Effect of Composition on the Oxygen Tracer Diffusion in Transparent Yttrium Aluminium Garnet (YAG) Ceramics

The grain boundary structure and oxygen tracer diffusion in transparent yttrium aluminum garnet (YAG) ceramics varying from 2% excess of Y₂O₃ to 0.5% excess of Al₂O₃ were studied. The characterization of the specimens is as follows: (i) For the Y₂O₃-excess specimen, a second phase (yttrium aluminum perovskite: YAP) containing silicon in the grain boundary was found, (ii) For the Al₂O₃-excess specimen, both aluminum-rich particles (alumina) and a silicon-rich segregant layer were observed in the grain boundary. The volume diffusion of the oxygen tracer is little influenced by the excess composition. In contrast, the grain boundary diffusion of the oxygen tracer is suppressed in the Y₂O₃-excess specimens, compared to Al₂O₃-excess specimens. These differences are thought to result from the chemical reaction between the second phase and the intergranular liquid phase during the sintering.     

Effect of Oxygen Partial Pressure on the Dielectric Properties and Microstructures of Cofired Base-Metal-Electrode Multilayer Ceramic Capacitors

The dielectric properties, dopant distributions, and microstructures of BaTiO₃-based multilayer ceramic capacitors (MLCCs) sintered in H₂–N₂–H₂O atmospheres with     =10^−7.5 Pa (BMX-7.5) and     =10^−9.5 Pa (BMX-9.5) were studied, and the effects of oxygen partial pressures were analyzed. Dielectric measurements showed that BMX-7.5 had a lower dielectric constant at temperatures above 20°C, but a higher dielectric constant at temperatures below 10°C when compared with BMX-9.5. The coexistence of core–shell and core grains was observed in bright field (BF) transmission electron microscopy images in both types of capacitors. Triple-point and grain boundary phases were observed more frequently in BMX-9.5 than in BMX-7.5, and energy-dispersive X-ray spectrometer point-by-point analysis revealed that these second phases contained high concentrations of dopants such as Si, Y, and Ca. The dopant concentration in the shell regions in BMX-7.5 was higher than that in similar regions in BMX-9.5. Smeared and twisted grain boundaries with fringes observed in both types of MLCCs indicated that the shell regions in both samples were formed either by diffusion of foreign ions into BaTiO₃ or by crystallization of grain boundary and triple-point liquid phases. It was deduced that the partial pressure of oxygen in the sintering atmosphere influenced the microstructures, dopant distributions, and core–shell ratios of the grains in these materials.     

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.     

Chemical Reactions in the Processing of MoSi2 Carbon Compacts

Hot-pressing of MoSi₂ powders with carbon at high temperatures reduces the siliceous grain boundary phase in the resultant compact. The chemical reactions in this process were examined using the Knudsen cell technique. A 2.3 wt% oxygen MoSi₂ powder and a 0.59 wt% oxygen MoSi₂ powder, both with additions of 2 wt% carbon, were examined. The reduction of the siliceous grain boundary phase was examined at 1350 K and the resultant P(SiO)/P(CO) rations interpreted in terms of the SiO (g) and CO (g) isobars on the Si-C-O predominance diagram. The MoSi₂+ carbon mixtures were then heated at the hot-pressing temperature of 2100 K. Large weigh losses were observed and could be correlated with the formation of a low-melting eutectic and the formation and vaporization of SiC.     

Discontinuous Dissolution of Iron Aluminate Spinel in the Al2O3–Fe2O3 System

When sintered 85Al₂O₃–15Fe₂O₃ (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO₃ and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe₂O₃ from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.     

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.     

Structure, Properties, and Impedance Spectroscopy of CaCu3Ti4O12 Ceramics Prepared by Sol–Gel Process

CaCu₃Ti₄O₁₂ (CCTO) ceramics with high dielectric constant (2–4 × 10⁴) and low loss (0.04) were prepared by the sol–gel process and sintered at 1050°C for different times. The sintering time has a sensitive influence on the values of the dielectric constant and nonlinear coefficient. Tailored dielectric constant and nonlinear coefficient can be obtained by selecting a suitable sintering time according to different desired device application. The result of current–voltage characteristics and Cole–Cole plots in a broad temperature range (60–400 K) provide more effective evidence of the high dielectric constant supported by the grain boundary barrier layer (GBBL) capacitors model. Below 150 K, the GBBL capacitors effect weakens and gradually disappears with further decrease of temperature, thus leading the dielectric constant to decrease rapidly. Two values of grain activation energy acting at different temperature for each sample were obtained.     

