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.     

Electrical Conduction in β-Bi2O3 Doped with Sb2O3

Electrical conduction in tetragonal β-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, ionic transference number, and Seebeck coefficient. The β-Bi₂O₃ doped with 1 to 10 mol% Sb₂O₃ was stable up to 600°C and showed an oxygen ionic and electronic mixed conduction, where the electron conduction was predominant at low oxygen pressures. The oxygen-ion conductivity showed a maximum at 4 mol% Sb₂O₃, whereas the activation energy for the ionic conduction remained unchanged for 4 to 10 mol% Sb₂O₃-doped specimens. These results were interpreted in terms of the oxygen vacancy concentration and the distortion of the tetragonal structure. The electron conductivity and its oxygen pressure dependence decreased with increasing Sb₂O₃ content. The fact that Sb⁵⁺ is partially reduced by excess electrons in heavily doped β specimens at low oxygen pressures is explained.     

(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.     

Effect of Bismuth of Oxide Content on the Sintering of Zinc Oxide

Densification and microstructure de velopment in Bi₂O₃-doped ZnO have been studied with a special emphasis on the effect of the Bi₂O₃ content. A small amount of Bi₂O₃ in ZnO (0.1 mol%) retarded densification, but the addition of Bi₂O₃ to more than 0.5 mol% promoted densification by the formation of a liquid phase above the eutectic temperature (∼740°C). The liquid phase increased grain-boundary mobility, which was responsible for the formation of intragrain pores and the decrease in the sintered density. The increase in the Bi₂O₃ content increased the probability of the formation of skeleton structure, which reduced the grain growth rate and the sintered density.     

Influence of V2O5 Additions to 5Li2O–1Nb2O5–5TiO2 Ceramics on Sintering Temperature and Microwave Dielectric Properties

The effects of the addition of V₂O₅ on the sintering behavior, microstructure, and microwave dielectric properties of 5Li₂O–1Nb₂O₅–5TiO₂ (LNT) ceramics have been investigated. With low-level doping of V₂O₅ (≤3 wt%), the microstructure of the LNT ceramic changed from a special two-level intergrowth structure into a two-phase composite structure with separate grains. And the sintering temperature of the LNT ceramics could be lowered to around 900°C by adding a small amount of V₂O₅ without much degradation in microwave dielectric properties. Typically, better microwave dielectric properties of ɛ_r=41.7, Q × f =7820 GHz, and τ _f =45 ppm/°C could be obtained for the 1 wt% V₂O₅-doped ceramics sintered at 900°C.     

Determination of Inversion Temperature of Sb2O3-Doped BaTiO3 Positive Temperature Coefficient of Resistivity (PTCR) Ceramics by the Finite Difference Method

The effect of the cooling rate on the PTCR (positive temperature coefficient of resistivity) characteristics of 0.1 mol% Sb₂O₃-doped BaTiO₃ ceramics has been investigated. Resistances both below and above the Curie temperature were increased by slow cooling, which indicated that the resistive layer width at the grain boundary increased as the cooling rate decreased. Concentration profiles of the Ba vacancies as a function of distance from the grain boundary have been simulated by the finite difference method. The inversion temperature of the 0.1 mol% Sb₂O₃-doped BaTiO₃ system was determined to be 1160°C from the measured electrical properties and computed concentration profiles.     

The Influence of Sb2O3 Addition on the Properties of Low-melting ZnO–P2O5 Glasses

The influence of 0–16 mol% Sb₂O₃ substitution for P₂O₅ on the properties of ZnO–P₂O₅ glasses has been investigated. It was shown that Sb₂O₃ could participate in the glass network and thermal stability of the glasses decreased with increasing Sb₂O₃ content. Glass transition temperature T _g, softening temperature T _s, and water durability all decreased firstly (up to 6 mol% Sb₂O₃ added) and then increased. Substitution of 12 mol% Sb₂O₃ led to a 16°C decrease in T _g and 30°C decrease in T _s, and weight loss of the glass was only 0.42 mg/cm², which is ∼11 times lower than that of the glass without Sb₂O₃ after immersion in deionized water at 90°C for 1 day. The glass containing 12 mol% Sb₂O₃ might be a substitute for Pb-based glasses in some applications.     

