Transparent magneto-optical Ho2O3 ceramics: Role of self-reactive resultant oxyfluoride additive and investigation of vacuum sintering kinetics

Polycrystalline transparent magneto-optical Ho2O3 ceramics were successfully fabricated by vacuum sintering using the self-reactive resultant HoOF as a sintering aid. They had an in-line transmittance of 72.0% at 1550 nm and the wavelength-dependent Verdet constants of −182 ± 8, −46 ± 3, and −22 ± 2 rad/Tm at 633, 1064 and 1550 nm (approximately 1.3 times those of terbium gallium garnet ceramics), respectively. The HoOF additive was generated by pyrolyzing Ho(OH)2F from the chemical reaction between the layered Ho2(OH)5NO3 compound and NH4F in the liquid phase. The role of the HoOF additive and the sintering kinetics of the Ho2O3 ceramic were systematically studied. The results show that: (1) the proper amount of F− additive helps to obtain an oxide powder with a narrow particle distribution and significantly improves the optical quality of the sintered body; (2) HoOF induces a transient second phase to effectively suppress the grain growth in the Ho2O3 ceramics during sintering; (3) the diffusion mechanism is primarily controlled by grain-boundary diffusion around 1400 °C; (4) the activation energy of the grain-boundary-controlled diffusion was determined to be approximately 545 kJ/mol via the grain growth method.