Synthesis of new rare earth containing ternary laminar Sc2PbC ceramic

In this paper, we combine rare earth element Sc and lead-containing MAX to predict a new ternary layered MAX phase Sc₂PbC. Sc₂PbC with a purity of 87.40 wt% was successfully synthesized by spark plasma sintering, and its crystal structure and atomic positions were obtained by X-ray diffraction (XRD). The Rietveld refinement results show that Sc₂PbC belongs to a typical MAX phase P6₃/mmc (No.194) space group with lattice parameters of a = 3.4003(7) Å and c = 14.7475(3) Å, where Sc atom is located at (1/3, 2/3, 0.57958), Pb atom is located at (1/3, 2/3, 1/4), and C atom is located at (0, 0, 0). At the same time, we observed the typical MAX phase layered microstructure in Sc₂PbC bulk using a scanning electron microscope (SEM). In addition, the proportion of elements measured by energy dispersive spectroscopy (EDS) is within the allowable error range, which confirms that Sc₂PbC is a new MAX phase compound.     

Determination of New α-312 MAX phases of Zr3InC2 and Hf3InC2

Present work successfully synthesized two new α-312 MAX phase compounds of Zr₃InC₂ and Hf₃InC₂ using spark plasma sintering. The crystal structure and microstructure of two new compounds were characterized by combining X-ray diffraction (XRD) and scanning electron microscopy (SEM). With typical crystal structure of MAX phase, the obvious layered features on the fracture surface of Zr₃InC₂ and Hf₃InC₂ grains were observed. The lattice parameters of these two new MAX phase compounds were confirmed as a = 3.3515(3) Å, c = 20.2515(9) Å for Zr₃InC₂, and a = 3.3370(3) Å, c = 19.9560(1) Å for Hf₃InC₂, respectively. Also, the atomic positions of Zr₃InC₂ and Hf₃InC₂ were determined as M1 at (0, 0, 0), M2 at (1/3, 2/3, 0.12774[Zr]/0.12455[Hf]), In at (0, 0, 1/4), and C at (1/3, 2/3, 0.57087[Zr]/0.54894[Hf]). Two new sets of XRD patterns of Zr₃InC₂ and Hf₃InC₂ were collected.     

Reactive spark plasma sintering of Ti3SnC2, Zr3SnC2 and Hf3SnC2 using Fe,Co or Ni additives

This work studied the effect of adding 10 at% Fe, Co or Ni to M-Sn-C mixtures with M = Ti, Zr or Hf on MAX phases synthesis by reactive spark plasma sintering. Adding Fe, Co or Ni assisted the formation of 312 MAX phases, i.e., Ti₃SnC₂, Zr₃SnC₂ and Hf₃SnC₂, while their 211 counterparts Ti₂SnC, Zr₂SnC and Hf₂SnC formed in the undoped M-Sn-C mixtures. The lattice parameters of the newly synthesized Zr₃SnC₂ and Hf₃SnC₂ MAX phases were determined by X-ray diffraction. Binary MC carbides were present in all ceramics, whereas the formation of intermetallics was largely determined by the selected additive. The effect of adding Fe, Co or Ni on the MAX phase crystal structure and the microstructure of the produced ceramics was investigated in greater detail for the case of M = Zr. A mechanism is herein proposed for the formation of M₃SnC₂ MAX phases.     

Thermal explosion synthesis of first Te-containing layered ternary Hf2TeB MAX phase

With the new discovery of layered boride compounds M₂AB (M = Zr, Hf, Nb; A = S, Se) with the typical Cr₂AlC-type structure, MAX phases have been successfully expanded from carbides and nitrides to borides. However, only five MAX phase borides have been synthesized at present, which means that the research on MAX phase borides is still in its infancy. Therefore, the exploration of new MAX phase borides is necessary and can provide a solid basis for future research. In this paper, we describe the discovery of the first tellurium (Te)-containing layered ternary compound Hf₂TeB using combinatorial methods of thermal explosion synthesis, XRD, SEM, and HRTEM analyses. This new MAX phase crystallizes with a Cr₂AlC-type structure with the space group of P6₃/mmc, and the lattice parameters are a = 3.60475 Å, c = 13.12663 Å, respectively, and atomic positions are Hf at (1/3, 2/3, 0.57505), Te at (1/3, 2/3, 1/4), and B at (0, 0, 0).     

