Composites are, in general, a rapidly evolving and growing technical field with a very wide range of applications across the aerospace, defence, energy, medical and transport sectors as a result of...

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Selection, processing, properties and applications of ultra-high temperature ceramic matrix composites, UHTCMCs – a review

Abstract Since the 1960s, a new class of Si-based advanced ceramics called polymer-derived ceramics (PDCs) has been widely reported because of their unique capabilities to produce various ceramic materials (e.g., ceramic fibers, ceramic matrix composites, foams, films, and coatings) and their versatile applications. Particularly, due to their promising structural and functional properties for energy conversion and storage, the applications of PDCs in these fields have attracted much attention in recent years. This review highlights the recent progress in the PDC field with the focus on energy conversion and storage applications. Firstly, a brief introduction of the Si-based polymer-derived ceramics in terms of synthesis, processing, and microstructure characterization is provided, followed by a summary of PDCs used in energy conversion systems (mainly in gas turbine engines), including fundamentals and material issues, ceramic matrix composites, ceramic fibers, thermal and environmental barrier coatings, as well as high-temperature sensors. Subsequently, applications of PDCs in the field of energy storage are reviewed with a strong focus on anode materials for lithium and sodium ion batteries. The possible applications of the PDCs in Li-S batteries, supercapacitors, and fuel cells are discussed as well. Finally, a summary of the reported applications and perspectives for future research with PDCs are presented. 

Si-based polymer-derived ceramics for energy conversion and storage

Abstract Three strategies were proposed to prolong the service life of continuous fiber-reinforced silicon carbide ceramic matrix composite (CMC-SiC), which served as thermal-structure components of aeroengine at thermo-mechanical-oxygenic coupling environment. As for some thermal-structure components with low working stress, improving the degree of densification was crucial to prolong the service life, and the related process approaches were recited. If the thermal-structure components worked under moderate stress, the matrix cracking stress ( σ mc ) should be improved as far as possible. The fiber preform architecture, interface shear strength, residual thermal stress, and matrix strengthening were associated with σ mc in this review. Introducing self-healing components was quite significant with the appearance of matrix microcracks when CMC-SiC worked at more severe environment for hundreds of hours. The damage can be sealed by glass phase originating from the reaction between self-healing components and oxygen. The effective self-healing temperature range of different self-healing components was first summarized and distinguished. The structure, composition, and preparation process of CMC-SiC should be systematically designed and optimized to achieve long duration target. 

Long-term ceramic matrix composite for aeroengine

Reversible protonic ceramic electrochemical cells (R‐PCECs) are emerging as ideal devices for highly efficient energy conversion (generating electricity) and storage (producing H2) at intermediate temperatures (400–700 °C). However, their commercialization is largely hindered by the development of highly efficient air electrodes for oxygen reduction and water‐splitting reactions. Here, the findings in the design of a highly active and durable air electrode are reported: high‐entropy Pr0.2Ba0.2Sr0.2La0.2Ca0.2CoO3−δ (HE‐PBSLCC), which exhibits impressive activity and stability for oxygen reduction and water‐splitting reactions, as confirmed by electrochemical characterizations and structural analysis. When used as an air electrode of R‐PCEC, the HE‐PBSLCC achieves encouraging performances in dual modes of fuel cells (FCs) and electrolysis cells (ECs) at 650 °C, demonstrating a maximum power density of 1.51 W cm−2 in FC mode, and a current density of −2.68 A cm−2 at 1.3 V in EC mode. Furthermore, the cells display good operational durabilities in FC and EC modes for over 270 and 500 h, respectively, and promising cycling durability for 70 h with reasonable Faradaic efficiencies. This study offers an effective strategy for the design of active and durable air electrodes for efficient oxygen reduction and water splitting.

An Efficient High‐Entropy Perovskite‐Type Air Electrode for Reversible Oxygen Reduction and Water Splitting in Protonic Ceramic Cells

High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing together scientists and engineers from physics, chemistry, and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the critical importance of understanding a material’s crystallinity, composition, and mixing length scales in determining its true definition.

