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First-order Raman scattering of the MAX phases Ta4AlC3, Nb4AlC3, Ti4AlN3, and Ta2AlC
Abstract
Herein, we report on the Raman spectra of the following ternary hexagonal carbides and nitrides (MAX phases): Ta4AlC3, Ta2AlC and Ti4AlN3. We also present the Raman-active modes of α- and β-Ta4AlC3, Nb4AlC3 and Ti4AlN3, – also referred to as the 413 MAX phases – as predicted from first principles calculations using density functional theory. We compare the obtained experimental and calculated results with previous studies on Ta2AlC and Ti4AlN3. The vibrational behavior associated with the Raman-active modes for the 413 phases has been identified for the first time. In general, the agreement is good between theory and experiment. The experimental and calculated results indicate that the modes at low wavenumbers - dominated by the Al atoms - are a weak function of chemistry and the differences in energy can be traced to variations in the reduced mass. The modes at higher wavenumbers are dominated by the C and N atoms and show a strong dependence on the unit cell chemistry, with the Ta–C bond being stiffer than the Nb–C bond, which is in turn stiffer than Ti–N. Copyright © 2011 John Wiley & Sons, Ltd.
... 19,37 The Raman spectra of Ti 4 Al(C 1−y N y ) 3 ( Figure 3c) are similar to Ti 4 AlN 3 spectra, with 10 Raman-active modes. 28,31 The peak at 180 cm −1 corresponds to E 2g mode with in-plane oscillations of Ti and C/N atoms and a minor contribution of Al atoms; the peaks at ∼210 and ∼400 cm −1 represent A 1g outof-plane vibrations of Ti and C/N atoms, and the peak at 230 cm −1 corresponds to E 1g vibration that includes in-plane modes of Ti and C/N atoms. 31 The peaks in the 600 cm −1 range would have a combination of all the vibrational modes described above. ...
... 28,31 The peak at 180 cm −1 corresponds to E 2g mode with in-plane oscillations of Ti and C/N atoms and a minor contribution of Al atoms; the peaks at ∼210 and ∼400 cm −1 represent A 1g outof-plane vibrations of Ti and C/N atoms, and the peak at 230 cm −1 corresponds to E 1g vibration that includes in-plane modes of Ti and C/N atoms. 31 The peaks in the 600 cm −1 range would have a combination of all the vibrational modes described above. 31 The peak at 550 cm −1 blue-shifts with the increase in nitrogen content (y) from 549 cm −1 in y = 0.733 samples to 559 cm −1 in the y = 1 sample. ...
... 31 The peaks in the 600 cm −1 range would have a combination of all the vibrational modes described above. 31 The peak at 550 cm −1 blue-shifts with the increase in nitrogen content (y) from 549 cm −1 in y = 0.733 samples to 559 cm −1 in the y = 1 sample. Interestingly, with the increase of the nitrogen content, the intensity of the 550 cm −1 peak increases and the second peak at 580 cm −1 becomes more pronounced. ...
... An analysis of the mode symmetries and of the relative atomic displacements allows for a deeper understanding of the changes in the Raman wave numbers with RE element. Similar studies were performed for a number of MAX phases, where Raman-active vibrational modes are measured and calculated ab initio [35][36][37][38][39][40]. In all cases, the low-wave-number modes, that ranged from 50 to 300 cm −1 were assigned to the vibrations of the M and A atoms; the X atoms vibrate at higher wave numbers, that range from 500 to 650 cm −1 , and [010] and, (c) and (d) [103] zone axes. ...
... In order to understand the effects of mass and bond stiffness on the Raman peak positions, we used the conventional expression for vibrational wave numbers, assuming harmonic oscillation, given by [35] ...
... where K is the bond stiffness and m red is the reduced mass of the atomic masses of the species involved in the vibrations. Because the C atoms are light, and as typical of the MAX phases in general [35][36][37][38][39][40], they tend to form strong covalent bonds with Mo, and their vibrations are exclusively located in the high-wave-number (ω 500 cm −1 ) range. Due to the lower symmetry of the i-MAX phases, this is the first time C atom vibrations are involved 053609-7 FIG. 5. Dependence of both experimental and calculated wave numbers on rare-earth atomic mass, for the 33 Raman-active modes. ...
Herein, we report on the growth of single crystals of various Mo2/3RE1/32AlC (RE=Nd,Gd,Dy,Ho,Er) i-MAX phases and their Raman characterization. Using first principles, the wave numbers of the various phonon modes and their relative atomic displacements are calculated and compared to experimental results. Twelve high-intensity Raman peaks are identified as the fingerprint of this new family of rare-earth containing i-MAX phases, thus being a useful tool to investigate their corresponding composition and structural properties. Indeed, while a redshift is observed in the low-wave-number range due to an increase of the rare-earth atomic mass when moving from left to right on the lanthanide row, a blueshift is observed for most of the high-wave-number modes due to a strengthening of the bonds. A complete classification of bond stiffnesses is achieved based on the direct dependence of a phonon mode wave number with respect to the bond stiffness. Finally, STEM images are used to confirm the crystal structure.
... • La surface (groupements terminaux) des MXènes contenant du molybdène sont similaires en termes de composition chimique. Dans la phase (413), Mo2Ti2AlC3, il y a dix modes de vibration (3A1g + 3E1g + 4E2g) actifs en Raman [268]. Dans le spectre correspondant (cf. ...
... La phase Mo 2Ga2C (A) inspirée de la référence[267]. Les phases(312) (B) et (413) (C) adaptées respect ivement des références[269] et[268].XXVI Annexes du Chapitre IV : Influence de l'élément M (M = Mo et/ou Ti) du MXène Mn+1XnTx sur sa chimie et ses propriétés de surface catalytiques d'un matériau sont obtenues selon la nature du matériau. Mais ces dernières peuvent être optimisées par différents paramètres en électrocatalyse. ...
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.
... Within the MAX phases family, Ti 2 AlC is among the most studied material and several groups have investigated its structural, elastic, electronic, and transport properties [8][9][10][11][12]. In particular, the phonon spectra of this material have been measured by first-order Raman scattering [13,14]. The electronic structure and dielectric properties have been investigated using local-probe electron energy loss spectroscopy (EELS) [15]. ...
... Recently, the partial phonon density of states and mean-squared displacement parameters of the Ti 2 AlC compound were studied using the density functional theory (DFT) within a generalized gradient approximation GGA [19]. Thus, although significant progress has been made in the experimental description of the vibrational properties and electron-phonon coupling (EPC) constants of Ti 2 AlC [13,16], no rationalization of these properties has yet been conducted from the ab initio point of view. The purpose of this paper is thus to provide a comprehensive description of the lattice dynamics, electron-phonon interaction, and electrical transport properties of Ti 2 AlC, taken as a model compound for 211 MAX phases, through the investigation of the Eliashberg spectral function α 2 F (ω), EPC parameter, and transport spectral function α 2 tr F (ω). ...
A linear-response method to the density functional theory is used to derive lattice dynamics, transport spectral function and electron-phonon coupling (EPC) constant of Ti2AlC, a member of the very large class of nanolaminated conducting ceramics named MAX phase. By coupling ab initio calculations with the semi-classical Boltzmann transport theory for electron-phonon scattering, the experimentally observed anisotropic electrical transport properties of Ti2AlC are rationalized. Our results indicate that in Ti2AlC, because of the weak dependence of the EPC constant〖 λ〗_(tr,α) (α =xx and zz) on the crystallographic direction, the anisotropy of ρ(T) results from the anisotropy of the Fermi-surface. These conclusions, in contrast with those obtained on Ti3SiC2 (another member of the MAX phase family) using a similar approach, establish a correlation between the nanolaminated structure of the MAX phases and the origin of their transport properties anisotropy.
... The D and G bands corresponds to defects as well as conventional graphitic vibrations in graphene [22]. While in Ta 4 C 3 T x MXene the peaks at around 183.7 cm − 1 , 248.9 cm − 1 and 650 cm − 1 corresponds to the vibration modes ω 3,4,5 , ω 6,7 , and ω 8,9,10 due to the presence of metal Ta, carbon and terminations T x [72]. A shift and intensity change of these modes can be observed due to the removal of Al in etching as well as sheet exfoliation and variation in termination attachment due to bonding with other materials [73]. ...
With the need of renewable energy sources electrocatalytic water splitting has been developed greatly. Meanwhile , it is very crucial to choose a suitable electrocatalyst for designing advanced catalytic systems to fulfill the requirements of the modern world. Two dimensional (2D) materials (graphene/MXene) and their modified phases have displayed excellent species as electrocatalysts. In this report we synthesized 2D Ta 4 C 3 T x MXene and zero-dimensional (0D) nitrogen-doped graphene quantum dots (GQD) to further prepare GQD/Ta 4 C 3 T x MXene nano-heterostructures using cost effective and easy approaches. The supporting characterizations helped in evaluating the good quality synthesis of nano-heterostructure. All these 1.5%GQD/MXene, 3%GQD/MXene, 5% GQD/MXene, 7%GQD/MXene and 10%GQD/MXene heterostructures demonstrated enhanced OER and HER catalytic activities. With varying distribution of GQD (1.5%-10%) over Ta 4 C 3 T x sheets, 5%GQD/MXene has been found to be the most efficient bifunctional electrocatalyst for overall water splitting. Moreover, GQD addition enhances the active sites as well as the faster reaction kinetics to achieve a low charge transfer resistance of 80 Ω, an onset potential of 1.5 V as well as performance stability up to 97% for extended time periods. Hence, this study offers insight for tuning MXene phases with dense structure and large flakes using an effective approach in order to overcome the challenges associated with advanced water splitting applications.
