Effect of Titania and Lithia Doping on the Boundary Migration of Alumina under an Electric Field

Boundary migration under an electric field was investigated for pure, TiO₂-doped, and Li₂O-doped Al₂O₃ specimens. Boundary migration rates in TiO₂-doped and Li₂O-doped Al₂O₃ specimens were much faster compared with that of pure Al₂O₃. In all specimens, the migration rate was observed to depend on the applied bias direction. Compared with pure Al₂O₃, the dependence of boundary migration on bias direction became more pronounced in TiO₂-doped Al₂O₃ but less pronounced in Li₂O-doped Al₂O₃. The results were explained in terms of the variation of grain sizes, mobility, and electrostatic potential of boundaries because of doping.     

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.     

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.     

Effect of Rare-Earth Dopants on Mechanical Properties of Alumina

We report here about the effect of rare-earth dopants on the improvement of room-temperature mechanical properties of alumina. Rare-earth ions (RE = Yb³⁺, Er³⁺, and La³⁺) of different ionic radii in a minimum concentration of 1000 ppm were added as dopants individually to high-purity alumina and densified by pressureless sintering. High strength of about 700 MPa was attained for Yb-doped alumina sintered at 1400°C. And, high toughness of about 7.0 MPa·m^1/2 was attained for Er- and La-doped alumina samples.     

Effect of Nd2O3 Doping on the Densification and Abnormal Grain Growth Behavior of High-Purity Alumina

The densification behavior and microstructural development of high-purity Al₂O₃ doped with different levels of Nd₂O₃ were investigated. Dopant levels ranged from 100–1000 ppm (Nd/Al atomic ratio). The densification behavior of the doped powders was studied using constant heating rate dilatometry. It was found that neodymium additions inhibited densification, with a corresponding increase in the apparent activation energy. The level of grain-boundary segregation was studied using high-resolution analytical electron microscopy. At dilute concentrations, the degree of neodymium grain-boundary excess was found to be consistent with a simple geometrical model relating this quantity to the overall dopant concentration and average grain size. For certain combinations of dopant level and heat treatment, supersaturation of the grain boundaries was observed, which was found to correlate with the onset of abnormal grain growth. Possible explanations for this behavior are discussed.     

Effect of External Electric Field on the Grain-Growth Behavior of Barium Titanate

The effect of an external electric field on the grain-growth behavior of acceptor Mg-doped, undoped, and donor Nb-doped BaTiO₃ ceramics was investigated. The acceptor-doped and undoped specimens showed enhanced grain growth at the positive-biased region. On the other hand, for the highly donor-doped specimens, grain growth was enhanced in the negative-biased region. The results have been explained in terms of defect polarization and the consequent change in the boundary potential. It has been suggested that liquid penetration into grain boundaries is critically dependent on the boundary potential.     

Influence of codoping with Hf and La on grain-boundary transport in alumina

The role of reactive elements (RE) is an important topic in understanding the oxidation behavior of high-temperature alloys. In this work, the influence of codoping alumina with two different RE elements (500 ppm Hf + 500 ppm La) was studied. The kinetics of oxygen grain-boundary (GB) transport were studied at 1400°C using metallic nickel particles as markers. The results were compared with data obtained on the corresponding singly doped compositions; alumina-500 ppm La, and alumina-500 ppm Hf. The results showed that singly doping with La did not have any benefit compared to undoped alumina, whereas singly doping with Hf resulted in a slowing of transport by a factor of ~7. The behavior of the codoped sample was very similar to that of the singly doped Hf composition. For all the studied compositions, atomic scale characterization using high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography (APT) revealed strong segregation of the dopant ions to the alumina grain boundaries. In the codoped sample, APT revealed evidence of oxygen excess and aluminum depletion at the GB core.     

