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Pure and Nb-substituted CaCu3Ti4−xNbxO12+x/2 (CCTO, x=0, 0.02, 0.1, 0.2, 0.4) ceramics were prepared by a conventional solid-state sintering, and their electric and dielectric properties were investigated using an impedance analyzer. A single-phase CCTO was obtained up to x=0.2 Nb substitution and the lattice parameter increased with Nb substitution concentration. While the grain size decreased with Nb substitution, the resistivity of the grain boundary decreased. The dielectric constant increased with Nb substitution, and the highest value of ∼420 000 was observed in the x=0.2 Nb-substituted specimen at 10 kHz. The obtained electric and dielectric properties in Nb-substituted CCTO were discussed in terms of the internal barrier layer capacitor model, particularly focusing on a ratio of thickness of the grain boundary region to grain size.

With the development of nanotechnology, more attention and care should be paid to grain-boundary structure and its structural transition in order to understand the behavior of polycrystalline materials with grain sizes down to nanometer levels. Here, we report direct evidence suggesting a correlation between a grain-boundary structural transition and a change in the electrical property, using a strontium titanate (SrTiO3) bicrystalline grain boundary as a model system. The electrical properties of grain boundaries in SrTiO3 play a critical role in barrier-layer devices, such as capacitors and varistors. [1] It is believed that the electrical behavior depends on the double Schottky barrier established by the interface charge and the associated space charge across the grain boundary. [2–4] The interface charge arises from the segregation of point defects, which form because the energies to move anions and cations to the grain boundary are different. [5] The consequent electrostatic potential obstructs the transport of charge carriers across the grain boundary. For pure, undoped SrTiO3, Kim et al., [6] using electron energy loss spectrometry (EELS), showed that the ratio of Ti to O concentration in various boundaries is higher than in the bulk, indicating that the grain boundaries are enriched in Ti or deficient in O. Browning et al. [7] reported that a ∑5 grain boundary (∑ denotes the reciprocal of the fraction of common lattice points of the adjoining grains) is segregated by oxygen vacancies. Klie and Browning [8] reported the segregation of oxygen vacancies at a

The interface atomic structure was proposed to have a critical effect on the microstructure evolution during sintering of ceramic materials. In liquid-phase sintering, spherical grains show the normal grain growth behavior without exception, while angular grains often grow abnormally. The coarsening process of spherical grains with a disordered or rough interface atomic structure is diffusion-controlled, because there is little energy barrier for atomic attachments. On the other hand, kink-generating sources such as screw dislocations or two-dimensional (2-D) nuclei are required for angular grains having an ordered or singular interface structure. Coarsening of angular grains based on 2-D nucleation mechanism could explain the abnormal grain growth behavior. It was also proposed that densification process is closely related to the interface atomic structure. Enhanced densification by carefully chosen additives during solid state sintering was explained in terms of the grain boundary structural transition from an ordered to a disordered open structure.

In a recent work,1 we have reported the optimization of the spark plasma sintering (SPS) parameters to obtain dense nanostructured 3Y-TZP ceramics. Following this, the present work attempts to answer some specific issues: (a) whether ZrO2-based composites with ZrB2 reinforcements can be densified under the optimal SPS conditions for TZP matrix densification (b) whether improved hardness can be obtained in the composites, when 30 vol% ZrB2 is incorporated and (c) whether the toughness can be tailored by varying the ZrO2–matrix stabilization as well as retaining finer ZrO2 grains. In the present contribution, the SPS experiments are carried out at 1200°C for 5 min under vacuum at a heating rate of 600 K/min. The SPS processing route enables retaining of the finer t-ZrO2 grains (100–300 nm) and the ZrO2–ZrB2 composite developed exhibits optimum hardness up to 14 GPa. Careful analysis of the indentation data provides a range of toughness values in the composites (up to 11 MPa·m1/2), based on Y2O3 stabilization in the ZrO2 matrix. The influence of varying yttria content, t-ZrO2 transformability, and microstructure on the properties obtained is discussed. In addition to active contribution from the transformation-toughening mechanism, crack deflection by hard second phase brings about appreciable increment in the toughness of the nanocomposites.

Lead zirconium titanate (PZT) ceramic specimens were prepared by liquid phase sintering with excess PbO. By the addition of a small amount of MgO, the grain shape was changed from spherical to angular. When SiO2 was further added, twin was induced in a few grains, which grew abnormally during heat treatment. Through the electron backscatter diffraction analysis and the observation of three-dimensional grain morphology, the abnormally grown large PZT grains were determined to be penetration twinned. Abnormal grain growth was suggested to be because of reentrant edges formed at the twinned grains.

