To investigate the homogeneous nucleation and growth process of nanoparticles in vapor phase, interferometric observation was attempted for the first time to the gas evaporation method, which has been a commonly accepted physical production method of nanoparticles. Tungsten oxide was evaporated by electrical heating of a tungsten wire in a mixture gas of Ar and O2. WO3 nanoparticles were formed via homogeneous nucleation and growth during a gas cools following a thermal convection produced by the evaporation source. The degree of supersaturation for nucleation was extremely high, 6.6×106, which was determined from the interferogram. Surface free energy of WO3 at 1100 K was calculated based on the classical nucleation theory and was 1.38×103 erg cm-2, which is within the reported values. A part of the difference between actual formation rate of produced nanoparticles, which were determined based on a transmission electron microscope, and calculated values based on the classical nucleation theory were well explained if we adopt the idea of coalescence growth.
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