Fabrication of Cr2AlC powder by molten salt electrolysis at 850 °C with good oxidation resistance

Journal Pre-proof Fabrication of Cr2AlC powder by molten salt electrolysis at 850�°C with good oxidation resistance Pengjie Liu, Mengjun Hu, Liwen Hu, Mingzhu Yin, Hongfei Wu, Meilong Hu PII:

S0925-8388(20)30366-2

DOI:

https://doi.org/10.1016/j.jallcom.2020.154003

Reference:

JALCOM 154003

To appear in:

Journal of Alloys and Compounds

Received Date: 3 November 2019 Revised Date:

20 January 2020

Accepted Date: 22 January 2020

Please cite this article as: P. Liu, M. Hu, L. Hu, M. Yin, H. Wu, M. Hu, Fabrication of Cr2AlC powder by molten salt electrolysis at 850�°C with good oxidation resistance, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Pengjie Liu: Liu Data curation, Writing Original draft preparation. Mengjun Hu: Hu Methodology, Software. Mingzhu Yin, Hongfei Wu, Meilong Hu: Hu Writing- Reviewing and Editing, Visualization. Liwen Hu: Hu Conceptualization, Validation

Fabrication of Cr2AlC powder by molten salt electrolysis at 850

with good oxidation resistance

Pengjie Liu, Mengjun Hu, Liwen Hu*1, Mingzhu Yin, Hongfei Wu, Meilong Hu*2 1. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China 2. Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and Advanced Materials, Chongqing University, Chongqing 400044, China * Corresponding author: [email protected] (Liwen Hu) [email protected] (Meilong Hu)

Abstract The Cr2AlC is a typical ternary intermetallic who possesses unique properties such as good high temperature oxygen resistance, excellent mechanical strength, and so on, making it a potential ceramic candidate to be utilized under a relatively strict circumstance. In this paper, a straightforward and cheaper process was proposed to synthesize high quality and uniform micron-size Cr2AlC, via electrochemical reduction of Cr2O3/Al2O3/C mixed powder at only 850℃. It is demonstrated that Cr2AlC was formed by the following three steps. Firstly, Cr2O3 was reduced to metallic Cr and react with carbon to form CrnCm. Then formed CrnCm will react with reduced alumina to Cr2AlC. The SEM-EDS images confirmed the homogeneous distribution of Cr, Al and C in micron-size Cr2AlC powder. And the TEM and TG results further demonstrate the excellent layered structure and good oxidation resistance of Cr2AlC powder. This work realizes and provides a low-cost and scalable low temperature

1

technique to prepare homogeneous MAX phase powders for commercial applications. Key words: molten salt electrolysis; MAX phase; Cr2AlC ceramic; layered structure

1. Introduction MAX phase has raised more and more concerns in recent years due to its unique properties of combined properties that bridges the gap between ceramics and metals [1,2]. This kind of compounds possess many excellent properties of both ceramics and metals owing to their peculiar layered structure and the mixed metallic-covalent bonds nature, [3,4] such as high mechanical strength, good oxidation resistance at high temperature, high electrical conductivity, excellent thermal conductivity and irradiation resistance etc. [5-7] In Mn+1AXn phase, M represents an early transition metal, A represents a III/IV-group element and X could be C(Carbon) or N(Nitrogen). And n can range from 1 to 3. Various methods have been proposed to synthesize MAX phases in previous research, such as self-propagating high temperature synthesis (SHS), hot pressing (HP), and pulse discharge sintering (PDS) etc. [8-17]. However, all these methods are commonly performed under extreme conditions such as high temperature and high pressure. And pure metals are always needed as the raw materials to synthesis of MAX phases and the 2

production

of

pure

metals

(such

as

Titanium)

is

usually

an

energy-consuming and high cost process. Therefore, a novel and cheaper method was proposed to prepare MAX phase by electrochemical deoxidation of metal oxide/carbon. In recent decades, Cr2AlC has raised more and more concerns owing to its excellent oxidation resistance and mechanical strength. Another distinctive feature of Cr2AlC is its layered structure with alternating atomic layers of chromium carbide and aluminum monolayers, which makes it to be a potential candidate for energy storage systems [19]. This ternary transition metal aluminum carbide was first discovered in the 1960s [20] and was further confirmed during the investigation of the ternary Cr2AlC phase diagram [21]. Cr2AlC was initially fabricated by a magnetron sputtering method [22]. However, this method is still far from industrial production of Cr2AlC MAX phase, mainly because of the high cost of this procedure [19]. Therefore, a simple and green method with low cost is in badly need to synthesize Cr2AlC [8]. The Fray-Farthing-Chen (FFC) Cambridge process, which utilized the solid metal oxides/carbon as the raw material, is a potential alternative technology for the facile synthesis of MAX phase under moderate condition [23]. This method employs the simple oxides as cathode precursor and graphite rod (or inert anode) as an anode, and the experiment was conducted at a relative low temperature at atmospheric 3

