Influence of ordered carbon‐vacancy networks on the electronic structures and elastic properties of Nb4AlC3−x

Carbon‐vacancy‐bearing Nb₄AlC_3?x has the best high‐temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb₄AlC_3?x have been identified to be ordered, establishing an ordered compound Nb₁₂Al₃C₈. Here, the spatial distribution of the ordered vacancies and their influences on bonding characteristics and elastic properties are unraveled by thoroughly comparing Nb₁₂Al₃C₈ and vacancy‐free Nb₄AlC₃. In Nb₁₂Al₃C₈, the carbon vacancies break only relatively weak Nb–C bonds and form ordered equilateral triangular carbon‐vacancy networks (OETCVNs) to maximize the bond strengthening effect. The networks slightly shift partial and total density of states toward the Fermi energy level, and bring about a feature of “de‐metallization”. Meanwhile, the presence of OETCVNs results in the softening of elastic modulus, decreasing of the anisotropy of Young's modulus, yet increasing that of shear modulus. These results shed lights on the carbon‐vacancy ordering behavior of MAX phases, and provide an opportunity to tailor their electronic structures and elastic properties through defect engineering.     

Experimental synthesis and density functional theory investigation of radiation tolerance of Zr3(Al1‐xSix)C2 MAX phases

Synthesis, characterization and density functional theory calculations have been combined to examine the formation of the Zr₃(Al_1–xSi_x)C₂ quaternary MAX phases and the intrinsic defect processes in Zr₃AlC₂ and Zr₃SiC₂. The MAX phase family is extended by demonstrating that Zr₃(Al_1–xSi_x)C₂, and particularly compositions with x≈0.1, can be formed leading here to a yield of 59 wt%. It has been found that Zr₃AlC₂ ‐ and by extension Zr₃(Al_1–xSi_x)C₂ ‐ formation rates benefit from the presence of traces of Si in the reactant mix, presumably through the in situ formation of Zr_ySi_z phase(s) acting as a nucleation substrate for the MAX phase. To investigate the radiation tolerance of Zr₃(Al_1–xSi_x)C₂, we have also considered the intrinsic defect properties of the end‐members. A‐element Frenkel reaction for both Zr₃AlC₂ (1.71 eV) and Zr₃SiC₂ (1.41 eV) phases are the lowest energy defect reactions. For comparison we consider the defect processes in Ti₃AlC₂ and Ti₃SiC₂ phases. It is concluded that Zr₃AlC₂ and Ti₃AlC₂ MAX phases are more radiation tolerant than Zr₃SiC₂ and Ti₃SiC₂, respectively. Their applicability as cladding materials for nuclear fuel is discussed.     

Ti3SiC2层状陶瓷的研究进展

Ti₃SiC₂层状陶瓷兼有金属和陶瓷的许多优良性能,具有高的热导率和电导率,易加工,同时具有良好的抗热震性、抗氧化性和高温稳定性。综合评述了Ti₃SiC₂陶瓷的结构、性能以及制备工艺,并对其在耐火材料行业中的应用前景进行了展望。     

High‐Temperature Neutron Diffraction,Raman Spectroscopy,and First‐Principles Calculations of Ti3SnC2 and Ti2SnC

Herein, we report—for the first time—on the additive‐free bulk synthesis of Ti₃SnC₂. A detailed experimental study of the structure of the latter together with a secondary phase, Ti₂SnC, is presented through the use of X‐ray diffraction (XRD), and high‐resolution transmission microscopy (HRTEM). A previous sample of Ti₃SnC₂, made using Fe as an additive and Ti₂SnC as a secondary phase, was studied by high‐temperature neutron diffraction (HTND) and XRD. The room‐temperature crystallographic parameters of the two MAX phases in the two samples are quite similar. Based on Rietveld analysis of the HTND data, the average linear thermal expansion coefficients of Ti₃SnC₂ in the a and c directions were found to be 8.5 (2)·10^?6 K^?1 and 8.9 (1)·10^?6 K^?1, respectively. The respective values for the Ti₂SnC phase are 10.1 (3)·10^?6 K^?1 and 10.8 (6)·10^?6 K^?1. Unlike other MAX phases, the atomic displacement parameters of the Sn atoms in Ti₃SnC₂ are comparable to those of the Ti and C atoms. When the predictions of the atomic displacement parameters obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Sn atoms. In the case of the Ti and C atoms, the agreement is more qualitative. We also used first principles to calculate the elastic properties of both Ti₂SnC and Ti₃SnC₂ and their Raman active modes. The latter are compared to experiment and the agreement was found to be good.     

