Low-Temperature Molten Salt Synthesis and the Characterisation of Submicron-Sized Al8B4C7 Powder

Submicron-sized (~200 nm) aluminium boron carbide (Al8B4C7) particles were synthesised from Al, B4C and carbon black raw materials in a molten NaCl-based salt at a relatively low temperature. The effects of the salt type/assembly and the firing temperature on the synthesis process were examined, and the relevant reaction mechanisms discussed. The molten salt played an important role in the Al8B4C7 formation process. By using a combined salt of 95%NaCl + 5%NaF, an effective liquid reaction medium was formed, greatly facilitating the Al8B4C7 formation. As a result, essentially phase-pure Al8B4C7 was obtained after 6 h of firing at 1250 °C. This temperature was 350–550 °C lower than that required by the conventional direct reaction and thermal reduction methods.

To fabricate high-performance Al 8 B 4 C 7 -based bulk ceramics, high-quality Al 8 B 4 C 7 powder often needs to be used. In this regard, several synthesis methods/techniques have been developed to date, among which the thermal reduction and direct reaction methods have been investigated most extensively. In the former, Al or carbon (C) is often used as a reducing agent [15][16][17][18], so inexpensive and readily available boron-containing oxides can be used as a boron source to replace the much more expensive element boron (B) or B 4 C. However, a high synthesis temperature (1700-1800 • C) is required to complete the formation reaction. Furthermore, some by-products/intermediate phases such as Al 2 O 3 , Al 4 O 4 C and Al 2 OC often remain in the final product powder. In addition, the product particles generally have relatively large sizes and suffer from heavy agglomeration. For example, Zhu et al. [15] and Deng et al. [16] prepared Al 8 B 4 C 7 powder containing secondary phases of Al 2 O 3 , Al 2 OC and Al 4 O 4 C at 1700 • C by using B 2 O 3 (or Na 2 B 4 O 7 ·10H 2 O), Al and C as raw materials. By using similar raw materials and a higher temperature (1800 • C), Cui et al. [17] prepared hexagonal micro-platelets of Al 8 B 4 C 7 containing minor Al 2 OC. On the other hand, Lee et al. [18] prepared Al 3 BC 3 via a complex route using Al(OH) 3 , B 2 O 3 and phenolic resin as raw materials. Despite the use of phenolic resin instead of solid C powder, the synthesis temperature still remained as high as 1725 • C.
In contrast to the thermal reduction method, no reducing agent is required for the direct reaction method. The raw material assemblies commonly used by this method include: (1) Al 4 C 3 and B 4 C; (2) Al, B and C; and (3) Al, B 4 C and C. Unfortunately, this method also suffers from similar drawbacks to those of the thermal reduction method, i.e., high synthesis temperature (1600-1800 • C) [11,13,14,[19][20][21], and relatively large sizes of product particles with heavy agglomeration among them. For example, Inoue et al. [22] synthesised Al 8 B 4 C 7 powder via a respectively direct solid-solid reaction between Al 4 C 3 and B 4 C at 1800 • C, and double stage reactions between Al, B and C initially at 1400 • C and then at 1830 • C. Several other researchers, e.g., Gao et al. [21], Hashimoto et al. [14] and Wang et al. [13], also synthesised Al 8 B 4 C 7 powder at 1600-1800 • C by using Al, B 4 C and C as raw materials.
To overcome the drawbacks of the two main synthesis techniques stated above, it is necessary to develop alternative techniques. As a response to this, in the present work, a molten salt synthesis (MSS) method, used previously to prepare oxide and binary carbide powders [23][24][25], was further developed and extended to synthesise high-quality submicron-sized Al 8 B 4 C 7 powder at a much lower temperature, from Al, B 4 C and C starting materials. As-prepared Al 8 B 4 C 7 powder was characterised, and the effects of key processing factors such as firing temperature and salt type/assembly on MSS were investigated. Based on the experimental results, the synthesis/formation mechanism of Al 8 B 4 C 7 was discussed.

