Synthesis of neodymium aluminide by aluminothermic reduction of neodymium oxide
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I.
INTRODUCTION
BINARY rare-earth aluminides are materials of great scientific and technological importance because of their magnetic properties. Neodymium aluminides are no exception to that. In the Nd-Al system, there are six intermetallic compounds, and many of them have become quite popular for scientific investigation and have attracted the attention of materials scientists. The most common method of preparation of Nd-Al alloy is by melting the component metals according to the desired ratio followed by homogenization. However, these alloys can be prepared by other methods as well, which use more economic neodymium compounds such as fluoride, chloride, and oxide as starting raw materials. Zviadadze et al.[1] prepared Al-Nd alloy by aluminothermic reduction of NdF3 in the presence of excess Al at a temperature of 1050 7C to 1115 7C under vacuum using the NdF3/Al molar ratio from 1:11 to 1:34. A first-order reaction kinetics with an apparent activation energy of 334.8 kJ/mol was established. The Al-10 wt pct Nd master alloy showed nonuniform distribution of the components and remelting was required for homogenization. Fused-salt electrowinning can also be used for the preparation of Al-Nd alloy. Peng et al.[2] made this alloy in a bath containing NaCl-KCl and 30 pct NdCl3 at a temperature of 700 7C using molten Al as the cathode. High-temperature thermodynamic data (DG71000) show that Ca, Y, and heavy lanthanides can reduce Nd2O3, but Al cannot.[3] Calciothermic reduction of Nd2O3 was reported by Sharma and Seefurth.[4] However, because of the formation reaction of the intermetallic compound between Al and Nd, which involves sufficient reduction of free energy, the overall reaction between Al and Nd2O3 becomes energetically feasible. Differential thermal analysis (DTA) of the reactant mixture (Nd2O3 1 Al) can be used to ascertain this. A similar result was reported[5] earlier in case of the aluminothermic reduction of Gd2O3 in the presence of exA. BISWAS, I.G. SHARMA, and G.B. KALE, Scientific Officers, Materials Science Division, and D.K. BOSE, Head, Uranium Extraction Division, are with the Bhabha Atomic Research Centre, Mumbai-400085, India. Manuscript submitted November 12, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
cess Al. As there is only a marginal difference between the free energy (DG7298) values of Gd2O3 (1730.589 kJ/mole) and Nd2O3 (1720.336 kJ/mole), it, therefore, seems possible to prepare neodymium aluminide by aluminothermic reduction of Nd2O3 in the presence of excess Al under vacuum or in protective atmosphere. In the present investigation, an attempt was made to do the same in the presence of excess Al in an induction furnace as well as by the thermit process along with the Fe2O3-Al mixture. II.
EXPERIMENTAL
High-purity Nd2O3 (99.9 wt pct) and Al (99.99 wt pct) powders were used for the experiments. The Nd2O3 powder was supplied by Indian Rare Earths (Mumbai, India) and the Al powder was obtained from ALCOA (Alcoa Center, PA). Particle sizes of the reactant powders were measured usi
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