In situ reaction mechanism of MgAlON in Al–Al2O3–MgO composites at 1700°C under flowing N2

The Al–Al2O3–MgO composites with added aluminum contents of approximately 0wt%, 5wt%, and 10wt%, named as M1, M2, and M3, respectively, were prepared at 1700°C for 5 h under a flowing N2 atmosphere using the reaction sintering method. After sintering, the Al–Al2O3–MgO composites were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that specimen M1 was composed of MgO and MgAl2O4. Compared with specimen M1, specimens M2 and M3 possessed MgAlON, and its production increased with increasing aluminum addition. Under an N2 atmosphere, MgO, Al2O3, and Al in the matrix of specimens M2 and M3 reacted to form MgAlON and AlN-polytypoids, which combined the particles and the matrix together and imparted the Al–Al2O3–MgO composites with a dense structure. The mechanism of MgAlON synthesis is described as follows. Under an N2 atmosphere, the partial pressure of oxygen is quite low; thus, when the Al–Al2O3–MgO composites were soaked at 580°C for an extended period, aluminum metal was transformed into AlN. With increasing temperature, Al2O3 diffused into AlN crystal lattices and formed AlN-polytypoids; however, MgO reacted with Al2O3 to form MgAl2O4. When the temperature was greater than (1640 ± 10)°C, AlN diffused into Al2O3 and formed spinel-structured AlON. In situ MgAlON was acquired through a solid-solution reaction between AlON and MgAl2O4 at high temperatures because of their similar spinel structures.


Introduction
Magnesia-based refractories with good resistance to alkali slag and high-ferric slag possesses poor resistance to thermal shock and slag penetration, which restrains their development and broad application [1][2][3]. The most effective method to solve this problem is to composite the oxide with non-oxides. The resulting composites typically have high hot strength, good thermal shock resistance, and excellent corrosion resistance to metallurgical slag [4][5][6][7]. In the 1970s, the Japanese introduced graphite into magnesia and prepared magnesia-carbon refractories, which prolonged the service life of refractories for electric furnaces and converters to ten times the service life of the original oxide refractories [8]. However, carbon easily solves into molten steel, resulting in the steel's carburization [9]. In addition, the use of phenolic resin or an organic binder leads to a series of environmental problems. Thus, researchers began to investi-gate non-oxide-oxide systems without carbon.
In 1959, Yamguchi and Yanagida [10] reported the possibility of a spinel-type phase in the Al 2 O 3 -AlN system. Over the next several years, other researchers confirmed that a spinel-type phase named aluminum oxynitride (AlON) indeed exists in this system [11][12][13][14]. As a new ceramic material, AlON offers researchers additional options in oxide-non-oxide systems [15][16][17]. It has great potential as a carbon-free, environmentally friendly refractory. AlON has two crystal structures [15,18]: wurtzite-structured AlN-polytypoids and spinel-structured AlON. However, AlON is only stable at (1640 ± 10)°C or higher temperatures and under a certain oxygen partial pressure and nitrogen partial pressure [19]. Because many of the foreseen applications of AlON correspond to this temperature region, this instability poses serious drawbacks on the actual applicability of the AlON non-oxide system. Therefore, extensive efforts have been devoted to making AlON stable at lower temperatures, leading to the discovery that the addition of MgO or MgAl 2 O 4 could lower the stable temperature to 1640°C; the newly formed solid-solution is named magnesium aluminum oxynitride spinel (MgAlON) [20].
MgAlON has been reported to exist at both room temperatures and high temperatures [21] and to possess excellent slag resistance and thermal shock resistance. MgAlON composites have therefore become the subject of intensive research interest. Yang et al. [22] studied MgAlON-bonded magnesia and spinel composites. Pichlbauer et al. [20] synthesized MgAlON-bonded magnesia refractories at 1800°C using MgO as the aggregate and AlN-MgO-Al 2 O 3 mixing powder as the matrix. The common synthesis methods mainly adopt AlN as the starting material [23][24], which requires harsh processing conditions, restricting the industrial application of MgAlON-bonded magnesia refractories. The issue of how to prepare MgAlON at lower temperatures has become a key point. Therefore, in this work, metal aluminum powder was introduced into the Al 2 O 3 -MgO system and MgAlON-bonded magnesia refractories were obtained at 1700°C. The composites after calcination were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to investigate the formation mechanism of MgAlON.

