Grain Growth Control in Sb2O3-Doped Zinc Oxide

Microstructure development in Sb₂O₃-doped ZnO was studied to evaluate the influence of inversion boundaries (IBs) on ZnO grain growth. In general, the addition of Sb₂O₃ is believed to inhibit the ZnO grain growth via the formation of spinels and IBs, but we have shown that even the conditions of exaggerated grain growth can be created in this system. We designed an experiment for diffusional doping of ZnO under slightly increased partial pressure of Sb₂O₃. In the high-concentration regime we observed no spinels, and yet the ZnO grains were small and inhibited in growth, while in the low-concentration regime we found huge grains, several times larger than normal ZnO grains, showing an obvious exaggerated growth. By controlling the number of nuclei with IBs we can design coarse-grained microstructures even with Sb₂O₃ doping, which has far-reaching implications in the production of low-voltage varistor devices.     

(Pr, Co, Nb)-Doped SnO2 Varistor Ceramics

The effect on microstructure and electrical properties of (Co, Nb)-doped SnO₂ varistors upon the addition of Pr₂O₃ was investigated by scanning electron microscopy and by determining I – V , ɛ– f , and R – f relations. The threshold electric field of the SnO₂-based varistors increased significantly from 850 to 2280 V/mm, and the relative dielectric constants of the SnO₂-based varistors decreased greatly from 784 to 280 as Pr₂O₃ concentration was increased up to 0.3 mol%. The significant decrease of the SnO₂ grain size, from 4.50 to 1.76 μm with increasing Pr₂O₃ concentration over the range of 0–0.3 mol%, is the origin for the increase in the threshold voltage and decrease of the dielectric constants. The grain size reduction is attributed to the segregation of Pr₂O₃ at grain boundaries hindering the SnO₂ grains from conglomerating into large particles. Varistors were found to have a superhigh threshold voltage and a comparatively large nonlinear coefficient α. For 0.15 mol% Pr₂O₃-doped sample, threshold electric field and nonlinear coefficient α were measured to be 1540 V/mm and 61, and for 0.3 mol% Pr₂O₃-doped sample, V and α were 2150 V/mm and 42, respectively. Superhigh threshold voltage and large nonlinear coefficient α qualify the Pr-doped SnO₂ varistor as an excellent candidate for a high voltage protection system.     

Transmission Electron Microscopy Characterization of Hot-Pressed ZrB2 with MoSi2 Additive

Microstructure of the hot-pressed ZrB₂ with MoSi₂ additive was investigated by transmission electron microscopy (TEM). The effect of MoSi₂ addition on the microstructure of the ceramic was assessed. For the pure ZrB₂, the microstructure consisted of the equiaxed ZrB₂ grains and a few elongated ZrB₂ grains. For the ZrB₂ with MoSi₂ additive, the microstructure consisted almost entirely of equiaxed ZrB₂ grains. A few dislocations were present in the ZrB₂ grains. In addition, high-resolution TEM observations showed that the intergranular amorphous phase was absent at two ZrB₂ grain boundaries in the ZrB₂ with MoSi₂ additive.     

Microstructure of Varistors Prepared with Zinc Oxide Nanoparticles Coated with Bi2O3

Zinc oxide (ZnO) nanoparticles coated with 1–5 wt% Bi₂O₃ were prepared by precipitating a Bi(NO₃)₃ solution onto a ZnO precursor. Transmission electron microscopy showed that a homogeneous Bi₂O₃ layer coated the surface of the ZnO nanoparticles and that the ZnO particle size was ∼30–50 nm. Scanning electron microscopy showed that ZnO grains sintered at 1150°C were homogeneous in size and surrounded by a uniform Bi₂O₃ layer. When the ZnO grains were surrounded fully by Bi₂O₃ liquid phases, further increases in the ZnO grain size were not affected by the Bi₂O₃ content. This predesigned ZnO nanoparticle structure was shown to promote homogeneous ZnO grains with perfect crystal growth.     

