Aerosol synthesis of silicon nanoparticles with narrow size distribution—Part 2: Theoretical analysis of the formation mechanism

This work investigates the mechanisms which lead to the formation of silicon nanoparticles with narrow size distributions by means of population balance modeling. The model accounts for the full aerosol process, including chemical reaction, nucleation from supersaturated vapor, growth and agglomeration. The results are in good agreement with experimental data. The effects of the process parameters temperature, silane concentration and reactor total pressure are systematically investigated. The simulation allows an in-depth insight into the particle formation mechanism and reveals the key requirements which are necessary for the generation of narrow particle size distributions. In this mechanism, only a short nucleation burst occurs, while surface growth plays the dominant role in silane precursor consumption. A key role is attributed to condensation, because the numerical calculations can only reflect the experimental observations, if the condensation mechanism is included in the model.     

On the reaction mechanism of MoS42− with nitric oxide

MoS₄^2? (TTM) reacts in deaerated neutral phosphate buffered aqueous solutions with NO. Three consecutive reactions are observed. The two first ones obey pseudo first order rate laws with observed rates proportional to [NO]. The third reaction obeys a first order rate law. Nitrite is one of the final products.The mechanism proposed involves thiol nitrosation via a radical mechanism followed by hydrolysis of the S–N bond. The findings indicate that if MoS₄^2? will be used as a drug, it will not affect considerably the NO concentration in the system.     

An update on the mechanism of the graphene–NO reaction

Cognizant of the key experimental facts from studies of carbonaceous solids ranging from soot to graphite, we performed a quantum chemistry study of the interaction of NO monomer or dimer with one or more zigzag sites. Thermodynamic and kinetic results were used to examine two alternative mechanisms proposed in the literature, and to compare them with the graphene–O₂ reaction mechanism. The chemisorption stoichiometry similarities are striking; but the differences, especially regarding the intermediate role of N₂O, have important practical implications. Monomer chemisorption on an isolated site is a dead-end and temporarily inhibiting process, similar to that of formation of a stable C–O surface complex in the graphene–O₂ reaction. When two sites are available, successive monomer adsorption eventually leads to N₂O formation subsequent to parallel reorientation of the first NO molecule. If three contiguous sites are available, N₂ and CO are the principal products. Chemisorption of the dimer provides a straightforward path to N₂ and CO₂ when one site is available and to N₂ and CO when two sites are available. The formation of N₂O is also feasible in this case, both during adsorption and desorption; in the adsorption phase it is very sensitive to the details of the electron pairing processes.     

Kinetics and mechanism of cubic boron nitride formation in the AlN–BN system at 6 GPa

The kinetics of hBN-into-cBN transformation at 6 GPa and 1770, 1880 and 1990 K in the presence of AlN have been studied. The transformation proceeds without liquid phase. It has been established that a limiting stage of the transformation is diffusion of boron and nitrogen atoms in wurtzitic aluminum nitride. Activation energy of the transformation is 170±40 kJ/mol. On the basis of our results we conclude that hexagonal BN dissolves in aluminum nitride solid solution and supersaturates it with respect to cBN, and then cubic boron nitride precipitates from the supersaturated solution of BN in AlN. An AlN–BN system phase diagram is proposed.     

On the prediction of graphene’s elastic properties with reactive empirical bond order potentials

The elastic properties of graphene as described by the reactive empirical bond order potential are studied through uniaxial tensile tests calculations at both zero temperature, with a conjugate gradient approach, and room temperature, with molecular dynamics simulations. A perfect linear elastic behavior is observed at 0 K up to ≈0.1% strain. The Young’s modulus and Poisson’s ratio obtained with this potential are of ≈730 GPa and 0.39, respectively, with little chirality effects. These values differ significantly from former estimations, much closer to experimental values. We show that these former values have certainly been obtained by neglecting the effect of atomic relaxation, leading to a severe inaccuracy. At larger strains, an extended apparent linear domain is observed in the stress–strain curves, which is relevant to Young’s modulus calculations at finite temperature. Our molecular dynamics simulations at 300 K have allowed obtaining the following, chirality dependent, apparent Young’s moduli, 860 and 761 GPa, and Poisson’s ratios, 0.12 and 0.23, for armchair and zigzag loadings, respectively.     

