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Mo2TiAlC2: A new ordered layered ternary carbide
Abstract
Herein we report on the synthesis of a new layered ternary carbide, Mo2TiAlC2, that was synthesized by heating an elemental mixture at 1600 °C for 4 h under an Ar flow. Its hexagonal, a and c lattice parameters were calculated via Rietveld analysis of powder X-ray diffraction patterns to be, respectively, 2.997 Å and 18.661 Å. High-resolution scanning transmission electron microscopy showed that this phase is ordered, with Ti layers sandwiched between two Mo layers in a M3AX2 type ternary carbide structure.
... These behaviors made the aforementioned set of materials ideal for a variety of prospective uses in contemporary technologies and industries [7]. The number of MAX phases that are currently known has reached 150 [8].Numerous MAX phase materials have been synthesized in recent years [9][10][11][12][13][14], and their physical characteristics have also been studied [15][16][17][18][19]. More than fifty M 2 AX compounds have been investigated to date, along with five M 3 AX 2 compounds (Ti 3 SiC 2 , Ti 3 AlC 2 , Ti 3 GeC 2 , Ti 3 SnC 2 and Ta 3 AlC 2 ) and seven M 4 AX 3 compounds (Ti 4 AlN 3 , Ti 4 GeC 3 , Ti 4 SiC3, Ta 4 AlC 3 , Nb 4 AlC 3 , V 4 AlC 3 , and Ti 4 GaC 3 ). ...
... Being hexagonal crystal structure, the compound Nb 4 AlC 3 has only five independent elastic constants (C 11 , C 12 , C 13 , C 33 and C 44 ) and one dependent elastic constant which is C 66 (i.e., C 66 =(C 11 -C 12 )/2). Before preceding further, the mechanical stability of the hexagonal Nb 4 AlC 3 compound should be checked with the well-known Born and Mouhat stability conditions [40] which is grounded on the stiffness constants (C ij ):C 11 > 0, C 33 > 0, C 44 > 0, C 11 -C 12 > 0, (C 11 + C 12 )C 33 -2C 13 2 > 0. However in order to show mechanically stable under pressure, a hexagonal crystal system should satisfy the following conditions [30]: ...
This article discusses the physical properties of the Nb 4 AlC 3 MAX phase compound under the influence of pressure ranging from 0 to 100 GPa via the calculation of structural, mechanical, thermal, and optoelectrical properties by utilizing the density functional theory (DFT). Under the studied pressure range, the phase Nb 4 AlC 3 is mechanically stable and various elastic moduli show a linear increase with pressure; while the machinability and dry lubricity show opposite cases. Nb 4 AlC 3 shows brittle nature up to 20 GPa pressure, and after 20 GPa, Nb 4 AlC 3 transits to ductile, and then the ductility increases with the further increase of pressure. Metallic feature is ensured from the band structure and density of states calculations as the valence band cross the Fermi level. The average reflectivity is more than 60% (from 0 to 100 GPa) which makes a capable solar heating reducing agent. The Debye temperature, minimum thermal conductivity, and melting point of the Nb 4 AlC 3 compound have a linear increase response with pressure and the application of this compound as favorable thermal barrier coating (TBC) material.
... And there are two additional impurities, Mo 2 C and Al 8 Mo 3 in the obtained products at 1700 K, using the stoichiometric starting compositions. Also, annealing the non-stoichiometric starting compositions, the predicted major impurities are also listed in Table 1 1: 2 and 2: 2: 1.3: 2.7, respectively [25,26]. Although, few minor impurities were observed in the X-ray diffraction patterns of the as obtained sample, their identities ...
... were not fully resolved in ref [25]. Later on, the crystal structures and experimental preparation procedures have been revisited for Mo 2 TiAlC 2 and Mo 2 Ti 2 AlC 3 in Ref. [18]. ...
The reaction thermodynamics for synthesizing the “312” and “413” o-MAX phases using the powder metallurgy are investigated using a linear programing optimization algorithm based on the high-throughput first principles phonon calculations. The validity and reliability of the current methodology are verified by correctly predicting the impurities in four experimentally known o-MAX systems including Cr-Ti-Al-C, Cr-V-Al-C, Mo-Sc-Al-C and Mo-Ti-Al-C. The formability of each investigated o-MAX phase is evaluated by means of formation enthalpy and formation Gibbs free energy in a temperature range from 0 K to 1700 K. It is revealed that the thermodynamic stability of the “413” o-MAX structure is no better than that of the “312” phase. The formability of “413” o-MAX is also reduced at high sintering temperature, compared to that of “312” phase. The optimal synthetic routes are predicted for all thermodynamically stable “312” and “413” o-MAX phases. It is found that most o-MAX phases considered could be prepared as the single phase using the non-conventional synthetic routes from the aspect of reaction thermodynamics. Few of them including Cr2TaAlC2, Nb2HfAlC2, Nb2TaAlC2, Nb2Hf2AlC3, Nb2Ta2AlC3, Mo2V2AlC3 and Mo2Ta2AlC3 are predicted to be either destabilized at high temperature or overwhelmed by the most competing side reaction.
... Similar to this, (Mo 2 Ti)AlC, (Mo,Ti) 2 AlC 3 , (Mo 2 ,Sc)AlC 2 , and (Ti 2 ,Zr)AlC 2 . Anasori et al. 141 have referred to MAX phases as o-MAX phases. It is interesting to note that Al is the A-element and C is the X-element in every identified o-MAX phase to date. ...
MAX phases and their MXene compounds have received significant attention owing to their extensive potential applications. The quality and purity of the MAX phase guarantee the desired quality of the MXene product, which is essential for a variety of applications, including energy storage, catalysis, and electrical devices. Due to the purity, quality, complex structure, and unavailable commercial pure MAX powders, it is frequently required to have sophisticated synthesis and characterization techniques for the expected MAX products. Many researchers entering this field seek a comprehensive approach to the synthesis and characterization of MAX phases. Despite this, a significant portion of existing reviews have overlooked the synthesis and characterization methods specific to MAX phases, particularly when addressing MXenes. Consequently, this review aims to offer a thorough overview of the various synthesis methods and characterization techniques that are often required for MAX phases. In this review, various synthesis techniques, including their advantages and disadvantages, have also been discussed. Characterization techniques, especially x‐ray diffraction (XRD), were found to be quite critical for new researchers. However, the integration of other techniques such as scanning electron microscopy, transmission electron microscopy, x‐ray photoelectron spectroscopy, and infrared analysis enhances and complements the findings obtained through XRD. The review also underscores the challenges associated with MAX phase synthesis and proposes potential solutions, emphasizing the assessment of their suitability across a broad spectrum of applications. Overall, this review serves as a comprehensive resource and guide for researchers engaged in the exploration and application of MAX phases, emphasizing the essential techniques of synthesis and characterization in harnessing their massive potential.
... The high crystallinity degree of MXene firmly combines with crystal Pt nanoparticles, which is favorable for the structural stability of catalysts during the harsh ORR process. In Figure 5f, the interplanar spacing of about 0.299 and 0.226 nm in the region of MXene and Pt correspond well with MXene(100) 60 and Pt(111) domains, respectively. The interfacial region is marked by two yellow lines. ...
Proton exchange membrane fuel cells suffer from performance degradation caused by the oxidation and corrosion of carbon supports. Metal−platinum interactions can enhance the catalytic activity and stability due to compositional and geometric effects. Two-dimensional MXenes have been intensively studied in recent years and are expanding rapidly in both types and applications. Recently, ORR performances of dual-transition metal MXenes with abundant surface Mo vacancy have been demonstrated as efficient electrocatalysts toward oxygen reduction reaction in an alkaline solution. In this study, the electrochemical performances of Mo-based MXenes-supported Pt electrocatalysts toward oxygen reduction reaction are investigated in an acidic solution. Compared with the Pt/C catalysts, Pt/Mo 2 TiC 2 F x exhibits improved durability, with a 28.7 mV negative shift of the half-wave potential after continuous scanning of 5000 cycles, which is superior to Pt/C with an 84.3 mV negative shift. The proton-exchange membrane fuel cell with Pt/Mo 2 TiC 2 F x as the cathode catalyst also shows excellent activity and stability with a 0.7% potential fade after 9 h. According to the morphology evolution during oxygen reduction, the excellence in electrochemistry is due to the coherent interface between Pt and MXene, which provides a high binding force between Pt nanoparticles and the supporting materials.
