Tomoya Hagiwara

Effects of Microstructure on the Curie Temperature in BaTiO3–Ho2O3–MgO–SiO2 System

In response to the demand for multilayer ceramic capacitors (MLCCs) with stable capacitance in a wide temperature range, a material with high Curie temperature (Tc) has recently been being developed. In this study, we investigated the effects of microstructure on the Tc for the BaTiO3–Ho2O3–MgO–SiO2 system with various Ho and Si contents. As the Ho/Si ratio increased, the secondary phase (Pyrochlore) increased; further, the tetragonality of the BaTiO3 phase at 125 °C increased, and the Tc shifted toward higher temperatures. A transmission electron microscope equipped with energy dispersive X-ray spectrometer (TEM–EDS) analysis revealed that the core-shell structure is the key to understand this Tc shift: a thin shell with a high concentration of Ho was the most promising microstructure for a high-Tc material in this composition system. We discussed the mechanism of the Tc shift from the viewpoints of both microstructure and crystal structure.

Effect of Ho/Mg Ratio on Formation of Core-shell Structure in BaTiO3 and on Dielectric Properties of BaTiO3 Ceramics

Ceramics were prepared from hydrothermally-synthesized BaTiO3 (100 mol%), BaSiO3 (1.5 mol%), MnO (0.1 mol%) with Ho2O3 (0.5 mol%) and MgO (0.25–1 mol%). The compositions with Ho/Mg molar ratios of 4:4, 4:2, 4:1 were selected for examination. The ceramic samples were sintered at 1340°C (PO2<10-9 Pa) and, then, annealed at 1000°C (PO2<30 Pa). The distribution of Ho and Mg in the samples was examined by transmission electron microscope (TEM) equipped with an element analyzer (EDS). Measurements of temperature dependence of dielectric permittivity over -55–155°C showed that the sample with a Ho/Mg ratio of 4/4 showed the least change of dielectric permittivity with temperature compared to the sample with a high Ho/Mg ratio of 4/1. The material with a Ho/Mg ratio of 4/1 is characterized by the deepest Ho diffusion into BaTiO3 and the slowest decrease of Ho concentration that is considered to result in a formation of a shell with considerable amounts of low-concentrated Ho-containing solid solutions.

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.

Polarization System and Phase Transition on (Li,Na,K)NbO3 Ceramics

The polarization system and phase transition on Li0.055(Na0.50K0.50)0.945NbO3 (LNKN) ceramics were investigated to determine the piezoelectric properties, relative permittivities and X-ray diffraction (XRD). The piezoelectric properties and relative permittivities were strongly dependent on the poled state, which poling temperatures were 25 or 150 °C. The XRD did not exhibit orthorhombic–tetragonal phase transition around the phase transition temperature because no orthorhombic phase was observed below the phase transition temperature. The LNKN ceramics had multiple poled states, the origin of the multiple poled states was considered to be attributable to the multiple spontaneous polarization (Ps) direction on perovskite structure. From these results, we proposed that the phase transition on LNKN was not an orthorhombic–tetragonal phase transition but a monoclinic-tetragonal phase transition. The monoclinic perovskite structure [Z=1, Pm, a=3.9825(5) A, b=3.9438(4) A, c=4.0217(5) A, β=90.292(3)°] can provide multiple Ps directions, such as [101]pc and [001]pc.

Piezoelectric Properties of (Li, Na, K)NbO3 Ceramics with Monoclinic System

The Relationship between the Piezoelectric Properties and Crystal Lattice Deformation and Distortion Resulting from the Li Substitution in LiXNa0.52K0.48-XNbO3, Alkaline Niobate-Based Perovskite Ceramics of (Li, Na, K)NbO3, Was Investigated. the Lattice Parameters of the Sample with a Li Content X = 0.05 and Having a Monoclinic System with Space Group PM Were a = 3.9789(6) Å, B = 3.9385(5) Å, C = 4.0134(6) Å, and B = 90.305(4)º. the ET33/e0, Kr, and -D31 Values of the Sample by Poling in the Monoclinic System Were 450, 44.6%, and 57 PC/N, Respectively. on the other Hand, the ET33/e0, Kr, and -D31 Values of the Sample by Poling in the Tetragonal System Were 600, 38.7%, and 56 PC/N, Respectively. the Remarkable Piezoelectric Properties of these Ceramics Can Be Attributed only to the Low Symmetry of their Monoclinic System, which Is a Subgroup of aMm2 and P4mm. Li0.05Na0.52K0.43NbO3 Ceramics with a Monoclinic System Were Observed to Have the Unique Piezoelectric Characteristics.

Occupational sites of sm in BaTiO3 analyzed by rietveld method and EXAFS

BaTiO 3 solid solutions containing rare-earth elements, which are used for capacitors with multi-layer structure, were prepared and their crystal structure was examined in order to determine preferential solubility site of rare-earth ions in perovskite structure. Since the ionic radius of Sm is in an intermediate range between those of Ba and Ti, both A and B-site replacements can be considered to occur. Rietveld refinement analysis and extended X-ray absorption fine structure (EXAFS) analysis using synchrotron data were carried out to obtain the knowledge on the occupation site in the BaTiO 3 perovskite structure. These results proved that Sm is soluble in both A and B sites.

Solubility of Ho Ions in Ho and Mg Co-doped BaTiO3 Analyzed by Rietveld Method and EXAFS

Rietveld refinement and extended X-ray absorption fine structure (EXAFS) analysis on the synchrotron radiation X-ray data of BaTiO3-based shell phase powder specimens with Ho and Mg co-dopants were carried out to obtain the knowledge on the occupation site and its quantitative occupancy fraction in BaTiO3 perovskite structure. Ho ions occupied preferentially A-site in BaTiO3 and the occupation ratio was same in any doped compositions. Ho─O atomic distance in A-site is increased with increasing Ho doping in contract to that in B-site.

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.