Thermal barrier coating (TBC) materials play important roles in gas turbine engines to protect the Ni-based super-alloys from the high temperature airflow damage. High melting point, ultra-low ...thermal conductivity, large thermal expansion coefficient, excellent damage tolerance and moderate mechanical properties are the main requirements of promising TBC materials. In order to improve the efficiency of jet and/or gas turbine engines, which is the key of improved thrust-to-weight ratios and the energy-saving, significant efforts have been made on searching for enhanced TBC materials. Theoretically, density functional theory has been successfully used in scanning the structure and properties of materials, and at the same time predicting the mechanical and thermal properties of promising TBC materials for high and ultrahigh temperature applications, which are validated by subsequent experiments. Experimentally, doping and/or alloying are also widely applied to further decrease their thermal conductivities. Now, the strategy through combining theoretical calculations and experiments on searching for next generation thermal insulator materials is widely adopted. In this review, the common used techniques and the recent advantages on searching for promising TBC materials in both theory and experiments are summarized.
High entropy materials (HEMs, e.g. high entropy alloys, high entropy ceramics) have gained increasing interests due to the possibility that they can provide challenge properties unattainable by ...traditional materials. Though a large number of HEMs have emerged, there is still in lack of theoretical predictions and simulations on HEMs, which is probably caused by the chemical complexity of HEMs. In this work, we demonstrate that the machine learning potentials developed in recent years can overcome the complexity of HEMs, and serve as powerful theoretical tools to simulate HEMs. A deep learning potential (DLP) for high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)C is fitted with the prediction error in energy and force being 9.4 meV/atom and 217 meV/Å, respectively. The reliability and generality of the DLP are affirmed, since it can accurately predict lattice parameters and elastic constants of mono-phase carbides TMC (TM = Ti, Zr, Hf, Nb and Ta). Lattice constants (increase from 4.5707 Å to 4.6727 Å), thermal expansion coefficients (increase from 7.85×10-6 K-1 to 10.58×10-6 K-1), phonon thermal conductivities (decrease from 2.02 W·m-1·K-1 to 0.95 W·m-1·K-1), and elastic properties of high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)C in temperature ranging from 0 °C to 2400 °C are predicted by molecular dynamics simulations. The predicted room temperature properties agree well with experimental measurements, indicating the high accuracy of the DLP. With introducing of machine learning potentials, many problems that are intractable by traditional methods can be handled now. It is hopeful that deep insight into HEMs can be obtained in the future by such powerful methods.
High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Although in the infant ...stage, the emerging of this new family of materials has brought new opportunities for material design and property tailoring. Distinct from metals, the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering. Aside from strengthening, hardening, and low thermal conductivity that have already been found in high-entropy alloys, new properties like colossal dielectric constant, super ionic conductivity, severe anisotropic thermal expansion coefficient, strong electromagnetic wave absorption, etc., have been discovered in HECs. As a response to the rapid development in this nascent field, this article gives a comprehensive review on the structure features, theoretical methods for stability and property prediction, processing routes, novel properties, and prospective applications of HECs. The challenges on processing, characterization, and property predictions are also emphasized. Finally, future directions for new material exploration, novel processing, fundamental understanding, in-depth characterization, and database assessments are given.
Layered ternary borides, which also named as 'MAB' phases, are close structural analogs to the 'MAX' phases and believed to be more ductile and resistant to oxidation than binary borides. Although ...attentions have been drawn on their atomic arrangement and ground-state elastic properties, their thermal expansions and maintainability of mechanical properties at high temperatures have not been fully understood. Herein, with the combination of density functional theory (DFT) calculations and quasi-harmonic approximation (QHA), the mechanical and thermal properties of two MAB phases, MAlB (M = Mo, W), at ground-states and high temperatures were thoroughly investigated. The effects of transition metals on the structure, elastic anisotropy, thermal expansion, and temperature-dependent mechanical properties are discussed in detail. Possible improvement of high temperature mechanical properties of MoAlB by substitution of Mo by W is proposed.
•Detail electronic structure and chemical bonding of MAlB phases are disclosed.•The direction-dependence and temperature-dependence of mechanical properties of MAlB phases are investigated.•The effects of transition metals on the structure and elastic properties are discussed.
