Most ReadPolymer-derived ceramics are prepared via forming precursors through the polymerization of tiny molecules and cracking at high temperatures. Compared to conventional ceramics, their advantage lies in an ability to precisely control the microstructure and crystalline phase composition through the design of the molecular structure and elemental composition of the precursor and subsequent thermal treatment, thereby producing the optimal final properties. Among these, SiBCN ceramics stand out within the polymer-derived ceramics due to their flexible molecular structure designability. This enables the in-situ formation of multi-phase synergistic loss systems incorporating SiC, BN and graphitic carbon, coupled with a unique oxidation resistance mechanism, which excel particularly within polymer-derived ceramic systems. However, SiBCN ceramics primarily exist in an amorphous state at lower temperatures (i.e., < 1400 ℃), thus limiting their application in electromagnetic wave absorption. The paper was to introduce Ti nanopowder during the ceramicization process to catalyze the formation of nano-dielectric crystals such as SiC, TiC, and crystalline graphite. These crystals could enhance the dielectric imaginary part of SiBCN ceramics, thereby strengthening their electromagnetic wave attenuation capabilities.
For the synthesis of polymer precursor, tetrahydrofuran (THF)-methylvinyl dichlorosilane and borane dimethyl sulfide complex were mixed into a three-neck flask and conducted in argon for 24 h. Also, methyl dichlorosilane and hexamethyldisilazane were introduced, and the reaction was continued at the ambient temperature for 24 h. Subsequently, the mixture was then heated from room temperature to 170 ℃ for amide copolymerization reaction. After holding at this temperature for 3 h, vacuum distillation was performed, and filtrated for three cycles, thus producing a pale yellow polyborosilazane (PBSZ). For the synthesis of SiBCN ceramibs, polyborosilazane (PBSZ) was placed in a tube furnace and heated to 280 ℃ for 2 h to fully cure the precursor. The cured sample was subjected to ball grinding. The resultant ground powder was mixed with Ti nanopowder at different Ti mass contents (i.e, 0%, 5%, 10%, and 15%), and then was ground to produce different composite powders, . The composite powders were pressed into discs with the diameter of φ20 mm. The discs were heat-treated in a vertical tube furnace(i.e., firstly heating at 800 ℃ for 1 h, and thenheating at 1000 ℃ for 2 h) to allow enough molecular diffusion for TiC crystal formation, resulting in SiBCN ceramics.
The analysis of the four-component doped ceramics reveals that Ti nanoparticles doping positively affects both the phase composition and dielectric properties of SiBCN ceramics. The XRD patterns indicate that pure SiBCN ceramics remain amorphous after heat treatment at 1000 ℃, whereas the addition of Ti nano-particles promotes the formation of TiC crystals within the ceramics, thereby enhancing their crystalline properties. The SEM and TEM images demonstrate that varying the nano-Ti doping content alters the microstructure of SiBCN ceramics. Nano-Ti addition promotes the formation of a porous structure within the ceramics and facilitates the growth of crystals such as TiC and carbon nanotubes, enriching the phase composition of the ceramics. Varying Ti nanoparticles doping contents alters SiBCN's electromagnetic wave absorption and loss capabilities. Compared to pure SiBCN ceramics, Ti nanoparticles doping confers higher electromagnetic parameters and lower reflection loss, and 10% Ti nanoparticles-doped SiBCN exhibits the optimum electromagnetic wave absorption performance. The incorporation of Ti nanoparticles optimizes the ceramic structure, with synergistic interactions among various crystals and structural components, thus enhancing the overall performance.
This study demonstrated that doping Ti nano-particles into SiBCN ceramic could enhance the ceramic dielectric loss and impedance matching qualities. Ti nano-particles enhanced the low-temperature crystallization property of SiBCN. The crystallinity and microstructure of SiBCN ceramics could be adjusted by varying the nano-Ti doping content. The ceramics heat-treated at 1000 ℃ could develop porous architectures, TiC, and crystalline phases such as crystalline carbon. Ti nanoparticles improved the electromagnetic wave attenuation properties of SiBCN. The formation of TiC and carbon nanotubes, along with the heterogeneous interfaces formed with the amorphous matrix, could boost the electromagnetic wave attenuation performance of SiBCN ceramics. The crystallinity of SiBCN ceramics and the presence of abundant atomic defects resulted in a significant polarization loss, thereby enhancing the ceramic's electromagnetic wave absorption capability. At Ti nanoparticles content of 10%, the RLmin value of SiBCN ceramics at 6.24 GHz achieved -44.5 dB, with an EAB as high as 3.43 GHz, indicating that adding Ti nanoparticles could effectively enhance the electromagnetic wave absorption capacity of low-temperature heat-treated SiBCN ceramics.
