Microstructural and chemical characterization of the system CaO‐Al2O3 using environmental scanning electron microscopy ( ESEM )

Abstract

The calcium and alumina oxide is a highly interesting functional materials. In the binary compound 12CaO·7Al 2 O 3 has a unique crystal structure as a nano‐sized cage with free oxygen anions randomly distributed inside the cages. C12A7 has gained much attention for potential applications in various fields, such as ion conducting solid electrolyte, field and ion emitters, oxidizing catalyst and as a transparent conductive oxide (TCO) in flat panel displays, solar cells and energy conservation (smart windows) devices [1, 2]. The functional properties of electrides are strongly depend on the microstructure. There is in the literature some information about the influence of specific surface area and impurities of alumina on the sintering behaviour of an alumina material. But in this study, the objective is to determine if the various processing techniques have an influence on the microstructure evolution of materials made from alumina and calcium oxide. Post‐fabrication C12A7 samples were subjected to heat treatment using different processing methods in order to achieve desired crystal structure. The final products were prepared by sintering (1) and melted in an electric furnace (2). Crystalline and amorphous phases were obtained. The aim of the present study was to evaluate the microstructure of CA ceramics under different melting conditions. The microstructure characteristics were analyzed by means of optical microscopy (OM), X‐ray diffraction (XRD), energy dispersive X‐ray spectroscopy (EDXS) and environmental scanning electron microscopy (ESEM). For scanning electron microscopy (SEM) examination, sintered specimens were cut and polished. ESEM allows the direct examination of specimens without a conductive metallic coating. The application of ESEM could have advantages in the study of ceramic samples due to a reduction in preparation time. The microstructures of bulk materials were observed before and after sintering, and the effect of solution processing parameters and the formation of surface defects was studied. Microstructural differences were observed for the same composition materials under the different processing conditions (Fig. 1). The morphological and compositional properties of the final C12A7 samples are highly affected by their structure and crystallisation.