Sintering and Properties of Nb 4 AlC 3 Ceramic

The chapters this book include emerging new techniques on sintering. Major experts in this field contributed to this book and presented their research. Topics covered in this publication include Spark plasma sintering, Magnetic Pulsed compaction, Low Temperature Co-fired Ceramic technology for the preparation of 3-dimesinal circuits, Microwave sintering of thermistor ceramics, Synthesis of Bio-compatible ceramics, Sintering of Rare Earth Doped Bismuth Titanate Ceramics prepared by Soft Combustion, nanostructured ceramics, alternative solid-state reaction routes yielding densified bulk ceramics and nanopowders, Sintering of intermetallic superconductors such as MgB2, impurity doping in luminescence phosphors synthesized using soft techniques, etc. Other advanced sintering techniques such as radiation thermal sintering for the manufacture of thin film solid oxide fuel cells are also described.


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 Nb 4 AlC 3 . 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 1700 o C, with a heating rate of 15 o C/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 Nb 4 AlC 3 was www.intechopen.com prepared using the starting materials with the molar ratio of Nb : Al : C = 4 : 1.  Figure 2 shows the X-ray diffraction patterns of the samples sintered at 1500-1700 o C 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 1500 o C, the phases in the sample were NbC, Nb 2 AlC, Nb 4 AlC 3 , C, Nb 2 Al, Al 3 Nb, and Nb 3 Al 2 C ( Fig. 2(a)). As the temperature was raised to 1550 o C, only NbC, Nb 2 AlC, Nb 4 AlC 3 , and Al 3 Nb were detected in the sample ( Fig.  2(b)). C, Nb 2 Al, and Nb 3 Al 2 C were completely consumed. The formation of Nb 2 AlC was probably associated with the reactions in equations (1) When the temperature increased to 1600 o C, the amount of Nb 4 AlC 3 increased with the consumption of Nb 2 AlC and NbC ( Fig. 2(c)). Possibly, the reaction occurred as following: When the temperature reached 1650 o C, the diffraction peaks of NbC disappeared. The main crystalline phase was Nb 4 AlC 3 , together with small quantities of Nb 2 AlC and Al 3 Nb ( Fig.  2(d)). When a higher temperature of 1700 o C was used, the final sample contained only Nb 4 AlC 3 and Al 3 Nb (Fig. 2(e)). All diffraction peaks of Nb 2 AlC disappeared. The decomposition reaction could be described as:  Figure 3 shows the X-ray diffraction pattern of single phase Nb 4 AlC 3 . All the diffraction peaks corresponded to Nb 4 AlC 3 . The crystal structure of Nb 4 AlC 3 prepared by the present method was Ti 4 AlN 3 -type. No impurity phases were detected.  www.intechopen.com

Microstructure
The etched surface of Nb 4 AlC 3 was shown in Fig. 4. Plate-like Nb 4 AlC 3 grains distributed irregularly with a few equiaxed grains. The average grain size of Nb 4 AlC 3 was 50 µm in length and 17 µm in width.   [4]. Figure 5 shows the electrical conductivity and electrical resistivity of Nb 4 AlC 3 in the temperature range of 5-300 K. With the increasing temperature, the electrical conductivity of Nb 4 AlC 3 decreased from 3.35 × 10 6 Ω -1 ·m -1 to 1.33 × 10 6 Ω -1 ·m -1 . The electrical resistivity of Nb 4 AlC 3 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, r 2 , of 0.99:

Properties evaluation
where 0 ρ 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 Nb 4 AlC 3 was measured as 7.2 × 10 -6 K -1 in the temperature range of 200-1100 o C. Figure 6 shows the temperature dependences of molar heat capacity and thermal conductivity of Nb 4 AlC 3 . The molar heat capacity increased linearly with increment of temperature, which fitted a third-order polynomial. The molar heat capacity of Nb 4 AlC 3 approached to a plateau above 1227 o C. At room temperature, the molar heat capacity of Nb 4 AlC 3 was determined as 158 J·(mol·K) -1 . A least square fitting the temperature dependence of thermal conductivity for Nb 4 AlC 3 was described as: ⋅ 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 25 o C.  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 Nb 4 AlC 3 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 Nb 4 AlC 3 grain (Fig. 7(c)). Additionally, the measured shear strength of Nb 4 AlC 3 was 116 MPa. The low shear strength implied good damage tolerance and easy machinability of Nb 4 AlC 3 .   Fig. 8(b). Below 1400 o C, a slight linear decrease of Young's modulus of Nb 4 AlC 3 was observed with increasing temperature. Whereas, a break was seen at a temperature between 1400 and 1500 o C. Similar turning points were also observed at 1200-1300 o C for Nb 2 AlC, 800-900 o C for β-Ta 4 AlC 3 , and 800-900 o C for Ta 2 AlC (Fig. 8(a)). Corresponding to the accelerated decrease of Young's modulus, the mechanical damping of Nb 4 AlC 3 also started to increase at 1400 o C ( Fig. 8(b)

