Investigation Of Composition And Morphology Of YAG Nano-powders And Its Antisite Defects

Neodymium doped Yttrium Aluminum Garnet (Nd:YAG) transparent ceramics are of promising materials with comparable potential application scale to glass gain materials. The thermal conductivity and mechanical properties of the YAG transparent ceramics are more prominent than that of its single crystals. Ceramic materials are the second generation of laser materials for solid state laser technology.Powders with high quality are the basis for fabricating ceramics with high quality. Thus, the synthesis of YAG powders has attracted extensive attention ever since the successful accomplishment of the YAG laser ceramics. Nowadays, the high quality laser ceramics have been achieved via various methods, however, due to uncertainty of chemical mechanism, nucleus formation, growth process, and surface fundamental in the nano-YAG synthesis process, the batch production of laser ceramics is still suspended. Moreover, yttrium might substitute aluminium in the YAG crystal to form antisite defects. These defects are considered to play important roles in the reduction of high laser performing of the crystals. However, the confirmation of the existence and concentration of antisite defects are still suspended, especially in powders.Thus, this work concludes the exploration for new synthesis process of YAG powders, and investigation of the fundamentals in the normal co-precipitation process with NH4HCO3, such as mechanism of precipitation, phase transition, morphology evolution, and so on, to try to uncover the reaction mechanism, in order to make it clear for the controllable synthesis of good YAG powders. Meanwhile, this study investigates the antisite defects in the non-stoichiometric YAG powders, trying to find ways to modify the antisite concentration.The main contents of this study are as following:1. YAG nanoparticles synthesized via nitrate decomposition and their low temperature densification behavior(1) To avoid the use of precipitation, and to form precursor with homogeneous ion distributions, the mixed nitrate decomposition process was used.Pure YAG powders were obtained from calcination of mixed nitrates at850℃. The powders were in size of20-30nm, highly sintering active, easy to assemble and grow up. Micro-ceramics with scale up to20μm with grains in sizes of1-2μm were obtained at the calcination temperature of1000℃, which is about700℃lower than the traditional sintering temperature of YAG ceramics.Due to the homogenous distribution of Y and Al elements, YAG structure could be formed without obvious existence of Hexagonal-YAlO3(YAH). The nano-scale powders supplied large gibbs free energy difference△G between adjacent particles, and pushed the nano-particles growing up. That is to say that the20-30nm nano-particles are the main reason for the low temperature densification. Moreover, the bridge-oxygen formed between ultra-fine particles could make the adjacent particles closer, and was considered as anther reason for the densification of YAG ceramics at low temperature.(2) Based on the significantly different potential between substrate and nano-particles, and the low temperature densification phenomenon, the epitaxial growth of single YAG at the solid state was proposed. Nitrate-citric acid sol solution was used as the precursor. With the deposition-decomposition-sintering process, the Nd:YAG films were achieved on YAG (111) single crystal substrates. With500℃heat treatment, the homogeneous amorphous oxides formed on the substrates. After crystallization at1100℃, the films with oriented direction have been proved with XRD measurement. With the obviations of the surface and interface section with AFM and SEM, the interface between film and substrate was found. After1700℃heat treatment, the interface disappeared.(3) Nitrates decomposition process supplies a simple route for synthesis of fine YAG powders. With the overcome of aggregation by using dispersion agencies, this route could realize the batch synthesis of homogenous YAG powders. The epitaxial growth of single YAG at the solid state is a method for growth of single crystal at low temperature. It is another route for crystal growth, beside traditional gas, liquid epitaxial growth. It could be applied for the fabrication of high-melt-point crystals, as well as doped YAG with gradient doping concentration.2. Modulation of composition and morphology of the YAG precursors obtained by co-precipitation process(1) The normal and reversed titration processes of co-precipitation method were performed to synthesize YAG precipitate. By comparing the variation of Y/Al ratio during the titration process, combined with the variation of pH in the reaction system, the formation of YAG precipitate with normal and reversed titration processes was proposed. The distribution of yttrium and aluminium ions was discussed.The investigation suggests that the precipitate