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Electric DC current dependence on the superplastic flow in densified, fine-grained tetragonal ZrO2 polycrystal was systematically investigated. An optimal current density for large tensile ductility existed; the maximum elongation of 135% was achieved under the current density of 250 mA∙mm-2 at a furnace temperature of 1000 oC and an initial strain rate of 1×10-3 s-1. Application of DC current decreased the flow stress, while that simultaneously accelerated the grain growth in tetragonal ZrO2 polycrystal. The reduced flow stress and improved ductility can be explained not only by Joule heating but also by enhanced atomic diffusion or accelerated grain boundary sliding.

1 mol% Zn2+-doped Y2O3 powder was consolidated utilizing spark plasma sintering (SPS) by systematically varying the heating schedule, sintering temperature, heating rate, holding time at the sintering temperature, and loading stress. Transparent, Zn2+-doped Y2O3 polycrystals were successfully produced at the heating rate of 2 °C/min, sintering temperature of 890 °C, holding time of 30 min, and loading stress of 150 or 170 MPa. The highest transmittance in the Zn2+-Y2O3 bodies at a wavelength of longer than 600 nm was comparable to those in the undoped SPSed Y2O3 in the literature. Employing a sintering temperature and loading stress higher than the optimum values led to coarsened pore and grain sizes, resulting in a decreased transparency. Low heating rate at temperatures above 700 °C was desirable for the attainment of high transparency. The grain boundary segregation of Zn2+ cations effectively contributed to the reduction of the sintering temperature required for attainment of transparency in the Y2O3 by SPS.

Y2O3-stabilised ZrO2 (YSZ) ceramics have been used for various engineering applications since Garvie et al. discovered phase-transformation toughening in 1975. The performance of YSZ ceramics depends on the YSZ microstructure. In the present review, the tetragonal-to-cubic phase transformation and grain growth are first discussed, organised according to existing hypotheses of microstructure-development mechanisms in YSZ during sintering. We demonstrate that the phase transformation and grain growth can be most reasonably understood by a grain boundary segregation-induced phase transformation (GBSIPT) mechanism and that the solute-drag effect of Y3+ ions segregating along the grain boundaries, respectively. Next, the Al2O3-doping effect is discussed with emphasis on the microstructure-development behaviour in a small amount of Al2O3-doped YSZ, which is widely used in engineering applications. We further discuss the effect of GBSIPT on low-temperature degradation resistance and the effect of grain s...

Consolidated, monoclinic ZrO2 polycrystal was produced from undoped ZrO2 powders in air by flash sintering at the sintering temperature of 1350°C for 5 minutes or 3 hours under an applied DC electric field of 175 V/cm. When the ZrO2 was heated under the applied DC field, the electric current of the specimen steeply increased at the furnace temperature of 1335°C below the sintering temperature of 1350°C. When the furnace temperature was decreased from the sintering temperature of 1350°C to room temperature, volumetric expansion associated with tetragonal-to-monoclinic phase transformation gradually took place at the furnace temperature from 1000°C to 750°C, and monoclinic ZrO2 body was remained consolidated even at room temperature in both specimens. In contrast, conventionally sintered ZrO2 without applying DC field exhibited the abrupt volumetric expansion at about 1000°C, and shattered. SEM observation revealed the presence of grain-boundary second phase in the flash-sintered specimen for 3 hours, which is a possible origin of keeping a bulk body at room temperature. The thinner second phase is considered to be formed also in the flash-sintered specimen for 5 minutes, although the formation of the phase could not be observed clearly by SEM observation. On the other hand, XRD measurements showed that directions of the monoclinic ZrO2 grains were oriented along the applied DC field after the isothermal flash sintering for 3 hours while the grain alignment could not be observed in flash-sintered specimen for 5 minutes. The alignment of ZrO2 grains observed in the isothermal flash sintering is considered to be closely related to the preferential direction of oxygen ionic conduction and the second phase formed along grain boundaries.

In BaTiO3, flash-sintering associated with a surge of the specimen electric current sometimes results in an inhomogeneous microstructure including Ti-excess secondary phases because of discharging. We applied field-assisted sintering technique (FAST) under precisely controlled specimen current that was set just below the threshold value for the occurrence of flash event for BaTiO3, to avoid the occurrence of the discharging. As a result, uniform and fine-grained compacts were obtained without any secondary phases. A relative density of approximately 92% was achieved under FAST condition of 100 V/cm with a limiting current of 72 mA and soaking time of 3 hours at 1070°C. The voltages during sintering under a constant current of 72 mA were found to decrease during the soaking process. Electron energy loss spectroscopy revealed the generation of excess oxygen vacancies at/near grain boundaries. The excess oxygen vacancies induced by application of DC electric fields were confirmed to reduce the voltages and to retard the shrinkage rate in a final sintering stage.