Tremendous advances in high-speed communications technologies have enabled modern services, including live 4K video streaming, real-time remote surgery, artificial intelligence (AI), the Internet of Things (IoT), virtual reality (VR), cloud storage, and social media. The further development of these digital platforms will inevitably require increased data transmission rate, which significantly exceed the current capabilities of existing high-speed communications systems. To meet the ever-growing data traffic, it is necessary to develop and implement new advanced solutions. Multi-wavelength transmission technology is considered one of the most promising approaches, which can potentially increase a bandwidth of transmission data over optical fiber systems by utilizing an extended range of wavelengths (from O- to U-band), where the optical loss of conventional single-mode fiber is below 0.2-0.3 dB/km. However, the success of this approach depends on the development of new amplification technologies, as traditional optical amplifiers based on fibers doped with rare earth ions, especially Er3+ ions, are inherently incapable of providing effective amplification beyond the C+L telecom bands. This has spurred research into promising amplification media, which began more than 20 years ago.
Bismuth (Bi)-doped fibers (BDFs) are a unique active medium suitable for optical amplifiers and lasers operating in a spectral range of 1.15-1.78 μm. The progress achieved in the development of BDFs and optical devices based on them gives hope that multi-band technologies capable of operating over the entire available spectral range can be successfully implemented in the near future. This is confirmed by the presence of commercially available devices developed by a number of telecom companies, as well as the start of implementation of bismuth-doped fiber amplifiers (BDFAs) for O-, E-, and S-band data transmission over optical communication systems. However, the progress achieved in the development of BDFA and BDF lasers was due not only to the solution of applied problems, but also to a deeper understanding of the fundamental principles of formation of bismuth active centers (BACs) and their physical nature. This review presents the main achievements in terms of optical characteristics of Bi-doped materials (crystals, ceramics, bulk glasses and optical fibers) and devices developed using these materials. This highlights that the structure and chemical composition of the glass matrix strongly influence the resulting optical properties of these media. Some fabrication strategies such as the modulation of topological order, coordination engineering, smart confined doping, and direct cluster control, and novel approaches for performance analysis (for example, "hidden potential") of BDFs are emphasized and discussed. The peculiar properties of bismuth active centers (BACs), in particular, optical anisotropy and "dark precursors", characterizing their structure and possible process leading to their formation are considered. Also, this review evaluates novel designs of BDFs, especially, heterogeneous glass-core fibers, which can be used for solution of the practical problems. For instance, such designs can be useful for developing a broadband flattop optical amplifier with adopted characteristics. In addition to bismuth-doped materials, this review includes the mainstream results in BDFAs for advanced optical technologies, summarizing the obtained results over two decades. Despite the significant progress the prospects for commercial production of BDFs remain uncertain that primarily due to difficulties in the reproducibility of Bi-doped fiber parameters and the high level of unsaturable loss in highly Bi-concentrated fibers. Addressing these challenges is essential to advancing commercialization and ensuring rapid deployment of this technology.
Bismuth-doped fibers (BDFs) have already proven themselves as active materials that can be used to develop optical devices with unique characteristics in previously inaccessible spectral ranges. Optical amplifiers based on these active fibers exhibit high gain and low noise across all telecommunication spectral bands (from O- to U-band), while BDF-based lasers offer the benefits of high efficiency and wide wavelength tunability. However, existing research still faces significant challenges in achieving a reliable technology for reproducing the parameters of BDFs, as well as in fabricating optical fibers with increased Bi concentrations and low unsaturable losses. Developing a high-gain, ultra-wideband amplifier that can be effectively integrated into existing communication systems remains a challenge. Future research should focus on balancing cost, energy efficiency, and device performance, which can be partially addressed by optimizing the BDF design and the device itself. All of this is necessary to meet the growing demand for high-speed data transmission over fiber-optic communication systems, which is crucial in the context of rapidly evolving artificial intelligence technologies. In this regard, the ability to utilize all available telecommunications bands appears very promising. We believe that progress in this area will undoubtedly lead to the development of optical communication systems with significantly increased bandwidth, where bismuth-doped optical amplifiers are key components. Moreover, thanks to ongoing advances in optical materials and process technology, bismuth-doped fiber technology can pave the way for efficient, reliable, and scalable solutions for next-generation fiber-optic systems.
Abstract The downstream glass industry in South Korea exhibits a distinctive structure compared with that of other non-Asian countries, as its semiconductor and display sectors are tightly integrated with the glass materials supply chain. In this review, the distinct features of glass-related industrial research and development in South Korea are addressed by taking two representative applications of glasses, i.e., glass core substrate for semiconductor packaging applications and ultra-thin glass for use as cover window of flexible display modules. In the case of glass core substrate and glass interposer, the inherent brittleness of glassy materials imposes constraints across the process flow, requiring precise control over laser irradiation, chemical etching, redistribution layer formation, and singulation. Ultra-thin glass needs to be durable during repeated deformations, so that its thickness is typically on the order of several tens of micrometers. At such thickness levels, resistance to mechanical deformations becomes a primary concern, and therefore processing steps from cutting, chamfering, and healing to chemical strengthening play a decisive role in determining mechanical reliability and optical clarity of ultra-thin glass. Flexible display devices with other form factors featuring multi-foldable, slidable or rollable capability would be realized along with advancement of processing technologies associated with ultra-thin glass.
