Lanthanum hexaaluminate is a potential candidate for thermal barrier coatings due to its unique lamellar structure and excellent thermophysical properties. In this work, lanthanum hexaaluminate was prepared by a solid-state reaction synthesis at 1600 ^oC, and the effects of aluminum source type and molding method on the phase composition and microstructure of the powder were studied. It can be seen that the synthesis efficiency of alumina as aluminium source is higher than that of aluminium hydroxide. However, the flake structure is more obvious when aluminium hydroxide is used to synthesize aluminium hydroxide. In addition, the process of compacting green compact can effectively improve the synthesis efficiency of LaAl₁₁O₁₈, but it will also affect the formation and growth of grains. Consequently, a high yield of LaAl₁₁O₁₈ powder with a particle size of 3 μm and aspect ratio of 9.88 can be obtained by compacting aluminum hydroxide as the aluminum source

Keywords: Lanthanum hexaaluminate, Crystalline grain growth, Plate-like structure, Thermal barrier coatings

Hexaaluminate, as a new type of inorganic composite material, has been widely used in the nuclear industry, catalysis, electronics, superconductivity, new energy and other industries because of its special layer structure and good high-temperature performance. These materials exhibit a stable phase composition up to 1600 ^oC and exceptional resistance to sintering and thermal shock, which makes them attractive materials for several applications as ceramics, matrices for immobilization of radioactive elements, catalysts for high-temperature applications, superionic conductors, and luminescent and laser materials, among others [1-5]. From the crystal structure point of view, hexaaluminate is a hexagonal layered crystal formed by alternately stacking of mirrored spinel structural units and conductive mirror surface layers along the c-axis. It belongs to the hexagonal P63/mmc spatial group and is expressed in the general formula AAl₁₁O_17-x or AAl₁₂O_19-x. Among them, A is an alkali metal (such as Na, K, etc.), an alkaline earth metal (such as Mg, Ca, Sr, Ba, etc.), or a rare earth metal (Ln³⁺ = La, Pr, Nd, Nd, etc.), or large cations (such as Sm, Eu, Gd, etc.).
According to the ion radius, charge and number of large cations, the crystal structure of hexaaluminate is divided into β-Al₂O₃ (AAl₁₁O_17-x) and magnetite (AAl₁₂O_19-x) [4, 6]. Hexaaluminate is considered one of the most promising high-temperature materials due to its mosaicibility, excellent activity and high-tempera- ture performance, especially in the field of thermal barrier coatings [7, 8]. Compared with the traditional Yttrium oxide partially stabilized zirconium oxide (YSZ), hexaaluminate solves the defects of YSZ such as phase transition, higher modulus and higher ablation resistance [9]. For example, LaMgAl₁₁O₁₉/YSZ double-layer com- posite thermal barrier coating formed by the introduc- tion of Hexaaluminate exhibits better high-temperature strength and thermal shock resistance than traditional YSZ [10,11].
Among many hexaaluminate materials, LaAl₁₁O₁₈ has attracted much attention due to its high melting point, low thermal conductivity and low cost. In addition to being widely used in the preparation of fluorescent materials [12] and catalysts [13], it is also used to improve the mechanical properties of alumina ceramics because of its good compatibility with Al₂O₃ [14].
Guo et al. [15] found that adding an appropriate amount of LaAl₁₁O₁₈ into ZIRCONIA-TOUGHENED alumina ceramics (ZTA) can improve the fracture strength and fracture toughness of ZTA ceramics through crack bridging and crack deflection effects.
Negahdari et al. [16] effectively improves the fracture toughness, hardness and elastic modulus of alumina ceramics by in-situ formation of LaAl₁₁O₁₈ in Al₂O₃ ceramics. In addition, more and more preparation methods are being used to further improve the performance of LaAl₁₁O₁₈, such as the combustion synthesis method [17, 18], coprecipitation method [19], sol-gel method [20,21].
However, up to now, few reports have been reported on the effects of preparation methods and raw materials on the microstructures of LaAl₁₁O₁₈. For this purpose, LaAl₁₁O₁₈ was synthesized from Al₂O₃ and aluminum hydroxides using two forming processes. The effects of different aluminum sources and forming processes on the phase composition and morphology of LaAl₁₁O₁₈ powders were also investigated.

