Microstructural Development in SnO2-Doped ZnO–Bi2O3 Ceramics

The effects of adding small quantities of SnO₂ to the basic ZnO–Bi₂O₃ varistor composition were studied in terms of phase reactions, microstructural development, and the formation of inversion boundaries. Scanning and transmission electron microscopy studies showed that the inversion boundaries, triggered by the addition of SnO₂, cause anisotropic grain growth in the early stages of sintering. ZnO grains that include inversion boundaries grow exaggeratedly, at the expense of normal grains, until they dominate the microstructure. Higher additions of SnO₂ lead to an increase in number of grains with inversion boundaries and to a more fine-grained microstructure. The increasing amount of secondary phases is also related to a higher level of SnO₂ addition; however, the influence of these phases on ZnO grain growth is subordinate to the role of inversion boundaries.     

Low-Fire Processing of ZrO2–SnO2–TiO2 Ceramics

Effects of a liquid-phase-sintering aid, BaCuO₂+ CuO (BCC), on densification and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics have been investigated. The densification kinetics of ZST are greatly enhanced with the presence of 2.5–5 wt% BCC, but become retarded when the amount of BCC increases further. At a given BCC content, moreover, slower densification kinetics are observed with a larger particle size of ZST. The above results are attributed to a chemical reaction taking place at the interface of BCC/ZST during firing. The ZST dissolves into BCC, forming crystalline phases of ZrO₂, SnO₂, CuO, and BaTi₈O₁₆ which reduce the amount of BCC flux available for liquid-phase sintering. The crystallization kinetics become more significant, compared with densification kinetics, with increasing the amount of BCC and the particle sizes of ZST. For samples with 2.5–5 wt% BCC, a high relative sintered density is obtained at 1000°C and the resulting microwave ceramics have a dielectric constant and a value of Q at 7 GHz in the ranges of 35–38 and 2800–5000, respectively.     

Solid Solubility of GeO2 in SnO2

Subsolidus equilibrium relations were reexamined in the SnO₂-rich portion of the system GeO₂-SnO₂ (60 to 98 mol% SnO₂). A limited solid solution based on SnO₂ was confirmed; 4mol% GeO₂ dissolved in SnO₂ at 1250 C, and glass was formed on quenching over the compositional range above 4mol% GeO₂. Phase relutions in the system are discussed with reference to the polymorphism of GeO₂ and its glass-forming ability, and a possible phase diagram is given .     

Knoop Microhardness Anisotropy of Single-Crystal Cassiterite (SnO2)

The Knoop microhardness anisotropy of single-crystal cassiterite (SnO₂) was measured on the (100), (110), (001), and (111). That anisotropy is depicted as the microhardness profiles for those planes. The results are addressed first in terms of the elastic anisotropy of SnO₂ and then on the basis of the effective resolved shear stress (ERSS), the latter an approach initially advanced by Brookes and co-workers. The load dependence of the Knoop microhardness is also evaluated in terms of the classical Meyer's law for which it is demonstrated that the Meyer's law coefficient and Meyer's law exponent are related.     

High-Temperature Thermoelectric Properties in the La2−xRxCuO4 (R: Pr, Y, Nb) Ceramics

La_{2− x} R _x CuO₄ (R: Pr, Y, Nb) polycrystalline ceramics have been prepared by a spark plasma sintering process. Analysis of the microstructure and phase composition shows that pure La₂CuO₄ ceramics with a high density can be obtained, and no impurity phases appear. All the samples exhibit a metal–semiconductor transition behavior at ∼750 K, which could be ascribed to the decreasing charge density as a result of loss of oxygen atoms and various conduction mechanisms in these La₂CuO₄-based ceramics. Our results indicate that the La_1.98Y_0.02CuO₄ ceramics show large thermoelectric power ∼1.0 × 10⁻³μW·(m·K²)⁻¹, and the evaluated ZT can reach 0.17 at 330 K, which potentially make them useful for thermoelectric applications.     

