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Frequency dependence of the densification behavior of undoped Y2O3 sintered by the AC-flash sintering was systematically investigated at 500 V·cm−1 over a frequency range from 0.05 Hz to 1 kHz. The Y2O3 bodies sintered under an AC field showed a uniform microstructure, without an asymmetric grain size distribution between the electrodes. Almost fully-densified Y2O3 body was consolidated at 1 kHz exhibited a relative density greater than 99 % and an average grain size of 1.6 μm. The almost full densification probably resulted from the high input power at the relatively high onset temperature of 1300 °C at this frequency. The temperature dependence of the power dissipation during the AC-flash sintering experiments can be ascribed to the periodic fluctuations of the specimen temperature at low frequencies and to the phase shift between the applied field and the specimen current at high frequencies.

We investigated the photoluminescence (PL) properties of 3 mol%Y2O3-ZrO2 (3YSZ) polycrystals. The 3YSZ polycrystals were prepared by an isothermal flash sintering technique in which an alternating electric field was applied at a predetermined furnace temperature. After flash sintering, clear, visibly blue PL at 2.82 eV (440 nm) was confirmed under an ultraviolet light (3.4 eV (365 nm)); however, no visible PL was observed from 3YSZ polycrystals conventionally sintered using the same raw powders. Thus, the emission of blue PL was induced by the electric field applied during sintering. The obtained PL band was related to excitation at 3.87 eV (320 nm), which is possibly assigned to the singly ionized associated oxygen vacancy defects (AOD + centers). Furthermore, the blue emission was stable after high temperature annealing at 1400 °C for 1 h in air. We possibly attribute the emission of this PL to the stabilization of AOD + centers by the electric field loading during flash sintering.

The experimental densification kinetics of 7.8 mol% Y2O3-stabilized zirconia was analyzed theoretically during isothermal sintering in the final stage. By taking concurrent grain growth into account, a possible value of the grain-size exponent n was examined. The Coble’s corner-pore model recognized widely was found not to be applicable for explaining the densification kinetics. The corner-pore model of n = 4 shows a significant divergence in the kinetics at different temperatures. Microstructural observation shows that most pores are not located at grain corners and have a size comparable to the surrounding grains. The observed pore structure is similar to the diffusive model where single pore is surrounded by dense body. The diffusive model combined with theoretical sintering stress predicts n = 1 or n = 2, which shows a good consistence to the measured densification kinetics. During sintering of nano-sized powder, it is found that the densification kinetics can be explained distinctively by the diffusive single-pore model.

The orientation-dependent micromechanical properties of nontransformable tetragonal (t') zirconia, which diffusionlessly transformed from the fluorite cubic phase and does not show stress-induced phase transformation, were characterized via pseudo-single crystal micropillar compression and electron microscopy. The t' zirconia sample was obtained via atmospheric plasma spraying of 4.5 mol% yttria-stabilized zirconia (YSZ) powders into liquid nitrogen and consolidated into a bulk state via hot pressing at 1100°C. Dense and cylindrical micropillars were fabricated using a focused ion beam from pseudo-single crystalline regions, which had a nanodomain microstructure of three t' variants partitioned by {1 0 1}c twin boundaries with 90° symmetry, and were compressed using a flat-end diamond indenter. Near- c compressions were attributed to ferroelastic domain switching and subsequent {1 0 1}c and/or {1 1 1}c slips. In ferroelastic deformation, a certain t' variant diminished and a binary domain microstructure developed with c axes perpendicular to the compressive direction. Near- c compressions were governed by {0 0 1}c slips accompanied by strain hardening with negligible ferroelasticity, resulting in buckling deformation with rotational kinking. Crack-tolerant plasticity was ascribed to ferroelastic toughening, where t' variants with c axes across crack planes developed to relieve stress concentrations around the crack tips in both orientations. In contrast, pseudo-cleavage fractures on low-index planes were observed in near- c compressions. Crack-tolerant behavior was not observed in the cubic counterpart with a domain-free microstructure (8.0 mol% YSZ), which demonstrated catastrophic fractures. Hence, ferroelastic toughening is viewed as the origin of enhanced toughness in t' zirconia.

This study systematically investigated the electric direct current (DC) dependence of the plastic flow behavior in a densified tetragonal ZrO2 polycrystal (TZP). Consequently, we observed an optimal current density for tensile ductility. The maximum elongation of 135% was achieved under the current density of 250 mA∙mm−2 at a furnace temperature of 1000 °C and an initial strain rate of 1 × 10

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.

