Showing papers in "Journal of the American Ceramic Society in 2018"

(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics with low thermal conductivity

Abstract: A novel high‐entropy carbide ceramic, (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C, with a single‐phase rock salt structure, was synthesized by spark plasma sintering. X‐ray diffraction confirmed the formation of a single‐phase rock salt structure at 26‐1140°C in Argon atmosphere, in which the 5 metal elements may share a cation position while the C element occupies the anion position. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C exhibits a much lower thermal diffusivity and conductivity than the binary carbides HfC, ZrC, TaC, and TiC, which may result from the significant phonon scattering at its distorted anion sublattice. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C inherits the high elastic modulus and hardness of the binary carbide ceramics.

Silicon oxycarbide glasses and glass-ceramics: “All-Rounder” materials for advanced structural and functional applications

Abstract: Silicon oxycarbides can be considered as being carbon‐containing silicates consisting of glass networks in which oxygen and carbon share bonds with silicon. The carbon‐for‐oxygen substitution in silicate glass networks has been shown to induce significant changes in the network connectivity and consequently strong improvements in the properties of the silicate glass network. For instance, SiOC glasses exhibit Young's moduli, hardness values, glass transition, and crystallization temperatures which are superior to those of vitreous silica. Moreover, the silicon oxycarbide glass network exhibits unique structural features such as reduced mass fractal dimension and nano‐heterogeneity, which significantly affect and/or dictate its properties and behavior. In the present Review, a consideration of the current state of the art concerning the synthesis, processing, and various structural and functional properties of silicon‐oxycarbide‐based glasses and glass‐ceramics is done. Thus, the synthesis of silicon oxycarbides starting from macromolecular precursors such as polysiloxanes or alkoxysilanes‐based sol‐gel systems as well as current advances related to their processing will be critically reviewed. In addition, various structural and functional properties of silicon oxycarbides are presented. Specific emphasis will be put on the intimate correlation between the molecular architecture of the precursors and the structural features and properties of the resulting silicon oxycarbides.

High strain (0.4%) Bi(Mg2/3Nb1/3)O3-BaTiO3-BiFeO3 lead-free piezoelectric ceramics and multilayers

Abstract: The relationship between the piezoelectric properties and the structure/microstructure for 0.05Bi(Mg2/3Nb1/3)O3-(0.95-x)BaTiO3-xBiFeO3 (BBFT, x = 0.55, 0.60, 0.63, 0.65, 0.70, and 0.75) ceramics has been investigated. Scanning electron microscopy revealed a homogeneous microstructure for x < 0.75 but there was evidence of a core-shell cation distribution for x = 0.75 which could be suppressed in part through quenching from the sintering temperature. X-ray diffraction (XRD) suggested a gradual structural transition from pseudocubic to rhombohedral for 0.63

Synthesis and thermophysical properties of RETa3O9 (RE = Ce, Nd, Sm, Eu, Gd, Dy, Er) as promising thermal barrier coatings

Abstract: Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa3O9 compounds have a defect-perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa3O9 bulk ceramics were fabricated via solid-state reaction. The crystal structure was characterized by X-ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermal physics properties of RETa3O9 were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients and high-temperature phase stability. The thermal conductivities of RETa3O9 are very low (1.33-2.37 W/m.K, 373-1073 K), which are much lower than YSZ and La2Zr2O7; and the thermal expansion coefficients range from 4.0×10-6 K−1 to 10.2×10-6 K−1 (1273 K), which is close to La2Zr2O7 and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high-temperature phase stability with these oxides, RETa3O9 ceramics were promising candidate materials for TBCs. This article is protected by copyright. All rights reserved.

Deep-red photoluminescence and long persistent luminescence in double perovstkite-type La2MgGeO6:Mn4+

Abstract: A novel non-rare-earth doped phosphor La2MgGeO6:Mn4+ (LMG:Mn4+) with near-infrared (NIR) long persistent luminescence (LPL) was successfully synthesized by solid-state reaction. The phosphors can be effectively excited using ultraviolet light, followed by a sharp deep-red emission peaking at 708 nm, which is originated from 2Eg → 4A2g transition of Mn4+ ions. The luminescent performance was analyzed by photoluminescence (PL) and photoluminescence excitation (PLE) spectra. The crystal field parameters were calculated to describe the environment of Mn4+ in LMG host. The LPL behaviors as well as the mechanisms were systematically discussed. This study suggests that the phosphors will broaden new horizons in designing and fabricating novel NIR long phosphorescent materials.