With an emphasis on the application-oriented research activities directly or indirectly related to glass materials led by South Korean universities, four different topics are introduced in this review: 1) a thermodynamic database for glass-forming systems, 2) mold flux systems used in steel casting, 3) glasses doped with quantum dots and/or perovskite nano-crystals, and 4) chalcogenide glasses for thermal imaging. For each topic, recent advances are described in conjunction with their basic concepts and future perspectives. The CALPHAD (CALculation of PHAse Diagrams) thermodynamic database for glass-forming system and glassmaking process has been actively developed to assist glass research and the manufacturing process. In addition, a multi-component diffusion model for glass systems is being actively studied for various kinetic simulations. Recent research focuses on the expansion of the database to address key issues relevant to the decarbonization of the glassmaking process such as water solubility in glass, new redox agents, refractory wear in glass melting furnace, and electrode materials for an electric melting furnace. However, the limited availability of phase diagram and thermodynamic data pose challenging issues for the database development relevant to the decarbonization process.
In order to meet the demands for advanced glass lubricants for continuous casting of steel, investigations of structural understandings and their effects on non-Newtonian rheology, thermal properties, and glass forming ability are being actively pursued. In particular, application of mixed alkali effect and Mie scattering is believed to be an effective design principle to enhance lubrication without deteriorating heat transfer controlling ability. In consideration of the necessity of increasing scrap recycling to achieve carbon neutrality during steel manufacturing, further investigations should be done on rheological behavior of supercooled silicate glass lubricant melts in order to mitigate the cracking on continuously cast steels with a greater number of accumulated impurities.
Quantum dots and perovskite nanocrystals are still widely studied for next generation color converters or phosphoric materials for high picture quality displays including micro-LED display and anti-counterfeit applications, thus offering high chances for robust quantum dot or perovskite nanocrystal embedded glasses. However, several issues such as sub-micron sized glass powders with stable PL-QY, Pb-free perovskite nanocrystals, and glass materials for direct driving LED remain as challenging issues for their future applications.
The thermal imaging market is expected to steadily expand with a conspicuously steep inflection arising in the automobile industry caused by the regulations mandating employment of the autonomous emergence braking system. It is noteworthy that the cost of LWIR image sensors based on microbolometer arrays is decreasing rapidly to meet the demands of the automotive industry, and in this regard a group of well-qualified chalcogenide glasses will be promising as molded lenses for the high-resolution thermal cameras. Specifically, those chalcogenide glasses consisting of environmentally less harmful and relatively cheaper constituents would be preferred in the civilian sectors.
Across diverse application domains, glass materials are engineered to meet increasingly stringent criteria with regard to performance, reliability, and sustainability. It is worth mentioning that the most advanced display modules and semiconductor chips are currently manufactured mostly in East Asia. This geographic imbalance provides unique opportunities and challenges for both of the glass industry and academia. New applications such as ultra-thin glass for flexible display modules and glass core substrate for semiconductor packaging will present unprecedented functionalities and benefits to glass materials.
Coagulation is a critical step in the water treatment process. As an important component of coagulation technology, the development and application of coagulants have always been a core focus of the industry. To address the challenges posed by complex water quality and improve the efficiency of coagulation, titanium-based coagulants are evolving from single-component formulations towards composite formulations. By optimizing synthesis conditions (such as molar ratio, alkalinity, reaction temperature, and reaction time) to alter the structure of single-component titanium-based coagulants, composite titanium salt coagulants can be prepared. These composite variants not only effectively overcome the limitations of single titanium salts and combine the coagulation characteristics of multiple reagents but also achieve efficient removal of various pollutants through synergistic effects between components. Consequently, composite titanium salt coagulants demonstrate superior coagulation performance.
This article provides a comprehensive review of the preparation, classification, application, current challenges, and future development strategies of composite titanium salt coagulants. It begins with an overview of current preparation methods, including slow alkali titration (SAT), electrodialysis (ED), stepwise/copolymerization methods, and the sol-gel method. Each technique has its unique characteristics in terms of control precision, product performance, and suitability for large-scale application. The SAT method is simple to operate, low-cost, and easily scalable, making it the most commonly used method for laboratory and industrial preparation of composite titanium salt coagulants. The ED method allows precise control over hydrolysis and polymerization processes, producing products with excellent performance; however, its higher cost and operational complexity have so far limited its application to laboratory and pilot-scale stages. Copolymerization/stepwise polymerization is suitable for preparing titanium salt-silicate composite coagulants with controlled structures, while copolymerization can produce titanium salt-metal salt composites with stronger synergistic effects. The sol-gel method can prepare dry gel coagulants that are convenient for storage and use, combining both coagulation mechanisms and adsorption. However, this technology is still in the laboratory research stage, and its cost and control techniques are key factors for future large-scale application.
