Optimization of Mullite Synthesis in Arc Discharge Plasma and Comparison of Ceramics Sintering Based on Natural Raw Material and Pure Oxide

The development of hydrogen metallurgy and production of special steels place increased demands on thermomechanical and chemical properties of refractories. This work addresses a pressing scientific and technical challenge relating to the development of promising mullite-containing materials for high-temperature applications under extreme energy conditions. Purpose: Arc synthesis plasma of mullite-containing materials. Methodology/approach : The best parameters of mullite synthesis in arc discharge plasma at 80–90 A current and 15–25 s exposure, are detected empirically, ensuring the formation of monolithic spherical products with the lowest number of defects. X-ray diffraction and computed tomography are used to identify the isomorphic substitution mechanism, leading to the formation of non-stoichiometric corundum-based solid solutions. A comparative analysis of sintering ceramics based on natural raw materials and pure oxide shows fundamental differences in the compaction mechanism. It is shown that ceramics made of pure oxides compacts through the volume diffusion, forming the equiaxed structure (grain size: 3.2 ± 0.5 μm), while natural materials sinter through a liquid-phase mechanism, forming acicular mullite crystals. The sintering temperature is 1400 to 1500 °C for pure oxide ceramics. The main density increase is achieved in this temperature range, while natural materials require temperatures of 1500 to 1550 °C for an intensive compaction. Research findings: Arc synthesis plasma of mullite-containing materials shows that extreme plasma conditions (5000–7000 °C) induce complex physicochemical processes, while the best current ranges between 80 and 90 A. It ensures the formation of monolithic spherical products with the lowest number of defects. Parameters are determined for the energy impact on the structure, phase composition and properties of the final product. Practical implication : The obtained results have practical significance for the creation of energy-efficient ceramic materials with specified structural and functional properties.