The work examines the unique nanostructure of carbon nanoparticles deposited from sooting premixed flames with flame temperatures exceeding 2200 K. This flame temperature regime has previously been shown to transition from typical soot formation conditions to a regime whereby the flame-form carbon adopts a nanostructure considerably more ordered than soot. Graphenic carbon deposits observed by High-resolution TEM (HRTEM) are reported here corroborating previous Raman spectroscopy evidence. The use of premixed stretchstabilized flames enables particle production in the high-temperature regime under a flow field amenable to lowdimensional flame modeling. Although the flame flow configuration is relatively simple, three sample preparation methods are used to assess the representation of true carbon properties as they exist in the flame. HRTEM imaging is carried out on carbon particle samples prepared by rapid-insertion deposition, aerosol dilution probe deposition and carbon particle film deposition. Images from rapid-insertion samples show amorphous particles in the lightly sooting flame and turbostratic particles in the heavy sooting flame. There is trace evidence of graphenic structure in rapid-insertion samples but the most striking particles on the TEM grid are graphite nanocrystals presumably formed by a new artificial crystallization process. HRTEM images of particles collected over time by diluted aerosol deposition and film deposition show clear graphenic structures. Overall, the carbon nanostructure observed by HRTEM is a mixture of amorphous, turbostratic and graphenic carbon lattices depending on the flame condition and sampling method. The current work highlights potential impacts of higher flame temperatures and higher equivalence ratio on deposited flame-formed carbon. Namely, graphenic particle structure is observed in rapid-insertion deposition samples but graphene portions are most abundant in aerosol dilution and carbon particle film deposition samples. This may indicate that graphene structures grow on the deposition surface over time.

Conventional carbon black production occurs by pyrolysis after heavy aromatic feedstock is injected into the post-combustor region of furnace black reactors. The current work examines the conversion of the coal tar distillate in turbulent spray flames to demonstrate a more compact reactor configuration. Coal tar distillates diluted in toluene is atomized and burned in a standardized flame spray synthesis configuration, known as SpraySyn. Flame conditions are characterized by thermocouple, soot pyrometry and image analysis and product particle properties are examined by TEM and Raman spectroscopy. The measured flame temperature corresponds to the range of temperatures used in the furnace black process, but the current synthesis includes oxidizing conditions and faster residence times. The resulting carbon black particles are aggregates with primary particle sizes on the small end of the carbon black size spectrum, according to analysis of TEM images. Carbon black, formed under a range of flame temperatures, show Raman spectra with features resembling typical carbon black materials. Conversion of coal tar distillate to carbon black by direct flame synthesis may be a scalable method to produce high-surface area grades without a conventional pyrolysis reactor stage.

A complementary computational and experimental study is carried out on the formation of ultrafine particulate matter in premixed laminar methane air flames. Specifically, soot formation is examined in premixed stretch-stabilized flames to observe soot inception and growth at relatively high flame temperatures common to oxygen enriched applications. Particle size distribution functions (PSDF) measured by mobility sizing show clear trends as the equivalence ratio increases from Φ = 2.2 to Φ = 2.4. For a given equivalence ratio, the measured distribution decreases in median mobility particle size as the maximum flame temperature increases from approximately 1,950–2,050 K. The median mobility particle size is 20 nm or less for all flame conditions studied. The volume fraction decreases with increasing flame temperature for all equivalence ratio conditions. The Φ = 2.2 condition is close to the soot inception limit and both number density and volume fraction decrease monotonically with increasing flame temperature. The higher equivalence ratio conditions show a peak in number density at 2,000 K which may indicate competing soot inception processes are optimized at this temperature. Flame structure computations are carried out using detailed gas-phase combustion chemistry of the Appel, Bockhorn, Frenklach (ABF) model to examine the connection of the observed PSDF to soot precursor chemistry. Agreement between measured and computed flame standoff distances indicates that the ABF model could provide a reasonable prediction of the flame temperature and soot precursor formation for the flames currently studied. To the first order, the trends observed in the measured PSDF could be understood in terms of computed trends for the formation of benzene, naphthalene and other soot precursors. Results of the current study inform particulate matter behavior for methane and natural gas combustion applications at elevated temperature and oxygen enriched conditions.