In an effort to refine the composition and properties of existing hard metals, (W,Ti)C–20 wt% Co cermets have been developed. The present research reports the mechanical and tribological properties of these novel materials. Single-step as well as two-stage and three-stage sintering experiments were conducted on the cermets, processed from the (W,Ti)C solid solution powders. For property comparison, premixed WC/TiC powders were used to fabricate a reference (W,Ti)C–20 wt% Co cermet material. Higher sintered density (9.57 g/cm3) as well as better elastic modulus (467 GPa) and hardness (∼17 GPa) were obtained after three-stage sintering of solid solution powders. In order to evaluate the tribological properties, the fretting wear experiments (mode I, linear relative tangential displacement) were performed against bearing steel for varying normal load in the range of 2–10 N. The experimental results reveal that the steady-state coefficient of friction (COF) varies between 0.50 and 0.65, and a lower COF is recorded at 10 N load for cermets processed from solid solution powders. Under varying tribological conditions, the cermets sintered from solid solution powders exhibit low wear depth (∼1–4 μm) and lower wear rate (7 × 10−7–18 × 10−7 mm3·(N·m)−1 when compared with cermet prepared from the premixed WC/TiC starting powders (wear rate ∼14 × 10−7–22 × 10−7 mm3·(N·m)−1). The wear rate data are critically evaluated based on the phenomenological models. Broadly, abrasive wear is the dominant wear mechanism, and limited contribution from localized spalling of tribolayer and tribochemical wear was also observed.

Conductivity profiles of nitrogen-graded zirconia from nitridation of 2 mol% yttria-doped tetragonal zirconia polycrystals (2Y-TZP) were determined using microcontact impedance spectroscopy. Local conductivity variations were interpreted in terms of nitrogen concentration and the morphological distribution of the tetragonal precipitates in the two-phase region of nitrogen partially stabilized zirconia (N-PSZ). Nitrogen incorporation in the stabilized zirconia was found to strongly decrease the local conductivity below 400°C, even though the oxygen vacancy concentration increased nominally. This behavior is ascribed to the strong vacancy–vacancy interaction leading to vacancy ordering. The strong interaction is indicated by the formation of ordered-vacancy phases in nitrogen-doped zirconia. Percolation of elongated tetragonal precipitates of lesser nitrogen concentration and thus of higher conductivity explains the conductivity anisotropy observed by microcontact measurements as well as the large effective dielectric constants.

The influence of TiO 2 additives on the sintering behavior of In 2 O 3 ceramics has been investigated. TiO 2 increases the densification rate, decreases the grain growth during the intermediate stage of sintering, and hinders the pore/ boundary breakaway that can affect the final stage of sintering. For a given grain size, TiO 2 shifts the grain size/density trajectory toward higher densities. TiO 2 mainly acts by a second-phase mechanism, but it also may decrease the decomposition rate of In 2 O 3 .

The low-temperature degradation of zirconia (ZrO 2 ) that was doped with 3 mol% yttria (Y 2 O 3 ) (3Y-TZP) was prevented by the heat treatment of sintered specimens in nitrogen. The heat treatment of sintered specimens resulted in a surface layer that was stabilized by nitrogen ions, whereas the interior was only slightly affected by the heat treatment. X-ray diffractometry and transmission electron microscopy analyses revealed that the stabilized surface layer consisted of cubic grains with tetragonal precipitates. Although the presence of the surface layer decreased the strength of the sintered 3Y-TZP, the strength of nitrified specimens was maintained when low-temperature annealing was applied.

For large-grained manganese zinc (MnZn) ferrite ceramics, grain misorientation determined by electron backscatter diffractions and grain-boundary resistance measured using microcontact impedance spectroscopy have been correlated. The degree of oxidation of grain boundaries and, hence, the barrier height depends on the overall grain-boundary network as well as on the individual boundary structure; therefore, a statistical analysis has been performed based on several hundreds of local measurements. When the boundaries are divided into low- and high-resistance groups, statistically significant differences in rotation axis and angle distributions are found. The misorientation distribution of the low-resistance boundary group is suggested to reflect the low-energy configurations of boundary planes in MnZn ferrites.