Based on compositional differences, composite titanium salt coagulants can be classified into several types: titanium salt-metal salt, titanium salt-silicate, and titanium salt-organic polymer composite coagulants. Titanium salt-metal salt composite coagulants mainly include liquid or conventional composite titanium salt coagulants prepared by techniques such as SAT and ED, as well as dry gel-form composite titanium salt coagulants prepared by the sol-gel method. Both types of coagulants form titanium-containing bimetallic or multimetallic composite systems by combining titanium salts with metal salts such as aluminum, iron, and zirconium, thereby incorporating the coagulation advantages of multiple metals. Titanium salt-silicate composite coagulants are formed by copolymerizing/stepwise polymerizing titanium salts and polysilicic acid (PSiA). These coagulants combine the charge neutralization capacity of titanium salts with the adsorption and bridging ability of PSiA, significantly enhancing coagulation efficiency. Titanium salt-organic polymer composite coagulants are a category of composite titanium-based coagulants formed by combining titanium salts with organic polymer compounds (such as polyacrylamide, chitosan, starch, etc.). These coagulants integrate the highly efficient charge neutralization capacity of titanium salts with the adsorption and bridging capabilities of organic polymers, thereby significantly improving coagulation performance. Composite coagulants effectively overcome the problems associated with single titanium salts, such as significant pH fluctuations and poor storage stability. They demonstrate superior performance compared to traditional coagulants in specific areas, including ultrafiltration pretreatment, sludge conditioning, treatment of low-temperature and low-turbidity water, and removal of micropollutants. The sludge generated from their use can serve as a raw material for producing TiO2, providing a new pathway for resource recovery in water treatment processes and highlighting their unique application prospects. However, their adaptability in real water bodies, economic feasibility, and long-term ecological safety still require systematic evaluation.
Finally, this study outlines the challenges faced in transitioning composite titanium salt coagulants from laboratory research tolarge-scale engineering applications and proposes corresponding strategies to address them.
Although composite titanium salt coagulants demonstrate excellent performance, there are still some issues and challenges that need to be addressed. Future research can focus on the following aspects: (a) Enhancing Adaptability to Real Water Bodies. Currently, the complex composition of various actual water bodies imposes higher demands on the application of composite titanium salt coagulants. To ensure stable and efficient coagulation performance under complex water quality conditions, future efforts should focus on developing novel composite titanium salt coagulants with stronger specificity and broader applicability, thereby meeting the needs of more complex and demanding application scenarios. (b) Lower- Cost Preparation of Composite Titanium Salt Coagulants. At present, the preparation of composite titanium salt coagulants largely relies on complex processes such as SAT, ED, and sol-gel methods, which hinders their large-scale application. Future research should aim to develop simpler, greener, and more economically viable preparation pathways—for instance, utilizing industrial by-products or waste materials as raw materials—while optimizing the preparation process to reduce energy consumption and production costs. (c) Ensuring Ecological and Health Safety. Current research on the toxicity assessment of titanium-based coagulants has predominantly focused on TiCl4 and Ti(SO4)2. Future studies should strengthen investigations into titanium residue and its ecotoxicological effects during the use of composite titanium salt coagulants. Meanwhile, it is essential to employ a wider range of aquatic organisms (e.g., fish, shellfish, and aquatic plants) for ecotoxicity studies, and to further explore the migration, transformation, and long-term accumulation of residual titanium in water bodies and organisms. This will provide a scientific basis for comprehensively safeguarding water environments and human health.
With the continuous improvement of aero-engine thrust-to-weight ratio and turbine inlet temperature, the performance limitations of conventional superalloys become increasingly prominent. SiCf/SiC ceramic matrix composites (CMCs) can be core candidate materials for hot-section components due to their excellent high-temperature mechanical properties and low density. As a key technology to realize the engineering application of CMCs, compatible abradable/environmental barrier coatings (A/EBCs) that can simultaneously achieve gas path sealing, high-temperature protection and abradable performance become a research focus in the field of advanced aero-engine sealing technology. This review represents the research progress of such coatings from three dimensions, i.e., material design, microstructural regulation, and performance evaluation, while analyzing key technical challenges and development trends. In terms of material system design, conventional yttria-stabilized zirconia (YSZ) abradable coatings suffer from thermal expansion mismatch with SiCf/SiC CMCs, which are prone to failure, while conventional solid lubricants undergo oxidative degradation at > 1200 ℃. It is urgent to develop new matrix materials with a high thermal stability, a water vapor-oxygen corrosion resistance and a thermal expansion compatibility. Multi-layer structure is the main design to realize functional synergy, and the interface matching and thermal expansion adaptability between layers are a key to the service durability. The introduction of negative thermal expansion materials provides an idea to solve the mismatch problem. In addition, the construction of material system matching for multi-layer coatings and the compatibility analysis of interlayer interfaces/multiphase interfaces also become important aspects in the design of abradable/environmental barrier coating systems.
