Investigation Of Micro-structure Control Of Nd:YAG Nanopowders And Colloid Chemistry Properties

Laser material is the central part and infrastructure of laser technology. As a new generation of solid-state laser materials, transparent laser ceramics are of great significance with respect to national security and sustainable economic development. Nd:YAG laser ceramic is the best solid state laser gain media that is significantly improved by the development of nano-science and technology. Great progress had been made recently on the preparation of laser ceramics while many physical and chemical mechanisms happened during the preparation process remains unknown. People can not make good control on the nanosize powder with high purity, perfect phase structure, mono-dispersed distribution on both size and morphology. That has become the main obstacle for the preparation of laser ceramics with high performance. What more, slip casting method has been demonstrated to be highly suitable for preparation of laser ceramics with high transmittance while it has not been investigated in a comprehensive way. That becomes another inhibitor for the preparation of laser ceramics. In this paper, methods with prospective batch productive ability such as chemical co-precipitation method and urea precipitation method are resorted to detailed investigation to prepare Nd:YAG precursors. Nucleation and growth of the Nd:YAG precursors and the affection of different dispersants were comprehensively studied. Controllable Nd:YAG powders with good sintering ability are prepared. Preparation of the Nd:YAG green body in slip casting method was also studied.1. Control of the microstructure of the Nd.YAG nano-powder(1) Ammonium hydrogen carbonate and urea co-precipitation method were employed to prepare the Nd:YAG precursor. Nucleation, growth of the Nd:YAG precursors and its evolution in the following calcination process were studied. Special attention was paid on the effect of (NH4)2SO4 to the size, morphology and phase of the Nd:YAG powder during different precipitation procedures. According to our study: added in the solution of metal ions have little affection on the morphology of the precursor and the calcined powder and its sintering ability when the Nd:YAG precursor was prepared via ammonium hydrogen carbonate co-precipitation method. While the (NH4)2SO4 added in the precipitant solution has profound affection on the nucleation and aggregation of the precursor which would further affect the morphology and dispersion of the precursor. And the decomposition of (NH4)2SO4 at high temperature (1000-1200℃) would contribute to the powder with good dispersion ability and high purity. That would improve the sintering ability of the calcined powder. It is found that the sintering ability of the nano powder was not only determined by its diminished size. Powder with skeleton structure also has good sintering ability.(2) Modification of the SO42- absorbed surface of the Nd:YAG precursor was proposed. Mechanism of the modification was studied by monitoring the electrokinetic and spectroscopic properties of the precursor. It is found that the OH-was replaced by SO42- and that lead to the formation of the spheric aggregation of the precursors. The modification process would also affect the microstructure of the precursor in the calcination process and the transparency of the Nd:YAG ceramics.(3) Nucleation and growth of the Nd:YAG precursors prepared by urea precipitation method were studied along with the investigation on the morphology,calcination of the precursor and the sintering ability of the prepared powder. Al(NO3)3,Y(NO3)3,Nd(NO3)3 were prepared into the stock solution and precipitated in the presense of urea. Zeta potentiometer and FT-IR were employed to measure the affection of SO42- to the surface of the precursors. SO42- ions were found to be the guidance of the formation of the skeleton structure according to the TEM result. XRD result showed that the addition of (NH4)2SO4 would lead to higher formation temperature of YAG crystalline structure. That should be attributed to the inhibition of (NH4)2SO4 to the diffusion of elements at high temperature. And it is found that sample with 4.56 wt% (NH4)2SO4 added have uniform particle size and best sintering ability. In conclusion, (NH4)2SO4 is a crucial factor for the control of the microstructure of the precursor and calcined powder. Affection of the urea concentration on the microstructure of the precursor was also studied. It was found from the sample with [M]=0.08 that the best morphology and sintering ability could be achieved by setting the [U]/[M] at 40 with 4.56 wt% (NH4)2SO4 added in the stock solution.(4) Antisite defects were analyzed when the Nd:YAG powders were prepared from unstoichiometric ingredient. That provides theoretical support for the preparation of stoichiometric powder. It was found that YAG crystalline structure was remained when the excess of Al is less than 20%. Al take the position of Y and the antisite defects were formed. XRD, HRTEM, ICP result were also in accord with the conclusions above. Directed evidence was given by the result of EXAFS showing that the Y was substituted by Al, sometimes by two Al atoms. That indicates the existence of antisite defects in the Nd:YAG powder. These results are different from the antisite defects found in YAG single crystal.Impurity phase showed up when the Y is excessively added into the system. The impurity phase with rich Y would decompose into YAP, YAM etc. EXAFS results showed that the Octahedral of Al-O could be substituted by Y insignificantly.In conclusion, excess of Al in the system would contribute to the stability of YAG crystalline structure than the excess of Y. That is distinctive to those antisite defects existed in YAG single crystal. 2. Colloidal chemical properties of the Nd:YAG powderThe electrokinetic properties of Nd:YAG nanopowder were investigated. The effect of pH of the slurry on the electrokinetic properties was discussed. The optimal pH value needed to obtain a stable Nd:YAG nanoparticle slurry was 10.5. NH4PAA can improve the stability of the slurry, and the optimal amount is about 2.6 wt%. Slip with solid loadings of 65 wt% was send to rheological test. Green body with uniform and high density was achieved via vacuum grouting method.Electrokinetic, rheological properties of Nd2O3, Al2O3, Y2O3 slurry and microstructure of these green bodies were also investigated to support the preparation of Nd:YAG ceramic via solid state reaction method. Result showed that the colloidal chemical properties of these slurries were analogous to that of the Nd:YAG slurry but dispersant needed for the mixed powder was much more. That can be attributed to the larger particles existed in the mixed powder. Amount of dispersant at about 5.2 wt% and solid loadings of 53 wt% were the best consideration for slurry with good rheological and green body with uniform density.Microstructure and colloidal chemical properties of the Nd:YAG powder were comprehensively studied in this paper. Innovative result of this paper would provide more infrastructure work for the preparation of transparent ceramics.