Sintering of In2O3 has been carried out in air to full density. Because of the difference in densification between the agglomerates and the matrix, large interagglomerate pores were observed to form at the initial stage of sintering. Such pore formation could be prevented by applying a small external pressure which resulted in the beneficial rearrangement of agglomerates.

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.

Sintered tetragonal-zirconia polycrystals (TZP) were embedded in a zirconium nitride powder bed and heat-treated at various temperatures. Surface layers of the TZP specimens were transformed to a stabilized cubic-zirconia by nitrogen incorporation, and their thickness was observed to increase by a parabolic rate law. The nitrogen diffusivity was evaluated from the temperature dependence of the nitridation rate. The microstructure of the nitrided layer was composed of different zones of equiaxed and columnar grains. The columnar grains were developed along the nitrogen flux lines. The observed microstructural evolution was explained in terms of nucleation and growth kinetics of nitridation.

The effect of an external electric field on the grain-boundary migration in Al2O3 ceramics has been investigated. The boundary migration is dependent on the direction and magnitude of the applied bias, and the observed boundary migration behavior is attributed to the presence of an electrostatic potential that inherently forms at the grain boundaries of Al2O3 ceramics. The results give experimental evidence that the boundary phenomena in oxide ceramics are related to the grain-boundary potential.

Alumina specimens with small amounts of CaO and TiO 2 were prepared and their microstructural evolution during sintering was investigated. Because of the appearance of a liquid phase during sintering, a duplex microstructure of a few abnormal grains and fine matrix grains was obtained when the CaO + TiO 2 content was small (≤0.04 wt%). When the CaO + TiO 2 content was relatively high (≥0.1 wt%), many grains grew and impinged upon each other. As a result, a rather uniform and homogeneous microstructure was observed.

Cavities formed in ruby (99.46Al 2 O 3 .0.54Cr 2 O 3 ) by the healing and annealing of indentation cracks at 1600°C are more equiaxed than similar cavities in sapphire. Surface energies for the observed facet planes (R, S, and A) relative to the surface energy of the basal plane, C, were γ A/C = 1.00 ± 0.03, γ R/C = 1.05 ± 0.07, and γ S/C = 1.02 ± 0.04, with the uncertainty representing 95% confidence limits. Thus, the surface energies of all observed facets were statistically indistinguishable. Unlike sapphire, P-plane facets were not observed. The substantial rounding of the cavities in ruby indicated that portions of the Wulff shape were above the roughening transition temperature. Thus, even though Cr 2 O 3 and Al 2 O 3 form ideal solutions, Cr 3+ ions are sufficiently surface active to modify the relative free energy of the surfaces.

Pb(Mg 1/3 Nb 2/3 )O 3 -35 mol% PbTi0 3 (PMN-35PT) specimens with a 5 mol% excess PbO were prepared by excessive heat treatment at 1150°C to induce abnormal grain growth. Through electron backscatter diffraction analysis and the observation of a three-dimensional morphology, the abnormally grown PMN-35PT grains were found to be twinned crystals with penetration characteristics. The morphology of the PMN-35PT twinned crystal was crystallographically analyzed. The abnormal grain growth of PMN-35PT is suggested to be due to preferential growth at the reentrant angles formed by twins.

Abnormal grain growth (AGG), which occurred during the heat treatment of Pb(Mg1/3Nb2/3)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.

The effect of TiO 2 /SiO 2 addition on the grain growth of alumina was reinvestigated. TiO 2 promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO 2 and SiO 2 resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.

The effect of an external electric field on the grain-growth behavior of acceptor Mg-doped, undoped, and donor Nb-doped BaTiO 3 ceramics was investigated. The acceptor-doped and undoped specimens showed enhanced grain growth at the positive-biased region. On the other hand, for the highly donor-doped specimens, grain growth was enhanced in the negative-biased region. The results have been explained in terms of defect polarization and the consequent change in the boundary potential. It has been suggested that liquid penetration into grain boundaries is critically dependent on the boundary potential.

The present contribution reports some interesting and new results obtained while developing yttria-stabilized tetragonal zirconia (Y-TZP) using spark plasma sintering (SPS). The experimental results clearly showed that ZrO2-nanoceramics with high hardness(∼14.5 GPa) can be processed at a lower sintering temperature of 1200°C in a short time (5 min). Another important result is that the newly developed Y-TZP nanoceramics, compared with the conventional sintered TZP, exhibit better fretting wear resistance against bearing steel. The intergranular fracture and the grain pullout were observed as the major wear mechanisms of the zirconia nanoceramics.

