Evidence of fluorescent carbon nanoparticles produced in premixed flames by time-resolved fluorescence polarization anisotropy

In this work we report on the analysis of combustion-generated nano-organic carbon (NOC) particles by means of time-resolved fluorescence polarization anisotropy (TRFPA). NOC samples were collected in water from nonsooting regions of ethylene/air laminar flames. The size of collected particles was determined with subnanometer accuracy by exploiting femtosecond laser pulses as an excitation source. Moreover, the TRFPA measurements were realized by selecting narrow-wavelength bands within the fluorescence spectrum. With this procedure, the ensemble-averaged size of particles emitting in the selected fluorescence band was determined, to provide additional information on composition and spectroscopic properties of the investigated particles. In particular, the NOC samples consisted of two groups of particles, preferentially emitting in two distinct wavelength bands: smaller particles, with diameter d=1-1.5 nm, mostly fluorescing in the UV (λ∼300-470 nm), and larger particles, d>2 nm, fluorescing in the visible (λ∼490-580 nm). The results of this work strongly support the attribution of UV/visible fluorescence generally detected in flames to nanoparticles. They also evidence that nanoparticles undergo a chemical transformation during the growth process, which produces a red shift in the fluorescence spectrum.     

Study of laminar combustion characteristics of gasoline surrogate fuel-hydrogen-air premixed flames

This paper investigated the effects of hydrogen addition to gasoline surrogates fuel-air mixture on the premixed spherical flame laminar combustion characteristics. The experiments were carried out by high speed Schlieren photography on a constant-volume combustion vessel. Combining with nonlinear fitting technique, the variation of flame propagation speed, laminar burning velocity, Markstein length, flame thickness, thermal expansion coefficient and mass burning flux were studied at various equivalence ratios (0.8–1.4) and hydrogen mixing ratios (0%–50%). The results suggested that the nonlinear fitting method had a better agreement with the experimental data in this paper and the flame propagation was strongly effected by stretch at low equivalence ratios. The stretched propagation speed increased with the increase of hydrogen fraction at the same equivalence ratio. For a given hydrogen fraction, Markstein length decreased with the increase of equivalence ratio; flame propagation speed and laminar burning velocity first increased and then decreased with the increase of equivalence ratio while the peaks of the burning velocity shifted toward the richer side with the increase of hydrogen fraction.     

Morphology and electrical properties of conductive carbon coatings for cathode materials

Many interesting cathode materials, such as LiFePO₄, LiMnPO₄, LiFeBO₃ or the recently discovered Li₂FeSiO₄ and Li₂MnSiO₄, exhibit extremely low electronic conductivity (<10⁻⁹ S cm⁻¹). A very efficient way for improving the electronic transport in such materials is supposed to be the preparation of carbon coatings around individual active particles. Despite the increasing number of reports on preparation of various carbon coatings, neither the formation mechanism nor the detailed coating properties have been explained satisfactorily. The present paper is an attempt to find a clear correlation between the synthesis parameters, the resulting coating morphology and, finally, its electrical properties. As a substrate material for deposition of coatings, more or less monodisperse TiO₂ particles in various sizes were used. As a carbon precursor, citrate was used because it had given excellent results in our previous investigation of the LiFePO₄ system. It is shown that citrate precursor delivers pretty good conductivity (ca. 30 S cm⁻¹) after a 10 h heat treatment at 700 °C or higher. The conductivity percolation threshold can be reached already at 1.5 vol.% of carbon, while the plateau conductivity of the whole composite is about 0.1 S cm⁻¹. At that level, the carbon phase is supposed to form a well-distributed 3D electrical network within the composite.     