Cation grain‐boundary diffusivity in SiO2‐ and MgO‐doped Al2O3

Cation grain‐boundary diffusion in undoped and aliovalent‐doped Al₂O₃ is characterized using Cr₂O₃ as a chemical tracer. The compositional depth profiles measured by secondary ion mass spectrometry are fitted to the Whipple‐LeClaire model. The results indicate that cation grain‐boundary diffusivity is insensitive to MgO and SiO₂ dopants between 1100°C and 1300°C.     

Monazite Coatings on Fibers: I, Effect of Temperature and Alumina Doping on Coated-Fiber Tensile Strength

Monazite was continuously coated onto Nextel 720 fibers, using an aqueous precursor and in-line heat treatment at 900°–1300°C. Some experiments were repeated with alumina-doped precursors. Coated fibers were heat-treated for 100 h at 1200°C. Coatings were characterized by optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. Coated-fiber tensile strengths were measured by single-filament tensile tests. The precursors were characterized by X-ray diffractometry, differential thermal analysis/thermogravimetric analysis, and mass spectrometry. Coated-fiber tensile strength was lower for fibers coated at higher deposition temperatures. Heat treatment for 100 h at 1200°C decreased tensile strength further. The coatings were slightly phosphate-rich and enhanced alumina grain growth at the fiber surface, but phosphorus was not detected along the alumina grain boundaries. Fibers with alumina-doped coatings had higher tensile strengths than those with undoped coatings after heat treatment for 100 h at 1200°C. Alumina added as α-alumina particles gave higher strengths than alumina added as colloidal boehmite. Alumina doping slowed monazite grain growth and formed rough fiber–coating interfaces after 100 h of heat treatment at 1200°C. Possible relationships among precursor characteristics, coating and fiber microstructure development, and strength-degradation mechanisms are discussed in this paper.     

Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

High-purity Al₂O₃ compacts were doped with 0–350 ppm (by weight) of MgO using a liquid immersion technique and equilibrated at temperatures between 1700° and 2000°C under hydrogen. The solubility limits of MgO in Al₂O₃ at temperatures of 1720° and 1880°C were very low, ∼75 and 175 ppm, respectively. Variation of MgO solubility with temperature could be represented by the equation, ln Mg/Al = 3.80–2.63 × 10⁴/ T . The small MgO solubilities were understood by the high enthalpy (326 kJ/mol) of solution. The results of this study suggested that previous investigations on sintering and grain-growth mechanisms in MgO-doped Al₂O₃ were probably not done in single-phase Al₂O₃ solid solutions. However, the conclusions on sintering and grain-growth mechanisms in prior research work in MgO-doped A₂O₃ may be correct. The effects of SiO₂ impurity and grain size on MgO solubility are discussed. Previous grain-growth experiments in MgO-doped Al₂O₃ are described that demonstrate the clearest evidence for grain-boundary mobility controlled by a solid-solution mechanism.     

Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The grain-growth behavior of Al₂O₃ compacts with small contents (≤10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO₂, or CaO + TiO₂. The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na₂O + SiO₂, CaO + SiO₂, SrO + SiO₂, or BaO + SiO₂. However, an addition of MgO to the Al₂O₃ doped with CaO + SiO₂ resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO₂ doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.     

Grain Boundary Sliding Microdisplacement Measurements During the Creep of Alumina

Grain boundary sliding (GBS) is thought to be the principal driving force for the nucleation, growth, and coalescence of grain boundary cavities during compressive creep of polycrystalline ceramics. It has been shown theoretically that stochastic GBS gives rise to continuous cavity nucleation and transient cavity growth and coalescence, eventually leading to crack formation and failure. This paper will show through experimental measurements, using stereoimaging techniques, that GBS is in fact stochastic. Also, mode II GBS, in-plane grain rotation, and in-grain shear displacements, strains, and strain rate measurements during creep of Lucalox Al₂O₃ will be presented. These displacements, measured on a machine vision system, will be presented in terms of the surrounding microstructural constraint and their lack of angular relation to the compressive load axis.     

