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)(2)F from the chemical reaction between the layered Ho-2(OH)(5)NO3 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 degrees C; (4) the activation energy of the grain-boundary-controlled diffusion was determined to be approximately 545 kJ/mol via the grain growth method.