Carbon contamination during the SPS processing was investigated in the spinel, alumina and zirconia. The carbon contamination changes with the SPS conditions and the target materials. At the high heating rate of 100 °C/min, the contamination occurred over the entire area in the spinel, but only around the surface areas in the alumina and zirconia. For the spinel, the contamination is sensitive to the SPS parameters, such as the heating rate and loading conditions, but less sensitive to the sintering temperature. This suggests that the carbon contamination was caused by evaporation of CO gas from the carbon paper/dies. At the high heating rates, the carbon evaporation is enhanced due to the rapid heating, and then, the evaporated CO gases are encapsulated into the closed pores during the heating process and remain in the matrix. The carbon contamination can be suppressed by a high temperature loading even at the high heating rate.

Carbon contamination from the carbon paper/dies during spark-plasma-sintering (SPS) processing was examined in the MgAl 2 O 4 spinel. The carbon contamination sensitively changes with the heating rate during the SPS processing. At the high heating rate of 100 °C/min, the carbon contamination having organized structures occurred over almost the entire area from the surface to deep inside the SPSed spinel disk. In contrast, at the slow heating rate of 10 °C/min, the carbon contamination having disordered structures occurred only around the surface area. The carbon phases transform into high pressure CO/CO 2 gases by post-annealing in air and lead to pore formation along the grain junctions. The pore formation significantly occurs at the high heating rate due to the large amount of the contaminant carbon phases. This suggests that if once the carbon contamination was formed in the materials, it is very difficult to remove the carbon phases from the materials.

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.

High-strength structural ceramics with sub-micron grain sizes can exhibit superplasticity, but the superplasticity appears at high temperatures and low strain rates. Low temperature and high speed superplastic flow in structural ceramics was achieved by applying a strong electric field above a threshold value during deformation. The application of a direct-current field of 190 V·cm− 1 led to superplastic deformation in Y2O3-stabilized tetragonal ZrO2 polycrystal with a total tensile elongation of > 150% at 800 °C and an initial strain rate of 2 × 10− 3 s− 1. The field-activated plasticity was attributed to highly-accelerated self-diffusion induced not only by temperature rise but also by a field effect.

This study systematically investigated the electric direct current (DC) dependence of the plastic flow behavior in a densified tetragonal ZrO2 polycrystal (TZP). Consequently, we observed an optimal current density for tensile ductility. The maximum elongation of 135% was achieved under the current density of 250 mA∙mm−2 at a furnace temperature of 1000°C and an initial strain rate of 1 × 10−3 s−1. The diffusional mass transport in the TZP was accelerated by the combination of Joule heating and applied current effect, resulting in a reduced flow stress and an improved ductility.

The orientation-dependent micromechanical properties of nontransformable tetragonal (t’) zirconia, which underwent a diffusionless transformation from the fluorite cubic phase and does not exhibit a stress-induced phase transformation, were characterized via pseudo-single crystal micropillar compression and electron microscopy. The t’ zirconia sample was obtained via atmospheric plasma spraying of 4.5 mol% yttria-stabilized zirconia (YSZ) powders into liquid nitrogen and consolidated into a bulk state via hot pressing at 1100°C. Dense and cylindrical micropillars were fabricated using a focused ion beam from pseudo-single crystalline regions, which exhibited a nanodomain microstructure of three t’ variants partitioned by {1 0 1}c twin boundaries with 90° symmetry. These micropillars were compressed using a flat-end diamond indenter. Near- c compressions were attributed to ferroelastic domain switching and subsequent {1 0 1}c and/or {1 1 1}c hard slips. In ferroelastic deformation, a certain t’ variant diminished, and a binary domain microstructure developed with c axes perpendicular to the compressive direction. Near- c compressions were governed by {0 0 1}c soft slips accompanied by strain hardening with negligible ferroelasticity, which resulted in buckling deformation with rotational kinking. In both the hard- and soft-slip orientations, ferroelastic toughening was observed with certain t’ variants awaken around the crack tips. Contrarily, cleavage fractures subsequent to yielding were observed in near- c compressions. In the cubic counterpart with a domain-free microstructure (8.0 mol% YSZ), ferroelastic toughening was not observed. Hence, it is viewed as the origin of enhanced toughness in t’ zirconia.

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.

