Abstract

Abstract

Abstract

Layered ternary carbides contain alternative stacking of structural slabs in the unit cells. Many mechanical and structural features are inherited with respect to their binary carbide counterparts, and some novel properties also appear because of new chemical bonds and atomic coordination at the boundaries of different slabs. In this review, we highlight important recent achievements, which focused on theoretical prediction, microstructure characterization, preparation, and macroscopic properties of newly developed layered ternary transition-metal carbides. These results provide insights into understanding the relationship between the structure (including crystal structure, chemical bonding, and microstructure) and the properties of these layered ternary carbides and further highlight their technological applications as high-temperature and ultrahigh-temperature structural materials.

Abstract

The phase stability and mechanical properties of Ti₂AC (A=Ga, Cd, Sn, In, and Pb) have close relationship with the behavior of group-A element atoms in these compounds. This letter shows that although the Al, Ga, and In or Si, Sn, and Pb are in the same group, the migration energy and vacancy formation energy of group-A atom in Ti₂GaC/Ti₂InC or Ti₂SnC/Ti₂PbC are noticeable lower than those in Ti₂AlC or Ti₃SiC₂ . The present results are helpful for a better understanding of easy out diffusion and self-extrusion of group-A atoms in these compounds.

Abstract

Experimental and thermodynamic studies of the hydrothermal oxidation behavior of Ti₃Si_0.9Al_0.1C2 powders were performed at 500–700^◦C undera hydrostatic pressure of 50MPa. Titanium, silicon and aluminum were selectively extracted from Ti₃Si_0.9Al_0.1C₂ during hydrothermal oxidation,resulting in the formation of oxides and disordered carbon. A comparative investigation with Ti₃SiC₂ disclosed the evident influence of Al dopanton the hydrothermal oxidation process, i.e. delaying the phase transformation from anatase to rutile, promoting the formation of carbon, thecrystallization of silica and decomposition of Ti₃Si_0.9Al_0.1C₂. The corresponding mechanism was discussed.

Abstract

The joining of titanium aluminum carbides has been successfully performed at high temperature and low oxygen partial pressure. The mechanism of the bonding is attributed to the preferential oxidation of Al atoms in the titanium aluminum carbides at low oxygen partial pressure, which leads to the formation of an Al₂O₃ layer through the joint interface. The specimens joined at 1400 ^◦C exhibit a high flexural strength of 315±19.1MPafor Ti₂AlC and 332±2.83MPa for Ti₃AlC₂, which is about 95% and 88% of the substrates, respectively, and the high flexural strength can beretained up to 1000 ^◦C. The high mechanical performance of the joints is attributed to the similar density and thermal expansion coefficient valuesof Al₂O₃ to those of the Ti₂AlC and Ti₃AlC₂ substrates. It indicates that bonding via preferential oxidation at low oxygen partial pressure is a practical and efficient method for Ti₂AlC and Ti₃AlC₂.

Abstract

The native point defects and mechanism of accommodating deviations from stoichiometry of Si₂N₂O crystal have been investigated using atomistic simulation techniques. This work firstly provides a reliable classical interatomic potential model derived from density functional theorycal culations. The force-field parameters well reproduce the crystal structure, elastic stiffness, and dielectric constants of Si₂N₂O. It is expected that the force-field parameters are useful in future investigations on Si₂N₂O by molecular dynamic simulation. The calculated formation energies for native defects suggest that intrinsic disorder in stoichiometric Si₂N₂O is dominated by antisites and a degree of oxygen Frenkel defect may also exist in this system. In nonstoichiometric Si₂N₂O, the calculated reaction energies indicate that excess SiO₂ or Si₃N₄ is most likely accommodated by the formation of antisite in the lattice. And we also find that SiO₂ excess is energetically more favorable than Si₃N₄ surplus in Si₂N₂O.

