Effects of Magnesium Oxide on Grain-Boundary Segregation of Calcium During Sintering of Alumina

The advantage of certain amounts of MgO addition in alumina sintering has been realized, and it is common practice. In an attempt to understand the role of MgO in the presence of CaO in commercial-grade alumina, grain-boundary segregation of Ca was investigated by scanning Auger electron microprobe (SAM) using an ultrapure alumina after controlled doping of CaO and/or MgO. The commercial-grade alumina, which usually contains a small amount of CaO, is difficult to sinter to high density. The pure alumina composition (<99.999%) gives "clean" boundaries when it is sintered under "clean" conditions. As the powder was doped with 100 ppm of CaO and sintered at 1300° to 1500°C, all of the grain boundaries were enriched by Ca as observed by others. However, it was also discovered that some of the grain boundaries are enriched by an exceptionally high concentration of Ca. Such a large variation of Ca contents depending on the grain-boundary facets disappeared when samples were codoped with small amounts of MgO. The results suggest that MgO plays a beneficial role in controlling the anisotropic segregation of Ca to various interfaces including grain boundaries and pore surfaces during sintering of alumina. MgO thus enhances chemical homogeneity of commercial-grade alumina.     

Oxygen Grain-Boundary Diffusion in Polycrystalline Mullite Ceramics

Oxygen tracer diffusivities of low- and high-alumina mullite ceramics (72 wt% Al₂O₃, 28 wt% SiO₂ and 78 wt% Al₂O₃, 22 wt% SiO₂, respectively) were determined. Gas/solid exchange experiments were conducted in an atmosphere enriched in the rare stable isotope ¹⁸O, and the resulting ¹⁸O isotope depth distributions were analyzed using SIMS depth profiling. The investigation showed that grain-boundary diffusivities for both mullite ceramics were several orders of magnitude higher than mullite volume diffusivity. Activation enthalpies of oxygen diffusion were 363 ± 25 kJ/mol for the low-alumina and 548 ± 46 kJ/mol for the high-alumina materials. Because the glassy grain-boundary films were not identified, the differences between the low- and high-alumina materials might be explained by different impurity concentrations in the grain boundaries of the two materials.     

Forces between Aluminum Oxide Grains in a Silicate Melt and Their Effect on Grain Boundary Wetting

Experiments in which the disperson and flocculation behavior of aluminum oxide grains in a Ca-Al-Si-O silicate liquid are examined have been used to determine the nature of the forces between oxide grains in a silicate melt. From observations of the way in which the melt penetrates grain boundaries in a high-purity polycrystalline aluminum oxide, it is concluded that at short ranges the interaction between most grains is strongly repulsive. The formation of aggregated groups of grains in settling experiments, however, indicates that at larger separations the forces between grains become attractive and hence that there is an energetic minimum in the interaction between grains. The results are discussed in relation to models for the stability of intergranular films in liquid-phase-sintered ceramics. Microstructural observations made on the melt-penetrated aluminas are used to make a quantitative estimate of the fraction of grain boundaries that are wetted by the melt. The observations indicate that large clusters of grains connected by unwetted grain boundaries exist in liquid-phase-sintered aluminas.     

Inhomogeneity of Grain-Boundary Resistivity in Calcia-Stabilized Zirconia

Impedance spectroscopy with millimeter-scale electrodes was used to determine the local grain-boundary resistivity (ρ_gb) of a 1-mol%-Al₂O₃-doped CaO-stabilized ZrO₂ (CSZ) specimen. A much higher ρ_gb 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.     

Effects of Polarity on Grain-Boundary Migration in ZnO

The migration of boundaries between single crystal and polycrystalline ZnO was investigated using single crystals with well-defined crystallographic faces. The migration rate of the basal (0001) planes through polycrystalline ZnO depended on the crystallographic polarity of the basal plane. Grain boundary migration in the [0001] direction was much faster than in the [000[Onemacr]] direction. Migration rates of boundaries in nonpolar directions were somewhat slower than that in the [0001] direction. Slow growth in the [000[Onemacr]] direction and rapid growth in nonpolar directions found in the present study help to explain the anisotropic growth of the grains that contain head-to-head inversion boundaries in ZnO varistor ceramics.     

