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The dependence of the friction and wear behavior of MAB-phase Fe2AlB2 against GCr15 steel counterpart on the sliding speed (0.8–6.4 m/s) and normal force (20–60 N) is investigated. The friction coefficient (0.3–0.7) decreases with increasing sliding speed and normal force, however, with some abnormalities for low sliding speeds of 0.8–1.6 m/s in the latter case. In contrast, overall increasing wear rates ((0.5–2.5) × 10-5 mm3/(N·m)) are observed, with significantly high values at 0.8 m/s and 40–60 N. Surface oxidation is identified due to the high temperature. At 0.8–1.6 m/s, the surface tongue patterns and material migration are both indicative of adhesive wear, with the contribution of fatigue wear becoming more significant for sliding speeds of over 3.2 m/s. At 6.4 m/s and 40 N, the wear changes from deformation-controlled by shallow spalling to fracture-controlled by deep failure.

The high-temperature flexural and compressive strengths, and thermal shock behavior in water of Fe2AlB2 were investigated from room-temperature (RT) to 1000 °C. The flexural strength varies in a narrow range of 200–250 MPa from room temperature to 1000 °C, without evident plastic deformation. The compressive strength of Fe2AlB2 decreases gradually from 1992 ± 176 MPa at RT to 1482 ± 127 MPa at 600 °C. However, the further increasing temperature results in quicker decrease of the compressive strength to 245 ± 7 MPa at 1000 °C although no plastic deformation is present in the temperature range of 600–800 °C. The brittle-ductile transition temperature (BDTT) is higher under flexure (>1000 °C) than compression (800–900 °C), which is attributed to the higher shear stress under compression. The water-quenched flexural strength exhibits features consistent with the quasi-static propagation of “long initial cracks”, with a critical temperature difference of 200–300 °C. The deduced cracks contribute to the decreasing retained strength. The uniaxial compress during hot pressing results in a weak anisotropy of mechanical properties.

The Si@Ti3C2 MXene nanocomposite was prepared in this paper by simply ultrasonic mixing of commercially available nanosized Si and Ti3C2 MXene. The introduction of Ti3C2 makes the aggregation of silicon nanoparticles relieved. Electrochemical measurements show that the nanocomposite maintains a reversible capacity of 188 mAh·g−1 at 0.2 A·g−1 after 150 cycles and exhibits improved capacity retention, which are significantly better than those of pure Si. The enhanced cycling stability and rate capability are attributed to the good electrical connection between silicon and Ti3C2. This study provides a new available matrix and guidance for the applications of MXenes in Si-based anode LIBs. Keywords: Silicon, Si@Ti3C2 nanocomposite, MXene, Li-ion batteries

Herein, a new nanocomposite was synthesized via in-situ growth of Fe3O4 nanoparticles on MXene Ti3C2 multilayer to improve the electrochemical performance of anodes by integrating the merits of transition metal oxide and Ti3C2, where further surface modification of Fe3O4@Ti3C2 nanocomposites by carbon coating was introduced here. The nanocomposites exhibited excellent electrochemical performance in Li-ion storage when used as the anode materials, which benefited from the combination of the high capacity of magnetite and favorable electrical conductivity of Ti3C2. The optimized Fe3O4@Ti3C2-2.5 (a mass ratio of 1.1) showed a high reversible capacity of 342.9 mAh·g−1 at 1C, which exceeded the theoretical capacity of bare Ti3C2 monolayer (320 mAh·g−1), and an impressive rate reversibility. TEM presented that the carbon layers were homogeneously coated on the surface of nanocomposites with a thickness of approximately 1 nm. The electrochemical measurement showed that C-coated Fe3O4@Ti3C2-2.5 presented enhanced cycling performance (382.9 mAh·g−1 at 1C and 236.7 mAh·g−1 at 5C) and cycling stability. This work presents a promising route for preparation of transition metal oxides@MXene nanocomposites as well as further surface modification for advanced Li-ion storage.

The presented work compared the etching behavior between combustion synthesized Ti 3 AlC 2 (SHS-Ti 3 AlC 2 ) and pressureless synthesized Ti 3 AlC 2 (PLS-Ti 3 AlC 2 ). Because the former had a more compact structure, it was harder to be etched than PLS-Ti 3 AlC 2 under the same conditions. When served as anode material for Li-ion batteries, SHS-Ti 3 C 2 showed much lower capacity than PLS-Ti 3 C 2 at 1 C (52.7 and 87.4 mAh g −1 , respectively) due to the smaller d -spacing. Furthermore, Potentiostatic Intermittent Titration Technique (PITT) was used to determine the Li-ion chemical diffusion coefficient ( D Li + ) of Ti 3 C 2 in the range of 10 −10 − 10 −9 cm 2 s −1 , indicating that Ti 3 C 2 could exhibit an excellent diffusion mobility for Li-ion.

