High pressure gas-solid interstitial modification of Nd(Fe,M)/sub 12/ (M=Ti,V,Mo) with H and N atoms

Abstract

Since the discovery of the improvement of magnetic properties in interstitially modified rare earth @)transition metal (T) compounds [I], many studies have been performed to investigate the nitrogenation behavior of R-T compounds with ThMnlrtype structure [2, 31. However, the mechanisms involved are not yet completely understood and different nitrogen absorption behaviors and stability ranges have been observed for different substitutions [4]. We studied the hydrogenation and nitrogenation behavior of NdFellTi, NdFelonVl,z5 and NdFelasMo1.5 at room temperature starting pressures between 1 bar and 30 bar by differential scanning calorimetry @SC) and x-ray diffraction (XRD). The hydrogenation behavior is an important parameter for the HDDR process and was also studied by temperature pressure analysis (TPA). Fig. 1 shows DSC traces of a NdFelo.JMol,5 powder sample (particle size < ISpm) heated up to 740T with 5 Wmin in a pure nitrogen atmosphere. At 1 bar starting pressure, one dominant exothermic peak is observed at 4WC, which, according to the results of XRD, can be attributed to the interstitial modification. A smaller peak at a temperature of 664°C indicates the decomposition reanion of the NdFelo.5Mol sNY phase with ThMn12-type structure into a mixture of a-Fe and an amorphous phase. When the starting pressure is raised to 5 bar, the fust peak shifts to a temperature of 449OC, and the second exothermic peak becomes more important, now appearing at €&Soc. Both peaks appear at even lower temperatures if the pressure IS further increased to 10 bar and 30 bar. Fig. 2 shows the XRD diagrams of the samples after the DSC run Fig. Za shows the pattern of the parent compound. High scattering intensities result from a-Fe and an amorphous phase when the N2 gas pressure in the DSC is 1 bar, but the decomposition reaction is incomplete and a residue of the ThMnlz-type structure is left (b). No intensities orighting fiom NdN were found When the N2gas pressure is raised, the decomposition reaction into a-Fe and an amorphous phase becomes complete (c-e). In both compounds NdFellTi and N~F~Io.RVI,ZS, the decomposition reaction of the ThMno-type structure into a-Fe and an amorphous phase also o c w s , but at slsnificantly lower temperatures (551°C and 607°C at I bar and 527T and 567°C at 30 bar, respectively). Additionaly, in these compounds the decomposition reaction is complete at a pressure of 1 bar. The interstitial modification and the decomposition reaction are irreversible in the case of N. For H, both reactions are reversible. Mer the hydrogen induced disproportionation process, a recombination of the ThMnn-type structure was successfully performed. Fig. 3 shows the number of H atoms per formula unit during a heat treatment in HZ atmosphere at a pressure of 1 bar for the three alloys under ipvestigation. The increase above 600T indicates the disproportionation process. In the case of NdFeloJMols this reaction is not completed at 86OOC. The reported results can be used to optimize the interstitial modification with nitrogen and the HDDR processing of the three compounds, and help to better understand the shuctural changes that o m during nitrogenation