Discontinuous Dissolution and Grain-Boundary Migration in Al2O3-Fe2O3 by Oxygen Partial Pressure Change

When sintered 95Al₂O₃-5Fe₂O₃ (wt%) specimens constituting corundum grains and iron aluminate spinel precipitates were annealed under high oxygen partial pressure (P_o2) where only a corundum phase is stable, fast dissolution of particulate spinel precipitates occurred, together with the migration of corundum grain boundaries. Behind the migrating boundaries, a corundum solid solution enriched with Fe₂O₃ formed. Discontinuous dissolution (DD) of particulate spinel precipitates thus occurred by P_o2 increase. In contrast, when 95Al₂O₃-5Fe₂O₃ specimens constituting only corundum grains were annealed under low P_o2 where both corundum and spinel phases are stable, grain boundaries migrated without spinel precipitation, leaving behind a corundum phase depleted of Fe₂O₃, similar to chemically induced grain-boundary migration (CIGM) observed during solute depletion. The volatilization of Fe₂O₃ appeared to cause the boundary migration without precipitation. The observed CIGM and DD would suggest various possibilities of microstructure control in other oxide systems through oxygen partial pressure change.     

Effect of PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.     

Resistance Degradation in Y(Cr,Mn)O3–Y2O3 Composite NTC Ceramics in Hostile Environments

A series of composite, negative temperature coefficient (NTC) ceramics were carefully processed with compositions based on the Y(Cr,Mn)O₃+Y₂O₃ system and these were investigated for resistance stability in hostile environments. This specific system is of interest for high-temperature automobile thermistors, however either through the processing or in use of these, materials can be exposed to reducing atmospheres at temperatures around 900°–1000°C. The thermochemical processes at intermediate temperatures and low     <10⁻¹⁰atm can influence the resistance of the given ceramics. Through an impedance analysis it is determined that the resistance increase is associated primarily with a grain boundary resistance increase. The grain and grain boundary elements are modeled through parallel constant phase element and resistance equivalent circuits connected in series. Possible origins of the defect chemistry being controlled through high-temperature processes at the sintering are partial Schottky reactions that are compensated through a superoxidation reaction on cooling and aging. The reduction process reversed the superoxidation reaction and transited the grain boundary surfaces to ionically compensated B-site vacancies with oxygen vacancies.     

Cubic-Formation and Grain-Growth Mechanisms in Tetragonal Zirconia Polycrystal

The microstructure in Y₂O₃-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°–1500°C was examined to clarify the role of Y³⁺ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y₂O₃ concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grain-boundary faces in Y-TZP and Y³⁺ ions segregated at their grain boundaries over a width of ∼10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y³⁺ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y³⁺ ions segregating along the grain boundary, respectively.     

Hot Isostatic Pressing of Si3N4 Powder Compacts and Reaction-Bonded Si3N4

Hot isostatically pressed silicon nitride was produced by densifying Si₃N₄ powder compacts and reaction-bonded Si₃N₄ (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X-ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si₃N₄ starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.     

The Role of TiN in the Intergranular Phase-Forming Process in TiN-Dispersed Si3N4 Nanocomposites

Internal interfaces in two ceramic systems, monolithic Si₃N₄ (SN) and TiN-dispersed Si₃N₄ nanocomposite (STN), were characterized by analytical transmission electron microscopy (TEM). In monolithic SN both MgO and Y₂O₃ dopants are preferentially hosted by the vitreous intergranular phase in pockets at triple grain junctions (TJ), whereas in STN composites the highest dopant concentrations were observed in grain and phase boundaries. The width of grain boundary films, as revealed by high-resolution TEM imaging, varied between ≈0.8 nm in monolithic SN and ≈1.0–1.2 nm in STN. Intergranular films with increased width ≈1.8 nm were detected in SN–TiN phase boundaries. Although no enrichment of Ti could be detected in the intergranular phase, it appears that the presence of TiN dispersants indirectly contributes to the intergranular phase formation. It is assumed that TiO₂ impurities sitting on TiN particle surfaces react with the matrix phase, resulting in a more oxidic nature of intergranular films due to increased SiO₂ supply in intergranular regions. Phase-specific Si-L_2,3 energy-loss near edge structure features, which could serve as fingerprints for phase identification, were observed in spatial-difference electron energy-loss spectra from grain boundary films and TJ pockets.     

Influence of Microstructure on the Electrical Properties of Iron-Substituted Calcium Titanate Ceramics

CaTi_0.8Fe_0.2O_3–δ ceramics with grain sizes that varied from 2 to 10 μm were obtained and studied using SEM, TEM, Mössbauer spectroscopy, impedance spectroscopy, and electrochemical oxygen permeability measurements. Smaller grains developed a core–shell microstructure that consisted of a pure CaTiO₃ core and an iron-rich microdomain structure at the shell. The effect of grain size on electronic conductivity was negligible. The ionic conductivity was higher for the ceramics with core–shell grains, which suggested that fast oxygen transport along microdomain walls may have occurred. For the homogeneous ceramics, the ionic conductivity decreased with decreased grain size, in which case the grain boundary represented an additional resistance, probably because of the depletion of oxygen vacancies.     