Optical, thermal and structural characteristics of carbon nanoparticles embedded in ZnO and NiO as selective solar absorbers

Selective solar absorber coatings of carbon embedded in ZnO and NiO matrices on aluminium substrates have been fabricated by a sol–gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings. The surface morphology of the samples was studied by scanning electron microscopy (SEM). Cross-sectional high-resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples. Chemical composition analysis was done by energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The crystal structure of ZnO and NiO samples was also investigated with an X-ray diffraction (XRD) technique. Samples were subjected to an accelerated ageing test for 95 h, with condensation at relative humidity of 95% and at a climate chamber temperature of 45 °C. The thermal emittances of the samples were 6% for the ZnO and 4% for the NiO matrix materials. The solar absorptances were 71% and 84% for ZnO and NiO samples, respectively. The SEM revealed a smooth featureless surface for both C–ZnO and C–NiO samples. Some C–NiO samples showed dentritic features. X-HRTEM, EDS and EELS studies revealed a nanometric grain size for both types of samples. The C–ZnO and C–NiO coatings contained amorphous carbon embedded in nanocrystalline ZnO and NiO matrices, respectively. Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix. The accelerated ageing tests produced performance criterion (PC) values of 0.15 and 0.054 at 95 h for the C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have better solar selectivity behaviour than the C–ZnO counterparts.     

Occurrence and characterization of carbon nanoparticles below the soot laden zone of a partially premixed flame

An experimental study has been performed to detect the occurrence of nanosized carbon particulates below the soot laden zone of a co-flowing partially premixed flame. Samples have been extracted from different points across the flame and passed through DI water. Absorption and fluorescence spectroscopies have been performed with the collected water suspensions. The occurrence of carbon nanoparticles is evident across the inner flame front. In addition, evidence of naphthalene has also been found inside the inner rich premixed flame. The concentration of naphthalene decreases while that of the carbon nanoparticles increases as the inner flame front is reached. The stability of the nanoparticles in the sample has been ensured by observing that the change in fluorescence quantum yield from the sample over a long duration is small. The band gap energy has been evaluated using the absorption data to characterize the likely structures of the particles in the collected suspension. Two kinds of particles having different zones of band gap energy are found in the flame. Dynamic light scattering measurements show that the particle size grows with the increase in height in the lower part of the flame. While, at 3 and 6 mm elevations the particles are observed to be below 2.5 nm in diameter, the particles at 10 mm elevation are found in the size range of 2.5-5.5 nm.     

A comparative study of TiO2 nanoparticles synthesized in premixed and diffusion flames

Previous studies have been shown that synthesis of titania （TiO2） crystalline phase purity could be effectively controlled by the oxygen concentration through titanium tetra-isopropoxide （TTIP） via premixed flame from a Bunsen burner. In this study, a modified Hencken burner was used to synthesize smaller TiO2 nanoparticles via short diffusion flames. The frequency of collisions among particles would decrease and reduce TiO2 nanoparticle size in a short diffusion flame height. The crystalline structure of the synthesized nanoparticles was characterized by X-ray diffraction （XRD）, transmission electron microscope （TEM）, Barrett-Joyner- Halenda （BJH） and Brnnauer-Emmett-Teller （BET） measurements. The characteristic properties of TiO2 nanoparticles synthesized from a modified Hencken burner were compared with the results from a Bunsen burner and commercial TiO2 （Degussa P25）. The results showed that the average particle size of 6.63 nm from BET method was produced by a modified Hencken burner which was smaller than the TiO2 in a Bunsen burner and commercial TiO2. Moreover, the ruffle content of TiO2 nanoparticles increased as the particle collecting height increased. Also, the size of TiO2 nanoparticles was highly dependent on the TTIP loading and the collecting height in the flame.     

