Fabrication And Performance Of Terbium Based Garnet Magneto-optical Transparent Ceramics

Optical isolators based on the non-reciprocal Faraday effect are critical devices to prevent back-reflected light from returning to the laser cavity in laser systems.Currently,terbium gallium garnet?TGG?is the most common magneto-optical material used in the visible and near-infrared isolators due to its excellent optical transparency and high Verdet constant.However,the growth of TGG single crystal is a difficult task due to the serious vaporization of Ga₂O₃ from the molten substances.On the other hand,although it is possible to grow the TGG crystal,several defects limit the further improvement on the scale,which results in the difficulty with its application in large aperture-needed Faraday isolator.Compared with TGG single crystal,TGG ceramics can effectively reduce the vaporization of Ga₂O₃ since its fabrication temperature is lower than the melting point.Furthermore,terbium aluminum garnet?TAG?has a higher Verdet constant and better thermal property,and it is thought to be a more promising magneto-optical material for the high-power laser systems.However,it is extremely difficult to grow TAG single crystal from the melt for its incongruent melting nature.Thanks to the lower fabrication temperature,TAG transparent ceramics can avoid the incongruent melting problem since it can be produced without melting process.Additionally,polycrystalline ceramic materials have many other advantages such as large scalable sizes and low time cost,excellent mechanical and thermal properties.In this dissertation,TGG ceramics were fabricated by air sintering and successive HIP post-treating from the TGG nanoparticles synthesized via the co-precipitation technique.In addition,TGG ceramics were doped to optimize the magneto-optical property.Finally,the sintering process of the TAG ceramics were systematically investigated.The main contents of this dissertation are as follows:1)The morphology and sinterability of co-precipitated TGG powders were optimized by controlling the calcination temperature.The powder calcined at 1100^oC retained the TGG phase purity and displayed good dispersity,which was suitable for the sintering of transparent ceramics.Considering the vaporization of gallium oxide,TGG ceramics were sintered by air presintering and HIP post-treatment.The TGG ceramics pre-sintered at 1500°C for 3 h in air and then HIP post-treated at 1550°C for3 h under 150 MPa argon gas showed the best optical quality and the in-line transmittance was 80.3%at 1064 nm,which was near to the theoretical value?81.3%?.TGG ceramics showed the similar Verdet constant compared to the TGG crystals(-134rad·T^-1·m^-1),and the Verdet constant of the TGG ceramic was about-136 rad·T^-1·m^-1at 632.8 nm.2)The influences of ammonium hydrogen carbonate to metal ions molar ratio?R value?on phase composition and morphology of the TGG powders were investigated systematically.The results showed that the second phase existed in the powder with R=3.2,which formed scattering center in the TGG ceramics and led to the low transmittance.By optimizing the R value to 4.0,the TGG powder showed the best dispersity,giving rise to ceramic with the best optical quality.With the increase of R value to 4.4,the calcined powder presented serious agglomeration,resulting in a large number of pores in the TGG ceramics.3)The magneto-optical performance of TGG ceramics was optimized by rare earth ion doping.The results showed that the powders doped with different rare-earth ions were pure TGG phases,and the morphologies were similar.The doped ceramics showed high optical transmittance and excellent magneto-optical property.The Verdet constant at 632.8 nm of the doped ceramics was measured to be-143 rad·T^-1·m^-1,which was 5%higher than that of the TGG ceramic.4)The influences of vacuum sintering temperature on the optical quality of TAG ceramics was systematically studied,and the influences of sintering aid on the optical properties and annealing properties of TAG ceramics were investigated.It was found that the optical quality of TAG ceramics was gradually improved with the increase of the temperature of vacuum sintering.The TAG ceramics with sintering aid vacuum pre-sintered at 1720°C for 20 h and then HIP post-treated at 1700°C for 3 h under 200 MPa argon gas showed the best optical quality and the in-line transmittance was 75.3%at1064 nm.The results showed that the sintering aid TEOS can help improve the optical quality of TAG ceramics and inhibit the change of valence states of Tb³⁺.The Verdet constant of the TAG ceramics at 632.8 nm was-178 rad·T^-1·m^-1,which is 33%larger than that of the commercial TGG single crystals(-134 rad·T^-1·m^-1).