Abstract

We performed ab initio calculations for monovacancy formation and migration in Ti2AlC. Carbon and aluminum vacancies have almost equally low formation energies, respectively, at (Ti- and Al-rich) and (Ti- and C-rich) growth conditions, wherein both defects exhibit a high equilibrium concentration and structural tolerance to large off-stoichiometry in Ti2AlC. In contrast, V_Ti has the highest formation energy at all possible conditions. The intrinsic migration energies of various vacancies are determined to be in the sequence Em(V_Al) < E_m(V_Ti) < E_m(V_C).

Abstract

The interaction of Ti3SiC2 with H₂O at 50 MPa and 500–700 ^oC was investigated. Thermodynamic calculations were also employed to analyze the reactions. During hydrothermal oxidation, Ti and Si were selectively oxidatively extracted from Ti3SiC2, resulting in the formation of TiO2, SiO2 and amorphous-sp²-disordered carbon. This phenomenon was attributed to the unique bonding and structural characteristics of Ti3SiC2.

Abstract

Silicon pack cementation has been applied to improve the oxidation resistance of Zr2Al3C4. The Si pack coating is mainly composed of an inner layer of ZrSi2 and SiC and an outer layer of Al2O3 at 1200 °C. The growth kinetics of silicide coating at 1000–1200 °C obey a parabolic law with an activation energy of 110.3 ± 16.7 kJ/mol,  which is controlled by inward diffusion of Si and outward diffusion of Al. Compared with Zr2Al3C4, the oxidation resistance of siliconized Zr2Al3C4 is greatly improved due to the formation of protective oxidation products, aluminosilicate glass, mullite, and ZrSiO4.

Abstract

In this study, a well-dispersed γ-Y2Si2O7 ethanol-based suspension with 30 vol%solid loading was prepared by adding 1 dwb% polyethylene imine dispersant, which allows feeble magnetic γ-Y2Si2O7 particles with anisotropic magnetic susceptibility to rotate in a 12 T strong magnetic field during slip casting, resulting in the development of a strong (202) texture in green bodies. Pressureless sintering gives rise to more pronounced grain growth in the textured sample than in the untextured sample prepared without the magnetic field due to the rapid migration of the grain boundaries of the well-oriented grains, which was revealed by constant-heating-rate sintering kinetics. It was found that the use of two-step sintering is very efficient not only for inhibiting the grain growth but also for enhancing the (202) texture. This implies that controlled grain growth is crucial for enhancing texture development in γ-Y2Si2O7.

Abstract

Thermal properties, namely, Debye temperature, thermal expansion coefficient, heat capacity, and thermal conductivity of γ-Y2Si2O7, a high-temperature polymorph of yttrium disilicate, were investigated. The anisotropic thermal expansions of γ-Y2Si2O7 powders were examined using high-temperature X-ray diffractometer from 300 to 1373 K and the volumetric thermal expansion coefficient is (6.68±0.35) 10 ^-6 K ^-1. The linear thermal expansion coefficient of polycrystalline γ-Y2Si2O7 determined by push-rod dilatometer is (3.90±0.4) 10 ^-6 K ^-1, being very close to that of silicon nitride and silicon carbide. Besides, c-Y2Si2O7 displays a low-thermal conductivity, with a j value measured below 3.0W. (m . K) ^-1 at the temperatures above 600 K. The calculated minimum thermal conductivity, jmin, was 1.35 W. (m . K) ^-1. The unique combination of low thermal expansion coefficient and low-thermal conductivity of γ-Y2Si2O7 renders it a very competitive candidate material for high temperature structural components and environmental/ thermal-barrier coatings. The thermal shock resistance of  γ-Y2Si2O7 was estimated by quenching dense materials
in water from various temperatures and the critical temperature difference, △T_c, was determined to be 300 K.

Abstract

Large-scale and long-time molecular-dynamics simulations are used to investigate the temperature dependences of elastic properties for amorphous SiO2. The elastic moduli increase in a temperature range up to 1600 K and decrease thereafter. The anomalous behaviour in elasticity is explained by analysing the changes of atomic-scale structure with respect to increment of temperature. The mechanism originates predominantly from distortion of the SiO4 tetrahedra network in low-temperature ranges. At an elevated temperature range, thermal-induced Si–O bond stretching dominates the process and leads to normal temperature dependence of elastic properties.

Abstract

Y2SiO5 has potential applications as functional–structural ceramic and environmental/thermal barrier coating material. As an important grainboundary phase in the sintered Si3N4, it also influences the mechanical and dielectric performances of the host material. In this paper, we present the mechanical properties of Y2SiO5 including elastic moduli, hardness, strength and fracture toughness, and try to understand the mechanical features from the viewpoint of crystal structure. Y2SiO5 has low shear modulus, low hardness, as well as high capacity for dispersing mechanical damage energy and for resisting crack penetration. Particularly, it can be machined by cemented carbides tools. The crystal structure characteristics
of Y2SiO5 suggest the low-energy weakly bonded atomic planes crossed only by the easily breaking Y–O bonds as well as the rotatable rigid SiO4 tetrahedra are the origins of low shear deformation, good damage tolerance and good machinability of this material. TEM observations also demonstrate that the mechanical damage energy was dispersed in the form of the micro-cleavages, stacking faults and twins along these weakly bonded atomic planes, which allows the “microscale-plasticity” for Y2SiO5.

Abstract

We use first-principles calculations to study the energetics of intrinsic defects in Ti₂AlC and the effect of N or O impurity atoms on the generation of Al vacancies. The insertion of impurity atoms lowers the vacancy formation energy of its neighboring Al. The formation of Al vacancies is related to the experimental observations of growth of AlN or Al₂O₃ nanowires and nanofibers on the surface of Ti₂AlC. Since the growth of these nanostructures is controlled by the generation and migration of intrinsic defects, we propose that a tunable method for synthesis of such nanostructures is possible by controlling impurities.