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The challenge of producing dense fine-grained high-entropy ceramics is hereby addressed by a combined cold and conventional pressureless sintering method for the first time. It provides nearly fully dense high-entropy pyrochlores with better microstructural homogeneity, and only half the grain size of conventional pressurelessly sintered samples with identical density. Both the finer and more homogeneous microstructures benefit better mechanical properties in hardness and reduced Young's modulus. The kinetics and activation energies of both densification and grain growth were quantified and discussed to understand how cold sintering affects the microstructural evolution during the following pressureless sintering. More importantly, cathodoluminescence analysis suggested that cold sintering could cause more defects on the surface of the original particles, which could promote the grain boundary diffusion and have drag effects on grain boundary motion. The lessons learned here offer scientific understanding and technological guidance towards pressureless sintering of dense bulk high-entropy ceramics. [ABSTRACT FROM AUTHOR]

The challenge of preparing dense ultrafine-grained high-entropy ceramics through conventional pressureless sintering is hereby addressed by a simple two-step sintering method. Ultrafine-grained (Ce 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2) 2 Zr 2 O 7 high-entropy zirconate with 99.2 % theoretical density and 135 nm grain size has been fabricated by pressureless two-step sintering for the first time. Compared to the conventional sintering, two-step sintering provides finer grain sizes and better microstructural uniformity, which yield excellent comprehensive mechanical properties with high hardness of 16.0 GPa, large elastic modulus of 262 GPa and high fracture toughness of 2.8 MPa·m1/2. It is believe that pressureless two-step sintering can be utilized to prepare other high-entropy ceramics with fine grain size and high quality. [ABSTRACT FROM AUTHOR]

Core-rim structure has distinct advantage to improve the performances of KNN-based piezoceramics. Whereas the state of the core-rim structure is difficult to control during sintering. Here, the core-rim structured 0.96(K 0.51 Na 0.47 Li 0.02)(Nb 0.8 Ta 0.2)O 3 -0.04CaZrO 3 (KNLNT) ceramics were both obtained by conventional sintering (CS) and rapid sintering (RS). KNLNT ceramics prepared by rapid sintering exhibit more outstanding controllability on grain growth and core-rim structure and can hold the core/rim size ratio in a stable and favorable level due to extended effective range of grain boundary diffusion in densification. Benefiting from the controllable microstructure, the RS method prepared samples show excellent performance. The unipolar strain value of RS-1240 (S max =0.252%) is 2.07 times as much as CS-1110 (S max =0.122%). Large strain, low hysteresis and low dielectric permittivity features make the core-rim structured KNLNT ceramics a potential material for pulse drive applications and demonstrate that the manual precise control of core-rim structure could create many possibilities on materials design. [Display omitted] [ABSTRACT FROM AUTHOR]

Skorohod and Olevsky's constitutive relation for viscous sintering (SOVS) is employed to simulate the shrinkage and relative density evolution of ceramics during the sintering process. While this model effectively simulates shrinkage and density evolution, the original formulation of the SOVS model does not explicitly incorporate grain growth or microstructural evolution. This paper introduces an enhanced version of the SOVS model, incorporating a novel calibration method to obtain the temperature dependence of the viscosity parameter. It considers grain growth as well as the evolution of the microstructure by adding new parameters. Simulations were conducted using Code_Aster, an open-source finite element software, in conjunction with MFront, a tool for formulating material behaviour equations. Dilatometer tests were performed with probes of porcelain stoneware composition to calibrate the model. The predictions of dimensional shrinkage and relative density evolution showed a high accuracy of the model (> 95%), enabling its use in the manufacturing process. [ABSTRACT FROM AUTHOR]

Bi 1.6 Pb 0.4 Sr 2 Co 2 O 8 samples have been textured by the Laser Floating Zone (LFZ) process using Nd:YAG, and CO 2 laser radiation. Using different wavelengths resulted in significant structural and microstructural modifications. Powder XRD patterns showed that the thermoelectric phase is the major one in both cases. Microstructural studies revealed that all samples presented the same phases but with much lower content of secondary ones in those processed with the CO 2 laser. Electrical resistivity showed different behavior for the two types of samples, being in general, lower for the CO 2 grown rods. Seebeck coefficient is lower for the CO 2 grown samples up to 300 °C, and higher in the high-temperature range, reaching 240 μV/K at 650 °C, which is one of the highest values obtained so far in these compounds. Moreover, thermal conductivity at 600 °C for these samples (0.93 W/K m) is among the lowest reported in the literature. As a consequence, ZT values at 600 °C reached 0.42 in CO 2 textured materials, about two times higher than the obtained in Nd:YAG ones. This value is among the highest reported so far in the literature, and is comparable to the performance attained for the same composition containing nanoparticles addition. All these properties, combined with the fact that the processed materials can be directly integrated into thermoelectric modules, render them highly attractive for industrial production. [ABSTRACT FROM AUTHOR]

Preparation of high-entropy ceramics (HECs) requires long duration at high temperatures, which limits their wide application. In this study, high-entropy (La 0.2 Ce 0.2 Nd 0.2 Sm 0.2 Eu 0.2) 2 Zr 2 O 7 ceramics was prepared at low temperatures of 762–885 °C in very short time of 60 s using the flash sintering. The effects of electric field strength and current limit on the densification and grain growth, as well as the phase transformation were systematically investigated. Both density and grain size were increased with the increase of the electric field strength and current limit. The phase transformation from defective fluorite to pyrochlore occurred at large current limit. The flash-sintered bulks exhibited good mechanical properties. This study demonstrates that flash sintering is an effective method for rapid preparation of HECs at low temperatures. Moreover, the microstructure of HECs can be controlled by adjusting the electric field strength and current limit. [ABSTRACT FROM AUTHOR]