Sample Preparation
Al, B 4 C and C were mixed in the stoichiometric molar ratios of 8:1:6 (1.35:0.35:0.53 g in a powder batch) corresponding to Equation (1), and then they were further combined with 20 g binary salt of 95%NaCl + 5%NaF in an agate mortar. The mixed powder batch was contained in a graphite crucible covered with a graphite lid, and then it was placed in an alumina tube furnace protected by flowing argon (Ar). The furnace was heated to a target temperature between 1100 and 1250 • C (at 5 • C/min to 1000 • C, then 3 • C/min to 1200 • C and finally 1 • C/min to the target temperature) and held at the temperature for 6 h. 8Al To study the effects of salt type/assembly on the formation of Al 8 B 4 C 7 , two other types of salts (NaCl, and 97.5%NaCl + 2.5%NaF) were used, as well as the binary salt NaCl-NaF stated above, to form the reaction media. They were then compared.
In addition, to assist in clarifying the relevant reaction/formation mechanisms, the following supplementary experiment was also carried out, and the resultant samples were similarly characterised (Section 2.3 below). In the first test, Al and B 4 C (1.35 and 0.35 g) in the molar ratio of 8:1 (referred to as Al-B 4 C sample) were heated in 20 g of 95%NaCl + 5%NaF at 1250 • C for 6 h. The reacted mass was further combined with 0.53 g C (so the molar ratio of Al:B 4 C:C = 8:1:6) and reheated at 1250 • C for 6 h in the identical salt. In the second test, Al 4 C 3 (prepared via the reaction of stoichiometric amounts of Al and C in 20 g of 95%NaCl + 5%NaF at 1150 • C for 6 h) was combined with B and C (the molar ratio of Al/B 4 C/C = 8:1:6) (referred to as Al 4 C 3 -B-C sample) and fired at 1250 • C for 6 h.
Some of the samples after firing were placed immediately in a desiccator to avoid the hydration of Al 4 C 3 in them prior to characterisation, whereas the other fired samples were subjected to repeated hot water washing to leach out the residual salt, followed by overnight oven-drying at 100 • C.

Sample Characterisation
Phases in fired samples were identified by powder X-ray diffraction (XRD) analysis (Bruker D8 advance reflection diffractometer, Karlsruhe, Germany). The diffractometer was operated at 40 mA and 40 kV using Ni-filtered CuKa radiation. The scan rate was 2. The microstructure and morphology of the as-prepared product powder were observed using a scanning electron microscope (SEM Nova Nanolab 600, FEI Company, Hillsboro, OR, USA) and a JEM 2100 transmission electron microscope (TEM, 200 kV).  (2)). As shown in Figure 2, AlOOH, i.e., Al 4 C 3 , decreased with the increase in the corresponding firing temperature. It disappeared upon increasing the firing temperature to 1250 • C, verifying the completion of the formation reaction at this temperature ( Figure 2d).

Supplementary Experiment for Mechanism Clarification
Shown in Figure 3 are the XRD patterns of the Al-B4C sample after the first-stage firing, and the patterns after subsequent re-firing with C in the 95%NaCl + 5%NaF salt. After the first-stage firing (Figure 3a), Al3BC was formed as the main phase, along with some AlB2. However, after the secondstage firing with C, Al8B4C7 became the primary phase (Figure 3b), suggesting that the Al3BC that formed in the sample after the first-stage firing was converted into Al8B4C7. Figure 4 further presents the XRD pattern of the Al4C3-B-C sample after 6 h of firing in the 95%NaCl + 5%NaF salt at 1250 °C , revealing the formation of the primary phase of Al8B4C7, as well as minor residual C and Al2O3. The minor Al2O3 detected in this case (also in Figure 3b) was likely a result of the decomposition of