Effects of Bismuth Sesquioxide on the Characteristics of ZnO Varistors

The nonlinearity of ZnO varistors is significantly influenced by the Bi₂O₃ and Sb₂O₃ contents, as well as by the phase composition of the Bi₂O₃. Degradation of the current-voltage characteristics due to the applied voltage is not always lowered by the β—γ transition of the Bi₂O₃ phase. Lattice parameter determinations and stress analyses suggest that the Bi₂O₃-rich phase in multigrain junctions causes mechanical strain at the grain boundary which may play an important role in the current-voltage characteristics of ZnO varistors.     

Microstructure and Nonlinear Properties of Microwave-Sintered ZnO-V2O5 Varistors: I, Effect of V2O5 Doping

The modification of the densification behavior and the grain-growth characteristics of the microwave-sintered ZnO materials, caused by the incorporation of V₂O₅ additives, have been systematically studied. Generally, the addition of V₂O₅ markedly enhances the densification rate, such that a density as high as 97.9% of the theoretical density and a grain size as large as 10 µm can be attained for a sintering temperature as low as 800°C and a soaking time as short as 10 min. Increasing the sintering temperature or soaking time does not significantly change the sintered density of the ZnO-V₂O₅ materials but it does monotonously increase their grain size. Varying the proportion of V₂O₅ in the range of 0.2-1.0 mol% does not pronouncedly modify such behavior. The leakage current density ( J _L) of these high-density and uniform-granular-structure samples is still large, which is amended by the incorporation of 0.3 mol% of Mn₃O₄ in the ZnO materials, in addition to 0.5 mol% of the V₂O₅ additives. Samples that are obtained using such a method possess good nonohmic characteristics (α= 23.5) and a low leakage current density ( J _L= 2.4 10^-6 A/cm²).     

Inner Electrodes for Multilayer Varistors

In the present study, Pt or AgPd metal is used as the inner electrode for Bi₂O₃-doped ZnO multilayer varistors (MLV). The growth of the ZnO grains is constrained by the presence of the inner electrodes. The Pt inner electrodes are chemically inert to Bi₂O₃-doped ZnO. The Bi₂O₃ could react with Pd to form PdBi₂O₄. The Bi₂O₃-rich liquid also tends to wet the AgPd electrode. The size of ZnO grains in the MLV/AgPd specimen is larger. The ZnO grains in the MLV/AgPd specimen can even grow to a size larger than the layer thickness at the expense of electrode continuity.     

Grain Growth of Zinc Oxide During the Sintering of Zinc Oxide—Antimony Oxide Ceramics

Grain growth in a high-purity ZnO with systematic additions of Sb₂O₃ from 0.29 to 2.38 wt% was studied for sintering in air from 1106° to 1400°C. The results are discussed and compared with previous studies of pure ZnO and ZnO with Bi₂O₃ additions in terms of the kinetic grain growth expression: Gⁿ – Gⁿ ₀= K ₀ t exp(— Q/RT ). Additions of Sb₂O₃ inhibited the grain growth of ZnO and increased the grain growth exponent ( n -value) to 6 from 3 for pure ZnO and 5 for the ZnO—Bi₂O₃ ceramic. The apparent activation energy for the grain growth of ZnO also increased to about 600 kJ/mol from 220 kJ/mol for pure ZnO and 150 kJ/mol for the ZnO—Bi₂O₃ ceramics. Both the grain growth exponent and the activation energy were independent of the Sb₂O₃ content. Particles of the Zn₇Sb₂O₁₂ spinel were observed on the grain boundaries and at the grain triple point junctions. It was also observed that the Sb₂O₃ additions caused twin formation in each ZnO grain. It is concluded that both the Zn₇Sb₂O₁₂ particles and the twins are responsible for the ZnO grain growth inhibition by Sb₂O₃.     

