Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

Abnormal grain growth (AGG) in alumina with anorthite liquid has been observed with varying anorthite and MgO contents, at 1620°C. When only anorthite is added to form a liquid matrix, the grain–liquid interfaces have either flat or hill-and-valley shapes indicating atomically flat (singular) structures. The large grains grow at accelerated rates to produce AGG structures with large grains elongated along their basal planes. This is consistent with the slow growth at low driving forces and accelerated growth above a critical driving force predicted by the two-dimensional nucleation theory of surface steps. With increasing temperature, the AGG rate increases. The number density of the abnormally large grains increases with increasing anorthite content. The addition of MgO causes some grain–liquid interfaces to become curved and hence atomically rough. The grains also become nearly equiaxed. With increasing MgO content the number density of the abnormally large grains increases until the grain growth resembles normal growth. This result is qualitatively consistent with the decreasing surface step free energy associated with partial interface roughening transition.     

Effect of Sintering Atmosphere on Grain Shape and Grain Growth in Liquid-Phase-Sintered Silicon Carbide

We investigated the effects of the sintering atmosphere on the interface structure and grain-growth behavior in 10-vol%-YAG-added SiC. When α-SiC was liquid-phase-sintered in an Ar atmosphere, the grain/matrix interface was faceted, and abnormal grain growth occurred, regardless of the presence of α-seed grains. In contrast, when the same sample was sintered in N₂, the grain interface was defaceted (rough), and no abnormal grain growth occurred, even with an addition of α-seed grains. X-ray diffraction analysis of this sample showed the formation of a 3C (β-SiC) phase, together with a 6H (α-SiC) phase. These results suggest that the nitrogen dissolved in the liquid matrix made the grain interface rough and induced normal grain growth by an α→β reverse phase transformation. Apparently, the growth behavior of SiC grains in a liquid matrix depends on the structure of the grain interface: abnormal growth for a faceted interface and normal growth for a rough interface.     

Normal and Abnormal Grain Growths in BaTiO3 Fibers

The grain growth mechanisms along the BaTiO₃ fibers were studied between 1150°C and 1250°C. The normal grain growth always reached a stagnant stage after certain heat‐treatment duration caused by the surface pinning effect. However, the abnormal grain growth (AGG) was not pinned by such surface effect, and can grow continuously. The confined normal grain (or matrix grain) size provides the driving force for AGG. The fiber diameter has an important influence on the grain growth behaviors. Submicrometer fibers have relative small stagnant grain sizes, resulting in large driving force for AGG. Abnormal grain growth occurred below 1200°C in the submicrometer diameter fibers, but was not observed at the same temperature in the fibers with diameter of above 1 μm. Due to the large AGG driving force, large number densities of abnormal grains were observed in submicrometer fibers, resulting in “bamboo‐like” microstructure. Fibers with diameters of 1–2 μm were able to be converted into single crystal fibers up to several tens of micrometers due to the relative small AGG driving forces.     

Effect of Ba6Ti17O40/BaTiO3 Interface Structure on {111} Twin Formation and Abnormal Grain Growth in BaTiO3

A structural transition of Ba₆Ti₁₇O₄₀/BaTiO₃ interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin-assisted abnormal growth of faceted BaTiO₃ grains was also obtained.     

Abnormal grain growth in (K,Na)NbO3-based lead-free piezoceramic powders

Abnormal grain growth (AGG) is frequently observed in sintered (K, Na)NbO₃ (KNN)-based piezoceramics. However, in the present study, abnormal grain growth was unexpectedly discovered in calcined KNN-based powders. To explain the phenomenon, three well-established models that account for the AGG in sintered ceramics were discussed, including (a) liquid-phase-assisted grain growth, (b) two-dimensional nucleation grain growth, and (c) complexion coexistence. However, the AGG in calcined powders was concluded to be none of them, but a consequence of the A-site compositional inhomogeneity in the K₂CO₃-Na₂CO₃-Nb₂O₅ ternary system. Since repeated calcination and ball milling have low efficiency on solving AGG and the accompanied compositional inhomogeneity, abnormal grains were found to coexist with normal grains at a very high calcination temperature, that is, 1000°C. The compositional inhomogeneity is believed to be remaining even after sintering and consequently deteriorate the comprehensive performances, which might be a determinant for the unstable reproduction of KNN-based piezoceramics.     