In order to discuss the influence of carbon contamination on the transmission, the effect of pre- and post-annealing treatments was investigated in the spark-plasma-sintered (SPSed) MgAl2O4 spinel. During the SPS process, the carbon phases transformed from CO32−, which is pre-existing in the powder, remains along grain junctions and caused discoloration. The pre-annealing of the starting powder can reduce the impurities and improve in-line transmission Tin by 10%. Post-annealing can remove the discoloration from the spinel, but it changes the spinel to whitish color with the annealing temperature and degraded Tin. This degradation can be explained by the increase of a scattering coefficient caused by the pore. During the post-annealing, the carbon phases generate high pressure CO/CO2 gas by reacting with oxygen and forms many pores along the grain boundaries. This suggests that reducing the carbon contamination is important for attaining highly transparent spinel by the SPS processing.

Low temperature degradation (LTD) behavior was examined in 3 mol% Y 2 O 3 -stabilized tetragonal ZrO 2 polycrystals (Y-TZP) doped with 0–1.1 mol% Al 2 O 3 sintered at 1500 °C. 0.12 mol% Al 2 O 3 -doped Y-TZP exhibited monolithic microstructure without dispersion of Al 2 O 3 grain, while second-phase Al 2 O 3 particles were observed in the Y-TZP sintered bodies doped with 0.54–1.1 mol% Al 2 O 3 . In the Y-TZP doped with 0.12 mol% Al 2 O 3 , neither amorphous nor second phase was present along the grain boundaries, but Y 3+ and Al 3+ ions segregated in the vicinity of the grain boundaries. The tetragonal-to-monoclinic ( T → M ) phase-transformation rate during the accelerated LTD test significantly decreased at 0.12 mol% Al 2 O 3 , but the phase-transformation rate slightly increased with the increasing doping amount of Al 2 O 3 . The suppressed T → M phase-transformation in Al 2 O 3 -doped Y-TZP is explained in terms of decrease in grain-boundary free energy of tetragonal phase owing to Al 3+ ions’ grain-boundary segregation.

Superplasticity in fine-grained oxide ceramics has been generally elucidated on the basis of their experimental strain rate-flow stress relationship and phenomenological analysis of cavity nucleation and growth. It has been widely accepted that the high temperature superplastic flow and failure in ceramics is significantly influenced by the atomic structure and chemistry of grain boundaries. Such phenomenon cannot be explained based on the classical phenomenological analysis. Our research group has therefore proposed to establish a new research field, grain boundary plasticity, to describe the superplastic deformation related to the grain boundary atomic structure. This paper aims to point out the importance of the atomistic analysis of grain boundary to develop new superplastic ceramics.

We systematically investigated the phase transformation and grain-growth behaviors during sintering in 2 and 3 mol% Y2O3-stabilized tetragonal ZrO2 (2Y and 3Y) and 8 mol% Y2O3-stabilized cubic ZrO2 polycrystals (8Y). In particular, grain-boundary segregation and grain-interior distribution of Y3+ ions were examined by high-resolution transmission electron microscopy (HRTEM)- and scanning transmission electron microscopy (STEM)-nanoprobe X-ray energy dispersive spectroscopy (EDS) techniques. Above 1200°C, grain growth during sintering in 8Y was much faster than that in 2Y and 3Y. In the grain boundaries in these specimens, amorphous layers did not present; however, Y3+ ions segregated at the grain boundaries over a width of about 10 nm. The amount of segregated Y3+ ions in 8Y was significantly less than in 2Y and 3Y. This indicates that the amount of segregated Y3+ ions is related to grain growth behavior; i.e., an increase in segregated Y3+ ions retards grain growth. Therefore, grain-growth behavior during sintering can be reasonably explained by the solute-drag mechanism of Y3+ ions segregating along the grain boundary. In 2Y and 3Y, the cubic-phase regions were formed in grain interiors adjacent to the grain boundaries and/or the multiple junctions in which Y3+ ions segregated, which can be explained by a grain boundary segregation-induced phase transformation (GBSIPT) mechanism.

An existing defect model for dynamic grain growth is modified in order to incorporate the role of grain-boundary sliding on the generation of defects during sintering. On the basis of the concept of enhanced defect generation, the unusual grain growth of alumina during spark plasma sintering is discussed in correlation with the densification caused by grain-boundary sliding. The modified model explains well the experimentally observed dependence of grain size on pressure, heating rate and loading schedule.

A grain-boundary sliding model is developed to characterize the shrinkage behavior of pores in the intermediate stage of sintering under compressive hydrostatic pressure. From an analysis of the relative sliding between grains, the bulk viscosity, densification rate and the shrinkage rate of pores are predicted for a dense matrix polycrystal containing spherical pores. Comparison with a continuum model shows that whereas the densification behavior in the present discrete model approaches the prediction of the continuum model for large pores, the occurrence of grain-boundary sliding is limited and the deviation from the continuum model increases significantly for small pores. It is also shown that the grain-size dependence of the densification rate observed experimentally is consistent with the model prediction and verifies the validity of the present model in the intermediate sintering stage.