Supplementary Experiment for Mechanism Clarification
Shown in Figure 3 are the XRD patterns of the Al-B4C sample after the first-stage firing, and the patterns after subsequent re-firing with C in the 95%NaCl + 5%NaF salt. After the first-stage firing (Figure 3a), Al3BC was formed as the main phase, along with some AlB2. However, after the secondstage firing with C, Al8B4C7 became the primary phase (Figure 3b), suggesting that the Al3BC that formed in the sample after the first-stage firing was converted into Al8B4C7. Figure 4 further presents the XRD pattern of the Al4C3-B-C sample after 6 h of firing in the 95%NaCl + 5%NaF salt at 1250 °C , revealing the formation of the primary phase of Al8B4C7, as well as minor residual C and Al2O3. The minor Al2O3 detected in this case (also in Figure 3b) was likely a result of the decomposition of  Figure 3 are the XRD patterns of the Al-B 4 C sample after the first-stage firing, and the patterns after subsequent re-firing with C in the 95%NaCl + 5%NaF salt. After the first-stage firing (Figure 3a), Al 3 BC was formed as the main phase, along with some AlB 2 . However, after the second-stage firing with C, Al 8 B 4 C 7 became the primary phase (Figure 3b), suggesting that the Al 3 BC that formed in the sample after the first-stage firing was converted into Al 8 B 4 C 7 . Figure 4 further presents the XRD pattern of the Al 4 C 3 -B-C sample after 6 h of firing in the 95%NaCl + 5%NaF salt at 1250 • C, revealing the formation of the primary phase of Al 8 B 4 C 7 , as well as minor residual C and Al 2 O 3 . The minor Al 2 O 3 detected in this case (also in Figure 3b) was likely a result of the decomposition of AlOOH formed from the quick hydration of Al 4 C 3 by the moisture in the atmosphere during the sample processing. AlOOH formed from the quick hydration of Al4C3 by the moisture in the atmosphere during the sample processing.   Figure 5 demonstrates the effect of salt type/assembly on the Al8B4C7 formation. In the case of using NaCl (Figure 5a), only minor Al8B4C7 was formed, but large amounts of AlOOH were detected in the water-washed sample, indicating the presence of large amounts of intermediate Al4C3 in the original fired sample. This implied the limited accelerating effect of NaCl on the Al8B4C7 formation. However, when small amounts (0.5 g, i.e., 2.5%) of NaF were combined with NaCl, Al8B4C7 became the main phase, although some AlOOH (i.e., Al4C3 in the original fired sample) was still detected (Figure 5b). This indicated the great accelerating effect of the NaF addition on the overall synthesis  AlOOH formed from the quick hydration of Al4C3 by the moisture in the atmosphere during the sample processing.   Figure 5 demonstrates the effect of salt type/assembly on the Al8B4C7 formation. In the case of using NaCl (Figure 5a), only minor Al8B4C7 was formed, but large amounts of AlOOH were detected in the water-washed sample, indicating the presence of large amounts of intermediate Al4C3 in the original fired sample. This implied the limited accelerating effect of NaCl on the Al8B4C7 formation. However, when small amounts (0.5 g, i.e., 2.5%) of NaF were combined with NaCl, Al8B4C7 became the main phase, although some AlOOH (i.e., Al4C3 in the original fired sample) was still detected (Figure 5b). This indicated the great accelerating effect of the NaF addition on the overall synthesis  Figure 5 demonstrates the effect of salt type/assembly on the Al 8 B 4 C 7 formation. In the case of using NaCl (Figure 5a), only minor Al 8 B 4 C 7 was formed, but large amounts of AlOOH were detected in the water-washed sample, indicating the presence of large amounts of intermediate Al 4 C 3 in the original fired sample. This implied the limited accelerating effect of NaCl on the Al 8 B 4 C 7 formation. However, when small amounts (0.5 g, i.e., 2.5%) of NaF were combined with NaCl, Al 8 B 4 C 7 became the main phase, although some AlOOH (i.e., Al 4 C 3 in the original fired sample) was still detected (Figure 5b). This indicated the great accelerating effect of the NaF addition on the overall synthesis process. Upon further increasing the NaF amount to 1 g (i.e., 5%), AlOOH (i.e., Al 4 C 3 ) disappeared and essentially phase-pure Al 8 B 4 C 7 was formed (Figure 5c). The above results indicated that the optimal salt type/assembly in the present work was 95%NaCl + 5%NaF. process. Upon further increasing the NaF amount to 1 g (i.e., 5%), AlOOH (i.e., Al4C3) disappeared and essentially phase-pure Al8B4C7 was formed (Figure 5c). The above results indicated that the optimal salt type/assembly in the present work was 95%NaCl + 5%NaF.  Figure 6 presents SEM and TEM images of Al8B4C7 particles synthesised in 95%NaCl + 5%NaF at 1250 °C for 6 h, revealing their irregular morphologies and average size of about 200 nm. The particles overall were dispersed well, though some were agglomerated together. The average size of the particles was much smaller, and their dispersion was much better than it was when the conventional synthesis techniques were used [15][16][17][18]. The lattice interlayer spacing (one of the insets in Figure 6) was measured as around 0.29 nm, which corresponds to the (111) plane of hexagonal Al8B4C7. This, in addition to the selected area electron diffraction (SAED) pattern (the other inset in Figure 6) and the XRD results in Figures 1 and 2, verified that the synthesised particles were Al8B4C7.  Figure 6 presents SEM and TEM images of Al 8 B 4 C 7 particles synthesised in 95%NaCl + 5%NaF at 1250 • C for 6 h, revealing their irregular morphologies and average size of about 200 nm. The particles overall were dispersed well, though some were agglomerated together. The average size of the particles was much smaller, and their dispersion was much better than it was when the conventional synthesis techniques were used [15][16][17][18]. The lattice interlayer spacing (one of the insets in Figure 6) was measured as around 0.29 nm, which corresponds to the (111) plane of hexagonal Al 8 B 4 C 7 . This, in addition to the selected area electron diffraction (SAED) pattern (the other inset in Figure 6) and the XRD results in Figures 1 and 2, verified that the synthesised particles were Al 8 B 4 C 7 .

