Grain growth - process for the preparation of emulsions with utra thin tabular grains of a high bromide - content

摘要

A grain growth process is claimed which provides a tabular Ag halide emulsion with a Br content greater than 50 mol.%. The av. equiv. circular dia. of the grains is increased while their av. thickness is maintained at less than 0.07 mu m. The process involves: (1) preparing an aq. dispersion contg. at least 0.1 wt.% of Ag in the form of Ag halide grains with a Br content of at least 50 mol.% and an av. thickness of less than 0.06mu m; the dispersion has a pH of 1.5 to 8 and a stoichiometric excess of br to Ag ions limited to a pBr of 1.5; (2) adding a phenol with at least two iodo substits. as a grain growth modifier, the phenol being incapable of reducing the Ag halide grains provided in step (1); and (3) holding the dispersion at 40[deg] C until the av. equiv. circular dia. is at least 0.1mu m greater than that of the grains in step (1), and greater than 50% of the total grain projected area is accounted for by tabular grains with (III) major faces, an av. aspect ratio of at least 5 and an av. thickness of less than 0.07 mu m.