Alumina nanoparticles were directly synthesized by spray pyrolysis using radiofrequency (RF) plasma. The mist that was generated from the aqueous solution of aluminum nitrate by using an ultrasonic vibrator was continuously pyrolyzed in the RF plasma. Scanning-electron-microscope and transmission-electron-microscope images showed that as-prepared alumina nanoparticles exhibited spherical morphology with non-aggregation. The particle size and geometrical standard deviation of the alumina nanopowders obtained at 3000°C were 80 nm and 1.41, respectively. The average particle size of the alumina nanopowders decreased with increasing pyrolysis temperature. The average particle size and particle size distribution of the alumina nanopowders were independent of the concentration. X-ray diffraction revealed that as-prepared alumina nanopowders were crystallized to £-alumina. The crystallinity of the as-prepared alumina nanopowders increased with increasing pyrolysis temperature. BET revealed that the specific surface area (SSA) increased with increasing pyrolysis temperature. The as-prepared alumina nanopowders had a high SSA of 100 m 2 /g at 3,000°C.

Silver powders are important for the production of silicon solar cells and low-temperature co-fired ceramics. In this study, a silver powder was produced at a rate of 2 kg/min through water atomization with a water pressure of 80 MPa and water flow rate of 220 L/min. Scanning electron microscopy analysis showed that the silver particles produced were aggregated and had various types of morphologies such as spherical, elliptical, and irregular. The volume average particle size and its standard deviation were 8 and 4.1¯m, respectively. Powder X-ray diffraction revealed that the as-prepared silver powder had a single phase with high crystallinity. The oxygen content in the silver powder measured by electron probe micro analysis was 50 ppm. The silver powder was classified to the size of¯2.5¯m, and the shrinkage of the silver paste was 25.5% at 900°C, comparable to that of the powder obtained by chemical reduction. Electrical measurements revealed that the specific resistivity of the silver paste sintered at 900°C for 60 min was 2.11 © 10 ¹8 ³ m. The silver paste exhibited lower specific resistivity when the sintering temperature was high and sintering time was long.

Silver powders were produced at rate of 2 kg/min by water atomization with the pressure of 80 MPa. SEM observation showed that the silver particles had various types of morphologies such as spherical, irregular and coarse. The volume average particle size and standard deviation determined by particle size analyzer was 8.2 μm and 4.15 μm, respectively. XRD revealed that as-prepared powders crystallized to silver with high crystallinity. TMA revealed that the shrinkage of as-prepared silver powders was about 25% after classified to 2.5 μm and then comparable with those obtained by chemical reduction method. The specific resistivity of silver paste obtained by silver powders derived from water atomization was superior to that obtained by silver powders derived from chemical reduction method.

Silver powder was produced through water atomization at the high pressure of 80 MPa. Before the water atomization, silver was melted at 1330°C under nitrogen atmosphere. Scanning electron microscope observation showed that the as-prepared silver powder had various particles with morphologies such as spherical, irregular and coarse. The volume average particle size (D 50 ) of as-prepared silver powder determined by laser photo-scattering method was 7.7 μm. D 50 was influenced by the condition of atomization angle and drain rate. D 50 decreased with decreasing to atomization angle and drain rate. Electron probe micro analysis revealed that the oxygen content in as-prepared silver powder was 50 ppm. The specific resistivity of silver paste decreased to 1.75 × 10 −8 Ω·m with increasing the sintering temperature. When silver paste was sintered at 900°C for 10 min, the microstructure of silver paste was densified with the large grain growth.

A large-scale spray pyrolysis apparatus in which the mist is pyrolyzed by a flame using a gas burner was developed for the mass production of Ag powder. Forty ultrasonic vibrators with a frequency of 1.6 MHz generated 4 dm 3 /h of mist. The thermal stability of the ultrasonic vibrators was improved using water-cooling. Spherical Ag powder was continuously produced, and the yield of the powders remained above 95% over 60 hours of continuous production with starting solution concentrations of up to 1.5 mol/dm 3. Scanning electron microscopy (SEM) analysis showed that the particles were sub-micrometer in size with a dense microstructure, and they did not aggregate. Powder X-ray diffraction (XRD) demonstrated that the as-prepared Ag powder exhibited high crystallinity. The particle size of the Ag powder was approximately in agreement with the predicted size, calculated under the assumption that one Ag particle was formed from one droplet of mist. Moreover, 7 kg of Ag powder was produced over 12 h at 1.5 mol/dm 3. This apparatus exhibited good reliability for the continuous mass production of Ag powder.