Yttrium aluminum garnet (YAG) has good optical properties and has been widely used in engineering. The preparation of conventional YAG powder is costly due to its high synthesis temperature, so lowering the synthesis temperature is the most effective way to save cost. The ultrafine oxide composite powder was rapidly synthesized using the microwave hydrothermal method at a low temperature. The effect of temperature and its performance. Differential thermal analysis and XRD phase analysis showed the mixed solution of Al(NO₃)₃ and Y(NO₃)₃ at pH=9 and C(Al³⁺)=0.42 mol·L⁻¹. SEM, zeta potential, and particle size analysis revealed that the microwave hydrothermal reaction temperature was 170 ^oC, and the high-purity ultrafine YAG powder was calcined at 928 ^oC, which was lower than conventional calcination temperature. The synthesized ultrafine spherical YAG powder had a small particle size and a uniform distribution.

Keywords: Ultrafine YAG powder, Synthesis temperature, Composite powder, Microwave hydrothermal method

Ceramics, as an important part of industrial ceramics, has been extensively investigated because of its high-temperature performance. The most common ones are Al₂O₃ ceramics, SiC ceramics, and Si₃N₄ ceramics [1-3], but the sintering temperature of several ceramics is usually high, accompanied by high cost, long preparation period, and limited existing research. YAG is a complex oxide formed by the reaction of Y₂O₃ and Al₂O₃, which belongs to the cubic system and has a garnet structure [4]. YAG ceramics have excellent high-temperature mechanical properties and chemical stability. YAG powder, which is the main material of YAG ceramics, has been widely explored. In optics, YAG is the best choice for the selection of laser matrix materials [5-6].
YAG is prepared mainly through hydrothermal, coprecipitation, mixed calcination, and sol-gel methods, et al. [7-10]. These synthetic methods have their own advantages and disadvantages. Although the cycle of ultrafine YAG powder preparation via coprecipitation is short, it results in large locally precipitated particles because of the influence of diffusion time when a precipitant is added during precipitation. The hydrothermal method yields ultrafine YAG powder with a small particle size and a uniform distribution, but the pre-   paration cycle is longer and the error is larger than those of other methods.
Coprecipitation and hydrothermal methods result in YAG powder with good dispersibility. However, in comparison with coprecipitation, the hydrothermal method has the advantages of yielding YAG powder with a small particle size and a uniform distribution, but the preparation cycle is long and costly [11-12]. Therefore, finding a method with a short period, low cost, and excellent powder performance is the current research direction concerning ultrafine YAG powder preparation. In terms of the shortcomings of the hydrothermal method, this study adopts the microwave hydrothermal method to help prepare ultrafine YAG powder. This method results in ultrafine YAG powder with a small particle size, a uniform distribution, and a considerably shortened preparation cycle [13-15].
In this paper, ultrafine YAG powder was prepared through microwave hydrothermal synthesis technology with a considerably shorter preparation cycle than the traditional hydrothermal method, and the process conditions in the YAG ultrafine powder synthesis process were started. The lowest synthesis temperature in the range was used to analyze the variables and determine the optimum conditions for the lowest synthesis tem-   perature of ultrafine YAG powder to reduce the sintering temperature and cost of YAG ceramics. The findings are also the innovation of this paper.