The densification behavior during the isothermal sintering of 8YSZ was examined in the initial and intermediate stages of sintering. In the initial stage, a difficulty in evaluating the densification behavior arises from the transition of the stable pore structure and the limitation of the theoretical two-sphere model. In the intermediate stage, a linear relationship with a slope of −1/2 is observed between the densification rate and the time. An empirical equation of the densification kinetics is proposed and found to be valid in a wide density range. At a relative density of 0.6–0.73, the activation energy is 688 kJ/mol. Rapid grain growth is observed at a relative density of 0.73–0.8 and >0.9 for the isothermal sintering at 1200–1300 °C and 1400–1500 °C, respectively. The rapid grain growth reduced significantly the densification rate. The densification mechanism and grain growth behavior are also discussed.

1 mol% Ni 2+ -doped Y 2 O 3 shows flash-sintering at DC fields greater than 300 V/cm. When compared to the undoped Y 2 O 3 , the onset temperature for the flash event is reduced by about 200 °C due to the Ni-doping. The electric conductivity during the sintering experiments for the Ni-doped Y 2 O 3 begins to increase above those of the undoped Y 2 O 3 at temperatures greater than 700 °C, then steeply rises corresponding to the flash event. Electron energy loss spectrometry (EELS) measurements indicated that Y 2 O 3 is highly reduced by the combination of the Ni-doping and flash-sintering as well as by conventional sintering in a reducing atmosphere. The applied electric field must involve the formation of oxygen anion vacancies and liberated electrons, and consequently, enhances the diffusional mass transport and electronic conduction in the Y 2 O 3 . The enhanced flash-sintering in the Ni-doped Y 2 O 3 must be attributed to an avalanche of the vacancy-electron pairs facilitated by the Ni-doping.

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.

Discoloration of spark-plasma-sintered spinel was investigated by spectroscopic techniques and transmission electron microscopy. The discoloration is explained by the combination of carbon contaminations and lattice defects (color centers), which are introduced in the spinel matrix depending on the spark-plasma-sintering conditions. For low heating rates of ⩽10 °C min−1, the trace carbonate CO3 pre-existing in the starting powder remained as glassy carbon within the matrix, irrespective of the sintering temperature. For a high heating rate of ⩾50 °C min−1, additional carbon contamination occurred by evaporating the carbon phases from the carbon papers and graphite dies during the heating process, and tended to be enhanced by the increasing heating rate. The present data indicate that the color center (F+-center) may be generated by the formation of oxygen vacancies, which are mainly introduced by dislocation motion depending on the sintering conditions. Since the rate of sintering, namely the deformation rate, increased with the heating rate, the concentration of the dislocation-related color centers increased with the heating rate, but decreased with the sintering temperature due to the bleaching of the oxygen vacancies. For the present spinel, the discoloration due to the carbon contamination and the formation of F+-centers deteriorates the light transmission, depending on the sintering conditions.

Textured Ti 3 AlC 2 ceramic was successfully fabricated by a strong magnetic field alignment (SMFA) technique followed by spark plasma sintering (SPS). About 15 vol.% Al 2 O 3 particles were formed in situ during the process. The unique combination of excellent bending strength of 1261 MPa (// c axis) and fracture toughness of 14.6 MPa m 1/2 (⊥ c axis) was achieved. Also, the high electrical and thermal conductivities were determined as 1.0 × 10 6 Ω −1 m −1 (// c axis) and 25.3 W (m K) −1 (⊥ c axis), respectively.

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.

Conventional sintering of undoped Y 2 O 3 requires temperatures above 1400 °C for a few hours. We show that it can be sintered nearly instantaneously to nearly full density at furnace temperature of 1133 °C under a DC applied field of 500 V/cm. At 1000 V/cm sintering occurs at 985 °C. The FLASH event, when sintering occurs abruptly, is preceded by gradually accelerated field-assisted sintering (FAST). This hybrid behaviour differs from earlier work on yttria-stabilized zirconia where all shrinkage occurred in the flash mode. The microstructure of flash-sintered specimens indicated that densification was accompanied by rapid grain growth. The single-phase nature of flash-sintered Y 2 O 3 was confirmed by high-resolution transmission electron microscopy. The non-linear rise in conductivity accompanying the flash led to Joule heating. It is postulated that densification and grain growth were enhanced by accelerated solid-state diffusion, resulting from both Joule heating and the generation of defects under the applied field.

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.

Transparent, nanocrystalline hydroxyapatite with an average grain size of 170 nm was fabricated by spark plasma sintering. The superplastic flow behavior of the hydroxyapatite specimen was examined at temperatures ranging from 950 to 1050 °C. The hydroxyapatite specimens exhibited superplasticity under the test conditions examined. A maximum elongation of 486% was achieved at 1000 °C and an initial strain rate of 1.0 × 10 –4 s −1 . The excellent tensile ductility in transparent, nanocrystalline hydroxyapatite was attributed to the very fine grain size and pore-free microstructure.

