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

Given the downsizing and high integration of laser diodes in recent decades, Yb: Sr 5 (PO 4 ) 3 F (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 Y 2 O 3 , YAG, Lu 2 O 3 , 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.