Preparation of Sm₂Fe₁₇N_x Powders with Ball Mill in NH₃ and their Magnetic Properties

Abstract

The preparation of Sm₂Fe₁₇N_x magnetic powders by mechanical grinding (MG) in NH₃ and subsequent nitrogenation heat treatment without crystallization was carried out. A vibration ball mill with high mechanical energy and a rotary ball mill with low mechanical energy were employed for MG. The effects of ununiform distortion introduced by MG into the powders on their magnetic properties were investigated.
Using a vibration ball mill, the powders after MG and subsequent nitrogenation had very poor rectangularity on their demagnetization curves. Using a rotary ball mill in order to decrease the effects of strain, the powders after MG for 288ks-324ks absorbed sufficient nitrogen of about 35000ppm. In spite of sufficient nitrogen content, their magnetic properties were far from those of hard magnetic materials. The powder with less strain was prepared by MG for 36ks and subsequent nitrogenation. This powder showed good rectangularity on the demagnetization curve, that is, high remanence of about 1 Wb⋅m^-2 and high maximum energy product of about 129 kJ⋅m^-3. These values are much better than those of powders prepared with a vibration ball mill. Magnetic properties are very sensitive to the distortion introduced by MG. Therefore decreasing distortion of powders is essential to obtain excellent properties as a hard magnetic material.