Magnetic and morphological characterization of bulk Bi2Fe4O9 derived by reverse chemical co-precipitation: A comparative study of different sintering methods

This paper has focused on various sintering routes for the preparation of bulk Bi2Fe4O9 (BFO) ceramic synthesized via thereverse chemical co-precipitation. The influence of different types of sintering, including the conventional sintering (CS), two-step sintering (TSS) and microwave sintering (MS) with different heating cycles on the magnetic and morphological properties of BFO bulk samples was studied in detail. The density of green compact was about 60% of the theoretical density and rose to 84, 85% and 91% for the CS, TSS and MS, respectively. The results of X-ray diffractometery (XRD) confirmed that all the samples have an orthorhombic crystal structure, which belongs to the space group pbam. Nanopowders morphological characteristics were examined by the transmission electron microscopy (TEM) and the average particle size was 72 nm. Also, the morphology of bulk samples was studied using a field emission scanning electron microscope (FESEM). The average grain sizes in the sintered samples were about 260, 295 and 210 nm for the best heating cycles of CS, TSS and MS, respectively. Based upon FESEM micrographs, it was found that the MS method because of the low sintering time and good heating gradient has the slightest grain growth. In addition, the line-scan spectroscopy showed the accumulation of Bi ions and vacancies of oxygen near the central line of grain boundaries in the CS and TSS methods, which is due to the long sintering time and high diffusion rate of bismuth ions. The M-H curves of the sintered samples measured through the vibrating sample magnetometery (VSM) confirmed antiferromagnetic behavior in all samples. The surface and grain boundary uncompensated spins led to the emergence of magnetization in Bi2Fe4O9ceramics. Generally, a decrease in the particle size and/or an increase in grain boundaries and porosity resulted in more uncompensated spins, thereby improving the magnetization.