In the aspect of microstructural regulation, improving porosity is a main way to obtain excellent abradability, but there is a prominent contradiction among abradability, erosion resistance, corrosion resistance and thermal stability. Excessive or uneven porosity, as well as high-temperature sintering and closure, will lead to the performance degradation and early failure. The core challenge is to realize the precise regulation of multi-scale pore structure and the multi-performance synergy balance. In terms of performance evaluation, the existing test devices have high cost and poor universality, and it is difficult to simulate the real multi-field coupling service environment. The lack of perfect preparation and evaluation standards restricts the engineering and standardized development of CMC-compatible coatings. Finally, the development trends of A/EBCs are prospected, providing a reference for the research and development of high-temperature sealing technology and coating system for advanced aero-engines.
In summary, with the increasing service temperature of aero-engines, the abradable/environmental barrier coatings (A/EBCs) that match SiCf/SiC ceramic matrix composites (CMCs) become a key research direction. This review represents the research progress of A/EBCs in material design, microstructural regulation and performance evaluation, and points out that the current challenges mainly include thermal expansion mismatch between conventional coating materials and CMC substrate, poor high-temperature stability of lubricants, difficult balance between multi-scale pore structure and multi-performance, and lack of standardized evaluation systems and test standards suitable for multi-field coupling service environment. In the future, the research and development of A/EBCs should focus on the multi-objective synergistic design of material composition, multi-scale microstructure and performance evaluation system. It is necessary to strengthen the analysis of failure mechanism under multi-physical field coupling environment, develop new high-temperature stable matrix and lubricant materials, realize the precise regulation of multi-scale pore and interface structure, and establish a standardized preparation and performance evaluation system. Through the breakthrough of the above key technologies, the comprehensive performance and service durability of A/EBCs will be effectively improved, so as to promote the leapfrog development of high-temperature sealing technology and provide an important support for the performance improvement of next-generation aero-engines.
Quartz ceramics (SiO2) possess unique properties such as low thermal expansion, excellent chemical stability, and outstanding dielectric performance, making them widely used in semiconductor manufacturing, optoelectronic devices, and high-frequency electronic components. Traditional sintering of quartz ceramics typically requires temperatures above 1000 ℃, which inevitably induces polymorphic transformations from α-quartz to β-quartz or cristobalite, hindering the preparation of single-phaseα-quartz ceramics. Recently, the cold sintering process (CSP) has emerged as a promising low-temperature densification route for ceramics, utilizing transient liquid phases to induce "dissolution-precipitation" or interfacial reaction mechanisms. However, for low-solubility ceramics such as quartz, CSP often fails to achieve full densification and crystallization due to insufficient dissolution kinetics and weak interfacial reactivity. The critical scientific problem addressed in this work is how to effectively trigger the amorphous-to-crystalline phase transition of low-solubility quartz at low temperature, thereby enabling the preparation of denseα-quartz ceramics.
This study systematically investigates the cooperative effects of transient solvent alkalinity, sintering temperature, and uniaxial pressure on the amorphous-to-α-quartz phase transition during CSP. A transient alkaline liquid phase is introduced to regulate interfacial reactions and crystallization kinetics, aiming to provide a theoretical basis and technical strategy for the low-temperature processing of low-solubility ceramics.
Amorphous mesoporous silica (SBA-15) powders were used as the starting material. The powders were homogeneously mixed with different transient solvent phases: deionized water (neutral), 5 mol·L-1 NH3·H2O (weak alkaline), and NaOH solutions of varying concentrations (0.5-10.0 mol·L-1, strong alkaline). Approximately 0.4 g of powder was thoroughly ground with the liquid phase in a mortar and then loaded into a 10 mm diameter steel die. Uniaxial pressures ranging from 200 MPa to 600 MPa were applied, while the sintering temperature was varied between 200 ℃ and 350 ℃. Heating was conducted at a rate of 15 ℃·min-1 and held for 40 min at the target temperature. After natural cooling, the sintered pellets were mechanically polished for further characterization.
The density of the cold-sintered ceramics was calculated by dimensional and weight measurements using a vernier caliper and electronic balance, and relative density was determined based on the theoretical density of quartz (2.2 g·cm-3). Phase composition and structural evolution were analyzed using X-ray diffraction (XRD, Cu Kα radiation, λ = 0.154 06 nm, 50 kV, 100 mA, scanning range 10 °-90 °, step size 0.01). Fourier-transform infrared spectroscopy (FTIR-ATR) was used to identify bonding characteristics and confirm phase transitions. Microstructural evolution and fracture features were observed by field-emission scanning electron microscopy (FE-SEM). The mechanical properties of the sintered ceramics were evaluated by Vickers hardness tests (3 kgf load, 10 s dwell), flexural strength using the modified small punch (MSP) method, and fracture toughness (KIC) calculated by the Anstis equation. Poisson's ratio and Young's modulus were determined by ultrasonic measurements.