The electrochemical properties of MXene Ti3C2 multilayer for Li-ion batteries were improved greatly by vacuum calcination, after systematically evaluating its thermal stability in different atmosphere in details. In air, the as-prepared Ti3C2 could not be oxidized up to 429.9 °C and the rutile-TiO2 would remain as the oxidation product at 1200 °C. The surface functional groups especially F groups can be eliminated by heat treatment. After vacuum calcination at 400 °C, the Ti3C2 show much higher capacities due to the removal of OH groups (126.4 mAh·g−1 at 1C), and exhibited excellent rate capability. Besides, the formation of TiO2 nanoparticles at 700 °C further increases the first coulombic efficiency (62%) and capacity retention after 100 cycles (97%). In contrast, the dense microstructures of resulting TiCx formed after calcination at 1000 °C results in the worst electrochemical properties. This paper presented a relatively simple and easily scalable post-treatment for improving the electrochemical properties of MXene, and demonstrated a great potential of Ti3C2 of using as anode material for Li-ion batteries.

Mn+1AXn phases (MAX phases for short with M: transition metal, A: A group elements, X: C or N, and n = 1–3) have attracted considerable attention due to the unique combination of the ceramic- and m...

Polycrystalline Fe2AlB2 bulk including minor Al2O3 is synthesized by reactive hot pressing from Fe, Al, and B powders at 1200 °C and 30 MPa for 30 min, with a relative density of 96%. The present approach enables a markedly reduced holding time compared with previous studies. The derived Fe2AlB2 shows an electrical resistivity of 2.27±0.01 μΩ·m, Vickers hardness of 10.2±0.2 GPa, flexural strength of 232±25 MPa, compressive strength of 2101±202 MPa, fracture toughness of 5.4±0.2 MPa·m1/2 and work of fracture of 117±12 J/m2. No dominant indentation cracks are observed, indicating that Fe2AlB2 may be quite damage tolerant. Interestingly, a non-catastrophic failure is present in the SENB test, with a high work of fracture. The energy-absorbing mechanisms in inhibiting crack formation are delamination and pullout of Fe2AlB2 grains. This article is protected by copyright. All rights reserved.

Density functional theory is used to provide theoretical insights into the ternary nanolaminated and layered transition metal boride (MAB phase) of MoAlB, with calculations of crystal structure, electronic structure, lattice dynamics and elastic properties, including a corresponding hypothetical MAX phase compound Mo2AlC for comparison. The calculated atomic configuration matches well with experiment. The metal-like electronic structure contributes to the physical origin of the high electrical conductivity of MoAlB. Strong covalent bonding is present between the B atoms, as well as between the Mo and B atoms, and significantly the much weaker Al Al bonds are consistent with the high fracture toughness and damage tolerance seen in MoAlB. With increasing pressure, the shrinkage is highest along the b axis, and lowest along the c axis. From the calculated second-order elastic constants, the bulk moduli B, shear moduli G, Young's moduli E and Poisson ratio μ are 207 GPa, 137 GPa, 336 GPa and 0.23, respectively. The G/B ratio of 0.66—similar in magnitude to values in MAX phases –demonstrate similarities in properties between MAB and MAX phases. Lattice dynamics are examined in detail, with 9 Raman-active modes and 6 infrared-active modes identified and analyzed in terms of their atomic motion and wavenumbers.

The thermal shock behavior of Ti2AlC synthesized by means of self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti2AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al2O3. Although the flexural strength decreases quickly in the temperature range of 300-500 °C, no discontinuous decrease in the retained strength is observed in Ti2AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti2AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared to the first quench. Quenching at 300 °C and above, voids after the pullout of grains and cracks are present, which however are absent in the un-quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock. This article is protected by copyright. All rights reserved.

The oxidation behavior of nonstoichiometric Ti2AlCx (x = 0.69) powders synthesized by combustion synthesis was investigated in flowing air by means of simultaneous thermal gravimetric analysis-differential thermal analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscope/energy dispersive spectroscopy, with an effect of powder size. The oxidation of the fine Ti2AlC powders with the size of about 1 μm starts at 300 °C and completes at 980 °C, while with increasing the powder size around 10 μm the corresponding temperature increases to 400 and 1040 °C, respectively. The oxidation of nonstoichiometric Ti2AlCx (x = 0.69) powders is controlled by surface reaction in 400–600 °C, and mainly diffusion in 600–900 °C, with the corresponding oxidation activation energy of 2.35 eV and 0.12 eV, respectively. In other words, the critical temperature of changing oxidation controlling step is around 600 °C. The oxidation products were mainly rutile-TiO2 and α-Al2O3. The tiny white flocculent particles of α-Al2O3 appeared on the surface of fine Ti2AlC powders and increased with increasing the oxidation temperature.