pressure. The electrochemical reduction has been leveraged to prepare a diversity of MAX phase in recent years. In this paper, Cr2AlC has been successfully prepared by molten salt electrolysis with the utilization of Cr2O3/Al2O3/C as raw material and the following issues have also been figured out, such as the suitable time duration for electrolysis, the appropriate mole ratio of the precursor , and the formation route of Cr2AlC. It can be found that excessive amount of alumina oxide was necessary to guarantee the purity of Cr2AlC as part of reduced alumina will lose during electrolysis. The formation route was also investigated by examining the electrolytic product at different time during electrolysis. And the results indicate that Cr2AlC was formed by following steps: first Cr2O3 reduced to Cr and Cr will react with carbon to form CrnCm, then CrnCm will react with Al to form Cr2AlC.The obtained Cr2AlC powder was uniformly distributed in micro-size with a layered structure. This work also provides a low-cost and scalable low temperature technique to prepare homogeneous Cr2AlC powders for commercial applications.

2. Material and methods 2.1 materials and methods All raw reagents utilized in this experiment, including Cr2O3 (≥99.0%, ~2µm), Al2O3 (≥99.0%, ~18nm), graphite powder (≥99.85%, ~30nm), and 4

PVB binder (Isopropyl alcohol, Polyvinyl alcohol and Polyethylene glycol) were supplied by ChengDu Chron Chemicals Co., Ltd (Analytical grade). CaCl2 (≥96.0%), was supplied by Sinopharm Chemical reagent Co., Ltd. The mixture of Cr2O3, Al2O3 and graphite powder with different mole ratio, together with PVB binder was prepared by ball milling for 12 hours. Then approximately 0.5g of the milled mixture was pressed under a stable pressure of 10 MPa for 10 min, forming a cylindrical pellet with a diameter of 10 mm [8]. Then the pellet was wrapped by a stainless steel mesh and then tied to a stainless steel electrode to be employed as the cathode while a spectral pure graphite rod with a diameter of 10 mm was used as anode. Pure anhydrous CaCl2 was added into a cylindrical alumina crucible (inner diameter: 7 cm, height 15 cm) and heated to 300

for 12 hours in a

vacuum oven to remove moisture and then performed as electrolyte. Before designed electrolysis started, a pre-electrolysis was conducted at 2.8 V between the Mo rod cathode and graphite anode for at least 12 hours to remove moisture and redox-active species from molten CaCl2 [8]. Then the constant voltage electrolysis was conducted between graphite rod anode and the assembled Cr2O3/Al2O3/C cathode at 850

with high purity argon

gas continuously introduced into the furnace to maintain an inert atmosphere during the electrolysis. As the mole ratio of oxide/carbon has a significant influence on the final product, and the specific information of the mole ratio of oxide/carbon 5

used in each experiment was shown in Table 1. Sample

Precursor ratio

T/

Voltage/V

S-1 S-2

1Cr2O3/0.75Al2O3/1C 1Cr2O3/0.5Al2O3/1C

S-3

1Cr2O3/0.5Al2O3/0.75 850 3.1 22 C Table 1 Contrast experiments with different ratio of oxide/carbon.

850 850

Electrolytic time/h

3.1 3.1

22 22

The electrochemical workstation (Prinston 2273) was used to impose an electrolytic voltage of 3.1 V and the current-time curve was recorded. (The experiment was conducted at 3.1 V which is lower than the decomposition potential of CaCl2 (3.21 V). After electrolysis, the cathode was cooled down to room temperature in the vertical furnace under the continuous flow of argon gas. Then the cathode was taken out from the furnace, and washed with deionized water for several times to remove the residual CaCl2. After washing, the product was dried at 80

in a drying

oven for further characterization. In order to systematically investigate the reaction routine for preparing Cr2AlC, the washed intermediate products were selective examined by XRD. Moreover, the oxidation resistance of the as-prepared

Cr2AlC

ceramic

powder

was

then

evaluated

by

thermogravimetric. About 25 mg synthesized Cr2AlC powder was placed in an alumina crucible and heated to 1300

with a heating rate of 10

min-1

in flowing air to determine the onset oxidation temperature of the Cr2AlC ceramic powder.