Experimental section
Metal aluminum powder, tabular alumina, α-Al 2 O 3 micropowder, fused magnesite micropowder, and sintered magnesia powder were used as the raw materials, and ther-mosetting phenolic resin was used as the binder. The ratio of aggregate to matrix was 65:35, and the formulations of M 1 , M 2 , and M 3 are specified in Table 1. The batched materials were mixed in a mixer for 30 min and shaped under 300 MPa by a hydraulic machine into specimens of 40 mm × 40 mm × 125 mm. After drying at 300°C for 24 h, three specimens (M 1 , M 2 , and M 3 ) were placed in a crucible and heat treated. The heat treatment was conducted at 580°C for 8 h and then 1700°C for 5 h in a graphite tube furnace under flowing nitrogen. The matrix composition based on the AlN-Al 2 O 3 -MgO ternary phase diagram plotted by Willem et al. [17] is shown in Table 2, and the chemical composition of the raw materials is shown in Table 3. XRD, SEM, and EDS were used to analyze and characterize the composites after calcination. XRD analyses were conducted using an X-ray diffractometer (PANalytical, X'pert powder; working voltage: 40 kV; current: 40 mA) employing Cu K α radiation; the sample were scanned in the 2θ range from 10° to 90° in steps of 0.013° with a scanning time of 2 min. SEM analysis was performed using a scanning electron microscope (FEI, X-3500N). The marked areas in Fig. 1 are enlarged in Fig. 2. These enlargements reveal that, compared with the characteristic peaks of the spinel phase in the XRD pattern of specimen M 1 , those in the XRD patterns of specimens M 2 and M 3 shift toward larger angles and their diffraction intensity increases with increasing aluminum addition. Therefore, the spinel phase in specimens M 2 and M 3 is deduced to be MgAlON for the following reasons. First, MgAlON can exist above 1400°C under flowing nitrogen with a low oxygen partial pressure [19]. Second, it has smaller crystal lattice parameters than MgAl 2 O 4 and its diffraction peaks shift toward those of MgAl 2 O 4 . We also observed that the content of MgAlON increases with increasing aluminum addition. Cannard et al. [24] noted that AlN can accommodate MgO at high temperatures. Thus, aluminum in specimens M 2 and M 3 are completely nitridized into AlN, which then further reacts with Al 2 O 3 and MgO to form MgAlON and AlN-polytypoids (containing some MgO) after being soaked at 580°C and fired at 1700°C.   2. XRD patterns of the marked areas (a, b, c) in Fig. 1. Fig. 4. The MgAlON phase is well known to form as octahedral particles under good crystal growth conditions.

Reaction mechanism
The melting point of aluminum is 660°C. When temperatures are higher than 660°C, the nitridation reaction of Al is restrained. Thus, soaking at 580°C for 8 h was carried out to allow more aluminum be nitridized into AlN, avoiding the transformation to Al(g), Al 2 O(g), or Al 2 O 3 at high temperatures [25], which will influence the synthesis of MgAlON or AlN-polytypoids. Therefore, after the soaking stage at 580°C for 8 h, aluminum is nitridized into AlN and the original Al-Al 2 O 3 -MgO system transforms to an AlN-Al 2 O 3 -MgO system. As temperature rises, Al 2 O 3 diffuses and migrates into AlN crystal lattices, forming AlN-polytypoids such as 12H, 21R, and 27R [26]. The reaction equation can be expressed as AlN 2 3 Al Al Al O 2Al + 3ON V      (· represents 1 unit positive charge, ′ represents 1 negative charge). When the temperature rises higher, AlN diffuses and migrates into Al 2 O 3 , forming γ-AlON; the reaction equation in this case is expressed as 2 3 Al O  Fig. 1, SEM images in Fig. 3, and the EDS results in Table 4. In Figs. 3(d) and 3(f), some cyclic structures are observed, which are deduced to be aluminum metal. Aluminum powder easily aggregates because of its small particle size and high surface energy. This aggregation leads to an incomplete nitridation at 580°C. At higher temperatures, Al(s) transforms into Al(g) and Al 2 O(g) and escapes.

Conclusions
In this work, Al-Al 2 O 3 -MgO composites (MgAlON as a bonding phase) were prepared at 1700°C for 5 h under a nitrogen atmosphere using fused magnesia, α-Al 2 O 3 , tabular alumina, metal aluminum, and sintered magnesia as raw materials. The composites after calcination were characterized and analyzed by XRD, SEM, and EDS. The results are summarized as follows: (1) Specimen M 1 was composed of MgO and MgAl 2 O 4 . Compared with specimen M 1 , specimens M 2 and M 3 contained MgAlON, whose content increased with increasing aluminum addition.
(2) Under a nitrogen atmosphere, MgO, Al 2 O 3 , and Al in the matrix of specimens M 2 and M 3 reacted to form MgA-lON and AlN-polytypoids, which combined the particles and the matrix together and imparted the composite with a dense structure.
(3) The mechanism of MgAlON synthesis is described as follows. Under an N 2 atmosphere, the partial pressure of oxygen is quite low; thus, when the Al-Al 2 O 3 -MgO system was soaked at 580°C for an extended period, aluminum metal was transformed into AlN. With increasing temperature, Al 2 O 3 diffused into AlN crystal lattices and formed AlN-polytypoids; by contrast, MgO reacted with Al 2 O 3 to form MgAl 2 O 4 . When the temperature was greater than (1640 ± 10)°C, AlN diffused into Al 2 O 3 and formed spi-nel-structured AlON. In situ MgAlON was then acquired through a solid-solution reaction between AlON and MgAl 2 O 4 at high temperatures because of their similar spinel structures.
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