PROCESS DESIGN AND MATERIALS DEVELOPMENT FOR HIGH-TEMPERATURE APPLICATIONS Comparison of MnO 2 and ZnO Additives on Zircon Decomposition and Formation of Solid Solution HUDSA MAJIDIAN, 1,2 LEILA NIKZAD, 1 and MOHAMMAD FARVIZI 1 1.—Ceramic Department, Materials and Energy Research Center, Alborz, Iran. 2.—e-mail: h-majidian@merc.ac.ir In this paper, the effects of MnO 2 and ZnO additives on zircon decomposition were investigated. The substitution of Mn and Zn into zircon lattices and the formation of solid solutions are discussed. The additives were added at 0– 4 wt.% to zircon. The phase composition, physical properties, and microstructural changes for the sintered samples were characterized. Results of Rietveld refinement analysis showed that 1 wt.% of both additives retarded the decomposition of zircon. Further addition of additives slightly accelerated the zircon decomposition and improved sample densification. X-ray diffraction peaks of zircon were shifted to higher angles by the addition of MnO 2 , whereas ZnO did not alter zircon peaks. Energy dispersive spectroscopy analysis de- tected the presence of Mn and Zn in zircon grains. ZnO has a lower solubility than MnO 2 in zircon, leading to the formation of a higher fraction of secondary phases, in turn leading to the suppression of grain growth. INTRODUCTION Zircon (ZrSiO 4 ) is an interesting ceramic material due to its high stability, good corrosion resistance, high melting point, good hardness, low thermal conductivity, and excellent thermal shock resis- tance. Zircon, as a common refractory material, is widely used for high-temperature applications and industries such as iron and steel production, energy technology, and protective coatings. 1–3 The decomposition of zircon at elevated temper- atures is one of the most challenging topics in the production of zircon refractories and composites. The decomposition temperature is important for the prediction of lifetimes of zircon and zirconia-based refractories. 4 The decomposition temperature of zircon depends on several factors, such as purity and fineness of the zircon. 1 This temperature is reported to be about 1673 ± 10°C, which is based on heat treatment experiments conducted on natural and synthetic zircon raw materials with known grain sizes and impurity levels, as well as zircon single crystals. 2 Zircon decomposition has been investigated to increase the working temperature of zircon refrac- tories. Using zircon for high-temperature applica- tions strongly depends on accurate knowledge about its thermal stability. There are some studies on the thermal stability of zircon without any additives. 4–6 However, research is limited regarding the influ- ence of oxides on the decomposition of zircon. Anseau et al. 7 measured the degree of zircon decomposition and found that it starts between 1525°C and 1550°C and accelerates at 1650°C. Pavlik et al. 8 studied the decomposition of pure and impure zircon and reported that impure zircon decomposed after 8 h at 1285°C, while the decom- position of pure zircon particles was still incomplete. Sarkar and Baskey 5 evaluated the effect of using 1 wt.% of different additives (alumina, magnesia, titania, and iron oxide) on the densification and decomposition characteristics of zircon. The results of their study indicated that these additives had a negligible effect on the decomposition of zircon, but they improved densification behavior. The forma- tion of the solid solution was not considered in their studies. In another study, the effects of adding 1 wt.% of MgO, Fe 2 O 3 , MnO, TiO 2 , CaO, Al 2 O 3 , and Cr 2 O 3 additives on the sintering temperature of zircon were investigated. The results showed that Fe 2 O 3 had a beneficial effect on decreasing the (Received January 18, 2020; accepted August 17, 2020) JOM https://doi.org/10.1007/s11837-020-04331-0 Ó 2020 The Minerals, Metals & Materials Society