3.1 The effect of heat treatment on the crystallization of chromium-containing CMAS glass-ceramics
In this part, the crystallization situation under different heat treatment temperature and holding time was studied.
3.1.1 Heat treatment temperature
Figure 3 shows the macroscopic morphology of glass ceramics when the heat treatment temperature is 780°C and 1090°C. Obviously, as the heat treatment temperature increased, the crystal phase gradually precipitated, the samples becomed denser, as well the glass luster gradually disappeared. The bubbles in the Fig. 3 may indicate that the viscosity of the formula is too high at high temperature, which makes the melt spread slowly, and the internal bubbles cannot be discharged in time.
Fig.4 shows the XRD test results of glass ceramics prepared at different heat treatment temperatures. As the heat treatment temperature increased, it can be seen from the figure that the crystallization was best at 1090°C. Compared with other diopside-based glass-ceramics, the higher crystallization temperature may due to the higher crystallization temperature of anorthite. For samples with a heating temperature of 1090°C, the crystallization peaks with an XRD scanning angle of less than 30° were all anorthite, which suggested that the crystallization peak temperature of anorthite is higher. In the semi-quantitative analysis, the relative content of diopside and anorthite is approximately 4:6, which implies that anorthite is easier to precipitate than diopside during the crystallization process.
3.1.2 Holding time
As mentioned earlier, the holding temperature of 1090°C was determined. Afterwards, the influence of different holding time on crystallization was studied.
Figure 5 shows the XRD test results of samples with holding time of 1h, 2h, 3h and 4h. It can be seen that the crystallization of the 1h sample is not complete, and the crystallization peak is not obvious. Table 2 is the semi-quantitative analysis results of samples with holding time: 2h, 3h and 4h. Among them, SiO2 was only detected in the samples of 2h, and not detected in the samples of 3h and 4h. This may be due to the incomplete crystallization in 2h, resulting in a little part of SiO2 not being reacted. In addition, the production of Mg2Si2O6 was also detected with the extension of the holding time. Figure 6 shows the ratio of anorthite and diopside in XRD semi-quantitative analysis under different holding times. It can be seen that with the extension of the holding time, more anorthite is precipitated. Figure 7 shows the SEM images with holding time of 3h and 4h respectively, the crystal grains gradually grow up with the extension of holding time.
Table 2
Semi-quantitative analysis of XRD results for different holding time
|
Diopside
|
Anorthite
|
Mg2Si2O6
|
SiO2
|
2h
|
36
|
44
|
-
|
19
|
3h
|
33
|
51
|
18
|
-
|
4h
|
15
|
67
|
16
|
-
|
3.1.3 Chromium content
Based on the above chemical composition of the basic glass, samples with chromium content of 2%, 3%, 4% and 5% (wt%) were prepared. According to the above experimental results, the heat treatment system was 1090°C for 4 hours.
Figure 8 shows the XRD schematic diagram of the samples, Table 3 shows the semi-quantitative analysis of the XRD results, and Fig. 9 shows the ratio of the precipitation of anorthite and diopside in the semi-quantitative analysis of the samples. It can be seen from the figures that the main crystal phase of the sample does not change with the increase of chromium content. When the chromium content of the sample increased to 2%, a new phase of MgAl2O4 appeared. When the chromium content continues to increase, the ratio of the phases in the sample does not change.
Table 3
Semi-quantitative analysis of samples with different chromium content after four hours of heat preservation
|
Diopside
|
Anorthite
|
SiO2
|
Mg2Si2O6
|
MgAl2O4
|
Cr3Si6
|
1%
|
15
|
67
|
0
|
18
|
0
|
0
|
2%
|
23
|
43
|
10
|
17
|
6
|
1
|
3%
|
24
|
44
|
7
|
18
|
7
|
1
|
4%
|
23
|
44
|
9
|
19
|
6
|
0
|
5%
|
24
|
44
|
8
|
20
|
5
|
0
|
With the increase of chromium content, a large number of bubbles appeared on the sample. Mills thinked that it is possible that the unreacted SiO2 in the silicate system generates a large number of silicon tetroxide interconnected crystal structure, which leads to the increase of melt viscosity [28].
Fig.10 is an SEM picture of the sample. The chromium content of the samples was 2%, 3%, 4%, and 5% from left to right and top to bottom. It can be seen from the picture that with the increase of chromium content, relatively fine and uneven crystal grains appear in the sample, which is also consistent with the decreasing trend of the strength of the main crystal phase in the XRD test results. Fig.11 is a partial enlarged view of a sample with a chromium content of 5%. It can be seen that some of the crystal grains in the sample show a tendency to merge.
Shuai Zhang et al. [20] pointed out that this observation can be explained in terms of two aspects. Firstly, the characteristic behavior of Cr3+ in a crystal field is supposed to determine the solubility of Cr2O3. Briefly, Cr3+ possess a higher OPSE (octahedral site preference energy) than common transition metal ions such as Fe2+ and Mn3+ (Cr3+=195523 J/mol, Fe2+= 16328 J/mol, Mn3+= 105926 J/mol) [29], which suggests that Cr3+ preferentially occupies the octahedral position of the crystal field.