Microstructure of As-Prepared Al 8 B 4 C 7 Powder
Materials 2019, 12, x FOR PEER REVIEW 6 of 10 process. Upon further increasing the NaF amount to 1 g (i.e., 5%), AlOOH (i.e., Al4C3) disappeared and essentially phase-pure Al8B4C7 was formed (Figure 5c). The above results indicated that the optimal salt type/assembly in the present work was 95%NaCl + 5%NaF.  Figure 6 presents SEM and TEM images of Al8B4C7 particles synthesised in 95%NaCl + 5%NaF at 1250 °C for 6 h, revealing their irregular morphologies and average size of about 200 nm. The particles overall were dispersed well, though some were agglomerated together. The average size of the particles was much smaller, and their dispersion was much better than it was when the conventional synthesis techniques were used [15][16][17][18]. The lattice interlayer spacing (one of the insets in Figure 6) was measured as around 0.29 nm, which corresponds to the (111) plane of hexagonal Al8B4C7. This, in addition to the selected area electron diffraction (SAED) pattern (the other inset in Figure 6) and the XRD results in Figures 1 and 2, verified that the synthesised particles were Al8B4C7.

Further Discussion and Reaction/Synthesis Mechanisms
Upon increasing the firing temperature above their melting/eutectic points, NaCl (melting point: ~714 °C) and NaF (melting point: ~743 °C) interacted with each other, forming a liquid medium in which Al slightly dissolved [26,27]. The dissolved Al diffused rapidly through the liquid medium onto the surfaces of C and B4C, and then reacted with them to form Al4C3, and Al3BC + AlB2, according to Equations (3) and (4), respectively. 4Al + 3C= Al4C3 (3) 9Al + 2B4C = 2Al3BC + 3AlB2 (4) AlB2 = Al + 2 B Since AlB2 is not thermodynamically stable at >1000 °C [28], the AlB2 formed from Equation (4) decomposed, forming Al and B (Equation (5)) in the molten salt [28]. The detection of Al4C3/AlOOH indicated the occurrence of Equation (3) at the test temperatures (Figure 1a-c and Figure2a-c), and the detection of Al3BC and AlB2 in the Al-B4C sample (Figure 3) indicated the occurrence of Equation (4). The AlB2 phase detected in this case is believed to be formed upon cooling from the Equation between the residual Al and B in the salt. The B formed from Equation (5) at the test temperatures also slightly dissolved in the molten salt [29,30] and then diffused through the molten salt onto the surface of the Al4C3 formed from Equation (3), forming Al8B4C7 according to Equation (6). As shown in Figure 4, Al8B4C7 was formed in the fired Al4C3-B-C sample, indicating that the original Al4C3 reacted directly with the B dissolved in the salt, to form Al8B4C7.
According to Figure 3a, if no carbon was present, the intermediate Al3BC formed from Equation (4) appeared to be stable. However, when C was present, it was readily converted into more stable Al8B4C7 (Figure 4). This also explained why no Al3BC was found in the samples whose XRD patterns are shown in Figures 1 and 2. The mechanism by which it was transformed into Al8B4C7 in the molten salt was not clear, but a plausible mechanism could be considered as follows: when C was present, it reacted with the Al in the molten salt to form Al4C3, which further reacted with the B in the molten salt to form Al8B4C7. The consumption of Al and B in the molten salt might have led to the decomposition of Al3BC and thus the additional formation of Al8B4C7 according to Equation (7).
The overall reaction processes/mechanisms described above can also be used to explain the effects of firing temperature and salt type/assembly on the MSS process. With an increase in the firing

Further Discussion and Reaction/Synthesis Mechanisms
Upon increasing the firing temperature above their melting/eutectic points, NaCl (melting point: 714 • C) and NaF (melting point:~743 • C) interacted with each other, forming a liquid medium in which Al slightly dissolved [26,27]. The dissolved Al diffused rapidly through the liquid medium onto the surfaces of C and B 4 C, and then reacted with them to form Al 4 C 3 , and Al 3 BC + AlB 2 , according to Equations (3) and (4), respectively.
4Al + 3C= Al 4 C 3 9Al + 2B 4 C = 2Al 3 BC + 3AlB 2 (4) Since AlB 2 is not thermodynamically stable at >1000 • C [28], the AlB 2 formed from Equation (4) decomposed, forming Al and B (Equation (5)) in the molten salt [28]. The detection of Al 4 C 3 /AlOOH indicated the occurrence of Equation (3) at the test temperatures (Figures 1a-c and 2a-c), and the detection of Al 3 BC and AlB 2 in the Al-B 4 C sample ( Figure 3) indicated the occurrence of Equation (4). The AlB 2 phase detected in this case is believed to be formed upon cooling from the Equation between the residual Al and B in the salt. The B formed from Equation (5) at the test temperatures also slightly dissolved in the molten salt [29,30] and then diffused through the molten salt onto the surface of the Al 4 C 3 formed from Equation (3), forming Al 8 B 4 C 7 according to Equation (6). As shown in Figure 4, Al 8 B 4 C 7 was formed in the fired Al 4 C 3 -B-C sample, indicating that the original Al 4 C 3 reacted directly with the B dissolved in the salt, to form Al 8 B 4 C 7 .
According to Figure 3a, if no carbon was present, the intermediate Al 3 BC formed from Equation (4) appeared to be stable. However, when C was present, it was readily converted into more stable Al 8 B 4 C 7 (Figure 4). This also explained why no Al 3 BC was found in the samples whose XRD patterns are shown in Figures 1 and 2. The mechanism by which it was transformed into Al 8 B 4 C 7 in the molten salt was not clear, but a plausible mechanism could be considered as follows: when C was present, it reacted with the Al in the molten salt to form Al 4 C 3, which further reacted with the B in the molten salt to form Al 8 B 4 C 7 . The consumption of Al and B in the molten salt might have led to the decomposition of Al 3 BC and thus the additional formation of Al 8 B 4 C 7 according to Equation (7).
The overall reaction processes/mechanisms described above can also be used to explain the effects of firing temperature and salt type/assembly on the MSS process. With an increase in the firing temperature, the solubilities of Al and B in the molten salt were increased, and their diffusions in the molten salt accelerated. Consequently, Equations (3)-(7) were greatly facilitated. Therefore, the overall formation reaction (Equation (1)) was considerably accelerated (Figures 1-4). When a single salt of NaCl was used, there was only limited formation of Al 8 B 4 C 7 in the sample after 6 h of firing at 1250 • C (Figure 5a). However, when small amounts of NaF (2.5%) were added to NaCl, much more Al 8 B 4 C 7 was formed (Figure 5b). Upon further increasing NaF to 5%, the formation reaction was completed, and essentially phase-pure Al 8 B 4 C 7 was obtained (Figure 5c). This can be explained as follows. Al and B have very limited solubility in molten NaCl [31], so Equations (3)-(7) proceeded very slowly in it. However, when NaF was added to NaCl, the solubilities of Al and B in the binary salt were increased significantly, which led to great acceleration of Equations (3)-(7), i.e., the overall formation reaction (Equation (1)).
Thanks to the strong accelerating effect of the NaCl-NaF binary salt discussed above, essentially phase-pure Al 8 B 4 C 7 particles were successfully prepared at 1250 • C. This synthesis temperature was 350-550 • C lower than that required by the conventional synthesis routes [11,[13][14][15][16][17][18][19][20][21], demonstrating the great advantage and feasibility of the MSS technique developed in this work.

Conclusions
A low-temperature molten salt synthesis technique was developed to synthesise high-quality Al 8 B 4 C 7 particles. The main conclusions can be drawn as follows.

1.
Al 8 B 4 C 7 particles with an average size of about 200 nm were successfully synthesised after 6 h of firing in NaCl-NaF at 1250 • C, from Al, B 4 C and C starting powders. They were essentially phase-pure and generally well-dispersed.

2.
Compared with the temperature required by a conventional synthesis technique, the synthesis temperature (1250 • C) in the present work was significantly lower (350-500 • C lower), owing to the great accelerating effect of NaCl-NaF salt. 3.
Al 8 B 4 C 7 particles were formed via the following mechanisms: at the test temperatures, NaCl and NaF interacted with each other, forming a liquid medium in which Al slightly dissolved. The dissolved Al diffused rapidly through the molten salt onto the surfaces of C and B 4 C, reacting with them to form Al 4 C 3 , and Al 3 BC + AlB 2 , respectively. AlB 2 is not stable at >1000 • C, so at the test temperatures, it decomposed into B and Al. The newly formed B also slightly dissolved in the salt, diffused onto the surface of the Al 4 C 3 formed earlier, and reacted with it to form Al 8 B 4 C 7 , which consumed Al and B in the salt, making the Al 3 BC formed earlier decompose into additional Al 8 B 4 C 7 , Al and B.

Conflicts of Interest:
The authors declare no conflict of interest.