Crystal growth of electroluminescent ZnS:Cu,Cl phosphor and its TiO2 coating by sol–gel method for thick-film EL device

Bigger-sized spherical particle of ZnS:Cu,Cl to be employed for electroluminescent (EL) device has been synthesized using a new eutectic mixture as flux. The best composition of the flux was the mixture of BaCl2·2H2O, MgCl2·6H2O and NaCl in the mole percentage ratio of 13.8:39.9:46.2 and when the total amount of the mixture was 6% of the total weight of ZnS. The phosphor was synthesized firing at higher temperatures in two steps. First step firing at 1150 °C gave the hexagonal phase of the ZnS phosphor particles. Low intensity ball-milling of the phosphor pastes in solvents converted hexagonal phase partially to the cubic phase of the phosphor, which is an essential step. Mixing with copper sulfate or copper(I) halides and magnesium chloride and then firing (second step) at 750 °C gave a phosphor with better luminescent characteristics and converting to almost 100% cubic phase. The phosphor particles having size >0.020 mm size were sieved and coated with TiO2 made by sol–gel process using titanium(IV)-isopropoxide as a precursor. The phosphor particles were coated twice with the TiO2 sol and finally calcined at 400 °C in nitrogen atmosphere. The EL devices were fabricated with the synthesized phosphor using a screen-printing method.