Effect of Hf 4+ Concentration on Oxygen Grain‐Boundary Diffusion in Alumina

The kinetics of oxygen grain‐boundary diffusion in alumina was studied as a function of HfO₂ concentration. The oxidation of Ni marker particles to NiAl₂O₄ spinel was utilized to delineate the position of the oxidation front. The HfO₂ doping levels spanned the solubility limit, ranging from 100 to 2000 ppm (Hf:Al concentration). For each dopant level, the parabolic rate constant (k) was determined for oxidation anneals carried out at 1400°C. Relative to undoped alumina, HfO₂ doping slowed the oxygen transport kinetics by a factor of approximately 3–8, depending on the dopant concentration. At 1400°C, the solubility limit of HfO₂ was found to be between 100 and 200 ppm. The results showed that the level of benefit saturated at the dopant level corresponding to the solubility limit (sol_lim), where 100 ppm < sol_lim < 200 ppm. The results of the transport experiments were also examined with respect to the fractional grain‐boundary coverage (f), as opposed to overall HfO₂ content. An approximate linear relationship between the rate of oxygen transport and f was observed, which can be rationalized in terms of a site‐blocking model.     

Influence of Grain and Grain‐Boundary Resistances on Dielectric Properties of KNbO3 Under Small DC Bias Field

Lead‐free piezoelectric potassium niobate (KNbO₃) system was synthesized by conventional solid‐state ceramic route. Rietveld analysis of X‐ray diffraction data of this system revealed that the sample crystallized in pure orthorhombic perovskite phase at room temperature. SEM micrograph of this system depicted presence of grains having diffuse brick structure with an average grain size of 500 nm. Dielectric properties of KNbO₃ ceramic were investigated under different DC bias voltage in a broad frequency (from 20 Hz to 1 MHz) and temperature (from 200°C to 500°C) ranges in its three crystalline phases. The dielectric constant was found to increase with increasing bias field in all three phases. The loss tangent of this system was found to increase first, and then it becomes constant with increasing bias field. These properties have been explained in terms of variation of grain and grain‐boundary resistances with bias field.     

Toward Knowledge‐Based Grain‐Boundary Engineering of Transparent Alumina Combining Advanced TEM and Atomistic Modeling

Transition element dopants (e.g., Y, La) are commonly used as sintering aids in polycrystalline alumina ceramics, which segregate to the grain boundaries and control the grain‐boundary mobility. However, due to the extremely thin (<2 nm) layer of segregated dopants, the experimental characterization of the segregated alumina grain boundaries is a complex task. Computational studies have focused only on tilt grain boundaries, which are only a small fraction in a sintered alumina sample. In this study, a quantitative characterization of the segregation of Y and La at general high angle grain boundaries in transparent alumina is carried out using a unique combination of advanced TEM and near coincidence grain‐boundary atomistic simulations. The result show that high angle grain boundaries may lead to enhanced grain growth in comparison to symmetric tilt twin grain boundaries due to the reduced configuration entropy for dopant segregation and higher order grain‐boundary complexions. On the other hand, multidoping with different dopants was shown to be more beneficial than single doping due to its contribution in increasing the configurational entropy for segregation. The advanced TEM analysis showed Y and La distributions and concentrations on a series of general grain boundaries in very good agreement with the atomistic simulations. This validation of atomistic modeling technique used in this study means, as it a generic method, can be used as a predictive tool to design ceramic microstructure and properties.     

Grain‐Boundary and Thermally Stimulated Current Characteristics of Y2O3‐Doped ZnO Varistor

The doping of rare‐earth oxides can greatly improve the electrical characteristics of ZnO varistors. Thermally stimulated current (TSC) characteristic test, capacitance voltage (C–V) characteristic test, scanning electron microscope (SEM) test, and voltage current (V–I) test were carried out to study the influence of Y₂O₃ content on the electrical properties of ZnO varistors in this study. The results show that the grain size decreases while the voltage gradient increases as the Y₂O₃ content is increased. The reaction of Y₂O₃ with other additives leads to the decrease in grain‐boundary defects, which accounts for the decrement of barrier height, donor density, and surface state density. The trap level and trapped charge of ZnO varistors decrease as the Y₂O₃ content is increased from 0.3 to 0.9 mol%, which means the shallow traps inside ZnO varistors reduce, and the Y₂O₃ additive can greatly improve the TSC characteristic of ZnO varistors.     

