Production of Nanometer-Sized Metal Oxide Particles by Gas Phase Reaction in a Free Jet. I: Experimental System and Results

In a study of the effect of process conditions and material properties on aerosol characteristics, nanosized metal oxide particles were produced by injecting precursors as a free jet into a methane-air flame. Primary particle size increased with volume loading, solid state diffusion coefficient, and maximum temperature. Larger particles were also obtained by decreasing the jet velocity. The number of particles per agglomerate increased with volume loading and decreased with solid state diffusion coefficient and maximum temperature. Metal oxides with diffusion coefficients ranging over several orders of magnitude produced different sized particles under the same process conditions (temperature profile and aerosol volume loading). Niobium oxide (largest diffusion coefficient) formed the largest particles with geometric volume mean diameters between 5.7 and 33.7 nm. Titania (mid-range diffusion coefficient) and alumina (lowest diffusion coefficient) formed particles with geometric volume mean diameters ranging from 3.8 to 21.3 nm and 2.8 to 10.7 nm, respectively. The geometric standard deviation for the metal oxide particles was about 1.2. The properties of the primary particles and agglomerates depend on the characteristic collision and coalescence times. The collision time was controlled by varying the aerosol volume loading from 10 - 7 to 10 - 6. The coalescence time depends strongly on the solid state diffusion coefficient which ranged over several orders of magnitude as the jet temperature changed. Maximum jet temperatures from 1050 to 1920 K were obtained by adjusting the precursor jet and flame gas exit velocities from 4.8 to 53.2 mls and 0.14 to 0.51 mIs, respectively. The mass production rate ranged from 0.05 to 1.0 glh for a jet orifice of 1.2 mm.