Non-isothermal crystallization kinetics and stability of leucite and kalsilite from K2O-Al2O3-SiO2 glasses

The crystallization mechanisms and elemental stability of leucite and kalsilite formed from K2O-Al2O3-SiO2 glasses were investigated by X-ray powder diffraction (XRD), X-ray fluorescence (XRF), Raman spectroscopy and differential scanning calorimetry (DSC). Glass samples with compositions along the leucite-kalsilite tie-line were produced by melt processing; and were then heat treated at 850ºC, 950ºC and 1250ºC for times ranging from 5 minutes to 1000 hours. Kalsilite is an unstable phase that behaves as an intermediate precursor to leucite. Crystalline materials in which kalsilite is the major phase lose potassium upon prolonged heat treatment (1000 hours at 1250ºC), in contrast to those with leucite, in which little or no compositional alteration is detected. The formation of leucite from stoichiometric kalsilite is accompanied by the formation of potassium doped alumina. The activation energies for leucite and kalsilite crystallization, determined via application of the Kissinger equation to thermal analysis data, were 579 kJ/mol and 548 kJ/mol respectively. Finally, production of pure leucite can be achieved with more favourable crystallization kinetics when starting with off-stoichiometric compositions

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

Thermal conductivity of refractory glass fibres

In the present study, the current international standards and corresponding apparatus for measuring the thermal conductivity of refractory glass fibre products have been reviewed. Refractory glass fibres are normally produced in the form of low-density needled mats. A major issue with thermal conductivity measurements of these materials is lack of reproducibility in the test results due to transformation of the test material during the test. Also needled mats are inherently inhomogeneous, and this poses additional problems. To be able to compare the various methods of thermal conductivity measurement, a refractory reference material was designed which is capable of withstanding maximum test temperatures (1673 K) with minimum transformation. The thermal conductivity of this reference material was then measured using various methods according to the different standards surveyed. In order to compare different materials, samples have been acquired from major refractory glass fibre manufacturers and the results have been compared against the newly introduced reference material. Materials manufactured by melt spinning, melt blowing and sol–gel have been studied, and results compared with literature values