Grain growth kinetics and polytype transformation play a critical role in determining the final microstructure and properties of SiC ceramics, particularly during the intermediate stage of sintering when the microstructure evolves most significantly. This study investigates the impact of pressureless induction heating on grain growth and polytype transformation during the intermediate sintering stage. By analyzing grain growth kinetics and its relationship with polytype transformation, the mechanisms governing these processes are examined to provide insights for controlling the microstructure and optimizing the properties of SiC ceramics. Rapid densification was observed during the temperature ramp, with the onset of the intermediate stage shifting to higher temperatures due to the high heating rate (200 degrees C/min). Isothermal heating at 1975 degrees C, 2000 degrees C, and 2025 degrees C resulted in significant grain growth and morphological changes within 10 min of heating. The results show that the grain-growth behavior was attributed to polytype transformation, anisotropic grain growth, boron additives, and electromigration. The activation energy for grain growth was 222 +/- 38 kJ/mol, which is lower than typical values for solid-state processes. These findings provide valuable insights for optimizing the sintering of SiC ceramics in advanced applications.

We report the fast vapor-liquid-solid growth of silica nanowires through active oxidation and corrosion of a passivated silicon carbide (SiC) substrate. Silica nanowires are formed from SiO gas generated during the active oxidation of SiC and supplied from the corrosion region. SiO gas is continuously absorbed by Si-Mo nanoparticles formed in the passivated region, leading to the production of ultralong nanowires in large quantities. This study demonstrates a nonequilibrium chemical pump effect that enhances silica nanowire growth by coupling the active oxidation of SiC with silica-oxide-assisted nucleation on the well-passivated SiC substrate.

To assess the potential risk of fuel matrix degradation, this study examines the oxidation behavior of the reaction-sintered SiC containing residual Si (Si-RS-SiC) during air-ingress accidents. Si-RS-SiC samples were subjected to oxidation tests in air and under 1 ppm O2 flow using thermogravimetric analysis (TGA). The morphology and thickness of the silicon oxide layer were investigated using scanning electron microscopy with energy-dispersive X-ray spectroscopy. The Si-RS-SiC exhibited exceptional resistance to air oxidation up to 1400°C, forming a uniform oxide layer of 20 μm thickness with a pure Si layer (2 μm thick) between SiO2 and SiC at this temperature. At lower temperatures, the Si-RS-SiC formed oxide layers with irregular thickness, which became thicker and more uniform as the temperature increased. Additionally, TGA tests in 1 ppm O2 showed that the passive/active transition behavior of Si-RS-SiC shifts in a preferable direction compared to previously reported Si-less RS-SiC, indicating that the presence of residual Si significantly enhances the corrosion resistance.

To immobilize and solidify carbonate-based Advanced Liquid Processing System (ALPS) sediment wastes, this study introduces a chemical transformation process that converts these wastes into calcium and magnesium phosphate phases, followed by densification using a novel cold sintering press (CSP) technique. Simulated calcium-magnesium carbonate slurries were treated with phosphoric acid to synthesize calcium and magnesium phosphates, which were then sintered at 300-500 degrees C using CSP. The effects of the calcium-to-magnesium ratio, strontium incorporation, and sodium chloride addition on phase composition and CSP densification were investigated. Dense bulk samples were successfully fabricated and characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and the Archimedes method. The results showed that the chemical transformation process led to the formation of whitlockite and newberyite, with the calcium-to-magnesium ratio determining the relative proportions of these phases. Strontium was effectively incorporated into the whitlockite crystal structure, while newberyite enhanced densification through a dehydration-driven process. Sodium chloride had no effect on chemical transformation and was not found in the final solid product. These results show that direct conversion of calcium-magnesium carbonate slurries to whitlockite-based phosphate ceramics, followed by CSP, enables stable solidification, making this method promising for ALPS sediment waste management.