Yttrium silicates (Y-Si-O oxides), including Y
2
Si
2
O
7
, Y
2
SiO
5
, and Y
4·67
(SiO
4
)
3
O apatite, have attracted wide attentions from material scientists and engineers, because of their ...extensive polymorphisms and important roles as grain boundary phases in improving the high-temperature mechanical/thermal properties of Si
3
N
4
and SiC ceramics. Recent interest in these materials has been renewed by their potential applications as high-temperature structural ceramics, oxidation protective coatings, and environmental barrier coatings (EBCs). The salient properties of Y-Si-O oxides are strongly related to their unique chemical bonds and microstructure features. An in-depth understanding on the synthesis - multi-scale structure-property relationships of the Y-Si-O oxides will shine a light on their performance and potential applications. In this review, recent progress of the synthesis, multi-scale structures, and properties of the Y-Si-O oxides are summarised. First, various methods for the synthesis of Y-Si-O ceramics in the forms of powders, bulks, and thin films/coatings are reviewed. Then, the crystal structures, chemical bonds, and atomic microstructures of the polymorphs in the Y-Si-O system are summarised. The third section focuses on the properties of Y-Si-O oxides, involving the mechanical, thermal, dielectric, and tribological properties, their environmental stability, and their structure-property relationships. The outlook for potential applications of Y-Si-O oxides is also highlighted.
The electronic structure, stability, chemical bonding and mechanical properties of 3d, 4d and 5d transition metal diboride TMB2 were investigated using first-principles calculations based on density ...functional theory. All the primary chemical bonds, i.e., metallic, ionic and covalent have contributions to the bonding of TMB2. The number of valence electrons of transition metals or the valence electron concentration (VEC) of TMB2 has strong effects on the lattice parameters, stability and mechanical properties of TMB2. Both lattice constants a and c decrease with VEC, but c decreases faster than a, which is attributed to the enhanced TM d–B p (sp2) bonding. Bulk modulus B of TMB2 increases continuously with VEC due to the enhanced TM d–B p (sp2) and TM dd bonding. Shear modulus G increases with VEC, reaching a maximum at VEC = 3.33, and then decreases with further increase of VEC. YB2 and MnB2 have low Young's modulus and are predicted to have good thermal shock resistance. According to Pugh's criterion (G/B < 0.571), MnB2, MoB2 and WB2 are predicted as ductile or damage tolerant ultrahigh temperature ceramics (UHTCs).
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•The electromagnetic wave absorption properties of TMCs (TM=Ti, Zr, Hf, Nb and Ta) and high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C were investigated for the first time.•Superior EMW ...absorbing properties were reported for the first time.•Mechanisms for the superior EMW absorbing properties, were reported for the first time.•Most importantly, this work opens a new window to design single phase high performance EMW absorbing materials by dielectric/magnetic loss coupling in carbides, nitrides and possibly borides.
Electromagnetic wave (EMW) absorbing materials play a vital role in modern communication and information processing technologies to inhibit information leakage and prevent possible damages to environment and human bodies. Currently, most of EMW absorbing materials are either composites of two or more phases or in the form of nanosheets, nanowires or nanofibers in order to enhance the EMW absorption performance through dielectric loss, magnetic loss and dielectric/magnetic loss coupling. However, the combination of complex shapes/multi phases and nanosizes may compound the difficulties of materials processing, composition and interfaces control as well as performance maintenance during service. Thus, searching for single phase materials with good stability and superior EMW absorbing properties is appealing. To achieve this goal, the EMW absorbing properties of transition metal carbides TMCs (TM=Ti, Zr, Hf, Nb and Ta) and high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C which belong to ultrahigh temperature ceramics, were investigated in this work. Due to the good electrical conductivity and splitting of d orbitals into lower energy t2g level and higher energy eg level in TMC6 octahedral arrangement, TMCs (TM=Ti, Zr, Hf, Nb and Ta) exhibit good EMW absorbing properties. Especially, HfC and TaC exhibit superior EMW absorbing properties. The minimum reflection loss (RLmin) value of HfC is −55.8 dB at 6.0 GHz with the thickness of 3.8 mm and the effective absorption bandwidth (EAB) is 6.0 GHz from 12.0 to 18.0 GHz at thickness of 1.9 mm; the RLmin value of TaC reaches −41.1 dB at 16.2 GHz with a thickness of 2.0 mm and the EAB is 6.1 GHz with a thickness of 2.2 mm. Intriguingly, the electromagnetic parameters, i.e., complex permittivity and permeability are tunable by forming single phase solid solution or high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C. The RLmin value of high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C is −38.5 dB at 9.5 GHz with the thickness of 1.9 mm, and the EAB is 2.3 GHz (from 11.3 to 13.6 GHz) at thickness of 1.5 mm. The significance of this work is that it opens a new window to design single phase high performance EMW absorbing materials by dielectric/magnetic loss coupling through tuning the conductivity and crystal field splitting energy of d orbitals of transition metals in carbides, nitrides and possibly borides.