Towards the perfect graphene membrane? – Improvement and limits during formation of high quality graphene grown on Cu-foils

We investigated the structure and crystalline quality of monolayer graphene grown by hydrogen and methane chemical vapor deposition (CVD) on polycrystalline Cu foils. Our data show that the high temperature hydrogen pretreatment of the Cu foil has to be performed at a sufficiently high H₂ pressure in order to avoid graphene (g) formation already during the pretreatment, which limits the achievable domain size during subsequent growth in the CH₄/H₂ mixture. Methane–hydrogen CVD sustains g growth but induces the faceting of the Cu substrate. Characterization by low energy electron microscopy evidenced a staircase Cu substrate morphology of alternating (4 1 0) and (1 0 0) planes interrupted by (n 1 1) type facets. The g flakes cover the staircase shaped support as a coherent layer. The polycrystalline film mostly contains rotational domains that are preferentially, but not strictly, aligned with respect to the stepped support surface. The substrate induced corrugated morphology occurs also underneath large single crystalline flakes and is transferred to suspended membranes, produced by etching the Cu underneath the graphene. Thus, membranes manufactured from g-Cu are non flat. This explains their reported softened elastic response and the formation of so called nanorippled graphene after transfer from the Cu support which deteriorates its electrical conductivity.     

The activation mechanism of oxalic acid on γ-alumina and the formation of α-alumina

Converting the γ phase into the α phase completely is necessary in the presintering stage of industrial alumina (Al₂O₃), which requires high temperature and energy consumption. To reduce the presintering temperature, γ-Al₂O₃ was activated by oxalic acid. XRD, ²⁷Al-MAS-NMR and TG-DSC were used to characterize the γ - alumina before and after activation, and the phase transformation was studied. The formation temperature of α-Al₂O₃ decreased to 1029 °C for oxalic acid activated γ-Al₂O₃, and the α-fraction was 100% for activated γ-Al₂O₃ at 1300 °C. After oxalic acid activation, the diffraction peak intensity of γ-Al₂O₃ decreased significantly; the results of ²⁷Al-MAS-NMR suggested that octahedral [AlO₆] in γ-Al₂O₃ was easier than tetrahedral [AlO₄] to be attacked by oxalic acid, and the formation of pentavalent [AlO₅] with higher reaction activity, which was in favour of the lowering formation temperature of α-Al₂O₃. The dissolution concentration of Al increased after oxalic acid activation, and the dissolution process was controlled by surface reactions. Oxalic acid mainly attacked the octahedral aluminium in γ-Al₂O₃ and extracted Al as three complexes of [Al(C₂O₄)]⁺, [Al(C₂O₄)₂]^- and [Al(C₂O₄)₃]^3-. Oxalic acid activated γ - Al₂O₃ with a lower phase transformation temperature has broad application prospects in the alumina industry.     

Features of formation of the contact zone in the glaze — Ceramics system

The features of formation of the contact zone in the glaze — ceramics system in relation to the time-temperature parameters of the heat treatment are considered. The distribution of the cations Si⁴⁺, Ca²⁺, Zr⁴⁺, and Al³⁺ in the opacified glaze coating and the ceramic substrate is studied. It is found that diffusion processes proceed due to viscous flow of the glaze glass.     

On the mechanism of CO adsorption on a silica‐supported ruthenium catalyst

The adsorption of CO at room temperature on a Ru/SiO₂ catalyst has been studied by means of FTIR spectroscopy. Spectral evidence for formation of water molecules and a quantity of very dispersed ruthenium on the catalyst surface during CO adsorption was found. On the basis of these experimental results a new reaction scheme for the interaction of CO with a silica‐supported ruthenium catalyst is proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the interpretation of XPS spectra of metal (Pt,Pt–Sn) nanoparticle/graphene systems

This Letter discusses the X-ray photoelectron spectroscopy (XPS) results of an article on Pt and Pt–Sn nanoparticles dispersed on graphene nanosheets. I argue that the authors’ interpretation is largely unwarranted because it neglects the spin–orbit multiplicity and the branching ratio of XPS signals arising from electron levels with l > 0, and the fact that XPS peaks of given electron levels and given chemical species possess precise full with at half maximum (FWHM) values, and binding energy (BE) values. I suggest an interpretation which offers a more accurate insight into the chemical composition and the catalytic properties of these nanoparticle systems.     

