Glass-ceramic of cordierite type - from materials contg. oxides of magnesium, aluminium and silicon with addition of nucleating agent

摘要

Prodn. of glass-ceramic of the cordierite type by melting together starting materials contg. MgO, Al2O3 and SiO2, chilling the melt to effect solidification of glass, heating the glass to form cordierite-type crystals embedded in a glass phase, is improved by introducing into the material up to 3 wt. % of a nucleating agent selected from Mo, Ta, Zr, Ng, Ti, Li and/or As, or their oxides, or cpds. capable of liberating the oxides at elevated temps., or LiF. Pref. the solid glass is comminuted (particle size =0.004 mm), mixed with a comminuted nucleating agent, and the mixt. is heated at a temp. at which cordierite-type crystals surrounded by the glass phase are formed. The cordierite crystals should have the formula 2MgO.2Al2O3.nSiO2 (n = 5-8, pref. 5) These refractory materials are suitable for high-temp. application e.g. in gas turbines, for applications, involving temp., fluctuations such as heat exchangers, for other purposes, e.g. in radar construction. The coefft. of thermal expansion is low (1.60 x 10-6 degrees C-1) due to the high content of the cordierite-type crystals (=80 wt. %). The preliminary grinding of the glass, mixing it with comminuted nucleating agent and heating the mixt. results in uniform formation (throughout the whole material) of a great number of small cordierite-type crystals. High mechanical strength can be achieved.