The development of novel high-temperature structural and multifunctional thermal protection materials for harsh environment applications, such as high-temperature oxidation, severe thermal shock, ...ablation by combustion gas flow etc., is one of the urgent needs of the modern aerospace industry. Ceramic matrix composites such as Cf/(C, SiC, Si3N4), SiCf/ZrB2, SiCp/(Si3N4, HfB2) have received much attention in recent years. Coincidently, metastable silicoboron carbonitride (Si-B-C-N) ceramics and corresponding matrix composites stand out from all recent materials offering great potential at high temperatures due to their high microstructural stability and excellent high-temperature properties including resistance to oxidation, thermal shock and ablation. Using inorganic powders (such as Si, C, B, BN, etc.) instead of organic precursor as raw materials, the inorganic processing route based on mechanical alloying (MA), one of the non-equilibrium processing technique, coupled with sequential sintering, although apparently very ‘hard’ compared to the ‘soft’ polymer precursor method, is actually a simple and effective way to prepare monoliths with the uniform microstructures and superior properties. It has been used to obtain dense Si-B-C-N monoliths and structural parts stable at high temperatures providing new experimental data and therefrom a more detailed understanding of the intrinsic properties of metastable Si-B-C-N materials, benefitting progress towards engineering applications. This review summarizes the state-of-the-art research in Si-B-C-N ceramics and their matrix composites obtained by the inorganic processing route in the last decade compared with those of precursor-derived counterparts, including material design and preparation, microstructural features and evolutionary process, mechanical and thermophysical properties, resistance to oxidation, thermal shock and ablation, and the mechanisms of oxidation, ablation and crystallization of amorphous Si-B-C-N ceramics. Future trends for Si-B-C-N relevant materials are also pointed out.
Cold sintering process (CSP) is an extremely low‐temperature sintering process (room temperature to ~200°C) that uses aqueous‐based solutions as transient solvents to aid densification by a ...nonequilibrium dissolution‐precipitation process. In this work, CSP is introduced to fabricate microwave and packaging dielectric substrates, including ceramics (bulk monolithic substrates and multilayers) and ceramic‐polymer composites. Some dielectric materials, namely Li2MoO4, Na2Mo2O7, K2Mo2O7, and (LiBi)0.5MoO4 ceramics, and also (1−x)Li2MoO4−xPTFE and (1−x)(LiBi)0.5MoO4−xPTFE composites, are selected to demonstrate the feasibility of CSP in microwave and packaging substrate applications. Selected dielectric ceramics and composites with high densities (88%‐95%) and good microwave dielectric properties (permittivity, 5.6‐37.1; Q × f, 1700‐30 500 GHz) were obtained by CSP at 120°C. CSP can be also used to potentially develop a new co‐fired ceramic technology, namely CSCC. Li2MoO4−Ag multilayer co‐fired ceramic structures were successfully fabricated without obvious delamination, warping, or interdiffusion. Numerous materials with different dielectric properties can be densified by CSP, indicating that CSP provides a simple, effective, and energy‐saving strategy for the ceramic packaging and microwave device development.
•High-entropy boride-carbide two-phase ultrahigh temperature ceramics are made.•This work further extends the emerging field of high-entropy ceramics.•A novel reactive spark plasma sintering route ...produces > ∼99 % dense specimens.•A thermodynamic relation governing the equilibrium phase compositions is discovered.•The hardness is higher than the weighted average of the two high-entropy phases.
A series of dual-phase high-entropy ultra-high temperature ceramics (DPHE-UHTCs) are fabricated starting from N binary borides and (5-N) binary carbides powders. > ∼99 % relative densities have been achieved with virtually no native oxides. These DPHE-UHTCs consist of a hexagonal high-entropy boride (HEB) phase and a cubic high-entropy carbide (HEC) phase. A thermodynamic relation that governs the compositions of the HEB and HEC phases in equilibrium is discovered and a thermodynamic model is proposed. These DPHE-UHTCs exhibit tunable grain size, Vickers microhardness, Young’s and shear moduli, and thermal conductivity. The DPHE-UHTCs have higher hardness than the weighted linear average of the two single-phase HEB and HEC, which are already harder than the rule-of-mixture averages of individual binary borides and carbides. This study extends the state of the art by introducing dual-phase high-entropy ceramics (DPHECs), which provide a new platform to tailor various properties via changing the phase fraction and microstructure.
Temperature‐stable, medium‐permittivity dielectric ceramics have been used as resonators in filters for microwave (MW) communications for several decades. The growth of the mobile phone market in the ...1990s led to extensive research and development in this area. The main driving forces were the greater utilization of available bandwidth, that necessitates extremely low dielectric loss (high‐quality factor), an increase in permittivity so that smaller components could be fabricated, and, as ever in the commercial world, cost reduction. Over the last decade, a clear picture has emerged of the principal factors, that influence MW properties. This article reviews these basic principles and gives examples of where they have been used to control microwave properties and ultimately develop new materials.
