EFFECT OF CHANGING THE SUBSTITUENT OXIDE AND ITS AMOUNT ON THE THERMAL STABILITY TOWARDS CRYSTALLIZATION OF GLASSES OF FORMULA (2.5-X)CaOX/3M2O32SiO2 (M= La, Y, IN, Ga)

Crystallisation behaviour of CaO-M2O3-SiO2 glasses (M=La, Y, In, Ga) is studied using a recently proposed dimensionless parameter, which can be easily calculated from thermoanalytical data. This parameter proves useful in comparing the stability of glass toward crystallisation. The stability to crystallisation increases as M2O3 is substituted for CaO. Moreover, the higher the ionic field strength, Z/R-2, of the substituting cation, the greater the effect. This can be ascribed to: (a) the decrease in density of nonbridging oxygens when a network former oxide (Al2O3, Ga2O3) is introduced (Z/R-2 > 5 Angstrom(-2)); (b) the increase of the transverse bond strength (the greater the higher the ionic field strength of the cation) when a network modifier oxide is introduced (Z/R-2 < 5 Angstrom(-2)).