2.2 Characterization 6

The electrolytic process was conducted with the utilization of an electrochemical workstation (PARSTAT 2273). The cathodic products were characterized by X-ray diffraction spectroscopy (XRD, D/MAX 2500PC), scanning electron microscopy (SEM, JEOL-JSM-7800F, FEI NOVA NANOSEM 400), transmission electron microscopy (TEM, FEI TECNAI G2 F20), energy-dispersive X-ray analysis (EDS, Oxford INCA Energy 350) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The oxidation resistance of the as-prepared Cr2AlC ceramic powder was then evaluated by thermogravimetric analyzer (TG, Setsys Evolution 2400, SETARAM, France).

3. Results and discussion 1-Cr2AlC

2-Cr7C3

3-Cr5Al8 1

1Cr2O3/0.75Al2O3/1C

Intensity (a.u.)

1 1

1 1

1

1 1 1

1

1 1

1

1Cr2O3/0.5Al2O3/1C

2 1 2 2 1 1

22

1

1

1 21

1

1Cr2O3/0.5Al2O3/0.75C 1 1

10

20

30

3 1

3

11

1

40

50

3

60

1

1 1

70

80

90

2θ (degree) Figure 1. XRD patterns of the product obtained from the precursor of oxide/carbon in different mole ratio after electrolysis for 22 hours. 7

Before the experiments started, the possible reactions between the reactants were firstly investigated by theoretical calculation at experimental temperature 850 . And the results are shown as below: Cr2O3 + C = 2CrO + CO (∆Gθ=149 KJ/mol)

(1)

Cr2O3 + 3C = 2Cr + 3CO (∆Gθ=202 KJ/mol)

(2)

3Cr2O3 + 13C = 2Cr3C2 + 9CO (∆Gθ=409 KJ/mol)

(3)

Al2O3 + 3C = 2Al + 3CO(g) (∆Gθ=687 KJ/mol)

(4)

(the statistics were calculated by Factsage at 850 ) It is observed that the calculated standard Gibbs free energies of the mentioned reactions at 850℃ area all greater than zero, suggesting that the chromium sesquioxide and aluminium oxide cannot react with the carbon directly at 850℃. The mole ratio of Cr2O3, Al2O3 and C has great influence on the composition of final products. Therefore, three samples with different mole ratio of Cr2O3, Al2O3 and C were assembled as the cathode to be electrolyzed for 22 hours and Fig. 1 shows the XRD patterns of them. The XRD pattern of sample S-2 with mole ratio of Cr2O3, Al2O3 and C to be 1:0.5:1 according to the atomic ratio of Cr2AlC, shows that Cr2AlC could be obtained after 22 hours’ electrolysis, together with a proportion of undesired Cr7C3 impurity. The formation of Cr7C3 may be due to the loss of aluminum during electrolysis. Although the melting temperature of Cr2O3 and Al2O3 are 2435℃ and 2054℃, which are higher than the experimental 8

temperature. However, the melting temperature of intermediate reduced product aluminum is only 660 , which is lower than the conducted experimental temperature. When the aluminum oxide was deoxidized through the electrochemical reduction, the aluminum can hardly stay in situ to react with chromium and carbon to form Cr2AlC. Similar results have also been found in previous work during the synthesis of Ti3AlC2 by molten salt electrolysis [8], in which the undesired impurity can also be observed when the stoichiometric ratio of oxides/carbon precursor is applied based on the final Ti3AlC2 phase. Therefore, the researchers tried to adjust the molar ratio of oxides/carbon in order to prepare pure Ti3AlC2, and the results demonstrated that less carbon employed can guarantee the purity of Ti3AlC2 phase. It is stated that the relatively pure Ti3AlC2 could be obtained with the mole ratio of TiO2, Al2O3 and C to be 3:0.5:1.5 rather than 3:0.5:2 [24]. In order to get pure Cr2AlC powder, the cathode assembled by Cr2O3,

Al2O3 and C with mole ratio of 1:0.5:0.75 was also employed and marked as sample S-3. The Cr5Al8 impurity can also be detected in the product as observed in XRD pattern. The formation of Cr5Al8 may be attributed to the deficiency of carbon. Then the mole ratio of Cr2O3, Al2O3 and C was slightly increased to be 1:0.75:1, and the results demonstrated that only pure Cr2AlC could be obtained, as seen in XRD pattern in Fig. 1. The results demonstrated that the mole ratio of Cr2O3, Al2O3 and C is 9

an important factor in order to prepare Cr2AlC powder by molten salt electrolysis. The results implied that the deficient carbon can’t improve the purity of Cr2AlC powder, while excessive Al2O3 is necessary to guarantee the purity of the synthesized Cr2AlC powder.