Contamination of sensors, electrical contacts, and optics by volatile cyclic siloxanes leads to their dysfunction and shortens their service life. Application of mesoporous adsorbents can be effective to remove volatile siloxanes from the environment, potentially making them useful for protecting sensitive equipment. However, the practical application of mesoporous adsorbents in powder form may not be suitable due to handling and containment problems. In the present work, cold sintering of mesoporous silica powders with addition of sodium silicate water solution is applied to fabricate highly effective adsorbent for volatile siloxanes. The cold-sintered mesoporous silica pellets are characterized through nitrogen gas adsorption, scanning electron microscopy, low-angle X-ray diffraction, and Fourier-transform infrared spectroscopy (FTIR). Hexamethylcyclotrisiloxane (D3) serves as a model substance to assess the adsorption capability toward volatile cyclic siloxanes under static adsorption test conditions at room temperature. Furthermore, thermal desorption of D3 is investigated using thermogravimetry with in-situ gas analysis employing mass spectrometry. Results show that the cold sintering method preserves the mesoporous structure while transforming mesoporous silica powders into bulk pellets, resulting in an enhanced adsorption capacity for D3 compared to raw silica powder. Furthermore, the temperature required for siloxane desorption significantly increases indicating chemical adsorption mechanism and possible D3 ring-opening polymerization. FTIR spectroscopy reveals the significant involvement of hydroxyl groups in the adsorption and desorption processes of D3 on mesoporous silica.

The development of new low-carbon emission methods for the synthesis and processing of materials is an urgent task today. Here, we present a rapid, efficient, and low-energy-consuming synthesis method for high-yield Ti3AlC2 MAX phase powders. Our method is based on induction heating-assisted combustion synthesis (IH-CS), where the heat of the reaction is used as a driving force of the process, and the conditions of the process are finely controlled using IH at the same time. Ti3AlC2 MAX phase was synthesized using TiC0.67 and Al powders as starting materials. The introduction of TiC0.67 reduced the total exothermic effect of the reaction, facilitating better control of the reaction process. IH was used to ignite the reaction and to hold the temperature after ignition at an optimal value to complete the synthesis process. The effects of holding temperature, Al, and Si addition on Ti3AlC2 yield were investigated. As a result, Ti3AlC2 MAX phase powders containing only .7 wt% of the secondary phase were synthesized in a very short time. Our findings suggest that IH-CS is a highly effective and promising alternative for the facile fabrication of high-purity MAX phase powders.

Silicon carbide fiber-reinforced silicon carbide (SiCf/SiC) composites have been expected to be used as next-generation heat resistant structural materials with high reliability. The fiber/matrix interface for SiCf/SiC composites plays an important role for toughening and strengthening, and it is important to form the optimal interphase for SiCf/SiC composites. In this study, hexagonal-boron nitride (h-BN), which shows better oxidation resistance than carbon, was selected as the interphase for SiCf/SiC composites, and h-BN coating was formed on low-conductive SiC fibers by the electrophoretic deposition (EPD) method using flaked h-BN suspension prepared by wet-jet milling. Unidirectional SiCf/SiC composites with the BN interphase formed by the EPD method were prepared by polymer impregnation and pyrolysis (PIP) method, and their mechanical properties were evaluated. The uniform h-BN interphase was successfully formed on the low-conductive SiC fibers by EPD when thin polypyrrole (Ppy) coating, that is, conductive polymer, was formed on the SiC fibers. The h-BN coating became denser by heat-treatment at 1000degree celsius, and the h-BN coating deposited tightly on the SiC fibers was confirmed. It was demonstrated that the SiCf/SiC composites with the uniform and relatively dense h-BN interphase formed by EPD using flaked h-BN particles exhibited pseudo-ductile fracture behavior with large amount of fiber pullout and higher fracture energy.

