Masahito Furukawa

Enhanced piezoelectric response of BaTiO3–KNbO3 composites

The piezoelectric response of solvothermally synthesized BaTiO3 (BT)–KNbO3 (KN) composites (the nominal BT/KN ratio was 1) with distinct interfaces was investigated. The x-ray diffraction pattern showed two distinct peaks began to merge into a singular broad peak at a two-theta position between (200) and (002) tetragonal-related peaks of BT. The transmission electron microscopy observation showed a heteroepitaxial interface region between BT single-crystal particles and deposited KN crystals. The large-field piezoelectric constant was 136 pC/N, which was three times larger than that of a sintered 0.5BT–0.5KN composite. The enhanced piezoelectric response was attributed to the strained epitaxial interface region.

Preparation of Barium Titanate–Potassium Niobate Solid Solution System Ceramics and Their Piezoelectric Properties

Barium titanate (BaTiO3, BT)–potassium niobate (KNbO3, KN) solid solution system ceramics were prepared by normal sintering, and their piezoelectric properties were measured. The phase diagram of the BT–KN system was investigated by dielectric measurement and X-ray diffraction (XRD). The Curie temperature (TC) was almost constant at approximately 130 and 430 °C for ceramics with KN contents below and above 50 mol %, respectively. At room temperature, two phases, ferroelectric tetragonal and ferroelectric orthorhombic, coexisted in the composition range from 0.7BT–0.3KN to 0.1BT–0.9KN. This system was found to be a limited solid solution system that involves BT- and KN-rich phases. Their piezoelectric properties were investigated on the basis of the electric-field dependence of strain measurements. For ceramics with KN contents below 30 mol %, the apparent piezoelectric constant (d33) decreased approximately with increasing KN content, whereas above 30 mol %, the apparent d33 increased as a function of KN content.

Thermal Reliability of Alkaline Niobate-Based Lead-Free Piezoelectric Ceramics

Alkaline niobate-based (K,Na,Li,Ba,Sr)(Nb,Ta,Zr)O3 (ANSZ) ceramics were prepared by conventional sintering process, and their piezoelectric properties were measured. The Curie temperature (TC) of the ANSZ ceramic was 278 °C. The electromechanical coupling factor (kp) and the piezoelectric constant (d33) were 45.5% and 295 pC/N, respectively, and the dielectric permittivity (e33T/e0) was 1610 at room temperature. The thermal reliability was tested by 1000 cycles of thermal shock with temperatures ranging from -55 to 125 °C. The change in d33 of ANSZ was compared with that of commercial Pb(Zr,Ti)O3 after 1000 cycles of thermal shock. The phase transition behavior of ANSZ was investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC) simultaneous measurement. No appreciable endothermic peak accompanied by a polymorphic phase transition was observed on the DSC curve. The crystalline structure was investigated in detail by XRD measurement with high-energy synchrotron radiation. The coexistence of two ferroelectric phases in a wide temperature range (from -175 to 150 °C) was observed.

Poling Field Dependence of Piezoelectric Properties and Hysteresis Loops of Polarization versus Electric Field in Alkali Niobate Ceramics

The DC poling field dependence of piezoelectricity was investigated to evaluate the mechanism of domain alignment in lead-free ceramics of the form of (1-x)(Na,K,Li,Ba)(Nb0.9Ta0.1)O3–xSrZrO3 (x=0–0.07) by comparison with those of Pb(Zr,Ti)O3 (PZT), PbTiO3 (PT), and BaTiO3 (BT) ceramics. Poling was conducted at 150 °C for 30 min while varying the poling field (E) between ±4.0 kV/mm. By increasing x from 0 to 0.07, the relative dielectric constant (er), electromechanical coupling factor in the planar mode (kp), frequency constant in the kp mode (fcp), and piezoelectric strain constant (d33) vs E plots showed domain clamping at a specific E. E was the coercive field estimated from the DC poling field dependence. The changes in er, kp, fcp, and d33 with E became smaller at x=0.06–0.07, because of the proximity to the paraelectric phase. The ceramics with x=0, such as PT or BT ceramics, show similar er, kp, fcp, and d33 vs E plots. The maximum kp (48%) and d33 (307 pC/N) were obtained for x=0.05 with the lowest fcp of 2964 Hzm, as shown in the er, kp, fcp, and d33 vs E plots for ceramics such as tetragonal hard PZT ceramics. Since domain alignment in the ceramics was accompanied by the deformation of crystals as a result of applying the poling field, it was clarified that the lead-free ceramics must have a high kp and high d33 to realize a low fcp, which corresponds to a low Youngs modulus. In addition, the optimal ceramic composition was obtained in the typical domain-clamping state from the poling field dependence. Furthermore, it was found that a higher kp was realized at a larger remnant polarization and a smaller coercive field in a symmetrical hysteresis loop of polarization vs electric field (P–E hysteresis loop), because of the easy alignment of ferroelectric domains by applying a poling field. The results of the poling field dependence were also supported by results of expansion strain measurement.

