On the Mechanisms of Heterogeneous Recombination of Nitrogen and Oxygen Atoms

Abstract

The problem of the heterogeneous recombination of nitrogen and oxygen atoms is considered. The processes influencing the results of measurements of the recombination probability are analyzed. This study presents the authors' data on the heterogeneous recombination of atoms in the temperature range of 300–3000 K and pressures of 0.01–50 hPa (mbar). The probabilities of the heterogeneous recombination of O and N atoms on the surface of quartz are measured using the method of resonance fluorescence spectroscopy (RFS) under strictly controlled conditions at temperatures of 300–1000 K and pressures of 0.01–10 hPa in reactors at the Institute of Biochemical Physics (IBCP). The pressure and temperature regions where recombination occurs predominantly according to the Langmuir–Hinshelwood or Rideal–Eley scheme are determined. In experiments at the VAT-104 TsAGI installation in the temperature range of 1000–3000 K and pressures of 5–50 hPa, the effective values of the rate constant of the joint heterogeneous recombination K_w of nitrogen and oxygen atoms are determined using measurements of specific heat flows. Coatings with a surface layer similar in composition to quartz and a number of high-temperature ceramics based on hafnium (zirconium) borides are studied. Studies of ceramics show that heterogeneous recombination also occurs at temperatures of 2500–3000 K. A new mechanism of the heterogeneous recombination of nitrogen and oxygen atoms is considered. Under the influence of a high-speed plasma flow, the ceramics are oxidized and a layer of hafnium (zirconium) oxide polycrystals is formed. The observed jump in temperature by ≈1000 K and heat flux up to 4–5 times is caused by the catalytic activity of the tetragonal and cubic phases of HfO₂ (ZrO₂) polycrystals. The high catalytic activity of the oxide layer is apparently explained by a new recombination mechanism related to the incorporation of nitrogen and oxygen atoms in the crystal lattice (formation of a solid solution).