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A new crosslinking reaction between two pentagonal rings around the periphery of planar pericondensed aromatic molecules is proposed and its impact on soot nanoparticle formation explored. The reaction mechanism was computed, using density functional theory, between an aryl-type σ-radical on arim-based pentagonal ring attacking another rim-based pentagonal ring. A hydrogen migration allowed for the formation of a double bond forming a planar aromatic penta-linked hydrocarbon (APLH) complex, recently experimentally observed. The clustering of this planar species is compared with a pericondensed polyaromatic hydrocarbon (PCAH) and an aromatic aryl-linked hydrocarbon (AALH) using molecular dynamics and metadynamics. Similar clustering is found for the investigated species compared with a peri-condensed structure of similar mass indicating enhanced physical interactions after forming the crosslink. Finally, a further crosslink is possible between the unsaturated pentagonal ring sites forming an aromatic rim-linked hydrocarbon (ARLH) complex of considerable stability. This was con firmed by simulating the stable molecular dynamics of such a complex with on-the-fly quantum forces from a quantum semi-empirical method, revealing possible reactions under ame conditions that might play a role in soot nucleation.

The self-assembly and structure of nanoparticles containing curved polycyclic aromatic hydrocarbon molecules (cPAHs) are investigated using molecular modelling. These polar fullerene-like molecules are receiving increased attention recently due to the steric and electronic properties caused by the inclusion of five-membered ring(s) within their hexagonal lattice. In this work, the curPAHIP potential is extended to describe the interactions between large cPAHs. It is then used within molecular dynamics simulations to produce nanoparticles containing cPAHs. Structural and energetic metrics, including diameter, density, intermolecular spacing, coordination number, alignment angle, radial distance, and energy value, are used to analyse systems containing cPAHs of different sizes and ratios, and containing flat PAHs or ions. Homogeneous cPAH particles are more tightly packed than their flat PAH counterparts, with large cPAHs displaying stacked columnar configurations absent in nanoparticles containing small cPAHs. Mixing cPAHs of different sizes disrupts the ordered mesophase and forms a core-shell structure in which the larger molecules make up the core and the smaller molecules comprise the shell, although this partitioning is less distinct compared to flat PAHs. In addition, the presence of flat PAHs and ions within cPAH nanoparticles promotes distinct arrangements dominated by weak dispersive interactions and strong electrostatic interactions, respectively.

The growth of carbonaceous materials was studied using a Kinetic Monte Carlo model that captures the growth and oxidation of six-member and partially-embedded five-member rings. A novel algorithm was used to resolve the migration of partially-embedded five-member rings around the edges of molecules. Circumcoronene molecules were grown at 1500 K and 1 atm in the presence of varying mole fractions of atomic and molecular oxygen and constant mole fractions of hydrogen and acetylene. The parameter space of the study covered the mole fraction of atomic and molecular oxygen in the ranges: 10-8 ≤ XO ≤ 10-1 and 10-6 ≤ XO2 ≤ 10-1. Four regions of carbon growth associated with different carbonaceous products were identified. Graphene was formed in the presence of high mole fractions of atomic oxygen (10-4 < XO ≤ 10-2). Fullerenes were formed in the presence of low mole fractions of atomic oxygen and high mole fractions of molecular oxygen (XO ≤ 10-4 and 10-2 < XO2 ≤ 10-1). Low mole fractions of both atomic and molecular oxygen (XO ≤ 10-4 and XO2 ≤ 10-2) resulted in structures that became curved as time progressed. The highest mole fractions of atomic oxygen (XO > 10-2) produced small structures due to oxidation of the molecules. The production and consumption of partially-embedded five-member rings appear to explain the formation of the observed structures. The oxidation of partially-embedded five-member rings leaves behind armchair sites that grow to form large and flat structures that resemble graphene. Formation and subsequent embedding of partially-embedded five-member rings result in curved structures that resemble fullerenes.

This paper presents a systematic study of the reactivity of polycyclic aromatic hydrocarbons (PAH), identifying crosslinks that permit the combination of physical π-stacking interactions and covalent bonding. Hybrid density functional theory was used to identify the location of reactive sites on PAHs using the average local ionization potential. The bond energies formed between these various reactive sites were then computed. σ-Radicals were found to be the most reactive, forming bonds with other radicals and some reactive closed shell edge types. Partially saturated rim-based pentagonal rings were found to form localized π-radicals with high reactivity. This site, in addition to resonantly stabilized π-radicals, was found to be capable of bonding and stacking, which is explored for a variety of larger species. Localized π-radicals on rim-based pentagonal rings, in particular, were found to form strongly bound stacked complexes, indicating a potentially important role in soot formation.

The formation of carbon nanoparticles in flames involves a nucleation step that remains poorly understood. Experimentally, carbon nuclei formation is known to depend strongly on the electrical aspects of combustion but modes of interaction between charged species in the flame and carbon precursors have yet to be found. We present evidence for flexoelectrically polarised aromatics contributing to carbon nanoparticulate nucleation. We imaged the nascent nanoparticles using high resolution transmission electron microscopy, which revealed that the majority of aromatics in the early carbon nanoparticles are fullerene-like and curved. The curvature induces a significant molecular flexoelectric dipole moment in the polyaromatic hydrocarbons. This electric polarization allows these molecules to strongly interact with chemi-ions produced during combustion, which we demonstrate using electronic structure calculations. The results indicate that the physical interaction between fullerene-like polar aromatics and chemi-ions is critically assisting the nucleation, and opens a new route to reduce pollution and improve flame-produced nanomaterials.

