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Coatings 2017, 7(7), 85; https://doi.org/10.3390/coatings7070085

Article
Oxidation Behavior and Mechanism of Al4SiC4 in MgO-C-Al4SiC4 System
1
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 10083, China
2
State Key Laboratory of Advanced Metallurgy, Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Received: 25 May 2017 / Accepted: 19 June 2017 / Published: 23 June 2017

Abstract

:
Al4SiC4 powder with high purity was synthesized using the powder mixture of aluminum (Al), silicon (Si), and carbon (C) at 1800 °C in argon. Their oxidation behavior and mechanism in a MgO-C-Al4SiC4 system was investigated at 1400–1600 °C. XRD, SEM, and energy dispersive spectrometry (EDS) were adopted to analyze the microstructure and phase evolution. The results showed that the composition of oxidation products was closely related to the atom diffusion velocity and the compound oxide layer was generated on Al4SiC4 surface. In addition, the effect of different CO partial pressure on the oxidation of Al4SiC4 crystals was also studied by thermodynamic calculation. This work proves the great potential of Al4SiC4 in improving the MgO-C materials.
Keywords:
Al4SiC4; oxidation; MgO-C bricks

1. Introduction

Magnesia-carbon (MgO-C) bricks are a kind of typical refractories, and are mainly composed of MgO and graphite. They have been playing a vital role in the fields of converters, electric furnaces and molten steel refining, etc. [1,2,3], owing to the nice combination of the perfect properties of the corresponding components (i.e., high temperature resistance and great basic slag resistance as well as low wettability to molten steel). Despite these advantages, graphite’s susceptibility to oxidation is the major weakness of carbon-containing refractories, leading to the degradation of brick properties in service. To solve this problem, antioxidants such as Al powder, Si powder, and Al-Si alloys as well as borides including B4C, ZrB2, etc. are adopted to decrease the oxygen partial pressure, reducing the detrimental effect of oxidizing gas on the network structure of carbon and graphite. Therefore, the study of antioxidants is as important as the crystallization, composition, and structure of magnesite and graphite in MgO-C. In view of the antioxidants, recent studies have been focused on their form, phase reaction, and structure evolution in MgO-C, as well as the influence of their types on the decarburized layer and the slag corrosion resistance [3,4,5,6].
Among these antioxidants, borides are seldom used due to their high costs and potential harmful impact on some steels. Al powder is the most common antioxidant, and it can react with graphite and carbon to form Al4C3 during application. Then, this resulting product can improve the strength of MgO-C bricks at high temperature, which is of significant importance for MgO-C in scour resistance of molten steel [7]. However, Al powder as an antioxidant still has certain defects. On one hand, the formation process of Al4C3 would bring in a substantial volume effect, limiting the adoption amount of Al powder at around 3 wt % [8]. On the other hand, Al4C3 always easily hydrates to form Al(OH)3, and this reaction also leads to large volume expansion, which is very detrimental to MgO-C bricks [9].
It should also be noted that with the purpose of improving the quality of steel, the carbon content in MgO-C bricks is required to reduce (C ≤ 6wt %). The following problem is the great decline of thermal stability, slag penetration resistance, and oxidation resistance, etc. due to high thermal expansion coefficient (14 × 10−6 °C) of MgO, which is the most important issue for low-carbon MgO-C bricks [10,11,12].
To solve the above problems of low-carbon MgO-C bricks, different introduced forms of carbon and brick structures are mainly studied, in which activated carbon and nanocarbon having relatively high specific surface area are the research focus. The introduction of these carbons can reduce the expansion of magnesite particles and improve the thermal stability and slag penetration resistance by enveloping magnesite particles. However, the micronization of carbon leads to weak oxidation resistance [12,13,14,15]. In addition, the adoption amount of Al powder is also limited because low-carbon MgO-C bricks cannot stand much of a volume effect brought by the formation of Al4C3.
Therefore, it is desirable to solve two important problems of traditional MgO-C bricks. The first is how to introduce the non-metal antioxidants that possess low volume effect and good oxidation resistance. The second is how to realize low carbon while retaining excellent slag penetration resistance.
Al4SiC4 is the most stable and valuable phase in the Al-Si-C system, and has many excellent properties, such as high melting point (around 2080 °C [16]) and great hydration resistance [17,18,19]. In addition, although Al4SiC4 is easily oxidized at high temperature, its oxidation product can form protective layer and prevent subsequent reaction. These advantages make Al4SiC4 a prospective modified material in MgO-C bricks.
Some investigations of the effect of Al4SiC4 on the properties of MgO-C bricks have been carried out. Zhang et al. [20,21] studied the oxidation behavior of Al4SiC4 in CO and the effect of Al4SiC4 addition on the carbon-containing refractories. They found that the Al2O3-SiO2 layer would form on the refractory surface after oxidation, and prevented the further oxidation of internal refractory. Li et al. investigated the properties of Al4SiC4 on MgO-C bricks. The results showed that MgO-C bricks were oxidized to generate magnesia-alumina spinel (MgAl2O4). During this process, the volume expansion was able to reduce the porosity of MgO-C bricks, and a protective layer was formed. In addition, Al4SiC4 could improve the stability of MgO-C bricks from high temperature to room temperature [22]. Wang et al. added Al4SiC4 powders synthesized by self-propagating chemical method to carbon-containing refractories, which indicated that Al4SiC4 could improve the property of original matrix and make up the shortage of Al-containing additives [23].
Although the above works have been conducted, there still remains some unclear questions; i.e., how does the oxidation of Al4SiC4 happen in the MgO-C-Al4SiC4 system? How do the elements of Al, Si, etc. transfer and diffuse? What will happen between components in diffusion scale and different MgO-C matrixes? The understanding of the above problems is crucial to optimizing and improving the effect of Al4SiC4 in MgO-C bricks. Therefore, in this work, the oxidation behavior and mechanism of perfect Al4SiC4 crystals in a MgO-C-Al4SiC4 system are investigated, laying a solid foundation for the breakthrough of traditional and low-carbon MgO-C bricks.