Direct Determination of Grain Growth Behavior in Zinc Oxide with Added Barium Oxide

Grain growth behavior in ZnO with 4 to 12 wt% BaCO₃ (3.12 to 9.59 wt% BaO) added was studied by a new method, in which the sintered specimen was treated with boiling water to separate the grains by dissolving the grain-boundary phase. The separated grains were examined with SEM to obtain quasi-three-dimensional information on the grain shape, size, and size distribution. The grains had a rounded shape, and the size distribution was found to be lognormal.     

Grain-Boundary Wetting-Dewetting in z= 1 SiAlON Ceramic

The grain-boundary structure of a model SiAlON polycrystal with nominal composition Si₅AlON₇ was characterized by transmission electron microscopy (TEM) both in an equilibrium (as-processed) state at room temperature and after quenching from elevated temperature. In addition, low-frequency (1–13 Hz) internal friction data were recorded as a function of temperature, showing a pronounced grain-boundary sliding peak positioned at 1030°C. High-resolution transmission electron microscopy (HRTEM) of the equilibrated low-temperature microstructure revealed residual glass only at multigrain junctions, but no amorphous intergranular films were observed. The detection of clean interfaces in the as-processed sample contradicts the internal friction data, which instead suggests the presence of a low-viscosity grain boundary phase, sliding at elevated temperatures. Therefore, a thin section of the as-sintered material was heated to 1380°C and rapidly quenched. HRTEM analysis of this sample showed, apart from residual glass pockets, wetted grain boundaries, which is in line with the internal friction experiment. This wetting-dewetting phenomenon observed in z = 1 SiAlON is expected to have a strong impact not only on high-temperature engineering ceramics but also on geological, temperature-activated processes such as volcanic eruptions.     

Microstructure and Grain-Boundary Effect on Electrical Properties of Gadolinium-Doped Ceria

The microstructural evolution and grain-boundary influence on electrical properties of Ce_0.90Gd_0.10O_1.95 were studied. The nanoscale powders synthesized from a semibatch reactor exhibited 50% green density and 92% sintering density at 1200°C (∼200°C lower than previous studies). Impedance spectra as a function of temperature and grain size were analyzed. The Ce_0.90Gd_0.10O_1.95 with finest grain size possessed highest overall grain-boundary resistance; this contribution was eliminated at temperatures >600°C, regardless of grain size. The grain conductivity was independent of grain size and was dependent on temperature with two distinct regimes, indicative of the presence of Gd'_Ce− V _o^∘∘ complexes that dissociated at a critical temperature of ∼580°C. The activation energy for complex dissociation was ∼0.1 eV; the value for the grain-boundary was ∼1.2eV, which was size independent.     

Grain-Boundary Migration in Nonstoichiometric Solid Solutions of Magnesium Aluminate Spinel: II, Effects of Grain-Boundary Nonstoichiometry

The grain-boundary chemistry of magnesium aluminate spinel solid solutions MgO· n Al₂O₃ in which grain growth measurements were reported in part I has been investigated in order to understand the mechanism of grain-boundary migration. It is found that although segregation of impurity Ca and Si is common, much larger deviations in grain-boundary stoichiometry are present. There is an excess of Al and O relative to Mg at grain boundaries in all compositions. Grain-boundary migration appears to be rate-limited by solute drag from intrinsic defects accommodating lattice nonstoichiometry, rather than by extrinsic solutes, consistent with the observed impurity tolerance of grain-boundary mobility. Different rate-limiting defects are proposed for magnesia-rich and alumina-rich spinels.     

