Formation of a random array of fine silver particles from a silver film: Preparation of the frequency selective screen

In the investigation for preparing frequency selective screen which reflects near-infrared rays and transmits radio waves, we found a phenomenon in which a continuous silver film deposited on sheet glass changes into an array of silver particles on heating. A silver film changes first into a layer consisting of connective networks, then into worm-like particles, and finally into a random array of isolated particles. The silver particles have a hemisphere shape, and the area occupied by particles is about 1/4 of the area of the original film. The average diameter of the bottom of particles linearly increases with increasing thickness of the original silver film. The growth pattern of silver particles was described by an equation derived based on the assumption that fluctuations in the film thickness cause silver atoms to migrate to the position of a larger thickness, that is, the elevated area. Simulation of the process indicated that worm-like particles completely change to a random array of isolated particles when the ratio of the number of spaces occupied by Ag atoms to the total number of spaces on a computed cross section is decreased to about 1/4, and intervals between the position of larger thickness increase with increasing thickness of the original silver film while the above ratio is kept constant. These indications were experimentally confirmed.