J. Thomas McKinnon

Intense, hyperthermal source of organic radicals for matrix-isolation spectroscopy

We have incorporated a pulsed, hyperthermal nozzle with a cryostat to study the matrix-isolated infrared spectroscopy of organic radicals. The radicals are produced by pyrolysis in a heated, narrow-bore (1-mm-diam) SiC tube and then expanded into the cryostat vacuum chamber. The combination of high nozzle temperature (up to 1800 K) and near-sonic flow velocities (on the order of 104 cm s−1) through the length of the 2 cm tube allows for high yield of radicals (approximately 1013 radicals pulse−1) and low residence time (on the order of 10 μs) in the nozzle. We have used this hyperthermal nozzle/matrix isolation experiment to observe the IR spectra of complex radicals such as allyl radical (CH2CHCH2), phenyl radical (C6H5), and methylperoxyl radical (CH3OO). IR spectra of samples produced with a hyperthermal nozzle are remarkably clean and relatively free of interfering radical chemistry. By monitoring the unimolecular thermal decomposition of allyl ethyl ether in the nozzle using matrix IR spectroscopy, w...

Supramolecular BioNanocomposites: Grafting of Biobased Polylactide to Carbon Nanoparticle Surfaces

Novel carbon nanostructures are attracting increasing interest and the combination of graphitic substrates with grafted biodegradable polymers may ultimately be of interest in a variety of biomedical and sensing applications. Here, a novel graphitic nanosubstrate, carbon nanospheres derived from cellulose, is functionalized with polylactides (PLA) using an established thionyl chloride intermediate scheme; the resulting supramolecular bionanocomposite is 97% from renewable resources. In addition, a direct ‘grafting from’ approach is utilized to grow polylactide chains on multi-walled carbon nanotubes (MWCNT). In the latter case, unlike previous approaches, the ring-opening polymerization is initiated directly from a hydroxyl bearing surface. Verification of the covalent attachment and characterization of the grafted layer are accomplished via a variety of techniques and methods. Even after repeated washing, thermal gravimetric analysis clearly shows the presence of a grafted layer, which decomposes at approximately 300°C, a value characteristic of PLA; it is found that 20 mg m–2 of PLA is grafted to the MWCNT and 3.9 mg m–2 of PLA is grafted to the carbon nanospheres. Solubility tests clearly show the graphitic structures have been fundamentally altered in their physiochemical properties; they become highly soluble in chloroform after the grafting reaction is complete. Transmission electron microscopy provides evidence of a 2–3 nm thick polymer layer. Finally, Fourier transform infrared spectroscopy shows several characteristic peaks of PLA including the ester group at 1760 cm–1.

Dimerization of cyclopentadienyl radical to produce naphthalene

Report on initial findings on the pyrolysis of triphenylmethylcyclopentadiene and their relevance to the flame chemistry of the cyclopentadienyl radical

A modeling study of the mechanisms of flame inhibition by CF3Br fire suppression agent

This paper presents the methodology and results of a modeling study undertaken to identify how Halon 1301 (CF 3 Br) interacts with a flame at the level of elementary reactions. The main purpose is to isolate the chemical kinetics and physical phenomena associated with flame inhibition mechanisms. The specific interest in the study of halons is to further understand how CF 3 Br functions as an effective flame suppresant so that this information can be used to identify replacement agents. As a results of this work, both the ways in which CF 3 Br molecules directly participate in flame inhibition and the contributions of the Br and CF 3 fragments have been identified. The predominant mechanisms include: (1) trapping of H-atoms diffusing in the direction opposite to the convective flow through the reaction H+CF 3 Br=CF 3 +HBr, thus reducing the level of chain branching in the preheat zone, (2) consumption of H-atom through the reaction H+HBr=H 2 +Br, (3) endothermic cooling of the flame by the unimolecular dissociation of CF 3 Br, and (4) terminating radicals in the preheat zone by the reaction Br+HO 2 =HBr+O 2 . The efficacy of the various mechanisms is quantitatively ranked by the computed decrease in the laminar flame speed relative to an undoped ethylene/air base case.

High-temperature UV-visible absorption spectral measurements and estimated primary photodissociation rates of formaldehyde, chlorobenzene and 1-chloronaphthalene

Measurements of the high-temperature UV-visible absorption spectra of formaldehyde, chlorobenzene and 1-chloronaphthalene and estimates of their high-temperature photodissociation rates are presented. The photodissociation rate estimates were based on the measured absorption spectra and an assumed direct terrestrial spectral radiant flux for a 1.5 air mass atmosphere. The results indicate a strong dependence of the solar photodissociation rate on the temperature.