Crystal Structure and Elastic Properties of Hypothesized MAX Phase‐Like Compound (Cr2Hf)2Al3C3

The term “MAX phase” refers to a very interesting and important class of layered ternary transition‐metal carbides and nitrides with a novel combination of both metal and ceramic‐like properties that have made these materials highly regarded candidates for numerous technological and engineering applications. Using (Cr₂Hf)₂Al₃C₃ as an example, we demonstrate the possibility of incorporating more types of elements into a MAX phase while maintaining the crystallinity, instead of creating solid solution phases. The crystal structure and elastic properties of MAX phase‐like (Cr₂Hf)₂Al₃C₃ are studied using the Vienna ab initio Simulation Package. Unlike MAX phases with a hexagonal symmetry (P6₃/mmc, #194), (Cr₂Hf)₂Al₃C₃ crystallizes in the monoclinic space group of P2₁/m (#11) with lattice parameters of a = 5.1739 Å, b = 5.1974 Å, c = 12.8019 Å; α = β = 90°, γ = 119.8509°. Its structure is found to be energetically much more favorable with an energy (per formula unit) of ?102.11 eV, significantly lower than those of the allotropic segregation (?100.05 eV) and solid solution (?100.13 eV) phases. Calculations using a stress versus strain approach and the VRH approximation for polycrystals also show that (Cr₂Hf)₂Al₃C₃ has outstanding elastic moduli.     

Enhanced high-temperature strength in textured (Ti1/3Zr1/3Hf1/3)B2 medium-entropy ceramics via strong magnetic field

This study prepared textured (Ti_1/3Zr_1/3Hf_1/3)B₂ medium-entropy ceramics for the first time that maintain enhanced flexural strength up to 1800°C using single-phase (Ti_1/3Zr_1/3Hf_1/3)B₂ powders, slip casting under a strong magnetic field, and hot-pressed sintering methods. Effects of WC additive and strong magnetic field direction on the phase compositions, orientation degree, microstructure evolution, and high-temperature flexural strength of (Ti_1/3Zr_1/3Hf_1/3)B₂ were investigated. (Ti_1/3Zr_1/3Hf_1/3)B₂ grain grows along the a,b-axes, resulting in a platelet-like morphology. Pressure parallel and perpendicular to the magnetic field direction can promote the orientation degree and hinder the texture structure formation, respectively. Reaction products of W(B,C) and (Ti,Zr,Hf)C between (Ti_1/3Zr_1/3Hf_1/3)B₂ and WC additive can efficiently refine the (Ti_1/3Zr_1/3Hf_1/3)B₂ grain size and promote grain orientation. (Ti_1/3Zr_1/3Hf_1/3)B₂ ceramics doped with 5 vol.% WC yielded a Lotgering orientation factor of 0.74 through slip casting under a strong magnetic field (12 T) and hot-pressed sintering at 1900°C. Furthermore, cleaning the boundary by W(B,C) and introducing texture can enhance the grain-boundary strength and improve its high-temperature flexural strength. The four-point flexural strength of textured (Ti_1/3Zr_1/3Hf_1/3)B₂-5 vol.% WC ceramics was 770 ± 59 MPa at 1600°C and 638 ± 117 MPa at 1800°C.     

Synthesis of novel Zr-rich 312-type solid-solution MAX phase in the Zr-Ti-Si-C system

A novel 312-type MAX-phase solid solution series in the Zr-Ti-Si-C system has been synthesized by the vacuum carbosilicothermic reduction method using mixtures of TiO₂, ZrO₂, SiC, and Si powders as starting materials. The upper limit for Zr content in metal sublattice of the synthesized (Zr,Ti)₃SiC₂ MAX phase solid solutions was found to be as much as approximately 66 at%, closely corresponding to a hypothetical quaternary Zr₂TiSiC₂ MAX phase. A wide miscibility gap inside the interval of Zr content in metal sublattice ranging between 22 at% and 55 at% was found. Crystal structure of the synthesized MAX-phase solid solutions was studied by HR-STEM/HAADF and XRD Rietveld analyses. The lattice constants were determined to be linearly correlated with Zr content as predicted by Vegard's law. A significant inhomogeneity in distribution of metal atoms similar to that of out-of-plane ordered quaternary MAX phases has been established for both Ti-rich and Zr-rich MAX-phase solid solutions.     

