Patent ID: 6485562
Filing Date: 2002-11-26
Classification: C30B

Abstract:
A process for producing single silver halide crystals having improved IR transmission over a medium IR range (5-15 microns) comprising the steps of:(a) preparing a first silver halide and a second silver halide in the form of their individual precursor salts by precipitating said silver halides from an aqueous solution; (b) combining said first silver halide and said second silver halide precursor salts in a predetermined ratio to form a mix; (c) placing said mix in a mixing ampoule made of a heat-resistant glass and having a longitudinal axis; (d) filling said mixing ampoule with a selected atmosphere; (e) closing said mixing ampoule and placing said ampoule into an upper high temperature zone of a two-zone vertical furnace at a temperature where said salt mixture is fused into a melt of a solid solution of silver halide, and maintained for a sufficient time to allow said melt to stir; (f) allowing said mixing ampoule to sink from said upper zone to a lower zone of said furnace at a rate to effect an oriented crystallization of said melt; (g) cutting off a contaminated end loaded with forced-out impurities from a thus-grown ingot of solid solution, leaving a purified portion; (h) further purifying said ingot of solid solution by placing an ampoule made of a heat-resistant glass having a conical shaped bottom into said upper zone of said furnace, then placing an upper and a lower capillary ampoule which terminate below by a thin open capillary, sequentially above said ampoule having said conically shaped bottom; (i) placing said purified portion of said ingot of solid solution into said first capillary ampoule where said ingot is subjected to melting in air, filtering said melt from the upper capillary ampoule into said lower capillary ampoule, from which said melt is filtered in vacuum into said ampoule with said conically shaped bottom; (j) subjecting said melt to an oriented crystallization in the air by moving said ampoule into said lower zone of said furnace at a downward rate sufficient to effect an oriented crystallization; (k) cutting off a contaminated end of a resulting ingot, creating a purified portion; (l) repeating steps (h) through (k) until a desired purity is achieved; (m) placing said purified portion into an ampoule for final crystal growth and evacuating said ampoule to a residual pressure, then filling a volume thus-liberated with a halogenating vapor; (n) sealing said ampoule and placing said ampoule into said upper zone of said furnace to melt down silver halide; (o) raising the temperature of said upper zone by 30-50 C. and maintaining at this level for a selected time to homogenize said silver halide melt; (p) moving said ampoule containing said silver halide melt into said lower zone of said furnace at a travelling rate sufficient for final crystallization to occur, until crystallization of said silver halide melt is complete; and (q) annealing a thus grown silver halide crystal by cooling said crystal in said lower zone of said furnace at a selected rate down to room temperature.