| 其他摘要 | Layered ternary Mn+1AXn compounds (where M is an early transition metal, A is an A group element, X is C or N, and n = 1-3) combine the merits of ceramics and metals. They have low hardness, machinability, high Young’s modulus, high strength, excellent damage tolerance and thermal shock resistance, as well as high electrical conductivity and thermal conductivity, promising for high temperature applications. In the last ten years, although a large number of compounds of this MAX family have been investigated, many questions still exist. For example, what are the properties of other ternary compounds, and whether new MAX phases exist etc.. Therefore, based on the previous works on MAX phases, we focused on the investigations of three systems of Ta-Al-C, Nb-Al-C, and V-Al-C, synthesizing Ta2AlC, -Ta4AlC3, Nb4AlC3, V2AlC, and V4AlC3 by an in-situ reaction/hot pressing method, in which -Ta4AlC3, Nb4AlC3, and V4AlC3 were three new MAX phases. Additionally, we investigated the physical and mechanical properties of Ta2AlC, -Ta4AlC3, Nb4AlC3, and V2AlC. The relative descriptions are as follows:
(A) Ta2AlC ceramic was fabricated by in-situ reaction/hot pressing of Ta, Al, and C powders. The reaction path and effects of initial composition on the purity were investigated. It was found that Ta2AlC formed through the reactions between AlTa2 and graphite, or among Ta5Al3C, TaC, and graphite at 1500-1550oC. By modifying the molar ratio of the initial Ta, Al, and C powders, single-phase Ta2AlC was prepared at 1550oC under an Ar atmosphere with an optimized composition of Ta : Al : C = 2 : 1.2 : 0.9. The lattice parameter and a new set of X-ray diffraction data of Ta2AlC were obtained. In addition, Ta2AlC was reported unstable above 1600oC and decomposed to -Ta4AlC3, and then to TaCx.
(B) Dense bulk Ta2AlC ceramic was synthesized by an in-situ reaction/hot pressing method using Ta, Al, and C powders as initial materials. The average grain size of Ta2AlC is 15 m in length and 3 m in width. The physical and mechanical properties were investigated. Ta2AlC is a good electrical and thermal conductor. The flexural strength and fracture toughness of Ta2AlC were measured to be 360 MPa and 7.7 MPa•m1/2, respectively. The typical layered grains contribute to the damage tolerance of this ceramic. After indentation up to 200 N at the tensile surface of the beam specimens, no obvious decrease of the residual flexural strength was observed. Even at above 1200oC, Ta2AlC still retains a high Young’s modulus and shows excellent thermal shock resistance.
(C) Bulk -Ta4AlC3 ceramic was prepared by an in-situ reaction synthesis/hot pressing method using Ta, Al, and C powders as starting materials. The lattice parameter and a new set of XRD data were obtained. The physical and mechanical properties of -Ta4AlC3 ceramic were investigated. -Ta4AlC3 is a good electrical and thermal conductor. The flexural strength and fracture toughness are 372 MPa and 7.7 MPa•m1/2, respectively. Typically plate-like layered grains contribute to the damage tolerance of -Ta4AlC3. After indentation up to 200 N load, no obvious degradation of the residual flexural strength of Ta4AlC3 was observed, demonstrating the damage tolerance of this ceramic. Even at above 1200oC in air, -Ta4AlC3 still retains a high strength and shows excellent thermal shock resistance.
(D) Nb4AlC3, a new compound belonging to the MAX phases, was obtained by annealing bulk Nb2AlC at 1700oC. The crystal structure of Nb4AlC3 was determined by combined x-ray diffraction, high-resolution transmission electron microscopy, and ab initio calculation. It was reported that Nb4AlC3 follows the Ti4AlN3-type crystal structure.
(E) Bulk Nb4AlC3 ceramic was prepared by an in-situ reaction/hot pressing method using Nb, Al, and C as initial materials. The reaction path, microstructure, physical and mechanical properties of Nb4AlC3 were systematically investigated. 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 Ω-1•m-1 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, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa•m1/2. 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 remain the ambient strength value without any degradation up to the maximum measured temperature of 1400oC.
(F) Dense bulk V2AlC ceramic was fabricated by an in-situ reaction/hot pressing method using V, Al, and C powders as initial materials. The grain size of as-prepared V2AlC was temperature-dependent. With increasing temperature from 1400 to 1700oC, the mean grain size of V2AlC increased from 49 m in length and 19 m in width to 405 m in length and 106 m in width. The samples prepared at 1500oC exhibited the highest flexural strength and fracture toughness, while those sintered at 1400oC showed the highest compressive strength. In addition, the Young’s modulus of V2AlC could retain up to 1200oC.
(G) V4AlC3, a new MAX phase, was synthesized by reactive hot pressing of V, Al, and C powder mixture at 1700oC. Using a combination of Rietveld refinement with X-ray diffraction data and ab initio calculations, the crystal structure was determinated. It is found that V4AlC3 has the Ti4AlN3-type crystal structure. |
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