Dresden 2009 – scientific programme

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MA: Fachverband Magnetismus

MA 25: Micro- and Nanostructured Magnetic Materials II

MA 25.15: Talk

Wednesday, March 25, 2009, 18:30–18:45, HSZ 403

Structure and magnetic properties of HDDR Nd₂Fe₁₄B powders — •Konrad Güth, Juliane Thielsch, Oliver Gutfleisch, and Ludwig Schultz — IFW Dresden, Institute for Metallic Materials, Postfach 270116, D-01171 Dresden, Germany

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.