The Th 1- x Sc x O 2- x /2 (0 ≤ x ≤ 0.1) ceramic system with high density was originally prepared via a co-precipitation route and simple sintering method. XRD analysis indicates that the Th 1- x Sc x O 2- x /2 ceramics exhibited a single cubic fluorite structure and a slight increase of cell parameters. SEM images show that Sc doping contributed a decreasing pores and more uniform grain distribution. The calculated relative densities could reach up to a value higher than 99.0% for x = 0.01 during the isothermal process while the maximum actual relative density was only about 77.4% during the non-isothermal sintering process. And the analysis of grain growth kinetics demonstrates that Th 1- x Sc x O 2- x /2 was a solid solution with a high solubility of Sc for x = 0.01 and a low solubility of Sc for both x = 0.05 and x = 0.1, which resulted in a strongly slow grain growth for x = 0.1 due to the solute drag effect. The sintering mechanism investigation also illustrates that low amount (x = 0.01) of Sc doping could significantly enhance the densification sintering of ThO 2 -based ceramics and reduce the sintering temperature by nearly 150 °C. Such strategy has issued the bottleneck problem existed in the densification sintering of ThO 2 -based ceramics at a lower temperature without any other special sintering atmosphere, which even could be applicable for other ceramics difficult to sinter. [ABSTRACT FROM AUTHOR]

We investigated the effect of very high pressure on the sintering temperature, phase transition and the grain growth during Spark Plasma Sintering (SPS) of a 15 nm TiO 2 nanopowder. Using in situ synchrotron X-ray diffraction during sintering at 1.5 and 3.5 GPa, we followed the evolution of the crystalline phases and the crystallite size as a function of temperature. In comparison, in the laboratory, SPS experiments were performed on two original facilities: A Paris-Edinburgh press and a high-pressure module adapted to standard SPS equipment. We studied the effect of the pressure on the sintering in the range 76 MPa to 3.5 GPa. We have shown that highly dense nanostructured ceramics can be prepared under very high pressure at low sintering temperatures. At 1 GPa, we limited the grain growth to an average size of 233 nm by heating at only 560 °C, and achieved a relative density of 98 %. [ABSTRACT FROM AUTHOR]

Preparation of Gd 2 Zr 2 O 7 nanoceramics using a simple and low energy-consuming approach is significant for their wide range of applications. In this work, flash sintering (FS) and two-step flash sintering (TSFS) were first utilized to prepare Gd 2 Zr 2 O 7 nanoceramics. The effects of electric field intensity and current limit on the densification and grain growth were investigated. The grain size was decreased with the increase of electric field intensity. Meanwhile, both grain size and density were increased with the increase of current limit. The Gd 2 Zr 2 O 7 nanoceramics with an average grain size of 98 nm and a relative density of 92.8% were prepared by FS. By contrast, the Gd 2 Zr 2 O 7 nanoceramics with the higher relative densities of 96.3–97.5% and the smaller average grain sizes of 85–90 nm were prepared by TSFS. Compared to FS, TSFS can achieve suppressed grain growth and fast densification. The as-prepared Gd 2 Zr 2 O 7 nanoceramics showed excellent hardness and fracture toughness. The sample breakage caused by current localization can be avoided by controlling the current ramp after the occurrence of the flash event. [ABSTRACT FROM AUTHOR]

This paper aims at elucidate the role of calcium as sintering aid for YAG ceramics elaborated by a reaction-sintering process. From structural and microstructural analyses, the solubility limit of calcium was found to be close to 0.065 ± 0.015 at% at 1700 °C. For calcium content higher than this value up to 1 at%, CaAl 12 O 19 crystallized precipitates with a melting temperature close to 1850 °C were observed. Whatever its content, calcium segregates at grain boundaries as evidenced by STEM and NanoSIMS chemical analyses at nanometric scale, and strongly decreases densification and grain growth kinetics of YAG ceramics. This result was explained by a solute-drag mechanism due to the segregation of complex C a 2 + - V O.. clusters with low mobility at grain boundaries. Finally, calcium appears as a very efficient sintering additive to limit grain growth in YAG, thus avoiding the formation of intragranular porosity that is a critical point for the manufacturing of highly transparent ceramics. [ABSTRACT FROM AUTHOR]

Pure and low α-Al 2 O 3 /SiO 2 co-doped ceria specimens were prepared by conventional ceramic processing using powder metallurgy. This study investigated the effect of co-doping on the microstructural and structural properties of cerium dioxide was investigated by means of optical microscopy, scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The co-addition of alumina and silica promoted liquid phase sintering and significantly contributed to grain growth, even in the small concentrations of the present study (1 and 2 wt%). A glassy look phase precipitated at the grain boundaries, a characteristic feature of liquid phase sintering. The addition of dopants to the formulation greatly enhanced the grain growth process without disturbing the CeO 2 structure. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate a hybrid cold sintering/spark plasma sintering (CSP-SPS) process to densify ZnO ceramic with controlled grain growth. The densification of ZnO is initially activated at 85 °C, and high densities (>98%) are achieved at 200–300 °C in only 5 min with a low assisted pressure of 3.8–50 MPa. The microstructure of ZnO grains experiences a mild coarsening from ~205–680 nm during the CSP-SPS. In comparison, a much higher temperature (>770 °C) is required to sinter ZnO ceramic via SPS, and the grain size exhibits an obvious overgrowth to ~10 µm. The calculated apparent activation energy of grain growth using CSP-SPS is 69.3 ± 6 kJ/mol, which is much lower than that of SPS samples with 296.8 ± 59 kJ/mol. In addition, the conduction mechanism of the CSP-SPS and SPS samples is investigated using impedance spectroscopy. Overall, CSP-SPS is promising for the fabrication of fine ceramics with mild sintering conditions. • A hybrid cold sintering/spark plasma sintering (CSP-SPS) is proposed to densify ZnO ceramics. • CSP-SPS enables highly dense ZnO ceramics at 200–300 °C for only 5 min under 3.8–50 MPa. • ZnO grains experience a mild coarsening from ~205–680 nm during the CSP-SPS. • The apparent activation energy of grain growth using CSP-SPS is only 69.3 ± 6 kJ/mol. [ABSTRACT FROM AUTHOR]

