Electronic structures, elastic properties, and minimum thermal conductivities of cermet M3AlN
Graphical abstract
- 1.
Young׳s moduli of anti-perovskite Ti3AlN, Zr3AlN, and Hf3AlN in full space.
- 2.
Electron density differences on crystal planes (1 0 0), (2 0 0), and (1 1 0) of anti-perovskite Zr3AlN.
Introduction
Reports on the outstanding hardness and abrasive properties of transition metal ternary systems have generated scientists׳ interests [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Among them, a kind of cermet, named MAX, is successfully developed incorporating both metals and ceramics properties, such as high hardness, high elastic modulus, corrosion resistance, oxidation resistance, damage tolerance, excellent machinability, and electrical conductivity. In the chemical formula “MAX”, M represent a few groups of early transition metals, A stand for IIIA or IVA group elements, and X is C or N atom. Among them, nearly a hundred kinds of monophase ternary nitrides and carbides have been reported by Nowotny et al. [2] as early as 1960s, which are known as M2AX, M3AX2, and M4AX3 phases. And the subsequent studies proved the existence of advanced MAX phases, such as M5AX4 M6AX5 M7AX6 [3], [4], [5].
Recently a new ternary cermet Ti3AlN was found at the temperature of 1273 K [17]. Other than Mn+1AlNn of hexagonal structures, Ti3AlN has cubic structure with two prominent advantages, i.e. not easy to slip and more closely packed when attached to material surfaces. That means they possess stronger adhesive force and do not easily fall off. Kanchana [18] has investigated the mechanical properties of this material by first principle calculation, and pointed out that Ti3AlN is more ductile than Ti3Al. The results are attributed to the less covalent Ti–N hybridization. It is known that Ti, Zr, and Hf belong to the same group in the elements table. As the physical properties of cubic TiN, ZrN, and HfN have been discussed widely [19], it is easy to associate that Ti3AlN, Zr3AlN and Hf3AlN may manifest some similar properties. Unfortunately, their elastic or thermal properties are not completely clear. Therefore, the applications of Ti3AlN, furthermore Zr3AlN and Hf3AlN (of course the latter two are not founded experimentally yet), are far from implementation. Hence it is necessary to study them together, collect facts, conclude some rules, and predict the possible properties theoretically. The preliminary research is very important for the potential applications. By contrast with cubic TiN, ZrN, and HfN, an in-depth research on cubic Ti3AlN, Zr3AlN, and Hf3AlN aims for emerging that Al atoms affect the physical properties of Ti–N, Zr–N, and Hf–N systems, thereby providing ideas for modifying experiments on the relevant materials.
Section snippets
Calculations
The lattice structure of cermet M3AlN is anti-perovskite belonging to Pm-3m, space group where the N atom occupies the position 1b, the Al atom occupies the position 1a, and the M atom occupies the position 3c (Fig. 1).
The first principles calculation was performed with the CASTEP code [20] based on DFT [21]. The electronic exchange-correlation terms were described by Ceperley–Alder–Perdew–Zunger method under local density approximation (LDA) [22] and Perdew–Burke–Ernzerhof method under
Electronic structures
Fig. 2 shows the energy band structures of Ti3AlN, Zr3AlN, and Hf3AlN, where the red dotted lines represent the Fermi level. In these band structures with no energy gaps, the energy bands pass Fermi levels from conduct bands to valence bands, which indicate the existence of free electrons. By analyzing densities of states (Fig. 3), it is primarily attributed to Ti 3d, Zr 4d, and Hf 5d states electrons, respectively. Moreover, the energy bands near the Fermi level determine the electric
Conclusion
Electronic structures, elastic properties, and the minimum thermal conductivities of Ti3AlN, Zr3AlN, and Hf3AlN were calculated based on DFT. The electronic structures show that there are no band gaps between conduct bands and valence bands, and exhibit metallicity in ground states. Hence, the density of states of all orbital electrons between transition metal and Al atoms as well as between transition metal and N atoms overlapped in the valence band, which corresponds to the difference
Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (Grants nos. 11102169, 51101130, and 51171156).
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