Hirokazu Chazono

Base-Metal Electrode-Multilayer Ceramic Capacitors: Past, Present and Future Perspectives

Multilayer ceramic capacitor (MLCC) production and sales figures are the highest among fine-ceramic products developed in the past 30 years. The total worldwide production and sales reached 550 billion pieces and 6 billion dollars, respectively in 2000. In the course of progress, the development of base-metal electrode (BME) technology played an important role in expanding the application area. In this review, the recent progress in MLCCs with BME nickel (Ni) electrodes is reviewed from the viewpoint of nonreducible dielectric materials. Using intermediate-ionic-size rare-earth ion (Dy2O3, Ho2O3, Er2O3, Y2O3) doped BaTiO3 (ABO3)-based dielectrics, highly reliable Ni-MLCCs with a very thin layer below 2 µm in thickness have been developed. The effect of site occupancy of rare-earth ions in BaTiO3 on the electrical properties and microstructure of nonreducible dielectrics is studied systematically. It appears that intermediate-ionic-size rare-earth ions occupy both A- and B-sites in the BaTiO3 lattice and effectively control the donor/acceptor dopant ratio and microstructural evolution. The relationship between the electrical properties and the microstructure of Ni-MLCCs is also presented.

dc-Electrical Degradation of the BT-Based Material for Multilayer Ceramic Capacitor with Ni internal Electrode: Impedance Analysis and Microstructure

The impedance of a BaTiO3 (BT)-based multilayer ceramic capacitor with a nickel internal electrode (Ni-MLCC) was investigated by measuring the frequency domain at various temperatures. All the obtained impedance data could be successfully fitted to a 4-RC section electrical equivalent network. The 4-RC section electrical equivalent network was successfully correlated to the microstructure: the core, the shell, the grain boundary, and the ceramic/internal electrode interface regions. Based on this electrical equivalent network, the electrical properties including the Curie-Weiss law, the current-voltage characteristics, and dc electrical degradation, were well explained. A model for the degradation behavior for BT-based Ni-MLCC with thin active layer thickness was proposed.

X7R Multilayer Ceramic Capacitors with Nickel Electrodes

Electrical properties and microstructures of a holmium-doped (Ba1.01Mg0.01)O1.02(Ti0.98Zr0.02)O2 system were studied. Additions of Ho2O3 had little effect in preventing the dielectrics from reducing at high temperature, but the resistivity at low temperature increased with increasing amount of Ho2O3 when treated in oxidizing atmosphere at the cooling stage. From transmission electron micrograph (TEM) observation, it was noted that the microstructure exhibited a grain core-grain shell structure. Newly developed X7R multilayer ceramic capacitors with Ni electrodes revealed highly reliable electrical properties.

Dielectric Properties of BaTiO3-Based Ceramics under High Electric Field.

The dielectric properties under a high electric field (ac-field) of BaTiO3-based ceramics with core grains, shell grains and core-shell grains were compared with those of multilayer ceramic capacitors (MLCCs) with these three kinds of grains. The MLCCs with the X7R specification had a core-shell structure, and the relative dielectric permittivity (er) of the dielectric layers in the MLCCs increased with increasing ac-field. Similar behavior was observed in the MLCCs consisting of only cores, indicating that the core predominantly determined the dielectric properties of MLCCs under high ac-fields. The dielectric properties of MLCCs and ceramic plates consisting of only shell grains showed that the shell was the relaxor ferroelectrics. A slight change in the shell composition yielded a large shift of the peak temperature of er. The shell improved the temperature stability of er at low temperatures under low ac-fields. In a ceramic plate with relatively large BaTiO3 grains (approximately 3 µm), the maximum er was observed at a moderate ac-field, which was explained from the electric displacement vs electric field hysteresis curves of ferroelectric BaTiO3. The MLCCs and ceramics plates with fine BaTiO3 grains (0.4 to 0.5 µm) showed similar dielectric behavior to the MLCC with the core-shell structure. The size effect of BaTiO3 played an important role in determining the temperature stability of er. For future MLCCs with very thin dielectric layers, a microstructure with fine BaTiO3 grains and grain boundary layers of the shell was proposed.

Dielectric properties of BaTiO 3 –BaZrO 3 ceramics under a high electric field

Multilayered ceramic capacitors (MLCCs) with BaTiO 3 –BaZrO 3 (BTZ) dielectric layers were fabricated, and the dielectric permittivity of the BTZ layers with different thicknesses in MLCCs was measured. The dielectric permittivity of the BTZ ceramic disk was also measured under various ac electric fields. The variation in the dielectric behaviors with the thickness of BTZ layers in MLCCs was explained by the ac-field dependence of dielectric permittivity observed in the BTZ ceramic disk. The ac-field dependence of dielectric permittivity of BTZ was markedly observed below the temperature of a broad maximum in the dielectric permittivity versus temperature (є versus T ) curve. It was found that the temperature of the broad maximum shifted to the low-temperature side and the peak shape became asymmetric with increasing ac field. These changes in the dielectric properties under high ac fields were explained by a model of relaxors with the concept of the formation of polar microregions (PMRs) and the freezing of fluctuating ipoles in PMRs.