... However, theoretical energy values are often computed to be lower than the experimental ones by 10-25%. First-order Raman spectra for the 413 phases were both measured and predicted by DFT for Ti 4 AlN 3 [97], Nb 4 AlC 3 [15] and Ta 4 AlC 3 [101]. In these phases, ten Raman active modes (3A 1g +3E 1g +4E 2g ) were highlighted with good agreement between theory and measurements, and then used to infer the M-X bond stiffness. ...
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibit a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M-C bonds in high-density MC slabs, and relatively weak M-A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other materials properties makes it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... The induced structural surface moieties during etching and delamination processes resulted in peak broadening with an increased interlayer d-spacing. 27 Addition of Ti 4 N 3 T x into the binary nanocomposite further resulted in increased peak intensities. The observed optical phonon modes of WO 3 , In 2 O 3 and Ti 4 N 3 T x in the nanocomposite suggested the effective development of 3-TWI-30 which indicated a clear fabrication of a ternary nanocomposite (Fig. S2a †). ...
Achieving high photoelectrochemical conversion efficiency requires the logical layout of a composite photocatalyst with optimal charge separation and transfer with ideal light harvesting capabilities to enhance the photocatalytic performance, and...
... The chemical bonds within the MAX phase are of ionic metallic-covalent nature, and also anisotropic. [35] The bonds between the layers are weaker than the intra-layer bonds. The phonon of atomic vibrations of transition metal and Al below the bandgap (Raman shift of 200-300 cm −1 ) corresponds to the weak bonds between Nb and Al and the phonon of atomic vibrations of C above the bandgap (Raman shift of 550-700 cm −1 ) corresponds to the strongest bonds between Nb and C ( Figure 5). ...
In search for novel materials to replace noble metal‐based electrocatalysts in electrochemical energy conversion and storage devices, special attention is given to a distinct class of materials, MAX phase that combines advantages of ceramic and metallic properties. Herein, Nb4AlC3 MAX phase is prepared by a solid‐state mixing reaction and characterized morphologically and structurally by transmission and scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, nitrogen‐sorption, X‐ray diffraction analysis, X‐ray photoelectron and Raman spectroscopy. Electrochemical performance of Nb4AlC3 in terms of capacitance as well as for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is evaluated in different electrolytes. The specific capacitance Cs of 66.4, 55.0, and 46.0 F g⁻¹ at 5 mV s⁻¹ is determined for acidic, neutral and alkaline medium, respectively. Continuous cycling reveals high capacitance retention in three electrolyte media; moreover, increase of capacitance is observed in acidic and neutral media. The electrochemical impedance spectroscopy showed a low charge transfer resistance of 64.76 Ω cm² that resulted in better performance for HER in acidic medium (Tafel slope of 60 mV dec⁻¹). In alkaline media, the charge storage value in the double layer is 360 mF cm⁻² (0.7 V versus reversible hydrogen electrode) and the best ORR performance of the Nb4AlC3 is achieved in this medium (Tafel slope of 126 mV dec⁻¹).
... A 1 g (o2, o3) and E 1 g o5 are Raman active modes of Ta 2 C-E layers; E 2 g (o1, o4) are attributed to Ta 2 AlC layers as per previous reports. 37,38 The vibration involving Al, i.e., E 2 g (o1, o4) is suppressed in Ta 2 C-E due to the absence of aluminium or exchange of Al with lower atoms because of basic etching. This confirms the complete etching of Al from the MAX phase. ...
One of the eco-sustainable sustainable ways to generate hydrogen, which is considered a clean energy source, involves the electrochemical process. On the other hand, identifying the most effective heterogeneous catalyst is crucial for amine synthesis from nitro compounds in order to determine the optimum catalytic activity, stability, and reusability. Herein, we report the preparation of a 2D tantalum carbide MXene via the fluorine-free etching method and its application in catalysis (nitro reduction) and electrocatalysis (HER and nitro reduction). It is well-known that due to their layered structure, MXenes show better catalytic activities, and hence they find applications in catalysis. Ta2AlC (MAX phase) and Ta2C (MXene) were evaluated for their electrocatalytic activity in the hydrogen evolution reaction. The electrocatalyst Ta2C MXene exhibited overpotentials of 223 mV to reach 10 mA cm⁻² current density when tested under standard HER conditions (0.5 M H2SO4), and furthermore, the electrocatalytic reduction behavior of Ta2C MXene towards the electrochemical reduction of 4-NP was tested in 1 M KOH. Galvanostatic electrolysis was performed in a divided cell using Ta2C modified carbon cloth as the cathode, and Pt as the anode separated by a Nafion membrane, and it showed 72% product conversion and 96% faradaic efficiency. The progress of the galvanostatic electrolysis was monitored using ex situ Raman and UV-Vis spectroscopy. Similarly, the reducing behavior of Ta2C towards the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH4 was tested. The Ta2C MXene displayed improved catalytic activity with pseudo-first-order kinetics. Ta2C nanoparticles completely reduced all three nitro compounds (10 mL each), 4-NP (216 μM-17 min), 2,4-dinitrophenol DNP (163 μM-25 min) and 2,4,6-trinitrophenol TNP (131 μM-36 min). Furthermore, in all these studies, the Ta2C MXene exhibited improved catalytic performance and stability.
... ω 1 , ω 2 , ω 3 , ω 4 . are associated with Titanium (Ti) and Aluminium (Al),whereas two more intense modes of ω 5 ,ω 6 were reported in literature corresponding to the carbon sub lattice due to the ultra pure systems (Lane et al., 2012).which were absent in our synthesised sample due to the presence of the mixed phases of T iC and Al 2 O 3 . ...
... 42 The rst peak ($250 cm À1 ) in MAX was broadened and became less intense aer etching treatment, indicating either the removal of Al or exchange of Al with some lighter atoms such as O, F, N. 43 The 2 nd peak at 920 cm À1 , associated with C, was also broadened and downshied. 44,45 The presence of C in the Raman spectrum suggests that sharper and intense peaks are most likely due to Nb carbide ordered phase. 46 The last two peaks in Raman spectra represent the D-and G-bands. ...
Owing to the tremendous energy storage capacity of two-dimensional transition metal carbides (MXenes), they have been efficiently utilized as a promising candidate in the field of super-capacitors. The energy storage capacity of MXenes can be further enhanced using metal dopants. Herein, we have reported the synthesis of pristine and nickel doped niobium-carbide (Nb2C) MXenes, their computational and electrochemical properties. Upon introduction of nickel (Ni) the TDOS increases and a continuous DOS pattern is observed which indicates coupling between Ni and pristine MXene. The alterations in the DOS, predominantly in the nearby region of the Fermi level are profitable for our electrochemical applications. Additionally, the Ni-doped sample shows a significant capacitive performance of 666.67 F g-1 which can be attributed to the additional active sites generated by doping with Ni. It is worth noting that doped MXenes exhibited a capacitance retention of 81% up to 10 000 cycles. The current study unveils the opportunities of using MXenes with different metal dopants and hypothesize on their performance for energy storage devices.
... We synthesize the mixed metal nitride MXenes by first applying our oxygen-assisted etching process to generate exfoliated Ti 4 N 3 T x MXene. Figure 1a shows the X-ray diffraction (XRD) patterns for Ti 4 AlN 3 MAX phase (gray pattern), [11,[35][36][37] Figure 1. a) X-ray diffraction patterns for Ti 4 AlN 3 MAX phase (gray pattern), oxygen-assisted molten salt fluoride treated Ti 4 AlN 3 (cyan pattern), and exfoliated Ti 4 N 3 T x MXene (black pattern). ...
2D early transition metal carbide and nitride MXenes have intriguing properties for electrochemical energy storage and electrocatalysis. These properties can be manipulated by modifying the basal plane chemistry. Here, mixed transition metal nitride MXenes, M‐Ti4N3Tx (M = V, Cr, Mo, or Mn; Tx = O and/or OH), are developed by modifying pristine exfoliated Ti4N3Tx MXene with V, Cr, Mo, and Mn salts using a simple solution‐based method. The resulting mixed transition metal nitride MXenes contain 6–51% metal loading (cf. Ti) that exhibit rich electrochemistry including highly tunable hydrogen evolution reaction (HER) electrocatalytic activity in a 0.5 m H2SO4 electrolyte as follows: V‐Ti4N3Tx > Cr‐Ti4N3Tx > Mo‐Ti4N3Tx > Mn‐Ti4N3Tx > pristine Ti4N3Tx with overpotentials as low as 330 mV at −10 mA cm−2 with a charge‐transfer resistance of 70 Ω. Scanning electrochemical microscopy (SECM) reveals the electrochemical activity of individual MXene flakes. The SECM data corroborate the bulk HER activity trend for M‐Ti4N3Tx as well as provide the first experimental evidence that HER results from catalysis on the MXene basal plane. These electrocatalytic results demonstrate a new pathway to tune the electrochemical properties of MXenes for water splitting and related electrochemical applications. Mixed transition metal nitride MXenes M‐Ti4N3Tx (M = V, Cr, Mo, or Mn) are prepared using a simple solution‐based method. These M‐Ti4N3Tx exhibit highly tunable electrochemical behavior based on the transition metal M including hydrogen evolution activity from the basal planes as measured by scanning electrochemical microscopy.
