Showing papers in "Journal of the American Ceramic Society in 2019"

First-principles study, fabrication, and characterization of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramic

Abstract: The formation possibility of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramic (HHC‐1) was first analyzed by the first‐principles calculations, and then, it was successfully fabricated by hot‐pressing sintering technique at 2073 K under a pressure of 30 MPa. The first‐principles calculation results showed that the mixing enthalpy and mixing entropy of HHC‐1 were −0.869 ± 0.290 kJ/mol and 0.805R, respectively. The experimental results showed that the as‐prepared HHC‐1 not only had an interesting single rock‐salt crystal structure of metal carbides but also possessed high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it exhibited extremely high nanohardness of 40.6 ± 0.6 GPa and elastic modulus in the range from 514 ± 10 to 522 ± 10 GPa and relatively high electrical resistivity of 91 ± 1.3 μΩ·cm, which could be due to the presence of solid solution effects.

Modification of NdNbO4 microwave dielectric ceramic by Bi substitutions

Abstract: Funding information Key Basic Research Program of Shaanxi Province, Grant/Award Number: 2017GY129; Fundamental Research Funds for the Central University; National Key Research and Development Program of China, Grant/Award Number: 2017YFB0406301 Abstract In the present work, Bi was used to substitute for Nd in the NdNbO4 ceramic and pure fergusonite solid solution was formed within 20 mol. % substitutions. Microwave dielectric permittivity of the (Nd1-xBix)NbO4 (x ≤ 0.2) ceramics increased linearly with x value due to the larger ionic polarizability of Bi than Nd. Excellent microwave dielectric properties with a permittivity (εr) ~22.5, a Qf (Q = quality factor, f = resonant frequency) ~50 000 GHz, and a TCF ~−9 ppm/°C were obtained in the (Nd0.9Bi0.1)NbO4 ceramic. This method might work in other fergusonite‐type rare‐earth ortho‐niobates.

Charge compensation mechanisms in Li-substituted high-entropy oxides and influence on Li superionic conductivity

Abstract: The charge compensation mechanisms that occurs when Li + substitutes a 2+ element in superionic conductor (MgCoNiCuZn)O high entropy oxide have been studied using a combination of thermogravimetric analysis and X-ray photoemission spectroscopy. Depending on the concentration of Li + in the compound, the charge compensation involves first partial oxidation of Co 2+ into Co 3+ for Accepted Article This article is protected by copyright. All rights reserved. low fraction of Li + , and then a combination of both the oxidation of cobalt and the formation of oxygen vacancies for large fraction of Li + .

Autogenous and drying shrinkage of alkali-activated slag mortars

Abstract: Shrinkage of alkali‐activated slag (AAS) cement is a critical issue for its industrial application. This study investigated the mechanisms and effectiveness of shrinkage‐reducing agent (SRA) and magnesia expansive agent on reducing autogenous and drying shrinkage of AAS mortars that were activated by liquid sodium silicate (LSS) solution with modulus (SiO2/Na2O molar ratio) of 0‐1.5. The results showed that the autogenous shrinkage of AAS mortars increased with the increase of LSS modulus from 0 to 0.5, then decreased as modulus increased up to 1.5. The drying shrinkage consistently increased with the increase in the modulus of LSS. The oxyalkylene alcohol‐based SRA could significantly reduce the autogenous and drying shrinkage of AAS mortars while the magnesia expensive agent was comparatively less effective. The autogenous shrinkage of AAS mortars was inversely proportional to the internal relative humidity, while the drying shrinkage was more related to the mass loss of samples. Mathematical models were established to describe the autogenous and drying shrinkage behavior of AAS mortars.

Hydration mechanisms of hybrid cements as a function of the way of addition of chemicals

Abstract: This research was funded by the Spanish Ministry of the Economy and Competitiveness and FEDER under research projects BIA2016‐76466‐R and RTC2016‐4872‐S.