Figure 2 a) SEM image of the precursor materials. Figure 2 b)-d) SEM images of Cr2AlC obtained after 22 hours’ electrolysis from the sample S-1, S-2 and S-3 at 850 .

Fig. 2 presents the SEM images of the sample before and after molten salt electrolysis conducted at 3.1 V for 22 hours. From the SEM image of the precursor (Fig. 2 a)), Cr2O3 particles in a diameter of about 2µm are evenly distributed while Al2O3 in a diameter of about 18nm and graphite in 10

a diameter of ~30nm are also uniformly distributed. The SEM image of Cr2AlC obtained after 22 hours’ electrolysis from sample S-1 is displayed in Fig. 2b). As shown in Fig. 2b, the Cr2AlC particles is in uniform distribution with a diameter of about 2 µm which is in accordance with the XRD pattern, in which Cr2AlC was the only observed phase. Corresponding to the XRD patterns in Fig. 1, the SEM images of the obtained products from sample S-2 and S-3 are also shown in Fig. 2c) and 2d). Fig. 2c presents the characterizations of the product obtained from precursor of Cr2O3, Al2O3 and C with mole ratio of 1:0.5:1 after 22 hours’ electrolysis, in which two different phases are observed, with a small fraction phase demonstrated to be Cr7C3, which is in accordance with XRD results. Similarly, the SEM images of the product obtained from precursor of Cr2O3, Al2O3 and C with mole ratio of 1:0.5:0.75 after 22 hours’ electrolysis are shown in Fig. 2 c) and 2 d). It is demonstrated that two particles with different size can be seen with the smaller particle demonstrated to be Cr5Al8. In this view, the most suitable mole ratio of Cr2O3, Al2O3 and C to prepare pure Cr2AlC should be 1:0.75:1. The excessive aluminum can assure the purity of the Cr2AlC product in this work.

11

Figure 3. EDS mapping of Cr2AlC synthesized in molten salt. Figure 3 b)-d) are corresponding to elements of chromium, aluminum and carbon.

To further investigate into the element distribution of the Cr2AlC powder, EDS spectrum are also conducted and shown in Fig.3. The Cr2AlC powder in about 2 µm could be seen clearly in the SEM image, while the EDS spectrum indicated that the Cr, Al and C elements are homogeneously distributed in the Cr2AlC powder with no obvious boundary among these elements, which means chromium, aluminum and carbon have formed a stable and uniform compound. Generally, the preparation of Cr2AlC powder by molten salt 12

electrolysis follows a complicated process, including the oxygen-ions transfer process and in-situ interaction change during the electrolysis [25]. These processes always result in the phase change during the electrolysis. In order to figure out the phase transmission process before Cr2AlC powder was formed, the intermediate products were examined by terminating the electrolysis at the designed time.

Figure 4. The current-time curve conducted during electrochemical reduction of sample S-1 in molten CaCl2.

Fig. 4 shows the current-time curve recorded during 22 hours’ electrochemical reduction of S-1. As shown in the figure, the current declined rapidly in the first hour and decreased to about 0.5A slowly in the next 9 hours. The current turns to be steady after 10 hours’ electrolysis, 13

which indicated that the electrochemical reduction could almost be finished in about 10 hours. According to the current-time curve, the intermediate phases during electrolysis were examined by terminating the electrolysis at 1, 3, 6 and 10 hours, in order to learn more about the forming process. The XRD pattern of the intermediate products obtained after 1, 3, 6 and 10 hours’ electrolysis are shown in Fig. 5. It is evident that the oxides (Cr2O3 and Al2O3) and carbon are gradually transformed into Cr2AlC, as presented in

Fig.

5.

Intermediate

phase

CaCr2O4

(PDF#

48-0203),

Ca4Al2O6C12·10H2O (PDF# 31-0245) etc., as well as carbide Cr7C3 (PDF# 11-0550) and Cr3C2 (PDF# 35-0804) coexist in the product obtained after 1,3and 6 hours’ electrolysis.