... Based on our visualization of the vibrational modes, see Fig. 6(b), we conclude that (i) the low-frequency modes (<220 cm −1 ) correspond to Ce and Mo vibrations, (ii) the Al vibrations are located in the midfrequency range (>220 cm −1 ), and (iii) the high-frequency modes (>500 cm −1 ) mainly involve the C and Mo atoms. This is congruent with the inverse proportionality between the vibrational frequency and the atomic mass, reported previously for other MAX phases [22]. ...
Rare-earth-based (RE) nanolaminates have attracted attention recently because of their complicated magnetism and their potential as precursors for strongly correlated two-dimensional materials. In this work, we synthesized a class of nanolaminates with a Mo4RE4Al7C3 chemistry, where RE = Ce or Pr. Powder samples of both phases were characterized with respect to structure and composition. Single crystals of Mo4Ce4Al7C3 were used for magnetization measurements. The crystal structure was investigated by means of x-ray diffraction and scanning transmission electron microscopy. Magnetization analysis reveals a ferromagnetic ground state with a Curie temperature of ∼10.5 K. X-ray absorption near-edge structure provides experimental evidence that Ce is in a mixed-valence state. X-ray magnetic circular dichroism shows that only the Ce atoms with 4f1 configuration occupying one of the two possible sites are ferromagnetically coupled, with a saturation moment of ∼1.2μB per atom. We thus classify Mo4Ce4Al7C3 as a ferromagnetic, mixed-valence compound.
... The rutile is presumably not observed in XRD because it is too fine-grained and/or has low phase content. Apart from [40]) are not observed in our Raman spectroscopy results. The reason may be that the structure of Ti 3 AlC 2 experienced some disorder, and that some of the Raman peaks are quite weak [41]. ...
Ti3AlC2 tends to partially decompose into TiC phase during deposition by traditional thermal spray techniques, preventing their use in surface anti-corrosion applications. Here, Ti3AlC2 coatings were synthesized using liquid plasma spraying (LPS). Although the average temperature of particles measured in LPS was higher than 2200 K, enough to decompose Ti3AlC2 phase, the resulting sprayed Ti3AlC2 particles were intact. This is probably due to formation of a protective oxide on the surface in the high-temperature steam. The phase purity of Ti3AlC2 coating was high when using water as solvent, but low with a solvent of a mixture of water and alcohol. Different pH values of the solutions influence the phase purity of Ti3AlC2 coatings. The alkaline solutions show detrimental effect on the conservation of Ti3AlC2 phase. The mechanism of improved structural integrity of Ti3AlC2 phase at high temperature through LPS was revealed by microstructural and compositional analysis.
... Since it is quite difficult to prepare single crystal sample, most of the Raman spectroscopic studies of MAX phases focused on the unpolarized Raman. [8][9][10][11][12] Mercier et al. 13 synthesized Ti 3 SiC 2 single crystal, but studied only the unpolarized Raman scattering. All the symmetry assignments of the Raman modes of MAX phases were based on the lattice dynamics calculations (LDCs). ...
Micro-Raman spectroscopic study and lattice dynamics calculations were conducted to study a recently identified layered ternary carbide, Ti5Al2C3. The experimental Raman shifts were remarkably consistent with the calculated values. Polarized Raman spectrum was collected in the polycrystalline sample, which confirmed the theoretical symmetry assignment of the Raman modes. In addition, the atomic vibrations of the peaks at 192 cm−1, 311 cm−1, and 660 cm−1 were identified to be the combination of the counterparts in Ti2AlC and Ti3AlC2.
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.
MXenes are a new family of layered 2D materials with potential over various niche applications. This chapter focuses on the synthesis and characterization methodologies for the MAX phases and MXenes. Different phases (211,312,413) and their structures were explained in detail. A detailed discussion is carried out on the behavior of the material such as metal or semiconductor, the role of surface functionalization (O, OH, F) on MXene surface and their influence on the material properties, the role of defects, advantageous, or disadvantageous, and their electrochemical performance as a supercapacitor electrode.
Selective and solvent-free aerobic oxidation of toluene as a volatile organic compound to benzaldehyde is of immense industrial significance, which is limited by harsh chemical conditions. Developing novel catalysts for overcoming this limitation has attracted the interest of researchers. In this work, a highly stable two-dimensional V4AlC3 MAX phase with layered-like polycrystal structures was synthesized successfully. X-ray diffraction, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy were applied to assess the structure of the MAX phase catalyst. Moreover, the morphology of the synthesized MAX phase was studied using scanning electron microscopy and transmittance electron microscopy. The prepared multi-layered V4AlC3 MAX phase represented a high catalytic activity for solvent-free aerobic oxidation of toluene to benzaldehyde. Up to 18.4% conversion and 84.2% selectivity to benzaldehyde was obtained within only 4 h under air pressure condition. Furthermore, the radical-based mechanism of the oxidation was confirmed by electron paramagnetic resonance by following the produced organic free radical species during reactions.
MXene‐transition metal dichalcogenide (TMD) heterostructures are synthesized through a one‐step heat treatment of Nb2C and Nb4C3. These MXenes are used without delamination or any pre‐treatment. Heat treatments accomplish the sacrificial transformation of these MXenes into TMD (NbS2) at 700 and 900 °C under H2S. This work investigates, for the first time, the role of starting MXene phase in the derivative morphology. It is shown that while treatment of Nb2C at 700 °C leads to the formation of pillar‐like structures on the parent MXene, Nb4C3 produces nano‐mosaic layered NbS2. At 900 °C, both MXene phases, of the same transition metal, fully convert into nano‐mosaic layered NbS2 preserving the parent MXene's layered morphology. When tested as electrodes for hydrogen evolution reaction, Nb4C3‐derived hybrids show better performance than Nb2C derivatives. The Nb4C3‐derived heterostructure exhibits a low overpotential of 198 mV at 10 mA cm−2 and a Tafel slope of 122 mV dec−1, with good cycling stability in an acidic electrolyte. The conversion of niobium carbide MXenes into niobium sulfide/carbide hybrids by sulfidation can be controlled to obtain specific nanoarchitecture. The obtained electrode materials show promising performance for hydrogen evolution reactions, with important differences between Nb4C3 and Nb2C derivatives.
Recently, MXene has set off a great research boom in the optoelectronic field benefiting from its strong conductivity, eminent broadband absorption, and tailorable electronic/optical properties. In this paper, two-dimensional tantalum carbide (Ta4C3) MXene nanosheets were successfully synthesized based on a two-step liquid exfoliation strategy. Firstly, the role of surface termination with different adsorption structures on the electronic and diverse optical properties of Ta4C3Tx MXene were firstly investigated theoretically via first-principles with density functional theory. Interestingly, the simulation revealed that the oxidized and hydroxylated Ta4C3 MXene featured significantly enhanced optical absorption properties in the near-infrared (NIR) band compared with the pristine bare Ta4C3 nanosheets. Besides, the third-order nonlinear optical (NLO) characteristics of the surfaced terminated Ta4C3 nanosheets were measured by the typical Z-scan techniques at 1 μm. The maximum effective nonlinear absorption coefficient was determined to be -0.78 cm/GW, indicating the great potential as a nonlinear material in NIR band. The nonlinear refractive index, real and imaginary parts of the third-order NLO susceptibility were also obtained. Furthermore, the endowment as an excellent broadband optical modulator was strongly acknowledged utilizing Ta4C3 MXene as saturable absorbers for NIR mode-locking operation at 1044 nm and 1557 nm, respectively. Both highly stable dissipative soliton and traditional soliton pulses were generated. Our results demonstrated the excellent broadband nonlinear absorption in NIR regime of terminal functionalized Ta4C3 MXene and might open a new avenue to the Ta4C3-based advanced ultrafast photonic devices.
Ta4AlX3 (X=B, C, N) MAX phase ceramics have been examined using first principles calculations in this study. Ta4AlX3 MAX phase ceramics have hexagonal crystal structure and the formation energies have been determined for the optimized crystal structures. The elastic constants of Ta4AlX3 MAX phase ceramics have been determined and these constants satisfy the mechanical stability criteria. In addition, the mechanical properties such as bulk modulus, shear modulus, etc. have been obtained to reveal the detailed properties of these compounds. The anisotropic elastic properties have been visualized in both 3D and 2D. Moreover, the thermal properties of Ta4AlX3 MAX phase ceramics such as thermal expansion coefficient, heat capacity etc. have been studied in 0 to 1000 K temperature range and 0 to 40 GPa pressure range. In this study, Ta4AlB3 has been considered for the first time along with Ta4AlC3 and Ta4AlN3 compounds and the effect of X atom to the properties of these compounds have been discussed in detail.
