Synthesis of micron particles with Fe–Fe4N core–shell structure at low-temperature gaseous nitriding of iron powder in a stream of ammonia

Abstract

Synthesis of the single-phase γ′-Fe₄N on the surface of the micron-sized particles of iron at low-temperature gaseous nitriding of carbonyl iron powder in a stream of ammonia is studied. It is shown that synthesis of particles with such structure is possible with simultaneous control of the number of process parameters: temperature, degree of dissociation of ammonia, and treatment time. It is found that, at temperature T = 400°C and nitriding potential of the atmosphere r_N ≈ 1.3 atm^−1/2, the shells with a thickness of about 1 μm are formed on the particles within ~15–20 min and the powder consists of the γ-Fe₄N phase within ~60 min of treatment. The mechanisms of formation of microparticles with a core–shell structure are considered. A qualitative model for the thermochemical treatment of the micron iron powder with consideration of the diffusion processes of the transport of ammonia molecules in the pore space of the powder and atomic nitrogen diffusion inside the particles is developed. Geometric and dimensional effects at nitriding of iron powders are discussed.