Effect of Glass Additions on BaO–TiO2–WO3 Microwave Ceramics

The effect of glass addition on the properties of BaO–TiO₂-WO₃ microwave dielectric material N-35, which has Q = 5900 and K = 35 at 7.2 GHz for samples sintered at 1360°C, was investigated. Several glasses including B₂O₃, SiO₂, 5ZnO–2B₂O₃, and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B₂O₃ to N-35, when sintered at 1200°C, had the best dielectric properties: Q = 8300 and K = 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO–B₂O₃ glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO–B₂O₃.     

Densification Behavior in Microwave-Sintered Silicon Nitride at 28 GHz

Si₃N₄ powders were sintered using a 28 GHz gyrotron source, with Y₂O₃, Al₂O₃, and MgO as sintering aids, in an attempt to investigate the effect of microwave radiation on densification behavior. The microwave-sintered samples were compared with identical samples produced by conventional pressureless sintering. The effect of sintering on the microstructural development and grain growth of the samples was assessed using scanning electron microscopy. Phase transformation behavior was assessed using X-ray diffractometry. In the microwave-sintered samples, densification and α→β transformation occurred at temperatures ∼200°C lower than those of the conventionally sintered samples. More importantly, at comparable stages of densification, the microstructures of the microwave-sintered and conventionally sintered samples were significantly different, with the microwave-sintered samples showing the development of elongated β grains at a much earlier stage of the α→β transformation. It was concluded that the effect of microwave radiation on sintering was not simply a decrease in sintering temperatures, but in possibly a different sintering mechanism, clearly related to localized heating within the grain-boundary phase.     

Compositional Effects on the Liquid-Phase Sintering of Praseodymium Oxide-Based Zinc Oxide Varistors

The influence of the formation of eutectic liquid phases during sintering of the ternary varistor system ZnO-Pr₆O₁₁-Co₃O₄ was studied. The temperature at which the samples were observed to be liquid-phase sintered was found to depend on the nominal batch composition. The appearance of the liquid phase had a significant effect on the final microstructure as well as on the electrical properties of the varistors. Small additions of ZrO₂, purposely added or resulting from ball-mill contamination, were found to affect moderately the properties of the liquid phases observed in this system.     

Effects of Glass Additions on (Zr,Sn)TiO4 for Microwave Applications

The effects of glass additions on the properties of (Zr,Sn)TiO₄ as a microwave dielectric material were investigated. The (Zr,Sn)TiO₄ ceramics with no glass addition sintered at 1360°C gave Q = 4900 and K = 37 at 7.9 GH_z. Several glasses, including SiO₂, B₂O₃, 5ZnO–2B₂O₃, and nine commercial glasses, were tested during this study. Among these glasses, (Zr,Sn)TiO₄ sintered with ZnO-B₂O₃–SiO₂ (Corning 7574) showed more than 20% higher density than that of pure (Zr,Sn)TiO₄ sintered at the same temperature. A 5-wt% addition of SiO₂, to (Zr,Sn)TiO₄, when sintered at 1200°C, gave the best Q : Q = 2700 at 9 GHz. Results of XRD analysis and scanning electron microscopy and the effect of glass content are also presented.     

Microwave Dielectric Properties of Li2TiO3 Ceramics Doped with ZnO–B2O3 Frit

A type of new low sintering temperature ceramic, Li₂TiO₃ ceramic, has been found. Although it is difficult for the Li₂TiO₃ compound to be sintered compactly at temperatures above 1000°C for the volatilization of Li₂O, dense Li₂TiO₃ ceramics were obtained by conventional solid-state reaction method at the sintering temperature of 900°C with the addition of ZnO–B₂O₃ frit. The sintering behavior and microwave dielectric properties of Li₂TiO₃ ceramics with less ZnO–B₂O₃ frit (≤3.0 wt%) doping were investigated. The addition of ZnO–B₂O₃ frit can lower the sintering temperature of the Li₂TiO₃ ceramics, but it does not apparently degrade the microwave dielectric properties of the Li₂TiO₃ ceramics. Typically, the good microwave dielectric properties of ɛ_r=23.06, Q × f =32 275 GHz, τ_f = 35.79 ppm/°C were obtained for 2.5 wt% ZnO–B₂O₃ frit-doped Li₂TiO₃ ceramics sintered at 900°C for 2 h. The porosity was 0.08%. The Li₂TiO₃ ceramic system may be a promising candidate for low-temperature cofired ceramics applications.     