Soot formation in premixed laminar flames is examined for a canonical set of flames burning C6 hydrocarbon fuels. Particle mobility size and flame temperature measurements are complemented by flame structure calculations using detailed flame chemistry. Specifically, the evolution of the detailed soot particle size distribution (PSDF) is compared for n-hexane, n-hexene, 2-methylpentane, cyclohexane and benzene at a carbon-to-oxygen ratio of 0.69 and maximum flame temperature of 1800 K. Under this constraint, the overall sooting process is comparable as evidenced by similar time resolved bimodal PSDF. However, the first inception of particles and the persistence of nucleation-sized particles with time are depend upon the structure of the parent fuel. For the given conditions, the fastest onset of soot is observed in cyclohexane and benzene flames and the observed evolution of the PSDF also shows that nucleation-sized particles disappear sooner in cyclohexane and benzene flames. Flame structure computations incorporating detailed chemistry show a clear connection between the early onset of soot particles as fuel specific routes to PAH formation are predicted in the pre-flame region of the cyclohexane and benzene flames. These observations illustrate the impact of alkane, alkene, cycloalkane and aromatic fuel structure on soot formation in premixed flames. Analysis of soot particle morphology by atomic force microscopy indicates that most of size distribution is composed of aggregates. Simple aggregate mobility diameter analysis shows the spherical assumption taken to interpret the mobility diameter does not impact the PSDF number density result but the inferred volume fraction for aggregates deviates by up to an order of magnitude depending on the morphology assumptions adopted.

A complementary experimental and modeling study is reported here for nucleation of manganese oxide nanoparticles in premixed stagnation flames. The current synthesis occurs at relatively high flame temperature and low precursor loading. Thermodynamic analysis based on the postulated nucleation process, Mn(g) + O-2(g) -> MnO(s), is carried out to quantify precursor supersaturation and potential impacts of the Kelvin effect on particle formation. Nucleation and growth are analyzed based on the computed temperature-time-oxygen history in the postflame region. Agreement between measured and computed flame position for the base flame and precursor doped flames indicates that the manganese methylcyclopentadienyl tricarbonyl precursor does not inhibit flame chemistry for the conditions currently studied. Particle size distributions measured by mobility particle sizing and TEM images show reasonable agreement. Moreover, the measured particle size is predicted much more closely by a nucleation-limited mechanism rather than the size predicted by coagulation-limited growth.

Soot formed in flames hotter than conventional combustion applications is expected to undergo unique formation processes and develop a carbon structure distinct from typical soot. A complementary experimental and modeling approach is reported here to assess flame temperature and equivalence ratio effects for soot formed in the higher-temperature regime (1950 K < Tf < 2250 K). Computations using three separate combustion chemistry models show that predictions of polycyclic aromatic hydrocarbon (PAH) concentration profiles for the higher-temperature flames are more sensitive to the choice in mechanism rather than specific flame conditions. As for material properties, the Raman signatures transition from a typical soot spectrum to features observed in disordered sp2 carbon materials. The defect distance extracted from the Raman bands nearly doubles from values typically reported for soot as the flame temperature exceeds 2200 K. Higher concentrations of gas-phase precursors may facilitate development of an ordered carbon structure as indicated by the relatively high defect distance observed for the highest equivalence ratio series. Particle size distributions measured by mobility sizing show size and yield of soot decreases with increasing flame temperature and the bimodal distribution falls within the ultra-fine range for all flame conditions. This is especially promising if the significant transformation in carbon structure inferred from the evolution in Raman spectra enables development of functional high-surface area sp2 carbon materials. Namely, the current observations indicate that the flame-formed carbon structure evolves towards high-defect sp2 carbon with size and carbon structure that can be tuned to some extent.

The dataset presented in this article is linked to the research article titled “Evolution in size and structural order for incipient soot formed at flame temperatures greater than 2100 K” [1]. The research article discusses the systematic evolution of flame formed carbon in premixed stagnation flames with flame temperatures hotter than conventional combustion applications. The effect of the growth environment on particle size, structure, composition and properties are studied. The flame temperature (1950 K < Tf,max < 2250 K) and equivalence ratio (Φ = 2.4, 2.5, and 2.6) are methodically varied to analyze impact on insipient soot while maintaining a comparable particle residence time (tp ~ 15 ms). This article presents the data acquired for this systematic study. The data presented herein provides fundamental observations suitable for development of soot formation theory and modeling. Characterization of material properties and morphology are also relevant to potential applications of functional carbon nanomaterials. Raman spectra are measured for carbon films deposited from the flames, soot particle size distributions are obtained by aerosol sampling from the flames and soot radiative emissions are measured in-situ by color-ratio pyrometry. Deconvolution of Raman peaks is carried out to extract information on carbon bonding and structural order. Flame temperature is extracted from the measured color-ratio field making assumptions for the soot optical dispersion exponent.