2. Materials and Methods

2.1. Preparation of Al4SiC4 Crystals

During the preparation process, commercial grade Al powder (≥99.99 wt %) with an average particle size of 100 μm, Si powder (≥99.9 wt %) with an average particle size of 75 μm, and graphite (≥99.85 wt %) with an average particle size of 30 μm were used as raw materials. They were all purchased from Sinopharm Chemical Reagent Beijing Co. Ltd., Beijing, China. The powders with a molar ratio of 4:1:4 corresponding to the chemical composition of Al4SiC4 were mixed in a planetary ball mill using alcohol as medium at the rate of 100 rpm for 24 h. Then, the mixture was dried at 80 °C for 24 h and pressed into column (Ф25 mm × 30 mm) under a pressure of 30 MPa. Finally, the compacted column was placed in a graphite crucible and heated to 1800 °C for 3 h in 99.99% argon with a flow rate of 0.2 mL/min. After cooling to room temperature, large-scale Al4SiC4 product was obtained. At last, the Al4SiC4 product was ground into powder for the oxidation experiment.

2.2. Evolution of Al4SiC4 in MgO-C-Al4SiC4 System

Analytically pure MgO powder (wt % > 98%), graphite powder (wt % > 99.85%) and synthesized Al4SiC4 crystals were mixed according to mass ratio of 40:15:45. Herein, the relatively higher content of Al4SiC4 was chosen to magnify the experiment and better study the oxidation behavior of Al4SiC4 in the MgO-C-Al4SiC4 system. Besides, 10 wt % phenolic resin was also added as binding agent. Subsequently, the mixture was pressed into column (Ф25 mm × 30 mm) under a pressure of 30 MPa, followed by drying at 110 °C for 12 h. Then, the dried sample was placed into an electric furnace and calcined at 1400–1600 °C for 4 h under carbon-buried condition in air, respectively. Finally, the sample was cooled naturally for subsequent analysis.

2.3. Characterization

The phase composition of the Al4SiC4 crystals and calcined MgO-C-Al4SiC4 specimens was characterized using X-ray Diffraction (XRD; D8 Advance, Bruker, Germany) at a scanning rate of 0.02°/min in the scanning range of 10°–90°. The surface microstructure of Al4SiC4 crystals and cross-sectional oxidation microstructure of calcined MgO-C-Al4SiC4 specimens were analyzed using a scanning electron microscope (SEM, nova™ nano 450, FEI Company, Hillsboro, OR, USA) equipped with an energy dispersive spectrometer (EDS, EDAX-TEAM™, EDAX, Mahwah, NJ, USA).

