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.     

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.     

Transformational Plasticity in Bi2O3

Transformational plasticity associated with the monoclinic-to-cubic phase transition at 730°C in Bi₂O₃ was observed and characterized. This phenomenon is explained in terms of Greenwood and Johnson's model of internal stress-induced deformation, proceeding, in this instance, by a time-dependent, grain-size-sensitive creep mechanism, probably grain-boundary sliding. Criteria are proposed for choosing other prospective transformationally plastic ceramics; they are met by Bi₂WO₆, which also exhibits extensive transformation plasticity.     

Effect of Sb2O3 on Thermal Properties of Glasses in Bi2O3–B2O3–SiO2 System

Glasses with compositions 50Bi₂O₃– x Sb₂O₃–10B₂O₃–(40– x ) SiO₂ ( x =0, 1, 3, 5, 8, 10) have been prepared by conventional melt quench technique. Substitution of Sb₂O₃ for SiO₂ exerted an obvious effect on properties of glasses, especially, increased glass transition temperature ( T _g) and crystalline temperature ( T _c) greatly. Results of infrared transmission spectra attributed the effect to the formation of new bridging bonds of Sb–O–B and Sb–O–Si in glass network.     

Effects of Excess Bi2O3 on the Ferroelectric Behavior of Nd-Doped Bi4Ti3O12 Thin Films

Effects of excess Bi₂O₃ content on formation of (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) films deposited by RF sputtering were investigated. The microstructures and electrical properties of BNT thin films are strongly dependent on the excess Bi₂O₃ content and post-sputtering annealing temperature, as examined by XRD, SEM, and P – E hysteresis loops. A small amount of excess bismuth improves the crystallinity and therefore polarization of BNT films, while too much excess bismuth leads to a reduction in polarization and an increase in coercive field. P – E loops of well-established squareness were observed for the BNT films derived from a moderate amount of Bi₂O₃ excess (5 mol%), where a remanent polarization 2P _r of 25.2 μC/cm² and 2E _c of 161.5 kV/cm were shown. A similar change in dielectric constant with increasing excess Bi₂O₃ content was also observed, with the highest dielectric constant of 304.1 being measured for the BNT film derived from 5 mol% excess Bi₂O₃.     

Para-to-Ortho H2 Conversion on Gd, Y, Gd2O3, and Y2O3

The mechanism of parahydrogen conversion was studied on Gd₂O₃ and Y₂O₃ powders and on Gd and Y evaporated metal films at low and high temperatures (77° to 90°K and 298° to 418°K). Absolute rates of conversion are compared to theoretical values for 3 possible reaction mechanisms, and it is concluded that a paramagnetic vibrational mechanism is operative on Gd₂O₃, Gd, and Y. On Y₂O₃ the reaction rate is enhanced by additional surface paramagnetic sites. The portion of the surface which is active is ∼1 for the metals and ∼0.01 for the oxides.     

Kinetics of Solid-State Reaction of Bi2O3 and Fe2O3

Solid-state reactions of equimolar mixtures of Bi₂O₃ and Fe₂O₃ from 625° to 830°C and their kinetics were investigated. The reaction rates were determined from the integrated X-ray diffraction intensities of the strongest peaks of the reactants and products. The activation energy for the formation of BiFeO₃ was 96.6±9.0 kcal/mol; that for a second-phase compound, Bi₂Fe₄O₉, which formed above 675°C, was 99.4±9.0 kcal/mol. Specific rate constants for these simultaneous reactions were obtained. The preparation of single-phase BiFeO₃ from the stoichiometric mixture of Bi₂O₃ and Fe₂O₃ is discussed.     

Dielectric Properties of the Fluorite-like Bi2O3–Nb2O5 Solid Solution and the Tetragonal Bi3NbO7

Our analysis of the microwave dielectric properties of the δ-Bi₂O₃–Nb₂O₅ solid solution (δ-BN_ss) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb₂O₅. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb₂O₅. At the same composition there is a discontinuity in the grain size of the δ-BN_ss ceramics. For the sample containing 25 mol% Nb₂O₅ two structural modifications were observed. A single-phase tetragonal Bi₃NbO₇, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ-BN_ss homogeneity range. Compared with the δ-BN_ss the Bi₃NbO₇ ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.     

Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20

High-performance Ba₂Ti₉O₂₀ ceramics are attracting great attention, but their formation mechanism still is somewhat unclear. The present investigation shows that the formation of Ba₂Ti₉O₂₀ can be promoted strikingly by the participation of Bi₂O₃ and Al₂O₃. The effect of Bi₂O₃ on the formation of Ba₂Ti₉O₂₀ is attributed to the fact that migration of the involved reactants is accelerated by liquid which forms from the melting of Bi₂O₃ above 830°C. This migration, however, is not the only rate-limiting factor. A high potential-energy barrier, resulting from stress that arises along the crystal-structured layers, also heavily restricts the formation of Ba₂Ti₉O₂₀. The participation of Al₂O₃, on the other hand, can reduce the height of this potential-energy barrier and effectively improve the kinetics of the formation of Ba₂Ti₉O₂₀ by causing the formation of BaAI₂Ti₆O₁₆ crystals; these crystals intergrow with Ba₂Ti₉O₂₀ crystals and result in decreased stress.     

The System Bi2O3-SiO2

The Bi₂O₃-rich side of the system Bi₂O₃-SiO₂ was studied with powder X-ray diffraction and differential thermal analysis. In the composition 6Bi₂O₃. x SiO₂, the metastable γ phase (bcc) was observed to exist over the range of 0 < x ≤ 1. In most of the compositions studied, metastable phases of water-quenched melts transformed into another metastable phase before reaching stable phases. A modification of the phase diagram is proposed.     

Liquid-Phase Sintering of MgO–Bi2O3

Liquid-phase sintering of MgO-5 wt% Bi₂O₃ was studied by loading dilatometry. The ratio of the creep viscosity to the densification viscosity (∼1.8) and the sintering stress remained nearly constant in a wide density interval. These results, together with results on several other systems, indicate that the constancy of the sintering stress during densification may be a general phenomenon, regardless of densification mechanism.     

Transformational Superplasticity in the Bi2O3-Sm2O3 Eutectoid System

Transformational superplasticity associated with the eutectoid reaction in the Bi₂O₃-Sm₂O₃ system was observed and characterized in hypoeutectoid and hypereutectoid compositions. Transformational strain varied linearly with applied stress and exhibited a stress-axis intercept, or threshold stress, that is related to the proeutectoid microconstituent. Results are explained quantitatively using the analysis by Greenwood and Johnson and a creep mechanism.     

Comment on "Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20"

in a recent article of the Journal , Yu et al .¹ reported their experimental results on the effect of Al₂O₃ and Bi₂O₃ on the formation mechanism of Sn-doped Ba₂Ti₉O₂₀. They claimed that both Al₂O₃ and Bi₂O₃ can dramatically assist the formation of Sn-doped Ba₂Ti₉O₂₀ but are based on different mechanisms. They concluded that first, Bi₂O₃ melts above 830°C and accelerates the migration of the involved reactants to form Ba₂Ti₉O₂₀; second, Al₂O₃ can reduce the height of the potential energy barrier of the formation of Ba₂Ti₉O₂₀ due to the intergrowth of BaAl₂Ti₆O₁₆ phase. They explained their results from a point of view that the formation of Ba₂Ti₉O₂₀ is controlled by (1) the migration of reactants to the interfaces and (2) the height of the potential-energy barrier of the reaction at the interfaces. However, based on their results, we feel their conclusions are incautious and may be misleading, as will be discussed later.     

Bi2O3 Vaporization in Microwave-Sintered ZnO Varistors

Vaporization of Bi₂O₃ in microwave-sintered ZnO varistors is discussed in this study. The Bi₂O₃ vaporization of ZnO varistors sintered by a conventional electric furnace is also studied for comparison. The results show that the Bi₂O₃ vaporization in microwave-sintered ZnO varistors is more homogenous from the surface to the inside of the sample, which results from the special thermal gradient inside the microwave-sintered samples, and we also find out that the Bi₂O₃ vaporization directly affects the electrical properties of ZnO varistors. Microwave-sintered samples exhibit more excellent electrical properties than the conventional ones because the homogenous Bi₂O₃ vaporization leads to more uniform microstructures.     

