Cathodo-luminescence of Samarium as Applied to the Structural Studies of Alumina. II. On the Alumina Produced from Hydrous Alumina Gels

Abstract

Abstract Hydrous alumina gels were prepared from both solutions and crystals of a number of aluminium salts, in the former case by precipitation with ammonia and in the latter by direct immersion in concentrated ammonia liquor. Samarium ions were impregnated in these hydrous alumina gels or in their transition phases resulting from the previous thermal treatment of various temperatures. The feature of the cathodo-luminescence spectrum was investigated on the samarium-bearing alumina produced from the above-mentioned samples by a final heat treatment. Spectrographic measurements of the cathodo-luminescence reveal that the heating of the hydrous alumina gels derived from a solution of aluminium nitrate results in their conversion to the α-alumina, while the heating of those from solutions of aluminium chloride and sulphate does not give the genuine α-modification. This was confirmed by the fact that the most intense band of a particular luminescence prototype III as designated by one of the present authors was observed in the cathodo-luminescence spectra. The Υ-α transformation of alumina occurs at temperatures far below its transition temperature when a suitable amount of ammonium nitrate, the by-product of the neutralisation reaction, was caused to remain by heating the precipitated hydrous alumina gel, which was derived from a solution of aluminium nitrate at a moderately elevated temperature. Similar remarkable results were, however, not obtained with hydrous alumina gels derived from solutions of aluminium chloride and sulphate, and also with the gel derived from a solution of aluminium nitrate, if the latter gel had been made free from the reaction by-product. An addition of ammonium nitrate to the purified alumina gels can no more regenerate the ability to accelerate the Υ-α transformation. It is interesting to note that hydrous alumina gels obtained by immersing crystals of aluminium nitrate enneahydrate, aluminium chloride hexahydrate, aluminium sulphate octodecahydrate and ammonium alum in concentrated ammonia liquor for a sufficiently long time behave themselves, on being heated, in a somewhat similar way to those obtained by precipitation from the solutions of the foregoing aluminium salts.