**Sintering and Properties of Nb4AlC3 Ceramic**

Chunfeng Hu1, Qing Huang1, Yiwang Bao2 and Yanchun Zhou3 *1Ningbo Institute of Material Science and Technology, Chinese Academy of Sciences, Ningbo 2State Key Laboratory of Green Building Materials, China Building Materials Academy, Beijing 3Ceramic and Composites, Aerospace Research Institute of Materials and Technology, Beijing, China* 

#### **1. Introduction**

140 Sintering of Ceramics – New Emerging Techniques

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Layered ternary compounds, M*n*+1AX*n* (where M is an early transition metal, A is an A group element, X is C or N, and *n* = 1-3), also called the MAX phases, are layered carbides or nitrides crystallizing with hexagonal symmetry structure [1,2]. These ceramics combine the characteristics of metals and ceramics such as high strength and modulus, low density, good electrical and thermal conductivity, easy machinability, damage tolerance, and resistance to thermal shock and high temperature oxidation. To date, more than 50 M2AX compounds (not list for brevity), five M3AX2 compounds (Ti3SiC2, Ti3GeC2, Ti3AlC2, Ta3AlC2, and Ti3SnC2), and seven M4AX3 (Ta4AlC3, Ti4AlN3, Ti4SiC3, Ti4GeC3, Nb4AlC3, V4AlC3, and Ti4GaC3) were identified. For 413 phases, it has been determined that there are two kinds of atomic stacking sequences along [0001] direction. Ti4AlN3, Ti4SiC3, Ti4GeC3, α-Ta4AlC3, Nb4AlC3, and V4AlC3 have the same Ti4AlN3-type crystal structure with atomic arrangement of ABABACBCBC along [0001] direction. Only β-Ta4AlC3 was determined to have the ABABABABAB atomic arrangement along [0001] direction. In detail, the difference of atomic arrangements between β-Ta4AlC3 and α-Ta4AlC3 lay in the diversity of atomic positions. The atomic positions of β-Ta4AlC3 were described as Ta1 at (4*f*) (1/3, 2/3, 0.05524), Ta2 at (4*f*) (2/3, 1/3, 0.16016), Al at (2*c*) (1/3, 2/3, 1/4), C1 at (2*a*) (0, 0, 0), and C2 at (4*e*) (0, 0, 0.11125). While, the atomic positions of α-Ta4AlC3 were determined as Ta1 at (4*f*) (1/3, 2/3, 0.05453), Ta2 at (4*e*) (0, 0, 0.15808), Al at (2*c*) (1/3, 2/3, 1/4), C1 at (2*a*) (0, 0, 0), and C2 at (4*f*) (2/3, 1/3, 0.10324).

Ti4AlN3-type Nb4AlC3 was firstly discovered by heat treating Nb2AlC at 1700oC, and the crystal structure was determined using a combined technique of *ab initio* calculation and high resolution scanning transmission electron microscopy. Additionally, the single phase Nb4AlC3 could be synthesized by hot pressing and spark plasma sintering. The microstructure, electrical, thermal, and mechanical properties of as-prepared Nb4AlC3 were systematically described.

Sintering and Properties of Nb4AlC3 Ceramic 143

prepared using the starting materials with the molar ratio of Nb : Al : C = 4 : 1.1 : 2.7. The

green compact was held at 1700oC for 60 minutes under a pressure of 30 MPa.

1500oC NbC, Nb2AlC, Nb4AlC3, C, Nb2Al, Al3Nb, Nb3Al2C

Table 1. Phase compositions of the samples sintered at the temperatures range from 1500 to

Figure 2 shows the X-ray diffraction patterns of the samples sintered at 1500-1700oC using initial powders with the molar ratio of Nb : Al : C = 4 : 1.3 : 2.7. The identified phase compositions of the samples were listed in Table 1. At 1500oC, the phases in the sample were NbC, Nb2AlC, Nb4AlC3, C, Nb2Al, Al3Nb, and Nb3Al2C (Fig. 2(a)). As the temperature was raised to 1550oC, only NbC, Nb2AlC, Nb4AlC3, and Al3Nb were detected in the sample (Fig. 2(b)). C, Nb2Al, and Nb3Al2C were completely consumed. The formation of Nb2AlC was

When the temperature increased to 1600oC, the amount of Nb4AlC3 increased with the consumption of Nb2AlC and NbC (Fig. 2(c)). Possibly, the reaction occurred as following:

When the temperature reached 1650oC, the diffraction peaks of NbC disappeared. The main crystalline phase was Nb4AlC3, together with small quantities of Nb2AlC and Al3Nb (Fig. 2(d)). When a higher temperature of 1700oC was used, the final sample contained only Nb4AlC3 and Al3Nb (Fig. 2(e)). All diffraction peaks of Nb2AlC disappeared. The

Figure 3 shows the X-ray diffraction pattern of single phase Nb4AlC3. All the diffraction peaks corresponded to Nb4AlC3. The crystal structure of Nb4AlC3 prepared by the present

*Nb Al C Nb AlC* 2 2 + = (1)

3 2 <sup>2</sup> *Nb Al C NbC Nb AlC* + = 2 (2)

<sup>2</sup> 4 3 *Nb AlC NbC Nb AlC* + = 2 (3)

2 43 3 9 3 24 *Nb AlC Nb AlC Al Nb Nb* = ++ (4)

Temperature Phase compositions

1550oC NbC, Nb2AlC, Nb4AlC3, Al3Nb

1600oC NbC, Nb2AlC, Nb4AlC3, Al3Nb

probably associated with the reactions in equations (1) and (2):

1650oC Nb2AlC, Nb4AlC3, Al3Nb

1700oC Nb4AlC3, Al3Nb

decomposition reaction could be described as:

method was Ti4AlN3-type. No impurity phases were detected.

1700oC [4].
