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Grain boundaries, ubiquitous in real materials, play an important role in the mechanical properties of ceramics. Using boron carbide as a typical superhard but brittle material under hypervelocity impact, we report atomistic reactive molecular dynamics simulations using the ReaxFF reactive force field fitted to quantum mechanics to examine grain-boundary engineering strategies aimed at improving the mechanical properties. In particular, we examine the dynamical mechanical response of two grain-boundary models with or without doped Si as a function of finite shear deformation. Our simulations show that doping Si into the grain boundary significantly increases the shear strength and stress threshold for amorphization and failure for both grain-boundary structures. These results provide validation of our suggestions that Si doping provides a promising approach to mitigate amorphous band formation and failure in superhard boron carbide.

The AlON transparent ceramic cantilever beams containing twin and grain boundaries were fabricated by focused ion beam (FIB) technique. The deformation behaviors were investigated by measurement of the load and displacement dependence of cantilever beams from micro bending tests. Young’s modulus of AlON transparent ceramics was calculated from load and displacement curves, the results of which were consistent with results of previous works. Moreover, the bonding strengths of twin lamella boundary, twin boundary and normal grain boundary in AlON transparent ceramics were 5.00 GPa, 5.05 GPa and 4.81 GPa respectively.

The reactivity between rare-earth (RE-) oxide stabilized ZrO2 or HfO2 thermal barrier coatings (TBCs) and a calcium-magnesium-aluminum-silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top-coating system. Thermodynamic calculations of the phase assemblage of the (1−x) ZrO2-xY2O3 coating materials and CMAS melt are compared with the experimental findings. CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total RE-oxide content higher than 16 mol% in the reaction zone for the ZrO2 system. In general, samples with nominal compositions ZrO2-9Dy2O3, HfO2-7Dy2O3, ZrO2-8Y2O3, HfO2-6Er2O3, ZrO2-9.5Y2O3-2.25Gd2O3-2.25Yb2O3, and ZrO2-30Y2O3 exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions.

Zirconium diboride (ZrB2) was formed into dense complex shapes using freeze casting as a near-net-shaping technique. Aqueous-based formulations were compared with nonaqueous (cyclohexane) based formulations in terms of rheological behavior, particle packing in the green body, sintered density, macroscale porosity, and cracking. The influence of particle solids concentration and freezing rate was investigated. The aqueous formulations were found to be deficient in that they produced macroscale porosity that could not be eliminated during sintering resulting in low density and large pores in the final shaped objects. The nonaqueous-based system was able to produce complex shaped objects with significantly reduced macroscale porosity. The higher concentration of solids in the nonaqueous-based formulations was primarily responsible for the reduced macroscale porosity and enabled higher sintered densities (up to 90%-91.5% of theoretical density for fast freezing). The microstructure of the ZrB2 formed at fast freezing rates and high solids content typically had isolated pores in the order of 5-10 μm in size, mainly found along grain boundaries (grain sizes between 20 and 50 μm). Although this rapid freezing produced denser components, it tended to produce objects with internal cracks. When slower freezing rates were used, intricate complex shaped objects could be produced without cracks but their density was only between 65% and 80% of theoretical density.

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.

Chemical coating, an effective doping modification method, was employed to fabricate fine-grain BaTiO3-based ceramics. Based on the consideration of subsequently using base metal as inner electrodes in multilayer ceramic devices, green bodies are generally sintered in reducing atmosphere, which generates more charged point defects and thus affects the electric properties. According to the elements distribution analysis, Al element is greatly enriched in the grain boundary and shell region. Coating Al2O3 achieves not only a smaller grain size and narrower distribution but also a higher breakdown strength, discharge energy density and energy efficiency at ambient temperature. In addition, temperature dependences of dielectric and energy storage properties under a same field were also investigated. Over the whole measuring temperature range, the sample with Al2O3 remains higher discharge energy density and energy efficiency.

