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The grain growth behavior in the 89Na 1/2 Bi 1/2 TiO 3 -11BaTiO 3 system has been studied at 1100 and 1200 °C, and under two-step sintering at these two temperatures. When the powder compacts were sintered at 1100 and 1200 °C, the growth behavior was distinctively abnormal and moderately abnormal, respectively. The difference can be explained in terms of the coupling effect between the maximum driving force for the growth of the largest grain and the critical driving force for appreciable migration of the grain boundary. In two-step sintering, abnormal grain growth was suppressed in the second step at 1100 °C. This beneficial effect of two-step sintering for grain growth suppression is found to be the result of a grain size distribution right after the first sintering step narrower than that of the sample conventionally sintered at 1100 °C. These results of single and two-step sintering support the general applicability of the principle of microstructural evolution. [ABSTRACT FROM AUTHOR]

In conjunction with a previous investigation [Jung SH, Yoon DY, Kang SJL. Acta Mater 2013;61:5685], we have investigated the correlation between grain boundary structure and grain growth behavior in a model Ni system. Ultrafine Ni powder compacts of 180nm size were sintered in a wide range of temperatures from 500 to 1150°C and for various times in wet H2. Between 500 and 600°C, abnormal grain growth (AGG) readily occurred, with the formation of cube-shaped grains. Between 650 and 900°C, grain growth behavior was stagnant up to 6000min due to impingement of rapidly grown abnormal cube-shaped grains during heating. At 950°C, some of the impinged abnormal grains suddenly grew after 20min, showing secondary AGG behavior. As the sintering temperature was increased further, the grain growth behavior became quite normal. These changes in grain growth behavior with increasing temperature were accompanied by the structural transition of grain boundaries from fully faceted to partially faceted and defaceted. The observed grain growth behavior with respect to the grain boundary morphology is explained in terms of a coupling effect of the maximum driving force for grain growth and the critical driving force for appreciable migration of faceted boundaries. The present experimental results appear to support the validity of the previously suggested mixed control model of grain growth and the principle of microstructural evolution in polycrystals. [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]

The effect of abnormal grain growth on the formation of amorphous films at grains boundaries was studied in a model system BaTiO3. 0.4 mol% TiO2-excess BaTiO3 powder compacts were sintered at 1380°C for various times up to 16 h. During the sintering, abnormal grains formed. With the growth of the abnormal grains, amorphous films formed and eventually thickened up to 19.2 nm at grain boundaries. The film formation is attributed to the accumulation of Ti solutes at grain boundaries with the grain growth, while the film thickening is mostly caused by the redistribution of liquid at triple junctions. Extended annealing of the 16-h-sintered sample at 1350°C for 15 days resulted in a thinning of the film to nearly 1.7 nm without a change in the grain size, showing an equilibrium thickness. This result demonstrates that the film thickness observed during the growth of the grain may not be the equilibrium thickness. The result further suggests that the shape of the abnormal grains, even when equiaxed, can differ from the equilibrium shape. [ABSTRACT FROM AUTHOR]

The effect of abnormal grain growth on the formation of amorphous films at grains boundaries was studied in a model system BaTiO3. 0.4 mol% TiO2-excess BaTiO3 powder compacts were sintered at 1380°C for various times up to 16 h. During the sintering, abnormal grains formed. With the growth of the abnormal grains, amorphous films formed and eventually thickened up to 19.2 nm at grain boundaries. The film formation is attributed to the accumulation of Ti solutes at grain boundaries with the grain growth, while the film thickening is mostly caused by the redistribution of liquid at triple junctions. Extended annealing of the 16-h-sintered sample at 1350°C for 15 days resulted in a thinning of the film to nearly 1.7 nm without a change in the grain size, showing an equilibrium thickness. This result demonstrates that the film thickness observed during the growth of the grain may not be the equilibrium thickness. The result further suggests that the shape of the abnormal grains, even when equiaxed, can differ from the equilibrium shape. [ABSTRACT FROM AUTHOR]

Grain growth behavior and solid-state single crystal growth (SSCG) in the Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 (PMN–35PT) system have been investigated with varying Li2O/PbO ratios. The effect of dislocation density on crystal growth has also been studied. For SSCG, a BaTiO3 single-crystal seed was embedded in a polycrystalline PMN–PT matrix. During annealing, a PMN–PT single crystal grew from the seed at the cost of the small matrix grains. Addition of Li2O dopant first enhanced and then reduced abnormal grain growth in the matrix. In the 2 mol% Li2O and 6 mol% PbO excess PMN–PT samples annealed at 1200°C, considerable single-crystal growth occurred without formation of abnormally large grains in the matrix. Increasing the dislocation density in the BaTiO3 seed crystal resulted in enhanced growth of single crystals. These results were explained in terms of interface reaction-controlled nucleation and growth, based on crystal growth theories. [ABSTRACT FROM AUTHOR]

