Low-Temperature Fabrication of Plate-like α-Al2O3 with Less NH4F Additive

Fluorinated compounds are effective mineralization agents for the fabrication of plate-like α-Al2O3. However, in the preparation of plate-like α-Al2O3, it is still an extremely challenging task to reduce the content of fluoride while ensuring a low synthesis temperature. Herein, oxalic acid and NH4F are proposed for the first time as additives in the preparation of plate-like α-Al2O3. The results showed that plate-like α-Al2O3 can be synthesized at a low temperature of 850 °C with the synergistic effect of oxalic acid and 1 wt.% NH4F. Additionally, the synergistic effect of oxalic acid and NH4F not only can reduce the conversion temperature of α-Al2O3 but also can change the phase transition sequence.

Until now, many strategies have been attempted to synthesize plate-like α-Al 2 O 3 , such as hydrothermal methods, melt salt methods, and chemical vapor deposition [13,[16][17][18]. Regardless of the methods used, calcination is a necessary step for the fabrication of platelike α-Al 2 O 3 . Since the phase transition from transitional alumina to α-Al 2 O 3 involves drastic bond breaking and remaking, the thermal process usually requires temperatures greater than 1200 • C [19]. These temperatures not only impose higher requirements for the preparation equipment of α-Al 2 O 3 , but they also cause substantial mass transfer, leading to rapid growth of α-Al 2 O 3 particle size [20]. Worse, when the sintering temperature exceeds the critical value, plate-like α-Al 2 O 3 perhaps converts into particle-like grains [8]. Therefore, it is critical to reduce the synthesis temperature of α-Al 2 O 3 .
It is well known that fluorine-containing compounds can significantly reduce the synthesis temperature of α-Al 2 O 3 , and they are also considered effective mineralization agents for the fabrication of plate-like α-Al 2 O 3 [5,7,8,14,[21][22][23][24][25][26][27][28][29][30][31]. Hence, coupled with the low cost and simple process of α-Al 2 O 3 , the addition of fluorine-containing mineralization agents to alumina precursors has been used by many researchers for the preparation of plate-like α-Al 2 O 3 . For example, Tian et al. [8] synthesized highly pure plate-like α-Al 2 O 3 from a boehmite precursor with 5 wt.% NH 4 F. Similarly, Wang et al. [21] also successfully prepared plate-like α-Al 2 O 3 by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) adding 5 wt.% AlF 3 . They thought that the formation of the intermediate compound AlOF accelerated the mass transfer of Al 3+ and improved the growth rate along the crystal orientation of <1010>. Nevertheless, a shortcoming of these studies was the excessive use of F content, even as high as 20 wt.% in some cases [31]. When the concentration of F is excessive, both gaseous and particulate fluoride is toxic to plants and animals, and a series of diseases, such as fluorosis of bone and cancer, can be caused [24,32]. In this context, an extremely challenging objective is finding an effective strategy to not only reduce the concentration of F but also to maintain a low synthesis temperature in the process of preparing α-Al 2 O 3 with fluoride as an additive. Currently, increasing interest has been focused on this aspect, but the existing research has been mostly theoretical, with almost no experimental research.
In our previous report [33], oxalic acid was confirmed to be helpful in reducing the resultant temperature of α-Al 2 O 3 by 100 • C. Therefore, in this paper, we proposed for the first time adding oxalic acid and NH 4 F as additives in the preparation process of plate-like α-Al 2 O 3 . The synergistic influences of oxalic acid and NH 4 F on the crystal transition order of Al(OH) 3 were studied, and the optimum F content was explored. Simultaneously, the action mechanism of oxalic acid and NH 4 F during the transformation was clarified based on the experimental results.

Experimental Procedures
The preparation process mainly involved the following starting materials: analytically pure Al(OH) 3 (>99.8%), analytically pure oxalic acid (>99.0%), analytically pure ammonium fluoride (NH 4 F, >98.0%), and deionized water. Except for deionized water, which was home made in the laboratory, other raw materials were obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All raw materials were directly adopted without any further purification.