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.     

Densification and Phase Development of Spodumene–Cordierite Glass Powder Mixtures

The sintering and crystallization of spodumene-cordierite glass-ceramics that are made from mixtures of Li₂O-Al₂O₃-SiO₂ (LAS) and MgO-Al₂O₃-SiO₂(MAS) glass powders were investigated. Pure LAS and MAS powders have good sinterability. However, the densification of LAS was drastically reduced when small amounts of MAS were added. When larger amounts of MAS were added, the amount of densification further increased. The decrease in the Li₂O content in the LAS glass promoted the densification of the mixed glass samples. The above-mentioned results can be explained by examining the crystallization temperature, which is influenced by the interactions between the LAS and MAS glass particles. The lower the temperature of crystallization, the less sintering occurred. For the sintered samples, the phase that crystallized from the MAS glass was alpha-cordierite, and that which crystallized from the LAS glass was ß-spodumene or high-quartz solid solution, depending on the Li₂O content in the LAS glass.     

Low-Temperature Sintering Barium Titanate-Based X8R Ceramics with Nd2O3 Dopant and ZnO–B2O3 Flux Agent

To demonstrate that barium titanate-based ceramics could be sintered at a low temperature thus reducing the cost of capacitor production, systematic investigation has been made on the structure and dielectric properties of barium titanate-based X8R ceramics, doped with various Nd₂O₃ content and different ZnO–B₂O₃ solution as the sintering aids. The dielectric ceramic powder with good permittivity and low dielectric loss were obtained at a sintering temperature of 900°C, meeting X8R specifications. Transmission electron microscopy and EDS analysis shows a high concentration of Nd element in the boundary regions, which verifies the beneficial role of Nd in facilitating the formation of core-shell structure. The results also suggest that the developed BaTiO₃-based ceramics may serve as a promising candidate for fabricating cheap multilayer capacitors with pure Ag as inner electrode.     

Finite Size Effect on the Sinterability and Dielectric Properties of ZnNb2O6–ZBS Glass Composites

ZnNb₂O₆ (ZN) is a columbite-structured niobate compound showing excellent dielectric properties and comparatively low sintering temperatures (∼1200°C). Hence it is a good candidate for possible low-temperature cofired ceramics (LTCC) applications. In the present investigation, ZnNb₂O₆ was synthesized in the form of micrometer-sized powder using a conventional solid-state ceramic synthesis route as well as in the form of nanosized powder by a polymer complex method. The finite size effect of ZN particles on sinterability and microwave dielectric properties of sintered pellets was evaluated. The phase formation was confirmed from the X-ray diffraction (XRD) analysis. The particle size distribution of the nanoparticles was found to be of the order of 18–20 nm by using high-resolution transmission electron microscopy analysis and 30 nm by analyzing the XRD patterns using Debye Scherrer's formula, after correcting for the instrument broadening effects. A ZN–60ZnO–30B₂O₃–10SiO₂ (ZBS) composite was made by adding predetermined amounts of glasses. The microstructures of the sintered pellets of ZN and ZN–ZBS composites were examined using scanning electron microscopy and analyzed using image analysis. The nano-ZN–ZBS composites were sintered to 93% of the reported density at 925°C/2 h, with microwave dielectric properties of ɛ_r=22.5, Q × f ∼12 800 GHz, and τ_f=−69.6 ppm/°C, emerging as a potential material for possible LTCC applications.     

