Journal of the American Ceramic Society最新文献

This study investigated the nonisothermal crystallization kinetics behavior of SiO₂–TiO₂–CaO–BaO–Al₂O₃-based fluorine-free mold fluxes during high-Ti steel continuous casting. By analyzing the thermodynamic characteristics of crystallization and characterizing the precipitated crystalline phases, this study examined the influence of three key component ratios—CaO/SiO₂ (0.95–1.25), CaO/BaO (1.00–3.00), and SiO₂/TiO₂ (2.40–5.20)—on the crystallization mechanism, crystal growth mode, and precipitated phases of the mold flux under heating conditions. The results indicate that the nonisothermal crystallization kinetics involves a complex mechanism exhibiting dynamic evolution. High CaO/SiO₂ ratios resulted in elevated crystallization rates, whereas high CaO/BaO ratios accelerated crystal growth. High SiO₂/TiO₂ ratios increased the degree of depolymerization of the network structure, inhibiting three-dimensional crystal growth. Furthermore, BaAl₂Si₂O₈ and CaTiO₃ crystalline phases primarily precipitated under medium temperatures (800–900°C), whereas CaTiO₃ and Ca₂Al₂SiO₇ were predominant under high temperatures (1000–1100°C). Different from previous studies focusing on single oxides, this work provides full-scale insights into the synergistic effects of multiple component ratios on nonisothermal crystallization behavior, offering both mechanistic understanding and practical guidance for optimizing fluorine-free mold fluxes in high-Ti steel continuous casting.

The modulation of electromagnetic parameters is crucial for enhancing the electromagnetic wave (EMW) absorption performance of materials. Here, a ternary MAB-phase solid solution, namely (Fe_1/3Mn_1/3Cr_1/3)₂AlB₂ (T-FMCAB), was synthesized for the first time via rapid joule heating, overcoming the limitations associated with the synthesis of conventional MAB phases. The EMW dissipation capacity of Fe₂AlB₂ was enhanced through the incorporation of both Cr and Mn at the Fe sites: Cr introduces strong magnetic moments, boosting the magnetic loss of the system, while Mn enhances its dielectric loss by promoting interfacial/dipole polarization through charge transitions between Mn²⁺ and Mn³⁺. Defect engineering achieved through Cr/Mn co-doping and the joule-heating treatment further enhances dielectric and magnetic losses: the lattice distortions arising from the size mismatch among the constituent elements suppress magnetic moment switching, thereby increasing magnetic hysteresis loss, and promote the formation of defect dipoles, which dissipate energy through enhanced dipole relaxation, leading to greater dielectric loss. The prepared T-FMCAB exhibits exceptional performance: a minimum reflection loss of −58.38 dB at an thickness of 1.47 mm and a maximum effective absorption bandwidth of 4.0 GHz. Additionally, a reduction in radar cross-section of more than 30 dB m² is also observed. This work establishes a novel paradigm for developing next-generation ultrathin, broadband, high-performance EMW absorbers.

This study investigates spatial autocorrelation of fiber fracture in ceramic matrix composites, with a focus on statistical characterization of fracture surfaces. While conventional micromechanical models assume global load sharing, experimental evidence increasingly supports the presence of local load sharing (LLS), leading to clustering of fiber breaks. To better understand spatial autocorrelation under LLS conditions, this work applies three geostatistical techniques: Global Geary's C statistic, semivariograms, and Local Geary's C statistic. Simulated fracture surfaces with prescribed levels of spatial autocorrelation are used to evaluate the effectiveness and limitations of each method. Results show that Global Geary's C effectively detects autocorrelation across a range of fiber counts and autocorrelation lengths, even in only partially correlated surfaces. Semivariograms provide insight into the range and strength of correlation but are sensitive to composite size and sample variability. Local Geary's C enables identification of spatially localized clusters of autocorrelated breaks, although interpretation requires careful thresholding. The study establishes guidelines for applying these techniques and demonstrates their utility in diagnosing spatial fracture patterns. The resulting tools pave the way for more detailed analysis of real fracture surfaces, with implications for understanding failure mechanisms and guiding micromechanical modeling.

Porous geopolymer spheres were prepared using slag, metakaolin, and zeolite as raw materials via a direct molding method, followed by in situ growth of nano-CuO on their surface. The effects of metakaolin/slag ratio, foaming agent concentration, surfactant type, and different calcination temperatures on the microstructure, porosity, mechanical properties, and photocatalytic performance of the composite spheres were investigated. It was found that at a slag content of 75%, hydrogen peroxide concentration of 3%, and calcination temperature of 400°C, the spheres exhibited good integrity and a more uniform pore size distribution. The CuO/geopolymer composite spheres exhibited a 71.1% photocatalytic degradation efficiency for methyl violet under 120 min of UV irradiation. The material also showed excellent reusability, maintaining 50.3% efficiency after nine cycles. Remarkably, the degradation efficiency was significantly enhanced to 98.1% through synergistic catalysis with H₂O₂. The synergistic catalytic degradation mechanism was thoroughly investigated. The preparation of composite catalysts by loading nano-CuO semiconductors onto a solid waste–based geopolymer matrix represents a relatively simple and environmentally friendly approach, showing promising development potential in wastewater treatment applications.

Al₂O₃, as a representative structural ceramic, suffers from intrinsic brittleness that severely limits its engineering applications. ZrO₂, as a common toughening phase, has long been used to toughen Al₂O₃ ceramic matrices. Although the phase transformation toughening mechanism of ZrO₂ can effectively improve the fracture toughness of Al₂O₃ ceramics, it typically leads to a significant reduction in hardness. In this study, we innovatively introduced high-entropy carbide (HEC) as a hardening phase combined with ZrO₂ toughening phase to construct a novel Al₂O₃–ZrO₂–(HfNbTaTiZr)C functionally graded ceramic system, achieving an excellent combination of surface hardness (19.49 GPa) and fracture toughness (7.40 MPa·m^1/2). Mechanistic investigations reveal that the thermal expansion coefficient mismatch between HEC and ZrO₂ induces beneficial compressive residual stress fields in the surface layer. Besides, the phase transformation toughening mechanism of ZrO₂ further enhances the fracture toughness, and the introduction of the HEC phase effectively compensates for the hardness loss caused by ZrO₂ addition. This study provides a new strategy to overcome the hardness-toughness trade-off in conventional toughening approaches and establishes a promising pathway for developing advanced structural ceramics, demonstrating significant potential for engineering applications requiring both high hardness and toughness.

Understanding the interplay between structural, electronic, and magnetic properties is essential for unraveling the unique magnetic phenomena and spin-driven ferroelectricity in BaYFeO₄ and related compounds. Herein, we present a comprehensive study on the effects of cobalt doping on the structural, electronic, and magnetic properties of BaYFe_1−xCo_xO₄ polycrystals (x = 0–0.15), using a combination of X-ray and neutron powder diffraction, Raman spectroscopy, X-ray photoemission spectroscopy, and magnetic measurements. The initial orthorhombic Pnma crystal structure remains stable upon Co doping, though local structural changes arise due to the complex spin states of Co³⁺ ions. These ions adopt low-spin and high-spin configurations depending on their coordination, respectively, while Fe³⁺ ions retain a high-spin state. All samples exhibit an incommensurate collinear spin-density wave antiferromagnetic order at the Néel temperature T_N1, which, evolves into a cycloidal antiferromagnetic order at a lower T_N2. These long-range magnetic orders are gradually suppressed with increasing Co content. Additionally, the onset temperature of short-range magnetic correlations—emerging well above T_N1—decreases with increased doping. These findings highlight the significant influence of Co doping on both intrachain and interchain magnetic interactions, governing the overall magnetic behavior of BaYFe_1−xCo_xO₄. The underlying mechanisms are discussed in detail.

Superplasticizers play dual regulatory roles during early Portland cement hydration they retard hydration kinetics while enhancing paste fluidity. However, research gaps remain in quantitatively modeling the retardation behavior of superplasticizers and in establishing mechanistic linkages between the time-dependent rheological effects in cementitious systems and the corresponding hydration processes under superplasticizer incorporation. In this study, a theoretical model was developed to predict the performance of polycarboxylate superplasticizers (PCEs) in Portland cement hydration. On the basis of obstruction theory and complexation reaction kinetics, the effects of adsorption and complexation were simulated to examine the retarding performance of cement hydration with PCE addition. Using rheological principles, mechanistic linkages between rheological evolution and hydration progression were established. Calorimetry and rheological experiments were conducted on Portland cement systems with varying PCE dosages. The experimentally measured hydration heat flow and yield stress data showed excellent agreement with the model predictions: the error in predicting the time to peak hydration heat release was within 10.46%; the overall error of the hydration heat release curve was within 17.31%; and the overall error of the yield stress curve was within 6.13%. This model not only provides theoretical guidance for admixture formulation based on engineering application requirements but also establishes a fundamental theoretical basis for future research on admixtures.

Dislocations in perovskite oxides have drawn increasing research interest due to their potential for tuning functional properties of electroceramics. Open questions remain regarding the stability of dislocations under strong externally applied electric fields. In this study, we investigate the dielectric breakdown strength of nominally undoped SrTiO₃ crystals after the introduction of high-density dislocations. The dislocation-rich samples are prepared using the Brinell scratching method, and they consistently exhibit lower dielectric breakdown strength as well as a larger scatter in the breakdown probability. We also study the impact of an electric field on the introduction and movement of dislocations in SrTiO₃ crystals using Brinell indentation coupled with an electric field of 2 kV/mm. No visible changes in the dislocation plastic zone size, depth, and dislocation distribution are observed under this electric field. Based on the charge state of the dislocations in SrTiO₃ as well as the electrical and thermal conductivity modified by dislocations, we discuss the forces induced by the electric field to act on the dislocations to underline the possible mechanisms for such dislocation behavior.

In alkali-free aluminosilicate glasses, alkaline earth metal cations serve either as charge compensators for [AlO₄]⁻ units or as network modifiers by disrupting the silicon‒oxygen network to generate non-bridging oxygens, and/or by facilitating the transformation from [BO₃] to [BO₄]. The preference of different alkaline earth cations for either charge compensation or network modification is influenced by their ionic sizes and charge density, also known as cation field strength. This study investigates how variations in the mean modifier cation size (MCS) affect the structural and property changes in boro-aluminosilicate glasses. Using Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR analysis of ²⁹Si, ²⁷Al, and ¹¹B, together with Raman spectroscopy and X-ray photo-electron spectroscopy measurements (Al_2p and B_1s), we examine structural changes related to MCS and compare well-annealed glass samples with those rapidly quenched in water. Our findings reveal a negative correlation between the Q⁴ species in the silicon group and the [BO₄] fraction in the boron group as a function of MCS. [AlO₄] species dominates in local structure of Al, with minor [AlO₅] species found in samples of smaller cation, especially for magnesium containing one. Smaller cation, especially for Mg²⁺ ion, acts less as a compensator for [AlO₄] and [BO₄]⁻, and less as a modifier to Si group than larger cations, and it leads to more disturbed [AlO₄] units and a pronounced non-uniform structure. Mechanical and thermodynamic properties of glass show a decreasing trend with increasing MCS. Viscosity measurements are employed to explore how liquid fragility varies with the MCS. However, a higher fragility index, indicating a more “fragile” liquid is found for glasses with smaller MCS. The contrasting behavior between bulk glass and molten glass may be attributed to differences in effective network connectivity.

The core-shell structured nanopowders of Ce_0.9Y_0.1O_2-δ@xBaCe_0.9Y_0.1O_3-δ (YDC@xBCY, x = 0.6, 0.8, 1, 1.2, and 1.4) were synthesized via a co-precipitation method and then sintered in air atmosphere at 1600^°C to obtain dense ceramic pellets. XRD, SEM, and EDS analyses confirm that the YDC@BCY core-shell structure remains in bulk ceramics after sintering. Among all samples, YDC@1BCY exhibits the highest total conductivity in air (1.40 × 10⁻⁴ to 1.05 × 10⁻² S/cm in 350°C–550°C). In a 10% H₂-90% N₂ atmosphere, the proton conductivity of YDC@xBCY peaks at x = 0.8. With the aid of the core-shell structure design, YDC/BCY heterogeneous interface networks are constructed, and the interconnected high conductive YDC/BCY heterogeneous interface networks facilitate ionic transport within the sample. The core-shell design offers a promising strategy for improving the ionic conductivity of doped CeO₂–BaCeO₃-based composite ceramics.