Nitrogenation studies of Nd2Fe17 alloys

In the present investigation, the kinetics of nitrogenation of Nd2Fe17 alloys were studied in order to optimize the magnetic properties of the pure Nd2Fe17 alloy by the introduction of nitrogen interstitials. The nitrogenation kinetics was followed in the temperature interval 1173-1473 K by the use of the thermogravimetric (TG) technique. The nitrogenated alloys were prepared from a Nd2Fe17 master alloy by heat treating in different partial pressures of nitrogen under well-controlled conditions. The results of the TG experiments showed that there was an initial incubation time before the onset of nitrogenation. The length of the incubation period was found to be a function of temperature and the partial pressure of nitrogen in the gas. The nitrogen used was of high purity (99.9999% N2). The gas purification system adopted in the present work could effectively remove traces of reducing as well as oxidizing impurity gases in the system. In order to remove the traces of CO, CH4 and H2, (as well as other hydrocarbons) if present, the gas was passed through a column of Cu turnings kept at 973 K. The lower portion of the Cu column was surface oxidized. The reducing gases got oxidized passing through this column and CO2 and H2O were formed. Further, the oxygen impurity in the gas (ca. 10-2 Pa) could react with the remaining reducing gases. These products were absorbed by columns of ascarite and dehydrite, respectively. Remaining oxygen impurity in the gas could then be removed by passing the gas through a furnace containing Mg turnings at 773 K. The rate of nitrogen uptake after this period was found to increase with increasing temperature. After an initial rapid nitrogenation period, the reaction rate was found to slow down, possibly due to nitrogen atoms diffusing into the bulk. The reaction was found to be rapid, and the incubation time was conspicuously absent, when nitrogen gas was used without the purification steps mentioned earlier. Oxygen impurity in the gas was found to have a strong influence on the reaction kinetics. Heat capacity measurements, as well as magnetic hysteresis measurements, were also conducted. The heat capacity of the Nd2Fe17 master alloy, as well as that of the nitrogenated alloys, was determined in the temperature interval 293-773 K by the use of the differential scanning calorimetric technique. From the experimental heat-capacity data the Curie temperatures of the alloys were derived. Compared to the master alloy, significantly incr