To understand the potential for use of the Hf–Al–C ternary compounds, (HfC)nAl3C2 (Hf2Al3C4 and Hf3Al3C5) and (HfC)nAl4C3 (Hf2Al4C5 and Hf3Al4C6) were investigated using density functional theory, including crystal structure, electronic structure, compressibility, and elastic properties. The theoretical density of (HfC)nAl3C2 (4.10–4.16 g/cm3) is higher than that of (HfC)nAl4C3 (3.92–3.98 g/cm3), due to the smaller number of lighter Al–C layers. With increasing numbers of Hf–C layers, the Hf–C and Al–C bond lengths remain almost unchanged. In none of the compounds is there a gap around the Fermi energy (Ef), which implies they are metal-like conductors. With increasing pressure, there is greater shrinkage along the c axis than the a axis. The bond stiffness increases with increasing pressure. In general, (HfC)nAl3C2 has higher elastic stiffness than (HfC)nAl4C3, with the moduli increasing with the number of Hf–C layers. The Hf–Al–C compounds as well as the brittle Zr–Al–C compounds all have low shear moduli/bulk moduli ratio (G/B) from 0.71 to 0.78, suggesting that the G/B ratio is not always a suitable measure of ductility.

Density functional theory is used to investigate the elastic and optical properties as well as the crystal and electronic structures of two-dimensional Ti2CT2 and Ti3C2T2 (T = F, O, and OH) MXene monolayers. It is found that the elastic stiffness, optical response, crystal structure and the electronic structure show strong dependence on the surface terminated groups often formed with MXene during the etching process. The elastic stiffness maintains only with the surface termination of O atoms, but a large degradation is present in the surface terminations of F and OH atoms. The low adsorption and reflectivity in the range from infrared to ultraviolet rays account for the high transmittance of Ti3C2T2 that has been experimentally observed, and it is predicted that Ti2CT2 will have higher optical transmittance in this range. The calculations also demonstrate the presence of the optical bandgap in Ti2CO2, which renders its potential applications in optical and electronic devices.

A systematic study on lattice dynamics of Mn + 1AlCn (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 Nb3AlC2 contributes to the composition gap in Nb-Al-C system (Nb2AlC and Nb4AlC3 do appear in experiments, but there are no experimental reports on Nb3AlC2). The full set of Raman-active and IR-active modes in the 211, 312, and 413 Mn + 1AXn phases is identified, with the corresponding Raman and IR wavenumbers. The 211, 312, and 413 Mn + 1AXn 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−1 in Ta4AlC3. The Raman wavenumbers of M2AlC 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 M3AlC2 and M4AlC3. Copyright © 2015 John Wiley & Sons, Ltd.

The effect of transition metal (M) and M–C slabs ( n ) on crystal structure and equilibrium properties of Al-containing MAX-phase carbides M n +1 AlC n ( n = 1–3) was investigated. There is a linear relation between lattice parameters and Goldschmid diameter of M atoms. For a given n , c / a ratios always fall into a narrow range. The theoretical density almost increases with increasing VEC (valence electron concentration), n and d-electron shell number. In general, the formation energy and cohesive energy increase with the increase of VEC. The bond length decreases with increasing VEC. The bond length of 3d compounds is much higher than those of 4d and 5d compounds. Among M–C bonds, the lowest bond length is present in the M–C bonds with the M atoms near to Al atoms. Overall, the M–Al bond angle decreases with increasing d-electron shell number. In addition, M–C bond angles increase with increasing the corresponding M–C bond length. Overall, the total density of states at Fermi energy increases with increasing VEC and n , but decreases from 3d, 4d to 5d for M 3 AlC 2 , and M 4 AlC 3 . In addition, a weak M1–M2 bond was found in M 4 AlC 3 with VEC = 5 and 6.

Ti2AlC bulk pellets, Φ150 × 25 mm2 in size, were fabricated by self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing (SHS/PHIP). The growth morphology and microstructures were investigated in detail by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The used SHS/PHIP process results in some unusual microstructures. The relative density increases to over 98% when increasing the size of the used pellet. The as-fabricated Ti2AlC has many lattice defects, contributing to its low phonon thermal conductivity. Interestingly, many nanopores were present in Ti2AlC grains. Of much importance, a high density of vacancies contributes to the nonstoichiometry of Ti2AlC synthesized by the SHS/PHIP process. Ti2AlC synthesized by the SHS/PHIP process is deformed with high residual stress, due to fast cooling and grain growth.

