**3.1 Synthesis procedure**

Commercial powders of niobium (99%, -200 mesh), aluminum (99%, -300 mesh), and graphite (99%, -200 mesh) were used as starting materials. Firstly, the molar ratio of Nb : Al : C = 4 : 1.3 : 2.7 was selected for investigating the reaction path of Nb4AlC3. Excess Al and less graphite were used because Al might lose during high temperature processing and Cdeficient existed in most of Al-containing MAX phases. The powders were dryly mixed in a resin jar, ball milled for 12 hours, and then sieved. The mixed powders were uniaxially pressed at 5 MPa to form the green compacts in a BN-coated graphite die. Afterwards, the green compacts were heated to 1500, 1550, 1600, 1650, and 1700oC, with a heating rate of 15oC/min in a flowing Ar atmosphere. The samples were held at target temperatures for 60 minutes under a pressure of 5 MPa, and then cooled down to room temperature with the furnace cooling rate. After composition optimization, single-phase dense Nb4AlC3 was

Figure 1 compares the projection of atoms (Figs. 1(a) and (b)) and the Z-contrast image of Nb4AlC3 (Fig. 1(c)), wherein the layer stacking sequence of Nb and Al atoms along the [0001] direction was directly shown. In one Nb-C slab, the number of Nb layers was four, i.e., the regular atomic arrangement was four Nb atoms layers per Al atoms layer alternately stacking along the [0001] direction. The Nb layers were separated by close packed Al atom (0001) planes. The Z-contrast image confirmed that the Nb4AlC3 crystallized in the Ti4AlN3-type crystal structure. It basically consisted of two units: nonstoichiometric NbC0.75 slab and Al atomic plane. The atom arrangement of Nb and Al

Fig. 1. (a) and (b) Atom arrangement of Nb4AlC3. (c) High-resolution Z-contrasting TEM

Commercial powders of niobium (99%, -200 mesh), aluminum (99%, -300 mesh), and graphite (99%, -200 mesh) were used as starting materials. Firstly, the molar ratio of Nb : Al : C = 4 : 1.3 : 2.7 was selected for investigating the reaction path of Nb4AlC3. Excess Al and less graphite were used because Al might lose during high temperature processing and Cdeficient existed in most of Al-containing MAX phases. The powders were dryly mixed in a resin jar, ball milled for 12 hours, and then sieved. The mixed powders were uniaxially pressed at 5 MPa to form the green compacts in a BN-coated graphite die. Afterwards, the green compacts were heated to 1500, 1550, 1600, 1650, and 1700oC, with a heating rate of 15oC/min in a flowing Ar atmosphere. The samples were held at target temperatures for 60 minutes under a pressure of 5 MPa, and then cooled down to room temperature with the furnace cooling rate. After composition optimization, single-phase dense Nb4AlC3 was

**2. Crystal structure** 

was ABABACBCBC [3].

image after FFT filtering of Nb4AlC3 [3].

**3. Hot pressing** 

**3.1 Synthesis procedure** 


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.

Table 1. Phase compositions of the samples sintered at the temperatures range from 1500 to 1700oC [4].

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

$$\text{Nb}\_2\text{Al} + \text{C} = \text{Nb}\_2\text{AlC} \tag{1}$$

$$Nb\_3Al\_2\mathbb{C} + Nb\mathbb{C} = 2Nb\_2Al\mathbb{C} \tag{2}$$

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:

$$Nb\_2Al\text{C} + 2Nb\text{C} = Nb\_4Al\text{C}\_3\tag{3}$$

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 decomposition reaction could be described as:

$$2Pbb\_2AlC = 3Nb\_4AlC\_3 + 2Al\_3Nb + 4Nb$$

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

Sintering and Properties of Nb4AlC3 Ceramic 145

The etched surface of Nb4AlC3 was shown in Fig. 4. Plate-like Nb4AlC3 grains distributed irregularly with a few equiaxed grains. The average grain size of Nb4AlC3 was 50 μm in

Fig. 4. Scanning electron microscope (SEM) micrograph of etched surface of Nb4AlC3 [4].

Fig. 5. Temperature dependences of electrical conductivity and electrical resistivity of

electrical resistivity was obtained with a coefficient of determination, r2, of 0.99:

 β

0

 ρ

ρμ

Figure 5 shows the electrical conductivity and electrical resistivity of Nb4AlC3 in the temperature range of 5-300 K. With the increasing temperature, the electrical conductivity of Nb4AlC3 decreased from 3.35 × 106 Ω-1·m-1 to 1.33 × 106 Ω-1·m-1. The electrical resistivity of Nb4AlC3 increased linearly above 50 K, indicating a metallic characteristic. Fitting the resistivity in the temperature range from 50 to 300 K, the temperature dependence of

( ) ( ) . [ . ( . )] Ω⋅ = − Δ = − *m T* 1 7 133 1 0 0025 273 15 −*T* (5)

**3.2 Microstructure** 

length and 17 μm in width.

**3.3 Properties evaluation** 

Nb4AlC3 [4].

Fig. 2. X-ray diffraction (XRD) patterns of initial powders with the molar ratio of Nb : Al : C = 4 : 1.3 : 2.7 sintered at (a) 1500oC, (b) 1550oC, (c) 1600oC, (d) 1650oC, and (e) 1700oC [4].

Fig. 3. XRD pattern of Nb4AlC3 prepared using initial powders with the molar ratio of Nb : Al : C = 4 : 1.1 : 2.7 [4].

#### **3.2 Microstructure**

144 Sintering of Ceramics – New Emerging Techniques

Fig. 2. X-ray diffraction (XRD) patterns of initial powders with the molar ratio of Nb : Al : C = 4 : 1.3 : 2.7 sintered at (a) 1500oC, (b) 1550oC, (c) 1600oC, (d) 1650oC, and (e) 1700oC [4].

Fig. 3. XRD pattern of Nb4AlC3 prepared using initial powders with the molar ratio of Nb :

Al : C = 4 : 1.1 : 2.7 [4].

The etched surface of Nb4AlC3 was shown in Fig. 4. Plate-like Nb4AlC3 grains distributed irregularly with a few equiaxed grains. The average grain size of Nb4AlC3 was 50 μm in length and 17 μm in width.

