Synthesis of cordierite by crystallization from MgO–Al2O3–SiO2 tempered glass

Abstract

Physicochemical research is conducted to examine the phase formation under dynamic and static annealing of MgO-Al 2 O 3 -SiO 2 tempered glass (composition range adjacent to stoichiometric cordierite). Thirty glass compositions are examined in this system and the range is determined over which single-phase ceramic materials with the lowest possible thermal expansion coefficient are synthesized under specific crystallization conditions.Melt crystallization provides materials based on alumina compounds, including mullite (3Al 2 O 3 ⋅ 2SiO 2 ), forsterite (2MgO ⋅ SiO 2 ), and cordierite (2MgO ⋅ 2Al 2 O 3 ⋅ 4SiO 2 ) with uniform chemical and phase compositions and controlled content of impurity phases. Two processing options may be used: (i) forming of parts from molten glass and their subsequent annealing (T ≥ 1000°C) for crystallization and (ii) production of granulated glass, its grinding, forming, and annealing using conventional ceramic processing.Single-phase α-cordierite with low thermal linear expansion coefficient (TLEC) and high thermal resistance was produced by cooling a cordierite melt at a rate of 1.5 °C/sec to 900-1100°C with subsequent annealing at these temperatures for four to five days [1]. To promote the crystallization of cordierite and decrease the volume content of the glass phase, oxide additions are used to form crystallization nuclei in glass: 3-5 wt.% ZrO 2 [2], 0.1-15 wt.% Nb 2 O 5 , Ta 2 O 5 [3], ≤10 wt.% TiO 2 [4]. This leads to a 25% decrease in annealing time, and the content of crystalline cordierite reaches 98-99 vol.%.The thermal linear expansion coefficients of samples representing stoichiometric cordierite ceramics and cordierite-based solid solutions are 2.0 ⋅ 10 -6 °C -1 over a range between 25 and 1000°C [5]. A substantial amount of the glass phase that had no admixtures of alkaline or alkaline-earth metals (ratios 1 : 1 : 4, 1 : 1 : 5, and 1 : 1 : 6 containing to 81% Si 2 ) increased the thermal expansion coefficient from 2.0 to 3 ⋅ 10 -6 °C -1 . The high content of crystalline cordierite (95-97%) resulted from arc melting of a cordierite charge consisting of serpentinite, quartz sand, and alumina [6]. When glass was heated at a rate of 10 °C/min, differential thermal analysis (DTA) revealed an exothermic effect of cordierite crystallization with a peak at 1000°C. There are no data on the thermal expansion of crystallized cordierite samples. In contrast to conventional negative attitude to alkaline oxide admixtures, it was established in [7] that about 7.56 wt.% K 2 O introduced into cordierite glass in accordance with the formula 4MgO ×

3D Computer Models of the Ag-Sb-Sn and MgO-Al2O3-SiO2 T-x-y Diagrams

“…It is used during the study and the description of the forsterite, periclase, spinel, corundum and mullite refractory materials. Its compounds, such as the forsterite 2MgO · SiO 2 , the alumina-magnesia spinel MgO · Al 2 O 3 , the cordierite 2MgO·2Al 2 O 3 ·5SiO 2 , have significant electrical insulating properties and are promising as thermoresistant ceramics [1][2][3][4].…”