Thermal plasma sintering of forsterite ceramics

Abstract

To date, new synthesis methods of forsterite (Mg2SiO4) ceramics are being searched for refractory industry. The limiting factor of Mg2SiO4  synthesis is its high melting point of 1890 °C, which makes it difficult to conduct experimental studies.The paper proposes a new method of forsterite synthesis based on thermal plasma. The plasma source is an effective medium for heating and melting refractory materials. For the Mg2SiO4synthesis, such initial materials are used as natural and sub-standard raw materials (silica sand sifting, microsilica, magnesite). The degree of crystallinity varies from 10 to 98 %, which allows studying the formation of the phase composition at different parameters.According to powder X-ray diffraction patterns, at a MgO/SiO2stoichiometric ratio of 1.34, the obtained melting products consist up to 90 % Mg2SiO4and the X-ray amorphous phase content varies from 5 to 12 %. After isothermal exposure, the amorphous phase is represented by silicon dioxide in the polymorphic modification of cristobalite. According to scanning electron microscopy observations, the formation of hexagonal close-packed crystals occurs on the surface of the ceramic sample, with the size ranging from 180 to 250 µm. The elemental composition of the crystal consists of ~ 38.60 wt. % O, ~ 28.54 wt. % Mg, ~ 26.92 wt. % Si, which matches the theoretical composition of Mg2SiO4. A detailed analysis of a single crystal shows that the surface structure consists of acicular microcrystals arranged symmetrically, but at the same time overlapping each other, thereby forming a lattice structure. The growth angle between the cubic microcrystals tends to ~87–94 degrees.