Ultra-high temperature ceramics Fahrenholtz, William G; Wuchina, Eric J; Lee, William E ...
2014., 2014, 2014-10-10, 2014-09-29
eBook
"This book will capture historic aspects and recent progress on the research and development of ultra-high temperature ceramics. This will be the first comprehensive book focused on this class of ...materials in more than 20 years. The book will review historic studies and recent progress in the field. The intent is to provide a broad overview and critical analysis rather than focus on the latest scientific results. The content will include synthesis, powder processing, densification, property measurement, and characterization of boride and carbide ceramics. Emphasis will be on materials for hypersonic aerospace applications such as wing leading edges and propulsion components for vehicles traveling faster than Mach 5, but will also include materials used in the extreme environments associated with high speed cutting tools and nuclear power generation"-- Provided by publisher. "This book provides a snapshot of the current state-of-the-art in the processing, densification, properties, and performance of boride and carbide ceramics. The book contains contributions from leading experts who have active research in ultra-high temperature ceramics"-- Provided by publisher.
Ferrites are the most widely used microwave absorbing materials to deal with the threat of electromagnetic (EM) pollution. However, the lack of sufficient dielectric loss capacity is the main ...challenge that limits their applications. To cope with this challenge, three high-entropy (HE) spinel-type ferrite ceramics including (Mg
0.2
Mn
0.2
Fe
0.2
Co
0.2
Ni
0.2
)Fe
2
O
4
, (Mg
0.2
Fe
0.2
Co
0.2
Ni
0.2
Cu
0.2
)Fe
2
O
4
, and (Mg
0.2
Fe
0.2
Co
0.2
Ni
0.2
Zn
0.2
)Fe
2
O
4
were designed and successfully prepared through solid state synthesis. The results show that all three HE MFe
2
O
4
samples exhibit synergetic dielectric loss and magnetic loss. The good magnetic loss ability is due to the presence of magnetic components; while the enhanced dielectric properties are attributed to nano-domain, hopping mechanism of resonance effect and HE effect. Among three HE spinels, (Mg
0.2
Mn
0.2
Fe
0.2
Co
0.2
Ni
0.2
)Fe
2
O
4
shows the best EM wave absorption performance, e.g., its minimum reflection loss (RL
min
) reaches −35.10 dB at 6.78 GHz with a thickness of 3.5 mm, and the optimized effective absorption bandwidth (EAB) is 7.48 GHz from 8.48 to 15.96 GHz at the thickness of 2.4 mm. Due to the easy preparation and strong EM dissipation ability, HE MFe
2
O
4
are promising as a new type of EM absorption materials.
Nano-laminated Ti
3
AC
2
(A = Si, Al) are highlighted as nuclear materials for a generation IV (GIV) reactor because they show high tolerance to radiation damage and remain crystalline under ...irradiation of high fluence heavy ions. In this paper, the energetics of formation and migration of intrinsic point defects are predicted by density functional theory calculations. We find that the space near the A atomic plane acts as a point defect sink and can accommodate lattice disorder. The migration energy barriers of Si/Al vacancy and Ti
Si
anti-site defects along the atomic plane A are in the range of 0.3 to 0.9 eV, indicating their high mobility and the fast recovery of Si/Al Frenkel defects and Ti-A antisite pairs after irradiation. This layered structure induced large disorder accommodation and fast defect recovery must play an important role in the micro-structural response of Ti
3
AC
2
to irradiation.
Interstitial configurations, formation energies, and migration paths of intrinsic point defects of Ti
3
AC
2
(A = Si or Al) were investigated by a first-principles method.
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