Dependence of Grain Growth and Grain-Boundary Structure on the Ba/Ti Ratio in BaTiO3

The grain-growth behavior and grain-boundary structure in titanium-excess BaTiO₃ depend on the amount of excess titanium at 1250° and 1300°3C. With excess titanium, abnormal grain growth (AGG) occurs and the grain boundaries are mostly flat or faceted with hill-and-valley shapes. With 0.5 at.% excess titanium, the large grains have flat {111} faces forming singular grain boundaries parallel to {111} double twins. With excess-titanium content between 0.1 and 0.3 at.%, the abnormal grains appear to have polyhedral shapes with {100} faces. These flat or faceted grain boundaries are expected to have singular structures, and hence AGG can occur by the step growth mechanism. When the excess-titanium content is decreased to 0, the grain boundaries become curved, indicating a rough atomic structure, and normal grain growth occurs.     

Shape Dependence of the Coarsening Behavior of Niobium Carbide Grains Dispersed in a Liquid Iron Matrix

In niobium carbide–iron (NbC-Fe) specimens where the grains were faceted, abnormally large grains appeared during coarsening. Normal and uniform grain growth occurred when the grain shape was changed to a spherical morphology by the addition of a small amount of boron. The results have been discussed, in terms of a coarsening mechanism, depending on the atomic structure of the interface. For faceted grains with an atomically smooth interface structure, the coarsening was suggested to occur via two-dimensional nucleation and a lateral-growth mechanism. For spherical grains with an atomically rough interfacial structure, diffusion was suggested to control the coarsening process.     

Effect of TiO2 addition on grain shape and grain coarsening behavior in 95Na1/2Bi1/2TiO3-5BaTiO3

Grain coarsening behavior in the 95Na_1/2Bi_1/2TiO₃-5BaTiO₃ 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.     

Effect of step free energy on delayed abnormal grain growth in a liquid phase-sintered BaTiO3 model system

The changes in solid/liquid interface structure and grain growth behavior with oxygen partial pressure (PO₂) were systematically studied during liquid-phase sintering of 8TiO₂/2SiO₂-added BaTiO₃. As the PO₂ 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 PO₂ 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 PO₂, the incubation time necessary for AGG also increased. Finally, for samples sintered in air, AGG did not occur even after 100 h. 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.     

Effect of the Liquid-Forming Additive Content on the Kinetics of Abnormal Grain Growth in Alumina

Alumina specimens with various amounts of CaO and SiO₂ (1:2 ratio) were prepared, and their abnormal grain growth (AGG) kinetics were investigated. A plot of the area fraction covered by abnormal grains versus log (sintering time) had a sigmoidal shape with an apparent incubation period before the onset of AGG. The overall kinetics of AGG was similar to that of a phase transformation controlled by nucleation and growth. The incubation time and the end point of AGG were strongly dependent on the amount of liquid-forming additives. Correspondingly, the final microstructure was affected by the liquid content: a large grain size and a high aspect ratio at low liquid content and a small grain size and a low aspect ratio at high liquid content.     

Coarsening Behavior of Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S) Grains Dispersed in a Clinker Melt

Microstructural evolution during the heat treatment of cement clinker was investigated. Two model specimens, which consisted of faceted tricalcium silicate (C₃S) and spherical dicalcium silicate (C₂S) grains dispersed in a liquid matrix, were prepared with 5 wt% of large seed particles. The seed particles of faceted C₃S grains grew extensively, whereas those of the spherical C₂S grains grew rather slowly, relative to the matrix grains. As a consequence, C₃S grains exhibited a bimodal size distribution that was typical of exaggerated grain growth, whereas C₂S grains retained a uniform and normal size distribution. These results suggest that the growth of faceted C₃S grains was controlled by the interface atomic attachment, such as two-dimensional nucleation, and that of spherical C₂S grains was controlled by diffusion through the liquid matrix. The dependence of growth mechanisms on grain morphology has been explained in terms of the atomistic structure of the solid/liquid interface.     

