Synthesis of Ni base nanoparticles by W/O emulsion combustion

Spray pyrolysis is one of the methods for fine particle production [ 1 , 2 , 3 ] and a droplet of precursor solution or slurry is a template of a produced particle. Many studies for development of fine particle production using the method have already been reported. A spherical droplet of precursor solution or slurry almost decides the size of the produced particle when these droplets do not break and/or unite themselves. Therefore, spray methods to produce fine and uniform droplets have been studied on and mass production methods of these droplets have been developed. [ 4 ] Electrostatic spray [ 5 , 6 , 7 ], ultrasonic spray [ 8 , 9 ] and low-pressure spray methods [ 10 , 11 ] are suitable for uniform nano-particle production since fine and uniformly sized droplets are obtained. On the other hand, it has been difficult to generate a large amount of fine droplets using these spray methods. Therefore, there is a need to produce a large amount of nano-particles more effectively and industrially. Then, we proposed production method for the ultrafine particle by the combustion of W/O emulsion prepared by ultrasonic homogenizer [ 12 ]. In this method, a large amount of W/O emulsion that included fine and uniform droplets of a metallic salt solution can be fed to a reactor at a time. The emulsion is burned and high-temperature combustion field is formed in the reactor. As a result, fine water phases of the aqueous solution are dried, decomposed and sintered by thermal energy of combustion in a short time. Although particle production by W/O emulsion combustion has already been reported [ 13 , 14 , 15 ], the W/O emulsions have been prepared by simple stirring and the size of water phases is not small enough for nano-particle production of less than 1 m [ 16 ]. Furthermore, there has been little reporting of the relationship between particle morphology and water phase structure in the emulsion.