Recent Progress In The Synthesis Of Calcium Lanthanum Sulphide Optical Ceramic

Abstract

Calcium lanthanum sulphide has been identified as a material worthy of detailed investigation for infrared window applications. Sintering of a pure sulphide powder to closed porosity followed by hot isostatic pressing to achieve full density appears to be the most promising synthesis route. In the present work an evaporative decomposition of solution (EDS) powder synthesis technique has been utilised. A mixed nitrate solution was sprayed through a hot furnace to yield mixed oxide precursor powder which was collected at the end of the furnace. This powder was then fired in H2S gas to synthesise a fine powder of calcium lanthanum sulphide raw material for ceramics processing. The evolution of the powder synthesis and ceramic processing techniques has enabled a continued improvement in ceramic quality. In 1981 very dark almost black CaLa2S4 not transmitting in the IR was typical of the material being synthesised by hot pressing. By 1983 a very dark brown CaLa2S4 partially transmitting in the visible and IR but showing extrinsic SO3 and SO4 absorptions was synthesised by hot pressing followed by annealing in H2S. In 1985 extrinsic absorption free lemon yellow coloured CaS45La2S355 (mole%) material transmitting in the visible and IR but showing some visible and near IR scatter has been synthesised by sintering in H2S followed by hot isostatic pressing (HIP) in argon.