The origin of WC/WC grain boundaries in liquid-phase-sintered WC-Co alloys has bet, n investigated in a WC-Mo2C-Co model system using coarse WC polygrain powder. The evolution of grain shape during liquid phase sintering was able to be identified by observing a growth laver that contained Mo. During liquid phase sintering, most of the grain boundaries in the powder were penetrated by a Co liquid but some of them were not. Electron backscattered diffraction analysis confirmed that some boundaries in the powder, in particular. Σ2 twist boundaries and Σ97 special boundaries, remained intact during liquid phase sintering, These experimental results confirm that the grain boundaries of WC grains in liquid-phase-sintered WC-Co alloys originated from those present in the starting powder. [ABSTRACT FROM AUTHOR]

Ever since the development of a late stage sintering model by Coble in 1961, densification at the final stage of solid state sintering has been explained basically by Coble''s model for lattice diffusion. For grain boundary diffusion, equations based on Herring''s scaling law were later developed and utilized. The present paper examines previous sintering models, in particular, Coble''s sintering model, and presents a new diffusion model and kinetic equations. The new model accounts for the role of grain boundaries and the diffusion area in densification in contrast to Coble''s model. Sintering kinetics have been calculated and the relative contributions of lattice and grain boundary diffusion evaluated. It has been shown that the densification mechanism can vary with pore size change during sintering, unlike the conclusions of previous investigations. The results have been highlighted as a sintering diagram. [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 donor segregation in grain boundary regions and its effect on grain growth in SrTiO[sub 3], SrTiO[sub 3] powder compacts were doped with Nb[sub 2]O[sub 5] and sintered in air or in hydrogen. The Nb-doped SrTiO[sub 3] sintered in air did not show any detectable Nb segregation at the grain boundary region while an appreciable segregation was observed in the spacecharge region of the sample sintered in H[sub 2]. The observed donor segregation in H[sub 2] suggests a negative grain boundary charge and compensating positive space charge, which are the opposite to those in air. The negative grain boundary core was attributed to the segregation of inherently present acceptor impurities and the trapping of electrons at grain boundaries. In the H[sub 2]-sintered sample, where the added Nb ions were segregated in the space-charge region, the grain growth was suppressed. This result may indicate that the grain growth suppression in H[sub 2] is due to the Nb solute drag of the boundary motion and the reduction in Ti vacancies. [ABSTRACT FROM AUTHOR]

The current-voltage characteristics with acceptor segregation at grain boundaries have been investigated in H2-sintered SrTiO3. Al-added SrTiO3 was sintered in H2 and then annealed in air for selective oxidation of grain boundaries. The samples showed nonlinear current-voltage characteristics, and both the breakdown voltage and nonlinearity coefficient increased with Al concentration. An energy dispersive spectroscopy analysis revealed that Al ions were segregated at the grain boundary, suggesting the formation of Schottky barriers at the boundary. The present results thus point toward the possibility of modifying the grain boundary composition in SrTiO3 as well as fabricating effective switching devices by H2 sintering. [ABSTRACT FROM AUTHOR]

The current-voltage characteristics with acceptor segregation at grain boundaries have been investigated in H2-sintered SrTiO3. Al-added SrTiO3 was sintered in H2 and then annealed in air for selective oxidation of grain boundaries. The samples showed nonlinear current-voltage characteristics, and both the breakdown voltage and nonlinearity coefficient increased with Al concentration. An energy dispersive spectroscopy analysis revealed that Al ions were segregated at the grain boundary, suggesting the formation of Schottky barriers at the boundary. The present results thus point toward the possibility of modifying the grain boundary composition in SrTiO3 as well as fabricating effective switching devices by H2 sintering. [ABSTRACT FROM AUTHOR]

By doping a few atomic percent of 3d-block cations, we demonstrate that the high dielectric response in CaCu3Ti4O12 can be reduced by a factor of ∼103 at room temperature. Each of the added dopants shows its own preferential substitution on either Cu or Ti sites. The dopants that act as acceptors have a critical impact on the disappearance of the electrostatic potential barrier at grain boundaries, resulting in drastically decreased permittivity values of <90 without voltage dependence. The present doping experiment directly shows that the potential barrier at internal interfaces is a key factor for the peculiar dielectric phenomena. [ABSTRACT FROM AUTHOR]

The effect of grain boundary structure, either rough or faceted, on diffusion-induced grain boundary migration (DIGM) has been investigated in BaTiO3. SrTiO3 particles were scattered on the polished surfaces of two kinds of BaTiO3 samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H2 for those with rough boundaries. In the BaTiO3 samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain-boundary migration hardly occurred in the BaTiO3 samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted. [ABSTRACT FROM AUTHOR]

