Optimized synthesis of mullite ceramics in thermal plasma

Abstract

   Mullite has low thermal expansion coefficient, thermal conductivity and excellentresistance to thermal shock, and plays therefore an important role in the synthesis of modern ceramic materials. The paper presents the experimental results of mullite ceramics synthesis using thermal plasma. The ceramic samples are obtained from natural raw materials such as boehmite and silica sand in the stoichiometric Al2O3/SiO2 ratio of 1.86, 3 and 5.67. It is shown that the optimal operation mode of the arc plasma torch in the synthesis of mullite-containing samples from an 8 g charge weighing includes 80 A current, 100 V voltage, and 60 s exposure time. With such technological parameters, the melting product is a hemisphere with diameter 23 mm and thickness 11 mm, which indicates to a complete transition of the initial components to the molten state. The synthesized ceramic samples are characterized by the crystalline phase of mullite 3Al2O3 ·2SiO2. At the same time, at the Al2O3/SiO2 ratio of 1.86, cryptocrystalline diffraction maxima are observed at 2θ = 53–54° and 69–76° due to the increased content of SiO 2 . According to morphological studies performed for the Al 2O3/SiO2 ratio of 3, which is the traditional stoichiometric ratio for the mullite phase synthesis, the matrix of the synthesized sample is represented by the amorphous phase penetrated by filamentary single crystals of mullite with the average length of 3.3 μm and 0.86 μm thickness. It is found that the transition layer between these phases has a glass-ceramic structure, which is consistent with the results of the X-ray phase analysis.