Grain Growth-Controlled Giant Permittivity in Soft Chemistry CaCu3Ti4O12 Ceramics

We report a dielectric constant of up to 5.4 × 10⁵ at room temperature and 1 kHz for CaCu₃Ti₄O₁₂ (CCTO) ceramics, derived from multiphase powders (coprecipitation products), made by a "chimie douce" (coprecipitation) method, and then sintered in air. The sintered products are pure-phase CCTO ceramics. The high dielectric constant is achieved by tuning the size of grains and the thickness of grain boundaries. The grain growth is controlled by varying the concentration of excess CuO in the initial powder (calcined coprecipitation products) between 1 and 3.1 wt%. The dielectric constant of pure CCTO ceramics increases with the initial CuO concentration, reaching its maximum at 2.4 wt% of CuO. A further increase of excess CuO in powders results in a permittivity decrease, accompanied by the formation of CuO as a separate phase in the sintered products. The unusual grain growth behavior is attributed to a eutectic reaction between CuO and TiO₂ present in the initial powder.     

Low-Temperature Sintering and Microwave Dielectric Properties of Anorthite-Based Glass-Ceramics

TiO₂ nucleated anorthite-based glass-ceramics were fabricated from glass powders. After sintering and crystallization heat treatment, various physical properties, including apparent bulk density and water absorption, were examined to evaluate the sintering behavior of anorthite-based glass-ceramic. Results showed that the complete-densification temperature for specimens was as low as 900°C. Sufficient crystallization was achieved by subsequently raising the firing temperature to 950°C, and the dielectric quality factor was promoted to the maximum value. Contents of nucleating agent (TiO₂) played an important role in the dielectric constants. The crystallinity was controlled by raising the firing temperature at a constant heating rate. The degree of crystallization affected the dielectric properties of sintered glass-ceramics. At the resonant frequency of 10 GHz, anorthite glass-ceramics with 5 wt% TiO₂ possessed the lowest permittivity of 8 and exhibited appropriate dielectric properties as compared with those with B₂O₃ and 10 wt% TiO₂.     

Characteristics of High-Q Microwave Dielectric Ceramics Nd(Co1/2Ti1/2)O3 With CuO Addition

We report the microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by the conventional solid-state route. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibits a mixture of Co and Ti showing a 1:1 order in the B site. Lowering the sintering temperature (as low as 1260°C) and promoting the densification of Nd(Co_1/2Ti_1/2)O₃ ceramics could be effectively achieved by adding CuO (up to 0.75 wt%). At 1350°C, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% CuO addition possess a dielectric constant (ɛ_r) of 27.6, a Q × f value of 165 000 GHz (at 9 GHz), and a temperature coefficient of resonant frequency (τ_f) of −20 ppm/°C. By comparing with pure Nd(Co_1/2Ti_1/2)O₃ ceramics, incorporating additional CuO helps to render a dielectric material with a higher dielectric constant, a smaller τ_f value, and a 20% dielectric loss reduction, which makes it a very promising candidate for applications requiring low microwave dielectric loss.     

Direct-Current Field Dependence of Dielectric Properties in Alumina-Doped Barium Strontium Titanate

The dielectric properties of (Ba_0.6Sr_0.4)TiO₃ and Al₂O₃-doped (Ba_0.6Sr_0.4)TiO₃ have been characterized. The grain size of the specimen is maximum for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 1 wt% Al₂O₃. The density and the real part of the relative dielectric constant each decrease as the Al₂O₃ content increases. The loss factor is minimum for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 2 wt% Al₂O₃. The dielectric constant of the specimens decreases as the applied dc field increases. The influence of the dc field on the loss factor is much less than that on the dielectric constant. The tunability is ∼24% for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 1 wt% Al₂O₃.     