Synthesis,microstructure, physical and mechanical properties of phase-pure entropy-enhanced (Nb0.8Ti0.05Ta0.05V0.05M0.05)4AlC3 (M = Hf,Zr) ceramics

The first 413-phase entropy-enhanced (Nb_0.8Ti_0.05Ta_0.05V_0.05M_0.05)₄AlC₃ (M = Hf, Zr) (EEMAX_Hf and EEMAX_Zr) ceramics were successfully consolidated by spark plasma sintering (SPS) using Nb, Ti, Ta, V, Zr, Hf, Al and graphite as initial materials. The formation of solid solution with five transition metals at the M sites of hexagonal M₄AlC₃ unit cell was confirmed by elemental analyses. Compared with pure Nb₄AlC₃, both the electrical and thermal conductivities of the entropy-enhanced ceramics showed a slight decrease, which is attributed to the lattice distortion and the increasing lattice defects that prevents the transfer of electrons and phonons. On the other hand, the mechanical properties of entropy-enhanced ceramics were greatly enhanced compared to pure Nb₄AlC₃. The measured fracture toughness of EEMAX_Hf and EEMAX_Zr ceramics were 8.2 MPa·m^1/2 and 10.0 MPa·m^1/2, respectively, which were increased by 18.8% and 44.9% compared to Nb₄AlC₃. The compressive strength of EEMAX_Hf and EEMAX_Zr ceramics were 987 MPa and 1187 MPa, respectively, being 92.0% and 130.9% higher than that of Nb₄AlC₃, respectively. EEMAX_Hf and EEMAX_Zr ceramics also possessed the higher Vickers hardness of 6.8 GPa and 7.4 GPa, respectively.     

Spectroscopic properties and thermally stable orange-red luminescence of Sm:Zr:LiNbO3 and Sm:Hf:LiNbO3 for white LED applications

LiNbO₃ crystals activated by Sm³⁺ and co-doped with Zr⁴⁺ (Sm:Zr:LN) or Hf⁴⁺ (Sm:Hf:LN) were prepared by the Czochralski method. Detailed investigation on spectroscopic properties was conducted on the frame of Judd-Ofelt (J-O) theory. The J-O intensity parameters Ω_i (i = 2, 4, 6), fluorescence branching ratios and radiative lifetime of excited level ⁴G_5/2 were determined. Furthermore, the thermal stability of the strong orange-red emissions obtained under near-UV excitation in both crystals was evaluated. As high as 100% and 97% of integrated intensities at room temperature in Sm:Zr:LN and Sm:Hf:LN respectively were retained at 423 K, demonstrating the suppressed thermal attenuation. The temperature sensing performance based on fluorescence intensity ratio strategy was degraded at higher temperatures with relatively low sensitivities, while the shift of CIE chromaticity coordinates of Sm:Zr:LN and Sm:Hf:LN in the orange-red region was insignificant, demonstrating the color constancy with increasing temperature. With the efficient and thermally stable orange-red luminescence, Sm:Zr:LN and Sm:Hf:LN could serve as promising candidate materials for near-UV excited white light-emitting diodes.     

B-site acceptor doped AgNbO3 lead-free antiferroelectric ceramics: The role of dopant on microstructure and breakdown strength

B-site aliovalent modification of AgNbO₃ with a nominal composition of Ag(Nb_1-xM_x)O_3-x/2 (x = 0.01, M = Ti, Zr and Hf) was prepared. The effects of dopants on microstructure, dielectric, ferroelectric and conduction properties were investigated. The results indicate that the introduction of acceptor dopant does not lead to grain coarsening. Zr⁴⁺ and Hf⁴⁺ doping are beneficial to stabilize the antiferroelectric phase of AgNbO₃. Among all the samples, Ti⁴⁺ doped AgNbO₃ has the minimum resistivity while Hf⁴⁺ doped AgNbO₃ has the maximum resistivity, therefore, Hf⁴⁺ doped AgNbO₃ has high BDS. The XPS results indicate that the conduction behaviour is associated with the concentration of oxygen vacancies. This work hints that acceptor dopant is also effective on the microstructure control and chemical modification of AgNbO₃-based ceramics.     

