Processing considerations for the combustion synthesis of yttrium aluminum garnet powders

Abstract

Combustion synthesis leverages the exothermic reaction between a fuel and an oxidizer to provide the energy needed to form crystalline refractory oxides. In many cases combustion syntheses are less energy intensive and require fewer processing steps than traditional ceramic oxide processes. However, the properties of materials produced by combustion syntheses can depend on many processing parameters, such as the type of fuel used, the fuel to oxidizer ratio, coordinating characteristics of the fuel, the rate of heating the reactant solution. This work reports the results of process optimization and materials characterization studies of three ceramic oxides: yttrium aluminum garnet (YAG), lanthanum zirconate (LZO) and yttrium borate (YBO). Cerium and/or europium dopants were incorporated into these three host lattices to enhance their photoluminescent (PL) properties. The effects of process parameters on the crystallinity, composition, and PL emission intensity of the synthesized materials were investigated. We show that mixing urea and citric acid fuels decreases the PL intensity of YAG:Ce1% compared to using either pure urea or pure citric acid. Characterization of the material found that the material synthesized using only citric acid contained the most YAG phase and that the decrease in PL intensity is likely due to dopant segregation. We show that the heating rate used to raise the reactant solution to the ignition temperature also affected the crystallinity of the synthesized material. Thermogravimetric analysis and differential scanning calorimetry identified a heating rate that produced the maximum heat of combustion. Transmission electron micrographs of the synthesized material show that heating rates affect the morphology of the material. The YBO process optimization involved adding additional fuel and oxidizer to compensate for the lack of a boron-based oxidizer and included a post-synthesis anneal. The effect of proton irradiation on the PL of these materials was tested. The PL of YAG and LZO was not affected by a fluence of 1x10^16 protons/cm^2, however the PL intensity of YBO decreased with doses of 1x10^14 protons/cm^2 and above.