Thermal Stability and Crystallization Kinetics of MgO–Al ₂ O ₃ –B ₂ O ₃ –SiO ₂ Glasses

To support commercialization of the MgO–Al ₂ O ₃ –B ₂ O–SiO ₂ ‐based low‐dielectric glass fibers, crystallization characteristics of the relevant glasses was investigated under various heat‐treatment conditions. The study focused on the effects of iron on the related thermal properties and crystallization kinetics. Both air‐cooled and nucleation‐treated samples were characterized by using the differential thermal analysis/differential scanning calorimeter method between room temperature and 1200°C. A collected set of properties covers glass transition temperature ( T _g ), maximum crystallization temperature ( T _p ), specific heat (Δ C _p ), enthalpy of crystallization (Δ H _cryst ), and thermal stability (Δ T = T _p — T _g ). Using the Kinssiger method, the activation energy of crystallization was determined. Crystalline phases in the samples having various thermal histories were determined using powder X‐ray diffraction (XRD) and/or in situ high‐temperature XRD method. Selective scanning electron microscope/energy‐dispersive spectroscopy analysis provided evidence that crystal density in the glass is affected by the iron concentration. Glass network structures, for air‐cooled and heat‐treated samples, were examined using a midinfrared spectroscopic method. Combining all of the results from our study, iron in glass is believed to function as a nucleation agent enhancing crystal population density in the melt without altering a primary phase field. By comparing the XRD data of the glasses in two forms (bulk versus powder), the following conclusions can be reached. The low‐dielectric glass melt in commercial operation should be resistant to crystallization above 1100°C. Microscopic amorphous phase separation, possibly a borate‐enriched phase separating from the silicate‐enriched continuous phase can occur only if the melt is held at temperatures below 1100°C, that is, below the glass immiscibility temperature. The study concludes that neither crystallization nor amorphous phase separation will be expected for drawing fibers between 1200°C and 1300°C in a commercial operation.