Cryogenic Microwave Dielectric Properties of Sintered (Zr0.8Sn0.2)TiO4 Doped with CuO and ZnO

The effect of CuO on the sintering temperature, microstructure, and the microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) has been investigated. The microwave dielectric properties of the ZST ceramics have been measured at cryogenic temperatures (15–290 K). The crystallite sizes of the sintered ZST ceramics are in the 30–50-nm range. The addition of CuO effectively reduced the sintering temperature to 1300°C due to the liquid-phase effects. The addition of CuO did not cause any secondary phases up to 1.5 wt% of CuO. It is found that the quality factor ( Q ) of the sample without CuO decreased with an increase in temperature, whereas the samples with the addition of CuO up to 1.0 wt% showed less dependence on temperature at cryogenic temperatures. The microwave dielectric properties of the ZST ceramics measured at cryogenic temperatures exhibited a Q factor of 15 000 for pure ZST and 11 800 for ZST with 0.5 wt% of CuO at 15 K. The increase in Q factor at cryogenic temperatures can be attributed to the reduction in both intrinsic and extrinsic losses in the ZST ceramics.     

Low-Temperature Sintering (Ba0.6Sr0.4)TiO3 Thick Film Prepared by Screen Printing

(Ba_0.6Sr_0.4) TiO₃ thick films doped with glass slurry were fabricated by the screen-printing technique. The dielectric properties and the sintering mechanism were investigated. The films can be sintered at 600°C. The dielectric constant is 88 and the dielectric loss is 0.002 with a tunability of 23.86% under 100 kV/cm. Higher dielectric constant and tunability were obtained in the samples sintered at higher temperatures. The highest tunability is 61.12% under 150 kV/cm in the sample sintered at 800°C. The low sintering temperature and dielectric loss of the glass-doped thick films make them potential candidates for LTCC and microwave tunable devices.     

Preparation of Dense Ta2O5-Based Ceramics by a Coated Powder Method for Enhanced Dielectric Properties

A hybrid solution-coated powder method was used to reduce the sintering temperature of TiO₂-modified Ta₂O₅ ceramics. This simple and effective method allowed chemical homogenization and densification to take place below a densification-impeding phase transformation, something that has not been achieved by traditional mixed oxide processing. Combining the coated powder method with a two-stage heat treatment (by which specimens were first densified and then transformed into the high dielectric constant H-Ta₂O₅ phase) enabled the realization of dense and homogeneous 92Ta₂O₅–8TiO₂ ceramics with dielectric constant values up to 280 at room temperature.     

A New BaO-TiO2 Compound with Temperature-Stable High Permittivity and Low Microwave Loss

The dielectric properties of ceramics in the TiO₂-rich region of the BaO-TiO₂ system were investigated. In the composition range between BaTi₄O₉ and TiO₂, another compound, Ba₂Ti₉O₂₀, can be obtained when calcining and sintering conditions are controlled carefully. When dense and single-phase, this ceramic has excellent dielectric and physical properties. At 4 GHz, the dielectric K = 39.8, Q = 8000, and τ _K (temperature coefficient of dielectric constant) =−24 ± 2 ppm/°C.     

(Bi1/2Na1/2)TiO3–Ba(Cu1/2W1/2)O3 Lead-Free Piezoelectric Ceramics

(Bi_1/2Na_1/2)TiO₃ with 0–6 mol% Ba(Cu_1/2W_1/2)O₃ (BNT-BCW), a new member of the BNT-based group, has been prepared following the conventional mixed oxide route. The compacted bodies were sintered at 1130°C for 2 h to get dense ceramics. The addition of BCW into BNT ceramics facilitated the poling process because of a reduction in leakage current. 0.995BNT·0.005BCW ceramics exhibit a relatively high piezoelectric constant ( d ₃₃= 80 × 10⁻¹² C/N) and a relatively low dielectric loss (tan δ= 1.5%). Increased amount of BCW was found to increase the dielectric constant and loss of BNT-BCW ceramics and to suppress the grain growth. During sintering, some BCW diffuses into the lattice of BNT to form a solid solution and some remains on the grain boundaries.     

Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Ceramics via a Soft Mechanochemical Route

Single-phase perovskite 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN–0.1 PT) from a stoichiometric mixture of starting materials was synthesized by applying a mechanochemical technique to the stage of a precursor. A stoichiometric mixture of PbO, TiO₂, Mg(OH)₂, and Nb₂O₅ was milled for 60 min and heated at temperatures as low as 850°C for 4 h to obtain a single phase. The maximum dielectric constant of the samples from the milled mixture increased as the sintering temperature increased, with the remarkable grain growth, and attained 24600 at 1200°C. In contrast, poor densification and coexistence of the pyrochlore phase were observed on the samples from the nonmilled mixture. Further observation suggested that the pyrochlore phase concentrated near the surface during sintering and then migrated into the PbZrO₃ packing powder, leading to a pyrochlore–free phase at 1250°C. The dielectric constant of the latter ceramics was explained by the series mixing rule for the dielectric constant of a diphasic solid.     

Microwave Characteristics of (Zr, Sn)TiO4 and BaO-PbO-Nd2O3-TiO2 Dielectric Resonators

The microwave characteristics of two dielectric resonator materials were investigated. This research included (Zr, Sn)TiO₄, a material having the characteristics of a dielectric constant K = 38, Q = 7000 at 7 GHz, and temperature coefficient of resonant frequency τ _f, = 0 ppm/°C. The investigation determined the relations between the dielectric loss and micro-structures of this ceramic. Analysis by X-ray microanalyzer made it clear that the addition of Fe₂O₃ increased the dielectric loss of this ceramic because the Fe ions diffused into the grain. The other material investigated was BaO-PbO-Nd₂O₃-TiO₂, a ceramic having a dielectric constant of K = 88, Q = 5000 at 1 GHz, and τ _f= 0 ppm/°C. As this ceramic has a very high dielectric constant, it is useful for applications at frequencies <1 GHz.     

Atmosphere Control of Interface Migration and Its Effect on Dielectric Property of CuO-Infiltrated Strontium Titanate

The control of interface migration and its effect on dielectric properties of SrTiO₃ based materials have been investigated. SrTiO₃ specimens with or without Nb₂O₅ doping were prepared, and the atmospheres of sintering and heat treatment were varied during the processing. It has been demonstrated that the change in defect structure at the interface of Nb-doped SrTiO₃ grains by atmosphere change was the cause of interface migration observed during CuO infiltration or heat treatment. The interface migration caused by sintering in a reducing atmosphere and infiltration in air could be suppressed by preoxidizing grain interfaces before the CuO infiltration in air. The suppression of migration increased the effective dielectric constant of the material.     

Dielectric Properties and Reliability of Zn0.95Mg0.05TiO3+0.25TiO2 MLCCs with Different Pd/Ag Ratios of Electrodes

In order to study the micromechanism of silver migration that influences the dielectric properties and reliability of Zn_0.95Mg_0.05TiO₃+0.25TiO₂ (ZMT") with 1 wt% 3ZnO–B₂O₃ multilayer ceramic capacitors (MLCCs), various silver (Ag)–palladium (Pd) ratios of conductors were used as inner electrodes. It was found that the electrical resistance of a MLCC sample with pure Ag as inner electrodes was degraded drastically to compared with the Ag/Pd inner electrodes at measuring temperatures ranging from 25°C to 175°C. It may be explained that the pure Ag migrates easily into the dielectric layer along the grain boundary during co-firing. The ZMT" MLCCs exhibited increasing dielectric constant and insulation resistance considerably with increasing sintering temperature. Moreover, the results also indicate that Ag diffusion changes the dielectric properties and decreases the breakdown voltage. A ZMT" MLCC with a high Ag content in the inner electrode exhibits poor reliability, and the effect of Ag⁺ migration is markedly enhanced when the activation energy of the ZMT" dielectric is considerably lowered due to the excessive formation of oxygen vacancies and the semiconducting Zn₂TiO₄ phase when Ag⁺ substitutes for Zn²⁺ during co-firing.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

Effect of TiO2 on Initial Sintering of Al2O3

Alpha alumina with additions of TiO₂ sintered more rapidly than "pure" alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO₂ in Al₂O₃. For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The "solubility limit" increased with decreasing particle size, indicating an excess surface concentration of TiO₂. The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO₂ concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient.     