Experimental procedure
In this experiment, the effects of different preparation processes (mixed powder and green body) and different aluminum sources (α-Al₂O₃ and Al(OH)₃) on lanthanum hexaaluminate were investigated by two preparation strategies. The flow of the first method is as follows: first, the raw materials are weighed according to the formula shown in Table 1, and then fully blended in the star ball mill. A certain amount of the blended raw material powder is then batched in a corundum crucible and placed in a muffle oven at 1600 ^oC for 5 h. The second method differs from the first one by adding a molding process based on the first method, that is, the raw material is fully mixed, and then pressed into a cylindrical green billet with a diameter of 15 mm (20 MPa for 2 min), and then put into the furnace for heat treatment. To prevent the deterioration of lanthanum oxide absorbed by air from affecting the accuracy of experimental measurements, the lanthanum oxide is treated at 1100 ^oC for 5 h before weighing [22].
The starting raw materials, in this work, include α-Al₂O₃ (≥99.9%, micron-sized, Shanghai Aladdin Bio- chemical Technology Co., Ltd., Shanghai, China), Al(OH)₃ (analytical-grade purity, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China), and La₂O₃ (analytical-grade purity, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China).

Characterization
X‐ray powder diffraction (XRD, Bruker D8-Advance, Germany) was used to characterize the phase composition of powder samples, with a scan speed of 4 ^o·min⁻¹ (2θ = 15^o – 85^o). A scanning electron microscope (SEM, Hitachi S-4800, Japan) was applied to record the micromorphology of the samples, and energy dispersive spectroscopy (EDS) was used to qualitatively analyze the micro-constitution of the samples. In addition, the relative amounts of LaAlO₃ in different samples were calculated by equation (1) [23].



Where M_LaAlO3 is the relative amount of LaAlO₃; I_x is the intensity of a given peak of LaAlO₃ or LaAl₁₁O₁₈.

