Non-isothermal crystallization kinetic studies on MgO–Al2O3–SiO2–TiO2 glass

doi:10.1016/j.jnoncrysol.2007.04.008

Journal of Non-Crystalline Solids

Volume 353, Issues 24–25, 15 July 2007, Pages 2383-2391

https://doi.org/10.1016/j.jnoncrysol.2007.04.008 Get rights and content

Abstract

Thermal stability and crystallization kinetics of the glass 21% MgO, 21.36% Al₂O₃, 53.32% SiO₂ and 4.11% TiO₂ (mol%) has been studied using differential thermal analysis (DTA), dilatometry and X-ray diffraction (XRD). Glass in both bulk and frit forms were produced by melting in platinum crucible at 1600 °C for 1–2 h. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 340 kJ mol⁻¹ and 498 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Crystallization of bulk glass was carried out at various temperatures and for different time durations in the range of 850–1000 °C. The influence of the addition of TiO₂ on the crystallization sequence of the glass was experimentally determined and discussed.

Introduction

Between the end members of the SiO₂–MgAl₂O₄ system there is a point corresponding to the ternary compound Al₃Mg₂(AlSi₅O₁₈), a cordierite showing complex polymorphism. Cordierite glass-ceramics belong to an important class of advanced technological materials having a wide range of applications [1], [2], [3]. They have not only unique feature of machinability, but also superior electrical insulation, ultrahigh vacuum compatibility, high thermal stability, low thermal conductivity and good mechanical strength [1], [3], [4], [5], [6], [7], [8]. These glass-ceramics are interesting candidates for a number of elevated temperature applications where good high temperature creep resistance coupled with a high resistance to thermal shock are required [9], [10].

To achieve the fine grained microstructure in a glass-ceramic which favors the development of outstanding physical properties, it is necessary to ensure that the nucleation of the crystallization process occurs with in the body of the glass. For cordierite glasses homogeneous nucleation is difficult if not impossible. However, the deliberate inclusion of nucleation catalysts in the glass greatly assists in the control of the crystallization processes and permits wider range of glass compositions to be converted into satisfactory glass-ceramics.

The most efficient nucleating agents in this type of glass-ceramic are zirconium dioxide (ZrO₂) and titanium dioxide (TiO₂), which are widely used as a mixture. TiO₂ is soluble in molten glasses but during cooling or subsequent reheating, large numbers of submicroscopic particles are precipitated and these can be utilized to assist the development of major crystalline phases. The total content of these agents is in order of 3–5 wt% in order to get an efficient and fast nucleating process, which results in high nucleation density and transparent glass-ceramic. However, ZrO₂ has some disadvantages as compared with TiO₂. In particular, zirconia is not very soluble in silicate melts and it is difficult to incorporate more than 3% or 4% of this oxide in solution. On the other hand, TiO₂ is quite soluble in silicate melts since 20% or more of this oxide can be dissolved and it has the additional advantage of markedly lowering the viscosity of the glass melt [9].

Although a number of studies have been reported aimed specifically at examining the relationship between phase separation, nucleation and crystallization in the cordierite glasses [11], [12] and in determining the kinetic parameters [13], [14], more comprehensive data is required before a clearer understanding of the crystallization processes can emerge. In view of the above-mentioned perspective, the aim of the present work is to investigate the thermal properties, the crystallization kinetics and the crystallization sequences of the MgO–Al₂O₃–SiO₂ system glass containing TiO₂ as nucleating agent by means of dilatometry, differential thermal analysis (DTA) and X-ray diffraction (XRD).

Section snippets

Materials and experimental procedure

Powders of technical grade SiO₂ (purity >99.5%) and of reactive grade MgCO₃, Al₂O₃ and TiO₂ were used. Homogeneous mixtures of batches (∼100 g), obtained by ball milling, were preheated at 900 °C for 1 h for decarbonization and then melted in platinum crucible at 1580°C for 2 h, in air.

Glasses in bulk form were produced by casting the melts on preheated bronze moulds and subsequent immediate annealing at 550 °C (i.e., close to the transformation temperature T_g) for 1 h. Glasses in frit form were

Thermal stability

A parameter usually employed to estimate the glass stability is the thermal stability (ΔT) [15], which is defined by $Δ T = T_{c} - T_{g} .$

The bigger difference between T_c and T_g indicates the more stability for glasses.

Saad and Poulain [16] obtained another criterion, the thermal stability parameter: $S = (T_{p} - T_{c}) (T_{c} - T_{g}) / T_{g} .$

The thermal stability parameter S reflects the resistance to devitrification after the formation of the glass. In Eq. (2), (T_p − T_c) is related to the rate of devitrification transformation of

Physical properties

The investigated composition was suitable for easy casting after 2 h of melting at 1580 °C, resulting in homogenous transparent, bubble-free glass and with no crystalline inclusions, as was also confirmed by X-ray analysis afterwards. The values of the density of the bulk and powder glass were calculated to be 2.66 ± 0.006 g/cm³ and 2.67 ± 0.02 g/cm³, respectively. The measured mean particle size of the fine glass powder was 3.19 μm.

Dilatometry

The thermal expansion curve of the cast-annealed bulk glass is plotted

Discussion

Fig. 5 shows the plots of $\ln (T_{g}^{2} / β$ ) (curve a) and ln(β) (curve b) versus 1/T_g for the glass powder displaying the linearity of the equations used. The values of the activation energy obtained for the glass transition are 622 kJ mol⁻¹ (plot a) and 640 kJ mol⁻¹ (plot b), respectively. The values of fragility index (F) and thermal stability parameter (S) for glass MAS-T for various heating rates are listed in Table 1. The value of F for the glass under investigation is near to KS limit. This indicates

Summary

Thermal behavior and crystallization kinetics of the glass MAS-T with composition 21.21% MgO, 21.36% Al₂O₃, 53.32% SiO₂ and 4.11% TiO₂ (mol%), has been studied by DTA and XRD. The value of glass fragility index, F indicates that the glass is formed from a kinetically stable liquid. Activation energy for glass transition was determined to be 622 and 640 kJ mol⁻¹ by two different approaches. Crystallization activation energies for μ-cordierite and α-cordierite were determined to be 340 and 498 kJ mol

Conclusions

To conclude, a comparatively low value of the activation energy of crystallization for α-cordierite along with the two and one-dimensional growth of mechanism for μ- and α-cordierite, respectively, has been observed with the addition of 5 wt% of TiO₂. This shows that TiO₂ favorably influences the nucleation of crystalline phases in the glass system by fine-scale glass-in-glass phase separation leading to the decrease in the activation energy of crystallization. The mechanism of bulk nucleation

Acknowledgement

A. Goel is indebted for the research fellowship grant from CICECO.

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Non-isothermal crystallization kinetic studies on MgO–Al2O3–SiO2–TiO2 glass

Abstract

Introduction

Section snippets

Materials and experimental procedure

Thermal stability

Physical properties

Dilatometry

Discussion

Summary

Conclusions

Acknowledgement

J. Mater. Process. Technol.

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

Physica B

J. Non-Cryst. Solids

Acta Mater.

J. Non-Cryst. Solids

Anal. Chem.

J. Am. Ceram. Soc.

J. Mater. Sci.

J. Mater. Sci. Lett.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Mater. Sci.

J. Am. Ceram. Soc.

Bull. Mater. Sci.

Glass-Ceramics

Non-isothermal crystallization kinetic studies on MgO–Al₂O₃–SiO₂–TiO₂ glass