We conducted compressional, tensile, and torsional creep experiments on fine-grained forsterite plus Ca-bearing pyroxene aggregates. A distinct microstructure with aggregation of the same phase in the direction of compression was formed in our samples after all the experiments. The stress–strain rate relationship, grain-size dependent flow strength, and the achievement of large tensile strain all indicate that samples underwent creep due to grain boundary sliding (GBS). As a result of GBS, grain-switching events allow dispersed phases to contact grains of the same phase and orient in the direction of compression. We identify similar aggregated microstructures in previously reported micrographs of polymineralic granite-origin ultramylonites. Mineral phase mixing through GBS, which helps to retain fine grain size in rocks due to grain boundary pinning, has been speculated to occur during formation of mylonites. However, our results contradict this hypothesis because mineral aggregation through GBS promotes demixing rather than mixing of the mineral phases. GBS processes alone will not promote a transformation of well-developed monomineralic bands to polymineralic bands during mylonitization.

Nanocrystalline tetragonal ZrO2 polycrystals (TZP) have been fabricated by the pressureless sintering of recently developed tetragonal ZrO2 powder containing 5.69 mol% YO1.5 and 0.60 mol% AlO1.5. The average grain sizes were 160 nm in the TZP sintered at 1150°C for 10 h and 150 nm in the 0.25 mol% GeO2-doped TZP sintered at 1100°C for 100 h. The TZP and Ge4+-doped TZP-sintered bodies were essentially single-phase materials, and neither the amorphous layer nor the second-phase particle was observed along the grain boundary faces. High-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and nanoprobe energy-dispersive X-ray spectrometer (EDS) measurements revealed that the Y3+, Al3+ and Ge4+ cations tend to segregate in the vicinity of the grain boundaries in the TZP-sintered bodies. The TZP and Ge4+-doped TZP exhibited an elongation to failure of more than 100% in the temperature range of 1150°C–1300°C and initial strain rate range of 1.4 × 10−5 s−1 to 1.0 × 10−2 s−1. For instance, an elongation to failure in the Ge-doped TZP reached about 200% at 1150°C and 1.4 × 10−5 s−1. The nanocrystallization reduced the lower limit of the superplastic temperature of conventional, submicron-grain TZP materials by 150°C. The improved ductility of the TZP at low temperatures was essentially attributed to the reduced grain size.

Microstructure development during sintering in 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) was systematically investigated in two sintering conditions: (a) 1100-1650°C for 2 h and (b) 1300°C for 0-50 h. In the sintering condition (a), the density and grain size in Y-TZP increased with the increasing sintering temperature. Scanning transmission electron microscopy (STEM) and nanoprobe X-ray energy dispersive spectroscopy (EDS) measurements revealed that the Y3+ ion distribution was nearly homogeneous up to 1300°C, i.e., most of grains were the tetragonal phase, but cubic-phase regions with high Y3+ ion concentration were clearly formed in grain interiors adjacent to the grain boundaries at 1500°C. High-resolution transmission electron microscopy (HRTEM) and nanoprobe EDS measurements revealed that no amorphous or second phase is present along the grain-boundary faces, and Y3+ ions segregated not only along the tetragonal-tetragonal phase boundaries but also along tetragonal-cubic phase boundaries over a width below about 10 nm, respectively. These results indicate that the cubic-phase regions are formed from the grain boundaries and/or the multiple junctions in which Y3+ ions segregated. We termed this process a “grain boundary segregation-induced phase transformation (GBSIPT)” mechanism. In the sintering condition (b), the density was low and the grain-growth rate was much slow. In the specimen sintered at 1300°C for 50 h, the cubic-phase regions were clearly formed in the grain interiors adjacent to the grain boundaries. This behavior shows that the cubic-phase regions were formed without grain growth, which can be explained by the GBSIPT model.

A grain-boundary-diffusion model is developed to understand the densification behavior of pores in the final stage of sintering under compressive hydrostatic pressure. From analysis of the diffusive model, the bulk viscosity, densification rate and shrinkage rate of pores are predicted for a dense matrix polycrystal containing spherical pores, and compared with the existing experimental results and models. A transition in the sintering mechanism is predicted from the different pore-size dependence of the shrinkage rate between the diffusive and the viscous flow models. The transition effect is experimentally confirmed by the appearance of a downward inflection in the size distribution of pores during sintering. The upward inflection observed experimentally in the cavity-size distribution after superplastic deformation is also explained by the transition of the mechanism.

The sintering behavior of Y2O3 doped with 1 mol% of Ca2+, Mg2+, Mn2+, Ni2+, Sr2+ or Zn2+ was investigated by pressureless sintering in air at a sintering temperature in the range 900–1600 °C. The sintering temperature required for full densification in Y2O3 was reduced by 100–400 °C by the cation doping, while undoped Y2O3 was densified at 1600 °C. The most effective dopant among the examined cations was Zn2+. The grain growth kinetics of undoped and cation-doped Y2O3 was described by the parabolic law. The grain boundary mobility of Y2O3 was accelerated by doping of the divalent cations. High-resolution transmission electron microscopy (HRTEM) observations and nano-probe X-ray energy dispersive spectroscopy (EDS) analyses confirmed that the dopant cations tended to segregate along the grain boundaries without forming amorphous layers. The improved sinterability of Y2O3 is probably related to the accelerated grain boundary diffusion owing to the grain boundary segregation of the dopant cations.