This experimental design allows for a systematic investigation of how transient solvent alkalinity, temperature, and pressure cooperatively affect densification and amorphous-to-crystalline transformation during CSP of quartz.
The phase composition of the sintered bodies was strongly influenced by the type and concentration of transient solvent. Without a liquid phase or with neutral water, the sintered samples remained largely amorphous and exhibited low relative density (~80%). Weak alkaline NH3·H2O increased compaction and relative density (~92%) but failed to trigger phase transformation. In contrast, strong alkaline NaOH solutions (≥3 mol·L-1) effectively promoted the dissolution of Si-OH surface species, forming soluble silicate intermediates. These intermediates subsequently underwent reprecipitation and recrystallization under external pressure and temperature, leading to complete transformation into α-quartz at 300 ℃ and 500 MPa. XRD and FTIR confirmed the disappearance of the amorphous broad peak and the emergence of α-quartz characteristic double peaks at 798 cm-1 and 778 cm-1, indicating a complete amorphous-to-α-quartz transition.
A clear alkalinity-dependent phase transition sequence was identified: amorphous → keatite (0.5 mol·L-1 NaOH) → keatite +stishovite (1 mol·L-1) → keatite + α-quartz (3 mol·L-1) → α-quartz (≥5 mol·L-1). Simultaneously, relative density increased from 71% (no solvent) to 95.7% (10 mol·L-1 NaOH). SEM revealed that strong alkalinity produced well-defined grain boundaries and uniform microstructures, while weak or neutral conditions resulted in porous, poorly bonded networks.
The phase transition was also sensitive to sintering temperature and pressure. At 200 ℃, no crystallization occurred even at 600 MPa. Crystallization initiated at 250 ℃ and 300 MPa, and complete α-quartz formation occurred at ≥300 ℃ and ≥400 MPa. Increasing pressure facilitated particle rearrangement, pore elimination, and enhanced atomic diffusion at the interface, accelerating phase transition. A comprehensive temperature-pressure-phase diagram was established, clearly delineating the non-crystalline, partially crystalline, and fully crystalline regions.
Mechanical properties were strongly correlated with microstructure and phase composition. The α-quartz ceramics cold-sintered with 5 mol·L-1 NaOH exhibited a Vickers hardness of 5.1 GPa, Young's modulus of 67.8 GPa, fracture toughness of 0.98 MPa·m1/2, and flexural strength of (58 ± 7) MPa. These values represent increases of 30%, 40%, and 110% in hardness, modulus, and toughness, respectively, compared to the amorphous samples. The enhanced mechanical properties are attributed to the formation of well-bonded crystalline interfaces that enable efficient stress transfer and crack deflection, unlike the disordered amorphous structure.
This work demonstrates a controllable strategy to induce amorphous-to-α-quartz transformation in low-solubility silica ceramics through the regulation of transient solvent alkalinity during cold sintering. By introducing strong alkaline NaOH solutions (≥3 mol·L-1), the activation energy for crystallization can be significantly reduced, enabling complete transformation at 300 ℃ under 500 MPa. The critical crystallization threshold was identified at 250 ℃ and 200 MPa, and a detailed temperature-pressure-phase diagram was established to illustrate the transition pathways. The resulting ceramics achieved a relative density above 95% and exhibited excellent mechanical performance, including a Vickers hardness of 5.1 GPa, Young's modulus of 67.8 GPa, fracture toughness of 0.98 MPa·m1/2, and flexural strength of (58 ± 7) MPa. These results clearly indicate that alkaline regulation during CSP not only enables precise control of phase structure but also produces dense, mechanically robust α-quartz ceramics at dramatically reduced sintering temperatures. This approach provides both fundamental insights and practical guidance for the low-energy fabrication of advanced low-solubility ceramic components.
ZrB2-based multiphase ceramics are representative ultra-high temperature ceramics (UHTCs). Their service temperature significantly exceeds the sintering temperature, often requiring substantial amounts of SiC as a sintering aid to achieve the densification and enhance the oxidation resistance. The transition metal carbides (MCs) are superior sintering aids for UHTCs, effectively removing the oxygen-impurity and improving high-temperature strength. From some projects supported from the National Natural Science Foundation of China, our studies focus on the effect of MC on controlling the multiphase microstructure of UHTCs. The results obtained reveal the reactive-sintering mechanism engaging the high-viscous liquid-phase and explore the mutual-solution behavior in multiphase ceramics along with the structure-property relationship. The quantitative characterization for microstructures indicates the dominance of bora-carbide sintering-melts on reactive-sintering and densification, and on regulating the multi-level evolution of high-solution microstructures. MC additives are transformed into ZrC grain boundary phases via the sintering-melt, and its exchange-reaction with the primary phase governs the multiphase relationship. The melt as a transient liquid enables a bi-solubility of M to create prevalent core-rim structures. In the later stages of sintering, ZrC second-phase precipitates with a higher solubility of M.