The effect of grain boundary structure, either rough or faceted, on diffusion-induced grain boundary migration (DIGM) has been investigated in BaTiO3. SrTiO3 particles were scattered on the polished surfaces of two kinds of BaTiO3 samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H2 for those with rough boundaries. In the BaTiO3 samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain-boundary migration hardly occurred in the BaTiO3 samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This reply addresses some fundamental questions related to estimated sintering pressures. The additional contribution of grain boundary energy to sintering pressure varies depending on the calculation method and assumption, unlike the comment by Wakai. For the grains adjacent to a pore with non-equilibrium shapes, the pressures at the pore surface and grain boundary need to be clarified to estimate the sintering pressure properly. Application of the energy variation method to the local geometry also suggests a need for further consideration. [Copyright &y& Elsevier]

Systematic control of electrical properties is a challenging task in polycrystalline materials having significant electrical heterogeneity. In particular, as most aliovalent dopants in oxides usually play a single role of either a donor or an acceptor, controlling the respective electronic conductance of grains and grain boundaries in a separate manner by simple doping is not easily achieved. Using the polycrystalline perovskite Ca 1/4 Cu 3/4 TiO 3 , the grains of which have relatively high conductance and the grain boundaries reveal much lower conductance, we demonstrate the donor–acceptor bifunctionality of dysprosium (Dy) and its impact on the variation of electrostatic potential barriers at grain boundaries. DC current–voltage measurements and impedance spectroscopy analyses consistently show that Dy selectively affects the grain-boundary conductance without a substantial change of conductance in the grains. Atomic-scale scanning transmission electron microscopy also visualizes the distinct site occupancy of Dy at the Ti columns in the grain-boundary region in contrast to the Dy occupation at the Ca columns in the bulk grains, directly showing that Dy is a donor in the bulk and simultaneously an acceptor in the grain-boundary region. In addition to providing an efficient way of tuning the electrostatic potential barriers, this bifunctional effect of Dy emphasizes the significance of understanding the correlation between the atomic-level site occupancy of dopants and the overall electrical properties. [ABSTRACT FROM AUTHOR]

Abstract: During annealing of ultrafine nickel powder compacts, abnormal grain growth (AGG) occurred with the formation of cube-shaped grains. The observed AGG is similar to the late-stage abnormal grain growth (LS-AGG) that is commonly observed in electrodeposited (ED) Ni sheets. The cube shape of the abnormal grains with {100} facets was identical to that of late-stage abnormal grains in ED Ni. As the temperature increased, the time to the appearance of abnormal grains decreased. After impingement of abnormal grains, little further grain growth occurred, indicating stagnant grain growth behavior. The formation of abnormal grains in the present study, however, cannot be explained by the previously suggested AGG mechanisms, i.e. the S phase wetting mechanism and initial texture mechanism, because no amorphous phase containing S was present and no texture developed in our samples. In contrast, the observed grain growth behavior can be well explained in terms of the coupling effect of the maximum driving force for grain growth and the critical driving force for appreciable migration of faceted boundaries. It is concluded that the AGG in ultrafine-grained Ni occurs via mixed migration mechanisms, i.e. diffusion and interface reaction-controlled, of faceted boundaries with respect to the driving force. [Copyright &y& Elsevier]

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]

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. [ABSTRACT FROM AUTHOR]

Abstract: A non-linear relationship between boundary migration and its driving force for faceted grain boundaries is demonstrated in a BaTiO3 model system using the solid-state single-crystal growth technique. BaTiO3 (0.4mol.% TiO2) samples with different grain sizes were prepared by sintering powder compacts in H2 for various times. Single-crystal seeds with {100} and {210} orientations were joined to the sintered samples and annealed in air for various times up to 20h. During the air annealing, the matrix grain size did not change, indicating that the driving force for the growth of the single-crystal seeds remained constant. Under driving forces that exceeded a critical value, the seed crystals grew into the polycrystals. The critical driving force was lower and the growth rate was ∼10 times higher for the {100} crystal compared with the {210} crystal. These results demonstrate the presence of a critical driving force for the migration of a faceted boundary and the remarkable effect of the crystallographic orientation on the boundary migration. The observed nonlinear migration behavior of faceted boundaries is similar to that of faceted solid/liquid interfaces. [Copyright &y& Elsevier]

Grain boundaries (GBs) significantly affect the properties of materials. In an effort to examine the phenomena at GBs, many model boundaries, typically symmetric tilt GBs, have been investigated. However, the geometries of symmetric tilt GBs are too restricted to represent commonly occurring interlace phenomena properly in polycrystalline materials. Thus, a method of applying density functional theory (DFT) to asymmetric GBs has long been desired. Here, we present a simple geometric method and a new GB model with two surfaces which make it possible to characterize an asymmetric tilt GB and calculate the GB energetics. Our method can be extended to study other geometric asymmetric interfaces in various materials. The proposed technique thus paves the way for DFT-related studies of asymmetric interfaces. [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]