... During the synthesis by acid etching, the oxygen-based functional groups attached to the surface of Ti 3 C 2 MXene. 26 Other impurities, Si, Zn and Cu, are also present in the composition in minute amounts. From Table 1, it is seen that the mass percent obtained for titanium atoms is 67.79%, whereas for carbon atoms it is 15.67%. ...
This work explores the characteristics of two-dimensional (2D) titanium carbide (Ti3C2 MXene) and
utilizes first-principles study to weigh its potential for supercapacitor applications. Scanning electron
microscopy images confirm the layered morphology of the MXene, and energy-dispersive X-ray
spectroscopy (EDX) analysis supports the extraction of aluminum from the MAX phase. Fouriertransform
infrared (FTIR) spectroscopy confirms the presence of oxygen-based functional groups on the
surface of the MXene and X-ray diffraction (XRD) patterns validate its hexagonal crystalline structure.
Cyclic voltammetry (CV) analysis reveals the presence of redox peaks, indicating the pseudocapacitive
behaviour of the fabricated electrode. Additionally, galvanostatic charge–discharge (GCD) measurements
yield a calculated specific capacitance of 370 F g�1. Electrochemical impedance spectroscopy (EIS) substantiates
the low impedance resulting from the layered structure via the adequate adsorption/
desorption of cations. The utilization of first-principles density functional theory (DFT) calculations
reveals the merging of conduction and valence bands, signifying effective conductivity. Both the total
and partial density of states cross the Fermi level, indicating a highly efficient charge mobility process.
The combination of prominent surface redox reactions and excellent conductivity contributes to the
superior specific capacitance of the fabricated electrode. Overall, these results highlight the excellent
electrochemical properties of the Ti3C2 MXene electrode, declaring it as a promising candidate for
supercapacitor applications.
... Usually, high temperature and long annealing time and pressure are required for the synthesis of the Mo 2 TiC 2 Al MAX phase (4 hours at 1600°C). 44 Our results have shown that the Al excess in the reaction mixture can promote the MAX phase (Mo 2 Ti 2 -AlC 3 , Mo 2 TiAlC 2 ) formation process at a lower temperature, probably generating Al-rich metal-Al alloys (intermediates) with a lower melting point (e.g. MoAl 4 with T melt = 1130°C, and TiAl 3 with T melt = 1340°C 45 ). ...
MXenes, a family of two-dimensional (2D) transition metal carbides, have been discovered as exciting candidates for various energy storage and conversion applications, including green hydrogen production by water splitting. Today, these materials mostly remain interesting objects for in-depth fundamental studies and scientific curiosity due to issues related to their preparation and environmental stability, limiting potential industrial applications. This work proposes a simple and inexpensive concept of composite electrodes composed of molybdenum- and titanium-containing MAX phases and MXene as functional materials. The concept is based on the modification of the initial MAX phase by the addition of metallic Ni, tuning Al- and carbon content and synthesis conditions, followed by fluoride-free etching under alkaline conditions. The proposed methodology allows producing a composite electrode with a well-developed 3D porous MAX phase-based structure acting as a support for electrocatalytic species, including MXene, and possessing good mechanical integrity. Electrochemical tests have shown a high electrochemical activity of such electrodes towards the hydrogen evolution reaction (HER), combined with a relatively high areal capacitance (up to 10 F cm⁻²).
... In fact, the precursor compounds can also be nitrides, thus a general formula with a chemical composition of M n+1 AX n is used for the presentation [39]. Furthermore, M n+1 AX n could form solid solutions with more complicated chemical compositions, where different elements are located at the sites of M, A, and X, resulting in the formation of many more MXenes [40][41][42]. ...
The traditional von Neumann computing architecture has relatively-low information processing speed and high power consumption, being difficult to meet the computing needs of artificial intelligence (AI). Neuromorphic computing systems, with massively parallel computing capability and low-power consumption, have been considered as an ideal option for data storage and AI computing in the future. Memristor as the fourth basic electronic component besides resistance, capacitance and inductance, could be the most competitive candidate for neuromorphic computing systems benefiting from the simple structure, continuously adjustable conductivity state, ultra-low power consumption, high switching speed and compatibility with existing CMOS technology. The memristor devices with applying MXene-based hybrids have attracted significant attention in recent years. Here, we introduce the latest progress in the synthesis of MXene-based hybrids and summarize the potential applications of MXene-based hybrids in memristor devices and neuromorphological intelligence. We explore the development trend of memristor constructed by combining MXenes with other functional materials and emphatically discuss the potential mechanism of MXenes-based memristor devices. Finally, the future prospects and directions of MXene-based memristors are briefly described.
... Cr 2 TiAlC 2 and V 2 CrAlC 2 can be summarized as a kind of quaternary MAX phase (M , M ) n+1 AX n , and the elements in the M position of this kind of quaternary MAX phase are composed of two transition elements, which may have better performance [12]. Since the new layered quaternary ceramic Mo 2 TiAlC 2 was reported, its highly ordered structure has attracted a lot of attention [13]. ...
Mo, TiH2, Al and graphite elemental powders were used as starting materials for the activation reaction sintering process, which was employed to fabricate porous Mo2TiAlC2. The alteration of phase constitution, volume expansion, porosity, pore size and surface morphology of porous Mo2TiAlC2 with sintering temperatures ranging from 700 °C to 1500 °C were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and pore size tester. Both the pore formation mechanism and activation reaction process at each temperature stage were investigated. The experimental results illustrate that the sintered discs of porous Mo2TiAlC2 exhibit obvious volume expansion and pore structure change during the sintering process. Before 1300 °C, the volume expansion rate and porosity increase with the increment of temperature. However, with the sintering temperature above 1300 °C, the volume expansion rate and porosity decrease. At the final sintering temperature of 1500 °C, porous Mo2TiAlC2 with a volume expansion rate of 35.74%, overall porosity of 47.1%, and uniform pore structure was synthesized. The pore-forming mechanism of porous Mo2TiAlC2 is discussed, and the evolution of pressed pores, the removal of molding agents, the decomposition of TiH2, and the Kirkendall effect caused by different diffusion rates of elements in the diffusion reaction are all accountable for the formation of pores.
... In 2015, Anasori et al. successfully synthesized an ordered Mo-based MAX phase, Mo 2 TiAlC 2 , for the first time. Such a layered structure can consume crack propagation energy under stress conditions through twisting and delamination [98]. Based on this, Lin et al. conducted a study on enhancing the mechanical properties of the Mo-12Si-8.5B ...
Mo-Si-B alloys are a crucial focus for the development of the next generation of ultra-high-temperature structural materials. They have garnered significant attention over the past few decades due to their high melting point and superior strength and oxidation resistance compared to other refractory metal alloys. However, their low fracture toughness at room temperature and poor oxidation resistance at medium temperature are significant barriers limiting the processing and application of Mo-Si-B alloys. Therefore, this review was carried out to compare the effectiveness of doped metallic elements and second-phase particles in solving these problems in detail, in order to provide clear approaches to future research work on Mo-Si-B alloys. It was found that metal doping can enhance the properties of the alloys in several ways. However, their impact on oxidation resistance and fracture toughness at room temperature is limited. Apart from B-rich particles, which significantly improve the high-temperature oxidation resistance of the alloy, the doping of second-phase particles primarily enhances the mechanical properties of the alloys. Additionally, the application of additive manufacturing to Mo-Si-B alloys was discussed, with the observation of high crack density in the alloys prepared using this method. As a result, we suggest a future research direction and the preparation process of oscillatory sintering, which is expected to reduce the porosity of Mo-Si-B alloys, thereby addressing the noted issues.