INFLUENCE OF THE MICROSTRUCTURE EVOLUTION ON ELECTRICAL PROPERTIES OF MULTILAYER CAPACITOR WITH NI ELECTRODE

The influence of the microstructure evolution on electrical properties as a parameter of the firing temperature was studied for materials in the BaTiO3(BT)–MgO–Ln2O3 (Ln=Ho and Dy) system. The sintering behavior and the formation of the core-shell structure were dependent on the kind of doped rare earth elements. The stability of the core-shell structure and electrical properties of Dy doped specimens against the firing temperature were much lower than those of the Ho doped specimens. Especially, the Dy doped disk specimens fired at more than 1320°C, in which the core-shell grains were destroyed as judged by the differential scanning calorimetry (DSC) measurement and the transmission electron microscopy (TEM) observation, exhibited characteristic electrical properties. The electrical properties of the Ho doped multilayer capacitor (MLC) specimen were superior to those of the Dy doped one. It was found that the microstructure had a definite influence on the electrical properties, such as the temperature dependence of the dielectric constant and the capacitance aging behavior under an unloaded field.

Effect of Mn Addition on dc-Electrical Degradation of Multilayer Ceramic Capacitor with Ni Internal Electrode

The effect of Mn addition on the microstructure and electrical properties, especially on the dc-electrical degradation, of the X7R-type multilayer ceramic capacitor with Ni internal electrode (Ni-MLCC) with thin active layers was investigated. As the amount of Mn increased, grain growth was suppressed, and the temperature characteristic (TC) curve was flattened. I–V characteristic measurements revealed that nonlinearity coefficient (α) at a high electric field of more than 10 V/µm was decreased, and the lifetime during the highly accelerated lifetime testing (HALT) under 20 V/µm was improved, as the Mn content increased. It was found that Mn addition caused the change of the electrical properties of the grain boundary (GB). The effect of Mn on dc-electrical degradation during HALT was investigated by introducing impedance measurement at elevated temperatures from the microstructural view point. The roles of Mn on dc-electrical degradation during HALT were proposed.

Mechanism of Capacitance Aging under DC Electric Fields in Multilayer Ceramic Capacitors with X7R Characteristics

Capacitance aging under DC electric fields has been studied on multilayer ceramic capacitors (MLCCs) with X7R characteristics. The capacitance change with time was divided into two consecutive stages. The first stage was due to the nonlinear permittivity of dielectrics and it should not be involved in the aging phenomenon. The second stage depended on the MnO content, grain size and firing conditions of the dielectrics. The aging was markedly depressed above the Curie temperature. From the behavior of the second stage, it was concluded that the capacitance aging is caused by the 90° domain switching in BaTiO3 in the core. An equation that represents the aging behavior in the second stage was presented and the mechanism that explains the capacitance aging was proposed. The first stage was due to the nonlinear permittivity and the second stage was due to the domain switching, but the domain switching was also included in the first stage if the domain walls were moved by the first application of a DC field. The change in the aging behavior with the intensity of the DC field could be explained by separating capacitance change into that due to the nonlinear permittivity and that due to the domain switching according to the mechanism proposed in this study. Finally, a methodology for handling the capacitance aging under DC fields was proposed.

Effect of milling process on core-shell microstructure and electrical properties for BaTiO3-based Ni-MLCC

Abstract The effect of the process parameter in the milling process on the microstructural evolution and the electrical properties of multilayer ceramic capacitor (MLCC) samples were investigated in the BaTiO 3 (BT)–Ho 2 O 3 –MgO system. The microstructure for MLCC samples fired at 1320°C was dependent on the degree of damage for BT given by the milling process, judging from the field emission scanning electron microscopy observation and differential scanning calorimetry measurement (DSC). The mean grain size ( D 50 ) determined from the chemically etched samples decreased as the damage increased. The endothermic peak of DSC profile at around 125°C was broadened and the peak area decreased as the damage increased. Furthermore, the electrical properties were dependent on the degree of damage. The dielectric constant for MLCC samples decreased and the peak of dielectric constant at around room temperature shifted to a higher temperature as the damage increased. It was found that the MLCC sample showed the small leakage current and long mean lifetime as the degree of damage increased.

Electric Conduction of Thin-Layer Ni-Multilayer Ceramic Capacitors with Core–Shell Structure BaTiO3

The electric conduction mechanism for multilayer ceramic capacitors with Ni internal electrodes (Ni-MLCCs) was investigated, utilizing impedance spectroscopy (IS) and thermally stimulated current (TSC) measurement techniques. A modified 4RC equivalent circuit model was proposed to analyze the IS data for the Ni-MLCCs. This model revealed that electrode/ceramics interfaces (E/C-I) and grain boundaries (GBs) have a Schottky type conduction mechanism controlling the leakage behavior at low electric field. The Schottky barrier height at E/C-I and surface level height at GB were calculated being 1.43 and 1.06 eV, respectively. The Ni-MLCCs showed a tunneling conduction occurs with high dc electric fields of more than 10 V/µm. The onset electric field for the tunneling conduction shifted toward high electric fields as the Mn content of the capacitors increased. TSC measurements revealed that a low Mn content resulted in high mobile oxygen vacancies concentration in the Ni-MLCCs. Mn also played a role in preventing oxygen vacancies from migrating to cathode electrodes, which resulted in a long lifetime for the Ni-MLCCs.