The electrical, thermal, and mechanical properties as well as the effect of the temperature of large-scale Ti2AlC bulk synthesized by self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing were investigated in detail. With increasing temperature, the lattice defects contribute to the decreasing phonon thermal conductivity, and the electrical resistivity increases linearly from room temperature (RT) to 900 °C. The RT flexural strength, compressive strength, fracture toughness, work of fracture, and Vickers hardness were measured to be 606 ± 20 MPa, 1057 ± 84 MPa, 6.9 ± 0.2 MPa m1/2, 158 ± 12 J/m2, and 4.7 ± 0.2 GPa, respectively. With increasing temperature, the flexural and compressive strengths both keep almost unchanged in the zone of brittle failure, but decrease sharply as the plastic deformation occurs. The brittle-plastic transition temperature under flexure (900–950 °C) is higher than compression (700–800 °C). Interestingly, a non-catastrophic failure is observed in the SENB test, with the high work of fracture (158 ± 12 J/m2).

The microstructure and the electrical, thermal, and mechanical properties of bulk Ti2AlC synthesized by self-propagating high-temperature combustion synthesis with pseudo hot isostatic pressing (SHS/PHIP) were investigated in detail. The plate-like Ti2AlC grains distribute irregularly, with the grain size of around 6 μm in length and 1 μm in width. With increasing temperature, the electrical resistivity increases linearly from room temperature (RT) to 900°C, but the thermal conductivity decreases slightly. The RT electrical resistivity and thermal conductivity are 0.40 ± 0.03 μΩ·m and 27.0 W·(m·K)−1, respectively. The electronic component of the thermal conductivity is the dominant mechanism at all temperatures, and the phonon contribution almost can be neglected above 873 K. With increasing temperature, the flexural strength increases first, then decreases above 550°C, at which it reaches the maximum value of 539 ± 36 MPa. The brittle-to-plastic transition temperature falls in the temperature range of 750°C–950°C. The Ti2AlC synthesized by SHS/PHIP process exhibits an anisotropic compressive strength. The work of fracture of Ti2AlC is estimated to be 200 ± 7 J/m2, which is much higher than that of traditional ceramics.

The newly discovered MAX phase, Ti4GaC3, can exist in one of three polymorphs, α, β and γ, all with the space group P63/mmc. The Ti and Ga (underlined) atomic arrangements are, respectively, AB A BACB C BC, AC A CACA C AC and AB C BACB A BC. Herein first-principles calculations are used to investigate the phase stabilities, electronic structures, elastic properties and compressibilities of the three polymorphs. Since the α- to γ-phase transition only involves shuffling of the A-atoms, it occurs much more easily than those to β-Ti4GaC3 despite the fact that the latter is thermodynamically less stable than γ-Ti4GaC3. For α-Ti4GaC3, the total density of states, TDOS, around the Fermi energy, Ef, lies in a local minimum; for the two other polymorphs, the TDOS is near a local minimum. The electrons occupy all the bonding states for α-Ti4GaC3, but the bonding states are partially occupied for both β- and γ-Ti4GaC3. Both bond stiffness and bond angle play an important role in the compressibility. In general, with increasing pressure, all the bonds become shorter, and the rate of increase in bond stiffness also increases. The bulk moduli of the α-, β- and γ-polymorphs were calculated to be 178, 174 and 169 GPa, respectively. The corresponding theoretical densities are 5.14, 5.12 and 5.11 g cm−3.

The phase stability, electronic structure, compressibility, optical and elastic properties of two polymorphs of Ti3SnC2 were investigated using first-principle calculation. α-Ti3SnC2 is confirmed to be the preferred equilibrium phase under high pressure and high temperature. The electronic structure calculations reveal that the Ti and C atoms form a strong Ti1-C-Ti2-C-Ti1 covalent bond chain while the bonding between Ti1 and Sn is relatively weak. In the low frequency range from radio waves to visible light, Ti3SnC2 behaves similarly with TiC. This material exhibits anisotropic compressibility under hydrostatic pressure: it is more compressible along the c-direction than along a-direction, related to the different bond stiffness and bond angle changes under high pressure. The second-order elastic coefficients were calculated. For the α-phase, the bulk, B, shear, G, and Young's moduli, E, are calculated to be 169.4, 124, and 197.4 GPa, respectively. The low G/B ratio partially explains why Ti3SnC2 is relatively soft and damage tolerant.