Mechanisms of Flame Quenching by Chlorine in Well-Stirred Reactorś

An evaluation of chlorine inhibition effects on the hydrocarbon combustion process was made using an elementary reaction mechanism consisting of 305 reactions and 77 species. The evaluation was conducted, by adding 1% (mole basis) elemental chlorine to fuel-lean and fuel-rich, C2H4 air mixtures, and running numerical simulations based on a perfectly-stirred reactor configuration. The evaluation included analysis of: chlorine-catalyzed free radical recombination cycles, carbon flux pathways, principal chain branching reactions, free radical flux pathways, chlorine flux pathways, and reaction exotherrnics. Additionally, the evaluation incorporated the use of a hypothetical molecule “deuterium” that possessed thermodynamic properties similar to hydrogen and chemical properlies similar to chlorine. From this evaluation, it was determined that both chemical and thermal effects result from the addition of chlorine. The primary chemical effect results from suppression of the O-atom concentration. The primary the...

The interaction of water mists and premixed propane-air flames under low-gravity conditions

A preliminary investigation of the effect of water mists on premixed flame propagation in a cylindrical tube under low-gravity conditions has been conducted to define the scientific and technical objectives of the experiments to be performed on the Space Shuttle and International Space Station microgravity environments. The inhibiting characteristics of water mists in propagating flames of propane-air mixtures at various equivalence ratios are studied. The effects of droplet size and concentration on the laminar flame speed are used as the measure of fire suppression efficacy. Flame speed and propagation behavior are monitored by a video camera. Reduced gravity is obtained with an aircraft flying parabolic trajectories. Measurements and qualitative observations from the low-gravity experiments clearly show the effect of water mist on flame speed abatement, flame shape, and radiant emission. For lean propane-air mixtures, the flame speed increases at first with low water-mist concentrations and then decreases below its dry value when higher water-mist volumes are introduced in the tube. This phenomenon may be due in part to the heating of the unburned mixture ahead of the flame as a result of radiation absorption by the water droplets. For rich propane-air mixtures, similar behavior of flame speed vs. water concentration is encountered but in this case is mostly due to the formation of cellular flames. At high water loads both lean and rich flames exhibit extinction before reaching the end of the tube.A preliminary investigation of the effect of water mists on premixed flame propagation in a cylindrical tube under low-gravity conditions has been conducted to define the scientific and technical objectives of the experiments to be performed on the Space Shuttle and International Space Station microgravity environments. The inhibiting characteristics of water mists in propagating flames of propane-air mixtures at various equivalence ratios are studied. The effects of droplet size and concentration on the laminar flame speed are used as the measure of fire suppression efficacy. Flame speed and propagation behavior are monitored by a video camera. Reduced gravity is obtained with an aircraft flying parabolic trajectories. Measurements and qualitative observations from the low-gravity experiments clearly show the effect of water mist on flame speed abatement, flame shape, and radiant emission. For lean propane-air mixtures, the flame speed increases at first with low water-mist concentrations and then decrea...

Modeling study of water-mist as flame suppressant

Presence of buoyancy in terrestrial experiments intricately links fluid mechanics with the chemistry of the fire suppression agent operation. Decoupling these two factors allows for the chemical effects to be understood. The objectives of this study are to use the unique, non-buoyant properties of microgravity to discern the fundamental governing parameters involved in the selection and use of post-Halon generation fire suppression agents. One such fire suppression agent is the fine water mist (less than 200 μm). The information on the effects of water mist on combustion is limited to the study of large-scale fire systems developed for specific practical applications. Little fundamental information exists on interaction of a water mist and the propagating flame. We have studied the water mist interaction with a premixed CH4/Air flame using the CHEMKIN code. Our mechanism consists of over 200 elementary reactions most of which come from the GRI 2.1 mechanism for methane combustion. Water droplet evaporatio...

The water-mist fire suppression experiment: Recent ground-based results and planned space experiments

An investigation of the effect of water mists on premixed flame propagation in a cylindrical tube under reduced-gravity conditions has been conducted to define the scientific and technical objectives of the experiments to be performed in the Space Shuttle and International Space Station microgravity environments. The inhibiting characteristics of water mists in buoyancy-free propagating flames of propane-air mixtures at various equivalence ratios are investigated. The effects of droplet size and concentration on the laminar flame speed and flame shape are used as the measure of fire suppression efficacy. Reduced gravity is currently obtained in the NASA KC-135 aircraft flying parabolic trajectories. Two different types of flame behavior are found depending on the mixture stoichiometry. In the case of lean C3H8-air mixtures, the flame speed initially increases with low water mist concentrations and then decreases below its dry value when higher water mist volumes are introduced in the tube. For rich C3H8-a...