As one of the rising 2D materials, niobium‐carbide (Nb2C, well‐known as a member of MXene family) has attracted considerable attention owing to its unique physical and chemical properties. In this work, few‐layer Nb2C nanosheets (NSs) with large (≈255 nm) and small (≈48 nm) lateral dimensions are obtained via a combination of selective etching and liquid cascade centrifugation. Their relaxation time and photophysics process are systematically investigated by transient absorption spectroscopy, and the size effect is demonstrated by phonon‐bottleneck mechanism. Ultrafast fast relaxation time (37.43 fs) and slow relaxation time (0.5733 ps) are observed due to the symmetric structure and metallicity of Nb2C NSs. The nonlinear optical properties of Nb2C NSs are studied by Z‐scan technique, and both saturable absorption and reverse‐saturable absorption are observed. According to first principle calculations, these phenomena can be attributed to the special band structure of Nb2C near the Fermi level, where two‐photon absorption or multiphoton absorption may occur under the irradiation of long wavelength light. These intriguing results suggest that few‐layer Nb2C NSs can be used as building blocks for broadband ultrafast photonics and optoelectronic devices and also hold the potential for breakthrough developments in these fields.
Hexagon-like MAX-phase V4AlC3 single crystals grown by a high-temperature flux method were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDX). We report, for the first time, the first-order Raman spectra (RS) of V4AlC3 single crystals experimentally and theoretically. Via the combination of the results of thermogravimetric analysis, differential scanning calorimetry, XRD, FE-SEM, and EDX, the oxidation performance and mechanism of V4AlC3 single crystals between 300 and 1473 K in air were clarified. Importantly, we carefully investigated the room-temperature corrosion behaviors of V4AlC3 single crystals in concentrated acids [HCl, H2SO4, hydrofluoric acid (HF), and HNO3] and alkalis (NaOH and KOH). V4AlC3 single crystals are stable in concentrated HCl, H2SO4, and NaOH but unstable and even dissolved completely in concentrated KOH and HNO3. In particular, our XRD, RS, FE-SEM, and EDX results have confirmed that HF can dissolve the Al layers of V4AlC3 single crystals but cannot corrode V4C3 layers at room temperature, which eventually led to the formation of macroscopic V4C3T
x
MXene. This reported approach of macro-sized V4C3T
x
MXene can be adapted for obtaining other macroscopic MXenes and will inspire plenty of theoretical and experimental investigations to explore their intrinsic nature and applications, especially for electronic and photonic applications.
Conventional fluorescent probes are either easily photobleached or non-biodegradable, which often leads to the unstable fluorescence signal output and long-term biological toxicity. Therefore, the development of novel fluorescent materials with both excellent photostability and biodegradability is of great significance for further broadening their application in numerous research fields. In this work, Nb2C quantum dots (Nb2C QDs) with pristine crystallographic structures of Nb2C MXene phases and surface oxygen-containing species are synthesized by an ultrasound assisted physicochemical exfoliation in tetrapropylammonium hydroxide. Detailed analyses indicate that the Nb2C QDs not only possess excellent chemical- and photo-stable fluorescence emission but also achieve successful application in fluorescence sensing of heavy metal ions and fluorescence imaging. More importantly, it is confirmed that the Nb2C QDs present high biocompatibility and unique biodegradation property responsive to human myeloperoxidase, implying the application safety of Nb2C QDs in vivo. Under the background of ever-growing and stringent requirements for biosafety and technical stability, the chemical- and photo-stability, biocompatibility, biodegradability, and most importantly the promising fluorescence sensing/imaging characteristic of the obtained Nb2C QDs may argue well for their future applications in environmental monitoring, biomedical diagnosis, visual display, and anti-counterfeiting.
The formation energies and formation volumes of the Ta, Al, and C vacancies in Ta 2 AlC have been calculated by using first-principles method. The results have shown that the vacancy formation is energetically most favorable for the C atom with formation energy of 1.72 eV. The formation energies of Ta and Al vacancies are 3.44 eV and 3.52 eV, respectively. The electronics properties show that the Ta vacancy has a clear effect on the conductivity of the Ta 2 AlC. This result indicates that Ta 2 AlC is a good candidate material for high-temperature and nuclear applications.
Electrochemical supercapacitors are hybrids of a capacitor and battery that rely on materials capable of storing charges via pseudocapacitive reactions in addition to conventional electrostatic double-layer charge storage. MXenes, a relatively new class of two-dimensional (2D) transition metal carbides and nitrides, are ideal candidates for supercapacitors due to their high electronic conductivity, high surface area, and ability to store charges via pseudocapacitive mechanisms. Nitride MXenes such as Ti2NTx are predicted to have higher pseudocapacitance than carbide MXenes but have not been explored experimentally. Here, we report on the synthesis, characterization, and pseudocapacitive charge storage mechanism in the Ti2NTx nitride MXene. Successful formation of nanolayered Ti2NTx MXene is characterized by XRD, SEM, and N2 physisorption analyses. The identity of the surface terminating groups Tx are assigned to primarily O and/or OH based on Raman, FTIR, and STEM-EELS. When tested in various electrolytes, the nanolayered Ti2NTx MXene exhibits pronounced reversible redox peaks and high areal capacitances (~1350 μF cm–2 in 1 M MgSO4 aqueous electrolyte) well exceeding that expected from a double-layer charge storage (~50 μF cm–2) showing that charge is stored in the Ti2NTx MXene via a pseudocapacitive mechanism. We report a trend in the capacitance as a function of cation as follows: Mg²⁺ > Al³⁺ > H⁺ > Li⁺ > Na⁺ > K⁺, that matches theoretical predictions. Remarkably, nanolayered Ti²NTx MXene exhibits >200 F g–1 capacitance over a 1.0 V range in the Mg-ion electrolyte, and the capacitance increases to 160% of its initial value after 1000 cycles owing to the 2 e– process and the distinctive multilayer adsorption characteristic of the Mg²⁺ cation on the Ti2NTx MXene. These findings identify Ti2NTx MXene as a new pseudocapacitive material that possesses high capacitance and wide working voltage in a safe and environmentally friendly Mg-ion electrolyte.
A relatively new class of two-dimensional (2D) materials called MXenes have garnered tremendous interest in the field of energy storage and conversion. Thus far nearly all MXenes reported experimentally have been described as metals, with a lone report of a mixed-metal carbide phase exhibiting semiconducting character. Here, we report the optical, electrocatalytic and electrical properties of the 2D Ti4N3Tx MXene (Tx = basal plane surface terminating groups) and show this material exhbits both metallic and semiconducting behavior. We provide complete structural characterization of exfoliated Ti4N3Tx MXene and assign Tx = O and/or OH and find that this material is susceptible to surface oxidation. Optical experiments indicate that the exfoliated Ti4N3Tx MXene forms a hybrid with a thin surface oxide layer resulting in visible light absorbtion at energies greater than ~2.0 eV and an excitation wavelength-dependent defect-state emission over a broad range centered at ~2.9 eV. As an electrocatalyst for the hydrogen evolution reaction, the exfoliated Ti4N3Tx shows an overpotential of ~300 mV at –10 mA cm–2 and a Tafel slope of ~190 mV/decade. Finally, we observe a clear semiconductor-to-metal transition at ~90 K from temperature-dependent transport measurements under 5 T magnetic field likely resulting from the thin oxide layer. These results unveil the intriguing optical, electrocatalytic, and electrical properties of this 2D Ti4N3Tx MXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.
The large-dimensional and rigid ceramic bulks fabricated by high-temperature solid-phase reaction and sintering have never been considered for possibly entering and circulating within the blood vessels for biomedical applications, especially on combating cancer. Here, it is reported for the first time that MAX ceramic biomaterials exhibit unique functionalities for dual-mode photoacoustic/computed tomography imaging and are highly effective for in vivo photothermal ablation of tumors upon being exfoliated into ultrathin nanosheets within atomic thickness (MXene). As a paradigm, 2D ultrathin tantalum carbide nanosheets (Ta4C3 MXenes) with nanosized lateral sizes are successfully synthesized based on a two-step liquid exfoliation strategy of MAX phase Ta4AlC3 by combined hydrofluoric acid (HF) etching and probe sonication. The structural, electronic, and surface characteristics of the as-exfoliated nanosheets are revealed by various characterizations combined with first-principles calculations via density functional theory. Especially, the superior photothermal-conversion performance (efficiency η of 44.7%) and in vitro/in vivo photothermal ablation of tumor by biocompatible soybean phospholipid-modified Ta4C3 nanosheets are systematically revealed and demonstrated. Based on the large family members of MXenes, this work may offer a paradigm that MXenes can achieve the specific biomedical applications (here, theranostic) providing that their compositions and nanostructures are carefully tuned and optimized to meet the strict requirements of biomedicine.
The Mn+1AXn phases (MAX phases for short, M: transition metal, A: A group elements, X: C or N, and n = 1-3) have attracted much attention due to the unique combination of the ceramic- and metal-like properties. The density functional theory (DFT) has emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of them with the interesting properties. After the beginning with a brief introduction of the MAX phase and DFT, we review the DFT study on this class of materials, including crystal structure, electronic structure, point defects, lattice dynamics and related properties, phase stability, compressibility, and elastic properties. Comparison between the theoretical values and available experimental ones shows that they are in decent agreement for most part, especially in the lattice constants, elastic properties, and compressibility. At the end of this paper, this review is concluded with an outlook of future research on DFT study of MAX phases, major challenges to be met and possible solutions in some cases.
