The presence or absence of gas phase species during combustion of aluminum nanoparticles (n-Al) is a crucial observable in evaluating competing theories such as a diffusive oxidation mechanism and the melt dispersion mechanism. Absorption spectroscopy was used to probe the ground state of aluminum monoxide (AlO) and Al vapor in order to quantify the amount of Al and AlO present under conditions where these species were not observed in emission previously. Absorption measurements were made during combustion of nanoaluminum and micron-sized aluminum in a heterogeneous shock tube. AlO was detected in absorption at temperatures as low as 2000 K in n-Al combustion, slightly below the limit seen in micro-Al combustion. Al vapor was detected during n-Al combustion at temperatures as low as 1500 K, significantly lower than in micro-Al combustion, suggestive of a gas phase component. The detection limit for Al vapor was 1 Â 10 12 cm À3 . The gas phase component was much weaker than that seen in 10 lm Al combustion. A comparison with n-Al in an inert environment did not show Al vapor at temperatures below 2300 K, even though the equilibrium concentration of Al from particles at that temperature were several orders of magnitude higher than the detection limit. This suggests a nearly pristine oxide coat that inhibits the production of Al vapor in appreciable quantities without reaction. These results are contrary to predictions of the melt dispersion mechanism, which should result in the generation of aluminum vapor from high-energy Al clusters produced from n-Al particles that spallate from mechanical stresses under rapid heating. This should further be independent of the bath gas.

The recombination of allyl radicals (C3H5), generated from the dissociation of 1,5-hexadiene or allyl iodide dilute in krypton, has been investigated in a diaphragmless shock tube using laser schlieren densitometry, LS, (900-1700 K, 10 ± 1, 29 ± 3, 57 ± 3, and 120 ± 4 Torr). The LS density gradient profiles were simulated and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C3H5I → C3H5 + I and C3H5 + C3H5 → C6H10 were obtained and showed strong fall-off. Second order rate coefficients for allyl radical recombination were determined as k(1a,124Torr) = (2.6 ± 0.8) × 10(55)T( -12.995) exp(-8426/T), k(1a,57Torr) = (1.7 ± 0.5) × 10(60)T( -14.49) exp(-9344/T), and k(1a,30Torr) = (7.5 ± 2.3) × 10(66)T( -15.935) exp(-10192/T) cm(3) mol(-1)s(-1). The contribution of a disproportionation channel in allyl radical reactions was assessed, and the best agreement was obtained with no more than 5% disproportionation. Notably, because both the forward and back reactions of C6H10 ⇌ C3H5 + C3H5 were measured, utilizing two different precursors, the equilibrium constant of this reaction could be found, suggesting an entropy of formation of 1,5-hexadiene, 87.3 cal mol(-1 )K(-1), which is significantly smaller than that group additivity predicts, but larger than other reference literature values.

The recombination and disproportionation of allyl radicals has been studied in a single pulse shock tube with gas chromatographic measurements at 1-10 bar, 650-1300 K, and 1.4-2 ms reaction times. 1,5-Hexadiene and allyl iodide were used as precursors. Simulation of the results using derived rate expressions from a complementary diaphragmless shock tube/laser schlieren densitometry study provided excellent agreement with precursor consumption and formation of all major stable intermediates. No significant pressure dependence was observed at the present conditions. It was found that under the conditions of these experiments, reactions of allyl radicals in the cooling wave had to be accounted for to accurately simulate the experimental results, and this unusual situation is discussed. In the allyl iodide experiments, higher amounts of allene, propene, and benzene were found at lower temperatures than expected. Possible mechanisms are discussed and suggest that iodine containing species are responsible for the low temperature formation of allene, propene, and benzene.

A miniature high repetition rate shock tube with excellent reproducibility has been constructed to facilitate high temperature, high pressure, gas phase experiments at facilities such as synchrotron light sources where space is limited and many experiments need to be averaged to obtain adequate signal levels. The shock tube is designed to generate reaction conditions of T > 600 K, P < 100 bars at a cycle rate of up to 4 Hz. The design of the apparatus is discussed in detail, and data are presented to demonstrate that well-formed shock waves with predictable characteristics are created, repeatably. Two synchrotron-based experiments using this apparatus are also briefly described here, demonstrating the potential of the shock tube for research at synchrotron light sources.

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Pyrometry measurements of clouds of high-temperature particles require an estimate of the spectral dependence of the particle emissivity. Common assumptions for this dependence range from " 2 to " constant. Depending upon the assumption used, there is uncertainty in the temperature of 100 s to a 1000 K in hightemperature clouds. Such errors are not apparent in goodness of fit of spectral data. A heterogeneous shock tube was used to measure the emissivity of aluminum oxide in an inert environment as a function of temperature (2000-3500 K), wavelength (0:55-0:95 m), and particle diameter (50 nm-10 m). In micro-sized alumina particles, the spectral dependence upon temperature transitions from decreasing with wavelength to increasing with wavelength with the dependence being roughly gray at about 3000 K. Because of local minima in the " vs curve, a power-law ( n ) dependence is insufficient to describe the emissivity. However, if such a dependence is assumed, n transitions from 1:4 to 0.5 as temperature increases from 2500-3500 K. Nano-sized alumina particles exhibit an even stronger spectral dependence. At 2678 K, n is approximately 1:2 but reaches as high as 2.1 at 3052 K. Considering optical depth issues, there is merit in gray emissivity approximations for high-temperature (3000-3300 K) particles typical of aluminum particle combustion. Nomenclature C = power-law coefficient c n = third-order law coefficients, n 0, 1, 2, 3 I = spectral intensity, W m 2 m 1 n = power-law exponent Q a = spectral absorption efficiency Q s = spectral scattering efficiency T = temperature, K t L = optical depth " = spectral emissivity = wavelength, m 0 = reference wavelength, 1 m Subscripts bb = Planck blackbody cal = calibration source exp = experimental p = particle

Tunable synchrotron-sourced photoionization time-of-flight mass spectrometry (PI-TOF-MS) is an important technique in combustion chemistry, complementing lab-scale electron impact and laser photoionization studies for a wide variety of reactors, typically at low pressure. For high-temperature and high-pressure chemical kinetics studies, the shock tube is the reactor of choice. Extending the benefits of shock tube/TOF-MS research to include synchrotron sourced PI-TOF-MS required a radical reconception of the shock tube. An automated, miniature, high-repetition-rate shock tube was developed and can be used to study high-pressure reactive systems (T > 600 K, P < 100 bar) behind reflected shock waves. In this paper, we present results of a PI-TOF-MS study at the Advanced Light Source at Lawrence Berkeley National Laboratory. Dimethyl ether pyrolysis (2% CH3OCH3/Ar) was observed behind the reflected shock (1400 < T5 < 1700 K, 3 < P5 < 16 bar) with ionization energies between 10 and 13 eV. Individual experiments have extremely low signal levels. However, product species and radical intermediates are well-resolved when averaging over hundreds of shots, which is ordinarily impractical in conventional shock tube studies. The signal levels attained and data throughput rates with this technique are comparable to those with other synchrotron-based PI-TOF-MS reactors, and it is anticipated that this high pressure technique will greatly complement those lower pressure techniques.