Recent progress and challenges of transparent AlON ceramics
Ceramics and their composites have been widely investigated for various applications due to their unique chemical and physics properties [1−7]. Among them, transparent ceramics have an extensive application in the business domain and the military industries due to their outstanding optical, physical, and mechanical properties [8−10]. Among the transparent ceramics, transparent aluminum oxynitride (AlON) ceramics have been considered as one of the most important ceramics in the domes, infrared and visible windows, and transparent armors, etc [11−13]. Compared with the single crystal sapphire, which is well-known as the hardest transparent ceramics, the polycrystalline AlON ceramics have similar characteristics on strength, hardness, and optical properties, but offer more flexibility in size and shape [14,15]. Therefore, AlON ceramics have attracted a growing investigation. γ-AlON is a solid solution of Al₂O₃ and AlN [16,17]. Many methods have been explored to prepare AlON powder or AlON ceramics, such as solid-state reaction , carbonization method for Al₂O₃[19,20], chemical vapor deposition , sol−gel method [22,23], and solution combustion synthesis . The band gap of AlON was measured to be 6.2 eV . TU et al  employed a first-principles density functional theory (DFT) to study the on-site preference of Al vacancy and Natoms in γ-AlON. The band gap and bulk modulus structural model of γ-AlON, as the local structure of the Al₂₃O₂₇N₅ were calculated to be 3.99 eV of N atoms and Al vacancies in γ-AlON is not and 200.9 GPa, respectively. Given a wide band clear. The properties of γ-AlON are displayed in gap together with low photon energy and high Table 1 .
Termal stability, AlON ceramic has been explored as a phosphor matrix. As an upconversion photo- luminescence (UCPL) phosphor, AlON can be doped with various rare earth elements, such as Eu₂+ , Yb₃+, Tm₃+ , and Ce₃+ . Recently, the glasses based on the low-cost AlON combined with the 4-dimethyl-amino-N-methyl-4- stilbazoliumtosylate (DAST) layer  and a VO₂ thin film on the transparent AlON  were found to have potential application in smart windows. The ALON(5)−DAST(90)−ALON(5) outperforms industry-standard commercial window glasses with the remarks of cheapest, lightweight, and toughest . Furthermore, the Ti6Al4V was successfully prepared on the AlON ceramic through an active element brazing method, and the composite exhibited outstanding mechanical properties . It should be noted that new sintering additives of H₃BO₃  and earth elements (Sc, La, Pr, Sm, Gd, Dy, Er, and Yb)  were different from the conventional Y₂O₃, La₂O₃, and MgO. Although the improved methods, new sintering additives, more complex earth elements doping and fresh explored application, etc, have been extensively developed, a systematic, targeted and up-to-date summary is still lacking [11,13,36,37]. Furthermore, some unsolved problems and new challenges of the AlON ceramics hinder their commercial promotion and application. Therefore, this article takes the latest and critical review of transparent AlON ceramics in terms of preparation methods, sintering additives, sintering technologies, the challenges and development prospects.
Table 1 Properties of γ-AlON 
|Poisson ratio, μ||0.24|
|Thermal conductivity/(W·m−1 ·K−1 )||12.6|
|Refractive index (Λ=4.0 μm)/%||1.66|
AlON has a cubic spinel structure with a space group of Fd3m [38,39]. As shown in Fig. 1 , N and O atoms are situated at the 32e sites, and Al atoms are located at the 16d and 8a sites. Based on experimental results and theoretical
In 1964, the first phase diagram of binaryAl₂O₃−AlN composition was published by LEJUS . Then, MCCAULEY et al [44,45] reported a more complete phase equilibrium diagram of the pseudo-binary Al₂O₃−AlN composition under the flowing nitrogen at 1.013×105 Pa, as shown in Fig. 2 . Besides the experimental determination, calculations, the constant anion structure model of AlON could be described by the formula of Al(64+x)/3V(8−x)/3O32−xNx, where 2≤x≤5 [39−42]. However, it is difficult to confirm the sensible some researchers have tried to calculate the AlON stability region and the pseudo-binary Al₂O₃−AlN system based on the experimental data and the available thermodynamic data of the phase equilibrium diagram [36,46−49]. However, the phase segregation happening in the experiments is still unable to be modified because of the less experimental information.