formed in the normal titration process is not homogenous. The distribution of Y and Al exhibits gradient. The reverse titration process performs as a tiny reactor of the normal titration process, but it could realize co-precipitation process, and obtain precursor with homogenous Y and Al distribution.Normal titration process is a process with increasing pH. The Al-precipitate forms first, and makes the visibly turbid solution a sol. A small amount of Y-precipitate forms under these conditions due to the higher solubility product of Y-precipitate. With the increase in pH, more Y-precipitate forms, and adheres to the surface of the existing Al-precipitate, forming the YAG precursor precipitate. Reversed titration process is a process with decreasing pH. Considering the droplet of ion solution and its neighborhood to be a small reaction system, the reaction process should be quite similar to that formed via the normal titration co-precipitation method. Fortunately, due to the agitation, the tiny droplet would form a Y and Al co-precipitate shell, keeping the overall composition stoichiometric. The Y and Al ions precipitate on the shell and form hollow particles, then driven by high surface energy and electrostatic attraction, these tiny particles assemble, forming the final precipitate. This homogeneity in such a small scale makes the formation of YAG easier.(2) Based on the synthesis of YAG precursor with reversed titration process, the effect of dosage of NH4HCO3on the morphology and composition of precursor were discussed. The high dosage of NH4HCO3is good for the first formation of precipitate, but would cause changes for the morphology and elemental distribution, which are bad for the phase transition.After the formation of precipitate, the chemical environment affects the composition and morphology of the as-obtained precipitate. High pH is good for the co-precipitation process, but it also brings trouble to the stoichiometry and homogeneity of the precipitate during the aging process. The effect makes that the reaction from Y and Al precipitates into YAG needs more energy and time due to the matter transfer. (3) The reversed titration process could realize the phase transition at low temperature. The dosage of NH4HCO3is the key to modify the composition and morphology of precursor.3. Effect of the powder size on the sintering property and the densification process(1) The YAG powders were obtained by calcining the precursors obtained via co-precipitation process at900,1000,1100,1200and1300℃for2h. The as-obtained powders are in sizes of30,40,80,100and200nm.The real-time measurement of the high temperature thermal expansion measurement was performed to identify the densification processes of the compacts. It is noticed that with the increase of particle size, the starting and ending temperatures of the shrinkage process increase. The powder calcined at900℃was not confined by the rule. That is due to the low crystallinity of the powder. The compact with30nm powder has a total shrinkage30%larger than that with200nm powder. The powders with similar particle sizes exhibit similar aggregation and sintering activity, which imply the important role of particle size playing in the sintering process.The sintering results prove that the samples made from the powder calcined at low temperatures readily exhibit abnormal grain growth and pore envelopment. With the increase of calcination temperature, the obtained ceramics becomes better. In this study, the ones calcined at1300℃with particle size of200nm showed the best result of all.(2) Taking the powder with particle size of200nm for example, different sintering states at different temperatures were investigated with SEM. It is suggested that low sintering temperature is good at the initial period of sintering, while high sintering temperature is needed to form ceramics with full density.4. Antisite defects of the non-stoichiometric YAG powdersSpecially synthesized non-stoichiometric YAG powders YxAl5O12(x=3.4,3.2,3.1,3.0,2.85,2.65) were used for the measurements of XANES, XPS, powder-XRD and NPD, as well as NMR. Low antisite defect concentrations in the powders were identified, and it was suppose to be a good chance for the fabrication of bulk materials with low antistite defects.Combined neutron and X-ray diffraction investigation on cation antisite defects was performed on specialized synthesized YxAl5O12yttrium aluminum garnet nanopowders to try understanding the defect chemistry in YAG system. No clear evidence was observed for the exists of YAl,16a, YAl,24c and AlY,24d antisite defects in these specially synthesized samples. It was found that the nonstoichiometry with extra amounts of Y or Al could only cause the formation of different phases such as YAM, YAP, or θ-Al2O3and α-Al2O3in addition to the main YAG phase. YAG materials containing low level or even no antisite defects may be achieved through low temperature synthesis process.