3. Results and Discussion

3.1. Characterization of Synthesized Al4SiC4 Crystals

Large-scale yellow Al4SiC4 powder was obtained in argon at 1800 °C. The reason for the choice of this temperature is that lower synthesis temperatures would introduce SiC as an impurity phase while higher ones could lead to the decomposition of Al4SiC4 [24]. The corresponding XRD pattern of Al4SiC4 is given in Figure 1. From Figure 1, it can be seen that the peaks are sharp, and the main characteristic peaks correspond well to Al4SiC4 (PDF card no. 35–1072), indicating the synthesis of highly pure Al4SiC4. Meanwhile, it should be noted that the peak intensity at (0010, 2θ = 41.62°) of the synthesized sample is far higher than standard spectral line, which is attributed to the special orientation distribution in Al4SiC4 crystals. Figure 2 shows the micromorphology of the synthesized Al4SiC4. It can be observed that Al4SiC4 nuclei grow to form a hexagonal crystal structure that has smooth surfaces and intercrosses each other. The plate-like Al4SiC4 crystals have diameters of 10–30 μm and thickness of 2–3 μm.

3.2. Evolution of Al4SiC4 in the MgO-C-Al4SiC4 System

Carbon-buried oxidation experiments of the MgO-C-Al4SiC4 bricks carbon-buried at different temperatures were conducted in order to analyze the evolution of Al4SiC4 in the MgO-C-Al4SiC4 system, and the XRD patterns of oxidized products are illustrated in Figure 3. It can be seen that with increasing oxidation temperature, the relative intensity of the characteristic peaks of Al4SiC4 decreased. Meanwhile, a new MgAl2O4 phase appeared and its relative intensity of characteristic peaks gradually increased, indicating that the oxidation products of Al2O3 and MgO react with each other to generate MgAl2O4. Besides, a small amount of SiC peaks can also be detected at 1600 °C, suggesting the formation of SiC in MgO-C-Al4SiC4 at higher temperature. To get more knowledge of the phase composition, quantitative analysis using X-ray diffraction Rietveld refinement method in the TOPAS software was also carried out, and the results of different phase contents are given in Table 1. As can be seen, the phase amount of Al4SiC4 decreased while the MgAl2O4 content increased with increasing temperature.
Micromorphologies of Al4SiC4 oxidized at different temperature in MgO-C-Al4SiC4 system are depicted in Figure 4, in which middle parts (marked by a green point), two outer layers (marked by a red point), and outmost parts (marked by a blue point) represent unreacted Al4SiC4 crystals, oxide layer, and MgAl2O4 particles existing in the matrix, respectively. With the combination of EDS results at different areas, it can be seen that the oxidation of Al4SiC4 took place at all temperatures and the oxidation thickness increased with increasing temperature, suggesting more severe oxidation of Al4SiC4 in MgO-C system at higher temperature.
To further understand the oxidation process of Al4SiC4 crystals, the cross-section and element distribution of the samples after oxidation at 1600 °C was investigated, as shown in Figure 5. After reaction at 1600 °C, the oxidation of Al4SiC4 crystal was obvious. The outer oxide layers were mainly composed of Mg, Al, and O, while the internal crystals mainly consisted of C, Si, and a lesser amount of Al. This shows that Al4SiC4 is unstable, accompanied with a rapid migration of Al from interior to exterior. Compared with the Al element, the migration rate of Si is relatively smaller. Such migration behaviors led to the formation of Al2O3 and SiO2 on the surface of Al4SiC4. At the same time, some MgAl2O4 phase generated by Al2O3 and MgO also appeared on the Al4SiC4 surface due to abundant MgO in the MgO-C matrix. This can explain why no mullite was detected in XRD while massive MgAl2O4 particles exist on the surface of Al4SiC4.
The micromorphological evolution of MgAl2O4 in the MgO-C-Al4SiC4 system was also observed, and the results are shown in Figure 6. With increasing temperature, both the amount and grain size of MgAl2O4 increased. These MgAl2O4 particles not only increase the density of samples by filling in the pores, but also provide benefit to the improvement of slag corrosion and permeation resistance, which is in line with the previous report [22].