It is well known that ceramics feature grains, grain boundaries, and porosity, etc (Fig. 3)[50,51]. As mentioned before, AlON ceramics possess an isotropic cubic lattice structure, which is one of the significant reasons that they can be optically transparent. Among the light-scattering sources, the porosity is the most important factor to determine whether ceramics can be transparent or not. The minimizing porosity should be greater than 99.9% of theoretical density, and the size of pores at the grain boundaries should be smaller than the wavelength of light or should not exist. Grain boundaries are an unavoidable presence in ceramics and have a considerable impact on transparency. So, grain boundaries with high quality and grains with smaller and uniform sizes are expected to obtain high-transparency AlON ceramics. Using the sintering additives can usually eliminate the residual pores during sintering, but it will give rise to new scattering centers of light in ceramics, the secondary phase, and the inclusions. As the two important light-scattering sources, the porosity and the grain boundaries should be reduced as much as possible. SHAHBAZI et al  described the transparent ceramics, effective parameters on transparency, Mie theory, and Fraunhofer theory in detail.
To date, many methods have been reported to prepare the AlON power or AlON ceramics, such as solid-state reaction [18,52−55], carbonization method from Al₂O₃[19,56−61], chemical vapor deposition [21,62], and sol−gel method [22,63]. Most of the studies focused on the solid-state reaction of Al₂O₃ and AlN at a high temperature and the carbonization method for Al₂O₃ reduction.
The solid-state method is a simple and conventional approach for the preparation of many compounds. One of the greatest advantages of the solid-state reaction at a high temperature is that raw materials can be effortlessly obtained. The reaction of Al₂O₃ and AlN for AlON formation can be described as 5AlN+9Al₂O₃→ Al₂₃O₂₇N₅[13,64]. The highly pure Al₂O₃ and AlN powers are available in the market and can be directly used to fabricate AlON powers or even the translucent AlON ceramics. The one-step preparation of the AlON ceramics can not only significantly reduce the sintering cost but also simplify the sintering process as well as easily achieve large-scale production. However, the powders may be aggregated or mixed inhomogeneously, resulting in the poor transparency of the AlON ceramics. Meanwhile, the high-purity ultrafine AlN is expensive, which increases the manufacturing cost. As shown in Fig. 4(a), MCCAULEY and ORBIN  firstly prepared the translucent AlON disc and presented a refined high-temperature phase diagram of the AlON along the pseudordinary Al₂O₃−AlN composition joint. The liquid-phase sintering process was employed to produce transparent AlON ceramics by PATEL et al . The α- Al₂O₃ in the range of 27−30 mol.% was firstly mixed with AlN. Then, the mixture was pressed into pellets after ball milling. The pellets were sintered at 1950−2025 °C for 10−60 min, and part materials could form a liquid phase to promote the sintering at this stage. Next, the system temperature fell by 50−100 °C and was kept for another 8−20 h to further improve the density and transparency. CHEN et al  firstly synthesized a phase of pure AlON:Ce3+ power at 1780 °C in nitrogen, then the full dense and transparent AlON:Ce3+ ceramics were achieved by liquid- phase-assisted pressureless sintering at 1900 °C for 20 h (Figs. 4(a) and (b)). Besides direct synthesis of used method to prepare AlON powders and then the translucent AlON ceramics by a solid-state employed to produce AlON ceramics. The major method, LI et al  used Al₂O₃ and AlN powders advantage of this method lies in the low cost