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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

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.

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.

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.

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 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.