Preparation And Characterization Of Neodymium-doped Yttrium Aluminum Garnet (nd: Yag) Laser Ceramics

Nd-doped yttrium aluminum garnet (YAG:Nd) is the most important four-level solid-state laser host, and has been used widely in various areas. Recently high transparent ceramic laser materials have received great attention since the quality of ceramic laser materials has been improved dramatically using nano-crystal technology and non-press vacuum sintering method. YAG ceramic is a promising material for large-size solid-state lasers, as a very good substitute candinate to YAG single crystal. Moreover, YAG ceramics are potential advanced structural materials in view of their high creep and oxidation resistance at high temperatures and theirsuitable heat conductivity. YAG powders doped with rare-earth metal elements can be used as ultra-short afterglow phosphors for cathode ray tubes and high-resolution displays.In the present work, the relationship of the composition, processes conditions, structure with properties of Nd:YAG synthesized were mainly investigated, moreover the effect of Nd:YAG ceramics preparing processes on its properties was explored. Meanwhile some creative work have been developed, e.g. the reaction conditions in the co-precipitation method and solvo-thermal synthesis, mechanism, dispersion, shaping and sintering process. The main points are as follows.The chemical co-precipitation method shows some advantages in many aspects such as the simple synthesis process, low cost potential for mass production and superior powder properties. Firstly, YAG nano-sized powders were prepared by co-precipitation method low costly. The effects of the precipitants, concentration of metal ions and mother solutions, heat- treatment temperature, composition of precursors on the formation of YAG and powder properties were investigated. The results show that the precursor composes of metal hydroxides and nitrate using NH₄OH as the precipitant. However, the precursors prepared are mainly compound of carbonate using NH₄HCO₃ as the precipitant. The sinter-ability of the former is worse than that of the later, because its chemical components are more homogeneous than those of the former and contain some carbonate. Moreover pure YAG phase can be directly formed at 900 °C without any intermediate phases, if the low concentration of metal ions and proper PH value are adopted. In contract, single YAG phase is obtained at high heat-treatment temperature if the reactant concentration is high resulting from the heter-geneous of the components of precipitates and aggleration of particles.The formation mechanism of YAG phase was investigated by the terms DTA/TG, XRD, IR and so on. The results show that the Y3+ and Al3+ may precipitate uniformly in the form of compound salt, the precursor keeps at atom level during the mixing process. The reaction temperature needed for the formation of pure YAG phase is much lower than that of solid-state reaction due to the transition distance which YJ and Al bonding is discreased. During the process of calcining, Al3+ and Y3+ combined with OH' ions form the basic crystal growth units [Al,Y(0H)6], OH" anions existed in the octahedral close-packed structure . In the case, the chain consists of these octahedral units that are bonded through corner, edge and face. As the calcination temperature rises, the hydrogen will be lost resulting in a decrease in the basic unit, and some bridged oxygen may break up and subsequent changes in the co-ordination of the cations. The cations A1J+ can be located in the oxygen octahedral or tetrahedral sites. However, the radius of Y3+ is larger than that of Al3+, it usually occupies the oxygen octahedral or dodecahedron. YAG garnet structure can be considered to be a perfect oxygen coordination polyhedral network that consists of [AIO4] tetrahedral, [A1O6J octahedral and [YOg] dodecahedron.Solvo-thermal method is popularly used because the ceramic power can be synthesized under a moderate reaction condition. Pure phase YAG powders with spherical shape and non-aggregation are synthesized using a cheap and harmless mixed solvent ( ethanol and water) for the first time. The results show that the increase of ethanol content in the mixed solvent is beneficial to the formation of YAG at a relatively low temperature. When the ratio of ethanol to water is equal to or exceeds 3:1, YAG powder can be synthesized at 280°C³00°C for 2h, which consists of well dispersed and nearly-spherical fine grains (60nm on average) with a relatively narrow grain size distribution. The synthesizing reaction for long time promotes the growth of grains and result in the agglomeration of grains. YAG phase is directly