The Ionisation of Smokes in Flames

Abstract

Many workers have contributed to the theory describing the thermal ionisation of small particles and the major findings of these investigations are discussed here. Very little has been previously published, however, on the experimental investigation of the ionisation and this was the primary object of the present work. The construction of the measuring system is described, together with a discussion of the principles involved in its use. There follows a thermodynamic investigation of the chemical stabilities and volatilities of some metals and their oxides in flames and the conclusion is drawn that many of the particles observed in flames are in disequilibrium with regard to their vapour pressures. An attempt is made to discover a relation for the rate of evaporation of smoke particles, but the approach is elementary and not very successful. The smokes which have been used for ionisation studies are uranium oxide, formed from aerosol droplets of uranyl nitrate solution, and nickel, formed by the addition of nickel carbonyl vapour to flames. The ionisation of these is shown to be consistent with the general form of the thermionic emission theory as applied to small particles, and work functions are obtained for both materials. Iron carbonyl was shown not to give rise to a smoke in flames at 2400 deg.K and above. The ionisation of gaseous iron is discussed briefly, and it is demonstrated that the majority of the iron is present in a combined form in the flames, the most probable compound being iron monoxide.