Characterization and optimization of the coercivity-modifying nitrogenation and re-calcination process for strontium hexaferrite powder synthesized conventionally

Strontium hexaferrite powder synthesized conventionally in-house from stron tium carbonate and hematite (Fe2O3) without using additives has been treate d in a static nitrogen atmosphere and subsequently calcined in static air. The phase identification studies by means of X-ray diffraction (XRD) and th ermal magnetic analysis (TMA) indicated the decomposition of the strontium hexaferrite and the reduction of the resultant iron oxide (Fe2O3) during th e reaction with nitrogen. High-resolution scanning electron microscopy (HRS EM) studies show that the reduction occurring during nitrogenation results in the conversion of some of the large grains into much finer sub-grains. S trontium hexaferrite, Fe3O4, and Sr7Fe10O22 were the main phases obtained a fter reduction. However, weak traces of other phases, such as Fe2O3, were a lso detected. The hexaferrite phase re-formed on subsequent calcination. Th e magnetic measurements indicated a significant decrease in the intrinsic c oercivity during nitrogenation due to the formation of Fe3O4. However, afte r a re-calcination process, the remanence and maximum magnetization (i.e., magnetization at 1100 kA/m) exhibited values close to the initial values be fore treatment, but the value of the intrinsic coercivity was higher than t hat prior to nitrogenation. Examination of the re-calcined microstructure s howed that this could be attributed to the fine grains that originated from the fine sub-grain structures formed in the powder particles during nitrog enation. The optimum time, initial gas pressure, and temperature of nitroge nation and the optimum temperature of re-calcination were investigated usin g a vibrating sample magnetometer (VSM), XRD, and HRSEM. The optimum temperature for nitrogenation was 950 and 1000 degrees C for re -calcination. The optimum time and initial nitrogen pressure were 5 h and 1 bar, respectively. The highest intrinsic coercivity obtained after re-calc ination was similar to 340 kA/m. (C) 1999 Kluwer Academic Publishers.