formed by the dissolution, dehydration and precipitation of precursor. This is due to the solubility of the precursor in the mixed solvent can decrease dramatically in the supercritical state. This allows the solution to reach the critical nucleation value for the YAG crystallites and promotes the formation of YAG grains at a much lower temperature than that of other methods.The methods of non-agglomeration are performed based on the study on the agglomeration mechanism of the synthesis YAG powders by co-precipitation method. The results show that the agglomeration of YAG nano-particles can be improved byvremoving water molecules and OH" adsorbed on the precursor particle using a define dispersant in the precipitation and the precursor ultrasonically washed with absolute ethanol. Meanwhile, the result indicates that washing the precursor with organic solvent can increase the dispersity of nano-particles only at the mother salt concentration is low, if not.Slip casting process is used extensively for ceramic forming, but it is important to prepare a stable slurry in order to obtain the body with high green density and homogenous microstructure. The stability of the water-based YAG slurry was studied by a serious of experiment in which the reaction conditions such as PH, types and content of dispersant as well as solid phase volume are adjusted. The results indicate that the water based YAG slurry appears a good stability when the PH is 9 and the amount of ammonium polyacrylic acid (NH4-PAA) is 3.5wt.%. The YAG particle can be dispersed in an aqueous solution by electro-steric stabilization. The green body with high average packing density and microstructure homogenously can be prepared when the solid phase amount is 60%.The investigations of the sintering of two kinds of YAG powders described above show that the powders prepared by co-precipitation are of good sinter-ability because of its highly reactivity and good dispersity, and the sinterability of powder prepared with NH4HCO3 as the precipitant is better than that prepared with NH4OH. Between the three forming methods, isostatic pressing yields green compacts with higher and more uniform density in a wider variety of gemometries than those of uniaxial pressing and slip casting techniques.Microstructure of ceramic and densification are connected with the sintering temperature, holding time and the additives in the sintering process.The largest relative density of lat%Nd: YAG body sintered from 1700°C¹750°C for 2h⁵h in vacuum can reach 99.75% in this work. The transmittance of sintered body measured over wavelength region 400nm¹000nm is about 60%,which is lower than that of 0.9at% Nd:YAG single crystal. The main reason is that sintered body contains a little volume of pore. Relationship between relativity density with sintering and time is in accordance with the kinetics equation of sintering. The TEOS additives can improve the sintering of YAG ceramic. The sample with 0.5wt.%TEOS is suitable for an uniform microstructure. If the amount of TEOS added is more than 3wt.% some residual impurities can beremained at grain boundaries, the transparency of ceramics will be further decrease.Compared with transmission spectra of Nd:YAG single crystal, the intensity of Nd:YAG ceramic is lower than that of single crystal. This indicated that Nd:YAG ceramic contains internal in-homogeneities which can serve as scattering sites which reduce the transparency to translucency. For example the existence of pores, impurities and structure defects, which have the different index of refraction from that of the matrix, thus they serve as scattering centers to reduce the transparency of ceramics. Another thing concerned in ceramic is the scattering loss caused by grain boundaries, if the grain boundary width is too large or impurities co-existed, which can reduced the transparency resulting from the energy loss at grain boundaries such as reflection and scattering. Profiles of the fluorescence and absorption sprctra of l.lat%³at% Nd:YAG powders and ceramics are consistent with 0.9at% Nd:YAG single crystal. When the Nd content ranges from 1.1 at% to 3at%, the fluorescence and absorption intensity of Nd:YAG powders and ceramics are larger than those of 0.9at% Nd:YAG single crystal because of the differing Nd content. These conclusions are also consistent with those of the effect the Nd content on YAG crystal structure.Generally speaking, some important results are obtained either in preparation of nano-sized powder, formation mechanism, dispersant, forming of green body or sintering. Some of the results are creative and useful, which can be a part of fabrication of high transparent ceramic in future.