Introduction Elastic Constants Young's Moduli and Shear Moduli Poisson's Ratios Bulk Moduli Extrema in Elastic Properties Effect of Temperature on Elastic Properties Raman Spectroscopy Infrared Spectroscopy Summary and Conclusions References
Here, we report, for the first time, on the first-order Raman spectra of the layered Mo-based ternaries: MoAlB, Mo2Ga2C and Mo2GaC. Polycrystalline samples were fabricated, and well-defined Raman spectra were recorded. When the experimental peak positions were compared with those predicted from density functional theory, good agreement was obtained, indirectly validating both. Furthermore, all modes in the three compounds were symmetry assigned. Copyright
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
We report on the synthesis of the first two-dimensional transition metal nitride, Ti4N3-based MXene. In contrast to the previously reported MXene synthesis methods - in which selective etching of a MAX phase precursor occurred in aqueous acidic solutions - here a molten fluoride salt is used to etch Al from a Ti4AlN3 powder precursor at 550 °C under an argon atmosphere. We further delaminated the resulting MXene to produce few-layered nanosheets and monolayers of Ti4N3Tx, where T is a surface termination (F, O, or OH). Density functional theory calculations of bare, non-terminated Ti4N3 and terminated Ti4N3Tx were performed to determine the most energetically stable form of this MXene. Bare and functionalized Ti4N3 are predicted to be metallic. Bare Ti4N3 is expected to show magnetism, which is significantly reduced in the presence of functional groups.
Herein, we report—for the first time—on the additive-free bulk synthesis of Ti3SnC2. A detailed experimental study of the structure of the latter together with a secondary phase, Ti2SnC, is presented through the use of X-ray diffraction (XRD), and high-resolution transmission microscopy (HRTEM). A previous sample of Ti3SnC2, made using Fe as an additive and Ti2SnC as a secondary phase, was studied by high-temperature neutron diffraction (HTND) and XRD. The room-temperature crystallographic parameters of the two MAX phases in the two samples are quite similar. Based on Rietveld analysis of the HTND data, the average linear thermal expansion coefficients of Ti3SnC2 in the a and c directions were found to be 8.5 (2)·10−6 K−1 and 8.9 (1)·10−6 K−1, respectively. The respective values for the Ti2SnC phase are 10.1 (3)·10−6 K−1 and 10.8 (6)·10−6 K−1. Unlike other MAX phases, the atomic displacement parameters of the Sn atoms in Ti3SnC2 are comparable to those of the Ti and C atoms. When the predictions of the atomic displacement parameters obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Sn atoms. In the case of the Ti and C atoms, the agreement is more qualitative. We also used first principles to calculate the elastic properties of both Ti2SnC and Ti3SnC2 and their Raman active modes. The latter are compared to experiment and the agreement was found to be good.
A systematic study on lattice dynamics of M n + 1 AlC n (n = 1–3) phases using first-principle calculations is reported, where the Raman-active and infrared-active (IR) modes are emphasized. The highest phonon wavenumber is related to the vibration of C atoms. The 'imaginary wavenumber' in the phonon spectrum of Nb 3 AlC 2 contributes to the composition gap in Nb-Al-C system (Nb 2 AlC and Nb 4 AlC 3 do appear in experiments, but there are no experimental reports on Nb 3 AlC 2). The full set of Raman-active and IR-active modes in the 211, 312, and 413 M n + 1 AX n phases is identified, with the corresponding Raman and IR wavenumbers. The 211, 312, and 413 M n + 1 AX n phases have 4, 6, and 8 IR-active modes, respectively. There is no distinct difference among the wavenumber ranges of IR-active modes for 211, 312, and 413 phases, with the highest wavenumber of 780 cm À1 in Ta 4 AlC 3. The Raman wavenumbers of M 2 AlC phases all decrease with increasing the d-electron shell number of transition metal M. However, this case is valid only for the Raman-active modes with low wavenumbers of M 3 AlC 2 and M 4 AlC 3 .
This work investigates the pressure dependence of the structural, mechanical, magnetic, and electronic properties of Cr4AlN3 using a first-principles method. Analysis confirms that the alpha-Cr4AlN3 phase is more stable than the beta-Cr4AlN3 phase and that Cr4AlN3 is more compressible along the a direction than along the c direction. The calculated elastic anisotropic factors indicate that Cr4AlN3 has a higher degree of anisotropy than other M(4)AX(3) compounds, that Cr4AlN3 is mechanically stable in the pressure range of 0 -60 GPa, and that its elastic constants and bulk modulus monotonously increase as pressure increases. Additionally, beta-Cr4AlN3 has better mechanical properties than alpha-Cr4AlN3, while the G/B value of alpha-Cr4AlN3 is currently the lowest among all reported M(n+1)AX(n) compounds. These results also show that the ground state of Cr4AlN3 is ferromagnetic, and the magnetic moments of Cr4AlN3 decrease as pressure increases. The electronic structure calculations indicate that the appearance of the ferromagnetic state in Cr4AlN3 derives mainly from the 3d electrons of the Cr atoms, and that the Cr-N bonding is stronger than the Cr-Al bonding. Finally, analysis shows that Cr4AlN3 does not undergo a structural phase transformation at high pressure (10-60 GPa).
The aim of this paper is to provide an overview of advances in the field of Raman spectroscopy as reflected in articles published each year in the Journal of Raman Spectroscopy as well as in trends across related journals publishing in this research area. The context for this review is derived from statistical data on article counts obtained from Thomson Reuters ISI Web of Knowledge by year and by subfield of Raman spectroscopy. Additional information is gleaned from presentations featuring Raman spectroscopy presented at the International Conference on Advanced Vibrational Spectroscopy in Kobe Japan in August 2013 and at SCIX 2013 sponsored by the Federation of Analytical Chemistry and Spectroscopy Societies in Milwaukee, Wisconsin, USA, October 2013. Papers published in the Journal of Raman Spectroscopy in 2012 are highlighted in this review and reflect topics and advances at the frontier of Raman spectroscopy, a field that is expanding rapidly as a sensitive photonic probe of matter at the molecular level in an ever widening sphere of novel applications. Copyright (c) 2013 John Wiley & Sons, Ltd.
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.
In this paper, in situ Raman spectra of Ta2AlC are measured in the temperature range of 80–500 K at ambient pressure. The frequencies of the Raman modes decrease with increasing temperature, which have been explained by the anharmonic and thermal expansion effects. The line-width of E2g (ω3) mode increases at elevated temperatures, which is found to be due to the anharmonic phonon–phonon scatterings. On the other hand, the line-widths of E2g (ω1) and A1g (ω4) modes decrease continuously with increasing temperature, which is explained by the electron–phonon couplings of these two phonon modes with the Ta 5d electrons. The electron–phonon coupling strengths are obtained both in experiments and density functional calculations. Finally, Ta2AlC is predicted to be a new superconductive MAX phase. Copyright © 2014 John Wiley & Sons, Ltd.
The compression behavior of the hexagonal MAX phases M2AlC
( M=Ti , V, Cr, Nb, and Ta)—selected because they represent two
series; a horizontal series in which the atomic number of the M element
increases from 23 to 25 and a vertical series where the M element (V,
Nb, or Ta) belongs to the VA group—were measured as a function
of quasihydrostatic pressure up to 55GPa , using a synchrotron
radiation source and a diamond-anvil cell. No phase transformations were
observed in any of these compounds. The contractions for the Ti- and
V-containing compositions were higher along the c axis than along the
a axis; the opposite was true for Cr2AlC and
Nb2AlC . In Ta2AlC , the shrinkages in both
directions are almost identical. For V2AlC the bulk modulus
K0 is 201±3GPa . As V is substituted by Nb,
K0 increases by 4%. The K0 of Ta2AlC
, 251GPa , is the highest reported for any MAX phase to date. For
Ti2AlC , K0 is 186±2GPa . As Ti is
substituted by V, K0 increases by 8%. Surprisingly, the
substitution of Ti or V by Cr leads to a reduction in K0 to
165±2GPa . With the notable exception of Cr2AlC , the
agreement between experimental and calculated K0 values is
acceptable. The presence of C in these structures appears to have a
stabilizing effect on the M-Al bonds, presumably by attracting
electrons to the M-X bonds.
We have grown single-crystal thin films of Ti2GeC and Ti3GeC2 and a new phase Ti4GeC3, as well as two new intergrown MAX-structures, Ti5Ge2C3 and Ti7Ge2C5. Epitaxial films were grown on Al2O3(0001) substrates at 1000 °C using direct current magnetron sputtering. X-ray diffraction shows that Ti–Ge–C MAX-phases require higher deposition temperatures in a narrower window than their Ti–Si–C correspondences do, while there are similarities in phase distribution. Nanoindentation reveals a Young’s modulus of 300 GPa, lower than that of Ti3SiC2. Four-point probe measurements yield resistivity values of 50–200 μΩcm. The lowest value is obtained for phase-pure Ti3GeC2(0001) films.
