Formation of calcium phases in the polycrystalline α-alumina ceramics

Official URL: https://risc01.sabanciuniv.edu/record=b2993558_(Table of contents)

The distribution of impurities, intergranular films, and second-phase particles, in addition to the composition and structure of the phases, all influence the characteristics of ceramic materials. Changes in the microstructure of calcium doped α-Al2O3 (alumina) with increasing sintering temperatures of 1300°C, 1400°C, and 1550°C were studied using electron microscopy techniques. To find the early stages of formation of calcium hexaaluminate, the sintering process was also performed for the calcium-doped alumina samples at 1300°C for various sintering time ranges between 1h to 10 h. A first principle calculation was done after studying the phase composition and microstructure evolution of the sintered specimen. Variations in the average grain sizes were related to the excess calcium concentrations (ΓCa). Analytical electron microscopy was used to study these calcium doped alumina ceramics. The alumina grains were homogeneous and equiaxed at all sintering conditions. As the sintering time and temperature increased, the calcium- ii rich grain morphologies elongated. An X-ray energy dispersive spectrometer attached to a scanning electron microscope showed the precipitation of calcium-rich second phases. These precipitates were identified as different calcium aluminate phases like mono calcium aluminate (CA), calcium dialuminate (CA2), and calcium hexaluminate (CA6) by X-ray energy dispersive analysis. Calcium precipitates at high temperatures appeared to be large and elongated grains. On the other hand, at low sintering times, tiny grains were detected as the calcium rich grains. The results demonstrate that a small amount of CA6 forms at 1300°C in the reaction system of Al2O3 doped with Ca sintered for 4 h, and more significant amounts form at higher times and temperatures. The reaction of formation mechanism is as regards: CaO.2Al2O3 (CA2) combines with 4Al2O3 results in CA6. According to the CaO-Al2O3 phase diagram, there is a diffusion process of Al3+ cations in Al2O3 towards CA2 to generate CA6.