In a Pb(Mg 1/3 Nb 2/3 )O 3 -35 mol% PbTiO 3 (PMN-35PT) specimen with 5 mol% excess PbO, the pores trapped inside grains by heat treatment in a double cycle at 1150°C have revealed the coarsening pattern of the grains. Using this technique, the coarsening process of abnormal large grains with typical characteristics of penetration twins could be determined. The results suggest that two-dimensional nucleation and lateral growth enhanced at the reentrant edges are the cause of zigzag-shaped Σ3 incoherent twin boundaries.

In a 15-mol%-CaO-stabilized zirconia (CSZ) specimen, the grain boundaries at the surface region exhibited a lower electrical resistance than those at the inner region. However, when up to 1.5 mol% Al2O3 was added, the grain boundaries at the surface showed a higher resistance. These results were explained in terms of redistribution of the intergranular-liquid phase during CSZ sintering.

A sapphire single crystal was put into anorthite powders containing Cr 2 O 3 and then heat-treated at 1600°C. The resultant morphologies of the sapphire-anorthite-liquid interfaces were corrugated and quite different depending on the crystallographic orientation of the sapphire; all the planes perpendicular to the C-plane showed fingerlike morphologies, while long parallel segments were detached from the basal C-plane. The results were interpreted in terms of coherency-strain-induced instability of the solid-liquid interface (ISLI) caused by the alloying of Cr 2 O 3 into sapphire. The difference in microstructural evolution depending on crystallographic orientation was explained in terms of strong anisotropy of the solid-liquid interface characteristics.

Boundary migration under an electric field was investigated for pure, TiO 2 -doped, and Li 2 O-doped Al 2 O 3 specimens. Boundary migration rates in TiO 2 -doped and Li 2 O-doped Al 2 O 3 specimens were much faster compared with that of pure Al 2 O 3 . In all specimens, the migration rate was observed to depend on the applied bias direction. Compared with pure Al 2 O 3 , the dependence of boundary migration on bias direction became more pronounced in TiO 2 -doped Al 2 O 3 but less pronounced in Li 2 O-doped Al 2 O 3 . The results were explained in terms of the variation of grain sizes, mobility, and electrostatic potential of boundaries because of doping.

The microstructural evolution during sintering of Al 2 O 3 was investigated to determine the role of MgO additive, particularly when its concentration is very low (

Changes in the microstructure and dielectric properties with the variation of the donor/acceptor ratio in BaTiO3 ceramics were investigated. In donor-rich specimens, a liquid that appeared during sintering did not penetrate into grain boundaries. However, in the acceptor-rich specimens, the grains were separated by a liquid film during sintering. The much higher mobility of the liquid film than that of the grain boundaries was suggested to cause extensive grain growth in acceptor-rich BaTiO3. The macroscopic homogenization of dopants because of grain growth in acceptor-rich specimens resulted in changes in the dielectric properties.

A thermodynamic approach is used to derive a defect chemistry formulation at the grain boundary of an ionic solid. Considering the elastic and/or electrostatic interactions between charged defects, the equilibrium electrostatic potential and the concentration of charged defects at the grain boundaries can be quantitatively predicted. The obtained result has general applicability at all levels of defect concentrations. Defect concentration at the boundary has been determined to critically depend on the interactions between defects, electrostatic potential, and segregation. These interactions, either acting individually or coupling with each other, lead to a nonuniform defect distribution near the grain boundary. Calculations also show that the grain-boundary segregation of an aliovalent solute is greatly affected by doping through the electrostatic or elastic interactions. The variation of the grain-boundary characteristics with the processing parameters, such as dopant concentration and temperature, is discussed in relation with the proposed model.

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.

From microstructural observations, we have shown that the domain structures of BaTiO3 ceramics are stabilized by aging. Once stabilized, the original domain structures reappear even when the stress conditions during phase transformation are changed. The pinning of the domain band walls as a result of a space charge is suggested to cause the stabilization of domain structures.