Densification of Calcia-Stabilized Zirconia with Borates

Densification studies of submicrometer ZrO₂ powders stabilized with 6.5 wt% CaO (CSZ) showed borate additions (1 to 10 wt%) to be effective sintering aids. Estimated densities >99% of theoretical were obtained on sintering at 1200°Cfor 4 h with 2 wt% B₂O₃ or 5 wt% CaO·2B₂O₃ additions to the CSZ powders. Average grain sizes obtained were typically <1 μm. Partial development of a monoclinic ZrO₂ phase was observed in the sintered samples. The amount of this phase varied from ∼7 to 75 wt% and was approximately linearly dependent on the additive concentration. The effect was most marked for the B₂O₃ additions. Development of the monoclinic phase was attributed to progressive leaching of Ca from the CSZ phase by B₂O₃, in effect partially destabilizing the ZrO₂.     

Effect of Dispersant Concentration on Preparation of an Ultrahigh Density ZnO–Al2O3 Target by Slip Casting

The rheological behavior of concentrated ZnO–Al₂O₃ aqueous suspensions has been studied in order to obtain an ultrahigh-density ZnO–Al₂O₃ composite ceramic target by slip casting. The influence of the mass fraction of polyacrylic acid (PAA) on the fluidity of slurries and the density and strength of the green and sintered bodies was investigated. The slurries exhibited a near-Newtonian flow behavior and had a lower viscosity with 0.3 wt% PAA. The excess of PAA enhanced the green strength and the density and strength of the sintered bodies. An ultrahigh density sintered body (>99.7% theoretical density) could be obtained after pressureless sintering at 1400°C. The Al species were well distributed in the sintered bodies, which showed a homogeneous, defect-free microstructure with no abnormal grain growth.     

Grain Growth of ZnO during Bi2O3 Liquid-Phase Sintering

Grain growth of ZnO during the liquid-phase sintering of binary ZnO–Bi₂O₃ ceramics has been studied for Bi₂O₃ contents from 3 to 12 wt% and sintering from 900° to 1400°C. The results are considered in combination with previously published studies of ZnO grain growth in the ZnO–Bi₂O₃ system. For the Bi₂O₃ contents of the present study, the rate of ZnO grain growth is found to decrease with increasing Bi₂O₃. Activation analysis, when combined with the results of similar analyses of the previous studies, reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-Bi₂O₃-content region of nearly constant activation energy values of about 150 kJ/mol, further Bi₂O₃ additions cause an increase of the activation energy to about 270 kJ/mol. consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of Bi₂O₃ changes from one of a phase-boundary reaction at low Bi₂O₃ levels to one of diffusion through the liquid phase at about the 5 to 6 wt% Bi₂O₃ level and above.     

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.     

Effects of Doping Trivalent Ions in Bismuth Borate Glasses

Bismuth borate glasses from the system: 40Bi₂O₃–59B₂O₃–1Tv₂O₃ (where Tv=Al, Y, Nd, Sm, and Eu) and three glasses of composition: 40Bi₂O₃–60B₂O₃, 37.5Bi₂O₃–62.5B₂O₃ and 38Bi₂O₃–60B₂O₃–2Al₂O₃ were prepared by melt quenching and characterized by density, UV-visible absorption spectroscopy and differential thermal analysis (DTA) studies. Bismuth borate glasses exhibit a very strong optical absorption band just below their absorption edge. Glasses were devitrified by heat treatment at temperatures above their glass transition temperatures and the crystalline phases produced in them were characterized by Fourier transform infrared (FTIR) absorption spectroscopy and X-ray diffraction (XRD). Bi₃B₅O₁₂ was found to be the most abundant phase in all devitrified samples. DTA studies on glasses and FTIR and XRD analysis on crystallized samples revealed that very small amounts of trivalent ion doping causes significant changes in the devitrification properties of bismuth borate glasses; rare-earth ions promote the formation of metastable BiBO₃–I and BiBO₃–II phases during glass crystallization.     

