In order to clarify the rate and mechanism of the formation of Sm–Fe intermetallic compounds by the reduction-diffusion (R-D) process, three kinds of experiments were carried out: (1) the reduction of pellets of Sm₂O₃ powder mixed with CaH₂, (2) the R-D reaction for the pellets which were composed of Sm₂O₃, CaH₂ and Fe (powder or wire) at 1300, 1400 and 1500 K, and (3) diffusion experiment by using Sm–Fe diffusion couples with Mo wire as a marker at 963 K.
The reduction of Sm₂O₃ uniformly proceeded in the pellets; more than 90% of Sm₂O₃ was reduced at 1400 and 1500 K in a reaction time less than 1 ks, though about 80% at 1300 K at a reaction time of 1 ks. During the R-D reaction, a Sm₂Fe₁₇ layer was formed on the Fe surface. The thickness of the Sm₂Fe₁₇ layer grew according to a parabolic rate law at 1300 K. At 1400 and 1500 K, however, the Fe dissolved in a liquid Sm or Sm–Ca melt, since the reduction rate of Sm₂O₃ was high. The experiment with the diffusion couple revealed that Fe diffused faster than Sm in the SmFe₂ layer.