Investigation of the mechanism for the separation of nitrogen—oxygen mixtures on carbon molecular sieves

Sorption separation of nitrogen-oxygen mixtures by carbon molecular sieves, e.g. to produce nitrogen using the pressure swing adsorption principle, proceeds under the condition of non-equilibrium. Experimental data based upon measurements of sorption equilibria and kinetics for nitrogen and oxygen on several samples of carbon molecular sieve particles in the temperature range 0–50°C suggest the existence of a surface barrier in the pore mouths of slit-like micropores for these particular samples. The conclusion on this phenomenon is supported by the results of canonical and grand canonical Monte Carlo simulations. Both types of analysis give a deeper insight into the complex non-equilibrium separation process.     

Fischer–Tropsch synthesis with ethene co-feeding: Experimental evidence of the CO-insertion mechanism at low temperature

Experiments were performed at both normal and rather extreme Fischer–Tropsch Synthesis (FTS) operating conditions over a typical cobalt-based catalyst, with the aim of exploring if aspects of the reaction mechanism could be elucidated. The results show that CO reacted when co-feeding C₂H₄ with syngas, while CO did not react with H₂ in absence of C₂H₄, under extremely low-temperature conditions (140°C). The adsorbed CO and C₂H₄ may behave as monomers and initiators, respectively, and react with each other to form long chain hydrocarbons. It suggests that the C C bond coupling precedes the C O bond dissociation, which is consistent with the CO-insertion mechanism. C_3–6 product distribution with a feed of H₂/CO/C₂H₄ at low temperature followed the same trends in terms of normal FTS product distribution. The observed FTS-type chain growth reaction that occurs at abnormally low temperatures (140°C) when co-feeding C₂H₄ may provide new insights into the chemistry of FTS.     

Self-propagating high-temperature synthesis of advanced ceramics in the Mo–Si–B system: Kinetics and mechanism of combustion and structure formation

The goal of this work is to investigate the combustion mechanisms of reactions in the Mo–Si–B system and to obtain ceramic materials using the SHS method. It is concluded that the following processes are defined by the SHS for Si-rich Mo–Si–B compositions: silicon melting, its spreading over the surfaces of the solid Mo and B particles, followed by B dissolution in the melt, and formation of intermediate Mo₃Si-phase film. The subsequent diffusion of silicon into molybdenum results in the formation of MoSi₂ grains and molybdenum boride phase forms due to the diffusion of molybdenum into B-rich melt. The formation of MoB phase for B-rich compositions may occur via gas-phase mass transfer of MoO₃ gaseous species to boron particles. The stages of chemical interaction in the combustion wave are also investigated. The obtained results indicate the possibility of both parallel and consecutive reactions to form molybdenum silicide and molybdenum boride phases. Thus the progression of combustion process may occur through the merging reaction fronts regime and splitting reaction fronts regime. Molybdenum silicide formation leads the combustion wave propagation during the splitting regime, while the molybdenum boride phase appears later. Finally, targets for magnetron sputtering of promising multi-phase Mo–Si–B coatings are synthesised by forced SHS compaction method.     

The doping of reduced graphene oxide with nitrogen and its effect on the quenching of the material’s photoluminescence

N-doped graphene (NG) was synthesized by annealing reduced graphene oxide (RGO) in an ammonia atmosphere. The dependence of the nitrogen content on the annealing temperature and the type of doping of NG were investigated. The photoluminescence (PL) properties of the RGO and NG samples were studied. The results show that RGO exhibits strong ultraviolet (UV) PL at 367 nm. The PL of RGO can be quenched by doping it with N and the quenching efficiency depends on the pyridine N content.     