Microstructural Evolution During Sintering with Control of the Interface Structure

This paper reports recent theoretical perspectives and experimental results on microstructural evolution during sintering in terms of the interface structure, which is either rough (atomically disordered) or faceted (atomically ordered). The paper presents theoretical predictions and calculations of grain growth during liquid-phase sintering based on crystal growth theories. It is shown that various types of grain growth behavior, which may be normal, abnormal, or stagnant, can appear as a result of the coupling effects of the maximum driving force for growth and the critical driving force for appreciable growth. The predictions are also shown to be valid in the case of solid-state sintering. A number of experimental observations showing the effect of some critical processing parameters have been found to be in excellent agreement with the predictions. Principles of microstructure development (grain growth control) during sintering are suggested. In addition, the effect of the interface structure on densification is briefly described and discussed.     

Grain-Growth Behavior during Stepwise Sintering of Barium Titanate in Hydrogen Gas and Air

To study the effect of oxygen partial pressure on grain growth in BaTiO₃, TiO₂-excess samples have been sintered in air with and without a prior H₂ heat treatment. Without prior H₂ treatment, abnormal grain growth occurs below and above the eutectic temperature ( T _e). An introduction of H₂ treatment before air sintering, however, increases the average grain size and suppresses the formation of abnormal grains during subsequent air sintering below and above T _e. This H₂ treatment effect has been explained in terms of a decrease of the driving force for the growth of faceted grains below a critical value for formation of abnormal grains. The observed grain-growth behavior under various atmospheres demonstrates the possibility of having various microstructures via control of oxygen partial pressure and initial grain size.     

Phase-field simulation of platelike grain growth during sintering of alumina

The toughness of alumina can be improved by utilizing the in situ formation of platelike anisotropic grains during sintering, that is, abnormal grain growth (AGG). Computer simulations of AGG may be effective to realize the conditions for obtaining the desired self-composite microstructure. In the first part of this study, sintering experiments of high-purity alumina powders were conducted to confirm the effects of powder size distribution as well as the amounts of additives. In the second part, a phase-field method for simulating the platelike grain growth was proposed. The large platelike grains were reproduced when the critical driving force of coarsening was set up. The incubation time of AGG was also observed in the case of the narrow size distribution. Although the morphology of the platelike grains did not exactly agree with the experimental observations, a possibility of the present method as a computational tool for simulating platelike AGG was verified.     

Abnormal grain growth in DC flash sintered 3-mol% yttria-stabilized zirconia ceramics

The origin of nonuniform microstructure and abnormal grain growth (AGG) was investigated in flash sintered 3 mol% yttria-stabilized zirconia (3YSZ) ceramics. The microstructural homogeneity decreased with increasing direct current (DC) density and with dwell time in a flash state, eventually resulting in AGG in the specimen core, the first observation of AGG in 3YSZ. Abnormal grains up to 100 μm in size emerged when the DC density was ≥160 mA/mm², and the specimen's density exceeded 99% of theoretical, starting from the cathode and propagating toward the anode. The results are discussed by comparison with established mechanisms and previous experimental evidence concerning AGG in oxides, focusing on the possible effects of the electrochemical reduction at the cathode end of the specimen.     

Bi-templated grain growth maximizing the effects of texture on piezoelectricity

Templated grain growth is beneficial for piezoelectric materials, the properties of which become the best in their single crystalline form. Nevertheless, a textured ceramic prepared by a templated grain growth technique often fails in exhibiting as good properties as expected in single crystals even with a high degree of orientation factor. Here, we propose a new strategy for maximizing texturing effect by suppressing the growth of untextured matrix grains. The textured ceramics made by our method, so-called bi-templated grain growth, are featured by ultrahigh piezoelectric properties (d₃₃ = ～1,031 pC/N, d?g = ～59,000, k_p = ～0.76). A special emphasis is on the achieved electric-field-induced strain of 0.13 % at 1 kV/mm, which is as high as that of single crystals. This work demonstrates that not only the degree of texture but also the coarsening of untextured matrix grains should be well-controlled to best exploit the templated grain growth technique.     