Current grain growth models have evolved to account for the relationship between grain boundary energy/mobility anisotropy and the five degrees of grain boundary character. However, the role of grain boundary networks on overall growth kinetics remains poorly understood. To experimentally investigate this problem, a highly textured Al 2 O 3 was fabricated by colloidal casting in a strong magnetic field to engineer a unique spatial distribution of grain boundary character. Microstructural evolution was quantified and compared to an untextured sample. From this comparison, a prevalence of (0001)/(0001) terminated grain boundaries with anisotropic networks were identified in the textured sample. These boundaries and their networks were found to be driving grain growth at a faster rate than predicted by models. These findings will allow better modelling of grain growth in real systems by experimentally exploring the impact thereon of grain boundary plane anisotropy and relative energy/mobility differences between neighboring boundaries. [ABSTRACT FROM AUTHOR]

This work demonstrates that large pores promote abnormal grain growth, a commonly observed but poorly understood behavior. To investigate this phenomenon, calcia doped aluminum oxide is used as a model ceramic because of its known propensity for abnormal grain growth. Controlled porous microstructures are fabricated using polymer microspheres with nominal diameters approximately an order of magnitude larger than the average grain diameters. Electron backscattered diffraction maps indicate that the introduction of large pores (∼3 vol.%) promotes abnormal grain growth, increasing the total abnormal grain area by more than 60 % when sintered at 1600 °C for 4 h in air. The differences between samples with and without large pores provide insight into the initiation of abnormal grain growth, which is needed to predict microstructure evolution in complex systems with such defects. [Display omitted] [ABSTRACT FROM AUTHOR]

The target of the present research is to fabricate single-phase β-Sialon porous ceramics by self-propagating high-temperature synthesis (SHS). Samples with composition of Si 6-z Al z O z N 8-z (z = 0.5) were prepared by the spontaneous reaction of initial mixture of Si, Si 3 N 4 , Al, and Al 2 O 3 at high-pressure N 2 gas atmosphere. The impacts of Si content on the porosity, phase composition, microstructural morphology, and flexural strength of the fabricated materials were studied in detail. Anticipative target was achieved by adjusting Si content of initial materials and resultant reaction temperature of SHS process. Sample prepared with 35 wt% Si presented single phase of β-Sialon, homogeneous and elongated morphology, and high flexural strength of 150 MPa. To the best of our knowledge, it is the first report to synthesize single-phase β-Sialon porous ceramics by such a rapid and cheap method. [ABSTRACT FROM AUTHOR]

One of the strategies for improving alumina ceramics' strength and optical properties is to eliminate grain growth during sintering. In this work, a casting process from water suspension was applied to prepare bulk alumina specimens doped with zirconia inclusions in the amount up to 0.5 wt.%. The effect of pure monoclinic (m-ZrO 2) or cubic zirconia (ZrO 2 stabilised with 8 mol.% Y 2 O 3) on the sintering and alumina grain growth up to 1600 °C was studied. The results demonstrate that even a negligible addition of zirconia particles (0.2 wt.%) reduces grain growth after sintering up to 1400 °C. At the sintering temperature of 1600 °C, significant grain growth was observed for both pure and doped alumina ceramics. However, the careful analysis of the grain size distribution revealed narrower distribution for zirconia-doped alumina compared to pure alumina ceramic. The Vickers hardness decreases with the increasing sintering temperature due to grains' growth independently on the load applied. The indentation fracture toughness of samples sintered at 1400 °C does not significantly depend on the presence of inclusions added in amounts up to 0.5 wt.%. [ABSTRACT FROM AUTHOR]

• The Gd 2 Zr 2 O 7 -based ceramic waste form with only ∼15 nm was succcessfully prepared by a high pressure (5GPa/520 °C) sintering method for its thermodynamic stability. • At the peak damage area, obvious grain growth (2–3 times in grain size) and big Kr bubbles (10–68 nm) were firstly observed in the nano-grained sample. • The formation mechanism of Kr bubbles in the nano-grained ceramic is on account of grain boundary diffusion and migration induced by the accumulation of the injecting Kr ions and irradiation defects. In this work, 0.5TRPO•0.5Gd 2 Zr 2 O 7 ceramic with an average grain size of only ∼15 nm was prepared by a high pressure (5 GPa/520 °C) sintering method. Phase evolutions and microstructure changes of the as-fabricated super nano and micron-grained ceramics under a high-dose displacement damage induced by 300 keV Kr2+ ions were investigated. The results show that the super nano-grained ceramic has low degree of amorphization, obvious grain growth (2–3 times in grain size) and big Kr bubbles (10–68 nm) formation after irradiation. The micron-grained ceramic was severely amorphized after irradiation and many microcracks were formed parallel to its surface. The formation mechanism of Kr bubbles in the super nano-grained ceramic is on account of grain boundary diffusion and migration induced by the accumulation of the injecting Kr ions and irradiation defects. Nevertheless, microcracks formed in the micron-grained sample are caused by the accumulation of Kr atoms. [ABSTRACT FROM AUTHOR]

Dense tantalum carbide (TaC) ceramics were prepared using TaC nanopowder via spark plasma sintering (SPS). The effects of the sintering temperature and applied pressure on the densification and grain growth behaviour of TaC ceramics were investigated. The results showed that high temperature and pressure promoted sintering densification, while their increase caused an increase in the grain size of TaC ceramics. A highly dense TaC ceramic (∼97.19%) with a fine grain size of 2.67 μm was obtained by sintering at 1800 °C for 10 min under 80 MPa. The Vickers hardness, Young's modulus and fracture toughness were 15.60 GPa, 512.66 GPa and 3.59 MPa·m1/2, respectively. The densification kinetics were investigated using a creep deformation model. Diffusion and grain boundary sliding were proven to be the dominant densification mechanisms based on the stress and grain size exponents combined with the microstructural characteristics. The apparent activation energy of the mechanism controlling densification was 252.94 kJ/mol. [ABSTRACT FROM AUTHOR]

