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Abnormal grain growth (AGG), where a small number of grains grow to sizes much larger than the neighboring matrix grains, is a frequent occurrence in liquid phase sintering of ceramics and cermets. As AGG can be detrimental to the material properties, a considerable amount of research on the nature, causes and suppression of AGG has been carried out. In this review, we outline the mixed control theory of grain growth and the principle of microstructural evolution that have been developed by Kang and coworkers over the last two decades. The theory and the principle, which are based on theories of crystal growth from a liquid, state that grain growth behavior is controlled by the nature of the solid‐liquid interfaces, either atomically rough (macroscopically rounded) or smooth (macroscopically faceted). For grains with atomically rough solid‐liquid interfaces, growth is controlled by diffusion of solute through the liquid phase and normal grain growth always occurs. For grains with faceted solid‐liquid interfaces (or a mixture of rough and faceted interfaces), growth is interface reaction‐controlled and diffusion‐controlled below and above a critical driving force for growth, respectively. Depending on the relative values of the critical driving force for growth Δgc and the maximum driving force for the largest grain in the system Δgmax, pseudo‐normal, abnormal, and stagnant grain growth can take place. Based on this theory and principle, we present strategies for suppressing AGG by adjusting Δgc and Δgmax to avoid AGG and examples of the successful use of these strategies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The structural transition of grain boundaries in BaTiO3 and related grain growth behavior have been investigated with changing Nd2O3 dopant concentration and oxygen partial pressure. A correlation among total vacancy concentration, grain boundary structure and grain growth behavior was found. When the total vacancy concentration was low by any means, grain boundaries were well faceted and abnormal grain growth occurred in the presence of {111} double twins. However, as the total vacancy concentration increased, the fraction of faceted grain boundary gradually decreased and the formation of {111} twins was suppressed. Further increase in the total vacancy concentration resulted in a change in grain boundary structure from faceted to rough and suppression of abnormal grain growth. These observed changes in grain growth behavior, including a grain growth anomaly with donor doping, can therefore be attributed to a change in boundary mobility with grain boundary structure, which was governed by the total vacancy concentration. [Copyright &y& Elsevier]

Abstract: (K0.5Na0.5)NbO3 is a potential lead-free piezoelectric ceramic, but often suffers from abnormal grain growth. Previous work on BaTiO3 and SrTiO3 has shown that abnormal grain growth can be suppressed by controlling the sintering atmosphere. In the present work, (K0.5Na0.5)NbO3 was sintered in atmospheres ranging from O2 to H2 and the effect on grain growth behaviour studied. Sintering in reducing atmospheres causes a delay in the onset and a reduction in the amount of abnormal grain growth. The effect of sintering atmosphere on grain growth behaviour can be explained using the 2D nucleation-controlled theory of grain growth. Changes in the grain shape during sintering in reducing atmospheres indicate a reduction in the edge free energy of (K0.5Na0.5)NbO3 caused by an increase in the concentration of oxygen vacancies. This decreases the critical driving force necessary for rapid grain growth and causes a transition from abnormal to pseudo-normal followed by abnormal grain growth. [Copyright &y& Elsevier]

For its analysis and understanding, sintering has been conventionally assumed to occur in proportion to driving force [J. Appl. Phys. 36 (1955) 1205; Acta Metall. 13 (1965) 227; Mater. Sci. Eng. 48 (1981) 53; Ceramic Processing and Sintering, Marcel Dekker, New York, 2003, p. 470]. This investigation, however, shows for the first time that the conventional assumption does not apply to faceted boundaries and that the microstructure can be frozen for faceted boundaries. A structural transition from rough to faceted was induced in BaTiO3, by changing oxygen partial pressure. As long as the boundary was rough, continuous grain growth and densification occurred with sintering time, in agreement with the conventional assumption. With the onset of the structural transition from rough to faceted, however, grain growth and densification rates were reduced and became zero at the completion of the transition. This result demonstrates that critical driving forces are present for grain growth and densification for faceted boundaries, unlike the conventional understanding. The result also suggests that the formation or maintenance of faceted boundaries is crucial to inhibit grain growth and thus to produce ultrafine-structured materials. [Copyright &y& Elsevier]

