Structure and magnetic properties of HDDR Nd2Fe14B powders

The HDDR process (hydrogenation disproportionation, desorption, recombination) is a unique method to produce highly coercive powders for bonded permanent magnets. The starting material is a Nd-rich Nd2Fe14B alloy. The first step starts with the absorption of hydrogen atoms at room temperature which fill the vacancies and cause expansion of the lattice. This large stress causes decrepitation of the alloy resulting in a powder with a particle size of several micrometers. An additional heating of the alloy at 840 C under 0.3 bar hydrogen pressure leads to the disproportionation of the Nd2Fe14B phase into a very fine mixture of neodymium hydride, α-iron and Fe-boride. Desorption of hydrogen and recombination can be induced at high temperatures, leading to the recovering of the original Nd2Fe14B phase but with a dramatically refined grain size (200-300 nm). Phase analysis and grain size determination of the powders are characterized by XRD using Rietveld analysis. High resolution scanning electron microscopy (HR SEM LEO 1530 GEMINI) was used to study the microstructure. Magnetic properties were investigated by vibration sample magnetometry (VSM) with a maximum magnetic field of 9 T at room temperature. Prior to the VSM measurement the Nd2Fe14B powders were aligned applying a magnetic field of 2 T during cold compaction.