2- Cr7C3

1- Cr2AlC

3-Cr3C2

6-CaCr2O4 7-Al2O3

5-Ca3Al2O6·H2O

Intensity (a.u.)

1

9-C 10-Ca3Al2(OH)12

0 4 00 0 4 0 4

10

6

10

00 10 9 10 008 55 6 5 65

20

1

1

1

1 1

1

10 3 4 443 3 31033333 33 3 3 3 0 10 4 4 2 0 0 10 10 8 4 8 47 7 66 6 2 74

10 02 2

0 4

30

8-Cr2O3

0-Ca4Al2O6Cl2·10H2O

1 1

1

4-Ca2Al(OH)7·3H2O

10 0 27 2 46

40

6h

3

3h

2 9 8 8 2 65 7 2

50

10 h

0 7 65

60

22

70

80

1h 90

2θ (degree) Figure 5. XRD patterns of the product synthesized from the precursor of 14

1Cr2O3/0.75Al2O3/1C after 1, 3, 6 and 10 hours’ electrolysis, respectively.

The XRD pattern of the product obtained after 10 hours’ electrolysis, indicates that pure Cr2AlC could be obtained after 10 hours’ electrolysis, which is in accordance with the current-time curve whose current turned to be steady after 10 hours’ electrolysis.

Figure 6. XPS spectrum of Cr2AlC synthesized in molten salt after electrolysis for 22 hours.

To further investigate into the surface composition, XPS could be employed. The X-ray photoelectron spectroscopy (XPS) showed in Fig.6 can not only be used to confirm the component of elements but also used to analysis the bonding sates of Cr2AlC. The XPS survey spectra revealed Cr, Al, C and O peaks can be observed in the sample. In both high-resolution 15

Cr and C spectra, the Cr-C bond can be detected in the Cr 2p spectrum at 574.4eV and C 1s spectrum at 283.0eV while the Cr-Al and C-Al bonds are absent from the Cr 2p and C 1s spectrum. However, the Al-Al bond can be observed in Al 2p spectrum at 73.2eV. Therefore, the layered Cr2AlC is constructed by alternating atomic layers of chromium carbide and aluminum monolayers, which is coincident to the peculiar layered structure of Cr2AlC. The XPS analysis also indicated that the bonds of Cr-O, C-O and Al-O are existing in the Cr 2p, Al 2p and C 1s spectra respectively, which may be attributed to small amount of residual oxygen in the Cr2AlC phase after electro-deoxidation for 22 hours.

Figure 7. Schematic of the molten salt electrochemical cell and the mechanism of the electrochemical reduction.

Based on the above information, the possible reaction mechanism for preparing Cr2AlC phase by molten salt electrolysis can be summarized in Fig. 7, It shows the scheme of the electrolytic cell and the reaction mechanism.

It

can

be

concluded

that

the

metal

oxide

was 16

electro-deoxidized firstly with oxygen in the metal oxide being ionized at the cathode, dissolved in the molten salt electrolyte and then discharged at the anode [26]. Oxygen ion would react with carbon to generate CO or CO2 gas at the anode. Ultimately, the carbonization occurred when the transition metal elements react with the carbon at the cathode [27]. In order to figure out the reaction sequence, the theoretical decomposition potential of Cr2O3 Al2O3 are also calculated and showed based on the following formula:

E=

−∆G ZF

(5)

Where E is the theoretical decomposition potential, ∆G is the Gibbs free energy at 850℃; F is the faraday constant and Z is the transfer electron number. The electro-deoxidation reactions of the metal oxides are shown as below: 2Cr2O3 = 4Cr + 3O2 (∆Gθ=-1671105J/mol)

(6)

2Al2O3 = 4Al + 3O2 (∆Gθ=-2641237J/mol)

(7)

ECr2O3 =

1671105 = 1.44v 12 *96485

(8)

E Al2O3 =

2641237 = 2.28v 12 *96485

(9)

The theoretical decomposition potential of Cr2O3 (1.44 V) is lower than that of Al2O3 (2.28 V), which means that Cr2O3 is deoxidized firstly and the chromium would be carbonized later. According to the 17

thermodynamic calculation, the carbonization of the chromium occurred spontaneously at 850 ℃. 7Cr + 3C = Cr7C3 (∆Gθ=-196 KJ/mol)

(10)

This can explain why the chromium carbide is formed before Cr2AlC was formed. Then the aluminum would react with the chromium carbide to form Cr2AlC. The formation of Cr2AlC may follow the sequence: Cr2O3 → Cr + C → CrmCn + Al → Cr2AlC.

