Gas-phase production of gold-decorated silica nanoparticles.

Gold-decorated silica nanoparticles were synthesized in a two-step process in which silica nanoparticles were produced by chemical vapor synthesis using tetraethylorthosilicate (TEOS) and subsequently decorated using two different gas-phase evaporative techniques. Both evaporative processes resulted in gold decoration of the silica particles. This study compares the mechanisms of particle decoration for a production method in which the gas and particles remain cool to a method in which the entire aerosol is heated. Results of transmission electron microscopy and visible spectroscopy studies indicate that both methods produce particles with similar morphologies and nearly identical absorption spectra, with peak absorption at 500-550 nm. A study of the thermal stability of the particles using heated-TEM indicates that the gold decoration on the particle surface remains stable at temperatures below 900 °C, above which the gold decoration begins to both evaporate and coalesce.

Gas-phase production of multifunctional composite nanoparticles by photoinduced chemical vapor deposition

As these results indicate, photo-CVD coating is a robust process that allows for the creation of core-shell nanoparticles. In the present work we demonstrated that photo-CVD can effectively coat Fe2O3 particles with silica for purposes of biological applications. TDMA results combined with TEM images indicate that all particles are effectively coated and that particle coating thicknesses can be tuned to desired thickness depending on the application. In addition, the ability to vary coating properties and to coat high concentrations of particles makes this technique of interest for industrial production where uniform properties are needed for large quantities of particles [2]. Copyright © 2010 by ASME.

Cold-gas chemical vapor deposition to identify the key precursor for rapidly growing vertically-aligned single-wall and few-wall carbon nanotubes from pyrolyzed ethanol

Vertically-aligned carbon nanotubes (VA-CNTs) were rapidly grown from ethanol and their chemistry has been studied using a "cold-gas" chemical vapor deposition (CVD) method. Ethanol vapor was preheated in a furnace, cooled down and then flowed over cobalt catalysts upon ribbon-shaped substrates at 800 °C, while keeping the gas unheated. CNTs were obtained from ethanol on a sub-micrometer scale without preheating, but on a millimeter scale with preheating at 1000 °C. Acetylene was predicted to be the direct precursor by gas chromatography and gas-phase kinetic simulation, and actually led to millimeter-tall VA-CNTs without preheating when fed with hydrogen and water. There was, however a difference in CNT structure, i.e. mainly few-wall tubes from pyrolyzed ethanol and mainly single-wall tubes for unheated acetylene, and the by-products from ethanol pyrolysis possibly caused this difference. The "cold-gas" CVD, in which the gas-phase and catalytic reactions are separately controlled, allowed us to further understand CNT growth. © 2012 Elsevier Ltd. All rights reserved.

Validation of a lattice Boltzmann model for gas-solid reactions with experiments

A lattice Boltzmann method is used to model gas-solid reactions where the composition of both the gas and solid phase changes with time, while the boundary between phases remains fixed. The flow of the bulk gas phase is treated using a multiple relaxation time MRT D3Q19 model; the dilute reactant is treated as a passive scalar using a single relaxation time BGK D3Q7 model with distinct inter- and intraparticle diffusivities. A first-order reaction is incorporated by modifying the method of Sullivan et al. [13] to include the conversion of a solid reactant. The detailed computational model is able to capture the multiscale physics encountered in reactor systems. Specifically, the model reproduced steady state analytical solutions for the reaction of a porous catalyst sphere (pore scale) and empirical solutions for mass transfer to the surface of a sphere at Re=10 (particle scale). Excellent quantitative agreement between the model and experiments for the transient reduction of a single, porous sphere of Fe 2O 3 to Fe 3O 4 in CO at 1023K and 10 5Pa is demonstrated. Model solutions for the reduction of a packed bed of Fe 2O 3 (reactor scale) at identical conditions approached those of experiments after 25 s, but required prohibitively long processor times. The presented lattice Boltzmann model resolved successfully mass transport at the pore, particle and reactor scales and highlights the relevance of LB methods for modelling convection, diffusion and reaction physics. © 2012 Elsevier Inc.

Methane chemistry involved in a low-pressure electron cyclotron wave resonant plasma discharg

A low-pressure methane plasma generated by electron cyclotron wave resonance was characterized in terms of electron temperature, plasma density and composition. Methane plasmas were commonly used in the deposition of hydrogenated amorphous carbon thin films. Little variation in the plasma chemistry was observed by mass spectrometry measurements of the gas phase with increasing electron temperature. The results show that direct electron-impact reactions exert greater influence on the plasma chemistry than secondary ion-neutral reactions.