Preparation and electrical conductivity of KTaO3-based proton conductor

KTaO₃ and KTa_0.9M_0.1O_3-α (M = Ti, Hf, Zr) were prepared by solid state reaction at 1330 °C for 2 h and characterized by x-ray diffraction. The AC impedance technique was used to analyze the sintered solid electrolytes in 1%H₂/Ar and dry air atmosphere. Among KTa_0.9M_0.1O_3-α (M = Ti, Hf, Zr), KTa_0.9Zr_0.1O_3-α displays the highest conductivity in 1%H₂/Ar atmosphere. The carriers transport numbers of solid electrolytes were measured by concentration cell method. The results show KTa_0.9Zr_0.1O_3-α is a pure proton conductor below 525 °C. Stability tests show that KTa_0.9Zr_0.1O_3-α has good chemical stability against CO₂ and H₂O.     

Optimized phase compositions and microwave dielectric properties of low loss Ca2Sn2Al2O9-based ceramics by M4+ substitution

The traditional solid-state reaction method was used to prepare Ca₂Sn_2−xM_xAl₂O₉ (M = Ti, Zr, and Hf) ceramics. Then, the impact of an M⁴⁺ substitution of Sn⁴⁺ on the phase transition, crystal structural parameter, and microwave dielectric properties of Ca₂Sn_2−xM_xAl₂O₉ (0 ≤ x ≤ 0.4) ceramics were investigated. Ti⁴⁺ could not replace the Sn⁴⁺ of Ca₂Sn₂Al₂O₉ due to its small ionic radius, and the Al-based second phases of Ca₂Sn_2−xTi_xAl₂O₉ ceramics were confirmed by the X-ray diffractometer and EDS map scanning results. With the Zr⁴⁺ and Hf⁴⁺ substitutions of Sn⁴⁺, the SnO₂ and CaSnO₃ second phases of Ca₂Sn₂Al₂O₉ ceramic were inhibited, and the Ca₂Sn_2−xM_xAl₂O₉ (M = Zr and Hf) (0.05 ≤ x ≤ 0.2) single-phase ceramics with orthorhombic structure (Pbcn space group) were obtained. New MO₂ (M = Zr and Hf) and CaAl₂O₄ second phases appeared in the Ca₂Sn_2−xM_xAl₂O₉ (M = Zr and Hf) (0.3 ≤ x ≤ 0.4) ceramics, and their contents increased gradually with the increase in x. The Ca₂Sn_2−xM_xAl₂O₉ (M = Zr and Hf) (0.05 ≤ x ≤ 0.2) ceramics exhibited high Q × f because of their pure phase compositions, and the Q × f of Ca₂Sn₂Al₂O₉ ceramic was improved to 77 800 GHz (12.6 GHz) in the Ca₂Sn_1.9Zr_0.1Al₂O₉ ceramic. The Q × f values of Ca₂Sn_2−xM_xAl₂O₉ single-phase ceramics were mainly controlled by r_c (Sn/M–O) and r_c (Al–O). The τ_f values of single-phase Ca₂Sn_2−xM_xAl₂O₉ ceramics were related to octahedral distortions. The Zr⁴⁺ and Hf⁴⁺ substitution of Sn⁴⁺ optimized the phase compositions and microwave dielectric properties of the Ca₂Sn_2−xM_xAl₂O₉ ceramics, and the Ca₂Sn_1.9Zr_0.1Al₂O₉ ceramic sintered at optimal temperature exhibited excellent microwave dielectric properties (ε_r = 8.67, Q × f = 77 800 GHz at 12.6 GHz and τ_f = −69.8 ppm/°C).     

Thermal Properties of Hf‐Doped ZrB2 Ceramics

The effect of Hf additions on the thermal properties of ZrB₂ ceramics was studied. Reactive hot pressing of ZrH₂, B, and HfB₂ powders was used to synthesize (Zr_1?x,Hf_x)B₂ ceramics with Hf contents ranging from x = 0.0001 (0.01 at.%) to 0.0033 (0.33 at.%). Room‐temperature heat capacity values decreased from 495 J·(kg·K)^?1 for a Hf content of 0.01 at.% to 423 J·(kg·K)^?1 for a Hf content of 0.28 at.%. Thermal conductivity values decreased from 141 to 100 W·(m·K)^?1 as Hf content increased from 0.01 to 0.33 at.%. This study revealed, for the first time, that small Hf contents decreased the thermal conductivity of ZrB₂ ceramics. Furthermore, the results indicated that reported thermal properties of ZrB₂ ceramics are affected by the presence of impurities and do not represent intrinsic behavior.     