... They have similar properties to metals, including high conductivity, low hardness, machinability, resistance to thermal shock, and damage tolerance. Up to date, there are more than 80 MAX phase elements that have already been synthesized [7][8][9][10][11][12] and the physical characteristics are also investigated. [13][14][15][16][17] The M 2 AX phases with M = (titanium, vanadium, chromium, niobium, tantalum, zirconium, hafnium), A = (aluminum, sulfur, Tn, arsenic, indium, gallium), and X = (nitrogen, carbon) are extensively studied conceptually and experimentally owing to their attractive properties, such as physical as well as optical and dynamical stability. ...
We characterize the physical feature of M2TlC (M = Ti, Zr, and Hf) MAX phase ternary carbides applying density functional theory. Along with previously calculated structural, elastic, and electrical properties, Vickers hardness, dynamical stability, and optical feature of the phases are also calculated. The Pugh ratio and Poisson’s ratio show the compounds’ brittleness in conjunction with their potent directional covalent bonds and combination of ionic contributions. The metallic character of the phases is supported by the Fermi level overlap of the conduction band and valence band. The examined materials have moderate hardness, according to the Vickers hardness, with the Hf2TlC combination having the highest value of 2.60 GPa. A number of well-known phenomena are used to compute and thoroughly analyze the optical characteristics. Notably, all investigated compounds show reflectivity above 44% up to 12.0 eV energy, which is encompassed by the infrared and visible regions. The compounds could therefore be used, in practice, as a coating material to lessen solar heating.
... 11 Two different transition metal atoms are chemically ordered on the base surface, and the larger sized M2 element atom layer pushes the M1 layer atoms toward the Al layer. 12,13 The formula of i-MAX phases can be described as (M1 2/3 M2 1/3 ) 2 AX, and to date, there are tens of compounds of (Mo 2/3 Sc 1/3 ) 2 AlC, 14 (Mo 2/3 Y 1/3 ) 2 AlC, 14 (V 2/3 Zr 1/3 ) 2 AlC, (Cr 2/3 Sc 1/3 ) 2 AlC, (Cr 2/3 Y 1/3 ) 2 AlC, (W 2/3 Sc 1/3 ) 2 AlC, (Mo 2/3 Sc 1/3 ) 2 GaC, and (Mo 2/3 Y 1/3 ) 2 GaC discovered. [15][16][17] In previous works, Cui et al. have synthesized polycrystalline i-MAX (Mo 2/3 Y 1/3 ) 2 AlC (space group C2/c) ceramic by pressureless sintering. ...
In this paper, the i‐MAX phase (Mo2/3Y1/3)2AlC ceramic with high purity of 98.29 wt% (1.13 wt% Y2O3 and 0.58 wt% Mo2C) and high relative density of 98.59% was successfully synthesized by spark plasma sintering (SPS) at 1500°C with the molar ratio of n(Mo):n(Y):n(Al):n(C) = 4:2:3.3:2.7. The positions of C atoms in the crystal of (Mo2/3Y1/3)2AlC were determined. Microstructure and physical and mechanical properties of (Mo2/3Y1/3)2AlC ceramic were systematically investigated. It was found that the obtained (Mo2/3Y1/3)2AlC ceramic had an average grain size of 32.1 ± 3.1 μm in length and 14.2 ± 1.7 μm in width. In terms of physical properties, the measured thermal expansion coefficient (TEC) of (Mo2/3Y1/3)2AlC was 8.99 × 10⁻⁶ K⁻¹, and the thermal capacity and thermal conductivity at room temperature were 0.43 J·g⁻¹·K⁻¹ and 13.75 W·m⁻¹·K⁻¹, respectively. The room temperature electrical conductivity of (Mo2/3Y1/3)2AlC ceramic was measured to be 1.25 × 10⁶ Ω⁻¹·m⁻¹. In terms of mechanical properties, Vickers hardness under 10 N load was measured as 10.54 ± 0.29 GPa, while flexural strength, fracture toughness, and compressive strength were determined as 260.08 ± 14.18 MPa, 4.51 ± 0.70 MPa·m1/2, and 855 ± 62 MPa, respectively, indicating the promising structural applications.
... Additionally, two chemically ordered MAX phases were synthesized in 2014 and 2017: the o-MAX phases (312 or 413), which exhibit out-of-plane ordering, and the i-MAX phase 211, which demonstrates in-plane ordering. In contrast, the out-of-plane ordered MXene consists of a single or double layer of the M element sandwiched between layers of another M element [44]. The two M elements are plane ordered while in the i-MAX phase [45], and more recently, the i-MAX phase based on molybdenum has also been found, respectively [46]. ...
Energy storage is becoming a critical issue due to the diminishing availability of fossil fuels and the intermittent nature of current renewable energy sources. As a result, thermal management (TM) and thermal energy systems have gained significant attention due to their crucial roles in various industries. Among the different TM materials, MXenes, a member of the transition metal carbide/nitride family, have emerged as a promising material due to their unique 2D nanostructure, changeable surface chemistry, high electrical/thermal conductivity, light absorptivity, and low infrared emissivity. This review outlines the synthesis methods of MXenes and their various features and applications in thermal management. These 2D materials exhibit outstanding optical and thermal properties, making them suitable for thermal energy generation and storage. The study also covers the potential applications of MXene in the desalination industry, hybrid photovoltaic thermal systems, solar energy storage, electronics, and other thermal management related industries. The findings suggest that MXene-based TM materials have remarkable features that significantly influence thermal energy storage and conversion and present opportunities for further research in efficiently using these materials.
... Dans la classification des matériaux, les phases MAX présentent une singularité étant donné qu'ils combinent à la fois des propriétés des métaux et des céramiques [6]. Ces À titre d'exemple, des solutions solides à base de Ti/Mo ont été synthétisées telles que Mo2TiAlC2 par Anasori et al. [8] où les atomes de molybdène se retrouvent sur les couches externes des feuillets (cf. Figure.I. ...
L’hydrogène est un vecteur énergétique envisagé pour remplacer les sources énergétiques issues des ressources fossiles. Il est prévu de l’utiliser à grande échelle dans des piles à combustible H2/O2. Néanmoins, il est nécessaire de le produire à un très haut degré de pureté et dans des conditions respectueuses de l’environnement. Le moyen le plus efficace pour obtenir de l’hydrogène « vert » est d’utiliser l’électrolyse de l’eau, processus nécessitant l’élaboration d’électrodes peu coûteuses, actives et stables. En particulier, il est nécessaire de trouver de nouveaux catalyseurs efficaces sans métaux nobles, coûteux et/ou rares. La nanostructuration des matériaux est une voie de plus en plus explorée pour répondre à ces critères. En particulier, les matériaux bidimensionnels présentant des conductivités électroniques élevées constituent une famille de nanomatériaux très prometteurs. En effet, ils présentent généralement des surfaces spécifiques très élevées et des propriétés électroniques et/ou catalytiques différentes de celles de leurs analogues massifs. Parmi ceux-ci, les MXènes, des carbonitrures de métaux de transition, découverts en 2011, sont une classe de matériaux 2D en pleine expansion en raison de leurs caractéristiques intrinsèques (versatilité chimique, conductivité électronique très élevée, hydrophilie) qui leur confèrent des propriétés très variées pour de nombreuses applications, et pour l’électrocatalyse en particulier. Au cours de cette thèse, des MXènes de formule Mn+1XnTx ont été synthétisés à partir de leur analogue tridimensionnel Mn+1AXn (ou M est du titane et/ou du molybdène, A est de l’aluminium et X est du carbone et/ou de l’azote), appelé phase MAX, et à partir de Mo2Ga2C par exfoliation de l’aluminium ou du gallium par attaque acide. Dans un premier temps, une attention particulière a été portée sur l’influence de la composition du milieu exfoliant afin de contrôler à la fois la nature de T (groupements terminaux à la surface des MXènes issus de l’étape d’exfoliation), la nature des espèces interfoliaires insérées, la structure, la macrostructure, l’état d’oxydation de surface et l’état de délamination. L’influence de l’élément M dans le MXène sur ses propriétés électrocatalytiques a ensuite été étudiée via la préparation de MXènes mixtes à base de Ti et Mo. Une autre étude a porté sur l’influence de l’élément X dans le MXène, en remplaçant totalement ou partiellement le carbone par de l’azote, du soufre ou du bore, l’objectif étant de modifier l’environnement électronique du métal M et donc ses propriétés d’adsorption/désorption des intermédiaires réactionnels lors de l’acte catalytique. Enfin, les caractéristiques des MXènes ont été mises à profit pour élaborer des composites dans lesquels le MXène joue le rôle de support d’espèces actives à base de Ni et Fe ou de co-catalyseur pour les réactions de dégagement d’oxygène (OER) et d’hydrogène (HER), réactions mises en jeu dans un électrolyseur. À chaque étape, les nouveaux matériaux obtenus ont été caractérisés par de nombreuses techniques (DRX, XPS, microscopie, spectroscopie Raman,…) et évalués en électrocatalyse. Ce travail a ainsi permis de proposer deux composites à base de MXène : MoS2@Mo2CTx et NixFey@Mo2CTx, très actifs et stables en milieu électrolytique alcalin pour l’HER (cathode) et l’OER (anode) respectivement, en vue de leur utilisation dans un électrolyseur alcalin. Compte-tenu de la richesse de la chimie de surface des MXènes, ce travail offre de nombreuses perspectives pour obtenir des électrodes toujours plus efficaces, et au-delà, pour les autres applications envisagées avec les MXènes.
