Ultra-high-temperature application of MXene: Stabilization of 2D Ti_(3)C_(2)T_(x) for cross-scale strengthening and toughening of 3D TiC

Transition metal carbide/nitride cores within MXenes make them considerably useful for ultra-high-temperature reinforcement.However,extensive research on Ti_(3)C_(2)T_(x) MXene has revealed its tendency to undergo a phase transition to TiCy at temperatures above 800℃due to high activity of a superficial Ti atomic layer.Herein,spark plasma sintering of Ti_(3)C_(2)T_(x) and TiC is performed to prevent the Ti_(3)C_(2)T_(x) phase transition at temperatures up to 1900℃through the fabrication of composites at a pressure of 50 MPa.Using a focused ion beam scanning electron microscope to separate layered substances in the composites and examining selected area diffraction spots in a transmission electron microscope enabled identification of non-phase-transitioned MXene.First-principles calculations based on density functional theory indicated the formation of strong chemical bonding interfaces between Ti_(3)C_(2)T_(x) and TiC,which imposed a stability constraint on the Ti atomic layer at the Ti_(3)C_(2)T_(x) surface.Mechanical performance tests,such as three-point bending and fracture toughness analysis,demonstrated that the addition of Ti_(3)C_(2)T_(x) can effectively improve the cross-scale strengthening and toughening of the TiC matrix,providing a new path for designing and developing two-dimensional(2D)carbides cross-scale-enhanced three-dimensional(3D)carbides with the same elements relying on a wide variety of MXenes.

Ultra-fast synthesis and thermodynamic analysis of MoAlB by self-propagating high-temperature combustion synthesis

MoAlB as a typical member of MAB phases has attracted much-growing attention due to its unique properties.However,the low production of MoAlB powders limits its further development and potential applications.In the present work,the ultra-fast preparation of high-purity MoAlB powders in a few seconds is achieved by self-propagating high-temperature synthesis(SHS)using a raw powder mixture at an atomic ratio of Mo:Al:B=1:1.3:1.SHS reaction mechanism is obtained by analyzing the corresponding composition changes of starting materials.Furthermore,the thermodynamic prediction for the SHS reaction is consistent with the present experiments,where the preparation of MoAlB also conforms to two common self-propagating conditions of the SHS.The enthalpy vs.temperature curve shows that the adiabatic temperature of the reaction decreases with the amount of excuse Al increasing but increases when pre-heating the reactants.Also,this thermodynamic calculation provides a new idea for the preparation of other MAB phases by the SHS.

DFT-assisting discovery and characterization of a hexagonal MABphase V_(3)PB_(4)

A 314-type MAB phase V_(3)PB_(4)with hexagonal crystal structure is synthesized by self-propagating high temperature combustion synthesis(SHS),with the help of the full first-principles predictions for the phase stability and adiabatic combustion temperature of SHS.Using XRD and TEM,V_(3)PB_(4)crystallizes in the space group of P6m2,with the lattice parameters a?3.030Åand c=9.148Å,of much interest,well with the predicted one.Furthermore,the electronic structure,chemical bonding,and elastic properties of hex-V_(3)PB_(4)are predicted by first-principles.No bandgap around Fermi energy indicates its electronic conductor.And the strong covalent bonding is present between the B and V atoms with,significantly,much weaker V-P bond.With the help of the theoretical model of bond stiffness,the significantly high ratio of bond stiffness of weakest bonds to the strongest ones(0.873)of hex-V_(3)PB_(4)indicates its poor damage tolerance and fracture toughness.The high bond stiffness results in its high moduli in comparison with other MAB phases.As the number of inserted P atoms increases,the engineering elastic modulus decrease,without the price of an increase in density.

Theoretical screening,intrinsic brittleness and thermal properties of the S-containing MAX carbides and borides

To respond the recent experimental advances,the phase stability,mechanical properties,phonon as well as infrared-and Raman-active modes,thermal expansion and heat capacity were investigated by density functional theory for the S-containing MAX carbides and borides(M fromⅢB toⅧB),of importance,well consistent with the available experimental results.After examining the thermodynamic competition with all the competing phases and intrinsic stability by their lattice dynamics,18 MAX phases were screened out from 138 ones.Using the“bond stiffness”model as well as the associated criterion for damage tolerance and fracture toughness,the ratio of bond stiffness of weakest M-S to the strongest MX bonds(k_(min)/k_(max))over 1/2 indicates their intrinsic brittleness of all S-containing MAX phases except Nb4SC3.Including the contributions from phonon and electrons,their linear thermal expansion coefficients[(8.1e13.6)×10^(-6)K^(-1),300-1,300 K]and heat capacities(Cp)as a function of temperature are predicted.Of much interest,a well-established relationship between molar C_(p)of the MAX and MX phases is theoretically deduced in the present work.

Density-functional-theory predictions of mechanical behaviour and thermal properties as well as experimental hardness of the Ga-bilayer Mo_(2)Ga_(2)C

Mo_(2)Ga_(2)C is a new MAX phase with a stacking Ga-bilayer as well as possible unusual properties.To understand this unique MAX phase structure and promote possible future applications,the structure,chemical bonding,and mechanical and thermodynamic properties of Mo_(2)Ga_(2)C were investigated by first-principles.Using the“bond stiffness”model,the strongest covalent bonding(1162 GPa)was formed between Mo and C atoms in Mo_(2)Ga_(2)C,while the weakest Ga–Ga(389 GPa)bonding was formed between two Ga-atomic layers,different from other typical MAX phases.The ratio of the bond stiffness of the weakest bond to the strongest bond(0.33)was lower than 1/2,indicating the high damage tolerance and fracture toughness of Mo_(2)Ga_(2)C,which was confirmed by indentation without any cracks.The high-temperature heat capacity and thermal expansion of Mo_(2)Ga_(2)C were calculated in the framework of quasi-harmonic approximation from 0 to 1300 K.Because of the metal-like electronic structure,the electronic excitation contribution became more significant with increasing temperature above 300 K.

Precipitation mechanism and microstructural evolution of Al2O3/ZrO2(CeO2) solid solution powders consolidated by spark plasma sintering

It is difficult to synthesize Al2O3/ZrO2 solid solution because of its low solubility under equilibrium solidification conditions.In this work,a new combustion synthesis combined with water atomization(CS-WA)method was developed to prepare supersaturated Al2O3/ZrO2(CeO2)solid solution powders.The ultrahigh cooling rate supplied by CS-WA greatly extends solid solubility of Al2O3 in ZrO2.The precipitation mechanism of solid solution was investigated by systematic heat treatments.The initial temperature of the metastable phase decomposed into Al2O3 and ZrO2 is 1050℃,and it could be completely precipitated at 1400℃in 0.5 h.The precipitated ZrO2 particles were uniformly dispersed in Al2O3 matrix and grew into submicron scale at annealing temperature of 1450℃.Subsequently,together with detailed microstructure,phase composition,as well as mechanical properties were collaboratively outlined to discuss spark plasma sintering(SPS)behavior.The solid solution precipitated Al2O3 matrix and ZrO2 particles during the SPS process.Partial ZrO2 particles were uniformly distributed within Al2O3 matrix,while the residuary ZrO2 located at the grain boundaries and formed special transgranular/intergranular structure.The average size of nanoscale transgranular ZrO2 particles was only^11.5 nm.The compact ZrO2 toughened Al2O3 nano ceramic(N-ZTA)exhibits excellent mechanical properties.This work provides a guidance to produce nanostructured ZTA with high performance.