Particle nucleation and growth of crystalline manganese oxide nanoparticles was examined in a complementary experimental and modelling study. Gas-to-particle conversion occured in a flame-assisted chemical vapor deposition process whereby a premixed stagnation flame drove the high-temperature synthesis. The structure of the stagnation flame was computed using pseudo one-dimensional and axisymmetric two-dimensional methods to assess the accuracy of using a faster similarity-based calculation for flame-deposition design. The pseudo one-dimensional computation performs reasonably well for the narrow aspect ratio stagnation flow currently studied as evidenced by reasonable agreement between the measured flame position and both computational methods. Manganese oxide nanoparticles having II, II-III, III or IV oxidation states were observed depending on the flame conditions. These observations may be explained by size-dependent equlibria between nano-scale manganese oxide and surrounding gas-phase oxygen. Local equibrium was assessed during the particle temperature-oxygen-time history to gain insight into oxide formation in the flame. Analysis of the saturation ratio for formation of condensed MnO in the flame indicates that nucleation may be limited by a thermodynamic barrier. This nucleation mechanism is supported by measured particle sizes smaller than what would be expected from a coagulation limited growth process. Nanocrystalline MnO, reported here for the first time by flame synthesis, was obtained in oxygen lean flames. MnO2 is the phase predicted to be thermally stable as the particles approach the deposition surface, yet other metastable oxide phases were produced in many of the flames examined. In fact, MnO2 was only observed in the smallest particle size conditions which may indicate that high cooling rates limit phase equilibrium to less massive particles.

See also: CrystEngComm (2020) 22, 5509-5521"Formation of nanocrystalline manganese oxide in flames: oxide phase governed by classical nucleation and size-dependent equilibria" by Shruthi Dasappa and Joaquin CamachoSynthesis of manganese oxide nanoparticles was carried out in premixed stagnation flames. The deposition surface was stationary to enable rigorous comparison to flame structure computations using pseudo one-dimensional and full two-dimensional calculations. The pseudo one-dimensional assumption taken by OPPDIF to calculate the flame structure performs reasonably well for the narrow aspect ratio stagnation flow currently studied. Agreement between the measured flame position and both computational methods was within 0.25 cm. The variation in manganese oxidation state in the products observed here is shown to be caused by competing oxidation and particle nucleation processes. Complimentary experimental and modeling studies of flame synthesis were carried out under well-defined boundary conditions to examine this competition. Manganese oxide products having II, II,III, III and IV valence were observed depending on the flame conditions. Unlike iron oxide and titanium oxide systems, complete oxidation to MnO2 is only observed for the most oxidizing growth conditions. The first observation of phase pure MnO by flame synthesis is reported here. Manipulation of the balance between manganese oxidation and particle nucleation through control of the time-temperature-oxygen history may enable selective synthesis of manganese oxide nanoparticles with tailored morphology and oxidation state.

The Cover Feature depicts the role of oxygen desorption in the phase preference of titanium oxide (TiO2) nanoparticles synthesized in a flat flame. The rutile–anatase phase equilibrium is impacted by surface oxygen coverage. A more generalized thermodynamic approach is proposed to account for the effect of O2 desorption on surface energy and crystal phase equilibrium. More information can be found in the Full Paper by H. Wang et al. on page 180 in Issue 2, 2018 (DOI: 10.1002/cphc.201700962).

Real jet fuels are complex mixtures of many organic components, some of which are aromatic compounds. Towards the high-temperature end of the distillation curve, some of the fuel components are multi-ring compounds. A small amount of these high molecular weight species in the fuel could impact soot nucleation in practical engines especially when the fuel is injected as a spray. This work aims to highlight the variation of the sooting propensity of jet fuels as a function of distillate fractions and to examine the validity of a surrogate fuel in emulating soot production from real fuels. Particle size distribution functions and soot volume fractions are studied in a series of laminar premixed stretch-stabilized ethylene flames doped with Jet A, its various distillate fractions, and the 2nd generation MURI surrogate. Soot formation as a result of doping real jet fuel and its distillate fractions is also investigated in counterflow and coflow diffusion flames. The results show that the higher-boiling distillates mostly influence soot nucleation and produce substantially more soot in nucleation controlled flames than the light molecular fraction and jet fuel as received, while such an effect is seen to be small in flames where soot production is controlled by surface growth. The potential impact of distillate fractions on soot nucleation propensities is discussed.

