Synthesis of YAG and Other Rare Earth Aluminates
A requirement for making YAG crystals is a homogeneous, high-purity starting material for use in the Czochralski technique. This means that the application of mechanical mixing and solid-state diffusion techniques are limited. Accordingly, a variety of synthesis techniques have been developed, most of which are sol-gel based.
The sol-gel technique is attractive because it allows for molecular mixing of constituents and results in chemical homogeneity. Typically calcining occurs at temperatures well below those required for solid-state synthesis resulting in amorphous nanocrystalline YAG samples. The citrate-based technique is commonly used for YAG synthesis [14, 110, 111]. The starting materials are aluminum and yttrium nitrates, which are soluble in water. Citric acid is added to the aqueous solution of the stoichiometric mixtures of nitrates (3:5 Y to A for YAG) to act as a chelating agent that is to stabilize the solution against hydrolysis or condensation. Ammonia is added to reduce excess acidity. The sol-gel process requires formation of an organic polymer framework, independent of the mineral species in solution. In the citrate technique, the polymer framework is based on acrylamide, which is easily soluble in water. Polymerization is initiated by free radicals and radical transfer agents. Transparent gels are obtained by heating to temperatures of ~80°C. The organic components and water are removed by placing the gel in a ventilated furnace and heating. Temperatures of 800°C result in amorphous YAG, while higher temperatures result in nanocrystalline YAG. Doping of YAG with other rare earth elements (e. g., Nd, Eu, or Tb) for phosphor or laser applications can be achieved by addition of the appropriate nitrate at the solution stage.
A translucent solution is produced by stoichiometric mixtures of aluminum isopro – poxide and yttrium acetate in butanedicol and by adding glycol instead of water in the autoclave (the so-called glycothermal technique) heated to 300°C. Adding ammonium hydroxide solution causes particles of YAG to precipitate.
There are some alternatives to sol-gel based on combustion synthesis [112-114]. The motivation of combustion synthesis is similar to that for sol-gel; a homogenous high-purity product. Coprecipitation of organo-metallic products is followed by addition of a fuel, such as urea or glycerin. When heated, combustion occurs causing localized formation of YAG.