The multi-levelled control of multiphase microstructures by the reactive-melt is analogous to "dissolution-reprecipitation" process for liquid-phase sintering in the transformable microstructures of silicon-based ceramics, with silicate-melts and glassy phases at grain boundaries. In contrast to the monolithic ceramics of high-entropy MB2 and MC, the multi-levelled solid-solutions and the associated multiphase microstructures of MⅠB2-MⅡC UHTCs offer ample and novel routes for comprehensive control, better optimization and further enhancement in high-performance UHTCs. The coherent hetero-interfaces created from the multi-levelled solutions via solid-state phase-separations and their interconnected dislocation networks can further improve the high-temperature strength, and those phase-boundaries, grain-boundaries, and solute-segregates allow a precise control over the multiscale semi-coherent microstructures. The research on this synergistic evolution of intergranular phases and sintering-melts at high temperatures along with the multiphase transformation has a promising potential for future advancements in ceramic genomes and levelled structure-property relationship for multiphase UHTCs governed by solid-solutions as enthalpy-regulation.
High-temperature vibration sensors are indispensable key components for the health detection of core equipment in fields such as aerospace and nuclear energy. The BiScO3-PbTiO3(BS-PT) system has attracted much attention due to its high Curie temperature (TC≈450 ℃) and excellent piezoelectricity (d33≈450 pC/N). However, the poor insulation properties of this material hinder its application in high-temperature vibration sensors because high electrical resistivity (ρ) and a long time constant (τ) are critical to prevent thermal runaway and ensure signal integrity. Manganese (Mn) doping is a commonly used modification method for piezoelectric ceramics. Previous studies on Mn-doped BS-PT were controversial regarding the valence state distribution and substitution positions of Mn ions, which could not be conducive to the design of high-temperature piezoelectric ceramics with the collaborative optimization of multiple electrical parameters. Therefore, this work was to clarify the defect chemical mechanism associated with manganese doping through refined structural characterization combined with electrical performance analysis, and to obtain the modified BS-PT piezoelectric ceramic components suitable for high-temperature vibration sensors.
0.365BiScO3-0.635PbTiO3-x% MnO2 (BSPT-x% MnO2, x=0.00, 0.01, 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00) ceramics were synthesized by a conventional solid-state reaction method. The powders were firstly calcined at 800 ℃ for 2 h and then sintered at 1050 ℃ for 2 h. The phase composition was analyzed by X-ray diffraction (XRD). The rietveld refinements were performed using a software named GSAS. The microstructure and elemental distribution were examined by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). The average grain size was estimated by a software named Nano Measurer. The Mn valence states were determined by X-ray photoelectron spectroscopy (XPS). For electrical measurements, poled samples (120 ℃, 5 kV/mm, 30 min) were used. The piezoelectric coefficient (d33) was measured by a model CAS ZJ-6A quasi-static meter. The electromechanical coupling coefficient (kp) was measured by a model Agilent 4294A impedance analyzer. The temperature-dependent dielectric properties were measured by a model Agilent E4980A LCR analyzer. The high-temperature DC resistivity (ρ) was measured by a model Keithley 6517B high-resistance electrometer. The in-situ d33 was measured by a model Julang TZFD-600 variable temperature quasi-static d33 measurement system.
The Mn doping mechanism and high-temperature performance of BS-PT ceramics are systematically clarified. The XPS results confirm the coexistence of Mn2+ and Mn3+. To quantitatively verify the substitution site, the rietveld refinement reveals a non-monotonic evolution of unit cell volume. Based on the EDS evidence of Sc segregation without Ti precipitation, Mn ions preferentially substitute for B-site Sc3+. The dominant aliovalent substitution introduces defect dipoles accompanied with strong local random electric fields, significantly enhancing a relaxor behavior, while triggering a "hardening" effect that reduces tanδ and εr. The decoupling of piezoelectric and dielectric properties is achieved in specific compositions due to the grain boundary effect compensating for the hardening effect, especially obtaining the optimal piezoelectric voltage constant (g33) at the component with x of 1.00. For high-temperature capabilities, the optimal composition (x=1.00) demonstrates a superior stability, with in-situ d33 variation remaining within 20% up to 400 ℃. The thermally stable defect dipoles effectively trap oxygen vacancies, leading to a high resistivity of 109 Ω·cm and an enhanced time constant of 0.072 s at 350 ℃. Consequently, the ceramic with x of 1.00 exhibits a high g33 of 0.012 V·m/N when evaluated at a unified service temperature of 350 ℃, which is 50% higher than that of the undoped counterpart. These results indicate that the modified ceramic achieves an optimal balance of sensitivity and insulation for high-temperature vibration sensors.