© 2016 The trajectories of a single nitrogen molecule resulting from a series of collisions with coronene molecular clusters of varying size are determined numerically by means of classical molecular dynamics simulations at two system temperatures, corresponding to the clusters being in solid and liquid state. The observed bimodality of the residence time distributions that corresponds to a combination of specular and diffuse molecular scattering tends to disappear with increasing temperature due to the more rapid rearrangements of the coronene cluster constituent molecules in the liquid state. The mean residence time decreases with increasing system temperature and appears to be independent of the coronene cluster size within the cluster size-range considered here. The recorded trajectories of the nitrogen probe are relatively tortuous, on average one order of magnitude longer than the shortest path connecting the impact and desorption points. The vast majority of the sites visited during the nitrogen molecule residence period correspond to the atoms at the edge of coronene molecules, mainly hydrogens. The intermolecular cohesive forces between the molecules cause that the coronene clusters are impenetrable by the nitrogen probe at temperatures below their thermal dissociation point.

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University. A technique has been developed and applied to monitor corrosion current transients, in phase with a cyclic stress. applied in reverse bending at 24 Ez to AISI 316 stainless steel plate immersed in selected aqueous media. The amplitude and waveform of this fluctuating current component, together with the measurement of the corrosion characteristics during fatigue, yield information on the nature of the stress-environment interaction. In conditions where the metal surface was passive, a mechanism is proposed, which involves the repeated transient depassivation of the surface at persistent slip bands, leaving ultimately to premature crack initiation. In conditions where the metal surface was active (transpassive) a mechanism is proposed, which involves the preferential dissolution at slip steps with resultant enhancement in plastic strain and premature crack initiation. This study is funded by the Science Research Council.

This paper investigates the impact of cyclic fuels on the nanostructure, nucleation and overall production of soot in an n-heptane (C7H16) laminar coflow diffusion flame. The fuels selected to dope the n-heptane flames are cyclopentene (C5H8), cyclohexene (C6H10) and methylcyclohexane (C7H14). These fuels were chosen for their differences in their structure and sooting tendency. The flame structure was studied with Differential Mobility Spectrometry (DMS) for particle size distribution determination, two-colour ratio pyrometry to calculate the soot volume fraction and soot temperature. The soot nanostructure was investigated using Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). The addition of cyclic fuels was found to promote the formation of soot nanoparticles earlier in flames. In addition, the soot volume fraction was increased significantly by the addition of the cyclic fuels, especially by the addition of cyclopentene. The addition of 20% of cyclopentene increased the soot volume fraction by a factor of 2. HRTEM results suggest a significant influence of cyclopentene on the soot nanostructure; cyclopentene addition promotes the incorporation of five-membered rings (pentagonal rings) leading to highly curved fringes. This suggests cyclopentene could be used as a fuel to promote curvature in different carbonaceous structures to modify their properties.

A novel, modified coflow burner was developed to study the effect of charge injection from a non-thermal plasma into three helium-diluted laminar coflow diffusion ethylene flames. The frequency of the high voltage (HV) signal was varied to control the ion concentration (charge) injected into the flames. Optical emission spectroscopy was used to characterize the non-thermal plasma while a bias plate methodology was used to gauge the relative amount of charge generated. For different HV signal frequencies, the laser-induced fluorescence of OH, chemiluminescence of CH*, and laser-induced incandescence of soot in flames were measured. The OH and CH* measurements showed that the flames retained the classic flame shape with charge injection. Significant soot reduction was observed at low HV signal frequencies, corresponding to an increase in charge injection. Notably, at low HV signal frequency, soot reduction in highly concentrated (60%) ethylene flame is three times lower than the less concentrated (32%) ethylene flame. This can be attributed to the decrease in the injected charge to soot precursor concentration ratio when the concentration of ethylene in the flame is increased. These results demonstrate that the current system is a promising candidate for studying the charge effect from non-thermal plasma on soot formation in laminar coflow diffusion flames.

© 2018 The Combustion Institute. In this paper, we consider the impact of polar curved polycyclic aromatic hydrocarbons (cPAH) on the process of soot formation by employing electronic structure calculations to determine the earliest onset of curvature integration and the binding energy of curved homodimers. The earliest (smallest size) onset of curvature integration was found to be a six ring PAH with at least one pentagonal ring. The σ bonding in the presence of pentagons led to curvature, however, the π bonding strongly favored a planar geometry delaying the onset of curvature and therefore the induction of a flexoelectric dipole moment. The binding energies of cPAH dimers were found to be of similar magnitude to flat PAH containing one or two pentagons, with an alignment of the dipole moments vectors. For the more curved structures, steric effects reduced the dispersion interactions to significantly reduce the interaction energy compared with flat PAH. Homogeneous nucleation of cPAH at flame temperatures then appears unlikely, however, significant interactions are expected between chemi-ions and polar cPAH molecules suggesting heterogeneous nucleation should be explored.

In this paper, we answer the question of whether polar curved aromatics are persistently polar at flame temperatures. We find, using electronic structure calculations and transition state theory, that the inversion barriers of curved aromatics (cPAH) 0.9–1.2 nm in diameter are high and that they are not able to invert over the timescales and at the high temperatures found in sooting flames. We find a transition for smaller curved aromatics between 11–15 ( ≈ 0.8 nm) rings where the increasing strain introduced from the pentagonal ring increases the inversion barrier leading to rigidity. We then performed ab initio quantum molecular dynamics to find the molecular dipole fluctuations of a nanometre sized cPAH at 1500 K. We found the bending mode of the bowl shaped molecule gave rise to the largest fluctuations on the dipole moment by ± 0.5–1 debye about the equilibrium value of 5.00 debye, indicating persistent polarity. We also observed binding of a chemi-ion at 1500 K over 2 ps, suggesting the molecular dipole of cPAH will be an important consideration in soot formation mechanisms. National Research Foundation (NRF) Accepted version This project was supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme.