Facile synthesis of hollow Ti3AlC2 microrods in molten salts via Kirkendall effect

The microstructure and morphology of Ti3AlC2 powders not only affect the preparation of Ti3C2 MXene but also have a great influence on their potential applications, such as microwave absorbers, alloy additives, or catalytic supports. However, the synthesis of Ti3AlC2 powders with desired microstructure and morphology remains a challenge. Herein, hollow Ti3AlC2 microrods were prepared for the first time in NaCl/KCl molten salts by using titanium, aluminum, and short carbon fibers as starting materials. It was found that the short carbon fibers not only performed as carbon source but also acted as sacrificial template. Furthermore, it was revealed that TiC and Ti2AlC were initially formed on the surface of carbon fibers. The subsequent reactions between the outer Ti, Al and the inner carbon were dominated by the Kirkendall effect which gave rise to the formation of a hollow structure. Based on this mechanism, hollow Ti3AlC2 microspheres and a series of hollow TiC, Ti2AlC, and V2AlC powders were also successfully fabricated. This work provides a facile route to synthesize hollow MAX phases and may give enlightenment on preparing other hollow carbide powders via the Kirkendall effect in the molten salts.

1 Introduction  MAX phases represent a large family of inherently nanolaminate, hexagonal, and early transition-metal carbides or nitrides, which exhibit relatively high fracture toughness, easy machinability, oxidation resistance, and good electrical conductivity [1,2]. These particular properties make them attractive to many fields such as electrical contacts, catalysis, heat elements, protective coatings, and high-temperature structural applications [3][4][5]. Especially, with the discovery of MXenes in 2011 [6], the MAX phases have captured growing attention since they are the only precursors of MXenes [7,8]. Over the past ten years, the studies of MAX phases mainly focus on the development of new phases. For instance, a series of MAX phases, such as Ti 2 ZnC, Ti 3 ZnC 2 , Ti 2 AuC, Ti 3 Au 2 C 2 , Mo 2 Ga 2 C, V 2 SnC, Sc 2 SnC, Zr 2 SeC, and Ti 2 InB 2 , were successfully synthesized [9][10][11][12][13][14][15][16]. In addition, lots of MAX phase solid solutions such as o-MAX, i-MAX, and even medium-/highentropy MAX were also explored [17][18][19][20][21][22]. These works not only greatly enrich the members of the MAX phase family, but also provide diversified precursors for the preparation of MXenes.
In addition to serving as the precursors of MXenes, another potential application of MAX phase powders is microwave absorbers [23,24]. In this case, the morphology and microstructure of microwave absorbers are of great significance. For example, low-dimensional morphologies, hollow or hierarchical structures are proved to be beneficial to the improvement of microwave absorption performance [25][26][27]. However, it is difficult to fabricate the MAX phase powders with the above-mentioned microstructures since the conventional synthesis methods usually involve high-temperature solid reactions and pulverizations, which leads to the formation of irregular powders. Therefore, although plenty of MAX phases and their solid solutions have been explored, preparing the MAX phase powders with specific morphologies and hollow structures remains a big challenge.
Different from the conventional synthesis methods, molten salt synthesis (MSS) provides a liquid medium that can accelerate the reactions between the precursors and thus reduces the synthesis temperature [28][29][30]. Another advantage of MSS is that the powder materials can be obtained directly by water washing after reactions. This grinding-free method gives the possibility for the regulation of powder morphology. In recent years, several MAX phases including Cr 2 AlC, V 2 AlC, Ti 2 AlC, Sc 2 SnC, Ti 3 SiC 2 , Ti 3 AlC 2 , and V 2 (A x Sn 1−x )C (A = Fe, Co, Ni, Mn) solid solutions have been synthesized successfully by the MSS at relative low temperatures [13,19,23,29,[31][32][33][34][35]. In these works, the obtained MAX phase powders are irregular aggregates that consist of submicron-/nano-grains, and the morphology evolution during the MSS has been overlooked. However, it is demonstrated that, the formation of TiC in chloride molten salts is controlled by "carbon-template growth" mechanism, which means that the morphology of obtained TiC depends on the morphology of carbon precursors during the MSS [36,37]. Since TiC is an important interphase during the formation of Ti 2 AlC or Ti 3 AlC 2 , it is anticipated that the morphologies of these phases may be regulated by using the carbon precursors with different shapes.
To verify this assumption, Ti 3 AlC 2 powders were synthesized in the NaCl/KCl molten salts by using Ti, Al, and short carbon fibers as precursors. As expected, the rod-like Ti 3 AlC 2 powders are obtained. However, the hollow structure is beyond our anticipation. Thus, the reaction process and morphology evolution are investigated, and the formation mechanism of the hollow structure is proposed.
The starting materials were mixed with a molar ratio of Ti : Al : C : NaCl : KCl = 3 : 1.2 : 2 : 4 : 4, and the mixtures were ball-milled in ethanol for 10 h by using zirconia balls (Φ = 5 mm) as milling media. The obtained slurry was dried under a vacuum at 60 ℃ for 2 h and then transferred to alumina boats. Next, the boats were placed into a tube furnace and heated to 1250 ℃ with a heating rate of 4 ℃/min in a flowing Ar atmosphere. After holding for 9 h, the temperature was decreased to 600 ℃ at a rate of 2 ℃/min, and then naturally cooled to room temperature. The obtained samples were washed with deionized water to remove the residual NaCl/KCl salts and then dried at 80 ℃ for 12 h. To investigate the reaction mechanism during the MSS, the mixed powders were heated to 750-1250 ℃ without holding time and cooled to ambient temperature quickly.
The phase compositions were tested by an X-ray diffractometer (D8-Advance, Bruker, Germany) with Cu Kα radiation (λ = 0.15406 nm) in the range of 5°-70°. The purity of obtained Ti 3 AlC 2 was estimated via Rietveld refinement. The morphologies and chemical compositions of obtained Ti 3 AlC 2 powders and starting precursors were characterized via a field emission scanning electron microscope (Apreo S, FEI, USA) equipped with an energy dispersive X-ray spectrometer (EDS, NORAN System 7, Thermo Scientific, USA). To www.springer.com/journal/40145 obtain the internal structures, the obtained powders were embedded into the epoxy resin and cut into pieces, and then polished mechanically. On the other hand, to reveal the formation mechanism of the hollow structure, the unwashed products after reactions at different temperatures were also polished directly by Ar ions (697 Ilion Ⅱ, Gatan, USA) and examined by the SEM and EDS.

Results and discussion
Figure 1(a) shows the XRD pattern and Rietveld refinement of the sample synthesized at 1250 ℃ for 9 h. It can be seen that the obtained product is composed of Ti 3 AlC 2 and a small amount of TiC. The relative content of Ti 3 AlC 2 reaches up to 97.8%, indicating the high purity of the MSS method. Figure 1(b) shows that the obtained Ti 3 AlC 2 powders exhibit a uniform rod-like morphology. However, when observing the internal structures from the cross-section images displayed in Figs. 1(c) and 1(d), it is surprising that the obtained Ti 3 AlC 2 powders represent a hollow structure.
To reveal the formation of a rod-like and hollow structure, the microstructures of short carbon fibers are firstly observed and shown in Figs. S1(a) and S1(b) in the Electronic Supplementary Material (ESM). The statistical analysis of diameters for short carbon fibers and Ti 3 AlC 2 microrods is also presented in Figs. S1(c)-S1(e) in the ESM. Obviously, the short carbon fibers have a uniform rod-like morphology, and their inside is solid, which indicates that the hollow structure of Ti 3 AlC 2 microrods is formed during the synthesis process. Based on the statistical analysis of diameters shown in Figs. S1(c)-S1(e) in the ESM, the average sizes of short carbon fibers and Ti 3 AlC 2 microrods along the radial direction are 7.05 and 13.17 μm, respectively. In addition, it should be noted that the internal diameter of Ti 3 AlC 2 powders is 6.62 μm, which is very close to the diameter of short carbon fibers (7.05 μm). It means that the formation of Ti 3 AlC 2 during the MSS may be initiated on the surface of short carbon fibers and extend radially outward. These results indicate that the short carbon fiber not only performed as carbon source but also acted as sacrificial template during the formation of Ti 3 AlC 2 .
To further clarify the formation mechanism of the hollow structure of Ti 3 AlC 2 during the MSS, the starting mixtures were heated to 750-1250 ℃ without holding time, and the XRD patterns of the representative samples are shown in Fig. 2(a). It is apparent that,  when the reaction temperature is 750 ℃, the main phases in the product include Ti, Ti 3 Al, TiAl, and a very small amount of Ti 2 AlC. With the temperature increasing to 850 ℃, the content of Ti 2 AlC in the product increases slightly. Meanwhile, a trace of TiC can be observed. When the temperature rises from 850 to 1050 ℃, the content of Ti 2 AlC and TiC increases continuously, while the content of Ti, TiAl, and Ti 3 Al decreases gradually. Until 1150 ℃, only Ti 2 AlC and TiC are residual in the product. As the temperature reaches up to 1250 ℃, the Ti 3 AlC 2 phase is present except for Ti 2 AlC and TiC. From the above phase analysis, it is demonstrated that, the interphases, such as Ti 3 Al, TiAl, TiC, and Ti 2 AlC, show up before the formation of Ti 3 AlC 2 during the MSS process of Ti 3 AlC 2 , which is consistent with the previous literature [33]. Figures 2(b)-2(g) show the SEM images and elemental mappings of unwashed samples which were synthesized at 950-1250 ℃ and directly polished by Ar ions. As can be seen from Fig. 2(b), the Ti and Al elements have a similar distribution in molten salt matrix. Especially, a thin layer of Ti and Al elements is observed along the surface of carbon fiber, indicating the formation of TiC/Ti 2 AlC shell according to the analysis of XRD pattern. As the temperature increases to 1150 ℃ , most of the Ti and Al elements are distributed along the surface of carbon fiber (Fig. 2(c)), which means that the reactions among Ti, Al, and C in the molten salts is a process wherein Ti and Al atoms diffuse to the surface of carbon fiber. Figures 2(d)-2(g) display the microstructural evolution as the temperature increases from 950 to 1250 ℃. It is obvious that the gap between the inner carbon fiber and the outer TiC/Ti 2 AlC shell becomes larger as the reactions proceed, finally resulting in the formation of the hollow structure. It should be noted that plenty of small particles are present inside the gaps as shown in Figs. 2(e)-2(g). According to the element mappings (Fig. S2 in the ESM), these small particles are NaCl/KCl mixtures which may play an important role on the transportation of carbon atoms from the inner carbon fiber to the outer TiC/Ti 2 AlC shell. These small particles may come from the molten salts that diffused into the surface of carbon fiber when the TiC/Ti 2 AlC shell has not been completely formed at the initial stage of the reactions. This speculation can be confirmed by the element mappings of Na, K, and Cl in short carbon fibers after being treated in the NaCl/KCl molten salts (Fig. S3 in the ESM).
Based on the above analysis, the underlying formation mechanism of hollow Ti 3 AlC 2 microrods is shown in Fig. 3. Firstly, when the temperature exceeds 700℃, NaCl, KCl, and Al have been melted. With the temperature increasing, the Ti particles begin to dissolve in the molten salts (Ti, Al, and NaCl/KCl molten salts are not shown in Fig. 3). Once the Ti and Al atoms diffuse to the surface of carbon fiber, the reactions among Ti, Al, and C are occurred and give rise to the formation of TiC and Ti 2 AlC on the surface of carbon fiber. As the reactions proceed, the carbon fiber is gradually covered by a shell of TiC/Ti 2 AlC mixtures, which hinders the direct contact between the outer Ti, Al and the inner carbon fiber (Fig. 3(b)). In this situation, the reactions among Ti, Al, and C are determined by the atomic diffusion across the TiC/Ti 2 AlC shell. From the Ti-C binary phase diagram, the composition of titanium carbide (TiC y ) can vary from TiC 0.48 to TiC [38], which means that a large number of C vacancies can exist in TiC lattice, and TiC could be recognized as a buffer pool for the diffusion of C atoms. On the other hand, the theoretical calculation shows that the Ti vacancies have the highest formation energy in Ti 2 AlC lattice [39], indicating the sluggish diffusion of Ti atoms in Ti 2 AlC. Therefore, once the TiC/Ti 2 AlC shell is formed on the surface of carbon fiber, the subsequent reactions involve a process in which the C atoms continuously diffuse outward across the TiC/Ti 2 AlC shell and react with the Ti and Al atoms on the outer surface ( Fig. 3(c)). This is a typical Kirkendall effect that can lead to the formation of the hollow structure as reported in the previous literature [40]. When all Ti and Al in the molten salts react with C from the carbon fiber, the hollow TiC and Ti 2 AlC shell is totally formed (Fig. 3(d)). As the temperature increases, the hollow TiC and Ti 2 AlC shell transforms into the hollow Ti 3 AlC 2 microrods through the in-situ reaction between TiC and Ti 2 AlC (Fig. 3(e)). Based on this synthesis mechanism, the hollow Ti 3 AlC 2 microspheres and a series of hollow TiC, Ti 2 AlC, and V 2 AlC powders are also synthesized (Fig. S4 in the ESM).
To evaluate the advantages of the hollow structure of Ti 3 AlC 2 , the microwave absorption properties were investigated. Figure 4 shows the reflection loss (RL) and effective absorption width (EAB) of Ti 3 AlC 2 (45 wt%)/paraffin hybrids. It can be seen that the RL and EAB values can reach −53.48 dB (10.72 GHz, 2.30 mm) and 4.56 GHz (9.92-14.48 GHz, 2.1 mm), respectively. The above results are better than those in the reported literature in which the mass ratio of MAX phase powders is as high as 70% (Table S1 in the ESM). The reason for the enhancement of microwave absorption properties may be attributed to the hollow and rod-like structure, which is beneficial to the impedance matching and dielectric loss. A further in-depth investigation is needed in the future work. The hollow Ti 3 AlC 2 microrods with high purity were successfully synthesized in the NaCl/KCl molten salts by using titanium, aluminum, and short carbon fibers as starting materials at 1250 ℃ for 9 h. It was found that the short carbon fibers not only performed as carbon source but also acted as sacrificial template. The formation mechanism of the hollow structure could be ascribed to the Kirkendall effect for which the interphases of TiC and Ti 2 AlC played a role in the buffer pool for carbon diffusion. The obtained hollow Ti 3 AlC 2 microrods exhibit good microwave absorption properties. Based on this mechanism, the hollow Ti 3 AlC 2 , Ti 2 AlC, V 2 AlC, and TiC powders were also successfully synthesized. This work is the first attempt to synthesize the micro-sized hollow MAX phases via the Kirkendall effect in the molten salts and may shed light on preparing other hollow carbides powders.
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