Elastic,mechanical, electronic,and defective properties of Zr–Al–C nanolaminates from first principles

By means of first principles calculations, Zr–Al–C nanolaminates have been studied in the aspects of chemical bonding, elastic properties, mechanical properties, electronic structures, and vacancy stabilities. Although the investigated Zr–Al–C nanolaminates show crystallographic similarities, their predicated properties are very different. For (ZrC)_nAl₃C₂ (n = 2, 3, 4), the Zr–C bond adjacent to the Al–C slab with the C atom intercalated in the Zr layers is the strongest, but the one with the C atom intercalated between the Zr layer and Al layer is the weakest. In contrast, the situation for (ZrC)_nAl₄C₃ (n = 2, 3) is just the opposite. For Zr–Al–C nanolaminates, the calculated bulk, shear and Young's modulus increase in the sequence of Zr₂AlC < Zr₃AlC₂ < Zr₂Al₄C₅ < Zr₃Al₄C₆ < Zr₂Al₃C₄ < Zr₃Al₃C₅ < Zr₄Al₃C₆. The (ZrC)_nAl₃C₂ (n = 2, 3, 4) series exhibit the most outstanding elastic properties. In the presence of the external pressure, the bulk and shear moduli exhibit a linear response to the pressure, except for Zr₂AlC and Zr₃AlC₂, both of which belong to the so‐called MAX phases. The two materials also exhibit very distinct properties in the strain‐stress relationship, electronic structures and vacancy stabilities. As the intercalated Al layers increase, the formation energy of V_Zr and V_Al increases, while the formation energy of V_C decreases.     

Significant Enhancement of the Dielectric Constant through the Doping of CeO2 into HfO2 by Atomic Layer Deposition

Films of CeO₂ were deposited by atomic layer deposition (ALD) using a Ce(mmp)₄ [mmp = 1‐methoxy‐2‐methyl‐2‐propanolate] precursor and H₂O reactant. The growth characteristics and film properties of ALD CeO₂ were investigated. The ALD CeO₂ process produced highly pure, stoichiometric films with polycrystalline cubic phases. Using the ALD CeO₂ process, the effects of Ce doping into an HfO₂ gate dielectric were systematically investigated. Regardless of Ce/(Ce + Hf) composition, all ALD Ce_xHf_1?xO₂ films exhibited constant growth rates of approximately 1.3 Å/cycle, which is essentially identical to the ALD HfO₂ growth rates. After high‐temperature vacuum annealing at 900°C, it was verified, based on X‐ray diffraction and high‐resolution cross‐sectional transmission electron microscopy results, that all samples with various Ce/(Ce + Hf) compositions were transformed from nanocrystalline to stabilized cubic or tetragonal HfO₂ phases. In addition, the dielectric constant of the Ce_xHf_1?xO₂ films significantly increased, depending on the Ce doping content. The maximum dielectric constant value was found to be nearly 39 for the Ce/(Ce + Hf) concentration of ~11%.     

Polyaminocarboxylate promoted synthesis of Hafnium/ Zirconium substituted anion excess In2O3: Structure,optical and electrical conductivity properties

The current study aimed to generate Hf/Zr substituted In₂O₃ with the ultimate aim of realizing a potential transparent conducting oxide. We applied a co-complexation method to bring the reactively dissimilar In and Hf/Zr together in one oxide network. We prepared an EDTA complex containing an equimolar concentration of In and Hf/Zr and examined their characteristics with FTIR and TG-DSC traces. Rietveld refinement results of calcined complexes and their Raman spectra confirmed the formation of anion excess bixbyite structure for (In_1-xM_x)₂O_3+δ (M = Hf, Zr, and x = 0.50). The lattice expanded after substituting with Hf/Zr, and the optical bandgap increased from 2.87 eV (In₂O₃) to 3.20–3.60 eV. The high percentage reflectance in the visible region and absorbance in the UV region fulfilled some of the prerequisites of transparent conducting oxide. Electrical resistivity reduced up to two orders in magnitude with increasing temperature for Hf and Zr incorporated In₂O₃.     