In situ formation mechanism of aluminum oxycarbonitride in the unoxidized zone of Al–Al2O3 composites

The Al–Al₂O₃ composites were prepared by fused alumina, α-Al₂O₃ micropowders, and metal aluminum powder. The samples with 3% carbon black (N330), 3% resin powder and α-Al₂O₃ micropowder, 10% α-Al₂O₃ micropowder were named S1, S2, and S3, respectively. They were oxidized at 1500 °C for 3 h in a box-type electric furnace, and then the unoxidized areas were analyzed by X-ray diffraction and scanning electron microscopy. It was found that the aluminum oxycarbonitride formed in situ during the oxidation resistance test inhibited further oxidation of S2. The in situ formation mechanism of aluminum oxycarbonitride in the unoxidized zone is believed to be 7Al + 3Al₃C₄ + 12AlN + 4Al₂O₃ = 12Al₃CON, which is also verified and proven by XRD, SEM and EDS in this work. The oxidation depth of S2 is 46.7% lower than that of S3. Sample S2 hardly has any linear change after fired at 1500 °C × 3 h in coke, which benefits to improve the volume stability and prolongs the service life. Among the three batches, S2 exhibits the minimum creep rate, from 0% to 0.026% at 1500 °C, and the HMOR at 1500 °C in a N₂ atmosphere of S2 is 58 MPa. The Al–Al₂O₃ composites combined with resin strengthened by in situ formed aluminum oxycarbonitride give the composites excellent high-temperature strength owing to the fiber-intersected reinforced microstructure of Al₃CON crystals at high temperature. In the Al–C–N–O system, it was found that the general formula of aluminum oxycarbonitride is expressed by Al_4n+m(C,O,N)_3n+m, and the (n, m) values are (0, 3) for Al₃CON.     

Studying the mechanism of the interaction between silicon nitride, oxygen, and nitrogen in heating and the processes of defect formation in Si3N4 — gas systems

The method of precision thermal massometry is used to study the interaction between silicon nitride, oxygen, and nitrogen in nonisothermal heating at 300 -1300 K and a partial gas pressure of 19,998.3 Pa (150 Torr). The behavior of Si₃N₄ heated in forevacuum is studied. A special computer analysis of the experimentally obtained dependencies has provided kinetic equations for analyzing and determining the mechanisms of these processes and computing their activation energy. The experimental data are used for creating a hypothesis that the nitrogen sublattice of Si₃N₄ is dispersed before the beginning of its interaction with oxygen and nitrogen.     

Evaporation — a key mechanism for the thaumasite form of sulfate attack

Understanding the mechanisms leading to chemical attack on concrete is crucial in order to prevent damage of concrete structures. To date, most studies on sulfate attack and thaumasite formation are based on empirical approaches, as the identification of associated reaction mechanisms and paths is known to be highly complex. In this study, sulfate damaged concrete from Austrian tunnels was investigated by mineralogical, chemical and isotope methods to identify the reactions which caused intense concrete alteration. Major, minor and trace elemental contents as well as isotope ratios of local ground water (GW), drainage water (DW) and interstitial solutions (IS), extracted from damaged concrete material, were analyzed.Locally occurring GW contained 3 to 545 mg L^− 1 of SO₄ and is thus regarded as slightly aggressive to concrete in accordance to standard specifications (e.g. DIN EN 206-1). The concrete linings and drainage systems of the studied tunnels, however, have partly suffered from intensive sulfate attack. Heavily damaged concrete consisted mainly of thaumasite, secondary calcite, gypsum, and relicts of aggregates. Surprisingly, the concentrations of dissolved ions were extremely enriched in the IS with up to 30,000 and 12,000 mg L^− 1 of SO₄ and Cl, respectively. Analyses of aqueous ions with a highly conservative behavior, e.g. K, Rb and Li, as well as ²H/H and ¹⁸O/¹⁶O isotope ratios of H₂O of the IS showed an intensive accumulation of ions and discrimination of the light isotopes vs. the GW. These isotope signals of the IS clearly revealed evaporation at distinct relative humidities. From ion accumulation and isotope fractionation individual total and current evaporation degrees were estimated. Our combined elemental and isotopic approach verified wetting–drying cycles within a highly dynamic concrete-solution-atmosphere system. Based on these boundary conditions, key factors controlling thaumasite formation are discussed regarding the development of more sulfate-resistant concrete and concrete structures.