Local Lattice Distortion in High-Entropy Carbide Ceramics

Using the ab initio calculations, we study the lattice distortion of HfNbTaTiVC5, HfNbTaTiZrC5 and MoNbTaTiVC5 high-entropy carbide (HEC) ceramics. Results indicate that the Bader atomic radius and charge transfer in HECs is very close to those from binary carbide. The degree of lattice distortion strongly depends on the alloying element. The Bader atomic radius can excellently describe the lattice distortion in HEC. Further, the corresponding atomic radius and formation enthalpy of binary carbides may be indicators to predict the single-phase HECs.

An Overview of Parameters Controlling the Decomposition and Degradation of Ti-Based Mn+1AXn Phases

A critical overview of the various parameters, such as annealing atmospheres, pore microstructures, and pore sizes, that are critical in controlling the decomposition kinetics of Ti-based MAX phases is given in this paper. Ti-based MAX phases tend to decompose readily above 1400 °C during vacuum annealing to binary carbide (e.g. TiCx) or binary nitride (e.g. TiNx), primarily through the sublimation of A elements such as Al or Si, forming in a porous MXx surface layer. Arrhenius Avrami equations were used to determine the activation energy of phase decomposition and to model the kinetics of isothermal phase decomposition. Ironically, the understanding of phase decomposition via exfoliating or selective de-intercalation by chemical etching formed the catalyst for the sensational discovery of Mxenes in 2011. Other controlling parameters that also promote decomposition or degradation as reported in the literature are also briefly reviewed and these include effects of pressure and ion irradiations.

Thermal Stability of MAX Phases

The susceptibility of MAX phases to thermal dissociation at 1300-1550 °C in high vacuum has been studied using in-situ neutron diffraction. Above 1400 °C, MAX phases decomposed to binary carbide (e.g. TiCx) or binary nitride (e.g. TiNx), primarily through the sublimation of A-elements such as Al or Si, which results in a porous surface layer of MXx being formed. Positive activation energies were determined for decomposed MAX phases with coarse pores but a negative activation energy when the pore size was less than 1.0 μm. The insights for tailor-design of MAX phases with controlled thermal stability and intercalated MXenes for energy storage are addressed.

Decomposition Kinetics of MAX Phases in Extreme Environments

MAX phases are remarkable materials but they become unstable at elevated temperatures and decompose into binary carbides or nitrides in inert atmospheres. The susceptibility of MAX phases to thermal dissociation at 1300-1550 °C in high vacuum has been studied using in-situ neutron diffraction. Above 1400 °C, MAX phases decomposed to binary carbide (e.g., TiCx) or binary nitride (e.g., TiNx), primarily through the sublimation of A-elements such as Al or Si, which results in a porous surface layer of MXx being formed Positive activation energies were determined for decomposed MAX phases with coarse pores but a negative activation energy when the pore size was less than 1.0 µm. The kinetics of isothermal phase decomposition at 1550 °C was modelled using a modified Avrami equation. An Avrami exponent (n) of < 1.0 was determined, indicative of the highly restricted diffusion of Al or Si between the channels of M6X octahedra. The role of pore microstructures on the decomposition kinetics is discussed.

Growth and Property Characterization of Epitaxial MAX-Phase Thin Films from the Tin+1(Si, Ge, Sn)Cn Systems

Epitaxial Mn+1AXn phase (n=1, 2 or 3) thin films from the chemically related Ti-Si-C, Ti-Ge-C, and Ti-Sn-C systems were grown on Al2O3(0001) substrates at temperatures in the region of 700-1000 oC, using d.c. magnetron sputtering from individual sources. In addition to growth of the known phases Ti3SiC2, Ti3GeC2, Ti2GeC, and Ti2SnC the method allows synthesis of the new phases Ti4SiC3, Ti4GeC3, and Ti3SnC2 as well as the intergrown structures Ti5A2C3 and Ti7A2C5 in the Si and Ge systems. Characterization by XRD, TEM and nanoindentation show similarities with respect to phase distribution, mechanical, and electrical properties, particularly pronounced when comparing Si and Ge. The Ti-Sn-C system is, however, the most liable system with respect to surface segregation of the A-element. This causes less favorable growth of MAX phases as seen by a preferential growth of the binary carbide TiC and metallic Sn. Nanoindentation on films from the Ti-Si-C and Ti-Ge-C systems shows large plastic deformation with extensive pile up. The typical thin film hardness is 20 GPa, and the Young’s modulus in the region of 320 GPa. The four-point probe resistivity is low for all systems, but differs depending on materials system and phase, with values of 25 μcm for Ti3SiC2, and 17 μcm for Ti2GeC.

Doped Amorphous SiC, Mixed Carbide and Oxycarbide Thin Films by a Liquid Route

ABSTRACTA new family of disordered materials called amorphous covalent ceramics (ACC) has been obtained by pyrolysis of metal-organic polymers. The transformation from polycarbosilane to amorphous and microcrystalline SiC has been studied. New a-SiC:H thin films have also been prepared by the polymer solution. Electrical and optical properties of this new a-SiC have studied. Amorphous binary carbide and oxycarbide have also been prepared by crosslinking polycarbosilane and metal alkoxides. The p-n control of this new a-SiC:H has been achieved, which also lead to the possible device applications.