Structural and Thermophysical Properties of (PbO)0.5(SiO2)0.5 Glasses: Effect of SnO2 Incorporation

¹¹⁹Sn and ²⁹Si solid-state nuclear magnetic resonance studies on lead silicate glasses containing different amounts of SnO₂ confirmed that tin exists in the glass as distorted SnO₆ polyhedra and there is no direct interaction between tin and silicon structural units. Transmission electron microscopic studies have established that tin structural units are uniformly distributed in the glass. Significant changes in the values of glass transition temperature, microhardness, and thermal expansion coefficient with SnO₂ incorporation into the glass have been attributed to the increased rigidity of the glass network brought about by the replacement of weaker Pb–O linkages with stronger Sn–O linkages.     

Nature of Abnormal Grain Growth in (Ba, Nb)-Doped TiO2 Ceramics and Effect of Crystal Phases in Starting Powders

The effect of crystal phase in the starting powder of (Ba, Nb)-doped TiO₂ ceramics on abnormal grain growth was investigated. The abnormal grain growth took place mainly during the cooling period after the second phase precipitated. The extent of abnormal grain growth is higher in an anatase powder sample than in a rutile powder sample and is highest at an intermediate rate of cooling for both powders. The nature of abnormal grain growth is suggested to be the Ba flow resulting from the concentration gradient of Ba between neighboring grains as the second phase precipitated. The reason for the greater abnormal grain growth in the anatase powder sample is proposed to be the higher dissolution of Ba in the grain which can be explained by the more open crystal structure of anatase with respect to rutile. The amount of second phase on the surface is greater than that in the interior, especially in the anatase powder sample. Characteristic features of the structure of anatase were calculated.     

Electrical Conductivity of Heat-Treated SnO2 Films

Variations in electrical conduction which were caused by heat treatment of SnO₂ films vapor-deposited on glass were studied. In the range 20° to 230°C, the relative importance of semiconduction processes attributed to 3 types of donors (probably O vacancies, interstitial Sn ions, and Cl ions) was determined as a function of heat treatment. A decrease in conductivity at constant temperature above 230°C was ascribed to grain growth of the SnO₂ crystals deposited initially.     

Spinodal Decomposition in Tetragonal Systems: SnO2-Tio2

Expressions are given for the coherency-strain elastic free energy of systems with tetragonal symmetry in the framework of the continuum theory. This theory is applied for SnO₂-TiO₂ and the results are compared with known experimental observations. Calculations for other tetragonal systems are also presented. Finally, the results of these calculations for systems of tetragonal symmetry are discussed in the light of the Cahn and Hilliard theory of spinodal decomposition.     

Hot Isostatic Pressing of Reactive SnO2 Powder

Tin(IV) oxide (SnO₂) crystallizes at room temperature by adding hydrazine monohydrate ((NH₂)₂· H₂O) to a hydrochloric acid solution of tin, followed by washing and drying. Well-densified SnO₂ ceramics (99.8% of theoretical) with an average grain size of 0.9 μm have been fabricated by hot isostatic pressing for 2 h at 900°C and 196 MPa. Their Vickers hardness and bending strength are 14.4 GPa and 200 MPa, respectively. They exhibit an electrical conductivity of 2 × 10⁻³−9 × 10⁻³ S·cm⁻¹ at room temperature.     

The Ternary Systems BaO–TiO2–SnO2 and BaO-TiO2-ZrO2

An investigation of the ternary systems BaO-TiO₂-SnO₂ and BaO-TiO₂-ZrO₂ led to the discovery of two new compounds belonging to the system BaO-TiO₂. These compounds, Ba₂Ti₉-O₂₀ and Ba₂Ti₉O₂₀, are stabilized by minute additions of SnO₂ or ZrO₂. The known compound BaTi₂O₅ can be obtained only from the molten phase and decomposes below 1300°C. into Ba₂Ti₅O₁₂ and BaTiO₂. In these systems no ternary compounds are found. The ternary phase diagrams can be divided into regions with high and low dielectric losses, which are in accordance with the phase relations. Tables with crystallographic data of the new compounds are included.     