SiO2 coating of silver nanoparticles by photoinduced chemical vapor deposition.

Gas-phase silver nanoparticles were coated with silicon dioxide (SiO2) by photoinduced chemical vapor deposition (photo-CVD). Silver nanoparticles, produced by inert gas condensation, and a SiO2 precursor, tetraethylorthosilicate (TEOS), were exposed to vacuum ultraviolet (VUV) radiation at atmospheric pressure and varying temperatures. The VUV photons dissociate the TEOS precursor, initiating a chemical reaction that forms SiO2 coatings on the particle surfaces. Coating thicknesses were measured for a variety of operation parameters using tandem differential mobility analysis and transmission electron microscopy. The chemical composition of the particle coatings was analyzed using energy dispersive x-ray spectrometry and Fourier transform infrared spectroscopy. The highest purity films were produced at 300-400 degrees C with low flow rates of additional oxygen. The photo-CVD coating technique was shown to effectively coat nanoparticles and limit core particle agglomeration at concentrations up to 10(7) particles cm(-3).

Hydrogenated amorphous silicon and silicon nitride deposited at less than 100° C by ECR-PECVD for thin film transistors

A systematic study has been made of the growth of both hydrogenated amorphous silicon (a-Si:H) and silicon nitride (a-SiN) by electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD). In the case of a-SiN, helium and nitrogen gas is injected into the system such that it passes through the resonance zone. These highly ionised gases provide sufficient energy to ionise the silane gas, which is injected further downstream. It is demonstrated that a gas phase reaction occurs between the silane and nitrogen species. It is control of the ratio of silane to nitrogen in the plasma which is critical for the production of stoichiometric a-SiN. Material has been produced at 80°C with a Si:N ratio of 1:1.3 a breakdown strength of ∼6 MV cm-1 and resistivity of > 1014 Ω cm. In the case of a-Si:H, helium and hydrogen gas is injected into the ECR zone and silane is injected downstream. It is shown that control of the gas phase reactions is critical in this process also. a-Si:H has been deposited at 80 °C with a dark conductivity of 10-11 Ω-1 cm-1 and a photosensitivity of justbelowl 4×104. Such materials are suitable for use in thin film transistors on plastic substrates.

On the photoluminescence of a-C:H films

a-C:H films deposited from methane and helium mixture in a capacitively coupled rf plasma were found to show photoluminescence (PL) with peak intensities at energies far above the Taue gap of these films. Apart from the PL the films were investigated with respect to their IR and UV/VIS absorption properties as well as their Raman spectra were examined. The ultraviolet (UV) and blue luminescence from hard a-C:H thin films are explained by incorporation of polycyclic hydrocarbons from gas-phase reactions in the methane helium plasma into the film.

Effects of the biodiesel fuel physical properties on the swirl stabilised spray combustion characteristics

An increasin g interest in biofuel applications in modern engines requires a better understanding of biodiesel combustion behaviour. Many numerical studies have been carried out on unsteady combustion of biodiesel in situations similar to diesel engines, but very few studies have been done on the steady combustion of biodiesel in situations similar to a gas turbine combustor environment. The study of biodiesel spray combustion in gas turbine applications is of special interest due to the possible use of biodiesel in the power generation and aviation industries. In modelling spray combustion, an accurate representation of the physical properties of the fuel is a first important step, since spray formation is largely influenced by fuel properties such as viscosity, density, surface tension and vapour pressure. In the present work, a calculated biodiesel properties database based on the measured composition of Fatty Acid Methyl Esters (FAME) has been implemented in a multi-dimensional Computational Fluid Dynamics (CFD) spray simulation code. Simulations of non-reacting and reacting atmospheric-pressure sprays of both diesel and biodiesel have been carried out using a spray burner configuration for which experimental data is available. A pre-defined droplet size probability density function (pdf) has been implemented together with droplet dynamics based on phase Doppler anemometry (PDA) measurements in the near-nozzle region. The gas phase boundary condition for the reacting spray cases is similar to that of the experiment which employs a plain air-blast atomiser and a straight-vane axial swirler for flame stabilisation. A reaction mechanism for heptane has been used to represent the chemistry for both diesel and biodiesel. Simulated flame heights, spray characteristics and gas phase velocities have been found to compare well with the experimental results. In the reacting spray cases, biodiesel shows a smaller mean droplet size compared to that of diesel at a constant fuel mass flow rate. A lack of sensitivity towards different fuel properties has been observed based on the non-reacting spray simulations, which indicates a need for improved models of secondary breakup. By comparing the results of the non-reacting and reacting spray simulations, an improvement in the complexity of the physical modelling is achieved which is necessary in the understanding of the complex physical processes involved in spray combustion simulation. Copyright © 2012 SAE International.