The equilibrium shape of internal cavities in sapphire was determined through the study of submicrometer internal cavities in single crystals. Cavities formed from indentation cracks during annealing at 1600°C. Equilibrium could be reached only for cavities that were smaller than is approximately100 nm. Excessive times were required to achieve equilibrium for cavities larger than is approximately 1μm. Five equilibrium facet planes were observed to bound the cavities: the basal (C) {0001}, rhombohedral (R) {1¯012}, prismatic (A) {12¯10}, pyramidal (P) {112¯3}, and structural rhombohedral (S) {101¯1}. The surface energies for these planes relative to the surface energy of the basal plane were γR = 1.05, γA = 1.12, γP = 1.06, γS = 1.07. These energies were compared with the most recent theoretical calculations of the surface energy of sapphire. The comparison was not within experimental scatter for any of the surfaces, with the measured relative surface energies being lower than the calculated energies. Although the prismatic (M) {101¯0} planes are predicted to be a low-energy surface, facets of this orientation were not observed.

Abnormal grain growth (AGG) during the sintering of Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 was investigated. When a small amount of SiO2 was added, large abnormal grains with the penetration twin characteristic were observed to appear. AGG, in this case, is because of the twin plane re-entrant edges that provide coarsening advantage. When TiO2 was added, on the other hand, the abnormal grains were simple cubic in morphology with well-developed {100} planes. In this case, the coarsening advantage is suggested to be because of grain boundary re-entrant edges.

The densification behavior and mechanical properties of SiC-30TiC (in volume percent) composites prepared with Cr3C2 additive were investigated. By hot-pressing a SiC-30TiC-lCr3C2 specimen at 1950°C, 98.5% of theoretical density was achieved and the specimen exhibited a fracture strength of 750 MPa. For the SiC-30TiC-10Cr3C2 specimen, (β-α transformation of SiC was observed to occur during hot-pressing and in situ growth of elongated α-SiC grains resulted in an increase of fracture toughness. Micro-structural observations using high-resolution TEM indi-cated that no liquid phase was present at the interfaces.

Diffusion-induced grain-boundary migration (DIGM) in Ba(Zn1/3Nb2/3)O3 (BZN) ceramics was investigated with small (3.0 μm) and large (31. 4 μm) grain size specimens. The specimens were embedded in Nb2O5 or ZnO powders and then heat-treated at 1250° and 1310°C, respectively. The grain boundaries of the small grain size specimens were immobile, while those of the large grain size specimens migrated away from their centers of curvature. From the observed difference in migration behavior depending on grain size, the magnitude of the driving force for the DIGM was estimated.

Pore–boundary separation in ZnO and 99.95ZnO·0.05Bi2O3 (in mol%) specimens during sintering at 1200°C was investigated. In pure ZnO specimens, pores were attached to the grain boundaries and disappeared during the final stage of sintering. In the Bi2O3-doped specimens, on the other hand, many pores were separated from the boundaries and trapped inside the grains. Observation using transmission electron microscopy showed that a thin layer of Bi2O3-rich phase existed at the boundaries in the Bi2O3-doped specimens. The pore separation in 99.95ZnO·0.05Bi2O3 specimens was explained in terms of the dihedral angle change and the high mobility of a liquid film boundary.

Microstructural evolution during sintering of alumina powder compacts prepared by cold isostatic pressing (CIP) was monitored. For CIP, rubber molds lubricated with silicone oil were used so that a very small amount of impurity was introduced to the surface of the powder compacts. During sintering at 1600°C, grain growth in the surface region was inhibited up to sintering for 1 h, but subsequently abnormal grain growth occurred. In the inner region, however, the grains grew uniformly without abnormal grain growth. Impurities that initially drag the boundary migration but form liquid at the end are suggested to cause abnormal grain growth.

Heavily doped barium titanate (BaTiO3) powders, one with a donor-rich and the other with an acceptor-rich composition, were prepared. After sintering, the donor-rich specimen exhibited a fine-grained microstructure but significant grain growth occurred in the acceptor-rich specimen. A stable dielectric behavior was observed only in the donor-rich fine-grained specimen over the temperature range studied. However, in both specimens, an undoped core region several hundred nanometers in size was detected. The core-shell structure appeared to be maintained in BaTiO3 under the conventional sintering conditions.

When a BaTiO3 powder compact was sintered in air, the abnormal grain growth (AGG) was observed to occur at the surface of the specimen. The BaO evaporation from the surface and consequent formation of eutectic liquid is suggested to be the cause of AGG. When the evaporation of BaO during sintering was prevented by embedding the specimen in the same powder, no AGG was observed to occur.