The Role of Li2CO3 and PbO in the Low-Temperature Sintering of Sr, K, Nb (SKN)-Doped PZT

It is demonstrated that the use of ∼0.9 mol% Li₂CO₃ (LC) as a sintering aid for Sr, K, Nb modified Pb(Zr_{1− x} ,Ti _x )O₃ (PZT) ceramics is effective only in the presence of excess PbO (∼2 mol%). It is shown that LC and PbO react to form the compound, Li₂PbO₃ (LPO) which has a melting temperature of ∼836°C. Using dilatometry, we were able to correlate shrinkage during heating of a green ceramic to the melting of the LPO. Consequently, complete densification and sizeable grain growth are achieved by solution-precipitation of the ceramic through the liquid phase. Importantly, sintering is not particularly effective with such small additions of either LC or PbO alone. In confirmation of this model, the LPO compound was presynthesized and used as the only sintering aid in the same PZT composition. The densification behavior of this mixture compared well with the case of separate additions.     

Habit Modifications of SnO2 Crystals in SnO2–Cu2O Flux System in the Presence of Trivalent Impurity Cations

SnO₂ single crystals have short prismatic habits bounded by well-developed {110} and {111} faces in a pure SnO₂–Cu₂O flux system. When trivalent cations are added to the system, the habits drastically change to needle, acicular, or whisker forms with large aspect ratios. The addition of trivalent cations also greatly increases the nucleation rate and drastically decreases the crystal size. SEM observations and EPMA investigations reveal that the flat {111} faces transform to rounded or rough { hkl } faces by the addition of trivalent cations. This roughening transition of {111} faces, keeping {110} faces unchanged, is the cause of drastic habit modification that is attributed to the breaking of the periodic bond chain in {111} faces by impurity cations.     

Grain Size-Microcracking Relation for NaZr2(PO4)3 Family Ceramics

The grain size-microcracking relation was examined for low thermal expansion NaZr₂(PO₄)₃ family ceramics. By measurements of the strength, Young's modulus, thermal expansion, and grain size of polycrystalline ceramics sintered at appropriate conditions, the critical grain size for microcracking was determined. The critical grain size was proportional to the inverse square of the maximum thermal expansion difference.     

Preparation of Necklace-Structured TiO2/SnO2 Hybrid Nanofibers and Their Photocatalytic Activity

TiO₂/SnO₂ nanonecklace-structured hybrid nanofibers have been prepared via an electrospinning method. These hybrid nanofibers are characterized with SnO₂-rich beads and pure TiO₂ chains. It is found that TiO₂ in the beads shows a rutile structure, and the one in the chains is entirely composed of anatase phase. This novel microstructure enhanced the photocatalytic activity, as well as its ideal recyclable character. We believe that this fire-new type of nanofiber may potentially serve as a new generation photocatalyst in environmental remediation.     

Effect of Impurity Cations on the Growth and Habits of SnO2 Crystals in the SnO2-Cu2O Flux System

The effects of a small amount of cations with valence states ranging from mono- to hexavalent on the growth and habits of SnO₂ crystals growing in the SnO₂-Cu₂O flux system were systematically investigated. Trivalent cations having ionic radii closer to that of Sn⁴⁺ had a conspicuous effect upon the size and habits of SnO₂. Pentavalent cations with ionic radii similar to those of Sn⁴⁺ had some effect, whereas cations of other valence states showed no effect at all.     

A Comparative Study of Thermal Conductivity in ZnO- and SnO2-Based Varistor Systems

We performed a comparative study of electrical and thermal properties of ZnO- and SnO₂-based varistor. The electrical properties of commercial ZnO-based varistor are equivalent to that found in SnO₂-based varistor system. In spite of this, the SnO₂ showed a thermal conductivity higher than commercial samples of ZnO-based varistor, which allied with its simpler microstructure and lower dopant concentration is a remarkable result that point out to the use of this system to compete commercially with ZnO-based varistor devices.     

Slow Crack Growth Behavior in Transformation-Toughened Al2O3-ZrO2(Y2O3) Ceramics

Significant increases in the critical fracture toughness (K _IC ) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO₂ particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO₂ particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO₂ phase transformation by tailoring the Y₂O₃ stabilizer content not only increases the critical fracture toughness (K_IC) but also the K _Ia to initiate slow crack growth. For example, crack velocities ( v )≥10^–9 m/s are obtained only at K _Ia≥5 MPa.m^1/2 in transformation-toughened ( K _IC=8.5 MPa.m^1/2) composites vs K _Ia≥2.7 MPa.m^1/2 for comparable velocities in composites where the transformation does not occur ( K _IC=4.5 MPa.m^1/2). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (≥1 μm) monoclinic ZrO₂ particles which reside along the intergranular crack path.