A new lead-free piezoelectric system of (1 - x)(K.sub.0.48Na.sub.0.52+y)(Nb.sub.0.95Sb.sub.0.05)O.sub.3-x(Bi.sub.0.8La.sub.0.2).sub.0.5(Na.sub.0.8Li.sub.0.2).sub.0.5ZrO.sub.3 (1 - ...x)KNa.sub.yNS-xBLNLZ were prepared using the conventional solid-state sintering method. The effects of BLNLZ content and Na excess on the microstructure and electrical properties of the ceramics were investigated. It was found that an enhanced dielectric, ferroelectric, and piezoelectric behavior has been attained by successfully constructing the rhombohedral-tetragonal phase boundary in the ceramics with x = 0.04. The small Na excess can greatly improve ceramics properties due to the increase of abnormal grain growth caused by the formation of a small amount of liquid phase. As a result, the ceramic with x = 0.04 and y = 0.004 has optimum electrical properties of d.sub.33 ~470 pC/N, k.sub.p ~50 %, 2P.sub.r ~32.3 muC/cm.sup.2, and 2E.sub.c ~14.2 kV/cm, together with a Curie temperature of ~210 °C. The giant d.sub.33 of this ceramic system could be comparable with the most of the reported results about the alkali niobate-based lead-free piezoceramics. It was believed that such an excellent piezoelectricity of this material system will promote the development of KNN-based lead-free ceramics.
Dense (Hf, Zr, Ti, Ta, Nb)C high‐entropy ceramics were produced by hot pressing (HP) of carbide powders synthesized by carbothermal reduction (CTR). The relative density increased from 95% to 99.3% ...as the HP temperature increased from 1750°C to 1900°C. Nominally phase pure ceramics with the rock salt structure had grain sizes ranging from 0.6 µm to 1.2 µm. The mixed carbide powders were synthesized by high‐energy ball milling (HEBM) followed by CTR at 1600°C, which resulted in an average particle size of ~100 nm and an oxygen content of 0.8 wt%. Low sintering temperature, high relative densities, and fine grain sizes were achieved through the use of synthesized powders. These are the first reported results for low‐temperature densification and fine microstructure of high‐entropy carbide ceramics.
•Nine compositionally-complex fluorite oxides (CCFOs) are made and investigated.•CCFOs exhibit reduced thermal conductivity and increased cubic phase stability.•Lower thermal conductivity is achieved ...in medium-entropy non-equimolar CCFOs.•High modulus and hardness retain in CCFOs with reduced thermal conductivity.•Non-equimolar CCFOs exhibit amorphous-like T-dependent thermal conductivity.
Using fluorite oxides as an example, this study broadens high-entropy ceramics (HECs) to compositionally-complex ceramics (CCCs) or multi-principal cation ceramics (MPCCs) to include medium-entropy and/or non-equimolar compositions. Nine compositions of compositionally-complex fluorite oxides (CCFOs) with the general formula of (Hf1/3Zr1/3Ce1/3)1-x(Y1/2X1/2)xO2-δ (X = Yb, Ca, and Gd; x = 0.4, 0.148, and 0.058) are fabricated. The phase stability, mechanical properties, and thermal conductivities are measured. Compared with yttria-stabilized zirconia, these CCFOs exhibit increased cubic phase stability and reduced thermal conductivity, while retaining high Young’s modulus (∼210 GPa) and nanohardness (∼18 GPa). Moreover, the temperature-dependent thermal conductivity in the non-equimolar CCFOs shows an amorphous-like behavior. In comparison with their equimolar high-entropy counterparts, the medium-entropy non-equimolar CCFOs exhibit even lower thermal conductivity (k) while maintaining high modulus (E), thereby achieving higher E/k ratios. These results suggest a new direction to achieve thermally-insulative yet stiff CCCs (MPCCs) via exploring non-equimolar and/or medium-entropy compositions.
Ultra-high temperature ceramics (UHTCs) are generally referred to the carbides, nitrides, and borides of the transition metals, with the Group IVB compounds (Zr & Hf) and TaC as the main focus. The ...UHTCs are endowed with ultra-high melting points, excellent mechanical properties, and ablation resistance at elevated temperatures. These unique combinations of properties make them promising materials for extremely environmental structural applications in rocket and hypersonic vehicles, particularly nozzles, leading edges, and engine components, etc. In addition to bulk UHTCs, UHTC coatings and fiber reinforced UHTC composites are extensively developed and applied to avoid the intrinsic brittleness and poor thermal shock resistance of bulk ceramics. Recently, highentropy UHTCs are developed rapidly and attract a lot of attention as an emerging direction for ultra-high temperature materials. This review presents the state of the art of processing approaches, microstructure design and properties of UHTCs from bulk materials to composites and coatings, as well as the future directions.
Zirconium diboride (ZrB2) was densified by pressureless sintering using <4‐wt% boron carbide and/or carbon as sintering aids. As‐received ZrB2 with an average particle size of ∼2 μm could be sintered ...to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as‐received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition‐milled ZrB2 with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB2 was not possible without the removal of oxygen‐based impurities on the particle surfaces by a chemical reaction.
Electrospinning is a remarkably simple method for generating nanofibers of polymers. When combined with conventional sol–gel processing, it provides a versatile technique for producing ceramic ...nanofibers with either a solid, porous, or hollow structure. This article presents a brief overview of recent progress in preparation of ceramic nanofibers by electrospinning, with a focus on an introduction to experimental procedures and analysis of several technical issues that are vital for a successful electrospinning experiment. We also highlight the unique capabilities of this technique in processing ceramic materials into nanostructures, and illustrate some potential applications of these nanostructures.
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