Showing papers in "Journal of the American Ceramic Society in 2017"

Current understanding and future research directions at the onset of the next century of sintering science and technology

Abstract: 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.

Cements in the 21st Century: Challenges, Perspectives, and Opportunities.

Abstract: In a book published in 1906, Richard Meade outlined the history of portland cement up to that point1. Since then there has been great progress in portland cement-based construction materials technologies brought about by advances in the materials science of composites and the development of chemical additives (admixtures) for applications. The resulting functionalities, together with its economy and the sheer abundance of its raw materials, have elevated ordinary portland cement (OPC) concrete to the status of most used synthetic material on Earth. While the 20th century was characterized by the emergence of computer technology, computational science and engineering, and instrumental analysis, the fundamental composition of portland cement has remained surprisingly constant. And, although our understanding of ordinary portland cement (OPC) chemistry has grown tremendously, the intermediate steps in hydration and the nature of calcium silicate hydrate (C-S-H), the major product of OPC hydration, remain clouded in uncertainty. Nonetheless, the century also witnessed great advances in the materials technology of cement despite the uncertain understanding of its most fundamental components. Unfortunately, OPC also has a tremendous consumption-based environmental impact, and concrete made from OPC has a poor strength-to-weight ratio. If these challenges are not addressed, the dominance of OPC could wane over the next 100 years. With this in mind, this paper envisions what the 21st century holds in store for OPC in terms of the driving forces that will shape our continued use of this material. Will a new material replace OPC, and concrete as we know it today, as the preeminent infrastructure construction material?

Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics

Abstract: With the cold sintering process (CSP), it was found that adding acetic acid to an aqueous solution dramatically changed both the densities and the grain microstructures of the ZnO ceramics Bulk densities >90% theoretical were realized below 100°C, and the average conductivity of CSP samples at around 300°C was similar to samples conventionally sintered at 1400°C Frequently, ZnO is also used as a model ceramic system for fundamental studies for sintering By the same procedure as the grain growth of the conventional sintering, the kinetic grain growth exponent of the CSP samples was determined as N = 3, and the calculated activated energy of grain growth was 43 kJ/mol, which is much lower than that reported using conventional sintering The evidence for grain growth under the CSP is important as it indicates that there is a genuine sintering process being activated at these low temperatures and it is beyond a pressurized densification process

Local structure of the MgxNixCoxCuxZnxO(x=0.2) entropy-stabilized oxide: An EXAFS study

Abstract: Entropy-stabilized oxides (ESOs) provide an alternative route to novel materials discovery and synthesis. It is, however, a challenge to demonstrate that the constituent elements in an entropy-stabilized crystal are homogeneously and randomly dispersed among a particular sublattice, resulting in a true solid solution with no evidence of local order or clustering. In this work, we present the application and analysis of extended X-ray absorption fine structure (EXAFS) on the prototype ESO composition MgxNixCoxCuxZnxO (x=0.2). In so doing, we can quantify the local atomic structure on an element-by-element basis. We conclude that local bond lengths between metal and oxygen vary around each absorbing cation, with notable distortion around the Cu–O polyhedra. By the second near neighbor (i.e., the cation-cation pair), interatomic distances are uniform to the extent that the collected data can resolve. Crystal models that best fit the experimental scattering data include cations that are distributed randomly on an FCC sublattice with minimal positional disorder, with an interleaved FCC anion sublattice with oxygen ions displaced from the ideal locations to accommodate the distortions in the cation polyhedra. Density functional theory calculations of the ESO system yield a significant broadening in the positional distribution for the oxygen sublattice compared to that for the cation sublattice for all peaks, showing consistency with the conclusion from the experimental data that the distortion from an ideal rock salt structure occurs primarily through disorder in the oxygen sublattice.

