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Open Access Research Article Issue
Layered metal carbo–selenide Nb2CSe2 with van der Waals interlayer coupling
Journal of Advanced Ceramics 2025, 14(1): 9221008
Published: 17 January 2025
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The stacking structure of Nb2CSe2, a newly synthesized layered metal carbo-selenide, was elucidated by scanning transmission electron microscopy. Nb2CSe2 features Se−Nb−C−Nb−Se quintuple atomic layers. These layers are stacked in Bernal mode. In this mode, Nb2CSe2 crystallizes in a trigonal symmetry (space group P 3¯m1, No. 164), with lattice parameters of a = 3.33 Å and c = 18.20 Å. Electronic structure calculations indicate that the metal carbo-selenide has Fermi energy crossing the bands where it touches to give a zero gap, indicating that it is an electronic conductor. As evidenced experimentally, the electrical conductivity is as high as 6.6×105 S·m−1, outperforming the counterparts in the MXene family. Owing to the layered structure, the bonding in Nb2CSe2 with an ionic formula of (Nb1.48+)2(C1.74−)(Se0.61−)2 is highly anisotropic, with metallic–covalent–ionic bonding in intralayers and weak bonding between interlayers. The layered nature is further evidenced by elastic properties, interlayer energy, and friction coefficient determination. These characteristics indicate that Nb2CSe2 is an analog of molybdenum disulfide (MoS2), which is a typical binary van der Waals (vdW) solid. Moreover, vibrational properties are reported, which may offer an optical identification standard for new ternary vdW solids in spectroscopic studies, including Raman scattering and infrared absorption.

Open Access Research Article Just Accepted
Stabilizing high-entropy MAX phases by incorporating tin element
Journal of Advanced Ceramics
Available online: 03 January 2025
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High-entropy MAX phase ceramics demonstrate exceptional potential in various fields, including physics, mechanics, and energy storage, due to their diverse compositions and outstanding properties. However, synthesizing stable high-entropy phases presents significant challenges blocked by the considerable differences in the physical and chemical properties of the complex elements. In this study, we added low-melting-point metal tin (Sn) as an additive to facilitate the formation of solid solutions. The cohesion energy and formation enthalpy of the Sn-containing system are negative, which maintains the thermodynamic stability of the system, and the incorporation of Sn decreases the mixing enthalpy of the target high-entropy MAX phase and inhibits the formation of competing phases. The addition of Sn increases the lattice parameter and improves the structural stability by increasing the lattice distortion of octahedral M6X and prism M6A, which facilitates the successful synthesis of single-phase high-entropy MAX bulk materials. In addition, the high-entropy MAX phases with Sn added still retain good mechanical and physical properties. This study provides a novel approach to the synthesis and application of high-entropy MAX phase materials, which has the potential to contribute to advancements in multiple technological fields.

Open Access Research Article Issue
High-entropy rare earth stannate ceramics: Acid corrosion resistant radiative cooling materials with high atmospheric transparency window emissivity and high near-infrared solar reflectivity
Journal of Advanced Ceramics 2024, 13(5): 630-640
Published: 22 May 2024
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In response to the development of the concepts of “carbon neutrality” and “carbon peak”, it is critical to developing materials with high near-infrared (NIR) solar reflectivity and high emissivity in the atmospheric transparency window (ATW; 8–13 μm) to advance zero energy consumption radiative cooling technology. To regulate emission and reflection properties, a series of high-entropy rare earth stannate ceramics (HE-RE2Sn2O7: (Y0.2La0.2Nd0.2Eu0.2Gd0.2)2Sn2O7, (Y0.2La0.2Sm0.2Eu0.2Lu0.2)2Sn2O7, and (Y0.2La0.2Gd0.2Yb0.2Lu0.2)2Sn2O7) with severe lattice distortion were prepared using a solid phase reaction followed by a pressureless sintering method for the first time. Lattice distortion is accomplished by introducing rare earth elements with different cation radii and mass. The as-synthesized HE-RE2Sn2O7 ceramics possess high ATW emissivity (91.38%–95.41%), high NIR solar reflectivity (92.74%–97.62%), low thermal conductivity (1.080–1.619 W·m−1·K−1), and excellent chemical stability. On the one hand, the lattice distortion intensifies the asymmetry of the structural unit to cause a notable alteration in the electric dipole moment, ultimately enlarging the ATW emissivity. On the other hand, by selecting difficult excitation elements, HE-RE2Sn2O7, which has a wide band gap (Eg), exhibits high NIR solar reflectivity. Hence, the multi-component design can effectively enhance radiative cooling ability of HE-RE2Sn2O7 and provide a novel strategy for developing radiative cooling materials.

