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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
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.

Research Article Issue
Intercalation−deintercalation design in MXenes for high-performance supercapacitors
Nano Research 2022, 15 (4): 3213-3221
Published: 22 November 2021
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MXene is a new intercalation pseudocapacitive electrode material for supercapacitor application. Intensifying fast ion diffusion is significantly essential for MXene to achieve excellent electrochemical performance. The expansion of interlayer void by traditional spontaneous species intercalation always leads to a slight increase in capacitance due to the existence of species sacrificing the smooth diffusion of electrolyte ions. Herein, an effective intercalation−deintercalation interlayer design strategy is proposed to help MXene achieve higher capacitance. Electrochemical cation intercalation leads to the expansion of interlayer space. After electrochemical cation extraction, intercalated cations are deintercalated mostly, leaving a small number of cations trapped in the interlayer silt and serving as pillars to maintain the interlayer space, offering an open, unobstructed interlayer space for better ion migration and storage. Also, a preferred surface with more −O terminations for redox reaction is created due to the reaction between cations and −OH terminations. As a result, the processed MXene delivers a much improved capacitance compared to that of the original Ti3C2Tx electrode (T stands for the surface termination groups, such as −OH, −F, and −O). This study demonstrates an improvement of electrochemical performance of MXene electrodes by controlling the interlayer structure and surface chemistry.

Open Access Review Issue
From structural ceramics to 2D materials with multi-applications: A review on the development from MAX phases to MXenes
Journal of Advanced Ceramics 2021, 10 (6): 1194-1242
Published: 10 November 2021
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MAX phases (Ti3SiC2, Ti3AlC2, V2AlC, Ti4AlN3, etc.) are layered ternary carbides/nitrides, which are generally processed and researched as structure ceramics. Selectively removing A layer from MAX phases, MXenes (Ti3C2, V2C, Mo2C, etc.) with two-dimensional (2D) structure can be prepared. The MXenes are electrically conductive and hydrophilic, which are promising as functional materials in many areas. This article reviews the milestones and the latest progress in the research of MAX phases and MXenes, from the perspective of ceramic science. Especially, this article focuses on the conversion from MAX phases to MXenes. First, we summarize the microstructure, preparation, properties, and applications of MAX phases. Among the various properties, the crack healing properties of MAX phase are highlighted. Thereafter, the critical issues on MXene research, including the preparation process, microstructure, MXene composites, and application of MXenes, are reviewed. Among the various applications, this review focuses on two selected applications: energy storage and electromagnetic interference shielding. Moreover, new research directions and future trends on MAX phases and MXenes are also discussed.

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