Thermochemical Modeling of Oxide Glasses

A modified associate species approach is used to model the liquid phase in oxide systems. The relatively simple technique treats oxide liquids as solutions of end-member and associate species. The model is extended to representing glasses by treating them as undercooled liquids. Equilibrium calculations using the model allow the determination of species activities, phase separation, precipitation of crystalline phases, and volatilization. In support of nuclear waste glass development, a model of the Na₂O–Al₂O₃–B₂O₃–SiO₂ system has been developed that accurately reproduces its phase equilibria. The technique has been applied to the CaO–SiO₂ system, which is used to demonstrate how two immiscible liquids can be treated.     

Grain-Boundary Viscosity of Preoxidized and Nitrogen-Annealed Silicon Carbides

Internal friction experiments were conducted on a model SiC polycrystal prepared from preoxidized (high-purity) SiC powder. This material contained high-purity SiO₂ glass at grain boundaries in addition to a free-carbon phase, which was completely removed upon powder preoxidation. Comparative tests were conducted on a SiC polycrystal, obtained from the as-received SiC powder with the addition of 2.5 vol% of high-purity SiO₂. This latter SiC material was also investigated after annealing at 1900°C for 3 h in a nitrogen atmosphere. Electron microscopy observations revealed a glass-wetted interface structure in SiC polycrystals prepared from both as-received and preoxidized powders. However, the former material also showed a large fraction of interfaces coated by turbostratic graphite. Upon high-temperature annealing in nitrogen, partial glass dewetting occurred, and voids were systematically observed at multigrain junctions. The actual presence of nitrogen could only be detected in a limited number of wetted interfaces. A common feature in the internal friction behavior of the preoxidized, SiO₂-added and nitrogen-annealed SiC was a relaxation peak that resulted from grain-boundary sliding. Frequency-shift analysis revealed markedly different characteristics for this peak: both the magnitude of the intergranular glass viscosity and the activation energy for grain-boundary viscous flow were much higher in the nitrogen-annealed material. Results of torsional creep tests were consistent with these findings, with nitrogen-annealed SiC being the most creep resistant among the tested materials.     

Exuding Liquid from Grain Boundaries in Alumina

Bicrystals of alumina with anorthite glass in the boundary were prepared by hot pressing. Annealing of the bicrystals leads to the migration of the intergranular liquid to the free surface of the sample. It is proposed that the migration is driven by the difference in the wetting behavior of the free surface and the boundary.     

Kinked Grain Boundaries in Alumina Doped with Y2O3

Polycrystalline alumina specimens with and without MgO doping show smoothly curved grain boundaries after heat treatment at 1400°C indicating their rough structure. When heat-treated at 1400° and 1500°C for 24 h after packing in an alumina–YAG powder mixture, many grain boundaries (without any liquid phase) develop kinks of large and small scales as observed by scanning electron microscopy and transmission electron microscopy. The addition of Y₂O₃ at concentrations close to the solubility limit is thus shown to induce the grain boundary transition to singular structures.     

Dihedral Angles in Magnesia and Alumina: Distributions from Surface Thermal Grooves

Dihedral angles, Ψ_s, from surface thermal grooves were measured using a metal reference line technique for polycrystalline MgO, undoped Al₂O₃, and MgO-doped Al₂O₃. The values of Ψ_s span the following ranges: 89° to 116° for MgO at 1520 K, 76° to 166° for undoped Al₂O₃ at 1870 K, and 90° to 139° for MgO-doped Al₂O₃ at 1870 K. For all three systems, the median Ψ_s values are 105° to 113°, implying that the median γ_gb/γ_s is 1.1 to 1.2, in contrast to metals where γ_gb/γ_s ranges typically from 1/4 to 1/2. The widths in Ψ_s distributions were different for the three materials with the width increasing in the following order: MgO, MgO-doped Al₂O₃, undoped Al₂O₃. For all three materials, the grainboundary grooves and their corresponding Ψ_s were not symmetrical with respect to the surface normal. The asymmetry for MgO was due to the pinning of the grain boundaries by the surface thermal grooves. The range of inclination angles of the grain boundary to the surface was a function of Ψ_s, with the maximum inclination angles of ∼13°, in quantitative agreement with theory.     

