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Ceramics is one of the oldest materials prepared and used by human beings. Ancient ancestors started to make pottery by forming and burning clay since the earliest civilizations. Ancient people started to fabricate burnt claywares since %6500 B.C., and clayware as a commercial product was available by %4000 B.C. [1] In modern society, as one of the largest branches in material family, ceramics have been developed and used in almost every fields of human beings' lives. In some senses, ceramics and metals possess complementary properties; generally, ceramics show the advantages of high strength, high hardness, high chemical stability, high wear resistance, whereas ceramics typically show intrinsic brittleness and poor plasticity due to their covalent and/or ionic chemical bonding; metals have good plasticity, high toughness, and high thermal and electronic conductivity; however, they typically show lower chemical stability and lower hardness than ceramics. It would be marvelous for a material to combine the properties of ceramics and metals. MAX phases are the materials that meet the challenge.MAX phases are a family of nanolaminate ternary nitrides and carbides, which were first discovered by Nowotny and his colleagues in Vienna. [2] In the beginning, MAX phases have a general formula of M nþ1 AX n (n ¼ 1-3), where M is a transition metal, A is an element from group A, and X is either carbon or nitrogen. The n value could be 1, 2, or 3, and the phases are named as 211, 312, and 413 MAX phases, respectively. These MAX phases show a hexagonal crystal structure (P63/mmc), consisting of M 6 X octahedra separated by A atomic layers. [3] M─X bonds in M 6 X octahedra are strong covalent bonds, whereas the M─A bonds between M 6 X and A atomic layer are much weaker, especially in shear. This unique bonding structure is analogous to graphite, and it is the special bonding structure that endows MAX phases properties of both ceramics and metals. On the one hand, MAX phases are stiff, lightweight, chemically stable, and oxidation resistant, which are the features of ceramics; on the other hand, they exhibit good electric and thermal conductivity as well as excellent machinability and damage tolerance, which are the features of metals.Unfortunately, MAX phases had not attracted enough attention in both academia and industry until 1996 when Barsoum et al. [4] synthesized and characterized Ti 3 SiC 2 . Ever since then, material researchers become more interested in this material. [5] In the past two decades, many MAX phases have been synthesized and tested to show highly unusual properties, [6] which will be discussed with detail in the following sections. The intensive extent of the researches on MAX phases can be demonstrated by the published papers (Figure 1). A remarkable jump can be observed between 1998 and 1999, and after that, the number of published papers increases steadily, reaching as high as 980 papers in 2019 (Figure 1a). From another aspect, the citation of the milestone paper published by Barsoum [4] also signific...
Ceramics is one of the oldest materials prepared and used by human beings. Ancient ancestors started to make pottery by forming and burning clay since the earliest civilizations. Ancient people started to fabricate burnt claywares since %6500 B.C., and clayware as a commercial product was available by %4000 B.C. [1] In modern society, as one of the largest branches in material family, ceramics have been developed and used in almost every fields of human beings' lives. In some senses, ceramics and metals possess complementary properties; generally, ceramics show the advantages of high strength, high hardness, high chemical stability, high wear resistance, whereas ceramics typically show intrinsic brittleness and poor plasticity due to their covalent and/or ionic chemical bonding; metals have good plasticity, high toughness, and high thermal and electronic conductivity; however, they typically show lower chemical stability and lower hardness than ceramics. It would be marvelous for a material to combine the properties of ceramics and metals. MAX phases are the materials that meet the challenge.MAX phases are a family of nanolaminate ternary nitrides and carbides, which were first discovered by Nowotny and his colleagues in Vienna. [2] In the beginning, MAX phases have a general formula of M nþ1 AX n (n ¼ 1-3), where M is a transition metal, A is an element from group A, and X is either carbon or nitrogen. The n value could be 1, 2, or 3, and the phases are named as 211, 312, and 413 MAX phases, respectively. These MAX phases show a hexagonal crystal structure (P63/mmc), consisting of M 6 X octahedra separated by A atomic layers. [3] M─X bonds in M 6 X octahedra are strong covalent bonds, whereas the M─A bonds between M 6 X and A atomic layer are much weaker, especially in shear. This unique bonding structure is analogous to graphite, and it is the special bonding structure that endows MAX phases properties of both ceramics and metals. On the one hand, MAX phases are stiff, lightweight, chemically stable, and oxidation resistant, which are the features of ceramics; on the other hand, they exhibit good electric and thermal conductivity as well as excellent machinability and damage tolerance, which are the features of metals.Unfortunately, MAX phases had not attracted enough attention in both academia and industry until 1996 when Barsoum et al. [4] synthesized and characterized Ti 3 SiC 2 . Ever since then, material researchers become more interested in this material. [5] In the past two decades, many MAX phases have been synthesized and tested to show highly unusual properties, [6] which will be discussed with detail in the following sections. The intensive extent of the researches on MAX phases can be demonstrated by the published papers (Figure 1). A remarkable jump can be observed between 1998 and 1999, and after that, the number of published papers increases steadily, reaching as high as 980 papers in 2019 (Figure 1a). From another aspect, the citation of the milestone paper published by Barsoum [4] also signific...
Metrics & MoreArticle Recommendations CONSPECTUS: MXenes, among other two-dimensional (2D) materials such as graphene, hexagonal BN, transition metal dichalcogenides (TMDs), 2D metal−organic frameworks (MOFs), and covalent organic frameworks (COFs), are the fastest growing class discovered thus far. The general formula of MXenes is M n+1 X n T x , where M, X, and T x represent an early transition metal (Ti, V, Nb, Mo, etc.), C and/or N, and the surface functional groups (typically, O, OH, F, Cl), respectively, and n can be between 1 and 4. MXenes as a class of materials have extraordinary properties, such as high electrical conductivity, nonlinear optical properties, solution processability, scalability and ease of synthesis, redox capability, and tunable surface properties, among others; the specific properties, however, depend on their chemistry. Since the initial report of the first MXene in 2011, the research community has primarily focused on Ti 3 C 2 T x , and the amount of research work to investigate its synthesis and properties has increased exponentially over the years. In materials science, alloying is a useful way of synthesizing new materials to improve the properties of a class of materials. Advancement of steel and synthesis of inorganic semiconductors can be regarded as some of the major historical advancements in the concept of alloying. Thus, just one year after the initial report of MXenes, the first solid-solution MXene, (TiNb) 2 CT x , was reported, which demonstrates the inherent chemical tunability of this class of materials.MXenes have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites, presenting promising routes for tailoring their properties. X-site solid-solutions include carbonitride MXenes and are the least studied class of MXenes to date. Comparatively, multi-M MXenes have acquired significant attention, leading to the extreme example of high-entropy solid-solution MXenes. By using multiple M elements, a significant expansion of the structural and chemical diversity is possible, giving rise to novel chemical, magnetic, electronic, and optical properties that cannot be accessed by single-M MXenes. Solid-solution MXenes represent the largest and most tunable class of MXenes; solid-solution MXenes are those that have multiple metals that are randomly distributed on their M sites with no distinct chemical ordering. Using multiple M elements in MXenes, it is possible to synthesize novel MXene structures that cannot be produced otherwise, such as M 5 X 4 T x MXenes. Based on their chemistry, it is possible to rationally control the electronic, optical, mechanical, and chemical properties in a way that no other class of MXenes can. In some cases, the resultant property is linearly related to the chemistry, such as the electrical conductivity, while in other cases the properties are nonlinear or emergent: optical properties, enabling these MXenes to fulfill roles that no other MXene, or 2D material, can.In this Account, we discu...
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