The main raw materials were aluminum nitrate Al(NO₃)·9H₂O (analytical grade), yttrium nitrate Y(NO₃)· 6H₂O (analytical grade), and ammonia water. Analytically pure Al(NO₃)₃·9H₂O (purity: 99.99%) was weighed with a beaker, deionized, dissolved, and stirred thoroughly with a magnetic stirrer to obtain Al (NO₃)₃ at a concen-   tration of 2.0826 mol·L⁻¹. The pure Y(NO₃)₃·6H₂O (purity: 99.99%) solid was weighed into a beaker, dissolved in deionized water, and fully dissolved by stirring with a magnetic stirrer to obtain an Y(NO₃)₃ solution with a concentration of 2.4866 mol·L⁻¹. The precursors of YAG, Al(OH)₃, and Y(OH)₃ were obtained by adjusting the pH of the mixed solution of Al(NO₃)₃-Y(NO₃)₃ by using the aqueous ammonia solution to carry out precipitation reaction and obtain Al(OH)₃ and Y(OH)₃. Different concentrations of Al(NO₃)₃ and Y(NO₃)₃ mixed solutions were prepared according to a Y:Al ratio of 3:5 and adjusted to different pH values by using a pH-adjustable aqueous ammonia solution. The mixed precipitates of the YAG precursors Al(OH)₃ and Y(OH)₃ obtained after precipitation were subjected to microwave hydrothermal decomposition by using a microwave hydrothermal synthesizer to obtain a mixed precipitate of Al₂O₃-Y₂O₃. Thereafter, the obtained mixed precipitate was filtered to obtain white composite powder that was then subjected to differential thermal analysis. The temperature of the YAG crystal phase was determined using a DSC-TG curve via a STA 449 F3-type comprehensive thermal analyzer (NETZSCH, Germany) and calcined in a programmed furnace to analyze the temperature. After calcination, the phase was conducted by using a D8 Advance X-ray powder diffractometer (Bruker, Germany). Surface morphology and particle size (D₅₀) were examined using a Zetasizer Nano Zeta potential and particle size analyzer (Malvern, UK), respectively.

Effect of pH on the properties of YAG ultrafine powder
The prepared mixed solution of C(Al³⁺)=0.42 mol· L⁻¹ and C(Y³⁺)=0.25 mol·L⁻¹ was adjusted to different pH values. The microwave hydrothermal temperature was 180 ^oC and the holding time was 5 min (Table 1). White precipitated powder was filtered and dried to obtain a composite powder.
Fig. 1 shows the XRD pattern of the YAG precursor in a Cu target with a source of Ka-ray (λ = 1.5405 nm) and a scan range of 2θ from 10° to 80°. The XRD pattern of the YAG precursor after thermal decom-   position by microwave. The YAG precursor did not decompose into an oxide at a hydrothermal temperature of 180 ^oC (Fig. 1).
Fig. 2a is a DSC curve of YAG precursor after microwave hydrothermal decomposition in an air atmosphere at a heating rate of 15 K/min from 25 ^oC to 1200 ^oC. An endothermic peak appeared at approximately 150 ^oC, and this observation was caused by the dehydra-    tion decomposition reaction of Y and Al hydroxides and the evaporation of adsorbed water. An exotherm occurred at approximately 940 ^oC. The peak did not lose weight because of the crystallization of the amorphous powder into the YAG phase. Fig. 2b illustrates the synthesis temperature and pH of YAG obtained by preparing the ultrafine YAG powder under different pH conditions. The relationship curve shows that the tem-   perature of the YAG crystal phase initially decreases and then increases as the pH of the mixed mother salt solution increases. At pH=9, the temperature of the YAG crystal phase is relatively low, and the tem-   perature is 933.9 ^oC.
Fig. 3 is the XRD pattern of the YAG precursor after calcination. At pH=8 and 9, pure cubic phase ultrafine YAG powder can be synthesized, but the low calcination temperature is observed at pH=9. ultrafine YAG powder and high crystallinity. At this time, Y:Al=3:5 may result in the lower potential energy of YAG synthesis and lower synthesis temperature. When pH=10, the intermediate phase Y₄Al₂O₉ (YAM) appears, and Y:Al= 2:1 mainly because a certain content of AlO²⁻ is formed at a high concentration of OH⁻ so that the ratio of Y:Al is greater than 1, leading to the appearance of the intermediate YAM phase of YAG during calcination. When pH=7, the YAM phase appears because of the low concentration of OH⁻, and Al³⁺ cannot be completely precipitated, so the Al(NO₃)₃-Y(NO₃)₃ mixed salt solution is not completely converted into hydroxide precipitate [16-18]. The ratio of Y:Al does not reach 3:5, resulting in the occurrence of the intermediate YAM phase of YAG during calcination. At pH=9 of the mixed mother salt solution, the pure cubic phase ultrafine YAG powder was synthesized at a low temperature (933.9 ^oC) under the condition of Y:Al=3:5.
 
