate thermal conductivities of two kinds of SiGe heterostructure nanowires (NWs), core(Si)/shell(Ge) and core(Ge)/(Si) NWs, using different interaction potentials between core and shell atoms. The influence of the proportion of core particles on the overall thermal conductivity of NWs is studied as well. Simulation results demonstrate that thermal conductivities of each kind of NWs with strong potential between core and shell atoms are higher than those of their counterparts with weak interaction between Si and Ge atoms. It is also found that thermal conductivities of both kinds of Si/Ge heterostructure NWs reduce with the decrease of the proportion of core atoms when the shell is not very thick.

Sorbitol transformation in aqueous medium: Influence of metal/acid balance on reaction selectivity

The effect of the metal/acid balance of Pt/SiO2–Al2O3 catalysts on the catalytic transformation of sorbitol in aqueous medium was investigated, using four mechanical mixtures of Pt/SiO2–Al2O3 and SiO2–Al2O3 (100:0; 50:50; 25:75; 0:100). The results show that the metal/acid balance has a strong impact on catalytic performances, tuning the yields in alkanes, alcohols, ketones and heterocycles. An optimum in the metal/acid balance is obtained with the 50:50 mechanical mixture: when the metallic function is introduced in higher quantity, the C–C cleavage pathway is favoured and leads to an important CO2 yield; on the contrary, a too low proportion of platinum atoms induces a lack of metallic sites for the hydrogenation reaction. Nevertheless many hydrocarbons containing 1–4 carbon atoms are still obtained with the best balanced catalyst, evidencing some C–C cleavage activity. Hydrocarbon selectivity could be improved by new more hydrogenating metallic phases and new supports with a stronger acidity in water.