Lithium titanate ($Li_2TiO_3$) is known to be a low-activation ceramics for use in tritium breeding. Calcium titanate ($CaTiO_3$) is one of the promising materials for the disposal of high level radioactive wastes because it can immobilize alkali-earth elements and lanthanides by forming solid solutions. A method for preparing fine-crystalline titanates powders that is based on the combustion reaction is reported. This process is an exploitation of an exothermic and usually very rapid chemical reaction to form final reaction products. The combustion and sintering behaviors of the synthesized powder with various fuel and compositions on the phase formation were investigated. Additionally, in order to investigate the effect of the fuel on the microstructure and reactivity, titanates powder were prepared by the ultrasonic mist combustion process.
Among various compositions of fuels, the stoichiometric composition of glycine and mixture of urea and citric acid were produced the single phase of $Li_2TiO_3$. The primary particles had ultra-fine and equiaxed crystalline with the particle size of about 20 nm. Typical micrographs of the synthesized $Li_2TiO_3$ powders using glycine formed the foamy agglomerated with pores, which is resulted from the evolved gas during the combustion reaction. The main exothermic peak and 95% of weight loss were occurred at 230℃ that is associated with the redox reactions of fuel and oxidizer. The synthesized powders using glycine could be sintered to the 80% TD at relatively low temperature (< 1,000℃) and short sintering time (< 4 hours).
When glycine and mixtures fuels with stoichiometric composition were used as a fuel, the solid solutions of $CaTiO_3$ with La, Gd and Ce were easily prepared by the combustion process. No evidence of the existence of second phases was observed at composition of up to 50mol.% of La. The combustion synthesized pellets has a good sinterability with the linear shrinkage of more than 25% up to 1500℃, while that of the solid state reacted was less than 10%. To produce the single $Ca(Ln)TiO_3$ phase, the fuel must possess both the carboxylic acid group and amine group such as glycine or mixture of urea and citric acid.
The formation mechanism of the hollow droplet-derived particles in the ultrasonic pyrolysis and dense particles in the ultrasonic mist combustion process were suggested. The droplets are spherical in shape when they are formed, and then water begins to evaporate from the surface of the droplet. When the surface concentration of the solutes exceeds the solubility limits, calcium and titanium nitrate precipitates is formed firstly in high-number nanoscaled-nuclei from its supersaturated sites, and then precipitated particles consisting of nanometer primary particles were formed and the solid nuclei start to pyrolysis. The hollow $CaTiO_3$ particles were produced in the ultrasonic pyrolysis process (without fuel). However, in the ultrasonic mist combustion process (with fuel), after evaporation of water, the droplet has viscous honey-like form of mixture of Ca, Ti nitrate and fuel. Increasing temperature of droplet, the spontaneous ignitions between nitrate and fuel occurred within the droplet, and then the continuous combustion reactions occurred inside the droplets. Therefore, the small and dense aggregates of the nanocrystals with low porosity had been obtained due to the local combustion reactions occurred inside droplet.
On the fracture surface for 30 mol.% La-doped $CaTiO_3$ powders prepared by the ultrasonic pyrolysis, more than 80% of the particles had a large void within an individual particle with an average particle size of 1.5 ㎛ and void size of 0.5 ㎛. The particle prepared by ultrasonic mist combustion had particle size of about 1.0 ㎛ in diameter and showed dense fracture morphology.
In the ultrasonic pyrolysis process, hollow particles have remained as a permanent large void in the final product after sintering, and result in the decrease of the sintered density as well as chemical property such as leaching behaviors in hydrothermal conditions had been degraded. The ultrasonic mist combustion process proposed in this research, had been proved to be a better method.