3.3. Thermodynamic Analysis of the Oxidation Process

Under carbon-buried conditions at high temperature, Al4SiC4 in the MgO-C-Al4SiC4 system is mainly confronted with CO, N2, and less Mg(g). When CO exists, Al4SiC4 is always oxidized according to the following equations:
Al 4 Si C 4 ( s ) + 6 CO ( g ) = 2 A l 2 O 3 ( s ) + SiC ( s ) + 9 C ( s )
3 Al 2 O 3 ( s )   +   2 SiC ( s )   +   4 CO ( g )   =   Al 6 Si 2 O 13 ( s ) + 6   C ( s )
To identify the possibility of the above reactions, the stability region of the solidification phase of Al4SiC4 according to thermodynamic data of Al4SiC4 and JANAF (Joint Army-Navy-NASA-Air Force) data under different CO partial pressure is depicted as shown in Figure 7 [25,26]. From Figure 7, it can be concluded that the product of Al4SiC4 is dependent on temperature and CO partial pressure. In addition, under the condition of low oxygen partial pressure, the reaction of MgO may also take place as follows:
2 MgO ( s )   =   2 Mg ( g )   +   O 2 ( g )
2 C ( s )   +   O 2 ( g )   =   2 CO ( s )
MgO ( s ) + C ( s ) = Mg ( g ) + CO ( g )
As is known to all, open and closed pores simultaneously exist in MgO-C bricks. The oxidation behavior of Al4SiC4 crystals at different positions may be different. To clarify this, oxidation experiments of Al4SiC4 under both open-pore and closed-pore systems were conducted.
Under an open-pore system, according to Equation (4), oxygen in the air accompanied with excessive carbon will mostly transfer to CO at temperature above 1000 °C. So, the total partial pressure of CO and N2 is close to 1 atm while the content of Mg(g) is negligible; i.e., PCO + PN2 = 105 Pa. Thus, the equilibrium gas composition is as follows:
φ ( CO ) = P C O P × 100 ,   φ ( N 2 ) = P N 2 P × 100
Besides,
n N n O = φ ( N 2 ) φ ( O 2 ) = 79 21 = 2 n N 2 2 n C O = 2 φ ( N 2 ) φ ( CO ) = 3.76
Combining Equations (6) and (7) obtains φ ( CO ) = 34.72 ; i.e., P C O P = 0.3472 .
From the red line in Figure 7, when the CO partial pressure was 0.3472, the abscissa was −0.46 and corresponding temperature was 1483 °C. Below 1483 °C, Al4SiC4 stayed in the Al6Si2O13-C phase area, indicating the entire transformation of Al4SiC4 to mullite. However, the characteristic peaks of mullite could not be detected in XRD (Figure 3), which is possibly attributed to two points. That is, (1) little amount or poor crystalline (glass phase) of mullite; (2) massive MgAl2O4 particles on the product surface weaken the intensity of mullite. When the temperature was above 1483 °C, phase equilibrium was in the Al2O3-SiC-C area and Al4SiC4 was oxidized to Al2O3 and SiC. Under this condition, the SiC was stable, suggesting the decrease of oxygen partial pressure above 1483 °C.
As for the closed pores in samples, they are a relative concept because the refractory itself is not a very dense material. With increasing temperature, the air in the pores will be replaced with CO(g) and CO2(g). In this situation, equilibrium gas composition is almost determined by specific material components. In such a closed system that is at high temperature with excessive carbon, the gas in the pores mainly originates according to Equations (3)–(5). In the gas composition, the content of oxygen is rather smaller than that of CO(g) and Mg(g).
Assuming the total pressure as P, one can obtain,
P C O + P Mg = P
Substituting Equation (8) into Equation (5) yields PCO/P = 0.50, and the corresponding abscissa is −0.301. At the same temperature, the CO partial pressure in closed pores is higher than that in open pores. Besides, Figure 7 also shows that the stable region of Al2O3 and SiO2 in closed pores was above 1516 °C, which is nearly same as the open system. Therefore, in closed pores, the oxidation products are Al2O3 and SiC at 1600 °C, while they are Al6Si2O13 and Al2O3 at 1400–1500 °C.
From the above, after oxidation at 1600 °C, SiC will be generated in both open and closed pores and can be detected in XRD. Meanwhile, at 1500 °C, SiC forms only in open pores, and lower temperature is not good for its formation, resulting in no characteristic peak shown in XRD.
In the MgO-C-Al4SiC4 system, open and closed pores have no obvious influence on the oxidation behavior of Al4SiC4, owing to little difference in CO partial pressure. Importantly, Al2O3 will have a priority to form on the Al4SiC4 surface and further react with MgO to generate MgAl2O4. As a result, MgAl2O4 can fill in the pores in materials, which provides a benefit to the improvement of the properties of MgO-C bricks.

