Heat and moisture resistance of divalent metals substituted La-Hexaaluminates prepared by reverse microemulsion

Abstract

Lanthanum hexaaluminates (LHAs) substituted with divalent metals (Mg, Zn) were successfully prepared by the reverse microemulsion (RM) method and high-temperature treatment. Phase composition, microstructure, porosity characteristics, and their thermal evolution were investigated using X-ray diffractometry, scanning electron microscopy, N2 isothermal sorption and thermogravimetry-differential scanning calorimetry, respectively. The thermal stability of substituted hexaaluminates was also evaluated in both dry and moisture atmosphere at 1200-1600 °C. Experimental results showed that the RM method promoted the formation of pure and well crystallized magnetoplumbite hexaaluminates with high specific surface areas at temperatures 1100-1200 °C. For both obtained hexaaluminates, their phase composition and nanometer-sized crystallites were retained at temperature as high as 1600 °C in dry air. However, LaMgAl11O19 presented severe deterioration in flowing water vapor at 1300 °C including about 50% weight loss and the remarkable crystalline structure change, whereas the substitution of Zn enabled the LaZnAl11O19 to possess superior moisture resistance. According to the partial charge model of cations in aqueous media, a “hydrolysis-corrosion” mechanism was proposed to describe the degradation of hexaaluminates in high-temperature water vapor. It was revealed that the high-temperature moisture resistance of substituted LHAs was inversely proportional to the hydrolysis tendency of substituted metal cations.