Transparent hexagonal Yb: S-FAP ceramics: Investigation of HP sintering kinetics on densification process and microstructural evolution

Abstract

Given the downsizing and high integration of laser diodes in recent decades, Yb: Sr5(PO4)3F (S-FAP) crystal materials are predicted to be employed as an all-solid laser medium to produce more energy for inertial nuclear fusion. The effects of various sintering conditions on the microstructure evolution and properties of 2 at. % Yb: S-FAP transparent ceramics by hot pressing sintering were systematically investigated in this work, which including sintering temperature, sintering pressure, heating rate, and holding time. When other sintering variables are held constant, the results reveal that a sintering temperature of up to 1050 °C results in visible grain development, substantial optical scattering, and even a significant carburizing impact at 1100 °C. Excessive sintering dwell time will result in grain development, and a high sintering holding time will result in the emergence of intergranular pores, lowering the optical quality of ceramics. The optimal heating rate range for as-synthesized ceramic is found to be 5–15 °C/min. Moreover, the grain development process of Yb: S-FAP transparent ceramics is determined to be grain boundary diffusion, with an activation energy of 239.4 kJ/mol during hot pressing sintering. Meanwhile, the as-prepared sample has excellent mechanical properties such as Vickers hardness of 7.47 ± 0.045 GPa, which are highly competitive when compared to other Yb-doped optical ceramic media such as Y2O3, YAG, Lu2O3, and so on. Finally, the transmittance was interpreted based on the microstructures of the as-synthesized ceramic samples and the Rayleigh-Gans-Debye scattering theory.