Formation of Model Grain Boundary by Bonding Two Surfaces of Sintered Cu-ferrites and its Properties

Abstract

To elucidate the microstructure, sintering process and developement of second phase at the grain boundary, model grain boundary was prepared and observed, by contacting two polished surfaces of sintered spinel Cu0.75Fe2.25O4+x and annealing. Because the sintered specimen was grown its grains extremely (above several hundreds microns diameter), little grain growth and little boundary migration occurred during annealing for bonding. Therefore, the bonded part could be cleaved again along the new grain boundary, and the both sides of boundary surface were observable.The oxygen content of the specimen was controlled by previous annealing, and the bonding was carried out between 1100°C and 1180°C. The bonding was nearly complete at the conditions, in which specimen was oxidized and the bonding was little when it was reduced (Fig. 1, 4). It was confirmed that the new grain boundary was not different from the grain boundary of the inner part of sintered body (Fig. 2, 3).The crystallographic orientation of the grains at the bonded boundary was determined by measuring the orientation of α-Fe2O3 crystallites, which developed in the spinel crystals by oxidation. Various step configurations were observed at the cleaved boundary surfaces (Fig. 5, 6). The relation between step configuration and crystallographic orientation of the grains at the boundary was plotted in the stereographic projection diagram (Fig. 7). It is confirmed that the (111) plane is one constituent of the step, and it seems to be stable at the grain boundary (Fig. 8).The process of developing α-Fe2O3 or CuFeO2 crystallites at the grain boundary of spinel was observed, when specimen was oxidized or reduced. It was confirmed from the observation of the cleaved boundary surfaces that the both crystallites developed along the (111) plane of spinel. The various configurations of developed crystallites were related to the orientation of the spinel crystal grains at the boundary (Fig. 9, 10).