Microstructure development during sintering in 3 mol% Y 2 O 3 -stabilized tetragonal zirconia polycrystal doped with a small amount of Al 2 O 3 was investigated in the isothermal sintering conditions of 1300–1500 °C. At the low sintering temperature at 1300 °C, although the density was relatively high, the grain-growth rate was much slow. In the specimen sintered at 1300 °C for 50 h, Y 3+ and Al 3+ ions segregated along grain boundaries within the widths of about 10 and 6 nm, respectively. In grain interiors, the cubic-phase regions were formed by not only a grain-boundary segregation-induced phase-transformation mechanism but also by spinodal decomposition. The grain-growth behavior was kinetically analyzed using the grain-size data in 1300–1500 °C, which indicated that the grain-growth rate was enhanced by Al 2 O 3 -doping. These phase-transformation and grain-growth behaviors are reasonably explained by the diffusion-enhanced effect of Al 2 O 3 -doping.

When the sliding of the grain boundary containing hexagonal particles is accommodated by grain-boundary diffusion, we evaluate the sliding rate and the stress distribution on the boundary, by taking the particle rotation and the intrinsic boundary viscosity into account. The sliding rate was obtained by the energy-balance method, and the particle-rotation rate by a condition of minimum energy-dissipation. With increasing boundary viscosity, the rotation rate increases and then decreases after a maximum. The sliding rate is enhanced by the particle rotation, and decreases with the boundary viscosity.

High temperature creep and superplastic flow in high-purity, polycrystalline oxide ceramics is very sensitive to a small amount of doping by various oxides. The doping effect is attributed to change in grain boundary diffusivity owing to grain boundary segregation of the doped cations. The doping effect on the grain boundary diffusivity is caused mainly by change of chemical bonding state in the vicinity of the grain boundary segregated with the doped cations. In other words, controlling of grain boundary nanostructure based on the doping process will be a useful way to develop new high-performance functional ceramics in the near future.

We evaluate the sliding rate and stress distribution on the boundary when the sliding of grain boundary containing particles is accommodated by grain-boundary diffusion, by taking the particle rotation and the intrinsic boundary viscosity into account. The particle rotation enhances the sliding rate, and can occur in the reverse direction with respect to the grain-boundary sliding. We investigate the sliding behavior for various particle shapes and boundary viscosities. A similar analysis is also conducted for the shear deformation of regular hexagonal grains.

The doping amount and temperature dependence of superplastic flow in a TiO 2 - and/or GeO 2 -doped tetragonal ZrO 2 polycrystal (TZP) were investigated in the doping range of 0.2–8 mol.% and in the temperature range of 1200–1550 °C. While the tensile ductility in the TZP is significantly improved by the co-doping of TiO 2 and GeO 2 , there is an optimum combination of doping amount and temperature for enhancing the tensile ductility. The present study also shows that the flow stress decreases with an increase in the doping amount, but this decrease levels off with a 2–3 mol.% addition of GeO 2 or (TiO 2 –GeO 2 ). The data for the flow behavior and thermal groove experiment indicated that TiO 2 and/or GeO 2 doping enhances the grain boundary diffusion of zirconium cations and reduces the grain boundary energy, respectively. These effects of grain boundary segregation can be regarded as the cause of the improved high-temperature ductility of (TiO 2 –GeO 2 )-doped TZP.

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO 2 and 2 mol% GeO 2 on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO 2 polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti 4+ and Ge 4+ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.

The isothermal sintering behavior in 3 mol% yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) was investigated to clarify phase-separation and grain-growth mechanisms during sintering. In the Y-TZP sintered at 1300°C for 2 h, the Y3+ ion distribution of grain interiors in Y-TZP was nearly homogeneous, but Y3+ ions segregated along grain boundaries within a width of about 10 nm. When the holding time increased from 2 to 50 h, the cubic-phase regions with high Y3+ ion concentrations were clearly formed in the grain interiors adjacent to the grain boundaries, though the average grain size hardly increased. This result shows that the cubic-phase regions were formed without grain growth, which can be explained by the grain-boundary segregation-induced phase transformation mechanism. In the Y-TZP sintered at 1500°C for 2 h, the cubic-phase regions were already formed, and both of the cubic-phase region and average grain size increased with increasing holding time. This grain-growth behavior can be interpreted by the third-power growth low derived based on the solute drag theory, which indicates that the cubic-phase regions do not effectively act as the pinning points.

Superplasticity in fine-grained materials has been generally analyzed on the basis of their experimental strain rate-flow stress relationship. The phenomenological analysis based on a constitutive equation is effective for understanding the overall flow and fracture behavior and to speculate on the rate-controlling mechanism of superplastic flow. However, it has been recently pointed out that the high temperature superplastic flow and failure in ceramics is significantly influenced by the atomic structure and chemistry of grain boundary. Such phenomenon cannot be explained based on the classical phenomenological analysis. Our research group has therefore proposed to establish a new research field, grain boundary plasticiry, to describe the superplastic deformation related to the grain boundary quantum structure. This paper aims to point out the importance of the quantum structure analysis of the grain boundary to understand the high temperature plasticity in ceramics.

