External additive for toner for electrophotography, toner for electrophotography, double-component developer for electrophotography, image-forming process using the toner, and image-forming apparatus using the toner

摘要

An external additive for a toner for electrophotography which contains oxide fine particles which contain silicon, in which the oxide fine particles have a primary particle diameter of 30 nm to 300 nm in number average, a standard deviation sigma of a particle size distribution of the primary particle diameter satisfies a relation of: R/4<=sigma<=R, in which the R expresses the primary particle diameter, the oxide fine particles are substantially spherical having a circularity SF1 defined as equation (1) of 100 to 130 and a circularity SF2 defined as equation (2) of 100 to 125; SF1=(L2/A)x(pi/4)x100 equation (1) SF2=(P2/A)x(1/4pi)x100 equation (2), in the equations, "L" expresses the absolute maximum length of the oxide fine particles; "A" expresses a projected area of the oxide fine particles; and "P" expresses a maximum perimeter of the oxide fine particles.