Influence of silane flow on structural, optical and electrical properties of a-Si:H thin films deposited by hot wire chemical vapor deposition (HW-CVD) technique

The electrical, structural and optical properties of hydrogenated amorphous silicon (a-Si:H) films deposited from pure silane (SiH₄) using hot wire chemical vapor deposition (HW-CVD) technique are systematically studied as a function of silane flow rate between 5 and 30 sccm. We found that the properties are greatly affected by the silane flow rate over the range we studied. The device quality a-Si:H films with a photosensitivity >10⁵ were deposited by HW-CVD at a deposition rate >10 Å s⁻¹ using low silane flow rate. However, a-Si:H films deposited at higher silane flow rate and/or higher deposition rates show degradation in their structural and electrical properties. The FTIR studies indicate that the hydrogen bonding in a-Si:H films shifts from mono-hydrogen (Si–H) to di-hydrogen (Si–H₂) and (Si–H₂)_n complexes when films were deposited at higher silane flow rate. The hydrogen content in the a-Si:H films increases with increase in silane flow rate and was found to be less than 10 at.%. The Raman spectra show increase in disorder and the Rayleigh scattering with increase in silane flow rate. The optical band gap also shows an increasing trend with silane flow rate. Therefore, only the hydrogen content cannot be accounted for the increase in the optical band gap. We think that the increase in the optical band gap may be due to the increase in the voids. These voids reduce the effective density of material and increase the average Si–Si distance, which is responsible for the increase in the band gap. Silane flow rate of 5 sccm, appears to be an optimum flow rate for the growth of mono-hydrogen (Si–H) bonded species having low hydrogen content (4.25 at%) in a-Si:H films at high deposition rate (12.5 Å s⁻¹), high photosensitivity (10⁵) and small structural disorder.     

Onset of cellular instabilities in spherically propagating hydrogen-air premixed laminar flames

Using high-speed Schlieren and Shadow photography, the instabilities of outwardly propagating spherical hydrogen-air flames have been studied in a constant volume combustion bomb. Combustion under different equivalence ratios (0.2 ∼ 1.0), temperatures (298 K ∼ 423 K) and pressures (1.0 bar ∼ 10.0 bar) is visualized. The results show that flames experience both unequal diffusion and/or hydrodynamic instabilities at different stages of propagation. The critical flame radius for such instabilities is measured and correlated to the variations of equivalence ratio, temperature and pressure. Analysis revealed that equivalence ratio affects unequal diffusion instability via varying the Lewis number, Le$Le$; increased temperature can delay both types of instabilities in the majority of tests by promoting combustion rate and changing density ratio; pressure variation has minor effect on unequal diffusion instability but is responsible for enhancing hydrodynamic instability, particularly for stoichiometric and near-stoichiometric flames.     

Stability enhancement of ozone-assisted laminar premixed Bunsen flames in nitrogen co-flow

Ozone (O₃) is known as one of the strongest oxidizers and therefore is widely used in many applications. Typically in the combustion field, a combination of non-thermal plasma and combustion systems have been studied focusing on the effects of ozone on flame propagation speeds and ignition characteristics. Here, we experimentally investigated the effects of ozone on blowoff of premixed methane/air and propane/air flames over a full range of equivalence ratios at room temperature and atmospheric pressure by using a co-flow burner and a dielectric barrier discharge. The results with ozone showed that a nozzle exit jet velocity at the moment of flame blowoff (blowoff velocity) significantly increased, and flammability limits for both fuel-lean and rich mixtures were also extended. Ozone had stronger effects of percent enhancement in the blowoff velocity for off-stoichiometric mixtures, while minimum enhancements could be observed around stoichiometric conditions for both fuels showing linear positive dependence on a tested range of ozone concentration up to 3810 ppm. Through chemical kinetic simulations, the experimentally observed trends of the enhancement in blowoff velocity were identified as a result of the modification of the laminar burning velocity. Two ozone decomposition pathways of O₃ + N₂ → O + O₂ + N₂ and O₃ + H → O₂ + OH were identified as the most controlling steps. These reactions, coupled with fuel consumption characteristics of each fuel determined the degree of promotion in laminar burning velocities, supporting experimental observations on blowoff velocities with ozone addition.     