Anisotropic Abnormal Grain Growth in TiO2/SiO2-Doped Alumina

The effect of TiO₂/SiO₂ addition on the grain growth of alumina was reinvestigated. TiO₂ promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO₂ and SiO₂ resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

Differences in Wetting Characteristics of Bi2O3 Polymorphs in ZnO Varistor Materials

Detailed analysis of the microstructure of grain boundaries, especially triple-grain and multiple-grain junctions, in ZnO varistor materials has been performed using transmission electron microscopy. Different polymorphs of Bi₂O₃ are shown to exhibit different wetting properties on ZnO interfaces. Recent investigations suggest that the equilibrium configuration consists of crystalline Bi₂O₃ in the triple-grain and multiple-grain junctions and an amorphous bismuth-rich film in the ZnO/ZnO grain boundaries. The present investigation supports this suggestion for δ-Bi₂O₃ and also adds to the microstructural image and wetting properties of α-Bi₂O₃.     

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Grain growth in a high-purity ZnO and for the same ZnO with Bi₂O₃ additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ— G ⁿ₀= K ₀ t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn²⁺ lattice diffusion mechanism. Additions of Bi₂O₃ to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi₂O₃ content. The preexponential term K ₀ was also independent of Bi₂O₃ content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi₂O₃ ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi₂O₃-rich liquid phase.     

Discontinuous Dissolution and Grain-Boundary Migration in Al2O3-Fe2O3 by Oxygen Partial Pressure Change

When sintered 95Al₂O₃-5Fe₂O₃ (wt%) specimens constituting corundum grains and iron aluminate spinel precipitates were annealed under high oxygen partial pressure (P_o2) where only a corundum phase is stable, fast dissolution of particulate spinel precipitates occurred, together with the migration of corundum grain boundaries. Behind the migrating boundaries, a corundum solid solution enriched with Fe₂O₃ formed. Discontinuous dissolution (DD) of particulate spinel precipitates thus occurred by P_o2 increase. In contrast, when 95Al₂O₃-5Fe₂O₃ specimens constituting only corundum grains were annealed under low P_o2 where both corundum and spinel phases are stable, grain boundaries migrated without spinel precipitation, leaving behind a corundum phase depleted of Fe₂O₃, similar to chemically induced grain-boundary migration (CIGM) observed during solute depletion. The volatilization of Fe₂O₃ appeared to cause the boundary migration without precipitation. The observed CIGM and DD would suggest various possibilities of microstructure control in other oxide systems through oxygen partial pressure change.     

Effect of Powder Coating on Stabilizer Distribution in CeO2-Stabilized ZrO2 Ceramics

The phase and microstructure relationship of 12 mol% CeO₂-stabilized ZrO₂ ceramics prepared from coated powder was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersed X-ray spectroscopy (EDS). As compared with the sample prepared with co-precipitated method, which exhibited a similar grain size distribution, the EDS analysis revealed that the powder coating induced a wide distribution of CeO₂ solubility, which decreases monotonically with the increase of grain size. This variation of stabilizer content from grain to grain rendered many large grains in the monoclinic phase. Stronger cerium segregation to grain boundaries was observed between large grains, which often form thin amorphous films there. The inhomogeneous CeO₂ distribution keeps more tetragonal ZrO₂ grains close to the phase boundary to facilitate the transforming toughness. Addition of an Al₂O₃ precursor in coated powders effectively raises the overall CeO₂ stabilizer content in the grains and preserves more transformable tetragonal phase in the microstructure, which further enhanced the fracture toughness. The dependence of CeO₂ solubility on grain size may be explained in a simple coating-controlled diffusion and growth process that deserves further investigation.     

Computer Simulation of Kinetics of Spinodal Decomposition in the Tetragonal TiO2–SnO2 System

Computer simulation was carried out for the kinetics of spinodal decomposition in the tetragonal TiO₂–SnO₂ system on the basis of a nonlinear diffusion equation. A time evolution of the microstructure and the effect of coherent strain on the separated two phases were investigated by Langer's approximate method and the finite difference method. It was shown that the composition fluctuations develop in the first stage of the spinodal decomposition, and the formation of interface and the grain growth appear in the second stage. The local stress field and the local strain field with the coherence of the lattice were calculated. Subsequently the appearance of the interface dislocations in the (100) and (010) planes was demonstrated to occur in the third stage. Physical interpretation was given to the experimental observations for the tetragonal TiO₂–SnO₂ system on the basis of those calculations.     