The representation of atomic configurations through cluster correlations, along with the cluster expansion approach, has long been used to predict formation energies and determine the thermodynamic stability of alloys. In this work, a comparison is conducted between the traditional cluster expansion method based on density functional theory and other potential machine learning models, including decision tree‐based ensembles and multi‐layer perceptron regression, to explore the alloying behavior of different elements in multi‐component alloys. Specifically, these models are applied to investigate the thermodynamic stability of triple transition‐metal (Ti−Mo−V)3C2${\rm (Ti-Mo-V)}_3{\rm C}_2$ MXenes, a multi‐component alloy in the largest family of 2D materials that are gaining attention for several outstanding properties. The findings reveal the triple transition‐metal ground‐state configurations in this system and demonstrate how the configuration of transition metal atoms (Ti, Mo, and V atoms) influences the formation energy of this alloy. Moreover, the performance of machine learning algorithms in predicting formation energies and identifying ground‐state structures is thoroughly discussed from various aspects.
By means of density functional theory, the energy‐loss near‐edge structure (ELNES) of carbon K‐edge of Mo2TiAlC 2 and corresponding MoTiC 2 Mxene at orientational‐independent condition is dealt with. Compared to the MAX (M is transition metal, A is an elment from group 13–16, X is C or N) phase, the energy separations increase between the main spectral features at the C K edge of Mo 2 TiC 2 MXene owing to the structural change and decreased bond length. The dispersions of the C K edge in both systems are similar to p‐symmetry densities of states. It is indicated that the source of the first fine structure at the C 1 s edge in both phases mainly comes from the electron transfer to p x + p y ‐like character. The other fine structures result from the transition to hybridization of p z and p x + p y states with the prominent contribution of p x + p y ‐like character. Moreover, the comparison of C K‐edge ELNES spectra in three Mo‐based compounds reveals that, ongoing from Mo 2 TiAlC 2 to Mo 2 TiC 2 and then to Mo 2 C, the energy position of the fine structures is shifted to higher energies (blueshifted), due to the quantum confinement effects and the change of the chemical environment around the excited carbon.
The field of modern biosensor development holds immense promise for clinical diagnostic. Projections indicate that the biosensor market will exceed $28 billion by 2024. In recent years, the integration of 2D materials with optical biosensors has emerged as a cutting-edge research area, owing to the unique optical, electrical, electrochemical, and physical properties of 2D materials. These materials offer an exceptionally high density of active sites over large areas, making them highly suitable for biochemical sensing applications. Optical biosensors have numerous advantages over conventional analytical techniques, including real-time, direct, and label-free detection of a wide range of biological and chemical compounds. 2D material-equipped optical biosensors have outperformed conventional sensors in terms of sensitivity and detection limitations. Significant progress has been made in the biomedical and healthcare applications of 2D materials, with research concentrating on biomimicry systems, brain interfaces, wearable technologies, and optogenetics, among other areas. These advancements might transform clinical diagnosis and enhance patient care. The importance of environmentally friendly techniques for the development and use of optical biosensors based on 2D materials is emphasized in this study. This study provides a picture of the current status of the subject and establishes the groundwork for future research endeavors that are in line with ecological responsibility by fusing technology improvements with sustainability considerations.
Since the discovery, MXenes has gotten a special attention as 2D functional materials. The advanced applications are directly linked to its synthesis approaches, terminal groups (O, OH and F) and...
The MAX phases are a class of nanolaminated materials composed of an early transition-metal (M), an A-group element (A) and C, N, B and/or P (X). Progress in MAX phase research in recent years has increased their number from the original 50 or so, to more than 300 phases. Since half of the 342 MAX phases have been discovered after 2018, an overview of the progress made in the field is timely. Currently, 28 M elements, 28 A elements, and 6 X elements have been incorporated in the MAX phases, alloys included. We further categorize MAX phases based on the synthesis route used to make them; if made via a one-step approach in bottom-up synthesis or formed through elemental replacement reactions in top-down synthesis. This classification is also correlated to theoretical phase stability predictions, that in turn, can be used to identify novel synthesizable MAX phase compositions as well as to suggest suitable synthesis routes. Furthermore, using phase stability predictions we identify 182 new theoretically stable MAX phases awaiting experimental confirmation. Notably, as MAX phases are precursors for MXenes, the dramatically increased interest in the latter for a large host of potential applications renders the former even more valuable.
Zn-based rechargeable energy devices showed more advantages, including safety, abundance, and high volumetric/gravimetric capacities. MXenes have been evaluated as valuable emerging 2D materials due to their thermal/chemical stabilities, conductivities, flexible mechanical properties, and unique topological features. However, the recent trends in MXenes for Zn-based rechargeable energy devices have rarely been reviewed. This review article presents a comprehensive summary of the latest developments in the design and synthesis of MXene materials intended for utilization as electrodes in Zn-based energy storage devices. Specifically, the focus is on their application in Zn-ion supercapacitors, Zn-ion batteries, Zn-air batteries, and Zn-halide batteries. Firstly, we have deliberately discussed the synthesis of MXenes by summarizing the latest reported techniques but giving the weightage of the initial synthetic methods. Further, the discussion on nano-engineering of active sites revealed that surface termination followed by defect engineering is an emerging strategy to improve the performance of MXenes. The role of machine learning in the synthesis of MXenes is also summarized by establishing the structural activity relationship. In the next section and sub-sections, we have outlined the recent advances in the MXenes as electrode materials for Zn-based energy storage devices. Each section is arranged according to the synthesis strategies to clarify the structural activity relationship in each sub-section and provide a suitable basis for the researchers to design and synthesize targeted materials instead of conventional hit-and-trial methods. Finally, concluding remarks and future perspectives are discussed to offer new directions in targeted MXenes synthesis for energy storage devices.
One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium‐based MXenes, with more than 70% of publication‐related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M‐based MXenes (M‐MXenes), where M stands for non‐Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non‐Ti MXene outperform standard Ti‐MXene in several applications. There is many advancement in top‐down as well as bottom‐up production of MXenes family members, which allows for exact control of the M‐characteristics MXene NMs to contain cutting‐edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M‐MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group‐(III–VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non‐Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation.
