Journal of the American Ceramic Society最新文献

To meet the needs for higher energy efficiency and a wide operating temperature range of electric vehicles, the low-loss MnZn ferrites in a wide temperature range have been developed by optimizing the Fe content and the oxygen partial pressure (P_O2) during the sintering process in this work. For the optimal sample, power loss at 300 kHz/100mT is 204 kW/m³ at 25°C and remains below 290 kW/m³ in the wide temperature range from -10 to 120°C. The loss separation method was employed to clarify the effects of the Fe content and P_O2 on power loss. The equivalent circuit model has been employed to fit the complex impedance and it is found that the increase of P_O2 enhances both the grain resistance R_g and the grain boundary resistance R_gb. The enhancement of R_gb is mainly responsible for the reduction of eddy current loss and consequently power loss. Dielectric permittivity is as large as about 15000 in this series of samples due to the electric polarization at the rich grain boundaries. Dielectric loss is very low between -50 and 150°C and has little contribution to the energy loss.

The crack initiation load of freshly fractured surfaces for silica glass was evaluated with ball indentation. The fracture surfaces were formed during subcritical crack growth in the regions I, II, and III of the stress intensity factor (K_I)—crack velocity (V) curve. From the K_I–V curve, we linked the obtained fracture surfaces with the ones in the regions I, II, and III. It was found that the crack-forming probability was the lowest for the fracture surface formed in the region III of the K_I–V curve. In order to understand the controlling factors of the crack formation, some properties which are topography, relative nonbridging oxygens (NBO), hydrogen concentrations, and Si–O three- or four-membered ring structures, of the fracture surfaces were measured by atomic force microscopy, X-ray photoelectron spectroscopy, dynamic secondary ion mass spectroscopy, and Raman spectroscopy, respectively. No distinct difference in NBO and hydrogen concentrations nor the ring structures were found among the fracture surfaces formed in different regions in the K_I–V curve. The peak-to-valley height of the fracture surface, however, decreased with increasing crack velocity. It is concluded that the roughness or topography of the freshly fractured surface is one of the controlling factors which reduce the intrinsic strength of silica glass.

MgO–C refractories are of paramount importance in the converter side blowing system, requiring outstanding oxidation resistance under harsh conditions including high temperature, oxygen atmosphere, and high-speed airflow. In this study, MgSiN₂ phase reconstruction was used to improve the oxidation resistance of MgO–C refractories, as well as the mechanical properties and oxidation resistance of MgO–C refractories were evaluated. The results indicated that the cold modulus of rupture of the sample with 9 wt% MgSiN₂ was increased by 93.3% compared with the MgO–C refractories without MgSiN₂. After oxidation tests, the oxidation index and rate constant (k) of the sample with 9 wt% MgSiN₂ were reduced by 38.9% and 35.3%. Furthermore, incorporating MgSiN₂ facilitated the formation of layered dense structures consisting of plate-like Mg-Sialon and MgO–Mg₂SiO₄–MgAl₂O₄. This structural optimization effectively inhibited oxygen diffusion and reaction within the material, resulting in gradual oxygen potential mitigation.

Andalusite-based refractories are characterized by superior thermal shock resistance. Although the demand for alternative raw materials is increasing, it is proving difficult to replicate the thermal shock behavior of andalusite with alternative materials. The physico-mechanical properties of andalusite refractories are well-studied. However, comprehensive chemical and microstructural analysis is still needed to fully understand factors influencing the thermal shock resistance. This investigation highlights the use of complementary microstructural techniques (3D digital microscopy, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and micro-X-ray fluorescence) to analyze crack propagation as well as glass bridge formation and composition in aluminosilicate-based refractory castables. Andalusite-based refractories are compared with chamotte 45 and chamotte 60, before and after a thermal shock, as well as after an additional firing. Factors influencing the thermal shock resistance of aluminosilicate refractory castables include (1) crack shape, size, occurrence, and propagation (2) glass bridge formation, size, occurrence, and composition, and (3) aggregate-matrix interactions.

Equal-period modulated metal/ceramic multilayers have shown promise in enhancing the toughness of ceramic thin films. However, this toughness enhancement typically comes at the sacrifice of hardness, limiting their potential applications. To tackle this issue, this study designed and fabricated two gradient-structured multilayer variations using Ta/TaB₂: one with a higher ceramic layer fraction near the surface (M2) and the other with a converse structure (M3). A conventional equal modulation period Ta/TaB₂ multilayer film (M1) served as a reference. M2 exhibited superior performance, with a 30% hardness increase and significant toughness enhancement compared to M1. Conversely, M3 experienced failure due to excessive thermal stress from its unique gradient structure. Finite element simulations revealed that M2's structure could alleviate in-plane stress and enhance loading uniformity, thus enhancing the film's toughness. These findings suggest that a well-designed gradient structure holds promise for concurrently improving the hardness and toughness of metal/ceramic multilayer films.

