Research On Characteristics And Key Technology Of Nd:YAG Laser Ceramics

With the rapid development of ceramic preparation technology, Nd:YAG transparent ceramics as solid-state laser materials, since then, have widely used in industrial, medical, defense and other fields. In this thesis, Nd:YAG transparent ceramics were fabricated by solid-state reaction and vacuum sintering method with high-purity powder of Al₂O₃, Y₂O₃ and Nd₂O₃ as raw materials. The phase, microstructure and property of Nd:YAG transparent ceramics were characterized by X-ray diffraction, scanning electron microscope, spectrophotometer, thermal conductivity analyzer, respectively. The key technology, Laser performance and thermal properties of Nd:YAG laser ceramics were investigated, which provided an experimental basis for domestic Nd:YAG laser ceramic.Nd:YAG ceramics were fabricated by solid-state reaction with high-purity Al₂O₃, Y₂O₃ and Nd₂O₃ as raw materials, different content TEOS as additives. It was found that the sample sintering at 1750℃ for 30 h with 0.3wt% TEOS exhibited good microstructure, but there were still a few pores in the grains, the grain boundaries are clean and the average grain size is about 30μm. The transmittance of the sample has reached 72.32% at 1064 nm.The ceramics were fabricated with different content Mg O and TEOS. It was found that the sample sintering at 1730℃ for 30 h with 0.3wt% TEOS and 0.3wt% Mg O had high density, clear grain boundaries and homogeneous grains. The transmittance of the sample at 1064 nm reached 78.46%, which was higher than that of the sample only adding TEOS. Mg O could inhibit grain growing, increase the density of grain boundary which provide more channels to eliminate the porosity. So the sample with Mg O has higher transmittance.By changing the ratio of B³⁺ to Si4+（0.5, 1.0, 2.0） with a constant summation, the effects of B₂O₃ and Si O₂ as compound sintering aids on microstructure and transmittance of the sample were investigated. With the increase of B³⁺:Si4+, the density and transmittance at 1064 nm of sample have decreased. When B³⁺:Si4+=0.5, the sample sintering at 1700℃ for 30 h exhibited highest density and transmittance（76.87%） with homogeneous grains and clear grain boundaries. B₂O₃ could bring down the viscosity of the liquid phase to improve the densification rate of the sample, which made the relative density of the sample to approach to theoretical density rapidly. So B₂O₃ as additive can reduce sintering temperature of the ceramics.Nd:YAG powders were synthesized successfully by co-precipitation method, sol- gel method and solid-state reaction method. The ceramics were fabricated with precursor powders. By comparing the microstructure and measuring the transmittance of the sample, the solid-state reaction method has been chosen for preparation of transparent ceramics. Because of improving bisque forming method and optimizing sintering system, thick size Nd:YAG laser ceramics were fabricated by solid-state reaction method with purity Al₂O₃, heat-treatment Y₂O₃ and Nd₂O₃ as raw materials. It was found that the relative density of bisque by cold isostatic pressing was 51.6% which is higher than that of bisque formed by bidirectional-axis pressing. On the basis of bisque forming, gradient-doping of Nd³⁺concentration was achieved by design of the mould. Because of their various particle sizes, sintering activity and specific surface area, Y₂O₃ powders obtained by different methods induced different YAG phase synthesis temperature. The synthesis temperature of YAG phase has risen successively by 50℃ from chemical precipitation to 1200℃ heat treatment. Formed by cold isostatic pressing method, bisques containing Y₂O₃ powders obtained by different methods were sintered at 1730℃ for 30 h. It was found that the microstructure of samples were different obviously. The sample with Y₂O₃ powders（sintering at 1200℃ for 2h） exhibited homogenous grain size（around 15μm）, low porosity and 78.46% transmittance at 1064 nm. Changing sintering system（1℃/min heating rate and 30 h holding time）, the porosity was eliminated totally. the transmittance of the sample at 1064 nm and 400 nm is 83.59%, 80.82% respectively, which has met the transparency requirement as laser material.Nd:YAG transparent ceramic was end-pumping with parallel-plane resonator and obtained 1.54 W laser output at 1064nm（pump threshold 1.15 w, light conversion efficiency 18.4% and slope efficiency 21.3%）. Meanwhile, thermal diffusion coefficient, specific heat value of the samples with different transmittance and grain size were measured by thermal conductivity meter（Nanoflash, LFA—447） and then the thermal conductivity was calculated. It was found that thermal conductivity decreased with increase of porosity（decrease of transmittance） and increased with increase of grain size（decrease of grain boundary density）. This work has provided a parameter index for preparing ceramic specimens with high thermal conductivity.