Reaction mechanisms of lithium garnet pellets in ambient air: The effect of humidity and CO2

Abstract: Li7La3Zr2O12 (LLZO) has been reported to react in humid air to form Li2CO3 on the surface, which decreases ionic conductivity. To study the reaction mechanism, 0.5-mol Ta-doped LLZO (0.5Ta–LLZO) pellets are exposed in dry (humidity ~5%) and humid air (humidity ~80%) for 6 weeks, respectively. After exposure in humid air, the formation of Li2CO3 on the pellet surface is confirmed experimentally and the room-temperature ionic conductivity is found to drop from 6.45×10−4 S cm−1 to 3.61×10−4 S cm−1. Whereas for the 0.5Ta–LLZO samples exposed in dry air, the amount of formed Li2CO3 is much less and the ionic conductivity barely decreases. To further clarify the reaction mechanism of 0.5Ta–LLZO pellets with moisture, we decouple the reactions between 0.5Ta–LLZO with water and CO2 by immersing 0.5Ta–LLZO pellets in deionized water for 1 week and then exposing them to ambient air for another week. After immersion in deionized water, Li+/H+ exchange occurs and LiOH H2O forms on the surface, which is a necessary intermediate step for the Li2CO3 formation. Based on these observations, a reaction model is proposed and discussed.

Viscosity of glass-forming systems

Abstract: As one of the most important properties of glass-forming liquids, viscosity has drawn significant attention in both glass manufacturing and fundamental research. We review the recent scientific progress in viscosity of glass-forming systems, including both the liquid and glassy states. After the Vogel-Fulcher-Tammann (VFT) equation was introduced, many more efforts have been made to develop more accurate models to describe the temperature dependence of viscosity. In addition to the VFT equation, we also discuss three other viscosity models, viz., the Adam-Gibbs, Avramov-Milchev, and Mauro-Yue-Ellison-Gupta-Allan models. We compare the four viscosity models in terms of their theoretical underpinnings and ability to fit measured viscosity curves. The concept of fragility and the universality of the high-temperature viscosity limit are also discussed. Temperature-dependent constraint theory is introduced in detail as a powerful tool for predicting the composition dependence of viscosity. Some examples of the application of this approach to predict the glass transition temperature and fragility of various glass systems are shown. Topological constraint theory is not only of scientific interest, but also has important industrial applicability. We also discuss the thermal history dependence of viscosity in the glassy state. Some phenomenological models are briefly reviewed, while the main focus is given to the modified Mauro-Allan-Potuzak model, which can accurately predict the nonequilibrium viscosity as a function of temperature, thermal history, and composition. The correlation of viscosity with elasticity is described in terms of the shoving model. Some theoretical implications of the various viscosity models are discussed, including the concepts of the Kauzmann paradox and the ideal glass transition. Some of the evidence against the existence of these phenomena are discussed. We also review the link between glass relaxation and viscosity, that is, emphasizing that the viscosity equations presented in this review can also be used to model different types of relaxation effects based on the Maxwell relation.

Cold sintering process: A new era for ceramic packaging and microwave device development

Abstract: Cold sintering process (CSP) is an extremely low-temperature sintering process (room temperature to ~200°C) that uses aqueous-based solutions as transient solvents to aid densification by a nonequilibrium dissolution-precipitation process In this work, CSP is introduced to fabricate microwave and packaging dielectric substrates, including ceramics (bulk monolithic substrates and multilayers) and ceramic-polymer composites Some dielectric materials, namely Li2MoO4, Na2Mo2O7, K2Mo2O7, and (LiBi)05MoO4 ceramics, and also (1−x)Li2MoO4−xPTFE and (1−x)(LiBi)05MoO4−xPTFE composites, are selected to demonstrate the feasibility of CSP in microwave and packaging substrate applications Selected dielectric ceramics and composites with high densities (88%-95%) and good microwave dielectric properties (permittivity, 56-371; Q × f, 1700-30 500 GHz) were obtained by CSP at 120°C CSP can be also used to potentially develop a new co-fired ceramic technology, namely CSCC Li2MoO4−Ag multilayer co-fired ceramic structures were successfully fabricated without obvious delamination, warping, or interdiffusion Numerous materials with different dielectric properties can be densified by CSP, indicating that CSP provides a simple, effective, and energy-saving strategy for the ceramic packaging and microwave device development