Design of Efficient Ce<Subscript>x</Subscript>M<Subscript>1−x</Subscript>O<Subscript>2−δ</Subscript> (M = Zr,Hf, Tb and Pr) Nanosized Model Solid Solutions for CO Oxidation

Abstract  

Nanosized Ce_xM_1−xO_2−δ (M = Zr, Hf, Tb and Pr) solid solutions were prepared by a modified coprecipitation method and thermally treated at different temperatures from 773 to 1073 K in order to ascertain the thermal behavior. The structural and textural properties of the synthesized samples were investigated by means of X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), BET surface area, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (RS) techniques. The catalytic efficiency has been performed towards oxygen storage/release capacity (OSC) and CO oxidation activity. The characterization results indicated that the obtained solid solutions exhibit defective cubic fluorite structure. The solid solutions of ceria–hafnia, ceria–terbia and ceria–praseodymium exhibited good thermal stability up to 1073 K. A new Ce_0.6Zr_0.4O₂ phase along with Ce_0.75Zr_0.25O₂ was observed in the case of ceria–zirconia solid solution due to more Zr⁴⁺ incorporation in the ceria lattice at higher calcination temperatures. The reducibility of ceria has been increased upon doping with Zr⁴⁺, Hf⁴⁺, Tb^3+/4+ and Pr^3+/4+ cations. This enhancement is more in case of Hf⁴⁺ doped ceria. Among various solid solutions investigated, the ceria–hafnia combination exhibited better OSC and CO oxidation activity. The high efficiency of Ce–Hf solid solution was correlated with its superior bulk oxygen mobility and other physicochemical characteristics.     

Oxygen octahedra distortion-induced multiferroic properties of Er and Zr co-doped BiFeO3 nanoparticles

The present work unveiled the distortion of oxygen octahedra influencing magnetic and magnetoelectric properties of novel Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) polycrystalline nanoparticles by sol–gel route. X-ray diffraction patterns analysis reveals that pristine BiFeO₃ and doped BiFeO₃ are crystalized in the rhombohedral structure (R3c). The Fe–O–Fe bond angle of Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) varies between 141° and 159.62° as the concentration of Er (via Bi site) and Zr (via Fe site) ions increases in BiFeO₃. As a result, the tilt angle of oxygen octahedra and the canting angle of spiral spin arrangement increase. Hence, the maximum magnetization varies between .03144 and .37558 emu/g in Er and Zr co-doped BiFeO₃ system. The number of electrons per unit cell of Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) lies between 733.38 and 831, respectively. Further, the number of coherently diffracting domains increases from 3.07 to 5.21, and then it decreases when Er and Zr are increased in BiFeO₃. Consequently, the magnetoelectric coupling coefficient varies between .0265 and .2511 mV/cm Oe, respectively. Particularly, Bi_0.95Er_0.05Fe_0.98Zr_0.02O₃ shows enhanced magnetic and magnetoelectric behaviors compared to other samples.     

Polymorphism of M3AlX Phases (M = Ti,Zr, Hf; X = C,N) and Thermomechanical Properties of Ti3AlN Polymorphs

M₃AlX (M = Ti/Zr/Hf, X = C/N) compounds are promising high‐temperature structural ceramics. However, their interesting polymorphism, thermomechanical stabilities, and thermal and mechanical properties were not fully understood. In this work, the polymorphisms of M₃AX phases are investigated by combining first‐principles and lattice dynamics calculations. Only Ti₃AlN shows polymorphic transition between the cubic and orthorhombic phases at around 1105 K; but other M₃AlX phases do not display similar polymorphic phase transition. Furthermore, the temperature‐dependent heat capacity, thermal expansion, and elastic stiffness of Ti₃AlN polymorphic phases are reported for the first time to explore the relationship between crystal structures, and mechanical and thermal properties. Ti₃AlN polymorphs show anisotropic thermal expansion and elastic stiffness; and the orthorhombic Ti₃AlN is suggested as a promising damage tolerant nitride, which has similar properties with the previously reported Zr₃AlN and Hf₃AlN.     

Oxyacetylene ablation of (Hf0.2Ti0.2Zr0.2Ta0.2Nb0.2)C at 1350 − 2050 °C

Ablation resistance of a multi-component carbide (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta)₂O₅, (Hf, Zr)TiO₄, and (Hf, Zr)O₂ were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O_x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics.