Study On Micro-structure Control Of Nd:YAG Laser Transparent Ceramic

Nd:YAG transparent ceramics have the potential to replace Czochralski grown single crystals in high power laser and laser fusion energy applications owe to their similar laser slope efficiency and threshold. Ceramic fabrication processes may be able to meet the size requirements for current and future high energy laser applications. Ceramic processing also allows for relatively easy manufacture of complex shapes and composite laser designs that may further enhance performance. However, there are some common scattering cores in transparent ceramics:residual porosity, grain boundaries with second phases, second phase particles, which can limit the usefulness of Nd:YAG ceramic as laser hosts. For this reason, it is important to study scattering in Nd:YAG ceramic materials.In this paper, the Nd:YAG nano-powders were synthesised by co-precipitation. In order to obtain the powders with high dispersion and sintered properties, the relationship of the synthetic condition, composition, structure of Nd:YAG powders was mainly investigated. The colloidal behavior of the aqueous suspensions of neodymia, yttria, and alumina mixed powders using NPAA and DA as dispersants were studied. The slurry with high solid loading and low viscosity was obtained. Moreover, the effect of Nd:YAG ceramic sintering temperature, dwell time and sintering aid on its micro-structure properties were explored.The raw powders of Nd:YAG are the base unit of controlling the micro-structure. The properties of disperse, the pure degree, the size of particle of Nd:YAG precursors have dramatic effect on the micro-structure of sintered ceramic. The properties of powders were affected by pH, metallic cation concentration and dispersant. It was found that the pH of 8.0 was suitable for the preparation of the Nd:YAG nanopowders. Sulfate ions were found to play an important role in the preparation of yttrium and aluminum precursor and nanosized powders. A small amount of sulfate ions effectively modified the precursor morphology and influenced the dispersion and the phase transformation of the precursor during the calcining process. Moreover, YAG phase started to develop without an intermediate phase when the precursor was calcined at about 900℃. Further calcination of the powders at 1100℃ showed the continued refinement of peak shapes and intensities, indicating crystallization completion and crystallite growth of YAG powder with increasing temperature.Slip casting is used extensively in ceramic forming. In order to obtain a green body with high green density and homogenous microstructure, it is important to prepare a stable slurry with high solid loading. The effects of ph, dispersant and solid loading on the stability of the water-based YAG slurry was studied. The particles with dispersant exhibited a higher negative zeta potential than the particles without dispersant at the same pH ranging from 4.5 to 10 because of the absorption of dispersant on the powder. The optimum additive content for obtaining the minimum apparent viscosity was 0.8% and 0.4% for NPAA and DA, respectively. The slurry with 64-66% solid loading and 0.8% NPAA as dispersant or with 70% solid loading and 0.4% DA as dispersant were suitable to casting.The effect of sintering temperature and time on the micro-structure of Nd:YAG ceramic beyond 1600℃ have been studied. Lots of pore were observed in grain boundaries of the Nd:YAG ceramic sintered at 1650℃ without sintering aid. The amount of pore decreased and the grain grew up with the increasing temperature. However, abnormal grain growth and several pores trapped into the grain were observed in the ceramic sintered at 1760℃. Obviously, the addition of 0.1% MgO as sintering aid to Nd:YAG inhibits the grain growth during sintering and allows the sintering process to proceed to theoretical density. MgO is segregated as solute at the grain boundaries where it then exerts a drag on grain-boundary motion. The segregation of solute causes a decrease in the grain boundary mobility, and then inhibits the discontinuous grain growth. The densification rate was significantly enhanced by both increasing the TEOS content and the sintering temperature. The Nd:YAG ceramic with full dense and pore-free microstructure was prepared with 0.5% TEOS. The TEOS addition plays a benefic role in the grain growth. A liquid phase begins to form around 1700℃ in TEOS doped YAG. Furthermore, the concentration of Si4+ in grain boundary can reduce grain growth rate and hamper abnormal grain growth. Silicate and alumina phase would be observed at the grain boundary when the TEOS above 1%.The Nd:YAG laser transparent ceramics with high transmittance were prepared by controlling the micro-structure of raw material, green body and sintered body. An oscillation experiment was performed on a cw laser by diode laser excitation system using the fabricated ceramics. The experimental results indicated a max output power of 1.4 W.