of the as raw materials to rapidly synthesize single-phase raw materials and the feasibility for industrial AlON powers firstly via a solid-state method. Then, production. However, the sintering conditions are the prepared AlON powders were ground into fine complex, and it is hard to precisely control the mole AlON powders, as shown in Figs. 4(d) and (e). The ratio of Al₂O₃ to C, and the AlON is easily transparent AlON ceramic was produced by decomposed into Al₂O₃ and AlN in the N2pressureless sintering the obtained fine AlON atmosphere at a high temperature. All of these powders, and the in-line transmittance of the AlON might result in impure AlON powders. ceramic was as high as 84.3% (d100 mm × 1 mm) at JIN et al  firstly fabricated an Al₂O₃ / 3.7 μm (Figs. 4(f) and (g)). urea-formaldehyde-resin nanocomposite precursor, which was used to produce the core-shell Al₂O₃/C
The carbothermal reduction and nitridation carbothermal nitridation mixture，as shown in Figs. 5(a−c). During the process, the carbon layer (CRN) approach was firstly used to produce on the Al₂O₃ particle surface was found to strongly the compound of AlON by YAMAGUCHI and retard the coalescence and growth of the Al₂O₃ YANAGIDA in 1959 . The CRN is the most particles. Finally, the transparent AlON ceramics with maximum in-line transmittance above 80% at 2000 nm can be achieved by the two-step carbothermal nitridation method in nitrogen at 1950 °C for 8 h (Fig. 5(d)). SHAN et al  reported that both a bimodal (~1.1 μm and ~2.2 μm) and a unimodal (~1.1 μm) AlON powders could be obtained by using a ball mill of the as-prepared AlON powder via the CRN method (Figs. 5(e) and (f)). They found that the AlON powder with bimodal particle size distribution (PSD) possesses fast densification during the sintering process, and excellent transparent AlON ceramics with up to 82.1% infrared transmittance at ~3600 nm was pressureless sintered in nitrogen at 1820 °C for 2.5 h (Fig. 5(g))
The fine and pure γ-AlON powders were successfully prepared by YUAN et al  via a combinational method (Figs. 6(a) and (b)). They further used the γ-AlON powders to produce AlON ceramics and studied the effect of the twin lamellas on their mechanical strength (Figs. 6(c−j)) . They found that the twin lamellas and boundaries rise with the increase of the average grain size in the large-sized AlON ceramics, which provided a promising approach to enhance the transparent ceramics with large grains.
Up to now, there are also other methods explored to synthesize the AlON powder or ceramics. For example, ASPAR et al  prepared the AlON compound using ammonia, trimethyl- aluminium, and nitrous oxide by a chemical vapor deposition (CVD) method. It was found that the temperature and pressure have a significant effect on the equilibrium compositions by modifying the quantity of CO present. IRENE et al  also applied the CVD method to produce AlxOyNz films on silicon. Importantly, the phase can be controlled by adjusting the ratio of NH₃/CO₂ gas and preparation temperature KIM et al  developed a low-temperature sol−gel based approach to obtain an Al-O-N system, although it may be difficult to handle the nitride precursor of hydrazine in this process. Some other nitriding agents were explored in their further investigation. KIKKAWA et al  fabricated AlON via ammonia nitridation of an oxide precursor, which was produced by peptizing a glycine gel with the aluminum nitrate. In addition, a plasma reactor has been designed to synthesize AlON nanopowders according to the interaction of Al powder with ammonia and air in a thermal nitrogen plasma  The phase, chemical, and dispersal compositions of the prepared nano- powders are correlated with the plasma process parameters and the reactor design.
