Synthesis of Al4SiC4 powders via carbothermic reduction: Reaction and grain growth mechanisms

Highly pure Al4SiC4 powders were prepared by carbothermic reduction at 2173 K using Al2O3, SiO2, and graphite as raw materials. The obtained Al4SiC4 powders owned hexagonal plate-like grains with a diameter of about 200–300 μm and a thickness of about 2–6 μm. Based on the experimental results, the reaction of Al4SiC4 formation and grain evolution mechanisms were determined from thermodynamic and first-principles calculations. The results indicated that the synthesis of Al4SiC4 by the carbothermic reduction consisted of two parts, i.e., solid–solid reactions initially followed by complex gas–solid and gas–gas reactions. The grain growth mechanism of Al4SiC4 featured a two-dimensional nucleation and growth mechanism. The gas phases formed during the sintering process favored the preferential grain growth of (0010) and (1 ī0) planes resulting in formation of hexagonal plate-like Al4SiC4 grains.


Introduction
Silicon-based non-oxide ceramic materials such as SiC and Si 3 N 4 are widely used for high temperature applications due to their excellent high temperature properties. Exposing those silicon contained ceramics to oxygen containing atmosphere at high temperature leads to formation of a protective SiO 2 oxidation scale which can sufficiently prevent the further oxidation inwards [1,2]. However, the formed SiO 2 protective scale will be degraded significantly under certain atmospheric vapors as alkali vapor and/or water vapor due to the formation of volatile Si(OH) 4 phase [3]. For example, the oxidation kinetics of SiC ceramics increased with enhanced water vapor content and the oxidation rates were approximately one order of magnitude higher than the one observed in the dry oxygen in the 1200-1400 ℃ temperature range [4]. Thus the application of the silicon-based non-oxide ceramic materials under severe conditions is challenged.
Ternary carbide Al 4 SiC 4 has received extensive attention because of its salient properties such as high melting point (≈ 2080 ℃), low density (3.03 g/cm 3 ), relative high toughness (3.98±0.05 MPa·m 1/2 ), high compressive strength (260 GPa) as well as remarkable  resistance to oxidation and hydration at high temperature [5][6][7][8][9]. It is worth noting that under high temperature oxidation condition, a dual protective layer consisting of inner mullite and outer alumina is formed on the Al 4 SiC 4 surface which can improve the oxidation resistance even in water vapor atmosphere [5,9,10]. However, the application of Al 4 SiC 4 is confined due to the difficulty in fabricating highly pure samples. Hence, most of reported research work focused on the synthesis of this ternary ceramic. There are mainly three kinds of powder mixtures used to synthesize the Al 4 SiC 4 powders or bulk, i.e., (1) using aluminum, silicon, and carbon powders as raw materials [11][12][13], (2) using Al 4 C 3 and SiC powders as raw materials [14,15], and (3) using Al 2 O 3 , SiO 2 , and C as raw materials by carbothermic reduction [16][17][18][19][20]. The former two reactant mixtures require expensive and active metal powders or carbides as starting materials. In addition, Al 4 C 3 powders are also very easy to hydrate which often leads to sample pulverization [21]. In recent years, carbothermic reduction has become a promising synthesis route for high temperature ceramics because of its low requirements on starting materials. Both industrial raw materials such as commercial alumina and quartz and natural minerals such as sintered bauxite and kaolin can be adopted as starting materials, leading to great industrialization potential [19,20,[22][23][24][25]. Some attempts have been done to synthesize highly pure Al 4 SiC 4 by carbothermic reduction [17]. Furthermore, it has been found that the synthesized Al 4 SiC 4 powders through carbothermic reduction usually have plate-like grains [16,17]. For the reaction mechanism, Lee et al. [17,18] reported that gas-solid reactions occur in the initial stage and gas-gas reactions in the latter stage by carbothermic reduction by using Al(OH) 3 , SiO 2 , and phenolic resin as starting materials. However, Yu et al. [20] argued that some solid-liquid reactions also occur as well during the synthesis and proposed that the metal phases such as Al and Si with low melting points are formed and wrap the Al 4 SiC 4 particles, which accelerate the nucleation of Al 4 SiC 4 grains. Hence, the reaction mechanism for synthesis of Al 4 SiC 4 is still unclear. Furthermore, the gas phase formation during carbothermic reduction may play important roles in grain growth of Al 4 SiC 4 . However, limited work has been done to investigate the microstructural evolution of Al 4 SiC 4 phase during the synthesis process by carbothermic reduction.
In the present work, highly pure Al 4 SiC 4 powders were prepared by carbothermic reduction. The reaction mechanism was proposed in combination of experimental results and the thermodynamic calculation. Based on the microstructure analysis, the growth mechanism of the hexagonal plate-like Al 4 SiC 4 grains was further evaluated using first-principles calculation.