Colloidal Processing: Enabling Complex Shaped Ceramics with Unique Multiscale Structures

Abstract: Colloidal processing of fine ceramic powders enables the production of complex shaped ceramics with unique micro and macro structures which are not possible to produce via conventional dry processing routes. Because of this enhanced structural control and shaping capabilities, colloidal processing has been exploited to produce ceramic components with ever increasing complexity and functionalities. In this review, we revisit some of the research efforts on this topic to highlight its relevance and growing importance for the advanced manufacturing of functional ceramics. Selected examples of colloidal systems with increasing level of complexity are discussed to showcase the wide range of structures that can be generated through wet processing approaches. The historical development and background knowledge pertaining to colloids and surface interactions is first briefly reviewed. The major colloidal shape forming and additive manufacturing processes that utilize colloidal pastes and inks are then reviewed, highlighting the control of suspension rheology needed in these techniques. Next, methodologies that combine suspended particles with a pore-forming phase are discussed as a means to produce porous ceramic materials. Further control over the interactions between anisotropic particles and their alignment in suspensions can be gained via externally applied fields (such as magnetic) to produce texturally aligned green bodies. This leads to bioinspired ceramics that can programmably morph into complex shaped objects upon sintering. Hierarchical porous structures with high mechanical efficiency are also shown as an example of the multiscale designs that can be generated through advanced colloidal processing. As drying of ceramic bodies is an inevitable consequence of wet colloidal processing, the current understanding of this critical processing step is reviewed. Finally, the gaps in knowledge in these fields are discussed to provide our perspective on where the field may support advances in ceramics in the future.

Molecular and submolecular scale effects of comb-copolymers on tri-calcium silicate reactivity: Toward molecular design

Abstract: The impact of organic compounds on the processing and reactivity of inorganic materials has been a source of inspiration for materials scientists for decades and continues to trigger novel and innovative applications in a broad range of disciplines. However, molecular design of such compounds to reach targeted properties remains challenging, particularly for reactive and multicomponent systems. This outstanding challenge is met here by combining a model cement, hosting different coupled reactions of dissolution, nucleation and growth, together with comb-copolymers that offer large and well-controlled variations of their molecular architecture. We show that silicate reactivity is affected by a combination of molecular and submolecular scale effects of these polymers. The first can be described by scaling laws from polymer physics, whereas the second involves specific chemical interactions. In particular, the ability of these polymers to hinder dissolution appears to be crucial, something for which strong experimental evidence is provided.

Band‐Gap Engineering and Enhanced Photocatalytic Activity of Sm and Mn Doped BiFeO3 Nanoparticles

Abstract: Bi1-xSmxFe1-yMnyO3 (BSFMO, x = 0.0, 0.05; y = 0.0, 0.05, 0.10, 0.15, 0.20, 0.25) nanoparticles were synthesized by using double solvent sol–gel method. Photocatalytic activity was investigated under UV and visible-light illumination. The structural, morphological, and optical properties were analyzed by X-ray diffraction, scanning electron microscopy, and UV-vis spectroscopy respectively. The crystallite size of BiFeO3 (BFO) decreases from (57.3–17.2 nm) with the increase in Sm and Mn-doping concentration. The surface morphology shows that the pure and Sm-doped BFO nanoparticles are irregular in shape but changes to spherical shape after Mn-doping up to 25%. The band-gap engineering of BFO nanoparticles is achieved by co-doping of Sm and Mn. The band-gap of BFO could be tuned successfully from 2.08–1.45 eV, which may be due to the distortion induced in Fe-O octahedron and the rearrangement of molecular orbitals. These results give rise to enhanced photocatalytic activity by degradation of organic dyes (MB, CR, and MV) under the visible-light illumination.