More than a decade after the discovery of MXene, there has been a remarkable increase in research on synthesis, characterization, and applications of this growing family of two-dimensional (2D) carbides and nitrides. Today, these materials include one, two, or more transition metals arranged in chemically ordered or disordered structures of three, five, seven, or nine atomic layers, with a surface chemistry characterized by surface terminations. By combining M, X, and various surface terminations, it appears that a virtually endless number of MXenes is possible. However, for the design and discovery of structures and compositions beyond current MXenes, one needs suitable (stable) precursors, an assessment of viable pathways for 3D to 2D conversion, and utilization or development of corresponding synthesis techniques. Here, we present a critical and forward-looking review of the field of atomic scale design and synthesis of MXenes and their parent materials. We discuss theoretical methods for predicting MXene precursors and for assessing whether they are chemically exfoliable. We also summarize current experimental methods for realizing the predicted materials, listing all verified MXenes to date, and outline research directions that will improve the fundamental understanding of MXene processing, enabling atomic scale design of future 2D materials, for emerging technologies.
It is a review article on isomorphous replacements of Inorganic compounds
MAX phases are layered solids with unique properties combining characteristics of ceramics and metals. MXenes are their two‐dimensional siblings that can be synthesized as van der Waals‐stacked and multi‐/single‐layer nanosheets, which possess chemical and physical properties that make them interesting for a plethora of applications. Both families of materials are highly versatile in terms of their chemical composition and theoretical studies suggest that many more members are stable and can be synthesized. This is very intriguing because new combinations of elements, and potentially new structures, can lead to further (tunable) properties. In this review, we focus on the synthesis science (including non‐conventional approaches) and structure of members less investigated, namely compounds with more exotic M‐, A‐, and X‐elements, for example nitrides and (carbo)nitrides, and the related family of MAB phases.
Providing the reader with an up-to-date digest of the most important current research carried out in the field, this volume is compiled and written by leading experts from across the globe. It reviews the trends in electrochemical sensing and its applications and touches on research areas from a diverse range, including microbial fuel cells, 3D printing electrodes for energy conversion and electrochemical and electrochromic colour switching in metal complexes and polymers.
Coverage is extensive and will appeal to a broad readership from chemists and biochemists to engineers and materials scientists. The reviews of established and current interests in the field make this book a key reference for researchers in this exciting and developing area.
The heterostructures of different two-dimensional (2D) materials have garnered significant attention recently as emerging energy conversion and storage systems. Combining the highly conductive and surface-active 2D MXene with the multifunctional...
Since the first report on Ti3C2Tx in 2011, the number of reports on the 2D transition metal carbides and nitrides, known as MXenes, has increased rapidly. Most of the reports...
To realize sustainable development, more and more countries forwarded carbon neutrality goal. Accordingly, improving the utilization efficiency of traditional fossil fuel is an effective strategy for this great goal. Keeping this in mind, developing thermoelectric devices to recover waste heat energy resulted in the consumption process of fuel is demonstrated to be promising. High performance thermoelectric devices require advanced materials. MXenes are a kind of 2D materials with a layered structure, which demonstrate excellent thermoelectric performance owing to their unique physical, mechanical, and chemical properties. Also, substantial achievement has been gained during the past few years in synthesizing MXene based materials for thermoelectric devices. In this review, the mainstream synthetic routes of MXene from etching MAX were summarized. Significantly, the current state and challenges of research on improving the performance of MXene based thermoelectrics are explored, including pristine MXene and MXene based composites.
Rapid industrialization and urban development release large amounts of toxic pollutants such as nitrate (NO3⁻) and CO2, which cause not only environmental issues but also offer reasons of sickness worldwide. Several methods have been devised to purify air and water in the past 2 decades but went futile because of high cost, low-performance, and cause secondary pollutants. Converting NO3⁻ and CO2 photochemically and electrochemically into energy-rich molecules and agro-boosters is an innovative strategy that can help with environmental remediation and meet the world's growing energy needs. However, these approaches require highly active, selective, and long-lasting catalysts. In this context, researchers have studied several smart and multifunctional materials for the reduction of NO3⁻ and CO2 pollutants into valuable chemicals. Among them, MXenes, a class of 2D materials composed of carbonitrides, carbides, and nitrides of transition metals, have gained attention because of their remarkable physico-chemical, mechanical, and electrochemical properties. However, quantization of MXenes for photo and electrocatalytic NO3⁻ and CO2, reduction is required, and the lessons learned must be applied to future MXene-based materials. This article focuses primarily on the photocatalytic and electrocatalytic conversion of NO3⁻ and CO2 to value-added products, highlighting MXene-based catalysts, reaction intermediates, links between the tuneable properties of MXenes and their catalytic activities, research hurdles, and future prospects.
By first-principles total-energy calculations, we investigated the thermodynamic stability of the MAX solid solution MoxV4-xAlC3 in the 0 ≤ x ≤ 4 range. Results evidence that lattice parameter a increases as a function of Mo content, while the c parameter reaches its maximum expansion at x = 2.5. After that, a contraction is noticed. Mo occupies VI sites randomly until the out-of-plane ordered Mo2V2AlC3 alloy is formed. We employed the Defect Formation Energy (DFE) formalism to evaluate the thermodynamic stability of the alloys. Calculations show five stable compounds. At V-rich conditions and from Mo-rich to Mo-moderated conditions, the pristine V4AlC3 MAX is stable. In the region of V-poor conditions, from Mo-rich to Mo-moderated growth conditions, the solid solutions with x = 0.5, 1, and 1.5 and the o-MAX Mo2V2AlC3 are thermodynamically stable. The line profiles of the Electron Localization Function and Bader charge analysis show that the V-C interaction is mainly ionic, while the Mo-C is covalent. Also, the exfoliation energy to obtain a MXene layer is ~ 0.4 eV/Å2. DFE also shows that MXenes exfoliated from the MAX phase with the same Mo content and atomic arrangement are thermodynamically stable. Our results get a deeper atomic scale understanding of the previously reported experimental evidence.
By first-principles total-energy calculations, we investigated the thermodynamic stability of the MAX solid solution Mo x V 4−x AlC 3 in the 0 ≤ x ≤ 4 range. Results evidence that lattice parameter a increases as a function of Mo content, while the c parameter reaches its maximum expansion at x = 2.5. After that, a contraction is noticed. Mo occupies V I sites randomly until the out-of-plane ordered Mo 2 V 2 AlC 3 alloy is formed. We employed the defect formation energy formalism (DFE) to evaluate the thermodynamic stability of the alloys. Calculations show five stable compounds. At V-rich conditions and from Mo-rich to Mo-moderated conditions, the pristine V 4 AlC 3 MAX is stable. In the region of V-poor conditions, from Mo-rich to Mo-moderated growth conditions, the solid solutions with x = 0.5, 1, and 1.5 and the o-MAX Mo 2 V 2 AlC 3 are thermodynamically stable. The line profiles of the Electron Localization Function and Bader charge analysis show that the V-C interaction is mainly ionic, while the Mo-C is covalent. Also, the exfoliation energy to obtain a MXene layer is ~ 0.4 eV/Å ² . DFE also shows that MXenes exfoliated from the MAX phase with the same Mo content and atomic arrangement are thermodynamically stable. Our results get a deeper atomic scale understanding of the experimental evidence by Pinto and coworkers [J. Mater. Chem. A 8 (2020) 8957].
Seven novel inherently nanolaminated transition metal carbides, (Cr2/3R1/3)2AlC (R = Gd, Tb, Dy, Ho, Er, Tm, Lu) with R in-plane chemical ordering in the carbide sheet, have been synthesized and found to be crystallized in Cmcm orthorhombic structure with monotonously decreased lattice parameters due to lanthanides constrictions. Linear antiferromagnetism is found for R = Gd and gradually changed to paramagnetism for R = Er, Tm and Lu through nonlinear magnetic configurations with gradually decreased ordering temperature. Compared with Cr2AlC benchmark, the Vickers hardness is significantly higher, leading to higher compressive fracture strength and better wear resistance. After compression at high temperature, nanoscale co-existence of the C2/c monoclinic and orthorhombic structure is observed, which is caused by the random stackings and high-density slidings within ab plane. The further first-principle calculations confirm the close thermal stability of both two structures.