Sintering of Lithium Zinc Ferrites with Alkoxide-Derived Borosilicate Liquid Phase

Lithium zinc ferrites (Li_0.3Zn_0.4Mn_0.05Fe_2.25O₄) of > 97% theoretical density were prepared with alkoxide-derived borosilicate sintering additive. A sol-gel technique was used to incorporate the borosilicate phase prior to sintering. The effects of borosilicate composition and additive quantity on microstructural development and densification are reported. Direct current resistivity, dynamic hysteresis, and microwave property data are discussed in terms of processing parameters and resultant microstructures. The microstructure of the sintered ferrites was analyzed using SEM and TEM. Heterogeneities including regions of liquid-phase coalescence and intragranularly included glass phase were observed in some cases. These microstructural features were shown to have deleterious effects on microwave magnetic properties when compared to those of commercial ferrites of the same nominal composition sintered with Bi₂O₃ additive.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

BiNbO4-Based Glass–Ceramic Composites for Microwave Applications

The effect of a bespoke glass sintering aid, 0.3Bi₂O₃–0.3Nb₂O₅–0.3B₂O₃–0.1SiO₂ (BN1), developed from the base ceramic composition, BiNbO₄ (BN), on the sinterability, microstructure, and microwave (MW) dielectric properties of BN ceramics has been investigated. Densities >97% theoretical could be achieved at 1020°C for samples with up to 15% BN1 additions. The resulting microstructure was composed of BN laths surrounded by a residual glass phase that contained small fibrous crystals. Some evidence of dissolution of BN crystals was observed. Optimum properties were exhibited for samples with 15 wt% of glass addition sintered for 4 h at 1020°C with a relative permittivity ɛ_r=38, a MW quality factor Q × f ₀=17 353 at 5.6 GHz, and a temperature coefficient of resonant frequency τ_f=−10 ppm/°C. The high Q × f ₀, ɛ_r, and low τ_f, coupled with a relatively low sintering temperature, suggest that the use of bespoke glass sintering aids of this type may have great potential for the fabrication of MW ceramics.     

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Nanometer-sized zinc aluminate (ZnAl₂O₄) particles were synthesized from heterometal alkoxides, [ZnAl₂(OR)₈], possessing an ideal cation stoichiometry for the ZnAl₂O₄ spinel. ZnAl₂O₄ is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl₂O₄. ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl₂O₄ possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl₂O₄ and ZnS suggested that the hybridization of O 2 p and Zn 3 d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.     

Effects of B2O3 Addition on the Dielectric and Ferroelectric Properties of Ba0.7Sr0.3TiO3 Ceramics

The effects of B₂O₃ addition on the sintering behavior and the dielectric and ferroelectric properties of Ba_0.7Sr_0.3TiO₃ (BST) ceramics were investigated. The dielectric and ferroelectric properties of a BST sample with 0.5 wt% B₂O₃ sintered at <1150°C were as good as those of undoped BST sintered at 1350°C, and the dielectric loss was better. When >1.0 wt% B₂O₃ was added to BST, the overdoped B₂O₃ did not form a liquid phase or volatilize; it remained in the samples and formed a secondary phase that lowered the sintering behavior and the dielectric and ferroelectric properties of the BST.     

Change of Crystal Phases and Microstructure of Amorphous Si-C-N Powder by Hot Pressing

An amorphous Si-C-N powder with Y₂O₃ and Al₂O₃ powder as sintering additives was hot-pressed at 1900°C for 120 min in a nitrogen atmosphere. Changes in the crystalline phases and microstructure of the amorphous Si-C-N powder during sintering were investigated by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The defects at the fracture origins of the sintered bodies after bending tests also were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). XRD showed that alpha-Si₃N₄ was formed initially from the amorphous Si-C-N by 1530°C, which then transformed to ß-Si₃N₄ at 1600°C. Also, a slight formation of crystalline SiC occurred during the transformation from alpha- to ß-Si₃N₄, and it increased after the transformation was completed at 1900°C. TEM revealed that many SiC nanoparticles were incorporated into ß-Si₃N₄ grains after the transformation from alpha- to ß-Si₃N₄ at 1600°C. They were located at the triple points of the grain boundaries of ß-Si₃N₄ after continued Si₃N₄ grain growth at 1900°C. Besides the SiC nanoparticles, large agglomerations of carbon or SiC particles of 20-60 µm size were observed by SEM and EPMA at the fracture origins of the sintered bodies after the bending tests.