Li 2 O–ZrO 2 –SiO 2 –Al 2 O 3 (LZSA) glass ceramic systems are usually obtained from powder technology to obtain materials with a low thermal expansion coefficient (CTE). However, in these cases, there is a high residual porosity. An alternative to reduce the porosity involves the production of monoliths. Nevertheless, there is still a lack of crystallisation kinetics and the final properties of glass ceramic monoliths are affected such as electrical properties. This study aims to evaluate the electrical behaviour as function of the crystalline layer thickness formed on the monolith surface of a 17.7Li 2 O · 5.2ZrO 2 · 68.1SiO 2 · 9.0Al 2 O 3 (molar basis) glass ceramic LZSA composition. Monoliths thermally treated at 750, 800, and 850 °C were chosen to evaluate based on the range of the crystalline layer growth. Electrochemical impedance spectroscopy was used for the electrical characterisation of LZSA glass and the glass ceramics. The resistivity increased with increasing thermal treatment temperature due to the formation of lithium-based crystalline phases. The electrical conductivity at 25 °C of the glass ceramic thermally treated at 850 °C decreased to 1.4 × 10−13 S cm−1 from 8.7 × 10−11 S cm−1 for LZSA glass. Based on the electrical behaviour, monoliths thermally treated at 850 °C can be considered potential for dielectric industrial applications. [ABSTRACT FROM AUTHOR]

This study reports the effects of composition and the presence of residual template particles on the electromechanical properties of random and textured CuO-doped Pb(Yb 1/2 Nb 1/2)O 3 -Pb(Mg 1/3 Nb 2/3)O 3 -PbTiO 3 ceramics. Fully textured PYN-PMN-PT ceramics were produced with [001]-oriented barium titanate (BT) particles. The strain of all textured compositions was improved by 40–82 %, and the coercive field was increased by −1 kV/cm relative to ceramic compositions. Textured 20PYN-46PMN-34PT ceramics had the best combination of properties including a high E c of 9.8 kV/cm and a low strain hysteresis of 8.5 %. Rayleigh analysis of textured PYN-PMN-PT ceramics showed that more rhombohedral compositions have greater strain because of improved extrinsic and intrinsic contributions to the piezoelectric response. These results demonstrate that textured PYN-PMN-PT is a promising material for high power transducers, however compositions must be sufficiently on the rhombohedral side of the morphotropic phase boundary to compensate for the phase-shifting effects of residual stresses from residual BT template particles. [ABSTRACT FROM AUTHOR]

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C f /C) composite for spark plasma sintering, two kinds of commercially pure α-Al 2 O 3 powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al 2 O 3 powder and 1400 °C for the 3 μm Al 2 O 3 powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al 2 O 3 compact can reach ∼80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth. [ABSTRACT FROM AUTHOR]

The effect of different alkali halides additives (LiF, LiCl, NaF, NaCl, KF, KCl) on the densification of spinel ceramics prepared via spark plasma sintering (SPS) is systematically studied. Significant differences are found between different additives considering the onset temperature of densification, grain growth and the possibility of densification up to transparency. It is confirmed that LiF is the best sintering additive as far as densification is concerned. It is found, however, that NaF has a quite similar effect on densification, while it leads at the same time to significant grain growth inhibition (contrary to LiF which generally promotes grain growth). Moreover, it is found that LiCl promotes grain growth to a very similar degree as LiF, thus indicating that the Li+ ion is responsible for exaggerated grain growth. Finally, the mechanism of LiF in the densification of spinel ceramics is revisited and discussed. [ABSTRACT FROM AUTHOR]

Miniaturization, high frequency capability, low loss, and matching impedance play critical roles in new generation antennas. Although ferrite ceramics show a great potential as antenna substrate materials, some challenges still remain associated with their application to high performance antennas. In this work, we synthesized Bi3+-substituted NiCuZn ferrite to adjust the microstructure, magnetic, and dielectric properties at low sintering temperatures. It is shown that Bi3+ substitution not only enhances grain growth and compactness of NiCuZn ferrite at low sintering temperatures, but also effectively tailors the permeability, permittivity, and loss factor. In particular, NiCuZn ferrite doped with 0.025 wt%, of Bi 2 O 3 possesses excellent performance with high permeability (~29.1), high n factor (~22.76), appropriate matching impedance (Z factor = 1.28), and low loss tangents (tanδ μ = 0.00853, tanδ ε = 1.22683 × 10-4). In addition, the dielectric loss is significantly reduced and becomes as low as magnetic loss at high frequencies; the typical tanδ ε reduces to 1.94 × 10-6, and tanδ ε / ε ´ is greatly decreases to 0.137 × 10-6 at 50 MHz. Thus, Bi3+-adjusted NiCuZn ferrites open up new opportunities for applications in high-frequency LTCC antennas. [ABSTRACT FROM AUTHOR]

The influence of sample size on microstructure, density, and onset temperature of the flash event was investigated for 3 mol% yttria-stabilised zirconia (3YSZ). Pellet samples with different height and constant diameter were flash sintered under an AC electric field. The larger the samples were, the lower was their flash onset temperature. Furthermore, a more heterogeneous microstructure was verified on larger samples followed by lower density. The influence of the samples' height on the thermal runaway and the thermal gradient within the samples are discussed as responsible for these effects. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The dependence of grain size on the heating rate has been investigated for alumina ceramics prepared via spark plasma sintering (SPS). For this purpose, the local grain size has been determined via position-dependent microscopic image analysis, using two independent grain size measures (mean chord length and Jeffries grain size). For alumina ceramics prepared with heating rates between 5 and 100 °C/min (pressure 80 MPa, maximum temperature 1300 °C) it is found that for higher heating rates the grain size is smaller. However, the microstructural non-uniformity is so large that any grain size determination that does not take this non-uniformity into account becomes meaningless, because grain size gradients from the specimen periphery to the center are larger than the differences in grain size due to different heating rates. Temperature and pressure gradients are discussed as the most plausible reasons for the microstructural non-uniformity. [ABSTRACT FROM AUTHOR]

For the preparation of transparent spinel ceramics it is common practice to use LiF as a sintering additive to achieve transparency. However, it is well known that in this case the grain size exhibits a significant increase compared to pure spinel ceramics, which can lead to a deterioration in mechanical properties. The findings of this paper indicate that when NaF is used as a sintering additive for the preparation of spinel ceramics via spark plasma sintering (SPS) translucent materials with a high level of densification can be obtained without observable grain growth. It is shown that the grain size after SPS at 1500 °C with 1 h dwell is essentially the same as the primary particle size of the spinel powder, whereas pure spinel ceramics prepared via SPS under the same conditions exhibit grain growth by approximately a factor 5. [ABSTRACT FROM AUTHOR]