The liquid-phase sintering behavior and microstructural evolution of x wt% LiF aided Li2Mg3SnO6 ceramics (x = 1-7) were investigated for the purpose to prepare dense phase-pure ceramic samples. The grain and pore morphology, density variation, and phase structures were especially correlated with the subsequent microwave dielectric properties. The experimental results demonstrate a typical liquid-phase sintering in LiF–Li2Mg3SnO6 ceramics, in which LiF proves to be an effective sintering aid for the Li2Mg3SnO6 ceramic and obviously reduces its optimum sintering temperature from ~1200°C to ~850°C. The actual sample density and microstructure (grain and pores) strongly depended on both the amount of LiF additive and the sintering temperature. Higher sintering temperature tended to cause the formation of closed pores in Li2Mg3SnO6-x wt% LiF ceramics owing to the increase in the migration ability of grain boundary. An obvious transition of fracture modes from transgranular to intergranular ones was observed approximately at x = 4. A single-phase dense Li2Mg3SnO6 ceramic could be obtained in the temperature range of 875°C-1100°C, beyond which the secondary phase Li4MgSn2O7 ( 1100°C) appeared. Excellent microwave dielectric properties of Q × f = 230 000-330 000 GHz, er = ~10.5 and τf = ~−40 ppm/°C were obtained for Li2Mg3SnO6 ceramics with x = 2-5 as sintered at ~1150°C. For LTCC applications, a desirable Q × f value of ~133 000 GHz could be achieved in samples with x = 3-4 as sintered at 875°C.

A series of SnOx-Sb2O3 thin film varistors were fabricated through hot-dipping tin oxide films deposited by radio frequency magnetron sputtering in Sb2O3 powder at varied temperatures in air. With the hot-dipping temperature (HDT) increased from 200 to 600 °C, the nonlinear coefficient (α) of the samples increased first and then decreased, reaching the maximum at 500 °C, which was mainly determined by the completeness of high-resistant Sb2O3 layer at tin oxide grain boundary and the chemical composition of tin oxide films. Correspondingly, the leakage current (IL) decreased first and increased later. The breakdown electric field (E100mA) decreased constantly with increasing HDT. The SnOx-Sb2O3 film varistors prepared at 500 °C exhibited the optimum nonlinear properties with the maximum α of 10.88, the minimum IL of 36.3 mA/cm2, and an E100mA of 0.0188 V/nm. The obtained nanoscaled film varistors would be promising in electrical/electronic devices working in low-voltage. This article is protected by copyright. All rights reserved.

Mutliferroic and magnetoelectric gallium ferrite (GaFeO3) is plagued by substantial electrical leakage in polycrystalline form. Here, we report on understanding the conduction mechanism in gallium ferrite ceramic samples vis-a-vis processing conditions. The results show that oxygen annealed samples exhibit minimum electrical leakage as compared to air or nitrogen annealed samples suggesting the role of oxygen vacancies on electrical conduction. Detailed time and temperature dependent impedance spectroscopy analysis of the samples showed higher activation energy of conduction in oxygen annealed samples than in air or nitrogen annealed samples. The lower activation energies of 0.3-0.4 eV in nitrogen/air annealed samples were attributed to higher oxygen vacancy concentration while oxygen annealed samples with low oxygen vacancy concentration exhibited higher activation energy of ~0.50 eV (high frequency i.e. grain) and 0.98 eV (low frequency i.e. grain boundary), latter due to superior level of oxygenation at the grain boundaries. Further, X-ray photoelectron spectroscopy revealed that the oxygen vacancies are compensated by the valence fluctuation between Fe2+/Fe3+ ions whose extent is higher in air/nitrogen annealed samples than in oxygen annealed samples. The conduction mechanisms that could be active are most likely to be double ionization of oxygen vacancies and hopping between Fe2+ to Fe3+ states, latter especially in oxygen deficient samples. This article is protected by copyright. All rights reserved.

