Carbon–carbon (C–C) composites are attractive materials for hypersonic flight vehicles but they oxidize in air at temperatures >500°C and need thermal protection systems to survive aerothermal ...heating. We investigated using multilayers of high‐temperature ceramics such as ZrB2 and SiC to protect C–C against oxidation. Our approach combines pretreatment and processing steps to create continuous and adherent high‐temperature ceramic coatings from infiltrated preceramic polymers. We tested our protective coatings at temperatures above 2600°C at the National Solar Thermal Testing Facility using controlled cold‐wall heat flux profiles reaching a maximum of 680 W/cm2.
Conversion of inorganic-organic frameworks (ceramic precursors and ceramic-polymer mixtures) into solid mass ceramic structures based on photopolymerization process is currently receiving plentiful ...attention in the field of additive manufacturing (3D printing). Various techniques (e.g., stereolithography, digital light processing, and two-photon polymerization) that are compatible with this strategy have so far been widely investigated. This is due to their cost-viability, flexibility, and ability to design and manufacture complex geometric structures. Different platforms related to these techniques have been developed too, in order to meet up with modern technology demand. Most relevant to this review are the challenges faced by the researchers in using these 3D printing techniques for the fabrication of ceramic structures. These challenges often range from shape shrinkage, mass loss, poor densification, cracking, weak mechanical performance to undesirable surface roughness of the final ceramic structures. This is due to the brittle nature of ceramic materials. Based on the summary and discussion on the current progress of material-technique correlation available, here we show the significance of material composition and printing processes in addressing these challenges. The use of appropriate solid loading, solvent, and preceramic polymers in forming slurries is suggested as steps in the right direction. Techniques are indicated as another factor playing vital roles and their selection and development are suggested as plausible ways to remove these barriers.
Ferroelectric perovskites such as BaTiO3 and Pb(Zr,Ti)O3 are well-suited for a variety of applications including piezoelectric transducers and actuators, multilayer ceramic capacitors, thermistors ...with positive temperature coefficient, ultrasonic and electro-optical devices. Ferroelectricity arises from the long-range ordering of elemental dipoles which determines the appearance of a macroscopic polarization and a spontaneous lattice strain. The confinement of a ferroelectric system in a small volume produces a perturbation of the polar order because of the high fraction of surface atoms and ferroelectricity vanishes when the size of the material is reduced below a critical dimension. This critical size is of a few nanometres in the case of epitaxial thin films and of 10−20 nm for nanoparticles and nanoceramics. The change in properties with decreasing physical dimensions is usually referred to as size effect. Thin films and ceramics are particularly prone to show size effects. A progressive variation of dielectric, elastic and piezoelectric properties of ferroelectric ceramics is already observed when the grain size is reduced below ≈10 μm, i.e. at a length scale much larger than the critical size. In this case it is more appropriate to refer to scaling effects as they are not related to material confinement.
The aim of this contribution is to review the current understanding of size and scaling effects in perovskite ferroelectric ceramics and, in particular, in BaTiO3. After a short survey on the intrinsic limits of ferroelectricity and on the impact of particle/grain size on phase transitions, the role of interfaces such as ferroelectric/ferroelastic domain walls and grain boundaries in scaling of dielectric and piezoelectric properties will be discussed in detail. Multiple mechanisms combine to produce the observed scaling effects and the maximization of the dielectric constant and piezoelectric properties exhibited by BaTiO3 ceramics for an intermediate grain size of ≈1 μm. The broad dispersion of experimental data is determined by spurious effects related to synthesis, processing and variation of Ba/Ti ratio. Furthermore, we will consider these size effects, and other properties in relation to the downsizing the modern multilayer BaTiO3 based capacitors.
