Influence of the time of interaction of an alloy based on SmCo5 with low-pressure hydrogen on the phase composition

Abstract

By the methods of differential thermal analysis and X-ray phase analysis, we have investigated the inter- action of an alloy based on SmCO5 with additives of the SmCO 3 phase under initial hydrogen pres- sures of 0.3, 0.4, 0.5, and 0.6 МРа at temperatures of 713 and 913 K with holding for 2 and 5 h. At 713 K, only the compound SmCO 3 interacts with hydrogen with the formation of unidentified phases. In holding at 913 K under a pressure of 0.3 and 0.4 МРа for τ = 2 and 5 h, the alloy partially dispro- portionates into samarium hydride and cobalt. It has been established that the complete disproportiona- tion occurs in holding at pressures of 0.5 and 0.6 МРа and for τ = 2 and 5 h. In vacuum, at 1223 K, disproportionation products recombine in the initial phases. The use of the hydrogenation-disproportionation-desorption-recombination (HDDR) process for obtaining highly coercive magnetic powders based on Nd 2Fe14B and Sm 2Fe17N x alloys gives positive results (1, 2). Its features were investigated in single-phase compounds of the samarium-cobalt system (3-5), and the improve- ment of their magnetic properties was found. Due to the high thermodynamic stability of samarium-cobalt al- loys, hydrogen under a relatively high pressure and milling in it were used. The conditions and phase composi- tion of HDDR products in commercial alloys based on the compounds SmCO5 (6) and Sm 2Co17 (7) were also studied. In particular, it was shown that an alloy based on SmCO5 disproportionates into samarium hydride and cobalt under a pressure PH2 ≈ 3-5 MPa in conventional and solid hydrogenation- disproportionation (solid HD) at ∼ 853 K. The use of hydrogen of such high pressure is dangerous, and, for this reason, it is important to determine the conditions of disproportionation under a lower pressure. It was shown (8) that, in heating of an alloy based on SmCO5 (containing a SmCO 3 impurity phase) in hydrogen at PH2 = 0.3-0.75 MPa, several phase transformations proceed. By X-ray phase analysis (XRPA), it was established that, at a temperature close to room temperature, hydrides form on the base of phases that are alloy components. At 413 K, the transformation of the SmCO 3 impurity phase occurs, and, at 523 K, hydride of the SmCO5 phase decomposes. At 838 K, the alloy partially disproportionates into SmH 2± x and Co, and at temperatures of 1058 and 1188 K, samarium hy- dride, which formed during disproportionation, decomposes. The influence of holding the alloy-hydrogen sys- tem for 0.5-5 h at a temperature above 1173 K on the interaction products was also found. However, the condi- tions of complete disproportionation of the SmCO5 ferromagnetic phase were not determined. With regard for the fact that the SmCO5 phase partially disproportionates under a low pressure at a temperature of ∼ 838 K and that hydrogen-initiated phase transformations can occur within a long period of time (9), the kinetics of the inter- action in the KS37 alloy-hydrogen system (with a hold for 2 and 5 h) was investigated at 713 and 913 K, i.e., up