Effect of Concentration on the Properties of YAG Ultrafine Powder
Fig. 4a shows the DSC curve of the synthetic powder. An endothermic peak appears at approximately 150 ^oC, and this peak is caused by the decomposition of the hydroxide of Y and Al and the adsorbed water. An exothermic peak appears at approximately 940 ^oC, and no loss of weight is observed because of the crystalli-   zation of the amorphous powder into the YAG phase. Fig. 4b shows the synthesis temperature of YAG and the Al³⁺ concentration in the mixed parent salt solution. The relationship curve (Fig. 4b) illustrates that the temperature of the YAG crystal phase initially decreases and then increases as the concentration of Al³⁺ in the mixed mother salt solution increases. At 0.52 mol·L⁻¹ concentration, the YAG crystal phase is produced at a lower temperature, and the temperature is 929.8 ^oC.
Fig. 5 shows the XRD pattern of the YAG precursor after calcination. When the Al³⁺ concentration in the mixed salt solution is less than 0.52 mol·L⁻¹, a pure cubic-phase ultrafine YAG powder can be produced, and the Al³⁺ concentration in the mixed salt solution is 0.42 mol·L⁻¹, which is synthesized. The ultrafine YAG powder has a strong peak and a high degree of crystallinity. When the concentration of Al³⁺ in the mixed salt solution is greater than or equal to 0.52 mol·L⁻¹, the YAM phase is produced, and the synthesis temperature of the ultrafine YAG powder increases as the concentrations increase. In the reaction of a dilute solution system, Al³⁺ precipitates earlier than Y³⁺ does because Ksp of the solubility products Y³⁺ and Al³⁺ is different. However, the formed Al(OH)₃ cannot be nucleated, making Y³⁺ precipitate in the outer layer of Al³⁺ to form a unique inclusion structure in high con-   centration systems. This phenomenon may be due to the incomplete conversion of Al³⁺ to Al(OH)₃ precipi-   tation or AlO²⁻, resulting in the precipitation of Al(OH)₃ in the solution [19-21]. Less, eventually Y:Al=2:1, and the YAM phase is produced after calcination. Therefore, under the same conditions, when the concentration of Al³⁺ in the mixed salt solution is 0.42 mol·L⁻¹, a pure cubic-phase ultrafine YAG powder can be synthesized at a calcination temperature of 931.9 ^oC.
Fig. 6 is a comparison of the particle size of the ultrafine YAG powder prepared at different Al³⁺ concentrations. Fig. 6 shows that the particle size of the prepared ultrafine YAG powder also increases as the concentration of the reaction mother salt solution increases. Thus, the higher the concentration is, the higher the formation temperature of the YAG crystal phase will be.
Fig. 7 shows the XRD pattern of the YAG precursor after microwave hydrothermal decomposition. YAG precursors have crystal phases of γ-Al₂O₃ and Y₂O₃ under the experimental parameters of the microwave hydrothermal temperatures of 190 ^oC and 200 ^oC and the holding time of 5 min, respectively.
Fig. 8a is the DSC curve of the powder. An endothermic peak appears at about 150 ^oC because of the decomposition of the hydroxide of Y and Al and the evaporation of the adsorbed water. An exothermic peak appears at approximately 940 ^oC, and no weight loss is observed because of the crystallization of amorphous powder into the YAG phase. Fig. 8b shows the relationship between the synthesis temperature of YAG and the temperature of microwave hydrothermal reaction. When the temperature of microwave hydro-   thermal reaction is 170 ^oC, the YAG crystal phase produces a low temperature of 927.7 ^oC.
