The preparation of alumina ceramics is usually sintered under atmospheric pressure, and the sintering temperature is usually above 1 800 ℃ to achieve densification of alumina. High temperature and long-term heat preservation often make the grains of alumina ceramics overgrow, and it is impossible to predict what defects and porosity that occur inside the crystal, resulting in the decline of the comprehensive properties of alumina ceramics. Previous work predominantly focuses on studying the stable nano α-phase alumina (α-Al₂O₃) sintering process, microstructure, and abnormal grain growth, but few studies on the variations in microstructure and defects during the sintering process of metastable γ-phase alumina (γ-Al₂O₃), let alone use CaO as an additive in nanoceramic alumina.

This study aims to apply positron annihilation lifetime spectroscopy combined with X-ray diffraction (XRD) to investigating the impact of calcium oxide (CaO) addition on the sintering process of Al₂O₃ nanoceramics.

Firstly, γ-phase nanoscale alumina powder with an average grain size of 20 nm, 99.99% pure and nano-calcium oxide powder with an average grain size of 160 nm, 98% pure was used as raw ceramic materials. Tablet samples of Al₂O₃ nanoceramics and Al₂O₃ nanoceramics doped with CaO (with a mass fraction of 1% CaO) were pressured under consistent experimental conditions. Then, the samples were sintered from 500 ℃ to 1 100 ℃ for 2 h, and cooling to room temperature. XRD was used to characterize the phases and compounds of the sample, and field emission scanning electron microscope (SEM) was employed to observe the microscopic morphology of the sample cross-section, accompanied by an EDS spectrometer for elemental composition analysis. Finally, a radioactive isotope ²²Na with intensity of about 0.5×10⁶ Bq was used as positron source and fast-fast coincidence positron lifetime spectroscopy was applied to characterizing the sample defects. The total count of each lifetime spectrum was over 10⁶, and all samples were measured for 2~3 times. The lifetime spectrum data were fitted and analyzed by LT(V9) program. The average value was taken as the result after spectral deconvolution.

The results of XRD and SEM show that the sintering process of CaO/Al₂O₃ nanoceramics is divided into two stages, no phase transition occurred from room temperature to 900 ℃, while significant phase transition occurred from 900 ℃ to 1 100 ℃. The addition of a small amount of CaO (such as 1% mass fraction) is uniformly distributed in the Al₂O₃ matrix at first, and with the increase of sintering temperature, it reacts with the Al₂O₃ to form a second phase and transform into a liquid phase at higher temperature. Analysis results of positron annihilation lifetime spectrum show that the size and number of vacancy clusters and micropores in CaO/Al₂O₃ nanoceramics before and after the phase transition are different from those of Al₂O₃ nanoceramics. In CaO/Al₂O₃ nanoceramics, vacancy clusters and micropores are more likely to form as the temperature rises, some micropores gradually merge to form macroscopic pores before phase transition whilst micropores in Al₂O₃ nanoceramics disappear with phase transition and grain growth, leaving only a small amount of micropores on the sample surface.

The addition of a small amount of CaO suppresses the growth of Al₂O₃ grains during the sintering process, resulting in a more uniform and denser structure. Simultaneously, the addition of CaO within Al₂O₃ nanoceramics induces the formation of a liquid phase. This alters the mass transfer mechanism from solid-state diffusion to liquid-phase flow, thereby delaying the phase transformation process of Al₂O₃ nanoceramics during sintering and leading to the formation of macroscopic pores.