In the present work, we report the processing of ultrahard tungsten carbide (WC) nanocomposites with 6 wt% zirconia additions. The densification is conducted by the spark plasma sintering (SPS) technique in a vacuum. Fully dense materials are obtained after SPS at 1300°C for 5 min. The sinterability and mechanical properties of the WC–6 wt% ZrO2 materials are compared with the conventional WC–6 wt% Co materials. Because of the high heating rate, lower sintering temperature, and short holding time involved in SPS, extremely fine zirconia particles (∼100 nm) and submicrometer WC grains are retained in the WC–ZrO2 nanostructured composites. Independent of the processing route (SPS or pressureless sintering in a vacuum), superior hardness (21–24 GPa) is obtained with the newly developed WC–ZrO2 materials compared with that of the WC–Co materials (15–17 GPa). This extremely high hardness of the novel WC–ZrO2 composites is expected to lead to significantly higher abrasive-wear resistance.

The shrinkage process of large pores during hot isostatic pressing (HIP) of presintered Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3}-ZrO{sub 2} ceramics has been investigated. Large pores were observed to collapse by grain-boundary sliding, and then small pores resulting from the misfit of flowed grains disappeared, mainly by diffusion. Due to the high resistance to grain-boundary sliding, the pores in the Al{sub 2}O{sub 3}-ZrO{sub 2} ceramics were hard to eliminate.

Liquid-forming impurities (CaO + SiO2) were placed on the top of undoped and MgO-doped Al2O3 powder compacts, and the microstructural evolution and the extent of liquid penetration during heat treatment were investigated. Compared with the undoped specimen, abnormal grain growth was limited and the impurities penetrated more deeply in the MgO-doped specimen. The homogenization of the microstructure and the enhanced liquid penetration in MgO-doped specimens could be explained in terms of an interface roughening or a modification of the liquid-(glass-) phase characteristics, such as viscosity.

Fully dense MgB2 bulk specimens (∼higher than 99% dense) were prepared using spark plasma sintering (SPS) at 1250°C for 15 min. Microstructure analyses revealed that faceted MgO particles of ∼8% volume fraction were dispersed in the MgB2 matrix. A sharp superconducting transition with an onset temperature of 38.5 K was confirmed by both magnetization and resistivity measurements.

SiC powder compacts were prepared with Al2O3, Y2O3, and CaO powders. By two-step sintering, fully dense nanostructured SiC ceramics with a grain sizes of ∼40 nm were obtained. The grain size–density trajectories are compared with those of conventional sintering processes.

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.

In a WC-Co specimen, the shape of WC crystals was a triangular prism with truncated corners. When VC was added to inhibit grain growth, the crystal shape changed to a triangular prism without truncation. This shape change was related to the variation of edge energy, which has a significant influence on the coarsening process of WC grains.

A quantitative method to evaluate the degree of whisker alignment in axisymmetric composite materials was developed. The angular distribution of the whiskers was analyzed by measuring the aspect ratios of the whiskers observed on a planar section. However, due to the large difference in the probability of whiskers being detected on the planar section (depending on whisker length and degree of alignment), the angular distribution of the whiskers observed on the planar section was significantly different from the actual distributions of the whiskers in three dimensions. Three-dimensional angular distributions were evaluated by comparing the aspect ratios observed on the planar section with those calculated under the assumption that the whisker angle fell in a Gaussian distribution with average angle of 90[degree]. By this method, the degree of preferential alignment is expressed as the standard deviation of the Gaussian distribution. This quantification of whisker alignment is useful in analyzing the mechanical behaviors of composite materials reinforced with elongated particles.

The stability of a calcium-aluminum-silicate liquid film between two near-basal plane surfaces of sapphire at 1650°C was studied. Samples were prepared having an average basal misorientation across the interface of 6–7° about < 〈1010〉. The interfaces varied in orientation from 0° to ∼38° to the [0001] direction. Three types of interfaces were observed: faceted, solid-liquid interfaces; low-angle grain boundaries consisting of aligned arrays of dislocations; and boundaries consisting of alternating regions of dislocations and faceted solid-liquid interfaces. The type of interface observed depended on the orientation of the interface and could be predicted by using a construction based on Wulff shapes. Because the type of interface depends on crystal alignment and interface angle, these results suggest an absolute method of determining the surface free energy of wetted boundaries.

Impedance spectroscopy with millimeter-scale electrodes was used to determine the local grain-boundary resistivity (ρgb) of a 1-mol%-Al2O3-doped CaO-stabilized ZrO2 (CSZ) specimen. A much higher ρgb at the specimen surface was observed compared with that of the specimen interior. The result was mainly due to the difference in the dihedral angle of the intergranular phase, which was ∼20° at the surface and ∼100° at the interior.