To obtain high-transparency AlON ceramics, the sintering additives should be added to eliminate the residual pores during sintering, which are the scattering center of the light. Nowadays, various sintering additives for AlON, such as Y₂O₃, La₂O₃, MgO, SiO₂, and CaCO₃, have been widely investigated [67−69,75−80]. According to the types of the sintering additives, we summarized the typical transparency results of AlON ceramics with various sintering additives, as shown in Table 2. For example, SHAN et al  reported that the in-line transmittance of AlON ceramic (3 mm in thickness) is 82.1% at a wavelength of 3600 nm with 0.5 wt.% Y₂O₃. SiO₂ was firstly employed as the sintering additive for the AlON ceramic (Fig. 7(b)) . They found that the in-line transmittance of AlON ceramic is up to 86% (3.5 mm in thickness) at 2000 nm and is not sensitive to the additive concentration with 0.15−0.55 wt.% SiO2. Some researchers used two types of sintering additives to produce high-transparent AlON ceramics. WANG et al  used 0.12wt.% Y₂O₃−0.09wt.% La₂O₃ as co-additives for the AlON ceramics, obtaining a transmittance of 80.3% (2 mm in thickness) at 400 nm (Fig. 7(a)). They reported that Y₃+ and La₃+ have a synergistic effect on the grain growth with the Y₃+ improving the mobility of grain boundary and promoting the grain growth while the La₃+ inhibited the grain growth. JIN et al  sintered AlON ceramics employing three types of sintering additives without pressure, composited by 0.1 wt.% MgO, 0.08 wt.% Y₂O₃, and 0.025 wt.% La₂O₃, and achieved a transmittance of 81% (1 mm in thickness) at 1100 nm. Recently,Y₂O₃−La₂O₃− MnO as a composite sintering additive to fabricate the transparent AlON ceramics was investigated by WANG et al  (Fig. 7(d)). The solubility limits of the sintering additives in AlON were studied by MILLER and KAPLAN  using wavelengthdispersive spectroscopy mounted on a scanning electron microscope. They found that the solubility limits of La, Y, and Mg in AlON at 1870 °C were (498±82)×10−6, (1775±128)×10−6, and >4000×10−6, respectively.
In addition to the conventional sintering additives of Y₂O₃, La₂O₃, and MgO, new sintering additives of H₃BO₃ based ternary composite  and earth elements  were also investigated. As illustrated in Figs. 8(a) and (b), various rare earth elements (Sc, La, Pr, Sm, Gd, Dy, Er, and Yb) were systematically explored as a sintering additive for transparent AlON ceramics, respectively. It was found that the AlON ceramics with 0.1 wt.% Pr-nitrate presented the highest transmittance of ~80% by two-step pressureless sintering (Fig. 8(c)), indicating that the rare earth elements can be a promising alternative sintering additive. More recently, using a Y₂O₃−MgAl₂O₄−H₃BO₃ as the co-sintering additive, YANG et al  obtained AlON ceramic with a transmittance of 81% (4 mm in thickness) at 600 nm by one-step reactive sintering when the H₃BO₃ content was 0.12 wt.% (Fig. 8(d)).