The evolution in carbon particle size and carbon bonds was observed with increasing flame temperature for a fixed growth time (t(p) similar to 13 ms) and equivalence ratio (Phi = 2.5) in set of sooting premixed stagnation flames. For carbon formed in each flame, detailed particle size distribution functions (PSDF) and Raman spectra (excitation energy of 2.33 eV) were measured as the maximum flame temperature increased from 1911 K < T-f,T-max < 2263 K. The PSDF steadily decreased in size and narrowed in width as the flame temperature increased, a trend which reversibility in precursors is expected to cause for carbon particles formed at elevated temperatures. Several features of the Raman spectra were used to analyze carbon bonds on the flame-formed carbon with increasing flame temperature. Typical features of Raman spectra corresponding to soot were observed for carbon products formed with T-f,T-max = 1911 K. The widths of the overall G (sp(2) in ideal graphite) and D (sp(2) defects) Raman bands narrowed significantly in spectra from elevated temperature flame carbon. The intensity of the amorphous band (D3) relative the G band decreased. For carbon products of the lower temperature flames, a relatively wide band in the vicinity of 1600 cm(-1) encompassing the G band was observed. As the flame temperature increased, two separate peaks were observed in this region; the G band and a separate band in the vicinity of 1620 cm(-1) The characteristic distance between defects was estimated to grow from 1 nm to 2 nm for carbon products formed at T-f,T-max = 1911 K and 2260 K, respectively. The corresponding surface area observed at these conditions was on the order of 600 nm(2) and 200 nm(2) per particle, respectively, which indicates the relative area of ordered structures on the particle increases. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

In previous studies we used a tubular probe to sample incipient soot formed in premixed flat flames and determine the particle size distribution by scanning mobility particle sizing (SMPS). Comparison of the spatially resolved soot volume fractions by SMPS and by thermocouple particle densitometry suggests that the probe causes a positional offset of 0.35cm in comparison to TPD, and indicates that the particle size distributions measured is influenced by flame perturbation by the probe. Moreover, the flame boundary conditions are difficult to define. In the present work, we propose a burner-stabilized stagnation-flow (BSSF) flame approach, whereby the particle probe is imbedded in a watercooled flat plate which acts both as a sample probe and flow stagnation surface. As the boundary conditions of this flow configuration can be completely defined experimentally, the flame was simulated with a quasi 1-D stagnation flow code considering radiative heat losses by CO2 and H2O. It was found that the simulated and measured gas temperature profiles compare well. The evolution of the soot size distributions obtained by the BSSF approach was found to be similar to that of the tubular probe. * Corresponding author: haiw@usc.edu Associated Web site: http://ignis.usc.edu Proceedings of the 6th U.S. National Combustion Meeting Introduction Probe sampling with scanning mobility particle sizing (SMPS) is now routinely used to follow the size evolution of soot formed in flames [1-17]. Using this technique, soot nucleation and mass/size growth may be closely investigated by resolving spatially the detailed particle size distribution (PSDF). Studies in premixed flat flames and well stirred reactors have been reported. They covered a broad range of experimental conditions and investigated the effect of temperature, fuel structure, and equivalence ratios on the detailed processes close to and beyond soot inception. Particles as small as 1.6 nm can be probed [11, 16], and the same technique has been applied to the analysis of metal oxide and other nanomaterial formation in flames [9, 18-20]. Results obtained from the probe samping/SMPS technique also aided fundamental soot model development and validation (see, e.g., [2, 17, 21-23]) and yielded useful information about the elementary processes associated with soot formation. One of the key observations is that the PSDF of incipient soot is persistently bimodal [16]. The cause for bimodality is the competition between particle inception and particleparticle coagulation [2]. It was shown that the shape of size distributions and its evolution can provide a wealth of kinetic information about elementary sooting processes [21]. Despite its increasingly widespread use, the probe sampling/SMPS technique has two fundamental drawbacks. First, care must be taken to minimize particle loss in the sampling probe [3]. The loss mechanisms include diffusion of the particles to the probe walls and coagulation among particles. In general, preventing particle losses requires rapid and large dilution of the flame sample by a cold, inert gas stream – a problem that can be solved with a proper probe design [3][13, 16]. The second drawback is more difficult to deal with, and indeed it has not been properly addressed. Probe sampling is inherently intrusive. An earlier study [3] demonstrated that the use of a tubular probe placed horizontally across a premixed flat flame (see, Figure 1) causes a significant drop in the local flame temperature near the sampling point, and this influence extends several millimeters from the probe. In addition, the probe also introduces local flow stagnation, leading to a longer reaction time than that in a burner-stabilized flame free of solid objects. More recently, probe perturbation and its influence on soot growth kinetics has also been discussed by Sgro et al. [15]. This problem is also expected to occur in molecular beam mass spectrometry studies of low-pressure premixed flat flames, in which a conical nozzle is inserted into the flame to sample flame species (see, e.g., [24, 25]). Figure 1. Schematic illustrating a previous tubular probe sampling technique [2, 3].