Sintering of (1 mol%Ni2+-1 mol%Er3+) or (1 mol%Mn2+-1 mol%Er3+) -doped Y2O3 polycrystal was investigated by conventional sintering at the sintering temperature in a range from 1200 to 1600°C. The sintering temperature required for full-densification decreased from 1600 to 1300°C by the doping. The average grain size in Ni2+ or Mn2+-doped Y2O3 is twice as large as that in undoped Y2O3 at the examined sintering temperature. HRTEM observations and EDS analyses with a nano-sized probe indicated that the doped transition metal ions segregated along the grain boundaries without amorphous layers and secondary phase particles. The segregated transition metal ions probably enhance the grain boundary diffusion, and the sinterability of Y2O3 was accordingly improved.

The densification behavior during sintering of 0.1 mol% TiO 2 -doped Al 2 O 3 was measured in either an air or N 2 + 5%H 2 gas atmosphere at the sintering temperature of 1573―1673 K. The grain boundary diffusivity was evaluated from the densification rate. High-resolution transmission electron microscopy (HRTEM) and nano-probe energy-dispersive X-ray spectroscopy (EDS) analyses revealed that the doped Ti cations segregate in the vicinity of the grain boundaries in the A1 2 0 3 . An electron energy loss spectroscopy (EELS) investigation indicated that the valence state of Ti in the Al 2 O 3 sintered in the reducing atmosphere was close to +3. The grain boundary diffusivity in undoped A1 2 0 3 was insensitive to the atmosphere, but was enhanced by the grain boundary segregation of Ti 4+ . The grain boundary diffusivity of alumina in the reducing atmosphere was, however, retarded by the Ti 3+ -doping. The retarded diffusivity by Ti 3+ -doping must be related to the lack of aluminum vacancies and the large ionicity of Ti-O compared to Al-O.

Microstructural developments during sintering in 2 and 3 mol% Y 2 O 3 -stabilized tetragonal zirconia polycrystals (2Y- and 3Y-TZPs) and 8 mol% Y 2 O 3 -stabilized cubic zirconia (8Y-CSZ) were systematically investigated in the sintering temperature range of 1100–1500 °C. Above 1200 °C, grain growth in 8Y-CSZ was much faster than that in 2Y- and 3Y-TZPs. In the grain-boundary faces in these specimens, amorphous layers did not exist; however, Y 3+ ions segregated at the grain boundaries over a width of ∼10 nm. The amount of segregated Y 3+ ions in 8Y-CSZ was significantly less than in 2Y- and 3Y-TZPs. This indicates that an increase in segregated Y 3+ ions retards grain growth. Therefore, grain growth behavior during sintering can be reasonably explained by the solute-drag mechanism of Y 3+ ions segregating along the grain boundary. The segregation of Y 3+ ions, which directly affects grain growth, is closely related to the driving force for grain-boundary segregation-induced phase transformation (GBSIPT).

The microstructures in 3 mol% Y2O3-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1100°-1650°C were investigated to clarify cubic-formation and grain-growth mechanisms. The cubic phase in Y-TZP appeared at 1300°C and its mass fraction increased with increasing sintering temperature. High-resolution transmission electron microscopy (HRTEM) and nanoprobe X-ray energy dispersive spectroscopy (EDS) measurements revealed that no amorphous layer existed along the grain-boundary faces in Y-TZP, and Y3+ ions segregated not only along the tetragonal-tetragonal phase boundaries but also along tetragonal-cubic phase boundaries. Scanning transmission electron microscopy (STEM) and nanoprobe EDS measurements revealed that the Y3+ ion distribution was nearly homogeneous up to 1300°C, but cubic phase regions with high Y3+ ion concentration clearly formed inside grains at 1500°C. These results indicate that cubic phase regions are formed from the grain boundaries and/or the multiple junctions in which Y3+ ions segregated. We termed such a new diffusive transformation phenomenon “grain boundary segregation-induced phase transformation (GBSIPT)”. The grain-growth mechanism is controlled by the solute-drag effect of Y3+ ions segregating along the grain boundary.

was estimated from the densification behavior during the final stage sintering. The densification rate was monitored from room temperature to the sintering temperature using a laser-scanning micrometer, which allows in-situ, non-contact measuring of the specimen's dimensions. The grain boundary diffusivity in Al 2 O 3 is sensitively affected by the doped cation which segregates at the grain boundaries. A first-principle molecular orbital calculation indicates that the doping effect on the grain boundary diffusivity is related to the ionicity in Al 2 O 3 ; the grain boundary diffusivity correlates well with the net charge of the Al and O ions in the cation-doped Al 2 O 3 .

For steady-state deformation caused by grain-boundary diffusion and viscous grain-boundary sliding, the creep rate of regular polyhedral grains is analyzed by the energy-balance method. For the microstructure, the grain-grain interaction increases the degree of symmetry of diffusional field, resulting in a decrease of the effective diffusion distance. Meanwhile, the viscous grain-boundary sliding is found to decrease the creep rate. The present analysis reveals that the grain-size exponent is dependent on the grain size and the grain-boundary viscosity: the exponent becomes unity for small grain sizes and/or high viscosity, while it is three for large grain sizes and/or low viscosity.