Open Access Research Article Issue
Microstructure, elastic/mechanical and thermal properties of CrTaO4: A new thermal barrier material?
Journal of Advanced Ceramics 2024, 13(3): 373-387
Published: 29 March 2024
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CrTaO4 (or Cr0.5Ta0.5O2) has been unexpectedly found to play a decisive role in improving the oxidation resistance of Cr and Ta-containing refractory high-entropy alloys (RHEAs). This rarely encountered complex oxide can effectively prevent the outward diffusion of metal cations from the RHEAs. Moreover, the oxidation kinetics of CrTaO4-forming RHEAs is comparable to that of the well-known oxidation resistant Cr2O3- and Al2O3-forming Ni-based superalloys. However, CrTaO4 has been ignored and its mechanical and thermal properties have yet to be studied. To fill this research gap and explore the untapped potential for its applications, here we report for the first time the microstructure, mechanical and thermal properties of CrTaO4 prepared by hot-press sintering of solid-state reaction synthesized powders. Using the HAADF and ABF-STEM techniques, rutile crystal structure was confirmed and short range ordering was directly observed. In addition, segregation of Ta and Cr was identified. Intriguingly, CrTaO4 exhibits elastic/mechanical properties similar to those of yttria stabilized zirconia (YSZ) with Young’s modulus, shear modulus, and bulk modulus of 268, 107, and 181 GPa, respectively, and Vickers hardness, flexural strength, and fracture toughness of 12.2±0.44 GPa, 142±14 MPa, and 1.87±0.074 MPa·m1/2. The analogous elastic/mechanical properties of CrTaO4 to those of YSZ has spurred inquiries to lucrative leverage it as a new thermal barrier material. The measured melting point of CrTaO4 is 2103±20 K. The anisotropic thermal expansion coefficients are αa = (5.68±0.10)×10−6 K−1, αc = (7.81±0.11)×10−6 K−1, with an average thermal expansion coefficient of (6.39±0.11)×10−6 K−1. The room temperature thermal conductivity of CrTaO4 is 1.31 W·m−1·K−1 and declines to 0.66 W·m−1·K−1 at 1473 K, which are lower than most of the currently well-known thermal barrier materials. From the perspective of matched thermal expansion coefficient, CrTaO4 pertains to an eligible thermal barrier material for refractory metals such as Ta, Nb, and RHEAs, and ultrahigh temperature ceramics. As such, this work not only provides fundamental microstructure, elastic/mechanical and thermal properties that are instructive for understanding the protectiveness displayed by CrTaO4 on top of RHEAs but also outreaches its untapped potential as a new thermal barrier material.

Open Access Research Article Issue
Electron-irradiation-facilitated production of chemically homogenized nanotwins in nanolaminated carbides
Journal of Advanced Ceramics 2023, 12(6): 1288-1297
Published: 05 June 2023
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Twin boundaries have been exploited to stabilize ultrafine grains and improve mechanical properties of nanomaterials. The production of the twin boundaries and nanotwins is however prohibitively challenging in carbide ceramics. Using a scanning transmission electron microscope as a unique platform for atomic-scale structure engineering, we demonstrate that twin platelets could be produced in carbides by engineering antisite defects. The antisite defects at metal sites in various layered ternary carbides are collectively and controllably generated, and the metal elements are homogenized by electron irradiation, which transforms a twin-like lamellae into nanotwin platelets. Accompanying chemical homogenization, α-Ti3AlC2 transforms to unconventional β-Ti3AlC2. The chemical homogeneity and the width of the twin platelets can be tuned by dose and energy of bombarding electrons. Chemically homogenized nanotwins can boost hardness by ~45%. Our results provide a new way to produce ultrathin (< 5 nm) nanotwin platelets in scientifically and technologically important carbide materials and showcase feasibility of defect engineering by an angstrom-sized electron probe.