Nano-scale titanium oxide (TiO2 ) is a material useful for a wide range of applications. In a previous study, we showed that TiO2 nanoparticles of both rutile and anatase crystal phases could be synthesized over the size range of 5 to 20 nm in flame-assisted chemical vapor deposition. Rutile was unexpectedly dominant in oxygen-lean synthesis conditions, whereas anatase is the preferred phase in oxygen-rich gases. The observation is in contrast to the 14 nm rutile-anatase crossover size derived from the existing crystal-phase equilibrium model. In the present work, we made additional measurements over a wider range of synthesis conditions; the results confirm the earlier observations. We propose an improved model for the surface energy that considers the role of oxygen desorption at high temperatures. The model successfully explains the observations made in the current and previous work. The current results provide a useful path to designing flame-assisted chemical vapor deposition of TiO2 nanocrystals with controllable crystal phases.

The evolution of the nascent soot particle size distribution function (PSDF) was determined by mobility sizing for two series of atmospheric pressure premixed ethylene flames in the burner stabilized stagnation flame configuration. The first series of flames has an equivalence ratio of 1.8, corresponding to conditions just above the sooting limit. The second series has an equivalence ratio of 2.5 and is quite heavy in soot production. In each series, six flames were tested in which the cold gas velocity is varied to obtain flame temperatures ranging from 1559 to 1941 K. The temperature profiles were carefully determined and the comparison to pseudo-one dimensional simulations was satisfactory. It was found that the evolution of the PSDFs with respect to flame stoichiometry, temperature and growth time is consistent with the understanding of kinetic competition during soot formation. Finite rate kinetic limitations are observed at lower temperatures and thermodynamic reversibility occurs at higher temperatures. The observed PSDF features are highly sensitive to competition among the various processes of soot formation, from nucleation to coagulation and gas–surface reactions. The PSDFs are mostly bimodal with both nucleation and coagulation mode particles present. The evolution of the PSDF indicates a strong contribution to the mass of coagulation-mode by the nucleation-mode particles. The measured PSDFs offer comprehensive, canonical data sets useful for testing models of soot formation.

Probe sampling of soot particles in laminar premixed flames is a common method for characterizing nascent soot formation. Probe intrusiveness into the flame can introduce significant uncertainty in interpretation of experimental data and comparison with numerical results. The aim of the present work is to study the probe-induced effects on soot sampling in a burner-stabilized stagnation (BSS) flame by numerical simulations. The thermophoretic effect is investigated first under non-reactive conditions. The relevant model formulation was tested against experimental data from the literature. Soot size distributions and global properties in the burner stabilized-stagnation flame configuration are studied using both a one-dimensional stagnation flow model and two-dimensional axisymmetric simulations using detailed kinetics and transport. A benchmark burner-stabilized stagnation flame fed with ethylene (Camacho et al., 2015 ) was employed as the target for detailed investigation, focusing on the quantification of the orifice flow effect on the soot size distribution. The results show that the orifice flow can introduce a notable impact on the local flow field, temperature, and particle residence time. Soot measurements have to be shifted some millimeters upstream from the stagnation surface because of the impact of the orifice on the local flow and temperature field. The extent of the spatial shift was quantified by comparing one-dimensional stagnation flow and two-dimensional axisymmetric simulations. The results showed that the spatial shift is weakly dependent on fuel chemistry, but it exhibits stronger dependencies on burner to stagnation separation, pressure drop across the orifice, unburned gas velocity, and the orifice diameter. The extent of spatial shift is parameterized with respect to these experimental parameters.

Evolution of particle size distribution function (PSDF) was studied in premixed stretch-stabilized flat flames for the first time. The purpose is to demonstrate that stretch-stabilized flames can broaden the experimental flame condition space for studying soot formation in a pseudo-one dimensional flow configuration. PSDFs were measured in three series of atmospheric-pressure ethylene–oxygen–argon flames with maximum temperatures around 1980, 2000, and 2160K. The measured PSDFs show a strong effect of flame temperature as nucleation and growth of soot is found to be suppressed towards high temperatures. Simulations using a population balance soot model show reasonably good agreement with the 1980 and 2000K series of flames, but it significantly overpredicts the number density and size of soot in the highest temperature flame. Numerical tests suggest that the discrepancy can be caused by high-temperature reversibility in surface growth and other processes.