The microstructure in a small amount of Al2O3-doped Y2O3-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1100–1650 °C was examined to clarify the effect of Al3+ ions segregated at grain boundaries on cubic-formation and grain-growth processes. The sintering rate in Y-TZP was remarkably enhanced by Al2O3-doping. In addition, at temperatures >1500 °C, grain growth remarkably proceeded, and the fraction of the cubic phase increased in comparison with that of undoped Y-TZP. High-resolution electron microscopy and nanoprobe x-ray energy dispersive spectroscopy measurements revealed that no amorphous layer existed along the grain-boundary faces in Al2O3-doped Y-TZP and that Y3+ and Al3+ ions segregated at grain boundaries over widths of ∼10 and ∼6 nm, respectively. At 1100 °C, Al3+ ions started to segregate at grain boundaries, and the segregation peak of Al3+ ions increased as the sintering temperature increased. Cubic-formation and grain-growth behaviors in Al2O3-doped Y-TZP were reasonably interpreted by taking into account the effect of Al3+ ions segregating along grain boundaries.

Superplastic behaviors were investigated for fine-grained yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) from a viewpoint of GeO2 or TiO2 doping. It was found that both dopants enhance the ductility in TZP. In particular, elongation to failure of 988% could be obtained in 3Y-TZP co-doped with 2mol%GeO2 and 2mol%TiO2. In addition, it was revealed that lower flow stress did not always give a larger elongation in this system. On the other hand, the strong segregation of dopants along grain boundaries was confirmed by high-resolution electron transmission microscopy study (HRTEM) with X-ray energy dispersive spectrometer (EDS). The unique phenomena observed in the relation between flow stress and ductility is closely related to the strong segregation of dopants along grain boundaries in this system. The grain size at fracture is determined by covalency because of the dopant segregation. It could be concluded that elongation to failure is closely related to a balance between grain size at fracture and grain growth rate during deformation.

The doping dependence of high temperature plastic flow behavior in TiO 2 -doped, GeO 2 -doped and Tio 2 and GeO 2 co-doped tetragonal ZrO 2 polycrystal (TZP) was systematically examined at 1400°C in air under an initial strain rate of 1.3 x 10 -4 s -1 . The amount of dopant cations was in the range of 0.2-4 mol%. The flow stress in the TZP decreases with the increasing doping amount in TiO 2 and/or GeO 2 -doped TZP, but levels off over a 3 mol% addition in the GeO 2 -doped and TiO 2 -GeO 2 co-doped TZP. The doping dependence of the flow stress in the TiO 2 -GeO 2 co-doped TZP is close to that in the GeO 2 -doped TZP rather than to that in the TiO 2 -doped TZP; the flow stress in the GeO 2 -doped and TiO 2 -GeO 2 co-doped TZP is significantly lower than that in the TiO 2- doped TZP. The doped cations tend to segregate in the vicinity of the grain boundaries in TZP. The doping dependence of the flow stress is probably related to the amount of the grain boundary segregation of the dopant cations; the grain boundary segregation of the dopant cations is supposed to enhance the grain boundary atomic diffusion in the TZP, and hence the flow stress consequently decreases. On the other hand, elongation to failure in the TZP is highly improved by the TiO 2 and/or GeO 2 doping. The improved high temperature ductility of the TZP is attributed to the flow stress reduction due to the TiO 2 and/or GeO 2 doping.

The microstructure in 2.9 mol% Y 2 O 3 -stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1100°-1650°C was examined to clarify the role of Y 3+ ions on the cubic-formation and grain growth processes. The cubic phase in Y-TZP stared to appear at 1300°C and the fraction of the cubic phase increased with the increasing sintering temperature. Scanning transmission electron microscopy and nanoprobe X-ray energy dispersive spectroscopy (EDS) measurements revealed that the Y 3+ ion distribution in the grain interiors in Y-TZP was nearly homogeneous up to 1300°C and cubic phase regions in the grain interiors were formed clearly over 1300°C. The cubic phase region in the grain interior was extended as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer existed along the grain-boundary faces in Y-TZP, and Y 3+ ions segregated at their grain boundaries over a width of ∼10 nm. The segregation peak of Y 3+ ions was clearly seen at 1300°C, and above this temperature, Y 3+ ions segregated at the grain boundaries not only between tetragonal grains but also between tetragonal and cubic grains. These results show that cubic phase regions started to be transformed from the grain boundaries and/or the triple junctions in which Y 3+ ions segregated. The cubic-formation mechanism in Y-TZP can be reasonably explained from the viewpoint of the Grain Boundary Segregation-Induced Phase Transformation model, and the grain-growth behavior is probably controlled by the solute drag effect of Y 3+ ions segregating along the grain boundary.