Phase composition analysis was performed with a D/max2500 X-ray diffractometer (Rigaku Industrial Corporation, Tokyo, Japan) using Cu Kα radiation (40 kV, 40 mA) at 2θ angles of 10-90 • . Fourier transform infrared (FTIR) spectroscopy (Nicolet 380) was characterized using a spectrograph (Thermo Electron, Houston, TX, USA). A Nova Nano 450 scanning electron microscope (SEM, FEI Company, Hillsboro, OR, USA) was used to examine the microscopic morphologies of all specimens. X-ray photoelectron spectroscopy (XPS) was performed using an Escalab 50XI photoelectron spectrometer (VG Scientific Ltd., St Leonards, UK). Particle size distribution was collected by a Malvin laser particle size analyzer (Mastersizer 3000, Marvin Pan-alytical, Almelo, The Netherlands). Thermal gravimetry and differential scanning calorimetry (TG-DSC) curves were collected using a simultaneous TG-DTA/DSC apparatus (STA2500, NETZSCH-Gerätebau GmbH, Free State of Bavaria, Germany) under an ambient atmosphere with a heating rate of 10 • C/min up to 1300 • C.

Results and Discussion
Since the analytic results of A1 were applied in our previous reports [33], only A2-A5 were analyzed in this paper. To characterize the compositions of A2-A5, the spectra of A2-A5 were recorded using the FTIR technique, as shown in Figure 1. Figure 1a exhibits the FTIR spectra in the infrared region of 400-4000 cm −1 . It was clear that the absorption peaks exhibited by A2-A5 were greatly similar to those of A1 reported in our previous work [33]. The only difference was that, with the increase in NH 4 F content, the absorption peak at around 3200 cm −1 , corresponding to the stretching vibrations of N-H bonds, began to gradually appear, especially for A5. Among these absorption bands presented by A2-A5, the absorption bands centered at 3621, 3526, 3456, and 3393 cm −1 can be assigned to the stretching vibrations of hydroxyl groups in Al(OH) 3 and adsorbed water [34]. The intensive bands shown at 1704, 1415, and 1297 cm −1 were related to the stretching vibration of carboxyl COO − , implying that a surface complex (Al 2 C 2 O 8 H 4 ), as we reported previously, had been formed due to the interaction of oxalic acid and Al(OH) 3 [33]. Regarding the bands in the 400 to 1200 cm −1 interval (local amplification in Figure 1b), the weak band observed at 915 cm −1 was caused by the deformation of surface hydroxyls. The vibration bands at 1022, 970, 803, 751, 666, 560, 517, 451, and 423 cm −1 were associated with the stretching and bending of Al-O bonds [35]. With the target of understanding the effects of F − ion on the structure of Al(OH) 3 , the changes in the spectra attributed to Al-O bonds between A2 and A5 were carefully observed. Interestingly, these vibration bands were identical in position and shape except for the intensity, revealing that F − ions did not bond with Al 3+ ions and O 2− ions on the surface of Al(OH) 3 , perhaps because of the surface of Al(OH) 3 was covered with hydroxyl groups.

Results and Discussion
Since the analytic results of A1 were applied in our previous reports [33], only A2-A5 were analyzed in this paper. To characterize the compositions of A2-A5, the spectra of A2-A5 were recorded using the FTIR technique, as shown in Figure 1. Figure 1a exhibits the FTIR spectra in the infrared region of 400-4000 cm −1 . It was clear that the absorption peaks exhibited by A2-A5 were greatly similar to those of A1 reported in our previous work [33]. The only difference was that, with the increase in NH4F content, the absorption peak at around 3200 cm −1 , corresponding to the stretching vibrations of N-H bonds, began to gradually appear, especially for A5. Among these absorption bands presented by A2-A5, the absorption bands centered at 3621, 3526, 3456, and 3393 cm −1 can be assigned to the stretching vibrations of hydroxyl groups in Al(OH)3 and adsorbed water [34]. The intensive bands shown at 1704, 1415, and 1297 cm −1 were related to the stretching vibration of carboxyl COO − , implying that a surface complex (Al2C2O8H4), as we reported previously, had been formed due to the interaction of oxalic acid and Al(OH)3 [33]. Regarding the bands in the 400 to 1200 cm −1 interval (local amplification in Figure 1b), the weak band observed at 915 cm −1 was caused by the deformation of surface hydroxyls. The vibration bands at 1022, 970, 803, 751, 666, 560, 517, 451, and 423 cm −1 were associated with the stretching and bending of Al-O bonds [35]. With the target of understanding the effects