On weakest link theory and Weibull statistics

Abstract: This communication addresses some common misconceptions about weakest link theory and Weibull statistics as they pertain to strength distributions of brittle fibers. After describing the nature of the problem, the flaws in ensuing proposed models of strength distributions are highlighted and discussed. A way forward that obviates the problems is suggested.

Composition and temperature dependence of structure and piezoelectricity in (1−x)(K1−yNay)NbO3-x(Bi1/2Na1/2)ZrO3 lead-free ceramics

Abstract: Lead-free piezoceramics with the composition (1-x)(K1-yNay)NbO3-x(Bi1/2Na1/2)ZrO3 (KNyN-xBNZ) were prepared using a conventional solid state route. X ray diffraction, Raman spectroscopy and dielectric measurements as a function of temperature indicated the coexistence of rhombohedral (R) and tetragonal (T) phase, typical of a morphotropic phase boundary (MPB) as the BNZ concentration increased and by adjusting the K/Na ratio. High remnant polarization (Pr = 24 μC/cm2), piezoelectric coefficient (d33 = 320 pC/N), effective piezocoefficient (d33* = 420 pm/V), coupling coefficient (kp = 48%) and high strain (S = 0.168%) were obtained at room temperature but significant deterioration of Pr, d33* and kp were observed by increasing from room temperature to 160 °C (17.5 μC/cm2, 338 pm/V and 32%, respectively) associated with a transition to a purely T phase. Despite these compositions showing promise for room temperature applications, the deterioration in properties as a function of increasing temperature poses challenges for device design and remains to be resolved.

Direct ink writing of ceramic matrix composite structures

Abstract: We present the development of an ink containing chopped fibers that is suitable for direct ink writing (DIW), enabling to obtain ceramic matrix composite (CMC) structures with complex shape. We take advantage of the unique formability opportunities provided by the use of a preceramic polymer as both polymeric binder and ceramic source. Inks suitable for the extrusion of fine filaments ( 30 vol% for a nozzle diameter of 840 μm) were formulated. Despite some optimization of ink rheology still being needed, complex CMC structures with porosity of ~75% and compressive strength of ~4 MPa were successfully printed. The process is of particular interest for its ability to orient the fibers in the extrusion direction due to the shear stresses generated at the nozzle tip. This phenomenon was observed in the production of polymer matrix composites, but it is here employed for the first time for the production of ceramic matrix ones. The possibility to align high aspect ratio fillers using DIW opens the path to layer-by-layer design for optimizing the mechanical and microstructural properties within a printed object, and could potentially be extended to other types of fillers. This article is protected by copyright. All rights reserved.

Electrical and hydrogen reduction enhances kinetics in doped zirconia and ceria: I. grain growth study

Abstract: The kinetics of mass transport is central to ceramic processing and device stability. In this work, the effect of electrical and hydrogen reduction on the grain growth behavior of doped zirconia and ceria has been investigated. Faster grain growth has been observed under reducing conditions in all cases. The results firmly establish that a depressed local oxygen potential can enhance cation kinetics in fluorite-structured oxide ceramics. Meanwhile, a large electrical current can generate a sharp, spatially varied oxygen potential profile, creating a graded microstructure with a dramatic grain size transition across the length of the sample.

The fracture toughness of inorganic glasses

Abstract: Measuring the fracture toughness (KIc) of glasses still remains a difficult task, raising experimental and theoretical problems as well. The available methods to estimate KIc are reviewed, with emphasis on their respective advantages and drawbacks. In view of our current understanding, this analysis gives precedence to the SEPB method. The ultimate glass strength, the critical flaw size, and the indentation load for the onset of crack initiation are discussed, in the light of the fundamentals of fracture mechanics and classical background regarding the mechanics of brittle materials. Analytical expressions were further proposed to predict the fracture energy and fracture toughness of glasses from different chemical systems from their nominal compositions. The theoretical values were compared with the experimental ones, as obtained by self-consistent methods when available. The agreement observed in most cases suggests that measured KIc values correspond to the crack propagation regime (as opposed to the crack initiation threshold), and supports previous investigations in glasses and ceramics, which showed that a crack tip is nearly atomically sharp in these materials (but for metallic glasses). Some ideas to design tougher glasses are finally presented.

