Effects of ZrO2 on thermal stability and crystallization of K2O-Al2O3-SiO2 glasses

The thermal stability and crystallization behaviour of two refractory glass-forming compositions in the K2O-Al2O3-SiO2 ternary system were studied without ZrO2 additions (samples KAS1 and KAS2, with compositions on the tie‐lines from KAlSiO4 to Al2O3 and from KAlSi2O6 to Al2O3, respectively) and with ZrO2 additions (samples KAS1-Z and KAS2-Z). X-ray diffraction (XRD) was used to determine the crystalline phases formed during the heat treatment of the glasses. All as-prepared glasses were amorphous, except KAS2, with a weak SiO2 phase attributed to imperfect melting of starting materials, which disappeared upon heat treatment. Full crystallization of kalsilite (KAlSiO4) was observed in sample KAS1 after only 5 minutes’ heat treatment at 1250oC; for sample KAS2, a minor phase transformation from γ-Al2O3 to α-Al2O3 was observed after 10 minutes’ heat treatment at 1250oC; the relict SiO2 phase disappeared and the major phase leucite (KAlSi2O6) began to form after 30 minutes in sample KAS2. Upon addition of ZrO2 (sample KAS2-Z) the transformation from γ-Al2O3 to α-Al2O3 was delayed and observed after 30 minutes’ heat treatment at 1250oC. Meanwhile, the promotion of kalsilite and leucite crystallization was observed in samples KAS1-Z and KAS2-Z, respectively. Differential scanning calorimetry (DSC) was used to determine characteristic temperatures and crystallization activation energies (Ea) for each glass. However, there was no clear correlation between crystallization tendency (glass-forming ability) and Ea for these glasses, which exhibit high crystallization tendency. Multiple glass stability parameters (Hrubý KH, Weinberg KW, Lu & Liu KLL) were calculated based on characteristic temperatures and a further criterion (Hu k) was calculated, based on Ea. Promotion of the major phase (kalsilite and leucite) crystallization by the addition of ZrO2 was confirmed through these criteria.