The current work presents the results of an experimental investigation of gas-phase combustion synthesis of silica (SiO 2 ) particles using a multi-element diffusion flame burner (MEDB, a Hencken burner). Silane (SiH 4 ) was added to hydrogen/oxygen/argon (H 2 /O 2 /Ar) flames to produce SiO 2 nanoparticles at various burner operating conditions ( ϭ 0.47-2.16). To characterize the burner performance, temperature measurements were made using water absorption spectroscopy and uncoated, fine-wire thermocouples. The results demonstrated the non-premixed flow arrangement of the fuel tubes and oxidizer channels of the MEDB provided uniform, ϳ1D conditions above the surface of the burner, with temperature variations of less than Ϯ3% in the transverse direction (parallel to the surface of the burner) for elevations above the mixing region (z ϭ 0 -7 mm), extending to heights Ն 30 mm. At heights above the mixing region, approximately constant axial temperatures are also observed. Silica particle formation and growth were examined for comparison with current understanding of the physical mechanisms important in combustion synthesis of SiO 2 . The particle properties were determined using transmission electron microscope (TEM) imaging. Geometric mean diameters of the primary particles varied from d p ϭ 9 to 18 nm. The current study demonstrates the utility of the MEDB in providing a controlled environment for fundamental studies of gas-phase combustion synthesis phenomena, as well as offering broad flexibility in experimental design with control over process variables such as temperature field, particle residence time, scalable reactant loading, and particle precursor selection.
Results of an experimental study of the application of frequency-modulated UV laser absorption spectroscopy to silica (SiO 2 ) particle-forming flames (SiH 4 /H 2 /O 2 /Ar) are presented. An argon-ion pumped ring-dye laser system in the rapid wavelength scanning configuration was used to obtain multiple line shape profiles of the R 1 (7) and R 1 (11) , respectively). Temperature and OH mole fraction were determined by a best fit of a convolved Voigt absorption profile to the data. Measurements were made in the multiphase regions of silane/hydrogen/oxygen/argon flames, verifying the applicability of the diagnostic approach to combustion synthesis systems. Absorption measurements were taken over a range of particle environments found at increasing heights above the burner surface (5-20 mm) and equivalence ratios ( ס 1.0 and 1.2). The experimental data were compared with thermocouple measurements, equilibrium, and one-dimensional modeling simulations. The results of the study successfully demonstrate OH UV absorption spectroscopy as a highly sensitive and accurate (uncertainties less than %01ע in the current work) diagnostic approach for in situ measurements of temperature and OH mole fractions in combustion synthesis flames.
Water absorption spectroscopy has been successfully demonstrated as a sensitive and accurate means for in situ determination of temperature and H2O mole fraction in silica (SiO2) particle-forming flames. Frequency modulation of near-infrared emission from a semiconductor diode laser was used to obtain multiple line-shape profiles of H2O rovibrational (v1 + v3) transitions in the 7170-7185-cm(-1) region. Temperature was determined by the relative peak height ratios, and XH2O was determined by use of the line-shape profiles. Measurements were made in the multiphase regions of silane/hydrogen/oxygen/ argon flames to verify the applicability of the diagnostic approach to combustion synthesis systems with high particle loadings. A range of equivalence ratios was studied (phi = 0.47 - 2.15). The results were compared with flames where no silane was present and with adiabatic equilibrium calculations. The spectroscopic results for temperature were in good agreement with thermocouple measurements, and the qualitative trends as a function of the equivalence ratio were in good agreement with the equilibrium predictions. The determinations for water mole fraction were in good agreement with theoretical predictions but were sensitive to the spectroscopic model parameters used to describe collisional broadening. Water absorption spectroscopy has substantial potential as a valuable and practical technology for both research and production combustion synthesis facilities.
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