The effect of alumina content on the thermomechanical and structural propertied of machineable magnesium aluminosilicate glasses and their glass ceramics

摘要

Magnesium aluminium silicate glasses and their machineable glass ceramics have been prepared with molar amounts of Al_2O_3 varying from 7-6 to 14-7 percent, keeping the total Mg content in the form of MgF_2 and MgO almost constant at around 25 mol percent. The base glasses were prepared by melting, quenching and annealing and converted into glass ceramics by controlled crystallisation at temperatures up to 950 deg C. Glass transition temperatures, microhardnesses and average thermal expansion coefficients have been determined for these samples. The structural aspects of these samples have been studied by using 29 Si and 2''AINMR spectroscopy and the crystalline phases present in the glass ceramics have been identified by powder x-ray and 29Si NMR spectroscopy. It is shown that for lower concentrations of Al_2O_3 the crystalline phases present in the glass ceramics are MgSiO_3 and fluorophlogopite and that at approx =12 mo I percent of Al_2O_3, fluorophlogopite phase is the predominant phase. Further increases in alumina content leads to the formation of magnesium silicate (forsterite, Mg_2SiO_4) and at 14-7 mol percent Al_2O_3 the only crystalline phase present is Mg_2SiO_4.