The tensile creep of coarse-grained, CG, Ti3SiC2 samples, in the 1000–1200°C temperature, T, and 10 MPa to 100 MPa stress, σ, ranges, respectively, was studied. The creep behavior is characterized by three regimes: an initial, a secondary where the creep rate is at a minimum, εmin, and a tertiary regime. In the intermediate regime εmin is given by: εmin(s−1)=εoexp17±1σσo2.0±0.1exp−458±12 kJ/molRTwhere σ0=1 MPa and ε0=1 s−1. The times to failure are given by: tf (s)=exp(−2±0.3) εmin−1. The results presented herein confirm that: (a) dislocation creep is the dominant mechanism; (b) the high plastic anisotropy of Ti3SiC2 results in large internal stresses during creep; (c) the response is dictated by a competition between the rates of generation and dissipation of these internal stresses; (d) at higher temperatures and/or lower strain rates the internal stresses can dissipate and the behavior is more ductile. Furthermore, in the tertiary creep regime, the creep appears to occur by a combination of dislocation creep and the formation of cavities and cracks. The coarse-grained samples have lower creep rates than their fine-grained (3–5 μm) counterparts, and their times to failure are longer. The latter is partially attributable to the ability of the larger grains, whose basal planes are normal to the applied load, to form tenacious crack bridges by delamination and kink band formation, in addition to the bridges that occur when the basal planes are parallel to the applied load.
One of the design goals of the neutron time-of-flight (TOF) diffractometer HIPPO (High Pressure–Preferred Orientation) at LANSCE (Los Alamos Neutron Science Center) was efficient quantitative texture analysis. In this paper, the effects of the HIPPO detector geometry and layout on texture analysis, particularly the shape and dimensions of the detector panels, are investigated in detail. An equal-channel angular-pressed (ECAP) aluminium sample with a strong texture was used to determine the methodological limitations of various methods of quantitative texture analysis. Several algorithms for extracting the orientation distribution function (ODF) from the TOF spectra are compared: discrete orientations at arbitrary positions, harmonic methods in Rietveld codes (MAUD and GSAS) and discrete methods in MAUD. Because of the detector geometry, the sharpest texture peaks that can be represented are 12–15° in width, resulting in an optimal texture resolution of 25–30°. Due to the limited resolution and incomplete pole-figure coverage, harmonic expansions beyond L = 12 (where L is the maximum degree of the harmonic expansion) introduce subsidiary oscillations, which are consistently identified as artifacts. Only discrete methods provide a quantitative representation of the texture. Harmonic methods are adequate for a qualitative description of the main texture component. The results of the analysis establish HIPPO as an efficient instrument to determine preferred orientations in relatively short measuring times.
Thin films of Mn+1AXn layered compounds in the Ti-Si-C system were deposited on MgO(111) and Al2O3(0001) substrates held at 900°C using dc magnetron sputtering from elemental targets of Ti, Si, and C. We report on single-crystal and epitaxial deposition of Ti3SiC2 (the previously reported MAX phase in the Ti-Si-C system), a previously unknown MAX phase Ti4SiC3 and another type of structure having the stoichiometry of Ti5Si2C3 and Ti7Si2C5. The latter two structures can be viewed as an intergrowth of 2 and 3 or 3 and 4 M layers between each A layer. In addition, epitaxial films of Ti5Si3Cx were deposited and Ti5Si4 is also observed. First-principles calculations, based on density functional theory (DFT) of Tin+1SiCn for n=1,2,3,4 and the observed intergrown Ti5Si2C3 and Ti7Si2C5 structures show that the calculated difference in cohesive energy between the MAX phases reported here and competing phases (TiC, Ti3SiC2, TiSi2, and Ti5Si3) are very small. This suggests that the observed Ti5Si2C3 and Ti7Si2C5 structures at least should be considered as metastable phases. The calculations show that the energy required for insertion of a Si layer in the TiC matrix is independent of how close the Si layers are stacked. Hardness and electrical properties can be related to the number of Si layers per Ti layer. This opens up for designed thin film structures the possibility to tune properties.
Bulk samples of Ti4AIN3 were fabricated by reactive hot isostatic pressing (hipping) of TiH2, AlN, and TiN powders at 1275 °C for 24 hours under 70 MPa. Further annealing at 1325 °C for 168 hours under Ar resulted in dense, predominantly single-phase samples, with <1 vol pct of TiN as a secondary phase. This ternary nitride, with a grain size of ≈20 μm on average, is relatively soft (Vickers hardness 2.5 GPa), lightweight (4.6 g/cm3), and machinable. Its Young's and shear moduli are 310 and 127 GPa, respectively. The compressive and flexural strengths at room temperature are 475 and 350 MPa, respectively. At 1000 °C, the deformation is plastic, with a maximum compressive stress of ≈450 MPa. Ti4AlN3 thermal shocks gradually, whereby the largest strength loss (50 pct) is seen at a ΔT of 1000 °C. Further increases in quench temperature, however, increase the retained strength before it ultimately decreases once again. This material is also damage tolerant; a 100 N-load diamond indentation, which produced an ≈0.4 mm defect, reduces the flexural strength by only ≈12 pct. The thermal-expansion coefficient in the 25 °C to 1100 °C temperature range is 9.7±0.2 × 10-6 °C-1. The room-temperature electrical conductivity is 0.5 × 106 (Ω · m)-1. The resistivity increases linearly with increasing temperature. Ti4AlN3 is stable up to 1500 °C in Ar, but decomposes in air to form TiN at ≈1400 °C.
We report on the one-phonon Raman scattering spectra from the following M2AC MAX-phase ternary carbides: Ti2AlC, V2AlC, Cr2AlC, Nb2AlC, Ta2AlC, Ti2InC, Hf2InC, V2GeC, Cr2GeC, V2AsC, and Nb2AsC. We also report the results of calculations of the Γ-point, Raman-active phonon energies for these phases based on density functional theoretical simulations, including the effect of the k-point sampling on the convergence of phonon energies. Good agreement between all measured and calculated Γ-point Raman-active optical phonon energies is obtained.
Bulk Ta4AlC3, a new layered-ternary carbide in the Ta–Al–C system, was synthesized and characterized. Transmission electron microscopy investigations on this new phase are reported here. Selected area electron diffraction and convergent beam electron diffraction analyses indicated that this ternary carbide crystallized with the space group P63/mmc. Atomic-scale microstructures of Ta4AlC3 were achieved by means of high-resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. The experimental crystal structural parameters agreed well with the theoretical values obtained using density-functional calculations.
The microstructure of oxide scales formed on Ti2AlC surface after oxidation at 1200 degrees C is investigated. The results reveal an inhomogeneous morphology of the oxide scales, i.e. islands of coarse rutile grains pile up on fine-grained Al2O3 layer. Local cross-sectional observations show that the rutile islands grow on top of cavities in the parent material. A model is proposed to describe the resulting microstructure after oxidation. These results are relevant in optimizing the self-healing conditions of Ti2AlC at high temperature.
Bulk samples of Ti4AIN3 were fabricated by reactive hot isostatic pressing (hipping) of TiH2, AlN, and TiN powders at 1275 °C for 24 hours under 70 MPa. Further annealing at 1325 °C for 168 hours under Ar resulted
in dense, predominantly single-phase samples, with <1 vol pct of TiN as a secondary phase. This ternary nitride, with a grain
size of ≈20 µm on average, is relatively soft (Vickers hardness 2.5 GPa), lightweight (4.6 g/cm3), and machinable. Its Young’s and shear moduli are 310 and 127 GPa, respectively. The compressive and flexural strengths
at room temperature are 475 and 350 MPa, respectively. At 1000 °C, the deformation is plastic, with a maximum compressive
stress of ≈450 MPa. Ti4AlN3 thermal shocks gradually, whereby the largest strength loss (50 pct) is seen at a ΔT of 1000 °C. Further increases in quench temperature, however, increase the retained strength before it ultimately decreases
once again. This material is also damage tolerant; a 100 N-load diamond indentation, which produced an ≈0.4 mm defect, reduces
the flexural strength by only ≈12 pct. The thermal-expansion coefficient in the 25 °C to 1100 °C temperature range is 9.7±0.2
× 10−6 °C−1. The room-temperature electrical conductivity is 0.5 × 106 (Θ · m)−1. The resistivity increases linearly with increasing temperature. Ti4AlN3 is stable up to 1500 °C in Ar, but decomposes in air to form TiN at ≈1400 °C.
We report on the Raman spectra of Ti3SiC2 (312), M2AlC (211) (M=Ti, V, Cr, and Nb) and Ti4AlN3 (413), as representative compounds from the family of Mn+1AXn phases. Intense and narrow first-order Raman peaks are observed, and we present an analysis of the spectra based on symmetry considerations and from results of first-principles calculations of phonon frequencies. The agreement between experimental and calculated mode energies is excellent. The identification of the modes enables application of Raman scattering as a diagnostic tool for the detailed study of the structural and physical properties of this family of compounds and their engineered solid solutions.
