Microstructure Regulation And Performance Optimization Of Co:MgAl₂O₄ Transparent Ceramics

Co:MgAl₂O₄ is an efficient saturable absorber for 1.5μm eye-safe solid-state laser,by which can acquire high-power 1.5μm pulse laser through passive Q-switching technology.The damage threshold to human eyes of 1.5μm laser is very high.The transmission loss in optical fibers is relatively low,which corresponds to a low loss transmission window.In addition,its transmission performance in air is good.Therefore,1.5μm laser is widely used in laser communication,laser ranging,laser radar and other fields.At present,the main saturable absorbers are Co:MgAl₂O₄ single crystal and Co:MgAl₂O₄ glass-ceramics.The difficult preparation of Co:MgAl₂O₄ single crystals by the high melting point of MgAl₂O₄ and the defects of Co:MgAl₂O₄ glass-ceramics make their application is limited.Due to the advantages of Co:MgAl₂O₄transparent ceramics in thermal conductivity,structure control and cost,it has great potential in the application of saturable absorbers.However,the research on Co:MgAl₂O₄ transparent ceramics is still immature,and the systematic exploration is urgently need.Based on the above background,the purpose of this topic is to prepare high-quality Co:MgAl₂O₄ transparent ceramics.The Co:MgAl₂O₄ transparent ceramics were firstly synthesised from commercial powders by solid-state reaction sintering.And then,Co:MgAl₂O₄ transparent ceramics were prepared with co-precipitated nanopowder by non-reaction sintering.The influence of the pre-sintering temperature,precipitant and doping concentration on powder and ceramic properties were systematically studied.The main research contents are as follows:（1）In the research of reaction sintering,Co:MgAl₂O₄ transparent ceramics were prepared from commercial MgAl₂O₄ and CoO powders by vacuum pre-sintering and HIP post-treatment at 1800 ^oC.The ceramic sample pre-sinterd at 1600 ^oC and HIP post-treated shows the highest transmittance,which are 75%at 1700-2400 nm and 64%in visible band.The ground state absorption cross section at 1540 nm is 2.9×10^-19 cm².However,due to the transformation of Co²⁺into Co³⁺,the absorption peak appears at407 nm,which is unfavorable to the non-linear absorption performance of ceramics.（2）In the research of non-reactive sintering,Co:MgAl₂O₄ nano-powders were prepared by co-precipitation method,and the influence of pre-sintering temperature of ceramics was investigated.The results show that the ceramic pre-sintered at 1650 ^oC and HIP post-treated at 1800 ^oC shows the best optical quality.The average grain sizes of transparent ceramics after pre-sintering and HIP post-treatment are 10.2μm and 16.8μm.The transmittance of ceramics HIP post-treated are 81.4%and 85.9%at 400 and900 nm,respectively.The ground state absorption cross section at 1540 nm is 6.18×10^-19 cm².（3）The influence of precipitation on the properties of Co:MgAl₂O₄ powders and ceramics were investigated.The results show that the dispersibility of powder prepared with 0.5 L ammonium carbonate as precipitator is relatively good.The ceramics pre-sintered at 1650 ^oC and HIP post-treated at 1800 ^oC shows the best properties,whose in-line transmittances at 400 nm and 900 nm are 81.4%and 85.9%,respectively.The grain sizes pre-sintered and HIP post-treated are 15.1μm and 20.5μm,and the ground state absorption cross section at 1540 nm is 7.56×10^-19 cm².（4）The effect of Co doping concentration on the absorption performance of Co:MgAl₂O₄ transparent ceramics was investigated.The results show that the Co doping concentration has little effect on the morphology of the powders and ceramics.The absorption coefficient of the ceramics increases with the increase of the Co doping concentration,which indicates that the non-linear absorption performance is enhanced.