Chemical kinetics of yttrium ionization in H 2O 2N 2 flames

Abstract

Trace amounts (≤10 −6 mol fraction) of yttrium were introduced into six H 2O 2N 2 flames in the temperature range 1820–2400 K at atmospheric pressure. Metallic ions were observed by sampling the flames through a nozzle into a mass spectrometer, and were measured as profiles of ion concentration vs. distance (i.e. time) along the flame axis. The ions observed conform to a series YO + · nH 2O ( n = 0−3); YO + is barely detectable, Y(OH) 2 + with n = 1 is the dominant ion, and the higher hydrates are produced by cooling that occurs during sampling. The major yttrium neutral species are the oxide-hydroxide OYOH (>80%) and the oxide YO (<20%). By the addition of 0.25 mol % of CH 4 and a trace of potassium, it was possible to boost the ionization level appropriately to make electron-ion recombination of Y(OH) 2 + the dominant process; values of the recombination coefficient (9.7 ± 1.5) T −2.3±0.6cm 3molecule −1s −1 were obtained. A radical species such as H in these flames can overshoot its equilibrium concentration by a factor γ of 100 or more in the reaction zone but γ decays downstream towards equilibrium. Neutral OYOH and YO, whose concentrations depend on γ, are linked by the balanced reaction YO + H 2O = OYOH + H having an equilibrium constant K = exp( −49,000 RT ) . This leads to a bond energy value D 0(OYOH) = 449 ± 20 kJ mol −1. Analysis of the ion profiles as a function of γ is helpful in ascertaining the ion formation processes. Yttrium ions are produced by the chemi-ionization reaction of OYOH with H; because the ionization energy of YO is low, a small contribution (<1%) might arise from collisional (thermal) ionization of YO. The rate constant for chemi-ionization was found to be (2.7 ± 0.8) × 10 −12 exp(−22452/T) cm 3 molecule −1 s −1. This rate constant should be regarded as a lower limit because a fraction of the total yttrium, presumably small, may be present in these flames as solid particles.