This work clarified the Mn doping mechanism in BS-PT ceramics. The results of correlative XPS, Rietveld refinement, and EDS analysis confirmed that Mn ions could preferentially substitute for B-site Sc3+. The dominant aliovalent substitution induced a hardening effect, while the recovery of d33 was dominated by grain size restoration. The optimal composition (x=1.00) exhibited a robust stability with d33 variation within 20% at 400 ℃. The thermally stable defect dipoles could obtain a high resistivity (109 Ω·cm) and time constant (0.072 s) at 350 ℃. Meanwhile, a superior piezoelectric voltage coefficient (g33) of 0.012 V·m/N was achieved at 350 ℃, which was 50% higher than that of the undoped counterpart, validating its potential for high-temperature sensors.
Continuous silicon carbide fiber reinforced silicon carbide ceramic matrix composites (referred to as SiCf/SiC composites) become the preferred high-temperature and lightweight thermal structure materials for high thrust-to-weight ratio aero engines due to their excellent high-temperature properties. However, their poor high-temperature oxidation and corrosion properties limit their long-life use. It is effective to improve the oxidation and corrosion properties of SiCf/SiC composite matrix through matrix modification, and the commonly used modified materials mainly include self-healing components, network intermediate oxides and rare-earth silicates.
The self-healing components form the liquid phase of B2O3 or borosilicate glass phase in the process of oxidative corrosion that has a certain fluidity and viscosity at a high temperature, which can fill the cracks and holes in the matrix. The volume expansion caused by oxidation makes the small defects heal when the oxidizing medium diffuses in the cracks as the oxidation phase, which can effectively block the diffusion of the oxidizing medium to the key areas that are easy to oxidize such as fibers and interfaces, thus enhancing the oxidation resistance of the material and extending its service life. The network intermediate oxide can absorb the "free oxygen" in the borosilicate glass melt and change its coordination from [MO6] to [MO4], reconstructing the silicate network damaged by water vapor erosion and maintain its integrity, and significantly improving the stability of the self-healing glass phase under high temperature water vapor conditions and the oxidation and corrosion properties of composites. Rare-earth silicate itself has excellent antioxidant corrosion properties, and the rare-earth elements migrate to the near surface of the matrix to form an antioxidant layer during the oxidative corrosion process, which inhibits the penetration of oxidative corrosion medium into the matrix and the reaction between borosilicate glass and water vapor, reduces the generation of gaseous substances, and slows down the oxidative corrosion of the matrix to a certain extent.
There are two main ways to introduce modified materials into the matrix, i.e., one refers to the direct introduction of antioxidant corrosive materials without changing their phases during use, such as directly introducing rare-earth silicates into the matrix, and another refers to the introduction of the precursor of the modified material or substances containing modified elements into the matrix, and the target modified material is obtained through chemical reactions during the oxidative corrosion process, usually in this way the introduction of self-healing components and oxidation corrosion phases into the matrix. In addition, the synthesis process of modified materials mostly adopts CVI, PIP, RMI, SI, etc., and their advantages and disadvantages of different modification processes are different. For instance, the preparation temperature of CVI process is low, but the densification cycle is long and the cost is high. The RMI process is simple, the cycle is short, and the composite material with a high density is prepared, but the melting temperature is high, which is easy to heat damage to the fiber and interface. Besides a single process, composite processes are mostly used to achieve complementary advantages, introducing modified materials into the matrix and realizing the densification of composite materials.
It is effective to introduce self-healing components, network intermediate oxides and rare-earth silicates into the SiCf/SiC composite matrix to fill the pores and cracks of the material via generating self-healing components during the oxidative corrosion process, and forming a dense oxide layer on the surface of the material to resist further erosion by the oxidizing medium, thus improving the water and oxygen corrosion resistance of the composites. However, there are still some challenges. Firstly, the research on the oxidation corrosion mechanism of composite materials is still in-depth, and there is a lack of data on oxidation kinetics, oxidative corrosion rate and oxidative corrosion depth, and the basic research on the damage evolution mechanism of materials in different environments is still relatively weak. It is thus necessary to construct a complete and reliable performance database of SiCf/SiC composites modified with different substrates, clarify the oxidative corrosion damage mechanism, and provide design parameters and theoretical support for the practical application of composites. Secondly, the effective temperature range of a single modified substance to improve the oxidative corrosion performance of composites is limited. The temperature range of B group is below
1000 ℃, and the temperature range of Al group is 1000-1300 ℃. The synergistic effect of multiple modification strategies is explored via introducing multiple modified substances into the matrix of composites at the same time. It is expected to achieve the oxidation and corrosion properties of SiCf/SiC composites in a wide temperature range and achieve a long-life cycle protection. Finally, it is also worth to develop new preparation processes, such as nano-infiltration and transient eutectic (NITE), or use hybrid processes to achieve material densification and improve the performance of SiCf/SiC composite substrates while modifying them.
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