Study On The Spectral Characteristics Of Cr³⁺ Doped MgO·nAl₂O₃ Transparent Ceramics

Magnesium aluminate spinel transparent ceramic is an optical material with excellent comprehensive properties,which has unique properties such as high transmittance,high temperature stability and corrosion resistance that can’t be obtained by other materials.The MgO-Al₂O₃system has a wide solid solution range,and MgO·n Al₂O₃transparent ceramics with better optical or mechanical properties can be obtained by adjusting the stoichiometric ratio,which can meet the application requirements of transparent armor,infrared window,night vision system and other harsh environments.In addition,the spinel structure contains many tetrahedral/octahedral vacancies,which is an ideal matrix material doped with rare earth or transition metal ions,by doping can expand its application in the fields of optics,solid-state display,and solid-state lasers.In this paper,Cr³⁺doped MgO·n Al₂O₃transparent ceramics were prepared by the combination of pressureless pre-sintering and hot-pressing sintering,the excellent comprehensive properties of magnesium aluminate spinel and the fine luminescence properties of Cr³⁺ions were successfully combined.This ceramic has excellent optical transparency and sharp red emission of Cr³⁺,which is expected to be applied to white LED and indoor plant cultivation.The effects of rare earth sintering additives,and non-stoichiometric ratio（n value）on the microstructure and spectral characteristics of Cr³⁺doped MgO·n Al₂O₃transparent ceramics were studied.（1）Using La₂O₃or Gd₂O₃as sintering aids（≤4.0 wt%）,the prepared ceramics were all single-phase MgAl₂O₄spinel.However,due to the large ionic radius of La³⁺and Gd³⁺,they were more inclined to occupy the six-coordinated center,and Al³⁺ions occupy the four-coordinated center,resulting in lattice shrinkage.In addition,both La₂O₃and Gd₂O₃inhibit the grain growth,which were helpful to obtain transparent ceramics with fine grains,uniform size and dense structure.Both additives help to improve the transmittance,and the best transmittance was obtained when the amount of La₂O₃was 0.8 wt%.When Gd₂O₃was used as an additive,a small amount of Gd₂O₃helps to improve the transmittance of the ultraviolet-visible band,and a large amount of Gd₂O₃improves the transmittance of the infrared band significantly.The quantum yield of the ceramic sample without additives was0.128,the quantum yield of the ceramic with Gd₂O₃（0.8 wt%）as the additive was slightly increased to about 0.175,and the quantum yield of the ceramic with La₂O₃（0.8 wt%）as the additive was greatly increased to 0.262.In a word,La₂O₃or Gd₂O₃as sintering aids can improve the transmittance and quantum yield of ceramics.（2）The sintering aids of Y₂O₃-R₂O₃（R=La,Gd）（total amount of 2.4 wt%）also obtained the ceramics with single phase,fine grain and uniform size of magnesia-alumina spinel.Among them,the ceramic samples prepared with（0.8 wt%La₂O₃-1.6 wt%Y₂O₃）as sintering aid have great transmittance（the transmittance at 1000-2500 nm was higher than 76%,and the highest was 80%at 1446 nm）and good quantum yield（0.219）.The（1.2 wt%Gd₂O₃-1.2wt%Y₂O₃）ceramics also have good transmittance（more than 75%at 1000-2500 nm,up to78%at 1438 nm）,excellent quantum yield（0.279）and the longest fluorescence lifetime（10.38 ms）.（3）When n=0.9,the prepared ceramics contain a second phase of MgO,when 0.95≤n≤1.2,the prepared ceramics were single-phase magnesium aluminate spinel,and the grain size gradually increases with the increase of n value.Ceramics with 0.95≤n≤1.1 have good transmittance,and the transmittance decreases before or after that due to the influence of second phase or carbon pollution.With the increase of n value,the proportion of ⁴A₂-⁴T₁and⁴A₂-⁴T₂transitions of Cr³⁺ions in the excitation spectrum gradually decreases,and the proportion of R-line emission also decreases due to the decrease of the number of Cr³⁺ions occupying the octahedral position in the emission spectrum.By optimizing the preparation process of Cr³⁺doped MgO·n Al₂O₃transparent ceramics（1.2≤n≤2）,Al-rich ceramics with better performance were obtained.The transmittance of n=1.2 ceramics at 1000-5500 nm band exceeded 75%,and the transmittance of n=1.5 ceramics in the 1000-5500 nm band exceeded 80%,and the fluorescence lifetime reached 12.24 ms.