Synthesis of Sphere GdN for Magnetic Refrigeration

Spherical GdN material with diameter of 0.85 ∼ 1.0 mm was synthesized by the hot isostatic pressing (HIP) method aiming at the application to the magnetic refrigeration for hydrogen liquefaction. Conditions of the HIP nitridation starting from metallic Gd spheres were investigated to obtain the spherical GdN materials free from unwanted deformation and/or cracks. We found that slower temperature rising rate, 10°C/h, around the melting point of Gd metal, 1300-1600°C, is favorable because it would allow the metallic Gd core to transform into nitride by reacting with nitrogen diffusing from the surface. We have successfully synthesized spherical GdN material of 300 g, which is enough to operate a test device for the magnetic refrigeration. Oxidization degree of this spherical GdN material was investigated by exposing it to air with some controlled humidity. It was found that only 2.5 % of the GdN was degraded after 56 hour exposition to air dried with silica gel.