Colossal permittivity in TiO2 co‐doped by donor Nb and isovalent Zr

Abstract: Effect of isovalent Zr dopant on the colossal permittivity (CP) properties was investigated in (Zr+Nb) co-doped rutile TiO2 ceramics, i.e., Nb0.5%ZrxTi1-xO2. Compared with those of only Nb-doped TiO2, the CP properties of co-doped samples showed better frequency-stability with a lower dielectric loss. Especially, a CP up to 6.4 × 104 and a relatively low dielectric loss (0.029) of x = 2% sample were measured at 1 kHz and room temperature. Moreover, both dielectric permittivity and loss were nearly independent of direct current bias, and measuring temperature from room temperature to around 100 °C. Based on the XPS, the formation of oxygen vacancies was suppressed due to the incorporation of Zr. Furthermore, it induced the enhancement of the conduction activation energy according to the impedance spectroscopy. The results will provide a new routine to achieve a low dielectric loss in the CP materials. This article is protected by copyright. All rights reserved.

One-pot combustion synthesis and efficient broad spectrum photoactivity of Bi/BiOBr:Yb,Er/C photocatalyst

Abstract: A highly efficient broad spectrum responsive Bi/BiOBr:Yb,Er/C ternary composite was synthesized by a simple one-pot combustion method using nitrates and citric acid as raw materials. Experimental results show that Er3+/Yb3+ were successfully doped into BiOBr lattice, and metallic Bi nanoparticles and carbon species were formed simultaneously. Compared with pure BiOBr and Bi/BiOBr/C, as-synthesized Bi/BiOBr:Yb,Er/C ternary photocatalyst is highly responsive in the UV-visible-NIR range, and possesses the best photodegradation performance for Rhodamine B, phenol, and imidacloprid under visible, NIR, or solar light irradiation, which can be attributed to the synergetic effects of surface plasma resonance of metallic Bi, up-conversion transition of Er3+ and heterojunctions (Bi/BiOBr, Bi/C, and BiOBr/C). Moreover, a plausible mechanism was given, the main active species, and photostability of samples were studied. The solar light photoactivity and influences of pH value and anions (Cl−, SO42−, CO32−, HCO3−, and NO3−) were also investigated. This study highlights the advantages of synergetic effects of SPR, up-conversion and heterojunctions, which provides a useful guide toward the rational design of broad spectrum (UV-visible-near infrared) photocatalysts.

Effects of calcium on setting mechanism of metakaolin-based geopolymer

Abstract: Geopolymer setting is seen to be substantially accelerated by addition of calcium and the objective of this study was to determine the mechanism for this effect by examining metakaolin geopolymers with and without calcium Solid-state 27Al NMR tests were used to examine the dissolution extent both qualitatively and quantitatively Solid-state 29Si NMR tests were conducted to determine the amount and structure of each phase Prior to the quantitative tests, chemical extractions were used to facilitate assignment of peaks in each spectrum On addition of calcium, it was found that both the rate and the extent of metakaolin dissolution were enhanced Accelerating dissolution increases the Al concentration in solution, thus reducing Si/Al available for geopolymer gel formation and further accelerating the gel formation to cause faster setting Although C-A-S-H was observed in the calcium mix, no evidence indicated that it is directly involved in setting This article is protected by copyright All rights reserved

Controllable τf value of barium silicate microwave dielectric ceramics with different Ba/Si ratios

Abstract: Ba1+1/M[Si2O5+1/M] low-permittivity microwave dielectric ceramics are prepared using the conventional solid-state method. Pure phases are obtained in barium silicates with M=1, 3, 4, 5, and ∞, except for M=7, in which two phases, Ba5Si8O21 and BaSi2O5, are observed. As the complexity of the crystal structure described by the Shannon information per reduced unit cell increases, the τf value tends to change from a negative to a positive value, except for M=5, which has the highest complexity. A single Ba5Si8O21 phase with er anomaly peak at −180 °C exhibits a rare positive τf value (+25 ppm/°C), which is a novel temperature compensator. This article is protected by copyright. All rights reserved.

