Newly Synthesized Ta‐Based MAX Phase (Ta_1−xHf_x)₄AlC₃ and (Ta_1−xHf_x)₄Al_0.5Sn_0.5C₃ (0 ≤ x ≤ 0.25) Solid Solutions: Unravelling the Mechanical, Electronic, and Thermodynamic Properties

Abstract

The possession of combined properties of metals and ceramics defines the MAX phase materials as an integrated version of metal-ceramics. [1] The large family of MAX phase nanolaminates contains an early transition metal as M; an element belonging to groups 12-16 as A, and either C or N as X. The electrical conductivity, thermal conductivity, machinability, and mechanical strength of MAX compounds are similar to those of metallic materials, but at the same time they possess excellent corrosion and oxidation resistance and mechanical properties at high temperature similar to those of ceramics. Such outstanding combinations put MAX phases into an extraordinary class of materials [2] and make them potential candidates for use in a long list of applications, such as in sensors, electrical contacts, and especially in high-temperature engineering, superconductivity, fuel cells, the nuclear industry, and spintronics. [3-9] The laminated structure containing A layers (Sn, Al, Ge, etc.) in between the M n þ 1 X n sheets (e.g., TaC) is the key factor for the hybrid properties of metals and ceramics [10,11] in MAX compounds. The aforementioned physical properties and structural features of MAX phase materials have attracted substantial research effort from the scientific community. So far, about 80 distinct compounds of ternary MAX phases have already been synthesized [2] out of a possible 665 viable MAX phases (M n þ 1 AX n , n ¼ 1-4) indicated by extensive phase stability investigations. [12-24] Discovering novel MAX phase solid solutions from the existing list opens a new door for research, and extends the long list of applications of the MAX phase materials. The solid solutions can be produced by mixing of M, A, and/or X elements with each other. Such an approach not only enlarges the number of MAX phase members, but also significantly enhances the decisive properties such as resistance to oxidation, [25] fracture toughness, [26] strength, [25] and self-healing properties, [27] which enhance the scope of their potential applications. Nowotny et al. [28] have reported MAX solid solutions with the compositions (Ti 0.5 Nb 0.5) 2 AlC, (Ti 0.4 Ta 0.6) 2 AlC, (V 0.65 Ta 0.35) 2 AlC, (V 0.5 Nb 0.5) 2 AlC, (Nb 0.6 Zr 0.4) 2 AlC, and (Nb 0.8 Zr 0.2) 2 AlC. A considerable improvement in compressive strength has already been reported for (Ti 1Àx V x) 2 AlC [29] and Ti 2 AlC 1Àx N x. [30] Similar prospects motivate researchers to investigate other MAX phase solid solutions: (Zr,Nb) 2 (Al,Sn)C, [31] (Zr,Ti) 2 (Al,Sn)C, [32] (Zr 1Àx Ti x) 2 AlC, [33,34] (Zr,M) 2 AlC[M ¼ Cr, Ti, or Mo] and Zr 2 (Al,A)C [A ¼ S, As, Sn, Sb, and Pb], [35,36] (Zr,Ti) 3 AlC 2 , [37] and (Ti 1Àx , Mo x) 2 AlC. [38] Moreover, Naguib et al. [39] synthesized the solid solutions (Ti 0.5 V 0.5) 3 AlC 2 , (Nb 0.5 V 0.5) 2 AlC, (Nb 0.5 V 0.5) 4 AlC 3 , and (Nb 0.8 Zr 0.2) 2 AlC and have listed 60 known solid solutions till date.

DFT insights into i-MAB phase, Mo4Y2Al3B6: a potential thermal barrier coating and solar heat reducing material

“…Typically, the nature of bonding is determined by observing the accumulation and depletion of electronic charges, visualized through positive or negative electron density. The accompanying scale delineates high and low electronic charge density with red and blue colors, respectively [36,40,41]. In figure 5, it is evident that a significant accumulation of charges occurs between two B atoms, suggesting the formation of a stronger B-B covalent bond within the Mo 4 Y 2 Al 3 B 6 compound.…”