Figure 8. SEM images of Cr2AlC obtained from the electrolysis for 10 hours and 22 hours.

The microstructure of the product obtained after 10 hours’ and 22 hour’s electrolysis was compared with their SEM images shown in Fig.8. The SEM images indicate that the particle size of Cr2AlC obtained after 22 hour’s electrolysis is a little bigger and brighter than the product obtained after 10 hours’ electrolysis. The reason may be the better electrical conductivity as lower oxygen content in sample after longer time electrolysis. Besides, there is no distinct difference between the products obtained after 10 hours and 22 hours. 18

Figure 9 a). TEM images of Cr2AlC particles. Figure 9 b)-e) EDS mapping of Cr2AlC Figure 9f)-h). TEM and HRTEM images of the particle’s microstructure.

Fig. 9 displays the representative microstructure of Cr2AlC obtained from 1Cr2O3/0.75Al2O3/1C precursor. The result indicates that the particle size of Cr2AlC is about 2 µm as shown in Fig. 9a), which is in accordance with SEM. The particle size of the as-prepared Cr2AlC is much smaller than that reported by Tian et al [28] (around 30 µm). The EDS obtained in TEM images can also prove that the Cr2AlC is a compound with chromium, aluminum and carbon uniformly distributed. MAX phase has attracted extensive interests due to its promising properties in two-dimensional materials owing to their peculiar layered structure. Cr2AlC, the only layered ternary carbide identified in the Cr-Al-C system, has received increasing attention in recent years [29] Fig. 9f further indicates that Cr2AlC ternary intermetallic has a noticeable layered 19

structure. Additionally, HRTEM images of the Cr2AlC micron particles show obvious lattice fringes reveal that the Cr2AlC powder is fine-grained [30] as displayed in Fig. 9 g)-h). The layered structure constructed with

alternating atomic layers of chromium carbide and aluminum monolayers has been proved by the XPS spectrum.

150

TG/%

140 130 120 110 100 0

200

400

600

800

1000

1200

Temperature/ Figure 10. TG curves for the oxidation of Cr2AlC powders at a heating rate of 10

min-1 in air.

Owing

to

the

peculiar

layered

structure

and

the

mixed

metallic-covalent bonds nature, Cr2AlC possesses many excellent properties of both ceramic and metal. TG analyses were conducted to evaluate the oxidation kinetics of Cr2AlC ceramic powder. The oxidation behavior of Cr2AlC was characterized by TG in an Al20

2O3

crucible. Fig. 10 shows the TG curve for the oxidation behavior of

Cr2AlC powder at a heating rate of 10

min-1 in air. The TG curve

revealed that the starting oxidation temperature for Cr2AlC powder is about 800

, which is 400

higher than that of other ternary transition metal

aluminum carbides [29]. Better high temperature oxidation resistance indicates that the Cr2AlC can be utilized as an excellent ceramic material under a relative strict circumstance.

21

Conclusions Cr2AlC has been successfully synthesized from Cr2O3/Al2O3/C mixture precursors by molten salt electrolysis at 850 . The main conclusions are summarized as below. 1. The excessive aluminum is necessary to prepare pure Cr2AlC, and the relatively pure Cr2AlC could be obtained by using Cr2O3, Al2O3 and C precursor with mole ratio of 1:0.75:1. 2. The formation of Cr2AlC follows the route that Cr2O3 was deoxidized and carbonized firstly, then the formed chromium carbide would react with the reduced aluminum to produce Cr2AlC, which can be proved by both XRD and thermodynamic calculation. 3. The XPS and TEM results exhibit that the obtained Cr2AlC MAX phase compound is the layered ternary carbide with alternating atomic layers of chromium carbide and aluminum monolayers. 4. The Cr2AlC powder obtained from electrolytic process is an ultrafine compound that exhibits good oxidation resistance below 800

.

Acknowledgements The authors are grateful to the National Natural Science Foundation of China (No. 51804056), the Fundamental Research Funds for the Central Universities (Project No. 2019CDXYCL0031) and Fundamental and Frontier

Research

Project

of

Chongqing,

China 22

(cstc2019jcyj-msxmX0230).

References [1] Gonzalez-Julian J, Mauer. G, Vassen. R, Cr2AlC MAX phase as bond coat for thermal barrier coatings: Processing, testing under ther mal gradient loading, and future challenges, J. Am. Ceram. Soc. (201 9), https://doi.org/10.1111/jace.16935. [2] Radovic BM, Barsoum MW, MAX phases: bridging the gap betw een metals and ceramics, Am Ceram Soc. Bull. 92 (2013) 20-27. [3] Eklund. P, Rosen. J, Persson. PerO. A, Layered ternary Mn+1AXn phases and their 2D derivative MXene: an overview from a thin-film perspective, J. Phys. D-Appl. Phys. 50 (2017) 113001, https://doi.org /10.1088/1361-6463/aa57bc. [4] Thornberg J, Halim J, Persson PerO. A, Rosen J, Synthesis of (V2/3Sc1/3)2AlC i-MAX phase and V2−xC MXene scrolls, Nanoscale, 11 (2019) 14720-14726, https://doi.org/10.1039/c9nr02354b. [5] X.H Wang, Y.C Zhou, Microstructure and properties of Ti3AlC2 prepared by the solid–liquid reaction synthesis and simultaneous in-sit u hot pressing process, Act. Mater. 50 (2012) 3141-3149, https://doi.o rg/10.1016/s1359-6454(02)00117-9. [6] Y.W. Bao, J.X. Chen, X.H. Wang, Shear strength and shear failur e of layered machinable Ti3AlC2 ceramics, J. Eur. Ceram. Soc. 24 (2 23

004) 855-860, https://doi.org/10.1016/s0955-2219(03)00328-5. [7] Imtyazuddin. M, Mir. AH, Radiation resistance and mechanical pr operties of magnetron-sputtered Cr2AlC thin films, J. Nucl. Mater. 52 6 (2019), https://doi.org/10.1016/j.jnucmat.2019.151742. [8] Li. SS, Zou. XL, Hu. Y, Lu. XG, Xiong. XL, Xu. Q, Cheng. H W, Zhou. ZF, Electrochemical Reduction of TiO2/Al2O3/C to Ti3AlC2 and Its Derived Two-Dimensional (2D) Carbides, Elec. Chem. Soc. 1 65 (2017) 97-107, https://doi.org/10.1149/2.0181803jes. [9] L. M. Peng, Preparation and properties of ternary Ti3AlC2 and its composites from Ti-Al-C powder mixtures with ceramic particulates, J. Am. Ceram. Soc. 90 (2007) 1312-1314, https://doi.org/10.1111/j.15 51-2916.2007.01517.x. [10] A. Zhou, C.A. Wang, Z. Ge, and L. Wu, Preparation of Ti3AlC2 and Ti2AlC by Self-Propagating High-Temperature Synthesis, J. Mate r. Sci. Lett. 20 (2001) 1971-1973, https://doi.org/10.1023/A:101314712 1618. [11] J.-H. Han, S.-S. Hwang, D. Lee, and S.-W. Park, Synthesis and mechanical properties of Ti3AlC2 by hot pressing TiCx/Al powder mix ture J. Eur. Ceram. Soc., 28 (2008) 979-988, https://doi.org/10.1016/j.j eurceramsoc.2007.09.015. [12] Y. Zou, Z. Sun, S. Tada, and H. Hashimoto, Synthesis reactions for Ti3SiC2 through pulse discharge sintering TiH2/Si/TiC powder mi 24

xture, Scr. Mater. 43 (2007) 968-975, https://doi.org/10.1016/j.materres bull.2007.04.028. [13] Y. Zou, Z. Sun, S. Tada, and H. Hashimoto, Synthesis Reactions for Ti3AlC2 Through Pulse Discharge Sintering Ti/Al4C3/TiC Powder Mixture, Scr. Mater., 55 (2006) 767-770, https://doi.org/10.1016/j.scri ptamat.2006.07.018. [14] C. Yang, S. Z. Jin, B. Y. Liang, G. J. Liu, and S. S. Jia, Synth esis of Ti3AlC2 ceramic by high-energy ball milling of elemental pow ders of Ti, Al and C, J. Mater. Process. Tech., 209 (2009) 871-875, https://doi.org/10.1016/j.jmatprotec.2008.02.056. [15] Y. Zou, Z. M. Sun, H. Hashimoto, and L. Cheng, Synthesis rea ctions for Ti3AlC2 through pulse discharge sintering Ti/Al4C3/TiC pow der mixture, Script Mater. 55 (2006) 767-770, https://doi.org/10.1016/j. scriptamat.2006.07.018. [16] S.B. Li, H.X. Zhai, G.P. Bei, Y. Zhou, and Z.L. Zhang, Formati on of Ti3AlC2 by mechanically induced self-propagating reaction in T i–Al–C system at room temperature, Mater. Sci. & Tech. 22 (2006) 6 67-672, https://doi.org/10.1179/174328406X91050. [17] A. Zhou, C.A. Wang, and Y. Huang, Synthesis and mechanical properties of Ti3AlC2 by spark plasma sintering, J. Mater. Sci. 38 (20 03) 3111-3115, https://doi.org/10.1023/A:1024777213910 [18] Amr M. Abdelkader, Molten salts electrochemical synthesis of C 25