Microwave Dielectric Properties of 5Li2O–0.583Nb2O5–3.248TiO2 Ceramics with V2O5

The effects of V₂O₅ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O–0.583Nb₂O₅–3.248TiO₂ (LNT) ceramics have been investigated. With addition of low-level doping of V₂O₅ (≤2 wt%), the sintering temperature of the LNT ceramics could be lowered down to around 920°C due to the liquid phase effect. A secondary phase was observed at the level of 2 wt% V₂O₅ addition. The addition of V₂O₅ does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the excellent microwave dielectric properties of ɛ_r=21.5, Q × f =32 938 GHz, and τ_f=6.1 ppm/°C could be obtained for the 1 wt% V₂O₅-doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as an internal electrode.     

Electrical Conductivity of PbO-P2O5-V2O5Glasses

The dc conductivities (α) of PbO-P₂O₅-V₂O₅ glasses containing up to 80 mol% V₂O₅ were measured at T = 100°C to T = 10°C below the glass transition temperature. Dielectric constants at 1 MHz, densities, and the fraction of reduced V ion were measured at room temperature. The conduction mechanism of glasses containing >10 mol% V₂O₅ was considered to be small-polaron hopping, as previously reported for other vanadate glasses. The temperature dependence of α was exponential, with α= (αo/ T ) exp(− W/kT ). When the V₂O₅ content was ≥50 mol%, W decreased and α increased with increasing V₂O₅ content, and the adiabatic approximation could be applied. In the composition range between 10 and 50 mol% V₂O₅, α increased with increasing V₂O₅ content, but W varied little. In this region, the hopping conduction was characterized as nonadiabatic. The effect of dielectric constants and V ion spacing on W is discussed.     

Influence of Process Variables on Microstructure and V-I Characteristics of Multicomponent ZnO-Based Nonlinear Resistors

Studies have been made of the effect of process variables such as calcination, initial particle size, and postsinterin on density, microstructure, V-I characteri stics, and energy -handling capability of a ZnO-based composite: 87.325 wt% ZuO + 0.1% Nb₂O₅+ 3.5% Sb₂O₅+ 6% Bi₂O₃+ 0.55% CoO + 0.7% MnO₂+ 0.9% Cr₂O₃+ 0.9% NiO + 0.025% Al(NO₃)₃. The nonlinear characteristics were found to be improved by the calcination of a ZnO + Nb₂O₅ mixture at 1200°C for 1h. The postsintering heat treatment was found to rejuvenate V-I characteristics of the degraded sample to a greater extent. Further, the postsintered sample containing Nb₂O₅, prepared by using a calcined charge of particle size around 1 μm, was found to possess better V-I characteristics and energy-handling capability compared to the samples prepared without the calcination step.     

Electrical Properties of ZnO-Bi2O3 Ceramics

The nonlinear volt-ampere characteristics and small-signal ac capacitance and resistance of sintered ZnO containing 0.5 mol% Bi₂O₃ were measured. Many of the electrical properties are related directly to the microstructure, which consists of conductive ZnO grains separated by a continuous amorphous Bl₂O₃, phase. The origin of the nonlinear conduction in the intergranular phase was confirmed by experiments with evaporated thin films. The proposed conduction mechanism in varistors containing ZnO and Bi₂O₃ is a combination of hopping and tunneling in the amorphous phase.     

Depolymerization of GeO2 and B2O3-GeO2 Network Glasses by Sb2O3

Binary Sb₂O₃-GeO₂ glasses containing 45 mol% Sb₂O₃ and ternary Sb₂O₃-B₂O₃-GeO₂ glasses containing 50 mol% GeO₂ were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined, and their colors and high-temperature viscosities were estimated visually. Small amounts of Sb₂O₃ (∼10 mol%) appear to perturb neither the Ge-O-Ge network nor those B-O-Ge networks with small B/Ge ratios (∼0.2). The B-O-Ge networks with larger B/Ge ratios (∼1.0) depolymerize in the presence of even less Sb₂O₃. Amounts of Sb₂O₃ >10 mol% appear to depolymerize the Ge-O-Ge and Ge-O-B networks progressively, possibly with the formation of chains. A structurally sensitive ir isofrequency contour technique developed for ternary glass systems was applied successfully to these Sb₂O₃-B₂O₃-GeO₂ glasses. These contours can thus readily detect significant network depolymerization in the absence of the usual network modifiers.     