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 Nb 4 AlC 3 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 www.intechopen.com 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 700 o C, it took 5 minutes to heat the sample. Between 700 and 1400 o C, a heating speed of 50 o C/min was adopted.
Above 1400 o C, the heating speed was set as 10 o C/min. The annealing temperatures were selected as 800, 1000, 1200, 1400, and 1600 o C, respectively. The vacuum degree was 7-10 Pa. The holding time was 2 minutes.  Figure 10 shows XRD patterns of samples sintered from ambient temperature to 1600 o C 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 800 o C, Al 3 Nb was detected in the sample by XRD (Fig. 10(b)). The melting point of aluminum was 660 o C, which meant that the melting aluminum probably combined niobium to form Al 3 Nb during the heating process: When increasing the temperature up to 1000 o C, Nb 2 Al and Nb 2 C appeared in the sample with the consumption of Al 3 Nb (Fig. 10(c)). The reaction could be possibly described as: Additionally, Nb 2 C was formed due to the reaction between Nb and carbon black: As temperature increased to 1200 o C, the diffraction analysis showed that new phases of NbC and Nb 2 AlC became the main phases in the sintered sample ( Fig. 10(d)). The amounts of Nb 2 Al and Nb 2 C also increased with the consumption of Nb, Al, and Al 3 Nb. Probably, the formation of NbC was ascribed to the reaction: Nb 2 AlC was probably formed due to the reaction between Nb 2 Al and carbon black. The reaction equation was described as: When the temperature increased up to 1400-1600 o C, the existed phases in the samples were only NbC, Nb 2 AlC, and Nb 4 AlC 3 . In hot pressing, it was found that Nb 3 Al 2 C existed in the sample sintered by hot pressing at 1500 o C. 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 1400 o C, Nb 4 AlC 3 was detected in the prepared sample ( Fig. 10(e)). Probably, it was ascribed to the following two equations: It was supposed that carbon black has been completely consumed at this temperature. When the temperature rose to 1600 o C, Nb 4 AlC 3 became the main phase with the consumption of Nb 2 AlC and NbC (Fig. 10(f)).
In order to get single phase Nb 4 AlC 3 , the sintering temperature was further increased. Figure 11 shows the effect of annealing temperatures on the synthesis of Nb 4 AlC 3 . When the annealing temperature was 1620 o C, only Nb 4 AlC 3 and Nb 2 AlC 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 1650 o C, less Nb 2 AlC could be detected in the sample by XRD (Fig. 11(b)). However, Al 3 Nb appeared again: When the annealing temperature was increased up to 1665 o C, more Al 3 Nb was formed and a small quantity of NbC also appeared in the sample (Fig. 11(c)). NbC was from the decomposition of Nb 4 AlC 3 due to the loss of Al. Up to 1680 o C, the amount of NbC increased and Al 3 Nb disappeared in the sample (Fig. 11(d)). The optimized annealing temperature in present work was 1650 o C.  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 Nb 4 AlC 3 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 Nb 4 AlC 3 , there were only a small quantity of Nb 2 AlC and a trace of Al 3 Nb 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 Nb 2 AlC and Al 3 Nb by modifying the holding time. However, when prolonging the holding time from 2 to 4 minutes, though Al 3 Nb has disappeared, a plenty of NbC appeared in the sample and Nb 2 AlC 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 Nb 4 AlC 3 . Figure 14 shows the X-ray diffraction pattern of as-prepared Nb 4 AlC 3 . The www.intechopen.com primary phase was Nb 4 AlC 3 and a few amount of Nb 2 AlC and Al 3 Nb existed in the sample. The impurities of Nb 2 AlC and Al 3 Nb were less than 6 wt%.    Figure 15 shows the etched surface and fracture surface of Nb 4 AlC 3 . 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, Nb 4 AlC 3 grains exhibited the multiplex damage modes, such as transgranular fracture, intergranular fracture, kink bands, and delaminations. www.intechopen.com

Properties evaluation
The thermal expansion and technical thermal expansion coefficient of Nb 4 AlC 3 sample were shown in Fig. 16. With increasing temperature, the thermal expansion of Nb 4 AlC 3 showed the linear change. Fitting the thermal expansion in the temperature range from -128 to 282 o C, the temperature dependence of thermal expansion was obtained with a coefficient of determination, 2 r , of 0.99:  www.intechopen.com Figure 17 shows the temperature dependent electrical conductivity and electrical resistivity of Nb 4 AlC 3 in a temperature range of 25-827 o C. With increasing temperature, the electrical conductivity decreased corresponding to the increase of electrical resistivity. The measured electrical conductivity of Nb 4 AlC 3 at 25 o C was 2.25 × 10 6 Ω -1 ·m -1 , higher than that of hot pressed sample (1.33 × 10 6 Ω -1 ·m -1 ), which might be due to the existence of Nb 2 AlC (3.45 × 10 6 Ω -1 ·m -1 ) and Al 3 Nb. The electrical resistivity of Nb 4 AlC 3 increased with a linear rule below 300 o C. Fitting the electrical resistivity in the temperature range of 25-300 o C, the temperature dependent resistivity could be obtained with a determination coefficient of 0.99: www.intechopen.com The measured Vickers hardness of as-prepared Nb 4 AlC 3 was 3.7 GPa, which was close to the value of hot pressed Nb 4 AlC 3 . Figure 18 displays the three cycles load versus depthof-microindentation of one Nb 4 AlC 3 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 ZrB 2 . 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 ZrB 2 , the smaller elastic recovery indicated the more effective energy dispersive capability. Fig. 18. Typical load vs. depth of indentation response of one Nb 4 AlC 3 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 ZrB 2 (hexagonal structure) [5]. www.intechopen.com The ambient flexural strength of Nb 4 AlC 3 was tested as 455 MPa, higher than that of hot pressed Nb 4 AlC 3 (346 MPa), which might be ascribed to the finer grain size. When the samples were tested at 1000 and 1400 o C, the flexural strength of Nb 4 AlC 3 were 297 and 230 MPa, respectively. The decrease of flexural strength might be ascribed to the existence of Al 3 Nb in the samples, which caused the plasticity deformation of bars at high temperatures.

Summary
In this chapter, bulk Nb 4 AlC 3 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 -  Other advanced sintering techniques such as radiation thermal sintering for the manufacture of thin film solid oxide fuel cells are also described.