Singular Grain Boundaries in Alumina and Their Roughening Transition

The shapes and structures of grain boundaries formed between the basal (0001) surface of large alumina grains and randomly oriented small alumina grains are shown to depend on the additions of SiO₂, CaO, and MgO. If a sapphire crystal is sintered at 1620°C in contact with high-purity alumina powder, the grain boundaries formed between the (0001) sapphire surface and the small alumina grains are curved and do not show any hill-and-valley structure when observed under transmission electron microscopy (TEM). These observations indicate that the grain boundaries are atomically rough. When 100 ppm (by mole) of SiO₂ and 50 ppm of CaO are added, the (0001) surfaces of the single crystal and the elongated abnormal grains form flat grain boundaries with most of the fine matrix grains as observed at all scales including high-resolution TEM. These grain boundaries, which maintain their flat shape even at the triple junctions, are possible if and only if they are singular corresponding to cusps in the polar plots of the grain boundary energy as a function of the grain boundary normal. When MgO is added to the specimen containing SiO₂ and CaO, the flat (0001) grain boundaries become curved at all scales of observation, indicating that they are atomically rough. The grain boundaries between small matrix grains also become defaceted and hence atomically rough.     

Growth of (Na0.5K0.5)NbO3 single crystals by abnormal grain growth method from special shaped nano-powders

Using the composite powders of (Na_0.5K_0.5)NbO₃ (NKN) nano-particles and nano-rods as starting materials, the NKN single crystals were prepared by abnormal grain growth (AGG) method. The morphology evolution and the formation mechanism in the crystal growth process were investigated in detail. The results revealed that the average size and the apparent quantity of abnormal grains increased gradually with the increase of sintering temperature. The biggest NKN single crystals with size of about 3 mm were obtained at 950 °C for 2 h. Though the nano-particles and nano-rods have the same composition, the driving forces are distinctively different due to the diversity of grain morphology. The nano-rods have the large driving forces especially at high sintering temperature, which plays a dominant role in facilitating the formation of NKN single crystals during AGG process.     

Effect of Grain Coalescence on the Abnormal Grain Growth of Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 Ceramics

Abnormal grain growth (AGG), which occurred during the heat treatment of Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) with excess PbO, was investigated. AGG has been suggested to be the consequence of grain coalescence that results in the formation of Σ3 coincidence site lattice and low angle grain boundaries. Because of reentrant edges appearing at the ends of these boundaries, the coarsening rate of grains was significantly enhanced and AGG occurred.     

Nucleation and growth kinetics of graphene layers from a molten phase

Nucleation and growth of graphene layers from Ni–C melts were investigated. It is shown that upon cooling of supersaturated liquids, graphite will grow either with flake or sphere morphology depending on the solidification rate and degree of supersaturation. At small solidification rates, graphite crystals are normally bounded by faceted low index basal and prismatic planes which grow by lateral movement of ledges produced by 2D-nucleation or dislocations. At higher growth rates, however, both interfaces become kinetically rough, and growth becomes limited by diffusion of carbon to growing interface. The roughening transition from faceted to non-faceted depends on the driving force and nature of growing plane. Due to high number of C–C dangling bonds in prismatic face, its roughening transition occurs in smaller driving force. As such, at intermediate rates, the prismatic interfaces become rough and grow faster while the basal plane is still faceted, leading to formation of flake graphite. At higher growth rates, both interfaces grow with a relatively similar rate leading to initiation of graphite sphere formation, which later grow by a multi-stage growth mechanism. An analytical model is developed to describe the size and morphology of graphite as a function of solidification parameters.