SYNTHESIS OF YAGCR PHOSPHORS BY PRECIPITATION FROM ALUMINUM AND YTTRIUM SULFATE-SOLUTIONS

Luminescent chromium doped yttrium aluminum garnet (YAG:Cr) powders were prepared using yttrium and aluminum (hydrous) oxide precursor particles produced by precipitation from aqueous sulfate solutions in urea. The rate of precipitation of yttrium hydrous oxide particles was observed to be slower than that of the aluminum hydrous oxide particles in the two component solution (yttrium plus aluminum sulfate). However, as single components the yttrium hydroxide nucleation occurred at a faster rate. The pH of the single aluminum and yttrium sulfate solution must be greater-than-or-equal-to 2.9 and greater-than-or-equal-to 5.5, respectively for precipitation to occur. Precipitation in a two component mixture is controlled by the acidic aluminum sulfate solution and homogeneous co-precipitation was not achieved. We have sped up the reaction by seeding a yttrium sulfate solution with aluminum hydroxide particles to promote rapid heterogenous nucleation of yttrium hydrous oxide. Heat treatment of the resulting precursor powders to 1200-degrees-C yielded YAG powders with some unreacted alumina and yttria.