Molten salt dynamic sealing synthesis of MAX phases (Ti3AlC2, Ti3SiC2 et al.) powder in air

Since the synthesis of non-oxidized ceramic and alloy powders requires both high temperature and oxygen insulation conditions, here we demonstrate a cost-efficient molten salt sealing/shielded synthesis method with dynamic gas tightness. Compared to conventional synthesis method, it can prevent the loss of reaction materials at high temperature, cut off the connection between reacting material and outside air, and does not require long-time ball milling mixing treatment or provision of applied pressing before or during heating. Only low-cost salts (e.g., NaCl, KCl), a few minutes of raw material mixing, and regular heating molds are required to obtain high-purity (>96 wt%), micron-sized Ti₃AlC₂ and Ti₃SiC₂ powders with narrow size distribution, which significantly decreased the complexity and production costs in the synthesis process. The effect of temperature and raw material content on the products were investigated. The mechanism of diffusion reaction between reactants in molten salt environment was analyzed. The new method developed here was also applicable to Ti₂AlC, V₂AlC and Cr₂AlC MAX phases, as well as provided new ideas for the preparation of other MXenes precursors with certain stoichiometric ratios, air-sensitive materials and nanopowders.     

Helium effects and bubbles formation in irradiated Ti3SiC2

Ti₃SiC₂ is a potential structural material for nuclear reactor applications. However, He irradiation effects in this material are not well understood, especially at high temperatures. Here, we compare the effects of He irradiation in Ti₃SiC₂ at room temperature (RT) and at 750 °C. Irradiation at 750 °C was found to lead to extremely elongated He bubbles that are concentrated in the nano-laminate layers of Ti₃SiC₂, whereas the overall crystal structure of the material remained intact. In contrast, at RT, the layered structure was significantly damaged and highly disordered after irradiation. Our study reveals that at elevated temperatures, the unique structure of Ti₃SiC₂ can accommodate large amounts of He atoms in the nano-laminate layer, without compromising the structural stability of the material. The structure and the mechanical tests results show that the irradiation induced swelling and hardening at 750 °C are much smaller than those at RT. These results indicate that Ti₃SiC₂ has an excellent resistance to accumulation of radiation-induced He impurities and that it has a considerable tolerance to irradiation-induced degradation of mechanical properties at high temperatures.     

Ti_3SiC_2可加工材料的研究现状

Ti3SiC2陶瓷具有很好的高温强度、热稳定性和耐腐蚀性能,同时它还具有很好的导电、导热能力,优良的可加工性,又具备金属良好的高的抗氧化性、抗热震性和高温塑性、良好的自润滑性。本文对其结构、性能以及制备方法和应用前景进行了综合评述。     

Composition‐ and Pressure‐Induced Relaxor Ferroelectrics: First‐Principles Calculations and Landau‐Devonshire Theory

We report calculations with first‐principles density‐functional theory and Landau–Devonshire theory that provide an atomic‐scale mechanism for the composition‐ and pressure‐induced relaxor ferroelectrics. A multiphase with coexisted cubic structures (MPCCS) is found to correspond to any of the composition‐ and pressure‐induced relaxor ferroelectrics. On the other hand, a normal ferroelectric without relaxor behavior is structurally characterized by a single phase. Furthermore, the presence of the MPCCS in a composition‐ and pressure‐induced relaxor increases the degrees of freedom of relaxors and no energy barriers are involved for the rotations of the polarization direction, leading to high electromechanical coefficients.     

Srn+1TinO3n+1 (n=1, 2) microwave dielectric ceramics with medium dielectric constant and ultra‐low dielectric loss

Sr_n+1Ti_nO_3n+1 (n=1, 2) ceramics with tetragonal Ruddlesden–Popper structure were prepared via a standard solid‐state reaction process, and their microstructures and microwave dielectric properties were investigated systematically. The phase composition, grain morphology, and densification behavior were explored using X‐ray diffraction (XRD) and scanning electron microscopy (SEM). Outstanding microwave dielectric properties were achieved in the present ceramics: ε_r=42, Q × f=145 200 GHz, τ_f=130 ppm/°C for Sr₂TiO₄, and ε_r=63, Q × f=84 000 GHz, τ_f=293 ppm/°C for Sr₃Ti₂O₇, respectively. The present ceramics might be expected as excellent candidates for next‐generation medium‐permittivity microwave dielectric ceramics after the further optimization of τ_f value.     