Open Access Research Article Issue
Fabrication of multi-anionic high-entropy carbonitride ultra-high-temperature ceramics by a green and low-cost process with excellent mechanical properties
Journal of Advanced Ceramics 2023, 12(6): 1258-1272
Published: 29 May 2023
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As a new category of ultra-high-temperature ceramics (UHTCs), multi-anionic high-entropy (HE) carbonitride UHTCs are expected to have better comprehensive performance than conventional UHTCs. However, how to realize the green and low-cost synthesis of high-quality multi-anionic HE carbonitride UHTC powders and prepare bulk ceramics with excellent mechanical properties still faces great challenges. In this work, a green, low-cost, and controllable preparation process of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)CxN1−x powders is achieved by sol–gel combined with the carbothermal reduction/nitridation method for the first time. The as-synthesized (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)CxN1−x powders possess high compositional uniformity and controllable particle size. In addition, the obtained bulk ceramics prepared at 1800 ℃ exhibit superior fracture toughness (KIC) of 5.39± 0.16 MPa·m1/2 and high nanohardness of 35.75±1.23 GPa, elastic modulus (E) of 566.70±8.68 GPa, and flexural strength of 487±41 MPa. This study provides a feasible strategy for preparing the high-performance HE carbonitride ceramics in a more environmentally friendly and economical manner.

Open Access Research Article Issue
Construction of hydrangea-like core–shell SiO2@Ti3C2Tx@CoNi microspheres for tunable electromagnetic wave absorbers
Journal of Advanced Ceramics 2023, 12(4): 711-723
Published: 24 March 2023
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Ti3C2Tx MXene shows great potential in the application as microwave absorbers due to its high attenuation ability. However, excessively high permittivity and self-stacking are the main obstacles that constrain its wide range of applications. To tackle these problems, herein, the microspheres of SiO2@Ti3C2Tx@CoNi with the hydrangea-like core–shell structure were designed and prepared by a combinatorial electrostatic assembly and hydrothermal reaction method. These microspheres are constructed by an outside layer of CoNi nanosheets and intermediate Ti3C2Tx MXene nanosheets wrapping on the core of modified SiO2, engendering both homogenous and heterogeneous interfaces. Such trilayer SiO2@Ti3C2Tx@CoNi microspheres are "magnetic microsize supercapacitors" that can not only induce dielectric loss and magnetic loss but also provide multilayer interfaces to enhance the interfacial polarization. The optimized impedance matching and core–shell structure could boost the reflection loss (RL) by electromagnetic synergy. The synthesized SiO2@Ti3C2Tx@CoNi microspheres demonstrate outstanding microwave absorption (MA) performance benefited from these advantages. The obtained RL value was −63.95 dB at an ultra-thin thickness of 1.2 mm, corresponding to an effective absorption bandwidth (EAB) of 4.56 GHz. This work demonstrates that the trilayer core–shell structure designing strategy is highly efficient for tuning the MA performance of MXene-based microspheres.

Open Access Research Article Issue
Zr2SeB and Hf2SeB: Two new MAB phase compounds with the Cr2AlC-type MAX phase (211 phase) crystal structures
Journal of Advanced Ceramics 2022, 11(11): 1764-1776
Published: 05 November 2022
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The ternary or quaternary layered compounds called MAB phases are frequently mentioned recently together with the well-known MAX phases. However, MAB phases are generally referred to layered transition metal borides, while MAX phases are layered transition metal carbides and nitrides with different types of crystal structure although they share the common nano-laminated structure characteristics. In order to prove that MAB phases can share the same type of crystal structure with MAX phases and extend the composition window of MAX phases from carbides and nitrides to borides, two new MAB phase compounds Zr2SeB and Hf2SeB with the Cr2AlC-type MAX phase (211 phase) crystal structure were discovered by a combination of first-principles calculations and experimental verification in this work. First-principles calculations predicted the stability and lattice parameters of the two new MAB phase compounds Zr2SeB and Hf2SeB. Then they were successfully synthesized by using a thermal explosion method in a spark plasma sintering (SPS) furnace. The crystal structures of Zr2SeB and Hf2SeB were determined by a combination of the X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The lattice parameters of Zr2SeB and Hf2SeB are a = 3.64398 Å, c = 12.63223 Å and a = 3.52280 Å, c = 12.47804 Å, respectively. And the atomic positions are M at 4f (1/3, 2/3, 0.60288 [Zr] or 0.59889 [Hf]), Se at 2c (1/3, 2/3, 1/4), and B at 2a (0, 0, 0). And the atomic stacking sequences follow those of the Cr2AlC-type MAX phases. This work opens up the composition window for the MAB phases and MAX phases and will trigger the interests of material scientists and physicists to explore new compounds and properties in this new family of materials.