Experimental details
Powders of analytically pure alumina (α-Al 2 O 3 > 98.1%, average grain size: 10 μm), analytically pure silica (quartz > 98.8%, average grain size: 25 μm), and graphite (carbon content > 99.0%, average grain size: 30 μm) were purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China, and adopted as raw materials. The raw materials were mixed according to the stoichiometric mole ratio of Al 2 O 3 :SiO 2 :C = 2:1:12 (the ratio is based on the overall reaction: +12C=Al SiC +8CO ) and ball milled for 24 h on a planetary ball mill with alumina grinding-balls using ethanol as the dispersive medium. This process is in favor of the uniform mix of the raw materials. Then the mixture was dried and cold compacted into a cylinder with 25 mm in diameter under a pressure of 30 MPa. The cylinder samples were placed into a graphite crucible with a cover and calcined at 2173 K for 8 h with a heating rate of 10 K/min in a graphite electric furnace. The furnace was vacuumed in advance and then highly pure Ar gas (purity ≥ 99.995%) was introduced into the furnace at the flow rate of 0.2 L/min. Finally, the furnace was cooled naturally to room temperature and the yellow product was obtained. The phases were identified by X-ray diffraction (XRD; D8 Advance, Bruker, Germany). The morphology and composition of the product were examined by scanning electron microscopy (SEM; Nova Nano 450, FEI, USA) with energy dispersive spectrometer (EDS; TEAM, EDAX, USA). The structure was further characterized using transmission electron microscopy (TEM; JEM-2010, JEC, Japan) with the selected area electron diffraction (SAED).
The surface energy of Al 4 SiC 4 grains at (0010) and (110) planes was calculated by first-principles calculation (FPC) based on the density functional theory (DFT). Firstly, the unit cell of Al 4 SiC 4 and the slab models of the crystal planes with a space layer thickness of 15 Å were established by Materials Studio 7.0 software. Then the surface energy of the crystal was calculated by Cambridge Serial Total Energy Package module (CASTEP) [26]. For the calculation, the ultrasoft pseudopotential based on general gradient approximation (GGA) was adopted [27]. The cut-off energy of the plane wave ( cut E ) was 450 eV. The sampling k-point mesh in the first irreducible Brillouin zone was generated by Monkhorst-Pack method, using a 6×6×1 grid for each structure. The convergence accuracy of the iteration was 5×10 6 per atom. The whole energy calculation was conducted in the reciprocal space.

1 Phase and microstructure characterization
The XRD pattern of the obtained product via carbothermic reduction of Al 2 O 3 /SiO 2 /C powder mixture at 2173 K for 8 h is shown in Fig. 1. Most of the diffraction peaks is consistent with the standard card of Al 4 SiC 4 (PDF No. 35-1072) and the relative X-ray intensity ratio of the (0010) and (1 10) planes ( (0010) (1 10) / I I ) of Al 4 SiC 4 is much higher than the standard PDF card, indicating a preferential grain growth orientation [28]. In addition, the presence of a small amount of 4H-SiC and carbon in the final product may be due to the intermediate phase formation during the carbothermic reduction process and residual carbon powders from the raw materials, respectively.
The SEM images of the synthesized product shown in Fig Figure 3(a) shows a TEM image of one Al 4 SiC 4 particle and the corresponding SAED pattern pattern is shown in Fig. 3(b). The SAED pattern was obtained by tilting the specimen into the respective zone axis (ZA) in independent alignment experiments. The SAED pattern in Fig. 3 (1 10) facet [29]. Since the Al 4 SiC 4 grains grew into hexagonal plate eventually, it is proposed that the grown rate of plane (0010) is slower than the one of plane (1 10) during the carbothermic reduction process.
For the grain morphology, the Al 4 SiC 4 grains with several kinds of morphologies are found in the crucible, which may mean the different growth mechanisms of Al 4 SiC 4 during the growth process (see Fig. 4). Figure  4(a) shows the morphology of Al 4 SiC 4 grain which has a smooth edge. Several shallow growing striations are clearly present on the surface of the grain. With the growth proceeding, grain growth along the (1 10) plane became dominated showing a step-like growth ( Fig. 4(b)). The height of each stacked grain is about 2 μm as shown in the inset of Fig. 4(b). Eventually, the development of grain becomes complete because of the presence of straight grain edges and the sharp grain corners (Fig. 4(c)) which exist as the dominant morphology of Al 4 SiC 4 in the final product. Only when the edge of the original Al 4 SiC 4 grain becomes thicker, the more reactant gases will deposit on the lateral (1 10) plane. Then the edge of the grain will present the hexagonal crystal growth habit, which makes the edge change from smooth to straight. Such kind grain evolution will be further discussed using 2D nucleation and growth mechanism in the later part.