of F − ion on the structure of Al(OH)3, the changes in the spectra attributed to Al-O bonds between A2 and A5 were carefully observed. Interestingly, these vibration bands were identical in position and shape except for the intensity, revealing that F − ions did not bond with Al 3+ ions and O 2− ions on the surface of Al(OH)3, perhaps because of the surface o Al(OH)3 was covered with hydroxyl groups. The SEM pictures of A2-A5 are illustrated in Figure 2. As shown in Figure 2a, the microstructure of A2 mainly consisted of granular structures, in which a small number o sheets were found, as shown by the red arrow. The formation process of granular particles was discussed in our previous research [33]. As the content of NH4F increased, the microstructures of A3-A5 exhibited little change compared with those of A2 (See Figure 2b-d) The illustration in Figure 2d perfectly presents the morphology of granular particles. The SEM pictures of A2-A5 are illustrated in Figure 2. As shown in Figure 2a, the microstructure of A2 mainly consisted of granular structures, in which a small number of sheets were found, as shown by the red arrow. The formation process of granular particles was discussed in our previous research [33]. As the content of NH 4 F increased, the microstructures of A3-A5 exhibited little change compared with those of A2 (See Figure 2b-d). The illustration in Figure 2d perfectly presents the morphology of granular particles.  [5]. For calcination at 800 • C, except for the peaks emerging at 700 • C, some shallow peaks at 2θ = 25.6 • , 35.1 • , 43.3 • , and 57.5 • , indexed as α-Al 2 O 3 , can be detected. As the temperature increased to 850 • C, instead of the peaks of χ-Al 2 O 3 , many characteristic peaks ascribed to κ-Al 2 O 3 can be observed, and κ-Al 2 O 3 became the major phase coexisting with α-Al 2 O 3 . With the further increase in calcination temperature, the peak intensity of κ-Al 2 O 3 gradually decreased. At 950 • C, the peak of κ-Al 2 O 3 disappeared, and the pattern only had the peak of α-Al 2 O 3 . From the above results, for sample A2, the phase transition process was: Al(OH) 3 → χ-Al 2 O 3 → κ-Al 2 O 3 → α-Al 2 O 3 ; and the synthetic temperature of α-Al 2 O 3 was 950 • C, in agreement with our previous study [33]. Regarding A3-A5, the synthesis temperatures of α-Al 2 O 3 were 900, 850, and 800 • C, re-spectively, which were all lower than those of A1 and A2. In addition, with the growth of NH 4 F content, χand κ-Al 2 O 3 were replaced by γand θ-Al 2 O 3 in the polycrystalline transformation of Al 2 O 3 , respectively. Surprisingly, except for γ-Al 2 O 3 , no trace of other transitional forms of Al 2 O 3 were detected before α-Al 2 O 3 was obtained for A5.   Figure 3a, it can be clearly observed that, at 700 and 750 °C, the XRD pattern of A2 presented four peaks at 2θ = 37.3°, 42.8°, 46.3°, and 67.0°, corresponding to χ-Al2O3 and κ-Al2O3 [5]. For calcination at 800 °C, except for the peaks emerging at 700 °C, some shallow peaks at 2θ = 25.6°, 35.1°, 43.3°, and 57.5°, indexed as α-Al2O3, can be detected. As the temperature increased to 850 °C, instead of the peaks of χ-Al2O3, many characteristic peaks ascribed to κ-Al2O3 can be observed, and κ-Al2O3 became the major phase coexisting with α-Al2O3. With the further increase in calcination temperature, the peak intensity of κ-Al2O3 gradually decreased. At 950 °C, the peak of κ-Al2O3 disappeared, and the pattern only had the peak of α-Al2O3. From the above results, for sample A2, the phase transition process was: Al(OH)3 → χ-Al2O3 → κ-Al2O3 → α-Al2O3; and the synthetic temperature of α-Al2O3 was 950 °C, in agreement with our previous study [33]. Regarding A3-A5, the synthesis temperatures of α-Al2O3 were 900, 850, and 800 °C, respectively, which were all lower than those of A1 and A2. In addition, with the growth of NH4F content, χ-and κ-Al2O3 were replaced by γ-and θ-Al2O3 in the polycrystalline transformation of Al2O3, respectively. Surprisingly, except for γ-Al2O3, no trace of other transitional forms of Al2O3 were detected before α-Al2O3 was obtained for A5. Aiming to further study the influences of oxalic acid and NH4F on the transform sequence of α-Al2O3, specimen A5 was fired at 800 °C with a heating rare 5 °C/mi different dwell times (10, 30, 60, and 120 min). The XRD images are illustrated in F 4. As shown in Figure 4a, the intense diffraction patterns attributed to θ-Al2O3 wer Aiming to further study the influences of oxalic acid and NH 4 F on the transformation