Phase‐dependent radiation‐resistant behavior of BaTiO3: An in situ X‐ray diffraction study

The radiation‐resistant response of BaTiO₃ in the tetragonal and rhombohedral phases on exposure to 100 MeV Ag⁷⁺ ion irradiation was investigated by in situ X‐ray diffraction (XRD) at room temperature (300 K) and low temperature (25 K), respectively. This study revealed that the BaTiO₃ in rhombohedral phase retained crystallinity up to an ion fluence of 1×10¹⁴ ions/cm², whereas tetragonal phase amorphized at much lower fluence viz. 1×10¹³ ions/cm². The in situ XRD along with Raman spectroscopy studies revealed that BaTiO₃ in rhombohedral phase is more radiation resistant than that of tetragonal phase. The density functional theory (DFT) calculations confirmed higher bond strength of rhombohedral phase as compared to tetragonal phase, which supported the experimental result of higher radiation stability of rhombohedral phase. The theoretical predictions on high‐temperature phase will be of relevance to the nuclear waste applications.     

Enhanced thermoelectric performance of n‐type Bi2O2Se by Cl‐doping at Se site

The n‐type polycrystalline Bi₂O₂Se_1?xCl_x (0≤x≤0.04) samples were fabricated through solid‐state reaction followed by spark plasma sintering. The carrier concentration was markedly increased to 1.38×10²⁰ cm^?3 by 1.5% Cl doping. The maximum electrical conductivity is 213.0 S/cm for x=0.015 at 823 K, which is much larger than 6.2 S/cm for pristine Bi₂O₂Se. Furthermore, the considerable enhancement of the electrical conductivity outweighs the moderate reduction of the Seebeck coefficient by Cl doping, thus contributing to a high power factor of 244.40 μ·WK^?2·m^?1 at 823 K. Coupled with the intrinsically suppressed thermal conductivity originating from the low velocity of sound and Young's modulus, a ZT of 0.23 at 823 K for Bi₂O₂Se_0.985Cl_0.015 was achieved, which is almost threefold the value attained in pristine Bi₂O₂Se. It reveals that Se‐site doping can be an effective strategy for improving the thermoelectric performance of the layered Bi₂O₂Se bulks.     

Porosity effect on microstructure,mechanical, and fluid dynamic properties of Ti2AlC by direct foaming and gel‐casting

In this study, Ti₂AlC foams were fabricated by direct foaming and gel‐casting using agarose as gelling agent. Slurry viscosity, determined by the agarose content (at a fixed solids loading), as well as surfactant concentration and foaming time were the key parameters employed for controlling the foaming yield, and hence the foam porosity after sintering process. Fabricated foams having total porosity in the 62.5‐84.4 vol% range were systematically characterized to determine their pore size and morphology. The effect of the foam porosity on the room‐temperature compression strength and elastic modulus was also determined. Depending on the amount of porosity, the compression strength and Young's modulus were found to be in the range of 9‐91 MPa and 7‐52 GPa, respectively. Permeability to air flow at temperatures up to 700°C was investigated. Darcian (k₁) and non‐Darcian (k₂) permeability coefficients displayed values in the range 0.30‐93.44 × 10^?11 m² and 0.39‐345.54 × 10^?7 m, respectively. The amount of porosity is therefore a very useful microstructural parameter for tuning the mechanical and fluid dynamic properties of Ti₂AlC foams.     

DFT Predictions of Crystal Structure,Electronic Structure,Compressibility, and Elastic Properties of Hf–Al–C Carbides

To understand the potential for use of the Hf–Al–C ternary compounds, (HfC)_nAl₃C₂ (Hf₂Al₃C₄ and Hf₃Al₃C₅) and (HfC)_nAl₄C₃ (Hf₂Al₄C₅ and Hf₃Al₄C₆) were investigated using density functional theory, including crystal structure, electronic structure, compressibility, and elastic properties. The theoretical density of (HfC)_nAl₃C₂ (4.10–4.16 g/cm³) is higher than that of (HfC)_nAl₄C₃ (3.92–3.98 g/cm³), due to the smaller number of lighter Al–C layers. With increasing numbers of Hf–C layers, the Hf–C and Al–C bond lengths remain almost unchanged. In none of the compounds is there a gap around the Fermi energy (E_f), which implies they are metal‐like conductors. With increasing pressure, there is greater shrinkage along the c axis than the a axis. The bond stiffness increases with increasing pressure. In general, (HfC)_nAl₃C₂ has higher elastic stiffness than (HfC)_nAl₄C₃, with the moduli increasing with the number of Hf–C layers. The Hf–Al–C compounds as well as the brittle Zr–Al–C compounds all have low shear moduli/bulk moduli ratio (G/B) from 0.71 to 0.78, suggesting that the G/B ratio is not always a suitable measure of ductility.     

