The use of electrical probes in flame plasmas

Method of measuring charge distribution of nanosized aerosols

In this paper, we present the development of a method to accurately measure the positive and negative charge distribution of nanosized aerosols using a tandem differential mobility analyzer (TDMA) system. From the series of TDMA measurements, the charge fraction of nanosized aerosol particles was obtained as a function of equivalent mobility particle diameter ranging from 50 to 200 nm. The capability of this new approach was implemented by sampling from a laminar diffusion flame which provides a source of highly charged particles due to naturally occurring flame ionization process. The results from the TDMA measurement provide the charge distribution of nanosized aerosols which we found to be in reasonable agreement with Boltzmann equilibrium charge distribution theory and a theory based upon charge population balance equation (PBE) combined with Fuchs theory (N.A. Fuchs, Geofis. Pura Appl. 56 (1963) 185). The theoretically estimated charge distribution of aerosol particles based on the PBE provides insight into the charging processes of nanosized aerosols surrounded by bipolar ions and electrons, and agree well with the TDMA results.

Mass spectrometry of flames

A quadrupole mass spectrometric analyser for use in flame ionization is described, together with its application to the ions produced by traces of sodium, added as chloride, in premixed hydrogen-air flames. Negative ions (Cl¯ and OH¯) appear to be of subsidiary importance in the main ionization processes, which rather proceed through direct production of electrons. Nevertheless, it is established that the appearance of Cl¯ ions is consistent with dissociative attachment HCl+ e ¯ → Cl¯+H, rather than by three-body attachment Cl+ e ¯+ M → Cl¯+ M . Hydroxyl ions probably are formed in the same way. The ratios of the amounts of A + (alkali) and A + . H 2 O (alkali monohydrate) ions have been measured in the cases of lithium, sodium and potassium. Equilibrium constants for the reaction A + .H 2 O ⇌ A + + H 2 O are obtained and heats of monohydration of 44, 28 and 17 kcal/mole deduced respectively for A ≡ Li, Na, K. Small amounts of H 3 O + appear in the reaction zone because of traces of hydrocarbon impurities in the flame gas supplies. These are severely reduced by alkali addition, apparently by an exchange reaction A +H 3 O + → A + +H 2 O+H, with a rate constant of 1.1 x 10 -8 cm 3 /s at 2000 °K, rather than by recombination with electrons produced by direct ionization of alkali H 3 O + + e - → H 2 O+H. In contrast with the steady production of A + ions in the burnt gases by reactions of the type A + M → A + + e ¯+ M , there is an anomalously high rate of production in the reaction zone. This appears difficult to explain without introduction of processes involving ‘hot’ electrons.