Recently, Raj et.al. have developed a very unique sintering technique, called flash-sintering [1]. According to their report, fully densified ZrO2-3mol%Y2O3 ceramic bulks were successfully obtained only at 800°C for 5sec. Considering the conventional sintering condition around 1500°C for a few hours necessary to obtain ZrO2-3mol%Y2O3 ceramic bulks, their sintering technique is very attracting from a viewpoint of sintering temperature, soaking time and further the physical phenomena. The flash-sintering is a technique that green compacts were heating under application of high electric field. When furnace temperature reaches at a critical temperature, the electric current abruptly increases and the compact sinters near full density with a very high shrinkage rate. So far, a few studies about flash-sintering were reported for Y2O3 [2], SrTiO3, MgO-Al2O3. To understand the detail mechanism of flash-sintering, more case studies must be necessary. In this study, we focused BaTiO3 widely used for electro-ceramics, which has not been investigated from a viewpoint of flash-sintering.Green compacts were prepared from BaTiO3 raw powders (0.1-m, 99.9%, SAKAI chemical industry Co. Ltd., Lot. No.1308607) after uniaxially pressed at 100MPa into a rectangular shape with 2x10x30mm(3). The green compacts were suspended into a box type furnace by Pt-wires with Pt-based paste. Then, the furnace temperature was raised at 300°C/h under application of electric field ranged from 25V/cm to 350V/cm with monitoring the specimen current. After sintering, the shrinkages, microstructure of the sintered compacts were investigated.Sintering rates at all electric fields were found to be accelerated by applying electric field in BaTiO3. The appearance of abrupt current increment was confirmed over the application of 75V/cm. For example, a density of green compact reached about 90% relative density of BaTiO3 only at 1020°C for 1min at 100V/cm. However, the final shrinkages were revealed to decrease with an increase in electric fields, which is very different from the case of ZrO2-3mol%Y2O3 and Y2O3 ceramics. This fact means that application of high electric fields does not effectively operate for enhancement of shrinkage rates in the case of BaTiO3. In contrast, only gradual current increment was observed at 25V/cm, which is categorized in field-assisted sintering (FAST) process. The density of the green compact at 25V/cm was more than 95%.To investigate the mechanism of the decrease in a total shrinkage with electric fields, the microstructure of flash-sintered compact was observed. As a result, it was found that discharge occurs during flash-sintering process, indicating that the input power due to high electric fields does not work effectively. A typical example of the microstructure near the discharge area is shown in Fig. 1. Fig. 1 is a TEM bright field image taken from BaTiO3 flash-sintered at 100V/cm. As seen in the image, the formation of a secondary phase along the grain boundary can be clearly seen. Diffractometric and EDS analysis have revealed that the secondary phase is BaTi4O9, one of compounds between BaO and TiO2 system. By discharging, grain boundaries partially melt and a part of Ba vaporizes to form BaTi4O9 with cooling. To investigate flash-sintering behaviors, it was concluded that FAST process play an important role to enhance the shrinklage rate in the case of BaTiO3.jmicro;63/suppl_1/i19/DFU048F1F1DFU048F1Fig. 1.TEM bright field image of a secondary phase and the electron diffraction pattern taken from the secondary phase.

High temperature plastic flow or grain boundary failure in oxide ceramics such as Al2O3 and tetragonal ZrO2 polycrystal (TZP) is sensitive to small levels of doping by various cations. For example, high temperature creep deformation in fine-grained, polycrystalline Al2O3 is highly suppressed by 0.1 mol% lanthanoid oxide or ZrO2-doping. An elongation to failure in superplastic TZP is improved by 0.2–3 mol% GeO2-doping. A high-resolution transmission electron microscopy (HRTEM) observation and an energy-dispersive X-ray spectroscopy (EDS) analysis revealed that the dopant cations tend to segregate along the grain boundaries in Al2O3 and TZP. The dopant effect is attributed to change in the grain boundary diffusivity due to the grain boundary segregation of the dopant cations. A molecular orbital calculation suggests that ionicity is one of the most important parameters to determine the high temperature flow stress, and probably, the grain boundary diffusivity in the oxide ceramics.

The grain boundaries in high-purity oxide ceramics, such as Al 2 O 3 and TZP, are often free from glass phase, and the high-temperature plastic flow or grain boundary failure is sensitive to small levels of doping by various cations. For example, the high-temperature creep strain rate in fine-grained, polycrystalline Al 2 O 3 is highly retarded by 0.1 mol% Lu 3 + or Zr 4 + -doping. The elongation to failure in superplastic TZP is improved by 0.2 - 3 mol% Ge 4 + -doping. Such a dopant effect is attributed to changes of the grain boundary diffusion due to the segregation of dopant cation along the grain boundaries. Quantitative analysis on the atomic structure and chemical bonding state along the grain boundaries by high-resolution electron microscopy, energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy and molecular orbital calculations will provide theoretical guiding principles to design high-performance oxide ceramics in the near future.

Superplastic behavior in a fine-grained, GeO2-doped 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) with the dopant level of 0.2 to 3 mol% was examined at 1400°C under an initial strain rate of 1.3×10-4 s-1. Microstructure and chemical composition at the grain boundaries were examined by high-resolution transmission electron microscopy (HRTEM) combined with an X-ray energy dispersive spectrometer (EDS). No secondary phase was observed along the grain boundaries, though EDS analysis indicated the segregation of Ge cations along the grain boundaries. The Ge content at the grain boundaries tends to increase with increasing the total amount of GeO2 addition, but saturate over the doping level of 2 mol%. Dependence of flow stress reduction on the total amount of GeO2 addition has a good correlation with Ge content at the grain boundaries. This fact indicates that the GeO2-doping effect on the flow stress in 3Y-TZP is caused mainly from the grain boundary segregation of Ge cations.