Spray flame structure of rapeseed biodiesel and Jet-A1 fuel

The spray combustion characteristics of rapeseed methyl esters (RME) were compared to Jet-A1 fuel using a gas turbine type combustor. The swirling spray flames for both fuels were established at a constant power output of 6 kW. The main swirling air flow was preheated to 350 C prior to coaxially enveloping the airblast-atomized liquid fuel spray at atmospheric pressure. Investigation of the fundamental spray combustion was performed via measurements of the fuel droplet sizes and velocities, gas phase flow fields and flame reaction zones. The spray flame droplets and flow fields in the combustors were characterised using phase Doppler anemometry (PDA) and particle imaging velocimetry (PIV) respectively. Flame chemiluminescence imaging was employed to identify the flame reaction zones. The highest droplet concentration zone extends along a 30 angle from the symmetry axis, inside the flame zone. Only small droplets(<17 μ) (<17 μm)are found around the centreline region, while larger droplets are found at the edge of the spray outside the flame reaction zone. RME exhibits spray characteristics similar to Jet-A1 but with droplet concentration and volume fluxes four times higher, consistent with the expected longer droplet evaporation timescale. The flow field characteristics for both RME and Jet-A1 spray flames are very similar despite the significantly different visible characteristics of the flame reaction zones. © 2013 Elsevier Ltd. All rights reserved.

A framework for the analysis of thermal losses in reciprocating compressors and expanders

This article presents a framework that describes formally the underlying unsteady and conjugate heat transfer processes that are undergone in thermodynamic systems, along with results from its application to the characterization of thermodynamic losses due to irreversible heat transfer during reciprocating compression and expansion processes in a gas spring. Specifically, a heat transfer model is proposed that solves the one-dimensional unsteady heat conduction equation in the solid simultaneously with the first law in the gas phase, with an imposed heat transfer coefficient taken from suitable experiments in gas springs. Even at low volumetric compression ratios (of 2.5), notable effects of unsteady heat transfer to the solid walls are revealed, with thermally induced thermodynamic cycle (work) losses of up to 14% (relative to the work input/output in equivalent adiabatic and reversible compression/expansion processes) at intermediate Péclet numbers (i.e., normalized frequencies) when unfavorable solid and gas materials are selected, and closer to 10-12% for more common material choices. The contribution of the solid toward these values, through the conjugate variations attributed to the thickness of the cylinder wall, is about 8% and 2% points, respectively, showing a maximum at intermediate thicknesses. At higher compression ratios (of 6) a 19% worst-case loss is reported for common materials. These results suggest strongly that in designing high-efficiency reciprocating machines the full conjugate and unsteady problem must be considered and that the role of the solid in determining performance cannot, in general, be neglected. © 2014 Richard Mathie, Christos N. Markides, and Alexander J. White. Published with License by Taylor & Francis.

Temperature measurements of the bluff body surface of a Swirl Burner using phosphor thermometry

Flames are often stabilised on bluff-bodies, yet their surface temperatures are rarely measured. This paper presents temperature measurements for the bluff body surface of the Cambridge/Sandia Stratified Swirl Burner. The flame is stabilized by a bluff body, designed to provide a series of turbulent premixed and stratified methane/air flames with a variable degree of swirl and stratification. Recently, modellers have raised concerns about the role of surface temperature on the resulting gas temperatures and the overall heat loss of the burner. Laser-induced phosphorescence is used to measure surface temperatures, with Mg4GeO6F:Mn as the excitation phosphor, creating a spatially resolved temperature map. Results show that the temperature of the bluff body is in the range 550-900 K for different operating conditions. The temperature distribution is strongly correlated with the degree of swirl and local equivalence ratio, reflecting the temperature distribution obtained in the gas phase. The overall heat loss represents only a small fraction (<0.5%) of the total heat load, yet the local surface temperature may affect the local heat transfer and gas temperatures. © 2014 The Combustion Institute.