Phase Transformation And Magneto-Optical Properties Of Tb₂O₃ Ceramics With High Verdet Constant

Magneto-optical transparent ceramics are a new type of inorganic transparent optical functional material with optical transparency comparable to that of single crystals and glass,but with the advantages of adjustable composition and structure of polycrystalline ceramics.Magneto-optical materials with a high Verdet constant are key components for the development of high-performance optical isolators,which can significantly shorten the magneto-optical medium and thus enable the development of integrated and miniaturized opto-isolators.Terbium（Ⅲ）Oxide（Tb₂O₃）has no absorption peak in the range 500 nm-1400 nm and its Verdet constant is more than 3.5 times that of commercial terbium gallium garnet single crystals(Tb₃Ga₅O₁₂,TGG).The bottlenecks in the preparation of Tb₂O₃ magneto-optical materials are:（1）the oxidation of trivalent terbium oxide powder in an air atmosphere,even at room temperature,resulting in the loss of magneto-optical properties;（2）the reversible phase transition of terbium oxide around 1600℃,2167℃ and 2370℃,which makes it difficult to obtain large-sized bulk crystal materials by conventional melt methods（shrinkage and collapse to powder in the cooling stage）.To address the above problems,this thesis chose a transparent ceramic preparation scheme to first circumvent the two high temperature phase transition points near 2167℃,2370℃.The component design,transparent preparation and magneto-optical properties of terbium oxide magneto-optic transparent ceramics were investigated,with the following conclusions:（1）High optical quality Tb₂O₃ transparent ceramics with yttrium oxide and lutetium oxide as phase stabilizers and lanthanum oxide as sintering aid,respectively,were prepared by a wet chemical co-precipitation combined with reduced hydrogen atmosphere sintering using ammonia bicarbonate and ammonia as composite precipitants and nitrate solution as mother liquor.（2）La₂O₃ was used a sintering aid to dope into yttrium stabilized Tb₂O₃ ceramics,and it was demonstrated that La₂O₃ can promote the sintering densification of the ceramics.With increasing content of La₂O₃,the number of pores and grain size of as-fabricated(Tb_0.8Y_0.2xLa_x)₂O₃ ceramics both decrease,and the pore size decreases from micrometer to submicron,and the transparency was greatly improved.The in-line transmittance of(Tb_0.8Y_0.16La_0.04)₂O₃ ceramics sintered at 1600℃ for 15 h was measured to be 73.3%,with an average size of approximately 13 μm.（3）Optimize the composition of phase stabilizers.Lutetium,which is closer to terbium in atomic mass,was selected as a phase stabilizer instead of yttrium,and(Tb_xLu_0.96-xLa_0.04)₂O₃ ceramics with different lutetium contents were prepared by nanocrystalline powders combined with pressureless sintering in hydrogen atmosphere technique（NC-PLSH）.The in-line transmittance at 1400 nm is 79%,which is close to the theoretical value of 80.6%.（4）Optimize the composition of sintering aids.Lanthanum oxide and zirconium oxide were used as composite sintering aids to inhibit grain growth on the basis of promoting sintering in order to inhibit the phase transformation of Tb₂O₃.The design of(Tb_0.95（Zr+La）_0.05)₂O₃ ceramics with different zirconia and lanthanum oxide content components shows that the 2at.%La₂O₃+3at.%ZrO₂ doped Tb₂O₃ ceramics have a good optical quality,with a linear transmittance of 76.3%obtained at 1600℃ and an average grain size of about 4 μm（5）Magneto-optical properties of the as-preapred transparent ceramics was tested by extinction method,and the Verdet constant of the(Tb_0.8Lu/Y_0.16La_0.04)₂O₃ ceramic at 633 nm was measured to be-354 rad/（T · m）,which is 2.64 times that of TGG single crystal.To further improve the magneto-optical properties,the phase transition of the Tb₂O₃ ceramics was stabilized using a composite sintering aid to increase the terbium content without using a phase stabilizer,and the Verdet constants of the lanthanum plus zirconium doped Tb₂O₃ ceramics were measured to be-652,-432 and-127 rad/（T·m）at 532,633 and 1064 nm,respectively,which are 3.31,3.17 and 3.46 times that of TGG single crystal.The length of a TGG single crystal requires 20 mm to obtain a 45° Faraday rotation when it is applied at 1064 nm under a magnetic field intensity of 1 T.However,the minimum required length for Tb₂O₃ ceramics sample requires only 5.8 mm.