https://aip.scitation.org/doi/pdf/10.1063/5.0122727

What is in a name: Defining “high entropy” oxides

High entropy perovskite oxides (HEPOs) have been proposed to serve as improved cathode materials for solid oxide fuel cells (SOFCs); however, the larger compositional design space introduced by HEPOs urges for a better understanding of the correlation among the composition, phase stability and resulting properties of HEPO cathodes. In this work, a series of LaMnO3 based HEPOs (HEALMOs) were designed systematically to investigate the effect of the A site high entropy composition on the structure and thermochemical/electrical properties of HEALMO materials. The results show that the high entropy effect manifests itself on top of the conventional doping effect. First of all, neither the Goldschmidt tolerance factor nor the cation size difference can be used simply to predict the formation ability of single-phase HEALMOs. Meanwhile, HEALMOs may exhibit higher crystallographic symmetry with much higher cation size differences and at Goldschmidt tolerance factor values deviating more largely from 1. Secondly, while high-temperature stability including both resistance to elemental segregation and chemical compatibility with 8YSZ is affected by the A site cation size difference in a similar way to that of conventional perovskite oxides, HEALMOs show much enhanced stability at larger A site cation size differences. Finally, high entropy contributes to the maintenance of electrical conductivity in the high temperature range. The optimum HEALMO with the composition of (La0.2Nd0.2Sm0.2Ca0.2Sr0.2)MnO3 exhibits a combination of excellent high-temperature stability and good electrical conductivity, highlighting its great potential as a promising cathode material for SOFCs.

Tailoring high-temperature stability and electrical conductivity of high entropy lanthanum manganite for solid oxide fuel cell cathodes

In this work, three-dimensional (3D) Cf/SiBCN composites were fabricated by polymer infiltration and pyrolysis (PIP) with poly(methylvinyl)borosilazane as SiBCN precursor. The 3D microstructure evolution process of the composites was investigated by an advanced X-ray computed tomography (XCT). The effect of dicumyl peroxide (DCP) initiator addition on the crosslinking process, microstructure evolution, and mechanical properties of the composites were uncovered. With the addition of a DCP initiator, the liquid precursor can cross-linking to solid-state at 120 °C. Moreover, DCP addition decreases the release of small molecule gas during pyrolysis, leading to an improved ceramic yield 4.67 times higher than that without DCP addition. After 7 PIP cycles, density and open porosity of the final Cf/SiBCN composite with DCP addition are 1.73 g·cm−3 and ∼10%, respectively, which are 143.0% higher and 30.3% lower compared with the composites without DCP addition. As a result, the flexural strength and elastic modulus of Cf/SiBCN composites with DCP addition (371 MPa and 31 GPa) are 1.74 and 1.60 times higher than that without DCP addition (213 MPa and 19.4 GPa), respectively.

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Microstructure and mechanical properties of 3D Cf/SiBCN composites fabricated by polymer infiltration and pyrolysis

Mechanical properties and microstructure evolution of 3D Cf/SiBCN composites at elevated temperatures

Using liquid poly(methylvinyl)borosilazanes (PMVBSZ) as precursor, carbon fiber reinforced SiBCN matrix composites (Cf/SiBCN) were fabricated by a modified polymer infiltration and pyrolysis (PIP) process. With dicumyl peroxide added as cross-linking agent, the PMVBSZ could be solidified at a low temperature of 120 °C, leading to a high ceramic yield of ~70%. The cross-linking mechanism and ceramization processes of the precursor were investigated in detail. Moreover, a modified infiltration technology was developed, which improved the efficiency and protected the precursor against moist air during PIP. Consequently, the obtained Cf/SiBCN composites had an oxygen content of around 1.22 wt%. Benefiting from the high ceramic yield and high efficiency of the modified PIP, Cf/SiBCN composites with an open porosity of ~10% and uniform microstructure were obtained after only 7 cycles of PIP. The flexural strength and fracture toughness of the derived Cf/SiBCN composites were 371 MPa and 12.9 MPa·m1/2, respectively. This work provides a potential route for the fabrication of high performance Cf/SiBCN composites.

/pdf/3d-c-f-sibcn-composites-prepared-by-an-improved-polymer-39o51692so.pdf

3D C-f/SiBCN composites prepared by an improved polymer infiltration and pyrolysis

Ablation behavior of 3D-Cf/SiBCN composites with (PyC/SiC)3 multi-layers as interphase was investigated in plasma ablation flame at a heat flux of 4.02 MW m−2. During ablation, amorphous SiBCN matrix is firstly oxidized and forms a borosilicate glass, in which B2O3 is volatilized as the ablation going on. At the ablation center with higher temperature, SiC and Si3N4 grains are precipitated from the amorphous SiBCN matrix. And then, these SiC and Si3N4 grains are oxidized, forming a certain viscous silica rich protective coating on the sample surface. Moreover, it is revealed that the SiC layers in (PyC/SiC)3 multi-layers interphase can protect the carbon fibers from ablation damage effectively. Compared with the Cf/(PyC)SiBCN composites, much improved ablation resistance is achieved for the Cf/(PyC/SiC)3SiBCN composites. The linear and mass recession rates of the Cf/(PyC/SiC)3SiBCN composites are as low as 1.7 ± 0.4 μm s−1 and 0.0427 ± 0.0053 mg mm−2s−1, which are ∼43 % and ∼21 % lower than those of Cf/(PyC)SiBCN.

Improved ablation resistance of 3D-Cf/SiBCN composites with (PyC/SiC)3 multi-layers as interphase

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