Fabrication And Properties Of Terbium Aluminum Garnet (Tb₃Al₅O₁₂)based Magneto-optical Transparent Ceramics

Terbium aluminum garnet(Tb₃Al₅O₁₂,TAG)has high optical transmittance and large Verdet constant in the visible and near-infrared spectral regions,and its Verdet constant is about 30%⁵0%higher than that of the commercial TGG single crystal.Therefore,it is considered as one of the most ideal candidates for the Faraday isolators.However,the growth of TAG single crystal is extremely difficult due to its incongruent melting nature,which leaves it far from practical application.The fabrication process of ceramics can avoid the incongruent melting problem,which provides a good way to prepare TAG material with excellent properties.In addition,compared with the single crystals,TAG transparent ceramics have the advantages of large size,good thermal shock resistance and high fracture toughness,which have good prospects.Up to now,TAG transparent ceramics are usually fabricated by the solid-state reaction method,which is heavily dependent on the properties of the raw powders.In this dissertation,we prepared TAG powders and transparent ceramics through a variety of routes and systematically investigated the related properties of the powders and ceramics,which was aimed to obtain highly transparent TAG magneto-optical ceramics.In addition,rare earth ions doping was adopted to improve the magneto-optical property of TAG ceramics.The main contents of this dissertation are as follows:1)TAG transparent ceramics were fabricated by the solid-state reaction and vacuum sintering using commercial oxide powders as raw materials.It was found that the commercial Tb₄O₇ powders were agglomerated,which had low sinterability.Pure TAG phase was obtained when the sintering temperature reached 1700?.For the TAG ceramic vacuum sintered at 1700? for 20 h,the in-line transmittance was only⁵0%in the region of 710-1590 nm,and the Verdet constants at 632.8 nm was-179.4rad·T^-1·m^-1.The sample sintered at 1725? for 20 h turned into glass phase,and its transmittance exceeded 80%in the region of 600-1500 nm.The Verdet constants at632.8 nm for the TAG glass sample was-179.3 rad·T^-1·m^-1.The thermal depolarization of the TAG glass was 2×10^-44 at maximum laser power of 91 W,which corresponded to the isolation ratio of 37 dB.2)Tb₄O₇ nanopowders were synthesized by the precipitation method using ammonium hydrogen carbonate as the precipitant and terbium nitrate as raw material.The influence of calcination temperature on the composition,phase and morphology of the Tb₄O₇ powders was studied.It was found that as the calcination temperature increased,the content of water,carbonate and other impurities in the powders gradually decreased and the average particle size of the powders increased.XRD result indicated that some impurities of Tb₂O₃ existed in the obtained Tb₄O₇ powders.The powder calcined at 1200? was obviously agglomerated,which was not suitable for sintering.The influence of calcining temperature of the powder on the optical property and microstructure of TAG ceramics was studied.It was found that the TAG ceramic sample sintered from 1000?-calcined Tb₄O₇ powders showed the best transparency,and its transmittance was above 55%in the 500-1480 nm region.The average grain size of the sample was 13.9?m.The existence of Tb₂O₃ in the Tb₄O₇powders led to the mixed powders deviating from stoichiometric ratio.The superfluous Al₂O₃ was eventually retained in the ceramic samples,which limited the improvement of optical quality of the TAG ceramics.3)TAG nanopowders were synthesized by the reverse co-precipitation method using ammonium hydrogen carbonate as the precipitant,terbium nitrate and aluminum nitrate as raw materials.The influence of calcination temperature and R value(precipitant to metal ions molar ratio,NH₄HCO₃/M³⁺)on the properties of the powders and ceramics were systematically investigated,and it was confirmed that1100? was the optimal calcination temperature and R=4.0 was the optimal R value.The performances of the TAG powders and ceramics were further optimized by ball milling and adding TEOS as sintering aids.The dynamic light scattering?DLS?result of the powders indicated that the agglomeration degree of the ball-milled TAG powders was weakened and the particle size distribution was narrower.For the TAG ceramic sintered from ball milled powders with 0.5 wt%TEOS,its transmittance was greatly enhanced,and the transmittance was about 80%in 500-1500 nm region,reaching 81.4%at the wavelength of 1064 nm,which was close to the theoretical transmittance of TAG?82.4%?.The Verdet constant at 632.8 nm was measured to be-179.8 rad·T^-1·m^-1,indicating that TEOS had little effect on the Verdet constant of TAG ceramics.This is the first time that highly transparent TAG ceramics were successfully fabricated via a wet-chemical route,to the best of our knowledge.4)The influence of rare earth ions(Ce³⁺?Pr³⁺?Ho³⁺)doping on the properties of TAG powders and ceramics was studied.It was found that the doping ions had little effect on the phase and morphology of the powders.The Ce:TAG and Pr:TAG ceramics with different doping concentrations showed high optical transmittance,and the Verdet constants of them were higher than that of the TAG ceramic.The Verdet constant at 632.8 nm of the 2at%Ce:TAG ceramic was measured to be-196.2rad·T^-1·m^-1,which was 9%higher than that of the TAG ceramic.The 0.5at%Ho:TAG transparent ceramic exhibited excellent optical property,and its optical transmittance exceeded 80%in the region of 500-1500 nm?except the absorption bands?,reaching81.9%at the wavelength of 1064 nm.The Verdet constant of the 0.5at%Ho:TAG ceramic at 633 nm was-183.1 rad·T^-1·m^-1,slightly higher than that of the TAG ceramic.