MoSi2- and WSi2-based electroconductive ceramic composites were fabricated using 40-80 vol% fine- and coarse-Al2O3, and ZrO2 particles (refractory oxides) after sintering in argon. Their chemical and thermal stability was tested between 1400°-1600°C for up to 48 h. X-ray diffraction analysis showed the formation of secondary 5-3 metal silicide (Mo5Si3, W5Si3) and silica phases on the grain boundaries and surface. The fraction of the W5Si3 (11.4-38.8 vol%) was significantly higher than that of the Mo5Si3 (3.3-7.3 vol%) in the composites after annealing at 1400°C for 48 h. The rates of grain growth in the composites (0.013-0.023 μm/h) were highly decreased by a grain boundary pinning effect. This effect was relatively better with addition of the coarse-grained oxides due to their more homogeneous distribution throughout the microstructure. The percolation threshold for the composites was generally above 20 vol% silicide addition except for the MoSi2-Al2O3 (fine-grained), which exhibited 8.8 and 8.1 S/cm electrical conductivity at 900° and 1000°C, respectively. At 60 vol% silicide content, MoSi2-Al2O3 (coarse-grained) and WSi2-Al2O3 (fine-grained) showed higher electrical conductivity (126-128 S/cm) at 900°C. The density, porosity level, particle distribution, intrinsic conductivity of silicide phase, particle size, and fraction of the secondary 5-3 silicide phase highly influenced their electrical properties. This article is protected by copyright. All rights reserved.

The primary characteristic of nanopowders is the high surface area and consequently high fraction of atoms on the interfaces, which changes the energy of the system. The additive distribution in the nanopowder interfaces is a fundamental aspect to control the energy, particle size, and final properties of nanopowders. In this work, the surface excess was determined using a selective lixiviation method, where a low-water-soluble oxide, SnO2, was used as the matrix, and a high-water-soluble oxide, ZnO, was used as the additive. The XPS analysis confirmed that ZnO segregated on SnO2 surfaces. However, after acid lixiviation the same analysis showed an undetectable surface concentration of ZnO. The evaluation of the nanostructure change and surface composition enables us to calculate the heat of segregation for the grain boundary(∆Heg^gb = -47.2 kJ mol-1)and surface(∆Hseg^s=-36.4 kJ.mol-1) and the interface energy reduction because of segregation. At low ZnO concentrations, the additive solubilizes in the bulk and promotes particle growth. However, the segregation to the grain boundary and surface determines the relative stability of each interface, which promotes hard agglomeration and particle size stabilization at intermediated ZnO amounts. At high ZnO concentrations, the surface segregation stabilizes the solid-gas interface and decreases the agglomeration and final particle size. This article is protected by copyright. All rights reserved.

The thermal shock behavior of Ti2AlC synthesized by means of self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti2AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al2O3. Although the flexural strength decreases quickly in the temperature range of 300-500 °C, no discontinuous decrease in the retained strength is observed in Ti2AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti2AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared to the first quench. Quenching at 300 °C and above, voids after the pullout of grains and cracks are present, which however are absent in the un-quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock. This article is protected by copyright. All rights reserved.

Consolidated, monoclinic ZrO2 polycrystal was produced from undoped ZrO2 powders in air by flash sintering at the sintering temperature of 1350°C for 5 minutes or 3 hours under an applied DC electric field of 175 V/cm. When the ZrO2 was heated under the applied DC field, the electric current of the specimen steeply increased at the furnace temperature of 1335°C below the sintering temperature of 1350°C. When the furnace temperature was decreased from the sintering temperature of 1350°C to room temperature, volumetric expansion associated with tetragonal-to-monoclinic phase transformation gradually took place at the furnace temperature from 1000°C to 750°C, and monoclinic ZrO2 body was remained consolidated even at room temperature in both specimens. In contrast, conventionally sintered ZrO2 without applying DC field exhibited the abrupt volumetric expansion at about 1000°C, and shattered. SEM observation revealed the presence of grain-boundary second phase in the flash-sintered specimen for 3 hours, which is a possible origin of keeping a bulk body at room temperature. The thinner second phase is considered to be formed also in the flash-sintered specimen for 5 minutes, although the formation of the phase could not be observed clearly by SEM observation. On the other hand, XRD measurements showed that directions of the monoclinic ZrO2 grains were oriented along the applied DC field after the isothermal flash sintering for 3 hours while the grain alignment could not be observed in flash-sintered specimen for 5 minutes. The alignment of ZrO2 grains observed in the isothermal flash sintering is considered to be closely related to the preferential direction of oxygen ionic conduction and the second phase formed along grain boundaries.