Open Access Research Article Issue
Electronic structure, bonding characteristics, and mechanical behaviors of a new family of Si-containing damage-tolerant MAB phases M5SiB2 (M = IVB–VIB transition metals)
Journal of Advanced Ceramics 2022, 11(10): 1626-1640
Published: 27 September 2022
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MAB phases are layered ternary compounds with alternative stacking of transition metal boride layers and group A element layers. Until now, most of the investigated MAB phases are concentrated on compounds with Al as the A element layers. In this work, the family of M5SiB2 (M = IVB–VIB transition metals) compounds with silicon as interlayers were investigated by density functional theory (DFT) methods as potential MAB phases for high-temperature applications. Starting from the known Mo5SiB2, the electronic structure, bonding characteristics, and mechanical behaviors were systematically investigated and discussed. Although the composition of M5SiB2 does not follow the general formula of experimentally reported (MB)2zAx(MB2)y (z = 1, 2; x = 1, 2; y = 0, 1, 2), their layered structure and anisotropic bonding characteristics are similar to other known MAB phases, which justifies their classification as new members of this material class. As a result of the higher bulk modulus and lower shear modulus, Mo5SiB2 has a Pugh’s ratio of 0.53, which is much lower than the common MAB phases. It was found that the stability and mechanical properties of M5SiB2 compounds depend on their valence electron concentrations (VECs), and an optimum VEC exists as the criteria for stability. The hypothesized Zr and Hf containing compounds, i.e., Zr5SiB2 and Hf5SiB2, which are more interesting in terms of high-temperature oxidation/ablation resistance, were found to be unfortunately unstable. To cope with this problem, a new stable solid solution (Zr0.6Mo0.4)5SiB2 was designed based on VEC tuning to demonstrate a promising approach for developing new MAB phases with desirable compositions.

Open Access Research Article Issue
Tunnel-structured willemite Zn2SiO4: Electronic structure, elastic, and thermal properties
Journal of Advanced Ceramics 2022, 11(8): 1249-1262
Published: 25 July 2022
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Willemite Zn2SiO4 crystallizes in such a way that Zn and Si are tetrahedrally coordinated with O in an ionic-covalent manner to form ZnO4 and SiO4 tetrahedra as the building units. The tetrahedra are corner-sharing, of which one SiO4 tetrahedron connects eight ZnO4 tetrahedra, and one ZnO4 tetrahedron links four ZnO4 tetrahedra and four SiO4 tetrahedra. The unique crystallographic configuration gives rise to parallel tunnels with a diameter of 5.7 Å along the c-axis direction. The tunnel structure of Zn2SiO4 definitely correlates with its interesting elastic and thermal properties. On the one hand, the elastic modulus, coefficient of thermal expansion (CTE), and thermal conductivity are low. Zn2SiO4 has low Vickers hardness of 6.6 GPa at 10 N and low thermal conductivity of 2.34 W/(m·K) at 1073 K. On the other hand, the elastic modulus and CTE along the c-axis are significantly larger than those along the a- and b-axes, showing obvious elastic and thermal expansion anisotropy. Specifically, the Young’s modulus along the z direction (Ez = 179 GPa) is almost twice those in the x and y directions (Ex = Ey = 93 GPa). The high thermal expansion anisotropy is ascribed to the empty tunnels along the c-axis, which are capable of more accommodating the thermal expansion along the a- and b-axes. The striking properties of Zn2SiO4 in elastic modulus, hardness, CTE, and thermal conductivity make it much useful in various fields of ceramics, such as low thermal expansion, thermal insulation, and machining tools.

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