Fabrication of Transparent Grain‐Oriented Polycrystalline Alumina by Colloidal Processing

Grain‐oriented polycrystalline alumina (PCA) ceramics with very high, nearly single‐crystal transparency were successfully fabricated through colloidal processing in a high magnetic field followed by hot isostatic pressing. The c‐axis of the produced material was oriented along the direction parallel to the magnetic field. The transmittance of the specimen with a thickness of 0.8 mm was 78% at a wavelength of 650 nm, close to the reported value of 84.5% for sapphire and more than 10% higher than the largest transmittance magnitude ever reported for PCA ceramics. In addition, the Na salt of 4,5‐dihydroxy‐1,3‐benzenedisulfonic acid (named “Tiron”) was found to be a highly effective dispersant for processing high‐purity materials.     

Sintering and Grain Growth of Alumina

The sintering and grain growth of alumina as a function of the type of impurity (oxide) added was studied. One weight per cent of the impurity was added to a commercial Be-grained alumina or was coprecipitated with iilcl₃ to form hydroxides. Fired shrinkage, bulk density, and apparent porosity measurements were used to correlate sintering with grain growth as observed by the petrographic and electron microscope. Some additives which increased grain growth were believed to enter into solid solution with alumina and to strain the lattice sufficiently to increase material transport greatly. Other additives were believed to produce a glassy phase which would greatly increase surface difusion and resulting grain growth. Some additives decreased grain growth because of the relatively large vapor phase produced by the impurity upon heating which may have been sorbed on the alumina; other additives may have Wed anion vacancies to reduce material transport or may have produced complex anions whose flow or diffusion may have been impeded.     

The Influence of Cation Additives on Grain‐Boundary Mobility in Yttrium Aluminum Garnet (YAG)

Pure yttrium aluminum garnet (YAG) nano‐powders, doped with 1.4 at.% of La₂O₃, ZrO₂, MgO, Nb₂O₅, and SiO_2, were vacuum sintered to full density and subjected to grain growth at 1700°C, for up to 15 h. The YAG intrinsic grain‐boundary (GB) mobility, determined from the pure fully dense YAG specimens, was 2.9 × 10^?16 [m³·N^?1·s^?1]. All dopants, except for La₂O₃, increased the GB mobility compared to pure YAG (La₂O₃ didn't cause any significant change in YAG's GB mobility). All GBs were found to be free of secondary phases or intergranular films. These findings differ from numerous publications where dopants were found to either inhibit grain growth by solute drag, or to enhance grain growth due to liquid phase sintering. It was found that the GB mobility systematically increased with the decrease in the dopant cation radius. Moreover, it seems that vacancy population plays an important role in determining the GB mobility of YAG.     

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.     

The Lowered Dielectric Loss and Grain‐Boundary Effects in La‐doped Y2/3Cu3Ti4O12 Ceramics

The effects of La concentration on the electrical conductivity and electric modulus of Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics (0.00 ≦ x ≦ 0.20) were investigated in detail. Proper amount of La substitution in Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics made the dielectric loss decreased. When x = 0.10, Y_2/3?0.10La_0.10Cu₃Ti₄O₁₂ ceramics exhibited the highest grain‐boundary resistance (0.893 MΩ) and the lowest dielectric loss (about 0.025 at 1 kHz), meanwhile the samples exhibited a relatively high dielectric constant above 6000 over a wide frequency range from 40 Hz to 1 MHz. The decreased dielectric loss was attributed to the enhanced grain‐boundary resistance. With the increase in La concentration, the dielectric relaxation behaviors correlated with the grain‐boundary effects were significantly enhanced. By La doping, the activation energies for the conduction in grain boundaries were slightly depressed, and the activation energies for the relaxation process in grain boundaries were slightly changed. Based on the activation values, it can be concluded that the doubly ionized oxygen vacancies had substantial contribution to the conduction and relaxation behaviors in grain boundaries.     