Autonomous high‐temperature healing of surface cracks in Al2O3 containing Ti2AlC particles

In this work, the oxidation‐induced crack healing of Al₂O₃ containing 20 vol.% of Ti₂AlC MAX phase inclusions as healing particles was studied. The oxidation kinetics of the Ti₂AlC particles having an average diameter of about 10 μm was studied via thermogravimetry and/or differential thermal analysis. Surface cracks of about 80 μm long and 0.5 μm wide were introduced into the composite by Vickers indentation. After annealing in air at high temperatures, the cracks were filled with stable oxides of Ti and Al as a result of the decomposition of the Ti₂AlC particles. Crack healing was studied at 800, 900, and 1000°C for 0.25, 1, 4, and 16 hours, and the strength recovery was measured by 4‐point bending. Upon indentation, the bending strength of the samples dropped by about 50% from 402 ± 35 to 229 ± 14 MPa. This bending strength increased to about 90% of the undamaged material after annealing at 1000°C for just 15 minutes, while full strength was recovered after annealing for 1 hour. As the healing temperature was reduced to 900 and 800°C, the time required for full‐strength recovery increased to 4 and 16 hours, respectively. The initial bending strength and the fracture toughness of the composite material were found to be about 19% lower and 20% higher than monolithic alumina, respectively, making this material an attractive substitute for monolithic alumina used in high‐temperature applications.     

Y5Si2B8: A theoretically predicted new damage‐tolerant MAB phase with layered crystal structure

Layer structured MAB phases (M=transition metal, A=III_A or IV_A group element, B is boron) are promising ternary borides for high and ultrahigh‐temperature applications. Herein, a new MAB phase Y₅Si₂B₈ consisting of alternative stacking of YB₄ and Y₃Si₂ slabs along the [001] direction is investigated. Density functional theory (DFT) calculations on the electronic structure and chemical bonding reveal that this new MAB phase has diverse chemical bonding and properties similar to MAX phases. The strong covalent bonds in the two‐dimensional B network on (001) plane in the YB₄ slab and between Si atoms on (002) plane in the Y₃Si₂‐slab warrant the high stiffness (E_x=288 GPa) of Y₅Si₂B₈ in the ab plane and the weak Y2‐Si and Y1‐B2 bonds that connecting the YB₄ and Y₃Si₂ slabs underpin the low Young's modulus in [001] direction (E_z=200 GPa). The low shear deformation resistance is due to the presence of the metallic bond and the weak bond within the B₆ octahedral. The possible slip systems are {001}< 100> and {110}< 111> . Based on the low shear modulus (G = 104 GPa) and Pugh's ratio G/B, Y₅Si₂B₈ is predicted as a damage tolerant MAB phase. Y₅Si₂B₈ is also predicted electrically conductive and the conductivity is higher in directions parallel to ab plane. In addition, temperature‐dependent phonon and electron heat capacity are predicted based on the electron and phonon density of states analysis.     

Effects of Al substitution with Si and Sn on tribological performance of Ti3AlC2

Since their discovery, the MAX phases have elicited engineering interest as potential choices for wear resistant parts. One such compound is Ti₃AlC₂ with nano-layered structure, low density (4.25 g/cm³), good oxidation resistant and self-lubrication properties. The purpose of this investigation was to evaluate the dominant effect of the A-site solid solution elements addition on dry sliding characteristics of Ti₃AlC₂ against 0.45% C steel (S45C) disk, a material which is widely used in wear-critical applications such as impeller, gear and axles. Dry sliding tribological behaviors of hot-pressed Ti₃Al_0.94C₂, Ti₃Al_0.78Sn_0.22C₂ and Ti₃Al_0.67Si_0.28C₂ solid solutions were conducted using a block-on-disk type tester at surface sliding speed range from 10 to 30 m/s and in the normal load range from 20 to 80 N. The results show that friction coefficient of Ti₃Al_0.94C₂ is higher than that of Ti₃Al_0.78Sn_0.22C₂, but lower than that of Ti₃Al_0.67Si_0.28C₂. However, the change in wear rate as a function of normal load for different sliding speed shows almost reversed trend. Difference in the phase composition of friction films were found to be responsible for observed change in tribological behaved after partial substitution of Al in Ti₃AlC₂ with Sn and Si. This study shows that friction coefficient of Ti₃AlC₂ can be adjusted from 0.2 to 0.38 by partially substituting Al with Si and/or Sn.     