r2AlC, J. Eur. Ceram. Soc 36 (2016) 33-42 https://doi.org/10.1016/j.je urceramsoc.2015.09.003. [19] E.I. Zamulaeva, E.A. Levashov, E.A. Skryleva, T.A. Sviridova, P h.V. Kiryukhantsev-Korneev, Conditions for formation of MAX phase Cr2AlC in electrospark coatings deposited onto titanium alloy, Surfac e & Coatings Tech. 298 (2016) 15-23, https://doi.org/10.1016/j.surfcoa t.2016.04.058. [20] Jeitschko. W, Nowotny. H, Benesovsky. F, Ti2AlN eine stickstoff haltige H-Phase, Monatshefte für Chemie, 94 (1963) 1198-1200, https: //doi.org/10.1007/BF00905710. [21] Schuster JC, Nowotny H, Vaccaro C, The ternary systems: Cr-A l-C, V-Al-C, and Ti-Al-C and the behavior of H-phases (M2AlC), J S olid State Chem. 32 (1980) 213, https://doi.org/10.1016/0022-4596(80) 90569-1. [22] Schneider JM, Sun ZM, Mertens R, Uestel F, Ahuja R, Ab initi o calculations and experimental determination of the structure of Cr2 AlC, Solid State Comm. 130 (2004) 445-449, https://doi.org/10.1016/j. ssc.2004.02.047. [23] AM. Abdelkader, Fray. DJ, Electrochemical synthesis of hafnium carbide powder in molten chloride bath and its densification, J. Eur. Ceram. Soc. 32 (2012) 4481-4487, https://doi.org/10.1016/j.jeurcerams oc.2012.07.010. 26

[24] M. A. Pietzka and J. C. Schuster, Summary of constitutional dat a on the Aluminum-Carbon-Titanium System, J. Phase Equilib. 15 (1 994) 392, https://doi.org/10.1007/BF02647559. [25] X. Zou, K. Zheng, X. Lu, Q. Xu, and Z. Zhou, Benefits to ene rgy efficiency and environmental impact: general discussion, Faraday Discuss. 190 (2016) 161-204, https://doi.org/10.1039/C6FD90030e. [26] G.Z. Chen, D.J. Fray, and T.W. Farthing, Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride, Nature, 407 (2000) 361-364, https://doi.org/10.1038/35030069. [27] K. S. Mohandas, Direct electrochemical conversion of metal oxi des to metal by molten salt electrolysis: a review, Min. Proc. & Ext. Met. (2013) 195-212, https://doi.org/10.1179/0371955313Z.0000000006 9 [28] Tian. W, Wang. P, Zhang. G, Kan. Y, Li. Y, Yan. D, Synthesis and thermal and electrical properties of bulk Cr2AlC, Script. Mater. 5 4 (2006) 841-846, https://doi.org/10.1016/j.scriptamat.2005.11.009. [29] Z.J. Lin, M.S. Li, J.Y. Wang, Y.C. Zhou, High-temperature oxid ation and hot corrosion of Cr2AlC, Act. Mater. 55 (2007) 6182-6191, https://doi.org/10.1016/j.actamat.2007.07.024. [30] Yang. LX, Wang. Y, Zhang. HL. Liu. HJ, Zeng. CL, A simple method for the synthesis of nanosized Ti3AlC2 powder in NaCl–KCl molten salt, Mater. Research Lett. 7 (2019) 361-367, https://doi.org/10. 27

1080/21663831.2019.1613695.

28

Highlights A

straightforward

and

cheaper

process,

electrochemical

deoxidation of Cr2O3/Al2O3/C powder mixture at relative low temperature is applied to synthesis high quality, uniform micron-size particles of a typical MAX phase, Cr2AlC. The Cr2AlC powder shows excellent layered structure and good oxidation resistance. The

synthesis

electrochemical

pathway reduction,

for in

Cr2AlC situ

electrochemical compounding processes.

powder

follows

carbonization

and

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.