In this paper we report on the tensile creep behavior of fine-grained (3–5 μm) Ti3SiC2 in the 1000–1200°C temperature, T, range and the 10–100 MPa stress, σ, range. The creep behavior is characterized by three regimes a primary, quasi-steady state and a tertiary. In the quasi-steady state range and over the entire range of testing temperatures and stresses, the minimum creep rate, , is given by:where σ0=MPa and s−1. The times to failure, tf, were fitted to an expression of the form:where t0=1 s. Interrupted creep tests show that volume-conserving plastic deformation is the dominant source of strain during the secondary creep regime, while cavities and microcracks are responsible for the acceleration of the creep rate during the tertiary creep regime. Like in ice, large internal stresses, especially at high stresses, are developed in Ti3SiC2 during deformation, as a consequence of its extreme plastic anisotropy. The response of Ti3SiC2 to stress appears to be determined by a competition between the rates of accumulation and relaxation of these internal stresses.
In this paper we report on the synthesis and characterization of Ti3GeC2, Ti3Si0.5Ge0.5C2, and Ti3Si0.75Ge0.25C2 solid solutions. Polycrystalline, fully dense, predominantly single phase samples of Ti3Si0.5Ge0.5C2, Ti3Si0.75Ge0.25C2, and Ti3GeC2 of varying grain sizes were fabricated by reactive hot isostatic pressing (HIP) or hot pressing of Ti, C, SiC, and Ge powders. Based on the lattice parameter measurements we conclude that the extent of solid solubility in Ti3(SixGe1−x)C2 ranges for x=0 to, at least, x=0.75. Since the hardness values of both solid solution compositions (2.5±0.2 GPa) were in between those of Ti3SiC2 (3.0±0.3 GPa) and Ti3GeC2 (2.2±0.5 GPa) we conclude that no solid solution strengthening occurs in this system. All samples explored in this work were quite damage tolerant and thermal shock resistant. A 300 N Vickers indentation in a 1.5 mm thick, four-point bend bar decreases its strengths by anywhere from 25 to 35%. Quenching in water from 1000 °C reduces the four-point flexural strength by 10 to 20%; i.e., it is not catastrophic. Notably, the post-quench flexural strength of the coarse-grained Ti3Si0.5Ge0.5C2 samples was ≈25% higher than the as-received bars. Increasing the Ge content resulted in a decrease in the compressive strengths. The ultimate compressive strengths of fine-grained Ti3Si0.5Ge0.5C2 samples, decreased monotonically from room temperature to ≈950 °C. And while failure was brittle at room temperature, above 1000 °C the loss in strength was more severe, but the deformation was more plastic.
Recent high resolution transmission electron microscopy and electron probe X-ray microanalysis show that the compound originally thought to be Ti3Al2N2 is Ti4AlN3. In this paper we report on the crystal structure determination by Rietveld refinement on neutron and X-ray powder diffraction data. Ti4AlN3 crystallizes with a hexagonal unit cell, space group P63/mmc, and with lattice parameters a = 2.9880(2) and c = 23.372(2) Å. The stacking sequence is such that every four layers of Ti atoms is separated by a layer of Al atoms. The N atoms occupy octahedral sites between the Ti atoms making up a network of corner shared octahedra. This compound is closely related to other layered, ternary, machinable, hexagonal nitrides and carbides, namely, M2AX and M3AX2, where M is an early transition metal, A is a A-group element and X is either C and/or N.
Single crystals of Ga-containing MAX-phases TiGaC, TiGaC, and CrGaC were grown from a metallic melt generated by an excess of Ga. This technique allows the crystal growth at different temperatures to control the product distribution. Compounds developed were TiGaC and TiC at 1500 deg. C, and TiGaC and TiGaC at 1300 deg. C. Crystal structures were refined from single crystal data. TiGaC and CrGaC were previously known, and belong to the CrAlC type as well as the solid solutions VGa{sub 1-x}AlC and CrGa{sub 1-x}AlC. TiGaC is one of the few 413-phases (P6/mmc, a=3.0690(4) A, c=23.440(5) A) and the first Ga-containing representative. The crystal structures of MAX-phases are intergrowths of layers of an intermetallic MGa in a hexagonal stacking sequence with carbidic layers (MC){sub n} of the NaCl type. The thickness of the layer depends from the value of n. The results of the structure refinements also demonstrate that also the structural details follow this description. - Graphical abstract: Single crystals of Ga-containing MAX-Phases (TiC){sub n}(TiGa) (n=1, 3) were grown from a metallic melt including and characterised by X-ray diffraction. TiGaC is one of the few 413-phase and the first containing Ga.
Ta4AlC3, a new member of the Mn+1AXn-phase family, has been synthesized and characterized (n=1–3; M=early transition metal; A=A-group element; and X=C and/or N). Phase determination by Rietveld refinement of synchrotron X-ray diffraction data shows that Ta4AlC3 belongs to the P63/mmc space group with a and c lattice parameters of 3.10884 ±0.00004Å and 24.0776±0.0004Å, respectively. This is shown to be the α-polymorph of Ta4AlC3, with the same structure as Ti4AlN3. Lattice imaging by high-resolution transmission electron microscopy demonstrates the characteristic MAX-phase stacking of α-Ta4AlC3. Three modes of mechanical deformation of α-Ta4AlC3 are observed: lattice bending, kinking and delamination.
The tetragonal to orthorhombic ferroelastic phase transition between rutile- and CaCl2-type SiO2 at high pressures is studied using first-principles calculations and the Landau free-energy expansion. The phase transition is systematically investigated in terms of characteristic phonon modes with B1g and Ag symmetries, shear moduli, transverse-acoustic mode, rotation angle of the SiO6 octahedra, spontaneous symmetry-breaking and volume strains, and enthalpy. The results show that these physical behaviors at the transition are well described using the Landau free-energy expansion parametrized by the first-principles calculations.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
V4AlC3, a new MAX phase, was synthesized by reactive hot pressing of a V, Al, and C powder mixture at 1700°C. Using a combination of Rietveld refinement with X-ray diffraction data and ab initio calculations, the crystal structure was determinated. It was found that V4AlC3 has a Ti4AlN3-type crystal structure. The lattice constants are a=0.29310 nm and c=2.27192 nm. And the atomic positions are V1 at (4f) (1/3, 2/3, 0.0544), V2 at (4e) (0, 0, 0.1548), Al at (2c) (1/3, 2/3, 1/4), C1 at (2a) (0, 0, 0), and C2 at (4f) (2/3, 1/3, 0.1080).
The mechanical and thermodynamic stabilities of M4AlC3 (M = V, Nb and Ta) and Ti4AlN3 polymorphs were investigated by means of the first-principles pseudopotential total energy method. All compounds satisfied the Born criteria for mechanical stability, but had different thermodynamic stabilities. Only Ta4AlC3 showed a possible polymorphic phase transformation driven by thermodynamic competition. The present theoretical results support the polymorphism of Ta4AlC3 in experimental synthesis and explain the underlying thermodynamic mechanism. Polymorphism was excluded from V4AlC3, Nb4AlC3 and Ti4AlN3.
Cold-pressed α-Ta4AlC3 powders were annealed up to 1750 °C to test first-principles predictions of α–β phase-stability reversal at 1600 °C. Up to 1600 °C, the α-Ta4AlC3 samples were stable with no indications of any α–β transformation, as shown by the strong characteristic X-ray diffraction peaks of α-Ta4AlC3 and the zigzag stacking observed by transmission electron microscopy. These results show that, in this experimental situation, high temperature alone is not sufficient to cause the α–β transformation.Graphical abstractIn this study, stability of α-Ta4AlC3 is investigated to test a hypothesized thermodynamically driven α–β phase transformation. It is found that the α phase is stable up to 1600 °C, with impurities and point defects most likely increasing the stability of the α-Ta4AlC3.Highlights► Cold-pressed α-Ta4AlC3 powders are annealed up to 1750 °C to test first-principles predictions of α–β phase-stability reversal at 1600 °C. ► The α-Ta4AlC3 samples are stable up to 1600 °C, with no indications of any α–β transformation. ► Transmission electron microscopy shows zig-zag stacking sequence characteristic of α-Ta4AlC3, as well as tantalum oxide impurities. ► The XRD patterns suggest that defects such as vacancies or antisites may increase the stability of α-Ta4AlC3.
Direct atomic resolution observations of the layered stacking characteristics of TaCx slabs and Al atomic planes in ternary Ta–Al–C carbides were achieved. Layered ternary Ta–Al–C compounds have diverse structures. A previously unknown Ta6AlC5 carbide, as well as intergrown Ta2AlC–Ta4AlC3 and Ta4AlC3–Ta6AlC5 structures were identified. Theoretical lattice parameters and bulk modulus of Ta2AlC, Ta3AlC2, Ta4AlC3, and Ta6AlC5 are presented. Furthermore, the Ta–C bonds are much stronger than the Ta–Al bonds in ternary Ta–Al–C carbides, which accounts for the enhancement of bulk modulus with increasing Ta–C layers.