Scavenging of Siliceous Grain-Boundary Phase of 8-mol%-Ytterbia-Stabilized Zirconia without Additive

The grain-boundary conductivity (ς_gb) of 8-mol%-ytterbia-stabilized zirconia increased markedly with heat treatment between 1000° and 1300°C with a slow heating rate (0.1°C/min) before sintering. The extent of the ς_gb improvement was the same or larger than that via Al₂O₃ addition. The heat treatment did not affect the grain-interior conduction when sintered at 1600°C, while Al₂O₃-derived scavenging significantly did, given the larger increment of total conductivity in the heat-treated sample. The formation of a silicon-containing phase in a discrete form was suggested as a possible route of scavenging the resistive phase from the correlation between average grain size and ς_gb.     

Effect of Strong Anisotropy in Grain-Boundary Energy on Boundary Mobility in Abnormally Grown Grains

The growth of a large crystal into a fine-grained polycrystal of the same phase is considered through a two-dimensional model. The macroscopic driving force for growth depends only on the average energy of the grain boundaries of the polycrystalline matrix. The analysis reveals a dependence of the local driving force and effective mobility on the orientation of the interface between the large crystal and the matrix. The orientations with very low energy relative to the neighboring orientations also have low effective mobility. Applications are found in modeling of abnormal grain growth in ceramics and in experiments designed to measure the mobility of grain boundaries.     

Corrosion Behavior of Alumina Ceramics in Caustic Alkaline Solutions at High Temperatures

The corrosion rate and changes in the microstructure and fracture strength of alumina ceramics (93.0% Al₂O₃ and 99.5% Al₂O₃) were studied in 0.1 m to 25 m NaOH solutions at 150°C to 200°C, where m = mol/(kg of H₂O). The attack of the caustic alkaline solution started at the grain boundaries. Consequently, the corrosion resistance increased with decreasing SiO₂ content in Al₂O₃ ceramics, and the corrosion resistance of 99.5% pure Al₂O₃ was similar to that of Si₃N₄ ceramics. Since large pits are formed by corrosion, the surface area increased first and the apparent corrosion rate increased with time in the initial stage of the corrosion. The corrosion rate of Al₂O₃ increased linearly with increasing NaOH concentration, and the activation energy was 102 kJ/mol. The fracture strength of corroded Al₂O₃ decreased monotonically as the degree of dissolution of alumina increased.     

Equivalent Circuit Model in Grain-Boundary Barrier Layer Capacitors

Electrical properties of BaTiO₃-based capacitors are investigated. A new model is developed to explain the frequency response of the impedance of grain-boundary barrier layer (GBBL) capacitors. This model takes into consideration the dipole polarization effect and provides a simple and effective approach to evaluate the performance of GBBL capacitors with various dopants and sintering in different atmospheres. When sintered in a reducing atmosphere, doped BaTiO₃ exhibits a higher dielectric constant and a relatively stable dieletric constant with respect to the frequency response and temperature dependence. Also, smaller grain resistivity is obtained with addition of both Dy₂O₃ and Nb₂O₅.     

Effect of Electric Field on the Migration of Grain Boundaries in Alumina

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.     

Segregation Effects at Grain Boundaries in Fluorite-Structured Ceramics

The atomic-scale structure, composition, and chemistry of grain boundaries in two fluorite-structured ceramic materials were characterized by a combination of Z -contrast imaging and electron energy-loss spectroscopy (EELS). In the case of a symmetric 24° [001] tilt bicrystal of yttria-stabilized-zirconia (YSZ), a shift in the zirconium M -edge onset and a change in the yttrium and zirconium M -edge ratios at the boundary indicate an increase in the number of electrons in the boundary plane. A detailed study of the structure and composition indicates that this is caused by an increase in the number of oxygen vacancies in the grain boundary core that is partially compensated by yttrium segregation. Studies of grain boundaries in an industrial Gd-doped ceria ceramic reveals similar changes in vacancy/dopant profiles indicating that these effects may be generic to grain boundaries in fluorite-structured materials.