Herein electromagnetic induction is used to densify one of the hardest-to-sinter non-oxide ceramics, boron carbide. Power on for less than 5 min allowed densification of boron carbide to a relative density of 95% without application of sintering additives and external mechanical pressure. No cracks were found in the densified samples despite fast heating and cooling rates; Vickers' hardness was 30 GPa, Young's modulus was 322-415 GPa and fracture toughness was 3.38 similar to 4.82 MPa center dot m(1/2). A gradient of densification was observed, with a higher degree of densification observed closer to the surface. The obtained results suggest that very fast pressureless densification of boron carbide was achieved as a result of acceleration of atomic diffusion and dislocation motion under eddy currents induced in boron carbide under high-frequency magnetic field at high temperatures.

Al4SiC4 shows excellent heat resistance, thermal shock resistance, machinability, and oxidation resistance. We focused on Al4SiC4-based ceramics with SiC as a non-oxide matrix for ceramic matrix composites for aircraft jet engines. In this study, monolithic Al4SiC4 and Al4SiC4/SiC ceramics were fabricated by hot-pressing, and a corrosion test against molten calcium-magnesium-alumino-silicate (CMAS) was conducted at 1350(degrees)C for 12-100 h in air, and their corrosion behavior was investigated. Scanning electron microscopy and energy-dispersive X-ray spectroscopy results revealed that severe damage was not observed at the interface between CMAS and the samples after the CMAS corrosion test. The recession of Al4SiC4-100, -10, and SiC-100 after corrosion for 100 h was 80-90 mu m, and that of Al4SiC4-50 was the highest of all samples and the value was 130 mu m. The dissolution behavior of the oxidation layer into molten CMAS via a corrosion reaction was dependent on the composition of both the sample and the oxidation layer, the thickness, and the microstructure of the oxidation layers. The dominant mechanism of reaction between CMAS and Al4SiC4-100, -90, and -50 samples was concluded to be the dissolution of the oxidation products, while in SiC-100 and Al4SiC4-10 samples, the dominant reaction was determined to be direct corrosion of the surface with CMAS.

The application of silicon carbide (SiC) at high temperatures faces a significant challenge due to its corrosion through active oxidation. Therefore, understanding the kinetics of the passive/active transition becomes crucial for evaluating SiC's integrity. This study focuses on investigating the transition and its dependence on temperature, oxygen concentrations, and flow rates. Experiments were conducted using reaction -sintered SiC, subjecting it to temperatures up to 1400 celcius in a thermogravimetric apparatus. The SiC samples were exposed to an Ar/O2 mixed gas with low oxygen concentration, while varying the flow rates from 50 to 300 mL/min. The results confirmed the occurrence of an active -to -passive transition at higher flow rates, validating a theoretical model. Additionally, a new model was proposed to describe the passive -to -active transition caused by SiO2/SiC interface reaction, observed during 22 h of oxidation at 1400 celcius/0.3 ppm O2. This work enhances our understanding of SiC oxidation behavior under extreme conditions, which is crucial for engineering applications of SiC including innovative nuclear reactor fuels.

The oxidation behavior of IG-110, a graphite core component, was investigated at temperatures ranging from 400 to 1000 degrees C in a 10 ppm Ar/O-2 flow to simulate the oxidation process between the graphite core component and helium coolant with low O-2 concentrations employed in advanced High-Temperature Gas-cooled Reactors (HTGRs). The results reveal that IG-110 undergoes significant mass loss at temperatures above 700 degrees C, resulting in total mass changes of -1.5%, -5.3%, and -9.0% at 700, 800, and 1000 degrees C, respectively, during a 10-hour oxidation period. No significant mass loss is observed below 600 degrees C. To understand the oxidation mechanism of IG-110 under low O-2 concentrations, we propose a kinetic model as the current chemical kinetic-controlled model does not fully explain the oxidation behavior observed in our research. Our analysis shows lower estimated reaction rates compared to studies at higher O-2 concentrations; the activation energy values exhibit good agreement. The proposed kinetic model sheds light on the oxidation mechanism of IG-110 under 10 ppm Ar/O-2 flow. This study provides new insights into the oxidation behavior of graphite core components in HTGRs and highlights the importance of controlling the O-2 concentration in the helium coolant to prevent severe degradation of SiC-matrix fuel compacts.