Microstructural observations of damage around indentations in Ti3SiC2 are presented. The Vickers hardness decreased with increasing load and asymptotically approached 4 GPa at the highest loads. No indentation cracks were observed even at loads as high as 300 N. Preliminary strength versus indentation plots indicate that, at least for the large-grained material (is approximately100 μm) studied here, Ti3SiC2 is a damage-tolerant material able to contain the extent of microdamage to a small area around the indent. The following multiple energy-absorbing mechanisms have been identified from scanning electron micrographs of areas in the vicinity of the indentation: diffuse microcracking, delamination, crack deflection, grain push-out, grain pull-out, and the buckling of individual grains.
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.
The structure and chemistry of what initially was proposed to be Ti3Al2N2 are incorrect. Using high-resolution transmission electron microscopy, together with chemical analysis, the stoichiometry of this compound is concluded to be Ti4AlN3-delta (where delta = 0.1). The structure is layered, wherein every four layers of almost-close-packed Ti atoms are separated by a layer of Al atoms. The N atoms occupy ∼97.5% of the octahedral sites between the Ti atoms. The unit cell is comprised of eight layers of Ti atoms and two layers of Al atoms; the unit cell is hexagonal with P63/mmc symmetry (lattice parameters of a= 0.3 nm and c= 2.33 nm). This compound is machinable and closely related to other layered, ternary, machinable, hexagonal nitrides and carbides, namely M2AX and M3AX2 (where M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen).
Using a synchrotron radiation source and a diamond anvil cell, we measured the pressure dependence of the lattice parameters of a recently discovered phase, Ta 4 Al C 3 . This phase adopts a hexagonal structure with the space group P63/ mmc ; at room conditions, the a and c parameters are 3.087(5) and 23.70(4) Å , respectively. Up to a pressure of 47 GPa , no phase transformations were observed. Like Ta 2 Al C , but unlike many related phases such as Ti 4 Al N 3 , Ti 3 Si C 2 , Ti 3 Ge C 2 , and Zr 2 In C , the compressibility of Ta 4 Al C 3 along the c and a axes are almost identical. The bulk modulus of Ta 4 Al C 3 , 261±2 GPa , is ≈4% greater than that of Ta 2 Al C . Both, however, are ≈37% lower than the 345±9 GPa of TaC.
Nb4AlC3, a new compound belonging to the MAX phases, was discovered by annealing bulk Nb2AlC at 1700 °C. The crystal structure of Nb4AlC3 was determined by combined X-ray diffraction, high-resolution transmission electron microscopy and ab initio calculations. It was reported that Nb4AlC3 follows the Ti4AlN3-type crystal structure. The lattice constants are a = 0.31296 nm, c = 2.41208 nm and the atomic positions are Nb1 at 4f (1/3, 2/3, 0.0553), Nb2 at 4e (0, 0, 0.1574), Al at 2c (1/3, 2/3, 1/4), C1 at 2a (0, 0, 0) and C2 at 4f (2/3, 1/3, 0.1086).
Advanced thin films for today's industrial and research needs require highly specialized methodologies for a successful quantitative characterization. In particular, in the case of multilayer and/or unknown phases a global approach is necessary to obtain some or all the required information. A full approach has been developed integrating novel texture and residual stress methodologies with the Rietveld method (Acta Cryst. 22 (1967) 151) (for crystal structure analysis) and it has been coupled with the reflectivity analysis. The complete analysis can be done at once and offers several benefits: the thicknesses obtained from reflectivity can be used to correct the diffraction spectra, the phase analysis help to identify the layers and to determine the electron density profile for reflectivity; quantitative texture is needed for quantitative phase and residual stress analyses; crystal structure determination benefits of the previous. To achieve this result, it was necessary to develop some new methods, especially for texture and residual stresses. So it was possible to integrate them in the Rietveld, full profile fitting of the patterns. The measurement of these spectra required a special reflectometer/diffractometer that combines a thin parallel beam (for reflectivity) and a texture/stress goniometer with a curved large position sensitive detector. This new diffraction/reflectivity X-ray machine has been used to test the combined approach. Several spectra and the reflectivity patterns have been collected at different tilting angles and processed at once by the special software incorporating the aforementioned methodologies. Some analysis examples will be given to show the possibilities offered by the method.
Herein, we report on the compressive creep behavior of hot isostatically pressed (HIPed) fine-grained (FG) and coarse-grained (CG) Ti3SiC2 in the 1100–1300 °C temperature range. The creep behavior is characterized by three regimes, a primary, quasi-steady state and a tertiary. At lower stresses, the creep rates of the two microstructures are comparable suggesting that dislocation creep is operative. At ≈2, the stress exponents in the quasi-steady state regime are comparable to those measured in tension; the creep rates in compression, however, are roughly an order of magnitude lower. At relatively high stresses and/or temperatures, the stress exponents of the FG samples increase dramatically and the creep rates of the CG samples are higher than their FG counterparts. Both observations suggest a change of mechanism from dislocation creep to possibly sub-critical crack growth, in which delaminations play an important role. This conclusion is bolstered by post-deformation microstructural analysis that shows evidence for sub-critical crack growth. The minimum creep rates of pressureless sintered Ti3SiC2 samples were roughly an order of magnitude higher than HIPed samples, with comparable grain size strongly suggesting that some form of grain boundary related deformation, such as decohesion and/or sliding, is playing an important role in the sintered samples.
2D Ti 3C 2 nanosheets, multilayer structures, and conical scrolls produced by room temperature exfoliation of Ti 3AlC 2 in HF are reported. Since Ti 3AlC 2 is a member of a 60+ group of layered ternary carbides and nitrides, this discovery opens a door to the synthesis of a large number of other 2D crystals.
The nucleation and growth of oxide scale at the early stages of oxidation of Ti3AlC2 ceramics was studied via oxidizing at 1100 °C in air for short times (≤900 s). The nucleation of nanosized Al2O3 particles mainly occurs at the ledges of the fractured lamellar Ti3AlC2 grains as well as on the {0001} basal surfaces. The Al2O3 nuclei mainly grow along these ledges to form oxide strings, and then spread on the terraces and the {0001} basal surfaces. An oxide layer consisting predominantly of nanosized α-Al2O3 forms after oxidizing for 180 s. The formation of lenticular hexagonal pores in Ti3AlC2 grains is attributed to the faster consumption of Ti, Al and C atoms along <112¯0> direction than along <0001> direction. With further oxidation, rutile-TiO2 particles form on top of the α-Al2O3 layer, and grow to form a rutile-TiO2 layer. Further oxidation leads to the formation of pores underneath the primary α-Al2O3 layer. In this porous layer both Al2O3 and TiO2 were present with a preference for Al2O3 to stay adjacent to the inward moving interface of Ti3AlC2 substrate.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Single crystals of the new ternary carbides Ta4AlC3 and Ta3AlC2 were synthesized from molten aluminum and characterized XRD, EDX, and WDX measurements. Crystal structures were refined for the first time on the basis of single-crystal data. Both compounds crystallize in a hexagonal structure with space group P63/mmc and Z = 2. The lattice constants are a = 3.1131(3) A and c = 24.122(3) A for Ta4AlC3 and a = 3.0930(6) A and c = 19.159(4) A for Ta3AlC2. The crystal structures can be explained with a building block system consisting of two types of partial structures. The intermetallic part with a composition TaAl is a two layer cutting of a hexagonal closest packing. The carbide partial structure is a fragment of the binary carbide TaC (NaCl type). It consists of three (Ta4AlC3) or two layers (Ta3AlC2) of CTa6-octahedra linked via common corners and edges. Both compounds are members of the series (TaC)nTaAl. The crystal quality of Ta3AlC2 is improved by using a Al/Sn melt for crystal growth leading to small quantities of Sn in the crystal: Ta3Al1-xSnxC2, x approximately 0.04. On the basis of reliable data a detailed discussion of structural parameters is possible. According to the building principle structure models can be developed for the whole series (MX)nMM' including coordinates for all atoms.
Single crystals of V2AlC and the new carbides V4AlC3-x and V12Al3C8 were synthesized from metallic melts. V2AlC was formed with an excess of Al, while V4AlC3-x (x approximately 0.31) and V12Al3C8 require the addition of cobalt to the melt. All compounds were characterized by XRD, EDX, and WDX measurements. Crystal structures were refined on the basis of single-crystal data. The crystal structures can be explained with a building-block system consisting of two types of partial structures. The intermetallic part with a composition VAl is a two-layer cutting of the hexagonal closest packing. The carbide partial structure is a fragment of the binary carbide VC1-x containing one or three layers. V2AlC is a H-phase (211-phase) with space group P63/mmc, Z=2, and lattice parameters of a=2.9107(6) A, and c=13.101(4) A. V4AlC3-x (x approximately 0.31) represents a 413-phase with space group P63/mmc, Z=2, a=2.9302(4) A, and c=22.745(5) A. The C-deficit is limited to the carbon site of the central layer. V12Al3C8 is obtained at lower temperatures. In the superstructure (P63/mcm, Z=2, a=5.0882(7) A, and c=22.983(5) A) the vacancies on the carbon sites are ordered. The ordering is combined to a small shift of the V atoms. This ordered structure can serve as a structure model for the binary carbides TMC1-x as well. V4AlC3-x (x approximately 0.31) and V12Al3C8 are the first examples of the so-called MAX-phases (MX)nMM' (n=1, 2, 3), where a deficit of X and its ordered distribution in a superstructure is proven, (MX1-x)nMM'.
- Matthies