Preparation And Properties Of(Tb_1-xLu_x)₂O₃ Magneto-Optical Transparent Ceramics

As a key component of magneto-optical devices,magneto-optical materials,which can eliminate the transmission of reflected light and ensure the stability of the laser source,are widely used in laser,optical communication and other fields.The main properties of magneto-optical materials include Verdet constant,thermal conductivity,optical quality and so on.And terbium oxide(Tb₂O₃),whose Verdet constant is three times higher than that of Tb₃Gd₅O₁₂(TGG),is a promising magneto-optical material.These magneto-optical materials with high Verdet constant can greatly minimize the length of optical isolators and reduce the necessary magnetic field strength,which can meet the requirements for practical applications.However,due to the oxidation behavior and the phase transformation process of Tb₂O₃,the preparation is very difficult.In this work,phase transformation process of Tb₂O₃ has been investigated in different atmosphere at first.And finally,it illustrates that the influence of the oxidation behavior and phase transition of Tb₂O₃.It is noted that when heating in air,Tb₂O₃ is firstly oxidized to TbO_1.81 at 500? and then deoxidized to TbO_1.714 at600? and Tb₂O₃ at 988? at elevated temperature.Furthermore,the most stable phase at room temperature is not Tb₂O₃,but TbO_1.81.Therefore,Tb₂O₃ should be isolated from oxygen and sintered in an oxygen-free atmosphere.As for the phase transition of Tb₂O₃ under high temperature,when the temperature rises above 1600? in oxygen-free atmosphere,the phase transition of Tb₂O₃ from cubic phase to monoclinic phase happens.And this phase transition is reversible.When the temperature drops to room temperature,the monoclinic phase Tb₂O₃ under high temperature returns to cubic phase so that the Tb₂O₃ samples crack seriously.Consequently,in order to prepare Tb₂O₃ transparent ceramics,the primary consideration is how to inhibit the phase transition of Tb₂O₃.To solve this problem,Lu₂O₃ is selected as a solid solution in this work.And the(Tb_1-xLu_x)₂O₃ ceramics were prepared by vacuum pre-sintering combined with hot isostatic pressing(HIP)post-treatment.As expected,the phase transition of Tb₂O₃during the sintering process is effectively inhibited by Lu₂O₃ solid solution and the obtained(Tb_1-xLu_x)₂O₃ ceramics show good thermal conductivities and Verdet constant.Moreover,the densification process and microstructure evolution of(Tb_1-xLu_x)₂O₃ ceramics with different Lu₂O₃ concentrations were studied.After the optimization of sintering process,the transmittance of(Tb_1-xLu_x)₂O₃ ceramics(x=0.1?0.75)was improved ultimately.The densification behavior and microstructure evolution of(Tb_0.5Lu_0.5)₂O₃ ceramics were further studied as the sintering temperature increased.Finally,the in-line transmittance of the polished(Tb_0.5Lu_0.5)₂O₃ ceramics with a thickness of 3 mm is 78.9%at 1064 nm and 78.1%at 633 nm,which is close to the theoretical transmittance of(Tb_0.5Lu_0.5)₂O₃ ceramics.The thermal conductivity of(Tb_0.5Lu_0.5)₂O₃ ceramics at room temperature is 5.88 W/m·K,which is a bit lower than that of common Tb₃Al₅O₁₂(TAG)and higher than that of(Tb_0.5Y_0.5)₂O₃ ceramics as reported previously.The Verdet constant measured at 633 nm is 224.33 rad·T^-1·m^-1at room temperature,which is 30%higher than that of TAG and almost equal to that of(Tb_0.5Y_0.5)₂O₃ ceramics.These performance is good enough so that the novel(Tb_1-xLu_x)₂O₃ ceramics have potential application value.In addition,the effects of the ZrO₂ sintering aid on Tb₂O₃ and(Tb_0.5Lu_0.5)₂O₃ceramics were studied.As for Tb₂O₃ ceramics,when the sintering temperature rises to1700?,the