Evolution of the microstructure during the early stages of sintering barium hexaferrite nanoplatelets

Abstract

Nanoparticles usually exhibit a specific structure and composition, which can influence the development of the microstructure during their sintering. Barium hexaferrite nanoplatelets have a specific, iron-rich structure defined by the termination at the surfaces with the S blocks of their SRS*R* hexaferrite structure (S and R represent a cubic (Fe6O8)2+ and a hexagonal (BaFe6O11)2− structural block, respectively). The unsubstituted and Sc-substituted hexaferrite nanoplatelets were hydrothermally synthesized and fired at different temperatures. A combination of morpho-structural analyses (XRD, SEM, TEM, and aberration-corrected STEM) and magnetic measurements was used to reveal the evolution of the microstructure during sintering. During the initial stages of sintering the nanoplatelets thicken predominantly by the fusion of individual original nanoplatelets. Due to the Fe-rich surfaces of the nanoplatelets, the fusion growth results in an inhomogeneity that leads to the formation of planar defects in the grains and the precipitation of Fe2O3 as the secondary phase. In the Sc-substituted hexaferrite grains, superstructural compositional ordering was detected for the first time. The Sc substitution caused exaggerated grain growth in barium hexaferrite ceramics sintered at 1300 °C.