Soot graphitic order in laminar diffusion flames and a large-scale JP-8 pool fire

High-resolution transmission electron microscopy (HRTEM) has been performed on soot samples collected from two smoking laminar ethylene diffusion flames (one steady and one unsteady) and from the active-flaming region of a 5-m diameter JP-8 pool fire. The motivation for this study is to improve the understanding of the influence of soot microstructure on its optical properties. The soot sampling positions in the steady ethylene flame correspond to locations of maximum soot mass growth, partial soot oxidation, and quenched oxidation along a common streamline. Visual examination of the HRTEM images suggests that the graphitic crystalline layers of soot undergo increased densification along the sampled streamline in the steady laminar flame. Quantitative image analysis reveals a small decrease in the mean graphitic interlayer spacing (d₀₀₂) with increasing residence time in the high-temperature region. However, the differences in the mean interlayer spacing are far smaller than the spread of interlayer spacings measured for any given soot sample. Post-flame samples from the unsteady ethylene flame show interlayer spacing distributions similar to the lower region of the steady flame. The soot samples from the pool fire show little evidence of oxidized soot and have interlayer spacings similar to the unsteady ethylene flame. Previous research in the carbon black field has demonstrated a direct relation between the graphitic interlayer spacing and the optical absorptivity of the carbon. Consequently, the current HRTEM results offer support to recent measurements of the dimensionless extinction coefficient of soot that suggest that the optical absorptivity of agglomerating soot shows only minor variations for different fuels and flame types.     

Synthesis and characterization of Pt nanoparticles on sulfur-modified carbon nanotubes for methanol oxidation

Pt nanoparticles supported on carbon nanotubes (Pt/CNTs) have been synthesized from sulfur-modified CNTs impregnated with H₂PtCl₆ as Pt precursor. The dispersion and size of Pt nanoparticles in the synthesized Pt/CNT nanocomposites are remarkably affected by the amount of sulfur modifier (S/CNT ratio). The results of X-ray diffraction and transmission electron microscopy indicate that an S/CNT ratio of 0.3 affords well dispersed Pt nanoparticles on CNTs with an average particle size of less than 3 nm and a narrow size distribution. Among different catalysts, the Pt/CNT nanocomposite synthesized at S/CNT ratio of 0.3 showed highest electrochemically active surface area (88.4 m² g⁻¹) and highest catalytic activity for methanol oxidation reaction. The mass-normalized methanol oxidation peak current observed for this catalyst (862.8 A g⁻¹) was ∼ 6.5 folds of that for Pt deposited on pristine CNTs (133.2 A g⁻¹) and ∼ 2.3 folds of a commercial Pt/C (381.2 A g⁻¹). The results clearly demonstrate the effectiveness of a relatively simple route for preparation of sulfur-modified CNTs as a precursor for the synthesis of Pt/CNTs, without the need for tedious pretreatment procedures to modify CNTs or complex equipments to achieve high dispersion of Pt nanoparticles on the support.     

Effect of ferrocene addition on sooting limits in laminar premixed ethylene-oxygen-argon flames

The critical sooting C/O ratio was measured for a series of atmospheric-pressure, laminar premixed ethylene-oxygen-argon flames doped with a small amount of ferrocene. Comparisons of the doped and undoped flames show marked increases in sooting tendency for flames doped with ferrocene. Under the same flame conditions, the critical C/O ratios in doped flames were uniformly lower than those of undoped flames. A five-step reaction mechanism of ferrocene decomposition, leading to the formation of the cyclopentadienyl, was proposed. Detailed kinetic modeling of the experimental flames showed little changes in the flame temperature and the aromatic concentration upon ferrocene doping. The experimental and computational results support the early suggestion that in premixed flames the increase in sooting tendency due to ferrocene addition is the result of induced nucleation by iron oxide nanoparticles. These particles provide a surface to initiate soot surface growth.     