Nanocrystalline Zirconia Surface-Doped with Alumina: Chemical Vapor Synthesis, Characterization, and Properties

Nanocrystalline zirconia with a grain size of ∼5 nm was surface-doped with 3 and 30 mol% alumina by chemical vapor synthesis using two sequential hot-wall reactors. The powders were characterized by high-resolution transmission electron microscopy, X-ray diffractometry (XRD), and nitrogen adsorption. Aqueous dispersions were studied by zeta-potential measurements and photocorrelation spectroscopy. Green and sintered ceramic bodies were investigated by high-resolution scanning electron microscopy, XRD, and nitrogen adsorption. Zirconia surface-doped with 3 mol% of alumina displays substantial changes in dispersability and sinterability compared with pure nanocrystalline zirconia.     

Microstructure of 96% Alumina Ceramics: II, Crystallization of High-Magnesia Boundary Glasses

Development of the grain-boundary microstructure with heat treatment at 800° to 1500°C was examined for a group of 96% Al₂O₃ ceramics containing a high-MgO boundary phase. Using a combination of analytical and conventional electron microscopy techniques, eight different crystalline phases were detected at the boundaries following annealing. Despite the extensive devitrification, however, a residual glass remained in all samples examined, and is believed to be continuous.     

Effects of Zirconium Doping on Grain-Boundary Bonding in Alumina-Silicon Carbide Composites

In this work, 800 ppm of Zr⁴⁺ dopants were added to Al₂O₃-5 vol% SiC particle composite. Zr⁴⁺ doping led to a weak Al₂O₃ grain-boundary bonding so that the fracture mode changed from transgranular in undoped composite to intergranular in Zr⁴⁺-doped composite. The fracture mode change increased the fracture toughness of the composite. Transmission electron microscopy and energy-dispersive spectroscopy examinations revealed that the weak grain-boundary bonding in the doped composite was caused by the segregation of Zr⁴⁺ and Si⁴⁺ ions at the Al₂O₃ grain boundary.     

Characterization and Tribological Investigation of Sol–Gel Titania and Doped Titania Thin Films

Titania (TiO₂) and doped TiO₂ ceramic thin films were prepared on a glass substrate by a sol–gel and dip-coating process from specially formulated sols, followed by annealing at 460°C. The morphologies of the original and worn surfaces of the films were analyzed with atomic force microscopy (AFM) and scanning electron microscopy. The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of TiO₂ and doped TiO₂ thin films sliding against Si₃N₄ ball were evaluated on a one-way reciprocating friction and wear tester. The AFM analysis shows that the morphologies of the resulting films are very different in nanoscale, which partly accounts for their tribological properties. XPS analysis reveals that the doped elements exist in different states, such as oxide and silicate, and diffusion took place between the film and the glass substrate. TiO₂ films show an excellent ability to reduce friction and resist wear. A friction coefficient as low as 0.18 and a wear life of 2280 sliding passes at 3 N were recorded. Unfortunately, all the doped TiO₂ films are inferior to the TiO₂ films in friction reduction and wear resistance, primarily because of their differences in structures and chemical compositions caused by the doped elements. The wear of the glass is characteristic of brittle fracture and severe abrasion. The wear of the TiO₂ thin film is characteristic of plastic deformation with slight abrasive and fatigue wear. The doped TiO₂ thin films show lower plasticity than the TiO₂ thin film, which leads to large cracks. The propagation of the cracks caused serious fracture and failure of the films.     