Cold sintering process of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte

Abstract: The recently developed technique of cold sintering process (CSP) enables densification of ceramics at low temperatures, i.e., <300°C. CSP employs a transient aqueous solvent to enable liquid phase-assisted densification through mediating the dissolution-precipitation process under a uniaxial applied pressure. Using CSP in this study, 80% dense Li1.5Al0.5Ge1.5(PO4)3 (LAGP) electrolytes were obtained at 120°C in 20 minutes. After a 5 minute belt furnace treatment at 650°C, 50°C above the crystallization onset, Li-ion conductivity was 5.4 × 10−5 S/cm at 25°C. Another route to high ionic conductivities ~10−4 S/cm at 25°C is through a composite LAGP - (PVDF-HFP) co-sintered system that was soaked in a liquid electrolyte. After soaking 95, 90, 80, 70, and 60 vol% LAGP in 1 M LiPF6 EC-DMC (50:50 vol%) at 25°C, Li-ion conductivities were 1.0 × 10−4 S/cm at 25°C with 5 to 10 wt% liquid electrolyte. This paper focuses on the microstructural development and impedance contributions within solid electrolytes processed by (i) Crystallization of bulk glasses, (ii) CSP of ceramics, and (iii) CSP of ceramic-polymer composites. CSP may offer a new route to enable multilayer battery technology by avoiding the detrimental effects of high temperature heat treatments.

Early stage dissolution characteristics of aluminosilicate glasses with blast furnace slag‐ and fly‐ash‐like compositions

Abstract: The authors would like to thank Nanocem (www.nanocem.org), an industrial-academic research network for sustainable cement and concrete research, for their financial support. The use of the ToF-SIMS and XPS facilities at iNano, Arhus University is much appreciated and particularly the support offered by John Hansen. Special thanks to Ruben Snellings for providing one of the glass samples and valuable discussions.

Niobium and divalent-modified titanium dioxide ceramics: Colossal permittivity and composition design

Abstract: Colossal permittivity (CP) (er=104~105) is attained in (A1/3Nb2/3)xTi1-xO2 (A=Ba2+, Ca2+, Zn2+, Mg2+) ceramics. Here, (Ca1/3Nb2/3)xTi1-xO2 material was studied as a typical example, and effects of Ca and Nb on their microstructure, dielectric properties and stability were studied. Both backscattering and elements mapping strongly confirmed the formation of secondary phases due to the addition of Ca and/or Nb. Secondary phases-induced by Ca cannot affect dielectric properties of the ceramics when low Ca and Nb contents were doped, while secondary phases formed by Ca and Nb strongly affected their dielectric properties in a high doping level. In particular, their dielectric properties can be well modified by the optimization of sintering temperatures. In addition, the (Ca1/3Nb2/3)xTi1-xO2 ceramics with x=0.01 exhibited the optimum dielectric properties (er=130500 and tan δ=0.19). Electron-pinned defect-dipoles may be suitable to explain CP phenomenon of this work. We believed that this profound investigation can benefit the development of new TiO2 ceramics as a CP material.

Structural and emission properties of Eu3+-doped alkaline earth zinc-phosphate glasses for white LED applications