4. Conclusions

Al4SiC4 crystals with diameter of 10–30 μm and thickness of 2–3 μm were synthesized at 1800 °C using Al, Si, and graphite powders as raw materials. The oxidation behavior and mechanism of Al4SiC4 in MgO-C-Al4SiC4 system was investigated. At high temperature, Al4SiC4 was unstable where the migration rate of Al element was greater than that of Si. Thus, at first, Al2O3 would form on the Al4SiC4 surface and then react with MgO existing in the matrix to generate massive MgAl2O4 particles. Si element exists either in the form of SiC within Al2O3 scale or in the form of Al6Al2O13 on the Al4SiC4 surface. Thermodynamic calculation was also carried out to further analyze the oxidation mechanism, verifying a slight influence of open and closed pores on the oxidation of Al4SiC4 in MgO-C materials.

Acknowledgments

The authors express their appreciation to the National Nature Science Foundation of China of No. 51572019 and the National Science Foundation for Excellent Young Scholars of China of No. 51522402.

Author Contributions

Huabai Yao and Jialin Sun conceived and designed the expeiments; Huabai Yao, Xinming Xing performed the experiments and analyzed the data; Enhui Wang and Bin Li contributed analysis tools. Huabai Yao, Junhong Chen and Xinmei Hou wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD pattern of Al4SiC4 powders and corresponding standard spectral line.
Figure 1. XRD pattern of Al4SiC4 powders and corresponding standard spectral line.
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Figure 2. SEM image of Al4SiC4 powders.
Figure 2. SEM image of Al4SiC4 powders.
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Figure 3. XRD patterns of the MgO-C-Al4SiC4 bricks reacted at different temperature: (a) 1400 °C; (b) 1500 °C; (c) 1600 °C.
Figure 3. XRD patterns of the MgO-C-Al4SiC4 bricks reacted at different temperature: (a) 1400 °C; (b) 1500 °C; (c) 1600 °C.
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Figure 4. Structure evolution of Al4SiC4 crystals at different temperature: (a) 1400 °C; (b) 1500 °C and (c) 1600 °C.
Figure 4. Structure evolution of Al4SiC4 crystals at different temperature: (a) 1400 °C; (b) 1500 °C and (c) 1600 °C.
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Figure 5. The energy dispersive spectrometry (EDS) mapping analysis of the cross-section oxide scale of Al4SiC4 crystal at 1600 °C. (a) the enlarged SEM image of the cross-section; (b)~(f) the distribution of respective element with different color.
Figure 5. The energy dispersive spectrometry (EDS) mapping analysis of the cross-section oxide scale of Al4SiC4 crystal at 1600 °C. (a) the enlarged SEM image of the cross-section; (b)~(f) the distribution of respective element with different color.
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Figure 6. Morphology of product MgAl2O4 in MgO-C-Al4SiC4 system reacted at different temperature: (a) 1400 °C; (b) 1500 °C and (c) 1600 °C.
Figure 6. Morphology of product MgAl2O4 in MgO-C-Al4SiC4 system reacted at different temperature: (a) 1400 °C; (b) 1500 °C and (c) 1600 °C.
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Figure 7. Stability region of solidification phase of Al4SiC4 oxidized under different conditions.
Figure 7. Stability region of solidification phase of Al4SiC4 oxidized under different conditions.
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Table 1. Phase compositions of the oxidized samples at different temperature calculated based on Rietveld.
Table 1. Phase compositions of the oxidized samples at different temperature calculated based on Rietveld.
TemperaturePhase Content (wt %)
MgOCAl4SiC4MgAl2O4SiC
1400 °C25.7810.448.2355.55
1500 °C4.2111.266.9477.59
1600 °C11.754.9577.725.58

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