Chemical kinetic considerations for postflame synthesis of carbon nanotubes in premixed flames using a support catalyst

Multiwalled carbon nanotubes (MWCNTs) on a grid supported cobalt nanocatalyst were grown, by exposing it to combustion gases from ethylene/air rich premixed flames. Ten equivalence ratios (?) were investigated, as follows: 1.37, 1.44, 1.47, 1.50, 1.55, 1.57, 1.62, 1.75, 1.82, and 1.91. MWCNT growth could be observed for the range of equivalence ratios between 1.45 and 1.75, with the best yield restricted to the range 1.5-1.6. A one-dimensional premixed flame code with a postflame heat loss model, including detailed chemistry, was used to estimate the gas phase chemical composition that favors MWCNT growth. The results of the calculations show that the mixture, including the water gas shift reaction, is not even in partial chemical equilibrium. Therefore, past discussions of compositional parameters that relate to optimum carbon nanotube (CNT) growth are revised to include chemical kinetic effects. Specifically, rapid departures of the water gas shift reaction from partial equilibrium and changes in mole fraction ratios of unburned C₂ hydrocarbons to hydrogen correlate well with experimentally observed CNT yields.     

Effect of boron doping in the carbon support on platinum nanoparticles and carbon corrosion

Carbon supported catalysts can lose their activity over a period of time due to the sintering of the nanometer-sized catalyst particles. The sintering of metal clusters on carbon supports can occur due to the weak interaction between the metal and the support and also due to the corrosion of carbon, especially in fuel cell electrocatalysts. The sintering may be reduced by increasing the interaction between the metal and the support and also by increasing the corrosion resistance of carbon supports. In an effort to mitigate the growth of the nanoparticles, carbon-substituted boron defects were introduced in the carbon lattice. The interaction between the Pt nanoparticles on the pure and boron-doped carbon supports was examined using X-ray photoelectron spectroscopy (XPS). The results indicate that the interaction between the Pt nanoparticles and the boron-doped carbon support was slightly stronger than the interaction between the Pt nanoparticles and the pure carbon support. Also, by using accelerated aging tests, the boron-doped system was found to be more resistant to carbon corrosion when compared to the pristine carbon-supported Pt catalyst.     

Wide range tuning of electrical conductivity of RF sputtered CdO thin films through oxygen partial pressure variation

The effect of oxygen partial pressure variation on the electrical conductivity and the optical transparency of CdO thin films, deposited through RF magnetron sputtering were studied in detail. Thin films of CdO have been deposited through radio frequency magnetron sputtering of a prefabricated CdO target at a fixed pressure 0.1 mbar and at a substrate temperature 523 K. It was found that the electrical conductivity of the CdO films could be varied over three decades for a variation of oxygen partial pressure of 0–100%, without introducing any extrinsic dopants. X-ray diffraction (XRD) studies showed that the films were polycrystalline in nature with a preferential orientation along (1 1 1) plane. Compositional information was obtained by X-ray photoelectron spectroscopic studies. This wide range of variation of electrical properties was explained through the oxygen vacancies formation.     

Numerical and experimental study of the combustion of acetic acid in laminar premixed flames

Experimental mole fraction profiles of chemical species (stable, radical and intermediates) have been measured in three CH₃COOH/O₂/Ar flat premixed flames burning at low pressure (50 mbar) and with equivalence ratios equal to 0.77, 0.9 and 1.05, respectively. The experimental setup used to determine the structure of one-dimensional laminar premixed flames consists of a molecular beam mass spectrometer system (MBMS) combined with electron impact ionisation (EI). Experimental results have highlighted an important role played by ketene (CH₂CO) as an intermediate during the combustion of acetic acid. In order to simulate the experimental results, a sub-mechanism concerning the combustion of CH₃COOH and CH₂CO has been built and added to a global mechanism recently proposed. This ensures a reasonably good modelling for the structures of acetic acid flames.     