Deformation and Grain Growth of Low-Temperature-Sintered High-Purity Alumina

Through close control over green-state powder processing, pure alumina ceramics of 0.5-μm grain size were obtained by sintering at 1250°C. The static grain growth of this material was modest at temperatures below 1300°C. However, dynamic grain growth occurred rapidly during superplastic deformation. Therefore, although the ultrafine-grained alumina exhibited rather low initial flow stress at relatively low deformation temperatures, dynamic grain-growth-induced strain hardening gave rise to high flow stress causing cavitation and cracking. As a result, superplastic deformation could not be achieved for the ultrafine-grained pure alumina.     

Densification and Grain Growth of Porous Alumina Compacts

Densification and grain growth of porous alumina compacts during various high-temperature processes were investigated. Experimental data were obtained for densification and grain growth of alumina powder during hot pressing. A set of constitutive equations was proposed based on the constitutive equations by Helle et al. ¹ for hydrostatic response and by Rahaman et al. ² for deviatoric response. Theoretical results from the proposed constitutive equations were compared with various experimental data for alumina powder compacts in the literature, including pressureless sintering, sinter forging, and hot pressing. The proposed model well predicts the densification and grain growth of alumina compacts.     

Densification and Grain Growth of Fe-Doped and Fe/Y Codoped Alumina: Effect of Fe Valency

The effect of iron and iron/yttrium codoping on the densification and grain growth of ultra high-purity (99.995%) fine-grained alumina has been studied. The experiments were carried out under both oxidizing (flowing air) and reducing conditions (N₂/H₂ mixture, p O₂∼5.1 × 10⁻¹⁴). For studies carried out in air, relative to undoped alumina, the addition of 1000 ppm Fe was found to reduce the densification rate by a factor of 5 and also retard the grain growth rate. This result, which was consistent with tensile creep data obtained in a separate study, was attributed to the retardation of grain-boundary diffusive processes by segregating Fe(III) ions. In contrast, under reducing conditions the 1000 ppm Fe- doped samples exhibited an increase in the densification rate of 2.5 orders of magnitude over that of the undoped samples. In the case of the codoped compositions (1000 ppm Fe/1000 ppm Y), for heat treatment in air, the densification behavior did not differ significantly from that of samples singly doped with Y (1000 ppm). However, under reducing conditions, the presence of the Fe²⁺ in the samples appeared to compensate for the retarding effect of the yttrium, such that the densification rate of the codoped samples was comparable with that of the undoped material. A mechanism based on compensating point defects is invoked to rationalize the more rapid kinetics under reducing conditions.     

Surface Diffusion of Oxygen Transport Membrane Materials Studied by Grain‐Boundary Grooving

Mass transport mechanism responsible for grain‐boundary grooving during thermal annealing of polished ceramics of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) and La₂NiO_4+δ (LN) was revealed by atomic force microscopy. Surface diffusion mechanism was confirmed for both materials by the evolution of the grain‐boundary width (w) with annealing time (t), and the surface diffusion coefficient was determined from the slope of w versus t^1/4 following the theory by Mullins. An Arrhenius temperature dependence of the surface diffusion was observed, and the activation energy was determined to be 220 ± 30 and 450 ± 30 kJ/mol, respectively, for BSCF and LN. The surface diffusion data are discussed with respect to similar data for other oxide materials and cation and oxygen anion diffusion in BSCF and LN. Finally, the dihedral angle for both LN and BSCF was determined, and these are typical in the range reported for other oxide materials.     

Influence of Grain Size on the Indentation-Fatigue Behavior of Alumina

The surface fracture behavior of a high-purity, high-density alumina, as a function of grain size (3, 5, and 9 μm), was investigated using an indentation-fatigue technique. Increasing the grain size reduced the threshold for crack nucleation, reduced the resistance to surface spalling, and increased the volume of materials lost per spalling event. These results are explained in terms of residual stresses and fatigue damage.     

Influence of Grain Size on the Grinding Response of Alumina

The effect of grain size on the grinding response, i. e., grinding forces, surface roughness, and grinding-induced subsurface damage, is investigated in a series of alumina ceramics with the average grain size ranging from 3 to 35 μm. The grinding forces are measured as a function of depth of cut in surface grinding. It is found that the grinding forces decrease as the grain size is increased from 3 to 9 μm. But at larger grain sizes, the grinding forces are independent of the grain size. Subsurface damage in grinding is observed using a bonded-interface sectioning technique. The subsurface damage is found to consist of intragrain twin/slip bands and intergranular microcracks. The density of grinding-induced subsurface microcracks increases with the grain size. In addition to using optical microscopy on the sections of the ground specimens, a nondestructive thermal wave measurement technique is used directly on the ground surfaces for the detection of grinding-induced subsurface microcracks. The grain size dependence of the microcrack density estimated from the thermal images is found to agree with the results obtained using the bonded-interface technique.     