Preventing severe corrosion incidents caused by air ingress accidents in high-temperature gas-cooled reactors (HTGRs) while improving heat removal efficiency from the core is of paramount importance. To enhance both safety and efficiency, a sleeveless silicon carbide (SiC)-matrix fuel compact has been proposed. This study evaluates the 10-hour oxidation of reaction-sintered SiC (RS-SiC)-matrix fuel compact under the conditions of an air ingress accident within the temperature range of 1000 to 1400 °C. The oxidation tests were conducted in a stagnant air environment without flow. As a result, it is demonstrated that RS-SiC exhibits exceptional resistance to air oxidation up to 1400 °C, as shown by the thermogravimetric analysis (TGA), with minimal mass loss due to the oxidation of free carbon. Scanning electron microscopy with energy-dispersive X-Ray spectroscopy (SEM–EDX) analysis reveals that the morphology and thickness of the SiO2 layer formed on the RS-SiC surface vary with temperature. At 1400 °C, uniform oxide layer thickness ranging from 1.59 to 4.10 μm and localized nodule-like oxide formations of approximately 10 μm are observed. In contrast, at 1000–1200 °C, thinner oxide layers are identified, indicating that oxide growth accelerates at higher temperatures. The oxidation rates measured provide insights into the mechanisms of oxide growth.

Boron carbide (B4C) powders with defined stoichiometry, high crystallinity, minimal impurity content, and a fine particle size are imperative for realizing the exceptional properties of this compound in advanced high-technology applications. Nevertheless, achieving the desired stoichiometry and particle size using traditional synthesis methods, which rely on prolonged high-temperature processes, can be challenging. The primary objective of this study is to synthesize fine B4C powders characterized by high crystallinity and a sub-micron particle size, employing a fast and energy-efficient method. B4C powders are synthesized from elemental boron and carbon in a high-frequency induction heating furnace using the electromagnetic induction synthesis (EMIS) method. The rapid heating rate achieved through contactless heating promotes the ignition and propagation of the exothermic chemical reaction between boron and carbon. Additionally, electromagnetic effects accelerate atomic diffusion, allowing the reaction to be completed in an exceptionally short timeframe. The grain size and crystallinity of B4C can be finely tuned by adjusting various process parameters, including the post-ignition holding temperature and the duration of heating. As a result, fine B4C powders can be synthesized in under 10 min. Moreover, these synthesized B4C powders exhibit oxidation onset temperatures higher than 500 °C when exposed to air.

Nanoporous graphene (NPG) materials have the pronounced electrochemical stability of the seamless graphene structures developed over the 3D space. We revisited the Raman spectra of nanoporous carbons (NPCs) synthesized using θ-/γ-Al2O3 templates and NPGs converted from NPCs by annealing at 1800 ºC to identify the type and density of defects. We found that both the NPCs and NPGs mostly consist of single-layered graphene with a few single vacancies and Stone‒Wales defects. The density of vacancy defect per hexagon in the graphene sheet is estimated to be 10-2 for NPCs, while the annealing reduced the value to 10-3‒10-4 for NPGs. This supports the outstanding chemical and electrochemical stability of the novel porous carbon materials.

The effects of solid solution, grain size and porosity on the thermal conductivity of aluminum (Al)- and boron (B)added porous SiC ceramics with in-situ grain growth were investigated. Al-added porous SiC ceramics showed higher porosity, higher solubility of Al in SiC lattice, and lower thermal conductivity with the range of 8.5 - 18.2 W/m center dot K. The higher solubility of Al in SiC lattice was considered as the reason of their low thermal conductivity. B-added porous SiC ceramics have lower porosity, lower solubility of B in SiC lattice, and higher thermal conductivity with the range of 30.6 - 79.0 W/m center dot K. The high thermal conductivity of B-added porous SiC ceramics would result from the low solubility of B in SiC lattice and the low porosity. The effect of grain size was found to increase the phonon mean free path for B-added SiC ceramics.