Oxygen Isotope Exchange between Gaseous Phase Enriched with ¹⁸O Isotope and Nanocrystal Oxides LaMnO_3+δ Obtained by Severe Plastic Deformation

Abstract

The kinetics of the isotope exchange between gaseous oxygen enriched with the 18O isotope and two LaMnO3+δ oxide samples – a nanopowder and a bulk nanocrystal – has been studied. The 18O isotope concentration has been measured by the acceleration nuclear microanalysis method. The coefficients of the volume and the nanograin boundary self-diffusion of oxygen have been evaluated at 500 °C. They are equal to 3.5·10 −20 and 1.5·10 −13 cm2/s, respectively.

Oxygen Isotope Exchange in Nanocrystal Oxides

“…A similar correction of theoretical expressions for C(t) was made while processing the experimental data for other oxides. (Gizhevskii et al, 2008): 1 -560 0 ; 2 -500 0 ; 3 -400 0 ; 4 -300 0 . Dots -experimental data; lines -calculation in the model for simultaneous realization of relaxation and diffusion mechanisms.…”

“…The oxide LaMnO 3+δ is a convenient model object for optimization of the OIE technique (Gizhevskii et al, 2008) on nanopowders. In practice, there often occurs uncontrolled doping and diffusion characteristics of oxides are known to be extremely sensitive to the presents of impurities.…”

“…Therefore, in our OIE studies on nanopowders we measured the average concentration of 18 О atoms in a large assembly of powder particles (Gizhevskii et al, 2008); the same assembly was annealed in oxygen enriched with 18 О isotopes. In view of the destructive character of SIMS measurements, this technique can scarcely be used to determine the average oxygen isotope concentration in powders.…”

“…Finally, the OIE model version providing for simultaneous implementation of the relaxation and diffusion mechanisms (Gizhevskii et al, 2008) was considered. In this case, the C(t) dependence is found by numerical calculations.…”

“…A fundamentally different "diffusion" model for the description of OIE (Gizhevskii et al, 2008) is used for processes characterized by the very high surface reaction rate and by comparatively slow volume diffusion of oxygen in the oxide particle. Here, for spherically shaped particles it is easy to obtain…”