Abstract

The surface chemistry and dispersion properties of aqueous Ti₃AlC₂ suspension were studied in terms of hydrolysis, adsorption, electrokinetic, and rheological measurements. The Ti₃AlC₂ particle had complex surface hydroxyl groups, such as Ti–OH, 5Al–OH, and OTi–(OH)₂, etc. The surface charging of the Ti₃AlC₂ particle and the ion environment of suspensions were governed by these surface groups, which thus strongly influenced the stability of Ti₃AlC₂ suspensions. PAA dispersant was added into the Ti₃AlC₂ suspension to depress the hydrolysis of the surface groups by the adsorption protection mechanism and to increase the stability of the suspension by the steric effect. Ti₃AlC₂ suspensions with 2.0 dwb% PAA had an excellent stability at pH=~5 and presented the characteristics of Newtonian fluid. Based on the well-dispersed suspension, dense Ti₃AlC₂ materials were obtained by slip casting and after pressureless sintering. This work provides a feasible forming method for the engineering applications of MAX-phase ceramics, wherein complex shapes, large dimensions, or controlled microstructures are needed.

Abstract

A polysilazane coating was prepared on polyimide substrate by the polymeric precursor method to protect
space materials from attack of atomic oxygen (AO) and vacuum ultraviolet radiation (VUV) in low earth orbit
(LEO) environment. Erosion kinetics, surface morphologies, and surface composition investigation indicate
that this polysilazane coating possesses excellent AO resistance, and displays low shrinkage tendency in AO
exposure. The erosion yield of the prepared polysilazane coating is 3.5× and 2.1×10⁻²⁶ cm³/atom during AO exposure and simultaneous AO and VUV exposure, respectively. The low AO erosion yield of the polysilazane coating results from the formation of a SiO2-rich layer on its top surface during exposure, and the VUV radiation is able to create free radical sites on the exposed surface, promoting the formation of a uniform SiO₂-rich surface layer.

Abstract

Polysiloxane/SiO₂ hybrid coatings have been prepared on Kapton flms by a sol-gel process. The erosion resistance of polysiloxane/SiO₂ (20 wt pct) coating was evaluated by exposure tests of vacuum ultraviolet radiation (VUV) and atomic oxygen beam (AO) in a ground-based simulation facility. The experimental results indicate that this coating exhibits better AO resistance than pure polysiloxane coating. The erosion yield (E_y) of the polysiloxane/SiO₂ (20 wt pct) hybrid coating is about 10 ^-27 cm³/atom, being one or two orders of magnitude lower than that of polysiloxane. VUV radiation can affect the erosion process greatly. Under simultaneous AO and VUV exposure, the value of E_y of the polysiloxane/SiO₂ (20 wt pct) hybrid coating increases by 39% compared with that under single AO exposure.

Abstract

Near-fully dense Ti₃Si(Al)C₂/Ti₅Si₃ composites were synthesized by in situ hot pressing/solid–liquid reaction process under a pressure of 30MPa in a flowing Ar atmosphere at 1580 ^◦C for 60 min. Compared to monolithic Ti₃Si(Al)C₂, Ti₃Si(Al)C₂/Ti₅Si₃ composites exhibit higher hardness and improved wear resistance, but a slight loss in flexural strength (about 26% lower than Ti₃Si(Al)C₂ matrix). In addition, Ti₃Si(Al)C₂/Ti₅Si₃ composites maintain a high fracture toughness (K_IC = 5.69–6.79MPam^1/2). The Ti₃Si(Al)C₂/30 vol.%Ti₅Si₃ composite shows the highest Vickers hardness (68% higher than that of Ti₃Si(Al)C₂) and best wear resistance (the wear resistance increases by 2 orders of magnitude). The improved properties are mainly ascribed to the contribution of hard Ti₅Si₃ particles, and the strength degradation is mainly due to the lower Young’s modulus and strength of Ti₅Si₃.

