3.1. Crystalline Phase and Kinetics Analyses
Figure 1 shows the DSC curves of the BG0–BG4(A0–A4) samples at a heating rate of 10 °C/min. As the Al
2O
3 content increased, the glass transition temperature (T
g) increased, the first crystallization peak (T
p1) values increased first and then decreased, and the second crystallization peak (T
p2) values generally increased. It could be explained that Al
3+ in the glass can connect the glass network with four coordinated [AlO
4], increasing the degree of polymerization of the glass network. On the other hand, Al
3+ can also exist with six coordinated [AlO
6], which plays a role in disrupting the glass network and reducing the degree of polymerization of the network [
19]. For BG0-BG3 samples, Al
3+ connected to a glass network in the form of [AlO
4] and the degree of polymerization of the glass network increased, which is not conducive to the diffusion of ions during the crystallization process, leading to an increase in the glass transition temperature and first crystallization peak temperature. Due to the precipitation of crystalline phases on the surface of glass powder, the viscosity of the glass increases, hindering the further diffusion of ions and hindering crystallization, resulting in an increase in the second crystallization temperature [
20,
21]. Notably, the first crystallization peak of the BG4 sample abnormally decreased, and that is because Al
3+ exists with six coordinated [AlO
6], which reduces the degree of network polymerization, facilitates ion diffusion during crystallization, and is more conducive to the reduction in crystallization temperature.
The heat treatment of the samples included two main stages: nucleation and crystallization [
13]. The nucleation temperature (T
N) is usually 50–100 °C higher than the glass transition temperature [
22] (T
N = T
g + 50 °C in this study). The BG0–BG4 samples were heat treated at T
p1 and T
p2, respectively. The glass ceramics (GC0–GC9) were obtained according to the heat treatment conditions, as shown in
Table 5.
Figure 2 shows the XRD patterns of the BG0–BG4 samples. It can be observed that there was almost no diffraction peak, except just an amorphous hump around 30º. This result indicates that there were no crystals precipitated in basic glass without heat treatment.
Figure 3 shows the XRD patterns of the GC0–GC9 samples. All samples obtained have the same crystal phase composition, consisting of two crystal phases: gehlenite (Ca
2Al
2SiO
7 PDF#35-0755) and wollastonite (CaSiO
3 PDF#27-0088). Simultaneously, the diffraction peak intensity of gehlenite is strong, while the diffraction peak intensity of wollastonite is relatively weak, indicating that gehlenite is the main crystalline phase, while wollastonite is the secondary crystalline phase. With the increase in the Al
2O
3 content, the diffraction peak intensity of wollastonite phase gradually weakened, while the diffraction peak intensity of gehlenite phase gradually strengthened.
It can be explained that Al
3+ directly participates in the formation of gehlenite, so an increase in Al
2O
3 content is beneficial for the precipitation of gehlenite. On the other hand, as a network intermediate, Al
2O
3 can replace Si
4+ with [AlO
4] to participate in the composition of glass networks and crystal phases and can also damage the network structure of glass in the form of [AlO
6] [
6,
19]. With the increase in the Al
2O
3 content, Si
4+ in the glass network structure decreased, and the increased [AlO
6] damaged the glass network, reduced the degree of network polymerization, and is conducive to the precipitation of gehlenite with a lower anionic polymerization degree. On the other hand, the increase in Al
2O
3 content leads to more Al
3+ replacing Si
4+ in the network, which is also conducive to the formation of [Al
2SiO
7] anion groups, thereby promoting the precipitation of gehlenite. Due to the large precipitation of gehlenite phase in glass ceramics, the silicon–oxygen content in the glass decreases, and the anionic groups forming the secondary crystalline phase decrease, resulting in a decrease in the wollastonite phase content. The results of XRD showed that the precipitation of gehlenite and wollastonite phase is in a competitive relationship, and the addition of alumina promotes the precipitation of gehlenite phase and hinders the precipitation of wollastonite phase. The chemical reaction equations for this process are as follows:
The effects of Al
2O
3 on the crystallization kinetics of glass ceramics were analyzed by the non-isothermal DSC method at different heating rates. The DSC curves of the BG0–BG4 samples at different heating rates are shown in
Figure 4. The crystallization temperatures of the BG0–BG4 samples at different heating rates are listed in
Table 6. It is clear that the T
p1 and T
p2 values increased with the increase in the heating rate, which is due to the later heating and melting for the glass system at a high heating rate, resulting in an increase in the temperature peak.
