Porous silica microspheres, -2 . 2~ in average diameter, were encapsulated with magnetite (F404) by ferrite plating in an aqueous solution of FeCl* at 65°C. Using the resultant ferrite-capsulated particles as ultrasonographic contrast agents (in which air trapped in the pores scatters ultrasonic waves), we tried to visualize still and flowing waters in holes and a channel perforated in agar blocks by using a 7 . m B-mode echograph Clear image enhancement was successfully obtained only when the ferrite-capsulated particles were evaporated in air before they were dispersed in the waters. This is because, during ferrite plating in the aqueous solution, the pores were permeated with water, which was expelled by the evaporation and, then, substituted by air when the particles were exposed to air again.
This paper is the first report of sonochemistry (or application of power ultrasound to stimulate chemical processes in liquids) combined with ferrite plating - a wet chemical method to synthesize ferrite films from aqueous solution. Due to the ultrasonic enhancement, we have successfully encapsulated polyacrylate spheres, ~0.25µm in diameter, with magnetite in a aqueous solution of FeCl2, at 65°C. This broke through the lower limit (~0.3µm) of the size of the particles to be encapsulate with magnetite without applying the ultrasound waves. The magnetite coating has magnetization (92emu/g) which is the same value as that reported for bulk samples, being increased from that (65emu/g) obtained without the ultrasonic enhancement
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