Subsolidus Phase Relations in the Ga2O3-In2O3-SnO2 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃–SnO₂ system were studied by X-ray diffraction over the temperature range 1250–1400°C. At 1250°C, several phases are stable in the ternary system, including Ga₂O₃( ss ), In₂O₃( ss ), SnO₂, Ga_{3− x} In_{5+ x} Sn₂O₁₆, and several intergrowth phases that can be expressed as Ga_{4−4 x} In_{4 x} Sn _{n −4}O_{2 n −2} where n is an integer. An In₂O₃–SnO₂ phase and Ga₄SnO₈ form at 1375°C but are not stable at 1250°C. GaInO₃ did not form over the temperature range 1000–1400°C.     

Low Breakdown Voltage Varistors by Grain Boundary Diffusion of Molten Bi2O3 in ZnO

Diffusion of molten Bi₂O₃ into the grain boundaries of sintered, alumina-doped (0.23 and 0.7 mol%) ZnO pellets resulted in varistors with breakdown voltages in the 3–5 V range and nonlinearity coefficients of 10–24. The varistors were fabricated by spreading a thin layer of Bi₂O₃ powder on the surface of ZnO pellets and heating the combination to various temperatures (860–1155°C) and different times. The highest nonlinearity coefficients (20–24) and lowest breakdown voltages (3–5 V) were recorded in samples annealed at 860°C for 35 min. Longer annealing times and/or higher temperatures resulted in progressively higher breakdown voltages. Eventually the devices became insulating, which was attributed to the formation of an insulating Bi₂O₃ layer between the grains. Separate wetting experiments have shown that the penetration of Bi₂O₃ into ZnO grain boundaries was a strong function of alumina doping —the penetration rate was decreased by a factor of 5–7 as the ZnO was doped with as little as 0.2 mol% alumina. It is this slowing down of the penetration of the ZnO grain boundaries that is believed to be critical in the development of the low breakdown voltages observed.     

Pore–Boundary Separation Behavior during Sintering of Pure and Bi2O3-Doped ZnO Ceramics

Pore–boundary separation in ZnO and 99.95ZnO·0.05Bi₂O₃ (in mol%) specimens during sintering at 1200°C was investigated. In pure ZnO specimens, pores were attached to the grain boundaries and disappeared during the final stage of sintering. In the Bi₂O₃-doped specimens, on the other hand, many pores were separated from the boundaries and trapped inside the grains. Observation using transmission electron microscopy showed that a thin layer of Bi₂O₃-rich phase existed at the boundaries in the Bi₂O₃-doped specimens. The pore separation in 99.95ZnO·0.05Bi₂O₃ specimens was explained in terms of the dihedral angle change and the high mobility of a liquid film boundary.     

Influence of the Addition of Bi2O3 on the Grain Growth and Magnetic Permeability of MnZn Ferrites

Additions of Bi₂O₃ were used to promote grain growth and to increase magnetic permeability during sintering of MnZn ferrites. The results showed that small additions of Bi₂O₃ of <0.05 wt% remarkably increase the permeability of MnZn ferrites. On the other hand, addition of 0.05 wt% Bi₂O₃ induced the formation of a microstructure composed of giant grains with trapped pores embedded in a normal microstructure. The permeability of these samples showed a pronounced secondary maximum in permeability. At still higher Bi₂O₃ concentrations, above 0.2 wt%, the grain growth was retarded and a normal microstructure appeared; however, the magnetic permeability was strongly reduced.     

Grain Growth Control and Dopant Distribution in ZnO-Doped BaTiO3

ZnO additions to BaTiO₃ have been studied in order to determine the role of this dopant on sintering and microstructure development. As a consequence of a better initial dopant distribution, samples doped with 0.1 wt% zinc stearate show homogeneous fine-grained microstructure, while a doping level of 0.5 wt% solid ZnO is necessary to reach the same effect. When solid ZnO is used as the dopant precursor, ZnO is redistributed among the BaTiO₃ particles during heating. Since no liquid formation has been detected for temperatures below 1400°C in the system BaTiO₃-ZnO, it is proposed that dopant redistribution takes place by vapor-phase transport and grain boundary diffusion. Shrinkage and porosimetry measurements have shown that grain growth is inhibited during the first step of sintering for the doped samples. STEM-EDX analysis revealed that solid solubility of ZnO into the BaTiO₃ lattice is very low, being strongly segregated at the grain boundaries. Grain growth control is attributed to a decrease in grain boundary mobility due to solute drag. Because of its effectiveness in controlling grain growth, ZnO appears to be an attractive additive for BaTiO₃ dielectrics.     