sequence of α-Al 2 O 3 , specimen A5 was fired at 800 • C with a heating rare 5 • C/min for different dwell times (10, 30, 60, and 120 min). The XRD images are illustrated in Figure 4. As shown in Figure 4a, the intense diffraction patterns attributed to θ-Al 2 O 3 were observed for A5 thermally treated at 800 • C for 10 min, as well as some shallow patterns indexed as α-Al 2 O 3 . With the prolongation of calcination time, α-Al 2 O 3 gradually became the main phase. Moreover, the grain size of as-received α-Al 2 O 3 also displayed an upward trend according to the Scherrer formula, as illustrated in Figure 4b. By combining the results obtained from Figure 3c,d and Figure 4, the phase transformation sequence of A4 and A5 was Al(OH) 3 → γ-Al 2 O 3 → θ-Al 2 O 3 → α-Al 2 O 3 , which was different from that of A1 and A2. Aiming to further study the influences of oxalic acid and NH4F on the transforma sequence of α-Al2O3, specimen A5 was fired at 800 °C with a heating rare 5 °C/min different dwell times (10, 30, 60, and 120 min). The XRD images are illustrated in Fi 4. As shown in Figure 4a, the intense diffraction patterns attributed to θ-Al2O3 were served for A5 thermally treated at 800 °C for 10 min, as well as some shallow patt indexed as α-Al2O3. With the prolongation of calcination time, α-Al2O3 gradually bec the main phase. Moreover, the grain size of as-received α-Al2O3 also displayed an upw trend according to the Scherrer formula, as illustrated in Figure 4b. By combining th sults obtained from Figure 3c,d and Figure 4, the phase transformation sequence o and A5 was Al(OH)3 → γ-Al2O3 → θ-Al2O3 → α-Al2O3, which was different from th A1 and A2. S. J. Wilson [36] suggested that the difference in polycrystalline transition seque was usually related to the difference in starting materials. In accordance with the re S. J. Wilson [36] suggested that the difference in polycrystalline transition sequences was usually related to the difference in starting materials. In accordance with the results of FTIR, the crystal structure of Al(OH) 3 was not altered by the introduction of NH 4 F. Therefore, it can be speculated that the structure of the reagent changed during the sintering process. After all, NH 4 F had a low decomposition temperature, beyond which NH 4 F decomposed into NH 3 and HF gas. The presence of HF gas perhaps plays a significant role in the structural changes of the sintered materials. Figure 5 demonstrates the high-resolution XPS spectra of the transitional Al 2 O 3 received from A5 at a calcination temperature of 500 • C. Notably, in addition to the characteristic peaks of Al 2p, Al 2s, and O 1s as expected, an exclusive peak ascribed to F 1s can also be detected (Figure 5a). The O 1s region can be attributed to the contributions of the binding energy of O-Al at 530.8 eV and Al-O-H at 532.4 eV (Figure 5b). However, the binding energy of O-Al and Al-O-H exhibited here was higher than that reported in the other literature, which may be caused by the existence of F. With respect to Al 2p spectrum, two peaks centered at 74.2 eV and 75.6 eV, corresponding to the binding energy of Al-O and Al-F, respectively were observed (Figure 5c). Accordingly, the individual line presented one peak at 685.7 eV belonging to the binding energy of F-Al (Figure 5d). The results of XPS confirmed that F existed on the surface of the calcined materials. Adopting Al(OH) 3 as a starting material, the first metastable phase received was χ-Al 2 O 3 , which inherited the arrangement of oxygen anions in the crystal structure of Al(OH) 3 , and Al 3+ ions occupied the octahedral position in the hexagonal oxygen layer. In comparison, the stacking of χ-Al 2 O 3 in the c direction demonstrated extremely obvious disorder [37,38]. Consequently, it turned out that Al(OH) 3 showed a significant disorder along the c axis during its dehydration, which may be an opportunity for gaseous HF to change the structure of the starting regent in calcination. The gaseous HF working with Al 3+ ions on the surface of the precursor in calcination might cause the collapse of the dis-ordered Al-O octahedron, resulting in the formation of γ-Al 2 O 3 so that the polycrystalline transformation sequence changed to Al(OH) 3 tion demonstrated extremely obvious disorder [37,38]. Consequently, it turned out tha Al(OH)3 showed a significant disorder along the c axis during its dehydration, which ma be an opportunity for gaseous HF to change the structure of the starting regent in calcina tion. The gaseous HF working with Al 3+ ions on the surface of the precursor in calcinatio might cause the collapse of the disordered Al-O octahedron, resulting in the formation o γ-Al2O3 so that the polycrystalline transformation sequence changed to Al(OH)3 → γ Al2O3 → θ-Al2O3 → α-Al2O3.    