High Power Characteristics of (Ca,Ba)TiO3 Piezoelectric Ceramics with High Mechanical Quality Factor

(Ca0.1Ba0.9)TiO3 + x wt % MnCO3 [x=0, 0.2, 0.5, 1.0] (CBT-Mx) ceramics were prepared by conventional sintering, and their dielectric and piezoelectric properties were measured. The mechanical quality factor (Qm) of CBT was markedly enhanced by Mn addition and reached 2400 when x=0.5. The dielectric loss tangent (tan δ) was 0.0023 at room temperature. The Curie temperature (TC) was 133 °C. Then, the high power characteristics of CBT-M0.5 were investigated. The maximum vibration velocity (v0) of CBT-M0.5 was 1.1 m/s and was higher than that of Pb(Zr,Ti)O3-based ceramics (Hard-PZT). The heat generation of CBT-M0.5 was smaller than that of Hard-PZT at the same v0. The resonant frequency (fr) of CBT-M0.5 was stable up to v0 of 1.1 m/s. Additionally, the temperature stability of fr was investigated in the practical application temperature range (from -20 to 80 °C). The amount of change in fr for CBT-M0.5 was larger than that for Hard-PZT below 10 °C.

Poling Field Dependence of Ferroelectric Properties in Alkali Bismuth Titanate Lead-Free Ceramics

The DC poling field dependence of ferroelectric properties was investigated to evaluate the mechanism of domain alignment in lead-free ceramics of the forms of (1-x)(Na0.5Bi0.5)TiO3–x(K0.5Bi0.5)TiO3 (x=0.08–0.28), 0.79(Na0.5Bi0.5)TiO3–0.20(K0.5Bi0.5)TiO3–0.01Bi(Fe0.5Ti0.5)O3 and (1-x)(Na0.5Bi0.5)TiO3–xBaTiO3 (x=0.03–0.11). From the DC poling field (E) dependence of ferroelectric properties, asymmetrical shapes in piezoelectric constants vs ±E caused by nonuniform mobility of domain alignment in the application of positive and negative poling fields were observed in the small amount substitution by (K0.5Bi0.5)TiO3 (x0.11) and BaTiO3 (x0.03). On the other hand, symmetrical shapes in piezoelectric constants vs ±E were observed in higher piezoelectricity with lower frequency constant accompanied with typical domain clamping. The effects to improve ferroelectric properties by Bi(Fe0.5Ti0.5)O3 and by BaTiO3 substitutions for (Na0.5Bi0.5)TiO3 were also proved by the measurement of P–E hysteresis loops.

Microstructure Control of Barium Titanate – Potassium Niobate Solid Solution System Ceramics by MPB Engineering and their Piezoelectric Properties

Effect of the microstructural homogeneity of 0.5 BaTiO3 - 0.5 KNbO3 (0.5BT-0.5KN) solid solution ceramics on the dielectric and piezoelectric properties was investigated. Microstructure of a sample prepared by a conventional sintering method was homogenous, and the room temperature crystal structure was assigned to cubic Pm3m symmetry and therefore the sample was paraelectric. On the other hand, microstructure of samples prepared by a two-step sintering method was inhomogeneous, that is, it was made of BT and KN grains. The large electric field piezoelectric constant d33* increased with increasing interface area.

Nanostructure Control of Barium Titanate--Potassium Niobate Nanocomplex Ceramics and Their Enhanced Ferroelectric Properties

Barium titanate (BaTiO3,BT)–potassium niobate (KNbO3,KN) nanocomplex ceramics with various KN/BT molar ratios were prepared by a solvothermal method. From a transmission electron microscopy (TEM) observation, it was confirmed that the KN layer thickness on BT particles was controlled from 5 to 40 nm by controlling KN/BT molar ratios. Their dielectric constants were measured at room temperature and 1 MHz, and the maximum dielectric constant of 370 was measured for the BT–KN nanocomplex ceramics with a KN thickness of 22 nm. TEM observation revealed that at a KN thickness below 22 nm, the BT/KN heteroepitaxial interface was assigned as a strained interface, while at 40 nm, the interface was assigned as a relaxed one. These results suggested that the strained heteroepitaxial interface could be responsible for the enhanced dielectric properties.

Enhanced Piezoelectric Properties of Barium Titanate-Potassium Niobate Solid Solution System Ceramics by MPB Engineering

Barium titanate (BaTiO3, BT) - potassium niobate (KNbO3, KN) solid solution system (0.5BT-0.5KN) ceramics with various microstructures were prepared by conventional sintering method and two-step sintering method using BT and KN nanoparticles. Their microstructures were investigated using X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM), and it was confirmed that two ferroelectric phases, i.e., BT-rich tetragonal and KN-rich orthorhombic phases, always coexisted for all ceramics, which suggested that 0.5BT-0.5KN ceramics had “pseudo-morphotropic phase boundary (MPB)” structure. Thus, the control of the interface area between two phases was important to enhance piezoelectric property. Finally, their piezoelectric property was measured, and the apparent piezoelectric constant d33* increased with increasing interface area.

Enhanced Piezoelectric Properties of Lead-Free Piezoelectric Materials by Microstructure Control

Barium titanate (BaTiO3, BT)—potassium niobate (KNbO3, KN) solid solution system (0.5BT-0.5KN) ceramics with various microstructures were prepared by two-step sintering method, and their piezoelectric properties were investigated. For 0.5BT-0.5KN ceramics, two phases, ferroelectric tetragonal and ferroelectric orthorhombic, coexisted in different grains at room temperature, owing to the limited solid solution system. The volume fraction of interface region between BT-rich tetragonal and KN-rich orthorhombic grains was controlled by sintering temperatures, and increased with decreasing sintering temperatures. Apparent piezoelectric constant d 33* was measured using slope of strain vs. electric field curves. As the results, the d 33* increased with decreasing sintering temperatures, which revealed that interface region between tetragonal and orthorhombic grains could contribute to enhancement of piezoelectric properties.