To investigate the effect of dislocations on grain growth in polycrystals, two sets of experiments were performed using SrTiO3 single crystals and SrTiO3 powder compacts. In the first set, with single crystals embedded in 2.0-mol%-TiO2-exess powder compacts, the growth of the single crystal was not affected by dislocations at 1300 °C, below the eutectic temperature, while it was enhanced by dislocations at 1470 °C, above the eutectic temperature, where the grain boundaries are wetted by an amorphous phase. In the second set, with 0.5-mol%-Nb2O5-doped powder compacts, the single crystal grew considerably into the SrTiO3 matrix grains in the presence of an amorphous film between the grains at 1470 °C both in 95N2-5H2 and in air, similar to the case of the TiO2-exess samples. However, when annealed at 1470 °C in 95N2-5H2 after pre-annealing at 1250 °C in 95N2-5H2, the amorphous phase remained at triple junctions and did not penetrate the grain boundaries, implying that this boundary configuration also is thermodynamically stable above the eutectic temperature. In this case, growth of the embedded single crystal was insignificant in spite of the presence of dislocations. These experimental observations indicate that the growth of SrTiO3 is promoted by dislocations only when an intergranular amorphous film is present at grain boundaries. The apparent ineffectiveness of dislocations on grain growth promotion without an intergranular amorphous film is discussed in terms of a low fraction of facetted grain boundaries and grain boundary drag by triple junctions. [Copyright &y& Elsevier]

To fully densify a powder compact, we should avoid two things: (i) entrapment of insoluble gases within pores and (ii) entrapment of isolated pores within grains. This paper describes general directions for promoting full densification in view of the above two points. Emphasis is placed on ways to potentially prevent pore entrapment in terms of grain growth control. Currently available techniques that can enhance densification while suppressing grain growth are briefly described, and their major mechanisms are discussed. [ABSTRACT FROM AUTHOR]

The grain growth behavior of 0.95(Na0.5Bi0.5)TiO3 –0.05BaTiO3 (mole fraction, NBT–5BT) grains was investigated with excess Bi2O3 addition. The powder compacts of NBT–5BT were sintered at 1200 °C for various sintering times and with various amounts of Bi2O3 (0.1, 1.5, 4.0 and 10.0 mol%). When Bi2O3 was added to round-edged cubic NBT–5BT, the grain shape changed to a more faceted cube and the amount of liquid phase increased during sintering. A more faceted cubic grain shape indicates an increase in the critical driving force for appreciable growth of grains. However, obvious abnormal grain growth did not appear in any of the NBT–5BT samples with excess Bi2O3. The amount of liquid phase increased as the amount of Bi2O3 increased. Therefore, the rate of grain growth could be decreased by the increasing the distance for the diffusion of atoms. These observations allowed us to conclude that the growth of Bi2O3-excess NBT–5BT grains is governed by the growth of facet planes via the two-dimensional nucleation grain growth mechanism during changing grain shape and amount of liquid. [ABSTRACT FROM AUTHOR]

Abstract: The structural transition of grain boundaries in BaTiO3 and related grain growth behaviors have been investigated for a wide range of oxygen partial pressures . A correlation among , grain boundary structure and grain growth behavior has been found. At , all of the grain boundaries were faceted and abnormal grain growth occurred with the assistance of {111} twins as in previous investigations. As decreased below 10−11 atm the formation of {111} twins was inhibited. At a moderately low between 10−11 and 10−17 atm, grain growth was suppressed, although a small fraction, less than 10%, of the boundary was rough. With an increased fraction of rough boundaries on reduction of to 4×10−18 atm, abnormal grain growth was observed for the first time in the absence of {111} twins in BaTiO3. Further increases in the rough fraction to ∼80% changed the growth behavior to normal. These observations show that in BaTiO3, a variety of growth behaviors can result with reduction of : from abnormal with the assistance of {111} twins, stagnant without {111} twins, abnormal again without {111} twins to normal without {111} twins. The observed difference in growth behavior with and grain boundary structure has been explained in terms of the change in step free energy and the critical driving force for appreciable migration of the boundary. [Copyright &y& Elsevier]

Kinetic formation and distribution of amorphous films at grain boundaries during grain growth have been studied in a model ceramic system, BaTiO3, using single crystal/polycrystal bi-layers. Single crystal containing BaTiO3 powder compacts with different TiO2 additions (0.1, 0.4 and 1.0 mol%) were heat-treated at 1250 °C for 10 h in H2 and then annealed at 1350 °C, above the eutectic temperature, in air for up to 50 h. During the air-annealing the single crystals grew into fine matrix grains of ∼2.0 μm diameter while no grain growth occurred in the matrix. With the growth of the single crystal, an amorphous film formed at the boundaries between the crystal and the matrix grains, and its thickness increased. As the amount of TiO2 increased from 0.1 to 1.0 mol%, the film thickness increased about ten times. These experimental results show for the first time that: (i) grain growth in solid–liquid two phase materials can induce the formation and thickening of intergranular amorphous films at dry grain boundaries, and (ii) the film thickening is mainly due to the penetration of liquid pockets at triple junctions into the films formed during grain growth. [Copyright &y& Elsevier]