MAX phases and their two-dimensional siblings MXenes are large, and rapidly growing, classes of materials, that present a huge playground for theoretical and experimental (inorganic) chemists and transcend into multiple adjacent disciplines, such as materials science, solid-state physics, engineering, molecular chemistry and biomedicine. More than 155 MAX phases—layered ternary carbides or nitrides (e.g., Ti2SiC, Cr2AlN), that crystallize in a hexagonal crystal structure—have been synthesized so far and more combinations of early transition metals (M), main group elements (A), and carbon/nitrogen or both have theoretically been predicted (either as ternary compounds or quaternary solid solutions). This invites extensive and diverse activities in the field of materials syntheses, which enable the preparation of new members of the MAX phase family, lead to innovative processing methods to achieve clever MAX phase microstructures and morphologies, and with that open up the path to new functionalities of these metallic ceramics/ceramic metals.
Furthermore, they are the precursor for their 2D analogs (MXenes), that are obtained by selective removal of the A element (mostly Al and Ga) and subsequent delamination of the atomically-thin carbide/nitride sheets. Due to their 2D nature, MXenes exhibit many similarities to graphene while being conductive, mechanically robust, and chemically more flexible.
Recent studies reveal that the current consumption rate of fossil fuels may lead to their exhaustion in the coming years. Due to the depletion of non-renewable energy sources and the fitfulness of renewable energy sources, energy storage and management can be overviewed as a prime requisite. The inception of advancement in energy storage techniques and rechargeable batteries has eased the problem to a certain extent. With highly exceptional properties like inherent volumetric capacitance, outstanding conductivity, durability, low energy barriers for metal ion diffusion and presence of surface functional groups, MXenes debut the field of energy storage. MXenes are 2-D layers of transition metal carbides or nitrides, synthesized by the selective etching of Aluminium from layered MAX phases. The large interlayer spaces for ion intercalation permits alkali ions in these layers revealing unexpected properties that can be exploited for the energy storage in Alkali-ion batteries (AIBs) like Lithium/Sodium/Potassium/Magnesium/Calcium ion batteries. Electrochemical energy storage can also be visualised as a clean energy storage system. This review will traverse through the synthesis of MXenes, eminent properties, and the fabrication of MXene based nanomaterials and hybrids for electrochemical energy storage in various alkali-ion batteries. Finally, the possible challenges required to be highlighted for the future exploration of the research possibilities are also being discussed.
A multicomponent porous MAX phase (Ti0.25Zr0.25Nb0.25Ta0.25)2AlC has been successfully synthesized by using pressureless sintering of mixed elemental powders. The microstructure and phase composition of the samples sintered at various temperatures have been characterized by using SEM, XRD, EDS and other analyses, from which conclusions regarding the reaction and pore forming processes could be drawn. During the whole sintering process, the pores did mainly arise from the diffusion related reactions between Al and other elements at low temperatures (below 1200 °C), and the formation reaction of the MAX phase took place at higher temperatures (above 1200 °C). An exception is the clearance holes that were left from the pressing. The optimum sintering temperature for the final MAX phase (Ti0.25Zr0.25Nb0.25Ta0.25)2AlC was 1600 °C. A too high sintering temperature (1700 °C) caused a serious loss of Al atoms and a decomposition of the synthesized MAX phase.
NiAl–Ti3AlC2 and NiAl–Mo2TiAlC2 composites were fabricated in-situ using a vacuum hot-pressing sintering technique. The microstructure and high temperature tribological performance of the composites were studied. When compared with NiAl–Ti3AlC2 composite, the NiAl–Mo2TiAlC2 composite contained the Mo2TiAlC2 precipitated phases and some nano-precipitated phases of Mo2TiAlC2 distributed along the grain boundaries. Meanwhile, the high temperature tribological property of NiAl–Mo2TiAlC2 composite was more superior than that of NiAl–Ti3AlC2 composite. The worn surface of NiAl–Mo2TiAlC2 composite formed a dense and continuous lubricating layer of MoO3 when subjected to the friction test at 800 °C, which could effectively improve the overall tribological properties of the composites.
Since the discovery of MXenes, research has progressed tremendously to explore MXenes with new compositions apart from the conventional Ti3C2Tx. Recently, the MXene family further expanded with the discovery of ordered double transition metal MXenes, wherein the metal sites are occupied by two different transition metals, which can be arranged in‐plane or out‐of‐plane forming i‐MXenes or o‐MXenes, respectively. The feature of optimizing the properties of ordered double transition metal MXenes by precise engineering of the composition, number of metal layers, interlayer spacing, and surface functionalities is distinctive among the existing 2D materials. This review provides a brief overview of the theoretical and experimental studies on the ordered double transition metal MXenes to elucidate its structure and properties. In addition, the recent trends in the synthesis and the effects of fine‐tuning the composition, structure, and functional groups on their electrochemical performance are elaborated. The current challenges faced by these emerging MXenes and future research directions are also proposed.
Mo-12Si-8.5B alloy with layered Mo2TiAlC2 (a new MAX phase) additions, in order to improve its fracture toughness, were prepared by hot pressing. The composition analysis combined with microstructure features revealed that after hot pressing sintering, the Mo2TiAlC2 does not decompose but still maintains the layered structure and exhibits a good interface bonding with the matrix. The grain size of Mo-12Si-8.5B alloys was apparently refined by Mo2TiAlC2. The mechanical properties suggested that the Mo2TiAlC2 phase affects the hardness of Mo-12Si-8.5B alloy mainly through grain refinement. As the amount of Mo2TiAlC2 increased, the compressive strength and flexure strength of alloys improved nonlinearly which is caused by the relatively low strength of Mo2TiAlC2 and stress release due to its basal plane sliding, the enhanced strength was attributed to the particle strengthening and fine grain strengthening. The Mo2TiAlC2 phase exhibits a remarkably toughening effect on Mo-12Si-8.5B alloy. The fracture toughness of Mo-12Si-8.5B-2%Mo2TiAlC2 alloy increased by about 52% compared with the Mo2TiAlC2 free alloy. Except for the fine-grain toughening caused by Mo2TiAlC2 particles, the improvement of fracture toughness of Mo-12Si-8.5B alloy by Mo2TiAlC2 is mainly caused by the crack deflection, the steps formed by Mo2TiAlC2 fracture and basal plane (0001) sliding of Mo2TiAlC2.
MAX phases are frequently dominated as precursors for the preparation of the star material MXene, but less eye‐dazzling by their own potential applications. In this work, the electrocatalytic hydrogen evolution reaction (HER) activity of MAX phase is investigated. The MAX‐derived electrocatalysts are prepared by a two‐step in situ electrosynthesis process, an electrochemical etching step followed by an electrochemical deposition step. First, a Mo2TiAlC2 MAX phase is electrochemically etched in 0.5 m H2SO4 electrolyte. Just several hours, electrochemical dealloy etching of Mo2TiAlC2 MAX powders by applying anode current can acquire a moderated HER performance, outperforming most of reported pure MXene. It is speculated that in situ superficially architecting endogenous MAX/amorphous carbide (MAC) improves its intrinsic catalytic activity. Subsequently, highly active metallic Pt nanoparticles immobilized on MAC (MAC@Pt) shows a transcendental overpotential of 40 mV versus RHE in 0.5 m H2SO4 and 79 mV in 1.0 m KOH at the current density of 10 mA cm⁻² without iR correction. Ultrahigh mass activity of MAC@Pt (1.5 A mgpt⁻¹) at 100 mV overpotential is also achieved, 29‐folds than those of commercial PtC catalysts.