Low‐Temperature Dielectric Relaxations Associated with Mixed‐Valent Structure in Na0.5Bi0.5Cu3Ti4O12

We, herein, present comparative investigations on the Na_0.5Bi_0.5Cu₃Ti₄O₁₂ ceramic samples with and without 10 mol% excess of Na/Bi. The samples were prepared by the standard solid‐state reaction technique. The dielectric properties of the sample were investigated in the temperature (93–320 K) and frequency (20 Hz–10 MHz) windows. Three thermally activated dielectric relaxations observed in Na_0.5Bi_0.5Cu₃Ti₄O₁₂ with the activation energies of 0.104, 0.267, and 0.365 eV for the low‐, middle‐, and high‐temperature dielectric relaxations, respectively. Only the low‐temperature relaxation was observed in both Na and Bi excessive samples. X‐ray photoemission spectroscopy results revealed the mixed‐valent structures of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ in Na_0.5Bi_0.5Cu₃Ti₄O₁₂ sample, but only Ti³⁺/Ti⁴⁺ in Na and Bi excessive samples. Our results showed that the dielectric properties of the investigated samples are strongly linked with these mixed‐valent structures. The high‐ and low‐temperature relaxations were attributed to be a polaron‐type relaxation due to localized carriers hopping between Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺, respectively. The middle‐temperature relaxation is suggested to be a dipole‐type relaxation caused by the defect complex of bismuth and oxygen vacancies.     

Structural Evolution in Hollandite Solid Solutions Across the A‐Site Compositional Range from Ba1.33Ga2.66Ti5.34O16 to Cs1.33Ga1.33Ti6.67O16

Hollandite solid solutions along the A‐site compositional range from the pure barium end‐member Ba_1.33Ga_2.66Ti_5.34O₁₆ to the pure cesium end‐member Cs_1.33Ga_1.33Ti_6.67O₁₆ have been synthesized using a solid‐state reaction technique. The crystal structure of the hollandite across the entire compositional range remained in the I4/m space group. Structural evolution was resolved by neutron diffraction, total scattering data, and density functional theory calculations. A trend of decreasing thermodynamic stability with smaller tunnel cations was attributed to increased structural distortion observed in the system. In addition, the tunnel cations' local environment was studied in the eightfold coordinated oxygen cavities. Local binding features of the tunnel cations reveals that the hollandite structure can strongly stabilize tunnel cations, even at elevated temperatures up to 500 K.     

Structure and microwave dielectric properties of the Li2/3(1−x)Sn1/3(1−x)MgxO systems (x = 0‐4/7)

In this paper, the Li_2/3(1?x)Sn_1/3(1?x)Mg_xO (LSMxO) ceramic systems were prepared by solid‐state reaction using novel atmosphere‐controlled sintering (x = 0‐4/7). Pure Li₂SnO₃ was observed for x = 0, the Li₂Mg₃SnO₆ and Li₂SnO₃ coexisted for x = 1/7, and the coexistence of three kinds of phases was detected for x = 1/5 and 1/4, including Li₄MgSn₂O₇ impurity phase. Pure Li₂Mg₃SnO₆‐like phase with cubic rock salt structure in Fm‐3m space group was obtained in the range of 1/3‐4/7. All samples showed well‐dense and smooth microstructures. The microwave dielectric properties highly depended on the phase composition, bond valence, FWHM of Raman spectrum, Raman shift, average grain sizes, and octahedral distortion. The LSMxO ceramics sintered at 1250°C for 5 hours possessed excellent comprehensive properties of ε_r = 15.43, Q×f = 80 902 GHz and τ_f = +5.61 ppm/°C for x = 1/7. Typically, the LSMxO ceramics sintered at 1350°C for 5 hours showed a maximum Q × f of 168 330 GHz for x = 1/2.