Liquid‐phase sintering of highly Na+ ion conducting Na3Zr2Si2PO12 ceramics using Na3BO3 additive

Abstract: Na3Zr2Si2PO12 (NASICON) is a promising material as a solid electrolyte for all-solid-state sodium batteries. Nevertheless, one challenge for the application of NASICON in batteries is their high sintering temperature above 1200°C, which can lead to volatilization of light elements and undesirable side reactions with electrode materials at such high temperatures. In this study, liquid-phase sintering of NASICON with a Na3BO3 (NBO) additive was performed for the first time to lower the NASICON sintering temperature. A dense NASICON-based ceramic was successfully obtained by sintering at 900°C with 4.8 wt% NBO. This liquid-phase sintered NASICON ceramic exhibited high total conductivity of ~1 × 10−3 S cm−1 at room temperature and low conduction activation energy of 28 kJ mol−1. Since the room-temperature conductivity is identical to that of conventional high-temperature-sintered NASICON, NBO was demonstrated as a good liquid-phase sintering additive for NASICON solid electrolyte. In the NASICON with 4.8 wt% NBO ceramic, most of the NASICON grains directly bonded with each other and some submicron sodium borates segregated in particulate form without full penetration to NASICON grain boundaries. This characteristic composite microstructure contributed to the high conductivity of the liquid-phase sintered NASICON.

Polymer‐derived SiCN cellular structures from replica of 3D printed lattices

Abstract: In comparison with metals and polymers, ceramics and/or carbon are more difficult to process into well‐defined cellular architectures (e.g., cubic, tetrakaidecadehron, etc.) using Additive Manufacturing techniques. The present work reports a simple method for generating complex and precise SiCN ceramic lattices using a preceramic polymer and applying the replica approach to structures fabricated using stereolithography of plastic materials, with the associated ease of fabrication. Three‐dimensional printed plastic lattices impregnated with a polysilazane were converted to SiCN by pyrolysis at 1000°C in inert atmosphere. In spite of the high amount of mass loss (~58%) and volume shrinkage (~65%), the impregnated structures did not collapse during pyrolysis, leading to highly porous (total porosity ~93 vol%) components possessing suitable strength for handling and potential use as lightweight components.

Ionic occupation, structures, and microwave dielectric properties of Y3MgAl3SiO12 garnet‐type ceramics

Abstract: The Y3MgAl3SiO12 ceramics with pure phase were successfully synthesized by solid state sintering reaction method for the first time. Their microwave dielectric properties were investigated as a function of sintering temperature. Their microstructure characteristics and ionic occupation sites of tetrahedral and octahedral units were characterized and analyzed by SEM&EDS and Rietveld refinement of X-ray powder diffraction data. Crystal structure of Y3MgAl3SiO12 is isostructural to Y3Al5O12 with a cubic garnet structure and space group of Ia-3d, which contains YO8 dodecahedra, (Mg/Aloct)O6 octahedral and (Si/Altet)O4 tetrahedral units. The Qf and er values of different samples are strongly dependent on the distribution of grain sizes, grain sizes, and porosity. The samples sintered at 1550°C exhibit optimized microwave dielectric properties with relative permittivity (er) of 10.1, Qf values of 57,340GHz (at 9.5 GHz), and τf values of -32 ppm/°C. Such properties indicate potential application of Y3MgAl3SiO12 as microwave substrates. This article is protected by copyright. All rights reserved.

Modeling environmentally induced property degradation of SiC/BN/SiC ceramic matrix composites

Abstract: The degradation of SiC-based ceramic matrix composites (CMCs) in conditions typical of gas turbine engine operation proceeds via the stress-rupture of fiber bundles. The degradation is accelerated when oxygen and water invade the composite through matrix microcracks and react with fiber coatings and the fibers themselves. We review micromechanical models of the main rate-determining phenomena involved, including the the diffusion of gases and reaction products through matrix microcracks, oxidation of SiC (in both matrix and fibers) leading to the loss of stiffness and strength in exposed fibers, the formation of oxide scale on SiC fiber and along matrix crack surfaces that cause the partial closure of microcracks, and the concomitant and synergistic loss of BN fiber coatings. The micromechanical models could be formulated as time-dependent coupled differential equations in time, which must be solved dynamically, e.g., as an iterated user-defined material element, within a finite element simulation. A paradigm is thus established for incorporating the time-dependent evolution of local material properties according to the local environmental and stress conditions that exist within a material, in a simulation of the damage evolution of a composite component. We exemplify the calibration of typical micromechanical degradation models using thermodynamic data for the oxidation and/or volatilization of BN and SiC by oxygen and water, mechanical test data for the rate of stress-rupture of SiC fibers, and kinetic data for the processes involved in gas permeation through microcracks. We discuss approaches for validating computational simulations that include the micromechanical models of environmental degradation. A special challenge is achieving validated predictions of trends with temperature, which are expected to vary in a complex manner during use.. This article is protected by copyright. All rights reserved.