Fig. 4. Scanning electron microscope (SEM) micrograph of etched surface of Nb4AlC3 [4].

#### **3.3 Properties evaluation**

Fig. 5. Temperature dependences of electrical conductivity and electrical resistivity of Nb4AlC3 [4].

Figure 5 shows the electrical conductivity and electrical resistivity of Nb4AlC3 in the temperature range of 5-300 K. With the increasing temperature, the electrical conductivity of Nb4AlC3 decreased from 3.35 × 106 Ω-1·m-1 to 1.33 × 106 Ω-1·m-1. The electrical resistivity of Nb4AlC3 increased linearly above 50 K, indicating a metallic characteristic. Fitting the resistivity in the temperature range from 50 to 300 K, the temperature dependence of electrical resistivity was obtained with a coefficient of determination, r2, of 0.99:

$$
\rho(\mu \Omega \cdot m) = \rho\_0 (1 - \beta \Delta T) = 7.13 \, 3 [1 - 0.0025 (273.15 - T)] \tag{5}
$$

Sintering and Properties of Nb4AlC3 Ceramic 147

lower load, the bigger scatter was seen due to the anisotropic nature of grains. Above 50 N, the hardness value converged to 2.6 GPa. Therefore, the intrinsic hardness of Nb4AlC3 was 2.6 GPa. The morphology of the indent produced by a load of 10 N showed that no cracks initiated and propagated from the diagonals, and the material was pushed out around the indent (Fig. 7(a)). The microscale plasticity was associated with infragrain multiple basalplane slips between microlamellae, intergrain sliding, lamellae or grain push out, and microfailures at the ends of the constrained shear-slips (Fig. 7(b)). In addition, the zigzag crack propagation was observed in an individual Nb4AlC3 grain (Fig. 7(c)). Additionally, the measured shear strength of Nb4AlC3 was 116 MPa. The low shear strength implied good

Fig. 7. Vickers hardness of Nb4AlC3 as a function of indentation loads. The insets show (a) push-out, (b) delamination, kink, and basal plane slips, and (c) zigzag crack propagation

Figure 8(a) shows the temperature dependences of normalized Young's moduli of Nb4AlC3, Nb2AlC, β-Ta4AlC3, and Ta2AlC. The temperature dependences of mechanical damping, Q-1, of Nb4AlC3, Nb2AlC, β-Ta4AlC3, and Ta2AlC were shown in Fig. 8(b). Below 1400oC, a slight linear decrease of Young's modulus of Nb4AlC3 was observed with increasing temperature. Whereas, a break was seen at a temperature between 1400 and 1500oC. Similar turning points were also observed at 1200-1300oC for Nb2AlC, 800-900oC for β-Ta4AlC3, and 800- 900oC for Ta2AlC (Fig. 8(a)). Corresponding to the accelerated decrease of Young's modulus, the mechanical damping of Nb4AlC3 also started to increase at 1400oC (Fig. 8(b)). Above 1500oC, more rapid decrease of Young's modulus of Nb4AlC3 was observed with increasing temperature. The rapid decrease of Young's modulus started at 1300oC, 900oC, and 900oC, respectively, for Nb2AlC, β-Ta4AlC3, and Ta2AlC. The higher critical temperature for the rapid decrease of Young's modulus of Nb4AlC3 indicated that this material could be used at much higher temperatures. The Young's modulus of Nb4AlC3 could retain up to 1580oC with a loss of 21%. The modulus loss of Nb4AlC3 was 16% at 1400oC and 21% at 1580oC (Fig.

damage tolerance and easy machinability of Nb4AlC3.

in a grain in an indent under a load of 10 N [4].

where ρ<sup>0</sup> was the electrical resistivity at 273.15 K (μΩ·m), *T* the absolute temperature (K), and β the temperature coefficient of resistivity (K-1). The temperature coefficient of resistivity was 0.0025 K-1.

The thermal expansion coefficient of Nb4AlC3 was measured as 7.2 × 10-6 K-1 in the temperature range of 200-1100oC. Figure 6 shows the temperature dependences of molar heat capacity and thermal conductivity of Nb4AlC3. The molar heat capacity increased linearly with increment of temperature, which fitted a third-order polynomial. The molar heat capacity of Nb4AlC3 approached to a plateau above 1227oC. At room temperature, the molar heat capacity of Nb4AlC3 was determined as 158 J(molK)-1. A least square fitting the temperature dependence of thermal conductivity for Nb4AlC3 was described as:

$$\mathcal{X} = 1\,1.6 + 0.0064T\tag{6}$$

with r2 of 0.99. At 25oC, the thermal conductivity of Nb4AlC3 was deduced to 13.5 W·(m·K)-1. Up to 1227oC, the thermal conductivity of Nb4AlC3 increased to 21.2 W·(m·K)-1. The total thermal conductivity was associated with both the electron and phonon contributions:

$$
\mathcal{A}\_{\text{total}} = \mathcal{A}\_{\text{electron}} + \mathcal{A}\_{\text{phonon}} \tag{7}
$$

in which λ σ *electron* = *L T* <sup>0</sup> (Wiedmann-Franz Law), where σ was the electrical conductivity at the selected temperature *T* , and 8 2 <sup>0</sup> *L WK* 2 45 10 . − − = × ⋅Ω⋅ . At room temperature, the calculated result was 9.6 W·(m·K)-1, about 71% of total thermal conductivity. Therefore, the electrons mainly contributed to the conductivity at 25oC.

Fig. 6. Temperature dependences of molar heat capacity and thermal conductivity of Nb4AlC3 [4].