As a structural ceramic, MAX phases have attracted the most attentions because of light weight, excellent machinability, good thermal shock resistance and thermal stability at ultra-high temperature. Herein, for the first time we reported the thermoelectric property of Mo2Ti2AlC3, one of MAX phases. It has similar resistivity (~2.3 μΩ·m) and higher Seebeck coefficient (−15μV/K) compared with Pt-Rh alloy. Mo2Ti2AlC3 has higher power factor (~46% improvement) and much lower thermal conductivity (6.2 W/(m·K), one-third) compared with SiC thermoelectric ceramics at 700 °C. The ZT value is about 0.0138 at 700 °C, which is three times as high as that of SiC-7%Si3N4 thermoelectric composite. These suggest that Mo2Ti2AlC3 and other MAX phases have an important application prospect in the field of thermoelectric generator and temperature sensor at ultra-high temperature. Meanwhile, although Mo2Ti2AlC3 ceramic shows metallic transport behavior, the electric thermal conductivity far deviates from traditional Wiedemann-Franz law, indicating that there are strong correlations or couplings between electrons and 2-D layered lattices, and Mo2Ti2AlC3 is a new strong related system with spin-orbital coupling.
Nanolamellar MAX phase compounds (Cr0.5V0.5)n+1AlCn are formed with n = 1, 2 and 3, and their 300 K structure is studied in detail by high-resolution neutron diffraction. While the n = 1 compound is found to have complete disordering of vanadium and chromium in the metallic layers, the n = 2 and 3 compounds show strong tendency for these elements' ordering, with the layer in the 2a(0,0,0) site of (Cr0.5V0.5)3AlC2 fully occupied by vanadium. The thermal expansion dependency of temperature is also studied by neutron diffraction for 2 < T < 550 K, revealing a negligible thermal expansion below 100 K for all of the compounds.
The crystal structure of the newly synthesized quaternary MAX phase (Cr2/3Ti1/3)3AlC2 was systematically characterized by various techniques. The space group of (Cr2/3Ti1/3)3AlC2 is determined to be P63/mmc by a combination of selected-area and convergent-beam electron diffraction techniques. Rietveld refinements of the neutron diffraction and X-ray diffraction data show that in (Cr2/3Ti1/3)3AlC2, Ti and Cr are ordered with Ti in the 2a and Cr in the 4f Wyckoff sites of a M3AX2 lattice. It is interesting to find that when the order of the magnetic moment of Cr atoms is considered, the ferromagnetic configuration of (Cr2/3Ti1/3)3AlC2 becomes the ground state. Meanwhile, the Raman-active mode wavenumbers of (Cr2/3Ti1/3)3AlC2 were calculated, and the theoretical data are quite consistent with the experimental data, further proving the ordered crystal structure of this phase. The formation of (Cr2/3Ti1/3)3AlC2 with a unique crystal structure may be related to the distinctly different electronegativities and covalent radii of Cr and Ti atoms.
Of all the Mn+1AXn phases, the most resistant to oxidation in air in the 900–1,400°C temperature range are Ti2AlC, Ti3AlC2 and Cr2AlC. A literature review, however, shows that while many claim the oxidation kinetics to be parabolic, others claim them to be cubic. Whether the kinetics are parabolic or better is of vital practical importance. By carefully re-plotting the results of others and carrying out one oxidation run for ≈3,000 h at 1,200°C on a Ti2AlC sample, we conclude that the oxidation kinetics are better described by cubic kinetics and that even that conclusion is an approximation. Lastly, we present compelling evidence that the rate-limiting step during the oxidation of Ti2AlC is diffusion down the alumina scale grain boundaries.
The isothermal oxidation behavior of bulk Ti2AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS. The results demonstrated that Ti2AlC had excellent oxidation resistance. The oxidation of Ti2AlC obeyed a cubic law with cubic rate constants, kc, increasing from 2.38×10-12 to 2.13×10-10 kg3/m6/sec as the temperature increased from 1000 to 1300°C. As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner a-Al2O3 layer and a discontinuous outer TiO2 (rutile) layer formed on the Ti2AlC substrate. A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys. In addition, the scales had good adhesion to the Ti2AlC substrate during thermal cycling.
The more than 60 ternary carbides and nitrides, with the general formula Mn+1AXn—where n = 1, 2, or 3; M is an early transition metal; A is an A-group element (a subset of groups 13–16); and X is C and/or N—represent a new class of layered solids, where Mn+1Xn layers are interleaved with pure A-group element layers. The growing interest in the Mn+1AXn phases lies in their unusual, and sometimes unique, set of properties that can be traced back to their layered nature and the fact that basal dislocations multiply and are mobile at room temperature. Because of their chemical and structural similarities, the MAX phases and their corresponding MX phases share many physical and chemical properties. In this paper we review our current understanding of the elastic and mechanical properties of bulk MAX phases where they differ significantly from their MX counterparts. Elastically the MAX phases are in general quite stiff and elastically isotropic. The MAX phases are relatively soft (2–8 GPa), are most readily mac...
This work concentrates on revealing the oxidation-induced healing performance of Ti 2 AlC. The microstructures of three kinds of defects: notches on single crystal, surface cavity and crack, after healing by oxidation in air at 1200 C, were investigated in detail with the aid of scanning electron microscopy in conjunction with a focused ion beam cross-section milling technique. The notch-healing results demonstrate an effective healing effect due to the predominant Al 2 O 3 phase filling in the notch with a dense microstructure. The microstructure of oxides filled in the surface cavity depends on the size and shape of the cavity. The largest cavity that can be healed at 1200 C solely by Al 2 O 3 is about 10 mm in width. A multi-layered microstructure of oxide scales was observed on the cleavage plane of Ti 2 AlC grains, whereas the microstructure of oxides formed on the non-cleavage area was more complicated due to the anisotropic oxidation behavior of layered Ti 2 AlC. The oxidation driven healing mechanism and factors affecting the healing efficiency of Ti 2 AlC were discussed in detail.
In spite of intrinsic limitations, neutron powder diffraction is, and will still be in the future, the primary and most straightforward technique for magnetic structure determination. In this paper some recent improvements in the analysis of magnetic neutron powder diffraction data are discussed. After an introduction to the subject, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed. Next, we discuss the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure. The simulated annealing approach for magnetic structure determination is briefly discussed and, finally, some features of the program FullProf concerning the magnetic structure refinement are presented and discussed. The different themes are illustrated with simple examples.
In this comprehensive yet compact monograph, Michel W. Barsoum, one of the pioneers in the field and the leading figure in MAX phase research, summarizes and explains, from both an experimental and a theoretical viewpoint, all the features that are necessary to understand and apply these new materials. The book covers elastic, electrical, thermal, chemical and mechanical properties in different temperature regimes. By bringing together, in a unifi ed, self-contained manner, all the information on MAX phases hitherto only found scattered in the journal literature, this one-stop resource offers researchers and developers alike an insight into these fascinating materials.
Introduction Dislocations and Their Arrangements Kink Band Formation in Crystalline Solids Incipient Kink Bands Microscale Model for Kinking Nonlinear Elasticity Experimental Verification of the IKB Model Effect of Porosity Experimental Evidence for IKBs Why Microcracking Cannot Explain Kinking Nonlinear Elasticity The Preisach–Mayergoyz Model Damping Nonlinear Dynamic Effects Summary and Conclusions References
Herein we report on the oxidation of bulk Ti2GeC in air in the 600 degrees C to 800 degrees C temperature range. At 600 degrees C, and up to 500 h, the oxidation kinetics are sub-parabolic and the rutile TiO2 layer formed is adherent and protective. At 700 degrees C and 800 degrees C the kinetics are parabolic up to 100 h before becoming linear. X-ray diffraction confirmed the presence of two rutile structures, TiO2 and GeO2, and a hexagonal GeO2 on the oxidized surfaces. The oxidation occurs by inward diffusion of oxygen through a rutile (Ti1-xGex)O-2 solid solution with x <= 0.07. Additionally, and throughout the entire temperature range studied, an oxygen-induced decomposition - occurring predominantly at the oxide/carbide interface - results in the formation of an elemental Ge-rich phase. If the rutile layer is not protective, the latter oxidizes into hexagonal GeO2. (c) 2013 Elsevier B.V. All rights reserved.