Sol–Gel Processing and Characterization of Pure and Metal-Doped SnO2 Thin Films

A chloride-based inorganic sol–gel route was used for preparing pure and metal (osmium, nickel, palladium, platinum)-doped SnO₂ sol. SnCl₄ was first reacted with propanol, then the resulting compound was hydrolyzed and subsequently mixed with solutions of the metal dopants. The obtained sols were used for depositing thin films by spin coating or for preparing powders by solvent evaporation at 110°C. FTIR spectroscopy and thermal analysis of the powders revealed that chlorine still bound to tin stabilized the sol against gelation by hindering the condensation reactions. Film characterizations showed that platinum and palladium, unlike nickel and osmium, were likely to form nanoparticles in the SnO₂ lattice. This result was discussed with regard to the different ways that platinum and palladium, on one hand, and nickel and osmium, on the other, modified the growth of SnO₂ grains and the film roughness and morphology. Dopants that formed nanoparticles (platinum, palladium) resulted in the roughest film, while dopants that did not form particles (nickel, osmium) resulted in SnO₂ grain size very close to that of pure SnO₂.     

Microstructure and Grain Boundary Structure of Na+-Diffused (Sr,Ca)TiO3 Capacitor-Varistor Ceramics

The microstructure of (Sr,Ca)TiO₃ capacitor-varistor materials has been investigated by employing electron microscopy techniques (TEM, STEM, HREM, EDX, and EPA). The material is found to contain (Sr,Ca)TiO₃ grains (∼30 μm) having perovskite crystal structure with domains, a Na⁺-diffused layer at the grain boundaries which is dependent on thermal diffusion conditions, and multiple-grain junctions in which the Ti _n O_2n–1 Magneli phase coexists with an amorphous intergranular phase. In addition, wider grain boundaries (10–30 nm), thin grain boundaries (∼1 nm), and clean grain boundaries which are free from intergranular phase were observed, and the effects of different grain boundaries on the diffusion of Na⁺are discussed.     

Development of Nano-Composite Microstructures in ZrO2-Al2O3 via the Solution Precursor Method

Aqueous mixtures of zirconium acetate and aluminum nitrate were pyrolyzed and crystallized to form a metastable solid solution, Zr_{1- x} Al _x O_{2− x /2} ( x < 0.57). The initial, metastable phase partitions at higher temperatures to form two metastable phases, viz., t −ZrO₂+γ-Al₂O₃ with a nano-scale microstructure. The microstructural observations associated with the γ- →α-Al₂O₃ phase transformation in the t -ZrO₂ matrix are reported for compositions containing 10, 20, and 40 mol% A1₂O₃. During this phase transformation, the α-Al₂O₃ grains take the form of a colony of irregular, platelike grains, all with a common crystallographic orientation. The plates contain ZrO₂ inclusions and are separated by ZrO₂ grains. The volume fraction of A1₂O₃ and the heat treatment conditions influence the final microstructure. At lower volume fractions of A1₂O₃, the colonies coarsen to single, irregular plates, surrounded by polycrystalline ZrO₂. Interpenetrating microstructures produced at high volume fractions of A1₂O₃ exhibit very little grain growth for periods up to 24 h at 1400°C.     

Effect of Seeding Sr2KNb5O15 on the Phase Formation and Microstructural Development in Reactive Sintering of Sr2NaNb5O15 Ceramics

The phase formation, densification behavior, and microstructure development of Sr₂NaNb₅O₁₅ (SNN) ceramics in both 10 wt% acicular Sr₂KNb₅O₁₅ (SKN) seed-containing and unseeded systems were investigated in this work. SNN ceramics were reactively sintered from SrNb₂O₆ and NaNbO₃ powders. The results show that the acicular SKN seeds not only accelerate SNN phase formation but also promote the densification at lower temperature. In reactive sintering, the acicular SKN seeds prepared by the molten salt synthesis method can give rise to the formation of a liquid phase and provide the structural framework for the grain growth of ceramics, leading to the formation of large anisotropic grains (>80 μm) in ceramics sintered at 1340°C. However, there are no such large anisotropic grains obtained in the SKN-free system. Observation of the large anisotropic grain growth is explained by the liquid-phase-assisted growth mechanism. For comparison, the microstructure evolution in the system with 10 wt% SKN seed, which was prepared by the conventional mixed-oxide method and without acicular morphology, was also investigated to further support the new growth mechanism.