Figure 7 shows the indentation load dependence of Vickers hardness of Nb4AlC3. The insets were the Vickers indentation on the polished surface of Nb4AlC3 under a load of 10 N. With increasing load from 3 to 200 N, the hardness gradually decreased from 6.2 to 2.6 GPa. At a

The thermal expansion coefficient of Nb4AlC3 was measured as 7.2 × 10-6 K-1 in the temperature range of 200-1100oC. Figure 6 shows the temperature dependences of molar heat capacity and thermal conductivity of Nb4AlC3. The molar heat capacity increased linearly with increment of temperature, which fitted a third-order polynomial. The molar heat capacity of Nb4AlC3 approached to a plateau above 1227oC. At room temperature, the molar heat capacity of Nb4AlC3 was determined as 158 J(molK)-1. A least square fitting the

with r2 of 0.99. At 25oC, the thermal conductivity of Nb4AlC3 was deduced to 13.5 W·(m·K)-1. Up to 1227oC, the thermal conductivity of Nb4AlC3 increased to 21.2 W·(m·K)-1. The total thermal conductivity was associated with both the electron and phonon contributions:

calculated result was 9.6 W·(m·K)-1, about 71% of total thermal conductivity. Therefore, the

Fig. 6. Temperature dependences of molar heat capacity and thermal conductivity of

Figure 7 shows the indentation load dependence of Vickers hardness of Nb4AlC3. The insets were the Vickers indentation on the polished surface of Nb4AlC3 under a load of 10 N. With increasing load from 3 to 200 N, the hardness gradually decreased from 6.2 to 2.6 GPa. At a

 λ

σ

temperature dependence of thermal conductivity for Nb4AlC3 was described as:

λλ

*electron* = *L T* <sup>0</sup> (Wiedmann-Franz Law), where

the selected temperature *T* , and 8 2

electrons mainly contributed to the conductivity at 25oC.

λ

<sup>0</sup> was the electrical resistivity at 273.15 K (μΩ·m), *T* the absolute temperature (K),

the temperature coefficient of resistivity (K-1). The temperature coefficient of

= + 11 6 0 0064 . . *T* (6)

*total electron* = + *phonon* (7)

<sup>0</sup> *L WK* 2 45 10 . − − = × ⋅Ω⋅ . At room temperature, the

was the electrical conductivity at

where

in which

Nb4AlC3 [4].

λ

 σ

and β

ρ

resistivity was 0.0025 K-1.

lower load, the bigger scatter was seen due to the anisotropic nature of grains. Above 50 N, the hardness value converged to 2.6 GPa. Therefore, the intrinsic hardness of Nb4AlC3 was 2.6 GPa. The morphology of the indent produced by a load of 10 N showed that no cracks initiated and propagated from the diagonals, and the material was pushed out around the indent (Fig. 7(a)). The microscale plasticity was associated with infragrain multiple basalplane slips between microlamellae, intergrain sliding, lamellae or grain push out, and microfailures at the ends of the constrained shear-slips (Fig. 7(b)). In addition, the zigzag crack propagation was observed in an individual Nb4AlC3 grain (Fig. 7(c)). Additionally, the measured shear strength of Nb4AlC3 was 116 MPa. The low shear strength implied good damage tolerance and easy machinability of Nb4AlC3.

Fig. 7. Vickers hardness of Nb4AlC3 as a function of indentation loads. The insets show (a) push-out, (b) delamination, kink, and basal plane slips, and (c) zigzag crack propagation in a grain in an indent under a load of 10 N [4].

Figure 8(a) shows the temperature dependences of normalized Young's moduli of Nb4AlC3, Nb2AlC, β-Ta4AlC3, and Ta2AlC. The temperature dependences of mechanical damping, Q-1, of Nb4AlC3, Nb2AlC, β-Ta4AlC3, and Ta2AlC were shown in Fig. 8(b). Below 1400oC, a slight linear decrease of Young's modulus of Nb4AlC3 was observed with increasing temperature. Whereas, a break was seen at a temperature between 1400 and 1500oC. Similar turning points were also observed at 1200-1300oC for Nb2AlC, 800-900oC for β-Ta4AlC3, and 800- 900oC for Ta2AlC (Fig. 8(a)). Corresponding to the accelerated decrease of Young's modulus, the mechanical damping of Nb4AlC3 also started to increase at 1400oC (Fig. 8(b)). Above 1500oC, more rapid decrease of Young's modulus of Nb4AlC3 was observed with increasing temperature. The rapid decrease of Young's modulus started at 1300oC, 900oC, and 900oC, respectively, for Nb2AlC, β-Ta4AlC3, and Ta2AlC. The higher critical temperature for the rapid decrease of Young's modulus of Nb4AlC3 indicated that this material could be used at much higher temperatures. The Young's modulus of Nb4AlC3 could retain up to 1580oC with a loss of 21%. The modulus loss of Nb4AlC3 was 16% at 1400oC and 21% at 1580oC (Fig.

Sintering and Properties of Nb4AlC3 Ceramic 149

ethanol as the dispersant. After milling, the mixed powders were dried in air and sieved using a 100 mesh sieve. The obtained mixture was put into a graphite die with a diameter of 20 mm. A layer of carbon sheet (~0.2 mm thickness) was put in the inner of die for lubrication. A layer of heat isolation carbon fiber was used to wrap the die for inhibiting the rapid heat diffusion. The mixture was firstly cold pressed as a compact green. Then the green together with the die was heated in a spark plasma sintering facility (100 kN SPS-1050, Syntex Inc., Japan). The sintering temperature was measured by an optical pyrometer focusing on a hole in the wall of die. From ambient temperature to 700oC, it took 5 minutes to heat the sample. Between 700 and 1400oC, a heating speed of 50oC/min was adopted. Above 1400oC, the heating speed was set as 10oC/min. The annealing temperatures were selected as 800, 1000, 1200, 1400, and 1600oC, respectively. The vacuum degree was 7-10 Pa.

Fig. 10. XRD patterns of samples sintered at different temperatures using Nb, Al, and carbon black mixture powders with the molar ratio of 4 : 1.5 : 2.7: (a) ambient temperature, (b) 800oC,

Figure 10 shows XRD patterns of samples sintered from ambient temperature to 1600oC using Nb, Al, and carbon black mixture powders with the molar ratio of 4 : 1.5 : 2.7. The XRD data of initial mixture powder was shown in Fig. 10(a). Carbon black could not be detected, which might be due to the fine structure. When the temperature was increased to 800oC, Al3Nb was detected in the sample by XRD (Fig. 10(b)). The melting point of aluminum was 660oC, which meant that the melting aluminum probably combined niobium

(c) 1000oC, (d) 1200oC, (e) 1400oC, and (f) 1600oC [5].

to form Al3Nb during the heating process:

The holding time was 2 minutes.