Fig. 1 shows the XRD patterns of different samples after being calcined at 1600 ^oC for 5 h, and their main crystalline phases are LaAl₁₁O₁₈, and their secondary crystalline phases are LaAlO₃ and α-Al₂O₃. Compared with different aluminum source samples (such as samples S-O-P or S-OH-P), the relative intensity of the diffraction peak of LaAl₁₁O₁₈ in the XRD spectrum of sample S-O-P synthesized from alumina is higher, which means that the target phase content is higher, as shown in Fig. 1(a). Compared with different forming process samples (such as samples S-O-P or S-O-G), the XRD spectra of sample S-O-G synthesized from green samples have a lower relative intensity of the diffraction peaks of the intermediate products (LaAlO₃ and α-Al₂O₃), which means that the reaction is more complete as shown in Fig. 1(b).
From the reaction mechanism, alumina first reacts with lanthanum oxide and form LaAlO₃, then continues to react with LaAlO₃ and form LaAl₁₁O₁₈ [24]. In addition, existing studies have shown that the direct use of MgAl₂O₄ as raw material for the synthesis of LaMgAl₁₁O₁₉ has a higher synthesis efficiency than that of MgO [25]. Therefore, samples synthesized from green compacts have shorter distances to achieve solid-state reactions at high temperatures by atom diffusion and require lower activation energy, so the synthesis efficiency is higher. In addition, although the decompo- sition of Al(OH)₃ results in high activity γ-Al₂O₃, however, because it can only participate in the formation of LaAlO₃ (γ-Al₂O₃ is converted to α-Al₂O₃), so it does not affect the reaction between LaAlO₃ and LaAl₁₁O₁₈. In addition, to quantify the synthesizing effect of different samples, the content of LaAlO₃ in different samples was further calculated, as shown in Fig. 2. By comparing the experimental results, it can be seen that the content of LaAlO₃ in molded sample with alumina as aluminum source is the least and the synthesis efficiency is the highest.
Fig. 3 presents the SEM images of different samples after being calcined at 1600 ^oC for 5 h. For samples S-O-P and S-O-G, their grains show two micron-scale structures, plate-like and granular, and sample S-O-G has no obvious flaking of sample S-O-P (Fig. 3(a) and (c)). Combining the EDS results shown in Table 2 with corresponding XRD patterns, it can be seen that the lamellar grains of sample S-O-P are LaAl₁₁O₁₈, and the granular grains are LaAlO₃, which is an unresponsive intermediate phase. The granular grains of sample S-O-G are still LaAl₁₁O₁₈. Similarly, comparing the SEM images of samples S-OH-P and S-OH-G, it is found that the grain size of sample S-OH-P synthesized directly from powders is smaller (~1 μm), the plate grain thickness is relatively thin. The grain development of sample S-OH-P synthesized from compacted billet is higher (~3 μm). In addition, the EDS results of points C and E indicate that all lamellar grains are LaAl₁₁O₁₈, as shown in Table 2.
Combined with the growth mechanism of LaAl₁₁O₁₈ crystal, there are two reasons why the samples exhibit different microstructures. Firstly, as mentioned earlier, the crystallization behavior of LaAl₁₁O₁₈ with plate structure is determined by its own crystal structure. LaAl₁₁O₁₈ belongs to the structure of magnetite lead ore and is characterized by a layer of lanthanum oxide and an aluminum spinel (γ-Al₂O₃) sandwiched between the layers of lanthanum oxide (La₂O₃). La₂O₃ layer is a crystallographic mirror structure, with γ-Al₂O₃ layer symmetrically mirrored on both sides. In the La₂O₃ layer, O^2- is closely packed in a hexagonal shape, and La³⁺ is in a compact stacked structure of O^2-. In such a structure, LaAl₁₁O₁₈ is more likely to grow perpendicular to the c-axis because the diffusion of oxygen ions is inhibited, while growth along the c-axis is inhibited [26]. Secondly, the lamellar structure of sample S-OH-G synthesized from Al(OH)₃ as the aluminum source is more evident, which may be due to the free growth of LaAl₁₁O₁₈ along the c-axis due to the space required for the grain orientation growth of LaAl₁₁O₁₈ left by the decomposition of Al(OH)₃ at high temperature [27, 28]. Similarly, when α-Al₂O₃ is used as the aluminum source, the plate shape of sample S-O-P synthesized directly from the powder is more obvious than that of sample S-O-G synthesized from the pressed green compact.
Furthermore, Fig. 4 shows the aspect ratios of different samples calculated by statistics. The aspect ratio of LaAl₁₁O₁₈ powders synthesized with Al(OH)₃ as the aluminum source is larger. Considering the degree of crystallization, sample S-OH-G synthesized from com- pacted raw billet with Al(OH)₃ as the source of aluminum is the best choice for obtaining good micro-morphology.

In this work, LaAl₁₁O₁₈ powders were synthesized by solid-state reaction at 1600 ^oC for 5 h, and the influence of aluminum source and forming process on the phase composition and micro-morphology of the prepared LaAl₁₁O₁₈ powders was investigated. Based on the above results and analysis, the following conclusions can be drawn.
From the synthesizing efficiency, alumina is more efficient as the aluminum source than aluminum hydroxide, and the compacted billet is more efficient than the powder. Specifically, using alumina as the aluminum source, the content of the LaAlO₃ inter- mediate phase in the synthetic powder can be reduced from 63.9% (sample S-OH-P) to 43.7% (sample S-O-P). The content of the LaAlO₃ intermediate phase in synthetic powder can be further reduced from 43.7% to 4.5% (sample S-O-G) by green compaction.
From the grain morphology, it is more obvious to use aluminum hydroxide as the aluminum source for the flaking, and the compaction of the raw billet can inhibit the grain orientation growth. Generally speaking, aluminum hydroxide plus green compaction is the best choice. The grains of LaAl₁₁O₁₈ powder synthesized are fully lamellar with an aspect ratio of 9.88.

The authors declare that they have no competing interests.