Optimalizace procesu slinování a tepelného zpracování slitin na bázi Nd-Fe-B pro přípravu permanentních magnetů s vysokou koercivitou

Abstract

The optimization of heat treatment conditions of Nd-Fe-B-based permanent magnets, prepared using the strip casting method, was carried out with the aim to enhance their coercivity HcJ and maximal energy product (BH)max, and improve the shape of the demagnetization curve. The initial alloy with the chemical composition (for high-coercivity magnets) of Fe – 67,05; Nd – 19,50; B – 1,00; Dy – 6,00; Pr – 6,00; Cu – 0,15; Al – 0,30 (wt. %) in form of strips was submitted to hydrogen decrepitation and vibration milling. The obtained powder with the granulometry of 3-5 um was mixed with 2.5 wt. % NdH2 due to the improvement of liquid-phase sintering process and increasing in the coercivity as a result of the formation of continuous grain boundary. Structural characteristics and phase composition were investigated using scanning electron microscopy equipped with EDX microprobe, light optical microscopy equipped with digital camera and XRPD. The microhardness of the principal Nd(R)2Fe14B phase was measured. The magnetic properties were evaluated using automatic hysteresisgraph and vibrating-sample magnetometer. The maximal values of the coercive force HcJ of 1470 kA/m were achieved by the heat treatment at 500 °C. The increasing of this temperature to 550 °C led to a sharp drop in magnetic properties and microhardness. The specially designed Zeiss microscope was used to visualize the magnetic domain patterns on the surface of magnetic materials using the magneto-optical Kerr microscopy. The observed star-like domain structure indicates a good magnetic texture for permanent magnets. These data are of great importance for deciding the optimum heat treatment of Nd-Fe-B-based magnets.