Influence of Dopant Concentration on Creep Properties of Nd2O3-Doped Alumina

The microstructural features and tensile creep behavior of Al₂O₃ doped with Nd₂O₃ at levels ranging from 100 to 1000 ppm (Nd:Al atomic ratio) were systematically investigated. Compositional mapping, using both high-resolution scanning transmission electron microscopy and secondary ion mass spectroscopy revealed that, for all of the compositions studied, the Nd³⁺ ions were strongly segregated to the Al₂O₃ grain boundaries. Microstructural observations revealed that the solubility of Nd₂O₃ was between 100 and 350 ppm. Tensile creep tests were conducted over a range of temperatures (1200°–1350°C) and stresses (20–75 MPa). Both the stress and grain-size exponents were analyzed. In selected experiments, controlled grain-growth anneals were used to enable creep testing of samples of the same average grain size but different neodymium concentrations. Independent of dopant level, the neodymium additions decreased the creep rate by 2–3 orders of magnitude, compared with that of undoped Al₂O₃. The value of the apparent creep activation energy increased with increased dopant concentration and then saturated at dopant levels exceeding the solubility limit. Overall, the results of the present study were consistent with a creep-inhibition mechanism whereby oversized segregant ions reduce grain-boundary diffusivity by a site-blocking mechanism.     

Effect of Silica Surface Dopants on the Formation of Alumina/Aluminum Composites by the Directed Metal Oxidation of an Aluminum Alloy

This study focused on clarifying the effect of SiO₂ surface dopants on the formation of Al₂O₃/aluminum composites, especially on oxidation phenomena during the incubation period. The present results showed that a surface dopant decreased the incubation period of an Al-Mg-Si alloy, as well as that of an Al-Mg alloy, and that addition of an external surface dopant decreased the incubation period more effectively than did an internal alloying of silicon. A two-step oxidation process was also conducted. In the first step of the process, an aluminum alloy was oxidized without a surface dopant and cooled to room temperature during the incubation stage. In the second step, the same specimen was surface-doped with SiO₂ powder and reoxidized. The incubation time for the specimen subjected to the two-step oxidation process was the same as that for the single-step specimen oxidized with a surface dopant. The substantial decrease in the incubation period, especially for the Al-Mg alloy, is ascribed to interaction between the SiO₂ surface dopant and the MgO layer. This interaction made the MgO layer thinner and increased the number of magnesium vacancies in the MgO layer, thus providing an appropriate microstructure in the MgO layer for bulk-growth initiation.     

Singular Grain Boundaries in Alumina and Their Roughening Transition

The shapes and structures of grain boundaries formed between the basal (0001) surface of large alumina grains and randomly oriented small alumina grains are shown to depend on the additions of SiO₂, CaO, and MgO. If a sapphire crystal is sintered at 1620°C in contact with high-purity alumina powder, the grain boundaries formed between the (0001) sapphire surface and the small alumina grains are curved and do not show any hill-and-valley structure when observed under transmission electron microscopy (TEM). These observations indicate that the grain boundaries are atomically rough. When 100 ppm (by mole) of SiO₂ and 50 ppm of CaO are added, the (0001) surfaces of the single crystal and the elongated abnormal grains form flat grain boundaries with most of the fine matrix grains as observed at all scales including high-resolution TEM. These grain boundaries, which maintain their flat shape even at the triple junctions, are possible if and only if they are singular corresponding to cusps in the polar plots of the grain boundary energy as a function of the grain boundary normal. When MgO is added to the specimen containing SiO₂ and CaO, the flat (0001) grain boundaries become curved at all scales of observation, indicating that they are atomically rough. The grain boundaries between small matrix grains also become defaceted and hence atomically rough.     

Conversion of Polycrystalline Alumina to Single-Crystal Sapphire by Localized Codoping with Silica

The effect of a localized SiO₂ codoping on the conversion of polycrystalline, MgO-doped Al₂O₃ tubes to single-crystal sapphire was investigated. Codoping with SiO₂ before sintering intentionally triggered abnormal grain growth, which resulted in the full conversion of tube surfaces to single crystal without adversely affecting densification to a almost pore-free, translucent state. The degree of surface conversion was strongly dependent on experimental variables, which included furnace temperature and codoping amount. Surface-converted tubes had excellent physical properties, which included good thermal cycling resistance and optical properties superior to unconverted, polycrystalline Al₂O₃.