Phase Relations in the System Bi2O3-WO3

Phase relations in the system Bi₂O₃-WO₃ were studied from 500° to 1100°C. Four intermediate phases, 7Bi₂O₃· WO₃, 7Bi₂O₃· 2WO₃, Bi₂O₃· WO₃, and Bi₂O₃· 2WO₃, were found. The 7B₂O · WO₃ phase is tetragonal with a ₀= 5.52 Å and c ₀= 17.39 Å and transforms to the fcc structure at 784°C; 7Bi₂O₃· 2WO₃ has the fcc structure and forms an extensive range of solid solutions in the system. Both Bi₂O₃· WO₃ and Bi₂O₃· 2WO₃ are orthorhombic with (in Å) a ₀= 5.45, b ₀=5.46, c ₀= 16.42 and a ₀= 5.42, b ₀= 5.41, c ₀= 23.7, respectively. Two eutectic points and one peritectic exist in the system at, respectively, 905°± 3°C and 64 mol% WO₃, 907°± 3°C and 70 mol% WO₃, and 965°± 5°C and 10 mol% WO₃.     

Cubic-to-Tetragonal Displacive Transformation in Gd2O3–Bi2O3 Ceramics

Gd₂O₃-doped Bi₂O₃ polycrystalline ceramics containing between 2 and 7 mol% Gd₂O₃ were fabricated by pressureless sintering powder compacts. The as-sintered samples were tetragonal at room temperature. Hightemperature X-ray diffraction (XRD) traces showed that the samples were cubic at elevated temperatures and transformed into the tetragonal polymorph during cooling. On the basis of conductivity measurements as a function of temperature and differential scanning calorimetry (DSC), the cubic → tetragonal as well as tetragonal → cubic → teansition temperatures were determined as a function of Gd₂O₃ concentration. The cubic → tetragonal transformation appears to be a displacive transformation. It was observed that additions of ZrO₂ as a dopant, which is known to suppress cation interdiffusion in rare-earth oxide–Bi₂O₃ systems, did not suppress the transition, consistent with it being a displacive transition. Annealing of samples at temperatures 660°C for several hundred hours led to decomposition into a mixture of monoclinic and rhombohedral phases. This shows that the tetragonal polymorph is a metastable phase.     

The System Bi2O,—B2O3

The phase diagram for the system Bi₂O₃-B₂O₃ has been determined experimentally. The melting point of Bi₂O₃ has been redetermined as 825° C with an estimated overall uncertainty of about ±3°C, and the molal heat of fusion of Bi₂O₃, calculated from the slope of the liquidus curve, is 2050 cal per mole. The system contains a body-centered cubic phase of approximate composition 12Bi₂O₃·B₂O₃, which melts incongruently at 632°C. Four congruently melting compounds exist in the system: 2Bi₂O₃· B₂O₃·5B₂O₃, Bi₂O₃·3B₂O₃, and Bi₂O₃·4B₂O₃, with melting points, respectively, of 675°, 722°, 708°, and 715°C. The Bi₂O₃·4B₂O₃ compound exhibits a sluggish transformation at 696°C. Compositions containing up to 97.5 wt% (85 mole %) Bi₂O₃ can be partly or totally quenched to glass. Indices of the quenched glasses are greater than 1.74. A region of liquid immiscibility extends at 709°C from almost pure B₂O₃ to 19.0 mole % Bi₂O₃. The extent of immiscibility theoretically calculated agrees with the experimentally determined value when 1.20 A is used for the ionic radius of Bi³⁺.     

Low-Temperature Sintering, Densification, and Properties of Z-type Hexaferrite with Bi2O3 Additives

The effect of the addition of Bi₂O₃ on the densification, low-temperature sintering, and electromagnetic properties of Z-type planar hexaferrite was investigated. The results show that Bi₂O₃ additives can improve the densification and promote low-temperature sintering of Z-type hexaferrite prepared by a solid-state reaction method. The presence of Bi₂O₃ in the grain boundaries and the generation of Fe²⁺ degrade the initial permeability of the samples but make the quality factor and cut-off frequency increase. Various possible mechanisms involved in generating these effects were also discussed.