Abstract

Plane-wave pseudopotential total energy method was used to calculate the effects of impurity Li atom on crystal structure, electronic and dielectric properties of Si₂N₂O. It is proved that Li atom prefers to occupy interstitial site than to substitute the Si atomic site. In addition, the presence of interstitial Li atom leads to relaxation of internal coordinates of Si, N, and O atoms, and bring out a different X-ray diffraction (XRD) pattern compared with that of a pure Si₂N₂O. The result is helpful to understand the diversity of experimental XRD data for Si₂N₂O sintered with and without Li₂O additive. The theoretical polycrystalline dielectric constant of Li-doped Si₂N₂O is larger than that of a pure one, which can be attributed to a reduction of band gap. The mechanism is that interstitial Li atom provides extra electronic states at the bottom of conductive band.

Abstract

Dense Nb₂AlC ceramic was synthesized from NbC, Nb, and Al powder mixture at 1650 ^oC and a pressure of 30MPa for 90 min using an in situ reaction/hot-pressing method. The reaction kinetics, microstructure, physical, and mechanical properties of the fabricated material were investigated. A thermal expansion coefficient of ~8.1× 10 ^-6 K ^-1 was measured in the temperature range of 30^o–1050 ^oC. At room temperature a thermal conductivity of ~20 W. (m . K) ^-1 and a Vickers hardness of ~4.5 GPa were determined. The material attained Young’s modulus, four-point bending strength and fracture toughness of ~294 GPa, ~443 MPa, and ~5.9 MPa .m^1/2, respectively. The nanolayered grains with a mean grain size of 17 μm contributed to the damage tolerance of this ceramic. Quenching from 600^o, 800^o, and 1000^oC into water at room temperature resulted in decrease in bending strength from 443 MPa for the as-synthesized Nb₂AlC to 391, 156, and 149 MPa, respectively.

Abstract

We performed density-functional calculations of oxygen incorporation and diffusion in layered Ti₂AlC for a range of intrinsic- and impurity-element chemical potentials. In view of the thermal equilibrium coexistence between oxygen-dissolved Ti₂AlC and the oxide scale, a thermodynamic scheme is presented that allows the comparison of the relative stability of oxygen defects in different exterior environments. The calculations show that the oxygen atom favors substitution on carbon lattice sites (O_C) under oxygenlean conditions and high temperatures, whereas the occurrence of an oxygen interstitial in the aluminum atomic layer (I_O-tri) becomes more preferential in an oxygen-rich atmosphere and low temperatures. Interstitial oxygen (I_O-tri) diffusion via a metastable interstitial site (I_O-oct) has a comparatively low migration energy. The substitutional oxygen defect (O_C) diffuses by exchanging with neighboring carbon vacancy, which needs a relatively high diffusion barrier.

Abstract

Bulk Ti₃AlC ceramic containing 2.68 wt% TiC was prepared by an in situ reaction/hot-pressing route. The reaction path, microstructure, mechanical and thermal properties were systematically investigated. At room temperature Vickers hardness of Ti₃AlC ceramic is 7.8 GPa. The flexural strength, compressive strength, and fracture toughness are 182, 708 MPa, and 2.6 MPa .m^1/2, respectively. Its apparent Young’s modulus, shear modulus, bulk modulus and Possion’s ratio are 208.9, 83.4, 140.4 GPa, and 0.25 at room temperature. Apparent Young’s modulus decreases slowly with the increasing temperature, and at 1210 ^oC the modulus is 170 GPa. The average coefficient of thermal expansion of Ti₃AlC ceramic is about 10.1×10 ^-6 K ^-1 in the temperature range of 150^oC–1200 ^oC. Both the molar heat capacity and thermal conductivity increase with an increase in the temperature. At 300 and 1373 K, the molar heat capacities are 87 and 143 . J . (mol . K) ^-1, while the thermal conductivities are 8.19 and 15.6 W. (m . K) ^-1, respectively.