content of ZrO₂ needs to increase to 1.5 at%to stabilize the phase transition of Tb₂O₃.However,after 1600? HIP post-treatment,obvious scattering phenomenon could be observed in the double-sided polished Tb₂O₃ samples by our naked eyes.Through further observation under a polarizing microscope,it is found that there are non-cubic phases in this sample,and these non-cubic phases are t-ZrO₂(tetragonal ZrO₂),which could be confirmed by X-ray diffraction,element mapping analysis and Raman testing results.It indicates that the phase transition of Tb₂O₃under high temperature is induced by 1600? HIP post-treatment to a certain extent.Because the phase transition of Tb₂O₃ is reversible,the lattice rearrangement after the reversible phase transition contributes to the severe lattice distortion,and part of t-ZrO₂ is desolated.These internal inclusions cause serious scattering so that the in-line transmittance of the HIPed Tb₂O₃ ceramics decreases evidently.As for(Tb_0.5Lu_0.5)₂O₃ ceramics,the phase transition of Tb₂O₃ can be stabilized by Lu₂O₃solid solution to some extent,but the following 1600? HIP treatment can still induce this phase transition,which must be inhibited by ZrO₂.However,the inhibition of ZrO₂ on the phase transition of Tb₂O₃ is limited to HIP temperature.When the HIP temperature increases to 1700?,the internal non-cubic phases and stress cracks can be observed under the polarizing microscope even though ZrO₂ is added into the(Tb_0.5Lu_0.5)₂O₃ ceramic samples,which leads to the deterioration of the optical quality.However,unlike pure Tb₂O₃,ZrO₂ was not desoluted from(Tb_0.5Lu_0.5)₂O₃ ceramics due to the solid solution of Lu₂O₃.It indicates that the few non-cubic phase observed under the polarizing microscope should be the residual phase of Tb₂O₃,which was not fully transformed back to the cubic phase and left in the ceramics.Therefore,it shows that higher HIP temperature is an important factor of inducing the reversible phase transition of Tb₂O₃,which should be avoided as far as possible in the preparation process.In addition,the average grain size and thermal conductivity of(Tb_0.5Lu_0.5)₂O₃ceramics gradually decrease with the increase of ZrO₂ contents.After further optimization for the sintering process,the transmittance of(Tb_0.5Lu_0.5)₂O₃ ceramics doped with 0.75 at%ZrO₂ is 80.2%at 1064 nm and the thermal conductivity is 6.25W/m·K at room temperature,which is significantly improved compared to previous preparation.Finally,CeO₂ doped Tb₂O₃ and(Tb_0.5Lu_0.5)₂O₃ ceramics were also investigated.It is found that only CeO₂ could not stabilize the phase transition of pure Tb₂O₃ under high temperature,but Ce ions could enter the lattice of Tb₂O₃ and exist as Ce³⁺.As for the(Tb_0.5Lu_0.5)₂O₃ ceramics,with the increase of CeO₂ contents,the color of Ce:(Tb_0.5Lu_0.5)₂O₃ ceramics vacuum pre-sintered changes to dark red and the average grain size of Ce:(Tb_0.5Lu_0.5)₂O₃ ceramics increases gradually.Moreover,with the increase of Ce-doping,the Verdet constant of Ce:(Tb_0.5Lu_0.5)₂O₃ transparent ceramics slightly increases and its in-line transmittance curve obviously shifts to red light wavelength.It is contributed by CeO₂,which is doped into the(Tb_0.5Lu_0.5)₂O₃ceramics and existed as Ce³⁺.Ce ions ultimately lead to the absorption transition of 4f?5d in the 400 nm?500 nm band.Therefore,the samples show different colors.The transmittance curve of 1 at%Ce:(Tb_0.5Lu_0.5)₂O₃ transparent sample red shifts to580 nm,and the Verdet constant measured at 633nm is 241.36 rad·T^-1·m^-1,which is7.6%higher than that of undoped(Tb_0.5Lu_0.5)₂O₃ samples.After improving the optical qualities,it is expected to obtain practical applications.