8(a)). Therefore, the flexural strength of Nb4AlC3 might still possess a high value up to 1580oC. Figure 9 displays the temperature dependence of flexural strength for Nb4AlC3. Obviously, the flexural strength of Nb4AlC3 could retain up to 1400oC without any degradation.

Fig. 8. Temperature dependences of (a) normalized Young's moduli and (b) mechanical damping, Q-1, of Nb4AlC3, Nb2AlC, β-Ta4AlC3, and Ta2AlC [4].

Fig. 9. Temperature dependent flexural strength of Nb4AlC3 and β-Ta4AlC3 [4].

#### **4. Spark plasma sintering**

#### **4.1 Synthesis procedure**

Commercial powders of niobium (45 μm, 99.9%), aluminum (30 μm, 99.9%), and carbon black (20 nm, 99%) were used for investigating the synthesis of Nb4AlC3 using the SPS technique. For investigating the reaction path, niobium, aluminum, and carbon black powders with a molar ratio of 4 : 1.5 : 2.7 were weighed using an electrical balance with an accuracy of 10-2 g. The powders were put into an agate jar and milled for 12 hours using

8(a)). Therefore, the flexural strength of Nb4AlC3 might still possess a high value up to 1580oC. Figure 9 displays the temperature dependence of flexural strength for Nb4AlC3. Obviously, the flexural strength of Nb4AlC3 could retain up to 1400oC without any

 Fig. 8. Temperature dependences of (a) normalized Young's moduli and (b) mechanical

Fig. 9. Temperature dependent flexural strength of Nb4AlC3 and β-Ta4AlC3 [4].

Commercial powders of niobium (45 μm, 99.9%), aluminum (30 μm, 99.9%), and carbon black (20 nm, 99%) were used for investigating the synthesis of Nb4AlC3 using the SPS technique. For investigating the reaction path, niobium, aluminum, and carbon black powders with a molar ratio of 4 : 1.5 : 2.7 were weighed using an electrical balance with an accuracy of 10-2 g. The powders were put into an agate jar and milled for 12 hours using

**4. Spark plasma sintering** 

**4.1 Synthesis procedure** 

damping, Q-1, of Nb4AlC3, Nb2AlC, β-Ta4AlC3, and Ta2AlC [4].

degradation.

ethanol as the dispersant. After milling, the mixed powders were dried in air and sieved using a 100 mesh sieve. The obtained mixture was put into a graphite die with a diameter of 20 mm. A layer of carbon sheet (~0.2 mm thickness) was put in the inner of die for lubrication. A layer of heat isolation carbon fiber was used to wrap the die for inhibiting the rapid heat diffusion. The mixture was firstly cold pressed as a compact green. Then the green together with the die was heated in a spark plasma sintering facility (100 kN SPS-1050, Syntex Inc., Japan). The sintering temperature was measured by an optical pyrometer focusing on a hole in the wall of die. From ambient temperature to 700oC, it took 5 minutes to heat the sample. Between 700 and 1400oC, a heating speed of 50oC/min was adopted. Above 1400oC, the heating speed was set as 10oC/min. The annealing temperatures were selected as 800, 1000, 1200, 1400, and 1600oC, respectively. The vacuum degree was 7-10 Pa. The holding time was 2 minutes.

Fig. 10. XRD patterns of samples sintered at different temperatures using Nb, Al, and carbon black mixture powders with the molar ratio of 4 : 1.5 : 2.7: (a) ambient temperature, (b) 800oC, (c) 1000oC, (d) 1200oC, (e) 1400oC, and (f) 1600oC [5].

Figure 10 shows XRD patterns of samples sintered from ambient temperature to 1600oC using Nb, Al, and carbon black mixture powders with the molar ratio of 4 : 1.5 : 2.7. The XRD data of initial mixture powder was shown in Fig. 10(a). Carbon black could not be detected, which might be due to the fine structure. When the temperature was increased to 800oC, Al3Nb was detected in the sample by XRD (Fig. 10(b)). The melting point of aluminum was 660oC, which meant that the melting aluminum probably combined niobium to form Al3Nb during the heating process:

Sintering and Properties of Nb4AlC3 Ceramic 151

When the annealing temperature was increased up to 1665oC, more Al3Nb was formed and a small quantity of NbC also appeared in the sample (Fig. 11(c)). NbC was from the decomposition of Nb4AlC3 due to the loss of Al. Up to 1680oC, the amount of NbC increased and Al3Nb disappeared in the sample (Fig. 11(d)). The optimized annealing temperature in

Fig. 11. Effect of annealing temperature on the synthesis of Nb4AlC3: (a) 1620oC, (b) 1650oC,

In order to investigate the effect of initial composition on the synthesis of Nb4AlC3, the initial mixture powders with different molar ratios of Nb : Al : C = 4 : 1.1 : 2.7, 4 : 1.3 : 2.7, 4 : 1.4 : 2.7, and 4 : 1.5 : 2.7 were selected and sintered at the optimized temperature of 1650oC. Figure 12 shows the XRD patterns of sintered samples. With the increasing Al content in the initial compositions, the amount of NbC in the sample decreased continuously (Figs. 12(a)- (d)). The optimized composition for synthesizing Nb4AlC3 by hot pressing in a flowing argon atmosphere was Nb : Al : C = 4 : 1.1 : 2.7. Because of the high vacuum level in SPS furnace (7-10 Pa), Al element was easier to evaporate at high temperature. Therefore, more Al element was added into the compositions. When the initial composition of Nb : Al : C = 4 : 1.5 : 2.7 was used for preparing Nb4AlC3, there were only a small quantity of Nb2AlC and a trace of Al3Nb existing in the sample. Therefore, the optimized composition was selected as 4 : 1.5 : 2.7. Additionally, it was hoped to eliminate the impurities of Nb2AlC and Al3Nb by modifying the holding time. However, when prolonging the holding time from 2 to 4 minutes, though Al3Nb has disappeared, a plenty of NbC appeared in the sample and Nb2AlC couldn't be removed, as shown in Fig. 13. Therefore, the optimized holding time in

Based on the above investigations, the optimized parameters were used to synthesize dense bulk Nb4AlC3. Figure 14 shows the X-ray diffraction pattern of as-prepared Nb4AlC3. The

present work was 1650oC.