In BaTiO3, flash-sintering associated with a surge of the specimen electric current sometimes results in an inhomogeneous microstructure including Ti-excess secondary phases because of discharging. We applied field-assisted sintering technique (FAST) under precisely controlled specimen current that was set just below the threshold value for the occurrence of flash event for BaTiO3, to avoid the occurrence of the discharging. As a result, uniform and fine-grained compacts were obtained without any secondary phases. A relative density of approximately 92% was achieved under FAST condition of 100 V/cm with a limiting current of 72 mA and soaking time of 3 hours at 1070°C. The voltages during sintering under a constant current of 72 mA were found to decrease during the soaking process. Electron energy loss spectroscopy revealed the generation of excess oxygen vacancies at/near grain boundaries. The excess oxygen vacancies induced by application of DC electric fields were confirmed to reduce the voltages and to retard the shrinkage rate in a final sintering stage.

Sintering and accompanying microstructural evolution is inarguably the most important step in the processing of ceramics and hard metals. In this process, an ensemble of particles is converted into a coherent object of controlled density and microstructure at an elevated temperature (but below the melting point) due to the thermodynamic tendency of the particle system to decrease its total surface and interfacial energy. Building on a long development history as a major technological process, sintering remains among the most viable methods of fabricating novel ceramics, including high surface area structures, nanopowder-based systems, and tailored structural and functional materials. Developing new and perfecting existing sintering techniques is crucial to meet ever-growing demand for a broad range of technologically significant systems including, for example, fuel and solar cell components, electronic packages and elements for computers and wireless devices, ceramic and metal-based bioimplants, thermoelectric materials, materials for thermal management, and materials for extreme environments. In this study, the current state of the science and technology of sintering is presented. This study is, however, not a comprehensive review of this extremely broad field. Furthermore, it only focuses on the sintering of ceramics. The fundamentals of sintering, including the thermodynamics and kinetics for solid-state- and liquid-phase-sintered systems are described. This study summarizes that the sintering of amorphous ceramics (glasses) is well understood and there is excellent agreement between theory and experiments. For crystalline materials, attention is drawn to the effect of the grain boundary and interface structure on sintering and microstructural evolution, areas that are expected to be significant for future studies. Considerable emphasis is placed on the topics of current research, including the sintering of composites, multilayered systems, microstructure-based models, multiscale models, sintering under external stresses, and innovative and novel sintering approaches, such as field-assisted sintering. This study includes the status of these subfields, the outstanding challenges and opportunities, and the outlook of progress in sintering research. Throughout the manuscript, we highlight the important lessons learned from sintering fundamentals and their implementation in practice.

The dependence of energy storage properties on grain size was investigated in BaTiO3-based ferroelectric ceramics. Modified BaTiO3 ceramics with different grain size were fabricated by two-step sintering method from BaTiO3 powders doped with Al2O3 and SiO2 by aqueous chemical coating. For samples doped with ZnO sintering aid in addition to Al2O3-SiO2, the density and breakdown strength increased significantly. In general, samples with smaller grains have lower polarization but higher energy storage efficiency. Al2O3-SiO2-ZnO-doped samples with average grain size of 118±2 nm have an energy density of 0.83±0.04 J/cm3. Obvious segregation of doping elements in second phase and grain boundary was observed by TEM-EDS. Impedance spectroscopy further explains the relationship between microstructure and properties. Compared to common energy storage ceramics, the grain size of this low-cost ceramics sintered at relatively low temperature is small, and the pilot scale production has been well completed. All these features make the utilization in multilayer devices and industrial mass production possible. In addition, the obtained rules are helpful in further development of energy storage ceramics.

Niobium-doped, {100} textured, gradient-free, lead zirconate titanate (PZT) films were fabricated from solutions with different lead contents. Film lead content was controlled through changes in the average solution lead excess from 14.7% to 17 at.%. The low field dielectric response as well as the polarization-electric field hysteresis loops were not a strong function of lead content. However, films with lower lead contents in the precursor tended to withstand higher poling fields than films prepared from more lead-rich precursors. Although no residual PbO was observed at the grain boundaries, films prepared from more lead-rich solutions had higher levels of grain-boundary porosity, lower breakdown strengths, and lower threshold electric fields at which cracking was observed.