Effect of Liquid Content on the Abnormal Grain Growth of Alumina

Alumina specimens with small amounts of CaO and TiO₂ were prepared and their microstructural evolution during sintering was investigated. Because of the appearance of a liquid phase during sintering, a duplex microstructure of a few abnormal grains and fine matrix grains was obtained when the CaO + TiO₂ content was small (≤0.04 wt%). When the CaO + TiO₂ content was relatively high (≥0.1 wt%), many grains grew and impinged upon each other. As a result, a rather uniform and homogeneous microstructure was observed.     

Hardening in Creep of Alumina by Zirconium Segregation at the Grain Boundary

The effect of zirconium segregation on hardening in the creep of fine-grained alumina was studied by using the tensile creep test. To avoid the effect of zirconia particle dispersion on creep, 100-ppm-zirconium-doped alumina and 1000-ppm-zirconium-doped alumina were fabricated by using a zirconium-containing precursor. The scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy study revealed that the zirconium was segregated at the alumina grain boundary. Doping even as little as 100 ppm of zirconium caused the hardening effect. The creep rate was further reduced by increasing the amount of zirconium dopant. Although the stress exponent of 2 was not affected by zirconium segregation, the apparent activation energy of the creep was found to be increased, from 520 kJ/mol for undoped alumina to 670 kJ/mol for 100-ppm-zirconium-doped alumina and 760 kJ/mol for 1000-ppm-zirconium-doped alumina. It was suggested that grain-boundary sliding was accommodated by impurity-drag-controlled diffusional creep.     

Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO₂ coated CMCs at high impact speed showed localized damage without any shattering.     

Fabrication of Highly Textured Fine‐Grained α‐Alumina by Templated Grain Growth of Nanoscale Precursors

[0001] textured alumina ceramics with a fine grain size were fabricated between 1400°C and 1600°C via templated grain growth (TGG) using fine alumina platelets (~0.6 and ~3 μm diameter) aligned by tape casting in either a 50 nm α‐Al₂O₃ matrix powder, or in a seeded boehmite sol. The 3 μm templates could be readily aligned by tape casting in both matrices (orientation parameters r = 0.27 and 0.18, respectively), whereas 0.6 μm diameter templates were well aligned in the seeded boehmite sol only (r = 0.29). Improved alignment in boehmite sols is attributed to inorganic gelation, resulting in a strongly pseudo‐plastic rheology that preserves template alignment against the influence of Brownian motion. The in situ formation of fine α‐Al₂O₃ matrix after transformation in the seeded boehmite system results in a higher driving force for TGG and improves texture development. The combination of 3 μm templates with a seeded boehmite matrix results in extremely high texture qualities (texture fraction f = 0.97–0.99, r = 0.17) while maintaining a relatively fine grain size (5–10 μm in diameter and 1.5–3 μm in thickness). Although undoped samples can be fully textured at 1600°C, adding as little as ~0.25 wt% CaO/SiO₂ dopant improves TGG kinetics and yields full texture at 1400°C.     

Effect of a Liquid Phase on the Morphology of Grain Growth in Alumina

In this investigation we have studied how the presence of a liquid phase affects the grain morphology and grain growth kinetics in Al₂O₃ at 1800°C using the growth of both matrix grains and large spherical single-crystal seeds growing into the matrix. The growth rates of the matrix grains were found to decrease in the following order: undoped Al₂O₃, AI₂O₃ with anorthite, AI₂O₃ with anorthite and MgO, and Al₂O₃ with MgO. Except for the samples doped with MgO alone, the matrix grains were faceted and appeared tabular in polished sections. In samples containing anorthite both with and without MgO, the single-crystal seeds exhibit basal facets with continuous liquid films and slow growth in the 〈0001〉 relative to all other crystallographic directions. When only MgO is added, the growth of the single-crystal seeds was not isotropic; however, no faceting was observed. We discuss how anisotropic growth rates caused by the anorthite additions can stimulate discontinuous grain growth in Al₂O₃.