In the secondary regime of the tensile creep of Ti2AlC, made with commercial powders with a grain size of 14 ± 8 μm, the minimum creep rate is given by a power law, with a stress exponent of 2.5 ± 0.3 and an activation energy of 362 ± 88 kJ mol−1. Dislocation creep—with possibly grain boundary sliding—are presumed to be the dominant creep mechanism(s). The high failure strains (>15%) can be partially attributed to substantial grain kinking near the fracture surface and concomitant damage tolerance.
A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step-scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least-squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.
Recently the authors reported on the fabrication and characterization of a layered ternary compound Ti{sub 3}SiC{sub 2}, that was found to combine many of the best attributes of metals and ceramics. Like metals it is an excellent electric and thermal conductor, easily machinable, relatively soft, not susceptible to thermal shock and behaves plastically at higher temperatures. Like ceramics it is oxidation resistant, refractory, and, most importantly, it maintains its strength to temperatures that render the best superalloys available today unusable. Upon realizing the remarkable properties exhibited by Ti{sub 3}SiC{sub 2} the authors carried out a literature search for other compounds with the same chemistry. They found one only, namely: Ti{sub 3}GeC{sub 2} which has properties that are very similar to Ti{sub 3}SiC{sub 2}. The purpose of this paper is two-fold. The first is to demonstrate the similarities between the properties of the H-phases, Ti{sub 3}GeC{sub 2} and Ti{sub 3}SiC{sub 2}. The second is to present some compelling microstructural evidence that indicates that these compounds behave as polycrystalline nanolaminates. To that effect the authors fabricated and characterized the following H-phases: Ti{sub 2}AlC, Ti{sub 2}AlN, Ti{sub 2}GeC and as well as Ti{sub 3}GeC{sub 2}. In addition they fabricated V{sub 2}AlC, Ta{sub 2}AlC, and Nb{sub 2}AlC and tested their machinability.
Aluminium containing heat-resistant silicides and ternary carbides (Maxthal®), have been evaluated in terms of their high temperature behaviour in various atmospheres including vacuum. The novelty of these materials is in their ability to form a stable and adherent protective alumina scale at low partial pressures of oxygen as well as in air. Recently developed heating elements, for oxidising and reducing atmospheres, made of molybdenum aluminosilicide (Kanthal Super ER) have been evaluated by thermogravimetric analysis (TGA) studies in air and other atmospheres. The performance up to 1575 °C has been evaluated at vacuum levels down to 10−4 mbar. In addition to its remarkable thermal shock resistance, damage tolerance and machinability, Ti2AlC (Maxthal®) showed a parabolic and stable oxide growth rate up to 1400 °C in air. Cyclic testing confirmed that the oxide layer is adherent to the bulk. It has been concluded that both Maxthal® and Kanthal Super ER are capable of operating in air, up to 1400 and 1575 °C, respectively.
Polycrystalline bulk samples of Ti3SiC2 were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10−8 and 2 × 10−5 kg2-m−4.s−1. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 106Ω−1.m−1, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10−6°C−1, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti3SiC2 does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.
Die Struktur von Ti3SiC2 wird aus Einkristallaufnahmen bestimmt. Die Gitterparameter der hexagonalen Zelle sind:a=3,068,c=17,669 undc/a=5,759. Die Titan-Atome besetzen die Punktlagen 2a) und 4f) (zTi=0,135), die Silicium-Atome die Punktlage 2b) und die Kohlenstoff-Atome die Punktlage 4f) (zC=0,5675) in der Raumgruppe D
6h
4
–P63/mmc. Die Struktur gehrt zu den Komplexcarbiden mit oktaedrischen Bauelementen [T
6C].The crystal structure of Ti3SiC2 has been determined by means of single crystal photographs; the lattice parameters of the hexagonal cell were found to be:a=3.068,c=17.669 andc/a=5.759. The titanium atoms occupy the positions 2a) and 4f) (zTi=0.135), the silicon atoms 2b) and the carbon atoms 4f) (zC=0.5675) of the space group D
6h
4
–P63/mmc. The crystal structure type belongs to the class of complex carbides having octahedral groups [T
6C].
This article is a critical review of the Mn + 1AXn phases (“MAX phases”, where n = 1, 2, or 3) from a materials science perspective. MAX phases are a class of hexagonal-structure ternary carbides and nitrides (“X”) of a transition metal (“M”) and an A-group element. The most well known are Ti2AlC, Ti3SiC2, and Ti4AlN3. There are ~ 60 MAX phases with at least 9 discovered in the last five years alone. What makes the MAX phases fascinating and potentially useful is their remarkable combination of chemical, physical, electrical, and mechanical properties, which in many ways combine the characteristics of metals and ceramics. For example, MAX phases are typically resistant to oxidation and corrosion, elastically stiff, but at the same time they exhibit high thermal and electrical conductivities and are machinable. These properties stem from an inherently nanolaminated crystal structure, with Mn + 1Xn slabs intercalated with pure A-element layers. The research on MAX phases has been accelerated by the introduction of thin-film processing methods. Magnetron sputtering and arc deposition have been employed to synthesize single-crystal material by epitaxial growth, which enables studies of fundamental material properties. However, the surface-initiated decomposition of Mn + 1AXn thin films into MX compounds at temperatures of 1000–1100 °C is much lower than the decomposition temperatures typically reported for the corresponding bulk material. We also review the prospects for low-temperature synthesis, which is essential for deposition of MAX phases onto technologically important substrates. While deposition of MAX phases from the archetypical Ti–Si–C and Ti–Al–N systems typically requires synthesis temperatures of ~ 800 °C, recent results have demonstrated that V2GeC and Cr2AlC can be deposited at ~ 450 °C. Also, thermal spray of Ti2AlC powder has been used to produce thick coatings. We further treat progress in the use of first-principle calculations for predicting hypothetical MAX phases and their properties. Together with advances in processing and materials analysis, this progress has led to recent discoveries of numerous new MAX phases such as Ti4SiC3, Ta4AlC3, and Ti3SnC2. Finally, important future research directions are discussed. These include charting the unknown regions in phase diagrams to discover new equilibrium and metastable phases, as well as research challenges in understanding their physical properties, such as the effects of anisotropy, impurities, and vacancies on the electrical properties, and unexplored properties such as superconductivity, magnetism, and optics.
Complex carbides formed in ternary systems of a transition element (M), a B-group element (M′), and carbon and having a formula M2M′C (H-phase) or M3M′C (perovskite carbide) occur frequently. This reflects the simple geometry of the atomic arrangement of the metals and the filling mode by an interstitial stabilizer such as carbon or nitrogen. The phase relationship of the ternary combinations {Ti, Zr, Hf, V, Nb, Ta, Cr, Mn, and Ni}-aluminum-carbon was investigated. New complex carbides were found with the corresponding zirconium, hafnium, and tantalum combinations. The crystal structures in the case of Zr- and Hf-containing complex carbides can be characterized by a twelve-metal-layer sequence and by a ten-metal-layer sequence with carbon atoms again filling octahedral voids. The transition of structure types from TiC, Ti2AlC, Ti3SiC2, ZrAlC2, Zr2Al3C5, to Al4C3 is also discussed.
Crack healing of Ti3AlC2 was investigated by oxidizing a partially pre-cracked sample. A crack near a notch was introduced into the sample by tensile deformation. After oxidation at 1100 °C in air for 2 h, the crack was completely healed, with oxidation products consisting primarily of α-Al2O3 as well as some rutile TiO2. The indentation modulus and hardness of the crack-healed zone are slightly higher compared with those of the Ti3AlC2 base material. The preferential oxidation of Al atoms in Ti3AlC2 grains on the crack surface results in the predominance of α-Al2O3 particles forming in a crack less than 1 µm wide.
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