2 Reaction mechanism
It should be pointed out that some golden isolated plate-like grains were observed on the cover and wall of the crucible after the calcination of the powder mixture at 2173 K for 8 h. Those grains were much larger than the internal grains. This may be attributed to the grain development derived from the solid-gas reactions or gas-gas reactions during the grain growth [30]. Hence the gas phase formation during the carbothermic reduction process was determined by thermodynamic calculation.
Concerning the thermodynamic calculation, systems of Si-O-C and Al-O-C are applied for the present study, which are considered as the possible reactions involving the synthesis of Al 4 SiC 4 phase by carbothermic reduction. In the Si-O-C system, the possible solid phases are SiC and SiO 2 . The reaction between SiO 2 and C is following [16]: Equation (1) can reach thermodynamic equilibrium at the experimental temperature (2173 K) by adjusting the CO(g) partial pressure, i.e., r ( , )=0.
G P T  Thus, the relationship between the reaction equilibrium constant of Eq. (1) and the CO(g) partial pressure at 2173 K can be calculated as following: In the Si-O-C system, the possible gases are Si(g), Si 2 (g), Si 3 (g), SiO(g), SiO 2 (g), and Si 2 C(g). The related reactions and constants are listed in Table 1.
For the Al-O-C system, the possible solid phases are Al 2 O 3 , Al 4 O 4 C, and Al 4 C 3 . The reaction between Al 2 O 3 and C is following [16][17][18]  Al O C(s)+6C(s)=Al C (s)+4CO(g) In the Al-O-C system, the possible gases are Al(g), Al 2 O(g), AlO(g), and Al 2 O 2 (g). The related reactions and constants are listed in Table 2. Figure 5 shows the equilibrium partial pressures of all gases formed in the Al-Si-C-O system at 2173 K determined by thermodynamic calculation using FactSage 7.0 thermodynamic calculation software. At this temperature, the gas phases of SiO(g), Al(g), and Al 2 O(g) have relatively high partial pressures and were considered as the main reactant gases during the carbothermic reduction process.
For the reduction of SiO 2 phase, it will be reduced firstly and large amount of SiO(g) can be generated during this process. At the same time, the generated SiO(g) can immediately react with carbon and CO(g) to form SiC according to the following reactions [17,18]: SiO(g)+3CO(g)=SiC(s)+3CO (g) (5) However, when SiC solid phase is formed on the graphite, Eq. (4) will be hindered or become slow because the diffusion rate of the solid carbon and SiO(g) is reduced significantly due to the new formed SiC solid phase. Then, the reaction in Eq. (5) becomes dominated for the further producing of SiC phase. Therefore, the possible reaction route for synthesis of SiC can be ascribed to a gas-solid reaction and a gas-gas reaction.
Concerning the reduction of Al 2 O 3 phase, it was considered as the main source of the Al 4 C 3 and Al-containing intermediate phase. However, another intermediate phase (Al 4 O 4 C) is also observed during the conversion of Al 2 O 3 into Al 4 C 3 phase (Fig. 5). During the reduction reaction of Al 4 O 4 C, a large amount of Al(g) and Al 2 O(g) will be generated [31,32]. These Al-containing gases can react with carbon to form Al 4 C 3 as follows: Table 2 Reactions and related thermodynamic data in the AlOC system at 2173 K

Equation
Reaction constant at 2173 K 4 3 Al C (s)=4Al(g)+3C(s) Al O (s)+C(s)=2AlO(g)+CO(g) θ θ AlO(g) CO(g) 2log( / )+log( / )= 12.17 P P P P  2Al O(g)+5C(s)=Al C (s)+2CO(g) According to the previous study [33], the Al 4 SiC 4 can be formed by the solid reaction between SiC and Al 4 C 3 (Eq. (8)): 4 3 4 4 Al C (s)+SiC(s)=Al SiC (s) In addition to the reaction in Eq. (8), Al 4 SiC 4 may also be precipitated by the gas-gas reactions between Si and Al contained gas phases such as SiO(g), Al(g), and Al 2 O(g), etc. Although SiO 2 has been reduced into SiC phase, the gas phases of SiO(g) and Al 2 O(g) may also be generated through Eq. (9) [34]: 2 3 2 Al O (s)+SiC(s)=Al O(g)+SiO(g)+CO(g) (9) Because of the relative high heating rate, the main reactant gases can participate in the following gas-solid reactions (Eqs. (10) and (11)) for the formation of Al 4 SiC 4 during the reducing reaction: 4 4 4Al(g)+SiO(g)+5C(s)=Al SiC (s)+CO(g) 2Al O(g)+SiO(g)+7C(s)=Al SiC (s)+3CO(g) (11) However, Eqs. (10) and (11) may only occur at the initial stage to form Al 4 SiC 4 nucleus since the reaction rate will be decreased quickly because of the presence of solid Al 4 SiC 4 covering on the carbon surface. Therefore, another possible reaction route for the formation of Al 4 SiC 4 during the carbothermic reduction can be subjected to following gas-gas reactions (Eqs. (12) and (13)): 2Al O(g)+SiO(g)+11CO(g)=Al SiC (s)+7CO (g) (13) Hence, based on the thermodynamic calculation in the Al-Si-O-C system, the formation of Al 4 SiC 4 during the carbothermic reduction process was not confined to the solid-solid reaction but also from the gas-solid reactions and gas-gas reactions where some Al and Si containing intermediate gases such as Al(g), Al 2 O(g), and SiO(g) were involved in the reaction process.