Figure 6 shows the TG-DSC curves of Al(OH) 3 , A1, and A5. As presented in Figure 6a, the TG-DSC curves of Al(OH) 3 can be divided into two stages. The first stage, with a mass loss of 33.8% exhibited, two endothermic peaks at about 85 and 292 • C. These endothermic peaks were related to the evaporation of physically adsorbed water and the decomposition of Al(OH) 3 . In the second stage, weight loss of around 2% can be observed, corresponding to the removal of hydroxyl groups from the transition Al 2 O 3 surface. Furthermore, two exothermic peaks were displayed at 870 and 1120 • C, which were caused by the appearance of κand α-phases, respectively [39]. Similar to Al(OH) 3 , the TG-DSC curves of A1 also included two steps, two endothermic peaks and two exothermic peaks, as shown in Figure 6b. The difference was that the two endothermic peaks were decreased to 800 and 1050 • C, respectively, and the heat release increased when the α-phase appeared. The growth in heat release indicated that the nucleation amount of α-Al 2 O 3 increased. There were two main reasons for the decrease in the phase transition temperature and the increase in the nucleation number. One was that the complex generated by Al(OH) 3 and oxalic acid can act as seeds, providing low energy positions for heterogeneous nucleation. On the other hand, at greater than 400 • C, the number of transition Al 2 O 3 surface hydroxyl groups obtained from A1 was approximately twice that of the transition Al 2 O 3 obtained from Al(OH) 3 , which can be obtained from the mass loss in the second stage of TG-DSC. Of course, this outcome also indicated that, at greater than 400 • C, A1 released twice as many water molecules as Al(OH) 3 released. As is known, the size of metastable Al 2 O 3 must reach a critical crystal diameter before converting to α-Al 2 O 3 [40]. The water vapor produced by dehydroxylation contributes to the coarsening of metastable Al2O3 to reach the critical crystal size [41]. Moreover, the formation of vacancies by dehydroxylation is also beneficial to the rearrangement of the Al 2 O 3 lattice structure [42]. Therefore, A1 is Materials 2023, 16, 4415 7 of 11 more easily converted into α-Al 2 O 3 than Al(OH) 3 . In contrast with Al(OH) 3 and A1, for the TG-DSC curves of A5, the exothermic peak attributed to α-Al 2 O 3 nucleation that we cared about decreased to 940 • C and was more intense, related to the mass transfer of AlOF gas, as well as the reasons mentioned for A1. Additionally, C. Barth et al. [43] believed that, once α-Al 2 O 3 at a high temperature was exposed to water, Al(OH) 3 microcrystalline clusters would form on the surface of α-Al 2 O 3 . In our work, the XPS of transition Al2O3 mentioned above confirmed the presence of Al-F bonds, which might produce AlF 3 clusters at the position of Al-F bonds, providing nucleation sites for α-Al 2 O 3 , which was also responsible for lowering the resultant temperature of α-Al 2 O 3. In short, α-Al 2 O 3 can be obtained at a low temperature under the synergistic effects of oxalic acid and NH 4 F. Nevertheless, the synthesis temperature of α-Al 2 O 3 obtained by the results of TG-DSC was higher than that shown by XRD, which may be caused by the high heating rate and purging of air during the operation process of TG-DSC.  Figure 7 shows SEM images of α-Al2O3 obtained from A2-A5 at different sintering temperatures. As can be seen from Figure 7a, the produced α-Al2O3 of A2 at 950 °C displayed a granular microstructure. Different from A2, at the same temperature, a drumlike architecture of α-Al2O3 particles was obtained for A3, while the α-Al2O3 prepared by A4 and A5 presented a plate-like structure, as illustrated in Figure 6b,c. Meanwhile, by carefully comparing the obtained α-Al2O3 in Figure 6b,c, it can be seen that the particle diameter of α-Al2O3 gradually increased with the increase in NH4F content, while the thickness exhibited a decreasing trend. From the perspective of crystal growth, the microstructure of materials depends on the growth rate of the crystal plane. The remaining crystal surface is a plane with a slow growth rate. The inset in Figure 6b perfectly exhibits the crystal plane of α-Al2O3 obtained from A3 at 950 °C. According to a study [44], the side of the drum-shaped α-Al2O3 corresponds to the crystal plane of (2243), and the front side belongs to the (0001) plane. Different from the side of drum-shaped α-Al2O3, the side of the plate-like structure synthesized from A4 and A5 was ascribed with an (1120) plane. It was reported that F − ions can work with Al 3+ ions and adsorb on the crystal face of α-Al2O3. In α-Al2O3 unit cells, the (0001) plane has close-packed atoms, lending themselves to more bare Al 3+ ions on this crystal face. As a result, F − ions tended to adsorb on the (0001) plane,  Figure 7 shows SEM images of α-Al 2 O 3 obtained from A2-A5 at different sintering temperatures. As can be seen from Figure 7a, the produced α-Al 2 O 3 of A2 at 950 • C displayed a granular microstructure. Different from A2, at the same temperature, a drum-like architecture of α-Al 2 O 3 particles was obtained for A3, while the α-Al 2 O 3 prepared by A4 and A5 presented a plate-like structure, as illustrated in Figure 6b,c. Meanwhile, by carefully comparing the obtained α-Al 2 O 3 in Figure 6b,c, it can be seen that the particle diameter of α-Al 2 O 3 gradually increased with the increase in NH 4 F content, while the thickness exhibited a decreasing trend. From the perspective of crystal growth, the microstructure of materials depends on the growth rate of the crystal plane. The remaining crystal surface is a plane with a slow growth rate. The inset in Figure 6b perfectly exhibits the crystal plane of α-Al 2 O 3 obtained from A3 at 950 • C. According to a study [44], the side of the drum-shaped α-Al 2 O 3 corresponds to the crystal plane of (2243), and the front side belongs to the (0001) plane. Different from the side of drum-shaped α-Al 2 O 3 , the side of the plate-like structure synthesized from A4 and A5 was ascribed with an (1120) plane. It was reported that F − ions can work with Al 3+ ions and adsorb on the crystal face of α-Al 2 O 3 .
In α-Al 2 O 3 unit cells, the (0001) plane has close-packed atoms, lending themselves to more bare Al 3+ ions on this crystal face. As a result, F − ions tended to adsorb on the (0001) plane, limiting the deposition of α-Al 2 O 3 on this face and bringing down its growth rate, resulting in the retention of the (0001) plane [21]. Of course, this was also responsible for the larger size and thinner thickness of α-Al 2 O 3 obtained in the case of higher NH 4 F content. In general, a small primary grain size was helpful in improving the mechanical properties of ceramic bodies. Figure 7e,f illustrates the SEM images of α-Al 2 O 3 synthesized at 850 • C from A4 and at 800 • C from A5. Apparently, the produced α-Al 2 O 3 presented similar particle sizes. Considering the particle size of as-received α-Al 2 O 3 , the F content, and the resultant temperature, under the synergistic effects of oxalic acid and NH 4 F, mixing 1 wt.% NH 4 F was optimal. In summary, plate-like α-Al 2 O 3 can be successfully synthesized by the incorporation of 1 wt.% NH 4 F with the assistance of oxalic acid at 850 • C. Table 1 illustrates detailed information about the synthesis of α-Al 2 O 3 when fluorine-containing mineralizing agents are used. Compared with other reports, the fluoride content and synthesis temperature in this study were the lowest when the morphology of as-synthesized α-Al 2 O 3 was plate-like, confirming the validity of our work.
Materials 2023, 16, x FOR PEER REVIEW 9 of 12 temperature in this study were the lowest when the morphology of as-synthesized α-Al2O3 was plate-like, confirming the validity of our work.

Conclusions
In the current work, oxalic acid and NH 4 F were proposed for the first time as additives in the preparation of plate-like α-Al 2 O 3 . The results showed that plate-like α-Al 2 O 3 can be synthesized at a low temperature of 850 • C with the synergistic effects of oxalic acid and 1 wt.% NH 4 F. Both the synthesis temperature and the content of fluoride were lower than in other reports. With the increase in NH 4 F content, due to the adsorption of F − and the formation of AlOF gas, the morphology of the as-obtained α-Al 2 O 3 changed from granular to plate-like, and the resultant temperature presented a downward trend. Additionally, owning to the adsorption of HF on the starting materials in sintering, the transition order of α-Al 2 O 3 was changed from Al(OH) 3 → χ-Al 2 O 3 → κ-Al 2 O 3 → α-Al 2 O 3 to Al(OH) 3 → γ-Al 2 O 3 → θ-Al 2 O 3 → α-Al 2 O 3 .

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.