Table 2 Transparency results of AlON ceramics with various sintering additives
|Type of sintering additive||Y₂O₃ content/wt.%||La₂O₃ content/wt.%||MgO content/ wt.%||SiO₂ content/ wt.%||CaCO₃ content/ wt.%||Wavelength/ nm||Transmittance/%||Thickness/mm||Ref.|
Before sintering, green pellets of the AlON powders are usually formed by a dry process, including a uniaxial press under pressure and cold isostatic press, or by a wet process, including gel-casting [8,63,83]. Many sintering technologies have been explored to prepare AlON ceramics, such as pressureless sintering [56,58,67,68,77,79], vacuum sintering , hot-press , microwave sintering [85,86], spark plasma sintering [87−89] and hot isostatic pressing [75,76,78,90,91]. The advantages and disadvantages of the common preparation methods of AlON ceramic are shown in Table 3. Pressureless sintering is the most traditional sintering technology and is cost-effective for the mass production of AlON ceramics with various sizes and shapes. However, high sintering temperature, long sintering time, and sintering additives are generally required to obtain the high- transparent AlON ceramics. LI et al  reported a arge number of transparent AlON ceramics with dimensions of d100 mm × 1 mm by pressureless sintering at 1950 °C for 12 h under flowing N2 atmosphere in a graphite furnace. The in-line transmittance of the AlON ceramic (1 mm in thickness) is 84.3% at 3.7 μm wavelength with 0.5 wt.% Y₂O₃. Vacuum sintering is an effective sintering technology to eliminate gas from ceramics . PATEL et al  used high-purity Al2O3 and AlN powers as the raw materials to fabricate the translucent AlON ceramics at 2000 °C for 8 h and 32 MPa of pressure under hot-press, following at 1900 °C for more than 8 h in a vacuum. Hot-press (HP) sintering can be employed to apply axial pressure to accelerate the movement of powers and make the green pellet fully dense at a relatively low temperature. But the HP sintering is not suitable to prepare large and complex samples, and the production is high cost, and impurities and defects can be inevitably introduced. A post- annealing process is needed to remove carbon contamination . TAKEDA and HOSAKA  obtained transparent λ-AlON ceramic at 1900 °C for 1 h and 20 MPa of pressure under hot-press. Microwave sintering possesses high energy efficiency, cost-saving, low sintering temperature, strengthened reaction, and sintering rate. In the microwave process, the converted microwave energy can heat within the sample volume itself. CHENG et al  resented that the AlON sintered at 1800 °C for 1 h during the microwave process has a total transmission of 60%. Spark plasma sintering (SPS), also called the pulsed electric current sintering, can realize dense transparent ceramics with fine grains due to its short sintering time and low temperature with the aid of pulsed DC under pressure. So, the grain growth can be reduced. SHAN et al  produced high-transparent AlON ceramics by SPS at the low temperature of 1600 °C and the fast heating rates of 50−250 °C/min under the pressure of 60 MPa. The maximum transmittance of the as-obtained AlON ceramics (1.4 mm in thickness) is 80.6%.
Table 3 Advantages and disadvantages of common preparation methods of AlON ceramic
|Pressureless sintering||Simple process, suitable to prepare large and complex samples, low requirement on equipment, and high output||Low energy efficiency, and long sintering time|
|Vacuum sintering||Simple process, suitable to prepare large and complex samples, low requirement on equipment, and high output||Low energy efficiency, and long sintering time|
|Spark plasma sintering||High energy efficiency, low sintering temperature, short sintering time, and cost-saving||Not suitable to prepare large and complex samples, high requirement on equipment, and low output|
|Microwave sintering||High energy efficiency, low sintering temperature, short sintering time, and cost-saving||Not suitable to prepare large and complex samples, high requirement on equipment, and low output|
|Hot-press sintering||High transmittance, high density, and low residual pores||Not suitable to prepare large and complex samples, high requirement on equipment, low output, complex process, and high cost|
|Hot isostatic pressing||High transmittance, high density, and low residual pores||Not suitable to prepare large and complex samples, High requirement on equipment, low output, complex process, and high cost|
Hot isostatic pressing (HIP) is the most powerful sintering technology to achieve the maximum density and high-end optical transmitting ceramics by ultimately reducing residual pores in ceramics [8,11,93,94]. During high-temperature sintering, the HIP equipment can be applied by isostatic gas pressure. Figure 9 shows the schematic diagram of the microstructure model for pore elimination by the HIP [8,95]. Normally, it is significantly difficult to eliminate residual pores by other sintering technologies. An additional HIP procedure is required to eliminate the residual pores and increase the density and transmittance very close to the theoretical value.
This work was supported by the Jiangxi Provincial Natural Science Foundation, China (No. 20192BAB216009), the Science and Technology Planning Project of Hunan Province, China (No. 2019WK2051), and Science and Technology Project of Changsha, Hunan, China (No. kh2003023).