(c) 1665oC, and (d) 1680oC [5].

present work was 2 minutes.

$$3Al(l) + Nb(s) \to Al\_3Nb(s) \tag{8}$$

When increasing the temperature up to 1000oC, Nb2Al and Nb2C appeared in the sample with the consumption of Al3Nb (Fig. 10(c)). The reaction could be possibly described as:

$$Al\_3Nb(s) + 5Nb(s) \to 3Nb\_2Al(s) \tag{9}$$

Additionally, Nb2C was formed due to the reaction between Nb and carbon black:

$$2Nb(s) + C(s) \to Nb\_2C(s) \tag{10}$$

As temperature increased to 1200oC, the diffraction analysis showed that new phases of NbC and Nb2AlC became the main phases in the sintered sample (Fig. 10(d)). The amounts of Nb2Al and Nb2C also increased with the consumption of Nb, Al, and Al3Nb. Probably, the formation of NbC was ascribed to the reaction:

$$\text{Nb}(\text{s}) + \text{C}(\text{s}) \to \text{NbC}(\text{s}) \tag{11}$$

Nb2AlC was probably formed due to the reaction between Nb2Al and carbon black. The reaction equation was described as:

$$\text{Nb}\_2\text{Al(s)} + \text{C(s)} \rightarrow \text{Nb}\_2\text{AlC(s)}\tag{12}$$

When the temperature increased up to 1400-1600oC, the existed phases in the samples were only NbC, Nb2AlC, and Nb4AlC3. In hot pressing, it was found that Nb3Al2C existed in the sample sintered by hot pressing at 1500oC. Due to the initial composition difference (Nb : Al : C = 4 : 1.3 : 2.7), it might be due to the kinetics of phase formation. At 1400oC, Nb4AlC3 was detected in the prepared sample (Fig. 10(e)). Probably, it was ascribed to the following two equations:

$$\text{Nb}\_2\text{AlCl}(\text{s}) + \text{Nb}\_2\text{C}(\text{s}) + \text{C}(\text{s}) \to \text{Nb}\_4\text{AlCl}\_3(\text{s}) \tag{13}$$

and

$$\text{Nb}\_2\text{AlCl}(\text{s}) + 2\text{NbC}(\text{s}) \rightarrow \text{Nb}\_4\text{AlCl}\_3(\text{s}) \tag{14}$$

It was supposed that carbon black has been completely consumed at this temperature. When the temperature rose to 1600oC, Nb4AlC3 became the main phase with the consumption of Nb2AlC and NbC (Fig. 10(f)).

In order to get single phase Nb4AlC3, the sintering temperature was further increased. Figure 11 shows the effect of annealing temperatures on the synthesis of Nb4AlC3. When the annealing temperature was 1620oC, only Nb4AlC3 and Nb2AlC could be detected in the sample (Fig. 11(a)). NbC has been completely consumed during the reaction process (Eq. (14)). When increasing the temperature as 1650oC, less Nb2AlC could be detected in the sample by XRD (Fig. 11(b)). However, Al3Nb appeared again:

$$2\text{ } 9\text{Nb}\_2Al\text{Cl}\text{(s)} \rightarrow 3\text{Nb}\_4Al\text{Cl}\_3\text{(s)} + 2Al\_3Nb\text{(s)} + 4\text{Nb(s)}\tag{15}$$

When increasing the temperature up to 1000oC, Nb2Al and Nb2C appeared in the sample with the consumption of Al3Nb (Fig. 10(c)). The reaction could be possibly described as:

As temperature increased to 1200oC, the diffraction analysis showed that new phases of NbC and Nb2AlC became the main phases in the sintered sample (Fig. 10(d)). The amounts of Nb2Al and Nb2C also increased with the consumption of Nb, Al, and Al3Nb. Probably, the

Nb2AlC was probably formed due to the reaction between Nb2Al and carbon black. The

When the temperature increased up to 1400-1600oC, the existed phases in the samples were only NbC, Nb2AlC, and Nb4AlC3. In hot pressing, it was found that Nb3Al2C existed in the sample sintered by hot pressing at 1500oC. Due to the initial composition difference (Nb : Al : C = 4 : 1.3 : 2.7), it might be due to the kinetics of phase formation. At 1400oC, Nb4AlC3 was detected in the prepared sample (Fig. 10(e)). Probably, it was ascribed to the following two

It was supposed that carbon black has been completely consumed at this temperature. When the temperature rose to 1600oC, Nb4AlC3 became the main phase with the consumption of

In order to get single phase Nb4AlC3, the sintering temperature was further increased. Figure 11 shows the effect of annealing temperatures on the synthesis of Nb4AlC3. When the annealing temperature was 1620oC, only Nb4AlC3 and Nb2AlC could be detected in the sample (Fig. 11(a)). NbC has been completely consumed during the reaction process (Eq. (14)). When increasing the temperature as 1650oC, less Nb2AlC could be detected in the

sample by XRD (Fig. 11(b)). However, Al3Nb appeared again:

Additionally, Nb2C was formed due to the reaction between Nb and carbon black:

formation of NbC was ascribed to the reaction:

reaction equation was described as:

Nb2AlC and NbC (Fig. 10(f)).

equations:

and

<sup>3</sup> 3*Al l Nb s Al Nb s* () () () + → (8)

3 2 *Al Nb s Nb s Nb Al s* () () () + → 5 3 (9)

<sup>2</sup> 2*Nb s C s Nb C s* () () () + → (10)

*Nb s C s NbC s* () () () + → (11)

*Nb Al s C s Nb AlC s* 2 2 () () () + → (12)

*Nb AlC s Nb C s C s Nb AlC s* 2 2 4 3 () () () () + +→ (13)

<sup>2</sup> 4 3 *Nb AlC s NbC s Nb AlC s* () () () + → 2 (14)

<sup>2</sup> 43 3 9 3 24 *Nb AlC s Nb AlC s Al Nb s Nb s* ( ) → ++ () () () (15)

When the annealing temperature was increased up to 1665oC, more Al3Nb was formed and a small quantity of NbC also appeared in the sample (Fig. 11(c)). NbC was from the decomposition of Nb4AlC3 due to the loss of Al. Up to 1680oC, the amount of NbC increased and Al3Nb disappeared in the sample (Fig. 11(d)). The optimized annealing temperature in present work was 1650oC.