Fig. 9 illustrates the XRD pattern of the YAG precursor after calcination via microwave hydrothermal decomposition. The intermediate-phase YAM of YAG occurs when the hydrothermal temperatures are 190 ^oC and 200 ^oC, and only the pure cubic-phase ultrafine YAG powder can be produced at the hydrothermal temperatures of 170 ^oC and 180 ^oC, respectively, and the water is hot. The ultrafine YAG powder prepared at 170 ^oC has a lower synthesis temperature and a calcination temperature of only 927.7 ^oC. Therefore, at 170 ^oC, it can provide the energy required for the decomposition of the YAG precursor hydroxide precipitate into the corresponding oxide to form a crystal phase [22-24]. As the temperature continues to increase, the energy also increases, so the formed grains grow and the particle size becomes coarse. Consequently, the calcination temperature of the ultrafine YAG powder further increases. When the hydrothermal temperatures are 190 ^oC and 200 ^oC, the intermediate-phase YAM of YAG appears because the crystal phases of Al₂O₃ and Y₂O₃ appear in microwave hydrothermal reaction in the YAG precursor. Large amounts of Al₂O₃ and Y₂O₃ decompose, and solid phase reaction is formed during calcination. The intermediate-phase YAM of YAG inevitably occurs because the solid phase reaction in YAG is gradual. Under other conditions, at the micro-   wave water temperature of 170 ^oC, a pure cubic-phase ultrafine YAG powder can be synthesized at a low calcination temperature (927.7 ^oC).
Fig. 10 shows a chart of the analysis of the particle size of the ultrafine YAG powder, indicating that the particle size of the prepared ultrafine YAG powder also continuously increases as the microwave water tem-   perature increases because in the microwave. During the hydrothermal process, an increase in temperature causes the crystal grains to grow continuously, resulting in a large particle size of the synthesized ultrafine YAG powder and a high temperature of the crystal phase.
Fig. 11 illustrates a mixture of Al(NO₃)₃ and Y(NO₃)₃ calcined at 927.7 ^oC under the experimental conditions of pH=9, C(Al³⁺)=0.42 mol·L⁻¹, and a microwave hydrothermal reaction temperature of 170 ^oC. Fig. 11 also shows the scanning electron micrograph of the pure cubic-phase ultrafine YAG powder. The ultrafine YAG powder synthesized under the experimental con-   ditions is approximately spherical, and the particle diameter is approximately 200 nm. It can be seen from the figure that the particle size of the powder is very small and has a spherical shape. Due to the small particle size, the powder has a large specific surface area, a high specific surface energy, and is prone to agglomeration. Table 2 Table 3

YAG precursor oxide was synthesized using the microwave hydrothermal method. After calcination, the powder was analyzed, and the following conclusions were drawn: as the Al³⁺ concentration in the mother salt solution increased, the particle size of the prepared ultrafine YAG powder also increased. As the microwave hydrothermal temperature increased, the particle size of the prepared ultrafine YAG powder increased. With microwave hydrothermal synthesis, the process conditions of high-purity ultrafine YAG powder were pH=9, C(Al³⁺)=0.42 mol·L⁻¹, microwave hydrothermal reaction temperature=170 ^oC, and calcination=928 ^oC. The body had a particle size of about 200 nm, and its shape was almost spherical. This study provided a reference for the low-temperature synthesis of high-purity and high-performance ultrafine YAG powder.

The authors are thankful for the financial support provide by the Science and Technology Found of the Educational Department of Jiangxi Province, China (GJJ181107), Teaching Reform Research Fund for Higher Education of Jiangxi Province (JXJG-18-22-2), The 13th Five-Year Plan for Educational Science in Jiangxi Province (20YB259), The Science and Tech-   nology Support Project of Pingxiang City (PST2019-01) and the Science and Technology Found of Jiujiang University, China (2019D0207).