Effects of Soret diffusion on the exergy losses in hydrogen laminar premixed flames

The analysis of the exergy loss is an effective tool for evaluating second-law irreversibility in laminar flames. However, despite numerous studies underlining the importance of taking into account thermal diffusion in laminar flame studies, especially in hydrogen/air flames, this phenomenon is usually neglected in the exergy analysis of these flames. Therefore, this work investigates the effect of Soret diffusion on the exergy loss in laminar premixed flames for hydrogen/air mixtures using a detailed reaction mechanism and the multicomponent transport model. The study starts from conditions in which the importance of the Soret effect is well established in the scientific literature. It is found that, while the exergy losses directly due to the Soret effect are negligible, the Soret effect can appreciably affect the other exergy loss contributions and hence the total exergy loss. Hence, the Soret effect, unlike what has usually been assumed, is not negligible in flame calculations at least when this effect is known to affect laminar flame speed.     

Analysis of exergy losses in laminar premixed flames of methane/hydrogen blends

Combustion is the primary source for exergy loss in power systems such as combustion engines. To elucidate the exergy loss behaviors in combustion and explore the principle for efficiency improvement, the second-law thermodynamic analysis was conducted to analyze the energy conversion characteristics in laminar premixed flames of methane/hydrogen binary fuels. The sources causing exergy losses in laminar premixed flames included five parts, namely heat conduction, mass diffusion, viscous dissipation, chemical reactions and incomplete combustion, respectively. The calculations were conducted at both atmospheric and elevated pressures, with the equivalence ratio varying from 0.6 to 1.5 and the hydrogen blending ratio increasing from 0% to 70%. The results indicated that the total exergy loss firstly increased and then decreased with increased equivalence ratio, and reached the minimum value at the equivalence ratio of 0.9. This was primarily due to the trade-off relation between the decreased exergy loss from entropy generation and the increased exergy loss from incomplete combustion, as equivalence ratio increased. As the hydrogen blending ratio increased from 0% to 70%, the total exergy loss decreased by 2%. Specifically, the exergy loss from heat conduction decreased, primarily due to the decreased flame thickness. Moreover, the reactions with H₂, H and H₂O as reactants were inhibited, leading to decreased the exergy loss from chemical reactions. As pressure increased from 1 atm to 5 atm, the total exergy loss decreased by 1%, because the exergy losses induced by heat conduction and chemical reactions decreased as the flame thickness was reduced. The exergy loss from incomplete combustion also decreased, because elevated pressure inhibited dissociations and decreased the mole fractions of incomplete combustion products.     

Optical characterization of tandem absorber/reflector systems based on titanium oxide–carbon coatings

The use of chemical methods to produce coatings with good spectrally selective properties is described. Sol–gel and dipping techniques were combined to fabricate thickness-sensitive solar-selective absorbing coatings based on titanium oxide combined with carbon black (TiO₂/CB) and carbon nanotubes (TiO₂/CNT) on top of stainless-steel substrates. The low to high reflectance transition (λ_T) observed in the tandem system based on TiO₂/CB films was smooth and red shifted when compared to the sharp transition observed in systems based on mesoporous TiO₂/CNT films. Mesoporosity of the oxide was controlled by the use of polymeric surfactants with different molecular weights, causing a significant blue shift in λ_T as the molecular weight increases. These films were characterized by X-ray diffraction and UV–vis light absorption spectroscopy.     

Effect of high copper and oxygen concentrations on the optical and electrical properties of (CdTe)xCuyOz films

Thin films of (CdTe)_xCu_yO_z have been prepared by reactive RF cosputtering using high concentrations of copper and oxygen. The films were grown at 350 °C on glass and Si substrates. Under these conditions samples of amorphous nature were obtained with some clusters of Cu₂O for the larger concentrations of Cu and O used in this work. The largest band gap variation, from 3.5 to 1.4 eV, was obtained for the samples grown with an oxygen flow of 17 standard cubic centimeters per minute (sccm) in the growth chamber. The samples are highly resistive for most cases, but for high Cu concentrations resistivities of the order of 10³ Ω-cm were obtained in the case of films grown with a flow of 15 sccm of oxygen.