Fig. 11. Effect of annealing temperature on the synthesis of Nb4AlC3: (a) 1620oC, (b) 1650oC, (c) 1665oC, and (d) 1680oC [5].

In order to investigate the effect of initial composition on the synthesis of Nb4AlC3, the initial mixture powders with different molar ratios of Nb : Al : C = 4 : 1.1 : 2.7, 4 : 1.3 : 2.7, 4 : 1.4 : 2.7, and 4 : 1.5 : 2.7 were selected and sintered at the optimized temperature of 1650oC. Figure 12 shows the XRD patterns of sintered samples. With the increasing Al content in the initial compositions, the amount of NbC in the sample decreased continuously (Figs. 12(a)- (d)). The optimized composition for synthesizing Nb4AlC3 by hot pressing in a flowing argon atmosphere was Nb : Al : C = 4 : 1.1 : 2.7. Because of the high vacuum level in SPS furnace (7-10 Pa), Al element was easier to evaporate at high temperature. Therefore, more Al element was added into the compositions. When the initial composition of Nb : Al : C = 4 : 1.5 : 2.7 was used for preparing Nb4AlC3, there were only a small quantity of Nb2AlC and a trace of Al3Nb existing in the sample. Therefore, the optimized composition was selected as 4 : 1.5 : 2.7. Additionally, it was hoped to eliminate the impurities of Nb2AlC and Al3Nb by modifying the holding time. However, when prolonging the holding time from 2 to 4 minutes, though Al3Nb has disappeared, a plenty of NbC appeared in the sample and Nb2AlC couldn't be removed, as shown in Fig. 13. Therefore, the optimized holding time in present work was 2 minutes.

Based on the above investigations, the optimized parameters were used to synthesize dense bulk Nb4AlC3. Figure 14 shows the X-ray diffraction pattern of as-prepared Nb4AlC3. The

Sintering and Properties of Nb4AlC3 Ceramic 153

Fig. 14. XRD pattern of dense Nb4AlC3 synthesized using the optimized parameters under a

Figure 15 shows the etched surface and fracture surface of Nb4AlC3. Laminar grains could be clearly observed in the etched surface. The growth of grain did not show the preferable direction, i.e., textured microstructure. The mean grain size was determined as 21 μm in length and 9 μm in width. In the fracture surface, Nb4AlC3 grains exhibited the multiplex damage modes, such as transgranular fracture, intergranular fracture, kink bands, and

 Fig. 15. SEM micrographs of (a) etched surface and (b) fracture surface of dense Nb4AlC3

pressure of 30 MPa [5].

**4.2 Microstructure** 

delaminations.

sample [5].

primary phase was Nb4AlC3 and a few amount of Nb2AlC and Al3Nb existed in the sample. The impurities of Nb2AlC and Al3Nb were less than 6 wt%.

Fig. 12. Effect of initial element compositions (molar ratio of Nb, Al, and C) on the synthesis of Nb4AlC3: (a) 4 : 1.1 : 2.7, (b) 4 : 1.3 : 2.7, (c) 4 : 1.4 : 2.7, and (d) 4 : 1.5 : 2.7 [5].

Fig. 13. Effect of holding time on the synthesis of Nb4AlC3: (a) 2 minutes and (b) 4 minutes [5].

primary phase was Nb4AlC3 and a few amount of Nb2AlC and Al3Nb existed in the sample.

Fig. 12. Effect of initial element compositions (molar ratio of Nb, Al, and C) on the synthesis

Fig. 13. Effect of holding time on the synthesis of Nb4AlC3: (a) 2 minutes and (b) 4 minutes [5].

of Nb4AlC3: (a) 4 : 1.1 : 2.7, (b) 4 : 1.3 : 2.7, (c) 4 : 1.4 : 2.7, and (d) 4 : 1.5 : 2.7 [5].

The impurities of Nb2AlC and Al3Nb were less than 6 wt%.

Fig. 14. XRD pattern of dense Nb4AlC3 synthesized using the optimized parameters under a pressure of 30 MPa [5].

#### **4.2 Microstructure**

Figure 15 shows the etched surface and fracture surface of Nb4AlC3. Laminar grains could be clearly observed in the etched surface. The growth of grain did not show the preferable direction, i.e., textured microstructure. The mean grain size was determined as 21 μm in length and 9 μm in width. In the fracture surface, Nb4AlC3 grains exhibited the multiplex damage modes, such as transgranular fracture, intergranular fracture, kink bands, and delaminations.

Fig. 15. SEM micrographs of (a) etched surface and (b) fracture surface of dense Nb4AlC3 sample [5].

Sintering and Properties of Nb4AlC3 Ceramic 155

Figure 17 shows the temperature dependent electrical conductivity and electrical resistivity of Nb4AlC3 in a temperature range of 25-827oC. With increasing temperature, the electrical conductivity decreased corresponding to the increase of electrical resistivity. The measured electrical conductivity of Nb4AlC3 at 25oC was 2.25 × 106 Ω-1·m-1, higher than that of hot pressed sample (1.33 × 106 Ω-1·m-1), which might be due to the existence of Nb2AlC (3.45 × 106 Ω-1·m-1) and Al3Nb. The electrical resistivity of Nb4AlC3 increased with a linear rule below 300oC. Fitting the electrical resistivity in the temperature range of 25-300oC, the temperature dependent resistivity could be obtained with a determination coefficient of

( ) ( ) . [ . ( )] Ω⋅ = − Δ = − *m T* 1 0 44371 1 0 003048 298 −*T* (18)

<sup>0</sup> was the electrical resistivity at 298 K (μΩ·m), *T* the absolute temperature (K),

 the temperature coefficient of resistivity (K-1). However, the temperature dependent electrical resistivity showed the nonlinear increase above 300oC with the increment of temperature. At 827oC, Nb4AlC3 still had a high electrical conductivity of 0.76 × 106 Ω-1·m-1,

Fig. 17. Temperature dependence of electrical conductivity of Nb4AlC3 in a temperature

0

indicating the excellent high temperature conductive capability.

 β

 ρ

ρμ

range of ambient temperature and 827oC [5].