A novel phenomenon is found that the grain growth of aluminum-rich magnesium aluminate is suppressed to 5 μm, despite of high-temperature hot-isostatic-press at 1750 °C. For the transparent MgO-nAl 2 O 3 (n = 1.2, 1.5, 2.0), the transmittance and strength tended to be enhanced with increasing Al 2 O 3 content, having the highest values 80 % at 550 nm and 214.3 MPa for n = 2.0. This is due to the different microstructural evolution depending on the composition. When the excessive Al 2 O 3 was small, the inhomogeneous microstructure with bimodal grain size and lots of rod-shaped particles were observed. However, as the Al 2 O 3 amount increased, the microstructure became homogeneous with reducing the grain size and the rod-shaped particles. The microstructural and compositional analysis revealed that the rod-shaped particle were generated due to the trace potassium impurity as well as the different cationic diffusion rate, and the suppression of grain growth was induced by the severe Al segregation at the grain-boundaries. [ABSTRACT FROM AUTHOR]

Herein, Al 2 O 3 ceramics are sintered by oscillatory pressure sintering (OPS) and hot pressing (HP) at different temperatures. The density and grain size of OPS- and HP-processed Al 2 O 3 ceramics are measured and analyzed using existing models. Results reveal that OPS-processed Al 2 O 3 ceramics exhibit high density and small grain size likely due to plastic deformation. Moreover, OPS-sintered Al 2 O 3 ceramics render higher hardness and fracture strength than HP-sintered Al 2 O 3 ceramics, which can be ascribed to high density and small grain size, respectively. These results demonstrate that sintering behavior and mechanical properties of Al 2 O 3 ceramics can be improved by OPS. [ABSTRACT FROM AUTHOR]

In this study, Ca 3 Co 4 O 9 ceramics were successfully prepared using a cold sintering process at significantly low temperatures of approximately 150 °C. This new procedure was used to produce Ca 3 Co 4 O 9 ceramics, in particular, as this material is extremely sensitive to the synthesis and processing owing to its partial decomposition into Co 3 O 4 or Ca 2 Co 2 O 5 as a secondary phase. Ca₃Co₄O₉ powder was prepared through a solid-state reaction and calcined at 900 °C. For the cold sintering process, the prepared powders were uniaxially pressed into 12 mm diameter discs of approximately 1 mm thickness, at 350 MPa, while the temperature was isothermally maintained at 150 °C for 1 h. Subsequently, selected "as prepared" ceramics were re-sintered at 900 °C for 6 h or 24 h. The results indicated that bulk densities greater than 74% of the theoretical density were obtained for the cold sintered ceramics. Compared to conventionally sintered ceramics, the cold sintered ceramics exhibited greater electrical conductivity (6459 S/m) and reduced thermal conductivity (0.672 W/m K) at 600 °C. The highest Figure of Merit value obtained was 0.12 for the cold sintered ceramics, which is thrice that of conventionally sintered ceramics. The cold sintering process was observed to be efficient in increasing bulk densification and improving the thermoelectric properties of Ca₃Co₄O₉ ceramics compared to the conventional sintering method. [ABSTRACT FROM AUTHOR]

Pure h-BN ceramic specimens were prepared by hot-pressing under different sintering temperatures and pressures using ball milled h-BN powders composed of amorphous and nanocrystalline BN. Microstructures and thermal conductivities of these h-BN ceramic specimens were characterized and measured. Higher sintering pressure is more favorable to the preferred orientation growth of plate-like h-BN grains along the pressure direction, forming microstructures where the c -axes of h-BN grains are preferentially oriented perpendicular to the pressure direction. However, such microstructures can only be obtained at appropriate sintering temperature. Thermal conductivities of h-BN ceramic specimens are strongly related to their microstructures, especially the grain orientation. Growth mechanisms of h-BN grains were investigated. There is multi-area co-growth phenomenon around the grain boundaries composed of the basal planes of h-BN grains, which results in the formation of stacking faults in the as-grown h-BN grains. [ABSTRACT FROM AUTHOR]

The relationships between microstructures and mechanical properties especially strength and toughness of high-entropy carbide based ceramics are reported in this article. Dense (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C (HEC) and its composite containing 20 vol.% SiC (HEC-20SiC) were prepared by spark plasma sintering. The addition of SiC phase enhanced the densification process, resulting in the promotion of the formation of the single-phase high-entropy carbide during sintering. The high-entropy carbide phase demonstrated a fast grain coarsening but SiC particles remarkably inhibited this phenomena. Dense HEC and HEC-20SiC ceramics sintered at 1900 °C exhibits four-point bending strength of 332 ± 24 MPa and 554 ± 73 MPa, and fracture toughness of 4.51 ± 0.61 MPa·m1/2 and 5.24 ± 0.41 MPa·m1/2, respectively. The main toughening mechanism is considered to be crack deflection by the SiC particles. [ABSTRACT FROM AUTHOR]

Diffusion kinetics, processing, and microstructures of alumina ceramics have been of great scientific and technological interests. Here we reported the sintering kinetics and microstructure evolution in high-purity dispersed ultrafine α -Al 2 O 3 nanoparticles with superior sinterability. An extremely fine grain size of 34 nm with 99.6 % theoretical density and a uniform microstructure can be produced by two-step pressureless sintering. Systematic kinetic analysis further points out to a surprising insensitivity of α -Al 2 O 3 nanocrystalline ceramics to Fe and other impurities, which contrasts with prior reports in the literature. We propose this insensitivity is originated from extensive grain boundaries in nanocrystalline ceramics, which allows segregation of impurities and lowers their concentrations at grain boundaries. [ABSTRACT FROM AUTHOR]

