Koichiro Morita

Superplasticity in alumina enhanced by co-dispersion of 10% zirconia and 10% spinel particles

Abstract Superplastic deformation behavior is examined for Al 2 O 3 -based ceramics dispersed with 10 vol% ZrO 2 and 10 vol% spinel (MgO·1.3 Al 2 O 3 ) particles. The multiple-phase dispersion considerably decreases the rate of grain growth during deformation, leading to enhanced superplasticity (larger tensile elongation and higher strain rate). Maximum tensile elongation reaches 850% at a strain rate of 5.0×10 −4 s −1 and at 1500°C. Grain growth during deformation is found to follow a theoretical model based on a grain boundary diffusion mechanism. The creep parameters corrected for concurrent grain growth are 2.2 as the stress exponent, 3.2 as the grain size exponent and 751 kJ/mol as the activation energy. Spherical ZrO 2 particles embedded in elongated Al 2 O 3 grains in deformed specimens suggest that the deformation mechanism of the present material is strongly related to grain boundary diffusion. Being different from other superplastic aluminas, cavities in the present material tended to grow in the direction parallel to the stress axis.

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.

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.

Thermally Stimulated Current (TSC) Studies on Resistance Degradation of Ni-MLCC

To avoid the influence of spontaneous polarization, thermally stimulated current (TSC) measurements of BaTiO3-based multilayer ceramic capacitors with Ni internal electrodes were performed within the paraelectric phase. We observed two TSC peaks and inferred that one of them was related to the resistance degradation according to the electromigration of oxygen vacancies. Further, we proposed that the two peaks resulted from the relaxation phenomenon of the oxygen vacancies, which migrated under dc electrical field stresses, in the grains and over the grain boundaries.

Degradation of Resistance over Time for Multilayer Ceramic Capacitors

The lifetime determination model for multilayer ceramic capacitors (MLCCs) is discussed. The accumulation of oxygen vacancies on the cathode/ceramics interface by an electro-migration process is a concept accepted by many researchers. However, the lifetimes and leakage currents measured during a highly accelerated lifetime test (HALT) could not be explained by this concept. To investigate the mechanism, we used a polarity reversal method during the HALT, which provided information on the dominant process for the leakage current. Thermally stimulated current (TSC) measurement provided the relative number of oxygen vacancies both on the cathode/ceramics interfaces and the grain boundaries. Moreover, the microstructure of the MLCC samples was evaluated by both electric property measurements and direct observation. From these results, we concluded that the grain boundaries controlled the leakage current as well as the lifetime.

Relationship between Microstructural Evolution and Electrical Properties in Ba(Ti, Zr)O3-Based Materials for Ni-MLCC

The influence of the microstructural evolution induced by increasing firing temperature on the change in dielectric properties induced by re-oxidation treatment in the Ba(Ti, Zr)O3(BTZ)-Ho-Mn system was investigated. The microstructural observation of as-fired disk samples revealed gradual grain growth with firing temperature. From the temperature characteristic (TC) measurement, the shift to higher temperatures of dielectric maximum (Tmax) due to re-oxidation treatment became larger with grain growth although the chemical compositions were quite the same. It was found that grain growth accelerated the valence change of Mn by re-oxidation treatment and the substitution of dopants in a grain in this system. Microstructural evolutions, such as compositional distribution and the concentration of additives in a grain, had an influence on the changes in dielectric properties induced by re-oxidation treatment.

Crack-Like Cavitaton in a 0.3 wt% SiO2–Doped Tetragonal Zirconia

The microstructural evolution at grain boundaries during constant stress tensile creep at 1673 K is examined in a fine grained yttria-stabilized tetragonal zirconia (Y-TZP) doped with 0.3 wt% SiO 2 , where crack-like cavitation degrades superplasticity. A glass phase, which is not detected before deformation, is shown to precipitate at multiple grain junctions with a decrease in grain boundary area duc to concurrent grain growth. The resultant glass pockets occur more frequently near the front of the crack-like cavities and join each other through channels filled with the glass phase. Furthermore, some of them enclose cavity nuclei. From these observations, it can be concluded that the glass pockets act as the site of nucleation and growth of the crack-like cavities. Acceleration both in the glass pocket precipitation and in crack-like cavitation accompanying an increase in the initial grain size also supports this conclusion.

Fabrication of Composition-Graded K(Nb,Ta)O3 Crystals and Their Dielectric Properties

Composition-graded crystals of K(Ta,Nb)O 3 solid solution were prepared by non-equilibrium cooling of melts. The size of the crystals was 0.5 - 2.0 mm. The texture, composition distribution and temperature dependence of dielectric properties were characterized by X-ray diffraction, electron-microprobe analysis, polarizing optical microscopy and impedance analysis. The core was rich in Ta content which decleased gradually in the crystals from the core to the margin, while Nb content increased gradually from the core to the margin. The dielectric properties reflected the composition gradient which depended on the cooling process. The crystals obtained with a higher cooling rate exhibited smaller temperature dependence of dielectric constant.