The Kissinger equation [
23] was applied to determine the crystallization activation energy and relevant parameters:
where
α is the heating rate, T
p is the crystallization peak temperature, R is the gas constant,
is the frequency factor, and E
c represents the activation energy.
E
c can be obtained from the plot of ln (T
p2/
α) vs. T
p−1 for glass ceramics crystallization, respectively, shown in
Figure 5. The relevant parameters of the crystallization kinetic of the BG0–BG4 samples crystallized at T
p1 and T
p2 are listed in
Table 6, respectively.
In
Table 7, the correction coefficients (r
2) of the fitting curves to calculate E
c2 are close to 1, indicating good fitting linearity and reflecting the reliability of the calculated E
c2 data. However, the r
2 values to calculate E
c1 have a certain difference from 1, indicating a slightly poor fitting linearity. It is very likely that the calculation process of the Kissinger equation only involves the peak temperature of the crystallization exothermic peak and heating rate, and the angular coefficient, linear coefficient, and frequency factor of the curve were not considered. In
Figure 3, the crystallization exothermic peak shape of T
p1 is wide and not sharp, the angular relationship between peak shapes is unclear, and the linear relationship is poor, which indicates that the reliability of the calculated crystal activation energy is slightly poor.
The Ozawa equation [
24] can be also used to calculate the crystallization activation energy as follows:
where E
c is the crystallization activation energy, K
0 is the frequency factor,
α is the heating rate, C
2 is a constant, T is the corresponding crystallization temperature when the crystallinity is x, and
f(x) is a function of crystallinity. When the crystallinity (x) is a constant,
f(x) is a constant.
To obtain the relationship between crystallinity (x) and temperature (T), it is necessary to deduct the substrate of the crystallization exothermic peak of T
p1 and integrate for the crystallization exothermic peaks at different heating rates. As shown in
Figure 6 and the Ozawa equation, the crystallinity temperatures were determined when the crystallinity was x (x = 0.2, 0.3, ……0.8), and then linear fitting was operated for ln
α-1000/T
i, as shown in
Figure 7. According to the slopes of the fitted straight lines in
Figure 7, E
c1 can be calculated, and the results are listed in
Table 8.
As shown in
Table 8, E
c1/(RT) satisfies the condition of 20 ≤ E
c1/(RT) ≤ 60, ensuring the reliability of data calculation using the Ozawa equation. Finally, the E
c1 of the BG0–BG4 samples is 395, 407, 417, 419, and 431 kJ/mol, respectively, and the E
c2 of the BG0–BG4 samples is 252, 310, 306, 326, and 331 kJ/mol, respectively. The results showed that as the Al
2O
3 content increased, E
c1 increased, indicating that the addition of Al
2O
3 reduces the formation barrier of gehlenite and promotes its crystallization and increases the formation barrier of wollastonite and inhibits its crystallization (consistent with the analysis results in
Figure 3a). However, during the crystallization process, the activation energy is positively correlated with the viscosity of the glassy phase, and the addition of Al
2O
3 increases the viscosity of the glassy phase, showing an increase in crystallization activation energy. Due to the precipitation of crystalline phases, the viscosity of the glass increased, hindering the further diffusion of ions and crystallization, resulting in the inhibition of the subsequent crystallization process, manifested as an increase in the E
c2 values.
After obtaining E
c1 and E
c2 values, the crystallization index (n) could be calculated by applying the Augis–Bennett equation [
25].
where ΔΤ is the full width at half maximum of the exothermic peak, E
c is the crystallization activation energy, R is the gas constant, and T
p is the corresponding crystallization temperature.
If the n value is close to 1, this indicates the surface crystalline mechanism and one-dimensional growth in the crystallization process. If the n value is close to 2, that denotes the bulk crystalline mechanism and one-dimensional growth. If the n value is close to 3, that denotes the bulk crystalline mechanism and two-dimensional growth. If the n value is close to 4, that denotes the bulk crystalline mechanism and three-dimensional growth [
26,
27]. The Avrami parameters (n) for the crystal growth of the GC0–GC9 samples under various heating rates are shown in
Table 9, respectively.
As shown in
Table 9, the Al
2O
3 content has a significant impact on the crystallization process of glass ceramics. Among all the samples, the n values at T
p1 are between 2 and 4, indicating that the crystalline growth mechanism is bulk nucleation, shifting from three-dimensional to one-dimensional and then to two-dimensional growth. Meanwhile, the n values at T
p2 are between 3 and 4, indicating that the crystal growth mechanism is bulk nucleation, shifting from three-dimensional to two-dimensional growth.