0.99:

in which

and β ρ

#### **4.3 Properties evaluation**

The thermal expansion and technical thermal expansion coefficient of Nb4AlC3 sample were shown in Fig. 16. With increasing temperature, the thermal expansion of Nb4AlC3 showed the linear change. Fitting the thermal expansion in the temperature range from -128 to 282oC, the temperature dependence of thermal expansion was obtained with a coefficient of determination, <sup>2</sup> *r* , of 0.99:

$$\frac{\Delta L}{L\_0} = -0.18204 + 6.5483 \times 10^{-4} T \tag{16}$$

in which Δ*L* (m) was the length change at temperature *T* (K), and *L*0 (m) was the length of sample at 5oC (initial room temperature). The technical thermal expansion coefficient, α*tech*. (K-1), was defined as:

$$
\alpha\_{\text{techt.}} = \frac{1}{L\_{145K}} \frac{\Delta L\_K - \Delta L\_{145K}}{T\_K - 145} \tag{17}
$$

in which *L*145*<sup>K</sup>* (m) was the length of sample at -128oC, Δ*L*145*<sup>K</sup>* (m) the length change at - 128oC, and Δ*LK* (m) the length change at temperature *TK* (K). The calculated thermal expansion coefficient at 282oC was 6.7 × 10-6 K-1. The technical thermal expansion coefficient of Nb4AlC3 increased from -128 to about -73oC rapidly, which might be attributed to the nonlinear increase of instrument temperature. Above -73oC, the technical thermal expansion coefficient approached a constant.

Fig. 16. Temperature dependence of thermal expansion and technical thermal expansion coefficient of Nb4AlC3 in a temperature range of -128 and 282oC [5].

The thermal expansion and technical thermal expansion coefficient of Nb4AlC3 sample were shown in Fig. 16. With increasing temperature, the thermal expansion of Nb4AlC3 showed the linear change. Fitting the thermal expansion in the temperature range from -128 to 282oC, the temperature dependence of thermal expansion was obtained with a coefficient of

0 18204 6 5483 10 *<sup>L</sup> <sup>T</sup>*

in which Δ*L* (m) was the length change at temperature *T* (K), and *L*0 (m) was the length of sample at 5oC (initial room temperature). The technical thermal expansion coefficient,

*K K*

*L T*

in which *L*145*<sup>K</sup>* (m) was the length of sample at -128oC, Δ*L*145*<sup>K</sup>* (m) the length change at - 128oC, and Δ*LK* (m) the length change at temperature *TK* (K). The calculated thermal expansion coefficient at 282oC was 6.7 × 10-6 K-1. The technical thermal expansion coefficient of Nb4AlC3 increased from -128 to about -73oC rapidly, which might be attributed to the nonlinear increase of instrument temperature. Above -73oC, the technical thermal expansion

Fig. 16. Temperature dependence of thermal expansion and technical thermal expansion

coefficient of Nb4AlC3 in a temperature range of -128 and 282oC [5].

145 1

0

*tech*

α. 4

145

145 *K K*

*L L*

*<sup>L</sup>* . . <sup>Δ</sup> <sup>−</sup> =− + × (16)

Δ −Δ <sup>=</sup> − (17)

α*tech*.

**4.3 Properties evaluation** 

determination, <sup>2</sup> *r* , of 0.99:

(K-1), was defined as:

coefficient approached a constant.

Figure 17 shows the temperature dependent electrical conductivity and electrical resistivity of Nb4AlC3 in a temperature range of 25-827oC. With increasing temperature, the electrical conductivity decreased corresponding to the increase of electrical resistivity. The measured electrical conductivity of Nb4AlC3 at 25oC was 2.25 × 106 Ω-1·m-1, higher than that of hot pressed sample (1.33 × 106 Ω-1·m-1), which might be due to the existence of Nb2AlC (3.45 × 106 Ω-1·m-1) and Al3Nb. The electrical resistivity of Nb4AlC3 increased with a linear rule below 300oC. Fitting the electrical resistivity in the temperature range of 25-300oC, the temperature dependent resistivity could be obtained with a determination coefficient of 0.99:

$$
\rho(\mu \Omega \cdot m) = \rho\_0 (1 - \beta \Delta T) = 0.44371 [1 - 0.003048(298 - T)] \tag{18}
$$

in which ρ<sup>0</sup> was the electrical resistivity at 298 K (μΩ·m), *T* the absolute temperature (K), and β the temperature coefficient of resistivity (K-1). However, the temperature dependent electrical resistivity showed the nonlinear increase above 300oC with the increment of temperature. At 827oC, Nb4AlC3 still had a high electrical conductivity of 0.76 × 106 Ω-1·m-1, indicating the excellent high temperature conductive capability.

Fig. 17. Temperature dependence of electrical conductivity of Nb4AlC3 in a temperature range of ambient temperature and 827oC [5].

Sintering and Properties of Nb4AlC3 Ceramic 157

The ambient flexural strength of Nb4AlC3 was tested as 455 MPa, higher than that of hot pressed Nb4AlC3 (346 MPa), which might be ascribed to the finer grain size. When the samples were tested at 1000 and 1400oC, the flexural strength of Nb4AlC3 were 297 and 230 MPa, respectively. The decrease of flexural strength might be ascribed to the existence of Al3Nb in the samples, which caused the plasticity deformation of bars at high

In this chapter, bulk Nb4AlC3 ceramic was prepared by an *in situ* reaction/hot pressing method and spark plasma sintering using Nb, Al, and carbon as the starting materials. The reaction path was investigated. Additionally, it was found that when different sintering methods were adopted the final properties of ceramic were different. *Hot pressing*: The thermal expansion coefficient was determined as 7.2 × 10-6 K-1 in the temperature range of 200-1100oC. The thermal conductivity of Nb4AlC3 increased from 13.5 W·(m·K)-1 at room temperature to 21.2 W·(m·K)-1 at 1227oC, and the electrical conductivity decreased from 3.35 × 106 to 1.13 × 106 Ω-1·m-1 in a temperature range of 5- 300 K. Nb4AlC3 possessed a low hardness of 2.6 GPa and high flexural strength of 346 MPa. Most significantly, Nb4AlC3 could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580oC was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400oC. *Spark plasma sintering*: The coefficient of thermal expansion was measured as 6.7 × 10-6 K-1 from -128 to 282oC. The electrical conductivity was tested as 0.76 × 106 Ω-1·m-1 at 827oC, showing excellent high temperature conductivity. The Vickers hardness and flexural strength were measured as 3.7 GPa and 455 MPa, respectively. The micro-indentation evaluation indicated the anisotropic response of Nb4AlC3 grains, reflecting the anisotropic crystal structure. Additionally, the flexural strength could remain a high value of 230 MPa up to

