Fabrication And Properties Of Pentavalent/Hexavalent Oxide-Doped Yttria Transparent Ceramics

Transparent yttria ceramics（Y₂O₃）are widely used as laser host,missile dome,and infrared window materials owing to their high theoretical transmittance（400 nm and higher wavelengths>80%）,broad transparency range（0.2-8μm）,low infrared emissivity,outstanding refractoriness,and superior chemical stability.However,due to the high melting point（2430℃）of Y₂O₃,it is challenging work to prepare highly transparent yttria ceramics.In this study,highly transparent yttria ceramics were fabricated by vacuum pressureless sintering method with Nb₂O₅/Ta₂O₅ or WO₃/MoO₃ as sintering additives.The effects of the doping concentration on the phase,relative density,microstructure,optical properties,thermo-mechanical properties of samples were studied.Furthermore,highly transparent Ho:Y₂O₃ceramic samples were prepared,and the luminescence properties of the samples were studied.It was found that the transmittance of yttria ceramic samples by vacuum pressureless sintering can be effectively improved by the use of Nb₂O₅/Ta₂O₅ or WO₃/MoO₃ as sintering additives.With an increase in the doping concentration of Nb₂O₅/Ta₂O₅,from 0 to 1 at.%,the transmittance level of the ceramic samples increased and then decreased.When sintered at 1780℃for 14 h,the optimal doping concentration of Nb₂O₅ and Ta₂O₅ were 0.3 and 0.2 at.%,respectively.The transmittance of 0.3 at.%Nb₂O₅-doped sample was 72.4 and 81.7%at 400nm and 1100 nm,respectively.For the sample doped with 0.2 at.%Ta₂O₅,the transmittance was 70.1 and 81.9%at 400 nm and 1100 nm,respectively.The variations of the transmittance of WO₃/MoO₃-doped samples with the doping concentration was similar to those of the Nb₂O₅/Ta₂O₅-doped samples.The optimal doping concentration of WO₃ and MoO₃ were 0.05and 0.025 at.%,respectively.The transmittance of the sample doped with 0.05 at.%WO₃ at 400nm and 1100 nm was 77.1 and 81.7%,respectively.The transmittance of the sample doped with 0.025 at.%MoO₃ at 600 nm and 1100 nm was 66.8 and 70.1%,respectively.As revealed by microstructural analyses,the transmittance of the sample is strongly dependent on structure and number of residual pores.Nb₂O₅/Ta₂O₅ and WO₃/MoO₃ are efficient grain boundary migration inhibitors of Y₂O₃.A proper doping concentration can not only effectively decrease the grain size and avoid the formation of intragranular pores,but also provide sufficient driving force to eliminate intergranular pores,thereby improving the transmittance of yttria ceramics.Compared with the conventional sintering additives such as Zr O₂ and La₂O₃,the optimal doping concentrations of Nb₂O₅/Ta₂O₅ and WO₃/MoO₃ for yttria transparent ceramics are 1-3 orders of magnitude lower.Moreover,the fine grain size and low doping concentration of the yttria ceramics sample doped Nb₂O₅/Ta₂O₅ or WO₃/MoO₃ led to higher thermal conductivity,strengthened mechanical properties and better thermal shock resistance compared to the yttria samples with Zr O₂,La₂O₃ or other sintering aids.Therefore,the results indicate that Nb₂O₅/Ta₂O₅ and WO₃/MoO₃ are new and highly efficient sintering additives for transparent yttria ceramics.Highly transparent Ho:Y₂O₃ ceramics were prepared using 0.3 at.%Nb₂O₅ as a sintering additive.The transmittance levels of all samples with various Ho³⁺concentrations reached～81.5%（2 mm thick）at 1100 nm,which is close to the theoretical transmittance.The photoluminescence properties of Ho:Y₂O₃ transparent ceramic samples were studied.Under the excitation of 363 nm（ultraviolet）or 448 nm（blue）light,Ho:Y₂O₃ transparent ceramic samples showed green emission peaked at 550 nm.The emission intensity was strongly affected by the concentration of Ho³⁺ions,which reached its highest level in the sample doped with 1 at.% Ho³⁺.