Action mechanism of rare earth oxides in magnesia-calcium refractories

The properties of an element determine its performance, and rare earth elements are no exception. Their performance is closely related to their properties. The primary factors determining their physical properties (such as hardness, crystal structure, and melting point) are their atomic and ionic radii. Rare earth metals have high melting points that increase with increasing atomic number, though this trend is not very consistent. Rare earth elements typically lose their outer s and d orbital electrons, forming a +3 valence state, thus forming rare earth oxides. This +3 valence state is the characteristic oxidation state of rare earth elements. Rare earth oxides have melting points exceeding 2000°C and are nonvolatile. They are mixed conductive semiconductors with both electronic and ionic conductivity. Electronic conductivity refers to the conduction of electrons and holes, while ionic conductivity refers to the movement of oxygen ions within oxygen vacancies, essentially oxygen ion conduction.

In addition to using rare earth elements directly as matrix components or functional centers based on the optical and magnetic properties of 4f electrons, their chemical properties, such as their chemical reactivity and large ionic radius, can also be leveraged to modify the material’s microstructure, thereby improving its performance. Rare earth-doped functional semiconductor ceramics are a major example. Adding rare earth oxides to refractory materials not only enhances and improves the material’s inherent strength and toughness, but also reduces sintering temperatures and production costs.

Due to their non-toxicity, high efficiency, and unique physical and chemical properties, rare earth compounds are increasingly being used in a wide range of applications, evolving from primary applications in metallurgy, chemical engineering, and ceramics to advanced applications in high-performance composite materials such as hydrogen storage and luminescence. Research on the application of rare earth oxides in ceramic materials has attracted widespread attention. Studies have shown that the addition of rare earth oxides significantly improves the performance of ceramic materials, ensuring their quality and performance for diverse applications. Furthermore, rare earth oxides, as fluxes, can promote sintering, improve the ceramic’s microstructure, and provide doping and modification.

Rare earth oxides, as additives, improve the properties of refractory materials, demonstrating their unique and significant benefits in enhancing performance and imparting new functions. Adding small amounts of rare earth oxides increases the density of magnesia-calcium refractories, improving their density and corrosion resistance.

Rare earth oxides are used as additives in magnesia-calcium refractories to improve their sinterability, compactness, microstructure, crystalline phase composition, room-temperature flexural strength, and fracture toughness, thereby meeting market performance requirements for magnesia-calcium refractories. There are three main mechanisms for adding rare earth oxides to magnesium-calcium refractory materials. (1) Additives as flux can promote sintering. The sintering temperature of magnesium-calcium refractory materials is generally high, and there are many factors that are not conducive to densification during the sintering process. Adding rare earth oxides can solve this problem. Due to the unique properties of rare earth oxides, adding rare earth oxides to refractory materials can change their internal structure, thereby promoting the sintering of magnesium-calcium refractory materials. (2) Rare earth oxides can improve the microstructure of magnesium-calcium refractory materials. The addition of rare earth oxides can improve the internal microstructure of the refractory materials. This reduces the grain boundary migration rate, inhibits grain growth, and is conducive to the formation of a dense structure. (3) Doping modification of rare earth oxides. Doping rare earth oxides in the process of preparing refractory materials will cause the sample’s crystal form to change, thereby causing its volume to change. This change can greatly improve its bending resistance and toughness. Research on adding additives to improve and optimize the relevant properties of materials in the preparation process of refractory materials has always attracted people’s attention. In the current research, the main focus is on the problem that magnesia calcium sand raw materials are difficult to sinter and easy to hydrate. The main additives include ZrO2, Fe2O3, Al2O3, rare earth oxides, etc.