Temperature and strain rate dependence on high temperature elongation to failure in fine-grained ceramics is phenomenologically explained from grain growth behavior during deformation and the superplastic flow behavior. The elongation to failure at temperatures between 1573 and 1773 K was analyzed for 2 mol% TiO2 and 2 mol% GeO2 co-doped tetragonal zirconia polycrystal (TZP), which exhibits excellent high temperature ductility. The improvement in the high temperature ductility in TZP is attributed to dopant cation segregation in the vicinity of the grain boundaries. The phenomenological analysis revealed that co-doping of Ti and Ge cations increases the grain size at the time of failure, as a parameter to describe a limit of an accommodation process for superplastic flow. The parameter of the critical grain size at the time of failure correlates well with the value of overlap population in cation-doped TZP model cluster obtained from a first-principle molecular orbital calculation. The covalent bond at the grain boundaries plays a critical role in the high temperature tensile ductility of TZP.

The densification behavior during sintering in 0.1 mol% MgO-, MnO-, SrO-, LuO1.5-, TiO2-, ZrO2- or PtO2-doped Al2O3 was investigated at the sintering temperature of 1300–1500 °C in order to systematically examine the dopant effect on grain boundary diffusivity in Al2O3. The densification behavior was monitored from room temperature to the sintering temperature using a laser-scanning system, which allows in situ, non-contact measuring of the specimen’s dimensions. The grain boundary diffusivity in Al2O3 is sensitively affected by the dopant cation which segregates at the grain boundaries. The dopant effect on the grain boundary diffusivity is related to the ionicity in Al2O3; a lower energy level of the dopant element’s outer shells provides a higher value of diffusivity in the divalent or tetravalent cation-doped Al2O3. A first-principle molecular orbital calculation revealed that the grain boundary diffusivity correlates well with the net charge of the Al and O ions in the cation-doped Al2O3. The ionic bond strength in the vicinity of the grain boundaries dominates the high-temperature grain boundary diffusion in polycrystalline Al2O3.

High temperature plastic deformation in fine-grained, polycrystalline alumina often takes place by diffusional creep or grain boundary sliding in a fairly wide stress and temperature range. Under such conditions, the high temperature creep or plastic flow behavior in alumina is sensitively affected by small amount of cation-doping, even in the dopant level of 0.1 mol% or less. The dopant effect on the plastic deformation in alumina results from a change in chemical bonding state at the grain boundaries due to the dopant cations’ segregation along the grain boundaries. Grain boundary chemistry dominantly affects matter transport phenomena in the grain boundaries and the high temperature mechanical property in alumina.

Misorientation dependence of grain boundary energy and grain boundary sliding at high temperature were examined in cubic zirconia bicrystals with [1 1 0] symmetric tilt boundaries, which were fabricated by diffusion bonding method from two cubic zirconia single crystals. High-resolution transmission electron microscopy observation revealed that the grain boundary in cubic zirconia bicrystals was clean and atomically sharp without any void or grain boundary amorphous layer. Grain boundary energy of the tilt boundaries was estimated from the dihedral angles on thermal grooved surface measured with atomic force microscope techniques. The misorientation dependence of the grain boundary energy in cubic zirconia bicrystals shows similar tendency to that of fcc metal such as aluminum and copper. Grain boundary sliding associated with intragranular dislocation slip in cubic zirconia bicrystals was observed for all specimens. The amount of the grain boundary sliding showed a good correlation with the misorientation factor of each boundary. Grain boundary migration also took place accompanying with the grain boundary sliding. The observed grain boundary sliding and migration can be explained based on a dislocation mechanism for sliding which is based on the movement of lattice dislocations along the grain boundary by a combination of climb and glide.

Local atomic structures around the Lu3+ ion in a 0.1 mol% LuO1.5-doped fine-grained Al2O3, in which the doped Lu3+ ions segregate to the grain boundaries, was characterized by Lu L3-edge X-ray absorption fine structure. Structural parameters in LuO1.5-doped Al2O3 were determined by a curve-fitting method, and the results showed that six O2− ions coordinate with the Lu ion in LuO1.5-doped Al2O3. In addition, it was also revealed that the Lu–O interatomic distance in Lu-doped Al2O3 was close to that in Lu2O3, which was about 19% longer than the Al–O interatomic distance in undoped Al2O3. The present results indicated that the local atomic structures around Lu in Al2O3 are close to that in Lu2O3. It is thus supposed that atomic distances between Al3+ and O2− ions in the vicinity of Lu-segregated grain boundaries are shortened in comparison with that in undoped Al2O3. A first-principles molecular orbital calculation was performed for the [Al2O9]−12 model cluster, and the shortening of the Al–O interatomic dis...