Material Design And Fabrication Process Optimization Of YAG Transparent Ceramics

Solid-state lasers have been widely used in the military, scientific research, medical and industrial fields now. Laser material, as the main body of the solid-state laser, is the key and fundamental component in the development of laser technology. The conventional laser materials are single crystal and neodymium glass. But the processing of single crystal needs long time, and it’s hard to achieve large size or high doping concentration. The strength and thermal conductivity of neodymium glass are not high enough. All of these disadvantages can’t satisfy the further development of laser technology to a certain extent. Recently, YAG transparent ceramics have received much attention and developed rapidly because of their unexampled preponderance in preparation technology and properties compared with single crystal and neodymium glass. Major progresses were made in the preparation of YAG transparent ceramics in China, however, there is still some space to improve compared with the foreign advanced level. So the study on the composition optimization and fabrication process of YAG transparent ceramics is very necessary.In this work, YAG transparent ceramics were fabricated by solid-state reaction and vacuum sintering using oxide powders as starting materials. The influence of some important process parameters (composition, doping concentration, sintering aids, and dispersant) on the solid-state reactive sintering of YAG transparent ceramics as well as the phase composition, densification, microstructure evolution, grain growth kinetics and optical transparency of YAG ceramics were mainly investigated. The main contents are as follows:(1) Transparent yttrium aluminum garnet (Y3(1+x)Al5O12) ceramics with different deviations from stoichiometry were fabricated. It was found that for Al2O3-excess samples, the in-line transmittance and the average grain size decreased sharply with the increasing of Al2O3 content due to the Al2O3 secondary phase. For Y2O3-excess samples, when the excessive content was smaller than 0.7mol%, the in-line transmittance decreased slightly and the average grain size increased. However, when the deviation was larger than 0.7mol%, the in-line transmittance and the average grain size decreased due to the secondary phase. For the stoichiometric sample sintered at 1760℃ for 10h, the highest in-line transmittances of 83.1% at 1064nm and 80.2% at 400nm were obtained.(2) Er:YAG transparent ceramics with different doping concentrations were fabricated. For all the doping concentration, the samples exhibit pore-free structure and there is no secondary phase in the grain boundary or inner grain. Fully dense Er:YAG transparent ceramics with homogeneous grain size distributions around 20-23 μm were obtained by sintering at 1750℃ for 50h, all the samples have favorable optical transmittance and homogeneity, the in-line transmittances are all above 83% at the wavelength of 1200nm, and the porosity below 1.5vol ppm. The erbium concentration show little influence on the densification behavior and the microstructure evolution of the Er:YAG ceramics. While the intensity of the luminescence spectra peaks increase with the increase of the Er3+ concentrations. The calculated activation energies for grain growth of the 0.5,1.0,5.0, and 10 at%Er:YAG ceramics are 779,855,805, and 861kJ/mol, respectively.(3) YAG transparent ceramics with different contents of TEOS and La2O3 as sintering aids were fabricated. The influences of TEOS and La2O3 content on solid-state reactive sintering of YAG transparent ceramics were studied. The optimal additions of TEOS and La2O3 have been obtained. When sintered at 1720℃ for 20h, the sintered YAG ceramics with 0.1at%La2O3, 0.5wt%TEOS,0.05at%La2O3 and 0.5wt%TEOS have excellent optical transparency. The in-line transmittances are 81.3%,81.9%, and 81.0% at 1064nm and 78.2%,78.7%, and 79.2% at 400nm, respectively. The microstructures of the samples are homogeneous without pores and secondary phase. The average grain sizes are 13.6μm,32μm, and 22.5μm, respectively. The grain size is refined by adding La2O3 and pores can be easily eliminated with the increase of grain boundaries. The added TEOS forms liquid phase with YAG, which promotes the densification process and the elimination of the pores.(4) What’s more, the YAG transparent ceramics were also fabricated by using dispersants during the ball milling process. Utilizing the dispersants fine and homogeneous powder mixture can be obtained, and finally high-quality YAG transparent ceramics could be fabricated. It was demonstrated that coarse powders can be ground into fine particles and the pores can be eliminated more easily to obtain pore-free ceramics by adding 5wt%PEG400 as dispersant.