Grain coarsening normally occurs at the final stage of sintering, resulting in trapped pores within grains, which deteriorates the density and the performance of ceramics, especially for ultra-high temperature ceramics (UHTCs). Here, we propose to sinter this class of ceramics in a specific temperature range and coupled with a relatively high pressure. The retarded grain boundary migration and pressure-enhanced diffusion ensure the proceeding of densification even at final stage. A highly dense TaC ceramic (98.6 %) with the average grain size of 1.48 μm was prepared under 250 MPa via high pressure spark plasma sintering using a C f /C die at 1850 °C. It was suggested that the final-stage densification is mainly attributed to grain boundary plastic deformation-involved mechanisms. Compared to the usual sintering route using a high temperature (>2000 °C) and normal pressure (<100 MPa), this work provides a useful strategy to acquire highly dense and fine-grained UHTCs. [ABSTRACT FROM AUTHOR]

In this work, the effect of ZrO 2 -doping on the microstructure evolution and phase change of Tb 3 Al 5 O 12 (TAG) ceramics has been investigated systematically. Except for the transformation of Zr4+–Zr3+, a novel phenomenon is discovered in the final stage of sintering that a second phase of ZrO 2 is discharged to the surface of samples at the sintering temperature higher than 1550 °C, which is accompanied with a largely promoted grain growth at the same time. The discharge behavior is considered to contribute to the enhanced optical quality of TAG ceramics for the decreasing residual impurity in grain boundaries. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Zn 0.95 V 0.05 O ceramics, elaborated from milled ZnO and V 2 O 5 nanopowders, were sintered at 900, 1000 and 1100 °C for 1, 2, 4, 6, 10 and 14 h. The growth kinetics was studied identifying the grain growth exponent, the activation energy and the pre-exponential factor. The high V 2 O 5 concentration allowed a rapid grain growth at 900 °C only at the very first stages (t < 1 h). Meanwhile, at temperatures of 1000 and 1100 °C, the grain growth was extremely fast with a growth exponent of 0.72. The magnetic properties of the samples indicate that ferromagnetism exist in all samples in different magnitudes depending on the sintering conditions. In particular, the maximum magnetization was obtained on the sample sintered at 1100 °C for 14 h, despite the reduction of V concentration. Additionally, secondary paramagnetic phases were detected in the samples sintered at lower temperatures and shorter sintering times. [ABSTRACT FROM AUTHOR]

In this paper, the UV curable ZrO 2 -Al 2 O 3 composite ceramic pastes based on SLA-3D printing technology were synthetized firstly, and the corresponding ceramic green bodies were 3D printed, and finally the ZrO 2 -Al 2 O 3 composite ceramic parts were manufactured by a subsequent debinding and sintering process. Our work focuses on the investigation of the microstructure, hardness and fracture toughness of the printed ZrO 2 -Al 2 O 3 composite ceramics sintered at different maximum sintering temperature and holding time. In case of the sintering temperature of 1500 °C and holding time of 60 min, the actual density, hardness and fracture toughness of the ceramics can reach 3.75 g/cm3, 14.1 GPa and 4.05 MPa⋅m1/2, respectively. If the sintering temperature is higher than 1500 °C, the abnormal grain growth can occur, resulting in some more reductions in the mechanical properties of ceramics; while when the sintering temperature is lower than 1500 °C, the lower driving force makes the grains not fully developed, resulting in a low density of ceramics and worse mechanical properties. With the increase of holding time, the sintering driving force is promoted, making for the grain growth. However, when holding time is more than 60 min, the actual density of ceramics almost remains unchanged, and the mechanical properties of ceramics are not able to be improved further. [ABSTRACT FROM AUTHOR]

Control of grain coarsening during sintering of oxide mixtures remains a challenge to manufacture high-performance varistors. This article reports microstructural evolution, grain growth kinetics and nonlinear electrical properties of Er 2 O 3 (0–1.0 mol%) doped ZnO-V 2 O 5 based varistors sintered at 900–1300 °C for a duration ranging from 30 to 480 min. Microstructural characterizations revealed that raise in sintering temperature and time enhanced coarsening of ZnO grains while Er 2 O 3 doping inhibited the grain growth substantially with marginal slowdown of the densification process. Phenomenological grain growth kinetics analyses inferred that the magnitudes of grain growth exponent (3–5), as well as apparent activation energy (263–774 kJ mol-1), increased monotonically with Er 2 O 3 concentration establishing its effectiveness as a strong grain growth inhibitor. In Er 2 O 3 doped sintered pellets, the development of Er-rich and ErVO 4 spinel phases along the intergranular layers of ZnO was confirmed by microstructural characterizations like FESEM, EDS and XRD studies. These phases found to exert pinning effects causing retardation of grain coarsening. Judicial selection of Er 2 O 3 doping concentration (0.5 mol%) and sintering parameters (900 °C and 120 min) generated ZnO-V 2 O 5 varistor ceramic with excellent electrical (nonlinear exponent: 80 and breakdown field: 8715 V cm−1) properties. [ABSTRACT FROM AUTHOR]

Er3+-doped xK 0.5 Na 0.5 NbO 3 -(1-x)Ba 0.85 Ca 0.15 Ti 0.9 Zr 0.1 O 3 (x = 0.04, 0.05, 0.06 and 0.07) translucent ceramics have been first prepared by conventional pressureless sintering. Owing to the effective suppression of grain growth induced by both Er3+ and K 0.5 Na 0.5 NbO 3 , the ceramics possess dense and fine-grained microstructure. The XRD patterns demonstrate cubic-like phase structure with minimal optical anisotropy. The ceramics perform good photochormic (PC) properties under 407-nm light irradiation and superior reproducibility after heat treatment (200 °C, 5 min). And PC reaction is responsible for coloration and decoloration processes. The up-conversion photoluminescence before and after irradiation have been studied. With increasing the K 0.5 Na 0.5 NbO 3 content or elevating the sintering temperature of the ceramics, the luminescent quenching degree (ΔR t) gradually increases. The color centers induced by irradiation can be reversibly created or eliminated, resulting in invertible luminescent modulations. This work can guide other multifunctional PC transparent ferroelectric materials for optoelectronic applications. [ABSTRACT FROM AUTHOR]

