Abstract: The present invention relates to a metal powder containing a metal of Au, Ag, Cu, or an alloy thereof, having an average particle size of 0.1 ?m or more and 0.4 ?m or less, and purity of 99.9% by mass or more. In this metal powder, a presence ratio, in terms of the number of particles, of non-spherical particles in the metal powder having a ratio (b/a) of a minor axis “a” and a major axis “b” of 3 or more is 1% or less. As a method for producing such a metal powder, in a wet reduction method including a metal colloid synthesis step and a metal powder granulation step of granulating metal colloidal particles into a metal powder, surfactants having alkyl groups having carbon numbers falling within specified ranges are preferably used as first and second dispersants in the metal colloid synthesis step and the metal powder granulation step.

Abstract: A material to be treated containing solid particles in a liquid is stirred together with milling media, and irradiated with ultrasonic waves during stirring to finely mill the solid particles to nanometer size and disperse the solid particles in the liquid. The ultrasonic waves create cavitation in the liquid and upon decay of the cavitation, shock waves are produced that cause the solid particles to vigorously collide with one another and with the milling media and these collisions, together with the shearing forces created by collision of the milling media, can produce on a commercial scale nano particles having an average particle size of at most 100 nm.

Abstract: A material to be treated containing solid particles in a liquid is stirred together with milling media, and irradiated with ultrasonic waves during stirring to finely mill the solid particles to nanometer size and disperse the solid particles in the liquid. The ultrasonic waves create cavitation in the liquid and upon decay of the cavitation, shock waves are produced that cause the solid particles to vigorously collide with one another and with the milling media and these collisions, together with the shearing forces created by collision of the milling media, can produce on a commercial scale nano particles having an average particle size of at most 100 nm.