Acknowledgements: This work is supported by the "Chunlei" program in Ningbo Institute

[1] H. Nowotny, "Struktuchemie Einiger Verbindungen Der Ubergangsmetalle Mit Den

[2] M. W. Barsoum, "The MN+1AXN Phases: A New Class of Solids; Thermodynamically

[3] C. F. Hu, F. Z. Li, J. Zhang, J. M. Wang, J. Y. Wang, and Y. C. Zhou, "Nb4AlC3: A New Compound Belonging to the MAX Phases," *Scripta Mater*., 57, 893-6 (2007). [4] C. F. Hu, F. Z. Li, L. F. He, M. Y. Liu, J. Zhang, J. M. Wang, Y. W. Bao, J. Y. Wang, and Y.

C. Zhou, "*In Situ* Reaction Synthesis, Electrical and Thermal, and Mechanical

Elementen C, Si, Ge, Sn," *Prog. Solid State Chem.*, 2, 27 (1970).

Properties of Nb4AlC3," *J. Am. Ceram. Soc*., 91, 2258-63 (2008).

Stable Nanolaminates," *Prog. Solid State Chem.*, 28, 201-81 (2000).

of Material Technology and Engineering in China.

temperatures.

**5. Summary** 

1400oC.

**6. References** 

The measured Vickers hardness of as-prepared Nb4AlC3 was 3.7 GPa, which was close to the value of hot pressed Nb4AlC3. Figure 18 displays the three cycles load versus depthof-microindentation of one Nb4AlC3 grain whose basal plane was perpendicular to the surface. Inset was the loops of single cycle indentation on both perpendicular (PE) and parallel (PA) directions, in comparison with that of ZrB2. The three indentation cycles were all open without reversibility. However, the open scope, i.e., loop area, was decreasing with more cycles, which showed the slight harder behavior. Additionally, the indentation responses were different along PA and PE directions. Obviously, the indentation depth and loop area along PA direction were larger than those along PE direction. It was easier to form kink bands along PA direction because the top surface was unconstrained. Pop-in appeared during the indentation when along PE direction, which was probably due to the delaminations between basal planes. In comparison with the hexagonal ZrB2, the smaller elastic recovery indicated the more effective energy dispersive capability.

Fig. 18. Typical load vs. depth of indentation response of one Nb4AlC3 grain whose basal plane is perpendicular to the surface. Inset is the loops of single indentation on both perpendicular (PE) and parallel (PA) directions, in comparison with that of ZrB2 (hexagonal structure) [5].

The ambient flexural strength of Nb4AlC3 was tested as 455 MPa, higher than that of hot pressed Nb4AlC3 (346 MPa), which might be ascribed to the finer grain size. When the samples were tested at 1000 and 1400oC, the flexural strength of Nb4AlC3 were 297 and 230 MPa, respectively. The decrease of flexural strength might be ascribed to the existence of Al3Nb in the samples, which caused the plasticity deformation of bars at high temperatures.

### **5. Summary**

156 Sintering of Ceramics – New Emerging Techniques

The measured Vickers hardness of as-prepared Nb4AlC3 was 3.7 GPa, which was close to the value of hot pressed Nb4AlC3. Figure 18 displays the three cycles load versus depthof-microindentation of one Nb4AlC3 grain whose basal plane was perpendicular to the surface. Inset was the loops of single cycle indentation on both perpendicular (PE) and parallel (PA) directions, in comparison with that of ZrB2. The three indentation cycles were all open without reversibility. However, the open scope, i.e., loop area, was decreasing with more cycles, which showed the slight harder behavior. Additionally, the indentation responses were different along PA and PE directions. Obviously, the indentation depth and loop area along PA direction were larger than those along PE direction. It was easier to form kink bands along PA direction because the top surface was unconstrained. Pop-in appeared during the indentation when along PE direction, which was probably due to the delaminations between basal planes. In comparison with the hexagonal ZrB2, the smaller elastic recovery indicated the more effective energy

Fig. 18. Typical load vs. depth of indentation response of one Nb4AlC3 grain whose basal plane is perpendicular to the surface. Inset is the loops of single indentation on both

perpendicular (PE) and parallel (PA) directions, in comparison with that of ZrB2 (hexagonal

dispersive capability.

structure) [5].

In this chapter, bulk Nb4AlC3 ceramic was prepared by an *in situ* reaction/hot pressing method and spark plasma sintering using Nb, Al, and carbon as the starting materials. The reaction path was investigated. Additionally, it was found that when different sintering methods were adopted the final properties of ceramic were different. *Hot pressing*: The thermal expansion coefficient was determined as 7.2 × 10-6 K-1 in the temperature range of 200-1100oC. The thermal conductivity of Nb4AlC3 increased from 13.5 W·(m·K)-1 at room temperature to 21.2 W·(m·K)-1 at 1227oC, and the electrical conductivity decreased from 3.35 × 106 to 1.13 × 106 Ω-1·m-1 in a temperature range of 5- 300 K. Nb4AlC3 possessed a low hardness of 2.6 GPa and high flexural strength of 346 MPa. Most significantly, Nb4AlC3 could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580oC was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400oC. *Spark plasma sintering*: The coefficient of thermal expansion was measured as 6.7 × 10-6 K-1 from -128 to 282oC. The electrical conductivity was tested as 0.76 × 106 Ω-1·m-1 at 827oC, showing excellent high temperature conductivity. The Vickers hardness and flexural strength were measured as 3.7 GPa and 455 MPa, respectively. The micro-indentation evaluation indicated the anisotropic response of Nb4AlC3 grains, reflecting the anisotropic crystal structure. Additionally, the flexural strength could remain a high value of 230 MPa up to 1400oC.

Acknowledgements: This work is supported by the "Chunlei" program in Ningbo Institute of Material Technology and Engineering in China.