Highly transparent pure alumina (65.4%) with an average grain size of 200 nm was fabricated using high-pressure (>400 MPa) spark plasma sintering. For the first time, highly transparent alumina ceramics were produced using low cost carbon fiber composite for the pressing punches. The high pressure device was also used to fabricate fully dense nanostructured alpha alumina with average grain size of 107 nm. The developed low cost high pressure sintering apparatus might be applied to produce a wide range of pore free nanostructured materials.

An increase in the loading temperature during SPS processing can reduce the residual porosity in a spinel and thus attain a high transmission even at the high heating rate of 100 °C/min. This suggests that load controlling is an important factor as well as the heating rate and sintering temperature. Although the transmission is lower than the maximum value attained at the low heating rates of

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.

We show an optimum condition for high-pressure (400 MPa) spark-plasma-sintering (SPS) of transparent cubic (8 mol% yttria) zirconia. The obtained samples represent higher in-line transmittance compared to the existing SPSed zirconias, and maintain nano-grained structures. The role of oxygen defects in transparency of the zirconia is discussed.

The densification rate of a spinel during spark plasma sintering can be characterized by the decreasing stress exponent from ⩾4 to ≈1 as the density increases. Transmission electron microscopy observations showed that stacking faults introduced by partial dislocations are observed in a low-density region, but limited in a high-density region. These results suggest that the dominant densification mechanism changes with density, from plastic flow caused by a partial dislocation motion to diffusion creep as the density increases.

The microstructure and optical properties of MgO-doped alumina fabricated by spark plasma sintering were investigated. MgO doping to alumina exhibited no noticeable effect on the total forward transmission, but suppressed grain growth and enhanced in-line transmission. The analytical investigation of the scattering coefficients confirmed that MgO-doped alumina has the scattering characteristics of the homogeneous matrix dispersed with spherical scattering grains, and that the existing model of light scattering in an alumina polycrystal is valid within a limited range of the wavelength/grain size ratio. The investigation also led to experimental determination of the effective volume density of spherical scattering grains satisfying the Rayleigh–Gans theory.

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.

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 microstructure and optical properties are evaluated for alumina sintered by spark plasma sintering at temperatures between 1100 and 1550 °C. With increasing sintering temperature, grain growth and densification occur up to 1250 °C, and above 1300 °C, rapid grain growth and pore growth occur. Light transmission increases with the densification and decreases with the grain/pore growth. It is found that the total forward transmission and the reflection of light are related to the porosity and the pore growth, whereas the in-line transmission and the light absorption are related to the grain size and the defects, respectively. The relationships are explained by using the Mie scattering theory, model prediction and observed microstructural characteristics.

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.

Commercial alumina powder was densified by spark plasma sintering (SPS) at 1150 °C. During SPS processing, the effects of the heating rate were examined on microstructure and transparency. With decreasing heating rate, the grain size and the residual porosity decreased, while the transparency increased. At a heating rate of 2 °C/min, the grain size was 0.29 μm, and the in-line transmission was 46% for a wavelength of 640 nm. The mechanisms for the fine microstructure and low porosity at slow heating, which are conflicting with some existing results, were explained by considering the role of defect concentration and grain-boundary diffusion during densification.

Transparent spinel can be fabricated without any sintering aids by spark plasma sintering (SPS) processing for only a 20 min soak at 1300 °C. For heating rates

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).

Transparent alumina with a fine grain size (0.27 μm) was obtained by controlling the heating rate during spark plasma sintering processing. The alumina sintered at 1150 °C with a heating rate of 8 °C/min has a residual porosity of 0.03% and an in-line transmission of 47% for a wavelength of 640 nm. We show that a low heating rate has an effect on the densification and transparency of alumina for sintering at 1150 °C.

A nanocrystalline ZrO2–spinel composite with an average grain size of smaller than 100 nm can be synthesized by high-energy ball-milling followed by spark plasma sintering. Nanocrystallization can strengthen the ZrO2–spinel composite by a factor of 2.0–2.5; the maximum flexural strength of the nanocrystalline composite reached ≈2200 MPa.

High temperature superplasticity in 8 mol % Y 2 O 3 -stabilized cubic-ZrO 2 (8YCZ) was enhanced by TiO 2 addition. In particular, 8YCZ–15 mol % TiO 2 with an initial grain size of 2.5 μm exhibits a significantly lower flow stress and a larger elongation to failure than 8YCZ with a grain size of 2.8 μm.

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.