JOURNAL OF MATERIALS SCIENCE LETTERS 15 (1996) 2088 2090 Effect of excess samarium oxide on the preparation of Sm2Fe17 powder by calciothermic reduction A.VERMA, R. K. SIDHU, S. MAHAJAN*, O. P. PANDEY* Thapar Corporate Research and Development Centre, and * Thapar Institute of Engineering and Technology, Patiala 147 001, India The low curie temperature and poor temperature stability of neodymium-iron-boron (Nd2Fet4B) has resulted in the search for new high performance permanent magnets. It has been shown that nitro- genation of Sm2Fe17 gives the desired magnetocrys- talline anisotropy for practical magnets [1]. The nitrogenated compounds (Sm2Fet7Nx) hold promise by virtue of their higher curie temperature. Under equilibrium conditions, rhombohedral Sm2Fel7 does not seem to have any binary solubility of compo- nents and coexists with other binary phases. It is not easy to achieve the correct stoichiometry for the formation of single magnetic phase Sm2FelTNx. An excess of Fe produces soft magnetic properties, whereas an excess of Sm leads to the formation of SmFe3, which is unstable during nitriding and decomposes to form SmN and a-Fe [2]. Different techniques for alloy making, such as conventional melting, mechanical alloying and calciothermic reduction (reduction-diffusion process) have been reported [3]. Although some reports on the prepara- tion of Sm2Fel7 magnetic materials using reduction- diffusion (R-D) processes have been published, very little has been published on the process aspect of the calciothermic reduction technique [4-6]. Samarium oxide (99.9% purity) and atomized iron powder were used for the experiments. The particle sizes of Sm203 and Fe powder were below 10 ~m and 40 ~tm respectively. Sm203 and Ca were taken in excess of stoichiometry in view of their lower vapour pressures at the reduction temperature. The quantity of Sm203 was varied from 1.1 to 1.4, whereas Ca was taken 40% in excess of chemical equivalent. As the Sm203 content was varied from 1.1 to 1.4, the quantity of Ca was changed in such a way so as to maintain the calcium content in 40% excess. The constituents were mixed in a planetary ball mill for 30 min. The mixing was carried out using 10 mm diameter tungsten carbide balls at a speed of 160 rpm, keeping the ball to charge ratio at 5:1. The desired quantity of calcium granules of 1- 2 mm size was added to the charge at the end. The mixture was compacted in the form of 16 mm diameter pellets by applying a compaction pressure of 600 MPa. The pellets were charged in a tubular furnace, which was repeatedly evacuated and back- filled with high purity dry argon gas. The pellets were heated to 1423 K at the rate of 20 Kmin -~ in argon. After soaking for 4.0 h at this temperature, the pellets were furnace cooled to room temperature. 2088 The pellets were crushed in a vibration mill to obtain a coarse powder. The coarse powder was washed repeatedly with chilled deionized water. Subse- quently, the powder was washed with 5% (v/v) acetic acid to remove trace quantities of calcium. Finally, the powder was washed with acetone and dried under vacuum at 323 K. The X-ray diffraction (XRD) pattern of the dried powder was recorded using CuKa target and graphite as monochromator at a scanning speed of 2 ° min i. The morphologies of the starting Fe and Sm203 powders are given in Figs 1 and 2, respectively. The XRD pattern of the Sm203 and Fe mixture is shown in Fig. 3. Figs 4-7 show the diffraction patterns of the reduced and washed powders. It is evident from the diffraction pattern that the content of Sm2Fet7 phase gradually increases as the percentage of Figure 1 SEM micrograph of Fe powder. Figure 2 SEM micrograph of Sm203 powder. 0261-8028 © 1996 Chapman & Hall