Fabrication And Performance Of YAG Polycrystalline Transparent Ceramics

Yttrium aluminum garnet (YAG, Y3AI5O12), with a cubic crystal structure, has no birefringence effect, outstanding creep resistance and superior optical and mechanical properties. It is widely applied as solid laser materials, as well as high temperature visible and infrared windows. Compared with YAG single crystals, YAG polycrystalline ceramics have the advantages of the ability of fabricating into large size and doping with higher concentration of active ions, which are the key requirements for high power solid-state laser applications. The YAG ceramic is a promising substitute for YAG single crystals and has aroused intense interest in recent years. In this work, ultrafine YAG powders and highly transparent YAG ceramics were fabricated using low-cost metal salts as the raw materials.Fabrication of fully transparent YAG ceramics by the solid-state reaction method needs higher sintering temperature, therefore, it is essential to synthesize highly reactive Y2O3 and Al2O3 powders so as to reduce the sintering temperature effectively. Highly reactive Y2O3 andα桝-Al2O3 powders were synthesized by a chemical co-precipitation method and the pyrolysis of NH4A1(OH)2CO3, respectively, and the transparent YAG ceramics were fabricated by the solid-state reaction method. The mixed powders transform mainly into YAG phase after calcination at 1300℃for 2 hours, about 200℃lower than the conventional YAG formation temperature. The YAG ceramics sintered at 1700℃for 15 hours are fully transparent, with an average grain size of 12.7μm and an optical transmittance of 70% in the visible wavelengths. Fully transparent 1 at%Nd:YAG ceramics with an optical transmittance of 53% in the visible wavelengths were produced by the solid-state reaction method from the same Y2O3 andα-Al2O3 powders and a commercial Nd2O3 powder. The average grain size of the specimens after sintering at 1700℃for 15 hours is 5.75μm, significantly lower than that of the pure YAG ceramics. In order to improve further the transmittance of YAG ceramics, a Al2O3 powder composed of mixture ofθandγ-Al2O3 with much higher sinterability were synthesized by the pyrolysis of NH4A1(OH)2CO3. Fully transparent 1 at%Nd:YAG ceramics with much higher optical performance were fabricated. The transmittance of specimens sintered at 1700℃for 20 hours is 78% in the visible wavelengths and 82% in the infrared wavelengths, respectively.The densification process of YAG ceramics was investigated. The results show that both the formation velocity of YAG phase and the densification of YAG ceramics can be improved significantly by the addition of (NH4)2SO4 during the synthesis of Y2O3 powders by the chemical precipitation method. The grain growth of the YAG ceramics sintered in vacuum is consistent with the relationshipG3-G03=Kt, with an activation energy of 213.54 KJ·mol-1. The growth process is controlled by grain boundary diffusion with the presence of the liquid phase.Using the good dispersive effect of SO42- ions in both the formation and the calcination process of the precursor, highly reactive YAG nanopowders were synthesized by the co-precipitation method from low-cost Y2O3, NH4A1(SO4)2·12H2O and NH4HCO3 starting materials. It was found that comparatively low pH value (pH=7.2) and slow dripping speed (2ml·min-1) are beneficial to the increase of reactivity of the precursor powders, resulting in a well dispersed YAG powder with a particle size of 48 nm. Fully transparent YAG ceramic was produced from this YAG powder by vacuum sintering at 1700℃for 5 hours. The in-line transmittance of the YAG ceramic reaches 56% in the visible wavelength. In addition, the 1.1at%Nd:YAG ceramic was fabricated under the same conditions, and after vacuum sintering at 1700℃for 10 hours, its in-line transmittance is 57% in the visable wavelength.A novel homogeneous precipitation method was used to synthesize ultrafine YAG powders using yttrium and aluminum nitrate and sulphate as the raw materials and urea as a precipitant. After calcination at 1100℃for 2 hours, the precursor from the nitrate system transforms into YAG phase. However, the resulting YAG powders are severely agglomerated and irregular in shape. With the addition of appropriate amount of (NH4)2SO4 dispersant, properties of the YAG powder can be improved effectively, obtaining a YAG powder with regular shape, good dispersion and an average particle size of 0.5μm. Transparent YAG ceramics was produced by vacuum sintering from this YAG powder. The precursor from the sulfate system transforms mainly to a well dispersive YAG powder with approximately spherical morphology and an average particle size of 0.8μm. Transparent YAG ceramics were fabricated by vacuum sintering at 1700℃for 10 hours.