Effects of Antimony Oxide on the Characteristics of ZnO Varistors

The breakdown voltage, the upturn voltage, and the nonlinearity of the ZnO varistors are significantly influenced by the Sb₂O₃ and SiO₂ contents, as well as by the β→γ transition of the Bi₂O₃ phase. The lattice parameter of spinel is influenced by the coexisting Bi₂O₃ phase. Antimony oxide disperses into the powders of ZnO and other additives in the early stage of sintering, and finally gathers again as particles of spinel which may play an important role in the improvement of the nonlinearity by stressing the interfaces of two ZnO grains.     

Compositional Dependence of Phase Separation and Photoluminescence in Er-Doped Alkaline Borosilicate Glasses

Er³⁺-doped materials have attracted great attention. In this study, the effects of the kind of alkali (Li, Na, and K), Er₂O₃ concentration (up to 3 mol%), and P₂O₅ addition (0 and 2 mol%) on the phase separation, optical absorption, and photoluminescence (PL) of the alkali borosilicate glasses were investigated. The relationship between microstructure and optical properties of the glasses is discussed. It was found that the development of the droplet phase enhances both the light-scattering effect (enhancing the PL intensity) and the concentration-quenching effect (reducing the PL intensity). As a result, the variation of the PL intensity of the Er^{3+ 4}I_13/2→⁴I_15/2 transition with Er₂O₃ content is mainly caused by the conflict between the light-scattering effect and the concentration-quenching effect. The 1 mol% Er₂O₃-doped, P₂O₅-containing, sodium borosilicate glass has the optimum microstructure and thus the highest PL intensity.     

Structure and Properties of Low Phonon Antimony Glasses in the K2O–B2O3–Sb2O3–ZnO System

The monolithic glass-forming region of the low phonon and low softening point antimony glasses containing high Sb₂O₃ (40–75 mol%) in the novel quaternary K₂O–B₂O₃–Sb₂O₃–ZnO system has been found with the help of X-ray diffraction (XRD) analysis. The structure of a series of glasses with the general composition of (mol%) 15K₂O–15B₂O₃–(70− x )Sb₂O₃– x ZnO (where x =5–25) has been evaluated by infrared reflection spectral (FT-IRRS) analyses. All the glasses are found to possess a low phonon energy of around 600 cm⁻¹, as revealed by FT-IRRS. Their softening point ( T _s), glass transition temperature ( T _g), and coefficient of thermal expansion (CTE) have been found to vary in the ranges of 351°–379°C, 252°–273°C, and 195–218 × 10⁻⁷ K⁻¹, respectively. These properties are found to be controlled by their fundamental property, like the covalent character of the glasses, which is found to increase with an increase in Sb₂O₃ content. In addition, the devitrified glasses have been characterized by XRD and field emission scanning electron microscopy, which manifests the presence of nanozinc antimony oxide crystals with sizes of 21–43 nm. The exhibited properties have revealed that they are a new class of versatile materials.     

Sintering of Zinc Oxide Doped with Antimony Oxide and Bismuth Oxide

The phase change, densification, and microstructure development of ZnO doped with both Bi₂O₃ and Sb₂O₃ are studied to better understand the sintering behavior of ZnO varistors. The densification behavior is related to the formation of pyrochlore and liquid phases; the densification is retarded by the former and promoted by the latter. The pyrochlore phase, whose composition is Bi_3/2ZnSb_3/2O₇, appears below 700°C. The formation temperature of the liquid phase depends on the Sb/Bi ratio: about 750°C for Sb/Bi < 1 by the eutectic melting in the system ZnO—Bi₂O₃, and about 1000°C for Sb/Bi > 1 by the reaction of the pyrochlore phase with ZnO. Hence, the densification rate is determined virtually by the Sb/Bi ratio and not by the total amount of additives. The microstructure depends on the sintering temperature. Sintering at 1000°C forms intragrain pyrochlore particles in ZnO grains as well as intergranular layers, but the intragrain particles disappear at 1200°C by the increased amount of liquid phase, which enhances the mobility of the solid second phase.