The chemical bonding and elastic properties as well as the effect of atomic radii for A element in the Ti 3 A C 2 phases ( A = Si, Ge, and Sn) were studied by ab initio total energy calculations using plane-wave pseudopotential method based on DFT. The atomic radii of A element has a weak effect on the electronic structure. However, the bond stiffness was quantitatively examined, which shows that the bond stiffness is affected by the atomic radii of A element. The calculated results including lattice constants, internal coordinate, elastic modulus, sound velocity, and Debye temperature agree with experimental values very well. With the increase of atomic radii of A element from Si, Ge to Sn, the cohesive energy and elastic moduli as well as Debye temperature decrease, but the elastic anisotropy increases. This is related to the change of bond stiffness. It can be predicted that the fracture toughness of Ti 3 SnC 2 would be comparable with that of Ti 3 SiC 2 and Ti 3 GeC 2 .

The chemical bonding and elastic properties of Ti2CdC were investigated by means of a first-principles pseudopotential total energy method. The calculated results for the lattice constants and internal coordinate agree with experimental values very well. Ti2CdC is conducting, and the Cd-d states have little effect on the chemical bonding. The elastic properties were estimated from the individual elastic constants by Voigt approximation. The calculated shear-modulus of Ti2CdC, 70 GPa, is the lowest value among all MAX phases. The lower shear-modulus and shear-modulus-to-bulk-modulus ratio are related to the weaker Ti–Cd bond, which indicates the lower coefficient of friction. This suggests that Ti2CdC would be a potential electrical frictional material.

The electronic structure and elastic properties of the newly discovered ternary layered carbide Ti 4 GaC 3 were investigated by means of the first-principle plane-wave pseudopotential total energy calculation method based on density functional theory. The computed results, including lattice constants and internal coordinates, are in good agreement with experimental values. The elastic moduli of ideal polycrystalline Ti 4 GaC 3 were predicted from the individual elastic constants by Voigt approximation. The band structure shows that the electrical conductivity is metallic and anisotropic, with a high density of states at the Fermi energy. The elastic properties are anisotropic, related to the Ti–Ga bonds being relatively weaker than the Ti–C bonds.

In this study, the dense polycrystalline Ti2AlC was synthesized by self-propagating high-temperature combustion synthesis with the pseudo–hot isostatic pressing process (SHS/PHIP). The resultant phase purity is highly dependent on the mol ratio of raw powders. The Ti2AlC was densified by applying pressure after the SHS reaction. The resultant sample mainly contains typical plate-like nonstoichiometric Ti2AlCx (x = 0.69) with grain size of ∼6 µm. The sample shows the Vickers hardness of 5.5 GPa, highest flexural strength of 431 MPa, compressive strength of 1033 MPa, and fracture toughness of 6.5 MPa·m1/2. No indentation cracks in Ti2AlCx were observed, indicative of a damage material nature. The reaction mechanism for the formation of SHS/PHIP-derived Ti2AlC is also discussed based on differential thermal analysis and x-ray diffraction results.

We have computed the lattice constants, bulk modulus, and total- and partial-density of states of MAX phases Ti 2 InC, Zr 2 InC and Hf 2 InC in the hexagonal P6 3 / mmc space group by ab initio calculation. The deviations from the experimental values for lattice constants are below 1.6%. The bulk moduli are computed to be 128 GPa, 113 GPa, and 136 GPa, respectively. The Zr 2 InC has the lowest bulk modulus among all MAX phases studied to date, which is related to the weaker covalent interaction between Zr-d and C-s, C-p states.

The compressibility of Al-containing MAX carbides was investigated in details using first-principle calculations based on density functional theory. The bond stiffness and bond angle as a function of pressure were examined. The M-Al bond stiffness is about 1/3–1/2 of M-C bond stiffness. The M-C bond close to Al atoms has the highest bond stiffness in M{sub 3}AlC{sub 2} and M{sub 4}AlC{sub 3} phases, with the similar bond stiffness of the other two bonds in the latter. Generally, the bond stiffness of the strongest M-C bond increases with increasing VEC (Valence Electron Concentration), which also affects the bond stiffness of other bonds. Of most importance, the bulk moduli are 0.256 of the mean bond stiffness for three series. With increasing pressure, M-Al bond angle increases, but M-C bond angles decreases, which indicates that M-Al and M-C bonds shift towards basal plane and along c-axis, respectively. As a result, the compressibility becomes more difficult along c-axis than a-axis. Some abnormal phenomena in the compressibility of Al-containing M{sub n+1}AlX{sub n} phases with VEC = 6 are attributed to the thermodynamical instability of these compounds.