On the other hand, in a previous analysis of diopside, it has been found to have a chain silicate structure and its silicon oxygen backbone is [Si2O6][30]. The oxygen at the top of the silicon-oxygen chain forms a small octahedral space, whereas the oxygen at the bottom surface of the silicon-oxygen chain forms a large distortion octahedral space. As well, when Cr2O3 is added to the diopside system, Cr3+ cations can occupy the octahedral space, replacing Mg2+ to produce the dissolution effect. Similarly, Anorthite is a framework silicate mineral, its silicon-oxygen unit is [SiO4] tetrahedron, and part of it is replaced by aluminum oxide tetrahedron [AlO4], and excess negative charge appears. These excess negative charges must be neutralized by cations. Because of isomorphism, there may be more octahedral space between the silica chains of anorthite. At the same time, it means that the dissolution of chromium is higher than that of diopside.
Therefore, as the Fig. 12, the Cr3+ in the chromium-containing solid waste can occupy the octahedral space in the generated diopside and anorthite system, replacing Mg2+ and Al3+ to produce a dissolution effect. From the SEM point of view, with the increase of chromium content, the gradually appearing fine grains in the picture may indicate that with the entry of chromium ions, free Mg2+ and Al3+ combine with unreacted SiO2 to form MgAlO4 and Mg2Si2O6. And the intensity of the characteristic peaks in XRD decreases, because with the generation of new phases, the smaller and smaller crystal grains may be one of the reasons.
3.1.4 MnO and Fe2O3 doped
Three groups of samples as shown in the Table 4 were prepared under the above conditions (1090 ℃, 4h) to analyze the effects of MnO and Fe2O3 doping on crystallization.
Table 4
Chemical composition of MnO and Fe2O3 doping
Composition
|
CaO/wt%
|
SiO2/wt%
|
MgO/wt%
|
Al2O3/wt%
|
Cr2O3/wt%
|
MnO/wt%
|
Fe2O3/wt%
|
MnO
|
20.9
|
44.7
|
7.4
|
19.0
|
5
|
3
|
-
|
Fe2O3
|
20.9
|
44.7
|
7.4
|
19.0
|
5
|
-
|
3
|
MnO,Fe2O3
|
20.2
|
43.2
|
7.2
|
18.4
|
5
|
3
|
3
|
Fig.13 is the XRD test results of the samples, and table 5 is the semi quantitative analysis results of XRD. As can be seen from the figure, the doping of MnO and Fe2O3 had little effect on the main crystal phase. Both reduced the content of anorthite. In addition, the content of Mg2Si2O6 was slightly reduced and the relative content of MgAl2O4 is increased.
Table 5
XRD semi quantitative analysis
|
Diopside
|
Anorthite
|
SiO2
|
Mg2Si2O6
|
MgAl2O4
|
Undoped
|
24
|
44
|
8
|
20
|
5
|
MnO
|
30
|
33
|
13
|
16
|
9
|
Fe2O3
|
29
|
37
|
11
|
12
|
11
|
MnO, Fe2O3
|
29
|
38
|
10
|
16
|
6
|
Fig. 14 is the SEM picture of MnO, Fe2O3, MnO and Fe2O3 doped. Larger and collapsed particles can be seen in the pictures of doped MnO. In the picture of Fe2O3 doping, we can see uniform and very fine particles. The morphology of the samples doped with both of them is similar to that of Fe2O3. From the SEM images, both MnO and Fe2O3 make the crystal phase of glass ceramics more uniform.
3.1.4 Influence of heat treatment and chromium content on the chemical stability and hardness of glass-ceramics
As the Fig. 15 and Fig. 16, the chemical stability and hardness of the material were tested. Figure 15 shows the acid resistance of the material. This material had a good stability in an alkaline environment, and the loss rate data measured in a laboratory environment was almost zero, so it is not listed in the picture. It can be seen from Fig. 15 that compared with the holding time and the chromium content; the temperature has a greater impact on the acid resistance of the material. Because the next step at a lower temperature cannot fully crystallize the glass ceramics, may resulting in a higher weight loss rate. In the range tested in the article, the chromium content had a little effect on acid resistance. The addition of MnO and Fe2O3 decreased the acid resistance.
Fig.16 shows the effect of heat treatment temperature, time and chromium content on the hardness of glass-ceramics. As shown in the figure, as the heat treatment temperature and holding time increase, the hardness of the glass-ceramics also gradually promoted. In Fig.16(c), the chromium content of the sample with 1% chromium mass fraction was significantly higher than that of other samples with chromium content, and as the chromium content continues to increase, the chromium content had a little effect on the hardness of the sample. The addition of Fe2O3 promoted the hardness of the sample, while MnO decreased it.