Thermal Plasma Synthesis of Phosphor Nanoparticle

Advanced phosphors are currently of interest for applications in various emerging technologies, such as electronic printing jets, lighting and display devices and medical devices such as fluorescence markers. Compared to conventional phosphors, nanoparticle phosphors offer potential advantages with regard to reduced light backscatter losses and dispersibility in polymer substrates. With current phosphor synthesis techniques it is challenging to simultaneously control the particle morphology and phase to synthesize phosphor nanoparticles in a commercially viable manner. This study is focused on developing a cost effective method for bulk production of nanoscale phosphor powders through a single-step radio frequency (RF) plasma process. Synthesis of several phosphor material systems were studied for this purpose, including cerium-doped yttrium aluminum garnet (Ce:YAG), which is among the most commonly used phosphors for white LED applications. Previous studies have shown that it is difficult to produce the desired YAG phase without ex-situ annealing irrespective of the synthesis technique used. Preliminary efforts to directly synthesize Ce:YAG nanoparticles with the RF plasma system were not successful. Characterization results of the as-synthesized particles showed the presence of the perovskite (YAP) phase with broad size distribution ranging from hundreds of nanometers to a few microns.;Novel methods were developed to control the size and phase of the particles in the synthesis process. The experimental setup was modified by inserting a ceramic tube coaxially with the reactor inlet to incorporate a central annealing zone. Numerical modeling performed to determine the influence of the tube insertion on the thermofluid fields in the reactor indicates that the tube provides a more uniform high-temperature region, without flow recirculation. The tube is hypothesized to aid in size and phase control through selective particle collection and in-flight annealing. Ce:YAG particles synthesis experiments conducted with the tube-insertion setup indicate that phase and size control of the particles is possible to a certain extent, depending on the size of the tube. Characterization results of the synthesized particles showed that submicron sized YAG particles are synthesized as the majority phase through the 47.5 mm tube-insertion setup. Synthesis of other phosphor materials such as Ce:CaSc2O4 and Eu:SrAl2O4 also indicate that the tube insertion setup helps to control the particle size and phase partially and to improve the photoluminescence through in-flight annealing.;The precursor atomization process was studied with the aim of developing effective methods to control the size and phase of the particles in the synthesis process. A laser imaging technique was used to characterize the high density spray generated with the Tekna atomizer probe. Image processing results indicate that sprays generated with flow conditions similar to that used in experiments have broad droplet size distribution ranging from 10s to 100s of microns. An effective method studied to fragment water based droplets in the plasma is by adding an organic fuel such as urea. The urea decomposes exothermically in the high temperature region, thereby aiding in the secondary fragmentation and evaporation of the droplets. Smaller droplets and enhanced evaporation rates are hypothesized to increase the gas phase nucleation synthesis route of particles, leading to smaller particles. Experiments were performed with varying concentrations of urea in nitrate salts and water based precursors for synthesis of various phosphor materials. The results indicated that the particle size decreases with increasing urea concentrations. Using >7M urea concentration with 0.4M salt concentrations in the precursor results in primarily nano sized particles. Adding urea in the precursor was also observed to affect the phase of the particles by providing a reducing chemical environment in the synthesis.;Finally a novel precursor developed by Nitto Denko Tech. Corp. for the synthesis Ce:YAG particles was studied. Thermal analysis of the dried precursor was performed to study the YAG crystallization process. YAG nanoparticles were synthesized through the precursor atomization and annealing process. Diluted precursor was also injected in the plasma to achieve single phase YAG particles directly from the RF plasma system. However, the precursor synthesis method is expensive and time intensive. Therefore it was not used for synthesis of other phosphor materials.