When Fe2O3 was substantially dissolved into an (0001) alumina single crystal through a vapor phase at 1500 °C, recrystallization of the single crystal occurred. TEM observation showed that the nucleation of the recrystallized grains occurred through the formation of very small angle wall boundaries via a rearrangement of the misfit dislocations in the surface diffusion layer and their subsequent transformation into high angle grain boundaries. The dislocation wall mechanism appears, therefore, to be the nucleation mechanism of diffusion-induced recrystallization in single crystal alumina. However, an abrupt change in Fe2O3 concentration was observed across the boundary between the single crystal and the growing recrystallized grains, indicating that the growth of the recrystallized grains occurred by diffusion-induced grain-boundary migration. During extended annealing, the division of large recrystallized grains into small ones (repeated recrystallization) often occurred. The observed repeated recrystallization was attributed to the redistribution of dislocations formed in the grains where the solute concentration was non-uniform. [Copyright &y& Elsevier]

Change in interface morphology with ionic vacancy concentration and the correlation between interface structure and grain growth behavior in strontium titanate (SrTiO3) have been investigated using SrTiO3 single crystals and powder compacts. Under experimental conditions where SrTiO3 contains a negligible amount of ionic vacancies, the shape of the single crystal embedded in matrix grains was well faceted, showing a strong anisotropy in interfacial energy. However, as strontium or oxygen vacancies increased with the addition of an Nb2O5 donor dopant or reduction of oxygen partial pressure, the faceted shape changed to a smoothly curved rough one indicating that an interface roughening transition occurred and, as a result, the anisotropy in interfacial energy was considerably reduced. Grain growth behavior was also strongly dependent on the interface structures; while normal grain growth occurred when the interfaces were rough, abnormal grain growth behavior was observed in the samples with faceted interfaces. It appears, therefore, that the ionic vacancies in SrTiO3 can cause the interface roughening transition and change the resultant grain growth behavior. [ABSTRACT FROM AUTHOR]

Abstract: Grain coarsening behavior in the 95Na1/2Bi1/2TiO3–5BaTiO3 system has been studied as a function of the addition of TiO2. As the amount of added TiO2 was increased, the grain shape changed to a more faceted cube, indicating an increase in the step free energy of the facets, and hence a rise in the critical driving force for appreciable growth of grains. Grain coarsening behavior also changed from pseudo-normal to abnormal with an increasing TiO2 concentration and thus increased faceting. The pseudo-normal behavior observed in the system without TiO2 addition also changed to quite abnormal behavior during extended sintering. These observations support our theoretical prediction based on the coupling effects between the maximum driving force for growth and the critical driving force for appreciable growth. [Copyright &y& Elsevier]

Abstract: The changes in solid/liquid interface structure and grain growth behavior with oxygen partial pressure were systematically studied during liquid-phase sintering of 8TiO2/2SiO2-added BaTiO3. As the of the sintering atmosphere increased, the grain boundaries and solid/liquid interfaces showed increased faceting, indicating an increase in step free energy. This increase in and step free energy caused a change in grain growth behavior as a function of sintering time. When samples were sintered in H2, abnormal grain growth (AGG) occurred from the beginning, resulting in a coarse microstructure with a large average grain size. With increasing the incubation time necessary for AGG also increased. Finally, for samples sintered in air, AGG did not occur even after 100h. These changes in incubation time for abnormal grain growth demonstrate the effect of changing the step free energy on the microstructural development during liquid phase sintering of ceramic systems. [ABSTRACT FROM AUTHOR]

Abstract: The effect of an intergranular amorphous film on grain growth behavior has been studied in a faceted model system, BaTiO3. We prepared two kinds of samples with and without intergranular amorphous films but with the same grain size and density. During annealing the samples at 1350°C in air, abnormal grain growth occurred in samples with intergranular amorphous films while grain growth was inhibited in samples with dry boundaries, indicating the presence of a pinning force in samples with dry boundaries. To compare the mobilities of dry and wet boundaries, single crystal and polycrystal bilayer samples with or without amorphous films were prepared and annealed at 1340°C. In contrast to the observed grain growth behavior in polycrystals, the growth of the single crystal into the polycrystal with dry boundaries was faster than that into the polycrystal with wet boundaries, demonstrating the higher mobility of a dry boundary, unlike the conventional understanding. [Copyright &y& Elsevier]

Abstract: The structural relaxation and formation energies of intrinsic vacancies in cubic-BaTiO3 were studied by using a first-principles plane-wave-based pseudopotential calculation. The calculated defect formation energies and the atomic relaxations were compared with previously reported data for SrTiO3 [Tanaka T, Matsunaga K, Ikuhara Y, Yamamoto T. Phys Rev B 2003;68:205213]. It was found that the formation energies of the partial Schottky species, and , are the lowest in the oxidized condition. In contrast, as the oxygen chemical potential decreased, namely the reduced condition, the O vacancy becomes dominant in cubic-BaTiO3. Concerning the atomic relaxation, it was found that BaTiO3 and SrTiO3 show a difference in magnitude. It was also found that the overall vacancy formation energy in BaTiO3 is higher than that in SrTiO3. [Copyright &y& Elsevier]