Microwave sintering of ZnO-containing in-situ spinelized alumina-based castables

Matching industrial activity and environmental protection is one of the toughest challenges of the 21^st century. As high consumers of fossil-based materials, refractory manufacturers are consistently seeking processes that can provide higher energy efficiency. In this sense, using microwaves for ceramic sintering has shown significant advantages compared with the conventional route (based on electrical or natural gas heating), reducing lead time, enhancing mechanical properties, and demanding less power. However, this method has been underexplored for such application due to the lack of knowledge about refractory phases capable of effectively interacting with this type of electromagnetic radiation. Thus, this work addressed the potential of applying ZnO as a microwave absorber in Al₂O₃-based refractories, comparing microwave sintered castables with electric-heated ones thermally treated up to 1700 ºC. Although presenting a relatively high vapor pressure, the content of ZnO (11.4 wt%) remained stable at all temperatures applied in the electric-heated furnace, whereas significant evaporation was detected for samples microwaved above 1300 ºC. The mechanisms related to this behavior and strategies to avoid it are discussed. On top of that, the microwave sintering method showed benefits such as higher densified microstructure with open pores remaining homogeneously scattered in the matrix, enhanced mechanical properties at room temperature, 26-fold faster lead time, and roughly-estimated 70%-lower energy consumption. Thus, this sintering method might be a promising alternative to increase the efficiency of refractory manufacturing, lowering environmental impacts and production costs.