The low-temperature degradation and mechanical properties of 3Y-TZP- x YF 3 ceramics synthesized by a conventional solid-state reaction were investigated. The addition of YF 3 has a significant effect on the aging and mechanical properties of 3Y-TZP ceramics. The results show that the mechanical properties and aging resistance of 3Y-TZP- x YF 3 ceramics with slow grain growth are greatly improved when the content of YF 3 is maintained in a reasonable biphasic region (RBR) due to the dominant role of T-M (tetragonal to monoclinic). When the content of YF 3 exceeds 0.15 mol%, the mechanical properties and aging properties of ceramics in a saturated biphasic region (SBR) decrease due to the influence of T-C (tetragonal to cubic). More importantly, 3Y-TZP-0.15YF 3 ceramics have good aging resistance and mechanical properties, and YF 3 doping makes the application of 3Y-TZP in dental ceramics more meaningful. [ABSTRACT FROM AUTHOR]

Various numerical methods have been recently employed to model microstructure of ceramics with different level of accuracy. The simplicity of the models based on regular morphologies results in a low computational cost, but these methods produce less realistic geometries with lower precision. Additional methods are able to reconstruct irregular structures by simulating the grain-growth kinetics but are restricted due to their high computational cost and complexity. In this paper, an innovative approach is proposed to replicate a three-dimensional (3D) complex microstructure with a low computational cost and the realistic features for porous polycrystalline ceramics. We present a package, written in MATLAB, that develops upon the basic Voronoi tessellation method for representing realistic microstructures to describe the evolution during the solid-state sintering process. The method is based on a cohesive prism that links the interconnect cells and thus simulates the neck formation. Spline surfaces are employed to represent more realistic features. The method efficiently controls shape and size and is able to reconstruct a wide range of microstructures composed of grains, grain boundaries, interconnected (open) and isolated (closed) pores. The numerical input values can be extracted from 2D imaging of real polished surfaces and through theoretical analysis. The capability of the method to replicate different structural properties is tested using some examples with various configurations. Image 1 • Employment of a geometric analogy between Coble's model and Voronoi cells. • Fitting spline surfaces to generate more realistic geometry. • Simulation of neck formation. • Effective size and shape control. • Modelling open and closed porosity. [ABSTRACT FROM AUTHOR]

The Q × ƒ values of SmNbO 4 ceramics had been effectively optimized by MgO doping, and a series of high-Q SmNbO 4 -x MgO (0.5 ≤ x ≤ 4.0) ceramics had been successfully prepared with conventional solid reaction process. The phase composition, microstructure and microwave dielectric properties had been investigated with different content of MgO addition. Two phases, surrounded by each other which indexed as SmNbO 4 and MgO, had been detected in SmNbO 4 -x MgO ceramics. It was found that the grain growth of SmNbO 4 -x MgO ceramics could be well promoted by MgO additive, owing to the similar c/a values between the two crystal structures. Based on the structural characteristic of SmNbO 4 -x MgO ceramics, the variation in dielectric constant of SmNbO 4 -x MgO ceramics was verified to satisfy with the Lichtenecker logarithmic mixing rule. Importantly, the Q × ƒ values of SmNbO 4 -x MgO ceramics increased as a function of MgO content, which was analyzed to result from the improved microstructure. • The Q׃ values of SmNbO4 ceramics had been distinctly improved by MgO addition. • The influencing factors of microwave dielectric properties were investigated. • The phase composition, microstructure and element distribution of SmNbO4-MgO ceramics had been investigated. • A series of high-Q ceramics had been prepared successfully with conventional solid reaction process. [ABSTRACT FROM AUTHOR]

BaTiO 3 -based multilayer chip thermistors with the inner electrodes have been designed to meet the requirements of miniaturization and low resistance. Generally, they were prepared by the reduction-reoxidation method to avoid the oxidization of internal electrodes. Due to their limited efficiencies of oxygen adsorption during the process of reoxidation, positive temperature coefficient of resistivity (PTCR) jump of multilayer chip thermistors were difficult to reach a high value. In this work, we have investigated the influence of (Bi 0.5 Na 0.5)TiO 3 (BNT) addition and sintering temperature on microstructure and electrical properties of BaTiO 3 based multilayer chip thermistor ceramics. The measurement of grain size and density suggested that a moderate addition of BNT could facilitate grain growth when the ceramic is sintered at a suitable temperature. This phenomenon was attributed to the liquid Na-rich phase at a suitable temperature, and it was affected by the Na+-rich grain shell during the sintering process. Meanwhile, Bi3+ was found to increase the adsorption efficiency of oxygen at the grain boundaries, thus promoted PTCR jump. Multilayer chip thermistors ceramics with high PTCR jump of 3.55 × 103 and low RT resistivity of 18.4 Ω cm were obtained after sintering at 1125 °C with 0.3mol% BNT additive. [ABSTRACT FROM AUTHOR]

(1- x)ZnGa 2 O 4 - x TiO 2 (x = 0.05–0.20) composite ceramics were fabricated via traditional solid state method. The experimental and theoretical value of ɛ r , τ f and Q × f shows similar trend. Tunable microwave dielectric performance was achieved due to the effect of TiO 2 , especially for temperature coefficient and quality factor. The large negative temperature coefficient at resonant frequency (τ f) of ZnGa 2 O 4 was continuously tailored from negative (−70 ppm/oC) to positive (+13 ppm/oC) 2 For practical application, 15 mol% (about 6.9 V%) TiO 2 can adjust τ f value to near zero. The tailoring effect on quality factor (Q × f) was significant. Due to the smaller Q × f values of TiO 2 , the Q × f value decreases with the increasing content of TiO 2. The inhomogeneous microstructure results in the increasing difference between calculated and experimented Q × f value. In addition, compositing TiO 2 with ZnGa 2 O 4 ceramic not only promote the grain growth but also reduce its sintering temperature from 1400 °C to 1300 °C. The dielectric constant (ɛ r) was not significant affected by TiO 2 , only varied from 10.8 to 12.7. The ɛ r value of 12.3, Q × f value of 73,000 GHz (at 11.8 GHz) and τ f value of +3 ppm/°C were achieved for 0.85ZnGa 2 O 4 -0.15TiO 2 , sintered at 1300 °C for 2 h. Image 1 • The compositing effect of TiO 2 results in tunable microwave dielectric performance. • The large negative τ f continuously shifts from negative to positive. • Endows ZnGa 2 O 4 -TiO 2 with additional application potential as high quality MDCs. • ZnGa 2 O 4 -TiO 2 provides a good example of rational design to tailor τ f. [ABSTRACT FROM AUTHOR]