Effect of Internal Stress on Physical Temperature Characteristics of Cerium-Doped and Gadolinium-Doped Barium Titanate Ceramics

The effect of internal stress on the physical properties (the temperature dependence of the dielectric constant and the lattice constant, and the diffuseness (gamma)) of solid solutions of 0.97BaTiO₃-0.03Gd₂O₃:1.5TiO₂ and 0.97BaTiO₃-0.03CeO₂:1.5TiO₂ was investigated using various particle sizes of the starting BaTiO₃, pressure-dielectric spectroscopy, and high-pressure X-ray analysis. The grain size was almost proportional to the particle size of the starting BaTiO₃ and was inversely proportional to the internal stress. A sudden change in internal stress occurred at grain sizes of ∼2 µm for cerium-doped BaTiO₃ and ∼3 µm for gadolinium-doped BaTiO₃. With an increase in external pressure and a decrease in grain size, the lattice anomalies of the cubic-to-tetragonal transformation and two other factors (the tetragonal-to-orthorhombic and orthorhombic-to-rhombohedral transformations) shifted to lower and higher temperatures, respectively. The temperature dependencies of the dielectric constant and the lattice constant were less sensitive to external pressure in cerium-doped core-shell-grained BaTiO₃ than in gadolinium-doped homogeneous-grained BaTiO₃; this difference was attributable to internal stress. In cerium-doped BaTiO₃, gamma was related to internal stress.     

Low-Fired (Zn,Mg)TiO3 Microwave Dielectrics

A dielectric ceramic comprised of (Zn_{1- x} Mg _x )TiO₃ ( x = 0 to x = 0.5) with low sintering temperature and promising microwave properties was prepared by applying a semichemical synthesis route and a microbeads milling technique. X-ray diffractometry and thermal analyses results indicated that the phase stability region of the hexagonal (Zn,Mg)TiO₃ extended to higher temperatures as the amount of magnesium increased. The dielectric properties in this system exhibited a significant dependence on the sintering conditions, especially near the phase decomposition temperature. From 950°C, the temperature compensation characteristics occurred as the phase composition changed from hexagonal (Zn,Mg)TiO₃ to two phases: (Zn,Mg)₂TiO₄ and rutile. The magnesium content for zero temperature coefficient (tau_f) was ~3 mol% at 950°C; however, tau_f increased with the sintering temperatures because of the shift of the decomposition temperature.     

Titania as a Sintering Additive in Indium Oxide Ceramics

The influence of TiO₂ additives on the sintering behavior of In₂O₃ ceramics has been investigated. TiO₂ increases the densification rate, decreases the grain growth during the intermediate stage of sintering, and hinders the pore/boundary breakaway that can affect the final stage of sintering. For a given grain size, TiO₂ shifts the grain size/density trajectory toward higher densities. TiO₂ mainly acts by a second-phase mechanism, but it also may decrease the decomposition rate of In₂O₃.     

Grain Growth and Phase Transformation of Titanium Oxide During Calcination

Additions of NiO, CoO, MnO₂, Fe₂O₃, and CuO promote the anatase-rutile transformation and grain growth of TiO₂. Additions of Na₂O and WO₃ retard the transformation and have no effect on the grain growth. The addition of MoO₃ strongly promotes the grain growth but has only a slight effect on the phase transformation. Both the grain growth and transformation are promoted slightly by Cr₂O₃. The transformation is significantly affected by the method of preparing TiO₂. The grain size of TiO₂ heated in H₂ exceeds that of TiO₂ heated in O₂, in air, in argon, and in vacuum. The rate of transformation decreases with an increase in the partial pressure of oxygen of the atmosphere.     

Microstructures of SrTiO3 Internal Boundary Layer Capacitors During and After Processing and Resultant Electrical Properties

The microstructure of strontium titanate internal boundary layer capacitors at various stages in their processing was studied by transmission electron microscopy of rapidly quenched and normally cooled samples. Compositions containing excess TiO₂, Al₂O₃, and SiO₂ have a completely wetting liquid phase at the sintering temperature; during cooling Ti_nO_{2 n −1}, Magneli phases precipitate at multiple grain junctions. Diffused metal oxides and flux (Bi₂O₃, PbO, CuO, and B₂O₃) rapidly penetrate as a liquid phase along boundaries in postsintering heat treatment. This liquid phase disappears during slow cooling.