Dielectric ceramics with both excellent energy storage and optical transmittance have attracted much attention in recent years. However, the transparent Pb‐free energy‐storage ceramics were rare ...reported. In this work, we prepared transparent relaxor ferroelectric ceramics (1 − x)Bi0.5Na0.5TiO3–xNaNbO3 (BNT–xNN) by conventional solid‐state reaction method. We find the NN‐doping can enhance the polarization and breakdown strength of BNT by suppressing the grain growth and restrained the reduction of Ti4+ to Ti3+. As a result, a high recoverable energy‐storage density of 5.14 J/cm3 and its energy efficiency of 79.65% are achieved in BNT–0.5NN ceramic at 286 kV/cm. Furthermore, NN‐doping can promote the densification to improve the optical transmittance of BNT, rising from ∼26% (x = 0.2) to ∼32% (x = 0.5) in the visible light region. These characteristics demonstrate the potential application of BNT–xNN as transparent energy‐storage dielectric ceramics.
The polymer‐derived‐ceramic (PDC) route has been widely used to fabricate the transition‐metal carbides (TMCs). Previously reported works focused mainly on the synthesis of the single or binary TMCs, ...while the synthesis of the ternary or more component TMCs was rarely reported. Herein, a class of the ternary TMCs, namely (Nb1/3Zr1/3Ta1/3)C solid‐solution ceramics, was successfully synthesized via PDC route for the first time. The as‐synthesized ceramics exhibited the particle‐like morphology with an average particle size of ~250 nm and showed a single rock‐salt crystal structure of metal carbides. At the same time, they had high compositional uniformity from nanoscale to microscale. In addition, they possessed low‐oxygen impurity content of 0.79 wt% and moderate‐carbon impurity content of 8.98 wt%. Such work provides a novel route to fabricate the ternary or more component TMCs.
Thermal barrier coatings (TBCs) can effectively protect the alloy substrate of hot components in aeroengines or land-based gas turbines by the thermal insulation and corrosion/erosion resistance of ...the ceramic top coat. However, the continuous pursuit of a higher operating temperature leads to degradation, delamination, and premature failure of the top coat. Both new ceramic materials and new coating structures must be developed to meet the demand for future advanced TBC systems. In this paper, the latest progress of some new ceramic materials is first reviewed. Then, a comprehensive spalling mechanism of the ceramic top coat is summarized to understand the dependence of lifetime on various factors such as oxidation scale growth, ceramic sintering, erosion, and calcium-magnesium-aluminium-silicate (CMAS) molten salt corrosion. Finally, new structural design methods for high-performance TBCs are discussed from the perspectives of lamellar, columnar, and nanostructure inclusions. The latest developments of ceramic top coat will be presented in terms of material selection, structural design, and failure mechanism, and the comprehensive guidance will be provided for the development of next-generation advanced TBCs with higher temperature resistance, better thermal insulation, and longer lifetime.
Silicon carbide (SiC) ceramic and related materials are widely used in various military and engineering fields. The emergence of additive manufacturing (AM) technologies provides a new approach for ...the fabrication of SiC ceramic products. This article systematically reviews the additive manufacturing technologies of SiC ceramic developed in recent years, including Indirect Additive Manufacturing (Indirect AM) and Direct Additive Manufacturing (Direct AM) technologies. This review also summarizes the key scientific and technological challenges for the additive manufacturing of SiC ceramic, and also forecasts its possible future opportunities. This paper aims to provide a helpful guidance for the additive manufacturing of SiC ceramic and other structural ceramics.
The ability to pattern ceramic materials in three dimensions (3D) is critical for structural, functional, and biomedical applications. One facile approach is direct ink writing (DIW), in which 3D ...structures are built layer‐by‐layer through the deposition of colloidal‐ or polymer‐based inks. This approach allows one to design and rapidly fabricate ceramic materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. In this feature article, we present both droplet‐ and filament‐based DIW techniques. We focus on the various ink designs and their corresponding rheological behavior, ink deposition mechanics, potential shapes and the toolpaths required, and representative examples of 3D ceramic structures assembled by each technique. The opportunities and challenges associated with DIW are also highlighted.
PDF