Abstract: Quaternary alkaline earth zinc-phosphate glasses in molar composition (40 − x)ZnO – 35P2O5 – 20RO – 5TiO2 – xEu2O3 (where x=1 and R=Mg, Ca, Sr, and Ba) were prepared by melt quenching technique. These glasses were studied with respect to their thermal, structural, and photoluminescent properties. The maximum value of the glass transition temperature (Tg) was observed for BaO network modifier mixed glass and minimum was observed for MgO network modifier glass. All the glasses were found to be amorphous in nature. The FT-IR suggested the glasses to be in pyrophosphate structure, which matches with the theoretical estimation of O/P atomic ratio and the maximum depolymerization was observed for glass mixed with BaO network modifier. The intense emission peak was observed at 613 nm (5D0→7F2) under excitation of 392 nm, which matches well with excitation of commercial n-UV LED chips. The highest emission intensity and quantum efficiency was observed for the glass mixed with BaO network modifier. Based on these results, another set of glass samples was prepared with molar composition (40 − x)ZnO – 35P2O5 – 20BaO – 5TiO2 – xEu2O3 (x=3, 5, 7, and 9) to investigate the optimized emission intensity in these glasses. The glasses exhibited crystalline features along with amorphous nature and a drastic variation in asymmetric ratio at higher concentration (7 and 9 mol%) of Eu2O3. The color of emission also shifted from red to reddish orange with increase in the concentration of Eu2O3. These glasses are potential candidates to use as a red photoluminsecent component in the field of solid-state lighting devices.

Solid solution synthesis of tantalum carbide-hafnium carbide by spark plasma sintering

Abstract: Solid solutions of Tantalum carbide (TaC) and Hafnium carbide (HfC) were synthesized by spark plasma sintering. Five different compositions (pure HfC, HfC-20 vol% TaC, HfC- 50 vol% TaC, HfC- 80 vol% TaC, and pure TaC) were sintered at 1850°C, 60 MPa pressure and a holding time of 10 min without any sintering aids. Near-full density was achieved for all samples, especially in the HfC-contained samples. The porosity in pure TaC samples was caused by the oxygen contamination (Ta2O5) on the starting powder surface. The addition of HfC increased the overall densification by transferring the oxygen contamination from TaC surface and forming ultrafine HfO2 and Hf-O-C grains. With the increasing HfC concentration, the overall grain size was reduced by 50% from HfC- 80 vol% TaC to HfC-20 vol% TaC sample. The solid solution formation required extra energy, which restricted the grain growth. The lattice parameters for the solid solution samples were obtained using X-ray diffraction which had an excellent match with the theoretical values computed using Vegard's Law. The mechanical properties of the solid solution samples outperformed the pure TaC and HfC carbides samples due to the increased densification and smaller grain size.

Wide-range thermometry based on green up-conversion luminescence of K3LuF6:Yb3+/Er3+ bulk oxyfluoride glass ceramics

Abstract: Yb3+/Er3+ ions codoped bulk glass ceramics (GC) with embedded monoclinic K3LuF6 nanocrystals are reported for potential temperature-sensing application by using the fluorescence intensity ratio method. Such GC with good transparency and enhanced up-conversion were prepared by the simple conversional melt-quenching method and subsequent annealing process. Optical, structural, and temperature-sensing up-conversion properties were characterized systematically. Optical spectroscopy analysis confirms the incorporation of Yb3+/Er3+ into the K3LuF6 crystalline lattice, resulting in enhanced up-conversion luminescence. Compared to other Er3+-doped typical systems, Er3+ ions in K3LuF6 GC present large energy gap (870 cm−1) and high relative sensitivity (37.6 × 10−4 K−1 at 625 K), revealing that K3LuF6:Yb3+/Er3+ GC can be excellent candidates for optical thermometers.

A comprehensive sintering mechanism for TBCs‐Part I: An overall evolution with two‐stage kinetics

