Control of particle morphology in ultrasonic spray pyrolysis and its application to the preparation of phosphor materials

Abstract

An ultrasonic spray pyrolysis process was applied to the preparation of oxide phosphor particles. Various control methods of particle morphology were developed in spray pyrolysis. Colloidal seed-assisted spray pyrolysis was introduced, in which the stable colloidal solution prepared by liquid phase reaction was used as precursor solution. The $(Y_{1-x}Gd_x)_2O_3:Eu (1 ≤ x ≤ = 0)$ particles prepared from colloidal solutions containing small amounts of gadolinium (or yttrium) hydroxy carbonate sol as seeds had a spherical and filled morphology before and after post-treatment, while those prepared from aqueous solution were porous and hollow and the structure was broken after post-treatment. The colloidal seeds dispersed in the droplet acted as nucleation sites for salt precipitation and induced volume precipitation in the droplet. The size of the colloidal seeds affected the morphologies of phosphor particles. Large colloidal particle was favored for the formation of filled particle. The phosphor particles prepared from colloidal solution had higher crystallinity and photoluminescence (PL) characteristics. In the case of $Y_{2}O_{3}$ :Eu phosphor, PL intensity increased up to 16 %. The stability of colloidal particle was a critical factor determining morphologies and luminous properties of phosphors. The particles prepared from the stable $TiO_{2}$ colloidal solution obtained from TTIP (Titanium tetra-isopropoxide) had a spherical shape, filled morphology, pure $CaTiO_3$ phase, and highly improved photoluminescence characteristics, while those prepared from the unstable $TiO_2$ colloidal solution obtained by mixing nano-sized $TiO_2$ particles into the premixed solution had uneven morphology with rough surface, impurity phase, and poor luminescence characteristics because of the instability of colloidal solution. A flux agent was firstly introduced to the preparation of phosphor material by spray pyrolysis process. As a flux agent, metal halide such as LiC...