Development of a sol-gel technique for obtaining sintering activators for engobe coatings

Abstract

We have developed a technology for manufacturing the glass ligaments-activators to intensify the sintering of ceramic materials, specifically engobe coatings. The comparative analysis has been performed into the glass ligaments obtained through a conventional glass melting and by applying the sol-gel synthesis. Large efficiency of the latter has been established related to the reduced energy costs required to thermally treat a composition (rather than melting glass at 1,300–1,400 °C, gel is to be thermally treated to 600 °C) as well as to the improved homogeneity of the glass ligament (chemical formulation of the composition is averaged at a molecular level). Different sol-gel techniques for obtaining glass ligaments-activators have also been examined: a powder sol-gel technique without calcination; a sol-gel technique for the preparation of solutions of salts; a powder sol-gel technique with the composition melting. The specified techniques differ in the sequence and method for mixing the source components, while their dispersion in a gel-forming agent the ethyl silicate ETS-40 is mandatory. The chosen basic components of the activator were soluble salts Al(NO 3 ) 3 ·9H 2 O, Ca(NO 3 ) 2 and NaNO 3 . We have investigated pattern in change of the phase composition of glass ligaments at their heating to 600 °C. It has been established that all obtained compositions approach the amorphous state at temperatures of ~600 °C. Further heating of the examined glass ligaments to 1,000 °C leads to their intensive melting. In this case, for the first two techniques, the viscosity and surface tension of melts are less compared to the melt of conventional glass, and, therefore, the wetting capacity of glass ligaments-activators is higher. When introducing these sintering activators to the compositions of ceramic coatings, it was established that the most effective is the activator that was obtained by a technique for the preparation of salt solutions. It is the one that provides, after annealing at 1,170 °C, for the densest structure of the sintered coating with reduced water absorption (not exceeding 0.05 %) and high whiteness of the surface (87–88 %). The activators that we developed could become an alternative to conventional glass-ligaments whose melting requires considerable energy costs