Sintering Temperature Dependence of Grain Boundary Resistivity in a Rare-Earth-Doped ZnO Varistor

We present a rare-earth-doped ZnO ceramic with nonohmic electrical properties. Analysis of the microstructure and composition indicates that the ceramic is composed of the main phase of ZnO and the second phase of rare-earth oxides (e.g., Dy₂O₃, Pr₆O₁₁, Pr₂O₃). The average grain size is markedly increased from 3 to 18 μm, with an increase in the sintering temperature from 1150° to 1350°C. The corresponding varistor voltage and nonlinear coefficient decrease from 1014 to 578 V/mm, and from 15.8 to 6.8, respectively. The resistivity of grain and grain boundary evaluated by the complex impedance spectrum indicates that the resistivity of the grain is approximately constant (∼10³Ω), and the resistivity of the grain boundary decreases. The relative dielectric permittivity of the sintered ceramic samples is much larger than that of pure ZnO ceramic, which should be ascribed to the internal boundary layer capacitance effect.     

Al2O3/Ni Laminar Ceramics Shaped by Tape Casting and Electroless Plating

Tape casting and electroless plating were used to fabricate Al₂O₃/Ni laminar ceramic composites with close control of the thickness of the Al₂O₃ and Ni layers. Ninety-seven percent relative density, macrodefect-free composites were obtained by spark plasma sintering. In electroless plating solutions, the stable potential of grain boundary led to the first deposition of nickel on the grain boundary of Al₂O₃. Scanning electron microscopy, energy-dispersive X-ray, and X-ray diffraction were used to analyze the structure, elements distribution, and phase composition of the Al₂O₃/Ni laminar composites.     

Degradation Mechanism of Dielectric Loss in Barium Niobate Under a Reducing Atmosphere

We developed multilayer ceramic capacitors (MLCCs) of the type negative–positive zero (NP0) cofired with Cu inner electrodes using novel low-loss barium niobates. The excellent dielectric properties of ceramics and compatibility between electrodes and dielectrics under a reducing atmosphere led to low loss character in the Cu-MLCCs, even in the high-frequency single-layer capacitors. We also examine the atmospheric dependence on dielectric loss of barium niobates. The origin of degradation in dielectric loss under a highly reducing atmosphere is explained by the association of two loss mechanisms: the oxygen vacancies in NbO₆ octahedra and the subsequent formation of a reduced second phase: Ba₃Nb⁴⁺Nb⁵⁺₄O₁₅.     

Secondary Grain Growth of Li2O-A12O3-SiO2-TiO2 Glass-Ceramics

The kinetics of secondary grain growth in a Ti0₂-nucleated β-spodumene solid-solution glass-ceramic was studied. The thermal stability of the grains was excellent. Grain growth followed the cube-root-of-time law. The activation energy of the grain boundary migration was 55 ± 10 kcal/mol. Grain growth inhibition due to Ti0₂ precipitates and the residual glassy phase was closely examined. The excellent thermal stability of the grains is due to grain growth inhibition by the residual glassy phase, not by rutile precipitates. It is suggested that the diffusion of A²⁺, and probably the simultaneous diffusion of Li⁺, through the residual glass is the rate-limiting process for the grain boundary migration.     

High-Temperature Deformation and Microstructural Analysis for Silicon Nitride–Scandium (III) Oxide

Limited past studies have indicated that Si₃N₄ doped with Sc₂O₃ may exhibit high-temperature mechanical properties superior to Si₃N₄ systems with various other oxide sintering additives. High-temperature deformation of this system was studied by characterizing the microstructures before and after deformation. It was found that elements of the additive, Sc and O, exist in small amounts at thin grain boundary layers and within secondary phases at triple and multiple grain boundary junctions. The secondary phase is devitrified as crystalline Sc₂Si₂O₇. Deformation of the samples was dominated by cavitation rather than dislocation processes. Thus, the excellent deformation resistance of the samples at high temperature can be attributed to the high refractories and enhanced crystallization of a secondary phase.     

Silicon Nitride Based Ceramic Nanocomposites

Nanocomposites (Si₃N₄/SiC) were studied by combined high-resolution transmission electron microscopy and electron energy-loss spectroscopic imaging (ESI) techniques. In ESI micrographs three types of crystalline grains were distinguished: Si₃N₄ matrix grains (0.5 μΩ), nanosized SiC particles (<100 nm) embedded in the Si₃N₄, and large SiC particles (100–200 nm) at grain boundary regions (intergranular particles). Amorphous films were found both at Si₃N₄ grain boundaries and at phase boundaries between Si₃N₄ and SiC. The Si₃N₄ grain boundary film thickness varied from 1 to 2. 5 nm. Two kinds of embedded SiC particles were observed: type A has a special orientation with respect to the matrix, and type B possesses a random orientation with respect to the matrix. The surfaces of type B particles are completely covered by an amorphous phase. The existence of the amorphous film between the matrix and the particles of type A depends on the lattice mismatch across the interface. The mechanisms of nucleation and growth of Ω-Si₃N₄ grains are discussed on the basis of these experimental results.