Crystallization kinetics of bioactive glasses in the ZnO-Na2O-CaO-SiO2 system

The crystallization kinetics of Na2O center dot CaO center dot 2SiO(2) (x = 0) and 0.68ZnO center dot Na2O center dot CaO center dot 2SiO(2) (x = 0.68, where x is the ZnO stoichiometric coefficient in the glass formula) bioactive glasses have been studied using both nonisothermal and isothermal methods. The results obtained from isothermal XRPD analyses have showed that the first glass crystallizes into the isochemical Na2CaSi2O6 phase, whereas the Na2ZnSiO4 crystalline phase is obtained from the Zn-rich glass, in addition to Na2CaSi2O6. The activation energy (E-a) for the crystallization of the Na2O center dot CaO center dot 2SiO(2) glass is 193 +/- 10 and 203 +/- 5 kJ/mol from the isothermal in situ XRPD and nonisothermal DSC experiments, respectively. The Avrami exponent n determined from the isothermal method is 1 at low temperature (530 degrees C), and its value increases linearly with temperature increase up to 2 at 607 degrees C. For the crystallization of Na2CaSi2O6 from the Zn-containing glass, higher values of both the crystallization temperature (667 and 661 degrees C) and E-a (223 +/- 10 and 211 +/- 5 kJ/mol) have been found from the isothermal and nonisothermal methods, respectively. The Na2ZnSiO4 crystalline phase crystallizes at lower temperature with respect to Na2CaSi2O6, and the E-a value is 266 +/- 20 and 245 +/- 15 kJ/mol from the isothermal and nonisothermal methods, respectively. The results of this work show that the addition of Zn favors the crystallization from the glass at lower temperature with respect to the Zn-free glass. In fact, it causes an increase of E-a for the Na diffusion process, determined using MD simulations, and consequently an overall increase of E-a for the crystallization process of Na2CaSi2O6. Our results show good agreement between the E-a and n values obtained with the two different methods and confirm the reliability of the nonisothermal method applied to kinetic crystallization of glassy systems. This study allows the determination of the temperature stability field of the crystalline phases with the view of creating a different glass ceramic useful in the field of bioactive materials.