PARTICLE-SIZE DEPENDENCE OF THE PR2FE17 NITROGENATION PROCESS

The particle size dependence of the Pr2Fe17 nitrogenation process at 4 00-degrees-C is determined by the Mossbauer method. The experimental d ata are described in terms of a particle model consisting of spherical concentrical shells containing: (i) an expanded central core saturate d with 2 + epsilon atoms N per formula unit; (ii) an intermediate unni trided shell with a volume larger than the core by a factor of about 2 .4 and accommodating a stress/strain field produced by the expanded co re, and (iii) an unnitrogenated undeformed external shell. We observed that small particles (d less-than-or-equal-to 10 pm and most probably lower) tend to absorb higher nitrogen contents than the large ones. O ur results also indicate that some small particles, most probably sing le crystals, do not nitrogenate.