The viscosities of Ce-free and Ce-bearing (∼1.3 mol%, ∼6.5 wt.% Ce₂O₃) soda lime silicate (window glass) melts were measured with respect to oxidation state. Experiments were performed isothermally using a concentric-cylinder viscometer on melts equilibrated with successively reducing CO–CO₂ gas mixtures within a gas tight vertical tube furnace at 1 atm. Viscosity measurement and sampling were performed at the end of each melt reduction step. Further, viscosities in the glass transition temperature range were estimated using the shift factor method applied to glass transition temperature values determined using differential scanning calorimetry (DSC) measurements on quenched glasses. The Ce speciation at each stepwise melt reduction was probed using Ce L₃-edge X-ray absorption near-edge structure (XANES) spectroscopy, while structural information upon Ce addition and reduction was provided by Raman spectroscopy. The viscosities of these materials remain constant at this level of Ce addition and do not vary significantly with redox state at high temperature. Conversely, viscosity values in the glass transition temperature range increase upon both Ce addition and reduction. Our analysis, based on the glass composition analyses obtained via electron probe microanalyzer (EPMA), viscosity calculations, and observations of silicate structural changes, leads to the conclusion that the observed viscosity increase around the glass transition temperature is explained by the high ionic field strength of Ce ions as well as the polymerization behavior of the silicate matrix occurring during reduction of Ce. Because of its low concentration, resulting from its low solubility, Ce redox changes exert only minimal effects on the viscosity of this melt.

Oxide particles are considered to be one of the reasons for the stability of cellular porous Al foam. Thus, the contact angle (ө) of molten pure aluminum on oxide substrates, including Al₂O₃, CaO·2Al₂O₃ (CA2), CaO·6Al₂O₃ (CA6), mullite (3Al₂O₃·2SiO₂), and SiO₂ was measured in a vacuum heating microscope at a temperature of 950°C to determine the value of wettability of molten aluminum on various oxides. Thereafter, the measured contact angles followed the tendency as mullite (155°) > CA6 (152°) > MgAl₂O₄ (151°) > Al₂O₃ (149°) > SiO₂ (124°) > CA2 (100°). Furthermore, the cross-sectional area between molten aluminum and substrates was thoroughly analyzed, and the reaction products, such as the Al₂O₃ layer at the interface, have been detected. Thermo-Calc, with the combination of available databases, was used to perform thermodynamic simulations and to discuss and predict possible reactions under the current experimental conditions.

A simple solution casting technique was used to formulate films of polyvinylidenefluoride-co-hexafluoropropylene, polymethylmethacrylate, and Ti₃AlC₂ MAX phase polymer nanocomposites (PPT) with different filler content. The functional groups present in the PPT films were studied using the Fourier transform infrared spectroscopy and the nature of crystallinity was observed using X-ray diffraction technique. The morphology of the films was studied by scanning electron microscopy. The thermal analysis was done using the thermogravimetric analysis and differential thermogravimetry analysis. The tensile behavior of the prepared films was studied from the stress-strain graphs. The ultraviolet–visible spectra revealed their optical characteristics. The dielectric parameters of the polymer blend and nanocomposite films like dielectric constant, loss tangent, AC conductivity, and complex impedance spectroscopy with corresponding equivalent circuits were studied. A comparative study of the above parameters was done for both the polymer blend and nanocomposites to find out the advancement in the properties of the prepared nanocomposite compared with the polymer blend. The nanocomposite film having 3 wt% of Ti₃AlC₂ MAX phase filler loading exhibited the best properties among all the nanocomposites. The enhanced dielectric properties of the nanocomposites with mechanical and thermal resistance can be used for real-time efficient energy storage applications.

In this paper, macroporous high-entropy spinel oxide (HESO) (Fe_0.2Ni_0.2Co_0.2Mn_0.2Zn_0.2)₃O₄ monoliths were successfully fabricated via a sol–gel method followed by calcination. Appropriate polyacrylic acid and propylene oxide contents allow the formation of three-dimensional co-continuous xerogel monoliths, and the water/glycerol ratio controls the macropore size of monoliths. Subsequent calcination achieves the precipitation of HESO (Fe_0.2Ni_0.2Co_0.2Mn_0.2Zn_0.2)₃O₄ with a singular phase and exceptional structural stability. The macroporous HESO (Fe_0.2Ni_0.2Co_0.2Mn_0.2Zn_0.2)₃O₄ demonstrates remarkable performance in the oxygen evolution reaction (OER) with an overpotential of 333 mV at 100 mA cm⁻² and a Tafel slope of 43.2 mV dec^–1, surpassing that of RuO₂ (391 mV) under identical conditions. Furthermore, the catalytic stability of the HESO catalyst remains superior even after 24 h of testing. This process offers a promising avenue for the development of macroporous high-entropy oxide OER catalysts for overall water splitting.