Full densification of Y 2 O 3 is challenging and requires a very high sintering temperature (above 1700 °C). In this study, the effect of ZnO and TiO 2 dopants on its densification was investigated, showing that both dopants lowered the sintering temperature and improved the process. Moreover, ZnO promoted the grain growth, while TiO 2 inhibited it; hence, the ZnO TiO 2 co-doping and the change in the ZnO/TiO 2 ratio allowed the control of the sintered body microstructure while maintaining high densification. Since Y 2 O 3 has a higher plasma erosion resistance than conventional Si-based materials, the plasma dry etching resistance of the sintered Y 2 O 3 was also evaluated and found superior due to the improved densification and controlled grain size of the doped samples. [ABSTRACT FROM AUTHOR]

Abstract We studied crystallization, grain growth and electric properties of La 0.8 Sr 0.2 MnO 3 (LSM) ceramics which were produced using the microwave-treatment. While co-precipitated nanoparticles remain mainly amorphous, the microwave irradiated particles are crystallized into LSM and La 2 Mn 2 O 7 at 550 °C, due to higher dielectric polarizability of La. This, in turn, decreases the amount of the second phase La 2 O 3 in calcined powder and promotes the growth of perovskite grains during sintering at 1400 °C. Larger grains of LSM ceramics lower the activation energy of small polaron hopping from 0.35 eV to 0.24 eV and increases high-temperature electric conductivity. In addition, high crystallinity of LSM ceramics from the microwave-treatment suppresses a chemical reaction with ZrO 2 and NiO in a temperature range of 900 – 1100 °C under oxidizing and reducing ambiances. These results show that LSM ceramics from the microwave-assisted reaction meet requirements for an interconnect layer for solid oxide electrolysis cells. [ABSTRACT FROM AUTHOR]

Abstract It is difficult to obtain nanocrystalline ceramic bulks due to its high surface activity at high temperatures. In the study, in order to achieve both high density and ultrafine morphology, the plastic deformation was induced by an ultrahigh pressure at a deliberately selected temperature, which was much lower than the threshold temperature for rapid grain growth. According to the ultra-high pressure route, nanocrystalline 3YSZ ceramics without grain growth were fully densified at 900 °C under 1.5 GPa. Both direct microstructural observations and calculation results proved that the plastic deformation including high temperature yield and sliding played a dominant role in densification during the sintering process. [ABSTRACT FROM AUTHOR]

Abstract Highly transparent Zr:TAG (Zr:Tb 3 Al 5 O 12) ceramics were obtained by the solid-state reaction after vacuum sintering (up to 82.04% at 1064 nm). Laser-induced damage threshold of Zr:TAG ceramics were measured and compared to TGG (Tb 3 Ga 5 O 12) crystal and ceramic. Improved optical quality and exaggerated grain growth of TAG ceramics were observed at the same time due to ZrO 2 -doping, which is argued to be related to the valence change of Zr4+ → Zr3+ at high temperature. A detailed analysis of possible mechanisms was presented and discussed. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract Tungsten trioxide (WO 3) ceramics were prepared by firing Bi 2 O 3 -added WO 3 compacts with atomic ratios of Bi/W = 0.00, 0.01, 0.03, or 0.05, in which Bi 2 O 3 was mixed as a sintering agent. Dense ceramics consisting of remarkably grown WO 3 grains were obtained for Bi-containing samples with Bi/W = 0.01, 0.03, and 0.05. The grain growth was enhanced by the liquid phase of Bi 2 W 2 O 9 formed among the WO 3 grains while firing. The XRD patterns did not show evidence for Bi inclusion into the WO 3 lattice, but the SEM-EDX showed an intensive distribution of Bi into the grain boundaries. Electrical conductivity σ and Seebeck coefficient S were measured in a temperature range of 373–1073 K. The temperature dependences indicated that the Bi 2 O 3 -added WO 3 ceramics were n -type semiconductors. It was considered that the electron carriers were generated from oxygen vacancies included into the WO 3 grains. The thermoelectric power factors S 2 σ for the ceramics ranged from 1.5 × 10−7 W m−1 K−2 to 2.8 × 10−5 W m−1 K−2, and the highest value occurred at 970 K for the ceramic with Bi/W = 0.01. [ABSTRACT FROM AUTHOR]

Abstract ZrB 2 -based ceramics, reinforced with 25 vol% SiC whiskers (SiC w) as well as 0, 2.5, 5 and 7.5 wt% carbon nanoparticles (C np), were prepared by spark plasma sintering (SPS) at 1900 ºC under 40 MPa for 7 min in a vacuum environment. The influences of C np content on densification behavior, microstructure evolution, hardness and fracture toughness of ZrB 2 –SiC w ceramics were investigated. Compared to the carbon-free sample, the grain growth of ZrB 2 matrix was moderately decreased (~ 20%) after the addition of C np. The in-situ formation of B 4 C and ZrC phases was attributed to the elimination of surface oxide impurities through their chemical reactions with the C np additive. All composite samples approached their theoretical densities. A hardness of 21.9 GPa was obtained for ZrB 2 –SiC w sample, but the hardness values linearly decreased by the addition of soft carbon additives and reached 14.6 GPa for the composite doped with 7.5 wt% C np. The fracture toughness showed another trend and increased from 4.7 MPa m½ for the carbon-free sample to 7.1 MPa m½ for 5 wt% C np -reinforced composite. The formation of new carbides and the presence of unreacted C np resulted in toughness improvement. Various toughening mechanisms such as crack branching, bridging, and deflection were detected and discussed. [ABSTRACT FROM AUTHOR]