Abstract: A comprehensive sintering mechanism for lamellar thermal barrier coatings was reported experimentally and theoretically in this study. To begin with, an overall property evolution with two-stage kinetics was presented during thermal exposure. The increase in mechanical property at initial thermal exposure duration (stage-I) was much faster with respect to that in the following longer duration (stage-II). At the stage-I, the in situ pore healing behavior revealed that the significant faster sintering kinetics was attributed to the rapid healing induced by multipoint connection at the intersplat pore tips, as well as a small quantity of the narrow intrasplat cracks. At the following stage-II, the residual wide intersplat pore parts and the wide intrasplat cracks decreased the possibility of multiconnection at their counter-surfaces, resulting in a much lower sintering kinetic. Moreover, a structural model based on the microstructure of plasma sprayed YSZ coatings was developed to correlate the microstructural evolution with mechanical property. Consequently, the model predicted a two-stage evolutionary trend of mechanical property, which is well consistent with experiments. In brief, by revealing the pore healing behavior, this comprehensive sintering mechanism shed light to the structure tailoring toward the advanced TBCs with both higher thermal-insulating effect and longer life time.

Grain size effect and microstructure influence on the energy storage properties of fine-grained BaTiO3-based ceramics

Abstract: 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.

In-situ measurements of lattice expansion related to defect generation during flash sintering

Abstract: We report results from in-situ measurements of lattice expansion during flash sintering of 3 mol% yttria stabilized tetragonal zirconia taken at the Advanced Photon Source, Argonne National Laboratory. The expansion is anisotropic, with the relative expansion of the a-lattice constant exceeding that of the c-lattice constant. The anisotropic expansion cannot be explained by thermal expansion and is consistent with predictions from ab-initio calculations based upon the generation of vacancy-interstitial pairs of zirconium and oxygen.

Cold sintering process for ZrO2‐based ceramics: significantly enhanced densification evolution in yttria‐doped ZrO2

Abstract: We recently developed a novel technique of cold sintering process (CSP) to obtain dense ceramics at extraordinarily low temperatures. In this communication, we demonstrate the feasibility of applying CSP to zirconia-based ceramics. As exemplified by 3Y-TZP ceramics, a significantly enhanced densification evolution is observed. Water is simply utilized as a sintering aid to assist the ceramic densification under an applied external pressure. The low-temperature advantage of CSP outstands in contrast to the densification curves compiled from other sintering techniques. A gradual monoclinic-to-tetragonal phase transformation is revealed in correspondence to the densification development, as well as contributes to the mechanical hardness evolution. A Vickers Hardness reaches ~10.5 GPa after annealing the cold-sintered ceramics at 1100°C, which is comparable to those values reported in the previous studies at higher sintering temperatures. Such a sintering methodology is of significant importance as it provides a roadmap for cost-effective processing of zirconia-based ceramics and composites that enable broad practical applications.

Effect of Sintering Additives on Relative Density and Li-ion Conductivity of Nb-Doped Li7La3ZrO12 Solid Electrolyte

Abstract: Lithium ion conductors with garnet-type structure are promising candidates for applications in all solid-state lithium ion batteries, because these materials present a high chemical stability against Li metal and a rather high Li+ conductivity (10−3–10−4 S/cm). Producing densified Li-ion conductors by lowering sintering temperature is an important issue, which can achieve high Li conductivity in garnet oxide by preventing the evaporation of lithium and a good Li-ion conduction in grain boundary between garnet oxides. In this study, we concentrate on the use of sintering additives to enhance densification and microstructure of Li7La3ZrNbO12 at sintering temperature of 900°C. Glasses in the LiO2-B2O3-SiO2-CaO-Al2O3 (LBSCA) and BaO-B2O3-SiO2-CaO-Al2O3 (BBSCA) system with low softening temperature (<700°C) were used to modify the grain-boundary resistance during sintering process. Lithium compounds with low melting point (<850°C) such as LiF, Li2CO3, and LiOH were also studied to improve the rearrangement of grains during the initial and middle stages of sintering. Among these sintering additives, LBSCA and BBSCA were proved to be better sintering additives at reducing the porosity of the pellets and improving connectivity between the grains. Glass additives produced relative densities of 85–92%, whereas those of lithium compounds were 62–77%. Li7La3ZrNbO12 sintered with 4 wt% of LBSCA at 900°C for 10 h achieved a rather high relative density of 85% and total Li-ion conductivity of 0.8 × 10−4 S/cm at room temperature (30°C).