3 Grain growth mechanism
As shown in Fig. 6, Al 4 SiC 4 owns a P6 3 mc space group and a hexagonal crystal structure which can be described as layered structure stacked with the 4H-SiC type units and Al 4 C 3 type units alternative along the c-axis [35,36]. It has been reported that the development of the hexagonal grains depends on the grain growing rate of (0010) and (1 10) planes which is closely related to the surface energy. Thus, the surface energy of (0010) and (1 10) Here, surf E is the surface energy, slab E is the total energy of the slab per unit cell, bulk E is the energy per atom in an infinite crystal, n is the number of atoms in the slab unit cell, and A is the specific surface area of the slab unit cell.
The obtained surface energy along plane (0010) and plane (1 10) by FPC (Table 3) indicates that plane (0010) of Al 4 SiC 4 possesses much higher surface energy which is almost two times that of plane (1 10) . According to the Gibbs-Wulff theory, the plane with higher energy grows faster and eventually disappears   during growth [39]. Then the low surface energy plane will compose the main surface of the crystal. Based on this mechanism, a hexagonal prismatic Al 4 SiC 4 shall be obtained. However, the obtained Al 4 SiC 4 crystals in our work are more plate-like shape instead of hexagonal prismatic shape. Therefore the growth of plate-like Al 4 SiC 4 grain might be controlled by two-dimensional (2D) nucleation and growth mechanism [40]. The 2D nucleation and growth includes two different steps: one is the discontinuous 2D nucleation and the other is the continuous side expansion of the planes. The growth schematic diagram of plate-like Al 4 SiC 4 grain is illustrated in Fig. 7. In the beginning, the original graphite and some solid carbides formed through the solid-solid reactions or solid-gas reactions are worked as the substrates where the reactant gases such as SiO(g), Al(g), and Al 2 O(g) generated in this system can be absorbed on it. At the atom sites where the 2D nucleation barrier is low, some generated gas molecules can form short lived unions and have the chance to form embryos with the size equal to or larger than the critical nucleus size. These embryos then become the nucleus of the 2D growth of Al 4 SiC 4 . Subsequently the reactant gases continue to collide and be absorbed on the surface. When diffusing on the perfect area of the surface, the absorbed gases have weak combination with surface and exist extra broken bonds. Only at the kink or step sites, the gases can form stable bonds with surface atom, become more stable and favor the lateral growth and spreading. Once one layer of flat grain surface is achieved, a new grain nucleus will be formed on the incomplete layers as well as at old flat grain surface [41]. And such a kind of reactions between the substrates and gases will occur repeatedly during the carbonthermic reduction. Hence, the growth of the Al 4 SiC 4 grain is not continuous which also explains the growth striations and the stacking layer-by-layer structure formation shown in Fig. 4.
According previous studies [42,43], for the two-dimensional nucleation and growth of the hexagonal plate-like microstructure, a high gas supersaturation that reaches the requirement of c crit ( / ) p p of both the high-energy planes and the low-energy planes is required. Since large amounts of gas phases such as SiO(g), Al(g), and Al 2 O(g) can be generated during the carbothermic reduction process in this work, they could ensure sufficient gas supersaturation in this system that required for 2D nucleation and growth of both the high-energy plane (0010) and the low-energy plane (1 10) . The high supersaturation vapor pressure will ensure enough gases attached on plane (1 10) which favors the 2D nucleation and growth of the low-energy plane while the growth at high-energy plane (0010) has already been completed leading to formation of plate-like Al 4 SiC 4 grains as shown in Figs. 4(b) and 4(c).

Conclusions
Al 4 SiC 4 powders with hexagonal plate-like morphology were successfully prepared through carbothermic reduction. Based on the product morphology and grain surface growth characteristics, it is deduced that such gases as SiO(g), Al(g), Al 2 O(g), and CO(g) are crucial intermediates during the synthesis process. For the grain development, it follows the two-dimensional nucleation and growth theory which is closely related to the surface energy and gas supersaturation partial pressure.